US2706342A

US2706342A - Veneer drying methods

Info

Publication number: US2706342A
Application number: US125647A
Authority: US
Inventors: Edward H Willis
Original assignee: OSCAR C SUNDSBY
Current assignee: OSCAR C SUNDSBY
Priority date: 1949-11-04
Filing date: 1949-11-04
Publication date: 1955-04-19
Anticipated expiration: 1972-04-19

Description

United States Patent VENEER DRYING METHODS Edward H. Willis, Seattle, Wash., assignor of one-third to Oscar C. Sundsby, Seattle, Wash.

No Drawing. Application November 4, 1949, Serial No. 125,647

13 Claims. (CI. 34-95) This invention relates to veneer drying. More particularly, it pertains to a method for rapid heat-drying of rotary cut veneer, commencing with the wood substantially in the green state and continuing until dry, and the general object is to accomplish this rapid drying operation without rendering the veneer brash, causing objectionable cracking and warping of the veneer, well known major sources of waste. The method is to be distinguished, both as to purpose and effect, from the oil bath treatment of wood which is already substantially dried.

The difficulty of drying green veneer without causing such waste-producing defects becomes especially great in the case of plywood core stock rotary cut veneer which is cut from the log nearer and nearer to its center where the wood becomes more knotty and the curvature sharper. Difficulty with this source of waste has ben encountered in all previous drying techniques known to me, including those involving use of supporting rolls. Cracking and warping are especially prevalent in conventional kiln or tunnel drying because in that case the veneer, being first cut into short lengths for stacking, must be stretched out flat for drying. A stack of these flattened horizontal pieces is formed on a kiln truck, each piece being separated from those above and below it by a group of spacers or separator sticks.

In order to combat cracking and warping during drying it seemed necessary in the past, especially in kiln or tunnel drying, to carry out the operation slowly at low temperature. Even then, however, much wastage occurred, and at the low temperatures employed drying became prolonged and more expensive.

In the light of these difficulties, for which, to my knowledge, there has been no effective solution heretofore proposed, the primary object of my invention is to provide a veneer drying process which, at minimum cost, practically eliminates objectionable Warping and cracking during or as a result of drying of green veneer and at the same time enables drying at much higher temperatures and for a shorter time. 1

A further and related object is to accomplish the foregoing without rendering obsolete most existing or conventional drying apparatus in which the plywood industry has a large investment. Moreover, the apparatus or facilities required in addition to that already being used can be extremely simple and easily accommodated in existing installations. The initial cost and the continuing expense are both negligible by comparison with the savings effected.

In its broad aspect my invention resides in the discovery of a process uniquely advantageous for the rapid drying of green veneer, especially rotary cut soft wood veneer, such as Douglas fir, pine, spruce, cottonwood, etc. Since most hardwood veneers are sliced, instead of being rotary cut by a peeler lathe, the problem of instant concern will not ordinarily be sufficiently acute in that case to justify special drying practices.

My discovery essentially is that if a small quantity of oil is supplied to rotary cut veneer as a plasticizer before or during drying it will notbecome brash and crack or warp appreciably, but when dry will be pliable and flexible. It is discovered, moreover, that only comparatively minute quantities of oil need be carried to the veneer in order to obtain these results. For instance, by entraining a quantity of oil vapor in the heated air circulated through the dry kiln, or by applying oil to the separator sticks, either by spraying them or dipping their ends,

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before utilizing them in stacking the veneer sheets on trucks to enter the kiln, splitting and warping of the veneer during drying are greatly reduced, if not altogether eliminated.

Oil may be supplied to the veneer in various alternative ways, the word supply as herein used being intended as a generic term to include all herein described techniques and equivalents thereof by which oil molecules may be caused to come in contact with the veneer surface in accordance with the invention. One technique involves dipping the veneer sheet or strip end portions. The surface of the veneer may be sprayed or otherwise coated lightly, such as by brushing, either uniformly or in patches or zones, as may be convenient. Veneer-permeating oil vapor may be imparted to the kiln atmosphere by a process of evaporation of oil from troughs or other containers located preferably adjacent to the veneer stacks, or sprayed in a fine mist into the circulating air. Whether the stacks move slowly on trucks through the kiln, are shifted periodically, or remain stationary therein for drying in separate batches, is immaterial. Where applicable, similar techniques may be employed effectively if the veneer is carried continuously on rolls during drying.

The exact scientific theory explaining the plasticizing effect of the oil on the veneer discovered is unknown to me, but the effectiveness of the process, as determined by repeated tests, is undoubted. It is practically impossible to prescribe generally any minimum or maximum quantities of oil required, because some benefit is to be derived from comparatively minute, although appreciable, quantitics imparted to the veneer, while the benefit derived increases if a somewhat larger quantity of oil is imparted to the veneer. The veneer should not be saturated or completely coated with oil, however, or even infused with it to a substantial degree. On the contrary the oil application is essentially superficial in nature and the quantity of oil used should be sufficiently small to avoid any perceptible difference in the surface appearance or color of the dried veneer over that dried in the conventional way.

Consider, for example, the technique in which oil is imparted to the veneer solely from oil-dipped separator sticks used between the veneer sheets in stacking them on a kiln truck. These sticks are usually /2 inch to 1 inch thick and, in supporting standard 26 /2 inch x 50 inch veneer sheets, are usually three in number, arranged parallel transversely of the grain and length of the sheets, such sticks being spaced approximately equally. The sticks are dipped momentarily in oil, surplus oil is allowed to drain from them, and then the sticks are inserted between veneer sheets. During drying a sufficient quantity of oil is attracted by capillary action, or evaporation and absorption, to the veneer from the separator sticks alone to plasticize it enough to allay virtually all tendency for the wood to crack and warp, thus drastically reducing wastage.

Oil may be supplied or imparted more directly to the veneer for accomplishing the same purpose, for example, by suspension of oil in the atmosphere, or by spraying or brushing oil directly on the veneer, as mentioned. The oil will then be distributed over the wood surface by capillary action or by evaporation and absorption. The quantity of oil preferred depends, of course, on the variety of wood and the type of oil used. Moreover, the quantity used by a particular manufacturer of veneer will also depend on his concept of the desirable balance between reduction in cracking and warping of the veneer and the tolerable residue of oil in the wood for the purpose for which the veneer is to be used. In the case of core stock veneer for plywood, a certain amount of cracking is permissible whereas the surface layer stock must be free from cracks. Moreover, the quantity of oil required further depends in a given case upon the drying temperature, because when temperature is increased the wood tends to be rendered more brash and the plasticizing role of oil application in preventing cracking is correspondingly more important. In general, it may be said that any slight traces of oil found naturally in the atmosphere of a dry kiln will have no noticeable effect. However, an appreciable amount of oil deliberately supplied greases to the veneer by adding oil vapors to the atmosphere, or by applying liquid oil directly to the veneer as indicated, will produce the striking effect I have observed and described herein.

Concerning oils useful in practicing the method, some are more effective generally than others. In this regard, the properties of the particular wood, the drying ternperature and other factors influence the select1on. Dipping the separator sticks in any mineral 011 having an SAE viscosity rating of 40, for example, produces very satisfactory results in the drying of Douglas fir veneer at 340 degrees F., in a tunnel type dryer with such sticks interposed between the veneer sheets as spacers, but 1t is to be understood that lower or higher viscosity oils are also suited to the method. Moreover, numerous types of vegetable oils can be used, such as linseed, castor, soybean, olive, cottonseed and other vegetable oils, the controlling consideration being their ability to produce on the veneer the plasticizing effect described above. Moreover, oil sulphonated to render it water soluble may be utilized and may be preferred for veneer to be bonded with glues not compatible with oil. In fact, because of the wide diversity in the types of oils capable of use, and permissible variations in their manner of use effectively in the practice of my invention, it does not seem necessary or practically feasible to attempt listing them all or stating any definite viscosity or quantity control limits applicable. The described use of separator sticks as the oil carrier medium has proven to be an especially satisfactory technique.

Concerning drying temperatures, in the case of kiln drying, whereas Douglas fir veneer usually requires about 8 hours to be dried at temperatures of about 230 degrees R, my oil application veneer drying process enables halving this drying time, for example, by raising the drying temperature to the vicinity of 340 degrees F. without impairing the product. In fact the oil treated veneer thus dried is less brash and less warped than unplasticized veneer dried to an equally low moisture content at the lower temperature and for the longer time mentioned above.

In this specification the word veneer has been used throughout to designate the material to which the method is particularly applicable, and the connotation of the term is that usually understood and interpreted in the plywood industry. It designates thin wood sheets which in the manufacture of plywood vary ordinarily from one-tenth of an inch to one-fourth of an inch in actual thickness. Moreover, the invention is restricted to the process of drying substantially green veneer in which oil is made available for the wood at or near the beginning of the drying process, such that as the wood becomes heated and loses any appreciable amount of moisture, plasticizing occurs.

It will be understood that the mechanical procedure of drying, that is, whether the veneer is carried on rolls, or on trucks in a standard kiln or tunnel dryer, for instance, will be a factor influencing the choice of oil and application technique. Kiln or tunnel drying naturally lend themselves to a wider selection of methods for carrying oil to the veneer than that afforded by drying veneer in roll driers because in the latter case the drying can be and is, therefore, usually more rapid than kiln drying, because each veneer layer is unconfined and the sheets are not pressed flat during drying. Dissemination of oil to the wood during roll drier veneer drying should be more rapid or immediate, therefore, to have maximum effect. Consequently, direct application of oil to the veneer, such as by brush or spray application, preferably at spaced locations, is preferred in the case of drying veneer in roll driers, and more oil and/ or less viscous oil may be desdirable.

I claim as my invention:

1. The process of drying veneer sheets which comprises initiating heating of the veneer in an air atmosphere at a temperature above normal room temperature while at least a major portion of the surface area of the veneer is substantially free of added oil, and, during continued heating of the veneer, supplying for access to the surface of the veneer oil of a type normally liquid at room temperature, less volatile than Water and substantially immiscible with water, and thereby deterring checking and cracking of the veneer.

2. The process of drying veneer sheets which comprises applying to the surface of the veneer at spaced locations over an area substantially less than half the surface area of the veneer oil of a type normally liquid at room temperature, less volatile than Water and substantially immiscible with water, and heating the veneer in an air atmosphere at a temperature above normal room temperature prior to dissemination of such oil over a major portion of the surface area of the veneer.

3. The process of drying veneer which comprises heating the veneer in an air atmosphere at a temperature above normal room temperature and carrying in suspension oil of a type normally liquid at room temperature, less volatile than water and substantially immiscible with water, and thereby reducing materially the brashness of the veneer resulting from drying.

4. The process of drying substantially green, rotarycut veneer which comprises applying to a minor portion of the surface area of the veneer oil of a type normally liquid at room temperature, less volatile than water and substantially immiscible with water, and then heating the veneer in an air atmosphere at a temperature above normal room temperature during which heating the oil materially reduces the brashness of the veneer.

5. The process of kiln drying substantially green, rotary-cut veneer which comprises stacking the veneer sheets with separator strips interposed between successive sheets, which separator strips carry oil of a type normally liquid at room temperature, less volatile than water and substantially immiscible with water, and subjecting the stack of veneer sheets to an air atmosphere at a temperature above normal room temperature while oil from the separator strips reduces the brashness of the veneer.

6. The process of drying veneer which comprises placing in physical contact with the veneer an element of porous material carrying oil of a type normally liquid at room temperature, less volatile than water and substantially immiscible with Water, and heating the veneer in an air atmosphere at a temperature above normal room temperature while oil from such porous material element reduces the brashness of the veneer.

7. The veneer drying process which comprises placing rotary-cut veneer sheets in a drying chamber, supplying to such chamber air heated to a temperature considerably above the boiling temperature of water, and supplying to the wood of the veneer oil of a type normally liquid at room temperature, less volatile than water, and sustantially immiscible with water, said oil being supplied to the veneer in such relatively small amounts as to only spread over the surface layers of the veneer and not infuse into the veneer in any substantial degree, and thereby curtailing the brashness and preserving the flexibility of the veneer.

8. The process defined in claim 7, in which the oil is suspended in the air and carried into contact with the veneer by flow of the air over the veneer.

9. The process defined in claim 7, in which the oil is supplied to the veneer by application of the oil directly onto the surface of the veneer and over only a minor portion of the surface area of the veneer.

10. The process defined in claim 7, in which the oil is supplied to the veneer by placing in contact with such veneer a stick carrying the oil from which stick oil is transferred to the veneer.

11. The process defined in claim 7, in which the oil is an oil selected from the group consisting of mineral oil, linseed oil, castor oil, soy bean oil, olive oil and cottonseed oil.

12. The process defined in claim 7, in which the oil is mineral oil having an SAE viscosity rating of approximately 40.

13. The process of drying veneer sheets which comprises drying the veneer in an air atmosphere while at least the major portion of the surface area of the veneer is substantially free of added oil, and during such drying supplying oil of a type normally liquid at room temperature, less volatile than water and substantially immiscible with water, for access to the surface of the veneer and thereby deterring checking and cracking of the veneer.

References Cited in the file of this patent UNITED STATES PATENTS 259,986 Lynch June 20, 1882 1,059,820 Besemfelder Apr. 22, 1913 (Other references on following page) 6 UNITED STATES PATENTS 2,435,218 Hudson Feb. 3, 1948 2,435,219 Hudson Feb. 3, 1948 1,480,804 Fish Jan. 15, 1924 2,507,190 Barksdale May 9, 1950 1,517,473 Thelen Dec. 2, 1924 1,692,635 Smith Jan. 22, 5 FOREIGN PATENTS 1,72 ,708 Fowler Aug. 20, 7122%0 Dierks 12, 929 1001837 Austraha P 1937 1,8 66 Curtin Apr. 26, 1932 2,000,663 Darrah May 7, 1935 OTHER REFERENCES 2,106,468 Schroth Jan. 25, 1938 10 Progress Report #1, Chemical Seasoning, revised Octo- 2,216,775 Helson Oct. 8, 1940 ber 1945, published by West Coast Lumbermens Asso- 2,402,331 Kvalnes June 18, 1946 ciation, Stuart Building, Seattle 1, Washington. 21 pages.