Cost Effective Opacity – A Natural Alternative to Opaque Polymer

Flash calcined kaolin and opaque polymer are competing technologies that can exert a direct influence on the optical properties of interior decorative matte emulsion paint. Flash calcined kaolin’s intrinsically sealed voids and opaque polymer air voids both act as scattering sources that make a direct contribution to dry film opacity. Table 1 compares the typical properties of flash calcined kaolin, Opacilite and a commercially available opaque polymer. The aim of this feature is to provide a practical comparison in a good quality and an economic quality matte emulsion paint outlining the advantages of each formulation approach.

Table 1: Typical properties of Opacilite & Opaque Polymer

PropertyOpaciliteOpaque Polymer*
DescriptionFlash calcined kaolinHollow-sphere polymeric pigment
Solid content by weight: %10030
Solid content by volume: %-52.1
Specific gravity2.06Latex: 1.025
Dry polymer: 0.591
Average particle size:µm1.60.38
Void fraction: %2244
*ammonia & alkyl phenol ethoxylate free

Opacilite is produced by a ‘flash’ calcination process that results in the ‘explosive’ dehydroxylation of fine hydrous kaolin. The resultant dehydroxylated aluminium silicate contains numerous intrinsically sealed voids that make a direct contribution to both dry and wet opacity. During the calcination process the lamellar kaolin particles also sinter to form irregularly shaped aggregates with an external void structure. Figure 1 depicts a Focused Ion Beam (FIB) image of an Opacilite particle with evidence of the internal and external particle structure. Applying this technique to cross-section the extender particles provides a three-dimensional understanding of the flash calcined kaolin’s structure1.

Opaque polymer is a non-film forming, water-filled emulsion polymer that forms microscopic air voids as the contained water diffuses through the polymer shell on drying. These polymer micro voids have a fine particle size and narrow particle size distribution that can influence the state of dispersion of the titanium dioxide particles by acting as a primary pigment spacer2. In addition the opaque polymer particles have a relatively low specific surface area compared to equivalent volumes of titanium dioxide. Subsequently, the associated low binder demand facilitates formulating at higher pigment volume concentrations3. Dry opaque polymer also has a low product density and reduces the specific gravity of the coating4.

Testing the Concept

The opaque polymer tested has twice the void volume of Opacilite, however, when compared at equal volume concentration in high PVC matt emulsion paints, the flash calcined kaolin’s internal and external voids provide an opacity advantage over opaque polymer. Subsequently, only 60 to 65% by weight or 57 to 62% by volume of Opacilite is required to match the opacity of the opaque polymer containing paint. This reformulation technique is illustrated in Table 2, the good quality (GQ) matte emulsion paint series, where the opaque polymer in the Standard GQ paint is replaced with a combination of Opacilite and water. Experimental GQ1 is a 60% weight replacement and Experimental GQ2 is a 65% weight replacement of the opaque polymer.

Table 2: Good quality (GQ) matt emulsion paint formulations

Raw MaterialStd GQExp GQ1Exp GQ2
(%)(%)(%)
Tiona 59514.014.014.0
Supreme4.54.54.5
OXO Talc13.013.013.0
Omyacarb 5GU14.014.014.0
Opaque Polymer10.0--
Opacilite-6.06.5
Additional Water-4.03.5
Mowilith LDM 187115.015.015.0
Other Additives*1.91.91.9
Water27.627.627.6
Total100.0100.0100.0

*other additives include dispersant, defoamer, biocide & rheology modifiers

PVC (%)73.671.371.6
Specific Gravity1.4611.5251.531
Solids Content (%)57.160.160.6
Volume Solids (%)40.038.438.9

Only a 60% weight replacement is provided in the economic quality (EQ) matte emulsion paint series as illustrated in Table 3.

Table 3: Economic quality (EQ) matt emulsion paint series formulations

Raw MaterialStd EQExp EQ1
(中文) -(%)(%)
Tioxide TR924.004.00
ImerCarb 2L25.0025.00
ImerCarb 5L10.0010.00
Opaque Polymer7.00-
Opacilite-4.20
Additional Water-2.80
Axilat DS9108.008.00
Other Additives*2.642.64
Water43.3643.36
Total43.3643.36

*other additives include dispersant, defoamer, biocide & rheology modifiers

PVC (%)82.481.1
Specific Gravity1.3451.383
Solids Content (%)45.747.8
Volume Solids (%)28.627.3

Results

The good quality matte emulsion paint series results are presented in Table 4.

Table 4: Good quality (GQ) matt emulsion paint series results

Std GQ Exp GQ1 Exp GQ2
Rheology
Gel Strength: g.cm 49 48 55
1 rpm Brookfield (S6, 0.3s-1, ASTM D2196): Poise 1520 1080 1220
100 rpm Brookfield (S6, 30s-1, ASTM D2196): Poise 53.2 39.3 42.4
Krebs Stormer (ASTM D562): Kreb Units 125 116 119
Rotothinner (150s-1, ISO 2884/2): Poise 8.5 8.9 9.8
Cone and Plate (104s-1, ISO 2884/1): Poise 0.8 0.9 1.0
Dry Film Properties
Opacity @ 20m2/l (ISO6504/2): % 95.7 95.5 95.6
Spreading Rate @ 98% CR: m2/l 13.3 13.2 13.4
Dry Contrast Ratio at 75µm (ISO 2814): % 96.0 95.9 96.5
Dry Contrast Ratio at 100µm (ISO 2814): % 97.7 97.5 97.7
Dry Contrast Ratio at 150µm (ISO 2814): % 99.2 99.3 99.5
Wet Opacility at 150µm: % 99.0 99.2 99.4
Colour (ISO 7724/2): L* 97.0 96.6 96.6
a* -0.52 -0.51 -0.51
b* 1.48 1.79 1.85
Gloss at 85° (ISO 2813): Gloss Units 6.5 4.2 4.4
Stain Resistance (Gilsonite) : % 92 83 82
Mud Crack Resistance (Plasterboard): µm >1500 >1500 >1500
Wet Scrub Resistance (ISO 11998): µm 16 14 14

Compared to the standard good quality matte emulsion containing opaque polymer, both paints containing Opacilite, Experimental GQ1 and Experimental GQ2, have:

  • Reduced low shear viscosity
  • Similar mid-high shear viscosity and gel strength
  • Similar dry film opacity
  • Worse colour, DE=0.51 & 0.54 respectively
  • Significantly lower sheen
  • Lower stain resistance
  • Similar mudcrack resistance
  • Similar wet scrub resistance

No attempt has been make to match the low shear rheology profile of the Opacilite containing good quality matte emulsion by adjusting the rheology modifiers present in the formulations.

The application of the FIB technique to the paint film provides an insight into the internal structural differences of the opaque polymer and Opacilite containing paint films. The focussed beam of gallium ions produces a small trench in the paint film. When the sidewall is viewed at an oblique angle with an electron microscope, the particle interaction and pore structure can be studied.Figure 2 is a low magnification of the Standard GQ paint film containing opaque polymer. Since this coating is formulated above the critical pigment volume concentration (CPVC) the ‘light grey’ areas represent conductive binder and the ‘dark grey’ areas non-conductive pigments, extenders and air voids.

Figure 3 is a magnification of the area marked as Insert A. The distribution and pore size of the opaque polymer provide evidence that the opaque polymer is non-film forming and is functioning as a titanium dioxide spacer. The fine hydrous kaolin, also present in this formulation, is also providing a spacer function. Macro-air voids are formed at the interfaces of the binder and extender particles.The lamellar, high aspect ratio talc is providing a reinforcing function and ensures that the mudcrack resistance of both the opaque polymer and Opacilite containing paint are both greater than 1500 microns over unsealed plasterboard.

Figure 4 is a low magnification of the ExperimentalGQ1 paint film containing Opacilite. Figures 5 and 6 are magnifications of the areas marked as Insert B and C respectively. Although this paint has a marginally lower PVC, the position of the CPVC is lower than the standard paint and there is evidence of a greater concentration of macro air voids. This higher porosity explains the difference in stain resistance between the opaque polymer and the Opacilite containing paints. The flatter, irregular internal ‘micro’ void structure of Opacilite can be clearly seen in Figures 5 and 6. These sealed voids contribute to the wet opacity of the coating unlike the water-filled voids of the opaque polymer. The irregular particle shape of Opacilite also provides significantly greater matting efficiency than the opaque polymer.

Table 5: Economic quality (GQ) matt emulsion paint series results

The economic quality matte emulsion paint series results are presented in Table 5.

Rheology
Krebs Stormer (ASTM D562): Kreb Units 103 101
Rotothinner (150s-1, ISO 2884/2): Poise 5.4 5.6
Cone and Plate (104s-1, ISO 2884/1): Poise 0.7 0.8
Krebs Stormer (ASTM D562): Kreb Units 125 116
Dry Film Properties
Opacity @ 20m2/l (ISO6504/2): % 85.2 85.6
Spreading Rate @ 98% CR: m2/l 6.1 6.5
Dry Contrast Ratio at 75µm (ISO 2814): % 85.8 88.0
Dry Contrast Ratio at 100µm (ISO 2814): % 91.1 94.1
Dry Contrast Ratio at 150µm (ISO 2814): % 95.8 96.8
Colour (ISO 7724/2): L* 95.5 95.2
a* -0.34 -0.28
b* 1.29 1.73
Gloss at 85° (ISO 2813): Gloss Units 5.2 4.7
Stain Resistance (Gilsonite) : % 90 77
Mud Crack Resistance (Plasterboard): µm 640 >1500

 

 

Compared to the standard economic matt emulsion containing opaque polymer the paint containing Opacilite, Experimental EQ1 has:

  • Similar mid-high shear viscosity
  • Significantly higher dry film opacity
  • Worse colour, DE=0.54
  • Lower stain resistance
  • Significantly better mudcrack resistance

The paint reformulation with Opacilite has a higher porosity than the opaque polymer containing standard which contributes to the higher opacity.

Figures 7 and 8 are scanning electron microscope (SEM) images of the Standard EQ and Experimental EQ1 paint films. Comparing these images provides an overview of the macro void structure of each formulation approach. The irregular particle shape of Opacilite has a higher aspect ratio than opaque polymer and helps to reinforce the drying paint film. The greater differential between the CPVC and the PVC in the Opacilite containing paint results in an improvement in the mudcrack resistance over unsealed plasterboard.

Conclusion

A summary of the advantages of each formulation approach is provided in Table 6.

Flash calcined kaolin and opaque polymer are both technologies that can each influence the micro void structure of high PVC matt emulsions, however, considerably less Opacilite is required to match the opacity and scrub resistance. Typically, it is possible to replace the opaque polymer in a high PVC matt emulsion by just 60 to 65% of its weight of Opacilite and the balance with water. Admittedly, since opaque polymer is a non-film forming emulsion polymer it does provide better overall paint colour and a lower porosity. In the two examples discussed in this work, the DE values were just greater than 0.5, however, it could be argued that this difference is acceptable in some applications. A significant advantage of Opacilite’s irregular particle shape and intrinsically sealed micro voids is that Opacilite provides lower sheen and improved wet opacity. The greater differential between the CPVC and the PVC in the Opacilite reformulation reduces the internal stresses during drying and provides better mudcrack resistance. Finally, the overall formulation cost using Opacilite is lower, with estimate savings of at least €0.02/litre, irrespective of the matte emulsions titanium dioxide content.

Table 6: Summary Opacilite & Opaque Polymer advantages in high PVC matt emulsions

Opaque Polymer Opacilite
Similar dry opacity
Similar scrub resistance
Better colour Better mudcrack resistance
Better stain resistance Lower sheen
Better wet opacity
Lower formulation cost

References

[1] S J Mee, J R Hart, M Singh, N A Rowson, R W Greenwood, G C Allen, P J Heard and D R Skuse, The Use of Focused Ion Beams for the Characterisation of Industrial Mineral Microparticles, Applied Clay Science, Volume 39, Issues 1-2, April 2008, pp 72-77
[2] M Schwartz & R Baumstark, Waterbased Acrylates for Decorative Coatings, Vincent Network, 2001, 64-65
[3] R Lambourne & T Strivens, Paint and Surface Coatings: Theory and Practice, Williams Andrew Inc, 1999, 350
[4] D Gysau, Fillers for Paints, Vincent Network, 2006, 133-134

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