US3518105A - Panel-treating process - Google Patents

Description

United States Patent 3,518,105 PANEL-TREATING PROCESS Clilford T. McElroy, 3722 Locust Ave., Long Beach, Calif. 90807 Continuation-impart of application Ser. No. 572,321,

Aug. 15, 1966. This application Feb. 11, 1969, Ser.

Int. Cl. B41m /24; B44c 1/097; C23c 7/00 US. Cl. 117-46 2 Claims ABSTRACT OF THE DISCLOSURE Finish-treating plywood and cellulose-composition boards by coating a surface of said boards with a liquid coating material free of combustible solvents, then drying and singeing the applied coating by impingement of an open flame, followed by alternate smoothing and drying steps to successively dry and harden the coating to a lustrous finish.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my pending application bearing the same title, Ser. No. 572,321, filed Aug. 15, 1966, and abandoned as of Feb. 14, 1969.

BACKGROUND OF THE INVENTION Field of the invention Wood and board surface-finishing to preserve the surface thereof.

Description of the prior art McElroy 3,110,6'07 with respect to use of open-flame drying and fiber singeing, and Dimond 3,219,473 with respect to eliminating nibs essentially by rubbing.

SUMMARY OF THE INVENTION Existing finishing systems, when using water-reducible coatings, cannot economically dry out moisture entrapped in the panels. In these existing finishing systems, water is allowed to soak into the Wood. In the course of drying, the finish crusts over the still-wet panel. The longer it takes to remove the water, the deeper the water can penetrate into the panels and the greater the possibility that it may not completely evaporate. Only a long, costly, time-consuming process can remove the excess water. Failure to effectively do so creates a panel surface which, because of the entrapped water, will eventually crack, craze and/or roughen.

Applicants earlier process, as disclosed in his waterreducible finishing of plywood (Pat. No. 3,110,607), consisted of first coating the surface, then running it through burners to remove fuzz and evaporate water, and then sanding until the surface was smooth. While somewhat costly of materials, over a period of about three years, more than 150,000,000 square feet of prefinished panelling were produced.

In the search for methods that would lower costs, the method disclosed in the parent application, among others, was tested and used. The same, while continuing to show improved results in one or another respect, continued to present one problem: when the burners were turned on high, the finish would become completely dry and hardened so that the smoothing roll would not actually smooth the panel. If the burners were turned substantially lower, the water would have time to penetrate into the wood, so that after smoothing, the panels would be too damp, and total removal by singeing was not accomplished. Because of the individual variances of wood, Water penetration is never consistent, and it has been quite a problem to regulate the burners accurately enough so that uniformity of product could be maintained.

As a consequence, it became apparent that the drying and smoothing process required to be less abrupt and that the same required to be carried out on a progressive basis.

In summary, over a period of some six months, during which a twenty-foot pilot machine operating between 300 and 500 linear feet per minute was tested, altered, tested again, and again altered, samples were obtained that showed definite improvement in water-spotting resistance, smoothness and luster, over all other products heretofore obtained.

According to this invention, the following broad steps of the method thus achieved consist of direct coating by means of a direct coater 5, flame impinging by means of a first flame-impinging area 6, a :first smoothing unit 7, a second flame-impinging area 8 primarily for final removal of water from the coating material, a second smoothing unit 9 for flattening the coating over the singed wood filaments, a third flame-impinging area 10 to reduce the coating on the panel to a hot and highly moldable state, and a third smoothing unit 11 to finish the coating to a clean, smooth state and with good luster.

BRIEF DESCRIPTION OF THE DRAWING The figure is a diagrammatic view of the apparatus for carrying out the present process.

DESCRIPTION OF THE PREFERRED EMBODIMENT The direct coater 5 essentially comprises a preferably knurled doctor roll 15 in operative contact with a rubber applicator roll 16. The valleys between the knurls deposit predetermined amounts of finishing material on the surface of the applicator roll, the latter, in turn, depositing said material on the upper face of the panel 17. The panel is fed between the applicator roll 16 and a driven support roll to provide for the application of a unifom1- thickness of the coating over said panel surface.

The subsequent steps of the process and the results obtained thereby will be clarified by the following list of coatings that are free of combustible solvents, the same 'being exemplary of numerous coating aqueous solutions, copolymer latices, water-reducible and water-soluble resins, and lacquers, polyvinyl emulsions and aqueous copolymer dispersions. The list comprises:

(1) Ammonium salts in aqueous solution.

(2) Self-reactive vinyl acetate copolymer latex.

(3) Vinyl acrylic self-reactive copolymer latex.

(4) Vinyl acetate homopolymer (useful in formulating specialty coatings).

(5) Polar vinyl acetate copolymer solution (in water 'or ammonia).

(6) Water-reducible alkyd resins (without consolvent).

(7) Water-soluble lacquer (as temporary alkali-removable coating, experimental).

(8) Polyvinyl acetate emulsion (low molecular weight or high molecular weight).

(9) Hexamethoxymethylmelamine (only up to 10% in solution and in baked films with polyvinyl acetate or in partially hydrolyzed polyvinyl acetate; the hydroxyl groups acts as cross linkage sites).

(10) Hydrosol system (a new class of aqueous copolymer dispersons; this group also provide reactive sites for cross linking when a thermoset or curable system is desired).

(11) Thermosetting polymers (aqueous solution).

(12) Solvent-free nitrocellulose in latex and dispersion form.

(13) Water-soluble urea formaldehyde resins (up to 15% in solution).

In the above list, the common feature is the absence of non-aqueous solvents and the presence of water, it

being clear that nonusables are solvent-soluble acrylic copolymer water-thinnable vehicles, polyvinyl acetate maleate copolymers, and all other conventional solvent systems.

, Those materials that fall into conventional solvent systems, as above indicated, are not usable since the same would be driven off by the open flames. The materials hereinabove listed are free of such conventional solvents, and use water as their vehicle. It is their water that is driven off after it has served its purpose of carrying the material to the surface that the same will coat. The result is a coating that, chemically, is substantially the same when being applied to the applicator roll 16 and when' finally finished, except that in its initial state the same is in suspension in water, and in finished state is waterless and hardened by the smoothing operations.

The first flame impingement area 6 preferably comprises a burner 19 which produces an open flame 20 that is directed toward the top surface of the panel 17. Practice has demonstrated that the temperature of said flame should approximate 1400 F. to cause substantially instantaneous boiling off of most of the water from any of the above coating materials without causing bubbling, as the panel is moving, in the direction shown, at 300 to 500 fet per minute, approximately. The viscosity of the coating material is, thereby, increased accordingly and halting, or at least slowing, the process of further penetration into the panel of residual water. This flash effect of the burner takes place in a matter of two-tenths of a secondabout the time it takes for any point on the panel to move past the flame 20 at the above-indicated speed range. In this connection, close proximity of the burner 19 to the applicator roll 16 is desired, so the flasholf of the water may begin at the earliest time after application of the coating, thereby halting or at least slowing the absorption rate of the water into the pores of the panel.

The first smoothing unit 7 preferably comprises a polished steel smoothing roll 21 in contact with the coated side of the panel and between which and a support roll 22 the panel 17 moves. Said roll is heated in the approximate range of 550 to 600 F. It is desired that a difference of speed between the rolls 21 and 22 be maintained to keep the smoothing roll from becoming contaminated. To this end, the speed of the roll 21 is reduced relative to the support roll 22, and the latter roll is provided with a knurled or roughened surface. The resultant slippage between the roll 21 and the panel, as indicated, keeps the latter roll clean, i.e., clear of crusted coating material.

The second flame-impinging area 8 may be quite similar to the area 6, i.e., comprising a burner 19a producing an open flame 20a applied, as before, to the coated surface of the panel. Such water contained in the coating that has not been boiled off by the flame 19 will, ordi-. narily, be removed, without bubbling, from the coating by the flame 19a, leaving the surface of the coating somewhat roughened with the binder in the material in a raised condition as a result of the rapid evaporation of the water. During this second boiling off of water, fine filaments have been observed flashing off from the coating.

The second smoothing unit 9, similar to the unit 7, comprises a smoothing roll 21a and a support roll 22a, the former being operated at the mentioned heat in the approximate range of 550 to 600 F with suflicient drag or acceleration, as before, on the smoothing roller 21a to keep coating material from adhering to it, but at an increased pressure setting between the rolls 21a and 22a. Since, in effect, the coating, during the second boil-off of Water, is in the raised-up condition above mentioned, the coating is flattened over the residual singed wood filaments and substantially totally covers the same. The result provides a smooth surface on the coating.

The third flame-impinging area comprises a burner 19b providing an open flame b, substantially similar to the previously described flame-impinging areas 6 and 8.

Said flame 20b, at about 1400 F., is substantially as hot as theprevious flames, and will effect removal of any residual water that may have been trapped within the coating. Said flame causes release of said water by again roughening the coating, but to a lesser degree than before. Again, the moisture release is too rapid to cause bubbling. Also, additional wood filaments have been observed flashing off, the same being combined with small quantities of wood fibers caused by the breakdown of the cellulose structure of the panel due to the increased compression of the panel when moving between the rolls 21a and 22a. Activation of the natural resins may occur on the first coat on some substrates. This indicates excessive heat at this stage of the process, in which case the above-indicated heat for the flame 20b may be lowered to minimize this condition. In any case, the panel 17 leaves the area 10 with the coating thereon in a hot and highly moldable state as it reaches the third smoothing unit 11.

The latter unit comprises a smoothing roll 21b and a support roll 22b, the former, as before, being heated in the approximate range of 550 to 600 F. Unit 11 is similar to unit 9, except that the pressure or nip is somewhat increased. The previously described drag to avoid contamination of the smoothing roll is provided in unit 11 also. This last step will leave the coating clean, smooth and with good luster.

When coating with a thermoplastic material, the abovedescribed procedure with the panel moving at 300 feet per minute will result in a smooth, lustrous surface, free of all capilaries such as would allow water to be absorbed and, thus, get under the finished coating to create Water spots. It will be clear that the surface is sealed against water absorption.

With a coating of thermosetting material, instead of the three stages, as above, each comprising a flame and a smoothing function, that have been found satisfactory to seal thermoplastic coatings, two or more additional stages may be used, the latter stages for the purpose of providing time to effect the cure of the material. Such additional flame-impinging and smoothing roll areas are shown in the drawing in broken lines.

At panel speed of 500 feet per minute, five stages may be required with a thermoplastic coating, and proportionally more stages for a thermosetting coating. Regardless of the number of stages used for particular coating materials, the flame and smoothing areas, in the aggregate, constitute a drying area.

Whether this coater and smoother-dryer combination is used alone or in series will depend on the number of coats required to complete the product. The formulation of the finishing material-stains, sealer, filler, builder or ground coats, or top coatwill of necessity vary considerably. In general, the viscosity, composition of coatings used, amount of water to be removed, and curing time (if any) after removal of water, will affect nip and flame settings on dryers.

The flame dryers impinge a flame of about 1400 F. directly to the surface of the panels. The Water is removed so rapidly that the finish left tends to stand out rather than lie flat until the panel encounters the smoothing roll. The first sealer or stain coat generally is used at about five to ten percent solids to expedite singeing of the minute filaments that are projecting from the board. As the finishing process progresses, however, the solids might increase to approximately fifty percent, depending on the nature of the board being finished and the depth of coating required.

As the viscosity of the material being used increases, it is advantageous to lessen the pressure or open the nip on the first smothing rolls. This facilitates water release and distributes the material evenly over the panel without accumulation of any surplus on the trailing edge of the plywood, that would otherwise contaminate the rolls. In the case of high viscosity, there is an advantage to changing the direction of rotation of the second smoothing roll to accelerate rather than drag.

While the foregoing discloses what is now contemplated to be the best mode of carrying out the invention, the same is, of course, subject to modification without departing from the spirit and scope of the invention. Therefore, it is not desired to restrict the invention to the particular method steps or combination or sequence of method steps described.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

1. The process of coating a surface of a panel of cellulosic material that consists in:

(a) applying to said surface a resinous chemical coating suspended in water while said panel is in constant linear movement,

(b) subjecting said coating to the direct impingement of an open flame at a temperature approximating 1400 F. while said panel is in linear movement at a speed of from approximately 300 to 500 feet per minute, to boil ofi most of the water in the coating without bubbling and, simultaneously singe some of the cellulosic filaments and fibers on said panel surface,

(c) smoothing over the partly dried coating under rolling pressure suflicient to flatten the same, said smothing being obtained by passing said panel between opposed rotating rollers, the roller on the coated side of said panel being heated in the approximate range of 550 to 600 F. and having a rotary speed different from that of the roller opposed thereto minimize collection of resinous material of the coating thereon,

(d) at least once again, subjecting the partly dried and flattened coating to direct impingement of a second open flame similar to the first mentioned flame to boil off residual water in the coating, and following the same with a second flattening of the further dried coating by smoothing over the same between second opposed rollers similar to the first mentioned rollers.

2. The process as defined in claim 1 and subjecting the further dried and flattened coating to impingement of a third open flame at a temperature of approximately 1400 F. to still further dry the coating, and following the same with a third flattening of said still further dried coating.

References Cited UNITED STATES PATENTS 3,110,607 11/1963 McElroy a 1178 3,219,473 11/1965 Dimond l1765.2 XR

DAVID KLEIN, Primary Examiner U.S. C1. X.R. 117-652

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