Kaolinite is a mineral belonging to the family of aluminosilicates. It is usually referred to as "China Clay" because it was first discovered at Kao-Ling, in China. The term kaolin is generally used for a set of common clay minerals having significant amount of kaolinite.
Of specific commercial interest is the degree of crystallinity of Kaolin that controls the key film property that is brightness, whiteness, opacity, gloss, film strength. Kaolin's whiteness and laminar nature makes it extremely suitable for its broad use in coating industry as an extender.
In its hydrous or calcined forms, kaolin is capable of improving the optical, mechanical and rheological properties of a paint. These are widely used in range of PVC paints whether it's glossy, satin or matt paint. They deliver mainly three properties that is increased opacity, whiteness and scrub resistance. In a time when coating manufactures are keen to reduce the TiO2 cost burden, optimization without compromising the performance is a greater challenge, and Kaolin offers an optimum solution.
This paper describes various possibilities and some commercial aspects which Kaolin can create for both TiO2 optimization as well as performance enhancement.
Kaolin, Extenders, PVC, Particle size, Whiteness, TiO2 optimization
KAOLIN is hydrous Aluminium Silicate clay having platy structure with the chemical formula Al2Si2O5 (OH)4. It is crystalline in nature and known to have very fine particle size, which generally is a basic requirement for TiO2 spacing in coatings.
The kaolin structure can be depicted as a layer of silica rings joined to a layer of alumina octahedral through shared oxygen, as shown in Figure 1 .
Kaolinite particles have the shape of a hexagonal plate. In nature these plates occur in stacks that exhibit varying degrees of stacking. Because an individual kaolinite particle has an oxygen surface on one side and a hydroxyl surface on the other, it is strongly hydrogen bonded to the plates above and below it. Delamination in such cases is more difficult than for the other platy silicate fillers, talc and mica. Kaolin is further categorized into hydrous kaolin, delaminated kaolin and calcined kaolin. Basic properties of different types of kaolin clays are given in table 1 .
In coatings, use of Calcined Kaolin is highest followed by Hydrous and then delaminated. Most formulators explore kaolin clays incorporation with limited consideration w.r.t. fundamentals of selection and that is one of the key challenges to derive best mileage out of the available tool.
On the of Basis our in-house research on Kaolin we have made efforts to identify basic consideration to select and optimize TiO2 levels without any compromise.
Fundamental reasons for kaolin supporting optical performance
High PVC Paints
Paint products with CPVC more than PVC usually have lower TiO2 levels and higher extender quantities and lower binder content. In such low level binder containing products, opacity is achieved by the larger refractive index difference between media and TiO2 because of the presence of air voids . If selection of proper extender pack with optimum particle size is not selected then there is possibility of TiO2 particles crowding due to less volume available for TiO2. In such system this affects scattering of TiO2 particles resulting into inefficient optical performance of TiO2.
Kaolin with fine particle size when explored by replacing the coarser extenders reduces the crowding of TiO2 thereby uniformly distributing TiO2 particles within given volume. Light scattering of TiO2 particles get enhanced by such phenomena thereby increasing optical characteristics and efficiency.
Low / Mid PVC Paints
This category of paint with CPVC less than PVC has more amount of TiO2 and lesser amount of extenders. In such cases all the TiO2 and extender particles are covered by the emulsion in the system and opacity in such cases is mainly achieved by light scattering from TiO2 particles. In such TiO2 rich system there is possibility of crowding and agglomeration of TiO2 in the system which may result into poor scattering of light and decreased opacity.
Finer kaolin as compared to other coarser extenders in such cases resists the crowding of TiO2 and thereby their agglomeration and properly distributes the TiO2 particles. Because of which efficiency of TiO2 for light scattering is optimum which gives improved opacity and this creates opportunity to optimize the formulation cost thereby reducing some amount of TiO2 without touching upon the optical performance of coating. Kaolin vs Other extenders
Most of the extenders that are being incorporated in coatings are carbonates and talc. They are added in paint formulation together with the TiO2 pigment to achieve opacity, sheen and rheology of paint there by optimizing cost. Carbonates are in round shape whereas the kaolin possess plate like structure which is the major difference between both carbonates and kaolin clays which influences the final optical performance. In paint formulations the most effective ingredient to achieve opacity is TiO2. Many of the formulations contain TiO2 in large amount but due to overcrowding and improper dispersion the optical efficiency gets adversely affected because of which the optical performance gets deteriorated. Carbonates like calcium carbonate and talc are added to improve the overall optical efficiency of paint by spacing the TiO2 particles in paint film.
Carbonates when partially replaced with plate structured Kaolin with fine particle size and better whiteness shows the better spacing of TiO2 than carbonates because of their structure and morphology. Kaolin is known to have higher spreading rate than that of carbonates which makes them better option over the carbonates for TiO2 spacing. This advantage of kaolin creates opportunity to reduce the TiO2 in formulation and thereby reducing formulation cost and minimizing carbon footprint effects. Schematically spacing of TiO2 using kaolin can be represented as shown in figure 2
Titanium dioxide rutile grade received from Global supplier. Clays & Kaolin received from local supplier. Talc & Carbonates are received from local supplier.
Malvern particle size analyzer Mastersizer 2000 was used for particle size analysis of extenders and clays, (internal test method -IPSA-03). Diffuse Reflectance Meter – AIMIL was used for checking whiteness of different extenders & Kaolin (internal test method -APTM/RM/ P – 153). Erichsen Wet scrub tester was used for checking scrub resistance cycles of paint films casted on Laneta papers (internal test method GP-58). Gretag Macbeth Color-Eye 7000A spectrophotometer was used for checking contrast ratio and whiteness values of paint films.
Experimentation: Exploration studies of Kaolin in different range of products
Different waterborne coatings varying from interior wall finishes to exterior wall finishes comprises of varying amount of TiO2 which enables them to have specific optical and other performance properties so called sheen, scrub resistance, etc. The TiO2 demand is very high across the paint industry and TiO2 is known to have carbon footprint effect on environment which is case sensitive. Many technical routes are being used to cut down on TiO2 consumption to make cost effective and environmental friendly coating which is best for business. Kaolin is one of the best promising source of raw material which helps in optimizing TiO2 and enhancing the features of coatings. Current paper describes how kaolin having finer particle size and whiteness can be used for serving the following purposes.
1) Optimizing TiO2 to some level without affecting optical and overall coating performance
2) Enhancing the optical performance by reengineering the current extenders by kaolin thereby making cost effective formulations.
Selected Kaolin was analyzed for its distinguishable properties which are listed in table.
Characterization of finer & brighter kaolin vs Other extenders
Kaolin is best known for its better whiteness and particle which improves the optical performance of coatings. Selected Kaolin was comparatively studied for its particle size, oil absorption & whiteness with other extenders. Following are the test results of the same.
Tested Particle Size, Oil absorption and Whiteness values of different extenders are give in the table :
On the basis of above comparative properties of kaolin with other extenders different experiments regarding exploration of kaolin were planned.
Experiment 1: In Medium PVC Interior Formulation
The kaolin was explored in interior wall finish product with PVC in range of 45-50% having TiO2 levels from 10-15%. Fine particle sized clay is replaced by cost effective but much brighter kaolin on weight on weight basis. Formulation details are as following. The below experimental design with critical changes was tested for optical properties and scrub performance.
Experiment 2: In Higher PVC Interior Formulation
In the another interior wall finish product having PVC in range of 58-62%, TiO2 is reduced by 2% of total TiO2 present in standard formulation and this was done by replacing the coarser clay by brighter kaolin in experimental formulation with critical changes is listed below. The same experimental design was tested for optical performance and scrub resistance.
Experiment 3: In Higher PVC Exterior Formulation
In the exterior wall finish products specifically the undercoats or primers having PVC in the range of 68-72% containing very less amount of TiO2, finer and whiter kaolin was explored in place of coarser clay and TiO2 is reduced by ~ 15 %. The following experimental design with critical changes was tested for optical properties and adhesion test for different top coats which is the key test to primers.
Experiment 4: In Medium PVC Exterior Formulation
In the exterior wall top coats with high sheen products having PVC in the range of 40-45%, the finer and whiter kaolin was added without touching upon the TiO2 part in place of coarser carbonates by matching the oil absorption value of carbonates present in STD. Kaolin being high oil absorption material added in lesser percentages as compared to carbonates which are having lower oil absorption values. The following formula consists of critical changes that are done in STD formulation. The experimental design was tested for optical properties against STD design.
Experiment 5: In High PVC Exterior Formulation
In the exterior wall top coats with low sheen having PVC in the range of 70-75%,5% of total TiO2 is reduced and some part of talc in STD formulation is replaced by brighter kaolin. The following experimental formula with critical changes was studies for optical performance.
Results and discussions
Results of Experiment 1:
Even though finer clay (d50=1.8 micron) is replaced by the economical but brighter kaolin (d50=2.65) the opacity of experimental design was found to be comparable against STD as observed in figure 3 and scrub performance of paint film remains unchanged as shown in figure 4.This shows that even on replacing the finer clay by coarser but brighter kaolin the optical performance and mechanical features won't get affected.
Results of Experiment 2:
Optical performance of Experimental design is found to be comparable against std even though the 2% TiO2 is reduced as shown in figure 5. This shows that the lesser amount of kaolin as compared to coarser clay when added has improved the spacing of TiO2 particles which in turns helped in optimizing some TiO2 to get cost benefit. Mechanical property like scrub resistance didn't get affected by addition of kaolin as seen in figure 6.
Results of Experiment 3:
On optimizing TiO2 by 10% of total TiO2 in under coat and by adding the finer and brighter kaolin in place of coarser kaolin the experiment showed much better opacity and whiteness than STD as shown in figure 7 . Also the inter-coat adhesion performance of different top coats on experimental under coat is comparable to standard as shown in figure 8 .This shows that the Kaolin with much better whiteness and equivalent oil absorption as compared to coarser kaolin provided comparable optical performance.
Results of Experiment 4:
In the exterior wall product with high sheen when Kaolin is added in lesser quantity as compared to carbonates present in the STD formulation, the opacity was found to be comparable against standard this creates the an opportunity for formulators to design the product with low weight per litre with better optical performance and gives the chance to cut down on formulation cost. The optical performance attributes are shown in figure 9.The scrub performance of experimental design on kaolin against STD is shown in figure 10.
Results of Experiment 5:
In the exterior top coat high PVC product when partial amount of coarser talc is replaced by Finer brighter kaolin and 5% of TiO2 is optimized the opacity of experimental design found to be satisfactory as compared with STD design. This is well reflected from figure 11. Kaolin with fine particle size and better whiteness improved the overall TiO2 efficiency and scattering of light and hence improved the opacity and given the chance to optimize the TiO2 in the formulation. Scrub resistance performance of Experiment on kaolin against STD is shown in figure 12.
From the above research study, it can be concluded that by use of appropriate kaolin having distinguishable characteristics like finer particle size, better whiteness and its unique morphology it is feasible to optimize the wide range of formulations across PVC range.
Out of many options available, the selection of appropriate kaolin not only provides formulator to use TiO2 at optimum level but also leads to reducing the carbon foot print thereby providing more sustainable and green solution to the coating industries.
The above learnings can be extended to stiff, solvent based, powder free coatings as well towards TiO2 independency and contributing to more sustainable solution to environment.
- Kaolin clay-functional optical additive, PCI magazine August 2003.
- Kaolin clays, IMERYS Minerals, APCJ December 2017.
- International Journal of Chemical Engineering and Applications vol. 6, no. 5, pp. 331-340, 2015.
The authors express gratitude to Research and Technology Centre, Asian Paints Limited for providing opportunity to write this paper. We would like to acknowledge Mr. Rajiv Goel for encouragement in writing this paper. We would like to thank modern instrument lab members and VDL group at R&T Centre, Asian Paints Limited for characterization part.
Sagar Bangar, Robin Varghese, Gaurav Shukla
Asian Paints, R&T Centre, Plot No:c-3b/1.TTC Indus. Area,
Midc Pawane, Thane - Belapur Road, Navi Mumbai-400703
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