Journal of Polymer Research (2005) 12: 373–383 © Springer 2005DOI: 10.1007/s10965-005-1638-3

Compressive Properties and Curing Behaviour of Unsaturated Polyester Resinsin the Presence of Vinyl Ester Resins Derived from Recycled Poly(ethyleneterephthalate)

Ayman M. Atta1,∗, Sayed I. Elnagdy2, Manar E. Abdel-Raouf1, Shimaa M. Elsaeed1 and Abdel-AzimA. Abdel-Azim1

1Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt2Chemistry Department, Faculty of Science, Ain Shams University, Abasia, Cairo, Egypt(∗Author for correspondence; E-mail: khaled_00atta@yahoo.com)

Received 3 November 2004; accepted in revised form 1 February 2005

Key words: curing behaviours, epoxy resins, mechanical characteristics, poly(ethylene terephthalate), recycling, unsaturatedpolyesters

Abstract

Poly(ethylene terephthalate) waste was depolymerised in the presence of tetraethylene glycol and manganese acetate as acatalyst, so as to produce oligomers. An epoxy resin was then prepared by the reaction of these oligomers with epichloro-hydrin in presence of NaOH as a catalyst. New diacrylate and dimethacrylate vinylester resins were then synthesized byreaction 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 ascrosslinking 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 temperaturesfrom 25 to 55 ◦C. The compression properties of the cured resins, having different vinyl ester contents and different curetemperatures, were evaluated. Increasing the cure temperature and the vinyl ester content led to a pronounced improvementin the compression strength and Young’s modulus.

Introduction

Unsaturated polyesters are one of the most important matrixresins for organic matrix fiber composites. They are used inmarine, automobile, construction, sport and furniture appli-cations. In the development of polymer composites, muchattention has been paid to improving their mechanical prop-erties. This can be done by chemical or physical methods [1–3]. Chemical modification can be achieved by incorporatingof different diols (e.g., diethylene glycol, propylene glycol,triethylene glycol) or saturated acids (e.g., adipic acid) intothe polyester chain during the polycondensation reaction.One disadvantage of this method is the fact that a largequantity of these compounds must be used in order to have asufficient effect on the mechanical properties.

Composites based on slightly different resins, i.e. vinylester resins, are also finding new uses in military and com-mercial applications. The selection of cure conditions isimportant for an understanding of the factors that affect thecure processes. Moreover, recent studies have shown thatthe cure behaviour is affected by the presence of the fibrousreinforcement [4].

Recent investigations [5–8] have shown that the produc-tion of curable resins from recycled poly(ethylene tereph-thalate), (PET), is possible. PET waste is typically found in

scrap textiles and in beverage bottles that are collected afteruse in many locations and recycled for use in applicationsnot requiring contact with food. If resins such as vinyl esterresins could be made from waste PET, they could be usedto produce good-quality polymeric adhesives, coatings andpolymer concrete at a competitive cost. The recycling ofPET to form curable resins would also help to alleviate anenvironmental problem.

There are many ways of using PET waste, [9, 10]of which chemical recycling (e.g., hydrolysis, acidolysis,aminolysis and ammonolysis) is one of the more popu-lar. The first aim of the present study was to preparevinyl ester resins from depolymerised oligomers of PET us-ing tetraethyleneglycol (TEG), and to analyse the resultingmolecular structures.

As described below, the preparation was carried outby first reacting the depolymerised PET oligomers withepichlorohydrin to produce an epoxy resin. Chain termi-nated acrylic or methacrylic ester resins were prepared byreacting the epoxy resins, containing terminal epoxy groups,with acrylic or methacrylic acid in the presence of triphenylphosphite as a catalyst.

Secondly, we shall describe the modification of an un-saturated polyester resin by the prepared vinyl ester resinsin order to influence their curing behaviour and mechanical

374 Ayman M. Atta et al.

properties. The incorporation of a vinyl ester resin into theunsaturated polyester should improve the flexibility of thecured material. In this respect TEG was selected to mod-ify the chemical structure of the vinyl ester resin, so as toprepare a more flexible resin.

Finally, blending a vinyl ester with an unsaturated poly-ester resin could lower the cure time, which would be anadvantage in polymer concrete and coating applications.

Experimental

Materials

Commercial unsaturated polyester resin (UP) Distitron459 LV3 from Lonza (Italy) was used. The resin wasorthophthalic and supplied already dissolved in styrene.

PET waste was collected from beverage bottles. Theintrinsic viscosity (limiting viscosity) of PET in m-chlorophenol at 25 ◦C was found to be 0.015 dm3 Kg−1.The weight average molecular weight was determinedby gel permeation chromatography (GPC) to be close to63,200 g mol−1. The chemicals were supplied by the Aldrichchemical company.

Tetraethyleneglycol (TEG) was used as supplied.Epichlorohydrin (EC) was used after distillation. Manganeseacetate was used as the transesterification catalyst for the de-polymerization of PET. Acrylic acid (AA) and methacrylicacid (MA) were used without further purification.

Methyl ethyl ketone peroxide (MEKP) and cobalt octoate(CO) were used as initiator and accelerator, respectively.MEKP was supplied as a 50% paste in dimethyl phthalate.The CO was a 10% solution in styrene. High purity acetone,toluene and pyridine were used as received.

Techniques

Depolymerization of PETThe PET waste (beverage bottles) was reacted with TEG(1 : 1 wt%) in the presence of 0.5% (by weight) of man-ganese acetate based on the weight of PET. The reaction wascarried out in a nitrogen atmosphere at 200 ◦C for 4 h, andthen at 210–230 ◦C for 3 h. After completion of the glycol-ysis, the product was cooled to room temperature, dissolvedin a suitable quantity of methylene chloride and shakenvigorously with an equal volume of 5% aqueous sodium hy-droxide solution to remove any unreacted glycol and residualcatalyst. The organic layer was collected and washed severaltimes with water. Finally, purified oligomers were obtained,after removal of the solvent by rotary evaporation.

The purified oligomer with residual TEG was analysedto determine the hydroxyl value by the conventional aceticanhydride/pyridine method [11]. The purified oligomer ofglycolised PET waste with TEG was abbreviated here as GT.

Synthesis of Glycidyl Ether of Recycled PETGT (0.11 mol) was mixed with 1% (based on weight of GT)of BF3Et2O and heated at 40 ◦C. EC (1 mol) was gradually

added to the reaction mixture over a period of 2 h. The reac-tion mixture was heated at 65 ◦C for 30 min and after coolingto 40 ◦C, 0.22 mol of NaOH (as a 50% aqueous solution) wasadded to the reaction mixture. The temperature of the reac-tion mixture was raised to 80 ◦C for 30 min. The mixture wasthen cooled and neutralized with 50 ml of acetic acid (50%),after which 150 ml of toluene was added and the reactionmixture was filtered to remove sodium salts. The filtrate waswashed four times with 150 ml of hot (60–80 ◦C) water andthe toluene was evaporated off from the organic layer usinga rotary evaporator.

The glycidyl ether of the glycolysed PET (GTE) wasobtained as a viscous liquid and the epoxy content wasdetermined by a previously reported method [12].

Synthesis of Acrylate and Methacrylate Esters of GTEGTE (0.11 mol) was mixed with 0.2 mol of acrylic acid,AA or methacrylic acid, MA, 1% of triphenyl phosphite,and 0.05% of hydroquinone and then diluted with 5% oftoluene. The reaction temperature was raised to 95 ◦C andthe reaction was continued until 0.2 mol of water had beenremoved using a Dean and Stark separator. The synthesizedacrylate and methacrylate oligomers of GTE were purifiedby extraction with ethanol, distilling off the solvent underreduced pressure. The viscous products were washed withsodium bicarbonate, in aqueous Na2CO3 solution to removeunreacted acid, continuing until the washings became neu-tral. They were then dried over anhydrous sodium sulphate,filtered, and vacuum distilled. They had a yellow oily liquidappearance.

The diacrylate and dimethacrylate oligomers of GTEwere designated here as GTA and GTM, respectively. Thechemical structures of GTA and GTM were analysed by1HNMR.

Physical Measurements

The prepared compounds were dissolved in CDCl3 andanalysed using a Jeol 1HNMR spectrometer model JNM-EX(270 MHz) to determine their chemical structures.

Their molecular weights were measured using a GPC(Water Model 600 E).

The curing exotherms of the unsaturated polyester UPwere measured in the presence of GTA and GTM, usingMEKP as initiator and CO as accelerator, as described inprevious work [7]. The UP was dissolved in 40% (w/w) ofstyrene monomer and mixed with various weight percent-ages of GTA and GTM (0–20%). The curing exotherms weredetermined at temperatures ranging from 25–55 ◦C. Theconcentrations of MEKP and CO were 2% and 0.2% (w/w)with respect to the total weight of the cured samples [7].

The gel times of the cured unsaturated polyester in thepresence of styrene and vinyl ester resins can be also de-termined by measuring the change in the viscosity of themixture as a function of time at constant temperature andshear rate. The viscosity measurements were determined byusing a Brookfield Model Dv-111 programmable rheometer.The critical value for the viscosity (ηc) and time (tc) corre-sponding to the gel point were determined from the relation

Properties and Behaviour of Unsaturated Polyester Resins 375

between the initial viscosity, the viscosity of the UP and thevinyl ester resin mixtures, and the curing time. This proce-dure was repeated at temperature intervals between 25 and55 ◦C.

Mechanical Testing

The Young’s modulus in compression (EY) and compressivestrength (σMax) of UP with GTA and GTM, were measuredusing a Shimadzu Universal Testing Machine UH-A, Se-ries Model 2000 KN Computer-Controlled Hydraulic ServoSystem.

The mechanical properties were measured according toASTM D 695-44T and were calculated as the average valuefrom five measurements

Results and Discussion

Commercial vinyl ester resins are a mixture of styrene with amethacrylated epoxy compound based on bisphenol-A. Us-ing the recycled PET offers the possibility of a lower costsource for the latter component. The present study was in-tended to prepare vinyl ester resin from recycled PET waste.The first part of the work consisted of studying the depoly-merisation of PET using tetraethyleneglycol (TEG) and thereaction of its products with EC to produce epoxy resin. Wedecided to model the process and characterize the reactionproducts for each step of the process.

The second part of the synthesis was based on the re-action between the epoxy resin and acrylic or methacrylicacid in presence of triphenyl phosphite to produce vinylester resins having acrylate or methacrylate end groups.The reaction scheme is illustrated in Figure 1. Tong etal. [2] found that unsaturated polyesters based on bis(2-hydroxyethyl)terephthalate, maleic anhydride and ethyleneglycol were not compatible with styrene monomer. Theseauthors found it necessary to replace part of the ethyleneglycol by another type of glycol to enhance the com-patibility of the products with styrene monomer. Accord-ingly, in the present study, TEG was incorporated in thevinyl ester so as to increase the miscibility of the synthe-sized resins with styrene monomers. Schulze et al. [13]have reported the modification of unsaturated polyestersby poly(ethylene glycol) end groups in order to influencethe solution behaviour in styrene and to modify the me-chanical properties of the cured resin. The synthesis of theblock copolymer was carried out by reacting a carboxyl-terminated unsaturated polyester with various poly(ethyleneglycol) monomethyl ethers of molecular weights from 350to 2000 g/mol. The block copolymers could be easily di-luted in styrene to create curable resins. The conversion ofthe typical polar end groups to poly(ethylene glycol) endgroups should improve the flexibility of the cured mater-ial. The intermolecular chain interactions will also changeconsiderably.

Instead of hydrogen bonds, which are responsible for ag-gregation and the high viscosity of the resin in styrene, van

der Waals interactions are dominant. Accordingly we pre-sumed that the incorporation of TEG into the structure of thevinyl esters enhances their solubility in styrene monomer.

Synthesis of Vinyl Ester Resins from Recycled PET

PET was depolymerised with an excess of TEG, and man-ganese acetate (CH3COO)2Mn as a catalyst. The free glycolof the produced oligomers and its hydroxyl value, as de-scribed in the experimental section, were 16% and 340 mgKOH/g of polymer. The hydroxyl number after removing thefree glycol indicates that the extent of depolymerization wasconsiderable and that the resulting oligomers were mostlyterminated with hydroxyl groups.

The theoretical molar mass of the glycolysed PET prod-ucts could easily be calculated from the percentage of re-acted TEG. In fact, if one macromolecule of PET (Mw =63200) reacted with n molecules of TEG its molecularweight would become 63200/(1 + n). The number of re-acted molecules of TEG (n) was calculated from the numberof moles of the reacted TEG divided by the number of molesof PET. Thus, for the oligomer samples reacted for 8 h, 100 gof PET (= 1.58 mmol) would have reacted with 8 g of TEG(= 82.47 mmol) and the molecular weight should have beenlowered to (63200/(1 + 82.47/1.58) = 1188 g mol−1. Themolecular weight of the glycolised PET with TEG was de-termined by GPC, as described in the experimental section,and equaled 965 g mol−1. The product was characterizedby 1HNMR after purification, as indicated in the experi-mental section. The 1HNMR spectrum of glycolised PETwith TEG (not represented, for brevity) showed a singletpeak at 8.23 ppm, a peak at 3.87 ppm and a triplet peakat 4.45 ppm which represented (a) a p-substituted phenylgroup, (b) (–CH2CH2O–), and (c) –COOCH2– of the de-polymerised PET oligomer, respectively. The peak observedat 2.80 ppm in the spectrum of glycolised PET indicated thepresence of terminated OH groups in the chemical structureof the depolymerised PET oligomer.

The production of epoxy resins from recycled PET wasstudied in a previous investigation [5]. The formation ofepoxy groups by reaction of the glycolised PET with ECwas confirmed from the epoxy content and the 1HNMRanalysis. The spectrum is represented in Figure 2(a). Thedisappearance of the OH peak and the appearance of threenew peaks at 2.82 ppm (–CH2– epoxy), 2.94 ppm (–O–CH2epoxy), and 3.43 ppm (–CH epoxy) indicate the formationof a glycidyl ether group.

The epoxy equivalent weight (EEW) of liquid epoxyresins is defined as the weight in grams which containsone gram equivalent of epoxide. The epoxy content (EC)is expressed as equv/100 g, which is the reciprocal of EEWmultiplied by 100. A common chemical method of analysisfor EEW and EC is titration of the epoxide ring by hy-drogen chloride in acetic acid [12]. The epoxy equivalentweight of the epoxy resin was 2360 g/equiv, which indicatesthat the epoxy content was 2. This in turn indicates thata low molecular weight difunctional epoxy resin had beenproduced [5].

376 Ayman M. Atta et al.

Figure 1. Scheme of synthesis of GTA and GTM resins.

Vinyl ester resins are produced by the reaction betweenepoxy resins and acrylic or methacrylic acids, as illustratedin Figure 1. The chemical structure of the recycled vinylester resins GTA and GTM was confirmed by 1HNMR. The1HNMR spectra of vinyl ester resins based on acrylic (GTA)and methacrylic groups (GTM) were represented in Fig-ures 2(b) and 2(c), respectively. The appearance of strong

peaks at 1.91 ppm, 5.50 ppm and 6.10 ppm, representingCH3, Ha and Hb of the methacrylate group, indicates theformation of a dimethacrylate vinyl ester resin from recycledPET. The appearance of peaks at 5.70, 5.99 and 6.10 ppm,representing Ha, Hb and Hc of the acrylate group, indi-cates the formation of an acrylate vinyl ester resin fromrecycled PET.

Properties and Behaviour of Unsaturated Polyester Resins 377

(a)

(b)

(c)

(d)Figure 2.

378 Ayman M. Atta et al.

Curing Exotherms of UP and Vinyl Ester Resins VE

The curing exotherms of the mixtures based on commer-cial UP and vinyl ester resins (VE) diluted with styrenewere obtained by plotting the curing temperature as a func-tion of time. Since the amount of heat evolved upon curingdepends on the sample size and shape, it was desirableto consider this parameter. It was very important to useglass bottles of the same volume in all measurements inorder to achieve repeatability of the measurements and af-ford legitimate comparison between temperature/time plotsduring the curing process. The plots for the synthesized VE(GTA and GTM) resins, commercial UP dissolved in styrenemonomer (60% both resins/40% styrene) cured by 2 wt%initiator in presence of 0.2 wt% accelerator are shown inFigures 3 and 4.

The formulae were based on UP and styrene in the pres-ence of 0%, 5%, 10%, 15% and 20% VE (based on the totalweight of UP). The cure of this resin proceeds via free rad-ical bulk copolymerization. Previous studies of the curingbehaviour of vinyl–divinyl systems show that the initial cureof such a system begins with an induction period, due tothe presence of inhibitors and cage effects, followed by theformation of microgels [14, 15].

Microgels have been defined as domains of highcrosslink density dispersed in a pool of unreacted monomers.The crosslinks of such microgels do not contribute to theglobal network structure until they are incorporated into thegel phase. It has been reported that the formation of microgelis followed by their growth, coagulation into clusters, andlinking of the clusters to form the final morphology of thecured resin. Little information is available, pertaining to theeffect of VE on the curing rate, exotherm peak temperatureor mechanical properties of the cured resins. For this rea-son, the succeeding sections endeavour to add more detailedinformation concerning these subjects.

It is also important to know the exact chemical structureand molecular weight of the commercial UP. The averagemolecular weight of UP and the styrene content of com-mercial UP were determined using 1HNMR spectroscopy.Figure 2(d) shows the 1HNMR spectrum of the commercialUP resin. The peaks were assigned to their respective chem-ical moieties, as shown in Figure 2. The molecular weightof UP was determined from the integration ratio of the vinylgroup in the maleic peak area to the peak area correspond-ing to the OH– end groups. This ratio refers to the numberof repeating units (m) or the degree of condensation poly-merisation between phthalic anhydride, maleic anhyride andglycol [16]. The styrene content of UP was calculated fromthe ratio of styrene vinyl groups to that of –HC=CH– inthe UP. Using this approach, the number average molecularweight of UP, Mn, was determined to be 6720 g/mol, andthe resin was found to contain 45% styrene by weight. Thismolecular weight of UP corresponds to a value of m. Thisprocedure agrees with data obtained by Ziaee et al. [16].

The following section concerns the possibility of mod-ifying the curing behaviour of UP resins by changing theamounts and types of the prepared VE (GTA and GTM), atconstant concentrations of MEKP and CO, i.e. 2 wt% and

0.2 wt%, respectively. The cure of thermosetting resins isusually characterized bytwo key processes, gelation and vit-rification; gelation correseponds to the incipient formationof an infinite network of crosslinked polymer molecules, andvitrification involves a transformation from a liquid or rub-bery state to a glassy state as a result ofincreased networkdensity. The effect of temperature on the curing exothermsis also discussed.

The curing exotherms were determined at 25 ◦C, 35 ◦C,45 ◦C and 55 ◦C. The gelation curve, Figure 3, dividesthe time-temperature plot into four distinct states of thethermosetting-cure process: liquid, gelled rubber, ungelledglass and gelled stages. As curing proceeds, the viscos-ity of the system increases as a result of the increasingcrosslinking density of the resins, and the reaction becomesdiffusion-controlled and eventually is quenched as the mate-rial vitrifies [17]. After quenching, the yield can be increasedby raising the temperature. In the present system CO andMEKP react with each other to produce radicals. Theseradicals initiate the exothermic process with UP and thetemperature rises because the number of radicals is suffi-cient to make the rate of heat generation larger than theheat dissipation. Consequently, heat accumulation increasesthe reaction rate and the polymer molecular weight risessteadily throughout the reaction (the mixture remains in theliquid phase at this stage). The viscosity of the reaction in-creases due to the increasing polymer molecular weight andthe length of the crosslinks (gelled rubber phase). This in-crease in viscosity causes a decrease in the rate of curing, asdescribed in the above kinetic investigation, because of theretardation of diffusion. Not only the increasing viscosity ofthe resin reduces the mobility of linear units that have not yetreacted but also and primarily, by the incorporation of thoseunits into the three-dimensional network. Thus the diffusionof the linear units towards the increasing of their molecu-lar weights is inhibited to a greater extent than its diffusiontowards the crosslinking with network structure (ungelledglass). Hence the reaction temperature continued to increaseat a slow rate. At Tmax the rate of heat dissipation started tobecome larger than the rate of heat generation, and the tem-perature decreased. As the crosslinking was completed, thetemperature decreased rapidly due to the normal cooling ofthe hot body in the absence of further heat generation (gelledglass state).

The curing time should be inversely related to the re-action rate, because the gel point corresponds to the firstformation of an infinite network, and this usually occurs ata fixed conversion for the particular system. The maximumtemperature produced, Tmax, upon curing and the time re-quired to reach the maximium temperature or curing time,tmax, obtained from these plots are shown in Table 1. Tostudy the influence of VE content on the curing time andreaction rate of the styrene, UP and VE system, four differ-ent resin solutions for each VE were prepared. The weightpercentage of VE resin in these solutions ranged from 5to 20% based on UP. The data listed in Tables 1 and 2 in-dicate that, in all cases, the curing time reduced slightly withincreasing VE content. This can be attributed to increasing

Properties and Behaviour of Unsaturated Polyester Resins 379

(a)

(b)

(c)

(d)Figure 3. Curing exotherms of commercial UP with different concentrations of GTA at temperatures (a) 25 ◦C, (b) 35 ◦C, (c) 45 ◦C and (d) 55 ◦C.

numbers of double bonds and hence this facilitates the dif-fusion of styrene molecules towards the unreacted doublebonds embedded in the three dimensional network. On theother hand the type of VE affected the reaction and curingrate. This was indicated by the data listed in Tables 1 and 2,

which suggested that VE based on acrylate groups havelower tmax values than those containing methacrylate. Thisreflects the ability of styrene to react with divinyl monomersbased on acrylates more easily than with those based onmethacrylates. This will increase the reaction viscosity at

380 Ayman M. Atta et al.

(a)

(b)

(c)

(d)Figure 4. Curing exotherms of commercial UP with different concentrations of GTM at temperatures (a) 25 ◦C, (b) 35 ◦C, (c) 45 ◦C and (d) 55 ◦C.

the beginning of the reaction and lead to a decrease in therate of diffusion of styrene towards UP, and will thereforeincrease the curing reaction time. The cure of thermosettingresins is complicated by the vitrification, and the kinetics areaffected by the local viscosity, which in turn is a function ofthe extent of reaction and temperature. Thus the cessation

of reaction is not necessarily an indication that the reactionis complete, i.e., the reaction may have been quenched dueto vitrification. Subsequent exposure to temperatures greaterthan the temperature of cure could result in further reaction.

The cross-linking reaction was characterized by changesin viscosity as a function of time at a constant temperature

Properties and Behaviour of Unsaturated Polyester Resins 381

Table 1. Curing parameters of UP with different concentrations of GTA atdifferent temperatures

Curing parameters at different temperatures

GTA t1max T 1

max t2max T 2

max t3max T 3

max t4max T 4

max(wt%) (min) (◦C) (min) (◦C) (min) (◦C) (min) (◦C)

0 25 52.2 23 64.5 18 67.2 12 72.5

5 20 55.4 16 67.5 14 69.7 6.5 78.5

10 19 56.3 13 68.5 11 70.1 6 79

15 17 58.0 12 70 10 71.5 5.5 80

20 15 59.8 11 72.3 8 72.4 5 81

1At 25 ◦C; 2at 35 ◦C; 3at 45 ◦C; 4at 55 ◦C.

Table 2. Curing parameters of UP with different concentrations of GTM atdifferent temperatures

Curing parameters at different temperatures

GTM t1max T 1

max t2max T 2

max t3max T 3

max t4max T 4

max(wt%) (min) (◦C) (min) (◦C) (min) (◦C) (min) (◦C)

0 25 52.2 23 64.5 18 67.2 12 72.5

5 24 53.4 22 66 17 68 10 75

10 23 55.4 20 67.1 16 69.2 9 76.2

15 22 57.2 18 69.2 14 70.1 8 78.1

20 20 59.3 16 70 13 71.0 7 79.2

1At 25 ◦C; 2at 35 ◦C; 3at 45 ◦C; 4at 55 ◦C.

and shear rate. Variation of the viscosity with time yields thecritical value for the viscosity (ηc) and the time (tc) corre-sponding to the gel point (pot life) [18]. The relation betweenviscosity and reaction time of UP/GTM in the presence ofstyrene cured at 25 ◦C was plotted for representative sam-ples in Figure 5. The relation between viscosity of UP/VEmixtures and time is characterized in the first stage by aconstant mixture viscosity with increasing reaction times. Inthe second stage, a sharp increase in mixture viscosity wasobserved. The critical values evaluated for the viscosity andthe time at gel point for the cured UP and VE resins wereobtained according to the Liska method [19] and are listedin Tables 3 and 4. The gel time values agreed with the resultsreported on curing exotherms.

Figure 5. Variation of UP/GTM viscosity in presence of styrene with curingtime at 25◦C.

Table 3. Variation of the critical time and critical viscosity for cured UPresins with different concentrations of GTA, at different temperatures

Temperature (◦C)

25 35 45 55

GTA tc ηc tc ηc tc ηc tc ηc

(wt%) (min) CP (min) CP (min) CP (min) CP

0 22 330 20 280 17 220 11 190

5 18 500 15 470 13 440 11 380

10 17 550 12 490 10 470 5 410

15 16 580 11 510 9 490 4 440

20 14 630 10 550 7 520 3 480

Table 4. Variation of the critical time and critical viscosity for cured UPresins with different concentrations of GTM at different temperatures

Temperature (◦C)

25 35 45 55

GTM tc ηc tc ηc tc ηc tc ηc

(wt%) (min) CP (min) CP (min) CP (min) CP

0 22 330 20 280 17 220 11 190

5 21 380 19 320 16 280 9 220

10 20 450 18 370 14 315 8 280

15 19 490 16 420 12 360 7 310

20 17 500 14 470 11 400 6 360

Effect of Cross-Linking Temperature

To further investigate the curing behaviour of the UP, VE,styrene system, curing was carried out at a range of temper-atures with constant initiator and accelerator concentrations.The nature of the networks formed with vinyl ester, styreneand UP is difficult to understand and could be influencedby the reaction conditions such as temperature. The effectof temperature on the curing reaction was investigated. Inradical polymerisation induced by thermal decomposition ofan initiator, generally, as the polymerisation temperature in-creases, the dissociation rate of the initiator and the rates ofinitiation, propagation, and termination are enhanced [20].All these factors except the rate of termination increase therate of polymerisation; however, a further increment in thetemperature reduces the molecular weight of the polymer,due to an increase in the rate of termination and chain trans-fer, and increases the relative amount of the polymer chainends.

The temperature of reaction of the proposed system wasvaried from 25 to 55 ◦C. The data for cures at these tempera-tures are listed in Tables 1 and 2. It was observed that, as thecuring temperature increased the values of tmax decreased.We suggest that the increase in the reaction temperatureleads to a decrease in the viscosity of the reactants andconsequently affects the diffusion and reactivity ratios be-tween the reactants. The curing kinetics of styrene VE resinshas been studied at various temperatures. Ziaee et al. [21]

382 Ayman M. Atta et al.

Table 5. Compressive parameters of cured UP with different concentrations of GTA at different temperatures

Compressive parameters

Compressive strength (σmax) Young’s modulus (EY)

GTA (MPa) (GPa)

(wt%) σ 1max σ 2

max σ 3max σ 4

max E1Y E2

Y E3Y E4

Y

0 135.231 137.351 158.689 161.234 2.1134 2.2541 2.5364 2.6545

5 151.132 153.159 161.231 166.211 2.1121 2.2403 2.3511 2.5101

10 170.947 175.947 180.325 186.421 1.8603 1.9415 2.1811 2.3031

15 179.652 183.897 187.213 192.310 1.5531 1.7705 1.8233 1.9854

20 182.641 204.871 235.214 243.221 1.4024 1.6265 1.6852 1.8621

1Cured at 25 ◦C; 2cured at 35 ◦C; 3cured at 45 ◦C; 4cured at 55 ◦C.

Table 6. Compressive parameters of cured UP with different concentrations of GTM at different temperatures

Compressive parameters

Compressive strength (σmax) Young’s modulus (EY)

GTM (MPa) (GPa)

(wt%) σ 1max σ 2

max σ 3max σ 4

max E1Y E2

Y E3Y E4

Y

0 135.231 137.351 158.689 161.234 2.1134 2.2541 2.5364 2.6545

5 136.637 145.740 154.497 163.201 1.8343 1.9501 2.0012 2.1212

10 138.878 152.897 160.312 170.121 1.6083 1.7543 1.8232 2.0011

15 152.431 155.544 168.512 181.312 1.5012 1.5202 1.6562 1.8253

20 171.774 177.604 180.322 210.352 1.4113 1.4433 1.5203 1.6312

1Cured at 25 ◦C; 2cured at 35 ◦C; 3cured at 45 ◦C; 4cured at 55 ◦C.

reported that the reactivity ratios of VE and styrene wereaffected by curing temperature. The data in Figure 3 showdifferent behaviour as the reaction temperature increases. Atlow curing temperatures such as 25 and 35 ◦C, the rate wasinitially low but it passed through a broad maximum as thereaction progressed. The reaction then slowed and finallyceased before all of the monomer had been consumed.

The cessation of polymerisation prior to total monomerconsumption has been widely observed in the cure of glassforming networks [22–24] and is due to diffusion control ofthe curing process when the glass transition temperature (Tg)approaches the curing temperature (Tcure). At higher curingtemperatures, the final degree of cure is increased becausethe material can polymerise further before the Tg rises aboveTcure. As indicated in Figures 3(c, d) and 4(c, d), the curveswere too flat when the curing temperature was 45 and 55 ◦C.This indicates that the final degree of curing was high [23].Hence, a flat curing exotherm is equivalent to a uniform curereaction rate, which can facilitate the final degree of curingfor a thermosetting reaction system.

Mechanical Properties of Cured UP and VE Resins

It is of interest to measure the mechanical properties ofthe cured resins so as to decide potential the applications.The mechanical properties are quantified easily, since thetest methods are standardized and are easily related. Thederived values of the Young’s modulus (EY) in GPa andcompression strength (σmax) in MPa are given in Tables 5and 6. The data show that the value of EY increased as thetemperature of reaction increased. The increase in σmax on

increasing the VE content can be attributed to the increasedcontent of curable double bonds, thus greatly increasing thecrosslink density. This resulted in the cured UP/VE resinsbeing harder and more resistant to deformation upon theapplication of external stresses. The net effect is that themechanical strength was significantly improved as a result ofthe more chemically bonded structures. This result is an ex-pected effect of TEG end groups, which are not crosslinkedbut freely incorporated in the crosslinked resin network. TheTEG units act as internal plasticizers and reduce the over-all crosslink density [13]. The chain length of the glycolmolecules, used in constructing the VE resin, has affectedthe flexibility of this resin. The increase in the length of theglycol molecules certainly led to an increase in the lengthof the polymer segments. This in turn affects the intra- andinter-molecular reactions and yields lower energy barriersfor rotation, which enhance the flexibility of the resin. Thereduction in EY may be attributed to an increase in the lengthof the polymer segments. The previous argument may leadto speculation that increasing the molecular weight of theglycols decreases the cross-linking density. Consequently,the mobility of the polymer segment increases and hence anincrease in flexibility is manifested by a reduction in Young’smodulus.

The following discussion deals with correlating Young’smodulus with the structure of the VE chains. There are vir-tually no published data that relate the mechanical propertiesof UP, cured with the styrene/VE system, to their structure,so no comparison can be made. However, early experimentalwork in both flexible and rigid polyester systems [25] has

Properties and Behaviour of Unsaturated Polyester Resins 383

revealed that copolymerization of acrylic monomers with fu-maric unsaturation was sluggish. The resulting copolymerswere frequently inferior in physical properties. The widedifference in reactivity between GTA and GTM during thecrosslinking of the UPs may be explained by the stabilizationcaused by the inductive effect of the methyl group present inthe GTM structure.

To summarize, the rate of reaction of the UP radicalwith GTA is faster than with GTM, and the GTM radicalreacts faster with its own monomer than does the GTA rad-ical [26]. Accordingly, all GTM cross-linker radicals areused up before a significant number of styrene moleculescan be incorporated in the cross-linked UP [27]. So theprobability that homopolymerisation of styrene occurs dur-ing crosslinking is higher with GTM than GTA. It is wellestablished that EY decreases with homopolymerisation ofthe monomer [27]. This speculation agrees with the datalisted in Tables 5 and 6, which indicate that EY decreaseson incorporation of GTM in a network of cross-linked UP.The fact that the σmax values were higher for cross-linkedUPs in presence of GTA than GTM indicate the formationof a denser cross-linked network in presence of GTA thanwith GTM cross-linkers. This can be attributed to the higherreactivity of GTA towards UP radicals.

Conclusions

The following conclusions emerge from the above results:

(1) New vinyl ester resins based on recycled PET wereprepared from its oligomers with tetraethylene glycol.

(2) The curing of the vinyl ester resins with unsaturatedpolyester resins in presence of styrene indicates that thecuring times were reduced and the cure exothermic tem-peratures were increased by the incorporation of VE intothe cross-linked networks.

(3) The compression strength of the cured resins was im-proved by the incorporation of vinyl ester resins.

(4) The Young’s modulus and compression strength werehigher when vinyl ester resins were given acrylate ratherthan methacrylate end groups.

(5) It is convenient to add VE to UP resins, and it is betterto use acrylate than methacrylate resins.

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