Specialty compounder LATI Industria Termoplastici SpA (Vedano Olona, Italy) has created a new range of injection moldable “super structural compounds,” pairing high-temperature high-performance engineering thermoplastics polyphenylenesulphide (PPS), polyphthalamide (PPA), and polyetheretherketone (PEEK), with high-modulus carbon-fiber yarns. The result: the HM (High Modulus) family of compounds with 40% carbon-fiber loading for what LATI calls dramatically higher elastic modulus.
In the development of the compounds—Larton K/40 HM (PPS), Laramid D K/40 HM (PPA), and Larpeek 10 K/40 HM (PEEK)—LATI weighed a variety of resins, fibers, and compounding technologies. High-tenacity chopped-strand carbon fibers are traditionally applied in thermoplastic compounding to increase elastic modulus and load at break without making the materials brittle. LATI chose to experiment with high-modulus carbon-fiber yarns, saying the HM yarns show an elastic modulus that is almost twice that of HT grades.
LATI also considered the high shear stress and pressure within a compounding extruder that can break up fibers, ultimately affecting the strength they can impart to the composite. After several trials, LATI technicians altered the extrusion parameters and optimized machine layout, specifically addressing the carbon-fiber feeding position along the extruder barrel. In terms of parameters, LATI says the speed and profile of the plasticization screw have also been optimized.
Throughout the trials, LATI found it needed to deal with fiber-bundle buildup occurring during extrusion and molding, with the bundles hindering melt flow and leading to the carbonization of the polymer, machine jamming, and defective parts. HM fibers were added up to 45% by weight, and the melt’s viscosity was closely controlled to avoid poor molding and material degradation due to friction and shear. LATI was also careful to ensure uniform distribution of carbon fibers in the matrix.
Using ISO-R-527 tensile tests, the compounder determined the HM materials achieved tensile strengths above 40,000 MPa, with the PPS-based grade reaching 49,000 MPa. By comparison, LATI says a polyamide 6/6 reinforced 50% with glass fibers is three times “softer” than the best HM grade. In terms of load at break, HM compounds offer up to 280 MPa (tensile, as per ISO-R-527) for the PPA-based grade. LATI says competitive industrial structural compounds featuring 50-60% glass-fiber loading achieve around 220 MPa with a much higher density. —[email protected]