Rheology additives

The rheology of a paint material can be described by its viscosity and the dependence of this viscosity on parameters such as shear rate, time, etc. 

Newtonian liquids display no shear-rate dependence, their viscosity is constant over a wide range of shear rates. Only ideal liquids show this behavior. It is not found in coatings and it is also not desirable for coatings, because very low shear forces already will cause material flow leading to sedimentation and sagging (levelling, however, would be perfect in such a system). 

Rheological additives are employed to modify the flow behavior of coatings materials in order to get a more favorable rheology. In particular they are used to 

Thickeners, mainly cellulose derivatives (e.g., methyl cellulose, ethylhydroxy-propyl cellulose) or polyacrylates, are generally used in emulsion paints. Recently polyurethane thickeners (associative thickeners) with more favorable leveling properties are also increasingly used. 

A large number of rheological additives for solventborne systems are commercially available. Hydrogenated castor oils, pyrogenic silica, and modified montmoril-lonite clays (organoclays, e.g, bentonite) are preferred. 

---

Polymers of hydroxy carboxylic acids as a rheologic additive [1298]... 

MT Clarke. Rheological additives. In D Laba, ed. Rheological Properties of Cosmetics and Toiletries. New York Marcel Dekker, 1993, pp 55-152. 

The use of AMP as a rheology additive in paper coating is illustrated below. Table 1 gives formulation details. Figure 8 shows the colour viscosity at low shear rates and Figure 9 shows the colour viscoisty at high shear rates. The application was board basecoating at 62.5% solids. 

Adhesive and sealant manufacturers employ rheological additives for thickening and to prevent sag of their products. In practice, rheological additives may provide benefits in addition to viscosity or flow control. When properly formulated into adhesives and sealants, rheological additives can... 

Other mineral additives have been used for many years as functional extenders and fillers in adhesives and sealants. These include kaolin (hydrated aluminum silicate), bentonite (hectorite clay), talc (magnesium silicate), and attapulgite (hydrated magnesium aluminum silicate) additives.1213 Kaolin and talc are considered to be viscosity thickeners whereas attapulgite is more of a conventional thixotrope. They are considered to be very cost-effective rheological additives. 

Alexander P. Organic rheological additives. Manuf Chem 1986 57 81, 83-84. 

The remaining tetrachlorosilane is predominantly used for the production of firmed silica [20], which finds wide use in a variety of industrial applications. Most important are reinforcement of silicone elastomers as active filler and thickening of liquids as a rheological additive. A highly dispersed particle structure, high surface area and surface energy are the main characteristics of firmed silica. 

Special considerations fumed silica used as rheological additive addition of salts decreases solubility of resin in water... 

Block copolymers of EO and PO such as EO-PO-EO, which are formed by condensing EO onto polypropylene glycol, are useful rheological additives with applications in household cleaners (toilet bowl cleaners, gels for cleaning... 

Clearly, the response of fluids to an applied shear can be linear or nonlinear and depends on two major factors shear rate and structural or mechanical properties of the system, which in turn depend upon the interaction between the components including the rheological additives. It is the latter that primarily determines the flow properties of LADDs. The intent of this section is to discuss various rheological properties and test methods pertinent to LADDs. Interested readers are referred to Chapter 4, which deals with the rheology of complex liquids and suspensions, and other books [76-79] and review articles [80-82] covering this subject. Heywood [83] discusses the criteria for selecting various commercial viscometers. 

Rheological Additives. SAS serve as excellent rheological additives in a large number of liquid, pastelike, or thermoplastic systems and in solids. Between 0.1 and 2% of the additive is used for applications of this kind, whereas between 3 and 33% is used for the carrier applications discussed in the following section. Table VII lists the applications in which SAS are used as rheological additives. 

Macaloid . [Rheox] Magnesium aluminum silicate suspending agent thickener, emulsion stabilizer, rheological additive for creams and totions. 

Novaprint. [3-V Sigma Prodotti Chi-mici] Polyacrylic acid polymo thickener, rheological additive for textile printing applies. 

Nykon . eox] Tetra alkyl or trialkyl aryl anunonium smectites with ctxro-sion inhibitor gellant rheological additive, conosioo inhibitn. ... 

Polytrope. [Rheoxj Modified montmo-rillonite clay rheological additive, thickener for unsaturated polyester laminating resins. 

ThixatroL [Rheox] Rheological additives, thixotropes, gellants. 

Baragel . [Rheox] Gays rheological additive for greases PVC stabilizer antiplating agoit... 

Nykon . [Rheox] Tetra alkyl or trialkyl aryl ammonium smectites with erxro-sion inhibitor geliant rheological additive, corrosion inhibitor. 

The use of rheology additives such as clays, plant exudates and natural polymers to formulate paints dates back to ancient times. These materials are used to thicken the fluid, suspend dispersions of additives in the fluid and improve the stability of the ensuing dispersion as a function of temperature and shear history. This paper classifies cellulosic rheology modifiers with respect to their influence on paint properties related to rheological profiles. 

---