Daniel Leiter
Information
NFCs (Natural Fiber Composites) are a matrix of natural fibers and a polymer, which can be of both natural and petrochemical origin.
Natural Fibers
Natural fibers have been used as reinforcements in composites for several decades, as they – especially bast fibers – offer exceptional mechanical properties and ecological advantages. Hemp, for example, can be harvested after 2 to 5 months and binds approximately 9 to 15 tons of CO2 per hectare, equivalent to a young forest.1 The fibers are largely CO2-negative.
Hemp requires little water and pesticides and can be cultivated on every continent except the Arctic, favouring local fibre production.2 World production of flax is almost four times greater than total hemp production, as hemp cultivation is still banned in many countries because authorities do not distinguish between drug hemp and low-THC fiber hemp.3
From hemp straw 24% of technical (long) fibers can be extracted, which can be used to make non-wovens (commonly used for impact insulation) or technical reinforcement fabrics, which are usually embedded in conventional, synthetic resins and used as a substitute for glass fibers. 55% of hemp straw is shives, which can be used for sustainable insulation (e.g. bulk insulation or hempcrete) or for the production of particleboards.4 Short fibers can be used for papermaking or can be pressed to fiberboard.
Polymers
Synthetic resins and adhesives are widely used in various industries, including the wood composite industry. Synthetic adhesives used in plywood, particleboard and fiberboard contain and emit formaldehyde, which is a proven carcinogen.5 The most common natural polymers are polysaccharides like cellulose or starch; tannin, which can be extracted from selected barks, leaves and fruits; lignin, which is an abundant material and by-product of e.g. the paper industry and has mostly been burned without further use; and proteins.
The utilization of proteins in plastics and adhesives has a long history.
Pre-industrialization, besides wood, natural polymers such as horn, tortoise shell, amber and shellac, all of which are malleable when heated, were used to produce goods like combs or billard balls. The industrialisation of production and the ever-increasing demand for affordable goods has gradually pushed some elephant and tortoise species to the brink of extinction. This has incentivised scientists to research disposable materials based on abundant resources. One of the first semi-synthetic plastics discovered was Galalith, produced by mixing the animal protein casein with formaldehyde. 6
By denaturing casein in an alkaline solution, casein wood glue can be produced, which was used to produce plywood in the early 1900s. Even airplanes were made from casein plywood.7 Casein was also widely used to bind pigments - casein tempera - and was a common painting technique on canvas before the advent of oil paint.
There are already a number of recent design projects using casein wood glue to produce fiberboard or particleboard. One of these projects focuses on the use of waste milk - in Germany alone 2 MIO liters of milk are discarded each year, which can be a source of casein protein.8 Each litre of milk can yield around 29.5g of casein, which equates to around 150,000 litres of glue.
However, casein is an animal dairy product that has a significant carbon footprint unless derived from waste. In the inter-war period, the plywood industry began to use plant protein glues, which are very similar to casein and can achieve similar properties, although moisture resistance and bonding strength are said to be lower.9
The adhesive used was based on soy protein derived from soy flour. Soy flour is also an abundant material and a by-product of soy oil production, but it is mostly imported to Austria, which is why local researchers are also considering wheat, maize, pea or potato protein.10 Protein glues have recently become one of the most researched adhesives due to their promising properties. Compared to lignin, for example, protein glues have the advantage that they can be cold as well as hot pressed and dry at room temperature. There is therefore no need for expensive hot-pressing equipment.
Basic formulations of vegetable protein glues and casein glues contain no petrochemical or toxic ingredients and are therefore biodegradable under natural conditions (additives in some formulations may be toxic or petrochemical). Current research is focusing on bio-based cross-linkers to make soy adhesives more moisture resistant while maintaining the absence of petrochemical additives; formulations to use unprocessed soy flour, as SPI (soy protein isolate) is currently too expensive for industry-wide application; and alternative locally available protein sources.11 This project aimed to establish a material strength benchmark for vegetable protein adhesives and used SPI, which has been shown to have superior bond strength compared to other vegetable protein sources.