Preparation, characterization and properties of ...

Article

Preparation, characterization and properties of organoclay reinforced polyurethane nanocomposite coatings

Journal of Plastic Film & Sheeting 29(1) 56–77 ß The Author(s) 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/8756087912448183 jpf.sagepub.com

Gaurav Verma1, Anupama Kaushik1 and Anup K Ghosh2

Abstract A processing scheme is used to reinforce polyurethane coatings with an organoclay Cloisite 20A in three different weight percentages (1, 3 and 5 wt%). Transmission electron microscopy of Cloisite 20A-polymer dispersions shows exfoliation, intercalation and even agglomeration of clay layers. X-ray diffraction studies also indicate expansion in gallery spacing of organoclay layers in polyurethane coatings. Light scattering experiments shows the unimodal and bimodal particle size distributions for 3 and 5 wt% Cloisite 20A dispersions. Rheological behavior of these dispersions changes from quasi-Newtonian to strongly pseudoplastic fluid. The power law index values decrease from 0.46 (0 wt%) to 0.29 (5 wt%) indicating increased shear thinning in coating formulation due to addition of Cloisite 20A. Although gloss reduces by 33%, maximum temperature of degradation improves by 27 C in 5 wt% Cloisite 20A-PU coatings. A loading of 5 wt% Cloisite 20A enhances the mar resistance by 22%, while the char residue of polyurethane coatings increases by 6 times. Color changes (CIEL*a*b*) in polyurethane nanocomposite coatings are insignificant below an addition of 5 wt% organoclay. Fourier transform infrared analysis shows that Cloisite 20A has good chemical compatibility with polyurethane and there is no change in basic urethane structure of the coatings.

1

University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh, India Centre for Polymer Science & Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India

2

Corresponding author: Gaurav Verma, University Institute of Chemical Engineering & Technology, Panjab University, Chandigarh, India Email: [email protected]; [email protected]

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Keywords Polyurethane-organoclay, nanocomposite coatings, X-ray diffraction, transmission electron microscopy, dynamic light scattering, viscosity, mar resistance, Fourier transform infrared, CIEL*a*b*, thermal gravimetric analysis

Introduction Polymer-layered silicates nanocomposites have attracted great interest from the scientific and business community because of its versatile applications in automotive industry; packaging, film and bottling industry; wires and cable industry; electronics and plastics industry.1–4 A wide spectrum of polymers like polyamides, polycaprolactone,2,5 polypropylene,6 ethylene propylene diene methylene linkage rubber,7 poly(methyl methacrylate),8 polycarbonate,9 chitosan,3 polybutadiene,10 epoxy polymer resins11 and polyurethanes (PU)12–15 have been combined with layered silicates to form nanocomposites. The diverse applications of PU in form of elastomers/rubbers, plastics, foams, adhesives, sealants, biocompatible polymers and coatings make it one of the most promising polymers to be explored.16 Over the years PU has been synthesized using various combinations of its components. Numerous raw materials available for manufacturing PUs are diisocyanates, polyether or polyester polyols, diols and chain extenders.17 With this rich available chemistry, the use of PUs as coatings finds extensive applications in bridges, buildings, exteriors, interiors, aircrafts and automobiles.18 The performance of PU coatings is constantly under scan and monitoring so as to improve their properties especially thermal stability and mar resistance with minimum compromise on its inherent properties like gloss, flow viscosity and the basic chemical structure.18,19 Although research in this area is underway; the most plausible option for improving PU coatings is by using nano-additives like ZnO, TiO2, SiO2, CNTs, expanded graphite and layered silicates.19 Of these additives, layered silicates tend to improve the characteristics of PUs without adversely affecting its lucrative chemistry.20 The scientific, commercial and technical literature has many reports on PU-layered silicates nanocomposites.21–25 But few describe the practical usage of layered silicates specifically for coating applications.19,26 This may be because coatings have to be designed and carefully processed for its flow properties, thermal degradation, morphology and structure especially when fillers like layered silicates with high surface energy are added to it. The tendency of layered silicates to agglomerate during processing makes it difficult for coating technologists to freely use it.18,19 Layered silicates below the size of 500 nm may not impart any color characteristics to clear PU coatings and hence can be used in low quantities


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Journal of Plastic Film & Sheeting 29(1)

to improve specific properties like thermal stability and mar-resistance without adversely affecting the clarity and gloss of the coatings.18,19 The technology behind controlling the particle size of these fillers is still in its infancy. In this study an organically modified layered silicate Cloisite 20A is used to reinforce a PU coating formulation using a processing scheme (Figure 1). Although Cloisite 30B is known to have better compatibility with PU, recent reports on Cloisite 20A have shown good results with thermoplastic PU nanocomposites.14,27 The dearth of reports of this variant of clay especially for coatings has prompted us to undertake the preparation and characterization of these nanocomposites with thermosetting PU. The morphology and structure of these Cloisite 20A loaded PU formulations are characterized using X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Also dynamic light scattering (DLS) determines the particle size distribution of Cloisite 20A in the polymeric formulation. Rheology of the clay loaded polymeric dispersions is also tested and analyzed by using a parallel plate arrangement of a dynamic rheometer. Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) chemically characterizes the coatings. The vital optical properties like gloss and the color scale parameters (CIEL*a*b*) are also assessed. The mar resistance of these coatings is measured using an automated scratch tester. Finally the thermal stability is evaluated through the Thermal gravimetric analysis (TGA). This work hence explains the preparation, characterization and properties of the Cloisite 20A-PU nanocomposite coatings.

Experimental Recipe for coatings: materials A two-pack PU system consisting of a binder (Desmophen 680BA, a branched hydroxyl polyester with an equivalent weight ¼ 770) and a hardener (DesmodurÕ N 3390BA, a polyisocyanate with an equivalent weight ¼ 214) was obtained from Bayer Material Science, Germany. Mixed-xylene (Reliance Petrochemicals, India) and n-butyl acetate (Shell Chemical Company, USA) were also used as received. Modified layered silicate (Cloisite 20A with 90% of the dry particles having size 1 for complete curing. C20A-polymer suspensions are collected from this step prior to application of these samples for coatings. The complete preparation flow sheet is shown in Figure 1.

Cloisite 20A+m-xylene+n-butyl acetate 2 hr, 50°C

X + Binder

10min Cloisite 20A dispersions 10000rpm

Y + Hardener 5 min, 250rpm + Catalyst

4 hr, 50°C Y

X

PUCN

C20A polymer suspensions Ultrasonic Bath

Ultrasonic Bath

High shear homogenizer

Mechanical stirrer

Figure 1. Flow sheet for preparation of Cloisite 20A dispersions, suspensions and PUCN samples.

Figure 2. Actual picture of Cloisite 20A dispersions. The decreasing visibility of the black band-line in the background is for the visual assessment of the sample clarity.


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Application of coatings Mild steel (MS) panels 150 75 1 mm3 were used as substrates for the above prepared coating formulations (PUCN). These MS panels were spray coated with the coating formulation and were dried under ambient conditions. All tests on the samples were carried out only after a week to allow complete curing of the coatings.18 For XRD and TGA tests free standing PUCN film samples were obtained by depositing the formulations on highly glazed ceramic tiles.

Characterizations Morphological and structural C20A powder and PUCN film samples were subjected to XRD using a Philips XpertPRO 240 mm diffractometer; with an integrated germanium detector of CuKa source (wavelength ¼ 1.54 A˚), operating at 45 kV/40 mA. The equatorial scans in continuous mode were obtained from 2y ¼ 3 to 10 in steps of 0.0170 with a scan step time of 30 s. The d spacing is calculated as per the Bragg’s scattering relation (1) 2d sin ¼ nl

where

ð1Þ

d ¼ gallery spacing of the layered silicates n ¼ number of crystallites (here it refers to clay structures) l ¼ wavelength of the X-rays.

TEM was performed on organoclay suspensions (refer Figure 1 and 2) using a Hitachi high powered electron microscope (H-7500) operating at an accelerating voltage of 100 kV. Samples for TEM were prepared by drying diluted suspension on a gold grid plated with a thin layer of carbon (thickness around 200 nm). A drop of a dilute suspension of C20A in polymer matrix was deposited on the carbon-coated grids and allowed to dry at room temperature.15 TEM helped in visually assessing the distribution of layered silicates.

Particle size measurements The particle size distribution of layered silicates in PUCN was also measured for organoclay dispersions, using light scattering equipment (Malvern Zetasizer NanoZS with application software version 5.10). The scattered light was measured at different angles in the range of 60 –120 . The temperature was set to 25 0.1 C. The primary information given by


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DLS data are intensity distribution whereby the relative amount of each particle size is measured by the intensity scattered by all the particles of the considered size. The intensity distributions have been converted into volume distributions to obtain different mean average sizes. The size of a particle is calculated from the translational diffusion coefficient by using the Stokes– Einstein equation (2) d ðHÞ ¼ kT=3D

where

ð2Þ

d(H) ¼ hydrodynamic diameter of C20A in the binder D

¼ translational diffusion coefficient

k

¼ Boltzmann’s constant

T

¼ absolute temperature

H

¼ viscosity of binder

Rheological measurements Measurement of viscosity of the organoclay dispersions was done on Malvern Bohlin Instruments CVOR at a temperature of 25 C. Organoclay dispersions were pipetted on the stationary disk of a parallel plate arrangement placed at a gap of 400 mm for the rheological tests. The moving plate has a diameter of 20 mm.

Spectroscopic assessments FTIR-ATR was carried out on a Perkin Elmer synthesis monitoring system in ranges of 500–4000 cm1. A minimum of 10 scans at a resolution of 2 cm1 each was conducted on the PUCN samples. FTIR-ATR spectra are essential to confirm the completion of PU formation both for blank and PUCN samples. CIEL*a*b* (Commission International de l’ećlairage) color parameters on GretagMacbethTM Color-Eye 7000A apparatus was used to evaluate the differences between color points corresponding to blank PU (PUC0) and C20A-loaded PU (PUC1,PUC3,PUC5) coating samples. This in fact helps in estimating the color changes in the clear PU coatings due to addition of layered silicates. The FTIR-ATR and CIEL*a*b* color characterization is conducted directly on PUCN coated MS panels.

Coating gloss and dry film thickness Gloss in percentage was evaluated at 20 for PUCN using a Sheen 160T TriMicrogloss instrument according to ASTM D523.28 The values were averaged


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after ten readings in each case. Dry film thickness (dft) was measured on PUCN coated MS panels with Elcometer 456 coating thickness digital gauge as per ASTM D1186.29 The dft values were averaged after 10 readings in each case.

Mar resistance Mar resistance of the PUCN samples was measured with Automatic scratch hardness tester (Sheen Instruments) on PUCN-coated MS panels by varying loads from 1500 to 4000 g as per ASTM D5178.30

Thermal properties The thermal degradation of the PUCN film samples was studied using TGA 7.0 with Pyris software (Perkin-Elmer Inc., USA) under non-isothermal conditions at a constant heating rate of 20 C/min in an inert nitrogen atmosphere from 50 C to 600 C. Table 1. Sample codes, particle sizes and PUCN nanocomposite coatings properties Avg. particle size (nm) (TEM)a

Avg. particle size (nm) (DLS)b

Power law index, n

20 Gloss in % and dft 5 mm

Sample codes

Wt% C20A

PUC0

0

–

–

0.4613

98.6 (57)

PUC1

1

176

–

0.4501

92.3 (59)

PUC3

3

223

125

0.3263

88.5 (55)

PUC5

5

789

889

0.2926

65.0 (58)

Sample codes also designate the C20A suspensions/dispersions prepared during the processing scheme. a Sizes calculated from 100 measurements each for a sample using various TEM visualizations. b Particle size in case of DLS is the hydrodynamic diameter which is accrued to particles in constant motion.

Results and discussions XRD and TEM analysis The d-spacings of PUCN nanocomposite coatings were studied by XRD and shown in Figure 3. A small peak at 2y ¼ 3.7 corresponding to the basal spacing of 23.9 A˚ in C20A is observed.31 The characteristic diffraction peak for d001 spacing of the organically modified layered silicates almost disappeared in the PUC1 and PUC3, indicating that oriented layers were disrupted by PU chains leading to possible exfoliation. The absence of peak in XRD signifies that either the d001 spacing of the clay becomes too large or highly disordered such that it is


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non-detectable by XRD and fails to produce a Bragg’s diffraction peak.15 PUC5 shows a small peak, although it slightly shifts towards a smaller angle in comparison to the corresponding diffraction peak for C20A. It signifies an enhancement in basal spacing to only 24.6 A˚. This indicates that the PU molecules have possibly intercalated into the layers of the silicates in the nanocomposite samples resulting in exfoliation of some of the particles, while few particles remained as non-intercalated as seen by the presence of small peak in PUC5 and even PUC3 (Figures 4 and 5).31–33 Disappearance of Bragg diffraction peaks in the XRD patterns cannot be used as the sole evidence for the formation of an exfoliated structure. Several factors such as dilution of clay, peak broadening and preferred orientation make XRD characterization of polymer nanocomposites susceptible to errors.15 To corroborate the results obtained by XRD, TEM investigations were performed.

Relative Counts

C20A (001)

PUC5 PUC3 PUC1 PUC0 4

6

8

10

Position [°2Theta] (Copper (Cu)

Figure 3. XRD of C20A and PUCN nanocomposites. Arrow indicates direction of peak shifting. XRD: X-ray diffraction.

Figure 4 shows TEM images for PUC3 and PUC5; which depicts the particle size of C20A platelets in the polymer suspensions as also the state of dispersion. PUC3 has smaller stacks of platelets as compared to the relatively larger ones in PUC5. The large size of few of the platelets might be due to the presence of overlapping platelets at one position.11,15,33 PUC5 shows majority of large sizes of C20A which occurs due to collection of intercalated and non-intercalated particles (Figure 5), thus indicating the presence of agglomerated particles. Figure 5 shows the intercalated platelets for PUC5, in which the d spacing varies between 24.8 and 25.7 A˚, which is in close confirmation with the diffraction patterns observed in Figure 3. The TEM images actually indicate the sizes based on level of dispersion, C20A seems to be better dispersed in PUC3 than PUC5. The averages of sizes from various pictures of the samples using 100 measurements are tabulated in Table 1.


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Figure 4. TEM pictures of PUC3 and PUC5 samples. TEM: transmission electron microscopy.

Particle size distribution Figure 6 shows the particle size (hydrodynamic diameter) distribution plotted for PUC3 and PUC5 samples. For PUC3 dispersions about 90% particles are under 100 nm while only 10% lie between 500 and 1000 nm. On the contrary for PUC5 almost all particles lie in the range of 500–1000 nm. Hence two populations of particles are observed for PUC3 while only one is observed for PUC5. These results are in close coherence with the TEM analysis discussed in the previous section, although sizes from DLS are hydrodynamic diameters of equivalent spheres which accrue to particles in constant motion. Similar particle size distribution in polymer matrix has been shown by a classic investigation on clay–polymer nanocomposites.34

Figure 5. Intercalated and non-intercalated/agglomerated states of C20A in PUC5 samples.


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Table 2. Characteristic band assignments of C20A Wavenumber (cm1)

Symbol

Band description

dSi–O–Si nAl–O–Si dO–H nSi–O-Si,C–N ds(CH2) dH–O–H gsN–H ns(CH2) nasym(CH2) nOH

462-463 522 917 1038–1048 1469 1653 2800–2930 2852 2925 3400–3630

Stretching vibrations of Si–O–Si Bending vibrations of Al–O–Si Bending vibrations of O–H of Al–Al–OH Stretching vibrations Si–O–Si and C–N Scissoring vibrations C–H of CH2 Deformation vibrations H–O–H in H2O Stretching vibrations of N–H of ammonium salts Symmetric stretching vibrations C–H of CH2 Asymmetric stretching vibrations C–H of CH2 Stretching vibrations O–H of H2O

Rheological analysis The viscosity as function of shear rate of unfilled binder is compared with that of C20A filled binder. Viscosity increases with increase in weight percentage of C20A as shown in Figure 7. The rheological behavior of the binder changes from that of a quasi-Newtonian fluid to a strongly non-Newtonian pseudoplastic fluid with the addition of C20A.23 This pseudoplastic behavior indicates that layered silicate is responsible for aggregation of binder chains which behave as larger and entangled polymer molecules. Clay lamellar platelets may also act as weak and labile physical cross linking spots, comparable with the entanglements occurring in high molecular weight polymers, when their concentration is greater than 3 wt%.23 To substantiate the above findings the values of power law index ‘n’ were also calculated from the rheological data using the power law equation (3) where

¼ mðdg=dtÞn1

¼ shear viscosity (Pa s)

m

¼ consistency index

ð3Þ

dg/dt ¼ g the shear rate (s1) n

¼ power law index

For the power law index values of n ! 0 implies high shear thinning behavior and n ! 1 implies solutions approaching Newtonian fluid.


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These values of ‘n’ for different formulations are shown in Table 1. Clearly the shear thinning behavior is enhanced as the concentration of C20A in the binder mixture increases.35 This may occur due to presence of platelets in C20A loaded samples which initially show resistance to flow and gradually get oriented in direction of applied stress.

Figure 6. Size distribution of C20A particles in PUC3 and PUC5 dispersions.

Table 3. Characteristic band assignments of PUC0 and PUCN Wavenumber (cm1)

Symbol

564 587 651 765 1114 1215–1350 1379 1462 1500–1600

dN–C–N bCH2 oN–H dasymN–H nC–O–C nC–N, dN–H nC–N ds(CH2) dN–H þ nC–N þ nC–C

1600–1800 1719

nC¼O, nC–N, nC–C–N

2856 2926 3150–3700

ns(CH2) nasym(CH2) nNH/nOH

Band description Bending vibrations of N–C–N Bending vibrations C–H of CH2 Amide V Amide IV, N–H out of plane bending Stretching vibration of ester group Amide III, nC–N, N–H bending and C–Ca Stretching vibrations of C–N Scissoring vibrations C–H of CH2 Amide II: sensitive to chain conformation and intermolecular hydrogen bonding Amide I: stretching vibrations of C¼O, C–N stretching and C–C–N deformation vibrations Symmetric stretching vibrations C–H of CH2 Asymmetric stretching vibrations C–H of CH2 Stretching vibrations NH/OH


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Table 4. PUCN optical and thermal properties Sample codes PUC0 PUC1 PUC3 PUC5

L*

a*

b*

T5 wt% ( C)

Tdeg ( C)

Wdeg (%)

62.132 63.636 63.844 65.627

0.128 0.261 0.435 0.976

0.951 1.093 1.338 1.943

235 248 255 260

465 477 483 492

1 2 4 6

All L*a*b* have been ascertained for colorless PUCN coatings sprayed on mild steel panels, which are steel-grey in color.

FTIR-ATR and CIEL*a*b* color parameters Figure 8 shows the FTIR-ATR spectra for C20A, PUC0 and PUCN coatings. The spectral assignments for PUCN even with varying C20A loadings are broadly similar to PUC0, suggesting that the chemical functional groups of PU remain same.36,37 The peak assignments for C20A and PUCN samples are listed in Tables 2 and 3, respectively. For PUCN, the absence of peak at 2270 cm-1 (attributed to NCO of polyisocyanate) confirms the completion of urethane formation for all categories of PUCN. Hence the addition of C20A does not interfere with basic urethane reaction and inherent PU chemistry. Further for PUCN spectra the stretching vibrations for NH/OH lie between 3150 and 3700 cm1, while the asymmetric and symmetric stretching vibrations C–H of CH2 are observed at 2926 and 2856 cm1, respectively. The region between 1600 and 1800 cm1 involves the contribution of the C¼O stretching, the C–N stretching, and the C–C–N deformation vibrations. The carbonyl groups make this range considerably broader with multiple overlapping peaks (1600–1800 cm1). The 1510–1550 cm1 region is a mixed contribution of the N–H in-plane bending, the C–N stretching, and the C–C stretching vibrations. The stretching vibration of the C–N group is observed at 1200–1300 cm1. The NH out-of-plane deformation mode lies between 500 and 800 cm1 region. A very weak single band is observed at 860 cm1, originating either from the coupled vibration of the C–O stretching or the CH2 rocking modes. The strong infrared band assigned to the asymmetric stretching vibration of the C–N group is expected at 1040 cm1. This band almost overlaps with the very strong band at 1114 cm1, the C–O–C stretching vibration of ester groups in PU coatings. Due to addition of C20A peaks are observed around 463 and 523 cm1 for the stretching vibrations of Si–O–Si and bending vibrations of Al–O–Si, respectively.38 All the peak assignments of PUC0 and PUCN match though there might be slight shifting of peak positions, possibly due to hydrogen bonding and other interactions of PU chains with C20A platelets.12,38 The other peak assignments of C20A merge


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easily in the existing peaks of PU and hence can’t be differentiated as shown in Figure 8 (box).

Figure 7. Viscosity as a function of shear rate (log versus log ’ plots) for C20A-binder dispersions.

Addition of fillers in coatings may also cause chromatic changes, which are spectroscopically determined by the CIEL*a*b* parameters (Table 4).39 The values of L* lie between 0 (black) and 100 (white). Positive a* signifies red while negative values signify green; similarly þb* implies yellow while b* implies blue color. Figure 9 gives the changes in color parameters for PUCN with respect to PUC0 samples. Only at 5 wt% C20A loadings these values assume prominence, below this weight percent the changes are less significant. This shows that if the addition of clay is above 3 wt% (particularly when the processing scheme given in Figure 1 is used) the chances of chromatic changes in colorless PU coatings are more (Table 4). These changes though are not visible to the human eye.


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Relative transmittance / intensity

C20A PUC5 PUC3 PUC1

PUC0

3400

2400 Wavenumber [1/cm]

1400

400

Figure 8. FTIR-ATR spectra for PUCN and C20A samples. FTIR-ATR: Fourier transform infrared-Attenuated total reflectance.

Gloss and dry film thickness Table 1 shows the decrease in the gloss values with increase in C20A content, which can be attributed to increased surface roughness in PUCN coatings. The addition of ceramic filler to a soft polymeric matrix causes increased surface roughness. Larger is the particle size of the filler greater is the reduction in gloss values. Hence increased surface roughness is dependent on the particle size of the filler which has been inducted into the PU matrix. The dft is 55 5 mm. Addition of C20A does not adversely affect the dft.

Mar resistance Figure 10 shows the increased mar resistance values for various coating samples. The addition of C20A to PU led to increase in mar resistance as it acts as reinforcing filler to the PU matrix. The mar resistance is 2100 and 2200 g for PUC3 and PUC5 respectively. There is a possibility that C20A strengthens the cross-linking in the PU chains (though this aspect needs further in-depth investigation) and hence enhances the resistance to surface deformation.


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Figure 9. Changes in L*a*b* color values for PUCN with respect to PUC0 coating.

TGA analysis Figure 11 shows TGA scans for the PUCN samples. T5 wt% (initial degradation temperature at 5% weight loss), Tdeg (the maximum degradation temperature) and % char residue at degradation (Wdeg) are identified with help of dotted lines and are also shown in Table 4. Compared with the pure PUC0, the nanocomposites in this study especially PUC3 and PUC5 exhibited a delayed decomposition, and the thermal stability was largely improved by the presence of the dispersed C20A organoclay. Initial degradation temperature (T5 wt%) increased by 13, 20 and 25 C for 1, 3 and 5 wt% C20A addition, respectively. The final degradation temperature of 465 C also increased to 477 C, 483 C and 492 C for 1, 3 and 5 wt% C20A additions, respectively. Similarly the char residue doubled with addition of just 1 wt% C20A. The role of C20A as thermal insulator and mass transport barrier to prevent early thermal decomposition of PU is thus established.40 Hence C20A enhances the thermal stability of PU coatings by providing a barrier effect to mass transport phenomena which takes place during thermal degradation.


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Figure 10. Mar resistance values in grams for PUCN samples.

Figure 11. Thermal gravimetric scans in N2 atmosphere for PUCN samples. T5 wt%, and Tdeg, char residue at degradation (Wdeg) are shown through dotted lines. For purpose of clarity the scans are not overlapped.


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Conclusions PU coatings are reinforced with organoclay Cloisite 20A using a novel processing method. TEM and XRD studies show that varied sizes of clay exist in the matrix possibly due to intercalation, exfoliation and agglomeration in PUorganoclay nanocomposites. DLS size determination of C20A show a close approximation to the C20A morphology ascertained through TEM and XRD. Greater shear thinning behavior is induced with 3 wt% addition of organoclay in PU. Although FTIR confirms that functional groups in PU are hardly affected by presence of C20A reinforcements; but the improvements in thermal stability and mar resistance are due to distribution of the nano-sized organoclays in the PU matrix. Significant reduction in gloss and changes in color parameters occurs only above 3 wt% C20A loadings indicating that with this kind of processing scheme PUC3 shows optimal property combinations.

Funding The authors thank UGC research grant and DST Chandigarh for support.

Acknowledgement We are thankful to Sh. A. Girish (ex-ICI Paints) for a whole-hearted support he extended in carrying out various testing.

References 1. Advani SG. Processing and properties of nanocomposites, 1st edn. Singapore: World Scientific, 2007. 2. Lagaroń JM and Nuń˜ez E. Nanocomposites of moisture-sensitive polymers and biopolymers with enhanced performance for flexible packaging applications. J Plast Film Sheet 2012; 28(1): 79–89. 3. Lagaroń JM and Fendler A. High water barrier nanobiocomposites of methyl cellulose and chitosan for film and coating application. J Plast Film Sheet 2009; 25(1): 45–59. 4. Sanchez-Garcia MD, Ocio MJ, Gimenez E, et al. Novel polycaprolactone nanocomposites containing thymol of interest in antimicrobial film and coating applications. J Plast Film Sheet 2009; 24(3–4): 239–251. 5. Dabrowski F, Bourbigot S, Delbel R, et al. Kinetic molding of the thermal degradation of polyamide-6 nanocomposite. Eur Polym J 2000; 36(2): 273–284. 6. Liu X and Wu Q. PP/clay nanocomposites prepared by grafting melt intercalation. Polymer 2001; 42(25): 10013–10019. 7. Usuki A, Tukigase A and Kato M. Preparation and properties of EPMD–clay hybrids. Polymer 2002; 43(8): 2185–2189.


Verma et al.

73

8. Salahuddin N and Shehata M. Poly(methyl methacrylate)– montmorillonite composites: Preparation, characterization and properties. Polymer 2001; 42(20): 8379–8385. 9. Huang X, Lewis S, Brittain WJ, et al. Synthesis of polycarbonate-layered silicate nanocomposites via cyclic oligomers. Macromolecules 2000; 33(6): 2000–2004. 10. Nugay N, Kusefoglu S and Erman B. Swelling and static dynamic mechanical behavior of mica-reinforced linear and star-branched polybutadiene composites. J Appl Polym Sci 1997; 66(10): 1943–1952. 11. Kornmann X, Lindberg H and Berglund LA. Synthesis of epoxy–clay nanocomposites: Influence of the nature of the clay on structure. Polymer 42(4): 1303–1310. 12. Tien YI and Wei KH. The effect of nano-sized silicate layers from montmorillonite on glass transition, dynamic mechanical and thermal degradation properties of segmented polyurethane. J Appl Polym Sci 2002; 86(7): 1741–1748. 13. Maji PK, Das NK and Bhowmick AK. Preparation and properties of polyurethane nanocomposites of novel architecture as advanced barrier materials. Polymer 2010; 51(5): 1100–1110. 14. Pattanayak A and Jana SC. Synthesis of thermoplastic polyurethane nanocomposites of reactive nanoclay by bulk polymerization method. Polymer 2005; 46(10): 3275–3288. 15. Kaushik A, Ahuja D and Salwani V. Synthesis and characterization of organically modified clay/castor oil based chain extended polyurethane nanocomposites. Composites: Part A 2011; 42(10): 1534–1541. 16. Kro´l P. Synthesis methods, chemical structures and phase structures of linear polyurethanes. Properties and applications of linear polyurethanes in polyurethane elastomers, copolymers and ionomers. Prog Mater Sci 2007; 52(6): 915–1015. 17. Oertel G. Polyurethane handbook, 2nd edn. Munich: Hanser Publishers. 18. Paul S. Surface coatings science & technology, 2nd edn. Chichester: John Wiley & Sons Ltd. 19. Chattopadhyay DK and Raju KVSN. Structural engineering of polyurethane coatings for high performance applications. Prog Polym Sci 2007; 32(3): 352–418. 20. Ke YC and Stroeve P. Polymer-layered silicate and silica nanocomposites. Amsterdam: Elsevier, 2005. 21. Finnigan B, Martin D, Halley P, et al. Morphology and properties of thermoplastic polyurethane nanocomposites incorporating hydrophilic layered silicates. Polymer 2004; 45(7): 2249–2260. 22. Dimitry OIH, Abdeen ZI, Ismail EA, et al. Preparation and properties of elastomeric polyurethane/organically modified montmorillonite nanocomposites. J Polym Res 2010; 17(6): 801–813. 23. Corcione CE, Prinari P, Cannoletta D, et al. Synthesis and characterization of clay-nanocomposite solvent-based polyurethane adhesives. Int J Adhes Adhes 2008; 28(3): 91–100. 24. Chattopadhyay DK, Sreedhar B and Raju KVSN. Influence of varying hard segments on the properties of chemically crosslinked moisture-cured polyurethane-urea. J Polym Sci Part B: Polym Phys 2006; 44(1): 102–108.


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25. Cao X, Lee LJ, Widya T, et al. Polyurethane/clay nanocomposites foams: Processing, structure and properties. Polymer 2005; 46(3): 775–783. 26. Ahmadi B, Kassiriha M, Khodabakhshi K, et al. Effect of nano layered silicates on automotive polyurethane refinish clear coat. Prog Org Coat 2007; 60(2): 99–104. 27. Chavarria F and Paul DR. Comparison of nanocomposites based on nylon 6 and nylon 66. Polymer 2004; 45(25): 8501–8515. 28. ASTM D523 - 08. Standard test method for specular gloss. West Conshohocken, PA: ASTM, 2008. 29. ASTM D1186 - 93. Standard test methods for nondestructive measurement of dry film thickness of nonmagnetic coatings applied to a ferrous base. West Conshohocken, PA: ASTM, 1993. 30. ASTM D5178 – 98. Standard test method for mar resistance of organic coatings. West Conshohocken, PA: ASTM, 2008. 31. Voorn DJ, Ming W and van Herk AM. Polymer-clay nanocomposite latex particles by inverse pickering emulsion polymerization stabilized with hydrophobic montmorillonite platelets. Macromolecules 2006; 39(6): 2137–2143. 32. Xiong J, Zheng Z, Jiang H, et al. Reinforcement of polyurethane composites with an organically modified montmorillonite. Composites: Part A 2007; 38(1): 132–137. 33. Paul DR and Robeson LM. Polymer nanotechnology: nanocomposites. Polymer 2008; 49(15): 3187–3204. 34. Muzny CD, Butler BD, Hanley HJM, et al. Clay platelet dispersion in a polymer matrix. Mater Lett 1996; 28(4–6): 379–384. 35. Stefanescu EA, Stefanescu C, Daly WH, et al. Hybrid polymer–clay nanocomposites: A mechanical study on gels and multilayered films. Polymer 2008; 49(17): 3785–3794. 36. Dai X, Xu J, Guo X, et al. Study on structure and orientation action of polyurethane nanocomposites. Macromolecules 2004; 37(15): 5615–5623. 37. Chen TK, Tien YI and Wei KH. Synthesis and characterization of novel segmented polyurethane/clay nanocomposites. Polymer 2000; 41(4): 1345–1353. 38. Mishra AK, Chattopadhyay DK, Sreedhar B, et al. FT-IR and XPS studies of polyurethane-urea-imide coatings. Prog Org Coat 2006; 55(3): 231–243. 39. McLaren K. The development of the CLE 1976 (L*a*b*) uniform-colour space and colour difference formula. J Soc Dyers Colour 1976; 92(9): 338–341. 40. Choi WJ, Kim SH, Kim YJ, et al. Synthesis of chain-extended organifier and properties of polyurethane/clay nanocomposites. Polymer 2004; 45(17): 6045–6057.

Biographies Gaurav Verma currently working as an Assistant Professor (Polymers) and Co-coordinator in the Department of Nanoscience and Nanotechnology at Panjab University, Chandigarh. He is undergoing his PhD in the same institution in the area of Nanotechnology and Advanced Polymers/Coatings. Gaurav received his MTech from IIT Delhi, India, in 2001 in Polymers and


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his BE from SBSCET Ferozepur, Punjab, India, in Materials Science and Engineering. His research interests are in nanotechnology, materials, and polymer composites. He has recently received the coveted Young Investigator (CHASCO VENTION) CHASCON 2012 award apart from 4 Best Paper Awards in Nanotechnology between 2008 and 2010. Anupama Kaushik currently working as a Professor at Panjab University, Chandigarh. Dr Anupama received her PhD in the same institution in the area of Polyurethanes. She received her MBA, ME and BE from Panjab University, Chandigarh, in Chemical Engineering. She is the recipient of UGC Research Award and Career Award from AICTE-MHRD, India, and also ISBRI Fellowship from government. of India. Her research interests are in bio nanocomposites. She has about 17 research papers and 6 book chapters to her credit. Anup Ghosh currently working as a Professor at IIT, Delhi. Professor Ghosh received his Bachelor degree in Chemical Engineering from the Department of Applied Chemistry, Calcutta University, in 1980, MTech (1982) from IIT, Kanpur, and PhD (1986) from State University of New York at Buffalo, NY, USA. His areas of research includes rheology and processing of polymers, reactive processing, polymer blends and alloys, mixing and compounding, polymer reaction engineering, and computer aided modeling and simulation. He has been actively involved in carrying out sponsored and consultancy projects for various organizations. He has contributed significantly in his areas of research and published over 60 papers in international journals and conferences. Dr Ghosh is the recipient of Young Alumnus Award (2001) – Chemical Engineering Department, Calcutta University. He has also been the Reliance Chair Professor at CPSE, IIT, Delhi.


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Supplementary data

Figure S1a. Actual organoclay suspensions subjected to increasing high shear homogenizing time (minutes). Sonication time was fixed (240 minutes). The colour changes from light to dark and transparency also reduces considerably. The viscosity of the sample increases such that it almost turns into a gel after 20minutes of processing. Beyond this time samples are very difficult to process for formation of coatings.

Figure S1b. Actual organoclay suspensions subjected to increasing sonication time (minutes). Homogenizing time was fixed (10 minutes). Separation of layers, shown by double arrows was observed after 72 hours of standing in first two samples (60 and 120 minutes) only (from left). No subsequent separation/changes were observed even after 6 months in all other samples. TEM images show no major changes in particle sizes/dispersion level beyond 240 minutes. 240 minutes was found to be adequate for this particular system of PU-C20A. (refer Images below).


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Figure S2a. Relevant TEM images for above processing protocols (Figure S1a) for PUC5. Corresponding processing time given in box.

Figure S2b. Relevant TEM images for above processing protocols (Figure S1b) for PUC5. Corresponding processing time given in box.
