The use of TiO2-polymer composites to lower environmental impact and improve performance of waterborne paints
Andrew Trapani1, Marie Bleuzen2, Houshang Kheradmand2 and Anne Koller2
1The Dow Chemical Company, Dow Coating Materials, Valbonne, France,
2The Dow Chemical Company, Dow Coating Materials, Valbonne, France
INTRODUCTION
The primary white hiding pigment in high quality architectural coatings is titanium dioxide, due to the scattering power produced by its very high index of refraction.[1] The scattering power of TiO2 also depends on its particle size[2] as well as on the geometry of the paint film.[3] As TiO2 particles are crowded close together by the high concentration needed to achieve full opacity the scattering efficiency decreases as the effective scattering volumes of the individual particles overlap (see Figure 1).[4] The high levels required, combined with the cost of producing modern coatings grades of TiO2, typically make this pigment one of the most costly raw materials in a paint formulation. TiO2 is also one of the highest components of the total embedded energy required to produce coatings. In the past, improvement in the efficiency of this expensive and high energy ingredient has been achieved through use of polymeric microvoids,[5],[6]extender pigments,[7] and inorganic coatings on the TiO2 particle.[8] There have also been a variety of approaches to using polymer-TiO2 composites to enhance TiO2 efficiency.[9] Recently we developed polymer particles (pre-composite dispersion polymers) which adsorb strongly to the TiO2, and thus can enable higher TiO2efficiency than other adsorbing polymers.
Figure 1. Crowded TiO2(left) compared with TiO2spaced by a pre-composite, strongly adsorbing polymer (right). Note the reduced overlap of the scattering volumes of the individual TiO2particles when they are coated by the pre-composite polymer. The reduction in overlap yields higher hiding in the paint film.
Wet hiding of the freshly applied latex paint can vary dramatically from formulation to formulation. The Cleveland Society for Coatings Technology Technical Committee used the changes in hiding during film drying to observe differences in film formation,[10] and the impacts of pigment geometry,[11] polymer coalescence,[12] toning and film thickness,[13] and opaque polymer[14] on film formation. In the wet state the TiO2is dispersed in water and its concentration is about one-third of what it will be in the dry paint film. The lower index of refraction of water relative to the binder and extender components of the dry paint film contributes to higher efficiency of the TiO2in the wet paint. Crowding effects due to the packing of the pigment, extender, binder and opaque polymers are also not a factor in the initial wet hiding due to the low concentration of TiO2 in the wet paint. Microvoids, from either porosity above CPVC or opaque polymers, contribute significantly to the final dry hiding of the paints; but these voids have not yet developed in the wet paint.
Wet hiding effects are important commercially as professional painters often equate the quality of the paint with its wet hiding. Initially all the latex paints which contain TiO2 drop in scattering power as they dry due to two primary effects: (A) the index of refraction rises from that of water to an average of the binder and extenders, thus reducing the contrast with TiO2and lowering its scattering power; and (B) as the concentration of the TiO2increases and the other particles restrict its location, the dependent scattering effects from the TiO2crowding become significant. When the TiO2use level in a formulation is low and a large amount of >CPVC porosity is a major source of dry hiding, the initial hiding of the wet paint can be lower than the final dry hiding of the paint. This can make it difficult for the painter to properly judge the color and hiding during the application process. In somewhat higher quality paints formulated only slightly above CPVC the initial wet and final dry hiding and color are closer together, which makes it easier to judge the final hiding and color during application. Below the CPVC the initial wet hiding is considerably higher than the dry hiding. This gives an impression of high quality during application, but this drop in hiding on drying can result in difficulty judging the final hiding and color during application.
While wet hiding is a very sensitive probe for monitoring latex film formation through the drying phases, a number of other techniques have recently been used to help elucidate further details of latex film formation[15],[16],[17],[18]and pigment distribution.[19] This improved understanding of film formation has helped the coatings industry to lower VOC in formulations while delivering high performance paints. Lowering TiO2 usage via polymer-TiO2composites continues this effort to lower the environmental impact of the coatings.
EXPERIMENTAL RESULTS
Interactions of latex particles with TiO2have helped boost the scattering power of many paint formulations. But the efficiency of the TiO2scattering in the formulation has varied with changes in surfactants, dispersants and thickeners. And that scattering has sometimes varied with time due to slow equilibration of the paint. Recently we developed adsorbing latex particles which interact more robustly with the TiO2surface, making the final hiding of the paint less dependent on the additives in the paint.
Defining the necessary Pre-composite/TiO2Ratio. A key consideration in the formulation of the composite is the number of pre-composite particles which are needed to saturate the TiO2surface. If the TiO2is not saturated, then the pre-composite particle can potentially bridge between two TiO2particles. Bridging can continue, involving other pre-composite and TiO2particles to make very large aggregates. This pre-composite: TiO2ratio can be thought of in several metrics such as the number ratio, weight ratio or volume ratio of adsorbing latex particles per TiO2particle. One particularly convenient metric is the PVC of the TiO2in the composite, so we can treat the composite formulation similarly to the way we formulate paints.
One can determine the Critical Composite Ratio (CCR), the PVC where the pre-composite saturates the TiO2surface, by titrating TiO2into the pre-composite. A 20 PVC, 38.7 VS Composite was prepared in a small container, adding the following materials in order with continuous good mixing:
Pre-Composite A43.57 g
Defoamer0.10 g
Water6.77 g
TiO2 Slurry Q (70% TiO2)23.30 g
The Brookfield Viscosity (Spindle 4, 60 rpm) was determined. TiO2was added in aliquots, with three minutes of mixing between additions. The viscosity was determined at the end of each three-minute mix. The results may be seen in Figure 2.
Figure 2. Critical Composite Ratio (CCR) Titration of Pre-Composite A with TiO2 Slurry Q.
As the TiO2concentration increases, the number of saturated composite particles increases. Since the composite particle has a larger hydrodynamic volume than the individual pre-composite and TiO2particles which form it, the viscosity will climb modestly. Once we approach the critical composite ratio, the viscosity increase becomes dramatic as bridging occurs between unsaturated composite particles. So the CCR for this combination of Pre-Composite A (a pre-composite polymer with a pure acrylic composition) and TiO2Slurry Q is about 35 PVC. Typically the composite should be formulated somewhat below the CCR to minimize the possibility of grit formation.
Figure 3 shows Cryo-SEM images of composite particles. These composites were made with Pre-Composite B and TiO2Slurries Q (left) and R (right) at 33.4 PVC and 40 VS. The samples were diluted and nebulized onto a cold substrate. The small droplets were then rapidly frozen by immersion in liquid nitrogen cooled to -210°C (referred to as “Slushy” liquid nitrogen) and transferred into a Gatan Alto 2500 Cryo-SEM Prechamber mounted on a JEOL 6700 Field Emission Scanning Electron Microscope. The water was sublimed from the samples by warming them in the prechamber to -80°C and then coated with Au/Pd with a sputter coater. Note that the pre-composites are distributed uniformly over the surface of TiO2Slurry Q in a fairly loose arrangement, but there are few places on the TiO2 Q particles where an additional pre-composite particle could bind. TiO2R is less reactive than Q. In the right image of Figure 3TiO2R is not as saturated: there would be room for more pre-composite particles to bind.
Figure 3. Cryo-SEM of composite formed with Pre-Composite B and TiO2Slurries Q (left) and R (right).
Reformulating with the Pre-composite in a 28% PVC Paint. The pre-composite needed to saturate the TiO2surface is roughly half of the polymer volume in a conventional 30-40% total PVC paint. So there is typically room in the formulation for another polymer – the “letdown binder” – often the original binder in the starting point formulation. For example, Table 1 shows a comparison of a conventional acrylic semi-gloss formulation and one in which a TiO2 -polymer composite has been used, with a reduction of total TiO2 of 20%. In this example the composite was made as a separate intermediate and the extender was subsequently ground with the composite in the same vessel. This composite intermediate has a PVC of 32.7% and a volume solids of 38.1%. Since the pre-composite coats and stabilizes the TiO2, the dispersant level has been reduced to that needed by the extender.
Table 2 gives viscosity and appearance properties for paints made following the recipe in Table 1. Four very different conventional binders were used in this experiment: a premium acrylic binder W, an economy acrylic binder X, a PVA binder Y and an EVA binder Z.
Note that in all four letdown binders, the total thickener demand is lower in the composite paints. The TiO2-polymer composite has a higher hydrodynamic volume than the volumes of individual polymer and pigment particles it is made of, which typically reduces the level of thickeners needed to achieve the target viscosity profile of the paint.
It is particularly striking that although the conventional paint hiding varies widely among the four binders, the hiding and color in the composite paints is constant within the experimental error expected for S/mil, Tint Strength and Contrast Ratio. Note that the conventional formulation with acrylic X has a light rub-up problem, so the color acceptance issue breaks the otherwise strong agreement between S/mil and Tint Strength. Conventionally formulated paints vary widely in the TiO2 efficiency due to interactions of the pigment with the binders, thickeners, surfactants and dispersants in the formulation. TiO2-composite paints are much less sensitive to the loss of efficiency due to these complex interactions.
Table 1. Semi-gloss Conventional and TiO2 -Polymer Composite Paints.
Despite the 20% TiO2 reduction in the formulation, three of the four composite systems have higher hiding, so even more TiO2 could be removed to achieve a hiding match. In acrylic binder X, the hiding is slightly lower in the composite paint, and a hiding match would probably be found between 10 and 15% TiO2 reduction.
Table 2. Semi-Gloss viscosity and appearance results (formulations in Table 1).
In the reformulation shown in Table 1 there is a 5 PVC drop between the conventional and the composite formulations due to the 20% reduction in the TiO2 level. That drop in PVC contributes directly to the increase in the gloss values seen in Table 2. In a more practical reformulation the total PVC and Volume Solids could be held constant by increasing the levels of opaque polymer and extender. Since the opaque polymer contributes independently to the scattering of the paint, the TiO2-polymer composite technology can be combined with the use of opaque polymer to maximize TiO2 reduction in the paint. Opaque polymer is similar in particle size to TiO2 which gives it a similar gloss potential, so it is particularly useful in maintaining the gloss level while replacing the dry volume of TiO2 removed in the reformulation. A precise match can then be achieved by balancing the opaque polymer and extender levels to fine-tune the gloss.
Reformulations with Styrene-acrylic Pre-composites. The previous results were obtained with a pre-composite polymer with a pure acrylic composition, however the pre-composite polymer can be made with other chemistries such as styrene-acrylic (SA) or even vinyl-acrylic. Table 3 contains a reformulation of a 54% PVC conventional paint based on traditional SA dispersion polymer requiring coalescent. The composite paint is made with a pre-composite SA polymer that also requires coalescent. At this total %PVC and %PVC of TiO2 (originally 16,5%) the pre-composite SA polymer makes up about two-thirds of the total binder in the reformulated paint (i.e, one-third of the original, conventional SA polymer remains in the reformulated paint.) The paint making process for the composite reformulation in Table 3 was a standard grinding process followed by the straight-forward introduction of the pre-composite SA polymer and the let-down binder. No special manufacturing steps were introduced.
The original paint contains 16,5% by volume (dry) and 17,9% by weight (wet) TiO2. The reformulated paint contains 14% less TiO2 by weight (wet) and 15% less TiO2 by volume (dry). There is also less dispersant and thickener in the reformulated paint.
Table 3. 54% PVC Conventional SAand TiO2 -Polymer Composite SAPaints.
The basic dry film properties are compared in Table 4. We observe that the dry hiding of the composite SA paint, as measured by the contrast ratio method or Kubelka-Munk Scattering, is essentially the same as the original paint even though the composite SA paint contains about 15% less TiO2. The gloss properties and scrub resistance of the two paints are also essentially identical. Thus we see that the pre-composite technology works just as well when the pre-composite binder and the original binder are SA polymers (requiring coalescent).
Table 4. Basic dry film properties of the conventional and composite paints (SA chemistry, Table 3 formulations)
The pre-composite technology has also been examined in the context of coalescent-free (i.e., self-film-forming) paints. Table 5 contains an example of a self-filming, PVC 80%, TiO2 PVC 9% interior wall paint based on a conventional, soft SA polymer. This table also contains the reformulated paint based on a self-filming (soft) SA pre-composite polymer. Since this paint starts with a low TiO2 content of 9% PVC, the TiO2 in the original formulation is statistically less crowded and so the amount of TiO2 that can be removed (at equal hiding) is also reduced. Therefore, the pre-composite based reformulation shows a TiO2 reduction of about 10%. There is also a reduction in the amount of dispersant needed in the composite paint as the pre-composite polymer plays a role as pigment dispersant. We note that in this type of formulation, the amount of pre-composite needed to coat the TiO2 represents almost all of the binder in the reformulated paint. The paint making process for the paints in Table 5 is the same as that used for Table 3(traditional paint making process).
The resulting dry film properties for the paints described in Table 5 are shown in Table 6. As with the coalesced SA paints, in this category of high PVC, self-filming SA paints, the composite SA paint shows the same hiding (only determined via contrast ratio since at this PVC the films are too brittle for the Kubelka-Munk scattering method) even though the composite based paint contains 10% less TiO2. As one can see in Table 6, the gloss and scrub resistance remain essentially unchanged when reformulating with the self-filming SA pre-composite.
Table 5. 80% PVC Conventional Self-filmingSA and TiO2-Polymer Composite SA Self-filming Paints.
Table 6. Basic dry film properties of the conventional and composite paints (SA chemistry, Table 5 formulations)
The reformulations presented in Tables 1, 3 and 5 are quite varied. They cover a wide range of total paint PVC from 28% to 80% and a wide range of TiO2 PVC from 9% to 23%. These reformulations have been effective with all-acrylic and styrene-acrylic polymer compositions. Initial scouting results lead us to believe that the pre-composite technology will also be effective with vinyl-acrylic polymer compositions. This set of data demonstrates that the pre-composite technology can be a robust tool for lowering the TiO2 content of a paint without sacrificing hiding or other film properties.
Pre-composites and Wet Hiding. The wet hiding was measured in white paints on Leneta 5C opacity charts. Both flat and semi-gloss formulations were examined. Table 7 contains a general description of the formulations.
Table 7. Formulation descriptions for wet hiding experiments.
A second opacity chart was cut as a mask, with a 50 mm slot open for the reflectance readings. The paint was cast with a 75 micron Bird Applicator on the opacity chart and the mask was placed on the wet film with the open slot over the white portion of the chart. Y reflectance was measured with a portable Gardco 45°/0° Novo-Shade Reflectometer which was placed directly on the mask. At each point in time an average of three Y reflectance readings was recorded at points pre-marked on the mask.
The results in Figures 4 and 5 demonstrate that despite the 20% reduction in the TiO2 in these formulations, the reflectance with time is quite similar for both standard and composite paints. In other words, the paints are matched for both wet and dry hiding.