Physical and chemical treatments of surface
for improved adhesion of PVA Coating
Mohamed Ramzi Ammar*, Gilbert. Legeay**, Alain Bulou***, Jean- Fançois.Bardeau***
*IUT de Blois, Université François Rabelais, 3 place Jean Jaurès- C.S. 2903, 41029 Blois Cedex.
**Centre de Transfert de Technologie du Mans - 20, rue Thalès de Milet, 72000 Le Mans.
***Laboratoire de Physique de l'Etat Condensé UMR/ CNRS 6087 - Avenue O. Messiaen, Université du
Maine, 72000 Le Mans.
mohamed.ammar@univ-tours.fr, glegeay@cttm-lemans.com, alain.bulou@univ-lemans.fr,
jean-francois.bardeau@univ-lemans.fr,
RESUME: Les revêtements polymères peuvent apporter des propriétés intéressantes aux surfaces des
dispositifs médicaux. Des efforts significatifs sont entrepris par les chercheurs afin d'élaborer des revêtements
plus mince, plus fiable et multi-fonctionnels En effet, les surfaces peuvent être modifiées pour améliorer la
rentabilité, la longévité et la performance du dispositif.. Cependant, un certain problème d'adhérence revêtement
polymère/substrat est souvent rencontrée. L'objectif du présent travail est de montrer que les traitements de
surfaces peuvent améliorer fortement l'adhérence polymère/substrat. Dans notre étude, le substrat modèle a été
soumis à différents types de traitements de surfaces (chimique, physique et mécanique). Ensuite, des tests dans le
milieu aqueux sont effectués afin de caractériser l'influence de l'eau sur la stabilité de l'interface du revêtement
polymère/substrat.
MOTS-CLES: Revêtement polymère; Adhésion; Traitements de surface, Durabilité
1
Introduction
During the last two decades, significant advances have been made in the development of biocompatible
materials for biomedical applications. The goal of actual researches is mainly devoted to the elaboration and
characterization of artificial materials (Ball 1997) which could be in contact with biological medium to improve
physiologic reaction. However, for implants, the interface between the device and the human body can be critical
to both performance and therapeutic effectiveness. Careful choice of materials at this interface is therefore key to
the success of the device. Biocompatibility of this type can sometimes be achieved with materials used in bulk
form, but surface modification, especially in the form of a coating, provides a wider range of design options and
related performance.
Polymeric coatings, in particular, can be chemically, mechanically and physically tailored to produce the
desired combination of properties and have found widespread use for nonthrombogenic (Frost 2005), lubricous
(Thompson et al. 2005), anti-microbial (Carlson et al. 2006) and protective (Reitman et al. 2005) purposes. Alone, in
combination or as carriers for active pharmaceutical ingredients, polymeric coatings continue to enable an everexpanding market for implantable medical devices.
Among several choices of polymers, poly(vinyl Alcohol) (PVA), a water-soluble polymer, has been frequently
explored as an implant material in biomedical applications such as drug delivery systems, dialysis membranes,
wound dressing, artificial skin, cardiovascular devices and surgical repairs because of its excellent mechanical
strength, biocompatibility and non-toxicity (Chiellini et al. 2003) (Pereira et al. 2000) (Yamaokaet al. 1995) (Tang et al.
2007).
Beyond the bio-functional requirements, a successful coating must adhere to the device, be flexible and
strong enough to withstand the expected movement of the device, allow for sterilisation and be durable under the
conditions of its use. It was highlighted that surface quality strongly affects the adhesion between the polymer
and substrate (Cognard 2005). Many adhesion-enhancing techniques have been explored, such as mechanical
1
roughening, plasma treatment and electrochemical anodisation (Cognard 2005) (Dayss et al. 1999). Smith (Smith et
al. 1999) reported recently, that they could increase the adhesive force between substrate and a polymer matrix by
using silane coupling agent. In the present study, we investigate the effect of coupling mechanical, chemical and
physical parameters on the improved of system durability and stability.
What is used to characterise the durability is the extension of the test fracture after some time of exposition in
humid condition (Cognard 2005).
2
Experimental
Poly(vinyl alcohol) (PVA) with average molecular weight M ω = 95000 has been used. The polymer
was amorphous and atactic and it had degree of hydrolysis of 95 %. This product was purchased from Across
Organics, Belgium.
For a water-soluble hydrophilic polymer, the total amount of the sorbed or the added water may be extended
from zero to infinity corresponding to its action just like a plasticizer to a solvent. However, the addition of water
to PVA 95% is limited, no more water could be further sorbed as the swollen polymer reaching to a state of
equilibrium swelling, because of acetate rest that the polymer contains.
Aqueous 5% PVA solutions are prepared with distilled water in a covered beaker and heating in an oven at 70°C
during 10 hours.
3
Surface energy effect
In order to study hydrophilic/hydrophobic character effect of surfaces on polymer coatings adhesion, we
compared results obtained from silicon substrates (about 2 cm 2) cleaned with ethanol in ultrasonic bath (US)
during 5 min, and others exposed, thereafter, to UV/Ozone radiation (Tominaga et al. 2005) during 15 min.This
process removes hydrocarbons from the surface.
Hydrophilic/hydrophobic character has been ascertained by wettability with the contact angles measurements,
global surface energy and polar and apolar components (according to Owens-Wendt model). These
measurements released by goniometry classical methods with three liquids technique (formamide, diidomethane,
water) are given in table 1.
Treatment
US
US + UV
Days number before
unsticking
Contact angle (°)
Water drop
Surface energy
(mJ.m-²)
Dispersive
components
(mJ.m-²)
Polar components
(mJ.m-²)
Some
hours
28
1
61.1
69.1
27.5
27.2
33.7
42
2
Table 1: Observed values of contact angle (from three values) and surface energy calculated for silicon
substrates before and after UV/O treatment
Silicon surface becomes very hydrophilic after being exposed to UV/O radiation since the contact angle of
water drop is about 2°. It is typically accepted that contact angle measurements smaller than ≈5◦ is a signature of
a fairly flat and chemically uniform substrate (Packham 2003).
Drops of the same polymeric solution are deposited on different substrates. Interferometry was used to gain
accurate information on coating thickness (35 ± 3) µm.
2
Durability test of these layers in water at room temperature is followed by observing the day number that film
takes to come off completely. According to Table1 results, it seems that UV/ozone treatment enhances slightly
polymer coatings adhesion. However, this durability remains insufficient, and others treatments are needed.
4
Use of primary thin layer
Reflectivité Absolue
Absolute Reflectivity (a.u.)
In order to enhance polymeric coating adhesion, a primer nanometric layer of PVA on silicon substrate
is deposited by spin coating process. This technique allows an homogenous film elaboration by a controlled
parameters (rotation speed, acceleration, solution viscosity). Thickness of this layer (130 nm) has been verified
by X rays reflectivity as shown in figure 1. X-ray reflectivity data was collected on a X-Pert Philips
reflectometer working at the energy of the Cu Ka radiation (kα =0.154 nm)
10
1
0,1
0,01
1E-3
1E-4
1E-5
1E-6
Figure 1: X
curve of a
silicon
deposited by
process.
1E-7
0,0
0,1
0,2
0,3
-1
qz (Å )
rays reflectivity
primer layer on
substrate
spin-coating
Polymeric coating of about 35 µm thickness is then deposited in the same manner as previously.
Probably adhesion mechanism between polymeric coating and the primer layer is due to the interdiffusion
phenomenon of polymeric chains in the interface from both side polymers according to the “diffusion theory of
adhesion” (Voyutskii 1977) which accounted for the time and temperature dependence of the adherence between
miscible, soft, polymers. Recently, it was shown that amorphous or semi-crystalline, immiscible, polymers could
be stitched together by a film of copolymer having lateral chains interpenetrating in both materials 10. Durability
test in aqueous medium has been done and results are given in Table 2.
Treatments
US + PL
US + UV/O + PL
Days number before
unsticking
5±1
8±1
US : Substrate cleaning by ultrasonic bath
UV/O: Substrate treated by UV/Ozone
3
PL : Primer layer
Table 2: Evolution of day’s number before polymeric coating unsticking in functions of the use of primer thin
layer
This durability test shows that polymeric coatings deposited on substrates strengthened with a primer
layer stay on more than 5 days. Compared with Table 1, it appears that the use of primer layer influences system
durability. It’s certainly obvious that even the process polymeric coating affect adhesion improvement.
5
Effect of temperature treatment
Raman intensity (u.a.)
Performances enhancement of adhesion by using a primer layer encouraged us to study its treatment
temperature effect on adhesion. Three types of substrates were then treated in different vaccum temperatures
(60°C, 80°C and 100°C) around the glass transition temperature (Tg = 80°C). In order to verify composition thin
primer layer after temperature treatment, we study them with confocal Raman scattering performed with T64000
Jobin-Yvon multichannel spectrometer in simple configuration with 600 lines/mm grating coupled to a CCD
detector is used. The spectra were collected in the backscattering geometry, under microscope (X100 objective)
using the 514.5 nm wavelenth of Ar-Kr laser source with 10mW.
100°
C
22°
C
500
1000
1500
2000
2500
3000
3500
-1
Wavenumber (cm )
Figure 2: Raman spectra of primer layer at room temperature and other treated under vaccum untill 100°C
during 24h.
A film of polymer (thickness: 35 µm) is deposited on primer coat support. The durability of these treatments and
stability of interfacial strength are estimated by the stability of adhesion with supports coated by dipping in water
at room temperature (Table 3).
Treatment
Days number before
unsticking
US+UV/O
+PL (22°C)
US+UV/O
+PL (60°C)
US+UV/O
+PL (80°C)
US+UV/O
+PL (100°C)
8± 1
12 ± 3
36 ± 5
715+23
4
Table 3: Evolution of day’s number before polymeric coating unsticking, in function of temperature treatment
Figure 3 shows the relation between adhesion durability of micrometric coating according to the treatment
temperature. The results show that coating’s adhesion increases considerably with the increase of annealing
temperature. In fact, treatments at 100°C allow to keep the polymer coating adhered to the substrates and this
after more than 700 days of immersion in water.
800
700
Durability (days)
600
500
400
300
200
100
0
20
40
60
80
100
Treatment temperature (°C)
Figure 3: Durability test results of substrats treated by US, UV/O and PL (at various temperatures).
6
Mechanical roughening effect
In parallel to the studies undertaken on the effect of physicochemical treatment, the couple effect of a
mechanical roughening on silicon surface is estimated. Increase of surface roughness is thought to increase the
effective surface of contact between the substrate and the polymer (Cognard 2005). Results (Table 4) indicate that
mechanical roughening doesn't improve clearly system durability when polymeric coating is deposited directly
above or on the untreated primer layer. In other part analysis show that the heat treatment of the primer layer
associated to a mechanical roughening proves to reinforce adhesion properties since the number of days before
the layer unsticking exceeds 715 days actually.
Treatment
Days number
before unsticking
Etching
Etching
Etching
Etching
+ US+UV/O + US+UV/O+ + US+UV/O+ + US+UV/O+
PL (22°C)
PL (80°C)
PL (100°C)
715
4± 1
9± 1
143 ± 14
Table 4: Evolution of day’s number before polymeric coating unsticking functions of primary layer treatment
7
Conclusion
5
The science of the adhesion of coatings to surfaces is a complex one that contains a degree of mystery in
the sense that we cannot always explain why it works or does not work. Various process techniques available to
improve adhesion were discussed in this article. When the coater is satisfied that the surface is as clean as can be
achieved, it requires, as we showed, a primer thin layer to make it work. Our analyses show clearly that thermal
treatment of nanometric primer layer associated to a mecanical etching proved true very effective to consolidate
adhesion properties.
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