evaluation of durability of plastics used for gas...
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EVALUATION OF DURABILITY OF PLASTICS USED FOR GAS APPLIANCES
Takafumi Kawaguchi, Osaka Gas Co., Ltd.
Hiroyuki Nishimura, Osaka Gas Co., Ltd.
Fumiaki Miwa, Osaka Gas Co., Ltd.
Osamu Tsukada, Osaka Gas Co., Ltd. 1. INTRODUCTION
Plastics have come to be used instead of metals and their amount of use is increasing in gas
appliances for the purpose of manufacturing cost reduction and of making appliances lighter. Although
plastic materials have many advantages, their durability is not so good as those of metals and they tend
to degrade by light, heat and chemical agents. The evaluation of the lifetime of plastic parts is, therefore,
important for the purpose of improving the quality of gas appliances. When plastic materials are applied
to outer parts of gas appliances, the resistance to environmental stress cracking and heat need be
considered. The resistance to hot water and water hammer should be evaluated in applying plastic
materials to inner parts of hot water suppliers as well. The authors attempted to establish methods for
evaluating the resistance of plastic materials to various environments for the purpose of improving the
reliability and of reducing the maintenance costs of gas appliances. These methods of evaluation
developed in this study and evaluation results were summarized in CD-ROM entitled "Guidelines for
Designing and Evaluating the Plastic Parts of Gas Appliances." The authors also paid attention to the
mechanisms of degradation or failure because they are important and useful in determining the proper
evaluation methods. The results of investigation of failure and degradation mechanisms will be
presented as well as the evaluation methods which consist the Guidelines.
Gas appliances are often in contact with chemical agents such as detergents and oils in
kitchens or bathrooms. Plastics which are in contact with particular liquids fail under very low stress.
This phenomenon is characteristic of plastic materials and is called "environmental stress cracking
( ESC )". As ESC is a multifaceted phenomenon, it is difficult, at the stage of appliance design, to
predict the possibility of failure of plastic parts by ESC. Exterior parts such as knobs of switches or
casings are especially likely to be in contact with chemical substances. These parts have possibility to
fail by ESC due to chemical agents and stress caused by assembly or use. Besides the chemical
agents, hot water, steams and heat must be taken into consideration in the selection of plastic materials.
The problem of degradation of plastics and plastic composites due to heat and hot water is also
important. In general, heat affects the mechanical strength and the color of plastics. If the heat
resistance of plastics is not fully considered at the stage of appliance design, the plastic parts may fail
or its color may change in actual use.
Because of the high stiffness, GFRPs are widely used in many kinds of parts. GFRPs are also
actively applied to water or hot water supply services, and metals used for the materials for casing of
parts such as valves, pumps, and sensors are substituted by GFRPs. These parts are required to be
resistant to hot water, high static inner pressure, and the water hammer, which gives impact fatigue to
the parts. The evaluation of the resistance of GFRPs to water hammer is very important as well as the
resistance to hot water when long-term durability under end-use service conditions are concerned.
Water hammer is a phenomenon which is caused by the sudden shut of water flow. The maximum
pressure of water hammer is, in some cases, about ten times as large as the dynamic pressure of
stable water flow. In usual use of water supplying equipment, water hammer occurs several or more
times a day and in some cases, it may cause the failure of plastic parts.
2. GUIDELINES FOR DESIGNING AND EVALUATING THE PLASTIC PARTS OF GAS APPLIANCES
" Guidelines for Designing and Evaluating the Plastic Parts of Gas Appliances" contains the
items shown below.
- the performance of plastic materials under real use conditions
- the evaluation methods of plastic parts in gas appliances
- important points in designing and molding plastic parts
- the examples of failures of plastic parts in gas appliances
Special attention was paid to establishing “the evaluation methods of plastic parts” since
evaluation is quite important in maintaining the reliability of plastic parts, and the evaluation methods
includes the methods for evaluating the resistance of plastics to ESC, heat degradation, water hammer
and so forth.
In April 1999, the authors published the third edition of the guidelines. In the third edition, the
data of performance of plastic materials and the examples of failures of plastic parts were added, and
the evaluation methods of plastic parts were improved.
3. ENVIRONMENTAL STRESS CRACKING (ESC)
3-1. Importance of ESC
Because of the development of the polymer industry, plastic materials are now widely used in
many kinds of appliances. It is necessary that these materials are resistant to the environment in which
the appliances are used. It is known that polymers fail under very low stress, when they are in contact
with particular chemical agents. This phenomenon is called environmental stress cracking (ESC)
Woshinis and Wright investigated the instances of many failure cases of plastic parts in actual use, and
they conclude that about one-third of the plastic part failures were caused by ESC [1]. Particularly, outer
parts more frequently come into contact with many kinds of agents. The evaluation of the resistance of
plastics to ESC is, therefore, very important in material selection. ABS co-polymer is now widely used in
a variety of fields owing to their favorable cost/performance ratio. The advantages of ABS are its luster
and resistance to impact. ABS is, therefore, used mainly for housings of appliances.
There are many works which focus on the ESC of plastic materials and previous studies have
shown that some kinds of chemical agents such as organic solvents and surfactants cause ESC of ABS
[2,3]. Understanding of the ESC by solubility parameters of solvents and plastics has been remarkably
successful. Calculating hydrogen bonding parameters as well as solubility parameters or calculating
three components of solubility parameters have proven to be useful for predicting ESC of plastics.
There are also some reports focusing on the effect of viscosity of solvents on the ESC behavior of
plastics. Shanahan and Schultz reported that the viscosity of silicon oil affected the ESC behavior of
polyethylene especially at high stress [4]. Kambour and Yee investigated the influence of the viscosity
of agents on the ESC and reported that capillary flow of agents through the crazes is an important
factor which determines the behavior of crack propagation by ESC [5]. There are also some reports
on the ESC of ABS which show that certain types of chemical agents such as organic solvents and
surfactants cause this phenomenon.
In the present study, firstly, the resistance of plastics to ESC was evaluated by critical strain
using many kinds of agents which are for daily use, such as detergents, oils, and seasonings. The
special attention was paid to the mechanism of ESC of ABS caused by detergents which contain
non-ionic surfactants because they were found to cause the ESC of styrenic polymers very easily. The
dependence of the ESC of ABS on temperature and on the kind of surfactants was investigated by ECT
tests and by observation of the morphology of the crack tip with a Transmission electron microscope
(TEM) which elucidated the relation between the morphology and crack propagation behaviors. The
experimental results will be discussed in terms of permeability of surfactants which is the product of
solubility between polymers and surfactants, and diffusivity of surfactants into polymers. The solubility
and diffusivity were evaluated by the solubility parameters calculation and viscosity measurement of
surfactants, respectively.
3-2. Test Method for Evaluating ESC Resistance
We evaluated the resistance of plastic materials to environmental stress cracking by "critical
strain" for various kinds of agents. In this test, a plastic sample is fixed to the surface of an ellipsoidal
cylinder. The sample is coated with a variety of agents which are used in kitchens or bathrooms. The
load which is applied to the fixed sample varies with the position of the sample because the curvature
of the elliptical surface differs with the position. Critical strain is calculated from the position where
cracks or crazes appear under the minimum stress. If the critical strain is small, the plastic material
tested is not resistant to the test conditions.
Schematic and size of the fixture used in this study for obtaining the critical strain is shown in
Fig. 1. When a specimen was fixed on the surface of the fixture, the load applied to the specimen varies
with the position of the specimen because the curvature of the elliptical surface has different values at
different positions. Critical strain was calculated from the position where cracks or crazes appear under
the minimum strain using the equation below.
( ) 23
4
222
2 12
−
−
−
=
abax
abtε (1)
where ε is critical strain, t is thickness of the specimen, x is the position where cracks or
crazes appear under the minimum strain, and a and b are the length of the long and short axes of the
ellipse, respectively. Rectangular specimens (127mm×13mm×1.6mm) were prepared by injection
molding and were anealed at 80 ℃ for 24 hrs before testing. Specimens were fixed on the surface of
the ellipsoidal cylinder and were coated with each agent. The experiment was performed at 23℃ for 24
hours. The chemical agents used in this
study are mainly detergents, oils, and
seasonings which are mainly for daily use.
Most of the detergents for domestic use
contain several kinds of surfactants which
can be divided into three types, anioic
surfactants, cationic surfactants, and
non-ionic surfactants.
3.3.Test Results of ESC Resistance
The differences in critical
strain for different kinds of detergents
were significant. It was found that the
critical strain of ABS plastic for non-ionic
detergents is small compared with that
for other ionic detergents. We obtained
the critical strain of many kinds of
plastics for detergents, oils, and
seasonings and found that some kinds
of agents had great influence on some
plastics, especially on amorphous
plastics.
Table 1 shows the critical
strains of typical plastics used in gas
appliances. The value >1.5% means that
the critical strain was more than 1.5% or
no cracks or crazes due to ESC were
observed. It was found that the
x : The position where cracks or crazesappear under the minimum strain
Specimen coated with agentElliptical surface
xa = 38.1mm
b = 127.0mmx : The position where cracks or crazes
appear under the minimum strain
Specimen coated with agentElliptical surface
xa = 38.1mm
b = 127.0mm
Figure 1 Schematic of the fixture used in this study for
obtaining the critical strain.
Agents Main components PPS PET PBT PC
Detergent 1 anionic and nonionic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Detergent 2 anionic and cationic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Detergent 3 anionic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Detergent 4 cationic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Detergent 5 nonionic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Detergent 6 nonionic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Detergent 7 nonionic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Detergent 8 nonionic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Detergent 9 nonionic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Seasoning - > 1.5 > 1.5 > 1.5 > 1.5
Sugar solution sugar > 1.5 > 1.5 > 1.5 > 1.5
Saline solution salt > 1.5 > 1.5 > 1.5 > 1.5
Bathing agent1 Na2SO4,NaHCO3 > 1.5 > 1.5 > 1.5 > 1.5
Bathing agent2 NaHCO3,Na2CO3 > 1.5 > 1.5 > 1.5 > 1.5
Bathing agent3 nonionic surfactant > 1.5 > 1.5 > 1.5 > 1.5
Cooking oil fatty acid > 1.5 > 1.5 > 1.5 > 1.5(%)
Table 1 Critical strains of plastics measured in this
study.
detergents which include nonionic surfactants cause the ESC of amorphous styrenic polymers, such as
PS, AS, ABS (including ABS alloys) and modified-PPE. Other agents such as cooking oils and organic
bathing oils were also found to cause the ESC of these plastics. On the other hand, crystalline plastics
were found to be more resistant to ESC. Hence, special attention was paid to the ESC of ABS and
investigated the mechanism of the ESC of ABS as shown below. It need to be mentioned that this
method was standardized in the Guideline for measuring the resistance of plastics to ESC.
3.4.Investigation of Failure Mechanisms of ABS by ESC
3.4.1. Materials and Test Methods
It was found that the detergents which contain non-ionic surfactants cause ESC of ABS as
mentioned above. Therefore, the authors chose two non-ionic surfactants as model compounds for the
purpose of investigating mechanism of ESC of ABS. Two types of non-ionic surfactants (surfactants 1
and 2) were used to investigate the mechanism of ESC of ABS. One was a kind of
poly-oxyethylenealkylphenyllether, and the other was a kind of poly-oxyethylenealkylether. Their
molecular structures are shown below. Those surfactants were purchased from Aldrich Co., Ltd and
were used without dilution and further purification.
Surfactant 1 4-C8H17C6H4(OCH2CH2)12OH
Surfactant 2 C12H25(OCH2CH2)4OH
The viscosity of these two surfactants at different temperatures was measured with a
Cannon-Fenske viscometer.
Creep tests were performed under constant tensile load conditions and the time to failure was
measured. The specimens used for the creep tests were cut out from compression-molded sheets. The
creep tests were performed in air and the non-ionic surfactant, and the temperatures of the specimens
and the surfactant were kept constant (23℃) during the tests. The fracture surfaces of the specimens
were observed by SEM.
Edge Crack Tension (ECT) tests were performed in non-ionic surfactants under constant
loading conditions. ECT test is a one of the techniques to investigate the crack propagation behavior.
The tests utilizes rectangle specimens with crack at the edge of the specimen. Stressed are applied to
the specimen from the direction which is perpendicular to the crack. Temperatures of the specimens
and the surfactants were kept constant (23℃ or 50℃) during the tests. Crack length was measured by
a charge coupled device (CCD) camera. The specimens were cut from compression-molded sheets.
For the purpose of investigating the mechanism of fracture, the morphology of the fracture surface and
of the crack tip in the ECT tests was observed by a SEM and a TEM. Before performing TEM
observations, the specimens were immersed in an OsO4 solution for 24 hrs. The specimens for TEM
observation were cut out in a thickness of about 60nm.
3.4.2. Calculation Method (Estimation of Solubility Parameters of Surfactants and Polymers)
There are many reports about the methods for calculating the solubility parameters of
solvents and polymers. Although the solubility parameters which are based on the cohesive energy of
solvents and polymers, are useful for predicting the solubility of polymers, it has been suggested that
using three components of solubility parameters is more suitable for predicting the ESC of plastic. In
this method, the solubility parameter δ is expressed as follows.
δ2 = δ d2 + δ p2 + δ h2 (2)
where
δ d = contribution of dispersion forces to solubility parameter
δ p = contribution of polar forces to solubility parameter
δ h = contribution of hydrogen bonding to solubility parameter.
The interaction between solvents and polymers is estimated using the equation below. It is
usually understood that the smaller ∆ δ value gives stronger interaction between solvents and
polymers.
∆ δ = {(δ d,S - δ d,P ) 2+ (δ p S - δ p P) 2 + (δ h S
- δ h P )2}1/2 (3)
In this study, the calculation of the three components of solubility parameters was carried out
according to the method reported by Hoy [6]. In this method, the solubility parameters are calculated
using molar attraction function, its polar component, molar volume, and correction factor for non-ideality
for solvents and polymers. It is pointed out that the different methods for calculating the solubility
parameters proposed by Hoy and Krevelen gives the values close to each other and that they are more
accurate than the other methods. For the copolymer systems such as acrylonitrile-styrene (AS)
copolymer, each component of the solubility parameter can be calculated using the volume additive
rule. For example,
δ d,AS = δ d,A (1-ΦS)+ δ d S ΦS (4)
where ΦS is the volume fraction of polystyrene, and δ d,AS, δ d,A and δ d S, are the dispersion
forces components of solubility parameter for AS, polyacrylonitrile, and polystyrene, respectively.
3.4.3. Measurement of Time to Failure by Creep Tests
Figure 2 shows the results of the creep tests performed in air and the non-ionic surfactant.
The X-axis denotes the time to failure and the Y-axis denotes the stress applied to the specimen. The
time to failure in the non-ionic surfactant was shorter than that in air, and the relation between the stress
and the time to failure was rather complicated. When the stress applied to the specimen was small, it
had a tendency to rupture in a short time relative to air. The curve could be divided into three regions
(regions I, II, and III) as shown in Fig. 2.
When the racture surfaces of the specimens observed by SEM, The specimens in region I
showed ductile fracture and their
surfaces were similar to those of the
specimen ruptured in air. The fracture
surfaces in region III indicated typical
brittle fracture by ESC. A mixture of
brittle and ductile modes were observed
in region II. It is found from these SEM
images that the fracture mechanism
changed from ductile to brittle as the
stress applied to the specimens
decreased.
3.4.4. Observation of Crack Propagation Behavior by ECT Tests
When the ECT test was performed in surfactant 1 for σ= 8.2x105 Pa at 23℃, it was also found
that the curve of crack length can be divided into two regions (regions A and B). In region A, a crack
propagated rather rapidly, and the whitening zone ahead of the crack tip could not be recognized by the
CCD camera. In region B, crack propagation stopped, and the whitening zone ahead of the crack tip
was large, and it could be recognized clearly by the CCD camera. Following these steps, the crack
propagated with a repetition of region A and region B, and in the end, ultimate failure occurred.
The behavior of crack propagation found in the test performed in surfactant 1 for σ= 8.2x105
Pa at 50 ℃ was quite different from that performed at 23℃. In the test, the crack propagation rate was
nearly constant without the arrest of crack propagation, and the whitening zone ahead of the crack tip
was not observed. The total time to failure was rather short compared with that performed at 23℃.
When the ECT test was performed in surfactant 2 for σ= 8.2x105 Pa at 23℃ and 50℃, cracks
propagated with the alternation of region A and region B. Although the total time to failure was also
shorter when tested at a higher temperature, the crack propagation behavior was similar between the
results tested at different temperatures.
3.4.5. Observation of Morphology of the Crack Tip
To obtain the specimen for TEM observation, ECT tests were interrupted in region A and
region B. Figure 3 shows the typical TEM image of the crack tip in region A in ECT test performed in
surfactant 2 at 23℃. A small zone due to the massive craze which originated from the rubber particles
were observed near the crack tip.
Figures 4 shows the TEM images of the area near the crack tip in region B and the area far
from the crack tip, respectively. The image was typically observed in region B in ECT tests performed in
surfactant 2 at 23℃. Highly elongated rubber particles in the region surrounded by the crazed zone are
found in Fig. 4. In the region farther away from the crack front, many crazes were observed as shown.
Tim e to Failure (hr)
1
1 0
1 0 0
1 0 1 04
1 031 1 02
1 0-1
1 0-2
1 0-3
in S urfactant 2 (23℃)in air (23℃)
region I
region II
region III
Stress
(MPa
)Figure 2 The result of creep tests of ABS
in air and in surfactant 2.
Figure 3 The typical TEM image of the crack
tip in region A in an ECT test
performed in surfactant 2 at 23℃.
Figure 4 The typical TEM images of crack tip
in region B in an ECT test performed
in surfactant 2 at 23℃.
The structure of the damaged zone ahead of the crack tip was similar to that of PA/PPO alloy, which
indicated that both shear banding and crazing coexisted.
It was found from these results that the change in morphology at the crack tip corresponded
to the behavior of the crack propagation in the ECT test. When the local stress was lower in the initial
step of the ESC of ABS, the small massive crazed zone originated from the penetration of the non-ionic
surfactant. As the local stress ahead of the pre-crack tip was relatively high because of the crack
growth, the toughening due to the deformation of the rubber particles and the crazing occurred ahead
of the crack tip and resulted in the arrest of crack propagation.
3.4.6. Properties of the Surfactants and Critical Strain
To investigate the effect of the kind of surfactant on the crack propagation behavior, the
solubility parameters of the surfactants were calculated, and their viscosity was measured. The critical
strains of ABS for each surfactant were also measured using the same method mentioned above.
The calculated results of each component of the solubility parameters of surfactants and
polymers are shown in Table 2. Since the crazing in the AS matrix was found to be important in the ESC
of ABS, the value of the AS was calculated, instead of the value of ABS, and was used for the
estimation. In this calculation, it was assumed that the volume fraction of styrene was 0.6. The
calculated values of ∆δ between each surfactant and the AS are also shown in Table 2. It was found
that ∆δ between surfactant 1 and the AS is smaller than that between surfactant 2 and the AS
suggesting that surfactant 1 is more soluble with the AS matrix.
The three components of solubility parameters were also calculated using Krevelen’s method
which has the same order of accuracy as the method proposed by Hoy [6]. The results of the
calculation also supported the conclusion that the ∆δ between surfactant 1 and AS was smaller.
The critical strains of ABS measured for surfactant 1 and surfactant 2 were 0.2% and 1.2%,
respectively which suggests that when they were in contact long enough, surfactant 1 affected the ABS
more than surfactant 2.
The viscosity of the surfactants was
measured at 23℃ , 30℃ , and 50 ℃ . The
results of the viscosity measurement are
shown in Fig. 5. It was found that the change in
the viscosity of surfactant 1 was much larger
than that of surfactant 2. Although the
percentage decrease of the viscosity of two
surfactants are close to each other, the
absolute change in the viscosity, which is much
larger for surfactant 1, turned out to be
important in determining the crack propagation
mechanisms as described below.
3.4.7. Dependence of Crack Propagation Behavior on Temperature and on the Kind of Surfactant
In the previous parts, the mechanism of the ESC of ABS focusing on the dependence of crack
propagation behavior on the level of stress at the crack tip was investigated. The study revealed that
when the local stress at the crack tip was low in the initial step of the ECT test, a small crazed zone
appeared through the permeation of the non-ionic surfactant. When the local stress ahead of the crack
tip was relatively high because of the crack growth, the toughening due to the deformation of the rubber
particles and the crazing occurred ahead of the crack tip and resulted in the arrest of crack propagation.
In other words, the appearance of region B was mainly caused by stress at the crack tip. The structure
of the damaged zone ahead of the crack tip in region B was similar to that of
polyamide/polyphenyleneether alloy, which indicates that shear banding and crazing coexisted.
1
10
100
1000
10 20 30 40 50 60Temperature(℃)
Vis
cosi
ty(m
m2 /s)
Surfactant 1Surfactant 2
Figure 5 The temperature dependence of the
viscosity of the surfactant 1 and 2.
Srfactant A Srfactant B ASδp 11.66 10.45 11.60δh 18.59 19.09 11.47δd 19.77 20.60 16.64
(J/ml)1/2
Surfactant A - AS Surfactant B - AS∆δ 7.78 8.67
(J/ml)1/2
Table 1 Critical strains of plastics measured in this study.
From the results presented in this part, it was found that the rise of temperature had different
effects on the ESC of ABS. In the case of surfactant 1, of which viscosity changed greatly according to
the temperature, temperature rise had the effect not only of shortening the total time to failure, but also
of changing the dominant mode of crack propagation from a combination of region A and B to only
region B. On the other hand, in the case of surfactant 2, the rise of temperature had the effect of
shortening the total time to failure, but the crack propagation behavior did not change very much.
The temperatures at which the ECT tests were performed was low enough not to be affected
by the glass transition temperature of ABS which is often reported to be about 100℃. Because the
change in the viscosity of surfactant 1 by temperature is drastic, the change in crack propagation
behavior by temperature could be attributable to the change in viscosity in the case of surfactant 1. At
low temperature, because of the high viscosity, diffusion of the surfactant into the specimen was not
active, and, therefore, region B caused mainly by stress at the crack tip appeared. At high temperature,
the viscosity of surfactant 1 became rather low compared with the viscosity at low temperature, and it
allowed the active diffusion of surfactant 1 which was found to have essentially higher solubility through
the calculation of solubility parameters and critical strain measurement. The active permeation resulted
in the rapid crack growth.
In the case of surfactant 2, although the viscosity was lowered at high temperature, the
change was much smaller compared with that of surfactant 1. Since surfactant 2 has essentially lower
solubility into ABS as suggested from the calculation of solubility parameters, the change in viscosity
was not large enough to change the mode of crack propagation. The inactive diffusion resulted in the
occurrence of region B which was observed even at high temperature in the ECT test.
It has been pointed out that in some cases, the crack propagation behavior of polymers by
ESC can be understood by the linear fracture mechanics using the parameters such as stress intensity
factor or strain energy release rate. Although it was attempted to apply fracture mechanics to crack
propagation behavior found in this study, it was not successful because of the fact that in some cases
different crack propagation and crack arrest mechanisms appeared alternately at the crack tip, which
was not clearly related to the stress intensity factor at the crack tip.
3.4.8. Interpretation of Creep Rupture Curve by Mechanism of Fracture
The shape of the creep rupture curve in non-ionic surfactant was quite different from that in
air as show in Fig. 2. The appearance of the three regions in the creep rupture curve could be attributed
to the change of morphology of the crack tip which were observed by TEM..
In region III, the fracture surface was rather flat. This result indicated that the crack
propagation was mainly governed by ESC. In this region, the stress applied to the specimen was
relatively small, and the mechanism of fracture was same as region A in ECT tests. The crack
propagate rather quickly by the mechanism which is same as that of region A in ECT tests and the
specimen ruptured in relatively short time.
In region II, the mechanism of fracture was the same as region B in ECT tests. The higher
local stress at the crack tip induced the deformation of the rubber particles and the crazing ahead of the
crack tip. These change of morphology at the crack tip caused the toughening of the crack tip and
resulted in the arrest of crack propagation and the time to rupture was relatively longer than was
expected from the time to rupture in region III.
4. HEAT RESISTANCE It is important to keep the appearance of appliances in good condition since the appearance
is one of the important factors for the customers, which determines the value of appliances. As a result
of the efforts to reduce the manufacturing cost of gas appliances, the outer parts are sometimes made
from plastic materials. Therefore, the authors attempted to establish a methods to evaluate the
methods to quantifying the change of color of outer parts of gas appliances as follows.
4.1. Test Methods for Evaluating Heat Resistance
Heat resistances of plastics were evaluated by mechanical strength and color. The
mechanical strength was mainly evaluated by yield strength in tensile tests. The colors of plastics were
determined by measuring the hue and the brightness. The colors of plastics are indicated by
three-dimensional positions in color space according to the hues and brightness.
4.2.Test Results of Evaluating Heat Resistance
It was found that the yield strength of the heat-resistant ABS plastic did not change in this
experiment. On the other hand, prominent change in the color was observed The color change of the
plastic, if the experimental conditions were appropriate, was found to be expressible by Arrhenius'
equation and accelerated evaluation was thus possible. In general, if the degree of color change
exceed 3, the human eye can recognize the change in color.
Detecting the change in color became more of a problem at an early stage of the experiment,
than detecting the change in the mechanical strength, so that the evaluation of the color change is
more important than that of the mechanical strength. It was also found in this study that the change of
color was caused by the oxidation of rubber, which is added for the purpose of improving the fracture
toughness of the polymer.
5. HOT WATER RESISTANCE Recently, the casing of parts in appliances for hot water supply such as valves, pumps, and
sensors have been made of plastics instead of metals for the purpose of reducing cost and making
appliances lighter. Glass fiber-reinforced plastics (GFRPs) are normally used for those parts because
their internal pressure is sometimes high. These parts are required to be resistant to hot water, high
static inner pressure, and water hammer.
There are some reports on the hot water resistance of GFRPs or carbon fiber (CF)-reinforced
plastics [7]. These reports focused on GF-reinforced epoxy, CF-reinforced epoxy, and
polyetheretherketone, and GF-reinforced vinylester. Those reports are mainly focused on the
properties of thermoset composites or the thermoplastic composites reinforced by continuous fibers. It
has been revealed that that the degradation of these composites is attributable to the degradation of
the interface between reinforcement and matrix resin or degradation of the matrix resin itself. There are
also some reports on the water hammer resistance of GFRPs. These reports are mainly focused on the
performance of virgin materials to water hammer.
In this study, the hot water resistance of the short glass fiber-reinforced polyphenyleneether
(GFPPE), polyphenylenesulfide (GFPPS), and glass bead-reinforced polyoxy-methylene (GBPOM)
was studied by measuring the change in tensile strength and water-hammer fatigue resistance when
they were immersed in hot water. For comparison, hot water immersion tests were also performed on
materials which were not reinforced. The surfaces and the fracture surfaces of the specimens were
observed by SEM and the cause of the change in tensile strength and in water-hammer fatigue
resistance was investigated. Acoustic emission analysis which is useful to detects events such as the
breakage of glass under stress fibers was also used to investigate the cause of change in strength. In
those studied, special attention was paid to the change in the state of bonding before and after the hot
water immersion.
5.1. Measurement of Change in Tensile Strength
Figure 6 shows the change in the tensile
strength of the GFRP specimens. It was found that
the strength of the specimens decreased when they
were immersed in hot water. The tensile strength of
GFPPS showed a drastic change, and its strength
after 9000 hrs of hot water immersion was only
about 57% of its initial strength. The tensile
strengths of GFPPE and GBPOM after 9000 hrs
of hot water immersion was 88% and 87% of their
initial strengths, respectively.
On the other hand, when the same tests were performed for specimens which were molded
out of non-reinforced plastics, they did not exhibit any decrease in the strength but a slight increase.
The evaluation of standard deviation of the tensile tests data showed that the changes in the strength
were more than statistical variation of the data. Therefore, it was concluded that the tensile strength of
those neat resins increased.
When the weight of GFRPs during the hot water immersion test was measured, it was found
that the weight of GBPOM increased rapidly at the early stage of the test and it decreased gradually.
0
50
100
150
200
0 2000 4000 6000 8000 10000
Immersion Time (hr)
Tensi
le S
trengt
h(M
Pa)
Figure 6 The change in tensile strength of
pecimens of GFRPs during hot water.
The decrease of the weight could be attributable to the hydrolysis of POM resin.
5.2. SEM Observations
When the SEM observations were performed for the specimens before and after 9000hrs of
hot water immersion tests, it was found that the surface of the GFPPE specimens after the hot water
immersion test was rather rough compared with that before the test. Debonding between the glass fiber
and the PPE matrix and surface cracks were observed. These debonding and surface cracks seemed
to be caused by the residual stress on the surface. On the surfaces of the GFPPE specimens after the
hot water immersion tests, flow marks could be recognized clearer than before the tests. The
appearance of the flow marks seemed to be attributable to the debonding between the glass fiber and
the matrix resin, which resulted in the clear appearance of the glass fiber on the surface.
The SEM observations were performed for the tensile fracture surfaces of GFPPS before the
hot water immersion test and after 9000 hrs of the hot water immersion test. It was found that there
were significant difference in the amount of matrix resin remained on the glass fibers. Before the hot
water immersion tests, the glass fibers were covered with matrix resin. On the other hand, it was found
that only a little amount of matrix resin remained on the surface of the glass fibers after hot water
immersion tests. Those differences in the SEM images are also reported by other investigators and
they are attributed to the difference in the bonding between matrix and reinforcements. The results of
the SEM observations shown suggests that the bonding between matrix and fibers was relatively good
before hot water immersion. SEM image after hot water immersion tests, on the other hand, suggests
that the bonding was pretty poor as a result of the deterioration of the interface adhesion.
5.3. Discussion on the Change of Strength
5.3.1. Neat Polymers
As mentioned above, the tensile strength of neat specimens increased. Although PPE and
PPS are reported to undergo photo-oxidation, it is reported that those two polymers are quite stable for
thermal oxidation. Although POM is known to undergo thermal oxidation, it would be reasonable to
conclude that the effect of thermal oxidation on the tensile strength was not significant because the hot
water immersion tests were carried out in closed systems without introducing fresh air. It has been
reported by many investigators that mechanical properties of some polymers, such as polypropylene,
polyethylene, poly(ether ether ketone), PPS, polycarbonate / poly(ether ester) copolymer blend, and so
forth, can be improved by annealing of specimens. In most of those studies, the improved mechanical
properties are attributed to the change in the molecular level structures such as the change in the
crystallinity. The increased tensile properties observed in this study could be attributed to the same
phenomena due to the change in the molecular level structures.
It is reported that polyolefin polymers such as polyethylene, exhibit 3 failure stages which
show 3 different types of failure modes (ductile failure (stage I), brittle failure (stage II), and brittle failure
with the chemical degradation of polymer (stage III ) under constant stress creep conditions at elevated
temperatures. Stage III is usually observed after longer period of time, for example, from 10000 to more
than 100000 hrs for medium density polyethylene at 60-80℃. The stage III type failure is caused by the
loss of antioxidant and results in a drastic degradation of the mechanical properties. Although the
specimens of neat polymers (PPE, PPS, and POM ) examined in this study did not exhibit mechanical
degradation, there is a possibility that they show degradation of mechanical properties due to chemical
degradation if those hot water immersion tests were performed for longer period of time than in this
study. The degradation of neat polymer, if it occurs, would result in the more drastic change in tensile
strength of the reinforced polymers because both interface degradation and matrix degradation will
affect the tensile strength.
5.3.2. Reinforced Polymers
Special attention was paid to the change in the tensile strength of GFPPS because its change
in tensile strength was most remarkable in the hot water immersion test. From the SEM observation of
the tensile fracture surface, it was found that the bonding between the glass fiber and the matrix PPS
resin decreased significantly during hot water immersion. The authors concluded that the change in the
tensile strength of GFPPS was attributable to the deterioration of the interface between the glass fiber
and the matrix PPS resin. The conclusion was also supported by the facts that the tensile strength of
the PPS specimens, which were not reinforced, was minimal compared with that of reinforced ones.
The experimental results of the acoustic emission tests also lead to the same conclusion that the
decrease of the strength is attributable to the degradation of interface.
It was also found that the change in the weight of the GFPPS specimens was small during the
hot water immersion test. It was concluded from the results that the deterioration of the interface
between the glass fiber and the matrix PPS resin was caused by a small amount of vaporized water
which penetrated the specimens
Although the tensile strength of GFPPE was found to be small in this study, the debonding
between glass-fibers and matrix, and surface cracks which were found in the SEM observation could
affect the long-term performance of the material such as resistance to creep fracture. These defects at
the surface could shorten the time to crack initiation when the material is subjected to stress for along
time.
The hydrolysis of POM which was observed in this study also affect the long-term
performance of the material which would lead to the significant degradation of material performance.
5. RESISTANCE TO WATER HAMMER Because of the high stiffness, GFRPs are widely used in many kinds of parts. GFRPs are also
actively applied to water or hot water supply services, and metals used for the materials for casing of
parts such as valves, pumps, and sensors are substituted by GFRPs. These parts are required to be
resistant to hot water, high static inner pressure, and the water hammer, which gives impact fatigue to
the parts. The evaluation of the resistance of GFRPs to water hammer is very important as well as the
resistance to hot water when long-term durability under end-use service conditions are concerned.
The resistance of GFRPs to fatigue is also of great importance because they are often used
for parts that are subject to cyclic loading. There have been many reports on the fatigue of GFRPs [8],
and they have shown some of the fatigue properties of GFRPs, such as stress-log cycle life, fatigue
crack propagation, and so forth. Impact fatigue properties of GFRPs sometimes become an issue when
these materials are used in parts which suffer from water hammer. The usage of polymers or polymer
based composites for water supplying systems is increasing. Consequently, the interest in the
resistance of polymer or polymer based composites to water hammer is increasing, and there have
been some reports on this subject. On the other hand, there are few basic study reports on the impact
fatigue properties of GFRPs and of non-reinforced plastics. Those reports mainly focus on the small
number of cycles of impact fatigue. Although precise data on the resistance of polymers and polymer
based composites to large number of cycles of impact fatigue are important in evaluating the long-term
performance of those materials, they presently remain unknown.
In this study, mechanical behavior of GFRPs and of a glass-bead reinforced plastic was
studied through impact fatigue in uni-axial and multi-axial loading conditions for the purpose of
providing basic data of impact fatigue properties of those materials. The materials were characterized
by basic mechanical tests such as tensile tests, and drop weight tests. In the case of fatigue tests,
special attention was paid to the effect of loading mode and of interval time between loading on fracture
behavior. Acoustic emission (AE) measurements were performed during the fatigue tests and the
relationship between AE and cycles to failure in fatigue tests were investigated. Attention was also paid
to the effect of interval times between loading, and the cause of the difference in the cycles to failure,
which strongly depended on the loading conditions such as interval time and loading mode (uni-axial vs.
multi-axial) was studied. Acoustic velocity measurements and OM observations was used to investigate
the mechanisms of damage development which, in turn, determine the resistance of the materials to
fatigue. Special attention was paid to the relation between the features of damage developed in the
specimens, interval time, loading mode, and cycles to failure.
As mentioned above, water hammer is a phenomenon which is caused by the sudden shutoff
of the flow of water. The maximum pressure of the water hammer depends on the amount of water flow,
the structure of the piping, the kinds of the material used for the piping, and so forth. The maximum
pressure of the water hammer is, in some cases, about ten times as large as that of the pressure of
water at stable flow. In daily use of the water supplying equipment, the water hammer occurs several or
more times a day, and it causes impact fatigue of the material. The detail of water hammer can be
found elsewhere.
Rawles et al. evaluated the time to failure of glass reinforced polyester and vinyl ester under
static load after applying water hammer pressure to those specimens [9]. They reported that the
damage caused by water hemmer results in the decrease of the time to failure under static load. Ho et
al. studied the effect of molding conditions on the impact fatigue life of
polycarbonate/acrylonitrile-butadiene-styrene blends, and they reported the optimized molding
conditions for the material under impact fatigue. Those studies are mostly focused on the small number
of fatigue cycles.
In this study, the resistance of the GFRPs to the water hammer for a wide range of fatigue
cycles was studied, and the structure of the fracture surface was closely investigated to study the
fracture mechanism of GFRPs by the water hammer. The effects of the specimen thickness and of the
stress concentration, which are important in designing parts, were also investigated by using
specimens which are different in their thickness and in their shape.
5.1. Accelerated Water Hammer Tests It was found in the water hammer experiments, that the relation between the maximum
pressure of water hammer and cycles to failure was linear, as has been reported for the fatigue of many
kinds of plastics.
In the accelerated method, as shown in Fig. 7, the evaluation of the resistance of plastic
materials to water hammer is based on "standard point" and "standard line" which are determined by
the conventional durability standard for water hammer and experiments in this study, respectively. The
number of cycles of the standard point is 100000 and water hammer pressure of that is 2.0MPa. The
water hammer pressure of the standard point was determined from 0.5MPa of water supply pressure
and 1.5MPa of increase of pressure due to water hammer. In the determination of the standard point,
the durability standard for
water hammer determined by
Japan Water Works
Association was considered.
Standard line is the line drawn
through the standard point and
its inclination is the same as
that of the experimental
results. By testing plastic parts
at higher water hammer
pressure, it is possible to
estimate the resistance of the
parts at the pressure of the
standard point. Thus, by this
method, it is possible to make
the time required for water
hammer test short. The detail of the results performed in the study is shown in the following parts.
1
10
5
1 10 104 105
cycles to failure
WH maximumpressure(MPa)
standardline
experimentalresult 2.0MPa
100000cycles
standard point
the example of acceleratedtest conditions
Figure2 Water hammer test results and the accelerated test method.
4.0MPa・20000cycles
103102
Figure 7 Water hammer test results and the
accelerated test method.
5.2. The Detail of the Test Methods for Evaluating Water Hammer Resistance
The test apparatus used for evaluating the resistance of GFRPs to the water hammer was
equipped with a water tank, a pump to circulate water, a electric valve to generate the water hammer,
pressure gauges, and water leak detectors which detect the fracture of the specimens. The pressure
caused by the water hammer was applied to one surface of the specimens through a branched piping.
The maximum pressure by water hammer was controlled by changing the amount of water flow. The
electric valve was opened for 5 seconds which was long enough for the flow of the water to become
stable, and then it was shut for 1 second which was also long enough for the water hammer pressure
was attenuated. In the conditions of the experiment, the water hammer was applied to the specimens
every 6 seconds. The fracture of specimens was detected by the leakage of water through a crack by
water leak detectors. When the fracture of the specimens was detected, the test apparatus was shut off
immediately so that the water hammer pressure ceased. The cycles to failure were measured at
different maximum pressure.
For water hammer fatigue tests performed in this study, various type of specimens were
prepared to investigate the effect of thickness and shape on the water hammer resistance. The test
specimens (type I, II and III) used in this study are shown schematically in Fig. 8. The type I and II
specimens are disk-shaped specimens with the diameter of 60mm and were cut from the
compression-molded or injection-molded sheets. The type I and type II specimens were different in
their thicknesses, and the thickness of type I specimens was 1.6mm, and that of type II specimens was
2.4 mm. The type III specimens which have ribs on one surface were directly injection molded. The
diameter of the type III specimens was also 60 mm, and its thickness was same as that of type I
specimens (1.6mm). A kind of pipe joint (type IV specimen) which has complicated structure was also
prepared by injection molding with GFPPE and GFPPS, and the resistance to water hammer was
studied.
In the tests for type III
specimens, the pressure of the
water hammer was applied to either
surface of one with the reinforcing
rib, and the tests were performed for
both cases. The specimens were fixed
between two jigs with holes (diameter
= 30mm) in the center of the jigs. For
the case of type IV specimens, the specimens were connected between the straight piping using joints,
and the pressure of the water hammer was applied to the specimen from the inner surfaces of the
specimens. The temperature of the water was kept at room temperature during the tests.
Type I(t=1.6)
R0.5
Type II(t=2.4) Type III(t=1.6)
Figure 8 Geometry of the type I, II and III specimens
for water hammer fatigue tests used in this
study.
5.3. Evaluation of the Resistance of the GFRPs to Water Hammer
When the water hammer pressure was applied to type I and type II specimens of GF-PPE
and GF-PPS, crack was observed at the center of the specimen at first, and it propagated towards the
edge of the specimen. The next crack was usually found to propagate perpendicular to the first one. For
the case of GB-POM, which was reinforced by glass beads, the specimens broke into many pieces at
one time without indicating the propagation of the crack.
Figure 9 shows the resistance of the GFRPs (molded by compression) to the water hammer.
X-axis denotes the cycles of the water hammer to failure, and the y-axis denotes the maximum
pressure caused by the water hammer. Circles, squares, and triangles in Fig. 9 show the results of the
experiment performed for GFPPE, GFPPS, and GBPOM, respectively. It was found that the relation
between the maximum pressure of the water hammer and cycles to failure was linear, which is often
reported for the fatigue of many kinds of plastics. It was also found that the resistance of GFPPE to the
water hammer was inferior to those of GFPPS and GBPOM.
For the injection-molded specimens, GFPPE also showed poor resistance compared to
GFPPS and GBPOM. It was also found that the resistance of the injection-molded specimens was
superior to that of compression-molded specimens.
Figure 10 shows the results of the water hammer tests that evaluated the effect of the
thickness and shape of the specimen. Circles, squares, and triangles in Fig. 10 show the results of the
experiment performed for type I, type II, and type III specimens molded with GFPPS, respectively. The
results of type I and type II specimens in Fig. 10 are the ones for injection molded specimens. The thick
solid line in the figure shows the result of the calculation which was performed by the finite element
method (FEM). In this calculation, the resistance of the type II specimens were estimated by the FEM
from the experimental results for type I specimens.
Comparing the test results for type I and type II specimens, the resistance of the type II
specimens were superior to that of type I specimens, due to the type II’s increase of the thickness. The
0.1
1
10
10 100 1000 10000 100000Cycles to failure
WH
max
imum
pre
ssure
(M
Pa)
Type I
TypeII
Calculation for Type II
Type III (R=0.5)
0.1
1
10
10 100 1000 10000Cycles to failure
WH
max
imum
pre
ssure
(M
Pa)
GFPPE
GFPPS
GBPOM
Figure 10 The resistance of the different types
of specimens to the water hammer
(GF-PPS).
Figure 9 The resistance of the compression
molded GFRPs to the water hammer.
resistance of the type II specimens, however, was not as good as that expected from the FEM
calculation results which are shown by thick a solid line in Fig. 10. The difference between the results of
experiments and of the calculations for type II specimens was attributable to the plain strain states of
the type II specimens, which allow less deformation of the specimens when the pressure of the water
hammer was applied to the specimens.
Although type III specimens were reinforced by the rib, the water hammer resistance was
inferior to those without the rib when the pressure was applied to the surface without rib. In that case,
the fracture originated at the foot of the rib where the stress was concentrated. The smaller number of
cycles to failure of the specimens could be attributed to the stress concentration when the pressure
was applied to the surface without a rib. The result was the same as those reported by Nishitani et. al
[10]. They reported that when a specific joint for water supply, which is made of polyvinylchloride, was
subjected to water hammer, the fracture originated at the point where stress was most concentrated. It
was also found in this study that when the pressure was applied to the surface with the rib (type III
specimens), the cycles to failure was almost the same as those of type I specimens which have the
same thickness as that of type III specimens.
When the same tests were performed for the type IV specimens molded with GFPPE, it was
found that the origin of the fracture of type IV specimen was also where stress was most concentrated
because of the design. Although the resistance of these specimens was different, the inclination of
these lines were almost the same. Therefore, when a datum at a pressure for a part with a complicated
structure is obtained, the data for specimens with simple shape could be applied to the expectation of
the other data for specimens with complicated structure.
5.4. Investigation of the fracture surface
When SEM observations were performed for tensile fracture surface of GFPPS, it was found
that some glass fibers surrounded by matrix resin around the fibers are observed at the fracture surface
as mentioned above. It was also confirmed from the investigation of cross section of the tensile fracture
surface of GFPPS by optical microscope. In this cross section, the glass fibers were found to stick out
from the fracture matrix surface. This observation result indicates that during the crack propagation
which resulted from tension, both the breaking of glass-fibers and pull-out of glass-fibers from the
matrix resin occurred.
On the other hand, when SEM observations were performed for the fracture surface of the
type I specimen (GFPPS) fractured by the water hammer, it was found that the structure of the fracture
surface was quite different from that of tensile fracture surface. It was rather flat and hardly any glass
fibers were observed at the fracture surface. This kind of fracture surface was also observed for the
fracture surface of the type II and III specimens, and for the specimens molded with GFPPE.
When the cross section of the water-hammer fracture surface was observed by optical
microscope, it was found that all the glass fibers were found to break at the fracture surface. From the
observation mentioned above, it was found that when the crack propagated through the specimen, the
glass-fibers broke at the fracture surface without being extracted from the matrix resin. The appearance
of the characteristic fracture surface which resulted from the water hammer was attributable to the
breaking of the glass fiber at the fracture surface. In the optical microscope observations, it was also
observed that the deformation of the matrix occurred at the edges of the glass fibers.
Optical microscope observations of the cross section of the fracture surface was also
performed for GBPOM fractured by the water hammer . The material was reinforced by glass beads as
mentioned above. For this specimen, it was found that the matrix deformation has its origin at the
interface of the glass beads and the matrix resin. In these two materials, the damage was dissipated by
the deformation of the matrix, and it resulted in the longer lives when subjected to water hammer.
On the other hand, an OM image of the cross section of the fracture surface of GFPPE which
had less cycles to failure compared with GFPPS and GBPOM, did not exhibit the deformation of the
matrix, suggesting that there is no effective mechanisms which absorb the energy applied to the
specimens by water hammer.
5. Conclusion Recently metal parts in gas appliances are substituted by plastic parts. The appropriate
evaluation of plastic parts are, therefore, becoming more and more important. The above mentioned
evaluation method and other methods are now used as the standard method to evaluate the durability
of plastic parts in gas appliances. It was also found that the investigation of the failure and degradation
mechanisms were quite important in establishing the test methods, since the acceleration of the tests
need to be done in a proper way based on the failure and degradation mechanisms of the materials.
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