Fibers

Fibers of nylon, polyester, and other thermoplastics are made by extruding molten material through tiny holes in a spinneret. The resulting fibers are cooled before coiling. The strengths of nylon, polyester, polypropylene, and high-density polyethylene fibers are increased greatly by stretching them by 400 to 500% in tension (drawing) to orient the molecules parallel to the fiber axis.

Fiber strength is often quoted in terms of grams force per denier (g force/denier). A denier is defined as grams mass per 9000 m of fiber. Figure 19.1 shows the relative strengths of various fibers. Kevlar fibers are much stronger, having strengths of about 22 g force/denier.

Example Problem 19–1:

Develop an equation for converting tenacity in g force/denier to MPa.

Solution: Let T be the tenacity in g force/denier. Then T[g force/(g mass/9000 m)]ρ(g/cm3)(100 cm/m)(980.7 × 10-7N/g-force)(1002cm2/m2) = 883 × 103 (Tρ) Pa. Tensile strength is 0.883Tρ MPa.

Fabrication of Porous Foams

Natural cellular materials include sponges and wood. Foams of polymers, metals, and ceramics can be made by numerous methods. Many foams are produced by gas evolvution. Inert gasses such as CO2 and N2 may be dissolved under high pressure and released by decreasing the pressure. Gas bubbles may also be formed by chemical decomposition or chemical reaction. Polyurethane foam is made by reacting isocyanate with water to form CO2. Mechanical beating also will produce foams. Foamed materials such as styrofoam can be formed by bonding together spheres that have been previously foamed.