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The structure of porous fibre-reinforced thermoplastics yields unique performance. The use of these materials is growing rapidly in automotive applications, that benefit from their unique balance of properties. To support this growth, Azdel has commercialized a second generation of SuperLite® products.
(Published on November-December 2008 – JEC Magazine #45)
THOMAS EBELING, TECHNOLOGY DIRECTOR, AZDEL INC.
Automotive and transportation manufacturers and designers are constantly seeking material solutions for both interior and exterior parts that improve design freedom, reduce weight and/or reduce cost, while maintaining or improving part aesthetics.
Solid fibre-reinforced thermoplastics
These solutions must also align with trends for decreased component build size, reduced part production cost (via part consolidation and automation), reduced environmental impact, and improved comfort and safety (especially improved acoustics). The challenge to material providers is to find ways to deliver valued performance improvements at reduced cost. With crude oil and gas prices at all-time highs, the enhanced performance-to-weight ratio of fibre-reinforced thermoplastic composites is attracting greater attention from transportation designers looking to reduce part weight. Historically, these composites were produced via compounding, which resulted in short-fibre pellets. Performance was limited by the surviving fibre length and the ability to manage injection flow fields. Technical advances enabling the production of long-fibre pellets and the production of parts from these pellets have extended the performance and applications of fibre-reinforced thermoplastics. Similarly, the development of glass mat thermoplastics (GMT), direct long fibre thermoplastics (DLFT), and unidirectional pultruded continuous fibre composites (and associated part production processes) have enabled the production of parts with longer fibres. These advances have extended the performance and application space of composites. As the fibre length is increased, the material stiffness and strength tend to increase, but it also becomes more difficult to produce intricate parts. For each of these material types, there is a range of commercially-available materials which provides a broad spectrum of performance and cost.
Porous fibre-reinforced thermoplastics
For the aforementioned fibre-reinforced thermoplastics, low or no porosity is generally desired both for part performance and for aesthetic reasons. More recently, new fibre-reinforced thermoplastic materials and parts have been developed that capitalize on the benefits of a porous structure. The use of these materials is growing in interior trim applications such as headliners, sunshades, door panel trim, and parcel shelves. The unique structure of these materials provides surprisingly good impact performance that is insensitive to temperature and impact speed, a very low coefficient of thermal expansion, and good sound-absorbing or noise-dampening capabilities. They also display the high strength and stiffness-to-weight ratio typical of long fibre-reinforced composites. These porous materials make it easier to use low-pressure thermoforming processes that in turn minimise equipment and tooling costs. In the part-forming process, aesthetically pleasing textiles or clothes are typically adhered to the composite.
For automotive interior trim applications, Azdel, Inc. offers a portfolio of products named SuperLite® . These materials are typically sold in large sheet form, in weights ranging from 600 to 2,000 grams per square metre, and generally come with functional cover materials that provide adhesion, barrier, ease of handling, and enhanced acoustic performance. The materials consist of a network of randomly-oriented fibres coated with, and held together by, thermoplastic resin. A distinguishing attribute of materials with these structures is that they “loft” or expand in the thickness direction when heated. This expansion is due to the softening of the thermoplastic resin which allows the fibres (previously bent or constrained as a result of the formation process) to return to a straightened orientation (Fig. 1).
The lofting capability enables part manufacturers to produce high-quality aesthetic parts, produce variable-thickness parts, and also “design in” part performance by variable tuning of thickness.
The typical thickness dependence of material performance is shown in Fig. 2. Engineering properties – such as flexural strength or flexural modulus – show the decreasing performance with increasing part thickness. This is to be expected for a material with a fixed areal density (basis weight) as increasing the thickness simply means incorporating more air in the structure, thereby increasing porosity. The engineering properties, being normalized by the increasing cross sectional area, will naturally decrease as the thickness is increased. When considering the absolute load-carrying capacity of the material (stiffness or peak load), however, the trend is quite different and exhibits an optimum thickness for maximum load-carrying capacity. This optimum can be explained by the presence of two competing mechanisms: the initial increase is due to a structural effect of increased part thickness and the subsequent reduction is due to localized weaknesses or instabilities with increased porosity. By selecting the basis weight of the material and choosing the moulded thickness, a part designer may tailor the stiffness, strength, and acoustic performance of the part without changing materials.
Second-generation product portfolio
In order to extend the value proposition of SuperLite®, Azdel has introduced a second generation of products offering improved acoustics, improved mechanical performance, weight reduction, reduced part costs, reduced environmental impact, and improved functionality. These materials include the SuperLite XLT® portfolio, VolcaLite® portfolio, and a portfolio of surfacing materials for improved functionality. The SuperLite XLT® products exhibit more than twice the lofting capability as the original SuperLite® portfolio. By leveraging this enhanced performance to increase moulded part thickness, customers are able to produce parts with both increased sound absorption, and increased strength and stiffness. Depending on the design objectives, this performance can then be leveraged for increased part performance, reduced weight and cost, or a combination of both (Table 1).
Emerging requirements for end-of-life disposal have created increased costs for the use of fibreglass-containing parts in some regions of the world. These costs are related to the use of incineration furnaces for material disposal. Because of the proximity of furnace operating temperatures and fibreglass melting temperatures, the fibreglass tends to coat the furnace liners thus increasing disposal costs. Fortunately, there are a number of viable alternatives to glass fibres, and porous fibrereinforced thermoplastic production methods are flexible enough to use a variety of fibrous materials. Although there are many technically feasible options, the advantages and disadvantages are different for each (Table 2). In response to customer feedback, Azdel has commercialized Volca-Lite®, a basalt-based product. The performance of this product matches that of SuperLite® and the higher melting temperature eliminates incineration cost issues. Basalt is a naturallyoccurring mineral quarried from volcanic rock, with globallydistributed deposits. Traditionally, it has been used as crushed rock in construction and roads, and in casting processes (tiles or slabs). Chemical components are similar to those of E- and Sglass, but the composition is different. Basalt fibres are relatively high in strength and modulus, and have high temperature and corrosion resistance.
As mentioned earlier, the surfacing materials play an important role in adhesion to aesthetic fabrics and textiles. As automotive standards increase for appearance, cost, acoustics and durability, the performance of these surfacing materials may limit performance. The third area of Azdel’s second-generation product enhancement is the improvement of functional surfacing materials in order to deliver performance enhancement for adhesion, high temperature environments, sound absorption, air flow resistance, squeak and rattle resistance, and part formability.
Recently, the inherent performance advantages of porous thermoplastic composites have inspired a significant amount of investigation into their utility for structural applications. For years, there have been attempts to create thermoplastic materials to meet the performance requirements of horizontal body panels. Previous attempts, however, have not been successful as a number have failed due to inadequate dimensional tolerances and weight reduction. Extensive investigation suggests that the combination of porous fibre-reinforced thermoplastic coupled with high-strength skins will be able to meet these needs. The IXIS® technology is being developed with the aim of offering a material capable of producing parts with a stiffness, weight, and coefficient of thermal expansion similar to aluminium, along with a class-A finish and energy absorption which will meet future pedestrian safety standards. Figure 3 shows the layered construction which uses SuperLite® as the core of the product. The figure also shows a micrograph of the cross section through the layers. Full-scale prototype hood part production using IXIS® material and subsequent part testing have shown that this new material offers the potential for producing composite automotive hoods.
The performance provided by the unique structure of porous fibre-reinforced thermoplastics has advantages for both interior and exterior automotive applications. Surprisingly good impact performance, a very low coefficient of thermal expansion, and good sound-absorbing or noise-dampening capabilities are the unique performance attributes of these materials.