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7. Specialty Additives for Water Borne Systems

The Water Borne coatings are suitably compounded with a range of additives , in order to modify the process ability. These additives are required in smaller proportions . Here are some details .

1.Wetting Agents also referred as Rheology Modifiers orSurfactants

2.Defoamers3.Dispersants

Wetting Agents / Rheology Modifers / Surfactants

To understand how a surfactant or wetting agent works in awater borne system , it is important to understand theproperties of Water. A water molecule consists oftwohydrogen atoms and one oxygen atom. Water is a dipolarsubstance, which means it hasboth negative and positive polar ends .

The unique properties of water are due to this polarity,which allows it to form bonds with a variety of polarmolecules. However, water and other polar molecules willnot bond with nonpolar molecules. In fact, water and otherpolar substances will be repelled by nonpolar molecules.

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An example would be trying to mix water with oil orgrease . Chemical bonds cannot be formed between water andoil. however if a surfactant , which has both polar and nonpolar portions , is added to water , the water is no longerrepelled. The non polar portion of the wetting agent bondswith the nonpolar oil, and the polar portion bonds with thewater, apparently allowing the two to mix.

Surface Tension

A significant problem of water is surface tension. Surface tension is a Tendency of the surface molecules of a liquid to be attracted towards the center of the liquid body. It is because of Surface Tension that a single droplet of water on wax paper appears like it has a tense , elastic membrane surrounding it. A small sewing needle will float on the surface of pure water because the needle is not heavy enough to break the surface tension of water. However, if a wetting agent is added to water , the Surface

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tension ( that is the inter active tension of water molecules to each other ) is reduced , and the needle will sink.

When wetting agent is added to water , the polar portion ofthe wetting agent bonds with water molecule while the non polar portion bonds with the non polar liquid or solid molecule , thus allowing the non polar molecule to wet. As long as the wetting agent bonding is sufficient enough the non polar molecule , will not be water repellent. However one must note that this is a temporary bond.

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Surface Tension Comparison  Pure LiquidsWater 72 mN/mSolvents ( Organic ) 25-40 mN/m

Solvent BorneCoatings/ Adhesives 25-35mN/m Water Borne Coatings/ Adhesives 25-35mN/m

Surface Energy of Metals 70-110 mN/mPlastics 22- 45 mN/mSilicone 20-35 mN/m

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Coated Liners

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Surfactant Characteristics to be considered for Optimized performance in Formulations.

Surfactant typeWetting Efficiency or EffectivenessWater solubility limitCloud PointPour PointFoam Control

Wetting agents Lower Surface tension to enable substrate Wetting.

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Surface Tension arises from intermolecular attraction in the liquid.

Surface Tension Impact on Water Borne Coatings

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Lowering of Surface Tension helps Rapid Substrate wetting or coverage Higher coating speed and process efficiency reduction in coating Defects better wetting of dirty substrates

The applications that require lower Surface Tension arePressure Sensitive Adhesives Spray able coatings Laminationadhesives

Surface Tension is reduced when surfactant molecules assemble at the air/liquid interface

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Many industrial processes apply coatings at high speeds, creating large surface areas very quickly For uniform substrate wetting, surfactants have to migrate rapidly to those new interfaces

Techniques to Measure Rheological Effects

Ford Cup Brookfield Viscometer

Page10Inorganic Rheology Modifer

The most common types of modified and unmodified inorganic rheology modifiers are clays, bentonite clays, organoclays, and treated and untreated synthetic silicas. Most inorganicthickeners and rheology modifiers are supplied as powders. If they are properly dispersed into a coating, they usually function as suspending or gelling agents and some have secondary utility as extender for pigments. Usually the viscosity of the formulation decreases with time under constant shear conditions as its gel structure is broken down. If

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the shear is removed, the coating gradually recovers to itsoriginal viscosity. Certain grades or mineral types are useful for thickening aqueous systems and others for solvent based coatings. Utility in one media or the other is mostly a function of the thickener’sparticle surface, which can be organically modified to render it hydrophobic for solvent based coatings.Inorganic rheology modifiers are sometimes added to aqueous formulations as secondary thickeners to improve the anti-sag, anti-settling, anti-synerisis and anti-spattering properties of a coating.

Organic Rheology ModifiersOrganic rheology modifiers are more diverse than inorganic ones. They can be subdivided into products based on naturalrawmaterials, like cellulose or xanthan and products based on synthetic organic chemistry, like polyacrylates or polyurethanes. Again, the synthetic products can be subdived into associative and non-associative rheology modifiers:

Non-associative rheology modifiersAct via entanglements of soluble, high molecular weight polymer chains (“hydrodynamic thickening”). The effectiveness of anon-associative thickener is mainly controlled by the molecular weight of the polymer. Formulations thickened non-associatively have pseudoplastic rheology with highly elastic properties.This produces good stabilization against settling and low sagging with even high build coatings. Non-associatively thickened systems often exhibit limited flowability.

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The high molecular weight of the polymers can sometimes lead to compatibility problems such as flocculation.

Associative rheology modifiersThicken by non-specific interactions of hydrophobic end-groups of a thickener molecule both with themselves and with componentsof the coating. They form a so called “physical network” (fig. 8): In contrast to non-associative types, associativethickeners interact with the polymer dispersions used as the binder. The hydrophobic terminal and side groups of these thickeners combine to form networks that serve to increase viscosity.