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Synthesis, characterization, and comparison of polyurethane dispersions based on highly versatile anionomer, ATBS, and conventional DMPA.

Abstract Polyurethane dispersions (PUDs) are one such class of coatings that comply well with stringent VOC regulations. In this study, PUDs were prepared as aqueous dispersions using a new highly versatile crosslinkable anionomer, 2-acrylamido 2 methyl-propanesulfonic acid (ATBS), which is commercially available, yet not explored by the coating industry. The influence of this novel anionomer on the performance properties of polyurethane resins was evaluated by comparing performance/properties of this newly synthesized PUD with dispersion based on conventional anionomer, DMPA. Analyses of their physico-chemical and thermo-mechanical properties revealed that PUD based on ATBS showed comparatively much better thermal, mechanical, and chemical and coating properties than the PUD based on DMPA anionomer.

Keywords Aqueous dispersion, ATBS, Coating, Prepolymer

Introduction

The stringent EPA regulations and the environmental concern of the coating industry have brought about significant changes in the types of coating systems which comply well in keeping the environment clean. Waterborne coatings have been one of the major technologies which helped the industry to reduce emission of solvents. They have been incorporated in sophisticated and very demanding end uses such as automotive coating and less demanding general industrial applications as well. Many important advances have taken place in recent years in terms of polymeric backbone, polymeric dispersant, polymeric thickeners, and polymeric colloidal stabilizers. Center to the utilization of polymers in aqueous medium is the fact that certain polar groups are capable of conferring water stability or water dispersibility to an otherwise water insoluble polymer. The conventional method to render polyurethane (PU) polymer dispersible in water is with built-in hydrophilic group, either ionic type or nonionic type. A commonly used and an industrially important class of waterborne PUs is ionic type among which anionic type is dominant, where PU ionomers possess pendant acid groups (anionomer) or tertiary amines (cationomer) incorporated into their backbone. These groups are neutralized to form internal salt prior to dispersion. (1) These anionic groups contribute positively to the mechanical strength and the elastomeric character of the materials. However, the hydrophilic nature of an ionic group makes the material have poor resistance and solvent and results in slow evaporation of water during drying. The other drawback of water-borne polyurethanes is their relatively high cost as compared to solvent-based products. (2), (3) One method to improve the performance is to form multiphase structure in the polyurethane dispersions (PUDs) through introduction of another polymer. A number of examples of such type are described in many technical papers, (4-7) in which acrylate monomers are incorporated by free radical graft polymerization to enhance performance of PUDs. These acrylic counter parts have special advantages such as low cost, high gloss, good weatherability, wide range of glass transition temperature, excellent chemical resistance, easy thickening, and so on. Therefore, they are the most attractive candidates to be incorporated into the PUDs.

ATBS, 2-acrylamido 2-methylpropanesulphonic acid, is an anionic monomer with wide spectrum of properties apart from its hydrogen-bonding capability and polyelectrolyte behavior in aqueous solutions. (8) It is a relative strong acid that has had a wide variety of applications (as a acid or salt), which includes packaging films, foam stabilizers, photographic materials, and water absorbents. Copolymer of ATBS with ethylene dimethacrylate has been used to make contact lenses, poly (ATBS-graft-styrene) gives self reinforced hydrogels, and in coatings industry, soap-free emulsion was prepared with ATBS as reactive emulsifier (9); hence, it should be of interest how it behaves in the graft or crosslinked networks with polyester. To account for the hydrophilicity and ionic character of ATBS and its effect on the properties of PUD, it is graft copolymerized onto unsaturated polyester polyol using azoisobis butyro nitrile (AIBN) as a free radical initiator. Consequently, unsaturated polyester and ATBS tend to crosslink. The crosslinking probably occurs through the removal of a hydrogen atom from one of the chains by energetic free radicals. This leaves a radical site on main chain and the hydrogen atom produced will abstract another hydrogen atom from an adjacent chain. The two radical sites left on the adjacent chains can then recombine to form a crosslink. (10) Therefore, grafting has been used as an important technique for modifying physical and chemical properties of polymers. (11) The shielding effect produced due to unique geminal dimethyl structure of ATBS offers the excellent hydrolytic stability to the synthesized polymer resin. (8)

Thus, the primary objective of this paper was to synthesize ATBS grafted polyester polyol and to prepare PUD from it. Due to synergistic effect, the ATBS-modified PUD is expected to exhibit the benefits of both the systems and presence of sulfonic acid groups would result in a strong polyelectrolyte character. In this study, we have attempted to introduce ATBS as a potential substitute for DMPA, which is a commonly used ionic moiety in conventional PUD.

Figure 1 gives the structure of ATBS and Table 1 records the specifications of technical parameters. ATBS was characterized by FT-IR and [.sup.1]H-NMR spectroscopy.

[FIGURE 1 OMITTED]
Table 1: Technical parameters of ATBS

Parameters                     Specifications
Appearance                     White powder
Nonvolatile matter             99.0% min.
Moisture                       0.30% max.
Color APHA (25% aq. solution)  100 max.


Considering the current importance and future potential of this relatively new but commercially available monomer in the coating industry, it was highly desirable to achieve a much better understanding of its role in PUDs. Therefore, it was thought worthwhile to investigate thermochemical, mechanical, and coating properties of PUDs and compare their properties with those of conventional PUD.

Experimental

Materials

Azelaic acid (AZA) LR (99%), maleic anhydride LR (98%), triethyl amine LR (99%), trimethylol propane (TMP, 98%), neopentyl glycol (NPG, 99%), and catalyst AIBN were procured from s.d. fine-chem (Mumbai, India). Dimethylol propionic acid (DMPA) (99%) and isophoron di isocyanate (IPDI) were purchased from Aldrich, USA. TMP was dried under vacuum at 1 mmHg and 85[degrees]C for 5 h before use. Triethyl amine (TEA) and N-methyl-2-pyrollidinone (NMP) (s.d. fine-chem, India) were dried over 3 [Angstrom] molecular sieves for 7 days. Ethylene diamine (EDA, 98%) was purchased from Fluka, Switzerland, and used as such without any further purification. A highly versatile monomer 2-acrylamido 2-methylpropane-sulfonic acid (ATBS, 99%, CAS No. 15214-89-8,) was procured from Vinati Organics Limited, Mumbai, India, and Catalyst Fascat 4100 (Butyl stannoic acid with 56.85% Sn) was kindly provided by Tarapur Coatings & Adhesives, Boisur, India. Solvents used in the titration were procured from s.d. fine-chem (Mumbai, India), and dried over 3 [Angstrom] molecular sieves before use. The emulsifying agent, defoamer, and biocides were supplied by KTECH, India.

Synthesis of polyester polyol (POLY 1)

A hydroxylated polyester polyol (POLY 1) with alkyd constant ~1.00 and hydroxyl value 106 mg KOH [g.sup.-1] (Mn = 1050) (12) (ASTM D 1957-286) was synthesized using azelaic acid, maleic anhydride, neopentyl glycol, and trimethylol propane (branching monomer) according to the formulations given in Table 2. A resin reactor equipped with thermometer, mechanical stirrer, Dean-Stark apparatus, and nitrogen inlet was charged with diacid, diol, and triol in the mole ratio of 0.4:0.5:0.08. Polyesterification was carried out in the presence of a catalyst, Fascat 4100 (0.05 wt% based on total weight of monomers), under a slow stream of [N.sub.2] to avoid oxidation due to atmospheric oxygen. The charge was initially heated to 120[degrees]C and thereafter increased with small increments of 20[degrees]C per hour until it finally settled at 180[degrees]C. The reaction was continued till the acid value dropped below 10 mg KOH [g.sup.-1]. Upon achieving the desired acid value, the temperature was brought down to 120[degrees]C, and the calculated quantity of maleic anhydride (0.1 mole) was added. The charge was further heated at 165[degrees]C until the acid value decreased to <5 mg KOH [g.sup.-1] (ASTM D 1639-90). The progress of reaction was solely monitored from acid value and the quantity of water of esterification accumulated during the course of reaction. Finally, the polyester polyol produced in this manner (Fig. 2) was discharged into a glass-stoppered bottle and was placed in a vacuum desiccator before the onset of further reactions. Table 3 displays the characteristics of newly synthesized polyester polyol (POLY 1).

[FIGURE 2 OMITTED]
Table 2:  Designing parameters for polyester polyol

Polyester polyol  AZA (g)  MA (g)  NPG (g)  TMP (g)

     POLY 1        75.20    9.80    52.00    11.17

Polyester                     Parameters
 polyol

           Alkyd constant     Excess        OH        Average
           K = [m.sub.0]     hydroxyl     excess   functionality
             /[e.sub.A]    content ratio     %    Fav = [e.sub.0]
                           R = [e.sub.B]            /[m.sub.0]
                            /[e.sub.A]

POLY 1     ~1              1. 250         25      2.0803

[m.sub.0], Total moles; [e.sub.0], total equivalents; [e.sub.A],
equivalents of acids; [e.sub.B], equivalents of glycols

Table 3: Characteristics of synthesized POLY 1

              Property                        POLY I

Physical properties
Molecular weight                     1050
Nominal functionality                2.08
Physical state                       Clear viscous liquid
Color (visual observation)           Off white
Solids (wt%)                         100

Chemical properties
Hydroxyl number (mg KOH [g.sup.-1])  106.27 (theoretical)
                                     106.85 (found)

Acid number (mg KOH [g.sup.-1])      <5


Preparation of aqueous polyurethane dispersion (waterborne polyurethane, PUD 1)

A typical anionic polyurethane dispersion, identified as PUD 1, was prepared by prepolymer mixing method (13) in two steps: synthesis of NCO-terminated prepolymers and preparation of dispersions by introducing anionic centers to aid dispersions (Fig. 3). Basic formulation of PUD 1 is given in Table 4. Isocyanate terminated prepolymer was prepared by reacting POLY I from the previous step with DMPA dissolved in NMP (5 wt% based on the total reaction mass) in a 500 mL four-necked round bottom flask fitted with mechanical stirrer, thermometer, nitrogen gas inlet, and reflux condenser. The mixture was heated on heating mantle at 80[degrees]C under nitrogen atmosphere for about 30 min. After complete mixing, IPDI and catalyst DBTDL (0.05 wt% based on total solids) were slowly added to the flask to maintain the reaction temperature at 85[degrees]C. The reaction proceeded until the amount of residual isocyanate groups reached a theoretical end point, calculated on the basis that all hydroxyl groups had reacted with isocyanate groups. The NCO content of the prepolymer was determined by dibutylamine back titration method. (14) Upon obtaining the theoretical NCO value, the prepolymer was cooled to 60[degrees]C, and the stoichiometric amount of TEA dissolved in NMP was added to it and stirred for 1 h to ensure complete neutralization of carboxylic group of prepolymer. The resultant polyurethane anionomer was then dispersed in water under high-speed stirring and desired molecular weight was achieved by adding EDA as a chain extender. For stabilization of dispersion, the emulsifying agent USOL K-98 (0.9% of total mass), defoamer, and biocides (0.1% of total mass) (KTECH, India) were added to aqueous dispersions. PU dispersion thus obtained had the 30 wt% solid contents.

[FIGURE 3 OMITTED]
Table 4: Composition of PUD 1 and MOD 1

Sample  POLY      POLY 1:DMPA      POLY 1: ATBS    IPDI (g)  TEA   EDA
        1 (g)  equivalent ratio  equivalent ratio            (g)   (g)

PUD 1   25.0           1:1                -          8.84    0.62  1.00
MOD 1   25.0             -               1:1         8.84    0.62  1.00


Typical procedure for preparation of acrylate modified aqueous polyurethane dispersion (MOD 1)

A polyester polyol was added to a 500 mL four-necked round bottom flask equipped with the same accessories as used in the preparation of PUD. Then calculated quantities of ATBS and initiator AIBN (0.05% based on weight of ATBS) were added to the polyol (Table 4). The mixture was kept at 80[degrees]C for 2 h while vigorous stirring and grafting were allowed to take place. After 2 h, IPDI and catalyst DBTDL (0.05 wt% based on total solids) were added drop-wise from a dropping funnel over a period of 1.5 h at 90-95[degrees]C into the reactor. The reaction was allowed to continue at this temperature for another 2 h until the isocyanate (NCO) content reached the desired value. The NCO content of the prepolymer was determined by dibutylamine back titration method. The prepolymer thus obtained was cooled to 60[degrees]C and the neutralizing solution of TEA dissolved in NMP was fed in slowly over 1 h. An aqueous dispersion of PU was obtained by adding the PU prepolymer to water under highspeed stirring in an ordinary steel reactor for about 15 min. After dispersion, the appropriate amount of EDA was added to perform the chain extension reaction. For stabilization of dispersion, the emulsifying agents USOL K-98 (0.9% of total mass), defoamer and biocides (0.1% of total mass) (KTECH, India) were added to aqueous dispersions. PU dispersion thus obtained had the 30 wt% solid contents. The modified PUD was abbreviated as MOD 1. The typical reaction scheme is shown in Fig. 4.

[FIGURE 4 OMITTED]

The general characteristics of PUD 1 and MOD 1 are discussed in Table 5.
Table 5: General characteristics and coating properties of
polyurethane dispersions

              Property                        PUD 1    MOD 1

General characteristics
Particle size (nm)                           72.8     83.3
Viscosity at 30[degrees]C (B4 ford cup) (s)  27       38
pH                                           8.0      8.1
Solids (wt%)                                 30.0     30.0

Mechanical properties
Hardness
   Pencil                                    5B       2H
   Shore A                                   69       89
Adhesion (Crosshatch)                        100%     100%
Flexibility (conical mandrel 1/4")           Passes   Passes
Impact resistance (in-lb)                    160/160  160/160
Direct/Reverse
Tensile (mPa)                                41.58    55.10
Elongation at break %                        530      480

Resistance to chemicals
3% Sulfuric acid (a)                         G        G
3% Sodium hydroxide (a)                      P        F

Resistance to solvents
Methyl ethyl ketone (MEK) (b)                28       100
Acetone (b)                                  40       70

G, Good; P, Poor; F, Fair

(a) 24 h immersion test
(b) Number of double rubs that the coating sustained without
any damage


Preparation of films

Films were prepared by casting the newly synthesized samples onto a Teflon plate at room temperature, followed by drying at ambient temperature for 48 h, and at 100[degrees]C for 2 h. This trend of drying is just for slow drying. It is also possible to evaporate the solvent at a fixed temperature (either room or elevated temperature). After demolding, the films were stored in a desiccator at room temperature for further studies.

Characterization

Fourier transform-infrared (FT-IR) spectroscopy

The IR spectra of polyurethane dispersion (PUD 1) and acrylate modified polyurethane dispersion (MOD 1) were obtained on a Perkin Elmer FT-IR spectrometer using NaCl pellet. Being in the form of thick syrup, a thin film of resin was cast over NaCl block.

[.sup.1]H-NMR

[.sup.1]H-NMR spectra of ATBS and MOD 1 were recorded using a Brucker 300 MHz NMR spectrophotometer at ambient temperature. DMSO-d6 was used as a solvent.

Particle size analysis

Particle size and viscosity are the important parameters in deciding the end use industrial applications of aqueous PUDs. Particle size was measured using Malvern Instrument India Ltd. Type Zetasizer 1000 HS and viscosity by B4 ford cup. All the measurements were carried out at 25[degrees]C.

Thermogravimetric analysis (TGA)

The decomposition profile of PUD film samples were thermogravimetrically analyzed using Diamond Perkin Elmer analyser. Film samples ranging from 4 to 6 mg were placed in a platinum sample pan and heated from 30 to 800[degrees]C, under [N.sub.2] atmosphere at a heating rate of 10[degrees]C [min.sup.-1], and the weight loss and temperature difference were recorded as a function of temperature.

Differential scanning calorimetry (DSC) analysis

Glass transition temperature of samples were measured using differential scanning calorimetry (DSC), on a NETZSCH DSC200 PC, using aluminum crimped pans under [N.sub.2] flow at 20 mL [min.sup.-1]. To erase the thermal history effects from the samples, the temperature was equilibrated at 150[degrees]C at the beginning of each experiment. The measurements were carried out between -100 and +150[degrees]C at a heating rate of 10[degrees]C [min.sup.-1].

Morphological properties (SEM analysis)

The morphology of the fractured surfaces of PUD 1 and MOD 1 was investigated by SEM to study the compatibility between polyurethane and polyacrylate phases.

Mechanical properties

The samples were applied onto previously degreased mild steel and glass panels using RDS USA make bar coater (50 [micro]m film thickness). Coated panels were then allowed to air dry at room temperature in fully ventilated atmosphere and were subjected to testing only after 7 days to ensure the full maturation of coated films. Pencil hardness and indentation hardness (by shore A durometer) were determined by ASTM 3363-74 and D 2240-86. A crosscut adhesion was employed as per ASTM D 3359-2002 to study the adhesion. Flexibility was measured using conical mandrel (1/4") bent test as per ASTM D 522-939. Impact resistance was measured on Falling Block Impact Tester (Komal Scientific, India) as per ASTM D 2794. Tensile strength and elongation tests were carried out on a computerized tensile testing machine, "Tensilon" (R&D Electronics, India), as per ASTM D 638. Molecular weight of the hydroxylated polyester polyol was evaluated by end group analysis. (15)

Chemical resistance

Chemical resistance tests were carried out by immersing coated glass panels (dry film thickness 40 [micro]m) into water, acid, and alkali solutions according to ASTM D 1647-89. The edges of the glass panels were coated with wax in order to prevent migration through the edges. The panels were then dipped into water, 3% (w/w) sulfuric acid solution, and 3% (w/w) sodium hydroxide solution, and were examined for the change in visual appearance after 24 h.

Solvent resistance

The solvent resistance was carried out as per the "Double Rubs" method using a piece of white cotton cloth (ASTM D 5402-93). The solvents used were methyl ethyl ketone and toluene. The result reported was the minimum number of double rubs at which the films were observed to fail or else reach 100, which was the maximum number of double rubs carried out.

Results and discussion

Infrared spectroscopy (IR)

Figure 5 shows the IR spectra of ATBS, PUD 1, and MOD 1. In the IR spectrum of ATBS, the band corresponding to the OH group in sulfonic acid is found at 2946 [cm.sup.-1]. The C=C stretching band of vinyl group occurs at 1614 [cm.sup.-1]. As geminal dimethyl group occurs at an internal position, a doublet is observed near 1373 [cm.sup.-1] region. Doublets are observed for geminal dimethyl groups because of interaction between the in phase and out of phase [CH.sub.3] bending of the two methyl groups attached to a common carbon atom. The band at 924 [cm.sup.-1] is due to methyl rocking vibration. In addition to these typical bands, we found the sulfonic acid ([SO.sub.2]) asymmetric and symmetric bands at 1248 and 1077 [cm.sup.-1], respectively, and C=C frequency of vinyl group at 978 [cm.sup.-1]. The sharp bands arising between 1000 and 900 [cm.sup.-1] are due to =C-H wagging in vinyl group of ATBS.

[FIGURE 5 OMITTED]

In the IR spectrum of ATBS modified polyurethane dispersion (MOD 1), the following bands are observed: the characteristics band of [NH.sub.2] at 3357 [cm.sup.-1], the stretching band of C=O at 1738 [cm.sup.-1], and the asymmetric and symmetric bands of [SO.sub.2] at 1247 and 1058 [cm.sup.-1], respectively, which confirms the graft polymerization of AMPS in the unsaturated polyester polyol. The amine salt of sulfonic acid absorbs strongly at 1170 [cm.sup.-1]. The bands at 2932 and 2859 [cm.sup.-1] may be attributed to asymmetric and symmetric stretching modes of [CH.sub.3] and [CH.sub.2] groups, respectively. A band at 1547 [cm.sup.-1] in PUD 1 and at 1564 [cm.sup.-1] in MOD 1 may be ascribed to combination of C-N stretching and N-H out of plane bending vibration. The absence of band due to NCO at 2270 [cm.sup.-1] confirms complete conversion by the curing agent.

It should be stressed that the effectiveness of the reaction between the monomers can be confirmed because the C=C frequency of 978 [cm.sup.-1], characteristic of the vinyl monomers, has disappeared and the bands in the region of 1000-900 [cm.sup.-1], which are due to =C-H wagging, are also no longer present in MOD 1. (8)

[.sup.1]H-NMR spectroscopy

The [.sup.1]H-NMR spectrum of ATBS showed the following signals: 1.442 ppm (s, 6H, -2[CH.sub.3]), 2.996 ppm (s, 2H, -[CH.sub.2]), 6.10 ppm (m, 2H, -[CH.sub.2]), and 8.2 ppm (s, 1H, NH).

The MOD 1 illustrated the peaks with chemical deviation of 1.83-1.88 ppm, corresponding to the hydrogens in the main chain. The [CH.sub.2] group bonded to sulfonic acid showed the signal at 3.8 ppm. The signal for the protons of vinyl group of ATBS is not detected at 5.3-6.1 ppm, which confirms the graft polymerization of vinyl group of ATBS onto polyester polyol backbone of MOD 1.

Particle size

The mean intensity average particle size diameter of PUD 1 and MOD 1 was shown in Figs. 6a and 6b, respectively, from which, it can be seen that the particle size distribution of MOD 1 became broader and shifted to larger particle size. The increase of particle size (Table 5) was primarily due to branching caused by grafting of vinyl group of ATBS onto polyester backbone of MOD 1. The strong electrostatic repulsions among sulfonate anions ([-SO.sub.3.sup.-]) could have resulted in the expanded network. Since branching probability increased, the distribution of the particle size was broadened, as expected.

[FIGURE 6 OMITTED]

Viscosity

Table 5 shows that the viscosity of the MOD 1 is much higher than PUD 1. This rapid rise in viscosity probably arose from main chain entanglements and interchain crosslinking in MOD 1 (H-bonds).

Thermogravimetric analysis

The TGA curves of PUD 1 and MOD 1 are shown in Fig. 7. The thermal stability can be evaluated by using the initial decomposition temperature (IDT) and the thermal indexes [T.sub.20] (20%) and [T.sub.50] (50%) of weight loss (Table 6). Thus, from the thermal indexes as a criterion of thermal stability, it can be inferred that ATBS modified polyurethane dispersion (MOD 1) had the higher thermal stability than PUD 1, indicating a synergistic effect. This behavior can be attributed to the stronger interactions during grafting of ATBS onto polyester backbone of MOD 1. Figure 7 reveals that the thermal degradation of MOD 1 occurs in three stages: decomposition of amide group (urethane linkage), degradation of sulfonic groups, and breakdown of polymer backbone. (16) At 600[degrees]C, the char residue of MOD 1 is 10.2%, whereas PUD 1 is completely degraded at 600[degrees]C. Thus, it can be concluded that ATBS incorporation has pronounced effect on the thermal degradation behavior of PUDs.

[FIGURE 7 OMITTED]
Table 6: [T.sub.g] and TGA data of polyurethane dispersions

Polymer    [T.sub.g]   IDT (a)   [T.sub.20]    [T.sub.50]   % Residue
  type   ([degrees]C)           ([degrees]C)  ([degrees]C)      (d)
                                     (b)           (b)

PUD 1     -39.1          232         285           387          0
MOD 1     -2.7, 29.3     265         300           395         10.2

(a) Initial degradation temperature
(b) Temperature at which 20% weight loss occurred
(c) Temperature at which 50% weight loss occurred
(d) % Char left at 600[degrees]C


DSC analysis

The glass transition temperature of the PUD 1 and MOD 1 was determined to observe the compatibility and interaction between the polymers (Table 6). The PUD 1 shows the single glass transition temperature at -39.1[degrees]C, whereas MOD 1 has two glass transition temperatures (Fig. 8). The one at -2.7[degrees]C corresponds to the flexible segments in polyurethane and the other at 29.3[degrees]C corresponds to the rigid segment in polyurethane, which may be ascribed to microphase separation between the flexible and rigid segments. However, such type of microphase separation is helpful to improve the mechanical properties of polyurethane dispersions.

[FIGURE 8 OMITTED]

SEM analysis

The SEM technique was used to investigate the morphology of the fractured surfaces of the polyurethane dispersions, PUD 1 and MOD 1. The results of SEM showed obvious differences existing between the surface of PUD 1 and MOD 1 (Fig. 9). Microgram of MOD 1 clearly indicates the coexistence of a dual phase, which appears to be cocontinuous as well. However, under same conditions, a homogenous morpholohy is seen in case of PUD 1. Thus, it can be concluded that different morphologies are due to the different interactions taking place, owing to different anionomers in PUD 1 and MOD 1.

[FIGURE 9 OMITTED]

Hardness

Shore A and pencil hardness observations (Table 5) confirmed the increase in hardness of MOD 1 as compared to PUD 1. The increase in film hardness is most likely because the [T.sub.g] of MOD 1 (29.3[degrees]C) covers a significantly wider and higher temperature range than PUD 1. Thus, at the measurement temperature (27[degrees]C), a greater percentage of the polymer exists in hard and glassy state.

Adhesion and flexibility

From Table 5, it can be seen that both PUD 1 and MOD 1 showed excellent adhesion and good flexibility.

Impact resistance

The excellent impact resistance of dried films could be attributed to very tough films resulting from both PUD 1 and MOD 1. Toughness is one of the inherent characteristics of vinyl group containing acrylates as well as of the polyurethane resins (Table 5).

Tensile behavior

Figure 10 illustrates the tensile behavior of films obtained from PUD 1 and MOD 1. The tensile strength and elongation at break were 41.58 MPa and 530%, respectively, for the films of PUD 1 and 55.10 MPa and 480%, respectively, for the film from MOD 1 (Table 5). As expected, (17) the weaker acid strength of the carboxylate groups led to formation of weaker physical crosslinks in the carboxylated ionomers and decreased tensile properties in comparison to the sulfonated ionomers.

[FIGURE 10 OMITTED]

Chemical resistance

It is apparent from Table 5 that both of the coatings perform well in 3% acid solution. However, the effect of ATBS modification on resistance of PUDs is very evident from enhanced alkali resistance of MOD 1 as compared to PUD 1.

Solvent resistance

It can be seen from Table 5 that MOD 1 has much better solvent resistance than that of PUD 1. This increased solvent resistance is most likely due to shielding effect of geminal dimethyl group of ATBS, which is used to modify PUD.

Conclusion

The study has highlighted the differences in properties of PUDs that could be achieved by incorporating an anionic monomer, ATBS, against conventional PUD based on DMPA. The acrylate-grafted PUD exhibited a comparatively higher alkali resistance and improved thermal stability. The particle size and viscosity of MOD 1 were higher than that of PUD 1, which confirmed the presence of additional crosslinkages and interchain forces in MOD 1. The MOD 1 based on ATBS shows significant enhancement in mechanical and chemical properties over that of PUD 1 prepared from DMPA. Hence, it can be concluded that, using ATBS as an hydrolysable anionomer, it is possible to design most desirable and high performance PUDs for the specific end use applications.

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[C] FSCT and OCCA 2009

V. D. Athawale ([??]), M. A. Kulkarni

Department of Chemistry, University of Mumbai, Vidyanagari, Mumbai 400 098, India e-mail: vilasda@yahoo.com
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Author:Athawale, Vilas D.; Kulkarni, Mona A.
Publication:JCT Research
Article Type:Report
Geographic Code:9INDI
Date:Mar 1, 2010
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