Polyolefins get tough with metallocenes.
When it comes to impact modification of polyolefins today, it seems there's nothing like a metallocene. Polyolefin elastomers and plastomers (POEs and POPs, as some call them) based on single-site catalysis have already become strong challengers to traditional EPDM and EPR modifiers, as well as for ULDPE or VLDPE, in TPOs and other high-impact blends (see PT, July '94, p. 46). There are only three domestic suppliers of metallocene-based POEs and POPs - Dow Plastics (whose prod-cuts are trade-named Affinity), DuPont Dow Elastomers (Engage), and Exxon Chemical (Exact). Their ultra-low-density linear PE copolymers and terpolymers are distinguished by narrow and highly reproducible distribution of both molecular weight and comonomer composition. These resins' claims to fame as modifiers include, superior stiffness/toughness balance, high clarity, and ease of compounding and handling due to their pellet form.
Metallocene-based resins are attracting most interest for modifying PP in durable goods ranging from automotive TPOs to filled and unfilled compounds for appliances, power tools, and medical devices. POPs and POEs are also finding use as modifiers for polyolefin foams, LLDPE packaging films, and LLDPE injection molded parts. Suppliers say they are also looking at applications in rotomolding LLDPE and in post-consumer reclaimed HDPE. Functionalized versions for toughening engineering thermoplastics such as nylon 66 and PBT are also in development (see PT, June, '96, p.23).
WHAT'S AVAILABLE NOW
Exxon offers four out of its 26 grades of Exact plastomers (density range is 0.865 to 0.915 g/cc) specifically for impact modification. All are ethylene butene copolymers. Exact 4042, 4033, 3035, and 4041 are in the 70-90[cents]/lb range and are typically used at 10-30% levels.
Exact 4033, with 0.880 density and MI of 0.8 g/10 min, is the most popular modifier grade of Exact, particularly for high-flow TPO formulations aimed at injection molding of large, complex, and thin-walled parts, says sales development manager Fred Steininger. In a typical automotive bumper composition, a TPO based on Exact 4033 shows up to 25% higher flow and lower shrinkage (0.30-0.55% versus 0.35-0.70%) than a traditional HMW EPR while maintaining the same stiffness and low-temperature ductility.
Grades such as Exact 4042 and 3035 are being used to modify nucleated/clarified PP random copolymers for food-storage containers that require low-temperature toughness and clarity. Studies show that adding up to 40% Exact 4042 to a clarified random copolymer reportedly causes no increase in haze, while only 10% of 4042 produces nearly a fivefold increase in total instrumented impact energy at -10 C. Exxon also recently released data showing that another plastomer, Exact 3024, prevents radiation embrittlement while preserving clarity and heat resistance (for autoclave sterilization) of clarified PP homopolymers used in syringes and other medical devices (PT, July '96, p. 11).
Outside of PP, Exxon's plastomers are being investigated for modifying LLDPE packaging films. Steininger says blown-film producers have achieved 40% improvement in dart impact strength, 25 [degrees] C lower heat-seal initiation temperatures, and 25-50% lower haze by adding Exact plastomers. Figure 1 shows that two terpolymers (Exact 3033 and 3034) are more effective than two butene copolymers (Exact 3028 and 4011) in raising dart impact strength of hexene LLDPE (0.918 density) blown film. Similar results were observed for tensile strength. Exact 3033 was also the best performer in improving hot tack and heat-seal strength of an octene VLDPE [ILLUSTRATION FOR FIGURE 2 OMITTED]. Also, Exact grades such as 4033, 4049, and 4041 are being used to improve softness, strength, and compression recovery in EVA and PE foams.
DuPont Dow Elastomers offers four of its 23 Engage POEs for impact modification. Included are general-purpose Engage 8100, high-performance 8150 for maximum ductility and strength, and 8200 for higher flow. These ethylene-octene copolymers have densities of 0.868 to 0.870 and sell in the 80-95[cents]/lb range. Recently launched Engage 8180 is an improved version of 8150 in terms of low-temperature impact properties. Typical use levels for all four products are 15-30%.
The recently formed DuPont Dow Elastomers venture took over marketing and development of what had been Dow's Engage POEs, while Dow retained the Affinity POPs. All are part of a continuum of products made with Dow's Insite metallocene technology. DuPont Dow global business manager Robert Bernacki explains that products with densities under 0.913 are designated Engage, and anything above that are labeled Affinity. Grades for packaging, rotomolding, and fibers are tagged Affinity even if lower than 0.913 density.
Affinity resin modifiers are sold primarily to PE film extruders. The market for Engage POE modifiers is mainly PP resin producers, compounders, and film extruders in certain markets - automotive, wire and cable, and non-packaging injection molded parts. DuPont Dow says selection of the POE as an impact modifier in PP requires a balancing act. Depending on the type and molecular weight of the PP, the POE selection must give the desired impact performance while maintaining a balance of stiffness, heat distortion, and processability.
TPO ingredient qualifications are complicated by differences in low-temperature impact tests specified by different customers - either -30 C dart impact or -30 C notched Izod. In general, TPOs in the U.S. require high flow, high modulus, high heat, and high dart impact at -30 C. In this case, homopolymer PP with Engage 8100 is an appropriate combination, according to DuPont Dow Elastomers. In Europe and the Pacific, the same requirements apply, but impact is typically specified as -30 C notched Izod. In that case, portions of the homopolymer PP must be replaced with copolymer PP, and a higher-performance modifier like Engage 8180 or 8150 is more appropriate, the company advises [ILLUSTRATION FOR FIGURE 3 OMITTED].
Dow Plastics' Affinity octene copolymers for impact modification include SM 1300 (0.902 g/cc) and SM 1350 (0.913 g/cc), both with an MI of 30. They are aimed primarily at injection molded, thin-walled LLDPE and PP packaging such as dairy containers, cups, reusable crates, and pallets. Adding 15% SM 1300 to a 60 MFR PP in a container application raised the failure height 50% in frozen drop tests from 4 ft up to 6 ft. The blend also showed modest increases in tensile strength and elongation, while density was reduced 0.2% and secant modulus dropped 17%.
BENEFITS BEYOND TOUGHNESS
Compounders using metallocene polyolefins for impact modification of PP and TPOs say improvements go beyond impact enhancement to better knit-line strength [ILLUSTRATION FOR FIGURE 4 OMITTED], clarity, flow, and mar resistance. Combining metallocene polyolefins with new high-performance PPs allows compounders to formulate automotive TPOs with properties previously unattainable, opening up new applications in door panels, instrument panels, and more.
Metallocene-based modifier resins dramatically improve impact and stiffness, says product manager Robert Heinold at A. Schulman. A new family of PP compounds based on metallocene modifiers reportedly has notched Izod impact of 5 ft-lb/in. and modulus of 420,000 psi - eight times higher impact resistance than a conventional PP with equivalent stiffness. They also exhibit low shrinkage and very low CLTE of 2.4 x [10.sup.-5] in./in./[degrees] F - less than half that of conventional PPs with equivalent toughness, Heinold says. Whereas a 60-in.-long part made with unmodified PP can shrink up to 1.25 in., the new PP shrinks only 0.25 in., he notes. The 5-ft-lb/in. PP compound has the same shrinkage as ABS, while a new grade with notched Izod of 3 ft-lb/in. and modulus of 460,000 psi exhibits one-third the shrinkage of ABS.
Applications such as long, intricate instrument panels are gaining other benefits from metallocene modifiers. Due to their high toughness, these impact-modified PPs are not sensitive to the stress risers in such parts. They also have ductile low-temperature impact.
Polyolefin foam maker Sentinel Products Corp., which has been marketing two families of 100%-plastomer foams, now offers two families of polyolefin foams composed of 30-60% metallocene modifier with PP, PE, or EVA. CEO Scott Smith says the metallocene modifiers typically have less than 0.890 g/cc density. The modified foams include MPO (Metallocene Polyolefin Foam) used in adhesive tapes, EKG pads, and surgical-grounding pads. "We are delivering flexible foams with superior elongation, tensile strength, and adhesion characteristics that can replace EVA, EPDM, and PVC foams," says Smith. MPO at 14-pcf density is used in bottle-cap gaskets to replace 28-pcf EVA foam, he notes. Another application is computer packaging, where MPO as low in density as 1.2 pcf replaces 2.5-pcf conventional LDPE foams.
Another modified foam was developed by Sentinel along with Ferro Corp.'s Filled and Reinforced Plastics Div., Evansville, Ind., for auto interiors. FMP (Foamed Metallocene PP) combines POE with a PP blend of homopolymer and both block and random copolymers, resulting in a soft-feel, closed-cell foam sheet that's flexible, high-heat resistant, and dimensionally stable. One use is a foam substrate for an all-PP instrument panel.
Meanwhile, Ferro has demonstrated that an unidentified metallocene-based ethylene-octene copolymer gives better impact and stiffness balance than ULDPE in g-p and high-crystallinity PP homopolymers or copolymers containing 30% talc or calcium carbonate. In stiffness and impact balance, the metallocene-modified calcium carbonate compounds approach super-high-impact ABS, while talc-filled blends are said to approach medium-impact, high-heat ABS.
Ferro found similar benefits in 30% glass-filled PP homopolymer and copolymer. Stiffness/impact balance was brought into the range of glass-reinforced styrenics like SMA and PPO/HIPS.
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|Author:||Sherman, Lilli Manolis|
|Date:||Aug 1, 1996|
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