ABSTRACT

The amount of particleboard produced in the United States in 1971 was more than 20 times that produced in 1956. Use has been primarily re­stricted to interior applications. However, the development of exterior grades will create new possibilities for particleboard as structural components.

To insure correct application where the material is now being used and to provide the information necessary for developing uses in new areas, data on the basic engineering properties of particleboard are needed. Information on the effects of moisture content, rate of loading, repeatedloading, and long-term loading is also needed. This report presents the results of evaluating strength and elastic properties in tension, compres­sion, bending, and shear of nine commercial particleboards conditioned to equilibrium moisture content at 75° F., and 64 percent relative humidity.Included For comparison are properties of the standard Forest Products Laboratory particleboard of urea-bonded Douglas-fir. All 10 particleboards can be classified as medium-density grade.

Properties closely related to each other, determined by least squaresregression, were interlaminar shear strength and internal bond strength,shear modulus and modulus of elasticity, tensile strength and edgewiseshear strength, and tensile strength and modulus of rupture. Relatingboard properties to manufacturing variables was complicated by the combi­nations of wood species and by the processing methods used. Variabilityof the properties of nine of the 10 particleboards was not considered excessive if compared to similar data from commercial hardboards. The greater variability of the tenth board was related to variation in density, to large particle, and to panel thickness.

FOREWORD

It is both possible and practical to use engineering methods to designstructural elements, components, and assemblies with almost any material if sufficient information is available on the strength and the elastic properties of that material as well as its behavior under load. This also applies to particleboard. The information required for particleboardincludes basic properties in tension, compression, bending, and shear; variability; effect of moisture content and temperature on strength;effect of rate and duration of load; creep and influence of different exposures on creep; and effect of repeated loads (fatigue).

This report is the first in a series to provide the basic data for establishing design stresses. The plan involves including a broad rangeof typical commercial particleboards so that the fundamentals developedwill apply to all of the boards. The particleboards included for this work were carefully selected to be representative of process, type of particle, kind of formation (homogeneous, three layer, graduated), kind of binder, and species. Thus, when the research is completed, the hopeis that acceptable design values can be established based on a valid mean value for pertinent properties of a specific particleboard or a group of particleboards and for variability factors if atypical.

BASIC ENGINEERING PROPERTIES OF PARTICLEBOARD'

By J. DOBBIN McNATT, Technologist

2 FOREST PRODUCTS LABORATORY; FOREST SERVICE

U. S. DEPARTMENT OF AGRICULTURE

INTRODUCTION

Published reports on mechanical and physical properties of particleboard are numerous. However, substantial gaps exist in the data of engineering properties since most studies were limited, and not all properties have been thoroughly investigated. For example, Hunt (6)3 carried out an extensive literature search, cited 66 references, and concluded, "there are no references available that report tensile modulus, shear modulus, and Poisson's ratio of particleboard."

To reinforce the information available, and to fill in some of the gaps, the Forest Products Laboratory (FPL) evaluated the strength and the elastic properties in tension, compression,bending, and shear of nine commercial particleboards and the standard FPL Douglas-fir urea-bonded particleboard. Basic properties after standard conditioning are reported in this work.

RESEARCH MATERIAL

Descriptions of the FPL particleboard and the nine commercial particleboards (identifiedby letter code) are given in table 1. The commercial boards were selected as representativeof thicknesses, species, resin type, and particle type now used in particleboard production.Eight of the nine particleboards were manufactured in the United States; board N (1/4-in. homogeneous, phenolic), in Canada. The board from Canada was included because the particle type was different from that typically used in the United States. It had a large flake, and was produced for general construction, such as for roof and wall sheathing.

Selection of the panels at the plant was randomized so that they were picked at different times of the day from different openings in the press, and the selection was spread over a 2-week period. Six 4- by 4-foot sections were shipped to the Forest Products Laboratory where they were cut into nominal 2-foot squares, and marked for identification. Fifteen FPL particleboard panels, 22 by 26 inches, were made at the Laboratory.

1Presented at the 7th Particleboard Symposium, Washington State University, Pullman, Wash., March 27-29, 1973.

2 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 3 Underlined numbers in parentheses refer to Literature Cited at the end of this paper.

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Tab le 1 .- - Par t ic leboards e v a l u a t e d f o r fundamental eng inee r ing p r o p e r t i e s o f p a r t i c l e b o a r d

Each commercial board was to be marked by the manufacturer to indicate the forming direc­tion as manufactured; however, only two boards, M and N, were marked. "Parallel" and "perpendicular" directions for products A, B, C, H, J, and K were chosen to correspond with the direction of sanding scratches on each panel. This may or may not correspond to the forming direction of the particle mat dependent on handling procedures from the press in the individual plants. Panels of board L were not sanded, and since forming direction was not marked, parallel and perpendicular directions were arbitrarily chosen.

RESEARCH METHODS

One of the four 2-foot squares was randomly selected from each of the six panels of each commercial board. Specimens were prepared for the following tests (numbers in parenthesesindicate the number of evaluations for each particleboard type):

1. Static bending (12)Equilibrium moisture content Specific gravityModulus of elasticityModulus of rupture

2. Tension parallel to surface (12)Tensile strengthModulus of elasticity

3. Compression parallel to surface (12)Compressive strengthModulus of elasticity

4., Tension perpendicular to surface (6)Internal bond strength

5. Interlaminar shear (12)Shear strengthShear modulus

6. Edgewise shear (12)Shear strength

7. Plate shear (6)Shear modulus

Specimens for tests 1, 2, 3, 5, and 6 were made both parallel and perpendicular to the formingdirection of the commercial products to note any differences in properties in the two directions.

One specimen of each type was prepared from each of the 15 FPL particleboard panels. Only one direction was evaluated for the FPL particleboard since previous tests indicated the board was "square" in regard to properties.

Before testing, all specimens were conditioned to equilibrium moisture content at 75" F. and 64 percent relative humidity [specified in ASTM D 1037-72 (3) for testing wood-base panelproducts]; the conditions are consistent with those for testing solid wood. Specimens for tests 1 through 6 were tested according to procedures specified in ASTM D 1037-72. Plate shear tests were made as specified in ASTM D 3044-72 (1) for determining the shear modulus of plywood associated with shear distortions in the plane of the panel. The plywood test method

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was used because ASTM D 1037-72 does not include a standard method of evaluating this important property for wood-base panel products. The test method has been used extensively at the Forest Products Laboratory for determining the shear modulus of plywood, solid wood, and hardboard.

The plate shear specimens tested were square with the length of the sides equal to 32 times the nominal panel thickness. This meant that the full-size 24-inch squares were used as plateshear specimens of the 314-inch-thick particleboards. The 518-inch-thick specimens were 20 inches square; the 112-inch-thick specimens, 16 inches square; and the 114-inch-thick specimens, 8 inches square.

After cutting the specimens for plate shear evaluations from the material of 1/4- and 1/2-inch thickness, sufficient material remained to cut the specimens for the remaining six evaluations (fig. 1). Since the plate shear test is nondestructive, the remaining six specimens could he cut from the 5/8- and 3/4-inch-thick plate shear specimens as shown in figure 1.

ANALYSIS OF RESULTS

Strength and elastic properties in tension, compression, bending, and shear for the nine commercial particleboards and the standard FPL particleboard after conditioning at 75° F. and 64 percent relative humidity are summarized in tables 2, 3, and 4 as follows:

Table 2 Static bending properties, plus moisture content and specific gravity values.

Table 3 Internal bond strength and tension and compression parallel to surface properties.

Table 4 Shear properties.

Mechanical Properties in Relation to Paras and Perpendicular Directions

All of the values for particleboard C (5/8-in. homogeneous, urea) were greater in the parallel direction, but all of the differences between parallel and perpendicular values were less than 10 percent. Other commercial products had certain properties with differences of more than 10 percent in the two directions; however, the differences were not consistent for all properties of the product. For instance, interlaminar shear strength of H (3/4-in. graduated, phenolic) was 13 percent higher in the parallel direction, but edgewise shear strength was 12 percent higher in the perpendicular direction. For this reason, strength and elastic values in tables 2, 3, and 4 for the commercial products are averages of specimens in both the parallel and the perpendicular directions.

Equilibrium Moisture Content and Specific Gravity

Equilibrium moisture content, the least variable of all of the properties evaluated, rangedfrom 8.0 percent for particleboard H to 9.8 percent for board C. All coefficients of varia­tion were 4 percent or less, and averaged approximately 2-1/2 percent. Average moisture con­tent of all boards combined was 9.0 percent. This compares with a value of approximately12 percent for solid wood at the same temperature-humidity conditions.

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Figure 1.--Schemes for cutting test specimens from particleboards with codes for identifying type of test to be made on each specimen: B, static bending; C, compression parallel to surface; Gp1, plate shear; Se, edgewise shear; Si, interlaminar shear; T, tension parallel to surface; and IB, tension perpendicular to surface (internal bond). M 140 863

Table 2.--Static bending properties of particleboard

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Table 3.--Internal bond strength and tension and compression parallel to surface strength of particleboard

Table 4.--Shear properties of particleboard

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Average specific gravities of the 10 particleboards, based on dimensions at test and oven-dry weight, ranged from 0.60 for board (114-in. homogeneous, phenolic) to 0.77 for H. All boards were within the medium-density range as defined in the commercial standard (37 to 50 lb. per cu. ft.) (7).

Variability of Properties and Relation to Particle Size

Standard deviations and coefficients of variation for all of the strength and the elastic properties of each of the 10 particleboards are included in tables 2, 3, and 4. As noted in the tables, these values are based on 12 evaluations in bending, 12 each in tension and com­pression parallel to surface, and 12 each in interlaminar and edgewise shear, and only six each in internal bond strength and plate shear modulus. The coefficients of variation, cal­culated from this limited number of evaluations, indicated that particleboard N had much greater variability than did the other boards. This is to be expected if particle size is considered. Boards with small particles contain a large number of particles and wood-resin interfaces in critical sections of the standard-size test specimen so that variability is relatively small.

Board N was made from large flakes, approximately 2 inches along the grain and up to 2 inches wide. If such large particles are used, only a few particles and wood-resin inter­faces are in the critical section of a test specimen such as in testing tension parallel to surface. Thus variability is expected to be greater than if a small particle is used.

In some tests, such as interlaminar shear and plate shear, large areas are stressed so that particle size has less influence, and variability of boards with large or small particlesshould be more comparable. Less variability was actually found. Coefficients of variation for interlaminar shear and plate shear properties for board N were within the range of values exhibited by the other particleboards.

It should also be noted that board N was only 1/4 inch thick, whereas the other materials were 1/2 inch thick or thicker. In tests such as tension and compression parallel to surface and edgewise shear, areas under critical stress were at least twice as large for the other boards as for board N. Coefficients of variation would be expected to be less for the thicker materials.

A condition similar to that of board N exists with commercial construction plywood.Strength-reducing characteristics occur in such a manner that results are extremely variable if small specimens are used. To reduce this variability to meaningful values more applicable to construction use, full-size (4- by 8-ft.) panels are tested in bending. Similarly, edgewise(panel) shear values are determined by a two-rail shear specimen that has a stressed area of 8 by 24 inches (2).

The phenomenon of apparent high variability of the properties for particleboard from largeparticles if small specimens are used for testing should not be a deterrent to use of this type of board. Analysis of tests of prototype structures that use boards with large particleswill establish the validity of using the mean values determined from tests on small specimens.

Relationships Among Strength and Elastic Properties

As expected from previous experience, certain strength and elastic properties of the par­ticleboards were closely related. For example, both interlaminar shear strength and internal bond strength are a measure of the bond quality between particles in the "core" of the panel,because all failures for both types of loading occurred in the core of the panel. Figure 2

Figure 2.--Relationship of interlaminar shear strength to internal bond strength of particleboard. M 140 864

is a plot of two sets of data. Values for internal bond strength, IB, are from individual test specimens; values for interlaminar shear strength, Si, the average of the one parallel and one perpendicular specimen from each 24-inch-square sample. Because relationships were determined between properties of matched specimens for each of the different products, no adjustments were needed in specific gravity before plotting the data. Specific gravity, of course, is a factor in strength and elastic properties. But for the products included in this study, differences in specific gravity did not affect the relationships between board proper­ties as expressed by equations (1) through (5). The equation calculated by least squaresregression that relates interlaminar shear to internal bond strength is the following:

(1)

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--

The standard error of estimate S y·x

, is an estimate of the variation of y about the regres­

sion line. The coefficient of determination, r 2 , is the fraction of the total variation of y that is explained by the regression line. If all points fell exactly on the regression line,

S y·x

, would be zero and r2

would be one (unity),

Also expected was the good correlation between shear modulus from the plate shear test and modulus of elasticity in bending. Individual shear modulus values, G

p1 , are compared with the

average of one parallel and one perpendicular bending modulus value, Eb

, in figure 3. The

least squares regression equation for these data is the following:

(2)

F igure 3. Relat ionship of bending modulus of e l a s t i c i t y t o p l a t e s h e a r modulus of p a r t i c l e b o a r d . M 140 865

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--

Since all particleboard bending specimens failed in tension on the lower surface, modulus of rupture, R, should correlate well with tensile strength parallel to surface, T. Figure 4 shows this to be true, Each modulus of rupture and tensile strength value is the average of one parallel and one perpendicular specimen. The equation of the line through these data is the following:

( 3 )

M 140 866 F igu re 4. Relationship of tensi le s t r e n g t h t o modulus o f r u p t u r e f o r p a r t i c l e b o a r d .

A r e l a t i o n s h i p w a s a l s o ob ta ined between edgewise s h e a r s t r e n g t h , Se, and t e n s i l e s t r e n g t h

p a r a l l e l t o s u r f a c e T. To ge t a s t r a i g h t - l i n e r e l a t i o n s h i p , s h e a r s t r e n g t h w a s p l o t t e d on a l i n e a r scale i n t h e y d i r e c t i o n and tensi le s t r e n g t h , on a l o g a r i t h m i c s c a l e i n t h e x d i r e c­t i o n ( f i g . 5 ) . Each v a l u e of t e n s i o n and of s h e a r i s t h e average of one p a r a l l e l and one p e r p e n d i c u l a r specimen. The r e g r e s s i o n equa t ion f o r t h e l i n e through a l l p o i n t s ( f i g . 5 , i s t h e fo l lowing :

(4)

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Figure

those

5.--Relationship of edgewise shear strength to tensile strength of particleboard.Regression line through all points; , regression line for all data except for

of board N.) M 140 867

Some data points for board N (1/4-in. homogeneous, phenolic) fall a considerable distance from the regression line compared to the data for the other nine particleboards because of the high degree of variability of the properties of N. If a regression equation is calculated for all data points except for those of N, the correlation between the tensile strength and shear strength is improved without significantly changing the relationship between the two properties.

The equation of the line representing this new relationship (fig. 5, is the following:

(5)

Comparison of Engineering PropertiesAmong Particleboards

All commercial particleboards were compared for strength and for elastic properties in tension, compression, bending, and shear. In comparing the relative rank of all of the par­ticleboards for all properties determined, boards H and K were most consistent. Board (3/4-in. graduated, phenolic) had the highest value in nine of the 11 properties, and was

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second in the remaining two, Board K (5/8-in. three-layer, urea), however, had the lowest values in eight of the properties, tied for lowest in one, and was second lowest in two. No particular manufacturing or raw material variable apparently accounts for this. Board H was the heaviest of all the boards (specific gravity = 0.77), but the difference in density be­tween it and board C (5/8-in. homogeneous, urea) (specific gravity = 0.76) was insignificant. The most inconsistent particleboard was N, which ranked first in two properties, third in two, fifth in one, sixth in three, eighth in one, and last in two.

Particleboards J and K are almost the same board except J is bonded with a phenolic adhe­sive and K with a urea adhesive. (Specific gravity of J was somewhat higher than that of K; 0.72 and 0.68.) Strength and elastic properties of J ranged from 6 to 45 percent higher than the corresponding properties of K, and averaged 23 percent higher. Part of this difference is, of course, explained by the difference in density. Another possible contributing factor is the reported superior efficiency in glue-bond formation for phenolic resins used under commercial conditions (5). Bryan and Schniewind reported that the bending strength and stiffness of a commercial three-layer particleboard bonded with phenolic resin was approximately 10 percent higher than that of the same product bonded with urea resin (4).

Manufacturing processes for boards L and M (both 3/4-in., three-layer, urea) are essentially the same. The main difference between the two is the species used as the raw material; L was from jack pine and northern hardwoods; M, from southern pine and southern hardwoods. Strength and elastic properties of M averaged just over 20 percent higher than did those of L, and ranged from 5 to 43 percent higher. The largest differences in properties between L and M occurred in the properties related to bonding between particles in the core material: Internal bond strength, 43 percent; interlaminar shear strength, 39 percent; and interlaminar shear modulus, 37 percent. The slightly higher specific gravity of M (0.65) compared to that of L (0.62) could account for some but not all the differences in properties.

Relationships between board properties and manufacturing variables were difficult to separ­ate because of different board densities and different kinds and combinations of wood species, raw material forms, and processing. Attempts were made to relate board properties to manufac­turing variables, but no clear relationships could be established from these limited data.

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SUMMARY

Strength and elastic properties in tension, compression, bending, and shear of nine commer­cial particleboards and one laboratory-made particleboard were evaluated by methods of test specified in ASTM Standards D 1037-72 and D 3044-72. The results of these evaluations are as follows :

1. Equilibrium moisture contents of the particleboards at 75° F. and 64 percent relative humidity ranged from 8 to 10 percent, and averaged 9 percent. This compares to an averageequilibrium moisture content of approximately 12 percent for solid wood at the same conditions.

2. All particleboards included here fell within the range of the "medium-density" defined by Commercial Standard CS 236-66 (37 to 50 lb. per cu. ft.).

3. There were no large consistent differences in board properties between the parallel and perpendicular relationships to the forming direction of the panel as manufactured.

4. Most coefficients of variation of the strength and the elastic properties for particle­board N were larger than were those for the other boards evaluated. Since board N was manu­factured from large flakes, greater variability than in the other boards could be expectedbecause critical sections in small standard specimens contain relatively few particles and wood-resin interfaces. Furthermore, board N was only half as thick as the other particle­boards; in most tests, critical areas were twice as large for the thicker materials. Greater variability is to be expected for the thinner product N.

5. Relationships between certain strength and elastic properties of the particleboards,determined by least squares regression, are given by the following equations:

Between interlaminar shear strength (Si) and internal bond strength (IB),

(1)

Between shear modulus from the plate shear test (Gp1

) and bending modulus of elasticity (Eb),

(2)

Between tensile strength parallel to surface (T) and modulus of rupture (R),

(3)

Between edgewise shear strength (S ) and tensile strength parallel to surface (T),

(5)

6. The relationships between board properties and manufacturing variables were complicatedby various board densities and various kinds and combinations of wood species, raw material forms, and processing methods; hence, no specific relationships could be found.

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LITERATURE CITED

1. American Society for Testing and Materials 1972. Standard method of test for shear modulus of plywood. ASTM Designation D 3044-72.

Philadelphia, Pa.

2. 1972. Standard methods of testing plywood in shear through-the-thickness. ASTM

Designation D 2719-71, Method C. Philadelphia, Pa.

3. 1972. Standard methods of evaluating the properties of wood-base fiber and particle

panel materials. ASTM Designation D 1037-72a. Philadelphia, Pa.

4. Bryan, E. L., and Schniewind, A. P. 1965. Strength and rheological properties of particleboard as affected by moisture

content and sorption. Forest Prod. J. 15(4): 143-148.

5. Carroll, M. 1963. Efficiency of urea- and phenol-formaldehyde and particleboard. Forest Prod.

J. 13(3): 113-120.

6. Hunt, Michael O'Leary1970. The prediction of the elastic constants of particleboard by means of a structural

analogy. Ph. D. thesis, N.C. State Univ., Raleigh, N.C.

7. U.S. Department of Commerce 1966. National Bureau of Standards. Commercial Standard CS 236-66. Mat-formed wood

particleboard.

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U.S. GOVERNMENT PRINTING OFFICE: 1980-653-729