Cellulose ethers are an inseparable element of dispersion paints, especially with medium-high and high PVC where the proportion of polymer dispersion is not too high. They are one of the most commonly used wall and ceiling dispersion paint thickeners on the EMEA and Asia-Pacific markets. One of the basic and very common cellulose thickeners is HEC (hydroxyethyl cellulose), which, depending on the degree of substitution, provides sufficient rheological properties for most wall paints. HEC thickeners contribute to a spattering tendency in dispersion paints when rolled on.
The answer to this problem is to be the hydrophobic modification of the HEC particles, thanks to which the associative effect of building viscosity will be so effective that the problem of splashes will be eliminated. The following case study examines whether this is actually happening and how it affects the viscosity of the paint.
Cellulose ethers for study
Two cellulose ethers – HEC were used for the case study. The first was the standard HEC grade and the second was the hydrophobically modified HEC, both with retarding swelling time. The studies used ethers from one producer.
The HEC efficiency was compared in the standard formulation as below. The formulation is based on simple raw materials to thoroughly understand the effect of ether alone on the interaction with the basic raw materials in the medium-high PVC dispersion paint.
The paints were prepared using a laboratory dissolver in accordance with the laboratory’s internal procedures.
The samples were compared for four parameters: Brookfield apparent viscosity (ASTM D2196 method A), Anti-Sag Index (ASTM D4400) and spatter resistance (ASTM D4707).
The graph below shows the differences in the apparent viscosity of paint samples prepared with the same amount of HEC and hydrophobically modified HEC with the same viscosity index (manufacturer’s symbol).
The graphs show a 4-fold difference in apparent viscosity. The associative thickening effect in favor of the hydrophobically modified HEC is clearly evident.
Anti-Sag Index was determined for the white sample (base) and tinted with 3% Chroma-Chem Red 254 WAB (Chromaflo). The purpose of preparing a color sample was to indicate whether the addition of a colorant would change the tendency to run off vertical surfaces.
The photos show the differences in the paint on the standard HEC type and the hydrophobically modified HEC. Base paint based on standard HEC type has an Anti-Sag Index 26% lower than base paint based on hydrophobically modified HEC. On the other hand, the tinted paint on the standard HEC type has the Anti-Sag Index lower by over 50% than the paint with the hydrophobically modified HEC type and by over 30% from its base before adding the colorant.
The most interesting parameter for the effective operation of the associative mechanism is the spatter resistance during roller coating, called spatter resistance and tested in accordance with ASTM D4707 using a standardized notched spool roller.
The test results show a clear difference in paint resistance to spattering in favor of hydrophobically modified HEC. The rating scale from 0 (poor result) to 10 (no spattering) places the paint on the standard HEC grade with rate 6, and the hydrophobically modified HEC grade with rate of 9.
The presented results of the case studies show a clear effect of the associative thickening mechanism for hydrophobically modified HEC, and this effect is clearly better than that of the standard HEC grade. Improving the rheological properties in the form of the Anti-Sag Index increased for both the white and tinted base, as well as the elimination of spattering during roller application make paints based on hydrophobically modified HEC the subject of further formulation development by latex paint manufacturers and combinations with other rheological additives (HEUR, HASE, etc.).