Resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds and can be derived from natural or synthetic sources. Plants secrete resins for their protective benefits in response to injury, and the resin protects the plant from insects and pathogens. Resins have a wide range of applications, including surface protection and stand-alone structures themselves. The combination of a resin with a reinforcement material, such as glass fiber or carbon fiber, has given rise to composites. Some of the more general qualities deemed beneficial are adhesion, flame retardancy, and mechanical strength. There are different types of resins, including epoxy resins, polycarbonate resins, polyamide resins, polyethylene resins, and acrylic resins. Composite resins generally consist of a resin polymer matrix, inorganic filler, coupling reagent, coloring agent, and initiator. Natural resins may be classified as spirit-soluble and oil-soluble, and among the former are balsams, turpentines, and mastics, while the oil-soluble resins include rosin, copals, amber, Oriental lacquer, and cashew-nutshell oil.