PLYWOOD- OAK-COTTONWOOD MINIMUM CURE TIME USDA FOREST SERVICE RESEARCH PAPER FPL

Transcription

1 OAK-COTTONWOOD PLYWOOD- MINIMUM CURE TIME USDA FOREST SERVICE RESEARCH PAPER FPL U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WIS.

2 ABSTRACT In bonding oak-cottonwood exterior plywood with a low-solids phenolic adhesive, how much can cure time be reduced and still produce acceptable bonds? Satisfactory results were achieved with a cure time of 4 minutes. Hot stacking might reduce the time in the press even more.

3 OAK-COTTONWOOD PLYWOOD- MINIMUM CURE TIME By RONALD W. JOKERST, Technologist and JOHN F. LUTZ, Technologist Forest Products Laboratory, 1 Forest Service, U. S. Department of Agriculture INTRODUCTION For several months we have been investigating the technical possibilities of producing an exterior grade of plywood using a mixture of high- and low-density hardwoods. To have any chance of industrial acceptance, the product must be competitive, costwise, with comparable grades of softwood plywood. E. Paul Craft 2 in 1971 described an experimental plant run and economic analysis of the production of construction grade plywood from grade 3 oak logs. From the economic standpoint, the production of plywood from this material appeared quite good, if the technical problem of developing suitable adhesives and bonding techniques could be developed. However, Craft was unable to meet, consistently, the wood failure requirements for exterior-type plywood as set forth in the Product Standard PS In prior work on this problem 3 we found we could meet the PS 1-66 requirements for wood failure, on an oak-cottonwood combination, by using a 30 percent solids, furafil-extended, hot-press phenolic adhesive. However, the cure times used were twice those recommended for similar constructions of southern pine plywood. In this study we wished to obtain an estimate of how much we could reduce the cure time, and still obtain bonds of acceptable quality. Acceptable quality in this case was defined as averaging 90 percent wood failure after vacuum pressure soaking and testing in shear while wet, as described in PS The requirements were purposely set high because we felt we had better control in the laboratory than would be possible under production conditions, and therefore, should expect better results. STUDY PROCEDURE Selection and Preparation of Veneer The veneer was selected from material cut from three red oak logs and three cottonwood logs from southern Wisconsin. The veneer was peeled 1/8 inch thick using lathe settings known to give good quality veneer. The sheets were clipped to a 28-inch width, in such a manner as to give clear straightgrained material. After clipping, the veneer was dried at 320 F in a roller veneer dryer, and then 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Craft, E. Paul. Construction Grade Plywood From Grade 3 Hardwoods-An Industrial Possibility. Forest Prod. J. 21(2). Feb Lutz, John F., and Jokerst, R.W. If We Need It- Construction Plywood From Hardwoods is Feasible. Plywood and Panel Mag. 14(9): Feb

4 stored at 80 F and 30 percent relative humidity until used. Gluing Procedure Before gluing the dried veneer was clipped into 14- by 14-inch sheets. The panel layup used was oak faces and backs with a cottonwood core randomly selected from the stock available. The only variable to be investigated in this study was cure time. Therefore, platen pressure was held constant at 175 pounds per square inch, platen temperature at 300 F, and closed assembly time at 9 minutes. Open assembly time was minimal. The spread rate was 80 to 83 pounds of adhesive per thousand square feet of double glueline. A roll-type spreader applied adhesive to the cottonwood cores only. This procedure gave good results in the previous work. The panels were weighed before and after spreading to determine the amount of adhesive actually applied. After the correct period of assembly time had elapsed, the three-ply panel was inserted into the hot press and heat and pressure applied for 5, 4, 3, or 2 minutes. A total of four panels were made at each cure time. After removal from the hot press, the panels were placed in an insulated box to retard the rate of cooling. After cooling, the panels were returned to the 80 F and 30 percent relative humidity room to equalize. Preparation of Test Specimens After the panels reached equilibrium at the conditions, they were trimmed to about 12 inches square. A stain was applied to one edge to determine the direction of the lathe checks in the core. A 3-1/4-inch strip was then cut from one side, and standard 3-1/4-by 1-inch plywood shear test specimens were cut and kerfed so lathe checks would be pulled close during shear testing. A total of 11 shear test specimens were taken from each panel. Testing of Specimens Prior to testing, all specimens were subjected to the standard vacuum-pressuresoak as described in PS 1-66, and tested in shear while still wet. We did not subject any specimens to the boil-dry-boil exposure described in PS 1-66 because we were primarily looking for undercured bonds. (Boiling may advance the cure of the adhesive and thus mask the effect of time in the hot press.) The total load at failure and the estimate of the percent wood failure was recorded for each specimen tested. RESULTS AND DISCUSSION Table 1 summarizes the results of the study, and table 2 shows the results by panels within a treatment. The U.S. Product Standard for softwood plywood, PS 1-66, specifies that exteriortype plywood must average 85 percent wood failure to be acceptable. For the purposes of this work, we maintained a 90 percent average wood failure in the laboratory to provide a margin of safety for industrial production. The results were encouraging. The adhesive manufacturers recommended a 3-1/2-minute cure time in conjunction with a minimum of 4 hours of hot stacking for southern pine under the same set of conditions. We obtained satisfactory bonds with a 4-minute cure time without the beneficial effect of hot stacking. It would seem probable that with hot stacking, as recommended by the manufacturers, cure time could be reduced another 30 seconds; this would equal the recommendations for three-ply 3/8-inch exterior-type southern pine plywood. The fact that we obtained better results with a 2-minute cure than with a 3-minute cure is unexplainable at this time. The work was carefully reviewed for errors but none were discovered. The panels were placed in an insulated box upon removal from the hot press. However, it is unlikely that this could be compared to hot stacking as far as advancing the cure of the adhesive. The only heat in the box was from the panels themselves: since they were relatively small and were added to the box at the rate of about one every 15 minutes, it is doubtful that the temperature was high enough to do any measurable good. 2

5 Table 1. --Summary of results by treatments 1 Table 2. --Results by panels within treatment 1 3

6 CONCLUSIONS It would appear that it is possible to bond oak-cottonwood combinations with a 30 percent solids, furafil-extended, hot-press phenolic well enough to meet the requirements for exterior-type plywood, as specified in PS The encouraging factor is that this was accomplished using a cure time only 30 seconds longer than that recommended for southern pine plywood manufactured under similar conditions. We believe that, had we been able to employ the 4-hour hot stacking as recommended by the adhesive manufacturer, we could reduce the cure time to the 3-1/2 minutes used with southern pine exterior-type plywood. U.S. GOVERNMENT PRINTING OFFICE /