Starch based loose-fill foams

Mechanical testing on the foam is performed to measure the compressive stress and the resiliency. The results from diese tests are shown in table 2, in which the properties of extruded polystyrene foam (XPS) and commercial starch-based loose-frll foams (Eco-foam and Mater-Bi) together with EPS loose-fill foam (Pelaspan Pac) are added for conq>arison. The values of XPS are obtained from tests on typical XPS retail packaging trays. The table shows that the compressive stress reached with potato starch foam is comparable with that of XPS. Through the cell structure of the potato starch foam (high cell density, very small cells) a good resiliency can be obtained, although pure starch plastics exhibit brittle fracture behavior. This brittle fracture still is present on the microscopic scale of the individual cells but due to the cell density, the foam exhibits resiliency on macroscopic scale. 

The combination of starch with a water soluble polymer such as PVOH (or polyalkylene glycols) has been widely considered since 1970 [100]. Since the early 1990s these compositions have been mainly studied for starch-based loose-fill production as a substitute for expanded PS [101-107], using compositional water as expanding gas. In this kind of blends, not only natural starch but also modified ones such as hydroxy propylated high amylose starch can be used, especially to improve foam resilience and density [101-105]. 

American Excelsior Company, Eco-Foam, The Original AH Natural Starch Based Loose Fill, Arlington, Tbx., undated. 

Fang Q, Hanna MA. Functional properties of polylactic acid starch-based loose-fill packaging foams 1. Cereal Chem 2000 77 779. 

Leading producers with well established products in the market are Novamont, National Starch (main Novamont partner and licensee in the sector of loose-fills and of foamed sheets), and Biotec. Following the recent start-up of its third line dedicated to the production of Mater-Bi film grades in Terni, Novamont s internal production capacity is of 20000 tons/year. The total capacity, including the network of licensees in the sector of loose fills, is of about 35000 tons/year. The technology for the production of starch-based loose fills is licensed together with National Starch and Chemical Co. 

Starch-based materials represent the largest class of biodegradable polymer with 44,800 tonnes (including loose-fill foam packaging) consumed in 2005. Excluding loose-fill, starch-based materials amounted to 21,700 tonnes in 2005. Polylactic acid (PLA) is the second largest material class with 35,800 tonnes in 2005, followed by synthetic aliphatic-aromatic copolyesters with 14,000 tonnes. The embryonic PHA category amounts to around 250 tonnes. 

Table 2 shows also that while Eco-Foam and Mater-Bi foams have comparable properties to PS-based loose-fill materials, the properties of potato starch foams are comparable with XPS. This indicates that potato starch foams could be suitable for XPS-like applications. 

Mitrus y Moscicki (2014) showed, for example, that the appUcalion of extrusion-cooking technique to process starch-plasticizer mixtures can be one of the most economical and efficient ways to produce TPS loose-fill foams. Then extrusioncooking technique can be successfully employed for starch-based foams production. 

Application of food extruders gives much better results in processing of starch-based materials than conventional plastic extruders due to the plant origin of the biopolymer. Most of the experimental works which apply extmsion-cooking for the production of starchy loose-fill foams started in the Department of Food Process Eng., Lublin University of Life Sciences in 2012. Their objective is to achieve commercially acceptable biodegradable products based on locally produced potato, corn- and wheat starch, which can replace popular expanded polystyrene loose-fill foam products. Results of the first phase of this study are presented in their work (Mitrus and Moscicki 2014). 

The combination of starch with a water soluble polymer such as PVOH and/or polyalkylene glycols has been widely considered since 1970 [157]. Recently, the system, thermoplastic starch/PVOH has been studied mainly for producing starch-based loose fillers as a substitute for expanded PS [158-164], As an example, Lacourse and Altieri developed a technology based on hydroxy propylated high amylose starch containing small amounts of PVOH for improving foam resiliency and density [158-162]. In this case loose fill was produced directly by a twin-screw extruder. Recently more advanced processes and alloys have been developed which have resulted in foams with lower foam densities (8-6 kg/m ) and better performance [165-167]. Other applications of modified starch/PVOH can be in the sector of sheet extrusion/thermoforming. 

Although the use of starch in loose-fill products gives advantages in the form of biodegradability and environmental protection, these products have been criticized for their imperfection in relation to EPS loose-fiU products. EPS- and starch-based foams have differences, but the differences do not compromise performance. 

One of the first applications of biodegradable materials is based on the cooked, extruded, and expanded starch known from the food and chemical sectors (Fig. 14.23). Starch is cooked with water in the extruder and chemically modified as necessary or mixed with plasticizers, then expanded to a starch foam and dried. The extrudate is ground so that the functional properties thus created can be used in the food/chemicals sector. The foamed, cut, and dried extrudate is the end product for loose-fill packaging applications. The degree of expansion is a measure of the foam texture. It increases strongly with product temperature at the die, helped by a higher specific mechanical energy input. However, both measures increase the water-solubility of the product. 

Loose-fill packaging was one of the first successful areas of application for starch-based biodegradable polymers. Loose-fill starch-based foam is used for packaging consumer products as an alternative to polystyrene and polyethylene. While, biodegradable plastics have made some inroads into these markets, the future prospects for their growth in loose-fill are not so exciting as they are in some other areas of packaging. 

The properties of foams made from pure starch are limited however mechanical properties are poor and starch foam is very sensitive to changes in relative humidity. To improve these characteristics, starch is chemically and physically modified or blended with other additives or polymers. This approach has proven to be very effective. Various starch-based foams have been introduced in the market, mainly in loose fill applications ... 

Polylactide has high potential in agricultural applications such as mulch films either in pure form or blended with other polymers such as thermoplastic starch. In addition, polylactide is used in compostable yard bags and dog poop bags. Lactic acid based hot-melt adhesives have also been developed (90). Foamed PLA could be used as structural protective foams, loose-fill packaging and insulation material as an alternative for expanded polystyrene (EPS). While packaging is currently the high volume application... 

The main use of thermoplastic starch alone is in foam applications. Starch-based foams have been found to be an effective alternative to PS foam in loose-fill protective packaging. Starch-based foams offer the advantage that they are readily biodegradable if they escape into the environment. They also offer superior antistatic properties. Starch-based foam, however, has some disadvantages it is brittle and the density is higher than PS. Foamed, starch-based articles are prepared by heating starch in an extruder in the presence of water with subsequent extrusion. 

One of the most crucial properties in packaging applications is bulk density. A comparison of bulk densities of expanded polystyrene loose-fill with starch-based foams is given in Table 6.1. 

These products differ with respect to composition and method of manufacture. Foam and bulk densities, which are higher than those of EPS-based foams by factors of two to three, are attributable to the density of starch, which is 50% higher than that of polystyrene homopolymer, and to the direct water-to-steam expansion process, which creates a predominately open cellular structure that stops foam expansion. Starch-based foam loose-fill is very hygroscopic. The foam densities of starch-based products significantly increase, between 10 and 30%, after conditioning at high humidity. 

Generally, extrusion techniques can be successfully employed for production of starch-based foams. The physical properties of loose-fills, such as density, porosity, cell structure, water absorption characteristics, and mechanical properties, are highly dependent on the raw materials and additives. The mechanical behavior of foamed pellets can be adjusted effectively by controlling the cell structure through use of different additives. At room temperature and 50% relative humidity, some mechanical properties, such as compressive strength or compressive modulus of elasticity, are comparable to those of commercial EPS foams. 

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