The various forms of wood utilization represent an extremely large and diverse
market for adhesives (Lambuth, 1994). The wood industry
has diversified into the production of downstream products, such as composite
furniture products and engineering woods products, including plywood, medium
density fiberboards and particleboards. Wood adhesives and resins have greatly
contributed to the construction and housing industries for about a century and
will continue to play an important role in this field. In early of this decade
the worldwide wood adhesive consumption was 13.3 million tons and total sale
value reached more than $6 billion (Seller, 2001). At
present, formaldehyde-based adhesives such as Phenol-Formaldehyde (PF) and Urea-Formaldehyde
(UF) and Melamine-Urea-Formaldehyde (MUF) resins are used predominantly as adhesive
for production of wood composites (Bono et al., 2006,
However, the emission of formaldehyde, especially from the breakdown of UF
resins in wood composites, poses a great hazard to human health because formaldehyde
is human carcinogen. On daily life, formaldehyde is emitted into the air from
composite wood products at manufacturing plants, fabrication facilities, home
construction sites, remodeling construction, goods transport, lumberyards and
through windows, doors and ventilation systems in homes and other buildings
when unreacted formaldehyde is released from urea-formaldehyde resins (International
Agency for Research on Cancer, 2004).
In addition, formaldehyde-based adhesives are derived from non-renewable petrochemicals
and natural gas and hence still have toxic chemical problems associated with
their manufacture. Unfortunately, there is no method to degrade them at rate
comparable to our current rate of consumption. The problem of non-biodegradability
is highlighted by overflowing landfills, polluted marine waters and unsightly
litter (Huang, 1995).
These problems and harmful effects of formaldehyde emission have lead to increased
efforts in research towards developing formaldehyde-free, environment-friendly,
safer, biodegradable green alternatives, particularly the sustainable ones based
on yearly renewable plants (Peijs, 2002).
Green chemistry is the environmentally benign chemical synthesis with attractive
economics and performance. The synthesis schemes are designed in such a way
that there is least pollution to the environmental and the waste products are
minimum (Ahluwalia and Kidwai, 2004). There has been renewed
interest in the development of protein based wood adhesives in recent years
for improving the strength and water resistance of wood composite panels bonded
with protein-based adhesives. Researchers prepared adhesives with alkali (NaOH)-
and trypsin-modified soy proteins. They found that the bond strength and water
resistance of the modified soy protein adhesives were enhanced compared with
those of unmodified soy protein adhesives. Sun and Bian also found that urea-modified
soy protein adhesives were more water-resistant than those modified by alkali
(Sun and Bian, 1999).
Huang and Sun (2000a,b) investigated
adhesive properties of soy proteins modified with different concentrations of
urea and Guanidine Hydrochloride (GH). The results indicated that both urea
and GH concentrations had significant effects on the extent of protein unfolding
and consequently, on adhesive properties. Compared to the unmodified protein,
the modified proteins also exhibited higher shear strengths after incubating
with two cycles of alternating relative humidity, zero delamination and higher
remaining shear strengths after three cycles water soaking and drying. These
results indicate that soy proteins modified with urea and GH enhance water resistance
as well as adhesive strength. Partly unfolded protein molecules with a certain
amount of secondary structure may be desirable for protein adhesion (Huang
and Sun, 2000a). Polyamidoamine-epi-chlorohydrin (PAE) resin has been found
to be excellent curing agent for soybean protein. A patented technology based
on soybean flour and the PAE resin has been successfully used in the commercial
production of plywood and particleboard (Li et al.,
2004). Although, various research groups have tried to synthesize these
proteins in genetically modified bacteria, their commercial use is limited owing
to high production costs (Acosta, 2007).
A novel adhesive based on Soy Flour (SF), Maleic Anhydride (MA) and polyethyleneimine
(PEI) has been developed (Li and Huang, 2008). However,
the usage of soy flour as a resin ingredient will compete with the production
of food and animal feed. Alternatively we should look at the possibility of
utilization of palm kernel cake as an ingredient of plant protein based resins.
Palm kernel cake is by-product of palm kernel oil industry. The production of
palm kernel cake well over 5000 million tons annually, this quantity is expected
to rise further due to the current development of biodiesel demand (Sumathi
et al., 2008). Unlike soya bean, palm kernel cake contains 20% protein
which lowers than the protein contains of soya bean. Therefore, protein content
enhancement of palm kernel is required in order of utilizing palm kernel as
ingredient of protein based resins. The protein content enhancement of palm
kernel can be done by adding protein extract of palm kernel itself, soya protein
extract or protein extract from micro-algae. Here in this paper, palm protein
content was enhanced by adding soya extract at various ratios to produce resin
to be used as wood adhesive.
MATERIALS AND METHODS
The experimental study was conducted according to the production of soya protein
based resin (Li and Huang, 2008). The study involved
was the reaction of selected chemicals such as polyetheleneimine (PEI) and Maleic
Anhydride (MA) with soya protein in alkali conditions. In this research soya
protein is replaced with palm kernel or the mixture of palm kernel and soya
protein. Therefore the experimental study involves the selection of materials
(i.e., palm kernel cake, PEI and MA), preparation of palm kernel, production
of wood adhesive, production of type II plywood and the test of wood adhesive
on type II plywood.
Selection of materials: The chemicals used for production of wood adhesive was 50 wt.% aqueous PEI solution (Mw = 750,000), Maleic Anhydride and NaOH. These chemicals were purchased from Sigma-Aldrich. Materials for production of type II plywood was Red-Meranti veneer, it was provided by Shin Yang Chemical Sdn. Bhd. For the enhancement of palm kernel protein content, soya protein extract with 84 wt.% protein was selected. This soya protein extract was purchased from local market in the form of high protein soy drink. Palm kernel cake by-product of palm kernel oil production was purchased from local Palm Oil Mill.
Preparation of palm kernel: The palm kernel cake obtained from Palm Oil Mill was contained of trace oil. The trace oil was removed to avoid interference during the production of wood adhesives. The trace oil removal was conducted using soxhlet solid-liquid extraction technique. Prior the oil removal the palm kernel cake was grinded to form fine particle for higher extraction efficiency. Iso-propanol was selected as an extraction solvent. The extraction was conducted at solvent boiling temperature and was left running for 10 h. The palm kernel cake was then dried in oven for removal of trace solvent.
||Summary of shear strength and delaminating test of type II
plywood produced using various formulation of palm kernel based wood adhesive
Production of wood adhesives: The production of wood adhesive was involving the reaction of protein with polyetheleneimine and maleic anhydride at alkali condition. Sodium hydroxide was used for providing alkali conditions. Prior the reaction oil free palm kernel cake powder was mixed with soya protein extract at various ratios. The mixture then was added into alkaline solution of polyetheleneimine and maleic anhydride. The reaction was left for 5 min before it was used as adhesive for making type II plywood. The ratios of materials and chemicals used for the production of wood adhesive was recorded and presented in Table 1.
Production of type II plywood: The production of type II plywood was conducted using Red-Meranti 300x300x3.3 mm veneer. In order to get consistence result veneer was maintain at 10% moisture content and the equal amount of wood adhesive were used at every plywood produced. The adhesive was applied onto two sides of a core veneer using a glue spreader. The spread rate of the adhesive was 23 mg cm-2. The adhesive-coated core veneer was stacked between two uncoated face veneers. The grain directions of the two adjacent veneers are perpendicular to each other. The unfinished plywood was left at room temperature for 5 min, before it was cold pressed 9 kg cm-2 for 20 min. Once reaches 20 min the unfinished plywood was removed from the cold press device and left free 5 min before it was transferred to hot press device. During hot pressed, pressure at 9 kg cm-2 and temperature of 135-140°C were applied. The process was conducted for 5 min. Once the hot press completed, the plywood product was released from the device and was stored at room environment for 24 h before it was tested for its shear strength and water resistance.
|| Dimension of plywood test pieces for bonding test
Performance test of wood adhesive
Shear strength: The shear strength of type II plywood produced
here was determined by bonding test according to the Japanese Agriculture Standard
(JAS) for structural plywood. Total of nine plywood test pieces (25x80 mm) were
tested for every trial plywood panel produced. The dimension of plywood test
pieces is shown in Fig. 1. Prior the test, the test pieces
were soaked in a hot water bath at 60°C for 3 h and followed by soaking
it at cold water bath at room temperature. Once the test pieces reached cold
state, then it were tested for shear strength. The test was conducted using
Jiing Koou HT-8311C bonding testing machine while the plywood test pieces were
wet. According to the standard, any plywood panel having the shear strength
less than 0.7 MPa is considered fail.
Water resistance: The water resistance was determined by immersion delaminating test (soaking test). This test was also adopted from the Japanese Agriculture Standard (JAS) for structural plywood. Here, six of plywood test pieces with size of 75x75 mm were immersed in hot water bath at 70°C for 2 h and followed of drying in the oven at 60°C for 3 h. After the drying procedure, test pieces were inspected for the delamination. The plywood produced here will be considered fail if any of the test piece delaminated.
RESULTS AND DISCUSSION
The formulation details of wood adhesive developed in the experimental study is summarized in Table 1. This table is also present the shear test, delaminating test and overall test results. Out of fifteen formulations, only three were pass overall test. These formulations are unmixed soya protein extract with 0.5 N NaOH, Mixture of (50% palm kernel and 50% soya protein extract) with 1.0 N NaOH and mixture of (70% palm kernel and 30% soya protein extract) with 1.5 N NaOH.
The result of overall test shows that palm kernel cake can be used for production of protein based resin or wood adhesive. In certain cases the mixture of palm kernel cake with soya protein extract produces better performance wood adhesive compared to unmixed soya protein extract. However the mixture palm kernel cake with soya protein extract required higher alkali concentration in order to produce better performance wood adhesive. The alkali concentration effects on the performance of wood adhesive can be observed in Fig. 2. This figure is clearly shows that the shear strength of all plywood produced out of formulations which include palm kernel cake was rise with the increases of alkali concentration. The result also show the higher ratio of palm kernel cake to soya protein extract will require higher alkali concentration to produce better performance wood adhesive. This phenomena may due to the solubility of palm kernel cake is increases with the increase of alkali concentration.
The formulation with non-added soya protein to palm kernel cake was unable to produce usable wood adhesive. This show that the protein content of palm kernel has to be enhanced before it can be used for protein based wood adhesive production. The effect of wood adhesive performance on the addition of soya protein onto palm kernel cake can be observed in Fig. 3. This Fig. 3 show that there is optimum amount of soya protein extract can be added to palm kernel cake to get the best wood adhesive performance.
Furthermore, PEI and MA are also essential components for the final adhesive
networks. In the study of Li and Huang (2008), it was
demonstrated that MA first react with Pei to form amide-linked maleyl groups
that further reacted with amino group in soy flour (SF) and PEI during a hot-press
of making plywood panels. In this study, PKM and SP were used to replace the
SF as ingredient for making PKM-SP-PEI-MA adhesive. In palm kernel wood adhesive,
PKM and SP might contain some water-soluble carbohydrates that would reduce
the water resistance of adhesive bonds.
||Shear Strength of plywood with various alkali (NaOH) concentration
||Shear strength of plywood wit various ratio of PKM to soy
The PEI-MA adduct might coat or bundle water-soluble carbohydrates, thus minimizing
their negative effects on the water resistance. Hydroxyl groups of the carbohydrates
might also react with PEI-MA adduct via Micheal addition although hydroxyl groups
are weaker nucleophiles than amino groups, thus further reducing the negative
effects of water-soluble carbohydrates on the water resistance (Li
and Huang, 2008).
The research presented in this study can be concluded that palm kernel cake can be used for protein based resin production. However due to the lower protein content of palm kernel cake, protein enhancement of palm kernel cake is required before it can be used for protein based resin or wood adhesive production. Protein content enhancement of palm kernel cake can be made by addition of palm kernel protein extract or soya protein into palm kernel cake. Using palm kernel cake as an ingredient of protein based resin will compliment the usage of soya protein.
Authors would like to extend their sincere gratitude and appreciation to the R and D management team of Shin Yang Chemical Sdn. Bhd. Sepanggar Bay, Sabah for providing the materials and facilities for conducting this research work.