Die Angewandte Makromolekulare Chemie 64 ( 1 977) 235-238 ( N r . 999)

The University of Helsinki, Department of Wood and Polymer Chemistry, Malminkatu 20, 00100 Helsinki 10, Finland

Short Communication

Determination of Specific Heat of Phenol Formaldehyde Resol Resins by Differential Scanning Calorimetry

Vaino A. Era, Antti Mattila, and J. J. Lindberg

(Received 21 March 1977)

SUMMARY: The specific heat, Cp, was determined by DSC on a series of resol-type phenol formalde-

hyde resins with varying phenol formaldehyde molar ratio. The extrapolated Cp values at 25°C vary between 1.181-1.206kJ.kg-'K-' and the dCp/dT ratio was found to be 0.0042kJ.kg-' K-' in the temperature range of 70-125°C.

Introduction

Although phenolic resins have been in practical use for over 60 years, there exists scant literature data with respect to the temperature dependence of the specific heat, Cp, of these polymers. Von Meysenbug' has measured Cp of the novolac and resol-type phenol formaldehyde (PF) resins in the temperature range of 2&140°C. Warfield et al.z determined the specific heat of the resol-type phenolic polymer by DSC in the temperature interval of 5&200"C. In both studies the measured samples were of commercial origin and were not characterized by any chemical or physical methods. The present report is concerned with the application of DSC in evaluating the specific heat, Cp, of model compounds of resol-type PF resins as a function of tempera- ture.

Experimental

Resol type phenolic resins were prepared at varying phenol formaldehyde molar ratios, preparation number in paranthesis; P/F = 1 :0.8 (I), 1 : 1 (2), 1 : 1.4 (3), 1 : 1.7 (4) and 1 :2.0.(5), in the presence of 0.1 mol NaOH. The mixtures were condensed for

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V. A. Era, A. Mattila, and J. J. Lindberg

5 h at 45°C. For DSC measurements the samples were cured by gradually heating up to maximum temperature 177°C.

The infrared spectra were measured with a Perkin-Elmer Model 457 spectrophotometer using KBr pellets on which a film from acetone solution was deposited.

A Perkin-Elmer differential scanning calorimeter, type 1 B, was used for measuring the specific heat, The calibration of the instrument was performed by measurement of the heat of fusion of indium. Specific heat measurements were carried out on the cured samples in the temperature range 7&200"C. A sapphire disk was used as an internal standard. The weights of the samples were 25-30mg. The heating rate was 16 "C/min. The specific heat was calculated according to the following equation:

amplitude (sample) weight (sapphire) amplitude (sapphire) weight (sample) Specific heat (sample) = x specific heat (sapphire)

Results

IR Characterization

The infrared absorption bands (cm-') of PF resins were found to be as follows: 3330, 2900, 1700, 1610, 1470, 1360, 1220, 1150, 1110, 1010, 890, 820, 760.

Very strong bands are printed in bold type. Major peak assignments are shown in Tab. 1.

In all samples the characteristic vibrations of resol-type PF resins were observed4.

Tab. 1. Assignment of absorption bands3.

Wave number (cm-') Assignment Remarks

3 330 2900 1610 1360 1220 1010 890 820 760

phenolic OH and methylol aromatic benzene ring in plane, phenolic OH phenolic OH methylol isolated H adjacent 2 H adjacent 3 ,4 H

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Specific Heat of' Phenol Formaldehyde Resins

Specific heat

In Tab. 2 are shown the Cp values of model compounds extrapolated to 25°C.

Tab. 2. Specific heat, Cp, of resol-type PF resins.

Sample Density' Molar Cp in kJ. kg-' K- ' in g/cm3 at 25°C P/F ratio at 25°C

1.27 1.26 1.26 1.26 1.27

1 :0.8 1.1 81 1:l.O 1.189 1:1.4 1.181 1:1.7 1.197 1 :2.0 1.206

As is seen from Tab. 2 the extrapolated Cp values of the model compounds at 25°C vary between 1.181-1.206kJ-kg-' K-'.

The Cp values obtained are typical for amorphous polymers e.g. for poly- styrene with Cp= 1.195 kJ-kg-'-K-' '.

In Fig. 1 a plot of the Cp of the PF resin with 1 : 1.7 molar P/F ratio as a function of temperature is presented. The relationship seems to be linear in the temperature range of 70-125°C and for all polymers the ratio of dCp/dT=0.0042 kJ.kg-K-' was found which is close to that of 0.0046 kJ. kg- ' . K-* obtained for linear amorphous polystyrene6. Recently Warfield et aL2 measured the temperature dependence of commercial resol-type PF resin in the temperature range of 4&125"C and their value for

Fig. 1. Specific heat of resol-type PF resin (P/F = 1 : 1.7) as a function of temperature.

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V. A. Era, A. Mattila, and J . J . Lindberg

dCp/dT = 0.0096 kJ . kg- K - 2 is much greater than that obtained in our study. The discrepancy may be due to the different chemical composition of PF samples which refers to the different densities found in the materials. It should be pointed out that the modifiers and the impurities such as oxidized species found in commercial samples exert a considerable effect on the thermo- dynamic properties of resol-type PF resins' giving irregularities in the lattice structure of the cured polymers. The inflexion point in the curve (Fig. 1 ) at about 125 "C is much weaker than observed previously in the Cp measure- ments ofcommercial PF resins2. This in turn points to the structural differences of the investigated samples.

C . M. F. von Meysenbug, Kunststoffe 47 (1957) 482 R. W. Warfield, E. G. Kayser, J. Macromol. Sci. Phys. 11 (1975) 325 M. Yamao, S. Nakai, S. Tanaka, Nippon Kagaku Kaishi 5 (1972) 919 Hummel/Scholl, Atlas der Kunststoff-Analyse Band 11, 1968, Carl Hanser Verlag, s. 115 ISOR 1183 R. W. Warfield, M. C . Petree, J. Polym. Sci. 55 (1961) 497

Chem. 50 (1976) 43 ' V. A. Era, J. J. Lindberg, A. Mattila, L. Vauhkonen, T. Linnahalme, Angew. Makromol.

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