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Modelling the effects of a radiation induced polymer impregnation on the moisture of wood-polymer composites
Radiat. Phys. Chem. Vol. 34, No. 5, pp. 739-742, 1989 Int. J. Radiat. AppL Instrum., Part C
0146-5724/89 $3.00+ 0.00 Copyright © 1989PergamonPress plc
Printed in Great Britain. All rights reserved
MODELLING THE EFFECTS OF A RADIATION INDUCED POLYMER IMPREGNATION ON THE MOISTURE OF WOOD-POLYMER COMPOSITES FREDDY Y. C. BOEY and LAWRENCEH. L. CHIAt Nanyang Technological Institute, School of Mechanical and Production Engineering, Nanyang Avenue, Singapore (Received 21 February 1989)
Abstract--The adverse effect of moisture diffusion on the properties of wood has been one of the main weaknesses of wood. Using a gamma irradiation method, wood-polymer composites have been produced which exhibit significant improvement in mechanical properties like compression, creep deformation and creep rupture particularly at high humidity. It has been thought that the impregnation of polymer into the wood has affected the moisture diffusion in the wood, so that its adverse effects on the mechanical properties has been reduced. In this report the apparent diffusion coefficients of a Ramin wood impregnated with varying amounts of polymethyl methacrylate (PMMA) were determined using a Fick's law approach. An initial linear relationship was found for impregnation of up to 70% PMMA, after which the diffusion coefficientlevelsoff to a maximum value, for the three environmental relative humidity levels of 40, 60 and 90(_ 5)%. The phenomenon could be explained by means of a cylindrical model with the polymer added as an internal layer onto the wood cell wall. The average maximum reduction in the value of the diffusion coefficient was about 60%.
INTRODUCTION
Previous work (Boey et al., 1985, 1987; Chia et al., 1978, 1988; Teoh et al., 1987) has resulted in a wood-polymer composite (WPC) based on local tropical woods using a gamma irradiation method which exhibited significant improvement in compression and bending strengths, as well as creep deformation and creep rupture. The improvement in creep deformation and creep rupture were even greater at higher humidity levels (Chia et al., 1988). This seems to suggest that one of the main contributing factor of the impregnated polymer was to reduce the mobility of the chemically bonded water by reducing the apparent moisture diffusivity (D) of the composite wood as a whole. It is commonly known that the mechanical properties of wood are significantly affected by its moisture content (Cizek, 1968; Raczkowski, 1965; Schaffer, 1972). A higher moisture content (M) would result in a lower ultimate stress, lower stress at proportional limit, lower modulus but a higher failure strain. A loaded wood beam which would experience little or no creep would, at the same load, suffer significant and increasing creep strain at increasing moisture levels (M). Any change in M--whether increasing or decreasing--would also increase creep deformation. There also exists a stress limit above which these changes in M in a constantly loaded beam would eventually cause creep rupture of the beam. Such a situation, termed as mechano-sorptive creep, has tPresent address: Department of Chemistry, National University of Singapore, Singapore.
been shown to contribute a far greater portion of the observed apparent creep. Experiments have been done to analyse the effects of the polymer impregnation on the moisture diffusivity (D) of the wood, with the analysis and results of the work presented in this report.
ANALYSIS Moisture diffusion below the Fibre Saturation Point (FSP) in wood comprises of the diffusion of water vapour through the void structures of the wood cell wall, as well as the internal diffusion of bond water within the cell walls. Combination of both components then gives the apparent diffusion coefficient (D). Recent reports (Nelson, 1986; Bramhall, 1976) have questioned the validity of assuming the moisture concentration gradient to be the driving force for diffusion, which leads to using Fick's Law as a basis for analysis. Alternative driving forces in terms of spreading pressure gradient (Nelson, 1986) and vapour pressure (Bramhall, 1976) has been subsequently proposed. Notwithstanding the controversy, what is not disputed in both cases is the path of entry of the water molecules from the ambient environment into the voids and penetration of the molecules into the wood cell wall through the cell cavities and pit membrane pores. Since the present report is concerned primarily with the effect of polymer impregnation on the overall apparent diffusivity of the wood, to avoid complications, the diffusivity measured in these experiments was the apparent diffusion co-
739
740
FREDDYY. C. BOEYand LAWRENCEH. L. CrtiA
efficient (D), which has been so far satisfactorily analysed using Fick's law (Choong and Skaar, 1972; Nadler et al., 1985). For this reason, the following analysis has been based on Fick's law. In order to facilitate the analysis of the moisture diffusion process within the WPC, we make the following assumptions: (i) the transverse diffusivities Dy, D: are negligible in comparison to the longitudinal diffusivity D, (see Fig. 1) (ii) the polymers impregnated are anisotropic in the x-direction. It is well known that the transvers diffusivity of wood is much less than the longitudinal diffusivity (Dinwoodie, 1981). This is particularly so for hardwoods. To ensure that this was the case during the experiments, the transverse areas of the wood specimens were all coated with an epoxy coating that has a much lower moisture absorption capacity than the wood itself. Thus taking the first assumption, we can reduce the problem to a one-dimensional problem. Assuming constant ambient temperature (T) and humidity (h), we can then use the standard Fick equation to model the diffusion process: 3c 8 ~c a t = ax [Ox] ~x'
(1)
The actual weight of moisture moving through the area A in time t is then de equations (3) and (4) then give: m = g2A (c, - Co) x/Dxt
(5)
7~
The diffusivity D z can therefore be determined by determining the initial slope of the plot of w vs x/t. For t = 0 , e0 = 0 . At this time, assuming Dy = Dz = O. The moisture content M ( % ) is given as M
w - w o× 1 0 0 = m - x 100 WO
WO
2 ×g2A ~ - - c~
(6)
Wo
Since w0 = ( p >
is the dry
4c~ ~ pl n
M -
(7)
The maximum moisture content M. is given as M a = W- -~--xW o wo
c. x 100. (8) 100 = M . / w o × 100 = -A 1/A 1 p
Hence (7) and (8) give with the initial and boundary conditions: C=Co+__ <~0 0 < x < l C=Ca'3i->O
X=0;
M
t x=l
"
(2)
Where 1 is the strength of the specimen and c is the moisture concentration. The diffusivity D x is in fact a function of both temperature and humidity levels (Dinwoodie, 1981; Avramidis and Siau, 1987). That is, D x = Dx(T, h). By keeping the temperature constant, and assuming that the diffusion process does not significantly alter the temperature within the wood specimens, we then need not consider Dx = Dx(T). If D~ is taken to be dependent significantly on the moisture concentration, that is D~ -- D~(h), the analysis will be extremely complex. To circumvent this difficulty, all lots of specimens (each lot representing a specific polymer impregnation weight amount) were identically tested in similar humidity levels, so that their apparent diffusion coefficient (D~) can be comparably determined. D~ is then assumed to be independent of the changing moisture concentration in the wood specimens during the tests. For equations (1) and (2), if we consider the case of 1 --, oo (Jost, 1960), it can be shown that: C
C0
c~ - co
=1 - erf(~x~X \2x/O~t j
where erf represents an error function.
(3)
4M. x / ~ t 1
n
According to equation (9), a plot of M vs ~ yield an initial linear slope whose value is
(9) will
4M. x / ~ z 1
n
so that once Ma is known, Dx is also determined. Ma is of course the maximum moisture increase (weight percentage) attained.
EXPERIMENTAL P R O C E D U R E
Ramin, a local tropical medium hardwood, was used for the experiments. Specimens measuring 10 x 10 x 150ram were first oven dried at 102 _+ 2°C to constant weight and then coated with an external layer of epoxy resin on the circumferential sides, leaving the two ends uncoated. They were then put into an environmental chamber that held constant temperature (+0.5°C) and humidity (_+ 5%), with a constant air circulation by a closed ventilating system. The diffusivity was then determined by the measurement of the weight change (from the original uncoated dried wood weight) with time. The same wood specimens were then redried in the oven and subsequently impregnated with M M A monomers by immersion in a impregnation chamber for 24 h, and polymerized by irradiation at 2 3 _ I°C by a ~°Co
Moisture impregnation of wood-polymer composites
741
3~ L~
90 4- 5% RH
° 21"
x- % . .
=o
40-1-5% RH
|
x
X'x~i"~L" i 0
/
I 20
I 40
I 60
I 80
I '1O0
f 120
0
20
40
60
80
100
i 120
Weight percentage polymer impregnation
Weight percentage polymer impregnation Fig. I. Results of' the apparent longitudinal coefficient (Dx) at 90% (+5%) r.h.
Fig. 3. Results of the apparent longitudinal coeffident (Dx) at 40% (+5%) r.h.
source of 5800 Ci activity and at an average dosage of 0.3 mrad/h for 15 h. Details of the impregnation system and process can be found from earlier publications (Boey et aL, 1985, 1987; Chia et al., 1978). In order to vary the amount of impregnation, the wood specimens were separated into different lots. Each lot was then placed into a fume chamber for a different period of time to evaporate away proportional amounts of the M M A monomers prior to irradiation. All tests were performed at 23 + 0.5°C and at 90, 65 and 40% r.h. The percentage of polymer impregnated was determined by dividing the weight difference between the impregnated wood and the untreated and dry wood, by the weight of the untreated and dry wood.
tion. The Fisher F-test value obtained for all three cases, when compared to standard F-test table at 1%0, were highly significant, implying at 99% probability on the correlation of Dx and the polymer impregnated. The constant term, which represents the value of Dx for untreated Rafiain wood, average 2.17 x 1 0 - 6 c m 2 s - i , indicating that the wood is very permeable to moisture diffusion. The value obtained for Dx is generally higher at higher humidity levels. The minimum values obtained for Dx, as seen in Figs 1-3, were 1.1, 1.0 and 0.5 x 10-6cm: s -~ for 90, 65 and 40% ( + 5 % ) r.h. respectively, which give an average of 0.87 x 10 -6 cm 2 s -1. This represents a reduction of about 60% in the value of the diffusion coefficient for the untreated wood. For fibre reinforced composites (Shen and Springer, 1981; Springer and Tsai, 1967), have shown that the longitudinal diffusivity (Dx) of a unidirectional fibre composite material can be written as:
R E S U L T S AND DISCUSSION
Figure 1-3 show the results of the apparent logitudinal diffusion coefficient (Dx) at 90, 65 and 40% ( + 5 % ) r.h. respectively, and plotted against the weight percent of polymer impregnated. In all three cases, an apparent linear relationship was obtained up to a maximum percentage of impregnation, after which the curves flattens out. This maximum percentage impregnation ranged from 75 to 80% of polymer impregnation. The results of a linear regression analysis are shown in Table 1. In all three cases the coefficient of determination (R:) exceeded 0.89, indicating that at least 89% of the variation in the value of Dx is caused by the increase in polymer impregna-
6 5 ! 5% RH
x
0
I 20
I 40
I 60
I 80
I I O0
I 120
Weight percentoge polymer impregnation Fig. 2. Results of the apparent longitudinal coefficient (Dx) at 65% (+5%) r.h.
Ox = Din(1 - Vf) -t- Of Yr.
(10)
Dra, Df and Vf are the matrix diffusivity, fibre diffusivity and the fibre volume fraction respectively. In the case of wood-polymer composite, equation (10) cannot be identically applied. This is because whereas for normal fibre composites addition of any fibres displaces an identical volume amount of matrix, in WPC the polymers simply fill up the air voids within the wood structure without displacing any wood material. The diffusivity of the polymer layer when compared to that for wood can be considered as negligible. It can be visualized that addition of the polymers would reduce the void volume or airspace in the wood. Such a reduction would then significantly reduce the apparent diffusion coefficient, as diffusion is more rapid in the cell cavity air space than through the cell wall. The increase in polymer impregnation would proportionally reduce the cell cavity air space, and should therefore correspond to a proportional decrease in the apparent diffusivity Dx. Such a proportionality assumes that the apparent diffusion coefficient is dominantly dependent on the diffusion through the cell cavity air space, as against the diffusion through the wood cell wall itself.
FREDDYY. C. BOEY and LAWRENCEH. L. CrllA
742
Table I. Statistical analysis of the results of the apparent longitudinal diffusion co-efficient(D0 at 90, 65 and 40% (_+5%) r.h., from 0 to 70% polymer weight impregnation Relative humidity
Degreeof freedom
Coefficientof determination
40 60 90
6 6 7
0.928 0.889 0.934
Constant Standarderror term estimate 1.91 2.27 2.34
1.27E-1 1.56E-1 0.92E-1
Slope
Fisher test
- 1.59E-2 - 1.63E-2 - 1.56E-2
64.2 39.9 93.5
shows a scanning electron fractograph of a W P C specimen similar to those used in the diffusion measurements. The Ramin wood cell wall cavities are seen to be filled partially or fully by the polYmer impregnated into it. Figure 5 shows the same features, but at a higher magnification. When all the cell cavity air space is filled with the polymer (that is, 7 = 1), from equation (11), D x will reach a constant value. This would explain the minimum value obtained in Figs 1-3. CONCLUSION
Fig. 4. Scanning electron fractograph of the WPC specimen. Cell cavities have been partially or fully filled by the impregnated polymer. Based on the above description, equation (10) can be modified to give: Dx=(l -7)Da+YOp+Dw
(11)
where 7 is the fraction obtained by dividing the volume of polymer present with the volume of cell cavity air space. If all the cell cavity air space is occupied by the polymer, 7 = 1. D a is the diffusion coefficient within the cell cavity airspace, Dp is diffusion coefficient within the polymer and D w the diffusion coefficient within the wood cell wall itself. Assuming that Dpis negligible in comparison to Da and Dw, equation (11) then provides a basis for the linear relationship obtained earlier between the apparent diffusivity (Dx) and the percentage of polymer impregnated. The fraction 7 used in equation (11) assumes that the polymer impregnated could partially or fully occupy the cell cavity air space. Figure 4
Fig. 5. As Fig. 4, but at a higher magnification.
The apparent diffusion coefficients of a Ramin w o o d - p o l y m e r composite were determined at 3 humidity levels for varying amount of polymer impregnation, using a Fick's law approach. An initial linear relationship was obtained for up to 70% polymer impregnation at 40, 65 and 90 (_+5%) relative humidity levels, after which the diffusion coefficient tapers off to a minimum value. This phenomenon was explained by assuming that the polymer so impregnated reduced the available cell cavity air space, when then reduced the apparent diffusivity. REFERENCES
Avramidis J. and Siau J. F. (1987) Wood Sci. Technol. 21, 249-256. Boey F., Chia L. and Teoh S. H. (1985) Radiat. Phys. Chem. 2,6, 415. Boey F., Chia L. and Teoh S. H. (1987) Radiat. Phys. Chem. 29, 337. Bramhall G. (1976) Wood Sci. 8(3), 153-161. Chia L., Boey F. and Teoh S. H. (1978) Radiat. Phys. Chem. 29, 25. Chia L., Boey F. and Teoh S. H. (1988) Radiat. Phys. Chem. 32, 671~75. Choong E. T. and Skaar C. (1972) Wood Fiber 4, 80-86. Cizek L. (1968) Holz als Roh- und Werkstoff 26(11), 416. Dinwoodie J. M. (1981) Timber--Its Nature and Behavior, p. 41. Van Nostrand-Reinhold, New York. Jost W. (1960) Diffusion in Solids, Liquid and Gases. Academic Press, New York. Nadler K. C., Choong E. T. and Wetzel D. M. (1985) Wood Fibre Sci. 17(3), 404-423. Nelson R. M. Jr (1986) Wood Sci. Technol. 20, 125 135, 235-251. Raczkowski J. (1965) For. Prod. J. 15, 260. Schaffer E. L. (1972) Wood Fiber 3(4), 232. Shen C. H. and Springer G. S. (1981) In Environmental Effects o f Composite Materials, (Edited by Springer G. S.) pp. 15-33. Technomic, Westport Conn. Springer G. S. and Tsai S. W. (1967) J. Compos. Mater. 1, 166. Teoh S. H., Boey F. and Chia L. (1987) Radiat. Phys. Chem. 29, 201.
0146-5724/89 $3.00+ 0.00 Copyright © 1989PergamonPress plc
Printed in Great Britain. All rights reserved
MODELLING THE EFFECTS OF A RADIATION INDUCED POLYMER IMPREGNATION ON THE MOISTURE OF WOOD-POLYMER COMPOSITES FREDDY Y. C. BOEY and LAWRENCEH. L. CHIAt Nanyang Technological Institute, School of Mechanical and Production Engineering, Nanyang Avenue, Singapore (Received 21 February 1989)
Abstract--The adverse effect of moisture diffusion on the properties of wood has been one of the main weaknesses of wood. Using a gamma irradiation method, wood-polymer composites have been produced which exhibit significant improvement in mechanical properties like compression, creep deformation and creep rupture particularly at high humidity. It has been thought that the impregnation of polymer into the wood has affected the moisture diffusion in the wood, so that its adverse effects on the mechanical properties has been reduced. In this report the apparent diffusion coefficients of a Ramin wood impregnated with varying amounts of polymethyl methacrylate (PMMA) were determined using a Fick's law approach. An initial linear relationship was found for impregnation of up to 70% PMMA, after which the diffusion coefficientlevelsoff to a maximum value, for the three environmental relative humidity levels of 40, 60 and 90(_ 5)%. The phenomenon could be explained by means of a cylindrical model with the polymer added as an internal layer onto the wood cell wall. The average maximum reduction in the value of the diffusion coefficient was about 60%.
INTRODUCTION
Previous work (Boey et al., 1985, 1987; Chia et al., 1978, 1988; Teoh et al., 1987) has resulted in a wood-polymer composite (WPC) based on local tropical woods using a gamma irradiation method which exhibited significant improvement in compression and bending strengths, as well as creep deformation and creep rupture. The improvement in creep deformation and creep rupture were even greater at higher humidity levels (Chia et al., 1988). This seems to suggest that one of the main contributing factor of the impregnated polymer was to reduce the mobility of the chemically bonded water by reducing the apparent moisture diffusivity (D) of the composite wood as a whole. It is commonly known that the mechanical properties of wood are significantly affected by its moisture content (Cizek, 1968; Raczkowski, 1965; Schaffer, 1972). A higher moisture content (M) would result in a lower ultimate stress, lower stress at proportional limit, lower modulus but a higher failure strain. A loaded wood beam which would experience little or no creep would, at the same load, suffer significant and increasing creep strain at increasing moisture levels (M). Any change in M--whether increasing or decreasing--would also increase creep deformation. There also exists a stress limit above which these changes in M in a constantly loaded beam would eventually cause creep rupture of the beam. Such a situation, termed as mechano-sorptive creep, has tPresent address: Department of Chemistry, National University of Singapore, Singapore.
been shown to contribute a far greater portion of the observed apparent creep. Experiments have been done to analyse the effects of the polymer impregnation on the moisture diffusivity (D) of the wood, with the analysis and results of the work presented in this report.
ANALYSIS Moisture diffusion below the Fibre Saturation Point (FSP) in wood comprises of the diffusion of water vapour through the void structures of the wood cell wall, as well as the internal diffusion of bond water within the cell walls. Combination of both components then gives the apparent diffusion coefficient (D). Recent reports (Nelson, 1986; Bramhall, 1976) have questioned the validity of assuming the moisture concentration gradient to be the driving force for diffusion, which leads to using Fick's Law as a basis for analysis. Alternative driving forces in terms of spreading pressure gradient (Nelson, 1986) and vapour pressure (Bramhall, 1976) has been subsequently proposed. Notwithstanding the controversy, what is not disputed in both cases is the path of entry of the water molecules from the ambient environment into the voids and penetration of the molecules into the wood cell wall through the cell cavities and pit membrane pores. Since the present report is concerned primarily with the effect of polymer impregnation on the overall apparent diffusivity of the wood, to avoid complications, the diffusivity measured in these experiments was the apparent diffusion co-
739
740
FREDDYY. C. BOEYand LAWRENCEH. L. CrtiA
efficient (D), which has been so far satisfactorily analysed using Fick's law (Choong and Skaar, 1972; Nadler et al., 1985). For this reason, the following analysis has been based on Fick's law. In order to facilitate the analysis of the moisture diffusion process within the WPC, we make the following assumptions: (i) the transverse diffusivities Dy, D: are negligible in comparison to the longitudinal diffusivity D, (see Fig. 1) (ii) the polymers impregnated are anisotropic in the x-direction. It is well known that the transvers diffusivity of wood is much less than the longitudinal diffusivity (Dinwoodie, 1981). This is particularly so for hardwoods. To ensure that this was the case during the experiments, the transverse areas of the wood specimens were all coated with an epoxy coating that has a much lower moisture absorption capacity than the wood itself. Thus taking the first assumption, we can reduce the problem to a one-dimensional problem. Assuming constant ambient temperature (T) and humidity (h), we can then use the standard Fick equation to model the diffusion process: 3c 8 ~c a t = ax [Ox] ~x'
(1)
The actual weight of moisture moving through the area A in time t is then de equations (3) and (4) then give: m = g2A (c, - Co) x/Dxt
(5)
7~
The diffusivity D z can therefore be determined by determining the initial slope of the plot of w vs x/t. For t = 0 , e0 = 0 . At this time, assuming Dy = Dz = O. The moisture content M ( % ) is given as M
w - w o× 1 0 0 = m - x 100 WO
WO
2 ×g2A ~ - - c~
(6)
Wo
Since w0 = ( p >
is the dry
4c~ ~ pl n
M -
(7)
The maximum moisture content M. is given as M a = W- -~--xW o wo
c. x 100. (8) 100 = M . / w o × 100 = -A 1/A 1 p
Hence (7) and (8) give with the initial and boundary conditions: C=Co+__ <~0 0 < x < l C=Ca'3i->O
X=0;
M
t x=l
"
(2)
Where 1 is the strength of the specimen and c is the moisture concentration. The diffusivity D x is in fact a function of both temperature and humidity levels (Dinwoodie, 1981; Avramidis and Siau, 1987). That is, D x = Dx(T, h). By keeping the temperature constant, and assuming that the diffusion process does not significantly alter the temperature within the wood specimens, we then need not consider Dx = Dx(T). If D~ is taken to be dependent significantly on the moisture concentration, that is D~ -- D~(h), the analysis will be extremely complex. To circumvent this difficulty, all lots of specimens (each lot representing a specific polymer impregnation weight amount) were identically tested in similar humidity levels, so that their apparent diffusion coefficient (D~) can be comparably determined. D~ is then assumed to be independent of the changing moisture concentration in the wood specimens during the tests. For equations (1) and (2), if we consider the case of 1 --, oo (Jost, 1960), it can be shown that: C
C0
c~ - co
=1 - erf(~x~X \2x/O~t j
where erf represents an error function.
(3)
4M. x / ~ t 1
n
According to equation (9), a plot of M vs ~ yield an initial linear slope whose value is
(9) will
4M. x / ~ z 1
n
so that once Ma is known, Dx is also determined. Ma is of course the maximum moisture increase (weight percentage) attained.
EXPERIMENTAL P R O C E D U R E
Ramin, a local tropical medium hardwood, was used for the experiments. Specimens measuring 10 x 10 x 150ram were first oven dried at 102 _+ 2°C to constant weight and then coated with an external layer of epoxy resin on the circumferential sides, leaving the two ends uncoated. They were then put into an environmental chamber that held constant temperature (+0.5°C) and humidity (_+ 5%), with a constant air circulation by a closed ventilating system. The diffusivity was then determined by the measurement of the weight change (from the original uncoated dried wood weight) with time. The same wood specimens were then redried in the oven and subsequently impregnated with M M A monomers by immersion in a impregnation chamber for 24 h, and polymerized by irradiation at 2 3 _ I°C by a ~°Co
Moisture impregnation of wood-polymer composites
741
3~ L~
90 4- 5% RH
° 21"
x- % . .
=o
40-1-5% RH
|
x
X'x~i"~L" i 0
/
I 20
I 40
I 60
I 80
I '1O0
f 120
0
20
40
60
80
100
i 120
Weight percentage polymer impregnation
Weight percentage polymer impregnation Fig. I. Results of' the apparent longitudinal coefficient (Dx) at 90% (+5%) r.h.
Fig. 3. Results of the apparent longitudinal coeffident (Dx) at 40% (+5%) r.h.
source of 5800 Ci activity and at an average dosage of 0.3 mrad/h for 15 h. Details of the impregnation system and process can be found from earlier publications (Boey et aL, 1985, 1987; Chia et al., 1978). In order to vary the amount of impregnation, the wood specimens were separated into different lots. Each lot was then placed into a fume chamber for a different period of time to evaporate away proportional amounts of the M M A monomers prior to irradiation. All tests were performed at 23 + 0.5°C and at 90, 65 and 40% r.h. The percentage of polymer impregnated was determined by dividing the weight difference between the impregnated wood and the untreated and dry wood, by the weight of the untreated and dry wood.
tion. The Fisher F-test value obtained for all three cases, when compared to standard F-test table at 1%0, were highly significant, implying at 99% probability on the correlation of Dx and the polymer impregnated. The constant term, which represents the value of Dx for untreated Rafiain wood, average 2.17 x 1 0 - 6 c m 2 s - i , indicating that the wood is very permeable to moisture diffusion. The value obtained for Dx is generally higher at higher humidity levels. The minimum values obtained for Dx, as seen in Figs 1-3, were 1.1, 1.0 and 0.5 x 10-6cm: s -~ for 90, 65 and 40% ( + 5 % ) r.h. respectively, which give an average of 0.87 x 10 -6 cm 2 s -1. This represents a reduction of about 60% in the value of the diffusion coefficient for the untreated wood. For fibre reinforced composites (Shen and Springer, 1981; Springer and Tsai, 1967), have shown that the longitudinal diffusivity (Dx) of a unidirectional fibre composite material can be written as:
R E S U L T S AND DISCUSSION
Figure 1-3 show the results of the apparent logitudinal diffusion coefficient (Dx) at 90, 65 and 40% ( + 5 % ) r.h. respectively, and plotted against the weight percent of polymer impregnated. In all three cases, an apparent linear relationship was obtained up to a maximum percentage of impregnation, after which the curves flattens out. This maximum percentage impregnation ranged from 75 to 80% of polymer impregnation. The results of a linear regression analysis are shown in Table 1. In all three cases the coefficient of determination (R:) exceeded 0.89, indicating that at least 89% of the variation in the value of Dx is caused by the increase in polymer impregna-
6 5 ! 5% RH
x
0
I 20
I 40
I 60
I 80
I I O0
I 120
Weight percentoge polymer impregnation Fig. 2. Results of the apparent longitudinal coefficient (Dx) at 65% (+5%) r.h.
Ox = Din(1 - Vf) -t- Of Yr.
(10)
Dra, Df and Vf are the matrix diffusivity, fibre diffusivity and the fibre volume fraction respectively. In the case of wood-polymer composite, equation (10) cannot be identically applied. This is because whereas for normal fibre composites addition of any fibres displaces an identical volume amount of matrix, in WPC the polymers simply fill up the air voids within the wood structure without displacing any wood material. The diffusivity of the polymer layer when compared to that for wood can be considered as negligible. It can be visualized that addition of the polymers would reduce the void volume or airspace in the wood. Such a reduction would then significantly reduce the apparent diffusion coefficient, as diffusion is more rapid in the cell cavity air space than through the cell wall. The increase in polymer impregnation would proportionally reduce the cell cavity air space, and should therefore correspond to a proportional decrease in the apparent diffusivity Dx. Such a proportionality assumes that the apparent diffusion coefficient is dominantly dependent on the diffusion through the cell cavity air space, as against the diffusion through the wood cell wall itself.
FREDDYY. C. BOEY and LAWRENCEH. L. CrllA
742
Table I. Statistical analysis of the results of the apparent longitudinal diffusion co-efficient(D0 at 90, 65 and 40% (_+5%) r.h., from 0 to 70% polymer weight impregnation Relative humidity
Degreeof freedom
Coefficientof determination
40 60 90
6 6 7
0.928 0.889 0.934
Constant Standarderror term estimate 1.91 2.27 2.34
1.27E-1 1.56E-1 0.92E-1
Slope
Fisher test
- 1.59E-2 - 1.63E-2 - 1.56E-2
64.2 39.9 93.5
shows a scanning electron fractograph of a W P C specimen similar to those used in the diffusion measurements. The Ramin wood cell wall cavities are seen to be filled partially or fully by the polYmer impregnated into it. Figure 5 shows the same features, but at a higher magnification. When all the cell cavity air space is filled with the polymer (that is, 7 = 1), from equation (11), D x will reach a constant value. This would explain the minimum value obtained in Figs 1-3. CONCLUSION
Fig. 4. Scanning electron fractograph of the WPC specimen. Cell cavities have been partially or fully filled by the impregnated polymer. Based on the above description, equation (10) can be modified to give: Dx=(l -7)Da+YOp+Dw
(11)
where 7 is the fraction obtained by dividing the volume of polymer present with the volume of cell cavity air space. If all the cell cavity air space is occupied by the polymer, 7 = 1. D a is the diffusion coefficient within the cell cavity airspace, Dp is diffusion coefficient within the polymer and D w the diffusion coefficient within the wood cell wall itself. Assuming that Dpis negligible in comparison to Da and Dw, equation (11) then provides a basis for the linear relationship obtained earlier between the apparent diffusivity (Dx) and the percentage of polymer impregnated. The fraction 7 used in equation (11) assumes that the polymer impregnated could partially or fully occupy the cell cavity air space. Figure 4
Fig. 5. As Fig. 4, but at a higher magnification.
The apparent diffusion coefficients of a Ramin w o o d - p o l y m e r composite were determined at 3 humidity levels for varying amount of polymer impregnation, using a Fick's law approach. An initial linear relationship was obtained for up to 70% polymer impregnation at 40, 65 and 90 (_+5%) relative humidity levels, after which the diffusion coefficient tapers off to a minimum value. This phenomenon was explained by assuming that the polymer so impregnated reduced the available cell cavity air space, when then reduced the apparent diffusivity. REFERENCES
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