Starch-plastic blends

According to filler theory, connectivity can be achieved at lower values when the filler form is plates rather than spheres. Depending on the proportions of the plates and whether or not an inactive phase is included in the blend, connectivity can be achieved at 8 to 16% (v/v) filler (4). The starch-plastic blends developed by Otey (2) have a laminate structure when the starch content is under 30% by volume (Figure 1) and the threshold for microbial attack on these materials is under 13% starch by volume (Figure 2). This low threshold value can be explained by considering the LDPE as a non-conductive (enzyme-impermeable) phase combined with a conductive phase of starch-EAA complex. 

Non-invasive Microbial Growth on Starch-plastic Blends... 

Figure 3. Postulated mechanism for microbial decay of starch-plastic blends.

Figure 7. Mineralization of SC/Starch/Plasticizers Blends with Variable Amounts of Crosslinker...

Kinetic Model for Degradation of Starch—Plastic Blends mth ControDed-Release Potential... 

The Potential of Starch-Plastic Blends as Controlled Release Formnlations. 

Since incorporating starch into a hydrophobic matrix retards the starch degradation rate (2), it should be possible to use starch-plastic blends in lieu of the starch alone as controlled release formulation of pesticides. The release of pesticides which are either adsorbed to (Figure la) or covalently bonded to (Figure lb) the starch could be controlled primarily by the rate of starch degradation. 

Enzyme Assays. Starch digestion from blends by porcine a-amylase (Sigma) was determined by measuring soluble product formation by the phenol-sulfuric acid method (12). Incubations were conducted in 20 mL of 0.05 M phosphate buffer (pH 7.0) containing 8 pieces of 1 x 2 cm starch-plastic blend and 2 /iL mL merthiolate to inhibit microbial catabolism of digestion products (13). Merthiolate did not inhibit en2yme activity or interfere with product assays. Sufficient enzyme was added to give a 100 to 200 units mL solution. Incubation temperature was 35° C. Mixtures were shaken at 50 to 70 rpm on a rotary shaker. 

Biodegradability of the Starch in the Blends. The availability of starch to contact with amylase in starch-plastic blends is a prerequisite for starch digestion. The release rate of non-bioavailable starch granules which were completely occluded by plastic is simply zero, unless the plastic matrix is broken by other means. Hence, the discussion about the kinetics of starch degradation in the following sections refers to that portion of starch which was biodegradable. 

The mathematic model provides a reliable means to predict the starch release kinetics of controlled release formulations made from starch plastic blends. The predicted release of starch digestion products from the blends as a function of time was plotted in Figure 5a and Figure 5b. Correspondence between model-derived values and experimental results was excellent in nearly all cases. 

ZHANG ETAL. Degradation of Starch-Plastic Blends... 

The work presented here constructed a general model of starch-plastic blends as potential controlled release formulations. This model provided a practical method of predicting the kinetics of the starch digestion and product release from starch-plastic blends, thus the kinetics of pesticide release is predictable if the pesticides are either adsorbed or covalently bonded to the starch. The model was developed for starch-plastic blends. It should be adaptable to other blends of incompatible polymers, so long as one of the polymers is susceptible to enzymatic l drolysis. 

---