Surface fluorination of polypropylene

Journal of Fluorine Chemistry 98 (1999) 107±114

Surface ¯uorination of polypropylene 1. Characterisation of surface properties F.J. du Toita,*, R.D. Sandersonb

a Poli®n, PO Box 1928, Secunda 2302, South Africa Institute for Polymer Science, University of Stellenbosch, PO Box X1, Matieland 7602, South Africa

b

Received 20 October 1998; accepted 9 April 1999

Abstract Polypropylene (PP) was exposed to various ¯uorine-gas mixtures and the ¯uorinated PP surfaces were characterised by means of X-ray photoelectron spectroscopy, Rutherford backscattering, attenuated total re¯ectance infrared spectroscopy, solid±liquid contact angles and thermogravimetric analysis. The surface wettability and surface tensions of PP, as functions of ¯uorination and oxy¯uorination times, were also determined and discussed. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Surface ¯uorination; Polypropylene; Fluorination; Oxy¯uorination; Wettability

1. Introduction The surface properties of polymers such as adhesion, friction 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. Polyole®ns require some form of pretreatment that will create, on them, surfaces with a satisfactory degree of adhesion for such purposes as printing, bonding and coating. The poor adhesion ability of polyole®ns stems from their low surface tensions and insuf®cient chemical functionality of the surface. In industry, a number of pretreatment techniques are used to activate polymer surfaces in order to improve their adhesion properties; these techniques include plasma modi®cation, surface-graft polymerisation, chemical reaction and ¯ame treatment. The adhesion of polyole®ns such as polyethylene can be greatly increased by treating them with ¯uorine gas mixtures [1]. Early work on the surface ¯uorination of polyethylene to improve its chemical stability was done by Rudge [2] and Joffre [3] in the mid-1950's. It was ascertained that a ¯uorinated polyethylene ®lm accepts printing ink much more readily than does an untreated ®lm. Since then, Air Products have commercialised surface ¯uorination with the Airopack process [4]. During this process the permeation of non-polar liquids through high density polyethylene containers is reduced by exposing the polymer surfaces to a *Corresponding author.

¯uorine/nitrogen gas mixture during blow moulding. Several studies have been directed at improving the barrier properties of high density polyethylene by ¯uorination, but very little has been reported about the ¯uorination of other polyole®ns, such as polypropylene. This paper describes the results of a study which focused on the exposure of polypropylene to various ¯uorine gas mixtures and the characterisation of the resultant ¯uorinated surfaces. In recent years, several advanced surface-active analytical techniques have been developed which now permit sophisticated characterisation of polymer surfaces. In this study, X-ray photoelectron spectroscopy, Rutherford backscattering, attenuated total re¯ectance infrared spectroscopy, the determination of solid±liquid contact angles and thermogravimetrical analysis were used to characterise the ¯uorinated polypropylene surfaces and to determine ¯uorination kinetics. Results reported in the present paper will later be used to interpret adhesion properties of ¯uorinated polypropylene surfaces; this will be the subject of a following paper [5]. During surface ¯uorination, ¯uorine reacts highly exothermically with the surface of a hydrocarbon polymer by a free-radical chain reaction mechanism [6]. This results in the formation of a very thin, partially ¯uorinated surface layer, the thickness of which is controlled by the diffusion of the ¯uorine gas through this layer. While the surface properties of hydrocarbon polymers are changed dramatically, the bulk of the polymer remains unchanged. The nature and the thickness of this ¯uorocarbon layer depend on reaction

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 1 - 3

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F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 107±114

variables such as ¯uorine concentration, reaction time and reaction temperature. The direct ¯uorination reaction is best controlled by dilution of the ¯uorine. Lagow and Margrave [6] reported on the importance of diluting ¯uorine gas with an oxygenfree inert gas to prevent fragmentation of the carbon backbone. Nitrogen and helium have been used extensively [7,8]. It is however still believed by many research groups that oxidation always accompanies ¯uorination, since commercial ¯uorine is known to contain oxygen as an impurity [9,10]. During oxy¯uorination, hydrocarbon polymers are simultaneously ¯uorinated and functionalised by signi®cant amounts of oxygen in the ¯uorinating reaction mixture or by the use of oxygen as a reactive diluting gas [6]. There remains, however, uncertainty about the mechanism by which oxygen is introduced into these surfaces and about the nature of the functionalisation. The existence of acid ¯uoride groups and peroxides as well as crosslinking of the surface during oxy¯uorination have been reported [8,11]. 2. Experimental 2.1. Materials and surface treatment Polypropylene ®lm (30 mm thick) was obtained from POLIFIN (RSA) and exposed to dilute ¯uorine gas mixtures containing 10% ¯uorine, in either nitrogen or oxygen, in a reactor from which all traces of oxygen had been removed [12]. Constant ¯uorine-gas ¯ow rates of 20 cm3minÿ1 were maintained during experiments and the reaction temperature was kept constant at 308C. 2.2. Thermogravimetry A thermobalance [13], modi®ed to be resistant to corrosive gases, was used for measuring reaction rates. The progress of the ¯uorination and oxy¯uorination reactions was monitored by measuring the mass increases which resulted from the substitution of hydrogen with ¯uorine. A polypropylene sheet, 1 mm thick, was used for kinetic studies. 2.3. Surface characterisation 2.3.1. X-ray photoelectron spectroscopy (XPS) XPS spectra were recorded on a VG ESCALAB MK 2 spectrometer. Linear background subtraction and leastsquares procedures were used to determine the peak positions, line widths and peak areas. The samples were coated with a thin layer of gold and the Au 4f7/2 peak at 84 eV was used to correct for surface charging. 2.3.2. Attenuated total reflectance infrared spectroscopy (ATR) Infrared spectra were recorded on a Nicolet Magna 550 spectrophotometer equipped with an ATR attachment. A

Table 1 Liquid surface tensions of the probe liquids, water and methylene iodide Surface tension (mN mÿ1)

Probe liquid Water Methylene iodide

T

d

p

72.8 50.8

22.1 44.1

50.7 6.7

ZnSe crystal was used as the re¯ecting element and the angle of incidence of the infrared beam was 458. 2.3.3. Surface wettability Contact angle measurements provide the most sensitive means for characterising polymer surfaces and provide Ê of a surface, whereas information about the outer 5±10 A Ê of a surface. ATR± XPS is used to sample the outer 50 A infrared studies pertain to the outer micron and more, varying with the wavelength. Advancing and receding contact angles were measured with a CAHN Dynamic Contact Angle Analyser using the Wilhelmy plate technique. Average advancing contact angles for water and methylene iodide were used to calculate the surface tensions of the treated and untreated polymers by using the harmonic-mean method that was developed by Wu [14]. The surface tensions of water and methylene iodide used are given in Table 1. 2.3.4. Rutherford backscattering (RBS) Rutherford backscattering is a fast, non-destructive technique for determining elemental depth pro®les in solids. The sample to be analysed is bombarded with a monoenergetic ion beam and the elastic scattered particles are measured with a silicon surface-barrier detector at a ®xed backward angle. The polymer samples were irradiated with 2.00 MeV alpha particles. Typical beam currents of approximately 1 nA were used and the scattered alpha particles were measured at an angle of 1658 to the direction of incidence of the beam. In order to prevent charge build-up on the targets during irradiation, the samples were pre-coated, by evaporation in vacuum, with a layer of gold, approximately 10 nm in thickness. The thicknesses and stoichiometries of elemental components in the samples were determined by comparing the observed spectra with spectra obtained from the RBS simulation programme, RUMP [15]. 3. Results and discussion 3.1. Thermogravimetry Fluorine gas reacts with hydrocarbons as follows: 

RH ‡ F2 ! R ‡ HF ‡ F



(1)

In the absence of other reactive species, chain propagation occurs with each reaction site consuming a reactive particle

F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 107±114

Fig. 1. Mass increase of a polypropylene film as a function of fluorination time.

while generating another radical: 



R ‡ F2 ! RF ‡ F

(2)

It has been found, and reported in previous work, that polymers with propylene in the structural unit had a higher ¯uorination rate than polyethylene [8,16]. Reaction rates of the ¯uorination of polypropylene sheets in which oxygen (oxy¯uorination) and nitrogen (¯uorination) were used as diluting gases are presented in Fig. 1. The rate of the ¯uorination reaction in the presence of oxygen was much slower than that of a ¯uorination reaction in which nitrogen was used as the diluting gas. This is to be expected since oxygen, which is very reactive towards radicals, inhibits the reaction by reacting with the radicals that are formed during ¯uorination (reaction (1)) to form peroxy radicals which are much less reactive than is the R radical: 



R ‡ O2 ! ROO



Increases in the mass of a polypropylene sheet as a function of treatment time, using O2 and N2 as diluting gasses, are shown in Fig. 1. Increases in the mass of a polypropylene sheet, as a function of oxy¯uorination time, are shown in Fig. 2. During the oxy¯uorination of polypropylene, an increase in the reaction temperature from

109

Fig. 3. The effect of heat treatment on fluorine-treated polypropylene.

308C to 708C and then to 908C resulted in decreased mass increases; which is not in agreement with results obtained for the oxy¯uorination of polyethylene [8]. Although it is expected that higher reaction temperatures would lead to an increased rate of radical formation, the release of reaction products of oxy¯uorination from the reacting surface at increased temperatures could explain why the mass increase was less than that found at 308C. In separate experiments, polypropylene sheet was ¯uorinated (F) and oxy¯uorinated (OF) for 1 h at 308C. Immediately after each ¯uorine treatment, the samples were heated in a nitrogen atmosphere, at a rate of 108C/min. While there was no change in mass of untreated polypropylene, heat treatment of the treated surfaces resulted in a small mass loss in the case of ¯uorinated polypropylene and a signi®cantly larger mass loss in the case of the oxy¯uorinated polypropylene. This is clearly seen in Fig. 3. A previous study has shown that hydrogen ¯uoride, as it is formed, desorbs from the polymer samples [8]. The loss of mass from the ¯uorinated sample could therefore be attributed to the loss of hydrogen ¯uoride since, after treatment for 1 h, the reaction front would have penetrated signi®cantly into the polymer substrate and by-products such as hydrogen ¯uoride would now need to diffuse out from the ¯uorinated matrix. The signi®cantly higher mass loss from the oxy¯uorinated surface also points to the possibility of the etching of the polypropylene surface during oxy¯uorination. As the temperature is increased, low molecular mass products become volatile. 3.2. Structure of the treated surfaces

Fig. 2. Mass increase as a function of oxyfluorination time.

3.2.1. After fluorination ATR±infrared spectra of untreated, oxy¯uorinated and ¯uorinated polypropylene are shown in Fig. 4. A broad CF absorption band appears at 1200 cmÿ1. No indication of oxygen-containing functionalities was obtained for either the reference sample or the ¯uorinated samples. A broad scan XPS spectrum of untreated polypropylene, Fig. 5,

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F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 107±114

Fig. 4. ATR±infrared spectra of untreated, oxyfluorinated and fluorinated polypropylene.

Fig. 5. Broad scan XPS spectrum of untreated polypropylene.

F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 107±114

111

Fig. 6. Carbon 1s XPS spectrum of fluorinated polypropylene.

does, however, show the presence of some oxygen functionality which could result from surface oxidation (a small oxygen peak was recorded). During ¯uorination, the propylene units at the surface of the material were replaced by a complex structure, the XPS spectrum of which is shown in Fig. 6. Even after long ¯uorination times, no indication of per¯uorination (±CF3) was observed and functionalisation was limited to ±CHF and CF2 groups. 3.2.2. After oxyfluorination Oxygen was incorporated in the surface during oxy¯uorination and was observed as acid ¯uoride groups (±COF) at 1850 cmÿ1 in an ATR±infrared spectrum (Fig. 4). In the presence of atmospheric moisture or water, these groups hydrolysed to the corresponding acid groups (1750 cmÿ1). It is clear from Fig. 7 that the acid ¯uoride absorption band decreased while there was a simultaneous increase in the absorption band of the hydrolysed groups, with time (at ambient conditions in atmospheric moisture). The hydrolysis reaction rate could be studied by recording infrared spectra at different reaction-time intervals and determining relative peak sizes of acid groups at 1750 cmÿ1 (see Fig. 8). The hydrolysis of oxy¯uorinated polypropylene was initi-

ally fast, but levelled off after about 4 h. After 40 h of exposure to atmospheric conditions, some unhydrolysed groups still remained. In comparison with other surface modi®cation techniques, ¯uorine atoms penetrate the surface of polypropylene to relatively great depths, the extent of which depends upon treatment conditions. The plateau in the hydrolysis curve may indicate the inability of atmospheric moisture to reach the unreacted groups deep in the polymer sample, especially since the barrier properties of the ¯uorinated and oxy¯uorinated layers are well known. Rutherford backscattering was used to determine the relative amounts of oxygen and ¯uorine incorporated during the oxy¯uorination of polypropylene (after hydrolysis). As can be seen in Fig. 9, throughout the duration of the oxy¯uorination reaction and the increase in layer thickness with oxy¯uorination time, ¯uorine and oxygen were incorporated in equal amounts. These results indicated that, together with the formation of acid groups, signi®cant ¯uorination also occurred during oxy¯uorination. 3.2.3. Surface wettability Advancing and receding contact angles of methylene iodide and water, as a function of ¯uorination treatment

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F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 107±114

Fig. 7. Hydrolysis reaction of oxyfluorinated polypropylene (ATR±infrared analysis).

Fig. 8. Hydrolysis rate of oxyfluorinated polypropylene.

F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 107±114

Fig. 9. Ratio of fluorine and oxygen atoms in oxyfluorinated polypropylene throughout duration of oxyfluorination reaction.

times, are presented in Table 2. Short ¯uorination times resulted in surfaces with slightly increased wettability. When the ¯uorination time was increased, however, the water contact angle increased again, reaching values similar to those for untreated polypropylene after about 1 h of ¯uorination. The wettability of polypropylene could be signi®cantly increased by oxy¯uorination, resulting in water contact angles which were as low as 418 (compared to 1008 for the untreated polymer). The large increase in wettability upon oxy¯uorination, even after a treatment time of 1 min, is signi®cant, since in any envisaged commercial applications one has to evaluate the improved wettability of polypropylene against the cost of its ¯uorination, which is to a large extent in¯uenced by the ¯uorination time. Even short ¯uorination times resulted in large changes in receding contact angles. Values for the receding contact angles of ¯uorinated polypropylene were much lower than those for the untreated polymer. The relatively large hysterTable 2 Water and methylene iodide contact angles on treated and untreated polypropylene Treatment

adv Water

rec Water

adv Methylene iodide

Control 10% F2/O2 1 min 5 min 15 min 30 min 60 min

100

89

60

58 48 44 41 48

31 27 30 31 33

62 59 64 63 64

10% F2/N2 1 min 5 min 15 min 30 min 60 min

82 84 86 89 99

42 43 48 56 58

66 68 69 70 73

Pure F2 10 min

95

32

±

113

Fig. 10. Surface tension of fluorinated polypropylene as function of fluorination time.

esis values (the difference between advancing and receding contact angles) for a ¯uorinated surface probably resulted from surface roughening during ¯uorination. Degrees of hysteresis were even larger when the same ®lm was exposed to pure ¯uorine. Kranz et al. [17] reported that the microroughness of a ¯uorinated polypropylene surface was signi®cantly more pronounced than that of untreated surfaces . They attributed this to the larger van der Waals radius of ¯uorine compared to that of hydrogen and to the exothermic nature of the ¯uorination reaction. Compared to ¯uorination, oxy¯uorination resulted in much lower hysteresis values. Surface tensions of polypropylene as functions of ¯uorination and oxy¯uorination times are shown in Figs. 10 and 11, respectively. Although the polypropylene molecule is non-polar, the total surface tension included a small polar component which probably resulted from surface oxidation, as was earlier demonstrated with XPS results. The dispersive component of the surface tension was considerably lowered during ¯uorination, and the polar component initially increased to almost 14 mN mÿ1 during partial ¯uorination of the surface layer. As the ¯uorination time was increased, the polar value decreased until values of the total

Fig. 11. Surface tension of oxyfluorinated polypropylene as function of oxyfluorination time.

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F.J. du Toit, R.D. Sanderson / Journal of Fluorine Chemistry 98 (1999) 107±114

surface tension that were much lower than those of the untreated polymer were obtained. This showed that, apart from the creation of a thicker layer, the outer part of the ¯uorinated layer (as determined by contact angle studies) changed continuously with ¯uorination time, with longer ¯uorination times resulting in a tendency towards per¯uorination of the surface. The inconsistency in these and the XPS results probably resulted from the difference between the depths of penetration which resulted from the use of the two techniques. Although it is expected that oxygen, if it is present in the reaction mixture, would be incorporated in the surface during ¯uorination, no evidence of such functionalisation could be detected with advancing contact angles. Lagow et al. [18] reported that the oxygen-containing functionalities that form during oxy¯uorination are stable, even at one atmosphere pressure of pure ¯uorine. It is therefore not possible to explain the turning point in the contact angle plot at longer ¯uorination times. It is believed that the initial increase in wettability upon ¯uorination (or increased polar component of the surface tension) results from dipole moments of the partially ¯uorinated polar `molecules' rather than from oxidation accompanying ¯uorination. The introduction of oxygen in small amounts into the surface, during the much slower oxy¯uorination reaction, resulted in a dramatic increase in the polar component of the surface tension: from 2 to 39 mN mÿ1. In addition, oxy¯uorination resulted in a decrease in the dispersive component of the surface tension. The total surface tension increased from 30.5 to 56.5 mN mÿ1. Exposure times of as short as 1 min, to ¯uorine concentrations of 10%, led to an increase in the total surface tension by more than 15 mN mÿ1. The authors have previously shown that the surface of oxy¯uorinated high density polyethylene changed dramatically when it was heated to 1008C [19]. When exposed to such temperatures, the surface tension of high density polyethylene decreased to almost that of the untreated polymer and the mechanism of this inversion was the migration of surface polar groups into the bulk of the polymer rather than a chemical change. It is clear from the present study that, upon heat treatment, the surface of polypropylene reacted differently to the surface of high density polyethylene; signi®cant changes occurred at the polypropylene interface upon heating. However, compared with other surface modi®cation techniques such as plasma

treatment, the inversion of ¯uorine-treated polyole®n surfaces is slow. This, together with the fact that ¯uorine gas can nowadays be safely handled in large quantities and ¯uorination can be done cost effectively, means that surface treatment with ¯uorine gas mixtures offers a viable and very effective alternate route to modifying polymer surfaces. Acknowledgements The ®rst author wishes to thank Poli®n for sponsoring the research. The Atomic Energy Corporation of South Africa is gratefully acknowledged for providing the ¯uorination facilities and Dr. P.A.B. Carstens for his valuable assistance. Dr. Len Olivier of the Department of Chemistry, University of Stellenbosch is thanked for his assistance with Rutherford Backscattering analyses. The CSIR is thanked for assistance with XPS studies. Dr. Margie Hurndall of the Institute for Polymer Science is thanked for her assistance with ®nalising this manuscript. References [1] I. Brass, B.M. Brewis, I. Sutherland, R. Wiktorowics, Int. J. Adhes. Adhes. 11 (1991) 150. [2] A.J. Rudge, British Patent 710 523 (1954). [3] S.P. Joffre, United States Patent 2 811 468 (1957). [4] D.D. Dixon, D.G. Manly, G.W. Recktenwald, United States Patent 3 862 284 (1975). [5] F.J. du Toit, J. Fluorine Chem., to be submitted. [6] R.J. Lagow, J.L. Margrave, Prog. Inorg. Chem. 26 (1979) 1439. [7] J. Shimada, M. Hoshino, J. Appl. Polym. Sci. 19 (1975) 1439. [8] R.D. Sanderson, F.J. du Toit, P.A.B. Carstens, J.B. Wagener, J. Therm. Anal. 41 (1994) 563. [9] L.J. Hayes, J. Fluorine Chem. 8 (1976) 69. [10] J.D. Le Roux et al., J. Membrane Sci. 90 (1994) 37. [11] T. Volkmann, H. Widdecke, Kunststoffe 79 (1989) 8. [12] F.J. du Toit, M.Sc Thesis, University of Stellenbosch, 1991. [13] E. Gimzewski, Thermochim. Acta 84 (1985) 7. [14] S. Wu, in: Polymer Interface and Adhesion, chapter 3, Marcel Dekker, New York, 1982, p. 98. [15] L.R. Doolittle, Nucl. Instrum. Method. B 9 (1985) 344. [16] F.J. du Toit, Surface Modification of Polymers using Elemental Fluorine, Ph.D. Thesis, University of Stellenbosch, 1995. [17] G. Kranz et al., Int. J. Adhes. Adhes. 14 (1994) 243. [18] J.L. Adcock, R.J. Lagow, S. Inoue, J. Am. Chem. Soc. 100 (1978) 1948. [19] F.J. du Toit, R.D. Sanderson, W.J. Engelbrecht, J.B. Wagener, J. Fluorine Chem. 74 (1995) 43.