Long-glass-fiber-reinforced polypropylene is proving its worth in a wide range of automotive applications.
By David Pahl
Automotive Marketing Specialist
Global Verton Product Manager
LNP Engineering Plastics, a GE Plastics Co.
Edited by Jean M. Hoffman
The use of polypropylene (PP) has grown in many markets over the last few years — especially automotive. One of the driving forces behind PP's acceptance by car designers is its "monomaterial construction." For example, instrument panel (IP) parts now come from a number of olefin PP resins, including longglass-fiber (LGF) PP for the structure, talc-filled PP for ductwork, and TPO (thermal plastic olefin) for the outer skin. Monomaterial construction encourages recyclability and part consolidation reduces weight and complexity, and permits volume material purchases.
Cost is another factor. PP has found a new home in many interior, exterior, and underhood applications at the expense of high-cost engineering resins such as ABS, polyurethane, and nylon. With improvements in catalysts and processes, PPs can compete in more-demanding applications — giving designers a low-cost alternative to some engineered resins with comparable impact, stiffness, and chemical-resistance properties.
One recent advancement in LGF PP is Verton MTX concentrate. The heat-stabilized masterbatch is 75% LGF by weight and available in 13 and 25-mmlong pellets. It is specifically formulated for machine-side blending using neat PP. Verton MTX is based on commingled-fiber technology called Twintex from Saint-Gobain Vetrotex America Inc., Valley Forge, Pa. (www.twintex.com) The PP resin fiber and glass-fiber filaments commingle into one roving, which is then processed under heat and pressure and pelletized to size.
Commingling differs from existing manufacturing methods for LGF PP pellets, including pultrusions/melt impregnation and wire-coating technologies. Twintex lets the glass and PP fibers intermix, giving good glass-to-resin dispersion within the consolidated pellets. Combining resin and glass in the composite matrix can boost glass loadings to 75% by weight, giving Verton MTX higher loadings than both melt impregnation and wirecoating processes. Further, physical properties match pultruded LGF PP materials at a 20% lower cost.
A recent test compares the performance properties of a fully compounded, heat-stabilized 30% LGF PP (Verton MFX-7006 HS) to a 75% LGF PP masterbatch diluted to a comparable 30% glass fiber fill. The test helps verify that key performance properties are not compromised by the masterbatch approach. For the test, the dilution resin was a 30-MFI (meltflow index) homopolymer. A third compound consisting of additional compatibilizers and heat stabilizers was also added to the blend to match the final loadings of these additives in the Verton MFX7006 HS control. Properties were tested with and without the third compound to determine the effect that lower heatstabilizer (HS) and compatibilizer contents have on mechanical properties of the blend.
When tested under ASTM standards, the tensile strength, flexural strength, and flexural modulus of the three-component blend differed less than 5% versus the fully compounded 30% LGF PP control. The blend without the HS and compatibilizer components had 11% lower tensile strength.
Impact testing showed that the three-component masterbatch blend varied less than 1% from that of the control in notched Izod and less than 8% on falling dart total energy tests. Both notched Izod and falling dart impact increased slightly in the blend without the added HS and compatibilizer compound.
Additional tests were with blends targeting glass loadings of 20, 30, and 40% by weight. The 40% glass blend was also tested without the HS and compatibilizer. As with the other tests, the 40% masterbatch compound showed higher tensile properties with the heat stabilizer and compatibilizer. Likewise, the 40% blend without the HS/compatibilizer addition had better impact strength.
CUSTOM LGF PP
Other dilution resins can produce additional benefits. Diluting the 75% LGF PP with various unfilled PP resins altered mechanical properties. For example, strength and impact properties differ between 75% LGF PP masterbatchs diluted with a homopolymer PP and similar masterbatchs diluted with a high-impact copolymer PP.
Tests used the homopolymer based LGF PP concentrate diluted with a 30-MFI homopolymer PP as a control. The first copolymer PP was diluted with a 65-MFI, heat-stabilized, nucleated, and high-impact copolymer. The second was diluted with a 100-MFI nucleated, high-impact copolymer.
Mechanical tests revealed that the copolymer blends had 30 to 40% higher notched Izod strength compared to the homopolymer blend. And falling dart impact energy improved 14% using the copolymer blends. Strength and stiffness properties, however, remained on par with the equivalent homopolymer blend.