Thermoplastics

Thermoplastics melt, and they are formed in heat, a process that can be repeated at will. For example in the case of plastic bags, the polyethylene is delivered in small granules to the processor, molten and stripped to foils. Good blending with synthetic plastics needs meltability and high purity of lignin, a requirement that many lignins for example lignosulfonates and unmodified kraft lignin do not satisfy, but is achieved with certain organosolv and soda lignins.

The preparation of useful polymeric materials containing a substantial amount of lignin has been the subject of investigations for more than thirty years. Typically, lignin could be blended into polyolefin or polyesters by extrusion up to 25-40% (w/w) with still satisfying mechanical characteristics.

A better understanding of lignin structure, in particular the empirical study of the factors governing the supramolecular structure of kraft lignin complexes, led to new lignin-based thermoplastic blends formulations. Hence, methylated or ethylated kraft lignins are now readily plasticized in homogeneous blends with 30-35% appropriate aliphatic polyesters while maintaining sufficient mechanical properties.

Novel lignins like the high purity soda and organosolv lignin, can be mixed more easily into polar industrial thermoplastics to up to 60%, thus lowering the cost for blends of expensive polymers like polyamides, polyesters, PHB etc as well as composites. Recently, polyvinyl chloride blends of organosolv lignin (Alcell) utilized in a highly filled calcium carbonate flooring formulation were shown to have good mechanical properties as well as other advantages (low VOC emissions) compared to controls.

In order to increase the compatibility between lignin and other hydrophobic polymers, different approaches have been used. The chemical modification of lignin through esterification with long chain fatty acids was reported recently. Interestingly esterification with an unconventional enzymatic reaction gave better products than the classical Schotten-Baumann acylation with stearoyl chloride and an alkali solution.