Enhancing Biopolymers: Additives Are Needed for Toughness, Heat Resistance & Processability
Plastics are going “green,” but they will need some help to get there. Biodegradable polymers derived from renewable resources are attracting lots of interest and publicity, but that enthusiasm is counterbalanced by persistent questions of availability, cost, performance, and processability. All these issues are inter-related: Increasing demand will lead to more capacity, which will presumably lead to lower prices. But the foundation is market demand, which ultimately depends on whether biopolymers will have the performance properties and processability to compete with existing non-renewable plastics.
Bioplastics are much like other plastics in that they need help from additives to address inherent weaknesses in processability and physical properties. Some weaknesses stand out particularly in the current generation of biopolymers: On the performance side, they tend to be limited in impact strength and heat resistance. On the processing side, they often need enhancements in melt strength and sometimes thermal stability or lubricity to overcome a tendency to stick to processing equipment.
COMPLEX MARKET PICTURE
The search is on for additives for several promising biopolymers made via bacterial fermentation of plant sugars or starches. The best known in this area is polylactic acid (PLA) from NatureWorks. There are also polyhydroxyalkanoates (PHAs), a family of polyesters that includes polyhydroxy butyrate (PHB) and polyhydroxy butyrate-valerate (PHBV), which are being developed by Metabolix, Meredian, and China’s Tianan Biologic. There are also synthetic and non-renewable—but still biodegradable—PHB and PHBV from Novomer, Ecoflex copolyester from BASF, and Kuredux polyglycolic acid (PGA) from Kureha PGA LLC.
Then there is another whole class of biomaterials: naturally derived starch-based resins such as Novamont’s Mater-bi and resins from Plantic in Australia (supplied here by DuPont as Biomax TPS). Thermoplastic starch polymers are also available in variety of compounded blends, such as with PLA (from Cereplast), with polyethylene or polypropylene (from Cereplast and Cerestech), or with BASF’s Ecoflex (from Novamont).
A big challenge for additive suppliers is the current lack of availability for many biopolymers, as most are either still in the pilot-plant stage or else essentially sold out (like PLA). Some suppliers complain of having a hard time getting “more than a few grams” of some biopolymers to experiment with.
Biopolymers also challenge additives suppliers with questions of where to focus their R&D resources. Lee Rieth, global marketing manager for Milliken Chemical, says his company is screening various commercial and developmental nucleating agents and clarifiers for potential application in biopolymers. “The challenge is justifying how much resources to dedicate to a market that is still very small.” Even if biopolymers fulfill their growth projections for the forseeable future and multiply their market share fivefold, he says, “The biopolymer market for additives would still be relatively small.”
According to Jim Gray, PolyOne’s director of biomaterials business development, “The whole picture is very complex and people have to understand and define their environmental objectives. On one hand, you have biopolymers like PLA, PHA, and starch-based resins. On the other, you have biodegradable but not bio-derived polymers like Ecoflex, which are being blended with biopolymers to enhance processing and properties. And then there are renewable versions of conventional polymers, such as biobased PE and PVC based on ethanol from sugar cane.” (These last are being developed in Brazil—see Learn More.)
Additives for biopolymers are being investigated on three levels. The first involves traditional additives that have no adverse effect on health or the environment and do not compromise resins’ compliance with compostability standards. Second are “renewable” additives derived from natural sources, but not necessarily biodegradable, for use in durable products. Third are additives that are both renewable and biodegradable, which are a good fit for single-use or short-lived products.
SHIFT TO RENEWABLES
When additives are added to biopolymers and other plastics intended for composting, they must meet standards for compostable plastics such as ASTM D6400 and its European Union counterpart, EN 13432. However, industry sources see a shift of interest in the marketplace from compostability to renewability, owing in part to the lack of composting infrastructure in the U.S., as compared with Europe, Japan, and even China. “The point is to achieve more sustainability and minimal environmental impact versus petrochemical-based polymers,” says Jim Lunt, formerly of NatureWorks and now a technical and business development consultant who represents Tianan Biologic Material in North America. He foresees increasing use of biopolymers in blends to decrease the environmental “footprint” of conventional polymers. Lunt projects that by 2011, 40% of bioplastics usage will be in only partially renewable compositions.
Adds Charles Hamilton, president of Novomer, “Our main focus in to reduce fossil-fuel content and to offer higher performance alternative polymers that can go head to head with traditional plastics.” Its synthetic PHV and PHBV resins are based partly on carbon monoxide and carbon dioxide. The company is exploring ways to enhance these resins through additives and copolymerization.
Though relatively small-scale, composting is not to be ruled out as a market in the U.S. “It will take a while before we have the infrastructure to fully utilize end-of-life biodegradability of these polymers,” says PolyOne’s Gray. “But there are niche opportunities for commercial composting of single-use products made of compostable materials such as plates, cups, cutlery, packaging, and the bags in which they are collected.”
Supporting this view is Frank Ruiz, research director of Heritage Plastics, Inc., a compounder and sister company of film extruder Heritage Bag Co. in Carrollton, Texas. According to Ruiz, about 40% of the inquiries his firm receives are for their BioTuf compostable trash liners, especially from supermarkets, restaurants, hotels, hospitals, schools, and other large institutions that can send food scraps to an industrial composting facility at one-third the cost of landfilling them.
Two months ago, Metabolix announced that it will supply Heritage with Mirel PHA resin for a new family of BioTuf compostable blown and cast film products. (Up to now, BioTuf was made of PLA and BASF’s Ecoflex.) Heritage Bag will use Mirel in compostable trash bags, can liners, and kitchen compost bags. But due to current supply limitations, Heritage is initially blending only a small amount of Mirel with PLA or Ecoflex.
As it has done with PE film resins, Heritage Plastics is adding talc and calcium carbonate fillers to PHA to reduce cost and modify the films’ stiffness and toughness. Heritage also uses vegetable-based slip and antiblock agents. Ruiz sees a need for new additives that meet compostability standards and have adequate performance for other PHA film applications. Examples include uv stabilizers for mulch film and clarifiers for overwrap or other clear packaging of PHA or other biopolymers that lack the inherent clarity of PLA.
DEFINING ADDITIVE NEEDS
Some biopolymer producers are offering their materials uncompounded, requiring customers to incorporate additives, while others offer fully compounded products that may only need addition of colorant. Some of the latter suppliers were reluctant to discuss the additives they use in their compounds or denied that they are particularly necessary. According to Blake Lindsey, president of Meredian, which acquired Procter & Gamble’s Nodax PHA technology last Fall, “The technology allows us to build in appropriate melt strength, heat resistance, impact strength, barrier properties, and clarity without additives. Processors will primarily have to add only colorant.”
On the other side is NatureWorks, which sells mainly unmodified PLA. The company is well versed in the types of additives needed for optimal processing of PLA and for improving its end-use properties. For example, in two-stage stretch-blow molding, special carbon blacks have been developed to increase the speed and uniformity of reheating in the oven prior to blowing. Denesting additives are useful in PLA rigid sheet extrusion to improve the destacking of nested parts after thermoforming. Antistats are also used in PLA films, as are a variety of processing aids that can reduce die pressures and motor loads on extruders. Also important are rheology modifiers in the form of polymer chain extenders that improve melt strength in foam and blown film.
NatureWorks sources emphasize the importance of impact modification due to PLA’s inherent brittleness. NatureWorks says its customers have used impact modifiers from Arkema, Rohm and Haas, DuPont, and Chemtura. They also say customers use antioxidants (such as from Ciba or ColorMatrix) and UV stabilizers. Because of PLA’s high clarity, colorants have not been in high demand so far. The only limitation in pigment choice that the company notes is that they should not contain heavy metals, which would defeat the purpose of using an eco-friendly resin. Carriers for additives and colorants are typically low-viscosity PLA grades that ensure good dispersion.
Other biopolymer suppliers that aim to provide virgin versions of their polymers include Tianan Biologic, Metabolix, and Meredian. Lunt says Tianan Biologic has been acquiring a good grasp of the types of additives needed—notably melt-strength enhancers and heat stabilizers to increase the processing window, nucleating agents, plasticizers, antioxidants, impact modifiers, and fillers. Additives for PHBV can come from existing chemistries. Says Lunt, “We are not bound by renewably sourced additives, as long as they have no adverse effect on health or the environment.”
Both Metabolix and Meredian say they will only use biobased additives with their resins. “In this way, we are somewhat limited currently,” says Metabolix director of technology Sally Kline. She is looking for processing aids and lubricants to enhance flow properties for injection molding and melt strength for improved draw in films and thermoforming. PHA could also use impact modifiers, slip/antiblock agents, plasticizers, and colorants. Kline cites a need for nucleating and clarifying agents. “While Mirel PHA is crystalline, it is very important to control both the speed of crystallization and the crystal size. If you change the crystallinity, a clarifying agent will ensure that you won’t cloud the transparent PHA.”
Also focused on biobased additives is DaniMer Scientific, a sister company of Meredian, which supplies modified-PLA extrusion and molding resins, as well as a line of PLA-based impact modifiers, processing aids, and color concentrates. It uses renewable materials to “functionalize” PLA, says Meredian’s Lindsey, employing plant-based bio-polyester technology from another sister company, Seluma Technologies.
Starch-based biopolymers from Novamont, Cereplast, Cerestech, and Plantic are sold as finished compounds with additives already incorporated. Cerestech mainly provides compounding know-how to licensees, who produce and sell the materials. Its first licensee is Innovative Compounding.
According to Lunt, additives required by starch-based biopolymers generally include flow additives and melt-strength enhancers. Among the latter is BASF’s Ecoflex, which reportedly adds melt strength and flexibility to these resins and allows them to be used in blown film. Novamont’s Mater-Bi starch compounds utilize Ecoflex as an additive/modifier, leaving them with an estimated 50% to 70% renewable content. Novamont’s new business development manager Stefano Falco says the main applications for Mater-bi are flexible, non-durable items such as bags, mulch film, and food packaging. The only additives that processors need for such uses are color concentrates or processing aids.
Shanna Moore, global business director for sustainable packaging at DuPont Packaging & Industrial Products, says Biomax TPS injection grades (60% to 90% renewable content), are primarily for short-life applications in lawn and garden products and outdoor sporting goods. Moore says the only additives currently used are colorants with DuPont’s Elvaloy EMA as a carrier.
A few companies have developed conventional and biobased additives to enhance processing of biopolymers such as PLA, PLA blends, PHA, Ecoflex, and starch resins. PolyOne, for example, offers its OnCap Bio family of slip, antiblock, and mold-release agents. Says Gray, “Many of these emerging biopolymers are tacky and tend to stick to themselves and to metal surfaces during processing. So we offer a range of additives to control or eliminate sticking for improved processing and handling efficiencies.” PolyOne also offers antistat concentrates to ease product handling and add shelf appeal.
Low melt strength, which can hinder extrusion, blow molding, foaming, and deep-draw thermoforming, is another common limitation of PLA and other biopolymers. Recognizing this need, Arkema recently came out with Biostrength 700 acrylic-copolymer processing aid, which reportedly can double the melt strength and extensibility of PLA at a 4% loading while maintaining transparency. Commercial development manager Peggy Schipper says the improved melt strength and melt elasticity provide a wider processing window for extrusion. Biostrength 700 reportedly shows the same effect on PLA with 75% regrind, and on biopolymer blends.
Rohm and Haas marketing manager Rob Martin says his company will soon launch an acrylic melt-strength enhancer for PLA. It may hold promise for other biopolymers. PolyOne also is evaluating a number of rheology modifiers to improve melt strength.
Melt strength is essential to good cell structure in foams, which are an area of keen interest for biopolymers like PLA (see Learn More). Says Mike Reedy, president of blowing agent supplier Reedy International, “There have been challenges in foaming PLA as well as other biopolymers, as most are crystalline, and the foaming agents release water.” Water tends to degrade biopolymers in the melt phase and cause further loss of melt strength and physical properties. Reedy has just introduced two new foaming agents designed for PLA and PET. Safoam RPC-20MS1 and 20MS2 are combination endothermic foaming agents and melt-strength enhancers. “We have shown them to work in low-density PLA applications such as meat trays, and we also see potential for medium-low-density foamed PLA deli, margarine, or ice-cream containers,” says Reedy.
Kirk Jacobs, North American business director for Clariant Additive Masterbatches, says the company has developed a special version of its Hydrocerol endothermic foaming agent in a PLA carrier. “Since PLA is very sensitive to acid, we have incorporated a moisture and acid scavenger that is both reactive and absorbent. We have used similar chemistry for polycarbonate foams,” Jacobs notes.
One way to enhance melt strength for foams and other uses is with Clariant’s CESA-extend chain extender, an epoxy-functional styrene/acrylic oligomer provided as a masterbatch in a variety of carrier resins. Originally developed to restore the molecular weight or I.V. of recycled PET and nylon, CESA-extend can re-link polymer chains that have broken due to thermal, oxidative, and hydrolytic degradation. Recently it has shown great promise in PLA, and similar results are expected for PHA.
When 2% of CESA-extend was added to NatureWorks’ PLA 4042D, the average molecular weight was raised by 49%, indicating branching extension of the polymer chains and higher molecular weights. After modification, PLA’s elastic modulus decreased by about 20% while its elongation was raised by 50%. Clariant says these effects make it possible to use direct gas injection to produce a nucleated foam structure with small cells, smooth surface, and up to 15% weight reduction.
CESA-extend appears to cause a change in PLA’s rheology from its typical Newtonian behavior to some degree of shear-thinning, non-Newtonian behavior after chain extension. This effect and higher melt strength assist in blown film extrusion, according to Hendrik Kammler, global head of Clariant Masterbatches’ additives market segment. CESA-extend permits running less “noisy” film at higher speeds, permits doubling the bubble size, and maintains better bubble-size uniformity.
Other Clariant developments for biopolymers include the new CESA-natur family of additive concentrates for improved processability, such as anti-stats and processing aids. A new slip-additive masterbatch produces a coefficient of friction in PLA that is close to the values achieved with modern synthetic waxes, according to Kammler.
Ampacet has developed slip and antiblock concentrates for PLA and PHA. It is working with chain extenders and chain-entanglement agents to develop melt-strength enhancers for PLA, says business manager Brian McKinley.
DuPont reports that its Elvaloy copolymers work well as processing aids to help various types of biopolymers feed better during injection molding. In addition, its Biomax Strong 100 and 120 ethylene copolymers, developed to toughen PLA, also act as processing aids that significantly reduce screw torque and improve melt stability.
Key issues for improved physical properties of biopolymers include impact modification, heat resistance, and barrier performance. Secondary issues include UV, antioxidant, and antifog properties
PolyOne has a range of impact-modifier masterbatches, including those in its new OnCap Bio line for opaque and transparent biopolymer systems.
DuPont’s Biomax 100 and 120 improve toughness and reduce brittleness in PLA rigid molded and thermoformed parts. At 1% to 3% loadings, they reportedly outperform competitive tougheners with little effect on transparency.
Paraloid PMA 500 acrylic impact modifier from Rohm & Haas is used to boost impact in PLA and is deemed likely to work well in other biopolymers.
Arkema offers new Biostrength 130 acrylic modifier, which retains adequate transparency for translucent PLA applications, and Biostrength 150 MBS-type modifier for greater toughening in opaque applications. Arkema’s Schipper says a new clear acrylic-based impact modifier for PLA and possibly other biopolymers will be launched soon. Also on the radar is possible development of biobased impact modifiers.
Clariant is aiming to expand its CESA-natur family with “natural” impact modifiers and metal deactivators for PLA. The company already has added natural antioxidants like vitamin E to the line, as well as aroma masterbatches based on natural oils.
PolyOne continues to explore technologies to enhance the heat resistance of biobased resins, particularly PLA and starch blends. Says Gray, “While we have not identified any unique additive solutions, we have developed polymer blends containing PLA that can raise the HDT from around 135 F to around 181 F. However, the non-PLA blend components are neither bio-derived nor biodegradable, and the system is opaque.” PolyOne is also working with PHBV resins that offer improved barrier and heat resistance. (Mirel PHA from Metabolix, for example, has HDT up to 276 F.)
DuPont plans to introduce soon Biomax Thermal 120, a proprietary heat-distortion modifier that will allow PLA thermoformed parts to withstand hot transport and storage.
For UV resistance, both Clariant and PolyOne have developed biobased UV-stabilizer masterbatches that protect the contents of transparent biopolymer packaging.
PolyOne offers additive masterbatches that control moisture fogging of the interior surfaces of transparent biobased packaging.
In addition to traditional pigments that can be used in biopolymers, bio-derived colorants are now available from at least four companies. Clariant’s Renol-natur color concentrates are derived mainly from plants and include red, orange, yellow, and green, with blue in the final stages of development. These colors are very earthy and organic-looking, and some have excellent clarity, though their lightfastness is not as high as traditional colorants. Various biopolymers can serve as carriers for these masterbatches.
PolyOne’s OnColor Bio color concentrates and liquid colorants are based in part on sustainable raw materials. The concentrates use biopolymer carriers such as PLA, PHA, modified starch compounds, and biodegradable polyesters. Opaque colors are available for all these biopolymers, but transparent colors are also available for PLA.
Teknor Color recently launched color concentrates for PLA resins and blends aimed at packaging, bags, liners, and other extruded or molded products. Three series of colorants are offered for clear or opaque bottles, film, sheet, profiles, and injection molded items. The carrier resins are either PLA or compatible polyesters (including PET). Organic and inorganic pigments are used, depending on the end-use requirements. “The ‘all-natural’ pigments produced from plants are more expensive and may be less consistent. Also, the colors are not as vibrant, leading to fewer color options,” notes director of marketing John Politis.
Ampacet has a line of custom color masterbatches for a wide range of biopolymers. They are formulated from sustainable sources, including primarily organic-based pigments.