Feasibility of Producing a High- Yield Laminated Structural Product:

Transcription

1 Feasibility of Producing a High- Yield Laminated Structural Product: U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WISCONSIN

2 FOREWORD This publication is another in a series of reports on the progress of a U.S. Forest Products Laboratory research team in estimating the technical and economic potential of producing a laminated structural product by a log-toproduct system Team members collectively defined the anticipated problems and sought practical solutions. Individual team members, and their respective contributions to this effort, were: E. L. Schaffer, Engineer--team leader, economic analysis R. W. Jokerst, Forest Products Technologist--adhesive, b on d quality, laminating R. C. Moody, Engineer--laminated material strength properties, structural performance C. C. Peters, Mechanical Engineer--rotary cutting, log yield L. Tschernitz, Chemical Engineer--drying, economic analysis J. Zahn, Engineer--structural performance Overall results are contained in Feasibility of Producing a High-Yield Laminated Structural Product--General Summary, published in 1972 as U.S.D.A. Forest Service Research Paper FPL Reports on individual phases of the study, such as the current one, are also being released. Another of these is: Feasibility of Producing a High-Yield Laminated Structural Product: Strength Properties of Rotary Knife-Cut Laminated Southern 1972, U.S.D.A. Forest Service Research Paper FPL 178.

3 This report summarizes the findings of research on the laminating portion of a process for rapidly converting raw material to a finished product. The laminating step is unique in that the residual heat of the drying operation is used to accelerate adhesive cure. The system appears to be not only fast and simple, but flexible enough to be fitted into a continuous production system It should be possible to use a wide variety of low- and intermediate-temperaturesetting adhesives satisfactorily on a wide range of species. Additional savings should accrue through the reuse of the heat of drying and the fact that it is unnecessary to surface the laminae before bonding. I

4 CONTENTS Page Introduction Past Work Difference of FPL Process FPL Hot Laminating Process Cooling Period Adhesive Reaction Continuous Process Assembly Time Lamination Procedure Gluing Rotary-Cut, Press-Dried Material Glueline Temperature Type of Adhesive Effects of Cooling and Assembly Time Measurements of Bond Quality Conclusions II

5 Feasibility of Producing a High-Yield Laminated Structural Product: RESIDUAL HEAT OF DRYING ACCELERATES ADHESIVES CURE By R. W. JOKERST, Technologist 1 FOREST PRODUCTS LABORATORY, U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE INTRODUCTION In 1968 a decision was made -at the Forest Products Laboratory to intensify the investigation of systems for converting logs directly into finished products. The primary goal was to develop a conversion system that would produce a higher yield of finished product for a given input of raw material than is currently possible. A group of scientists with varied backgrounds cooperatively developed a process concept which appears to fulfill these requirements. The process also exhibits the additional benefit of producing a product in a matter of minutes, as compared to days or weeks with standard procedures. Briefly, the process requires that the raw materialbeknife cut rather than sawn and then rapidly dried and laminated. The key to the entire laminating process is the use of the residual heat of the drying operation to accelerate cure of the adhesive used in laminating. This report discusses the work to date in the laminating of hot, knife-cut, press-dried wood (usually designated as FPL press-lam). Past Work The use of heat stored in the wood to accelerate adhesive cure is not new. In 1946, Taylor 2 of the Tennessee Valley Authority described a process for the continuous production of laminated oak flooring; one aspect was the preheating of the oak strips to accelerate adhesive cure. In 1956, Marra 3 reported on a method of rapid laminating using preheated lumber. In 1958, McKean and Smith, 4 using Marra s work as a base, developed a pilot plant process for making laminated 2 by 4 s from 1 by 4 s. The process was reported to be technically and economically feasible, but not enough 1 by were available to keep the plant in production. Several other laminating operations employing the stored-heat concept are known, but very little 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Taylor, R. Brooks. A continuous process for manufacture of laminated lumber. Tennessee Valley Authority. Apr Marra, G. Rapid laminating of lumber without high-frequency heat. Forest Prod. J. 6(3). Mar McKean, H. B., and Smith, J. W. Pilot plant laminated 2 by 4's. Forest Prod. J. 8(8): 19A-23A

6 has been published concerning them. Difference of FPL Process The primary difference from other methods is that the Forest Products Laboratory process does not add heat specifically to accelerate adhesive cure. The FPL process takes advantage of the residual heat of the drying operation. Residual heat thus makes it unnecessary to use a hot press or high-frequency unit to accelerate adhesive cure and can, in effect, be considered a substantial cost savings. FPL HOT LAMINATING PROCESS The actual laminating of the hot, knife-cut wood has proven surprisingly simple; problems that have arisen do not appear to be of great consequence. The hot laminating procedure is similar in many respects to normal laminating procedures, except for the speed at which the material must be assembled and placed under pressure. The sequence of the laminating operation can best be explained and described by dividing it into four time intervals: (1) The prespread, or cooling period, begins with the removal of the laminae from the press dryer and ends as the adhesive is applied. (2) The open assembly period begins with the application of the adhesive and ends as thelaminae are assembled to form the desired construction. (3) The closed assembly period includes the time from the placement of the laminae in position to just before application of external pressure to the construction. (Throughout the remainder of this paper the total assembly time will include the total elapsed time in the intervals (1), (2), and (3).) (4) The pressure period is the length of time the laminated construction must remain under externally applied pressure while the adhesive cures sufficiently to be handled and machined. This is strictly an inline system and what is done in any one step directly affects what can and must be accomplished in succeeding steps. Cooling Period Of the four time intervals, the cooling period is the only one over which there is a degree of control, and that only in one direction. The only purpose for having a cooling period is to allow the wood temperature to decrease so the laminations can be assembled and pressure applied before the adhesive cures. If the assembly can be laid up, and the pressure applied rapidly enough, there is no need for a cooling period. Once the adhesive is applied to the hot laminae, the maximum allowable time of open and closed assembly and the minimum pressure period are established. Since the total heat available in the wood appears to be relatively constant for a given drying schedule and laminae thickness, the minimum time required for laminating is preset--it cannot be reduced. By lengthening the cooling period and allowing the wood to cool before applying the adhesive, the allowable open and closed assembly time can be increased, but the required time under pressure must also be increased. The increase in presstime required depends on the glueline temperature attained. Adhesive Reaction Other factors can be introduced into the system that tend to increase flexibility from the standpoint of timing. One of these is the method of applying the adhesive. If the adhesive is applied to the hot wood in a thin, continuous film, the allowable assembly time is materially shortened. If, however, the adhesive is extruded from a ribbon-type spreader, speed of assembly becomes less critical. The reason is that, when the adhesive is laid down in beads, only a relatively small portion directly contacts the hot surface of the wood. The bulk of the adhesive takes longer to heat up and is thus slower to react. With one type of adhesive, assembly time was increased by adding 10 percent, by weight, of additional water to the adhesive formulation. The addition of the water retarded the reaction of the adhesive and did not appear to reduce the quality of the bonds to any noticeable degree. 2

7 Continuous Process The intention so far has been to view the bonding as one part of a continuous process. This is not entirely realistic at this time because a continuous press dryer does not exist. However, a standard multiopening-type hot press is available commercially and probably would work quite well. The problem that immediately comes to mind is that, at the end of the drying cycle, the laminating phase would be deluged with a mass of hot, dry laminae which would be impossible to handle properly. One answer to this would be to discharge the dryer into a heated chamber from which the laminae could be removed as needed. To check this possibility, material was dried as usual, placed in heated storage at 200 F. for periods up to 2 hours, and then laminated. No particular difficulties were encountered, and bond strength and quality did not appear to be reduced. Assembly Time Contrary to standard gluing operations, we found the length of open assembly time in hot gluing can be considerably longer than the period of closed assembly. There are two reasons for this: First, in closed assembly the beads of adhesive tend to be spread out in a thin film, and thus the adhesive heats up rapidly and begins to react quickly. Secondly, the heat radiating from the surfaces of the hot laminae is trapped in the area of the glueline during closed assembly, again tending to increase adhesive temperature in the glueline and cause it to react more rapidly. Therefore, when a construction of several laminations is laid up, it is best to extend the period of open assembly and keep the period of closed assembly as short as possible to reduce precure. LAMINATION PROCEDURE Gluing Rotary-Cut, Press- Dried Material The material laminated in this process was rotary cut and ranged from 1/4 to 1/2 inch in thickness. To date, three species--northern red oak, southern pine, and Douglas-fir--have been successfully laminated. In all cases, essentially the same techniques have been used. The thick veneer goes directly from the lathe to the press dryer and is then laminated; no intermediate machining or surfacing step is necessary. In some early work, attempts to bond knife-cut, press-dried southern pine using roomtemperature-curing methods were unsuccessful. The problem seemed to be due to surface roughness. Figure 1, which illustrates a two-ply construction that was bonded hot, may explain why surface roughness is not a problem when the gluing is done hot. Apparently, the hot wood is plastic enough and deforms sufficiently under external pressure to form a well-mated bonding surface. Normally the adhesive is spread on the loose side of the veneer; in constructions of more than two plies, a loose and a tight surface are bonded together. The loose surface absorbs a large amount of adhesive, and it seems necessary to use a heavy spread (60 to 70 lb. per 1,000 sq. ft.). Another point of interest concerning the pressdried laminae is the effect of moisture content. Because of the way in which the wood is dried, the surface moisture content is essentially zero, regardless of the final average moisture content of the laminae. This means that the condition of the wood at the glueline is practically the same regardless of the average wood moisture content. However, the average wood moisture content does have a pronounced effect on what can be done. When wood is dried in a hot platen press, a very steep moisture gradient is set up between the surfaces in contact with the heated platens and the center of the wood. As soon as the driving force of the external heat is removed, the moisture remaining in the center of the wood begins to migrate back toward the surface. This migration does two things: First, it raises the moisture content at the surface; and second, it rapidly cools the wood surface, thus lengthening the curing time. If the wood can be handled rapidly enough--spread, assembled, and placed under pressure--it is possible to bond wood with an average moisture content of 30 percent and above. However, when the average wood moisture content is much above 20 percent, speed of assembly becomes very critical, and the possibility of starved joints increases. FPL 179 3

8 Figure 1.--End view of 1/2-inch, rotary- cut and hot- glued southern pine showing deformation that occurred in pressing, resulting in a better mating of the two surfaces. M Glueline Temperature Since the key to the entire laminating process is the use of the residual heat of drying to accelerate adhesive cure, everything possible must be learned about the temperatures in the glueline and how they are affected by process variables. With this information, it is possible to estimate which adhesives have a possibility of working, how much time is available to laminate, and how long external pressure must be applied to the assembly. All of these factors are important in designing and coordinating the entire FPL press-lam process. Figure 2 is a set of regressions showing the effect of the length of the total assembly time on the maximum temperature reached in the glueline. These data were obtained by inserting a thermocouple into the glueline during closed assembly, and continuously recording the temperature through the period of pressure application and for at least 3 minutes after removal of pressure from the assembly. Normally the temperatures recorded increased rapidly to a peak and then gradually began to decrease. The maximum temperatures usually within 15 to 20 seconds after application of pressure. The difference in A and C is due to differences in final moisture content to which the wood was dried. Comparing the two curves indicates that the expected maximum glueline temperature for wood dried to 6 percent moisture content is about 16 F. higher than wood dried to 12 percent, other things being equal. In this case, the slope of the two curves was identical. In figure 2, B and D are the expected glueline temperatures 2 minutes after the maximum temperature is reached. It appears that, in southern pine at least, glueline temperature decreases on the average about 3" F. to F. per minute after reaching the maximum temperature. With these four curves, it is possible to estimate the approximate average 4

9 Figure 2.--This family of curves describes the effect of increasing assembly time on glueline temperature for 1/2-inch southern pine press dried at 375' F. (A) Maximum glueline temperature for wood dried to 6 percent average moisture content. (B) Temperature in the glueline 2 minutes after the maximum temperature was reached. (C) Maximum glueline temperature for wood dried to 12 percent average moisture content. (D) Temperature in the glueline of the 12 percent wood 2 minutes after the maximum temperature was reached. M glueline temperature over a 2-minute period; from this we can estimate the time an assembly must remain under pressure while the adhesive cures. Glueline temperature measurements were taken in five Douglas-fir hot-laminated constructions. The recorded temperatures were different from those in southern pine. In Douglas-fir, instead of decreasing after reaching a peak temperature, the readings remained relatively constant over about 5 minutes. Since drying temperatures, wood moisture contents, and adhesives were the same, the difference would seem to be related to species. The information in figure 2 also makes it possible to predict if an adhesive may work in the system It is quite obvious that an adhesive requiring temperatures above 230 F. for any length of time is not likely to performwell unless assembly times can be cut substantially. However, if the wood were dried at a higher platen temperature or briefly exposed to radiant heaters after leaving the dryer, higher glueline temperatures perhaps could be obtained and some of the high-temperaturecuring adhesives used. Type of Adhesive It appears that many room-temperature- or intermediate-temperature-curing adhesives are possibilities, but the majority of the research has been with a room-temperature-curing phenol resorcinol. However, exploratory work using a FPL 179 5

10 melamine-urea and a crosslinking polyvinyl emulsion has also shown promise. A hot-press plywood phenolic adhesive to which a small percentage of technical-grade resorcinol was added gave rather variable results and more work is needed to perfect it. The indications are that a wide variety of adhesive types can be used successfully. The final choice depends upon the cost and durability requirements of the product to be manufactured. EFFECTS OF COOLING AND ASSEMBLY TIME In a preliminary study, the length of the four intervals in the hot-laminating process were varied to investigate their effect on bond strength. This study was conducted on 1/2-inch, rotary-cut southern pine press-dried to 12 percent moisture content at a platen temperature of 375 F. The laminating time schedules used are shown in table 1. These time schedules were based on past experiences in exploratory work; the primary objective was to develop a feeling for the problem so a more complete study could be designed. The adhesive used was a room-temperaturecuring phenol resorcinol. The analysis of variance shown in table 2 indicated that, within the range of times investigated, the length of the cooling period was inversely related to bond strength and was highly significant statistically. The length of open assembly and the period of pressure application were not significant at the 0.05 level of testing, but were at the 0.1 level of testing and are therefore felt to be of secondary importance. Within the limits investigated in the study, the length of closed assembly could not be considered to have had an effect on bond strength. The overall indications are that by reducing the total assembly period the shear strength of the bonds will be improved. Increasing the pressure period would tend to compensate for longer assembly times or lower glueline temperatures. In general, it appears that the higher the glueline temperature, the stronger the bonds formed. 6

11 Table 1.--Summary of data for shear specimens 1 Timing began when laminae were removed from the press dryer. 2 Each value shown is the average of 24 shear tests. FPL 179 7

12 Table 2.--Results of the analysis of shear strength data for specimens tested without exposure **Significant at 0.01 level of testing. 8

13 MEASUREMENTS OF BOND QUALITY Bond quality was measured using a modification of the standard shear block described in ASTM D However, since laminae thickness in this case was 1/2 inch or less, it was necessary to reduce the overall size of the test specimen to prevent bending and peel forces from becoming the predominate forces on the glueline. In so doing, the possibility of direct comparison of historical values for solid-sawn, laminated southern pine was sacrificed. However, referring again to table 1, and in particular totreatments 1 through 8, the shear strength does not appear to be too far below what would be expected of solidsawn material. All of these results are from 1/2-inch material glued with phenol resorcinol and conditioned to approximately 12 percent moisture content before testing. In a further effort to define the quality of the bonds being formed, specimens of the hot-glued southern pine were subjected to the three-cycle accelerated-aging exposure as described in ASTM D The exposure failed to bring out any weaknesses in the adhesive bond (table 1). Shear strength declined by approximately 20 percent on the average, but the percentage of wood failure remained essentially the same. This would indicate that the decrease in strength was due to degradation of the wood and not the adhesive bond. Part of the decrease was due to the nature of the material being bonded. The thick-cut southern pine was heavily checked during cutting and these checks allowed the wood to move internally without putting much stress on the glueline. However, this internal movement does tend to propagate the existing checks and weaknesses induced in cutting and press drying. 5 American Society for Testing and Materials. ASTM D905. Test for strength properties of adhesives in shear by compression loading. Philadelphia, Pa. 6 American Society for Testing and Materials. ASTM D2559. Standard specification for adhesives for structural laminated wood products for use under exterior (wet use) exposure conditions. Philadelphia, Pa. FPL

14 CONCLUSIONS Based on the observations made and the data collected, the following conclusions can be drawn regarding the laminating of hot, knife-cut, pressdried wood: (1) The process of rapidly bonding knife-cut, press-dried wood is simple and flexible enough to allow it to be fitted into a continuous production system, (2) Presurfacing of the laminae prior to bonding is not required when the gluing is done hot. This results in a savings in wood, equipment, and time. (3) A wide variety of adhesive types capable of curing at low or intermediate temperatures should be suitable for use in this process; the primary consideration need only be the durability desired in the final product. (4) The hot-gluing process appears to be suitable for a wide range of species. It has performed well on northern red oak, southern pine, and Douglas-fir with only minor adjustments in procedure being required. (5) Glueline temperature appears to be predictable if the drying schedule and total assembly time are known. With this information and a knowledge of the adhesive properties, the required presstime can also be predicted. (6) The hot-gluing process will tolerate a wide range of wood moisture contents because the bonding surfaces, from the standpoint of moisture content, are quite similar after press drying. 10