Long long-fiber thermoplastic (LFT)-polypropylene (PP) compounds are extensively employed in automotive applications. They are now taking on a new challenge in a solar application that requires both non-halogenated flame retardance and colorability.
The need to develop these new FR LFT-PP grades arose from the requirement of TULiPPS Solar B.V. (Eindhoven, The Netherlands), a startup company working on a new composite-based solar photovoltaic (PV) module system for flat roofs in which the composites are used in place of aluminum components. New regulations are anticipated in many geographies requiring all plastic materials used on roofs to be flame retarded. Since the composites used in TULiPPS' module system needed to be formulated to offer an extremely demanding target service life of 25 years or longer, company founder and managing director, Paul Stassen wanted to ensure the units would comply not only with current regulations but also with future requirements. That presented an interesting challenge, since the composite module system needed both to be flame retardant as well as lightweight and thin, and therefore required high stiffness and strength plus excellent long-term durability - properties that can be affected by the non-halogenated additives package.
"What we are trying to make LFT-PP polymers do for the TULiPP's module is a breakthrough that has never been done before - not even in the demanding automotive industry where these materials got their start," says Stassen. "Meeting any one of these criteria would have been challenging enough, but meeting all of them at once - high but balanced mechanicals, high flame retardance without use of brominated flame retardants or antimony, good processing, good aesthetics, and service life that is two-and-a-half-times longer than automotive specifications - really provided us with major challenges as well as opportunities. Balancing stiffness and toughness with FR was particularly difficult, because everything you do to improve halogen-free flame retardance usually has a negative effect on mechanicals, processing, and service life."
Stassen looked to his previous automotive-composites contacts for a creative compounder with a lot of formulation experience, particularly in the area of flame retardance. He chose to partner with RESIN (Products & Technology) B.V. (Enschede) to find the right recipe to meet his project's demanding performance specifications. Frans Haafkens, managing director at RESIN notes, "With our new halogen- and antimony-free, ECO-compliant flame retardant recipes for polypropylene, we have achieved the difficult balance of good processing and mechanical properties at an attractive cost. Although it was a lot of work, these new thermoplastic formulations are well positioned to serve demanding market segments such as automotive, consumer electronics, composite pallets and other materials-handling components, as well as roof-mounted solar PV systems."
Meanwhile, recognizing that the solar panel module system would need excellent outdoor weathering for several decades in addition to meeting required mechanical and FR requirements, Stassen also partnered with QolorTech B.V. (Vaassen, The Netherlands), a supplier of custom-color masterbatches, to develop a series of unique colorant packages for the new LFT-PP compounds that would not interfere with the efficiency of the reinforcements. The goal here was to develop three "industrial" colors (grey, green, and blue). Like basic automotive black and unpigmented natural/off-white grades, these new colors would need to look good long term despite exposure to heat, ultraviolet (UV) radiation from sunlight, and other forms of weathering that can affect aesthetics and, more importantly, lead to a deterioration of performance. In the automotive industry, nearly all of the glass-reinforced LFT applications are black or neutral (unpigmented), but as LFT moves into non-automotive markets, it will be necessary to offer a better and broader color palette.
"Colorant packages need to be formulated as carefully as other components of the compound because they can inadvertently contribute to a loss of mechanical properties and service life," explained QolorTech's managing director, Michiel De Jong. "We put our pigment know-how to work in helping formulate unique color masterbatches for these new FR-LFT grades and that knowledge produced materials with good aesthetics and excellent long-term UV protection that do not cause property loss. Of course, having a broader color palette will attract customers from a wider range of industries."
Once a thermoplastic compound was developed and tested on small-scale equipment, it was necessary to identify a commercial molder who could evaluate the compound on production-scale compounding equipment to ensure it processed properly and, once the final formulation work was completed, who would be able to produce the large panel structures commercially for TULiPPS. For this work, voestalpine Plastics Solutions (Roosendaal and Putte, The Netherlands), the largest automotive LFT molder in the Netherlands, was chosen as the fourth development partner. Not only was the company highly experienced at working with both pelletized LFT and inline compounded (ILC) direct-LFT (D-LFT), but it also had an excellent reputation for quality, and used the latest inline compounding as well as injection and compression molding technologies.
Speaking about their involvement in the project, Huibjan Braafhart, sales director of voestalpine Plastics Solutions noted, "We were impressed by how well these formulations processed, with good flow, no discoloration, and achieving excellent wet out of glass, which can be a problem with FR compounds. Based on the excellent mechanical properties that we measured, this obviously was not a problem with these materials. The unique cost/performance profile of the new flame-retarded LFT-PP grades makes them suitable for large industrial parts in a number of industries and we see enormous commercial potential for these types of thermoplastics. In addition, we are pleased to be a part of this development team, which fits very well with our corporate growth strategy."
Formulation work on the new FR LFT-PP compounds took close to two years to complete, but hopefully will shortly pay dividends for all the companies involved. In-house testing by voestalpine showed the FR-LFT grades - in natural, automotive black, and the three new colors of grey, green, and blue - in many cases actually have higher mechanical properties than those of the non-FR base resin, which is an unusual situation. All compounds featured 30% glass reinforcement, were produced on commercial-scale automotive-industry D-LFT equipment, and were molded on commercial-scale compression presses, and test specimens were subsequently cut from the larger molded plaques. voestalpine used a D-LFT charge pattern that yields a very isotropic distribution of glass in the test plaques. While this yields values that are more conservative than otherwise would be expected in the direction of flow, they are also more representative of the results that would be expected from real industrial parts rather than idealized test specimens.
FR testing to European EVN 1187-2002 standards for roofing-membrane materials was conducted by an independent test laboratory (BDA Dakadvies B.V., Gorinchem, The Netherlands) and mechanical testing was performed by voestalpine Plastics Solutions in the company's internal testing lab using their standard automotive-black LFT-PP grade as the baseline/control.
The new materials are already commercially available from RESIN (Products & Technology) B.V. under the trade name ECO-FORTE. They will be custom formulated and supplied ready to use in a granular/pelletized form factor for direct compounding or dilution in a D-LFT process. All formulations contain a high-quality UV stabilization package for protection against the effects of outdoor weathering.
TULiPPS' Stassen adds, "It's funny to think about the automotive composites industry applying its materials and processing expertise to develop the next generation of solar PV modules, but that's exactly what happened here. Our work represents some real contributions to the state-of-the-art and it will shortly help new industries that haven't yet experienced the benefit of using LFT composites."‑[email protected]