H. A. CLARK
AND
K.
R. HOFFMAN
Dow Corning Corp., Midland, Mich.
Data on hydroxyl group condensation gives an insight into the cure of silicone resins when used for the production of silicone-glass laminates. The hydroxyl groups present in a silicone resin hydrolyzate do not all have the same condensation reaction rate. An indication of the amount of more reactive hydroxyl groups can be made b y utilizing a special acid number reagent, which i s described. Acid numbers determined by its use are shown to be related to the gelling rate of the resin. The types of glass cloth commonly used in making silicone-glass laminates have measurable surface activities which also affect the gelling rate of silicone resins. Two empirical methods for predicting this effect are outlined and their results compared. One method i s based on the acid number solution used for the resins; the other, on the gelling rate of actual resin glass mixes.
HE struggle for materials to meet military demands many times causes the practical aspects of an end product to forge ahead of a complete understanding of the basic processes by which the product is obtained. Such has been the case with silicone laminating resins. Miniat,irization, high speeds, and protection against over-loads have made the outstanding electrical and high temperature structural properties of Eilicone-glass laminates familiar to many design engineers concerned with equipment which mcst operate under adverse temperature and humidity conditions. During this time, research and development have been aimed, not only toward improved products, but toward better understanding and control of the variables involved in commercial manufacture. Because of the properties desired, silicone resins and glass fibers are natural complements to each other. To date, glass fabrics have been used almost exclusively as reinforcement with silicone resins. From the beginning, this has created a problem because the sizings which are necessary to allow spinning and weaving of glass fabrics have a deleteiious effect on the properties of silicone-glass structures The processes used for removal of the size result in a variety of effects on the glass fiber strength and the natuie of the fiber surfaces left exposed. Production of a silicone glass laminate involves the impregnation of the glass fabric, precure of the glass-resin combination to the B stage, and lamination at the desired temperature and pressure. The key to successful fabrication lies mainly in a better understanding of n-hat happens both during precure and during final setting. The resin is chemically reactive and it is this reactivity which permits attainment of the plastic stage necessary for lamination, followed by thermosetting. However, even a partial understanding of the reactions which take place is not possible until the chemically dynamic glass surfaces in the structure are also considered. This behavior of the glass surfaces which causes many anomalies in the fabrication of silicon-glass laminates may or may not affect other resins; however, the reactions of some of the hydroxyl groups present in silicone resins are analogous to those of hydroxyl groups on a glass fiber surface. 104
Experimental
+
The reaction of B,SiCla-, H20 goes through the R,,Si(OH)d-, step but normally over 90% of these hydroxyl groups are condensed during hydrolysis and subsequent treatments in preparation for practical uses. The relatively small proportion of hydroxyl groups which remain, however, are the basis for the cure of present commercial resins. If the hydroxyl content of a silicone resin hydrolyzate is determined by conventional methods such as the Zerewitinoff ( 1 , I,4), or infrared methods, only one kind of hydroxyl appears to be present, There is, however, strong evidence that not all these hydroxyl groups have the same reactivity. If such a simple resin formulation as equimolar +SiCls/RIeSiC13/MezSiC12is hydrolyzed with an excess of water and the assumption is made that no monomers survive the initial condensation, then it can be shown structurally that five potentially different types of hydroxyl groups may be present, Me 1. -0-Si-00 H
H 0
2. -O--Si--I1Ie
H
0
Me 3. -0-Si-OH hle
H
If two hydroxyl groups remain on the same silicon, this silicon must be terminal; similarly a dimethyl substituted silicon must be terminal if it holds a hydroxyl. Both the position of the hydroxyl and the nature of the other three substituents on the silicon atom affect both the condensation rate and the nature of the resultant polymer. That there are differences in these hydroxyls is easily and quickly demonstrated by the determination of acid numbers on hydrolyzates which have already been washed completely free of chlorides. By ASTM definition, the acid number of an organic solvent or solution is defined as the milligrams of potassium hydroxide necessary to neutralize the acidity of one gram of sample.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
THERMOSETTING RESINS The procedure outlined in ASTM D 664-52 calls for the solution of the sample in a 50:50 mix of benzene and isopropyl alcohol followed by electrometric titration using 0.001N sodium hydroxide or potassium hydroxide. In the adaptation of this method to silicone resins and resin hydrolyzates a number of changes have been made. 1. The benzene portion of the standard solution was changed to toluene because of its much lower toxicity and vapor pressure. 2. High viscosity resins were poorly soluble in the solution so the alcohol was chapged to n-butyl alcohol. 3. Although electrometric determination of the end point was satisfactory for essentially nonpolar aromatics and aliphatics it did not give good results on silicone resin hydrolyzates containing varying amounts of olar hydroxyl groups; therefore, visual determination using {romocresol purple waa adopted. Add 20 mg. dry bromocresol purple per quart of mixed solvent. 4. Because of poor solubility of water in the 50:50 toluene/ n-butyl alcohol mix, the alkali solution was changed to 0.001N alcoholic potaasium hydroxide. It is usually necessary to adjust the solution to neutrality before use.
If +zSi(OH)zor MezSi(0H)z are dissolved in this acid number solution they show no acidity. If, however, +Sicla is hydrolyzed under mild conditions using ethyl ether as the solvent and a mixture of ice and water, hydrolyzates have been produced with acid numbers as high as 5.30. Numerous determinations of both hyllrolyzable and total chloride have been made to confirm that this acid number is not due to residual chlorine on silicon. When a simple resin formulation, such as the equimolar one described, is hydrolyzed in toluene a t room temperature, hydrolyzates with acid numbers above 1.0 are often produced. This acid number may be reduced in a number of ways, Several d the simpler methods are stirring with dehydrating agents such as calcium carbonate or activated alumina, or heating. To establish definitely that this acid number was not proportional to the total hydroxyl content of the resin hydrolyzate the following experiment was conducted. A resin hydrolyzate was made a t 35% resin solids in toluene. One portion was taken for analysis and the balance stirred at room temperature with 5% of precipitated calcium carbonate (based on solid resin). After 4 hours stirring, another portion of the hydrolyzate solution was withdrawn and filtered. The balance waa filtered after 8 hours stirring. Table I shows the acid number and percentage hydroxyl values for the three samples. These values are percentages based on the resin solids. Included in the tables are calculated values showing the percentages of loss during treatment of each of these constituents.
Table 1.
Acid Numbers and Hydroxyl Contents of ChlorideFree Resin Hydrolyzate
Hydrolyzate Treatment, Hr. None 4 8 a
Acid NO. 1.11 0.54 0 06
Acid No., Loss
%
..
51 95
OHa, % 4.15 3.55 3.15
Hydroxy 1 Loss, %
..
14 24
Zerewitinoff method.
These data show that loss of those hydroxyl groups which give rise to the acid number proceeds a t a much higher rate than that of the over-all hydroxyl content. Another property which was determined on these resin solutions was their gel time a t 150" C. as measured by placing a 15mi. sample of the resin solution in an oil bath at 150' C. and stirring with a glass rod until the resin gelled. Table I1 relates these values to the acid numbers. January 1956
A portion of the solution which had been treated 8 hours was dried on a glass plate at room temperature. The gel time of the 100% resin so obbained was determined on a hot plate at 175' C. which is a commonly recommended molding temperature for silicone laminates. Under these conditions, the gel time was found to be 45 seconds. Commercial silicone laminates are normally made with catalyzed resins which have a B stage gel time of 15 seconds to 3 minutes, so it now appeared that it might be possible to make a silicone laminating resin which would gel rapidly enough so that a silicone glass laminate could be made without catalyst. A larger batch of hydrolyzate was treated 8 hours with calcium carbonate and used to impregnate 181-112 glass cloth (8-shaft satin weave fabric with the organic sizing material removed by ignition) to the extent of 35% resin/65% glass. When this layup vas pressed at 175" C. the resin did not gel. Much of the resin eventually ran out of the lay-up and no laminate was obtained.
Table
II.
Acid Numbers and 150' C. Gel Times of Resin Hydrolyzates
H drolyzate deatment, Hr. None 4
,8
$aid NO.
1.11 0.54 0.06
150' C. Gel, Seconds 615 270 225
This particular shipment of 181-112 glass cloth had been analyzed and an alkaline number of 0.39 determined by boiling a sample of the the glass 3 hours in distilled water and titrating with 0.01N hydrochloric acid. Alkali ions migrate to the surface of glass on heating and may be removed by water. Unless contaminated by washing with acid solutions, the glass employed for resin-glass laminates gives an alkaline reaction when extracted by hot water. Due to this fact, the generally accepted method of evaluating the reactivity of a cleaned glass surface han been by means of alkalinity tests similar to the one described above. As investigation proceeded, a further dilemma was created when it was found that active alkali such as potassium hydroxide even in quantity as low as one part per million would shorten the gel time of a silicone resin solution. On the basis of these data it becomes apparent that 1. The alkali of the glass is not available as a catalyst in the nonaqueous solution of the resin 2. The chemical activity of some ingredient of :he glass surface tends to inhibit the gelation of the si'1'icone resin
Eventually, a piece of the 181-112 cloth was dropped into the acid number solution for nonaqueous systems. An immediate acid reaction was shown by the indicator and all the samples of heat-cleaned glass available in the laboratory reacted in the same manner to a greater or lesser degree. Thus, the shortening of gel time obtained by removing the acidic hydroxyls from the resin had been reversed by acidic hydroxyls on the glass surface (5). The color changes of the indicator varied for different samples of glass cloth and a quantitative expression of the magnitude of this acid effect seemed possible. Titrating solutions such as alcoholic potassium hydroxide and triethanolamine in n-butyl alcohol were tried. Differences between various glass samples were found, but the values were not reproducible. The rate of titration seemed to affect the end point and under the most carefully controlled titrating conditions the results varied from day to day. The most consistent results were obtained on glass stored in a constant temperature/constant humidity room. The variable results are thought to be best explained by differences in the absorptive power of different glass fibers, by absolute humidity changes, and contaminants on the glass surface which
INDUSTRIAL AND ENGINEERING CHEMISTRY
105
THERMOSETTING RESINS upset the normal reaction of the glass surface and the acid number solution. Regardless of the fact that no quantitative value has yet been placed on the acidity of uncontaminated, cleaned glass cloths, much useful information can be obtained. A laminator may, from a prediction of how a given glass cloth will affect the curing rate of a silicone resin, determine more closely the curing conditions necessary to produce a consistent B stage material. The information is obtained as follows: 3 grams of glass cloth is cut into small pieces and placed in a 125-ml. Erlenmeyer flask; 50 ml. of the acid number solution is added, and the flask stoppered. Any color change is observed while the contents of the flask are gently swirled around for 1 to 2 minutes. The samples are re-examined after 30 minutes as the initial color change occasionally is modified upon standing. Table I11 shows the observations made by this method on a number of glass cloths. A more functional evaluation of the effect of glass cloth on the cure of silicone resins may be made by actually gelling a silicone resin in the presence of glass fibers from the cloth. The best resin for such a test is an uncatalyzed resin hydrolyzate which is high in hydroxyl content such as DOWCorning 2104. It is common practice in industry to determine the gel time of such a resin by putting a test tube containing approximately enough resin solution to yield 10 grams of solid resin into a 250" C. oil bath and stirring with a glass rod until the resin gels. If the 10-gram resin sample is carefully weighed into the test tube taking the amount of solvent into consideration, then the addition of 0.5 gram of finely cut glass cloth will yield a 5% suspension of glass in resin after the solvent has been driven off. The gel time determined on this mix gives an indication of the effect of that particular glass cloth on the gelling rate of the resin. The choice of 5% glass is arbitrary. The effect of the glass on the gel time is proportional to the amount of glass surface exposed to the resin. An ultimate value cannot be put on the factor causing the change in gel time because of mechanical difficulties. If much more than 5% chopped glass fiber is added to the resin, it becomes impossible to determine a n accurate gel time because of the high viscosity of the mass.
181-112 181-112 181-washed 181-136 261-112 181-Volan 181-114 181-washed
B'
C D
E F ' G H I ,
Definitely acid-no
change on standing
shich the estimates of acidity from Table I11 are compared to the 250" C. gel times of 2104 resin containing 5% of the same glass cloth. The validity of the comparison is obvious. Its importance arises if we consider the problem of the fabricator who tries to make silicone-glasslaminates from Cloth No, 2 using an amount of catalyst recommended for a normal cloth such as Cloth No. 5. As might be anticipated, cloth which has been heat cleaned is much more consistent than washed cloth on which the possibility of chemical contaminants is much greater, If catalysts, which are themselves slightly acidic or neutral, are employed for silicone-glass laminates, it is possible that the normal inhibiting effect of the glass fibers is never overcome. On the other hand, alkaline catalysts may react with or be neutralized by the fibers to the extent that none remains to catalyze the resin.
Table IV.
Gel Time (250" C.) of Dow Corning 2104 with 5%, b y Weight, Chopped Glass Cloth
Cloth
Acidity Estimate (Table 111)
250' C. Gel,
Min.
Conclusions Silicone resin hydrolyzates have an acid number due to some of the hydroxyl groups that remain on the silicon atoms, The hydroxyl groups that cause this acid number may be removed xithout removing a proportionate amount of the total hydroxyl groups present. A method has been developed for the evaluation of this acid number. The higher this acid number is, the slower the cure rate of the resin vdl be. Properly cleaned glass cloth, which is uncontaminated by cleaning reagents, may have an acid reaction in organic mediums. This reaction can be shown both by use of the acid number solution used for resins, and by determining the gel time of a silicone resin with glass present. Two methods for the evaluation of this effect of glass cloth on the cure of silicone resins are outlined and compared. Both methods are empirical, but should be of real value to the fabricator of silicone-glass structures. literature Cited
Neither the acid number method nor the gel time method yields a positive, measurable value for the effect of a glass cloth on the cure of a silicone resin, but relative comparisons are definitely possible. The comparability of the methods is shown in Table IV in
106
(1) Soltus, A , Mikrochemie 11, 107-25 (1936). (2) Tschugaeff, L., Be?. 35, 3912 (1902). (3) Weyl, W. A, PTOC. Am. SOC.Testing Materials 46, 1506 (1946). (4) Zerewitinoff, T., Ber. 40, 2023 (1907). RECEIVED for review March 3, 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
-4CCEFTED
July 20, 1955.
VOl. 48, No. 1
