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Moulded pulp
1-1.5 usd/kg
Moulded pulp uses the same wood-based raw materials as paper production, plus a range of plants fibres such as bagasse (sugarcane pulp), bamboo, reed, wheat straw and palm. Many types of agricultural by-products are suitable, as well as waste from other types of raw material production. Also recycled material, such as old corrugated container (OCC). The type of fibre often depends on geography, with seasonal and locally available fibres being preferable due to the relatively lower price. These additional fibres help to improve the mechanical properties and have cost advantages over traditional wood pulp.
Moulded pulp is produced using various techniques, which will affect the end result. The simplest plain moulding technique, known as one-cast, uses a wire mesh for the mould. The pulp is drawn onto the mesh and the water sucked out to produce relatively thick walled parts (6-8 mm). This technique makes use of recycled paper and card. It is used for industrial and lower grade, relatively heavy packaging.
Transfer moulding adds an additional step in the process, also called after-pressed, whereby the moulded pulp is compressed on the inside to produce a smooth surface and thinner wall section (3-5 mm). The superior surface properties and strength are utilised for a range of electronic, household and product packaging. Things like egg cartons and medical products (urinals) are produced in large volumes and at high speed with this process. After transfer moulding, it is possible to further enhance the smoothness of the surface, add debossed logos and other features, by hot pressing.
Thermoforming produces thin walled parts between 0.5-1.5 mm. It is a highly automated process unlike traditional wet pulp forming. Once the water has been removed the pulp is hot pressed to produce complex parts with smooth faces in as little as 60 seconds. It is typically used to make take out food clamshells, drinks cups, salad bowls, pasta bowls, and high end inserts for cosmetics, healthcare and electronic packaging. There are many variations on this technology and it is rapidly becoming more efficient, with cycle times well below 30 seconds, in an effort to make it cheaper.
Sustainability concerns
Raw material generates polluting by-products
Potentially toxic in use
Forever chemicals
Like paper, moulded pulp uses renewable and potentially very sustainable raw materials, which are biodegradable and compostable end of life. However, a lot depends on production and the consumption of chemicals, water and energy. As well as the pulp liquor and associated impacts, several different additives and additions are used, such as to make the process more efficient (like increasing wet strength), enhance the properties of the moulded pulp (moisture resistance, printability and so on), and create a barrier layer on the surface. While it is entirely possible to make safe, non-toxic and compostable pulp, this depends on selection of additives and coatings.
The most concerning are fluorocarbon additions such as polyfluoroalkyl substances (PFAS), which are widely used with pulp to provide a cheap solution to achieve oil and water barrier properties. Also known as ‘forever chemicals’, these hazardous ingredients are found in many industrial materials and applications, and do not degrade or break down – they are shown to be extremely persistent in both the environment and in the human body. In fact, it can already be found in the blood of people and animals all over the world, and small amounts in some food products. There are many different types of PFAS and the most commonly studied are perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) – also known as C8, for their eight-carbon chain structure. These two in particular, with their long chains, are considered so dangerous that production has now been largely phased out in the US and there is a trend among global manufacturers to replace them with shorter chain PFAS or non-PFAS products. The eye-opening documentary, The Devil We Know, looks at allegations of health hazards from these chemicals, in particular around the Dupont Teflon manufacturing facility in West Virginia, US.
The need for PFAS can be eliminated through fibre development, or it can be replaced with natural polymer additives and waxes. Examples include polylactic acid (PLA), nanocellulose, lignin (already naturally present in plant fibre) and alkyl ketene dimer (AKD).
AKD is a common commercial chemical used as a hydrophobic sizing agent in paper and pulp. Sizing is used to reduce a paper’s tendency to absorb water when dry. This helps to make printing and painting more efficient, because the colour remains on the surface and is not absorbed. AKD also helps to make moulded pulp water resistant.
Containers that are exposed to heat and moisture – such beverage cups and ready meal packaging – may require a thin plastic barrier layer. Regular oil-based plastics such as polypropylene (PP), polyethylene (PE) and polyethylene terephthalate (PET), polyester, are commonly used. Of course, these make it impossible to recycle the pulp product along with other paper based products. Alternatives include non-toxic bio-based polymers, such as starch-based and PLA films. There is an obvious conflict between biodegradability and the effective storage of liquids and foodstuffs. However, grades of PLA exist that can tolerate oil and water at 100 degC, and still remain compostable end of life. This is achieved by balancing the water-vulnerable chemistry of the biodegradable plastic with film thickness.
