Note: This is the second article of a three part series covering bioplastics (1) trends, (2) material/process advances and (3) applications.

Microbiologists, rather than materials scientists, are developing new plastics building blocks. New technologies, such as converting plant sugars directly into polylactic acid (PLA) in a single step rather than the current two-step process, will reduce manufacturing costs, and bring new bio-based products to market. To further enhance overall performance and durability of bioplastics, additives, blends and alloys are increasingly used.

The increasing number of biopolymers gives more choices for blend processing and will lead to more innovation — blending/alloying is an important route to improving biopolymer properties because fermentation by living creatures doesn’t lend itself readily to making grade changes in the ways that petrochemical synthesis does. This also results in hybrid resins that combine biomaterials with conventional resins.

Let’s take a look at recent bioplastic material and process advances.

First, bioplastic feedstock advances have produced new plastic bio-based building blocks. A good example here is isosorbide renewable diol. Isosorbide is a biobased chemical made from starch that is a sustainable, non-toxic diol for polymers and can be used in a wide range of applications, including bisphenol A (BPA) — free polycarbonate and bio-based copolyesters. Although isosorbide, as a derivative of sorbitol, is not a new product, it is one that for a long time was considered too complicated to purify for chemical industrial use.

Roquette Frères, a global leader in the starch manufacturing industry, conducted major research and development efforts to develop technology for using isosorbide in the polymer industry. The company spent five years investigating the use of isosorbide as a renewable diol for various polymers and biopolymers. The company collaborated with Mitsubishi Chemical on its development and commercialization of Durabio, a durable biobased isosorbide polycarbonate that combines the benefits of polycarbonate (PC) and polymethylmethacrylate, or acrylic (PMMA).

Mitsubishi Chemical
Durabio chemical structure (left) and comparison properties (right).

The plastic has excellent optical properties, combined with high-impact strength and bio-based content. Mitsubishi has upgraded its existing polycarbonate facilities at its Kurosaki Plant in Japan and produces Durabio there for global customers with an annual production of 20,000 tonnes.

Next, bioplastic processed blends, alloys and compounds have brought us rayon fiber reinforced bio-based polyamide (PA). A novel combination of rayon fiber and bio-based high-performance polyamides commercialized by Evonik provide a high biocontent composite with good reinforcement. Rayon is a bio-based fiber produced from regenerated cellulose from wood residue. Although adding reinforcing glass fibers improves polymer mechanical properties, in the case of bio-based polymers, the use of glass as a reinforcement system lowers the bio-content, reducing the ecological advantage.

Two PA grades (Terra HS and Terra DS) from the biopolyamide Vestamid Terra product family form the matrix of these new composites, namely:

  • Terra HS is a PA6.10 containing approximately 60% renewable raw materials
  • Terra DS is a 100% bio-based PA10.10 that offers properties between standard shorter-chain PA6 and high-performance, long-chain PA12

Evonik
Terra HS PA6.10 (left) and Terra DS PA10.10 (right) chemical structures.

Both Terra HS and Terra DS are produced, at least in part, from the non-edible castor oil plant. Both are available as glass fiber-reinforced grades with glass content of 30 to 65 percent. While this use of glass effectively lowers biocontent, the use of natural fibers has typically resulted in significant deterioration of reinforcing performance, and can cause unpleasant odor in the end product, as well as lower product thermal stability and low resistance to moisture.

The Vestamid Terra family of polyamides has been certified as bio-based per DIN ISO 10694; 1996-08 using C14 carbon trace analytics to verify the carbon bio-based origin and by the USDA’s Biopreferred program.

Vestamid Terra with rayon fibers retains the high biocontent along with excellent reinforcing potential. Rayon, a viscose fiber, is obtained entirely from wood residues (by dissolving pulp), and is therefore also based on renewable raw materials. Compared with glass fiber-reinforced PA systems, the new rayon-reinforced biopolyamide offers significantly improved carbon balance. For example carbon dioxide savings for a rayon fiber system of PA1010 with a fiber content of 30 percent are 57 percent higher than for a 30 percent glass fiber reinforced PA66.

Finally, bioplastic composites have advanced into the marketplace in the form of "Thrive" cellulose fiber-reinforced polypropylene. A family of natural fiber reinforced polypropylene composite based on "Thrive," a specially engineered cellulose fiber extracted from trees developed by Weyerhaeuser. For example, thermoplastics compounder RTP Company has developed and process optimized an Eco Solution product portfolio based on "Thrive" cellulose fiber-reinforced compounds.

Advantages of "Thrive" cellulose fiber compared to glass fiber reinforcement:

  • Renewable content to meet sustainability objectives
  • Lower energy requirements during processing
  • Specific gravity reductions of 6-9 percent at like loadings
  • Faster molding cycle time (up to 30 percent faster)for medium to thick walled parts
  • Less abrasive, reducing tool wear

Advantages of ‘Thrive’ cellulose fiber compared to other natural fibers like wood, hemp, and sisal or natural fillers like wood flour and wheat straw:

  • Higher strength and stiffness
  • reliable supply
  • Consistent color
  • Low odor
  • Superior processability

Automotive parts, office furniture, household goods, small and large appliances, industrial goods and consumer personal products are seen as potential applications for cellulose fiber reinforced polypropylene (PP) composites.

Weyerhaeuser
"Thrive" cellulose fiber (left) and cellulose versus glass fiber PP property comparison (right).

Weyerhaeuser, a global leader in cellulose fiber technology, is also marketing cellulose fiber reinforced PP composites under the Thrive DV Series. Cellulose fiber loadings of between 10 and 40 percent are possible, blended with virgin or recycled PP and compatibilizer.

Melt Flow indices range between 5 and 35. The composite is available both as ready-to-mold composite pellets and in masterbatch form for custom compounders. In addition to the PP blends currently available, Weyerhaeuser has worked with customers to blend cellulose fibers with a range of fossil-fuel derived and bio-based polymers, including acrylonitrile butadiene styrene (ABS), low density polyethylene (LDPE), high-density polyethylene (HDPE), polyvinyl chloride (PLC), PLA and other bioderivative polymers.

Thermoplastic compounder RheTech Inc. has added agave and coconut fiber to its RheVision line of biocomposite reinforced polyolefins (wood, rice hulls, flax), and also expanded the product line to include a wood fiber-reinforced biocomposite using a sugar cane-based HDPE matrix supplied by Braskem. Agave fiber, a byproduct from production of tequila, adds impact strength/stiffness and imparts a unique fibrous aesthetic quality.