Properties of Unsaturated Polyesters

The dynamic behavior of both cured and undercured polyesters, as well as relating the molecular structures to the bulk properties or chemical 

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.  

The creep, stress intensity factors,tensile,elasticity, and dynamic mechanical relaxation properties of cured unsaturated polyesters have received considerable study. The tensile strength (TS) and tensile elongation (TE) at 25-120°C of a styrene crosslinked propylene glycol maleate phthalate polyester follows the following relationship 

From studying the unsaturated polyesters of polyalkylene glycol maleate adipates/ it was found that the tensile strength at break (y) of several polyesters was related to the concentration of double bonds in the copolymer (x), the conversion of the crosslinking monomer styrene (A), and its initial concentration (Co) by the following empirical equation. 

The compression, impact, and fracture properties of many cured unsaturated polyester resin formulations have been studied and reviewed.The compressive strengths of the rigid cured polyesters are of the order 15,000-30,000 psi. Cyclic glycols and specialty fumarate resins tend to raise this value, while aliphatic saturated acids lower the compressive strength. For aliphatic acids, the impact strength is proportional to the acid chain length. 

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From the investigations reported, it may be concluded that the concentration dependences of the physical and mechanical properties of unsaturated polyesters formed in the presence of surfactants are of extremal character. This extremal character corresponds essentially to the same dependences of surface tension, turbidity, and viscosity of system with surfactant present. 

Lee SC, Mariatti M (2008) The effect of bagasse fibers obtained (from rind and pith component) on the properties of unsaturated polyester cranposites. Mater Lett 62 2253-2256... 

Table 24.1 Physicomechanical properties of unsaturated polyester-isocyanate resins...

Gunduz G, Erol D, Akkas N (2005) Mechanical properties of unsaturated polyester-isocyanate hybrid polymer network and its E-glass fiber reinforced composites. J Compos Mater 39 1577-1589... 

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. 

Melot,D., Escaig, B., Lefebvre, J. M., Eustache, R. R., Laupretre, F. (1994). Mechanical Properties of Unsaturated Polyester Resins in Relation to their Chemical Strcucture.-2. Plastic Deformation Behavior. J. Polymer Sci. PartB Polymer Phys., 32(11), 1805-1811. 

The properties of unsaturated polyesters are also modified by a host of additives, such as thixotropic agents, chemical thickeners, fillers or extenders, fire-retardant chemicals, glass-fiber, and other reinforcement. A discussion of these areas is beyond the scope of this book. Most of this technology is disclosed in the extensive polyester patent literature. However, many books and review articles on polyester, " as well as manufacturers product literature, give good coverage of the subject. 

Miyagawa, H., Mohanty, A.K., Burgueno, R., Drzal, L.T., Misra, M., 2006. Characterization and thermophysical properties of unsaturated polyester-layered silicate nanocomposites. Journal of Nanoscience and Nanotechnology 6, 464—471. 

Thermosetting unsaturated polyester resins constitute the most common fiber-reinforced composite matrix today. According to the Committee on Resin Statistics of the Society of Plastics Industry (SPl), 454,000 t of unsaturated polyester were used in fiber-reinforced plastics in 1990. These materials are popular because of thek low price, ease of use, and excellent mechanical and chemical resistance properties. Over 227 t of phenoHc resins were used in fiber-reinforced plastics in 1990 (1 3). PhenoHc resins (qv) are used when thek inherent flame retardance, high temperature resistance, or low cost overcome the problems of processing difficulties and lower mechanical properties. 

This is also known as Bulk Moulding Compound (BMC). It is blended through a mix of unsaturated polyester resin, crosslinking monomer, catalyst, mineral fillers and short-length fibrous reinforcement materials such as chopped glass fibre, usually in lengths of 6-25 mm. They are all mixed in different proportions to obtain the required electromechanical properties. The mix is processed and cured for a specific time, under a prescribed pressure and temperature, to obtain the DMC. 

For example, the molecular weight of unsaturated polyesters is controlled to less than 5000 g/mol. The low molecular weight of the unsaturated polyester allows solvation in vinyl monomers such as styrene to produce a low-viscosity resin. Unsaturated polyesters are made with monomers containing carbon-carbon double bonds able to undergo free-radical crosslinking reactions with styrene and other vinyl monomers. Crosslinking the resin by free-radical polymerization produces the mechanical properties needed in various applications. 

Variation of Maleate to Vinyl Ether Ratio. Variation of the stoichiometric ratio of vinyl ether double bond to maleate double bond should produce more flexible films due to plasticization by dangling end groups but also may reduce cure speed and mechanical properties. The curing of unsaturated polyester F with DVE-3 using 4% Darocur 1173... 

Table 21.2 provides a general guide to the effect that chemical constituents can have on unsaturated polyester resin end-use performance. The routes to change an unsaturated polyester resin for a particular application is normally apparent and there can be several pathways available to achieve the desired properties. As the unsaturated polyester resin markets are highly competitive, raw material cost usually reduces the number of available routes available to the formulator. As noted previously, there are three main types or families of unsaturated polyesters, namely general purpose orthophthalic, isophthalic and DCPD resins. However, within each of these families, there are hundreds of variants that incorporate these various chemical constituents in different combinations and permutations to achieve the desired results. 

Polycondensation of a diol with a dicarboxylic acid, either of which may contain a double bond, results in an unsaturated polyester. For this purpose suitable starting compounds are maleic anhydride and 2-butylene-1,4-diol.These can also be used mixed with saturated dicarboxylic acids or diols (copolycondensation) in order to vary the number of double bonds per macromolecule and thereby the properties of the polyester. Unsaturated polyesters are generally prepared by melt condensation.The resulting products are often viscous or waxy substances of relatively low molecular weight. 

Alkyds. Alkyd resins consist of a combination of unsaturated polyester resins, a monomer, and fillers. Alkyd compounds generally contain glass fiber filler, but they may also include clay, calcium carbonate, alumina, and other fillers. Alkyds have good heat, chemical, and water resistance, and they have good arc resistance and electrical properties. Alkyds are easy to mold and economical to use. Postmolding shrinkage is small. Then-greatest limitation is extremes of temperature (above 175°C) and humidity. 

Lecithin has some catalytic or cocatalytic effects in multiphase systems because of its surface-active properties. Lecithin is reported to be useful as an emulsifier in the curing of aqueous dispersions of unsaturated polyesters (337). The products are more easily removed from their molds and have improved mechanical properties when lecithin is used. In a fermentation application, 1.5% soybean lecithin acts as an inducer in the preparation of cholesterol esterase using a strain of Pseudomonas bacteria (338). Aside from its role as a catalyst, initiator, or modifier, lecithin may have ancillary uses in catalyst systems as part of a protective coating (339). 

Table 9.1 highlights the properties, products and applications of unsaturated polyesters. 

The properties of cured polyesters are affected by their structural elements. These elements include the structure and ratio of acids and glycols, the type of end-group present, the type of unsaturation and the distribution of various repeating units. 

Polymer concretes show excellent mechanical properties and chemical resistance compared with conventional cement concretes. Polymer concretes can be cured quickly by the use of curing agents. Thus, the applications of polymer concretes are being increased. One of the popular polymers for polymer concretes is unsaturated polyester (UPE) resin. The properties of UPE resin can be modified by changing its molecular features. For the synthesis of the resin, phthalic anhydride or isophthalic acid as well as maleic anhydride can be employed to modify the mechanical properties or hydrothermal resistance. Terephthalic acid which is also used for the synthesis of poly ethylene terephthalate (PET) enhances the thermal resistance of the cured UPE resin. However, the synthesis of unsaturated polyester resin from terephthalic acid is difficult. One method to synthesize unsaturated polyester from terephthalic acid is the use of recycled PET. 

The nonacrylic monomer styrene is used as a reactive solvent in the polymerisation of coating systems on the basis of unsaturated polyesters. Styrene has a penetrating and unpleasant odour and a strong irritant effect. In spite of this substitute, products such as 2-phenyl-l-propene (a-methylstyrene), n-vinylpyrrolidone and vinyltoluene have not become established owing to their clearly inferior application properties. 

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