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Surface modification of EVA copolymer by UV treatment

Article in International Journal of Adhesion and Adhesives · April 2005

DOI: 10.1016/j.ijadhadh.2004.06.001

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 ARTICLE IN PRESS

International Journal of Adhesion & Adhesives 25 (2005) 139–145

Surface modification of EVA copolymer by UV treatment

Mar!ıa Dolores Landete-Ruiz, José Miguel Mart!ın-Mart!ınez*
Adhesion and Adhesives Laboratory, Department of Inorganic Chemistry, University of Alicante, 03080 Alicante, Spain
Accepted 2 June 2004

Available online 20 July 2004

Abstract

The adhesion between a polychloroprene adhesive and an ethylene vinyl acetate copolymer containing 12 wt% vinyl acetate
(EVA12) was improved by treatment with UV radiation. Changes in the EVA12 surface due to UV radiation treatment were
assessed using contact angle measurements, ATR-IR spectroscopy, XPS, SEM and T-peel tests. Adhesion was evaluated from T-
peel tests of treated EVA12/ polychloroprene adhesive +5 wt% polyisocyanate. Treatment of EVA12 with UV radiation increased
its wettability, carbon–oxygen polar moieties were produced and roughness was created. As a consequence, adhesion of EVA12 to
polychloroprene adhesive +5 wt% polyisocyanate was enhanced by treatment with UV radiation longer than 5 min. A mixed failure
(mainly cohesive failure in EVA12) was obtained.
r 2004 Elsevier Ltd. All rights reserved.

Keywords: A. Rubbers; B. Plastics; Surface treatment; C. Contact angles; Infrared spectra

1. Introduction reasonably faster, economical and environmentally

friendly. Thus, UV-ozone treatment has been used to
Ethylene vinyl acetate (EVA) copolymers are com- increase the wettability of poly(ethylene terephtalate)
monly used in the footwear and toy industries due to (PET), polyethylene (PE), polypropylene (PP) and
their chemical inertness, light weight, flexibility and different rubbers (vulcanized styrene-butadiene—SBR-
toughness. They are also employed as foams, commonly , unvulcanized styrene-butadiene—SBS) [6–11].
used in applications where a high flexibility is required, The photons produced by UV/ozone irradiation have
such as midsoles and insoles in sport shoes. In some of sufficient energy to break most C–C bonds and also can
these applications, EVA copolymers need to be bonded induce chain scission and crosslinking on polymer
to different substrates using adhesives. However, adhe- surfaces [6,12–14]. Additionally, these photons can
sion is poor in EVA copolymers due to their low surface interact with oxygen to form ozone and also with ozone
energy and, therefore, a surface treatment is necessary. to produce atomic oxygen radicals. Both ozone and
Corona discharge and low-pressure gas plasma have atomic oxygen radicals can react with polymer surfaces
been shown to be effective in oxidizing polymer surfaces, to remove low weight contaminants, cleaning and
increasing their surface energy and creating surface modifying surfaces [6,8,12,15,16]. Moreover, UV radia-
roughness [1–4]. tion can incorporate oxygen moieties in treated surfaces
Traditionally, a treatment with UV-ozone has been to form highly polar groups such as hydroxyl, carbonyl
used as a means of removing organic contaminants from and/or carboxyl moieties [9].
different polymer surfaces [5]. More recently, UV-ozone Walzak and coworkers [6] have established that the
has been developed as an alternative surface treatment degree of modification in polypropylene is governed by
to other traditional radiation methods. This method is the oxygen formed by ozone degradation under UV
radiation. However, PET is directly affected by UV
*Corresponding author. Tel.: +34-96-5903977; fax: +34-96- radiation, which causes chain scission, and the mod-
5909416. ifications are independent of the oxygen concentration
E-mail address: jm.martin@ua.es (J.M. Mart!ın-Mart!ınez). in the gas phase [15]. On the other hand, UV treatment

0143-7496/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijadhadh.2004.06.001
 ARTICLE IN PRESS
140 M.D. Landete-Ruiz, J.M. Mart!ın-Mart!ınez / International Journal of Adhesion & Adhesives 25 (2005) 139–145

incorporates oxygen into the surface of polyethylene [7]. 2.2.2. Contact angle measurements
New C–O, CQO and COOH/COOR moieties are Contact angle measurements were carried out in a
incorporated, and as a result, the contact angle values thermostated chamber (25 C) of a Ramé Hart 100
decrease and better adhesion properties are found. goniometer. Drops (4 ml) of deionized and doubly
Different rubbers have improved wettability and distilled water and methylene diiodide were placed on
higher peel strength values after treatment with UV the EVA surface using a micrometric syringe. Advan-
radiation, and the extent of the modifications increased cing contact angles (tilting plate method) were measured
with increasing the length of treatment [11,17]. immediately after UV treatment. The experimental error
However, treatment of EVA with UV radiation has was 72 .
not been described in the literature. We showed [18] that
UV radiation-treated polyolefin foams containing dif- 2.2.3. ATR-IR spectroscopy
ferent amounts of EVA copolymer had improved The attenuated total multiple reflection technique was
wettability and polar moieties were produced on the employed (ATR-IR). A Nicolet FTIR 5DXB spectro-
surface, but adhesion was not improved. Therefore, in meter provided with a thallium bromo-iodide crystal
this study the surface energy of an EVA copolymer was was used (KRS-5). The incident angle of the IR
increased by treatment with UV radiation, and the effect radiation was 45 . Two hundred scans were obtained
on its adhesion properties to polychloroprene adhesive and averaged at a resolution of 4 cm 1. The ATR-IR
was considered. spectra of EVA12 surfaces before and after UV
treatment were obtained to evaluate the chemical
changes produced within a surface thickness of about
2. Experimental 5 mm [20].

2.1. Materials 2.2.4. X-ray photoelectron spectroscopy (XPS)

A VG Scientific, Microtech Multilab spectrometer
The EVA copolymer (EVA12) was supplied by with a MgKa X-ray source (1253.6 eV) operating at
Repsol Qu!ımica, S.A. (Santander, Spain). This copoly- 15 keV and 300 W and using a pass energy of 50 eV
mer was received as pellets and moulded using a was used. The pressure inside the analysis chamber
Margarit JSW injection machine to obtain test samples was held below 5  10 8 Torr during the course of
of 150  60  2 mm3. These samples were cut into the analysis. Square sample pieces (5 mm  5 mm)
150  30  2 mm3 pieces for adhesion tests. Two iden- were used and cooled using liquid nitrogen. The
tical treated EVA12 samples were used to produce measurements were made at a take-off angle of 45 .
adhesive joints. Survey scans were taken in the range 0–1200 eV and
Adhesive joints of UV treated EVA12 were made narrow scans (20 eV) were obtained on significant peaks
using a two-component solvent-based commercial poly- in the XPS survey spectra. Binding energies were
chloroprene adhesive (Telcopren 3003) containing referenced to the C1 s at 285.0 eV. Multicomponent
5 wt% polyisocyanate (Desmodur RFE); both were C1 s photopeaks were curve fitted using 30% Gaussian/
supplied by Composan Adhesivos, S.A., Alicante, Lorentzian function with a full-width at half-maximun
Spain, and Bayer AG, Leverkusen, Germany, respec- (FWHM) of 1.80 eV.
tively. Polyisocyanate was added as a crosslinking agent
just before adhesive application [19]. 2.2.5. Scanning electron microscopy (SEM)
A JEOL JSM-840 microscope was employed to
2.2. Experimental techniques analyze the morphological modifications of EVA12
treated with UV radiation. Samples were gold coated
2.2.1. UV reactor before analysis. The energy of the electron beam was
The UV radiation treatment was carried out with a 20 kV.
low-pressure vapour grid mercury lamp, manufactured
by American Ultraviolet (USA). The UV lamp was 2.2.6. T-peel strength
placed inside a box made of UV-resistant polycarbo- An INSTRON 4411 instrument (peel rate=100 mm/
nate; an exhaust fan was used to avoid both the local min) was used. Adhesive joints using two similarly
concentration of ozone during treatment and the treated EVA12 samples (150  30  2 mm3) were pro-
heating of the lamp. The lamp operates at the duced. The polychloroprene +5 wt% polyisocyanate
wavelength of 254 nm and provides a power of adhesive solution was applied with a brush and the
10 mW/cm2 taken at a distance of 2.54 cm from the solvent was allowed to evaporate at room temperature
lamp. The distance between the UV source and the for 30 min. The dried solid adhesive films on the EVA12
EVA12 was set to 2 cm. The length of the treatment was surfaces (about 50 mm) were then melted at 75 C under
varied between 1 and 7.5 min. IR irradiation and immediately placed in contact under
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M.D. Landete-Ruiz, J.M. Mart!ın-Mart!ınez / International Journal of Adhesion & Adhesives 25 (2005) 139–145 141

a pressure of 0.8 MPa for 12 s. Adhesion was measured 50

72 h after joint formation. Five replicates for each
adhesive joint were carried out. The error was less than 40 γs
0.7 kN/m. The failed surfaces obtained after T-peel test

Surface energy (mJ/m2)

were analyzed using ATR-IR spectroscopy to precisely
assess the locus of failure in the joints [21]. 30 γ sd

20
γ sp
3. Results and discussion
10
The effectiveness of the UV treatment depends on
different experimental conditions like UV source–
0
sample distance, length of treatment, radiation power
0 2 4 6 8
and wavelength of the UV lamp, and ozone concentra-
Length of treatment (min)
tion [6,9]. In this study, the length of treatment was
varied between 1 and 7.5 min, and the distance between Fig. 2. Total (gS), dispersive (gsd ) and polar (gsp ) surface energy
components values on EVA12 treated with UV radiation.
the EVA12 samples and the lamp was set to 2 cm.
The water advancing contact angle values on EVA12
decrease by increasing the length of treatment with UV
0.4 As-received
radiation (Fig. 1). Water advancing contact angles are
higher than methylene diiodide contact angles due to the Abs 0.2
non polar-nature of EVA12 surface. In fact, after UV
0.2 UV-1 min
treatment the methylene diiodide contact angles have
Abs

0.1
almost a constant value near 55 .
Surface energy was calculated by using the Owens and 0.0
0.2 UV-5 min
Wendt approach (Fig. 2) [22, 23]. A progressive increase
Abs

0.1
in surface energy with the length of the UV treatment is
0.0
observed, due predominantly to the development of the
0.10 UV-7.5 min
polar component of the surface energy.
Abs

0.05
The decrease in contact angle values (and the
0.00
subsequent increase in surface energy), can be ascribed
4000 3000 2000 1000
to modifications in the surface chemistry and/or
Wavenumber (cm-1)
morphology of EVA12. ATR-IR spectroscopy and
XPS were employed to analyze the chemical changes Fig. 3. ATR-IR spectra of the as-received and UV-treated EVA12.
in EVA12 surfaces after UV treatment. Fig. 3 shows the
ATR-IR spectra of the as-received and UV-treated
EVA12. The ATR-IR spectra of the as-received EVA12
show the typical bands of vinyl acetate at 1739, 1242, 1023 and 606 cm 1, and ethylene at 2919, 2846, 1467,
1375, and 725 cm 1. The UV treatment for more than
1 min decreases the relative intensity and increases the
100 width of the band at 1739 cm 1 due to CQO bond of
vinyl acetate groups, indicating the formation of new
90 oxygen-containing moieties on the EVA12 surface.
Contact angle (degrees)

80
Table 1 shows the elemental composition obtained
using XPS on the as-received and UV-treated EVA12.
70 The as-received EVA12 is mainly composed of carbon
and a small amount of oxygen. The O/C ratio (0.05) is
60
smaller than expected (O/C=0.11) according to the
50 vinyl acetate content in EVA12; this can be attributed to
a heterogeneous distribution of vinyl acetate domains on
40
Water Methylene diiodide the EVA12 surface. A treatment of EVA12 with UV for
30 5 min is necessary to significantly increase the amount of
0 2 4 6 8 oxygen on the surface, and the UV treatment of EVA12
Length of treatment (min) for 7.5 min produces the highest O/C ratio (0.11). Table
Fig. 1. Advancing contact angle values on EVA12 treated with UV 2 shows the percentages of carbon species obtained from
radiation. C1 s curve fitting of as-received and UV-treated EVA12.
 ARTICLE IN PRESS
142 M.D. Landete-Ruiz, J.M. Mart!ın-Mart!ınez / International Journal of Adhesion & Adhesives 25 (2005) 139–145

As-received EVA12 shows the typical moieties of EVA due to the formation of CQO groups are created on
copolymer : 285.0 eV (C–C, C–H), 285.6 eV (CH3– the treated EVA12 surface [24,25]. The creation of
(CQO)–O), 286.6 eV (HC–O–(CQO)–), and 289.2 eV these carbon–oxygen moieties is in agreement with
((CQO)–O). The UV treatment decreases the the trend in advancing contact angle values and
hydrocarbon content and slightly increases the C–O ATR-IR spectra, and indicates that the UV treatment
moieties content. Furthermore, new moieties at 288.0 eV increases the wettability of EVA12 due to oxidation
processes.
The modifications in surface morphology of EVA12
Table 1 by treatment with UV radiation were analyzed using
XPS elemental composition on the as-received and UV-treated EVA12 SEM. SEM micrograph (Fig. 4) of the as-received
EVA12 shows some ruffles on the surface due to the
Sample Atomic% O/C
injection process. These ruffles on the EVA12 are
C O removed by treatment with UV radiation for 1 min,
As-received 95.2 4.8 0.05 and by increasing the length of treatment to 5 min some
UV - 1 min 95.2 4.8 0.05 cracks and surface heterogeneities are created. The UV
UV - 5 min 92.8 7.2 0.08 treatment of EVA12 for 7.5 min renders a smoother
UV - 7.5 min 89.8 10.2 0.11 surface (ablation).
As a consequence of the chemical and morphological
Table 2 modifications produced by UV treatment, improved
C1s species on as-received and UV treated EVA12 adhesion in EVA12/polychloroprene +5 wt% polyiso-
cyanate adhesive joints is achieved (Fig. 5). The adhesive
C1s species Length of treatment (min)
joint produced with the as-received EVA12 shows a
0 1 5 7.5 low peel strength value due to its poor wettability
C–C, C–H 84.8 79.7 76.9 75.5 and lack of surface chemistry and roughness. The
C–CO or C–C in polyethylene 8.0 10.9 11.1 11.3 UV treatment for 1 min increases the peel strength
C–O 6.1 7.4 9.1 9.3 values, and the highest value is reached by treatment
CQO — 1.6 2.3 2.4 for 5 min (in agreement with the improvement in
(CQO)–O 1.1 0.4 0.6 1.5
wettability and surface chemistry). For more extended

Fig. 4. SEM micrographs of the as-received and UV-treated EVA12.

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M.D. Landete-Ruiz, J.M. Mart!ın-Mart!ınez / International Journal of Adhesion & Adhesives 25 (2005) 139–145 143

As-received EVA12 / PCP + 5 wt% polyisocyanate joint

4 A surface
0.2
T-peel strength (kN/m)

3 0.1

Absorbance
2 0.0
0.15 P surface

1 0.10

0.05
0
0 2 4 6 8 0.00
Length of treatment (min) 4000 3000 2000 1000
(a) Wavenumber (cm-1)
Fig. 5. T-peel strength of as-received and UV treated EVA12/
polychloroprene +5 wt% polyisocyanate adhesive joints.
EVA12 / PCP + 5 wt% polyisocyanate joint
A surface
0.10

0.30 C-Cl 0.05

Absorbance
CH2
0.25 C=C 0.00
0.15 P surface
Absorbance

0.20
Tackifier
0.15 =CH 0.10
0.10
0.05
0.05 N=C=O
0.00 4000 3000 2000 1000
(b) Wavenumber (cm-1)
4000 3000 2000 1000
Wavenumber (cm-1)
EVA12 / PCP + 5 wt% polyisocyanate joint
Fig. 6. ATR-IR spectra of polychloroprene adhesive +5 wt% poly-
isocyanate. A surface

0.10

0.05
length of treatment (7.5 min), the peel strength slightly
Absorbance

0.00
decreases. P surface
The loci of failure of the adhesive joints were assessed
0.10
by analyzing the failed surfaces after peel test using
ATR-IR spectroscopy. Different bands in the ATR-IR 0.05
spectra allow to differentiate the EVA12 and the
0.00
adhesive. Fig. 6 shows the ATR-IR spectrum of the
polychloroprene adhesive +5 wt% polyisocyanate. The 4000 3000 2000 1000
most typical bands are QC–H stretching of aromatic (c) Wavenumber (cm-1)
ring (3025 cm 1), C–H stretching of methylene groups Fig. 7. ATR-IR spectra of the failed surfaces obtained after peel test of
(2930 and 2867 cm 1), CQC stretching (1660 cm 1), C– EVA12/polychloroprene +5 wt% polyisocyanate adhesive joints.
Cl stretching (830 cm 1), aromatic–COOH vibration of Joint produced with: (a) As-received EVA12; (b) EVA12 treated with
UV radiation for 5 min; (c) EVA12 treated with UV radiation for
the tackifier in the adhesive (1745 cm 1), and asym- 7.5 min.
metric stretching due to NQCQO group of the
isocyanate (2269 cm 1).
The locus of failure is different in the adhesive joints polychloroprene adhesive and the EVA12 surfaces,
produced with the as-received and UV-treated EVA12 and thus, an adhesion failure is obtained. However,
(Fig. 7). In this study, the A surface corresponds to the for the joint produced with EVA12 treated with UV for
adhesive surface and the P surface to the polymer 7.5 min, the ATR-IR spectrum of the A surface (Fig. 7c)
surface. The failed surfaces obtained in the joints shows typical bands of adhesive and EVA12 and, thus, a
produced with the as-received (Fig. 7a) and UV-1 and mixed failure (in a thin layer of the UV treated EVA12)
5 min treated EVA12 (Fig. 7b) correspond to the is produced.
 ARTICLE IN PRESS
144 M.D. Landete-Ruiz, J.M. Mart!ın-Mart!ınez / International Journal of Adhesion & Adhesives 25 (2005) 139–145

moieties. Longer UV treatment produces some degrada-

tion and a weak thin layer on the surface of treated
EVA12 is obtained, and thus the peel strength value
decreases. However, a mixed locus of failure (mainly
cohesive in EVA12) is obtained.

Acknowledgements

Financial support from Research Spanish Agency

MCYT (Projects MAT2002-02463 and PETRI 95-0578-
OP) is acknowledged. M.D. Landete-Ruiz thanks the
Generalitat Valenciana for granting her Ph.D. Thesis.
The authors also thank Repsol Qu!ımica, AIJU, Com-
posan Adhesivos, S.A. and Bayer AG for supplying
some of the materials used in this study.

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