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Development of hyperbranched polyester polyol-based waterborne anticorrosive coating.

Abstract Water-soluble hydroxyl functionalized hyperbranched polyester (WH-HP) resin was synthesized using pentaerythritol, pyromellitic dianhydride, phthalic anhydride, and 1,1,1-tris(hydroxymethyl) propane. The synthesized WH-HP was characterized by spectroscopic (FTIR and NMR) as well as thermal (TGA and DSC) analysis. WH-HP and hexa methoxy methyl melamine (HMMM) resins were mixed in different mole ratios (1:2.5, 1:5, 1:7.5, and 1:10), coated on mild steel panel, and cured by baking at 110[degrees]C for 1 h. These coating compositions were evaluated for their mechanical and anticorrosive properties. It was observed that coating composition having 1:5 mol ratio (WH-HP:HMMM) has shown very good mechanical and anticorrosive properties compared to the rest.

Keywords Waterborne, Hyperbranched polymers, Acid value, Anticorrosive coatings

Introduction

Application of organic coatings is the most common and economical method to protect metallic structures from corrosion. They consist of polymeric resins, pigments, additives, and solvents. Basically, solvents are used to reduce the viscosity and increase the solubility of paint for application. During drying/curing of paints, solvents, which constitute volatile organic compounds (VOC), are released into the atmosphere, resulting in environmental pollution. The environmental hazards associated with VOCs have led the government regulations restricting them. The options available for reducing VOCs are powder coatings, waterborne coatings, UV-curable coatings, and high solid coatings. Efforts continue to decrease the amount of volatile organic compounds (VOC) present in conventional solventborne coatings. (1-8) Use of water as solvent or thinner is the recent trend in the field of organic coatings, and several waterborne paints for decorative purpose have been developed. (9,10) However, there is a strong need to develop VOC-free paints which can be used for industrial coating applications to provide chemical resistance and anticorrosive properties.

In the field of polymer science, hyperbranched/dendritic polymers have gained much attention due to their extraordinary properties. The dendritic polymers mimic the hydrodynamic volume of spheres in solution and shows low viscosity in the melt, even at high molecular weight, due to the lack of restrictive interchain entanglements. (11-14) For coating applications, this special relationship between molecular weight and viscosity should be highly useful in terms of the environmental issues and cost in order to formulate low VOC coatings. Hyperbranched alkyds (HBAs) have been reported as promising resins for development of low VOC coatings. Chemical modifications can be easily carried out on them for generating watersoluble hydrophilic salt-like structure. Recently, Naik et al. (15) reported the preparation of hyperbranched urethane alkyd polymer for formulating low VOC weather-resistant coatings. Bat et al. (16) reported the synthesis of hyperbranched air drying fatty acid-based resin having good flexibility, abrasion resistance, and adhesion to metal substrate. Johansson et al. (5) reported a high solid coating formulation using highly branched polymers which exhibit higher solubility and low melt viscosity compared to their linear counterparts. Dutta et al. (17) developed a low-cost stoving paint containing oil-based polyester and melamine resin with better mechanical and corrosion resistance properties. Allauddin et al. (18) reported the synthesis and characterization of melamine-cured hyperbranched polyester-epoxy hybrid coating. They observed that addition of HMMM and epoxy groups increased flexibility and toughness of the coating films. These resins are expected to provide good corrosion protection in marine environment when used in protective paint formulations due to their higher number of functional groups and branched structure. Therefore, hyperbranched polymers are considered as promising resin materials in reducing VOCs of the paint formulations. (19-23)

Despite the availability of many polyol-based waterborne paint formulations, not many reports are available on the use of them for anticorrosive applications. In the present work, a second generation hydroxylfunctionalized hyperbranched polyester (WH-HP) resin was synthesized and characterized. Waterborne coating was formulated with synthesized WH-HP and HMMM and their mechanical and corrosion resistance properties were studied in detail.

Materials and experimental methods

Materials

Pentaerythritol (PE), pyromellitic dianhydride (PDA), and 1,1,1 tris(hydroxyl methyl)propane (TMP) were purchased from Sigma Aldrich, India. Phthalic anhydride (PA) and p-toluene sulphonic acid (pTSA) were procured from Merck, India. Hexa methoxy methyl melamine resin (HMMM, Cymel 303LF) was received from Cytec Industries, India. All the chemicals were used as received without any further purification.

Synthesis of polyester polyol (PP)

The procedure for the synthesis of polyol from pyromellitic dianhydride with pentaerythritol was described by Kanai et al. (24) Pyromellitic dianhydride (0.2 mol), pentaerythritol (0.8 mol), pTSA (0.003 mol), and DMF (100 ml) were well mixed in a four-necked round bottom flask equipped with nitrogen inlet, Dean & Stark assembly, thermometer, and mechanical stirrer. This reaction mixture was heated slowly to 200[degrees]C. The progress of reaction was monitored by checking the acid value and the amount of water collected. The reaction mixture was then cooled to room temperature and dissolved in acetone and methanol mixture (1:1). The product was precipitated by addition of sufficient amount of dried petroleum ether. (25) The solid product was filtered and dried under vacuum at 60[degrees]C. The reaction scheme is shown in Fig. 1. The hydroxyl value of polyester polyol is 927.

Synthesis of carboxyl-functionalized hyperbranched polyester (C-HP)

Synthesis of C-HP was carried out by reacting the polyol (0.045 mol) with phthalic anhydride (0.54 mol), pTSA (0.003 mol), and xylene (50 ml) in a four-necked round bottom flask fitted with nitrogen inlet, thermometer, and mechanical stirrer. The mixture was heated slowly to 140[degrees]C. The reaction was monitored by observing the disappearance of anhydride peak by FTIR spectroscopy. The reaction was stopped by cooling the reaction mixture to room temperature and purified by dissolving in acetone and methanol mixture (1:1). The product was precipitated by addition of sufficient amount of hexane, filtered, washed, and dried. The reaction scheme is shown in Fig. 2. The acid number of C-HP is 224.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Synthesis of water-soluble hydroxyl-functionalized hyperbranched polyester resin (WH-HP)

Synthesis of hydroxyl-functionalized hyperbranched polyester resin was carried out by using C-HP (0.04 mol), l,l,l-tris(hydroxylmethyl)propane (TMP) (0.53 mol), pTSA (0.003 mol), and xylene. The reaction mixture was taken in a four-necked round bottom flask mentioned in "Synthesis of polyester polyol (PP)" section. The reaction mixture was heated slowly to 140[degrees]C. FTIR spectroscopy was employed to monitor the progress of reaction. The reaction was stopped after the disappearance of the acid peak in FTIR spectroscopy and acid value close to zero. The reaction scheme is shown in Fig. 3. The hydroxyl value of WHHP is 327.

[FIGURE 3 OMITTED]

Preparation of coating compositions

WH-HP resin containing 45% solid was prepared by slowly adding butyl cellosolve as a co-solvent and sufficient amount of preheated (70[degrees]C) deionized water into 100% solid WH-HP resin at about 90[degrees]C for 15-20 min under stirring. After addition of water, stirring was continued for 90 min more. The prepared WH-HP contains 45% solid (resin), 2% butyl cellosolve, and rest is deionized water (53%). The particle size of WHHP resin is 0.4-6 [micro]m and it has 6 month stability under ambient storage conditions. Clear coating compositions were prepared by mixing WH-HP (45% solid) with HMMM resin in different mole ratios (1:2.5, 1:5, 1:7.5, and 1:10) at room temperature. The coating composition was applied on burnished mild steel panel by brush and after 15 min flash-off time at ambient conditions, specimens were subjected for baking at 110[degrees]C for 1 h. The baked specimens were left at ambient conditions for 24 h and were evaluated for their mechanical and corrosion resistance properties. The coating thickness is 75 [+ or -] 10 [micro]m.

Characterization

Acid value

Acid value of intermediates & WH-HP was determined as per ASTM D 1639-90 by titrating resin sample against standard alcoholic KOH using potassium hydrogen phthalate as a primary standard. Acid value (A) was calculated using following equation:

A= VK/S x N (1)

where V is volume of KOH solution required for titration of the specimen (mL), K is weight of KOH per milliliter of KOH solution (mg), S is specimen weight (g), and N is nonvolatile content of the material expressed as a decimal fraction.

Characterization of resins

FTIR spectra of compounds were recorded using Thermo Electronic Corporation, Model-NICOLET6700 spectrophotometer. FTIR spectra of solid samples were recorded by making sample-KBr pellets and the liquid samples were recorded by coating on KBr disc. The FTIR spectra were recorded in the 400-4000 [cm.sup.-1] range with a resolution of 4 [cm.sup.-1] and 16 scans per spectrum. 3H and [sup.13]C NMR spectra were recorded using Bruker 500 MHz nuclear magnetic resonance spectrometer.

Melting point of compounds was determined using TA Instruments, Model-Q100 differential scanning calorimeter. Measurements were carried out at a heating rate of 10[degrees]C/min under nitrogen atmosphere. Thermal stability of polymers was studied using TA Instruments, Model-Hi-Res. TGA 2950 thermogravimetric analyzer at a heating rate of 10[degrees]C/min under nitrogen atmosphere up to 800[degrees]C.

Physical and mechanical properties of coated films were studied by various tests: (i) adhesion strength by pull-off method (IS: 101-Part 5/Sec 2), (ii) scratch resistance (BS 3900), (iii) impact resistance (ASTM D 2794), (iv) flexibility-bend test (ASTM D 522), (v) solvent resistance (ASTM D5402-06).

Gel content

The gel content of the WH-HP coatings was evaluated as per ASTM D 2765-01. WH-HP coating films were weighed and then immersed in the extracting solvent (xylene) at 110[degrees]C for 1 h. After the extraction, the remaining WH-HP coating films were removed, dried, and reweighed. The gel content was determined using the following equation:

Gel content = [W.sup.1]/W x 100, (2)

where W is the initial weight of the sample (g) and [W.sub.1] is the weight of the sample after extraction (g).

Gel permeation chromatography (GPC) analysis

The relative values of weight average ([M.sub.w]) and number average ([M.sub.n]) molecular weights of the PP, C-HP, and WH-HP resins were determined using Waters Instruments, Model 2690, USA GPC with styragel column and Waters 2410 as refractive index detector. The samples were dissolved in tetrahydrofuran and the run was performed at 30[degrees]C. Linear polystyrene was used as a calibration standard.

Particle size analysis

WH-HP polymer was completely dispersed in water containing a drop of surfactant (Triton X-100) and particle size was measured using Master Sizer 2000, Malvern particle size analyzer.

Resistance to corrosion under different exposure conditions

The anticorrosive properties of developed coatings were assessed by evaluating their resistance to corrosion in salt spray and humidity cabinet as per methods described in ASTM D1654 and ASTM D1748-10. The coated and edge-sealed mild steel panels of size (150 mm x 100 mm x 1.5 mm), were exposed to salt spray test and humidity cabinet.

Electrochemical impedance spectroscopy (EIS)

EIS has been used to study the corrosion mechanism of developed coatings. The EIS measurement was carried out using the Gamry CMS 300 electrochemical impedance system. A three electrode cell assembly having coated panel (area 22.5 [cm.sup.2]) as working electrode, large Pt mesh as counter electrode, saturated Calomel electrode as reference electrode, and 100 ml 3.5% NaCl solution as electrolyte was employed. A sine wave of 10 mV (r.m.s) was applied across the cell. The measurements were made in the frequency range of 50 kHz-0.02 Hz. EIS study was carried out on samples after different exposure intervals at ambient temperature.

Results and discussion

Polyester polyol (PP) was synthesized and further modified with phthalic anhydride to develop carboxylic acid-functionalized hyperbranched polyesters (C-HP). Second generation water-soluble hydroxyl-terminated hyperbranched polyester (WH-HP) was prepared by reacting C-HP with TMP. The products were characterized by FTIR, [sup.1]H and [sup.13]C NMR spectroscopy, and GPC analysis.

FTIR analysis

The FTIR spectrum of C-HP is presented in Fig. 4. The reduction in the absorption value of hydroxyl stretching band at 3440 [cm.sup.-1] is due to the reaction of hydroxyl group in the polyol with phthalic anhydride and also the formation of carboxylic acid and ester groups. The product formation is also confirmed by a broad stretching band at 1746 [cm.sup.-1] which is related to the ester group. The broadness of this peak may be due to the presence of more than one stretching frequency. To separate and identify the peaks, the peak at 1746 [cm.sup.-1] was deconvoluted using G/L + G method. The deconvoluted spectrum (Fig. 5) shows two distinct peaks at 1749 and 1694 [cm.sup.-1] which are mainly due to ester and carboxylic acid groups, respectively. This confirmed the presence of the carboxyl acid groups in the synthesized product. FTIR spectrum of synthesized WH-HP using C-HP and TMP (Fig. 4) shows the reappearance of broad hydroxyl stretching band at 3440 [cm.sup.-1] indicating the formation of hydroxyl-terminated polyester. Formation of WH-HP was also assessed by measuring its acid value. The calculated acid value for WH-HP was below 10 indicating the reduction of acid groups in WH-HP.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[sup.1]H and [sup.13]C NMR analysis

In order to further confirm the polymer structure, [sup.1]H and [sup.13]C NMR techniques were employed. [sup.1]H and [sup.13]C NMR of C-HP are not recorded due to poor solubility in CD[Cl.sub.3] and DMSO. The [sup.1]H NMR spectrum of WH HP is shown in Fig. 6. The resonance frequency at 7.95-8.1 ppm is due to the aromatic proton of both pyromellitic anhydride and phthallic anhydride. The presence of -OC[H.sub.2]- group is noticed at 4.4 ppm resonance frequency which is from pentaerythritol moiety. The resonance peak at 3.2 ppm is assigned to methylene group of pentaerythritol moiety. The peak between 0.8 and 1.3 ppm is due to the presence of aliphatic group (-C[H.sub.2]C[H.sub.3]) in TMP. [sup.13]C NMR spectrum of WH-HP along with the peak assignments is shown in Fig. 7. The spectrum shows different peaks corresponding to different distinguishable carbon atoms, indicated in the structure. The peaks at 170-175, 130-135, 65, 38-43, 23, and 10 ppm are due to --COO--, aromatic ring carbons, -C[H.sub.2]O-, tertiary carbon, -C[H.sub.2], and -C[H.sub.3] groups. The above evidence confirms the formation of WH-HP polymer and also shows the presence of pentaerythritol, pyromellitic anhydride, and phthallic anhydride moieties in WH-HP polymer molecule.

Thermal properties of films

Figure 8 shows the DSC thermograms of polyol (PP), C-HP, and WH-HP-hyperbranched resin with heat flow direction. The glass transition temperature ([T.sub.g]) of polyol and C-HP was calculated by double tangent method and found to be 59.02 and 57.13[degrees]C. The lower [T.sub.g] value for C-HP compared to polyol is due to the partial hydrogen bonding between carboxylic acid groups present in C-HP. Also the molecular weight of C-HP is higher than the polyol indicating more amorphous nature of C-HP compared to polyol which is also cause for lowering of [T.sub.g] of C-HP. On reacting C-HP with TMP for the synthesis of WH-HP, [T.sub.g] further reduces to 23.14[degrees]C indicating the presence of a large number of methylene groups in main and side chains of polymer and also an increase in molecular weight. The higher flexible ester linkages (ester content) in WH-HP than C-HP increased localized segmental mobility, thus lowering [T.sub.g] of the molecule. The thermal stability of WH-HP was analyzed by thermogravimetric analysis and found to be stable up to 350[degrees]C (Fig. 9).

Gel permeation chromatography (GPC) analysis

Table 1 shows the values of number average molecular weight ([M.sub.n]), weight average molecular weights ([M.sub.w]), and polydispersity index (PDI) of PP, C-HP, and WH-HP resins. The results show that the polydispersity index of resins varies from 2.5 to 3.2. The polydispersity index is much higher than the theoretical value of corresponding dendrimers (PDI = 1). However, PDI is lower compared to the linear polymers (PDI = 4-5). (15,26) The molecular weights of PP, C-HP, and WH-HP resins were determined using linear polystyrene as the standard and GPC measurements depend on the radius of gyration. So, the exact values might be higher than the observed molecular weights for the PP, C-HP, and WH-HP resins.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Gel content

Coating films were prepared by curing WH-HP polymer with HMMM in different mole ratios and gel content of the films was estimated. It is observed that the gel content of the cured films with mole ratio of 1:5 was found to be higher than other compositions (Table 2). In the 1:5 molar ratio, -OC[H.sub.3] functional group in HMMM completely reacts with OH group of WH-HP and forms high molecular weight crosslinked networks. This results in high gel content, whereas, at mole ratio above 1:5, HMMM is excess. Hence, OH of WH-HP may not be available for all the reactive functional groups in HMMM. Few of the sites may be left untreated and some other may undergo self-condensation leading to the formation of smaller networks, thus leading to lower gel content. (27,28)

[FIGURE 9 OMITTED]

Particle size distribution of WH-HP polymer

Particle size distribution of WH-HP was determined by dispersing them in aqueous media. Figure 10 shows the particle size distribution of WH-HP resin. The particle size ranges from 0.4 to 10 [micro]m. However, most of the particles fell in the size range around 4-6 [micro]m. Significant amount of WH-HP particles with particle size less than 1 pm was also observed.

[FIGURE 10 OMITTED]

Mechanical properties

Adhesion, impact, flexibility, and scratch resistance properties of WH-HP-HMMM coating film with varying mole ratios were tested and the respective results are given in Table 2. Addition of HMMM to WH-HP improved coating adhesion, and the maximum adhesion was noticed for a composition containing WH-HP-HMMM with the ratio of 1:5. The increased adhesion of this coating is due to the great affinity of large number of polar groups present in the WH-HP polymer, towards the polar metal substrate. At mole ratio above 1:5, the excess HMMM forms low molecular weight -OC[H.sub.3]-terminated networks due to nonavailability of OH groups in WH-HP. Due to low molecular weight chains, the formed films are brittle in nature as well as have poor adhesion and mechanical properties. Also the excess HMMM undergoes self-condensation and contributes to the poor mechanical properties of the film. (27,28) The above evidences are supported by the high impact resistance of the coating film containing WH-HP-HMMM in the ratio of 1:5 compared to other compositions. This coating film also showed excellent scratch resistance compared to the other compositions which also confirms higher crosslink density, resulting from curing of WH-HP and HMMM mixture.

Solvent resistance

Solvent resistance of cured WH-HP-HMMM mixture coating films were tested against various solvents as per ASTM D5402-06 and presented in Table 3. It is observed that all coating films showed good resistance towards polar, aromatic, and nonpolar solvents due to high crosslink density which is evident from the high gel content values (Table 2). Among the coating compositions, the coating containing WH-HP-HMMM in the ratio of 1:5 showed best resistance to all polar as well as nonpolar solvents compared to the other compositions. In coating compositions above 1:5 mol ratio, the unreacted HMMM reduces the solvent resistance of the coating films by leaching from the coating.

Anticorrosive properties

Anticorrosive properties of WH-HP and HMMM mixture coating were evaluated by following two methods.

Accelerated corrosion test

Anticorrosive properties of developed coatings were evaluated as per ASTM D1748-10 and ASTM D1654 by exposing to humidity cabinet as well as salt spray chamber and the respective results are presented in Tables 4 and 5 and Fig. 11. After 300 h of exposure to humidity cabinet, first blistering spots were noticed on mild steel panel coated with WH-HP-HMMM mixture in the ratio of 1:5. At lower HMMM content (2.5 mol ratio), the coated panel showed blistering and corrosion spots after 150 h of exposure. In the coating composition with mole ratio above 1:5, the blistering and corrosion spots were noticed after 225 h. To support the above evidences, the mild steel panels coated with mixture of WH-HP-HMMM were exposed in a salt spray chamber and the results are given in Table 5. The intact film of panel coated with 1:5 mol ratio of WH-HP-HMMM mixture showed neither corrosion nor coating defects on the main body of the panel and free from corrosion up to 600 h in salt spray exposure conditions. However, rusting was observed at cross scribe but it had not spread, suggesting very impressive performance of the coating under saline conditions, whereas spreading of rusting at cross scribe after 600 h for 1:2.5, 1:7.5, and 1:10 coating compositions was observed. This is attributed to the good adhesion and high crosslink density of the coated film. At higher HMMM content, formation of low molecular weight polymer chains takes place. This results in the ingress of water in the coated film and subsequent reduction in the corrosion resistance of coating.

Electrochemical impedance spectroscopy (EIS)

EIS technique was employed to obtain quantitative information about the protective nature of the coatings. Experiments were conducted on coated panels after 1, 5, 10, 15, 20, 25, and 30 days of exposure. To analyze the EIS plots of different coatings, two time constant equivalent circuit model shown in Fig. 12 (inset) was used. [R.sub.s], [R.sub.p], and [R.sub.po] are the solution, polarization, and pore resistances. Cc and Cd| are the capacitance of coating and double layer. The experimental and the simulated results are in good agreement with each other (Fig. 12).

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

The polarization resistance calculated from the Nyquist plots was plotted with respect to exposure periods for various coatings and shown in Fig. 13. The [R.sub.p] of all the coatings after 1 day immersion were close to [10.sup.9] [OMEGA] [cm.sup.2]. On increasing the immersion period, [R.sub.p] of all the compositions decreased. However, the decrease in [R.sup.p] was deep for 1:2.5 and 1:10 compositions. Bierwagen reported (29) that polarization ([R.sub.p]) value for an excellent coating lies between [10.sup.9] and [10.sup.10] [OMEGA] [cm.sup.2], whereas for a good coating it lies between [10.sup.6] and [10.sup.9] [OMEGA] [cm.sup.2]. The coating is considered poor if [R.sub.p] value is less than [10.sup.6] [OMEGA] [cm.sup.2]. The initial Rp value of 1:2.5 mol ratio coating was 3.3 x 109 11 [cm.sup.2] and it reduced to 5.2 x [10.sup.6] H [cm.sup.2] after 20 days of exposure and further decreased to 4.1 x [10.sup.5] [OMEGA] [cm.sup.2] after 30 days of exposure, which indicates that coating has lost its protective nature after 30 days of exposure.

The [R.sub.p] value of 1:7.5 mol ratio coating was found to be 5.2 x [10.sup.7] [OMEGA] [cm.sup.2] after 30 days' exposure. The gradual decrease in [R.sub.p] value for this coating during exposure period may be attributed to the loss of barrier properties due to permeation of electrolyte. However, the [R.sub.p] values for coating 1:5 mol ratio composition show lesser decrease compared to the rest of the composition even after 30 days of exposure indicating that this coating will be suitable for long-term corrosion protection.

The capacitance of coating is a measure of water permeation into the coating. The capacitance of the coating increases on water permeation due to the charge developed at the coating electrolyte interphase. The water uptake of coating in terms of volume fraction of water absorbed (W) was calculated from coating capacitance (Cc) using following equation (30):

W = log [[C.sub.1]/[C.sub.0]]/log 80 (3)

where [C.sub.t] is coating capacitance after time "r" and [C.sub.0] is coating capacitance after 5 days of exposure. The dielectric constant of water is 80. The relative volume fraction of water absorbed by the various WH-HPHMMM coatings is plotted against the immersion period and shown in Fig. 14. It is observed that water uptake of WH-HP-HMMM coating with mole ratios of 1:5, 1:7.5 is less compared to the rest of the compositions. The lesser water uptake of 1:5 mol ratio composition may be attributed to the formation of high crosslinked intact film on metal substrate resulting in enhancement of barrier property of the WH-HP-HMMM coating.

Conclusions

Water-soluble second generation hyperbranched resin was synthesized and characterized by FTIR and NMR spectroscopy. Glass transition temperature and thermal stability of polymer showed formation of watersoluble hydroxyl-functionalized hyper branched polymer (WH-HP). Coating prepared using WH-HP resin and HMMM in the mole ratio of 1:5 showed good adhesion, mechanical, and anticorrosive properties compared to the rest of the compositions. The excellent solvent and corrosion resistance shown by this composition (1:5) is due to the higher crosslink density and the great affinity of large number of polar groups present in WH-HP towards the polar metal surface. The developed polymer can have application in the area of eco-friendly waterborne decorative as well as anticorrosive coatings.

DOI: 10.1007/s11998-015-9720-1

A. P. Singh, C. Suryanarayana, R. Baloji Naik, G. Gunasekaran ([mail])

Naval Materials Research Laboratory, Shil-Badlapur Road,

P.O. - Anandnagar, Ambernath 421506, India

e-mail: gunanmri@gmail.com

Acknowledgment Authors thank Dr. R. S. Hastak, Director, NMRL for his valuable suggestion and also permission to carry out this work. We would also like to thank Mr. N. G. Malvanker, NMRL for his help in testing the samples.

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Table 1: Molecular parameters of PP, C-HP, and WH-HP resins

Average molecular weight               PP       C-HP     WH-HP

[M.sub.n]                              279      886      1242
[M.sub.w]                              698      2482     3976
Polydispersity ([M.sub.w]/[M.sub.n])   2.5      2.8      3.2

Table 2: Properties of WH-HP-HMMM coating

Tests performed                               WH-HP/HMMM mole ratio

                                           1:2.5   1:5    1:7.5   1:10

Gel content                                85      95     90      85
Adhesion strength (Pull-off method)        10.8    13.5   11.8    11.2
MPa (IS: 101-Part 5/Sec 2)
Scratch hardness (tolerance wt in kg)      2.1     2.5    2.2     2
  (BS 3900)
Impact resistance (1 kg/m) (ASTM D2794)    Fail    Pass   Pass    Fail
Flexibility (Bend test, by 1/4"), ASTM     Fail    Pass   Pass    Fail
  D522

Table 3: Solvent resistance of WH-HP-HMMM coating

Solvents                      Number of double rubs (50)
                           (lifting and softening of film)

                            1:2.5    1:5    1:7.5    1:10

Butanol                     No       No     No       No
Methyl ethyl ketone (MEK)   Yes      No     Yes      Yes
Xylene                      No       No     No       No
Toluene                     No       No     No       No
Hexane                      No       No     No       No

Table 4: Evaluation of anticorrosive properties of WH-HP-HMMM
coating using humidity cabinet test

Time          Resistance to corrosion (Humidity cabinet)
period (h)            Rating: 0-Severe corrosion
                    10-no corrosion/no blistering

             1:2.5        1:5          1:7.5        1:10

75           10           10           10           10
150          9            10           10           10
225          8            10           9            9
300          7            9            8            8

Table 5: Evaluation of anticorrosive properties of WH-HP-HMMM
coating using salt spray test

Time            Resistance to corrosion (Salt spray)
period (h)           Rating: 0-Severe corrosion
                   10-no corrosion/no blistering

             1:2.5        1:5          1:7.5        1:10

150          10           10           10           10
300          9            10           10           10
450          8            10           9            9
600          8            10           9            8
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Author:Singh, Ashish Pratap; Suryanarayana, C.; Naik, R. Baloji; Gunasekaran, G.
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Jan 1, 2016
Words:5372
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