Maleic acid grafted low density polyethylene for thermally
sprayable anticorrosive coatings
S.K. Singh a , S.P. Tambe a , A.B. Samui a , V.S. Raja b , Dhirendra Kumar a,∗
a
b
Naval Materials Research Laboratory, Shil-Badlapur Road, P.O. Anandnagar, Ambernath 421506, India
Corrosion Science and Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India
Abstract
Low density polyethylene (LDPE) coatings are being used in an ever growing range of applications. However, non-polar characteristics of
LDPE make its adhesion poor to metal substrate and so become less suitable for anticorrosive coating application. An anticorrosive coating can
therefore be developed if LDPE is modified with requisite polar groups. In the present work, a polar group has been introduced in polyethylene
matrix by grafting maleic acid. Grafted LDPE was characterized by chemical method, Fourier transform infrared spectroscopy (FTIR), differential
scanning calorimetry (DSC), thermo-gravimetric analysis (TGA), melt flow index (MFI), particle size analysis and hot stage optical microscopy.
The presence of carbonyl peak of high intensity, high acid value and low melt flow index value confirmed grafting of maleic acid on LDPE.
Change in crystallization behaviour of LDPE has been noticed after grafting. Grafted LDPE was applied on grit blasted mild steel surface by flame
spray technique and adhesion study showed improved adhesion of grafted LDPE than LDPE. The coated panels were evaluated for resistance to
corrosion in salt spray, humidity cabinet and seawater. The corrosion resistance of modified LDPE was also studied by AC Impedance technique.
Grafted LDPE showed satisfactory corrosion resistance. Grafted LDPE was pigmented with red iron oxide and pigmented composition has shown
improved resistance to corrosion in laboratory tests.
Keywords: Low density polyethylene; Radiation; Grafting; Flame spray; Adhesion; Corrosion resistance
1. Introduction
Protective coatings are employed to protect steel structures
from corrosion due to exposure in marine atmosphere, such as
salt spray, high humidity, ultraviolet radiation and other weather
conditions. These coatings function by providing a barrier between the surface and corrosion inducing elements. Key attributes in the performance of protective coatings are, therefore,
good adhesion to substrate, a low permeability to ions, water,
gases and other corrosive species of the service environment.
Factors that influence the choice of a coating system include the
forms of corrosion encountered, exposure conditions and type
of structure to be protected [1].
Protective coatings are generally formed on substrate by
application of liquid paint at ambient temperature. During the
drying process, solvents are released from such coatings which
pollute the environment. Hence, it would be worthwhile con-
sidering a coating system which does not utilize solvents. One
of the examples of such type of coatings is thermally sprayable
organic coatings, which have attracted attention of corrosion engineers in view of several advantages [2]. Thermally sprayable
organic coatings are thermoplastic in nature and they are 100%
solid materials. Application of these coatings does not result
in release of volatile organic compounds. These coatings form
tough and firmly adherent barrier coatings. Film formation of
these materials is virtually instantaneous, unlike other coatings,
which may require several hours or even days to become
suitable for service. Further more, they may also be applied at
relatively low ambient temperatures. Since, these coatings are
thermoplastic, they can be repaired easily in situ by remelting
or applying additional material. Thermally sprayable organic
coatings can have excellent chemical resistance depending on
polymer type. These coatings are generally resistant to damage
caused by impact and abrasion [3].
Thermally sprayable organic coatings are formed by thermally induced liquification of polymer powders and subsequent
deposition on a substrate where cooling and solidification occurs. Polymers may be thermally sprayed as powders by either
21
plasma or flame spray techniques. Plasma spray uses a hot ionized gas as thermal source, while flame spray process melts the
polymer feed-stock in an oxygen-fuel gas flame. Both methods
employ compressed air to atomize and propel the molten polymer to the substrate.
Polyethylene, polypropylene, fluoropolymers [4] and thermoplastic polyesters are some of the polymers which can be
sprayed by thermal spray process and are expected to provide
longer service life to marine structures. In our work, we have selected low density polyethylene (LDPE) as candidate material
for thermal spray application because of its low cost and a wide
range of physical and chemical properties.
Polyethylenes are of high commercial interest due to their
wide range of physical and chemical properties. However, their
use in polymer blends of technological interest has been limited
due to their typical non-polar character leading to poor adhesion.
To overcome this deficiency and to facilitate compatibilization
with polar polymers, polyethylenes have been chemically modified through functionalization or grafting. This process introduces polar groups onto the polymer main chain as pendant
units or short-chain branching [5]. It can be achieved by copolymerization of new monomers or by modification or blending of
existing polymers [6].
Grafting of preformed polymer is an important method
for preparation of polymers with suitable functional groups
[7]. Grafting involves covalent coupling of species, usually
monomers or chain extenders, onto an existing polymer backbone. Reactive sites for grafting can be generated by mechanical
processing, by chemical initiation, by photochemical activation
(UV-radiation) and by high-energy radiation (electron beam or
gamma radiation). Grafting processes may be carried out under simultaneous treatment conditions, with all components being present during the creation of reactive sites. Alternatively,
grafting can be performed via pre-activation process. Reactive
sites are generated in the absence of the modifying species, but
subsequently exposed to modifying species under some predetermined and controlled condition [8]. Recently, other methods
have been developed such as atom transfer radical polymerization [9,10] or “living” radical polymerization [11,12]. These
methods allow in controlling the length of grafts since they act
more efficiently on the kinetics of the chain growth.
Maleic anhydride (MAn) is one of the widely used vinyl
monomers for graft modification of polyolefins. It has been
grafted to polyethylene by mechanical, free radical, ionic and
radiation techniques [13]. The reason for use of maleic anhydride as monomer for graft modification can be attributed to
chemical reactivity of the anhydride functionality. Rangnathan
et al. [14] reported that performance of maleic anhydride graft
modified polymers in adhesion and compatibilization can be expected to depend on the chemical nature and microstructure of
the graft. Gaylord et al. [15–17] reported formation of single and
oligomeric grafts during grafting of maleic anhydride onto polyolefins. In addition to these grafts, bridges consisting of single
and oligomeric maleic anhydride units between polymer chains
were proposed. Formation of free poly(maleic anhydride) was
also observed, which could be reduced by using additives that
suppressed formation of homopolymer.
Considerable studies have been carried out on maleic anhydride [13–18] grafting of polyethylene. But authors have
not come across any reported work where maleic acid (MAc)
has been taken as starting material for grafting on low density
polyethylene (LDPE). Maleic acid has been reported [19] to improve adhesion of coating due to its high polarity. The aim of the
present work is to introduce polar groups in polyethylene matrix
by grafting maleic acid using ␥-radiation to improve the adhesion of LDPE to metal surface and use the modified LDPE for
development of thermally sprayable organic coating for marine
environment.
2. Experimental
2.1. Materials
Low density polyethylene (LDPE) powder obtained from
IPCL (India), has a density of 0.93 g/cm3 , average particle size of
267 ␮m and melting point of 103 ◦ C. LDPE powder was sieved
in laboratory and powder having particle size less than 200 ␮m
was used for grafting. MAc monomer used in the study, having 99.5% purity and melting point 136–141 ◦ C, was supplied
by S.D. Fine Chem (India) and was used as received. Chemical
reagents used for titration were of AR grade. Red iron oxide
pigment was obtained from ZIGMA International (India).
2.2. Grafting of MAc on LDPE
Hundred grams of LDPE powder of less than 200 ␮m particle
size was exposed under atmospheric conditions at different levels of radiation in a ␥-radiation chamber using Co-60 as source.
Immediately after irradiation, powder was transferred to a threenecked flask containing 350 ml aq. MAc solution (10%, w/v).
The reaction mixture was maintained at 80–90 ◦ C for 3.5 h under
nitrogen atmosphere. Irradiated powders were agitated continuously by using a magnetic stirrer. After completion of reaction,
the powder was separated by filtration, transferred to hot distilled
water and stirred for about 30 min and filtered. The powder was
washed thoroughly with hot water to remove any free maleic
acid. Grafted LDPE powder (LDPE-g-MAc) was dried under
reduced pressure at 80 ◦ C for 16 h before characterization.
2.3. Pigmentation of LDPE and grafted LDPE
Red iron oxide was used for pigmentation of LDPE and
grafted LDPE. A blend of red iron oxide and resin, in 1:4 ratio,
was prepared by extrusion method. The blend was extruded at
140 ◦ C in single screw extruder at 50 RPM for 10 min. After
extrusion, blend was passed through a cryogrinder to achieve
required particle size, suitable for thermal spray application.
2.4. Characterization
2.4.1. Acid value and percent grafting
Acid value and percent grafting were determined by titrating
grafted product. 0.5 g of grafted sample was suspended in 25 ml
of 0.1N NaOH solution (aq.). Reaction mixture was stirred for
22
30 min and then it was kept for 24 h. Solid from mixture was
removed by filtration. It was washed several times with distilled
water and washings were added to filtrate. Unreacted NaOH was
determined by titrating the filtrate and blank against 0.1N HCl
solution, using phenolphthalein as indicator.
2.4.2. Melt flow index
Melt flow index measurements were carried out as per ASTM
D-1238, using Tinius Olsen meltflow Indexer (Model MP600).
As melting points of LDPE and LDPE-g-MAc are 103 and
105 ◦ C, respectively, measurements were carried out at 113 ◦ C.
Table 1
Extent of maleic acid grafting on LDPE
Radiation dose (kGy)
Extent of grafting (±0.1) (wt.%)
15
30
60
1.9
2.0
2.4
and air mixture was used as fuel gas. Compressed air was used
as powder carrier gas.
2.4.3. Infrared spectroscopy
LDPE and grafted LDPE samples were converted into thin
films by hot pressing at 130 ◦ C and were directly mounted on the
frame of spectrophotometer. FTIR spectra of films having identical thickness were recorded by employing a Perkin-Elemer
spectrophotometer (Model 1600 series). The FTIR spectrum
of grafted LDPE was also taken after application by flame
spray.
2.4.8. Adhesion strength
LDPE, LDPE-g-MAc and pigmented compositions were applied on grit blasted mild steel surface by flame spray method
for determination of adhesion strength. Adhesion strength was
determined as per method described in Indian Standard IS: 101
(pull-off method). The method was modified by reducing the
area of one dolly to facilitate the debonding of coating from one
side. For each coating, five specimens were tested and average
has been reported as adhesion strength.
2.4.4. Thermal properties
Melting points of polymeric powders were determined by using a differential scanning calorimeter (TA Instruments, Model
Q 100). DSC measurement was carried out with a ramp of
10 ◦ C/min maintained during heating scan under nitrogen atmosphere. Cooling scan was performed at a rate of 1 ◦ C/min
after holding the sample at 110 ◦ C for 30 min. Thermal decomposition temperature was studied by thermo-gravimetric analyser (TA Instruments, Model Hi-Res. TGA 2950) at a heating
rate of 10 ◦ C/min under nitrogen atmosphere to a temperature
of 800 ◦ C.
2.4.9. Evaluation of anticorrosive property
The anticorrosive property of LDPE, LDPE-g-MAc and pigmented compositions was evaluated by assessing their resistance
to corrosion in salt spray, humidity cabinet and seawater as per
method described in IS: 101. Clean and rust free mild steel panels of size 150 mm × 100 mm × 1.5 mm were grit blasted and
coated by flame spraying with one coat of LDPE and LDPE-gMAc at an average film thickness of 250 ␮m and pigmented compositions were applied at an average film thickness of 175 ␮m.
The edges were sealed by dipping the coated panels in molten
wax before exposure.
2.4.5. Particle size analysis
Particle size analysis was carried out on Mastersizer-2000
of Malvern Instruments by dispersing small quantity of powder
in aqueous and non-aqueous medium in presence of dispersing
agent.
2.4.10. Electrochemical impedance spectroscopy (EIS)
EIS has been used to study the behaviour of flame sprayed
LDPE and LDPE-g-MAc coatings immersed in 3.5% NaCl solution. The impedance measurements were carried out using
Gamry CMS 300 electrochemical impedance system. A sine
wave of 10 mV (rms) was applied across the cell. The exposed
area was 5.3 cm2 . The measurements were made in a frequency
range of 50 kHz to 0.02 Hz. Measurements were taken after
every 10 days using three electrodes system having saturated
calomel electrode (SCE) as reference electrode.
2.4.6. Hot stage microscopy
Crystallization behaviour of LDPE and LDPE-g-MAc was
studied by taking optical micrographs on a Hot stage microscope (Leica Model DMLD). In this experiment, samples were
heated from room temperature to 130 ◦ C at a heating rate of
20 ◦ C/min to get clear melt. Cooling scan was performed at a
rate of 5 ◦ C/min after holding the sample at 130 ◦ C for 2 min.
Cooling was continued till a temperature of 107 ◦ C and the sample was maintained at this temperature for 10 min to facilitate
the crystal growth. After this temperature no change in crystallization was observed.
2.4.7. Thermal spray application
LDPE, LDPE-g-MAc and pigmented compositions were applied on grit blasted mild steel surface by “Powderjet 86 PII”
flame spray gun and the powder feed unit PF 700 equipped with
powder hopper marketed by MEC, Jodhpur (India). Acetylene
3. Results and discussion
Irradiation generate free radicals in LDPE powders [8,20].
Some of the radicals present in powders participate in grafting
on LDPE. In this study, an attempt was made to graft MAc
directly to avoid additional step of hydrolysis of MAn to MAc
as done in conventional methods [11–16]. LDPE-g-MAc, thus
obtained was characterized by various techniques.
Table 1 shows variation in percent grafting at different radiation doses. As expected, extent of grafting increased with
radiation dose. Maximum grafting of 2.4 wt.% was observed for
60 kGy dose.
23
Fig. 1. FTIR spectrum of (a) LDPE, (b) irradiated LDPE and (c) LDPE-g-MAc.
Fig. 1 shows FTIR spectra of LDPE under different conditions. Grafting of MAc is expected to result in the presence
of carboxylic group and hence, the peak corresponding to this
group expected to appear at 1714 cm−1 in the FTIR spectrum.
Spectrum a corresponding to LDPE has a very small peak at
1714 cm−1 . Irradiated LDPE (spectrum b) and LDPE-g-MAc
(spectrum c) exhibit sharp peaks at 1714 cm−1 indicating presence of carboxyl group. However, intensity of peak for LDPE-gMAc is higher compared to that of irradiated LDPE. It has been
reported in literature [21] that exposure of LDPE to ␥-radiation
in air leads to formation of carbonyl groups due to oxidation. To
ascertain the observation that peak appearing at 1714 cm−1 in
spectrum of LDPE-g-MAc is due to grafting of MAc, acid values
of LDPE and LDPE-g-MAc were determined. Acid value (AV)
and percent grafting were calculated using following equations:
Acid value (AV) =
56.1 × (Vb − Vs ) × NHCl
W
(1)
where Vb is the titre value for blank, ml; Vs the titre value for
sample, ml; NHCl the normality of hydrochloric acid; W is the
weight of the sample.
Percent grafting =
EMAc × 100
Eg − EMAc
(2)
where EMAc is the equivalent weight of maleic acid and Eg is
the equivalent weight of grafted LDPE.
Higher acid value of LDPE-g-MAc (Table 2) compared to
LDPE indicated grafting of MAc onto LDPE. The grafting of
MAc to LDPE is further established by melt flow index results,
shown in Table 2. It is expected that introduction of polar group
in LDPE matrix will lead to reduction in melt flow index due to
hydrogen bonding.
Fig. 2 shows FTIR spectrum of LDPE-g-MAc before and
after flame spray application. In both the cases, carboxyl group
peak appears at 1714 cm−1 which indicates that LDPE-g-MAc
Fig. 2. FTIR spectrum of LDPE-g-MAc (a) before and (b) after flame spray
application.
has retained carboxyl groups even after application by flame
spray.
DSC curves in Fig. 3 show melting points of unexposed
LDPE, irradiated LDPE and LDPE-g-MAc at 103, 103.5 and
105 ◦ C, respectively. This change in melting point after grafting
can be attributed to the presence of polar carboxyl groups. However, the effect is small as the grafting level is very low. Exposure
to ␥-radiation does not affect the melting point and crystallization temperature of LDPE. Cooling thermogram shows crystallization peak for LDPE at 99 ◦ C, whereas for LDPE-g-MAc,
peak appeared at 97 ◦ C. The downward shift of crystallization
peak for LDPE-g-MAc can be attributed to hindrance by the
polar groups generated by grafting of MAc.
Fig. 4 gives TGA thermograms of LDPE and LDPE-g-MAc.
Decomposition of LDPE and LDPE-g-MAc is seen to start at
410 ◦ C showing that the degradation behaviour of LDPE has not
changed due to grafting. Low grafting is not expected to change
much of the thermal degradation behaviour. The large difference
between the melting (i.e., 105 ◦ C) and degradation temperatures
(i.e., 410 ◦ C) of LDPE-g-MAc makes this resin suitable for flame
spray application.
Particle size analysis carried out in aqueous medium shows
a marginal increase in the size of LDPE from 187 to 192 ␮m
due to grafting.. This increase in particle size was not observed
Table 2
Acid value and melt flow index of LDPE samples
Polymer
Acid value, mg of KOH/g
Melt flow index g/10 min
(113 ◦ C, 2.16 kg)
LDPE
Irradiated LDPE
LDPE-g-MAc
0.6
0.6
23.0
2.633
1.710
1.096
Fig. 3. DSC curves of (a) LDPE, (b) irradiated LDPE and (c) LDPE-g-MAc.
24
Fig. 6. Adhesion strength of (a) LDPE, (b) LDPE-g-MAc, (c) red iron oxide
pigmented LDPE and (d) red iron oxide pigmented LDPE-g-MAc.
Fig. 4. TGA curves of LDPE and LDPE-g-MAc.
when analysis was carried out in non-aqueous medium. Hence,
this increase in particle size is mainly due to the swelling of
the grafted material in aqueous medium assisted by the polar
COOH group.
The extent of crystallization of LDPE and LDPE-g-MAc can
be seen in optical micrographs presented in Fig. 5(a and b), respectively. Samples were annealed at temperature above melting
point before commencing cooling scan. Slow cooling enabled
crystallization of both samples. It can be seen that at 107 ◦ C the
crystal growth in LDPE sample is quite prominent and crystal
size is also found to be large. In case of LDPE-g-MAc, crystal
growth occurs at this temperature but the crystal size is smaller
compared to that seen in LDPE. This may be due to a reduced
stacking in case of LDPE-g-MAc. It is obvious that presence of
MAc can introduce hydrogen bonding in grafted LDPE and due
to this reason only fine crystals resulted.
Fig. 6 shows the adhesion strength of flame sprayed LDPE,
grafted LDPE, red oxide pigmented LDPE and grafted LDPE.
It is seen that grafting with maleic acid enhances adhesion of
LDPE to mild steel surface due to introduction of polar groups
in the polymer matrix. Pigmentation with red iron oxide does
not affect the adhesion strength of LDPE but a slight decrease in
the adhesion strength in the case of grafted LDPE has occured.
The decrease in adhesion strength is possibly due to adsorption
of some of the polar groups from maleic acid grafted LDPE on
red iron oxide surface and hence less number of polar group
were available for interaction with substrate.
Grit blasted mild steel panels coated with LDPE, LDPE-gMAc, red iron oxide pigmented LDPE and LDPE-g-MAc were
exposed to salt spray, humidity cabinet and seawater for 13
weeks to evaluate their resistance to corrosion. The results are
given in Table 3. It is evident from the results that the grafted
LDPE exhibit higher resistance to corrosion as compared to
ungrafted LDPE under all exposure conditions. The onset of
corrosion in LDPE was noticed after 3 weeks of exposure and
it increased with time, whereas in case of LDPE-g-MAc, first
corrosion spot was observed after 11 weeks of exposure. Results shown in Table 3 indicate that pigmentation improved the
corrosion resistance of these two resins. Improved corrosion re-
Fig. 7. Coating resistance of (a) LDPE and (b) LDPE-g-MAc.
Fig. 5. Hot stage optical micrographs of (a) LDPE and (b) LDPE-g-MAc.
25
Fig. 8. SEM of flame sprayed (a) LDPE and (b) LDPE-g-MAc.
Table 3
Corrosion resistance of flame sprayed coatings
Polymer
LDPE
LDPE-g-MAc
LDPE + Red iron oxide
LDPE-g-MAc + Red iron oxide
a
Performancea (after 13 weeks)
Salt spray
Seawater
immersion
Humidity
4
7
5
7
2
7
7
9
4
8
7
9
10, no corrosion; 0, severe corrosion.
sistance of pigmented LDPE-g-MAc compared to pigmented
LDPE may be attributed to the better interaction between red
iron oxide and LDPE-g-MAc due to their polar nature, leading
to compact film formation, thus reducing the ingress of ionic
species towards the metal substrate.
Fig. 7 shows the change in coating resistance of LDPE and
LDPE-g-MAc with time. At the initial stages of experiment,
both polymers possess high resistance and after 30 days LDPE
showed a sharp drop in resistance and after 50 days of exposure
resistance has further dropped down and it has gone below the
allowable level (i.e., less than 106 cm2 ) [22]. Though, LDPE
is hydrophobic in nature, the poor adhesion of LDPE to metal
surface may be one of the reasons for onset of corrosion. In the
case of LDPE-g-MAc, a drop in coating resistance was noticed
after 50 days, but its resistance remained higher than allowable
level even after 75 days of exposure. It is evident from Fig. 8(a
and b) that both the coatings are porous but the pores in LDPEg-MAc are lesser in number and small in size than LDPE. This
may be the reason for improvement in resistance to corrosion of
LDPE-g-MAc. The EIS results are also in agreement with the
results obtained by accelerated exposure tests (Table 3), where
LDPE-g-MAc has shown better resistance to corrosion than
LDPE.
4. Conclusion
Maleic acid was grafted to LDPE by pre-activation method
using ␥-radiation. The dose was varied to achieve different
level of grafting. Maximum grafting of 2.4% was achieved
at 60 kGy dose. Grafted LDPE has shown modified proper-
ties compared to LDPE. FTIR has shown generation of carboxyl group in grafted LDPE which led to an acid value of
23. Grafting has not affected thermal and mechanical properties
of LDPE to appreciable extent. However, a noticeable change
in crystallization behaviour has been observed after grafting.
Reduced crystalline growth of grafted mass was observed in
optical microscope. Grafted LDPE has shown improved corrosion resistance compared to LDPE resin. Pigmentation with
red iron oxide has further improved the corrosion resistance of
LDPE-g-MAc.
Acknowledgements
The authors sincerely thank to Dr. J. Narayana Das, Director for his keen interest, constant encouragement and for giving permission to publish this paper. The authors also thank
to Dr. P.C. Deb, Emeritus Scientist, Naval Materials Research
Laboratory for his critical comments, suggestions and direction
in completing the work. Assistance of Mr. N.G. Malvankar of
this laboratory is gratefully acknowledged in carrying out FTIR
analysis.
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