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Development of Wood-based Materials Bonded with Citric Acid

* Research Institute for Sustainable Humanosphere, Kyoto University, Japan ** Graduate School of Advanced Integrated Studies in Human Survivability Kyoto University, Japan

Kenji Umemura*, Shuichi Kawai**

+ =

Wood element Synthetic resin adhesive (UF, PF, IC etc. ·····> Fossil resources)

Wood-based material

Non fossil resources based (natural) adhesive

— Adhesives are always required when manufacturing wood-based materials. —

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Development

Future global environment & Potential shortage of fossil resources

Reduction of the consumption of synthetic resin.

Natural Wood Adhesives

Saccharide based

Protein based

Aromatic based

• Starch • Cellulose etc.

• Animal glue • Casein • Soy protein • Blood albumin etc.

• Lignin • Tannin etc.

Oil based • Castor oil • Canola oil etc.

Others • Natural rubber • Liquefied wood etc.

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When preparing adhesives, various methods were performed.

Research trends in preparation of natural wood adhesives

① Utilization of natural substances in conventional synthetic resins.

②  Improvement of adhesion performance by adding synthetic compounds.

③  Improvement of adhesion performance by chemical modification.

④ Combination of ①, ② and ③.

Chemical substances derived from fossil resources are used. In addition, some harmful and toxic chemical substances are used.

Tannin, Lignin + Formaldehyde ·····> PF resin.

Addition of isocyanate resin (pMDI)

Starch, Soy protein ········> Some chemical modifications

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～ Policy for development of new natural adhesives ～

1.　Non-use of chemical substances derived from fossil resources.

2. Non-use of toxic and harmful substances.

Strong acid compounds, Strong alkaline compounds, Formaldehyde compounds, Amine compounds, etc.

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Synthesized resins, Aromatic compounds etc.

【 Citric acid 】

v  Citric acid exists in lemons and limes.

v  It is used in many foods and soft drinks.

v  Organic carboxylic acid containing three carboxyl groups.

v  White crystalline powder, water soluble, weak organic acid.

v  It is commercially produced by fermentation of glucose.

CH2 - COOH

HO-C - COOH

CH2 - COOH -­‐

-­‐

M.W.:  192,  M.P.:  153℃  

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wood consists of approximately 50% of cellulose thereare many similarities with cotton textiles, which mainlyconsist of cellulose - up to 98%. For that reason theagents that have shown good results in textile finishingapplications can be applied in chemical modification ofwood as well. Textile or wood is usually impregnatedwith emulsions or solvents of the applied agents. The re-action of chemicals and wood usually occurs at highertemperatures. Cross-linking chemicals reacting withhydroxyl groups reduce the hygroscopicity of wood andthe tendency to swell or shrink (Rowell et al, 1988; Ro-well 1991; Yasuda and Minato, 1994; Ashaari et al,1990). One of the reactants often applied is DMDHEU(1.3-dimethylol 4.5-dihidroxy ethylene urea). At highertemperature the N_methylole reactants form ether linka-ges accelerated with Lewis acid catalyst (MgCl2). Woodmodified by DMDHEU showed great improvement inwood stability, while tensile strength was reduced (Mi-litz, 1993; Xie et al, 2005). One of the problems that li-mit its usage is formaldehyde release at higher tempera-tures which can be toxic, potentially carcinogen and cau-se dermatitis (Solja~i} and Katovi}, 1988).

In late 80s the research for non-formaldehyde fi-nishes in cotton textiles focused on polycarboxylicacids (PCA). Welch and Andrews (1988) reported that1,2,3,4-butanetetracarboxylic acid (BTCA) is an effec-tive cross-linking agent for cotton cellulose. NaH2PO2

proved to be the best catalyst (Bischof Vukusic et al,2002; Schramm et al, 2002).

The mechanism involved is a two-step esterifica-tion. In the first step anhydride is formed, while in thesecond, this cyclic anhydride reacts with hydroxyl gro-ups (Figure 1).

Bischof Vukusic et al. (2006) showed that CAand BTCA crosslink with wood and reduce swelling

and shrinking of wood. They also showed that the di-mensional stability of wood achieved in this way iscomparable to that achieved with DMDHEU. In thisstudy NaH2PO2 was used as a catalyst in CA and BTCAsolutions, because it already proved to be the best ca-talyst for PCA modification of cotton cellulose (Bi-schof Vukusic et al, 2002; Schramm et al, 2002).

This research was aimed at establishing the effectof replacing one costly catalyst (NaH2PO2) with a lessexpensive one (NaH2PO4) on dimensional stabilisationof wood. The second aim was to shorten the time of cu-ring while achieving comparable anti swelling effi-ciency (ASE) of wood.

2 MATERIALS AND METHODS2. MATERIJALI I METODE

The samples with dimensions 20!20!10 mm(T!R!L) were cut from quarter-sawn air dried boardsof fir (Abies alba Mill.) and beech wood (Fagus sylvati-ca L.). They were signed in succession from one end fordifferent treatments according to Figure 2. There were10 replicates of samples for each treatment. After airdrying and conditioning at 20 °C and 65% relative humi-dity, the samples to be modified (T, S and Z) were impre-gnated with the specific CA solution, and control sam-ples (KT and KZ) were impregnated with distilled water.CA solutions were water solutions of 6.9% of CA and6.5% catalyst. One solution catalyst contained NaH2PO2

and the other NaH2PO4. The impregnation cycle consi-sted of a 5-minute initial vacuum of 2 kPa. The vacuumvessel was then filled with specific treating solution(control distilled water) and maintained under the samevacuum for 3 hours, followed with an 18-hour soaking atatmospheric pressure. The samples were then drained,

24 DRVNA INDUSTRIJA 60 (1) 23-26 (2009)

[efc, Trajkovi}, Hasan, Katovi}, Bischof Vuku{i}, Fran~i}: Dimensional stability... ........

Figure 1 Cross-linking via ester linkages of CA and BTCA with cellulosic chains (Schramm, 1999a).Slika 1. Umre`avanje CA i BTCA s lancima celuloze preko esterskih veza (Schramm, 1999a)

Catalyst: NaH2PO4 etc. Temperature: 140 – 180 ˚C Reaction time: 2 – 10 hours

Dimensional stability of wood

Previous research on citric acid

Non-formaldehyde cross-linking reagent

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•  Wood-based moldings using citric acid as an adhesive

•  Particleboards using citric acid as an adhesive

Bonding properties of ･･･････

1.  Citric acid and wood powders were mixed in a plastic beaker. 2.  The mixture powder was poured into a mold. 3.  The mold was hot-pressed at 200˚C and 4MPa for 10min.

Preparation of wood-based molding

Citric acid powder

Ligocellulosic powder

Plastic beaker

Mold

Mixing

Hot press

200˚C, 4MPa, 10min

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Photo1. Wood-based molding bonded with citric acid. �Photo1. Wood-based molding bonded with citric acid.

Evaluation of wood-based molding

◆ Static 3-point bending test Specimen size: 80×10×4-6mm Loading speed: 5mm/min

◆ Repeated boiling treatment (boiling water immersion for 4h, drying at 60˚C for 20h in an oven, boiling water immersion for 4h and vacuum- drying at 60˚C for 15h. )

Specimen size: 20×20mm The weight change was measured at each stage.

All of the infrared spectra were obtained with an FT-IR spectrometer by using the KBr disk method.

◆ FT-IR measurement

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◆ Charpy impact test Specimen size: 80×10×4-6 mm (flatwise direction, un-notched sample)

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▶ Bending properties ◀

The wood-based moldings with citric acid had good bending properties.

0

5

10

15

20

25

30

35

40

45

Bark Bark+

Citric acid

Wood Wood+

Citric acid

Specific

MO

R (

MP

a/(

g/c

m3))

0

1

2

3

4

5

6

Specific

MO

E (

GP

a/(

g/c

m3))

Specific MOR

Specific MOE

Fig.1. Bneding porperties of wood-based moldings.

21.2

3.5

38.1 4.9

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▶ Impact strength ◀

Wood-based moldings with citric acid had good impact strength.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Bark Wood

Impa

ct s

treng

th (k

J/m

2 )

Fig.2. Impact strength of wood-based moldings.

Bark+

Citric acid

Wood+

Citric acid

0.21

1.2

1.2

0.27

　

v  Some elusion occurred during boiling treatment.

v  The molding kept the form even

w h e n a r e p e a t e d b o i l i n g treatment.

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-30-20-10

010203040506070

Control Boilingfor 4h

Drying at60ºC

Boilingfor 4h

VacuumDrying

Wei

ght c

hang

e (%

)

Bark+Citric acid Wood+Citric acid

Fig.3. Weight changes of the wood-based moldings in a repeated boiling treatment.

▶ Repeated boiling treatment ◀

Abs

orba

nce

Wavenumber (cm-1)

4000 500100015002000250030003500

Fig.4. Infrared spectra of wood powder (a) and molding after repeated boiling treatment.

(a)

(b)

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Bonding mechanism of citric acid

Citric acid-COOH + HO-Wood

Citric acid- COO -Wood

The ester linkages between carboxyl groups of citric acid and h y d r o x y l g r o u p s o f w o o d component brought adhesiveness causing good physical properties.

Formation of ester linkages

1736 cm-1

▶ FT-IR spectra ◀

Fig.4. Infrared spectra of wood powder (a) and molding after a repeated boiling treatment (b).

Particleboard bonded with citric acid and sucrose

Preparation of particleboard

Wood particle

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Forming Hot Press Citric acid + sucrose

water solution

Citric acid Sucrose

200˚C, 10min

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Evaluation of particleboard (JIS A 5908)

◆ Static 3-point bending test Specimen size: 200×30×9 mm Loading speed: 10 mm/min

◆ Thickness swelling (TS) test

◆ Internal bond strength (IB) test Specimen size: 50×50×9 mm Loading speed: 2 mm/min

Specimen size: 50×50×9 mm Water immersion for 24h at 20˚C

18

0

5

10

15

20

25

0

1

2

3

4

5

6

Citric acid / Sucrose (wt%)

100/0 75/25 50/50 25/75 0/100

Fig.5. Effects of citric acid / sucrose ratio on

bending properties.

MO

R (

MP

a)

MO

E (

GP

a)

10.7

3.3

4.4

20.1

▶ Bending properties ◀

In the ratio of citric acid to sucrose of 25 / 75, the MOR and MOE showed the highest values.

This is comparable to the standard of the 18 type of Japanese Industrial standard (JIS) A 5908.

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IB s

treng

th (M

Pa)

Citric acid / Sucrose (wt%)100/0 75/25 50/50 25/75 0/100

Fig.6. Effect of citric / sucrose ratio on IB strength.

0.0

0.2

0.4

0.6

0.8

1.0

1.21.1

▶ IB strength ◀

The mechanical properties of the boards was greatly affected by the ratio, and the optimum ratio of citric acid to sucrose was 25 / 75.

The strength was much higher than the requirement (> 0.3MPa) of the 18type of JIS A 5908.

The addition of sucrose brought marked improvement of the bond strength between particles.

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100/0 75/25 50/50 25/75 0/100

120

100

80

0

20

40

60TS (%

)

Citric acid / Sucrose (wt%)

Fig.7. Effect of citric / sucrose ratio on TS.

33.8

20.0

▶ TS ◀

In the case of a ratio of citric acid to sucrose of 25 / 75, the value of TS exhibited lowest value of 20.0% Suitable addition of sucrose in the adhesion system would contribute to the inhibition of the thickness change of the board.

Conclusions

v  The wood-based molding with citric acid had good mechanical properties and good water resistance.

v  Judging from the FT-IR spectra, the ester linkages between

citric acid and wood components were recognized. The chemical bond contributes to the adhesion mechanism.

v  The addition of sucrose was very effective for enhancement

of physical properties of particleboard.

v  The optimum ratio of citric acid to sucrose was 25 / 75.

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Perspective on development of sustainable wood-based materials using citric acid as an adhesive.

Wood-based molding Particleboard

Citric acid（ + Saccharide ） Bio-resources

Sustainable wood-based materials

Plywood

Lignocellulosic materials ・Wood ・Agricultural waste ・Herbaceous plant

Hot-press

Fiberboard

Nonuse of toxic and harmful substances

Natural and safe substances

Thank you for your attention.

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