Push is on to commercialize biodegradable lactide polymers.

There are no commercial uses as yet, but a half-dozen suppliers in the U.S. and Japan are betting that a new generation of polylactide polymers will be a hit. Polylactic acid (PLA) resins are true "bio-plastics" in that they are usually derived from agricultural products and their monomers can be produced by microbial fermentation. These melt-processable thermoplastics are completely nontoxic and fully compostable, reverting through biological action to their basic constituents - carbon dioxide and water.

PLAs are part of a new wave of biodegradable plastics that are being readied for market. Not only have starch-based resins re-entered the North American market (see PT, Feb. '94, p. 13 and this month's Industry Newsfocus), but a growing list of firms are developing polyglycolides, cellulose-derived resins, and various synthetic polyesters. Some suppliers think PLAs could be among the least expensive of the bunch, once they become fully commercial.

The interest in PLAs and other biodegradable resins is driven largely by growing activity in municipal composting and by concerns over marine disposal of shipboard waste. In a compost site, PLAs can degrade completely within a month. The resin is also digested by microbes in seawater. On the other hand, it is not water soluble and is only slowly degraded by water in the absence of micro-organisms. PLA can be recycled by chemical conversion back to lactic acid and then subsequently repolymerized.

PLAs can be engineered for a fairly broad range of mechanical properties (elongation values, for example, from 6% to 500%) through varying the molecular weight or "L" and "D" isomer ratios (and thus the crystallinity). Other options include adding fillers such as talc or copolymerizing with other degradable materials such as glycolic acids or caprolactone. Suppliers say PLA properties can be made to mimic those of polystyrene, polyethylene, or flexible PVC. PLA could substitute for those non-biodegradable resins in injection molded disposable cutlery, blow molded bottles, extruded film for lawn or leaf bags, and extruded straws, foams, or sheet. PLA sheet could be thermoformed into lids, trays, tubs, or clamshells to package produce, baked goods, or deli take-out foods. Other possible uses are extrusion coating on paper for drink cups and other packaging, and nonwoven fibers for diapers, hygiene products, mulch cover, or medical garments.

COST BREAKTHROUGH

Polylactides - mainly in the form of lactide-glycolide copolymers - have been used commercially for many years in medical uses such as resorbable sutures and bone screws. Penetration of other markets has been blocked by prices in the range of $500 to $2000/lb. In the last few years, however, several new technologies have emerged for making lower-cost PLAs. Also newly available is high-purity lactic acid with an isomer ratio suitable for these processes.

Three U.S. firms are actively developing PLAs. Cargill, Inc. in Minneapolis has spent seven years developing EcoPLA (pronounced "Eco-play") resin, which is derived from fermented corn sugar. Last spring, Cargill's Corn Milling Div. opened the world's first large-scale semi-works plant for PLA plastics in Savage, Minn. It has a capacity of 8-12 million lb/yr. Cargill plans to open a commercial plant of 100-300 million lb/yr in Blair, Neb., by 1998, says applications development leader Christopher M. Ryan. EcoPLA is developmentally priced at $3-5/lb, but Ryan expects it to sell for $1/lb or less in commercial production.

Two other firms have been developing PLA for four to five years. One is Ecochem in Newport, Del., a 50/50 joint venture of DuPont Co. and ConAgra, Inc. The other is The Chronopol Co. in Golden, Colo., a subsidiary of Golden Technologies Co., Inc., which is the R&D arm of ACX Technologies, a spinoff from the Adolph Coors Co.

In January, the two firms created a 50/50 joint venture called Ecopol L.L.C., which will exploit their combined know-how in polymerization, resin formulation, processing, and applications. Those joint resources include over 50 patents and two pilot plants, each capable of producing some tens of thousands of pounds per year.

Says Ecochem president Mark Montgomery "We're on the steep part of the learning curve. Every day we're acquiring significant knowledge." Neither he nor Chronopol president Dan Verser is willing to forecast commercial timing or pricing for Ecopol's PLA.

Ecopol's resin property data are not yet available. Ecopol is aiming for specifically engineered applications rather than commodities, according to Montgomery. He notes that processing trials of PLA have been conducted on commercial-scale equipment for injection and blow molding, blown film, and extrusion coating.

At least three Japanese firms are also actively developing PLA - Dainippon Ink & Chemicals, Inc., Mitsui Toatsu Chemicals, Inc., and Shimadzu Corp., all of which have offices in N.Y.C. All three are said to be planning new pilot plants. Shimadzu has operated a 200,000-lb/yr facility for its "Lacty" resin since 1992. The firm has developed textile fibers and is working with Mitsubishi Plastics Industries Ltd. to develop packaging film and sheet.

PROPERTIES & PROCESSING

PLA homopolymer in its neat form is an amorphous polymer resembling crystal PS. It's crystal-clear, glossy, stiff, and brittle. It has high moisture permeability and better ESCR in contact with fatty foods than does PS. Cargill's Ryan says EcoPLA's high strength has permitted downgauging plates and cup lids normally made of HIPS by 30% while still passing all customer tests. A panel of taste testers gave EcoPLA good grades for organoleptics, according to Ryan.

INJECTION MOLDED EcoPLA(a)

                            PLA-Neat   Filled   Toughened

Ult. Tens. Str., psi        7700       8400     3600
Elongation [at] Break, %    4.1        2.0      40
Tens. Yield Str., psi       8900       8700     4800
Tens. Modulus, kpsi         500        1000     300
Flexural Strength, psi      12,000     -        -
Notched Izod Impact,
ft-lb/in.                   0.3        0.3      3.9
Glass-Transition
Temp., F                    140        140      140
Specific Gravity            1.25       -        -

a Cargill data.

Brittleness and low heat-distortion temperature (around 130 F at 66 psi) are the unmodified product's two main performance limitations. Both can be remedied by various means. Flexible film suitable for compost bags can be produced by copolymerization with caprolactone or by some other modification. (Cargill is conducting blown film trials.) PLA's slow crystallization rate (like unmodified PET) can be remedied through nucleation (e.g., with talc) or post-annealing of molded or extruded products. Cargill reports that EcoPLA can develop up to 40% crystallinity, thereby raising its HDT up to 240-250 F - well above that of most PS - without loss of clarity.

Modifiers can raise EcoPLA's impact and tear strengths above normal HIPS levels, Ryan claims, though toughening does sacrifice clarity. At impact strength equal to HIPS, EcoPLA reportedly gives higher tensile strength and modulus. Ryan adds that high-molecular-weight EcoPLA can produce thermoformed containers with living hinges that show higher flex-crack resistance than PS.

Cargill reports that EcoPLA's rheology is similar to that of PS [ILLUSTRATION OMITTED]. Processing temperature and melt viscosity also resemble those of PS. EcoPLA reportedly extrudes and thermoforms similarly to HIPS. Injection molding cycle times are now being optimized to compare favorably with PS.

Ryan notes that resin modification can remedy two limitations of neat PLA - low melt strength and high neck-in during extrusion coating. PLA has several attractive features for extrusion coating: higher clarity and gloss than PE, higher stiffness (which might permit use of thinner paper), lower coating temperature (425 F), low coefficient of friction, and ability to be printed without pretreatment (owing to PLA's polar nature). PLA is also heat sealable.

EcoPLA SHEET PROPERTIES(a)

                            Neat    Toughened
                            PLA    PLA-A   PLA-B   HIPS-A   HIPS-B

Thickness, mils             7.2    7.0     8.0     6.0      7.0
Ultimate Tensile, psi       7700   6100    5200    3100     3800
Ultimate Elongation, %      5.0    7.2     18      25       22
Tensile Yield, psi          8700   7500    6900    3400     4200
Tensile Modulus, kpsi       510    -       400     260      360
Tear Strength, lb           0.11   0.11    0.3     0.17     0.15
Impact Strength,(b) ft-lb   0.20   0.30    0.55    0.68     0.19

a Cargill data.

b Pendulum or Spencer impact, ASTM D 3420.

One processing limitation is that PLA requires predrying to very low moisture levels - similar to PET - in order to prevent hydrolysis and lowering of molecular weight. Cargill recommends drying for 4 hr at 120-210 F to bring moisture levels to 150 ppm (0.015%) or less before processing. If kept dry, PLA can survive 10 extrusion passes with almost no detectable change in tensile strength, according to Cargill's regrind studies.