Injection moulding thermoplastic starch

The first commercial product made of injection-moulded thermoplastic starch was the drag-delivery capsule Capill, and further products are gradually appearing, e g. golf tees, cutlery, plates, and food containers [43]. In addition, extrusion has been applied to produce rigid foams, suitable for loose-fill packaging. 

Figure 7.10 shows the microhardnesses of several injection moulded starch samples processed at different temperatures with different initial water content. There is an increase in H with the injection moulding temperature in the range 80-110 °C, from 120 MPa to 140 MPa (Balta Calleja etal, 1999). These microhardness values are notably higher than those found for conventional injection moulded thermoplastic polymers like PE (50-60 MPa) (Rueda et al 1989 Balt Calleja et al., 1995). 

In recent years starch, the polysaccharide of cereals, legumes and tubers, has acquired relevance as a biodegradable polymer and is becoming increasingly important as an industrial material (Fritz Aichholzer, 1995). Starch is a thermoplastic polymer and it can therefore be extruded or injection moulded (Balta Calleja et al, 1999). It can also be processed by application of pressure and heat. Starch has been used successfully as a matrix in composites of natural fibres (flax, jute, etc.). The use of starch in these composites could be of value in applications such as automobile interiors. An advantage of this biopolymer is that its preparation as well as its destruction do not act negatively upon the environment. A further advantage of starch is its low price as compared with conventional synthetic thermoplastics (PE, PP). 

The introduction of gelatinized starch expanded considerably the use of starch as a polymeric material, but this technology based on water processing was limited by the necessity of more than one step, some of which were discontinuous. Nonetheless, these developments showed that it was possible to produce blends of synthetic thermoplastic materials with gelatinized starch, affording new prospects of the utilization of starch as a plastic material in the production of films and injection-moulded goods. 

The thermoplastic starch beads are foamed on a Demag D60 injection molding machine equipped with a standard PE screw (compression ratio 1 2). Beads with the desired moisture content (10-15% b.w.) are fed into the injection imit, melted and injected in the same fashion as in a regular injection molding process. The exception, however, is that the melt is injected into free air instead of a mould. The temperature used for foaming ranges from 150 to 200°C. 

Mondragon et al ° reported that unmodified and modified NR latex were used to prepare thermoplastic starch/NR/MMT nanoeomposites by twin-screw extrusion. After drying, the nanoeomposites were injection moulded to produce test specimens. SEM of fractured samples revealed that chemical modification of NR latex enhanced the interfacial adhesion between NR and thermoplastic starch (TPS), and improved their dispersion. X-ray diffraction (XRD) showed that the nanoeomposites exhibited partially intercalated/exfoKated structures. Surprisingly, transmission electron microscopy (TEM) showed that clay nanoparticles were preferentially intercalated into the rubber phase. Elastic modulus and tensile strength of TPS/NR blends were dramatically improved from 1.5 to 43 MPa and from 0.03 to 1.5 MPa, respectively, as a result of rubber modification. 

Poly(butylene adipate-co-terephthalate) (PBAT) Some of its main applications are films (mulch, containers, bags), filaments, thermoformed and injection moulded products, and blown bottles. Two products in the market are Ecoflex (BASF, 14,000 t per year) and MaterBi (former EasterBio/Eastman, now Novamont, 15,0001 per year). For some applications, PBAT has a very low stiffness, and may be mixed with PITB or PEA, for example. It may also be mixed with thermoplastic starch [69, 71]. 

In 1980s a breakthrough occurred by processing starch at approximately its natural water content (15%) in a closed volume at temperatures above 100°C. Using conventional injection moulding, glassy, amorphous, thermoplastic starch (TPS) polymers (Tg 60 C) were obtained with moduli similar to those of polypropylene and high-density polyethylene. 

Water is required in the manufacture of both thermoplastic starch and thermoplastic protein. For starch, it ensures gelatinization and the formation of a continuous thermoplastic phase. In proteins, it lowers the glass transition and denaturing temperature, allowing processing. However, dry resin ensures dimensional stability directly after injection moulding and over time. 

This is a thermoplastic starch made from potatoes, wheat, maize or tapioca. The applications are found in food packaging, dog toys and veterinary accessories, and for injection moulding of complex parts. 

Ratto et al. (1999) has examined poly butylenes sueeinate adipate (PBSA) terpolymer/granular starch composites for blown films and showed they could produce good film tensile properties and control biodegradation with granular starch addition. Thermoplastic starch/cellulose fibre extrudates and injection moulded products were examined by Funke et al. (1998) and they showed a reduction in water adsorption with increasing fibre content. Work by Halley et al. (2001) examined the use of thermoplastic starch-polyester blends for use in mulch film applications noting excellent field performance and biodegradability for these materials. 

Modified processing techniques have also been useful for thermoplastic starch polymers. Recent work by Martin et al. (2001) and Martin and Avemous (2002) has examined the use of coextruded sheet processing to produce polyester/thermoplastic wheat starch/polyester multilayer films. It was found that adhesion strength between the layers and stability of the interface were crucial properties in controlling the final performance properties of the films. Work by Sousa et al. (2000) has also examined use of the novel shear controlled orientation injection moulding (SCORIM) process to control morphologies and provide tensile property increases of thermoplastic starch/synthetic blends. 

Some commercial thermoplastic starch polymer based products were highlighted in Table 6.1, and some of them can be examined in more detail in this section. Probably one of the first starch based products developed was the National Starch expanded starch foam packaging material ECO-FOAM . ECO-FOAM materials are derived from maize or tapioca starch and include modified starches. This relatively short-term, protected-environment packaging use is ideal for thermoplastic starch polymers. National starch now has additional thermoplastic starch materials, blends and speciality hydrophobic thermoplastic starches for a range of apphcations including injection moulded toys, extruded sheet and blown film apphcations. [http //www.eco-foam.com/ loosefill.asp]. 

Novamont has been developing thermoplastic starch based polymers since 1990. Mater-Bi polymers are based on thermoplastic starch-blend technologies and product apphcations include biodegradable mulch films and bags, thermoformed packaging products, injection moulded items, personal hygiene items and packaging foam, [http //www.novamont.com/]... 

Rodenberg Biopolymers produce Solanyl a thermoplastic starch based biopolymer which is focused on injection moulding apphcations. Solanyl is... 

The impact properties of injection-moulded Mater-Bi (R) were increased by 30% when the matrix was reinforced with miscanthus fibres (Johnson et al, 2003). Key factors in determining composite properties were the temperature of the extruder barrel and the extruder screw speed. A summary of plant fibre-thermoplastic starch composite mechanical properties is presented in Table 8.4. 

Starch is the most commonly used agricultural raw material. Starch is inexpensive, widely available and relatively easy to handle. All-starch bioplastics are made from thermoplastic starch and formed with standard techniques for synthetic polymer films such as extmsion or injection moulding. The use of... 

Averous, L., Fauconnier, N., Moro, L and Fringant, C. (2000) Blends of thermoplastic starch and polyesteramide processing and properties. Journal of Applied polymer Science, 76,1117-28. Mani, R. and Bhattacharya, M. (1998) Properties of injection moulded starch/ synthetic polymer blends. Ill Effect of amylopectin to amylose ratio in starch. European Polymer Journal, 34 (10), 1467-75. Mani, R. and Bhattacharya, M. (2001) Properties of injection moulded blends of starch and modified biodegradable polyesters. European Polymer Journal, 37, 515-26. 

Oniszczuk, T. (2006) Effect of parameters of injection moulding process on the structural properties of thermoplastic starch packaging materials. PhD Thesis. Lublin Agricultural University, Poland. 

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