Polymers are used for the fibers in nonwoven webs and for the resins and additives used to bond, coat, and provide other properties to the fiber formed web.
In meltblown, spunbonded, and electrospinning processes the polymer is the funda- mental raw material supplied to the process and the forming of filament fibers is an intermediate stage in the forming of the web. Web properties for these processes are determined by the appropriate choice of polymer based on its properties and selection of the process conditions for forming that polymer into a fibrous form.
On the other hand, formation of webs by the air laid, dry laid and wet laid processes begins with a raw material that is already in fiber form. These are shorter nonfilament
104 Handbook of Nonwoven Filter Media
types of fibers provided by some form of outside supplier. The choice of fiber proper- ties is the guiding criterion in the design of the web.
Organic fibers and resin binders are composed of polymers. Their properties and processability are very much dependent on the chemistry and properties of the poly- mers from which they were made. A listing of the structures of many of the poly- mers used for nonwoven materials is tabulated in Table 4.1.
Natural fibers such as wood pulp, cotton and other vegetable fibers are composed of cellulose, a natural polymer structure. Animal fibers such as wool and hog’s hair are composed of keratin, also a natural polymer.
Important properties of polymers are melting point, glass transition temperature (Tg), and the degree of polymerization. According to Stille(90.1) (p. 31), the glass transition tem- perature is the temperature at which a polymer loses its hardness or brittleness, becomes more flexible, and takes on rubbery or leathery properties. Notable changes in specific volume, thermal conductivity, refractive index, stiffness, heat content, and dielectric loss are apparent. The degree of polymerization, dp, describes the molecular size of the poly- mer. It is the number of repeat units in the polymer chain (Stille(90.1) (p. 4)).