Unsaturated prepolymers

Unsaturated prepolymers crosslink with peroxides or sulfur. This unsaturation can be present in the backbone or in the pendant groups. Vulcanization or crosslinking of elastomers with sulfur or peroxides is discussed in Chapter 8. 

Irgacure 184 is a highly efficient non-yellowing photopolymerization initiator which can be used to initiate the photopol5mieriza-tion of unsaturated prepolymers, such as acrylates, in combination... 

P-373. To 100 g of the unsaturated prepolymer were added 30 g of styrene and 1.3 g benzoyl peroxide. The mixture was stirred and castings made between glass plates sealed with rubber gaskets (to prevent monomer evaporation) and cured overnight at 85°C. 

Scheme 8.21 Synthesis of the citric acid based unsaturated prepolymers from citric acid 1,8-octanediol and glycerol l,3 diglycerolate diacrylate or bis(hy-droxypropylfumarate) (A) acrylated prepolymers (B) fumarate-con-taining prepolymers.

Polyethylene Glycol-Based Membranes Brylev et al. [149] synthesized new AEMs based on polyethylene glycol (PEG). They prepared amorphous anionconducting polyether networks by cross-linking a quaternary ammonium salt with an unsaturated prepolymer (Rg. 5.16). The synthesis took place in two steps. The first step was a polycondensation between PEG and 3-chloro-2-chloromethyl-l-propene. Second, simultaneous cross-linking and functionalization took place. The chloride conductivity of these polymers was on the order of 1 X 10 S/cm at 25 C and no result in terms of hydroxyl conductivity was given. 

Polyester-based networks are typically prepared from polyester prepolymers bearing unsaturations which can be crosslinked. The crosslinking process is either an autoxidation in the presence of air oxygen (alkyd resins) or a copolymerization with unsaturated comonomers in the presence of radical initiators (unsaturated polyester resins). It should also be mentioned that hydroxy-terminated saturated polyesters are one of the basis prepolymers used in polyurethane network preparation (see Chapter 5). 

Unsaturated Maleate/O-Phthalate/1,2-Propanediol Polyester Prepolymer... 

Example 11. One-Component, Moisture-Cure Polyurethane Sealant. This example is of a low-hardness, high-elongation, moisture-curable polyurethane sealant. The material is based on a low-%NCO prepolymer made from 4,4 -MDI and a low-unsaturation (low-monol-content) Acclaim polyol from Bayer. It is adapted from (a) J. Lear et al., Adhesives Age, February 1999, pp. 18-23 and (b) B. Lawrey, et al., presented at UTECH 2000, The Hague, The Netherlands, March 30, 2000, Crain Communications London, 2000. 

Ultralow-monol polyols, 223 Ultrapek, 327, 328 Ultraviolet (UV) radiation, 209 Ultraviolet spectroscopy, 490 Unimolecular micelle, 58 United States, phenolic production in, 375 Unsaturated maleate/O-phthalate/ 1,2-propanediol polyester prepolymer, 101-102 Unsaturated polyester resins (UPRs), 19, 29-30, 58-59... 

There are numerous possible monomers that can be used in the backbone of the polyester prepolymer. However, typical monomers are 1,2-propanediol, as just mentioned, with maleic acid (usually added as the anhydride) to provide the sites of unsaturation, and phthalic acid (again usually added as the anhydride) to act as the second of the two diacid species. The structures of these latter two substances are shown in Figure 4.1. 

For the preparation of the prepolymer, a mixture corresponding typically to mole ratios of diol unsaturated acid saturated acid of 3.3 2 1 is polymerised by stirring the ingredients together at elevated temperatures, normally 150-200 °C. The slight excess of diol is to allow for possible evaporation... 

Fumaric and itaconic acids are also used as the diacid component. Most reaction formulations involve a mixture of a saturated diacid (iso- and terephthalic, adipic) with the unsaturated diacid or anhydride in appropriate proportions to control the density of crosslinking (which depends on the carbon-carbon double-bond content of the prepolymer) for specific applications [Parker and Peffer, 1977 Selley, 1988], Propylene glycol, 1,4-butanediol, neopentyl glycol, diethylene glycol, and bisphenol A are also used in place of ethylene glycol as the diol component. Aromatic reactants are used in the formulation to improve the hardness, rigidity, and heat resistance of the crosslinked product. Halogenated reactants are used to impart flame resistance. 

Sec. 9-2a). The crosslinking process is referred to as drying and is directly dependent on the content of unsaturated fatty acid. The crosslinking reaction involves chemical reactions different from those involved in prepolymer synthesis. 

The crosslinking of unsaturated polyesters (Sec. 2-12a) is carried out by copolymerization [Selley, 1988]. Low-molecular-weight unsaturated polyester (prepolymer) and radical initiator are dissolved in a monomer, the mixture poured, sprayed, or otherwise shaped into the form of the desired final product, and then transformed into a thermoset by heating. Styrene is the most commonly used monomer. Vinyltoluene, methyl methacrylate, diallyl phthalate, a-methylstyrene, and triallyl cyanurate are also used, often together with styrene. 

Crosslinks can be controlled by the number of unsaturated sites in the polyester prepolymer. Theoretically if each molecule has only two reaction sites, then infinite, almost linear, chains could be obtained. Hence, average functionability and molecular weight distribution in the prepolymer are extremely important. Plasticizers can be used to advantage in adjusting the average properties of the binder as obtained in the solid propellant formulation. 

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... 

This approach can be illustrated by unsaturated polyesters based on an almost equimolar combination of maleate and phthalate of propylene glycol, crosslinked by styrene (45 wt%) (Mortaigne el al., 1992). Six samples differing by the prepolymer molar mass were analyzed. The chain-ends concentration, b, was determined by volumetric analysis of alcohols and acids in the initial reactive mixture. Then, the system was cured, elastic measurements were made in the rubbery state at Tg + 30° C, and the shear modulus G was plotted against chain-ends concentration (Fig. 14.7). The following relationship was obtained ... 

Heating of a mixture of BPA/DC and BMI at 100-160 °C results in a formation of prepolymers, known as BT Resins of Mitsubishi Gas Chemical Co. [57]. A broad assortment of BT Resins has been developed. Some of the BT Resins are modified with epoxide resins, unsaturated monomers etc. 

Cold curing unsaturated resin was obtained in the following way. A mixture of BPA, BPA monocyanate and BPA dicyanate was cyclotrimerized and reacted with BMI. The obtained prepolymer with phenolic hydroxyls (cf. Scheme 9) was then treated with epichlorohydrin and alkali. The phenolic hydroxyls were thus transformed into glycidyl ether groups. Then the addition of methacrylic acid to the epoxy groups was carried out. The obtained vinyl ester (epoxyacrylate) type resin was dissolved in styrene and cured with the usual benzoyl peroxide/dimethylaniline system [131],... 

In general, group X in the prepolymer could be a number of possible photolabile groups. Benzoin alkylethers are usually superior to other photosensitizers studied for the photopolymerization of allylamines Therefore, prepolymers are prepared in which group X is derived from an unsaturated benzoin derivative of which several suitable structures are shown ... 

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