THE RESOURCE CENTRE FOR BUILDING EXCELLENCE
REPRINT
NO. 100 (1990)
Effect of Humidity on Physical and
Mechanical Properties of
New Zealand Wood Composites
P.J. Watkinson
N.L. van Gosliga
Effect of humidity on
physical and mechanical properties
of New Zealand wood composites
P.J. Watkinson
N.L. van Gosliga
Abstract
The effects of a range of moisture contents (MCs) induced by different relative humidities have been studied
for tempered hardboard, urea-formaldehyde-bonded flooring particleboard, and medium density fiberboard (MDF)
commercially available in New Zealand. MCs, dimensional changes, and mechanical properties were recorded after
the MCs were close to equilibrium. There were significant
differences between the control humidity and the other
humidities pooled for modulus of elasticity (MOE), modulus of rupture (MOR), internal bond strength, and compressibility. However, for tempered hardboard, controls
were only different from the other humidities for MOE
and compressibility. The effects of MC on physical dimensions, MOR, and MOE were similar to those reported
by other researchers for particleboard and hardboard, but
no literature could be found to compare with MDF mechanical properties. Some implications of this study for
current New Zealand building practices are discussed. In
particular, a t a high humidity of 95 percent, which may
be found in a damp crawl space, the MOE of flooring particleboard is lowered by 47 percent to 63 percent.
In common with all materials composed mainly of cellulose, the properties of wood-based composites depend
on moisture content (MC). MC changes induced by atmospheric moisture changes significantly alter both mechanical and physical properties of composites.
The major objective of this work was to find how the
physical and mechanical properties of commercial New
Zealand composites change with static humidity. No work
on the effect of static humidity on the mechanical properties of composites could be found for commercially produced
New Zealand composites or similar composites using Pinus
radiata. However, these effects have been studied for composites from other species in the United States (7,ll-13),
the United Kingdom (6), and Japan (17).
New Zealand has recently become one of the larger
medium density fiberboard (MDF) manufacturers in the
world (4) with an estimated 1987 production of 2 to 3 x
FOREST PRODUCTS JOURNAL
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NO. 718
lo5m3.The majority of MDF is exported. To date there is
no literature on the effect of static humidity on the fundamental properties of modulus of rupture (MOR), modulus
of elasticity (MOE), and internal bond (IB) for MDF.
Previous studies on particleboard have indicated that
mechanical properties, for example, MOR, MOE, and IB,
often have a maximum within the 5 to 10 percent MC
range (7,9,17). In all cases, these properties decrease with
MC above this range (7,9,11,13,17). At high humidities,
about 90 percent, significant reductions in MOR, MOE,
and IB occur compared to medium humidities of 50 to 65
percent (7,11,13,17). Particleboard bonded with melamineurea-formaldehyde, phenol-formaldehyde, and sulphite
liquor yielded higher IB retention and lower thickness
swelling (TS) than urea-formaldehyde-bonded particleboard a t high humidity (6).
Significant reductions in MOE, MOR, and IB of hardboard are found a t high humidity (90%) compared to medium humidity (12).
This sample of previous studies shows that structural
uses of composites need to include MC as a significant
variable when calculating mechanical properties like
strength and stiffness. Because of the large use of ureaformaldehyde particleboard in structural applications in
New Zealand and the growing interest in structural uses
of MDF, it is timely to examine the effects of moisture on
mechanical effects in more detail. In New Zealand, about
75 percent of new houses used flooring particleboard in
1981(10). Ventilation in underfloor spaces (crawl spaces)
is mandatory, but even so, floors are either directly exposed to the high humidity of the crawl space or separated
The authors are, respectively, Materials Scientist and Technician, Building Res. Assoc. of New Zealand, Private Bag, Porirua, New Zealand. The authors appreciate the assistance from
D.P. Krouse, statistician, who advised on the interpretation of
the results, and the Association acknowledges the cooperation
of Canterbury Timber Products Ltd., Panelcorp Industries Ltd.,
and Fletcher Wood Panels Ltd. This paper was received for publication in April 1989.
Forest .Products Research Society 1990.
Forest Prod. J. 40(7/8):15-20.
TABLE 1. - Exposure conditions.
Relative
humidity.
.",
Temperatureb
1%)
\
I~ D -,
IT
25
65
85
95
22
20
23
23
Method
Saturated solution (CH,COOK)
Constant climate rwm
Saturated solution (KC])
Saturated solution (K.SOJ
Nominal measured values.
bThe difference between equilibrium MC at 20°C/65 percent RH and
23'C/65 percent RH is negligible (12).
from it by perforated foil insulation. Commonly, crawl
space humidity is higher than outdoor humidity. Crawl
space humidities in New Zealand are high since outdoor
humidities are high, with the main centers of Auckland
and Wellington having average mean hourly humidities
of 77 and 81 percent, respectively (15).
For nonstmctural uses of composites, MC is a n important consideration, since it normally dominates the inservice changes in physical properties such as dimensions.
Common nonstmctural uses in New Zealand include furniture, internal joinery and fittings, and ceiling and internal wall linings that usually do not perform bracing functions. A study of the moisture-induced dimensional changes
of composites will help to assess the suitability of composites in the environmental conditions imposed by new uses.
Materials
The timber species used in all composites in this study
was predominantly Pinus radiata and the adhesive was
urea-formaldehyde for boards A, B, and C. Two brands of
flooring particleboard were used; board A and board B
were both nominally 20 mm in thickness with a mean
density of 690 or 730 kg/ms and had faces of long, thin
flakes and a core of more coarse flakes. The MDF, board C,
was nominally 18mm in thickness, 725 kg/m8in density,
and essentially homogenous. It is the standard grade and
is commonly used for furniture. The oil-tempered hardboard, board D, was manufactured by the wet process and
was nominally 6 mm in thickness with a density of about
1020 kglm? This is commonly used in wet areas as wall
linings.
All composites were obtained direct from their manufacturer. Only a single sheet of each board type was used
in order to reduce variability.
Methods
Number of replicates
Guidelines for the number of replicates n for a given
test method and environment were calculated from a function (18) requiring the coefficient of variation of the property and the difference in property arising from the difference in humidity.
Estimates of the coefficients of variation for MOE,
MOR, and IB were obtained using the test methods described later a t 65 percent humidity and estimates of the
differences arising from the differences in humidity were
found from McNatt (13). These guidelines, together with
the constraint imposed by using one single sheet per board
type, led to the following replicate sizes in each environment: MOE and MOR: 10 replicates; IB: 20 replicates;
MC: 13replicates, linear expansion (LE)and TS: 16 replicates; compressibility: 9 replicates. Half the replicates for
MOE, MOR, and LE were tested normal to the direction
of manufacture and half were tested parallel to the direction of manufacture.
Conditioning
All samples were first conditioned to 65 percent humidity so that dimensions a t this control condition could
be found for all samples. They were then randomly assigned to one of the exposures (Table 1); the samples leR
a t 65 percent humidity were designated as controls. The
actual humidity of each of these exposures was monitored
using a Novasina MIK 2000 hygrometer with a n enCS-3
sensor calibrated with a two-pressure humidity generator.
Four exposure conditions were provided by a constant
climate room and three cabinets. Each cabinet contained
a saturated salt solution with two oscillating fans (nominal
air velocity 1.7 mls) to ensure a homogenous distribution
of humidity (Table 1).
Before the samples were conditioned, initial conditioning trials were performed to estimate the time for all the
composites to reach about 95 percent of the way from the
initial equilibrium MC to the final equilibrium MC in
these exposure conditions. The increase in mass was monitored and fitted to:
m(t) = m, - (m, - m,) exp[-UtJ
[ll
where:
m(t) = mass of a composite sample a t time t
m, = equilibrium mass
m, = initial mass
t, = time constant
The time required for 95 percent approach to equilibrium
MC is three times the time constant.
Physical properties
Test methods for physical properties (MC, TS, and LE)
of all composites used the British Standards Specification
BS 5669:1979 (2) with the following modifications: conditioning times were determined by the aforementioned
criteria; humidities were nominally 25, 85, and 95 percent and temperatures were near 22'C; MC determinations used sample dimensions of 75 mm by 75 mm rather
than 100 mm by 100 mm.
Mechanical properties
Test methods for the mechanical properties of particleboard and MDF (MOR, MOE, IB, and compressibility)
used BS 5669:1979 (2) with the following modifications:
conditioning times, humidity, and temperature were modified as in the test for physical properties. Similarly, for
hardboard, test methods used BS 1142:Part 1:1971 (3)
apart from conditioning. BS 5669:1979 (2) was used for
compressibility tests. Properties were determined a t the
MC resulting from equilibration a t the final humidity.
An examination for fungi was made on the samples
tested a t the highest humidity.
Results a n d discussion
Physical properties
A standard analysis of variance was performed to assess the effect of humidity on physical properties, and the
analysis was significant a t the 1percent level for every
TABLE 2. - Effect of MC on mechanical properties of New Zealand wwdbased composites a& the control environment of 65 percent humidity and 20°C.'
Board
code
MOR~
MC
11.14
(0.06)
10.33
(0.06)
9.61
(0.06)
B
C
0.682
(0.034)
0.377
(0.031)
0.701
(0.031)
3600
(12).
3140
(60)
3040
60)
25.3
(0.8)
23.3
(0.6)
41.0
(0.7)
..
Compressibility
(mm)
0.264
(0.005)
0.270
(0.003)
0.204
(0.006)
-------------- (MPa) --------------
(%)
A
IB
MOE~
Linear Expansion (%)
Board A
...... .. Board
*.-.- Board C
'Standard errors of the means are given in parentheses.
Dimensions at the time of test are used to calculate MOR and
-
MOE.
Moisture Content (%)
26
24 22 20 1816-
3
0
22%
-Board A
I
I
I
I
I
I
I
I
1
I
4
6
8
10
12
14
16
18
20
22
I
24
1
Moisture content ( O h )
/
,f
:i'
. ...... Board B
Board C
----.
Board 0
,, ,
I
2
Figure 2. - The effect of MC on LE. The 95 percent confidence
limits are given for the mean values.
:,..
Thickness Swelling (%)
16
-
1412
10
-
-
-
-.-.-
2-L,
0
,
10
20
30
40
r
50
,
8
60
,
I
70
,
,
1
80
4
Board A
a,.
....... Board B
/
Board C
*
I
1
90
11
Relative Humidity (%)
Figure 1. - The effect of RH on MC. The 95 percent confidence limits are given for the mean values.
property. In addition, to establish whether properties at
the control environment (65%humidity) are, overall, different from other environments, analyses of properties at
the control environment were contrasted with other environments pooled. These were significant at the 1percent
level for every property except LE with board D.
Moisture content. - The MC data, presented in Table 2
and Figure 1, were obtained following preconditioning at
65 percent humidity. Board D had a lower MC for a given
humidity than the other boards and one factor contributing to this result is the relatively large amount of hygrophobic oils in oil-tempered hardboard. All these MC data
lie on or within the MC versus humidity hysteresis loops
for commercial New Zealand composites similar to the
.
ones in this study (5).
Linear expansion and thickness swelling. - Although
the LE and TS data (Figs. 2 and 3)cannot be directly compared to data from other researchers on composites because of the different combinations of humidities used,
the LE changes per MC change and the TS changes per
MC change can be compared. In the range of MCs represented by 65 to 95 percent humidity in the present work,
the LE versus MC plots are close to linear, and similarly
for TS, although the linear approximation is not as good.
For urea-formaldehyde-bondedparticleboards, the following comparisons can be made with the LE change per
MC change. Values from the present work of 0.036 (board
A) and 0.043 (board B) are similar to other work on New
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Moisture content (%)
Figure 3. -The effect of MC on TS. The 95 percent confidence limits are given for the mean values.
Zealand boards (20),slightly lower than commercial United
States boards (ll), and much higher than a laboratorymade board (11)that had a value of 0.007. In the present
work, the TS change per MC change of 0.92 and 1.22 for
particleboards is slightly higher than other reported values on urea-formaldehyde particleboard (0.75 to 0.86 for
commercial United States boards), and similar to a laboratory-made board of 1.2 (11).
For tempered hardboard, the LE change per MC change
in the present work is 0.024 and is similar to other findings on New Zealand tempered hardboard (14). Also in
this work, the TS change per MC change in tempered
hardboard is 0.94, which is slightly lower than the value
of 1.2 found by other researchers (14).
Mechanical properties
A standard analysis of variance of mechanical properties showed that all properties varied with humidity at
the 1 .percent level of significance. Analyses contrasting
properties at the control environment (65%humidity) with
all other environments pooled were all significant a t the
1percent level for boards A, B, and C. Only compressibility and MOE were significant a t the 5 percent level
for board D.
Board D had the smallest variation in mechanical
properties of all the boards, probably as a result of having
the smallest variation in MC.
Table 2 lists the mechanical properties of the composites a t the control environment. MOR and MOE are depicted in Figures 4 and 5 respectively, as a percentage
of values a t the control environment, using dimensions a t
the time of test. IB is depicted in Figure 6 as a percentage
of values determined in the control environment and compressibility is depicted in Figure 7.
There were no wood decay fungi in the samples a t
highest humidity. Therefore, the reduction in MOR, MOE,
and IB and the increase in compressibility a t high MC
is attributed to mechanical breaking of binder-to-wood
bonds (6,8), less intertwining of wood particles (17), and
binder degradation (17)for particleboard, and to softening
of amorphous cellulose and hemicellulose for hardboard (1).
The rationale for the changes in these mechanical properties for MDF is similar to that for particleboard, since
they both use the same binder.
MOR, MOE, and IB. -The percentage changes in
MOR, MOE, and IB with MC have a roughly linear relationship with a negative slope, the slope approaching zero,
or possibly showing a maximum in a few cases, a t lower
MCs. This is consistent with the results from other researchers with particleboard (7,13,17) or hardboard (12),
who found a linear negative slope from 10 to 20 percent
MC, the slope approaching zero below 5 to 10 percent, and
sometimes having a broad maximum near 4 to 7 percent
MC.
Table 3 shows the percentage retention of MOR, MOE,
and IB a t the same increase in MC, as reported in several
different previous studies. To increase the number of comparisons, MOR and MOE are reported using dimensions a t
the control environment, unlike our data on MOR and
MOE elsewhere in this paper. These comparisons are only
approximate, being derived from interpolations of the published data. The percentage retentions of MOR and MOE
Modulus of Rupture (Percent of Controls)
Internal Bond Strength (Percent of Controls)
120
130
I
1
I
-
-.....
Board A
.......... Board B
- Board C
Board D
-
30i
20/
0
30
I
0
2
4
I
I
6
8
I
I
10
I
12
14 16 18
Moisture content (%)
I
20
22
24
Board D
,
2
,
,
I
,
4
6
8
I
26
b
*---,
10
12
14
16
18
'd
20
22
24
26
Moisture content (%)
Figure 6. -The effect of MC on I6 (percent of controls).
Figure 4. - The effect of MC on MOR (percent of controls using
dimensions at the time of test).
Modulus of Elasticity (Percent of Controls)
120
I
A
Compressibility (Percent of Controls)
-
-.....
Board A
.......... Board S
- Board C
Board D
-
500
..........
*....
Board A
Board B
Board C
Board D
.
.
40
a.
30
I
0
2
4
6
I
8
10
12
I
I
14
16
18
20
22
24
26
Moisture content (%)
Figure 5. - The effect of MC on MOE (percent of controls using
dimensions at the time of test).
0 1,. ,
0 2
,
,
4
,
6
,
,
8
.
, .
10
,
12
14
16
18
20
22
24
26
Moisture content (%)
Figure 7. -The effect of MC on compressibility (percent of
controls).
TABLE 3. - Comparison of percent retention of mechanical properties for
different wood-based composites.
Type of
composite
Particleboard
Percent retention
of propertyb
Source and detailsa
New Zealand, flooring, average of
boards A and B.
(11)United States, average of two
boards.
(17) Japan.
(17) Japan, melamineformaldehyde-bonded.
(13) Experimental, average of three
boards, structural, phenolformaldehyde-bonded.
(7) Experimental, (No. 769 board),
similar density to boards
A and B.
MOR
82
MOE
83
IB
75
77
82
--
74
80
70
74
---
76
72
--
..
--
51
Hardboard
New Zealand, tempered, Board D.
(12) United States, tempered,
average of six boards.
84
88
86
78
80
69
MDF
New Zealand, board C.
69
75
67
.
Unless otherwise stated, all particleboard and MDF has a urea-formaldehyde binder, and all boards were commercially available.
The property retention equals the property at 9 percent MC divided by
the property at 16 percent MC as a percentage for all particleboard and
MDF. The property retention equals the property at 4 percent MC divided
by the property a t 11 percent MC as a percentage for all hardboard.
.
for New Zealand particleboards (boards A and B) are similar to or slightly higher than the other United States (11)
and Japanese particleboards (17). The retention of IB for
New Zealand particleboards (75%)is higher than for particleboard made in the laboratory (51%)(7). The retention
of MOR, MOE, and IB for New Zealand commercial temb r e d hardboard (board D) is similar to that for commercial
U.S. tempered hardboard (12). No previous literature on
MDF could be found on the effect of MC on the MOR, MOE,
or IB of MDF. However, retention of MOR and MOE for
MDF@oard C) is slightly lower than for New Zealand
particleboard or hardboard.
For all boards e x c e.~A.
t MOR. MOE, and IB decrease
monotonically with increasing MC, showing no maximum
as found by some other researchers (7,9,17). Althmgh for
board A, MOR and IB a t the lowest humidity are lower
than a t controls. these board A ~
. r o.~ e r t iate sthe lowest
humidity were not significantly different to the controls a t
the 5 percent level. On the other hand, properties at the
lowest humidity were significantly higher than the controls at the 5 percent level for: board A, MOE; board B,
MOE, MOR, and IB; board C, MOR and MOE; and board
D, MOR, MOE, and IB.
Compressibility. - In New Zealand, without specific
design, building practice allows up to two stories to be
built on top of a residential floor. Since the load-bearing
walls are put on top of the floor, some compression of the
floor occurs. Thus, there is some interest in the compressibility of flooring particleboard, and the effect of MC on
compressibility. The compressibility of boards A, B, and C
increased dramatically above 13 to 15percent MC, reaching values about 2.5- to 6-fold larger a t 20 to 23 percent
MC compared to 6 to 12 percent MC. Although only used
as an overlay for flooring, board D showed no significant
variation in compressibility with MC a t the moisture levels attained.
FOREST PRODUCTS JOURNAL
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Practical implications
Some practical implications of moisture-induced dimensional changes include: 18 mm thick MDF used as a
washbasin cabinet in a bathroom starting a t 65 percent
humidity and ending a t 95 percent humidity will expand
as much as 3 mm over a 1m length. Tempered hardboard
used as a shower lining substrate that experiences a humidity increase from 65 to 95 percent humidity expands
as much as 1.7 mm over'a 1 m width of material. This
expansion needs to be accommodated by jointers, or sheet
buckling may occur.
The single most important practical implication of this
humidity study for current New Zealand building practices is found in the reduction in MOE for flooring particleboard a t high humidity. At 95 percent humidity, these
boards have an MOE 37 to 53 percent of the value a t 65
percent humidity. The high MC of 20 to 23 percent found
by conditioning to 95 percent humidity represents a worst
case that may be achieved when the floor has a vapor barrier on top, for example a sheet PVC floorcovering, and
a damp crawl space directly below it. This floor may yield
noticeably larger deflections from foot traffic than one
conditioned a t 65 percent humidity. Field studies of the
MC of flooring particleboard would show how important
this effect is. A survey that included New Zealand houses
with relatively damp crawl spaces (19) yielded MCs of
about 19 to 28 percent in the timber strip flooring, corresponding to flooring particleboard MCs (5)of about 14
to 24 percent. Thus, there are some floors with a high
enough MC to cause significant reductions in the MOE of
particleboard.
At high humidities of 95 percent, flooring particleboard can be reduced in thickness by as much as 1.7 mm
under 1.4 MPa pressure. The pressure exerted by gravity
from two stories of a standard residential building via the
bottom plate onto the floor will normally be less than the
compressibility test pressure of 1.4 MPa if the timber bottom plate resting on the floor is perfectly flat. However,
the pressure exerted on the floor can become similar to
the test pressure if the plate is bowed.
Conditioning
The times required for 95 percent approach to equilibrium MC were as much as 12 weeks. Using this guideline,
the conditioning time in the humidity cabinets was chosen
to be a t least 16 weeks. Conditioning times not only depend on the type of board and thickness but also on the
experimental details, since these depend on temperature
and air velocity over the board. The mass change per day
after 12 weeks was 0.01 percent.
McNatt(l1) found that up to 45 days of conditioning
was needed to comply with the ASTM D 1037 criterion of
0.05 percent mass change per day. Although in McNatt's
experiments, extending the conditioning time from 30 to
90 days at 90 percent humidity changed the equilibrium
MC from 13.9to 16.1percent, the only significant mechanical property change was for retention of MOR (using dimensions a t time of test), which dropped from 77 percent
to 64 percent (11).
Fiber orientation
None of the boards tested had a specific orientation of
timber components parallel to the board edges. However,
since the direction of manufacture will introduce some
residual orientation in the timber components, this effect
was examined.
Following the example of Saito and Hashimoto (16),
D, is defined as the ratio P, divided by P, as a percentage,
where P, is the dimensional change or mechanical property normal to the direction of manufacture and similarly,
P, is the property parallel to the direction of manufacture.
The only case where a property exhibits a significant
orientation effect and the effect is large, is LE for board
D (D, = 185%). Therefore, fiber orientation needs to be
accounted for when measuring LE for board D. LE normal
to the residual fiber orientation is larger than parallel
orientation. This is analogous to the moisture movement
normal to the grain in timber being larger than movement
parallel to the grain. Previous studies (14)on oil-tempered
hardboard similar to board D also showed a large difference
in LE depending on fiber orientation (D, is 167%).
Fungal examination
The samples most likely to have the highest concentration offungi, namely the ones at 95 percent humidity,
were covered in mold fungi (including Penicillium sp. and
Aspergillus sp.) after 16 weeks of conditioning, but had
no wood decay fungi (Basidiomycetes).No fungi were observed on the surface or in the core of samples conditioned
at 65 percent humidity.
Conclusions
Conditioning using different static humidities gave
statistically significant changes in MC, LE, TS,MOR,
MOE, IB, and compressibility in flooring particleboard,
MDF, and tempered hardboard commercially available in
New Zealand. For every property, the 65 percent humidity
controls were significantly different from other humidities (pooled) at the 1 percent level, except for tempered
hardboard. In this latter case, LE, MOR, and IB were not
significantly different at the 5 percent level.
Conditioningtimes required for 95 percent approach to
equilibrium MC were as much as 12 weeks for the boards
under the experimental conditions used.
The LE changes with MC changes and TS changes
with MC changes are similar to other work on hardboard
and urea-formaldehyde particleboard.
Retention of MOR, MOE, and IB with MC are similar
to other work on particleboard and hardboard. The exception to this trend is that IB retention of particleboard is
higher than the other reported value. Retention of MOR
and MOE for MDF is slightly lower than for particleboard
or hardboard, but no comparable literature on MDF could
be found.
Compressibility of flooring particleboard and MDF
increases greatly above 13 to 15 percent MC.
The MOE can be reduced by 47 to 63 percent at high
humidity (95%)compared to controls for flooring particleboard, representing the upper limits of humidity found in
a damp crawl space. Field studies on the MC of flooring
particleboard would quantify the practical importance of
this effect and an existing study on timber floors indicates
that some floors could have a high enough MC to cause
significaht reductions in particleboard MOE.
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3.
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).
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-.~roperties.
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E f f e c t of humidity on phy
sical and mechanical p r o p
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