In a more and more competitive environment coating materials must be able to be optimally used. The desired layer thickness should be applied in an optimum way in a single work step with simple processing behavior and good levelling. With wall paints often the first impression counts, when stirring. Also a reduced spattering tendency and sufficient sagging must be adjusted in order to stand out from competitive products.download >
Paints are used to achieve additional surface functions. The application spectrum ranges from traditional surface protection against aging and corrosion up to special optical or functional properties, such as complex metallic coatings in automotive industry or self-cleaning surfaces. A great deal of work is put into the development of paints to obtain protective, optical or functional properties. However, in the course of development the rheological behaviour of the coating is often neglected, although it affects its entire life cycle including manufacturing, storage up to easy application by the end user (figure1).
Often the viscosity of a coating is adjusted to a desired level with only one standard thickener at a specific shear rate, usually with flow times from the ISO cup or with spindle viscometers.
However, the rheological profile of paints, which describes the viscosity with different shares, cannot be adjusted sufficiently this way. Viscosities at different shear rates have very different technical implications in the life cycle of a coating (figure 2). A complete rheology profile can be recorded only with expensive rheometers. Therefore they are still not available in every research laboratory.
In general the classic thickeners based on cellulose and acrylates have strong pseudo plastic rheology profiles. The thickening mechanism is based on long, entangled molecular structures.
Due to their voluminous chemical structure they are thickening the aqueous phase. These thickeners show good sagging resistance, but often it is associated with poor flow and high elasticity causing spattering. Additionally water resistance is adversely affected; this is especially important for anti-corrosion coatings.
TAFIGEL® PUR -
Associative polyurethane thickener
The thickening mechanism of associative polyurethane thickeners is completely different. Due to their hydrophobic-hydrophilic-hydrophobic structure they form aggregates in aqueous systems (micelles), whereby e.g. one thickener molecule can be part of two or more micelles linking them physically this way. The hydrophobic end groups of several thickener molecules are forming micelles among each other. Beyond this they interact with the hydrophobic surfaces of the binder dispersion or with other hydrophobic ingredients of the formulation. This association causes the immobilization of the formulation components and thus causes thickening (see figure 3)
Under shear the associative network between the polyurethane thickeners and the hydrophobic components of the systems –this are mainly the dispersion binder– is destroyed temporarily. Once shear energy is taken away the associative network rebuilds after a short time, therefore levelling properties are much better than with classic cellulose or acrylic thickeners.
In the same way with polyurethane polymers other hydrophobic and hydrophilic components of the formulation are stabilized. The hydrophobic as well as the hydrophilic polymer segments of the thickener can interact with pigments and can provide additional wetting and dispersing effects. Colour acceptance in tinting is increased, settling tendency and rub-out problems are reduced by the interaction with pigments and fillers. Furthermore, in general the molecular weight of polyurethane thickeners is significantly lower than the molecular weight of other thickener types such as acrylic thickeners and cellulose ether (Table 1). In combination with the lower elastic properties it leads --in comparison to cellulose ethers- to a significantly decreased spattering tendency of dispersion paints (figure 4).
The chemical design of the hydrophobic-hydrophilic -hydrophobic segments of a polyurethane thickener allows to vary the associative behaviour and to adjust the desired rheology profile within a wide range (see figure 5).
For example: weak hydrophobic end groups and longer polymer chains tend to lead to Newtonian flow behaviour , as in the case of TAFIGEL ™ PUR 45,55,80,82 or 85 .
This way the viscosity at high shear rates can be targeted increased. Excellent levelling, high gloss and the so-called brush drag are obtained with these thickeners, this means, that the paint transfer with a single brush stroke is increased.
On the contrary , with strong end groups and short polymer chains (as in the case of TAFIGEL ™ PUR 60, 61, 64 or 65) strong pseudo plastic rheology profiles are obtained. Dispersion paints adjusted with these rheology modifiers have a high sagging resistance and an excellent spray ability. A typical application is, for example, the industrial thick layer spray coating for door and window frames.
Medium pseudo plastic polyurethane thickeners like TAFIGEL ™ PUR 40, 41,48, 50, 54 or 52 have an universal rheology profile.
They can be used for brush, roller and for spray applications. They develop their particular strength in combination with cellulose thickeners in wall paints. While the cellulose derivative with its water retention capacity provides the required open time, TAFIGEL ™ PUR improves levelling and reduces spattering drastically, for example, at brisk roller application.
Table 2 gives a selection support for thickener types to be used in various applications for construction and industrial sectors.
The associative polyurethane thickeners can be combined easily with each other as well as with all known thickeners types, often with synergistic effects. Their particular strengths are the specific adjustable rheology profiles.
They have better levelling properties than classical thickeners, and higher gloss levels are achievable. Further advantages are: the higher water and scrub resistance and the higher resistance against bacteria. Open time and storage stability can be improved with a clever combination with cellulose thickeners.
The selection of a suitable thickener depends crucially on application field and application technique. With a proper selection and a clever combination the rheological behaviour can be adjusted appropriately and user-friendly. In a more and more competitive environment this is an ideal way of: Creating Additive Value!