The development of operational methods for the manufacture of products from thermoplastic starches using classical polymer technology is a major challenge. Processes designed for synthetic polymers could potentially produce starch plastics that act as supplements to existing synthetic products, but starch plastics have not yet become widely introduced as consumer goods due to major drawbacks such as brittleness, retrogradation, and volume relaxation processes. The glass transition temperature (Tg) is an important parameter that gives information about the retrogradation behavior and mechanical properties of the material. Starches from different sources behave differently when extruded with the plasticizer glycerol. The amylose/amylopectin ratio influences the Tg, with products having higher percentages of amylose being more flexible. Addition of plasticizers such as water, glycols, sugars, and amides can lower the Tg and make the thermoplastic starch more rubber-like. Injection-molded specimens experience considerably higher shrinkage than those made by pressing, but the glassy polymer offers the capability to dissipate energy in situations of shocks on short timescales, making the polymer less brittle.
Plasticizers
Plasticizers are compounds that increase the flexibility of polymers by reducing the interactions between their molecules. They have low viscosities and temperature coefficients of viscosity, and are often polymers with low molar mass, resulting in greater free volume and increased mobility of polymer chains. This leads to a lower glass-transition temperature (Tg) and less brittle polymers. The plasticizer needs to be perfectly mixed on a molecular scale to achieve a homogeneous mixture, and should have a high boiling point to avoid evaporation during further processing and storage. The amylose/amylopectin ratio in starch also affects the properties of thermoplastics made from it. This article describes and discusses the effects of plasticizers, amylose/amylopectin ratio, and mechanical properties of four different sources of starch (potato, pea, wheat, and waxy maize). The goal is to gain better insight into the behavior of starch materials during processing.
Extrusion of Thermoplastic Starch
The extrusion process of thermoplastic starch from four different sources (potato, pea, waxy maize, and wheat) using an intermeshing co-rotating twin-screw extruder. Glycerol was added to the starch and gelatinization was achieved using a screw speed of 110 rpm and a linearly increasing wall temperature from 45-150°C. Test specimens were produced by injection-molding and pressing, and the resulting products were stored in airtight bags in dark, cool surroundings for analysis within two days. Water was added to the pea starch to prevent degradation during extrusion.
Test Methods
Specimens were prepared with an injection-molding machine and tested for tensile and impact strength using standard ASTM methods. The study tested at least five specimens of each type of material for tensile tests and at least ten specimens for impact tests. The unnotched type of specimens was used for impact tests due to the brittleness of some materials. The study also included synthetic polymers for comparison. The water content of most specimens was around 9-10% w/w moisture.
Dynamic Mechanical Thermal Analysis (DMTA)
The Dynamic Mechanical Thermal Analysis (DMTA) technique was used to measure the glass transition temperatures of the batches. The technique involves monitoring the modulus of a polymer against the frequency of an oscillating deformation of a sample bar at different constant temperatures. The Rheometric Solids analyzer RSA II was used for the DMTA tests, and the specimens were made by pressing with a width of 6mm and thickness of 1.5mm. The temperature of each sample was raised by 2°C min-1 and the angular frequency was 6.28 rad s-1. The glass transition temperature was determined by observing the peaks in the dissipation factor (tan δ = E″/E′) around the phase transitions.
Glass Transition Temperature
Glycerol acts not only as a plasticizer but also as a solvent, forming a more homogeneous mixture. Higher percentages of amylose increase the temperature region between Tg and the flow point, while higher percentages of amylopectin increase Tg values. Waxy maize has a higher Tg than pea starch due to its higher amylopectin content. Glycerol has a greater impact on materials containing more amylopectin.
Tensile Strength
Study on the effects of glycerol content and starch source on the tensile strength and elongation of starch-based materials. Samples with low glycerol content break in a brittle fashion, while those with higher glycerol content show increased elongation and decreased stress. Pea starch has low tensile strength but large elongation at break due to its low amylopectin content. Potato, wheat, and waxy maize become very brittle at low levels of glycerol due to their higher amylopectin content. The aging of starch results in increased moduli and tensile strengths but decreased elongations due to further retrogradation. The effects of moisture content on tensile strength are also discussed. Further research is needed to explain the difference in tensile strength between wheat and waxy maize starch.
Impact Strength
The impact strengths of polymer samples with glycerol are low but their toughness and impact strength should increase due to the lowered glass transition temperature (Tg). However, there is a difference in behavior between slow deformations and short timescale deformations, such as impact tests. The DMTA tests show that the broad peaks of tan δ may be responsible for the low impact strength. The limited mobility of the chains causes the material to distort during slow deformations, but impact tests will cause a brittle fracture at this point.
Shrinkage
The injection-molded sample bars experience shrinkage in the injection direction and swelling in the width due to the elongation flow that causes the chains to be oriented in the injection direction. The chains realign in a helix configuration, which causes shrinkage in the injection direction and swelling in the width. Shrinkage becomes faster with increasing glycerol fraction due to decreased local viscosity and increased mobility of the chains. The shrinkage of waxy maize starch takes much more time than that of other starches due to the large proportion of amylopectin. Pressed specimens shrink less than injection-molded specimens because there is no elongation flow present, resulting in less chain relaxation.
Conclusion
The addition of glycerol to extruded starches lowers their glass transition temperature, which can increase impact strength but only up to a certain point. Glycerol also decreases modulus and tensile strength but increases elongation. Higher amylose/amylopectin ratios have similar effects. Injection-molded thermoplastic starch products with glycerol contents above 20% experience significant shrinkage due to elongation flows during injection.