Water Based Coatings
Additives -Thickeners and associative thickeners
Mr Veeramani shares his knowledge in this Column devoted exclusively to Water Based Coatings
LET'S now go to the next additive called THICKENERS. This is also an important additive which plays a vital role in bringing a uniform consistency of Water based Paints. Thickeners have mainly two functions, they increase the viscosity thereby, reducing the Pigment mobility to prevent settling and they change the flow and levelling characteristics. The most commonly used thickeners are derivatives of Carboxy Methyl Cellulose and Hydroxy Methyl Cellulose. However they suffer from certain disadvantages viz: poor flow and levelling, sensitive towards enzyme degradation. Thickening agents at moderate shear increase the viscosity and increase the coatings resistance to flow during stirring, mixing and pouring. Rheology modifiers are necessary additives to prevent sagging and spatter during application. They also help to retain supply viscosity, provide anti-sag control, water retention/open time, spatter and scrub resistance.
One of the very important functions of Cellulosic Thickener is to prevent sagging of Paint Film; pictorially sagging can be represented as follows and one can perceive how sagging will look like: Sagging of Paint Film represents poor quality of the Paint and a most undesirable quality by applicators. This increases their labour cost and painted surface will not be uniform and will be seen with patches.
Cellulosic thickeners with high molecular weight chains swell with water and give high structural type viscosity. They are available in large viscosity ranges to suit the requirements. Paints thickened with HEC show remarkable colorant compatibility and storage stability. Cellulosic thickeners can be used by themselves or in combination with other thickeners to achieve specific Rheology characteristics. They are produced with a variety of molecular weights in different grades with a 2% solution viscosity is approx. 10 cps to 1,00,000 cps. Most of the cellulosic ether suppliers offer variants with surface treated so that the ether does not absorb moisture and hydrate causing the powder to gel immediately in water causing large agglomerates. The surface treated materials permits complete dispersion of the powder before dissolution. The Hydration time depends on the temperature and the pH of the water.
Various Cellulosic thickeners are, Methyl Cellulose (MC), Hydroxy Ethyl Cellulose (HEC), Ethylhydroxy Ethyl Cellulose (EHEC), Hydroxypropyl Cellulose (HPC), Hydroxypropyl methyl Cellulose (HPMC) & hydrophobically modified hydroxyethyl Cellulose (HMHEC). The last one, provide excellent sag resistance, flow and levelling, gloss and offer good resistance to pigment settling.
The thickening mechanism of the Cellulosic thickeners in a Paint system can be typically illustrated as follows:
Structure of hydroxy ethyl cellulose (HEC)
Waterborne Latex Paints unlike the solvent based paints
with inherent viscosity require a thickener to yield viscosity that will hold the paint ingredients in a homogeneous state and also contribute to its Rheological properties. This is because in latex paints, the binder is in dispersed phase and the basic viscosity is extremely low. Viscosity which is an important property in latex paints, controls critical application properties such as Brushability flow and levelling etc.
The requirement of Rheological profile for latex paints thus is very complex with compromise between low shear viscosity and high shear viscosity. On one hand, a certain level of low shear high viscosity is required to prevent sagging, but at the same time, if it is not quite low enough, flow and levelling will be insufficient and brush marks in dry film will be retained. On the other hand, a certain level of high shear high viscosity is required to produce film build up. However, high shear viscosities that are too high will lead to wrist/arm fatigue of the painter. In order to attain such a complex Rheological Profile and strike a perfect balance between the low and high shear viscosities, thickeners or Rheology Modifiers are being used in latex paints.
Organo clay thickeners
Schematic 3: Incorporation of small amountof organoclay improves antisagging and antisettling [roperties in paints and inks
Another thickener known as Organo Clay Thickener was in vogue for some time.
- They are rheology control agents
- Swell in water to produce a gel
- Which is incorporated in the paint manufacture
- Provides excellent anti-sag control
- Maintains good levelling properties
- Provides good pigment suspension
Over a period of time these thickeners became obsolete in view of higher costs and gave way to new generation thickeners.
Formulating of water based paints with cellulosic thickeners
Most commonly used in coatings is HYDROXY ETHYL CELLULOSE (non-ionic)) and Carboxy Methyl Cellulose (anionic), which are now being used almost for about four decades and still continues to be the favourite choice of the Formulators. These thickeners are in powder form and when added to water, with constant stirring and once when dispersed, the pH of the water is made alkaline which helps the hydration and swelling of the polymer. The process of thickening is through hydration in water to form thick solutions via macromolecular enlargement and hydrogen bonding with water molecules. When a gel is formed, surfactants and other additives are added, followed by pigments, then the required amount of binder is added and the paint batch is completed.
Formulating paint with associative thickeners
It was in 1973 a new type of thickener was introduced to the Paint Industry known as Associative Thickeners. These thickeners have primarily improved flow and levelling, by a novel thickening mechanism called association thickening. Associative thickeners are low molecular weight, hydrophobically modified water soluble co-polymers composed of both water soluble and water insoluble components. The combination of both hydrophobic and hydrophilic segments in one molecule gives the polymer surfactant properties.
Associative thickeners are formulating tools which allow the paint chemist to design latex paints with superior application, appearance and protective properties. Application advantages include more uniform coverage and hiding due to greatly improved levelling, greater film build in brush or roller application, improved spray patterns and faster bubble release and much reduced roller spattering. Besides, being synthetic polymers, the problems of microbial degradation often encountered with natural products are avoided. These Associative Thickeners are high in cost and are often used in high end Paints.
Alkali swellable associative thickeners
Alkali swellable associative thickeners are produced by emulsion polymerisation, to obtain easy to handle low viscosity lattices. They are mainly of two types, hydrophobically modified acrylic alkali swellable (HASE) and hydrophobically modified
Aquatech ethoxylated urethane alkali swellable (HEURASE). Urethane based non-ionic associative thickeners – HEUR, a significant breakthrough in the control of Rheology for latex systems was achieved with the introduction of non-ionic associative thickeners. There is another category of thickener hydrophobically modified Hydroxy ethyl cellulose (HMHEC). It has typically a molecular weight of 3, 00,000 which is substantially lower than the conventional cellulosic, which is mainly responsible for better flow and levelling.
Now let's turn our attention to the Mechanism of association thickening in order to understand it better so that, a formulator will know where to use this or conventional cellulosic thickeners. The mechanism based on associative thickening is proposed in which hydrophobic segments of the polymer are used to build up a network of micelles linked to each other as well as associate with other hydrophobic such as latex particle, pigment hydrophobic surfaces, surfactants and dispersants. The general principle of a thickening mechanism is that, under quiescent conditions, a continuous network is set in throughout the body of latex paint, involving all the components having hydrophobic character and result in association thickening.
The Rheology of latex paints thickened with associative thickeners is closer Newtonian type, which is the primary requirement for better flow and levelling. Unlike the shear thinning behaviour exhibited by high molecular weight thickeners such as cellulosics, associative thickeners give higher viscosities at higher shear rates. However, they can also be designed to provide varying degree of shear thinning behaviour by blending different associative thickeners with stronger and weaker associative tendencies to get optimum rheological properties.
While concluding this topic I am sure that the readers, by now, would have a very clear idea of using conventional thickeners and other thickeners like Associative thickeners in the formulations. I strongly suggest to the Formulators to take care of the above into consideration while formulating Paint. Few trials with varying quantities of these thickeners should be run, to achieve the best combination for the Paint they require to make.
- Award winning Article “Emerging Trends in Rheological Modifications of latex Paints” 1996 By: Girish A.Bhagwat
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