Abstract Cathodic electrodeposition (CED) behavior and film
properties of coating binders as modified polyepoxide resins based on
cardanol, a constituent of Cashew Nut Shell Liquid (CNSL), have been
studied. These coating binders were synthesized by cpoxidation of
cardanol-formaldehyde novolacs (CN.sub.s) with cpi-chlorohydrin and
subsequently modified with secondary amine to make them water-thinnable
by neutralization with acetic acid, and suiTable for CED. Laboratory
synthesized coating binders, modified, epoxidized cardanol-formaldehyde
novolacs (MECNs), were cathodically electrodeposited on pretrealed MS
panels to investigate their electrodeposition behavior and film
properties of deposited coating binders. Among the nine MECNs prepared
by varying molar ratios of cardanol to formaldehyde as well as ECN to
DEtOA, only seven MECNs([MECN.sub.1], [MECN.sub.2], [MECN.sub.3],
[MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9]) were
water-soluble and electrodcpositable, whereas [MECN.sub.4] and
[MECN.sub.5] were not water soluble, and hence not considered for
further studies. These seven MECNs were used for the study of their
electrodeposition characteristics, such as electrodeposition yield (ED
yield), coulombic yield (CY), dry film thickness (DFT), and properties
of their deposited films. The most suiTable molar ratio of ECN:DEtOA for
the preparation of MECNs was found to be 1:1. The CN prepared by using
cardanol and formaldehyde in the molar ratio of 1:0.7 was used for the
preparation of [ECN.sub.2] and [MECN.sub.2] prepared from this was found
to be the most suiTable in terms of deposition behavior and overall film
properties.The film of [MECN.sub.2] electrodeposited at constant voltage
(100 V) was found to be the best in terms of film properties along with
corrosion resistance, as it passed 800-h exposure to salt spray
atmosphere. [MECN.sub.2] was optimized for its electrodeposition
characteristics like ED yield as 3.62 mg/[cm.sup.2], CY as 35.87 mg/C,
and DFT as 25.26 [micro]m. Through a wide cure window, the films of
[MECN.sub.2] were found to be self-curable at an optimum cure schedule
of 160[degrees]C/30 min. The eleclrodeposited films of MECNs had good
physical, chemical, and corrosion resistance properties, but
demonstrated low resistance to xylene, in particular. The study
emphasized the electrodeposition behavior and film properties of the
prepared MECN resins as binders for CED coating formulations, which were
self-curable without using any external crosslinkcr. The self-curing of
the deposited films was achieved via a novel self-curing mechanism,
i.e., one molecule chemistry through anionic polymerization. The
prepared epoxide resins as MECNs could be cathodically eleclrodeposited
as primer coat for the protection of metallic substrates against
corrosion.
Keywords Cathodic electrodeposition, Electrodeposition yield,
Coulombic yield, Dry film thickness, Modified epoxidized
cardanol-formaldehyde novolacs
Introduction
Cathodic electrodeposition (CED) is the most important technique
used presently in the automotive industry for priming vehicle bodies. It
is also applied for domestic appliances, agricultural
equipments/implements, metal toys, jewelry, nuts, bolts, etc. (1) and
has had tremendous economic impact, especially in the area of corrosion
protection. CED permits the application of protective coatings on such
areas of a workpiece, which are not accessible to spray application. (2)
CED also offers high resistance of films to alkali, excellent adhesion
of electrodeposited films to metal substrates and, as a consequence of
these, enhanced corrosion resistance. CED is free of the major drawbacks
of anodic electrodeposition ((AED).sub.1) such as excessive substrate
dissolution and film discoloration.(3-5)
The main resin compositions (2), (6-9) used today in CED coatings
are modified epoxy resins and blocked polyisocyanates like TDI, HMDI,
IPDI, etc. However, the use of polyisocyanate inevitably generates
blocking agent at elevated temperatures. The weight loss caused by
deblocking and subsequent evaporation of the blocking agent from the
coating systems usually amounts to 10-15 wt% based on the cured
coatings, which is not negligible either from the point of view of
material cost or the air pollution. (10) The use of polyisocyanates as
crosslinkers, in a co-reacted form in existing systems also poses a risk
of gelation during the CED resin synthesis. Now, with increasing concern
for environmental protection and due to stringent regulations to ensure
the same, it has become a must for industry today to find eco-friendly
substitutes for all objectionable materials like phenols and volatile
organic compounds (VOC) in organic coatings.
However, the development of low-cost, self-curable coatings and
coatings curable at reduced temperatures still remains an important
challenge in the field of organic CED coatings. To this end, the
development of eco-friendly coating compositions for CED, cardanol,
obtained from Cashew Nut Shell Liquid (CNSL) as an eco-friendly
substitute for phenols, was used for the preparation of novolac resins.
(11-14) Cardanol is a natural alkyl phenol present in CNSL which has a
meta-substituted (C.sub.15) unsaturated chain containing 1, 2, or 3
double bonds at the 8, 11, or 14 positions, (15) and it is a potential
source for biomonomers.
CNSL having cardanol as a natural phenol is considered to be less
toxic and more eco-friendly as compared with ordinary phenol. (16), (17)
The lower cost of cardanol (US$ 333/MT) (18) when compared with that of
ordinary phenol (19) (US$ 2,154-2,177/MT) turns out to be an added
advantage for the commercial use of CNSL.
In this research study, an attempt has been made to evaluate CED
behavior of the prepared coating binders (modified epoxidized
cardanol-formaldehyde novolacs, MECNs) in terms of electrodeposition
characteristics, such as electrodeposition yield (ED yield), coulombic
yield (CY), and dry film thickness (DFT). Finally, the electrodeposited
films, after curing, were evaluated for their physical properties,
chemical resistance and corrosion protection performance.
Experimental
Materials
The materials used in the experimental study include laboratory
synthesized cationic binders as MECNs from cardanol. Reagents such as
acetic acid, sodium hydroxide, sulfuric acid, methyl ethyl ketone,
xylene, sodium chloride, methanol (Qualigens Fine Chemicals, Mumbai
India) of L. R. grade and mineral turpentine oil (MTO) of commercial
grade were used for evaluation of chemical resistance properties of
cured films. Laboratory prepared double-distilled water was used
throughout the experimental study.
Evaluation of film properties
The following instruments/equipments were used for the evaluation
of properties of the cured films of the electrodeposited resins:
Scratch hardness tester (Sheen, UK)
Pencil hardness tester (Mitsubishi Uni., Japan)
Flexibility tester (Conical Mandrel) (Sheen, UK)
DuPont impact tester (Ueshima Scisakusho Co. Ltd., Japan)
Salt spray cabinet (Sheen, UK)
DFT meter (Elcometer, England)
Triglossometer (Elcometer, England)
General mechanisms of cathodic electrodeposition
The fundamental aspects of the electrodeposition process and resin
compositions have been reported by many authors in the past. (20-23)
As CED is mainly employed where corrosion protection is the main
concern, and as the epoxies, by virtue of their chemical constitution,
exhibit excellent corrosion resistance properties, it was, therefore,
planned to combine both epoxide resins and CED technique to have a
synergistic performance.
Thus, in this study, it was decided to develop a true self-curable,
waterborne epoxy based system, which could be used for developing
coating formulations suiTable for CED of films.
Chemical reactions involved in the preparation of water-soluble CED
binders based on prepared epoxy resin are shown in Scheme 1.
Experimental design: cathodic electrodeposition of binders
The synthesis part of CED coating binders used for this study can
be accessed through our forthcoming publication. However, a brief
outline of the synthesis is described here.
CED binders were synthesized in the laboratory in a sequence of
steps, as, first of all, cardanol-formaldehyde novolac resins (CNs) were
prepared by reacting cardanol with formaldehyde in different molar
ratios. The prepared CNs were epoxidized by reaction with
epichlo-rohydrin to produce epoxidized cardanol-formaldehyde novolacs
(ECNs). Further, ECNs were modified by reaction with secondary amine as
diethanolamine (DEtOA) to produce ECN-DEtO A adducts having tertiary
amino group(s) into their molecules to make them water-soluble and
suiTable for CED, on neutralization with acetic acid. True
self-curability of the coaling films has been achieved via anionic
polymerization. Characteristics of the prepared CED coating binders as
MECNs arc presented in Table 1.
[ILLUSTRATION OMITTED]
Preparation of test panels
Pretreated mild steel panels (150 mm x 50 mm x 0.25 mm) were used
as test panels. They were prepared as per the standard procedure
described in ASTM D 609. (24) They were prepared by following a sequence
of steps, viz. mechanical cleaning (by 400# abrasive paper), degreasing,
rinsing, pickling with a hydrochloric acid solution at 1:1 dilution (30
s), rinsing with distilled water, phosphating as zinc phosphate based
conversion coating at 60[degrees]C for 3 min, followed by chromate
passivation and finally rinsing.
A phosphating chemical composition (having zinc phosphate,
phosphoric acid, nickel nitrate, etc.) with the brand name of Nipaphos
3020M, a proprietary composition of M/S Nipa Chemicals Ltd., Chennai,
India, was dissolved in DI water to make an aqueous solution of 20 g/L
of zinc phosphate concentration, and was used for the purpose of
phosphate treatment of mild steel panels.
Preparation of electrolytic composition
The required amount of adduct was weighed in a 500 mL glass beaker
with a glass rod and Teflon coated magnetic bar. Approximately 100 mL of
double distilled water was added to the paste. The adduct present in the
paste was neutralized by adding drop wise from a burette the aqueous
solution of acetic acid (0.5N) under thorough agitation. The pH of the
solution was monitored continuously during neutralization. The
concentration of this solution was adjusted by the addition of double
distilled water.
Cathodic electrodeposition of coating binders derived from cardanol
as MECNs
A glass beaker (500 mL capacity) was filled with 350 mL of the
electrolytic composition (CED coating liquid). A weighed metallic panel
(the cathode) and a stainless steel anode of the same size were immersed
up to a depth of 50 mm into the solution. After giving electrical
connections to the electrodes, the required time of deposition was set
in the timer of ED rectifier (Allied Industries, Mumbai). The voltage
was then increased to the required value in 5-10 s (the ramp time) with
the help of the voltage regulator of the rectifier. The "peak
current" and the "drop in current with time" were noted
with the progress of film deposition at constant voltage. An ampere
meter connected in series was used for this purpose. The voltage was
applied for a preset time for deposition. The coulombs were read on
coulometer of the rectifier, and recorded. The electrodeposition was
performed at five constant voltages as 50, 100, 150, 200, and 250 V.
The panel with freshly deposited film was removed from the
electrodeposition bath (the electrolytic com-position) and rinsed under
a mild current of running tap water, followed by a distilled water
rinse.
The panel having the electrodeposited film (wet and uncured) was
then hung inside the oven maintained at the predetermined temperature
for the desired time, to effect film cure. The panel with the dried and
cured film was taken from the oven, cooled to room temperature in a
desiccators, and reweighed to determine the difference in the weight of
the cathodically electrode-posited film (dried/cured). The area of the
deposited film was measured on both sides of the panel. Using values of
the weight of deposited film, area of the deposited film and the
coulombs passed; the values of electrodeposited yield (in mg/[cm.sup.2])
and CY (in mg/C) were calculated.
Results and discussion
Cure characteristics of deposited coating binders as MECNs
To determine the suiTable molar ratio of DEtOA to ECN for the
preparation of ECN-DEtOA adducts (MECNs), the effect of molar ratio of
each ECN to DEtOA on its cure characteristics was studied. Each ECN,
viz., [ECN.sub.1], [ECN.sub.2], [ECN.sub.3], and [ECN.sub.4] was reacted
(modified) with nine different molar ratios of DEtOA as shown in the
fifth column of Table 1. The molar ratio of DEtOA per mole of ECN was
varied from 1 to 2.5 mol in view of the maximum number of epoxy groups
available in the ECN molecule (epoxy functionality of ECN), i.e., 2.04,
2.20, 2.92 and 2.12 for [ECN.sub.1], [ECN.sub.2], [ECN.sub.3], and
[ECN.sub.4], respectively. The curability was checked by baking the film
at a fixed temperature (range 100-180[degrees]C) and fixed time (range
30 or 60 min). Resins, not curing at the highest temperature during 60
min, were considered incurable. Cure schedules of 100-180[degrees]C by
20[degrees]C increments and 30-min duration were used as shown in Table
2. No cure for any film was observed at or below 120[degrees]C while
[MECN.sub.2] and [MECN.sub.8] cured slightly at 140[degrees]C and fully
cured at 160[degrees]C. MECN, and [MECN.sub.6] required further higher
temperature (180[degrees]C) for their complete curing. The curing
temperature of MECNs seems to be dependent on their epoxy functionality
with at least one cationic amino group present in their molecule. MECNs
having higher epoxy functionality have been observed to cure at lower
temperatures.
Optimization of the electrodeposition parameters
The prepared modified epoxidized cardanol-formaldehyde novolacs
(epoxy-amine adducts), viz., [MECN.sub.1], [MECN.sub.2], [MECN.sub.3],
[MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9], were
neutralized with acetic acid and dissolved in distilled water to result
in the solution to be used for electrodeposition experiments. The
electrodeposition, using the solutions having optimized bath parameters
such as concentration (10%, w/w) and pH (5.5), was carried out at room
temperature at applied voltages of 50, 100, 150, 200, and 250 V for 60 s
as deposition time. In fact, deposition time was varied up to 90 s, and
it was found that almost all adducts attained the highest DFT in 60 s.
The electrodeposition characteristics such as ED yield, CY, and DFT of
the deposited and cured film of adducts were determined/ measured, and
the values are given in Table 3.
Current density
The values of current densities (i, expressed in mA/[cm.sup.2]), at
the optimized voltages of 50, 100, 200, 250, 250, 100, and 200 V for
adducts, such as [MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6],
[MECN.sub.7], [MECN.sub.8], and [MECN.sub.9], respectively, vs times of
deposition have been plotted in Fig. 1.
[FIGURE 1 OMITTED]
The graph shows that the current density, having a high initial
(peak) value, dropped very rapidly during the initial period of
deposition, as is evident by a steep curve corresponding to the
deposition time of less than 30-40 s, and it attained its lowest
(constant) value in 60 s, as shown in Fig. 1.
Electrodeposition yield
The voltage applied across the electrolytic cell (electrodeposition
bath) was varied from 50 to 250 V. The values of ED yield, CY and DFT
for chosen adducts at different voltages are tabulated in Table 3. It
can be seen from the Table that the ED yield of adduct [MECN.sub.1]at 50
V was the lowest (1.34 mg/[cm.sup.2]) which increased to 4.06
mg/[cm.sup.2] at 200 V. On further increasing the applied voltage, the
ED yield did not increase. Similarly, there was no significant increase
in DFT with an increase in applied voltage after 200 V. The maximum ED
yield and DFT that could be obtained by varying the applied voltage for
[MECN.sub.1] were 4.06 mg/[cm.sup.2] and 25.26 [micro]m, respectively at
200 V. At an applied voltage of 250 V, the deposited film was found to
be rough due to film rupture, as was observed by sudden upsurge of
current. Thus, the voltage of 50 V was considered to be a suiTable
applied voltage for the CED of adduct [MECN.sub.1] to produce smooth
films with suiTable ED yield. On the same lines, the suiTable applied
voltages for the optimized deposition of adducts as [MECN.sub.2],
[MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9]
were found to be 100, 200, 250, 250, 100, and 200 V, respectively, on
the basis of outstanding film properties and smoothness.
The number of cationic amino groups and the hydrophobic content of
the molecule are the key factors and go hand-in-hand to govern the
micelle size. It has been mentioned by several researchers (25), (26)
that the molecule having net higher hydrophobicity would aggregate to a
larger extent to form larger micelles. In this context, net
hydrophobicity refers to the degree of the overall dominance of
hydrophobicity over hydrophilicity.
In our system, which is unique in itself, hydrophilicily and
hydrophobicity would be caused by number of cationic amino groups and
the number of long C (15) (hydrocarbon) chain, respectively, that are
present on the molecules. The overall dominance of hydrophobicity would
result in larger micelle size. The number of hydrophobic C (15) chains
per molecule has been calculated for all MECNs, as tabulated in Table 4.
Based on the above data, it is clear that the increasing micelle
size will observe the following trend:
[ILLUSTRATION OMITTED]
Our experimental results also confirm that the MECN in group (I),
having smallest micelles, would deposit suitably at the lowest voltage
(50 V). Accordingly, MECNs in group (II) would deposit at 100 V, group
(III) at 200 V and group (IV) at 250 V.
The ED yield and DFT for MECNs were determined/measured, and they
were found to behave in the same manner, as shown in Table 3. Identical
trends of these two ED characteristics confirm that the two terms, viz.,
ED yield and DFT, used to indicate the film-build, manifest the same
feature. However, measurement of both the characteristics is useful, as
ED yield can be conveniently measured quickly and very accurately in the
laboratory, by weighing the panels before deposition and after curing
the deposited film on the panels. On the other hand, the film-build on
large articles, as encountered in commercial productions, is measured by
DFT only.
Figure 2 shows that, for all MECN resins, the average ED yield is
dependent on molecular weight. The lowest molecular weight resin,
[MECN.sub.1], had an ED yield of only 2.32 mg/[cm.sup.2], whereas the
higher molecular weight resins [MECN.sub.6] and [MECN.sub.7] had ED
yields of 4.92 and 4.84 mg/[cm.sup.2], respectively. Molecular weight
and the number of amino groups are key factor in controlling film-build.
[FIGURE 2 OMITTED]
Coulombic yield
Coulombic yield (CY) of all adducts was found to be independent of
the time of deposition and remained constant for all adducts, whereas
the ED yield increased steadily with time of deposition up to 60 s. From
Table 3, it can be observed that the value of CY is almost constant in
the range of 25.85-26.70 mg/C for [MECN.sub.1], 35.85-36.65 mg/C for
[MECN.sub.2], 34.10-34.75 mg/C for [MECN.sub.3], 37.56-38.43 mg/C for
[MECN.sub.6],39.50-40.85 mg/C for [MECN.sub.7],35.31-36.73 mg/C for
[MECN.sub.8] and 33.31-34.87 mg/C for [MECN.sub.9] with the increase of
applied voltage from 50 to 250 V. Independence of columbic yield on
deposition time can be explained well by Faraday's laws of
electrolysis, based on the fact that the mathematical form of laws have
built-in time term in the expression, as the coulomb is current
multiplied by time. As a matter of fact, CY of a material (adduct) is
nothing but its electrochemical equivalent as per Faraday's laws of
electrolysis.
However, the higher values of CY for all adducts, obtained
experimentally, as compared to that of the theoretical values, i.e.,
calculated by Faraday's laws, can be attributed to a number of
facts such as the self-polymerization of adduct during preparation,
micelle formation of neutralized adduct molecules in the electrolytic
solution and partial neutralization of the adducts by acetic acid.
Acetic acid, being a weak acid ([pK.sub.a] = 4.76) might have not been
able to react fully with tertiary amino groups of the adduct due to
steric hindrance. As a consequence, some of the tertiary amino groups
would have remained shielded and un-neutralized, to increase the value
of electrochemical equivalent of the neutralized adducts, i.e., micelle
formation in the electrodeposition bath. Secondly, as a consequence of
micelle formation following physical and/or chemical aggregation, the
adductcations as individual ions might not have migrated to deposit,
rather aggregates/clusters of adductcations could have been dragged as a
single cation and deposited on the cathode.
Dry film thickness
The thickness of the dried/cured films (called Dry Film Thickness,
DFT) was measured at a number of equidistant points on the deposited
film using a digital electromagnetic induction film thickness gauge. The
readings (4 or 5 in number) were used to calculate the average film
thickness. DFTs of the films have been given in Table 3.
The average rates of film growth of adducts [MECN.sub.1],
[MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8],
and [MECN.sub.9], were calculated and found to have fallen in the range
of 0.33-0.44, 0.41-0.46, 0.40-0.48, 0.36-0.50, 0.41-0.65, 0.40-0.48, and
0.54-0.56 um/s, respectively. In fact, the film deposition was rapid in
the beginning and slowed down with the progress of deposition. The film
growth almost stopped beyond 60 s of deposition time because of the
near-complete insulation of the substrate by the deposited film. Thus,
the optimum time for deposition was considered to be 60 s.
Kinetics of film growth during cathodic electrodeposition
To investigate the kinetics of growth of film during CED, resin
adducts as MECNs prepared from ECN and DEtOA at 90[degrees]C for 3 h of
reaction time were used. Pierce (27) proposed a model for the kinetics
of film growth which relates DFT (denoted by letter ([delta]) with time
of deposition (t)). The mathematical expression for the relation of
([delta]) with (t) can be given as
[delta] = [[2c[[sigma].sub.F]V].sup.1/2][t.sup.1/2]
where c = coulombic efficiency,[[sigma].sup.F] = specific
conductivity of the deposited film, and V = applied voltage.
From the equation, it can be appreciated that the slope of the
straight line portion of the curve obtained by plotting [sigma] vs
[t.sup.1/2] (Figs. 3-9) should be equal to
[(2c[[sigma].sub.F]V).sup.1/2] which can give the value of coulombic
efficiency, c (in [cm.sup.3]/C), if the specific conductivity,
[[sigma].sub.F] (in [mu]S/cm) is known. From Figs. 3-9, slopes of the
straight line plots of [MECN.sub.1], [MECN.sub.2], [MECN.sub.3],
[MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9] have been
calculated to be 2.6, 3.19, 3.83, 3.96, 4.78, 3.01 and 4.47
[micro]m/s1/2 at 50, 100, 200, 250, 250, 100, and 200 V, respectively.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Specific conductivity of the deposited film Ideally, the
conductivity of the electrodeposited film of adduct should be zero,
i.e., the film should be totally nonconductive, as it is composed of
electrically non-conductive organic material (cardanol-derived
epoxy-amine adduct). In practical situations, however, a freshly
deposited film has pores filled with the same electrolyte from which the
film was being deposited. As a result, current conduction can take place
through these pores. In general, the conductivity of the film is
regarded as the conductivity of electrolyte filled in pores of the film.
(26) If we consider these pores as conductors of electricity, then
Ohm's law should hold true with respect to resistance or
conductance.
In equation (2), [delta] is the pore depth which would obviously be
equal to the wet film thickness. According to equation (2), the current
density (i) and the field strength (V/[delta]) across the deposited
films should be linearly proportional:
The values of specific conductivities ([sigma].sub.F) of the films
have been calculated with the help of equation (2) using the values of
current density (i) and field strength (V/[delta]), and these have been
found to be 0.0313, 0.0179, 0.0185, 0.0219, 0.0252, 0.0257, and 0.0193
[micro]S/cm for [MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6],
[MECN.sub.7], [MECN.sub.8], and [MECN.sub.9], respectively. By using
these values of [[signa].sub.F] and the values of slope of ([delta] vs
[t.sub1/2] curves, in accordance with equation (1) and from Figs. 3-9,
the values of coulombic efficiency have been calculated and found to be
0.0215, 0.0284, 0.0198, 0.0142, 0.0181, 0.0175, and 0.0258 [cm.sup.3]/C,
respectively, for the above-mentioned MECNs.
Measurement of gloss
Gloss of cured films of all the MECNs was measured at a number of
equidistant points on the deposited film using a digital triglossometer
(Elcometer) at 60[degree] as per standard test method ASTM D
[523-99.sub.28]The reported values are the averages of four or five
readings to determine the accurate value of gloss. The values of gloss
of the deposited films of all the MECNs are given in Table 5.
The gloss generally increased with the increase in voltage from 50
to 250 V because thicker films were deposited on the substrate at higher
voltages. It is also observed that lower molecular weight resin has
higher gloss value as compared to the resins having higher molecular
weight.
Evaluation of film properties
Based on the curability of MECNs, the applied films of MECNs, viz.,
[MECN.sub.1], [MECN.sub.2], [MECN.sub.6], and [MECN.sub.8] were selected
to be used in this part of the study. These films were evaluated for
their physical and chemical resistance properties after proper curing as
per their selected cure schedules as mentioned in Table 2, and maturing
the cured films for 4 days at room temperature. The films were applied
on pretreated mild steel panels for evaluation of physical properties,
while glass panels were used for determining the chemical resistance
properties.
Physical properties
The films were evaluated for physical properties such as adhesion,
(29) flexibility, (30) hardness, (31), (32) and impact resistance. (33)
The results on physical properties of the films of different MECNs have
been tabulated in Table 6.
Adhesion of films to mild steel panel was measured by cross-hatch
tape test in which all the MECNs showed 100% adhesion, i.e., no square
was lifted. It probably is due to the presence of a large number of
hydroxy1 groups per mole of MECN.
In the measurement of flexibility, films of all MECNs passed 1/4
in. mandrel bend test, but films of [MECN.sub.6] and [MECN.sub.8] failed
on 1/8 in. Based on this qualitative measurement, it can be said that
the films had reasonably good flexibility due to the presence of
[C.sub.15] side chain in cardanol. Films of [MECN.sub.6] and
[MECN.sub.8] were found to be slightly inferior, probably, due to their
higher initial molecular weight than that of [MECN.sub.1] and
[MECN.sub.2].
Hardness of the films of MECNs can also be said to be good, as
indicated by scratch hardness values of 1200, 1500, 1400, and 1800 g for
[MECN.sub.1], [MECN.sub.2], [MECN.sub.6], and [MECN.sub.8],
respectively, and pencil hardness as 2H-3H. This probably can be
attributed to the higher degree of polymerization effected by the
presence of a number of epoxy groups, acting synergistically to the
higher degree of hydrogen bonding resulting from the considerable amount
of hydroxy1 groups present in the MECNs molecules.
Impact resistance of a film is generally governed jointly by two
factors, viz., adhesion to the substrate and the film flexibility. The
failure of a film against impact (rapid deformation) is determined by
the net effect of the two, i.e., the predominance of one factor over the
other. The impact resistances of the films of [MECN.sub.1] and
[MECN.sub.2]were found to be excellent, as the films of these resins
passed impact tests of 50 and 45 cm, respectively, by 1000 g of weight.
The rest of the MECNs have lower impact resistances of 25-35 cm,
probably due to the higher film hardness. Thus, some of the prepared
MECNs produced films with optimum combinations of adhesion and
flexibility.
Chemical resistance properties
The films of MECNs were evaluated for their chemical resistance
properties (34) by immersing the coated films in a variety of chemicals
such as acid, alkali, salt, organic solvents, and distilled water.
Conditions of the films were observed after every hour in the beginning,
and then after 4, 8, 12, and 24 h, and subsequently after 2 days, 4
days, 1 week, 2 weeks, 3 weeks, 4 weeks, and finally 1 month, 2 months,
3 months, etc. Chemical resistances of the films in terms of their
conditions after 1-week and 3-month exposures are given in Tables 7 and
8, respectively.
The alpha-numerical codes have been assigned to the condition of
the exposed films, for brevity in presentation, as given below:
NE = No Effect; FD - Fully Dissolved; SG = Slight Loss in Gloss; S
= Swelling of film, ranging from 1S to 10S, where 1S is minimum
swelling, i.e., maximum resistance of films to the chemical, and 10S is
maximum swelling, i.e., minimum resistance to the chemical; L = Degree
of Film Lift Off, ranging from 1L to 10L, where 1L is minimum film
lifting, i.e., maximum resistance to the chemical, and 10L is maximum
film lifting, i.e., the lowest resistance to the chemical; D = Degree of
Dissolution of Film, ranging from 1D to 10D, where 1D is minimum film
dissolution, i.e., maximum resistance to the chemical, and 10D is
maximum film dissolution, i.e., the lowest resistance to the chemical.
Resistance to acid ([H.sub.2][SO.sub.4]) solution
The films of all the four resins, viz., [MECN.sub.1], [MECN.sub.2],
[MECN.sub.6], and [MECN.sub.8] immersed into acid solution (IN sulfuric
acid), remained unaffected for 1 week, but after 3 month's
immersion of the films of these MECNs, films became swollen to varying
degrees, and lost gloss. This behavior of the films can be because the
films were composed of alkaline resin (a tertiary amine), and hence the
films were attacked by the acid solution significantly.
Resistance to alkali (NaOH) solution
The films, due to their alkaline nature, were not affected
(attacked) by the alkali solution (1N sodium hydroxide) in a week of
film immersion. However, varying degrees of swelling (in the range of
2S-4S) of the film were observed for films of resin, viz., [MECN.sub.1],
[MECN.sub.2], [MECN.sub.6], and [MECN.sub.8] after 3 months of film
immersion. The films of these MECNs were lifted slightly from the dipped
edges of the panel after 3 months of film immersion. When the films were
removed from the panels (as free films) after 3 months of immersion, the
removed (free) films had good integrity and enough strength. This
indicates that the alkali could not affect the film-forming material
but, of course, it attacked at the film-substrate interface to detach
the film from the panel. The film of resin [MECN.sub.2] had the least
effect due to its good adhesion.
Resistance to salt (NaCl) solution
The salt solution (5% w/w, sodium chloride) did not cause any
damage to the films of all the resins in 1 week of immersion. However,
after 3 months of exposure, the films of MECNs observed loss in gloss,
swelling and film lift-off.
Resistance to solvents
Solvent resistance of the cured films of MECNs was checked in
mineral turpentine oil (MTO), xylene, methyl ethyl ketone (MEK), and
methanol as solvents. The effects of these solvents in 1 week and 3
months of immersion of the films are shown in Tables 6 and 7,
respectively.
From the results, it can be inferred that the films had dominating
nonpolarity due to the presence of C15 (hydrocarbon, nonpolar) chains in
the resin molecule. Accordingly, xylene as an aromatic hydrocarbon had
the most significant effect on the film and methanol (highly polar)
exhibited the least effect. The effect of MEK, modatory polar, had an
intermediate degree of effect.
Apart from the nonpolarity introduced by the C15 chain, the degree
of polymerization resulted in during film curing, which could be the
result of the number of cationic amino groups as well as the number of
unreacted epoxide groups in the resin molecule before curing.
Resistance to water
Films of all the resins designated as MECNs were totally unaffected
even after 3 months of immersion period in distilled water. This
indicates that all MECNs have excellent water (immersion) resistance at
ambient temperature, due to much higher polarity of water as compared to
that of the resin films, leading to a high order of noncompatibility of
the two.
Corrosion resistance: salt spray test
The electrodeposited films were exposed for 800 h in the standard
salt spray atmosphere; exposed films were assessed by the corrosion
creepage (in mm) across the scribe, number (density), and size of
blisters developed. The blisters were assessed as per ASTM B 117-94.
(35)
Table 9 show that, overall, the cathodically electrodeposited films
of all adducts exhibited excellent corrosion resistance properties, as
the corrosion creep-age across the scribe was very low, between 1.0 and
2.2 mm, along with small blister size (2-6 only) for all the adduct
films even after 800 h of continuous exposure to saline (salt spray)
atmosphere.
Conclusions
Based on this study, the following conclusions can be drawn:
(1.) Cardanol has been successfully incorporated within the resin
molecule synthesized for CED, and hence these resin systems are cost
effective, nontoxic and eco-friendly.
(2.) Cathodically electrodeposited films of the developed
polyepoxide resins (MECNs) are self-curable without use of any external
crosslinker. Nonisocyanate chemistry of curing of such polyepoxide
resins has been investigated via anionic polymerization mechanism.
(3.) Films of developed MECNs are cathodically electrodeposiTable
even at a lower voltage of 100 V, achieving good ED characteristics in
terms of ED yield, DFT, and CY, which were found to be 3.62
mg/[cm.sup.2], 25.26 pm, and 35.87 mg/C, respectively.
(4.) ED yield increased with the increase in the molecular weight
and number of tertiary amino group(s), but decreased with increase in
epoxy functionality of MECNs. The maximum ED yield of lowest molecular
weight resin ([MECN.sub.1]) was found as 4.06 mg/[cm.sup.2] and the
highest molecular weight resin ([MECN.sub.7]) had the ED yield as 9.75
mg/[cm.sup.2].
(5.) It was found through the studies on cure schedule that in
general higher molecular weight MECNs with higher epoxy functionality
and lower number of tertiary amino group(s) were cured at lower
temperature, and vice versa.
(6.) Based on the kinetic studies, specific conductivities
([[sigma].sub.F]) of the deposited films of MECNs have been calculated
and found to be 0.0313, 0.0179, 0.0185, 0.0219, 0.0252, 0.0257, and
0.0193 [micro]S/cm for [MECN.sub.1], [MECN.sub.2], [MECN.sub.3],
[MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9],
respectively.
(7.) With the increase in molecular weight of resins, the physical
properties of the films varied significantly in terms of hardness,
flexibility, and impact resistance. In fact, hardness increased with the
increase in molecular weight and epoxy functionality (decreased in
tertiary amino groups), whereas flexibility and impact resistance
decreased.
(8.) In general, higher molecular weight resins with higher
crosslink density due to more epoxy functionality demonstrated belter
resistivity against chemicals. Conceptually, e.g., [MECN.sub.6] having
higher molecular weight and less epoxy functionality (0.93) was affected
more than [MECN.sub.2] having higher epoxy functionality (1.21).
(9.) The corrosion resistance performance of electrodeposited films
of MECNs was found to be satisfactory, as all films demonstrated good
salt spray resistance (800 h).
In view of the self-curability, low-cost, and nontoxicity of MECNs,
as well as their good ED characteristics and film properties, it can be
expected that the developed resin systems through this investigation
would gain wide commercial acceptance.
Acknowledgments The authors wish to express their gratitude to the
All India Council for Technical Education (AICTE), New Delhi (India) for
the financial assistance provided for running the project entitled
"Development of eco-friendly and energy-efficient polymeric
coalings for cathodic electrodeposition" under its TAPTEC scheme
(AICTE File No. 8021/RID/NPROJ/TAP-12/2002-03). The authors also wish to
thank Satya Cashew Chemicals Ltd., Chennai (India) for providing free
samples of cardanol and CNSL for this study.
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Raju, P. Kumar (El)
Department of Oil & Paint Technology, Harcourl Butler
Technological Institute, Kanpur 208 002, India
e-mail: Pramod.hbti@yahoo.co.in
Raju
e-mail: raju_hbti25@yahoo.com
DOI 10.1007/s 11998 -011-9337-y
Table1:Characteristics of MECN resins: mode ratio,
epoxy functionality, and amine value
S. MECN resin Molar (a) Epoxy Molar ratio
No. ratio (C:F) functionality (ECN:DEtOA)
of ECN
(calcd.)
1 [MECN.sub.1] 1:0.6 2.04 1:1.0
2 [MECN.sub.2] 1:0.7 2.20 1:1.0
3 [MECN.sub.3] 1:0.7 2.20 1:1.5
4 [MECN.sub.4] (c) 1:0.8 2.92 1:1.0
5 [MECN.sub.5] (c) 1:0.8 2.92 1:1.5
6 [MECN.sub.6] (c) 1:0.8 2.92 1:2.0
7 [MECN.sub.7] (c) 1:0.8 2.92 1:2.5
8 [MECN.sub.8] (c) 1:0.9 2.12 1:1.0
9 [MECN.sub.9] (c) 1:0.9 2.12 1:1.5
S. Epoxy Amine value of [Conversion.sup.c]
No. functionality MECN resin (%)
of MECN (mgKOH / g)
(calcd.)
(Theoretical) (detd.)
1 1.04 59.49 58.53 98.38
2 1.20 50.17 49.65 98.96
3 0.70 71.89 61.72 85.85
4 1.92 47.18 45.74 96.94
5 1.42 67.78 68.54 98.17
6 0.92 86.70 85.77 98.92
7 0.42 104.15 95.83 92.01
8 1.12 50.44 40.40 80.09
9 0.62 72.26 58.33 80.72
S. Epoxy Watersolubility Selfcurability
No. functionality
of MECN
(actual)
1 1.041 Soluble Curable
2 1.21 Soluble Curable
3 0.85 Soluble Uncurable
4 1.98 Insoluble -
5 1.45 Insolubie -
6 0.93 Soluble Curable
7 0.46 Soluble Uncurable
8 1.40 Soluble Curable
9 0.77 Soluble Uncurable
(a) C:F is molar ratio of cardanol to formaldehyde used for
preparation of corresponding CN resin
(b) ECN to MECN, calculated by using theoretical and determined amine
values
(c) Resins had inadequate pot life, as they gelled in a short period
of time, and hence they were not used for application as films and
for evaluation of film propertiesTable 2: Cure temperature of modified epoxidized
cardanol-formaldehyde novolac (MECN) resins
S. MECN Temperature
No. resin (cure time: 30
min)
100[degrees]C 120[degrees]C 140[degrees]C
1. [MECN.sub.1] NC NC NC
2. [MECN.sub.2] NC NC SC
3. [MECN.sub.3] NC NC NC
4. [MECN.sub.6] NC NC NC
5. [MECN.sub.7] NC NC NC
6. [MECN.sub.8] NC NC SC
7. [MECN.sub.9] NC NC NC
S.
No. 160[degrees]C 180[degrees]C
1. SC c
2. c c
3. NC SC
4. SC c
5. NC SC
6. C c
7. SC SC
NC, not cured; SC, slightly cured; C, curedTable 3: Electrodeposition characteristics of MECN resins
electrodeposited at different applied voltages
S. MECN Applied ED Columbic DFT
No. resin voltage yield yield (mg / ([mU]m)
(V) (mg / C)
[cm.sup.2)
1. [MECN.sub.1] 50 1.34 26.31 20.08
100 2.32 25.85 22.86
150 2.92 26.11 24.40
200 4.06 25.88 25.26
250 4.06 26.70 25.56
2. [MECN.sub.2] 50 2.05 35.95 24.60
100 3.62 35.87 25.26
150 4.69 36.00 26.36
200 6.54 36.39 28.10
250 6.59 36.65 28.00
3. [MECN.sub.3] 50 2.19 34.54 24.26
100 4.04 34.75 26.65
150 5.40 34.10 27.46
200 6.42 33.92 28.96
250 6.67 34.49 28.88
4. [MECN.sub.6] 50 3.33 37.79 21.83
100 4.92 37.56 22.33
150 7.84 38.22 28.08
200 8.60 38.43 30.46
250 9.31 37.26 30.34
5. [MECN.sub.7] 50 2.94 40.05 24.76
100 4.84 40.72 28.84
150 8.34 39.73 35.67
200 9.46 40.85 36.34
250 9.75 39.50 36.87
6. [MECN.sub.8] 50 3.18 35.31 24.50
100 4.39 36.09 25.79
150 7.11 36.75 28.55
200 7.62 36.15 28.76
250 7.46 35.71 28.86
7. [MECN.sub.9] 50 - - -
100 4.27 33.59 32.80
150 6.61 33.31 33.10
200 7.84 34.67 33.30
250 7.86 34.87 33.68
S. No. MECN resin Film appearance (after curing)
1. [MECN.sub.1] Smooth
Smooth
Rough
FR
FR
2. [MECN.sub.2] Smooth
Smooth
Rough
Rough
FR
3. [MECN.sub.3] Rough
Rough
Rough
Smooth
Rough
4. [MECN.sub.6] Rough
Rough
Rough
Smooth
Smooth
5. [MECN.sub.7] Rough
Rough
Rough
Rough
Smooth
6. [MECN.sub.8] Rough
Smooth
Rough
Rough
FR
7. [MECN.sub.9] FND
Rough
Rough
Smooth
FR
FR, films ruptured drastically, hence not considered in study;
FND, film not deposited; deposition time: 60 s; bath
concentration: 10% (w/w)Table 4: Theoretical dominance of hydrophobicity over hydrophilicity
MECNs Molecular No. of No. of Hydrophilic
weight cationic [C.sub.15] cationic
of MECN group chain group
per mole per mol contente
(charge (2 + DP (equiv./
per n value kg)
molecule)
1. 943 1 2.0804 1.0604
[MECN.sub.1]
2. 1118 1 2.4608 0.8944
[MECN.sub.2]
3. 1170 1.5 2.5739 1.2820
[MECN.sub.3]
4. 1294 2 2.8434 1.5455
[MECN.sub.6]
5. 1346 2.5 2.9565 1.8573
[MECN.sub.7]
6. 1112 1 2.4478 0.8992
[MECN.sub.8]
7. 1164 1.5 2.5608 1.2886
[MECN.sub.9]
MECNs Hydrophobic
[C.sub.15]
chain
content
(equiv. /
kg)
1. 2.2061
[MECN.sub.1]
2. 2.2010
[MECN.sub.2]
3. 2.1999
[MECN.sub.3]
4. 2.1973
[MECN.sub.6]
5. 2.1965
[MECN.sub.7]
6. 2.2012
[MECN.sub.8]
7. 2.2000
[MECN.sub.9]
DP, degree of polymerization; n, number of repeat units in the moleculeTable 5: Gloss of electrodeposited films of MECN resins after curing
Applied Gloss measured at 60[degrees]
voltage
(V)
[MECN.sub.1] [MECN.sub.2] [MECN.sub.3] [MECN.sub.6]
50 88.90 67.43 63.16 48.90
100 93.96 68.30 64.56 52.64
150 94.47 68.06 66.37 55.60
200 95.73 67.93 67.76 58.93
250 85.4 68.30 69.40 66.13
Applied Gloss measured at 60[degrees]
voltage
(V)
[MECN.sub.7] [MECN.sub.8] [MECN.sub.9]
50 30.40 55.86 -
100 34.36 70.36 82.93
150 35.68 71.34 87.94
200 37.56 72.33 99.00
250 39.47 74.83 79.63Table 6: Physical properties of MECN resins
S. No. Property/test
(MECN.sub.1)
1. Adhesion (%)
Cross hatch (1 X 1 mm) tape test 100
2. Flexibility (Mandrel bend test)
(a) (1/4 in. mandrel) Pass
(b) (1/8 in. mandrel) Pass
3. Hardness
(a) Scratch hardness (g) 1200
(b) Pencil hardness (pencil grade) 2H
4. Impact resistance (cm) 50)
S. No. Result/value
(MECN.sub.2) (MECN.sub.6) (MECN.sub.8)
1.
100 100 100
2.
Pass Pass Pass
Pass Fail Fail
3.
1500 1400 1800
2H 3H 3H
4. 45 35 25Table 7: Conditions of the films of the prepared MECN resins after
1 week immersion in different chemicals
S. Resin Condition of the film after immersion for 1 week
[H.sub.2]S[O.sub.4] NaOH NaCI Xylene Methanol
No (1N) (1N) (5%)
1. [MECN.sub.1] NE NE NE 3D NE
2. [MECN.sub.2] NE NE NE 2D NE
3. [MECN.sub.6] NE NE NE 3D NE
4. [MECN.sub.8] NE NE NE 3D NE
S. MEK MTO Water
No (distilled)
1. NE NE NE
2. NE NE NE
3. NE NE NE
4. NE NE NE
Table 8: Conditions of the films of the prepared MECN resins
after 3 months immersion in different chemicals
S. Resin [H.sub.2]S[O.sub.4] NaOH NaCI Xylene Methanol
No. (1N) (1N) (5%)
1. [MECN.sub.1] 4S 3S SG 7D 2S
2. [MECN.sub.2] 3S 2S NE 5D 1.S
3. [MECN.sub.6] 4S 3S 3S 6L 2S
4. [MECN.sub.8] 5S 4S 6S 7L 4S
S. MEK MTO Water
No. (distilled)
1. 4D 3D NE
2. 2D 2D NE
3. 5D 3D NE
4. 4D 4D NETable 9: Salt spray test results of electrodeposited MECN resins
S. Resin Salt spray
No. resistance
Creepage Blister Blister
size
(mm) (grade) density
1. [MECN.sub.1] 2.2 6 Medium
dense
2. [MECN.sub.2] 1.5 4 Medium
3. [MECN.sub.6] 1.0 2 Medium
4. [MECN.sub.8] 1.5 4 Medium
dense