Spacing for better effects

Transcription

1 Spacing for better effects Influence of fine particle size extenders and entrapped air on utilisation of TiO 2 in emulsion paints. A number of interactions can occur between titanium dioxide and mineral extenders, which can effect a range of paint properties, such as rheology, durability, opacity and scrub resistance, some of which are not fully appreciated. This paper looks at one of these interactions, TiO 2 spacing by extenders, and the effect it has on the optical properties and scrub resistance of emulsion paints. Evaluation of the possible TiO 2 savings suggest that, while small reductions (ca. 5%) in TiO 2 loadings could be made with flash calcined extenders in certain paint systems, implementing greater savings, as suggested by the manufacturers, led to detrimental effects on several other important paint properties. Paul Frederick Dietz. TiO 2 pigments provide opacity and whiteness to paints and influence a wide range of other properties. However, the manufacturing processes of TiO 2 are relatively expensive, hence it is often one of the most expensive raw materials used in emulsion paints. Many techniques and additives have been developed to help improve the utilisation of TiO 2 and thus reduce the amount used whilst maintaining the performance properties of the paint. Extender manufacturers have been promoting their products as potential ways of saving TiO 2, often proposing savings as high as 30 vol.%. Fine particle size extenders save TiO 2 It is true to say that over a period of 30 or so years paint formulators have made use of smaller particle size extenders in their paints and significantly reduced their TiO 2 content. For example, a typical high quality matt paint of the 1970's would often contain in excess of 18 vol.% TiO 2. Through use of small size calcined clay extenders, and to some degree opaque polymers, this level has now been reduced to typically between 10 and 14 vol%. It is difficult to see, however, how modern paints could continue to benefit from extender manufacturers' continuing claims of potential savings of 20 to 30% TiO 2 by using their fine particle size extenders. Thus, this study looked at several current fine particle size extenders and some "new" flash calcined extenders to see how the manufacturers' claims hold up when the extenders are used in a relatively modern paint formulation. Improving TiO 2 use through spacing In decorative emulsion paints it has long been known that TiO 2 is often poorly dispersed and crowded by the extender and emulsion particles [1]. Indeed the higher the pigment and extender volume concentration the worse the crowding can become. It is well understood that by reducing the size of the extender particles used, the spacing of the TiO 2 can be significantly improved (Figure 1). Replacing a 6µm extender/filler with a 2µm calcined clay would, and did, lead to improved TiO 2 spacing, better opacity and to the possibility of reducing TiO 2 levels. However, to further improve the spacing to levels leading to significant improvements in scattering and opacity would ideally require the extender particle size to be an order of magnitude smaller at least. Figure 2, shows the theoretical effect on the TiO 2 nearest neighbour distance when extender particle size varies from 0.01 to 1.0µm. This modelling program, developed by Temperley et al. [2], assumes that there is random packing between TiO 2 and extender particles. Clearly, further spacing of the TiO 2 only begins to have a significant effect on opacity when the particle size of the extender reaches a size less than 0.5µm. The optimum TiO 2 spacing and maximum increase in scatter occurs with a particle size between 0.1 and 0.2µm. The majority of extenders which still claim to offer potential savings of 20 to 30% through TiO 2 spacing are well in excess of this particle size and therefore are unlikely to be able to have such an impact on opacity using this mechanism, unless some other property which could affect film porosity or scrub resistance, such as air entrapment, comes into play. pvc ladder with varying extender volume concentration A number of leading extender manufacturers were asked to provide fine particle size extenders which they would recommend for improving TiO 2 utilisation. Table 1 lists the ones used in the evaluation with the chemical type and particles sizes quoted by the manufacturers. Extender A was selected as the standard and represents a well known, and much used, calcined clay extender which is known to have improved the utilisation of the TiO 2 when substituting a larger filler type product. To evaluate the extender performance it was felt necessary to produce a pvc ladder with constant TiO 2 vc and varying extender vc. This evaluation allowed to determine the critical pigment volume concentration (cpvc) for each type of extender in this paint system. It would not, however, easily allow multiple extender paints to be tested and one therefore accepted that the evaluation would be limited and that one may lose any synergistic effects (good or bad) by using extenders in combination. Several paint properties were tested including opacity (contrast ratio dry and wet) measured using black and white charts and a number 6 K-Bar wire-wound applicator, colour (CIELab), resistance to Gilsonite oil-based stain and glycol opacity (reflectance after the application of a propylene glycol film). The glycol film removes the effects of increased porosity (dry hiding) and helps confirm the position of the cpvc for each extender. The performance was plotted against the extender vc and compared to that of the standard Extender A. Figures 3a and b show how three of the extenders performed in terms of dry opacity and glycol opacity. Extender flocculating performs worse Considering the relationship of the curves and the cpvc levels, shown in Figure 4, one can draw certain conclusions from their shape and direction. For example, both Extender A and Extender I in Figure 4 show a downward trend with increasing extender volume, which suggests the degree of crowding is also increasing. The decrease is steeper for Extender I - it is crowding the pigment more, producing a more rapid fall-off in opacity. Given the particle size of Extender I is 3.2µm this is as would be expected, since Extender A has a slightly smaller particle size and tighter distribution. We would thus consider Extender I to have a worse performance and not to offer any potential savings over Extender A. Extender H has a much smaller particle size of 0.18µm and, given the modelling data, should produce a much better opacity than Extender A. Figure 4 shows, however, that this is not the case. In fact the performance below the cpvc is worse than Extender I. The reason for the unexpected result is that the extender is badly flocculated, leading to a much larger effective particle size. It is therefore not surprising that the opacity is well down and that this extender is performing worse than Extender A.

2 Extender with air voids improves opacity It is thus important to note that the measured particle size of the extender may not truly reflect the particle size in application. It can not be stressed enough that in order to obtain the best TiO 2 utilisation both the extender and the TiO 2 must be well dispersed and stable. Extender F is of the "new" flash calcined family. The manufacturing process produces an extender particle containing air voids (both interconnected and discrete), which are claimed to help scatter light, thus improving the opacity of the paint film. The particle size is very similar to Extender A and so one would not expect to gain any advantage through a reduction in pigment crowding. Figure 4 shows that there is an improvement over and above the opacity generated using Extender A. This is most likely produced by the entrapped air which is off-setting the crowding effect. Ranking of extenders It is not possible to review all of the results obtained in this study. However, each property was carefully considered and compared to Extender A and the results used to rank the extenders in order of performance. Table 2 shows the results. Four extenders appeared to give an improved, and therefore potentially TiO 2 saving performance - Extenders C, D, E, and F, the latter two being flash calcined china clays. Examination of the four curves in Figure 5 shows that C and D are both gaining most of their advantage by reducing the cpvc of the paint, although small increases in opacity below the cpvc are also evident. The brightness of these two extenders is very high and their shape and size may make them reflect rather than scatter light, as is the case with some platelet type extenders. E and F, although reducing the cpvc slightly, still show an overall increase in opacity across the pvc ladder. Considering Extender E, the increase in opacity below the cpvc is quite substantial (at pvc 30%, Delta Contrast Ratio DCR = 1.5) and should allow for a reasonable reduction in TiO 2 loading. TiO 2 reductions of 20% are possible To determine how much TiO 2 could potentially be saved and to determine what, if any, detrimental effects this would have on the other paint properties, a series of paints were made with incremental reductions in TiO 2. A pvc of 30%, below the cpvc, was chosen to avoid complications of dry hiding. This pvc also showed a substantial improvement in opacity over Extender A. As Figure 6a shows, a reduction in TiO 2 vc of 20% is possible if a match in opacity is required. If, however, one consider the reflectance over black (Figure 6b), the equivalent reflectance is at a slightly lower saving of between 10 and 15% TiO 2 vc. This also suggests that some of the opacity advantage exhibited by Extender E (and F) is coming from absorption rather than scattering. Only the smallest level of TiO 2 reduction could be made without a significant decrease in wet opacity (Figure 7a). This confirms that TiO 2 is not spaced sufficiently well and also suggests that the voids within the flash calcined extender are not discrete and only begin to scatter light when the paint dries. Replacing TiO 2 gives poor scrub resistance A greater porosity in the paint with Extender E was confirmed by Mercury porosimetry. Whilst not completely ideal for assessing the total porosity of paint films, this method does appear to work well in systems where any voids are likely to be interconnected rather than discrete. The porosity of the paint made with Extender E is approximately 5% greater than that for Extender A. Also, most of this additional porosity appears to come in the size range between 0.1 and 1.0µm, the exact size range given for the voids within the flash calcined clays. This also covers the ideal range of void sizes for the scattering of light, although much of the additional porosity indicated is above 0.5µm in size. Given that a greater porosity was identified in the Extender E paint film the level of scrub resistance was examined to see if there were any adverse effects in reducing the TiO 2 level. Figure 7b shows that simply exchanging Extender A for Extender E has led to a slight decrease in scrub resistance. On reducing the level of TiO 2 and replacing it with Extender E, the scrub resistance continues to fall and fails the requirements of <5mg/cm ² loss when more than 5% TiO 2 vc is replaced. This again falls well short of the claimed 30% saving in TiO 2. Entrapped air effects on TiO 2 utilisation Flash calcined extenders with their air void structures can improve the opacity of emulsion paints through some additional scattering although it has been shown that the majority of the voids are not of a suitable size to preferentially scatter light. This suggests that there is another component in the equation that is adding to the opacity of the paint and is imparted by the extender. In comparison to a solvent-borne alkyd paint the TiO 2 pigment in an emulsion paint is usually flocculated and under-utilised and yet in many cases for an equal loading of TiO 2 the opacity of the emulsion paint can be equal to, or surpass that, of the alkyd. This apparent contradiction can be explained by the differences in refractive index of the two systems. Alkyd resins have a typical refractive index of ca. 1.51, an acrylic emulsion will be slightly lower at ca Modelling the scattering power of TiO 2 as a function of resin refractive index (RI) shows that this small difference is sufficient to produce an uplift of 0.5 units contrast ratio. Also, a level of 5% air, entrapped within the paint film, will approximately account for a similar reduction in refractive index and thus increase opacity. Opacity increases if more air is entrapped From earlier Mercury porosimetry work, it is believed that a typical emulsion paint contains between 5 and 15% of entrapped air mainly due to incomplete film coalescence. This quantity of air would account for an additional 1.5 units of contrast ratio and would be sufficient to offset the opacity loss due to flocculation when compared to the solvent borne alkyd paint. Table 3 indicates the relative increases in opacity that can be achieved by reducing the refractive index of the resin system used. If, upon addition of certain extenders, more air is entrapped, then this would reduce the perceived refractive index of the paint medium and thus increase the opacity. This effect is contributing to the opacity advantage seen when using Extender E. Further work is now being done to determine better the levels of air entrapped in emulsion paints, the "particle size" of the air voids, how these interact with the paint film and how, if at all, it is possible to control the inclusion of air in the paint. Acknowledgements The Author would like to express his thanks to Dr Les Simpson, John Temperley, Kathryn Fullerton and Frank Holden for their help in preparing this paper and carrying out the necessary laboratory work. He would also like to thank Mahomed Maiter, Group Marketing Director and Brian

3 Thomas, Group HR & Communications Director, for their support. References [1] L Cutrone, D Becherel, Huntsman Tioxide, Technical Report D9202GC "Interaction Between Fine Particle Extenders and Titanium Dioxide in Paints" [2] J. Temperley, M. J. Westwood,M. R. Hornby, L. A. Simpson, "Use of a Mathematical Model to Predict the Effects of Extenders on Pigment Dispersion in Paint Film", Journal of Coatings Technology, Vol.64 (1992) No.809, p Result at a glance A range of fine particle size extenders have been evaluated and there are some small improvements in TiO 2 utilisation. The claims of 20 to 30% saving in TiO 2 could not be fully substantiated. Current extenders with a particle size in excess of 0.5µm have been shown not to improve the spacing of TiO 2. Extenders with particle sizes of less the 0.5µm were not easily dispersed down to their primary size and tended to homo-flocculate creating large multi-particle extenders causing increased crowding and lower opacities. Flash calcined china clays did appear to offer better TiO 2 utilisation. The incorporation of air voids within the paint film by these extenders have a positive effect on opacity. The inclusion of air voids is thought to reduce the apparent refractive index of the medium and this reduction is contributing to the increase in scattering of the TiO 2 pigment. This paper was presented at the European Coatings Show, 7th Nürnberg Congress, 7 and 8 April 2003, Nuremberg/Germany. The author: ->Paul Frederick Dietz, Huntsman Tioxide, joined the company in He graduated in Chemistry atthe University of Teesside, since 1998 he has worked in Technical Service Department where, in his current role as Decorative Applications Manager, he has technical responsibility for UK, Nordic and French customer service and provides a supporting role in North America.

4 Figure 1: Effect of decreasing extender particle size on spacing of TiO 2 pigment.

5 Figure 2: Theoretical effect of reducing extender particle size on the TiO 2 nearest neighbour distance and scattering.

6 Figure 3: Opacity curves for 4 extenders over a pvc range: a) without application of glycol film, b) with application of glycol film.

7 Figure 4: Opacity curves showing crowding effect.

8 Figure 5: Extenders giving improved performance.

9 Figure 6: Reducing TiO 2 content using extender E: a) effect on contrast ratio, b) effect on equivalent reflectance..

10 Figure 7: TiO 2 savings with extender E: a) effect on wet opacity, b) effect on scrub resistance..

11 Figure 8: Mercury porosity with 30% pvc.

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