Polyurethane Thermoset networks

Thermoset materials are produced by the direct formation of network polymers from monomers, or by crosslinking linear prepolymers. Important thermosets include alkyds, amino and phenolic resins, epoxies, unsaturated polyesters and polyurethanes. Thermosetting polymers consist of two liquid components, one containing a resin and the other a hardener [1]. 

Thermosets consist of a network of interconnected chains whose positions are fixed relative to their neighbors. Such polymers do not flow when heated. Instead, when exposed to high temperatures, thermosets degrade into char. Examples of thermosets include some polyurethanes and epoxy resins. 

One alternative is to select precursors which form a gas as a reaction product in situ during the network formation of thermosets. However this approach is restricted to a very limited number of precursors reacting via a polycondensation mechanism to split off a gas. For example, flexible polyurethane foams are commercially produced using CO2 that is liberated as a reaction product of the isocyanate monomer with water [5]. Very recently, Macosko and coworkers studied the macroscopic cell opening mechanism in polyurethane foams and unraveled a microphase separation occurring in the cell walls. This leads to nanosized domains, which are considered as hard segments and responsible for a rise in modulus after the cell opening [6]. 

By means of chemical reactions thermosetting plastics form three-dimensional structures. In the example above the nitrogen compound urea reacts with formaldehyde (methanal), in which process three molecules combine and a molecule of water is formed. In this example two H atoms react, but all other H atoms ( ) enter into the same reaction. Since urea is a three-dimensional molecule, the network will also be three-dimensional. For instance switches and sockets are made of UF. Other thermosetting plastics are polyurethane PU (insulation) and melamine-formaldehyde MF (panels). 

Unsaturated resins are usually mixtures of vinyl monomers and prepolymers, such as unsaturated polyesters, polyurethane acrylates, and ac-rylated epoxides of the bisphenol A type. Polymerization of styrene-based resins involves the formation of a three-dimensional network via the cross-linking of prepolymer chains by styrene radicals. These standard thermoset resins are therefore characterized by great hardness (Shore D over 80, DIN 53505, arbitrary scale 0-100 based on the penetration of a needle point in the tested material), do not melt, and are not soluble in organic solvents. 

Thermosets, on the other hand, are polymers formed by the mixing and chemical reaction of fluid precursors into a mold once the precursors react, a crosslinked network that cannot flow anymore under heating is created therefore, reaction and molding into the final shape usually take place at the same time (by the RIM or reaction injection molding process). Examples of common thermosets are some polyesters, phenol-formaldehyde resins, epoxy resins, and polyurethanes, among others. Chapter 28 of this handbook elaborates on this topic. 

Many thermoset polymers of major commercial importance are synthesized by step-growth polymerization, as the case of unsaturated polyester, polyurethanes, melamines, phenolic and urea formaldehyde resins, epoxy resins, silicons, etc. In these systems, the crosslinking process, which leads to a polymer network formation, is usually referred to as curing. 

Polymers can be subdivided into three main categories. Thermoplastics, consisting of individual long-chain molecules, can be reprocessed products can be granulated and fed back into the appropriate machine. Thermosets contain an infinite three-dimensional network, which is only created when the product is in its final form, and cannot be broken down by reheating. Rubbers contain looser three-dimensional networks, where the chains are free to change their shapes. Neither thermosets nor rubbers can be reprocessed. Some polymers, such as polyurethanes, can be produced in both thermoplastic and thermoset variants. 

Upon mixing and subsequent hardening a three-dimensional polymeric network develops within the material, which is intimately combined with the three-dimensional stracture of the hardened cement paste. A variety of polymer dispersions may be combined with inorganic cements, as long as the polymeric material is sufficiently resistant to sustain the high-pH enviromnent of the cement paste. These may be thermoplasts, such as polyvinyl acetate, polyvirtyl chloride or polyacrylate thermosets, such as epoxides, polyesters, or polyurethanes and also elastomers, such as natural rabber latex or a butadiene-styrene copolymer. Polymer additions between 5% and 20% may be considered typical. 

The incorporation of nickel zinc ferrite particles into a commercial ester-based polyurethane network enabled the indirect magnetic actuation of thermosets... 

Several mechanisms are needed to explain the action of the many different phosphorus compoimds used as FRs. Some of these compounds decompose in the condensed phase to form phosphoric acid or polyphosphoric acid. They can promote charring. Char formation is further enhanced by cellulosics, polyurethanes, phenolics, epoxy resins and EVA copolymers, and there are catalysts that promote it. Phenol-formaldehyde polymers can be used as flame retardants themselves when combined with a more flammable thermosetting polymer to form an interpenetrating network. 

An important group of polymers are classified as being thermosetting materials (165) (see Thermosets). The chemistry associated with their formation is often the same as that used for thermoplastic materials, except that in the case of these polymers the monomers used have more than two functions per monomer unit. It is therefore possible to have two materials that ostensibly look chemically similar, but one is a thermoplastic and the other a thermoset. This situation is to be found in Polyurethanes (qv) where, depending on the functionality of the isocyanate or soft block used, the resiJting material may be a thermoplastic or a thermoset. In most cases it is desirable to monitor the cure process to avoid either too slow a formation of the three-dimensional network, and hence poor production efficiency, or too quick a cure with the possibility of excessive exotherms and possible degradation of the material. The formation of a network for a material that has a Tg above... 

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