US3833501A

US3833501A - Lubricating compositions

Info

Publication number: US3833501A
Application number: US00266697A
Authority: US
Inventors: J Elliott; I Crail; H Askew
Original assignee: Burmah Oil Trading Ltd
Current assignee: Burmah Oil Trading Ltd
Priority date: 1971-12-22
Filing date: 1972-06-27
Publication date: 1974-09-03
Anticipated expiration: 1991-09-03

Abstract

A LUBRICATING COMPOSITION COMPRISES A MAJOR PROPORTION OF A MINERAL OIL OF LUBRICATING VISCOSITY AND A MINOR PROPORTION OF A COMBINATION OF ADDITIVES (A) AND (B), ADDITIVE (A) BEING A NITROGEN-FREE RANDOM COPOLYMER OF AN ALKYL ACRYLATE COMPONENT AND AN ALKYL METHACRYLATE COMPONENT WHEREIN THE COPOLYMER IS COMPOSED OF UP TO 65% ALKYL ACRYLATE UNITS BASED ON THE TOTAL NUMBER OF ALKYL METHACRYLATE AND ALKYL ACRYLATE UNITS PRESENT IN THE COPOLYMER, THE ALKYL ACRYLATE COMPONENT IS METHYL ACRYLATE, ETHYL ACRYLATE OR A MIXTURE THEREOF AND THE ALKYL METHACRYLATE COMPONENT COMPRISES AT LEAST 50% ALKYL METHACRYLATE UNITS IN WHICH THE ALKYL GROUP CONTAINS FROM 1 TO 10 CARBON ATOMS, AND ADDITIVE (B) BEING AN ALKALINE EARTH METAL CONTAINING DETERGENT HAVING A TOTAL BASE NUMBER GREATER THAN 20.

Description

United States Tatent C U.S. Cl. 25233.4 11 Claims ABSTRACT OF THE DISCLOSURE A lubricating composition comprises a major proportion of a mineral oil of lubricating viscosity and a minor proportion of a combination of additives (a) and (b), additive (a) being a nitrogen-free random copolymer of an alkyl acrylate component and an alkyl methacrylate component wherein the copolymer is composed of up to 65% alkyl acrylate units based on the total number of alkyl methacrylate and alkyl acrylate units present in the copolymer, the alkyl acrylate component is methyl acrylate, ethyl acrylate or a mixture thereof and the alkyl methacrylate component comprises at least 50% alkyl methacrylate units in which the alkyl group contains from 1 to carbon atoms, and additive (b) being an alkaline earth metal containing detergent having a total base number greater than 20.

This application is a continuation-in-part of our copending application Ser. No. 213,165 filed on Dec. 28, 1971, now, U.S. Pat. No. 3,788,990 for lubricating compositions, more particularly lubricating compositions containing viscosity index improvers based on polymers of alkyl esters of unsaturated acids.

One of the most widely used groups of viscosity index improvers is based on copolymers of alkyl methacrylates, particularly copolymers of one or more relatively short chain alkyl methacrylates, such as butyl methacrylate, with one or more relatively long chain alkyl methacrylates, such as stearyl methacrylate.

However, the commonly used polymethacrylate viscosity index improvers display certain undesirable properties. In particular such polymethacrylates tend to derate the performance characteristics of simple mineral oil blends with regard to engine cleanliness, i.e. the polymethacrylates are a source of soot and dirt. This can be counteracted, at least to some extent, by adding a detergent to the mineral oil blends or by increasing the concentration of detergent if already present. However, it will be apparent that an alternative additive having comparable potency with regard to viscosity index properties to the polymethacrylates but having greater thermal stability, i.e. less tendency to form thermal degradation products such as soot, will enable the amount of detergent employed to be reduced or engine cleanliness at the same additive dosage to be improved.

Various alternatives to polymethacrylate viscosity index improvers have been suggested but these also suffer from attendant drawbacks. For example, styrene/methacrylate copolymers have greater thermal stability than polymethacrylates on the basis of laboratory evaluation techniques such as thermogravimetric analysis. Nevertheless, in practice, these copolymers tend to derate performance characteristics of lubricant compositions with regard to engine cleanliness because the pendant aromatic groups on the copolymers chains are a source of carbon deposition. Similarly, polyacrylates have been proposed, as alternatives to polymethacrylates, but have been found to be unstable in the presence of basic additives, particularly overbased detergents, commonly present in fully formulated mineral oil blends. Hydrolysis of the polyacrylate can occur accompanied by gel formation. This problem can be especially troublesome in the preparation of additive packages having a high content of additives and which are intended to be diluted with mineral oil before use.

Accordingly, the provision of an improved lubricating composition is a complex task requiring careful consideration of several mutually interacting factors. The lubricating compositions of the present invention, or at least the preferred embodiments thereof, have viscosity indices comparable to those of similar compositions containing polymethacrylate viscosity index improvers, improved characteristics with respect to engine cleanliness and superior stability at high temperatures. Furthermore, the additive packages of the present invention have a reduced tendency to form gels.

According to the present invention there is provided a lubricating composition comprising a major proportion of a mineral oil of lubricating viscosity and a minor proportion of a combination of additives (a) and (b), additive (a) being a nitrogen-free random copolymer of an alkyl acrylate component and an alkyl methacrylate component wherein the copolymer is composed of up to 65 alkyl acrylate units based on the total number of alkyl methacrylate and alkyl acrylate units present in the copolymer, the alkyl acrylate component is methyl acrylate, ethyl acrylate or a mixture thereof and the alkyl methacrylate component comprises at least 50% alkyl methacrylate units in which the alkyl group contains from 1 to 10 carbon atoms, and additive (b) being an alkaline earth metal containing detergent having a total base number greater than 20, preferably greater than 40.

It is to be understood that the percentages of monomer units set out above, and the percentages of monomer units referred to hereinafter, are, unless otherwise stated, equal to the mole percentages of polymerised monomer in additive (a).

The copolymer which constitutes additive (a) may be composed of from 5 to 40% alkyl acrylate units based on the total number of alkyl methacrylate and alkyl acrylate units. In a particularly preferred embodiment the copolymer is composed of from 10 to 30%, more preferably 15 to 25%, alkyl acrylates units based on the total number of alkyl methacrylate and alkyl acrylate units.

Additive (a) in the lubricating compositions of the present invention is preferably prepared by copolymerising one or more alkyl methacrylate monomers with methyl acrylate, ethyl acrylate or a mixture thereof. It is preferred that the alkyl groups containing from 1 to 10 carbon atoms of the alkyl methacrylate units comprising at least 50% of the alkyl methacrylate component of additive (a) are alkyl groups containing from 8 to 10 carbon atoms. The remainder of the methacrylate units, if any, may be higher alkyl methacrylate units such as C2348 units e.g. C or C methacrylate units.

The copolymer may be present in the lubricating composition of the present invention in the proportions in which viscosity index improver copolymers are commonly employed, for example 0.5 to 7.5%, preferably 1 to 5% by weight based on the total weight of the composition.

Additive (b) must have a basicity, expressed as total base number (TBN), greater than 20. Examples of additive '(b) are overbased alkaline earth metal salts, particularly calcium, barium and magnesium salts, of alkyl benzene and petroleum sulphonic acids. Such overbased salts may have a TBN of up to 500 to 600, for example 250 to 400. Other basic additives are overbased alkaline earth metal salts, particularly barium salts, of phosphosulphurised hydrocarbons, for example polyisobutylenes. These salts may be overbased by carbonation of an alkaline earth metal oxide or hydroxide suspended in a mineral oil blend containing the phosphosulphurised hydrocarbon in conventional manner and/or by admixture with overbased alkaline earth metal sulphonates of the type hereinbefore described. Such overbased salts may also contain an alkyl substituted phenol or metal phenate, optionally containing more than one alkyl substituent on the aromatic nucleus.

Alternatively the basic additives may consist of alkaline earth metal alkyl phenates, preferably having alkyl chains containing at least 9 carbon atoms, the corresponding sulphurised phenates or metal phenate sulphides or metal alkyl salicylates.

The basic additive concentrates may be employed in the lubricating compositions of the present invention in amounts of up to 15%, preferably 0.5 to 7.5%, more preferably 1.0 to 5.0% by weight based on the total weight of the compositions.

In another aspect the present invention also provides an additive package comprising a mineral oil having dissolved or dispersed therein from to 50% by weight, based on the total weight of the additive package of: additive (a) which is a nitrogen-free random copolymer of an alkyl acrylate component and an alkyl methacrylate component wherein the copolymer is composed of up to 65% alkyl acrylate units based on the total number of alkyl methacrylate and alkyl acrylate units present in the copolymer, the alkyl acrylate component is methyl "acrylate, ethyl acrylate or a mixture thereof and the alkyl methacrylate comprises at least 50% alkyl methacrylate units in which the alkyl group contains from 1 to carbon atoms, and from 5% to 90% by weight, based on the total weight of the additive package, of: additive (b) which is an alkaline earth metal containing detergent having a total base number greater than 20, preferably greater than 40.

It will be understood that the preferred features here inbefore described for additives (a) and (b) of the lubricating compositions of the present invention are also applicable to additives -(a) and (b) of the additives packages of the invention.

It will also be understood that the lubricating cOmposi tions and additive packages of the present invention may optionally contain additional conventional additives such as antioxidants; dispersants, especially the ashless dispersants of the succinimide type; corrosion inhibitors; pour point depressants and other conventional additives as desired.

The invention will now be illustrated with reference to the following examples:

Examples 1 to 5 Alkyl methacrylate/acrylate copolymers were prepared by the following techniques: I

Comonomers and a commercially available mineral oil polymerisation solvent of viscosity 31-33 cst. at 100 F. were mixed and purged with nitrogen for 14%, hours at a temperature of 85C maintained by an oil bath fitted with a thermostat. A solution of a mercaptan chain modifier was then added to the polymerisation mixture followed by a peroxide initiator and the internal temperature of the polymerisation mixture was allowed to rise to 90C where it was maintained constant. Further additions of initiator were added at half-hour intervals and the polymerisation finally stopped, 45 minutes after the addition of the last portion of initiator, by the addition of 5 drops of 2,4-dimethyl-6-t-butyl phenol.

In this manner the following preparations were carreid In this manner the following preparations were carried out:

Example 1 An approximately 40% by weight concentrate, in mineral oil, of a copolymer containing the following molar percentages of polymerised monome Percent Ethyl acrylate 50.0 Methyl methacrylate 2.5 C Alkyl methacrylate 30.0 C1248 alkyl methacrylate 17.5

This concentrate was prepared from the following reactants:

Ethyl acrylate 133 g. Monomer Mix A 317 g. Polymerisation Solvent 675 g. n-Dodecyl mercaptan 0.0 g. Lauroyl peroxide 0.3 g. 5 X 0.1 g. portion.

Monomer Mix A contained 5 molar percent methyl methacrylate, 60 molar percent C alkyl methacrylate and 35% C1248 alkyl methacrylate.

Example 2 An approximately 40% by weight concentrate, in mineral oil, of a copolymer containing the following molar percentages of polymerised monomer:

Percent Ethyl acrylate 20.0 Methyl methacrylate 4.0 C alkyl methacrylate 48.0 C alkyl methacrylate 28.0

This concentrate was prepared from the following reactants:

Ethyl acrylate 43 g.

Monomer Mix A 407 g.

Polymerisation Solvent 675 g.

n-Dodecyl mercaptan 0.1 g.

Lauroyl peroxide 0.5 g.+5 0.2 g. portions.

Example 3 An approximately 40% by weight concentrate, in mineral oil, of a copolymer containing the following molar percentages of polymerised monomer:

Percent Ethyl acrylate 40.0 Methyl methacrylate 3.0 C alkyl methacrylate 36.0 C1248 alkyl methacrylate 21.0

This concentrate was prepared from the following reactants:

Methyl acrylate 45.5 g.

Monomer Mix A 302.5 g.

Monomer Mix B 102.0 g.

Polymerisation Solvent 675 g.

n-Dodecyl mercaptan 0.05 g.

Lauroyl peroxide 0.35 g.+5 0.13 g. portions.

Monomer Mix B contained cetyl and stearyl methacrylates as the only monomers.

Example 5 An approximately 40% by weight concentrate, in mineral 011, of a copolymer containing the following molar This concentrate was prepared from the following reactants:

Ethyl acrylate 71 g. Monomer Mix A 236 g. Monomer Mix C 143 g.

Polymerisation solvent 675 g. n-Dodecyl mercaptan 0.05 g. Lauroyl peroxide 0.35 g.+5 0.13 g. portions.

Monomer Mix C contained 60 molar percent butyl methacrylate and 40 molar percent mixed cetyl and stearyl methacrylates.

Viscosity characteristics, gelling tendencies and dispersing properties of the products of Examples 1 to 5 were evaluated and the results obtained demonstrated the suitability of the products for use in conjunction with basic alkaline earth metal containing detergents.

The following Table 1 shows viscosity data for solutions respectively of the products of Example 4 and Example 5 in a 150 Solvent Neutral mineral oil having a viscosity index, without copolymer, of about 101 to 105. Corresponding measurements performed upon a conventional polymethacrylate viscosity index improver and upon certain acrylate/methacrylate copolymers used in the invention claimed in our copending application Ser. No. 213,165 are reported in said copending application and such data are reproduced hereinafter in Table 2 for comparison. Thus, in Table 2, the various polymer concentrates have the following compositions:

Polymer Concentrate A.A mineral oil concentrate (40% by weight solids) of a conventional polymethacrylate copolymer containing 60% butyl methacrylate units and 40% mixed cetyl and stearyl methacrylate units;

Polymer Concentrate G.-A mineral oil concentrate (40% by weight solids) containing 20% 2-ethylhexyl acrylate units and 80% mixed alkyl methacrylate units, the methacrylate units being derived from a mixture of 50% butyl methacrylate units and 50% mixed cetyl and stearyl methacrylate units;

Polymer Concentrate H.A mineral oil concentrate (40% by weight solids) of a copolymer as in Polymer Concentrate G except that it contains 20% butyl acrylate units in place of the 20% Z-ethylhexyl acrylate units; and

Polymer Concentrate J.A mineral oil concentrate (40% by weight solids) containing 60% butyl acrylate units and 40% mixed cetyl and stearyl methacrylate units.

From the results set out in Table 1 and Table 2 it will be noted that the acrylate/methacrylate copolymers as used in the invention claimed in the present application gave improved viscosity properties as compared with the conventional polymethacrylate viscosity index improver, Polymer A, and the acrylate/methacrylate copolymers as used in the invention claimed in our copending application Ser. No. 213,165.

In order to demonstrate the gelling tendencies of the additive combinations of the present invention a series of tests was carried out in which the polymer concentrates (a) forming the products of Examples 1 to 5 were respectively mixed with two overbased detergents (b) at room temperature and the consistency examined after standing for 24 hours. The results are given in Table 3. Similar tests in which component (a) was a polymethacrylate, a polyacrylate or an acrylate/methacrylate copolymer as used in the invention claimed in our copending application Ser. No. 213,165 are reported in said copending application and the results of certain of these tests are repeated hereinafter in Table 4 for comparison. Polymer Concentrates A, G and I, referred to in Table 4, are as described above in connection with Table 2, whereas Polymer Concentrates B, F, K and L have the following compositions:

Polymer Concentrate B.-A commercially available alkyl acrylate polymer concentrate (approximately 26% solids);

Polymer Concentrate F.A concentrate (approximately 40% solids) of a copolymer of about 22% methyl methacrylate units and 78% C alkyl methacrylate units;

Polymer Concentrate K.A concentrate (approximately 40% solids) of a copolymer containing 20% butyl acrylate units, 40% butyl methacrylate units and 40% mixed cetyl and stearyl methacrylate units; and

Polymer Concentrate L.A concentrate (approximately 40% solids) of a copolymer containing 50% 6 butyl acrylate units, 10% butyl methacrylate units and 40% mixed cetyl and stearyl metharcrylate units.

It will be seen from Tables 3 and 4 that mixtures containing a wholly acrylate polymer (Polymer Concenerate B) gave, in two instances out of three, very severe thickening, which did not occur in the other cases.

In order to evaluate the dispersancy characteristics of the additive combinations of the invention spot tests were carried out. Additives were blended with used SAE 30 engine oil which had conatined antioxidant as the only additive. The samples so obtained were stored for 6 hours at C., after which one drop of each sample was allowed to fall onto a sheet of chromatographic paper. This produced a spot. After being allowed to develop over a period of one week the spot usually became surrounded by a lighter coloured outer ring, the outer periphery of which indicated the sludge front attributable to the migration of dispersed sludge away from the central spot where the test sample had been absorbed into the paper. The specific dispersancy of each test additive was calculated using the equation:

Specific Dispersancy i X 100 wherein a! diameter of the central spot and D diameter of the sludge front.

The results of these tests are shown in Table 5 and demonstrate that the additive combinations employed gave comparable results in this test to a commercially available dispersant viscosity index improver despite the individual components of the combination showing relatively little dispersancy characteristics.

In order to further evaluate the dispersancy characteristics of the additive combinations employed in the present invention, paper strip chromatographic tests were carried out by the following procedure. Test samples were prepared by blending additives with an SAE 30 engine lubricant base oil. The samples were then mixed with an equal volume of the same base oil in which had been dispersed, by shaking for 24 hours, 4% Spheron 9-27 mp. particle size carbon black. 2 gms. of each of the resulting mixtures were heated at 100 C. for 5 hours in 20 gm. vials of 20 mm. diameter. Thereafter, a strip of Whatman filter paper Qualitative No. 1 was suspended with its base immersed to a depth of 1-2 mm. in the oil sample and chromatography was allowed to take place at 100 C. for 16 hours. The R value of each additive combination was calculated as the ratio of the distance travelled by the carbon black front to the distance travelled by the oil front. These results are also shown in Table 5 and demonstrate that the additive combinations employed in the present invention also exhibit dispersancy characteristics in this test, despite the individual components of the combination showing relatively little dispersancy characterisitcs.

Thus the results set out in Tables 1, 3 and 5 herein show that additive (a) as defined in the claims of the present application imparts viscosity characteristics comparable to those of similar compositions containing conventional polymethacrylate viscosity index improvers while having greater resistance to gel formation than polyacrylates in the presence of additive (b). Moreover, the additive combinations employed in the present invention are more elfective as dispersants in paper strip chromatographic tests than prior art combinations of alkyl polymethacrylates with basic detergent additives, despite the individual components of the combination showing relatively little dispersancy characteristics.

7 8 TABLE 2 COMPARATIVE VISCOSITY DATA acrylate component comprises at least 50% alkyl methacrylate units in which the alkyl Vlmslty (st) Viscosity group contains from 1 to 10 carbon atoms, and Polymer concentrate 100 F. 210 F. index (b) a detergent amount of an alkaline earth metal Polymer A 32 9 22 160 containing detergent having a total base number 58.36 9.30 152 greater than 20. $123 312 2. A lubricating composition according to claim 1 PolymerJ 57.04 9.04 149 wherein additive (21) comprises from 10 to 30% alkyl acryl ate units.

TABLE 3.-GELLING TESTS Additive (b) Weight Calcium overbased alkyl ratio benzene sulphonate *Hitec E627 is a commercially available basic detergent based on an over-based barium salt of a phosphosulphurised polyisobutylene and having a TBN of from 70 to 85.

TABLE 4.COMPARATIVE GELLING TESTS Additive (b) Weight Calcium overbased alkyl ratio benzene sulphonate Additive (a) (a) (b) ('lBN 300) Hitec E627 Polymer concentrate:

A No viscosity increase No viscosity increase.

do Do. do Small increase.

Very viscous"rubbery Small viscosity increase.

Small viscosity increase do N o viscosity increase Very v1scous-rubbery Hitcc E627 is a commercially available basic detergent based on an ovcrbased barium salt of a phosphosulphurised polyisobutylene and having a TBN of from 70 to 85.

TABLE 5.Dl'SPERSANCY TESTS Specific Comp n n Component dispersaney R; value (mean of (mean of Percent Percent duplicate duplicate Component dosage Component dosage tests) tests) 8.0 0.03 8.0 Calcium ovcrbased alkyl benzene sulphonate soap TBN 300) 3. 0 0. 17 8.0 Calcium overbascd alkyl benzene sulphonate (20% soap TBN 800) 3.0 43 0.21 8.0 Magnesium overbascd alkyl benzene sulphonate (TBN 400)- 2. 0 69 *0. 09 8.0 22 0.02 8.0 Calcium overbased alkyl benzene sulphonate (30% soap TBN 300) 3. 0 62 0. 16 8.0 Calcium overbased alkyl benzene sulphonate (20% soap TBN 300) 3. O 0. 32 8.0 Magnesium overbased alkyl benzene sulphonate (TBN 400) 2.0 68 0 10 8.0 13 0 01 8.0 Calcium overbased alkyl benzene sulphonate (30% soap TBN 300) 3.0 56 0. 26 8.0 Calcium overbased alkyl benzene sulphonate (20% soap IBN 300) 3. 0 78 0. 45 Do 8.0 Magnesium overbased alkyl benzene sulphonate (TBN 400) 2.0 44 0, 1o Commercially available dis- 8. 0 48 persant concentrate of alkyl methacrylate/N-vinyl-2- pyrrolidone copolymer.

Single test carried out.

We claim: 3. A lubricating composition according to claim 1 1. A lubricating composition consisting essentially of; 60 wherein the akyl groups containing from 1 to 10 carbon (i) a i l il of l b i ti i i d atoms of the alkyl methacrylate units comprising at least (ii) f 1 to 15% b eight based on h total 50% of the alkyl methacrylate component of additive weight of the composition of a combination of addi- (a) are alkyl groups contalmng from 8 to 10 Carbon atoms.

4. A lubricating composition according to claim 1 wherein substantially all alkyl methacrylate units comprising additive (a), other than the alkyl methacrylate units in which the alkyl group contains from 1 to 10 carbon atoms, are alkyl methacrylate units in which the tives consisting of:

(a) viscosity index improving amounts of a nitrogenfree random copolymer of an alkyl acrylate component and an alkyl methacrylate component wherein the copolymer is composed of from about 5% to 65% alkyl acrylate units alkyl group contains 12 to 18 carbon atoms based on the total number alkyl methacry' 5. A lubricating composition according to claim 1 coml e and alkyl acrylate unlts P 111 prising 1 to 5% by weight, based on the total weight of copolymer, the alkyl acrylate component is the composition, f additive Se ec from the group consisting of methyl 6. A lubricating composition according to claim 1 acrylate, ethyl acrylate and mixtures of methyl wherein additive (b) has a total base number greater acrylate and ethyl acrylate and the alkyl meththan 40.

7. A lubricating composition according to claim 1 wherein additive (b) is an overbased alkaline earth metal salt of a compound selected from the group consisting of alkyl benzene and petroleum sulphonic acids, the alkaline earth metal being selected from the group consisting of calcium, barium and magnesium.

8. A lubricating composition according to claim 1 wherein additive (b) is an overbased alkaline earth metal salt of a phosphosulphurised hydrocarbon.

9. A lubricating composition according to claim 1 comprising from 1.0 to 5.0% by weight of additive (b).

10. A lubricating composition consisting essentially of;

(i) a mineral oil of lubricating viscosity, and

(ii) from 1 to 15% by weight based on the total weight of the composition of a combination of additives consisting of;

(a) from 1 to 5% by weight based on the total weight of the composition of a nitrogen-free random copolymer of an alkyl acrylate component and an alkyl methacrylate component wherein the copolymer is composed of from 15 to 25% alkyl acrylate units based on the total number of alkyl methacrylate and alkyl acrylate units present in the copolymer, the alkyl acrylate component is selected from the group consisting of methyl arcrylate, ethyl acrylate and mixtures of methyl acrylate and ethyl acrylate and the alkyl methacrylate component comprises at least 50% methacrylate units in which the alkyl group contains from 8 to 10 carbon atoms, substantially all other alkyl methacrylate units being alkyl methacrylate units in which the alkyl group contains 12 to 18 carbon atoms, and

(b) a detergent amount of an alkaline earth metal containing deter-gent having a total base number greater than 40.

11. An additive package comprising a mineral oil having dissolved or dispersed therein from 5 to by weight, based on the total weight of the additive package of:

additive (a) which is a nitrogen-free random copolymer of an alkyl acrylate component and an alkyl methacrylate component wherein the copolymer is composed of from about 5% to alkyl acrylate units based on the total number of alkyl methacrylate and alkyl acrylate units present in the copolymer, the alkyl acrylate component is selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures of methyl arcrylate and ethyl acrylate and the alkyl methacrylate component comprises at least 50% alkyl methacrylate units in which the alkyl group contains from 1 to 10 carbon atoms, and from 5% to by weight, based on the total weight of the package of:

additive (b) which is an alkaline earth metal containing detergent having a total base number greater than 20.

References Cited UNITED STATES PATENTS 2,991,246 7/1961 Kleinholz 25233.4 X 3,350,308 10/1967 McMillen 252--33.4 X 3,368,971 2/1968 Retzloff 252--33.4 3,451,931 6/1969 Kahn et al. 25232.7

H. M. S. SNEED, Primary Examiner US. Cl. X.R. 252-56R