GB2045823A - A method of improving the water repellency of a cellulosic web - Google Patents

Abstract

A method of improving the water repellency of a cellulosic web, such as paper, involves treating the web with an alkyl alkoxysilane containing at least one methyl group and at least two lower alkoxy (1-2c) groups in the presence of a condensation catalyst, and curing the resulting treated web for a time sufficient to increase the water repellency thereof. The treatment can be carried out in the presence of a solvent or in the absence of a solvent.

Description

SPECIFICATION A method of improving the water repellency of a cellulosic web The present invention relates to the treatment of cellulosic webs, such as paper webs, for the purpose of rendering at least one surface thereof water repellent while retaining the inherent porosity and strength of the paper web.

There have been some disclosures in prior patent specifications regarding the treatment of various materials with organic silicon compounds in order to render them water repellent. For example, in United States Patent Specification No. 2,306,222 there is a disclosure of treating various materials, including paper, with an organo-silicon halide such as methyl silicon chloride in vapour form to render the body water repellent. It was hypothesised that the organo-silicon halide vapours react with an adsorbed film of water to form the corresponding silicol which is strongly adsorbed and water repellent, or results in the formation of a water repellent silicone.

United States Patent Specification No. 2,386,259 refers to the treatment of fabrics or paper to make them water repellent by treating such materials with the product obtained by the hydrolysis of a methyldihalogensilane.

United States Patent Specification No. 2,412,470 describes a process for treating a solid body to render it water repellent which involves treating the same with a mixture containing 2.8 to 99.2% by weight of trimethyl silicon chloride and 97.2 to 0.8% by weight of silicon tetrachloride.

United States Patent Specification No. 2,961,338 refers to a process of treating wool to render the same water repellent by reacting it with an organo-silicon halide in vapour form while the wool is at a relatively low moisture content.

United States Patent Specification No. 2,995,470 describes a continuous process for treating material with vapours of a waterproofing substance such as an organo-silicon halide wherein vapours of the treating reagent mixed with an inert carrier gas are introduced into an enclosed treating zone while a length of the material to be treated is passed continuously through the zone. Downstream from the reaction zone the spent vapours, including the by-products, are exhausted from the system. The objective is to remove the by-products as rapidly as possible so that they do not have an opporutnity to build up appreciably in concentration.

Finally, United States Patent Specification No. 3,856,558 suggests rendering cellulosic materials water repellent by contacting the same, while they have a water content between 2 and 7 weight percent, with vapours of a lower alkyl silicon halide which reacts with water to form a siloxane and maintaining the cellulosic material and halide in contact for between 0.1 and 8 seconds. The conditions are such that the cellulosic material being contacted is rendered water repellent and has a pH greater than 2.5. The objective of this technique is to eliminate the subsequent step of neutralising hydrogen chloride formed as a by-product of the alkyl silicon halide reaction with water.

While these disclosures appear in the prior art, there has not yet been produced a commercially acceptable method and composition for treating paper or other cellulosic products with silane solutions. In those instances in which an alkyl silicon halide is used, the disposition of the resulting hydrogen chloride vapour has posed serious problems because of the corrosiveness of the hydrogen chloride, the contamination of the atmosphere, and the weakening of the paper strength by its presence.

The treating method of the present invention involves treating a cellulosic web, for example of paper or paperboard, with an alkyl alkoxysiloxane, as such, or in pre-hydrolyzed form wherein it is essentially in the form of a hydroxysilane. The starting materials for the purposes of the present invention are identified by the following generic formula:

where A is H, CH3, or B and B is an OR group in which R is methyl or ethyl.

The method of the present invention can supplement or replace conventional wet end sizing systems. It may be applied to the dry web by means of a spray giving maximum efficiency of use of the treating chemical. The reaction by-products, consisting of a lower alcohol, are non-corrosive and of low toxicity. The effect of the treatment on paper is to produce a strong resistance to water while retaining the same porosity and strength characteristics of the untreated sheet. The treatment of the present invention can be applied to one or both sides of the sheet and the degree of water resistance can be controlled by controlling the severity of the treatment.

The alkyl alkoxysilane may be applied to the paper as such in the form of a spray, but is preferably introduced in the form of a solution in a suitable organic solvent such as a lower alcohol containing from 1 to 4 carbon atoms.

To cause the reaction between the alkoxysiloxane or its hydrolysis product to occur with the paper or other cellulosic base within reasonable times, it is desirable to include a catalyst in the treating mixture. Such catalyst may be a strong mineral acid, an alkali metal peroxide, or an organometallic catalyst dissolved in a non-reactive organic solvent.

After the spray application of the material to the web, the sprayed material can be cured by means of subjecting the same to a slightly elevated temperature of about 300"F (147"C) or so for 15 to 60 seconds, or by leaving it at room temperature for at least 30 minutes.

The invention includes within its scope a cellulosic web having improved water repellency on at least one surface thereof while retaining substantially all of its natural porosity toward gases, said surface comprising a silicone polymer produced by reaction of water in said web with a hydroxysilane which itself is the hydrolysis product of an alkyl alkoxysiloxane having the formula:

where A is H, CHB or B and B is an OR group in which R is methyl or ethyl.

The following is a detailed description of preferred methods according to the invention, reference being made to the accompanying drawing which is a somewhat schematic view of a continuous system which can be used for applying the waterproofing material to a travelling paper web, and collecting and re-using the by-products.

As previously indicated, in accordance with the present invention, a cellulosic web has its water repellency improved by treatment with an alkyl alkoxy siloxane or its hydrolysis product, the alkyl alkoxysiloxane having the following formula:

where A is H, CH3, or B and B is an OR group in which R is methyl or ethyl.

Examples of the alkoxysiloxanes coming under this generic expression are given below: A. Methyl trimethoxysilane:

E. Methyl dimethoxysilane:

These alkoxysilanes react with water and cellulosic hydroxyl groups to produce a silicone polymer. The only by-product produced is a lower alcohol formed by hydrolysis of the alkoxy group. The alcohol by-product is relatively easy to dispose of, in contrast to hydrogen chloride, for example. Moreover, the surroundings can tolerate more of the alcohol than hydrogen chloride, as evidenced by the fact that the Environmental Protection Agency puts a limit of about 0.5 parts per million on hydrogen chloride in the atmosphere, but the limit on methyl alcohol is of the order of 1,000 parts per million.

The moisture content of the paper or other cellulosic base is not critical as long as it is within a reasonably dry range of about 2 to 14% by weight. The reaction proceeds rapidly and to completion, particularly in the presence of a catalyst. The overall reaction is aiong the following lines:

silicone polymer The silicone polymer sticks to the cellulose fibres forming a hydrophobic surface which provides water repellency.

While the silane can be used as such, it is preferable to combine it with a diluting solvent so that it provides a solution of about 1 to 3% by volume of the silane. This provides the optimum coverage with the least waste. An average water repellent coating on paper can be produced with a solution containing 1.5 to 2% by volume of the silane. The solvent used to dilute the silane can be almost any volatile, an hydros non-reactive organic liquid. The solvents which appear to work best for the solution are the lower alcohols containing 1 to 4 carbon atoms, such as ethanol or methanol. Since the by-products are themselves alcohols, the use of alcohol solvents poses no additional disposal problem.

The speed and the extent of the polymerisation is dependent on the type of catalyst used. There are three different categories of catalysts which are useful to polymerise the silanes: strong acids, peroxide condensation catalysts, and organometallic condensation catalysts. Concentrated strong acids include materials such as sulphuric or nitric acid. Sulphuric acid appears to work best when added to the silane solution in the ratio of 1 ml of concentrated (95%) commercial acid to 250 ml silane solution. When the catalyst is added to the silane solution, a small amount of white precipitate may form due to water in the acid reacting with the silane. The paper is treated by spraying with the resulting solution and then curing.The curing can be accomplished either by heating at 3000F (148"C) for 15 to 60 seconds, or by allowing it to sit at room temperature for at least 30 minutes. Curing at room temperature is not very efficient because some silane loss will occur due to evaporation.

When treating the solution with a peroxide catalyst such as an alkali metal peroxide, the solvent can be the same as used in the case of a strong acid. In this technique, it is desirable that the silane be hydrolysed before the peroxide catalyst is added. To accomplish this, a molar ratio of water equivalent to the number of alkoxy groups on the silane is added to the solution. An acid such as glacial acetic acid is added in a proportion of 1 ml acid to 400 ml solution to catalyse the hydrolysis. The solution should sit for at least one-half hour to make sure that the silane is completely hydrolysed. The hydrolysis reaction proceeds as follows:

A peroxide can be added to the hydrolysis product in the ratio of about 1 gram peroxide to 500 ml solution.

The best peroxide catalysts are sodium peroxide and potassium peroxide. This solution may be applied as a spray and the paper cured for 5 to 30 seconds at 300"F (147"C) for 5 to 15 minutes at room temperature.

These solutions are somewhat basic and therefore raise the pH of the paper. This does not necessarily damage the sheet although it is desirable to hold the paper as close to its original pH as possible.

In treating with an organometallic catalyst, the solvent for the silane must be anhydrous and cannot be an alcohol. This is because the catalyst will react with water and alcohols preventing catalysation of the polymer. The solvent should be a non-reactive organic solvent such as benzene, pentane, trichloroethane, or trichloroethylene. Trichloroethylene is the preferred solvent because it is non-flammable and has a low toxicity. A typical catalyst is tetra-iso-propyl titanate (TPT). This catalyst is added to the solution in the ratio of 1 ml TPT to 200 ml solution. The solution is most advantageously applied by spraying. The polymerisation reaction is rapid and needs no curing or post-heating. Water repellency is imparted to the paper at the moment the spray contacts it.It has also been observed that during a period of several hours after the treatment there is a slight increase in the water repellency. This is due to short-chain silicone polymers joining to form longer chains, thus giving a slight increase in hydrophobicity. This solution does not affect the pH of the paper.

Organometallic tin compounds such as dibutyltin laurate, stannous oleate and tetrabutyl orthotitanate can also be used as catalysts. Tin compounds may be employed if the end use of the paper permits the presence of tin in the residual silicone.

Afurther description of the present invention will be made in conjunction with the attached drawing which illustrates rather schematicai ly a continuous system for treating paper with the improved process of the present invention.

In the accompanying figure, reference numeral 10 has been applied to a container of silane solution, usually in the form of a 1 to 3% by volume solution in a suitable organic solvent, but as will be hereinafter explained, the solvent need not always be present. The solution is withdrawn from the container 10 through a valve 11 where it is combined with a catalyst from a container 12. Metered amounts of the catalyst are provided from the container 12 through a valve 13 to be combined with the silane solution. The mixture of the two then passes through a conduit 14 into a pair of spray heads 15 and 16 which are arranged to treat opposite sides of a paper web 17.The paper web is guided over an upper guide roll 18, a lower guide roll 19, and a second upper guide roll 20 before it leaves the treating vessel generally indicated at reference numeral 21 in the drawings. Air forthe spray heads 15 and 16 is supplied from a suitable source of compressed air (not shown) through a valve 22 and is delivered to each of the spray heads.

The reaction between the catalyst-containing silane solution and the paper fibres is quite rapid. The only by-products of the reaction are the alcohol produced, the solvent used in the treating solution, and any unreacted silane. These products are directed by means of an effluent line 23 into a condenser 24. Here they are combined with additional amounts of alcohol which are recovered from the travelling web by means of vacuum boxes 25 and 26 positioned at the exit end of the vessel 21. The condensed materials, usually consisting mainly of alcohols, can then be recycled by means of a conduit 27 back into the silane solution container 10.

The following specific examples illustrate the manner in which the invention is carried out, and the improved results achieved.

Example I A sample of 100 Ib. bleached, unsized board was treated by spraying with a treating solution containing approximately 2% by volume methyl trimethoxysilane, with the balance methanol and a catalyst consisting of 1 ml of sulphuric acid per 250 ml of solution. The treated sample was then heated for 15 seconds in a 300"F (147"C) oven. The sample became quite water resistant.

Example II A sample of 100 Ib. bleached, unsized board was treated as in Example I, using dimethyl diethoxysilane as the treating agent. Results substantially the same as those in Example I were obtained.

Example lII Samples of 100 Ib. bleached, unsized board were treated by spraying with a solution containing approximately 2% by volume methyl trimethoxysilane, water in an amount of 3 moles of water for every mole of silane, acetic acid in an amount of 1 ml acid for 400 ml solution, in a lower alcohol solvent, using a catalyst of sodium peroxide in an amount of 1 gram per 500 ml solution. The samples were heated for varying periods of time in a 300"F (147"C) oven. All of the sheets developed similar water resistant properties.

Example IV A sample of 325 Ib. bleached blotting paper was treated with the same solution as in Example III and heated for 15 seconds in a 300"F (147"C) oven. The paper developed excellent water resistant properties, and was resistant to continuous running water.

Example V A sample of 325 Ib. bleached blotting paper was treated by spraying with a solution containing approximately 2% by volume methyl trimethoxysilane, dissolved in benzene, and containing TPT in an amount of 1 ml per 200 ml of solution. No heating was done. The treated sheet was very water resistant, including resistance to running water.

Example Vl The paper stock was treated as in Example V, but given only a very light spraying. The sheet was slightly water resistant to drops of water, but not to running water.

Example VII The paper was treated with the same composition as in Example V, but with a very heavy spraying. The sheet which resulted was very resistant to water under any conditions.

Example Vffl This example utilised the same composition and procedure as in Examples V and VII, but the paper was treated on one side only. Both lightly treated and heavily treated papers were resistant on the treated side, with the greatest resistance in the heavily treated sheet. The untreated side of the heavily treated sheet showed a very slight water resistance.

Example IX A sample of 100 Ib. N.S.S.C., unbleached board was treated by spraying with the composition described in Example V. No heating was done. The treated sheet was water resistant, although only slightly resistant to running water.

Example X The procedure here was the same as in Example IX, except the board was given only a light treatment. The resulting product was very slightly water resistant.

Example Xl This procedure and composition was the same as used in Example IX, only a heavy treatment was applied.

The resulting board was very water resistant, including resistance to running water.

Example XII The composition and method were the same as in Examples IX and Xl, except that the board was treated on one side only. The normally treated side was slightly water resistant, and the untreated side had no water resistance. When heavily treated, the sheet was very water resistant on the treated side and slightly resistant on the untreated side.

Example XIII A sample of newsprint was treated with the solution set forth in Example V. An increased water resistance was developed in the sheet.

Example XIV The silane may be applied to a moving web without the use of a diluting solvent. In this case, a catalyst such as TPT should be present in amounts of from 10 to 15% by volume of solution for maximum polymerisation efficiency. The application to the moving web must be in the form of a fine spray that atomises but does not vaporise the silane. The treating chamber should be well ventilated to remove any vaporised silane and titania by-product from the reaction.

The application of the silane solution is most easily controlled by applying it as a spray or in the form of an aerosol. The spray can be adjusted to give the degree of coating desired as well as an even application. The solvent remaining in the paper can be removed by means of a vacuum or by heating.

Physical testing was done on some of these samples produced according to the examples set forth above.

In the following table, "MD" refers to testing in the machine direction, and "CMD" refers to the cross machine direction. The Z-tensile strength tests were performed by applying a double-sided adhesive tape to the paper and stretching the same in an Instron machine. The brightness figures were derived from a General Electric brightness meter. The Unger oil test consisted in clamping a ring on the paper, pouring on oil under the ring, wiping off excess oil, and then weighing the sheet. The water absorption test was performed by applying about 0.1 ml of water to the paper, reflecting a light off it, and measuring the time elapsed until the reflection stops.

% Ave. Tensile Average Ave. Unger Oil Water Absorption (min) Sample Moisture pH Burst Z-tensile Bright Factor MD CMD Strength ness Wire Felt Wire Feit Untreated 7 8.960 14.52 - 30.7 35.20 84.8 167.0 148.1 0.23 0.23 IV 7 9.100 14.93 - 31.2 36.90 84.7 116.40 136.70 oo 53.00 V 7 7.450 15.69 69.6 30.8 35.00 83.3 106.80 123.40 oo oo VI 7 7.710 14.76 67.4 29.8 36.46 83.6 145.45 173.10 9.80 6.72 VII 7 7.900 13.10 62.2 29.3 32.56 83.6 105.85 79.25 23.07- > oo 11.16- > oo VIII normal 7 - 15.34 67.7 33.0 33.54 83.2 150.00 - 0.08 12.40 VIII heavy 7 - 14.86 68.6 30.5 32.93 84.1 129.70 - 0.33 50.25- > oo Untreated 9.59 7.320 11.35 25.4 11.9 66.71 11.40 121.00 91.85 0.70 0.73 IX 9.59 7.270 12.26 25.3 12.5 67.68 11.50 94.50 82.80 oo 48.35 X 9.59 7.240 12,81 22.4 11.5 64.76 11.75 94.50 87.80 1.25 1.08 XI 9.59 7.270 12.04 22.4 11.7 67.93 11.23 94.00 84.25 20.75 5.86 XII normal 9.59 - 12.93 23.4 12.8 67.07 11.50 100.40 91.90 0.53 2.30 XII heavy 9.59 - 12.23 25.0 12.0 64.88 11.20 87.90 74.50 2.15 8.72- > oo

Claims (20)

1. A method of improving the water repellency of a cellulosic web which comprises treating said web with an alkyl alkoxysiloxáne having the formula:

where A is H, CH3, or B and B is an OR group in which R is methyl or ethyl, in the presence of a condensation catalyst and curing the resulting treated web for a time sufficient to increase the water repellency of said web.

2. A method according to claim 1, wherein said alkyl alkoxysilane is methyl trimethoxysilane.

3. A method according to claim 1, wherein said alkyl alkoxysilane is methyl dimethoxysilane.

4. A method according to claim 1, wherein said alkyl alkoxysilane is methyl triethoxysilane.

5. A method according to claim 1,wherein said alkyl alkoxysilane is methyl diethoxysilane.

6. A method according to any of claims 1 to 5, wherein said alkyl alkoxysiloxane is dissolved in a volatile liquid solvent which is non-reactive toward said alkyl alkoxysiloxane.

7. A method according to claim 6, wherein said solvent is an aliphatic alcohol containing from 1 to 4 carbon atoms per molecule.

8. A method according to claim 6 or claim 7, wherein said alkyl alkoxysiloxane is dissolved in said solvent in an amount of from 1 to 3% by volume.

9. A method according to any of claims 1 to 5, wherein said alkyl alkoxysilane is applied to the web as a finely atomised spray in the absence of an added solvent.

10. A method according to any of claims 1 to 5, wherein said alkyl alkoxysilane is pre-hydrolysed prior to application to said web.

11. A method according to any of the preceding claims, wherein said catalyst is a strong mineral acid.

12. A method according to any of claims 1 to 10, wherein said catalyst is a peroxide condensation catalyst.

13. A method according to claim 12, wherein said peroxide is an alkali metal peroxide.

14. A method of improving the water repellency of a cellulosic web substantially as hereinbefore described with reference to any of Examples I to XIV.

15. A cellulosic web which has been treated by the method according to any of the preceding claims.

16. A cellulosic web having improved water repellency on at least one surface thereof while retaining substantially all of its natural porosity toward gases, said surface comprising a silicone polymer produced by reaction of water in said web with a hydroxysilane which itself is the hydrolysis product of an alkyl alkoxysiloxane having the formula:

where A is H, CH3 or B and B is an or group in which R is methyl or ethyl.

17. A cellulosic web according to claim 16, wherein said alkyl alkoxysiloxane is methyl trimethoxysilane.

18. A cellulosic web according to claim 16, wherein said alkyl alkoxysiloxane is methyl dimethoxysilane.

19. A cellulosic web according to claim 16, wherein said alkyl alkoxysiloxane is methyl triethoxysilane.

20. A cellulosicweb according to claim 16, wherein said alkyl alkoxysiloxane is methyl diethoxysilane.