How to… use plant-based ingredients to make polymers

Materials World magazine
10 May 2019

Gaia Biomaterials UK Packaging Technologist, Donald Beaton, discusses how to get the best results from plant-based ingredients during polymer production. 

Biodegradable and compostable plastics must be of high quality and performance in order to compete in the market. But switching out fossil-derived ingredients for plant-based ones can present the risk of product failure if not properly managed.

Gaia BioMaterials AB Sweden produces biodegradable biomaterials for use across a wide range of industries. As the company’s end-products include medical and food contact applications, the ingredients must be safe and reliable, as well as eco-friendly.

To achieve this, Gaia BioMaterials grows plant material as feedstock. Calcium carbonate is added to the plants to increase their pH interval to counteract acidification and control phosphorus levels, and raise their growth potential. The calcium carbonate also contains trace minerals, like silica, that add to the plants’ nutrition and mechanical strength, and provide resistance against fungal diseases. All of this boosts the yield of the crops.

The challenge

The use of calcium carbonate presents a problem. While calcium is a natural stabiliser it is also highly absorbent and has sharp-edged molecules, making it abrasive. In applications for polymer making, the mineral is often avoided because of its corrosiveness, which makes it an unpopular choice for use in injection and rotational moulding machines.

However, both of these challenges can be resolved by introducing vegetable oil to the mineral. Vegetable oil, among other materials, can be mixed with the salt to create a smoother structure and a polymer pellet that is more beneficial in manufacturing plastics, particularly with injection or rotational moulding.

The process

The calcium is first ground into a powder to reduce its sharp edges as much as possible. Next, this fine powder is combined with vegetable oil and extruded into a granular form. The oil coats the grains, removing their water absorbency capabilities during the binding process. The oil also acts as a lubricant to reduce the abrasiveness of the salt when in a moving machine.

This is then extruded with a mix of a biodegradable long-chain ester and sugar starch, which assists the chemical reaction forming pellets. The ester helps long chains form, enabling the salt-based material to reach pellets of 5-10mm in size. This is the last step in the primary preparation, as these pellets can be melted to make polymer products.

The compound has good barrier properties for oil and fats, and combining these four ingredients in different percentage ratios leads to a material that can either breathe and allow vapour to pass through, or act as an impermaeable vapour barrier.

The long-chain molecules of the ester, combined with the small molecule of the calcium, give a very good barrier that is superior to polyethylene.

Mixing the four ingredients in this way enables it to be produced for processing in film blowing, bottle blowing, thermoforming and injection moulding. All seeds used to grow the plants are GMO-free, ensuring all end-products are safe for food contact purposes.

Environmental credentials

Chemical company BASF is the ester supplier and has been working to eliminate the fossil element of the ester ingredient. This has been gradually reduced and is being replaced with a plant-based ester. Through volume and development it is anticipated the ester will be fossil-free within three years.

An independent lifecycle assessment by the European Union’s LIFE climate change funding programme is being done on the material at the moment. Early research has shown the material is carbon neutral.

Testing carried out by TUV AUSTRIA has shown the product turns to compost via microorganisms, leaving no nano or microplastics. It will degrade by 90% in 120 days to achieve the OK to compost certification. Food contact certificates have been supplied by Normpack for films, trays and bottles.