Resins are amorphous or semi-crystalline solids or highly viscous liquids that can be converted into plastics. These substances have a polymeric or semi-polymeric structure, and are usually of natural or (semi-)synthetic origin. They are often treated with plasticizer, stabilizer, filler, antioxidant, etc. before pelletized, extruded and molded into a myriad of shapes.
Resins encompass a great number of compounds with vastly different properties. For example, they can be flexible and ductile or hard and stiff. The oldest resins were derived from plants and animals. Today most natural resins have been largely replaced by synthetic resins, which can be divided into two major classes:
Thermoplastic resins can be repeatedly melted and solidified by heating and cooling. Generally, no chemical changes take place during molding so that the mechanical and physical properties stay the same. Thermoset resins, on the other hand, react during processing and are converted to cross-linked polymers, which can neither be remolded nor reprocessed (recycled) and which have very different properties than the resins.
The most important resins in order of demand (production volume) are polyethylene (HDPE, LDPE, LLDPE), polypropylene (i-PP), polyvinyl chlorides (PVC), polystyrene (PS), polyethylene terephthalate (PETE), and polyurethane (PUR).
The plastic and rubber industry uses a large variety of methods to convert resins into plastic products. The method most frequently used for processing resins is injection molding. Other important methods include casting, blow molding, thermoforming, compression and transfer molding, extrusion, and reaction injection molding.
Other industries define resins as a sticky substance that oozes out of trees when an injury occurs to a plant. These substances are usually extracted from fir and pine trees and are used in medicine, varnishes and other industries. A large portion of these resins are used as tackifiers in adhesives. Today many of these natural tackifiers have been replaced by synthetic tackifiers which are produced via cracking of hydrocarbons (naphtha) which are also called resins.