US3600123A - Modification of cellulosic textile materials with formaldehyde and cyanamide using ammonium salt catalysts - Google Patents

Abstract

A PROCESS FOR TREATING A CELLULOSIC TEXTILE MATERIAL TO IMPROVE THE DIMENSIONAL STABILITY AND CREASE RESISTANCE THEREOF WHEREIN FORMALDEHYDE AND CYANAMIDE ARE REACTED AT AN ELEVATED TEMPERATURE IN THE PRESENCE OF A CATALYST AND WATER, AND IN CONTACT WITH THE MATERIAL IS IMPROVED BY EMPLOYING AS THE CATALYST, A SALT SELECTED FROM THE GROUP CONSISTING OF AMMONIUM DIHYDROGEN PHOSPHATE, DIAMMONIUM HYDROGEN PHOSPHATE, TRIAMMONIUM PHOSPHATE, DIAMMONIUM SULFATE, DIAMMONIUM SULFITE, AMMONIUM SULAMATE, AND MIXTURES THEREOF.

Description

United States Patent 3,600,123 MODIFHCATION OF CELLULOSIC TEXTILE MA- TERIALS WITH FORMALDEHYDE AND CYAN- AMIDE USING AMMONHUM SALT CATALYSTS Hugh J. E. Campbell, Clarkson, Ontario, Canada, assignor to Ontario Research Foundation, Toronto, Ontario, Canada No Drawing. Filed Mar. 24, 1967, Ser. No. 625,602 Claims priority, application Great Britain, Mar. 30, 1966, 14,185/66, 14,186/66 Int. Cl. D06m 13/14, 13/36 US. Cl. 8-1163 8 Claims ABSTRACT OF THE DISCLOSURE A process for treating a cellulosic textile material to improve the dimensional stability and crease resistance thereof wherein formaldehyde and cyanamide are reacted at an elevated temperature in the presence of a catalyst and water, and in contact with the material is improved by employing as the catalyst, a salt selected from the group consisting of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, diammonium sulfate, diammonium sulfite, ammonium sulfamate, and mixtures thereof.

This invention relates to processes for modifying cellulosic textile materials, which processes can improve their dimensional stability as well as the ability of such materials to recover from deformation. More particularly, the invention relates to providing cellulosic textiles, including fibres, yarns, and particularly fabrics with an ability to recover from deformations such as creasing, wrinkling, crushing and the like, and also with a readiness to return to a configuration in which they are placed during treatment by a process embodying this invention. The invention can have particular application in the imparting of a so-called permanent crease tocotton fabrics. The invention further may be applied to fabrics containing a mixture of cellulosic fibres and synthetic fibres.

Cellulosic textile materials, such as cotton or viscose rayon fabrics, are known to possess poor resilience, i.e., resistance to creasing and wrinkling when subjected to localized external forces, and they require frequent pressing or ironing to restore them to the desired configuration. Cellulosic fabrics do not have good shape retention properties. For example, pleats and pant creases are difficult to retain during use and laundering of the fabric. In addition, these fabrics have poor dimensional stability, as is evidenced by their tendency to shrink and stretch during laundering.

Processes have been developed for improving the crease recovery properties and/or dimensional stability of cellulosic textile materials, which processes include treating or reacting them with various chemical reagents. One of the first known and most widely used methods involves treatment with N-methylol derivatives of certain nitrogen-containing organic compounds. Among those compounds which have achieved commercial importance are ureas, triazines and triazones. Other nitrogen-containing compounds which have been suggested and which have achieved a development.

At present, it is customary for mills to purchase the pre-formed N-methylol precondensate which is prepared from the raw materials as a separate operation. This may require several hours and careful control of temperature and pH.

Further, the impregnating or pad bath may be, in some cases, relatively unstable, and undue polymerization of the precondensate can render a bath inoperative 3,600,123 Patented Aug. 17, 1971 ICC in a short period of time. For example, With N-methylol dihydrazides, gelling of the pad bath may occur Within a very few minutes after mixing, depending upon concentration, temperature, and pH conditions.

Some of the disadvantages of processes for treatment of cellulosic textile materials can be partially controlled through judicious choice of the catalyst utilized. Although many types of catalysts have been proposed, those most commonly employed to set or fix N-methylol precondensates to cellulosic textiles are the metal salts of inorganic acids, such as magnesium chloride, zinc chloride and zinc nitrate. Of these, zinc nitrate often is preferred due to its relatively high efiiciency in catalyzing what is generally considered to be, at least in part, a cross-linking reaction with the cellulose.

It is an object of the present invention to provide a process for improving the deformation recovery and dimensional stability of cellulosic textile materials (which expression is hereinafter used to include textile materials formed of a mixture of cellulosic and synthetic fibres), while minimizing the disadvantages attending the prior art processes. This object is achieved in one embodiment of the invention by employing an aqueous solution containing cyanamide, formaldehyde and a particular catalyst.

The objects of the invention are accomplished by an improvement of a process wherein formaldehyde and cyanamide are reacted in the presence of a catalyst and in contact with a cellulosic textile material, the improvement being using, as the catalyst, an ammonium salt of an inorganic acid, the salt being selected from the group consisting of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, diammonium sulfate, diammonium sulfite, ammonium sulfamate, and mixtures thereof.

In one embodiment of the present invention, the cellulosic textile material is impregnated in a bath with an aqueous solution containing cyanamide, formaldehyde and one of the above-identified ammonium salts. The textile material then may be squeezed, e.g., in squeeze rolls or the like to remove any excess bath liquor. Satisfactory results have been obtained if the textile is squeezed to a wet pick-up of about 60% to about 130%, preferably about 70 to about The goods then are dried and cured at a suitable elevated temperature. The material can be dried and cured in separate steps or simultaneously. Catalyst residues then may be removed by washing the textile material. The aqueous treatment bath will have a pH generally between 2.5 and 4. Aqueous solutions of the above identified ammonium salts have pHs from about 4.5 to about 9. On addition of the formaldehyde to the aqueous solution, the pH drops to within the range 2.5 to 4. Addition of further ammonium salt to the solution does not affect the pH when a minimum amount, different for each salt, has been added, indicating an equilibrium has been reached. The pH of 2.5 to 4 for the precondensate solution is not sufficient to subject the textile to acidic degradation. The precondensate is a water-clear solution. It has been found that aldehydes other than formaldehyde do not generate the necessary acidity in the aqueous solution, and the present invention therefore is restricted to formaldehyde. The preparation of the pad bath may be carried out in any convenient manner. One method which can be employed involves adding formaldehyde, either as such or as a 37 to 40% formalin solution, methanol hemiformal, or methyl formcel, to an aqueous solution containing the cyanamide and catalyst. The concentrations of cyanamide and catalyst are such that after the addition of the calculated amount of formaldehyde, the reagent concentrations fall within the range desired.

Of course, other methods for impregnating the material with formaldehyde, cyanamide and the catalyst can be employed. Thus, a vapour technique may be used, for example. In this event, the textile material is impregnated with an aqueous solution of a mixture of cyanamide and one of the above listed ammonium salts, and any excess liquor may be removed by means of squeeze rolls or the like. The material then is exposed to the vapours of formaldehyde, either as formaldehyde per se, or volatilized from an aqueous solution of hemiformal, or volatilized from a methanolic solution of methanol hemiformal for a suitable period of time, or by heating a suitable polymeric form of formaldehyde, such as paraformaldehyde. The material then is dried, and cured at a suitable elevated temperature to effect chemical reactions with the cellulose substrate. The drying and curing may be carried out sequentially or simultaneously. The amount of ammonium salt used in the impregnation solution should be sufficient to produce a pH of 2.5 to 4 when the textile is contacted with the formaldehyde vapours. Generally, the minimum amount is the amount sufficient to produce about 0.5% concentration in the solution. The impregnation of the textile by the catalyst and cyanamidecontaining solution may be carried out on a batch or continuous basis. A single pad bath containing both cyanamide and catalyst is preferred, although two separate impregnation baths may be used. This latter technique suffers from the disadvantage that part of the compound padded on in the first bath can be lost by diffusion into the second bath. Any excess liquor may be removed with squeeze rolls, but care has to be taken not to remove so much liquor that low wet pick-ups result. Wet pickups in the range of about to about 130% (based on the dry weight of the textile) have been found satisfactory, and the preferred range is from about to about The exposure to formaldehyde vapour preferably takes place in a closed or substantially closed system for economic reasons. The exposure conditions, e.g., time, temperature, vapour concentration, pressure and humidity, should be such as to provide a sufficient formaldehyde pick-up (whether reacted or adsorbed) such that the final curing step wherein the fabric is baked will effect a chemically bonded N-methylol-cellulose system. Instead of using a closed vapour system, the formaldehyde vapour could be swept into a treatment chamber, allowed to impregnate the textile and react with the cyanamide and the residual vapour vented. It is preferable to use a continuous process such as a finishing procedure similar to that described by Francis and Staples in the Textile Research Journal, 33 (1963), 583. Thus, a roll of damp fabric, impregnated with a solution of cyanamide and catalyst is led through a vapour-tight slit into an oven containing formaldehyde vapour, the oven being maintained at a suitable temperature. After an appropriate dwell time, the fabric is led out of the oven via a similar exit slit and thence into a second oven maintained at the desired curing temperature. If anhydrous formaldehyde gas is used, such as is produced by depolymerization of paraformaldehyde, the exposure temperature can be raised to coincide with the normal curing temperature, thereby achieving exposure, drying and curing in one chamber. With a vapour phase process it is possible to exercise better control over the final properties of the finished material through manipulation of the conditions used to carry out the vapour phase process.

The curing of the fabric is effected at an elevated temperature above about C. and up to about 180 C. The length of time necessary for curing depends upon the curing temperature, shorter times being sufficient at high temperatures, and is usually about 1 to 20 minutes. The preferred curing conditions have been found to be about to about C. for about 2 to 10 minutes.

During the drying and baking, the fabric can be held flat or in any other desired configuration such as that for imparting a permanent crease to the fabric, and can be held either loose or under tension.

The treatment of cellulosic textile materials with cyanamide and formaldehyde has been proposed previous to the present invention. US. Pat. No. 2,234,905 (Tallis), issued Mar. 11, 1941, discloses the treatment of textile materials with the reaction product of cyanamide and formaldehyde. According to this patent, the cyanamide and formaldehyde are reacted under acid or neutral conditions in the absence of a catalyst, and the incorporation of the condensation product into the material is said to improve the afiinity of cellulose for dyestuffs. No mention is made of crease recovery, and it has been found that cotton fabric which has been treated with cyanamide and formaldehyde in the absence of catalyst shows yellow discolouration, which often is intensified on ironing, is strongly susceptible to chlorine damage, has a harsh boardy hand, is difficult to wet out, and processes poor crease recovery properties.

US. Pat. No. 2,247,353 (Auer), issued July 1, 1941, suggests the treatment of cellulosic textiles with cyanamide and formaldehyde to improve their crease and crush-resisting properties. According to this patent, the textile materials are impregnated with a solution of cyanamide and formaldehyde in the presence of an organic acid as a condensation catalyst. A series of tests were carried out on cotton fabrics using the process described in this patent, using a mixture of acetic acid and tartaric acid, a mixture of oxalic acid and formic acid, and tartaric acid alone as catalysts. The fabrics produced showed less than a satisfactory crease recovery improvement, prohibitively high strength loss and in some instances, noticeable yellowing of the fabric. Depending on the particular organic acid used, treated cellulosic fabrics may or may 1101: Show yellow discolouration combined with crease recovery improvement which ranges from poor to moderate. In this patent the acidity of reacting solution is provided by the organic acid, and the pH is generally high, e.g. 1.5 to 1.8 in Examples 1 and 2 of the patent, whereas in the present case, as indicated above, by employing the particular ammonium salts, pHs between 2.5 and 4, generally about 3.5, are produced. The high acidity of the Auer process can quickly give rise to excessive fabric damage, thereby giving rise to tensile strength loss of the fabric. By employing the particular ammonium salts enumerated above as catalysts, it is possible to produce a vastly superior product to that produced by this prior art process.

U.S. Pat. No. 2,530,261 (Morton and Ward), issued Nov. 14, 1950, sets forth the treatment of cellulosic textiles with cyanamide in the presence of a non-volatile strong mineral acid for the purpose of producing a material having improved fire-resistance and some attendant improvement in crease recovery. It is stated in the patent that formaldehyde may be optionally included in the cyanamide impregnating solution to inhibit tendering of the cellulosic fabric and to improve its colour. This patent discloses the inclusion of various oxygen-containing acids of phosphorus and sulfur, or their partially neutralized salts, in a. pad bath containing cyanamide and possibly formaldehyde. The purpose of the process of this patent is to impart a degree of fire-resistance to the cellulosic fabric, a property which is largely the result of the incorporation into the substrate cellulosic material of appreciable amounts of the acid in the form of cellulosic esters of the phosphate and sulfate type. The acids employed act as reactants and not solely as condensation catalysts for the cyanamide-formaldehyde reactions. The acid concentrations specified in this patent, i.e. 5 to 40% are considerably in excess of those usually employed to catalyze typical N-methylol type condensations.

In contrast to the process of this patent, the present invention involves the use of specific ammonium salts as catalysts for the reaction between cyanamide and formaldehyde, the catalysts being employed in a concentration generally about 0.5% to about 5% by weight, and preferably approximately 2%.

As pointed out previously, it is known to one familiar with the treatment of cellulosic textile materials that metal salts such as Zinc nitrate would be logicall catalysts to employ if cyanamide were to be used as a substitute for, say, urea or ethyleneurea. In such well-known systems as those containing dimethylol ethyleneurea, the relatively higher efiiciency of metal salts of the zinc nitrate type is reflected in the superior crease recovery improvement obtained and the fewer sites for subsequent chlorine retention which result from the treatment. In those known systems, ammonium salt catalysis has been found to be less effective. For example, in the case of the same N- methylol compound, it has been shown (Enders and Pusch, American Dyestutf Reporter, 49 (1960), that ammonium salts produce a relatively poorer crease recovery improvement than do metal salts, while at the same time giving rise to a greater degree of chlorine retention by the cellulosic textile.

In contrast to this, in the treatment of cotton and other cellulosic textile fabrics with cyanamide and formaldehyde, the particular ammonium salts employed in the present invention have been shown to be superior catalysts to the metal salts. The relative efiiciencies of a typical metal salt catalyst and a typical ammonium salt catalyst are indicated in the examples which appear hereafter.

US. Pats. Nos. 1,791,433 and 1,791,434 (Schmidt), both issued Feb. 3, 1931, indicate that aqueous solutions of cyanamide and formaldehyde may be reacted together under slightly acid, neutral or slightly alkaline conditions, i.e., between a pH 5 and 9. The product is a yellowywhite amorphous powder which is essentially insoluble in water and in organic solvents. It is soluble, however, in dilute acid. When the product is heated, it becomes deep yellow in colour. If the material produced in the process of US. Pat. No. 2,247,35 3 (Auer) without use of catalyst is heated, then the same deep yellow colour is observed. One purpose for which these condensates have been used is as a dye assistant for cellulosic and Wool fibres, as is indicated in the previously discussed US. Pat. No. 2,234,- 905 (Tallis).

Several proposals have been made in the past to condense formaldehyde with dicyandiamide for various enduses and, in one such proposal, formaldehyde and dicyandiamide was condensed in the presence of an ammonium salt as catalyst. The condensation product was used in conjunction with dimethylolurea, a well-known cross-linking agent in the production of crease resistant textiles, to produce crease resistance in the textile to which it was applied. The authors of this prior proposal state that comparable improvements in crease recovery could not be obtained by simply using dimethylolurea alone or the condensation product alone.

In contrast, by using a more water-soluble chemical than dicyandiamide, namely cyanamide, and one of the specific ammonium salts enumerated above, it is possible to obtain excellent crease recovery properties for a textile material without the necessity of employing dimethylolurea. I

It will therefore be evident that by employing certain specific ammonium salts; namely, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, diammonium sulfate, diammonium sulfite, ammonium sulfamate, or mixtures thereof as catalysts in the formation of a reaction product of cyanamide and formaldehyde, superior crease recovery properties are imparted to a textile material on which the reaction product is formed in accordance with this invention and no undesirable colouration is formed in the material either initially or on subsequent laundering or ironing. To indicate the uniqueness of these particular ammonium salts, there follows a list of ammonium salts, tested by the applicants, which, although they have aqueous solutions with a pH from 5 to 9 and form acid solutions with a pH from 2.5 to 4 in the presence of formaldehyde, yet, when used as catalysts in the processes hereinbefore described either result in a fabric having a lower degree of crease recovery, and/or some colouration in the textile material, either initially or on subsequent laundering or ironing, thereby rendering them unsatisfactory: ammonium sodium hydrogen orthophosphate, diammonium ortho phosphite, ammonium chloride, ammonium magnesium chloride, ammonium nitrate, ammonium hydrogen tetraborate, ammonium hydrogen carbonate, ammonium carbamate, ammonium oxalate, ammonium succinate, ammonium tartrate, ammonium formate and triammonium citrate.

The relative proportions of formaldehyde and cyana mide may vary considerably and aqueous pad baths containing sufficient material to give concentrations of about 1 to about 10% by weight of cyanamide and about 2 to about 5 molar equivalents of formaldehyde based on the amount of cyanamide present have been found satisfactory, but it has been found that optimum results are obtained when 4 molar equivalents of formaldehyde are used for each molar equivalent of cyanamide present, the amount of cyanamide being from about 3 to about 5% by weight.

The present invention is illustrated by the following examples.

EXAMPLE I summarized in the following Table I.

TABLE I Monsanto crease recovery angle (warp direction only), degrees Sample No. Catalyst Dry Wet 1 Diammonium hydrogen 139 131 phosphate. 2 Zine nitrate 110 107 3 None 111 112 Control 67 EXAMPLE II Bleached, desized 80 x 80 cotton fabric was padded to about 75% wet pick-up, based on the dry weight of fabric, from an aqueous solution containing 3% by Weight cyanamide, 8.6% by weight formaldehyde, and 2% by weight diammonium hydrogen phosphate. The pH of the bath about 20 minutes after its preparation was 3.5. The cotton fabric specimen then was pin framed, dried at to C. and finally baked for five minutes at to C. The fabric then was given an afterwash in warm soapy water, rinsed in water, ironed dry and conditioned. The finished fabric showed a high level of both dry and wet crease recovery in the warp direction, i.e., 136 and 131 respectively, compared with 85 and 67 for the untreated fabric, was of good white colour and possessed excellent body and handling characteristics. When tested for susceptibility to chlorine damage according to the AATCC standard test method, the fabric showed negligible loss.

EXAMPLES III TO VII The procedure of Example II was repeated using various catalysts in place of the diammonium hydrogen phosphate. The same quantities of cyanamide, formaldehyde and catalyst were employed. In each case, excellent crease recovery results were obtained. All specimens had a good white colour and possessed excellent body and handling characteristics.

The following Table II summarizes the results:

EXAMPLE VIII Samples of bleached desized 80 x 80 cotton fabric were padded to about 75% wet pick-up, based on the dry weight of the fabric, from an aqueous bath containing 3% by weight of cyanamide and 2% by weight of catalyst. The specimens each were pin framed and suspended over a dish containing 37% formaldehyde solution in a closed glass tank fitted with a motor driven fan to promote efficient vapour circulation. Vapour impregnation was carried out for a period of one hour at ambient temperature, after which the specimens were removed from the tank, dried at 100 to 120 C. and then cured for five minutes at 150 to 160 C. Finally, the fabrics were given an afterwash in warm, soapy water, were ironed dry, and then conditioned at 65% relative humidity and at a temperature of 70 F. The results are set forth in the following Table III:

TABLE III Monsanto crease recovery angle (warp direction only), degrees Sample No. Catalyst Diy \Vet 1 Diammoninm hydrogen 142 137 phosphate.

Zinc nitrate 121 115 3 Magnesium chloride 117 110 4 None 106 103 Control 85 67 This example clearly indicates the superior results which are obtained by use of the process of the present invention involving, as catalyst, the specific ammonium salt, diammonium hydrogen phosphate, as opposed to using the prior art catalysts, zinc nitrate and magnesium chloride or using no catalyst at all.

EXAMPLES IX TO XIII The procedure of Example VIII was repeated using various preferred catalysts of the present invention. The same quantities of cyanamide and catalyst were employed. In each case excellent crease recovery results were obtained. All specimens had a good white colour and possessed excellent body and handling characteristics. The following Table IV summarizes the results:

TABLE IV Monsanto crease recovery EXAMPLE XIV Bleached, desized 80 x 80 cotton fabric was padded to a 75% wet pick-up from an aqueous solution containing 3% cyanamide and 2% diammonium sulfate. The sample was suspended in a vessel containing formaldehyde vapour volatilized from a formalin solution at a temperature of 50 to 60 C. Following an exposure time of two minutes, the damp fabric was removed and cured for five minutes at 150 to 160 C. The finished fabric had a dry pick-up of 8.3% and showed dry and wet crease recovery angles of and 133 respectively, compared with 85 and 67 before treatment.

This example illustrates that by raising the temperature, the vapour treatment time can be considerably reduced.

EXAMPLE XV A pin framed cotton swatch of bleached, desized 80 x 80 cotton fabric, padded to 75% wet pick-up from an aqueous bath containing 3% by weight cyanamide and 2% by weight diammonium hydrogen phosphate, was placed in a small closed container into which anhydrous formaldehyde gas was subsequently admitted. Exposure was carried out for 5 minutes at a temperature of C. The weight gain of the sample was 43% and'the dry and wet crease recovery angles (warp) were 135 and 145 repsectively.

EXAMPLE XVI Example II was repeated using bleached, desized, mercerized 92 x 96 terylene-cotton (65:35) fabric. The wet pick-up was 90% and the dry pick-up 7.8%. The dry and wet crease recovery angles in the warp direction were 137 and 124 respectively, as compared with 125 and 121 for the untreated fabric. The treated fabric showed no strength loss.

The production of satisfactory crease recovery properties is usually an indication that genuine covalent crosslinking has been the predominant reaction rather than polymer deposition, and the above examples indicate that by employing certain ammonium salts of inorganic acids as catalysts, such cross-linking is achieved in the fabric.

What I claim as my invention is:

1. In a process for treating a cellulosic textile material wherein formaldehyde and cyanamide are reacted at an elevated temperature in the presence of a catalyst and water and in contact with said cellulosic textile material, the improvement wherein said cellulosic textile material is impregnated with an aqueous solution which contains 1 to 10% by weight of cyanamide, 2 to 5 molar equivalents of formaldehyde based on the amount of cyanamide, and from 0.5 up to 5% by weight of catalyst, excess solution is removed therefrom, and the material thus impregnated is dried and cured at an elevated temperature, and said catalyst is an ammonium salt of an inorganic acid, said salt being selected from the group consisting of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, diammonium sulfate, diammonium sulfite, ammonium sulfamate, and mixtures thereof.

2. The process of claim 1 wherein said curing takes place at a temperature from 120 C. to C. and for 2 to 10 minutes.

3. The process of claim 1 wherein said aqueous solution contains 3 to 5% by weight cyanamide, 4 molar equivalents of formaldehyde based on the amount of cyanamide, and 2% by weight of said ammonium salt.

4. The process of claim 1 wherein said cellulosic textile material is selected from the group consisting of cotton and a mixture of cotton and a polyester.

5. The process of claim 1 wherein suflicient of said excess solution is removed so that said cellulosic textile material has a wet pick-up of 70 to 100% by weight, based on the dry weight of said cellulosic textile material.

6. The process of claim 1 wherein said cellulosic textile material is selected from the group consisting of cotton and a mixture of cotton and a polyester.

7. The process of claim 6 wherein sufficient of said excess solution is removed so that said cellulosic textile material has a wet pick-up of 70 to 100% by Weight, based on the dry weight of said cellulosic textile material.

8. In a process for treating a cellulosic textile material wherein formaldehyde and cyanamide are reacted at an elevated temperature in the presence of a catalyst and water and in contact with said cellulosic textile material, the improvement wherein said material is impregnated with an aqueous solution which contains 1 to 10% by weight of cyanamide and from 0.5 up to 5% by Weight of catalyst, excess solution is removed therefrom, such impregnated fabric is exposed to the vapours of formaldehyde, and the material then is dried and cured at an elevated temperature, and said catalyst is an ammonium salt of an inorganic acid, said salt being selected from the group consisting of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, diammonium sulfate, diammonium sulfite, ammonium sulfamate, and mixtures thereof.

1 0 References Cited UNITED STATES PATENTS 1,791,433 2/1931 Schmidt 8--116.3X 2,247,353 7/1941 Auer 8116.3X 2,234,889 3/1941 Boulton et a1 8-116.3 2,234,905 3/1941 Tallis 8-116.3X 2,318,464 5/1943 Cameron et a1 8-116.3 2,530,261 11/1950 Morton et al. 8116.3

FOREIGN PATENTS 449,243 6/1936 Great Britain 8-116.3

GEORGE F. LESMES, Primary Examiner J. CANNON, Assistant Examiner US. Cl. X.R. 8-115.7