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Journal of Macromolecular Science, Part C

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Current Status of Unsaturated Polyester Resins

MONA MALIK , VEENA CHOUDHARY & I. K. VARMA

To cite this article: MONA MALIK , VEENA CHOUDHARY & I. K. VARMA (2000) Current Status ofUnsaturated Polyester Resins, Journal of Macromolecular Science, Part C, 40:2-3, 139-165, DOI:10.1081/MC-100100582

To link to this article: http://dx.doi.org/10.1081/MC-100100582

Published online: 07 Feb 2007.

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J.M.S.—REV. MACROMOL. CHEM. PHYS., C40(2&3), 139–165 (2000)

Current Status of UnsaturatedPolyester Resins

MONA MALIK, VEENA CHOUDHARY, and I. K. VARMA*Center for Polymer Science and EngineeringIIT Delhi, Hauz KhasNew Delhi 110016, India

1. INTRODUCTION .............................................................................................. 140

2. CLASSIFICATION OF UNSATURATED POLYESTER RESINS................ 141

3. SYNTHESIS OF UNSATURATED POLYESTER RESINS........................... 142

4. SIDE REACTIONS DURING POLYCONDENSATION ................................ 1444.1. Addition of Glycol Across the Double Bond (Ordelt Saturation) .......... 1444.2. Isomerization............................................................................................. 1454.3. Transesterification..................................................................................... 1454.4. Dehydration of α-Diols ............................................................................ 146

5. MODIFICATION OF POLYESTERS............................................................... 146

6. CHARACTERIZATION OF UNSATURATED POLYESTER RESINS........ 149

7. LOW-PROFILE ADDITIVES ........................................................................... 1497.1. Mechanism of Action ............................................................................... 1507.2. Selection Criteria ...................................................................................... 151

8. THICKENING AGENTS................................................................................... 152

*To whom correspondence should be addressed.

139

Copyright 2000 by Marcel Dekker, Inc. www.dekker.com

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140 MALIK, CHOUDHARY, AND VARMA

9. CURING OF UNSATURATED POLYESTER RESINS................................. 1539.1. Differential Scanning Calorimetry ........................................................... 1549.2. Effect of Additives on Curing Exotherm................................................. 1569.3. Infrared Studies......................................................................................... 1579.4. Dynamic Mechanical Thermal Analysis.................................................. 158

10. MECHANICAL PROPERTIES ......................................................................... 158

11. APPLICATIONS ................................................................................................ 161

REFERENCES ................................................................................................... 161

1. INTRODUCTION

Unsaturated polyester (UP) resins are linear polycondensation products based onunsaturated and saturated acids/anhydrides and diols or oxides. These resins are generallypale yellow oligomers with a low degree of polymerization. Depending on the chemicalcomposition and molecular weight (1200–3000 g/mol), these oligomers may be viscousliquids or brittle solids. The unsaturation in the backbone provides sites for reaction withvinyl monomers using free-radical initiators, thereby leading to the formation of a three-dimensional network. The solutions of unsaturated polyesters and vinyl monomers (reac-tive diluents) are known as UP resins. The dilute UP resin solution thus obtained has aviscosity in the range 200–2000 cps.

Polyester resins, because of their versatility and low cost, are widely used through-out the world. The commercial applications of the resins started in 1941. Unsaturatedpolyester laminating resins were first introduced on the market in 1946. UP resins arethe most widely used thermosets in polymeric composites. They can be processed overa wide temperature range. Glass-fiber-reinforced UP resins are extensively used in build-ing and construction; in the transportation, electric, and electronic industries; and forsanitary and domestic applications. The biggest use of these resins is in the constructionindustry, followed by transportation. The possibility of curing at room temperature andatmospheric conditions (i.e., ambient conditions) makes these resins very versatile mate-rials for fabrication of large structures such as boat hulls, automobile bodies, aircraftradomes, and the like. Approximately 2.1 million [1] tonnes of UP resins were used forstructural applications alone in 1997.

One of the major problems with these resins is their high shrinkage during thecuring reaction (>7%). The glass fiber reinforcement becomes raised from the surface ofthe molding because of the shrinkage, leading to a poor surface finish. By selection ofdifferent acids/anhydrides and glycols and changes in the ratio of saturated/unsaturatedcomponent and the reactive diluent, the nature of the final network can be varied signifi-cantly to meet different performance requirements. Addition of low-profile additives(LPAs) (thermoplastics) in the resin formulation compensates for the thermal and poly-merization shrinkage of UP resin, thereby leading to low-shrink or zero-shrink resinsystems. The advent of low-profile, low-shrink technology has allowed molding com-pounds to compete successfully with steel in exterior automotive applications.

Considerable work has been reported on the synthesis, characterization, curing be-havior, and properties of UP resins in the past [2–7].

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UNSATURATED POLYESTER RESINS 141

In this paper, an attempt is made to review the state of the art of this important classof thermoset resins. It deals with classification, synthesis, modification, characterization,curing, properties, and applications of UP resins. The main focus in this review articleis on the latest developments in this area.

2. CLASSIFICATION OF UNSATURATED POLYESTER RESINS

Polyester resins may be classified on the basis of their structure into the followinggroups: (1) ortho resins, (2) isoresins, (3) bisphenol-A fumarates, (4) chlorendics, and(5) vinyl ester (VE) resins.

Ortho resins, also known as general-purpose resins, are based on phthalic anhydride(PA), maleic anhydride (MA), or fumaric acid and glycols. PA is relatively low in priceand provides an inflexible link in the backbone. However, it reduces the thermal resis-tance of laminates. Limited chemical resistance and processability are other problemsassociated with these resins. Among the glycols, 1,2-propylene glycol is the most impor-tant. Due to the presence of the pendant methyl group, the resulting resins are less crys-talline and more compatible with commonly used reactive diluent (styrene) than thoseobtained using ethylene glycol, diethylene glycol (DEG), and triethylene glycol and giveproducts with inferior electrical properties. Resins with high heat and chemical resistanceare produced using neopentyl glycol or hydrogenated bisphenol-A.

Isoresins are prepared using isophthalic acid, MA/fumaric acid, and glycol. Theseresins are higher in cost than ortho resins and also have considerably higher viscosities,hence, a higher proportion of reactive diluent (styrene) is needed. The presence of higherquantities of styrene imparts improved water and alkali resistance to the cured resins.Isophthalic resins are thus of higher quality since they have better thermal and chemicalresistance and mechanical properties.

For bisphenol-A fumarates, the introduction of bisphenol-A in the backbone im-parts a higher degree of hardness and rigidity and improvement in thermal performance.They are synthesized by reacting ethoxy-based bisphenol-A with fumaric acid.

For chlorendics, to enhance flame retardancy, chlorine/bromine-containing anhy-drides or phenols are used in the preparation of UP resins. For example, reaction ofchlorendic anhydride/chlorendic acid with MA/fumaric acid and glycol yields the follow-ing resin with better flame retardancy than general-purpose UP resin. Other monomersused include tetrachloro or tetrabromophthalic anhydride. The bromine content must beat least 12% to make a self-extinguishing polyester [8].

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For VE resins, bisacryloxy or bismethacryloxy derivatives of epoxy resins containunsaturated sites only in the terminal position and are prepared by reaction of acrylicacid or methacrylic acid with epoxy resin (e.g., diglycidyl ether of bisphenol-A, epoxyof the phenol-novolac type, or epoxy based on tetrabromobisphenol-A). These resinswere first commercialized in 1965 by Shell Chemical Company under the trade name ofEpocryl. In 1966, Dow Chemical Company introduced a similar series of resins formolding purposes under the trade name of Derakane resins. The viscosity of neat resinsis high; hence, reactive diluent (e.g., styrene) is added to obtain a lower viscosity solution(100–500 poise). Notable advances in VE resin formulations are low-styrene-emissionresins, automotive grades with high tensile strength and heat deflection temperature,hybrid grades that balance performance and economy, and materials for corrosion resis-tance [9]. The effect of marine environment on a VE resin and its highly filled quartzcomposites has recently been reported [10].

3. SYNTHESIS OF UNSATURATED POLYESTER RESINS

Unsaturated polyesters have been synthesized by the reaction of unsaturated andsaturated dibasic acids or anhydrides with dihydric alcohols or oxides. These resins aremanufactured in heated cylindrical steel reactors equipped with a condenser to collectthe aqueous by-products. In the presence of oxygen, some discoloration may be pro-duced, which can be prevented by purging fresh nitrogen or carbon dioxide into the

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reactor. These gases must be free of oxygen. UP resins are synthesized by the fusion-melt process, which is used mainly for the condensation of liquid glycols (e.g., propyleneglycol) (PG) with dibasic acids having low melting temperatures such as PA, aliphaticdibasic acids/anhydrides (e.g., MA), and various halogenated intermediates. The rates ofpolyesterification reactions are influenced by the structure and stoichiometry of the gly-col and dibasic acid components [11]. Generally, a 5% excess of glycol is added in theinitial stages to prevent any glycol losses during polymerization. After adding the rawmaterials, the temperature of the reactor is increased to 100°C to start the exothermicreaction. Two main reactions are involved in this synthesis: (1) monoester formation and(2) polycondensation.

The monoester formation takes place in the temperature range 60°C–130°C, whilethe polycondensation takes place above 160°C. Since it is a reversible reaction, therefore,water coming out of the reaction has to be removed continuously. In this process, phthalicresins require 15 h at 190°C to attain satisfactory molecular weight. At this temperature,some glycol is also lost, along with water of condensation, which can be preventedeither by fractionating condenser systems or by adding acid catalysts [12] to promote theformation of volatile ethers that are lost as by-products. The molecular weight of UPresins is about 3000 g/mol or less at equilibrium state. Further increase in polycondensa-tion leads to gel formation. Several new approaches have been used to increase Mn to5000 g/mol. On curing, such resins give networks with better toughness and mechanicalstrength [13].

The extent of reaction is controlled continuously by checking the acid number.This is done by withdrawing some prepolymer samples from the reactor and titratingthem with 0.1 N alcoholic potassium hydroxide (KOH) solution. The acid number isrecorded as milligrams of KOH per gram of resin and is related to the molecular weightby the equation

MW = 56,100/Acid number (1)

When the acid number drops to 200, xylene is added into the reactor at 3–6 wt% of thetotal material content, and refluxing is started. Xylene forms an azeotrope with waterand accelerates its removal, which in turn increases the extent of reaction. The distillateis collected in a small receiver tank, in which xylene and water separate from each other.Xylene (upper phase) is continuously fed back to the reactor, and water (lower phase) isremoved from the bottom of the tank.

When the acid number has reached 135, the reactor temperature is increased to175°C–185°C. The reaction is carried out until the acid number has fallen to 50 ± 2.Then, cooling is started, and an inhibitor (hydroquinone) is added at 150°C. The activityof hydroquinones is enhanced by quarternary ammonium salts such as trimethylbenzylammonium chloride. Other inhibitors in use are 1,4-napthoquinones, chloranil, catecholderivatives, and the like. The inhibitor is usually added at 0.075 mol% of the total mono-mer content. When the temperature has fallen to 90°C–100°C, styrene is added andcooled as rapidly as possible to prevent polymerization of styrene. The resin thus pro-duced has a pale straw color, mainly owing to inhibitor (hydroquinone).

Isophthalic acid and terephthalic acid are insoluble in the initial glycol/anhydridemelt, which leads to agitation problems. Isophthalic acid reacts more slowly with glycolscompared to PA. In this case, PG and MA react at lower temperatures, while isophthalicacid remains inert. MA preferably forms esters with primary hydroxyl group on PG,

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leading to formation of PG maleate esters with a preponderance of terminal secondaryhydroxyl groups. Maleate ester derivatives do not homopolymerize during processing inspite of having reactive double bonds in MA [14]. The isophthalic acid is esterifiedabove 190°C. This condensation reaction proceeds much more slowly because of thenonavailability of primary hydroxyl groups. Subsequent reaction with isophthalic acidrequires temperatures of 240°C for prolonged periods to dissolve it. Due to these pro-longed heatings, maleate component gives rise to undesirable discoloration. Isophthalicresins attain higher molecular weights than phthalic resins due to the greater bondstrength of the esters. These resins are stable at higher temperatures and give polymerswith higher molecular weight. After polymerization is finished, the resin is cooled andblended with inhibited styrene. When cross-linked, their properties are superior to thoseof phthalic resins. A similar method has been used for preparation of halo and nonhalopolyesters from tetrachlorophthalic anhydride, tetrabromophthalic anhydride, and PAwith DEG and MA. A wide range of rigid, semirigid, semiflexible, and flexible UP resinsand chloropolyesters have been reported in the literature [15, 16].

VE resins are produced by the reaction of epoxy resins such as diglycidyl ether ofbisphenol-A or N,N-O-triglycidyl amino-phenols [17] or epoxy novolacs with acrylicacid/methacrylic acid at 100°C–120°C in the presence of catalyst (imidazole). To inhibitthe polymerization, hydroquinone is added, and the reaction is carried out in the presenceof air. The progress of the reaction is monitored by determining the acid number atregular intervals. The reaction is allowed to continue until the acid number reaches avalue below 10. The amber color resin is cooled to room temperature and diluted with avinyl monomer for further applications.

4. SIDE REACTIONS DURING POLYCONDENSATION

4.1. Addition of Glycol Across the Double Bond(Ordelt Saturation)

The maleate double bond in UP is highly electron deficient due to the presence ofelectron-withdrawing carbonyl groups. Addition of glycol across the double bond (Mi-chael addition) leads to reduction of unsaturation and branching, which leads to an uncer-tainty of chain end functionality. This reaction was first studied by Ordelt et al. [18–20].The Ordelt reaction leads to the formation of a branched structure and a change in thefunctionality of UP resins.

Formulations containing a higher concentration of MA result in the reduction of15% unsaturation in the final product, while fumaric acid retains a higher degree ofunsaturation when substituted in the same formulations. This leads to products of higherreactivities and superior properties. Modeling of unsaturated polyester prepolymer struc-ture has been done from a knowledge of fundamental molecular parameters, including

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hydroxyl and carboxyl indexes, Ordelt saturation extent, short- and long-chain branchdistribution, molecular polydispersity, and starting reactant composition [21, 22].

4.2. Isomerization

The strain energy across the double bond increases when MA, from its planarconfiguration of low conformational energy, changes into maleate diesters and oligomer.This increase is caused mainly because of steric hindrance. As the reaction temperatureincreases from 160°C to 180°C, maleate esters relieve the strain by transforming intomore planar trans-fumarate isomer. This isomerization, which may be partial, occursmainly in the presence of branched secondary glycols such as PG [23] or bulky aromaticdibasic acids, which subjects the maleate double bond to increased strain.

Glycol structure plays an important role in this isomerization, which is enhancedwhen bulky groups are close to the hydroxyls. It has been shown that linear glycols witheven-numbered carbon atoms cause higher isomerization than odd-numbered linear gly-col [24, 25]. Isomerization diminishes in going from ethylene glycol to 1,3-propyleneglycol, from around 65–70% to 15%. The fact that there is one more methylene groupin the 1,3-propylene glycol may be indicative of less spatial interaction between con-densed groups. The isomerization is reported to be 95% with 1,2-propylene glycol, 39%with 1,4-butylene glycol, 55% with DEG, and 35% with 1,6-hexamethylene glycol at180°C [26]. In 1,4-butylene glycol, there is one more methylene group, and a higherdegree of isomerization (in comparison with the polyester from 1,3-propylene glycol)was observed. It has been attributed to spatial interactions between ester groups in closeproximity. The spatial configuration of polyester chains is even more important thanmethylene groups within the glycol chain for cis-trans isomerization to occur. The con-densation reaction proceeds more rapidly and gives polymer in less time if it is betweenlinear ethylene and DEGs with MA. Maleate esters are slightly distorted from the planarconfiguration, which suppresses their ability to copolymerize with styrene monomers.The corresponding fumarate polymers show reactivities [27] almost 20 times those ofmaleate esters because they attain a planar configuration in the trans form. The mecha-nism and kinetics of this isomerization have been studied using 1H-NMR (nuclear mag-netic resonance) [28], infrared analysis, and polarographic techniques. The signals at 6.27and 6.85 ppm in NMR indicate the isomerization of maleate to fumarate. The rate of theisomerization reaction can be increased at lower temperatures by a cycloaliphatic aminecatalyst (e.g., morpholine [29]).

4.3. Transesterification

At high temperature, the polyesterification reaction is often accompanied by trans-esterification, which is in fact an alcoholysis or an acidolysis of the polyester chains byhydroxyl or carboxyl groups of monomers and/or macromolecules.

The transesterification would redistribute molecular mass and functional endgroups of UP molecules. It results in a statistical distribution of macromolecular unitsand chain ends. Some amount of transesterification reaction also takes place in the finalstages of manufacture, which modifies the final molecular distribution by limiting devel-opment of higher molecular weight polymers.

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4.4. Dehydration of �-Diols

Under the conditions of polyesterification reaction, dehydration of α-diol also takesplace, thereby leading to the formation of (1) DEG or dipropylene glycol from 1,2-ethanediol or 1,2-propanediol, respectively, and these dihydroxy compounds react withthe carboxylic species and modify the mechanical properties of the final product; and (2)compounds with a low boiling point, such as tetrahydrofuran (THF) in the case of 1,4-butanediol or cyclic ethers and propanal in the case of 1,2-propanediol in the presenceof strong acids.

Intramolecular ring closure between polymer end groups has been suggested in thepolyesterification reaction between MA and 1,5-pentanediol [30].

5. MODIFICATION OF POLYESTERS

Modification of polyesters has been done to improve fracture toughness, impactresistance, and solubility in water and to reduce Tg and viscosity of polymers. Toughnessof the polymers increases due to the presence of flexible units in the backbone, such aslong-chain diols (e.g., DEG, dipropylene glycol, triethylene glycol) or long-chain satu-rated acids (e.g., adipic acid). Newer UP resin technology capable of providing 50%lower styrene emission levels compared to standard resins has been developed [31].Partial replacement of MA with maleopimaric acid yielded UP resin with higher Vicatdistortion temperatures than usual commercial samples prepared from PA, MA, andPG, while mechanical properties such as modulus and strength were similar [32]. Anacyclic diene metathesis polymerization technique has also been used to prepare UPresins [33].

“All-trans” unsaturated polyesters from diesters of fumaric acid and a variety ofaliphatic and aromatic diols in organic solvents such as THF or acetonitrile have beenprepared using enzyme lipase at ambient temperatures [34–36]. These enzymes werepreviously used to catalyze the preparation of optically active polyesters [37–39]. In

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THF, low molecular weight oligomers were obtained, while in acetonitrile, the averagemolecular weights of the unsaturated all-trans polyesters were comparable with the val-ues obtained for industrially prepared products [40]. The presence of the trans doublebond facilitates cross-linking and has an important influence on the rheological and me-chanical properties of the UP and the cured end product. All-trans [41] enzymaticallyand chemically synthesized UPs are crystalline compared to the commercial amorphousUP. Thermal analysis of the samples of all-trans enzymatically and chemically preparedUP gave the same endotherms. The tensile strengths of enzymatically prepared polyesterswere similar to those for the industrial product.

UV-curable, waterborne unsaturated polyesters prepared using 60/40 (mol%) tri-mellitic anhydride, tetrahydrophthalic anhydride, and the equimolar mixture of ethyleneglycol, DEG, and PG showed balanced coating properties such as good tensile strengthand weatherability, as well as proper viscosity [42]. Unsaturated polyesters have alsobeen prepared using 1,4-bis(2-carboxyvinyl)benzene acid chloride and bisphenols or 4-hydroxycinnamic acid [43].

Preparation of fluorine-containing UP resin has been achieved using telechelicmacromers based on hydroxy-terminated perfluoro polyethers with different molecularweights [44].

Poly(ethylene glycol) (PEG) or liquid hydroxyl- or carboxyl-terminated rubbershave also been copolymerized with UP resin. PEGs with the number-average molecularweight Mn ranging from 600 to 1000 g/mol have been used [45–52]. Reaction of carboxy-terminated UP with various PEG monomethyl ethers of molecular weights from 350 to2000 g/mol was carried out at a high temperature (230°C) for several hours according tothe following reaction sequence [53]:

The block copolymer thus prepared had predominantly one PEG end group perpolyester segment.

Introduction of PEG end groups leads to phase-separated and partly crystallineblock copolymer. The Tg of the resins decreased with increasing length of the PEG endgroup. The solubility of styrene increased at room temperature. Processable resins withabout 20 wt% styrene content could be prepared.

Rheological changes during thickening and molding have been achieved [54] usinga modified UP resin and introducing a thermally breakable diketo group onto the UPresin by salt formation according to the following scheme:

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These groups are stable at room temperature, but can be broken at elevated temperatures.These modified resins showed a fast viscosity increase during thickening and a stableviscosity during room temperature storage.

Synthesis of a variety of novel para-linked aromatic liquid crystalline polyesterscontaining unsaturated fumaroyl units has been reported recently [55–58]. These polyes-ters were prepared by reaction of fumaroyl chloride with t-butyl, phenyl, and phenylethyl hydroquinones using both interfacial and melt polycondensation techniques[57].

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The melt polycondensation of hydroquinone derivative and fumaroyl chloride wascarried out at 145°C–175°C for several hours. The t-butyl–and phenyl ethyl–substitutedpolyesters were thermotropic nematic liquid crystals above their Tg, while the phenyl-substituted polyester showed a nematic texture above the melting point. The t-butyl–sub-stituted polyester formed a lyotropic phase in styrene above 50wt% polyester, whichcould be processed into a film and cross-linked with benzoyl peroxide. Investigation onthermotropic block copolyesters containing aliphatic unsaturated units have also beencarried out. Polymers with a rigid block content of 75% or above exhibited nematicphases after melting [59].

Specific properties of UP resins can be tailored using special additives. Low flammabil-ity of UP resins or systems have been developed by adding aluminum hydroxide [60, 61] andorganic compounds containing halogen or phosphorus [62, 63]. In UP systems containing sty-rene, the evaporation of styrene can be reduced by addition of pyrogenic silicic acid with hy-drophilic and hydrophobic end groups [64] or substituted succinic acid [65].

To eliminate the inhibiting effect of atmospheric oxygen on curing of UP resincoatings and to obtain a tack-free surface, paraffin wax is generally added. A relativelysmall amount of paraffin wax (0.1% in hard paraffin wax and approximately 1% in thecase of stearin) is sufficient for this purpose. The wax or stearin is initially soluble inUP resin coating, but as the curing advances, its solubility is reduced. It thus forms animpermeable surface layer that is a barrier to atmospheric oxygen and for evaporation ofstyrene from the resin formulation. LPAs (for reducing thermal shrinkage) and thickeningagents are described separately below.

6. CHARACTERIZATION OF UNSATURATEDPOLYESTER RESINS

The UPE resins are comprised of molecules of different molecular weights, chemi-cal compositions, and degrees of branching. A thorough characterization of resin is essen-tial for the quality control of resin manufacture and structure-property correlation inpolyester. Molecular characterization of resin has often been carried out by hydroxyl andacid number determination. However, these techniques are not capable of distinguishingdifferences between samples.

The unsaturated polyester resins have been characterized by gel permeation chro-matography (GPC) [66–68] and high-performance liquid chromatography (HPLC) [69,70]. Number-average molecular weights, determined by GPC with polystyrene (PS) cali-bration, were found to be in good agreement with values obtained by vapor phase os-mometry (VPO) and end-group analysis. GPC with UV detection at a pair of suitablewavelengths can determine the mole ratio of unsaturated acid and saturated acid as afunction of molecular weight. Conventional GPC with PS calibration provides good mo-lecular characterization of UP resins with a slight tendency to overestimate Mn and, inthe case of samples containing high molecular weight fractions, to underestimate Mwvalues. Studies on compositional sequence distribution in UP resins have been carriedout by 13C-NMR spectroscopy [71].

7. LOW-PROFILE ADDITIVES

There are several processing problems associated with neat UP resins. Some of theseare surface-quality flaws such as sink-mark formation, part-surface waviness, dimensionalcontrol problems such as warpage, and inability to reproduce the mold accurately.

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These problems are due to UP resin/styrene polymerization and thermal shrinkage.Polymerization shrinkage during cure has been measured by dilatometer [72, 73]. Theamount of shrinkage is reduced by addition of fillers or fibrous reinforcements, but isnot eliminated. Inclusion of 2–5% (by weight of the molding compound) low-profilethermoplastic additive (LPA) in the resin formulation [74] helps compensate for thepolymerization shrinkage. The weight percentage of LPA on the basis of organic resinalone is 7–20%. The function of LPA is to compensate for the thermal and polymeriza-tion shrinkage of the UP resin at minimum cost. The LPA does not participate in thefree-radical polymerization; however, it is known to have an effect on compound viscos-ity and thickening. Low-shrink and low-profile unsaturated polyester resins are com-monly used in several reactive processing manufacturing techniques, such as compres-sion molding of sheet molding compound (SMC) and bulk molding compound (BMC),thick molding compound (TMC), resin transfer molding (RTM), pultrusion, hand lay-up,and resin casting processes [75]. The performance of LPA is affected by the processingconditions, which in turn depend on the process used. For example, RTM is generally alow-pressure ( PMMA > PS. However, when the LPAcontent exceeded 10 wt%, PU was less effective in volume shrinkage control than PVAcor PMMA. For a neat UPE resin having a 2:1 molar ratio of styrene to polyester−C=C− bonds, the fractional volume shrinkage is around 7.2–7.4%. Adding 10 wt%of PVAc or PMMA reduced it to 2.8–4.0%. The extrapolated LPA concentration to givezero volume shrinkage is estimated to be around 15 wt%.

7.1. Mechanism of Action

The curing of thermosetting polyester leads to volume shrinkage, which is compen-sated by expansion of the thermoplastic phase and development of a void structure withinthis phase.

The action [72, 85, 86, 87] of LPAs is believed to involve the formation of a two-phase structure between LPA and UP resins. Polyester/styrene resin solutions containingPU or PVAc as the LPA are initially miscible. During curing, these thermoplastics be-come incompatible with the polyester/styrene copolymer due to a significant change inits polarity and phase separation. Phase separation occurs at the very beginning of thereaction [88–91].

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At low concentrations of PVAc, the final morphology is composed of discreteparticles of LPA dispersed in the cured UP matrix; at a concentration higher than acritical value, a co-continuous structure is obtained. An easy way to determine this criti-cal concentration is by the transition from translucent to cured opaque products. Whenthe reaction starts, the initiator decomposes to form free radicals, initiating polymeriza-tion, which links adjacent UP chains through connecting styrene monomers by bothinter- and intramolecular reactions. Unreacted styrene and UP resin also collect in thethermoplastic phase. As the polymerization continues, the temperature and degree ofpolymerization increases, causing a shrinkage of the UP phase. Polymerization shrinkagein the UP phase causes a large stress in the LPA phase, leading to formation of micro-voids in the LPA phase. The polymer coils become tightened to form the so-called micro-gel structure [92–95]. As the polymerization proceeds, the concentration of the microgelincreases continuously, leading to macrogelation. Due to the increase in temperature, thevolumes of LPA and unreacted monomer inside it increase, thereby compensating forpolymerization shrinkage. During the cooling process, the bulk coefficient of thermalexpansion of LPA and the UP phase are similar above Tg, but below Tg, UP shrinks muchless than LPA, creating more voids in the LPA phase. The subsequent microvoid forma-tion at the interphase between the LPA and the cross-linked UP phases, as well as insidethe LPA phase due to the microstress cracking, leads to the volume shrinkage compensa-tion [96]. The polyester microgel structure formation and curing behavior are stronglyaffected by the compatibility of LPA and UP resin. PVAc has better compatibility withthe UP resins than PMMA does. For UP resins mixed with LPAs, the thermoplasticLPAs may be considered as diluents. Increase of the diluent concentration favors theintramolecular cross-linking reaction of UP with styrene monomer and microgel forma-tion. Methyl methacrylate (MMA), contrary to PVAc, is not miscible with UP/styreneresin, but forms a relatively stable dispersion after thorough mixing. PMMA, along withstyrene and some UP, is dispersed as a 50–100 µm liquid globule in the continuousstyrene/UP phase. During the curing process, the continuous phase cross-links to form anetwork. Curing also takes place in the liquid globules, resulting in the precipitation offine particles coated with PMMA. Further heating and subsequent cooling lead to forma-tion of voids.

7.2. Selection Criteria

The factors considered in selecting an LPA are (1) molecular weight [97], (2)dipole moment, and (3) glass transition temperature Tg. The molecular weight of LPA isimportant. The shrinkage decreases significantly with increase in molecular weight ofLPA until an optimum value is reached. Further increase in molecular weight does notaffect the shrinkage. In the case of PVAc, the useful range is 10,000–250,000, preferablybetween 25,000 and 175,000. Introduction of a few carboxylic groups in LPA helps inthe maturation of SMC. Copolymerization of vinyl acetate or MMA with acrylic acid,methacrylic acid, or maleic acid yields copolymers with a controlled concentration ofacid groups.

The polarities of uncured and cured UP resin are significantly different. The dipolemoment of uncured UP resin is estimated to be 2.0–2.5, while for cured resin, a 0.2–0.8.LPA with a large dipole moment would be very compatible with the highly polar unre-acted resin; however, on cure, the LPA becomes increasingly incompatible with the resindue to the large difference in polarity. Hence, the difference in dipole moment would act

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as a driving force for phase separation. The glass transition temperature of LPA, if lowerthan the curing temperature, will be advantageous because the expansion would be in-creased in the rubbery state.

8. THICKENING AGENTS

The carboxylic acid end groups in unsaturated polyester resins are able to reactwith oxides and/or hydroxides of group II-A metals, such as MgO and Mg(OH)2, whichquite often will result in at least a 1000-fold increase in viscosity (up to >106 cp or 103Pas) in 2–3 days. Such a maturation or thickening process of UP resins is an essentialstep that precedes curing in the processing of SMC, for which the rate of thickening andthe resulting viscosity determine the moldability of the compound [77, 98].

The mechanism of the maturation process using MgO has been the subject ofextensive investigation [80, 99–101]. Several theories have been proposed to account forthis phenomenon. Some of these are as follows:

The dicarboxylic acid end groups of UP chain react with MgO, generating very highmolecular weight species, thereby resulting in a large increase in viscosity andchain entanglement.

Reaction of MgO and one polyester carboxylic end groups. The basic magnesium saltthen complexes with carbonyl or the −OH group of UP resin.

The reaction of basic salt of magnesium with the extra carboxylic acid end group,followed by aggregation of magnesium ions.

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9. CURING OF UNSATURATED POLYESTER RESINS

The curing reaction [93] of UP is a free-radical chain growth cross-linking poly-merization between the reactive diluent (e.g., styrene monomer) and UP resin. Polyestermolecules are the cross-linkers, while styrene serves as an agent to link the adjacentpolyester molecules. Depending on the temperature and other processing conditions,some of the species remain unreacted after curing in the form of residual monomer andsoluble polymer that do not contribute to the network structure.

A wide range of peroxides and azo and azine compounds can be used as initiators,depending on the curing temperature. For room temperature curing, as in the case oflarge hand lay-up structures, methyl ethyl ketone peroxide (MEKP) is used; for moderatetemperature (�60°C–90°C) curing, benzoyl peroxide is used. For hot press or oven cur-ing (�130°C–150°C) di-t-butyl peroxide or t-butyl perbenzoate is used. A mixture ofinitiators is used when a large temperature increase is expected.

To accelerate the decomposition of peroxides, some metal compounds, tertiaryamines, and mercaptans can be used. Cobalt naphthenate (CoNp) and cobalt octanoate(CoOc) are the most widely used accelerators.

ROOH + Co+2 → RO� + OH− + Co+3

ROOH + Co+3 → ROO� + H+ + Co+2

CoNp is used at about 0.01%. The excess of it causes too much acceleration, resultingin darkening color and bubble formation in the cured products. Cure characteristics ofUP resins are affected by the use of cobalt promoter [102]. A three-dimensional networkis formed as a result of copolymerization between the unsaturated monomer present inthe resin (e.g., styrene) and the maleate and fumarate double bonds of the polyester.Microgels are formed as a result of this copolymerization even at very low percentageconversion (�3–4%). The initial studies on copolymerization of UP resin with styrenerevealed an early onset of gelation and a decrease of final conversion, which was attrib-uted to segmental immobility in a cross-linked network [103]. Differences between theo-retical and experimental reactions of UP resin with different styrene/polyester ratios havebeen reported [104].

The occurrence of cyclization and the formation of compact microgel-like particlesat the very beginning of polymerization have been suggested [102, 105]. The networkstructure depended on the concentration and type of polyester resin. It was observed thatgel conversions of UP resins were much higher than those predicted by the classicalFlory-Stockmeyer theory [106]. This delay in cross-linking has been explained on thebasis of competition between intermolecular (network formation) and intramolecular (cy-clization) cross-linking reactions. Hild and Okasha [107], on the other hand, explainedthe delay in gelation as due to reduction of reactivity of polyester vinyl groups with theprogress of the reaction, while Minnema and Staverman [108] attributed it to the shield-ing of pendant vinyl groups.

The curing process of UP resins can be divided into four stages: induction, micro-gel formation, transition, and macrogelation [109]. In the induction period, the free radi-cals are consumed by the inhibitor, and very little polymerization takes place. In thesecond stage, spherical structures (microgel particles) with high cyclization and cross-

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link density are formed. In the transition stage, the (C=C) double bonds buried insidethe microgel undergo intramolecular cross-linking, while those on the surface react withmonomers or microgels. This results in growth of microgel. Finally, macrogelation takesplace by intermolecular microgels and microgel clusters. This stage is accompanied by asharp increase in viscosity.

Thus, the curing reaction of the UP-styrene system can be described in four steps:

1. Intermolecular cross-linking with or without linking through styrene monomers.2. Intramolecular cross-linking with or without linking through styrene monomer.3. Chain branching on the polyester molecule by styrene.4. Free styrene homopolymerization.

Although all reactions affect the curing kinetics, only the first two contribute tothe network formation. The intramolecular cross-linking reaction resulted in the retarda-tion of gelation by reducing the polyester coil sizes. The shrinkage of polyester coils dueto the intramolecular cross-linking reaction was observed by GPC [110].

The cross-linking reaction proceeds as inter- or intramicrogel reaction [111–115].The formation of microgels affects not only the cure behavior, but also the rheologicalchanges. Mazumdar and Lee studied the chemorheological changes using rheometry,electron spin resonance (ESR) spectroscopy, and dynamic light scattering goniometryand suggested a mathematical model to explain the shear effect on resin viscositychanges [116].

Studies on the morphology of UP and VE resins revealed a two-phase structureconsisting of microgels in the PS phase. The two-phase structure of VE matrices, unlikethat of UP resins, was organized rather than random structured. This may be the contrib-uting factor to the excellent hydrolytic stability of VE resins [117].

Various methods employed to study the curing behavior of UP resins are differen-tial scanning calorimetry (DSC) [118], infrared spectroscopy (IR) [119], gel time studies,dynamic mechanical thermal analysis (DMTA) [120], and electron spin resonance spec-troscopy [121]. The effects of temperature and the concentration of initiator, promoter,reactive diluent, additives, and thickening agents on the curing kinetics of UP resins werestudied by the above techniques over the whole conversion range. The cure characteris-tics of thermosetting resins such as VE resin are also affected by the presence of rein-forcements due to surface-resin interaction [122].

9.1. Differential Scanning Calorimetry

The curing of thermoset is a complex process in that several steps and differentreaction processes are involved. As a result of the overall process, heat accompanyingthe curing is liberated. Assuming that the cure reaction is the only thermal event, thereaction advancement is directly proportional to the rate of heat generation, and theultimate extent of conversion is proportional to the total heat that the system is capableof liberating, which can be measured by the DSC technique.

To determine the heat evolved in the total conversion of all reactive groups, onehas to associate an exothermic heat per mole of both the polyester >C=C< double bondsreacted and the reactive diluent (vinyl monomer) >C=C< double bonds reacted, provid-ing that these two reactive groups are the only ones that contribute to the overall exother-mic effect.

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The peroxide-initiated copolymerization of UP resin with solid monomers such aszinc acrylate was followed by gelation curves and DSC [123]. The effect of variousparameters, such as curing temperature, initiator concentration, promoter concentration,additives [124], and thickening agents [125, 126] has been studied by DSC techniques.

The curing of UP resin and VE resin depends on concentration, as well as thenature of reactive diluents. Styrene is still the main reactive diluent in these formulations.Apart from styrene, other unsaturated compounds, such as vinyl toluene, α-methyl sty-rene, diallyl phthalate, methacrylates, acrylonitrile (AN), and triallyl cyanurates, havealso been used. Partial replacement of styrene by AN reduces the resin cure [127]. TheDSC scans of VE resins having 4% benzoyl peroxide and containing 50% MMA (samplea) are shown in Fig. 1. A sharp exotherm was observed in the temperature range 100°C–155°C. The exotherm peak position was at 125°C. Partial replacement of MMA by vary-

FIG. 1. Differential scanning calorimetry scans of vinyl ester (VE) resins (recorded instatic air at 10°C/min) having 4% benzoyl peroxide and 50% methyl methacrylate (MMA)/glycidylmethacrylate (GMA) as the reactive diluent: (a) VE + 50% MMA; (b) VE + 50% GMA; (c) VE +20% GMA + 30% MMA; (d) VE + 40% GMA + 10% MMA.

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ing amounts of glycidyl methacrylate (30% wt/wt) decreased the characteristic curingtemperatures of VE resin.

Isothermal DSC scans recorded for a particular composition at various temperaturesshowed that the reaction occurs at a fast rate as the temperature increases. The inductiontime and the time of maximum rate are shorter with increasing temperature. The ultimateoverall conversion increases with temperature. In the beginning of the curing process, anearly azeotropic copolymerization mechanism dominates. At these initial stages, the cross-link density of the reaction system is not sufficiently high to influence appreciably thepropagation mechanism of the styrene and polyester vinylene groups. As the reaction pro-ceeds, the cross-link density increases to some extent so that the mobility of vinylene unitsin the large polyester molecules declines remarkably, while that of small styrene monomersis less affected. Consequently, the relative probability of the styrene propagation reactionis elevated, causing a higher conversion of styrene than of polyester vinylene.

Macrogel formation is the main feature in the curing of UP resins. At low tempera-tures, at which the cross-linking reactions are quite moderate, prior to the peak of theDSC rate profile, the intermicrogel cross-linking reaction is dominant over the intrami-crogel cross-linking reaction. In the later stage of the reaction following the peak of theDSC rate profile, intramicrogel reactions are more favorable than intermicrogel ones[128]. The intermicrogel reaction enhances polyester vinylene conversion more than sty-rene conversion since styrene tends to copolymerize with polyester vinylene with asmaller cross-link length of styrene. In contrast, the intramicrogel reaction is more favor-able with styrene conversion due to the extensive branching growth of styrene on polyes-ter >C=C< bonds inside the microgels with a larger cross-link length of styrene.

The kinetics of the cure of UP and VE resins obeys the autocatalytic rate equation

dα/dt = (k1 + k2α)m(1 − α)n

where α is extent of conversion, k1 and k2 are kinetic rate constants, and the overallreaction order m + n is generally considered equal to 2. Two model elementary rateequations for radical and monomer have been proposed to describe the free-radical po-lymerization of UP resin [129].The activation energy of the curing reaction has beendetermined using this expression and was found to be around 46 kJ/mol [130] for UPresins and 80 kJ/mol for VE resins [131]. The heat of polymerization of the double bondsin the polyester heavily depend on the type of prepolymer and have been reported in therange 54–104 kJ/mol [132, 133].

The mechanism and the kinetics of the cross-linking of UP resin are strongly af-fected by the presence of bismaleimide resins as coreactive monomers. Martuscelli andothers studied the structure of such an intercross-linked network; during the curing ofVE resins with styrene as a diluent, it was observed that polymerization was randominitially, but as the resin advancement took place, styrene homopolymerization tended todominate [134, 135]. Styrene monomer was found to react at a slower rate, yet it reacheda greater extent of conversion than VE resin [136]. The properties of cured resin dependon the initiator system [137].

9.2. Effect of Additives on Curing Exotherm

9.2.1. Low-Profile Additives

The three major reactions (styrene–polyester vinylene, styrene-styrene, and polyes-ter vinylene–polyester vinylene) in low-shrinkage resins are the same as those in purestyrene-UP copolymerization. The structure formation, however, can be different.

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Addition of the LPAs such as polycaprolactone diol (isocyanate terminated) oncuring kinetics revealed a decrease in exotherm [138].The final degree of conversion wasalso lower for resins containing LPAs. The curing reaction rate decreased with an in-crease in LPA concentration [139]. The compatibility of LPA with UP resin had a stronginfluence on the polyester microgel formation and the curing behavior. The curing reac-tion of UP resins with the addition of LPAs is an inhomogeneous free-radical chaingrowth cross-linking copolymerization. When the ratio of styrene to UP unsaturation waskept constant, PVAc did not have any effect on cure kinetics, maximum conversion, orTg. In the cure of UP resin, the overall reaction rate is a summation of the intramicrogelcross-linking reaction rate and intermicrogel cross-linking reaction rate. The diffusionbarrier effect of LPA may reduce the later reaction (intermicrogel cross-linking). Increaseof the LPA concentration facilitates the UP resins to form coils in the styrene monomerand to undergo an intramolecular reaction, which may cause a delay of gelation. Microgelformation is enhanced by such intramolecular reactions, which become more compact.Many pendant polyester vinyl groups may be trapped in the compact microgels thusformed. The radical concentration profiles obtained from isothermal and scanning ESRexperiments for uncured UP resin with and without PVAc were very different, indicatinga difference in reaction mechanism.

9.2.2. Thickening Agents

Curing of UP resin during thickening has been investigated using DSC and viscom-etry [125]. Thickening of UP resin with group II-A elements leads to formation of micro-domains of aggregates for polyester chain segments through linking and coordination ofMgO. In these microdomains, the molar ratios of styrene to PE would be lower than inthe original resin. This would lead to a reduction in reactivity of styrene and polyesterC=C bonds. It is generally observed that [140] in the cure of thickened UP resins, theheat of reaction decreases when compared with nonthickened ones. During the initialperiod of thickening, induction time and the time to reach the maximum rate increasedas the degree of thickening increased, while the trend was reversed in the later period ofthickening. The final C=C conversion was reduced when compared with those for theneat UP resin. Conversion with styrene was higher than that of polyester C=C bonds.The curing reaction of UP resin thickened with isocyanate-terminated polyester has beenexplained on the basis of microgel formation [141].

9.3. Infrared Studies

Reaction conversion can be determined by an IR technique from the consumptionof styrene >C=C< bonds at 912 and 992 cm−1 (C−H out-of-plane bending in CH2=CHR)and the consumption of polyester >C=C< bonds at 982 cm−1 (C−H out-of-plane bendingin trans CHR=CHR) characteristics of styrene and polyester, respectively. During thereaction, the styrene consumption can be determined easily from the peak area changeat 912 cm−1, but the consumption of polyester >C=C< bonds cannot be followed directlyfrom the peaks at 982 and 992 cm−1 because they overlap each other. A subtractionmethod has been used to separate the overlapping peaks [142, 143]. Final conversion ofmethacrylate end groups of VE resin diluted with styrene has been evaluated by Fouriertransform infrared (FTIR) spectroscopy [144]. The polyester/styrene cross-linking reac-tions at 26°C were studied by Urban et al. [145]. Their study indicated that, during thecross-linking reaction, styrene polymerized, forming atactic polystyrene segments that

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were an integral part of the polyester/styrene network. Dell Erba et al. [146] investigatedthe conversion profiles of both the styrene and polyester unsaturations in the temperaturerange 336–363 K using FTIR spectroscopy. The effects of comonomer compositions onthe curing kinetics of UP resins using DSC and IR have been reported by Huang andChen [112, 113].

9.4. Dynamic Mechanical Thermal Analysis

Small curing changes can be monitored easily by dynamic measurements [147].For the sample cured at room temperature [148], two apparent glass transition regions(at 65°C and 115°C) were exhibited in the DMTA trace, but only one transition wasobserved for the “fully” cured sample. This anomalous behavior was explained in termsof the influence of the additional cure that occurred during the DMTA experiment. Asthe sample is heated, it enters the glass transition region. As the temperature is increasedfurther, the thermal energy provides sufficient molecular mobility to recommence thecuring process, causing a shift in the transition region and an increase in the modulus.Similar behavior has been reported in polyesters based on bisphenol-A and has beenattributed to additional cure during the DMTA experiment.

The dynamic mechanical behavior of UP resins showed three relaxations: α, β,and γ, in order of appearance with decreasing temperatures. According to Tanaka [149]and Cook and Delatycki [150–152], α-transition occurs due to the activation of relativelylong-range motions involved in glass transition phenomenon. The β-transition is mainlydue to activation of motions in the polyester main chain or may be due to the relaxationof styrene-based bridging units, while the γ-transition may be related to some motion ofthe phenyl groups in the styrene sequences [153–155]. Dynamic mechanical spectros-copy combined with solid-state 13C-NMR has been used to characterize the molecularmotion responsible for the gamma and beta secondary relaxations in UPE networks. InDEG networks, the gamma transition is assigned to restricted phenyl group rotation inthe styrene cross-links, whereas the motion of molecular groups in the vicinity of theresidual maleic double bonds account for the β-transition [156].

10. MECHANICAL PROPERTIES

The rigidity of the prepolymer, the type and concentration of reactive diluent (vinylmonomer), and the cross-link density are the major contributing factors to the mechanicalstrength of the cured UP and VE resins [157–161]. An increase in cross-link densityresults in an increase in modulus, glass transition temperature, and decrease in strain tofailure and impact energy. Cross-linking in UP resin is due to unsaturation provided bymaleate/fumarate linkage in the backbone. The higher the concentration of these units,the higher is the cross-link density. The microstructure of the cured UP resin is heteroge-neous, consisting of highly cross-linked domains surrounded by less cross-linked areas[162–166]. The heterogeneity of the microstructure is influenced by the concentration ofthe catalyst (MEKP [167]), accelerator (cobalt octoate), and temperature of curing andpostcuring [168], which in turn affect the Tg of resins. The effect of chemical compositionand structure of UP resins on the miscibility, cured sample morphology, and mechanicalproperties of styrene/UP/LPA ternary systems has been reported recently [169].

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The amount and type of reactive diluent also influence processing characteristicsof UP resins and the properties of the cured resin. Styrene is the most commonly usedvinyl monomer in polyester resins. This is due to its low cost, high compatibility, andreactivity with the 1,2-disubstituted double bond of polyesters. The reactivity ratio ofstyrene and maleic/fumaric acid esters is about 0, indicating that this system has a ten-dency to form copolymers in which the sequence of the monomer unit alternates. Styreneimparts good mechanical properties and heat distortion resistance to cured resins. Thestyrene content in these resin formulations ranges from 30% to 60%. An increase instyrene content reduces the cross-link density, which leads to a reduction in tensile andflexural strength and modulus. Most general-purpose resins contain between 30% and35% styrene. An excessive amount of styrene increases the curing time and impartsstyrenelike properties to the cured UP resins. Isophthalate resins (Mn ranging from 484to 1712 g/mol) based on equimolar quantities of MA, isophthalic acid, PG, and DEGexhibited Tg from 251 to 267 K. By using styrene as the diluent (molar ratio of C=Cdouble bond in styrene and UP resin of 2–5) resins with different Tg could be prepared.A relatively high fraction of styrene gave a cured product with reasonably high Tg values(approaching pure PS) and low moisture absorption [170]. Small quantities of α-methylstyrene added to UP and VE resin acts as an exotherm peak depressant and increasescuring time [171–173]. Addition of vinyl toluene increases the cross-linking. It is alsoof high reactivity and is less volatile than styrene. Partial substitution of styrene byacrylates such as MMA or methyl acrylate improves weather resistance and adhesion toglass fibers.

The mechanical properties of UP resins can be improved by the addition of styreneand AN. AN copolymerizes readily with styrene and gives products with better mechani-cal properties. The hardness of the samples increases as small amounts of AN are addedand reaches a maximum point. Maximum increase in hardness is achieved at 40% styreneand 12% AN for PG-based products. Maximum increase in impact strength (IS) occursat 40% styrene and 20% AN for PG-based products. DEG-based products have a verysoft texture without AN. Their hardness increases at 40% AN composition. AN increasesthe solubility of the resin in styrene and hence prevents phase separation, which leads toimproved mechanical properties [174]. Resins with better heat resistance are producedusing trialkyl cyanurate, which is less volatile. However, it is less reactive than styrene.Diallyl isophthalate compositions, although more expensive, have better heat resistanceand can withstand temperatures up to 220°C.

The rigidity of the polyester backbone also affects the mechanical properties. Sub-stitution of tetrabromo or tetrachloro phthalate units in UP resins increases the molecularrigidity, which leads to an improvement of tensile strength and modulus [175, 176].Young’s modulus and compressive strength are influenced by the molecular weight ofthe glycol [177]. The isophthalic resin showed higher tensile and flexural properties thanthe ortho resins. Terephthalic acid provides a higher heat deflection temperature thanisophthalic acid. Cured BPA fumarate and chlorendics show brittle behavior with lowelongation at break.

VE resins combine the excellent thermal and mechanical properties of epoxy resinswith the ease of processing and rapid curing of polyester resins. Epoxide backbones ofdifferent molecular weights produce greater toughness and resiliency, solvent resistance,and heat resistance. The mechanical properties of various UP resins are given in Table 1.

UP resins are limited by their brittleness, especially when good impact behavior isrequired. This problem can be overcome by blending it with liquid rubber [178–180],

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TABLE 1

Mechanical Properties of Cured Unsaturated Polyester Resins

VE resinBPA bisphenol-A–

Property Ortho resin Iso resin fumarate Chlorendic based resin

Tensile strength,MPa 55 75 40 20 80

Tensile modulus,GPa 3.45 3.38 2.83 3.38 3.5–3.9

Elongation, % 2.1 3.3 1.4 — 4.0Flexural strength,

MPa 80 130 110 120 140Flexural modulus,

GPa 3.45 3.59 3.38 3.93 3.7Heat distortion

temperature, °C 80 90 130 140 100

which leads to the toughening of UP resins. This method has only limited success be-cause of poor solubility of the rubbery component in the unreacted resin, as well as thepoor chemical reactivity of rubber toward the polyester functionalities. The NH2 [181]end groups of a butadiene-AN copolymer show enhanced reactivity toward an unsatu-rated polyester matrix. Chemical modifications of rubbers [182] have been done to in-crease their reactivity with UP end groups. For example, hydroxy-terminated polybuta-diene was converted to isocyanate-terminated rubber, which could react with hydroxygroups of UP resin or an amino-terminated butadiene-acrylonitrile (ATBN) rubber wasconverted to maleimido end-capped rubber (ITBN) capable of reacting with ester doublebonds of UP resins. The reaction between ATBN and MA proceeds via two steps: (1)the formation of a maleamic acid intermediate and (2) cyclization via dehydrating agents.

Both the modulus and the yield stress decreased on addition of ATBN or ITBN,but this effect was less pronounced for the UP/ITBN blend. A substantial increase oftoughness was obtained in the UP/ITBN blend. This may be due to the chemical interac-tion among the components during the curing process, which leads to improved adhesionamong the phases.

Another example of toughening agent for UP resins is a polyisobutene rubber mod-ified by grafting succinic anhydride onto its end groups. Toughness increased to a largeextent when polyisobutylene–succinic anhydride was used as a modifier. The best resultswere achieved with the highest grafting degree of the rubber. Liquid polybutadiene andits copolymers with AN are immiscible with commonly used VE resins. Modification oflow molecular weight hydroxy-terminated polybutadiene through reaction with diisocya-nate and alcohols rendered the polymer more compatible and yielded a tougher curedproduct [183]. Nylon 6,6 oligomers, obtained from industrial wastes, have been used tomodify UP resins to prepare semi-interpenetrating polymer networks [184].

The increasing use of UP matrix composites exposes them to a variety of hygro-thermal conditions that can affect the (1) thermomechanical properties of a compositeand the matrix and (2) the deformation and residual stresses that result from thermal andswelling strain in the laminate.

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The absorption of moisture into a resin generally results [185] in a reduction of theglass transition temperature Tg and a reduction of the modulus and strength, coupled withan increase in strain to failure, a small increase in thermal expansion coefficient belowTg, and dilation (swelling). The effects of solvent exposure on the viscoelastic propertiesof resins (UP and VE resins) have been investigated by dynamic mechanical analysis.The chemical structure of the resin was found to be a determinant in the changes pro-duced after solvent exposure [186].

11. APPLICATIONS

Unsaturated polyester resins either in the form of pure resin or in the compoundedform with fillers find a wide range of applications. Their main applications are in theconstruction industry (nonreinforced or glass-fiber-reinforced products), automotive in-dustry, and industrial wood and furniture finishing.

Some of the important products based on UP resins are cast items such as pearlbuttons, knife and umbrella handles, and encapsulated electronic assemblies. Polyestercompounds have been formulated for the manufacture of bathroom fixtures. Floor tileshave been manufactured by mixing UP resin with fillers such as limestone, silica, chinaclay, and more.

The applications of UP resins in a variety of areas such as transportation, electricalappliances, and building and construction are largely due to development of bulk andsheet molding compounds using glass fibers.

ACKNOWLEDGMENT

We are grateful to the Naval Research Board, Government of India, for providingfinancial assistance for carrying out this research work.

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