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Surface fluorination of polypropylene
Journal of Fluorine Chemistry 98 (1999) 115±119
Surface ¯uorination of polypropylene 2. Adhesion properties F.J. du Toita,*, R.D. Sandersonb
b
a Poli®n, PO Box 1928, Secunda 2302, South Africa Institute for Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
Received 20 October 1998; accepted 9 April 1999
Abstract Surface ¯uorination is an interesting method of rendering surfaces more acceptable to adhesion. The adhesive properties of ¯uorinated and oxy¯uorinated propylene surfaces, using epoxy, polyester and epoxy vinyl ester adhesives, are described. Lap shear tests were carried out to determine the strength of the adhesive joints. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Polypropylene; Fluorination; Oxy¯uorination; Adhesion properties; Lap shear tests
1. Introduction The surface properties of polymers such as adhesion and permeation are largely in¯uenced by the structures of the polymer surfaces and have a strong in¯uence on the commercial applications of polymers. Applications for polypropylene and its copolymers have shown remarkable growth over the last few years. Many industrial applications require good adhesion properties and the inherent poor adhesion of polyole®ns necessitates pretreatment of their surfaces. In general, such treatments alter surfaces in one or more of the following ways:
by by by by
removing the weak boundary layer; changing the surface topography; changing the chemical nature of the surface; and/or modifying the physical structure.
Surface ¯uorination offers an interesting method of rendering polyole®n surfaces more susceptible to adhesion by, for example, printing ink, paints and coatings, lamination and conventional adhesives. When Schonhorn and Hansen [1] exposed a polyethylene surface to ¯uorine gas they recorded a great improvement in adhesion, even though the critical surface tension or wettability of the treated surface decreased (to resemble that of polytetra¯uoroethylene). It was believed that the weak boundary layer associated with polyethylene was eliminated during ¯uorination. *Corresponding author.
Brass et al. [2] studied the effects of ¯uorination on the adhesion of polyethylene and observed large increases in adhesion even when ¯uorine treatment resulted in surfaces with low surface energies. The reactions between elemental ¯uorine, diluted with nitrogen or oxygen, and the surface of polypropylene have been described by us [3]. The resulting chemical structures of the ¯uorinated and oxy¯uorinated surfaces and the surface property of wettability have also been discussed. Here we will report on the adhesive properties of these surfaces and results of lap shear tests carried out to determine the strengths of the adhesive joints. An epoxy, a polyester and an epoxy vinyl ester were used for these determinations. 1.1. Theoretical considerations of adhesion Reasons for the poor adhesion of polyole®ns and the effects of various modi®cation processes to improve this have been a source of controversy for many years. Bikerman [4] reported on the existence of a weak boundary layer at the polymer surface, implying that the main aim of modi®cation was therefore the removal of this ®ne layer of low molecular mass compounds from the surface. Brewis and Briggs [5] argued that the inadequate chemical functionality of polyole®ns was the cause of poor adhesion. Levine [6] related the critical surface tension of a polymer to the strength of the adhesive bonds and discovered that adhesion decreased with decreased critical surface tensions of wetting values; indicating the importance of an adhesive being able to wet the polymer surface. The surface tensions of nearly all adhesives
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 0 9 2 - 5
116
F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 115±119
are too large to effect the wetting of the surfaces of solids with low surface tensions. According to Levine [6] the most important aspect in the modi®cation of these surfaces is the chemical changes which result in an increase in polarity and surface free energy. Blais et al., [7] studied the correlation between the surface roughness of a polymer and adhesion. Increases in adhesive properties of polypropylene can, therefore also, be related to the mechanical anchoring of the adhesive in the pores or irregularities of the substrate. According to Haller [8] the atoms or molecules in any solid are held together by cohesion forces. After a break in a solid, the different fractions cannot be put together in such a way that the original state is restored. Reactivation of these cohesion forces would be possible only if the original intervals between the atoms could be re-established. The same applied to the joining of different materials. In practice, however, such convergence is prevented by surface roughness and weak-boundary layers that arise from the adsorption of impurities from the surroundings. Over the years, several different theories regarding the mechanism of adhesion have been developed: Mechanical interlocking: This mechanism is based on mechanical anchorage of the adhesive in pores and irregularities in the adherent. Adsorption and surface reaction: According to this theory, an adhesive molecule is attracted to a speci®c site on a solid surface and may even react with the surface. The attraction may result from van der Waals forces or speci®c donor±acceptor interactions. Diffusion: According to the diffusion theory, adhesion is achieved by the mutual penetration of both the adhesive and the substrate. Electrostatic attraction: Electrostatic forces can develop at an interface between molecules. These forces are attributed to the transfer of electrons across the interface, thereby creating positive and negative charges that attract one another. 2. Experimental 2.1. Materials
¯ushing with nitrogen, followed by evacuation, all oxygen was removed before the ¯uorine gas mixture was introduced into the reactor. Treatment times ranged between 1 and 40 min. Oxy¯uorinated samples were left in air for at least seven days to ensure complete hydrolysis. 2.3. Adhesive joint strength determination Adhesive properties of the pretreated samples were evaluated on an Instron tensiometer, using lap shear tests. A 5000 N load cell was used and the pulling rate was 5 mm/ min. Fluorine-treated polypropylene strips were glued to steel plates to prevent failure of the material on the outside of the adhesive joints. These steel plates were abraded and cleaned, and the polymer strips were glued to their surfaces with an epoxy adhesive. Samples were cured for 48 h at 508C and adhesive joints were prepared with the following joint dimensions, polyester: 26 mm15 mm and epoxy: 26 mm10 mm, and glue line thickness 0.16 mm. Each lap shear value reported is the average result of ®ve tests. 3. Results and discussion 3.1. Oxyfluorination The effects of oxygen/¯uorine gas mixtures on a polypropylene surface include improved wettability, a signi®cant increase in polarity and a roughening of the surface resulting from the exothermic nature of the reaction [3,9]. Lap shear results of oxy¯uorinated polypropylene, using three different adhesives, are given in Fig. 1. Great improvements in adhesion strengths were obtained for the polyester and the epoxy vinyl ester. There is no adhesion between the additives and untreated polypropylene. The highest lap shear value obtained was 12.8 MPa for the polyester adhesive. The fact that this was obtained after the shortest activation (oxy¯uorination) time was surprising. In general, the lap shear plots followed the same trends as did plots of
Polypropylene sheet (PP 1020, supplied by Poli®n, 2 mm thick), was used for adhesion studies. Three different adhesives were used: an epoxy (30/71), supplied by Prostruct; a polyester (N 7384 PA) and an epoxy vinyl ester (dion 9100) from NCS resins. 2.2. Surface treatment In two separate experiments, polypropylene sheet was ¯uorinated and oxy¯uorinated at 308C using 10% ¯uorine in nitrogen and oxygen, respectively. Prior to the ¯uorine treatment, samples were degreased with trichloroethylene and dried under vacuum at 508C for 24 h. By continuous
Fig. 1. Lap shear values of oxyfluorinated polypropylene using three different adhesives: an epoxy, a polyester and an epoxy vinyl ester.
F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 115±119
the changes in water contact angles and surface tensions with treatment time, i.e. signi®cant changes occurred at very short activation times and the effects soon levelled off with longer treatment times. (Refer to Part 1, The surface ¯uorination of polypropylene, in which the characteristics of the ¯uorinated and oxy¯uorinated polymers are discussed.) In disagreement with results reported by Kranz et al. [10], lap shear values for oxy¯uorinated surfaces correlated with treatment times and therefore with the oxygen content (as has been shown by the results of Rutherford backscattering) [7] and the polarity of the surface (polar component of the surface tension). In general, lap shear values increased with increased oxy¯uorination time. Lower lap shear values were obtained for the epoxy adhesive than for the polyester and the epoxy vinyl ester. It is believed that these lower values resulted from dif®culties in applying and testing the epoxy (glue line failure was experienced) rather than the epoxy being an inferior adhesive for the speci®c purpose. 3.2. Fluorination Fluorination was found to result in a complex surface structure of partially ¯uorinated propylene units, and although commercial ¯uorine contains oxygen as an impurity, no indication of the addition of oxygen-containing functional groups was observed here. Lap shear results for ¯uorinated polypropylene with a polyester adhesive are presented in Fig. 2. Although the wettability of ¯uorinated polypropylene was not much greater than of untreated polypropylene, lap shear results were surprisingly high, especially for the surface that was ¯uorinated for 20 min. (In fact, all treatment times resulted in improved surfaces.) The total surface tension of polypropylene decreased progressively with increasing ¯uorination time, but the polar components of the surface tensions were signi®cantly higher for all treated surfaces than they were for the untreated polymer (refer Part 1). This might explain the improved adhesion performance during ¯uorination, even though no correlation could be found between polarity of the surface and adhesion strength. However, this cannot be the only factor contributing to the improved adhesion
117
performance, since partially ¯uorinated commercial ¯uoropolymers such as polyvinyl ¯uoride and polytri¯uoroethylene have only `average' adhesion characteristics even although they are signi®cantly polar. Wetting, or rather the insuf®ciency of it, played a signi®cant role in the adhesion properties of polypropylene since, after evaluation of the a failed adhesive joint, it was clear that the polyester adhesive did not spread on the treated surface, but was con®ned to certain circular areas, considerably decreasing the actual joint dimensions. 3.3. Prediction of adhesion from thermodynamic compatibility Bistry and Penn [11] recently described a new method to predict adhesion at a solid/solid interface. Maximizing interfacial molecular adhesion between two solids is based on the matching of the surface energies of the two adhering solids. This is done by presenting the advancing and receding data in bar graph form and visually comparing the graphs of the different solids. Greater similarities in the bar graphs will correlate with better adhesion. This is consistent with the concept of thermodynamic compatibility, namely that solids with similar graphs have similar surfaces. Bar graphs for the treated and untreated surfaces are presented in Fig. 3. Each bar represents contact angle data for one liquid; the bottom of each bar is the cosine of the advancing contact angle, while the top is the cosine of the receding contact angle. The length of each bar is a measure of the contact angle hysteresis. The liquids used in the study are given in Table 1. Contact angles were measured with a CAHN dynamic contact angle analyser using the Wilhelmy Plate technique. Fig. 3 shows that oxy¯uorination renders polypropylene surface properties (as measured by surface tension) which are much the same as those of the polyester. The surface properties of ¯uorinated polypropylene also resemble those of the polyester more than those of the untreated polymer. This might also explain the improved adhesion that resulted from ¯uorination compared with the adhesion of the untreated polypropylene, even although the wettability was decreased. Although advancing contact angle studies cannot explain the good adhesive behaviour of ¯uorinated polypropylene, Table 1 Surface tensions of the liquids used for hysteresis studies
Fig. 2. Lap shear values of fluorinated polypropylene with a polyester adhesive.
Liquid
Surface tension (mN/m)
Water Glycerol Methylene iodide Bromonaphthalene Dimethyl formamide o-xylene
72.8 63.4 50.8 44.6 37.3 30.1
118
F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 115±119
Fig. 3. Hysteresis graphs of the polyester, untreated and treated polypropylene surfaces.
receding contact angles, often wrongly omitted in contact angle studies, may provide some answers. In addition to slight changes in advancing contact angles, ¯uorination resulted in large decreases in the receding contact angles and consequently large surface hysteresis of the polypropylene surface. Similar results were obtained for oxy¯uorinated surfaces, with the exception that oxy¯uorination had a much greater effect on the advancing contact angles. Roughening of the surface may contribute the close similarity between the hysteresis graph of the ¯uorinated surface and that of the adhesive, since surface roughness is (amongst other factors) a cause of contact angle hysteresis. However, although ¯uorination caused surface roughness and therefore increased the contact area between the substrate and the adhesive, Kranz et al. [10] reported that the growth in surface area upon ¯uorination was too small to explain the marked improvement in bonding strength. In addition to surface roughness, surface heterogeneity and surface restructuring are possible causes of contact angle hysteresis. It has been reported in an earlier paper by us [12] that high density oxy¯uorinated polyethylene surfaces reconstructed upon heat treatment. However, no such effect was observed for ¯uorinated surfaces. This observation, together with the absence of polar functional groups on ¯uorinated surfaces that could be oriented away from the surface to minimise its surface free energy (as was observed during oxy¯uorination), makes reconstruction upon heat treatment an unlikely cause of the large hysteresis of the ¯uorinated surfaces. Heterogeneous surfaces are constituted by domains of
different compositions and hence different wetting properties. A large number of metastable states are introduced, giving rise to large hysteresis values. In contact angle studies, advancing contact angles are sensitive to the less wettable part of a surface, while receding angles are very sensitive to small fractions of high surface energy materials [13]. Although no evidence of oxygen-containing functionalities could be detected with infrared spectroscopy or advancing contact angles, it is known that commercial ¯uorine contains oxygen as an impurity. In addition, several authors are convinced that oxidation always accompanies ¯uorination, even in atmospheres `substantially' free of oxygen [14]. Since it is the receding contact angles and not the advancing contact angles that are sensitive to small amounts of high energy material, the large hysteresis and improved adhesion of ¯uorinated surfaces may be explained by a slight oxidation of the polypropylene surface during ¯uorination. Because of the small amount of oxygen incorporated during ¯uorination, changes could be detected with receding contact angles studies, but not from advancing contact angle data nor from surface tension data, the latter being determined from advancing data. Acknowledgements The ®nancial support of Poli®n is gratefully acknowledged. Dr. Margie Hurndall of the Institute for Polymer Science is thanked for her assistance with ®nalising this paper.
F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 115±119
References [1] H. Schonhorn, R.H. Hansen, J. Appl. Polym. Sci. 12 (1968) 1231. [2] I. Brass, D.M. Brewis, I. Sutherland, R. Wiktorowics, Int. J. Adhes. Adhes. 11 (1991) 150. [3] F.J. du Toit, J. Fluorine Chem., submitted. [4] J.J. Bikerman, Ind. Eng. Chem. 59 (1967) 41. [5] D.M. Brewis, D. Briggs, Industrial Adhesion Problems, Orbital Press, Oxford, 1985. [6] M. Levine et al., Polym. Lett. 2 (1964) 915. [7] P. Blais, D.J. Carlsson, G.W. Csullog, D.M. Wiles, J. Colloid Interface Sci. 47 (1974) 636.
119
[8] W. Haller, in: Wolfgang Gerhartz (Ed.), Ullman's Encyclopedia of Industrial Chemistry, vol. A1, VCH Verlagsgesellschaft mbH, Weinheim, FRG, 1988. [9] F.J. du Toit, Ph.D. Thesis, Surface modification of polymers using elemental flourine, University of Stellenbosch, 1995. [10] G. Kranz et al., Int. J. Adhes. Adhes. 14 (1994) 243. [11] F.A. Bistry, L.S. Penn, Surf. Interface Anal. 5 (1983) 98. [12] F.J. du Toit, R.D. Sanderson, W.J. Engelbrecht, J.B. Wagener, J. Fluorine Chem. 74 (1995) 43. [13] M. Morra, E. Oschiello, F. Garbassi, Adv. Colloid Interface 32 (1990) 79. [14] L.J. Hayes, J. Fluorine Chem. 8 (1976) 69.
Surface ¯uorination of polypropylene 2. Adhesion properties F.J. du Toita,*, R.D. Sandersonb
b
a Poli®n, PO Box 1928, Secunda 2302, South Africa Institute for Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
Received 20 October 1998; accepted 9 April 1999
Abstract Surface ¯uorination is an interesting method of rendering surfaces more acceptable to adhesion. The adhesive properties of ¯uorinated and oxy¯uorinated propylene surfaces, using epoxy, polyester and epoxy vinyl ester adhesives, are described. Lap shear tests were carried out to determine the strength of the adhesive joints. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Polypropylene; Fluorination; Oxy¯uorination; Adhesion properties; Lap shear tests
1. Introduction The surface properties of polymers such as adhesion and permeation are largely in¯uenced by the structures of the polymer surfaces and have a strong in¯uence on the commercial applications of polymers. Applications for polypropylene and its copolymers have shown remarkable growth over the last few years. Many industrial applications require good adhesion properties and the inherent poor adhesion of polyole®ns necessitates pretreatment of their surfaces. In general, such treatments alter surfaces in one or more of the following ways:
by by by by
removing the weak boundary layer; changing the surface topography; changing the chemical nature of the surface; and/or modifying the physical structure.
Surface ¯uorination offers an interesting method of rendering polyole®n surfaces more susceptible to adhesion by, for example, printing ink, paints and coatings, lamination and conventional adhesives. When Schonhorn and Hansen [1] exposed a polyethylene surface to ¯uorine gas they recorded a great improvement in adhesion, even though the critical surface tension or wettability of the treated surface decreased (to resemble that of polytetra¯uoroethylene). It was believed that the weak boundary layer associated with polyethylene was eliminated during ¯uorination. *Corresponding author.
Brass et al. [2] studied the effects of ¯uorination on the adhesion of polyethylene and observed large increases in adhesion even when ¯uorine treatment resulted in surfaces with low surface energies. The reactions between elemental ¯uorine, diluted with nitrogen or oxygen, and the surface of polypropylene have been described by us [3]. The resulting chemical structures of the ¯uorinated and oxy¯uorinated surfaces and the surface property of wettability have also been discussed. Here we will report on the adhesive properties of these surfaces and results of lap shear tests carried out to determine the strengths of the adhesive joints. An epoxy, a polyester and an epoxy vinyl ester were used for these determinations. 1.1. Theoretical considerations of adhesion Reasons for the poor adhesion of polyole®ns and the effects of various modi®cation processes to improve this have been a source of controversy for many years. Bikerman [4] reported on the existence of a weak boundary layer at the polymer surface, implying that the main aim of modi®cation was therefore the removal of this ®ne layer of low molecular mass compounds from the surface. Brewis and Briggs [5] argued that the inadequate chemical functionality of polyole®ns was the cause of poor adhesion. Levine [6] related the critical surface tension of a polymer to the strength of the adhesive bonds and discovered that adhesion decreased with decreased critical surface tensions of wetting values; indicating the importance of an adhesive being able to wet the polymer surface. The surface tensions of nearly all adhesives
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 0 9 2 - 5
116
F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 115±119
are too large to effect the wetting of the surfaces of solids with low surface tensions. According to Levine [6] the most important aspect in the modi®cation of these surfaces is the chemical changes which result in an increase in polarity and surface free energy. Blais et al., [7] studied the correlation between the surface roughness of a polymer and adhesion. Increases in adhesive properties of polypropylene can, therefore also, be related to the mechanical anchoring of the adhesive in the pores or irregularities of the substrate. According to Haller [8] the atoms or molecules in any solid are held together by cohesion forces. After a break in a solid, the different fractions cannot be put together in such a way that the original state is restored. Reactivation of these cohesion forces would be possible only if the original intervals between the atoms could be re-established. The same applied to the joining of different materials. In practice, however, such convergence is prevented by surface roughness and weak-boundary layers that arise from the adsorption of impurities from the surroundings. Over the years, several different theories regarding the mechanism of adhesion have been developed: Mechanical interlocking: This mechanism is based on mechanical anchorage of the adhesive in pores and irregularities in the adherent. Adsorption and surface reaction: According to this theory, an adhesive molecule is attracted to a speci®c site on a solid surface and may even react with the surface. The attraction may result from van der Waals forces or speci®c donor±acceptor interactions. Diffusion: According to the diffusion theory, adhesion is achieved by the mutual penetration of both the adhesive and the substrate. Electrostatic attraction: Electrostatic forces can develop at an interface between molecules. These forces are attributed to the transfer of electrons across the interface, thereby creating positive and negative charges that attract one another. 2. Experimental 2.1. Materials
¯ushing with nitrogen, followed by evacuation, all oxygen was removed before the ¯uorine gas mixture was introduced into the reactor. Treatment times ranged between 1 and 40 min. Oxy¯uorinated samples were left in air for at least seven days to ensure complete hydrolysis. 2.3. Adhesive joint strength determination Adhesive properties of the pretreated samples were evaluated on an Instron tensiometer, using lap shear tests. A 5000 N load cell was used and the pulling rate was 5 mm/ min. Fluorine-treated polypropylene strips were glued to steel plates to prevent failure of the material on the outside of the adhesive joints. These steel plates were abraded and cleaned, and the polymer strips were glued to their surfaces with an epoxy adhesive. Samples were cured for 48 h at 508C and adhesive joints were prepared with the following joint dimensions, polyester: 26 mm15 mm and epoxy: 26 mm10 mm, and glue line thickness 0.16 mm. Each lap shear value reported is the average result of ®ve tests. 3. Results and discussion 3.1. Oxyfluorination The effects of oxygen/¯uorine gas mixtures on a polypropylene surface include improved wettability, a signi®cant increase in polarity and a roughening of the surface resulting from the exothermic nature of the reaction [3,9]. Lap shear results of oxy¯uorinated polypropylene, using three different adhesives, are given in Fig. 1. Great improvements in adhesion strengths were obtained for the polyester and the epoxy vinyl ester. There is no adhesion between the additives and untreated polypropylene. The highest lap shear value obtained was 12.8 MPa for the polyester adhesive. The fact that this was obtained after the shortest activation (oxy¯uorination) time was surprising. In general, the lap shear plots followed the same trends as did plots of
Polypropylene sheet (PP 1020, supplied by Poli®n, 2 mm thick), was used for adhesion studies. Three different adhesives were used: an epoxy (30/71), supplied by Prostruct; a polyester (N 7384 PA) and an epoxy vinyl ester (dion 9100) from NCS resins. 2.2. Surface treatment In two separate experiments, polypropylene sheet was ¯uorinated and oxy¯uorinated at 308C using 10% ¯uorine in nitrogen and oxygen, respectively. Prior to the ¯uorine treatment, samples were degreased with trichloroethylene and dried under vacuum at 508C for 24 h. By continuous
Fig. 1. Lap shear values of oxyfluorinated polypropylene using three different adhesives: an epoxy, a polyester and an epoxy vinyl ester.
F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 115±119
the changes in water contact angles and surface tensions with treatment time, i.e. signi®cant changes occurred at very short activation times and the effects soon levelled off with longer treatment times. (Refer to Part 1, The surface ¯uorination of polypropylene, in which the characteristics of the ¯uorinated and oxy¯uorinated polymers are discussed.) In disagreement with results reported by Kranz et al. [10], lap shear values for oxy¯uorinated surfaces correlated with treatment times and therefore with the oxygen content (as has been shown by the results of Rutherford backscattering) [7] and the polarity of the surface (polar component of the surface tension). In general, lap shear values increased with increased oxy¯uorination time. Lower lap shear values were obtained for the epoxy adhesive than for the polyester and the epoxy vinyl ester. It is believed that these lower values resulted from dif®culties in applying and testing the epoxy (glue line failure was experienced) rather than the epoxy being an inferior adhesive for the speci®c purpose. 3.2. Fluorination Fluorination was found to result in a complex surface structure of partially ¯uorinated propylene units, and although commercial ¯uorine contains oxygen as an impurity, no indication of the addition of oxygen-containing functional groups was observed here. Lap shear results for ¯uorinated polypropylene with a polyester adhesive are presented in Fig. 2. Although the wettability of ¯uorinated polypropylene was not much greater than of untreated polypropylene, lap shear results were surprisingly high, especially for the surface that was ¯uorinated for 20 min. (In fact, all treatment times resulted in improved surfaces.) The total surface tension of polypropylene decreased progressively with increasing ¯uorination time, but the polar components of the surface tensions were signi®cantly higher for all treated surfaces than they were for the untreated polymer (refer Part 1). This might explain the improved adhesion performance during ¯uorination, even though no correlation could be found between polarity of the surface and adhesion strength. However, this cannot be the only factor contributing to the improved adhesion
117
performance, since partially ¯uorinated commercial ¯uoropolymers such as polyvinyl ¯uoride and polytri¯uoroethylene have only `average' adhesion characteristics even although they are signi®cantly polar. Wetting, or rather the insuf®ciency of it, played a signi®cant role in the adhesion properties of polypropylene since, after evaluation of the a failed adhesive joint, it was clear that the polyester adhesive did not spread on the treated surface, but was con®ned to certain circular areas, considerably decreasing the actual joint dimensions. 3.3. Prediction of adhesion from thermodynamic compatibility Bistry and Penn [11] recently described a new method to predict adhesion at a solid/solid interface. Maximizing interfacial molecular adhesion between two solids is based on the matching of the surface energies of the two adhering solids. This is done by presenting the advancing and receding data in bar graph form and visually comparing the graphs of the different solids. Greater similarities in the bar graphs will correlate with better adhesion. This is consistent with the concept of thermodynamic compatibility, namely that solids with similar graphs have similar surfaces. Bar graphs for the treated and untreated surfaces are presented in Fig. 3. Each bar represents contact angle data for one liquid; the bottom of each bar is the cosine of the advancing contact angle, while the top is the cosine of the receding contact angle. The length of each bar is a measure of the contact angle hysteresis. The liquids used in the study are given in Table 1. Contact angles were measured with a CAHN dynamic contact angle analyser using the Wilhelmy Plate technique. Fig. 3 shows that oxy¯uorination renders polypropylene surface properties (as measured by surface tension) which are much the same as those of the polyester. The surface properties of ¯uorinated polypropylene also resemble those of the polyester more than those of the untreated polymer. This might also explain the improved adhesion that resulted from ¯uorination compared with the adhesion of the untreated polypropylene, even although the wettability was decreased. Although advancing contact angle studies cannot explain the good adhesive behaviour of ¯uorinated polypropylene, Table 1 Surface tensions of the liquids used for hysteresis studies
Fig. 2. Lap shear values of fluorinated polypropylene with a polyester adhesive.
Liquid
Surface tension (mN/m)
Water Glycerol Methylene iodide Bromonaphthalene Dimethyl formamide o-xylene
72.8 63.4 50.8 44.6 37.3 30.1
118
F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 115±119
Fig. 3. Hysteresis graphs of the polyester, untreated and treated polypropylene surfaces.
receding contact angles, often wrongly omitted in contact angle studies, may provide some answers. In addition to slight changes in advancing contact angles, ¯uorination resulted in large decreases in the receding contact angles and consequently large surface hysteresis of the polypropylene surface. Similar results were obtained for oxy¯uorinated surfaces, with the exception that oxy¯uorination had a much greater effect on the advancing contact angles. Roughening of the surface may contribute the close similarity between the hysteresis graph of the ¯uorinated surface and that of the adhesive, since surface roughness is (amongst other factors) a cause of contact angle hysteresis. However, although ¯uorination caused surface roughness and therefore increased the contact area between the substrate and the adhesive, Kranz et al. [10] reported that the growth in surface area upon ¯uorination was too small to explain the marked improvement in bonding strength. In addition to surface roughness, surface heterogeneity and surface restructuring are possible causes of contact angle hysteresis. It has been reported in an earlier paper by us [12] that high density oxy¯uorinated polyethylene surfaces reconstructed upon heat treatment. However, no such effect was observed for ¯uorinated surfaces. This observation, together with the absence of polar functional groups on ¯uorinated surfaces that could be oriented away from the surface to minimise its surface free energy (as was observed during oxy¯uorination), makes reconstruction upon heat treatment an unlikely cause of the large hysteresis of the ¯uorinated surfaces. Heterogeneous surfaces are constituted by domains of
different compositions and hence different wetting properties. A large number of metastable states are introduced, giving rise to large hysteresis values. In contact angle studies, advancing contact angles are sensitive to the less wettable part of a surface, while receding angles are very sensitive to small fractions of high surface energy materials [13]. Although no evidence of oxygen-containing functionalities could be detected with infrared spectroscopy or advancing contact angles, it is known that commercial ¯uorine contains oxygen as an impurity. In addition, several authors are convinced that oxidation always accompanies ¯uorination, even in atmospheres `substantially' free of oxygen [14]. Since it is the receding contact angles and not the advancing contact angles that are sensitive to small amounts of high energy material, the large hysteresis and improved adhesion of ¯uorinated surfaces may be explained by a slight oxidation of the polypropylene surface during ¯uorination. Because of the small amount of oxygen incorporated during ¯uorination, changes could be detected with receding contact angles studies, but not from advancing contact angle data nor from surface tension data, the latter being determined from advancing data. Acknowledgements The ®nancial support of Poli®n is gratefully acknowledged. Dr. Margie Hurndall of the Institute for Polymer Science is thanked for her assistance with ®nalising this paper.
F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 115±119
References [1] H. Schonhorn, R.H. Hansen, J. Appl. Polym. Sci. 12 (1968) 1231. [2] I. Brass, D.M. Brewis, I. Sutherland, R. Wiktorowics, Int. J. Adhes. Adhes. 11 (1991) 150. [3] F.J. du Toit, J. Fluorine Chem., submitted. [4] J.J. Bikerman, Ind. Eng. Chem. 59 (1967) 41. [5] D.M. Brewis, D. Briggs, Industrial Adhesion Problems, Orbital Press, Oxford, 1985. [6] M. Levine et al., Polym. Lett. 2 (1964) 915. [7] P. Blais, D.J. Carlsson, G.W. Csullog, D.M. Wiles, J. Colloid Interface Sci. 47 (1974) 636.
119
[8] W. Haller, in: Wolfgang Gerhartz (Ed.), Ullman's Encyclopedia of Industrial Chemistry, vol. A1, VCH Verlagsgesellschaft mbH, Weinheim, FRG, 1988. [9] F.J. du Toit, Ph.D. Thesis, Surface modification of polymers using elemental flourine, University of Stellenbosch, 1995. [10] G. Kranz et al., Int. J. Adhes. Adhes. 14 (1994) 243. [11] F.A. Bistry, L.S. Penn, Surf. Interface Anal. 5 (1983) 98. [12] F.J. du Toit, R.D. Sanderson, W.J. Engelbrecht, J.B. Wagener, J. Fluorine Chem. 74 (1995) 43. [13] M. Morra, E. Oschiello, F. Garbassi, Adv. Colloid Interface 32 (1990) 79. [14] L.J. Hayes, J. Fluorine Chem. 8 (1976) 69.