Page 1 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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Page 1
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.
Page 2
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.
Page 3
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.
Page 4
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