“Sustainability is the buzz-word throughout the packaging industry, and people are looking for new materials to meet sustainability goals,” says Suzanne Carroll, Rohm and Haas packaging marketing manager. Commercial biopolymer use is growing in packaging, where their ‘environmentally friendly’ image is being used by retailers to differentiate their product.

 

Biopolymers were originally pushed as biodegradable and compostable, which has led to confusion, because while most are degradable in industrial composting facilities, not all are degradable in home composting. The fact that some biodegradable polymers are produced from petroleum-based resins adds to the confusion. However, the industry trend is now to emphasize ‘bio’ as in biologically-produced, renewable raw materials. Although many note that biodegradability is still important, some are looking to recycling rather than composting as an end-of-life option. European Bioplastics estimates that global production capacities of bioplastics will increase from about 260 to 1500 kilotonnes from 2007 to 2011, and expects the share of bio-based and non-biodegradable resins to increase (see Figure 1).

 

“This trend brings biopolymers into more direct competition with resins such as PET, ABS, and polystyrene, and opens up more opportunities for additives to increase the functionality of biopolymers and to enter new markets,” says Thomas Weigl, managing director at Sukano. Additives are essential for improving properties as biopolymers grow not only in packaging applications but also in single-use applications such as cutlery, plates, and cups and eventually in durable goods, comments Vipul Joshi, marketing manager at Specialty Minerals.

 

Biopolymers used in packaging include polylactic acid (PLA), starch-based polymers, and others, with polyhydroxyalkanoates (PHA) now being commercialized. NatureWorks LLC, a business unit of Cargill, reports that NatureWorks^® PLA resins exhibit high gloss and clarity, similar to polystyrene, tensile strength and modulus like hydrocarbon-based thermoplastics, and a high barrier to odour and flavour. Drawbacks of unmodified PLA for packaging applications include brittleness, low melt strength, and low heat deflection temperature. Unmodified starch-based polymers have relatively good melt strength, but can be brittle. These limitations present an opportunity for additives.

In addition to some of the traditional polymer additives that are being used in biopolymers, first-generation additives developed specifically for improving processing and properties of biopolymers have been introduced, with more in development. To date, new additives have been targeted primarily for PLA, since it is the most widely used biopolymer, but suppliers are currently testing their additives in other bioresins. Suppliers of bioresin compounds, like DaniMer and Cereplast, offer compounds improved through proprietary formulations and processing. Processors using bioresins can dose some types of additives directly, or use additive concentrates. Twin-screw compounding extrusion is typically used for dispersing additives in the concentrate.

PLA and other biopolymers exhibit brittleness, which results in poor impact and tear resistance (see Figure 2). Reduced brittleness is important both for end-use and for manufacturing, because brittle breaks during thermoforming, for example, raise the potential for small, shattered pieces of sheet to contaminate the packaging. PLA brittleness can be reduced by combining with starch, plasticizers such as glycerol or sorbitol, or other degradable polyesters. For instance, BASF’s Ecovio resins are a combination of PLA with Ecoflex, a petroleum-based, biodegradable, aliphatic-aromatic copolyester. Mineral and polymeric additives are also being used to improve impact properties.

 

Specialty Minerals’ EMforce^® Bio is a high aspect ratio, carbonate-based engineered mineral designed to improve impact toughness of amorphous and semi-crystalline PLA for opaque applications. When dispersed in the polymer matrix, the mineral additive absorbs energy and slows crack propagation, thus toughening the biopolymer. Using 20-30% EMforce^® Bio dramatically improves toughness and impact resistance, the company found in a joint development programme with NatureWorks LLC. It also acts as a nucleating agent, which slightly increases crystallinity. EMforce Bio^® is fully compostable. Specialty Minerals is currently evaluating EMforce Bio^® in other biopolymers such as PHA and thermoplastic starch, says Mr. Joshi.

 

Arkema offers Biostrength^™ core-shell impact modifiers to improve impact strength of PLA with 4-7% additive. Translucent Biostrength^™ 130 is designed for clarity applications, while Biostrength™ 150 offers higher efficiency for opaque applications. A higher clarity impact modifier is close to introduction, says Peggy Schipper, commercial development manager for Arkema’s functional additives. She notes that although the current impact modifiers may not meet ASTM compostability standards at higher use levels, most customers seem more concerned about getting the properties they need from a renewable-resourced resin. In the long-term, as more polymer types are produced from renewable resources, polymeric impact modifier technology will likely become renewable-resource based and completely biodegradable, predicts Ms. Schipper. These impact modifiers typically do not have a significant effect on heat distortion characteristics.

 

Rohm and Haas’ Paraloid™ BPM-500 impact modifier is designed to improve impact strength and tear resistance of PLA while maintaining clarity. The additive uses nanoparticles that do not scatter light, achieving less than 10% haze at a 5% loading, reports the company. Compostability tests are in progress, but it is thought that the additive will not affect compostability when used at low levels.

 

DuPont™ Biomax® Strong 100 for non-food contact applications and 120 for food contact applications are ethylene copolymers that improve impact strength and toughness of both amorphous and crystalline PLA with addition of 1-5 wt.% additive. Cast sheets containing Biomax Strong exhibit improved cutting and trimming, can withstand repeated flexing and exhibit increased elongation at break, says the company. Biomax Strong also acts as a processing aid by stabilizing PLA viscosity against thermal degradation and reducing torque. The additive marginally reduces clarity, and can be dosed directly during processing. Biomax Strong is not biodegradable, but at low loading levels should not affect the compostability of articles made with it.

Sukano’s patented, FDA-approved impact-modifier concentrate, Sukano^® PLA im S550, improves the impact characteristics of PLA without reducing clarity, heat stability, or compostability. Using a bioresin carrier is essential for obtaining homogeneity and clarity when the concentrate is let down into the resin, as well as for guaranteeing compostability, says Mr. Weigl.

 

Novamont’s Mater-Bi® nanostarches, introduced at K2007, significantly increase impact and tensile strength while remaining biodegradable even in household composting environments, reports the company, which produces a range of Mater-Bi® biopolymers modified to suit different applications. Because starch-based polymers are hydrophilic and typically absorb water that acts as a plasticizer, the polymers can become more brittle in low-humidity environments. Nanostarch overcomes this problem and is effective even in low-humidity. The 100-200 nm starch particles are functionalized to be compatible with different matrices, pre-dispersed in various carriers, and supplied in pelletized form for use in various biopolymers.

 

While PLA has adequate melt strength for sheet, its relatively low melt strength limits its capability in processes that require higher melt strength, such as blown film extrusion and foaming. PLA has inherently low melt viscosity, and is also vulnerable to thermal, oxidative, and hydrolytic degradation, which causes chain scission and a further loss of molecular weight and lower viscosity, explains Kirk Jacobs, head of Clariant Masterbatches’ additive masterbatches for North America. Clariant’s Cesa^®-Extend reconnects the polymer chains and creates a branched network that improves melt strength and increases tensile strength properties. In film applications, a robust melt strength gives the potential for increased film line speed and greater productivity due to fewer line breaks, says Mr. Jacobs. In foam, greater melt strength allows development of smaller, more resilient foam cells. “Cesa-Extend opens up applications which have been difficult for PLA” says Mr. Jacobs. Cesa-Extend is a synthetic molecule supplied in a PLA carrier, and does not affect degradability at typical use levels of less than 2%, reports Clariant.

 

Arkema’s Biostrength™ 700 acrylic copolymer improves melt strength and enhances processability at levels of 1-4%. In clear, thermoformed packaging applications, the additive improves melt strength for quicker line start-up, allows increased wall uniformity which improves package strength, and maintains properties of regrind so that it can be used at higher levels. In the emerging market of foamed PLA, Biostrength 700 allows production of uniform, closed cells. Because it is a non-reactive product, it offers very consistent processing improvement, leading to lower material waste, says Ms. Schipper. Rohm and Haas is currently working on acrylic-based melt strength enhancers for PLA to improve fabrication performance.

 

 

PLA has high gloss and clarity, similar to that of polystyrene. However, PLA resins are also inherently sticky and require slip and antiblock additives to improve handling, such as easing de-nesting of thermoformed parts or preventing blocking in thin films. “One of the challenges of PLA is finding additives that do not decrease clarity. Effectiveness at low use levels is critical for maintaining clarity as well as compostability,” notes Mr. Jacobs.

Suppliers are beginning to introduce additive masterbatches with bioresin carriers to cover a range of additives that bioresins can use to enhance performance just as traditional resins do. For example, PolyOne’s OnCap^™ Bio Additives product line for bioresins includes antistat, slip, antiblock, UV protection, and blue-tone additive masterbatches. Sukano offers a portfolio of functional and optical additive masterbatches for PLA. Sukano^® PLA dc S511 highly-loaded concentrate includes slip and antiblock additives that maintain clarity, as well as an optical brightener, which is a blue-tone used to correct the yellowish tone of PLA. Variations include slightly blue for Europe, strongly blue for the US, and a natural tone for packaging products such as strawberries or for those who want the yellowish tone of PLA. Viba introduced Vibatan PLA Blue Antiox 97838 masterbatch, which increases polymer thermal stability during processing and controls yellowing caused by thermal exposure, says the company.

 

Clariant also offers masterbatches using conventional synthetic additives in a PLA carrier, including Cesa^®-bloc antiblock masterbatches and Cesa^®-light UV stabilizer masterbatches. Mr. Jacobs reports that Clariant is researching Cesa^®-natur masterbatches that would feature not only a bioresin carrier, but additives from natural sources. Among the products being evaluated are slip additive masterbatches, based on naturally occurring waxes that can produce a coefficient of friction (CoF) in PLA films similar to that achieved with synthetic waxes. Natural aromatic molecules are being studied as light-stabilizing masterbatches for biopolymers or as a UV filter to protect the contents inside of packaging made of bioplastics such as PLA. “The more we can do from natural sources, the better,” says Mr. Jacobs, who notes that while, in general, natural materials tend to have more limitations than synthetic materials, Clariant continues research and development efforts to obtain the best possible performance from natural materials.

Improving heat distortion or heat deflection temperatures to withstand higher processing and use temperatures for PLA is an area of continuing development. PLA’s low heat deflection temperature, around 50-60°C, can cause PLA packages or bottles to deform during storage, as well as cause sticky pellets in transportation and handling. PLA has grown so far in applications that don’t require high use temperatures, such as refrigerated food packaging, but researchers are looking at ways to improve temperature resistance. NatureWorks reports that cellulose fibres can increase the heat resistance of PLA, and compatibilized starch particles added to PLA can reduce cost and enhance modulus and heat resistance. Additives such as nucleating agents can give slight increases in heat resistance. Researchers are also looking further at fibre reinforcement. Industry experts note that, while additive approaches give incremental improvements, development of polymers with increased crystallinity or alloys of biopolymers with synthetic polymers are more likely to enable significant gains in heat resistance.

 

“The biopolymer industry has a continued need to provide a full range of products that meet different performance requirements. There is much space for growth of polymers and additives of different kinds,” concludes Catia Bastioli, chief executive officer of Novamont.