**3. Technology trends on composite materials with natural fibers**

The resins used in manufacturing composite materials can be thermosetting and thermoplastic. The thermosetting resins are predominant, while the thermoplastic resins play a minor role in the fabrication of advanced composites.

The thermosetting resins require the addition of a curing hardener. Once cured, the composite cannot be reformed. Widely used thermosetting resins include epoxies, polyurethanes, phenolic and amino resins and polyamides. The most

## *Composite Materials with Natural Fibers DOI: http://dx.doi.org/10.5772/intechopen.101818*

common employed are the epoxies because the potential for respiratory exposure is low due to their relatively high molecular-weight.

Another essential component of the cured resins is the hardener. Such compounds control the reaction rate and greatly influence the properties of the matrix. Some commonly used curing agents are the aromatic amines. Like epoxies, such hardeners have a low vapor pressure and are not a respiratory hazard. However, both epoxies and hardeners are a dermal hazard and can even permeate common protective gloves.

The authors of this chapter performed extensive experiments with epoxy resins and hardeners from Struers GmbH, Hanover, Germany. Their epoxy resins are suitable for the matrix of composite materials because they have low shrinkage and the adhesion to natural fibers is excellent. The hardened epoxy is duro-plastic and not affected by moderate heat or chemicals. The curing time is relatively long, but adhesion to most materials is excellent. They polymerize through a chemical reaction after being mixed in the correct proportions. Our experiments employed the polymer Epiclorhydrin in a ratio 15/2 with the hardener Triethylenetretamine. The hardened epoxy is duro-plastic and not affected by moderate heat or chemicals [16].

The curing of epoxy matrix depends on the amount of resin. If small amounts of resin are employed, the polymerization takes longer. However, favorable conditions exist for removing excessive heat generated by the chemical reaction. Larger amounts of epoxy will accelerate the curing process by storing heat due to the poor conductive properties of the system. The air bubbles developed during the curing are caused by higher than acceptable temperatures. Sticky or rubbery sample surfaces after curing indicate a process temperature too low and can be corrected by post-curing in ovens at temperatures below 50°C. The content of water in natural fibers can significantly influence the bond fiber-matrix and it can be reduced by combined exposure to heat at around 120°C for 2 hours and immersion in 10% NaOH for 4 hours. The immersion of fibers in NaOH is increasing the exposed surface, with beneficial effects for their adhesion to the matrix material.
