Blends, Interpenetrating Polymer Networks, and Gels of Unsaturated Polyester Resin Polymers With Other Polymers

CHAPTER

BLENDS, INTERPENETRATING POLYMER NETWORKS, AND GELS OF UNSATURATED POLYESTER RESIN POLYMERS WITH OTHER POLYMERS

6

Pragnesh N. Dave1 and Ekta Khosla2 1

Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, India 2Department of Chemistry, Hans Raj Mahila Maha Vidyalaya, Jalandhar, India

6.1 INTRODUCTION HISTORY The fact that some natural oils besides alkyd resins can be dried by certain additives and employed as coatings was realized long ago. This results from a polymerization reaction of the unsaturated moieties in ester molecules (Fig. 6.1). Carleton Ellis’ original patents with regard to polyester resins egressesed in the 1930s. While profit-making production started in 1941 by now reinforced with glass fiber for radar domes, also known to as radomes. Unsaturated polyester resins (UPRs) consist of two polymers, that is, a short-chain polyester bearing, polymerizable double bonds and a vinyl monomer. The curing reaction involves the copolymerization of the vinyl monomer with the double bonds of the polyester. During the process of curing, a threedimensional network is formed. UPR is yielded in this process, belonging to the thermoset class. UPRs have interesting applications in compression molding (sheet molding compounds), injection molding (bulk molding compounds), filament winding, resin transfer molding (RTM), pultrusion, and the hand lay-up process [1]. 85% of fiber-reinforced polymer (FRP) products such as aircraft parts, motor covers, belt guards, water-cooling towers, boats, architectural parts, chairs in ducts and other process equipment in chemical plants and paper mills, water pipes, and chemical containers are manufactured using polyesters, and they are used in offshore applications, construction, and the paint industry [2,3]. The determination of the gel time and curing time is a very significant step in the processing of UPRs while manufacturing composite products. The curing reaction should be accomplished in a governable manner to attain a high-quality product [4]. On account of the enormous amount of applications where flammability does not matter, such as underground mine bolts, pipes, and in many marine applications, the percentage of flame-retardant resins relative to all UPRs is less than Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00006-5 © 2019 Elsevier Inc. All rights reserved.

153

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HO−CH 2 CH 2−OH Ethylene glycol (Structure)

Propy lene glycol (Structure)

Neopentyl glycol (Structure)

Trimethylol propane (Structure)

Glycerol (Structure) FIGURE 6.1 Diols and triols used for unsaturated polyester resins.

2% (v) [5]. UPR, like other polymer materials, has a restriction in fire retardancy. UPR liberates volatile vapors that trigger the ignition of fire when it is thermally degraded [6]. The addition of halogen can improve the fire retardancy of composites, nonetheless the use of a halogen additive may create environmental problems due to halogen radical and halo acid production which is harmful to human health when burnt [7]. The pie chart in Fig. 6.2 shows the world consumption of UPRs (https://ihsmarkit.com/products/ unsaturated-polyester-resins-chemical-economics-andbook.html, accessed on August 1, 2018). Fig. 6.2 shows the worldwide consumption of UPRs, which are thermosetting polymers, broadly used in the preparation of polymer composites [8
10]. The enhanced use of these materials is due to their comparatively low cost, good compatibility, ease of processing with a variety of fillers, and large selection of diverse types of monomers. These polymers are distinguished for the excellent balance between their mechanical, electrical, and chemical properties as compared to other engineering polymers. Some limitations of unsaturated polyesters (UPs) are their lower mechanical and thermal properties, which restrict their usage for some applications. To overcome these imperfections various modified fillers have been added [8
10].

6.1 INTRODUCTION

155

FIGURE 6.2 Worldwide consumption of unsaturated polyester resins.

Conventional fillers serve the purpose of improving the mechanical properties and decreasing manufacturing costs, but their use is restricted due to the phase separation and agglomeration of filler particles, inching toward a drastic worsening of the material properties [11]. The incorporation of clay into UPRs can result in the expansion of their mechanical, thermal, barrier and chemical properties, wear resistance, and flame retardancy [12
16]. This can be achieved with less filler content than what is used in most conventional composites. Important patents filed in this research area have been typified in Table 6.1. The high degree of cross-linking in UPR makes these composites more fragile and they possess lesser impact strength, which, unfortunately, restrains their use in high-performance applications. Hence, improving the toughness of UPRs has been the foremost foundation of the work of scientists. For the modification of UPR—improved toughness, the main method includes the introduction of elastomeric, nanosized fillers for modifying the chemical structure and forming interpenetrating polymer networks (IPNs) [17
20]. To improve the impact strength, IPNs of commercial UP and polyurethanes (PUR) have been used. These materials are employed in reaction injection molding (RIM) technology [21,22]. UPs represent the main class of thermosetting molding resins. They are products of the condensation of saturated and unsaturated dicarboxylic acids or anhydrides with alcohols. Propylene glycol is the most commonly used alcohol and phthalic and maleic anhydride are the most common saturated and unsaturated anhydrides being used. Condensation products (reactive resin) form very durable structures and coatings when cross-linked with vinyl reactive monomers (e.g., styrene).

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Table 6.1 Some Industrially Important Interpenetrating Network Patents US Patent Number

Assignee

Vinyl chloride resin composition Latex paints Composite resin particles and the preparation thereof Copolyetherester elastomeric compositions Interpenetrating polymer network (IPN) of epoxy resin (ER), polyallyl polymer, and anhydride Maleic anhydride
ER prepolymer, (vinyl or isopropenyl) phenyl glycidyl ether and anhydride Semi-IPN for tougher and more microcracking resistant high temperature polymers

5132359 5124393 5115020 5112915 5110867

Mitsubishi Rayon Company, Ltd. Union Oil Company of California Nippon Paint Co., Ltd. General Electric Company Akzo NV

5106924

Westinghouse Electric Corp.

5098961

Polyurethane
poly(vinyl chloride) interpenetrating network Process for the preparation of stable interpenetrating polymer blends, comprising a poly(vinyl aromatic) polymer phase and a poly(alkylene) phase Porous membrane formed from IPN having a hydrophilic surface Novel damping compositions Microporous waterproof and moisture vapor permeable structures, including the processes of manufacture and useful articles thereof Epoxy
polyimide blend for low temperature cure, highperformance resin systems and composites Use of reactive hot melt adhesive for packaging applications Thermoplastic resin composition Embedded lens retroreflective sheeting with flexible, dimensionally stable coating Processible polyimide blends Vinyl chloride resin composition Impact-resistant resin composition Copolyetherester elastomeric compositions IPN of an aliphatic polyol(allyl carbonate) and ER IPN of blocked urethane prepolymer, polyol, ER, and anhydride Electron beam irradiated release film Thermoplastic polyester resin composition

5091455

The United States of America as represented by the Administrator of the National Aeronautics and Space Administration W. R. Grace & Co.-Conn.

Patent Title

5084513

Shell Internationale Research Maatschappij B.V.

5079272

Millipore Corporation

5066708 5066683

Rohm and Haas Company Tetratec Co., Aluminum Company of America operation

5021519

Aluminum Company of America

5018337

4996101 4994522 4994523 4992506 4957981 4923934

National Starch and Chemical Investment Holding Corporation Mitsubishi Rayon Co., Ltd. Minnesota Mining and Manufacturing Company Lockheed Corporation Mitsubishi Rayon Company Limited Mitsubishi Rayon Company Limited General Electric Company Akzo N.V. Todd A. Werner (Inventor)

4921882 4918132

Hercules Incorporated Mitsubishi Rayon Company Limited

5011887 5008142

6.2 CLASSIFICATION OF POLYESTER RESINS

157

Table 6.1 Some Industrially Important Interpenetrating Network Patents Continued

Patent Title Process of preparation for a new memory thermoplastic composition from polycaprolactone and polyurethane, products obtained by this process, and its use particularly in orthopedics Resin blends exhibiting improved impact properties Matrix
matrix polyblend adhesives and method of bonding incompatible polymers Impact modified poly(alkenyl aromatic) resin compositions Interpenetrated polymer films Enhanced melt extrusion of thermoplastics containing silicone IPNs Thermosetting composition for an IPN system Interpenetrating polymeric network comprising polytetrafluoroethylene and polysiloxane Thermosetting cyanate resin and the use thereof for the production of composite materials and IPNs

US Patent Number

Assignee

4912174

Laboratoires D’Hygiene et de Dietetique (L.H.D.)

4902737 4886689

General Electric Company Ausimont, U.S.A., Inc.

4882383

General Electric Co.

4845150 4831071

Foster-Miller Inc. ICI Americas Inc.

4766183 4764560

Essex Specialty Products, Inc. General Electric Company

4754001

Bayer Aktiengesellschaft

The properties of cross-linked resin can be altered by varying the type and amount of acids and glycols. UPRs are used in the production of sanitary-ware, tanks, gratings, fiber-reinforced plastics and fiber-filled plastic articles, pipes, and high-performance components for the marine and transportation industries such as in closure panels, body panels, fenders, boat hulls/decks, and other large glass fiber
reinforced parts. UPRs also find use in coatings and adhesives.

6.2 CLASSIFICATION OF POLYESTER RESINS Polyesters are broadly classified into unsaturated and saturated polymers. These are two broad divisions split up as: 1. Saturated: a. Fibers and films: These were based on the reaction of terephthalic acid with ethylene glycol and are linear, high molecular weight polymers which do not experience any cross-linking reactions. b. Plasticizers: These are completely saturated polyesters and are usually referred to as polymeric plasticizers. c. Polyester/polyurethanes: These are polyesters having high hydroxyl contents that react with various isocyanates to form polyurethane. These polyesters find extensive use as foams, elastomers, surface coatings, and adhesives.

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2. Unsaturated: a. Laminating and casting resins: These are based on dibasic acids and dihydric alcohols. The polyester unit formed must be capable of copolymerizing with a vinyl-type monomer, thereby giving a vinyl
polyester copolymer or cured polyester having a thermoset structure. b. Alkyds: In general, these are of the same types as those described in (a), although the glyptal (surface coatings)-types are modified with oils or fatty acids. This term was also used to describe a group of thermosetting molding materials based on the reaction of a dihydric alcohol with an unsaturated acid such as maleic acid in place of the conventionally used phthalic acid. Vinyl-type monomers are also essential to speedy cross-linking and curing and these are used as molding powders for compression and transfer molding techniques. On the basis of their structure, polyester resins may be classified into these groups: (1) ortho-resins, (2) iso-resins, (3) bisphenol-A fumarates, (4) chlorendics, and (5) vinyl ester (VE) resins. 1. Ortho-resins: These are based on maleic anhydride (MA), phthalic anhydride (PA) or fumaric acid, and glycols. They are also recognized as general-purpose resins. Characteristics: PA is comparatively low in cost and provides a rigid link in the backbone. Limitations: It reduces the thermal resistance of laminates, while limited chemical resistance and processability are other problems associated with these resins. Owing to the presence of the pendant methyl group, the resulting resins are less crystalline and more attuned with wideranging reactive diluents (styrene) compared to those obtained using ethylene glycol, diethylene glycol (DEG), and triethylene glycol and they yield products with inferior electrical properties. 2. Iso-resins: These are prepared using MA/fumaric acid, isophthalic acid, and glycol. Characteristics: These resins are higher in price than ortho-resins and also have substantially higher viscosities; therefore, a higher proportion of reactive diluents (styrene) are needed. Isophthalic resins are of a higher caliber as they have better thermal and chemical resistance and mechanical properties. Limitations: The presence of higher quantities of styrene imparts improved water and alkali resistance to cured resins. 3. Bisphenol-A fumarate resins: Characteristics: The introduction of bisphenol-A into the backbone bestows a higher degree of inflexibility and stiffness with improved thermal performance. By reacting ethoxy-based bisphenol-A with fumaric acid, they are synthesized.

n

6.2 CLASSIFICATION OF POLYESTER RESINS

159

4. Chlorendic resins: Characteristics: To boost flame retardancy, chlorine/bromine-containing anhydrides or phenols are exploited in the preparation of UPRs. For example, the reaction of chlorendic anhydride/chlorendic acid with maleic acid/fumaric acid and glycol yields resin with better flame retardancy than general-purpose UPR. Other monomers used, include tetrachloro- or tetrabromophthalic anhydride. The bromine content must be at least 12% to make a self-extinguishing polyester [23].

5. VE resins: Characteristics: Bisacryloxy or bismethacryloxy derivatives of epoxy resins (ERs) carry unsaturated sites only in the terminal position and are developed by the reaction of acrylic acid or methacrylic acid with ER (e.g., diglycidyl ether of bisphenol-A, epoxy of the phenol-novolac type, or epoxy based on tetrabromobisphenol-A). These resins were first marketed under the trade name of Epocryl in 1965 by Shell Chemical Company. In 1966, Dow Chemical Company, under the trade name of Derakane resins, introduced a similar series of resins for molding purposes. The viscosities of neat resins are high; hence, reactive diluents (e.g., styrene) are added to obtain low viscosity solutions (100
500 poise). Notable advances in VE resin formulations are low-styrene-emission resins, hybrid grades that balance performance and economy, automotive grades with high tensile strength and heat deflection temperatures, and materials for corrosion resistance [24]. The effect of presence of electrolyte on a VE resin and its greatly filled quartz composites have just been reported on [25].

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6.2.1 SYNTHESIS AND CHARACTERIZATION OF INTERPENETRATING POLYMER NETWORKS An IPN can be defined as a mixture of two cross-linked polymers when at least one of them is synthesized and/or cross-linked with another. If another polymer that is capable of cross-linking separately is added to a UPR, the physical properties can be enhanced dramatically. The components that make up an IPN are thermodynamically incompatible and a transition region of two phases is formed in such a system. The whole complex of IPN properties is determined by the accessibility and characteristics of this area. Other special types of such systems are also addressed as hybrid systems. The properties of an adhesive derived from the mixture of oligomers are similar to UPR and a prepolymer with end isocyanate groups (or macrodiisocyanate) based on polydiethylene glycol adipate of molecular weight 800 and Toluene diisocyanate (TDI) (a mixture of the 2,4- and 2,6-isomers in a ratio of 65:35), with the surfactant Alkyl triazole glycoside (ATG) added. The adhesive is cured by the effect of a redox system, for which MEKP(O) and cyanoacrylate (CN) are commonly used.

n

6.2 CLASSIFICATION OF POLYESTER RESINS

161

a. Polyurethanes: Polyurethanes are like UPR, compounds that simultaneously form a crosslinkable polyurethane which are added to poly glycols and diisocyanates. The rate of reaction of one component might be estimated to be reduced by the dilution effects of the other components. On the other hand, during free radical polymerization, the reaction may become diffusion controlled and a Trommsdorff effect (a self-acceleration of the by and large rate of the polymerization) emerges. When the polymerizing in mass becomes more viscous as the concentration of polymer increases, the mutual deactivation of the rising radicals is stalled, while the other basic reaction rates such as initiation and propagation remain constant. For a UPR
polyurethane system, the rate of the curing process is increased considerably in comparison to pure homopolymers. Validating reactions between the polyurethane isocyanate groups and the terminal UP carboxyl groups were suggested to possibly lead to the formation of amines (Eq. 6.1). R 2 N 5 C 5 O 1 R0 COOH-R 2 NHCO 2 O 2 CO 2 R0 -R 2 NHCO 2 R0 1 CO2

b.

c.

d.

e.

(6.1)

These amines may act as promoters of the curing process. Moisture, which does not influence the curing reaction of UPR, would also lead to the formation of amines by the reaction of the isocyanate groups with water. A tricomponent IPN system consisting of castor oil-based polyurethane components, acrylonitrile, and a UPR (the main component) was synthesized with the purpose of strengthening the unsaturated UPR. By incorporating urethane and acrylonitrile structures, the tensile strength of the matrix (UPR) decreased and the flexural and impact strengths were increased. Epoxides: Mixtures of UPR systems and ERs also form IPNs. Since a single glass transition temperature (Tg) for each IPN is observed, it is suggested that both materials are compatible. On the other hand, an interlock between the two rising networks was suggested because in the course of curing, a retarded viscosity increase was observed. A network interlock is indicated by a lower total exothermic reaction during simultaneous polymerization in comparison to the reaction of homopolymers. In bismaleimide-modified UP
ERs, the reaction of the UP with the ER could be established by IR spectral studies. The absorption of bismaleimide into the ER enhanced both the mechanical strength and thermal behavior of the ER. Vinyl ester resins: UPs modified with 30% of VE oligomer are tougheners for the UP matrix. The introduction of VE oligomer and bismaleimide into UPR improves its thermomechanical properties. Phenolic resins: An IPN consisting of a UPR and a resol-type phenolic resin showed improved heat resistance but also suppressed smoke, toxic gas, and heat release during combustion in comparison to a pure UPR. Organic
inorganic hybrids:

Organic
inorganic polymer hybrid materials can be synthesized using a UP and silica gel. First the UP is prepared and to this polyester the silica gel precursor is added, that is, tetramethoxysilane,

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methyltrimethoxysilane, or phenyltrimethoxysilane. Gelling of the alkoxysilanes was achieved at 60 C using HCl catalyst in UPR. It was established by nuclear magnetic resonance spectroscopy that the polyester hydrolyze during the acid treatment. Finally, the IPN was formed by the photopolymerization of the UPR. It is assumed that between the phenyltrimethoxysilane and the aromatic groups in the UPR pinteractions arise.

6.3 POLYURETHANE HYBRID NETWORKS The mechanical properties of UPR can be significantly enhanced by incorporating a polyurethane linkage into the polymer network. The mechanical properties also can be altered by the same techniques used in segmented polyurethanes. The basic concept is to use soft segments and hard segments. A polyester is prepared with an excess of diols and diluted with styrene as usual. Additional diols as chain extenders are mixed into the resin solution. 4,4-Diphenylmethane diisocyanate dissolved in styrene is used to form hybrid linkages. Desirable peroxides are added to begin the radical curing. The curing begins with a reaction between the isocyanates and the hydroxyl groups, thus forming the polyurethane linkage. Then the cross-linking reaction takes place. The mechanical properties of the hybrid networks were usually improved by the assimilation of a chain extender at room temperature. Hexanediol increased the flexibility of the polymer chains, ensuing in a higher deformation and impact resistance of the hybrid networks. Hybrid networks with ethylene glycol as the chain extender are stiffer. The synthesis and characterization of IPNs was carried out by blending two thermosets and this method is extensively used [26
29]. This has been shown to be a promising way to extend the range of properties of thermosets and, hence the applications of polymer products. A string of studies on IPNs have discovered enhanced mechanical properties [30]. Epoxy/poly vinyl acetate IPNs are branded for toughness [31]. Epoxy/polydimethylsiloxane IPNs show potential toughening and better impact and thermal strength [32]. Epoxy/acrylate IPNs are described to display improved elongation at break, toughness, modulus, and tensile strength [33
35]. Likewise, IPNs of polyurethane
polystyrene, polyacrylates and polybenzoxazine, polymethacrylate, and epoxy
amine networks are reported to have higher tensile strength and elongation at break, improved thermal and surface free energy, and exhibit gas barrier properties [36
38]. The usage of polymer blends to make new materials for specialty applications is becoming widely employed for a range of applications. IPNs belong to a category of polymer blends with special characteristics such as impact strength modification and pH sensitive hydrogels. In semi-IPNs only one of the constituents is cross-linked. Polyurethane
polyester IPNs have been synthesized to improve the flexibility of the resulting polymer network. In recent years, the blending of two thermosets through IPNs has been extensively studied [39
47]. Several studies on IPNs have revealed improved mechanical properties [48,49]. Lin et al. [50,51] investigated the chemorheology and kinetics of epoxy/UP IPNs.

6.4 CURRENT RESEARCH ON UNSATURATED POLYESTER

163

The enhanced cracking energy-absorbing capability of epoxy/acrylic IPNs has also been reported in the literature [48]. The entanglement of the two interlocked networks exhibited an improved toughness on the mechanical properties of epoxy/UP IPNs [52]. The physical and mechanical properties of PUR elastomers are related to the phase separation of hard from soft blocks, which depends on the solubility parameter of the blocks, crystallinity of the phases, temperature, and the sample’s thermal record [53]. Hence, the degree of phase separation, structural and dynamical heterogeneity, and morphology of the resulting phases (size, shape, orientation, and domain associations) have been extensively investigated. Their dependence on factors such as chemical composition, degree of crystallinity, molecular masses and the ratio of segments, the introduction and structure of the side chains or functional groups, the preparation method, and sample storage has been the subject of numerous studies. PUR can be used as an underwater acoustic absorption material and/or damping material [54
58]. The ability to absorb mechanical energy and convert it into heat is its uppermost around the Tg, where the rate (frequency) of mechanical action is equal to the rate (frequency) of coordinated molecular chain segment motion [59]. Therefore the damping capacity is connected to motional heterogeneity, which can be adjusted by varying the composition of soft and hard segments and their mutual ratio and the introduction of functional groups and side chains [54,60
65]. Damping capacity can be determined from dynamic mechanical properties. In two damping methods that are interesting from an engineering point of view, the values for the loss modulus and the loss factor tan(δ) are important. An extensional damping requires a high E value, while a constrained layer damping requires a high tan value [59]. With the aim of being efficient in the wide temperature and frequency ranges encountered in real damping applications, damping materials should exhibit a high tan value ( . 0.3) above the temperature range of no less than 60 C
80 C. PUR frequently shows an inadequately broad tan(δ) peak. Besides, the usage of PUR in structural materials can be hindered by its meager thermal stability and mechanical properties. So as to widen the temperature range with adequately high damping peaks and to obtain materials with adequate properties, polyurethanes can be combined with polymers possessing high modulus and strength through the preparation of IPNs [66,67]. Changes in the physical and mechanical properties of networks compared to the pure components depend on the degree of phase separation and morphology. The degree of phase separation in IPNs depend on numerous factors such as the miscibility, mutual ratio, crystallinity, and the Tg of the components, the degree of cross-linking of individual networks and the interconnection of networks, the method and conditions (temperature, pressure) of preparation, and the rate of forming [68]. The effect of blending on the damping ability of PUR IPNs with various polymer components such as ER [27
30,67
69] VE resin [67,69
72], UPR [73], acrylates [67,69,74
77], poly(vinyl chloride) (PVC) [67
69,78], and polystyrene [66,67,69,79] has been extensively prepared and characterized.

6.4 CURRENT RESEARCH ON UNSATURATED POLYESTER UPR is the class of superior and applied polymers, which find use in a number of diverse applications [80
82]. UPRs were selected primarily for making FRPs using any molding technique because of their ease of handling and fabrication and low cost as compared to ER.

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They are primarily used in compression molding (sheet molding compounds), injection molding (bulk molding compounds), pultrusion, resin transfer molding, filament winding, and the hand lay-up process [83]. About 85% of FRP products (like boats, car and aircraft components, and chairs) are manufactured using polyesters [84]. Different kinds of polyesters have been synthesized over the past few decades from various types of diols and diacid chlorides. Thermally stable products made of polyesters derived from isophthalic and terephthalic acids with bisphenol-A have successfully been commercialized [85]. These polyesters were normally hard to process because of their inadequate solubility in organic solvents and their melting temperature or high Tg relating to their rigid structures [86]. Therefore the growth of polyesters for use at high temperatures with improved solubility was an important goal. To improve the solubility and processability of polymers without the extreme loss of their high thermal stability, polar and flexible groups were introduced into the backbone of the polymers [87
91]. The insertion of bulky side chain affects solubility because this strategy produces a severance of chains and the lowering of the chain packing by way of molecular mobility leading to the enhancement of solubility concurrently [92
95]. (99) It has been identified that a large number of polymers containing heterocyclic rings in their main chain were resistant to high temperatures (98). New polyesters containing rigid segments such as pyridine rings that possess high Tg values and enhanced solubility in organic solvents were synthesized by researchers (99; 100). The challenge in synthesis of UPRs riveted on the enhancement of chemical inertness, barrier properties, low friction coefficient and low surface tension, solvent and high temperature resistant that in essence are transferred to other polymeric materials by blending or copolymerization. Some augmentation such as excellent resistance to corrosion, water and atmospheric agents, formulations for resins and foams, and several others were also present in patent literature [87]. Nowadays, the macroscopic properties of polymers and other complex materials are mainly interpreted by understanding the underlying microscopic phenomena. The temperature dependence of the polymer affects average relaxation time and the molecular mobility decide the properties at a particular temperature. Toward this end, an energy landscape model based on the nature of structural evolution in a super cooled liquid approaching the glassy state was developed. According to this scheme, relaxation behavior was considered as strong and fragile, depending on the rate with which the associated properties were modified as the temperature passes through the glass transition region [82]. The continuing search for polymers with improved or unusual properties guided a considerable level of interest in the behavior of so-called rigid-rod polymers. Such materials were of curiosity owing to their potential to form fibers of particularly high strength [80
82]. This rigid-rod polymer had achieved commercial success in a variety of applications, particularly those relying on its exclusive combination of high strength and low density [87]. Their lack of processability was a major drawback in the commercial exploitation of the many rigid-rod systems. However, the usage of UPR resin was used to overcome this drawback [90]. Polyester resins also called unsaturated copolyesters, are based on a polyester backbone in which both a saturated acid and an unsaturated acid are condensed with a dihydric alcohol [82]. An inspection of the scientific literature reveals that few unsaturated copolyesters based on the interaction of unsaturated diols and saturated acids were synthesized and studied [86]. Aromatic polyesters and copolyesters containing phenylindane units with Tg between 235 C and 253 C were

6.6 POLYMER GELATION AND VITRIFICATION

165

documented in the literature [87]. In the contemporary era, attempts have been made toward the syntheses of polymers containing chromophoric groups, for instance, the aromatic azo groups which can form a part of the main chain [89]. Therefore polymers that possess the azo group have potential use in a variety of applications [93
95]. The aromatic azo group, because of the existence of cis-trans isomerism and further effects on the photochromic properties of polymers, is of special interest. New UPs and copolyesters based on some dibenzylidenecycloalkanones and containing meta- and para-azo groups in the main chain were investigated relating to the synthesis and characterization of them. Through the interfacial polycondensation of various monomers, new interesting classes of linear UPs polyesters based on dibenzylidenecycloalkanones were synthesized [96]. The effects of a cycloalkanone ring in the polymer backbone on the properties of polymers were also studied.

6.5 POLYMER BLENDS IPNs are formed by two cross-linked polymers, so they both constitute a network, whereas semiIPNs are blends, in which only one of the constituents is cross-linked. Very little work has been reported in the area of producing IPNs with oligomers; the production of IPNs for practical applications has been described, but only a few papers in this respect have appeared in the literature. The industrial fabrication of most polymers involves the formation of oligomer as byproducts. This is not only a waste of resources but also a source of pollutants in landfills and water deposits. Also, for these IPN-like materials, the microstructure is essential, so that characterization can direct a better understanding of the synthesis
properties relationships.

6.6 POLYMER GELATION AND VITRIFICATION Since the beginning of this century, the polymer industry has been producing thermosetting resins (viscous liquids with the ability to harden permanently) owing to their excellent chemical, thermal, and mechanical properties along with their easy, controllable, inexpensive, and fast molding and production. More specifically, these thermosets are highly elastic (very ductile), strong (adequate stiffness but still quite tough), dimensionally stable, and resistant to heat and corrosion agents; which are all important requirements regarding finished products and their fabrication, processing, and use in several fields, for example, automobile and marine transportation (e.g., protective coatings, hulls, and auto bodywork compounds) and civil infrastructure construction (e.g., covers, bathroom components and fixtures, pipes, tanks, and fittings). These properties are the result of a highly cross-linked network composed by polymer chains and an additional monomer (solvent), which is highly reactive. In order to obtain this highly stable and strong network, the formation of covalent bonds is necessary between the polymer chains and the solvent monomers, which is only possible if they comprise functional groups that can react with each other such as the alkene groups (due to their carbon
carbon double bonds).

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FIGURE 6.3 Schematic illustration of a curing reaction and its steps, from unlinked chains to gel formation.

To accomplish this reaction between the functional groups, it is also necessary for an ion or a radical to be present, which are obtained by heating, heating and compression, or light irradiation of chemical compounds called initiators. Ions or radicals work like a trigger by breaking some double bonds, which generate other reactive free ions or radicals. These would be able to “attack” the other double bonds along the chains and, as a result, a copolymerization occurs between the initial polymer and the solvent, producing the final resin. This whole process is also known as a curing reaction and it is not as simple as it seems. It comprises two solidification phases—gelation and vitrification—and two more subreactions—polymer and solvent homopolymerization—which are worthy of consideration (Fig. 6.3). Gelation consists of a liquid (sol phase) to rubber (gel phase) transition controlled by the kinetics of the reaction, in which the resin molecular weight and viscosity increases considerably. After gelation, chains begin to lose their mobility (due to the increase of cross-linked network density) and a diffusion-controlled rubber
glass transition occurs, known as vitrification. This is a significant stage as it determines the rate and degree of the reaction conversion and enables some modifications to the structure and the properties of the final resin. Concerning the subreactions, it is also important to note that besides the copolymerization between the polymer and the solvent, covalent bonds are also created within the polymer chains or between the solvent monomers as shown in Fig. 6.4 These homopolymerization reactions have dissimilar kinetics and affect, in different ways, the macro- and micro-structure of the final cross-linked network. UPs have been extensively used in the biomedical and environmental fields. Due to their inherent biodegradability (ester linkages), biocompatibility, and cross-linking ability (carbon
carbon double bonds), they had developed into the most appropriate and expectant candidate for the production of resins which require not only superb physicochemical properties but also good biological properties. Classically, UPs comprise glycols and saturated and unsaturated acids as monomers. The types and amounts of monomers used define the composition and the properties of UPs: glycols and saturated acids are accountable for strength and thermochemical resistance in UPs; and unsaturated acids allow for the cross-linking of UPs in curing reactions due to their double bonds.

REFERENCES

167

FIGURE 6.4 Schematic illustration of inter (a.1) and intra (a.2) polymer homopolymerization and solvent monomer homopolymerization.

6.7 CONCLUSION AND FUTURE DIRECTIONS The prospects of UPRs in the global automotive composites market look to be full of opportunities in various applications including body panels, closure panels, grille opening reinforcement, heat shields, fenders, headlamp reflectors, pickup boxes, and others. The production of UPRs in the worldwide automotive composites market is predicted to grow at a compound annual growth rate (CAGR) of 5.3% from 2016 to 2021. The major agents of change for market growth are the rising demand for lightweight resources and the performance profit of reinforced composites over competitor materials. Properties such as high tensile strength, lightweight, ease of processability, and good corrosion resistance and surface tension make UPR composites ideal candidates for developing lightweight and fuel-efficient vehicles.

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