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Observations of peeling of a polyisobutylene-based pressure-sensitive adhesive
International Journal of Adhesion & Adhesives 18 (1998) 131—137
Observations of peeling of a polyisobutylene-based pressure-sensitive adhesive S+ren Flygenring Christensen!, Hanne Everland!, Ole Hassager", Kristoffer Almdal# ! Coloplast Research, Bakkega> rdsvej 406a, DK-3050 Humlebæk, Denmark " Department of Chemical Engineering, Building 229, The Danish Technical University, DK-2800 Lyngby, Denmark # Department of Solid State Physics, Research Center Ris~, PO 49, DK-4000 Roskilde, Denmark Accepted 18 July 1997
Abstract A pressure-sensitive adhesive (PSA) was prepared by mixing low- and high-molecular-weight polyisobutylenes (PIB). Peeling of the adhesive from polycarbonate was observed from the side and from below at three peel rates. A correlation between the peel force and a characteristic dimension of the peel front was obtained. According to the observations, the deformation of adhesive across the peel front is dominated by uniaxial extension. From model stretch experiments simulating the observed stretch history of adhesive, theoretical peel force values were obtained which agree well with the measured peel force values. ( 1998 Elsevier Science Ltd. All rights reserved. Keywords: A. pressure-sensitive; C. microscopy; C. rheology; D. stress distribution; peel test
1. Introduction Gent and Petrich approximated the deformation of an adhesive during peeling by uniaxial elongation at constant stretch rate and tried to predict the peel force from extensional rheometry, by using an integral of the form [1]: e. P"h f de
P
(1)
0
2. Experimental
where P is the peel force per width of peel strip, h is the thickness of the adhesive, e is the fractional extension at . break or detachment, and f is the tension as function of fractional extension, e. Connelly et al. evaluated Eq. (1) by using stress—strain data obtained at constant stretch velocity, i.e. decreasing stretch rate [2]. Criticising their work, Gupta suggested an expression presupposing knowledge of the actual shape of the adhesive/backing interface during peeling [3]: x" P" p(x) dx
P
where w is the width of the peeled strip, p(x) is the tension distribution across the peel front, x is the horizontal position in the peel front, and x is the position at which " the adhesive detaches from the substrate or ruptures cohesively. In the present work, Eq. (2) is used to predict the peel force in 90° peel tests. Information about p(x) is obtained in uniaxial stretch experiments simulating the observed stretching of the adhesive across the peel front.
(2)
0
0143-7496/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved. PII: S0143-7496(97)00037-7
A pressure-sensitive adhesive (PSA) was prepared by mixing low- and high-molecular-weight polyisobutylene (PIBs) (Vistanex-L80 and Vistanex-LM/MH, Exxon Chemicals) in the weight ratio 1:1 at 140°C. The molecular weight distribution was determined by size exclusion chromatography (Fig. 1). Test samples were made by cutting 0.5 mm thick sheets of the adhesive into strips and applying them to a backing tape (Tesa tape 4651, Beiersdorf AG). The samples were applied manually to smooth polycarbonate plates and left for 30 min. The peel force was measured with a specially designed peel stage mounted in a load frame
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Fig. 1. Molecular weight distribution of the adhesive as obtained from size exclusion chromatography
(Fig. 2). Bonding and peeling were performd at 23°C and 50% relative humidity. The peel front was observed through a microscope equipped with a charge-coupled device camera. Two series of peel tests were carried out as follows.
Fig. 2. Sketch of the experimental set-up. Note that the peel frame is attached to the crosshead and thus moves at the same speed at which the adhesive is peeled. Thus the peel front is kept in the same position throughout the test. (The microscope is set up for observing the peel front from below)
Series A—observations of the peel front from below: the peel front was observed from below and the peel force was measured at a peel rate of 100 mm min~1 using peel strips of various widths. Series B—side-view observations of the peel front: by using 2 mm wide peel strips, the peel front was observed from the side at the peel rates of 50, 100 and 200 mm min~1 as the peel force was being measured. Three tests were carried out at each rate. On the basis of the observations obtained in this series, the deformation of the adhesive across the peel front was simulated in a final series of experiments. Series C—uniaxial extension of the adhesive: rectangular samples of the adhesive were stretched at varying rate in the load frame, and the tensile force was measured. Fig. 3 shows how the peel force varies during a single peel test. At first the peel force builds up, until it reaches a value where the adhesive begins to detach from the substrate. This part of the curve is referred to as the ‘start-up region’. After this region follows the ‘operation region’ where the peel force varies moderately. All results reported here belong to the operation region. In series A the peel force reported is the average value in the steady-state region, as most often reported in peel tests. In series B, however, the peel force values are not average values, but single measurements, which can be correlated to single images.
Fig. 3. Measured peel force versus distance during one peel test. Substrate: Lexan' 9030 Sheet (uncoated), GE Plastics Structured Products
3. Results and discussion 3.1. Series A Figs. 4(a)—(d) show observations of the peel front from below by using peel strips of various widths. There is
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Fig. 4. The peel front seen through the substrate. The substrate is moving upwards, and the adhesive is being pulled away from the camera at right angles to the substrate. Peel rate: 100 mm min~1. Width of peel strip: (a) 12.5 mm; (b) 6 mm; (c) 3 mm; (d) 2 mm
a marked boundary effect when using a wide (12.5 mm) peel strip. This boundary effect gives rise to a secondary crack tip perpendicular to the primary crack tip, and shields the main peel front from being observed from the side. As the peel strip width is decreased the boundary effects move closer, and at 3 mm the peel morphology is totally dominated by the boundary effects. As the width is further decreased to 2 mm, the boundary effects ‘cancel out’, in the sense that the peel front, on a large scale, is uniform across the peel width. This means that side-view observations offer representative descriptions of the shape of the peel front along the whole width. It should be noted that the observed deformation in this case may not be representative of the experiments carried out with wider peel strips. To examine whether peeling of 2 mm strips is representative of peeling of wider strips, the average peel force, F, was plotted against peel strip width, w (Fig. 5). It is seen that the points are fairly well situated
on a straight line, and that the average peel force obtained with 2 mm strips is very close to the linear fit of F versus w. The linearity indicates that the boundary effect does not influence the average peel force significantly, and that the information gained from studying peeling of 2 mm wide strips can be used in modelling of peeling of wider strips. Interfacial failure was observed for all peel tests, except at the edges, where the boundary effects give rise to thin lines of adhesive residue [this is discernible in Fig. 4(c)]. 3.2. Series B Fig. 6 shows a side-view observation of adhesive peeled at 100 mm min~1. To quantify the size of the peel front a characteristic dimension, D, was measured: D is the shortest distance between the crack tip and the interface between the backing tape and adhesive.
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Fig. 5. Average peel force, F, against width of peel strip, w, at a peel rate of 100 mm min~1
Now, let d"(D!D )/D , where D is the thickness of 0 0 0 adhesive in its unstretched state. A plot of d versus the peel force, F, yields a fairly straight line in the range of peel rates examined (Fig. 7). Note that linear extrapolation to zero force yields a d value of about zero, i.e. at zero force there is no deformation. On the basis of this observation it is fair to assume linearity between force and deformation in the whole range of peel rates from 0 to 200 mm min~1. In two of the peel tests performed at 200 mm min~1, the peel front resembles qualitatively the peel front at 50 and 100 mm min~1, but in the third 200 mm min~1 peel test an instability occurred giving rise to filaments at the peel front (Fig. 8). This results in the highest values of F and d in Fig. 7. Instabilities are expected to arise at high peel rates, because the tension gradient across the peel front is steeper at higher rates [4], but other factors, such as the presence of inhomogeneities inside the adhesive or on the adhesive/substrate interface, may also give rise to instabilities.
Fig. 6. Side-view observation of the peel front. Peel rate: 100 mm min~1; width of peel strip: 2 mm
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135
3.3. Series C
Fig. 7. Dimensionless deformation versus measured peel force
Based on the side-view observations, the stretch history of the adhesive across the peel front was quantified in the following way. Consider an infinitely thin adhesive element spanning the height and width of the peel strip at x"0 (Fig. 9). At this point the height of the element is d . 0 This element is stretched between the backing and the substrate until it detaches from the substrate at x"x . " Assuming that the backing tape is ideally unstretchable, corresponding points on the backing/adhesive interface and the adhesive/substrate interface will be equally spaced. A discretized stretch velocity profile was determined by measuring the distances between corresponding points and by assuming constant stretch velocity within each interval. From this velocity profile the stretch rate profile was computed. Fig. 10 shows how the stretch rate, eR , varies across the peel front in each experiment. Fig. 11 shows the tension distribution across the peel front for three of the peel experiments as obtained from
Fig. 8. Side-view observations of the peel front. Peel rate: 200 mm min~1; width of peel strip: 2 mm
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Fig. 10. Stretch rate profiles for all nine observations
Fig. 9. The stretch history of adhesive across the peel front is quantified by sectioning the peel front, and measuring the distance between backing and substrate in each interval. Also shown is the assumed deformation of an infinitely thin adhesive element traversing the peel front
the corresponding stretch experiments. Two distinct differences are observed between the tension profiles reported here and the ones measured by Kaelble in 90° peeling of polybutadiene—polystyrene PSA from steel [5]. First, he observed a significant compression stress peak preceding the separation zone, which is absent in Fig. 11. This discrepancy may be explained by the following experimental difference. In Kaelble’s peel tests the compression stress peak was due to the resistance of the backing (Cellophane), which was thicker than the adhesive layer, against being bent. In the experiments reported here, the backing is very flexible and much thinner than the adhesive layer, thus it does not compress the adhesive to any observable extent. Second, following the compression zone, Kaelble observed a tension stress peak which he ascribed to micro cavitation within the adhesive. With one exception (Fig. 8) cavitation is not observed in the peel tests treated here, and thus no tensile stress peak is expected near x"0.
Fig. 11. Tension distribution across the peel front as obtained from stretch experiments corresponding to single side-view images of the peel front. The raggedness of the curves is due to the discontinuous velocity profiles used in the stretch experiments
As a rule of thumb, the rate of extension in a peel experiment is given by [1]: v eR " d 0
(3)
where v is the peel rate. Eq. (3) predicts the same stretch rate value of 3.3 s~1 for the 100 mm min~1 peel tests
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137
are mean values from two identical stretch experiments. Ideally the points should be on the diagonal. The deviations from the ideal are well within the range of experimental error.
4. Conclusion
Fig. 12. Comparison between measured and calculated peel force values
reported here as for the 90° peel tests reported by Kaelble. Comparison with Fig. 10 shows that, in respect to the present work, this value of eR , although too high, is of the right order of magnitude. Assuming this is also the case for the experiments reported by Kaelble, the rheology of the two systems are comparable regarding rate of deformation. On the basis of this observation it is notable that the tension at detachment is in the same range for the two systems: 3 to 4 atm. From the tension profiles obtained the integral in Eq. (2) was evaluated numerically for each observation. The peel force calculated from Eq. (2) is plotted against the measured peel force in Fig. 12. The calculated values
Observations of the peel front yield valuable information about the deformation of the adhesive during peel tests. Observations from below reveal distinct boundary effects when using wide peel strips. These boundary effects shield the main peel front from being observed from the side. This problem is overcome by using slim peel strips. Side-view observations suggest that the deformation should be approximated by uniaxial elongation at varying rate. Theoretical peel force values obtained from uniaxial stretching of the adhesive agree well with the measured values, indicating that this is a good approximation.
Acknowledgements The authors wish to thank Walther Batsberg at Department of Solid State Physics, Research Center Ris+, for performing the size exclusion chromatography.
References [1] [2] [3] [4] [5]
Gent AN, Petrich RP. Proc. R. Soc. 1969;310:433. Connelly RW, Parsons WF, Pearson GH. J. Rheol. 1981;25(3):15. Gupta RK. J. Rheol. 1983;27(2):171. Argon AS, Salama M. Mater. Sci. Eng. 1976;23:219. Kaelble DH. Trans. Soc. R. 1965;9(2):135.
Observations of peeling of a polyisobutylene-based pressure-sensitive adhesive S+ren Flygenring Christensen!, Hanne Everland!, Ole Hassager", Kristoffer Almdal# ! Coloplast Research, Bakkega> rdsvej 406a, DK-3050 Humlebæk, Denmark " Department of Chemical Engineering, Building 229, The Danish Technical University, DK-2800 Lyngby, Denmark # Department of Solid State Physics, Research Center Ris~, PO 49, DK-4000 Roskilde, Denmark Accepted 18 July 1997
Abstract A pressure-sensitive adhesive (PSA) was prepared by mixing low- and high-molecular-weight polyisobutylenes (PIB). Peeling of the adhesive from polycarbonate was observed from the side and from below at three peel rates. A correlation between the peel force and a characteristic dimension of the peel front was obtained. According to the observations, the deformation of adhesive across the peel front is dominated by uniaxial extension. From model stretch experiments simulating the observed stretch history of adhesive, theoretical peel force values were obtained which agree well with the measured peel force values. ( 1998 Elsevier Science Ltd. All rights reserved. Keywords: A. pressure-sensitive; C. microscopy; C. rheology; D. stress distribution; peel test
1. Introduction Gent and Petrich approximated the deformation of an adhesive during peeling by uniaxial elongation at constant stretch rate and tried to predict the peel force from extensional rheometry, by using an integral of the form [1]: e. P"h f de
P
(1)
0
2. Experimental
where P is the peel force per width of peel strip, h is the thickness of the adhesive, e is the fractional extension at . break or detachment, and f is the tension as function of fractional extension, e. Connelly et al. evaluated Eq. (1) by using stress—strain data obtained at constant stretch velocity, i.e. decreasing stretch rate [2]. Criticising their work, Gupta suggested an expression presupposing knowledge of the actual shape of the adhesive/backing interface during peeling [3]: x" P" p(x) dx
P
where w is the width of the peeled strip, p(x) is the tension distribution across the peel front, x is the horizontal position in the peel front, and x is the position at which " the adhesive detaches from the substrate or ruptures cohesively. In the present work, Eq. (2) is used to predict the peel force in 90° peel tests. Information about p(x) is obtained in uniaxial stretch experiments simulating the observed stretching of the adhesive across the peel front.
(2)
0
0143-7496/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved. PII: S0143-7496(97)00037-7
A pressure-sensitive adhesive (PSA) was prepared by mixing low- and high-molecular-weight polyisobutylene (PIBs) (Vistanex-L80 and Vistanex-LM/MH, Exxon Chemicals) in the weight ratio 1:1 at 140°C. The molecular weight distribution was determined by size exclusion chromatography (Fig. 1). Test samples were made by cutting 0.5 mm thick sheets of the adhesive into strips and applying them to a backing tape (Tesa tape 4651, Beiersdorf AG). The samples were applied manually to smooth polycarbonate plates and left for 30 min. The peel force was measured with a specially designed peel stage mounted in a load frame
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Fig. 1. Molecular weight distribution of the adhesive as obtained from size exclusion chromatography
(Fig. 2). Bonding and peeling were performd at 23°C and 50% relative humidity. The peel front was observed through a microscope equipped with a charge-coupled device camera. Two series of peel tests were carried out as follows.
Fig. 2. Sketch of the experimental set-up. Note that the peel frame is attached to the crosshead and thus moves at the same speed at which the adhesive is peeled. Thus the peel front is kept in the same position throughout the test. (The microscope is set up for observing the peel front from below)
Series A—observations of the peel front from below: the peel front was observed from below and the peel force was measured at a peel rate of 100 mm min~1 using peel strips of various widths. Series B—side-view observations of the peel front: by using 2 mm wide peel strips, the peel front was observed from the side at the peel rates of 50, 100 and 200 mm min~1 as the peel force was being measured. Three tests were carried out at each rate. On the basis of the observations obtained in this series, the deformation of the adhesive across the peel front was simulated in a final series of experiments. Series C—uniaxial extension of the adhesive: rectangular samples of the adhesive were stretched at varying rate in the load frame, and the tensile force was measured. Fig. 3 shows how the peel force varies during a single peel test. At first the peel force builds up, until it reaches a value where the adhesive begins to detach from the substrate. This part of the curve is referred to as the ‘start-up region’. After this region follows the ‘operation region’ where the peel force varies moderately. All results reported here belong to the operation region. In series A the peel force reported is the average value in the steady-state region, as most often reported in peel tests. In series B, however, the peel force values are not average values, but single measurements, which can be correlated to single images.
Fig. 3. Measured peel force versus distance during one peel test. Substrate: Lexan' 9030 Sheet (uncoated), GE Plastics Structured Products
3. Results and discussion 3.1. Series A Figs. 4(a)—(d) show observations of the peel front from below by using peel strips of various widths. There is
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Fig. 4. The peel front seen through the substrate. The substrate is moving upwards, and the adhesive is being pulled away from the camera at right angles to the substrate. Peel rate: 100 mm min~1. Width of peel strip: (a) 12.5 mm; (b) 6 mm; (c) 3 mm; (d) 2 mm
a marked boundary effect when using a wide (12.5 mm) peel strip. This boundary effect gives rise to a secondary crack tip perpendicular to the primary crack tip, and shields the main peel front from being observed from the side. As the peel strip width is decreased the boundary effects move closer, and at 3 mm the peel morphology is totally dominated by the boundary effects. As the width is further decreased to 2 mm, the boundary effects ‘cancel out’, in the sense that the peel front, on a large scale, is uniform across the peel width. This means that side-view observations offer representative descriptions of the shape of the peel front along the whole width. It should be noted that the observed deformation in this case may not be representative of the experiments carried out with wider peel strips. To examine whether peeling of 2 mm strips is representative of peeling of wider strips, the average peel force, F, was plotted against peel strip width, w (Fig. 5). It is seen that the points are fairly well situated
on a straight line, and that the average peel force obtained with 2 mm strips is very close to the linear fit of F versus w. The linearity indicates that the boundary effect does not influence the average peel force significantly, and that the information gained from studying peeling of 2 mm wide strips can be used in modelling of peeling of wider strips. Interfacial failure was observed for all peel tests, except at the edges, where the boundary effects give rise to thin lines of adhesive residue [this is discernible in Fig. 4(c)]. 3.2. Series B Fig. 6 shows a side-view observation of adhesive peeled at 100 mm min~1. To quantify the size of the peel front a characteristic dimension, D, was measured: D is the shortest distance between the crack tip and the interface between the backing tape and adhesive.
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SF. Christensen et al. / International Journal of Adhesion & Adhesives 18 (1998) 131—137
Fig. 5. Average peel force, F, against width of peel strip, w, at a peel rate of 100 mm min~1
Now, let d"(D!D )/D , where D is the thickness of 0 0 0 adhesive in its unstretched state. A plot of d versus the peel force, F, yields a fairly straight line in the range of peel rates examined (Fig. 7). Note that linear extrapolation to zero force yields a d value of about zero, i.e. at zero force there is no deformation. On the basis of this observation it is fair to assume linearity between force and deformation in the whole range of peel rates from 0 to 200 mm min~1. In two of the peel tests performed at 200 mm min~1, the peel front resembles qualitatively the peel front at 50 and 100 mm min~1, but in the third 200 mm min~1 peel test an instability occurred giving rise to filaments at the peel front (Fig. 8). This results in the highest values of F and d in Fig. 7. Instabilities are expected to arise at high peel rates, because the tension gradient across the peel front is steeper at higher rates [4], but other factors, such as the presence of inhomogeneities inside the adhesive or on the adhesive/substrate interface, may also give rise to instabilities.
Fig. 6. Side-view observation of the peel front. Peel rate: 100 mm min~1; width of peel strip: 2 mm
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135
3.3. Series C
Fig. 7. Dimensionless deformation versus measured peel force
Based on the side-view observations, the stretch history of the adhesive across the peel front was quantified in the following way. Consider an infinitely thin adhesive element spanning the height and width of the peel strip at x"0 (Fig. 9). At this point the height of the element is d . 0 This element is stretched between the backing and the substrate until it detaches from the substrate at x"x . " Assuming that the backing tape is ideally unstretchable, corresponding points on the backing/adhesive interface and the adhesive/substrate interface will be equally spaced. A discretized stretch velocity profile was determined by measuring the distances between corresponding points and by assuming constant stretch velocity within each interval. From this velocity profile the stretch rate profile was computed. Fig. 10 shows how the stretch rate, eR , varies across the peel front in each experiment. Fig. 11 shows the tension distribution across the peel front for three of the peel experiments as obtained from
Fig. 8. Side-view observations of the peel front. Peel rate: 200 mm min~1; width of peel strip: 2 mm
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Fig. 10. Stretch rate profiles for all nine observations
Fig. 9. The stretch history of adhesive across the peel front is quantified by sectioning the peel front, and measuring the distance between backing and substrate in each interval. Also shown is the assumed deformation of an infinitely thin adhesive element traversing the peel front
the corresponding stretch experiments. Two distinct differences are observed between the tension profiles reported here and the ones measured by Kaelble in 90° peeling of polybutadiene—polystyrene PSA from steel [5]. First, he observed a significant compression stress peak preceding the separation zone, which is absent in Fig. 11. This discrepancy may be explained by the following experimental difference. In Kaelble’s peel tests the compression stress peak was due to the resistance of the backing (Cellophane), which was thicker than the adhesive layer, against being bent. In the experiments reported here, the backing is very flexible and much thinner than the adhesive layer, thus it does not compress the adhesive to any observable extent. Second, following the compression zone, Kaelble observed a tension stress peak which he ascribed to micro cavitation within the adhesive. With one exception (Fig. 8) cavitation is not observed in the peel tests treated here, and thus no tensile stress peak is expected near x"0.
Fig. 11. Tension distribution across the peel front as obtained from stretch experiments corresponding to single side-view images of the peel front. The raggedness of the curves is due to the discontinuous velocity profiles used in the stretch experiments
As a rule of thumb, the rate of extension in a peel experiment is given by [1]: v eR " d 0
(3)
where v is the peel rate. Eq. (3) predicts the same stretch rate value of 3.3 s~1 for the 100 mm min~1 peel tests
SF. Christensen et al. / International Journal of Adhesion & Adhesives 18 (1998) 131—137
137
are mean values from two identical stretch experiments. Ideally the points should be on the diagonal. The deviations from the ideal are well within the range of experimental error.
4. Conclusion
Fig. 12. Comparison between measured and calculated peel force values
reported here as for the 90° peel tests reported by Kaelble. Comparison with Fig. 10 shows that, in respect to the present work, this value of eR , although too high, is of the right order of magnitude. Assuming this is also the case for the experiments reported by Kaelble, the rheology of the two systems are comparable regarding rate of deformation. On the basis of this observation it is notable that the tension at detachment is in the same range for the two systems: 3 to 4 atm. From the tension profiles obtained the integral in Eq. (2) was evaluated numerically for each observation. The peel force calculated from Eq. (2) is plotted against the measured peel force in Fig. 12. The calculated values
Observations of the peel front yield valuable information about the deformation of the adhesive during peel tests. Observations from below reveal distinct boundary effects when using wide peel strips. These boundary effects shield the main peel front from being observed from the side. This problem is overcome by using slim peel strips. Side-view observations suggest that the deformation should be approximated by uniaxial elongation at varying rate. Theoretical peel force values obtained from uniaxial stretching of the adhesive agree well with the measured values, indicating that this is a good approximation.
Acknowledgements The authors wish to thank Walther Batsberg at Department of Solid State Physics, Research Center Ris+, for performing the size exclusion chromatography.
References [1] [2] [3] [4] [5]
Gent AN, Petrich RP. Proc. R. Soc. 1969;310:433. Connelly RW, Parsons WF, Pearson GH. J. Rheol. 1981;25(3):15. Gupta RK. J. Rheol. 1983;27(2):171. Argon AS, Salama M. Mater. Sci. Eng. 1976;23:219. Kaelble DH. Trans. Soc. R. 1965;9(2):135.