embrittlement of nylon in arid environments

Steven Bowling

28 Foxwood cct. Wakerley QLD 4154

28/5/2013

Professor D.J. Mee Head School of Mechanical & Mining Engineering University of Queensland Queensland 4072

Dear Sir,

I hereby submit my Thesis titled “Embrittlement of Nylon in Arid Environments” for consideration as partial fulfillment of the Bachelor of Engineering degree.

All the work contained within this Thesis is my original work except where otherwise acknowledged.

I understand that this thesis may be made publicly available and reproduced by the University of Queensland unless a limited term embargo on publication has been negotiated with a sponsor.

Yours sincerely

Steven Hugh Francis Bowling Student ID: 40789972

Embrittlement of Nylon in Arid Environments

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Acknowledgements

First and foremost I’d like to express my gratitude to my supervisor Assoc Prof Rowan truss,

for guiding and mentoring me throughout the duration of my thesis.

I have also appreciated the time Glenda Zemanek took teaching me to use the optical

microscope and lending me the micrometre callipers.

Also I’d like to thank Ron Rasch for the excellent SEM session and for showing me the

preparation procedures and giving me a tour of the Hawken SEM facility.

I’m also appreciative of Rob Lude for supplying the Nylon 66 samples – which provided the

inspiration and basis for this thesis.


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Summary

Nylon 66 parts have been fracturing prematurely while in service in Perth, Australia. This

thesis investigates the causes behind their failure using a deductive investigate method

similar to that used in polymer forensics. Numerous possible failure modes were explored in

the literature, then optical microscopy and scanning electron microscopy (SEM) was

performed. Results showed brittle fracture, and supported generalized embrittlement rather

than hypothesis of stress concentrations and poor design. It is likely the hygroscopic

behaviour of Nylon played significant role in their failure, although further information about

these samples and their service conditions is required to be certain.


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Table of Contents

Acknowledgements ..................................................................................................................... i

Summary ..................................................................................................................................... ii

1.0 Introduction ..................................................................................................................... 1

1.1 Background .................................................................................................................... 1

1.2 Aim ................................................................................................................................. 1

1.3 Introduction to Nylon ..................................................................................................... 1

1.3.1 History ..................................................................................................................... 1

1.3.2 Chemistry and Synthesis ......................................................................................... 1

1.4 Investigative method ..................................................................................................... 4

1.5 Part specifications and nomenclature ........................................................................... 7

2.0 Fractography .................................................................................................................... 8

2.1 Effect of water content on fracture surface ................................................................ 12

3.0 Possible Failure Modes .................................................................................................. 14

3.1 Oxidation ...................................................................................................................... 14

3.2 Hydrolysis ..................................................................................................................... 15

3.3 Hygroscopic Behaviour ................................................................................................ 17

3.3.1 Rate of moisture diffusion ..................................................................................... 17

3.3.2 Solar radiation and moisture loss .......................................................................... 23

3.3.3 Outdoor exposure tests......................................................................................... 24

3.3.5 Degradation caused by humidity .......................................................................... 25

3.4 Environmental stress cracking ..................................................................................... 26

3.5 Other Chemical attack ................................................................................................. 27

3.5.1 Smog and ozone .................................................................................................... 27

3.6 Manufacturing faults .................................................................................................... 28

3.7 Design ........................................................................................................................... 30

3.8 Ductile-to-Brittle Failure Transition ............................................................................. 31

3.9 Mechanical Overload ................................................................................................... 32

3.10 Low Temperatures ..................................................................................................... 32

3.11 Fatigue ........................................................................................................................ 33

3.12 Thermal fatigue .......................................................................................................... 34

3.13 Plasticizer loss ............................................................................................................ 34


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3.14 Thermal degradation.................................................................................................. 35

3.15 Thermal oxidative ...................................................................................................... 35

3.16 Temperature and moisture combined ....................................................................... 36

4.0 Environment .................................................................................................................. 36

4.1 Introduction ................................................................................................................. 36

4.2 Rainfall .......................................................................................................................... 37

4.3 Humidity ....................................................................................................................... 38

4.5 Solar exposure .............................................................................................................. 40

4.6 UV Radiation ................................................................................................................ 41

4.7 Temperature cycling .................................................................................................... 42

4.8 Acid Rain ....................................................................................................................... 43

5.0 Optical Microscopy Inspections..................................................................................... 44

5.1 Morphology .................................................................................................................. 44

5.2 Factors Effecting Fracture ............................................................................................ 46

6.0 Scanning Electron Microscopy ...................................................................................... 49

6.1 Summary of Procedure ................................................................................................ 49

6.2 Results .......................................................................................................................... 49

7.0 Summary ........................................................................................................................ 56

8.0 Conclusions .................................................................................................................... 56

9.0 Recommendations ......................................................................................................... 58

10.0 References ..................................................................................................................... 61

11.0 Appendix ........................................................................................................................ 69


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Table of Figures

Figure 1 Nylon Synthesis (Adilia, 2006) ...................................................................................... 2

Figure 2 Nylon 6,6 repeat unit and the secondary bonding between polymer chains .............. 2

Figure 3 Stress-Strain graph for dry and conditioned Nyon 66 (BASF, 2003) ............................ 3

Figure 4 Nylon interacting with water (Howe, 2012) ................................................................. 3

Figure 5 General steps involved in failure analysis (Jensen, 2012) ............................................ 5

Figure 6 Basic Dimensions of sample ......................................................................................... 7

Figure 7 Dimensions of backbone cross section ........................................................................ 7

Figure 8 Nomenclature labelled on unbroken sample. On the rear side of each protrusion are

2 small flanges running the length, one on each side. ............................................................... 7

Figure 9 Scanning Electron Microscope (Calvier, 2008) ............................................................. 8

Figure 10 Images show the mirror region at point ‘A’ with brittle morphology like ridges

radiating out from this origin. (Dasari, 2006) ............................................................................. 9

Figure 11 Hackle morphologies (ASM, 2003) ............................................................................. 9

Figure 12 Low magnification SEM micrographs of impact-fracture surfaces of nylon 66. The

smooth region at site “A” is the crack initiation site and the region labelled “B” represents

hackle morphology. (Dasari, 2006) ............................................................................................. 9

Figure 13Schematic of a craze (Halary et al. 2011) .................................................................. 11

Figure 14 Figure A: Brittle fracture Hackles in Nylon 66, the arrow points in the direction of

crack origin. Figure B: Ductile bands in Nylon fracture. (Dasari et al. 2008) ........................... 11

Figure 15 Left: fatigue fracture surface (Brydson, 1999) Right: fatigue crack showing beach

marks (Marissen et al. 2005) .................................................................................................... 12

Figure 16 Fracture surface of Nylon 66 at different moisture levels. (a) 0%RH (b) 50%RH (c)

100%RH (Bretz at al. 1979) ....................................................................................................... 12

Figure 17 (a) Fatigue fracture at 50%RH (b) fatigue fracture at 100%RH, the arrest lines are

believed to be striations ........................................................................................................... 13

Figure 18 Hydrolysis of Nylon (New World Encylopedia, 2008) ............................................... 15

Figure 19 Loss of tenacity of Nylon (N) fibres as a result of exposure to 10% sulphuric acid,

compared with PET (T) (ASM, 2013) ........................................................................................ 15

Figure 20 (a) Is a microscopic image of the fracture surface, “ch” stands for chamfer which

acts as a notch. “c” stands for cusp which protruded up from the sample and was torn in a

ductile manner. Image (b) is an SEM micrograph which shows the curved “arcs”. Image (c) is


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a schematic of the fracture surface, while (d) is a composite of micrograph images to provide

better resolution. (Lewis, 2006) ............................................................................................... 16

Figure 21 Dimensions of backbone cross section .................................................................... 18

Figure 22 Water absorption at relative humidity for Nylon. (Brydson, 1999) ......................... 19

Figure 23 Diffusion coefficient for Nylon at various water content. Untreated Nylon is

marked (o) ................................................................................................................................ 20

Figure 24 Nylon 55 moisture absorption rate at 23c and 40-80%RH for Nylon with a shape

factor ω=2000 (ASM, 2013) ...................................................................................................... 21

Figure 25 (BOM, 2013) ............................................................................................................. 21

Figure 26 Rate of moisture loss of Nylon over “Drierite” desiccant (DuPont, 1995) ............... 22

Figure 27Moisture gain in wt% over time (Jia et al. 2004) ....................................................... 23

Figure 28 Effect of outdoor exposure on the percentage loss of breaking strength for Nylon

(shown as blue diamond), compared with other polymers. Red triangles is polyester, while

the pink squares and green circles are UVR finished Nylon and polyester respectively. (Vikas

et al. 2010) ................................................................................................................................ 25

Figure 29 Tensile strength with ageing time for Nylon in 100% relative humidity and argon.

Plot A has a linear time scale and plot B is a logarithmic scale. (Bernstein et al. 2005) .......... 26

Figure 30 Chemicals which cause Nylon to permanently loose stiffness and become weak.

(DuPont, 2005) .......................................................................................................................... 27

Figure 31 Ozone levels at Caversham in Perth (DEP, 1996) ..................................................... 28

Figure 32 Non-hygrscopic resins only collect moisture on the outside of the pellet, whereas

hygrscopic resins collect water on the inside as well. (Shah, V. 2007) .................................... 29

Figure 33 Stress concentration factor with fillet radius (DuPont, 1995) ................................. 30

Figure 34 Dimensional changes of Polyamides with varying moisture content (Hoffman,

2000) ......................................................................................................................................... 30

Figure 35 Ductile-Brittle transition ........................................................................................... 31

Figure 36 Effect of Temperature and Moisture on the Modulus of Nylon 6 and Nylon6,6

(Intech, 2012) ........................................................................................................................... 33

Figure 39 TGA of Nylon 66(Perkin Elmer, 2011). ...................................................................... 35

Figure 40 DSC of heat stabilized polyamide 66. (Crompton, 2010) ......................................... 36

Figure 41 (BOM, 2013) ............................................................................................................. 37

Figure 42 Mean relative 9am humidity for 1981-2010 ............................................................ 38


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Figure 43 Mean daily solar exposure (MJ/m2) ......................................................................... 40

Figure 44 Mean daily sunshine hours for Perth airport. Light bars are data for 1993-2013 and

dark bars are for the year 2012 only ........................................................................................ 41

Figure 45 UV only (SoDa, 2013) ................................................................................................ 41

Figure 46 Temperature differences for Perth Airport (BOM, 2013) ........................................ 42

Figure 47 Radiation fluxes per unit area of Earth’s surface for a simple greenhouse model

(Jacob, D. 1999) ........................................................................................................................ 43

Figure 48 Fracture surface no. 1 of sample C .......................................................................... 44

Figure 49 Fracture surfaces C2, E and B ................................................................................... 45

Figure 50 Images of fracture surfaces are labelled with the designated part letters for each

piece they occurred on. ............................................................................................................ 46

Figure 51 Samples G and C with weld lines highlighted ........................................................... 47

Figure 52 Fracture surface of sample H. Fracture occurred close to the base of protrusion

rather than at the hypothesised locations of stress concentrations. ...................................... 48

Figure 53 Fracture surface C1 ................................................................................................... 49

Figure 54 SEM photo on the left shows hackle morphology, sample on the right shows much

finer ridges called mist. Both photos were taken on surface C1 with same magnification. .. 50

Figure 55 Zoom on the mirror region of surface C1 shows an inhomogeneity. ...................... 50

Figure 56 Spectral results of inhomogeneity on fracture surface C1. The spectra show the

inhomogeneity is not pure Nylon 66; rather it has minerals in it. ........................................... 51

Figure 57 Ductile region of sample C1, showing small inhomogeneity ................................... 52

Figure 58 Spectral results of inhomogeneity in ductile cups on sample C1 ............................ 52

Figure 59 Sample A fracture surface of thin protrusion. .......................................................... 53

Figure 60 Sample A ................................................................................................................... 53

Figure 61 Fracture surface C2 ................................................................................................... 54

Figure 62 Voids on the mirror region of sample E. .................................................................. 55

Figure 63 Voids in the mirror region of sample E ..................................................................... 55

Figure 64 (Toray, 2013) ............................................................................................................. 59

Figure 65 CES materials section chart ...................................................................................... 59

Figure 66 CES materials selection chart ................................................................................... 60


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1.0 Introduction

1.1 Background

Nylon 6,6 clips used near a road close to Perth airport, Australia have been fracturing

prematurely during service. Rob Lude, the director of Plastic Essentials has sent 8 fractured

samples along with 3 unbroken samples for investigation.

1.2 Aim

The aim of this thesis is to investigate the cause/s of failure behind the fractures of these

Nylon 66 samples. Ideally the cause should be determined, or narrowed down as far as

possible based available information about these samples, resources, time and information

about Nylon failures in general.

1.3 Introduction to Nylon

1.3.1 History

Nylon was discovered in 1934 by Wallace Hume Carothers (1896–1937) while he was

working at DuPont (Smith, 1985). Instead of using the glycols and forming polystyrene,

Carothers and his team used amides in place of the glycols and ended up with polyamide.

Carothers boss, Elmer K. Bolton played a key role in the discovery and later came up with the

name – “Nylon”. Nylon started to be produced industrially by 1938, and the new Nylon

stockings were a sensation at the World’s Fair (CHF). So successful they were that years later

in 1945 riots broke out when DuPont was unable to keep up with demand (Smith, 1985).

1.3.2 Chemistry and Synthesis

The term “Nylon” refers to thermoplastic polymers from the polyamide family. Nylon 6,6 is

copolymer, its two monomers each contain 6 carbon atoms which what the “6,6”

designation refers to. As it’s a polyamide it’s made from an amide – hexamethylene diamine

- and the other monomer can be adipoyl chloride or Adipic acid. The two monomers

combine in a polycondensation reaction, the condensate depends on the second monomer

used, for example the reaction with adipoyl chloride results in an HCl condensate. This


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process is a step-growth polymerization, although nylon can also be made using chain-

growth polymerizations; in which case lactams would be used (Odin, 1981)

Figure 1 Nylon Synthesis (Adilia, 2006)

When the two monomers react, a pair of electrons on the nitrogen attacks the carbon on the

part of the carboxylic acid group (Aharoni, 1997). A new dimer forms which then reacts with

more amine and acid chloride monomers and so on the chain grows (Taylor, 2003).

The amide unit on the chain plays a very important role in the materials properties. The

nitrile groups are highly polar so strong intermolecular forces develop in the form of

hydrogen bonding between chains as shown in figure 6. This results in increased strength,

crystallinity and glass transition temperature.

Figure 2 Nylon 6,6 repeat unit and the secondary bonding between polymer chains

http://www.wellesley.edu/Chemistry/Chem105manual/Lab06/lab06.html


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The hydrogen bonding of the amide group is not only Nylons strength, but its Achilles heel as

well. The consequences of it could potentially be very relevant to this thesis. Water is also a

polar molecule and will hydrogen bond to these nitrile groups; this makes Nylon hygroscopic

(Snape, 2008). The absorbed water molecules increase the free space in the polymer matrix

– allowing the chains to move about more freely – so the water essentially acts as a

plasticizer. Nylon may pass tests for ductility in a humid test facility, but then when shipped

to dry climates for use can become brittle over time as the water migrates out. Nylon can

absorb up to 9% of its weight in water (Matweb, 2012) so its effect on the polymer poses a

challenge to manufacturers and users of nylon. The rate of moisture absorption is time,

relative humidity and temperature dependant. The effect of moisture on Nylon is so

pronounced that materials data sheets usually have two values for Nylon mechanical

properties – one for dry and one for wet (conditioned) (Snape, 2007).

Figure 3 Stress-Strain graph for dry and conditioned Nyon 66 (BASF, 2003)

Figure 4 Nylon interacting with water (Howe, 2012)


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1.4 Investigative method

The ideal approach to a case such as this is the investigative methods used in polymer

forensic engineering because it avoids subjective assumptions and instinctual bias in favour

of critical analysis of all possible options and deductive reasoning to derive conclusions.

Analogously, a criminal investigator must question all suspects rather than simply trying to

prove the investigators instinctual suspicions.

Polymer forensics is the investigation or study of a polymer failure which led to loss of

productivity, damage to the environment, property, harm to people etc. The goal is to

determine the cause of why the product failed, did not meet specifications or did not

perform as intended. Similar to criminal forensics, polymer forensics seeks to find, collect

and analyse evidence which might provide a clue as to the suspect failure mode. (Neil, 2012)

It is often used in industry to settle disputed between two or more parties; disputes can

arise when a product or structure fails and results in loss to either party. For this reason it is

often practiced by insurance companies evaluating claims, or from attorneys in liability suits.

(Thompson, 2012)

Research of similar failures is important to this investigative method as it can provide useful

insight into cause, although no two failures are ever exactly alike. In this case the failure did

not lead to catastrophic consequences such as loss of life, or even significant productivity

loss – but to determine the failure the same methods will still be applied in order to

rigorously determine the cause (Lewis, 2010).


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Figure 5 General steps involved in failure analysis (Jensen, 2012)

Therefore order to best ascertain the cause of failure, the failure analysis should be

conducted in a deductive procedure rather than simply running with the most intuitive

cause. It is best to consider all possibilities and then eliminate each one until the most likely

cause/s remains. This way investigators can be certain the process has been thorough. In

order to structure this deductive logic, a basic fault tree was constructed as part of the

preliminary report shown on the following page.


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1.5 Part specifications and nomenclature

An outline of major dimensions is provided in figure 6. The backbone has a “T-shaped” cross

section with dimensions in figure 7. A digital vernier calliper: Mitutoyo Absolute Digimatic

was used for measurements.

Figure 6 Basic Dimensions of sample

Figure 7 Dimensions of backbone cross section

The nomenclature used to describe different parts of the samples is shown in figure 8.

Figure 8 Nomenclature labelled on unbroken sample. On the rear side of each protrusion are 2 small flanges running the length, one on each side.


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2.0 Fractography

Fractography of polymers is a relatively new field which involves the study of fracture

surfaces as a critical tool for failure analysis (Parrington, 2002). Examining material fracture

surfaces can reveal information about failure mechanism, location of origin of fracture,

sequence of events, directions of crack propagation, environmental interactions, material

defects and information about stresses which occurred in the material. Hence, fractography

is vital for this thesis. Factors which can significantly affect fracture morphology in

thermoplastic polymers include: loading rate, temperature, moisture content and degree of

crystallinity. (Greenhalgh & Hiley, 2007)

Common analysis techniques include optical microscopy (OM), scanning electron microscopy

(SEM) and transmission electron microscopy (TEM) - for greater resolution of fine structures.

Nowadays modern SEM’s have sufficient resolution to study the fine structure of surfaces

which used to be studied with TEM. Atomic force microscopy (AFM) may also be used to

provide high resolution without the use of a vacuum. (Sawyer et al. 2008)

Figure 9 Scanning Electron Microscope (Calvier, 2008)

A few main macroscopically evident regions are worth studying for comparison to the

micrographs obtained in the lab (section 5). The smooth mirror region surrounding the crack

initiation site is often called a “mirror” region and is from the slow crack growth phase.


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Adjacent to this is the mist region, where the crack transitions from slow to fast growth and

begins to branch, this is evidenced by fine ridges or “mist”. (Parrington, 2002)

Figure 10 Images show the mirror region at point ‘A’ with brittle morphology like ridges radiating out from this origin. (Dasari, 2006)

As the crack accelerates and propagates further away from the initiation site, ridges or

hackles are formed; generally the fracture surface becomes rougher away from the point of

origin (Sawyer et al. 2008). These ridges are an important morphology for this investigation

because they radiate out from the fracture origin, thereby revealing its location. In SEM

image on the right of figure 10 it is easy to see these lines radiating out from the origin at

point ‘A’.

Figure 11 Hackle morphologies (ASM, 2003)

Figure 12 Low magnification SEM micrographs of impact-fracture surfaces of nylon 66. The smooth region at site “A” is the crack initiation site and the region labelled “B” represents hackle morphology. (Dasari, 2006)


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The basic fracture mechanics description is,

√

Where KIC is the fracture toughness, and Y is the geometry factor, σf is the stress at fracture

and ac is the critical crack size. The crack size at the moment of instability is expected to be

smaller at high stresses than at low stresses, this means the size of the mirror region is

inversely proportional to the square of the fracture stress. Theoretically the magnitude of

stress at fracture could be approximated by measuring the size of the mirror region (ASM,

2003) – however in practice the effects of temperature and environment would have to be

known and accounted for. This is not possible for this thesis as the times of fractures were

not recorded, so local environmental conditions at the exact time of fracture remain

unknown. Still this theory is worth considering when examining the fracture surfaces.

The fracture strength is dependent on material flaws, and as no two samples are exactly

alike it is not a well-defined bulk property and the flaws mean theoretical strength limits are

never attained (Strobl, G. 2007). Modelling the fracture of polymers can be quite complex, it

depends on intermolecular forces such as van der Waals, intermolecular covalent forces, and

crystallinity. Although with an estimate of bond energies and intermolecular forces,

estimates of theoretical strengths can still be attained (Strobl, G. 2007)

In the centre of the mirror region is a small inhomogeneity which can provide the crack

nucleation site (Ehrenstein, 2008). The mechanism for fracture in glassy thermoplastics is

called crazing. Despite being macroscopically brittle, on the microscopic level polymers are

ductile. This is fortunate because crazing means the toughness is not simply dependent on

overcoming the energy barrier to from new surfaces. When the craze begins to form,

polymer fibrils across the new surface support the load. Voids form at the tip of the crack,

and the material between the voids is drawn – analogously to many tiny tensile tests – each

of them absorbing energy as they are drawn out. The final fracture will take the path of least

resistance: down mid-rib of the craze. These crazes are thin, planar defects so they form a

very smooth flat region described above as the mirror region; in this region remnants of the

craze may be found. (ASM, 2003)


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Figure 13Schematic of a craze (Halary et al. 2011)

Figure 14 Figure A: Brittle fracture Hackles in Nylon 66, the arrow points in the direction of crack origin. Figure B: Ductile bands in Nylon fracture. (Dasari et al. 2008)

Ductile polymer morphology is usually not as smooth, but has tearing marks and “cups”.

Sometimes fibrils can be seen protruding from ductile ridges, a remnant from the tear. It is

often clearly visible without microscopic inspection due to stress whitening.

Discoloration in polymers can have a few causes; white discolouration is probably caused by

crazing or crystal formation from cold drawing (ASM, 2003). As the sample is cold drawn

polymer chains can line up and crystals grow which refract light differently and cause

discoloration. Small crystalline regions can also align up and the many boundaries between

amorphous and crystalline regions will scatter light and increase opacity. The other

explanation is crazing, when a semicrystalline polymer such as this is cold drawn the

amorphous spaces in-between crystals are strained causing small cracks or fissures to form.

This causes density differentials to increase, and along with it the bending and scattering of

light. Our perception of this reflected light is a “whitening” of the sample. (Rostaro et al.

2000).

http://www.sciencedirect.com/science/article/pii/S0008622309001201


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In contrast to these morphologies, a mechanical fatigue fracture surface would be more

regular, consisting of “beach marks” or “clam shell marks” due to the arrest lines from

repetitive loading (Brydson, 1999).

Figure 15 Left: fatigue fracture surface (Brydson, 1999) Right: fatigue crack showing beach marks (Marissen et al. 2005)

2.1 Effect of water content on fracture surface

A study by Bretz et al. 1979 made some interesting comparisons between Nylon 66 brittle

fracture surfaces for various moisture contents. Their findings are potentially useful because

the moisture content of the samples at time of fracture is unknown. Perhaps their

Fractography can shed light on how the moisture content effect fracture morphology

beyond the intuitive expectations due to moistures plasticizing effect. (Bretz et al. 1979)

Figure 16 Fracture surface of Nylon 66 at different moisture levels. (a) 0%RH (b) 50%RH (c) 100%RH (Bretz at al. 1979)

Their studies focussed on fatigue, they found fatigue crack growth rates for Nylon in

equilibrium with 50%RH to be 2.5 times slower than for those at 0% or 23%RH.

They found that tightly bound water, with a maximum of one water molecule per 2 amide

groups, toughens the Nylon; while loosely bound water (saturation) weakens it. They noticed


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a transition from terminal unstable crack growth for water contents less that 3wt% — to a

tearing mode of failure at saturation. They discovered in their fatigue tests that samples in

0%RH, 23%RH and 50%RH failed via rapid unstable crack propagation, while the sample in

100%RH failed in a more stable manner, albeit still rapidly.

Figure 17 (a) Fatigue fracture at 50%RH (b) fatigue fracture at 100%RH, the arrest lines are believed to be striations (Bretz et al. 1979)


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3.0 Possible Failure Modes

This section contains a review of current literature on the major possible failure modes, and

some investigation, discussion and calculations to complement and relate the literature to

this case.

3.1 Oxidation

Ultra-violet rays can cause free radicals to form which react with oxygen and lead to chain

scission and subsequently loss of molecular weight. This alters the mechanical properties of

the polymer, often leading to loss of strength, impact resistance and embrittlement.

(Massey, 2007)

Heat will also cause degradation via a similar mechanism; it’s quite possible that during

moulding the polymer melt was overheated for example. Anti-oxidants are added usually to

polymers to prevent oxidation; although it’s possible for these to fail or not be added by

mistake. These samples do appear to contain carbon black, an additive which improves UV

resistance and weathurability (Wellman, 2009).

To determine if the Nylon has oxidised, a characterisation technique such a Fourier

Transform Infra-red spectroscopy (FTIR) can be used. Oxidation will produce bonds like C=O

which will show up as an extra peak on the IR spectra. Another way to detect if UV radiation

has led to chain scission and loss of molecular weight is with a simple melt viscosity test, the

longer the polymer chains the greater the entanglements and hence viscosity.

Studies have shown the presence of TiO2 considerably enhances the degradation of Nylon in

sunlight (Lock, 1973). Titanium dioxide is a naturally occurring oxide with a wide range of

applications from paint to sunscreen. It’s plausible a worker has sunscreen residue on their

hands after applying it, and then proceeded to handle the Nylon parts – perhaps during

installation.


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3.2 Hydrolysis

Most step-growth polymers will hydrolyse in water; this reaction is presented in figure 18.

Nylon 66 is particularly susceptible to hydrolysis in the presence of an acid, so Nylon

products will fail by fracture when exposed to even minor quantities of acid. The acid

depolymerizes the Nylon meaning the molecular weight rapidly drops, and along with it the

toughness. Cracks then form and the product fails (Lewis, 2006).

Figure 18 Hydrolysis of Nylon (New World Encylopedia, 2008)

Figure 19 Loss of tenacity of Nylon (N) fibres as a result of exposure to 10% sulphuric acid, compared with PET (T) (ASM, 2013)

There is a relatively famous polymer forensics case in which a leaky battery dripped acid on

the fuel line of a van and the fuel pipe was hydrolysed. The broken pipe started leaking

diesel fuel onto the road, which caused the following car to lose traction and slide, it collided

head on with a semi-trailer and the female occupant sustained serious injuries. (Lewis, 2006)

(a) (b)


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Figure 20 (a) Is a microscopic image of the fracture surface, “ch” stands for chamfer which acts as a notch. “c” stands for cusp which protruded up from the sample and was torn in a ductile manner. Image (b) is an SEM micrograph which shows the curved “arcs”. Image (c) is a schematic of the fracture surface, while (d) is a composite of micrograph images to provide better resolution. (Lewis, 2006)

Figure 20(c) displays a schematic of the fracture surface. The crack originated near the

erosion zone at the point marked “o”, then travelled around leaving curved marks called

arcs. These are similar to beach marks found in fatigue surfaces, each successive arrest in the

crack front leaves a new mark. The larger arcs were from engine start-up while the smaller

ones from engine movement during vehicle operation. The crack fronts met on the other

side of the sample and a single crack advanced to the tip of the cusp until the pipe hung on

by only a small tab, this then tore in a ductile fashion.

The samples for this thesis were located near a road, so battery acid is one possibility

(especially after an accident has occurred) although acid can come from a variety of sources

including cleaning products, industrial chemicals and even acid rain.

A study into polyamide degradation found acid rain to cause changes in Nylons breaking

strength, elongation and molecular weight losses. They also observed a synergistic effect

between light and acid rain, leading to even greater loss in breaking strength and molecular

weight as a result of chain scission (Kyllo, K. 1984). (See section 4.8 for acid rain in Perth)

(c) (d)


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3.3 Hygroscopic Behaviour

As covered in the introduction (section 1.3) Nylon is a hygroscopic material; this is due to the

presence of hydrogen bonds (Miri et al. 2006). When in humid conditions Nylon will absorb

up to 9% of its mass in water; this water acts as a plasticizer. Then when the nylon is in an

arid environment, this water will migrate out over time thus leading to embrittlement. This

section focuses mainly of diffusion rates and diurnal humidity fluctuations, though moisture

is a common theme throughout this thesis.

3.3.1 Rate of moisture diffusion

Data abounds on rate of moisture sorption and desorption for Nylon, however the rate is

dependent on a couple of factors including relative humidity and equilibrium moisture

content. So there is a large variation in rates found in literature, furthermore is also

dependant on the specific part due to the effects of shape factor (surface area to volume

ratio), so generic literature values can be of little use.

Equations and constants have been developed to help estimate rate of water absorption for

Nylon 66 components.

Where D is the diffusion constant (m2/s) and C is the water present at a distance x from the

samples surface at time t.

The water absorption rate (%) after a given time t, is given by:

( )

√ √

Where is the equilibrium water absorption, V (m3) is the volume and S (m2) is the surface

area of the sample.

The time to reach equilibrium water absorption is given by,

(

)

Where is the shape factor,


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S is the surface area and V is the volume.

Figure 21 Dimensions of backbone cross section

The thicker section of the sample with the “T-shaped” cross section will be used for

calculations. Due to the cross section remaining uniform throughout the length, in a T-

shaped prism, the shape factor can be obtained easily from cross sectional area and

perimeter using micrometre callipers as,

and still retains the shape factor units of m-1.

The diffusion coefficient (D) for Nylon 66 at 20°C and 60%RH is 3.48×10-11m2/s (Toray, 2013),

and the equilibrium moisture content is approximately 2.5wt% (see figure 22). D is actually a

function of moisture content, however a single value is used in the literature by the slope of

the initial linear region of a Q/Q E vs. √ ⁄ , where Q is the mass of water lost or gained by

time t, over thickness L. Q E is for equilibrium water mass (Asada, T. 1963; Kawasaki, K. 1964)


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Figure 22 Water absorption at relative humidity for Nylon. (Brydson, 1999)

The time to reach 80% of equilibrium water absorption with a surrounding environment is

therefore given by,

( )

( ) ⁄

The time to reach 100% equilibrium (2.5wt %) is therefore 12.35hrs. This is unexpectedly

rapid, further calculations show this is primarily due mainly to the high shape factor. If a

shape factor value of 500 is trialled then time to reach full equilibrium works out to be 40hrs.

In other words, these samples gain moisture rapidly due to their high surface area-to-volume

ratio (as they are small and have complex geometry).

Another reason for the fast diffusion rate is the diffusion coefficient varies amongst the

literature; the value used above from the materials supplier was quite high. Kawasaki and

Sekita, 1964 found a much slower diffusion rate of ⁄ or roughly

⁄ (assuming average moisture content of 2wt %).


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Figure 23 Diffusion coefficient for Nylon at various water content. Untreated Nylon is marked (o) (Kawasaki, K. Sekita, Y. 2003)

If their value is used then the time to reach 80% equilibrium works out to be,

( )

( ) ⁄

Clearly there is massive variation in rates of absorption and desorption in the literature,

beyond even that expected from varying shape factors. The diffusion coefficient from the

materials suppliers is lower than most, and its possible they are bias about a product they

want to sell. Despite Kawasaki and Sekita to giving a more rigorous justification for their

coefficient values, theirs remains abnormally high. Tests on these specific parts would need

to be performed in order to be certain of desorption rates.


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Figure 24 Nylon 55 moisture absorption rate at 23c and 40-80%RH for Nylon with a shape factor ω=2000 (ASM, 2013)

The first set of results show that Nylon 66 with a high shape factor will re-absorb water from

the environment in a matter of hours. Humidity data recorded by BOM at 10min intervals

throughout the day was used to plot the chart in figure __. A polynomial of the 6th order was

used as a trend line.

Figure 25 (BOM, 2013)

Evidently humidity fluctuates with the diurnal cycle. This curve is typical of humidity in Perth;

a similar sinusoidal cycle repeats itself, although the curve will shift up or down depending

on the month and the period will change.

20

30

40

50

60

70

80

90

100

110

0:00 8:00 16:00 0:00

%RH

Time of Day

24hr Humidity for 20/05/2013


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The rate of moisture loss is not equal to the rate of absorption, even for thin samples and

the use of a desiccant the rate of loss is still measured in days (figure __). Additionally, the

high shape factor of these parts would mean they lose moisture more rapidly as well.

Figure 26 Rate of moisture loss of Nylon over “Drierite” desiccant (DuPont, 1995)

This means the Nylon does not dry out and crack from a diurnal cycle, it needs extended dry

periods to loose moisture. Even then, one 5-hour period of rain and the sample can return to

80% equilibrium.

If Kawasaki and Sekita’s results are more accurate, then the yearly humidity would need to

be considered. Mean annual humidity for Perth also follows a sinusoidal fluctuation,

although the wave period is year. On average the ‘dry’ months are about half of November

and March, and all of December, January and February.

In Perth the summer months have low humidity, and January and February average only 2-

3days of rain for the month (see section 4.2); so based on these calculations and data it is

both possible, and most likely these parts failed in the summer months.

When the correct diffusion coefficient for these samples is obtained, further comparisons

with climate can be made.


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Figure 27Moisture gain in wt% over time (Jia et al. 2004)

It must be noted that other factors will affect the rate of moisture loss including

temperature, sunlight and even wind. Also because the samples are black, sunlight can raise

their temperature above ambient and accelerate moisture loss beyond that which is

expected of the temperature recordings from weather stations.

3.3.2 Solar radiation and moisture loss

As sunlight hits the polymer samples, their temperature is elevated; this in turn drives

moisture out more rapidly. The heat transfer, Q, from radiation is given by,

Where σ is the Stefan-Boltzmann constant, T is the absolute temperature, A is the surface

area and ε is the emissivity. Polymers on average have an emissivity of 0.91, although the

carbon black filler in these Nylon parts would make their emissivity around 0.95 (Beardmore,

R. 2013). This means most of the solar radiation is absorbed. To calculate more accurately,

shape factor F, must be taken into consideration, along with the radiation lost from

remittance,

So the net gain the absorbed minus the emitted. There would also be loss via convection. In

addition to this, sunlight dries the outer surface of the polymer thereby creating a gradient

to drive diffusion towards equilibrium. In overcast regions of the world, condensation such

as dew may remain on the polymer longer and prevent or reverse drying.

44

BBBAAABABB TATAFQ


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Contrariwise, the higher temperatures from sunlight will soften the polymer. So the carbon

black can help prevent fracture from low temperature.

3.3.3 Outdoor exposure tests

Nylon degrades from heat, UV rays and other elements. Usually lab tests isolate one or two

factors and study those; however multiple factors can interact in a complex manner, so

analogously the whole is not the sum of its parts. Outdoor exposure tests are used to study

the combined effect of degradation in order to more closely model reality. Assuming

ordinary atmospheric conditions (23°C/60%RH), the equilibrium water absorption term in

the table is 2.5% for nylon 66.

Category Unit

Type Nylon 66

(CM3001-N)

Exposure time 0 years 1 year 1 year and 9 months

Tensile strength MPa

In a dry state 80 67 56

Equilibrium water absorption

50 48 46

Tensile breaking elongation

%



200＜ 35 30

Flexural strength MPa



65 62 -

Flexural modulus GPa

In a dry state 2.8 2.6 2.4


1.4 1.4 1.4

Izod impact strength

J/m

In a dry state 50 40～10 50～30


300 - -

Rockwell hardness R

scale



100 104 103

Table 1 Nylon 66 properties for exposure durations of wet and dry samples (Toray, 2013)

While most of the properties in this study remained relatively constant, one surprising find

was wet Nylon 66 looses 82.5% of its tensile breaking elongation in the first year, but in the

following 9 months this only climbs to 85% loss of original (Toray, 2013). So degradation

plateaus somewhat after the first year. Ideally longer studies should be done, although there

is an issue of practicality for experiments spanning many years.


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Other studies support the view of a plateau, one theory for this prosed by Vikas et al. (2010)

is that the outer layer degrades then has a shielding effect, preventing the inner material

from degrading to a similar extent.

Figure 28 Effect of outdoor exposure on the percentage loss of breaking strength for Nylon (shown as blue diamond), compared with other polymers. Red triangles is polyester, while the pink squares and green circles are UVR finished

Nylon and polyester respectively. (Vikas et al. 2010)

3.3.4 Miscellaneous effects of water

Nylon is subjected to stress ageing, whereby if help at a stress below the yield point a

temporary hardening occurs. Bubeck and Kramer 1971 found the kinetics of stress ageing to

be markedly increased with moisture content. They also found the breakup of the stress

aged structure is accelerated by moisture. (Bubeck, R. Kramer, E. 1971)

Water will also affect Nylons acoustic, electrical, chemical and optical properties (Jai, N.

Kagan, V. 2001)

3.3.5 Degradation caused by humidity

In a catch-22, Nylon degrades via a hydrolysis pathway from extended exposure to high

humidity. This is the reverse of the synthesis reaction, and is widely accepted as the known

‘textbook’ pathway for humidity degradation.

An accelerated ageing study using a time-temperature superposition and Arrhenius plot

found the rate of loss of tensile strength of Nylon in 100%RH to be significantly higher than

thermal-oxidative ageing. The study also revealed a link between oxidation and hydrolysis

previously unknown to researchers. When both oxygen and humidity were present the rate


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of degradation was considerably higher than for humidity alone (in the presence of argon).

(Bernstein et al. 2005)

Figure 29 Tensile strength with ageing time for Nylon in 100% relative humidity and argon. Plot A has a linear time scale and plot B is a logarithmic scale. (Bernstein et al. 2005)

3.4 Environmental stress cracking

Environmental stress cracking (ESC) mode of failure accounts for 15-30% of all thermoplastic

failures, making it the most common form of unexpected thermoplastic brittle failures

(Mark, 2004). ESC can cause the polymer to fail at stresses substantially lower than the

fracture or yield stress. Solvent can enter a small crack where it is absorbed and softens the

crack tip, thus allowing the crack to advance more easily and propagate to failure (Young,

2011). Typical chemicals which can cause ESC in Nylon include; acetone, methanol,

cyclohexanone, n-butanol (Wright, D. 1996), detergents (Bounamous, B. 2009), LiCl and LiBr

(Kenney, M. 1985). Nylons also become brittle in the presence of metal halide solutions such

as calcium, magnesium, lithium and zinc, chlorides, bromides, iodides and perchlorates

(Wright, D. 1996).

If the fracture surfaces were uncontaminated from handling then a sample could be taken

from the crack tip and tested for the presence of unwanted solvents. The environment in

which the Nylon was used could also be examined for any potential chemicals.


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3.5 Other Chemical attack

Exposure to certain chemicals can lead to temporary or permanent loss of mechanical

properties. Chemicals which Nylon 66 is not resistant to include: Chromic acid, Ethanoic acid,

Potassium permanganate, Phenol, Nitric acid and Hydrogen peroxide.

Figure 30 Chemicals which cause Nylon to permanently loose stiffness and become weak. (DuPont, 2005)

Nylon will also experience a temporary loss of stiffness, which is regained upon evaporation

of the following common solvents: acetone, ethyl alcohol, ethylene dichloride, isopropyl

acetate, methyl alcohol, methyl acetate, methyl ethyl ketone and trichloroethylene.

(DuPont, 2005)

3.5.1 Smog and ozone

A study by Mead et al. (1982) into the effects of humidity, smog and ozone of Nylon 66

found Nylon exposed to smog for six months lost 63% of its tensile strength compared to 7%

loss in ozone (Mead et al. 1982).

Perth is notorious for smog; thick smog has grounded planes at Perth airport (Rimrod &

Pickard, 2011) near where the samples were in service. The department of environmental

protection and the state energy commission of Western Australia carried out a 4 year study

called the Perth photochemical smog study (PPSS). The study found Perth’s daily smog is


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closely linked to an offshore wind pattern, where the sea breeze causes recirculation of

pollutants over Perth (DEP, 1996).

Phil Morgan from the department of environmental protection claims Perth’s air is among

the worst in the world (Loweth, A. 2000). Mr Morgan claims that during the summer, the

levels of photochemical smog occasionally breach the National air quality standards.

Photochemical smog is a noxious mixture of Aldehydes, Nitrogen oxides, Peroxyacetyl

nitrates (PAN), Tropospheric ozone and volatile organic compounds. These are all highly

reactive and oxidising. (DEP, 1996)

Figure 31 Ozone levels at Caversham in Perth (DEP, 1996)

The nearest monitoring station for the PPSS was Caversham, approximately 5kms from Perth

airport. Figure 31 shows ozone fluctuations over the year, peaking in the summer

3.6 Manufacturing faults

There are a wide range of manufacturing faults that could cause embrittlement. One way to

eliminate manufacturing would be to get in contact with the manufacturer and discuss if

there have been other faults with the batch, testing of other samples in the batch could be

performed or data could be gathered on the performance of other batches in the same

environment.

If the evidence is pointing to a manufacturing fault these are a few causes to consider which

include;

Contamination

Re-grind levels

Low mould temperatures or high

Lack of moisture conditioning

High moisture content in the resin

Weld lines


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melt temperatures

Incorrect cooling channels

Screw retraction time can effect

homogeneity of toughness

Poor quality raw material (resin)

Small gate

Voids which can cause loss of

structural integrity of the part

(DuPont, 1995 )

The hydroscopic behaviour of Nylon means drying prior to moulding is particularly

important. For non-polar polymers such as PVC, PE or PS they have a low affinity for

moisture so will only contain moisture on the surface. Conversely Nylon pellets will rapidly

absorb moisture in a matter of hours. If moisture is trapped in the resin pellet, and not

completely removed by the dryer prior to moulding then two major issues can arise. The first

is void, bubble and splay deformities; these are generally caused by trapped gas. The gas can

be air, water trapped in the pellet, volatiles from the resin, decomposing by-products or in

the case of a vacuum void there may not be any gas. If water is trapped in the resin pellets,

then heating during moulding could cause the water to vaporize and form a gas bubble in

the melt which solidifies upon cooling. (Bozzelli, 1986)

Figure 32 Non-hygrscopic resins only collect moisture on the outside of the pellet, whereas hygrscopic resins collect water on the inside as well. (Shah, V. 2007)

The other major problem is hydrolysis, to which Nylons are particularly susceptible. If the

material is processed while still wet, then the condensation polymerization reaction which

formed the Nylon can reverse itself. This results in a loss of molecular weight, as longer

chains are broken down into smaller molecules, this in turn results in severe loss of

mechanical properties. This process is not reversible, meaning re-grinding and re-moulding

the material will not recover it.


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3.7 Design

If the design incorporates a small fillet radius it produces a sharp corner in which stress

concentrates. Stress concentration factor is a function of fillet radius over thickness. Nylon

has a high notch sensitivity (Matweb, 2012) so this is especially important when designing

Nylon parts.

Figure 33 Stress concentration factor with fillet radius (DuPont, 1995)

Another design issue is the placement of weld lines in areas of high stress. Weld lines occur

when two solidifying melt fronts meet. The weld line is an area of weakness as the polymer

chains do not interpenetrate well, so they must be carefully positioned. Finite element

analysis could be performed to determine if these line up with areas of stress concentration,

and moulding software used to reposition them.

Loss of water content causes shrinkage which can sharpen a tight fillet radius even further.

Dimensional change can be significant for Nylon 66, reaching 2% dimensional change at

7wt% moisture content.

Figure 34 Dimensional changes of Polyamides with varying moisture content (Hoffman, 2000)


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3.8 Ductile-to-Brittle Failure Transition

The ductile and brittle failure stresses of Nylon both decrease over time, although the brittle

failure stress decreases more rapidly than the ductile failure stress. This produces a

transition region where the dominant failure mode switches from ductile to brittle.

Figure 35 Ductile-Brittle transition

It is possible the Nylon samples have crossed this transition region, and this could be the

cause of their brittle fracture. Another theory is that moisture effect this transition, this is

plausible because water affects almost every aspect of Nylons mechanical behaviour. For

example, lack of moisture in the Nylon could shift the line down or left so the transition to

brittle occurs after less time in service.

Unfortunately after extensive searching, a ductile to brittle transition graph for Nylon 66

(like the one in figure 35) was not found. Constructing one of these charts for Nylon 66 and

investigating the effect of relative humidity or moisture absorption would help prove or

disprove this failure theory. However this is not straightforward and would require time and

extensive experimental resources. Furthermore, the time that samples spent in service is

unknown.

Due to the long time required to perform ageing tests at service temperature, ageing may be

accelerated at high temperature then time-temperature superposition used to generate long

term data by shifting the curve. Common shift factors include Williams-Landel-Ferry and

Arrhenius law.


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3.9 Mechanical Overload

Failure can often be a simple case of subjecting the components to stresses greater than

they were designed for. The stresses in service can be difficult to predict so usually a safety

factor in incorporated, though this still may not compensate for excessively high stresses in

service.

To model areas of high stress and behaviour under load, it is recommended that a 3D CAD

model should be obtained or drawn and then input into a finite element analysis software

such as strand7. When load is applied during modelling, areas of high stress can graphically

displayed using colour coding. Based on the design of parts and most likely loading

scenarios, this engineer supposes that stress would concentrate in the two prongs, as they

have less cross sectional area on which to distribute load.

The usual failure mode for Nylon under high stress is yielding, so the brittle nature of these

fractures would still need to be explained.

3.10 Low Temperatures

Polymers like Nylon lose their ductility at low temperature and become brittle. Nylon 6 has a

glass transition temperature (Tg) of roughly 50°C. This means it gains much of its modulus

below this temperature, so during service at room temperature Nylon is in its glassy state.

However as the temperature keeps falling further below the glass transition, the modulus

still keeps increasing slightly as well. If the modulus is too high then the polymer is still and

inflexible so could fracture in a brittle fashion. Section 4 outlines temperatures for Perth

airport.


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Figure 36 Effect of Temperature and Moisture on the Modulus of Nylon 6 and Nylon6,6 (Intech, 2012)

3.11 Fatigue

The main types of fatigue in polymers are mechanical and thermal. Mechanical fatigue can

occur when Nylon is subjected to a lifetime of loading cycles. For Nylon, the lifetime is

mainly determined by the time under load, rather than the number of loading cycles

(Scheirs, 2000).

Jai and Kagan (2001) found the absorbed moisture to decrease fatigue crack growth rates,

they claim this is due to water enhancing chain mobility.

Conversely Toray (2013) found the endurance limit for Nylon 66 is actually lower at higher

moisture contents; table 3 shows a reduction in 30Hz endurance limit of 60% for Nylon at

2.5wt% moisture content.

Moisture content (wt%)

Temperature (°C)

Endurance limit

(MPa)

0 73 35

2.5 73 21

Table 2 (Toray, 2013)

Exposure to seawater and other salt solutions can affect the fatigue life of Nylon 66;

researchers found a 10% loss of strength compared with ambient air environments (Kenny et

al. 1985). Perth sits on the coast and experiences regular sea breezes, although contact with

salt water unknown.


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Rajeesh et al. provided an interesting schematic for crack propagation and molecular

orientation when comparing effect of humidity on the flexural fatigue and indentation

hardness of Nylon 6. Greater moisture contents increase polymer chain mobility which helps

inhibit crack propagation.

Figure 37 (Rajeesh et al. 2009)

3.12 Thermal fatigue

When a polymer is respectively stressed, such as in cyclical loading, some of the inelastic

deformation energy is transformed into heat and the temperature climbs. If the rate of heat

transfer to the surroundings is less than the rate of generation then the materials properties

can decline with raising temperature until it fails (ASM, 2003). Because Nylon is a

thermoplastic, the heat would most likely just raise it above the glass transition temperature

and make brittle fracture even less likely.

However changes in temperatures from diurnal cycles or other means could produce

residual stresses in the material which can accelerate premature fracture.

3.13 Plasticizer loss

The plasticizer added to the nylon may have degraded or leached out, leaving the Nylon

brittle and susceptible to cracking. No information is available at this point on the type of

plasticizer used in these samples.


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3.14 Thermal degradation

Overheating Nylon during moulding will cause crosslinking to occur and produce involatile

char (Holland, B. 2000). Thermalgravimetric analysis (TGA) of Nylon 66 shows it undergoes

thermal degradation beginning at 480°C (Perkin Elmer, 2011).

Figure 38 TGA of Nylon 66(Perkin Elmer, 2011).

3.15 Thermal oxidative

Thermal oxidative degradation is essentially thermal degradation in the presence of oxygen

(e.g. air). FT-IR spectroscopy and differential scanning calorimetry (DSC) is also used to

investigate thermal oxidative degradation. A DSC scan of Nylon 66 is displayed in figure __,

the endotherm at approximately 260°C is due to the crystalline melting. The temperature

oxidation begins is around 305°C – here an exothermic reaction shifts the baseline. The

oxidation peak is around 330°C (marked OPT on the chart). (Crompton, 2010)


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Figure 39 DSC of heat stabilized polyamide 66. (Crompton, 2010)

For more on oxidation see section 3.1.

3.16 Temperature and moisture combined

While temperature and moisture both have a similar effect on Nylon – increasing ductility

and reducing strength; Jai and Kagan (2005) found that temperature and moisture combined

did not result in more ductility that would allow it to deform further prior to failure. Rather,

the material failed sooner which suggests moisture and high temperatures combined can

deteriorate the Nylon (Jai, N. Kagan, V. 2001)

4.0 Environment

4.1 Introduction

This section will analyse the environmental conditions of the service parts. Conveniently the

Bureau of Meteorology (BOM) has a monitoring station at Perth airport, in close proximity to

where the samples were in service. The monitoring station is site number 009021 at a


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latitude of 31.93 °S and longitude of 115.98 °E, and elevation of 15m and data recording

since 1944. (BOM, 2013)

4.2 Rainfall

According to the Köppen-Geiger climate classification the climate of Perth is a classic

example of a Mediterranean climate (Kottek et al. 2006). It is temperate to subtropical;

summers are hot and dry while the winters are cool and wet (BOM, 2013). This high level of

rainfall during the winter months (>150mm rain) makes Perth the 4th wettest capital city in

Australia. Conversely the summers have very low rainfall (approximately 10-15mm rain),

figure 41 shows the summer months of January and February average only 2-3 days of rain

for the month. Usually this sporadic summer rainfall occurs in rapid thunderstorms,

interspersed by long dry periods.

Figure 40 (BOM, 2013)


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4.3 Humidity

Figure 41 Mean relative 9am humidity for 1981-2010

Reflecting the mean average rainfall data, the mean average relative humidity for Perth also

reaches a maximum in winter (80% RH) and a minimum in summer (50% RH).

1980-2010 data in figure 42 represents a typical year for Perth airport, evidently some years

are drier, 2010 is one example. As far into the year the data extends it, it already appears to

be abnormally low, though it is still not significantly lower than 50%RH.

Initially the mean 9am humidity data appears to counteract the prevailing theory of this

thesis about Nylons hygroscopic behaviour. However, mean 3pm RH data presents a


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different view; humidity reached 35% in the summers between 1981 and 2010, and did not

even reach 60%RH average for the wet winter months. Clearly there are large fluctuations in

humidity over the period of the day. In the morning when sunlight strikes dew the humidity

spikes, and by the afternoon evaporation of condensation has long since dried it out.

Comparing to the dry year in 2010 the average 3pm humidity drops to almost 25%RH

average for the start of the year – and although remaining data for the year is not yet

released to the public, the winter appears to average 50% – which is equivalent to the

summer average for mean 9am RH for 1981-2010.


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In addition to this, 2011 saw an all-time humidity record for Perth where RH reached 10% on

1/12/2011. Worth noting also is 2012 was a record year for highest humidity. (BOM, 2013)

4.5 Solar exposure

Solar exposure is the total solar energy radiating onto 1m2 of horizontal surface. The daily

values given in figure 43 are the total solar exposure for the day, typical values for global

exposure range from 1 to 35MJ/m2. The figures __ and __ show Perth receives high levels of

solar radiation, especially in the summer months. The effect of this radiation heating the

black polymer and accelerating moisture desorption is explored in section 3.3.2.

Figure 42 Mean daily solar exposure (MJ/m2)

During the summer Perth is exposed to >10hrs of sunshine per day. The Campbell-stokes

recorder measures only the duration of “bright” sunshine, which is less than “visible”

sunshine, so dawn and dusk are not recorded.


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Figure 43 Mean daily sunshine hours for Perth airport. Light bars are data for 1993-2013 and dark bars are for the year 2012 only

4.6 UV Radiation

As covered in section 3.1 of the literature review, UV rays cause polymer to degrade and lose

molecular weight. Figure 45 shows global UV radiation data, Perth is in the 160-170J/cm2

band.

Figure 44 UV only (SoDa, 2013)


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4.7 Temperature cycling

To study the natural temperature variations of the diurnal cycle a plot of the difference

between mean maximum and mean minimum temperatures was generated for each month

in figure 46.

Figure 45 Temperature differences for Perth Airport (BOM, 2013)

The largest temperature variations occur in the summer months; this could be attributed to

the drier climate in those months. The cloud cover creates a miniature greenhouse effect,

acting as a blanket and reducing radiation escape from the ground and air below the cloud.

At night the loud cover absorbs then re-emits the long wave radiation from the earth. This is

a function of temperature as per the Stefan-Boltzmann law describing black- body radiation.

(Frierson, D. 2012)

Where J* is the emissive power or irradiance and has dimensions of energy flux (energy per

unit time per unit area) and σ is the Stefan-Boltzmann constant which as derived from other

constants as,

Where h is Plank’s constant, k is the Boltzmann constant and c is the speed of light in a

vacuum.

0

2

4

6

8

10

12

14

16

11/11 12/11 2/12 4/12 5/12 7/12 8/12 10/12 12/12 1/13

ΔT (°C)

Date (mm/yy)

Mean annual temperature differences for Perth airport


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Figure 47 shows the radiation fluxes for this simple greenhouse model. Summer nights in

Perth lack sufficient low altitude warm temperature cloud cover to provide any substantial

back radiation.

Figure 46 Radiation fluxes per unit area of Earth’s surface for a simple greenhouse model (Jacob, D. 1999)

4.8 Acid Rain

Like many Australian cities, Perth is sparsely populated; and the city also has no other major

population centres nearby. Typically then one would expect the precipitation to be close to

5 (unpolluted acidity); however areas which contain smelters, power stations and other

industrial activities will usually record higher readings (CSIRO, 2011). The area surrounding

Perth airport is mostly residential, and some industry. The nearest power station is the coal

and gas-fired Kwinana Power Station located approximately 20kms away, its 4 turbines

generate approximately 420MW of electricity (Verve, 2013).

If the samples show signs of Hydrolysis then a study into the acid rain at Perth airport would

be recommended in order to determine if this is the cause.


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5.0 Optical Microscopy Inspections

5.1 Morphology

A Wild M8 stereo optical microscope was used to examine the fracture surfaces of the

broken samples A to G. Micrographs were taken using a Leica DFC295 digital camera fitted to

the microscope. Section 2.0 of literature review provides some comparative information on

polyamide Fractography.

This fracture surface in figure 48 is classic brittle fracture; there are fine ridges radiating from

the crack initiation site – thereby indicating the origin. In the bottom left corner of figure 48

is a very smooth region which is the mirror zone, adjacent to this can be seen very fine

ridges or “mist”. Further out, as the crack became faster and more violent the ridges become

rougher and large “hackles” were formed.

The morphology of this facture surface is indicative of classic brittle fracture.

Figure 47 Fracture surface no. 1 of sample C

C1


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Although most of the fracture surface is characteristic of brittle fracture, there is a ductile

tearing region seen in figures 49 and 49 as the region which appears to have been stress

whitened. This region does not mean the overall fracture is ductile, but is probably where

the remaining connecting portion was torn off. The protrusion adjacent to the white ductile

tearing zone supports this theory.

There are a number of other samples displaying similar fracture surfaces.

Figure 48 Fracture surfaces C2, E and B

E B

C2


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5.2 Factors Effecting Fracture

A potentially important observation was made during inspection, the fractures on the small

“prongs” all originated from the same edge. This suggests a possible design flaw or local

stress concentration.

Figure 49 Images of fracture surfaces are labelled with the designated part letters for each piece they occurred on.

Without more information on the service conditions of these parts, it remains uncertain how

they are loaded. Although based on the design, it is reasonable to assume the “hooks” are

loaded in tension. If this is the case then the bottom side of the sample in figure __ will be

loaded in compression and the top side in tension. The flange seen in figure 50 (opposite the

origin) would be on this side in compression; this explains why the fracture originated on the

opposite side.

That does not explain why it originated from the same edge for each sample, but what could

explain that is the sharp fillet radius. Although difficult to display, from handling the parts it

E G F1

F2


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is clear the sharpest corner on the whole sample occurs on the inside of the box-shaped hole

– to the side of the hook. This edge is significantly sharper than the rest, and is the same

edge in which all above fractures originated. This observation could be explained by the high

notch sensitivity of Nylon (see section 3.7).

Additionally, the presence of the box-shaped hole has meant the two “prongs” carry the

entire load. As they have a small area on which load is distributed, the resulting stresses are

higher – this would compound the problem further.

In a number of the samples there can be seen a transition in the morphology occurring

roughly 2/3rds of the way into the crack. This location corresponds to the presence of a weld

line, which may be the cause of this transition.

Figure 50 Samples G and C with weld lines highlighted

Despite these sharp edges, weld lines or thin prongs – there are fractures which seem to

have no simple cause and fracture in unlikely places; so this suggests the samples were just

very brittle at the time of failure. Figure 52 shows a fracture which occurred at the base of

protrusion from the backbone. This is a thick section with no sharp edges or weld lines.

This also doesn’t explain the fractures of the thicker “t-shaped” cross section backbone.

Considering their frequency it is indicative again of the samples being very brittle – rather

than some design issue.


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Figure 51 Fracture surface of sample H. Fracture occurred close to the base of protrusion rather than at the hypothesised locations of stress concentrations.

Overall the evidence from this lab seems to support general embrittlement, the fracture

surfaces are classic brittle fracture and not limited to areas of stress concentration or sharp

edge.


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6.0 Scanning Electron Microscopy

A scanning electron microscope (SEM) was used to inspect the fracture surfaces. The section

contains procedure, results and critical discussion and analysis.

6.1 Summary of Procedure

To ensure correct fitting in the SEM, chosen samples were cut down to approximately 5-

10mm size using a fine hacksaw. They were washed with ethanol to clean them, and then

mounted to Aluminium SEM stubs using 5 min Araldite. An SPI sputter coater was used to

coat the top fracture surface and sides of the samples by first exchanging the gas in the

vacuums chamber with argon then using platinum in the plasma state to coat the top and

two sides of the samples. The stubs were mounted in the SEM and calibrated using the

copper mount. The SEM used was a Philips XL30 with a Lanthanum hexaboride (LaB6)

emitter. The accelerating voltage was set to 15kV, and spot size set to 5 then micrographs

were taken of the fracture surfaces.

6.2 Results

Figure 52 Fracture surface C1


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Figure 53 SEM photo on the left shows hackle morphology, sample on the right shows much finer ridges called mist. Both photos were taken on surface C1 with same magnification.

The hackles and ridges were traced back to the mirror region, there an inhomogeneity was

noticed. It was hypothesized that this inhomogeneity could be the origin of fracture;

however a further zoom showed the fine ridges do not originate there. In fact they appeared

to avoid this region suggesting it may be tougher than the surrounding Nylon. Spectra results

of the inhomogeneity in figure 56 reveal it is not composed of pure Nylon 66, rather minerals

are present.

Figure 54 Zoom on the mirror region of surface C1 shows an inhomogeneity.


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Figure 55 Spectral results of inhomogeneity on fracture surface C1. The spectra show the inhomogeneity is not pure Nylon 66; rather it has minerals in it.

A further zoom on the mirror region showed the presence of small voids similar to those in

figure 62.

The ductile region of the surface was also explored; this is not the fracture origin so its

morphology tells us little about the true nature of the fracture. While photographing the

ductile “cups” a small inhomogeneity was noticed. The lines radiating out from this inclusion

suggest it was already present prior to fracture.


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Figure 56 Ductile region of sample C1, showing small inhomogeneity

This inhomogeneity shows a straight edge and appears to have a layered sheet structure.

The spectral results showed this was not Nylon 66. Only the carbon, oxygen and platinum

(coat) correspond with Nylon; however the oxygen peak on the spectra is larger than the

carbon. Also the presence of large magnesium and silicon peaks suggest this is an inorganic

mineral inclusion, perhaps clay. The sheet structure suggests it may be a sheet silicate.

This inhomogeneity would have been the origin of fracture for that local ductile cup only.

However its presence tells us something about the quality of the Nylon. If further inorganic

mineral inclusions can be found in brittle crack origins it may be theorized that its presence

contributed to the brittle behaviour of these fracture, however no such inclusions were

found.

Figure 57 Spectral results of inhomogeneity in ductile cups on sample C1


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Sample A revealed a similar overall morphology as the other brittle fractures. The hackles

and mist were traced back to fracture origin which was obstructed by Nylon, this may have

occurred post-fracture during handling or while still in service. The reason for the fracture

origin location remains unknown. The theory discussed in section 3.7 about the high notch

sensitivity of Nylon and the sharp fillet radius labelled “stress concentration” was not

confirmed. The weak weld lines (see section 3.7) were not the point of origin either.

Figure 58 Sample A fracture surface of thin protrusion.

On further inspection of the samples, there is another possible location of stress

concentration. The sharpest corner on the sample appears to be at the edge of the box-

shaped hole on the protrusions (see figure 50). The sharp edge would actually be to the left

of the one labelled “stress concentration” in figure 59. In addition to this, upon further

analysis on the SEM images, it appears the obstruction may not be the fracture origin at all.

The fine ridges pass this obstructed region and continue on.

Figure 59 Sample A


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Unfortunately this observation was not made during the SEM session so no higher

magnification images of the mirror region are available. Based on projection of the direction

of fine ridges, it’s plausible the fracture origin was in the south-west corner of figure 60 - the

corner with the highest notch radius on the whole sample, and this is on the protrusions,

which due to the box-hole have the smallest cross section on which to distribute mechanical

load – thereby furthering the stress concentration.

Figure 60 Fracture surface C2

In all the mirror regions of the samples, small voids were observed (see figure 62). 3 voids

were measured, they were: 485nm, 589nm and 820nm. The average size was approximately

600nm, and they were spaced 5-10µm apart. Figure 63 shows these voids on the mirror

region of sample E, the small cracks which can be seen are from the Pt coating cracking from

the concentrated electron beam.

It’s possible these voids occurred during moulding. If a small amount of water was trapped in

the resin then it may vaporize upon heating, form a bubble and then solidify into a void upon

cooling. It is unlikely these voids were caused by insufficient plastic; they are too small, too

numerous and too evenly distributed throughout the part. If the resin pellets did contain

water prior to moulding it is likely to be a minor quantity as Nylon pellets would almost

certainly undergo a drying process prior to moulding. If a hopper drier was used then the

degree of drying achieved will be dependent on moisture levels in the intake air, so a

seasonal variation can often be seen, and high quality drying is rarely achieved. With the use

of a dehumidifier then manufacturers claim less than 0.02wt% moisture content (Kenplas,

2013), although it’s plausible even this would be enough moisture to cause these tiny voids,

especially considering the volume expansion to steam.


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Voided areas have a greater susceptibility to fracture, and when too numerous can leave the

moulded part brittle (Bryce, D. 1997).

Unfortunately voids were not spotted in any region besides the mirror. This is partly because

they are only visible at >1000x magnification, becoming clear at 16,000x, and the largest

void spotted was less than a micron in diameter. It’s also possible the micro “cups” in figure

57 were local voids which stretched and warped during the ductile tearing.

Figure 61 Voids on the mirror region of sample E.

Figure 62 Voids in the mirror region of sample E


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7.0 Summary

Nylon’s chemistry, synthesis, general properties and behaviour were introduced and the

limited information available on these parts was collated. Then literature on numerous

possible failure modes was examined including: oxidation, hydrolysis, ductile-brittle

transition, fatigue and design flaws.

The optical micrographs revealed the origin of fracture; and this seemed to confirm the

hypothesis about stress concentrations and design. Micrographs showed clear signs of brittle

fracture, although revealed little about cause of fracture.

The SEM micrographs revealed some interesting inclusions; spectral analysis showed these

were not Nylon but rather they were likely of inorganic mineral composition.

These Nylon 66 samples all showed signs of classic brittle fracture.

8.0 Conclusions

There are a few main factors which could be the cause of failure, or could be contributing

factors which combine to cause failure. These include;

Exposure to arid environment for extended durations, Nylon is hygroscopic and

becomes brittle why dry.

Sharp fillet radius, Nylon has high notch sensitivity.

Poor design, location of sharp notch radius is on the section with smallest cross

sectional area on which to distribute load – mean higher stresses.

Micro voids, probably caused by water in the resin pellets, can lead to embrittlement.

These voids are also a sign that water was present during moulding which can lead to

degradation, although there was only a tiny quantity present.

The second and third factors listed help explain why the fractures on the prongs originated

on the same corner every time, but they don’t explain other fractures of thick sections with


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no stress concentrations. Because of this, the theory only explains the origin of fracture

location, not the reason these parts were brittle to begin with – so it can be ruled out.

The evidence collect does not support the hypothesis of stress concentrations or poor design

as the sole cause of failure, rather it is indicative of generalized embrittlement.

The cause of this embrittlement remains unknown; however an interesting observation was

made just prior to the culmination of this thesis. When handling the parts and attempting to

fracture them by hand, it proved impossible. The samples were extremely flexible and

would bend back upon themselves (>160° flexion) without fracturing, even a previously

fractured sample (definitely in service) was impossible to break. Humidity on this day was

94%RH.

This seems to reinforce the profound effect of Nylons hygroscopic behaviour, based on the

research in this thesis it is highly probable the samples were simply dry, became brittle, and

fractured.


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9.0 Recommendations

From the information available, the fault seems to be primarily due to inadequate material

selection. While the sharp edges contributed to failure locality, these were not the cause of

premature failure. Additionally, re-tooling the die to enlarge the sharp fillet radius would be

costly. Perhaps the easiest solution to prevent further failures would be an alternative

material, and then simply send any old Nylon 66 parts in stock to humid areas of the country

for service.

Possible additions to reduce the effect of Nylon 66 hygroscopic behaviour include:

Addition of a reinforcement– water absorption capacity and rate of sorption both

decrease with increasing glass fibre content. Additionally, glass filled Nylons have

been found to retain more of their properties at all moisture levels than unfilled

Nylons (ASM, 2003)

Nylon-rubber blends have improved ductility at low moisture contents. (Murphy, J.

2001)

Nano particle clay reduces Nylon’s moisture absorption capacity, and the diffusion

coefficient (Brady & Parsi, 2011)

Further additions of plasticizers, these will not migrate out in dry conditions like

water. Although the possibility of over plasticizing them should be investigated.

Increasing crystallinity may help because water is absorbed into amorphous regions

only

Crosslinking improves dry tensile strength (Brydson, 1999)

If none of the above are possible then perhaps consider an alternative choice of polymer. If

Nylon was needed for a particular reason then possibly an alternative type of Nylon may

suffice. While water is absorbed into other Nylons and plasticizes each of them, they don’t

all share the same capacity for moisture absorption. From figure __ it’s evident Nylon 66 is

the second worst performing in terms of moisture absorption, so its properties will change

much more significantly than a Nylon 11 or 12 when subjected to changing relative humidity.

Those Nylons generally have inferior mechanical properties to 66, so it is a question of

performance or longevity.


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Figure 63 (Toray, 2013)

Although it is still extremely difficult at this point to make a proper materials selection - as

information on the parts and their service function is still lacking - a general comparison of

various Nylons was conducted using the materials selection software CES. Based only on

fracture toughness and price, Nylon 66 seems a very good choice.

Figure 64 CES materials section chart

Conversely, when selecting based on water absorption, the PA66’s are high up on the chart.

Surprisingly the cast PA6 has a low water absorption, and still very high tensile strength.

Figure 65 shows it performs quite well with fracture toughness as well – similar to PA66


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(moulding). Perhaps a cast PA6 wouldn’t be a bad choice, the graph in figure 64 may have

used toughened PA6, it does not specify.

Figure 65 CES materials selection chart

Once the application of these samples is known, a full materials selection could be

conducted to select the alternative polymer. As part of that, equations can be derived to

give certain “weighting” to various properties such as price, moisture absorption, modulus,

fracture toughness then all included on the chart for selection.

If further investigation into these fractures is conducted the following are recommended:

Collect information about the samples:

o Part function

o Specific grade of Nylon, and any additives or fillers used

o Time spent in service prior to fracture

o Time of fracture, even the approximate time of year they failed

o Local environment (exposure to the elements such as direct sun or rain,

surrounding properties and use, any pesticides or chemicals nearby etc.)

Investigate the ductile-brittle transition of Nylon 66, and the effect of moisture

Conduct finite element analysis (FEA) to examine stress concentrations


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11.0 Appendix

embrittlement of nylon in arid environments

Documents

embrittlement of nylon

summary nylon

hygroscopic behaviour

generalized embrittlement

foremost id

optical microscopy

electron microscopy

excellent sem session