US3489633A - Plywood manufacture - Google Patents

Description

United States Patent 3,489,633 PLYWOOD MANUFACTURE Howard W. Holmquist, Seattle, Wash., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Dec. 30, 1966, Ser. No. 606,005 Int. Cl. C09 02; B32b 31/04 US. Cl. 156-312 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the manufacture of hotpressed plywood and particularly to the pro-press of a sembled panels.

In the manufacture of hot-pressed lywood, core stock veneers are wet spread with an adhesive composition and assembled with uncoated veneers to form a panel comprising a plurality of veneers in which the wood grains of adjacent veneers are disposed at right angles. The panel is subsequently hot-pressed to set up the adhesive and bond the veneers together. In conventional plywood mill procedure, a plurality of panels is laid up and stacked one on top of the other to form a press load containing from about 20 to 30 panels depending upon the capacity of the hot-press. Pre-press is the technique whereby the stacked panel assemblies are subjected to a preliminary cold pressure period prior to separation and loading into individual hot-press openings for final consolidation and cure. Pre-press is thus intended to establish a temporary bond of sufficient strength to facilitate panel handling before hot pressing. This is of commercial value because it (a) allows automatic press loading, (b) the consolidated panels can be loaded into smaller press openings which allow more platens for a given press frame, and (c) it significantly reduces panel rejects due to veneer misalignment during hot-press loading and closing opera tions. The adhesive is applied to the core stock as a mobile liquid and must be allowed to lose moisture through absorption or evaporation before a cold pressure pre-press bond can be established. Accordingly, the press load is allowed to stand for a period of time after last panel lay up. A typical mill pre-press cycle comprises about a 3 minute stand time after the last panel is assembled and then the press load is compressed for 4 minutes. Such pre-press cycle develops satisfactory pre-press bonds in the lower panels of the load but the upper panels still delarninate and adversely affect subsequent handling and loading of the hot-press. Since the lay-up of a press load requires from to minutes, the first panels assembled have considerably more time for the adhesive spread to lose moisture and become tacky than do the latter assembled panels. Therefore, it would be highly desirable to equalize the tack level of the adhesive throughout a press load of panels Without disrupting the normal mill assembly times or hinder the hot-press bonds.

Accordingly, typical objects of this invention are to provide (1) improved pro-press bond strength of assembled plywood panels, (2) improved method for prepressing assembled plywood panels, and (3) improved method for manufacturing hot-pressed plywood.

Other objects, aspects and advantages of this invention will become apparent to those skilled in the art upon further study of this disclosure and the appended claims.

3,489,633 Patented Jan. 13, 1970 In accordance with this invention, the improved technique for pro-pressing a press load of plywood panels comprises the steps of (a) applying a first cold compressive pressure to the press load immediately after the final panel lay-up for a period of time ranging from 30 to seconds, (b) releasing the pressure and allowing the press load to stand uncompressed for a period of time ranging from 2 to 5 minutes, and (c) applying a second cold compressive pressure to the press load for a period of time ranging from 30 to 90 seconds. The pre-press bonds obtained by this technique are uniform throughout the press load and stronger than those obtained by the present pre-pressing techniques.

Any of the adhesive compositions useful in the manufacture of plywood can be used as the adhesive composition when practicing this invention. Such adhesives are well known in the art and preferably contain an aqueous alkaline phenol-formaldehyde condensate. The customary relatively lengthly stand times of commercial adhesives required to achieve acceptable p-re-press bond strengths is considerably reduced by the present invention while still achieving superior pro-press bond strengths and maintaining the time tolerances of a plywood mill.

The following examples are given to illustrate the invention and such are not intended to limit the scope of the invention concept.

EXAMPLE I A medium grain Douglas Fir veneer is cut into 4%" x 5%" panels. Pairs of the panels are matched and one surface of one panel from each pair is spread with about 70 pounds per MDGL (thousand square feet of double glue line) of an adhesive composition prepared as follows:

Parts by weight Water at 70 F. 438

Unrefined potato starch 50 Mix two minutes 50% caustic soda solution 9 Mix six minutes Furafil 1 200 Mix five minutes 50% caustic soda solution 48 Mix 15 minutes Alkaline phenol formaldehyde resin 2 1250 Mix five minutes and cool the mixture to room temperatu-re.

A solid furfural by-product residue from the acid hydrolysis of a pentosan containing material selected from the group consisting of ground oat hulls and ground corn cobs, said residue substantially 'all passing a mesh standard soreen.

23 3r27resin as prepared in accordance with U.S. Patent Re.

The second panel of each pair is then placed across the coated surface of the first panel with its grain running at a 90 angle to that of the first panel and with the long dimension of each panel over-lapping the short dimension of the other panel on all sides. Each panel assembly is permitted to stand a short time (assembly time) after layup to simulate mill procedure. The panel is then cold pressed at 70 F. and psi. pressure (pre-press). The pressed assembly is then broken on a laboratory puller having a fixed base jig under which the overlapping ends of one panel are placed and a second, movable jig attached to a scale under which the overlapping ends of the second panel are placed. The assembly is then pulled apart, using a load rate of about 10 pounds per second, recording the pounds of pull at the break for each assembly. The comparative results of a conventional pre-press cycle and that of the instant invention are given below in Table I.

TABLE I CONVENTIONAL FEE-PRESS CYCLE Assembly Adhesive Percent Percent T me Pro-Press Wet Spread Adhesive Wood (mun) Time (min) (MD GL) Lbs. Pull Transfer Failure DUAL PRE-P RESS CYCLE Second First Aeration Pressure Adhesive Percent Assembly Pressure Time Period Wet Lbs. Percent Wood Time (111111.) (min.) (nun) Spread Pull Transfer Failure 1 min. 1 1 3 1 68 110 100 0 30 sec 1 2 3 1 70 60 100 0 A soft grain southern pine veneer thick is cut into i a series of 4%" x 5 /8" panels. Pairs of the panels are matched and one surface of one panel from each pair is adhesive can be increased by forced air circulation. The second cold pressure period consolidates the pro-press bond throughout the load.

What is claimed is: 1' 1. The method of pre-pressing plywood panels which comprises the steps of:

(a) applying a first cold compressive pressure to the panels for about 30 to about 90 seconds; (b) releasing said pressure for about 2 to about 5 mincoated with a commercial adhesive composition. The pairs 25 of veneers are placed together and tested as outlined in mes and anowmg Sald Panels to expand vertically; Example I. Comparative data including assembly and prei I press times and adhesive spread levels are given below in pplymg a Second cold compressive pressure to the Table II. panels for about 30 to about 90 seconds.

TABLE II CONVENTIONAL CYCLE Pre- Assembly Press Adhesive Percent Time Time Wet Spread Adhesive (min) (min) (MD GL) Lbs. Pull Transfer Percent Wood Failure 2 4 98 100 0 (wet glue line). i 4 90 100 0. 7 5 96 60 95 0 (some adhesive tack). 15 5 96 100 95 0.

DUAL PRESSURE CYCLE Adhesive Assembly First Open Second Wet Percent Time Pressure Time Pressure Spread Lbs. Percent Wood (111111.) (see) (min.) (min). (MD GL) Pull Transfer Failure This example shows the variation in pro-press bond strength throughout a press load of panels and the improved bond strengths obtained by this invention.

From the preceding disclosure, those skilled in the art will realize that the initial cold pressure period provided will (1) establish satisfactory pre-press :bonds in the lower portion of the load because those panels have had 10 to 15 minutes stand time and the adhesive is in a tacky stage and (2) will transfer about half of the adhesive fromthe spread cores to adjacent veneers in the upper portion thus causing the adhesive to develop tack at a faster rate through greater adsorption into the wood. The early establishment of transfer in the lower portion of the load will also tend to improve the final hot press adhesive bond. A normal pre-press cycle which holds the load 3 or 4- minutes before pressure may allow the bottom panels to become so dry they will not transfer and properly bond the adjacent plys during hot press. When the initial pressure is released, the upper portion of the load expands vertically due to some warpage of the veneers. This opening of the veneers allows for the adhesive to be aerated and lose moisture. The evaporation of moisture from the 2. The method of claim 1 wherein a plurality of panels are pre-pressed.

3. In the manufacture of plywood wherein a press load of panels is assembled pre-pressed and subsequently hotpressed, the improvement which comprises applying a first cold compressive pressure to the load immediately after the final panel lay-up for about 30 to about seconds, releasing said pressure and allowing said panels to stand for about 2 to about 5 minutes, and applying a second cold compressive pressure to the load for about 30 to about 90 seconds.

References Cited UNITED STATES PATENTS 1,125,973 1/1915 Davidson. 1,329,402 2/ 1920 Henning.

REUBEN EPSTEIN, Primary Examiner US. Cl. X.R.