The need for environmentally-friendly plastics has increased due to environmental threats caused by non-degradable plastics. Starch-filled polyethylene is the most cost-effective biodegradable plastic, but it often performs poorly because starch and PE don’t mix well. A new method called reactive blending can overcome this issue without requiring additional chemicals. By modifying the blend components, they can react during the blending process to improve compatibility between the two phases. This results in a small amount of graft polymer being formed, which improves the properties of the blend.
To make different types of plastic blend together better, people come up with methods called “compatibilization strategies.” One way to do this is by adding a compound called a “compatibilizer” that’s made by modifying at least one of the polymers in the blend. Some different approaches to making compatibilizers are:
- Changing the polyester by adding things like maleic anhydride, pyromellitic anhydride, polyacrylic acid, or telechelic polyester phosphate
- Changing the starch by adding things like polyglycidyl methacrylate or urethane functions
- Using coupling agents like peroxides or polyisocyanates to make the starch and polyester bond together
- Making copolymers, like a blend of starch and polyester
When two different types of polymers need to work together, they can be made to do so by a technique called grafting. A compatibilizer, which is a compound attached to a commercially available polymer, is added to granular starch and the polymer. The mixture is then heated and pressurized, so the grafting compound attaches to the starch covalently. Maleic anhydride is often used because it’s readily available. Adding plasticizer to the mix is not recommended because it makes the finished product worse. The compatibilizer is usually made of the same polymer as the primary polymer, such as polyethylene, polypropylene, or polystyrene. The concentration of the compatibilizer is typically around 10% of the starch, but it can go up to 80%. The starch binding compound should be around 5% of the overall weight of the compatibilizer. The resulting mixture contains 1-30% starch, 1-24% compatibilizer, and the remainder polymer. It is less expensive and more biodegradable than pure polyethylene and has similar mechanical properties.
Maleic anhydride (MA) is a compound that binds starch effectively and can be used in different compatibilizers at a level of 0.01 to 10 weight percent. The polymer component of the compatibilizer should ideally match the polymer mixed with starch granules. PE-g-MA is appropriate for PE, while PP-g-MA is suitable for polypropylene. SEBS-g-MA is a good compatibilizer for PE, PP, PS, PB, PET, PVC, and PVF. A European patent on biodegradable thermoplastic nanocomposite materials involves natural polymer/clay hybrid with polyolefin in the presence of compatibilizer and plasticizer. Corn starch is preferred, but starch from wheat, rice, and potato can be used. A suitable compatibilizer can be maleic anhydride-grafted polyethylene or another depending on the biopolymer. Plasticizers such as glycerin, formamide, and urea and/or polymers can be added. Suitable synthetic resins are Polyethylene oxide, low-density polyethylene, high-density polyethylene, polypropylene, and their combinations. The biodegradable nanocomposite materials are appropriate for producing agricultural covering mulch films, packaging materials, yoghurt containers, marketing bags, or waste containers for composting. The addition of starch decreases the tensile strength and elongation properties of polyethylene. However, using PEgMA as a compatibilizer can provide better tensile strengths and elongation values in films.
To make biodegradable thermoplastic nanocomposite granules, you:
- Dissolve a natural polymer like starch, chitosan, cellulose, or polylactic acid in a solvent.
- Adjust the pH of the solution to acidic.
- Add 2% to 50% natural clay based on the weight of the natural polymer.
- Stir the solution to get a natural polymer/clay nanocomposite precipitate.
- Melt blend the natural polymer/clay nanocomposite with a synthetic polymer like polyethylene oxide, polypropylene, or low-density polyethylene at a suitable temperature between 110 – 250°C.
- Granulate the material obtained in a pelleticizer.
This method produces transparent biodegradable polymers that have improved tensile strength and good elongation properties.
Yoo et al. studied the PE/starch blend system with a reactive compatibilizer (m-PE) and found enhanced interfacial adhesion and increased tensile strength. They also observed a chemical reaction at the interface, and the effect of the reactive compatibilizer on interfacial morphology was examined by SEM.
The addition of PE-g-MA to the blend improved the tensile strength and elongation at break, especially at higher starch contents. The interaction between starch and PE-g-MA was based on chemistry, and MA-g-LLDPE/starch blends showed better water resistance and biodegradability than LLDPE/starch blends.
Blends with more than 30% starch content supported heavy fungus growth and degraded more in soil environments than in mixtures with fungi inoculum. Biodegradation resulted in increased crystallinity in the tested blends.
Mani and Battachaya conducted a study on how adding maleic anhydride to starch/polymer mixtures during injection molding affects their properties. They used corn starches with varying amylose contents (0%, 25%, 50%, and 70%) and added 5% maleic anhydride-functionalized polymer to the blends. Adding maleic anhydride improved the mechanical properties of the blends, especially at higher amylose contents.
However, the properties of corn starch blends were not as good as those of potato starch blends. Starch/EVA blends had higher elongation values but lower strength compared to blends with LDPE and HDPE.