Unsaturated Polyester Prepolymer Research Articles

Enhanced performance of a bio‐based diluent with both vinyl and epoxide groups for unsaturated polyester resin applications

AbstractStyrene has been widely used as a reactive diluent for a variety of commercial unsaturated polyester resins (UPRs), but suffered from high volatility. In this work, we developed a bio‐based reactive diluent, 4‐vinylguaiacol glycidyl ether (DE4VG), via an epoxidation of 4‐vinylguaiacol (4VG). Excellent miscibility and high reactivity of DE4VG were demonstrated by a series of copolymerization tests with an unsaturated polyester prepolymer (named PMSD) synthesized from maleic anhydride, 1,3‐dihydroxypropane and succinic acid. While, significant advantages in terms of viscosity and volatility were confirmed. Additionally, DE4VG provides both vinyl and epoxide groups and the epoxide groups were cured using 4,4‐diamino diphenylmethane (DDM) to participate the network formation of the cured PMSD/DE4VG/TBPB/DDM resin, resulting in high glass transition temperature (Tg, 195°C), residual carbon (45.5%), tensile strength (41.75 MPa), and the elongation at break (4.5%), which were all significantly surpassed the cured PMSD/St/TBPB, PMSD/St/TBPB/DDM, PMSD/DE4VG/TBPB resin. This work provided a successful demonstration to replace styrene with a bio‐based reactive diluent in UPR preparation, not only overcame the high volatility of styrene, but also enhanced material performances of the cured resin.

Development of high‐performance partially biobased thermoset polyester using renewable building blocks from isosorbide, 1,3‐propanediol, and fumaric acid

AbstractResearch on biobased thermoset resins has been overlooked when compared with the rapid progress on biobased thermoplastics. The objective of this work was to develop unsaturated polyester prepolymers based on building blocks derived from renewable raw materials, namely, biobased isosorbide, 1,3‐propanediol, and fumaric acid, with petroleum‐derived phthalic anhydride. The prepolymers developed herein behaved as low‐molecular weight macromolecules (oligoesters), with Mn varying between 1.2 and 1.5 kDa, but achieved a high bio‐content of up to 87.1 wt%. The prepolymers were incorporated into reactive diluents comprising a blend of 2‐hydroxyethyl methacrylate and styrene, formulated to be eco‐friendlier and less toxic than typical styrene‐only incorporation approach, thus resulting in resins with viscosities between 750 and 950 cP. These resins are suitable for use in various fiber‐reinforced polymer production techniques, such as manual lamination, vacuum infusion, and pultrusion, having the benefit of presenting over 50 wt% of bio‐content in some formulations. Moreover, the crosslinked polyester resins (thermosets) exhibit comparable mechanical and thermomechanical behavior to their petrochemical‐based counterparts, with modulus of elasticity and tensile strength of up to 3.9 GPa and 62.1 MPa, respectively, and glass transition temperatures of up to 106°C, making them greener alternatives for high‐performance structural applications.

Highly Bio‐Based Unsaturated Polyester Resins with Improved Performance by Incorporating Isosorbide into the Polyester Prepolymer

AbstractHerein, the successful design of a highly bio‐based family of unsaturated polyester resins (UPRs) is presented, comprising of an isosorbide (ISO) incorporated unsaturated polyester prepolymer (UP) and a reactive diluent of 4‐vinylguaiacol acetyl ester (ACVG). A series of UPs are prepared by copolymerizing ISO with other green monomers, i.e., itaconic acid, ethylene glycol and oxalic acid, in a variation of ISO fraction. The UP/ACVG mixtures exhibit very low viscosity ranging from 2.8 to 3.7 Pa s. After curing at 135 °C to form a network, the resulting thermosets exhibit remarkable improvements in terms of glass transition (Tg, about 131 °C), storage modulus (2564–3342 MPa), and tensile strength (around 76.6 MPa), which are significantly better than the UP thermosets reported in previous works, The incorporation of ISO units into the UPs reveals a favorable effect on thermal stability of the thermosets as well. The best Td5% is measured to be 320.4 °C, and the volatile molecular information collected using a TGA‐GC/MS set suggests that the decomposition mechanism of the cross‐linked resins becomes heat‐resistant due to the ISO incorporation. Moreover, after being reinforced by cotton fabrics, an outstanding compatibility between the ISO containing UPRs and cotton fibers is found to be suitable for composite fabrication.

Improving flame retardancy and mechanical properties of halogen-free unsaturated polyester resin with diethylene glycol as comonomer

Unsaturated polyester resin (UPR) is a widely applied engineering material with drawbacks of high fire risks and brittleness. In this paper, low-cost diethylene glycol (DEG) was used as one of diols to react with saturated and unsaturated anhydrides for unsaturated polyester pre-polymers. Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, gel permeation chromatograph were used for studying their structures and differential scanning calorimetry, thermogravimetric analysis (TG) for studying their thermal properties. Incorporated DEG comonomer improves limited oxygen index values of cross-linked UPR from 20 to 26% as the mole proportion of DEG increases from 0 to 50 mol% in diols. Combustion test indicates that the UPR sample with 70 mol% DEG in diols is self-extinguishing with none drop, referring to nonflammable burning grade. What is more, mechanical properties are also increased greatly by comparing with typical UPR base. Pyrolysis behaviors of UPR in nitrogen atmosphere suggest that the polyesters incorporated with DEG will produce more char and residues during the degradation process. By comparing the pyrolysis behaviors of UPR having DEG with those of UPR adding ammonium polyphosphate in TG curves, the flame-retarding mechanism of DEG incorporation is probably due to residual char forming in temperature range of 500–800 °C, which is close to the burning temperature. This study suggests that DEG element increases effectively and environmental friendly the fire safety of UPR materials.

Morphology and performance of unsaturated polyester nanocomposites modified with organoclay and thermoplastic polyurethane

Unsaturated polyester (UPR)/thermoplastic polyurethane (TPU)/organoclay nanocomposites were prepared by melt compounding of thermoplastic polyurethane and unsaturated polyester prepolymer, and then mixing with the hybrids of styrene monomers and organoclay at ambient temperature. The crosslinking reaction eventually occurred through the unsaturated polyester prepolymer and styrene monomer. The morphology of the composites was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the impact strength of UPR/TPU/organoclay nanocomposites increases obviously; the cure shrinkage decreases markedly, the glass transition temperature is enhanced and an elastic response to the deformation is prominent at the temperature above 10°C.

Preparation and Surface Characterization of Highly Ordered Polymer/Graphite Nanosheet Composites

Graphite nanosheets obtained from sonicating expanded graphite were coated chemically with magnetite nanoparticles. The modified graphite nanosheets were dispersed in epoxy or unsaturated polyester prepolymer, and subjected to a magnetic field before the suspension solidified. Composites with graphite nanosheets of highly ordered alignment have been prepared via the described approach. Several techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy were used to characterize the modified graphite nanosheets and the polymer/graphite nanosheet composites.

Analysis and modeling of PVTX diagram of an unsaturated polyester resin, thermoplastic additive, and mineral fillers blend

AbstractWe studied the dynamic curing of a polymer blend containing an unsaturated polyester prepolymer, styrene, and a saturated polyester as a low‐profile thermoplastic additive during heating from 40 to 160°C with a constant rate (4 or 10 K/min). This work was performed by means of a homemade dilatometer that simultaneously determined conversion degree, volume, pressure, and temperature. The various phenomena inducing volume change were identified and then separated (thermal expansion, shrinkage, and shrinkage compensation). A model based on physical consideration was built that allowed an accurate fit of the experimental results. The influence of the pressure on all this phenomena was summarized and quantified. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1258–1267, 2003

Surface morphologies of composites based on unsaturated polyester pre-polymer

We report the study of surfaces of bulk molding compounds (BMC) based on miscible polymeric thermoset blends (TB)—unsaturated polyester, styrene and low profile additive (LPA)—containing fillers and glass fibers. In contrast to scanning electron microscopy (SEM) that identified a continuous organic layer at the BMC surface, atomic force microscopy (AFM) showed the existence of aggregates linked together to form a network at the micrometric scale. This indisputably demonstrated that phase separation took place at the surface of the BMC. The influence of TB was examined by comparing the surface morphologies of BMC and corresponding TB. Several cases were distinguished as a function of the TB composition. (1) Without LPA, the surface of the TB was continuous (no phase separation took place during curing) and the surface of BMC revealed the presence of aggregates resulting from a phase separation induced by the fillers. (2) For very low molecular weight LPA, aggregates randomly spread on islands surrounded by large holes were observed on the TB surface. These holes were shown to result from surface deformations induced by absence of shrinkage compensation. The corresponding BMC presented particles randomly spread on the surface. (3) The general case (higher molecular weight LPA) corresponded to similar TB and BMC surfaces morphologies with aggregates randomly spread on the surface. In this case, BMC roughness and morphology reflected the TB roughness and morphology. These observations led to the proposal of some considerations concerning the control of surface aspects of BMC.

Pyrolysis–gas-chromatographic study of a series of polyester thermosets

Thirteen different unsaturated polyester prepolymers have been synthesised in an attempt to establish a relationship between the structures of the commonly used polyesters and the pyrolysis products of their cross-linked polymers. All the polyesters contained maleic anhydride units as the unsaturated sites and all were cured using styrene as the cross-linking monomer. The variation in structure involved either the glycol component or the aromatic acid. The cross-linked polyesters were pyrolysed at 600°C and the products separated and identified using gas chromatography and gas chromatography–mass spectrometry. Many products were identified as arising from either the styrene cross-links or the polyester chains. The presence of cinnamate esters confirmed that degradation occurred across both distinct parts of the polymer network. The identity of some other products, particularly the phthalate esters, suggested that more than one degradation mechanism was occurring.

Synthesis and properties of UV-curable waterborne unsaturated polyester for wood coating

UV-curable waterborne unsaturated polyesters for wood coatings were prepared. The effects of different polyols and acids on the properties of the UV-curable waterborne unsaturated polyesters were investigated. Several different unsaturated polyester prepolymers were prepared from three different polyols [ethylene glycol (EG), diethylene glycol (DEG), and propylene glycol (PG)] and three different acids [tetrahydrophthalic anhydride (THPAn), terephthalic acid (TPA), and trimellic anhydride (TMAn)]. UV-curable coating materials were formulated from the prepolymers and 2-hydroxy-2-methylphenyolpropane-1-one as a photoinitiator with distilled water as a diluent. Trimethylolpropane diallyl ether was used as an air inhibitor of cure. The dynamic mechanical studies showed the properties of those unsaturated polyesters were well correlated with their glass transition temperature behaviors. It was found that the unsaturated polyester prepared with 60/40 (mol %) TMAn/THPAn and the equimolar mixture of EG, DEG, and PG showed balanced coating properties such as good tensile properties and weatherability, as well as proper viscosity (ca. 2500 cps) when using distilled water as a diluent. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 695–708, 1998

Relationship between the structure and hydrolysis rate of unsaturated polyester prepolymers

Polyester prepolymers based on binary, ternary or quaternary combinations of the following monomers: maleic anhydride (MAA), isophthalic acid (IPA), propylene glycol (PG), neopentyl glycol (NPG), ethylene glycol (EG) and diethylene glycol (DEG) were synthesized and immersed in water at 100 °C. The hydrolysis rate was determined by acid titration of chain ends; the rate constants were of second order and independent of hydrophilicity. Fumarates are intrinsically 5–20 times more reactive than phthalates. NPG, PG and DEG esters have very close reactivity. In contrast, EG esters are considerably more reactive. In copolymers, the whole hydrolysis rate cannot be deduced from the data relative to the corresponding homopolymers by a simple linear relationship; copolymers appear more stable towards hydrolysis than would be predicted. The rate constants obtained on model compounds were used to predict the hydrolysis behaviour of the corresponding styrene crosslinked networks. It appeared that the proposed method gives a good order of magnitude for the rate constant but with systematic overestimation.

Modeling of unsaturated polyester prepolymer structures. I. Chain branches and overall chain end numbers

A review on the synthesis of unsaturated polyester (UP) prepolymers showed that Ordelt saturation (reaction of hydroxyl groups with double bonds) is the major effect that may cause formation of chain branches and then result in an uncertainty of chain end number or functionality. A model has been derived in this work to estimate the number of chain branches and the number of chain ends of UP prepolymers from several fundamental molecular parameters, including hydroxyl and carboxyl indexes, Ordelt saturation extent, short- and long-chain branch distribution, molecular polydispersity, and starting reactant composition and also reasonable assumptions concerning the chain-branch lengths. Simulation results were compared with experimental ones. The influences of the mentioned molecular parameters on the chain branches and on the overall molecular chain ends are discussed in this work. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 133–145, 1997

Modeling of unsaturated polyester prepolymer structures. II. Hydroxyl and carboxyl functionalities

Based on the structural model being derived in Part I, a statistics approach has been used to further estimate separately carboxyl and hydroxyl functionalities of unsaturated polyester (UP) prepolymers directly from their molecular parameters. This extended model shows satisfactory results when comparing with experimental results of hydroxyl functionality. The influence of molecular parameters, such as hydroxyl and carboxyl indexes, Ordelt saturation extent, and molecular mass polydispersity, on the carboxyl and hydroxyl functionalities is discussed. Examples showing the important impact of synthesis deviation on the hydroxyl and carboxyl functionalities, by model estimation, and, finally, on the SMC thickening are also discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 147–156, 1997

Viscosities of unsaturated polyester resins: Combining effects of prepolymer structure, resin composition, and temperature

The effects of temperatures, styrene concentration, and molecular composition of unsat-urated polyester (UP) prepolymers on the viscosity of UP-styrene systems are studied. The viscosity of UP resins follows the Arrhenius-type expression with resin temperature as well as with the styrene molar fraction of the resin. The two Arrhenius correlations can be combined into a simple dual-Arrhenius equation by combination rule. This dual-Arrhenius equation comprises pseudo viscosity parameters of styrene monomer and UP prepolymer, respectively. The pseudo viscosity parameters of styrene monomer are experimentally found constant; while those of UP prepolymer seem to be number-average molecular weight correlated rather than weight-average molecular weight correlated. The UP prepolymers having different molecular structures show different pseudo viscosity parameters due to their own molecular interaction parameters. The pseudo viscosity parameters of UP'prepolymers should be determined experimentally for the time being. © 1996 John Wiley & Sons, Inc.

Hydrolytic stability of unsaturated polyester prepolymers

Polyester prepolymers were used as model compounds of the corresponding networks in a study of relationships between structure and hydrolytic stability. A comparison of various types of prepolymers allowed us to establish that the hydrolytic stabilities are in the order: propylene glycol (PG) < neopentyl glycol (NPG), maleic acid (MA) < isophthalic acid (PA). For industrial prepolymers, which are copolyesters based on combinations of the above monomers, it appears that the overall hydrolysis rate is lower than its theoretical value determined from a simple mixture rule from the individual rate constants determined from the experimental study of homopolyesters.

Structure and Molecular Weight Determinations of Unsaturated Polyester Prepolymers by Steric Exclusion Chromatography

Four model unsaturated polyester prepolymers and two industrial resins were studied by size exclusion chromatography (SEC). A calibration curve was set up with model resins and used to characterize the industrial unsaturated polyester resins. Weight average molecular prepolymer weights were determined from the new calibration curve. They were lower than those determined with polystyrene or epoxy calibration curves. The observation of SEC chromatographs proves the formation of diol-ISO-diol (where ISO = isophthalic acid) and MAA-diol-MAA structures (where MAA = maleic anhydride) in the beginning of polycondensation reactions. This gives information on the future structure of the UP network.

Structure and Molecular Weight Determinations of Unsaturated Polyester Prepolymers by Steric Exclusion Chromatography

Four model unsaturated polyester prepolymers and two industrial resins were studied by size exclusion chromatography (SEC). A calibration curve was set up with model resins and used to characterize the industrial unsaturated polyester resins. Weight average molecular prepolymer weights were determined from the new calibration curve. They were lower than those determined with polystyrene or epoxy calibration curves. The observation of SEC chromatographs proves the formation of diol-ISO-diol (where ISO = isophthalic acid) and MAA-diol-MAA structures (where MAA = maleic anhydride) in the beginning of polycondensation reactions. This gives information on the future structure of the UP network.

Gel Prediction of Unsaturated Polyester Resins from a Resin Molecular Aspect

The influence of chemical structure of unsaturated polyester (UP) prepoly mer on gelation of UP resins is studied by varying NPG content in the UP prepolymer backbone. Also, the effects of styrene concentration and solvent quality on gelation are evaluated. A corrected gelation model concerning free radical ionization reaction based on a microgel mechanism is proposed. The critical free radical concentration, Rc/f, needed to reach gelation state for each studied system is determined and is correlated with Flory interaction parameter, χ. A linear relationship of Rc/f and χ is obtained, which provides an opportunity to predict the gelation of UP resins from a resin molecular aspect.

Unsaturated polyester prepolymer from rosin

Unsaturated polyester prepolymer was synthesized from maleopimaric acid (MPA), diethylene glycol (DEG) and maleic anhydride (MA) in the molar proportions 1:4:3 respectively. MPA was prepared from natural rosin and MA. Kinetic studies of the reaction were performed, and the reactivities of MPA, MA and phthalic anhydride (PA) with respect to diethylene glycol were compared. Copolymerized with styrene, this prepolymer gave samples with higher Vicat distortion temperatures than usual commercial samples prepared from PA, MA and propylene glycol, while mechanical properties such as modulus and strength were similar.

Structure–property relationships for styrene crosslinked polyesters. I. Network structure and rubbery elastic modulus

AbstractTen unsaturated polyester prepolymers differing by the nature of the diol, by the maleate/phthalate molar ratio, or by the chain length, were crosslinked by about 40% by weight of styrene and then analyzed by IR spectrophotometry, solid state NMR, and rubbery elasticity measurements.The spectrometric measurements revealed a practically complete conversion of the styrene/fumarate copolymerization. The rubbery elastic modulus varies in a complex way with the presumed network structure.A non‐ideality factor f was defined from the current theories; f is a decreasing function of the average number of flexible bonds between crosslinks.The networks based on dimeric diols display a practically ideal behavior.The influence of dangling chains, corresponding essentially to polyester chain ends, is taken into account by the Graessley's formula in which trifunctional and tetrafunctional crosslinks are distinguished. A method for the estimation of the nature and weight fraction of dangling chains was proposed.