Flax Fibre Reinforcement in Composites

Discussing the advantages and disadvantages of flax fibre as a reinforcement in composites, suitable applications and practical considerations when working with flax.

In this video we take a closer look at flax fibre and its use in composites. I discuss what flax fibre is and what its advantages and disadvantages are, as well as how to best work with this material, a demonstration of which is provided with the resin infusion of a flax reinforced chair moulding.

Introduction to Flax Fibre

What is Flax Fibre?

Flax is a natural fibre that comes from the stem of the flax or linseed plant, which is grown and harvested specifically for its fibre. Flax fabric, commonly known as linen, has been produced for thousands of years and is in fact the oldest known textile.

What makes this fibre viable as a reinforcement material for us in composites is its long, continuous, cellulose fibres which – for a natural fibre - have excellent tensile strength and stiffness. Although there are other types of natural fibre, such as hemp and jute that can be used, flax generally offers the best mechanical properties of these materials which is why it is the most widely used natural reinforcement in composites.

Advantages and Disadvantages of Flax as a Reinforcement

Sustainable Production

One of the main reasons that flax is increasingly considered for use in composites is the growing emphasis on using more sustainable and environmentally-friendly materials throughout industry. Flax fibre requires between 2 and 5 times less energy to produce per unit weight than glass fibre, and, in fact, carbon emissions from its production are often claimed to be negative, owing to the carbon capture during its growth, so - in markets where customers appreciate or maybe even demand more environmentally conscious products - like outdoor sports and recreation equipment or home goods – flax reinforcement can be a desirable option.

Natural Appearance

When used to produce clear-coated or unpainted laminates, flax has a unique natural appearance. In its unidirectional format, laminated flax fibre looks very similar to wood grain, whereas in its woven format it has a dark brown woven textile appearance, quite different from that of conventional composite reinforcements, such as carbon or glass.

It is worth noting that when flax fibre is wetted out with resin, the colour changes to become considerably darker (as can be seen in the cured laminate sample images on the individual product listings).

Performance of Flax Fibre Compared to Glass Fibre

Flax fibre as a composite reinforcement is not without its compromises. Compared to conventional reinforcements such as glass or carbon fibre, flax does have considerably lower mechanical performance. Weight-for-weight, glass fibre is about four times stronger than flax and carbon fibre is about five times stronger. Stiffness too is similarly compromised.

Inevitably, this reduced mechanical performance does mean that at least some of the environmental benefit from the more sustainable production is negatively offset by the increased resources required to make and use a heavier component meaning that there is certainly a balance to be considered.

How to Work with Flax Fibre

Flax fibre can be used as a composite reinforcement is much the same way as more conventional reinforcements such as glass fibre and carbon fibre, however there are some differences in how the material behaves and is best processed.

Suitable Laminating Methods

Flax fibre can be used on its own or alongside other reinforcements in any laminating method, including hand layup, vacuum bagged hand layup, compression moulding, resin infusion and RTM. Flax reinforcement is also available (in limited formats) as a prepreg reinforcement for oven/autoclave cure.

In the case of open mould hand layup, the ‘springiness’ of flax fibre makes it difficult to laminate flax into more complex mould contours without the reinforcement bridging in corners and also results in quite a high resin uptake as the fibre ‘swells’ resulting in a higher resin content and reduced performance. For this reason, hand layup laminates are best vacuum bagged after lamination to ensure good consolidation and mould contact, and improved fibre fraction.

Suitable Resin Systems

Flax can be used with any conventional composites resin system, including epoxy, polyester and vinylester. Although polyurethane resins are rarely used as a laminating resin matrix, polyurethane resins should be avoided due to their moisture sensitivity.

High bio-based laminating epoxy or bio-based infusion epoxy make obvious choices when working with flax reinforcement due to their reduced environment footprint.

Thickness and Resin Uptake

Flax fibre makes for a particularly low density reinforcement and has a high resin uptake meaning that laminates will be considerably thicker, for a given weight of reinforcement, than would be expected for carbon or glass fibre.

In an open layup without any vacuum bag or press consolidation, flax will tend to swell and consume unnecessary resin meaning that to avoid excessive resin uptake, some form of positive pressure consolidation is highly preferable.

Typical fibre-to-resin ratio in a resin infused woven flax laminate will be 33% flax /66% resin (1:2 fibre ratio).

Typical resin infused laminate thickness for the various weights of woven flax are as follows:

200gsm = 0.44mm
300gsm = 0.65mm
550gsm = 1.2mm

Moisture Content

One consideration when working with flax is its moisture content. Like other natural materials, such as wood, flax fibre will absorb moisture from the air, increasing its moisture content. High moisture content in the fibre can affect its mechanical performance, react with the resin matrix and also cause off-gassing (boiling off) of the moisture under vacuum.

Where flax has been or may have been exposed to relatively high humidity, its moisture content can be reduced by oven drying or alternatively, when processing by resin infusion, a vacuum can be pulled and maintained on the dry laminate for several hours before infusion in order to boil off moisture in the reinforcement.

Resin Infusion

Vacuum resin infusion is one of the most practical and effective ways to manufacture flax fibre mouldings. The advantages of resin infusion when working with flax are good mould conformity, good consolidation (leading to higher fibre fraction) and the ability to vacuum dry the flax before infusion (see Moisture Content above).

Due to the excellent resin flow characteristics, woven flax can be infused without the need for a flow media (infusion mesh), reducing the consumables used in the process. The same would not be true for unidirectional reinforcement where - much like undirectional glass or carbon - resin flow is more restricted by the closely packed fibres, and therefore would still require an infusion mesh.

Available Formats of Flax Fibre

Easy Composites are the exclusive distributors of Eco-Technilin flax fibre reinforcement products in the UK and also sell these materials throughout the EU from our warehouse in the Netherlands and also worldwide through our export sales from the UK. Eco-Technilin are Europe’s largest manufacture of flax reinforcement products with a wide range of flax reinforcement formats available, including unidirectional, woven, non-woven, and even prepreg reinforcements available.

Unidirectional Tapes

These 100% unidirectional flax fibre reinforcements have no weaving or cross stitch and rely only on a lightweight binder to stabilise them. Visually, this results in a pure and simple appearance which is very similar to woodgrain, but does result in a more delicate reinforcement which is better suited to flat laminates. This type of unidirectional reinforcement is supplied in 400mm wide tapes and is available in 50, 110 and 200gsm weights.

Cross-Stitched Unidirectional Tape

In addition to the unsupported undirectional tapes, flax reinforcement is also available in a cross-stitched undirectional format which uses a light woven cross stitch to stabilise the fibre into a more drapable and easy-to-handle material. This cross-stitched UD reinforcement is available in 1000mm width at 180gsm weight.

Woven Flax Fibre

One of the most conventional formats for flax fibre reinforcement is a woven cloth. Woven flax works just like any other woven reinforcement, it is easy to handle and offers a good degree of drapeability.

Prepreg Flax

FLAXPREG is an uncured flax reinforced prepreg, meaning that the epoxy resin matrix has already been impregnated into the flax fibre, much like any other type of prepreg reinforcement. FLAXPREG is currently only available in 400mm width, 110gsm weight unidirectional format.

Non-Woven Flax Fibre Mat

These non-woven mats have a randomly oriented fibre structure resulting in a felt-like mat that is best suited to compression moulding. This type of non-woven flax mat is already used in volume production for non-structural components like automotive door-liners and parcel shelves.