More than just tough.
Users and formulators of impact-modified plastics today are looking for more than just toughness. They want ambient- and low-temperature toughness plus a better balance of stiffness and heat resistance. Toughness plus enhanced processability, clarity, or weatherability. Toughness plus lower stress whitening or hexane extractables. Fortunately for all concerned, whole new families of impact modifiers are coming onto the market promising just these sorts of benefits.
Interviews with virtually all major suppliers of impact modifiers turned up several incremental advances in the traditional areas of PVC modification and a few new developments for thermosets. But the big news today is in toughening polyolefins and engineering thermoplastics. Formulators of polyolefin compounds--particularly PP-based TPOs--are being offered new families of ultra-low-density linear PE copolymers as alternatives to traditional EPDM and EPR modifiers. They are copolymers and terpolymers made with new single-site metallocene catalysts. Functionalized versions for toughening engineering thermoplastics may be the next step for these new polyolefins.
Meanwhile, there's something else very new for engineering resins--namely, silicone-rubber modifiers that boast better weatherability than acrylic types and function at lower use levels.
EXCITEMENT IN POLYOLEFINS
New developments in polyolefin modifiers are coming thick and fast these days. Compounders are exploring the potential of metallocene-catalyzed olefinic plastomers and elastomers that have been commercialized in the past two years by Exxon under the Exact trade name and by Dow Plastics under the Affinity and Engage brands. These materials are challenging traditional elastomers and ULDPE or VLDPE copolymers such as Dow's Attane and Union Carbide's Flexomers.
Metallocene-catalyzed polyolefin plastomers and elastomers (POPs and POEs, as Dow calls them) are distinguished by their narrow and highly reproducible distribution of both molecular weight and comonomer composition. Low extractables for FDA applications is one resulting benefit. Another is said to be unprecedented combinations of toughness, stiffness, and heat resistance.
Numerous processing advantages also stand out. For one thing, the new modifiers are sold in dry, free-flowing pellet form. Whereas EPR and EPDM are frequently supplied in big blocks or bales suitable mainly for batch processing, pellets allow compounders to switch from batch to more efficient continuous compounding. Even when the traditional rubbers are available in pellet form they reportedly disperse with some difficulty, requiring twin-screw extruders. By contrast, the metallocene resins reportedly process well on single-screw extruders, and they can even be simply dryblended at the hopper of an injection machine, according to a paper at the recent SPE ANTEC in San Francisco by Exxon researcher T.C. Yu. The paper also noted that pelletized Exact modifier resins do not require exterior dusting agents or internal processing aids.
Another promising attribute of metallocene-based modifiers is that they impart higher clarity to PP blends. As noted in a Dow paper delivered at this year's SAE conference in Detroit, "Optical-clarity-grade impact-modified PP is not readily available in the marketplace." Exxon's Yu said in the ANTEC presentation that "impact plus clarity is not achievable with common impact modifiers such as EPR and polyisobutylene rubbers." Both Dow and Exxon says this tradeoff can be remedied by use of their new plastomers. As shown in Fig. 1, an Exact plastomer of 0.899 density toughens a clarified PP random copolymer with virtually no increase in haze.
Even suppliers of EPDM and EPR modifiers concede some advantages to metallocene resins. Says one large EPDM supplier, "Because of their clarity, we see lots of impact by these products in the film and packaging area. While we expect some substitution of EPDM and EPR by these products, we think this will largely occur in lower-performance applications. These products are not a direct drop-in substitute for EPDM in applications such as higher-performance TPOs and TPVs. You may need to use higher levels and the melt flow of the entire TPO must be reformulated when using these new materials."
Of Dow's seven Engage POEs currently on the market (all ethylene-octene copolymers), three are intended for use as impact modifiers in polyolefins (see PT, March '94, p. 17). Engage EG 8100 is a general-purpose impact modifier; EG 8150 is a high-performance modifier that offers maximum ductility and strength; and EG 8200 is designed for higher-flow impact modification. These products sell for around 80-95[cents]/lb.
According to Dow project manager Mike Levinson, Engage POEs are currently modifying mainly PP homopolymers, copolymers, and filled compounds for durable-goods applications ranging from appliances and power tools to automotive and non-automotive TPOs. However, he also notes increased interest in impact modification of EVA, LDPE, LLDPE, and HDPE.
Levinson says Engage materials are much more efficient tougheners at lower use levels than its own Affinity plastomers or non-metallocene VLDPE or ULDPE resins, owing chiefly to POPs' lower density. Moreover, there is said to be a dramatic difference in the low-temperature impact properties afforded by the POEs.
In addition, Levinson stresses the improved balance of stiffness, toughness, and heat resistance the new Engage products offer. Typical rubber modifiers tend to reduce stiffness and elevated-temperature performance of PP blends. Engage products reportedly minimize such tradeoffs because of their highly homogeneous nature and tendency toward finer and more uniform dispersion in the PP matrix, which also yields higher knitline strength--better even than in-reactor TPOs, according to Dow's data.
Dow also says this improved dispersibility allows use of higher-flow PP matrix resins while maintaining low-temperature ductility. Using a higher-flow matrix, Levinson explains, "means you can mold more parts per hour at reduced part thickness." As shown in Table 2, use of a POE modifier results in better flow of the TPO compound even without changing the PP matrix.
Dow also offers Affinity POP octene copolymers for impact modification in densities from 0.880 to 0.910. They're priced at 60-90[cents]/lb--competitive with Carbide's Flexomers.
In direct competition with Dow, Exxon is currently marketing four of its 26 grades of Exact plastomers for TABULAR DATA OMITTED impact modification. Exact 4042, 4033, 3035, and 4041 are said to toughen polyolefins without sacrificing stiffness. "We've done a lot of work with our family of ethylene-butene copolymers--which are highly compatible with PP--to improve the balance of toughness and stiffness properties," says Fred Steininger, director of sales development for Exact plastomers, which range from 0.865 to 0.915 g/cc.
These products are currently being blended with PP, EVA, LLDPE, LDPE, and HDPE in commercial applications such as injection molding, extrusion blow molding, blown and cast film, and profile extrusion. The two higher-density Exact grades are FDA compliant for use in food-contact applications. Exact grades sell for around 70-90[cents]/lb and are typically used at levels of 10-20%
WHAT COMPOUNDERS SAY
Teknor Apex Co. in Pawtucket, R.I., is one compounder that's working with Dow's Affinity plastomers and the newer Engage POEs. "We are still in the developmental stages and are currently doing some customer sampling of PP compounds that contain these modifiers," says marketing manager Tom Moccia. Initial customer feedback has been very good, he notes, adding that these materials--particularly Engage POEs--process very well, increase tensile properties by as much as 15-20%, and also appear to boost impact. End uses for which he sees the most potential include auto body cladding, weather stripping, and packaging closures.
Colonial Plastics in Dyersburg, Tenn., has also evaluated Dow's Engage POEs for PP impact modification. Says marketing manager Rick Shafer, "In TPOs, these materials provide opportunities not previously possible; they allow for continuous processing and allow us to extend certain properties in different directions. We have found that you lose less stiffness than you would with EPDM or similar modifier systems." An auto bumper pad made of a Colonial TPO containing an Engage POE is already commercial, and the company is exploring other applications.
TABLE 2--PROPERTIES OF TPO MODIFIED WITH DOW'S ENGAGE POE Blend of Blend of Neat Properties Engage POE EPDM PP Homopolymer MFR @ 230 C, g/10 min 20 20 Elastomer Density, g/cc 0.870 0.860 Elastomer Melt Index, g/10 min 1.5 0.3 Weight % of Elastomer 30 30 Blend Properties Young's Modulus, psi 167,400 145,700 Ultimate Tensile Strength, psi 3240 2650 Dynatup Impact Energy, ft-lb, -20 C 37.0 29.6
Boyd Ramsey, technical director at Thermofil, Inc., Brighton, Mich., says his company has tried both Exact and Affinity plastomers, as well as Union Carbide's Flexomers, in its glass-filled PP compounds and plans to continue work with the Engage POEs. Thermofil found all these products to be equivalent in overall performance for the applications in which they were tried. All three were used at levels of less than 10%, comparable to EPDM.
The recent MetCon '94 Worldwide Metallocene Conference in Houston (sponsored by Catalyst Consultants Inc., Spring House, Pa.) included three testimonials by compounders and end users concerning the promise of POP and POE modifiers. First, Dr. Lecon Woo, researcher at Baxter Healthcare in Round Lake, Ill., spoke favorably of low-temperature impact test results with a Dow Engage POE in PP. He noted that the metallocene resin was an effective toughener down to about -22 F, perhaps equivalent to EPR, and that pellets were indeed easier to process than EPR bales.
Second, Dr. Deen Chundury, technical director of Ferro Corp.'s Filled and Reinforced Plastics Div. in Evansville, Ind., reported on trials of a metallocene-based plastomer. (Although he didn't identify it, the plastomer matches the density and melt index of Exxon's Exact 4041.) Ferro tried modifying a PP homopolymer with 10% of either EPR, VLDPE, or the metallocene plastomer. In both unfilled and 20% talc-filled compounds, the PP/plastomer blend showed toughening capability comparable to the other blends at ambient and low temperatures. However, the plastomer blend had the highest melt flow in both filled and unfilled compounds, and the filled plastomer compound was also stiffer, stronger, and more heat resistant than the other filled compounds. (Improved mechanical properties were not evident in unfilled compounds.) Lower stress whitening was also seen in the plastomer blend, which Chundury attributed to enhanced compatibility relative to EPR and VLDPE. Chundury also considered economics in his conclusion that "PP/plastomer blends offer a good alternative to more expensive PP/EPR blends with respect to impact modification."
Chundury also said that Ferro substituted a metallocene plastomer for EPR in new grades of its Rxloy polyolefin alloys for medical tubing. He said that the metallocene modifier conferred improvements in contact clarity, kink resistance, and kink recovery, plus lower cost and higher extrusion rates. These benefits were obtained with minimal sacrifice of softness.
Also at the MetCon meeting, Bruce Petersen, v.p. of materials management at A. Schulman, Inc., Akron, Ohio, reported that in the last 18 months his company has made commercial quantities of Schulaflex TPOs using metallocene modifier resins from both Dow and Exxon. He said these resins "seem well positioned to carve out a place at the table in the thermoplastic elastomers compound market."
He noted that these modifiers confer excellent low-temperature impact and are easier to compound than conventional rubbers. He cautioned that metallocene resins are not simple drop-in replacements for EPR or EPDM: "You must rebalance the entire system." The economics are not clear cut, either--metallocene modifiers can be more or less expensive than EPR and EPDM, depending on the specific compound.
MORE FOR POLYOLEFINS
The metallocene story is far from over, and more new developments from this fertile catalyst technology are on the way. For example, firms such as Himont in Italy, DSM in Holland, and Exxon here in the U.S. are working on the metallocene-made EPDMs and EPRs. At MetCon, a speaker from Himont Italia noted that metallocenes can produce an "incredible variety" of such rubbers with a wide spectrum of compositions and properties. Like other metallocene resins, these can be made in pellet form and offer the expected benefits of narrow and predictable molecular-weight and composition distributions. The Himont speaker said metallocene catalysts can make either drop-in copies of existing EP elastomers or wholly new, innovative materials.
Meanwhile, metallocene resins are far from the whole story of what's new in polyolefin impact modification. According to Union Carbide business development manager John Brindel, use of Flexomer VLDPEs is rapidly increasing for modifying PP--particularly for TPOs--as well as HDPE and ethylene copolymers like EVAs and EEAs. "Flexomers are being used as impact modifiers or energy absorbers in TPOs, as well as for compatibilizing two different TPOs--for example, a TPO bumper fascia reground with a TPO instrument panel," Brindel reports.
In addition, Thermofil has tested Flexomers in glass-filled PP. Although EPDM outperformed some of the earlier Flexomers in low-temperature impact, Boyd Ramsey says newer Flexomer grades can outdo EPDM at temperatures down to -40 F.
Available in powder TABULAR DATA OMITTED or pellet form, these VLDPEs can be compounded continuously with single-or twin-screw extruders or with batch mixers. Flexomers have densities of 0.880-0.905 g/cc and melt indexes from 0.1 to 5.0. They sell for around 68-90[cents]/lb.
In other types of modifiers, Miles' Polysar Rubber Div. is close to commercializing its experimental EPM XF 004 elastomer (PT, July '92, p. 63) for TPOs and other PP modification. This low-viscosity EPR has a Mooney viscosity of 22 and melt flow of 3.0. In a 12-MFR PP, 30% of the modifier yielded a compound with MFR of 8.0, notched Izod impact of 4.5 ft-lb/in. at room temperature and 0.5 ft-lb/in. at -22 F, and 1% secant flex modulus of 125,000 psi. This EPR is also said to improve flow and gloss. It sells for about $1/lb.
According to Royalene EPDM market manager John Johnson, Uniroyal Chemical is focusing its efforts on developing "very customized EPDM products," particularly for TPOs and TPVs. They feature low coefficients of thermal expansion and improved paintability.
At Nova Polymers, a major thrust has been to come up with low-cost impact modifiers that enable recyclers to upgrade scrap or waste resins, according to president Roger Chapman. He says a high-Izod copolymer or a TPO can be produced from scrap PP homopolymer plus 10-15% loadings of Novalene EPDM-based modifier.
A recent addition to Nova's line is Novalene 6400-U, a low-cost EPDM-based product in free-flowing pellet form for use in PP homopolymer. Typical properties of a resulting blend of recycled homopolymer and 12% 6400-U include tensile strength of 3500 psi, flex modulus of 140,000 psi, and notched Izod impact greater than 2.5 ft-lb/in. The product can also be effective in upgrading HDPE regrind. The black-colored modifier sells for 65[cents]/lb. Sister product Novalene 7300-P for natural- and custom-colored products costs 75[cents]/lb.
Also EPDM-based is Novalene 7030, a new modifier supplied in a pelletized PP masterbatch. When blended at about a 30% level with homopolymer PP, it reportedly produces a paintable TPO with a notched Izod value of 10 ft-lb/in. at room temperature and 5 ft-lb/in at -20 F. It is designed for applications that require ductility at low temperatures.
NEW FOR ENGINEERING TPs
A novel class of impact enhancers for PVC and engineering resins has been introduced by Elf Atochem. Metablen S-2001 is based on silicone-rubber technology developed in Japan by the company's joint-venture partner Mitsubishi Rayon. Product manager John Wallace describes S-2001 as being in between acrylic and MBS impact modifiers in toughening performance. He explains that the silicone product mimics the weatherability of all-acrylic modifiers but with greatly improved impact retention as it weathers and with good low-temperature impact properties like MBS. S-2001 is for making weatherable ABS, polycarbonate, PC blends, and PET.
While it is higher priced (about $3/lb versus $1.10-1.20/lb for most all-acrylic modifiers), S-2001 is said to be highly efficient, requiring only about 60-70% of the use level of acrylic modifiers. "It's biggest claim to fame is that it provides excellent impact retention at temperatures as low as -4 F, versus all-acrylics at about 32 F," says market manager Art Van Norstrand. "In general, you can achieve 50-100% improved impact in both extrusion and injection molding at 3-8 phr."
Sources at Dow Corning, another major silicone producer, confirm that the company has similar technology that is being developed for impact-modifier use, though they offered no details.
Metallocene-made olefin copolymers may have a role to play in toughening engineering resins as well as polyolefins. Both Exxon and Dow indicate they are exploring the versatility of metallocene catalysis to incorporate reactive functionality in these olefinic copolymers. For example, they can be grafted with maleic anhydride, allowing them to bond to a nylon matrix. Incorporating functionality that would react with other engineering resins such as PBT, PET, and ABS is also being explored.
Uniroyal Chemical recently introduced Royaltuf X330, the latest addition to its line of modified EPDMs (PT, May '94, p. 16). This SAN-grafted version is designed to toughen filled and unfilled PBT and PET. Unusual for a rubber modifier, the product is said not to adversely affect stiffness and heat-distortion resistance. A blend of PBT with 25% X330 shows a notched Izod of 20 ft-lb/in. at room temperature and 2.5 ft-lb/in. at -40 F, plus flex modulus of 285,000 psi and HDT of 147 F at 264 psi.
Uniroyal also recently acquired the Polybond modified-polyolefins business of BP Performance Polymers, Inc. (PT, April '94, p. 77). Made by grafting acrylic acid or maleic anhydride onto a PP or PE backbone, these products are used as coupling agents and tougheners in glass-filled PP.
Core/shell modifier technology, originally for developed for PVC by companies such as Rohm & Haas, Elf Atochem, and GE Specialty Chemicals, is gaining considerable interest among compounders of engineering thermoplastics. These emulsion-polymerized products usually have a rubber core and a shell made of SAN, PMMA, or ABS, and are crosslinked into a sphere of 0.1-0.5 micron typical size. Sold in powder or pellets, these modifiers break down during compounding into the original submicron spheres that were formed in the emulsion.
"This technology is very sophisticated and very promising for impact modification of unfilled plastics," says George Niznik, v.p. and R&D director at LNP Engineering Plastics, Inc., Exton, Pa. "We see promise in core/shell technology beyond vinyl applications, such as in unfilled polycarbonate, SAN, and ABS/PC blends."
Rohm and Haas continues to fine-tune its core/shell engineering-thermoplastic modifiers for specific applications. Paraloid EXL-3361 acrylic modifier was introduced in 1992 (PT, July '92, p. 62). It is intended for durable and weatherable applications. Director of sales Joseph Sullivan says EXL-3361 has been shown to improve heat stability in both processing and heat aging of polycarbonate and PC blends. Moreover, its use has been expanded to include nylon and PET. Typical use levels are 10-20%.
Two other firms known particularly for PVC modification are directing R&D toward engineering resins. GE Specialty Chemicals is actively working to expand its product line for modifying ABS, SAN, and engineering resins. And Kaneka is working on new impact modifiers for polycarbonate, PBT, PET as well as some styrenics.
Shell Chemical is developing a line of liquid hydrocarbon-backbone saturated olefin polymers with hydroxyl and epoxy functionalities. They will have a variety of uses, including modifying TP polyesters, acrylics, and thermosets.
MODIFIERS FOR RECYCLE
Opportunities are growing for additives that can be used in the dual roles of impact modification and compatibilization of regrind and/or post-consumer scrap. GE Specialty Chemicals, for example, has found such applications for Blendex 338, an ABS-based impact modifier said to increase impact strength of SAN, ABS, and PVC at low loadings. It also reportedly achieves better initial color in SAN and ABS. The product was originally designed for low-temperature impact in rigid PVC, polycarbonate, and TP polyester. It has also proved to be an efficient modifier in SAN and ABS. Use levels run 10-50%. It sells for $1.35/lb tl.
Nova Polymers is now marketing Novalar P, a high-rubber-content modifier in pelletized concentrate form that allows recyclers to upgrade impact strength of ABS and PVC. "By providing it in a pellet form, it allows recyclers to use a single-screw extruder, whereas powdered modifiers required them to have twin-screw extruders," says Roger Chapman.
Some recent work with styrenic copolymer modifier resins also applies to recycle applications. Ellen Brindley, polymer modification development manager at Dexco Polymers, says its relatively new Vector line of SBSs is gaining use in HIPS and compatibilization of both prime alloys of styrenics and polyolefins and commingled recycle blends of these polymers. Dexco reports that a 60/30/10 blend of HIPS, HDPE, and Vector 6241-D shows a notched Izod impact of 5.1 ft-lb/in., as compared with 0.8 ft-lb/in. for an unmodified 70/30 HIPS/HDPE blend.
Firestone's Stereon 840A SBS has found use in upgrading PS regrind--both post-consumer and industrial scrap. This product is said to provide toughness, tear resistance, and flexibility when dryblended with LLDPE, PS and certain types of PP.
WHAT'S AHEAD FOR STYRENICS
For HIPS and ABS, Firestone has been fine-tuning and customizing grades of its Stereon SBS to provide enhanced gloss and ESCR, according to product manager Jay Lovrak. Other active programs include work on styrenic block copolymers that give improved impact resistance, driven by end-use requirements in both automotive and housing markets. Firestone is also developing SEBS elastomers, similar to Shell's Kraton G. (The need for a second source of these materials became particularly evident after the recent Shell fire and plant shutdown.) Explains Lovrak, "SEBS elastomers are the same basic polymer as SBS but the butadiene has been hydrogenated. As the double bonds have been removed, these products are very thermally stable and resistant to environmental stress cracking."
LOTS NEW IN PVC TOUGHENING
New developments in impact modification of PVC include products with improved efficiency, weatherability, and low-temperature performance. Of particular interest are two entirely new generic types of modifiers, as well as blends of impact modifiers and processing aids.
Besides engineering resins, higher-performance weatherable vinyls are targets for Elf Atochem's new Metablen S-2001 silicone rubber, discussed earlier. Besides much better weatherability than all-acrylic modifiers, S-2001 boasts better impact efficiency--especially at low temperatures. "We expect that it will allow for the use of PVC in areas where it could not previously be used, such as window profiles in cold climates, as well as in automotive and recreational vehicles," says Art Van Norstrand. He also hints that the company is developing a sister product that will offer lower cost in vinyl siding and window profiles.
GE Specialty Chemicals recently launched another brand-new category of weatherable impact modifier based on ASA (graft copolymer of SAN and acrylic rubber), which also confers high heat resistance. Advanced application development engineer Marty Grohman says the development of Blendex 975 was driven by the popularity of dark colors in PVC window profiles and, to a lesser extent, in siding. The new modifier is also aimed at automotive vinyl trim. Blendex 975 is used at loadings of 20-50%.
A weatherable all-acrylic modifier with enhanced low-temperature properties is also in development by Elf Atochem. Durastrength PD-1006 is said to have better toughening efficiency and low-temperature properties than the company's Durastrength 200 all-acrylic modifier for PVC profiles. Trials in two different tin-stabilized siding formulations have shown PD 1006 can be a part-for-part replacement for Durastrength 200 in twin-screw extrusion. In low-temperature saw-cutting tests, siding containing PD 1006 outperformed Durastrength 200 by generating fewer chips. Recommended typical use levels are 4-5 phr in extrusion and around 12 phr in injection molding.
Rohm and Haas has launched another new acrylic product, multifunctional Paraloid KAM-390, which incorporates both an impact modifier and a process aid. "We can blend these two in a liquid state to get the perfect ratio and then make it into a powder so that the different additives cannot segregate as they can in physical blends," explains marketing manager Jim Souder.
Similarly, Kaneka has focused recent R&D efforts on increasing the synergy of its existing impact modifiers with new process aids to provide better physical properties and low gloss. In the last year, the company has marketed its K-Blends line, which now consists of eight grades and includes blends of an impact modifier with one, two, or more process aids. "These are customized to particular needs and are being sold primarily to vinyl siding and window processors," says additives business manager Bruce Duerringer. Typical use levels are 4-5 phr. A three-component K-Blend typically sells for around $1.10/lb to a large-volume buyer.
Improved impact modifiers are also emerging for PVC injection molding and packaging markets. Rohm and Haas is tailoring products for custom injection molding, focusing on the challenges of improving toughness of the lower-molecular-weight PVC resins used in this application without sacrificing flowability. "One key area we have looked to address is the contribution of impact modifiers to increasing viscosity--normally a given when you add impact modifiers to a formulation. We can now say that you can get the same amount of impact modification but with lower viscosity," reports marketing manager Ken Rozkuszka. The new products, he says, have a different morphology while the chemistry remains either MBS or acrylic, depending on the balance of impact strength and weatherability needed.
In the packaging area, Rohm and Haas aims to improve toughening efficiency while preserving clarity and providing good dispersion in order to achieve low gel content. Rozkuszka says R&D is well under way on a developmental product that will address all three of these factors.
Clear packaging is also the target of new developments from Kaneka. It has a new line of clear ABS impact modifiers--J150, J210, J250--for injection molding and extruded or calendered film. Says Duerringer, "These products provide impact strength equivalent to our best rubber modifier--an opaque MBS material--but with very good clarity." Also new from Kaneka are two MBS modifiers for PVC bottles. B382 is specifically designed to be compatible with calcium/zinc heat stabilizers in water bottles. B582, which features improved chemical resistance, is designed for household chemicals containing aggressive oils like pine oil. Typical use levels of both products are 12-14 phr. They sell for around $1.25/lb tl.
Zeon Chemicals has commercialized three new NBR modifiers that were in development two years ago (PT, July '92, p. 62). According to national account manager Mike Rowland, Nipol DP5123P, DP5125P, and DP5128P give improved low-temperature and heat-aging performance plus chemical resistance to plasticized PVC. While NBR products have traditionally been aimed at flexible PVC, Zeon is now aggressively marketing them for rigid PVC as well.
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|Title Annotation:||new plastic impact modifiers|
|Author:||Sherman, Lilli Manolis|
|Date:||Jul 1, 1994|
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