Cellulose acetate (CA)
2.5-5 usd/kgCellulose acetate (CA) is a family of bio-based semi-synthetic plastics, some of which are biodegradable and home compostable at the end of their useful life. To be certified compostable, the product must break down into water, CO2, and biomass at a rate consistent with other biomaterials. Also, there must be no negative chemical effects on the final compost, which is ensured through eco-toxicity testing. Thermoplastic types are recyclable.
Cellulose is an abundant renewable material, found in plants, algae and some bacteria. The raw materials used for industrial cellulose acetate production, such as wood and cotton, are converted into useable monomer through a process of esterification. The pulp is processed with acetic acid and acetic anhydride in the presence of sulphuric acid to produce acetate flake from which products are made. All manner of cellulose containing materials may be used in the production of these bio-based plastics, including bamboo, eucalyptus and bacteria. While the choice can affect the efficiency of the process and overall sustainability, it will not affect the properties – these are determined more by the manufacturing process.
The disadvantage of cellulose acetates, is they need plasticiser to be useful. This additive is mixed with the polymer in order to make it more flexible, durable, and processable. A balance must be struck, because these advantages come at the cost of strength and hardness. In the past, phthalate esters were the most common. These are being phased out, especially in toys and food packaging, and replaced with alternatives due to the health risk associated with their use – they are considered harmful and carcinogenic to humans.
The type of plastic depends on the degree of substitution of hydroxyl groups with acetyl types. During the manufacture of triacetate (TAC) the cellulose is completely acetylated (>92%), whereas in regular cellulose acetate or cellulose diacetate, it is only partially acetylated. Partially acetylated types are used in fibre, textiles, filters, films, drug delivery, membranes and moulded products. Triacetate, used in film and fibre, is significantly more heat resistant than acetate and diacetate types – with increasing acetyl content, the permeability to gas and moisture decreases whereas the chemical resistance, heat resistance and stiffness increases.
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Cellulose acetate (CA) film, also called cellophane, is predominantly used in packaging for its excellent clarity combined with high rate of water vapour and gas transmission. This combination is good for packaging fruit and vegetables, and freshly baked bread – condensation (fogging) doesn’t form like with impermeable materials. It is combined with plasticiser to reduce brittleness and improve transparency, and water vapour transmission may be prevented with coatings. As a natural barrier to water, oils, grease and bacteria, it is also used in packaging things like cheese, tea, coffee and confectionary. It is the clear, crinkly material, such as used in sweet bags, cheese wrapping and cut flower decoration.
To a lesser extent now, but still occasionally, cellulose films are used in many other applications, such as photographic film, polarising films and stationary.
Companies seeking renewable packaging materials are replacing films like polyethylene terephthalate (PET), polyester, and polypropylene (PP), with cellulose. A major advantage of cellulose film is that it can be biodegradable, and home compostable, at the end of its useful life. While thermoplastics such as PET and PP are fully recyclable, flexible films from these materials cannot be recycled into materials with guaranteed suitable barrier properties for lightweight packaging. Therefore, compostable cellulose may offer some sustainable advantages.