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IRG/WP 11-40551
THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION
Section 4 Processes and Properties
Physical properties of Pinus radiata veneers modified with
hexamethoxymethyl melamine prepolymers
Chris Molloy, Wallace Rae, Scott Connor, Tamara Henderson, André Siraa
Zelam Ltd
Hudson Rd, Bell Block
New Plymouth, New Zealand
Paper prepared for the 42nd Annual Meeting
Queenstown, New Zealand
8-12 May 2011
IRG SECRETARIAT
Box 5609
SE-114 86 Stockholm
Sweden
www.irg-wp.com
Disclaimer
The opinions expressed in this document are those of the author(s) and
are not necessarily the opinions or policy of the IRG Organization.
2
Physical properties of Pinus radiata veneers modified with
hexamethoxymethyl melamine prepolymers
Chris Molloy, Wallace Rae, Scott Connor, Tamara Henderson, André Siraa
Zelam Ltd
Hudson Rd, Bell Block
PO Box 7142, New Plymouth 4341, New Zealand
ABSTRACT
Prepolymers containing hexamethoxymethyl melamine and either sucrose (Suc-HMMM) or
polyvinyl alcohol (PVA-HMMM) were prepared by acid catalysis under reaction conditions
optimised with respect to stiffening effect when introduced into Pinus radiata veneers by
vacuum impregnation and cured by hot pressing at 150 C. Maximum increases in MOE were
20% for Suc-HMMM (50% WPG), 22% for PVA-HMMM (14% WPG). A 1:1 mixture of the
two prepolymerisation reactions (P-H:S-H) produced a maximum 27% increase in MOE (40%
WPG) and was stable at ambient temperature for at least 54 days. Veneers vacuum-impregnated
with P-H:S-H increased the flatwise MOE and MOR by 32% and 19%, respectively, when used
as the two superficial veneers on both sides of 12-ply LVL. Edgewise MOE and MOR increased
by 19% and 11% respectively. Smaller increases in stiffness and strength were achieved when
four treated veneers were distributed throughout the LVL, or when LVL was produced using
twelve treated veneers. Veneers treated by immersion in P-H:S-H at various concentrations, then
used as a face veneer in plywood increased surface hardness significantly but formaldehyde
emissions of the most useful plywood products exceeded the limit of the F*** classification.
MDF manufactured from Pinus radiata fibres treated with P-H:S-H at 10%, 30% and 50% by
fibre weight showed a decrease in water-induced weight gain and swelling proportionate to the
prepolymer loadings.
Keywords: hexamethoxymethyl melamine, sucrose, polyvinyl alcohol, prepolymer, modified
wood, MOE, hardness
1. INTRODUCTION
Timber is highly anisotropic with excellent longitudinal strength but poor resistance to bending
and impaction in transverse directions. Further shortcomings of solid timber are flammability,
susceptibility to decay and insect damage, and deformation, the latter problems being
exacerbated by uptake of water. The introduction of glues, resins and waxes into engineered and
reconstituted wood products improves properties such as strength, hardness and dimensional
stability to match the performance requirements of specific applications. However, these
products may display new problems such as creep in medium density fibreboard (MDF).
Significant improvements may be achieved by modifying the wood component of timber or
wood products. Mechanism include solidification of polymers within the wood cell walls and/or
lumen (Norimoto et al. 1992), and derivitization, e.g. acetylation, or crosslinking of functional
groups, mainly hydroxyl groups within the cellulose, hemicellulose or lignin constituents of the
wood cell wall. Various resins have been used for crosslinking including furfuryl alcohol
polymers (Magalhães and da Silva 2002, Westin et al. 2004), melamine-formaldehyde
3
(Lukowsky 2002, Gindl et al. 2004) and dimethyloldihydroxyethylene urea (DMDHEU) (van
der Zee et al. 1998).
In an alternative approach a resin is prepolymerised with low molecular weight substances
compatible with wood cell polymers before use as a wood modification agent. Treatment of
radiata pine with prepolymers containing hexamethoxymethyl melamine (HMMM) and
maltodextrins increased the modulus of elasticity (MOE) of Pinus radiata sapwood blocks by
12% at a weight percent gain (WPG) = 40-60%, and increased hardness and biological resistance
(Franich and Anderson 1998). Torr et al. (2006) achieved a more useful 20% increase in MOE
using prepolymers containing HMMM and low molecular weight chitosan, and speculated that
the superior stiffening properties of polymers containing chitosan compared to maltodextrins
relates to the considerably greater tensile strength of the former. However chitosan is an
expensive raw material to use at the required 57% WPG.
In the present study we have taken a more simplistic approach and chosen substances with
known or likely affinity for wood cell wall polymers: a) polyvinyl alcohol (PVA), known for its
excellent wood adhesion properties based on strong hydrogen bonding to wood constituents, and
b) sucrose (table sugar), a cheap disaccharide, highly soluble by virtue of hydrogen bonding, and
considered likely to be compatible with wood. In this paper we discuss the preparation of
prepolymerised mixtures of these substances and their use to modify the physical properties of
Pinus radiata. We present examples demonstrating the strengths and limitations of this
approach to wood modification focusing on its application to engineered and reconstituted wood
products.
2. EXPERIMENTAL METHODS
2.1 Prepolymerisation reactions
Sucrose-HMMM prepolymer mixtures (Suc-HMMM) reported in Figures 1 and 3 were prepared
by dissolving (per kg) 333 g sucrose and 4 g boric acid in approximately 320 g hot water,
combining with 167 g HMMM (Luwipal 066, BASF) and 167 g ethanol (pre-mixed). p-
Toluenesulfonic acid (pTSA, 4 g) was added to catalyse the condensation reaction. The reaction
was maintained at 25 C for up to 24 h, terminated by adding 6 g 33% NH4OH, then combined
with 2 g non-ionic sufactant.
Polyvinyl alcohol-HMMM prepolymer mixtures (PVA-HMMM) reported in Figures 2 and 3
were prepared by mixing (per kg) 750 g 20% PVA, 100 g HMMM and 100 g ethanol (pre-
mixed), then adding 4 g pTSA to catalyse the reaction. The reaction was maintained at 25 C for
up to 240 min, terminated by adding 2 g 33% NH4OH, then combined with 2 g Surfynol 104A.
Subsequent experiments were conducted with a mixture containing PVA-HMMM (30 min) and
Suc-HMMM (24 h) in a 1:1 weight ratio. The PVA-HMMM component was prepared without
ethanol. Suc-HMMM was prepared using 10 g/kg CALFAX DBA-70 instead of ethanol, and 0.5
g/Kg silicone surfactant was used in the mixture instead of Surfynol 104A. The 1:1 mixture is
denoted as P-H:S-H.
2.2 Modification of veneers and MOE measurements
Rotary peeled Pinus radiata veneers (4.3 mm thick) were sawn along the grain to 145 mm in
length and 10 mm in width, and randomly divided into groups of six pieces. The width was
restricted to 10 mm because wider pieces tended to curl according the curvature of the log. The
4
initial weight and initial MOE of each piece was measured after conditioning for 5 days at 20 C
and 65% RH. For MOE measurements the deflection of veneers was kept well within the elastic
range and repeated five times to give an average MOE value. Each group was then treated with
the specified prepolymer mixture by vacuum impregnation (10 min vacuum (-90 kilopascals
gauge), admit solution under vacuum, 20 min atmospheric pressure, drain solution). After partial
drying for 1 hour in a fan oven at 30 C, veneer pieces were hot pressed for 12 min at 150 C to
complete polymerisation reactions and crosslinking to wood. The weight and MOE of each
piece was measured a second time after conditioning treated pieces for 5 days at 20 C and 65%
RH. At the conclusion of the experiment each piece was oven dried at 105 C for 20 hours,
cooled in a desiccator and weighed in order to calculate equilibrium moisture content (EMC)
after treatment. Δ MOE is defined in Eq. 1.
Δ MOE = (final MOE – initial MOE) / initial MOE x 100% (1)
2.3 Manufacture and testing of modified LVL lab boards
Pinus radiata veneers (350 mm x 350 mm x3.2 mm) were selected visually to obtain consistent
proportions and distribution of early wood/late wood content, grain, etc across the squares. The
squares were then labelled and cut in half along the grain to provide 350 mm x 175 mm x 3.2
mm matched pairs. One member of each pair was treated with P-H:S-H by vacuum
impregnation (15 min vacuum (-90 kilopascals gauge), admit solution under vacuum, 20 min
atmospheric pressure, drain solution), then dried for 18-24 hours in a fan oven at 30 C. Pairs of
LVL lab boards containing 12 veneers were made in such a way that the veneers at each position
were the original matched pairs with the treated member in one board and untreated in the other.
LVL was made with treated (T) and untreated (U) veneers arranged in the layup in three ways:
a) “superficial” T-T-U-U-U-U-U-U-U-U-T-T 4/12 veneers treated,
b) “distributed” T-U-U-U-T-U-U-T-U-U-U-T 4/12 veneers treated, and
c) “complete” T-T-T-T-T-T-T-T-T-T-T-T 12/12 veneers treated.
Veneers were laid up with PF resin (200 g/m2) and hot pressed (37 mm thickness, 30 min,
160 C). Four 350 mm x 30 mm lengths were cut from each board. The lengths from four
modified boards and their unmodified matched pairs were tested for MOE and MOR using a
Testometric Universal Testing Machine with either a flatwise bending test, as in scaffold
planking, or an edgewise bending test, as in a LVL beam. The “% Improvement” in MOE or
MOR is defined as:
% Improvement = (treated board – untreated board) / untreated board x 100% (2)
2.4 Properties of modified plywood manufactured in a mill operation
Pinus radiata veneers (2.5 mm x 1900 mm x 1000 mm) were treated with P-H:S-H by
submerging veneers on their sides in a dip tank for either 30 min or 60 min, with the treatment
solution at full strength or diluted with water to 66% strength or 33% strength. After draining
away the treatment solution, veneers were passed through a continuous veneer drier (80 C 8 min,
60 C 2 min, ambient 2 min), then laid up with three long bands and two cross bands to give 5-
ply plywood. Treated veneers were used on one or both faces. After cold pressing at 9kg/cm2
the panels were hot pressed at 120 C for 7 min, then sprayed with a formaldehyde scavenger on
exiting the press. Formaldehyde emissions were measured by JAS233 2003. Surface hardness
was evaluated by measuring indentation according to JIS K5600-5-3 using a 12 mm 500 g
5
weight dropped from heights of 400, 500 and 600 mm. Each test was conducted on two panels
at three positions and the data were averaged.
2.5 Manufacture and testing of modified MDF
To prepare MDF lab boards, dry Pinus radiata fibres (65 g) were pre-wetted by vacuum
impregnation with tap water at 60 C, drained of excess water then dried without further addition
(untreated) or combined with P-H:S-H (10%, 30% and 50% based on dry fibre weight). After
thorough mixing the fibres were dried for 24 h at 30 C, then circulated in a forced air blender
and combined with a mixture containing 9.75 g melamine-urea formaldehyde resin, 0.65 g
emulsified wax and 3.0 g water, that was sprayed into the blender with an airbrush. The fibres
were hot pressed at 170 C for 400 seconds to make MDF boards (105 mm x 105 mm x 6 mm).
Each board was sanded and divided into four 50 mm x 50 mm pieces, two of which were soaked
in 80 C water for 5 min and the other two soaked in 20 C water for 23 hours. Weights and
thicknesses (measured in 5 places per piece) were measured before and immediately after
soaking.
3. RESULTS AND DISCUSSION
3.1 Production and stiffening properties of Suc-HMMM
Aqueous solutions containing a polar substance like sucrose and a sparingly water-soluble amino
resin such as HMMM are inherently immiscible and unstable. Formation of a homogeneous,
albeit opaque, mixture at 25 C was facilitated by prior mixing of HMMM with an equal weight
of ethanol. Addition of catalytic amounts of pTSA produced a clear mixture within 10-15
minutes at 25 C and the reaction products became increasingly water-soluble over time,
indicating the formation of ether linkages between sugar and resin, among the possible
condensation reaction products. For convenience we term this process prepolymerisation to
distinguish it from further polymerisation reactions (i.e. curing) that occur when treated wood is
subjected to heat and pressure. Suc-HMMM prepolymer mixtures formed by reacting for 24 h
before adding neutralising amounts of ammonium hydroxide remained in a clear liquid state for
at least 3 months.
Aside from the practicalities of making a shelf-stable product, the importance of the extent of
Suc-HMMM prepolymerisation on wood strengthening properties is illustrated in a time course
experiment shown in Fig. 1. Control mixtures either without pTSA catalyst (monomers, no
catalyst), or containing a neutralising amount of ammonium hydroxide added prior to pTSA
(monomers + catalyst), prepared and used without delay to treat Pinus radiata veneers, produced
11% and 14% increases in MOE, as would be expected based on the reactivity of native
HMMM. Mixtures labelled as prepolymers in Fig. 1 were incubated under acidic conditions for
the times shown and produced a trend whereby the change in MOE increased from about 11% to
about 17% over the 24 h prepolymerisation time. Veneers treated with Suc-HMMM increased in
weight by about 50%, swelled by 5-10% and appeared to show a small reduction in EMC with
increasing prepolymerisation time. Veneers treated with all of the Suc-HMMM mixtures were
coated with a crusty deposit.
6
0%
5%
10%
15%
20%
25%
30%
Monomers,
no catalyst Monomers +
catalyst Pre-polymer,
5 min Pre-polymer,
15 min Pre-polymer,
45 min Pre-polymer,
90 min Pre-polymer,
180 min Pre-polymer,
360 min Pre-polymer,
24 hours
Δ MoE, Δ Volume & EMC
0%
10%
20%
30%
40%
50%
60%
70%
80%
WPG
Δ MoE Δ Volume EMC after WPG
Linear (Δ MoE) Linear (Δ Volume) Linear (EMC after)
Figure 1. Influence of prepolymerisation reaction time on the stiffening effects of sucrose-
hexamethoxymethyl melamine prepolymers on treated P. radiata veneers.
3.2 Production and stiffening properties of PVA-HMMM
Acid catalysed reaction of HMMM with PVA proceeds considerably more rapidly than with
sucrose. The reaction is accompanied by an easily observed increase in viscosity and
transformation of water-insoluble resin into water soluble products. A time course experiment
demonstrated that PVA-HMMM prepolymers formed after 15, 30 or 60 min reaction at 25 C
remained in a liquid state for 19 days at ambient temperature, whereas mixtures produced after
120 min and 240 min at 25 C solidified after 19 days (data not shown). The effect of time on the
wood stiffening properties of PVA-HMMM mixtures is shown in Fig. 2, where the Δ MOE
increased from about 15% to 22% with increasing prepolymerisation. This effect occurred in
spite of what appears to be a decreased uptake as evidenced by a reduction in WPG from about
23% to 14%. Presumably the reduced WPG is a reflection of the increasing viscosity over time
of the PVA-HMMM reaction products. Also, at 14-23% the WPG is considerably lower than the
~50% WPG of the Suc-HMMM prepolymers (Fig. 1). The EMC of all PVA-HMMM treated
veneers was about 1% lower than control veneers treated with a surfactant solution (Fig. 2).
Finally none of the PVA-HMMM treated veneers showed a crusty deposit.
7
4.0%
14.9%
17.9%
18.0%
21.0%
22.0%
-10%
0%
10%
20%
30%
Surfynol 104A 15 min 30 min 60 min 120 min 240 min
PVA – HMMM prepolymerisation time
Δ MoE Δ Volume EMC WPG
Figure 2. Influence of prepolymerisation reaction time on the stiffening effects of PVA-
hexamethoxymethyl melamine prepolymers on treated P. radiata veneers.
3.3 Shelf life and stiffening properties of PVA-HMMM : Suc-HMMM mixtures
In order to extend the storage properties of PVA-HMMM we investigated the effects of
combining PVA-HMMM prepolymers with shelf-stable Suc-HMMM. PVA-HMMM
prepolymers from a time course experiment with reaction termination points at 15, 30, 60, 120
and 240 min were combined with Suc-HMMM 24 h reaction products in a 1:1 ratio. The
mixtures corresponding to 15, 30 and 60 min PVA-HMMM prepolymerisation time remained in
a liquid state for at least 54 days at ambient temperature although there was a some opacity
indicating a degree of further polymerisation during storage. The 120 min mixture was a viscous
opaque liquid and the 240 min mixture an opaque gel after 54 days (data not shown). The wood
stiffening effects of these mixtures are illustrated in Fig. 3. Treatment of veneers with Suc-
HMMM produced an average ΔMOE of 18%. When mixed 1:1 with Suc-HMMM, the PVA-
HMMM reaction products from the prepolymerisation time course produced an average ΔMOE
that increased from 25% (15 min) to 27% (30 min) and declined thereafter. All of the
prepolymer mixtures produced a greater ΔMOE than Suc-HMMM alone. This enhanced
performance was accompanied by 39-33% WPG values that are about half way between that of
veneers were treated with Suc-HMMM alone (60% WPG) and veneers treated with PVA-
HMMM (23-14% WPG, Fig. 2). Treatment with the prepolymer mixtures also reduced the EMC
to 8.3-8.5% compared to veneers treated with Suc-HMMM alone (9.1%) and the untreated
control (11.7%, Figure 3). Furthermore veneers treated with the prepolymer mixtures did not
show the crusty deposit evident in veneers treated with Suc-HMMM alone. Mixtures containing
PVA-HMMM (30 min) and Suc-HMMM (24 h) in a 1:1 weight ratio were chosen for further
investigation and are designated henceforth as P-H:S-H (see section 2.1).
8
PVA-HMMM prepolymerisation time
0%
25%
27%
25%
22%
21%
18%
-10%
0%
10%
20%
30%
40%
Untreated 15 min 30 min 60 min 120 min 240 min Suc-HMMM
24h alone
Δ MoE, Δ Volume & EMC
-20%
0%
20%
40%
60%
80%
WPG
Δ MoE Δ Volume EMC after WPG
Figure 3. Influence of PVA-HMMM prepolymerisation reaction time on the stiffening effects of 1:1
mixtures of PVA-HMMM and Suc-HMMM.
3.4 Use of P-H:S-H in LVL
We explored the use of P-H:S-H to strengthen LVL using various layup arrangements of treated
and untreated veneers as described in section 2.3. To begin we considered the use of treated
LVL as scaffold planking where the load is applied flatwise on the board and where the I-beam
principle might provide additional strength. To this end we sandwiched eight untreated veneers
between two treated veneers top and bottom (“superficial” layup, section 2.3) and produced an
average 32% improvement in MOE for the four matched pairs and an average 19% improvement
in MOR when the LVL was tested flatwise (Fig. 4). There was some variation between boards
and the relationship between MOE and MOR was not very consistent. We attribute this
variability to the non-uniform raw material. Considerably greater variations are produced
without careful selection of veneers for the test.
9
0%
10%
20%
30%
40%
50%
60%
Board 1 Board 2 Board 3 Board 4
% Improvement
MoE MoR
Figure 4. Results of flatwise bending tests performed on Pinus radiata LVL prepared using the
“superficial” layup of P-H:S-H–modified and unmodified veneers.
Most LVL is used edgewise as a beam. To investigate the potential strengthening effects of
treated veneers in this situation we used the matched pairs approach to manufacture two sets of
LVL lab boards in which four of the twelve veneers were treated veneers – in the superficial and
the distributed layups – and one set in which all twelve veneers were treated (section 2.3).
Edgewise testing of the superficial arrangement produced reduced improvements in MOE and
MOR compared to flatwise testing (Table 1). This result is not surprising because the I-beam
principle is not expected to apply in the edgewise direction. Edgewise testing produced even
lower results for LVL when the four treated veneers were distributed throughout the LVL. This
might be explained by the likelihood that treated veneers would reach a lower temperature and
cure less completely when located internally than when located on the outside of the layup next
to hot press platens. LVL containing 100% treated veneers produced disappointing MOE and
MOR results (Table 1), with the pattern of rupture indicating poor adhesion of the PF resin.
These results illustrate a particular challenge for the modification of glued wood products. On
one hand treated veneers must be dried at temperatures cool enough to minimise further
polymerisation reactions (and consequent problems such as poor glue penetration into the
modified wood matrix), yet on the other hand drying must be warm enough to reduce the
moisture content to avoid steam blow during hot pressing.
10
Table 1: Edgewise and flatwise strengths of Pinus radiata LVL manufactured using three arrangements
of P-H:S-H–modified and unmodified veneers
Layup of
treated
veneers
Proportion
of treated
veneers
Flatwise test. Average %
Improvement + SD
Edgewise test. Average %
Improvement + SD
MOE
MOR
MOE
MOR
Superficial a
4/12
32.0 + 4.8
19.1 + 7.9
Superficial a
4/12
19.2 + 4.7
11.0 + 6.8
Distributed a
4/12
9.1 + 5.7
4.5 + 12.8
Complete b
12/12
4.9 + 10.0
-4.4 + 8.9
Data based on four a or five b pairs of lab boards
3.5 Use of P-H:S-M in plywood
To examine further the applicability of modified veneers in engineered wood products we
examined the potential for surface hardening of plywood for use in flooring. We conducted a
mill trial in which 5-ply plywood was produced with one or both face veneers pre-treated by
immersion in P-H:S-H. In addition the treatment solution was used at full strength, and diluted
with either one or two parts of water (section 2.4). All panels passed a JAS plywood type 1 bond
test indicating satisfactory glue adhesion. The extent of bow in treated plywood was also
satisfactory although plywood containing only one treated face bowed more than plywood with
both faces treated, e.g. veneers soaked for 60 min in 100% P-H:S-H produced the greatest bow
among the various treatments (e.g. 6.9 mm average lengthwise bow with one veneer treated, and
2.3 mm with both outer veneers treated). All treatment regimes produced similar relative
differences in the extent of bow in plywood with one or two treated faces. While it is fortunate
that the two treated panels counterbalance rather than augment each other, the addition of a
second treated veneer would be an expensive solution to problems of bow or warp. A valuable
increase in surface hardness was evident in all of the treated surface veneers (Table 2). Surface
hardening, measured as indentation after dropping a 500 g ball from a height of 400, 500 or 600
mm, was influenced most significantly by the strength of the P-H:S-H treatment solution with
the immersion time, 30 or 60 minutes, having a lesser contributory effect (Table 2).
Table 2: Surface hardness of plywood containing P-H:S-H-treated face veneers.
Strength of
P-H:S-H
Veneer
immersion time
Depth of indentation (mm)
400 mm
500 mm
600 mm
100%
60 min
0.69
1.01
1.13
30 min
0.87
1.04
1.17
66%
60 min
0.95
1.03
1.47
30 min
1.04
1.33
1.41
33%
60 min
1.30
1.48
1.74
30 min
1.31
1.21
1.43
Untreated
1.66
2.00
2.08
3.6 Formaldehyde
Formaldehyde emissions are a major consideration for wood products in an enclosed building
structure. The formaldehyde emissions were directly related to the loadings of prepolymer
treatment solution (Table 3). Among the options investigated only plywood containing one face
veneer treated with the 33% P-H:S-H mixture complied with Japanese Classification F****,
11
while plywood containing two faces treated at the 33% rate or plywood with one face treated
with the 66% mixture complied with the less stringent F*** classification (Table 3).
Table 3: Formaldehyde emission data for plywood containing one or two treated face veneers.
Strength of
P-H:S-H
Veneer immersion
time
Formaldehyde emission (mg/L)
Treated veneer on one
face
Treated veneer on
both faces
100%
60 min
1.83
0.99
30 min
0.57
2.83
66%
60 min
0.34 F***
1.07
30 min
0.43 F***
0.58
33%
60 min
0.25 F****
0.30 F***
30 min
0.16 F****
0.34 F***
F*** requires an average formaldehyde emission value <0.5 mg/L and F**** <0.3 mg/L.
3.7 Use of P-H:S-M in MDF
A key problem associated with reconstituted wood products is poor moisture and water
resistance and associated problems such as creep and swelling. To examine the potential for
wood modification in this situation, we prepared small MDF lab boards using Pinus radiata
wood fibres at various P-H:S-H loadings (section 2.5), and measured weight gain and increase in
thickness after soaking for 5 minutes in hot water or 23 hours in cold water (Table 3). The hot
soak data indicates improved water resistance commensurate with the amount of P-H:S-H
applied. Only the 50% loading of P-H:S-H, which produced a ~20% WPG for the final product,
gave a marked increase in water resistance under prolonged cold soak conditions.
Table 3: Soak testing of MDF made from P-H:S-H–modified and untreated Pinus radiata fibres
% P-H:S-
H
5 minute soak in 80 C water
23 hour soak in 20 C water
Average weight
gain
Average thickness
increase
Average weight
gain
Average thickness
increase
Untreated
41.16%
19.27%
16.60%
10.83%
10%
21.26%
16.01%
16.15%
10.65%
30%
12.80%
9.38%
16.28%
9.23%
50%
7.38%
6.72%
11.69%
7.48%
4. CONCLUSIONS
A wood modification product combining the wood strengthening and shelf-life properties of
water-soluble prepolymers produced by reacting HMMM with PVA and with sucrose produced
significant mechanical improvements when applied to Pinus radiata veneers and used in
engineered wood products such as LVL or plywood, or when used to make MDF. However, the
benefit is very much dependent on the application and the extent of wood modification. Thus,
for example, 12-ply LVL produced by modifying one third of the veneers and placing them top
and bottom demonstrated performance gains suitable for use in a scaffold planking product but
lesser gains when used as a structural beam. Increasing the proportion of treated veneers in the
experimental LVL introduced new problems such as delamination, at least with the materials and
12
manufacturing system we investigated. Significant surface hardening may be achieved by using
a single modified veneer in plywood but a product such as this is subject to strict formaldehyde
emission standards and only dilute, but nevertheless effective mixtures of wood modifier passed
the required classifications. Modification of wood fibres produced a more or less dose-
dependent increase in water-resistance in MDF. This product is used almost exclusively in
enclosed situations and the extent of fibre modification would need to be selected carefully to
ensure compliance with emission standards and achieving the required performance gain.
5. REFERENCES
Franich R A, Anderson K (1998): Densification of lignocellulosic material. US patent 5770319.
Gindl W, Hansmann C, Gierlinger N, Schwanninger M, Hinterstoisser B, Jeronimidis G (2004):
Using a water-soluble melamine-formaldehyde resin to improve the hardness of Norway Spruce
wood. Journal of Applied Polymer Science 93(4), 1900-1907.
Lukowsky D (2002): Influence of the formaldehyde content of waterbased melamine
formaldehyde resins on physical properties of Scots pine impregnated therewith. Holz als Roh-
und Werkstoff 60(5), 349-355
Magalhães W L E, da Silva RR (2002): Treatment of Caribbean pine by in situ polymerization
of styrene and furfuryl alcohol. Journal of Applied Polymer Science 91(3), 1763-1769.
Norimoto M, Gril J, Rowell R M (1992): Rheological properties of chemically modified wood:
relationship between dimensional and creep stability. Wood and Fiber Science 24(1), 25–35.
Torr K M, Singh A P, Franich R A (2006): Improving stiffness of lignocellulosics through cell
wall modification with chitosan-melamine co-polymers. New Zealand Journal of Forestry
Science, 36(1), 87–98.
van der Zee M, Beckers E P J, Militz H (1998): Influence of concentration, catalyst, and
temperature on dimensional stability of DMDHEU modified Scots pine. IRG document:
IRG/WP 98-40119. International Research Group on Wood Preservation, IRG Secretariat,
Sweden.
Westin M, Lande S, Schneider M (2004): Wood furfurylation process and properties of
furfurylated wood. IRG document: IRG/WP 04-40289. International Research Group on Wood
Preservation, IRG Secretariat, Sweden.
