Additives for recycling: maintaining value the second time around.
In the recycling trade, where brutal economic pressures never let up, it's easy to think of additives as an unnecessary compounding cost. So, even those ingredients considered vital in virgin resin formulations can seem like expensive frills on the second pass through the pelletizing line. Yet today's recycling markets demand reprocessed resins that meet the same quality standards as their virgin counterparts. Additives may be the best way to keep old resins acting and looking like new.
Some compounders already rely on stabilizers, lubricants, impact modifiers and other processing and performance additives to raise recycle to a new level of value--sometimes even above that of the original material. At the same time, additives can help offset the recycle stream's lack of homogeneity, bringing individual lots up to a consistent quality standard.
MRC Polymers, for instance, employs both these strategies. The company reprocesses engineering resins from industrial scrap and post-consumer sources, producing compounds that take on virgin in the most demanding automotive electronics applications, "Additives are absolutely essential to our business," says George Staniulis, product development manager. "We couldn't live without them." Straight recycled polymer, he explains, simply would not possess the physical properties or consistency required by these premium end-uses. Likewise, at Quantum Chemical's Resource Recovery Facility in Heath, Ohio, additives help preserve quality in HDPE recycling. "Using antioxidants is one way we differentiate our resins from our competitors," says acting manager Bruce Perlson.
And as resins start to go through more than once, additives will take on greater importance. "It's likely that additives will make more of a difference for more recyclers as recycled resins become recycled themselves," predicts Gerry Fisbbeck, v.p. at recycling giant Wellman Inc., Johnsonville, S.C.
Next to the mechanics of recycling technology--sorting, cleaning, grinding, and repelletizing--additives for reprocessing are still a relatively unexplored frontier. "Much of the attention so far has been on solving all these mechanical engineering problems," says Peter Solera, senior scientist at Ciba Additives. Adds Wellman's Fishbeck, "First, make sure the recycle is contaminant-free, and you're most of the way there. Then, additives can help you go further."
Over the past year, more and more additive suppliers have recognized the unique chemical requirements of reprocessed materials. The various antioxidants, light stabilizers, impact modifiers and other chemicals useful in recycling may be basically the same as those used with virgin, but blends specifically proportioned for recycling applications have emerged too. And from recent applications, more information has surfaced about the true benefits of additives for reclaim.
When polymer degradation poses the key threat to physical property maintenance, restabilization is the prime solution. "By restabilizing, you can protect properties and go back into high-value applications," says Joseph Puglisi, director of polymer products at Ciba Additives. Depending on the cause of polymer degradation and intended end use, many recycled thermoplastics can benefit from antioxidants, light stabilizers or both.
Prevailing antioxidant strategy for recycling many resins calls for hindered phenolic and phosphite blends--or less often, just one component. Because virgin and regrind suffer from the same degradation mechanisms, they both benefit from the same stabilization products, notes GE Specialty Chemicals lead chemist Suzanne Dietz.
The required loadings can shift upwards in recycling, though. For example, an antioxidant typically used at 220 ppm in a virgin polyolefin will often be added at 1000-5000 ppm to stabilize recycle, Dietz says. Optimum loadings and ratios of dual components vary widely because of such factors as the feedstock's residual additive levels and purity, Dietz points out. And individual resins have inherently different stabilization requirements, too.
For example, HDPE can suffer from either crosslinking or polymer chain scission during reprocessing, both altering the molecular-weight distribution. According to Dietz, these sorts of degradation manifest themselves as a shift in melt-flow index, potentially causing processing problems for subsequent users and a drop in physical properties.
As a way to stave off HDPE meltflow changes, whatever their direction, antioxidant blends can help. Multiple extrusion passes in Ge's labs show that HDPE's melt flow remains far more consistent with stabilization. To maintain melt-flow consistency, a mixture of Ultranox 626 phosphite and Ultranox 276 hindered phenolic did the trick in 100% post-consumer HDPE (see Fig. 1). Dietz also notes that the phosphite inhibited color development, yielding a yellowness index of only -0.46 versus +9.04 for the unstabilized HDPE reclaim.
Work by Ciba's newly formed Additives for Plastics Recycling Group in Switzerland shows similar improvements in recycled HDPE melt-flow consistency through the action of the company's Irganox phenolics and Irgafos phosphites. And in polypropylene, where degradation generates polymer chain scission, the group has demonstrated that phosphite/phenolic blends can buoy the MFI through multiple extrusions. In one experiment stabilized PP samples kept the same MFI after five extruder passes as an unstabilized reference sample retained after only two passes (see Fig. 2). At the same time, stabilization provided thermal stability far beyond that of the base material (Fig. 3).
And for both LDPE and HDPE, Quantum Chemical's USI Div. has included BHT antioxidants in its Spectratech line of additive concentrates formulated specifically for recycling.
PET can gain from increased stabilization, though its individual requirements differ from polyolefins. Dietz says PET stabilization should rely on higher phosphite levels to prevent molecular-weight, or I.V., loss and maintain acceptable color (Fig. 4). Also, PET recycling may be especially application-dependent. Wellman's Fishbeck notes that PET reclaimed for fibers needs only the "traditional kind" of additives--such as optical brighteners and titanium dioxide pigment for delustering. But for nascent value-added PET recycle applications like thermoforming sheet, the additives requirements grow. "There's no question that more additives are needed for thermoforming," Fishbeck says. "Different grades of performance are required and these correspond to differing additives requirements."
Stabilization's benefits hold true for engineering resins as well; and here, rheological consistency takes on a special importance. "We have seen drops in viscosity correspond to drops in mechanical properties. Antioxidants inhibit this molecular-weight degradation," says MRC's Staniulis, citing experience with polycarbonate, TP polyesters, nylon and nylon alloys. Moreover, for most of MRC's customers, consistent rheology is a non-negotiable prerequisite doing business. "Our resins can't vary more than 10% on viscosity versus shear rate from lot to lot," Staniulis notes. Some customers even demand rheology control to within three-sigma quality levels--a goal MRC says it meets.
Ciba's research work likewise demonstrates the potential of additives for saving physical properties of engineering resins (Fig. 5).
By contrast, PVC reprocessing doesn't appear to create special stabilization or additives needs, according to suppliers. "Most compounds contain enough additives for two or three cycles," says Peggy Holsopple, technical service leader at Elf Atochem. If they don't, all the recycled compound needs is an infusion of the same sorts of additives used in the first run, according to Don Brilliant, technical director at Witco Polymer Additives.
LET THERE BE LIGHT
Heat--whether from processing or end use--is not the only enemy of recycled plastics. Applications, such as outdoor furniture, that expose resins to sunlight or other sources of uv radiation can also threaten a resin's longevity. A typical uv stabilization system today would combine a hindered-amine light stabilizer (HALS) with a uv absorber to safeguard color, according to Ciba's Puglisi.
One Ciba weatherability study found that addition of 1000 ppm (0.1%) of the company's Tinuvin 770 HALS enabled samples of HDPE from five-year-old milk crates to achieve the same weathering resistance as the original HDPE. Without restabilization, crack formation in 100% recycled HDPE took place at less than 2000 hr; the stabilized sample withstood more than 8000 hr crack-free. Tensile impact strength of the unstabilized recycle also fell far short of the mark set by the restabilized HDPE.
Responding to the need to combat different types of degradation--thermal and uv--simultaneously, some new additive systems combine antioxidants and fight stabilizers in ratios optimized for recycling. Ciba recently introduced two such blends under the Recyclostab trade name. Already sold in Europe, these products should become available domestically in the third quarter, according to Puglisi. Both consist of hindered phenolics and phosphites, and one contains a light stabilizer. The Recyclostab 400 Series fits in applications needing long-term heat protection, especially for plastics from a mixed waste stream. And the Recyclostab 800 Series goes into applications demanding light stability as well.
Based on 15 years' experience in recycling, MA Polymers also sells additive packages optimized for reprocessing applications. The company can formulate its multicomponent antioxidant and light stabilization packages for injection molding, blow molding, sheet extrusion and thermoforming applications in a variety of resins.
HAVING AN IMPACT
Beyond using additives to restore or maintain physical properties at virgin levels, some compounders use a value-added strategy of upgrading recycled resins so they attain better physicals than the original. "We're not using any additives other than those with the ability to improve properties," says Michel Bitritto, manager of resource recovery and recycling at Hoechst Celanese. She points to glass-fiber reinforcement and impact modifiers as the two most obvious choices to achieve these goals.
One supplier, AmeriHaas, reports that its Paraloid EXL impact modifiers have gained a new recycling application--turning polycarbonate compact-disc scrap into a resin suitable for making the "jewel boxes," or disc holders. Formerly made from a black HIPS tray and clear ABS cover, these PC compact disc holders now have displaced the earlier resins at one customer's plant. At 5-7% loading levels, the impact modifier more than doubled the strength of the PC while allowing it to accommodate the aluminum metalized layer from scrapped compact discs.
MRC Polymers, which also uses compact discs as part of its feedstock, makes extensive use of impact modifiers, according to Staniulis. In two automotive applications--a wheel cover and an interior console component--improved impact strength made the difference between MRC keeping and losing the business.
GE Specialty Chemicals recommends its Blendex ABS modifier resins to improve the impact strength and color hold in recycled ABS, SAN, PVC, and blends of these resins.
Produced from a Nylon/PPE alloy, the MRC wheel cover points to yet another route to upgrading recycled resins--combining dissimilar materials to get the necessary properties. MRC also sells a recycle-based PC/PET blend called Stanuloy. With blends, the key to success or failure may lie in use of compatibilizing additives. "The whole field of compatibilization technology is an area of fertile ground for recycling," notes Staniulis.
Although the technology remains highly proprietary, a number of different chemicals and polymers have shown effectiveness as compatibilizers (see PT, Feb. '89, p. 67). One of the newer products is AmeriHaas' Paraloid EXL-4100 Series of acrylic-imide copolymers for compatibilizing nylon/ABS and PC/nylon systems. The company's Paraloid PM-800 is an acrylicpolyolefin graft copolymer that can be applied to polyethylene-based blends. "We've seen success with PM-800 in PC/PE blends," says national sales representative Greg Torchiana.
Apart from engineered blends, compatibilizers may also help elevate the normally low physical properties of a commingled waste stream. A paper at this year's SPE ANTEC in New Orleans by A.A. Adewole and M.D. Wolkowicz of Himont USA showed beneficial effects of Shell's Kraton FG1901X maleated MEBS copolymer on physical properties of a blend of PP industrial scrap with post-consumer HDPE. By varying Kraton levels between 1% and 7%, the authors achieved different balances of impact and elongational properties. BASF Corp . has come u with three developmental compatibilizers for mixed-plastic recycle. These RCM polymeric blends products are designed to enhance properties of PS/PE, PS/PP, and ABS/PP blends (see PT, Feb. '93, p. 14).
In trying to make blends or commingled material easier to compound, processing aids can play a role. Struktol Co. reports that a recycling customer is using its TR 044 fatty-ester process aid to produce improved nylon/PET dispersions from regrind by means of viscosity adjustments. The company also offers processing aids, such as TR 016, composed of a fatty acid metal salt and an amide. Because these have hydrogen sites available for bonding, explains product manager Don Hall, they can function both as flow aids and as coupling agents when blending recycled resins.
And Quantum notes that its Spectratech PM 11607 concentrate, containing a fluoropolymer processing aid, can help increase output rates and prevent melt fracture in recycled polyolefins.
Even recyclers using no other additives say they often use colorants to overcome appearance handicaps. Color comes in handy for masking the cosmetic effects of contamination, slight degradation, or the mottled appearance of commingled waste. "Even with stabilization, there's still a color shift, so you use color correctors," says MRC's Staniulis, citing blue tints to offset yellowness as one solution.
Quantum, for one, offers color concentrates specifically geared to recycling as part of the Spectratech line. According to market manager Ken Auer, an array of dark browns an blacks can uniformly color commingled resins at about 2.5% loadings. The company also offers recyclers a number of custom colors to boost appearance quality when black or brown won't do.
Foaming resins from commingled streams may be yet another way to improve both appearance and physical properties. Apart from potentially advantageous density reduction, foaming can eliminate warpage, improve impact strength, and impart more attractive surface, explains Auer. This way, lower-end applications like plastic lumber can gain esthetic and functional value that they would otherwise lack. Auer adds that several Quantum recycling customers use concentrates of azo foaming agents.
Additives not only solve problems for recyclers, they may create some as well. Cost is the biggest. One recycler, for example, has seen times when the carbon black for masking mixed-color mottling has cost much as the recycled resin feedstock itself.
Non-uniformity of the recycle stream presents another key hurdle, despite the leveling effects of additives. "A dilemma we see is the additive control issue," explains Wellman's Fishbeck. "Recycling consistency is variable to begin with, so if s hard to anticipate what additive levels are needed." And Hoechst's Bitritto cautions that additive-induced compatibility problems can plague mixtures of diversely stabilized and modified reclaim--even in a singleresin feedstream. In such cases, she says Hoechst's best additive solution so far has been "adding more virgin."
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|Article Type:||Cover Story|
|Date:||Jul 1, 1993|
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