Synthesis and Characterization of Thermoplastic Agro-polymers
Agro-polymers are produced by directly converting polymers from animal or plant matter into useful materials such as thermoplastics and thermosets. This chapter discusses the synthesis and characterisation of agro-polymers that exhibit thermoplastic properties. Synthesis of thermoplastic melts from biological macromolecules such as starch and protein normally requires water as a plasticiser. In the case of starch, water facilitates gelatinization of starch granules prior to thermoplastic processing, while for proteins the secondary and tertiary structure of proteins need to be disrupted and this is called denaturing. Additional plasticisers are used to improve processing for both materials by lowering the glass transition temperature and softening point. In the absence of plasticisers these are high relative to the degradation temperature, prohibiting processing. Water sensitivity is the most important characteristic of both thermoplastic starch and thermoplastic protein, caused by the many hydrophilic groups in their constituent monomers. Their mechanical and thermal properties are highly water dependent and change due to uptake or loss of moisture at different atmospheric moisture contents. This effect can be mitigated to a limited extent by choosing alternate plasticisers. Thermoplastic processing tends to increase the ordered beta sheet content of proteins, although this is not always the case. Thermoplastic starch has an amorphous structure immediately after processing but is known to retrograde. This is a time dependent recrystallisation which causes embrittlement. Agro-polymers’ mechanical properties are generally characterised as relatively poor when compared to synthetic engineering plastics. Regardless, these may be sufficient depending on their intended use in non-structural applications.