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Progress in Organic Coatings 67 (2010) 44–54

Contents lists available at ScienceDirect

Progress in Organic Coatings

journa l homepage: www.e lsev ier .com/ locate /porgcoat

olyester polyols for waterborne polyurethanes and hybrid dispersions

ilas D. Athawale ∗, Mona A. Kulkarniepartment of Chemistry, University of Mumbai, Vidyanagari, Mumbai 400098, India

r t i c l e i n f o

rticle history:eceived 13 May 2009eceived in revised form 12 August 2009ccepted 17 September 2009

eywords:ispersionarticle size

a b s t r a c t

In this study, environmentally friendly polyester based polyurethane dispersions (PUDs) were synthe-sized using various combinations of isophthalic acid, adipid acid and maleic anhydride (IPA-AA-MA).A triangular empirical model was employed to optimize total number of experiments for optimal per-formance of polyurethane dispersions. In addition to PUDs, polyurethane/acrylate hybrid dispersions(PU/AC) were synthesized using graft copolymerization method to enhance the performance/propertiesof PUDs and for potential cost benefit.

The influence of molar ratio and diacid type on the thermo-mechanical and physico-chemical prop-

olyurethane/acrylic hybridhermal stability

erties of PUDs and PU/AC hybrids was studied. Results revealed that hybrid sample based on IPA:MAexhibited superior performance properties in terms of thermal stability, Tg, hardness, chemical resis-tance and colloidal stability, though their solvent resistance was relatively poor. Interestingly, PUD basedon equimolar mixture of IPA:AA also exhibited better thermal and colloidal stability with intermediatehardness and chemical resistance properties between PUDs based on aliphatic and aromatic diacids.

Scanning electron microscopy (SEM) results indicated better polyurethane/acrylate compatibility inn IPA

hybrid dispersion based o

. Introduction

Water-based polyurethane dispersions (PUDs) are a rapidlyrowing segment of the polyurethane coating industry due to envi-onmental legislations such as the clean air act and also due toechnological advances that have made them an effective substi-ute for the solvent-based analogs. Conventional polyurethane (PU)s insoluble or undispersible in aqueous media. For polyurethaneo be dispersible in water, ionic or hydrophilic pendant groupsere incorporated in the polymer chain. Typically, waterborneolyurethane dispersions are prepared in the form of ionomershich contain pendant acid groups or tertiary nitrogen groups. Theotential ionic groups are neutralized or quaternized to form salts.hese ionic centers positively contribute to the mechanical strengthnd elastomeric character of the materials. Among polyurethaneispersions, polyester based PUDs have been widely used as a rep-esentative PU in coating and adhesive industries [1]. However, annherent problem with this type of PUs is their hydrolytic instabilityue to gradual hydrolysis of the ester linkages resulting in molec-

lar weight decrease and deterioration of physical properties [2].herefore, such type of PUDs cannot be a direct plug-in for solvent-ased polyurethanes. In order to obtain the optimum properties,hey have to be skillfully formulated. The composition of the poly-

∗ Corresponding author. Tel.: +91 22 66926492; fax: +91 22 66926492.E-mail address: vilasda@yahoo.com (V.D. Athawale).

300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2009.09.015

:MA, resulting in homogeneous phase morphology.© 2009 Elsevier B.V. All rights reserved.

mer backbone as well as the formulating ingredients (monomerselection) will have significant influence on film formation proper-ties [3].

In our opinion, the possible approach to address this problemis developing empirical models for optimum performance proper-ties of PUDs. Colloidal stability, pH, neutralization, hydrophobicity,steric factor are all thought to be important variables that controlthe rate of ester hydrolysis. Proper selection of resin intermediatescan minimize the tendency of polyester resins to undergo hydroly-sis when formulated into typical waterborne coatings. The aromaticand aliphatic dibasic acids are known to have numerous benefitsin the synthesis of resin for solventborne coatings. In the synthe-sis of PUDs, the more commonly used acids in combination withneopentyl glycol (NPG) and trimethylol propane (TMP) are isoph-thalic acid (IPA), terephthalic acid (TPA), adipic acid (AA), maleic(MA), phthalic (PA) and trimellitic anhydride (TMA). The diacidsselected in this work for the synthesis of polyesters are IPA, AA andMA. IPA, being an aromatic diacid, has highest dissociation con-stant and it exhibits hydrophobic nature whereas, AA has bettersolubility and much lower dissociation constant. Maleic anhydrideis one of the most widely used monomer for chemical modificationof resins. Considerable studies have been carried out on grafting of

MA on polymeric resins [4–6], but it has not been so far exploredas starting material, which introduces C C bonds on the poly-mer segment. PUDs obtained from these monomers are evaluatedfor their physico-chemical and thermo-mechanical properties. Wehave attempted to improve properties of polyurethane dispersions

V.D. Athawale, M.A. Kulkarni / Progress in

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based on total solids) were slowly added to the flask to main-

Fig. 1. Empirical triangular model for design of PUDs.

sing chemical modification through grafting acrylate monomers.hough grafting is a useful technique to modify the PU ionomers,et, it is generally limited to polyurethanes containing unsaturatedolyester segments as backbone. Therefore, PUDs containing MA

n their backbone have been used to graft acrylate monomers ontohe main chain.

The empirical design of PUDs synthesized in the present works represented in the form of an equilateral triangle in Fig. 1, in

hich the resin composition is represented as dots. Each apex ofhe triangle indicates 100% usage of the diacid in that resin compo-ition. The midpoint of each leg represents a 1:1 molar blend of twoiacids and the center point is a resin composition that contains anquimolar blend of all three diacids. Attempts to make 100% MAesin were unsuccessful due to high unsaturation and difficulty inchieving desired acid value; therefore, we could not synthesizeUD of 100% MA.

Six resins representing each dot on the triangle except for MAere synthesized using conventional melt condensation technique

nd used as soft segments for PU dispersions. The selected rawaterials are reacted together using a molar excess of diisocyanate

o form an NCO-terminated prepolymer. The polymerization is car-ied out in a water miscible cosolvent, e.g. N-methylpyrollidinoneNMP), as it acts as a process aid (for viscosity control) and also aoalescing aid (for low volatility) in film formation. An organo-tinatalyst such as dibutyltin dilaurate is often used to reduce reactionimes and formation of undesired side products. The dimethy-ol propionic acid (DMPA) is used to introduce hydrophilic centern polyester backbone. The main advantage of DMPA is that thearboxylic acid is sterically hindered and so preferentially reactsith the backbone through the hydroxyl groups. Thus, six differentolyurethane dispersions have been prepared from their corre-ponding polyester resins using prepolymer mixing method [7].ll resins were prepared with same diacid/glycol equivalent ratio.

t should be noted that the ratio of equivalents of polyester polyol,MPA and chain extender ethylene diamine (EDA, to increaseumber of hard segments) was kept same throughout for all the for-ulations so was the case with isocyanate index (NCO/OH). Change

n diacids resulted in variation in resin’s molecular weight, acidumber and hydroxyl number. By varying the nature of polyesters,spectacular change in properties was observed ranging from flex-

ble to hard and brittle films. In this study, we have attempted

o upgrade the performance of PUDs, considering the importance,eed of an hour and cost effectiveness. The most widespread com-ination is polyurethane/acrylic (PU/AC) hybrid latexes in whichcrylates are grafted onto macromolecular chain by copolymer-

Organic Coatings 67 (2010) 44–54 45

ization method. The intimate combination of two such polymernetworks results in limited phase separation, i.e., controlled mor-phology and leads to improved thermo-mechanical properties andresistance due to synergistic behaviour.

2. Experimental

2.1. Materials

Isophthalic acid (IPA) LR (98%), adipic acid (AA) LR (99%), maleicanhydride LR (98%), triethyl amine (TEA) LR (99%), trimethylolpropane (TMP, 98%), and neopentyl glycol (NPG, 99%) were pro-cured from s.d.fine-chem (Mumbai, India). Dimethylol propionicacid (DMPA) (99%), dibutyltin dilaurate (DBTDL) and isophoron dis-carnate (IPDI) were purchased from Aldrich, USA. TMP was driedunder vacuum at 1 mm of Hg and 85 ◦C for 5 h before use. Triethylamine (TEA) and N-methyl-2-pyrollidinone (NMP) (s.d.fine-chem,India) were dried over 3 Å molecular sieves for 7 days. Ethylenediamine (EDA), a water soluble initiator potassium persulfate (KPS)were purchased from Fluka, Switzerland and were used as suchwithout any further purification. Catalyst Fascat 4100 (butyl stan-noic acid with 56.85% Sn) was kindly provided by ‘Tarapur Coatings& Adhesives,’ Boisur, India. Solvents used in the titration wereprocured from s.d.fine-chem (Mumbai, India), and dried over 3 Åmolecular sieves before use. The emulsifying agent, defoamer andbiocides were supplied by KTECH, India.

2.2. Synthesis

2.2.1. Synthesis of polyester polyolsIn a four-necked round bottom flask equipped with mechan-

ical stirrer, Dean Stark assembly, thermometer and nitrogen gasinlet, predetermined quantities of glycols and diacids were chargedas per the formulations, which were designed at specified alkydconstant (K) and excess hydroxyl content (R) (Table 1). The tem-perature was initially raised to 120 ◦C and thereafter increased withsmall increments of 20 ◦C per hour until it finally settled at 230 ◦Cwhere the reactions were continued till the desired acid values(<10 mg KOH/g) (ASTM D 16392-90) and hydroxyl values (ASTMD 1957-286) were obtained (∼8 h). Polyesterification was carriedout in the presence of catalyst, Fascat 4100 (0.05 wt.% based on totalweight of monomers), under a slow stream of N2 to provide an inertatmosphere and to avoid oxidation due to atmospheric oxygen. Theprogress of reaction was solely monitored from acid value and thequantity of water of esterification accumulated during the courseof reaction. Finally, the polyester polyols thus produced were dis-charged into glass stopper bottles and were placed in vacuumdesiccator before the onset of further reactions.

2.2.2. Synthesis of polyurethane dispersions (PUDs)Anionic polyurethane dispersions were prepared by ‘prepoly-

mer mixing’ [7,8] method in two steps namely synthesis ofNCO-terminated prepolymers and preparation of dispersions byintroducing anionic centers to aid dispersions. The reaction schemefor the synthesis of polyurethane dispersion is illustrated inFig. 2. Isocyanate terminated prepolymer was prepared by reactingpolyester polyols from previous step with DMPA dissolved in NMP(5 wt.% based on the total reaction mass) in a 500 mL four-neckedround bottom flask fitted with mechanical stirrer, thermometer,nitrogen gas inlet and reflux condenser. The mixture was heatedon heating mantle at 80 ◦C under nitrogen atmosphere for about30 min. After complete mixing, IPDI and catalyst DBTDL (0.05 wt.%

tain the reaction temperature at 85 ◦C. The reaction proceededuntil the amount of residual isocyanate groups reached a theo-retical end point, calculated on the basis that all hydroxyl groupshad reacted with isocyanate groups. The NCO content of the pre-

46 V.D. Athawale, M.A. Kulkarni / Progress in Organic Coatings 67 (2010) 44–54

Table 1Composition and properties of polyester polyols.

Sample code Monomers AVa (mg/g of KOH) OHvb (mg/g of KOH) Molecular weightby titration

Alkydc constant Averaged

functionalityOHe excess %

Diacids Glycols

IPA AA MA NPG:TMP

Poly 1 – 0.5 – 0.5:0.08 1.20 118.10 950 1.083 2.000 25Poly 2 0.5 – – 0.5:0.08 6.09 98.85 1070 1.082 2.000 25Poly 3 0.25 0.25 – 0.5:0.08 6.22 96.13 998 1.083 2.000 25Poly 4 – 0.25 0.25 0.5:0.08 1.62 111.12 995 1.083 2.000 25Poly 5 0.25 – 0.25 0.5:0.08 6.00 89.81 1171 1.083 2.000 25Poly 6 0.16 0.16 0.16 0.5:0.08 5.14 102.62 1041 1.083 2.000 25

a Acid value in mg/g of KOH.

× 100

p[wsnpsEf(sbp

b Hydroxyl value in mg/g of KOH.c Alkyd constant = total moles/equivalents of acid.d Average functionality = total equivalents/total moles.e OH excess % = [(equivalents of glycols − equivalents of acid)/equivalents of acid]

olymers was determined by dibutylamine back titration method9]. Upon obtaining the theoretical NCO value, the prepolymersere cooled to 60 ◦C, and the stoichiometric amount of TEA dis-

olved in NMP was added and stirred for 1 h to ensure completeeutralization of carboxylic groups of prepolymer. The resultantolyurethane anionomer was then dispersed in water under high-peed stirring and desired molecular weight was achieved by usingDA as a chain extender. For stabilization of dispersion, the emulsi-ying agents USOL K-98 (0.9% of total mass), defoamer and biocides

0.1% of total mass) (KTECH, India) were added to aqueous disper-ions. PU dispersions thus obtained had the solid contents of ∼30%y weight. The feed composition and characteristics of resultingolyurethane dispersions are given in Table 2.

Fig. 2. Reaction scheme for the synthe

.

2.2.3. Preparation of polyurethane/acrylic (PU/AC) hybriddispersions

A predetermined quantity of PUDs prepared in previous stepalong with deionized water and initiator (0.03% KPS based on totalsolids) were taken in the same reactor as used for the synthesisof polyurethane dispersion. Subsequently, calculated weight per-centage of styrene, BA and AA were added to this system throughdropping funnel over a period of 2 h at 80 ◦C. The temperaturewas gradually raised to 85 ◦C and maintained for 2 h to ensure

complete reaction of all the free monomers. The resulting PU/ACemulsion hybrids thus prepared were stable dispersions with asolid content of ∼30% by weight. The reaction scheme for the syn-thesis of PU/AC hybrid emulsion is shown in Fig. 3. The feed ratios

sis of polyurethane dispersion.

V.D. Athawale, M.A. Kulkarni / Progress in Organic Coatings 67 (2010) 44–54 47

Table 2Feed compositions (g) and characteristics of polyurethane dispersions with variable dicarboxylic acid.

Sample Polyester polyol type Polyester polyol (g) DMPA (g) IPDI (g) TEA (g) EDA (g) Appearance Particle size (nm) pH Colloidal stability

CAT IPUD 1 Poly 1 25 3.46 8.52 0.62 0.929 Transparent 72.8 8.2 >6 monthsPUD 2 Poly 2 25 3.98 7.26 0.57 1.0 Milky white 83.3 7.5 ∼1 monthPUD 3 Poly 3 25 4.16 10.55 0.47 1.0 Transparent 77.5 7.9 >1 year

CAT IIPUD 4 Poly 4 25 3.37 9.13 0.50 0.98 Transparent 90.1 8.0 >1 yearPUD 5 Poly 5 25 2.86 8.20 0.60 1.0 Bluish white 69.3 7.3 >6 monthsPUD 6 Poly 6 25 3.22 8.85 0.58 1.0 Bluish white – 7.1 >3 months

NMP: 8 g; Water: variable; Solid contents: 30%; CAT: category.

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Fig. 3. Proposed scheme f

f PU/AC hybrids prepared from different PUDs are presented inable 3.

.3. Preparation of films

Films were prepared by casting the newly synthesized samplesnto a Teflon plate at room temperature, followed by drying at0 ◦C (24 h), at 60 ◦C (24 h), and at 70 ◦C (24 h). This trend of dry-

ng is followed essentially to ensure slow drying. It is also possibleo evaporate the solvent at a fixed temperature either room orlevated temperature. After demolding, the films were stored inesiccators at room temperature for further studies.

.4. Characterization

.4.1. Fourier transform infrared (FTIR) spectroscopy

The IR spectra of polyurethane dispersions, and hybrid resins

ere obtained on a Perkin Elmer FT-IR spectrophotometer. Beingn the form of thick syrup, a thin film of resin was cast over NaCllock.

able 3ecipe for CAT III PU/AC hybrid dispersions.

Ingredients PUD 4a PUD 5a PUD 6a

PUD PUD 4 PUD 5 PUD 6PUD/AC (% w/w) ratio 50:50 50:50 50:50Styrene/BA (% w/w) ratio 50:50 50:50 50:50Acrylic acid (g) 1.0 1.0 1.0KPS 1.0 1.0 1.0Particle size (nm) 188.4 151.1 –% solid 32% 30% 36%Colloidal stability 3 monthsa 4 months 2 daysa

AT III: category III.a Little precipitation.

synthesis of hybrid resin.

2.4.2. Particle size analysisParticle size is the important parameter in deciding the end use

industrial applications of aqueous PUDs. Particle size was mea-sured using Malvern Instrument India Ltd., Malvern Instrument,Type Zetasizer 1000 HS. The samples were diluted with deionizedwater to adjust solid content and directly placed in the cell. Themeasurements were carried out at 25 ◦C.

2.4.3. Thermogravimetric analysis (TGA)The decomposition profile of samples was thermogravimetri-

cally analyzed using ‘Diamond’ Perkin Elmer analyzer. Film samplesranging from 4 to 6 mg were placed in a platinum sample pan andheated from 30 to 800 ◦C, under N2 atmosphere at a heating rate of10 ◦C min−1 and the weight loss and temperature difference wererecorded as a function of temperature.

2.4.4. DSC analysisGlass transition temperature of samples were measured using

differential scanning calorimetry (DSC), on a NETZSCH DSC200 PC,using aluminum crimped pans under N2 flow at 20 mL min−1. Themeasurements were carried out between −100 ◦C and +150 ◦C at aheating rate of 10 ◦C min−1.

2.4.5. Morphological properties (SEM analysis)The morphology of the fractured surfaces of emulsion hybrids

were investigated by SEM to study the compatibility between

polyurethane and acrylate phases. Electron micrographs wereobtained on Cameca (France) Model SU-30 scanning electronmicrograph (SEM). Samples were prepared for SEM by freeze frac-turing them in liquid nitrogen and applying a gold coating ofapproximately 200 Å units by ion sputtering method. The gold-coated samples were mounted on the SEM stubs with silveradhesive paste.

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8 V.D. Athawale, M.A. Kulkarni / Progr

.4.6. Mechanical propertiesThe samples were applied onto previously degreased mild steel

nd glass panels using ‘RDS’ USA make bar coater (50 �m filmhickness). Coated panels were then allowed to air dry at roomemperature in fully ventilated atmosphere and were subjected toesting only after 7 days to ensure the full maturation of coatedlms.

Pencil hardness and indentation hardness (by shore A durome-er) were determined by ASTM 3363-74 and D 2240-86. A crosscutdhesion was employed as per ASTM D 3359-2002 to study thedhesion. Flexibility was measured using conical mandrel (1/4 in.)ent test as per ASTM D 522-939. Molecular weight of the hydrox-lated polyester polyol was evaluated by end group analysis [10].

.4.7. Chemical resistanceChemical resistance was checked according to the ASTM D 1647-

9. Glass panels coated with newly synthesized samples werellowed to dry for 3 days. The periphery of the glass panels wasoated with wax in order to restrain the migration of water underhe film from open ends. The panels were then dipped into 3% (w/w)ulfuric acid solution and 3% (w/w) NaOH solution, and the changen the appearance was monitored.

.4.8. Solvent resistanceThe solvent resistance test was carried out as per the “Double

ubs” method using a piece of white cotton cloth (ASTM D5402-). The solvents used were methyl ethyl ketone and toluene. Theesult reported was the minimum number of double rubs at whichhe films were observed to fail or else 100, which was the maximumumber of double rubs carried out.

. Results and discussion

For the sake of convenience, we have grouped all newly synthe-ized PUDs and hybrid PUD samples into three different categories:

CAT I: PUD 1, 2 and 3.CAT II: MA modified PUDs: PUD 4, 5, and 6.CAT III: acrylate modified hybrid PUDs: PUD 4a, 5a and 6a.

.1. Infrared spectroscopy (IR)

Fig. 4(a) shows the representative FTIR spectra of three differentolyurethane samples selected from each category: PUD 1 (basedn AA) from CAT I, PUD 4 (based on AA:MA) from CAT II and PUDa (based on acrylate modified IPA:MA) from CAT III.

Fig. 4(b) illustrates the FTIR spectra of another three samples:UD 2 based on IPA from CAT I, PUD 5 based on IPA:MA from CATI and PUD 5a based on acrylate modified hybrid polyurethane dis-ersion of IPA:MA from CAT III.

The IR spectra of samples PUD 2, 5 and 5a show the followingands: the characteristic absorption bands at ∼3310–3300 cm−1

nd 2925–2966 cm−1 indicate the stretching of hydrogen bond-ng of N–H band and –CH2 asymmetric stretching respectively. Theand at ∼730 cm−1 illustrates the presence of aromatic ring in allhe three samples of this set. The bands at 1724 cm−1 (amide I,C O), 1538–1545 cm−1 (amide II, ıN–H and �C–N), 1240 cm−1

amide III, �C–N and ıN–H) and 1136–1150 cm−1 (antisymmet-ic �C–O–C) confirm the formation of urethane group. Moreover,he absence of a band at 2270 cm−1 confirms that unreacted

CO groups are not present, which is a result of the fact thathen NCO reacted with active hydrogen of a hydroxyl functional

ompound (hydroxylated polyester polyol), urethane linkage wasormed. A new band at 1644 cm−1 is detected in case of MA mod-fied PUD (IPA:MA), which indicates the presence of C C bond

Organic Coatings 67 (2010) 44–54

in the polyester backbone of PUD 5. However, it is interestingto note that in case of hybrid resin, PUD 5a (acrylate modifiedIPA:MA), the absence of this characteristic band of unsaturation at1644 cm−1 confirmed the successful grafting of acrylate monomersonto reactive site of polyurethane dispersion. There is no appar-ent difference between absorption bands shown in Fig. 4(a) and(b) except band due to aromatic ring (Fig. 4(b)), which indicatesthat the similar chemical interactions occurred in the second setof polyurethane dispersion and its hybrid resin independent ofmonomer species involved in the synthesis. Since we are interestedin verification of occurrence of chemical grafting of acrylates ontopolyester backbone of polyurethane dispersion, it is necessary tocheck the absorption region of C C bond. The band at 1640 cm−1

in PUD 4 (AA:MA) indicates the incorporation of C C bond in itsmacromolecular chain, however, the absence of this band in hybridstructure of PUD 4a (acrylate modified AA:MA), is the evidence ofgrafting during copolymerization. The samples PUD 3, 6 and 6aexhibited similar absorption pattern as observed in case of rest ofthe PUDs and hybrids, hence need no discussion.

3.2. Particle size

The particle size of anionic polyurethane dispersion is primarilydependent on the functional monomers employed and mainly gov-erned by hydrophilicity of polyurethane. Smaller particles resultfrom increased hydrophilicity. Fig. 5 shows the particle size distri-bution of the PUDs and PU/AC hybrid resins. It can be seen fromTable 2 that the increased flexibility of the soft segment resultedin decreasing particle size. This can be explained as flexible parti-cles are more deformable in shear field, and thus at the dispersestage, the dispersed phase can be more easily broken into smallerones [11]. The PUD 2 (based on IPA) shows larger particle size ascompared to PUD 1 (based on AA), which may be due to the pres-ence of aromatic ring on its backbone. As expected, particle size77.5 nm of PUD 3 (based on IPA:AA), falls intermediary betweenPUD 1 and 2. On comparing PUD 1 with PUD 4 (based on AA:MA),PUD 1 (polyol containing AA as PUD backbone) is more flexible thanPUD 4 (polyol containing AA:MA as a backbone), therefore PUD 1exhibits smaller particle size (72.8 nm) than PUD 4, which bearsparticles of 90.1 nm size. The investigations of particle size of hybridresins show that grafting of acrylate components onto C C bondof polyester polyurethane dispersion results in a drastic increaseof average particle size. This must be primarily due to swelling ofgrafted material into aqueous medium. Unsaturated PU molecularchain contains C C double bond and many �-H atoms. When acrylicmonomers are polymerized in presence of unsaturated aqueousPUDs, these �-H atoms are abstracted by the free radicals fromdecomposition of initiator, resulting in grafting between unsatu-rated PU molecular chain and acrylic monomers. Thus, the resultingcomposite latex (PU/AC hybrid) contains urethane acrylic graftcopolymer, ungrafted PU, and ungrafted acrylic copolymer. Theeffect of addition of acrylate monomers on the properties of PU/AChybrid resins has been summarized in Table 3. In case of hybridresin PUD 4a (acrylate modified AA:MA) particle size increases from90.1 nm (PUD 4 without acrylate, Table 2) to 188.4 nm and so is thecase with polyurethane dispersion of MA modified IPA (PUD 5) andacrylate modified IPA:MA (PUD 5a), where particle size increasesfrom 69.3 to 151.1 nm. However, in this case the increase in particlesize is accompanied by little precipitation.

3.3. Colloidal stability

The colloidal stability of the particles, both during emulsionpolymerization and on subsequent storage of the dispersion is amatter of crucial importance. It is well known that particle sizehas the direct effect on the polyurethane dispersion stability [12].

V.D. Athawale, M.A. Kulkarni / Progress in Organic Coatings 67 (2010) 44–54 49

F : PUDP on P

Ttsea2tsw

ig. 4. FTIR spectra of polyurethane dispersions. (a) PUD 1: PUD based on AA; PUD 4UD 2: PUD based on IPA; PUD 5: PUD based on IPA:MA; PUD 5a: hybrid PUD based

he stability observations were made on the basis of phase separa-ion (water and resin layer) due to complete particle settlement. Aummary of these observations is presented in Table 2. The high-st stability was found for PUD 3 (based on combination of IPA:AA)

nd 4 (based on AA:MA), while the lowest stable system was PUD(based on IPA). It is well known that PUDs can be stabilized due

o the formation of electrical double layers between the ionic con-tituents, which are chemically bound to PU and their counter ions,hich migrate into water phase around the particles. The inter-

based on AA:MA; PUD 4a: hybrid PUD based on PUD 4 and acrylate monomers. (b)UD 4 and acrylate monomers.

ference of electrical double layers of different particles resulted inparticle repulsion, leading to the stabilization mechanism of dis-persions [12,13]. Some of the dispersions, specifically PUD 2 andPUD 6, started to phase separate after a short period of 1 month

and 3 months respectively, which may be due to lowering of pH,as a result of volatilization of neutralizing amine. These settled dis-persions were redispersible by properly maintaining the pH and bymechanical stirring of the coagulated dispersion. The hybrid resinsare relatively less stable than their PUD counter parts, which may

50 V.D. Athawale, M.A. Kulkarni / Progress in Organic Coatings 67 (2010) 44–54

s and

bpTob

3

tTi1f4rth4g

Fig. 5. Mean particle size distribution of some representative PUD

e because of their unusually high particle size when compared toolyurethane dispersions, leading to early sedimentation (Table 3).hus, colloidal stability of PUDs and their hybrid resins is a matterf crucial importance and can be achieved by careful selection ofuilding blocks of polyol backbone.

.4. Thermogravimetric analyses (TGA)

Thermogravimetric analysis was used to analyze decomposi-ion behaviour of cured films of PUDs and their hybrid coatings.he thermal decomposition data are given in the form of thermalndexes T10, T30 and T50 (i.e., the temperatures corresponding to0%, 30% and 50%) of weight loss in Table 4. TGA curves of PUDsrom CAT I (PUD 1, 2 and 3) and hybrid PUDs from CAT III (PUDa, 5a and 6a) are given in Fig. 6(a) and (b) respectively. The TGA

esults showed that among all PUDs from 1 to 6, PUD 3 (IPA:AA) ishe most stable and PUD 4 (AA:MA) is the least stable while CAT IIIybrid resins showed the stability trend as PUD 5a > PUD 6a > PUDa. The thermal degradation of PUD is a complex process; it is aeneral rule that the more easily formed urethanes are less sta-

their acrylate modified hybrid samples (PU/AC emulsion hybrids).

ble, i.e., more easily dissociated. The PUDs are thermally degradedthrough three basic mechanisms: the urethane bond dissociatinginto its starting components, i.e., alcohol and isocyanate; break-ing of the urethane bond with the formation of primary amine,carbon dioxide, and an olefin; and finally splitting the urethanebond into secondary amine and carbon dioxide [14]. Lower sta-bility and faster decomposition of PUD 2 (based on IPA) in theinitial stage of degradation may be due to the presence of a greateramount of aromaticity in polyester backbone which makes the PUDsusceptible to chain scission and relieves the structural crowding.However, in later stage (above 300 ◦C), it showed enhanced ther-mal stability. In case of CAT II PUDs (MA modified PUDs: PUD 4,5 and 6), the thermal stability turned out to be slightly inferiorto their unmodified PUD counterparts (conventional PUDs). How-ever, in the present study, the MA modified PUDs are desirable

due to their C C bonds. These PUDs bearing unsaturation allowfurther performance enhancement through graft copolymerizationwith acrylate monomers. Thus, with the thermal index as the cri-terion of thermal stability, it can be inferred that CAT III acrylatemodified polyurethanes (PU/AC hybrids) have higher stability than

V.D. Athawale, M.A. Kulkarni / Progress in Organic Coatings 67 (2010) 44–54 51

Table 4Tg and TGA data of polyurethane dispersions.

PUD Monomer composition Glass transition temperature from DSC TGA weight loss

T10a T30

b T50c % residued

CAT IPUD 1 AA −37.7 ◦C 244 ◦C 325 ◦C 370 ◦C 8PUD 2 IPA 27.9 ◦C, 66 ◦C 215 ◦C 300 ◦C 376.13 ◦C 0PUD 3 IPA:AA −1.8 ◦C 230 ◦C 315 ◦C 400 ◦C 8.2

CAT IIPUD 4 AA:MA −4.6 ◦C 208 ◦C 296 ◦C 344 ◦C 2.5PUD 5 IPA:MA −2.2 ◦C 200 ◦C 300 ◦C 345 ◦C 10PUD 6 IPA:AA:MA 11.5 ◦C 222 ◦C 320 ◦C 367 ◦C 10

CAT IIIPUD 4a Hybrid resins of AA:MA −0.1 ◦C, 129 ◦C 200 ◦C 302 ◦C 379 ◦C 2.8PUD 5a Hybrid resins of IPA:MA 71.3 ◦C 210 ◦C 332 ◦C 402 ◦C 8.1PUD 6a Hybrid resins of IPA:AA:MA 11.8 ◦C, 44.3 ◦C 287 ◦C 330 ◦C 387 ◦C 5

CAT: category.a Temperature corresponding to 10% weight loss.b Temperature corresponding to 30% weight loss.c Temperature corresponding to 50% weight loss.d % residue at 600 ◦C.

Fig. 6. TGA curves of PUD samples and emulsion hybrids. (a) CAT I dispersions PUD1, 2 and 3 and (b) CAT III hybrid dispersions.

their corresponding CAT I PUDs, indicating a synergistic effect. Thisbehaviour can be attributed to the presence of stronger interactionbetween the acrylate monomers and PUD backbone during the graftcopolymerization reaction. The char residues at 600 ◦C for all threecategories of polyurethane samples are presented in Table 4, whichshow the highest char content (10%) in case of PUD 5 and 6 whereas,the least char content in case of PUD 2 (based on IPA). Thus, it can beconcluded from TGA analyses that choice of monomers, their moleratios and hybridization with acrylate monomers have pronouncedeffect on the degradation behaviour of PUDs.

3.5. DSC analysis

The thermal transitions of polyurethane dispersions and theirhybrid emulsions are determined by DSC analysis and shown inFig. 7(a) and (b) respectively. The results of DSC analysis of PUDand hybrid samples are summarized in Table 4. Generally, it is saidthat Tg is directly proportional to crosslinking density and indirectlyproportional to chain flexibility. In our case, results are consistentwith this statement. From Fig. 7(a), it can be seen that amongst CATI PUDs, i.e., PUD 1, 2 and 3, the PUD 1 (based on AA) has the low-est Tg (−37.7 ◦C) and PUD 2 (based on IPA) depicts the highest Tg

(27.9 ◦C) whereas, PUD based on combination of IPA:AA (PUD 3) hasthe intermediate Tg value (−1.8 ◦C). This variation in Tg is attributedto nature of macromolecular chain structure of PUDs. In case ofPUD 5 (based on IPA:MA), addition of MA has reduced the Tg from27.9 ◦C (PUD 2) to −2.2 ◦C, which may be due to less restriction ofchain mobility in case of PUD 5 than PUD 2 (based on IPA) whereas,in case of MA modified linear PUD (PUD 4), Tg is shifted to highertemperature (−4.6 ◦C) compared to unmodified PUD 1 based onAA. (−37.7 ◦C), indicating impeded chain flexibility and less flexiblestructure of PUD 4. Apparently similar results are observed in caseof PUD 6 and 3. The higher Tg of PUD 6 based on IPA:AA:MA (11.5 ◦C)compared to unmodified PUD 3 based on IPA:AA (−1.8 ◦C) can alsobe associated with chain stiffening in polymeric chain. It can be seenfrom Fig. 7(b) and Table 4 that CAT III hybrid PUDs exhibit higherTg values than their corresponding PUD counterparts. The higherTg values of hybrid resins are attributed to their higher degree ofcrosslinking and entanglements, which may be because of grafting

of acrylate monomers onto their polyester backbones. The hybridresins PUD 4a and 6a based on AA:MA and IPA:AA:MA respectivelyexhibit two distinct Tg’s, one of the PU component and one of the ACcomponent. The lower Tg is attributed to the PU component and thehigher Tg is attributed to the AC component. However, unlike this,

52 V.D. Athawale, M.A. Kulkarni / Progress in Organic Coatings 67 (2010) 44–54

F

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better hardness as compared to their PUD counterparts. Among all

ig. 7. DSC curves of (a) polyurethane dispersions and (b) PU/AC emulsion hybrids.

he hybrid resin PUD 5a (based on IPA:MA) had a single Tg (71.3 ◦C),hich is substantially higher than other PUDs. A possible explana-

ion for this could be higher degree of miscibility between PU andC components, which are held together with numerous entan-lements and secondary intermolecular bonding forces. However,rimary reason is the grafting reaction, which occurred betweenU and AC components. During graft copolymerization, PU andC components are attached directly through primary bonds. Theybrid samples PUD 4a and 6a, exhibit two Tg value, which indi-ate that in these cases PU and AC components are not mixed atolecular level.

.6. SEM analysis

The morphology of the fractured surfaces of PU/AC hybrid sam-les was revealed by a scanning electron microscope (SEM), ashown in Fig. 8. SEM images show different particle morphologiesn all the three hybrid samples and the particle coagulation is alsoecognizable in case of the hybrid sample PUD 6a. In the micro-raphs, the bright isolated domains are acrylate phase and the darkontinuous phase is PU. The smooth and smaller domain size in casef PUD 5a (based on IPA:MA) is indicative of homogeneous mor-hology mixing of PU and AC phases on molecular level. However,

n case of PUD 6a (based on IPA:AA:MA), the micrograph showshe formation of aggregates due to joining of various domains,hich indicates the heterogeneous phase morphology. Some what

rregular, non-spherical sporadic spots and rare small spherulites

Fig. 8. Scanning electron micrographs of hybrid polyurethane dispersions (PU/AChybrids). PUD 4a: hybrid PUD based on AA:MA; PUD 5a: hybrid PUD based onIPA:MA; PUD 6a: hybrid PUD based on IPA:MA:AA.

are observed in case of PUD 4a (based on AA:MA)s, which can beattributed to its linear phase morphology.

3.7. Coating properties

3.7.1. Shore A and Pencil hardnessIt can be observed from Table 5 that all the hybrid samples (CAT

III) exhibit better hardness properties than their correspondingPUD counterparts (CAT I). PUD 5a has been found to have improvedfilm hardness compared to coatings based on PUD 4a and PUD 6a.Similarly PUD based on IPA (PUD 2) has the highest hardness thanall other PUD samples. The higher hardness in case of PUD 2 andPUD 5a can be attributed to their corresponding Tg values (27.9 ◦Cand 71.3 ◦C respectively). Thus, at the measurement temperature(27 ◦C) a greater percentage of these polymers exist in hard andglassy state and hence exhibit enhanced hardness properties. Therelatively higher hardness of PUD samples based on IPA (PUD 2, 3and 6) is due to occurrence of aromatic stiffening in their respectivepolymeric backbones.

3.7.2. Scratch hardnessThe data in Table 5 show that all the hybrid coatings showed

PUDs and hybrid samples tested, the samples PUD 2, 5a and 6ahave had relatively higher hardness. It can be inferred from theseobservations that higher scratch hardness is achieved with PUDfilms with rigid phenyl structure in their backbone [15]. However,

V.D. Athawale, M.A. Kulkarni / Progress in Organic Coatings 67 (2010) 44–54 53

Table 5Coating properties of polyurethane dispersions.

Property PUD 1 PUD 2 PUD 3 PUD 4 PUD 5 PUD 6 PUD/AC 4a PUD/AC 5a PUD/AC 6a

CAT I CAT II CAT III

Monomers in polyester backbone AA IPA IPA:AA AA:MA IPA:MA IPA:AA:MA AA:MA IPA:MA IPA:AA:MAHardness (Shore A) 81 96 92 88 80 93 91 100 98Pencil hardness H 4H 3H 2H 3H 4H 4H 6H 5HScratch hardness 1400 2000 1600 1500 1800 1700 1600 >2000 2000Adhesion (cross hatch) 100% 100% 100% 100% 100% 100% 100% 100% 100%Flexibility (conical mandrel 1/4 in.) P F P P P P P P P

Acid alkali resistancea

H2SO4, 3% UF UF UF UF UF UF UF UF UFNaOH, 3% HB UF SB FR FR FR SB UF SB

Solvent resistanceb

MEK 40 100 74 81 78 80 90 90 98Acetone 32 100 88 65 81 100 75 100 >100Toluene 40 100 81 45 86 100 62 >100 >100

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: fail; P: passes; UF: unaffected; HB: highly blushed; SB: slight blush; FR: film coma 3 months immersion test.b Number of double rubs that the coating sustained without any damage.

ncorporation of MA to such rigid structure (PUD based on IPA)esults in diminution of hardness, which is evident from hardness ofUD 2 (2000 g) and PUD 5 (1800 g). Generally, the linear structuresf the diacids in the polyester lead to polyurethanes with lowerardness [16], which can be confirmed from the hardness of PUD 1ased on AA (1400 g), which is the lowest hardness exhibited ever

n this study. Although the PUD sample based on AA:MA (PUD 4)ad slightly higher hardness and could bear the weight of 1500 get, it is inferior to those based on IPA:MA (PUD 5) and IPA:AA:MAPUD 6).

.8. Percentage adhesion

The samples from all the three categories (CAT I, CAT II and CATII) showed 100% adhesion on mild steel panel (Table 5).

.9. Flexibility

Although most of the PUDs and hybrid samples yield a hardlm as measured by Shore A and Pencil hardness, however, withhe exception of PUD 2, all the films possess a significant level ofexibility (Table 5). The failure of PUD 2 can be correlated to theighest isophthalic content, which reduces the chain flexibility andhe internal mobility [17].

.10. Chemical resistance properties

.10.1. Acid and alkali resistanceThe acid and alkali resistance of the different PUDs and their

ybrid samples is shown in Table 5. The data show that all theoating films are almost unaffected and offered good resistance toulfuric acid (3%, w/w) for the entire period of immersion. How-ver, upon their exposure to 3% (w/w) alkali solution, only PUD 2based on IPA) and PUD 5a (IPA:MA hybrid) remained completelynaffected whereas, PUD based on AA (PUD 1) showed loss in glossvisual observation) on first day and heavy blistering during thehird day. Another interesting observation in case of MA modifiedAT II type of PUDs (PUD 4, 5 and 6) is that they have compara-ively poor alkali resistance than their corresponding unmodifiedAT I PUDs (PUD 1, 2 and 3). In all the cases loss in gloss and change

n colour is observed after 5 days and complete removal of filmccurred only after a significant period of 1 month. In case of CATII, the hybrid samples PUD 4a and PUD 6a showed relatively betterlkali resistance as only slight swelling was observed during thehird month of their exposure to 3% (w/w) NaOH solution.

y removed.

3.10.2. Solvent resistanceFrom the data in Table 5, it can be seen that all the IPA based

films showed excellent solvent resistance, which can be attributedto the presence of phenyl ring in their backbone. PUD based onAA (PUD 1) was the most affected one and showed the least resis-tance to all the solvents used for Rub test. PUDs based on blendof aromatic and aliphatic diacids (PUD 3 and 6) provide better sol-vent resistance than their aliphatic counterparts (PUD 1 and PUD 4).The solvent resistance of hybrid coatings (PUD 4a, 5a and 6a) showsthat these coatings have better resistance to solvent, which may bebecause of increased branching during graft copolymerization ofsuch samples.

4. Conclusion

In this study, we have endeavored to synthesize environmen-tally friendly polyester based PUDs using various combinations ofdiacids such as IPA, AA and MA with NPG and TMP as glycol com-ponents. Empirical triangular model (Fig. 1) was used to optimizetotal number of experiments. In addition to range of conventionalPUDs (PUD 1–6), novel PU/AC hybrid resins (PUD 4a, 5a and 6a) havebeen synthesized using acrylate monomers to enhance the perfor-mance properties. The effects of the structures and mole ratios ofthe diacids on particle size, glass transition temperature and themechanical, thermal as well as chemical resistance properties ofPUDs and the hybrid resins were studied. On comparing the proper-ties of PUD 1, 2 and 3, the PUD 3 (IPA:AA) provided superior thermaland colloidal stability (∼1 year) with the intermediate Tg value(−1.8 ◦C), hardness and chemical resistance properties betweenPUDs based on aliphatic and aromatic diacids. Although, PUD basedon 100% IPA demonstrated the best thermal and chemical prop-erties, yet, due to its poor flexibility, brittle nature and limitedshelf life (1 month), PUD formulations with 100% IPA is not recom-mended. The aliphatic PUDs are comparatively expensive and theirperformance was inferior to that of aromatic PUDs. In this study,the cost/performance benefit was sought by grafting acrylates ontopolyester backbone of PUD through C C bonds introduced by incor-poration of monomer MA into its polyol composition. Comparisonof midpoints of legs of triangle viz. PUD 3, 4 and 5 (Fig. 1) showsthat the PUD 3 with equimolar mixture of IPA:AA offers optimum

performance in terms of thermo-mechanical and hardness prop-erties. Evaluation of various PUDs and hybrids demonstrate PUD5 as a promising polymer to formulate hybrid coatings, though,its shelf life is relatively less. The suggested model will providethe general guideline to analyze and evaluate effects of numerous

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4 V.D. Athawale, M.A. Kulkarni / Progr

ormulations and subsequent modifications on the properties ofolymer compositions and opens up avenues for more extensiveesearch investigations.

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