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Enter a new crop of enhanced HIPS.

Halfway between the low end of ABS and the best of polystyrene is a rapidly proliferating group of enhanced high-impact polystyrenes (HIPS) that some suppliers are calling "advanced styrenic resins," or ASRs. Enhanced HIPS grades have always set themselves apart from conventional HIPS by gloss and price. The newest ASR grades are distinguished by an unprecedented combination of high impact and high gloss. They also carry a significant premium over standard HIPS.

These resins are made differently. Whereas conventional HIPS contains upwards of 6-10% rubber by weight, with 1-8 micron rubber particle size, enhanced HIPS or ASR grades use 8-9% rubber, taking advantage of smaller particle size (0.1-1.5 micron particles).

Some of the newest enhanced HIPS grades use different ratios of small and large rubber particle sizes. Dow Plastics, Midland, Mich., which introduced its first ASRs late last year on a limited or semi-commercial basis (see PT, Feb. ,93, p. 73), says its process (for which Dow has applied for a patent) doesn't use ordinary butadiene rubber. Competitors speculate that Dow blends a variety of butadiene rubber together (conventional HIPS is made with low-cis butadiene rubber) and possibly other rubbers as well.

One resin company has made an enhanced HIPS material for at least a decade--BASF Corp., Parsippany, N.J. This material formerly was produced as part of the PS business of Mobil Chemical Co., but was made by a BASF process licensed to Mobil at the time. BASF's grade, ES 9524, has gloss in the 90s, notched Izod impact of 2.3 ft-lb/in., and Gardner impact of 280 in.-lb. (CIRCLE 26)

The target market for all these enhanced HIPS materials is refrigerator liners, small appliances, electronics and toys that currently use medium-impact ABS. A big advantage the enhanced HIPS grades offer processors is 20-30 /lb off the market price of medium-impact ABS. Most enhanced HIPS grades list at 70[cts.]/lb (they may actually sell for a nickel less), compared with 90 /lb for comparable general-purpose ABS. Standard HIPS runs 46-50[cts.].

Another big plus for these materials is improved processability and faster injection molding cycle times. Enhanced HIPS, like standard HIPS, processes at temperatures of 450-470 F, 50[degrees] cooler than for ABS. In addition, standard HIPS needs heated molds (140-150 F) to get high gloss, while enhanced HIPS can use molds at 90-100 F, though even higher gloss may be achieved by using heated molds. HIPS also doesn't need predrying like ABS.


The table on p. 27 shows comparative properties of some developmental and some recently introduced grades of enhanced HIPS, along with the original BASF enhanced HIPS and a medium-impact ABS.

Dow has brought two new grades to the customer sampling stage so far: XU-70016.01 for injection molding small appliances and XU-70019 for sheet extrusion for thermoforming refrigerator liners. XU70016 has a Gardner gloss of 92 in a 100 F mold and 98 in a 150 F mold. Gardner impact is 270 in.-lb at 73 F; notched Izod is 4-4.5 ft-lb/in. at 73 F and 1.6 ft-lb/in. at 0 F; and total-energy dart impact is 350 in.-lb at 73 F. (CIRCLE 27)

Chevron Chemical Co., Houston, introduced its first enhanced HIPS last year, Valtra HG200 (high gloss) with Gardner gloss of 91 (in a 150F mold) and notched Izod impact of 3.5 ft-lb/in.(see PT, Sept. ,92, p. 13). Chevron has since introduced a developmental RS300 (refrigerator sheet) grade with slightly higher impact and enhanced ESCR, unusual in any PS, but with low gloss. Some competitors speculate that RS300 may be alloyed with polybutene, PET or PBT for chemical resistance. Chevron is also developing a third new grade (D7023.04) with very high gloss at 96-97. (CIRCLE 28)

Huntsman Chemical Corp., Salt Lake City, last year commercialized XI 2124 high-gloss HIPS with Gardner gloss of 95 (in a 120 F mold) . A second experimental high-gloss HIPS, XFRI 2158 H2N, offers flame retardency (UL 94V-O at l/8 in.). This grade has 90 gloss, tensile yield of 3600 psi, 25% elongation, flexural modulus of 250 kpsi and notched Izod of 1.6 ft-lb/in. (CIRCLE 29)

Fina Oil & Chemical Co., Dallas, is in limited precommercial sampling with developmental grade CX7125 and expects to be commercial by year end. (CIRCLE 30)

Novacor Chemicals Inc.'s PS group in Leominster, Mass., is also developing a new family of ASR grades with 90-95 gloss and notched Izod impact of 3.2-4.2. The first, designated P-2292, is in limited customer sampling now and expected to be commercial by year-end. Novacor also plans new grades with improved scratch and chemical resistance, flame retardency and heat resistance. (CIRCLE 31)
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Title Annotation:high-impact polystyrenes
Publication:Plastics Technology
Date:Jun 1, 1993
Previous Article:Innovative molding cuts cost, weight of structural plastic parts.
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