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Laser-induced photochemical adherence enhancement
336
Applied
Laser-induced
photochemical
adherence
Surface
Science 46 (1990) 336-341 North-Holland
enhancement
J. Breuer, S. Metev, G. Sepold BIAS,
Bremen Institute
of Applied Beam TechnoloR), Klrrgenfurter Strore
2, D-2800 Bremen 3.7. Fed. Rep. CI/ Germmr
O.-D. Hennemann,
H. Kollek and G. Kriiger
IFAM,
of Applied Materials Science, Lesumer Heer.wmse
Received
Fraunhofer Instttute
29 May 1990: accepted
for publication
36, D-2X20 Bremen
77. Fed. Rep. of Germurn
17 July 1990
Some results are presented concerning the laser-induced photochemical enhancement of the adhesive bonding strength betwjeen polypropylene (PP) and adhesives on a resinous basis. The mechanism of the laser-activated processes is discussed. At some conditions a bonding strength enhancement of more than 5 times has been achieved.
1. Introduction
Connection of materials by adhesive bonding is a technique that becomes more and more important nowadays. This concerns especially syntetic materials like plastics. With some of them it is a big problem to get bonds with good quality and sufficient long-term stability. In this case a pre-treatment of the contact surfaces of the parts to be connected is necessary in order to improve the adhesion of the binding material. To some degree the bonding strength can be enhanced through mechanical, wet-chemical or plasma pretreatment of the workpieces leading to improvement of the surface moistening by the adhesive [1,2]. Most of these methods have some disadvantages as for example damaging of the surface, bad controllability and pollution of the environment. To overcome these disadvantages it is necessary to look for new surface treatment techniques. A way, in principle, to improve the adherence is to enhance the adhesion between the binding material and the solid surface. Although, the mechanism of the adhesion is not completely clarified [3], one can assume on the basis of general considerations that the activation of chemi0169-4332/90/$03.50
CC1990 - Elsevier Science Publishers
sorption at the solid/adhesive interface can positively influence the adherence. Such chemisorption processes could be initiated photolytically by UV-laser radiation of suitable wavelength. On the one hand, the laser radiation pre-treatment of the solid surface in an active gas atmosphere can result in a change in the physicochemical activity of the surface layers in relation to the adhesive. On the other hand, the direct action of the UV-laser light on the solid/adhesive interface can activate a chemical reaction between them leading to adhesion improvement. The aim of this paper is to present the first results of an experimental investigation of the influence of the photochemical laser radiation action on the adherence of polypropylene.
2. Experimental In the experiments polypropylene (PP) has been used as a sample material because of its poor adherence with conventional adhesives. The chemical structure of PP (fig. 1) is also suitable for laser activation due to the effective photochemical interaction of the UV-laser radiation with the (CC) and (C-CH,,) bindings [4].
B.V. (North-Holland)
J. Breuer et al. / Luser-induced
I.I>’ -CH
-CH,CHm-CH,CH
c_H
Fig. 1. Chemical
CH,
structure
1 CH
1 4l
of polypropylene.
The adhesive was a two-component resin AW106-Ciba Geigy. The chemical of the binding agent and of the hardener
epoxide structure are given
photochemrcal
adherence
337
enhumement
in fig. 2. The characteristic feature of the binding agent is the ring-shaped epoxy group. This ring opens when it comes to a reaction with the ammo group ((NH,) of the hardener during the hardening process [S] (fig. 2). The NH, group of the hardener reacts with the CH, group of the binding agent resulting in a splitting of the CHIP0 bond and in the formation of an OH radical in
H2N%NH2
V+%
c
-
Hardener
+
Epoxy-resin
example for a hardening process
I
addlllonol
P
? CH2
CH2
“hti
t-&-OH
h42
hi2 II--
a--
0
(8
N--
N--
l
Fig. 2. Chemical
structure
of an epoxy-resin,
a hardener
and an example
for a hardening
process
338
J. Breuer et al. / Laser-induced
photochemrcal
spectra with that of the unirradiated sample (fig. 4a) one can see that the main result of the laser radiation action on the sample surface is the formation of OH groups and C=O bonds in the PP structure. These groups are not present in the structure irradiated in helium, which means that they are formed in reactions between the PP and the surrounding gas atmosphere under laser radiation activation. The photolytic character of the activation process is confirmed by the dependence of the results on the laser wavelength. In particular, this is most clearly demonstrated with the samples irradiated in H,O vapour (fig. 4b). The height ratio of the OH and C=O components in the IR spectrum is also dependent on the laser wavelength (fig. 4~). Although, we did not make a quantitative analysis of the IR spectra, it seems from their qualitative comparison that the 308 nm wavelength infuences more strongly (in comparison to 248 nm) the formation of C=O bonds. The experiments have also shown that variation of the energy density below the damage threshold or variation of the pulse number results in quantitative changes of the obtained results but not in qualitative ones. The measurements of the tensile strength of the adhesive bond of PP samples have shown that the UV-laser pre-treatment under certain conditions enhances significantly (up to 5 times) its value. Some characteristic results of these measurements are presented in table 1. Enhancement of the tensile strength has been observed in both cases ~ with a significant number of OH and C=O groups on the pretreated surface and with UV-laser irradiation of the solid/adhesive interface through a previously deposited adhesive layer. In the first case, the im-
this place. This process takes place additively until a complete polymerization (hardening) of the adhesive is achieved [5]. The experimental set-up is shown in fig. 3. Prism-shaped PP samples with dimensions 85 x 25 X 4 mm3 were placed in a chamber with defined gas atmosphere (He, 0,, air and H,O vapour) and irradiated with pulsed excimer laser radiation with wavelengths of 248 and 308 nm and pulse durations of 30 ns. In order to obtain a uniform light flux density distribution in the irradiated zone a projection optical scheme has been used [6]. The light energy density in the treated zone of rectangular shape was varied from 0.1 J crnp2 up to the surface damage threshold of about 1 J cm-’ by an interference mirror attenuator. The number of the irradiation pulses was varied from 1 up to 200. In one experimental series the samples were irradiated through a thin layer of the adhesive deposited on the treated surface before irradiation. The result of the UV-laser radiation action on the sample surface has been investigated by FT-IR spectroscopy. After the laser pre-treatment (with or without presence of adhesive layer) every two samples irradiated under the same conditions were stuck together with an overlap of 12.5 X 25 mm2. After two days hardening at room temperature the tensile strength of the bond was determined. The measurement conditions were in accordance with international standards.
3. Results and discussion In fig. 4 some representative pre-treated samples are shown.
IR spectra Comparing
of the these
6
Fig. 3. Experimental
set-up:
1
laser:
adherence enhancement
ta
Air.
0,
beam-sphtter; 5 2 - attenuator; 3 mask: 4 objective; 7 chamber; 8 - ga+inlet; 9 - sample.
He, H,O-vap
energy-measurement;
6
projection
J. Breuer et al. / Laser-inducedphotochemical
Laser
339
adherence enhancement
parameters:
E.-density: No. of
O.BJ/cm’
shots:
10
P
I 3kaam36mzbmklmm ,Y,&. 2
‘-‘_‘. bz.8
h=308rm Gas He
X=240rm
h=248rm Gas
air
h=308rm
X=248rm
Fig. 4. FTIR spectra
of laser pre-treated
samples
of PP number
of pulses N = 10: energy density
E = 0.7 J cmd2
J. Breuer et al. / Laser-induced pholochemrcul
340 Table 1 Parameters NO.
of different
laser pre-treatments
2 3 4 5
measurements
Laser treatment Wavelength
1
and results of tensile strength
adherence mhor~~entettt
(nm)
Energy density (J/cm’ )
of allots
Untreated 24X 24X 308 308
0.2 0.9 0.4 0.9
200 200 200 200
Fig. 5. SEM mIcrographs
of fracture
surfaces represents
Number
after tensile strength measurement: (a) adheswe fracture 100 pm, (b) cohesive fracture (sample 3 in table 1).
Tensile strength (N/mm’)
0.x 1.3 5.0 1.5 3.6
(sample
4 in tahlc 1): marker
341
J. Breuer et al. / Laser-induced pholochemical adherence enhancement
provement of the adhesive bonding strength could be due to an introduction of electrically polarized groups (OH and C=O) into the PP structure leading to improvement of the physisorption (wetting) of the adhesive molecules [7]. Another possibility could be an enhancement of the chemical activity of the solid surface with respect to the adhesive due to the presence of chemically active OH and C=O groups resulting in initiation of chemisorption processes. In the second case, the enhancement of the tensile strength could be due to direct laser radiation activation of chemical reactions (chemisorption) between PP and adhesive. Notwithstanding the fact that the concrete mechanism of the adherence enhancement is not clear yet, the results obtained show that in all cases it is connected with an improvement of the adhesion. This is also shown by the SEM microphotographs (fig. 5) of the fracture surface after the tensile strength examinations. In the case of low tensile strength (No. 1, 2, 4 in table 1) the fracture has adhesive character (fig. 5a) while a suitable UV-laser pre-treatment (No. 3 in table 1) results in a fracture of cohesive character (fig. 5b).
4. Conclusion These first experiments with polypropylene have definitely shown that the adherence of some polymer materials can be significantly enhanced by
laser-induced photochemical reactions at the solid/adhesive interface. In order to clarify the particular nature of the activation mechanism more detailed experiments are needed. Such experiments are now in progress and we hope to publish some new results in the near future.
Acknowledgement This work was supported by the German Ministry of Research and Technology (BMFT) which the authors gratefully acknowledge.
References VI F. Dolezalek
and R. Hartmann, in: Proc. SURTEC Congr. (1981) p. 221. i21 C. Bischof and W. Possart, Adhlsion (Akademie-Verlag, Berlin, 1982). 108 (VCH, Oberur(31 W. Brockmann, DECHEMA-Monogr. sel, 1987). with Lasers, Springer I41 D. Bluerle, Chemical Processing Series in Materials Science, Vol. 1 (Springer, Heidelberg, 1986); H. Jellinek, Degradation and Stabilization of Polymers, Vol. 1 (Elsevier, Amsterdam, 1983) ch. 3. Grundlagen und Stand der Metallkleb151 W. Brockmann, technik (VDI, Diisseldorf, 1971). [61 S. Metev. S. Savtchenko and K. Stamenov, J. Phys. D 13 (1980) L75.
[71 K. Hauffe and S. Morrison, 1973).
Adsorption
(De Gruyter,
Berlin,
Applied
Laser-induced
photochemical
adherence
Surface
Science 46 (1990) 336-341 North-Holland
enhancement
J. Breuer, S. Metev, G. Sepold BIAS,
Bremen Institute
of Applied Beam TechnoloR), Klrrgenfurter Strore
2, D-2800 Bremen 3.7. Fed. Rep. CI/ Germmr
O.-D. Hennemann,
H. Kollek and G. Kriiger
IFAM,
of Applied Materials Science, Lesumer Heer.wmse
Received
Fraunhofer Instttute
29 May 1990: accepted
for publication
36, D-2X20 Bremen
77. Fed. Rep. of Germurn
17 July 1990
Some results are presented concerning the laser-induced photochemical enhancement of the adhesive bonding strength betwjeen polypropylene (PP) and adhesives on a resinous basis. The mechanism of the laser-activated processes is discussed. At some conditions a bonding strength enhancement of more than 5 times has been achieved.
1. Introduction
Connection of materials by adhesive bonding is a technique that becomes more and more important nowadays. This concerns especially syntetic materials like plastics. With some of them it is a big problem to get bonds with good quality and sufficient long-term stability. In this case a pre-treatment of the contact surfaces of the parts to be connected is necessary in order to improve the adhesion of the binding material. To some degree the bonding strength can be enhanced through mechanical, wet-chemical or plasma pretreatment of the workpieces leading to improvement of the surface moistening by the adhesive [1,2]. Most of these methods have some disadvantages as for example damaging of the surface, bad controllability and pollution of the environment. To overcome these disadvantages it is necessary to look for new surface treatment techniques. A way, in principle, to improve the adherence is to enhance the adhesion between the binding material and the solid surface. Although, the mechanism of the adhesion is not completely clarified [3], one can assume on the basis of general considerations that the activation of chemi0169-4332/90/$03.50
CC1990 - Elsevier Science Publishers
sorption at the solid/adhesive interface can positively influence the adherence. Such chemisorption processes could be initiated photolytically by UV-laser radiation of suitable wavelength. On the one hand, the laser radiation pre-treatment of the solid surface in an active gas atmosphere can result in a change in the physicochemical activity of the surface layers in relation to the adhesive. On the other hand, the direct action of the UV-laser light on the solid/adhesive interface can activate a chemical reaction between them leading to adhesion improvement. The aim of this paper is to present the first results of an experimental investigation of the influence of the photochemical laser radiation action on the adherence of polypropylene.
2. Experimental In the experiments polypropylene (PP) has been used as a sample material because of its poor adherence with conventional adhesives. The chemical structure of PP (fig. 1) is also suitable for laser activation due to the effective photochemical interaction of the UV-laser radiation with the (CC) and (C-CH,,) bindings [4].
B.V. (North-Holland)
J. Breuer et al. / Luser-induced
I.I>’ -CH
-CH,CHm-CH,CH
c_H
Fig. 1. Chemical
CH,
structure
1 CH
1 4l
of polypropylene.
The adhesive was a two-component resin AW106-Ciba Geigy. The chemical of the binding agent and of the hardener
epoxide structure are given
photochemrcal
adherence
337
enhumement
in fig. 2. The characteristic feature of the binding agent is the ring-shaped epoxy group. This ring opens when it comes to a reaction with the ammo group ((NH,) of the hardener during the hardening process [S] (fig. 2). The NH, group of the hardener reacts with the CH, group of the binding agent resulting in a splitting of the CHIP0 bond and in the formation of an OH radical in
H2N%NH2
V+%
c
-
Hardener
+
Epoxy-resin
example for a hardening process
I
addlllonol
P
? CH2
CH2
“hti
t-&-OH
h42
hi2 II--
a--
0
(8
N--
N--
l
Fig. 2. Chemical
structure
of an epoxy-resin,
a hardener
and an example
for a hardening
process
338
J. Breuer et al. / Laser-induced
photochemrcal
spectra with that of the unirradiated sample (fig. 4a) one can see that the main result of the laser radiation action on the sample surface is the formation of OH groups and C=O bonds in the PP structure. These groups are not present in the structure irradiated in helium, which means that they are formed in reactions between the PP and the surrounding gas atmosphere under laser radiation activation. The photolytic character of the activation process is confirmed by the dependence of the results on the laser wavelength. In particular, this is most clearly demonstrated with the samples irradiated in H,O vapour (fig. 4b). The height ratio of the OH and C=O components in the IR spectrum is also dependent on the laser wavelength (fig. 4~). Although, we did not make a quantitative analysis of the IR spectra, it seems from their qualitative comparison that the 308 nm wavelength infuences more strongly (in comparison to 248 nm) the formation of C=O bonds. The experiments have also shown that variation of the energy density below the damage threshold or variation of the pulse number results in quantitative changes of the obtained results but not in qualitative ones. The measurements of the tensile strength of the adhesive bond of PP samples have shown that the UV-laser pre-treatment under certain conditions enhances significantly (up to 5 times) its value. Some characteristic results of these measurements are presented in table 1. Enhancement of the tensile strength has been observed in both cases ~ with a significant number of OH and C=O groups on the pretreated surface and with UV-laser irradiation of the solid/adhesive interface through a previously deposited adhesive layer. In the first case, the im-
this place. This process takes place additively until a complete polymerization (hardening) of the adhesive is achieved [5]. The experimental set-up is shown in fig. 3. Prism-shaped PP samples with dimensions 85 x 25 X 4 mm3 were placed in a chamber with defined gas atmosphere (He, 0,, air and H,O vapour) and irradiated with pulsed excimer laser radiation with wavelengths of 248 and 308 nm and pulse durations of 30 ns. In order to obtain a uniform light flux density distribution in the irradiated zone a projection optical scheme has been used [6]. The light energy density in the treated zone of rectangular shape was varied from 0.1 J crnp2 up to the surface damage threshold of about 1 J cm-’ by an interference mirror attenuator. The number of the irradiation pulses was varied from 1 up to 200. In one experimental series the samples were irradiated through a thin layer of the adhesive deposited on the treated surface before irradiation. The result of the UV-laser radiation action on the sample surface has been investigated by FT-IR spectroscopy. After the laser pre-treatment (with or without presence of adhesive layer) every two samples irradiated under the same conditions were stuck together with an overlap of 12.5 X 25 mm2. After two days hardening at room temperature the tensile strength of the bond was determined. The measurement conditions were in accordance with international standards.
3. Results and discussion In fig. 4 some representative pre-treated samples are shown.
IR spectra Comparing
of the these
6
Fig. 3. Experimental
set-up:
1
laser:
adherence enhancement
ta
Air.
0,
beam-sphtter; 5 2 - attenuator; 3 mask: 4 objective; 7 chamber; 8 - ga+inlet; 9 - sample.
He, H,O-vap
energy-measurement;
6
projection
J. Breuer et al. / Laser-inducedphotochemical
Laser
339
adherence enhancement
parameters:
E.-density: No. of
O.BJ/cm’
shots:
10
P
I 3kaam36mzbmklmm ,Y,&. 2
‘-‘_‘. bz.8
h=308rm Gas He
X=240rm
h=248rm Gas
air
h=308rm
X=248rm
Fig. 4. FTIR spectra
of laser pre-treated
samples
of PP number
of pulses N = 10: energy density
E = 0.7 J cmd2
J. Breuer et al. / Laser-induced pholochemrcul
340 Table 1 Parameters NO.
of different
laser pre-treatments
2 3 4 5
measurements
Laser treatment Wavelength
1
and results of tensile strength
adherence mhor~~entettt
(nm)
Energy density (J/cm’ )
of allots
Untreated 24X 24X 308 308
0.2 0.9 0.4 0.9
200 200 200 200
Fig. 5. SEM mIcrographs
of fracture
surfaces represents
Number
after tensile strength measurement: (a) adheswe fracture 100 pm, (b) cohesive fracture (sample 3 in table 1).
Tensile strength (N/mm’)
0.x 1.3 5.0 1.5 3.6
(sample
4 in tahlc 1): marker
341
J. Breuer et al. / Laser-induced pholochemical adherence enhancement
provement of the adhesive bonding strength could be due to an introduction of electrically polarized groups (OH and C=O) into the PP structure leading to improvement of the physisorption (wetting) of the adhesive molecules [7]. Another possibility could be an enhancement of the chemical activity of the solid surface with respect to the adhesive due to the presence of chemically active OH and C=O groups resulting in initiation of chemisorption processes. In the second case, the enhancement of the tensile strength could be due to direct laser radiation activation of chemical reactions (chemisorption) between PP and adhesive. Notwithstanding the fact that the concrete mechanism of the adherence enhancement is not clear yet, the results obtained show that in all cases it is connected with an improvement of the adhesion. This is also shown by the SEM microphotographs (fig. 5) of the fracture surface after the tensile strength examinations. In the case of low tensile strength (No. 1, 2, 4 in table 1) the fracture has adhesive character (fig. 5a) while a suitable UV-laser pre-treatment (No. 3 in table 1) results in a fracture of cohesive character (fig. 5b).
4. Conclusion These first experiments with polypropylene have definitely shown that the adherence of some polymer materials can be significantly enhanced by
laser-induced photochemical reactions at the solid/adhesive interface. In order to clarify the particular nature of the activation mechanism more detailed experiments are needed. Such experiments are now in progress and we hope to publish some new results in the near future.
Acknowledgement This work was supported by the German Ministry of Research and Technology (BMFT) which the authors gratefully acknowledge.
References VI F. Dolezalek
and R. Hartmann, in: Proc. SURTEC Congr. (1981) p. 221. i21 C. Bischof and W. Possart, Adhlsion (Akademie-Verlag, Berlin, 1982). 108 (VCH, Oberur(31 W. Brockmann, DECHEMA-Monogr. sel, 1987). with Lasers, Springer I41 D. Bluerle, Chemical Processing Series in Materials Science, Vol. 1 (Springer, Heidelberg, 1986); H. Jellinek, Degradation and Stabilization of Polymers, Vol. 1 (Elsevier, Amsterdam, 1983) ch. 3. Grundlagen und Stand der Metallkleb151 W. Brockmann, technik (VDI, Diisseldorf, 1971). [61 S. Metev. S. Savtchenko and K. Stamenov, J. Phys. D 13 (1980) L75.
[71 K. Hauffe and S. Morrison, 1973).
Adsorption
(De Gruyter,
Berlin,