Plywood manufacture

United States Patent Office

3,489,633

Patented Jan. 13, 1970

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. C09j 5/02; B32b 31/04

U.S. Cl. 156—312 3 Claims

ABSTRACT OF THE DISCLOSURE

A method of making plywood of improved pre-press bond strength, using a dual pre-press cycle. Assembled plywood panels are cold compressed for about 30 to about 90 seconds, the pressure released for about 2 to 5 minutes, and the consolidated panels again cold compressed for about 30 to about 90 seconds. The consolidated panels are then hot-pressed.

This invention relates to the manufacture of hotpressed plywood and particularly to the pre-press of assembled panels.

In the manufacture of hot-pressed plywood, 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 operations. 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 delaminate and adversely affect subsequent handling and loading of the hot-press. Since the lay-up of a press load requires from 10 to 15 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 pre-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.

In accordance with this invention, the improved technique for pre-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 5 panel lay-up for a period of time ranging from 30 to 90 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 10 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 manu15 facture 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 20 required to achieve acceptable pre-press bond strengths is considerably reduced by the present invention while still achieving superior pre-press bond strengths and maintaining the time tolerances of a plywood mill.

The following examples are given to illustrate the in25 vention 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 30 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.43g

Unrefined potato starch 50

Mix two minutes 50% caustic soda solution 9 ⁴⁰ Mix six minutes

Furafil¹ 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 temperature.

A solid furfural by-produet 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 100 mesh standard screen.

^!A resin as prepared in accordance with U.S. Patent Re. 23,347.

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 175 p.s.i. 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.

3,489,633

TABLE I

CONVENTIONAL PRE-PRESS CYCLE

<td> Assembly</td><td></td><td colspan="2"> Adhesive</td><td></td><td colspan="2"> Percent</td><td> Percent</td>

<td> Time</td><td colspan="3"> Pre-Press Wet Spread</td><td></td><td colspan="2"> Adhesive</td><td> Wood</td>

<td> (min.)</td><td colspan="3"> Time (min.) (MDGL)</td><td> Lbs. Pull</td><td colspan="2"> Transfer</td><td> Failure</td>

<td> 4</td><td></td><td> 4</td><td> 70</td><td> 55</td><td></td><td> 100</td><td> 0</td>

<td></td><td></td><td colspan="2"> DUAL PRE-PRESS</td><td> CYCLE</td><td></td><td></td><td></td>

<td></td><td></td><td></td><td> Second</td><td></td><td></td><td></td><td rowspan="2"> Percent</td>

<td></td><td> First</td><td> Aeration</td><td> Pressure</td><td> Adhesive</td><td></td><td></td>

<td> Assembly</td><td> Pressure</td><td> Time</td><td> Period</td><td> Wet</td><td> Lbs.</td><td> Percent</td><td> Wood</td>

<td> Time</td><td> (min.)</td><td> (min.)</td><td> (min.)</td><td> Spread</td><td> Pull</td><td> Transfer</td><td> Failure</td>

<td> 1 min_______</td><td> 1</td><td> 13</td><td> 1</td><td> 68</td><td> 110</td><td> 100</td><td> 0</td>

<td> 30 sec_______</td><td> 1</td><td> 23</td><td> 1</td><td> 70</td><td> 60</td><td> 100</td><td> 0</td>

i Panels were laid open.

a Panels remain stacked.

The above example shows improvement in the pre-press bond strength is obtained with a shortened cycle time when using a dual pressure cycle in accordance with this invention.

EXAMPLE II

A soft grain southern pine veneer W thick is cut into a series of 4% x 5% panels. Pairs of the panels, are matched and one surface of one panel from each pair is coated with a commercial adhesive composition. The pairs 25 of veneers are placed together and tested as outlined in Example I. Comparative data including assembly and prepress times and adhesive spread levels are given below in Table Π.

adhesive can be increased by forced air circulation. The second cold pressure period consolidates the pre-press bond throughout the load.