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Face check development in veneered furniture panels.

Abstract

This project studied check develop in 1-foot square veneered furniture panels. Wood species used were flat-sliced red oak and hard maple cut on a half-round machine. The veneer flitches were clipped to 6-inch widths with the cathedral centered book matched and spliced. The resulting faces were 12 inches by the length of the veneer flitch. One set of veneer faces was conditioned at 35 percent relative humidity (RH) and 72[degrees]F. The second set of veneer faces was conditioned at 65 percent RH and 72[degrees]F. The panel core used was 11/16-inch southern pine particleboard. The particleboard core stock was all taken from the same bundle. The particleboard core was conditioned at 35 percent RH and 72[degrees]F. The adhesives used were urea-formaldehyde (UF) and precatalyzed cross-linked PVA. The assembly times were 1 minute and 10 minutes. The panels were first exposed for 3 weeks at 75 to 80 percent RH and then 3 additional weeks at 20 to 25 percent RH. This cycle was repeated three times. After every 3-week exposure, the panels were examined for the number of veneer checks. The panels were also evaluated for veneer checking as they were removed from the hot-press. Thus, the panels were evaluated seven times. The Student-Newman-Keuls test was used to evaluate the presence of face checks. The combination of factors resulting in the fewest face checks for red oak veneered panels was UF adhesive, loose side out, faces conditioned at 35 percent RH, and 1-minute assembly time. Six of the panels in this test were check free and two panels developed just one check each. For hard maple veneered panels, the combination of UF adhesive, tight side out, faces conditioned at 35 percent RH, and 1-minute assembly time resulted in no face check development.

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The face checking of hardwood veneered panels where appearance is important is a long-standing problem for both manufacturers and consumers (Batey 1955). Checking has been observed with all species. In some cases, checking is observed when the veneered panels are removed from the hot-press and in other cases veneer checks may not appear until months after production (Holcombe 1952, Jayne 1953). In other cases, checks may not appear at all. Although very large individual checks sometimes occur, face checking is more often in the form of numerous, very fine cracks (Feihl and Godin 1970). Manufacturers have indicated that they can experience very few or no problems from checking and then for no apparent reason, problems can develop. Because wood is a non-uniform and directional material, which changes dimensions with changing environmental conditions, and because there are numerous production variables, the problem is very complex.

The objective of this study was to quantify the number of veneer checks per panel as influenced by: 1) two different equilibrium moisture contents (EMCs) of the veneer before lay-up: 2) two different adhesives: 3) tight and loose side of the veneer: and 3) two different assembly times for the species of red oak and hard maple. After pressing, the veneered panels were conditioned for 3 weeks at 75 to 80 percent relative humidity (RH) and then for 3 additional weeks at 20 to 25 percent RH. This cycle was repeated three times. The panels were evaluated for checking after removal from the hot-press and at the end of each 3-week conditioning period for a total of seven times. Statistical methods were used to analyze the experimental data.

Literature review

Statement of the problem

All knife-cut veneer has a loose side and a tight side. The loose side contains checks that occur as the veneer sheet bends away from the knife. The side remaining on the workpiece is the tight side. The loose side is more likely to develop numerous fine checks, the depth and distribution of which are determined partly by the thickness of the veneer and partly by the precision and care employed in its production (Blackwell 1947, Newall 1953).

Knife checks form on the loose side of the veneer owing to the stresses built up in the wood near the edge of the knife. Nose bar setting, slicing temperature of the flitch, cutting velocity, log diameter, and the condition of the lathe knife are considered as the critical factors in veneer cutting (Lutz 1974a). One way to reduce the number and depth of knife checks is to increase the nose bar pressure. Since hot green wood is more pliable than cold green wood, knife checks can also be reduced by properly heating the log in the cooking vats (Limbach 1946, McMillin 1958, Feihl and Godin 1974, Lutz 1974a). However, logs which are either too cold or too hot, or at uneven temperature should be avoided (Feihl and Godin 1970). An increase of cutting speed causes less force on the nose bar, and results in more severe checks. High speed cutting of wet wood causes large frictional resistance and pressure over the cutting edge. The veneer bends and knife checks develop. The log diameter also has an effect on veneer strength in relation to the stresses developed. Excessive tangential stresses, particularly in smaller logs, are developed when the forming veneer sheet is bent, resulting in knife checks (McMillin 1958, Lutz et al. 1967, Cade and Choong 1969, Feihl and Godin 1970).

Loose-cut veneer has checks more than halfway through the thickness of the veneer sheet. Tight-cut veneer has checks less than halfway through the thickness of the veneer sheet (Jayne 1953).

Face checking in veneer is inevitable if moisture loss occurs in a dry environment, but its importance depends upon the size and distribution of the checks themselves (Knight 1947). The checks may vary from fractions of an inch to several inches in length. The reason for checking is that the moisture content (MC) of the face veneer and the panel core are different than the existing EMC. The difference subsequently causes a dimensional movement between the face veneer and the core. If the EMC were lower than the actual MC of the face veneer and the core, a stress would be formed that would pull the fibers of the face veneer apart perpendicular to the grain direction. Otherwise, the fibers of the face veneer would be compressed.

The factors that influence the formation and severity of veneer checking are listed in the following sections.

Veneer quality and species

It is generally accepted by the industry that well produced tight-cut veneer will result in less checking and loose-cut veneer will result in more checking in the finished panels. Some manufacturers select only the tightest cut veneer for use in critical areas such as tops or fronts of desks, dressers, and tables (Gilmore and Hanover 1990). However some experts suggest that very tight-cut veneer may be too stiff and it breaks easily in handling or when being pressed. It may be necessary for manufacturers to make a compromise between loose-cut and tight-cut veneer (Feihl and Godin 1970).

Those species of wood with smaller pores check less than wood with larger pores. This is because checking occurs more easily over deep knife checks and large pores that are less resistant to failure caused by stress concentrations (Forbes 1997). Lower density woods and those with less figure also tend to check less than wood with higher density and more figure (Gilmore and Hanover 1990).

The research conducted by Lutz (1956) showed that the density difference between springwood and summerwood, as well as between wood ray cells and adjacent tissue, and the thickness of the pores affect veneer check development.

To conserve materials, there is a tendency to use thinner veneer in Asia and Europe. Because less stress develops, thinner veneer can result in fewer checks at least for many species.

Veneer and core MC

When pressing, control of MC is the most important factor in preventing veneer checking (Australian Furniture Research & Development Institute Ltd. 1993, Lutz 1974b, Sparkes 1986, Forbes 1997). The face veneer and core should be conditioned to the most common environmental conditions in which the panel will be placed. For most areas of the United States, this translates to an MC of 6 to 8 percent. Veneer checks tend to appear along the knife checks parallel to the grain as veneer with excess moisture dries and shrinks. Some researchers indicate that veneer checks will develop more easily when the MC of the veneer is above 7 to 11 percent. The veneer simply dries and shrinks. Within the 7 to 11 percent MC range, the critical point for veneer checking is higher for straight-grained and tight-cut veneer (Yan and Lang 1958, Fine Hardwood Veneer Association, Sparkes 1986).

The shrinking and swelling of wood across the grain with changes of MC, is one of the most important factors in veneer checking (Yan and Lang 1958). Checks are formed when stress failures occur in the face of the veneer, caused by differential shrinking or swelling between the face veneer and the panel substrate, particularly when the face veneer and panel substrate have different EMCs after panel assembly (Blackwell 1947, Keith 1964, Lutz 1974b, Gilmore and Hanover 1990, Forbes 1997). The veneer is only restrained on its inner face; consequently, any shrinkage of the veneer imposes a tensile stress on its outer face, and thus a tendency to check. Conversely, the swelling of wood as a result of moisture absorption introduces a compressive stress on the outer face, and the restraint imposed by the glueline may impart a permanent compression set. The effect of this compression set is to cause a permanent reduction in size when the veneer re-dries. This reduction can only affect the outer surface, therefore, checking occurs. If the loose side of face veneer is exposed, the checks already present tend to increase in width after swelling and shrinking (Newall 1953).

Most of the time, the species of the veneer and the substrate board have a different hygroscopicity. The reduced hygroscopicity of manufactured panel products such as particleboard and medium density fiberboard (MDF) is of particular importance in panel manufacturing. For these reconstituted panel products, the EMC will be less than that of natural wood veneer at the same room temperature and the same RH. The appropriate MC for the veneer and the substrate may be slightly different since the two may have different EMCs for a given atmospheric condition. However, both should be conditioned to the same EMC (Gilmore 1983). They may also shrink or swell at different rates.

Panel components and construction

There is a relationship between the panel core and the face veneer in regard to check development. It is commonly thought that the tight side of the face veneer should be assembled outward if possible. However, when the face veneers are book-matched to produce a symmetrical pattern, one-half of the veneer must be placed loose side out. With this situation, well-produced tight-cut veneers should be used to minimize checking. Veneered panels can have an all-veneer construction or a thick core of lumber or particleboard, with or without crossbands. As the surface smoothness of reconstituted panel products has improved, there is a tendency to apply the veneer face directly to the panel core. Some researchers suggest that 5-ply construction is less subject to checking than 3-ply construction (Black well 1947, Keith 1964, Lutz 1974a, Gilmore and Hanover 1990, Forbes 1997). However, Gilmore (1983) reported that 3-ply construction is about as good as 5-ply construction with 1/10-inch hardboard or Yorkite crossbands under the conditions of 6 percent face veneer MC and an almost zero assembly time. The objective is to limit the amount of water entering and remaining in the face veneer while bonding (Forbes 1997).

Gluing procedures

There is a relationship between gluing procedures and sanding with respect to veneer check development. Adhesive spreading should be done to assure a uniform thickness of adhesive. Varying thickness of adhesive spread may result in different veneer thickness after sanding. Due to excess adhesive, localized areas of the veneer may be sanded through or made thinner than the rest of the veneer. In comparison to the thicker areas, thin veneer cannot resist the stress concentration caused by a moisture change in the panel and are likely to check. Therefore, a uniform thickness of adhesive spread is important.

Any moisture present in the adhesive will cause swelling of both the veneer and the panel core and there will be a tendency for the veneer to check along the grain as the panel re-dries to its original state. Therefore, high solids content adhesives with proper spread rate and short assembly times should be used (Newall 1953, Yan and Lang 1958. Sparkes 1986, Gilmore and Hanover 1990).

Poorly bonded areas are much more sensitive to checking. Poor bonding can be caused by inappropriate adhesive or gluing procedures. The partial pressure of moisture vapor in the void spaces of the porous wood material, the variations in interfacial contact of non-smooth veneer, and the shear stresses caused by differential swelling and shrinking of adjacent veneer as moisture migrates through them at the glueline during pressing are the main reasons for glueline weaknesses (Keith 1964, Zavala and Humphrey 1996). Shrinkage is less restrained over the areas of poor adhesion than over adjacent areas; the result is stress concentration, a fundamental cause of veneer checking. Therefore, the adhesive supplier's instructions should be followed precisely (Yan and Lang 1958, Lutz 1974, Australian Furniture Research & Development Institute Ltd. 1993).

Pressing procedures

Many researchers think that hotpressed panels are less prone to checking than cold-pressed panels, because excess moisture is evaporated from the face veneer in hot-pressing (Gilmore and Hanover 1990). However, hot-pressing may result in more telegraphing of any core irregularities through the face and warping if press times or temperatures are excessive (Yan and Lang 1958).

Assembly time is the period that elapses between adhesive spreading, assembly of components, and pressing. It is considered to have a significant effect on veneer checking. As the assembly time increases, the veneer and core absorb more moisture from the adhesive before the bonding process begins, and more checks may subsequently occur. Keeping assembly time within minutes is recommended (Yan and Lang 1958, Gilmore and Hanover 1990. Australian Furniture Research & Development Institute Ltd. 1993). However, excessive drying should also be avoided when using a hot-press.

Panel conditioning

Moisture and temperature imbalances are set up whether cold pressing or hot-pressing is used. Manufacturers have used two methods to condition panels. One method is to bulk stack panels coming from the hot-press to complete curing. Another method is to sticker hot-pressed panels to make them balanced and cool before further processing.

In general, cold-pressed panels have absorbed a considerable amount of moisture during curing. These panels should be stickered and conditioned until they reach a 5 to 8 percent MC for most applications (Yan and Lang 1958. Gilmore and Hanover 1990).

Sanding

One study has shown that about 40 percent of the original thickness of veneered faces is removed in sanding (Gilmore and Hanover 1990). Even if the face veneer is laid tight side out, excessive sanding may reduce the veneer thickness so that the knife checks of the loose side are exposed. The exposed side of the face veneer will determine the depth of sanding. If the tight side is out, sanding should be light. If the loose side is exposed, sanding should be heavy. In book-matching, sanding should be moderate (Yan and Lang 1958).

Finishing

Finishing materials are simply retarders for MC change. They only slow the process due to the low moisture vapor transmission characteristics (Gilmore and Hanover 1990. Australian Furniture Research & Development Institute Ltd. 1993, Forbes 1997). Finishes cannot be expected to bridge over checks in the veneer (Australian Furniture Research & Development Institute 1993). Water-based fillers, stains, and primers, which will increase the MC of face veneer, should not be used in finishing (Yan and Lang 1958). Water-based topcoats are common but not stains and primers. Regardless, adding water to veneer makes checks more likely.

Warehousing conditions

Furniture manufactured from veneered panels should be stored in a properly conditioned warehouse (Gilmore and Hanover 1990, Australian Furniture Research & Development Institute Ltd. 1993). If the panels pick up excess moisture, they may check after they subsequently dry out. In order to avoid moisture change, plastic wrapping can be effective (Forbes 1997). When veneered furniture is shipped, the manufacturer should make certain that the receiver understands the importance of proper storage.

Conditions in service

End-use conditions for furniture are different and varied. Extremely dry conditions can induce veneer checking even in properly made panels (Gilmore and Hanover 1990). Yan and Lang (1958) reported that checking was especially common in Canada due to the extremes in climate and the universal use of central heating. Conditions in the northern part of the United States are similar, where cold, dry outside air brought indoors and heated to 70[degrees]F can result in a very low EMC of 3 percent (Gilmore and Hanover 1990).

Warping

Warping is also a common problem with furniture panels. Yan and Lang (1958) indicated that most of the reasons for veneer checking are the same as the causes of warping, such as non-uniformity of the wood, differential moisture change among different parts of the panel, unbalanced surface construction, gluing procedures and adhesive types, assembly time, pressing procedures, improper panel construction and conditions, core material used, warehousing conditions, and others. The fundamental cause of warping is also the unequal wood movement during MC change. Therefore, preventing moisture change will not only eliminate veneer checking in veneered panels, but it will also help keep the panels flat. Moisture change in panels can be reduced by selecting stock at the correct initial MC. minimizing any additions of moisture, and finishing all faces and edges.

Materials and methods

Veneer

Two common species were selected for this study: 1) flat-cut red oak (Quercus sp.), a very common ring porous wood: and 2) hard maple (Aeer saccharum), a diffuse porous wood. A flitch for each species was obtained from an Indiana veneer company. The sheets were clipped to a 6-inch width with the cathedral centered, book-matched, and spliced to form 12-inch-wide sheets. Each individual sheet of veneer was then crosscut to 13-inch lengths. Specimens were coded while being cut and then randomized.

After cutting, one set of the veneer faces was conditioned at 35 percent RH and 72[degrees]F, resulting in an MC of 7 percent. The second set was conditioned at 65 percent RH and 72[degrees]F, which produced a wood EMC of 12 percent.

Backer sheets of comparable veneer were conditioned along with the faces. No crossbands were used.

Thickness measurements were made on the four corners of each individual sheet of veneer by using a micrometer after the 2-month conditioning period. The veneer meets the American National Standard for Hardwood and Decorative Plywood with a thickness standard of 1/38 inch (0.026 in.) and a tolerance range from 0.026 to 0.029 inch as suggested for most species used in the domestic market (ANSI 1995).

Using a microscope, the knife checks were determined to be 1/2 to 3/4 of the veneer thickness in red oak and 1/4 to 1/2 the thickness of the veneer in hard maple. Based on a study by Jayne (1953), the red oak is a loose-cut veneer and the hard maple is a tight-cut veneer.

Two methods were used to help identify both the loose and tight side of the book-matched veneer. The first method is the ink method. Indian ink was used to produce a black color on the veneer. The ink was brushed across the grain of the veneer to form a stripe on both sides. After the ink dried, the stripe section was cut across the grain with a sharp blade. The side where ink penetrated the veneer was the loose side: the side with no ink penetration was the tight side.

Another method is to use tape. A "Permacel" glue tape was applied by firmly pressing across the grain of the veneer to form a stripe on both sides of the veneer piece. Then the tape was snapped from the veneer surface with a quick pulling action. The side with more fibers on the tape was the loose side: the side with fewer fibers on the tape was the tight side.

Core

The panel core was 11/16-inch southern yellow pine particleboard with all stock taken from the same bundle. The sheets were conditioned at 35 percent RH and 72[degrees]F for 2 months before being cut into panels. Each sheet of particleboard was first ripped full length to obtain 12-inch-wide strips. In turn, each individual strip of particleboard was crosscut to a 12-inch length. These 12-inch by 12-inch specimens were randomly assigned to the veneer specimens.

Thickness measurements were made on the four corners of each individual 12-inch-square particleboard core by using a micrometer after the 2-month conditioning treatment. The particleboard meets the M-2 grade as specified in ANSI A208-1993 (NPA 1993). The particleboard MC was 5 percent after conditioning.

In comparison to MDF, particleboard is commonly used in veneer and panel construction because of its lower cost Surface properties of particleboard have been improved and quality veneered panels can be produced with single-ply construction.

Adhesive

A urea-formaldehyde with resin catalyst powder (UF) and a pre-catalyzed cross-linked polyvinyl acetate (PVA) were the two adhesives used. These are typical adhesives used in the industry and there are discussions over which one results in fewer face cheeks. The solids content of the UF adhesive was 59 to 61 percent and the PVA adhesive was 49 to 51 percent. The UF and PVA adhesives were applied with a fine-textured paint roller at the rate of 30 pounds per 1,000 ft.2 or 5 wet mils of thickness. A wet film thickness gauge was used to check the amount applied to each panel. The adhesive weighed 9 pounds per gallon. The panels were pressed at 150 psi and 250[degrees]F as recommended by the manufacturer.

Assembly time

Two assembly times were used for the UF and PVA adhesives: 1 minute and 10 minutes between the time the adhesive was applied to the core board and the press was closed. Wet veneer tends to curl. Thus, panels waiting to be pressed were dead stacked. A dummy panel was placed at the top and bottom with a consistent weight on the top.

Experimental design

The experimental design consisted of four factors: 2 adhesives (UF and PVA), 2 veneer sides (loose side and tight side), 2 veneer MCs (7% and 12%), and 2 assembly times (1 min. and 10 min.). This resulted in 16 combinations both for the red oak group and the hard maple group. Each combination was replicated eight times for a total of 128 samples. For each panel by veneer-side combination, the eight specimens were randomly assigned to the eight (two adhesives by two MCs by two assembly times) combinations of the other factors.

By taking 16 specimens, corresponding to each combination of the 4 factors, from each of the 8 panels of each species, the panel-to-panel variation is controlled in this split-plot design. The analysis of variance (ANOVA) separates out this variation and the result is that we have good power to detect effects due to the factors of interest. Because the analysis indicated that several interactions were significant, the Student-NewmanKeuls test with multiple comparisons procedure was used to interpret the results.

Exposure and measurement of checking

After pressing, the panels were set on racks with each panel equal distance from its neighbor. Panels were exposed for 3 weeks at 75 to 80 percent RH (wood EMC of about 15%) and then 3 more weeks at 20 to 25 percent RH (wood EMC of about 5%). These two treatments were repeated three times, resulting in most of the panels developing checks. The panels were evaluated immediately after pressing and then at the end of each of the 3-week conditioning periods. Thus, the panels were evaluated seven times.

The measurement system developed by Batey (1955) was used for evaluation. With this system, 11 lines spaced 1 inch apart and perpendicular to the veneer grain direction are drawn on each panel. If a check crossed a line once, it was counted once. If a check was long enough to cross two lines, it was counted twice, and so on. It was decided that the smallest check plainly visible with the help of a 16x magnifier under good light should represent the lowest limit size of checks counted.

Results

Red oak veneered panels

The ANOVA results for the red oak veneered panels with 128 observations are presented in Table 1.

From the results of the F-test, the main effects of adhesive, veneer side, MC, and assembly time were each significant at the 0.05 level. The interaction of veneer side and assembly time, and the interaction of MC and assembly time had significant effects on the veneer check development in red oak veneered panels.

Residual analysis was used in model adequacy checking. The plots of residuals by the factors of adhesive, veneer side, MC. and assembly time gave no indication of inequality of variance. From a normal probability plot of the residuals, there is no indication of non-normality. We conclude that the assumptions necessary for the validity of this model are satisfied.

The full model accounted for 80.4 percent of the variability.

The factor of veneer side (loose and tight) had the most significant effect, followed by adhesive, then assembly time, and finally MC (Table 1). The two-level interaction of veneer side and assembly time and the two-level interaction of MC and assembly time also had significant effects on veneer check development.

The Student-Newman-Keuls test was conducted on the red oak veneer check data for the 16 combinations. Seven groups of combinations resulted from the test and are labeled A-G. The combinations were not significantly different within the same group (Table 2).

Group G with combinations of 1111 (UF adhesive, loose side out, 7% MC, 1-minute assembly time), 1122 (UF adhesive, loose side out, 12% MC, 10-minute assembly time), 1211 (UF adhesive, tight side out, 7% MC, 1-minute assembly time), 1121 (UF adhesive, loose side out, 12% MC, 1-minute assembly time), 1112 (UF adhesive, loose side out, 7% MC, 10-minute assembly time), and 2111 (PVA adhesive, loose side out, 7% MC, 1-minute assembly time), had the least number of face checks as compared to the other six groups (A, B, C, D, E, and F).

Group A with combinations of 2222 (PVA adhesive, tight side out, 12% MC, 10-minute assembly time), 2212 (PVA adhesive, tight side out, 7% MC, 10-minute assembly time) and 2221 (PVA adhesive, tight side out, 12% MC, 1-minute assembly time), had the most number of face checks as compared to the other six groups (B, C, D, E, F, and G).

Considering Group G, the combination of UF adhesive, loose side out, 7 percent MC, and 1-minute assembly time resulted in the least number of face checks in red oak veneered panels. At the conclusion of the humidity cycling, only two of the eight panels in this test contained face checks. Each panel con tained just one check and these did not appear until the end of the last cycle. In all of Group G. PVA adhesive, and tight side of the veneer appeared only once, while 12 percent MC and 10-minute assembly time appeared twice.

Considering Group A, the combination of PVA adhesive, tight side out, 12 percent MC, and 10-minute assembly time resulted in the most number of checks in the red oak panels. Five out of the eight panels in this set were checked after the second 75 to 80 percent RH cycle. By the end of the cycling, the average number of checks per panel was 15.25 with a range of 4 to 20 checks.

Hard maple veneered panels

The ANOVA results for the hard maple veneered panels group of 128 observations are presented in Table 3.

From the results of the F-test, the main effects of adhesive, veneer side, MC, and assembly time were each significant at the 0.05 level. The effect of panel replicate was also statistically significant. The two-level interactions of adhesive and veneer side, adhesive and MC, veneer side and MC, and veneer side and assembly time had significant effects on the veneer check development. The four-level interaction of adhesive, veneer side, MC, and assembly time also had a significant effect on veneer check development in hard maple. Since the four-level interaction was significant, all the two-and three-level interactions are kept in the model, although they were not significant at the 0.05 level.

Residual analysis was used in model adequacy checking. The plots of residuals by the factors of adhesive, veneer side, MC, and assembly time gave no indication of inequality of variance. The plot of the residual versus the predicted value revealed no items of unusual interest. From a normal probability plot of the residuals, there is no indication of non-normality. The assumption of this model is satisfied.

The full model accounts for 90.7 percent of the experimental variability.

The factor of adhesive had the most significant effect, followed by veneer side, MC, and finally assembly time (Table 3). The factor of panel replicate had the least significant effect on veneer check development. Panel replicate stands for the effect which cannot be explained by the factors of adhesive, MC and assembly time. The two-level interactions of adhesive and veneer side, adhesive and MC, veneer side and MC, and veneer side and assembly time had significant effects. The four-level interaction of adhesive, veneer side, MC, and assembly time also had a significant effect on veneer check development.

The Student-Newman-Keuls test was conducted on the hard maple veneer check data for the statistical comparison of the 16 combinations. Four groups of combinations resulted from the test and the combinations were not significantly different in the same group (Table 4).

Group D with combinations of 1212 (UF adhesive, tight side out, 7% MC, 10-minute assembly time), 1221 (UF adhesive, tight side out, 12% MC, 1-minute assembly time), 1112 (UF adhesive, loose side out, 7% MC, 10-minute assembly time). 2211 (PVA adhesive, tight side out, 7% MC. 1-minute assembly time). 1111 (UF adhesive, loose side out, 7% MC, 1-minute assembly time). 1211 (UF adhesive, tight side out, 7% MC, 1-minute assembly time), 1222 (UF adhesive, tight side out, 12% MC, 10-minute assembly time), 2212 (PVA adhesive, tight side out, 7% MC, 10-minute assembly time), 1121 (UF adhesive, loose side out, 12% MC, 1-minute assembly time), and 2111 (PVA adhesive, loose side out, 7% MC, 1-minute assembly time) had the least number of face checks as compared to the other three groups (A, B, and C) (Table 4). Since the combinations of 1122 (UF adhesive, loose side out, 12% MC, 10-minute assembly time) and 2221 (PVA adhesive, tight side out, 12% MC, 1-minute assembly time) were not only in group D but also in Group C with apparent numeric difference from the rest of the combinations in Group D, the authors decided to allocate both combinations to Group C.

Group A with combinations of 2122 (PVA adhesive, loose side out, 12% MC, 10-minute assembly time) and 2121 (PVA adhesive, loose side out, 12% MC, 1-minute assembly time) had the most number of face checks as compared to the other three groups (B, C, and D).

Considering Group D, six of the combinations did not develop any veneer checks by the conclusion of the RH cycling process, three combinations developed one check each, and one combination developed four checks. There are a number of combinations of factors in this group. However, the factor of UF, 7 percent MC, tight side of the veneer, and 1-minute assembly time appeared most often.

Considering Group A, the factors for the poorest performing set of combinations were PVA, loose side out, 12 percent MC, and 10-minute assembly time. By the end of the cycling, the average number of veneer checks per sheet was 12.1 with a range of 4 to 21. The second worst combination had the same factors except assembly time decreased to 1 minute and the average number of checks was 8.5. At least some of the panels in each group checked after the first 20 to 25 percent RH cycle.

There is a common opinion that veneer should be tightly cut and placed with the tight side out during lay-up of the panel. However, the results from this experiment on red oak veneered panels demonstrates that fewer checks developed when the loose side was turned out.

Discussion

The factors of adhesive type, side of the veneer, veneer MC, and assembly time with their combinations account for 90 percent of the experimental variability in the red oak veneered panels and 95 percent of it in the hard maple veneered panels. Thus, these are the important variables.

Table 5 is a summary of the factors that caused the least and most number of checks. For red oak, the best combination was UF adhesive, loose side of the veneer turned out, 7 percent MC of the veneer, and a 1-minute assembly time. The factors were identical for hard maple except the tight side of the veneer was turned out. By contrast, for red oak, PVA adhesive, tight side out, 12 percent MC of the veneer, and a 10-minute assembly time resulted in the largest number of checks. The factors were identical for hard maple except the loose side was turned out.

Adhesive

The UF adhesive contained 59 to 61 percent solids while the PVA adhesive contained 49 to 51 percent solids. Thus, there is somewhat more water available for the wood to absorb with the PVA adhesive. UF adhesives are also rigid whereas PVA is somewhat flexible.

MC

Most wood products intended for interior use in North America are conditioned to about 6 to 8 percent MC. This MC is midway between seasonal wood MC highs and lows for a year. The veneer conditioned to 7 percent MC checked less than that conditioned to 12 percent. The veneer with 12 percent MC shrunk and checked as it was dried to lower MCs.

Assembly time

The assembly time of 1 minute generally resulted in less checking than 10 minutes. The 10-minute period allowed the veneer to absorb more moisture from the adhesive and swell before it was hot-pressed. Thus, as it lost MC and shrunk it checked more.

Species effect

The anatomical characteristics of red oak are distinctly different from hard maple. Red oak is a ring porous wood with very large pores that abruptly change to small-diameter pores in each growth ring. Red oak also has very large wood rays. In contrast, hard maple has small-diameter pores uniformly distributed across each growth ring and much smaller wood rays.

Large pores are also a factor in red oak veneer checking. Because of the large pores, the effective thickness of the veneer in these regions is reduced. Checks occur in both the loose and tight side of the veneer in these regions.

The wood rays are probably the most important factor in checking of red oak veneer. The radial and tangential shrinkage of wood rays in red oak is less than that of the adjacent tissue types (Mclntosh 1957), thus causing a stress point. A large number of checks in this study appear at the interface of the wood rays and the surrounding tissue.

The loose side of the veneer sheet probably checks less because the knife checks that are already present relieve any stress that develops. Therefore, checks between the ray and adjacent tissue do not develop as much and the panel appears less checked.

It has been shown in softwood research (Wang and Leng 1995) that a cylinder with fine needles on the surface can be used to treat the tight side of the veneer and balance the stresses on both sides. The fine ruptures along the rays on the tight side of red oak veneer could function in the same manner. However, this theory was not investigated in this study.
Table 1. -- Analysis of variance at the 0.05 level for veneer checking
in red oak panels. (*)

 Source DF F-value p-value

Adhesive-B* 1 46.20 0.0001

Veneer side-C* 1 64.22 0.0001

BC 1 0.02 0.89

Moisture content-D* 1 5.88 0.0185

BD 1 0.02 0.90

CD 1 3.18 0.08

BCD 1 0.25 0.62

Assembly time * 1 7.63 0.0078

BK 1 0.03 0.86

CK* 1 7.01 0.0105

BCK 1 0.11 0.74

DR* 1 8.10 0.0062

BDK 1 2.00 0.14

CDK 1 0.40 0.55

BCDK 1 0.77 0.41

Panel replicate-P 56 1.22 0.23

(*) B = adhesive (UF and PVA);f C = veneer side (loose and tight); D =
moisture content (7 and 12%); K = assembly time (1 and 10 minutes); P =
panel replicate; * = statistically significant at the 5 percent level.

Table 2.--Red oak veneer checking comparison of the 16 combinations in
rank order and the results of Student-Newman-Keuls test at the 0.05
level. (a)

Square root means of number of checks in red oak veneered panels
3.831 3.255 2.945 2.400 2.376 2.348 2.306 2.228 2.015 1.733
2222 2212 2221 1222 1212 2211 1221 2121 2112 2122
A_________________ E__________ B__________________________________________________ F___
 C____________________________________________________
 D____________________________________________

Square root means of number of checks in red oak veneered panels
3.831 1.559 1.081 1.020 0.758 0.478 0.250
2222 2111 1112 1121 1211 1122 1111
A____ E________________________
 F_______________________________
 C___
 D_________________
 G______________________________________

(a) The combinations of factors above the same line are not
statistically different at the 0.05 level. For the combination of
factors: first digit is the adhesive (1 = UF:2 = PVA: second
digit is the veneer side (1 = loose side: 2 = tight side):
third digit is moisture content (1 = 7%.2 = 12%):
fourth digit is assembly time (1 = 1 min:2
10 min.)

Table 3.--Analysis of variance at the 0.05 level for veneer checking in
hard maple panels. (a)

 Source DF F-value p-value
Adhesive - B* 1 46.70 0.0001
Veneer side - C* 1 46.08 0.0001
BC* 1 18.51 0.0001
Meisture content - D* 1 35.81 0.0001
BD* 1 17.06 0.0001
CD* 1 8.31 0.0056
BCD 1 0.16 0.69
Assembly time - K* 1 7.50 0.0083
BK 1 1.56 0.22
CK* 1 6.29 0.0151
BCK 1 0.40 0.53
DK 1 0.59 0.45
BDK 1 0.52 0.47
CDK 1 0.03 0.86
BCDK* 1 4.22 0.0447
Panel replicate - P* 56 2.79 0.0001

(a) B=adhesive (UF and PVA); C=veneer side (loose and tight): D=moisture
content (7 and 12%), K = assembly time (1 and 6 minutes); P = panel
replicate: * = statistically significant at the 5 percent level.

Table 4--Hard maple veneer checking comparison of the 16 combinations in
rank order and the results of Student-Newman-Keuls test at the 0.05
level. (a)

Square root means of number of checks in hard maple veneered panels
3.320 2.538 1.755 1.475 1.159 1.019 0.250 0.125 0.125 0.125
2122 2121 2222 2112 2221 1122 2111 1121 2212 1222
A__________ C______
 B__________ D_______________________________________

Square root means of number of checks in hard maple veneered panels
3.320 0.000 0.000 0.000 0.000 0.000 0.000
2122 1211 1111 2211 1112 1221 1212
A______
 D______________________________________
(a) The combinations of factors above the same line are not
statistically different at the 0.05 level. For the combination of
factors: first digit is the adhesive (1 = UF:2 = PVA:second digit
is the veneer side (1 = loose side:2=tight side); third
digit is moisture content (1 = 7%; 2 = 12%); fourth
digit is assembly time (1=1 min:2= 10 min.)

Table 5.--Summary of factors that resulted in the least and most number
of checks.

 Adhesive Veneer side Veneer moisture Assembly time
 content
Least no. of checks (%)

 Red oak
veneered panels UF Loose side 7 1 minute out

 Hard maple UF Tight side 7 1 minute
 veneered panels out

Most no. of checks

 Red oak veneered PVA Tight side 12 10 minutes
 panels out

 Hard maple PVA Loose side 12 10 minutes
 veneered panels out


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The authors are respectively. Professor and Former Graduate Student. Wood Res. Lab., Dept. of Forestry and Natural Resources, Purdue Univ., 1200 Forest Prod. Bldg., West Lafayette, IN 47907-1200; and Professor, Dept of Statistics, Purdue Univ., MATH Bldg., West Lafayette, IN 47907-2068. This paper is Journal Paper 16733 of the Purdue Univ. Agri. Expt. Station, David R. Webb Company. Heritage Hardwoods, Kimball International and Southeastern Adhesives Company all provided materials and/or technical assistance. This paper was received for publication in March 2002. Article No. 9448.

* Forest Products Society Member.

[c] Forest Products Society 2003.

Forest Prod. J. 53(10): 79-86.
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Date:Oct 1, 2003
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