Influence of rheology modifiers and coating parameters on the color-changing effects of textile coatings with multi-layered mica pigments

Materials

Sample preparation

Three coating formulations (batches of 400 g) were prepared, all of which contained a commercial binder (B), effect-pigment dispersion (EPD) and one type of RM (LDPSAP, HASE or HEUR; see Table 1). To prepare a homogeneous coating formulation, one part EP was dispersed in two parts of water before being mixed with a binder, using a plain stirrer for two minutes at 300 rpm at 23℃. The mixed formulation B:EPD had a pH of 9, sufficiently alkaline for addition of HASE. When adding EPD to the binder, the viscosity of the formulation decreased and this was compensated for by adding a RM to the formulation at 800 rpm for five minutes. The amount of RM added was adjusted in order to obtain three formulations with similar or slightly higher shear viscosities than the binder dispersion throughout the investigated shear range, from 33 up to 2000 s⁻¹ corresponding to the shear rates during the coating procedure, as seen in Table 2.

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Table 1.

Coating formulations, composition in weight%

Ingredients	Commercial name	Density	Coating formulations (composition in wt%)
(Abbreviation)	(Solids content in wt%)	[g/cm3]	B:EPD:LDPSAP	B:EPD:HASE	B:EPD:HEUR
Binder (B)	Performax 16297 G (35)	0.950	84.5	84.6	84.6
Effect pigment (EP)	Colorstream Pacific Lagoon T10-05 (100)	2.6	5	5	5
Water (D)	Distilled water (0)	0.998	10	10	10
Rheology modifier (LDPSAP)	Rheosol PTP (55)	1.1	0.5	–	–
Rheology modifier (HASE)	Acrysol TT-615 (30)	1.1	–	0.4	–
Rheology modifier (HEUR)	Borchi Gel L75N (48)	1.1	–	–	0.4

Effect pigment + Water (EPD).

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Table 2.

Coating parameters

Gap height (h) [µm]	Coating speed (v) [m/min]	Shear rate (v/h) [1/s]
100	0.6	100
200	0.6	50
300	0.6	33
100	12	2000
200	12	1000
300	12	667

By using the theoretical solids content for each ingredient given in Table 1, the coating pastes were calculated to contain approximately 35 weight% solids. This is in agreement with the solids content obtained by differentially weighing each coating paste before and after drying for 10 minutes at 100℃. By using the density of each ingredient and composition in weight% for each coating formulation, all coating formulations were calculated to have a density of 1.038 g/cm³.

The three different coating formulations were applied to the substrate in the warp direction of the woven fabric with a lab-scale knife-over-table coating method. Fabric samples of 20 cm × 50 cm were taped to an even surface and the coating formulation was manually applied using a film-applicator (ZUA 2000, Zehntner) with a width of 15 cm and gap settings of 100, 200 and 300 µm, as shown in Figure 1. To minimize any pressure variation on the fabric from the rolling bank, 50 cm³ of each formulation was evenly distributed in front of the knife. Two different coating speeds were used, 0.6 and 12 m/min, generating different shear rates depending on gap height, as shown in Table 2. The coated samples were first dried on stenter frames for 10, 15 and 20 minutes at 100℃, adjusted on the gap setting, and then thermofixed for five minutes at 150℃ in a low convection heat oven (D-36-03-EcoCell, Rycobel NV). Two replicas of each sample (3 × 2 × 3 × 2 samples) were produced, resulting in a total of 36 samples. OPEN IN VIEWER

Figure 1.

Lab-scale knife-over-table coating method.

Influence of rheology modifiers on the shear viscosity of coating pastes

In Figure 3, the viscosity versus shear rate is plotted logarithmically for all coating formulations described in Table 1, showing a pseudoplastic (shear-thinning) behavior typical of non-Newtonian fluids. The applied flow field induces alignment and disentanglement of the polymer molecules (in the coating formulations) in the flow direction, making them slip past each other more easily with increasing shear rates. ³⁶ The viscosity decreased at all used shear rates by adding the effect-pigment dispersion (EPD) to the binder (B) stabilized with HEC. HEC is only active in the water phase where it creates a network of entangled hydrated chains. The strength of this network depends on the number of entanglements in the water phase and since the concentration of HEC decreased in the B:EPD formulation the viscosity also decreased. The B:EPD system had a tendency for a more rapid phase separation during storage (lower shear rate) compared to formulations containing RMs, visually observed after three days’ storage. OPEN IN VIEWER

Adding small amounts of RMs (LDPSAP, HASE or HEUR) resulted in an increased viscosity over the measured shear rate range, enough to delay phase separation during storage. B:EPD:LDPSAP had the highest viscosity at lower shear rates. This formulation also had a steeper slope showing a somewhat more pronounced shear thinning behavior compared to HEUR and HASE having more similar viscosity curves, a result that can be correlated to their different thickening mechanism. LDPSAP polymers, which swell due to hydration (including the other polymer particles in the formulation), more easily align themselves with the shearing forces, whereas the hydrophobic associative networks created by HEUR and HASE resist the deformation forces better, resulting in a less pronounced shear thinning behavior.

B:EPD:HASE, compared to B:EPD:HEUR, provided only slightly higher viscosity values up to shear rates of 500 s^–1, and at higher shear rates, the curves intersected each other. B:EPD:HASE was also slightly less shear-thinning up to shear rates of 500 s^–1 compared to B:EPD:HEUR, as seen in Figure 3. This may be correlated to the fact that HASE (unlike HEUR) binds water to its polyelectrolytic main chain and thereby limits the mobility of the associate network in the formulation, whereas the HEUR network has a higher mobility in the water phase and provides less resistance to flow.

Influence of coating parameters on solids deposit

The influence of the RM, the gap height and the coating speed (shear rate) on the solids deposit is shown in Table 3. As expected, the solids deposit increased with increasing gap heights for all samples. The solids deposit increased almost in proportion with the coating gap; when doubling the gap height (from 100 to 200 µm) the solids deposit on samples increased by 72–109%. By increasing the gap height by 50% (from 200 to 300 µm) the solids deposit increased by 48–56%.

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Table 3.

Solids deposit obtained with the different coating formulations, gap heights and coating speeds, experimental and calculated values

Formulations	Coating speed [0.6 m/min]				Coating speed [12 m/min]
	Gap height [µm]	Shear rate [s−1]	Viscosity [Pa.s.]	Solids deposit [g/m2]	Shear rate [s−1]	Viscosity [Pa.s.]	Solids deposit [g/m2]
B:EPD:LDPSAP	100	100	2.45	30	2000	0.26	23
	200	50	4.39	54	1000	0.44	48
	300	33	6.14	82	667	0.60	75
B:EPD:HASE	100	100	2.70	29	2000	0.24	23
	200	50	3.96	50	1000	0.43	47
	300	33	5.27	75	667	0.60	71
B:EPD:HEUR	100	100	2.30	23	2000	0.26	23
	200	50	3.50	48	1000	0.44	43
	300	33	4.75	71	667	0.60	65

From Table 3, it is evident that, with exception of B:EPD:HEUR coated with a gap of 100 µm, a higher coating speed resulted in lower solids deposit, up to 23%. This difference in solids deposit was more pronounced at the smaller gap heights.

A trend of increasing solids deposit in the order of LDPSAP > HASE > HEUR, (except for samples coated with a gap height of 100 µm at the higher coating speed) can be noted from Table 3. Thus, there was an influence of the chosen RM on the solids deposit, which at least at the lower coating speed followed the viscosity differences between the formulations, that is, a higher viscosity was associated with a higher solids deposit. At the higher coating speed, the corresponding viscosity differences were only marginal. The reason for obtaining a higher solids deposit at a lower coating speed (at constant gap height) may be speculated on. At lower coating speeds (or lower shear rates), the viscosity is clearly higher, which could give rise to a higher hydrodynamically induced force under the knife. This force compresses the flexible substrate, allowing a thicker wet film to pass under the knife and hence giving rise to a higher solids deposit. ³⁷ It should be mentioned that the formulations showed no significant variations in density or surface tension, 1.04 g/m³ and between 41 and 42 mN/m, respectively, and are not considered to influence the solids deposit variations.

Influence of coating parameters on the surface structure and pigment orientation

The top-view and cross-section SEM images in Figure 4 are B:EPD:HASE samples coated at the higher speed (12 m/min). These were selected to represent the influence of gap heights on the surface appearance and the orientation of the pigment platelets. The type of thickener, coating speed and, consequently, the viscosity in general did not seem to influence the distribution or the orientation of the platelets. As seen in Figures 4(a)–(c), there was a clear improvement of the surface smoothness with increased gap height due to increased solids deposit and coating thickness. The higher solids deposit filled out the valleys between the yarns and leveled out surface irregularities from the woven structure and the multifilament yarn. OPEN IN VIEWER

In the backscattered electron images (see Figures 4(d)–(f)), the EPs are contrasted against the binder, showing their distribution and orientation within the dried coated layer. The layers were roughly estimated to have thicknesses of 8, 19 and 29 µm (by cross-section SEM images) corresponding to approximately 10% of the gap heights of 100, 200 and 300 µm, respectively. Within the 8 µm thick layer, the EPs appeared to form compact stacks close to the filament surfaces. Some of these stacks bridged two or more filaments within the yarn, and may possibly prevent the binder from interlocking around the filaments, thus leading to a reduced adhesion between the coating and fabric. The platelets within the thinner layer were not perfectly horizontally oriented, which can be a consequence of the drying process. When the water in the suspension (formulation) evaporates and also is drained into the fabric structure, the polymers coalesce. Subsequently, shrinkage of the film (wet coating) ¹⁰ takes place and forces the platelets to orient themselves along the curved surfaces of the filaments.

Generally (and as expected), higher amounts of pigments were observed within the thicker coating layers (Figures 4(e) and (f)). The pigment platelets are also more homogeneously distributed within the layers, some were close to the filaments, but without bridging them as was seen in the thinner layer. Furthermore, some pigment platelets appeared to be located rather close to the surface of the coating layer. Despite the semitransparent nature of the pigments, obviously the thicker layers should promote better coverage of the substrate surface. The pigment distibution within the layer here is suggested to be the result from the forming of a layered structure under the knife during fabric coating. ²² Assuming that the layers consist of polymers and platelets (oriented into the shear direction) with a continuous liquid phase, when the shearing stops, the viscosity of the wet layer instantly increases. This may to some extent prevent the pigment platelets from compacting or sedimenting down to the bottom of the coating layer and instead stabilize them in the distributed way seen in Figure 4(f). It is not unreasonable that this cause of events could lead to a more homogeneous distribution of the platelets in the thickness direction in case of the thicker layers.

As already mentioned, it was hard to visually detect (in SEM images at microscopic level) any significant effects on the distribution or the orientation of the platelets by the viscosity or coating speed. However, the coating speed (i.e. the shear rate) at a constant gap height influenced the solids deposit, which should have an impact on the amount of pigments within the layers.

The influence of the coating speed on the surface structures of HASE samples coated at 0.6 and 12 m/min with the gap set to 100 µm is shown in Figure 5. It was quite obvious that the sample coated at the lower speed, Figure 5(a), exhibited a smoother surface due to an increased solids deposit of 6 g/m² (Table 3) and probably a somewhat thicker coating layer when compared to the corresponding sample coated at 12 m/min (see Figures 5(c) and (d)). In those micrographs, the filaments and pigments are emphasized on the coated surface compared to the appearance exhibited in Figures 5(a) and (b). Since the difference in solids deposit between samples coated at different speeds decreased with increasing gap heights, it was difficult to distinguish differences in smoothness by SEM for these samples. OPEN IN VIEWER

The EPs are distinguished as the lighter gray non-symmetrical spots on the coated surfaces seen in the backscattered electron images in Figures 5(b) and (d). In Figure 5(b), the pigment particles are not as clearly revealed in the sample coated at the lowest speed, possibly because they are better covered by binder polymer.

Preliminary studies indicated that the mechanical properties of the coated samples essentially were influenced by the coated layer thickness generated by gap height. As expected, the fabric stiffness was found to increase with increasing layer thickness. This is possibly because increased amount of solids deposit interlaces filaments and yarns, which restrict fabric flexibility. Preliminary studies also indicated that the abrasion resistance of coated samples was lower for the thinner coated samples. For these samples the weight loss after 5000 Martindale cycles was slightly higher in comparison to the thicker coated samples. Further, when comparing the color differences before and after abrasion, the color differences were also greater for the thinner coated samples. The adhesion between polymeric binder and substrates may therefore be lower for thinner coated samples, possibly due to poorer interlocking around the filament and also that the stacks of EPs formed near the filament surfaces (cf. Figure 4(d)) may in fact lead to reduced adhesion, as previously discussed. On the other hand, within the thicker coated layer with increased pigment distribution, the affinity between pigment and binder as well as binder and substrate are suggested to be greater. The thermodynamic process that most certainly influences the pigment distribution and orientation may have an impact on the adhesion and thus also on the mechanical properties of the coated samples, and should therefore be assessed in a follow-up study.

Influence of coating formulation and gap height on color differences

Influence of coating speed on color differences

The influence of the coating speed on the total color differences (ΔE*) at various detection angles for samples coated with each type of formulation is shown in Figure 8. Generally, the L*, a* and b*- values indicated that samples coated at the lower speed (0.6 m/min) were slightly lighter and more chromatic. An increased solids deposit followed the same trend, generating lighter and more chromatic color changes (see Table 3, Figures 6 and 7). Here, the reference values used for the evaluation were the color coordinates for the samples coated at the lower speed. OPEN IN VIEWER

The highest ΔE*-values were noted for the B:EPD:LDPSAP and the B:EPD:HASE samples coated with a gap height of 100 µm (Figures 7(a) and (b)). This can be related to the solids deposit differences, which were higher between samples with smaller gap heights. The decreasing ΔE*-values with increasing gap heights can be related to the higher pigment concentrations within the increased coated layers, leading to increased covering of the substrate surface, resulting in smaller differences between low and high coated samples. It is also evident that the relative thickness difference of the coating layer between samples coated at low and high speeds was smaller when increasing the gap. The reason why the ΔE*-values were relatively small for the B:EPD:HEUR samples coated with a gap of 100 µm (and then increased with increasing gaps) could possibly be associated with the small differences in solids deposits between these samples.

The ΔE*-values tended to decrease with increasing gaps for B:EPD:LDPSAP and B:EPD:HASE samples in the angle range −15° to 25° and −15° to 45° angles, respectively (Figures 8(a) and (b)). This indicates that the greatest color differences between low- and high-speed coated samples should be easiest to detect at the −15° angle, which was also visually confirmed in diffuse illumination. Figure 8 shows that the ΔE*-values varied significantly depending on the solids deposit and the detection angle. However, it also points to the complexity in performing visual observations associated with angle-dependent color changes. Thus, it is difficult to speculate on whether or not the smaller ΔE*-values (below two units) between samples coated with the same gap heights can be perceived by the human eye.