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Chemical Corrosion Effecton Wood and Wood–PlasticCompositesM. A. Khan a , K. M. Idriss Ali a & M. U. Ahmed ba Radiation Chemistry Laboratory , Institute ofNuclear Science and Technology Atomic EnergyResearch Establishment , PO Box 3787, Savar, Dhaka,Bangladeshb Department of Chemistry , JahangirnagarUniversity , Savar, Dhaka, BangladeshPublished online: 22 Sep 2006.

To cite this article: M. A. Khan , K. M. Idriss Ali & M. U. Ahmed (1993) ChemicalCorrosion Effect on Wood and Wood–Plastic Composites, Polymer-Plastics Technologyand Engineering, 32:4, 355-365, DOI: 10.1080/03602559308019241

To link to this article: http://dx.doi.org/10.1080/03602559308019241

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P0LYM.-PLAST. TECHNOL. ENG., 32(4), 355-365 (1993)

CHEMICAL CORROSION EFFECT ON WOOD AND WOOD-PLASTIC COMPOSITES

M. A. KHAN and K. M. IDRISS ALI

Radiation Chemistry Laboratory Institute of Nuclear Science and Technology Atomic Er;ergy Research Establishment PO Box 3787, Savar, Dhaka, Bangladesh

M. U. AHMED

Department of Chemistry Jahangirnagar University Savar, Dhaka, Bangladesh

Abstract

Effect of chemical corrosion on the tensile strengths of five types of Bangladeshi timbers (kadom, simul, koroi, mango, and deb- daro) and their composites has been evaluated. Wood-plastic composites (WPC) formed by the gamma-radiation induction po- lymerization of butylmethacrylate (BMA) with those timbers show better resistance to chemical corrosion attack than the par- ent timbers. Enhanced mechanical properties such as tensile strength of the composites are also protected during corrosion.

INTRODUCTION

Wood is widely regarded as a versatile, useful, and valuable industrial and engineering material. As a matter of fact, it is difficult to suggest

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Copyright 0 1993 by Marcel Dekker, Inc

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356 KHAN, ALI, AND AHMED

another material possessing such diverse properties and basic utilities as wood. Although modern technology has been successful in replacing wood in many ways with light metals and plastics, it has never been possible to supercede the versatile utility of wood. In spite of this versa- tility, there are some inherent characteristics which tend to restrict its versatile and potential applications: These drawbacks are dimensional instability; low resistance to abrasion; ease of combustion and attack by fungus, microbes, and insects. The properties of wood have, thus, been improved in order to withstand these drawbacks through forma- tion of wood-plastic composites (WPC) (Khan and Ali, 1992a). The composites are normally formed not only by filling the void and porous spaces of wood but also by grafting the wood fiber with monomer units. The quality of a WPC depends on the type of monomers and timbers used as well as on the process of polymerization. Wood-plastic com- posite materials are being industrially manufactured on a wide scale in Europe, the USA, and Japan. These can be suitably used as walkway and flooring materials in chemical plants, since composites possess useful properties of resistance to chemical corrosion. The effect of chemical corrosion on composites of some wood has been studied (Spindler, Pateman, and Hills, 1973). The present investigation deals with chemical corrosion studies on the mechanical (tensile strength) properties of composites prepared with different timbers of Ban- gladesh.

EXPERIMENTAL

Five varieties of wood of Bangladesh were used and their physicome- chanical properties are given in Table 1. Samples of sizes 6.2 x 0.8 x 0.4 crn were selected from long, flat-grained planks and were uniformly polished with sandpaper. Free water was removed from the samples by heating them at 60°C for 24 h under vacuum at 50 mm Hg, and moisture content was found to be 10-13% w/w. Wood composites were prepared by using butyl methacrylate (BMA) without removing the added stabilizer from the monomer. The dose used was more than 1 Mrad below the point at which the stabilizer might inhibit the results. Wood samples were impregnated by soaking them for 15 h in the mono- merimethanol solution (90: 10, v/v), under vacuum at 50 rnm Hg. Sam- ples were wiped smooth with tissue paper, encapsulated in polythene bags, and irradiated with Co-60 gamma-rays (50 kCi) for a total dose

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CHEMICAL CORROSION EFFECT 357

TABLE 1 Physical and Mechanical Properties of the Woods Studied

Property"

Local d YM TS Vf name Botanical name (g/cm3) (Ksi) (Ksi) (cm3)

Kadom Anthrocephalus cadamba 0.36 129 2.12 0.76 Simul Salrnalia mnlabnrica 0.40 146 2.43 0.74 Koroi Samanea samane 0.44 155 2.84 0.71 Mango Mangifera indica 0.51 170 3.43 0.67 Debdaro Polythia longifolia 0.71 214 0.10 0.53

a d = density of wood; YM = Young's modulus; TS = tensile strength; Vf = free pore volume of the wood, calculated theoretically.

of 3 Mrad at 800 krad/h. Polymer loading was determined by the percent of weight gain of the sample.

Tensile Strength

This particular property was directly measured at room temperature under 70% to 80% humidity by using the tensile strength machine of Instron (model 101 1 , UK) integrated with a personal computer (Am- strad PC 1640 HD20, UK). The cross-head speed was 0.31 cm/min at a fixed gauge length of 1.54 cm. US standard testing methods (ASTM) were followed throughout the experiment.

Chemical Corrosion Test

Corrosive action by chemicals, acids, alkalis, salts, and solvents on wood and its composites can best be evaluated by soaking the sub- strates in them for a period until a constant gain in absorption is achieved. Flashing out the corrosive agents from the substrate with water may reveal the effect of corrosion on the substrate. This can be elaborated as follows:

Weight Gain

Duplicate samples of both composite and control were accurately weighed and placed in a closed but air-tight container of an appropriate

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358 KHAN, ALI, AND AHMED

solution which was either acid, alkali, inorganic salt, organic solvent, or oil. Samples were taken out of the container after every 7 days and wiped with tissue paper; the weights were recorded in order to monitor the gain in absorption of the solution. The sample was immersed again in the same solution. This process was continued until a constant weight was achieved by the sample at the equilibrium condition. The absorp- tion of solution was determined by the percent of weight gain at the equilibrium (Khan and Ali, 1922b). Results are given in Table 2.

Weight Loss

Samples were then taken out of the solution and washed several times with distilled water to bleach out the colored materials from the samples. This was done every day until the disappearance of the colored solution, estimated visually. Samples so treated were then dried at 105°C for 20 h, after which they were cooled at the ambient tempera- ture under normal condition. Their final weights were taken to deter- mine the corrosive effect on wood and its WPC; the final weight was determined until a constant weight of the corroded substrate was achieved. The equation used for weight loss determination is

% WL = lOO(W0 - WJW0

where W0 = dry weight of the sample; W, = dry weight after the treatment of washings; WL = % weight loss. Results are given in Table 2.

RESULTS AND DISCUSSION

Chemical corrosion tests were undertaken in order to investigate the effect of different chemicals on wood and WPC. These tests were per- formed with samples of five varieties of wood (kadom, simul, koroi, mango, and debdaro) and their WPC using a series of reagents in solu- tion. Among the reagents used for corrosive purposes there were five solvents, three acids (organic and inorganic) of different concentra- tions, one alkali solution of different strengths, six inorganic salts, and two edible oils. Samples of both treated (WPC) and untreated (reference wood) materials were kept soaked for more than a month in the reagent solution until there was maximum absorption of the solution in order

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CHEMICAL CORROSION EFFECT 359

TABLE 2 Weight Gain and Weight Loss by Wood and WPC on Chemical Corrosion

9% Weight gain, W; % Weight loss, W L ~

Reagents Wood WPC (PL) Rf Wood WPC Rf

Water Hydrogen peroxide Methanol Ethanol Xylene Acetic acid Hydrochloric acid (20%) Sulfuric acid (30%) Sulfric acid (10%) Sodium hydroxide (10%) Sodium hydroxide (5%) Sodium chloride (10%) Sodium chloride (5%) Ferrous sulfate (10%) Calcium chloride (10%) Copper sulfate (10%) Ferric chloride (10%) Soybean oil Mustard oil

Water Hydrogen peroxide Methanol Ethanol Xylene Acetic acid Hydrochloric acid (20%) Sulfuric acid (30%) Sulfuric acid (10%) Sodium hydroxide (10%) Sodium hydroxide (5%) Sodium chloride (10%) Sodium chloride (5%) Ferrous sulfate (10%) Calcium chloride (10%) Copper sulfate (10%) Femc chloride (10%) Soybean oil Mustard oil

Wood = Kadom 189 64 (54) 168 103 (30) 135 87 (22) I19 56 (22) 115 110 (22) 184 114 (17) 204 114 (25) 185 116 (27) 144 100 (18) 179 169 (20) 155 54 (35) 148 37 (70) 155 34 (35) 55 54 (22)

155 73 (47) 96 49 (52) 53 17 (74)

172 18 (33) 137 57 (50)

200 105 (47) 20 1 102 (13) 164 68 (78) 135 31 (78) 185 70 (20) 202 112 (20) 202 188 (09) 166 856 (97) 232 213 (09) 257 99 (92) 22 1 1 18 (92) 191 74 (47) 202 90 (92) 193 60 (22) 143 37 (92) 170 43 (68) 97 41 (68) 53 22 (73) 51 21 (73)

Wood = Simul

0.33 0.61 0.56 0.47 0.95 0.60 0.56 0.62 0.70 0.94 0.35 0.25 0.22 0.57 0.47 0.51 0.32 0.10 0.41

0.50 0.90 0.41 0.20 0.80 0.50 0.93 0.51 0.92 0.38 0.53 0.38 0.44 0.31 0.25 0.25 0.42 0.41 0.41

1.10 18.00 5.20

23.40 2.00 4.00

26.00 24.00 14.00 45.00 27.00 4.00 8.00 2.00 5.00 4.00

17.00 99.00 59.00

I .75 18.00 5.20

17.00 5.00 7.00

36.00 16.00 3.00

43.00 33.00 6.00

13.00 5.00 9.00 5.00

24.00 24.00 59.00

0.81 0.73 10.00 0.55 2.00 0.38 8.40 0.37 2.00 1.00 2.00 0.50

25.00 0.96 22.0 0.91 10.00 0.70 32.00 0.71 19.0 0.70 3.00 0.75 0.50 0.06 2.00 1.00 4.00 0.80 3.00 0.75

12.00 0.70 7.00 0.07

21.00 0.35

0.85 0.48 14.00 0.83 3.00 0.70 8.00 0.47 2.00 0.40 5.00 0.71

35.00 0.97 12.00 0.75 2.00 0.97

19.00 0.44 16.00 0.40 4.00 0.66 8.00 0.61 4.00 0.80 4.00 0.44 3.00 0.60

15.00 0.62 10.00 0.41 19.00 0.57

(continued)

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360 KHAN, ALI, AND AHMED

TABLE 2 Continued.

% Weight gain, W; % Weight loss, WLO

Reagents Wood WPC (PL) Rf Wood WPC Rf

Water Hydrogen peroxide Methanol Ethanol Xylene Acetic acid Hydrochloric acid (20%) Sulfuric acid (30%) Sulfuric acid (10%) Sodium hydroxide (10%) Sodium hydroxide (5%) Sodium chloride (10%) Sodium chloride (5%) Ferrous sulfate (10%) Calcium chloride (10%) Copper sulfate (10%) Ferric chloride (10%) Soybean oil Mustard oil

Water Hydrogen peroxide Methanol Ethanol Xylene Acetic acid Hydrochloric acid (20%) Sulfuric acid (30%) Sulfuric acid (10%) Sodium hydroxide ( 10%) Sodium hydroxide (5%) Sodium chloride (10%) Sodium chloride (5%) Ferrous sulfate (10%) Calcium chloride (10%) Copper sulfate (10%) Ferric chloride (10%) Soybean oil Mustard oil

Wood = Korai I04 102 (10) 127 86 (38) 73 55 (29) 67 37 (30) 75 36 (26)

104 51 (26) 105 93 (13) 106 98 (13) 107 98 (13) 153 128 (1 I ) 175 121 (10) 89 73 (10) 93 90 (92) 52 47 (13) 58 52 (11) 41 37 (09) 67 31 (09) 33 13 (35) 26 14 (35)

Wood = Mango

120 96 (14) 126 102 (29) 106 38 (44) 99 2344)

127 52 (29) 141 79 (29) I25 81 (38) I22 105 (32) I87 108 (38) 162 124 (12) 161 113 (16) 147 83 (14) I50 1 I6 (16) 92 66 (31)

150 44 (12) 102 50 (18) 93 37 (84) 90 70 (53) 70 19 (53)

0.98 0.67 0.75 0.53 0.48 0.49 0.88 0.92 0.91 0.83 0.69 0.82 0.97 0.90 0.90 0.90 0.47 0.39 0.53

0.80 0.81 0.36 0.20 0.41 0.56 0.64 0.86 0.57 0.76 0.70 0.56 0.77 0.71 0.29 0.49 0.39 0.77 0.27

0.97 40.00 6.00

16.00 10.00 7.00

29.00 15.00 15.00 43.00 29.00 6.00 2.00 5.00

21.00 5.00

21.00 20.00 14.00

2.50 23.00 12.90 10.50 3.00 5.00

3 I .OO 15.00 3 1 .OO 34.00 54.00 5.00 5.00 4.00

17.00 4.00

26.00 58.00 32.00

0.46 26.00 4.00 3.00 2.00 6.00

28.00 13.00 13.00 3.00

26.00 4.00 2.00 3.40 6.00 3.00 9.00 5.00 2.00

0.85 20.00 4.00 5.40 2.00 4.00

27.00 14.0 22.00 28.00 27.0 4.00 2.00 2.00 6.00 1 .OO

14.00 4.00 7.00

0.47 0.65 0.66 0.18 0.20 0.85 0.96 0.86 0.86 0.07 0.89 0.66 1 .oo 0.80 0.28 0.60 0.42 0.25 0.14

0.34 0.86 0.31 0.47 1.66 0.80 0.87 0.93 0.71 0.82 0.50 0.80 0.40 0.50 0.35 0.25 0.54 0.07 0.22

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CHEMICAL CORROSION EFFECT 361

TABLE 2 Continued. ~ ~ ~ ~ _ _ _

% Weight gain, W,. % Weight loss, WLO

Reagents Wood WPC (PL) Rf Wood WPC Rr

Water Hydrogen peroxide Methanol Ethanol Xylene Acetic acid Hydrochloric acid (20%) Sulfuric acid (30%) Sulfuric acid (10%) Sodium hydroxide (10%) Sodium hydroxide (5%) Sodium chloride (10%) Sodium chloride (5%) Ferrous sulfate (10%) Calcium chloride (10%) Copper sulfate (10%) Ferric chloride (10%) Soybean oil Mustard oil

Wood = Debdaro 137 71 (36) 0.51 131 100 (25) 0.76 105 57 (39) 0.54 99 IS (19) 0.15 99 79 (23) 0.79

153 100 (20) 0.65 126 104 (17) 0.82 144 100 (17) 0.69 117 102 (13) 0.87 225 144 (19) 0.64 167 54 (85) 0.41 126 57 (56) 0.45 127 29 (108) 0.23 101 89 (09) 0.88 130 43 (58) 0.33 122 50 (48) 0.41 100 50 (61) 0.50 65 18 (31) 0.32 55 27 (15) 0.49

I .33 15.00 5.00

43.00 6.00 8.00

33.00 30.00 17.00 53.00 32.00 5.00 4.00 4.00 6.00 5.00

21.00 24.00 24.00

0.19 0.14 12.00 0.80 4.00 0.80 2.40 0.04 4.00 0.66 7.00 0.87

26.00 0.78 27.00 0.90 15.00 0.88 40.00 0.80 21.00 0.65 4.00 0.80 0.34 0.08 3.00 1.75 5.00 0.83 3.00 0.60

13.00 0.62 2.00 0.08 5.00 0.21

" W, = 100 (We - WO)/WU; WL. = lOO(W0 - W,)/Wu. WO = oven dry weight, We = equilibrium weight on chemical treatment, W, = oven dry weight after treatment.

to attain its equilibrium weight. Percent weight gain of absorption of the reagent by both reference wood (C) and WPC samples was determined. Thus, the ratio (Rf) of the percent weight gain by the WPC to the percent weight gain by the reference wood (Rf = Wwpc/Wwood) is a measure of the absorption ability of the samples for the particular re- agent used. Results are given in Table 2. It is observed here that the WPC samples of all the five types of wood showed less absorption tendency toward all the corrosion reagents used. This signifies that the WPC product is a better material, to be considered in place of the corresponding wood for any particular application. The low-density wood and its composites have more absorption ability toward the solu- tion than the high-density wood and its WPC, respectively.

Similarly, weight loss of both WPC and reference wood samples caused by the corrosive effect of the reagents was also determined.

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362 KHAN, ALI, AND AHMED

Results are given in Table 2. Low ratio values of WPC samples (less than unity) indicate good resistance toward chemical attack. Treated samples (WPC) exhibited far less corrosive effect by the different types of reagents. This may be attributed to the fact that the composites are formed by filling up the void spaces with polymer which is generally inert toward chemical attack. Besides, the access of corrosive reagents into the void spaces is restricted by the presence of polymer/copolymer material built up into the spaces. Results of water absorption (Fig. 1) by the composite and the wood samples also show a similar trend.

6 0

- s - 4 0 c z w c z 0 u W K 3 e g 20 z

i 0 '

I I I 1 1 I

2 4 0 4 8 0 72 0

T I M E ( h r )

FIG. 1. Water absorption capacity of wood and WPC.

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CHEMICAL CORROSION EFFECT 363

Simul has density lower than mango and, thus, has more absorption capacity than the mango. This is due to the presence of more free volume space available with simul than with mango. This is reflected in their absorption profiles of water against time of soaking (Fig. 1). But this difference in absorption ability by both simul and mango is minimized when their composites are prepared and soaked in the same reagents.

The corrosive effect is dominant with acids, followed by alkali, in which almost all the samples (both composite and reference wood) became twisted, unlike in other reagents. Samples treated with acid also took on a blackish color; similar color change was noticed with ferric chloride solution. Nevertheless, inorganic salts also imparted cor- rosive effect on virgin samples and composites. Loss due to the corro-

TABLE 3 Tensile Strength (TS) Loss by Wood and WPC on Chemical Corrosion

Wood"

Sirnul Korai Mango Debdaro

Reagents T, T , T, T , T, T, T, T , T, Tw

Kadom _ _ _ ~ _ _ _ ~ ~

Water Hydrogen peroxide Methanol Ethanol Xylene Acetic acid Hydrochloric acid (20%) Sulfuric acid (30%) Sulfuric acid (10%) Sodium hydroxide (10%) Sodium hydroxide (5%) Sodium chloride (10%) Sodium chloride (5%) Ferrous sulfate (10%) Calcium chloride (10%) Copper sulfate (10%) Femc chloride (10%) Soybean oil Mustard oil

75 54

48 75 44 93 88 81 33 64 86 26 70 so 66 80 32 35

-

48 45 79 41 54 20 85 54 68 20 03 72 12 61 32 55 75 24 35

26 45 51 47 65 80 85 72 82 86 66 70 80 70 76 60

26 -

-

23 50 - 86 46 64 33 29 40 45 41 16

63 76 - 72 - 88 40 59 60 40 58 29 68 86

62 67 89 - 07 42 02 47

87 -

72 -

2s 73 46 2s 36 4

81 70 58 59 30 - - 68 86 64

40 22

-

89 88 53 43 86 63 - -

89 30

88 57 79 87 88

63 55

-

-

81 75 50 35 77 48 - - 81 22

85 51 52 74 74 91 56 31

-

65 80 61 76 60 49 94 90

50 70 69 52 65

81 95 69 60

-

-

62 78 51 47 50 45 89 87 86

64 55

63 75 71 91 47

-

-

-

%TS loss = 100 ( T S b - TSa)/Tsb, where TSb = TS before corrosion and TS, = TS after corrosion; T, = %TS loss by control wood and 7, = %TS loss by WPC.

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364 KHAN, ALI, AND AHMED

sive effect of various reagents is dependent on the extent of polymer loading as well as on the density of the wood substrate. Thus, debdaro has, in general, the least corrosive loss while loss with kadom is the maximum. It is also observed that iron salts are more corrosive than the copper ones.

Samples prepared for the corrosion test were dried at the ambient temperature and were then used for the tensile strength (TS) measure- ment. Percentage of TS loss was determined by the formula

where, TSb = tensile strength before corrosion and TS, = tensile strength after corrosion.

Results are given in Table 3 for all the five types of wood and their composites. Consistent with the weight loss obtained during corrosion attack, TS values of the untreated samples are also substantially re- duced in presence of acids; however, TS of composite samples are also reduced in presence of acids, but to a less extent. The extent of TS loss of the composites as well as of the untreated wood samples under the treatment of all other corrosive reagents is relatively small. TS measurements have again demonstrated the fact that the composite materials are less susceptible to chemical corrosion than the corre- sponding wood samples.

The present study on the corrosive effect on wood and its compos- ites, using a wide range of chemicals, acids, alkalis, salts, and organic solvents, illuminates facts useful in selecting types of wood for prepar- ing wood-plastic composites to be used for different purposes.

ACKNOWLEDGMENT

The authors extend their thanks to the International Atomic Energy Agency (IAEA) for providing chemicals and financial support for this work under the Technical Assistance Program BGD/8/008.

REFERENCES

Khan, M. A., and Ali, K. M. I . , Studies of physicomechanical properties of wood and wood-plastic composite (WPC), J . Appl. Polym. Sci. , 45, 167 (1992a).

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CHEMICAL CORROSION EFFECT 365

Khan, M. A . , and Ali, K. M. I . , Swelling behavior of wood and wood-plastic composite (WPC), Po1ym.-Plasr. Technol. Eng., 31 (3&4), 299 (1992b).

Spindler, M. W . , Pateman, R., and Hills, P. R., Polymer impregnated materials: The resistance of polymer-wood composites to chemical corrosion, Com- posite, Nov. 1973.

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