Polyester-styrene networks

Unsaturated polyester-styrene compositions were mixed with castor oil and TDI to form a polyurethane network (Example 6). Such compositions make electrical ballasts with excellent heat dissipation and also reduce... 

More recently, lightly crosslinked, suspension-sized particles have been evaluated. Chemically, networks based on polybutadiene, poly-styrene, or poly(vinyl chloride) have been suggested. These particles are swellable in the unsaturated polyester-styrene mix, but are substantially insoluble in it. On curing the polyester, an IPN is formed. 

N. Devia-Manjarres, J. A. Manson, L. H. Sperling, and A. Conde, Simultaneous Interpenetrating Networks Based on Castor Oil Polyesters and Polystyrene, Polym. Eng. Sci. 18(3), 200 (1978). Castor oil-polyester SIN with polystyrene. Polymer morphology of castor oil-polyester/styrene SINs. 

Unsaturated Polyester Systems. Coatings in the furniture industry rely heavily upon resin formulations containing unsaturated polyesters, styrene, and photoinitiator (3,4,13,43,44). The imsaturated polyesters are synthesized using step-growth polymerization (see Polyesters, Unsaturated). Upon illumination, the carbon-carbon double bond in the imsaturated polyester and st5Tene copolymerize to form a cross-linked network (eq. 6). Equation 6 shows a generalized reaction scheme for an imsaturated polyester system. 

Crosslinking polyurethanes in the presence of unsaturated polyester-styrene mixtures produce unique topologically interpenetrating networks. The cured mixtures exhibited one glass transition tem-... 

Unsaturated polyesters 44 containing multiple electron-deficient maleate and/or fumarate double bonds along the polymer chain are readily prepared by the reaction of maleic anhydride or fumaric acid with a diol. In many cases, other diacids or dianhydrides are also included as comonomers to improve the final mechanical properties. The polyester is combined with styrene monomer and photopolymerization is carried out using a radical photoinitiator. Due to the reactivity ratios of the two vinyl components, there is a tendency toward alternation. However, the length of crosslinks between the polymer chains can be controlled by the amount of excess styrene present. This allows appreciable control over the structure of the network polymer formed, and this has a considerable impact on its mechanical and chemical properties. Photocurable unsaturated polyester-styrene systems have found numerous uses, but the major applications are in wood... 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

IPNs are found in many applications though this is not always recognised. For example conventional crosslinked polyester resins, where the polyester is unsaturated and crosslinks are formed by copolymerisation with styrene, is a material which falls within the definition of an interpenetrating polymer network. Experimental polymers for use as surface coatings have also been prepared from IPNs, such as epoxy-urethane-acrylic networks, and have been found to have promising properties. 

Thermoplastic elastomeric behavior requires that the block copolymer develop a microheterogeneous two-phase network morphology. Theory predicts that microphase separation will occur at shorter block lengths as the polarity difference between the A and B blocks increases. This prediction is borne out as the block lengths required for the polyether-polyurethane, polyester-polyurethane, and polyether-polyester multiblock copolymers to exhibit thermoplastic elastomeric behavior are considerably shorter than for the styrene-diene-styrene triblock copolymers. 

According to this model the structure of the crosslinked polymer is not considered as a homogeneous infinite network of polyester molecules, connected by styrene bridges. Since the styrene is a poor solvent for polyester chains the copolymerization takes place at statistically placed radicals (R )85). 

For styrene-crosslinked unsaturated polyesters and vinyl esters, v is considerably higher v (20 5)10 4 KT1 and v/G —3. It is noteworthy that in such networks (Tg 100°C) the temperature interval between ambient temperature and Tg is a diffuse transition region in which tan 8 increases almost continuously with temperature. 

In the case of unsaturated polyesters, nondegraded samples made from a prepolymer of molar mass M and a styrene mass fraction s have a chain-ends concentration b = [2(1 — s)/M]p, where p is the density. If ve is the actual concentration of elastically active network chains, an ideal network would be obtained by welding each chain end to another one, leading to... 

Initiators are used to initiate the curing reaction at elevated temperatures. Cross-linking or polymerization occurs by a free radical mechanism in which the double bond of the polyester chain reacts with the vinyl monomer that is usually styrene, and this reaction provides a three-dimensional network that converts the viscous resin to a hard thermoset solid. The initiators added decompose at elevated temperatures thus providing free radicals to initiate the cross-linking. Peroxyesters and peroxyketals are the most common classes of peroxides used as initiators. 

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