Starch Based Bioplastics

(Thermoplastic Starch)

Properties and Applications

Starch is one of the most abundant biopolymers. It is completely biodegradable, inexpensive, renewable and can be easily chemically modified. Not surprising, bioplastics derived from (modified) starch have received increased attention as biodegradable alternatives to conventional petroleum-based plastics. Like cellulose, starch can be considered a condensation polymer because its hydrolysis yields glucose molecules. The cyclic structure of the starch molecules together with strong hydrogen bonding gives starch a rigid structure and leads to highly ordered crystalline and granular regions.

Starch in its granular state is generally unsuitable for thermoplastic processing. To obtain thermoplastic starch (TPS), the semi-crystalline starch granules have to be broken down by thermal and mechanical forces. Since the melting point of pure starch is considerably higher than its decomposition temperature, plasticizers such as water and/or glycols have to be added. The natural crystallinity can then be disrupted by vigorous mixing (shearing) at elevated temperatures which yields thermoplastic starch. However, this blend is still unsuitable as bioplastic due to its high moisture sensitivity and poor mechanical properties.

To overcome these drawbacks starch is chemically modified¹ and/or blended with other (bio)polymers to produce a tougher and more ductile and resilient bioplastic. For example, starch is often blended with natural and synthetic (biodegradable) polyesters such as polylactic acid, polycaprolactone, and polyhydroxybutyrate. To improve the compatibility of the starch/polyester blends suitable compatibilizers such as poly(ethylene-co-vinyl alcohol) (EVA) and/or polyvinyl alcohol (PVA) are often added which also improve the mechanical properties. These approaches do not compromise the biodegradability of starch and many of the compositions are fully compostable. They also have much improved impact resistance and dimensional stability. However, polyester-starch blends are less strong than chemically modified and cross-linked starch.¹ Chemical modifications also improves TPE's compatibility with other polymers. A common methode is to replace the hydrophilic hydroxyl groups (-OH) with hydrophobic (reactive) groups, for example by esterification or etherification.² The maximum degree of substitution per glucose unit (DS) is 3 which is equal to the number of hydroxyl groups on each glucose unit. However, commercial grades have typically a much lower DS value which is around 0.2 or lower.

Starch based bioplastics are mainly used for food packaging such as cups, bowls, bottles, cutlery, egg cartons, and straws. Other applications include disposable bags and trash liners as well as compostable films for agriculture.

Manufacturers & Distributors

¹The mechanical properties of starch are greatly improved by grafting functional monomers onto the polymer backbone and by subsequent crosslinking.

²Typical grafting and crosslinking agents are phosphoryl chloride, acid anhydrides, methacrylates, epoxies, epichlorohydrin, glyoxal, and acrylonitrile among many other compounds.