Polyesters, linear unsaturated properties

Table I. Composition and Properties of Linear Unsaturated Polyesters with Different Molecular Weights...

Poly(Propylene Fumarate) (PPF) is a linear, unsaturated, hydrophobic polyester (Structure 12) containing hydrolyzable ester bonds along its backbone. PPF is highly viscous at room temperature and is soluble in chloroform, methylene chloride, tetrahydrofuran, acetone, alcohol, and ethyl acetate [66]. The double bonds of PPF can form chemical crosslinks with various monomers, such as W-vinyl pyrrolidone, poly(ethylene glycol)-dimethacrylate, PPF-diacrylate (PPF-DA), and diethyl fumarate [67,68]. The choice of monomer and radical initiator directly influence the degradative and mechanical properties of the crosslinked polymer. Once crosslinked, PPF forms a solid material with mechanical properties suitable for a range of bone engineering applications. 

Styrene is the most widely used cross-linking monomer, being preferred because of its compatibility, low viscosity, ease of use and low price. Other materials are sometimes employed when special properties are required. For example, methyl methacrylate is used, often in conjunction with styrene, for the preparation of translucent sheeting. Diallyl phthalate (X) and triallyl cyanurate (XI) are used for heat resistant products. Partially polymerized diallyl phthalate (solid) is used as the cross-linking agent in moulding powders (the so-called alkyd polyester moulding powders) based on linear unsaturated polyesters. 

Unsaturated polyester resins can be formulated to be hard and brittle, tough and resilient, or soft and flexible. By the appropriate choice of intermediates, particularly to form the linear unsaturated resin, special properties can be built into the resin system. As pointed out earlier (Table 12.2), the starting materials used with MA in polyesterifications have different effects upon the extent of isomerization of maleate to fumarate, and this in turn has a strong influence upon the final properties of the cured materials, As shown in Table 12.2, the percentage of fumarate increases considerably as more sterically hindered glycols or aromatic acids are used in the formulations. ... 

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. 

This chapter covers fundamental and applied research on polyester/clay nanocomposites (Section 31.2), which includes polyethylene terephthalate (PET), blends of PET and poly(ethylene 2,6-naphthalene dicarboxy-late) (PEN), and unsaturated polyester resins. Section 31.3 deals with polyethylene (PE) and polypropylene (PP)-montmorillonite (MMT) nanocomposites, including blends of low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and high density polyethylene (HDPE). Section 31.4 analyzes the fire-retardant properties of nanocomposites made of high impact polystyrene (HIPS), layered clays, and nonhalogenated additives. Section 31.5 discusses the conductive properties of blends of PET/PMMA (poly (methyl methacrylate)) and PET/HDPE combined with several types of carbon... 

Polymers are normally classified into four main architectural types linear (which includes rigid rod, flexible coil, cyclic, and polyrotaxane structures) branched (including random, regular comb-like, and star shaped) cross-linked (which includes the interpenetrating networks (IPNs)) and fairly recently the dendritic or hyperbranched polymers. I shall cover in some detail the first three types, but as we went to press very little DM work has been performed yet on the hyperbranched ones, which show some interesting properties. (Compared to linear polymers, solutions show a much lower viscosity and appear to be Newtonian rather than shear thinning [134].) Johansson [135] compares DM properties of some hyperbranched acrylates, alkyds. and unsaturated polyesters and notes that the properties of his cured resins so far are rather similar to conventional polyester systems. 

Jash and Wilkie [86] reported that even when the fraction of clay was as low as 0.1 wt% the PBQiR in a cone calorimeter was lowered by 40 %. Lee et al. [87] demonstrated that incorporation of 6, 8 and 10 wt% of MMT into epoxy resin increased linearly the char yield firom 9.1 to 15.4 % reducing the thermal degradation of the epoxy matrix. Nazare et al. [88] studied the flammability properties of unsaturated polyester resin with nanoclays using cone calorimetry. The authors verified that the incorporation of 5 wt% of nanoclays reduces the PHRR by 23-27 % and THR values by 4-11 %. While incorporation of condensed-phase flame retardants (such as ammonium polyphosphate, melamine phosphate and alumina trihydrate) reduce the PHRR and THR values of polyester resin, the inclusion of small amounts of nanoclay (5 % w/w) in combination with these char promoting flame retardants causes total reductions of the PHRR of polyester resin in the range 60-70 %. Ammonium polyphosphate, in particular and in combination with polyester-nanoclay hybrids show the best results compared to other flame retardants. 

Unsaturated polyesters (UP) are by far the most widely used resins in the composite industry [49]. UP resins are used in the highest volume due to their relatively low cost and good combination of thermo-mechanical properties and environmental durability. Typical properties of a cast UP resin are reported in Table 8.2. A UP resin is usually obtained from unsaturated polyesters dissolved in a reactive monomer. Unsaturated polyesters are linear polymers obtained by a condensation reaction between three chemical species saturated aromatic acids, unsaturated acids (maleic anhydride, fumaric acid), and glycols... 

The bis(dienes) [19] were also useful as cross-linking agents for linear poly(dienes) and unsaturated polyesters. The cross-linking reaction, presumably a Diels-Alder type, was claimed to yield products that possessed an attractive combination of mechanical, electrical, and adhesive properties (22). 

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