Improving hiding power obtained by variation of fillers for interior emulsion paints

Improving hiding power obtained by variation of fillers for interior emulsion paints

This paper presents a study who emphasizes the importance of the choice of fillers and together with a performance quality of white pigment to help achieve those desired for interior emulsion paint. Thus, we studied various types of fillers (sulfates, silicates, calcium carbonate) with white pigments (zinc oxide, titanium dioxide) in a copolymer of ethylene vinyl acetate (EVA) and all these combinations analyzed and measured by values of hiding power, white and yellow index.

Keywords: coating emulsion, fillers, white pigments, interior emulsion paints

To obtain a quality and stable paint is necessary to establish the optimal technological parameters. Although, the basic principles can be found in the literature of World War period when Schou describes the possibility of obtaining emulsified paint binders based on triglycerides, emulsified paints are relatively new [1]. An emulsified paint systems development occurred only after the Second World War, when become available quantities of butadiene and styrene plants manufacturing synthetic rubber. These materials are combining to form butadiene-styrene latex, became able to make paint diluted with water. There are today three main types of latex used as binders for paints, namely: styrene-butadiene copolymers, poly-vinyl-acetate and acrylic polymers and copolymers. The term of latex based paint was given initially emulsified styrene-butadiene paints, but now it defines all polymer systems made with emulsified [2-3].

Materials and devices

The Base - it was use a base that includes the main components of paint: copolymer and ethylene vinyl acetate (EVA), cellulose-ether thickener, filler, preservative (biocide), water and ammonia solution. The Base's characteristics are presented in Table 1.

Table 1 - Characteristics of used Base

Water based paint is a nano dispersion of a solid component called Base and fillers in an aqueous medium [4]. To give a stability and other properties that hiding power, white and yellow index it is need to add foaming agent, dispersing agent, coalescent agent and fillers. The fillers used were: natural calcium carbonate (CCN), precipitated calcium carbonate (PCC), titanium dioxide, opaque polymers, sodium aluminum silicate, 14.5 SiO2 * 1.4 Na2O * Al2O3, mica, talc, kaolin, micronized barite, diatomite, BaSO4, silica. Optical properties of pigments and fillers are determined by the refractive index difference between air and pigment or filler. The more this difference is higher the hiding power is higher [5]. The substances with a refractive index greater than 1.7 are generally called white pigments. Bigger differences occur between air and fillers with an effect everywhere is an interface between pigment and air or between filler and air is a distinct increase of hiding power.

It assumes that the original recipe has 60% in weight fillers. This percentage will be distributed to each type of fillers depending on the composition, role and its ratio of the resin from Base.

Experimental part

Besides the Base, a water based paint requires another components like in Table 2:

Table 2 - The components of water based paint

The variants underlying of this study are presented in Table 3:

Table 3 - variants of paints

There have been twelve versions of compositions of paints that after tests carried out to define the influence of those components and their concentrations to opacity, white index and yellow index.

The quantities of fillers were based on data sheets that are recommended optimum dosage for each filler in the formula.
          These are theoretical formulations as a starting point for further study. Each variant can be analyzed separately and represents a small-scale study. There are not final formulations but only alternatives to be further developed and analyzed.

Main equipment

It was used a Byk Gardner Spectrophotometer for measurements dried paints films: Contrast Ratio (Opacity, Hiding Power), White Index and Yellow Index, Reflectance Index.

There also have been using these devices: Bar-Type applicator, Opacity Drawdown Charts, Brookfield Viscometer - measure viscosity by sensing the torque required to rotate a spindle at constant speed while immersed in fluid, the torque is proportional to the viscous drag on the spindle, thus the sample viscosity; Cup Volume; Oven at 105⁰C; electronic pH meter; Oven heated at 950⁰C; the Fineness of Grind Gage is used to indicate the fineness of grind or the presence of coarse particles or agglomerates in a dispersion and a Pfund Cryptometer which determines the wet hiding power of a coating within a few minutes.

Procedure

To obtain an interior latex paint similar with the one that meets the market requirement, called Standard (S), are proposed to be prepared using a simple laboratory dissolvers - Dispermat, twelve variants, a minimum amount of 1.5 kg for each variant, with the same errors in weighing (same electronic scale), the same batches of raw materials are kept constant, the same technology and policy input materials, while the mixing, the same labor worker. The variants obtained are stored in the same conditions of temperature and humidity in the laboratory (20 + / - 30⁰ C, 50%), and analyzed after 24 hours of preparation. The analysis of these variants will be made by the same labor worker in the same working conditions to eliminate as many possible errors.

The technological process is: the Base, half quantity of water, coalescent agent, half of antifoaming agent and dispersing agent, are mixed at medium speed of 400-500 rpm for 5 min, then add the fillers, using a high speed of 1000-1200 rpm, put the rest of water, allow the mixture for 1-2 hours, until obtain a fine paste (less than 50 microns using Fineness of Grind Gage), apply a thin layer on a glass plate to see the dispersion of the film, then reduce speed to 300-400 rpm, and add the difference of antifoaming agent, and left to a slow speed for 2 min.

Results and discussions

Paints formulations V1-V12 in liquid form were analyzed in terms of viscosity, density, content of volatile, resin content and organics, fineness, pH and hiding power measured as the specific consumption.

Table 4 contains the characteristics of these liquid formulations.

The Physical-chemical characterization of the Standard (S) sample with the. rest of the variants obtained is shown in Table 4.

Table 4 - Physical-chemical characterization for all variants

As content of the resin, all the twelve variants analyzed are clearly superior to Standard reference paint, demonstrating that this paint film will have a better adhesion to the substrate and also a high wet scrub resistance. The difference of viscosity between Standard and studied variants can be adjust by adding water, which will lead a decrease in resin content and implicit a less hiding power. A triple value of viscosity does not bother with anything, it can be reduced by adding water to the application as to have a good application, without suffering major differences in other characteristics. The fineness, the dispersion of fillers, have an important role in the formulation of paints, leading to a homogenization as good, close to that of a paste, with a good hiding power but also the adequate chemical stability. The fineness should be max. 50 microns, a smaller one will be better for paint.

The hiding power measured as the specific consumption, increases with the variation of fillers in formula, their lack of formula shown very clearly (eg V1), but not always a better hiding power lead to acceptance like a final version. It must be relevant with other characteristics such as density, fineness, solid content.

A pH between 8 and 9 lead a safe stability for paint, avoid the formation of bacteria and fungi that develop in aqueous medium at a pH below 8.

In conclusion, from all twelve variants studied and analyzed, just V2, V5, V6, V8 and V12 may be taken as variants using for another tests.

Characterization of dry paint film applied on Opacity Drawdown Charts

The only meaningful way to compare two different batches of paint is to have a dry film of uniform thickness and texture. Bar-Type applicators with which we applied wet film of 75 microns are used to determine hide or contrast ratio.  To measure the accurately of color and gloss we can apply 100 microns (or greater).

Each variant studied, including the reference one - Standard, were applied after 24 hours of manufacture, on the Test Chart, using a Bar Type applicator with 75 microns, let them to dry and compared visually but also measurement with graphic analysis.

All readings are standards with the International Commission of Illumination (CIE). In practice light sources are:
- Daylight, D65, C;
- The incandescent light;
- Fluorescent light F2, F11.

Contrast Ratio (Opacity, Hiding Power) variation - presented in Figure 1

Essential sales criteria for architectural paint are hiding power and yield. In other words: how many layers are necessary for complete coverage and how many cans will be needed?

Opacity is a measure for hiding power:

 Opacity (%) =  YBLACK   x 100 (%) (1)
                   YWHITE

Where: YBLACK = the value measured on black area on the chart;

YWHITE = the value measured on white area on the chart.

100% opacity means complete hiding, no differences can be seen between the drawdown over black and white.

Procedure
A uniform paint film is applied on a black / white contrast chart. After air drying the drawdown can be objectively evaluated using the BYK-Gardner spectro-guide. The operator is menu guided through the measurement procedure and the opacity value is displayed automatically in a second [6-7].

Figure 1 - Contrast Ratio variation

The contrast ratio value for Standard reference sample is not a very big one, 98.71%, and nine of twelve variants have higher contrast ratio values than this one, but this characteristic needs to be correlate with the rest of characteristics, like White Index, Yellow Index, so from all these nine variants only V5 variant has this condition and has 99.72% like contrast ratio value.

White Index variation - presented Figure 2

Figure 2 - White Index variation

White Index value for the reference sample Standard is 90.63. This is the target for our variants. Only V5 variant has a closer value with this Standard, 87.54.

Yellow Index variation - presented in Figure 3

The shades, fillers materials and hiding power substances are measured in quantitative terms the value of an index called yellow size. This is measured by the method described in DIN 6167 of reflectance values Rx, Ry, Rz with this formula:

(Rx-Rz)/Ry (2)

where: Rx = red filter reflectance, Rz = blue filter reflectance, Ry = green filter reflectance

Negative values for yellow index indicate a blue tempt, positive values - a yellowish tempt. It can easily see from reflectance gradient of the curve if there is a yellowish tempt (= positive increase) or blue tempt (= negative increase).

Figure 3 - Yellow Index variation

The Yellow Index value for Standard reference sample is 1.15. This is our target in this study. Only V5 has a closer value, 2.53. If this value is lower than the paint is less yellow, so our target is to have a better Whiteness Index and a less Yellowness Index.

Reflection values

The brightness, which is measured according to DIN 53163 at a wavelength of 520 nm, gives indication about the optical properties of the product, the scope and can give to user an estimating of purity product.

The term 'brightness' that is very important for paints manufacturers, also require a clarification. According with ISO 2470, the degree of brightness is measured at a wavelength of 457 nm that correlates best with the physiological impression of human eye, while at the 520 nm, the wavelength to measure the brightness, the eye presents high sensitivity.

The measurements differences - dL and dE- for all twelve variants compared with those of standard reference are presented in Table 5.

Table 5 - difference measurements CIE

The dE represent the error of sample tested against the standard, which is considered optimal to 0, the center axis. Can be accept an error around the center of max + / - 1 of these samples. So, V5 has an error until max. 1, which corresponds to the requirement and that can be easy changed to have a decrease value of dE. dL should have positive value, so the positive values are for V5 and V7 variants, closest to 0, but V7 falls because has a dE bigger than Standard.

Conclusions

All of twelve variants produced, analyzed and studied, only the V5 variant, is still the subject of another study, the same theme, but the improvement of this form to make an interior paint that rise to the market requirements of paint and varnish. In this case it can be said that sodium aluminum silicate, can successfully replace titanium dioxide, using a higher percentage than we did in this formula. An improvement of the paint may consist in maintaining constant the ratio of titanium dioxide and sodium aluminum silicate and adding some other types of fillers in combination with them.

One thing is certain, namely that: sodium aluminum silicate - partial replacement of titanium dioxide, offers the following advantages brought in a paint:
- Improves the optical properties (white index and high opacity);
- Reduces cost price of paint;
- Improves the rheology character of paint: a greater stability during storage, pH stabilize,

being also an anti-deposition agent; - Improves paint properties: resistance to water and to wet scrub and also a resistance to

atmospheric agents.

References

1. TALBERT, RODGER (2007). Paint Technology Handbook. Grand Rapids, Michigan, USA.
2. G. GEUSKENS, M. BORSU, C. DAVID, Eur. Polym. J. 8 (1972) 883.

J. LACOSTE, D.J. CARLSSON, J. Polym. Sci. A: Polym. Chem. 30 (1992) 493.

4. M. COPUROGLU, M. SEN, Polym. Adv. Technol. 16 (2005) 61.
5. GEUSKENS G, BORSU M, DAVID C. Eur. Polym. J. 1972; 8:
6. Federal Test Method Standard, Paint, Varnish, Lacquer and Related materials: methods of inspection, sampling and testing, March 22, 2001.
7. WESTLUND, H., and MEYER, G., "Applying Appearance Standards to Light Reflection Models"' Proceedings of SIGGRAPH'01, 501-510, (2001).