US3487109A

US3487109A - Manufacture of unsaturated aliphatic carboxylic acids

Info

Publication number: US3487109A
Application number: US699766A
Authority: US
Inventors: Naoji Kurata; Takehiko Matsumoto; Takashi Ohara; Kenzo Oda
Original assignee: Nippon Shokubai Co Ltd
Current assignee: Nippon Shokubai Co Ltd
Priority date: 1964-03-02
Filing date: 1968-01-23
Publication date: 1969-12-30
Anticipated expiration: 1986-12-30
Also published as: GB1036375A; DE1468428A1; DE1468428C3; FR1425871A; DE1468428B2

Description

United States Patent US. Cl. 260-533 14 Claims ABSTRACT OF THE DISCLOSURE A process for the manufacture of an unsaturated aliphatic carboxylic acid, comprising reacting in the vapor phase at a temperature ranging from 200 to 500 C. an wolefin selected from the group consisting of propylene and isobutylene with molecular oxygen in the presence of a catalyst selected from the group consisting of the following formulas:

W Sn Mo Te O and W Co 'Sn Mo Te O wherein v is from 0.1 to 6, v from 0.1 to 16, w from 1 to 5, x from 7 to 12.9, y from 0.1 to 5, z from 30 to 59 and z is 30-83, and sum of v and x is 13.

This is a continuation-in-part of US. application Ser. No. 413,299 filed Nov. 23, 1964, now abandoned.

This invention relates to the manufacture of unsaturated aliphatic carboxylic acids. More particularly, the invention pertains to the oxidation of propylene or isobutylene in the vapor phase with molecular oxygen to produce acrylic acid or methacrylic acid in a single step process.

There have been proposed in the prior art various processes for the conversion of a-olefins to the corresponding unsaturated carboxylic acids, which processes may be broadly classified into two categories, namely, the double step processing comprising oxidizing rx-OlCfins and then oxidizing the resultant unsaturated aldehydes into the desired unsaturated carboxylic acids; and the single step processing comprising manufacturing the desired unsaturated carboxylic acids directly from the starting aolefins without intermediate steps. From the commercial and industrial points of view, the latter process is manifestly desirable.

The aforesaid single step process, however, is not necessarily satisfactory. The order of conversion of the starting a-olefins to the corresponding unsaturated carboxylic acids is generally poor, and a considerable amount of unsaturated aldehydes is produced in admixture With the desired unsaturated carboxylic acids, so that the unreacted starting a-olefins and intermediate aldehydes should be recycled if a good yield of the desired unsaturated carboxylic acids is to be secured. For instance, in Japa nese patent publication No. 4,209 (1962), 14,562 (1963) and 19,260 (1963), British Patent 878,802 and Belgium Patent 602,472, there are disclosed varied processes for the manufacture of the desired unsaturated aliphatic carboxylic acids in a single step process. These processes, however, are not only poor in conversion of the starting a-olefins but also produce a considerable amount of unsaturated aldehydes as a byproduct, so that the resultant unsaturated aldehydes and unreacted olefins should be recycled for further conversion if the yield of the desired unsaturated carboxylic acids is to be improved. From the economic and industrial points of view, said recycling of 3,487,109 Patented Dec. 30, 1969 the unreacted u-olefins and unsaturated aldehydes is manifestly undesirable.

The major object of this invention is accordingly to provide a process for the manufacture of acrylic acid or methacrylic acid in a single step from propylene or isobutylene in a considerably high order of yield, namely, in a high order of conversion and selectivity, substantially necessitating no recycling of the unreacted olefins and the intermediate aldehydes.

Other objects and specific features of this invention will become apparent in view of the following:

The process of this invention for the manufacture of unsaturated aliphatic carboxylic acids comprises contacting in the vapor phase propylene or isobutylene with molecular oxygen in the presence of a catalyst of the formula:

W Sn Mo Te O wherein v is from 0.1 to 6, w from 1 to 5, x from 7 to 12.9, y from 0.1 to 5 and z from 30 to 59, and the sum of v and x is 13.

The present invention is based upon the discovery that the catalyst of the above formula enables the starting propylene or isobutylene to convert selectively in a single step into the corresponding unsaturated aliphatic car- :boxylic acids, acrylic acid or methacrylic acid, in a high order of selective conversion without recycling the unreacted a-olefins and the unsaturated aldehydes. The amount of the resultant unsaturated aldehydes and other byproducts is negligible, and the per pass yield of the desired unsaturated carboxylic acids is markedly improved.

The catalyst of this invention consists of tungsten, tin, molybdenum, tellurium and oxygen in the order as disclosed in the aforesaid formula. Lack of any one element as specified in the formula results in a reduction in conversion or selectivity, so that the objects of this invention cannot be attained. The known catalyst consisting of cobalt, molybdenum, tellurium and oxygen, for instance, invites a deterioration in conversion if selectivity is to be enhanced and vice versa, so that it is impracticable for said catalyst to improve the yield of the resultant unsaturated carboxylic acids in a single step. When the catalyst of this invention is employed, on the other hand, conversion and selectivity are both raised to a high order, producing in a single step the desired unsaturated carboxylic acids in high yields. The term conversion and selectivity as herein employed are defined as below:

Percent conversion:

Number of moles of starting olefin fed number of moles of unreacted olefin Number of moles of starting olefin fed Percent selectivity:

Number of moles of each final product Number of carbon atoms of each final pro duct Number of carbon atoms of starting olefin fed Number of moles of reacted olefin by atomic ratio and sum of W and M0 is 13. Preferable results are obtained in the atomic ratio of and sum of W and M0 is 13, the most desirable ratio being 1:l.9:l2:3:49 or thereabout.

There may be added to the catalyst of the aforesaid composition cobalt as one more metal component, forming the formula:

W Co 'Sn Mo Te O wherein v, w, x, and y are as defined before, v is from 0.1 to 16 and z is 30-83. The presence of cobalt as an auxiliary component makes it possible to attain a higher order of conversion and selectivity of the catalyst. Preferable atomic ratio of W:C:Sn:Mo:Te: is in the range of 0.2 3:1-11:1.2-4:l012.8:14:3672 and sum of W and M0 is 13, the most desirable being 1:7:1.9:12:3:59 or thereabout.

The catalyst of the above formula is prepared by any of the conventional methods, such as precipitation and evaporation to dryness or mixing each component metal in the form of oxide. The catalyst containing cobalt may further be prepared by adding cobalt in the form of a cobalt compound, such as cobalt oxide, to theW-Sn-Mo- Te-O catalyst.

The catalyst of the above formula may be regarded as a mixture of oxides of the component metals, or a mixture of a heteropoly acid salt, such as tin-tungsten-molybdate, and tellurium oxide, or a mixture of two heteropoly acid salts, such as tin-molybdate and tin-tungstate, and tellurium oxide. But the exact chemical structure of said catalyst is yet unknown. Whether the structure is known or not, however, it has been made known for the first time in the art by the present inventors that the aforesaid catalyst has a property to produce the desired unsaturated aliphatic carboxylic acids in markedly high yields.

The aforesaid catalyst of this invention is employed singly or in conjunction with a suitable support, such as silica, diatomaceous earth, alumina, silicon carbide, titanium oxide or zicronium oxide.

The starting olefins employed in this invention are propylene and isobutylene.

In the mechanism of the reaction involved in this invention, the starting tit-olefins are oxidized into the corresponding aldehydes and the resultant aldehydes are subsequently converted into the desired corresponding unsaturated aliphatic carboxylic acids. The catalyst of this invention can therefore be successfully applied to the oxilation of unsaturated aldehydes to produce the corresponding unsaturated carboxylic acids in high yields. Even if there are present in the feed stock a-olefins and the corresponding 0:,[3-11I1S2t11f8t6d aldehydes in admixture, said olefins or aldehydes are respectively converted into the corresponding unsaturated aliphatic carboxylic acids, so that there can also be employed as a feed stock a-0lefins containing the corresponding unsaturated aldehydes in various proportions. In this case a gas mixture of propylene and acrolein or a mixture of isobutylene and methacrolein is preferably employed to produce the corresponding acrylic acid or methacrylic acid in a yield as high as is the case with the single employment of propylene or isobutylene.

The range of amount of the oxygen to be fed to the reactor varies widely, but good results are obtained when oxygen is employed in the order of from 0.1 to moles, preferably from 3 to 12 moles, per mole of the starting a-olefin or the starting mixture of an a-olefin and the corresponding aldehyde. The reaction mixture of the starting olefin and oxygen may be diluted, where desirable, with an inert gas, such as carbon dioxide, nitrogen, saturated hydrocarbon or steam, so that air is advantageously used as an oxygen source from the economic point of view. The most desirable diluent is steam as conversion and selectivity are still more increased when the reaction gas is diluted with steam. The range of amount of said diluents to be applied is very wide. To secure the desirable effects, however, steam is employed in the order of from 1 to 60 moles, preferably from 5 to moles, per mole of the starting a-Olefin or the starting mixture of an OC-O'Ie' fin and the corresponding aldehyde.

The reaction temperature employed in this invention ranges from 200 to 500 C., preferably from 250 to 400 C. There is no appreciable efiect of pressure on reaction. Hence the reaction may be conducted under normal, increased or reduced pressure, but normal atmospheric pressure is preferred.

The time of contact of the gas mixture with the catalyst of the invention can be varied widely. Satisfactory results, however, are obtained in the range of from 0.1 to 20 seconds, preferably from 1 to 10 seconds. The term contact time as herein employed is defined as follows:

Contact time in second:

Apparent volume of the catalyst in the reactor Standardized volume of gas fed to the reactor per unit The catalyst bed employed in this invention may be fixed or fluidized, of which the fluidized bed is preferablebecause of its ability to prevent the reaction system from the occurrence of hot spots due to the exothermic reaction.

Besides the desired unsaturated aliphatic carboxylic acids, the gaseous product from the reactor contains a small amount of unreacted starting gases, unsaturated aldehydes, saturated carboxylic acids, carbon monoxide and carbon dioxide. The desired unsaturated aliphatic carboxylic acids are separated from said products in accordance with the known methods, such as condensation or extraction with water or other suitable solvents.

In order to afford a fuller understanding of the principles of this invention, there are provided the following examples which are illustrative only and not limiting the invention:

EXAMPLE 1 In 1,800 cc. of hot water were dissolved with stirring 55 grams of ammonium paratungstate of a formula (NH W O -3H O, 445 grams of ammonium paramolybdate of a formula (NH Mo O -4H O, and 104 grams of ammonium chloride. There were then added dropwise 123 grams of stannic chloride of a formula SnCl -3H O dissolved in 150 cc. of water. The mixture was heated with stirring for further 45 minutes, and the precipitates were filtered, washed with water and dried.

Whole amount of the resultant precipitates were heated with stirring in 460 cc. of water. To the resultant mixture were added 80.4 grams of metal tellurium dissolved in 280 cc. of nitric acid in 280 cc. of water. The mixture was heated and evaporated with stirring and the dried mixture was pelleted, dried and heated in air stream at 400 C. for 4 hours. The resultant catalyst was confirmed to have an atomic ratio of cc. of the catalyst were filled in a U-shaped stainless steel reactor, 25 mm. in internal diameter, and the reactor was dipped in a nitrate bath heated to 330 C. Into this reactor was introduced a gas mixture of 1 percent by volume of propylene, 60 percent by volume of air, and 39 percent by volume of steam, and reacted by contact for 1.8 seconds. The resultant reaction products were water scrubbed and analyzed. Results obtained were as follows:

Conversion of propylene:

Selectivity=Acrylic acid: 62.5%, acetic acid: 2.2%, acrolein: 7.0%, carbon dioxide: 12.5%, carbon monoxide: 12.3%, others: 3.5%.

EXAMPLE 2 Whole amount of the precipitates obtained by the same manner as in Example 1 were stirred in 460 cc. of water and to the solution were added 427 grams of cobalt nitrate of a formula Co(NO -6H O dissolved in 300 cc. of water. There were then added 80.4 grams of metal tellurium dissolved in 280 cc. of nitric acid in 280 cc. of water. The mixture was then heated with stirring for evaporation, pelleted, dried, and heated in air stream at 400 C. for 4 hours. The resultant catalyst was confirmed to have an atomic ratio of W:Co:Sn:Mn:Te:O=1:7: 1.9112359.

80 cc. of the catalyst were filled in a U-shaped stainless steel reactor, 25 mm. in internal diameter, and the reactor was dipped in a nitrate bath heated to 350 C. Into this reactor was introduced a gas mixture of 1 percent by volume of propylene, 60 percent by volume of air, and 39 percent by volume of steam, and reacted by contact for 1.8 seconds. Results obtained were as follows:

Conversion of propylene: 93.0%.

Selectivity=Acrylic acid: 70.5%, acetic acid: 2.0%, acrolein: 5.5%, carbon dioxide: 12.3%, carbon monoxide: 8.6%, others: 1.1%.

EXAMPLE 3 80 cc. of the catalyst of Example 2 were filled in a U-shaped stainless steel reactor, 25 mm. in internal diameter, and the reactor was dipped in nitrate bath heated to 340 C. Into this reactor was introduced a gas mixture of 1 percent by volume of propylene, 60 percent by volume of air, and 39 percent by volume of nitrogen, and reacted by contact for 1.5 seconds. Results obtained were as follows:

Conversion of propylene: 94.5%.

Selectivity=Acrylic acid: 55.2%, acetic acid: 3.0%, acrolein: 3.9%, carbon dioxide: 24.1%, carbon monoxide: 11.5%, others: 2.3%.

EXAMPLE 4 60 cc. of the catalyst of Example 2 were filled in a U-shaped stainless steel reactor, 25 mm. in internal diameter, and the reactor was dipped in a nitrate bath heated to 300 C. Into this reactor was introduced a gas mixture of 1 percent by volume of isobutylene, 60 percent by volume of air, and 39 percent by volume of steam, and reacted by contact for 0.9 second. Results obtained were as follows:

Conversion of isobutylene: 81.2%.

Selectivity=Methacrylic acid: 47.7%, acetic acid: 7.8%, methacrolein: 1.2%, carbon dioxide: 20.1%, carbon monoxide: 13.1%, others: 11.1%.

EXAMPLE 5 Whole amount of the precipitates prepared by the same manner as in Example 1 were stirred in 460 cc. of hot water. There were then added with stirring 427 grams of cobalt nitrate of a formula Co(NO -6H O dissolved in 300 cc. of water, and then 64.2 grams of metal tellurium dissolved in 224 cc. of nitric acid in 224 cc. of water. To the resultant mixture were further added 279 grams of anatase type titanium oxide of a particle size of from 0.2 to 0.4 micron, and the mixture was heated with stirring for evaporation, pelleted, dried and heated in air stream at 400 C. for 4 hours. The resultant catalyst was confirmed to have an atomic ratio of W:Co:Sn:Mo:Te: O:(TiO )=1:7:1.9:12:2.4:58:17.

Conversion of propylene: 91.0%.

Selectivity=Acrylic acid: 71.3%, acetic acid: 2.1%, acrolein: 7.0%, carbon dioxide: 11.0%, carbon monoxide: 7.6%, others: 1.0%.

EXAMPLE 6 Whole amount of the precipitates obtained by the same manner as in Example 1 were stirred in 460 cc. of hot water. There were then added 427 grams of cobalt nitrate of a formula Co(NO -6H O dissolved in 300 cc. of water, and 64.2 grams of metal tellurium dissolved in 224 cc. of nitric acid in 224 cc. of water. To the resultant mixture were further added 279 grams of zirconium oxide particles, and the mixture was heated with stirring for evaporation, pelleted, dried, and heated in air stream at 400 C. for 4 hours. The resultant catalyst was confirmed to have an atomic ratio of W:Co:Sn:Mo: Te:O:(ZrO )==1:7:1.9:12:2.4:58:11.

cc. of the catalyst were filled in a U-shaped stainless steel reactor, 25 mm. in internal diameter, and the reactor was dipped in a nitrate bath heated to 340 C. Into this reactor was introduced a gas mixture of 1 percent by volume of propylene, 60 percent by volume of air, and 39 percent by volume of steam, and reacted by contact for 1.8 seconds. Results obtained were as follows:

Conversion of propylene: 92.4%.

Selectivity=Acrylic acid: 70.0%, acetic acid: 2.4%, acrolein: 5.0%, carbon dioxide: 11.9%, carbon monoxide: 8.3%, others: 2.4%.

EXAMPLE 7 Whole amount of the precipitates obtained by the same manner as in Example 1 were stirred in 460 cc. of hot water. There were then added 427 grams of cobalt nitrate of a formula Co(NO -6H O dissolved in 300 cc. of water, and then 64.2 grams of metal tellurium dissolved in 224 cc. of nitric acid in 224 cc. of water. To the resultant mixture were further added 279 grams of diatomaceous earth, and the mixture was heated with stirring for evaporation, pelleted, dried and heated in air stream at 400 C. for 4 hours. The resultant catalyst wa confirmed to have an atomic ratio of W:Co:Sn:Mo:Te:O:(SiO )=1:7: 1.9: 12:2.4:58:22

80 cc. of the resultant catalyst were placed in a U- shaped stainless steel reactor, and the reactor was dipped in a nitrate bath heated to 360 C. Into this reactor was introduced a gas mixture of 1.5 percent by volume of propylene, 60 percent by volume of air, and 38.5 percent by volume of steam, and reacted by contact for 2.2 seconds. Results obtained were as follows:

Conversion of propylene: 89.8%.

Selectivity=Acrylic acid: 73.2%, acetic acid: 2.1%, acrolein: 7.0%, carbon dioxide: 10.2%, carbon monoxide: 6.7% others: 0.8%.

EXAMPLE 8 80 cc. of the catalyst of Example 2 were placed in a U-shaped stainless steel reactor, 25 mm. in internal diameter, and the reactor was dipped in a nitrate bath heated to 350 C. Into this reactor was introduced a gas mixture of 0.2 percent by volume of propylene and 1.0 percent by volume of acrolein, 60 percent by volume of air, and 38.8 percent by volume of steam, and reacted by contact for 1.8 seconds. Number of moles of each component in the resultant product on the basis of moles as total of the propylene and acrolein fed were as follows:

Acrylic acid: 79.0 moles, acetic acid: 1.4 mole, acrolein: 2.1 moles, carbon dioxide: 8.7 moles, carbon monoxide: 7.9 moles, others: 0.9 moles.

EXAMPLE 9 60 cc. of the catalyst of Example 2 were placed in a U-shaped stainless steel reactor, 25 mm. in internal diameter, and the reactor was dipped in a nitrate bath heated to 350 C. Into this reactor was introduced a gas mixture of 1.5 percent by volume of acrolein, 60 percent by volume of air, and 38.5 percent by volume of steam, and reacted by contact for 1.3 seconds. Results obtained were as follows:

Conversion of acrolein: 90.6%.

Selectivity=Acrylic acid: 90.3%, acetic acid: 2.2%, carbon dioxide: 3.7%, carbon monoxide: 1.8%, others: 2.0%.

EXAMPLE 10 Another series of runs were made employing the catalyst of Example 1 in which the amount of metal tellurium content was varied as specified in the subsequent table. Thus the catalysts of atomic ratio of W:Sn:Mo:Te:O=1:1.9:12:y:z, in which y and z were specified in the table, were obtained. Reaction conditions were identical to those employed in Example 1. Results obtained were as follows:

cobalt nitrate was varied as specified in the subsequent table. Thus the catalysts of atomic ratio of W:Co:Sn:Mo:Te:O=1:v':l.9:12:3:z'

in which v' and z are specified in the table, were obtained. Reaction conditions were identical to those employed in Example 2. Results obtained were as follows:

Catalyst No.

Metal tellurium added in grams percent of propylen Selectivity in percent:

Acrylic acid 5. 15. 18. Others 4.

EXAMPLE 1 1 Further series of runs were conducted employing the catalyst of Example 1 in which the amount of ammonium paratungstate and that of ammonium paramolybdate were varied as specified in the subsequent table. Thus the catalysts of atomic ratio of W:Sn:Mo:Te:O=v:1.9:x:3:49

in which v and x are specified in the table, were obtained.

Reaction conditions were identical to those employed in Example 1. Results obtained were as follows:

Catalyst No.

Ammonium paratungstate in grams Ammonium paramolybdate in grams.

Others EXAMPLE 12 Still further series of runs were performed employing the catalyst of Example 1 in which the amount of stannic chloride was varied as specified in the subsequent table. Thus the catalysts of atomic ratio of W:Sn:Mo:Te:O=l:w:12:3:z

in which w and z are specified in the table, were obtained.

Catalyst No.

Conversion in percent of propylene 70. 9 83. 7 100 Selectivity in percent:

Acrylic acid 44. 2 53.

Acetic acid. 2.

$3 o: a O Camp--10: wad en Stannic chloride added in grams Acroleiu .1 Carbon dioxide Carbon monoxide- EXAMPLE 13 A still another series of runs were conducted employing the catalyst of Example 2 in which the amount of Catalyst No.

v Conversion in percent Selectivity in percent Acrylic acid Acetic acid Acrolein.

EXAMPLE 14 Catalyst No.

Ammonium paratungstate in grams Stannic chloride added in grams Ammonium paramolybdate in grams Metal tellurium added in grams Conversion in percent of propylen Selectivity in percent:

Acrylic acid Acetic acid EXAMPLE 15 Another series of runs were conducted employing the catalyst of Example 2 in which the amount of ammonium paratungstate, cobalt nitrate, stannic chloride, ammonium paramolybdate and metal tellurium was varied as specified in the subsequent table. Thus the catalysts of atomic ratio of W:Co:Sn:Mo:Te:O=v:v'zwrxzyzz in which v, v, w, x, y and z are specified in the table, were obtained. Reaction conditions were identical to those employed in Example 2. Results obtained were as follows:

Catalyst No.

Ammonium paratungstate in grams 165 Cobalt nitrate added in grams. 671 Stannic chloride added in grams, 259 Ammonium paramolybdate in grams. 475 445 371 26. 8 80. 4 107 Metal tellurium added in grams 0. 2 1 3 v'.. 1 7 11 w 1. 2 1. 9 4 I- 12. 8 12 10 1h 1 3 4 z 45 59 72 Conversion in percent of propylene 82. 1 96. 4 83. 0 Selectivity in percent:

Acrylic acid- 53.1 70. 3 52. 3 Acetic acid- 4. 2 1. 4 2. 9 Acro1ein 20. 0 9. S 13. 3 Carbon dioxide 10.0 11. 6 15. 1 Carbon oxide 7. 7 6. 1 13. Others 5. 0 0. 8 2. 9

In view of the foregoing, it may be apparent that various changes and modifications can be made in the principles of this invention without departing from the scope and spirit of the same. It is accordingly requested that the invention be understood rather broadly except otherwise described.

We claim:

1. A process for the manufacture of an unsaturated aliphatic carboxylic acid, comprising reacting in the vapor phase at a temperature ranging from 200 to 500 C., an a-olefin selected from the group consisting of propylene and isobutylene with molecular oxygen in the presence of a catalyst selected from the group consisting of the following formulas:

W Sn Mo Te O and W Co 'Sn Mo Te O wherein v is from 0.1 to 6, v from 0.1 to 16, w from 1 to 5, x from 7 to 12.9, y from 0.1 to 5, z from 30 to 59 and z is 30-83, and sum of v and xis 13.

2. The process of claim 1, wherein said zx-olefin is propylene.

3. The process of claim 1, wherein said u-olefin is propylene, isobutylene.

4. The process of claim 1, in which said a-olefin is used in combination with the corresponding 0a,;3-111'1S31L1- rated aldehyde.

5. The process of claim 4, wherein said gas mixture consists of propylene and acrolein.

6. The process of claim 4, where said gas mixture consists of isobutylene methacrolein.

7. The process of claim 1, Where the atomic ratio of W, Sn, Mo, Te and O of said catalyst is and sum of W and Mo is 13.

8. The process of claim 1, wherein the atomic ratio of W, Co, Sn, Mo, Te and O of said catalyst is and sum of W and M0 is 13.

9. The process of claim 7, wherein the atomic ratio of W, Sn, Mo, Te and O of said catalyst is 10. The process of claim 8, where the atomic ratio of W, Co, Sn, Mo and Te of said catalyst is 11. The process of claim 1, wherein the reaction gas to be employed contains molecular oxygen in the range of from 0.1 to 20 moles per mole of the starting olefin.

12. The process of claim 4, wherein the reaction gas to be employed contains molecular oxygen in the range of from 0.1 to 20 moles per mole of the starting gas mixture of an a-olefin and the corresponding e o-unsaturated aldehyde.

13. The process of claim 1, wherein the reaction gas contains steam as diluent in the range of from 1 to moles per mole of the starting olefin.

14. The process of claim 4, wherein the reaction gas contains steam as diluent in the range of from 1 to 60 moles per mole of the starting gas mixture of an a-olefin and the corresponding 0:,[3-1111S3tl1l'3t6d aldehyde.

References Cited UNITED STATES PATENTS 3,240,806 3/1966 Bethell et al. 260--533 JAMES A. PATTEN, Primary Examiner D. STENZEL, Assistant Examiner US. Cl. X.R. 252439