Control of shrinkage and final conversion of vinyl ester resins cured in low‐temperature molding processes

Vinyl ester resin is a major thermoset polymer used in low‐temperature composite manufacturing processes such as the Seemann composite resin infusion‐molding process (SCRIMP). Volume shrinkage and residual styrene are important concerns for composites produced in such processes. A low‐shrinkage additive (LSA) is a typical agent added to control the volume shrinkage of vinyl ester resins during molding. In this study, the effects of LSA content and the temperature profile (the temperature gradient and peak temperature) on the volume shrinkage control of a vinyl ester resin were investigated. The reaction kinetics of the resin system were also studied. We achieved good volume shrinkage control if we raised the curing temperature slowly to allow sufficient time for phase separation and if the curing temperature reached a high value after phase separation to allow microvoid formation. On the basis of experimental results, we designed an improved SCRIMP to increase resin conversion, reduce resin shrinkage, and produce composites with better properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1486–1496, 2003     

Comparison of unsaturated polyester and vinylester resins in low temperature polymerization

Many composite products are produced at low temperatures in processes such as resin transfer molding (RTM), vacuum infusion molding (e.g., Seemann Composite Resin Infusion Molding Process—SCRIMP), and hand lay‐up. These processes are widely used for marine, civil infrastructure, transportation and defense applications. Unsaturated polyester and vinylester resins are two major resins used in these processes due to their low cost, good performance, and processibility. In this study, the reaction kinetics and rheological changes of these two resins cured at low temperatures were studied. Effects of resin type, initiator, promoter, inhibitor and retarder on the reaction kinetics and rheological behaviors were examined using a Differential Scanning Calorimeter (DSC) and a Rheometrics Dynamic Analyzer (RDA). A model was developed to quantify the effects of resin type, temperature, and different curing agents on the gel time for both polyester and vinylester resins cured at low temperatures. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1230–1242, 2001     

Low temperature cure of vinyl ester resins

Vinyl ester resins are well known for their versatility as a composite matrix. With the development of a promising room temperature molding technology, vacuum-assisted resin transfer molding, e.g. Seemann Composite Resin Infusion Molding Process (SCRIMP), the processability of vinyl ester resins at low temperatures has attracted considerable attention from the composite industry. The objective of this paper is to provide a better understanding of the reaction kinetics of this resin system at low temperatures. In this study, a differential scanning calorimeter (DSC) coupled with a Fourier transform infrared (FTIR) spectrometer was employed to measure the reaction profile of a vinyl ester resin with different promoter and styrene contents. A kinetic model based on the free radical co-polymerization mechanism was developed for simulating the reaction rates and conversions of styrene vinyl and vinyl ester vinylene groups. The model parameters were determined from several FTIR experiments under isothermal conditions. This model, in conjunction with heat transfer analysis, was able to successfully predict the temperature profiles during curing in two SCRIMP molding cases based on groove type resin distribution system.     

Effects of a chelating agent - 2,4-pentanedione on low temperature composite molding of vinyl ester and unsaturated polyester resins

A major concern in low temperature composite manufacturing processes is how to design and control the mold filling and curing time. Inhibitors or retarders are often used to prevent premature gelation and provide a sufficiently long time to fill the mold completely. However, the addition of these chemical species tends to result in a low mold curing rate and a low final resin conversion. In this study, a chelating agent 2,4-pentanedione (2,4-P) was used to manipulate resin gelation and curing. This agent is known to affect the catalytic activity of the promoters (i.e. metal compounds such as cobalt carboxylates) in the decomposition of initiators. It can function as either a retarder or a co-promoter in the co-polymerization of styrene/polyester and styrene/vinyl ester resins depending on the acidity of the resin system. Based on this observation, an improved room temperature vacuum-assisted resin transfer molding process was designed. This design allows 2,4-P to serve first as a retarder during mold filling to achieve a long gel time, it then as a co-promoter during curing to increase the curing rate. The 2,4-P also increases the resin conversion as the acidity of the resin increases.     

Electrically conductive thermosetting resins containing low concentrations of carbon black

A cured vinyl ester resin containing electrically conductive carbon black (CB) particles shows electrical percolation at very low CB concentration (<0.5 phr). CB particles have a strong tendency to agglomerate in a low‐viscosity resin, such as vinyl ester, unsaturated polyester resin, and epoxy resins. The agglomeration process in the low‐viscosity vinyl ester resin generates electrically conductive paths already in the resin's liquid state, which undergo partial fixation by room temperature curing and full fixation by hot postcuring. The fully cured castings containing CB concentrations above percolation are characterized by a constant, temperature‐independent conductivity, over a wide temperature range. The current–voltage relationships of the cured vinyl ester/CB castings obey a power‐law dependency. The presence of the continuous CB paths in the vinyl ester casting is clearly observed in fracture surfaces formed at 100°C. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1165–1170, 2000     

Compressive Properties and Curing Behaviour of Unsaturated Polyester Resins in the Presence of Vinyl Ester Resins Derived from Recycled Poly(ethylene terephthalate)

Poly(ethylene terephthalate) waste was depolymerised in the presence of tetraethylene glycol and manganese acetate as a catalyst, so as to produce oligomers. An epoxy resin was then prepared by the reaction of these oligomers with epichlorohydrin in presence of NaOH as a catalyst. New diacrylate and dimethacrylate vinylester resins were then synthesized by reaction of the terminal epoxy groups with acrylic and methacrylic acid in the presence of triphenyl phosphite as a catalyst. The chemical structures of the resulting vinyl ester resins were confirmed by ¹HNMR. The vinyl ester resins were used as crosslinking agents for unsaturated polyester resin diluted with styrene, using free radical initiator and accelerator. The curing behaviour of the unsaturated polyester resin, vinyl ester resins and styrene was evaluated at temperatures from 25 to 55 ^○C. The compression properties of the cured resins, having different vinyl ester contents and different cure temperatures, were evaluated. Increasing the cure temperature and the vinyl ester content led to a pronounced improvement in the compression strength and Young’s modulus.     

Thermokinetics of unsaturated polyester and vinyl ester resins

An experimental study was carried out to investigate the isothermal and non-isothermal curing kinetics of unsaturated polyester and vinyl ester resins, using differential scanning calorimetry (DSC). Emphasis was put on investigating the effect of low-profile additives on the curing kinetics of the thermo-setting resins. For the study, a general-purpose polyester resin and a vinyl ester resin were used, together with polyvinyl acetate (PVAc) as low-profile additive, benzoyl peroxide as initiator, and N,N-dimethyl aniline as promoter. It has been found that (1) the addition of the low-profile thermoplastic-additive decreases the rate of cure and, also, the final degree of cure of the resins, (2) the total heat of cure generated by isothermal cure is lower than that generated by non-isothermal cure, and (3) the resin/initiator mixture with promoter exhibits two major exotherm peaks during non-isothermal cure, but only a single exotherm peak during isothermal cure.     

High‐performance thermosets with tailored properties derived from methacrylated eugenol and epoxy‐based vinyl ester

A renewable chemical, eugenol, is methacrylated to produce methacrylated eugenol (ME) employing the Steglich esterification reaction without any solvent. The resulting ME is used as a low‐viscosity co‐monomer to replace styrene in a commercial epoxy‐based vinyl ester resin (VE). The volatility and viscosity of ME and styrene are compared. The effect of ME loading and temperature on the viscosity of the VE–ME resin is investigated. Moreover, the thermomechanical properties, curing extent and thermal stability of the fully cured VE–ME thermosets are systematically examined. The results indicate that ME is a monomer with low volatility and low viscosity, and therefore the incorporation of ME monomer in VE resins allows significant reduction of viscosity. Moreover, the viscosity of the VE–ME resin can be tailored by adjusting the ME loadings and processing temperature to meet commercial liquid molding technology requirements. The glass transition temperatures of VE–ME thermosets range from 139 to 199 °C. In addition, more than 95% of the monomer is incorporated and fixed in the crosslinked network structure of VE–ME thermosets. Overall, the developed ME monomer exhibits promising potential for replacing styrene as an effective low‐viscosity co‐monomer. The VE–ME resins show great advantages for use in polymer matrices for high‐performance fiber‐reinforced composites. This work is of great significance to the vinyl ester industry by providing detailed experimental support. © 2018 Society of Chemical Industry     

Vinyl ester and unsaturated polyester resins in contact with different chemicals: Dynamic mechanical behavior

A prior study has shown for a vinyl ester resin that the adequate choice of the curing conditions becomes completely necessary to reach the maximum glass transition temperature. In the same study, the variations produced in the dynamic mechanical properties because of exposure to different solvents were related to the chemical structure of both the resin and solvent. This investigation was undertaken in an attempt to analyze the effects of solvent exposure on the viscoelastic properties of resins currently used for applications in which the resin is kept in contact with the solvents. Several vinyl ester resins as well as various unsaturated polyester resins immersed in different liquids were investigated. The influence of exposure time to the solvent as well as that for the temperature were characterized using dynamic mechanical analysis. The chemical structure of the resin was found to be determinant in the changes produced after solvent exposure. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 21–28, 1999     

Silicone‐Organic Resin Hybrid Matrix Composites

Summary: Glass fabric reinforced hybrid matrix composites of a toughened silicone resin and a vinyl ester resin were fabricated and their properties investigated. The hybrid composites consisted of multi‐layers of fiber reinforced silicone resins and vinyl ester resins. The toughened silicone resin, a crosslinkable phenylsilsesquioxane resin with high thermal and thermal oxidation resistance but relatively low T_g, was chosen to be the outer layers. The vinyl ester resin, with better strength, toughness and a much higher T_g than the toughened silicone resin, was used as the inner layers. A co‐cure process proved to establish a strong interface between the two in a hybrid composite. The hybrid composites had better flammability properties and much lower short term moisture absorption than the vinyl ester composites. The strength and modulus retention of the hybrid composites at elevated temperatures was higher than the composites using any single resin as the matrix. For example, when tested at 150 °C the flexural modulus and strength values of a twelve layer composite, with eight inner vinyl ester resin layers and four silicone outer layers, were almost an order of magnitude higher than the composite using the silicone resin alone, and were significantly higher than the one using vinyl ester resin alone. The room temperature short beam shear strength of the hybrid composites was also higher. DMA revealed that the inter‐diffusion of reactive components between the two resins was probably responsible for this synergistic effect, resulting in an α transition temperature of 182 °C for the hybrid composite, higher than that of either the silicone resin (85 °C) or the vinyl ester resin (162 °C).

     

聚三唑酯树脂的合成及其性能

通过酯化反应合成丁二酸二炔丙醇酯(DPS)、间苯二甲酸二炔丙醇酯(DPIP)、对苯二甲酸二炔丙醇酯(DPP),与三官能团叠氮化合物(TAMTMB)反应,制备了3种热固性聚三唑酯(PTAE)树脂,研究了树脂的加工特性、固化行为、树脂固化物的力学性能,制备和表征了T700单向碳纤维增强PTAE树脂复合材料。结果表明,PTAE树脂具有良好的加工性能,可在较低温度(80℃)下固化;固化后的PTAE树脂的玻璃化转变温度(Tg)受主链结构影响,3种树脂的Tg均高于140℃,浇铸体弯曲强度高于170 MPa,T700单向纤维增强PTAE树脂复合材料的常温弯曲强度高于1500 MPa。     

Chemorheology of thermosetting resins. IV. The chemorheology and curing kinetics of vinyl ester resin

The rheological properties and curing kinetics of a vinyl ester resin have been determined during isothermal cure. Both steady and oscillatory shearing flow properties were determined using a cone-and-plate rheometer, and the curing kinetics were determined using a differential scanning calorimeter (DSC). Also determined were the rheological properties and curing kinetics of the resin when it had been thickened using magnesium oxide (MgO), in the presence of calcium carbonate (CaCO₃) as filler and polyvinyl acetate (PVAc) as low-profile additive. The steady shearing flow behavior observed with the vinyl ester resin was found to be very similar to that observed with a general-purpose polyester resin, reported in Paper I of this series [C. D. Han and K. W. Lem, J. Appl. Polym. Sci., 28 , 3155 (1983)]. However, a significant difference in the oscillatory shearing flow behavior was found between the two resins. We have concluded that dynamic measurement is much more sensitive to variations in resin chemistry than steady shearing flow measurement. DSC measurement has permitted us to determine the degree of cure as a function of cure time. By combining the rheological and DSC measurements, we have constructed plots describing how the viscosity increases with the degree of cure, at various isothermal curing temperatures.     

Curable resins based on recycled poly(ethylene terephthalate) for coating applications

Poly(ethylene terephthalate) waste was depolymerised in the presence of diethylene- or tetraethylene glycol and manganese acetate as a catalyst. An epoxy resin was then prepared by the reaction of these oligomers with epichlorohydrin in presence of NaOH as a catalyst. The produced oligomers were condensed with maleic anhydride and ethylene glycol to produce unsaturated polyester. The chemical structures of the resulting epoxy and unsaturated polyester resins were confirmed by ¹HNMR. The vinyl ester resins were used as cross-linking agents for unsaturated polyester resin diluted with styrene, using free radical initiator and accelerator. The 2-amino ethyl piprazine was used as hardener for epoxy resins. The curing behaviour of the unsaturated polyester resin, vinyl ester resins and styrene was evaluated at different temperatures ranged from 25 to 55 °C to calculate the curing activation energy of the system. The cured epoxy and unsaturated polyester resins were evaluated in coating application of steel.     

An investigation of vinyl–ester?styrene bulk copolymerization cure kinetics using Fourier transform infrared spectroscopy

A Fourier transform infrared (FTIR) spectroscopy technique was developed to investigate the effects of reaction temperature and reactant composition on the isothermal curing kinetics of commercial vinyl nester resins comprised of vinyl–ester monomer (dimethacrylate of diglycidyl ether of bisphenol A DGEBA) and styrene. This technique enables a more complete evaluation of the bulk copolymerization reaction of vinyl–ester styrene systems by monitoring the depletion of vinyl–ester and styrene double bonds independently. The results indicate that the rate of fractional conversion of styrene double bonds is initially less than that of vinyl–ester vinyl groups. However, styrene monomer continues to react after conversion of vinyl–ester double bonds has ceased. In addition, the overall extent of conversion was found to increase with increasing isothermal cure temperature, and it was observed that higher styrene concentration enhances final conversion of vinyl–ester double bonds and not styrene double bonds. Increasing styrene monomer concentration also resulted in lowering the apparent activation energy for the reaction of vinyl groups from both monomers as characterized by an empirical autocatalytic model used to fit the conversion results for styrene and vinyl–ester double bonds independently. The results of this work demonstrate that reaction temperature and resin composition significantly affect the cure behavior of vinyl–ester resins and provide insight into the development of the resulting network structure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1572–1582, 2000     

Glass‐transition temperature based on dynamic mechanical thermal analysis techniques as an indicator of the adhesive performance of vinyl ester resin

Vinyl ester resins are being used extensively as matrices in fiber‐reinforced polymer composite materials, but their use as a structural adhesive has been limited. Initial studies investigating the durability of a vinyl ester as a wood adhesive showed unsatisfactory performance in comparison with other adhesives. In this work, the glass‐transition temperatures (T_g's) of a vinyl ester and a E‐glass/vinyl ester composite material, fabricated by the Composites Pressure Resin Infusion System, were determined with dynamic mechanical thermal analysis. The results indicated that the resin cured under ambient conditions had a much lower T_g (～60°C) than the postcured material (～107°C). This suggested undercuring, that is, incomplete crosslinking, of the resin when it was cured at room temperature. E‐glass/vinyl ester samples, however, showed virtually no difference in T_g between room‐temperature‐cured and postcured samples. The exact reasons for this are not currently known but are thought to be both mechanical and chemical in nature. On the basis of the findings presented in this article, it can be concluded that if this vinyl ester resin is to be used as a structural adhesive, postcuring or formulation to ensure a high degree of crosslinking under ambient conditions is necessary. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2221–2229, 2005     

Spatially homogeneous gelation in liquid composite molding

On‐line mixing of the resin with its curing agents prior to injection into a mold is a common industrial technique for fabricating composite parts. For vinyl‐ester resins that cure via free radical polymerization, the concentrations of retarder, accelerator, and initiator are pre‐selected and cannot be changed during the injection. Hence, the resin that enters the mold the earliest has cured longer than the resin that enters the mold later, since the gel time for the resin is the same, owing to the fixed ratio of the curing agents. This approach leads to inhomogeneous cure of the resin and consequently to longer residence time of the resin in the mold. It requires an additional 50 to 75 percent of the filling time before a part can be de‐molded. In this study, it is shown that by adjusting the concentration of curing agents during the injection, a more homogeneous gel time throughout the mold can be achieved. The time to de‐mold is reduced to 18‐24 percent of the filling time. Sensors that measure the conductivity of the resin were used to detect the location and monitor the cure of vinyl‐ester. This approach could be extended to other resin systems to control the spatial curing of the resin in the mold.     

Effect of styrene on properties of vinyl ester resins,I

The paper describes the effect of styrene on the properties of bis(methacryloxy) derivatives of epoxy resins (vinyl ester resins). The styrene concentration in the resin was systematically varied between 20–60 wt.-%. The curing characteristics of vinyl ester resins changed only marginally on dilution with styrene. Dilution with styrene resulted in a decrease in tensile modulus and increase in elongation of neat resin castings and glass fibre reinforced laminates. The glass transition temperature of laminates was determined by dynamic mechanical analysis and was found to decrease with increasing styrene content.     

酚醛环氧乙烯基酯树脂碳纤维复合材料的制备及性能研究

把酚醛环氧乙烯基酯树脂分别与固化剂过氧化氢异丙苯(CHP)、过氧化苯甲酸叔丁酯(TBPB)、过氧化二异丙苯(DCP)进行混溶,模压成型,碳纤维层合板,进行了力学性能和热稳定性的研究。结果表明,研究的碳纤维-酚醛环氧基乙烯基酯树脂复合材料具有非常好的力学性能及热稳定性,弯曲强度达到1 400～1 900 MPa。     

Use of allyl‐functional benzoxazine monomers as replacement for styrene in vinyl ester resins

New vinyl ester systems are prepared using allyl‐functional benzoxazine monomers, 3‐allyl‐6‐methyl‐3,4‐dihydro‐2H‐benzo[e][1,3]oxazine (pC‐ala) or bis(3‐allyl‐3,4‐dihydro‐2H‐benzo[e][1,3]oxazin‐6‐yl)methane (BF‐ala), as reactive diluents for vinyl ester resins derived from an epoxy resin, diglycidyl ether of bisphenol A, instead of using styrene. Different initiators are used to investigate the copolymerization of allyl function from pC‐ala with vinyl function from vinyl ester resin prepolymer. The temperature dependence of viscosity is studied to demonstrate the retention of processability of the new vinyl ester resins. Dynamic mechanical and thermogravimetric analyses are used to investigate the dynamic mechanical properties and thermal stability of the new resins. Copyright © 2012 Society of Chemical Industry     

Curing kinetics and mechanism of novel high performance hyperbranched polysiloxane/bismaleimide/cyanate ester resins for resin transfer molding

Curing kinetics and mechanism determine the structure and property of thermosetting resins and related composites. The curing kinetics and mechanism of a novel high performance resin system based on hyperbranched polysiloxane (HBPSi), 2,2′‐diallylbisphenol A modified bismaleimide (BD), and cyanate ester (CE) resins for Resin Transfer Molding (RTM) technique were systemically studied by Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FTIR) spectra, and torque rheometer. Results show that the addition of HBPSi to BD/CE resin not only decreases the initial curing temperature and apparent activation energy, but also changes the curing mechanism, and thus the structure and properties of resultant crosslinked networks. An “Interpenetrating network (IPN)‐coupling structure” is proposed to be formed in the HBPSi/BD/CE system, which is different from traditional “IPN” structure in BD/CE resin. The simulation of curing reaction suggests that the variety of the curing activity leads to the difference between the curing behaviors of BD/CE and HBPSi/BD/CE resins, which is in good agreement with FTIR and DSC analyses. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011