Polymer Degradation and Stability 95 (2010) 1160e1166
Contents lists available at ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
Thermal stability of magnesium-rich primers based
on glycidyl carbamate resins
Neena Ravindran a, Dipak K. Chattopadhyay b, Autumn Zakula b, Dante Battocchi a, b,
Dean C. Webster b, *, Gordon P. Bierwagen a, b
a
b
Center for Surface Protection, North Dakota State University, NDSU Dept. 2760, P.O. Box 6050, Fargo, ND 58108-6050, USA
Department of Coatings and Polymeric Materials, North Dakota State University, NDSU Dept. 2760, P.O. Box 6050, Fargo, ND 58108-6050, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 April 2009
Received in revised form
19 April 2010
Accepted 21 April 2010
Available online 27 April 2010
Coatings of outstanding thermal stability were obtained by the combination of two novel technologies,
that of a magnesium-rich primer and a silane-modified glycidyl carbamate binder. While conducting
a study to evaluate the new binder system with respect to properties of the magnesium-rich primer,
during thermogravimetric analysis of samples, previously unobserved and unexpected properties were
noted. The samples transformed into an intact solid residue, with the amount of the residual char ranging
between 40 and 90% weight depending on the pigment volume concentration (PVC) of the magnesium
particles in the composition. It appears that the hitherto unobserved property is essentially a function of
the metallic pigment particles in the coating. The discovery of the exceptional thermal stability potentially increases the range of application for these primers and these can be further developed for use as
a thermal barrier coating.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Corrosion
Thermal stability of organic coatings
Thermal properties
Thermal barrier coating
Metallic primers
Magnesium-rich primer
Magnesium nitride decomposition
1. Introduction
Mg-rich primers (MRPs) were developed in response to a need
for chrome-free replacements for primers for aircraft alloys [1].
Currently, chromium-based materials are used to impart corrosion
protection properties either as pigments or in the form of
a pretreatment. The development of MRPs for Al2024 alloy is
a major paradigm shift in coatings technology with respect to
corrosion protection of this family of alloys. MRPs provide cathodic
protection to the substrate and the polymer matrix provides good
adhesion and barrier properties. Electrochemical behavior investigations of the system confirmed that the connecting magnesium
particles provide corrosion protection by two mechanisms: polarizing the aluminum cathodically and by offering barrier protection
from oxidation products [2]. Therefore, as opposed to the wellknown zinc-rich systems for steel substrates, wherein cathodic
protection can only be obtained at greater than the critical pigment
volume concentration (CPVC), cathodic protection can still be
obtained at a PVC lower than the CPVC in the case of the
* Corresponding author. Tel.: þ1 701 231 8709; fax: þ1 701 231 8439.
E-mail address: [email protected] (D.C. Webster).
0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2010.04.015
magnesium-rich system [2]. The binder system used in the initial
Mg-rich primer was a three-part system which included a silanepretreatment for the metal substrate, a hybrid binder and a silanised crosslinker [1].
Most of the research since this innovation has focused on
expanding the understanding of pigmentary properties and other
pigment modifications. This study focused on the evaluation of an
alternate binder system in order to enhance the barrier properties
and thereby significantly increasing the lifetime of the coating. The
binder chemistry under consideration is the glycidyl carbamate
(GlC) chemistry [3]. GlC chemistry has the potential of combining
polyurethane and epoxy chemistry into a single system. It has been
previously demonstrated that coatings based on GlC resins exhibit
outstanding chemical resistance and mechanical properties. For
this particular study, a silanised GlC resin was used as the binder
because it had many of the characteristics of the original binder
system. This original binder had an additional inorganic component
in the coating system which effectively made it a three-part system
[1]. However, the use of a silanised resin had the benefit of yielding
a simpler two-part coating system with potentially comparable or
better performance properties.
As this was meant to be a feasibility and screening study, the
focus was on obtaining trends of final coating properties due to
N. Ravindran et al. / Polymer Degradation and Stability 95 (2010) 1160e1166
variations in formulation parameters. However, a hitherto unobserved property in this system was discovered: the good thermal
stability of the coatings which has the potential to expand the range
of applications. The literature on thermally stable coatings is
illustrative of approaches to obtaining good thermal stability. Wu
et al. reported the thermal stability of coatings on 2024-T3 and
6061-T6 aluminum alloys using polydimethylsiloxane oligomer as
cross linking agent in epoxide-modified Ormosils (organically
modified silicates) [4]. It was found that the thermal stability of the
PDMS modified Ormosil was found to be lower than that of the
GPTMS-TEOS (3-glycidoxypropyltrimethoxysilane and tetraethoxysilane) Ormosil and this was attributed to the higher thermal
conductivity of silica causing enhanced degradation of the aliphatic
n-propyl segments and thereby reducing thermal stability. Nyambo
et al. described the concomitant addition of magnesium aluminum
undecenoate (MAU) and ammonium polyphosphate (APP) to
develop formulations meeting the requirements of flame retardancy [5]. They observed a synergistic effect when the two were
combined with a notable stabilization in the thermo-oxidation
degradation stage. In another approach described by Marini et al.
the thermo-oxidative stability of poly(ethylene terephthalate) and
low-density polyethylene films were improved by coating the films
with organic-inorganic coatings of different compositions [6]. Fang
et al. described the preparation of a halogen-free retardant silicone
rubber composite by using magnesium hydroxide sulfate hydrate
(MHSH) as a flame retardant and microencapsulated red phosphorus (MRP) as a synergist with improved flammability properties
[7]. Several studies have reported the use of organically modified
layered silicates to improve the thermal stability of organic coatings. For example, Gorrassi et al. reported that in case of polyurethane composites, coatings that contained the organomodified
clay showed a greater thermal stability than those without the clay
[8]. Improvement in thermal stability has been reported by the use
of MgO as an additive in NdeFeeB magnets and as a coating on
BaMgAl10O17:Eu2þ, blue phosphor [9,10].
In this study, we have evaluated the thermal properties of the
coating system using techniques such thermogravimetric analysis
and differential scanning calorimetry among others and this is the
focus of this short communication.
2. Experimental
2.1. Materials
GlC resins containing 10, 15 and 20% silanization (aminopropyltrimethoxy silane) of the free isocyanate groups were
synthesized. The details of the synthesis are outside the scope of this
study and were described in a separate publication [11], however,
the structure of the resin can be found in the schematic in Fig. 1.
Ancamide 2353 (AHEW ¼ 114), a modified polyamide, was obtained
from Air Products and Chemicals, Inc. (Allentown, PA, USA). Epikure
3164 (AHEW ¼ 256), an oligomeric polyamine and a product of
Hexion, was obtained from Miller-Stephenson Chemical Company,
Inc. (CT, USA) The magnesium-rich primer was made using
a 30e40 mm stabilized Mg particulate (Non-Ferrum-Metallpulver
GmbH, Austria), consisting of Mg covered with a thin layer of MgO,
intended to control the reactivity of magnesium. Methyl ethyl
ketone was purchased from Sigma Aldrich. All materials were used
as received without further purification.
2.2. Preparation of coatings
Formulations were prepared by mixing calculated quantities of
the GlC resin, magnesium filler particles and crosslinker, thinning
the resultant dispersion with methyl ethyl ketone and spraying the
1161
coating onto the substrate. The epoxide:amine active hydrogen
ratio was fixed at 1:1 and the PVCs of the formulations were fixed at
20, 30 and 40%. Epikure 3164 and Ancamide 2353 were used as
crosslinkers. The formulations for the screening study were
selected to determine the effect of increasing amount of silane, the
effect of PVC and the type of crosslinker. The formulations were
spray-coated onto Al2024-T3 (Q Panel Lab products, Cleveland, OH)
and glass substrates for further tests. The Al2024 substrates were
prepared by sanding (first using 220 grit sandpaper followed by
a 600 grit sandpaper) followed by a hexane wash. The coatings
were cured at ambient temperature for at least seven days. The final
dry film thickness ranged between 50 and 70 microns.
2.3. Nomenclature
The formulations have been designated as SXPYA or SXPYE
wherein, X ¼ % of NCO groups modified with silane and Y ¼ % PVC. PVC
is the pigment volume concentration and is calculated as follows:
%PVC ¼ 100 Vpigment =ðVpigment þ Vnonvolatile binder Þ
The samples without the PVC term are control samples which
do not contain pigments. These controls were included in the
evaluation to delineate the properties in terms of binder and
pigmentary contributions to the extent possible. The crosslinkers
Ancamide 2353 and Epikure 3164 are notated as A and E, respectively. For example, sample S10P40A indicates a system where the
binder contains 10% silane modification, the PVC of the coating is
40%, and Ancamide 2353 was used as the crosslinker.
2.4. Characterization
The coatings were evaluated for thermal properties using
differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), energy dispersive X-ray (EDAX), and X-ray photoelectron spectroscopy (XPS). DSC measurements were conducted using
a TA Instruments Q1000 series DSC. The testing method used was
a heat-cool-heat cycle. The samples were first equilibrated at
75 C and then subjected to a heat cycle at the rate of 10 C/min to
200 C, followed by cooling to 75 C and holding isothermally for
5 min, and final heating at a rate of 10 C/min to 250 C. Tg was
determined from the midpoint of the inflection in the final heating
cycle. TGA was conducted using a TA Instruments Q500 Thermogravimetric Analyzer. Samples were heated under house nitrogen
(flow rate ¼ 60 mL/min) from 25 C to 800 C, at a rate of 10 C/min.
A few samples were run under house air in order to observe the
behavior in air (flow rate ¼ 60 mL/min). The TGA runs of the
magnesium powder were carried out under different conditions of
house air, ultra high purity nitrogen and argon (flow rate ¼ 100 mL/
min) from 25 C to 850 C, at a rate of 20 C/min after being initially
held in the purge gas, isothermally for 60 min. A TGA experiment
using a combination of gases (house air and house nitrogen) was
carried out wherein the sample was heated under house nitrogen
(flow rate ¼ 100 mL/min) from 25 C to 850 C at a rate of
20 C/min. The sample was initially held isothermally at ambient
temperature for 60 min, followed by the temperature ramp and
then cooled down to 28 C, followed by switching of gas to house
air and held at 28 C for 60 min. XPS measurements were carried
out with a PHI Quantera XPS microprobe system. It was equipped
with an electron neutralizer gun and an Ar ion sputter gun.
The base pressure of chamber was less than 1 108 torr during
measurement. All the results were obtained with an Al Ka
(hn ¼ 1486.6 eV) X-ray beam at beam diameter of 200 microns. The
pass energy of analyzer was fixed at 55 eV and scanning step was
0.1 eV. The binding energy was calibrated by using Au 4f7/2 ¼ 84.0
and Cu 2p3/2 ¼ 932.67 eV. Scanning electron microscopy (SEM) was
1162
N. Ravindran et al. / Polymer Degradation and Stability 95 (2010) 1160e1166
O
O
C=O
NH2
N
Si
N
O
O
(CH2)6
O
(CH2)6
NH
O
NH
O
(CH2)6
Step 1
NH
HN
(CH2)6
N
O
(CH2)6
N
O
Si
O
..
N
NH
O
HN
(CH2)6
(CH2)6
C=O
C=O
N
N
C=O
C=O
HDI-Biuret
(Tolonate HDB-LV)
O
O
O
O
O
O
Si
NH
O
Amine crosslinker
Moisture cure
NH
NH
(CH2)6
(CH2)6
N
NH
OH
HN
O
O
N
O
Step 3
NH
HN
(CH2)6
(CH2)6
NH
NH
O
(glycidol)
(CH2)6
NH
O
O
Si
(CH2)6
O
Step 2
OH
O
O
OH
Crosslinked network
HN
(CH2)6
(CH2)6
NH
NH
O
NH
O
O
O
O
O
O
Silane modified BGC resin
Fig. 1. Schematic for the reaction and coating formation from HDB, APTMS and amine crosslinkers.
carried out with a Jeol JSM-6490LV (JEOL USA, Inc., Peabody,
Massachusetts) apparatus, equipped with an energy dispersive
X-ray analyzer (EDAX). The samples were mounted on aluminum
mounts and coated with gold using a Technics Hummer II sputter
coater (Anatech Ltd., Alexandria, Virginia). X-ray information was
obtained via a Thermo Nanotrace Energy Dispersive X-ray detector
with NSS-300e acquisition engine. In order to obtain an oxidized
sample, the coating on the Al2024 substrate was subjected to an
indirect flame, by exposing the backside of the coated panel to
a propane flame. X-ray powder diffraction (XRD) data was collected
using a Rigaku Ultima IV powder diffractometer in Bragg-Brentano
geometry, using Cu Ka radiation with a wavelength of 1.5406. The
samples were scanned from 5 to 80 2q, using a step size of 0.02
2q and a run time of 1 s/step.
3. Results and discussion
The thermal behavior of the magnesium-rich primer based on
silane-modified glycidyl carbamate resin was investigated. As this
was a screening study, selected formulations were evaluated to
understand the effect of parameters such as % silane, PVC and type
of crosslinker on performance properties. The glass transition
temperature (Tg) data obtained from differential scanning calorimetry is included Table 1.
Several conclusions can be drawn from the Tg data such as the Tg
varies with the type of the crosslinker which is as expected.
Table 1
Glass transition temperatures of coatings prepared from silane-modified BGC resins,
amine crosslinkers and magnesium pigment.
Sample
% Silane
PVC
Crosslinker
Tg ( C)
S10A
S10P20A
S10P30A
S10P40A
S15A
S15P40A
S10E
S10P20E
S10P30E
S10P40E
S15E
S15P30E
S15P40E
S20E
S20P30E
10
10
10
10
15
15
10
10
10
10
15
15
15
20
20
e
20
30
40
e
40
e
20
30
40
e
30
40
e
30
Ancamide2353
Ancamide2353
Ancamide2353
Ancamide2353
Ancamide2353
Ancamide2353
Epikure 3164
Epikure 3164
Epikure 3164
Epikure 3164
Epikure 3164
Epikure 3164
Epikure 3164
Epikure 3164
Epikure 3164
72.0
77.6
78.6
81.2
76.3
81.3
41.0
43.0
47.9
50.7
42.1
43.1
41.1
43.1
43.3
N. Ravindran et al. / Polymer Degradation and Stability 95 (2010) 1160e1166
S10P20A
S10P30A
S10P40A
S15P40A
S10P20E
S10P30E
S10P40E
S15P30E
S15P40E
S20P30E
110
100
90
Weight %
80
70
60
50
40
30
20
0
100
200
300
400
500
600
700
800
900
Temperature (°C)
Fig. 2. Thermogravimetric analysis curves in nitrogen for formulations with
PVC ¼ 20%, 30% and 40%.
S10A
S10P20A
S10P30A
S15P40A
100
80
Weight (%)
Coatings containing Ancamide 2353 had a higher Tg as compared to
those containing Epikure 3164 and this can be attributed to the
difference in chemical composition of the two crosslinkers. It was
also found that generally the introduction of the magnesium
particles resulted in an increase in the Tg value and that within the
compositions containing Mg, the Tg showed a proportionate
increase with PVC.
TGA experiments on samples in a nitrogen atmosphere showed
that there were three distinct stages in the decomposition process
as depicted in Fig. 2. The first reduction in weight below 200 C may
be attributed to the loss of volatiles. The weight loss from 200 C to
600 C is typically due to the random decomposition of the polymer
chains. Further, after the decomposition of the organic portion of
the coating, the magnesium pigment is directly exposed to the
elevated temperature, and a significant weight gain is observed.
Since the intended application for these coatings is under
ambient conditions, TGA runs were obtained for a few samples in
air including a control that did not contain Mg and is illustrated in
Fig. 3. A weight gain was also observed in air at elevated temperatures for all of the Mg-containing samples.
It was also observed that the residue was visually and
mechanically intact and looked strikingly similar to the material
prior to thermal exposure. However, cracks appeared in the material due to handling as described in the next section.
In order to understand the reactions leading to the weight gain,
the residue obtained from the thermogravimetric analysis in
nitrogen was analyzed using EDAX to study its composition. As
stated earlier, the residue looked intact in terms of appearance and
there were no visible signs of deterioration due to the thermal
exposure. Figs. 4 and 5 are SEM images of the residue mounted on
a carbon tape; the cracks which are visible appeared after pressure
was applied to make it adhere to the tape. High magnification
images of the residue are also included. The high magnification
images suggest a sintering of the coating material and this behavior
possibly contributes to the sample integrity post high temperature
exposure.
The elemental analysis scans in Figs. 6 and 7 indicate that the
major components of the residue are Mg and O both in case of 30%
PVC and 40% PVC coatings. Thus, it appears that an oxidation
reaction had occurred. The values of composition for Mg and O
obtained from elemental analysis from EDAX are listed in Table 2
and, from these results, the stoichiometric ratio of Mg:O was
1163
60
40
20
0
0
100
200
300
400
500
600
700
800
900
Temperature (°C)
Fig. 3. Thermogravimetric analysis curves in air for formulations with PVC ¼ 20%, 30%
and 40% and a control.
estimated to be approximately 1:1. More accurately, the Mg:O ratio
for S10P30E is 1:1.23 and for S10P40E is 1:1.22 obtained by
normalizing the atom% with respect to Mg. The higher value of
oxygen is expected and may be attributed to the oxygen present in
the resin and the oxygen associated with C or with Si.
The observation that the percent residual weight was equal to
a doubling of %PVC was further investigated through stoichiometric
calculations and we concluded that this was related to the Mg in the
composition being converted to MgO. The values in Table 3 provide
a comparison of the final weight of residue relative to the initial
weight of sample and to the PVC, showing that the residue weighs
twice the initial PVC.
To further confirm the composition of the residue, a sample
(S10P30A) coated on an Al2024 substrate was subjected to an
indirect flame as described in the characterization section. The
coating before and residue after exposure to the flame were
analyzed using XPS. The XPS findings are depicted in Fig. 8.
In the Mg 2p spectrum obtained with deconvolution from XPS,
two distinct signals were obtained before flame exposure corresponding to metallic magnesium and oxidized magnesium. After
exposure to the indirect flame, a single signal was obtained corresponding to oxidized magnesium. This provides further confirmation that the material degrades into a MgO residue.
The initial experimental findings led to several important
questions: (a) What is the difference in composition of the residue
when exposed to different environments at elevated temperatures?
(b) Does the binder have a role to play in the overall weight gain?
(c) Is the formation of magnesium nitride (Mg3N2) a possibility
under the given experimental conditions?
As seen in Figs. 2, 3 and 9, the weight gain phenomenon was
observed during TGA experiments under nitrogen as well under
atmospheric air at elevated temperatures. It is clear from Fig. 3 that
the weight gain is due to the presence of the magnesium pigment
because the sample S10A, consisting solely of binder resin, does not
show any weight gain at the end of the TGA cycle. Fig. 9 contains TGA
plots of magnesium powder samples under different environments
(atmospheric air, ultra high purity nitrogen and argon) and once
again weight gain is observed to varying extents in the presence of
nitrogen and air, but not in the presence of argon. Based on these
observations, we can conclude that the binder does not play a role in
the weight gain. We analyzed the residue of the samples from the
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N. Ravindran et al. / Polymer Degradation and Stability 95 (2010) 1160e1166
Fig. 4. (a) Residue of 10% Silane (aminopropyltrimethoxy silane) modified glycidyl carbamate resin with 30% PVC and crosslinker Epikure 3164 observed from SEM after TGA (in
nitrogen) at 800 C. (b) High magnification image of the same sample.
TGA runs under different environments using EDAX and the
compositions were again found to contain only Mg and O.
However, this still leaves us with the question of the origin of the
weight gain in the nitrogen atmosphere and thus the possibility of
formation of Mg3N2 must be considered. Since none of the experimental techniques that we applied to the study of the residues
detected nitrogen, we looked into the literature for previous studies.
In a study of the thermal behavior of 70- to 80-mesh samples of
chemically pure magnesium and aluminum in various atmospheres
(argon, nitrogen, oxygen and air), a TGA curve similar to Fig. 9 was
reported for magnesium as well as in a paper studying pyrotechnic
mixtures [12,13]. The paper by Markowitz clearly states that nitrogen
reacted with the metal and it occurred at much higher temperatures
than the melting point of Mg (650 ) and was found to be significant in
the range of 660e700 C [12]. The authors further hypothesized that
for both aluminum and magnesium in air, any nitride formed is
converted into the corresponding oxide which is thermodynamically
more stable in an oxygen containing environment. The decomposition of Mg3N2 to form MgO on exposure to moisture/oxygen as per
the reaction Mg3N2 þ 3H2O / 3MgO þ 2NH3 has also been reported
[14,15]. Mg3N2 can also form Mg(OH)2 in the presence of water as per
the reaction Mg3N2 þ 6H2O / 3 Mg(OH)2 þ 2NH3 [16]. Sercombe
and Schaffer, in an infiltration study, show the important role that the
reaction between nitrogen and Mg has in the infiltration process [15].
In another study, combustion products of aluminum, magnesium and
their aluminum-magnesium alloy powders were studied under
normal atmospheric conditions using X-ray diffraction. In case of
magnesium, magnesium oxide was found to be the predominant
product, followed by its nitride and hydroxide [17]. In a study of the
kinetics of magnesium nitriding by pure nitrogen it was found that
there are different nitriding mechanisms in operation at temperatures in the range of 385e650 C [18]. All of these studies confirm that
the formation of Mg3N2 is a distinct possibility under either atmospheric conditions or under nitrogen. However, the rapid decomposition of this nitride upon exposure to atmospheric moisture and/or
oxygen could be the reason why no nitrogen could be detected by the
analytical techniques employed. Within this system, some sources of
oxygen/moisture which could be present during the TGA experiment
are (a) the thin layer of oxygen of the surface of Mg pigment present
to control reactivity (b) moisture on the surface of the pigment
Fig. 5. (a) Residue of 10% Silane (aminopropyltrimethoxy silane) modified glycidyl carbamate resin with 40% PVC and crosslinker Epikure 3164 observed from SEM after TGA (in
nitrogen) at 800 C. (b) High magnification image of the same sample.
Metallic
Mg
1165
Mg 2p
Oxidized
Mg
56
54 52 50 48 46 44
Fig. 7. Scan from EDAX showing the elemental composition of the residue at 800 C
from TGA (in nitrogen) 10% silane-modified glycidyl carbamate resin with 40% PVC and
crosslinker Epikure 3164 sample.
Table 2
Composition of residue from TGA samples in a nitrogen atmosphere.
S10P30E
S10P40E
Element
Atom%
Element
Atom%
MgeK
OeK
40.84
50.52
MgeK
OeK
41.50
50.68
S10P20
S10P30
S15P40
Initial wt of
coating
sample, g
Actual
weight of the
residue at the end
of TGA runa, g
% final weight of residue
compared to the initial
weight of sample
PVC
0.2538
0.2104
0.2955
0.1000
0.1285
0.2372
39.40
61.07
80.27
20
30
40
54 52 50 48 46 44
Binding Energy (eV)
Fig. 8. Deconvoluted XPS spectrum before and after indirect flame exposure of
samples.
powder sample from the TGA run in ultra high purity nitrogen in
order to determine the composition and the result is depicted in
Fig. 10. The strong lattice spacings are obtained at d(Å) of 1.49, 2.10
and 2.43 and these indicate that MgO is the major constituent [17].
The reflection at 2.10 can also indicate the presence of Mg(OH)2.
The reflections corresponding to Mg3N2 were not obtained but the
detection of possible reaction products of Mg3N2 indicate that
a conversion could have occurred.
The final experiment was a TGA run under a sequential
combination of gases, that of house nitrogen followed by house air.
By this experiment we wanted to demonstrate that the predominant weight gain reaction in a nitrogen environment is the nitridation reaction and that there is a subsequent conversion into the
corresponding oxides on exposure to air. The plot in Fig. 11shows
two distinct regions of weight gain, first under the nitrogen environment and a subsequent weight gain under isothermal conditions (temp ¼ 28 C) on exposure to air. However, the weight gain is
of a higher magnitude than if it were a pure nitridation reaction at
the end of the house nitrogen exposure cycle on the basis of the %
weight residue. When we compare this % weight of the residue to
that obtained in Fig. 9 with an ultra high purity nitrogen gas, it
indicates that the house nitrogen is very likely contaminated with
some amount of oxygen. The higher weight gain may be explained
on the basis of formation of MgO in addition to Mg3N2. This is also
substantiated by the detection of MgO through the different
Table 3
TGA data for the coating samples in house nitrogen compared to the initial PVC.
Sample
Oxidized
Mg
56
Binding Energy (eV)
Fig. 6. Scan from EDAX showing the elemental composition of the residue @ 800 C
from TGA (in nitrogen) 10% silane-modified glycidyl carbamate resin with 30% PVC and
crosslinker Epikure 3164 sample.
Mg 2p
Intens ity (arb. un its)
Intens ity (arb. un its)
N. Ravindran et al. / Polymer Degradation and Stability 95 (2010) 1160e1166
Magnesium powder in air
Magnesium powder in ultra high purity nitrogen
Magnesium powder in argon
140
135
130
125
120
a
particles and (c) by product of silanol condensation of the resin. In
addition, the samples run under nitrogen in the TGA were removed
from the instrument and exposed to the atmosphere upon completion of the experiment. Therefore, it is highly likely that Mg3N2 was
formed at some stage during the thermal exposure cycle under
nitrogen.
While further investigation is warranted into the exact mechanism of decomposition, we carried out two additional experiments
to confirm our attribution of the weight gain of the sample in
a nitrogen atmosphere to the conversion of Mg3N2 to MgO/Mg
(OH)2. We ran an XRD scan on the residue of the magnesium
115
Weight %
Assumption is made that the binder does not make a significant contribution in
terms of weight gain as is evident from Fig. 3.
110
105
100
95
90
85
80
0
200
400
600
800
1000
Temperature (°C)
Fig. 9. Thermogravimetric analysis curve for magnesium powder under different
atmospheres.
1166
N. Ravindran et al. / Polymer Degradation and Stability 95 (2010) 1160e1166
Intensity (cps)
d(Å)2.10
3000
2500
Intensity (cps)
2000
1500
d(Å)1.49
1000
finding was the excellent thermal stability of the material at high
temperatures as obtained from thermogravimetric analysis.
As was stated earlier, studies have shown that MgO has been
used in formulations to improve the thermal stability of the
composition. Different experimental techniques used in this study
and stoichiometric calculations indicate the formation of MgO at
elevated temperatures. This change in composition combined with
the integrity of the coating at elevated temperatures present the
possibility for the coating to be used under high temperature
conditions (thermal barrier property).
Acknowledgements
d(Å)2.43
500
0
0
10
20
30
40
50
60
70
80
90
2θ ( º )
Fig. 10. XRD pattern of residue of the TGA sample of magnesium powder carried out in
ultra high purity nitrogen.
200
800
Weight
Temperature
180
Weight (%)
atmosphere switched from
140
house nitrogen to house air
400
120
References
Temperature (°C)
600
160
200
100
0
80
0
50
100
150
The authors would like to acknowledge Scott Payne of the USDA
microscopy laboratory at NDSU for characterization with EDAX, Jinhai
Wang of the department of Coatings and Polymeric Materials at
NDSU for characterization with XPS, Heidi Doctor for assistance with
the TGA experiments, and Brad Halverson and Eric Jarabek of the
Center for Nanoscale Science and Engineering at NDSU for characterization with XRD. We acknowledge Mark Hatzenbeller of the
department of Coatings and Polymeric Materials at NDSU for help
with spray coating panels. We also acknowledge the funding support
from AFOSR through fund number FA9550-05-1-0381.
200
Time (minutes)
Fig. 11. Thermogravimetric analysis curve for magnesium powder under a combination of gases (house nitrogen followed by house air in sequence).
techniques used in this study. Nevertheless, the finding of
a continued increase in weight on exposure to air supports our
assertion that the reaction that occurs on exposure to air plays
a significant role in the overall weight gain and is supported by the
experimental results and findings from literature studies.
The thermal stability behavior exhibited by the coatings is an
important finding from the perspective of overall coating properties. Mg-rich primers have been shown to exhibit outstanding
corrosion protection properties. The thermal stability attribute of
the coating system can further increase the range of applications
for the system. For example these coatings can be potentially
developed to be used in high temperature applications, in addition
to its established corrosion protection properties.
4. Conclusions
Silane-modified glycidyl carbamate resins were evaluated as
a potential binder for magnesium-rich primer system. It was found to
be a promising system for corrosion protection in the screening study.
However, in addition to the other thermal properties, an unexpected
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