Control of shrinkage and residual styrene of unsaturated ...

1 Control of shrinkage and residual styrene of unsaturated polyester resinscured at low temperatures: I. Effect of curing agentsXia Cao, L. James Lee*Department of Chemical Engineering, The Ohio State University, Columbus, OH 43210, USAR eceived 11 September 2002; received in revised form 6 December 2002; accepted 9 December 2002 AbstractIn low temperature molding processes, Control of resin shrinkage and residual monomer is an important concern. The presence of lowprofile additives (LPAs) can reduce the shrinkage of unsaturated polyester (UP)/ styrene (St) resins under proper processing conditions butmay increase the residual styrene content. A systematic study was carried out to investigate the effect of the initiator system and reactiontemperature on sample morphology, final resin conversion, and resin shrinkage of UP resins with LPA. It was found that the final conversionof the resin system could be improved by using dual initiators. The effect is more obvious at low temperatures.

2 Volume shrinkagemeasurements of the resin system initiated with dual initiators revealed that good LPA performance was achieved at low ( 358C) and high( 1008C) temperatures but not at intermediate ones. This can be explained by how temperature affects phase separation, reaction kineticsin the LPA-rich and UP-rich phases, micro-void formation, and thermal Elsevier Science Ltd. All rights : unsaturated polyester resins; residual styrene ; Volume shrinkage1. IntroductionUnsaturated polyesters are widely used in the compositeindustry. They can provide excellent mechanical andchemical properties, good chemical and weather resistance,and a low cost. Further advantages of unsaturated polyesterresins over other thermosetting resins are that they are easyto handle, can be pigmented, and can be easily filled andfiber reinforced in a liquid form. The cross-linking reactionbetween unsaturated polyester resins and vinyl monomers, styrene , allows one polymer chain to connect with otherpolymer chains, and to produce a three dimensionalnetwork, which converts the resin system from a viscousliquid into a hard, thermoset recent years, low temperature and low pressuremolding processes like resin transfer molding (RTM) andvacuum assisted resin transfer molding (VARTM) aregaining increased attention because of their low cost andease of operation.

3 An accelerator or promoter such as cobaltoctoate or naphthenate has to be added to the peroxideinitiator in order to induce chemical decomposition at lowtemperatures. In addition, other curing agents, such asinhibitors, retarders, and co-promoters, are also needed toadjust the resin pot life, gel time, and cycle time[1]. In roomtemperature processes without any external heating source,polymer chains become more difficult to move after gelationand the reaction becomes diffusion controlled. This impedesunsaturated polyester resins from achieving high finalconversion and low styrene residue when cured at lowtemperatures. Low profile additives (LPAs) are usuallyadded into the unsaturated polyester resin system tocompensate polymerization shrinkage during the moldingprocess. Although LPAs can provide shrinkage Control inunsaturated polyester resin systems, the reaction exothermmay decrease because LPA is a non-reactive component inthe system.

4 Consequently, it may further increase release of residual styrene from molded compositeparts creates problems for the environment and is the sourceof odor in many applications. residual styrene may alsoresult in blisters and voids on the surface of molded productsat elevated temperatures ( in the painting line). There-fore, detection and Control of styrene content in the fiber-reinforced unsaturated polyester resin are very important,especially in low temperature processes. Many researchers0032-3861/03/$ - see front matterq2003 Elsevier Science Ltd. All rights (03)00014-4 Polymer 44 (2003) 1893 *Corresponding author. Tel.: 1-614-292-2408; fax: ( Lee).have studied the effects of resin system, curing agent andreaction conditions on residual styrene of polyesters[2 4]by using various analytical instruments to measure theresidual styrene content and to determine physical proper-ties ( heat distortion temperature, flexural strength,compressive yield strength, and tensile strength) of curedresins.

5 Available methods to determine residual styrenecontent have included chemical degradation[5,6], refractiveindices[7], gas chromatography[8], infrared spectroscopy[9,10], and nuclear magnetic resonance (NMR) spectro-scopy[11]; infrared spectroscopy has been the mostcommonly used among them. The researchers found thatless styrene residue could be attained by adding moreperoxide, choosing flexible resins, and increasing thereaction exotherm. Most studies emphasized on theinfluence of resin flexibility and initiator type onthe residual styrene content of molding panels withoutaddressing reaction mechanism. In this study, an in-depth kinetic analysis is carried out to better understandthe reaction kinetics of unsaturated polyester resins withLPA under different curing agents cured at lowtemperatures. The volume shrinkage of the resin systemis also MaterialsAn unsaturated polyester resin, Aropol Q6585, providedby Ashland Chemical was used in this study.

6 It is a step-growth product of 1:1 maleic anhydride and propyleneglycol with an average of vinylene groups per mol-ecule and an average molecular weight of 1580 g mole21,containing 35% by weight of styrene . The low profileadditive used was Neulon-T plus, a modified carboxylatedpoly(vinyl acetate) from Union Carbide (now DowChemical). All of the samples to be tested were formulatedto provide a monomer double bond to unsaturated polyesterdouble bond ratio of octoate (6% cobalt octoate in mineral spirits,Pfaltz & Bauer) was employed as the promoter todecompose the initiator at low temperatures. Theinhibitor, 300 ppm benzoquinone (BQ, Aldrich), wasused to Control the curing process. The initiators used inthis study included a single component initiator, methylethyl ketone peroxide (MEKP, Hi-point 90, Witco), anda dual initiator system, MEKP/tert-butyl peroxy-benzoate (TBPB, Trigonox C, Akzo Noble).

7 Trigonox Cis a solution of 98%tert-butyl peroxybenzoate activate oxygen, while Hi-point 90 contains 38%peroxide with activate oxygen. All materials wereused as received without further purification in order tomimic industrial Instrumentation and Differential scanning calorimeter (DSC)The overall reaction rate was measured by a differentialscanning calorimeter (DSC2910, TA Instruments). Thesample was sealed in a hermetic aluminum sample pan thatcan withstand 2 atm internal pressure after sealing. Aftermixing the reactants, about 10 mg sample was placed in aDSC pan. Isothermal runs were conducted at pre-specifiedtemperatures ( 35, 60, 75, 1008C) for long enough timeuntil no further reaction exotherm could be detected. Thesamples were then followed by scanning from 30 to 3008 Cto determine the residual exotherm with a heating rate of58 Cmin21. The total heat of reaction during curing wascalculated from the area under both isothermal and residualscanning DSC curves and the reaction rate or conversionwas based on the total heat calculated by this Fourier transform infrared (FTIR) spectroscopyBecause it is difficult to get information by means ofDSC measurements to differentiate overlapped multiplereactions, a computer-assisted Fourier Transform Infrared(FTIR) spectroscope (Nicolet, Magna 550II) with aresolution of 4 cm21in the transmission mode was usedin this study for kinetic measurements of individual reactionof both St and UP CyC bonds.

8 FTIR has the ability toaccurately monitor the complex reactions based on spectrachanges of different functional groups. After the reactantswere mixed, one drop of mixture was placed between twosodium chloride plates, which were then mounted on asample holder located in the FTIR chamber. A temperaturecontroller was designed to maintain the reaction tempera-ture. Four consecutive, 10-s scans were taken at eachsampling time. The sampling interval was 1 s to 5 minduring the reaction, depending on the reaction rate. Eachmeasurement ended at a preset time. All IR spectra in thisstudy are shown in absorbance absorption is based on the fact that eachchemical group in a sample absorbs infrared radiation ofsome characteristic frequencies. The amount of lightintensity of transmission relative to the amount of lightintensity incident on the sample can be related directly to theconcentration of the absorbing species by Beer s lawAi bilCi 1 whereAiis the absorbance of species which can bedetermined from the peak height or peak area,biis theabsorptivity which is characteristic of absorbing speciesi,lis the pathlength (sample thickness), andCiis theconcentration of absorbing species 1shows typical FTIR spectra of an unsaturatedpolyester resin during reaction.

9 Consumption of styreneCyC bonds is indicated by changes of peak area at 912 and992 cm21(CH2yCH deformation), while consumption ofunsaturated polyester C C bonds is indicated by a peakarea change at 982 cm21(trans CHyCH deformation)[7].X. Cao, Lee / Polymer 44 (2003) 1893 19021894To compensate for the changes of thickness and opacity inthe sample during curing of the UP resin, the C H peak at2942 cm21was chosen as the internal standard to normalizethe interested spectra applying Beer s law to the quantitative analysis,the calibration curves for styrene CyC bonds and unsatur-ated polyester CyC bonds were established by preparing aseries of styrene -dibromomethane solutions and unsaturatedpolyester-dibromomethane solutions of known concen-tration. During calibration, the solutions were placedbetween two sodium chloride plates with a 25mm thickTeflon spacer to keep all the samples in the same calibration curves based on the change of peak area forstyrene-912 cm21, styrene -992 cm21, and UP- 982 cm21are shown inFig.

10 2. A linear relationship between the peakarea and monomer concentration was obtained for all threepeaks. The absorptivity for each peak can be determinedfrom the slope of the calibration the reaction system of unsaturated polyester andstyrene, the styrene consumption during the reaction can bedetermined easily from the peak area change at 912 cm21based on Beer s law. The styrene conversion (aSt) can thenbe determined according to the following equation:aSt 12 At A0 912 2 where A0and Atare the normalized absorbance of thefunctional group before the reaction and at reaction timet,respectively. However, the consumption of polyester CyCbonds cannot be followed directly from peak 982 cm21because it overlaps with peak 992 cm21as shown inFig. subtraction method was used to separate the overlappingpeaks[7]and the unsaturated polyester conversion (aUP)can be calculated according to the following equation:aUP 121BI 12 At A0 982 992 1 BI 2 12aSt 3 whereB b982/b992andI CUP=CSt 0which is the initialconcentration ratio of UP (CUP) and styrene (CSt)CyCbond.