ELECTRICAL RESISTIVITY AND CROSSLINKING
IN THERMOSETTING RESINS S. L E A R M O N T H A N D G E O F F R E Y P R I T C H A R D Chemistry Department, The University of Aston in Birmingham, Gosta Green, Birmingham 4, England
G E O R G E
A linear unsaturated polyester resin was copolymerized with styrene, using benzoyl peroxide-tertiary amine and cobalt naphthenate-cyclohexanone peroxide systems, in the range 45' to 7OoC. Volume resistivity was measured using a Wheatstone bridge, and the results were compared with spectroscopic studies on the chemical reaction between styrene and the fumarate bonds. The disappearance of absorption bands a t 9 1 5 and 9 8 5 cm.-' was measured, and the carbonyl band employed as a n internal standard. The logarithm of the resistivity showed a n approximately linear relationship with the extent of disappearance of reactive double bonds, and the over-all activation energy determined by electrical resistivity was 2 1.1 kcal. per mole, compared with 20.6 by refractometry. The relationship of resistivity to dynamic mechanical tests is mentioned.
THEdesirability of carrying out physical and chemical studies on the same polyester resin under identical curing conditions, to establish correlations between them, has often been stressed. Previous publications have described work on the commercial resin Beetle 4116, obtained from British Industrial Plastics, Ltd. Changes in torsional modulus and logarithmic decrement (Learmonth et al., 1968), hardness (Czerski et al., 1968), absorption of infrared radiation and refractive index (Learmonth and Pritchard, 1967), and Young's modulus (Learmonth and Pritchard, 1968) have been reported, and some preliminary correlations attempted. This work describes the use of electrical resistivity to follow the polymerization, and attempts to relate the results to previous measurements. Crosslinking of Polyester Resins. The reaction is an addition copolymerization between an unsaturated monomer (styrene) and the polyester. The original polyester is made from an unsaturated acid, a saturated acid, and a glycol, so that it contains unsaturated groups. The styrene reacts with these unsaturated groups by a free radical mechanism, forming an insoluble material built up of two types of interconnected chains: the original polyester chains, and the copolymer chains, which consist of styrene and maleic or fumaric units. If the glycol used is propylene glycol, the maleic groups will be almost entirely converted into the fumaric form. Volume Resistivity as a Cure Parameter. Over 30 years ago, Kienle and Race (1934) prepared alkyd resins and cured them, following the changes in resistivity during formation and subsequent cure. They attempted to correlate the results with the extent of reaction (during the formation stage only) by measuring the ester groups present. This work was taken up after the war by Fineman and Puddington, who studied the cure of phenolic resins (1947a) and of resorcinol-formaldehyde and polyester resins (1947b). Warfield and Petree subsequently measured the polymerization of diallyl phthalate (1959), epoxy (Warfield, 1958; Warfield and Petree, 1961), polyurethane (1961), and polyester (1961) systems and achieved a rough 124
I & E C PRODUCT RESEARCH AND DEVELOPMENT
qualitative correlation between the logarithm of the resistivity of the epoxy resin and the infrared absorption due to the epoxy groups a t 915 cm.-'. Magee and Rotariu (1960) found a correlation between the change in the intensity of the infrared peak, due to the epoxy group, and the change in the logarithm of the resistivity of epoxide polymers during cure. Delmonte (1959) also followed the epoxy cure by resistivity techniques, but concentrated on the dielectric properties. More recently, Judd (1965) studied the kinetics of the polymerization of unsaturated polyesters, using resistivity as an index of reaction rate. He compared the results obtained with those from established methods of estimating cure, such as acetone extraction and Barcol hardness (1966). Despite the growing volume of publications on resistivity measurements on polymerizing systems, no correlations appear to have been reported which link electrical and chemical measurements on polyester resins. The importance of making such studies was stressed by Aukward, Warfield, and Petree (1958) and other workers. Conduction in High Polymers. Electrical conduction is believed to be an ionic mechanism (Warfield, 1965), the ions originating from impurities, catalyst residues, unpolymerized monomer, and moisture. Even if the trace impurities are removed, further ionization can occur by background radiation or thermal dissociation. The conductivity of a polymer can be considerably lowered by purification, as Warner has shown (1948). Some workers have postulated electronic mechanisms for conduction (Simpson, 1950), and it has been suggested that in polyamides, protons may be the current-carrying media (McCall and Anderson, 1960). According to the most widely held views, conductivity decreases during polymerization owing to the gradual immobilization of the system, as crosslinks are formed. There is no sudden change a t the gel point, as the network is still fairly open to all except very large current carriers. If the polymerization is carried out rapidly, micelles or aggregates of highly crosslinked material in a matrix of
less crosslinked resin are formed (Erath and Spurr, 1959), and the resistivity is higher because ions are impeded by the micelles. Crystallinity also lowers the conductivity of polymers (Amborski, 1962). The relationship bet.ween resistivity and viscosity for polymer solutions has been compared with that holding for electrolytic solutions (Purdon and Morton, 1962), and it has been shown that electrical conduction and ion diffusion are closely analogous processes (Spiegler and Coryell, 1953). Experirnental Mixing. The resin was made from 1 mole of maleic anhydride, 3 of propylene glycol, and 2 of phthalic anhydride. It contained 38.7% by weight of styrene. The resin was mixed with 1% by weight of benzoyl peroxide and 0.007% by weight of N , N - p dimethyltoluidine (DMT), and de-aerated. Electrical Measurements. The resin was poured into a preheated parallel-plate cell, which was sealed with PVC tape and returned to a well-ventilated oven. The resistivity was measured using a Marconi 0.1% Universal bridge, TF 1313 A, until no further change took place or the limit o f the useful range of the bridge was approached, about 1014 ohm-cm. No measurements were made below 45" C., as this figure would have been exceeded. The parallelplate cell resembled that used by Judd (1965) and Delmonte (1959), with slight modifications. Squares of glass, 5 x 5 x 3/16 inch, were used; aluminum foil was the electrode material. I t was fixed to the glass plates by impact adhesive, and rolled continuously until the surface resembled that of a mirror. The electrode shape was circular, with nonoverlapping projecting tabs for connection to the bridge. Nitrile rubber gaskets approximately 0.10 inch thick were employed to separat.e the electrodes. By using this design, exotherms are minimized and cracks, bubbles, etc., are readily detected through the glass. Dielectric measurements were also recorded, and will be reported separately. Measurement of Extent of Reaction. The principle of the spectroscopic measurements has been given. The resin was cast between rock salt plates and the absorption a t the vinyl peak (915 cm.-') and the 9 8 5 - m - l peak measured, using the carbonyl peak as an internal standard. I n most cases, the cell was heated by a special heating jacket controlled by a voltage regulator and voltage stabilizer. The heating jacket was in contact with the rock salt plates a t all points of their circumference. (The thermocouple could be inserted into a hole in the rock salt itself, but this would not be appreciably nearer .the resin.) Variations in Formulation. For certain purposes the resistivity was monitored on systems other than the standard formulation given above. I n one case, the level of activator (DMT) was trebled, to allow comparison with some viscoelastic measurements made earlier in these laboratories. A cobalt naphthenate-cyclohexanone peroxide system (HCH) was also studied, and an isophthalic resin, from Scott Bader, Ltd., containing 3 moles of fumaric acid to 1 of isophthalic acid, with 4 of propylene glycol, was polymerized. Viscoelastic Measurements. Dominic (1967) used a Weissenberg rheogoniometer in these laboratories to measure the dynamic properties of the same polyester resin, Beetle 4116, polymerized a t various temperatures with 17' benzoyl peroxide and 0.021% DMT. The rheogoniometer allows the study of a material under oscillatory shear between a cone and a plate. If a" and a' are, respectively, the maximum angular displacement of plate and cone in radians, a"/a' is directly related t o the viscosity of the material and the dynamic shear modulus, b,y equations which Dominic quotes fully. Only the early stages of polymerization were studied, because of difficulties in the use of the rheogoniometer in the later stages as the resin shrank away from the platens. Dynamic Measurements in Postgelation Stage. The use of the torsional pendulum to measure dynamic shear modulus has been described (Learmonth et al., 1968). The information obtained was compared with the resistiv:ity in the postgelation stage. Discussion of Results
Figure 1 indicates that the resistivity of the polyester resin changes by 4 or 5 decades during polymerization, a much greater range than is obtained in most properties customarily measured. The increase in resistivity is carried
further by postcuring, and the maximum value is obtained by keeping the resin a t 100°C. for 24 hours. This increase, however, is much smaller than was obtained by Warfield and Petree (1961). An initial fall in resistivity a t the commencement of measurement is to be expected, as isothermal conditions are approached. However, the same effect is seen even when polymerization is carried out isothermally a t room temperature. I n comparison with the total change, this effect was small. Figure 2 shows the resistivity change for some other systems. The cobalt system, a t 45"C., showed a close similarity to the "standard" system of Figure 1, and did not ultimately reach an appreciably higher resistivity value. (Since the temperature coefficient of resistivity is negative, the final value for a system monitored a t 70°C. will be lower than for one monitored a t 45"C., although the degree of cure may be greater.) Changing the catalyst system does not seem to affect the resistivity, though the dielectric properties were somewhat altered. Increasing the proportion of styrene in the resin would be expected
5
15
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CURE TIME IN HOURS
Figure 1. Change in volume resistivity of unsaturated poly ester resins during cure
l
5
15
IO
CURE TIME IN HOURS Figure 2. Change in volume resistivity of four polyester resin systems during cure 0 Beetle 41 16, containing 0.75% cyclohexanone peroxide and 0.172% cobalt naphthenate by weight.
w Beetle 41 16, containing standard system used as described in text, a t 45" C. XResin RT4, containing isophthalic and fumaric acid 0.007% DMT and 1 % benzoyl peroxide; 65" C. --.-Beetle 4116, containing added styrene to conditions as
w
44.8% by weight. Same
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2.6
Figure 3. Relation between a " / a r and resistivity
/
12.7
Left Increase in resistivity compared with shear modulus os meosured on torsion pendulum a t 1 cycle per second
Right
Beginning of linear relotionship, beyond gel point,
between a resistivity expression and o '/a' obtained from Weissenberg rheogoniometer
0 c)
18
a"/a' to make longer styrene crosslinks of dimer, trimer, and tetramer, and so decrease the resistivity. The isophthalic resin has a very high proportion of crosslinking sites, through they are probably not all used. This would be expected to increase the resistivity. I n Figure 3, the relationship between the viscositydependent term a"/a' and the resistivity is indicated. I t must be regretted that a wider range of polymerization time was not available, especially as the early stages of the reaction were taken up with reaching isothermal conditions. However, it could be seen that u"/a' was not proportional to the logarithm of the resistivity. The onset of gelation a t about 25 minutes is followed by a period where a"/u' is linear with respect to log -p for as long as measurements were obtained. Here, p- is defined as the increase in log resistivity over its minimum value. I n Figure 3, for convenience, log 100 p- has been plotted, using 9.00 as the minimum value. The equation of the line obtained is
Log 100 -p = 1.82
(a") ~
U'
tometric value. Fairly close agreement is obtained, and if h = the pseudo-first-order rate constants
An apparent over-all activation energy of 21.1 kcal. per mole was obtained by resistivity, and 20.6 kcal. per mole by refractometry. Figure 5 shows the relationship between the logarithm of the resistivity and the extent of reaction of the unsaturated groups. For simplicity, no distinction is made here between styrene and polyester unsaturation. There are four curves, corresponding to (1) the standard system a t 4 j o C . , (2) the same system a t 70°C., (3) the aminecatalyzed system, but including 44.85 styrene by weight ( 4 j ° C . ) , and (4) the cobalt-HCH system a t 45°C. Considering 1 first, there is some resemblance here to the original curve of Kienle and Race (1934). I n both cases, the linearity of the curve is lost in the latter stages
0.018
The torsional modulus is also shown, plotted against the logarithm of the resistivity.
1.4-
Dependence of Rate on Temperature
Warfield (Warfield, 1965; Warfield and Petree, 1961) showed that, where resistivity plots were made during polymerization, the rate of reaction could be represented by ( d log p ) / d t max. On the other hand, Purdon and Morton (1962) followed the polymerization of styrene and found that p rather than log p was linear in relation to polymerization time. The present authors found log p to give a linear expression, and compared the Arrhenius plot obtained by using (d log p ) / d t max as an indication of rate of reaction, with the plot obtained by using the pseudo-first-order rate constants from refractometric (Learmonth and Pritchard, 1967) and infrared data, and with the plot obtained from observation of the viscometric gel point. The Arrhenius data are given in Figure 4. No attempt has been made to draw an actual Arrhenius plot from the spectroscopic data, since only two points were obtained, but these have been denoted by squares on the refractometric line. One coincides with the refrac126
I B E C PRODUCT RESEARCH A N D DEVELOPMENT
,
2.90
2.95
3.00
305
3.10
\
345
Figure 4. Arrhenius expressions obtained by various techniques Reciprocal gel time; 10' (d log p / d t ) max; lO'k
K
Y
Figure 5. Increase in resistivity as a function of extent of chemical reaction
3.
0 1
1. Standard system a t 45” C. 2. Standard system at 70” C. 3. Some system, 45” C., 44.8% styrene
z
4. Cobalt-cyclohexanone peroxide system at 45” C.
90
PER CENT OF ALL UNSATURATION REACTED
of reaction, but not to a very great extent. Where this involves an increased resistivity (over and above that expected), Purdon ancl Morton (1962) suggest that the point may have been reached where the geometry of the system no longer allows the larger current carriers to pass easily through the polymer chains, but they have to “drain” through. However, in all cases reported here the opposite tendency is found-namely, the resistivity does not increase as much as expected during the latter stages. While the upper limit of the usefulness of the TF 1313 A bridge was reached before the end of curve 1, it was not approached in curve 2, and so it seems that a lack of accuracy near the end of the reaction is not the whole answer. Chemical measurements were not made for the entire reaction in case 3, but a very close similarity to 1 is found over the range covered. A striking similarity was also found between curves 2 and 4, which correspond to systems of roughly similar reaction rate. Superficially 2 and 4 do not resemble 1 and 3, but they have substantially the same slope and level out in the same way. The displacement of the second pair of lines from the first is probably due to the fact that in (comparatively) rapid curing systems the time required to heat the contents of the parallel plate cell to oven temperature could make a very considerable difference to the state of the reaction a t a given time. This time lag would not apply to the spectroscopic measurements. Further work will reveal whether the faster reactions can, by the elimination of this factor, give correlation curves which do not show a delayed approach to linearity. If so, the.y may superimpose on the curves obtained for slow systems. The absolute values of the resistivity in all these experiments must be regarded with caution, since factors such as moisture content and the presence of other impurities make very great differences. Many resistivity measurements are made using apparatus which is a simple application of Ohm’s law, and a t variable field strengths. The field strength has been shown to affect the results (Warfield and Petree, 1963). Nevertheless, if standardized conditions are employed, this techinique is of value for studying polymerizations and estimating the completeness of cure. The electrical properties of fully cured polyester resins will be considered in a separate report.
changes in the infrared absorption spectrum of the resin. The use of the method to study rates of reactions of resins is convenient. As might be expected, there is a relationship between the resistivity and modulusdependent expressions, but this has to be investigated further. literature Cited
Amborski, L. E., J . Polymer Sci. 62, 331 (1962). Aukward, J. A. Warfield, R. W., Petree, M. C., J . Polymer Sci. 23, 199 (1958). Czerski, J., Learmonth, G. S., Tomlinson, M. J., J . Appl. Polymer Sci. 12 (31, 403 (1968). Delmonte, J., J . A p p l . Polymer Sci. 2 (4), 108 (1959). Dominic, C. J., M S c . thesis, University of Aston in Birmingham, England, 1967. Erath, E. H., Spurr, R. A., J . Polymer Sci. 35, 391 (1959). Fineman, M. N., Puddington, I. E., Can. J . Res. 25B, 101 (1947a). Fineman, M. N., Puddington, I. E., Ind. Eng. Chem. 39, 1288 (194713). Judd, N. C. W., J . Appl. Polymer Sci. 9 (5), 1743 (1965). Judd, N. C. W., R. A. E., Farnborough, England, Tech. Rept. 86261, 21 (1966). Kienle, R. H., Race, H. H., Trans. A m . Electrochem. SOC. 65, 87 (1934). Learmonth, G. S., Pritchard, G., S P E J 2 3 , 12, 46 (1967). Learmonth, G. S., Pritchard, G., S P E J 24, 11, 47 (1968). Learmonth, G. S., Pritchard, G., Reinhardt, J., J . A p p l . Polymer Sei. 12 (41, 619 (1968). Magee, C. B., Rotariu, %., unpublished work, 1960. McCall, D. W., Anderson, E. W., J . Chem. Phys. 32, 237 (1960). Purdon, J. R., Morton, M. J., J . Polymer Sci. 57, 453 (1962). Simpson, J. H., Proc. Phys. SOC.63, 86 (1950). Spiegler, K. S., Coryell, C. D., J . Phys. Chem. 57, 687 (1953). Warfield, R. W., S P E J 14, 39 (1958). Warfield, R. W., “Testing of Polymers,” J. V. Schmitz, Ed., Vol. 1, Chap. 8, p. 273, Interscience, Wiley, London, 1965. Warfield, R. W., Petree, M. C., J . Polymer Sci. 37, 3058 (1959). Warfield, R. W., Petree, M. C., Nature 199, 67 (1963). Warfield, R. W., Petree, M. C., S P E Trans. 1, 3 (1961). Warner, A. J., A S T M Bull. 153, 60 (1948).
Conclusions
Resistivity measurements on unsaturated polyester resins crosslinked with styrene have been correlated with
RECEIVED for review March 18, 1968 ACCEPTED September 23, 1968 VOL. 8 N O . 2 JUNE 1 9 6 9
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