Blending natural and synthetic polymers like PE or EVOH has gained significant interest in recent years. To maintain compostability, various biodegradable blends have been developed, which can be made into useful disposable products that degrade in specific environments. The mechanical properties of these blends depend on the adhesion of their different phases. Strong adhesion leads to good properties and reduced molecular mobility, while poor adhesion results in lower ultimate properties.
There are two types of experiments used to assess the viscoelastic properties of these blends over time: stress relaxation and creep. Stress relaxation determines the time decay of stress at a constant strain, while creep investigates the decrease in strain with constant stress. Stress relaxation experiments can predict the long-term mechanical behavior of materials from short-term tests.
Starch-based polymer blends, particularly those based on polyolefins, have been extensively studied. A biodegradable composition containing a high proportion of biodegradable starch has been proposed for LDPE blown films, which are useful for making trays for meat packaging. These trays are liquid-repellant but gas-permeable.
However, starch/polyolefin blends are generally incompatible, leading to larger phase domains, non-degradable residues, and secondary pollution. Modification of starch can produce smaller domain sizes in the blends, leading to improved mechanical properties and reduced environmental impact.
Thakore et al. studied the use of potato starch and starch phthalate blends with LDPE for injection molding. They found that starch phthalate was a better filler than starch because it provided a rough surface that improved adhesion and anchoring. However, blends containing loosely embedded starch granules showed poor tensile properties due to phase separation. On the other hand, LDPE/starch phthalate blends showed stath particles uniformly distributed and firmly embedded into the LDPE matrix, resulting in better mechanical, thermal, and morphological properties as well as better biodegradation. Increasing the percentage of biodegradable component in the blend decreased both tensile strength and elongation at break, but replacing some of the starch with stath improved these properties.
Martin and colleagues studied how the strength of adhesion between layers affects the properties of multilayer films. They used different types of polyesters (PCL, PBSA, PEA, PLA, and PHBV) and plasticizers (glycerol) as outer layers in a “polyester/plasticized wheat starch/polyester” film structure. Adding polyester layers improved the mechanical performance and moisture resistance of the plasticized wheat starch film. The best adhesion was seen with polyesteramide, while PLA and PHBV were the least compatible. Coating the films with polyester blends improved adhesion by up to 50%. The multilayers showed good water resistance and improved mechanical properties.
Walker et al. used a mechanical process called solid-state shear pulverization (SSSP) to create blends and composites of polyethylene (PE) and damaged starch granules, which reduces oxygen permeability. Processing a 70:30% PE:starch mixture with SSSP reduced permeability by 29%, compared to only 21% reduction with melt processing when starch particles were not damaged.
Liu et al. studied the effect of starch granule size on the crystallization behavior of starch-filled polypropylene (PP). They found that adding starch decreased the crystallization temperature and rate of PP, with larger starch granules having a greater effect. There was little interaction between starch and PP.
Averous conducted tests on PLS-based blends that can be used as fillers in petroleum polymers. Blending is a cost-effective way to obtain materials with improved properties compared to developing new polymers. Blends can also be used to test compatibility between different polymeric phases. In a separate study, Averous et al. found that adding small amounts of PCL to TPS (10%) improved the properties of TPS, such as resilience, moisture sensitivity, and shrinkage, which were otherwise weak.
Battachaya conducted experiments on blends of starch and synthetic polymers using an extruder. The blends contained 70% starch, 25% non-functionalized synthetic polymer (HDPE, LDPE, or EVA), and 5% functionalized polymer (HDPEMA, EMA, or EVAMA), with varying amounts of starch. A compatibilizer was added to all blends to promote compatibility. The blends were tested using injection molding, and results showed that blends with HDPE and LDPE had similar behavior to ductile polymers, while EVA blends were more rubbery. The moduli of the blends increased as starch content increased, while elongations at break decreased. Relaxation times were longest for LDPE blends and shortest for EVA blends. Amylose content in the blend affected relaxation time, as amylose interacts more with synthetic polymers.
Blends of recycled LDPE with corn starch have two environmental advantages:
- Virgin synthetic thermoplastic material can be substituted by post-consumer materials, and
- the end product can be biodegradable and cheap.
Recycled LDPE and corn starch blends were made using extrusion. The blends contained different amounts of starch (30-50%) and had weaker tensile strength and elongation but higher moduli than LDPE alone. Another study evaluated the mechanical properties of starch/PE blends with added EVA, EVAMA, and EMA and cellulose fibers. Corn starch/EMA blends had higher tensile strengths but were brittle. Corn starch/PE blends had higher tensile strength than wheat starch blends, but less elongation. Adding fibers increased tensile strength, but not flexural strength. Overall, starch/EMA blends had better mechanical properties than starch/EVAMA blends.
Park et al. investigated the properties of blends of potato starch with LDPE and APES plastics with the addition of an ionomer. They used different levels of starch in each blend and measured tensile properties. They also studied the biodegradation of the prepared films by microorganisms in compost soil mixtures. The results showed that as the starch content increased, the tensile strength values decreased due to poor compatibility between the hydrophobic plastics and hydrophilic starch. Better dispersion of starch in the matrix led to better mechanical properties and stiffness. However, adding starch resulted in smaller tensile strengths and decreasing elongation at break. The lack of good phase adhesion and poor dispersion were responsible for this. Microstructural morphologies showed that increasing starch loading decreased interfacial adhesion and homogeneity. The biodegradation of the blends suggests that microbes consume starch and create pores in the materials, leading to increased surface areas and susceptible groups for their biodegradation.
Petersen et al. studied biobased materials made by blending wheat and corn starch with commercial materials for food packaging and other uses. The starchy materials had lower tensile strength and were more permeable to water vapor compared to LDPE and HDPE. Increasing the starch content in LDPE-starch blends led to higher gas permeability. However, combining the biobased materials with edible coatings or multi-layer films could improve their properties and make them suitable for packaging highly respiring foods.
Averous et al. mixed wheat starch with plasticizers and polyester to make biodegradable materials. They found that adding more glycerol increased the flexibility of the material, but too much caused it to break easily. Adding polyesteramide improved impact resistance and other mechanical properties. Polyesteramide also helped reduce shrinkage and increase water resistance. Different combinations of these materials could be used for different applications, making biodegradable materials more affordable.