Degradation of an aliphatic aromatic copolyester film, poly(butylene adipate-co-terephthalate) or PBAT, under atmospheric, laboratory simulated solar radiation, phosphate buffer solution, manure, food and yard compost environments was investigated. Methodologies and equipment were developed to measure biodegradation of PBAT films in compost environments using a direct measurement respirometric system (DMR), and to determine the gel content in PBAT films due to crosslinking by differential scanning calorimetry and Fourier transform infrared spectroscopy. A response surface methodology (RMS) was used to design films for agriculture applications with the required optical, physical, mechanical, and biodegradable properties for exposure to different amount of solar radiation. Field and laboratory experiments were carried out to verify and corroborate the performance of the designed films. The performance of PBAT films in tomato production for two consecutive seasons was compared to commercial low density polyethylene mulch film. Changes in light transmission, mechanical properties, molecular weight, thermal behavior and functional groups of the films were used to investigate the effects of solar radiation on PBAT. Exposure of PBAT films to solar radiation produced crosslinked structures of combined phenyl radicals within the film structure. The behavior was replicated when the films were exposed to simulated laboratory UV radiation. The presence of crosslinked structures not only decreased the mechanical properties of the film due to embrittlement, but also hindered the biodegradation process by limiting the access of water and microorganisms to the polymer chains as confirmed by scanning electron microscope and the DMR system. To prevent the formation of the PBAT crosslinked structures from recombination of free radicals, a chain breaking antioxidant, butylated hydroxytoluene (BHT), was incorporated in the formulation. RSM was used to determine the optimal concentrations of carbon black (CB) and BHT for the design of mulch film. Light transmission, final tensile strength, final gel content, and the pattern of reduction of number average molecular weight (Mn) were selected as the response surfaces for determination of the film formulation. Twenty percent light transmission or less, a final tensile strength of at least 6.35 MPa, a maximum gel content of 0.30 g gel/g polymer, and a specific reduction pattern of Mn were established as the mulch films design criteria. A contour plot of suitable concentrations of CB and BHT for the formulation of mulch film for crop production in Michigan or regions with similar solar radiation was established. Furthermore, a model including CB and an antioxidant concentration with the four design criteria was developed for the formulation of mulch films for general exposure radiation. Therefore, the proposed model is not restricted by different crops or geographical regions.
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