Getting Tough On PLA
Plastics can't go "green" without a lot of help from biodegradable polymers -- in particular, polylactic acid (PLA). But will nagging concerns about PLA's toughness, melt strength and heat resistance keep it out of the spotlight as a replacement for petroleum-based resins? Not if additive suppliers can help it. Find out about the latest efforts to transform PLA from processing pushover into tough guy on the block.
February 1, 2010 by Mark Stephen, Editor
It’s a cliche acknowledged in every boxing movie from Requiem for a Heavyweight to Rocky V: in order to win, you have to get strong.
This is the challenge that biopolymers, the up-and-coming contenders in the continuing matchup of competing plastics materials, now have to overcome.
The green image attached to biodegradable polymers like polylactic acid (PLA) from leading biopolymer producer NatureWorks LLC, starch-based polymers and copolyesters make them attractive not only for single-use, short-lived packaging applications, but also for durable products like automotive, electronic and construction parts — and therefore, almost irresistible to plastics processors looking for new market opportunities.
Problem is, biopolymers are plagued with inherent weaknesses in physical properties, chief among them tendencies to be limited in impact strength, melt strength and heat resistance. PLA in particular is a problem. The best known and most commercially available biopolymer at present, PLA represents some 90 per cent of the market for bottles, packaging film and other disposable items, but remains in limited use because of persistent questions about performance.
“PLA has strong appeal in food packaging such as produce clamshells and deli trays, plus the potential for market penetration in a wide range of other applications,” said John Moisson, president of biopolymer distributor Jamplast Inc. “Unfortunately, PLA struggles in durable applications as a replacement for materials like high-impact polystyrene, ABS and polypropylene.”
There are signs, though, that this might change — and soon. In the race to make biopolymers stronger faster, the vast majority of new additives and impact modifiers are targeting PLA in efforts to bulk up its processability and market appeal.
Reduced brittleness in PLA 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.
A special high-aspect-ratio precipitated calcium carbonate from Specialty Minerals, called EMforce Bio, is said to reduce PLA’s brittleness. A recent joint development program between Specialty Minerals and NatureWorks LLC found that using 20 per cent to 30 per cent EMforce Bio in PLA can improve toughness and impact resistance; during dart impact testing — a traditional method for evaluating the impact strength or toughness of a plastic film — a 30 per cent loading of EMforce Bio in PLA achieved dart impact strength of 35 ft. per lb., versus 3 ft. per lb. for unmodified PLA.
Specialty Minerals is currently evaluating EMforce Bio in other biopolymers such as PHA and thermoplastic starch.
The Sustainability Additives group of Arkema Inc. offers Biostrength core-shell impact modifiers to improve impact strength of PLA. According to Peggy Schipper, commercial development manager for Arkema’s functional additives unit, translucent Biostrength 130 additives are designed for clarity applications, while Biostrength 150 additives offer higher efficiency for opaque applications. A higher clarity impact modifier is close to introduction, Schipper continued.
Hinting at the strong demand for biodegradable products on the marketplace right now, Shipper also noted that although the current impact modifiers may not meet the compostability standards at higher use levels set by the American Society for Testing and Materials, many customers don’t seem to care. “They’re more concerned about getting the properties they need from a renewable-resourced resin,” she said.
In the longer term, she continued, as more polymer types are produced from renewable resources, polymeric impact modifier technology will likely become renewable-resource based and completely biodegradable.
Rohm and Haas’ Paraloid BPM-500 impact modifier is designed to improve the impact strength and tear resistance of PLA without sacrificing clarity. The additive uses nanoparticles that do not scatter light, the company said, achieving less than 10 per cent haze at a five per cent loading. Compostability tests are in progress, said Rohm and Haas marketing manager Rob Martin, but the company doesn’t believe that the additive will affect compostability when used at low levels.
DuPont has also joined the fight against brittleness in PLA. The company’s Biomax Strong 100 for non-food contact applications and Biomax Strong 120 for food contact applications are ethylene copolymers that improve the impact strength and toughness of both amorphous and crystalline PLA. According to DuPont, cast sheets containing Biomax Strong exhibit improved cutting and trimming, can withstand repeated flexing and exhibit increased elongation at break. Biomax Strong also acts as a processing aid by stabilizing PLA viscosity against thermal degradation and reducing torque, the company said; 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, DuPont said.
MODIFYING MELT STRENGTH
Melt strength is essential to good cell structure in a plastics part. Low melt strength, which can hinder extrusion, blow molding and foaming, is another limitation of PLA. “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,” said Kirk Jacobs, head of Clariant Masterbatches’ additive masterbatches for North America.
The plus side: certain additives, when used in combination with chemical foaming agents, can reconnect short or broken PLA acid chains and restore them to a higher level. Additive masterbatches such as these are available from Clariant under the trade name CESA-extend, a synthetic molecule originally developed to restore the molecular weight of recycled PET and nylon, and now available in a PLA carrier. “The CESA-extend additive masterbatches reconnect the polymer chains and create a branched network that improves melt strength and increases tensile strength properties,” said Jan-Erik Wegner, a researcher at Clariant’s facility in Ahrensburg, Germany. In film applications, he continued, a robust melt strength gives the potential for increased film line speed and greater productivity due to fewer line breaks; in foam, greater melt strength allows development of smaller, more resilient foam cells. “CESA-extend opens up applications which have been difficult for PLA,” said Jacobs.
BASF’s Ecoflex reportedly adds melt strength and flexibility to PLA and other starch-based resins, and allows them to be used in blown film. Used most recently as an additive/modifier in Novamont’s Mater-Bi starch compounds, the result is an estimated 50 per cent to 70 per cent renewable content, according to Novamont new business developer Stefano Falco. The main applications for Mater-Bi are flexible, non-durable items such as bags, mulch film and food packaging, he added.
Also focused on raising melt strength in PLA is Arkema. The company’s Biostrength 700 acrylic copolymer is said to improve melt strength and enhances processability at levels of one per cent to four per cent. “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,” said Peggy Schipper. “In the emerging market of foamed PLA, Biostrength 700 allows production of uniform, closed cells.” Because it is a non-reactive product, she continued, it offers consistent processing improvement, leading to less material waste.
Finally, Rohm and Haas is currently working on acrylic-based melt strength enhancers for PLA to improve fabrication performance.
RAISING HEAT RESISTANCE
Improving heat disto
rtion or heat deflection temperatures to withstand higher processing and use temperatures for PLA are other areas of continuing development. PLA’s low heat deflection temperature — between 50 to 60C — can cause PLA packages or bottles to deform during storage, Jamplast’s John Moisson said, 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 broaden PLA use by boosting temperature resistance into the 90C range,” Moisson added.
Made from 95 per cent PLA, Spartech Plastics’ Rejuven8 Plus — initially developed for graphic art and printed applications — is said to work well in most thermoforming processes. “The unique alloy material has performance enhanced physical properties over standard PLA with impact properties similar to PET, and also raises the heat resistance properties to well over 70C,” the company said.
DuPont’s Biomax Thermal 300, a heat-stabilizing modifier for thermoformed packaging, ups the ante even higher. The Thermal 300 material is reported to be dimensionally stable up to 95C. “Its introduction extends the use of PLA to applications beyond chilled-storage packaging,” the company said. “Now PLA thermoformed packages can be stored shelf stable, shipped normally and even re-heated in the microwave without deformation.”
Arkema Canada Inc. (Burlington, Ont.) www.arkema-inc.com;1-800-567-5726;
BASF Canada (Mississauga, Ont.); www.plasticsportal.com;1-866-485-2273
Clariant Masterbatches Division (Toronto); www.clariant.masterbatches.com;1-800-265-3773
E.I. DuPont Canada Company (Mississauga, Ont.);
Jamplast Inc. (St. Louis, Mo.); www.jamplast.com;636-238-2100
Novamont North America (Ridgefield, Conn.); www.materbi.com;203-438-5904
Rohm and Haas (Philadelphia, Pa.); www.rohmhaas.com;1-877-288-5881
Spartech Plastics (Clayton, Mo.) www.spartech.com;314-721-4242
Specialty Minerals Inc. (New York, N.Y.); www.specialtyminerals.com;212-878-1800
Brenntag Canada (Toronto); 416-243-9615 Univar Canada (Toronto); 416-401-3340