Journal of Scientific & Industrial Research

Vol. 6 \ . June 2002. pp 444-448

Chemical Modification of Wood

C C Menon

Devi Nivas. Edathil Road.Tellicherry 670 \ 0 \

Received: 1 5 October 200 \ ; accepted: 25 February 2002

Three different processes are delineated. enabling the transformation of plantation wood like rubber into superior qual­ity equalling hardwood. These processes are based on: (i) Ligneous Reactive Diluent derived from coconut shell. (ii) Resin

composition based on cardanol. phenol. l inseed oil and formalin. and (iii) Catalysed linseed oil. The properties of the modi­fied wood and some other inferior grade timbers are given. The development of novel fibreboard. namely Weather-proof hardboard with remarkable strength and durability is also presented. Since the raw materials are derived from regenerative re­sources. the development constitutes an attractive solution to the raw materials crisis in the wood processing industry.

Introduction

The demands on the world's forest resources have been ever-increasing with the increase in popu­lation. Due to the consequent desperate efforts to marshall fuel, fodder, land, and materials of construc­tion the forests have been denuded at an alarming rate viz 40-50 mha/y.The velocity of denudation has been estimated to be an acre/so

This trend has, however, been reversed by the emergence of environmental movement in the 60s which served to highlight and impress the importance of integrity of biosphere for the survival of life on the planet. But the increasing restrictive measures en­forced for forest conservation have caused a sharp decline in the availability of forest resources required by the wood processing industry which i s in the throes of raw material crisis. Plantation wood, a regenerative resource grown outside the forest system has, therefore, emerged as an attractive raw material choice crucial for the continuity of operations in the wood processing industry. Rubberwood available to the extent of 1 .5 million cftly has, therefore, acquired the status of a viable raw material option.

The main drawback of rubberwood is its suscep­tibi lity to biodeterioration by the action of borers, fungi and termites. Boron compounds are generally used as preservatives. However, boron is not fixed in the wood and is liable to be leached. Boron treated rubberwood is hence unsuitable for exterior appli­cationl.3 •

Boron is an important trace element, crucial to the growth of plants and animals4. A particularly cu­rious aspect of boron is the narrow range between deficiency and toxicit/. For many organisms it is toxic at the level of 1 ppm while many tolerate it even at 1 5 ppm5. Seawater contains an average of 4-5 ppm and the permissible criterion for public water supply is I mgIL6. Boron toxicity has been reported when sewage containing boron was used as fertiliser and the phytotoxic effect of boron has been manifested by composts made from wood waste containing boron 7 . Water containing more than 4 mg/L of boron is unsat­isfactory for all crops. Recently, boron has been im­plicated in the testicular atrophy of mammals. Since the toxicity of boron is displayed at extremely low concentrations there is debate whether pollution con­trol measures at 1 mgIL levels are possible at reason­able cost8•

Therefore, an important step from the perspec­tive of wood science and forest products technology is the development of suitable processes to impart resistance to the biodecay of rubberwood without the use of boron. An enhancement of the attractiveness of the processes is evident if the developed processes cause collateral improvement of physical and me­chanical properties of rubberwood so as to upgrade it as a suitable substitute to durable hardwood.

Work with these dual objectives was undertaken by the utilisation of reactive chemical component from the plant kingdom as the principal compound. The impetus for the preference of reactant from plant kingdom was provided by consideration for conserva-

MENON: CHEMICAL MODIFICATION OF WOOD 445

tion of scarce resources l ike petroleum and the attrac­tive opportunities offered by the chemistry of lignin. Lignin is second only to cellulose with respect to abundance in the plant kingdom. A polyphenolic body it is recognised as 3-D polymer of 3-methoxy 4-hydroxy phenyl propane with a plethora of functional groups. The complexity of 3-D structure with su­pramolecular aspects invests the compound with wide differences in reactivity, depending on source. It is a byproduct of the paper industry with abundant avail­abil ity. Yet, i t has not made significant impact on thermosetting resin technology. But l ignin from co­conut shell was found to be ideally suited as coreac­tant for phenolic and urea resin adhesives deployed in the production of wood based laminates, being cost­effective without any adverse impact on qualit/. A process was hence developed, involving the vacuum impregnation of seasoned rubberwood with a me-

Table 1- Properties of rubberwood and chemically modified rubberwood (Process I )

Property Rubber Chemically

I Density g/cc

2 Volumetric shrinkage per cent

3 Modulus of rupture kg/cm2

4 Modulus of elasticity kg/cm2

5 Impact strength kgmlcm2

6 Tensile strength kg/cm2

7 Rockwell hardness

8 Screw holding capacity kg

wood modified rubber

0.5 1 1

1 .75

787.5

78625

0.35

633

1 60

1 75

wood

0.566

1 .32

997.5

97326

0.52

673

1 60

255

Table 2- Data relating to resistance of chemically modified rubberwood to weathering by exposure in open space for 90 d

at the peak of monsoon season

Property Prior to expo- After expo-sure sure

I Density g/cc 0.65 0.65

2 Volumetric shrinkage per cent 1 1 1 77 1009

3 Modulus of rupture kglcm2 97326 90989

4 Modulus of elasticity kg/cm2 620 552

5 Impact strength kgmlcm2 500 402

6 Tensile strength kglcm2 0.58 0.38

7 Rockwel l hardness 1 60 1 42

8 Screw holding capacity kg 255 204

lange of l igneous reactive diluent from coconut shell containing a small quantity of low molecular weight phenolic resin to function as bridging component9. The impregnated wood was then dried by a sequential operation. The resultant product, chemically modified rubberwood, was subjected to exhaustive testing for product quality. Resistance to biodeterioration was examined as per B IS 4873 and 4833. Test Report from Indian Plywood Industries Research and Train­ing Institute, Bangalore confirms conformity to na­tional standard regarding resistance to biodeteriora­tion. The physical and mechanical properties (Tables 1 -3) indicate significant improvements point­ing to upgradation and attainment of parity to durable hardwood. These findings were further confirmed by test results from Kerala Forest Research Institute, Peechi and Forest Research Institute, Dehra Dun.The process was adopted for industrial use. It is hoped that this technical development, fostering an alliance between coconut and rubberwood augurs well for the two ailing sectors of the plantation economy.

An extension of the work was the development of a diffusion process, instead of impregnation, using a resin system based on cardanol, an ingredient of cashew shell l iquid. Cocondensation of cardanol with phenol, l inseed oil, and formalin gives a resin syrup. Immersion of seasoned wood in the resin syrup fol­lowed by heat treatment gives chemically modified wood (Tables 4- 1 1 ) .

A further simplification of the work was with the util ization of a single ingredient namely l inseed oil . Linseed oil due to unsaturation is susceptible to oxidative polymerization when exposed to air. The process involves copolymerisation with atmospheric oxygen, addition polymerization, copolymerisation, cyclisation and even Diels-Alder reaction. Activation

Table 3 - The properties of densified chemically modified rubberwood with comparitive data of densified durable hardwood

Property Densified Densified hardwood chemically

modified rubberwood

I Density glcc 1 .2 1 . 1

2 Modulus o f rupture kglcm2 1 650 1 699

3 Impact strength kglcm2 0.3 1 0.3 1

4 Water absorption in 24 h per cent 2 5

5 Tensile strength kglcm2 1 375 1 250

6 Compressive strength kglcm2 843 1081

7 Modulus of elasticity kg/cm2 1 ,43,943 2,97.743

446 J SCI IND RES VOL 6 1 JUNE 2002

of the process can be accomplished using cobalt and manganese naphthenates.

The oil imbibed wood was dried by a sequential process. The product was tested as per IS standard . Property improvements were indeed tangible (Table 1 2). A 6 ft long dining table made with the material is displayed in the Indian Plywood Industries Research and Training Institute, Bangalore as anex­hibit. The product has remarkable resistance to biode­terioration as well as action of water. The properties of the product indicate suitabil ity for making beams, decking, axles, poles, tool handles, columns, ports, furniture and flooring.

Table 4 - Data relating to resistance of chemically modified rubberwood to decay fungi. molds and termites

Period of exposure: 1 2 Weeks

Decay tests Weight loss per cent

Test fungi Treated Untreated

Coriolellus Palustris 4.8 35

Poria mOlllicola \ .0 20

Corioles sangL/irius 1 .0 28

Corioles versicolor 2 32

Moldfungi 0.3 1 .35

Termite tests Period of exposure: 24 weeks

Test organism Weight loss per cent

Treated Untreated

Ondontolermes sp Nil 60 per cent

Result Treated samples are found to be resistant to decay fungi. molds and termites for the test period

Table 5 - Properties of rubberwood before and after transfor­mation by Process "

Property Before After

I Moisture content 1 3 per cent 1 1 per cent

2 Tensile strength 500 kg/cm2 80 1 kglcm2

3 Compressive strength 380 kg/cm2 5 1 4 kglcm2

4 Modulus of rupture 8 1 8 kg/cm2 974 kglcm2

5 Modulus of elasticity 1 0 1 882 kglcm2 99964 kg/cm2

6 Density 0.68 g/cc 0.73 glcc

7 Water absorption 24 h 38.8 per cent 40 per cent

8 Swelling Lengthwise 3 per cent 1 .5 per cent Breadthwise 2.7 per cent 1 .92 per cent Thicknesswise 6.5 per cent 2 .2 per cent

Materials and Methods

Phenol and formalin were procured from Hindu­stan Organic Chemicals Ltd, Udyogamandal , Cochin Caustic soda was obtained from Travancore-Cochin Chemicals Ltd, Udyogamandal , Cochin.Doubly boiled linseed oil was procured from OiIchem India, Indore.Cardanol was obtained from Card-Chern Ltd, Hyderabad.Cobalt and manganese naphthenates were obtained from Veekay Chemicals, Dombivili , Maha­rashtra.

Table 6 - Properties of Accacia before and after transformation by Process I I

Before After

I Density g/cc 0.94 0.96

2 Modulus of rupture kg/cm2 1 200 1 26 1

3 Tensile strength kg/cm2 528 9 1 8

4 Water absorption per cent i n 24 h 29.5 1 3

5 Compressive strength kg/cm2 3 85 663

6 Thickness swelling per cent 1 . 1 62 0.77

Table 7 - Properties of Mango (Mangifera Indica) before and after transformation by Process "

Density g/cc

Modulus of rupture kglcm2

Water absorption

Per cent in 24 h

Compressive strength kg/cm2

Before After

0.49 0.62

2 1 0 250

52 49

80 1 70

Table 8 - Properties of Ekaba before and after transformation by Process II

Before After

Density g/cc 0.57 0.64

Modulus of rupture kglcm2 560 7 1 4

Water absorption per cent in 50 1 4 2 4 h Tensile strength kg/cm2 268 534

Compressive Strength kg/cm2 282 3 1 7

Thickness swelling per cent 0.4 1 0.26

Table 9- Properties of Vaga (A. Lebbeck) before and after transformation by Process I I

Before After

Density g/cc 0.5 0.62

Modulus of rupture kg/cm2 34 1 344

Water absorption per cent in 24 h 53 50

Compressive strength kg/cm2 2 1 6 308

MENON: CHEMICAL MODIFICATION OF WOOD 447

Process II Ligneous reactive diluent was procured from Kontiki Chemicals and Pharmaceuticals Ltd, Balia­patam, Kannur. Samples of different seasoned woods were obtained from Western India Plywood Ltd, Baliapatam,Kannur.

Experimental Procedure

Process I

90 parts of cardanol, 9 parts of linseed oil, 14 parts of formalin, 2 parts of caustic soda dissolved in 3 parts of water were reacted at 90 °C until the resin syrup had a flow time of 15 s in a B-4 cup. It was then cooled and 8 parts of manganese naphthenate and equal quantity of cobalt naphthenate were added to obtain the resin syrup.

Seasoned wood was fed into a vacuum impreg­nation chamber. The impregnating solution consti­tuted of 90 per cent of reactive lignin diluent and 1 0 per cent of phenolic resin was fed into the chamber. The phenolic resin was made by alkali catalyzed reac­tion of 4.5 parts of phenol and 7.5 parts of formalin at 90 °C. Catalyst solution was made by dissolving 2 parts of caustic soda in 5 parts of water. The reaction time was I h when the resin syrup attains a flow-time of I Ss in a B-4 cup.

Table 10 - Properties of Thodayarn before and after transformation by Process I I

Before After

Density g/cc 0.5 0.62

Modulus of rupture kglcm2 341 344

Water absorption per cent in 24 h 53 50

Compressive strength kg/cm2 2 1 6 308

After the feeding of the impregnating solution the system was maintained for 8 h. The impregnated wood was then unloaded and left for drying in open air for 24 h. It was then fed into a drier where a se­quential drying was carried out for 8 h in the tempera­ture range 50-900 C. The dried wood was subjected to testing as per IS: 1 708 (P 1 to 1 8) - 1986.

Table 1 1 - Properties of weather-proof hardboard made using Process I I

Density glcc

Water absorption per cent after 24h soaking

Modulus of rupture kg/cm2

Water absorption per cent after one week in sea water

Table 1 2- Physical and mechanical properties of chemically modified rubberwood made using Process I I I

1 . 1 8

3 .6

669 along 622 across

1 3

Property Rubber wood Chemicaly modified rubberwood

1 Density 0.6928g1cc 0.7079 glcc

2 Moisture content 12 .43 per cent

3 Water absorbtion in 24 h

4 Shrinkage Lengthwise 0.64 per cent

Widthwise

Thicknesswise

5 Modulus of rupture

6 Compressive Strength

7 Tensile Strength

8 Block shear Strength

9 Screw holding strength

10 Nail holding strength

1 1 Hardness

12 Impact strength

1 3 Swelling Lengthwise

Widthwise

Thicknesswise

9.96 per cent

45.28 per cent

0.429 per cent

2.32 per cent

1 .835 per cent

766.3 kg/cm2

347.9 kglcm2

476.6 1 kg/cm2

46.32 kglcm2

248 kg

1 45 kg

1 05

0.45 kgrnlcm2

0.75 per cent

0.7 per cent

1 .23 per cent

1 8.48 per cent

1 .906 per cent

2.059 per cent

993.7 kglcm2

5 1 3.36 kg/cm2

724 kglcm2

1 1 1 .2 kglcm2

2 1 0 kg

100 kg

1 1 4

0.41 kgrnlcm2

0.367 per cent

0.7 1 per cent

1 .52 per cent

448 J SCI IND RES VOL 6 1 JUNE 2002

Seasoned wood was kept immersed in the resin syrup for 24 h .It was then taken out and left in open air for 7 d for drying. Afterwards the wood was sub­jected to sequential drying for 8 h at a temperature range of 50 to 1 000 C. The product was tested as per IS standards.

In the production of weather-proof hardboard, the standard hardboard was kept immersed in resin syrup for 24 h. It was then removed and left for dry­ing in air at room temperature for a week. It was then pressed at a pressure of 1 40 psi at 1 000 C for 4 min.

Process III

Linseed oil was mixed with 6 per cent of cobalt naphthenate solution and 8 per cent of manganese naphthenate solution. Seasoned rubber wood was kept immersed in the mixture for 8 h. The wood was then kept exposed to air for a period of 7 d. It was then subjected to a sequential drying operation in the tem­perature range 30- 1 00 dc.

Results and Discussion

Amongst the products developed weather-proof hardboard deserves special mention. It has remark­able resistance to weathering besides pocessing excel­lent strength parameters. A gabled-roof structure hav­ing an approximate area of 700sf using the product as roofing has withstood the onslaught of Kerala mon­soon for several years without any damage. Further the escalating cost of superior quality timbers needed for manufacture of marine plywood is a serious set­back to its consumption. An exterior grade fibreboard environmentally acceptable because of derivation from regenerative resource is an attractive substitute. Representation has been made for approval of the

I · to qua tty parameters .

The improvement of the durability of wood un­der conditions of exterior use can be achieved by the use of copper - chrome - arsenic combination devel­oped in India or by chloropyriphos. The wood treated with the inorganic combination on expiry of the pe­riod of util ity is usually recycled as landfi l l or incin­eration. Often, i t has l ikel ihood of reaching the hearth of the low-income group as fuel . The fumes generated on combustion containing arsenic may inpair respira­tory health. Further the ashes containing chromium and copper constitute a vector of land and water pol­l ution. As regards chloropyriphos recently some SllS­picions have been roused relating to biodegradability and toxicity. Therefore the need for cost - effective and environmentally more benign processes for pre­servative treatment of wood intended for exterior use has emerged. Hence the processes described using indigenous ingredients constitute an advance in the relentless quest in wood S&T in the sphere of pre­servative treatment.

References

1 . Lacko A G & Palardy J E, Proc Inst Wood Res (Michigan Technological University) 153 ( 1 992) 964/487, 2464.

2. Wil leitner R, Holz!orschung, 49 ( 1 995) 3 . 3. Horne R A, Chemistry of Environment(Wiley-lnterscience,

New York) 1 978, 393. 4. Lund H E, Industrial pollutioll control handbook (McGraw­

Hi l l , New York), 1 993, 4-3 1 . 5 . Lund H E, Industrial pollutioll control halldbook (McGraw-

Hi l l , New York) 1 993, 4-35. 6. Liversidge A, Aust Forest lnd J, 38 ( 1 975) 42. 7. Bradford A F, J Environ Qual, 4 ( 1 975) 1 23 . 8 . Menon C C, Proc Nat Semin - Process Util Plallt Timber

Bamboo ( IPIRTI, Bangalore) (July 1 998) pp 1 5 8- 1 63 . 9. Menon C C, Chemcon 2000, Indian Chem Eng Can/,

December 2000, Kolkata, Technical Session, Transcription, 2 (2000) CS 4 1 .

1 0. Agenda for CED20, Bureau of Indian Standards, November 1 999. Bangalore.