Polyester resin
Polyester resin, more precisely unsaturated polyester resin (UPR), is a thermosetting polymer synthesized via polycondensation of glycols such as propylene glycol with unsaturated dicarboxylic acids or anhydrides like maleic anhydride, resulting in a viscous liquid oligomer dissolved in a vinyl monomer like styrene that serves as both solvent and cross-linking agent.[1] Upon curing with free-radical initiators such as peroxides, the styrene undergoes copolymerization with the resin's carbon-carbon double bonds, yielding a rigid, cross-linked three-dimensional network with tunable mechanical, thermal, and chemical properties.[2] This process enables the formation of durable composites when reinforced with fibers like glass, making UPR one of the earliest commercialized matrices for such materials, with initial development tracing back to the 1930s and widespread industrial adoption by the 1940s for molding applications.[3] Key advantages include low viscosity for easy impregnation, cost-effectiveness relative to alternatives like epoxies, and compatibility with hand lay-up or spray-up fabrication, though curing often emits volatile styrene, prompting ongoing refinements in low-emission formulations.[4] Principal applications span marine vessels, automotive body panels, corrosion-resistant tanks, and building panels, where its balance of strength, stiffness, and weatherability supports high-volume production.[2]Definition and Classification
Chemical Structure and Types
Polyester resins are condensation polymers featuring ester linkages (-COO-) that connect dihydric alcohols (diols) and dicarboxylic acids or anhydrides, yielding linear chains with the general repeating structure [...-O-R-O-CO-R'-CO-...], where R derives from the diol and R' from the diacid. This structure arises from step-growth polymerization via esterification, eliminating water and forming the backbone.[5] Resins are classified as saturated or unsaturated based on the presence of carbon-carbon double bonds (C=C) in the polymer chain, which determine reactivity and applications. Saturated polyester resins employ fully saturated monomers, such as terephthalic acid with ethylene glycol to produce polyethylene terephthalate (PET), resulting in thermoplastic chains lacking reactive unsaturation for cross-linking.[6] These are used in coatings, films, and bottles due to their melt-processability and lack of double bonds, which prevent radical polymerization.[7] Unsaturated polyester resins (UPRs), the predominant type for thermoset composites, incorporate 5-50 mol% unsaturated dicarboxylic components like maleic anhydride or fumaric acid alongside saturated acids (e.g., phthalic or isophthalic anhydride) and diols (e.g., propylene glycol or neopentyl glycol), embedding isolated C=C bonds in the backbone for subsequent cross-linking with vinyl monomers such as styrene via free-radical mechanisms. UPRs are further differentiated by monomer composition: orthophthalic types use ortho-phthalic anhydride as the primary saturated acid for cost-effective general-purpose applications; isophthalic variants substitute meta-phthalic (isophthalic) acid to enhance chemical resistance and thermal stability; and specialized formulations incorporate dicyclopentadiene or chlorendic anhydride for improved hydrolytic or fire resistance.[8][9] These structural variations dictate properties like viscosity, cure speed, and mechanical performance post-cross-linking.[5]Unsaturated vs. Saturated Variants
Unsaturated polyester resins (UPRs) are synthesized through the polycondensation of unsaturated dicarboxylic acids, such as maleic anhydride or fumaric acid, with saturated dicarboxylic acids like phthalic anhydride and diols including propylene glycol or ethylene glycol, resulting in a polymer chain containing reactive carbon-carbon double bonds.[10] These double bonds facilitate cross-linking with vinyl monomers, typically styrene at 30-50% by weight, via free-radical polymerization initiated by peroxides, forming a rigid, three-dimensional thermoset network upon curing.[11] In comparison, saturated polyester resins are produced by esterification of fully saturated dicarboxylic acids, such as adipic or terephthalic acid, with excess polyols like neopentyl glycol, yielding linear or branched chains devoid of double bonds and thus lacking inherent sites for radical cross-linking.[12] Saturated variants require external cross-linking agents, such as melamine-formaldehyde or isocyanates, for thermosetting applications, often in coatings. The presence of unsaturation in UPRs imparts distinct properties post-curing, including tensile strengths of 50-100 MPa, flexural moduli up to 4 GPa, and heat distortion temperatures around 80-120°C depending on formulation, alongside good corrosion resistance to acids and solvents due to the dense cross-linked structure.[13] However, this rigidity can lead to brittleness, with elongation at break typically below 5%. Saturated polyesters, by contrast, exhibit greater flexibility, with glass transition temperatures often below 0°C and inherent chemical stability from the absence of reactive sites, making them suitable for applications demanding toughness over rigidity, though they generally possess lower thermal resistance without additional curing.[14] Cured UPRs demonstrate superior dimensional stability under load compared to uncured or differently cross-linked saturated variants, attributable to the covalent network formation.[15]| Variant | Key Structural Feature | Primary Curing Mechanism | Mechanical Properties (Post-Cure) | Thermal Resistance |
|---|---|---|---|---|
| Unsaturated | C=C double bonds in backbone | Free-radical with styrene/peroxides | High strength (50-100 MPa tensile), low elongation (<5%), rigid | HDT 80-120°C |
| Saturated | No double bonds, saturated chain | External agents (e.g., melamine, urethanes) | Flexible, higher elongation, lower modulus | Lower, Tg <0°C without cross-link |
Historical Development
Origins in the 1930s
The development of polyester resins, particularly unsaturated variants suitable for thermosetting applications, originated in the early 1930s through the pioneering work of American chemist Carleton Ellis. Ellis, a prolific inventor with over 800 patents in resins and plastics, focused on creating synthetic resins from polyhydric alcohols and unsaturated dibasic acids or anhydrides, enabling cross-linking reactions that produced durable, moldable materials. His efforts addressed the need for alternatives to natural resins, building on earlier polyester explorations but emphasizing unsaturation for polymerization control.[3][19] A key milestone was Ellis's U.S. Patent 1,897,977, granted on February 14, 1933, which detailed the production of artificial resins by esterifying glycerol (or similar polyols) with maleic anhydride or fumaric acid derivatives, yielding viscous liquids that could be cured into hard solids. This patent described resins with properties like solubility in styrene (later identified as an effective diluent and cross-linker) and resistance to water, marking the first systematic formulation of what became unsaturated polyester resins (UPRs). Ellis's approach involved heating the reactants under vacuum to minimize side reactions, producing oligomers with molecular weights around 1,000–2,000 daltons, which formed the basis for subsequent industrial scaling.[3][20][19] Concurrent research in the 1930s, including at DuPont under Wallace Carothers, advanced linear saturated polyesters, but these were primarily fiber-oriented and lacked the cross-linking potential of Ellis's unsaturated formulations. Ellis's resins demonstrated superior moldability and chemical stability compared to contemporaneous phenolics, though initial curing was inconsistent without peroxides or accelerators, limiting immediate commercialization. By the mid-1930s, additional patents refined Ellis's methods, incorporating phthalic anhydride for viscosity control and exploring styrene copolymerization, setting the stage for wartime applications. These innovations stemmed from empirical trial-and-error in organic synthesis, prioritizing causal mechanisms like double-bond reactivity for network formation over theoretical modeling.[20][21][22]World War II Applications and Early Commercialization
Commercial development of unsaturated polyester resins for molding commenced in the United States in 1941, following Carleton Ellis's 1933 patent, with the initial introduction of a heat-curing alkyl casting resin.[3] [23] This marked the transition from laboratory synthesis to industrial-scale production, driven by the resins' ability to form durable thermoset materials when cross-linked.[3] World War II accelerated applications of polyester resins, particularly when combined with glass fibers to create reinforced composites for military use. These materials offered reduced weight compared to metals while providing comparable strength and corrosion resistance, addressing demands for lighter aircraft and watercraft components.[24] [25] The first commercial deployments of glass fiber-reinforced polyester (GRP) occurred in aircraft parts, including ducting systems, where the composites enabled efficient fabrication and performance under operational stresses.[23] [26] Wartime necessities catalyzed the shift from experimental to production-scale manufacturing of fiber-reinforced plastics (FRP), with polyester serving as the primary matrix resin due to its room-temperature processability when diluted with styrene monomer.[25] Early commercialization extended beyond military confines by the mid-1940s, as surplus production capacity and technological refinements enabled civilian adaptations, such as cold-cure formulations introduced in 1946 for GRP laminates in boat hulls.[3] However, the foundational wartime innovations in resin formulation and composite layup techniques laid the groundwork for broader market penetration, establishing polyester resins as a cornerstone of the emerging reinforced plastics industry.[21]Post-1945 Expansion
Following World War II, unsaturated polyester resins transitioned from military applications, such as radar-transparent radomes reinforced with glass fabric, to widespread civilian uses, driven by the development of room-temperature "cold cure" systems. In 1946, styrene-containing polyester resins, catalyzed by peroxides, enabled easier processing without heat, facilitating the production of glass-reinforced polyester (GRP) laminates for boat hulls, vehicle bodies, and building panels.[3] This innovation spurred commercial expansion, as the resins' low viscosity, compatibility with fiberglass reinforcements, and cost-effectiveness relative to metals supported scalable molding techniques.[27] The marine industry exemplified early post-war growth, building on experimental fiberglass-polyester boats constructed as early as 1942 using Owens Corning fabrics and American Cyanamid resins; post-1945 production scaled rapidly for recreational and commercial vessels due to GRP's corrosion resistance and lightweight strength.[28] By the 1950s, companies including Owens Corning and DuPont commercialized integrated fiberglass-polyester systems, extending applications to automotive components and appliances. A landmark was the 1953 Chevrolet Corvette, which featured exterior body panels molded from unsaturated polyester resin reinforced with fiberglass, demonstrating viability for high-volume structural parts.[20] These advancements aligned with broader plastics production surging from 2 million metric tons globally in 1950, with thermosets like polyesters contributing to composite markets in construction and transportation.[29] In the 1960s, intensified competition lowered resin prices, accelerating adoption in bulk molding compounds (BMC) and sheet molding compounds (SMC) for electrical enclosures and consumer goods.[30] Vinyl ester variants, developed in the late 1950s and early 1960s by reacting epoxy resins with acrylic acids, offered improved chemical resistance for demanding environments, further diversifying polyester-based composites.[15] This era solidified unsaturated polyesters as the dominant matrix for reinforced plastics, establishing enduring markets despite challenges like styrene volatility.[27]Chemistry and Synthesis
Monomer Components and Polymerization
Unsaturated polyester resins, the predominant type used in thermosetting applications, are synthesized through the esterification of diols with a mixture of unsaturated and saturated dicarboxylic acids or anhydrides. Common unsaturated components include maleic anhydride or fumaric acid, which introduce polymerizable carbon-carbon double bonds into the backbone, typically comprising 20-30% of the acid content to enable subsequent cross-linking.[31] Saturated dicarboxylic acids or anhydrides, such as phthalic anhydride or isophthalic acid, balance rigidity and cost, often making up the remaining acid portion.[31] Diols like propylene glycol, ethylene glycol, or neopentyl glycol provide the alcohol functionalities for condensation, with propylene glycol favored for its flexibility and lower viscosity in the final resin.[31] This polycondensation reaction occurs at elevated temperatures (around 180-220°C) under acid catalysis, yielding a linear oligomer with a molecular weight of 1,000-3,000 g/mol and maleate/fumarate double bonds separated by saturated segments.[19] The oligomeric polyester is then dissolved in a vinyl monomer, most commonly styrene at 30-50% by weight, which serves as both a reactive diluent to reduce viscosity and a co-monomer for cross-linking.[11] Inhibitors like hydroquinone are added to prevent premature polymerization during storage. For saturated polyester resins, used in thermoplastic applications, the synthesis excludes unsaturated acids, relying solely on saturated dicarboxylic acids (e.g., adipic or terephthalic acid) and diols, resulting in higher molecular weight polymers without inherent cross-linking sites.[32] Polymerization of unsaturated resins proceeds via free-radical mechanism, initiated by peroxides such as methyl ethyl ketone peroxide (MEKP) or benzoyl peroxide, which thermally or redox-decompose to form radicals.[33] These radicals primarily add to styrene's double bonds, generating oligomeric styrene radicals that propagate by alternating copolymerization with the polyester's fumarate/maleate double bonds, due to their higher reactivity and electron-poor nature.[34] This leads to branching and eventual cross-linking, forming a rigid, infusible network as the styrene bridges multiple polyester chains; the process exhibits auto-acceleration (Trommsdorff effect) from reduced termination rates in the viscous medium.[34] Cure times vary from minutes to hours depending on temperature (typically 20-80°C) and initiator concentration (1-2% by weight), with gelation occurring when cross-link density reaches a percolation threshold.[19] Saturated variants polymerize differently, often via melt polycondensation or transesterification without radicals, yielding linear chains for extrusion or injection molding.[32]Cross-Linking Mechanisms in Unsaturated Resins
Unsaturated polyester resins achieve cross-linking through free radical addition copolymerization between the α,β-unsaturated ester double bonds (typically from maleate or fumarate units) in the linear polyester backbone and vinyl monomers, most commonly styrene. This process converts the initially soluble and thermoplastic resin into a thermoset network, rendering it insoluble and infusible. Commercial formulations generally incorporate 30-50% styrene by mass, which functions dually as a low-viscosity diluent and reactive co-monomer essential for network formation.[11][33] Initiation occurs via thermal or redox decomposition of organic peroxides, such as methyl ethyl ketone peroxide (MEKP) at 1-2% by weight, generating primary alkoxy or hydroxyl radicals that initiate chain growth. In propagation, these radicals predominantly add to the electron-rich styrene double bonds, forming a styryl radical; this radical then preferentially attacks the electron-deficient double bonds of the polyester due to favorable reactivity ratios (e.g., r_styrene ≈ 0.4, r_polyester ≈ 0.04-0.1), promoting alternating copolymerization. Cross-linking arises as propagating chains attach to multiple unsaturated sites across different polyester macromolecules, interconnecting them via polystyrene bridges or direct polyester-polystyrene sequences. Termination proceeds through radical combination, disproportionation, or chain transfer, though propagation dominates until high conversion.[33][34][35] The overall curing evolves heterogeneously, often in five kinetic stages: induction (minimal reaction), microgel formation (localized high-cross-link domains), transition (potential phase separation), macrogelation (network percolation), and post-gelation (continued reaction in constrained matrix). This heterogeneity stems from rapid initial polymerization around initiator sites, leading to clustered cross-links separated by less dense regions. Factors influencing cross-linking density include the molar ratio of styrene to unsaturated sites (typically 2-4:1), polyester unsaturation level (e.g., 20-50% maleic anhydride-derived units), temperature (higher values increase mobility and density up to diffusion limits), and initiator concentration.[34][36][37] Greater cross-linking density correlates with enhanced mechanical rigidity and thermal stability but can reduce elongation at break, as denser networks restrict chain mobility. For instance, increasing maleic anhydride content in the polyester raises unsaturation, thereby elevating effective cross-link junctions per volume and stiffening the cured resin. Inhibitors like hydroquinone (50-400 ppm) are added to control pre-mature gelation during storage, while accelerators (e.g., cobalt salts) enable room-temperature curing via redox systems.[37][38][33]Differences in Saturated Resin Production
Saturated polyester resins are synthesized via polycondensation of saturated dicarboxylic acids, such as isophthalic acid, terephthalic acid, or adipic acid, with dihydric alcohols like neopentyl glycol or ethylene glycol, deliberately excluding unsaturated acids like maleic anhydride that characterize unsaturated variants.[12] This monomer selection yields polymers without carbon-carbon double bonds in the main chain, prioritizing end-group functionality (e.g., hydroxyl or carboxyl) for alternative crosslinking mechanisms, such as with epoxy or melamine hardeners, rather than radical polymerization enabled by unsaturation.[39] The process unfolds in a batch reactor through esterification followed by polycondensation: acids and excess diols are charged and heated to form ester linkages with water byproduct removal via inert gas flow (e.g., nitrogen), typically at 190–220°C, often with catalysts like titanium or tin compounds to enhance reaction rates and minimize side reactions.[12] Polycondensation then builds molecular weight under vacuum to distill excess glycols, targeting specific acid numbers (e.g., for carboxyl-terminated resins) and viscosities, with batch durations of 24–30 hours excluding loading and unloading.[40] In contrast to unsaturated resin production, which incorporates unsaturation control and post-synthesis dissolution in styrene (30–50% by weight) for liquidity, saturated synthesis remains solvent-free, yielding solid or high-viscosity products often flaked for storage and later formulation into powders.[39] Post-reaction handling for saturated resins includes dilution in a separate kettle for quality sampling (e.g., evaporation testing for solids content), reactor washing with solvents, stirring for homogeneity, and filtration to eliminate unreacted particles or gels, ensuring clarity and fineness suitable for coatings.[39] Real-time monitoring via near-infrared spectroscopy or offline acid/viscosity checks addresses variability absent in unsaturated processes, where styrene addition stabilizes the prepolymer but introduces volatility and inhibition needs.[40] The resulting resins exhibit inherent stability against oxidation or premature gelling due to saturation, facilitating longer shelf lives but necessitating precise end-group balancing for thermoset applications like powder coatings baked at 180–200°C.[12]Physical and Chemical Properties
Mechanical Strength and Durability
Unsaturated polyester resins (UPR) exhibit moderate tensile strength in their cured, unreinforced state, typically ranging from 50 to 70 MPa, depending on formulation and curing conditions.[41] Flexural strength for similar unreinforced UPR can reach 44.65 to 119.23 MPa, reflecting the material's ability to withstand bending stresses before failure.[42] Compressive strength values are generally higher, often exceeding 80 MPa in neat resin forms, though exact figures vary with additives and processing; for instance, enhanced formulations with fillers have demonstrated compressive strengths up to levels supporting structural applications.[43] Impact resistance of UPR is relatively low without reinforcement, with Charpy impact energy measured at 3.5 to 6.5 Joules for standard laminating grades, indicating brittleness under sudden loads.[42] Hardness, assessed via Brinell methods, falls between 31.5 and 47.0 BHN, contributing to wear resistance in non-abrasive environments.[42] These properties improve significantly when UPR serves as a matrix in fiber-reinforced composites, where tensile and flexural strengths can exceed 100 MPa due to load transfer to reinforcements, but the inherent resin matrix limits overall toughness without such enhancements.[44] Durability of UPR under fatigue loading shows a static fatigue ratio approximately 10% lower than room-temperature baselines after extended exposure, such as 10 years, due to progressive microcracking and stress corrosion.[45] UV resistance is limited in orthophthalic variants, leading to surface degradation like chalking and yellowing upon prolonged outdoor exposure, though isophthalic grades offer improved weathering performance through better hydrolysis and corrosion resistance.[46] Natural weathering, combining UV radiation and thermal cycling, induces embrittlement by increasing crosslinking or chain scission, reducing mechanical properties over time; UV inhibitors in gelcoat formulations mitigate this by absorbing radiation and stabilizing the polymer backbone.[47] Overall, while UPR demonstrates adequate durability for indoor or protected applications, long-term outdoor use necessitates protective coatings or premium resin variants to maintain integrity against environmental stressors.[5]| Property | Typical Range (Unreinforced UPR) | Test Method/Reference |
|---|---|---|
| Tensile Strength | 50–70 MPa | ASTM D638[41] |
| Flexural Strength | 44.65–119.23 MPa | ASTM D790[42] |
| Impact Energy (Charpy) | 3.5–6.5 J | ASTM D256[42] |
| Hardness (BHN) | 31.5–47.0 | Brinell scale[42] |
Thermal, Chemical, and Electrical Properties
Cured unsaturated polyester resins exhibit glass transition temperatures (Tg) ranging from 60°C to 170°C, with specific measurements showing midpoints as low as 66°C in differential scanning calorimetry during initial heating cycles; this variability arises from differences in monomer composition, such as diol chain length and cross-link density.[48] Heat deflection temperatures under load typically span 50°C to 80°C for standard orthophthalic formulations, though higher values up to 100°C or more are achievable with isophthalic or neopentyl glycol modifications that enhance rigidity and reduce chain mobility.[49] Thermal degradation initiates above 250°C, with primary weight loss between 300°C and 400°C due to depolymerization and volatilization of styrene cross-links and ester breakdown products, limiting continuous service temperatures to 130–150°C.[5]| Thermal Property | Typical Range | Influencing Factors |
|---|---|---|
| Glass Transition Temperature (Tg) | 60–170°C | Cross-link density, diol type[48] |
| Heat Deflection Temperature (HDT) | 50–80°C (up to 100+°C specialized) | Acid anhydride type (e.g., isophthalic > orthophthalic)[49] |
| Initial Decomposition Temperature | >250°C | Cure completeness, fillers[5] |