Curing Studies on Vinyl Ester Resin Using Acrylates as Reactive Diluents

Krishna Kant, A. Mishra & J. S. P. Rai*

Department of Plastics Technology, Harcourt Butler Technological Institute, Kanpur 208 002, India

(Received 14 August 1990; revised version received 29 May 1991; accepted 16 June 1991)

Abstract: The curing behavior of bis(methacry1oxy) derivatives of diglycidyl ether of bisphenol A (vinyl ester resin) containing acrylates (methyl, ethyl and butyl acrylate) as the reactive diluents (40% w/w) were studied by DSC analysis. Data found in this study indicated that an appreciable curing rate is obtained a t lower temperature with increased concentrations of initiator.. The acid value, activation energy, Arrhenius factor and specific heat are discussed.

Key words: vinyl ester resin, curing behavior, DSC, reactive diluents.

INTRO D UCTlON

Bis(methacry1oxy) derivatives of diglycidyl ether of bisphenol A resin can be converted to a cross-linked network by free radical polymerization of vinylic double bonds. These resins can be used in pure form or may contain a vinyl type reactive diluent such as acrylates and styrene. There are a few reports on the effect of acrylates as reactive diluent,' - 3 and and substituted styrene7 on the properties of vinyl ester resins. In the present work, we report the highlights of our findings on preparation and curing kinetics of vinyl ester resin using methyl, ethyl and butyl acrylate as reactive diluents.

EXPERtM ENTAL

Epoxy resin (DER-331, Dow Chemical Company), methacrylic acid (E. Merck) and triphenylphosphine (Fluka AG) were used for the preparation of vinyl ester resin. Benzoyl peroxide and methyl, ethyl and butyl acrylate of LR grade (SDS) were used as initiator and reactive diluents.

The epoxide equivalent weight of epoxy resin was determined by the pyridinium chloride method.8

Vinyl ester resin was prepared using a 1 : 1 mole ratio of epoxy resin (epoxide equivalent weight: 190) and meth- acrylic acid in the presence of triphenylphosphine (1 phr

* To whom correspondence should be addressed.

based on the weight of epoxy resin) as catalyst for the esterification reaction. The reaction was carried out at 100f2"C in a nitrogen atmosphere for 100min. The control of the reaction after l00min became difficult as the product gelled.

The acid value of vinyl ester resin was determined by the method given by Korshak & Vinogradova.' The number average molecular weight (Mn) of vinyl ester resin was calculated by using the formula given by Han & Lem."

Curing of the sample

Nine samples were prepared by mixing the vinyl ester resin, acrylate monomer (three samples each of methyl, ethyl and butyl acrylate) and benzoyl peroxide in requisite amounts at 30°C. The details of these samples are given in Table 2. These samples were properly sealed and kept under refrigeration to avoid any premature polymerization.

A DuPont 910 Differential Scanning Calorimeter (DSC) system with 990 XR thermal analyzer was used to study the curing kinetics of the prepared samples (Ml-B3). The analyzer was linearly programmed for an initial temperature of 5OoC, program/heating rates of 5, 10, 15 and 20"C/min and the final temperature of up to 180°C.

The approximate activation energy ( E ) within k 3% accuracy was estimated by the method given by Ozawa"

189 Polymer internutionulO959-8 103/92/$05.00 0 1992 SCI. Printed in Great Britain

190 Krishna Kant, A. Mishra, J. S. P. Rai

TABLE 1. Acid value and number average molecular weight of vinyl ester resin

Sample Reaction Acid Number no. time value average

weight (min) molecular

(M")

121.6 - 1 0 2 10 92.5 121 0.8 3 20 75.0 1493.3 4 30 59.1 1895.1 5 40 56.2 1992.8 6 50 46.4 241 3.8 7 60 44.7 2505.6 8 70 33.7 3323.4 9 80 29.6 3783.7 10 90 26.1 4291.2 1 1 100 19.4 5773.2 12 103 19.0 5894.7

and the accurate values of E were obtained by further refinement of approximated values of E using a series of iterations until the two successive iterated values were almost identical.

The frequency factor ( Z ) has been calculated by using the following equation:

BE eEIR T Z=-

where: p = program/heating rate T = peak temperature R = gas constant E = activation energy

R T ,

The specific rate constant has been calculated from the Arrhenius equation.

The specific heat of the cured experimental samples at 60°C was calculated by the method given in DuPont's DSC manual.

RESULTS AND DISCUSSION

Table 1 shows the acid value and number average molecular weight (H,,) of vinyl ester resin with reaction time. The acid value decreases with the time whereas the number average molecular weight of the resin increases. The acid value which determines the progress of reaction was observed to be nonlinear in the initial stages of the reaction. This is because of the high concentration of reactive sites and greater possibility of association of acid and epoxide groups in the initial stages of reaction. Figure 1 shows a plot between acid value and reaction time and it is evident that a linear relationship exists between the acid value and reaction time in the latter part of the reaction, i.e. from - 55-60% conversion (taking the decrease in acid value from the initial value as a measure

1 2 O h L

I

-3000 g,

-2000

0 L

&I

QI

-1000 z f 0: 0 20 LO 60 80 100 120

Time(min) Fig. 1. Acid value vs reaction time (a) and number average

molecular weight vs reaction time (b) for vinyl ester resin.

of conversion) and onwards, which is a characteristic of such polyesterification reactions. A plot of M , and reaction time shows the opposite behavior in comparison with that observed between the acid value and reaction time. This plot is linear up to -60% conversion after which the nonlinear behavior is observed. This relation- ship between M,, and reaction time could be explained on the basis that the number average degree of polymeriz- ation has a square root relationship with the reaction time." It is also apparent from Fig. 1 that after -60% conversion (which occurs after 1 h of reaction), there is a sharp increase in the molecular weight of the resin and it follows the same course in the latter part of the reaction. The total reaction time, 103 min for the vinyl ester resin of acid value 19 was determined after several trials. The reaction became difficult to control at 100°C after 103 min as the product started to gel.

The structure of the vinyl ester resin prepared was confirmed by IR spectrum (Fig. 2). The presence of characteristic bands at 3490 cm- for -OH, 3050 cm - for -CH,-CH,, 1720 and 1610cm-' for -C=O, and 1630cm- for -C=C-, and the absence of an epoxide group band at 860-910 cm-' confirmed the structure of the prepared sample.

Curing studies

The typical dynamic DSC scans for the curing of vinyl ester resin containing 40wt% of methyl acrylate with initiator concentration of 1-3 phr at a heating rate of 10"C/min show that the exothermic peaks are in the temperature range of 85-100°C with the peak tempera- ture 95.8, 91.0 and 89~5°C and the onset temperature 85, 80 and 74°C for the experimental samples M M, and M, respectively. These results indicate a decrease in peak and

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Curing studies on vinyl ester resin 191

MICRONS 2 5 30 40 50 6.0 70 80 90 10 12 1L 16 18 20 253035&09

4000 3500 3000 2500 2000 1600 1600 1400 1200 1000 800 600 400 WAVE N U M BE R ( C M-l)

Fig. 2. IR spectrum of vinyl ester resin.

onset temperatures with the increase in initiator con- centration. Similar behavior was shown by the samples prepared with ethyl and butyl acrylates. DSC scans of samples (M1-B3) at heating rates of 5, 15 and 20"C/min also show the trend as observed at a 10"C/min heating rate. The thermograms show that the rate of curing reaction becomes sluggish in the latter stages of reaction whereas it is very much faster in the early stages of reaction. Due to this fall in the reaction rate in the latter part of the reaction i t became difficult to draw an exact baseline for the determination of total heat of reaction and the parameters related to it.

The plots of logp (heating rate) versus 1/T (peak temperature) and its slopes obtained by regression analysis were used to calculate the energy of activation. The values of activation energies for different systems (M1-B3) are given in Table 2.

The value of the activation energy of the curing reaction of vinyl ester resin with all the acrylates was found to decrease with increase in initiator concentration. At a particular initiator concentration the values of activation energy of the samples containing ethyl acrylate were low in comparison with that of samples containing methyl and butyl acrylate. The values of activation energy were found to be maximum in the case of samples

containing butyl acrylate (the bulkiest monomer used), but the reverse was observed in the case of ethyl acrylate. The reason for this behavior could be confirmed by the 'Q-e' scheme of Alfrey & Price,I3 which predicts the reactivity behavior of the monomers with reference to another monomer radical.

The values of the frequency factor for all the samples are given in Table 2. It did not change appreciably with the heating rate. The increase in initiator concentration reduces the value of the frequency factor which again confirms that the reaction will occur faster at lower temperature with an increase in curing agent con- centration. The value of 2 was maximum in the case of butyl acrylate and minimum in the case of ethyl acrylate.

The specific rate constant ( K ) calculated by using the values of E and Z gave a linear relationship between In K , and 1/T, suggesting that it follows the Arrhenius rate expression.

The specific heat (C,) of the cured samples (at 75°C for 2 h) are given in Table 2. The value of C, was maximum for butyl acrylate and minimum for methyl acrylate. This finding indicates that the T, of the methyl acrylate-based sample will be higher than that of ethyl and butyl acrylate- based samples. The value of specific heat decreases with the molecular weight of the reactive diluent.

TABLE 2. Results of curing kinetics of vinyl ester resin with different acrylates

Reactive Curing agent Activation Average value Specific diluent (wt%) energy of frequency heat

(40 wt%) (kcal/mol . ) factor (J/g I T )

1 22.73 2 22.1 2 3 20.39 1 20.91 2 20.07 3 19.1 9 1 24.43 2 23.96 3 23.1 3

2.42 x 1013 1 5 9 1013 1.46 x 10" 2.03 x 1 0l2 9.66 x 10" 3.38 x 10" 2.33 x 1 014

2.22 1014

7.31 x 1013

1.2285 - -

1.2750 - -

1.3661 - -

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192 Krishna Kant, A . Mishra, J. S. P. Rai

CONCLUSION

The acid value of vinyl ester resin decreases with the reaction time at 100°C and the nature of the reaction follows the conventional type of polyesterification reaction. The curing behavior shows that an appreciable curing rate is obtained at lower temperature with increased initiator concentrations.

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