Biodegradable polymers derived from renewable resources are attracting a significant amount of interest and publicity. As the drive to make plastics green goes full throttle, the enthusiasm is offset by unrelenting questions of availability, cost, performance and processability. Most of these are interconnected as increasing demand will lead to increased capacity, which will eventually lead to lower prices. The key factor, however, is market demand, which will eventually be influenced by the performance properties and processability of biopolymers as compared to conventional plastics.
The prominent limitations of the current generation of biopolymers, on the performance side is that they tend to be limited in impact strength and heat resistance, and on the processing side is that the enhancements in melt strength, thermal stability or lubricity overcome a tendency to stick to processing equipment. Like conventional plastics, biopolymers need additives to address inherent weaknesses in processability and physical properties. The major challenge that additive suppliers currently face is lack of availability for many biopolymers, as most are either still in the pilot-plant stage or sold out (like PLA).
Three levels of additives for biopolymers are being explored:
• Conventional additives that have no adverse effect on health or environment and do not compromise biopolymers compliance with compostability standards.
• Renewable additives derived from natural sources, but not necessarily biodegradable, for use in durable products.
• Additives that are both renewable and biodegradable, mostly 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, 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 US, as compared with Europe, Japan, and even China. By 2011, 40% of bioplastics usage will be in only partially renewable compositions. New additives that meet compostability standards and have adequate performance for other PHA film applications are required, including UV stabilizers for mulch film, clarifiers for overwrap, clear packaging of PHA, or other biopolymers that lack the inherent clarity of PLA.
Key issues for improved physical properties of biopolymers include impact modification, heat resistance and barrier performance. Secondary issues include UV, antioxidant, and anti fogging properties. Few developments are listed below:
Akin to PE film resins, Heritage Plastics is adding talc and calcium carbonate fillers, as well as vegetable-based slip and anti block agents to PHA to reduce cost and modify film stiffness and toughness.
NatureWorks sells mainly uncompounded PLA, but is well versed in the types of additives needed for optimal processing of PLA and for improving its end-use properties.
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, along with a range of processing aids that can reduce die pressures and motor loads on extruders. Rheology modifiers in the form of polymer chain extenders improve melt strength in foam and blown film. Impact modification is important due to PLA’s inherent brittleness, along with use of antioxidants and UV stabilizers. Though colorants are not in demand because of PLA’s high clarity, the company states that they should typically be low viscosity PLA grades that ensure good dispersion, and not contain heavy metals, as it would reverse the purpose of using an eco-friendly resin.
Starch-based biopolymers from Novamont, Cereplast and Plantic are sold as finished compounds already incorporated with additives. Cerestech mainly provides compounding know-how to licensees, who produce and sell the materials. Novamont's Mater-Bi starch compounds utilize Ecoflex as an additive/modifier, leaving them with an estimated 50-70% renewable content.
Additives required by starch-based biopolymers generally include flow additives and melt-strength enhancers. BASF’s Ecoflex adds melt strength and flexibility to biopolymers and allows them to be used in blown film.
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 introduced with Biostrength 700 acrylic-copolymer processing aid, which can double the melt strength and extensibility of PLA at a 4% loading while maintaining transparency.
Clariant has developed a special version of its Hydrocerol endothermic foaming agent in a PLA carrier.
To enhance melt strength for foams and other applications, Clariant’s CESA-extend chain extender, an epoxy-functional styrene/acrylic oligomer provided as a masterbatch in a variety of carrier resins, can be used. Originally developed to restore the molecular weight or IV of recycled PET and nylon, CESA-extend can re-link polymer chains that have broken due to thermal, oxidative, and hydrolytic degradation. After encouraging results with PLA, 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%. CESA-extend permits running less noisy film at higher speeds, permitting doubling of the bubble size and maintains better bubble-size uniformity. Other developments by Clariant 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.
Ampacet has developed slip and antiblock concentrates for PLA and PHA, and is working with chain extenders and chain-entanglement agents to develop melt-strength enhancers for PLA. DuPont’s Elvaloy copolymers work well as processing aids to help various types of biopolymers feed better during injection molding. 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. They also improve toughness and reduce brittleness in PLA rigid molded and thermoformed parts. At 1-3% loadings, they reportedly outperform competitive toughners with little effect on transparency.
Arkema’s Biostrength 130 acrylic modifier retains adequate transparency for translucent PLA applications, and Biostrength 150 MBS-type modifier for greater toughening in opaque applications.
PolyOne has a range of impact-modifier masterbatches, including those in its new OnCap Bio line for opaque and transparent biopolymer systems. PolyOne continues to explore technologies to enhance the heat resistance of biobased resins, particularly PLA and starch blends. It has developed polymer blends containing PLA that can raise the HDT from around 70 degreecC to around 90 degree C. However, the non-PLA blend components are neither bio-derived nor biodegradable, and the system is opaque. PolyOne is also working with PHBV that offer improved barrier and heat resistance. (Mirel PHA from Metabolix has HDT up to 140 C). 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. They 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 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.
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.