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An atomic force microscopy study of corona-treated polypropylene films
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Applied Surface Science 64 (1993) 197-203 North-Holland
”
:,:
‘h
. .. . . . .. . . . . ,:.::: p ,:,,:?:
applied
surface science
An atomic force microscopy study of corona-treated polypropylene films R.M. Overney
*, R. Liithi, H. Haefke,
J. Frommer,
E. Meyer,
H.-J. Giintherodt
Institute of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
S. Hild and J. Fuhrmann Institute of Physical Chemistry, University of Clausthal, Arnold-Sommerfeki-Strasse 4, D(W)-3392 Clausthal, Germany Received 28 April 1992; accepted for publication 20 October 1992
The surfaces of corona-treated isotactic polypropylene films have been investigated by atomic force microscopy. The occurrence of droplets on the film surfaces is related to the energy dose of the corona discharge. The sizes of these droplets correlate with the corona dose. The loss of adhesive strength of self-adhered polypropylene films can be explained on the basis of morphology changes during corona treatment. A comparative study of uniaxial and biaxial polypropylene films is presented.
1. Introduction Corona treatment is bne of the most applied techniques to improve the adhesion characteristics of polyolefin polymers. Isotactic polypropylene (PP) is a widely used synthetic polymer which requires for technical application some improvement by corona treatment [l]. The corona treatment produces polar chemical functional groups at the polymer surface [2,31. Contact-angle measurements and electron spectroscopy showed that electrons, emitted by the corona-induced photoeffect, are mainly responsible for the surface activation [4]. This surface activation leads to two important simultaneous chemical processes: (a) cross-linking of the polymer chains, i.e. immobilization of self-diffusive or interdiffusive polymer chain motions and (b) oxidative polymer chain degradation, i.e. creation of more or less mobile chemical degradation products on surface areas of the polymer. It is known that corona treatment can change the morphology of polymer films;
* To whom correspondence 0169-4332/93/$06.00
should be addressed.
owing to this the level of adhesion is affected [51. The adhesion can either increase or decrease depending on the potential bonding area and the presence of voids. This has been observed by self-adhesion experiments on corona-treated polyolefins [6,7]. Surface changes have been observed by X-ray photoelectron spectroscopy [8-101 and infrared (IR) spectroscopy [ 111. Here, we present a study on the correspondence between the results of peel experiments and changes of the morphology of corona-treated PP films documented by atomic force microscopy (AFM). Atomic force microscopes [12] are able to image surfaces of conducting and nonconducting materials by scanning a sharp tip, mounted on a very sensitive spring, over the sample surface. The forces acting on the probing tip deflect the cantilever-type spring. Forces of the order of lo-” to 10T6 N can be measured by this technique and a lateral resolution of a few Hngstriims can be achieved. The displacement of the tip is measured by electron tunneling [12], optical interferometry, beam deflection or capacitance measurements [13]. For this study an atomic force microscope of a beam deflection type was used.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
198
R.M. Ovemey et al. / AFM of corona-treated polypropylene films
AFM images (termed force micrographs) recorded on the micrometer scale.
were
2. Experimental Two kinds of isotactic PP films have been investigated. The films contain an anti-oxidant and a lubricant (calcium stearate) from the manufacturing process. For our investigations PP films with different morphologies have been used. (1) PP films have been stretched uniaxially during crystallization, leading to uniaxially oriented PP (OPP). The orientation of these films have been determined by IR dichroism measurements. A high orientation in the reference plane of the OPP films has been determined. (2) In order to achieve biaxially oriented PP (BOPP) the OPP films have been stretched perpendicularly to the previously produced orientation axis. As result of this two-step stretching process no preferred orientation in the BOPP films can be detected by IR dichroism measurements. The film thicknesses are determined to be 35 pm for OPP and 25 pm
for BbPP. By differential scanning calorimetry, OPP and BOPP films showed a degree of crystallinity of 67% and 72%, respectively. PP films were treated with a corona discharge apparatus (Ahlbrandt System GmbH) with a typical power of 1350 W and a frequency of 18 kHz. The gap width between electrode and grounded sample holder was 0.3 cm. The electrode was 12 cm wide and could be moved at various speeds from 0.25 to 15 cm/s. The intensity of corona discharge depends on this speed. The corona dose (CD) ranges between 7 and 112.5 J/cm2 and is defined as 1141: power of the corona discharge apparatus CDs
width of the electrode x speed
.
For self-adhesion peel experiments the samples were adhered immediately after corona treatment for 6 min at a temperature of 75°C and a pressure of 5 X lo5 Pa. The samples were oriented parallel to machine direction for the adhering. For adhesive strength measurements the Tpeeltest [15] has been used. The peeling velocity was kept at 0.1 mm/s.
Fig. 1. Force micrographs of polypropylene films. (a) Uniaxial polypropylene (OPP) films are characterized by stria-like surface structure which is aligned along the stretching direction. The mean width between striae is about 1 pm and their heights were determined to be about 50 nm. (b) Biaxial polypropylene (BOPP) films do not exhibit the characteristic striated surface structure of OPP films. Occasionally striae are still visible in some surface areas. Scanned surface areas: 100 X 100 pm* (a) and 90 X 90 pm* (b).
RiU. Overney et al. / AFM of corona-treated polypropylene
fhs
199
air. The applied force is of the order of 10 nN and kept constant during the measurements. Here, the AFM images are presented as unfiltered data with an uncertainty of 5% in lateral and 10% in vertical dimensions.
3. Results and discussion
Fig. 2. Force micrograph of an OPP film treated with a corona dose (CD) of 60 J/cm*. The uniaxial surface structure is more ramified than the untreated surface of the OPP film (cf. fig. la). Scanned surface area 100~ 100 pm’.
The atomic force microscope, which is used to document morphological changes of the coronatreated PP films, was a commercially available instrument with a 120 pm scanner. The film surfaces were scanned by a V-shaped force sensor (termed cantilever or, in short, lever) with integrated S&N, tip and spring constant of 0.12 N/m. All measurements have been performed in
Fig. la shows a force micrograph of an OPP film. Surface corrugations are imaged in grey scales, whereas white parts represent elevations and black parts represent grooves. The surface is characterized by a stria-like structure with a mean distance between two striae of less than 1 pm. The height of the striae are of the order of 50 nm and their longitudinal axes are aligned along the direction of the manufacturing process. Therefore, the striae reflect a high anisotropy of the surface. In contrast to it, BOPP films exhibit no significant alignment of surface structure (fig. lb). The parallel-aligned striae of the OPP film disappear by additional stretching procedures. Corona treatments act on the OPP and BOPP films quite differently. Whereas on the BOPP surface no obvious changes in morphology are detected optically, the surface of the OPP film undergoes a rearrangement during the second stretching. Fig. 2 shows an uniaxial PP film treated
Fig. 3. Force micrographs of an OPP film treated with a corona dose (CD) of 112.5 J/cm’. (a) Droplet-like features occur on the striated film surface. They are stable at the AFM scanning conditions used here. (b) Droplets of about 500 nm in diameter and 60 nm in height are imaged at higher magnification. Scanned surface areas: 20 X 20 pm2 (a) and 3 X 3 wrn2 (b).
R.M. Overney et al. / AFM of corona-treated polypropylene films
200
CD [J/cm*] Fig. 4. Changes in diameter (+) and height (0) of droplets as a function of corona dose (CD).
with a CD of 60 J/cm’. The surface structure is more ramified than in the untreated OPP film (cf. fig. la). This ramification can be observed at all corona dose stages up to 60 J/crn2. At a CD of 112.5 J/cm2, however, the parallel striated structure remains as in the untreated film (fig. 3a). In addition to the alignment of the striae droplet-like features appear on the film surface. The droplets are observed on the striated OPP films by using discharge doses between 22.5 and 112.5 J/cm*. At this high density of corona treatment, the OPP films turn milky to the eye. Fig. 3b
shows a typical droplet ensemble on an OPP film at higher magnification. The diameters of the droplets increase with increasing CD (fig. 4). Table 1 contains diameters and heights of these discharge-induced surface features. At a CD of 112.5 J/cm2 the largest droplets reached a dimension of more than 0.5 pm in diameter. Below a CD of 22.5 J/cm2 the size of the droplets is of the order of the surface roughness and provides a bad statistic. Additional information is reached with gel permeation chromatography (GPC) and attenu-
Table 1 Molar weight distribution (gel permeation chromatography, GPC), oxidation (attenuated total reflection, ATR), and diameters and heights of the droplets on OPP films as a function of the corona dose (CD) CD stage
Energy dose (J/cm21
GPC Molar weight distribution
ATR Oxidation (butanon a) 1726 cm-‘)
AFM Diameter of droplets (nm)
AFM Heights of droplets (nm)
0 1 2 3 4 5 6 7 8
0.0 7.5 10.0 15.0 18.0 22.5 36.0 60.0 112.5
1.94 1.92 1.95 2.0 2.0 1.95 1.98 1.92 1.08
0.000 0.075 0.093 0.185 0.202 0.297 0.428 0.820 1.042
182 200 312 500
10 17 25 64
a) Methyl ethyl ketone (butanon) is representative
AFM Droplets per 10 wrn’
128 66 35 27
for the oxidative degradation provided by ATR spectroscopy.
RM. Ovemey et al. / AFM of corona-treated polypropylene film
ated total reflection (ATR) infrared spectroscopy (cf. table 1). GPC provides a slight increase of the molar weight distribution with increasing the CD energy in the range of O-18.0 J/cm2. Above 18.0 J/cm2 the molecular weight of the surface molecules decreases with increasing CD. For example, at a CD energy of 112.5 J/cm2, the molecular weight is about 500 g/mol, i.e. about 16 carbon atoms per degraded molecule. That the molecular weight of the PP molecules in surfaces decreases during corona treatment has already been shown by different authors 116,171.The ATR spectroscopy provides that oxidation takes place during all stages of the corona treatment (cf. table 1). The increase but also the decrease of the molecular weight can be described by two oxidation processes. First, oxygen can be chemisorbed by the polymer chains which causes an increased molar mass as observed by ATP spectroscopy below 18.0 J/cm2. On the other hand, the reaction with oxygen can also cause degradation of polymer molecules which becomes dominant above 18.0 J/cm 2. GPC, ATR and AFM show that droplets on the PP surface, are built up by
201
degradation products of oxidation. A possible explanation for the occurrence of the droplets is local surface melting of degradation products during corona treatment. The surface temperature can easily be raised considerably because of the low heat conductivity of PP. Because of the increased surface tension the PP molecules form droplets. In fig. 5 the GPC and ATR data of corona-treated OPP are schematically compared. Films of BOPP do not exhibit such droplet-like features on being corona-treated in the same energy regimes although GPC and ATR provide qualitatively similar results as OPP (cf. table 2). Only surfaces treated with a CD of 112.5 J/cm2 show occasionally droplets aligned along remaining striae (fig. 6). We assume that the polar functional groups, which are created preferentially on the amorphous surface sites of the PP samples [18], physisorb the droplets. This would explain the differences in the density of the droplets on OPP and BOPP films (cf. degree of crystallinity in section Experimental). Fig. 7 shows the morphology of an OPP film treated with 112.5 J/cm2 and afterwards softly
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
0
20
40
60
80
100
120
Corona discharge [J/cm21 Fig. 5. Gel permeation chromatography (GPO and attenuated total reflection (ATR) infrared spectroscopy results are schematically drawn from table 1 of corona-treated OPP. With increasing corona dose (CD) GPC provides above 18.0 J/cm* a decreasing molar weight distribution. At CD 112.5 J/cm*, the molecular weight of degraded molecules is measured to be about 500 g/mol (corresponds to about a molecular chain of 16 carbon atoms per molecule). The ATR spectroscopy points out that oxidation takes place during all stages of CD.
R.M. Ovemey et al. / AFM of corona-treated polypropylene films
202
Table 2 Molar weight distribution and attenuated total reflection (ATR) spectroscopy on BOPP films as a function of the corona dose (CD) CD stage
Energy dose (J/cm*) 0
7.5 10.0 15.0 18.0 22.5 36.0 60.0 112.5
GPC Molar weight distribution
ATR Oxidation (butanon a) 1726 cm-‘)
1.80 1.85 1.85 1.82 1.94 1.93 1.90 1.81 1.08
0.000
0.067 0.004 0.021 0.123 0.182 0.406 0.619 0.737
a) Methyl ethyl ketone (butanon) is representative for the oxidative degradation provided by ATR spectroscopy.
wiped with a cotton tissue. No droplets remain on the film surface. However, under the scanning conditions used to image the films, it was not possible to remove the droplets by means of the AFM cantilever. Peel-experiments of self-adhered OPP films indicate an increasing of the peel-off force by increasing CD up to 18 J/cm2. At higher discharge energies the peel-off force decreases (fig. 8). These results are in good agreement with the morphological changes observed by AFM. The
Fig. 7. Force micrograph of an OPP film treated with a corona dose of 112.5 J/cm*. Using a cotton tissue to wipe the film surface the droplets are removed. Scanned surface area 20 x 20 pm2.
detection threshold for the appearance of droplets is at 22.5 J/cm2. The peel-off force of the samples treated with a CD of 112.5 J/cm2 is smaller than the peel-off force of the samples treated with the same corona dose and wiped afterwards. Therefore, we conclude that the self-adhesion of the OPP films strongly depends on the density and size of the discharge-induced droplets. For BOPP films the peel-off force increases monot-
-:P.Y I
-m-
OPP
-c-
BOPP
I
-
20
Fig. 6. Force micrograph of a BOPP film treated with a corona dose of 112.5 J/cm*. The density of the droplets is much less than on OPP tilms (cf. fig. 3b). The droplets tend to occur preferentially in the direction of the remaining striae. Scanned surface area 3.5 x 3.5 pm*.
40
60
80
loa
120
Comnadwe[J/cm21
Fig. 8. Normalized peel-off force as a function of corona dose (CD). On a CD of about 18 J/cm* a maximum in force is reached for OPP films, whereas BOPP films level off to a constant value at 20 J/cm*.
R.M. Ovemey et al. / AFM of corona-treated po&propylene films
onously within the CD range. At a CD of about 20 J/cm2 the force levels off to a constant value. This is the same region of CD in which OPP films reach the maximum peel-off force. The saturation is supported by the fact that no droplets appear on the film surfaces.
4. Summary Using AFM it is possible to relate the loss of adhesive strength of corona-treated PP films with the change of their morphology. Droplets are formed on the striated surface. Their diameters and heights increase with increasing CD. The adhesion between droplets and bulk is assumed to be the limiting factor for adhesion. Self-adhesion cannot be stronger and therefore the attractive component between bulk and droplets set the adhesion characteristics. Biaxial PP films which were also investigated by AFM showed a very decreased density of droplets on the surface. It corresponds with the peel experiments showing no maximum peel-off force. Gxidative degradation and local surface melting or sublimation is proposed as a possible explanation for the formation of the droplets.
Acknowledgments This work was supported by the Swiss National Science Foundation, the Kommission zur FSrderung der wissenschaftlichen Forschung (Switzerland) and the Arbeitsgemeinschaft indus-
trieller Forschungsvereinigungen 7706 (Germany).
203
e.V.,
Projekt
References [ll C.Y. Kim, J. Evans and D.A.I. Goring, J. Appl. Polym. Sci. 15 (1971) 1357, 1365. El W. Stiller and J. Friedrich, Z. Chem. 21 (1981) 91. [31 H. Yasuda, A. Akarma and T. Yasuda, J. Appl. Polym. Sci. 19 (1981) 1285. I41 H. Steinhauser and G. Ellinghorst, Angew. Makromol. Chem. 120 (1984) 177. El D.M. Brewis and D. Briggs, Polymer 22 (1981) 7. bl M. Stradal and D.A.I. Goring, J. Chem. Eng. 53 (1975) 427. [71 D.K. Owens, J. Appl. Polym. Sci. 19 (1975) 265. @I A.R. Blythe, D. Briggs, C.R. Kendall, D.G. Rance and V.J.I. Zichy, Polymer 19 (1978) 1273. 191 D. Briggs and C.R. Kendall, Polymer 20 (1979) 1053. ml Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy, Ed. D. Briggs (Heyden, London, 1977). illI D.J. Carlson and D.M. Wiles, Can. J. Chem. 48 (1970) 2397. iI21 G. Binnig, C.F. Quate and Ch. Gerber, Phys. Rev. Lett. 56 (1986) 930. H31 For a review, see: E. Meyer, H. Heinzelmann, D. Brodbeck, G. Qverney, R. Qvemey, L. Howald, H. Hug, T. Jung, H.-R. Hidber and H.-J. Giintherodt, J. Vat. Sci. Technol. B 9 (1991) 1329. 1141U. Zorll and E.-C. Schiitze, in: Kunststoffe in der Oberfl&c.hentechnik (Kohlhammer, Stuttgart, 1986) p. 95. Ml C. Bischof and W. Possart, Adhesion (Akademie Verlag, Berlin, 1983). 1161 S. Hild, Thesis, University of Clausthal (1992). [171 M. Strobel, C. Dunatov, J.M. Strobel, C.S. Lyons, S.J. Perron and M.C. Morgan, J. Adhes. Sci. Technol. 3 (1986) 321. lls1 D. Briggs, in: Surface Analysis and Pretreatment of Plastics and Metals, Ed. D.M. Brewis (Barking, Essex, 1980) p. 199.
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Applied Surface Science 64 (1993) 197-203 North-Holland
”
:,:
‘h
. .. . . . .. . . . . ,:.::: p ,:,,:?:
applied
surface science
An atomic force microscopy study of corona-treated polypropylene films R.M. Overney
*, R. Liithi, H. Haefke,
J. Frommer,
E. Meyer,
H.-J. Giintherodt
Institute of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
S. Hild and J. Fuhrmann Institute of Physical Chemistry, University of Clausthal, Arnold-Sommerfeki-Strasse 4, D(W)-3392 Clausthal, Germany Received 28 April 1992; accepted for publication 20 October 1992
The surfaces of corona-treated isotactic polypropylene films have been investigated by atomic force microscopy. The occurrence of droplets on the film surfaces is related to the energy dose of the corona discharge. The sizes of these droplets correlate with the corona dose. The loss of adhesive strength of self-adhered polypropylene films can be explained on the basis of morphology changes during corona treatment. A comparative study of uniaxial and biaxial polypropylene films is presented.
1. Introduction Corona treatment is bne of the most applied techniques to improve the adhesion characteristics of polyolefin polymers. Isotactic polypropylene (PP) is a widely used synthetic polymer which requires for technical application some improvement by corona treatment [l]. The corona treatment produces polar chemical functional groups at the polymer surface [2,31. Contact-angle measurements and electron spectroscopy showed that electrons, emitted by the corona-induced photoeffect, are mainly responsible for the surface activation [4]. This surface activation leads to two important simultaneous chemical processes: (a) cross-linking of the polymer chains, i.e. immobilization of self-diffusive or interdiffusive polymer chain motions and (b) oxidative polymer chain degradation, i.e. creation of more or less mobile chemical degradation products on surface areas of the polymer. It is known that corona treatment can change the morphology of polymer films;
* To whom correspondence 0169-4332/93/$06.00
should be addressed.
owing to this the level of adhesion is affected [51. The adhesion can either increase or decrease depending on the potential bonding area and the presence of voids. This has been observed by self-adhesion experiments on corona-treated polyolefins [6,7]. Surface changes have been observed by X-ray photoelectron spectroscopy [8-101 and infrared (IR) spectroscopy [ 111. Here, we present a study on the correspondence between the results of peel experiments and changes of the morphology of corona-treated PP films documented by atomic force microscopy (AFM). Atomic force microscopes [12] are able to image surfaces of conducting and nonconducting materials by scanning a sharp tip, mounted on a very sensitive spring, over the sample surface. The forces acting on the probing tip deflect the cantilever-type spring. Forces of the order of lo-” to 10T6 N can be measured by this technique and a lateral resolution of a few Hngstriims can be achieved. The displacement of the tip is measured by electron tunneling [12], optical interferometry, beam deflection or capacitance measurements [13]. For this study an atomic force microscope of a beam deflection type was used.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
198
R.M. Ovemey et al. / AFM of corona-treated polypropylene films
AFM images (termed force micrographs) recorded on the micrometer scale.
were
2. Experimental Two kinds of isotactic PP films have been investigated. The films contain an anti-oxidant and a lubricant (calcium stearate) from the manufacturing process. For our investigations PP films with different morphologies have been used. (1) PP films have been stretched uniaxially during crystallization, leading to uniaxially oriented PP (OPP). The orientation of these films have been determined by IR dichroism measurements. A high orientation in the reference plane of the OPP films has been determined. (2) In order to achieve biaxially oriented PP (BOPP) the OPP films have been stretched perpendicularly to the previously produced orientation axis. As result of this two-step stretching process no preferred orientation in the BOPP films can be detected by IR dichroism measurements. The film thicknesses are determined to be 35 pm for OPP and 25 pm
for BbPP. By differential scanning calorimetry, OPP and BOPP films showed a degree of crystallinity of 67% and 72%, respectively. PP films were treated with a corona discharge apparatus (Ahlbrandt System GmbH) with a typical power of 1350 W and a frequency of 18 kHz. The gap width between electrode and grounded sample holder was 0.3 cm. The electrode was 12 cm wide and could be moved at various speeds from 0.25 to 15 cm/s. The intensity of corona discharge depends on this speed. The corona dose (CD) ranges between 7 and 112.5 J/cm2 and is defined as 1141: power of the corona discharge apparatus CDs
width of the electrode x speed
.
For self-adhesion peel experiments the samples were adhered immediately after corona treatment for 6 min at a temperature of 75°C and a pressure of 5 X lo5 Pa. The samples were oriented parallel to machine direction for the adhering. For adhesive strength measurements the Tpeeltest [15] has been used. The peeling velocity was kept at 0.1 mm/s.
Fig. 1. Force micrographs of polypropylene films. (a) Uniaxial polypropylene (OPP) films are characterized by stria-like surface structure which is aligned along the stretching direction. The mean width between striae is about 1 pm and their heights were determined to be about 50 nm. (b) Biaxial polypropylene (BOPP) films do not exhibit the characteristic striated surface structure of OPP films. Occasionally striae are still visible in some surface areas. Scanned surface areas: 100 X 100 pm* (a) and 90 X 90 pm* (b).
RiU. Overney et al. / AFM of corona-treated polypropylene
fhs
199
air. The applied force is of the order of 10 nN and kept constant during the measurements. Here, the AFM images are presented as unfiltered data with an uncertainty of 5% in lateral and 10% in vertical dimensions.
3. Results and discussion
Fig. 2. Force micrograph of an OPP film treated with a corona dose (CD) of 60 J/cm*. The uniaxial surface structure is more ramified than the untreated surface of the OPP film (cf. fig. la). Scanned surface area 100~ 100 pm’.
The atomic force microscope, which is used to document morphological changes of the coronatreated PP films, was a commercially available instrument with a 120 pm scanner. The film surfaces were scanned by a V-shaped force sensor (termed cantilever or, in short, lever) with integrated S&N, tip and spring constant of 0.12 N/m. All measurements have been performed in
Fig. la shows a force micrograph of an OPP film. Surface corrugations are imaged in grey scales, whereas white parts represent elevations and black parts represent grooves. The surface is characterized by a stria-like structure with a mean distance between two striae of less than 1 pm. The height of the striae are of the order of 50 nm and their longitudinal axes are aligned along the direction of the manufacturing process. Therefore, the striae reflect a high anisotropy of the surface. In contrast to it, BOPP films exhibit no significant alignment of surface structure (fig. lb). The parallel-aligned striae of the OPP film disappear by additional stretching procedures. Corona treatments act on the OPP and BOPP films quite differently. Whereas on the BOPP surface no obvious changes in morphology are detected optically, the surface of the OPP film undergoes a rearrangement during the second stretching. Fig. 2 shows an uniaxial PP film treated
Fig. 3. Force micrographs of an OPP film treated with a corona dose (CD) of 112.5 J/cm’. (a) Droplet-like features occur on the striated film surface. They are stable at the AFM scanning conditions used here. (b) Droplets of about 500 nm in diameter and 60 nm in height are imaged at higher magnification. Scanned surface areas: 20 X 20 pm2 (a) and 3 X 3 wrn2 (b).
R.M. Overney et al. / AFM of corona-treated polypropylene films
200
CD [J/cm*] Fig. 4. Changes in diameter (+) and height (0) of droplets as a function of corona dose (CD).
with a CD of 60 J/cm’. The surface structure is more ramified than in the untreated OPP film (cf. fig. la). This ramification can be observed at all corona dose stages up to 60 J/crn2. At a CD of 112.5 J/cm2, however, the parallel striated structure remains as in the untreated film (fig. 3a). In addition to the alignment of the striae droplet-like features appear on the film surface. The droplets are observed on the striated OPP films by using discharge doses between 22.5 and 112.5 J/cm*. At this high density of corona treatment, the OPP films turn milky to the eye. Fig. 3b
shows a typical droplet ensemble on an OPP film at higher magnification. The diameters of the droplets increase with increasing CD (fig. 4). Table 1 contains diameters and heights of these discharge-induced surface features. At a CD of 112.5 J/cm2 the largest droplets reached a dimension of more than 0.5 pm in diameter. Below a CD of 22.5 J/cm2 the size of the droplets is of the order of the surface roughness and provides a bad statistic. Additional information is reached with gel permeation chromatography (GPC) and attenu-
Table 1 Molar weight distribution (gel permeation chromatography, GPC), oxidation (attenuated total reflection, ATR), and diameters and heights of the droplets on OPP films as a function of the corona dose (CD) CD stage
Energy dose (J/cm21
GPC Molar weight distribution
ATR Oxidation (butanon a) 1726 cm-‘)
AFM Diameter of droplets (nm)
AFM Heights of droplets (nm)
0 1 2 3 4 5 6 7 8
0.0 7.5 10.0 15.0 18.0 22.5 36.0 60.0 112.5
1.94 1.92 1.95 2.0 2.0 1.95 1.98 1.92 1.08
0.000 0.075 0.093 0.185 0.202 0.297 0.428 0.820 1.042
182 200 312 500
10 17 25 64
a) Methyl ethyl ketone (butanon) is representative
AFM Droplets per 10 wrn’
128 66 35 27
for the oxidative degradation provided by ATR spectroscopy.
RM. Ovemey et al. / AFM of corona-treated polypropylene film
ated total reflection (ATR) infrared spectroscopy (cf. table 1). GPC provides a slight increase of the molar weight distribution with increasing the CD energy in the range of O-18.0 J/cm2. Above 18.0 J/cm2 the molecular weight of the surface molecules decreases with increasing CD. For example, at a CD energy of 112.5 J/cm2, the molecular weight is about 500 g/mol, i.e. about 16 carbon atoms per degraded molecule. That the molecular weight of the PP molecules in surfaces decreases during corona treatment has already been shown by different authors 116,171.The ATR spectroscopy provides that oxidation takes place during all stages of the corona treatment (cf. table 1). The increase but also the decrease of the molecular weight can be described by two oxidation processes. First, oxygen can be chemisorbed by the polymer chains which causes an increased molar mass as observed by ATP spectroscopy below 18.0 J/cm2. On the other hand, the reaction with oxygen can also cause degradation of polymer molecules which becomes dominant above 18.0 J/cm 2. GPC, ATR and AFM show that droplets on the PP surface, are built up by
201
degradation products of oxidation. A possible explanation for the occurrence of the droplets is local surface melting of degradation products during corona treatment. The surface temperature can easily be raised considerably because of the low heat conductivity of PP. Because of the increased surface tension the PP molecules form droplets. In fig. 5 the GPC and ATR data of corona-treated OPP are schematically compared. Films of BOPP do not exhibit such droplet-like features on being corona-treated in the same energy regimes although GPC and ATR provide qualitatively similar results as OPP (cf. table 2). Only surfaces treated with a CD of 112.5 J/cm2 show occasionally droplets aligned along remaining striae (fig. 6). We assume that the polar functional groups, which are created preferentially on the amorphous surface sites of the PP samples [18], physisorb the droplets. This would explain the differences in the density of the droplets on OPP and BOPP films (cf. degree of crystallinity in section Experimental). Fig. 7 shows the morphology of an OPP film treated with 112.5 J/cm2 and afterwards softly
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
0
20
40
60
80
100
120
Corona discharge [J/cm21 Fig. 5. Gel permeation chromatography (GPO and attenuated total reflection (ATR) infrared spectroscopy results are schematically drawn from table 1 of corona-treated OPP. With increasing corona dose (CD) GPC provides above 18.0 J/cm* a decreasing molar weight distribution. At CD 112.5 J/cm*, the molecular weight of degraded molecules is measured to be about 500 g/mol (corresponds to about a molecular chain of 16 carbon atoms per molecule). The ATR spectroscopy points out that oxidation takes place during all stages of CD.
R.M. Ovemey et al. / AFM of corona-treated polypropylene films
202
Table 2 Molar weight distribution and attenuated total reflection (ATR) spectroscopy on BOPP films as a function of the corona dose (CD) CD stage
Energy dose (J/cm*) 0
7.5 10.0 15.0 18.0 22.5 36.0 60.0 112.5
GPC Molar weight distribution
ATR Oxidation (butanon a) 1726 cm-‘)
1.80 1.85 1.85 1.82 1.94 1.93 1.90 1.81 1.08
0.000
0.067 0.004 0.021 0.123 0.182 0.406 0.619 0.737
a) Methyl ethyl ketone (butanon) is representative for the oxidative degradation provided by ATR spectroscopy.
wiped with a cotton tissue. No droplets remain on the film surface. However, under the scanning conditions used to image the films, it was not possible to remove the droplets by means of the AFM cantilever. Peel-experiments of self-adhered OPP films indicate an increasing of the peel-off force by increasing CD up to 18 J/cm2. At higher discharge energies the peel-off force decreases (fig. 8). These results are in good agreement with the morphological changes observed by AFM. The
Fig. 7. Force micrograph of an OPP film treated with a corona dose of 112.5 J/cm*. Using a cotton tissue to wipe the film surface the droplets are removed. Scanned surface area 20 x 20 pm2.
detection threshold for the appearance of droplets is at 22.5 J/cm2. The peel-off force of the samples treated with a CD of 112.5 J/cm2 is smaller than the peel-off force of the samples treated with the same corona dose and wiped afterwards. Therefore, we conclude that the self-adhesion of the OPP films strongly depends on the density and size of the discharge-induced droplets. For BOPP films the peel-off force increases monot-
-:P.Y I
-m-
OPP
-c-
BOPP
I
-
20
Fig. 6. Force micrograph of a BOPP film treated with a corona dose of 112.5 J/cm*. The density of the droplets is much less than on OPP tilms (cf. fig. 3b). The droplets tend to occur preferentially in the direction of the remaining striae. Scanned surface area 3.5 x 3.5 pm*.
40
60
80
loa
120
Comnadwe[J/cm21
Fig. 8. Normalized peel-off force as a function of corona dose (CD). On a CD of about 18 J/cm* a maximum in force is reached for OPP films, whereas BOPP films level off to a constant value at 20 J/cm*.
R.M. Ovemey et al. / AFM of corona-treated po&propylene films
onously within the CD range. At a CD of about 20 J/cm2 the force levels off to a constant value. This is the same region of CD in which OPP films reach the maximum peel-off force. The saturation is supported by the fact that no droplets appear on the film surfaces.
4. Summary Using AFM it is possible to relate the loss of adhesive strength of corona-treated PP films with the change of their morphology. Droplets are formed on the striated surface. Their diameters and heights increase with increasing CD. The adhesion between droplets and bulk is assumed to be the limiting factor for adhesion. Self-adhesion cannot be stronger and therefore the attractive component between bulk and droplets set the adhesion characteristics. Biaxial PP films which were also investigated by AFM showed a very decreased density of droplets on the surface. It corresponds with the peel experiments showing no maximum peel-off force. Gxidative degradation and local surface melting or sublimation is proposed as a possible explanation for the formation of the droplets.
Acknowledgments This work was supported by the Swiss National Science Foundation, the Kommission zur FSrderung der wissenschaftlichen Forschung (Switzerland) and the Arbeitsgemeinschaft indus-
trieller Forschungsvereinigungen 7706 (Germany).
203
e.V.,
Projekt
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