Lectures part 2

Methods for cure monitoringManufacturing methods for

composites

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Characterisation methods for the crosslinking in thermosets

1. Physical methods2. Thermal methods

3. Spectroscopic methods

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Cure monitoring in composites

• The crosslinking of thermoset resins is a fundamental process in the manufacturing of composites

• This curing process must be done in a controlled manner in order to achieve good quality

• It is necessary to follow and check the the cross-linking reaction during and after the cure of the composite

• Important both in production and in R&D work

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Methods to measure the degree of cure in composites

1. Techniques based on changes in physical properties: hardness, dielectric constant, viscosity

2. Techniques based on changes in chemical properties: reaction heat, chemical structure

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Physical methods

1. Barcol hardness2. Gel time

3. Cure meters4. Rheology

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Barcol hardness• Measurement of the surface

hardness by pressing a needle into the laminate surface

• Easy and fast method for quality control in production

• 10-20 measurements needed per laminate for statistically reproductive results

• Glas fibers near surface can disturb the measurement

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Geltime measurement

• Mixing of curing resin in a beaker, time for gelling is detected by a stopwatch

• Most common method to characterise gel time by producers and end-users

• Simple and fast method, which can be used by any-one

• Subjective method• Must be done under standard

conditions

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Thermal Scanning Rheometer

Gel-time detection by a oscillating probe

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Rheological characterisation of the curing

• Most accurate characterisation of the curing process is obtained by studying the rheological properties

• Expensive instruments and skilled operators needed

Bohlin CVO rheometerMSK 20120213

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Rheological methods1. Steady shearing flow properties

• only in liquid phase• data before the gel point can be detected• can give too long gel times due to shear thinning

near the gel point

2. Dynamic shearing flow properties• oscillatory shearing flow• both in liquid,rubbery and glassy states• data after the gel point can be detected• Storage modulus G’ and loss modulus G’’ are

measured• At the gel point is G’ = G’’ (tan = 1)

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Harts 2

2

43

1 56

Steady shearing flowShear rate 1.17 s-1, 80 ºC, 2 wt-% BPO

Bohlin VOR, Cone & plate D = 30 mm, 5 º, 150 µm distance

Gel time 370 s, 6.2 min

Gel time 0/t -> 00 = viscosity at the start

t = viscosity at the time tMSK 20120213

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Oscillatory shearing flow1 Hz, 80 ºC, 2 wt-% BPO

Harts 2

Gel time 350 s (5.8 min)

Bohlin VOR, Cone & plate D = 30 mm, 5 º, 150 µm distance

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Thermal methods

Differential scanning calorimetry

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DSC• The exothermal heat H which is liberated in the

crosslinking reaction is detected by DSC• Results depends on:

• Heating rate (10 ºC/min)• Sample preparation• Sampling (10 - 15 mg)• Atmosphere (nitrogen most common)• Thermal history

• Commonly used for quality control of laminates

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Differential scanning calorimetry (DSC)

Perkin-Elmer DSC 7

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DSC Principle

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The glass transition temperature can be detected from the DSC scan after a dynamic or isothermal scan

Glass transition temperature

Unsaturated polyesters have Tg’s at 50 - 70 ºC

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Isothermal DSC scan

• A liquid thermoset is cured in the DSC, at the curing temperature

• Gives the total exothermal heat when assuming that all functional groups will react

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Residual reactivity

• A cured thermoset sample is analysed in the DSC

• Any unreacted components will react during the scan, which can be detected as an exothermal heat

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Degree of cureThe degree of cure t can be calculated from the isothermal scan and the residual reactivity scan:

(1) Htot = Hiso

(2) t = Ht / Htot

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Spectroscopic methods

Raman spectroscopy

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Benefits with Raman spectroscopy

• Well resolved spectra: simple calibration methods can be used

• No sample preparation: can be easily adapted to on-line measurements

• Weak water spectrum: works well with aqueous samples

• NIR and visible wavelengths: low-cost fibre optics can be used

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Process Raman spectrometer

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Spectral changes in a unsaturated polyester after crosslinking

ENDUR M 105 TB• Curing time 0.1 and 2 h• Sample thickness 0.5 cm• Measurement time 1 s• Laser effect 150 mW

1300140015001600170018001900-1000

0

1000

2000

3000

4000

5000

6000

7000

C=O

C=CArom. ring

CH(olef.)

CH(alif.)

RAMAN SHIFT (cm-1)

RA

MA

N I

NT

EN

SIT

Y

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The crosslinking reaction can be monitored

ENDUR M 105TB• 2 wt-% MEKP• Gel time 20 min, 23oC• Sample thickness 0.5 cm• Postcuring 50oC, 24h

10-1

100

101

102

103

104

-0.2

0

0.2

0.4

0.6

0.8

1

CURING TIME (h)

PE

AK

RA

TIO

(R

1620

/ R

1600

)

50oC, 24h

Gel time

100% cure

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Postcuring effects can be detected

ENDUR M 105TB• Curing time 30 h and 350 h• Postcuring 24h, 50oC• Measurement time10 s• Laser effect 150 mW

13001400150016001700180019000

0.5

1

1.5

2

2.5

3x 10

4

RAMAN SHIFT (cm-1)

RA

MA

N I

NTE

NS

ITY

C=OC=C

Arom. ring

CH(alif.)

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Laminates can also be analysed

M 105 TB and glass fiber

• The glass gives its own background

• Longer measurement time

200400600800100012001400160018002000-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

4

RAMAN SHIFT (cm-1)

RA

MA

N I

NT

EN

SIT

Y

HARTSI

HARTSI(50%) + LASIKUITU(50%)

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Gel coats

MAXGUARD, clear gel coat• 0.5 mm thickness• Measurement time 1 s, 30

s

10-1

100

101

102

103

104

-0.2

0

0.2

0.4

0.6

0.8

1

CURING TIME (h)

PE

AK

RA

TIO

(R

1620

/ R

1600

)

+ 24h, 50oC

100% cure

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The fibre-resin interaction

Surface treatmentFibre impregnation

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Fibre – resin impregnation

• A efficient and fast impregnation of the reinforcement by the resin is essential in all composite processing

• Can be achieved mechanically by rolling or by the use of external pressure

• Heating helps the impregnation• The impregnation is highly depending on the

resin and the reinforcement characteristics

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Effect of constituents on impregnation

Resin• Low viscosity enhances

resin flow• Fillers and additives can

enhance impregnation or make it more difficult

Reinforcement• Fibre surface treatment

enhances impregnation• A yarn with high twist is

more difficult to impregnate• Fabrics structure, a more

dense structure will make impregnation more difficult

• Flow layers will enhance impregnation

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The reinforcement impregnation occurs during the processing of the composite

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Manufacturing methods for composites

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Different manufacturing methods

Open methods• Hand lay-up• Spraying• Filament

winding• Compression

moulding• Pultrusion

Closed methods• Resin transfer

moulding• Vacuum bag

infusion• Vacuum infusion

with flexible tooling

• Vacuum infusion with rigid tooling

• Autoclave processing

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A mould is needed for the making of the composite product

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Prepreg• Preimpregnated fabrics:

– A fabric is preimpregnated with the resin, and cuit into desired size

– The prepreg is then processed by compression moulding, pultrusion or filmanet winding

– Both thermoset prepregs and thermoplastic prepregs (GMT) are available

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Processing methods

• Fiber impregnation by mechanical action– Hand lay up– Spray lay up– Filament winding

• Fiber impregnation by external pressure– RTM– Vacuum infusion– Autoclave, rubber bladder processing– Compression moulding (SMC and BMC)

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Hand lay up

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Mechanical impregnation of fibers

Hand lay up

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Reinforcement placement in mold

• Pre-cutting to desired shape

• Preforming by heat and pressure

• Core material and inserts can be attached

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Preformed glass fiber is oftem prepared in advantage with the desired shape

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Spray up lamination

CHOPPED ROVING + RESIN

SPRAYED LAYER

RESIN

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Equipment for spray up lamination

• Resin - peroxide mixing in spray

• Roving chopper• 108 bar output

pressure• Resin heating

possible• 200 l resin drums

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Filament winding

• Winding of impregnated reinforcement yarns onto a rotating mold (mandrel)

• A precise, highly efficient and automated process

• Only for closed geometries which can rotate such as pipes, pressure vessels

• Fiber volume fractions can be varied in a laminate

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Filament winding - process principle

4 degrees of freedom

Mandrel

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Filament winding

Polar winding pattern

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Filament winding

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Resin impregnation - dip through

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Resin impregnation – drum type

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Resin impregnation – closed type

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Mandrel removal after winding

Removable mandrel1. Treatment with release agent2. Release tape winding3. Water-soluble sand and salt mandrels4. Melting metal alloy mandrels5. Casted plaster mandrels

Integrated mandrel6. Termoplastic liner mandrel7. Metal liner mandrel

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Filament winding - product examples

Liner for smoke chimney6 m diameter

25 min production time

Wind mill blade8 m x 38 m

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Pressure vessels• Filament winding• Carbon fibers and

epoxy• Much lower weight

than metal• Not sensitive for

corrosive environments

• Better work safety and comfort

Fiber impregnation by external pressure

• Autoclave processing – fibre compaction under very high pressure in a chamber, commonly used for pre pregs

• Resin transfer moulding – fibre impregnation and compaction by a pressurised mould

• Vacuum infusion – fibre impregnation and compaction by external pressure (surrounding air)

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Autoclave processingUsed for consolidisation of prepregs by

high pressure and curing by heating

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Aircraft• Epoxy resins• Carbon, aramid

and glass fibers• Honey comb core

sandwich (Al)• Prepreg lay up

with autoclave cure

• Wings, control surfaces, hatches, covers, floors

JAS Gripen

SAAB 2000

Rubber bladder consolidation

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RTM - Resin transfer molding

• Resin injection into a closed mold, containing the reinforcement, with the aid of pressure

• Rigid, metal molds are used• Mold can be heated if necessary• Large, complex and highly integrated

components can be produced• Low investment and mold costs• Good work environment• Medium length to long length series

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RTM principle

Insertion of preformed

reinforcement

Resin injection by external pressure

Product demoulding

Curing under pressureHeating possible for resin curing

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RTM automated process

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Bicycle fram manufacture by bladder moulding process

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Vacuum infusion molding• Resin injection into a closed mold,

containing the reinforcement, with the aid of vacuum

• Processing at room temperature• Higher external pressure seals the mold• As upper mold can either a vacuum bag

or a rigid shell be used• Lower investment cost• Only one high quality surface is obtained

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