Synthesis and Characterization of an Acrylamide-Based Resin for Coating Application

Abstract

To synthesize an acrylamide-based resin, two functional acrylamide monomers, N-methylolacrylamide and N-butoxymethylolacrylamide, were prepared and copolymerized separately with two methacrylate esters: methylmethacrylate and butylmethacrylate. The resin derived from N-methylolacrylamide proved to be inadequate due to its instability. To adjust the necessary amount of the reagents needed for the synthesis, different concentrations of the initiator, benzoyl peroxide, and various concentrations of the molecular weight regulator, tertio-dodecyl mercaptan transfer agent, were tested by monitoring the resulting viscosity and conversion. The paint formulation containing N-butoxymethylolacrylamide co-monomer was characterized in terms of hardness, impact and embossing resistance, gloss and adhesion to a metal substrate. The performance properties, which were compared with those of a commercial paint composition, considered as a reference, were overall satisfactory.

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International Journal of
Polymeric Materials and
Polymeric Biomaterials
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Synthesis and Characterization
of an Acrylamide-Based Resin
for Coating Application
F. Riahi a , A. Bouaziz a , S. Benmesli a & R.
Doufnoune b
a Laboratoire des Matériaux Polymériques
Multiphasiques, Département de Génie des
procédés , Faculté des sciences de l'ingénieur,
Université Ferhat-Abbas , Sétif-Algérie
b Laboratoire de Physico-Chimie des Hauts
Polymères (LPCHP), Département de génie des
procédés , Faculté des sciences de l'ingénieur,
Université Ferhat-ABBAS , Sétif-Algérie
Published online: 09 May 2008.
To cite this article: F. Riahi , A. Bouaziz , S. Benmesli & R. Doufnoune (2008)
Synthesis and Characterization of an Acrylamide-Based Resin for Coating Application,
International Journal of Polymeric Materials and Polymeric Biomaterials, 57:7,
745-758, DOI: 10.1080/00914030801963424
To link to this article: http://dx.doi.org/10.1080/00914030801963424
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Synthesis and Characterization of an Acrylamide-Based
Resin for Coating Application
F. Riahi
A. Bouaziz
S. Benmesli
Laboratoire des Mate
´riaux Polyme
´riques Multiphasiques, De
´partement
de Ge
´nie des proce
´de
´s, Faculte
´des sciences de l’inge
´nieur, Universite
´
Ferhat-Abbas, Se
´tif-Alge
´rie
R. Doufnoune
Laboratoire de Physico-Chimie des Hauts Polyme
`res (LPCHP),
De
´partement de ge
´nie des proce
´de
´s, Faculte
´des sciences de l’inge
´nieur,
Universite
´Ferhat-ABBAS, Se
´tif-Alge
´rie
To synthesize an acrylamide-based resin, two functional acrylamide monomers,
N-methylolacrylamide and N-butoxymethylolacrylamide, were prepared and copoly-
merized separately with two methacrylate esters: methylmethacrylate and butyl-
methacrylate. The resin derived from N-methylolacrylamide proved to be inadequate
due to its instability. To adjust the necessary amount of the reagents needed for the
synthesis, different concentrations of the initiator, benzoyl peroxide, and various
concentrations of the molecular weight regulator, tertio-dodecyl mercaptan transfer
agent, were tested by monitoring the resulting viscosity and conversion. The paint
formulation containing N-butoxymethylolacrylamide co-monomer was characterized
in terms of hardness, impact and embossing resistance, gloss and adhesion to a
metal substrate. The performance properties, which were compared with those of a
commercial paint composition, considered as a reference, were overall satisfactory.
Keywords: acrylamide, acrylic resins, coatings, polymerization, synthesis
INTRODUCTION
The widespread usage of polymeric materials in different fields is
attributable to their great performance, low cost and ease of processing,
Received 10 January 2008; in final form 4 February 2008.
Address correspondence to Farid Riahi, Faculte
´des sciences de l’inge
´nieur,
Universite
´Ferhat-Abbas, Se
´tif-Alge
´rie 19000. E-mail: faridriahi@yahoo.com
International Journal of Polymeric Materials, 57:745–758, 2008
Copyright #Taylor & Francis Group, LLC
ISSN: 0091-4037 print=1563-535X online
DOI: 10.1080/00914030801963424
745
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making them reliable engineering materials. The variety of polymeric
materials also contributes to their great versatility of applications,
including plastics, rubber goods, adhesives and coatings. Among the
polymeric resins that have found success in the coating and paint
industry are the acrylic resins. Although they have been used for more
than 60 years, they are continuously developed to meet the changing
and ever-increasing demands made on them in paint technology [1].
Moreover, the wide interest concerning the preparation and handling
of acrylic resins results not only from the proper choice of adequate
monomers to be copolymerized, but also from the pertinent chemical
treatment required for the handling of their derivatives [2].
This article describes the step-by-step procedure that was followed
to prepare a formulation for a self-curing acrylamide-based resin and
the end use performance properties of the resulting paint composition.
A paint used for coating is a fluid substance which, once dried,
forms a thin layer that could be a varnish if transparent, or an opaque
spreadable paint film spread over a substrate to which it imparts qua-
lities of a esthetics as well as physical and mechanical properties such
as gloss and impact resistance [3]. A typical coating formulation
includes a polymeric binder, solvents and plasticizers, colorant and
other special purpose additives. The binder is the continuous matrix
which represents the continuous phase throughout which the other
ingredients are dispersed. The main properties of the coat film formed
are strongly governed by the intrinsic characteristics of the resin. The
solvents used in paint formulations are volatile liquids used mainly to
fluidize a composition. Except for a few cases where the solvent reacts
with the resin, it is generally eliminated by evaporation during the
coating film formation. Plasticizers are also fluid substances but are
non-volatile and are added to impart a plasticization effect in the com-
position of paints and varnishes. Other additives are used in order to
impart specific properties. For example, surfactants help improve the
wetting and the dispersion over the substrate. Thickening agents are
another category of additives which are used to increase the viscosity
of the paint and prevent the separation of the different ingredients [4].
The success of the preparation of the thermosetting grade of
acrylamide-based resins depends largely on the control of polymeriza-
tion conditions and kinetics. The kinetics of the polymerization of acry-
lamide are characterized by an unusually large rate constant for
the propagation reaction, a relatively low rate constant for termin-
ation, and very low transfer processes to monomer and to solvent.
As a consequence, high degrees of polymerization are possible, and
the conversion of monomer to polymer can be nearly complete within
relatively short reaction times. Extensive details concerning the
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chemistry of polymeric resins used in the paint industry, in addition
to the general considerations in paint formulation, can be found in
the literature [5–9].
EXPERIMENTAL
Materials
The different chemicals required for the polymerization, for the paint
composition, and for testing, butanol, xylene, paraformaldehyde, and
methacrylic acid (MAA), were all supplied by CYANAMID-France
and were used as supplied. The monomers, acrylamide, methyl meth-
acrylate (MMA), and butyl methacrylate (BMA), being of the highest
purity grades, were used without any further purification.
The thermosetting polyacrylamide resin that was considered for the
reference composition, trade name Uracron CS 103 XB, was obtained
from DSM Resins, The Netherlands.
Monomer Preparation and Polymerization
The preparation of N-methylolacrylamide and N-butoxymethylolacry-
lamide, and their copolymerization with MMA and BMA, were carried
out in a three-necked flask equipped with a stirrer and a reflux con-
denser. During each synthesis reaction, samples were taken at regular
time intervals and were characterized in terms of evolution of free for-
maldehyde and hydroxyl group percentage for the monomer prep-
aration, and in terms of viscosity and percentage conversion for the
resulting resin.
The technical approach that was followed is explained in the
Results section.
Testing Procedures
All the tests were carried out following the French standard AFNOR
NF T-30 procedures [10].
Percentage of OH Groups
This method gives the weight of hydroxyl groups contained in 100 g
of a sample. The experiment was performed for 3 tests among which
one was the reference. A solution of 25 ml of acetic anhydride was
added to a specimen and heated under agitation at 90C for 40 min.
To neutralize the excess acetic acid formed, distilled water was added
to the solution, which was then titrated by a 1 N NaOH solution in
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the presence of phenol phthalein indicator. The percentage OH was
calculated as
%OH ¼ðV1V2Þ17;05 f
P100 ð1Þ
where V
1
is the volume (ml) of NaOH solution added for the reference.
V
2
is the volume of the NaOH solution added for the test, f is a correction
factor (usually taken as equal to 1), and P is the sample weight (g).
Free Formaldehyde
This method is based on the principle that in the presence of water
the free formaldehyde reacts with sodium bisulphite, leading to the
formation of sodium hydroxide, which is then neutralized by sulfuric
acid solution. A sample of the methylolated monomer was put in an
Erlenmeyer flask to which ice was added to 2=3 volume. After adding
the color indicator under continuous agitation the mixture was neu-
tralized by a 0.5 N H
2
SO
4
solution until discoloration. Next, 50 ml of
a molar solution of Na
2
SO
3
was added and the mixture was titrated
again by 0.5 N H
2
SO
4
until a clear change in color occurs. The percentage
of free formaldehyde was calculated as
%free formaldehyde ¼VN3
Pð2Þ
where V is the volume (ml) of the 0.5 N H
2
SO
4
added, N normality, and
P the sample weight (g).
Iodine Index
The experiment, which is a measure of the double bonds in a mol-
ecule, was carried out for 3 tests among which one was the reference.
A test sample was placed in a 250 ml Erlenmeyer flask and 25 ml. of
carbon tetrachloride was added. While agitating, 25 ml of Wijs solution
(iodide tetrachloride and iodine) was added and the mixture left in the
dark for 2 h. Before performing titration by sodium thiosulfate, about
50 ml of distilled water was added in addition to starch powder as the
color indicator. The iodine index was calculated as
Iodine Index ¼ðV1V2ÞNf126:9
P100 ð3Þ
where V
1
is the volume (ml) of the thiosulfate added for the reference,
V
2
is the volume (ml) of the thiosulfate added for the test, f is the thio-
sulfate normality factor, N is the normality, and P is the specimen
weight (g).
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Solid Content
The percentage conversion was estimated from solid content
measurements performed on samples withdrawn from the polymer-
ization setup at regular intervals of time. It is calculated as the
ratio of the initial weight (g) of the sample to the final weight (g)
after drying.
Viscosity
The resin viscosity during the polymerization was measured by
means of a Brookfield viscometer run at variable speeds. All measure-
ments were carried out at room temperature.
Coat Film Perzos Hardness
A sample of a coat film, 20–30 microns thick, applied on a metal
plate was placed resting horizontally on two steel balls and then a
pendulum was freed to oscillate from an initial angle of 12. The Perzos
hardness is expressed in terms of the time (sec) it took for the pendu-
lum to oscillate between 12and 4.
Impact Resistance
This test consists of determining the minimum height from which a
given load of 1000 g falls freely and induces visual fracture on the coat
film. The impact resistance is expressed as the ratio of height (cm) to
the load (g).
Embossing Resistance
This test measures the resistance to fracture of a coat film subjected
to slow deformation by embossing the metal substrate from its back
side. The coated metal plate was tightely held between two steel rings
perpendicular to a plunger axis. Then, the plunger, having a diameter
of 20 mm was allowed to penetrate slowly at a constant speed until
fracture occurs. The apparatus is fitted with an optical microscope
that detects the appearance of cracks on the film surface. Embossing
resistance is expressed in terms of the plunger penetration depth
(mm) to cause fracture.
Gloss
Gloss was measured by means of a gloss meter at an angle of 60.
The apparatus was calibrated using a standard specimen with a gloss
value of 93.
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RESULTS AND DISCUSSION
Monomer Preparation
In order to functionalize acrylamide and provide a site for crosslink-
ing, the monomer was prepared by a methylolation reaction, i.e.,
reaction of acrylamide with formaldehyde:
ð4Þ
To follow the course of this reaction, the disappearance of formalde-
hyde and hydroxyl group formation were monitored. Figure 1 shows
the variations of the free formaldehyde and OH groups’ percentages
with the methylolation reaction time. The trends of these curves are
governed by kinetic considerations. They show that during the first
FIGURE 1 Evolution of the free formaldehyde and the hydroxyl groups with
methylolation reaction time.
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20 min, the reaction proceeds at a rate such that the formaldehyde is
rapidly consumed and methylolacrylamide is slowly formed. Then
for the rest of the reaction time, methylolation follows a steady rate
before reaching a constant level. This leveling off did not correspond
to a final state, since it turned out later that the functional monomer
formed was not stable as was evidenced from the change in its appear-
ance. Within two weeks’ storage at room temperature, this monomer
changed from a wax-like viscous liquid to a less-viscous milky one.
This instability could be attributed to the reversibility of the methylo-
lation reaction owing to the weakness of the NHCH
2
OH bond.
Conversely, since in the presence of a strong base such as NaOH,
which was used to adjust the pH value to around 8, acrylamide may
polymerize, it was therefore necessary to verify. But according to
the Iodine index variation, from 1.89 to 1.96 for acrylamide and N-
methylolacrylamide, respectively, it is clear that homopolymerization
was far from being the predominant reaction.
Because of the instability of N-methylolacrylamide, another func-
tional form of the monomer, N-butoxymethylolacrylamide, was
prepared by the etherification of the former:
ð5Þ
This reaction was carried out using an excess of n-butanol in an inert
acidic medium and driving out the formed water by means of an
azeotropic distillation.
Both this functional monomer and the resin resulting from its
polymerization were stable.
Polymerization with N-butoxymethylolacrylamide
The end-use performance of the resin and the appropriate choice of
the synthesis conditions were the main considerations that dictated
the selection of all ingredients, that is, the copolymerizing monomers,
initiator, molecular weight regulator, catalyst, and solvents. For
instance, the use of the methylmethacrylate (MMA) and buthylmetha-
crylate (BMA) combination was intended to result in a synergistic
effect with respect to the resin hardness. Benzoyl peroxide was chosen
as the initiator for practical reasons. It generally decomposes at a
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moderate rate between 60C and 80C, a temperature range safe
enough to help avoid premature crosslinking through the reaction
between the methylol groups, and to avoid also the loss of the solvents
and monomers through evaporation. Methacrylic acid (MAA) was used
because it catalyzes curing and enhances adhesion to substrates.
Finally, laurylic mercaptan was used since it is a common transfer
agent for the control of molecular weight, a major structural factor
that affects the resin properties.
Once the ingredients were selected, the next step was to find the
optimum concentration of each of them. Taking into account the fact
that viscosity and conversion, in addition to the mechanical and
physical properties of the resulting resin, are the limiting factor, we
have aimed to find the initiator concentration [I] and that of the
transfer agent [T.A.] that would offer the required system viscosity
with maximum conversion. In this context, the copolymerization of
N-butoxymethylolacrylamide with MMA and BMA was carried out
using the initial composition, which is shown in Table 1. In order to
adjust the conditions and estimate the optimum [I] and [T.A.] the sys-
tem viscosity and the corresponding conversion were monitored and
the results are shown in Figures 2, 3, and Figures 4, 5 for MMA and
BMA, respectively. The major comments to be made concerning the
synthesis are as follow:
First, the N-butoxymethylolacrylamide that was prepared did not
show any change throughout the entire progress of this work.
As for amido resins, the degree of etherification is the main factor
that affects the resin stability and paint properties as well as viscosity
and degree of polymerization. As the viscosity decreases, the degree of
polymerization increases and so does stability, consequently, reactivity
decreases and softer paint films would result. On the other hand,
TABLE 1 Initial Composition used to Estimate Initiator Concentration [I]
and Transfer Agent Concentration [T.A] Needed
Formulation with ingredient (g)
[I] constant [T.A.] constant
BMA MMA BMA MMA
N-butoxymethylolacrylamide
(50%in n-butanol)
20 20 20 20
Initiator 03 0.3 04–0.7 04–0.7
Xylene=butanol (60=40) 90 90 90 90
Transfer agent 0–0.7 0–1.4 0.5 1.2
MMA or BMA 83.3–82.6 83.3–81.9 82.7–82.4 82–81.4
MAA 3.2 3.2 3.2 3.2
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increasing the degree of etherification gives better gloss but lower
hardness. This is why a compromise should be found in order to
produce balanced properties. In this study the etherification to 80%
seemed to be adequate to achieve satisfactory resin characteristics.
Despite a slight reddish coloration, the resulting synthesized resin
exhibited an aspect similar to that of the reference resin. This color-
ation actually originated from the methacrylic acid itself.
One of the drawbacks of the resin based on this monomer was
the relative lower conversion compared to that obtained with
N-methylolacrylamide. In fact, conversion above 90%was not possible
as this limit marked the onset of gel effect (shown in Figure 2 with a
FIGURE 2 Copolymerization with MMA. Effect of transfer agent concen-
tration [T.A.] on viscosity and conversion (constant initiator concentration
[I] ¼0.6 mon wt%).
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dotted line). The maximum conversion (90%) that was reached
corresponded to a 45%solid content, meaning that the residual mono-
mers could have then affected drastically the performance of the resin
once used in application. One of the problems linked to this aspect was
in fact the appearance of orange skin due to the presence of volatile
substances which broke the continuity and the regularity of the flow
lines of the coating paint during its application. To remedy this prob-
lem different solutions are possible. These include the use of a heavier
diluent, such as xylene or diacetone alcohol. Another solution reported
by Savostianoff [4] and referred to as the ‘‘reflow’’ process, consists of
drying the film at 80C90C and polishing in order to get rid of
orange skin and redrying again at 140C to get a smooth surface.
The most reliable solution however, in order to avoid the formation
of any residual substance, would be to carry out the polymerization
FIGURE 3 Copolymerization with MMA. Effect of initiator concentration [I]
on viscosity and conversion (constant transfer agent concentration
[T.A.] ¼1.2 mon wt%).
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to complete conversion (as near to 100%as possible) by adjusting the
monomer and initiator concentrations as well as temperature.
Polymerization Formulation Optimization
Methacrylic acid is an important ingredient in the coat film formu-
lation, for it catalyzes the curing reaction and participates also in the
crosslinking formation. It was therefore necessary to find its adequate
concentration that would give optimum mechanical properties in terms
of hardness and impact resistance. Different concentrations were used,
fixing the other components at the specified quantities, and the corre-
sponding properties were monitored, as shown in Figure 6. It is shown
that hardness and impact resistance vary contrarily with respect to
methacrylic acid concentration, suggesting that a compromise should
FIGURE 4 Copolymerization with BMA. Effect of transfer agent concen-
tration [T.A.] on viscosity and conversion (constant initiator concentration
[I] ¼0.6 mon wt%).
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also be found. The optimum concentration of methacrylic acid that gave
satisfactory properties, compared to those of the reference formulation
was found to be 2 wt%.
The properties of the coat film that was made from the synthesized
resin are shown in Table 2. In addition to a high degree of gloss, the
coat paint showed a good adhesion to the metal substrate and a high
detergent resistance. Nevertheless, a major drawback, worth to be
pointed out, is that this resin was more sensitive to impact than the
reference formulation. This property may be enhanced through
the use of additives, including an external high molecular weight
plasticizer such as an epoxy resin or even a urea=formaldehyde resin,
that could be incorporated once the polymerization composition and
conditions adjusted.
FIGURE 5 Copolymerization with BMA, effect of initiator concentration [I]
on viscosity and conversion (constant transfer agent concentration
[T.A.] ¼0.5 mon wt%).
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FIGURE 6 Effect of Methacrylic acid content on Hardness and Impact resist-
ance of the coat film. Ingredients (g): N-butoxymethylolacrylamide-63%in n-
Butanol (32.4), Xylene=n-Butanol 60=40 (60=27.7), Initiator (1), transfer agent
(0.9), MMA (38), BMA (38).
TABLE 2 Butoxymethylolacrylamide-based Paint Properties Compared to
those of the Reference Composition
Property Synthesized resin Reference resin
Persoz hardness (sec) 213 245
Embossing (mm) 8.6 9.6
Impact resistance (cm=1000 g) 30–35 40–45
Gloss (degree) 71.39 88.60
Adhesion to a metal substrate Good Good
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CONCLUSION
This study dealt with the functionalization of acrylamide monomer
and its copolymerization with methylmethacrylate and butylmetha-
crylate. Different compositions were prepared in order to adjust the
initiator and molecular weight regulator concentrations.
The resulting resin was characterized in terms of paint property
requirements and the results were compared to those of a commercial
reference composition. After some adjustments in both the polymeriza-
tion conditions as well as the paint formulation, the synthesized resin
exhibited satisfactory properties including gloss, hardness, and resist-
ance to impact and to embossing.
For further completion of this work, it is recommended to investi-
gate the effect of the addition of an epoxydic or urea=formaldehyde
resin, and to correlate quantitatively the synthesized resin molecular
weight with the different ingredient concentrations.
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