Rheological studies of two component high build epoxy and polyurethane based high performance coatings

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1 DOI 0.00/s Rheological studies of two component high build epoxy and polyurethane based high performance coatings Ajit Deka, Nirmalya Dey Ó American Coatings Association & Oil and Colour Chemists Association 0 Abstract The work presented in this article involves the study of rheometric profile of several rheological additives in two-component (K) high build epoxy zinc phosphate primer and two-component high build aliphatic polyurethane topcoat. Viscosity profile and thixotropic behavior at different shear rates have been determined for both the paints using Physica MCR 30 Rheometer of Anton Paar. The valuable information derived from these measurements led to better insight into the influence of these rheological additives on important paint properties like flow and leveling, sag control, in-can settling during storage, etc. Rheometric results were also compared with the results obtained during the actual application of these experimental coatings on mild steel panels. From the rheological study it was concluded that the thickeners based on surfacemodified clay and organically modified castor oil derivative work well in epoxy zinc phosphate primer whereas polyurea-based thickener showed better results than other rheological additives in the case of the K polyurethane system. Measurement of low shear and high shear viscosity response of different thickeners helps in predicting storage and application behavior of these coatings which correlates well with the actual observation. Keywords Introduction Paint, Epoxy, PU, Rheology, High build High build paints are suspensions or heterogeneous dispersions of several types of inert solids and liquid A. Deka, N. Dey (&) Research & Technology Centre, Asian Paints Ltd, Plot No. C3-B/, TTC MIDC-Pawane, Thane-Belapur Road, Navi Mumbai 0005, India nirmalyadey@yahoo.co.in; nirmalya.dey@asianpaints.com additives on a polymeric network forming structures that are highly affected by shear. During in-can storage of paint the pigment particles should remain in suspension and this is accomplished by producing a weak gel using additives that induce high yield stress or high viscosity at very low shear rate. This characteristic also leads to another desirable effect, namely the nondrip behavior of the paint after its application. In addition, during the application of the paint by a brush, spray, or roller, it must show good shear thinning property to be readily laid on the surface without affecting flow and leveling property. Also one would like to achieve uniform coating thickness after application as much as possible without any sag or run down (downward pull by gravity) prior to drying. Thus, immediately after its application onto a surface, the paint film should start thickening again but at a slower rate such that adequate leveling may occur. In other words, an ideal paint should be a soft solid that becomes a viscous liquid during its application and finally after application it must resolidify at a controlled rate that is slow enough for adequate leveling to occur but fast enough to minimize the tendency of run down from the vertical surface due to the effect of gravity. Bosma et al. studied the role of sag control agents in optimizing sag and leveling of coating. They developed a method called the falling wave method to determine the sag and leveling behavior of wet paints during drying and curing in a quantitative manner. Most paint and polymeric systems contain some sort of structure or aggregates due to interaction between irregularly shaped particles having wide size distribution in suspension or association among branched and/ or highly entangled long chain polymeric molecules in solutions and melts. At rest these structures are oriented randomly corresponding to their minimum energy state. At low levels of externally applied stress, the system resists any deformation and strives to retain its network structure, thereby offering high resistance

2 to deformation which is exhibited in the form of high apparent viscosity and high yield stress. 3 Osterhold studied the rheological behavior of modern paints systems by oscillation tests and yield points measurement. He also studied rheological behavior of waterborne pearlescent/colored pigment systems by measuring the values of storage modulus (G ), loss modulus (G ), phase shift (d), and yield point. The measured values were compared with respective flop index values. Ettlinger et al. 5 noted that fumed silica had been successfully used for some decades to control the rheology of organic coatings by providing a strong thixotropic effect and a yield point. They learned that by surface modification with silanes the fumed silica content could be further optimized for modern coating systems. The authors reported the effect of surfacemodified fumed silica in solvent-based and waterborne coatings and their role in the improvement of the adhesion strength of these coatings. Rheology is defined as the study of the science of deformation and flow of materials under shear. Viscosity is an important material property that is directly related to the rheology of paint. Since the viscosity of material is strongly influenced by the conditions of measurement and the rate at which deformation is created, the study of shear-dependent viscosity change is of special importance for materials like paints for examining their behavior during storage and application. A rheometer is an important tool for evaluating the change in viscosity and related viscoelastic properties of material under varying shear rate. Viscometers, on the other hand, provide data at constant or over a small shear rate range for quick qualitative comparison of formulations but do not offer enough information on viscoelastic behavior of material. Figure illustrates the relationship between viscosity, shear rate, and other important material properties. In general, the three different regions of shear rates correspond to different classes of paint properties. While ultra low shear viscosity profile provides information on settling, sag, flow and leveling properties of material, mid shear viscosity correlates better with stirred consistency and in-package appearance. High shear viscosity correlates better with application properties like brush drag, spatter resistance, film build, etc. Work presented in this article describes the study of rheometric profile of various rheological additives using Physica MCR 30 rheometer in two paints namely high build K epoxy zinc phosphate (ZP) primer and high build K aliphatic polyurethane (PU) topcoat. Rheological properties are evaluated by using three different modes of rheology tests viz. flow and viscosity curve, three interval thixotropy test (3ITT), and amplitude sweep. The rheological findings of these experimental coatings are compared with the observed in-can physical properties (viz. viscosity, in-package stability) as well as application properties (viz. sag and leveling) in order to establish a correlation. Experimental Materials Materials for high build K epoxy zinc phosphate primer (ZP) Medium molecular weight type epoxy resin based on diglycidyl ether of bisphenol-a, procured from M/s Huntsman Advanced Materials (India) Pvt Ltd, was 000 Viscosity in Poise 00 0 Sagging Film build Spattering resistance High gloss Setting Flow and leveling 0. Brush Loading Brush drag Spreading Rolling Gel tester Brookfield Krebs-stormer cone & plate KU ICI Shear rate s Fig. : Rheology profile indicating various storage, applications, and appearance properties of paint at different shear rate

3 used as binder in epoxy ZP primer. The polyamide resin used as crosslinker was made in house based on proprietary composition. Urea formaldehyde type resin, used as flow and dispersing aid, was supplied by M/s Synthetic & Polymer Industries (India). Silicon oil used as wetting agents was supplied by Dow Corning Corporation, India. The rutile TiO was supplied by Dupont (India) Ltd. Zinc phosphate used as anticorrosive pigment was supplied by a local supplier. Extenders and solvents used in formulation were of local origin. All raw materials used were of commercial grade. Nine rheological additives used, and some physical parameters, are given in Table. Materials for high build K PU topcoat Acrylic polyol resin used as primary binder was synthesized in house based on proprietary technology. Aliphatic polyisocyanate used as crosslinking agents was supplied by Bayer Material Science. Rutile TiO used as true pigment was purchased from Dupont. Wetting and dispersing agent, extenders, and solvents were procured from local manufacturers. Eight different rheology modifiers were used as thickeners for studying the rheological properties of paint. All raw materials were used as such, as received from suppliers, without any further modification. Physical parameters of resins are given in Table. Methods Preparation of paints samples Samples of both K Epoxy ZP primers and K PU topcoats were processed in lab model Dispermat (high speed disperser) as per standard formulations. The details of formulation could not be disclosed due to proprietary nature. However, critical formulation parameters are shown in Table 3. Nine different samples of K Epoxy ZP primers were prepared by incorporating individual rheological additives at a concentration of 0.% on solid basis on total formulation. Similarly, eight samples of K high build PU topcoat were also prepared. Rheological characterization Rheological behavior of above samples was measured on Anton Paar Physica MCR 30 at temperature of 30 C. The rheometer was equipped with EC motor with low friction bearing, normal force sensor and a combination of rotational and oscillatory mode. The samples were used as such for this measurement without any further dilution. Well-stirred paints samples were taken in the measuring cup up to the mark shown for a definite volume. The cylinder and the bob were set to zero gaps prior to test run. Test programs were set up as per the specifications of individual tests. Three evaluation modes were used to illustrate the measurements of paint properties. FLOW CURVES MEASUREMENTS: In flow curve the viscosity of paints samples were measured as a function of increasing shear rate. The shear rate was varied from 0 to 5000 s. Rheometric profile of viscosity vs shear rate of nine samples of K epoxy ZP primer and eight samples of K high pigment volume concentration (PVC) PU paint were used to predict the processing ability, spreadability, and applicability. LEVELING AND SAGGING MEASUREMENTS: Leveling and sagging were measured with the help of 3ITT test. In this test materials were subjected to three different shear rates in a stepwise fashion. Initially the samples were subjected to low shear rate (0.00 s ) for 0 s and then for 30 s at high shear rate (5000 s ) and finally again for 0 s in low shear rate. The high shear rate in this test reflects the shear rate during application. By this process the structure breaks down and recovery pre and post-application of high shear can be measured. The percent recovery at given time after removal of high shear can be measured with the help of the inbuilt software in the instrument. The plot of viscosity vs time is used to predict the sagging Table : Physical and chemical properties of different rheological additives Additives Generic composition Density (g/cm 3 ) Particle size (lm) Organoclay I Organic derivative of bentonite clay. < Organoclay II Organic derivative of hectorite clay. < Organoclay III Organically modified hectorite clay.5 < Surface-modified clay Alkyl quarternary ammonium clay.5. 3 Fumed silica Hydrophobic fumed silica. Polyolefin Polyolefin wax 0. Not applicable Polyurea Modified polyurea.0 Not applicable Polyamide Urea modified polyamide 0. Not applicable Castor oil derivative Three-dimensional triglyceride.05 0 maximum

4 Table : Physical characteristic of resins Property Epoxy resin Acrylic polyol Polyamide Aliphatic polyisocyanate Molecular weight WPE (g/eq) Hydroxyl value (mg KOH/g) 5 5 Amine value (mg KOH/g) 0 30 NCO content (%).5 5 C Gardener scale Z3 Z R U Ford cup B (s) Nonvolatile content (%) C (g/cm 3 ).09 ± ± Table 3: Formulation parameters of K epoxy zinc phosphate primer and K polyurethane topcoat Parameters Epoxy zinc phosphate primer and leveling characteristics of the rheological additives in the paint systems. AMPLITUDE SWEEP: By oscillating the samples the change in storage modulus G (the elastic component) and loss modulus G (the flow component) as a function of increasing amplitude were measured in this test. The test was carried out at constant oscillatory frequency. This test explains the viscoelastic behavior of coating in terms of storage modulus (G ) and loss modulus (G ). The structural character of samples in LVE range was expressed by comparing the values of G and G. If G > G the samples exhibit gel character and if G > G the samples exhibit liquid character. 9 From this test incan antisettling properties of paints during long-term storage can be predicted in terms of G and G. Physical property measurement Polyurethane topcoat PVC (%) 0 33 Pigment content (wt%) Binder content (wt%) 5 33 NONVOLATILE MATTER MEASUREMENT (%NVM): %NVM of all batches were checked at 0 C for h and adjusted with suitable solvent composition to the theoretical results. IN-CAN STABILITY MEASUREMENT: The in-can stability was checked by keeping the paint samples in an oven at 53 ± C for 0 days. The paints samples were kept in 50 ml volume tin containers. After removal of samples from the oven, the degree of settling was checked by giving a rating as per ASTM D 9. The ratings were given in 0-0 scale depending upon the degree of settling. Application property measurement SAG RESISTANCE MEASUREMENTS: Burnished mild steel panels of inches 9 inches were properly degreased with xylene and dried under IR Lamp. The dried panels were sanded using 0 grit sand paper followed by washing with clean solvent and finally air dried. Both paints samples were applied on prepared MS panels by Erichsen model 5/ sag applicator. For ZP primers 50 to 500 lm wet film thickness (WFT) applicator slots were used and for PUs 30 to 300 lm applicator slots were used because the PU was a lower PVC based product than ZP primer. After application, the panels were kept vertically in such a way that the bands of different WFT would be horizontal to the surface. The higher WFT band was closer to the surface. Sag resistances of samples were rated based on the degree of run down and the limiting WFT at which the sample starts falling. Results and discussion Result and discussion for epoxy ZP primer with nine different rheological additives Flow curve measurement of epoxy ZP primer with nine different additives Paint and related materials are subjected to various types of shear rate and deformation during manufacture, storage, dilution, and application. Study of flow curves representing variation in viscosity at different shear rate provides interesting information on the rheological properties of paint. 9 Figure depicts the flow curves of nine samples of epoxy zinc phosphate primer with the different rheology modifiers described in Table. It is observed that in the shear zone <0.00 to 0.0 s which is generated by gravity and vibration,

5 000 Pa s Table : Low shear viscosities of epoxy zinc phosphate primer and K polyurethane topcoat with different rheological additives Rheological additives Low shear viscosity (PaÆs) Epoxy ZP primer K PU topcoat η /s 0000 Shear rate γ Fig. : Flow curves of epoxy zinc phosphate primer with different additives ( surface-modified clay, modified polyurea, 3 castor oil derivative, fumed silica, 5 modified clay III, modified polyamide, modified clay I, polyolefin, 9 modified clay II) the surface-modified clay-based ZP primer exhibits a higher peak viscosity (3590 PaÆs) than all other thickeners. High viscosity in low shear zone is a reflection of good antisettling property and stable pigment suspension in storage condition. This thickener may form a gel structure by edge-to-edge hydrogen bonding with the polar hydroxyl group of alcoholic solvents and epoxy binders which arrest pigment settlement in unstirred condition. This observation is supported by the development of high yield value that effectively counters the gravitational and vibrational forces associated during storage and transportation. The other clay-based ZP primers exhibit lower viscosity as compared to surface-modified clay which may be due the effectiveness of surface treatment during manufacturing of surface-modified clay. The surface modification may create a more open site to form hydrogen bonding or other chemical interactions with the binder, solvent, and pigments. Modified polyurea, castor oil derivative, and fumed silica also provide the viscosity of 300, 00, and 90 PaÆs, respectively, in this range as shown in Table. From the accelerated oven stability test as shown in Table 5 it was also observed that modified polyurea, castor oil derivative, and fumed silica thickener based epoxy ZP primers had same rating ( out of 0) in terms of antisettling behavior. This may be because of the minimum viscosity requirement to withstand the gravitational force acting on the pigment particles or to flocculate the pigment particles is lower than 90 PaÆs exhibited by fumed silica. Another reason may be due to 9 Castor oil derivative Organoclay I 0 3. Organoclay II 0.3 Organoclay III Polyurea Surface-modified clay Polyolefin Fumed silica Polyamide 0 ** ** Test was not performed due to incompatibility of additive Table 5: Oven stability test at temperature of 53 ± C 5 for 0 days Rheological additives Rating for pigment settling (0 = best, 0 = worst) Epoxy ZP primer K PU topcoat Castor oil derivative Organoclay I 0 Organoclay II Organoclay III Polyurea 0 Surface-modified clay Polyolefin Fumed silica Polyamide ** ** Test was not performed due to incompatibility of additive formation of stable structure which may not be affected by temperature during accelerated tests at 53 ± C. The probable reason for polyurea thixotropy may be attributed to the formation of network structure with binder as well as with the polar solvent. The hydrogenated castor oil has a typical three-dimensional structure and owes most of its functionality to the presence of the hydroxyl group in the fatty acid chain. At sufficient concentration, dispersion, and solvation, the polar bonding between the hydroxyl groups provides the desired structural network in the paints. 0 Christ and Bittner describe in their research that the interaction between the surface silanol groups of the fumed silica operates either directly or via liquid molecules acting as hydrogen bond to form a three-dimensional structure as flow barrier. They also provided a probable structure of fumed silica in which there are two OH groups attached with silicon in different plane and other two valances are occupied by methyl groups. Due to the high acidic nature of silanol groups, fumed silica may have a tendency to form strong hydrogen bonds with solvents/binders

6 having suitable hydrogen bonding sites. Ettlinger et al. 5 measured thickening effect of fumed silica in different solvents with different polarity. They found that the silicon dioxide agglomerates became progressively more solvated in liquid phase of increasing polarity and the space filling interaction between the silanol groups was disrupted. From their study it was observed that fumed silica required the polar group to form the network structure and was accordingly incorporated in the formulation. Organoclay III and modified polyamide thickener based epoxy ZP primers exhibit viscosities of 330 and 0 PaÆs, respectively, in low shear rate. These two additive based epoxy ZP primers provide poor pigment flocculation as compared to the top four additives viz., surface-modified clay, modified polyurea, castor oil derivative, and fumed silica which is also supported by the accelerated oven stability test. The organoclay I based epoxy ZP primer has a viscosity of 0 PaÆs. in low shear rate which indicates the poor pigment flocculation ability of this additive among all. However, the oven stability test shows good results for this additive. The probable reason may be due to the sustainability of the network structure formed by the additive with the solvent-binder composite system at high temperature. To provide greater clarity on this, further investigation is required. Time-dependent decomposition and regeneration of structure: rotational test (3ITT) Leveling and sagging appear to be in mutual opposition, i.e., leveling requires low viscosity, whereas sag prevention requires high viscosity. Control of sagging by the incorporation of high molecular weight polymer alone usually affects flow and leveling. Therefore it is not necessary to stop downward flow of paints completely to prevent sagging; in fact some flow is desirable for good leveling. Sag velocity can be reduced without sacrificing leveling by introducing both pseudo plasticity and thixotropy into the system. Thixotropy can be introduced by incorporation of the right type of rheology modifiers. The selection and dosage of these additives for a given coating depends on the type of resin-solvent present and the degree of thixotropy/pseudo plasticity required. Thixotropic coating has yield point, and unless the gravitational shear stress exceeds the yield stress, no flow will occur. In general yield point increases with the increase in concentration of thixotrope and degree of dispersion in the paint. However, excessive increase in yield point can cause the material difficult to pump and spray. In Fig. 3 the graph describes the viscosity of nine different additives based ZP primer in three different shear rate with time viz., low shear viscosity, high shear viscosity, and viscosity after removal of high shear. The most important aspect of this test is determining how fast the rheological additives try to recover their network structure after the removal of high shear. The faster the build-up of viscosity, the better are the η 000 Pa s s 00 Time t Fig. 3: 3ITT curves for epoxy zinc phosphate primer ( surface-modified clay, castor oil derivative, 3 polyolefin, fumed silica, 5 organoclay II, polyurea, modified polyamide, organoclay III, 9 organoclay I) sag resistance properties after application on a vertical surface. However, this may affect the leveling of the paint film. After removal of high shear rate, the surface-modified clay based ZP primer shows faster structure recovery than all other additives based ZP primers. The time-dependent structure regeneration after removal from high shear in 0 s for surfacemodified clay and castor oil derivative based ZP primer samples is found to be 3% and %, respectively. This indicates that the surface-modified clay and castor oil based derivative interact rapidly and give better antisagging property as compare to others. The interaction of surface-modified clay may be due to the formation of network structure with polar solvent and binder pigment in the system. The castor oil-based rheology modifier generally forms a network structure by swelling at higher processing temperature. The practical test by sag applicator for these two also has a similar observation and no sagging is observed up to 500 lm (WFT) for epoxy ZP primer (Table ). Cohu and Magnin 3 have given the correlation between leveling and structural recovery. They described that as leveling proceeds there was a competition between this structure rebuilding and leveling tendency. If the time of structure recovery is too short, the leveling is stopped by the reconstructed yield stress, and the residual amplitude (a a ) is not equal to zero, since the residual amplitude is directly connected with the yield stress. From their study it is clear that optimization of dosages of individual rheological additive is required to get optimum level of thixotropy. Organoclay I and organoclay III also showed very rapid structure regenerations after removal of high shear but lesser as compared to surface-modified clay. The rapid structure regenerations of these two indicate very good antisagging behavior. Fumed silica-based ZP

7 Table : Sag resistance of epoxy ZP primer and K polyurethane topcoat with different rheological additives Rheological additives Sag resistance (WFT in lm) Epoxy ZP primer K PU topcoat G 0 3 Pa Castor oil derivative Organoclay I Organoclay II Organoclay III 50 0 Polyurea Surface-modified clay Polyolefin Fumed silica Polyamide 50 ** G ** Test was not performed due to incompatibility of additive primer exhibits 5% structure recovery in 0 s after removal of high shear rate, which indicates very good antisagging behavior of the additive. The pattern of the curve also indicates the better leveling as compared to other additives based ZP primer % 000 Strain γ Fig. : Amplitude sweep curve for ZP primer ( surfacemodified clay, organoclay I, 3 polyolefin, castor oil derivative, 5 fumed silica, polyurea, organoclay III, organoclay II, 9 modified polyamide) Amplitude sweep Viscoelastic materials show a response that contains both in-phase and out-of-phase contributions. These contributions reveal the extent of solid-like and liquidlike behavior. As a consequence, the total stress response shows a phase shift d with respect to the applied strain deformation that lies between that of solids and liquids, 0 < d < p/. The viscoelastic behavior of the systems is characterized by the storage modulus, G (x), and the loss modulus, G (x), that indicate the solid-like and fluid-like contributions to the measured stress response, respectively. From Fig. organoclay II and polyolefin-based ZP primer showed G > G indicating their more fluid behavior. This is supported also by their inferior pigment suspending properties. To the contrary, other additives showed G > G and thus behaved more like an elastic solid than a viscous liquid. This resulted in better pigment suspending properties. Surface-modified clay-based ZP primer exhibit G > G and higher crossover point as compared with other additive based ZP primer, i.e., this additive behaves more like elastic in their viscoelastic range. The higher crossover point also indicates the high yield value. This result of surface-modified clay reflects better sedimentation stability of this paint during storage as well as in transportation. Higher yield points are also indicative of the ability of higher film thickness to build up. The higher yield point of surface-modified clay also indicates the formation of gel character by edge-to-edge hydrogen bonding. Organoclay I, castor oil derivative, and fumed silica-based ZP primers also have very good sedimentation stability as these paints also showed relatively higher yield point. The organoclay I showed high yield point and may be sustaining a network structure against increasing strain. Due to this, the pigment flocculation of the same was found to be very good in the stability oven test. Modified polyamide thickener has G > G, but the crossover point is in lower range, thus showing low sedimentation stability as compared to other additives. Result and discussion of polyurethane topcoat with eight different additives Flow curve measurement of polyurethane topcoat with eight different additives From Fig. 5, in shear rate range < s, modified polyurea-based thickener exhibits higher viscosity than all other seven rheological additives based polyurethane topcoats. Modified polyurea may form network structure by associating with polar groups present in the solvent and the medium. The PU topcoat based on this additive exhibits very good suspension stability of pigment particles with the liquid media due to high viscosity in low shear rate. This observation is corroborated by the absence of settling during hot box aging test. From Table, it is obvious that the castor oil derivative-based polyurethane topcoat exhibits lower viscosity as compared to the modified polyurea but higher than all other six rheological additives. All other additives based PU samples except these two have viscosity in the range of 0 PaÆs. For better clarity six additives which exhibited low viscosity in low shear are presented in the inset diagram in Fig. 5. Due to lower

8 0 Pa s Pa s 3 η η /s 0. Shear rate γ /s 0000 Shear rate γ Fig. 5: Flow curve of K polyurethane with different additives ( modified polyurea, castor oil derivative, 3 fumed silica, polyolefin, 5 modified clay I, surface-modified clay, modified clay III, modified clay II) viscosity in low shear the other additives were not found to be as effective in a K PU system in arresting pigment settlement on storage. From the accelerated oven stability test as shown in Table 5 it was also observed that modified polyurea shows the best result in antisettling characteristics (0 out of 0) among all eight rheological additive based PU topcoats. In the shear region 0.0 s, which is a leveling zone, the viscosities of all the additive-containing formulations exhibit low viscosity, which indicates very good flow and leveling after application. In the polyurethane system all claybased rheological additives showed very low viscosity in the low shear rate which may be due to the less reactive site present to form the gel structure in the paint system. Interaction of different rheological additives is discussed broadly in the flow curve of the epoxy ZP primer section. 3ITT for polyurethane topcoat As shown in Fig., the low shear rate 9 modified polyurea-based PU paint exhibits higher viscosity as compared to other additives based PUs. The modified polyurea binds pigment particles with the help of the polar media by secondary forces and breaks down during the high shear rate. The possible mechanism of modified polyurea is formation of hydrogen bonding with solvent and binder system to give the desired thixotropy. The high shear viscosity for all additives based PUs is within the range 0. PaÆs. The timedependent structure regeneration after removal of high shear rate of polyurea-based PU is higher as compared to other additives based PUs. Thus, the antisagging property of polyurea-based PU is better as compared to other additives based polyurethane topcoats which is confirmed by application in vertical surface by sag applicator. Organoclay I, fumed silica, and organoclay II based PUs showed slow viscosity recovery after η 000 Pa s 00 0 removal of high shear rate, which may be due to the less interaction with the solvent binder system present the formulation. Surface-modified clay and castor oil derivative-based PUs have the same low shear viscosity but the castor oil derivative-based PU has better structure regeneration as compared to surface-modified clay. The pattern of curves of organoclay I, fumed silica organoclay II, and surface-modified clay additive-based PUs indicate that after removal of high shear the structure regeneration due to network structure formation is slower initially but the interaction goes on for a long time. This type of viscosity recovery reflects the good leveling tendency s 300 Time t Fig. : 3ITT curves for K polyurethane topcoats ( polyurea, organoclay I, 3 fumed silica, organoclay II, 5 surface-modified clay, castor oil derivative, organoclay III, polyolefin)

9 after application of the paints since there is enough time for the composite system to level out. Low shear viscosity and viscosity after removal of high shear for organoclay III and polyolefin additive-based PUs is very low as compared to all other additives based polyurethane topcoats. There is less interaction for these two additives to form a network structure. These two additives showed very poor antisagging behavior during the application by sag applicator which is shown in Table. Amplitude sweep seeds due to heat sensitivity of the additive during the storage at 53 ± C. This was overcome by redispersing the paint under lower temperature conditions. 0 In the case of PUs, polyurea and surface-modified clay-based samples have very good antisettling behavior. The antisettling behavior measured by the oven stability test of polyurea and organoclay III based PU is shown in Fig. 9. Fumed silica, organoclay I, and organoclay II based PUs also have very good storage stability. Castor oil derivative shows moderate storage stability with seeding. Organoclay II has poor storage stability as compared to other additives. From Fig., polyurea and castor oil derivative-based PUs behave like solids in storage condition since G > G for both the cases. All other additives based PUs except these two behave like liquid in storage condition since G < G for all of them. Polyurea has higher crossover point than that of castor oil and all other additives based PUs. In the storage condition polyurea forms very good three dimensional networks with the help of polar media in the system which reflects solid-like behavior of this paint. Castor oil derivative has a typical three-dimensional triglyceride structure and provides structural network by formation of polar bond between the hydroxy groups. 9 Organoclay I based epoxy ZP primer exhibits very good storage stability at moderate temperature. Surface-modified clay, castor oil derivative, fumed silica, and polyurea have also very good storage stability which was tested by accelerated oven stability (rating out of 0). The settling characteristic of surfacemodified clay and organoclay III is shown in Fig.. The castor oil derivative-based epoxy ZP primer forms No settling in surface-modified clay based epoxy ZP primer Settling in organo clay III based epoxy ZP primer 0 Fig. : Settling property of surface-modified clay and organoclay III based ZP primer by oven stability test Pa G 0 G % 000 Strain γ Fig. : Amplitude sweep curve for PU ( polyurea, castor oil derivative, 3 fumed silica, surface-modified clay, 5 organoclay I, organoclay III, organoclay II, polyolefin) No settling in polyurea based PU Settling in organo clay III based PU Fig. 9: Settling property of polyurea and organoclay III based PU by oven stability test

10 Fig. 0: Antisag behavior of castor oil derivative-based PU Fig. : Antisag behavior of surface-modified clay-based ZP primer Fig. : Antisag behavior of organoclay III based PU Castor oil derivative and surface-modified claybased ZP primer passes sag resistance up to 500 lm WFT. Fumed silica, polyolefin, and organoclay I has sag resistance of up to 350 lm. Organoclay II exhibits remarkably lower sag resistance of 50 lm WFT. Sagging behaviors of some additives based epoxy ZP primer as well as K PU are given in Figs. 0,,, and 3. Castor oil derivative and polyurea-based PUs have sag resistance of up to 300 lm WFT which is shown in Fig.. Surface-modified clay and organoclay I show good performance in the moderate range. Organoclay III has sag resistance of 90 lm WFT. Fig. 3: Antisag behavior of organoclay III based ZP primer Conclusion The rheological measurements, the flow curve, 3ITT, and oscillatory amplitude sweep test indicate the viscoelastic behavior of various paint samples of K epoxy primer and K PU topcoat prepared with different rheological additives. The overall performance of surface-modified clay was found better than all other additives in case of K epoxy ZP primer. Similarly for K PU paints the polyurea-based thickener showed much better results in terms of

11 antisettling and antisagging behavior in comparison to other seven additives chosen for this study. The rapid recovery of structure after application in both cases contributed to better sag resistance but impaired the leveling property of these paints to some extent. However, this problem can be easily resolved by adjusting the application viscosity and optimizing the dosage of rheological additives in the paint. On the whole, the results of various rheological parameters obtained instrumentally correlate well with the observed storage and application properties for both paints. Acknowledgments The authors wish to thank Asian Paints management system for giving the opportunity to publish this work. The authors specially wish to thank Dr. Randhirshinh Parmar and Dr. B.P. Mallik for their continuous support from initial level of the activity. Furthermore, support received from all our colleagues and superiors, who contributed to this work, is gratefully acknowledged. References. Varela Lopez, F, Rosen, M, Rheological Effects in Roll Coating of Paints. Lat. Am. Appl. Res., 3 5 (00). Bosma, M, Brinkhuis, R, Coopmans, J, Reuvers, B, The Role of Sag Control Agents in Optimizing the Sag/Leveling Balance and a New Powerful Tool to Study This. Prog. Org. Coat., 55 P9 P0 (00) 3. Chhabra, RP, Richardson, JF, Non-Newtonian Flow and Applied Rheology: Engineering Applications, nd ed., p. 3. Butterworth-Heinemann/IChemE series, Oxford, 00. Osterhold, M, Rheological Methods for Characterizing Modern Paint Systems. Prog. Org. Coat., (000) 5. Ettlinger, M, Ladwig, I, Weise, A, Surface Modified Fumed Silica for Modern Coatings. Prog. Org. Coat., (000). Whittingstall, P, Paint Evaluation Using Rheology, RH059 TA Instruments Inc., New Castle, DE. Rheology Handbook: A Practical Guide to Rheological Additives, pp. 3,, Elementis Specialities Inc., Hightstown, NJ 9. Manuscript to the Lecture on Rheology, Part, 00, Anton Paar Gmbh, Ostfildern, Germany 0. OCCA, Surface Coating Materials and Their Usage, nd ed., p. 35, V-I Chapman and Hall, 93. Christ, U, Bittner, A, Rheology Control of Organic Coatings with New Hydrophobic Silicas. Prog. Org. Coat., 9 (99). Schoff, CK, Recent Advances in the Rheology of High Solids Coatings. Prog. Org. Coat., 9 0 (9) 3. Cohu, O, Magnin, A, The Leveling of Thixotropic Coatings. Prog. Org. Coat., 9 9 (99). Wyss, HM, Larsen, RJ, Weitz, DA, Oscillatory Rheology Measuring the Viscoelastic Behavior of Soft Material,