Design Concepts & Trends
Upgrading PLA Bioplastic to Meet Performance Demands
January 3, 2013
PLA compounds can create custom compounds for use in a wide range of applications while higher performance resins and innovative compounding will expand its property profile.
In recent years, the public’s concern over the depletion of the earth’s limited natural resources has increased. An important solution that has evolved within the heavily fossil fuel dependent plastics manufacturing sector has been the development of new thermoplastic polymers derived from bio-based, rapidly renewable resources. During a relatively short period of time, biopolymers have made inroads to replace petroleum-based plastics in a range of applications.
Commercial penetration for unmodified bioplastics has been largely limited to packaging and commodity applications due to performance limitations. Leading plastics compounders have been successful in using property enhancing additives to impart higher performance into biopolymers that allow them to be used for more demanding semi-durable engineered applications in a range of industries including the appliance market.
Among the obtainable performance improvements are greater impact resistance, higher strength, and better thermal capabilities along with value added features such as halogen-free flame retardance and permanent static control.
The emergence of these new biopolymer compounds can help processors and OEMs meet their corporate sustainability goals and the growing consumer demand for products that have a lower environmental impact.
What Are Bioplastics?
To fully grasp the development and use of these unique bio-based plastics, it’s important to have a firm understanding of the industry’s terms and definitions. Bioplastics are a form of plastics derived from renewable biomass sources such as corn, wheat, sugar cane, and sugar beets, unlike traditional plastics which are derived from petroleum products.
Since bioplastics are not derived from limited fossil fuel resources, they are considered an environmentally friendly alternative to plastics derived from petroleum. The production of bioplastics typically results in lower CO2 gas emissions and uses less energy compared to traditional plastics resulting in reduced environmental impact.
“Bioplastics” is a commonly used catchall term and can encompass many types of bio-based plastics, often containing blends of both renewable and fossil-fuel based carbon materials. The renewable carbon content is referred to as “bio-content” and the percentage of bio-content varies widely.
Commercially available bio-based polyester type resins include polylactic acid (PLA), polyhydroxyalkanoates (PHA) such as PHB and PHV and polytrimethylene terephthalate (PTT). PLA and PHA contain 100 percent bio-content while PTT has 37 percent. In the polyamide family of plastics, nylon 11 (PA) is 100 percent bio-based and nylon 6/10 (PA) is 62 percent.
PLA Offers Commercial Opportunities
Among these new biopolymers, PLA, 100 percent bio-based polyester, has gained the most attention because it is widely available in commercial-scale quantities and at relatively low cost.
From an environmental standpoint, the manufacture of PLA produces 60 percent less greenhouse gases and uses 50 percent less non-renewable energy than traditional polymers like polyethylene terephthalate (PET) and polystyrene (PS), according to NatureWorks LLC of Minnetonka, Minn., a major producer of PLA resin.
Materials produced using renewable content, like PLA, are also valued by industry certification agencies such as LEED, EPEAT, BIMFA and the USDA’s BioPreferred label, which can help increase a product’s marketability.
The potential commercial opportunities for unmodified PLA polymer, however, have largely been limited to commodity applications such as clamshell packaging, disposable utensils, grocery bags and disposable water bottles due to inherent low mechanical properties.
For PLA to take the next step into semi-durable applications, its properties must be improved to put it on par with petroleum-based polymers including high-impact polystyrene (HIPS), acrylonitrile-butadiene-styrene (ABS), polypropylene (PP), polycarbonate/ABS alloys (PC/ABS), nylons (PA) and polybutylene terephthalate (PBT).
Leading thermoplastic compounders have invested considerable resources to develop methods of upgrading PLA’s properties to engineering level performance by using the process of melt compounding, which mixes molten resin with additives, modifiers, reinforcements or other polymers to modify select characteristics.
While PLA displays inherently low mechanical properties in terms of impact resistance, strength, stiffness and heat deflection temperature (HDT), it can be up-engineered by compounders due to its favorable economics, ample supply, and ease of modification via compounding.
A limiting characteristic of PLA is that it does share the hydrolytic stability properties that are inherent to all polyesters, making these resins best suited for indoor applications, like many appliances, where the environment (temperature and humidity) is better controlled.
Melt Compounding Raises PLA Impact Resistance
Various polymer technologies allow PLA to overcome its deficiencies and permit this biopolymer to be considered for more semi-durable uses. In the resin production stage, upstream reactor technology to produce purer monomers will result in polymers with a higher melting point and greater crystallinity.
Advancements are also made possible by melt compounding PLA with other polymers and additives. PLA can be successfully compounded with impact modifiers to dramatically improve toughness to match that of HIPS, ABS and PC/ABS alloys.
Compounds are commercially available where adding 5 percent of a compatible impact modifier to PLA produces a bio-based compound that has approximately 95 percent renewable resource content and offers impact performance similar to popular HIPS products which are used in many semi-durable applications. Similarly, incorporating 10 percent impact modifier with PLA results in a compound that has impact performance comparable to popular ABS grades but with roughly 90 percent renewable resource content. Further, increasing the impact modifier to 15 percent produces a compound with greater than 80 percent renewable resource content that has impact performance similar to PC/ABS alloy, one of industry’s leading impact-resistant petroleum-based materials.
PLA can also be compounded with mineral reinforcements and impact modifiers to produce a material with the overall performance of ABS, which, due to its engineering level performance, has been the material of choice in many demanding appliance applications.
Upgrading PLA Thermal Performance
PLA can also be alloyed with other polymers to improve properties including both impact resistance and thermal performance. With an HDT @ 66 psi of 125 °F, a component manufactured from unmodified PLA could possibly have difficulty surviving shipment in a semi-trailer truck across hot southern U.S. climates in mid-summer. In order to allow PLA to be confidently used in semi-durable applications, its thermal capabilities must be improved.
Product development work by compounder RTP Company, for example, has shown that PLA can also be alloyed via compounding with several traditional polymers including PC, ABS, acrylic (PMMA) and polyethylene (PE) to produce polymer blends that have unique performance.
A PC/PLA alloy has appliance market potential for its significant performance. This blend offers performance similar to the versatile PC/ABS alloy in strength, impact resistance and HDT.
One of the reasons that PLA has such a low HDT is that the polymer, in its unmodified form, achieves very low levels of crystallinity during the course of a typical injection molding cycle and can be considered an amorphous polymer. To overcome this, nucleating packages have been developed that can be compounded into the polymer to speed up the rate and degree of crystallization during the injection molding process.
Using nucleators the HDT of PLA can be increased from 125 °F to 195 °F, comparing favorably to HIPS, ABS and PMMA. A nucleation package typically consists of a nucleating agent, such as a fine mineral or salt, which provides a site on which crystal growth can initiate.
Engineering Adds Strength to PLA
Incorporating glass fiber reinforcement is another important method to overcome the shortcomings of unmodified PLA by increasing its strength and stiffness with 10 percent to 40 percent loadings of chopped glass fiber.
Bio-based PLA’s performance, through compounding with glass fiber, can be raised to levels that fall predominantly between that of glass fiber reinforced PP and glass fiber reinforced PBT. A 30 percent glass fiber reinforced PLA compound features a tensile strength of 16,500 psi, a flexural modulus of 1,630,000 psi, and HDT @ 66 psi of 320 °F.
Testing has clearly demonstrated that PLA can be up-engineered to compete as a “drop-in” alternative to some of the most popular glass fiber reinforced thermoplastics in use today while still providing its bio-content benefits. Raw material costs for glass fiber reinforced PLA also fall between that of the glass fiber reinforced PP and that of glass fiber reinforced PBT.
PLA compounds can be easily processed in existing tools and equipment and its colorability plus resin-rich surface finish allow for its use in many types of branded semi-durable goods.
For the appliance industry, PLA compounds can be used to create custom compounds for use in a wide range of applications including housings, bezels, brackets, handles, knobs, buttons and lenses.
As more demanding application opportunities for bioplastics present themselves, higher performance resins and innovative compounding technology will be used to expand the property profile of PLA and other plastics produced from renewable resources to meet industry’s performance requirements.