BIODEGRADATION AND PHOTODEGRADATION OF POLYLACTIC ACID AND POLYLACTIC ACID/ POLYHYDROXYALKANOATE BLENDS NONWOVEN AGRICULTURAL MULCHES IN AMBIENT SOIL CONDITIONS

Agricultural mulch films, typically made of petroleum-based polyethylene, improve crop productivity by controlling weeds and providing microclimate. Extreme fragmentation of films imposes labor and disposal costs, not to mention environmental problems during and after service life. Although mulches made of biodegradable polymers such as cellulose, (nonbiobased) poly (butylene adipate-co-terephthalate) and polybutylene succinate are employed in the field, the fate of biodegradation of additives in mulches is still questionable in addition to service life until the harvesting. Nonwovens, made of biobased polymers such as poly (lactic acid) (PLA) and PLA-polyhydroxy alkanoate (PHA) blends, have been developed using nonwovens textile technology to address the poor mechanical properties and/or biodegradability of traditional mulch films. This dissertation focuses upon biodegradation of nonwovens mulches—spunbonds (SB) and meltblowns (MB)—retrieved from soil after 30 week and after exposure to simulated weathering. Soil amendments, color of mulches, presence of PHA in PLA-based mulches, soil temperature, and kinetics of biodegradation were evaluated in soil burial studies. The biodegradation of nonwovens mulches were characterized by the loss of tensile strength, depolymerization, disappearance of ester bond and decrease of glass transition temperature, melting temperature and enthalpy of fusion. At high moisture conditions, SB mulches of white and black color were recalcitrant to all the soil amendments determined from marginal depolymerization and insignificant loss of tensile strength. MB mulches, particularly PLA-PHA blends, underwent the greatest (~90%) loss of tensile strength among other physico-chemical losses. Although weathered SB mulches did not undergo physico-chemical changes during simulated weathering, the biodegradability test under composting condition (ASTM D5338) met the compostability standard (ASTM D6400) criteria ( > 60% biodegradation after 90 days). MB mulches, with the greatest biodegradation ( > 90% after 90 days) in ASTM D5338, is recommended for “Class II” material in ASTM WK 29802, the standard specification for biodegradability of agricultural plastics in soil.