Stain blocking mechanisms

From "EVALUATION OF STAIN BLOCKING PRIMER COATINGS WITH LOW VOLATILE ORGANIC COMPOUND (VOC) CONTENT and EFFECTS OF DRYING CONDITIONS ON THE PROPERTIES OF WATER REDUCIBLE COATINGS FOR POLYPROPYLENE A Project Report presented to the Faculty of California Polytechnic State University, San Luis Obispo"

http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1508&context=theses

Page 10.
Note: It's just a guess of the author of this blog but the epoxy resin ester developed by Cytec Company seems to be the Allnex product under Duroxyn commercial name

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2.1.2.2. Experimental Low VOC Stain Blocking Primer 

An additional experimental water based primer was included in stain blocking testing. This primer is formulated with an experimental water based cationic epoxy resin ester developed by Cytec Company.

Cationic resins based on epoxy chemistry are known to provide excellent barrier properties without the use of usually expensive reactive pigments, such as zinc oxide (Tsang, 11 2009; Brandt-Rothermel, 2010). These systems are polycationic systems created by protonation of functional amine groups within the polymer chain (Brandt-Rothermel, 2010). The system is then neutralized with volatile acids, such as formic, acetic, or lactic acid, and the polymer particles swell considerably.

Stain blocking can be achieved in two ways using cationic resins (Brandt-Rothermel, 2010):
(i) cationic groups can form ionic interactions with anionic staining molecules.
(ii) alkaline staining molecules can be neutralized and fixed by the acid groups.

Another important factor is the low pH of the primer, which prevents the diffusion of stains through the primer and into the topcoat (Brandt-Rothermel, 2010). As previously mentioned, adhesion is also important for improved stain blocking. The amine groups within the cationic polymer are able to hydrogen bond with the cellulose in wood, resulting in especially good adhesion to wood and a variety of other substrates (Brandt-Rothermel, 2010).

The cationic resin in the experimental primer is still in the testing stages and is not yet available in any paint formulations available for general purchase. It was included in this study because of manufacturer claims about outstanding tannin blocking capabilities and also to assess the potential of water based stain blocking technology. The experimental stain blocking primer was formulated by Cytec company according to Experimental Lab Formula # KS 101-4. This formulation was provided by Cytec, and can be seen below in Table 2.3. This primer will be referred to as the experimental primer for the remainder of this report."

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As mentioned earlier in this report, a near neutral pH and a fast drying time are important factors in preventing tannin staining of the topcoat. The experimental primer formulated using the Cytec resin claims a pH of 5.0-6.5 and a set-to-touch time of 15 minutes. Both of these qualities suggest that this primer may have excellent stain blocking capabilities against tannins.

The experimental primer was not used in all testing. The results are reported separately from those of the commercially available primers, and can be found in section 3.3. "


Ref.

Brandt-Rothermel, S., “Blocking Around the Clock,” Asia Pacific Coatings Journal, 22-23, August 2010.

Tsang, M., “New Waterborne Cationic Resins for Wood Primers,” Cytec Industries, Presented at: The Waterborne Symposium – Advances in Sustainable Coatings Technology, Feb. 18- 20, 2009.