Replacing polymer dispersion in recipes is a frequent procedure that precedes the market situation, e.g. lack of raw materials, the need to optimize prices or quality reasons, e.g. the desire to modify the formulation to improve the parameters of latex paint. Not every pure acrylic (PA) polymer dispersion brings appropriate properties to the formulation, such as scrubbability, weatherability or even ensuring the same gloss or binding pigments and fillers. So can they be replaced on a “plug n play” basis?
The multitude of acrylic monomers and methods of polymerization with the use of various surfactants allows to obtain countless dispersions of acrylic polymers as binders for water-based latex paints. Such a large number of binders on the market means that in order to choose the right one, a number of tests must be conducted. It is good if these tests are supported by properly prepared case studies and guides to the effectiveness of polymer dispersion in various formulas. Such tests constitute the daily work of The Spektrochem Research Laboratory, some of which are presented in this article.
In this article, we will discuss an example of case studies of differences between two pure acrylic polymer dispersions. This project was made on the basis of raw materials from the USA/Canada market.
Pure acrylic dispersions for studies
For studies, paints based on two pure acrylic (PA) polymer latex dispersions (formaldehyde and APEO-free ) with the following characteristics have been prepared. The names of the polymer dispersions have been hidden due to commercial restrictions.
Two formulations PVC ca. 35% (base without colorants) were prepared for the studies. The paints were tinted with VOC-free water-borne colorants. The paints were deliberately prepared in terms of the tolerance of colorant dosing to the bases in order to notice the effect of the added colorants on PVC drift and parameters not described in this article, and covering the project assumptions implemented for the client.
The paints were prepared on a laboratory scale from a previously prepared slurry containing dispersants, wetting agents, in-can biocides, defoamer and rutile titanium white, fillers and reinforcements. This slurry was let down to the polymer dispersion along with the remainder of the ingredients according to an internal Spektrochem procedure. Additional titanium dioxide slurry was added due to the necessity to bring the bases to a certain proportion of white pigment in the recipe.
The differences in MFFT were reduced in the formulation by lowering the film forming temperature to 0 °C with a VOC-free dibenzoate coalescent (coalescent curve obtained by ASTM D2354 test). Paint based on PA latex dispersion No. 1 has been colored green, and paint on PA latex dispersion No. 2 has been colored pink-purple.
Below is the characteristics of the basic constans data of formulations. All of these data include colorants involved in paints.
The paints were first prepared with the adjustment of viscosity at low shear rates (consistency) around 100-105 KU. Initial consistency means measuring immediately after preparing the paints. Measurement after 7 days is for samples stored under laboratory conditions @ 73.5 °C. The graph of changes in consistency is shown below.
PA latex paint #1 retains its consistency within the assumed overnight tolerance and after 7 days (100-110 KU). The paint based on the PA latex #2 polymer dispersion in the tested formulation exceeds the acceptable viscosity range, because after 7 days the consistency is 114.2 KU.
The prepared paints have gloss/sheen which allows them to be classified as velvet, however paint based on PA #1 polymer dispersion has a higher gloss @60° and higher sheen. PA #2 polymer dispersion paint does not have such a visible velvet finish.
Subsequent tests concern the print resistance (ASTM D2064) . The print resistance test consists in loading the prepared coatings with a special load-bearing kit exerting a pressure of 0.9 psi at 140 °F. The weighting set is set to medium wave cheesecloth . The trace remaining after removal of the load is assessed.
The chart below shows the results of the print resistance test. The differences between the coatings are clearly visible. The coating based on the polymer dispersion PA latex # 1 scored 3-4 on a scale from 0-10, which means fair (moderate amount of printing), while the coating based on the polymer dispersion PA latex # 2 scored 8-9 in the classification very good (very slight amount of printing).
Another test that clearly shows the differences between the polymer dispersions is wet scrub resistance. For the paints tested, the most demanding ASTM D2486 test was applied using a nylon brush and abrasive scrub medium.
In the picture from the scrubbility test, you can see the drastic difference between the coatings. The PA latex polymer dispersion paint #2 has a scrub resistance of 3,000 cycles to failure, and at this number of cycles the PA latex polymer dispersion #1 paint is completely scrubbed off the substrate.
The coating based on the PA latex #1 polymer dispersion showed a model abrasion as in the ASTM D2486 standard at approximately 670 cycles. Thus, the scrub resistance of PA latex dispersion paint #1 is approximately 22 % of the scrub resistance of PA latex polymer dispersion paint #2.
The last test we publish was the gloss retention of both paints under accelerated artificial weathering in the Q-UV chamber. The test was performed according to ASTM G154 cycle 7 (8 h of UVA-340 at 1.55 W/m2 @340 nm, 0.25h of water spray 7 L/min, 3.75 h of condensation at 50 °C).
Total exposure time 1,000 hours in a chamber with weekly control of pH, conductivity and TDS of demineralized water used for water spray and condensation (water conductivity was maintained at max. 3 µS / cm).
The coatings of paints based on the PA latex polymer dispersion #1 showed a 40 % reduction of gloss @60°. For comparison, PA latex #2 coatings showed a gloss loss @60° of less than 18 %.
The presented fragment of comparative studies of two pure acrylic polymer dispersions (PA latex # 1 and PA latex # 2) showed drastic differences between them. Seemingly similar polymer dispersions, the difference is apparently only in viscosity and MFFT are actually completely different raw materials for the production of latex paints. These differences are due in particular to hidden parameters resulting from the surfactants, monomers, synthesis method used, etc.
Each change of raw materials, and especially of the polymer dispersion, is associated with the need to have the appropriate guide materials for its proper use. Dedication of polymer dispersion to a specific type of fillers, PVC range, or recommending appropriate film forming aids is the key to providing the R&D departments of paint producers with appropriate knowledge. This is especially true when newcomers to the paint industry are working on their projects who need properly prepared starter recipes, formulation guides and case studies to show how to use raw materials correctly.