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A comparison of physical properties of
glass fibre epoxy composites produced by wetlay-up with autoclave consolidation and resintransfer moulding
D. Abraham, S. Matthews and R. McIlhaggerEngineering Composites Research Centre (ECRE), Department of Electrical and Mechanical Engineering, University of Ulster at Jordanstown, Shore Road,Newtownabbey BT37 0QB, Northern Ireland, UK
Comparisons are reported for composite samples of similar resin and fibre systems which were processed using thewet lay-up with autoclave consolidation and resin transfer moulding (RTM) by vacuum impregnation. Similardegrees of cure were obtained for laminates using the two methods of processing and the arising thermal andphysical properties (tensile, flexural, interlaminar shear strength (ILSS), void content and thickness variation)were measured. The fibre dominated properties (i.e. flexural and tensile strength) were found to be higher for theautoclaved samples due to the higher volume fraction arising from the superior compaction pressure, althoughwhen normalised on the basis of fibre volume fraction the results were similar. The matrix dominated ILSS valueswere higher for the RTM samples and this was attributed to improved wetting, reduced void content and a slightlylower degree of cure. Thermal analysis also indicated that the autoclaved (60% glass fibre by volume) compositeattained a slightly higher glass transition temperature than that achieved by RTM (50% fibre by volume) forsimilar cure times and cure temperatures. The significance of the results in an industrial context is discussed. ᭧1998 Elsevier Science Ltd. All rights reserved
(Keywords: composites manufacture; E. resin transfer moulding (RTM); E. autoclave)
INTRODUCTION
Aerospace composites manufacturing
The aerospace sector remains firmly rooted in traditional
autoclaving routes for processing the majority of
their composite parts. Autoclaving is a well understood
and mature technology which is capable of producing
material of consistently high quality, with high (greater than
55%) fibre volume fraction and low (less than 2%) void
content. Numerous studies of autoclaving have been
published, although most of these relate to pre-preg.
Stringer1 investigated autoclave consolidation for wet lay-
up carbon/epoxy laminates and reported a fibre volume
of 58% and less than 2% voidage by incorporating a
dwell period at the start of the cure cycle before applying
the consolidation pressure. Wenger2 has investigated
autoclave cure cycle optimisation for the facility used
in this investigation, varying the point of pressure
application, dwell temperature, dwell period and heat-upramp, and studying the quality of the final composite
component.
Aerospace resin transfer moulding
Despite the traditional use of the autoclave (almost
universally with pre-preg.), interest and confidence is
increasing for resin transfer moulding (RTM), usually
driven by materials costs. This has led to the production
of aircraft components such as radomes3, bullet fairings4,
and propeller blades5. Previous development studies for
structural components by aerospace operators includes
missile airframes6 and a highly loaded crank 7. With the
development of automated methods of preform produc-
tion8,9, parallel research on processing methods is aimed at
fast and reliable methods of impregnation with reduced
capital and running costs compared to either traditional wet
lay-up or vacuum bag and autoclave (pre-preg.) moulding.
RTM offers a useful alternative in several industrial sectors,
since it reduces the labour costs and environmental concerns
associated with wet lay-up and provides substantial savings
in materials costs when compared with the use of pre-pregs.
A further advantage of RTM is the wide range of processvariants, which range from relatively simple vacuum
impregnation processes requiring little by way of capital
Composites Part A 29A (1998) 795–8011359-835X/98/$-see front matter
᭧ 1998 Elsevier Science Ltd. All rights reserved.
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equipment to hybrid RTM/SRIM operations and their
associated high investment.
Vacuum driven processes
Recent years have seen significant activity in vacuum
infusion processes for composites manufacture. Althoughapplications of this process have existed since the 1940s,
increasingly stringent legislation in several countries
concerning the emission of volatiles in the workplace has
led to investigations and numerous production examples of
vacuum infusion as a replacement for open mould
processes. This technique uses vacuum to transfer the
liquid resin into the mould cavity containing the fibre
reinforcement. The cavity itself may be formed by two
matched tools or a single hard tool and a vacuum film. In
common with all fabrication processes, it is important to
maintain voidage in the final laminate as low as possible,
and since RTM (generally) involves no further consolida-
tion, the impregnation process needs to be controlled withsome care. Increasing void content is well known to be
detrimental to mechanical performance, as illustrated for
example by Judd and Wright10, who reported that an
increase in the void content of 1% resulted in a 7% decrease
of the short beam shear properties (up to 4% void content).
Several detailed reviews of void formation in RTM have
been presented11,12. Hayward and Harris13 investigated the
effects of vacuum assistance and reported a reduction
in voidage during positive pressure RTM. Abraham
and McIlhagger14 considered several injection strategies
and gating arrangements, and found these to affect
the degree of visual porosity and moulding translucency.
Qualitative comparison showed that vacuum impregnation
and peripheral gating produced the best quality
mouldings.
Clearly, the properties of vacuum impregnated mouldings
are also strongly dependent upon the fibre volume fraction
which is achieved. Unlike conventional RTM, which iscarried out in a fixed height cavity, the use of a flexible
membrane to close the cavity means that the degree of fabric
compaction (thus the fibre volume fraction) depends upon
the pressure difference across the membrane15.
Autoclaving versus RTM from a manufacturing perspective
Table 1 summarises the main features that characterise
autoclaving and RTM. It is evident that autoclaving is the
favoured method when the need for high product quality
dominates over production rate (as evidenced by its
continued dominance of aerospace manufacture), while
RTM appears to offer lower cost manufacture and entry tohigher volume markets, typified by the growing interest
from within the automotive sector.
In addition to the savings on materials costs compared
with pre-pregs and the improvements in the working
environment compared with wet lay-up, RTM offers
considerable advantages over either alternative, due to a
reduction in the labour content and higher production rates.
Operator exposure to liquid resins is also much reduced. An
approximate time analysis for the two (laboratory) pro-
cesses is shown in Table 2, which suggests that RTM offers
a significant time saving of 1.5 h (25%) over the equivalent
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796
Table 1 Main characteristics of RTM and autoclaving
Property RTM Autoclave
Production output Moderate (10 2–10 5 /annum) Low (10 1–10 2 /annum)Void content Low ( Ͻ 2%) Low ( Ͻ 2%)Labour intensity Low–moderate HighAchievable V f Moderate (30–65%) High (50–70%)Typical applications Low to medium volume, automotive parts, non-structural components
(e.g. automotive spoilers), structural items (e.g. propeller blades, missile boxes)Aerospace industry, Formula 1automotive, sporting goods
Table 2 Comparison of Wet lay-up with autoclave consolidation and RTM: breakdown by operations for laboratory scale processes
Wet lay-up and autoclave RTM
Production step Time (min) Production step Time (min)
Mould preparation H 22.5a Mould preparation H 22.5 a
Fabric cutting H (including preparation) 25 Fabric cutting H (including preparation) 25Resin mixing H I (including preparation) 25 Resin mixing H I (including preparation) 25Hand lay-up H I 120b Resin degassing 60Vacuum bagging H 30 Tool assembly þ testing H 30Autoclave cycle 192 Resin injection H 12Autoclave debagging H 10 Resin curing 99
Tool cooling 30Tool ejection and preparation for next run H 30
Total processing time 424.5 Total processing time 333.5Resin post-curing at 145ЊC 452 Resin post-curing at 145ЊC 368.4Total production time 876.5 Total production time 701.9Manual processing time (% of total production time) 232.5 (26.5%) Manual processing time (% of total production time) 144.5 (20.6%)Direct Contact with resin (% of total production time) 145 (16.5%) Direct contact with resin (% of total production time) 25 (3.6%)
aAutoclave and RTM mould preparation takes 90 min, but the preparation will endure a minimum of four production runs.bAutoclave hand lay-up takes two operators working for 60 min.H, these steps involve manual processing.I, these steps involve direct operator contact with the resin.
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autoclave processing time. Further gains are evident when
post-curing is taken into account. The major portion of the
time savings using RTM arises because the majority of the
initial processing takes place at a higher temperature than
for wet lay-up with autoclave consolidation; thus the
lengthy heat-up times are eliminated. RTM is also
significantly less labour intensive and eliminates almost1.5 h of manual processing time. The operator exposure to
wet resin is limited to the relatively short time required for
mixing. In contrast, the wet lay-up alternative involves
exposure for the entire laminating process (2 h). It must be
recognised that the data presented here are for research
laboratory processes, which are not necessarily
representative of conventional industrial practice.
Nonetheless it is likely that the general, rather than
the specific, findings read over to industrial
operations. Clearly, on an industrial scale, where batch
mixing would serve several mouldings, the level of
automation would be higher, and certain procedures
(e.g. preforming, resin degassing) could be conductedoff-line, the advantages of the closed mould process
would be magnified.
A quantitative comparison of RTM and autoclaving
In addition to the manufacturing requirements, it is useful
to compare the quality and physical properties arising from
RTM laminates with those of competing processes. Few
direct comparisons of autoclaving and RTM have been
reported. Hayward and Harris16 compared properties arising
from vacuum assisted RTM with several alternative
processes, although different resin systems were used ineach case. The main comparison here has been with an
autoclave route which, although used conventionally with
pre-pregs, was used in the present study with a dry fabric/
wet lay-up approach, in order that the same materials could
be used in both processes. Tensile, flexural and interlaminar
shear strength (ILSS) material tests were used to compare
the physical properties arising from the two processing
routes. In addition void contents, volume fraction of fibre
and the thickness variation within the laminate were
examined. The degree of cure, as determined using dynamic
mechanical analysis (DMA) and differential scanning
calorimetry (DSC)17,18, was also assessed although, in
order to minimise the influence on properties, cure timeswere adjusted in an attempt to achieve similar glass
transition temperatures T g.
In order to examine the potential of RTM composites as
replacements for laminates which are traditionally
autoclaved, the performance of two representative
materials has been compared. These were produced using
identical materials using laboratory equipment and
processes as defined below. Vacuum impregnation was
used for the RTM plaques in this work to achieve a glass
fibre volume fraction in the region of 50%. Parallel studies
using wet lay-up with subsequent autoclave consolidation at
12 bar pressure used a similar arrangement with a flexiblemembrane with the mould cavity evacuated to
approximately 1 bar vacuum.
EXPERIMENTAL DETAILS
Materials
The reinforcement material used was E-glass, 136 tex
yarn. This was woven by CS-Interglas Ltd. into a plain
weave structure of 7.1 ends/cm in the warp direction and7 picks/cm in the weft direction to give an areal density of
196 g/m2. Twelve fabric layers were cut from the roll stock
and stacked so that weft and warp were aligned in the same
direction for every layer. The fabric preform dimensions
were 355 ϫ 355 mm and 440 ϫ 168 mm for the RTM and
wet lay-up plaques respectively. The matrix was an epoxy
resin supplied by Ciba Polymers. This was a two-part
system of LY-564 resin and HY-2954 hardener which is
intended for both RTM and hand lay-up applications. The
mix ratio used (as recommended by the manufacturers) was
100:35 parts by weight resin to hardener.
Processing
The RTM technique consisted of first mixing the resin
and then degassing it for 1 h at 30ЊC to remove air
suspended in the liquid. It was then injected into the tool at
75ЊC at a supply pressure of 750 mbar (absolute). After
injection, the temperature was increased to 100ЊC at 0.64ЊC/
min (this relatively slow heating rate was a consequence of
the small oven and high thermal mass of the mould). The
resin supply pressure was increased to atmospheric and the
composite cured at this temperature for 1 h. The plaque was
then demoulded and subsequently post-cured at 145ЊC for
368 min. These temperature and pressure conditions areshown in Figure 1a. The tool, as seen in Figure 2, consisted
of an aluminium base containing thermocouples for process
monitoring and oven temperature control. Resin inlet and
outlet ports were also positioned on the base plate. In order
to achieve a good surface finish on both sides of the
moulding a flat metallic caul plate was used. The area of the
caul plate was slightly larger than the fabric to form a
peripheral gap which would permit the resin flow front to
surround the preform and form a peripheral gate. The tool
was sealed using a rubber gasket and nylon bag arrangement
to which a vacuum was applied. The purpose of the vacuum
was twofold; firstly vacuum only was used to impregnate the
dry fabric, and secondly to compact the fibre to obtain ahigher volume fraction.
The hand lay-up/autoclave technique consisted of mixing
the resin and then using it in 12 approximately equal
portions to impregnate each layer of the cut fabric. The resin
was poured onto a sheet of nylon film and rolled out to the
size of the fabric, which was then placed carefully onto the
thin film of resin. The fabric was then lightly rolled in order
to assist wet-out and then transferred to the tool base plate.
All 12 layers were treated in the same way and laid up on top
of each other. Release film was placed over the top fabric
layer followed by a metal caul plate and finally two layers of
breather cloth. The assembly was then vacuum bagged andtransferred to the autoclave, where it was subjected to the
temperature and pressure conditions shown in Figure 1b.
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The temperature was ramped at 4ЊC/min to 50ЊC, and then
held at this temperature for 1 h before being further ramped at
3ЊC/min to 100ЊC, where it was held for 1.5 h before beingfinally ramped down at 4ЊC/min to 40ЊC. As well as there
being a vacuum applied to the mould cavity, the autoclave
pressure was raised to 12 bar (gauge). After the thermal cycle,
the tool was removed from the autoclave and the flat
plaque demoulded before subsequent post-cure at 145ЊC for
452 min.
Preparation and analysis
The plaques were sectioned using a diamond wheel.
Specimens were then polished to the required dimensions
using 320 grit wet and dry paper. A grid size of 10 mm wasdrawn on the plaques to map thickness variation and the
thickness was measured at the line intersections using a
micrometer fitted with barrels of 6.8 mm in diameter. The
Composite Research Advisory Group (CRAG) standards for
tensile, flexural and ILSS methods 302, 200 and 100respectively were used19. The DMA method required a
60 mm ϫ 10 mm sample which was a flexure bar cut from
the composite plaques, whereas the DSC technique only
required a 10 mg sample weight.
Void content was calculated by comparing the composite
density calculated by resin burn off (to ASTM D2584-94)
and the value calculated by the liquid displacement method.
The latter used a Sartorius Density Determination Kit
(Model Number YDK 01-OD) coupled to a Sartorius MC1
series balance (Model Number RC 210 D). Prior to the tests,
the samples were conditioned in a humidity chamber at
23ЊC and 50% relative humidity for 40 h. Five samples were
taken from along a length of the mouldings, in order to give
a representative void content.
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798
Figure 1 (a) i, RTM temperature cycle; ii, RTM pressure cycle. (b) i, Autoclave temperature cycle; ii, Autoclave pressure cycle
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RESULTS
The properties of the RTM and wet-layup/autoclave plaques
are compared in Table 3. The results (Table 3b) show that
autoclave moulding achieved an extra 13.5% fibre volume
fraction due to the superior compaction pressure of 13 bar.
This produced increases in flexural and tensile strength and
modulus. However, it is worth noting that the RTM plaque
was compacted and impregnated at a fibre volume fraction
of 50% using only the pressure difference caused by
evacuating the mould cavity. It is also interesting that the
ILSS arising from the RTM technique was 70% higher than
that from the autoclave method. Since ILSS is generally
accepted to be a good indicator of matrix quality and void
content, this suggests that RTM more than adequately
matches the autoclave process in terms of matrix quality.
However the slightly lower degree of cure of the RTM
sample (Table 3c) is also likely to be a contributing factor
here. In order to determine the influence of the processing
method on the other mechanical properties, two approaches
were considered. The first was to normalise the properties
on the basis of the volume fraction of fibre. The nominal
fibre volume fraction was calculated from the tex, fibre
density and yarn spacing in the fabric (which provided the
reinforcement superficial density) and the volume of the
laminate, using the mean thickness values from Table 3b.
The normalised flexural strength and modulus for the
autoclaved laminates are 9% and 6% higher than those
achieved using RTM while in tension, the situation isreversed with the RTM laminates exhibiting increases of 4
and 8% in normalised strength and modulus respectively
over the autoclaved laminates. The second method involved
the estimation of tensile properties using a rule of mixtures.
This was done by considering the effective volume fraction
of fibre in either the warp or the weft direction (approxi-
mately 50% in each case) and neglecting the reinforcing
effect of the transverse fibres. The effective fibre volume
fraction in each of the two yarn directions was determined
by the following expression:
V f ¼nsestexs
10t c r
s
where V f is the fibre volume fraction expressed as a percen-
tage, ns is the number of plies, e s is the number of yarns per
metre, texs is the linear density in g/km, t c is the final com-
posite thickness (mm) and rs is the fibre density in kg/m3 .
The matrix properties (tensile strength and modulus)
were taken from the supplier data sheet and the fibre tensile
modulus was also taken from supplier information. The fibre
tensile strength was calculated by measuring the tensile
strength of one yarn. The constituent properties and the
warp direction fibre volume fractions used in the ROM
calculations are shown in Table 4.
For both processing techniques the laminate properties
were over-estimated. The estimated tensile strengths were
20% and 28% higher than the measured values for the RTM
and autoclaved plaques respectively while the equivalent
tensile moduli approached 50% error in both cases. This is
attributable largely to the simple nature of the ROM
approach and highlights the difficulty of predicting proper-
ties in woven structures. The simplifying assumptions used
in the calculation include; a non-crimp fibre architecture;
zero void content; continuous and unidirectional fibres (in
each yarn direction); that the load was applied along the
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Table 3 Laminate properties arising from wet lay-up with autoclave consolidation and RTM
Property RTM Autoclave RTM% differ-ence
(a) Mechanical PropertiesILSS (MPa) 17.73 (%CV ¼ 4.9), normalised ¼ 35.16 10.45 (%CV¼ 2.4), normalised ¼ 16.35 þ69.7%Flexural strength (MPa) 415.7 (%CV ¼ 3.8), normalised ¼ 824.4 571.2 (%CV¼ 5.3), normalised ¼ 893.8 ¹27.2%Flexural modulus (GPa) 19.63 (%CV ¼ 3.2), normalised ¼ 38.93 26.43 (%CV¼ 5.6), normalised ¼ 41.36 ¹25.7%Tensile strength (MPa) 274.0 (%CV ¼ 8.2), normalised ¼ 543.4 332.9 (%CV¼ 6.6), normalised ¼ 521.0 ¹17.7%Tensile modulus (GPa) 13.91 (%CV ¼ 7.9), normalised ¼ 27.58 16.36 (%CV¼ 10.5), normalised ¼ 25.60 ¹15.0%Calculated theoretical tensile strength (MPa) 348.3 421.6 ¹17.4%Calculated theoretical tensile modulus (GPa) 19.87 24.61 ¹19.3%
(b) Physical propertiesCalculated V f from mean thickness (%) 50.4 63.9 ¹21.1%Mean thickness (mm) 1.775 (%CV ¼ 4.3) 1.394 (%CV ¼ 2.1) þ27.3%Voidage (%) 1.507 (%CV ¼ 13.4) 1.566 (%CV ¼ 14.1) ¹3.8%
(c) Thermal propertiesT g (DMA) 145.0 (%CV ¼ 0.4) 155.6 (%CV ¼ 0.6) ¹6.8%T g (DSC) 142.9 (%CV ¼ 2.1) 149.9 (%CV ¼ 2.2) ¹4.7%
Figure 2 RTM tool construction
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entire length of the fibre; that the fibres and matrix wereperfectly bonded; that all fibres broke simultaneously after
application of the tensile load; and that there were no
variations in constituent properties or fibre volume fraction.
In practice, crimp and misalignment of the yarns during
handling means that some departures from the ideal
architecture are inevitable. Consequently, the properties
will be reduced compared to the estimated values. The use
of a plain weave, which maximises crimp, magnifies these
effects. Also, prior to separation, damage mechanisms such
as disbonding and delamination mean that the tensile load
will no longer be shared by the two phases in the simple way
assumed by the ROM. In addition, manufacturing induced
defects such as fibre breakage, matrix cracking and voidageall contribute to reductions in measured strength and
modulus.
Micrographs taken from representative areas of each
laminate type are shown in Figure 3. The effects of the
higher autoclave pressure are immediately apparent from
the high degree of compaction which is evident in both the
90Њ fibre bundles which are visible and in the lenticular 0Њ
tows running out of the section. Also, there are fewer visible
resin rich areas in the autoclaved sample, which is again
attributed to the higher pressure which caused nesting of the
plies and compaction of the tows. These two effects are
consistent with the higher fibre volume fraction of theautoclaved sample. It is also interesting to note that neither
micrograph displays any significant voidage, which con-
firms the quantitative results produced by the density
measurements at approximately 1.5% in both cases.
The results suggest generally that the only inferiority in
the vacuum impregnation technique, compared to the wet
lay-up and autoclave alternative for glass fibre/epoxy, arises
from the lower fibre fraction which was achieved. Clearly,
the fibre fraction could be raised (for vacuum impregnation)
by introducing a secondary consolidation process, although
it remains to be seen if such laminates can be filled
satisfactorily using the low pressure gradient which is
available from vacuum impregnation if the entire compac-
tion process is carried out beforehand. If this is not the case,
then a shift to conventional, pressure-driven RTM process is
indicated. Mould stiffness is critical in such processes to
control the fibre volume fraction and minimise the part
thickness variation. However, tooling costs are required to
be minimised to reduce the part cost and increase the
flexibility in production volumes. In this study, a flexible
upper tool was used. Atmospheric pressure was used to
compact and impregnate the fabric reinforcement.
Comparison of the thickness distribution compared with
that for an autoclave consolidated plaque (Table 3b) shows
that the coefficient of variation (CV) for the RTM plaquewas twice that from the autoclave. It seems reasonable to
expect corresponding variations in the fibre volume fraction
and mechanical properties within the plaque (particularly in
intensive properties such as the strength values). However,
no significant difference in the variability of properties is
evident from Table 3.
DMA and DSC were used to determine the T g of both
composites. DMA is often reported18 to give T g results that are
10ЊC higher than those given by DSC. Five tests on eachmoulding were completed for both methods and the mean
values are listed in Table 3c. The results from both techniques
suggest that the T g of the RTM composite is slightly lower than
that from the autoclave with the DMA results, suggesting a
greater difference between the two processes. This difference
arises not from any inherent differences in the processing
methods, but rather in the different cure schedules which were
used. The results indicate that the RTM post-cure cycle was
inadequate. This was attributed to the reliance on data from
simulated thermal cycles using a dielectric analyser (DEA) to
determine the post-cure requirements. The latter were
conducted using similar fibre volume fractions, whereas thedifferent consolidation pressures during the two processes
produced different volume fractions. Clearly, this factor will
Wet lay-up and resin transfer moulding: D. Abraham et al.
800
Figure 3 (a) Autoclave micrograph at 35 ϫ magnification. (b) RTMmicrograph at 35 ϫ magnification
Table 4 Constituent properties
Constituent Tensile strength (MPa) Tensile modulus (GPa) Warp V f (RTM) Warp V f (autoclave)
Fibre 1139 71 0.253 0.322Matrix 80 2.5 0.747 0.678
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influence the thermal properties and therefore the progression
of the cure cycle. The RTM plaques had a higher resin content
with lower thermal conductivity, a higher heat capacity, and
resulted in a lower degree of cure. The intrinsic differences
between DSC and DMA measurements must also be
considered. The former measures a thermochemical change
in the resin matrix, and the difference in T g measurements isdue only to the different states of cure present in the two
sample types. However, since the DMA measures a thermo-
mechanical effect, this is more likely to be influenced by the
fibre content, and this may magnify the apparent differences
between the two processes. It is also worth noting that the CV
for the DMA results is lower than that for the DSC. The
improved repeatability may be related to the effects of sample
size and the somewhat variable fibre content in the relatively
small DSC samples.
CONCLUSIONS
RTM, based upon vacuum impregnation, offers materials and
labour cost savings compared with conventional autoclaving,
operational and health and safety benefits compared to wet
laminating and reductions in tooling costs compared to normal
(positive pressure driven) RTM. This study has shown that
laminate quality from vacuum impregnation is comparable
with autoclave consolidation when the same materials are
used. However, the fibre fractions available from vacuum
impregnation are somewhat lower due to the lower con-solidation pressure and this influences the mechanical and
thermal properties of the laminate. A secondary consolidation
process would overcome this problem.
ACKNOWLEDGEMENTS
The authors would like to thank CS-Interglas for supplying
the glass fibre and also David Hurry of Ciba Polymers for
supplying the epoxy resin. Also, thanks are due to RoyCarton and Maurice Jamieson for their assistance in
processing and testing the samples respectively. Financial
support from the European Regional Development Fund
(ERDF) and the Technology Development Programme
(TDP) is also gratefully acknowledged.
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