Natural Materials in Composites

Natural materials are used in a variety of composites. Plant fibers like flax, sisal, hemp, jute, ramie and cotton contain cellulose which is a natural polymer with high strength and stiffness, made from long fibrous cells that creates a strong composite when combined with appropriate binders.

There are two most common types of natural plant fibers called bast and leaf fibers, depending on what type of plant it is and what part of the plant it comes from. Bast fibers consist of a wood core surrounded by stem. During the initial processing, the pectin inside the fiber is removed during retting process, which is the controlled rotting of the material. And then the fiber bundles are impregnated with resin. The composite comes out stronger is the material is pre-treated by boiling in alkalai to separate individual cells within the fibers.

Leaf fibers are coarser than bast fibers and commonly used for ropes and coarse textiles. Sisal is a good example of a leaf fiber, it's obtained from the agave plant and creates a composite with high stiffness and low odor unlike flax.

In order to make a composite compostable or recyclable it's important not to mix a binder and filler that won't separate, or if they won't then make both of the materials recyclable. The less materials that are used, the easier it is to separate them or re-use them.

Some natural binders inculde:

•Modified starch

•cellulose-esters

•Polylactide

There are many advantages and disadvantages to natural fibers. One major problem is that structurally natural composites don't tend to be as strong as synthetic composites.

Advantages:

•Low specific weight – results in higher specific strengh and stiffness (useful in bending stiffness)

•Renewable resource – production uses little energy (CO2 used, oxygen produced)

•Can be produced with low investment

•Friendly processing – little wear of tooling, no skin irritation

•Thermal recycling is possible (glass products cause problems in combustion furnaces)

•Good thermal acoustic and insulating properties

Disadvantages:

•Lower strength

•Variable quality

•Moisture absorption

•Restricted maximum processing temperature

•Lower durability (fiber treatments can improve this)

•Poor fire resistance

•Price can fluctuate from harvest results

Currently a lot of research is being done with using natural composites in the automotive industry for interior components using natural mat thermoplastic which is based on polyester or PP and fibers like

flax, hemp, or sisal. Some of the motives for using natural composites are price, weight reduction, and marketing.

One interesting material that we looked at more closely was Sisal, most commonly found in the subtropics and primarily exported out of Brazil. Sisal is traditionally used for rope and twine, but has many uses, including low-cost and specialty paper, dartboards, buffing cloth, filters, geotextiles, mattresses, carpets, handicrafts, wire rope cores, and Macramé. It is an environmentally friendly strengthening agent to replace asbestos and fiberglass.