Though the industry is shifting to green plastics, it will be some time till they gain respectable volumes because of several inter related factors such as availability, cost, performance and processability. As demand for biopolymers rises, it will lead to capacity expansions, which will in turn lead to lower prices. However, the trigger point for lower prices is market demand, which eventually depends on whether biopolymers will have the performance properties and processability to compete with conventional plastics. As biopolymers are produced in smaller quantities and mostly sold out, very limited availability to experiment with for the additive manufacturers poses a key challenge along with the current lack of availability as most biopolymers are still in the pilot-plant stage.
Several performance enhancing additives have been developed in recent years for biopolymers. 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.
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, a shift of interest has been seen from compostability to renewability in USA. This is partially because of lack of composting infrastructure in USA as compared with Europe, Japan and even China. However, compostable biopolymers are existent in USA on a small scale.
Processors require to incorporate additives in those biopolymer that are offered uncompounded, while other producers offer fully compounded products that may only need addition of colorant. A few companies have developed conventional and biobased additives to enhance processing of biopolymers such as PLA, PLA blends, PHA, Ecoflex, and starch resins.
Melt strength, which can hinder extrusion, blow molding, foaming, and deep-draw thermoforming, is a common limitation of PLA and other biopolymers. Recognizing this need, Arkema introduced Biostrength 700 acrylic-copolymer processing aid, which can double the melt strength and extensibility of PLA at a 4% loading while maintaining transparency. Melt strength is essential to good cell structure in foams, which are an area of keen interest for biopolymers like PLA. 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. They are combination endothermic foaming agents and melt-strength. PLA is very sensitive to acid, hence requires a moisture and acid scavenger that is both reactive and absorbent.
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%. 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. CESA-extend permits running less noisy film at higher speeds, permits doubling the bubble size, and maintain better bubble-size uniformity.
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.
Elvaloy copolymers of DuPont work well as processing aids to help various types of biopolymers feed better during injection molding. In addition, it’s 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 anti fog properties. PolyOne has a range of impact-modifier masterbatches, including those in its 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 1-3% loadings, they 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 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.
Ciba has added natural antioxidants like vitamin E to the line, as well as aroma masterbatches based on natural oils for biopolymers.
PolyOne continues to explore technologies to enhance the heat resistance of biobased resins, particularly PLA and starch blends. PolyOne offers additive masterbatches that control moisture fogging of the interior surfaces of transparent biobased packaging. For UV resistance, both Clariant and PolyOne have developed biobased UV-stabilizer masterbatches that protect the contents of transparent biopolymer packaging.
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.
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 light fastness is not as high as traditional colorants. Various biopolymers can serve as carriers for these masterbatches.
PolyOne’s 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 Apex 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 pigments produced from plants are more expensive and may be less consistent. Also, the colors are not as vibrant, leading to fewer color options.