Ecomaterials for wood preservation based on acrylic copolymers and lignin derivatives.

Dumitrescu, Lucia ; Manciulea, Ileana ; Sica, Mihaela 等

1. INTRODUCTION

Wood has been used as an engineering material because it is low in cost, renewable and strong and it requires low processing energy. Wood has many excellent properties that result from its exceptional combination of microstructural, ultrastructural and molecular features. The main drawback is its dimensional instability in the presence of moisture, which can affects also the biological stability.

Wood is formed basically of cellulose, hemicelluloses and lignin, with a minor amount of extraneous materials (tannins, terpenoids, starch, lipids, alkaloids, proteins, etc.).

These three principal constituents make up cell walls and are responsible for most of the physical and chemical properties of wood.

Wood is susceptible to degradation in a number of ways, including rot or decay, insect attack, fire and weathering. Each of these forms of degradation is essentially chemical in nature and can therefore potentially be inhibited by chemical means.

Wood preservation is a complex process involving a wide range of physical, chemical and biological factors, demanding much from the chemicals selected for this purpose (Militz, et al., 1997).

As an alternative to the protection of wood with toxic chemicals that inhibit the activity of deteriogenic organisms, research into chemically modifying the wood substrate as a means of preventing deterioration is a current area of study. The intention is to alter the chemical structure of wood components so that enzymes are unable to degrade the modified holocellulose and lignin, and yet to leave the physical and engineering properties of the material more or less unchanged (Militz, et al., 1997; Hon, 1996).

Based on the variety of functional groups (hydroxyls, carbonyls, carboxyls etc), etherification, esterification, alkylation, hydroxyalkylation, graft copolymerization, crosslinking and oxidation reactions can be conducted to produce ecomaterials with many practical applications (Hon, D.N. S. 1996; Hon, D.N.S.et al., 2001).

2. EXPERIMENTAL

Taking into account these aspects, as well as the main demand regarding the environmental protection, our research research was focused on the development of some new wood preservation agents based on acrylic copolymers with extracts obtained from the bark of poplar, pine and acacia trees and copper sulphate.

The ideea of the utilization of such lignin derivatives as reaction partners into the emulsion polymerization of acrylic comonomers is based on the observation that the third major wood component, lignin also possess biocide activity and was proved to be the major fixation site for certain wood preservation components, such as copper.

The extracts obtained from the bark of poplar pine and acacia trees have also been used as reactive comonomers in emulsion copolymerization of acrylic comonomers because they possess a reactive chemical potential, caused by the presence in the structure of lignin (as a major wood component) of some functional groups such as: phenolic and alcoholic hydroxyl, carbonyl, carboxyl etc (Hon, 1996; Hon, D.N.S.et al., 2001).

The chemical characteristics of the poplar, pine and acacia bark extracts used in the synthesis, determined following the methodology specific for lignin (Zakis, 1994) are presented in the Table 1.

Both the acrylic copolymers and the copolymers modified by copper sulphate and poplar, pine or acacia bark extracts have been obtained using a semicontinuous technique emulsion copolymerization.

The copolymerization processes have been done in a one litter glass reactor, equipped with a paddle type stirrer, thermometer and cooler, conforming the following standard recipe:

Monomer composition:

--ethyl acrylate, acrylonitrile, butyl acrylate, acrylic acid

--poplar, pine or acacia bark extracts

Emulsifying system:

--ionic and nonionic emulsifier

Initiation system:

--sodium persulphate

Dispersion medium:

--distilled water

--copper sulphate

Percentages of 8% from the acrylic comonomers have been substituted by bark extracts from poplar, pine and acacia trees extracts.

The acrylic comonomers emulsion, with or without bark extracts and copper sulphate, was dozed by a constant ratio, at 80[degrees]C, for 2.5-3 hours into the copolymerization autoclave. At the end of the process, the inner temperature was raised at 90[degrees]C and kept for an hour, in order to finish the copolymerization.

The chemical characteristics of the acrylic copolymers proposed as wood preservatives are presented in Table 2.

3. RESULTS AND DISCUSSIONS

Considering the biocide activity of acrylic monomers, copper sulphate and the major wood component, lignin, the obtained copolymers were biologically investigated and proposed as wood preservatives.

The wood preservation agents based on the acrylic copolymers (CP1) and the acrylic copolymers with bark extracts from poplar (CP2), pine (CP3) and acacia (CP4) were submitted to the biological testing.

The testing of the biocide activity of the wood preservation proposed agents consisted on treating the sapwood samples by immersion in the wood preservations agents for 30 minutes and exposing the treated samples against the attack of the following microorganisms: Chaetomium globosum Kunze, Paecilomyces varioti Brainier, Stachibotrys atra Corda, Alternaria tennuis Ness, Trichoderma viridae Person ex. Fries.

After 28 testing days, the samples were examined using an optic microscope, in order to establish the attack level of the above mentioned microorganisms.

The fungal growth was classified (according to STAS 8022/91) between 0 and 4, as following:

0 = no growth;

1 = trace of growth detected visually;

2 = slight growth or 5-20% coverage of the total test area;

3 = moderate growth or 20-50% coverage;

4 = plenty of growth or above 50% coverage.

The results of the biocide activity of the wood preservation proposed agents is presented in Table 3.

4. CONCLUSIONS

Our research has established the existence of some functional groups (hydroxyl, carbonyl, carboxyl) bonded into the structure of polymers cellulose, hemicelluloses and lignin, main components of the wood structure, which confer to these wood wastes a specific chemical reactivity.

As consequence, bark extracts from poplar, pine and acacia trees, in amount of 8% (based on the total monomer content) were used in the copolymerization of the acrylic comonomers: ethyl acrylate, butyl acrylate, acylonitrile, acrylic acid.

An interesting aspect of this copolymerization process is those regarding the emulsifying capacity of the bark extracts which improve the emulsion stability.

The new preservation agents present better tensile breaking strength and better water absorption of the copolymer film, essential condition for the biocide preservation which depends on the water content and on the easy accessibility of the microorganisms at the wood surface.

These aspects of the emulsion copolymerization of the acrylic comonomers with extracts from poplar, acacia and pine trees, as well as the biocide activity of the copolymers increase the interest in the new research field of emulsion copolymerization of classic monomers with lignin based derivatives and the possibility for obtaining some new wood preservation agents (Dumitrescu et al., 1996).

The biocide activity of the copolymers seems to be influenced by the lignin content of the bark extracts and varies in order: copolymer with acacia extract (CP4)>copolymer with pine extract (CP3)>copolymer with poplar extract (CP2)> acrylic copolymer (CP1).

The new ecomaterials based on acrylic copolymers and bark extracts, will assure a better preservation action than the chemical agents as such, because they also contain lignin derivatives with biocide activity (Dumitrescu et al., 1996).

Our future research will be dedicated to identify and use other wood wastes capable (due their chemical structure) to act as reaction partners in synthesis of bioprotection and finishing agents, which will confer a better access and compatibility with the wood structure.

5. REFERENCES

Dumitrescu L., Popa V. I., Baciu G.& Petrovici V.(1996). Wood preservation agents based on acrylic copolymers and metal complexed lignosulphonates. Bulletin of the Transilvania University of Brasov. Series B. 3(38): 6368. ISSN 1223-964X.

Hon, D.N.S. (1996). Chemical Modification of Lignocellulosic Materials. Mark Dekker, Publishers, New York, NY.

Hon, D.N.S., and N. Shiraishi. (2001). Wood and Cellulosic Chemistry. 2nd edition. Marcel Dekker Publishers, ISBN 0824700244, New York, NY.

Militz, H.; Beckers, E.P.J. & Homan, W.J. (1997). Modification of solid wood: research and practical potential.IRG Document WP/97-40098.

Zakis, G. F. (1994). Functional analysis of lignins and their derivatives. TAPPI Press, ISBN: 0-89852-258 7, Atlanta, Ga.

Tab. 1. The chemical characteristics of the poplar, pine and
acacia bark extracts

Characteristic Poplar Pine Acacia
 extract extract extract

Appearance brown brown Brown
 liquid liquid liquid

pH- value 4.90 4.72 3.80

Solids, % 45.00 38.50 40.50

Viscosity at 72.50 75.30 69.20
20[degrees]C,
cP

Cellulose, % 29.35 31.85 26.80

Lignin, % 33.55 34.80 36.75

Pentosans 20.20 12.55 18.25

Tab. 2. Chemical characteristics of the acrylic copolymers
proposed as wood preservatives.

 Tensile Water
Copolymer Viscosity breaking absorption
 at 20 strenghts of the film
 [degrees]C cP MPa (%) after

 24h 48h

CP1 36.5 2.5 17.4 37.5
CP2 38.0 3.0 15.5 35.0
CP3 41.8 3.4 14.5 30.6
CP4 52.5 3.8 12.0 25.5

Tab. 3. The results of the biological testing of the
proposed wood preservation agents.

Treatment type Degree of attack Note

Treatment with Samples attacked in 4
 CP1 extent of 80%

Treatment with Samples attacked in 2
 CP2 extent of 18%

Treatment with Samples attacked in 1
 CP3 extent less than 4%

Treatment with Samples attacked in 1
 CP4 extent less than 2%