Effectiveness of thickeners in preventing sagging

Latex paint formulations for architectural applications are intended for the manufacture of DIY paints and for applications by professional painters who apply them with a brush, paint rollers and various spraying techniques. During their application, most substrates are placed on vertical surfaces, such as walls, furniture fronts or fences. Application to such surfaces carries the risk of sagging due to gravity when the paint has not yet dried. In order to eliminate this phenomenon, thickeners are added, due to the effectiveness of their operation, I invite you to travel from the side of their research.

Introduction

Let’s start by defining what sagging is.

Sagging disposition – unwanted property of a coating material, which becomes visible during its application and drying, through sag formation. Sagging can be caused by a deficient formulation, for example too low viscosity, too slow solvent evaporation, or by processing defects like too low temperature/or too high film thickness [1].

Sagging resistance – capability of coating materials to form, after application of a defined wet film thickness or vertical surfaces, a well leveling film without producing sagging effects. Good sagging resistance normally requires the employment of rheological additives [1].

Formulations of latex paints, depending on the quality level, contain at least one type of thickener. Typically, this primary thickener is a rheology modifier providing a viscosity at a level that can be measured by a Brookfield (low-shear) or Stormer (mid-shear) viscometer. It is in these areas of shear velocity that sagging occurs, and more precisely at the contractual boundary between the upper low-shear range and the lower mid-shear range. Basic thickeners, such as hydroxyethylcellulose, work effectively in these shear-forces ranges, and therefore, in order to ensure the stickiness of the paint in the package (in-can feeling) and the reduction or elimination of sagging, thickeners of this type are the most common modifiers of latex paints.

However, there is never one thickener in higher-class paints. In order to adjust the rheological properties in the entire range of shear-forces, at least two or more thickeners are selected that interpenetrate the boundaries between low-, mid- and high-shear forces to ensure all rheological properties such as brush or roller application or various types of spraying, but also no sagging at the end of painting and when the paint layer is still wet [2]. Such paints then contain polyurethane thickeners (HEUR) with efficiency in various shear-forces ranges, and can also be supported by acrylic thickeners (ASE, HASE) or various advanced versions of basic thickeners, e.g. hydrophobically modified HEC [3].

The sagging test is performed under laboratory conditions in accordance with ASTM D4400 and its result is marked as the Anti-Sag Index which is the result of reading non-drip strips and calculating how successive strips that drip merge with each other.

Why does sagging occur in latex paints?

From the definitions it follows that the layer is too thick, which is quite obvious and we will omit these reasons in the article. We will focus on the reasons underlying the raw materials and formulation.

Open time as an extension to drying is not generally the cause of sagging in latex paints. Like slow evaporation of e.g. coalescents, it will also not be the main cause of sagging, because the coalescents evaporate for a long time, but in the phase when the coating is so dry that it is impossible to run down (drying above surface drying, tack-free, initial hard-dry, etc.) So where can you find the causes? The basic one is too low viscosity in the area between low and mid-shear forces, which is usually the result of using too low a thickener dose or the use of a thickener that does not work effectively in this range of shear-forces. It is also quite obvious.

The first reason for the occurrence of sagging is the insufficient associative effect of the thickener, which usually works in the area of ​​low- and mid-shear forces, but for some reason the applied dose, often despite increasing, does not provide the appropriate viscosity in this range. This is usually due to the associative interaction with the latex binder and surfactants that are present in the formulation. One thickener may be less effective at the same dose in a formulation with an altered latex binder, as shown in the photo below (Pic. 1).

Pic. 1 Effect of the associative effect of thickener at the same dose but in a formulation with two different binders

The above samples for roofing systems, more precisely liquid acrylic roof membranes with PVC ca. 40% based on two styrene-acrylic dispersions with a low glass transition temperature. The same HEUR thickener was used in the formulations in the same dose, as you can see, its effect is slightly different. The Anit-Sag Index as determined by ASTM D4400 in the range of 4-24 mils varies by 2.4 mils.

The following example is a latex (PVC ca. 32%) as a sample with an additional dose of 2% surfactant (based on total titanium dioxide and calcium carbonate fillers in slurry) on the right side, and control sample without additional dosage of surfactant (left). Due to the low viscosity at the low- and mid-shear boundary, the Anti-Sag Index (ASTM D4400) is also low, but the difference between the control sample and the additional surfactant is clearly visible. This is because the additional amount of surface active ingredients interferes with the action of the thickener at a level closer to the low-shear forces and the associative effect is also disturbed. This worsens sagging resistance.

Another example is also the liquid acrylic roof membrane, this time based on the same formulation with a base thickener HEUR effective in low-shear forces to adjust the Brookfield viscosity, but with an additional one to increase the viscosity in the mid-shear area, i.e. to measure Stormer viscometer. The sample on the left shows drip in thick layers obtained with applicator clearance above 20 mils, and the sample on the right performs much more efficiently in the exemplary formulation and does not sag at 24 mils. Here we are dealing with an example of sagging resistance disturbance caused by a less effective thickener in the low-shear forces mid-shear boundary region, with the same dosing in a formulation based on the same styrene-acrylic polymer emulsion.

Summary

Are these all the possibilities for which latex paints show sagging? Of course not. The reasons may also be a change in viscosity due to biological contamination, pH drift or the influence of coalescing agents on viscosity. Nevertheless, in most cases it is dictated by a disruption of the associative operation of the viscosity building mechanisms on the border of low- and mid-shear forces responsible for sagging. The decrease in viscosity may occur over time, e.g. during storage, as well as after tinting where TVS (tint viscosity stability) is disturbed. When using thickeners, pay attention to the recommendations and data provided by the manufacturer on the basis of research, technical reports and case studies that arise, among others. in the Spektrochem laboratory to facilitate the daily work of the recipe formulator.

At this point, I highly recommend my introductory article on the basics of thickener assessment, which was published by PCI Magazine in September 2022 [2].

References:
[1] Coatings from A to Z, Paolo Nanetti, Vincentz, 2006
[2] Best Practices for Selecting Rheological Additives in Latex Paints, Artur Palasz, PCI Magazine, September 2022 [link to e-article]
[3] Hydrophobically modified hydroxyethyl cellulose (hmHEC) – effective low-shear viscosity builder for latex paints, Artur Palasz, PPCJ, August 2022 [link to e-article]

Published by Artur Palasz

Scientist, paint formulator and testing expert.

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