I n d . Eng. Chem. Res. 1988,27, 1296-1300
1296
Finally, an economical evaluation of the process proposed in this investigation using kaolin P has been carried out for a production of 30.000 metric tonlyear. Taking into consideration a period of amortization of 10 years and an annual profit of approximately 15% of the total investment of 1700 X IO6 peseta ($14 x lo6), the resulting estimated price for the zeolite is 54 peseta ($0.43)/kg, (Ruiz, 1986). This price is comparable with other detergent builders.
Literature Cited Aranda, C. "Capacidad de intercambio ibnico de la zeolita 4 A Influencia de las condicioness de sintesis y de operacibn." Tesina de licenciatura, Universidad Complutense de Madrid, Madrid, 1982. Berth, P.; Jakobi, G.; Schmadel, E.; Schwuger, M. J.; Krauch, C. H. Angew. Chem. 1975,87, 115. Bosh, P.; Ortiz, L.; Schifter, I. Ind. Eng. Chem. Prod. Res. Deu. 1983, 22, 401. Burriesci, N.; Corigliano, F.; Saja, L.; Zipelli, C.; Bart, C. J. J. Chem. Technol. Biotechnol. 1983, 33A, 421. Burzio, F.; Pasetti, A. Rev. It. Sostanze Grasse 1983, 60, 7. Costa, E.; Sotelo, J. L.; Gutierrez, M. L.; Uguina, M. A. An. Quim. 1979, 75, 96.
Costa, E.; Lucas, A.; Uguina, M. A.; Ruiz, J. C. An. Quim. 1988, in press. Derleth, H.; Walter, L.; Bretz, K. H.; Kurs, A. Ger. Often Patent P2705088.0, 1977. Endres, R.; Drave, H. Ger. Often Patent P2852674.1., 1978. Ettlinger, M.; Ferch, H. Manuf. Chem. Aerosol News 1978,49, 51. Ferris, A. P. G.B. Patent 23049, 1977. Garside, J. Chem. Eng. Sci. 1985, 40, 3. Gresser, R. Fr. Patent APPL 82/10.368, 1982. Grove, C. S. Advances in Chemical Engeneering; Academic: New York, 1962; Vol. 3. Kostinko, J. A. Symposium of Advances in Zeolite Chemistry, Las Vegas, 1982. Layman, P. Chem. Eng. News 1984,23, 17. Rodrigo, L. C. "Sintesis de la zeolita A de sodio a partir de productos comerciales: Influencia de las variables." Tesina de licenciatura, Universidad Complutense de Madrid, Madrid, 1981. Ruiz, J. C. "Sintesis de zeolita 4A a partir de caolines." Tesis doctoral, Universidad Complutense de Madrid, Madrid, 1986. Stack, H. Eur. Chem. News 1983, 1. Schweitzer, P. Handbook of Separation Techniques f o r Chemical Engineers; McGraw-Hill: New York, 1979. Weber, H. Ger. Often Patent P 2715934.8, 1977.
Received for review J u n e 25, 1987 Revised manuscript received January 26, 1988 Accepted February 10, 1988
Water Repellent Efficacy of Wax Used in Hardboard Oscar H. H. HSU*and Howard S. Bender John M . Coates Research Center, Masonite Corporation, St. Charles, Illinois 601 74
An important property of commercial hardboard is its resistance to wetting and the penetration of water. T o accomplish a satisfactory degree of water resistance generally requires the use of a hydrophobic material called "size". Wax has been used as a water-repellent sizing agent in the hardboard industry for decades. Work was conducted to identify the mechanism of wax performance as an effective sizing compound. The chemical structure and molecular size were found to be the essential factors which determine the effectiveness of wax as a water repellent. The degree of branching and carbon chain length of the hydrocarbon affect water repellency. Furthermore, by heat treating the wax, its effectiveness can be increased. A simple technique to identify the efficacy of the wax is described.
Wood and paper products have traditionally used sizes to impart water repellency. For wood fiberboards and hardboards exposed to weather as siding and roofing products, high water repellency is essential to prevent wetting of rain into exposed faces and edges. In these products, hydrocarbon waxes derived from petroleum crude are the commonly used sizing agents. These waxes come from petroleum refining involving distillation at elevated temperatures and subatmospheric pressures to split the petroleum crude into distillates and unvolatilized residues. Some of the distillates are sufficiently high in molecular weight to yield semisolid waxes (slack waxes) when cooled to room temperature. The residue is solvent extracted to yield lube oils, waxes, and asphaltic tars. Slack waxes are a mixture of viscous oils and semisolid (paraffinic) waxes, whereas, petrolatum contains hard wax (microcrystalline waxes) mixed in semisolid paraffinic wax and viscous oils. Wide variations in color, hardness, melting point, melt viscosity, and water repellency have been observed by hardboard makers using these different waxes. Because of the criticality of water repellency to weatherability, we undertook a study to ascertain those attributes of hydrocarbon waxes that affect water repellency. The results of the investigation are described in this paper and involved 0888-588518812621-1296$0l.50/0
studies of the materials in bulk and at the molecular level. Roffael and May (1983) have discussed the influence of chain length, chain length distribution, and paraffinicity on the sizing behavior of paraffins in particleboard. We have extended our program beyond this point; molecular weight, molecular weight distribution, viscosity, melting point, oil content, and degree of branching are discussed. As a result of this work, a practical method for measuring water-repellent efficacy is introduced. Materials used in hardboard manufacture, in addition to wood, include substances which influence either the physical or mechanical properties of the final product. Sizing compounds are normally used to affect the interaction between water and wood, in particular, the cellulose and hemicellulose fraction. Products derived from wood through their reduction to fiber and subsequent processing to hardboard, paper, and similar materials retain this interaction with water. The interaction between the wood components and water causes profound dimensional and strength changes. A variety of materials have been employed by various manufacturers of hardboard to achieve water repellency. Consequently, a number of hardboard manufacturers have used various substances to impart water repellency to their product. These include petrolatum wax, tall oil pitch, fatty acids and their derivatives, 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1297 -r/ LG
Figure 1. Surface tension forces and contact angle for an idealized liquid drop on a solid.
vinsol, gilsonite, pulpex, terates, alum, resin, and many synthetic compounds such as ketene dimer and silane derivatives.
Chemical Composition of Petrolatums and Paraffin Wax As described above, the difference present in various grades and types of petrolatum wax can be traced back to differences in chemical composition. Petrolatum waxes are composed almost entirely of hydrocarbons. The variations, therefore, become a matter of different configurations of carbon and hydrogen in molecules containing from 20 to 70 carbon atoms per molecule and roughly twice as many hydrogen atoms. The literature emphasized that paraffin wax contained relatively high percentages of normal or slightly branched hydrocarbons, whereas microcrystalline petrolatum waxes contained substantial amounts of highly branched or cyclic hydrocarbons (Phillips, 1963). The n-alkanes of high-purity paraffin wax have been isolated and their properties are well-known. Very few of the branched and cyclic isomers have been separated, based on the fact that there are tremendous numbers of possible isomers in a complete molecule. By mathematic calculation, there may be 6.95 X 1013different isomers of tetracontane (C40H82) (Ferris, 1963). Examination by gas chromatography, infrared spectroscopy, high-pressure liquid chromatography, nuclear magnetic resonance, mass spectroscopy, and correlation of various physical properties does make it possible to establish the types of hydrocarbons present in the wax. The Role of Hydrocarbons in Imparting Water Repellency The wetting of a solid by a liquid can be analyzed by the Young-Duprey equation. The consequences of the intimate contact between a solid and liquid are shown in Figure 1 as predicted by the equation COS
e = (TSG - T S L ) / T L G
If the surface tension of the liquid is less than the surface tension of the solid, wetting and spreading of the liquid over the surface of the solid occurs. When the solid is porous to the liquid, the liquid will wet into the bulk of the solid, causing swelling if the solid is an elastic gel. Where the surface tension of the solid is much less than that of the liquid, the liquid will bead upon the solid. Zisman (1964) demonstrated that the surface tension of the solid can be determined by measuring the contact angle (e)as a function of liquid surface tension and extrapolating it to a value of one. The intercept is the critical surface tension of the solid. In the case of water (surface tension = 72 dyn/cm) which wets out readily on clean wood surfaces, we conclude that wood fiber has a surface tension greater than 72 dyn/cm. The role of water repellents in wood is to reduce the surface tension of the solid ( T S G ) to that extent necessary to preclude wetting by water. This can be accomplished by coating the surfaces of wood fiber with a thin film of size. Waxes have values of approximately 30 dyn/cm and are among the most cost effective for this purpose.
Characterization of Waxes Waxes are generally divided into two types, paraffin wax and microcrystalline wax. The former consists primarily of straight-chain n-paraffins of various chain length, whereas the latter contains a substantial amount of branched or cyclic components. It becomes extremely important to distinguish between them, as well as to identify the major ingredients which contribute to their water-repellent properties. Six commercially available waxes were selected as models. Melting point and melting point ranges, molecular weight and molecular weight ranges, oil content, viscosity, and the degree of branching were evaluated. Straight chain paraffin compounds with chain lengths of C14, CI6,C2,,, CZ2,C24,and CS2were also included in this investigation. The basic description of the six waxes is listed as follows: waxes A, microcrystalline B, paraffinic C, microcrystalline D, microcrystalline E, microcrystalline F, microcrystalline
description of waxes dark brown, hard, not greasy translucent, light brown, hard, not greasy light brown, soft, greasy dark, creamy, hard, not greasy dark, creamy, hard, not greasy amber, soft, greasy
In this investigation, the water-repellent efficacy test is used. This test is performed by dipping Whatman No. 4 filter paper into a carbon tetrachloride solution containing various amounts of wax. After air drying at room temperature until a constant weight was reached, contact angles were measured on the treated paper using a goniometer. The water-repellent properties of waxes were indicated by the contact angle. The higher the contact angles on the filter paper, the higher the water-repellent property of the wax. As mentioned previously, wax is a mixture of straight and branched chain hydrocarbons. They were separated by using a urea clathration technique (McMahon and Wood, 1963), which is based on the theory that straight chain hydrocarbons form clathrates with urea, while branched chains fail to do so. The drawback of this method is that the separation is not as complete as the theory might lead one to expect. However, it served our needs to differentiate and isolate them for investigation into the effects of efficacy.
Effect of Molecular Weight on Water Repellency Wax has a large variation in molecular weight and molecular weight distribution. The relationship between water repellency and molecular weight was investigated using straight chain paraffinic compounds with definite chain length as models. The results shown in Table I indicate that, at the addition level of 3%, molecular weights for CI8and less (MW 254) show no water-repellent properties. Significant water repellency was observed when the molecular weight of the hydrocarbon exceeded that for CZ4(MW 338). In general, by increasing the chain length of the waxes, the water-repellent property increased. The molecular weight and molecular weight distribution of selected commercial waxes were determined by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as a solvent and a microstyragel column. Number and weight average molecular weights and polydispersity were obtained for each of the individual waxes as shown in Table 11. Wax A, with the highest molecular weight, had the greatest water repellency, which agrees with the results of the model compound study. However, wax B, having the lowest molecular weight, also gave a high contact angle. This indicates that factors other than molecular weight are
1298 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 Table I. Water-Repellent Efficacy of S t a n d a r d Hydrocarbons w i t h Various Chain Lengths % hydrocarbonc absorbed in paper contact angle (3 min), deg hydrocarbon chain length before heating after heating" before heating after heating" 0 0 0 0 c 1 4 14 4.3 0 0 0 C16 16 4.9 0 0 (2.9 s ) ~ 0 Cl8 18 20 5.0 3.8 0 (9.6 s) 0 (3.1 s) CZO 22 5.4 5.4 0 (10.5 s) 0 (3.8 s) C2Z 24 5.4 5.4 0 (11.8 s) 62 c24 32 5.4 5.2 0 (13.0 s) 123 c32 5.4 5.4 120 125 cZZ + c24 + c32 22 + 24 + 32 controld 0 0 0 0 0 "Heated at 160 O F for 1 h. *Time for the water droplet to disappear on the paper surface. 'Dipped in 3% wax-CC1, solution. dNo wax, .. only CCl,. Viscosity (cp.)
Table 11. GPC Analysis of Waxes wax A
B C
D F
Mw
M-
P = M,.,lMn
698 437 589 592 655
609 427 556 556 340
1.146 1.023 1.059 1.065 1.923
contact angle" (3 m i d , deg 131.5 123.0 70.5 48.0 0
"Paper treated with 3% wax in CC14 solvent, water as droplet medium. Table 111. Oil Content a n d Melting Point of Waxes melting point, O F oil straight branched combines chain chain content, % wax A 14.55 177.8 204.8 129.2 B 1.21 132.8 136.5 145.4 170.6 93.2 29.34 C 141.6 22.11 D 147.7 186.0 96.0 23.78 E 147.0 165.2 98.0 13.16 F 143.2 168.8 89.0
important in contributing to wax repellency. In addition, wax F, with broad molecular weight distribution (p = 1.923), exhibited zero contact angle. This suggests that polydispersity is important in the ultimate performance of waxes.
411,
15 10
A
F D C
B T e m p e r a t u r e (F) Figure 2. Viscosity of various waxes.
fication for "oil" in wax lies in its ability to ensure the even distribution of high molecular weight hydrocarbons in the substrate. Wax B, a paraffin wax, contains very little "oil" (1.21%), while the remaining waxes have oil contents between 13% and 30% (Table 111). The substantial difference in oil content between waxes A and B demonstrates that oil content alone does not determine water repellency of wax. Waxes A and F have similar oil contents, but great differences in water repellency. This provides additional evidence that something other than "oil" causes the water repellency of wax.
Effect of Melting Point on Water Repellency Wax does not have a specific sharp melting point due to its complex nature. To determine the accurate melting point of these substances, differential scanning calorimetry (DSC) was employed from room temperature to 100 "C at a 20 OC/min heating rate using a 5-mg sample size (Giavarini and Poschetti, 1973). The melting points of straight chain and branched fractions were also obtained. The results, shown in Table 111,demonstrate that wax A has the highest melting point of the waxes evaluated, while wax B has the lowest. The results also indicated that the branched fragments have an approximately 80 O F lower melting point than its straight chain counterpart. The only exception was wax B, which had only a 12 O F variation, indicating wax B is relatively uniform in nature. The water-repellent properties of waxes A and B were both excellent. However, they represent the highest and lowest melting points, respectively. Thus, melting point has no effect on water repellency.
Effect of Viscosity on Water Repellency Because of differences in isomers, molecular weight, melting point, and oil content, among other factors, each wax possesses its own viscosity and viscosity profile. A highly viscous wax would have a higher surface tension than less viscous waxes at equivalent temperatures. High surface tension could cause difficulty in distributing wax, due to the low wettability of the substrate. The initial viscosity is a function of the melting point of the wax. However, the viscosity profile is associated with the nature of the wax. Figure 2 describes the viscosity profile of all waxes examined. Due to the substantial difference in melting points between them, the obvious difference in viscosity profile is seen between wax A and wax B. Wax A demonstrated a drastic reduction in viscosity at around 185 OF, slightly above its melting point. Wax B shows a reduction in viscosity at 135 OF. In general, paraffin waxes, such as wax B, have a lower viscosity, which is beneficial to wax distribution and results in hardboard with higher water-repellent properties.
Effect of Oil Content on Water Repellency Wax contains various amounts of "oil" depending upon the sources of crude oil and processing conditions. "Oil" is believed to have very little or no water-repellent properties. However, high oil content is present in low cost waxes, which most of the wood industry uses. The justi-
Effect of Degree of Branching on Water Repellency In an attempt to identify the difference in sizing efficiency between straight chain and branched chain hydrocarbons, the waxes were separated into two fractions by urea clathration (McMahon and Wood, 1963). Their
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1299 Table IV. Comparison between Percent Straight Chain and Contact Angles in Waxes 9i straight contact angle wax chain (3 m i d , dep A
B C
D E F control (no wax)
39 34 26 25 25 8
131.5 123.0 70.5 48.0 45.0 0
0
0
Table V. Water-Repellent Efficacy of Waxes amount of wax in filter contact angle (3 min), paper, g deg wax before heat after heat” before heat after heat“ A 0.0262 0.0199 131.5 131.3 B 0.0289 0.0213 123.0 130.6 C
D E
F control*
0.0263 0.0276 0.0283 0.0274 (0.0028)
0.0208 0.0216 0.0214 0.0225 (0.0084)
70.5 48.0 45.0 0
0
85.5 90.0 91.2 117.5 0
“At 160 O F for 1 h. b N o wax.
water-repellent properties were evaluated by the waterrepellent efficacy method. Table IV summarizes the results. Wax A contained the greatest amount of straight chain compounds and showed the highest water-repellent properties. Wax B, a paraffinic hydrocarbon, contains the next greatest amount of straight chain compounds and showed the next highest contact angle. Conversely, wax F is almost entirely made up of branched chains and has the lowest sizing efficacy. Thus, there is a direct relationship between the amount of straight chains in wax and the degree of water repellency. From this one-to-one relationship, one draws the conclusion that the major contribution to water repellency is the straight chain content. Large amounts of branched fraction, on the other hand, could harm sizing efficiency. In the selection of an appropriate wax for the purpose of reducing water absorption, the straight chain content of the wax should be seriously considered.
Effect of Heat Treatment on Water Repellency Table V demonstrates that heat treatment, the common practice in the wood industry to increase the water-repellent property of their products, will improve contact angles. The possible mechanisms for improving water repellency by heat treatment are, firstly, heat enhances the redistribution of wax and, secondly, heat changes the wax morphology. To examine the possibility of changes in crystallinity during heat treatment, two pieces of waxcoated glass were heated above the melting point of the wax and cooled in two different ways: one was cooled in a rapid fashion (quenching), and the other was cooled slowly (annealing). The difference in contact angles was then observed. This experiment showed no statistical difference in these processes (Table VI), indicating that heat treatment does not alter the bulk surface structure. Heating enhanced the redistribution of wax and is believed to be the major reason for the improvement of sizing efficiency. Critical Wax Content (CWC) The concentration of wax on filter paper can be changed by increasing or decreasing the concentration of wax in carbon tetrachloride. This resulted in changes in contact angles.
Table VI. Water-Repellent Efficacy of Waxes with Different Heating Processes % wax contact angle wax process absorption (3 min), deg A quenchinga 5.13 130.6 annealingb 4.88 131.9 B quenching 3.00 133.0 annealing 2.96 130.0 C quenching 3.48 95.0 annealing 3.44 93.0 F quenching 3.65 125.0 annealing 3.78 130.0 “Quenching: heat in an oven at 160 O F for 6 h and then removed and cooled to room temperature. Annealing: heated in an oven at 160 OF for 6 h and then allowed to slowly cool in the oven.
Table VII. Critical Wax Content wax before heat, % A 1.2
B
after heat, 7’0 0.8 0.8 2.6 2.8 2.8 3.9
1.0 4.5 5.1 5.4 6.4
C
D E F Contact Angle (Dee) 120
100 80 60 40
20 0
0
2
4
6
8
1
0
Wax in Paper ( X )
Figure 3. Effect of wax concentration on contact angle before heating.
-
Contact Angle
(De&!)
:
120 -
A
B
0 Wax in Paper ( X ) Figure 4. Effect of wax concentration on contact angle after heating.
The wax deposition on filter paper was reduced in small increments from approximately 8% (actual weight gain of the filter paper) and the contact angles were measured at each increment until they became nil (Figure 3). The minimum wax content on filter paper to sustain a contact angle was designated as the “critical wax content” (Table VII). This is another method of determining the sizing efficacy of wax. Following this approach, waxes A and B gave CWC values of 1.2% and 1.0%, while waxes C and F ranged from 4.5% to 6.4%. After heating, the CWC values decreased for all waxes (Figure 4 and Table VII),
1300 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 Table VIII. Properties of BoardsbUsing Waxes with Various Straight Chain and Branched Chain Fractions 24-h water soak % wax % wax % caliper % wt wax addition retention swelling gain B straight 2.7 13.1 27.2 3 C straight 3.0 13.6 26.3 3 F straight A straight 3 1.8 15.9 39.2 B as is 2.7 12.3 27.0 3 C as is 17.5 37.7 2.3 3 2.5 F as is 3 27.6 54.4 2.3 12.9 28.9 A as is 3 B branched“ 11.3 23.5 2.7 3 C branched” 2.8 3 30.5 53.1 2.3 F branchedn 44.4 68.6 3 2.6 A branched” 3 48.6 60.4 0.15 no wax 84.1 143.7 0 “Boards prepared in a different press cycle. *The wax was heated to melt and introduced it as a neat wax into the fiber-water slurry in a tank with agitation. The water temperature is maintained above the melting point of wax.
demonstrating the effectiveness of heat treatment for improving water repellency. These results can be explained in terms of the amount of straight chain hydrocarbons present in the wax (Table IV). Straight chain hydrocarbons have the ability to crystallize, or at least form ordered structure. Once a monomolecular straight chain hydrocarbon film forms on fiber, wetting of the fiber becomes difficult because of this ordered structure. Branched chain hydrocarbons do not pack as well and leave voids on the surface of the fiber which leads to wetting (Swanson, 1980). The CWC is directly proportional to the amount of straight chain hydrocarbon present. Heating has the effect of increasing the mobility of the straight chain hydrocarbons, leading to redistribution and improvement in sizing efficiency. Another interesting point is the abrupt change in contact angle over a very narrow concentration range. This is related to the ability of the straight chain hydrocarbon to form a monomolecular layer. Once total coverage is achieved, a rapid change in contact angle occurs. Evidently, small changes in wax concentration allow this to occur.
Water Repellency of Hardboards It was concluded from this investigation that molecular weight and degree of branching are the major contributors to water repellency. To confirm this, actual hardboard was prepared using these waxes, including the straight chain and branched chain fractions as sizing agents. The degree of water absorption and caliper swell was measured on these boards after a 24-h soak in water. Table VI11 shows that waxes A and B gave the lowest water absorption and caliper swell, demonstrating their superior sizing efficiency. As expected, the branching components gave very poor results, except for wax B. It was later found that the
branched fraction of wax B still contains substantial amounts of straight chain compounds due to incomplete separation by the urea clathration. Nevertheless, the results from these actual boardmaking processes confirmed the above experimental data.
Conclusions In summarizing the results, the following conclusions can be drawn: 1. Decreasing the amount of branched fraction in waxes has a positive effect on their sizing efficacy. 2. Sizing efficacy depends upon the chain length and molecular weight of a sizing agent. 3. Heat treatment of the sized product improved the water repellency because of enhanced wax distribution in the substrate. 4. The water repellent efficacy test proved to be an effective method to evaluate the water-repellent properties of waxes. 5. No direct relationship between melting point, viscosity, and water repellency was found. However, the distribution of wax in hardboard could be affected by melting point and viscosity. 6. Oil content of wax does not directly affect water repellency. Acknowledgment We acknowledge W. F. Krup, D. E. Rubis, and C. A. Barker for their technical assistance, as well as J. Barber of the U.S. Gypsum Research Laboratory for DSC analysis. We are indebted to Dr. H. A. Spalt for his helpful suggestions concerning this work. Registry No. CI4, 629-59-4; CI6, 544-76-3; Czo,112-95-8; Czz, 629-97-0; CZ4,646-31-1; C32,544-85-4; water, 7732-18-5.
Literature Cited Ferris, S. W. “Characterization of Petrolatum Waxes”. Proceedings of the ASTM-TAPPI Symposium on Petrolatum Waxes, 48th Annual Meeting of TAPPI, 1963, p 14. Giavarini, C.; Poschetti, F. “Characterization of Petrolatum Products by DSC Analysis”. J. Thermal Anal. 1973, 5 , 83. McMahon, G. S.; Wood, J. A. ”Identification of Some Chemical Components of Waxes and Their Effects on Performance Characteristics”. Proceedings of the ASTM-TAPPI Symposium on Petrolatum Waxes, 48th Annual Meeting of TAPPI, 1963, pp 18-94. Phillips, J. “Characterization of Petrolatum Waxes by the Behavior of Blends with Oil or Wax”. Proceedings of the ASTM-TAPPI Symposium on Petrolatum Waxes, 4 k h Annual Meeting of TAPPI. 1963: D 20. Roffael, E.; May,- H. A. “Paraffin Sizing of Particleboards”. Proceedings of the 17th Washington State University International Particle Symposium, 1983, pp 283-295. Swanson, J. W. “Mechanisms of Paper Wetting, Dynamic Wettability”. TAPPI, Sizing Short Course, 1980, Notes 1- 10. Zisman, W. A. Adv. Chem. 1964,43, 1. Received for review March 20, 1987 Accepted February 8, 1988
