Impact modifiers: multiplying roles.
The need for restoring lost properties to recycled resins and for compatibilizing commingled scrap are just two of the factors driving a rush to develop new impact modifiers--some based on new types of materials, including ones with reactive functionality.
The focus of much of the recent development has been on polyolefins and engineering resins--especially blends and alloys, where new modifiers are starring in the dual role of compatibilizer and toughener. Less emphasis has been placed on the traditional area of PVC. Yet another factor behind the recent developments has been the rapidly growing applications for TPO automotive exterior trim and interior parts such as arm rests and dashboards.
Compatibilizing isn't the only extra function impact modifiers are being asked to perform today. "The plethora of new products coming onto the market are designed as value-added products," says Jim Keeler, market development manager for additives at Zeon Chemicals (a spinoff from BFGoodrich). These modifiers provide what Keeler describes as a "basket of properties." Besides toughening, many of them also enhance other properties like low-temperature performance, heat aging and oil and chemical resistance. Compared with previous modifiers, many of the new entries also are said to improve flow and broaden a compound's processing window.
"We think there have been unmet needs that cried out for new products," says Ellen C. Brindley, polymer modification market development manager for Dexco Polymers, the new Exxon/Dow partnership in TP elastomers. Earlier this year, Dexco introduced seven grades of styrenic block copolymers that both toughen and compatibilize. Produced by a proprietary technology that reportedly offers unprecedented ability to tailor the product (see PT, April '92, p. 57), these modifiers are designed in part for upgrading the increasing amount of recycled resins being used today.
Dexco's new line of styrenic block copolymers (the first domestically produced competitors for Shell's Kraton) include Vector 6000-D, which doubles as a compatibilizer in styrenic/olefin blends (such as might occur in commingled recycle). Vector 6240-D is a diblock-free SBS with 4495 psi tensile strength; Vector 6201-D reportedly has good elasticity and processability; Vector 6115-D is an SIS diblock with 1500% elongation at break; Vector 6101-D is a diblock-free SIS with 38 Shore A hardness; Vector 6241-D is an SBS diblock (88 Shore A); and Vector 6010-D is a low-styrene medium-molecular-weight rubber.
Brindley says Dexco's high-styrene triblock grades can improve cycle times and flowability. The modifiers have melt indexes from 23-25 where many other modifiers have indexes around 12. And because its technology can yield a diblock-free, 100% triblock elastomer, Dexco says its modifiers can provide about 5-9|degrees~F higher heat-distortion temperature.
Dexco says it feels the unprecedented 100% conversion to triblock content of its elastomers is the key to their success. The best previous elastomers could hope to attain was 85%. Andre J. Uzee, polymer modification technical service and development leader at Dexco's Plaquemine, La., production plant, says even a minimum of 15% uncoupled diblock can adversely affect the tensile strength and elastic modulus of a neat polymer and negatively affect the degree of compatibilization of dissimilar materials.
Furthermore, this unique polymerization process also makes it easier to predict precise molecular weights, Uzee says, eliminating the byproduct production of halide salts and cutting out the risk of future discoloration or property degradation. Polymerization control also makes it feasible to tailor the polymer block interfaces, opening up the possibility of improving blending compatibility with other polymers.
Yet, Dexco representatives admit, there are a few drawbacks to these modifiers. Dexco's SBS chemistry provides less high-temperature processing stability than SEBS, and the Dexco products are not highly oil resistant.
Exxon Chemical Co., Houston, recently completed a 2-million-lb/yr plant to produce its new Bromo XP-50 elastomers (PT, Dec. '91, p. 9). These products reportedly have potential applications in impact modifying and compatibilizing blends of engineering resins. They also can be used to blend commingled recycled resins.
Bromo XP-50 elastomers are said to be unique brominated copolymers of isobutylene and paramethylstyrene that reportedly have good ozone resistance, heat stability and chemical reactivity. Exxon has several versions that show potential as reactive low-temperature flexibilizers and impact modifiers for nylon, PPO alloys, TP polyesters and other engineering thermoplastics, as well as urethanes and epoxies. The new rubbers' ability to graft to many polymers (including PS and PP) further suggests uses in blend compatibilization and in dynamically vulcanized TPO alloys.
Shell Chemical recently introduced a new functionalized Kraton SEBS elastomer that reportedly can produce "supertough" nylon 6. New Kraton FG 1921X gives a better balance of impact and tensile strength in nylon 6 than Kraton FG 1901X introduced in 1988. FG 1901X was the first in a series of SEBS elastomers functionalized with reactive maleic or succinic anhydride end groups that bond with nylon. This gives much improved results over standard Kraton grades. While FG 1901X supertoughens nylon 66, it does not work well in nylon 6 unless it is used together with an unfunctionalized Kraton G. In that case, the functionalized elastomer serves as a compatibilizer for the unfunctionalized modifier.
Rohm and Haas, traditionally specializing in PVC tougheners, has put its main R&D thrust lately into engineering thermoplastic modification. Its most recent entries here are Paraloid EXL-3657 (MBS) and EXL-3361 (acrylic). Both reportedly improve heat stability in both processing and heat aging of polycarbonate and PC blends (PT, Feb. '92, p. 13).
And Nova Polymers recently unveiled Novalar, a so-called "universal" modifier said to work with ABS, PC, PBT, PVC, polyurethane, epoxies and acrylics.
"Continued demand to improve performance in such applications as automotive and the desire among TPO suppliers to differentiate their products from the competition are important factors in helping expand the impact modifier market," says Bruce Duerringer, additives business manager for Kaneka Texas Corp. And Roger Chapman, president of Nova Polymers, sees plenty of opportunity "as more and more custom compounders will be formulating their own TPOs."
Despite the growing availability of reactor-made TPOs, suppliers like Chapman see a growing trend to further custom-tailor one's own polyolefins for application-specific hardness and other properties. New impact modifiers are arriving just in time.
Exxon's new brominated rubbers are one example. Another is an experimental grade of ethylene polypropylene oil-extended synthetic rubber, EPDM XG 006, introduced last fall by the Polysar Rubber Div. of Miles, Inc. for use in TPOs requiring low stiffness and high flow (PT, Dec. '91, p. 38). Miles says the particular paraffinic oil in this material, which has only 0.5% aromatic content, improves uv and oxidation resistance and color integrity. This white oil, Miles says, allows better color matching and provides enhanced color stability without additional stabilizers.
At the same time this modifier was introduced, Miles unveiled another ethylene-polypropylene modifier for TPOs and PP, EPM XF 004. Besides boosting impact strength, this elastomer reportedly improves flow and gloss. Miles says it is aimed at use in auto bumpers and fascia, gasketing, tubing and films. Neither of Miles' newest elastomers are fully commercial yet.
Some nontraditional materials finding roles in toughening polyolefins are the relatively new ultra-low-density linear PE copolymers--Dow's Attane and Union Carbide's Flexomers. These have been found to improve low-temperature impact resistance in large HDPE injected molded parts, company spokesmen say. There is also R&D work under way to investigate their suitability as the rubber phase in PP-based TPOs.
More New Products
For PVC: Kaneka Texas Corp. has introduced Kane-Ace F-20, an all-acrylic impact modifier for PVC siding that also provides low gloss. (Kaneka also will increase its capacity for MBS modifiers by 35 million lb/yr, or 64%, in late 1993.)
Three new ABS-type modifiers for PVC have been developed by Zeon Chemicals. These developmental products could be commercialized shortly. Nipol DP5123P is an uncrosslinked nitrile rubber for plasticized PVC; Nipol DP5125P is a partially crosslinked NBR copolymer said to improve oil and chemical resistance; and Nipol DP5128P is a partially crosslinked NBR that improves low-temperature performance and heat aging in plasticized PVC.
For thermosets: Besides the new brominated rubbers from Exxon, Zeon is working on an NBR powder for toughening epoxy composites.