Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 179-185
179
SYMPOSIA SECTION
I.
Symposium on "Phenolics Revisited, 75 Years Later" Louis A. Pilato, Chairman 186th National Meeting of the American Chemical Society Washington, DC, August 1983 (Continued from September 1984 and March 1985 Issues)
Intermolecular Hydrogen Bonding in Novolaks Zvonimir Katovif" and Miljenko Stefan16 Chromos, Chemical Research and Development Center, P.O.B.94, Zagreb, Yugoslavia
Intermolecular hydrogen bonding in novolak resins in the solid state (homoassociation)and with other hydrogenaccepting substances (heteroassociation) has been investigated by using differential scanning calorimetry to observe the changes in the glass-transition temperatures. I t has been demonstrated that solid novolaks are self-associated aggregates interactingthrough the polar hydroxyl groups of neighborlng molecules. Heteroassociatiinswith nitrogen containing substances make stronger hydrogen bonds at temperatures above the glass-transition temperature of novolak resin. Such a hydrogen bonding in the case of hexamethylenetetramineleads to the complete proton transfer and cleavage of the carbon-nitrogen covalent bond and can be considered a driving force for the curing reaction. An appropriate mechanism for the novolak curing reaction with hexamethylenetetramine is proposed.
Introduction Hydrogen bonding in phenolic resins, particularly in novolaks, has been of great interest to many workers in that field of research. Drumm and LeBlanc (1972) have reviewed the subject thoroughly. The majority of the research, however, has been done with resins in solutions, with exceptions being the works of Jones (1952) and Baltenene and Igonin (1967). Jones (1952) has described novolak resins as large associated complexes held together by a network of hydrogen bonds. The high increase in entropy of this resin during heating demonstrates a higher degree of order in the solid state. Similarly, Baltenene and Igonin (1967) have stressed that the glassy state of novolaks is mainly due to the relatively high content of polar hydroxy groups, rather than to the structure of the resin itself. They found by IR measurements that hydrogen bond weakening, which occurs at 60 "C, is independent of the molecular weights of novolaks. Recently, Fahrneholtz and Kwei (1981) have found that certain substituted novolak resins are compatible with a wide range of polymers. The latter are characterized by their capabilities of forming hydrogen bonds with the novolak. Such interactions were manifested by a shift of approximately 20 cm-' in the infrared stretching frequency of the carbonyl group and positive deviation of the glass-transition temperatures from the calrulated weight average values as determined by a DSC twhnique. The aim of this work is to present some data, based on thermoanalytical methods, that indicate the importance of intermolecular hydrogen bonding on the properties and reactions of novolaks. Here we would like to distinguish two types of intermolecular hydrogen bonding, namely: (1) homomolecular self-association of the novolak molecules 0196-4321/85/1224-0179$01.50/0
and formation of aggregates as described by Jones (1952) and Baltanene and Igonin (19671, and (2) heteromolecular association with other substances possessing hydrogenaccepting sites in their molecules. In the latter case we will consider the interaction of novolaks and phenols with hexamethylenetetramine (hexa): specifically chemical reaction between them, known as the curing process. Hydrogen bonds, although an order of magnitude weaker than covalent bonds, may in certain cases like this one be considered a driving force for the chemical reaction. Experimental Methods Novolak resins were prepared in the usual way using oxalic acid as a catalyst, formaldehyde as 37% formaline solution, and phenol as 90% solution in water. Devolatilization was conducted after condensing ingredients for 60 min at 100 "C, and neutralizing the catalyst with calcium carbonate. Molar proportions of phenol to formaldehyde were as indicated in the text. In some cases free phenol was removed by heating the resin under vacuum up to about 200 "C. Differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) were performed on Perkin-Elmer DSC 1B and DuPont TMA instruments, respectively. Heating rates were 16 "C/min for DSC experiments and 20 "C/min for TMA experiments, unless otherwise stated. Results and Discussion Thermograms of the TMA and DSC measurements of novolak resin are shown in Figure 1. From the TMA performed a t the lower sensitivity, two distinct ranges in the rate of penetration vs. temperature are noticeable. The first one, at approximately 95 "C, corresponds to the visually observed "melting" range of the resin, i.e., melting 0 1985 American Chemical Society
180
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2 , 1985
10,
5
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Figure 1. Thermograms of novolak resin: (a) TMA a t the sensitivity of 200 Wm/cm; (b) at the sensitivity of 10 Wm/cm; (c) DSC thermogram of the quenched-cooled sample.
the aggregation of powered resin particles into a continuous liquid phase. Preceding this but visible a t the higher sensitivity, a glass-transition temperature is seen from the change in the linear expansion coefficients. DSC measurements show good agreement between these two techniques as is evident from the abrupt change on the specific heat vs. temperature curve. The values for Tgof novolak resins are difficult to explain on the basis of initial segmental motion in the polymer chain. The number average molecular weight of the novolak resin is usually between 400 and 600 Da. The largest molecules will rarely exceed 20 phenolic nuclei interconnected through the methylene bridges. It is unlikely to suppose for such a relatively small molecules to exhibit segmental motion in that temperature range. For comparison, structurally similar polystyrene of the same size has a glass-transition temperature well below room temperature. The most plausible explanation is offered by Jones (1952). Based on viscosity measurements of melted novolak resin, he visualized the novolak structure as an ordered amorphous phase, where the number of hydrogen bonded neighboring molecules contributed to the apparent much greater size of molecules than that obtained from molecular weight measurements. During the sample heating the hydrogen-bonded structure is disrupted, causing the "segmental" movement of novolak molecules not associated within novolak aggregate. This is then evidenced through the
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observation of the glass-transition temperature seen by DSC and TMA. Relaxation processes as described by Petrie (1972) are also observed in novolak resins as an endothermic peak superimposed on the Tgof the quenchcooled sample (Figure 2). Here reorientation of novolak molecules takes place because the molecules are "frozen" by rapid cooling in a metastable state which is characteristic of the temperature above but not of the temperatures below Tg. During annealing the equilibrium state typical for the temperature below Tgis restored as a consequence of the orientation of polar hydroxyl groups. The ultimate result is the formation of a novolak network which gives the amorphous phase a partially ordered structure. On subsequent reheating an endothermic peak is observed as the result of the excess enthalpy trapped in the resin on cooling. The change of glass-transition temperature of novolak resin in relation to the molecular weight of the resin, i.e., phenol to formaldehyde mole ratio, is shown in Figure 3. This dependency is obvious, but quantitative correlations are not possible unless the exact influence of the unreacted phenol and residual water is established. Qualitatively,however, water influence on Tgcan be judged from the thermogram in Figure 4. Water content in this resin, of around 1.570corresponds , approximately to one water molecule per hydroxyl group. (Devolatilized resin picks up this amount of water, if stored under 100% relative humidity at room temperature for 4 days.) It can be seen that even after heating up to 130 "C a certain amount of water still remains bound to the resin. The same holds true for other oxygen-containing solvents. Hydrogen-bonding interaction of novolak resin with other substances which in their molecules have hydrogen-accepting sites like oxygen, nitrogen, or chlorine atoms is also evident from the increase in T gof their mixtures (Figure 5). Such intermolecular associations, although weak, could have a pronounced influence on the properties of such adducts, especially where the multiplicity of such bonds between neighboring molecules is possible. Figure 6 is an example of hydroxy modified polystyrene. Here, the introduction of one hydroxyl group per nine styrene molecules increases the glass-transition temperature for almost 20 "C, even though the number-average molecular weight is only one half of it. Such hydroxy-modified polystyrene is not, however, compatible with the phenolic novolak resin, as evidenced by two glass-transition temperatures. The possible explanation is that affinity for association through the intermolecular hydrogen bonding of the hydroxyl groups is much more favorable for the molecules of the same kind than between different ones. The situation changes if nitrogen atoms are introduced in polystyrene molecules instead of hydroxyl groups as evident
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 24,
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No. 2, 1985 181
mass. To the best of our knowledge it has not been explicitly said why phenols and novolaks react with hexa. It was taken for granted that these substances react at elevated temperatures without explaining why and how hexa is broken down and in what way ammonia is formed. In order to shopw that hydrogen bonding between phenol hydroxyl and nitrogen in hexa is responsible for the cleavage of carbon-nitrogen covalent bonds, we have chosen a structurally similar compound to hexa, 1,4-diazabicyclo[2.2.2]octane (Dabco),which does not react with novolak as a curing agent. In this way the thermograms can be interpreted more easily. Changes in Tgfor the two different ratios of novolak to Dabco (Figure 7) clearly illustrate the strength of the hetero-hydrogen bonding in these systems. For the case of 1:0.5 weight ratio of novolak to Dabco, which is approximately one hydroxyl group per nitrogen atom, there is an increase in Tgof almost 50 "C which is an unexpectedly high value. Such heteroassociation through hydrogen bonding in novolak-Dabco mixtures is in equilibrium with homoassociated intermolecular hydrogen bonding of novolak itself, as is evident from the change in the glass-transition temperature during annealing (Figure 8).
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by the presence of only one Tkin their blends. As a special case, in connection with heteroassociation of novolaks, we might consider reaction of novolaks with hexamethylenetetramine (hexa). Throughout the literature on phenolic resins, curing of novolaks with a hexa has always been stressed as a very complex reaction. Kopf and Wagner (1973) have demonstrated on model substances, like phenol-hexa adducts and 2,6-xylenol-hexa mixtures, the presence of hydroxybenzylamines as the first reaction products. Sojka et al. (1981) have measured by 13CNMR spectrometry the rate of growth and decay of these intermediates and subsequent formation of dihydroxydiphenylmethanes (DPM). Aranguren et al. (1982) gave a simplified scheme describing the curing reaction of novolak with hexa similar to that put foward by Kopf and Wagner (1983). It is based on the infrared spectra identification of the presence of certain groups and data on the amount of soluble fractions and nitrogen distribution between the soluble and insoluble fractions, as a function of the hexa
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It could be assumed that heteroassociation is kinetically controlled while homoassociation is a thermodynamically governed process. Changes in the glass-transition temperatures for novolak-hexa mixtures are shown in Figure 9. Here we can notice a far smaller increase in Tg, of around 9 "C, for an approximate 1:l hydroxyl to nitrogen ratio. This is due to the difference in the temperature treatment, which for hexa was 40 "C lower than with Dabco in order to prevent premature reaction with novolak. The thermogram of the mixture of novolak with 5% hexa (Figure lo), in addition to the glass-transition temperature region and relaxation endotherm for the equilibrated sample, show the well-known exotherm with a maximum at around 160 "C. This exotherm is usually associated with the curing reaction. Another broad exotherm beginning at around 100 "C is also noticeable which is later masked by the much larger previously mentioned curing exotherm. The latter exotherm is associated with the evolution of ammonia and is proportional to the amount of hexa used, as already mentioned by Aranguren et al. (1982) (Figure 11). The size of this exotherm, and consequently the amount of ammonia involved, depends on the time and the temperature at which the sample has been held (Figure 10). Under certain conditions even the complete omission of the exotherm, i.e., ammonia release, is possible (Figure 12). This is valid only if there is no mixing of the sample during the temperature treatment.
182
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985
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place with the formation of adequate ions. It could be indirectly reasoned, by making comparison to the novolak-Dabco case, that the weakening and subsequent cleavage of two methylene-nitrogen bonds >N-CH,-N< seems improbable, based on the results of Sojka et al. (1981), who showed that DPM are not found among the initial products in the phenol-hexa reaction. Kopf and Wagner (1983) came to a similar conclusion, so we wish to propose two consecutive and temperature dependent
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 183
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steps that occur in the novolak reaction with hexa: (1)a hydrogen-transfer step which includes the transfer of hy-
drogen from phenolic hydroxyl to hexa nitrogen with the formation of ionized species (2) a hydrogen shift and
184
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985
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Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 185-188
benzylation step which consists of the ring hydrogen shift to the oxygen anion and an attack of the so-formed ring carbon anion on the methylene group in hexa. This will result in the cleavage of carbon-nitrogen bond in hexa and formation of novolak benzylamine derivatives of the partially disrupted hexa cage structure It seems reasonable
185
attack on the secondary and later on the primary nitrogen that is formed, two routes are possible (Scheme I): one with tertiary amine derivatives of novolaks and hexahydrotriazine (route A) and another with hexahydrotriazine ring as the cross-linking point (route B). Reactions can certainly proceed as long as nitrogen-containing substances are present, but are especially dependent on the reaction condition and the mobility of the reacting species. Certainly a speculation that reactivity of hexa with novolaks and other hydrogen donors is possible, because of the proximity of two nitrogen atoms between the methylene group, has yet to be corroborated. Registry No. Dabco, 280-57-9; (phenol).(formaldehyde)(copolymer), 9003-35-4; water, 7732-18-5; melamine, 108-78-1; poly(ethy1ene terephthalate) (SRU),25038-59-9; poly(styreneco-4-hydroxystilbene), 94426-84-3; poly(styrene-co-4-vinylpyridine), 26222-40-2; hexamethylenetetramine, 100-97-0.
L i t e r a t u r e Cited Aranguren, M. I.; Borrajo, J.; Wllliams, R. J. J. J. Polym. Scl., Poly” Chem. Ed. 1882, 20, 311. Baltenene, Ja.; Igonin, L. A. Plast. Massy 1867, No. 7 , 32. Drumm, M. F.; LeBlanc. J. R. I n ”Step-Growth Polymerizations”, Solomon, D. H. Ed.; M. Dekker, Inc.: New York, 1972; pp 204-219. Fahrenholtz, S. R.; Kwei, T. K. Macromolecules 1981. 74, 1076. Jones, T. T. J. Appl. Chem. (London) 1852, 2 , 134. Kopf, P. W.; Wagner, E. R. J. Polym. Sci., Polym. Chem. Ed. 1873, 7 7 , 939. Leclercq, J. M.;Dupuis, P.; Slndorfy, C. Croat. Chem. Acta 1882, 55, 105. Petrle, S. E. B. J . Polym. S d . , Polym. Phys. Ed. 1972, 70, 1255. Sojka, S. A.; Wolfe, R. A.; Guenther, 0. D. Macromolecules 1881, 74, 1539.
Received for review May 2, 1984 Revised manuscript received September 24, 1984 Accepted November 21, 1984
to suppose that so depicted process is possible because the other nitrogen atom connected to the same methylene group is also hydrogen bonded. Proceeding in the same way, either by simultaneousor consecutive hydrogen attack on the remaining tertiary nitrogen in hexa, or hydrogen
Presented a t the Symposium on “Phenolics Revisited, 75 Years Later”, 186th National Meeting of the American Chemical Society, Washington, DC, August 1983.
Effect of Silane Coupling Agents on a Phenolic Cure Reaction Howard L. Price* and Jentung Kut Bendlx Advanced Technology Center, Columbia, Maryland 2 7045- 1998
The effect of silane coupling agents on the exothermic cure reaction of a phenolic used in composite materials has been measured. A model composite material of phenolic novolac and solid soda lime silica micropheres was used. Four different coupling agents were applied to the microsphere surface, and the reaction was carried out with a differential scanning calorimeter (DSC) as a microreactor. The main reaction peak temperature (approximately 440 K) and kinetics were not affected by the microsphere surface treatment. However, the surface treatment dld change the heat released during the reaction, with the amine and the chioroalkyl coupling agents having the largest effects. These measurements and a finite difference analysis technique were used to estimate the internal temperature distribution expected during isothermal molding of a 15-mm-thick phenolic-glass-fiber composite. These estimates suggested that the amine agent would help to keep the temperatures relatively low. However, for the chloroalkyl agent, the centerline temperature would rise sharply to some 80’ above the heated mold wall.
Introduction
The cure reaction of thermosetting polymers is influe n c d by mineral faem and fibers which are added in order to create a polymeric composite. The effect of additives on cure reaction been demonstrated by use of different
* Promatec Inc., Cincinnati, f
OH 45236. OAO Corporation, Greenbelt, MD 20770. 0196-4321/85/1224-0185$01.50/0
mineral fillers (Willard, 1974;Tishchenko et al., 1973) and glass fabric with a variety of surface treatments (Plueddemann, 1974). This effect arises from two sources: the “thermal mass” (PC,) of the nonreactive fillers/fibers represents a heat sink which reduces the temperatures reached during a reaction; the surface of the fillers/fibers can be an in situ catalyst which causes the reaction time and temperature to change under adiabatic conditions (Plueddemann, 1974). 0 1985 American Chemical Society
