Unsaturated Polyester Resins, Blends, Interpenetrating Polymer Networks, Composites, and Nanocomposites: State of the Art and New Challenges

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UNSATURATED POLYESTER RESINS, BLENDS, INTERPENETRATING POLYMER NETWORKS, COMPOSITES, AND NANOCOMPOSITES: STATE OF THE ART AND NEW CHALLENGES

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Anjali A. Athawale1 and Jyoti A. Pandit2 1

Department of Chemistry, Savitribai Phule Pune University, Pune, India 2School of Chemistry, Dr. Vishwanath Karad MIT World Peace University, Pune, India

1.1 INTRODUCTION Unsaturated polyesters (UPs) are synthetic copolymers having applications as fibers, plastics, composites, and coatings. Depending on the choice of monomers, initiators, curing agents, additives, and modifiers used, different varieties of products can be produced exhibiting a wide range of chemical and mechanical properties. The low cost involved in their production makes them attractive. Their main application is as matrices in the composite industry. Among the composites, fiber glass
reinforced composites are of prime importance.

1.2 TYPES OF UNSATURATED POLYESTER RESINS Based on their structure, unsaturated polyesters resins (UPR) can be classified as: (1) ortho resins; (2) iso resins; (3) bisphenol A fumarates; (4) chlorendics; or (5) vinyl ester (VE) resins.

1.2.1 ORTHO RESINS Ortho resins are also referred to as general-purpose polyester resins and are based on orthophthalic acid, namely, phthalic anhydride (PA), maleic anhydride (MA)/fumaric acid, and glycols. PA is relatively cheap and provides rigidity to the backbone. However, it has limited thermal and chemical resistance and processability. Among the glycols, resins formed using 1,2-propylene glycol (PG) are more important in comparison to other glycols. The pendant methyl group in PG lowers the crystallinity of resin and improves its compatibility with commonly used reactive diluents (such as Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00001-6 © 2019 Elsevier Inc. All rights reserved.

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CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

styrene). Neopentyl glycol or hydrogenated bisphenol A yields resins with high heat and chemical resistance.

1.2.2 ISO RESINS Iso resins are prepared using isophthalic acid, MA/fumaric acid, and glycol. They are relatively expensive and have considerably high viscosities. Hence, they require a large proportion of reactive diluent, which also imparts improved water and alkali resistance to cured resins. They find applications as gel barrier coats in marine environments since they have better thermal and chemical resistance and mechanical properties.

1.2.3 BISPHENOL A FUMARATES These are synthesized using ethoxy-based bisphenol A and fumaric acid. Though expensive, they exhibit superior chemical properties as well as corrosion resistance as compared to ortho and iso resins. The presence of bisphenol A in the backbone renders a higher degree of hardness and rigidity and improved thermal performance. Due to the reduced number of interior chain ester groups, their hydrolysis resistance is best among commercial unsaturated resins.

1.2.4 CHLORENDICS Chlorine/bromine-containing anhydrides or phenols are used for preparing chlorendics. They exhibit flame resistance along with good chemical and corrosion resistance. For example, the reaction between chlorendic anhydride/chlorendic acid and MA/fumaric acid and glycol yields resin with better flame retardancy than general-purpose UPR. Other monomers used include tetrachloroor tetrabromophthalic anhydride. The bromine content must be at least 12% in order to obtain a self-extinguishing polyester.

1.2.5 VINYL ESTER RESINS VE resins contain unsaturated sites only at the terminal position as bisacryloxy or bismethacryloxy derivatives of epoxy resins. They are prepared through the reaction of acrylic acid or methacrylic acid with epoxy resin (e.g., diglycidyl ether of bisphenol A (DGEBA), epoxy of the phenol novolac type, or epoxy based on tetrabromobisphenol A). These resins were first commercialized in 1965 by Shell Chemical Company under the trade name Epocryl [1]. In 1966 Dow Chemical Company introduced a similar series of resins for molding purposes under the trade name Derakane resins [1]. The viscosity of neat resins is high; hence, reactive diluents (e.g., styrene) are added to obtain solutions with lower viscosities (100
500 poise). Notable advances in VE resin formulations include low-styrene-emission resins, automotive grades with high tensile strength and heat deflection temperature, hybrid grades that balance performance and economy, and materials for corrosion resistance.

1.3 SYNTHESIS OF UNSATURATED POLYESTER RESINS

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1.3 SYNTHESIS OF UNSATURATED POLYESTER RESINS UP is often synthesized as a viscous liquid through the melt condensation of an aromatic dicarboxylic acid such as phthalic acid or anhydride with polyhydric alcohol and unsaturated dicarboxylic acid or anhydride. The viscosity of the reaction product/oligoester (OER) is reduced using a reactive diluent such as a vinyl monomer, usually styrene. Free radical copolymerization between styrene and the double bonds of UP results in a rigid three-dimensional cross-linked structure, which is a heterochain thermoset type of polymer. Methyl ethyl ketone peroxide (MEKP) is a standard catalyst that initiates the curing reaction in combination with a cobalt or cobalt-amine activator system/accelerator at room temperature. Other free radicals used for curing UPRs include benzoyl peroxide (BPO) or cumene hydroperoxide [2]. After synthesis, an inhibitor is added to the resin to provide a long storage life, fast cure, and to minimize catalyzed or uncatalyzed drift, undesirable colors, odors, or side effects. Hydroquinone, 4,4-dihydroxybiphenyl, and substituted catechols are some examples of inhibitors [3]. Attempts have been made by various researchers to tailor the mechanical, thermal, corrosion, and fire resistance properties of UPRs for various applications. Parker et al. suggested the use of isophthalic acid for improved mechanical properties and corrosion resistance [4]. A two-stage synthesis process was patented by Watanabe et al. to address the necessary improvements using dimethyl terephthalate instead of isophthalic acid [5]. Styrene, vinyl toluene, tert-butylstyrene, chlorostyrene, and diallyl phthalate have been used as reactive diluents. The effects of various concentrations of anhydride (PA and MA) on mechanical properties were reported by Thomas et al., with 60%
70% MA showing the best mechanical properties [6]. They also synthesized various formulations by varying both the anhydride and the alcohol concentration. A mixture of 60% MA with PA yielded an UPR with the best mechanical properties. However, resin with a higher proportion of PA was found to be tough and flexible. Similarly, diethylene glycol (DEG) increased the toughness, impact strength, and flexibility, which was lost on standing. Optimal properties are observed with a 20/80 ratio DEG/PG resin together with an equimolar amount of MA and PA [7]. UPRs have also been synthesized from bio-derived diesters of unsaturated diacids such as itaconic, succinic, and fumaric acids with various diols and polyols to afford resins of M n B480
477,000 and glass transition temperature (Tg) of between 230.1 C and 216.6 C with solubilities differing based on the starting monomers used [8]. Yoon et al. regenerated UPR after recycling cured UPR. The recycled UPR exhibited a faster curing rate than that of neat resin. A comparison of the mechanical properties of the neat resin and the mixtures (neat resin and recycled) revealed that although the properties of neat resin were superior, those of the mixtures were dependent on composition and were found to be suitable for many applications [9]. Different proportions of cobalt (Co) curing agent were used (0.05%
1%). An increase in the concentration of Co from 0% to 1% led to a decrease in curing time. This was reduced to half in the presence of 0.05% Co [10]. The effect of volume ratios of curing agents, viz., cobalt octoate as an accelerator and MEKP as an initiator, on gelation time and exotherm behavior of a UPR has also been studied. The gelation of the resin was found to correspond with the onset of an increase in temperature during resin curing. The gelation time was found to vary inversely with the concentrations of accelerator and initiator [11]. The viscosity of the liquid system was found to decrease with increasing temperature, but increased at the curing temperature. The quality of the cured UPR

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CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

was predicted on the basis of its fragility parameter (Mc). In the UPR
MEKP system, the smaller the Mc the larger the Tg and the better the heat resistance [12]. The curing behavior of UPR was studied using an experimental and theoretical model by Kosar and Gomzi [13]. The kinetic behavior of the curing system was investigated using both dynamic and isothermal measurements and a good agreement was established between the two (in terms of presented kinetic parameters and reaction heat). Heat generated from the cure reaction was measured in molds of cylindrical shape. The difference in heat conductivity between glass and copper was the main reason for the greater heat generated in the glass mold. Control over resin shrinkage of residual monomers is an important concern in low-temperature molding processes. The presence of low-profile additives (LPAs) can reduce the shrinkage of UPR/ styrene resins under proper processing conditions, but may increase the residual styrene content. A systematic study was carried out to investigate the effect of the initiator system and reaction temperature on the sample morphology, final resin conversion, and resin shrinkage of UPR with LPA. The results showed that the final conversion of the resin system could be improved by dual initiators, with the effect being prominent at low temperatures. The study on shrinkage control reported that good LPA performances were achieved at low (35 C) and high (100 C) temperatures, but worse performances were observed in the intermediate temperature range (e.g., 60 C
75 C) (Fig. 1.1). The final shrinkage is influenced by the effect of temperature on the morphology, the relative reaction rate in the LPA-rich and UPR-rich phases, and microvoid formation [14]. The sample morphology shows a two-phase cocontinuous structure at 35 C (Fig. 1.2). One is a particulate phase (LPA-rich) having loosely packed spherical particles with diameters ranging from 1 to 5 mm. The other phase is a flake-like region (UPR-rich) with domain sizes ranging from 10 to 20 mm. At a curing temperature of 60 C, a similar two-phase structure is observed, but it is no longer cocontinuous. The particulate region is smaller and becomes the dispersed phase with a domain 4

Volume shrinkage (%)

3 2 1 0 –1 –2 –3 35

60 75 Temperature (ºC)

100

FIGURE 1.1 Volume shrinkage of UP/St/LPA systems cured at different temperatures (3.5% LPA, 0.5% Co Oct., 1.3% MEKP, 0.4% TBPB, 300 ppm BQ).

1.3 SYNTHESIS OF UNSATURATED POLYESTER RESINS

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FIGURE 1.2 Morphology of St/UP/LPA samples cured at different temperatures (3.5% LPA, 0.5% Co Oct., 1.3% MEKP, 0.4% TBPB, 300 ppm BQ).

size of less than 20 mm, while the flake-like region forms the continuous phase. On increasing the temperature to 75 C and 100 C, the size of the particulate region is further reduced. The various morphological structures result in different interface areas, strongly affecting the shrinkage control. Commercial UPRs contain 30%
40% styrene by mass. The miscibility of resin and styrene depends on the resin composition. Phase separation is reported with an increase in styrene concentration. Thermal stability and mechanical properties are governed by the phase behavior of the mixture and can, therefore, be controlled by styrene content [15]. Dynamic mechanical analysis (DMA) tests have shown phase separation in cured resin with high styrene concentrations. Tg is also dependent on styrene concentration together with thermal stability and mechanical behavior [16].

1.3.1 LOW-STYRENE-EMISSION UNSATURATED POLYESTER RESIN Styrene has remained a preferred reactive diluent for adding to UP due to its cost and availability. It controls the viscosity and facilitates the curing of polyesters at room temperature. However, the use of styrene is associated with serious health problems such as respiratory diseases and skin

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CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

irritation. It is carcinogenic and also attacks the central nervous system on exposure over a long period of time, leading to possible headaches and depression. The minimization of styrene volatilization or its elimination using alternative monomers is being attempted to overcome these problems. The volatilization of styrene is reduced by paraffinic waxes which act as a barrier. However, the wax layer needs prior removal to avoid problems of adhesion to other parts. The ambient concentration of styrene vapor can be reduced using spray guns that can monitor the amount of resin sprayed. Since alternatives such as vinyl toluene, alpha-methylstyrene, and diallyl phthalate suggested for styrene also involve health hazards, Poillucci and Hansen proposed the use of bioderived limonene oil and petroleum-derived vinyl neodecanoate and vinyl laurates as other substitutes for styrene, but they exhibit limited chemical compatibility. The styrene content was reduced by 50% using trimethylolpropane diallyl ether [17]. Mariani used various cross-linking agents such as 2-hydroxyethyl acrylates (HEA) or a mixture constituted of diurethane diacrylate and styrene or HEA for frontal curing of UPR derived from the reaction of MA and 1,2 propanediol [18]. Zang et al. reported a benzyl end-cap-UP resin with low styrene emission using benzyl alcohol as the end-capper [19]. For nonhalogenated resins, a marked restriction in styrene emission is achieved by including long-chain alpha-olefins with 18
40 carbon (C) atoms without the addition of wax. These olefins on their own will not usually provide such a marked restriction in styrene emission, but will allow for the incorporation of a waxy compound in an amount sufficiently large to achieve the desired styrene emission restriction without incurring the expected disadvantages associated with the incorporation of such large amounts of waxy compound [20].

1.3.2 STYRENE-FREE COMPOSITIONS FOR CURABLE COATINGS When UPRs are used as coatings, styrene-free compositions are favored since volatile emissions by such compositions are expected to be low. An example of such a formulation consists of one comonomer selected from the (meth)acrylates of cycloaliphatic alcohols and optional comonomers could be tetrahydrofurfuryl (meth)acrylate, methoxypolyethylene glycol, mono(meth)acrylate, ethylene glycol dimethacrylate, and di(ethylene glycol) di(meth)acrylate while the curing can be done by radiation and/or through the peroxide or thermal routes. More specifically, curing can be performed by adopting a process comprising at least one step of radiation and/or peroxide curing [21]. Styrene-free UPR coatings cured by infrared radiations are described as containing an unsaturated ether component as well as saturated monohydric alcohol along with dicarboxylic acid and dihydric alcohol [22]. Also, radically curable styrene-free coatings are claimed to be composed of compounds containing a (meth)acryloyl group and/or vinyl ether groups along with paraffin, a plasticizer, and carbamic acid [23]. Styrene-free compositions are reported by using various reactive diluents singly or in combination such as 2-hydroxyethyl methacrylate, 2-hydroxy propyl methacrylate, 2-HEA, 2-hydroxypropyl acrylate, and related compounds [24]. UPR can also be obtained as a reaction product of at least one diol having 2
8 C atoms, one monoalcohol with at least one allylic unsaturation, and at least one saturated aliphatic monoalcohol having 4
10 C atoms or one aromatic monoalcohol having 7
10 C atoms. The coating or molding composition of such a UPR is curable by radiation and/or through the peroxide or thermal routes [25]. McAlvin reported UPR derived from biologically renewable resources and recycled materials,

1.3 SYNTHESIS OF UNSATURATED POLYESTER RESINS

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which are styrene-free and ultralow volatile organic compound (VOC) resins that provide matrix materials to produce more ecologically friendly composites [26]. A styrene-free UPR forming a stable dispersion in water has been reported. The modification was done by introducing polar hydrophilic groups such as carboxylic and sulfonic groups (sodium 5-sulfonatoisophthalic acid) into the resin molecule, which ensure good tolerance to water. Styrene has been replaced with the glycerol monoethers of allyl alcohol and unsaturated fatty alcohols as reactive built-in cross-linking monomers for resin modification [27].

1.3.3 MODIFICATION OF UNSATURATED RESIN FOR VISCOSITY CONTROL UPRs have replaced sheet metal in many applications such as in the automotive, electric, and home appliance industries as a consequence of their properties such as being light weight, having high strength, and their noncorrosive nature. UPR composite products are manufactured by compression molding in the form of sheet molding compounds (SMCs) or bulk molding compounds (BMCs), through injection molding in the form of BMC, resin transfer molding, casting, and hand layup. Chiu et al. attempted to develop UPR systems exhibiting viscosity profile properties such as rapid increase during maturation/thickening and mold filling so that they can be handled easily, have good fiber carrying characteristics, and long-term stability. For good material flow, a significant reduction in viscosity is required during molding which facilitates the complete filling of the mold as well as the complete wetting of the filler and other ingredients in the system by the UPR [28]. Fig. 1.3 shows the ideal viscosity profiles for SMCs and BMCs during molding. Chemically, thickening or “maturation” occurs by linking up various UPR molecules together to form polymer chains of considerably higher molecular weights. Usually, this is done by adding a Stable viscosity

Fiber carrying

Mold detail reproducibility

Easy handling Pressure Viscosity

Fast thickening

I

Shrinkage and dimentional control

II

III

IV

Time

FIGURE 1.3 Ideal viscosity profile for SMCs or BMCs during molding: (I) thickening; (II) storage; (III) mold filling; and (IV) curing.

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CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

di- or multifunctional compound to the system which couples two or more polyester molecules together via their terminal hydroxyl and/or carboxyl groups. As UPR molecules usually contain more than two functional groups, the actual product formed is a complex network of interconnected polymer chains rather than discrete individual chains. Compounds used for thickening UPRs are known as “thickening agents” or “maturation agents.” Two types of compounds are used as thickening agents. The first type comprises Group IIA metal oxides and hydroxides, for example, MgO [29]. Maturation with this type of agent occurs via the formation of ionic bonds through the reaction of MgO with the carboxylic acid end groups of polyester molecules. The other type of maturation agent is diisocyanate [30]. Diisocyanates operate by forming covalent bonds, specifically urethane linkages, with the terminal hydroxyl groups of polyester molecules. Each type of maturation agent has its own advantages and disadvantages. The maturation process with MgO-type agents is slow. They form ionic bonds which weaken at elevated temperatures encountered during molding. This results in a reduced compound viscosity and hence the desired material flow. Diisocyanate maturation agents exhibit rapid thickening. The covalent bonds formed with isocyanate-type thickeners do not weaken at molding temperatures and hence material flow is more difficult. MgO-type maturation agents are highly sensitive to humidity after maturation, whereas diisocyanates are not. A thermally breakable di-keto group can be introduced onto the UPR molecule before curing through salt formation. This group, along with the salt, may break at elevated temperature in most UPR molding operations and therefore reduce the compound viscosity upon heating; hence the desired amount of material flow is realized. Modified resins are further thickened with MgO or diphenyl diisocyanate. This exhibits a fast viscosity rise during molding and a stable viscosity during room temperature storage [28]. Molded articles made with conventional UPRs often exhibit poor surface finishes. This is probably due to shrinkage of the UPR during the molding operation. LPAs are used to overcome this problem. Along with LPAs, good material flow during molding is also necessary to obtain finishes of the highest quality. The reduced material flow encountered when diisocyanates are used as thickeners reduces the effectiveness of LPA in these systems, which in turn may lead to significant finish problems. One proposal to overcome this limitation is to use a combination of both MgO-type and diisocyanate-type thickeners in the same system [31].

1.4 UNSATURATED POLYESTERS RESIN BLENDS Polymer blends are made by the physical mixing of two or more different polymers or copolymers to produce a mixture with desirable mechanical and physical properties. Usually, the Tg of cured UPRs are high and their brittleness presents an obstacle for their use in engineering applications [32]. The mechanical, physical, and thermal properties of UPR can be improved by blending with other polymers or by reacting them with different additives or modifiers which generally form a second dispersed phase after the resin is cured. Blends show the demanded performance at low cost.

1.4.1 UNSATURATED POLYESTERS RESIN
ELASTOMER BLENDS The addition of elastomeric phases to UPRs usually improves their overall ductility over a wide range of temperatures, toughness, and impact resistances. Elastomers are blended with UPR before

1.4 UNSATURATED POLYESTERS RESIN BLENDS

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curing by physical and chemical methods. When blends are formed by the physical mixing of two or more polymers at least 5% of another polymer is necessary to form a blend. If the component polymers are miscible, a single-phase blend is obtained. If they are immiscible, a multiphase blend is formed. Even if rubber additives are soluble in uncured resin, phase separation during curing is advantageous since these blends are tougher than homogeneous blends [33,34]. The presence of elastomeric domains increases the absorption and dissipation of mechanical energy. Various mechanisms proposed for toughening by blending with rubber include the debonding of the rubber matrix interphase, energy absorption by rubber particles, matrix crazing, shear yielding, and combining shear yielding and crazing [32,33]. Essential characteristics of elastomers for toughening are [35]: 1. The presence of a sufficient number of polar groups to enhance solubility in the resin. 2. The elastomer should have a slow rate of cross-linking compared to the UPR used to facilitate the distribution of discrete elastomeric particles during cross-linking. 3. The weight of the elastomer should be relatively high. 4. The major part of the elastomer should be thermodynamically incompatible with resin.

1.4.1.1 Unsaturated polyesters resin
natural rubber blends Natural rubber is an elastic material present in latex from rubber trees. Its easy availability, low cost, and excellent physical properties such as good resilience, high tensile strength, superior resistance to tear and abrasion, good tack, and self-adhesion have led to its use in preparing blends. On the other hand, it has poor age resistance and oil resistance. Blends based on UPR from recycled poly(ethylene terephthalate) (PET) wastes with varying percentages (0%
7.5 wt.%) of liquid natural rubber (LNR) have been prepared by Hisham et al. They are found to exhibit good compatibility compared to commercial resins, but show higher Tg. A blend with 2.5 wt.% LNR rendered the highest strength and best dispersion of elastomer particles while commercial resin required 5% of LNR to achieve optimum properties [36]. Studies on the influence of the source of water and immersion time on the mechanical properties of UPR
natural rubber blends have revealed increases in the impact strength and strain rates and decreases in the Young’s modulus of polymer blends under identical conditions [33]. Hybrid proton exchange membranes as an alternative for Nafion in polymer electrolyte membrane fuel cell (PEMFC) were developed by Jimenez et al. UPR
natural rubber blends were prepared and subjected to the process of vulcanization and TiO2 was added as an inorganic load. The blends exhibited higher Young’s moduli and strains compared to commercial Nafion membranes. The water uptake as well as ion exchange capabilities of the vulcanized membrane was found to be superior [37]. Natural rubber latex (NRL) when blended with UPR in the presence of dispersion aids such as sodium lauryl sulfate (SLS), toluene, and ammonia led to an improvement in impact strength. However, the flexural strength decreased with NRL content in the blend (Fig. 1.4). The impact strength was highest when NRL and toluene were 15 phr and 20 wt.%, respectively [38].

1.4.1.2 Unsaturated polyesters resin-synthetic rubber blends Synthetic rubbers have also been used for blending with UPRs. Binary polymer blends of UPR and different weight ratios (0%, 5%, 10%, and 15%) of nitrile butadiene rubber (NBR) have been prepared by mechanical mixing using toluene as a solvent. However, they showed poor mechanical

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CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

FIGURE 1.4 Impact strength of pure UPR and UPR/NRL blends using different dispersion aids: (A) using SLS as a dispersion aid; (B) using toluene as a dispersion aid; and (C) using liquid ammonia as a dispersion aid.

1.4 UNSATURATED POLYESTERS RESIN BLENDS

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properties, except for their strain rates which were higher. The wear rates of the blends were found to decrease with increasing NBR content [39]. Cherian and Thachil prepared blends of UPR with elastomers bearing reactive functional groups such as hydroxy-terminated polybutadiene, epoxidized natural rubber, hydroxy-terminated natural rubber, and maleated nitrile rubber. These elastomers show better compatibility with resin and impart superior toughness, fracture resistance, and impact resistance as compared to unmodified elastomers [34]. The authors also synthesized UPR blends using two different strategies for incorporating rubber. The first method involves the dissolution of masticated elastomers such as natural rubber (NR), styrene butadiene rubber (SBR), NBR, butyl rubber (IIR), and chloroprene rubber (CR) in styrene followed by blending with UPR, while in the second method, elastomers are modified with MA and then dissolved in styrene and blended with UPR to get maleated elastomers. Blends having elastomers modified with MA show improved mechanical properties (toughness, impact resistance, and tensile strength) compared to unmodified rubbers (Tables 1.1 and 1.2). The performance of nitrile rubber is found to be far superior in

Table 1.1 Summary of Properties of UPR Modified With 0%
5% Elastomers Maximum Improvement Achieved (%)/Elastomer Concentration (%) Property

UPR

NR

SBR

NBR

CR

IIR

Tensile strength (MPa) Modulus ( 3 102 MPa) Elongation at break (%) Toughness (MPa) Impact strength ( 3 1022 J/mm2) Hardness shore D Abrasion loss (cc) Water absorption (%)

33.3 14.1 2.25 0.36 1.21 88 0.37 0.21

57.8/2.5 24.2/2.5 44.4/2.5 136/2.5 150/2.5 2 0.6/1 32.4/5 90.5/5

53.9/2 15.1/2 33.8/2 111/2 107/2 2 0.6/1 18.9/5 38.1/5

83.4/2.5 8.6/2.5 88.9/2.5 286/2.5 239/2.5 0/1 21.6/5 47.6/5

40.8/2.5 21.3/2.5 24.4/2.5 97.2/2.5 90.1/2.5 2 1.1/1 27.0/5 57.1/5

16.4/2 2.9/2 7.1/1 41.7/2 50.4/2 2 1.7/1 37.8/5 76.2/5

Table 1.2 Summary of Properties of UPR Modified With 0%
5% Maleated Elastomers Maximum Improvement Achieved (%)/Maleated Elastomer Concentration (%) Property

UPR

NR

SBR

NBR

CR

IIR

Tensile strength (MPa) Modulus ( 3 102 MPa) Elongation at break (%) Toughness (MPa) Impact strength ( 3 1022 J/mm2) Hardness shore D Abrasion loss (cc) Water absorption (%)

33.28 14.1 2.25 0.36 1.21 88 0.37 0.21

84.0/2.5 24.8/2.5 58.7/2.5 214/2.5 203/2.5 2 0.6/1 37.8/5 100/5

63.0/2 10.8/2 58.2/2 161/2 116/2.5 2 0.6/1 18.9/5 42.9/5

97.8/2.5 8.9/2.5 95.6/2.5 303/2.5 247/2.5 0/1 27.0/5 57.1/5

71.5/2.5 15.2/2.5 64.9/2 184/2.5 136 /3 2 1.1/1 32.4/5 71.4/5

35.0 /2 6.0/2 33.8/2 88.9/2 66.9/2 2 1.1/1 48.6/5 90.5/5

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comparison to all other rubbers [32]. Toughening agents like carboxyl and vinyl terminated nitrile rubbers as well as urethane rubbers have also been used for preparing blends. The incorporation of flexible polyorganosiloxane segments in the UPR network enhances its flexibility [34]. The thermal stability of toughened UPR
NBR improved on reinforcing it with slag powder. The sample modified with stearic acid showed better mechanical properties. A fire resistance test showed reduced mass loss when exposed to direct open flame [40]. Suspene et al. observed an improvement in the compatibility of a UPR
carboxyl-terminated butadiene-acrylonitrile rubber (CTBN)blend by exchanging 10% CTBN for epoxy-terminated nitrile rubber (ETBN) in a blend with 5 phr of rubber. In the resulting triblock copolymer a decrease in particle size of the dispersed rubbery phase from 12 to 5 μm was observed and the interfacial tension between UPR and CTBN is also reduced. The impact behavior of the triblock copolymer was enhanced due to a reduction in failures caused by the presence of large particles [41].

1.4.2 UNSATURATED POLYESTERS RESIN
PHENOL FORMALDEHYDE RESIN BLENDS UPRs are highly flammable and produce large quantities of smoke and toxic gases on burning. The flame resistance of UPR can be improved by adding flame-retardant additives or by blending it with other polymers such as phenolic formaldehyde resins (PF). The addition of additives usually leads to unfavorable reactions between the polymer and additives resulting in the deterioration of the mechanical properties of polymers to some extent. Blends of UPR and PF show good fire retardant abilities due to the high charring tendency of PF. PF is known to generate less toxic gases and smoke and leave a large amount of carbon residue [42]. Among phenolic resins, resoles and novolacs are important; on curing, they produce highly cross-linked thermally stable network structures, which on exposure to high heat or fire, char, thus producing relatively low levels of combustible volatiles [43]. Kandola et al. used ethanol-soluble epoxy and allyl-functionalized phenolic resoles to overcome the incompatibility of UPR and PF resins resulting from their chemical structures and curing mechanisms. They demonstrated an increase in compatibility with functionalization. Allyl-functionalized resole exhibited the best compatibility with UPR. A mechanism has been proposed for their decomposition and interactions and their effects on flammability based on thermal behavior and infrared spectroscopic analysis of volatile degradation products [44]. Mahadar et al. blended UPR with resole to produce materials with good flame retardancy. The blends showed good compatibility when compounded with kenaf fiber, which is a natural fiber. Although the thermal stability of the blends was improved, the mechanical properties were found to be slightly inferior [42]. Novolac resin was modified with free-radically curable methacrylate groups (M-Nov) with styrene to give a material with a higher Tg, better thermal and thermo-oxidative stabilities, and better flame retardancy than cured UPR, with an approximately 20% lower modulus at room temperature (RT) [43]. An alternative modification of novolac with the vinylbenzyl group to obtain a homogeneous, free-radically cocured phenolic/UPR blend with better flame retardancy than those made using M-Nov has also been attempted. The cured vinylbenzylated novolac and its cocured blends with UPR exhibited superior flame retardancy in comparison to cured UPR and have potential applications as matrix resins in glass-reinforced composite laminates, especially for marine structures [45].

1.4 UNSATURATED POLYESTERS RESIN BLENDS

13

1.4.3 UNSATURATED POLYESTERS RESIN
EPOXY RESIN BLENDS UPR and epoxy resin are miscible with each other and show good compatibility. Hybrid polymer networks (HPNs) based on UPR and epoxidized phenolic novolacs (EPNs) have been prepared through reactive blending. EPN shows good miscibility and compatibility with UPR. The blend shows substantially improved toughness and impact resistance along with better thermal stability. Blends with 5 wt.% of EPN exhibit the highest tensile strength [46,47]. HPNs were also synthesized by coreacting UPR with epoxidized cresol novolac and DGEBA. Cocured blended resins showed substantial improvements in toughness and impact resistance along with thermal stability and damping properties. The performance of the blends with EPN was found to be superior [47]. A new bioresin was produced by Mustapha et al. by blending UPR with epoxidized palm oil (EPO) in 10, 20, and 30 wt.%. The addition of EPO in UPR resin lowered the Tg at 20 wt.% loading of EPO, Tg decreased by 6 5 C, and the storage modulus decreased by about 20% in comparison to UPR. However, the impact properties increased with the amount of EPO added. EPO provides a rubbery phase and absorbs the energy applied by the impact loadings. Bio-based thermoset UPR blended with EPO may reduce the dependency on conventional composite matrix systems made from petrochemicals [48]. UPRs were prepared by reacting bisphenol A epoxy resin with various glutaconic acids using a base catalyst. They were functionalized by treatment with acryloyl chloride to yield acrylated polyesters (APEs). Blending of these APEs were carried out with styrene monomers. In comparison to APEs, these blends exhibited high curing temperatures, slow degradation of products (i.e., low weight loss), good chemical resistances, and good mechanical and electrical strengths [49].

1.4.4 UNSATURATED POLYESTERS RESIN
ESTERS BLENDS Ardhyananta blended VE and UPR containing aromatic benzene rings (10%
80 wt.%). The UPR/ VE blends were prepared by mechanical blending and cured at room temperature using 4% of MEPK in the absence of an accelerator. The mechanical and thermal properties of the blends were found to be superior [50]. Polymer blends of unsaturated polyether ester resins and dicyclopentadiene polyester resins yield cured thermoset resins having high tensile and flexural strengths. The coaction resulting from blended polymers provides an economic way to improve both the stiffness and strength of cured polyether ester resins [51]. Styrenated polyester resins were blended with poly(vinyl acetate) (PVAc). A cocontinuous phase morphology was observed in blends containing PVAc with concentrations $ 6% and styrene levels $ 40%. An increase in the styrene content from 20% to 80% resulted in the sharpening of the principal dynamic loss peak, and the peak temperature reached a maximum at a concentration of 40%. The change from particulate PVAc to cocontinuous structure was associated with a sharp drop in GBc and KBc. Parallel studies have shown this transition to be important in “low-profile” behavior [52].

1.4.5 UNSATURATED POLYESTERS RESIN
POLYSACCHARIDE BLENDS Modified UP was blended with cellulose and ethyl cellulose (5%
25%) at ambient conditions in the presence of MEKP as a curing agent. The blends showed compatibility with modified UP as a

14

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

result of the polar
OH groups in their structure. The results indicate that cellulose increases the impact strength, hardness, and dielectric constant and decreases the bending of the blends, while ethyl cellulose causes an increase in the impact strength, hardness, and bending but a decrease in the dielectric constant of the blends [53]. The work done by Salih et al. involves the blending of UPR with starch powder (0
3 wt.% fraction) with particle sizes less than 45 μm. The blends were further irradiated by UV acceleration. The UV irradiations had a noticeable effect on most of the mechanical properties of the blends. The mechanical properties were found to be a function of particle size and the dispersion of starch powder in the UP matrix. A significant decrease was observed in the ultimate tensile strength and elongation percentage with increasing weight fractions of starch powder, while the modulus of elasticity of the blend showed a significant increase. Other mechanical properties of the blends such as hardness, impact strength, fracture toughness, and flexural strength also increased with increasing weight fractions of starch powder (1%), except the flexural modulus at 1.5%, followed by a decrease at higher percentages of starch [54].

1.4.6 THERMOPLASTIC BLENDS Xanthos and Wan reported the melt blending of polypropylene (PP) with a nonconventional low molecular weight UPR (5:3 PP/UPR wt. ratio) in the presence of organic peroxide by reactive processing. The reacted blend exhibited a finer and more uniform morphology and different properties. The results indicate the possibility of the formation of block and/or graft PP/UPR compatibilizing copolymers [55]. UPR blends of different compositions were prepared with two different thermoplastics, polystyrene (PS) and polycarbonate (PC), by mixing solutions of the polymers in chloroform. A miscibility study of these solution blends was carried out using simple and inexpensive techniques. The UPR/PS blend was found to be miscible while the UPR/PC blend was immiscible [56]. Hydrogen bonding interactions between the two components in poly(ethylene oxide) (PEO)/ OER blends and PEO/cross-linked UPR blends were understood by Fourier transform infrared spectroscopy (FTIR) study. These hydrogen bonding interactions are responsible for the miscibility of the blends. The crystallization kinetics and morphology of PEO in the PEO/UPR blend was found to be dependent on cross-linking. At the crystallization temperature, the overall crystallization rate of PEO in the PEO/UPR blend was larger than that in PEO/OER blend [57]. Li et al. used an improved nuclear magnetic resonance (NMR) method to measure the interphase thickness and to interpret the phase behavior, miscibility, heterogeneous dynamics, and microdomain structure of a thermoset blend of UPR with a cotriblock polymer of PEO-block-poly(propylene oxide)-block-PEO (PEO-PPO-PEO). The results indicated that thermodynamic interaction between the block copolymer and the cross-linked thermoset resin is a key factor in controlling the phase behavior, domain size, and interphase thickness of these blends [58]. Although poly(ε-caprolactone) (PCL) was found to be miscible with uncured polyester/OER, it is partially miscible with crosslinked polyester resin (PER). The miscibility of PCL and OER/PER was found to be a consequence of intermolecular hydrogen bonding between the components of the blend. The importance of the contribution of entropy to the miscibility of thermosetting polymer blends is also shown from FTIR results. The spherulitic morphology of the blends was remarkably affected by cross-linking. Birefringent spherulites were observed in uncured PCL/OER blends, whereas a distinct pattern of extinction rings, which was absent both in the pure PCL or in the uncured PCL/OER blends, was apparent in the cross-linked PCL/PER blends [59].

1.5 INTERPENETRATING NETWORKS

15

1.4.7 SIMULATION STUDIES ON BLENDS Ruffier et al. performed a simulation to show the connection between voids scattered inside the UPR
polyvinyl acetate blend and the blend phase separation mechanism [60]. The effect of curing temperature on the morphology of UPR/styrene/PVAc blends with 5% and 10% PVAc cured between 75 C and 150 C was studied. A cocontinuous phase separated structure resulted for 10% PVAc. An insignificant change in morphology with curing temperature was observed for this composition [61]. A computer simulation model to analyze the reaction injection molding process of polyurethane and UP blends has also been proposed [62]. Mezzenga et al. modeled the free energy of mixing during polymerization in blends of UPR, styrene, and allyl ether functionalized hyperbranched polyesters. They combined the Flory
Huggins theory and gel permeation chromatography (GPC) molecular weight measurements during modeling. The cure behaviors of UPR, phenol, and UPR/ phenol blends were detected and simulated using differential scanning calorimetry (DSC) and DMA. Cure behavior was used to calculate and predict the cure rate, cure temperature, conversion, and changes in the Tg along with various cure orders in order to obtain the optimum parameters for processing [63]. With dynamic scanning, isothermal DSC procedures, and Borchardt
Daniels dynamic software, cure data for the UP resin were obtained; 90% of the conversion rate at 100 C being achieved after 15 minutes. However, for the phenol and UPR/phenol blends, gradually increasing the temperature was found to be the best for curing according to the DSC and DMA test results [64].

1.5 INTERPENETRATING NETWORKS OF UNSATURATED POLYESTER RESIN These are a relatively new type of polymer blends which consists of two or more cross-linked polymers in which at least one network is synthesized in the presence of the other. Although different modification processes are reported, the formation of interpenetrating polymer networks (IPNs) is a promising method.

1.5.1 UNSATURATED POLYESTERS RESIN
POLYURETHANE INTERPENETRATING POLYMER NETWORKS Mutually permeable semi-IPN-type networks consisting of UPR and polyurethane resin (PUR) (semi-IPN UPR/PUR) have been prepared using a new method of adding PUR to styrene. Both resins form dispersed phases with heterogeneous microstructures. PUR seems to affect the mechanical properties significantly, but the effect ceases on increasing the PUR content above 10%. The dynamic elasticity modulus depends only on composition [65]. IPNs with four different types of UPRs (commercially available unsaturated polyester (UPE), partially end capped UPE, OH-free, and having acetate linkages at the end) and PU were cured with UV. The reaction sequence was found to be an important factor in determining the phase mixing, phase morphology, and hence, the

16

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

mechanical properties of the IPNs. The simultaneous reaction of the two reacting systems resulted in a cocontinuous structure that provided enhanced tensile properties and impact strengths [66]. A series of BaTiO3 fiber and nanopowder unfilled and filled IPNs composed of polyurethane (PU) and UPR were prepared using a simultaneous polymerization process. The damping behaviors and degree of phase separation of the unfilled and filled IPNs were strongly dependent on the PU/ UP component ratios, types of filler, and the amount of nanopowder added. The filled IPNs exhibited synergistic effects on damping properties. Performing a polarizing process enhanced the properties. The temperature ranges exhibited excellent consistency of maximum dielectric loss and dielectric constant with damping loss factor [67].

1.5.2 UNSATURATED POLYESTERS RESIN
ACRYLATE INTERPENETRATING POLYMER NETWORKS Polyester
poly(ethyl acrylate-co-styrene) IPNs were synthesized using a two-step in situ sequential technique. Both semi- and full IPNs were synthesized. Poly(ethyl acrylate-co-styrene) acts as the rubbery phase and polyester as a hard phase. With increasing proportions of ethyl acrylate in the IPN, the elongation at break, toughness, and molecular weight between cross-links was higher, but tensile strength, modulus, tear strength, and density were lower. The full IPNs showed higher tensile strength, modulus, tear strength, density, and hardness, but lower elongation at break and toughness compared to semi-IPNs. The semi-IPNs showed higher toughness and elongation. The extent of cross-linking of the elastomer had a determining role in the mechanical property profile. The diameter of the domain depended on the amount of elastomer added [68]. Acrylate-modified PUR resin was first prepared and then added to UPR to obtain an IPN that could be cured at RT. An improvement in miscibility led to higher degree of penetration and entanglement, thus resulting in improved mechanical properties [69]. A series of semi-IPNs based on different compositions of an acyclic PET oligomer and UPR were prepared with styrene as a cross-linker, MEKP as a catalyst, and cobalt naphthenate as a promoter. The mixture was cured at RT. The tensile strength of the IPNs decreased, whereas the elongation at break increased with the concentration of PET oligomer [70].

1.5.3 UNSATURATED POLYESTERS RESIN
EPOXY RESINS INTERPENETRATING POLYMER NETWORKS Simultaneous IPNs based on epoxy (DGEBA) and UP were prepared using m-xylene diamine and BPO as curing agents. Single Tg suggested good compatibility of epoxy and UP. Interlock between the two growing networks led to a retarded viscosity increase. The hydroxyl end groups in the UP catalyzed the curing reaction of epoxy; leading to rapid increase in viscosity. Entanglement affected the cracking energy absorption and was reflected in an improvement in toughness [71]. The Tg of simultaneous IPNs was found to increase with the EP (epoxy polyester) content (10%
90 wt.%). IPNs containing higher EP contents exhibited higher values of tan δ(max.) (Fig. 1.5) and lower cross-linking densities in the rubbery state probably due to the plasticization effect of the EP component along with the heterogeneous network structure [2]. Studies on the curing kinetics of simultaneous UP/DGEBA IPNs showed lower total heat of reaction compared to that observed while curing pure resins. This could be an effect of network interlock that could not be compensated

1.5 INTERPENETRATING NETWORKS

17

0.7 0.6

100EP 10UPR:90EP 30UPR:70EP 50UPR:50EP 70UPR:30EP 90UPR:10EP 100UPR

tan δ

0.5 0.4 0.3 0.2 0.1 0 –135

–80

–25

30 85 140 Temperature (ºC)

195

250

FIGURE 1.5 tan δ versus temperature of BPO/THPA-cured IPNs, BPO-cured UPR, and THPA-cured EP.

completely by an increase in curing temperature. Incomplete curing in isothermal mode is caused by both network interlock and the vitrification of DGEBA. The rate constant for 50/50 of UP/ DGEBA was higher while the activation energy was lower presumably due to the catalytic environment provided by the hydroxyl end group of UP in the IPN [72,73]. A series of IPNs with excellent flame-retardant and damping properties were developed. The flexibility and range of thermal transition increased as the content of UPR increased in the IPNs while the homogeneity decreased. The heat resistance, damping, and mechanical properties were all improved simultaneously with the addition of plate-shaped carbon black (CB) into the UPR/epoxy IPNs [74]. Shin and Jeng also prepared UPR/epoxy IPNs. A series of IPNs based on UP/epoxy were developed. Phase separation was observed when the UPR content was higher than 30%. The best miscibility for IPN was obtained for a composition with similar amounts of hydrogen donors and carbonyl group [75]. From a kinetic study of EP/UPR it was found that the heat resistance of UP was enhanced with the addition of a flame-retardant or epoxy resin [76]. A series of translucent, compatible IPNs were prepared by Shaker et al. using an elastomeric amine-cured epoxy and UPR. A 45% increase in toughness was observed for one of the compositions. This was a reflection of the homogeneous distribution of the rubber component [77]. SemiIPNs of epoxy and UPR have been synthesized with different proportions of UPR (0%
50%). Trimethylenetetramine was used as a room temperature curing agent. IPNs with 11.1% of UPR exhibited improved mechanical properties. The blends were further modified by aromatic amines such as benzidine and diphenyl amine. The mechanical properties of the blend modified with diphenyl amine were found to be superior [78].

1.5.4 UNSATURATED POLYESTERS RESIN
PHENOL AND UNSATURATED POLYESTERS RESIN
NYLON INTERPENETRATING POLYMER NETWORKS IPNs of UPR and several phenolic resoles have been reported by Avendano et al. These IPNs were found to show both physical and chemical compatibility as they cocure such that they result in

18

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

cocontinuous IPNs. The participation of the allyl groups of resole in the cross-linking process of IPNs could be confirmed from the solid-state 13C-NMR spectra [79]. Novel semi-IPNs of UPR and nylon have been produced by mixing different amounts of Nylon 66 oligomers (residues of industrial Nylon 66 polymerization) into UPR and heating followed by cross-linking. Nylon 66 was obtained from industrial waste. Three important aspects of this work include (1) the possibility of producing new materials with improved impact strengths, (2) the plastifying effect of Nylon 66 oligomers on the UP resin, and (3) ecologically more important, the feasibility of reutilizing waste materials for producing engineering materials with tailored properties [80].

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES Composites are heterogenous materials made up of two or more chemically distinct constituents. The basic components are a reinforcement and a matrix. Each of these should have appropriate characteristics and function both individually and collectively so that composites attain the desired superior properties. The reinforcement contributes to the strength and modulus to the composite, while the main role of the matrix is to transmit and distribute stresses in the reinforcement. Reinforcements are of two types, namely particulate and fibers. Commercially, glass fiber (GF)reinforced polyester composites are important due to their high strength-to-weight ratio, low cost, and easy manufacturing methods. In comparison to particulate-filled composites, many fiber-filled composites are anisotropic with tremendous strength in one direction; although uniaxially oriented fiber composites have very high moduli in one direction, the other moduli are low. Therefore to get good properties in atleast two or three directions, fibers can be randomly oriented such that composites are nearly isotropic in a plane. In the case of fibers as a reinforcement, it also provides protection against both fiber aberration and fiber exposure to moisture or other environmental conditions.

1.6.1 UNSATURATED POLYESTERS RESIN
SYNTHETIC/GLASS FIBER COMPOSITES Commercial interest in GF-reinforced UPR composites is due to their high strength-to-weight ratio, and low cost. E-GF composites prepared using a hand layup technique with concentrations varying between 15 and 60 wt.% rendered excellent mechanical properties. Table 1.3 shows the improvement in the mechanical properties as a function of filler content [81]. Table 1.3 Effect of Glass Fiber of Fabricated Composites Contents on Tensile Strength No. Control sample GFRP 1 GFRP 2 GFRP 3 GFRP 4

Content (%)

Width (mm)

Thickness (mm)

Max. Load (N)

Yield Strength (MPa)

Tensile Strength (MPa)

0

19

5

2707.50

10.29

19.76

15 30 45 60

19 19 19 19

5 5 5 5

3504.30 4827.90 5370.35 7488.85

18.63 20.32 21.08 12.54

28.25 50.82 56.53 78.83

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES

19

Belaid et al. carried out thermal ageing studies on polyester fiberglass composites and reported a strong effect on mechanical properties. The Young’s modulus decreased with aging time; from 6% after 30 days to 55% after 120 days [82]. Pedroso et al. achieved significant improvements in the texture, flexural strength, and impact resistance of sheets of UPR GF composites by pressing and heating the sheets at 40 C and 50 C during curing [83]. Studies on the immersion of GFreinforced composites (GFRP) in seawater revealed significant water absorption initially, while soluble material extraction was higher later. The tensile and bending strengths showed decrease with prolonged immersion. Serious corrosion of the interface was observed in micrographs [84]. Mechanical properties such as density, ultrasonic velocity, shear modulus, except Poison’s ratio and elasticity modulus were reported to increase with increasing concentrations of GF (5%
25%) after ultrasonic treatment at 26 KHz [85]. Ferreira et al. reported a higher char yield for an aluminized E-GF composite compared to that of an unmetallized E-GF composite [86]. Surface functionalized chopped cellulose fibers (CFs) when added in small amounts (1%
3 wt.%) further enhanced the strength of composites [87]. The incorporation of methylene spacers in the backbone of UPR enhanced the strength of oil palm empty fruit bunch (OPEBF) as well as fiberglass
UPR composites. Composites based on six methylene spacers showed the highest strength as compared to UP composites based on two methylene spacers [88]. Dagwa and Ohaeri prepared composites of UPR together with banana empty fruit bunch fibers, GFs, and OPEBF particles, which showed a decrease in flexural strength with increases in banana fiber content, while with GFs it increased. A ternary composite with 5 wt.% OPEBF and 10 wt.% banana fiber/10 wt.% GF showed a high impact strength, that is, 55.556 J/m2, representing a 1568.67% improvement over virgin UP. A binary composite with 15 wt.% banana fiber and 5 wt.% GF showed the highest hardness (3.55 HV), representing a 136.67% improvement. Hardness is seen to be influenced by increase in banana fiber content; therefore, banana fiber could be considered for applications requiring high impact strengths such as some parts of automobile vehicles (Fig. 1.6) [89].

FIGURE 1.6 Hardness test values for banana and glass fiber polyester composites.

20

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

TiO2 particulate-filled GF-reinforced polymer composites were prepared by Moorthy and Manonmani with two different fiber lengths, 3 and 5 cm, by hand layup method. The TiO2 content was varied from 10 to 40 wt.%. The combined reinforcement yielded better mechanical properties with increased fiber length and particulate material. Chemical resistance was more pronounced in the 5 cm fiber length composites [90]. The toughness of GF/UPR composites was improved by adding low-molecular-weight polyisobutylene (1
12 wt.%), which was grafted onto MA and glycidyl methacrylate through the novel solvothermal method to improve compatibility [91]. GF/URP exhibited considerable chromatic changes upon UV exposure. Although the mechanical properties were slightly poor, especially in the immersion and condensation chambers, the durability tests proved the generally good behavior of this material under aggressive conditions [92]. Microfibril cellulose (0.3 wt.%) when added to GF
UPR composites improved its mechanical properties. The composites were prepared by hand layup and vacuum bagging method. The impact strength was increased by 19.6% and flexural strength increased from 192.40 Mpa to 208.63 Mpa by hand layup method. The tensile strength increased from 10.24% to 19.62% for samples prepared by hand layup and vacuum bagging, respectively as reported by Vu et al. [93]. Jiang et al. modified carbon fibers with two different functional polyhedral oligomeric silsesquioxanes (POSS) monofunctional (methacrylolsobutyl) and multifunctional (methacryl) POSS and mixed these with UPR to form composites. They showed significantly increased lamellar strength (62 and 67 MPa, increase of 31.9% and 42.6%, respectively) and interfacial shear strength (IFSS). The impact energy was also higher for modified CF composites in comparison with CF/UPR composites [94].

1.6.2 UNSATURATED POLYESTERS RESIN
NATURAL FIBER/PARTICLE COMPOSITES 1.6.2.1 Fibers The use of natural fibers in composites is increasing due to their light weight, nonabrasive, combustible, nontoxic, low cost, and biodegradable properties. The only limitation is their poor mechanical properties, and to overcome this synthetic fibers are used. Osman et al. observed that alkali treatment and the length of kenaf fibers affect the mechanical properties of composites [95]. The effect of water absorption on the flexural properties of kenaf fiber composites significantly reduced with the incorporation of recycled jute fibers [96]. In the case of natural fibers, it is commonly observed that alkali-treated fibers render superior properties in composites. The structural features of sugar palm fibers were found to be affected by alkali treatment using NaOH and an enhancement in IFSS was observed due to the internal morphological changes of the sugar palm fibers. Also, the effect of sugar palm fiber for both untreated and that treated with NaOH on single fiber strength and IFSS has been studied [97]. Cho et al. revealed that henequen fibers when subjected to surface treatment (1% and 6% NaOH) using two different methods, namely soaking and ultrasonic, exhibit drastic changes in topography (increased surface roughness and area). The change is strongly dependent on the treatment method and media used. The IFSS between the fibers and the matrix of the composites was

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES

21

appreciably improved by surface treatment. The topological and interfacial results were quite consistent with each other [98]. Coconut fibers (5
20 wt.%) treated with 10% NaOH provided a reinforcing property, tensile properties, and microhardness to the composites. A 10% loading gave the best reinforcing property, while a 15% loading exhibited the best microhardness [99]. Wood fibers also showed improved mechanical properties [100]. Chemically modified wood flour and wood fiber
UPR composites are reported to offer better performance under compressive loads [101]. Bagasse fiber
reinforced UPR composites gave optimum mechanical properties at 5
10 wt.% loadings [102]. The tensile and flexural properties of alfa fiber
reinforced composites improve with increasing concentrations of NaOH (1
7%) [103]. Acetic anhydride and styrene treatment of alfa fiber added into UPR increased the water resistance and mechanical properties of composites [104]. The thermal stability of modified polyester resin and jute and maize fiber composites is good [105,106]. Cat tail fiber
UPR composites exhibit improved tensile and flexural strengths, which increase with increasing fiber volume (0%
6.01%) [107]. An increase in water absorption with increasing fiber content (0
25 vol.%) in hemp fiber
UPR composites was observed by Rouison et al. Various fiber treatments were tested but none resulted in a substantial increase in the resistance to water absorption [108]. Studies of water absorption on the mechanical properties of hemp fiber (0
0.26 fiber volume)-reinforced composites have shown that the water uptake increased with increasing fiber volume simultaneously to decreases in tensile and flexural properties. Moisture-induced degradation was significant at elevated temperatures. The percentage of moisture uptake increased as the fiber volume fraction increased (0%
0.26%) due to the high cellulose content [109]. Balaji and Senthil Vadivu synthesized coir and cotton
reinforced UPR composites. The cotton fiber
reinforced UPR had better mechanical properties useful for packaging applications. Figs. 1.7
1.9 demonstrate the tensile, flexural, and impact strengths for samples (1
6) prepared with 80% polyester and coir and cotton fibers together with percentages varying from 20% to 0% at intervals of 4% and vice versa [110].

Tensile strength (MPa)

35 30 25 20 15 10 5 0 1

2

3 4 Sample no.

FIGURE 1.7 Tensile strength comparison of different composite materials.

5

6

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

Flexural strength (MPa)

22

18 16 14 12 10 8 6 4 2 0 1

2

3 4 Sample no.

5

6

5

6

FIGURE 1.8 Flexure strength comparison of different composite materials. 4.5 Impact strength (J)

4 3.5 3 2.5 2 1.5 1 0.5 0 1

2

3 4 Sample no.

FIGURE 1.9 Impact strength comparison of different composite materials.

Acetylating and cyano-ethylating treatment of luffa fibers enhanced the flexural strength and flexural modulus of the composites due to improved adhesion between the fiber and the matrix [111]. Comparative studies of the dielectric and electrical properties of chicken feather and kenaf fiber
reinforced UPR composites revealed a lower dielectric constant, dissipation factor, and loss factor for chicken fibers. The increase was higher at 40% fiber content. Experimental values were correlated with theoretical calculations [112]. Chemical modification was found to improve the adhesion of the fiber to the matrix, as well as the physicomechanical properties of sisal fiber, hemp fiber, and bamboo fiber
UPR composites [113
115]. Banana empty fruit bunch and sisal fiber
UPR composites also show superior mechanical properties [116,117]. The addition of flax fibers to UPR limits the role of styrene in network formation in the composite [118]. The effect of cold plasma and autoclave treatments on the mechanical properties of

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES

23

flax fiber
reinforced UPR composites has shown that plasma treatment improves fiber/matrix adhesion while autoclave treatment reduces water solubility in the fibers [119].

1.6.2.2 Particles The mechanical properties were improved when wood ash and microcrystalline cellulose (MCC) were added as fillers into UPR. 5% wood ash provided the best tensile strength and elongation at break while MCC provided the best tensile modulus together with a slight improvement in impact strength. However, both fillers had adverse effects on the flexural strength and modulus [120]. Guar gum is a natural polysaccharide that has been explored for various applications. Shenoy and D’Melo have shown that the inclusion of guar gum and its derivatives results in composites with increased solvent resistance and mechanical properties [121]. Coconut shell and snail shell powders (5%
50%) were added to UPR. The coconut shell particles improved the tensile properties while the snail shell composites showed better thermal properties. The maximum improvement in tensile elongation of the composites was 375%, while that in microhardness was 125% over virgin UPR. The maximum improvement in tensile strength of the composites was 140% over that of virgin UPR [122,123]. A comparison of the mechanical properties of charcoal and snail shell (particle size 635 μm, 0
30 wt.%)-reinforced UPR composites indicated superior properties for snail shell composites over charcoal composites [124,125]. Odusanya et al. evaluated the properties of hybrid seashell/snail shell
reinforced UPR and found the highest resistance before breakdown at 30 wt.% reinforcement compared to individual fillers [126]. Among as received fly ash and surface-treated particles used as fillers to obtain composites with UPR, both exhibit improvement in mechanical properties but certain properties were better in surface-treated fly ash composites [127]. The moisture absorption properties of modified linen
UPR composites also improved by 30.3% compared to unmodified resin [128].

1.6.3 UNSATURATED POLYESTERS RESIN
SYNTHETIC PARTICLE COMPOSITES 1.6.3.1 E-glass Polymer composites absorb moisture; however, composites of aluminized E-glass and UPR exhibited significantly reduced water absorption at RT. At high temperatures, these show the opposite behavior due to their unstable nature [129]. The mechanical properties of E-glass nonwoven matreinforced UPR composites improved after heat treatment below 100 C. The maximum tensile strength (200.6 MPa) was obtained for samples treated at 90 C. Water uptake increased with time and degraded above 150 C [130]. The flexural modulus of composites (UPR glass/carbon) increased while the flexural strength and izod impact decreased with increasing fiber content. The dynamic storage modulus also increased with increasing carbon and CaCO3 content and postcure temperature. Thermogravimetric analysis (TGA)showed increased weight loss with increasing carbon content in an N2 atmosphere, while the opposite trend was observed in air [131].

1.6.4 UNSATURATED POLYESTERS RESIN
METAL/METAL OXIDE COMPOSITES The electrical and thermal conductivity of copper-filled UPR composites has been investigated by Yaman and Taga; dendrite-shaped copper (Fig. 1.10) was used and the effects of both size (Fig. 1.11, fractional size groups) and content were studied. The thermal as well as electrical

24

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

FIGURE 1.10 Dendrite-shaped copper particle ( 3 500).

FIGURE 1.11 Copper powder particle size distribution.

conductivity increased with increasing filler content and particle size. The maximum thermal conductivity of the composite obtained experimentally was 4.72 W/m/K, an increase of 21 times that of neat UPR. The models of Maxwell and Budiansky exhibit convergence to the experimental results, particularly for lower (below 37% volumetric) filler content [132].

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES

25

The thermomechanical properties of UPR reinforced with ceramic Al2O3 particles (0
20 vol.% fractions) were improved [133]. UPR filled with 3 wt.% bentonite-modified silsesquioxanes (POSS) synthesized by Oleksy and Galina exhibited improved mechanical properties and flame resistance [134]. Composites of polyester resin reinforced with diorite, pulverized sandstone, and cornstalk were studied for their moisture absorption properties. The cornstalk
polyester composite was found to absorb more moisture than other composites. The diorite
polyester composite absorbed the least amount of moisture and the moisture decreased with increasing amounts of diorite filler [135]. Composites of UPR filled with silica (micro and nano), nano ZnO and chitin powder
filled UPR composites showed good solvent resistance together with better tensile properties. Hardness increased with the addition of silica, and ZnO, whereas it decreased in the presence of chitin [136]. Expanded polystyrene (EPS) as a waste material was incorporated as a filler into UPR. Tin and zinc oxides were added in different amounts (0
2 wt.%). The composites were fabricated as flat panel windows or glazing to replace glass. Results showed that both the EPS and metal oxide imparted higher flame retardancy to the composites, although each additive reacted differently with the polymeric matrix [137].

1.6.5 UNSATURATED POLYESTERS RESIN
GRAPHITE/CARBON COMPOSITES In the case of graphene and graphite UPR composites, each one has a different effect on the mechanical properties due to the difference in their chemical bonds. The amounts added were 0.05, 0.10, and 0.15 wt.%. Among the two, the storage modulus did not vary with graphene oxide (GO) content while graphite increased the storage modulus up to 65 C. The storage modulus, and loss moduli and Tg were higher for both composites compared to neat UPR (Figs. 1.12 and 1.13). The damping factor values were higher for UPR/graphene composites. Thermal degradation is not affected significantly. SEM showed graphene flakes, graphite particles, and dispersion degree [138].

FIGURE 1.12 Storage modulus and loss modulus versus temperature for polyester/graphene composites.

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

Polyester

Polyester+0.05 wt.% graphite

Polyester+0.01 wt.% graphite

Polyester+0.15 wt.% graphite

2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0

250 200 150 100 50

Loss modulus (MPa)

Storage modulus (MPa)

26

0 30

60

90

120

150

Temperature (ºC)

FIGURE 1.13 Storage modulus and loss modulus versus temperature for polyester/graphite composites.

Composites containing carbon (teak wood, ground nut, neem wood, and rose wood; 10
40 wt.%) prepared from environmental waste were fabricated using a casting technique. The composites are amorphous in nature as observed from X-ray diffraction (XRD). Teak wood carbon showed the best thermal stability. The inadequacy of the bond between the filler and the matrix leads to poor mechanical properties. A higher loading level leads to deterioration in properties. SEM showed crack initiation due to an increase in the agglomeration of particles [139]. Modeling studies of the curing process of two types of composites, Al-filled and CB-filled UPR, showed that the Al
UPR composite responded faster to heat input
induced curing and as such was able to cure faster than the polyester
carbon composite [140]. A bismaleimide
UPR composite exhibited an increase in the thermal index of the material, thus making it useful for high-temperature applications [141].

1.7 UNSATURATED POLYESTERS RESIN
NANOCOMPOSITES The main challenge in nanocomposite synthesis is achieving a uniform dispersion of the nanofiller thought the UPR matrix. Commercially available nanocomposites using UPR matrix are costly.

1.7.1 METAL/METAL OXIDE The results of nanocomposites of TiO2 (1% w/w)
UPR showed an interaction between the -OH groups on the TiO2 surface and the ester groups of the UPR leading to decreased crystallinity and hydrophilicity. Fig. 1.14A, C, E, and G shows micrographs of pure aromatic polyester (APE) and APE/TiO2 (1, 3, 5% w/w). A homogeneous distribution of the filler can be observed for the composite with 1% TiO2, whereas clusters are observed at higher concentrations of TiO2. Energydispersive X-ray spectra (EDS) of the nanocomposites (Fig. 1.14D, F, and H) evidenced an increase

1.7 UNSATURATED POLYESTERS RESIN
NANOCOMPOSITES

27

FIGURE 1.14 Micrographs of the pure polymer (A); APE with 1% (C); 3% (E); and 5% TiO2 (G). EDS spectra of the pure polymer (B); APE with 1% (D); 3% (F); and 5% TiO2 (H).

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CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

100

Weight (%)

80

(A) (B)

60

(C) 40

(D) (E)

20

(F) 0

100

200

300 400 500 Temperature (ºC)

600

700

800

FIGURE 1.15 TGA thermograms of UPR/alumina nanocomposites. (A) Pure polyester; (B) UPR/1% nanoalumina; (C) UPR/3% nanoalumina; (D) UPR/5% nanoalumina; (E) UPR/7% nanoalumina; (F) UPR/9% nanoalumina.

in the Ti peak compared to that of the pure polymer (Fig. 1.14B) and confirmed the incorporation of the TiO2 filler into the APE matrix. However, improvements in thermal properties and hardness are evidenced [142]. UP nanocomposites filled with nanoalumina (60
70 nm, 1
9 wt.%) were prepared by Baskaran et al. using a casting technique, which showed higher tensile, flexural, and impact strengths than pristine UPR. The storage modulus increased maximum up to 5 wt.% of filler loading level. The thermal stability of the nanocomposites was also higher with char yields increasing as a function of alumina concentration in the composites (Fig. 1.15) [143]. The effects of the particle size of nanoalumina and the concentration of the coupling agent used on the erosion resistance and mechanical and thermal properties of composites with UPR have been investigated. A higher particle size of nanoalumina and higher concentrations of the coupling agent showed increased erosion resistance along with overall increases in thermal and mechanical properties [144]. A new type of polyester-based composite material with enhanced flame retardancy has been developed by modifying the polymer with nanoalumina and microsilica particles. Experiments were based on Taguichi’s methodology. The material and processing parameters used had different effects on the properties. The composites showed better fire retardance when particles were added singly or in combination. Best fire resistance properties were obtained for a combination of 0
5% nano alumina or micro silica particles along with 1
15% phosphinate based flame retardant [145]. The results of Chen et al. have shown that surface-modified hydrophobic ZnO nanoparticles improved the thermal properties of composites with UPR. The tensile strength and bending strength increased by 91.4% and 71.3%, respectively, with a 3 wt.% addition of ZnO

1.7 UNSATURATED POLYESTERS RESIN
NANOCOMPOSITES

29

nanoparticles [146]. The mechanical properties of nanoiron particle (1
7 wt.%) reinforced epoxy/ polyester nanocomposites were investigated. Two highly dispersed nanoparticles, Fe2O3 and functionalized Fe2O3 (f-Fe2O3), were prepared using a chemical reduction method. The mechanical properties of the f-Fe2O3 composites were better than those of the Fe2O3 composites. The matrix became magnetically harder after the incorporation of nanoiron particles. Machine-generated results were compared and analyzed with system generated software analysis of variance (ANOVA) values. ANOVA seemed to reduce the P-values but the machine-generated values were greater than what were expected [147]. Rusmirovic et al. studied the mechanical properties of hybrid composite materials prepared using UPEs based on glycolyzates and chemically modified silica nanoparticles with vinyl reactive functionalities, namely vinyl, methacryloyl, and linseed oil fatty acid reactive residues were investigated. TEM confirmed that the silica nanoparticles formed domains of aggregates in the polymer matrix. The results reveal that the method of synthesis used for preparing UPR had a more pronounced effect on the dynamic mechanical properties as compared to the SiO2 particles [148]. Glass fiber reinforced composites when added with small amounts (1% and 2%) of fumed silica (FS) improved the tensile strength by 8% and 11%, respectively. The flexural strength also increased and the resin system underwent a transition from having a brittle to a ductile nature [149]. Surface-modified FS added to UPR exhibited a higher heat deflection temperature. The elastic modulus was enhanced while the tensile properties were unaffected. The strongest effect was found to be on the impact strength; modified silica resulted in a positive effect while unmodified silica had a negative effect [150]. Small amounts (1%
3%) of nanosilica (50
60 nm) when added to UPR improved the thermal and mechanical properties of the composites. An excess of 5% led to a decline in properties [151]. Nanocomposites based upon hexahydrophthalic anhydride-cured bisphenol A diglycidyl ether and layered silicates such as synthetic fluoromica (Fmica), purified sodium bentonite, and synthetic hectorite (5
10 wt.%) prepared by Zilg et al. showed enhanced toughness associated with the formation of dispersed anisotropic laminated nanoparticles consisting of intercalated layered silicates [152]. UP
POSS hybrid nanocomposites showed improvements in thermal properties in proportion to the proportion of functionalized POSS added [153].

1.7.2 OTHER INORGANIC FILLERS A nanosized calcium carbonate (CaCO3) filler (1
9 wt.%) with particle sizes between 50 and 60 nm was dispersed in a UPR matrix using a casting technique. Uniform dispersions were obtained below 5 wt.%, which enhanced their mechanical properties [154]. A nanocomposite gel coat system prepared using UPR with aerosil powder, CaCO3, and organoclay (1
3 wt.%) showed improved mechanical and water barrier properties. Improvements of 55%, 25%, and 30% were observed in tensile modulus, flexural modulus, and impact property while the Tg was slightly increased [155]. The addition of ramie cellulose nanofiber and CaCO3 as a reinforcement in a UPR matrix also resulted in the increased thermal stability and mechanical properties of the biocomposite as reported by Wahono et al. [156]. Al(OH)3 and Mg(OH)2 can be used as alternatives for fire retardant additives, which can improve the fire retardancy of composites. A combination of 40% Al(OH)3 and 10% Mg(OH)2 improved the thermal stability of the composite by reducing the mass loss rate to 4.9%/min and

30

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

total mass loss to 77%, while the tensile strength decreased to 64% and the hardness improved to 64.5%. The results of the morphology and mapping of the composite showed that Al(OH)3 was well dispersed while Mg(OH)2 had the tendency to agglomerate [157]. Pereira et al. investigated the flame-retardant properties of layered double hydroxides (LDHs) such as adipate-LDH (A-LDH) and 2-methyl-2-propene-1-sulfonate-LDH (S-LDH) when added to UPR and significant reductions in the UPR flammability (by 46% and 32%) were indicated by incorporating 1 wt.% of A-LDH and 5 wt.% S-LDH, respectively, followed by enhanced char formation while evolved smoke remained unchanged [158].

1.7.3 MONTMORILLONITE Preparation procedures also affect the properties of composites due to the chemical and physical reactions involved [159]. UPR
montmorillonite (MMT) clay composites prepared by Suh et al. through in situ free radical polymerization also have improved thermal and mechanical properties as compared with neat UPR due to them having more homogeneous dispersion and optimum amounts of styrene monomer molecules inside and outside the MMT layers at 1 wt.% loading [160]. At an MMT content of only 1.5 vol.%, the fracture energy, Cs, of the nanocomposite was doubled to 138 J/m2 as compared to the 70 J/m2 of the pure UPR [161]. Hassan et al. investigated the possibility of using an ammonium polyphosphate (APP)-filled UP/ phenolic/MMT blending system to obtain nanocomposites exhibiting excellent flame retardancy, thermal stability, and mechanical properties. The optimum APP content in the composites was 30 phr where the char formation on the surface of the composites forced an insulating carbon layer and resisted further escalation of fires. The optimum content of APP and MMT were 30 and 3 phr, respectively, to achieve the best balance of properties based on flame retardancy, thermal stability, and mechanical performance [162]. Tunisian nanoclay/UPR nanocomposites also have better mechanical and thermal properties. The degradation temperature was increased by 78 C with the addition of organic modification [163]. Romanzini et al. showed that the chemical modification of MMT (Cloisite) with compatible silanes, vinyltriethoxysilane, and γ-methacryloxy propyltrimethoxysilane helps in preventing agglomeration and enhances the interaction between MMT and UPR and hence, improves the thermal mechanical and fire retardancy [164]. The chemical resistance of organically modified MMT
UPR composites under aqueous conditions in acetic acid, nitric acid, hydrochloric acid, sodium hydroxide, aqueous ammonia, and sodium carbonate shows maximum weight gain/loss with increasing clay content. The Tg value was found to be maximum for the composites with the maximum clay content [165]. The solvent resistance of UPR
MMT-filled nanocomposites was studied in acetic acid using an equilibrium swelling method. The composites showed low diffusion coefficients. The diffusion coefficient, sorption coefficient, and permeation coefficient increased with increases in temperature for all the samples [166].

1.7.4 CLAY COMPOSITES The effects of variables such as clay type, clay content, and prepolymer
clay mixing type on the mechanical properties of UPR/clay nanocomposites including tensile strength and percentage elongation have been addressed by Johnson et al. Unmodified kaolinite clay and vinyl silane
modified

1.7 UNSATURATED POLYESTERS RESIN
NANOCOMPOSITES

31

clay were used; the clay type and mixing method were found to have a profound effect on the mechanical properties showing good improvement [167]. PEO was used as a new modifier to replace traditional ionic surfactants which present the problem of disintegration at high temperatures. The clay galleries changed to intercalated state in the nanocomposites and the properties of the nanocomposites were improved significantly with only 1 phr loading of organoclay [168]. UPR reinforced with nanosized clay Cloisite 30B (C30B) and CB were prepared using hand layup and open-molding techniques. The amounts of filler added varied between 0 and 10 wt.%. Mechanical strength was superior for C30B compared to CB due to its higher surface area. At 4% filler content the mechanical properties were optimum [169]. Kusmono and Ishak reported an exfoliated structure upon the addition of clay to UPR/GF composites at 2% loading while at 6 wt.% it formed an intercalated structure. The optimum loading was 2%, where tensile strength, flexural strength, and flexural modulus were approximately 13%, 21%, and 11%, respectively. The highest values for impact and fracture were obtained at 4 wt.% loading [170]. Studies of influence on the volume shrinkage of nanoclay composites showed a decrease in volume change while at higher nanoclay contents the reaction rate increased and induction time decreased [171]. Dhakal et al. reported a strong correlation between the nanomechanical properties and interlayer d-spacing of clay particles in the nanocomposite system. The incorporation of 1, 3, and 5 wt.% of layered silicate nanoclay into UPR showed an improvement in hardness of 29%, 24%, and 14%, respectively. The elastic modulus increased from 5393 MPa for neat polyester to 6646 MPa (23% increase) with 5 wt.% nanoclay [172]. Composites with various amounts (1, 3, and 5 wt.%) of different nanofillers such as Nanofil 116, Cloisite 30B, and Laponite RD exhibited slightly enhanced thermal stability as Nanofil 116 and Laponite RD content increased, they also imparted good strength and stiffness. In contrast, 1 wt.% of Cloisite 30B (UPC1) showed a higher degradation temperature and thermal stability than those with 3 and 5 wt.%. The compressive strength was also maximum at 1 wt.% [173]. The addition of halloysite nanotubes into UPR improved the flexural properties of the nanocomposites in dry conditions and after water-methanol exposure. A significant increase in surface roughness was also observed [174].

1.7.5 BENTONITE Bentonites were chemically modified with a surfactant (quaternary ammonium salt) through a cation exchange reaction and added to UPR as reinforcements. They improved the thermal and mechanical properties of the nanocomposites. Nanocomposites loaded with 7 wt.% showed better mechanical properties compared to unsaturated polyesters (PEs) filled with micrometer clay (40 wt.%). According to Motawie et al., the electrical conductivity was also improved [175]. No detrimental effect was observed on the barrier properties (water absorption), which is important for several applications. Phosphonium salts can be used instead of ammonium salts [176].

1.7.6 NATURAL FILLERS Isora nanofibrils (INFs) with a length of 300 nm, width of 20 nm, and an aspect ratio of 15 extracted from Helicteres isora by steam explosion showed a network-like structure. The high

32

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

aspect ratio of the nanofibrils led to an improved network in the polyester matrix, hence, resulted in good water absorption (73% decrease) and mechanical properties of the nanocomposites. The volume fraction of the constrained region was highest at an INF loading of 0.5 wt.% while the tensile strength increased by 57%. A constrained model was proposed to understand the role of the constrained region in enhancing the mechanical properties. The Tg was higher by 10 C [177]. The water absorption properties were also improved due to interfacial adhesion which prevents the penetration of water. At 90 C, as the INF loading increased the water uptake also increased. The mechanism of diffusion was found to be Fickain-type [178]. Nanocomposites of UPR and cellulose nanocrystals (CNCs) prepared by Kargarzadeh et al. showed that the crystallinity index of the CNCs was reduced after surface treatment. However, it did not impact on the size and aspect ratio of the rod-like nanoparticles. The tensile strength and stiffness of the composites improved in the presence of silane-treated CNCs (STCNCs). Interestingly, the impact energy increased significantly with the addition of untreated CNCs. The viscoelastic behavior and thermal degradation for both the CNC- and STCNC-reinforced nanocomposites were improved. The water absorption behavior of the UPR was found to decrease upon the incorporation of CNCs, and a further reduction was observed with STCNCs [179].

1.7.7 CARBON FILLERS Acid and ammonia functionalized multiwalled carbon nanotubes (MWCNTs) were coated with iron oxide (III) and used to obtain polymer/MWCNT nanocomposites with different contents (0.05, 0.10, 0.15, 0.20, and 0.25 wt.%). The viscosity was optimum for MWCNTs in the range between 0.15 and 0.20 wt.%. Fe functionalized MWCNTs exhibited the best mechanical behavior, while the electrical conductivity increased by three or four orders of magnitude with unfunctionalized MWCNTs [180,181]. Seyhan et al. investigated critical aspects related to the processing of nanocomposites made of CNTs with and without amine (NH2) functional groups and polyesters. The composites were processed using three roll milling and sonication techniques. Styrene evaporation from the polyester resin system was a critical issue for nanocomposite processing. CNT/polyester suspensions exhibited shear-thinning behavior, while polyester resin blends behaved as Newtonian fluid. Nanotubes with amine functional groups were found to have better tensile strengths [182]. GO and its derivatives with vinyl and alkyl functional groups (modified GO; mGO) were synthesized and dispersed into UPR to prepare nanocomposites. The mGO was easily dispersed in the UPR compared to the GO, even without sonication. A 55% improvement in fracture energy was obtained with little change in flexural strength or modulus with only 0.04 wt.% mGO. This high effectiveness renders mGO economically viable. Scanning electron microscopy suggests that mGO particles interact with the propagating crack; the main toughening effect being crack pinning [183]. The strength of UPR
MWCNT composites rose with the rising content of MWNTs (0.1
0.5 wt.%). Composites were prepared by solution dispersion and casting methods. The microcrystallinity, stiffness, and strength of nanocomposites rose with the rising content of CNT. A noticeable improvement was observed in the Tg, melting temperature (Tm), and enthalpy (ΔHm). The microcrystallinity of nanocomposites increased with increasing CNT content [184]. The dispersion of MWCNT in UPR was investigated by rheology and optical microscopy. The results revealed a percolation threshold of 0.097 vol.%; which is very close to the theoretical value

REFERENCES

33

of 0.085 vol.% expected for individually dispersed MWNT with an average aspect ratio of 590. The dispersions formed an open network with a fractal dimension of 1.28. The results were compared with single-walled carbon nanotubes (SWNT) and PS-modified MWNT (MWNT-PS). The SWNTs formed stronger networks than those of the MWNTs, but the MWNT-PS networks were weaker than those of the unmodified MWNTs [185]. The influence of MMT nanofiller on the mechanical properties of glass fiber recyclate (rGF) (25
40 wt.%, coarse and fine) reinforced UPR composites, have been studied by Hannan et al. The results showed that the addition of MMT further enhanced the tensile strength (14% in 40rGF
3MMT compared to nonhybrid 40rGF), optimal concentration of MMT is 2%
3% and rGF is 25%. Fiber/resin adhesion is better for low concentrations of MMT. Coarse rGF composites contain relatively larger aspect ratios and hence have better tensile properties [186]. A study on the thermal decomposition kinetics of UP and UPR composites reinforced with 2, 4, and 6 wt.% toner carbon nanopowder (TCNP) revealed faster rates of decomposition of the composites. The activation energy, reaction rate constant, and thermodynamic properties were lower in these composites. This enhancement is attributed to the nanosized iron content in TCNP, which enhances the pyrolysis reaction [187].

1.8 FUTURE CHALLENGES Some of the challenges include reducing dependency on petroleum-derived UPRs using sustainable methods by synthesizing bio-based UPs or blending them with other bio-based oils/polymers. The development of a new variety of UPRs using bio-modifiers and synthesizing biocomposites to replace existing composite materials are also gaining importance. Synthesizing UPRs using monomers obtained by recycling polymer waste is also an important task. Obtaining UPRs with optimum properties from sustainable sources and through recycling is a real challenge. Halogen-free flameretardant UPR (with improved mechanical properties) molded products with superior surface quality is also an important research area. Tailoring low-styrene-emission/styrene-free UPR compositions is necessary to save the environment and to minimize health issues associated with styrene emissions. The effect of thermal aging on different properties of cured resin also needs to be addressed.

ACKNOWLEDGMENT The authors are grateful for assistance from Ms. Vandana Mooss, Mr. Prakash Rathod, Mr. Sudhaker Satpal, and Mr. Yadnesh Kesari in the compilation of this chapter.

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FURTHER READING B.N. Raju, K. Ramji, V.S.R.K. Prasad, Mechanical properties of glass fiber reinforced polyester ZnO nanocomposites, Mater Today Proc. 2 (2015) 2817
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