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Processes and materials have been developed to successfully manufacture very large structural components using a novel liquid thermoplastic composite (TPC). This technology has been proven with the production of the world’s largest TPC product: a 13.5 m semiarticulated trailer.
DRMATTHEW TURNER, SENIOR COMPOSITES ENGINEER DAVID GOODWIN, DEVELOPMENT ENGINEER EPL COMPOSITE SOLUTIONS
Over 65% of all goods and freight in Europe are transported by road using flatbed, curtain-sided or boxed semitrailers. This accounts for approximately 10% of the total European energy consumption and contributes over 10% of total CO2 emissions.
The semi-trailers are predominantly manufactured by welding or bolting heavy steel sections together, which are then either galvanised or painted for corrosion protection.
With the increasing oil price and demand for commodities worldwide, the price of steel has risen significantly in recent years. Along with increasing fuel prices, this has led companies to look at alternative lightweight materials and construction techniques for the production of semi-trailers. One approach is to utilise composites with integrated design which offer the strength and stiffness of metals at a much reduced weight, coupled with excellent corrosion resistance.
A number of composite concepts have been developed around the world and some are currently in production. Whilst they invariably offer lightweight options to the problem, there are also disadvantages such as the use of expensive carbon composites or the requirement for a structural body to create a monocoque structure. Moreover, all current composite systems use thermosets which are difficult to recycle – a key requirement in the age of high commodity demand.
Whilst lightweight and fuel efficiency are very important factors to the road transport industry, research has shown that cost is a key driver and most road hauliers want a generic trailer using standard ancillary equipment to which they can tailor bodywork to suit their client’s needs.
With this in mind, a consortium of leading technology organisations has been working on the development of recyclable composites that offer high volume fractions and can be moulded in large sizes using inexpensive tools and processes. This consortium is supported by the EU under a Framework 6 programme and includes Ahlstrom Glass Fibre, BAE Systems, Basmiler, Cyclics Corp, EPL Composite Solutions and IKV.
In order to make the products environmentally sound, thermoplastics were chosen as the materials of choice due to their inherent natural recyclability. In the past, the main issues with thermoplastic composites have been either their low structural properties (in comparison with thermosets) due to low fibre loading, or their high cost in using exotic (expensive) polymers such as PEEK and PPS.
Liquid thermoplastics provide the benefits of thermoplastics (toughness, durability, impact resistance, recyclability) whilst using low-cost polymers and the ability to include more reinforcement due to the low viscosity of the resin. The issues with liquid thermoplastics in the past have been related to their limited moulding size and to their physical processing as these materials tend to be susceptible to processing conditions which can influence polymerization.
Therefore, a material and process investigation programme was the cornerstone of this project. Cyclics Corporation’s range of thermoplastic polyesters, CBT® (the cyclic form of polybutylene terephthalate, PBT), were used because of their very low viscosity during polymerization and negligible volatile organic compounds (VOCs) during processing. CBT® resin is produced from linear PBT through catalytic depolymerization which takes place at elevated temperatures, converting PBT into a lower molecular weight cyclic form or CBT® resin. A reversible reaction allows CBT to re-polymerize to PBT, an engineering thermoplastic providing excellent stiffness and superior chemical resistance.
Process development was carried out to limit intolerance of the base resin to moisture and other chemicals and to control the polymerization rate. A form of CBT was thus developed whereby the polymerization and closely-linked crystallization of the resulting PBT was engineered to a state that could be satisfactorily moulded in processes such as RTM, vacuum infusion and lowpressure bag moulding.
Closely matched to the development of the base resin was an investigation into compatibility with fibre reinforcements. Ahlstrom Glass Fibre Oy developed a range of sizings that did not affect the polymerization of PBT while providing a good bond between the glass fibre and PBT.
Whilst the raw glass fibre was useful for processes such as RTM, the aim of the project was to develop materials and processes for very large transportation components (larger than RTM is efficiently capable of making). Therefore, a prepreg material that incorporated the resin on a non-crimp glass fibre fabric was developed by Ahlstrom (see Figure 1).
Research in processing was critical to the successful production of components using the liquid thermoplastic composites. This research was carried out by IKV in Aachen, Germany and by EPL Composite Solutions in the UK. For smaller components, IKV developed processing routines for closedmould RTM and single-sided-tool vacuum infusion processes.
Teams concentrated on eliminating moisture from the process, as well as developing optimal injection and thermal profiles. Closed-mould RTM resulted in glass composites with volume fractions up to 63% and mechanical properties similar to industry-standard thermoset composites, i.e. 40 GPa flexural modulus and 850 MPa flexural strength. Moreover, these components have the added benefits of increased toughness, thermoformability and full recyclability.
In order to commercialise the process for the transport industry, very large components need to be manufactured economically. This requires using lowpressure moulding techniques which were developed by EPL Composite Solutions. Using the prepreg from Ahlstrom, EPL devised a means of achieving high levels of polymerization and thus high-volume-fraction components through careful control of processing variables. EPL also used equipment and technology that were readily available to many composite manufacturers. Hightemperature tooling is needed as the resin starts to polymerize at about 160°C. Statistical methods were applied to a range of variables in order to determine what affected the quality of components. This resulted in an optimized processing methodology capable of creating composites with flexural strengths in excess of 800 MPa.
Design and manufacture
With materials manufactured and processes developed, the task of translating the concept of a thermoplastic composite material into the reality of a 44-tonne, triaxle semi-trailer was undertaken by EPL
Composite Solutions. Having previously developed a thermoset 28- tonne trailer, EPL were able to quickly translate the properties of the TPC into a feasible design for a 13.5-m trailer (Figure 2) using a mixture of composites and mechanical design methodologies.
Care was taken at every level to ensure that the trailer design was rugged and suitable for end users. The ancillaries used in the trailer are industry standard and not special lightweight products. The trailer has been designed for the widest use possible, i.e. if a tyre blows, or an axle or landing leg need replacing, any supplier in the current industry will be able to repair or replace the damaged part.
All parts that require maintenance or replacement are easily at hand. The composite itself is designed to be easily repairable as it has been designed to the stage where sections can be cut from the trailer without affecting performance adversely.
With the design in place, tooling was then designed and manufactured by project partner Basmiler in Portugal. Due to the size and relative simplicity of the composite shape, the tooling was manufactured from metal although research also highlighted a range of feasible composite tooling options.
Using the prepreg manufactured by Ahlstrom and the processing techniques developed by EPL Composite Solutions, BAE Systems and EPL worked on the methodology to transfer lab-scale knowledge into the full-scale 13.5-m moulding. The trailer was designed around only three main mouldings and after some development (and the purchase of a 15-m oven), all three components were successfully manufactured via low-pressure moulding, the largest being the “hull” or base of the trailer which weighs over 1,000 kg and covers some 50 m² in area – the largest single, high-performance TPC moulding in the world. Investigation into joining techniques provided a simple and effective solution to create the monocoque trailer.
Ancillaries, electrics and pneumatics were installed and the resulting trailer is currently undergoing a range of lifecycle tests on roads and test tracks in the UK.
The trailer (Figure 3) has been measured and is currently producing weight savings of approximately 1,500 kg (generating over 700 litres of fuel savings per year) compared with the steel trailer that it has replaced. The trailer has the capacity to carry loads in excess of 30 tonnes and it only deflects by 8 mm when fully laden.
IKV performed tests on the materials and developed techniques to recycle the trailer at the end of its useful life. The resulting recyclate can be used in compression or injection moulding processes and possesses 80% of the properties of virgin stock material.
To enhance the environmental aspects of the design and to take advantage of the composite’s shaping ability, a computational fluid dynamics (CFD) analysis was performed using racing car technologies to reduce drag forces over the trailer and to improve its aerodynamic profile (Figure 4).
The aerodynamic profile of the monocoque dramatically reduces drag and generates even greater fuel savings, giving a combined fuel reduction of approximately 15% – typically around 7,000 litres of diesel per year. The composite trailer carries additional cost over the conventional steel version but the payback is expected to be less than 2 years.
EPL Composite Solutions Ltd. are seeking partners to develop and commercialise the composite trailer concept with a view to having a fleet on the road by 2013.
Here are few examples and update of these two techniques which can be seen in many application sectors now.