Abstract
The aim of this study was to find alternative starch plasticizers to glycerol that yielded a less tacky material in high-moisture conditions without leading to starch crystallization. A range of glycerol films containing different potential plasticizers (linear alkane diols) were therefore produced, and it was shown that 1,3-propanediol, in combination with glycerol, was a possible solution to the problem. Several additional interesting features of the starch films were however also revealed. The larger diols, instead of showing plasticizing features, yielded a variety of unexpected structures and film properties. Films with 1,6-hexanediol and 1,7-heptanediol showed an ultraporous film surface and near-isoporous core. The most striking feature was that starch films with these two diols moved/rotated over the surface when placed on water, with no other stimulus than the interaction with water. Films with 1,8-octanediol and 1,10-decanediol did not show these features, but there was clear evidence of a structure with phase-separated crystallized diol in a starch matrix, as observed in high-resolution scanning electron microscopy (SEM) images.
Highlights
The environmental problems associated with plastic waste, microplastics, and incineration of petroleum-based plastics necessitate a change whereby fossil-based plastics are replaced by bio-based plastics
Starch is an attractive biopolymer due to its abundance in nature and biodegradability, but it is brittle with a high glass-transition temperature and needs a plasticizer for typical plastic applications.[1]
Its efficiency in polar biopolymers has been documented in many papers, e.g., out of 30 tested potential plasticizers in wheat gluten, glycerol stood out as the most effective candidate, with the largest strain at break.[3]
Summary
The environmental problems associated with plastic waste, microplastics, and incineration of petroleum-based plastics necessitate a change whereby fossil-based plastics are replaced by bio-based (biodegradable) plastics. The plasticization efficiency of glycerol has made it a common additive in thermoplastic starch.[2] Its efficiency in polar biopolymers has been documented in many papers, e.g., out of 30 tested potential plasticizers in wheat gluten, glycerol stood out as the most effective candidate, with the largest strain at break.[3] In a recent article, we showed with molecular dynamics simulations on a starch (amylopectin) system with six potential plasticizers that its plasticizing effectiveness originated from a comparatively low overall degree of hydrogen bonding, especially with the polymer matrix, short hydrogen-bond lifetime, and molecular flexibility.[4] The high plasticization efficiency of glycerol, often leads to starch crystallization after gelatinization or with aging/storage time, thereby affecting, e.g., the transparency of cast films and coatings and the mechanical properties.[5,6] Under moist conditions, due to its high water miscibility and the resulting low viscosity,[7] glycerol migrates to the film surface, leading to a tacky material and altered film properties.[8] At high relative humidities, it can phase-separate with water.[9] Alkane diols (e.g., 1,2-ethanediol and 1,3-propanediol) are alternatives which are less effective as a plasticizer compared to glycerol but with less negative side effects (crystallization/tackiness).[10−12] 1,3-propanediol is known to lower the tackiness of glycerol in hygiene/lotion products.[13] A drawback with the smaller alkane diols (1,2-ethanediol and 1,3propanediol) is, their relatively low boiling point They tend to evaporate during high-temperature starch gelatinization. A number of unexpected results were obtained when investigating the larger diols, including the formation of films with ultraporous surfaces and near-isoporous cores, and films that rotated and moved when placed on a water surface (without any external stimulus)
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