The current global consumption of plastics is more than 200 million tons, with an annual growth of approximately 5%, which represents the largest field of application for crude oil. It emphasizes how dependent the plastic industry is on oil and consequently how the increase in crude oil and natural gas prices can have an economic influence on the plastic market. (It is becoming increasingly important to utilize the alternative raw materials. Until now petrochemical-based plastics such as polyethylene terephthalate (PET), polyvinylchloride (PVC), polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyamide (PA) have been increasingly used as packaging materials because their large availability at relatively low cost and because their good mechanical performance such as tensile and tear strength, good barrier to oxygen, carbon dioxide, anhydride and aroma compound, heat stability, and so on. But nowadays their use has to be restricted because they are not non-totally recyclable and/or biodegradable so they pose serious ecological problems. Plastic packaging materials are also often contaminated by foodstuff and biological substances, so recycling this material is impracticable and most of the time economically not convenient. As a consequence, several thousands of tons of goods, made of plastic materials, are landfilled, increasing every year the problem of municipal waste disposal.
The growing environmental awareness imposes on packaging films and processes both user-friendly and eco-friendly attributes. As a consequence, biodegradability is not only a functional requirement but also an important environmental attribute. The compostability attribute is very important for biopolymer materials because while recycling is energy-expensive, composting allows disposal of the packages in the soil.
By biological degradation, it produced only water, carbon dioxide, and inorganic compounds without toxic residues.
According to the European Bioplastics, biopolymers made with manufactured renewable resources have to be biodegradable and especially compostable, so they can act as fertilizers and soil conditioners. Whereas plastics based on renewable resources do not necessarily have to be biodegradable or compostable, the second ones, the bioplastic materials, do not necessarily have to be based on renewable materials because the biodegradability is directly correlated to the chemical structure of the materials rather than the origin. In particular, the type of chemical bond defines
whether and at which time the microbes can biodegrade the material. Several synthetic polymers are biodegradable and compostable such as starch, cellulose, and lignin, which are naturally carbon-based polymers.
Biodegradable polymer applications in food packaging
The field of application of biodegradable polymer in food-contact articles includes disposable cutlery, drinking cups, salad cups, plates, overwrap and lamination film, straws, stirrers, lids and cups, plates and containers for food dispensed at delicatessen and fast-food establishments. In the last few years, polymers that can be obtained from renewable resources and that can be recycled and composted, have garnered increasing attention. Also, their optical, physical, and mechanical properties can be tailored through polymer architecture so as a consequence, biodegradable polymers can be compared to the other synthetic polymers used in the fresh food packaging field, like the most commonly oriented polystyrene (OPS) and polyethylene terephthalate (PET).
Polymers used in making biodegradable packaging are classified into two groups:
- Natural polymers (such as polysaccharides and proteins)
- Polymer composites such as poly-caprolactone (PLA) and polylactic acid (PCL)
Combined polymers are divided into three categories according to the origin of their production:
- Polymers produced from microorganisms
- Polymers derived from biotechnology
- Polymers produced from petroleum derivatives
Types of polymers used in making biodegradable packaging
Polylactic acid (PLA):
One of the most promising biopolymers is the poly (lactic acid) (PLA) obtained from the controlled depolymerization of the lactic acid monomer obtained from the fermentation of sugar feedstock, corn, etc. which are renewable resources readily biodegradable. It is produced by the conversion of corn, or other carbohydrate sources, into dextrose, followed by a fermentation into lactic acid. It is a versatile polymer, recyclable and compostable, with high transparency, high molecular weight, good processability, and water solubility resistance. In addition, the production of PLA has been reported to result in 15% to 60% lower carbon emissions and 25% to 55% lower energy consumption than petroleum-based polymers. In general, commercial PLA is a copolymer between poly (L-lactic acid) and poly (D-lactic acid).
Cellulose:
Cellulose is the most widely spread natural polymer and is derived by a delignification from wood pulp or cotton linters. It is a biodegradable polysaccharide that can be dissolved in a mixture of sodium hydroxide and carbon disulfide to obtain cellulose xanthate and then recast into an acid solution (sulfuric acid) to make a cellophane film. Alternatively, cellulose derivatives can be produced by derivatization of cellulose from the solvated state, via esterification or etherification of hydroxyl groups. Especially these cellulose derivatives were the subject of recent research. Cellulose esters like cellulose (di)acetate and cellulose (tri)acetate need the addition of additives to produce thermoplastic materials. Most of them can be processed by injection molding or extrusion. Cellulose ethers like hydroxypropyl cellulose and methyl cellulose are water-soluble, except for ethyl cellulose and benzyl cellulose. Most of these derivatives show excellent film-forming properties but are too expensive for bulk use.
Starch:
Starch is a widely available and easily biodegradable natural resource. To produce a plastic-like starch-based film, high water content or plasticizers (glycerol, sorbitol) are necessary. These plasticized materials (application of thermal and mechanical energy) are called thermoplastic starch (TPS) and constitute an alternative to polystyrene (PS). Starch-based thermoplastic materials (e.g. blends of TPS with synthetic/ biodegradable polymer components, like polycaprolactone, polyethylene-vinyl alcohol, or polyvinyl alcohol) have been successfully applied on an industrial level for foaming, film-blowing, injection molding, blow molding, and extrusion applications.
Commercial polymer coming from the synthesis of oil-based monomer can be mixed with different percentages (10, 50, and 90%) of starch used as an additive. Depending on starch percentage and other materials like additives (coloring additives, flame retardant additives) the properties of these materials can vary a lot, becoming stable to unstable for example in hot/cold water.
Polyhydroxyalkanoates (PHA):
These polymers are produced in nature by bacterial fermentation of sugar and lipids. They can be thermoplastic or elastomeric materials, with a melting point between 40 and 180°C, depending on the monomer used in the synthesis.
These polymers, alone or in combination with synthetic plastic or starch give excellent packaging films. The most common type is polyhydroxy butyrate (PHB), coming from the polymerization of 3-hydroxybutyrate monomer, with properties similar to PP but stiffer and brittle. The copolymer polyhydroxybutyrate-valerate (PHBV), used as packaging material, is less stiff and tougher. The price is very high but it degrades in 5e6 weeks in a microbiology-active environment, giving carbon dioxide and water in aerobic conditions. In an anaerobic environment the in aerobic condition. In an anaerobic environment, the degradation is faster, with the production of methane.
References:
Siracusa V. ,Rocculi P., Romani S., and Rosa M (2008). Biodegradable Polymers for food Packaging: a review.Food Science & Technology 19: 634-643. https://doi.org/10.1016/j.tifs.2008.07.003
Kumar P., Sandeep K.P., Alavi S.,Truong V.D., Gorga G.E. (2010). Preparation and characterization of bio-nanocomposite films based on soy protein isolate and montmorillonite using melt extrusion. Journal of Food Engineering 100: 480–489. https://doi.org/10.1016/j.jfoodeng.2010.04.035
Intan S. M. A, Tawakkal, Marlene J, Cran, Joseph Miltz, and Stephen, Bigger W. 2014. A review of poly (Lactic Acid)-based materials for antimicrobial packaging. Journal of Food Science, 79: 1477-1490. https://doi.org/10.1111/1750-3841.12534
Peelman N, Ragaert P, De Meulenaer B, Adons D, Peeters R, Cardon L, Van Impe F, Devlieghere F. 2013. Application of bioplastic for food packaging. Trends in Food Science & Technology, 32:128-141. https://doi.org/10.1016/j.tifs.2013.06.003
