Glass reinforced polypropylene (PP) is a workhorse composite with a long history of use in semi-structural and select structural components in the automotive and a growing number of other transportation segments. Strong competition among suppliers has led to a proliferation of form factors and processing methods that continuously improve the cost/performance ratio, leading to exciting opportunities for automotive designers and processors. Our benchmark report gives an overview of the history and status quo of LFT, D-LFT, and GMT composites with PP matrices in the automotive industry.
Increasing fibre length and fibre volume fraction
Neat PP is a good commodity resin that is easy/forgiving to mould, provides broad chemical resistance and good toughness, has acceptable outdoor weatherability (when used with the right additives), can reduce part weight significantly (since its specific gravity is less than 1.0) and saves money. Being a thermoplastic, the material also is melt-reprocessable, so in-plant scrap is reusable and end-of-life parts can be recycled. On the other hand, it is not a particularly stiff material on its own, it is prone to brittle failure at low temperatures, its chemical resistance makes it a real challenge to paint or plate, and it cannot be used at temperatures much above 93°C for short-term spikes and 54°C for continuous use.
However, when you add fibre reinforcement to PP resin (glass, natural fibre, basalt, or something higher performing) you arrive at a composite material that shifts the mechanical performance curve upward while still holding part weight and cost in a modest range. This has made reinforced PP a perennial favourite in the automotive industry and lead to growing interest in other transportation segments, including mass transit, commercial truck, agriculture and even lawn & garden. Because of this, a great deal of research and development work has been focused on PP composites over the last 40 years – in terms of base resin, additives, and reinforcements – leading to four major categories of material/process combinations. These have shifted market share away from short-glass injection grades of higher temperature engineering thermoplastics, as well as thermoset composites like injection- or compression-moulded bulk-moulding compound (BMC) and compression-moulded sheet-moulding compound (SMC). PP composites even replace aluminium and steel when those materials are over-engineered in applications.
It started with short-glass fibre injection-moulded PP grades, which had injection moulding’s benefits of rapid cycle times, good surface finish, high design flexibility and parts consolidation opportunities. To improve stiffness and toughness, throughout the 1980s and 1990s resin producers, compounders, and additives suppliers worked together to enhance the fibre/matrix bond (via sizing and coupling agent advancements) and boost both fibre lengths and loading levels of glass reinforcement in precompounded pellets. In the composites industry, there is a long and well-established relationship between increases in reinforcement length and fibre-volume fraction (FVF), and improvements in mechanical performance of a reinforced polymer.
Gradually glass length in pelletised LFT was increased to about 13mm. LFT offered higher mechanical performance than short-glass PP and many engineering resins, yet was still injection mouldable, so capable of greater part complexity than parts produced in most other moulding processes. Major pelletised LFT-PP suppliers include Ticona Engineering Polymers (Florence, KY, USA), Sabic IP (Pittsfield, MA, USA), Dow Automotive (Midland, MI, USA), RTP Company (Winona, MN, USA), Chisso (Tokyo, Japan), and Plasticomp (Winona, MN, USA).
Eventually, a limit was reached on pellet length due to feed-throat restrictions in the compounding unit of the injection moulding machine. When glass exceeded 13mm, the fibres not only tangled and clogged equipment, but also broke, reducing their effectiveness at boosting mechanical performance. Efforts to work around these challenges led to development in the late 1990s/early 2000s of inline compounding (ILC), which allowed resin, additives, and reinforcements (fed from large spools) to be combined at press-side just prior to moulding. In this method, which came to be called direct-LFT (D-LFT), a log or charge of precompounded resin and reinforcement (with glass typically 20-40mm in length) exits the ILC unit, is weighed and cut, and the charge is placed (manually or with automation) into either an injection or compression moulding tool to form a part. With this process, the moulder takes on the role of compounder, able to control (but also becoming responsible for) the specific recipe of resin, additives, and reinforcement for a given application. ILC can save time and costs during product development since resin formulation can be changed on the fly as parts are moulded and tested, rather than having to wait for a custom formulation to be produced and delivered by a resin supplier.
Although the process requires special equipment (hence capital investment), users say they are able to produce parts at lower cost than conventional pelletised LFT and sheet-form composites like glass-mat thermoplastic (GMT). D-LFT injection moulding offers greater part complexity (but at shorter fibre lengths, hence lower mechanical performance); D-LFT compression moulding maintains higher glass length with less fibre breakage (hence higher mechanicals), but is not capable of filling as complex a part.
Key equipment suppliers include Dieffenbacher, Eppingen, Germany, Coperion Werner & Pfleiderer, Stuttgart, Germany and Composite Products, Winona, MN., USA on the compression moulding side and KraussMaffei, Munich, Germany and Plasticomp, Winona, MN, USA on the injection side. Fraunhofer Institute for Chemical Technology (ICT), Pfinztal, Germany was also involved in helping develop the process.
Improving mouldability & part complexity
While pelletised PP was evolving into LFT and eventually D-LFT, a parallel but opposite effort had been underway since the 1980s with another type of PP composite called glass-mat thermoplastic (GMT). First developed by PPG Industries, Pittsburgh, PA, USA, original GMT featured a randomly oriented, continuous glass fibre mat impregnated with PP and produced in a sheet form that was subsequently cut to customer-specified “blanks”. GMT parts were formed exclusively in compression moulding presses after preheating and stacking blanks inside the press. Traditional GMT parts were stiff (by thermoplastic standards) and properties were more balanced in multiple axes owing to the random orientation of the continuous glass. They also were exceptionally tough, had no shelf-life restrictions and processed faster than SMC (its initial competitor), since there was no need to wait for polymerisation and cross-linking. This led to lighter, more damage tolerant, and less costly parts than comparable thermoset composites or metal designs. Since moulding temperatures and pressures were relatively low compared with injection moulding (0.96bar; oven temp close to 200°C and mould temp 38-54°C depending on surface-finish requirements and part thickness), another benefit of the process was that functional or decorative layers could be added during blank layup in the tool reducing secondary finishing time and costs. Despite significantly higher mechanical performance, GMT was more costly and heavier than injection moulded thermoplastics.
As with any process, compression moulding had its own limitations. Unlike injection moulding, there is no way to mould a through-hole in compression moulded parts, and undercuts are problematic since tooling action (like slides) are challenging to employ given the equipment setup as the press opens and closes along a vertical rather than horizontal plane. More of an issue was traditional GMT’s extremely long and randomly oriented fibres, which restricted material flow in the tool and were unable to penetrate into tall, thin ribs and other intricate design features, leading to resin-rich areas that could fail prematurely. Hence part designs for traditional GMT were more restricted and featured far less complexity than those for injection moulding.
To address moulding and filling issues, GMT producers – which had grown to include PPG licensee, Quadrant Plastic Composites, Lenzburg, Switzerland (now partially owned by Mitsubishi Engineering-Plastics) and PPG’s Azdel, JV with GE Plastics (now Sabic IP, Pittsfield, MA, USA; Azdel is now owned by Hanwha L&C of Seoul, S. Korea) – began experimenting with chopped fibre mats and eventually phased out random/continuously reinforced grades. Although stiffness was sacrificed by going to a discontinuous reinforcement system, moulding was significantly improved and part complexity began to expand, leading to more opportunities in a broader range of applications, including more complex bumper beams, door-panel inners and hardware modules, front-end modules, underbody shields, trunk wells, trunk separators, instrument-panel carriers and liftgate panels.
Eventually, several specialised types of GMT also were developed. For areas of a part that required greater stiffness in a single axis (such as a bumper reinforcement beam), unidirectional (UD) glass reinforced GMT was developed for selective stiffening of parts. For semi-structural parts like automotive headliners, a lightweight (glass) reinforced thermoplastic (LWRT) form that lofted when heated (creating air pockets between fibres) was developed. This material allows density to be varied across the part simply by compressing some sections more than others, selectively boosting stiffness without changing tooling or adding mass. To avoid crushing the air pockets, it tends to be processed at lower pressures (7-10bar) via thermostamping rather than high-speed compression moulding like conventional GMT. For applications requiring significantly higher multiaxial mechanical performance, products reinforced with a variety of woven and non-woven textiles/fabrics (in glass, aramid, carbon or hybrid combinations) were developed. Like the UD laminates, they could be selectively added just to areas of a part requiring the highest properties to balance cost and performance. Variants on conventional chopped glass mat GMT included chopped natural fibre GMT, which reduced mass and lower a part’s carbon footprint, and a variant on LWRT that replaced fibreglass with chopped basalt fibre to deal with challenges that glass reinforced polymers posed in Japan’s energy-cycling furnaces. This reinforcement is said to offer mechanical performance approaching that of carbon fibre but priced lower than S-glass.
Balancing cost & performance with hybrid moulding
Efforts among thermoplastics resin producers to increase mechanical performance (at the expense of moulding complexity) by using longer fibres at higher FVFs, and parallel but opposite work by GMT suppliers to improve mouldability and reduce mass and cost (at the expense of stiffness/strength) by going to shorter, discontinuous fibres eventually merged with the development of inline compounding of D-LFT materials. Further work to improve performance of compression-moulded D-LFT led to the development of a hybrid moulding process that borrows a technique common to GMT – the ability to selectively combine layers of continuous-fibre (fabrics and multi-ply UD-glass laminates in a variety of orientations) with discontinuous-fibre (D-LFT) forms of composite of the same matrix in the tool. The technique, pioneered in the 2005-2006 timeframe by Fraunhofer and Dieffenbacher and called tailored D-LFT, offers the best balance of cost, weight, mouldability and mechanical performance. Use of continuous-glass products can boost FVFs from a standard 20-30% seen in conventional D-LFT to as much as 50-60% in local areas of a part, not only improving stiffness, strength, and impact but also boosting thermal stability and creep resistance. Meanwhile, the discontinuous fibre-portions of the material maintain flow where the ability to fill complex geometry like ribs is important. Although the tailored D-LFT process was developed in Europe for the automotive industry, itis not yet in wide use for commercial part production.
Given the evolution of PP composites, whose largest market was and still is automotive, and the decades-long competition among suppliers of various materials and process technologies, it is safe to assume that more innovation is yet to come. There has even been talk about applying ILC techniques to make GMT-type materials at the press, which could represent yet another performance improvement at lower cost. In fact, a variant on the process has already been applied to SMC. Called direct-SMC or direct-strand moulding, this hybrid process (pionered by Fraunhofer and Dieffenbacher) eliminates the need for the several-day maturation period required with conventional SMC and reduces the interval between compounding and moulding to minutes.