The appearance of a water film on the surface of latex paints is a common phenomenon. It is often passed over to the agenda, because after mixing this phenomenon subsides and the paint is not difficult to homogenize as in the case of settling. But what is this phenomenon and how to prevent it?
Syneresis is the release of small amounts of fluid from the particles suspended in the gel, associated with the coagulation process, leading to the formation of a concentrated gel and highly diluted sol. This is the term used to describe the release of the water layer onto the surface of liquid paints. It is not a settling process that can occur simultaneously with syneresis, but is caused by other mechanisms. Usually syneresis is not accompanied by settling, and the water layer consists not only of water but also of additives dissolved in it, such as dispersants or biocides.
Syneresis may occur spontaneously in the case of ineffectiveness of thickeners at low shear forces, which may also result in settling, however, most of the syneresis occurs under the influence of electrolytes lowering the solubility of compounds in the colloidal system.
The syneresis usually occurs after some time. Of course, in the case of incompatibility of the raw materials in the paint, the water layer may be visible after the night of paint preparation. Usually, however, this process takes from several weeks to several months. To expedite the laboratory tests to eliminate this phenomenon, a storage stability test according to ASTM D1849 is performed.
The liquid paint sample is placed in a sealed package (can) and placed in an incubator for one month at 52 °C (125 °F). For better observation of syneresis for the purposes of the test, glass packaging can be used, e.g. a laboratory glass jar to assess the progress of syneresis. For the first signs of syneresis, the test can be shortened to two weeks, or extended if positive results are obtained.
The ASTM D1849 test method gives a method of evaluating the settling in the paint at the end of the test, however, it does not classify the occurrence of syneresis. Of course, in many cases the mere occurrence of a small water layer disqualifies the sample, however, the classification of the results and their grading is needed to measure the effectiveness of modifications with the use of additives or their different doses (ladder tests). For the purpose of identifying different levels of syneresis and communicating with the recipients of test results, we have developed our own internal scale of syneresis degrees, which is used in our laboratory. The rating and evaluation method are shown in the table below.
The elimination of syneresis is an operation that requires a review of the entire latex paint formulation. Depending on the level of PVC, the reduction and elimination of syneresis can be performed by appropriately selecting dispersing and rheological additives. When selecting dispersing additives, it is important to base their selection and dose selection on the principle used to stabilize suspensions. This means the need to know the electric charge of dispersed pigments and fillers and the electric charge of the dispersing additive we will be using. Therefore, it is often necessary to use several dispersants. It is advantageous to prepare paints from slurries containing single fillers and pigments, which are then dispersed with certain types of dispersants ensuring their stabilization. With regard to rheological additives, it is preferable to use those that are effective at low shear forces, e.g. mineral thickeners (attapulgite, smectite clay).
It is very helpful to use appropriate case studies provided by raw material producers. Well-developed starter formulations and case studies discussing the methods of eliminating syneresis on reference formulas can be a very good tool to use the tips when transferring tests to your own formulations.