Composite Materials Research Progress

Major Trends in Polymeric Composites Technology 119
fibers available on the market today, which has been widely used both in soft body armor
applications, and as reinforcement for hard armor, helmets and electronic housing protection.
UHWMPE fibers, such as Spectra® and Dyneema®, are a type of ultra lightweight, highstrength
polyethylene fibers. High damage tolerance, non-conductivity and flexibility, a much
higher specific strength and modulus and energy-to-break, low moisture sensitivity, and good
UV resistance, all make this fiber a good aramid alternative. These fibers are typically used in
ballistic and high impact composite applications. Zylon® consists of a rigid chain of
molecules of poly(p-phenylene-2,6-benzobisoxazole, PBO. It has excellent tensile strength
and modulus. Fabrics made from Zylon ® are found in both ballistic and composite
applications.
In addition to the attractive mechanical properties of organic fibers, albeit limited in their
current form, it should be noted that they are highly valuable by the key industries of the U.S.
and offer the significant advantage in the avoidance of galvanic corrosion with aluminum and
certain other metals. With organic fiber composites, the corrosion threat is avoided and the
cost and weight benefits would be enormous to the aerospace and other industries if other
critical properties could be enhanced. In addition, there is the economic challenge now of the
growing applications for carbon fibers and the resultant shortages of carbon composite
materials, which may impact the economy of the US. Engineering conferences for the past
two years have focused on that increasing shortage and what technical and scientific
alternatives are available. How to make organic fibers a viable alternative to carbon fibers for
structural applications is often discussed in these forums within the genre of nanocomposites.
This category of research holds great promise to enable that technology sector and can
accelerate the fulfillment of the promise of multi-functional materials.
Typical characteristics of these organic fibers include non-polar chemical structures and
crystalline chains. It is these structural characteristics that impart the advanced mechanical
properties on to the fibers. For example, UHMWPE fiber obtains its high strength from the
straightening of long polymer chains by taking advantage of the strong covalent bonds in the
backbone of the monomer. The modulus of the fiber is proportional to the draw ratio which
controls the degree of crystallinity. The main benefits of a UHMWPE continuous fiber
include high specific strength and moduli, leading to a lower weight for a given design load.
The chemical neutrality of the fiber surface leads to a high degree of corrosion resistance;
there are no places to allow for a concentrated attack on the surface. In addition, the
anisotropic nature of the fiber allows for low coefficients of thermal expansion, meaning
dimensional stability of the finished composite product.
However, the non-polar chemical structure and resulting lack of reactive groups on the
organic fiber surface lead to low surface energy, and thus leads to difficulty in obtaining good
wetting and adhesion at the fiber/matrix interface. This low surface energy requires that the
matrix material be of an even lower energy to achieve sufficient wetting and adhesion,
ultimately realizing strong bond at the fiber/matrix interface. This results in the limited
applications of the organic fibers because many properties of the composites are determined
by the transfer ability of the fiber/matrix interface. To tackle the problem, various surface
treatments to improve the interfacial wetting and adhesion, are applied. There appears to be
an absence of a good means to alter the fiber without sacrificing its desirable properties. It is
concluded that novel cost-effective methods for improving the interfacial adhesion between
the organic fibers and the polymer matrix are vital to the full realization of their potential as
structural materials.