Modifiers lessen gloss, raise toughness.

Among the more interesting work done recently in polymer modifiers are three product introductions from Rohm & Haas Co., Philadelphia. Over the last few months, the company has developed and commercialized Paraloid EXL-5137, a gloss-reducing agent for engineering resins; Paraloid EXL-3657, an MBS-based toughener that dramatically improves the impact strength of polycarbonate systems; and Paraloid EXL-3361, an acrylic core-shell toughener that provides strong thermal and uv stability to polycarbonate and polycarbonate blends.

GETTING THE GLOSS OUT

The gloss-reducing agent, Paraloid EXL-5137, is an all-acrylic additive that is believed to be the first additive ever developed to give engineering resins a matte appearance. Rohm & Haas says Paraloid EXL-5137 is targeted for use with ABS, polycarbonate and polycarbonate/ABS blends. The demand for low-gloss engineering resins is growing as a flat finish becomes the preference for automobile interiors and electrical/electronics housings. Matte finish is also useful to help minimize visibility of weld lines.

More information is now available on this unusual modifier after initial details were released this spring (see PT, May '91, p. 174). To ensure consistently good results, the gloss modifer must be compounded or melt-blended prior to molding or extrusion, Rohm & Haas says. In 60 [degrees] gloss tests, a 10% loading dropped the gloss value of a polycarbonate from 90% (unmodified) down to 50%; a PC/ABS blend went from 80% to 40% gloss; and an ABS sheet dropped from 75% gloss to 30%. These low-gloss characteristics are consistent over a wide molding window, Rohm & Haas says.

On the other hand, use of Paraloid EXL-5137 can reduce impact strength. In 1/8-in. ABS sheet, Notched Izod impact strength fell from 9.7 ft-lb/in. for a sample with no gloss reducer to 4.2 ft-lb/in. with 5% modifier and 2.8 ft-lb/in. with 10% EXL-5137. Addition of an impact modifier can restore some but not all of the impact strength, Rohm & Haas says. Other mechanical properties of the ABS sheet were virtually unaffected, the company says. So far, Rohm & Haas has only released data on the modifier's performance on materials used indoors. A company spokeswoman says results of outdoor testing have not yet been compiled. Paraloid EXL-5137 costs about $2.60/lb.

NEW TOUGHENERS

Both Paraloid EXL-3657 and Paraloid EXL-3361 are said to provide significantly better impact performance after high-temperature processing and heat aging than earlier generations of tougheners. For example, the company compared toughness of 1/4-in. polycarbonate after oven aging for 65 hours at 284 F using the new modifier and two older MBS products, Paraloid EXL-3691 and EXL-3647, all at 8% loadings. New EXL-3657 yielded a notched Izod value of 10 ft-lb/in., while PC containing EXL-3691 registered a little over 6 ft-lb/in. and that with EXL-3647 came in at 3 ft-lb/in. Other benefits offered by Paraloid EXL-3657 include low-temperature toughness down to -40 F and thermal stability that can withstand processing temperatures in excess of 600 F. Paraloid EXL-3657 costs $1.71/lb.

New Paraloid EXL-3361, a stabilized acrylic core-shell toughener, can be used in polycarbonates and PC blends. Thermogravimetric analysis (TGA) indicates its improved heat stability, allowing users to process their materials at higher temperatures. The new modifier shows 5% weight loss at about 617 F, compared with around 563 F for Paraloid EXL-3330, an unstabilized acrylic toughener. At a 4% loading, 1/4-in. PC with Paraloid EXL-3361 has a notched Izod impact of 13.5 ft-lb/in. at 73 F. At 32 F and an 8% loading, toughness is 10 ft-lb/in. Other benefits of this toughener, which costs about 5-10[cents]/lb, more than unstabilized acrylics, include good uv stability and easy dispersion.

Both Paraloid EXL-3361 and Paraloid EXL-3657 are manufactured in pellet form.