The physical characteristics of packaging polymers are influenced by their chemical structures, molecular weights, crystallinities, and processing conditions. The specific physical characteristics required for packaging depend on the products being packed and the storage environments. Frozen products, for example, have specific packaging requirements, while foods require more stringent packaging than solid products.
Research is currently being conducted to assess the usability of starch-based materials for food packaging. A study by Holton et al. found that regular PE film and PE film with 5% maize starch content had no significant impact on the quality of packaged broccoli, bread, and beef stored under standard conditions. However, PE film with starch content did cause a significant decrease in elongation, likely due to interactions between the film and free radicals produced during lipid oxidation in frozen beef storage. Inconsistent results were observed in the cases of broccoli and bread packaging in starch film, suggesting that PE films with starch content should be used for packaging wet and dry low-lipid foods but not high-fat foods due to potential interactions with free radicals originating from lipid oxidation.
In response to increasing demand for environmentally friendly packaging, packaging suppliers have introduced various forms of biodegradable plastics made from plants, primarily corn. Some companies predict the market for biodegradable packaging will grow by approximately 20% annually, and bio-based packaging is increasingly being used as a replacement for petroleum-based plastics like PET, PE, and PP. Analysts believe that biodegradable packaging has a bright future due to growing environmental awareness and consumer power, as well as anti-pollution legislation.
Two types of materials are included in the group of biodegradable polymers: those made from renewable plant raw materials that do not easily decompose and those formed by chemical synthesis reactions that are easily decomposed and mineralized by microorganisms. Technical developments have allowed bioplastic materials to achieve the quality of conventional products made of mineral oil, and new trends involve combining commercialized biomaterials to create new functional characteristics and special benefits. Legislative support will also be a significant factor in the growth of the biodegradable packaging sector.
Compostable or biodegradable plastic is usually made from plant-based starch or fossil oil with additives that enable it to break down into CO2 and water. While compostable packaging breaks down more quickly than a banana skin, plastic packages or carrier bags take many years to do the same.
Starch-Based Packaging Materials
Starch-based plastics are commonly made from maize, sugar-cane, corn, and potato starch. Corn is the most widely used crop for producing biodegradable materials, which are used for packaging food products. These materials break down naturally in garden compost heaps, eliminating the need for packaging to be binned or bagged and sent to landfills.
High-amylose corn starch (HACS) can produce films with higher barrier properties and physical strength than films made from normal corn starch. InnoWare, located in Atlanta, USA, produces Expressions-ECO, effective and high-performing containers. They are more durable than typical compostable packagings and are capable of withstanding higher temperatures, which is important during transport and storage. Solid bases and lids are safe up to 49°C, whereas clear lids are safe up to 41°C. These containers are ideal for cold applications, such as salads, sandwiches, wraps, snacks, desserts, fruits, and more.
To create InnoWare’s Expressions-ECO food containers, corn is harvested and broken down into dextrose. Dextrose is fermented and distilled into lactic acid, which is then modified with other biobased materials to reinforce its molecular structure. The result is an eco-friendly resin that is converted into Expressions-ECO food containers. These containers are recyclable and entirely compostable, decomposing completely in 60-180 days without leaving toxic residue.
Belu Mineral Water launched its compostable bottle, the UK’s first bottle made from IngeoTM alternative bioplastic, in May 2006. IngeoTM bioplastics are materials made from plants, not oil. Belu’s “Bio-bottle” is the latest initiative from London-based Belu, an environmental initiative that contributes 100% of its net profits to clean water projects. The bottle can be commercially composted back to soil in 12 weeks, and Belu was formed in response to a challenge set by the UN’s Global Compact, a movement to engage the business community in solving global social and environmental problems.
Amcor and Plantic Technologies from Australia have teamed up to develop biodegradable, flexible plastic packaging for confectionery. Plantic provides its patented material, a plastic created from plants that dissolves rapidly on contact with water. Amcor intends to use the Plantic material to undertake trials of the resin in a commercial packaging film operation. Plantic materials have been used as rigid plastics in confectionery and biscuit trays. The goal of Plantic’s collaboration with Amcor is to develop a thin and durable plastic for the flexible packaging of food and confectionery, such as chocolate bar wrappers and overwrap.
Australian scientists have developed revolutionary packaging materials that are fully biodegradable. Shopping bags based on wheat starch blended with other biodegradable materials compost fully in around 30 to 60 days. These materials could also be used to pack vegetables, in place of polystyrene trays for baked goods, and for other purposes such as mulch film for farming and gardening.
Modified starches, such as cross-linked starch, substituted starch, acid-hydrolyzed starch, and pregelatinized starch, have several functional uses as viscosity modifiers, thickeners, texture enhancers, and flavor encapsulation agents in a host of products, including soups, sauces, bakery products, dairy products, and confectionery. Researchers are focusing efforts on utilizing starch as a biodegradable and virtually inexhaustible raw material for producing packaging materials, including films, foams, and molded packages.
The utilization of TPS for the production of biodegradable plastics has increased and has been the object of several studies in the last decade. However, TPS has other materials, such as polyethylene (PE), and to improve its mechanical properties, TPS has been combined with other polymers, such as polycaprolactone (PCL) or polybutylene adipate-co-terephthalate (PBAT). These combinations have resulted in TPS-based bioplastics with improved properties and performance, making them suitable for a wider range of applications, including food packaging.
In addition to starch-based bioplastics, other biodegradable and compostable materials are also being developed for food packaging. For example, cellulose-based materials, such as cellophane, have been used for food packaging since the early 1900s and are still in use today. More recently, cellulose nanofibers (CNF) have been investigated as a potential biodegradable and renewable alternative to petroleum-based plastics. CNF can be derived from various sources, including wood, plants, and bacteria, and have shown promise as a sustainable and biodegradable material for food packaging.
Another promising biodegradable material for food packaging is chitosan, a biopolymer derived from chitin, which is found in the shells of crustaceans such as shrimp and crabs. Chitosan has antimicrobial properties and has been shown to extend the shelf life of food products, making it a potential candidate for food packaging applications. However, chitosan is still in the early stages of development as a packaging material and more research is needed to determine its feasibility and effectiveness.
Overall, the development of biodegradable and compostable materials for food packaging is an important step towards reducing the environmental impact of plastic waste. While there are still challenges to be addressed, such as cost and scalability, the increasing demand for sustainable packaging solutions is driving innovation and progress in this field.
PLA-Based Packaging Materials
Applications of PLA include its use in compostable sugar-cane trays and in punnets or pallets. A starch derivative, PLA can be produced from maize and other plants and is a biodegradable, compostable plastic material.
The material is available in a range of blends and can be used in sheet or film form for a diverse range of products including food containers. PLA can be used for rigid thermoforms, films, labels, and bottles, but because of its biodegradable features it cannot be used for hot-fill and gaseous drinks such as beer or sodas.
PLA can also be used for non-carbonated beverages such as water, juices, and milk, as well as for edible oil products. It provides a flavor and aroma barrier comparable to that provided by PET and readily accepts coatings, inks, and adhesives. Its stiffness allows for a smaller thickness than is required with materials such as PET without any loss of strength. Heat seals can be made at temperatures as low as 80°C, resulting in faster packaging times and increased output.
Monolayer PLA bottles can be formed on the same injection-molding and stretch blow-molding equipment as used for PET, with no sacrifice in production rate.
Poly-L-lactic acid (PLLA) is formed by chemical condensation of lactic acid monomers; its breaking stress is about 47–70 MPa and its elongation 85–105%. PLLA could be used as packaging material, especially for garbage and shopping bags, wrappings, or fast-foods plates and cups.
Nowadays, packaging manufactured from PLLA mainly consists of grocery and rubbish bags, coatings, six-pack rings, and fast-food containers.
Purac has invested in new lactides that should potentially provide the food, pharma, and cosmetics industries with cheaper and more effective bioplastic packaging. Netherlands-based Purac, a producer of lactic acid for the food sector, has stated that the new lactides will contain L-(+)- and D-(−)-lactic acids. The new lactides will be sold to companies that will use them to make bioplastics that can withstand greater heat than ever before.
The new materials will be suitable for applications as diverse as hot-fill bottles, microwaveable trays, temperature-resistant fibers, and electronics, and will withstand temperatures of up to 175°C.
In order to solve the problem of the inherent brittleness of poly(lactic acid) (PLA), which is due to poor elongation at break and impact strength, researchers at Michigan State University have recently developed biodegradable materials based on nanoscale hyperbranched organic particles (i.e., Boltrorn TM H2004 from the Swedish firm Perstorp) or on a poly(lactic acid) (PLA) matrix (i.e., Biomer ® L9000 grade supplied by the German firm Biomer).
This innovative modified PLA material exhibits an improvement in elongation at break of about 800 to 1000% in relation to traditional PLA grades with minimal impact on tensile strength and modulus.
NEC Corporation has developed a biodegradable plastic composite that is suitable for personal computer case applications. The composite material is made from poly(lactic acid) with fibers of the kenaf plant, a herbaceous annual related to cotton, incorporated to provide both increased strength and heat resistance.
The material’s heat resistance, processability, and strength are comparable to those of fiber-reinforced polycarbonate. It can withstand temperatures up to 120°C before it begins to deform, which is 80% higher than normal poly(lactic acid). The material is also 70% stronger in terms of its ability to withstand bending forces. NEC plans to use the plastic in cases for its laptop computers and aims to have 10% of its line of laptops biodegradable by 2010.
NEC uses commercial PLA sourced from several producers. The company has secured a source of kenaf fibers from Australia and is working to lower the manufacturing cost of the plastic.
Scientists estimate that petroleum-based plastic products require thousands of years to decompose. In contrast, PLA degrades under commercial composting conditions in 75 to 80 days. The compost can then be used to fertilize the next year’s crop of corn, completing the cycle of a totally renewable resource.
Since 2000, when NatureWorks PLA was introduced, the European market has been an early leader in adopting renewable resource-based products. Heightened environmental awareness in European countries, including Italy, France, Germany, Belgium, and the UK, has been the key to bringing nature-based plastics into the mainstream.
New BioPeel peelable lid film can be used for trays and pots. The film is made from corn-based PLA from the US-based NatureWorks, part of Cargill, and one of the main movers behind the biodegradable packaging trend.
The concept behind NatureWorks is relatively simple. Cargill essentially “harvests” the carbon from corn, which plants remove from the air during photosynthesis and store in grain starches. This is achieved by breaking down the starches into natural plant sugars. The carbon and other elements in these natural sugars are then used to make PLA through a simple process of fermentation, separation, and polymerization. Packaging made from NatureWorks is therefore 100% nature-based.
PLA is biodegradable and compostable, and it can be free of genetically modified ingredients. PLA thus has wide appeal within the packaging industry, helping food packagers to meet EU waste targets. A few companies in the US (e.g., Naturally Iowa) have been using PLA for packaging of products such as organic milk.
BioPeel is a clear, peelable PLA lidding or flow-wrap film, suitable for use with chilled and frozen products such as fruit, vegetables, salads, and sandwiches. BioPeel has a broad sealing window and clarity, is available with antifog, and can be perforated. BioPeel can be tailored to seal and peel to any type of container.
The German-based BASF has also announced that it intends to launch a biodegradable plastic based on renewable raw materials to meet the growing demand for environmentally friendly packaging. BASF’s Ecovio plastic is made up of 45% PLA from NatureWorks. Ecovio can be used to produce flexible films from which biodegradable carrier bags or other packaging can be made. Food packaging for products such as yogurt can be produced if other components are added to Ecovio. Biodegradable plastics are completely degraded within a few weeks under composting conditions and may be produced from either petrochemical or renewable raw materials. The other component is the existing biodegradable plastic Ecoflex, which is derived from petrochemicals.
Huhtamaki Oyj, an international producer of consumption products packages, introduced BioWare products – disposals and one-use packages fabricated from renewable and compostable materials – as first on the market. BioWare is Huhtamaki Oyj’s range of biodegradable products. BioWare fresh produce containers (drink cups and salad trays) are made from NatureWorks corn starch biopolymers. The technology of NatureWorks PLA is based on the transformation of starch into natural sugars, with subsequent fermentation and separation. After composting, the products are transformed into water, carbon dioxide, and organic materials. They can be processed on standard technological lines.
Europackaging in the UK produces film bread bags using PLA, which allow steam to escape, enabling bakers to package hot items. Stanelco, also based in the UK, produces a natural, biodegradable food packaging named Starpol 2000, which is based on starch.
Treofan uses PLA to create Biophan, a packaging film with exceptional gloss, transparency, printability, and sealing properties. The company plans to market Biophan to the food, cosmetics, and office materials industries. Biophan is currently used for food packaging in the EU and for bottle labels in the US. When disposed of in an industrial composting plant, Biophan fully decomposes into carbon dioxide and water within 45 days.
Cellulose-Based Packaging Materials
Innovia Films produces the NatureFlex NVS line of films, which can be used for packaging of fresh foods. In addition to being biodegradable, the film is also compostable. It provides improved dimensional stability under chill conditions.
This high-gloss film with enhanced transparency has inherent anti-static properties and is semipermeable to moisture, providing good anti-mist properties. NatureFlex films (Figure 1.7) are made from cellulose, which is derived from wood pulp from plantations operating under good forestry management standards.
NatureFlex films also perform well on the packing line and have a wide heat sealing range – values from 70 °C to 200 °C are claimed. This means that the packaging film can be used on faster processing lines with no loss of seal performance.
NatureFlex films are stiffer and more oriented than some other biopolymers, which makes them suitable for use on standard flow-wrap and form-fill-seal equipment, the company claims. They are also static-free for easier handling.
Innovia is able to add specially formulated biodegradable coatings to the film, giving processors a choice of packaging with varying levels of thickness to meet the demands of different foodstuffs. The film is certified to meet both EU and US standards for compostable packaging.
The film is first printed with the compostable logo and reference numbers before being micro-perforated, in order to tailor gas permeability to the products’ requirements. The film can be used to flow-wrap a wide range of own-brand organic fruit and vegetables. NatureFlex NVS is currently available in 23 and 30 micron thicknesses.
Innovia Films has launched a metalized biodegradable film for the confectionery business. As the cost of plastics increases, biodegradable and recycled packaging may become viable alternatives for more food companies. NatureFlex NM is a unique cellulose-based film, made from renewable wood pulp and metalized in-house. The film has an excellent luster and sheen for pack presentation and can be printed with solvent, water-based, and UV inks. It is also static-free, even in contact with machinery or sugar dust. This anti-static performance, combined with the inherent stiffness of the film, also gives enhanced bag-fill.
Scientists in Montpellier, France have conducted research showing that biodegradable packaging material made from cellulose paper impregnated with wheat gluten boosts the shelf life of cultivated mushrooms. The packaging is biodegradable, gas-selective, and permeable, and it allows mushrooms to be stored at 20°C for four days. Synthetic films currently used to package mushrooms in trays do not solve the problems of moisture and carbon dioxide sensitivity. The poor permeability of synthetic films to water vapor causes condensation and the appearance of brown marks on the mushrooms. The use of paper as a substrate considerably improves the mechanical properties of the wheat gluten film while still ensuring its biodegradability.
Bioceta is a transparent granulate processed at 170°C, and modified by the addition of high amounts of liquid plant-based plasticizers to improve the complete biodegradability of acetylcellulose. Its forming can be accomplished by injection, pressing, or – in the case of film production – calendering or film blowing.
Pullulan-Based Packaging Materials
Pullulan is a water-soluble polymer composed primarily of maltotriose units linked in α-1,6 fashion. It is produced as an extracellular secondary metabolite of some fungi and was first commercialized in Japan as a food source due to its natural origin. Pullulan has also been accepted as a coating material for foods, providing transparent films of low oxygen permeability.
To obtain a film, a 1-20% aqueous solution of pullulan can be cast on a metal plate roller or molded with heat and pressure, similar to starch, if a suitable amount of water is added as plasticizer. The challenge in biodegradable packaging materials is the development of completely biodegradable polymers for films or laminates with properties similar to those of synthetic polymers.
For food applications, pullulan can be used as a wrapping and as an edible film with limited oxygen permeability. PHBV can be used as an outside flexible cover with limited moisture permeability, and the addition of pullulan to PHBV may reduce oxygen permeability and improve product biodegradability by increasing the PHBV surface after the solution of pullulan in water.
Other polysaccharides from cultivated plants, such as conjac flour from the perennial herb Amorphophallus grown in Asia, are also being studied for film and coating production. The flour comprises around 1.6 mannose units to 1 glucose unit with β-1,4 linkages and randomly distributed acetyl groups. Conjac flour can be mixed with aqueous glycerol solution and calendred into a film, and treatment with potassium or sodium carbonate can modify the film characteristics.
ADM and Metabolix, a biotechnology company that has developed a form of PHA, intend to begin production of what they call a new generation of high-performance natural plastics. PHAs are polymers that are synthesized in the bodies of bacteria fed with glucose in a fermentation plant, producing compostable and biodegradable plastics that are durable during use. ADM’s plant will produce PHA natural plastics that have a wide variety of applications in products currently made from petrochemical plastics, including coated paper, film, and molded goods.
Other Bio-Packaging Solutions
Zip-Pak intends to feature a new line of biodegradable zippers to allow film and bag converters to produce packaging that meets requirements while maintaining the convenience of a press-to-close format for easy storage. Brand owners can now specify zippered degradable packaging for food products such as produce, nuts, breads, and cheeses.
The packaging offered by RPC has been produced from PHAs – polyhydroxyalkanoates, polymers made from organic sugars and oils that break down in soil, composting installations, waste treatment processes, river water, and marine environments. The only products generated during decomposition are carbon dioxide and water. This means that, because these are the materials used to make the material, the life cycle is effectively a closed loop. The resulting moldings have, to date, proved to be far more heat-stable than the more familiar biodegradable polymer PLA, which, according to RPC, proves PHA’s suitability for the cosmetics packaging market. The successful application of PHAs indicates that fully biodegradable cosmetics packaging can be a reality.
An example of a commercial biodegradable polyester is Biopol – a copolymer of hydroxybutyric and hydroxyvaleric acids, achieved during fermentation of sugars from sugar beet in the presence of bacteria that transform glucose into polymer. Biopol is one of the polymers with an ideal biodegradability profile, decomposing into carbon dioxide and water. Because of its stiff nature, it is useful for bottles and canisters.
Partially Biodegradable Packaging Materials
In addition to completely biodegradable polymers, partly biodegradable polymers, such as mixtures of synthetic polymers with added starch, are also presented. The disadvantage of these materials is that only the starch is biodegradable and the rest is dissipated in the environment. The degradation of the synthetic film can be accelerated by means of starch used as filling.
Low Density Polyethylene (LDPE) blends containing up to 10% maize starch have been produced by conventional techniques and the end products were used in bags for shopping or rubbish .
A challenge in the field of biodegradable packaging development is the combination of truly biodegradable polymers into blends of films or laminate films with qualities as good as in synthetic laminates. For instance, for food use, products may be coated with pullulan, an edible material of very low oxygen permeability and utilized as an outer packaging of high flexibility, thus acting as a moisture barrier.
Pullulan film can be produced because it and the blending polymer can both undergo the same processing when melted under controlled moisture conditions. Pullulan as an additive reduces oxygen permeability. Some biopolymers based on polysaccharides are applied as coating materials or packaging films. They include starch, pullulan, and chitosan.
The Danish-based Danisco produces an additive from hardened castor oil and acetic acid. It is colorless, odorless, and completely biodegradable.
The Toronto-based Diamant originally marketed its polystyrene-based stretch film as a non-plasticized food wrap that was eco-friendly and recyclable. The company developed the technology over the course of ten years as an alternative to PVC. Bioplastics were formed to develop and acquire new products and technologies for the manufacturing of totally biodegradable products.
Diamant uses EPI Environmental Technologies’ TDPA (Totally Degradable Plastic Additive) product for manufacturing the pallet wrap. When incorporated into commodity plastic resins, such as polypropylene (PP), polyethylene (PE) and polystyrene (PS), the TDPA additives render the plastics degradable and ultimately biodegradable. This product will degrade and ultimately biodegrade, and once biodegrading is complete all that remains is carbon dioxide, water, and biomass, all which are part of the normal biocycle.
The Californian-based company Cereplast claims that its biopropylene resin is an industry first and could replace traditional polypropylene in the vast majority of applications. These bio-resins replace a significant portion of petroleum-based additives with materials such as starches from tapioca, corn, wheat, and potatoes, meeting the demand from consumers and manufacturers for sustainable plastics.
Also notable is Cereplast’s already existing line in compostable resins in which bio-based starch products, such as corn, wheat, and potato starches, replace nearly 100 percent of petroleum-based additives. This results in products that will compost in commercial facilities within 180 days, leaving no chemical residue.
Cereplast Compostables® resins are renewable, ecologically sound substitutes for petroleum-based plastic products, replacing nearly 100% of the petroleum-based additives used in traditional plastics (Figure 1.8). Cereplast Compostables® resins are starch-based, made from corn, wheat, tapioca, and potato starches that come primarily from the Midwest.
Resin manufacturing begins once the Cereplast production team has selected the right biopolymer matrix made from renewable, cost-stable resources. These biopolymers include poly(lactic acid) (PLA), soy proteins, PHA, PHBs, or starch from corn, wheat, or potatoes. The selected biopolymer is blended with other biodegradable components to reinforce its molecular structure through a proprietary process developed by Cereplast.
The blend is then polymerized and treated with nano-composites for surface optimization and further reinforcement. The entire green composite process is high-speed and low-cost. The final product is then packaged and shipped to converters, which are able to process the resin using traditional equipment. Cereplast manufactures ten grades of resins for various applications: injection-molding, extrusion, and more.
Cereplast proprietary resins are a cost-competitive alternative to traditional fossil-fuel-based resins. Cereplast can be substituted in a wide range of applications for manufacturing. There are currently 12 resin formulations available to manufacturers. All Cereplast resins are made from renewable resources and are certified as 100% compostable.
Cereplast Hybrid Resins TM products are made from bio-based materials, replacing 50% or more of the petroleum content in traditional plastic products with starches from corn, tapioca, wheat, and potatoes. These hybrid resins can be processed with the same cycle times as traditional plastics, but require less energy during production due to significantly lower machine temperatures. The first product in the Cereplast Hybrid Resins TM family, Biopropylene 50 TM (CP-BIO-PP-50), is a 50% petroleum and 50% starch-based resin with physical characteristics similar to traditional polypropylene. Biopropylene TM resin can be used in a variety of manufacturing processes, including injection molding, thermoforming, profile extrusion, and extrusion blow molding.
The Harvest Collection TM from Genpak (USA) is a new line of compostable plates, food containers, and cups made from annually renewable resources such as corn, rice, and wheat. These products will completely compost and biodegrade in a commercially run composting facility. The compostable products in the Harvest Collection TM line have the same look and feel as traditional plastic items but are made from renewable resources. The Harvest Collection includes a wide range of biodegradable products for food service applications, including food containers, cups, and dinnerware.
Starch mixed with polyethylene is also known as hydrodegradable material and meets both American Standards for Testing Materials and European norms for compostability, degrading in 60% under 180 days.
Polymers that utilize starch include polycaprolactone (PCL), polyvinyl alcohol (PVA), and poly(lactic acid) (PLA). These are designed to regulate microbial breakdown and only break down in the presence of microbes, heat, moisture, and proper aeration, conditions typically found in traditional compost piles.
Additive-based plastic bags, on the other hand, are traditional plastic bag films with a special chemical adjustment to facilitate faster breakdown under certain conditions. In the initial stages of decomposition, oxygen, light, heat, and/or stress react on a molecular level, fragmenting the film so that water can wet and surround its molecules, thereby making it biodegradable. Once microorganisms begin to decompose the material, it breaks down into CO2, water, and biomass.
These oxo-degradable/photodegradable plastic bags may simplify the process without causing significant changes to the consumer’s lifestyle. They are also less expensive. Nowadays, industrial processing of partly biodegradable polymers containing starch includes Mater-Bi by Novamont, Bioplast by Biotec, Greenpol and Eslon Green by Yukong LTD and Cheil Synthetics Int., Starpol 2000 by the UK-based Stanelco, Cargill’s NatureWorks, and many others.
Protective Loose-Fill Foams
As an alternative to popular polymer foams, such as expanded polystyrene (EPS), loose-fill foamed chips made from thermoplastic starch have become a successful application of starch-based materials in cushion packaging. These biodegradable loose-fill foams have competed well with EPS-based products, despite costing about 30% more, at approximately $21 per cubic meter delivered. Extruded foams based on starch and blends of starch with various additives have been patented and are commercially available. Extensive research has been done on the influence of extrusion conditions, moisture content, and composition on the physical properties of starch-based foams.
Generally, extruded starch foams are water-soluble and sensitive to moisture content, with modifi ed high-amylase starches producing the greatest expansion and lowest densities. Synthetic polymers, such as poly(vinyl alcohol) or polycaprolactone, have been blended with unmodified starches to produce foams with lower densities and increased water resistance.
While starch-based loose-fill products have advantages in biodegradability and environmental protection, they have been criticized for imperfections compared to EPS loose-fill products. 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 that creates an open cellular structure. Starch-based foam loose-fill is very hygroscopic, and foam densities increase significantly after conditioning at high humidity.
The compressive stresses of most starch-based foams do not differ significantly from those of EPS products. Chemically modified starches produce foams with good retention of compressive stress over a broad humidity range. The resiliencies of starch-based foams, with values between 69.5 and 71.2%, are about 10% lower on a relative basis than those of EPS foams. However, these products retain between 62 and 67% resiliency, even after conditioning at high humidity.
Fragmentation of both starch- and EPS-based foam amounts to 2 to 6 wt%, but starch-based foams break down into fine dust, whereas EPS-based foams break into large fragments.
All starch-based foams have significantly higher foam and bulk densities and open-cell and moisture contents than EPS-based foams. Both product types have similar compressive stresses, resiliencies, and friabilities.
Starch-based foams are more sensitive to changes in relative humidity and temperature than EPS-based foams, but the larger amounts of absorbed moisture do not compromise mechanical integrity. 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. Starch-based foams can be prepared from different starch sources, with 70% of the polystyrene being replaceable with biopolymer starch.
Functional starch-based plastic foams can be prepared from different starch sources, depending on their availability. Starch-based foams with polymer addition (e.g., PS, PM, PHEE) show improved properties in comparison with 100% starch foams. The addition of polymers significantly increases radial expansion and gives low-density foams.
Compressive strength depends primarily on foam density, and not on starch type or polymer structure. Friability is reduced when polymer is present in the foam. PLA/starch foams can be successfully prepared by using water as a blowing agent in the presence of talc, which acts as an effective nucleation agent.
Water is a good blowing agent for the PLA/starch system. Talc at 2% gives the PLA/starch foam a fine foam cell size and uniform cell size distribution. The addition of Mater-Bi® affects the foam expansion characteristics. High levels of MBI result in low radial expansions and high densities.
The resiliency improves as the levels of MBI and moisture contents are increased. The MBI-starch foams have the potential to be used as an environmentally friendly loose-fill packaging material.