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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.