RESONANCE ABSORPTION OF COHERENT INFRARED RADIATION BY THIN
POLYPROPYLENE FILMS AND ITS TECHNOLOGICAL APPLICATIONS
Edward Bormashenko*, Roman Pogreb, Avigdor Sheshnev, Semion Sutovski, Alexander Shulzinger,
Lior Nachum, Roi Kerbel
The College of Judea and Samaria, The Research Institute, Ariel, 44837, Israel
*
Corresponding Author: The College of Judea and Samaria, The Research Institute, 44837, Ariel,
Israel, Tel: 972-3-906 61 34, Fax: 972-3-936 68 73, E-Mail: polytris@netvision.net.il
Abstract
Tunable CO2 and non-tunable CO2 lasers were used for the irradiation of thin polypropylene
films. The wavelength of the IR radiation was adjusted in such a way that it coincided exactly with the
absorbance dip in the spectrum of the polypropylene pattern. In this way conditions of resonance
absorbance of IR radiation by polymer films were produced. Strong thermal effects in PP were
observed under irradiation at the resonance wavelength. The threshold value of IR power, which causes
thermal phenomena in irradiated films was established. Possibility of production of thin traces and
highly developed surface relieves was demonstrated. IR spectra of irradiated PP films were studied
with FTIR spectrometer. Changes in the spectra of PP films were established using correlation analysis.
The phenomenon could be applied for the obtaining of highly developed relieves, storage of
information in polymer films, welding of polymer materials.
PACS codes: 61.80.Ba; 78.40.Me, 81.05.Lg
Keywords: infrared, absorption, polypropylene, tunable CO2 laser, microlens, welding
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Introduction
Laser ablation of polymer substrates caused by UV-laser radiation was under intensive
investigation recently, photochemical and thermal aspects of the phenomenon were studied [1-3].
Y.Feng studied ablation of polypropylene substrates caused by UV laser irradiation, and signed cooccurrence of the thermal and photochemical processes [3]. At the same time very few reports were
devoted to the study of interaction of coherent IR radiation with polymer substrates [5,6].
Presented paper continues the series of investigations carried out by the authors in which
interaction of IR radiation produced by CO2 laser was under study [7-8]. The phenomenon of changes
in the absorbance spectra of polymers, induced by strong IR laser radiation was reported in these
works. This effect makes it possible to exert an influence on the structure of polymers using IR laser
radiation. The phenomenon was applied successfully for the contacting of ZnSe IR-optics elements
using thin polyethylene films [9]. It was shown by Ornelas-Rodriges and Calixto that CO2 laser
radiation allows direct laser writing of mid-infrared microelements on polyethylene [10].
We suggest that exposure of polypropylene (PP) films to IR radiation produced by tunable CO2
laser, and a study of the effects of interaction of IR radiation with PP film, will be of much scientific
interest. The chemical structure of polypropylene is:
-CH2-CHCH3
n
Polypropylene is distinguished by the dips in its absorbance spectra, which are located in the
nearest vicinity of 10.6 μm – wavelength generated by CO2 laser. These dips are well known as the dips
inherent for the stretch vibrations of the CH3 group [11]. This fact allows supply of the coherent IR
energy to PP directly in the bands of high absorption using tunable CO2 laser. Such exposure of PP film
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to IR radiation will be followed by the resonance absorbance of IR radiation, and strong thermal effects
could be expected. Numerous studies of radiation effects in PP have been carried out in order to
understand the mechanisms of the fundamental processes, and with a view to possible applications.
Photoluminescence spectra induced in PP by irradiation of photons from an ArF excimer laser was
studied by Ito [12]. Chemical modification of polypropylene induced by high-energy carbon ions was
studied by A.Saha [13]. Changes in the spectra of γ-irradiated polypropylene were studied by Dawood
and Miura [14]. They established that absorption intensity around 1700 and 3400 cm-1 increased for
irradiated sample, and that the change in the FTIR spectrum of PP rises with increased of irradiation
dose. Dawood and Miura concluded that the main effect of γ-ray irradiation on PP is the formation of
free radicals. The formation of free radicals in irradiated PP using electron spin resonance was studied
by Burlinska [15].
Experimental
Biaxially oriented PP films with a thickness of 20 μm were supplied by Dor Film Ltd, and their
infrared spectra were studied using Bruker Vector 22 FTIR spectrometer. We established that these
films demonstrated a strong absorption dip located between 10.7 and 10.1 μm with distinguished
maximal absorption at 10.3 μm (972 cm-1 see Fig. 1). We exposed PP samples to the IR radiation
produced both by non-tunable CO2 laser which generated at 10.6 μm, and tunable CO2 laser which was
adjusted to generation at 10.3 μm. In order to irradiate PP films exactly at this wavelength, we used the
optical bench described in Fig. 2. Coherent IR radiation produced by the tunable CO2 laser was directed
toward polymer film through a ZnSe plate, mirror, polarizer and iris diaphragm. The ZnSe plate was
used for the splitting of the laser beam. The intensity of the beam reflected from the ZnSe plate Ia was
measured by a precise IR power meter (detector A in Fig. 2). The intensity of the beam, which passed
through the ZnSe plate, polarizer, and diaphragm, irradiating the PP sample Ib, was measured by a
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second IR power meter (detector B in Fig. 2). The diaphragm with a circular aperture of 2-mm diameter
was placed in close contact with the PP film. The polarizer was used in order to regulate the intensity of
the IR radiation incident on the PP film (the polarizer prevented destruction of the film during the
adjustment procedure).
At the beginning of the experiment, the wavelength of the laser radiation was established
roughly in the vicinity of 972 cm-1. Then signals obtained by the detectors A and B were passed to the
Laser Star power/energy multi-channel monitor (Ophir Ltd), equipped with Starcom 32 software and
networked to the computer, and precise tuning of the system was carried out. We tuned the wavelength
of the laser in such a way that the minimal value of the ratio Ib/Ia was obtained. The minimum of Ib/Ia
indicated the fact that maximal absorption of the IR radiation by the polymer film was achieved, by
which we confirmed that we irradiated our sample precisely at the wavelength which corresponds to the
maximal absorption. We continued our experiment at the wavelength established by the abovementioned procedure. Then by use of the polarizer we increased the intensity of the IR radiation, which
passed through the film. We irradiated films statically and under pulling with constant speed in the
plane normal to laser beam.
3. Results and discussion
3. 1. Thermal deformation of PP films caused by CO2 laser radiation
PP substrates were irradiated with tunable CO2 laser at wavelength 10.3 μm under increasing
power of radiation. When the power density of IR radiation achieved P = 40 mW/mm2 a thermal
deformation of the samples took place and craters were formed on the surface of the substrate very
similar to those described in [3]. Very slight variation of the power density (from 40 to 45 mW/mm2)
caused dramatic changes in the structure of the crater (see Fig.3a-3b). Strong absorption at selected
wavelength explains this fact reasonably. We came to the conclusion, that the above-mentioned value
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of power is of a threshold nature (when power was lower than the threshold value no thermal effect was
observed). Then PP substrates were irradiated with non-tunable CO2 laser at wavelength 10.6 μm and
craters were formed in much the same way, but under higher threshold power densities (P = 140
mW/mm2). The high intensity of absorption of IR radiation at the resonance wavelength explains these
discrepancies reasonably (absorption of PP films at 10.6 μm is still high but three times lesser than
those at resonance wavelength 10.3 μm). Ornelas-Rodriguez and Calixto presented very close threshold
value (P = 110 mW/mm2) established for polyethylene substrates. The details of the craters surface
structure of the surfaces are presented in Fig. 4. We related the formation of surface defects to the
biaxial orientation of PP films. These defects make problematic the application of the phenomenon for
the fabrication of microlenses and microarrays [4, 10], but make possible another technological goals,
such as discussed below.
After defining of the parameters of IR radiation necessary for craters formation we pulled films
exposed to established threshold values with constant speed in the plane normal to laser beam. Thus
promising results were obtained. The formed surface relief and its details are presented at Fig. 5a-5b.
When pulling speed was increased we obtained very thin traces with characteristic diameter 1-10 μm
such as presented at Fig.7. We want to emphasize that the trace with the characteristic dimension lesser
than wavelength of incident laser beam (10.3 μm) could be obtained, this fact could be successfully
used for direct storage of information in PP films.
3.2. Changes in the spectra of PP films induced by coherent IR radiation.
IR absorption spectrum of the irradiated PP films was studied with optical bench presented in
Fig. 7. Spectra were taken by FTIR spectrometer under scanning of the samples surface using iris
diaphragm with a diameter of 500 μm with a step of 100 μm. Different authors3, 7-8 discussed changes
in the spectra of polymers induced by the laser radiation and related the effect to the thermal and
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photochemical processes in polymer which occurred in polymers under such irradiation. At the same
time analysis of the spectra of the structures presented in Fig. 3-5 is not a simple task, because of the
developed relief of the films. Different sections of the irradiated part of the film provide spectral data,
which alter with a thickness of the section. This problem is common for spectral study of all welldeveloped surfaces. We developed the method of correlation analysis of spectral data obtained when
samples with altered thickness are under investigation.
When the electromagnetic wave with frequency ω passes through the film with thickness d,
transmittance Tr(ω) is given according to:
Tr ( ) 
I ( )
 (1  2r ( )) exp( k ( )d )
I 0 ( )
(1)
where: I0(w) – signal absorbed by the detector in the absence of the sample, I(ω) – signal passed
through the sample, r(ω) – reflection coefficient (r << 1); k(ω) – absorption coefficient, and d is the
thickness of the film. For the absorbance Abs (ω) we obtain:
A( )   log Tr ( )   log( 1  2r ( ))  dk ( ) log e
(2)
It could be recognized from the equation (2) that, when properties of the film are constant, the
distinctions between different spectra are defined by the thickness of the sample only, and absorbance
depends on the thickness linearly for all frequencies. So when we apply Equation (2) for two different
frequencies, we can exclude d easily, and for two different frequencies we’ll obtain:
A( i )   ik A( k )   ik
(3)
where αik and βik depend on r(ωi), r(ωk), k (ωi), k (ωk) . It is clear from (3) that when the properties of
the sample don’t alter, absorptions at different wavelengths are tied by linear dependence and the
deviation of this dependence could be caused by non-uniformity of the film only. So when we try to
establish possible changes of in the chemical structure of the films, which demonstrate high developed
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relieves (such as described in Fig. 3-5), the following procedure should be adopted: functional
dependence of different “dips” of the spectra have to be studied, and its linear or non-linear character
have to be established.
It is reasonable to apply correlation analysis for this purpose. It is well-known fact that when
correlation coefficient between two random values is close to 1, these values are related one to another
by the linear dependence. Small correlation coefficients are indicative of absence of the dependence (or
possibly of its non-linear character). An(ω) will be considered as a random value which alters from one
point of the relief to another (n – number of the point, which corresponds to the number of the
spectrum). With the availability of N spectra the correlation coefficient Rik which relates two lines of
the spectrum A(ωi) and A(ωk) one to another is given by:
Rik  R( i ,  k ) 
1
N
N

[ An ( i )  A ( i )][ An ( k )  A ( k )]
[ DA( i ) DA( k )]
n 1
1
(4)
2
where A ( ) and DA(ω) defined as:
1 N
A ( )   An ( )
N n 1
1 N
DA( )   [ An ( )  A ( )] 2
N n 1
(4a)
(4b)
When one of the frequencies ωi, ωk is fixed Ri ( )  R( ,  i ) could be treated as a spectrum of
correlation coefficients for the frequency ωi.
Highly developed relief presented at Fig. 5 was scanned in two transverse directions: direction Y
coincides with the direction of the pulling and direction X is normal to the direction of the pulling (see
Fig. 5a). Fig. 8 demonstrates spectrum of correlation coefficients referred to the average absorbance in
the band 1700-2200 cm-1 R1700-2200 (where no peculiarities of the spectrum could be recognized)
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comparatively to the transmission spectrum of PP (curve A). Curve B presents set of the spectra
obtained when scanning was performed in the direction of the pulling (the diaphragm was located at the
center of irradiated area), curve C presents the same value obtained when scanning was performed
normally to the pulling direction.
It could be recognized that in the first case (direction of pulling and scanning are the same)
values of R(ω) are very close to 1, and changes in the spectrum R(ω) are random (curve B). In fact this
result is predictable, because properties of the film doesn’t change in this direction. When set of the
spectra obtained under scanning normal to the pulling direction is under study (curve C) we observe
very another picture. Bands of the spectrum in which R(ω) if far from 1 could be identified. It is
important that these bands coincide with the dips of the absorption spectrum of PP. Spectrum of
correlation coefficients practically retraces absorbance spectrum of PP. It could be concluded that
changes in the spectra of the pattern are closely tied with the changes in optical properties of irradiated
films, and it reasonable to connect these changes to the non-uniform, gaussian distribution of energy in
the incident laser beam: intensity of the beam is much stronger in the small area adjacent to the
centerline of the trace.
More detailed picture gives Fig. 9, which demonstrates of spectrum of correlation coefficients
R972 with absorption peak at 972 cm-1. Scanning in the pulling direction gives correlation coefficients
which are close to 1, as in the preceding case (curve B). Correlation analysis of the scanning in the X
direction gives valuable information about chemical processes, which take place under irradiation. It
can be seen that peaks at 1378 and 1454 cm-1 related to scissor vibrations of CH2 group give correlation
coefficients, which are close to 1. At the same time peaks at 1378 and 1454 cm-1 related to the vibration
of CH3 group give lower correlation coefficients. The complicated structure of the peaks at 1378 and
1454 cm-1 is worth noting as well. So we can come to conclusion that behavior of CH2 and CH3 under
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irradiation is different. This conclusion is consistent with a theory of thermal degradation of PP
described in [16]. Peaks at and 840, 998 and 1167 cm-1 are inherent for hybridized vibration of CH2 and
CH3 groups, so in spite of the fact that they give correlation coefficient close to 1, exact information
about behavior of these groups could not be extracted from the correlation analysis of the spectral data.
We concluded that resonance absorption of IR radiation will cause changes in the chemical
structure of PP similar to those revealed in polyethylene and polysulfone2,3, but under exposure to
much lower energy densities and free radicals necessary for the rise of the photoluminescence effect
may be formed6,9. We will now concentrate our efforts on the investigation of this effect.
3.3. Technological applications of the phenomenon.
Authors believe that very different applications of the effect under discussion are possible: very
thin traces, such as presented in Fig. 6 make possible laser marking of PP goods with very high density
of the marking procedure. Highly developed relieves such as presented at Fig. 4-5 allow various
technological applications as well, such as substrates for sputtering of thin layers of catalytic materials.
We concentrated our efforts at this stage of our investigation on the one of the possible
applications of the effect: laser welding of PP films. Changes in the spectra of irradiated films
discussed in paragraph 3.2 indicate on the changes in the chemical structure of irradiated films, which
results in the formation of polar chemical groups, on the other hand highly developed relief has been
obtained. It seems to be reasonable to conclude that such surface treatment of polymer substrates will
be effective for improvement of their adhesion properties. Bonding of non-polar polymer films (such as
polyethylene and polypropylene) to different substrates is more than problematic, and generally needs
expensive preliminary treatment. Well-developed surface of the laser treated polymer substrate will
alleviate bonding procedure, and use of tunable laser will be especially effective when the material to
be contacted with polypropylene is transparent at the wavelength of resonance absorption. For example
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we successfully welded PP films with polyethylene (LDPE) samples when scanning an IR beam over
the surface of a PE-PP-PE “sandwich” (see Fig. 10) using the optical bench described in Fig.
2.Whereas LDPE is distinguished by its extremely low adhesion properties. Practically all polymers
(including engineering types) are highly transparent at wavelength 10.3 μm, so resonance absorption of
IR laser radiation could be used as a general method for the welding of PP.
Conclusions
The presented paper demonstrates that relatively small powers of coherent IR radiation (4·10-2
W/mm2) cause significant thermal effects in PP films when supplied to the substrate at the resonance
wavelength. The thermal effect is of distinguished threshold nature. Highly developed surface relieves
were produced. Changes in the IR absorption spectra of irradiated films were studied by specially
developed method of spectral correlation analysis. The effect is of importance for direct storage of
information in polymer films, as well as laser welding of PP.
Acknowledgements
The authors wish to thank the Israeli Ministry of Science of Israel (Project No. 1461-2-00) and
the Ministry of Absorption for their support of this work. The authors thank Professor Abraham Katzir
for his inestimable support of our experimental activity, and Dr Zahava Barkay for supplying SEM
images of our samples.
References
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[2] T.Lippert, F.Raimondi, J.Wambach, J.Wei, A. Wokaun, Appl. Phys., A 69 (1999) S291.
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[3] Y.Feng, Z.Q.Liu, X.-S.Yi, Applied Surface Science, 156 (2000) 177.
[4] K. Zimmer, D.Hirch, F.Bigl, Applied Surface Science 96-98 (1996) 425.
[5] M.M.Radwan, Radiation Measurements, 33 (2) (2001) 183.
[6] Yu.Peng, Z. Cheng, Ya. Zhang, Ju.Qiu, Optical Engineering, 40 (12) (2001) 2822.
[7] E. Bormashenko, R. Pogreb, A. Sheshnev, E. Shulzinger, Ye. Bormashenko, A. Katzir, J. Opt. A:
Pure Appl. Opt. 3, (2001) 229.
[8] E. Bormashenko, R. Pogreb, A. Sheshnev, E. Shulzinger, Ye. Bormashenko, Pogreb Z., A. Katzir,
Polymer Degradation and Stability 72 (2001) 125.
[9]. E. Bormashenko, R. Pogreb, A. Sheshnev, S. Sutovski, Ye. Bormashenko, R. Pogreb, and A.
Katzir, Opt. Eng. 40 (9) (2001) 1754.
[10]. M. Ornelas-Rodriguez, S. Calixto, Opt. Eng. 40 (6) (2001), 921.
[11]. B. Wunderlich, H. Baur, Heat capacities of linear high polymers, Springer-Verlag Berlin,
Heidelberg, New York, 1970.
[12] T. Ito, T. Toyoda, N. Hirai, Y. Okhi, Transactions of the Institute of Electrical Engineers of Japan,
A, 121 A, No.9 (2001) 865-871.
[13] A. Saha, V. Chakraborty, S. Chintalapudi, Nucl. Instr. and Meth. in Phys. Res. B, 168 (2000) 245.
[14] A. Dawood, K. Miura, Polymer Degradation and Stability, 73 (2001) 347.
[15]. G. Burlinska, J. Bojarski, J. Michalik, Radiat. Phys. Chem., 47, No 3, (1996) 449.
[16] E. I. Kirilova, E.S. Shulgina, Ageing and stabilization of thermoplastic materials, Chemistry,
Leningrad, 1988.
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1.0
0.9
0.8
0.6
0.5
0.4
0.3
Transmittance
0.7
0.2
0.1
1250
1200
1150
1100
1050
1000
950
900
0.0
850
1/cm
Fig. 1. IR transmittance (a.u.) spectrum of the PP film (thickness 20 μm)
.
4 - 55
computer
Laserstar
power/energy
monitor
detector B
irradiated PP film
detector A
diaphragm
polarizer
tunable CO2 laser
mirror
ZnSe plate
Fig. 2. Optical bench used for the resonance irradiation of PP films
.
4 - 56
pulling
device
500 μm
Fig. 3a. SEM image of the “microlens” formed on the surface of PP film under resonance irradiation, (λ
= 10.23 μm), P = 4 10-2 W/mm2
Fig. 3b SEM image of the “microlens” formed on the surface of PP film under resonance irradiation, (λ
= 10.23 μm), power density P = 4.5 10-2 W/mm2
4 - 57
100 μm
Fig. 4. SEM image of surface details of PP film exposed to resonance IR irradiation
4 - 58
Fig, 5a. SEM image of the surface structure obtained when PP film was pulled with speed V = 0.7
mm/s, power density P = 0.14 W/mm2 (λ = 10.6 μm)
Y
Fig. 5b
X
Fig 5b.Image of surface details obtained with optical microscope equipped with CCD camera.
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Fig. 6. “Traces “ produced by laser beam in the PP film pulled with velocity 5mm/s,
P = 0.14 W/mm2 ( λ = 10.6μm )
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Detector connected to
FTIR spectrometer
diaphragm
source of
IR radiation
irradiated
sample
precise XYZ
translator
Fig. 7. Optical bench used for the study of IR spectra of irradiated samples.
4 - 61
.
.
C
B
.
.
.
.
(a
.u
.)
Absorbance
Correlation coefficient R
-
.
A
.
- .
Wavenumber cm -
Fig. 8. Spectrum of correlation coefficients of IR spectra of irradiated samples compared to absorption
spectra of PP (correlation with average absorption in the band 1700-2200 cm-1 is presented), curve A –
absorption spectrum of PP, curve B – spectrum of correlation coefficients for the sample scanned in Y
direction (see Fig. 5), curve C – spectrum of correlation coefficients for the sample scanned in X
direction.
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.
B
C
.
.
(a
.u
.)
Absorbance
Correlation coefficient R
.
.
A
.
Wavenumber cm -
Fig. 9. Spectrum of correlation coefficients of IR spectra of irradiated samples compared to absorption
spectra of PP (correlation with absorption at 972 cm-1 is presented), curve A – absorption spectrum of
PP, curve B – spectrum of correlation coefficients for the sample scanned in Y direction (see Fig. 5),
curve C – spectrum of correlation coefficients for the sample scanned in X direction
4 - 63
PP film
PE films
tunable CO2 laser
Fig. 10. Scheme for the welding of PE-PP-PE sandwiches using tunable CO2 laser.
4 - 64