Film blowing of thermoplastic starch

Abstract

Thermoplastic starch materials are often based on a combination of starch, glycerol and water. In the present study, two potato starch grades were employed; a native (natural) grade and an oxidised and hydroxypropylated grade of the native material, in order to produce the thermoplastic material. The primary aim of the study was to identify possible routes for film blowing thermoplastic starch on a laboratory scale by a suitable choice of processing conditions, amount of glycerol and moisture content. With an appropriate combination of these parameters, the thermoplastic material based on the modified starch could be film blown in a satisfactory manner. Film blowing of material based on the natural starch was significantly more difficult. The difficulties encountered were mainly related to a sticky surface of the film, insufficient tenacity and foaming. The processing window for thermoplastic starch (related to film blowing) is briefly outlined and discussed.

Introduction

In order to convert starch materials into films or coatings in a feasible manner, techniques that are energy-efficient in combination with a high productivity are desired. Preferably such processes should be based on already existing converting technology.

Films of starch can be cast from aqueous solutions or suspensions, but significant amounts of water have to be evaporated in order to obtain the film. This can be relatively energy-consuming. When converting starch in the molten state, products with different shapes can, in principle, be attained, including films (using extrusion). The evaporation step is then avoided and is replaced by a cooling phase. In order to achieve thin films such as those obtained with laboratory scale solution casting, the melt has to be adjusted or shaped after the extruder die. This can be done simply by pulling and squeezing the melt through calendering nips or by stretching the melt in two directions, as in film blowing.

Film blowing is a commonly used method for producing self-supporting plastic films. A hollow tube is extruded and then expanded by increasing the pressure inside the tube. The tensile properties of the melt are of great importance for the final result in both melt calendering and film blowing. Poor melt tenacity has been identified as one of the potential limitations when extruding (or processing) thermoplastic starch, i.e. starch blended with plasticizer and water, at elevated temperatures (Thunwall, Boldizar, & Rigdahl, 2006a). Melt tenacity is here defined as the ability of the melt to deform without rupture. A high melt tenacity would make it possible to produce thinner films and also lead to higher production rates. A recent study identified factors influencing the melt tenacity of thermoplastic starch and indicated some ways of improving it (Thunwall, Kuthanová, Boldizar, & Rigdahl, 2006b). A high plasticizer content and a material system based on oxidised and hydroxypropylated starch seemed here to be favourable.

In several studies on the film blowing of starch-containing materials, rather moderate quantities of starch have either been introduced in a synthetic polymeric system (Fishman et al., 2006, Jana and Maiti, 1999, Otey et al., 1987) or been used together with a biodegradable polymer, such as poly(vinyl alcohol), poly(caprolactone) or polyester (Halley et al., 2001, Matzinos et al., 2002). It can be concluded that films based on such blends can be produced by film blowing.

In recent work on thermoplastic starch-containing plasticizers together with montmorillonite nanoclay, it was reported that film blowing was not possible due to flashing of part of the plasticizer at the die lip which led to holes and collapse of the blown bubble (McGlashan & Halley, 2003). The problem was eliminated by the addition of 30% polyester to the thermoplastic starch and it was also noted that the nanoclay appeared to reduce the migration of the plasticizer towards the surface of the film. Problems with the migration or build-up of deposits in the die region have also been reported elsewhere, not only causing failure of the blown bubble but also resulting in sticking problems after the nip roll (Otey et al., 1987). The latter problem can perhaps be one reason for using sorbitol and urea (Buehler, Schmid, & Schultze, 1994) as a complement to or replacement for glycerol, which is the most common plasticizer in thermoplastic starch. Water is also a plasticizer for starch and can be regarded as a process aid since the viscosity of the melt is lowered with increasing water content. However, water restricts the upper processing temperature because steam generation in the material leads to bubbles and foaming which is not desirable in film blowing (Thunwall et al., 2006a).

In the present work, the film blowing of materials containing only starch, glycerol and water was investigated. The main objective was to establish that film blowing is possible with such materials and to estimate the processing window, i.e. the conditions which limit the processability. A high blow-up ratio (BUR), i.e. the ratio of the diameter of the die to that of the blown bubble, was here desirable and was interpreted as being an indication of good processing behaviour. A normal grade of potato starch with an amylose content of about 21% (Swinkels, 1985) and the same starch material but oxidised and hydroxypropylated were used.