Adhesion and Bonding Characteristics of Preservative-Treated Bamboo (Gigantochloa scortechinii) Laminates
A. Roziela Hanim,
This study were investigate the adhesion and bonding characteristics of bamboo (Gigantochloa scortechinii) strips and laminates treated with permethrin-based preservative (Light Organic Solvent-Based (LOSP) and Water-Based (WBP)) formulations, Tributyl Tin Oxide (TBTO) and borax. The bomboo culm were cut into strips and treated with those selected chemicals. The bamboo strips were then glued edge to edge to form a bamboo veneers before fabrication of the three ply perpendicular bamboo laminates. In this research the properties studied include wettability, buffering capacity, shear strength and wood failure. Untreated strips and bamboo strips which were boiled in water (100°C) were also tested for comparison purposes. Those strips treated with LOSP had higher contact angle (3°-9°) which reflects that the surface of the treated strips is less readily wetted. Whereas, borax-treated strips had the highest wetting rate where the value is 1°. In buffering capacity study shows that treated bamboo was more stable towards alkali. This is suggested that a buffering agent (Calcium carbonate) is required in the adhesive formulation to ensure sufficient curing of the resin. Preservative treatments on bamboo strips significantly affect shear strength and wood failure of the laminates. Shear and wood failure of the laminated bamboo were significantly reduced especially in the wet condition where, the range is 0 N mm-2 (WBP treated) to 0.65 N mm-2 (boiled-treated) when compared to untreated bamboo laminates (0.79 N mm-2). While, in dry condition test, the glue bond strength of were range from 0.64 N mm-2 (WBP-treated) to 2.04 N mm-2 (borax-treated). All chemicals and non-chemical treatment generally affects the glue strength of the bamboo laminates especially in wet condition test. In dry condition test there are slightly reductions in glue bond strength but the quality still meets the requirement in the British Standard Part 8: Specification for Bond Performance of Veneer Plywood.
Received: February 24, 2010;
Accepted: May 15, 2010;
Published: June 10, 2010
Bamboo is regarded as eco friendly plants that grows and matures quickly, has
a versertile used, unique appearance and potentially as an alternative raw material
for wood (Rafidah et al., 2010; Salleh,
1984). Bamboo is a highly renewable resource which may have 40-50 stems
in one clump, which adds up to 10 to 20 culms yearly. However, the low durability
of bamboo makes it render highly susceptible to fungi, insect and other deterioration
agents. According to durability classification (Anonymous,
1982), bamboos fall in class III (non-durable category) with little variation
in durability among different species. The starch content in bamboo plays an
important role in its durability and service life. Latif
et al. (1993) found that the durability of bamboo against deterioration
agents is strongly associated with the chemical composition. Bamboo also attractive
and tough and thus suitable for conversion to engineered products such as composites
and laminated boards (Latif et al., 1993; Razak
et al., 1998), ply bamboo (Anwar et al.,
2004). Gigantochloa scortechinii, known as Buluh Semantan is well
known and established in the Malaysian community. It was widely used for traditional
uses such as water pipes, poles, flooring and handicrafts. Anwar
et al. (2005) reported that bamboo such as G. scortechinii is
suitable for composite materials, laminated boards and plywood. The properties
of plywood made from G. scortechinii are extremely high in modulus of
rupture, bending strength and modulus of elasticity. The ply-bamboo strength
ranks as the highest among all the structural boards and even as good as the
solid wood of high-density commercial timber (Chen and Qin,
Like most lignocellulosic materials, bamboos have a low resistant to biological
degradation agents (Zaidon et al., 2000). Thus,
there is a need to treat bamboo in order to enhance the service life of the
product. The best way is through chemical treatments. Chemical preservation
ensures a longer service life for bamboo products and maintains its quality
(Satish and Dobriyal, 1992; Anonymous,
2006).The choice of preservatives would determine the success of the bamboo
treatment. Chemicals selected should possess insecticidal and fungicidal properties,
commercially available, cheap and environmentally friendly and will not affect
other properties of the treated bamboo.
Preservatives like Copper Chromated Arsenate (CCA) and boron compounds are
becoming less preference due to their toxicity hazards and are not environmentally
friendly. New and more environmental friendly preservative formulations are
sought to preserve bamboo. Pyrethroids are potential group of preservatives
that can be served as an alternative to boron and CCA. It is a synthetic form
of pyrethrin that possesses insecticidal properties. They are one of the least
poisonous insecticides to mammals (Ray, 1991; Tomlin,
1994). Some of the pyrethroids that have formulated into preservatives include
permethrin, cypermethrin and deltamethrin and they are available in the market
under different trade names. In earlier study, it has been found that a small
concentration of permethrin-based preservatives and Tributyltin Oxide (TBTO)
successfully increased the resistance of bamboo towards white rot (Pycnoporous
sanguineus) and termite (Coptotermes curvignathus) attacks. Since
the potential usage of bamboo is in lamination form, the adhesion and bonding
characteristics of bamboo laminated should then be investigated.
This study was undertaken to investigate the effect of permethrin-based and TBTO preservative treatments on gluing properties of bamboo (Gigantochloa scortechinii) strips bonded with Phenol Formaldehyde (PF) resin. Adhesion properties of untreated, water boiled and borax-treated strips were also evaluated.
MATERIALS AND METHODS
Materials: This study was conducted at wood working lab at the Faculty
of Forestry, Universiti Putra Malaysia. This research was started on March 2004
until August 2008. Three year-old Buluh Semantan (Gigantochloa scortechinii)
was used in this study. They were extracted from bamboo plantation research
plots managed by FRIM (Forest Research Institute of Malaysia) at Compartment
24, Chebar Besar Forest Reserve, Nami, Kedah.
||Chemical composition in preservatives used in this study
|Standard concentration recommended by the manufacturers
The natural stand of bamboo in this area has undergone silviculture treatment
to promote the growth of bamboo clumps and individual culms since 1988.
The chemicals used in the study were borax (sodium borate, 5% w/v), pyrethroid based preservatives; LOSP (a.i: tributyltin naphthenate, 3.5%, permethrin, 0.2%) and WBP (a.i: disodium octaborate, 10%; benzalkonium chloride, 2%; permethrin, 0.2%), tributyltin-oxide (TBTO, 1% w/v). The chemicals were selected based on the ability to provide both insecticidal and fungicidal properties, cheap, possess low mammalian toxicity, soluble in water and have an ability to retain the clear and light colored finish of the treated material. LOSP and WBP were natural based preservatives while TBTO and borax was selected based on the previous studies which has been used to treat wood. Boiling treatment of bamboo in water was also carried out for comparison purposes. Table 1 shows the chemical composition and concentration of active ingredients of pyrethroid based preservatives which is LOSP and WBP, TBTO and also borax, the active ingredients and concentration is a recommended by manufacturer.
Preparation of samples: Bamboo culms were split using splitter knife and were dried to a moisture content of about 10% in a kiln (32.5°C and 60% relative humidity) for two weeks. The epidermis at the outer and inner part of the splits were then removed by using a single face-planning machine and finally dressed into strips of dimensions 150 mm (length)x20 mm (width)x4 mm (thickness). The strips were then treated with the chemicals using vacuum-pressure impregnation method. A full vacuum (56 cmHg) was applied for 30 min and at the end of the evaluation period, an external pressure of 25 kPa was applied and the samples were left immersed for 30 min. The weights of the treated sample were measured before and after treatment for determination of chemical retention.
||Variation of pH with respect to addition of acid
Wettability properties of treated bamboo strips: Wetting behavior of
bamboo is to observe the movement of liquid to penetrate into the wood, when
the good wetting occurs, the contact angle become very small (Minford,
1991). Investigation of wetting properties of the material may help to determine
its bonding characteristic with adhesive. The contact angle was measured on
the inner and outer surfaces of the bamboo strips. Bamboo strips with dimension
of 25x25x4 mm were used for the evaluation of contact angle. Distilled water
was used by dropping it onto the surface using injection tube. The AB Lorentzen
and Watter (L-W) surface wettability tester was used to measure the contact
angle. Prior to determination of wettability, untreated and treated samples
were first conditioned at 35°C and 65% RH for about 2 days. The image of
the drops was captured by a microscope tube after dropping the droplet on the
solid surface. The diameter of contact angle and height of the droplet measured
from the glass plate with transparent measuring scale and tangent were calculated.
Time of penetration was also recorded.
Determination of buffering capacity of treated bamboo: Investigating
the acidity or pH and buffer capacity of bamboo is an important criterion of
its suitability for various applications. Knowledge of the pH and buffering
capacity of the raw material is important consideration to better understand
the effects of raw materials on the curing rate of resin used for panel manufacturing
(Cheng et al., 2004). According to Paridah
et al. (2001), the buffering capacity of wood helps to determine
the amount of buffering agent required in the adhesive to prevent changes in
pH at the glue line. Bamboo particle in green condition was ground into powder
form. Then, 10 g of bamboo powder was refluxed in 100 mL distilled water for
1 h (solution = 10% w/v). The mixture was filtered with filter paper and washed
with 100 mL distilled water. From the 5% distillates, 1 mL was diluted with
100 mL distilled water to produce solution of 0.05%. A final concentration (0.025%)
was made by diluting 25 mL distillates with 25 mL distilled water. About 50
mL was then used for the determination of initial pH. The solution was then
titrated with 0.01 N HCL and 0.01 N NaOH until the pH reaches pH 3.0 and pH
11. The procedure was repeated to other different chemical treatment. The amount
of HCL and NaOH consumed in the titration and pH level was recorded. A graph
(Fig. 1, 2), pH versus volume (mL) was plotted
to observe the change in pH.
Fabrication of bamboo laminates: In fabricating bamboo laminates, bamboo
strips were glued edge-to-edge using polyvinyl acetate (PVAc) resin to fabricate
150x12x 4 mm bamboo sheets. The bamboo veneer was then glued three-ply perpendicular
to the grain using phenol formaldehyde resin to produce a bamboo laminates.
Adhesive was formulated by mixing with some other components such as industrial
wheat flour, calcium carbonate and water. Adhesive formulation was prepared
by mixing the resin with filler, Industrial Wheat Flour (IWF), calcium carbonate
(CaCO3) and water. Table 2 was summarized the mixing
formulation of the resin used for fabricating three-ply bamboo laminates. The
glue was spread at a rate of 230-270 g m-2 Single Glue Line (SGL)
according to the method specified in British Standard: Part 8: Specification
for Bond Performance of Veneer Plywood, (Anonymous, 1986).
The glue mixture was applied using a scraper. Three ply bamboo laminates were
produced by assembling each bamboo sheet perpendicular to the adjacent ply.
||Variation of pH with respect to addition of alkali
||Mixing formulation for PF adhesive
|Malayan Adhesives and Chemicals Sdn. Bhd (2004)
The bamboo laminates were then pre-pressed at a specific pressure of 10 kg
cm-2 for 5 min. They were then pressed in a hot press at 140°C
for 7 min with a specific pressure of 14 kg cm-2. Finally, the laminates
were conditioned in a conditioning room at 25°C and 65% relative humidity
(12% EMC) prior to testing. British Standard (BS: Part 8: Specification for
Bond Performance of Veneer Plywood) (Anonymous, 1986)
was used to investigate the shear strength and glue line between composite elements
before and after exposure to cool and boiled cyclic condition.
RESULTS AND DISCUSSION
Wettability and contact angle: Wettability of bamboo indicates the rate
and how fast a liquid can wet and spread on it. In relation to contact angle
when good wetting occurs, the contact angle becomes very small. On the contrary,
if the contact angle is higher, there is lack of wetting. Contact angle of G.
scortechinii on the outer part was 9° for LOSP-treated strips, 4.03°
for TBTO-treated strips, 3° for WBP-treated strips, 2° for untreated
strips, 1° for boiled-treated and borax-treated strips respectively. Contact
angle for the inner part was 5° for LOSP-treated bamboo strips, 3° for
WBP-treated bamboo strips and 3° for TBTO treated bamboo strips, respectively.
There was no contact angle recorded for untreated, boiling and borax due to
||Contact angle (°) on the outer and inner layer surfaces
of G. scortechinii strips
|No. of samples, 10 for each treatment. Values in parentheses
are standard deviations
The smaller contact angle or complete wetting suggested that the surface was
easier to be wetted. Zhu (1995) also stated that higher
wettability resulted in poor bonding due to greater tendency for starved joints.
Thus, the adhesive used for wood has to be modified. In this study, some modification
on glue mixture was made to optimize the bonding process. In all cases, the
amount of filler was kept at 27% of the amount of resin. The contact angle of
untreated and treated bamboo strips are summarised in Table 3.
The highest contact angle value was found with LOSP-treated strips. This was
related to the chemical properties of LOSP, which is an organic solvent-based
fungicidal preservative (Anonymous, 2000). Hunt
and Garret (1967) also mention that hydrocarbon solvents would restrict
the penetration of liquid into the bamboo substrate. Tascioglu
et al. (2003) reported in their research about composite joist treated
with oil-borne preservatives had a low surface wettability due to the presence
of hydrocarbon solvent on the surface of the material.
The contact angle between the outer and inner surface of bamboo differs significantly.
From the results, regardless of the treatment, the outer layer of material was
easier to wet than the inner layer. This was due to the difference in specific
gravity between the two surfaces.
Vessel size also contributed to the variation of contact angle value. This
finding also supported by Sekhar and Bhartari (1960),
Razak et al. (1998), Jamaludin
et al. (1997) and Anwar et al. (2005),
the contact angle is strongly related to the size of vessel or pores. In comparison
to the vessel size for inner and outer part of the culm, the vessels size was
very small at the outer surface and become larger when it goes inward (Liese,
Buffering capacity: The results on buffering capacity shows that the
treated and untreated bamboo was more stable towards alkali (Fig.
1), the amount of 0.1N NaOH required to change the pH value range from 7.51
(TBTO)-8.14 (Boiling) to 11 was only 0.5 mL. Conversely, the amount required
to change the pH 7 to 3 was using 0.1 N HCL was 8.5 mL (Fig. 2).
Bamboo is sensitive to an alkali-based adhesive, such as PF, a buffer is required
in the adhesive formulation to ensure sufficient curing of the resin (Sakuno
and Moredo, 1996). The results imply that the adhesive formulation used
for bamboo laminates for all treatments would behave the same in terms of curing
rate as long as the pH value is within the alkaline region. Malonney
(1993) also stated that an acidic condition might affect the curing rate
or pressing time due to the combination of pH, buffer capacity and the existing
or potential of total free volatile acid content of the material it is suggested
that there is a needed for a longer pressing time to ensure sufficient curing
Shear strength and wood failure: Glue line shear strength is one of
the basic mechanical and durability indicators for laminated composite boards.
By using the British Standard (BS: Part 8: Specification for Bond Performance
of Veneer Plywood) (Anonymous, 1986), the shear strength
and bamboo failure between composite elements were assessed in Dry Condition
(DT) and after Cyclic Boil Treatment (CBR). In dry condition test, except for
borax treatment, the results show that the shear and wood failures of the bamboo
laminate were significantly reduced. The values of the shear strength were in
the range of 0.64 to 2.04 N mm-2, as compared to untreated bamboo
laminates (2.66 N mm-2). For wet condition test, the range was from
0.48-0.69 N mm-2 compared to 0.78 N mm-2 for untreated
bamboo laminates. The finding on the shear strength and bamboo failure at dry
and cyclic boil test is summarized in Table 4.
Anwar et al. (2005) and Zaidon
et al. (2000) revealed that the shear strength of untreated laminates
was relatively higher compared to treated laminates. In addition, Anwar
et al. (2005) in their findings found that the shear strength of
G. scortechinii plywood in dry condition was 3.4 and 1.7 N mm-2
in wet condition. Similarly, Zaidon et al. (2000)
found that the shear strength was 2.36 N mm-2 (parallel) and 2.68
N mm-2 (cross-ply laminates in wet condition) while in dry condition
the shear strength was 1.51 and 1.24 N mm-2, respectively. The percentage
of the shear strength reduction is summarized in Fig. 3.
Treatment with borax were found to reduce the shear strength of the laminates
from 2.66 to 2.04 N mm-2. i.e., 23% reduction in the shear strength
properties. While for TBTO-treated bamboo laminates the strength decreased by
32%, water boiled-treated laminates 44%, LOSP-treated by 45% and WBP-treated
by 76% when tested in dry condition.
||Shear reduction of treated bamboo laminates calculated against
||Mean shear strength and bamboo failure percentage of bamboo
|No. of samples, 70.±is standard deviation; Means within
a column with the same letter are not significantly different at p = 0.05.
1DT (Dry Condition Test), 2CBR (Cyclic Boiled Resistance)
The same trend happened when tested in wet condition. The shear strength of
the laminates decreased for all treatments. Boiling-treated laminates decreased
by 17%, borax-treated laminate by 21%, TBTO-treated laminates by 26%, LOSP-treated
39% and lastly, for WBP-treated laminates it was delaminated after exposure
to wet condition test.
Borates used as a preservative for wood composite panels bonded with phenol
formaldehyde (PF) resin often reduce resin gel time, not allowing the resin
to flow and cure sufficiently. This problem is related to the functional methlylol
groups on resin molecules and their interaction with borate ions (Sean
et al., 1999). This interaction is detrimental to bond performance
and ultimately reduces physical properties of the panel. Vick
(1990), investigated preservatives compatibility with PF resin for thirteen
non-acidic waterborne preservatives using retreated aspen veneers. The results
indicated that the borate containing preservatives tested caused poor bonds,
as did an emulsion of copper naphthenate.
Treatment on bamboo strips altered the shear strength and wood failure properties
of the bamboo laminates. In particular, WBP showed a tremendous reduction in
the glue bond shear strength and bamboo failure after exposing the bamboo laminates
to both dry and cyclic boiling test. The properties of the formulations and
treatment that were water-based or hydrocarbon-based may have influenced the
compatibility of the adhesive used in bonding the bamboo laminates. However,
the wood failure test of the treated material is not significantly different
compared to untreated bamboo laminates where the wood failure test ranged between
8% to 28% in the dry condition test and 2-4% in the wet condition test. However,
in the dry condition test, the glue bond quality of untreated and treated laminates
met the minimum standard requirements of the British Standards: Part 8: Specification
for Bond Performance of Veneer Plywood (Anonymous, 1986).
Shear strength and wood failure reflected on the glue bond quality of the laminated
bamboo. Whereas one of these values is high and the other is low, it indicates
that either the bamboo strips are of low strength quality or the adhesive bond
is poor. Anwar et al. (2005) stated that the
strength of glue line is also related to the glue mixture itself. Even though
the bamboo failure was relatively low (ranging 40-60%), this does not imply
the ply-bamboo is of inferior quality. This is because the adhesives were found
to have penetrated into the bamboo substantially, forming a good adhesion and
anchorage between the bamboo layers. Zaidon et al.
(2000) reported that shear strength is affected by grain orientation. However,
it was observed that dry wood failure percentage was relatively higher in the
parallel-ply laminates (75%) than in the cross-ply laminates. Strips arranged
perpendicular to each other experienced a relatively higher reduction in shear
strength after cyclic soaking in boiled water and after long soaking in boiled
water. As reported by Sulaiman et al. (2006)
shear strength of bamboo laminates was reduced by heat treatment, where oil
was used as the heating medium. The reason for the loss in strength may have
been due to the presence of oil in the cells. Presence of oil may reduce the
wetting of the surface, thus reducing the absorption of adhesive by the surface.
This effect may eventually reduce the adhesion to the surface. Formaldehyde
adhesives are usually water-borne resins such that the curing process is not
only polymerisation, but also the loss of water used as the solvent. Polymerization
process evolves too much water in the bond lines and this retards the reaction.
Insufficient water prior to polymerisation reduces the mobility of the resin
and limits collision needed for polymerization, in addition to limiting heat
transfer. Controls of both open and close assembly times are important in controlling
the penetration and water content in the bond line (Rowell,
The result on wettability shows that there a need to modify the adhesive formulation so that the adhesive can penetrate adequately into the bamboo substrate. The buffering capacity for all treatments behaves the same in terms of curing rate as long as the pH value is within the alkaline region. Chemicals and non-chemical treatments affected the bonding properties of laminates slightly. Shear strength was adversely affected by the chemicals treatments. In dry condition, except for borax acid-treated laminates, the shear strength of other treated laminates were significantly reduced. Boiling-treated laminates had superior shear strength compared to other treated laminates tested in wet condition. Among the preservatives, WBP affected the bonding properties most. The wood failure of treated laminates was not significantly different from the untreated. As a whole, the bonding property of the treated bamboo laminates surpassed the minimum requirement of British Standard: Part 8: Specification for Bond Performance of Veneer Plywood.
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