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Automotive structural parts require a high level of mechanical and design performance. In that sense, long-fibre thermoplastic (LFT) materials make thermoplastic composites more attractive than metal regarding automotive functions such as weight savings, cost reduction and recycling requirements.
PHILIPPE PARDO, LFT & TWINTEX® PRODUCT AND SALES DEVELOPMENT, SAINT-GOBAIN VETROTEX
(Published on July-August 2007 - JEC Magazine #34)
The increasing use of long-fibre thermoplastic composite materials in the industry to replace metal solutions is due mainly to their design flexibility and improved mechanical properties (good stiffness, high impact resistance even at low temperature, good creep behaviour, etc.). Short-fibre engineering thermoplastic resins have some limitations in replacing metal components: the mechanical properties of reinforced thermoplastics are directly linked to the fibre L/D ratio and for a specific glass fibre diameter, they are directly related to the residual fibre length of the reinforcement.
LFT is not just a material or a process. Rather, it is a global system combining thermoplastic resins, additives and reinforcing fibres (no longer considered as a filler) that covers component design and manufacturing process simulations as well. The success stories and technical feedback on the design of automotive structural parts (LFT front end carriers, for example) has led automotive OEMs to think about new functions like instrument panels, door modules, visible parts, etc. In the future, LFT materials will be used not only to provide a reinforcing solution, but also to add other properties like low VOC & fogging emissions, flame retardancy and UV resistance. Long-glass-fibre PP concentrate material like Twintex® pellets will make it possible to fulfil these requirements, while keeping an excellent cost/performance ratio and processing flexibility with standard injection units.
What is Twintex®?
In the past, few thermoplastic resins reinforced with continuous glass fibre were developed, due to the high viscosity of thermoplastic matrices. The first patents for these materials were taken out in the early 70’s. Now, various types of thermoplastic composite materials are available. One of these is Saint-Gobain Vetrotex’s Twintex®, a revolutionary dry prepreg produced by commingling continuous glass and PP filaments.
Twintex® is an ingenious low-cost solution for impregnating continuous glass fibres with thermoplastic resins. A high glass content (60% to 75% by weight) can easily be achieved in this TP composite material to reach a significant level of mechanical performance.
The material is available in different forms such as rovings, woven structures, consolidated plates, and long-glass-fibre PP concentrated pellets for dilution. Twintex® 75% pellet – the most concentrated pellet in the market – incorporates a TP additive package to meet the performance demands of the final parts.
The high commingling level achieved by the process makes it possible to ensure perfect impregnation of the glass fibres in the concentrate material. Being fully impregnated with PP resin, the glass fibres are protected during the plasticizing phase, resulting in longer residual glass fibres with better dispersion are produced using different manufacturing technology.
The Twintex® pellet dilution concept
To reduce their raw-material costs yet keep the mechanical properties required for their parts, manufacturers are now using diluted long-glass-fibre PP concentrated pellets to replace shortglass- fibre PA compounds or ready-to-use long-glass-fibre pellets. The higher the concentration of glass fibres in the compounded material, the less compounded material will be needed in the mix and the higher the productivity will be, because the other material needed for the dilution will be a pure-grade PP supplied by a resin producer. As the most concentrated pellets in the market, Twintex® pellets can provide customers with this very benefit, for excellent material quality and performance. As a glass fibre manufacturer, Saint-Gobain Vetrotex guarantees moulders constant quality for both the glass fibre and the sizing.
Simulation with LFT material
Lc = (σT*d) / (2*τ)
σT: Tensile strength of glass fibre filament
d: Filament diameter
τ: Shear strength of fibre-matrix interface
Lc: Critical fibre length
The behaviour of LGF TP materials is better understood today, thanks to accurate material data tuning for simulation tools, which makes it possible to optimise both composite parts design and the manufacturing process. This includes the final dimensional stability of the components.
Twintex® pellets: formulation flexibility to meet part specifications
In addition to the high glass-fibre content and the additive package range developed, Twintex® pellets allow moulders to tailor their dilution formulations in order to: