novel moisture-cured hyperbranched urethane alkyd resin for caoting application

5/20/2018 Novel Moisture-cured Hyperbranched Urethane Alkyd Resin for Caoting Application

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Novel moisture-cured hyperbranched urethane alkyd resinfor coating application

R. B. Naik, N. G. Malvankar, T. K. Mahato,

D. Ratna, R. S. Hastak

American Coatings Association 2014

Abstract A hyperbranched polyol (HBP) was syn-thesized using dipentaerythritol as a core material and2,2-bis(methylol) propionic acid as a chain extender.This was reacted with varying concentrations of soyafatty acid to make hyperbranched alkyd (HBA) resins.The HBA resins containing unreacted hydroxyl groupswere reacted with isophorone diisocyanate at NCO/OH ratio of 1.6:1 to make high solid hyperbranchedurethane alkyd (HBUA) resins. The excess NCOcontent in the HBUA resins was used to cure withatmospheric moisture, and thus moisture-cured HBUAcoatings were formed. The resins were characterizedby FTIR, and 13C NMR spectroscopic analysis. Aseries of such resins were made using different fatty

acid/isocyanate ratios with respect to the hydroxylgroups present in the HBP. The effect of compositionson the mechanical and weathering properties of thecured resins was investigated. It was observed thatthere was an optimum fatty acidisocyanate ratio interms of the requirements of solvent, mechanical andweathering properties of the resin. The requirement ofsolvents for formulating HBUA coatings is much lowercompared to linear alkyd-based coatings. The presentstudy reveals that the moisture-cured HBUA resinscan be used as a binder material in the field of low-pollution weather-resistance coatings.

Keywords Hyperbranched polymer, Fatty acid,Isophorone diisocyanate, Hyperbranched urethanealkyd, Coating

IntroductionThe recent trend in the field of organic coatings is todevelop coatings with low levels of volatile organiccompounds (VOCs).17 The VOCs are added to acoating formulation to reduce the viscosity of resin foreasy processing and uniform application. After appli-cation of the coating, the VOCs evaporate out causingenvironmental pollution. There has been a consider-able effort in the past years to reduce the VOC contentin organic coatings. The environmental hazards asso-ciated with VOCs have led to the government regula-tions restricting them and publicfeeling against them.The options for reducing VOCs2 are powder coatings,

thermal spray coatings, waterborne coatings, UV-curable coatings, and high solids coatings. Dendritic/hyperbranched polymers offer an alternative way toreduce VOCs. Over the last few years, syntheses of alarge number ofdendritic polymers have been reportedin the literature.8,9 They have potential applications ina variety of fields such as commercial coatings,biomedical, foams, catalysis, electrical materials, andso on.10 However, actual exploitation of dendriticpolymer technology is still in its infancy, especially inapplications such as organic coatings where largeamounts are required, due to the synthetic difficultiessuch as retaining solubility of the growing polymer and

maintaining regularity and order in the structure,which makes the technology extremely costly.Recently, dendritic polymers have been produced

by a new low cost hybrid synthetic process thatgenerates highly branched, polydisperse molecules.10

These materials are called hyperbranched polymers todistinguish them from more perfect monodispersematerials.11,12 Due to the compact three-dimensional(3D) structure of dendritic polymers, these moleculesmimic the hydrodynamic volume of spheres in solutionand also show low viscosity in the melt, even athigh molecular weight, due to the lack of restrictive

R. B. Naik (&), N. G. Malvankar, T. K. Mahato,D. Ratna, R. S. HastakNaval Materials Research Laboratory, Shil Badlapur Road,P.O. Anand Nagar, Additional Ambernath (East) 421506Dist-Thane, Maharashtra, Indiae-mail: [email protected]

J. Coat. Technol. Res., 11 (4) 575586, 2014

DOI 10.1007/s11998-013-9561-8

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interchain entanglements.1316 For coating applica-tions, this special relationship between molecularweight and viscosity should be highly useful in termsof the environmental issues and cost in order toformulate low VOC coatings.

Hyperbranched alkyds (HBAs) have been reportedas promising resins for development of low VOCcoatings. Bat et al.17 reported the synthesis of hyper-

branched air drying fatty acid-based resin having goodflexibility, abrasion resistance, and adhesion to metalsubstrate. Johansson et al.5 reported a high solidcoating formulation using highly branched polymerswhich exhibit higher solubility and low melt viscositycompared to their linear counterparts. Pettersson18

observed that HBAs have significantly lower viscositiesand rapid air drying properties. The coatings based onunmodified alkyd resins show considerable chalking,fading, and loss of gloss within a period of 1012 months in tropical climates due to intense UVradiation, thermal fluctuations, high humidity, andwind-driven salt spray. A number of modification

processes for alkyd resins with different individualmonomers like vinyl, acrylic, silicone, urethane, etc.,have been reported in the literature.19 Polyure-thanes2026 are known for their excellent physical andmechanical properties. A recently published review27

indicates the importance of introducing hyperbranchedpolyurethane for finetuning properties of polyurethanefor high performance coating applications. But, amajor drawback of polyurethane-based resins is thehigh cost of isocyanate and high viscosity of resin. Theproblems can be overcome by modifying the urethanewith alkyd.

In the present study, attempts have been made tosynthesize NCO-terminated HBA urethane resins

with high solid content. The excess NCO was curedwith atmospheric moisture to get moisture-curedhyperbranched urethane alkyd coatings. The synthesis,characterization of urethane alkyd resin, and its prop-erties will be discussed in the present paper.

Experimental

Materials

Soya oil (M/s Jayant Oil Mill, India), sodium hydrox-

ide, sodium chloride, anhydrous sodium sulfate, andhydrochloric acid (S.D. Fine Chem., India) were usedto produce the soya fatty acid (SOFA). 2,2-Bis(meth-ylol) propionic acid (BMPA) and dipentaerythritol(DPE) were procured from Aldrich, Germany. p-Toluene sulfonic acid (Merck, India) was used as acatalyst. Xylene (High Purity Laboratory ChemicalsPvt. Ltd., India) was used as an azeotropic solvent.Methanol and potassium hydrogen phthalate (Aldrich,India) were used for acid value determination. Cobaltoctoate and lead naphthenate (Globe Products, India)were used as driers. Dibutyl tin dilaurate (DBTL) and

isophorone diisocyanate (IPDI) were obtained fromAldrich, USA. All the chemicals were used as receivedwithout any further purification.

Preparation of fatty acid

Soya oil (900 g) was taken in a four-necked round

bottom flask fitted with a mechanical stirrer, a con-denser, and a thermometer. 500 ml NaOH solution wasprepared by dissolving 120 g of sodium hydroxide in amixture of water/methanol (1:1 v/v) and added slowlyunder stirring to the reaction mixture. After completeaddition, the reaction mixture was heated at 7580Cfor 3 h, and the reaction was continued until ahomogeneous mixture was formed. Thereafter, thereaction mixture was allowed to cool down to the roomtemperature. The reaction mixture was poured into asaturated NaCl solution and washed with water toremove glycerol and monoglyceride (formed duringthe reaction). Then, dry sodium sulfate was added to

remove the trace amount of water. The reactionscheme for the formation of SOFA from soya oil isshown in Fig.1.

Synthesis of hyperbranched polyol (HBP)

The procedure of synthesis of HBP from DPE whichwas used as the core molecule andBMPA as the chainextender described by Bat et al.17 was followed in oursynthesis. For the synthesis of second generation HBP,a mixture of DPE (38 g, 0.15 mol) and BMPA (362 g,2.7 mol) was taken in a four-necked 1000 ml roundbottom flask fitted with a mechanical stirrer, a con-

denser attached with a Dean and Stark apparatus, anda thermometer. Xylene (20 ml) was added as an

Fig. 1: Synthesis of soya fatty acid


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azeotropic solvent to remove the water formed duringthe esterification reaction. The reaction was carried outat 140C under nitrogen atmosphere, using p-toluenesulfonic acid as a catalyst. The reaction was continueduntil a theoretical amount of water was collected andacid value reached below 10 mg KOH/g sample. Thereaction scheme for the synthesis of the HBP is givenin Fig.2.

Synthesis of hyperbranched alkyd (HBA)

HBAs with different oil lengths and different hydroxylvalues were prepared from HBP and fatty acid with avarying concentration of free hydroxyl groups (10%,20%, 30%, 40%, and 50%) at the outer periphery ofHBP. The procedure to synthesize the HBA having30% free hydroxyl group is described below. A mixtureof HBP (52 g, 1 mol) and fatty acid (112 g, 16.8 mol)was taken in a four-necked round bottom flask fittedwith a mechanical stirrer, a condenser, a thermometer,and an inlet for nitrogen. 20 ml xylene was added as an

azeotropic solvent to remove water formed during theesterification reaction. The reaction mixture washeated at 220C for 4 h, and the reaction was contin-ued until the theoretical amount of water was col-lected. The reaction mixture was then allowed to cooldown to room temperature. The condensation product(water) was removed by azeotropic distillation withxylene. The acid value of the resins was determined inorder to monitor the extent of reaction. The reactionwas stopped when the acid value of the product wasless than 10 mg KOH/g of resin. The reaction scheme

of the synthesis of the HBA (30% free hydroxyl group)is given in Fig. 3. A similar procedure was followed tosynthesize HBA resins having varying concentrationsof free hydroxyl groups with different oil lengths andnamed as HBA-0, HBA-10, HBA-20, HBA-30, HBA-40, and HBA-50 based on their free hydroxyl group onHBP and is shown in Table1.

Synthesis of hyperbranched urethane alkyd(HBUA)

HBUA resins were prepared using the above-men-tioned HBA resins. The remaining hydroxyl groups ofthe HBA were reacted with IPDI. The procedure tosynthesize the HBUA-30 is described below. TheHBUA resins were synthesized by reacting HBA withIPDIat NCO/OH ratio of 1.6:1, as described by Savitaet al.28 An HBA (164 g) was taken into a four-neckedround bottom flask fitted with a mechanical stirrer, acondenser, a thermometer, and an inlet for nitrogen gas

purge. IPDI (35.21 g) was added drop wise to HBA atroom temperature, and after complete addition of IPDI,the DBTL catalyst (0.020.04 g) was added, and themixture was heated at 5060C for 4 h. The progress ofthe reaction was monitored by FTIRspectroscopy whichindicates the decrease in peak intensity of hydroxyl andisocyanate groups at 3400 cm1 and at 2230 cm1,respectively. The reaction scheme for the synthesis ofthe HBUA-30 is given in Fig.4. Similarly, other HBUAresins were prepared by varying free hydroxyl grouppresent on HBA and IPDI and named as HBUA-0,HBUA-10, HBUA-20, HBUA-30, HBUA-40, andHBUA-50 based on their free hydroxyl group on HBA.

Characterization

Characterization of resins

Acid value

The acid value was determined as per ASTM D 1639-90 by titrating the resin sample against standardalcoholic KOH using potassium hydrogen phthalateas a primary standard. Acid value (Z) was calculatedusing the following equation:

Z 56:

1 A 0:

1B

100;

where A is the Buret reading (volume of KOH (ml))and B is the weight of sample (g).

FTIR spectroscopy

FTIR spectrophotometer (Thermo Electron Corpora-tion Nicolet 5700) was used to characterize soya oil,

Fig. 2: Synthesis of HBP


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soya oil fatty acid, DPE, BMPA, HBP, HBA, andHBUA. FTIR spectra were obtained in transmittancemode at a resolution of 4 cm1 and 32 scan. The liquidsamples of each were spread over KBr pellets, andspectra were recorded.

NMR spectroscopy

13C NMR spectra were recorded using a NuclearMagnetic Resonance spectrometer (Bruker 500 MHz).The solution of DMPA and DPE in d6-DMSO and a

Fig. 3: Synthesis of HBA

Table 1: Composition of HBA resins

S. No. Resins HBP (mol) SOFA (mol) Oil length (%) Theoretical hydroxyl value

1 HBA-0 1.0 24 75

2 HBA-10 1.0 21.6 73 16

3 HBA-20 1.0 19.6 71 304 HBA-30 1.0 16.8 68 56

5 HBA-40 1.0 14.4 64 83

6 HBA-50 1.0 12 60 117


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solution of soya oil fatty acid, soya oil, HBP, andHBUA resin in CDCl3 were prepared with tetramethyl silane (TMS) as internal standard for themeasurement of NMR spectra.

Viscosity and volume solids measurement

The viscosity of the synthesized resins without solventwas measured by Haake Rotoviscometer (Model RT

Fig. 4: Synthesis of HBUA


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20) using a cone plate sensor system. The anglebetween the cone and plate was 1, and the viscositywas measured at a fixed shear rate of 50 s1 at 27C.The volume solids of the HBUA resins were measuredas per ASTM D 2697.

Gel permeation chromatography (GPC) analysis

The relative values of weight average (Mw) andnumber average (Mn) molecular weights of the HBPand HBUA resins were determined by GPC (watersInstruments, Model 2690, USA) using a styragelcolumn and Waters 2410 as refractive index detector.The samples were dissolved in tetrahydrofuran and therun was performed at 30C. Crosslinked polystyrenewas used as a calibration standard.

Characterization of cured films

Preparation of free films

Free films of hyperbranched urethane alkyd resinswere prepared by mixing 1.25 g of Pb-naphthenate,0.5 g of cobalt octoate, and required amount of xyleneto the 100 g of each HBUA (HBUA-30, HBUA-40,and HBUA-50) resins, separately. This solution waspoured on methyl cellulose precoated glass plates, andfilms were prepared using a motorized film applicator(Model No. 335/1 Erickson) using 300 lm clearanceblade. The HBUA-coated glass plates were kept in adust-free chamber for drying of the films. After 7 days,the films were peeled off from plates, washed thor-oughly under running water to remove methyl cellu-

lose film, and dried at an ambient temperature. Theresins (HBUA-0, HBUA-10, and HBUA-20) were notconsidered for further studies due to their poor dryingand film-forming properties.

Gel content

The gel content of the HBUA coatings was evaluatedas per ASTM D 2765-01. The gel content (insolublefraction) of HBUA coatings can be determined byextracting with solvents such as toluene or xylene.HBUA coating films were weighed and then immersed

in the extracting solvent (xylene) at 110

C for 1 h.After the extraction, the remaining HBUA-coatingfilms were removed, dried, and reweighed. The gelcontent was determined using the following equation:

Gel content 1 w w1

w1

100;

wherewis the initial weight of the sample (g) andw1isthe weight of the sample after extraction (g).

Determination of mechanical properties

Tensile strength and elongation at break of the freefilms were determined as per method given in ASTMD 882-97. Test specimens of size 100 mm915 mmwere cut from film and conditioned at 50% relativehumidity for 24 h before examination using a Uni-versal Tensile Testing Instrument (Lloyd, Model-

LR30K) under a strain rate of 20 mm/min. Tenspecimens were tested for each sample, and an averageresult of the five highest readings at peak load wasreported as the tensile strength. The strain values atbreaking point were used to obtain percent elongation.

Adhesion strength

Adhesion strength was determined as per the methoddescribed in Indian Standard IS: 101 (pull-off method).The method was modified by reducing the area of onedolly to facilitate the de-bonding of coating from oneside. Measurements were carried out using a UniversalTesting Machine (Lloyd, Model-LR30K). Differentcompositions were applied onto burnished mild steelspecimens (150 mm 9100 mm 91.5 mm) by brush ata dry film thickness of 105115 lm. For each coating,ten specimens were tested for adhesion strength andthe average of five highest readings has been reportedas adhesion strength.

Accelerated weathering test

Weathering resistance of HBUA resin was evaluatedas per method described in ASTM G-53 by exposingthe resin-coated aluminum panels (150 9 75 91 mm)to UV radiation (wave length 285315 nm) and highhumidity condition in a QUV-accelerated weatherom-eter. A weathering cycle, comprising 2 h of condensa-tion (temperature 45 5C) and 4 h of UV exposure(temperature 60 5C), was maintained during thestudy. Gloss measurements (initial and after exposure)were carried out at periodic intervals as per methoddescribed in BS No. 3900 (Part D5) using a multiheadgloss meter, Novogloss (Rhopoint) at 27C.

Results and discussion

FTIR spectroscopy

The FTIR spectra of the developed resins and precur-sor materials are shown in Figs.5a5d. The FTIRspectra of soya oil and SOFA are shown in Fig. 5a. Thecarbonyl (C=O) peak appearing at 1748 cm1 corre-sponds to the presence of ester linkages. However,


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after hydrolysis and purification, the ester group wascompletely consumed and acid was generated, whichwas confirmed by the appearance of carboxylic acidgroup peak at 1704 cm1 in the FTIR spectra ofSOFA. The FTIR spectra of HBP, BMPA, and DPEare presented in Fig. 5b. The broad and intense peaksseen at 3380 and 1731 cm1 in spectra of HBP are dueto the presence of hydroxyl and ester groups formedduring the esterification reaction of BMPA and DPE,

respectively. The decrease in peak intensity of hydro-xyl group at 3380 cm1 and absence of acid group peakat 1704 cm1, in HBA spectra (Fig.5c), confirm themodification of HBP with SOFA. The absence ofhydroxyl group peak at 3380 cm1 and the presence ofpeaks at 10501100 cm1 (CN stretching vibrations),1550 cm1 (CN stretching and NH bending vibra-tions) and 3410 cm1 (NH stretching vibration fromurethane group) indicated the formation of HBUA(Fig.5d).28

NMR spectroscopy

Synthesized resins were further characterized by 13CNMR spectroscopy, and the spectra are shown inFigs.6a6d. The change in d values observed fromspectra also confirms the formation of new bonds.From the 13C NMR spectra of SOFA (Fig.6a), it isobserved that peak appearing at 178 ppm is due to thepresence of COOH group, which confirms the forma-

tion of SOFA. HBP was also characterized by 13

CNMR, and the spectra are shown in Fig. 6b. It showstwo weak peaks at 174 and 173 ppm, indicatingformation of two types of ester linkages in HBP. Theappearance of peaks at 65 ppm for terminal CH2OH,at 71 ppm for CH2OCH2, and at 47 ppm forC(CH3)(CH2OH) (CH3) confirms the formation ofHBP. The reaction between HBP and SOFA resultedin shift of COOH peak from 177 ppm for acidfunctional group to 173 ppm (Fig.6c) due to the

SOFA

SOFA

HBA

HBP HBA

IPDl

HBUA

DPE

BMPA

HBP

4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500

4000 3500 3000 2500 2000 1500 1000 500

%T

ransmittance

%T

ransmittance

%T

ransmittance

%T

ransmittance

Soya oil

3500 3000 2500 2000

Wave number (cm1)

Wave number (cm1)

Wave number (cm1)

Wave number (cm1)

1500 1000

1748 cm1

3467 cm1

3380 cm1 3467 cm1

2258 cm1

2250 cm1

1712 cm1

1743 cm1

1704 cm1

1710 cm1

1731 cm13380 cm1

(a)

(c) (d)

(b)

Fig. 5: (a) FTIR spectra of soya oil, SOFA; (b) FTIR spectra of DPE, BMPA, and HBP; (c) FTIR spectra of HBP, SOFA, andHBA; and (d) FTIR spectra of HBA, IPDI, and HBUA


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(a)

(b)

210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 ppm

210 200 190

190

180

180

170

170

173 ppm

174 ppm

178 ppm

173 ppm

160

160

150

150

140

140

130

130

120

120

110

110

100

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10 ppm

ppm

(c)

Fig. 6: (a) 13C NMR spectra of soya fatty acid, (b) 13C NMR spectra of HBP, (c) 13C NMR spectra of HBA, and (d) 13C spectraNMR of HBUA


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formation of ester linkages, which confirms the forma-

tion of HBA. The HBA was further reacted with IPDI,and the appearance of characteristic peaks for ure-thane at 174.7 and 175 ppm (Fig.6d) confirms theformation of HBUA.

Viscosity and volume solid measurement

The viscosities of HBUA resins were determined asper the method described in the earlier section onViscosity and volume solids measurement section,and the results are presented in Table 2. It can be seenthat the viscosities of all the HBUA (without solvent)resins increased with increasing concentration of IPDI.This increase in viscosity can be attributed to anincrease in inter and intra molecular interactions anddegree of hydrogen bonding between the moleculesand atoms.29 Hydrogen bonds are formed betweenmolecules containing an electronegative atom possess-ing lone pairs of electrons (usually O, N, or F) andmolecules containing covalent bonds between hydro-gen and an electronegative atom (usually OH, NH,and SH). The polarized nature of the XH bond(X = O, N) results in highly electropositive hydro-gen, which is attracted toward bond formation with the

electron rich electronegative atoms. From HBUA-0 to

HBUA-50, degree of hydrogen bonding (NHO=C)is enhanced due to the increase in the hydrogen bondforming moieties (NH and O=C) in the polymerbackbone. These H-bonds increase viscosity, becausethey increase attractive forces between molecules,making them more resistant to flow. Therefore,viscosity from HBUA-0 to HBUA-50 was increased.After analyzing the data, it can be concluded that theviscosities of HBUA resins are lower than those ofconventional alkyd resins that do not have high solidcontent, as reported by Patel et al. (values between148,000 and 186,000 mPa s).30 These HBUA resinswere diluted in xylene to get solid content in the rangeof 8595%, and the results are shown in Table 2. TheseHBUA resins were applied on mild steel specimens,and their wet and dry film thicknesses were measured.It was observed that the wet film thickness of coatedpanels was in the range of 125135 lm and the dry filmthickness of cured HBUA coatings on mild steel was inthe range of 105115 lm. The difference between wetand dry film thickness is very low (20 lm) whichindicates the high solid nature of the developedcoatings. For conventional coatings, the difference inwet and dry film thickness is more than 40 lm. Thelower difference between wet and dry film thickness

200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 ppm

174 ppm

(d)

Fig. 6: continued

Table 2: Physical properties of HBUA coatings

S. No. Resins Viscosity (mPa s) Volume solid (%) Wet film thickness (lm) Dry film thickness (lm)

1 HBUA-0 4,100 95 125 120

2 HBUA-10 5,700 93 128 115

3 HBUA-20 6,400 90 128 115

4 HBUA-30 8,500 88 130 110

5 HBUA-40 9,400 86 135 110

6 HBUA-50 10,900 85 135 115

Viscosity reported is without solvent (xylene)


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indicates the high solid nature of the developedcoatings.

Gel permeation chromatography (GPC) analysis

Table3 shows the values of number average molecularweight (Mn), average molecular weights, and polydis-persity index (PDI) of HBP and HBUA resins. Theresults give clear evidence that the polydispersity indexof HBUA resins varies from 2.5 to 3.2. The polydis-persity index is much higher than the theoretical valueof corresponding dendrimer (PDI = 1). However, PDI

is lower compared to the same linear polymers(PDI = 45). The molecular weights of HBP andHBUA were determined using linear polystyrene asthe standard, and GPC measurements depend on theradius of gyration; so, the exact values might be higherthan the observed molecular weights for the HBP andHBUA resins.

Gel content

HBUA coating films were prepared as discussed inPreparation of free films section. During the cure

process, HBUA coatings form a 3D network due to thereaction between NCO-terminated HBUA and atmo-spheric moisture. The curing reaction of NCO-termi-nated HBUA resins with atmospheric moistureinvolves a series of polyaddition reactions. Watervapor which is present in the atmosphere reacts withthe NCO-terminated HBUA, producing carbamicacid. The carbamic acid is unstable at room temper-ature and decomposes into CO2 and primary amine.The primary amine reacts with NCO-terminatedHBUA and forms crosslinked hyperbranched polyure-thaneurea coatings. The evidence of a crosslinkednetwork is apparent from the evaluation of gel contentby extraction of HBUA coatings with xylene. From the

results (Fig.7), it is seen that the gel content of theHBUA coatings has approached 90%. This is due tothe high crosslink density obtained due to optimumcuring of the developed HBUA coatings.

Mechanical properties

Table3 represents mechanical properties of the dif-ferent compositions of HBUA coatings. It can be notedthat the mechanical properties of HBUA-0, HBUA-10,

and HBUA-20 were not determined, because on curingat room temperature (27C) they do not provide dryfilms. Hence, their mechanical properties could not beevaluated. HBUA-30, HBUA-40, and HBUA-50 oncuring at room temperature give flexible films withtensile strength value in the range of 810 MPa(Table2). HBUA-40 and HBUA-50 display only

marginally higher tensile strength compared toHBUA-30. The marginal increase in tensile strengthis associated with a significant reduction in flexibility(% elongation at break). The percent elongation atbreak recorded for HBUA-30, HBUA-40, and HBUA-50 are 55%, 50%, and 45%, respectively. As seen fromViscosity and volume solid measurement section,the viscosity of the HBUA resins is increased fromHBUA-30 to HBUA-50; hence, solvent requirement toformulate the coatings will be increased from HBUA-30 to HBUA-50. The solvent requirement will be leastfor HBUA-30. A lower solvent requirement allowshigher solids. From the above discussion, it is clear thatthe HBUA-30 is considered as the optimized formu-

lation, which is further supported by the data obtainedfrom viscosities. The increase in tensile strength anddecrease in elongation of coatings may be attributed tothree factors such as increase in inter- and intramole-cular interactions, degree of hydrogen bonding, andamount of cycloaliphatic moieties responsible forincrease in hard segment content10 which makes theHBUA coatings hard to stretch. The reason for thisincrease is that more hydrogen bonds per moleculeenable the formation of strong 3D networks betweenthe HBUA molecules. Hence, the increase in tensile

Table 3: Molecular parameters of HBUA resins

S. No. Average molecular weights HBP HBUA-0 HBUA-10 HBUA-20 HBUA-30 HBUA-40 HBUA-50

1 Mn 1,077 4,005 3,918 3,467 3,158 3,863 2,904

2 Mw 2,879 11,612 10,967 10,748 10,106 9,659 9,016

3 Polydispersity (Mw/Mn) 2.7 2.9 2.8 3.1 3.2 2.5 3.1

100

75

50

25

0HBUA-30

Gelcontent(%)

HBUA-40 HBUA-50

Fig. 7: Gel content of the HBUA coatings


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strength from HBUA-0 to HBUA-50 was observed.The mechanical properties of the developed HBUAcoatings were compared with silicon soya alkyd andsilicon acrylate soya alkyd coatings developed in theauthors laboratory and reported earlier,19 and thecomparison data are presented in Table2. It wasobserved that HBUA coatings have similar tensile

strength and superior adhesion strength than siliconsoya alkyd resin and silicon acrylate soya alkyd resin(Table4).

Adhesion strength

Adhesion is the key to the effectiveness of coatings.Without adhesion, a coating is merely a film on asurface like plastic wrap over a plate. Coating withpoor adhesion to the substrate will give less service life.Hence, the development of a coating which has goodadhesion to the substrate is promising to the research-

er. The pull-off adhesion test was carried out todetermine the adhesion of the developed HBUAcoating on mild steel substrates. Table3 shows thatthe adhesion strength of HBUA coatings tends toincrease with increasing IPDI content in the polymer.Previous results (Table3) clearly indicate very goodadhesion of HBUA coatings. The adhesion strength forall these three coatings was in between 8 and 11 MPa.The good adhesion can be attributedto the presence ofa higher number of polar linkages31 such as urethane(NHC=O) and ester (C in the polymer backbonewhich are clearly observable from Fig.4. The polargroups can form hydrogen bonding with metal and inbetween the resin molecules and thereby enhance the

adhesion strength of the coatings. Another importantfactor for good adhesion may be the molecularstructure of the resin which has numerous terminalfunctional groups in the polymer backbone.

Accelerated weathering test

During the service of a coating, gloss retention is ofgreat importance for maintaining the esthetic appear-ance of the painted surface for longer periods. In the

present work, HBUA-30, HBUA-40, and HBUA-50coatings were examined for their gloss retentionproperty by exposing the coated aluminum panels toa QUV-accelerated weatherometer. The test wascarried out as per method described in Determinationof mechanical properties section. The exposure studyresults are presented in Table4. After 300 h of

exposure, it was observed that there was a gradualreduction (Table4) in gloss for the coatings based onsoya alkyd and silicone soya alkyd resins, which, fromour earlier work, was known to be caused by UV-scission of ester groups.19 After 300 h, the HBUA-30,HBU-40, and HBUA-50 coatings had retained 76%,78%, and 80% gloss, respectively. The high glossretention characteristic of HBUA coatings is due to thepresence of UV-resistant cycloaliphatic urethane link-ages in the polymer backbone.

ConclusionA hyperbranched polyester-based NCO-terminatedurethane alkyd has been developed that can be curedunder atmospheric moisture to produce moisture-cured polyurethane coatings. These coatings shouldbe particularly useful in the areas where surfacepreparation and moisture removal are difficult andthe coating must be applied under humid conditions,for example, naval applications. Mechanical and glossretention properties of HBUA coatings are better thanconventional alkyds due to higher degree of crosslink-ing and compact structure. HBUA-50 and HBUA-40show marginal increases in mechanical properties

compared to HBUA-30. However, HBUA-40 andHBUA-50 require more solvent due to their higherviscosities. Therefore, HBUA-30 has the optimumcomposition considering the mechanical propertiesand high level of solids that are achievable. All theHBUA resins developed produced coatings at highersolids than did the conventional linear alkyds.

Acknowledgments The authors would like to thankDr. S.K. Singh of NMRL for providing guidance andencouragement during the work.

Table 4: Mechanical and weathering properties of HBUA resins

Resins Tensile strength

(MPa)

Elongation

(%)

Adhesion strength

(MPa)

Gloss retention (%) (after

300 h)

Soya alkyda 2 45 3 34

Silicone soya alkyda 9 70 6 50

Silicone acrylate soya

alkyd

a

5 75 6 80

HBUA-30 8 55 8 76

HBUA-40 9 50 10 78

HBUA-50 10 45 11 80

a The data for soya alkyd, silicone soya alkyd, and silicone acrylate soya alkyd are mentioned in reference ( 19)


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