Wood adhesives based on lignocellulosic materials.

Dumitrescu, Lucia ; Manciulea, Ileana ; Patachia, Silvia 等

1. INTRODUCTION

Lignocellulosic materials, important natural renewable resources, contain cellulose, hemicellulose and lignins and also posses many active functional groups susceptible to reaction such as: primary and secondary hydroxyls, carbonyls, carboxyls (esters), ether and acetal linkages, and sulfur-containing groups. Virtually every type of reagent capable of reacting with these functional groups can be applied to wood or sub-products derived from wood. Hence, based on the variety of functional groups, etherification, esterification, alkylation, hydroxyalkylation, graft copolymerization crosslinking, oxidation has been conducted to produce a series of products with many practical applications (Muzaffar et al., 2004).

The presence of lignin as a waste product in pulp mills has made it an attractive raw material for adhesives. As a major wood component, native lignin is insoluble in water. During the technical sulphite pulping, lignin becomes soluble in water, due to the partial degradation and introduction of sulfonic groups. When lignosulfonate is treated with strong mineral acid at elevated temperatures, condensations reactions leading to diphenylmethanes and sulfones take place. Hydroxybenzyl alcohol groups as well as sulfonic acid groups on the carbon alpha to the aromatic rings of the phenylpropane units of the random polymer react with the aromatic nuclei of other phenylpropane units in the presence of the strong mineral acids. This reaction, leading to diphenylmethane, is of the same type as the formation of phenolic resins from phenol and formaldehyde. Lignin also reacts with formaldehyde and can be cross-linked by it, in the same manner of synthetic polyphenol resins. The ability of phenol formaldehyde to capture more loads before the adhesive broken apart is probably due to the presence of lignin and phenol (Nihat et al., 2002).

2. EXPERIMENTAL

In order to partially substitute the phenol and formaldehyde, aluminum, calcium and iron lignosulfonates, were used in the synthesis of phenol-formaldehyde resins. Metal complexed lignosulfonates have also been used as reactive comonomers in the polycondensation reaction because they possess a reactive chemical potential, put into evidence by the presence in their structure of some functional groups like: phenolic and alcoholic hydroxyl, carbonyl, carboxyl, sulfonic. The chemical characteristics of the aluminum, calcium and iron lignosulfonates used into the synthesis are presented in Table 1.

Where:

LSAl = aluminum lignosulfonate

LSCa = calcium lignosulfonate

LSFe = iron lignosulfonate

Our research has been focused on the obtaining of some new adhesives based on aluminum, calcium and iron lignosulfonates, as partial substitutes for formaldehyde and phenol in phenol-formaldehyde adhesives.

2.1 Synthesis of the phenol-formaldehyde adhesives with lignosulfonates

Phenolic resins (polycondensation products of the reaction of phenol with formaldehyde) were the first true synthetic polymers developed commercially (Bousoulas et al., 2001). The characteristic that renders these resins invaluable as adhesives is their ability to deliver, at relatively low cost, water, weather, a and high-temperature resistance to the cured glue line of a joint bonded with phenolic adhesives. Phenols condense initially with formaldehyde at pH either acid or alkaline, to form a methylol phenol or phenolic alcohol, and then, dimethylol phenol.

The initial attack may be at 2-, 4-, or 6- position of the phenol molecules.

The second stage of the reaction involves the reaction of the methylol groups with other phenol or methylol phenol, leading first to the formation of linear polymers and then to the formation of hard-cured, highly branched structures.

Resols are obtained as a result of alkaline catalysis and an excess of formaldehyde. A resol molecule contains reactive methylol groups. Heating causes the reactive resol molecules to condense to form large molecules without the addition of a hardener. A typical phenolic resin was made in a glass reactor equipped with a turbine-blade agitator, a reflux condenser and heating and cooling facilities. Molten phenol, formalin (containing 37 to 40% formaldehyde) and water are charged into the reactor in molar proportions between: 1:1:1 and mechanical stirring was begun.

Where:

PF0 = standard phenol-formaldehyde resin;

PF1 = phenol-formaldehyde resin with LSAl

PF2 = phenol-formaldehyde resin with LSCa

PF3 = phenol-formaldehyde resin with LSFe

a = 10% aluminum, calcium, iron lignosulfonate

b = 15% aluminum, calcium, iron lignosulfonate

c = 20% aluminum, calcium, iron lignosulfonate.

Quantities of: 10%, 15%, 20% of aluminum, calcium and iron lignosulfonates were also added to the mixture of above mentioned monomers. To make a resol-type resin, used in wood adhesive manufacture, an alkaline catalyst, such as sodium hydroxide was added, and the reaction mixture was heated to 80-90[degrees]C for about 2-3 hours. Since the resol can gels into the reactor, the temperature was kept below 100[degrees]C. Tests have to be done in order to determine first the degree of advancement of the resin, and second, when the batch should be discharged. Such tests consist on the measurements of the gel time of the resin on a hot plate or at 100 [degrees]C in a water bath. Resins, that are water soluble and of low molecular weight, are finished at a low temperature, usually around 400 to 60[degrees]C. It is important that the liquid, water soluble resols, retain their ability to mix with water easily, since when they are used as wood adhesives they often require the addition of water to counterbalance the effect of the fillers added. The characteristics of the new adhesives, based on phenol, formaldehyde and aluminum, calcium and iron lignosulfonates are represented in Table 2. The FT-IR spectra (Figure 1.) were performed with a FTIR-Spectrometer model BX II (Perkin Elmer, 2005). Characteristic absorbances of the phenolic resin are assigned to identifying the components of the condensation reaction of phenol and formaldehyde with lignosulfonates. The observed wave number 1610 cm-1 corresponds to the aromatic ring vibration; 1451 [cm.sup.-1] indicates benzene ring obscured by -CH2- methylene bridge; 1058 corresponds to single bond C-O stretching vibrations of CH2OH group and 976 [cm.sup.-1] to 1,2,4-substituted benzene ring. These observed IR spectral frequencies indicate the formation of 2, 4, 6-trihydroxymethyl phenol as precursor of the resin (Poljansek et al., 2005).The IR spectrum of lignin shows absorption at 1506 [cm.sup.-1], 1411 [cm.sup.-1] and 1506 [cm.sup.-1]. The absorption bands for resins with lignosulfonates showed the presence of OH functional group at 3033-3136 [cm.sup.-1]. Besides, stretching vibration of aromatic compound can be seen in a wide adsorption band at 1506.64-1522.14 [cm.sup.-1] for all resins. The presence of these functional groups is the products of the reaction between phenol-lignin adduct and formaldehyde (Mohamad Ibrahim et al., 2007).

3. CONCLUSION

Our research was focused on the synthesis of some new adhesives based on phenol-formaldehyde resins with aluminum, calcium and iron lignosulfonates, as substitutes for toxic monomers phenol and formaldehyde.

[FIGURE 1 OMITTED]

The improvements achieved by using the lignosulfonates components in resins (due to the polyphenolic structure of lignin, and the presence of carbonyl groups), consist on decrease in adhesive viscosity (which will insure better wettability of wood particles), a better water resistance of finished boards, the decreasing of the reactivity (for phenol type resins), and the increasing of the pot life of these new kind of resins. The reason for their application has to be seen also in the lowering of costs, resulting from the difference in cost between monomers and lignin derivative, a raw material. The possibility of obtaining of new type of wood adhesives based on the lignin derivatives should to gain a great interest in the future, taking into account both, the need for using of some raw materials as reactants for organic synthesis, and also for medium protection, because it is possible to use a wide variety of lignocellulosic materials, low-quality wood species and sawdust, and low-value lignin products. Due to increasing economic and environmental issues concerning the use of petrochemicals, lignocellulosic materials will be relied upon as feedstock for the production of chemicals, fuels and biocompatible materials. Nowadays progress has been made in the development of new engineering materials from lignocellulosic residues, such as lignosulfonates. Expanding research in the field of renewable resource materials our future research will be dedicated to synthesis of new wood adhesives based on metal complexed lignosulfonates at laboratory and pilot scale.

4. ACKNOWLEDGEMENTS

We would like to be thankful to ANCS for the financial support through IDEI 839/2009 grant.

5. REFERENCES

Bousoulas, J.; Tarantili, P. A.& Andreopoulos, A. G. (2001). Resole resin as sizing agent for aramid fibres. Advanced Composites Letters, Vol.10, No. 5, (September--October 2001) 249-255, ISSN: 0963-6935

Mohamad Ibrahim, M.N; Ghani, A. Md. & Nen, N. (2007). Formulation of lignin phenol formaldehyde resins as a wood adhesive. The Malaysian Journal of Analytical Sciences, Vol 11, No. 1, (January - June 2007)213-218, ISSN 1394-2506

Muzaffar, A. K; Sayed, M. A & Ved, P. M. (2004). Development and Characterization of Wood Adhesive using Bagasse Lignin. International Journal of Adhesion & Adhesives, Vol. 24, No. 6, (December 2004) 485493, ISSN 0143-7496

Nihat, S. C & Nilgul O. (2002). Use of Organosolv Lignin in Phenol-Formaldehyde Resin for Particleboard Production, International Journal of Adhesion & Adhesives, Vol. 22, No. 6, (December 2004) 477-480, ISSN 0143-7496

Poljansek, I. & Krajnc, M. (2005). Characterization of Phenol-Formaldehyde Prepolymer Resin by In Line FT-IR Spectroscopy, Acta Chim. Slov., Vol.52, No. 3, (September 2005) 238-244, ISSN: 1580-3155

Tab. 1. Chemical characteristics of the metal complexed
lignosulfonates

Characteristic LSAl LSCa LSFe

Appearance brown brown Brown
 liquid liquid liquid

pH- value 3.07 4.72 2.24
Solids, % 33.99 38.58 35.47
Density at 200C, g/[cm.sup.3] 1.1400 1.0250 1.1500
Viscosity at 20[degrees]C, cP 66 70 68
Ash, % 2.69 12.57 5.28
Cation, % 7.62 9.20 6.70
Functional
groups, %:
-OH phenolic 18.74 11.22 13.35
-OH alcoholic 16.50 15.23 16.06
--carbonyl 2.66 1.35 9.35
--carboxyl 0.72 0.56 0.74

Tab. 2. The characteristics of the new adhesives based on phenol,
formaldehyde (PF) and aluminum, calcium and iron lignosulfonates

 Solids Viscosity Miscibility Reactivity
Resin % pH cP + water 160[degrees]C, s,

PF 0 45-50 8.5-11 100-200 2:1 180
PF1 a 47.5 9.5 158 2:1 150
PF1 b 48.5 10.0 170 2:1 130
PF1 c 50.5 10.0 180 2:1 120
PF2 a 47.6 10,5 164 2:1 145
PF2 b 49.0 10.0 170 2:1 135
PF2 c 51.5 10.0 180 2:1 125
PF3 a 46.5 10.5 166 2:1 145
PF3 b 48.5 10.5 176 2:1 130
PF3 c 50.5 10.0 184 2:1 120