Phenolic Resins and Composites

Abstract

Abstract One of the major contributions of phenolic resins is their use as a matrix in fiber‐reinforced composites to prepare materials with low fire‐smoke toxicity coupled with favorable economics and excellent properties. New formulations became available in the 1980s and were developed mainly to meet requirements of the mass‐transit fire code, which allowed the application of phenolic composites in new areas, such as in mass‐transit car interiors and architectural and marine components. These facts, along with their properties, led to a favorable conjunction for phenolic composites, making them widely used when the requirements of both public and worker safety are of primary importance. Currently, there are basically three types of phenolic resins: the traditional resoles and novolacs and the more recent polybenzoxazines. Lignin, a phenolic biopolymer present in wood and non‐wood plants, is a by‐product of the pulp and paper industry. The growing number of biorefineries in many countries, aiming to convert lignocellulosic wastes into fuels and chemical raw materials, will lead to the availability of large quatities of lignin, which gives it strategic importance as a substitute of phenol in phenolic resins. Aside from lignins, the condensed tannins, also obtained from renewable sources, are suitable to prepare a phenolic‐type polymeric matrix because of the presence of phenolic rings with a larger number of free positions where the electrophilic attack can occur. To decrease the dependence of phenol–formaldehyde (PF) resins on formaldehyde, and hence their formaldehyde emissions, phenolic–furfural (PFu) resins have been developed. In addition, the low vapor tension and low toxicity of glyoxal (OHC–CHO) solutions represent some of the advantages of using glyoxal instead of formaldehyde for many applications. Concerning the reinforcement of phenolic‐type composites, the properties required for a particular application, along with other questions such as cost, define the fiber to be used as reinforcement. The prerequisites for a given application define the fiber length, the diameter, and the need for surface modifications of the fiber, among other characteristics. In addition to the traditional fibers used (as glass, carbon, aramids), agro‐fibers can be mentioned, going by the growing interest this material has aroused. Lignocellulosic fibers are particularly interesting as plastic reinforcements because of their low cost, low density, process flexibility, and modest equipment requirement. These fibers are renewable, widely grown, moldable, anisotropic, nonabrasive, porous, viscoelastic, biodegradable, compostable, and reactive. Lignocellulosic fibers have many hydroxyl groups on the surface, mainly from cellulose and lignin, which may interact easily with the phenolic or lignophenolic polar matrices. The use of nanofibers or nanoparticles as reinforcement can be highlighted, as they can improve several properties of the related composite, such as thermal, flame, chemical, and moisture resistance, and decreased permeability. The set of properties of phenolic‐type matrices, as well as both the flexibility to make changes in their formulations and possible use of different reinforcements, in addition to their favorable cost/performance characteristics, place these materials in a prominent position in relation to other thermoset matrix composites.