Composite Materials

When two or more materials are mixed together, the resulting composite material very often has physical properties that are very different than the properties of the used composites.

Many technical textile products appear like textile composite materials, that consist of two or more materials of different nature, joined together by adhesion or cohesion (through a third material).

Their typical ways of appearing are:

• stratified composites (spread or assembled surfaces, lamé)
• matrix composites (e.g. bonded fiber fabric joined with a binding agent)

As far as structure is concerned, apart from spread textiles (textile supports, plastic surface or substrate), plastic substances with a textile content (plastic support, external coat made of textile material) deserve consideration. Components are bound together usually by adhesion (it?s a typical method for binding products).

There are then the composite structures: a flexible or rigid composite structure is made of a substrate of fabric fibers soaked and protected by a polymeric flexible or rigid matrix.

Reinforcement structures

• Fabrics
  • Unidirectional fabrics; rope strands of fibers oriented in one way only and lined on a plane are used.
  • Conventional weaved fabrics; the most part of most used fabrics are conventional weaves of filaments rope strands. The weaved structure blocks warp and weft filaments. Weft and warp filaments aren?t completely extended but, while alternatively overlapping, they bend increasing the final deformability of the fabric.

• Plane weaved and mat fabrics
  • A plane weave is used for fabrics that have to be soaked and covered by a polymeric matrix in order to remove filaments bending out of the plane of the laminate and to obtain a material that has more uniform elastic properties.
  • In this kind of structure, weft filaments are only leaned on the warp ones (and not weaved with them) and they are later sewn together through a very light filament.
  • Fibers can be placed in an orderly way (stretched and lined up) or in a disordered way (curved and not lined up like in mat). In this case, it?s difficult to foreseen what the mechanic characteristics of the resulting material can be.

• Fabrics with a weave on many axes
  • The use of multi axis fabrics is finalized to obtain a better resistance to tearing out and to cut stresses.
  • An example of fabric weaved on more axis is the three axis one, where filaments are weaved with angles of about 60°.

Materials used in fibrous reinforcements

Materials used to produce particularly resistant to traction and to plastic collapsing filaments, are both polymeric and inorganic. Among traditional materials more commonly used there are polyamides, polyesters, meta aramidic fibers and glass fibers, whereas among high performances materials recently developed there are para aramidic fibers, carbon fibers, high polyethylene modulus fibers and poly-eter-eter –ketone (PEEK) ones. These materials are different for their elastic and environment resistance characteristics and for their plastic collapsing.

Glass fibers

Thanks to their traction and tearing resistance, their high modulus and dimensional stability, glass fibers have been used for many years to produce fabrics and reinforcement materials for composites. They are gotten through warm spinning method of glasses made in a suitable way (usually, alumino- borate –silicate) according to the use and the environment where it will be used. The kinds of glass commonly used for fibers are kind E and kind S, with a density of about 2,6 g/cm³, with elastic modules of about 80 and 90 Gpa and break resistance of respectively 3,5 and 4,5 Gpa.

To obtain composites with good characteristics under stress, the break stretching of the fiber (3 and 6% in many composites) must be less and the rigidity must be more of those of the matrix. The transfer of matrix efforts to the fiber is bettered with the help of chemical coverings.

These doubling agents can better very much the mechanic characteristics of the resulting composite.

Carbon fibers

Carbon fibers are thin filaments made of elementary carbon with structures that vary from those of the amorphous carbon to those of the crystalline graphite. These fibers own very variable chemical and physical properties: as far as the elasticity or rigidity modules are concerned, e.g., they change from about 35 Gpa, that is the half of that of glass fibers or of aluminium, to 700 Gpa, more than three times that of steel. As the carbon density is low, the specific rigidity is very high.

Carbon fibers keep electric, thermal and chemical characteristics of carbon and they are often used as reinforcement in rigid polymeric composites. Usually, mechanical resistance and deformability don?t increase at the same rate as the rigidity increases. In specific applications where both high resistance and high rigidity are needed, fibrous reinforcements must be chosen, where these two characteristics are balanced. With the current production technologies, the greater resistance is gotten for fibers that have a rigidity between 210 and 300 Gpa.

Polyamides

One of the first polymeric materials produced has been the Nylon filament, a polyamide gotten through polycondensation of diamines and dicarboxil acids that can be linear or contain aromatic groups up to 85% of weight ( if contents of aromatics in the repetitive structure are higher than 85%, we talk about aramids). Nylon 6/6, for example, is gotten from a linear diamine and a linear dicarboxil acid with 6 carbon atoms. The reaction between amine and acid produces amide (NH-CO) that characterizes this group of materials. This macromolecule is very flexible, can rotate on every bond and produces polymer fibrils lined with amorphous areas and crystallites orientated to the ironing direction. Nylon has a big affinity for water and its resistance to ultraviolet radiations is not high but, if protected with a suitable covering, can reach an acceptable environment resistance. However, because of its low elasticity module (about 5 Gpa), of its tendency to plastic collapsing under laden and of the dimensional variations caused by water absorbing (fibers stretching in humid places and fiber shortening in dry places), this material gives problems in applications where fabric preliminary tension and dimensional stability are critical. The resistance of this fiber varies between 500 and 700 Mpa , but, like the elastic module, it is significantly reduced if there is absorbed humidity.

Polyester

Polyester fibers are gotten through spinning of an aromatic polymer gotten through polycondensation of terephthalic acid and of a dialcool (glycols). The most used polyester is polyethylene terephthalic (PET). The oriented fibers structure is similar to that of polyamides. Polyester contains an aromatic ring that makes it less flexible than polyamide macromolecules. PET fibers are in fact characterized by an higher elastic module, about 18 Gpa, and by break resistance like that of Nylon.

However extensibility, as well as elastic module, very much depends on the orientation level induced by the spinning process.

Ultraviolet radiations resistance of these fibers is very high and their sensibility towards humidity and plastic collapsing is very low.

These characteristics make them good for those applications where good dimensional stability characteristics are needed. Dimensional stability can be further bettered through thermal proceedings of annealing of the fibers exposed to traction.

Aramidic

Aromatic polyamides that contain aromatic groups higher than 85% are called aramidics. The first aramidic fibers were produced in the Sixties and have a poly-phenyldiamine-isoftalamide basis and are commericialized as Nomex. This fiber is good for those applications where high heat resistance is required. It has an elastic module that can be compared to that of polyester but it is less variable as far as temperature is concerned. These fibers are directly gotten from a polymerization process, because they can?t be fused, not even at temperatures higher than 400°C. Polymer in fact degrades before it fuses.

Aromatic polyamide fibers have also been synthesized. They have very high mechanical characteristics gotten through humid spinning of a liquid-crystalline solution of p-phenyldiamine and terephtalic chlorite polymerized in sulphuric acid: Kevlar.

Because of the monomers characteristics (e.g. the length of aromatic diamine), aramidic polymers with different mechanical characteristics can be obtained. Among the most common ones there are Kevlar 29 and 49. The elastic module of Kevlar 49 is 135 Gpa and its break resistance of 3,6 Gpa: this material results 5 times more resistant than a steel yarn of the same weight, because its stretching is of only 1,4 g/cm³. The highly anisotropic structure of these aramidic fibers makes them however very weak in other characteristics and they are only good for those applications where there are traction laden only.

Their resistance to compression is in fact very low. On the other hand, the same causes that give low resistance to compression are those that give to this material very high toughness ( aramidic fibers are used to produce structures that have an high resistance to impacts, such as, for example, the bullet resistant ones). The collapsing of these materials is always fibrillation in traction and, when subjected to flexion, they show a plastic collapse in the compressed area that allows the shift of the neutral axis, not letting the break point in the traction area be reached and increasing the fiber capacity of losing its shape. The high toughness that is a characteristic of these aramidic fibers, suggests its use in applications where high impact resistances are needed.

New formulations called Kevlar 149 are being studied and we expect that they can reach elastic modules of about 190 Gpa and resistances to traction of 3-4 Gpa. Composite materials made stronger with aramidic fibers show remarkable disadvantages in mechanical processing.

Polyethylene fibers with high module

Polyethylene fibers with high module are gotten through extrusion at a solid state of high density polyethylene that must be in a condition that can allow conversion of disordered polymeric segments to very much extended bars. This molecular structure allows to reach very high elastic modules that are close to the theoretic ones of oriented macromolecules. Particularly, modules of 170 Gpa and resistances of 2 Gpa are reached in a material with very low density, 0,97g/cm³. Its specific resistance can then result even higher than that of more advanced aramidic fibers. Being a matter of polyoilefinic macromolecules, however, adhesion to polymeric matrix of a different kind can be very shoddy.

Hybrids

With this word we mean connections of different kinds of fibers finalized to the balancing of some characteristics or weaknesses of individual materials.
Frequent examples are those where carbon fibers (very fragile but very rigid) and more ductile glass fibers are weaved together.
In the same way, in order to better the resistance to impact, hybrids with aramidic and glass fibers or aramidic and carbon ones are used. Possible kinds of connections, however, are many and they can be aimed to specific laden and environment conditions.