In terms of chemistry, resins and polymers are different because resins are compounds while polymers are macromolecules. The molecular weights of resins and polymers are also different; resins are smaller than polymers. In general, molecular weight is the weight of one molecule of a substance relative to the weight of an atom of the most abundant kind of the element carbon. All resins and polymers consist of chainlike molecules, although polymers have longer chains than resins. Both resins and polymers, however, may occur as natural or synthetic, although polymers are formed by a particular process called polymerization.
Resins can be molded into solid materials or spun into thread due to their ability to form thin, continuous films. They may be thick, viscous fluids or hard, brittle solids. The solubility of only a few resins to water makes them useful in areas where water resistance is important. Once known only as naturally available compounds, resins have diverse chemical composition and various applications.
Natural resins are collected from plant sources as exudates from living trees. Examples are myrrh and aloe, which are used in incense, medicines, and perfumes. Synthetic resins are products of technology in the 20th century. For instance, the modification of cellulose, a complex carbohydrate that was used in the production of smokeless gunpowder during World War I, has resulted in the production of a film-forming resin. This resin is extremely flammable, but it has remarkable qualities that include durability and quick drying in paints, inks, and wood coatings.
During World War II, there was a high demand for synthetic rubber that led to the development of synthetic resins such as latex. Other synthetic resins have been developed in later years, including vinyl toluene, urethanes, and polystyrene. These resins have particular qualities as plastics, films, or coatings. Some resins that have also been used in the military, such as acrylic resins, also became available for general use. They are stable and durable as alternative paint binders and as substitutes in automobile coatings and food packaging.
Meanwhile, polymers are a combination of thousands of smaller molecules called monomers. For instance, thousands of molecules of ethylene, a monomer, can be joined together to form a polymer called polyethylene. Polymers commonly occur in nature, although synthetic polymers have also been developed as basis of all synthetic fibers and plastics and are structured as linear, branched, or cross-linked.
Linear polymers, such as polyvinyl chloride and nylon, are composed of long simple chains of monomers. Some linear polymers are elastic because they have kinked chains that can straighten out then spring back as a result of the removal of force. The shorter chains of branched polymers are attached along the main chains. Cross-linked polymers are harder and less flexible due to the links between their chains.