European International Journal of Science and Technology ISSN: 2304-9693 www.eijst.org.uk
78
Epoxy/amine networks for coating 316L stainless steel: Preparation,
surface characterization, adhesive properties and water absorption
Filiberto González Garciaa*
, Alexandre Z.Simõesb, L. Rogerio O. Hein
b, Elson Longo da Silva
c
aInstitute of Pabsortionhysics and Chemistry, Federal University ofItajubá, Av. BPS. No. 1303, 37500-903,
Itajubá, MG, Brazil.
bFacultyofEngineeringGuaratingueta,Paulista StateUniversity, Ave. Dr. Ariberto Pereira da Cunha 333,
Guaratinguetá, 12516-410, São Paulo, SP, Brazil.
cPaulistaStateUniversity, InstituteofChemistry, Rua Francisco Degni, 55, Quitandinha,
14801907 - Araraquara, SP, Brazil.
*Corresponding Author Email: [email protected]
Sponsoring information:This work was financially supported by the Brazilian research funding agencies
FAPEMIG (Fundação do Amparo à Pesquisa do Estado de Minas Gerais, Brazil, processes APQ.01736-11
and APQ.00073-13) and CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnológico, Brazil).
Abstract: 316L stainless steel (SS) is widely used in the metallic stents manufacturing and in general is
considered to be a biocompatible material, but it is prone to corrosion in contact with biological
environments. We examined possible surface coating by using inert polymeric with antithrombotic
properties and low cytotoxicity. Thickness and roughness of the surface coating, adhesive properties and
water absorption of three epoxy thermosetting polymers were studied. Diglycidyl ether of bisphenol A
(DGEBA) epoxy monomer modified with reactive diluent and cured with 4,4’-diamino-3,3’-
dimethyldicyclohexylmethane (3DCM), isophorone diamine (IPD) and 4-methylpiperidina (4MPip)were
evaluated. Only the DGEBA/4MPip system formed a coating on polished and silanizated metal plates and
exhibits the better adhesive strength and low water adsorption. We conclude that the DGEBA/4MPip system
exhibits greater ability to form coatings surface on 316L SS with better adhesive properties and lower water
absortion.
Keywords: Surface coating; 316L stainless steel; epoxy/amine networks; adhesive properties; water
adsorption.
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79
1. INTRODUCTION
The percutaneous transluminal coronary angioplasty (PTCA) was introduced in the management of coronary
artery disease symptoms (Sheiban et al., 2008).Grüntzig and Myler performed the first coronary angioplasty
during coronary artery bypass graft surgery in 1977 (Newsome et al., 2008; Erbel et al., 2002). PTCA is
today the most frequent interventions in medicine and constitutes an important economic factor. PTCA is a
minimally invasive procedure to open up blocked coronary arteries, allowing blood to circulate unobstructed
to the heart muscle. This involves steering a balloon catheter to the blockage site and inflating it to compress
the deposit that obstructs the artery and to re-establish the blood flow. The most important complication
associated to this therapy is restenosis that is re-narrowing of the vessel, which occurs in 30 – 40% of
coronary lesions within 6 months (Serruys et al., 1994).To improve the clinical outcome of PTCA, bare
metallic stents have been used. Stents are small metal scaffolds that are placed on the balloon and then
expanded into the damaged artery at stenosissite. It is known that the use of these devices has reduced the
restenosis rate to 20 – 30%, they often increase the incidence of inflammation, thrombosis, and
fibromuscular proliferation (Mani et al., 2007).
316L stainless steel(SS) is widely used in the metallic stents manufacturing and in general is
considered to be a biocompatible material, but it is prone to corrosion in contact with biological
environments and causes the gradual release of metal ions, such as chromium, iron and nickel in the
surrounding tissue. This may trigger local immune response and inflammatory reaction, which in turn may
induce intimal proliferation (Kajzer et al., 2008; Köster et al., 2000). In cardiovascular stenting application,
a thin stainless steel wire could also lose its mechanical integrity as a result of corrosion. To improve the
performance of cardiovascular stents different types of protective coating based on stainless steel are being
developed. For example; inorganic coating such as two oxides (TiO and ZrO) and diamond-like carbon have
been studied by in vitro and in vivo essays exhibiting good biological properties (Mikhalovska et al., 2011).
Another possibility is a plasma-activated coating for metallic coronary stents that is durable, with stands
crimping and expansion, has low thrombogenicity and can covalently bind proteins, linker-free. It is know
that the enhancement of endothelial cell interactions in vitro has the potential to promote biointegration of
stents (Waterhouse et al., 2012).To improve the strength and corrosion resistance of 316L SS the cold spray
technique was used. In this work the stainless steel mixed with cobalt chromium alloy L605 powders has
been cold sprayed onto mild steel substrate (AL-Mangour et al., 2013).
Multiples investigations have been devoted to polymers and copolymers coating of bare metallic
stents. Polymer properties can be tailored by specific physico-chemical characteristics. For example a new
copolymer acrylic was investigated for stent coating which contain trifusal covalently attached to the
polymer backbone. This polymer is a powerful platelet aggregation inhibitor which has been used in the
development of stent coating with inherent anti aggregating modulation of platelets, as well as support for
the release of a antiproliferative drug to prevent restenosis (Rodríguez et al. 2010). The metallic surface
(alloy 316L) deposited by plasma from biological environment has lead to an ultra-thin, stable, cohesive and
adhesive plasma polymerized allylamine coating with high selective towards primary amine groups. The
coatings satisfy the adhesion and cohesion properties to be stable upon desionised water immersion and to
resist to stent expansion (Gallino et al., 2010).A new nanocomposite polymer based on polyhedral
oligomeric silsesquioxanes (POSS) and poly(carbonate-urea)urethane (PCU), which is an antithrombogenic
and a non-biodegradable polymer with in situ endothelialization properties has also been investigated. This
nanocomposite coated with nitinol(NiTi) surface can enhance surface resistance and improve
biocompatibility (Bakhshi et al., 2011). Copolymers of poly(e-caprolactone) (PCL), poly(ethylene glycol)
(PEG), and carboxyl-PCL(cPCL) were also employed as potential coronary stent coating materials with two
primary human coronary artery cell types: smooth muscle cells (HCASMC) and endothelial cells (HCAEC)
European International Journal of Science and Technology ISSN: 2304-9693 www.eijst.org.uk
80
and the copolymer with 4%PEG–96%PCL–0%cPCL was identified as the most appropriate coatingmaterial
(Crowder et al., 2012).Amphiphilic copolymers based on the copolymerization of hydrophilic and
hydrophobic moieties offer versatility in various biomedical material applications. Copolymer of dextran-
graft-poly (butyl methacrylate) was synthesized and characterized as coating for metallic endovascular stents
(alloy 316L). The resulting coating is smooth and uniform with neither cracks nor detachment after stent
expansion. Interestingly, surfaces coated with the copolymer greatly improve in vitro adhesion and growth
of endothelial cells (Dekaoui et al., 2012). However, an epoxy thermosetting polymer has never been
studying for coating 316L SS (e.g., stent material). The use of inertpolymeric coatings is attractive for bare
metallic coronary stents. This argument is supported by studies with human blood compatibility and Chinese
hamsters ovary cells which possess anti thrombotic properties (González Garcia et al., 2009; 2015) and no
signs of cytotoxicity (González Garcia et al., 2009).Moreover, epoxy thermosetting polymers its interest for
coating applications (Omrani et al., 2011; 2013) and have good adhesive properties to various metals,
including stainless steel (de Morais et al., 2007).However, this property decreases in water physiological
environment (humidity or water) over time as a result of water absortion by the polymeric matrix (Oudad et
al., 2012; González Garcia et al., 2011; Colombini et al., 2002).Another problem is related to the rigidity of
suchpolymers (Pascault et al., 2002; Clayton, 1988). However, a suitable selection of the monomers and
additives can be obtained in a wide range of biological properties, mechanical properties, adhesive strength
and water absortion. The study of these properties is in progress in our laboratories.
In this work, we examined the possibility of coating the surface of 316 Lstainless steel based
thermosete poxy polymers such asdiglycidyletherof bisphenol A modified with diepoxy aliphatic diluent
cured with cycloaliphatic amines. Optical microscope (OM) was used to examining the polished surface
plate, silanized and coated after cure process. The adhesive strength of epoxy formulation on 316LSS
adherend was evaluated in terms single lap shear using 316L SS adherends. Finally, the water absortion of
the thermosetting polymers was obtained by immersion at the physiological temperature (37 °C).
2. MATERIALS AND METHODS
2.1 Materials
The formulations used in this study are based on the diglycidyl ether of bisphenol A (DGEBA, DER 331
Dow Chemical Co. Ltda) with an equivalent weight of epoxy groups of 185.5 as determined by acid
titration, modified with 30 phr of diepoxy aliphatic diluent (1,4-butanediol diglycidyl ether, DGEBD), 30 g
of DGEBD per 100 g of DGEBA. The epoxy monomer was vacuum dried at 80 °C before use. The
cycloaliphatic amines used were4-methylpiperidine (4MPip, 96% Aldrich), 4,4'-diamino-3,3'-dimethyl-
dicyclohexylmethane (3DCM, 99% Aldrich) and isophorone diamine (IPD, 99% Aldrich) with an equivalent
weight of amine hydrogen groups 42.6 for IPD and 59.6 for 3DCM determined by potentiometric titration,
respectively (González Garcia et al., 2007). The cycloaliphatic amines were used as received.
2.2 Surface treatment and silanization
The 316L SS used was a commercial stainless steel VI 138 (specialty alloy, ASTM – F138) from Villares
Metals, Brazil. This metallic substrate was supplied with shape plates of side 1.0mlong, 10.0 mm wide and
1.2 mm thick with unpolished surfaces. Plates were used as carrier instead of real stents due the high costs.
The composition of such alloy is given in Table 1. In order to increase its adhesive properties, the metallic
adherend surfaces were prepared. The applied surface treatment consisted of the following steps: (1) Before
any application the plates were ultrasonically cleaned in acetone at room temperature for 5 min then dipped
in acetone at 60 °C for 5 minutes and dried by dabbing with absorbent paper and dry nitrogen flow. (2) The
surface of the plate was mechanically polished to a mirror like finish using 230, 300, 400, 500, 600, 1200
European International Journal of Science and Technology Vol. 4 No. 7 September, 2015
81
silicon carbide disks followed by 0.30 µm and 0.05 µm alumina oxide polishing (suspension in solution). To
remove polishing residuals, samples were cleaned with distillated water between each polishing step.
Samples were then dried by dabbing with absorbent paper. (3) The surface of the plates were silanizated: the
method consisted in a silane solution (0.12 % v/v) of 3-aminopropyl-trimethoxysilane (ATPS, 97% Aldrich)
prepared with 25%/75% v/v mixture of ethanol and distilled water according to the methodology reported in
the literature[Chovelon et al., 1995). The polished and silanizated metal plates were stored in a vacuum at
room temperature.
Table 1. Chemical composition in % (wt) of the specialty alloy IV 138 by Villares Metals supply.
C Mn Si Cr Ni Mo P S Cu N
0.025
max.
1.80 0.40 17.50 14.00 2.80 0.025
max.
0.003
max.
0.10
max.
0.10
max.
2.3 Viscosity and contact angle
On the surface of the plate treated, one drop of the epoxy formulation freshly prepared was deposited using
micropipette and the contact angle measured at room temperature. The used equipment was Kruss, model
FMMK2 Easy drop equipped with camera and software. Viscosity of epoxy formulations was measured
using a Rheometric, Physica Anton Paar (model MCM 301) at 25 °C with conical plates of 24.74 mm of
diameter and angle 1.012°.
2.4 Coating and curing
Three different epoxy matrixes based on diglycidyl ether of bisphenol A (DGEBA).The first one was cured
with 4-methylpiperidina. The second one and the third one were cured with 4,4’-diamino-3,3’-
dimethyldicyclohexylmethane and isophorone diamine, respectively. The first formulation was cured with 4-
methylpiperidine (4MPip) using 5 phr (5 g of 4MPip per 100g of DGEBA) with the following thermal cycle:
30 minutes at 60 ºC and later during 16 hours at 120 °C. The second and the third one were prepared by
carefully weighting the monomers at the stoichiometric proportion (epoxy/amine hydrogen e/a= 1). The
cured schedule was4 hours at 60 ºC for these two systems and for the formulations based on 3DCM and IPD
were post-cured at 180 ºC and 160 ºC during 2 hours, respectively. All mixtures were stirred for 2 minutes
and degased under vacuum over for 5 minutes at room temperature to remove trapped air. The silanized
metal plates surfaces were coated by immersion in the monomers mixtureat60 ºC. After immersion, the
metal plates were cured according to the schedule for each system and slowly cool down to room
temperature. After that, it was stored at room temperature in a vacuum system.
2.5 FTIR spectroscopy
Fourier transform infrared (FTIR) measurements of cured coatings were performed with the Perkin Elmer,
Spectrum 100 spectrometer equipped with universal ATR sampling accessory with a diamond crystal.
Spectra were recorded with a resolution of 4 cm-1
and at least 32 scans with the wave number range from
650 – 4000 cm-1
.
2.6 Differential scanning calorimetry (DSC)
The glass transition temperature (Tg) and the residual enthalpy (∆HR) of the cured coatings were investigated
in a differential scanning calorimetry (DSC) (Shimadzu, DSC 60). Samples of around 10.0 mg were
weighting in standard 40 µL aluminum pan and heat from 30 °C to 145 °C with a heating rate of 10 °C min-1
European International Journal of Scien
in nitrogen atmosphere with 50 mL min-
the initial changes on heat capacity (onse
2.7 Optically microscopy
The thickness and roughness of the po
metallic plates surface coated were o
technique was performed and allowed t
dimensional images using a opening hol
the focal plane. The optical microscope u
2.8 Preparation of lap shear specimens
The adhesive behavior was evaluated fo
adhesion test was carried out according t
1. In order to increase its adhesive pro
surface treatment which consists of th
acetone at room temperature for 5 min t
with absorbent paper and dry nitrogen f
bath at 60 °C for 10 minutes, rinsed with
method consisted in silane solution (0.1
25/75% (v/v) mixture of ethanol 95%
literature (Chovelon et al., 1995). Speci
applications, specific metallic mold was
by the design of the mold. After surface
shear joint. Each epoxy formulation wa
epoxy formulation was applied uniforml
specific metallic molds. The applied con
with uniform adhesive thickness, (0.2±
the tensile axis, chocks in the extremes
room temperature (22 ± 2) °C and relativ
Fig. 1.Dimensions of the adhesives joint
ence and Technology ISSN: 2304-9693
-1nitrogen flow rate. The glass transition tempera
set value).
polymer coating was determined byoptical mic
observed at ambient temperature. Confocal m
d to increase the contrast microscopic image and
ole pinhole, which leads to a high image definitio
e used in confocal mode was the Leica DCM3D.
ens
for mechanical tests using single-lap shear joint.
g to ASTM D1002. The geometry of adhesive join
roperties, the metallic adherend surfaces were p
the following steps: (1) Specimens were ultras
n then dipped in acetone at 60 °C for 5 minutes a
n flow. (2) The metal surface was chemical treate
ith distillated water and blown dry with nitrogen.
.12% v/v) of 3-aminopropyl-trimethoxysilane (A
and distilled water according to the methodo
cimens were stored in a glass dryer with silica g
as designed for adhesive joint. The layer thickne
ace treatment, metallic pieces were assembled for
as prepared and cured as mentioned in item coa
ly on both surfaces of the adherend with the sam
ontact pressure was kept constant, which allows
0.02) mm. To reduce the deviation of the adhe
es of the specimens were used. These specimens
tive humidity of (50 ± 5) % during 24 hours befor
ints of single lap shear using steel adherend (meas
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erature (Tg), is taken at
icroscopy (OM). The
l microscopy imaging
nd constructing three-
tion in thicker samples
t. For this purpose, the
oint is shown in Figure
prepared by applying
rasonically cleaned in
and dried by dabbing
ated in a sulfochromic
en. (3) The silanization
(ATPS) prepared with
dology reported in the
a gel. For the adhesive
ness can be controlled
for adhesive single-lap
oating and curing. The
mple introduced in the
s obtaining specimens
hesive layer, respect to
ns were maintained at
ore testing.
asured in mm).
European International Journal of Science and Technology Vol. 4 No. 7 September, 2015
83
2.9 Testing of the adhesive specimens
The adhesive strength of the single-lap shear joints was measured at room temperature in a same universal
testing machine under a 5 kN load cell. A crosshead speed of 3.0 mm/min was employed. The lap shear
strength was expressed in MPa. The adhesion tests were carried out at (22 ± 2) °C and relative humidity of
(50 ± 5) %. The average values were taken from at least eight specimens.
2.10 Water absorption
Test specimens (10.0 ± 0.2) mm long, (15.0 ± 0.2) mm wide and (3.2 ± 0.2) mm thick, were used for water
absorption tests, following the recommendations of ASTM D570 standard of test specimens for sheet
materials. The samples were removed from distillated water at (37.0 ± 0.2) °C, carefully wipped with a filter
paper and weighting on analytical balance to ± 0.01 mg. Three specimens were used per each epoxy
polymer. The mass water absorbed Ct (%) by the specimens was calculated with the following expression
(Equation 1):
���%� = � ���
��� ∗ 100 (1)
where: wt is the mass of the specimens at time t, w0 is the mass of the dry specimen. The average standard
deviation corresponds to the value less than 0.05% on the Ct (%) scale.
3. RESULTS AND DISCUSSION
3.1 Coating and curing
In this work we examined possible surface coating of 316L SS by using inert polymeric with antithrombotic
properties and low cytotoxicity. For this purpose metal plates were used. The plates were submitted to a
conventional mechanical polish method to simulate the surface of the stent and silanization treatment with
APTS to improve adhesive strength between metal substrate and polymeric coating. In previous work
(González Garcia et al., 2009; 2015),the epoxy/amine systems based on diglycidyl ether of bisphenol A
(DGEBA) and diglycidyl ether of glycerol (DGEG) epoxy monomers cured with cycloaliphatic amines
demonstrated that these epoxy thermosetting exhibit better blood compatibility and low cytotoxicity.
Therefore, three cycloaliphatic amines as curing agents were chosen.
After the immersion of the plates in the mixture of monomers, a thin film coating was formed on the
plates surface. At room temperature, the thin film is released from the steel and large drops in the center of
the plate were formed. This effect inhibits films formation based on DGEBA/IPD and DGEBA/3DCM
systems which indicates that the viscosity is an important factor to achieve the formation of a coating on the
surface of the plates. As shown in Table 2 the DGEBA/4MPip system exhibits the low viscosity and contact
angle. After curing schedule only the DGEBA/4MPip system formed a thin coating on the metal plates. The
other two formulations formed droplets which didn’t spread uniformly on the plates surface. Only the
DGEBA/4MPip system formed a thin coating on surface polished and silanizated metal plates.
Also, is possible to ensure the influence of the viscosity and consider the gelation and vitrification
during cure schedule. For the DGEBA/4MPip system, the cure schedule of 30 minutes at 60 °C and 120 °C
for 16 hours causes appreciably decrease on viscosity of the monomers mixture. Under these conditions, the
formation of a thin coating of the formulation on the surface of the metal plates is enhanced occurring during
the gelation and the vitrification processes. For the DGEBA/3DCM and DGEBA/IPD systems in the first
curing stage at 60 °C for 2 hours the viscosity of the monomers is low. Thus it appears that the cure schedule
have no significant contribution on the formation of thin coating layer on the surface of the plates.
European International Journal of Scien
Table 2. Viscosity and contact angle of
Epoxy formulations V
DGEBA/3DCM 0.
DGEBA/IPD 0.
DGEBA/4MPip 0.
3.2 FTIR spectroscopy
The FTIR spectrums of the coated pla
literature for diglycidyl ether of bisphen
(Lee & Neville, 1967). For instance, ab
groups, and bands at 2911, 2865 and
vibration to APTS, respectively. Bands
1508 cm-1
. The bands around 1236 and
bands at 1432, 1380, 1360 and 1294 an
between APTS silanol groups and 316L
vibration mode, respectively while the b
absortion band around 915 cm-1
attribu
suggest that all networks are completely
Fig. 2.FTIR spectra of DGEBA/4MPip (
3.3 DSC Measurements
DSC results of the DGEBA/3DCM netw
These results were obtained from both
considered in the first scan. The same Tg
that the networks are fully cured or th
maximum Tg value. Similar results wer
present in Table 3.
ence and Technology ISSN: 2304-9693
of the epoxy/amine system.
Viscosity (Pa s) Contact angle (°)
0.51 48.6 ± 2.4
0.44 36.6 ± 3.5
0.42 32.1 ± 1.5
plates are shown in Figure 2. Typical absortio
enol A bonded to an amine group in aliphatic co
absortionb and at around 3387 cm-1
is associated
d 2853 cm-1
belong to C–H stretching vibration
ds associated with the aromatic ring appear aro
and 1031 cm-1
is associated with C-O-C ether g
and 1432cm-1
can be assigned to the siloxane lin
6L SS hydroxyl groups (Wang et al., 2007) and
band at 827 cm-1
is assigned to the 1.4 substitut
ibuted to epoxy group vibration mode was evid
ly cross linked and there are no huge differences a
p (
____), DGEBA/IPD (
_ _ _) and DGEBA/3DCM (
_
etwork are presented in Figure 3. The Tg value o
th first and second scans and no residual enthal
g value from both scans and the absence of an ex
the same, we can infer that the three epoxy s
ere obtained for the other system. DSC results o
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ion bands reported in
compound were noted
ed with –OH hydroxyl
ional sp3 and skeleton
round 1605, 1580 and
r group. The absortion
linkage due to reaction
d methyl symmetrical
ution. Furthermore, no
idenced. FTIR spectra
s among them.
_ _ _) networks.
obtained was 125 °C.
alpy (∆HR) have been
exothermic peak prove
systems attaining the
of the all systems are
European International Journal of Scien
Fig. 3.DSC scans for cured system DGE
Table 3. Glass transitions temperatu
networks determined by DSC.
Networks First sc
DGEBA/4MPip 55.5
DGEBA/IPD 100.2
DGEBA/3DCM (30 phr) 125.0
3.4 Optically microscopy
Figure 4 and Table 4 show tree-dimensi
the coating surfaces from the 316L SS
roughness increased from 0.32 µm to 1.
surface coating metal plates. The silaniz
of coating surface was 1.97 µm. The su
smoother than the surface of the s
DGEBA/4MPip (Figure 4c), the rough
behaviors can be related to ATPS covale
groups and 316L SS hydroxyl groups
monomers mixture epoxy groups.
Table 4. Thickness (µm) and roughnes
Metallic surface Po
Thickness (µm)
Roughness (µm )
ence and Technology Vol. 4 No. 7
EBA/3DCM.
ature (Tg) of DGEBA/4MPip, DGEBA/IPD a
scan (T °C) Second scan (T °C)
55.3
100.3
125.1
sional OM images and the roughness of the poli
with DGEBA/4MPip system, respectively. Th
1.75 µm and 1.54 µm, respectively after the poli
izated surface is rougher than the coating surface
surface roughness of the polished 316L SS surf
silanized one (Figure 4b). After coating th
ghness slightly becomes smoother to that the s
lently linked to 316L stainless steel by reaction be
ps and subsequent reaction between APTS am
ess (µm) obtained from optical microscopy.
Polished Polished and
silanized
Co
DGE
- 1.39 1.
0.32 1.75 1.
September, 2015
and DGEBA/3DCM
lished, silanizated and
he root-mean-squared
lished, silanizated and
ace being the thickness
urface (Figure 4a) was
the surface with the
silanized one. These
between APTS silanol
amine groups and the
Coatedwith
EBA/4MPip
1.97
1.54
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86
(a) (b)
(c)
Fig, 4.OM images of (a) polished surfaces SS, (b) after silanizated surface SS, (c) after coating surface SS
with DGEBA/4MPip system.
3.5 Adhesive properties
The adhesive properties were evaluated in terms of single lap shear using 316 SS adherends chemical
treated. The glass transition temperature (Tg) and the adhesive properties values of three epoxy formulations
obtained from single lap shear joints tests is shown in Table 5. The Tg of the epoxy networks also presents
large influence on the adhesive behavior of the adhesive joints.
Table 5. Glass transition temperature (Tg) and adhesives properties of different epoxy adhesives
obtained from single lap shear tests.
Epoxy networks Tg(°C) Adhesive strength in lap shear
joints (MPa)
DGEBA/3DCM 125.0 14.6 ± 0.5
DGEBA/IPD 100.2 15.5 ± 0.4
DGEBA/4MPip 55.5 17.2 ± 0.4
European International Journal of Science and Technology Vol. 4 No. 7 September, 2015
87
The DGEBA/4MPip system has the lower glass transition temperature and as expected exhibits the
best adhesive properties. It is well known that low Tg values are results of lower cross linking density
(Pascault et al., 2002). This explained the adhesive behavior because is know that the lower crosslink
density improve the ability of the adhesive and thus increase the lap shear strength (Colombini et al., 2002;
Hu & Huang, 2005). Also, it is known that the viscosity influencing for adhesive strength of the joints
(Montions et al., 2007). The lower viscosity is an important factor to achieve surface coatings with adhesive
strength. The performance of adhesive properties is related to different structure of the epoxy networks by
changing the chemical structural of curing agent. This comes from the fact that the networks involved are
“closed” networks (Pascault et al., 2002), resulting from a single step polymerization mechanism and also
that stoichiometric ratio of monomers are reacted until attaining the maximum Tg value. In particular, the
DGEBA/4MPip system presents two different curing processes. The first one by step-growth
polymerizations were carried out for epoxy cross linking, and the second one by homo polymerization of
epoxy groups by anions mechanism allowing into generating polyether chains with flexible structure
(González Garcia et al., 2002). In those circumstances, the flexible epoxy network chains exhibit lower
crosslink and smaller value of Tg. From these results, we can infer the relationship between adhesive
behaviors with the chemical structure of the curing agent in the networks structure.
3.6 Water absorption
Solvent transport in organic polymer matrices is usually depicted as a two-step mechanism. The first step is
the solution of the solvent in the superficial polymer layer. This process, which can be considered almost
instantaneous in the case of water, creates a concentration gradient. The second step is the solvent diffusion
in the direction of the concentration gradient. This process may be described by a differential mass balance
(often called Fick’s second law) (Pascault et al. 2002, Apostol, 1974), which, in the one-dimensional case,
may be written as (Equation 2):
∆�
��= �
���
��� (2)
where: D is the diffusion coefficient and x the coordinate along the sample’s (L) thickness. For a membrane
shape sample (Apostol, 1974) the resolution of this differential equation gives (Equation 3):
�
�∞= 1 − ∑
�
������� ��∞
��� exp [�# ����/������
%�)] (3)
At short times, typically when C ≤ 0.5 C∞, this function can be well approximated by (Equation 4):
�
�∞= 1 −
�
��&
�'()�
*+� � (4)
where: Ct is the mass of water absorbed at time t, C∞ is the amount of water absorbed at saturation, L is the
thickness of the freestanding specimen and D is the diffusion coefficient.
It is thus usual to plot ln (1- Ct/C∞) vs. ln t. The linearity of the curve in its initial part can be
considered as validity criteria for the Fick’s law. The slope of the linear allows us to determine the diffusion
coefficient. To obtain the saturation value (C∞) and diffusion time (tD) it usual to plot weight gain vs. time.
In this plot the diffusion time, tD defined as the duration of the transient, can be arbitrary taken at the
intersection between the tangent at the origin and the asymptote and the saturation value (C∞) as considered
European International Journal of Science and Technology ISSN: 2304-9693 www.eijst.org.uk
88
as the maximum value of the weight gain. This point corresponds to the value where the absortion reaches a
stable value.
Figure 5 shows the experimental data for the weight gain vs. time curves. All epoxy polymers show
an almost linear relationship between the weight gain and the immersion time at the beginning of the
absorption process. This behavior is well described by Equation 4, showing that the initial stage of water
absorption behavior is governed by the Fick’s law. Therefore, the water concentration gradient is the driving
force that leads to water absorption in these epoxy polymers.
Fig. 5.The gain weight vs. time curve showing the water sorption behavior of the different epoxy adhesives.
The obtained values for C∞, D and tD are listed in Table 6. It can be highlighted that the diffusion
coefficient obtained is consistent with the values cited for other epoxy polymers (Pascault et al., 2002; Berry
et al., 2007; Maggana et al., 1999; Moy &Karasz, 1980). The diffusion coefficient, saturation value and
diffusion time for each epoxy network changed depend on the curing agent employed. The diffusion
coefficient (D) and saturation (C∞) values of the DGEBA/4MPip network is always lower than that of the
others epoxy networks. However, the diffusion time (tD) value is the largest. On the other hand, the
DGEBA/3DCM and DGEBA/IPD networks show the same diffusion coefficient (C), water saturation (C∞)
and diffusion time (tD) values. However, the structure-diffusivity relationships are not clearly established. It
is known that the curing agent affects the water absortion (Abdelkader&While, 2005). But reports based on
the diffusion kinetic suggest that in aliphatic diepoxide cured by aromatic diamine, the diffusion rate of
water is controlled by the strength of the polymer-water hydrogen bond (Tcharkhchi et al., 2000). In this
way, the DGEBA/4MPip network has a little hydrogen bond concentration (González Garcia et al., 2011)
because this network is formed by steps-growth polymerization and homopolymerization by anionic
mechanics as mentioned before. This structural characteristic makes difficult the diffusion rate of water. For
the DGEBA/3DCM and DGEBA/IPD polymers the hydrogen bond concentration are similar, but the first
network present more rigid structure leading to a relative lower diffusion coefficient (C), water saturation
(C∞) and time of diffusion (tD) values. This make the diffusion rate of the water in the DGEBA/3DCM
network more complicated.
European International Journal of Science and Technology Vol. 4 No. 7 September, 2015
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Table 6. Diffusion coefficient (D, mm2/s), saturation value (C∞, %) and diffusion time tD, days)
obtained from experimental weight gain vs. time curve of DGEBA/3DCM, DGEBA/IPD and
DGEBA/4MPip systems.
Epoxy networks DGEBA/3DCM DGEBA/IPD DGEBA/4MPip
C∞ (%) 1.497 1.522 1.227
D (mm2/s) 1.17 x 10-6
9.43 x 10-6
9.55 x 10-7
tD(days) 57.56 54.12 61.04
4. CONCLUSIONS
This study demonstrated that the cure schedule of the epoxy systems achieve high conversion. Only
DGEBA/4MPip system formed a thin coating on surface polished and silanizated of 316L stainless steel.
This epoxy system show slow viscosity and contact angle that could be applied to stents with complicated
shapes. In addition, this system shows better adhesive properties and exhibit low water absortion.
Accordingly, we believe that DGEBA/4MPip system can be potentially employed for coating in bare
metallic coronary stents.
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