Post on 10-May-2020
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PRODUCTION OF CLASS-8 TRUCK TRAILER BED USING cPBT
THERMOPLASTIC PREPREG & VACUUM-BAG PROCESSING
James Mihalich
Cyclics Corp.
Abstract
An ambitious, multi-year program was recently undertaken in Europe to improve the
sustainability of composites used in transportation – particularly with respect to the ability to
develop thick parts with large surface areas economically. The goal was to develop an
advanced composite material that has a thermoplastic matrix; is tough and durable; emits no
volatile organic compounds (VOCs) during production or in part use life; has a high strain to
failure as well as excellent fatigue, impact, chemical resistance, and hydrolytic stability; allows
the use of high fiber volume fractions to reduce component mass; is of low density and high
specific mechanical properties; and also is low cost and fully recyclable (melt-reprocessable) to
reduce scrap and improve material recovery at end of part life.
The program worked with a novel, highly reinforced thermoplastic composite based on cyclic
oligomers of polybutylene terephthalate (cPBT). This cPBT / fiberglass was used to produce
thermoplastic prepregs that were then evaluated in vacuum bag (VB) processes, while liquid
cPBT / fiberglass systems were assessed for their use in vacuum infusion (VI) and vacuum-
assisted resin-transfer molding (VARTM) – all forming processes traditionally used for
composites with thermoset (not thermoplastic) matrices. Once the best material / process
combination for the program was determined, and small-scale testing confirmed the finished
composite provided sufficient mechanical performance, the prepreg / vacuum bag process was
used to mold one of the largest thermoplastic parts ever produced: a 3-piece structural floor for
a flat-bed trailer for a Class 8 truck, which is the focus of this paper. .
While more work is needed to make this technology practical on production-scale
equipment, the project did demonstrate that manufacturers in the transportation segment now
have the opportunity to produce sizeable, high-quality, structural components with lower mass
and greater toughness, while simultaneously reducing environmental emissions, improving
worker safety, and allowing for recyclability and materials recovery (via melt reprocessing).
Increasing the Sustainability of Transportation Composites
There are many compelling reasons why more composites (and less steel and aluminum)
should be used in the global transportation market, starting with the opportunities to reduce
mass (and increase fuel economy), eliminate corrosion, reduce or eliminate paint, improve
damage resistance and long-term aesthetics, increase functionality, reduce assembly
operations, and lower costs. While neat and reinforced thermoplastics have come to dominate
passenger vehicle interiors and are gaining share under the hood, the majority of structural and
semi-structural automotive components molded in composites have long been dominated by
thermoset matrices, particularly for chassis and exterior body. Among composites, thermosets
hold even greater share in other transportation segments, ranging from marine and
aviation/aerospace to heavy-truck, agricultural, and mass-transit.
The author wishes to extend special thanks to team members, Giles Fryett –BAE; Roman Scholdgen, Lionel Winkelmann, & Jan Wessels –
IKV; Rami Haakana – Ahlstrom; Alfredo Correia – Basmiler; and Mat Turner & David Goodwin – EPL, for their assistance on this paper.
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Generally the dominance of thermoset over thermoplastic composites in structurally demanding
transportation applications has occurred because the former offers greater stiffness and
strength at comparable wall sections, can withstand higher temperatures under load before
sustaining creep, and provide higher thermal and chemical stability. This is a function both of
cross-link density and the typical ability to achieve higher fiber volume fractions of
reinforcements – particularly continuous fiber or fabric weaves – due to better wetout and
coupling between matrix and reinforcement in thermoset polymers. In virtually all cases,
thermoset resins used in structural composites applications begin life in liquid form, which either
is used to produce prepregs (as with epoxies for vacuum-bag or vacuum-infusion processes),
or B-stage semi-finished goods (as with unsaturated polyesters and vinyl esters for sheet-
molding compound (SMC)), or are designed to be compounded at the press (as with urethanes
for structural reaction-injection molding (SRIM) or unsaturated polyesters and vinyl esters for
bulk-molding compound (BMC)). The base resin’s initial very-low viscosity helps facilitate high
wetout of reinforcements and production of composites with higher fiber volume fractions that
are typically achieved in thermoplastics, thereby producing stiffer parts. In the case of liquid
resins, low initial viscosity also facilitates longer flow lengths, making it easier to produce large
parts at low pressures and lower energy requirements. And many thermosets can be cross-
linked at much lower temperatures and pressures than thermoplastics, and under isothermal
(constant-temperature) or near-isothermal conditions, further reducing energy usage and
simplifying processing equipment and tooling.
However, these advantage come at a price, as the deficiencies of thermoset composites
are often slower processing times (owing to the need to polymerize and cross-link the matrix),
much higher post-mold finishing steps (which can make piece cost high despite lower raw
material and processing costs), and challenges in fully automating many thermoset molding
processes, which again impacts costs, particularly at higher production volumes. Whether
supplied in liquid or solid prepreg / semi-finished sheet form, nearly all thermosets require
special storage, handling, and disposal prior to cross-linking owing to shelf-life and toxicity
issues. Further, environmental regulations on VOC emissions during processing necessitate
installation of air-handling equipment in processing facilities and special protective clothing for
workers, adding still more to production costs. And the tendency of these materials to continue
to emit VOCs during use life puts end users and the environment at risk – a fact that is
beginning to draw the attention of both governmental and non-governmental organizations,
each of which are starting to call for (or already legislating) tougher post-mold emissions
standards. Additionally, thermoset composites’ higher stiffness comes at the expense of impact
strength, making them inherently brittle. Last, the inability to melt-reprocess in-plant scrap, and
the lack of economically viable post-consumer recycling opportunities are of concern in Europe,
where tough end-of-life recycling requirements challenge all automakers. While many
thermosets can be reground and used as filler during composites processing (at low loading
levels) or during production of concrete or asphalt, most thermoset scrap is considered to have
little economic value.
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On the other hand, there are many opportunities for thermoplastic composites if the
aforementioned thermo-mechanical issues can be improved upon, as these matrices typically
emit little or no VOCs during processing (since they are fully polymerized as delivered) or during
subsequent use-life. They also tend to be lower density (helping further reduce part weight and
costs), there are many fully automated methods for processing them (albeit on higher cost,
higher pressure and temperature equipment), and cycle times can be significantly shorter.
Additionally, they require fewer post-mold finishing operations and offer greater in-mold
decoration options, they have higher toughness and impact strength (so are less prone to
damage or brittle failure), and they are fully melt-reprocessable, so both in-plant scrap and
material from end-of-life parts can be recaptured and recycled (and in most cases that material
has good economic value when reused). The challenge is to increase stiffness and strength via
higher fiber volume fractions without requiring high conversion pressures and temperatures,
which necessitate use of complex and costly processing equipment and tooling, and become
physically and financially impractical for very-large parts, particularly at low-to-moderate
production volumes.
Since the melt viscosities of fully polymerized thermoplastics are typically 500-1,000x higher
than that of most thermosets, achieving high fiber loadings without high pressures and
temperatures has been a long-standing challenge. However, there is a particular class of
thermoplastics called oligomers that represent only a few monomer units, rather than the
thousands and tens of thousands of monomers that conventional polymers contain. Most
plasticizers and paraffin wax are oligomers. There are also cyclic (ring-shaped) oligomers of
several common engineering thermoplastics, including polycarbonate, nylon 6, and
polybutylene terephthalate. These oligomers process like thermosets (since their initial melt
viscosity is very low and they can be processed isothermally), yet they reactively polymerize
(with nylon and PBT also crystallizing) to form parts that possess thermoplastic properties.
Among cyclic oligomers, polycarbonate offers very-high impact strength and good stiffness,
plus excellent optics. However, it is not commercially available and it has poor chemical
resistance, particularly to aromatic hydrocarbons and petroleum products that too often are
encountered in transportation applications. Hence this material was eliminated from
consideration. Reactive nylon 6 offers high toughness with good stiffness, plus good thermal
and broad chemical stability, but it is even more hydrolytically sensitive than conventional
polyamides, so is not a good choice for applications where dimensional stability is important –
particularly in humid environments. Polybutylene terephthalate, on the other hand, offers good
toughness, stiffness, and thermal stability, plus has broad chemical resistance and is only
moderately moisture sensitive during initial polymerization. Furthermore, cyclic PBT can
produce composites with higher glass loadings and with less dry spots than most
thermoplastics, which made the material of great interest for this project.
Assembling the Team & Setting the Goals
Since Europe (as a geography) has long struggled with very-high fuel prices, it has
traditionally been more aggressive in using composites to take weight out of vehicles in order to
increase fuel economy. Hence, about 5 years ago, a team was assembled in the United
Kingdom (UK) to study the viability of using glass-reinforced unsaturated polyester to produce
very-large parts via low-cost tooling and vacuum infusion. Called Roadlite, the program
produced a 2-axel, 10-m trailer for urban delivery, saving 400 kg, increasing stiffness 18%, and
reducing CO2 emissions by 400 kg / year vs. steel. That study formed the foundation of the
current work.
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Given the present opportunities in the transportation sector for composites in general, there
is great for a thermoplastic-composite alternative that could produce much larger parts
economically for low-volume production than injection- or compression-moldable thermoplastic
composites (e.g. long-fiber thermoplastics (LFT) and in-line compounded direct-LFT (D-LFT),
and glass-mat thermoplastics), plus offer weight reduction, greener processing, greater impact
strength, and the option of recycling vs. thermoset composites, while still meet the thermo-
mechanical requirements of structural parts. In order to create such a material, a team –
comprised of composites consultancy, EPL Composite Solutions (Loughborough, U.K.); specialty-papers and fiber-composites producer, Ahlstrom Corp. (Helsinki, Finland); cPBT resin
supplier, Cyclics Corp. (Schenectady, N.Y., U.S.A.); plastics processing institute, IKV1 (RWTH
Aachen, Germany); defense-contractor, BAE Systems (London, U.K.); and commercial truck
OEM, Basmiler (Viseu, Portugal) – was convened and a developmental program, partially
funded by the European Commission under Framework 6 was begun. Called “Cleanmould,”
the program’s goal was to develop a novel glass-reinforced PBT composite that could be used
to produce very-large surface-area, thick-section parts, which in turn could be processed on
inexpensive tooling or consumables, e.g., modified versions of the vacuum-bag, vacuum-
infusion, and VARTM processes. After all, advanced composites can produce very light and
stiff parts of tremendous size, but cannot do so inexpensively or quickly. A secondary goal of
the program was to see if a thermoplastic prepreg could be produced from cPBT, whose
prepolymerization viscosity is very low, allowing for high fiber wetout and therefore the
opportunity to produce composites with high fiber volume fractions, which in turn leads to parts
with high stiffness for a given wall thickness. .Assuming the concept could be proven out in
small-scale testing, a third goal was to produce a real-world transportation part in cPBT
composite: in this case, a previously steel and aluminum flatbed trailer.
Developing Resin & Reinforcement
About cPBT
Cyclic oligomers of (poly)butylene terephthalate (cPBT) are offered in 1- and 2-part
formulations, which are solid at room temperature. The standard form factor is a 1-part
precatalyzed pellet, although a second, uncatalyzed powder (plus separate catalyst) is also
available Upon heating above 190C and in the presence of a suitable catalyst, cPBT’s viscosity
quickly drops, its ring structures are broken and opened (via ring-opening polymerization) to
form short molecular chains, and – if the reaction is allowed to proceed far enough – then the
short-chain monomers are connected to each other via their functional end groups to form long-
chain polymers of PBT. These PBT chains can achieve high molecular weight – higher, in fact,
than is typically offered with commercial prepolymerized PBT in order to make the material easy
to process on conventional injection molding or extrusion equipment. However, with the
oligomeric form of PBT, since polymerization occurs in the tool after the reinforcement has been
wetout and the resultant composite has been shaped, pushing to higher molecular weight is not
only not an issue, but it actually becomes a benefit.
1 Institut fȕr Kunststoffverarbeitung, the Institute of Plastics Processing at RWTH Aachen University (IKV).
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With oligomers, application needs (for higher stiffness) drive molecular weight rather than
processing needs (to keep pressures and temperatures reasonable). The price paid for higher
molecular weight is, of course, the need to polymerize in the tool, which in turn means slightly
longer cycle times and different processing methods and equipment. What that buys, however,
is molded products with high stiffness and strength, good thermal stability (heat-deflection
temperature 200C and continuous-use temperature 100C), broad chemical resistance, and
excellent dimensional stability at lower weight, with minimal VOC emissions, and with an end-
product that can be melt-reprocessed (hence easily recycled).
Chain length / molecular weight (and therefore finished part properties) are impacted by a
number of factors, including conversion process, catalyst, temperature, rate of heat rampup,
moisture content of powder / pellet prior to processing, etc. Of interest for prepregs, the
polymerization reaction can be stopped partway through, although once halted, it cannot then
be restarted. Hence timing and careful temperature control are needed to allow the resin to
achieve good wetout of reinforcing fibers, but then quickly chilled so viscosity builds enough to
handle the resultant prepreg before polymerization actually begins. The polymerization process
is relatively rapid (from <1 to 6 min) and has no measurable exotherm. Actual cure time is
dependent on the catalyst used, part thickness, and the processing temperature. Obviously,
faster cure puts challenges on the processor’s ability to infuse the part well and shape it,
particularly in large, thick-sectioned designs, but the reaction can be quite quick. And unlike
most low-viscosity thermosets, cPBT produces no odor and no VOCs during processing,
making it a clean polymer to mold and handle, and a clean product for the end user and
environment.
The very-low viscosity of cPBT during melt processing means that it is ideal for use in low-
pressure forming processes such as casting, rotational molding (rotomolding), and a number of
conversion processes traditionally associated with thermoset matrices. Since pressures are
typically low during forming, molding equipment and tooling can be simpler and less costly, and
molded products have little or no residual stresses, so have excellent dimensional stability and
very-little post-mold warpage. Processing can be carried out at or near isothermal conditions,
since the polymer solidifies and crystallizes simultaneously at the processing temperature,
reacting and forming a solid as it continues to build molecular weight. This means that, unlike
conventional PBT, when molding cPBT it is not necessary to cool the tool to demold a part.
cPBT has traditionally been used to form highly reinforced thermoplastic composites; highly
reinforced castings for plug-assist and tooling-block applications; higher temperature
rotomolding resins; and as additives and compatibilizers for other polymer systems. Because of
its low melt temperature (<200C), and very-low initial melt viscosity (<40 cP), cPBT oligomers
can easily impregnates fibrous or particulate reinforcements, providing the opportunity to
develop composites with high fiber volume fractions2 (high loading levels), and therefore parts
with high mechanical properties, allowing the material to be used in applications where
traditionally only thermoset composites could compete. However, cPBT retains its
thermoplastic advantages of toughness (high impact strength and ductile failure), weldability,
thermoformability, and melt reprocessability (recyclability), and it does so at lower weight per
comparable wall section and mechanical performance than the thermosets it replaces.
2 Fiber volume fractions of 50% are easily achieved and values to 60% are not uncommon. However, to achieve values above
70% requires special processing and skill.
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Given cPBT’s unique properties (low initial viscosity, low melt temperature, high molded part
stiffness, and maintenance of thermoplastic benefits), the Cleanmould team worked initially to
select among all the commercial and developmental grades available for the material to find
several that would be well suited to produce highly loaded composites for structural
applications. Next, reinforcements were evaluated for compatibility with the matrix resin and the
applications being considered. Last, it was felt that if the resin could be pre-infused into the
fabric or fibrous reinforcement (as in a prepreg), that not only would it be easier to handle (in a
dry form) but that it would take little time to complete the infusion process, contributing to rapid
cycle times even in thick-section parts. Hence, the team had a goal to see if it would be
possible to create prepregs with cPBT and then determine which composite forming processes
such prepregs could be formed in to produce sizable composite structures for the transportation
market. Cyclics already had a commercial grade of cPBT that met program specifications and
favored production of the highest molecular weight and this grade was used subsequently in the
program.
Selecting an Appropriate Reinforcement System
Once the team had confidence in the matrix resin, it turned to the reinforcement system. A
number of different reinforcement types (fiberglass and carbon fiber) and forms (continuous /
unidirectional, chopped, and fabric weaves) were evaluated at the start of the program to
determine the effect of various reinforcements and sizing systems on base-resin processability.
Only the products selected for the vacuum-infusion processes are discussed here.
In the initial attempts to develop a resin / reinforcement system, Ahlstrom began with a non-
crimp woven stitched triaxial (+45°/-45°/0°) fiberglass fabric (complete with sizing and coupling
agents) that featured sparsely laid rovings and flow channels between roving layers to facilitate
infusion (see Figures 1a & 1b). Such a product could be used with either liquid forms of the
resin or could act as a carrier for solid compounds containing resin and catalyst and would
facilitate high flow rates and rapid infusion even under high vacuum compression. Two different
area weights were evaluated: a 1,300 g/m2 (gsm) product and a looser-weave 650 gsm
product, which used larger flow channels). They were supplied as wound sheets, which were
subsequently combined with resin in attempts to produce prepreg. These products are typically
also supplied with a light chopped-glass layer (veil) for stability in handling.
Figure 1a & 1b: Ahlstrom’s triaxial fiberglass fabric reinforcement with flow channels showing the two different fabric
weights tested.
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Development of cPBT Prepregs
Dry Mixing
The first attempt to produce cPBT prepreg was a low-tech dry-mixing process that involved
positioning rovings in a tool and then sprinkling powdered, 1-part (precatalyzed) resin over
them. For simplicity’s sake, the resin was applied via a drop spreader normally used to apply
lawn fertilizer (see Figure 2). Next, the glass / resin combination was heated and consolidated
in both a press and under a vacuum bag (30 min at 41.4 bar), after which the polymerized and
demolded parts were cut into plaques, which were tested to evaluate mechanical properties.
Figure 2: Initial attempt at producing prepreg via dry-mixing of reinforcement & resin. A drop spreader was used to
apply resin to reinforcement with limited success.
The initial (dry-mixing) application method turned out to have a number of problems. First, it
was not only difficult to distribute resin across the reinforcement but it was nearly impossible to
get it to adhere (since there was no heat source to provide limited melting of the powder).
Hence, most of it simply fell through the fabric or fell off to the side, and volume fraction could
not be accurately controlled across the width and length of the prepreg. This issue, in turn, also
meant that only flat prepreg could be produced, because any slope to the shape caused
powdered resin to fall off and collect at the bottom of the tool. While the hand-made prepreg
works well enough for small-scale lab tests, they are impractical for larger parts with shape, and
certainly would be undesirable in a commercial operation. However, despite the shortcomings
of this method, the initial work allowed Ahlstrom to evaluate different sizings and coupling
systems for the glass. Ultimately, a standard 1,200 gsm triaxial fabric was selected, and a sizing
/ coupling system typically used for epoxy reinforcements (R338) was found to be most suitable.
Since there were so many challenges with just dry-mixing glass and resin, and it was simply
impractically messy for a production environment and for a part with complex geometry, the
team determined that it would be necessary to develop a fabric / prepreg that could be stored
and handled by molders and that could be draped in tools with more complex geometry.
Although the team evaluated a number of different technologies, the brittle nature of
prepolymerized cPBT and glass, and the resin’s sensitivity to thermal stress, meant that only
two approaches proved successful, each of which are described below.
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Film Lamination
Film lamination (via the Meyer GmbH process) was the second method to be evaluated. It
was selected because it is a well-known process that is often used to produce prepreg on an
industrial scale with excellent prepreg consistency and repeatability of composite properties. It
also can be adjusted for a variety of fabrics. In this setup, the equipment heats powdered, 1-part
cPBT resin / catalyst and applies it as a thin film on one side of a woven fiberglass reinforcing
fabric before the resin is rapidly chilled and consolidated between rollers. Two types of prepreg
were manufactured with this method: the first was a triaxial fiberglass fabric (Ahlstrom’s 63051,
1,180 gsm, 60% fiber melt fraction (FMF)) and the second was a unidirectional fiberglass
(Ahlstrom’s (42024L, 1,200 gsm, 60% FMF). Figures 3-9 show the film lamination process
used to produce cPBT / glass prepreg for the program.
Figure 3: Powdered resin is loaded in the hopper.
Figure 4: Glass fabric is fed in from one end.
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Figure 5: Powdered resin is deposited onto the fabric while edges are cleaned of powder by vacuum.
Figure 6: Hot rollers melt the powder & initiate viscosity drop, facilitating coating of fabric. Before polymerization
can begin, chilled rollers consolidate and freeze-off resin on fabric.
Figure 7: Consolidated resin / fabric wound onto spool.
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Figure 8: Prepreg’s uncoated edges are trimmed off on both sides of the roll.
Figure 9: Final prepreg in roll form is removed from the line.
Because solid cPBT was applied to the reinforcement, and because the resin possesses
both a broad melting point and there is considerable variation in particle size, it was challenging
to quickly melt the solid resin and disperse it on the fabric without initiating polymerization,
which, once started and stopped, cannot be restarted again. That necessitated keeping melt
temperatures low, which made it difficult to fully infuse the sheet. Challenges getting good and
consistent coverage of resin across the reinforcement led to other problems during actual part
molding (which will be covered in later sections). Additionally, the film lamination process
yielded prepreg with resin on only one side, which necessitated a layup pattern that alternated
the resin-rich side to ensure good wetout of all layers.
While the resulting product was usable, the resin film proved to be thick, stiff, and brittle
(hence prone to breakage and flake-off), making it difficult to achieve good draping
characteristics in all but the simplest geometries, and also making it difficult to handle without
damage. Although prepreg from this process was used to mold the program’s test part, the
team knew going into the molding trials that a better option would need to be found if the
program were ever to gain commercial interest.
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Extrusion Coating
Toward the end of the Cleanmould project, another prepreg production trial was conducted
– this time at Polynov Ecole Polytechnique (Anjou, Québec, Canada). Here, an extrusion-
coating process on Davis-Standard equipment was used to create a more traditional prepreg
whose fibers were more thoroughly impregnated with resin in an amorphous state. Rather than
use a 1-part precatalyzed powder, as with the Meyer lamination process, at Polynov a twin-
screw extruder was used to melt neat (unmodified) cPBT resin in pellets form before metering in
liquid catalyst at the vent section of the screw. (The liquid catalyst provided greater process
flexibility, since it could be “turned on” during process startup and “turned off” during shutdown,
making it easy to flush the die at the end of a production run or in case of interruptions). The
now mixed and catalyzed compound was heated to 180C, then forced through a coat-hanger
die, which spread a film of molten resin over one side of the reinforcement fabric, which was
faced by a release liner on both sides (to prevent loss of liquid resin). The impregnated fabric
was then pulled through two temperature-controlled rollers (some of it run at 25C and some run
at 15C). The key was to quickly quench the polymer to keep the polymerization process from
starting. At the nip point, resin was forced under pressure, coating the top layer of fibers and
squeezing through gaps between the weave, which caused both sides of the fabric to be well
impregnated. After very rapid quenching, the resultant prepreg was still very flexible with good
drape and handling characteristics, making it better suited for use in molding complex
geometries. Figures 10-11 show the basic process setup.
Figure 10: Initial extrusion-coating setup showing hot-melt die delivering molten, catalyzed cPBT onto the
reinforcing fabric.
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Figure 11: By changing the angle of the die to achieve kiss-off against the fabric face, a second production trial led
to far better cross-machine resin distribution and eliminated a dripping problem seen in the setup in Figure 10.
During the trial at Polynov, this extrusion-coating method of delivering molten cPBT and
catalyst was used to produce a number of rolls of glass prepreg representing a variety of
coating weights. However, vs. the earlier film-lamination process, little improvement in coating
consistency across the surface of each roll was seen, leading to dry spots and spots that were
resin-heavy – likely because the type of die used and its initial position failed to create a melt
curtain (wall of flowing resin), which was necessary to achieve coating consistency. Although
the team felt the variation in coating thickness would probably even out during actual molding,
the desire was to have better control and consistency. That led to additional work with an
alternate die configuration where the head was placed flush against the fabric surface, much as
is done when applying hot-melt adhesives, dragging the die face across the fabric and
effectively creating backpressure (where previously there was none) and thereby forcing the
cPBT resin to flow across the die face. This led to significant improvements in cross-machine
coating consistency, which in turn stopped a dripping problem seen in the previous production
run.. While the process showed great potential for a commercial prepreg production method,
the available dies at Polynov were not well matched to cPBT’s low viscosity, and the program
did not have sufficient budget to commission a new die to be built. The team assumed that with
the correct die, the issues of inconsistent coverage would be eliminated. Additionally, it is
believed that Polynov’s unconventional setup coupling a hot-melt die to a thermoplastic extruder
might be exactly the combination that would work best for producing cPBT prepreg.
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Performance Evaluations
Small-Scale Testing
In parallel with the program to develop cPBT prepregs, EPL, IKV, and BAE Systems worked
together on another program to assess the viability of the resin in several thermoset molding
processes known to be able to produce large parts on inexpensive tooling. cPBT prepregs
developed in the Meyers film lamination process were evaluated in vacuum-bag consolidation,
while liquid versions of catalyzed cPBT plus dry reinforcements were evaluated in vacuum
infusion and vacuum-assisted resin-transfer molding.
As part of this work, standard test specimens were cut from plaques of cPBT composite
molded in all 3 processes and used to conduct small-scale laboratory tests to measure
mechanical properties (e.g. flexural strength and flexural modulus (ISO 14125), interlaminar
shear strength (ILSS, ISO 14130 and ISO 4585), and penetration impact (DIN 6603)). In
addition, accelerated-aging studies (at 100% humidity) were conducted on cPBT samples
submerged in deionized water at room temperature, 40C, 50C, 60C, and 70C for discrete
periods of time ranging from 0 to 256 days. The heat-aging studies compared flexural strength,
flexural modulus, and ILSS to evaluate loss of mechanicals over time and at elevated
temperature. Furthermore, small-scale testing of specimens cut from VARTM plaques of
competitive epoxy and unsaturated polyester – products traditionally used for large, structural
components for industrial and marine applications – were done to obtain comparative properties
in order to ensure cPBT would provide sufficient performance for the demonstration part.
Figure 12 looks at properties of cPBT composites (flex strength, flex modulus, and ILSS)
measured at 0o to the direction of the unidirectional layup in all 3 processes. Note that each
process achieved a different % volume fraction of glass reinforcement, with VARTM having the
highest at 63%, vacuum infusion at 57%, and vacuum bag at 55%. Figure 13 compares
flexural (bending) strength and modulus of cPBT (here identified as CBT) vs. unsaturated
polyester and epoxy produced by VARTM (with the same volume fraction as the cPBT
samples), while Figure 14 compares maximum force and energy absorbed during penetration
impact for these 3 materials. Figure 15 compares flexural strength and modulus, as well as
ILSS results of vacuum-bag cPBT specimens vs. vacuum-infused unsaturated polyester
(referred to as PE in the following graphs). Samples were cut at 0o & 90o to the unidirectional
glass reinforcement layer.
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Figure 12: Comparison of mechanical properties (flex strength plotted on Y axis; flex modulus & ILSS on Y’) for
cPBT composites from 3 different processes. Note each process achieved different % vol. fraction values for the
glass reinforcement, with vacuum bag being lowest (55%), VARTM the highest (63%), and vacuum infusion falling in
between (57%).
Figure 13: Flexural modulus and strength for epoxy, unsaturated polyester, & cPBT (CBT) composites molded in
the VARTM process with the same volume fraction of glass reinforcement.
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Figure 14: Maximum force & energy absorbed in penetration impact test for epoxy, unsaturated polyester, & cPBT
(CBT) composites molded in the VARTM process with the same volume fraction of glass reinforcement.
Figure 15: Flex strength, flex modulus, & ILSS results of vacuum-bag cPBT (CBT) specimens vs. vacuum-infusion
unsaturated polyester (PE) specimens cut at 0o & 90o to the unidirectional glass reinforcement layer (Day 0
properties).
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Process Evaluation & Large-Scale Testing
In addition to the small-scale laboratory testing of mechanical properties of specimens cut
from plaques, process evaluations were also carried out on both test plaques and a 3D molded
part that was the same design (at 1/6th-scale) as a critical component that had been identified
during the parallel design of the trailer bed. This work was used to help optimize processing
parameters for each molding method, as well as to select among the processes for the best
method of producing the demonstration part. During the processing studies, statistical methods
of experimental design were used to help determine the relationships between an array of
variables and the quality of the final component produced via vacuum-bag consolidation.
Results of this work yielded a “guidebook” for molding cPBT composites. Interestingly, the
studies showed that with a higher cure temperature (220C), the ramp-up rate became
insignificant. This was of major importance as the team considered whether vacuum=bag
processing of such a large part would be practical given that it would be heated in a relatively
slow convection oven. The work also showed that impact performance could be optimized by
changing the crystallization rate with controlled cooling or with higher processing temperatures.
While the specific details of these studies are outside the scope of the current discussion,
results of the comparison tests showed that cPBT composites could be used to produce parts
whose properties were comparable to the thermosets evaluated. As would be expected of a
thermoplastic, both flexural modulus and flexural strength of cPBT composites were better than
those of the thermosets. And while the epoxy composite studied did provide higher ultimate
properties, even in this early stage of development, the cPBT composites provided 80% of the
performance in a more damage-tolerant system that eliminated VOCs and offered recyclability
and lower specific gravity. Accelerated aging in 100% humidity indicated that despite a drop in
properties above 60C, the composite’s flexural modulus values remained high, which should
make it a good choice for use in stiffness-oriented designs like the trailer bed planned for the
program and that it should provide years of service. Although future work is needed to compare
the durability of cPBT composites to the benchmark thermosets, initial results looked promising.
Currently, EPL is continuing to investigate cPBT composites (primarily using VARTM).
Small-scale testing also showed that all 3 processes offered excellent and comparable
results with cPBT composites, although it equally demonstrated that the ultimate performance of
the cPBT laminates produced is heavily dependent on processing method and parameters.
Hence, it is also likely that – with refinement of equipment and methodology -- mechanical
performance of cPBT composites should be able to be boosted significantly from their current
levels. A recycling study also was conducted to ensure the cPBT composite could be melt-
reprocessed and reused at end of part life, which further demonstrates its sustainability vs.
competitive thermosets.
After evaluating cPBT composites in the 3 processing methods, vacuum-bag consolidation
was selected as the most appropriate process to mold the demonstration part owing to the
excellent properties of its laminates as measured in the small-scale testing protocols (above),
as well as the simplicity of the process and low cost of its tooling for even very-large parts,
which made it the simplest to scale up for the demonstration part. While VARTM achieved
higher volume fractions and lower cycle times, with vacuum infusion only slightly slower, both
were ruled out owing to pot-life issues for such a large part.
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Trailer Bed Validation Study
In order to prove that the new cPBT composites could be used in a real-world transportation
application, a single 13.6-m, 40-metric ton, tri-axle semi-articulated trailer was designed and
produced as part of the program. The design phase of the program began by deconstructing a
steel trailer built by Basmiler (see Figure 16) analyzing its construction, and calculating its
rigidity. After researching relevant highway regulations and hardware / mounting options, EPL
designed a lightweight composite monocoque structure, which offered all the benefits of the
steel system (carries the same payload in the same load area and provides the same stiffness)
but brought additional benefits of 30% lighter chassis (equating to 16% lower vehicle mass),
lower drag coefficient (5% reduction without crosswind and 13% with an 8o crosswind), excellent
corrosion resistance, and low-maintenance aesthetics. It was important to match the steel
trailer’s stiffness so a superstructure (e.g. box or curtain) could subsequently be attached and
would sustain the same load inputs as with the steel trailer. However, because the cPBT
composites are an order-of-magnitude less stiff than steel (~ 30 GPa vs ~200 GPa), it was
necessary to significantly increase the 2nd moment of area for the chassis cross-section through
clever design. To do this, the full space envelope permitted by practicality, legislation, and
aesthetics was exploited to increase the depth of the structure, thereby allowing lower cost,
lower modulus composite materials to be used without sacrificing stiffness.
As noted previously, limitations to the draping characteristics of the prepreg restricted the
design of trailer components to fairly flat and simple shapes with minimal changes in curvature.
However, with careful selection of tires, axles, and other hardware, a constant cross-section
design was created that both met the project’s structural requirements as well as ensured
manufacturing capability. Additionally, tooling geometry was kept simple to ensure technicians
could physically layup the stiff prepreg in the tool.
The trailer’s flat, smooth exterior creates low drag surfaces and further work on packaging trailer
hardware further reduced air-flow disruptions. Later in the program, a computational-fluid
dynamics (CFD) study was conducted to see if the drag coefficient of the trailer fitted with a box
superstructure could be reduced further. Virtual prototyping with additional aerodynamic
bodywork appendages (similar to existing solutions) was conducted. With no real reduction in
load space, the CAD model of an aerodynamically optimized cPBT trailer with an organic shape
to its underside and box superstructure showed that drag could be reduced an additional 12%
vs. steel with no crosswind, and 21% with an 8o crosswind – a value that should translate to
roughly a 10% direct fuel saving and therefore reduced operating costs. Even more significant,
a measured weight saving of 1.5 metric tons means the trailer has 1.5 metric tons of extra
carrying capacity without exceeding legal load limits. This helps reduce the number of trips
required to carry the same contents, possibly cutting emissions (and trips) by as much as 50%
(with attendant fuel savings and CO2 reductions).
Although further specifics about the design, analysis, and virtual prototyping of the trailer fall
outside the scope of this paper, the final design, shown in Figures 17 and 18, consisted of 3
major components – the hull, spine, and deck. At slightly more than 600 kg, the hull alone is
believed to represent among the largest single structural thermoplastic composite molding ever
produced, and when combined with the spine and deck, it would be the largest.
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Figure 16: Original steel trailer built by Basmiler whose design was deconstructed and became the basis for the
new all-thermoplastic-composite demonstrator.
Figure 17: New cPBT composite design developed by EPL to meet both the program’s structural requirements as
well as to be manufacturing capable with the prepreg system used.
Figure 18: Another view of the completed design for the all-thermoplastic composite trailer.
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At 12 m2, the hull is dimensionally the largest of the 3 components and also features the most
complex geometry. The “goose-neck” area of the hull features several radii, a double curvature,
and is the most highly stressed region since it represents the location where the trailer’s deep
load-bearing section transitions to its shallow front end, which, in turn, is where the trailer is
coupled to the tractor unit’s “fifth wheel.” The goose-neck section was considered to be the
most critical aspect of the trailer’s complete design and was the one most carefully investigated
during the previously mentioned processing studies. There, a 1/6th scale model of the section
was produced in cPBT using all 3 molding processes and evaluated for performance. Once
mechanical tests assured the team that the laminate quality was as good in the scale-model
moldings as in the flat plaques, and thus comparable with thermoset results, the program
moved to vacuum-bag production of a single full-size cPBT composite trailer.
Vacuum Bag Production of cPBT Trailer
Basic Process Steps
The basic process was that plies of cPBT prepreg were placed in the tool; the laminate was
bagged with inlet and outlet manifolds, then heated to 100C; air from a compressor was dried,
then sucked through the laminate stack via a high-flow pump with a goal of achieving as large a
flow of dry air as possible without disturbing the stack; after 2 h the purging of dry air was
stopped and a high-vacuum pump was engaged; the laminated was heated to 220C with a
dwell time of 1 h; after this, the laminate was allowed to cool down naturally; and the
consolidated, cured part was demolded. A schematic of the vacuum bag setup used in the
program can be seen below in Figure 19, and in Figures 20a & 20b, photographs of the novel
consumable layout required for the in-situ drying process after layup are shown.
Figure 19: Schematic of vacuum bag setup used to produce trailer components.
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Figures 20a & 20b: Novel consumable layout (under vacuum bag in steel tool on wheels (buck) for trailer
components during in-situ drying process in conventional convection oven.
Tooling
Throughout the program, a series of test tools were produced and evaluated in a variety of
materials to meet the demanding requirements of high glass-transition temperature (Tg, 200C),
dimensional stability / accuracy, low coefficient of thermal expansion (CTE, to ensure good part
release during cool down), good surface finish, vacuum integrity, and low cost. While
composite tooling could have been used, to achieve such large shapes in a matrix resin with
sufficient Tg would have been costly. Therefore, final prototype tooling for all 3 trailer
components was produced by Basmiler using 5-mm-thick sheet steel, which better matched the
CTE of PBT. Interestingly, as the steel tooling was heated with prepreg reinforcements inside,
the tool would expand rapidly, while the prepreg would expand only slightly. However, upon
cooling, the consolidated part shrank faster and to a greater degree than the steel tool, making
it easy to demold. Hence, the tooling for this program was cut 16-mm wider and 80-mm longer
than the target component’s dimensions. In addition, each tool was coated with a common, 75-
μ thick self-adhesive, fiber-reinforced polytetrafluoroethylene (PTFE) film supplied by Tygavac.
Aside from cleaning the surface of each tool with acetone prior to applying the PTFE film, no
further treatment was required. Figures 21 to 24 show different views of the trailer’s 3 molds, as
well as the buck in which parts were laid up and moved into the oven for drying, prior to
vacuum-bag consolidation.
Prepreg Layup
As previously noted, 2 types of prepreg were produced via the Meyer film lamination
process: one with triaxial glass and one with unidirectional glass. Both prepregs were
positioned with 0o direction running the length of the tool. Since this type of prepreg had resin
on only one side (and is dry on the other side), to promote part consistency, the direction of
resin-rich face was alternated in each layer of the layup stack. Both prepreg rolls were 1.27-m
wide, so insufficient to completely span the “deck” tool. Hence, each ply consisted of 3 pieces:
a bottom, a left-hand side, and a right-hand side. A “pyramid” arrangement was selected for the
overlaps. Plys 1 and 2 were cut to the same width, then 3 and 4 were trimmed by 100 mm (50
mm on each side) in order to create the pyramid shape shown in Figure 25. This strategy was
selected to reduce bridging in the radii.
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Figures 21 to 24: Technician cleans the tool surface of the “hull” (top left) and “spine” (top right) tools prior to
application of PTFE film to “deck” tool (bottom left). Layup buck (bottom right) holds tool with prepreg during layup
and later during in-situ drying.
Figure 25: Layup pattern for prepreg in the tool showing “pyramid” arrangement in the bottom.
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In addition, high-temperature consumables from both Aerovac and Tygavac were used.
Spray adhesive was applied locally to secure the breather fabric. Tacky tape was applied
straight to the PTFE-film on the tool surface. A thermocouple was adhered to the outside of the
bag via flash tape. Figure 26 shows the film prepreg being laid into the trailer hull tool.
Figure 26: Prepreg being laid up inside the critical “goose-neck” section of the hull tool, one of 3 used to produce
the demonstration trailer. Note the clamps and wooden tampers used.
It is interesting to note that the method, temperatures, and timescale used to mold the
critical section were all based on values used in the lab-scale work conducted at EPL. Only the
volumetric-flow parameter of dry air was increased to correspond with the increased volume of
the component.
The critical section was demolded as soon as the thermocouple read 100C, which in
retrospect was perhaps too soon as it may have led to some unnecessary shrinkage and
cracking. The component released from the tool with minimal effort and was slid down the tool
and out into the yard so it could be inspected in daylight, as shown in Figure 27. The biggest
problem noted after demolding was that significant distortion and shrinkage had occurred in the
critical goose-neck section of the test part, likely due to an unbalanced laminate and the high
CTE of the resin. While the distortion was significant in absolute numbers, on the low-tolerance
trailer, it was judged to be at an acceptable level. Some additional defects resulted from the
prepreg material being difficult to position in the tool, such as areas of bridging and creasing of
the reinforcement, and dry spots that resulted from resin falling off the brittle prepreg during
layup. Generally, these issues would exist with any heavy-weight fabric and likely could be fixed
either with intensifiers or spray adhesive, or by using prepreg with better drape properties, such
as that produced later in the program on David-Standard equipment.
Upon demolding, the full-scale parts also showed excellent consolidation throughout and
appeared to have a good level of rigidity. The general finish was quite good, although some
defects were noted.
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Figure 27: Full-size molding of critical section of the trailer.
Because the cPBT is self-releasing from the tool, it was not possible to leave the hull in the
tool after molding to facilitate subsequent bonding of the internal structure (bulkheads, bearers,
and spine) or the deck. Although there were also some issues scaling up certain equipment
from laboratory to commercial scale, the quality of the final component far exceeded
expectations – particularly in light of the early stage of development it actually represents. The
team learned a great deal during this program about working with cPBT composites on a large
sale. Further work should yield a step change in increased quality.
At 13.6-m long, 2.5-m wide, with thickness varying from 8-15 mm, a total area of 50 m2, and
weighing in at over 600 kg, the trailer hull molding represents the largest single thermoplastic
structural component that has been produced to date. At the time of writing, the prototype trailer
is awaiting a date for a shakedown test at the Motor Industry Research Association (MIRA,
Nuneaton, U.K.) proving ground. Additionally, Mi Technology has been subcontracted to
instrument the trailer with strain gauges and conduct a series of non-destructive static and
dynamic tests. Figures 28 & 29 show photos of the final trailer alone and mounted to a truck
cab and carrying a load.
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Figure 28: Complete trailer with axels, wheels, and mounting hardware installed.
Figure 29: Trailer mounted to truck cab and carrying a load.
Results & Conclusions
The Cleanmould program set out to develop a more sustainable thermoplastic composite
that could be used in conventional molding processes with low-cost tooling and consumables to
produce very-large, structural components for the transportation segment. The program
additionally sought to reduce weight and increase aerodynamics (to improve fuel economy),
and improve toughness / damage resistance – all while offering the same stiffness and load-
carrying capacity as the conventional unit it replaced. In so doing, the program developed a
new range of thermoplastic composites with high fiber volume fractions that are suitable for
processing on 3 common thermoset processing methods and that are more sustainable than
either steel or thermoset composites. Although they offer similar performance to common
thermoset composites used in transportation, the thermoplastic composites are slightly lighter,
emit no VOCs, have excellent fatigue, impact, weathering, and chemical resistance, and are
fully recyclable at end of life, helping reduce pollution and offering a more sustainable solution
that is compliant with the European Union’s (EU’s) EN2000/53/EC end-of-life directive. As the
first all-composite, fully recyclable, maximum payload, tri-axel semi-trailer, no functionality has
been lost and a great deal of additional benefit has been gained. For example, the 1.5-metric
ton weight reduction and aerodynamic improvements mean direct and immediate cost savings
to an operator through fuel savings. If deployed on a large scale, it would also help the EU
better meet its Kyoto Agreement targets for CO2 emissions, and would address many of the
problems currently facing European road haulage. While the initial prepreg production and part
processing methods have room for improvement, the program showed they have the ability to
produce a truly large and demanding part in a real manufacturing environment. Hence, the
Cleanmould program represents a significant achievement and a leap forward in the current
state of technology for transportation composites.