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Maleic acid grafted low density polyethylene for thermallysprayable anticorrosive coatings

S.K. Singh a, S.P. Tambe a, A.B. Samui a, V.S. Raja b, Dhirendra Kumar a,

a Naval Materials Research Laboratory, Shil-Badlapur Road, P.O. Anandnagar, Ambernath 421506, Indiab 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 ofLDPE 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 be-

tween the surface and corrosion inducing elements. Key at-

tributes 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 en-

gineers 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 ther-

mally induced liquification of polymer powders and subsequent

deposition on a substrate where cooling and solidification oc-

curs. Polymers may be thermally sprayed as powders by either

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plasma or flame spray techniques. Plasma spray uses a hot ion-

ized 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 poly-

mer to the substrate.

Polyethylene, polypropylene, fluoropolymers [4] and ther-

moplastic 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 se-

lected 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 mod-

ified through functionalization or grafting. This process intro-

duces polar groups onto the polymer main chain as pendantunits or short-chain branching [5]. It can be achieved by copoly-

merization 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 back-

bone. 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 un-

der simultaneous treatment conditions, with all components be-ing 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 prede-

termined and controlled condition [8]. Recently, other methods

have been developed such as atom transfer radical polymeriza-

tion [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 anhy-dride 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 ex-

pected to depend on the chemical nature and microstructure of

the graft. Gaylord et al. [1517] reported formation of single and

oligomeric grafts during grafting of maleic anhydride onto poly-

olefins. 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 an-

hydride [1318] 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 im-

prove 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 adhe-

sion 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), hasa density of 0.93 g/cm3, average particle size of

267m and melting point of 103 C. LDPE powder was sieved

in laboratory and powder having particle size less than 200 mwas used for grafting. MAc monomer used in the study, hav-

ing 99.5% purity and melting point 136141 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 200m particle

size was exposed under atmospheric conditions at different lev-

els of radiation in a -radiation chamber using Co-60 as source.

Immediately after irradiation, powder was transferred to a three-necked flask containing 350 ml aq. MAc solution (10%, w/v).

The reaction mixture was maintained at 8090 C for 3.5 h under

nitrogen atmosphere. Irradiated powders were agitated continu-

ously by using a magnetic stirrer. After completion of reaction,

thepowder was separatedby filtration, transferred to hotdistilled

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 andgrafted 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

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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 indexMelt 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.

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 iden-

tical 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.

2.4.4. Thermal properties

Melting points of polymeric powders were determined by us-

ing 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 at-

mosphere. Cooling scan was performed at a rate of 1 C/min

after holding the sample at 110 C for 30 min. Thermal decom-

position temperature was studied by thermo-gravimetric anal-

yser (TA Instruments, Model Hi-Res. TGA 2950) at a heating

rate of 10 C/min under nitrogen atmosphere to a temperature

of 800 C.

2.4.5. 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.

2.4.6. Hot stage microscopy

Crystallization behaviour of LDPE and LDPE-g-MAc was

studied by taking optical micrographs on a Hot stage micro-

scope (Leica Model DMLD). In this experiment, samples were

heated from room temperature to 130

C at a heating rate of20 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 107C and the sam-

ple was maintained at this temperature for 10 min to facilitate

the crystal growth. After this temperature no change in crystal-

lization was observed.

2.4.7. Thermal spray application

LDPE, LDPE-g-MAc and pigmented compositions were ap-

plied 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

Table 1

Extent of maleic acid grafting on LDPE

Radiation dose (kGy) Extent of grafting (0.1) (wt.%)

15 1.9

30 2.0

60 2.4

and air mixture was used as fuel gas. Compressed air was used

as powder carrier gas.

2.4.8. Adhesion strength

LDPE, LDPE-g-MAc and pigmented compositions were ap-

plied 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.

2.4.9. Evaluation of anticorrosive property

The anticorrosive property of LDPE, LDPE-g-MAc and pig-

mented compositions was evaluated by assessing theirresistance

to corrosion in salt spray, humidity cabinet and seawater as per

method described in IS: 101. Clean and rust free mild steel pan-

els of size 150 mm100 mm 1.5 mm were grit blasted and

coated by flame spraying with one coat of LDPE and LDPE-g-

MAcatanaveragefilmthicknessof250m andpigmented com-

positions were applied at an average film thickness of 175 m.

The edges were sealed by dipping the coated panels in molten

wax before exposure.

2.4.10. 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 so-

lution. 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.

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 [1116]. LDPE-g-MAc, thus

obtained was characterized by various techniques.

Table 1 shows variation in percent grafting at different ra-

diation doses. As expected, extent of grafting increased with

radiation dose. Maximum grafting of 2.4 wt.% was observed for

60 kGy dose.

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Fig. 1. FTIR spectrum of (a) LDPE, (b) irradiated LDPE and (c) LDPE-g-MAc.

Fig. 1 shows FTIR spectra of LDPE under different condi-

tions. 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 cm1 in the FTIR spectrum.

Spectrum a corresponding to LDPE has a very small peak at

1714 cm1. Irradiated LDPE (spectrum b) and LDPE-g-MAc

(spectrum c) exhibit sharp peaks at 1714 cm1

indicating pres-ence of carboxyl group. However, intensity of peak for LDPE-g-

MAc 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 cm1 in

spectrum of LDPE-g-MAc is dueto 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 theweight 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 cm1 which indicates that LDPE-g-MAc

Table 2

Acid value and melt flow index of LDPE samples

Polymer Acid value, mg of KOH/g Melt flow index g/10min

(113 C, 2.16kg)

LDPE 0.6 2.633

Irradiated LDPE 0.6 1.710

LDPE-g-MAc 23.0 1.096

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 unexposedLDPE, 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. How-

ever, the effect is small as the grafting levelis very low. Exposure

to -radiation does not affect the melting point and crystalliza-

tion temperature of LDPE. Cooling thermogram shows crystal-

lization 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., 105C) and degradation temperatures

(i.e., 410 C) of LDPE-g-MAc makes this resin suitable forflame

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

Fig. 3. DSC curves of (a) LDPE, (b) irradiated LDPE and (c) LDPE-g-MAc.

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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 polarCOOH group.

The extent of crystallization of LDPE and LDPE-g-MAc can

be seen in optical micrographs presented in Fig. 5(a and b), re-

spectively. 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 dueto 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

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.

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-g-

MAc, 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. Re-

sults 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.

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Fig. 8. SEM of flame sprayed (a) LDPE and (b) LDPE-g-MAc.

Table 3

Corrosion resistance of flame sprayed coatings

Polymer Performancea (after 13 weeks)

Salt spray Seawaterimmersion

Humidity

LDPE 4 2 4

LDPE-g-MAc 7 7 8

LDPE + Red iron oxide 5 7 7

LDPE-g-MAc + Red iron oxide 7 9 9

a 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 LDPE-

g-MAc are lesser in number and small in size than LDPE. Thismay 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 car-

boxyl 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 cor-

rosion 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, Direc-

tor for his keen interest, constant encouragement and for giv-

ing permission to publish this paper. The authors also thank

to Dr. P.C. Deb, Emeritus Scientist, Naval Materials ResearchLaboratory 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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