Engineering Resins | 8 MINUTE READ

Renewable PLA Polymer Gets 'Green Light' For Packaging Uses



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 Polylactic acid, first synthesized a half-century ago, has finally arrived as an alternative to PET, HIPS, PVC, and cellulosics in some high-clarity packaging roles. PLA is being used in candy wrap, optically enhanced films, and shrink labels. It's also showing up as the sealant layer in form-fill-seal coextrusions. The novel resin is forging roles in thermoformed cups and containers and is about to appear in single-serve drink bottles.

Early this year, Cargill Dow LLC started up the world's first full-scale PLA plant in Blair, Neb., capable of 300 million lb/yr. For the past two years, Cargill Dow has been seeding PLA markets from a 14-million-lb semi-works plant, also in Blair. The only other PLA suppliers are Mitsui Chemical and Shimadzu Chemical, both in Tokyo, Japan. Each supplies resin from small (under 1 million lb/yr) demonstration units and are focused exclusively on the Japanese market. Through a technology-sharing and marketing arrangement with Mitsui, Cargill Dow will supply Mitsui with resin to be sold in Japan under Mitsui's Lacea brand.

Until now, PLA had grabbed attention mostly because of its unusual nature: It is synthesized from processed corn, a renewable plant feedstock, and it biodegrades after use, if composted. Making PLA requires 30% to 50% less fossil fuel than polymers synthesized from hydrocarbons, and it thus reduces carbon dioxide emissions, Cargill Dow notes. These "green" benefits could provide users with a marketing edge. The payoff can be even more direct, as in helping users avoid a "green tax" on packaging in Germany or meet strict disposal regulations in Japan.

Today, Cargill Dow is putting more emphasis on PLA's cost-performance. "PLA competes by providing end-use benefits," says commercial director Jim Hobbs. PLA is an aliphatic polyester featuring high clarity and gloss, high stiffness (thus downgauging potential), and easy processability in most equipment.

Nonetheless, PLA faces hurdles, including its high density (1.25 g/cc) relative to PP and PS. It also has high polarity, making it difficult to adhere without tie-layers to non-polar PE and PP in multi-layer structures. Add to that poorer heat resistance than PET and limited barrier against moisture and gases. But the greatest stumbling block is PLA's cost, currently an average of $1.30/lb. However, new economies of scale and ongoing process improvements lead to Cargill Dow to anticipate PLA prices under a $1/lb in a year or so. Within two or three years, PLA may be cost-competitive with PET on a finished-product basis.

In the meantime, PLA delivers a balance of properties that has generated enthusiastic interest from some end-users. Hobbs cites PLA's excellent sealing performance, its paper-like twist and deadfold retention, and its good taste and aroma barrier. Cargill Dow's immediate strategy is to pursue high-value niches where the benefits of PLA performance and environmental advantages intersect.

A good example of PLA's early successes is a biaxially oriented PLA film made by Mitsubishi Plastics in Japan. This BOPLA film is laminated to paperboard in a golf-ball package for Dunlop Japan. The reverse-printed PLA film becomes the clear viewing window when a panel is die-cut out of the paperboard. BOPLA film laminates easily, and justifies Dunlop's "recyclable-compostable" claims.


PLA performs in films

Cargill Dow's continuous-process plant began making on-spec resin by mid-January, and a steady resin flow is expected in the pipeline within months. Meanwhile, Cargill Dow is diversifying and refining its slate of PLA resin grades. "Freedom to modify properties is broad," says application specialist Nicole Whiteman, who cites variables like molecular weight, melt flow, and crystallinity. Cargill Dow's NatureWorks PLA grades now include film, sheet, injection molding, and stretch-blow molding families.

In general, PLA is best suited to replacing clear materials that are at the high end of both the density and price range. Top candidates are PET (1.4 g/cc, 70¢/lb historical average price), flexible PVC (1.3 g/cc, 50¢/lb), and cellulosic (e.g., Cellophane, glassine) films.

Versus PP (0.9 g/cc, 35¢/lb) and HIPS (1.05 g/cc, 50¢/lb), PLA is clearly disadvantaged. Observers say these are unlikely targets at this time, save when the environmental push is exceptionally powerful.

Bob Springs, Cargill Dow's managing director for Europe, characterizes PLA as "in the same league as PET, PVC, OPS, and Cellophane" in terms of clarity (low haze) and gloss. Film-grade PLAs have 2.1% haze, which matches other top contenders for high-clarity packaging (see Table 1).

PLA's high stiffness is reflected in tensile modulus values of 480,000 psi for film and 500,000 psi for sheet extrusion grades. Springs estimates that such stiffness could allow about 25% downgauging versus Cellophane film and 15-20% versus HIPS sheet. He says PLA sheet runs at line speeds equivalent to those for HIPS, or 20% faster than PET sheet. PET and PLA offer equal downgauging.

Interest in PLA film is high among makers of candy twist wrap, multi-layer FFS structures (e.g., cereal and bakery wraps), and shrink films. Applications development has been under way at Hoechst Trespaphan GmbH in Neunkirchen, Germany; Teich Flexibles in Spondon, Derby, U.K.; and others.

Cargill Dow's film resins include NatureWorks 4031D and 4041D, products with similar properties but optimized for equipment currently used to orient PP and PET, respectively. NatureWorks 4060 is tailored for use as the sealant in multi-layer structures. A controlled-shrink grade, NatureWorks 4050, is aimed at pressure-sensitive bottle labels, sleeves, and other shrink films, where BOPLA offers potential for downgauging, improved printability, and superior optics versus OPS, PET, and PVC.

A plus for BOPLA films is their twist and deadfold retention. BOPLA films are equivalent to Cellophane and at least 20% better in this respect than competing thermoplastics (see graph). This explains BOPLA's successes in candy wrap, where it can also be downgauged versus Cellophane and PVC.

BOPLA also performs well as a barrier to flavor, aroma, and solvent molecules and grease penetration. Tests with d-limonene show flavor barrier of PLA equal to that of PET or nylon 6, the benchmarks for this property.

PLA is a polar material, and the natural surface energy of BOPLA films is 38 dynes/cm2. They accept a broad range of printing inks with or without corona or flame treatment.

"PLA is a high-performance sealant," states Cargill Dow's Whiteman, citing a Tg of 55 to 65 C, which means seal initiation occurs at around 80 C (176 F), equivalent to an 18% EVA sealant. PLA also seals well at high line speeds and offers high hot-tack strength. "The combination of low-temperature heat-sealability and flavor and aroma barrier provides opportunities for PLA," says Whiteman. She cites barrier food pouches, where PLA in the inner layer would minimize scalping while achieving an excellent seal.

All-PLA form-fill-seal coextrusions would potentially realize cost savings over current FFS structures, which are typically four to six layers of nylon or PET with polyolefin skins and non-polar tie layers. If high oxygen barrier is not required, a PLA/PLA coextrusion—with one layer optimized for sealability—would simplify the structure and reduce flavor scalping.

PLA's behavior in orienting equipment is said to be similar to PET's. A typical area stretch ratio is 9:1 to 16:1. Cargill Dow claims that BOPLA films can generally be run on PET orientation equipment without any hardware modification. PLA has also been run with some success on equipment suitable for BOPP (which typically has higher stretch ratios), but modification of that equipment is required. However, DMT Americas, an orientation equipment manufacturer, says its latest systems are flexible enough to run either BOPP or BOPLA.


Clear rigid packaging

Cargill Dow is also promoting PLA use in thermoformed and blow molded rigid containers. For sheet, the company offers a more amorphous grade (NatureWorks 2000D) for clear clamshells and cups. A more crystalline offering (2100D) is aimed at food-service ware and other opaque uses.

"We have 14 rigid packaging programs in Europe ready to go," says Springs. In very environmentally sensitive nations such as Austria and Sweden, fast-food retailers have launched PLA versions of salad containers, drink cups, and yogurt tubs.

Iper, a food retailer in Milan, Italy, has adopted PLA in place of HIPS for a line of short-shelf-life hinged containers for fresh salad, fruit, and pasta. One motive is to market a "natural" product in a "natural" package. PLA is also breathable, which improves freshness and reduces fogging. Equally important is PLA's superior appearance and potential for downgauging versus HIPS, along with its ability to run on existing extrusion lines and tools.

For stretch-blow molding, Cargill Dow recently launched NatureWorks 7000D as a candidate for replacing PET in niches requiring excellent opticals, high stiffness, and good flavor and aroma retention. The supplier claims PLA has enough melt strength and stretchability to be "close to a drop-in" substitute for PET in standard reheat stretch-blow machines (e.g., those of Sidel). Output rates are said to be comparable to those with PET. However, cycle times for injection molding PLA preforms are longer than for PET.

A limiting factor for PLA is its relatively poor barrier to water vapor, oxygen, and CO2 (Table 2). PLA's water-vapor transmission rate is significantly higher than for PET, PP, or PVC. PLA's oxygen barrier is better than that of HIPS but is still significantly lower than those of PET, PP, and PVC. Consequently, PLA's role in blow molding is at present restricted to short-shelf-life products, viscous liquids (e.g. edible oils), dry goods, and refrigerated items.

In the U.S., a single-serve dairy container has been prototyped for Cargill Dow by Plastics Technologies Inc., a specialist in bottle design and process development. According to Cargill Dow, PLA has a somewhat lower viscosity than PET and works well in filling complex blow molds. In some designs, PTI recommends reinforcing the bottle base. Cargill Dow expects global commercialization this year of single-serve containers for products with five- to 10-day shelf life.

Looking ahead, Cargill Dow is working to improve PLA's properties and tackle its deficits. In blow molding, for instance, it is developing a system of PLA bottle, cap, and label grades that would provide synergistic benefits.

In order to enhance PLA's moisture- and gas-barrier properties, technologies like nanocomposites, aluminum metalizing, and glass or other barrier coatings are being investigated. Cargill Dow is also exploring foaming as a way to minimize PLA's density disadvantage versus PP and HIPS.

Finally, Cargill Dow is developing a new-generation PLA production method that would draw on biomass feedstocks instead of processed corn. If successful, this would cut the cost of making PLA and increase its environmental benefits.