Flax, hemp and jute fibres can reinforce a resin or glue matrix where glass fibre is unavailable, and flax laminates are reported as comparable to low-grade fiberglass at lower cost, with hemp good for impact. Their weaknesses are water uptake, which swells the fibre and breaks its bond to the matrix, and rot, so the fibre must be well retted, dried hard and fully sealed by the matrix. Without tested figures, treat any natural-fibre laminate as lower in strength than glass until a coupon proves otherwise.
Part of Fiber-Reinforced Composites
Natural fibre composites are laminates and moulded parts whose reinforcement is bast fibre from plants such as flax, hemp or jute, bound with a resin, glue or natural binder.
Why Plant Fibre Works And Where It Fails
Bast fibre is long strands of cellulose, strong along its length, and cheap and growable. The structure that makes it strong also makes it a poor partner for resin: cellulose absorbs water, so the fibre swells and shrinks with humidity, and a water-loving fibre bonds poorly to a water-hating resin. A damp laminate loses the bond first.
The Advanced Materials article lists three options:
- Flax: strength comparable to low-grade fiberglass at lower cost.
- Hemp: good impact resistance, widely growable.
- Bamboo: strips replace steel rebar in low-load concrete. See Concrete Composites.
The Advanced Materials article gives no tensile figures for these, and the corpus does not either. Do not quote any; test your own coupons with Tensile Testing.
Preparing The Fibre Is Most Of The Work
A reinforcement needs to be clean, straight, dry and free of woody bits. Use the fibre preparation already described:
- Ret to remove the pectin glue so the fibre separates cleanly. Retting Bast Fibers gives water retting at 15-20 °C taking 7-14 days for flax and hemp, and warns that over-retting weakens the fibre.
- Break and scutch the dried stalks, then hackle to clean parallel line fibre, as in Flax Processing.
- Dry thoroughly before wetting with resin. Wet fibre gives a weak bond and a poor cure.
- Keep the fibre straight and parallel, in a woven cloth or aligned sheet. Do not use fibre prepared as cordage: twist and spinning oils reduce both stiffness and bonding.
Choose The Matrix For The Fibre
The Advanced Materials article suggests cashew nut shell liquid, pine resin or animal glue for low-temperature applications, and synthesised polyester or epoxy for higher performance. Pine Resin and Adhesives cover the natural binders. Match matrix to environment:
| Matrix | Moisture | Notes |
|---|---|---|
| Hide glue | Poor | Dissolves in water and must stay below 70 °C |
| Pine resin | Moderate | Softens in heat |
| Casein | Water-resistant, suited to outdoor use | See Casein Glue |
| Polyester or epoxy | Good | Needs a source of resin; see Epoxy Systems |
Seal every cut edge, because the end of a fibre is a wick.
Worked Example: A Test Coupon Plan
Make three coupons 25 mm wide with a 3 mm thick laminate: one in dry conditions, one soaked 24 hours, and one soaked then dried. Break each in a tensile test and compare. Cross-section area = 25 × 3 = 75 mm². If the dry coupon breaks at 6.0 kN, stress = 6,000 / 75 = 80 MPa. If the soaked one breaks at 3.0 kN, it is 3,000 / 75 = 40 MPa, half the dry value. That ratio, not the dry number, tells you whether to use the material outdoors. (Example values only.)
Resin, mould and dust
Natural binders such as pine resin are flammable when hot, and synthetic resins give off irritating fumes while they cure. Work in ventilation, and wear a mask when sanding. Rotting damp fibre grows mould whose spores harm lungs, so dry it before storing it.
Common Mistakes
- Using rope fibre — wrong preparation and weak bond.
- Ignoring humidity — a laminate that passed a dry test fails when wet.
- Leaving cut edges unsealed — water wicks in.
What’s Next
- Learn next: Laminated Composites — layering for strength in more than one direction
- Combine with: Concrete Composites — bamboo and plant fibre in cement