PROPERTIES OF PARTICLEBOARDSAT VARIOUS HUMIDITY

CONDITIONS

U.S.D.A. FOREST SERVICERESEARCH PAPERFPL 2251974

U.S. DEPARTMENT OF AGRICULTUREFOREST SERVICEFOREST PRODUCTS LABORATORYMADISON, WIS. 53705

Abstract

The changes in properties of particleboardthat result from changes in moisture contentshould be determined to avoid possibleproblems related to linear expansion andthickness swelling and to loss in load-carryingcapacity and stiffness. Property-moisturerelationships are particularly important ifmaterial is used as structural elements that aresubjected to adverse moisture exposures. In thisreport the physical and mechanical propertiesare determined for five commercial par-ticleboards manufactured primarily fromplaner shavings and for a board made at theForest Products Laboratory from random-widthDouglas-fir flakes, 1 inch long and 0.015 inchthick. All were conditioned at humidity ex-posures between ovendry and 90 percentrelative humidity.

Linear expansion of the commercial boardswas considerably greater than that of the FPLboard. Thickness swelling of the FPL board wasgreater than that of the commercially producedparticleboards.

Ovendrying reduced most load-carrying-capacity and stiffness properties of the particle-boards. The major exception to this reductionwas compressive strength. Conditioning speci-mens to equilibrium moisture content at 80and 90 percent relative humidity reduced load-carrying capacity and stiffness below the valuesof matched specimens at 30 and 65 percentrelative humidity. The effect of exposure timeup to 90 days at 90 percent relative humidityon values of properties was slight. The urea-bonded particleboards behaved essentially asthe phenolic-bonded boards at all humidityconditions used in this study.

PROPERTIES OF PARTICLEBOARDSAT VARIOUS HUMIDITY CONDITIONS

ByJ. DOBBIN McNATT, Technologist

Forest Products Laboratory1

Forest ServiceU.S. Department of Agriculture

Introduction

The effect of changes in moisture content isan important factor to consider in transformingthe basic engineering properties of par-ticleboard into design stresses for efficientstructural application, particularly if exteriorexposure is likely.

This report presents results of evaluatingthe effects of changes in equilibrium moisturecontent (EMC) on strength and elastic proper-ties in tension, compression, bending, and shearof five commercial particleboards and a par-ticleboard made at the Forest ProductsLaboratory (FPL). Also measured was linear ex-pansion and thickness swelling between 30 and90 and between 50 and 90 percent relativehumidity.

This is the second in a series of reports toprovide the basic information necessary to es-tablish design stresses for particleboard. Basicengineering properties at standard conditions of75° F and 64 percent relative humidity of ninecommercial particleboards and the FPL boardwere presented in the first report (6).2

Research Material

The nine commercial particleboards in-cluded in the first report of this series wereselected as representative of thickness, species,resin types, particle types, and processingmethods used in particleboard production whenthe study was initiated. The five commercialproducts included here were selected to coverthe range of particleboard types included inthe first report (6). The five commercial par-ticleboards and the FPL board are described intable 1.

Commercial material was selected at themanufacturing plants over a 2-week period andwas randomized so that panels were taken atdifferent times of the day from different.openings in the press. Six 4- by 4-foot sections ofeach material were shipped to the ForestProducts Laboratory where they were cut intonominal 2-foot squares and marked for iden-tification. Fifteen FPL particleboard panels,22 by 26 inches, were made at the Laboratoryfrom random-width Douglas-fir flakes, 0.015inch thick and 1 inch long.

Research Methods

Specimens were prepared for testing instatic bending, tension parallel to surface, com-pression parallel to surface, interlaminar shear,and edgewise shear at the following eight ex-posures:

(1) Ovendry (dried in an oven at 218°F for 3 days)

(2) 80° F-30 percent relative humidity(R.H.) for 30 days

(3) 80° F-65 percent R.H. for 30 days(controls)

(4) 80° F-80 percent R.H. for 30 days(5) 80° F-90 percent R.H. for 30 days(6) 80° F-90 percent R.H. for 45 days(7) 80° F-90 percent R.H. for 60 days(8) 80° F-90 percent R.H. for 90 days

1Maintained at Madison, Wis., in cooperation with theUniversity of Wisconsin.

2 Italicized numbers in parentheses refer to LiteratureCited at end of report.

Table l.--Description of particleboards evaluted at different humidity exposures

1 Average of 3

bending,

3 compression,

and

3 interlaminar shear

specimens;

based on dimensions after conditioning 30 days at 80° F and

65 pct

relative humidity and

ovendry

weight.

Exposures 6, 7, and 8 were included todetermine the effect of time of exposure at 90percent relative humidity on strength andelastic properties of the particleboards and tonote any difference in behavior of urea- andphenolic-bonded boards; i.e., possiblebreakdown of urea resin due to exposure up to90 days at high humidity.

One of the four 2-foot squares was random-ly selected from each of the six panels of eachcommercial board (a total of 5 x 6 = 30 squaresselected). Six of the 22- by 26-inch FPL par-ticleboard panels were randomly selected from

six samples selected for each commercialproduct, eight static bending and eight in-terlaminar shear specimens were prepared asshown in figure 1. (One specimen for each of theeight exposures listed above.) From the remain-ing three samples of each commercial product,eight tension parallel, eight compressionparallel, eight edgewise shear specimens, andone linear expansion specimen were prepared asshown in figure 2. The same number ofspecimens were prepared from similar layoutsfor the FPL board. Except for conditioning,specimens were evaluated as specified in ASTM

the 15 that were made. From each of three of the Standard D 1037 (1).

STATIC BENDING90 (90)

90 (60)

90 (45)

90 (30)

8 0

6 5

3 0

O.D.

Figure 1 .—Layout of static bending (in varied lengths depending on thickness)and interlaminar shear (nominally 2 by 6 in.) specimens on 24- by 24-inchcommercial particleboard samples. (Numbers outside parentheses in-dicate relative humidity (R.H.); numbers in parentheses, days of exposureat 90 pct R.H.; and O.D., ovendry.) (M 141 802)

3

Figure 2.—Layout of tension (2 by 10 in.) and compression (1 by 4 in.) parallelto surface and edgewise shear (3-1/2 by 10 in.) specimens on 24- by 24-inch commercial particleboard samples. (Numbers outside parentheses in-dicate humidity condition; numbers in parentheses, days of exposure at 90pct R.H.; and O.D., ovendry.) (M 141 803)

Analysis of Results

Linear Expansion and Thickness Swelling

Linear expansion and thickness swelling ofthe six different particleboards were determinedbetween 50 and 90 and between 30 and 90 per-cent relative humidity. Specimens were firstconditioned to equilibrium at 30 percentrelative humidity, then were measured; subse-quently they were measured after conditioningto equilibrium at 50 and at 90 percent relativehumidity. Values are presented in table 2, ex-pressed as a percent of specimen dimensions at50 percent relative humidity. The moisture con-tent at each relative humidity condition was

also determined (table 2).For each humidity range, product M ex-

hibited the least linear expansion of the com-mercial boards. M was a three-layer board withthe faces of disk-flaked roundwood, whereas theraw material for the other commercial productswas primarily planer shavings. Furtherevidence of the superior performance of boardsfrom flakes is shown by the values for linear ex-pansion of the FPL board, which are less thanone-third those of the best commercial board M.

Gatchell et al. (3) and Heebink (4) alsoreported much less linear expansion fromboards from flakes compared to boards fromsawdust, slivers, or planer shavings. Lehmann(5) expressed linear expansion, α, as a percentper percent change in particleboard moisture

4

Table

2.--Linear

expansion

and

thickness

swelling of particleboard

between

30 and

90 and

50 and

90 percent

relative humidity

1 (R

.H.)

Table

3.--Percent

moisture content

of particleboard

at different

relative humidity (R.H.) exposures

at 80°

F1

content between equilibrium at 30 and 90 per-cent relative humidity and 90°F. He found thatthe linear expansion of boards from long thinflakes averaged about one-third that of boardsfrom short hammermilled particles. Suchsland(7) also expressed linear expansion of selectedcommercial particleboards in terms of αbetween equilibrium at 40 and 90 percentrelative humidity. He reported expansion coef-ficient values for medium-density conventional-type boards of 0.0284 to 0.0636 percent per per-cent. A similar analysis for the linear expansioncoefficient α, was made here for the change from50 to 90 percent relative humidity; the five com-mercial boards have values from 0.038 to 0.055percent per percent (table 2). Suchsland (7)reported a much smaller value, 0.0088 percentper percent, for an all-flake aspen par-ticleboard. This compares to a value of 0.0072percent per percent for the FPL all-flakeDouglas-fir board.

By contrast, thickness swelling of the FPLboard was substantially greater than wasthickness swelling of the five commercial par-ticleboards. The swelling was about 10 percent,or approximately 1-1/2 to 2 times that of thecommercial boards. Thickness swelling of theboard from long thin flakes in Lehmann’s study(5) was also about 1-1/2 times that of the boardfrom short hammermilled particles. As ex-pected, the commercial board with the facelayers of disk-cut flakes, M, swelled more inthickness than did the other four commercialproducts. Suchsland (8) suggests that therelatively large thickness. swelling of boardsfrom large flakes is caused by an inherentlylarge horizontal density variation compared toboards from smaller particles.

Equilibrium Moisture Content

Static bending specimens were used to ob-tain values for equilibrium moisture content(EMC) for the six particleboards at 30, 65, 80,and 90 percent relative humidity. Also obtainedwere changes in moisture content at 90 percentrelative humidity as the duration of exposuretime was increased from 30 to 45 to 60 and final-ly to 90 days. At the 30, 65, and 80 percentrelative humidity exposures, EMC was attainedafter 30 days or less. According to the ASTMStandard D 1037, Note 38, (1) practicalequilibrium is reached when “the specimenis neither gaining nor losing moisture contentmore than 0.05 percent by weight in a 24-hourperiod.” At 90 percent relative humidity, how-ever, between 30 and 45 days were requiredto attain EMC. Table 3 presents the moisture

7

contents of the six particleboards at the sevenhumidity exposures.

The EMC data are presented graphically infigure 3 with the average EMC data for solidwood from the Wood Handbook (10). Moisturecontent after 45 days’ exposure was plotted asthe EMC’s for the particleboards at 90 percentrelative humidity. The relative position of thesetwo curves must be interpreted with some cau-tion. The data for solid wood from the WoodHandbook are based on drying wood from thegreen condition whereas the data for par-ticleboard are based on the material picking upmoisture on the first adsorption to reach EMC.Further cycling between low and high relativehumidity as Suchsland (7) has shown wouldraise the adsorption EMC values at a givenrelative humidity.

At 30 percent relative humidity, theaverage EMC of the six particleboards equaledthat of solid wood (6.0 pct). At 65 percentrelative humidity, the average EMC of the par-ticleboards (9.4 pct) was about four-fifths of thevalue for solid wood (11.7 pct). At both 80 and90 percent relative humidity, the average EMCof the particleboards (11.6 and 15.8 pct, respec-tively) was about three-fourths that of solidwood (15.7 and 20.2 pct, respectively). TheEMC values for the six particleboards in thisstudy were within the range of values reportedby Suchsland (7) for commercial par-ticleboards. Similar values have been reportedby Chow (2) and Terentiev (9). Suchsland’swork indicates that the lower EMC of par-ticleboard compared to that of solid wood is atleast in part due to the heat used to dry the par-ticles and the heat used to cure the adhesiveduring hot pressing.

Conditioning Effectson Engineering Properties

Engineering properties for the sixparticleboards at the eight conditioning ex-posures are presented in tables 4 through 11.

Stiffness in compression parallel to surfaceis not reported because the gage recordingdeformation with load did not operate correctlyfor some specimens. Data for stiffness in com-pression that were obtained indicated that itwas affected by EMC changes in much the samemanner that stiffness in tension was affected.

Strength and elastic properties, based onload and deformation data and on the dimen-sions of the specimens at time of test, arepresented in the upper part of each of tables 4through 11. Values were calculated using the or-dinary engineering equations that assume the

Figure 3.—Comparison of equilibrium moisture content of solid wood with thatof particleboard at four relative humidities (C, H, J, K, and M = commercialboards; FPL = Forest Products Laboratory board). (M 141 804)

material is homogeneous. But some par-ticleboards, such as three-layer boards, are nothomogeneous, thus the calculated values are anapparent property-e.g., an apparent modulusof elasticity in bending. These values, true orapparent, serve as a common and usefulreference to other investigators; therefore, theyare reported on the basis described.

Although the values in the upper part ofthe tables reflect changes in conditioning, theyare probably not the most practical forengineering design. Particleboard is likely to beat a moisture content of 6 to 9 percent or aboutequivalent to conditioning at a relative humidi-ty of 30 to 65 percent when placed in a struc-ture. When subjected to higher relativehumidities in the structure, moisture contentwill increase and strength and modulus ofelasticity will decrease. Thickness will also in-crease, however, so that the change in load-carrying capacity and stiffness is likely to besignificantly less than the decrease in max-imum stress and modulus of elasticity.

Load-carrying capacity and stiffness ratios,expressed in terms of the control values, arepresented in the lower half of tables 4 through11 and are summarized in table 12. Basicstrength and elastic properties of particleboardconditioned at the standard 65 percent relativehumidity can be used for design and, where ap-propriate, corresponding ‘corrections can bemade for load-carrying capacity and stiffness.

The ratios for load-carrying capacity andstiffness ratios from the tables are plotted forthe various relative humidities in figures 4through 11 and are summarized in figures 12and 13. Values plotted at 90 percent relativehumidity are those for the 45-day exposure.

With two exceptions, load-carrying capaci-ty and stiffness of the particleboards in bend-ing, tension, and shear decreased with oven-drying. Properties of ovendry specimens wereusually 10 to 20 percent lower than properties ofmatched specimens at 30 percent relativehumidity. The two exceptions were the ovendryload-carrying capacity in tension of J, whichwas 10 percent higher than the value at 30 per-cent relative humidity (fig. 6), and the ovendryinterlaminar shear stiffness of H, which was es-sentially the same as the value at 30 percentrelative humidity (fig. 10). There are no ap-parent reasons for these exceptions.

For all of the particleboards evaluated,compressive load-carrying capacity wasaffected much less by ovendrying than were theother engineering properties. Ovendry com-pressive load-carrying capacity of the six par-ticleboards averaged less than 5 percent lowerthan the corresponding values at 30 percentrelative humidity, and ranged from 7 percentlower to 4 percent higher.

At 30 percent relative humidity, all averageproperties except load-carrying capacity inbending were equal to or as much as 14 percent

8

higher than the control values at 65 percentrelative humidity (figs. 12 and 13). Load-carrying capacity in bending averaged 95 per-cent that of the controls. This agrees withvalues obtained by Chow (2). At 80 percentrelative humidity all properties averaged 85 to95 percent that of the controls, with only in-terlaminar shear stiffness less than 90 percent.Of the specimens conditioned to equilibriummoisture content at 90 percent relative humidi-ty, all but interlaminar shear stiffness averagedabout 70 to 80 percent of the controls.Interlaminar shear stiffness averaged 52 percentof the controls.

Figures 14 and 15 show the close agreementbetween bending data for commercial par-ticleboards in this study and data from anearlier study on commercial particleboards byNordenson at the Forest Products Laboratory.Data on engineering properties of par-ticleboards at different relative humiditiesreported by Terentiev (9) are also quite similarto those presented in this report.

Although the foregoing comparisons are ofinterest to determine effects of ‘near limits ofconditioning, i.e., ovendry and 80 to 90 per-cent relative humidity, most interior applica-tions for particleboard will be between 30 and 65percent relative humidity. Examination of thedata or of individual plots for load-carryingcapacity in bending, tension, compression, andshear indicates values for the 30 percent ex-posure are almost always equal to or greaterthan values at the 65 percent exposure. Thelowest values were in bending and ranged from90 to 100 percent of the values at 65 percentrelative humidity. Similarly, stiffness valuesat 30 percent relative humidity were consisten-tly larger than those at 65 percent relativehumidity except for the bending stiffness ofthe FPL board. Thus, for practical purposes,the standard data obtained from tests afterconditioning at 65 percent relative humiditywill usually be satisfactory in engineeringdesign for interior applications.

Effect of Exposure Timeat 90 Percent Relative Humidity

on Engineering Properties

Figure 16 shows that average load-carryingcapacity of the six particleboards after 90 days’exposure at 90 percent relative humidity wasabout 5 percent below that after 30 days’ ex-posure and about 3 percent below that when thespecimens reached EMC after 45 days’ ex-posure. Figure 17 shows that stiffness wasaffected even less by exposure time up to 90

days at 90 percent relative humidity. Nodifferences were noticeable in the effect of theseexposure times on the urea-bonded boards com-pared to that on the phenolic-bonded boards.

Summary and Conclusions

The relationship between equilibriummoisture content (EMC) at different relativehumidity exposures and the physical andmechanical properties of particleboard is sum-marized in the following discussion.

Linear Expansionand Thickness Swelling

The ranges of linear expansion for five com-mercial particleboards, manufactured primari-ly from planer shavings, between 50 and 90 andbetween 30 and 90 percent relative humiditywere 0.30 to 0.37 and 0.40 to 0.49 percent,respectively. Linear expansion of a par-ticleboard made at the Forest ProductsLaboratory from l-inch long, 0.015-inch thick,random width Douglas-fir flakes was 0.06 per-cent between 50 and 90 percent relative humidi-ty and 0.12 percent between 30 and 90 percentrelative humidity.

The ranges of thickness swelling for thecommercial boards, measured on the samespecimens, were 4.8 to 6.8 percent between 50and 90 percent relative humidity and 5.6 to 7.6percent between 30 and 90 percent relativehumidity. Thickness swelling of the FPL boardwas 9.9 percent between 50 and 90 percentrelative humidity and 10.9 percent between 30and 90 percent relative humidity.

The significance of these data is that par-ticleboards from larger flakes are far superior inlinear expansion to conventional planershaving-type particleboards. However, theplaner-shaving-type boards are substantiallybetter in thickness swelling than are boardsmade from larger flakes.

Equilibrium Moisture Content

At 30 percent relative humidity the averageEMC of the six particleboards evaluated wasthe same as the average EMC of solid wood.However, at higher humidities, the EMC of par-ticleboard was less than the EMC of solid wood.

A major factor contributing to the lowerEMC’s of particleboard at the higher relativehumidities compared to those of solid wood is

9

Table 4.--Effect of ovendrying and varied conditioning on bending strength of

particleboard 1

1Each value based on average of 3 specimens.2Control specimens.3Based on dimensions of specimens at time of test.4Based on maximum load.

Table 5.--Effect of ovendrying and varied conditioning on bending stiffness of

particleboard 1

3Based on dimensions of specimens at tine of test.4Based on slope of load-deformation curve.

1Each value based on average of 3 specimens.2Control specimens.

1 0

Table 6.--Effect of ovendrying and varied conditioning on the tension

parallel-to-surface strength of particleboard

1Each value based on an average of 3 specimens.2Control specimens.3Based on dimensions of specimens at time of test.4Based on maximum load.

Table 7.--Effect of ovendrying and varied conditioning on tension parallel-to-surface

stiffness of particleboard

1Each value based on an average of 3 specimens.2Control specimens.3Based on dimensions of specimens at time of test.4Based on slope of load-deformation curve.

11

Table 8.--Effect of ovendrying and varied conditioning on compression parallel-to-surface

strength of particleboard

1Each value based on an average of 3 specimens.2Control specimens.3Based on dimensions of specimens at time of test.4Based on maximum load.

Table 9.--Effect of ovendrying and varied conditioning on the interlaminar shear strength

of particleboard

3Based on dimensions of specimens at time of test.4Based on maximum loads.

1Each value based on an average of 3 specimens.2Control specimen.

1 2

Table 10.--Effect of ovendrying and varied conditioning on interlaminar shear

stiffness of particleboard1

3Based on dimensions of specimens at time of test.4Based on maximum loads.

1Each value based on an average of 3 specimens.2Control specimens.

Table 11.--Effect of ovendrying and varied conditioning on edgewise shear strength of

particleboard 1

1Each value based on an average of 3 specimens.2Control specimens.3Based on dimensions of specimens at time of test.4Based on maximum loads.

13

Table 12.--Summary of load-carrying capacity and stiffness of particleboards at different conditionsexpressed as a percent of control values at 65 percent relative humidity

1Values with dashes are range of average values for 6 particleboards. Numbers in parentheses arecombined averages for the 6 particleboards.

14

Figure 4.—Load-carrying capacity in bending of six particleboards ovendriedand conditioned to equilibrium at four relative humidities (C, H, J, K, andM = commercial boards; FPL = Forest Products Laboratory board).

Figure 5.—Bending stiffness of six particleboards ovendried and conditionedto equilibrium at four relative humidities (C, H, J, K, and M = commercialboards; FPL = Forest Products Laboratory board). (M 141 806)

15

Figure 6.—Tension parallel to surface load-carrying capacity of six particle-boards ovendried and conditioned at four relative humidities. (M 141 807)

Figure 7.—Tensile stiffness of six particleboards ovendried and conditioned toequilibrium at four relative humidities. (M 141 808)

1 6

Figure 8.—Compression parallel to surface load-carrying capacity of six par-ticleboards ovendried and conditioned to equilibrium moisture content atfour relative humidities. (M 141 809)

Figure 9.—Interlaminar shear load-carrying capacity of six particleboardsovendried and conditioned to equilibrium moisture content at four relativehumidities. (M 141 810)

17

Figure 10.—Interlaminar shear stiffness of six particleboards ovendried andconditioned to equilibrium moisture content at four relative humidities.(M 141 811)

Figure 11.—Edgewise shear load-carrying capacity of six particleboardsoveridried and conditioned to equilibrium moisture content at four relativehumidities. (M 141 812)

18

Figure 12.—Load-carrying capacity of six particleboards ovendried and con-ditioned to equilibrium moisture content at four relative humidities.(M 141 813)

Figure 13.—Stiffness of six particleboards ovendried and conditioned toequilibrium moisture content at four relative humidities. (M 141 814)

19

Figure 14.—Comparison between data by Nordenson at the Forest ProductsLaboratory and the data in this study by McNatt for load-carrying capacityin bending of particleboards. (M 141 815)

Figure 15.—Comparison between data by Nordenson at the Forest ProductsLaboratory and the data in this study by McNatt for bending stiffness ofparticleboards. (M 141 816)

2 0

Figure 16.—Effect of exposure time at 90 percent relative humidity on load-carrying capacity of six particleboards. (M 141 817)

Figure 17.—Effect of exposure time at 90 percent relative humidity (R.H.) onstiffness of six particleboards. (M 141 818)

21

the heat used to dry the particles before formingthe mat and the heat to cure the adhesive whenpressing the mat. It should also be noted thatthe data on solid wood are based on drying fromthe green condition, i.e., desorption data,whereas the particleboard values are data ob-tained during the first adsorption. Because ofthis, the quantitative conclusions relating to theinfluence of relative humidity on a propertymust be taken with some caution.

Effects of Conditioning on

Engineering Properties

For purposes of engineering design, com-parisons were made among the various exposureconditions using load-carrying capacity (max-imum loads) and stiffness (slope of load-deformation curves) rather than strength andelastic properties based on dimensions of thespecimens at time of test.

Values for ovendry bending, tension, andshear load-carrying capacities averaged10 to 20 percent lower than did those of thecontrols. Values for ovendry compressionaveraged about 5 percent higher than did thoseof the controls. At 30 percent relative humidityonly values for load-carrying capacity in bend-

ing averaged lower than those for the controls.The values for the other properties were equalto or as much as 10 percent higher than thoseof the controls. At 80 percent relative humidity,values for all properties were about 10 percentlower than those for the controls. At 90 percentrelative humidity, bending, tensile, and edge-wise shear values were about 20 percent lowerthan those for the controls, and compressiveand interlaminar shear values were about 30percent lower.

Values for stiffness in bending, tension, andinterlaminar shear generally reacted the sameas those for load-carrying capacity with changesin EMC except at 90 percent relative humidity,values for interlaminar shear stiffness werealmost 50 percent below the control valuescompared to stiffness values in bending andtension, which were about 80 and 75 percentthose of the controls respectively.

Values for load-carrying capacity after 90days’ exposure at 90 percent relative humiditydecreased about 5 percent below those after 30days’ exposure and about 3 percent below thosewhen they reached EMC after 45 days’ ex-posure. Stiffness was affected even less.

No particular particleboard, either urea-bonded or phenolic-bonded, behaved substan-tially different from the others at any of theeight different exposures used in this study.

2 2

Literature Cited

2 3 U.S. GOVERNMENT PRINTING OFFICE 1974-650-246/2