A resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are found in the cells of softwoods and some flowering plants, as well as in the cuticles of insects. They are insoluble in water but soluble in organic solvents, essential oils, and cold alcohols.
Commercially available resins include amber, copal, rosin (colophony), dammar gum, ebonite, mastic gum and shellac. These natural resins have largely been replaced by synthetic polymer resins such as Bakelite and epoxy resin.
The word “resin” has been applied in a second sense to any natural or artificial compound consisting of a non-crystalline material with high molecular weight; this includes many semi-solid organic substances such as: asphalt, coal tar pitch and certain hardening oils used in making varnishes.
In this article we will focus on the first definition of resin: a solid or highly viscous substance derived from plants that can be converted into polymers.
Plant resins are typically produced two ways: exudation and accumulation. Exudation occurs when wounds to the bark or leaves of a plant cause it to ooze out a sticky substance that helps protect the plant from further damage. This happens most frequently in response to insect attacks but can also result from physical damage (such as being struck by hail) or fungal infections. Once exuded, this sticky material quickly dries and forms an hard coating over the wound site. Some examples of plants that produce exudative resins are pine trees (from which pine resin is derived), spruce trees (from which spruce resin is derived) and various species of flowering plants such as Euphorbia lathyris (caper spurge) .
Accumulation type resins are not secreted in response to wounding but rather occur naturally within the plant tissue itself without any external stimuli. Many conifers produce these types of resins for example: Pinus sylvestris (Scots pine), Abies sibirica (Siberian fir)and Picea sitchensis(Sitka spruce). The majority of flowering plants do not produce accumulative type resins with notable exceptions including some membersof the genus Pistacia(mastic trees). Unlike exudative resinous substances, accumulative ones tend to be brittle solids rather than sticky liquids at ambient temperatures