:s, the process that produces
linoleum. Crosslinking generally involves covalent bonds that join two polymer chains. The term
curing refers to the crosslinking of
thermosetting resins, such as unsaturated
polyester and
epoxy resin, and the term
vulcanization is characteristically used for
rubbers. When polymer chains are crosslinked, the material becomes more rigid. The mechanical properties of a polymer depend strongly on the cross-link density. Low cross-link densities increase the viscosities of
polymer melts. Intermediate cross-link densities transform gummy polymers into materials that have
elastomeric properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as
phenol-formaldehyde materials. derived from
bisphenol A diglycidyl ether. Free-radical polymerization gives a highly crosslinked polymer. In one implementation, unpolymerized or partially polymerized
resin is treated with a
crosslinking reagent. In
vulcanization, sulfur is the cross-linking agent. Its introduction changes
rubber to a more rigid, durable material associated with car and bike
tires. This process is often called sulfur curing. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Chemical covalent cross-links are stable mechanically and thermally. Therefore, cross-linked products like car
tires cannot be recycled easily. A class of polymers known as
thermoplastic elastomers rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as
snowmobile tracks, and
catheters for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains that act as cross-links are reversible, so can be reformed by heat. The stabilizing domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters. is a cross-linking agent: the
siloxy groups link to silica and the
polysulfide groups vulcanize with
polyolefins.
Alkyd enamels, the dominant type of commercial oil-based paint, cure by oxidative crosslinking after exposure to air.
Physical cross-links In contrast to chemical cross-links, physical cross-links are formed by weaker interactions. For example, sodium
alginate gels upon exposure to calcium ions, which form ionic bonds that bridge between alginate chains.
Polyvinyl alcohol gels upon the addition of
borax through hydrogen bonding between
boric acid and the polymer's alcohol groups. Other examples of materials which form physically cross-linked gels include
gelatin,
collagen,
agarose, and
agar agar. ==Measuring degree of crosslinking==