Zinc borate flame retardants

Abstract

Unsaturated polyesters resins (UPRs) comprise an important class of thermoset polymers. They are used extensively due to their low cost, easy processability, good corrosion resistance, and availability in a variety of grades. During their curing process they do not emit any volatile by-products; this feature makes them more attractive. They are classified into five different types based on structure. The properties of UPRs can be tailored by varying the type and ratio of monomers and curing agents used. Generally, aromatic groups improve hardness and stiffness while aliphatic chain components increase flexibility. However, as they contain polyester linkages, they are susceptible to water, have high glass transition temperatures (Tg), and have poor fire resistance. Although they have stable structures and good mechanical and thermal properties compared to other thermosets, they are not suitable for advanced applications. Hence, they have been modified as blends, interpenetrating polymer network (IPNs), composites, nanocomposites, and others. The blending of polymers results in the formation of new materials which combine the useful properties of each constituent polymer. UPRs have been blended with elastomers (e.g., nitrile butadiene rubber, maleated nitrile rubber, carboxyl-terminated butadiene acrylonitrile rubber, etc.,) to improve their impact strength and other mechanical properties. Usually, fire properties are enhanced by blending with phenol formaldehyde resins. UPR blends with epoxy resin show improved toughness and impact properties. In some blends, superior thermal and damping properties are observed. IPNs are blends of two or more polymers in a network form, with one of the polymers synthesized and/or cross-linked in the immediate presence of the other(s). Although they are not bonded covalently with each other, they have partial or total physical interlocking between them. Semi-IPNs and full IPNs of UPR with other polymers such as polyurethanes, epoxy, nylons, phenol formaldehyde resin, and polyacrylates are discussed. Composites are made by combining two dissimilar materials at a macroscopic level in such a way that the resultant material is conferred with properties superior to those of any of its components. Of the two, one is the continuous phase, known as the matrix, and the other is the reinforcement. These components neither take part in any chemical reaction nor do they dissolve or completely merge into each other, yet they still remain strongly bonded and maintain an interphase between them. The reinforcement and matrix are complimentary to each other. They make use of each other’s properties in such a way that the composite exhibits enhanced properties. The reinforcing material may be in the form of fibers, particles, or flakes, and the polymer constitutes the matrix. Glass-reinforced composites are most popular, but apart from these, natural fibers such as sugar palm fibers, coconut fibers, henequen fibers, cotton fibers, hemp fibers, kenaf fibers, etc., have been used. Composites with metals, metal oxides, carbon fibers, and graphite have also been investigated. They have superior mechanical, chemical, thermal, and electrical properties. Nanocomposites consist of nanofillers dispersed in a polymer matrix with the reinforcement having a nanoscale structure (at least one dimension less than 100 nm) with a high aspect ratio (surface to volume ratio). A smaller size of filler leads to an exceptionally large interfacial area between the matrix and reinforcement compared to conventional composites. Polymer nanocomposites show significantly enhanced properties at much lower filler loading rates, which ultimately results in lower component weight and can simplify processing. In general, nanofillers are broadly classified on the basis of their geometries as particles, layered, or fibrous materials. Examples of particle-type nanofillers are carbon black, metal oxides (ZnO, Al2O3, and TiO2), silica nanoparticles, and polyhedral oligomeric silsesquioxanes. Carbon nanotubes (CNTs) and cellulose nanofibrils are examples of fibrous materials, while nanoclays and montmorillonite are layered materials. UPR nanocomposites show improved thermal, mechanical, electrical, water absorption, solvent resistance, flame retardancy, and volume shrinkage properties. Modification or functionalization of polymers and/or fillers leads to further enhancements in the properties of these materials. They are widely used in several industrial applications such as the automotive, construction, marine, and electrical fields, as well as in coatings, among others. Major challenges faced by the composite industry include reducing dependency on petroleum sources for the synthesis of UPRs by recycling polymer waste and making use of sustainable sources, the elimination of halogens in order to achieve fire retardancy, and the replacement of styrene to overcome environmental and health hazards.