US3481763A - Method for controlling the penetration of a siloxane resin into fibrous substrate - Google Patents

Description

United States Patent 3,481,763 METHOD FOR CONTROLLING THE PENETRA- TION OF A SILOXANE RESIN INTO FIBROUS SUBSTRATE Shibley A. Hider and Walter Kitaj, Toledo, Ohio, assignors to Owens-Illinois, Inc., a corporation of Ohio No Drawing. Filed July 13, 1966, Ser. No. 564,721 Int. Cl. B44d 1/092 U.S. Cl. 117-60 9 Claims ABSTRACT OF THE DISCLOSURE In the process of impregnating a fibrous article, such as paper sheets, woven or non-woven mats of cellulosic fibers, synthetic fibers, inorganic fibers, metallic fibers, and the like, with an organopolysiloxane resin and the resin is subsequently cured thereon, the method of con trolling the pick-up and penetration of the resin into the fibrous article wherein the article is first treated with a basic solution having a pH of from 8 to 14, is dried, and then impregnated with the resin. The more basic the solution, the less the pick-up and penetration of the resin into the article.

This invention relates to a method of controlling the penetration of an organopolysiloxane resin into a fibrous substrate. More specifically, this invention is concerned with a process for pretreating a fibrous substrate in such a way that the penetration of an organopolysiloxane resin into said sheet can be carefully controlled.

organopolysiloxane resins have been utilized as coating and treating media for various fibrous substrates. For example, see U.S. Patents Nos. 2,646,373 and 3,095,902, wherein various organopolysiloxane resins are utilized to increase the Wet strength and water repellency of paper. Regardless of these disclosures, the prior art does not teach a method for controlling the pick-up and penetration of an organopolysiloxane coating into a fibrous substrate. In accordance with the process of this invention, it is possible to pretreat a fibrous substrate in such a way that the penetration and pick-up of an organopolysiloxane coating can be carefully controlled.

The primary object of this invention is a process whereby the penetration and pick-up of an organopolysiloxane resin into a fibrous substrate can be carefully controlled.

Another object of this invention includes the production of a superior organopolysiloxane coating on a fibrous substrate.

Finally, the objects of this invention include all the other novel features which will be obvious from the specification and claims at hand.

Generally, the process of this invention entails the pretreatment of a fibrous substrate with a solution which has a carefully regulated pH. It has been found in accordance with this invention that the pH of this pretreated solution has a direct bearing on the pick-up and penetration of any subsequently applied organopolysiloxane coating.

Generally, when the pH of the pretreated solution ranges from about 1 to about 7, the organopolysiloxane resin completely penetrates the fibrous substrate.

3,481,763 Patented Dec. 2, 1969 "ice When the pH of a pretreated solution ranges from about 8 to about 11, the penetration of the subsequently applied organopolysiloxane coating is limited and the coating tends to produce a semi-gloss surface.

When the pH of the pretreating solution ranges from about 11 to about 14, the organopolysiloxane has no tendency to penetrate the fibrous substrate and hence a coated surface having excellent gloss is produced. When the pH of the pretreated coating solution is in the upper ranges, the pick-up of the organopolysiloxane coating is minimal. Pick-up naturally bears a relationship to a degree of penetration. Any acidic or basic compound can be utilized as a pretreating agent in accordance with this invention, provided its pH is within the desired range. Due to their cost advantage, inorganic compounds are preferred as pretreating agents.

On the acidic side, preferred pretreating agents ar acids such as hydrochloric, sulfuric, nitric, acetic, etc. Preferred basic pretreating agents are the alkali and alkali earth metal hydroxides and amines, for example, sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide, diethanaloamine, tetraethylenepentamine, tetraethylammoniumhydroxide, etc.

The invention can be utilized to control the penetration of a plurality of organopolysiloxane coating resins into various substrates. Examples of these organopolysiloxanes are: vinyltriethoxysilane, ethyltriethoxysilane, propyltriethoxysilane, phenyltriethoxysilane, aryltriethoxysilane, and various substituted compounds thereof, etc.

The process as taught in this invention is particularly adapted to controlling the penetration and pick-up of organopolysiloxane resins which are produced by the hydrolysis and condensation of at least one compound embraced by Formula I (I) n (4-n) wherein T independently generally represents a member such as alkyl, cycloalkyl, alkenyl and aryl. More specifically, T is independently a member such as alkyl, e.g., methyl, ethyl and propyl through hexyl (both normal and isomeric forms), cyclopentyl, cyclohexyl, vinyl, and the normal and isomeric forms of propenyl through hexenyl and phenyl, Z independently represents an alkoxy group (e.g., methoxy through heptoxy), and n is 1.

In Formula I, as given above for substituent Z, alkoxy groups are preferred, Alkoxy groups of less than 5 carbon atoms are especially advantageous, because: the rate of hydrolysis can be inconveniently slow when the organic hydrolyzable radical(s) have a higher molecular weight (i.e., more carbon atoms).

The terms hydrolysis product and condensation product as used in thepreceding paragraph and elsewhere in this specification, and in the appended claims, are intended to include Within their meaning the cohydrolysis and co-condensation products that result when mixtures of silicon-containing starting reactants are employed.

Specific examples of compounds as represented by Formula I which are adapted for use in this invention are methyltrialkoxysilanes such as methyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, methyltri(l-propoxy)silane, methyltri(Z-propoxy)silane, methyltri( 2-methyl-2-propoxy) silane, methyltri( l-butoxy) silane,

and methyltri(2-butoxy)silane; examples of phenyltrialkoxysilanes are phenyltrimethoxysilane, phenyltriethoxysilane, phenyltri(1-propoxy)silane, phenyltri(2-propoxy) silane, phenyltri(2-methyl-2-propoxy)silane, phenyltri(1- butoxy) silane, and phenyltri(2-butoxy) silane.

A preferred organosilane monomer adapted for use in this invention consists essentially of a compound represented by Formula II (llz s CH Si-O-CrHr Ja s The concentration of water in the initial hydrolysiscondensation reaction mixture of the compound as represented by Formula II above should be in the range of from about 1.5 moles to about 10 moles of water per mole of silane reactant. Preferably the water to total silane molar ratio is from 1.5 to 5.0 moles, still more preferably from 1.5 to 3.0 moles, of water per mole of total silane. Assuming methyltrialkoxysilane as the only monomer, its complete hydrolysis and condensation can be represented as CH Si(OR) +3H O CH Si(OH) -+3ROH mCH Si(OH) 3 (CH SiO )m+1.5mH O or overall mCH Si (OR) 1.5mH O (CH SiO )m+ 3 mROH where m is a. number corresponding to the degree of polymerization and is greater than 1. Thus the lower theoretical limit of water is 1.5 moles per mole of silane. The solid heat-softenable organopolysiloxane resins of this invention can be prepared at this concentratioin. However, a further decrease in water content of the reaction mixture leads to polymers which are rubbery and soft, presumably due to incomplete hydrolysis and condensation. If the quantity of water is in the range of about 1.5 moles to 5.0 moles of water per mole of silane monomer, the alkanol by-product formed during hydrolysis acts as a solvent for the other products and.

reactants, and the initially heterogeneous reaction mixture becomes clear and homogeneous. This homogeneity is desirable, since it prevents resin precipitation and allows more uniform control of resin formation. If the water:silane ratio substantially exceeds 5:1, the alkanol formed is insufficient to convert the aqueous medium to a solvent for the reactants and products; therefore, resin precipitation can occur. Insolubility of resinous products at higher water concentrations can be overcome by adding a water-miscible organic solvent such as ethanol, etc. However, at water concentrations above about moles of water per mole of silane monomer, gel formation can occur even if suflicient organic solvent is added to make the reaction mixture homogeneous. The exact upper limit of the waterzsilane ratio will depend on such factors as silanes used, acid content, time and temperature, etc. Thus it cannot be set forth precisely, but can be determined by routine test in each case.

Some alkanol by-product must be retained in the readtion mixture during initial hydrolysis and condensation. It is believed that the alkanol formed in the manner indicated by the equations above slows the overall rate of hydrolysis-condensation. This control of the rate of resin formation prevents gel formation and allows preparation of homogeneously highly cross-linked polymers with good dimensional stability. If the by-product alkanol concentration is allowed to fall substantially below 1.5 moles of alkanol per mole of silane monomer (assuming complete hydrolysis according to the above equations), gel formation occurs. This limit can vary slightly with particular conditions and materials used.

To avoid gelation and effect hydrolysis and polysiloxane formation of the compound represented by Formula II at a conveniently rapid rate, the acidity of the initial hydrolysis-condensation reaction mixture must be maintained within certain limits hereinafter set forth in detail.

A means of purifying the starting silane monomer II as described above and monomers III and IV as will be described hereinbelow, to insure the right acidity, is distillation from admixture of the monomers with a reagent which will convert acidic species present to nonvolatile compounds. Thus distillation of the monomers from admixture with alkali-metal alkoxides such as sodium ethoxide or methoxide or aqueous dilute alkali or aqueous alkali-metal carbonate is usually suitable. The methods involving aqueous media are of less advantage when the monomer contains silicon-bonded methoxyl groups, because these species hydrolyze rapidly, causing substantial quantities of monomer to be lost during purification. It has also been found that some commercial monomers initially treated by this procedure to give materials of suitably low acidity later increase in acidity during hydrolysis, causing gelation.

A particularly preferred method of purification which avoids these difiiculties is distillation from a metal hydride that is preferably lithium aluminum hydride. The hydride destroys all active hydrogen species present, thus reducing acidity, and reduces esters such as those described above, preventing subsequent increase in acidity during hydroly- SIS.

It will be apparent that the actual nature of the various acidic species in commercial methyltrialkoxysilanes and their reaction mixtures cannot always be specified. For convenience, acidity is expressed herein, unless otherwise specifically stated, as parts by weight of HCl per million parts by weight of methyltrialkoxysilane, plus water, or, as abbreviated, p.p.m. hydrochloric acid or p.p.m. HCl. However, it is to be understood that this language is not intended to imply that HCl is the only or even one of the acidic species present. Acid content of the monomer alone, when determined, was measured as follows:

To 25 ml. of toluene was added 13 drops of a 0.04% methanol solution of bromcresol purple, and the resultant mixture was titrated to a blue-violet endpoint with 0.02 N potasssium hydroxide. A 10.0 ml. sample of methyltrialkoxysilane was pipetted into the solution thus obtained, and the resultant mixture was titrated to the same blueviolet endpoint with 0.02 N potassium hydroxide; a similar 10.0 ml. sample of methyltrialkoxysilane was rapidly weighed. Under these conditions, acidity of the monomer alone is calculated as A-729V/S, where A is acid content in parts by weight (grams) of HCl per million parts by weight (grams) of methyltrialkoxysilane (assuming entire sample is the silane), V is volume of alkali used in second titration described, and S is weight of sample in grams.

Initial hydrolysis-condensation is conveniently carried out by placing in a flask pure water, methyltrialkoxysilane, whose acid content has been suitably adjusted by one of the means just described, and optionally up to 5 mole percent, based on the total hydrolyzable silanes, of one or more compounds of the formula T SiOR as previously defined, also purified, if necessary, and heating the resultant mixture under reflux. The initially cloudy reaction mixture clears on heating, usually within an hour, because alcohol formed as a hydrolysis by-product dissolves the other components of the mixture. A suitable degree of hydrolysis-condensation is usually obtained if reflux is allowed to proceed for about one to four hours after the mixture clears. This step can be carried out at lower temperatures, but the rate is substantially slower.

The upper limit of permissible acid content during this initial hydrolysis-condensation is that beyond which gel formation occurs. The lower limit is determined by the desired reaction time. In general, the minimum reaction time to obtain satisfactory products is about one hour of reflux. Maximum and minimum allowable acid contents vary with the ratio of methyltrialkoxysilane and water used. The lower theoretical water content is Y/2, where Y is the average number of alkoxy groups attached to silicon throughout the mixture. Thus, when methyltrialkoxysilane is the sole silane constituent, the theoretical lower molar ratio of silane:water is 1115. When the molar silane:water ratio is 111.5, the minimum allowable acid content is about 50 parts of HCl per million parts of total methyltrialkoxysilane and water, and the maximum is about 650-700 parts on this same basis. When the molarzsilanezwater ratio is 1:3.0, the minimum allowable acid content ranges from a small positive amount which may be a very slight trace less than 1 p.p.m. HCl, e.g., 0.1-0.01 p.p.m. HCl; or it may be from 1 up to about 5 p.p.m. HCl or a little higher such as parts.

These limits have been carefully established but are necessarily subject to minor variation in each case, for several reasons. First, polymer formation by its nature will not proceed identically in any two runs and the particular mode of polymerization can alter slightly the acid sensitivity of the system. Second, use of other alkoxysilanes as comonomers in amounts previously specified can reduce acid sensitivity, since methyltrialkoxysilanes are most acid labile, but the effect will generally be small. Third, extremely small quantities of impurities in a given sample, impractical to remove, can alter acid sensitivity slightly. These factors, however, affect only the maximal and minimal extremes of allowable acid content, and the major portion of the suitable area indicated will be unchanged.

It is usually most convenient to reduce the acid content of the monomers to about zero weight part per million HCl by one of the methods previously described and, if necessary or desirable (as it usually is), then adjust the acidity of the initial reaction mixture by adding acid to the water used in the calculated amount required to impart the desired acidity to the starting mixture. Although generally, any acidic material soluble in the reaction mixture can be used, organic acids such as phenol and formic acid may sometimes be advantageous because they retard subsequent oxidation of the reactants.

Another preferred organopolysiloxane for use in accordance with this invention is a mixture of compounds as represented by Formulae III and IV:

wherein C H is phenyl.

During 'the in situ hydrolysis and polymerization, compounds III and IV link together by conjoint hydrolysis and condensation to form a copolymer. The molar ratio of the compound as represented by Formula III to the compound as represented by Formula IV can be from 1:10 to 10:1, with a more preforred ratio being about 1:5 to 5:1. A most preferred composition is produced by the hydrolysis and condensation of about 2 moles of the compound as represented by Formula III with about one mole of the compound as represented by Formula IV.

To avoid premature gelation of the resins, the quantity of acid in the reaction mixture must be below about 0.01 mole of acid per mole of hydrolyzable silanol precursor. Thus, it may be of the same order of magnitude as hereinbefore described with reference to the production of organopolysiloxanes from silanes of the kind embraced by Formulae I and II. Similarly, a solvent, e.g., ethanol, can be added to render the reaction mixture homogeneous.

The preferred water concentration for consideration in the production of copolymers made from monomers represented by Formulae III and IV above is from about 1.5 to about 3 moles, with a most preferred concentration being about 3 moles for every mole total of silane monomer (RO) present in the reaction mixture.

A further variation in the procedure can be achieved by hydrolyzing individually a hydrolyzable methyltrialkoxysilane and a hydrolyzable phenyltrialkoxysilane, and then combining the resultant organopolysiloxanes to form the initial reaction mixture described above. It is believed that the product formed in this way is a block copolymer of the constituent organopolysiloxanes.

The subject monomeric organosilane compounds that are represented by Formulae I, II, III and IV can be converted into the desirable organopolysiloxane coatings of the present invention by the following general procedure. The organosilane compound or compounds are hydrolyzed and partially condensed at a temperature of from about 50 to about C. for a period of time of from about 1 to about 10 hours, in the presence of at least a trace of acid and at least about 1.5 moles of water per mole of silane. This hydrolysis of corrpounds as represented by Formulae I to IV above is carried out in the presence of water as discussed above. The reaction conditions are then changed from a reflux to distillation and the temperature is maintained constant for a period of time of from about 1 to about 30 minutes to effect the removal of the by-product alcohol and excess water and thereby concentrate the solution of the partial condensation product of the above described reaction. The concentration step effects the further condensation of the liquid organopolysiloxane partial condensation product. The concentrated organopolysiloxaue product is then precured (advanced in cure without gelation) at a temperature of from about to about 250 C. for a period of time of from about /2 to about 24 hours to provide a liquid siloxane partial condensation product that is capable of being further cured to a thermoset polymer. This precured product is then cured at a temperature of from about 90 to about 200 C. for a period of time of from about 4 to about 168 hours.

The composition and preparation of the monomeric organosilane compounds and their subsequent polymerization and copolymerization, whereby there is obtained a concentrated, precured siloxane partial condensation product, is described in copending US. patent application, Ser. No. 306,344, now abandoned filed Sept. 3, 1963, US. patent application, Ser. No. 370,684, now abandoned filed May 27, 1964, US patent application, Ser. No.

520,893, filed Jan. 17, 1966, and US. patent application Ser. No. 545,579, now Patent No. 3,395,117 filed Apr. 27, 1966, these applications having an assignee that is common with the assignee of this application.

The formulation, polymerization and application to appropriate substrates of the monomeric organosilane compounds and organopolysiloxane compounds in accordance with this invention can be carried out in the presence of a solvent such as methanol, ethanol, butanol, acetone, ethyl acetate, benzene, xylene, toluene, ethylene glycol monomethyl ether, ethylene glycol monomethyl ether acetate, ethylene glycol ethyl ether, ethylene glycol ethyl ether acetate, ethylene glycol ethyl butyl ether, ethylene glycol butyl ether acetate, ethylene glycol dibutyl ether, ethylene glycol monohexyl ether, ethylene glycol monophenyl ether, etc.

It is to be noted that the invention at hand can utilize solutions of the above described organopolysiloxane resins or the powdered forms of these resins. Likewise, prepolymer and precured forms of these resins can be utilized.

This invention can be utilized to control the penetration of organopolysiloxane resins into a plurality of sub- I strates, which comprise woven or nonwoven mats of woods, softwoods, and woody annual plants such as balsam fir, eastern hemlock, jack pine, eastern white pine, red pine, black spruce, red spruce, white spruce, tamarack, cyprus, quaking aspen, American beech, paper birch, yellow birch, eastern cottonwood, sugar maple, silver maple, yellow poplar, black cherry, white oak, bagasse, hemp, cotton and jute; mixtures of these cellulosic materials can also be used.

Examples of suitable synthetic fibers are those that are formed from polyamides, polyesters, polyaldehydes, polyolefins, acrylics, etc.

The invention at hand can also utilize inorganic fibers such as fibers formed from glass, asbestos, titanium dioxide, etc.

Fibers of metals such as iron, copper, aluminum, zinc, etc. can also be utilized.

The process of this invention is particularly adapted to controlling the penetration and pick-up of organopolysiloxane resins into paper and paperlike products.

Examples of paper and paperlike products which are adapted for use in accordance with this invention are alpha, cellulose, kraft pine liner, corrugated media, bleached hardwood, etc. The invention at hand can also utilize substrates such as wood, and wood products, i.e., chipboard, slate board, etc.

The following examples will illustrate the subject invention. These examples are given for the purpose of i1- lustration and not for purposes of limiting this invention. (All parts percent are given by weight unless otherwise specified.)

Example I A southern pine kraft pulp sheet was immersed momentarily in a .1 percent hydrochloric acid solution which has a pH of 1.1. The pretreated kraft sheet was then airdried. A uniform coating of a partially cured powdered organopolysiloxane resin was then applied over the pretreated dried kraft sheet. The powdered organopolysiloxane resin was prepared by the hydrolysis and condensation of two moles of methyltriethoxysilane with one mole of phenyltriethoxysilane.

The powdered coated pretreated kraft surface was then exposed to a temperature of about 300 C. at a pressure of 150 p.s.i.g. for a period of time of 25 minutes. The

its

organopolysiloxane resin penetrated the kraft substrate to produce a finished product which had a limited degree of flexibility and non-gloss surface.

Example II The coating procedure as discussed in Example I was repeated except that the pretreating solution was a .1 percent solution of toluene sulphonic acid which had a pH of 2.5. Again, the organopolysiloxane resin completely penetrated the fibrous substrate and produced a coated article of limited flexibility which had a non-gloss surface.

Example III The coating procedure in Example I was repeated. However, a pretreated solution which comprised a .l percent amino-acetic acid solution having a pH of 7.2 was utilized. The resulting coated sheet had properties which were identical to those discussed in regard to Example II.

Example IV Example V Using the coating procedure of Example I, a kraft sheet was coated with a pretreating solution which comprised a .1 percent solution of diethyl ethanol amine which 8 had a pH of 10.6. The finished coated surface had physical properties which were identical as those discussed in Example IV.

Example VI Using the coating procedure as taught in Example I, a pine kraft substrate was pretreated with .a .1 percent solution of sodium hydroxide which had a pH of 11.8. In this case the organopolysiloxane resin did not penetrate into the pine kraft substrate but instead produced a surface having excellent gloss. The resulting article was somewhat brittle due to the fact that the organopolysiloxane did not penetrate into the pine kraft substrate. The resin pick-up was minimal.

Example VII A kraft pulp sheet was immersed in a 0.5% NaOH solution. After oven drying, the sheet was treated with a 25% prepolymer solution in methanol, or organopolysiloxane. The resin was prepared by the hydrolysis and condensation of two moles of methyltriethoxysilane and one mole of phenyltriethoxysilane, as discussed in Example I. The solvent was evaporated and the resin cured at 280 C. at p.s.i.g. for 25 minutes.

The resulting sheet had similar gloss as the sheet described in Example VI.

What is claimed is:

1. In the process of treating a fibrous substrate by applying thereto an organopolysiloxane resin and subsequently curing said resin on said substrate, the improvement wherein the pick-up of the organopolysiloxane resin by said substrate and the depth of penetration of said resin into said substrate are controlled, said improvement consisting essentially of the steps of (l) applying to said fibrous substrate, prior to the application of said organopolysiloxane resin, a basic solution having a pH of from about 8 to about 14, the amount of pick-up and depth of penetration of said resin being dependent on the pH of said solution, the higher the pH of the solution, the lower the amount of pick-up and pentration, and (2) drying said solution-containing substrate prior to applying to said substrate said organopolysiloxane resin.

2. The process of claim 1 wherein the substrate consists of sheet formed from pine kraft pulp.

3. The process as defined in claim 1 wherein said basic solution is a solution of a member selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide, diethyl ethanolamine, tetraethylenepentamine and tetraethylammonium hydroxide.

4. The process of claim 1 wherein the organopolysiloxane resin is the product of hydrolysis and condensation of at least one monomer coming within the scope of the formula 3 and (OC2H5 3 7. The process of claim 6 wherein the organopolysiloxane resin is the hydrolysis and condensation product of two moles of CH Si(OC H and one mole of 8. The process of claim 1 wherein organopolysiloxane resin is utilized in a powdered heat-softenable, solventsoluble form.

9 10 9. The process of claim 1 wherein the pH of the basic 2,691,604 10/1954 Priest 11760 solution is from about 10 to about 13. 2,780,560 2/1957 Hanley 117-60 3,002,848 10/1961 Clark 117-5.5 X R f r n s Cited 3,083,118 3/1963 Bridgeford 117-47 UNITED STATES PATENTS 5 3,146,121 8/1964 Turner 1176O 1,825,178 9/1931 Coghill 11760 X 32:5 2,251,296 8/1941 Shipp 11760 2,321,072 6/1943 Eiseman 117-60 MURRAY KATZ, Primary Examiner 2,375,998 5/1945 McGregor et a1. 2 3 259 10 1945 Norton 117 154 X 10 RAYMOND M. SPEER, ASSIStaIIt Exammer 2,428,608 10/1947 Bass. 2,441,320 5/1948 Hyde, 2,460,795 2/1949 Warrick 2 117161 11721, 47, 49, 54, 148, 155, 161

3 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5, ,7 5 Dated December 2, .1969

llhlblL'j/ A. Hider and Walter Kitaj It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

601. 2, line 46 change the comma to a period Col. 5, line 56,

concencrecioin should be --concentration--; line 6 "adtion" should be --action--. Col. 5, line 15, insert --part-- between "1 and up"; line 62, preferred" should be --preferred- Col. 6, line 48, "flled" should be --filed--. Col. 8, line 59, pencration" should be penetration--; line 58, "aloxy" should be --alKoxy--; line 65, last number in line should be a sub 5 instead of a super 5.

:MMD Mia. QEALEP oars-m I Attest:

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