Alkoxysiloxanol Modified Surfaces

This invention relates to alkoxysiloxanols and more particularly to alpha-alkoxy-omega-siloxanols. Such materials contain one relatively more reactive group (OH) and one relatively less reactive group (alkoxy); for this reason they have long been sought as intermediates in the synthesis of siloxanes. They have not, however, been available by any process known heretofore.

Several processes that might have been expected to lead to alkoxysiloxanols have instead produced other products. For example, linear and cyclic methylpolysiloxanes heated with methanol in the presence of potassium hydroxide gave instead trimethylalkoxysilanes and dimethyldimethoxysilanes (U.S. Pat Nos. 3,746,982 and 2,826,599). Cyclic trisiloxanes heated with n-octyl alcohol and toluenesulfonic acid in xylene gave dioctyloxytrisiloxanes and water as the principal products, even when the reaction was stopped at an early stage. [ See Sprung and Guenther J. Org. Chem. 26, 552 (1961)].

It is another object of this invention to provide alkoxysiloxanols. Another object of this invention is to provide alpha-alkoxy-omega-siloxanols. It is also an object of this invention to provide a method of producing alpha-alkoxy-omega-siloxanols in high yield and in a high state of purity. It is a further object of this invention to provide novel antistructure agents, coupling agents and filler-treating agents.

These objects, and others which will become apparent from the following description, are achieved, generally speaking, by contacting a cyclic polysiloxane with an alcohol having up to 20 carbon atoms under relatively mild conditions to form a compound of the general formula R'O(R.sub.2 SiO).sub.x H, wherein R and R' are organic radicals and x is an integer of at least 2 and preferably from 2 to 10. In some cases satisfactory results are obtained without a catalyst. In other cases it is advantageous to employ a weak base of a weak acid as a catalyst.

The cyclic polysiloxanes that may be used in the practice of this invention have the general formula (R.sub.2 SiO).sub.y. The radicals represented by R in this formula are hydrocarbon radicals, halogenated hydrocarbon radicals or cyanoalkyl radicals having from 1 to 8 carbon atoms. Suitable radicals include alkyl radicals such as methyl, ethyl, propyl, butyl, or hexyl and fluorinated derivatives thereof; alkenyl radicals such as vinyl or allyl; and aryl radicals such as phenyl or tolyl and chlorinated derivatives thereof. It is preferred that at least half of the radicals be methyl radicals. Very good results are obtained it all of the radicals are methyl.

The number of units, y, in the cyclic polysiloxane is at least 3 and may be as high as 10. Generally the fastest and cleanest reaction occurs when y is 3. However, very good results are also obtained when y is 4 5 5 or even more. Suitable cyclic polysiloxanes thus include those of the general formula [ (CH.sub.3).sub.2 SiO]y, where y is from 3 to 10, particularly when y is 3, 4 or 5. Other cyclic polysiloxanes that may be used include those having groups other than methyl; for example, trimethyltrivinylcyclotrisiloxane, tetramethyltetravinylcyclotetrasiloxane,, heptamethylvinylcyclotetrasiloxane, trimethyltriethylcyclotrisiloxane, trimethyltriphenylcyclotrisiloxane, hexaphenylcyclotrisiloxane, and the like.

Alcohols of almost every description may be used in the practice of this invention. Long-chain or short-chain alkyl, cycloalkyl, alkenyl and aralkyl alcohols and substituted derivatives thereof having up to 20 carbon atoms, including allyl alcohol and benzyl alcohol may be used. Substituted alcohols such as ethanolamine, 2-methoxyethanol, and 2-chloroethanol may also be used. Best results are usually obtained with the short-chain primary and secondary alcohols having up to 4 carbon atoms, e.g. methyl, ethyl, n-propyl, isopropyl, n-butyl, or isobutyl alcohol. The cleanest and fastest reactions, with fewest side reactions, occur with methyl alcohol.

The general reaction may be expressed by the equation R'OH + (R.sub.2 SiO).sub.y.fwdarw.R'O(R.sub.2 SiO).sub.x H. In this equation R' represents the radical of an alcohol having up to 20 carbon atoms. Generally x is equal to y. Under certain conditions, x may have values greater than or less than y. It is preferred to work with a large excess of the alcohol, or the reaction may be very slow. Mole ratios of alcohol to cyclic polysiloxane should be between 2:1 and 50:1, preferably at least 5:1. Ratios higher than 50:1 may be employed, but offer no special advantage.

Reaction temperature is not critical. A reasonable rate of reaction can usually be achieved at room temperature or below. Often it is preferred to operate at somewhat higher temperatures, but generally not much over 100.degree.C. The reflux temperature of the alcohol is ofen a satisfactory operating temperature. If higher temperatures are desired, the reactions may be conducted under moderate pressure, but this is not usually necessary.

In some instances, as in the reaction of methanol with a cyclic trisiloxane, no catalyst is required, and the reaction proceeds at a satisfactory rate as the reflux temperature of the methanol.

When a catalyst is desired a weak acid or a weak base may be added, as indicated above. Strong acids such as toluenesulfonic acid and strong bases such as sodium methcxide are commpletely unsatisfactory as they caue unwanted cleavage and equilibration reactions. Even moderately strong acids such as oxalic acid (pK.sub.a 1.23) cause rapid decomposition of the alkoxysiloxanol that is produced and are of borderline utility. For optimum utility the catalyst should have a pK.sub.a or pK.sub.b above 1.5, corresponding to an acid or basic dissociation constant below 0.03. Maleic acid (pK.sub.a 1.83) and phosphoric acid (pK.sub.a 2.12) are about the strongest acids that can be used with saftey. Even so they must be quickly neutralized when the desired reaction has been essentially completed. In general, any organic or inorganic acid or base may be used if its pK.sub.a or pK.sub.b lies between 1.5 and 10. Extremely weak acids or bases with pK values above 10 are relatively ineffective.

In order to eliminate the neutralization step, it is advantageous to use an acid or base that is volatile, so that it can be removed by distillation. A catalyst that decomposes into harmless by-products on heating is also desirable. Suitable acid catalysts include formic acid (pK.sub.a 3.75), acetic acid (pK.sub.a 4.75), propionic acid (pK.sub.a 4.87), malonic acid (pK.sub.a 2.83), succinic acid (pK.sub.a 4.16), and cyanoacetic acid (pK.sub.a 2.45).

Suitable bases include the primary, secondary, and tertiary aliphatic amines, which have pK.sub.b values in the range of about 1.95 (diisopropylamine) to 4.26 (trimethylamine); ammonia (pK.sub.b 4.75); ethanolamine and its alkyl derivatives; pyrrolidine, piperidine, and their homologs; morpholine (pK.sub.b 5.4), N-methylmorpholine (pK.sub.b 6.5), and N-ethylmorpholine (pK.sub.b 6.2); ethylenediamine (pK.sub.b 4.07) and its n-alkyl derivatives; piperazine (pK.sub.b 4.1) and dimethylpiperazine (pK.sub.b 5.8); pyridine (pK.sub.b 8.77); and aromatic amines such as dimethylamine (pK.sub.b 8.94). Ammonia and the more volatile amines are particularly preferred because of their easy removal.

Salts of weak acids and bases may also be used, but they are less easily removed than the acids and bases listed above, and are therefore not usually preferred.

The catalysts listed above are effective at relatively low concentrations. Concentrations up to five percent may be used, but the preferred range is from 0.01 to 1.0 percent.

In order to purify the alkoxysiloxanols the excess of alcohol is removed by distillation at atmospheric or reduced pressure. Unreacted cyclic polysiloxane, if any, is best removed by vaccum distillation. The alkoxysiloxanol left in the distillation pot at this stage is often pure enough for most purposes. Further purification, if desired, may be achieved by distilling the alkoxysiloxanol at reduced pressure. Products of essentially 100 percent purity can thus be obtained.

The alkoxysiloxanols of this invention are useful as chemical intermediates, as antistructure agents in silica-filled elastomers, and as agents for reducing the surface reactivity of inorganic fillers, especially siliceous fillers. Suitably treated fillers may be obtained by heating untreated fillers with alkoxysiloxanols, preferably in the range of 50.degree. to 200.degree.C. The hydrophobic fillers thus obtained are very useful in the preparation of high-strength silicone elastomers.

Alkoxysiloxanols that contain vinyl groups, e.g. 5-methoxytrimethyltrivinyltrisiloxan-1-ol, are particularly useful as coupling agents between inorganic materials such as fillers and fibrous reinforcing agents, e.g. glass fibers, and organic polymers, especially those that are cured by free-radical or vinyl-addition reactions. Examples include silica-reinforced elastomers of various types and glass-reinforced polyesters.

The following examples are offered by way of illustration, but not by way of limitation. In these examples the dimethylsiloxane unit, (CH.sub.3).sub.2 SiO, is represented by the symbol D, and the methylvinylsiloxane unit, CH.sub.3 C.sub.2 H.sub.3 SiO, is represented by the symbol D.sup.v. All parts are by weight unless otherwise specified.

Hexamethylcyclotrisiloxane (D.sub.3) (22.2 parts) was dissolved in 120 parts of methanol and heated at reflux for four hours. Analysis by gas chromatography showed, in addition to methanol, 91.0 percent 5-methoxy-hexamethyltrisiloxan-1-ol (CH.sub.3 OD.sub.3 H), 7.5 percent unreacted D.sub.3, and 1.5 percent 3-methoxy-tetramethyldisiloxan-1-ol (CH.sub.3 OD.sub.2 H), the latter indicating a slight amount of additional cleavage of the trisiloxanol. There was no evidence of symmetrical siloxanes such as dimethoxytrisiloxane or a trisiloxanediol. On distillation at reduced pressure a nearly pure fraction of CH.sub.3 OD.sub.3 H was obtained boiling at 86.degree.C. (15 mm). Absorption in the near infrared showed strong, sharp OH peaks at 2700 nm. and 2900 nm. Nuclear magnetic resonance showed the group ratios CH.sub.3 (Si) 6.0, CH.sub.3 O 1.1, OH 1.0; theoretical 6:1:1.

Ten parts of 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane (D.sup.v.sub.3) was mixed with 70 parts of methanol and 0.35 part of formic acid and allowd to stand at room temperature for 4 days. The methanol was then removed under vacuum below room temperature, and the remainder was distilled at 1.6 mm., giving 8.1 parts of a product boiling at 78.degree.-89.degree.C. Analysis by gas chromatography of the product showed 7.6 percent of 3-methoxy-1,3-dimethyl-1,3-divinyldisiloxan-1-ol (CH.sub.3 OD.sup.v.sub.2 H), 21.5 percent of D.sup.v.sub.3, 68.9 percent of 5-methoxy-1,3,5-trimethyl-1,3,5-trivinylsiloxan-1-ol (CH.sub.3 OD.sup.v.sub.3 H), and 2.0 percent of CH.sub.3 OD.sup.v.sub.4 H.

Octamethylcyclotetrasiloxane (D.sub.4) (11 parts) was heated to reflux with 60 parts of methanol and 0.4 part of formic acid for 16 hours. Gas chromatographic analysis showed 29.7 percent of 7-methoxyoctamethyltetrasiloxan-1-ol (CH.sub.3 OD.sub.4 H), 1.0 percent of CH.sub.3 OD.sub.3 H, 0.1 percent of CH.sub.3 OD.sub.2 H, 68.7 percent of unreacted D.sub.4, and 0.5 percent of a volatile compound, possibly CH.sub.3 ODH. The rate of formation of CH.sub.3 OD.sub.4 H is thus about 2 percent per hour at 65.degree.C, with very little by-product.

One part of decamethylcyclopentasiloxane (D.sub.5) was mixed with 6 parts of methanol and 0.07 part of cyanoacetic acid and kept at room temperature for 48 hours. At the end of this time 6.7 percent of the D.sub.5 had been converted to 9-methoxydecamethylpentasiloxane-1 -ol (CH.sub.3 OD.sub.5 H) with no by-products detectable at a level of 0.02 percent. This is a conversion of 3.4 percent per day.

A similar reaction was carried out with 0.15 part of di-n-butylamine. A smaller amount of CH.sub.3 OD.sub.5 H was produced, along with significant amounts of CH.sub.3 OD.sub.4 H, CH.sub.3 OD.sub.3 H, and CH.sub.3 OD.sub.2 H. In this example the acid catalyst appears to give fewer by-products.

One part of D.sub.3 was mixed with 7 parts of ethyl alcohol and 0.05 part of formic acid at room temperature. In six hours gas chromatography showed the following (in addition to ethyl alcohol): unreacted D.sub.3 71.0 percent, C.sub.2 H.sub.5 OD.sub.3 H 18.3 percent, a more volatile by-product 3.8 percent, and a less volatile byproduct 6.9 percent.

Eleven parts of D.sub.3, 60 parts of n-propyl alcohol and 0.4 part of formic acid were heated at reflux (95.degree.C.) for 1 hour, producing a major amount of C.sub.3 L.sub.7 OD.sub.3 H and minor amounts of two less volatile materials.

Eleven parts of D.sub.3, 60 parts of methanol, and 0.6 part of acetic acid were heated at reflux (65.degree.) for 2 hours. At this time 98 percent of the D.sub.3 had been converted to CH.sub.3 OD.sub.3 H, with only traces of by-products (CH.sub.3 OD.sub.2 H and CH.sub.3 OD.sub.4 H). In comparison with a similar reaction without a catalyst (Example 1) it is clear that the reaction in the presence of acetic acid is not only faster but produces fewer-by-products.

Example 7 was repeated with 0.4 parts of formic acid in place of acetic acid, and the reaction was about 96 percent complete in 12 minutes, again with practically no by-products. When reflux was continuted for 90 minutes, small amounts of CH.sub.3 OD.sub.2 H, CH.sub.3 OD.sub.4 H and CH.sub.3 OD.sub.6 H were formed.

Example 7 was repeated with 0.5 part of N,N'-dimethylpiperazine as a catalyst. The reaction was 80 percent complete in 10 minutes with only traces of by-products. After 2.5 hours of reflux significant amounts of CH.sub.3 OD.sub.2 H, CH.sub.3 OD.sub.4 H, CH.sub.3 OD.sub.5 H, and CH.sub.3 OD.sub.6 H were formed, CH.sub.3 OD.sub.3 H still being the major product.

Solutions of 9 parts of D.sub.3 in 60 parts of methanol were prepared at room temperature. To these were added 0.5 part of cyanoacetic acid, 0.15 part of ammonia, and 0.3 part of di-n-butylamine. All were effective catalysts and produced 90 percent yields of CH.sub.3 OD.sub.3 H in less than 30 minutes. In each case significant by-products appeared only after several hours.

Identification of the minor ingredients in the above examples (1 - 12) was made on the basis of gas chromatography. A Varian Aerograph Model 700 Gas Chromatograph was used. The column used has the following description:

The reaction times given below are those actually measured. They were reproducible within 1 percent using the above column, although another column might have given somewhat different values. However, the important consideration is relative, rather than absolute retention times. Thus it is known that in a homologous series the ratio of retention times is constant from one member to the next.

It is evident from the data above that the effect of an added D unit in a methoxysiloxanol is almost identical to its effect in the known series of cyclic siloxanes. This regularity provides an invaluable aid to identification. The same is not true in the series HOD.sub.3 H, CH.sub.3 OD.sub.3 H, C.sub.2 H.sub.5 OD.sub.3 H, C.sub.3 H.sub.7 OD.sub.3 H, in which the homologous change involves a relatively small part of the molecule

A. One hundred parts of a silicone gum (containing 0.1 percent of methylvinylsiloxane) was mixed with 10 parts of CH.sub.3 OD.sub.3 H and 36 parts of fumed silica (Cab-O-Sil Hs-5) in a Sigma mixer at 250.degree.F. No difficulty was encountered and a smooth compound was obtained.

B. A similar compound was prepared with only 6 parts of CH.sub.3 OD.sub.3 H. Some mixing difficulty was encountered, but a smooth compound was eventually obtained.

C. A reference compound was prepared from 100 parts of the same gum, 16 parts of a standard "softener" (antistructure agent) commposed of a linear polydimethylsiloxane containing 2.5 percent of OH groups, and 36 parts of Cab-O-Sil HS-5. Attempts to prepare similar compounds with less than 16 parts of the standard softener were unsuccessful because of structure build-up. Thus it is apparent that CH.sub.3 OD.sub.3 H is approximately twice as effective, on a weight basis, as the standard softener.

Samples of each of the above were successfully cured by heating with dichlorobenzoyl peroxide (1.1 percent of a 50-percent paste, 5 minutes at 240.degree.F.). The following physical test data were obtained after a 16-hour postcure at 450.degree.F.

It can be seen from these data that the physical properties are approximately equivalent, in general. However, the better (lower) compression set values of the elastomers containing the methoxysiloxanol are clearly evident.

Six drops of CH.sub.3 OD.sub.3 H were applied to the surface of a clean glass plate. After 10 minutes at room temperature the surface was washed off with acetone and found not be water repellent. A second glass plate was treated with six drops of CH.sub.3 OD.sub.3 H and heated 15 minutes at 105.degree.C. The liquid had evaporated and the surface was found to be somewhat water repellent; water drops on the surface formed a contact angle of about 60.degree.. A third glass plate was treated with 6 drops of CH.sub.3 OD.sub.3 H and heated for 30 minutes at 150.degree.C., whereby it became water repellent; water drops formed contact angles of about 70.degree. on the surface.

A fumed silica (Cab-O-Sil MS-7) was mixed with one tenth its weight of CH.sub.3 OD.sub.3 H and allowed to stand for 16 hours at room temperature. It was not visibly altered and was easily dispersed in water. A similarly treated silica heated for one hour at 110.degree.C. in a closed container became highly hydrophobic and could not be dispersed in water.

A precipitated calcium polysilicate (Hi-Sil 404) (1.0 part) was heated with 0.15 part of CH.sub.3 OD.sub.3 H in a closed bottle at 95.degree.C. for 2 hours, at the end of which it was completely hydrophobic.

Although specific examples are mentioned and have been herein described, it is not intended to limit the invention solely thereto but to include all the variations and modifications falling within the spirit and scope of the appended claims.