US2936796A - Paper-makers' dryer felt - Google Patents

Description

May 17, 1960 T. HINDLE ETAL 2,936,796

PAPER-MAKERS' DRYER FELT Filed July 3, 1956 2 Sheets-Sheet 2 Male/@Kaff Mama' bfi/de l I 7a/ward Have United D States Patent-O 2,936,796 PAPER-MAKERS DRYER FELT Thomas Hindle and Edward Race, Blackburn, England,

assignors -to Scapa Dryers Limited, Blackburn, Eugland, a corporation of England Application Juiy s, 1956, serial No. `595,616

16 claims. (ci. 139-426) This invention pertains to paper-makers dryer felts, which term is explained below and which are endless bands or belts of woven textile fabric employed for supporting and conveying the paper web delivered by a papevmaking machine while the web is moved through a drying zone or zones; the ypresent invention relating more especially to va felt of novel construction having a much longer useful life than felts as heretofore constructed.

Paper-makers dryer felts are of great length and for use in making certain types of paper may be very wide. Such felts must be woven by the use of special looms and because of their size and thickness, great quantities of yarn are used in their construction, sorthat the total cost of such a felt to the paper manufacturer is substantial, often running into many thousands of dollars. On the other hand, `such felts Aare subjected during use to very exacting conditions, being called upon to run without intermission for twenty-four hours a day; to flex rapidly about heated cylindrical drums and felt rolls of relatively small diameter; to withstand the effects of moisture; to withstand the degrading action of certain chemicals which are invariably present in the paper web, and of the Vabrasive wear and oil contamination to which they may be subjected especially at their marginal portions. -At the same time, they must be operative to draw away the moisture from the paper web and rapidly to ice of the customary cotton upon the theory that, because rof the well-recognized resistance of asbestos to the eiects von that section of the dryer part of the paper machine to be clothed. In recent years, other materials than wool, cotton and asbestos have been suggested as useful in the manufacture of dryer felts; for example, some of the synthetic fibers such as nylon or Daeron.

Although the use of some of these other materials in the manufacture of dryer felts has been proposed, they have not, as yet, superseded cotton, since cotton does possess certain desirable basic characteristics for use in such felts. For example, it is relatively cheap; it is of high tensile strength; it possesses reasonable abrasion resistance and has a relatively high modulus of elasticity under low load so that it can be made into fabrics of good dimensional stability. On the other hand, from the yield it in liquid form or dissipate it in the form of vapor; A

to provide a substantially uniform and smooth surface upon which the paper web rests and such as Ywill not impart any appreciable pattern etect, for example of the weave structure, to the wet paper web.

In the early daysof .paper-making and paper-making machines, thesel felts were of wool and were subjected to operations during manufacture, which doubtless justiiied the use of the term felt in this connection. However, these wool felts were gradually superseded by cotton, and were at first of single -ply cotton duck, but such single ply fabrics were in turn, superseded by multi-ply fabrics which afforded a greater degree of softness and smoothness, which are highly desirable qualities, especially `when the felt is to lbe, used in the manufacture of the finer grades of paper. The expressionY (fply) Vis used herein not in its limited sense as'regards the construction of weave, but to define layers in which one material predominates compared 'with another orothers, it being obvious that there must be some migration of Vsuch other ful life was shortened by degradation of the cotton so as Y to be much shorter than desir-able, from the paper-makersv p standpoint owing to the high cost of replacing such a felt.

Such degradation wasassumed to be` due to the heat, and in the hope of extending the useful life of the felt,

y it was proposed` to employ asbestos for a part,.at least,

-heat and acid resistance.

dryer felt standpoint specically, cotton has some undesirable characteristics, in particular its poor resistance to heat, especially under moist conditions. Moreover, in the presence of Weak acids and/ or trace amounts of iron (often encountered in the paper-drying operation), the degradation of cotton in the presence of moisture and heat is greatly accelerated. Furthermore, cotton, as compared with most of the other natural fibers, has poor iiexural endurance. As compared with cotton, certain synthetic fibers, notably nylon and Dacron, are superior in strength, abrasion resistance, ilexural endurance and In these respects, nylon has the greater abrasion resistance and ilexural endurance ,while on the other hand Daeron is outstandingly superior to nylon in its resistance to heat and in its ability to resist attack by acids at high temperatures.

Thus, it appears that these particular synthetics, nylon vor Daeron, `might well be usedin the manufacture of dryer felts; for instance, by making dryer felts Wholly of nylon or Daeron. yOn the other hand, the initial cost of felts made wholly of nylon or Dacron is much higher than those made wholly of cotton or cotton and asbestos because of the cost of the raw materials employed. Experience has shown that the increased service life of such all-synthetic felts is not always suicient to justify the substitution of all-synthetic felts for those of the materials heretofore commonly used. Thus, it has been suggested that -a practical solution of the problem might be found in using a combination of the synthetic iibers with cotton or cotton and asbestos. It was iirst suggested that the mere blending of nylon or Daeron staple ber with cotton would provide a long-wearing dryer felt, but it was soon realized that since cotton fibers are'degraded by heat oracid, so that they break into shorter fibers as they are vcontinuously iiexed about the rolls of a papermaking machine, the mere incorporation of a small percentage of synthetic fibers would not greatly retard the disintegration of the cotton yarns. Further experiment showed that desirable results could be obtained by twisting continuous lament yarns of synthetic material with cotton yarns in such a manner that the filament yarns would lform open helical wrappings around a core of cotton, and that if the pitch of the synthetic helix is not greater than half the length of the individual cotton fibers, vthe yarn will tend to maintain its integrity even though the 121/2 ofthe synthetic material, representing a cost inass-aree crease of only 15% or 16%, as compared With all-cotton felt, the increase in the life of the felt was far in excess of that required to compensate for the added cost of manufacture. Such a fabrici'is more fully disclosed in thle pgegnt to Hindle and Hall, No. 2,882,933, dated April 2 `As above noted, the incorporation of asbestos yarns in a dryer felt has been very Widely adopted because the inclusion of the asbestos was supported to impart added resistance to degradation of the felt structure by the action lof heat. It is true that asbestos-cotton felts have Shown greater dimensional stability and a longer life than the all-cotton felt, but recent experimental investigation appears -to indicate that the longer life of the asbestos-cottn felt, as compared with the all-cotton felt, is not due to the fact that asbestos itself is substantially unaffected byv heat, but that certain other properties of the asbestos are responsible for the improved results. Customary practice in the use of asbestos in a dryer felt (it being noted that yarns made wholly of asbestos fibers are too weak toI be used in the manufacture of a heavy textile fabric) is to blend from to 20% of cotton or other cellulose fiber with the asbestos to assist in the spinning and weaving operations. Even such yarns are relatively weak and it is not uncommon to use asbestos yarn with a cotton core. such that the asbestos yarn may contain up to 30%. or more of cellulosic fibers, representing up to 90% of the total strength of the yarn. Customarily, the asbestos yarns are so interwoven that they appear on the Working face of the felt, that is to say, the face on which the paper web rests, and consequently the cotton component of these composite yarns is fully exposed to the heat and moisture conditions, so that the cotton degrades rapidly and there is a substantial loss of strength. Thus,

'to which the integrity of the back cotton Warp yarns is maintained, and thus the cotton yarns in the back of the fabric should be protected so far as possible from the effects of the heat to which the face of the fabric is eX- posed At first glance, it might be thought that the asbestos at the face of the fabric would protect the co-tton at the back of the felt, but it is a proven fact that asbestos is a poor heat insulator as compared with most other fibrous materials, in spite of the fact that asbestos is commonly employed as an essential component in many insulating fabrics. In the course of investigation, an allcotton felt was compared with a felt in which all of the filling yarns were asbestos and the warps of the Working face of the felt were asbestos (that is 'to say, asbestos reinforced with cotton), these felts being subjected, at their Working faces to a temperature of 350 F. Rather astonishingly, it was found that these asbestos-cotton felts sustained a greater loss in strength than the all-cotton felt and that the loss in strength of the face Warp yarns of these asbestos-cotton felt Was as great as the loss in strength of the face warp yarns of the cotton felt. Obviously, this was due to the fact that virtually the entire strength of the asbestos face yarns resided in their cotton components. To overcome this loss in strength in the asbestos face yarns, it was proposed to substitute Dacron for the cotton in the composite asbestos yarns, but this arrangement still failed to produce optimum results because asbestos is not a good heat insulator but on the other hand is, in fact, a good conductor of heat as compared with organic fibres, so that the cotton warp yarns of the fabric were degraded from 50% to 100% more than those of an all-cotton felt under similar conditions of treatment.

-Apart from degradation by heat and by mechanical causes, there are two agencies which play a major part in` thel destruction of a papermakers felt particularly of a It wasY ,4 cotton felt or a cotton containing felt though affecting other materials, including the synthetic materials aforesaid, to Ya lesser degree. One is acid or even a mildly acid condition and the other is iron in some of its compounds. Thus, the rate of degradation of cotton at 212 F. and a pH value ofV 4.5 is twenty times that of cotton at 212 F. and a pH value of 6.5,. Moreover, in the presence of as littler as one part in 10,000 of ferrie oxide, cotton at 212 F.. and at a pH value of 6.5 is degraded manyv times more rapidly than iron-free cotton under the same conditions. It thus appears that they major contribution of asbestos to the lengthening of the service life of a dryer felt is its heretofore unrecognized ability to protect the cotton or other component of the felt from degradation by acid and by the hydrolytic decomposition which also affects other material than cotton and is catalyzed by the ever-present traces of iron compounds.

4low as' 2.5 and a sulphate content as high as 2.4%. The rate of degradation of cotton at high temperatures increases very rapidly with decrease in pH value and it is believed that acid degradation plays at least as great a part as the heat in the destruction of cotton dryer felts When asbestos is present in the felt, it shows a preferential absorption of acid, the acid being takenV up exclusively by the magnesium oxide component of the asbestos with the formation of water soluble magnesium sulphate. It has also been shown by experiment that the asbestos is capable of rendering any iron compounds harmless to the cotton. Thus, samples ofall-cotton felt and an asbestos-cotton felt were treated (every four hours for a period of 48 hoursyby alternately wetting them in a solution of an iron salt containing one part in 10,000

of iron and then drying at a temperature of 220 F.,

iron An asbestos-cotton felt free from added iron 23 An asbestos-cotton felt containing a small amount of added iron 25 That is, the loss in strength in the presence Yof iron and absence of asbestos is four times the loss when iron is not present; and, when both asbestos and iron are present, the iron causes no appreciable increase in the rate of degradation of the cotton.

The present invention is based upon the above discoveries and provides adryer felt which, in respect to cost of production, as compared with increased service life, swholly practical. Briefly stated,'the dryer felt of the present inventionconsists of textile yarns interwoven to form a multi-ply fabric, said fabric comprising a top or face layer for contactwith the paper web, which is predominantly of a material which is a good heat insulator, and which is substantially unaffected by exposure to moisture, to acid, to iron and to the temperatures encountered in the use of ajdryer felt; an intermediate layer integrally joined' to the top or face layer and underlyandwhich is of a tensile strength 'comparable'to that of Acotton and which is capable of accepting all of the moisture1passed to it from the face of thejfelt, and which is capable of readily releasing this moisture as vapor. Preferably, the face layer is of Daeron, amaterial which can, by capillarity, accept v ery high quantities of water, but .which has such a low'moisture regain that it cannot always .hold the water it accepts; and, which is virtually unaffected by, but will not absorb either acid or trace quantities ofy iron. Therefore, the back layer of the felt should becomposed of yarns of a material capable of accept- `in g all vof the moisture which passes through the felt from its front face and which is capable of readily releasing this moisture in the form of vapor. Cotton has these latter characteristics as well as substantial tensile strength, and is desirable for the back face of the felt.. Since Dacron does not absorb acid or trace quantities of iron, i-t doesnot protect the cotton back from the effects of these reagents, althoughl it does have high insulating value and thus protects the cotton back from heat degradation. To protect the cotton from the action of acid and iron, the intermediate layer should be of a character such as to absorb the acid and nullify the eects of minute quantities of iron, and asbestos provides these characteristics. Y

Jn accordance with the present invention, warp-faced fabricsv have been woven comprising t'wo, three or four plies, wherein the exposed surface. layer of the face ply consists predominantly of Dacronr warp yarns; the back ply warps are of cotton predominantly, preferably reinforcedwith helically wound synthetic filament yarns, and all of the filling yarns are asbestos yarns which constitute ythe intermediate layer between the face layer of Dacron andthe back layer of cotton. Also, weft-faced fabrics have been woven in which the exposed face weft lyarns are of Dacron; the center and back weftl yarns are of asbestos and the whole of the warp-yarns are of cotton, preferably reinforced -with helically-wound synthetic filamentryarns.' No particular weave structure appears to be necessary to obtain highly desirable results. The expression asbestos yarns is t used above and hereinafter to mean a yarn madeof asbestos fibres suitably reinforced by other fibres, such as cotton. The superiority in heat resistance of this new type of Dacron-faced, asbestos-cotton filled dryer felt over conventional asbestos-cotton feltsmay be illustrated by the following example wherein a three-layer felt, according to the present invention, was compared with a conventional vasbestos felt of identical weave structure, by subjecting the working face of each ofV them to the same heat ,conditions for a given time. Under these circumstances, the conventional asbestos-cotton felt lost 48% of its initial strength, whereas the Dacron-faced felt, according Ito thepresent invention, lost 7%. The face warp of the conventional asbestos felt lost 71% of its initial strength whereas the Dacron face warp of the felt of the present Vinvention lost 9%. The cotton back of the asbestos felt Ylost'55%, While the cotton Vback of the Dacron-faced felt lost 12%.

In the accompanying drawings,which are wholly diagrammatic- Y Fig. l is a fragmentary diagrammatic plan view of a section of a paper-makers dryer felt embodying the present invention; Y'

Fig. 2 is a fragmentary diagrammatic transverse section on the-line 2-2 of Fig. l, illustrative of a felt ernfhodyng the essential characteristics of a felt constructed according to the present invention;

` Fig. 3 is a diagram illustrative of a weavestructure, which gives a substantially warp-faced fabric, such as may be employed in a three-plyfelt embodying the invenf tion',

Fig. 4 is a view of the same kind as that of Fig. 3, but illustrating another weave structure which givesja weftgfaced fabric. t

i Figs. 5, 6 and .7 are views similar to Fig.- 2 but illustrative of other embodiments of the invention;

Fig. 8 is a side elevationL showing a composite yarn comprising asbestos reinforced with cotton; and

Fig. 9 is a view similar to Fig. 8, showing a composite yarn comprising cotton reinforced with nylon.A

Referring to Fig. l, the character B designates the body portion of a felt made according to the present invention; While the characters M designate the marginal portions of the felt and which may, if desired, be of the construction and materials more fully described in the patent to Hall, No. 2,612,190, September30, 1952, and which provides high resistance, for the marginal portions of the felt, in particular, to the effects of abrasion and exposure to oil. However, this particular marginal construction is optional, withv respect to the felt of the present invention.

Referring to Fig. 2, which is a diagrammatic transverse section through the body portion of the felt, and without any attempt to indicate actual weave structure, the numeral 10 designates the exposed face of the top layer of a three-layer fabric, the numeral 11, the intermediate layer, and the numeral 12, the exposed face of the back or rear layer. As illustrated in Fig. 2, the exposed face of layer 10 consists of Dacron combined with a certain percentage of nylon. The relative proportions of nylon and Dacron in this face layer will depend upon the specific conditions to which the felt will be subjected during use. Thus, for example, when excessive moisture is present or there is severe mechanical abrasion, or the felt rolls are of unusually small diameter, the nylon should be in major proportion. On the other hand, when acid conditions lower than pH 4.5 are encountered, or when the felt runs very hot and unusually dry, then the Dacron should be in major proportion.

tAs illustrated in Fig. 2, the intermediate layer 11 consists of asbestos reinforced with cotton. Thus, it may comprise a yarn consistingV of a cotton core 13, Fig. 8,

rwith a Wrapping 14 of asbestos roving or the like, or it may consist of any other desirable or usual composite Ayarn in which asbestos is reinforced inA one way or another with a stronger fiber. The back layer 12 of this fabric of Fig. 2 is shown as consisting of cotton yarn reinforced with nylon.

While the arrangement illustrated in Fig. 2 suggests that the asbestos be confined to the intermediate layer 11, it is to be understood that a greater proportion of asbestos may be employed if desired, for example all of the lling yarns of layers 11 and 12 may be of asbestos yarns, or the filler yarns of all three layers may be asbestos.

vInFig. 5, the face layer 10a is shown as consisting wholly of Dacron, while the intermediate layers 11a and fthe back layer 12a are like those of Fig. 2. In Fig. 6, Vthe face layer 10b is of Dacron; the back layer 12bis of cotton, while the intermediate layer is of asbestos yarn, reinforced with cotton, while in Fig. 7, the face layer 10c is of Dacron, the back layer 12e` is of cotton and theintermediate layer 11e` is of asbestos yarn.

Referring to Fig. 3, there is shown an actual weave 'structure of aj three-ply, warp-faced woven fabric in which the surface layer Vof the upper ply 10 is comprised of the warps 1, 2 and 3. Y The warps` 1, 2, and 3 are of Dacron, for example 5s/3 (cotton count) spun Dacron.

` Thev warps 4, 5, v6, 7 and 8 are composed of, for example,

8s/ 6 cotton plied with two filament yarns, each of 210 total denier, of nylon. The warps 2 and 3 form stitches ,integrally uniting the face ply 10 and the intermediate v ply 11, while the warps 6 and 7 form stitches Whichiintegr-ally unite the intermediate ply 11 and back ply.` 12. In this fabric all of the ller yarns a, b and c are of asbestos; or, more strictly speaking, asbestos spun around acore of 6s cotton or spun nylon to a resultant count of 0.8s. The Dacron warps employed in this structure are highlyresist-aut to degradation by heat or acid and .575i

securely bind in the iiller yarns a, b and c. The filler yarns, being of asbestos, act to neutralize the effects of acid and/or iron upon the cotton warps `4, 5, 6, 7 and 8.

In the fabric of Fig. 4, which is also a three-ply fabric, the face layer 10x is comprised of weft yarns, and the whole of the warp is composed of 40 ends per inch of Ss/ 6 cotton reinforced with two filament nylon yarns each of 2l() total denier. The filler yarns a are of 5s/ 3 Daeron. The filler yarns b and c of asbestos; for example, asbestos containing 10% to 15% of cotton spun laround a core of 20s/3 cotton to a resultant count of 0.8s.

When nylon is employed as a reinforcement for cotton, it is preferable so to combine the cotton and nylon strands that the nylon forms an open helix or helices embracing the cotton as illustrated in Fig. 9, and although the asbestos yarn may consist of an asbestos strand wrapped about a cotton or nylon core, such composite yarn may alternatively itself comprise an asbestos core encased in an open jacket consisting of helical windings of the reinforcing synthetic material.

Since, as above noted, the particular reason for employing the asbestos is to provide a barrier which preferentially absorbs any acid present and neutralizes the latter, and also to protect the cotton yarns from the degrading effect of minute traces of iron, it is contemplated that the proportion of asbestos in the felt may be substantially reduced as compared with the customary amount, without, in any, lessening the service life of the felt. Thus, the asbestos fillers in the bottom and intermediate plies, for example, might be replaced under certain circumstances, with cotton or in particular cotton reinforced with nylon, or with some other suitable organic ibrouskrnaterial, thus reducing Weight and dif# ficulties in manufacture.

When herein reference is made to the face ply of the felt as, for example, of Daeron or as predominantly of Dacron, itis to be understood that this terminology is to be interpreted as meaning that the exposed surface layer, at least, of the said ply is predominantly of Dacron, although the ply itself may also comprise -a substantial percentage of yarns of other material, for example, asbestos.

While certain desirable embodiments of the invention have herein been suggested by way of example, it is t be understood that the invention is broadly inclusive of any and all equivalents and modifications coming within Athe terms of the appended claims.

We claim:

1. A paper-makers dryer felt consisting of textile yarns interwoven to form a multi-ply fabric, said fabric comprising a top or face layer whose exposed surface, which contacts the paper web, is predominantly of an organic material which is substantially unaffected by exposure to the acids, compounds of iron, or the temperatures en- ACPOllntered in the use of a dryer felt and which is a good heat insulator, an intermediate `layer integr-ally joined to the face layer and underlying the latter, said intermediate layer comprising a material which reacts'chemically with acids initiallypresent in the paper to neutralize the same, and a back layer integrally joined to the intermediate layerand which comprises predominantly a material having a tensile strength, moisture absorption and modulus of elasticity similar to those of cotton.

2. A paper-makers dryer felt according to claim l wherein the heat insulating material which forms the exposed surface of the top layer is synthetic material, and the acid neutralizing material of the intermediate layer is asbestos.

43. A paper-makers dryer felt according to claiml wherein the exposed surface of the face layer comprises predominantly Daeron, the rear layer comprises predominantly cotton, and the intermediate layer comprises a material operative to shield the cotton of the rear layer from the effects of acid and/or iron in the paper web.

4. Apaper-makersdryer felt according to'claim 3` esserne the filler yarns of all plies are of asbestos reinforced with an organic filamentous material.

6. Apaper-makers dryer felt according to claim 1 wherein the exposed surface of the face layer is predominantly of Daeron in the form of warp yarns, the exposed surface of the rear layer is formed of warp yarns predominantly of cotton, and the filler yarns in each of the several plies are of asbestos reinforced with an organictilamentous material.

7. A paper-makers dryer felt according to claim 1 and comprising more than two plies, wherein the face ply is a warp-faced fabric in which the warps are of Dacron, the intermediate layer comprises wefts of asbestos reinforced with cotton, and some, at least, of the Daeron VWarps of the face layer form stitches at intervals which unite the face and intermediate layers.

8. A paper-makers dryer felt according to claim 1 Awherein the face ply is a weft-faced fabric in which the `face wefts are of Daeron, the intermediate and back wefts are of asbestos and -all of the warps comprise cotton.

9. A paper-makers dryer felt according to claim 8 wherein all of the warps are of cotton reinforced with a filamentous material.

10. A paper-makers dryer felt according to claim l wherein the' exposed surface of the face vlayer consists of yarns of a synthetic material which is a heat insulator superior to asbestos and which is substantially inertl as respects the action of moisture, high temperature, acid or iron to which the felt is exposed during normal use, the warps of the rear ply are predominantly of cotton, and means interposed between the exposed surface of the front ply and the rear ply which is operative to shield Vcotton comprised in the rear ply from the action of acid Y or iron penetrating from the face ply.

, Y 11. A paper-makers dryer felt according to claim -10 wherein the means for shielding the cotton of the rear ply comprises asbestos contained in the filling yarns in certain at least of said plies.

12. A paper-makers dryer felt consisting of textile yarns intervwoven to form a multi-ply fabric, said fabric comprising a face ply, the surface of which contacts the paper when the felt is in use, whose warps are predominantly of Daeron reinforced with nylon, and the Vwefts which form an intermediate layer are of asbestos reinforced with nylon.

13. A paper-makers dryer felt consisting of textile yarns interwoven to form a multi-ply fabric, said fabric comprising a face layer, which contacts the paper web when the felt is in use, whose exposed surface ispredominantly of a synthetic material which is highly resistant to heat within the range to which the papermakers dryer felt is normally exposed, which is a good heat insulator, which is non-absorbent as respects acid or traces of iron commonly present in the paper web, and which is readily capable of transmitting moisture from the paper web to an intermediate layer of the felt, the rear layer of the felt comprising predominantly organic mateal which is highly absorbent of moisture, and a material within the structure of the `felt, which is operative to neutralize any acid and to shieldthe rear layer from the effects of any iron compounds held in suspension or solution in the moisture which istransmitted from the frontlayer to the rear layer.

14. A lpaper-makers dryer felt consisting of textile yarns interwoven to form a multi-ply fabric, the exposed surface ofthe face layer, which contacts the paper web when the felt is in use, being predominantly of an organic material which is a good heat insulator, certain of the yarns comprised in the rear layer of the fabric being of cotton, and a layer of moisture-pervious material which reacts between the exposed surface of the facev layer and said cotton yarns for shielding the latter from the degrading action of any acid and/or iron compounds to which the felt is exposed during use.

15. A paper-makers dryer-felt comprising textile yarns interwoven to form a multi-ply fabric, said fabric havv ing a top or face layer whose exposed surface, which contacts the paper web when the felt is in use, is pre-v comprises asbestos, and a back layer integrally joinedto the intermediate layer and which comprises predominantly cotton.

16. A paper-makers dryer-felt comprising textile yarnsY interwoven to `form a multi-ply fabric, said fabric having a paper-contacting surface which is predominantly of Daeron, -an intermediate layer comprising asbestos and av rear layer comprising predominantly cotton.

Y References Cited in the le ofthis patent UNITEDVV STATES PATENTS 2,152,415 Krasse1t Mar. ze, 1939 2,506,667 Hall May 9, 1950 2,540,874 Geddings Feb. 6, 1951 2,542,297 Sunbury et al. Feb- 20, 195,1 2,604,689 Hebeler July 29, 1952 FOREIGN PATENTS 133,874 Australia Aug. l5, 1949 610,171 Great Britain Oct. 12, 1948 710,078 Great Britain June 9, 1954

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