More on that new polyphthalamide.

More on That New Polyphthalamide

Last month (p. 13), we reported some initial details on a brand-new generic category of crystalline engineering thermoplastic, to which ASTM has accorded the name polyphthalamide (PPA). The opaque white resins is trade-named Amodel by Amoco Performance Products, Inc., Atlanta. Shown in the table on p. 47 are properties of Amoco's six initial commercial grades, plus two representative carbon-fiber reinforced compounds from RTP Co., Winona, Minn. Amoco's prices range from $2.35/lb tl for 40% mineral-filled to $2.96/lb for glass-filled grades. At 13.1-15.3 [cents]/cu in., this puts them in about the same range as PPS.

MOLDS LIKE A NYLON

Based on RTP's evaluation of the material, RTP product development engineer Jeff Otto describes PPA as "basically a crystalline aromatic nylon," with chemical resistance similar to that of nylon 66, but less than one-third as much water-absorption tendency and much higher heat resistance--higher even than PPS. Glass-transition temperature is 253-275 F, depending on test method.

Equally good news, according to RTP's Otto, is that PPA molds very much like nylon in terms of flow and cycle time. Melting point is 590 F, and Amoco's recommended injection molding melt temperatures are 610-650 F. A mold temperature of 275-300 F, Otto says, will provide maximum crystallinity and a resin-rich part surface. If exposed to moisture, PPA must be dried for 6-16 hr at 175 F. "Like nylon, PPA will discolor if dried at temperatures much above 175 F," Otto reports.

In addition to the properties shown in the table, Amoco reports continuous-use temperatures of 302-365 F at 5000 hr and 266-329 F at 20,000 hr. Rockwell hardness ranges from 125 for unfilled to 127 for some compounds. Mold shrinkage is 12-16 mil/in. unfilled, which reportedly can be reduced to 0.5-1 mil/in. in highly filled compounds. RTP reports unnotched Izod impact strength (1/8 in.) of 3.2 ft-lb/in. unfilled, 6-17 ft-lb/in. for 10-50% glass-filled compounds, and 4-15 ft-lb/in. for 10-40% carbon-fiber compounds. RTP's 50% glass-filled 4009 grade offers a flex modulus of 2.4 million psi, together with 2.4 ft-lb/in. notched Izod. Note that mechanical properties after moisture conditioning are somewhat lower.

As for electrical properties, Amoco and RTP report values in the range of [10.sup.15] to [10.sup.16] ohm-cm for filled and unfilled material. Dielectric constant can range from 4.2 to 4.6 at 60 Hz and 3.9-6.1 at 1 MHz. Dissipation factor ranges from 0.005-0.006 at 60 Hz and 0.009-0.027 at 1 MHz. Amoco cites Comparative Tracking Index (CTI) values of 550 v.

At press time, Amoco released data on additional grades, including AF-1133 VO, a 33%-glass, flame-retardant version that meets UL 94V-0 at 1/32 in. Also brand new are two unreinforced injection grades, AD-1000 (see table) and impact-modified ET-1000. The latter has a notched Izod impact of 20 ft-lb/in. (2 ft-lb/in. at -40 F vs. 0.8 for AD-1000), yield elongation of 6.8%, density of 1.13, flex modulus of 330,000 psi, and HDT at 264 psi of 248 F. In development are extrusion and blow molding grades. Amoco has multimillion-lb/yr Amodel capacity and is engineering an even larger plant. [Tabular Data Omitted]