Prepolymer thermoset epoxy

Liquid crystalline thermoset epoxy prepolymer Not applicable Nematic to isotropic 176... 

Crosslinked cyanate ester systems typically exhibit higher glass transition temperatures (T S 250°C), lower moisture absorption and lower dielectric constants than conventional epoxy thermosets. Hence by mixing dicyanates or their corresponding prepolymers with epoxy resins, and then co-curing, the desired level of property improvements can be achieved. Such hybrid thermosets have been used in printed circuit board manufacture. 

Typical epoxy resins used to formulate epoxy adhesives have at least two epoxy rings, usually at the ends of a relatively short-chain prepolymer. The epoxy groups then are reacted with other epoxy groups in a chain-growth polymerization or with another curative in a step-growth polymerization to produce a polymer network, which can be either thermoplastic or thermoset, The polymer linkages created by reaction of the epoxy ring are polar... 

Note 2 The term is sometimes used not only for prepolymers of thermosets, but also for cured thermosets (e.g., epoxy resins, phenolic resins). Use of the term for cured thermosets is strongly discouraged. 

The chemical structure of the epoxy matrix constituent as well as processing are reported to strongly influence 11 -I3> the thermoset network and hence the properties and durability of the crosslinked polymer 11 ,4-16). The cure of a reactive prepolymer involves the transformation of low-molecular-weight reactive substances from liquid to rubber and solid states as a result of the formation of a polymeric network by chemical reaction of some groups in the system. Gelation and vitrification are the two macroscopic phenomena encountered during this process which strongly alter the viscoelastic behavior of the material. 

Structopendant Prepolymers. The reactive or functional groups are distributed along the prepolymer chain. Examples are the epoxies, unsaturated polyesters, and the thermoset acrylics. 

Liquid crystalline thermoset system 1. Liquid crystalline thermoset prepolymer dihydroxy methylstilbene epoxy (Mn, 3600) 2. Curing agent 4,4 -methylene dianiline ... 

Many commercial thermosets are quite often used as complex mixtures of several co-reacting monomers and prepolymers, specifically formulated to suit a given end-use application. For example, in coating and adhesive applications, often mixtures of different epoxies, differing in chemical structure and/or molecular weights, are used as required. Even two different thermoset-... 

As previously mentioned, several commercial hybrid thermosets are known to be co-reacting thermosets, i.e. when the mixture of two different thermosetting monomers or prepolymers is cured, there is a simultaneous graft or co-reaction between the components along with the crosslinking reactions. These systems may therefore be considered as co-polymerizing thermosets and not as true blends. Examples of such systems are phenolic novolak/epoxy resin (or epoxy novolaks) ... 

Rubber toughened epoxy resins are the well known examples of impact modified thermosets utilizing reactive rubbery prepolymers. Epoxy resins can be toughened or flexibilized by any one of the following types of oligomeric reactive elastomers ... 

A thermoset is a polymer that, when heated, imdeigoes a chemical change to produce a cross-linked, solid polymer. Thermosets usually exist initially as liquids called prepolymers they can be shaped into desired forms by the application of heat and pressure, but are incapable of undergoing repeated cycles of softening and hardening. Examples of thermosetting polymers include urea-formaldehyde, phenol-formaldehyde, and epoxies. 

Oriented PLC thermosets based on epoxy chemistry and with the mesogens in the backbone chain have been prepared by aligning an oligoether prepolymer either mechanically or in a magnetic field (13.5 T) prior to and during curing [135]. The temperature associated with a drop in order parameter increased with increasing crosslink density. Samples with few crosslinks showed a drop in order parameter at 130 C... 

Precipitate can also form fiom a step reaction. Precipitation polymerizations of prepolymers to form crosslinked, thermoset polymers is a very common commercial reaction of the epoxy resins and phenol-methanal, network polymers covered in this book. These reactions work best when the reaction is slow, mildly exothermic, and undergoes a viscous to solid transformation late in the synthesis. 

Nanocomposites of thermoset polymers like unsaturated polyester and epoxy can be fabricated by this method. In this process, the monomers/ prepolymers are allowed to intercalate the layer spacing of the clay platelets. Polymerization will then be initiated either by the application of heat or radiation or by introducing suitable organic initiator. The intercalated monomer swells the clay and during polymerization increases the interlayer spacing and results in the formation of intercalated or exfoliated nanocomposites. 

Siloxanes that are extremely heavily cross linked can be thought of as thermosets, which have some interesting glass-like properties.Copoly-meric analogues have also been prepared, by combining siloxanes with epoxy resins,or with polybenzoxazine prepolymers. 

These hydroxy-terminated polyesters may have applications as prepolymers for other high molecular weight polymers. These could condense with diisocyanates to produce polyurethane foams [44], either flexible or rigid. They also may have compatibility with epoxy resins and polyester to give rise to polyester epoxy resins, polyester-polyamide [41], and polyester-polyether copolymers. The further condensation with some unsaturated monomer may result in thermoset film formers. They can be used as a mildness additive in metal-working lubricants. 

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]. 

In the two thermosets seen above the curing of the prepolymer was carried out by a thermal process. It is also possible to use a chemical reagent for the curing reaction. In the case of the epoxy resins this principle is employed. 

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