Metolat & Edaplan LA - Additives for Surfâ¦ - Lawrence Industries

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Additives for Surface and Substrate In addition to inhomogeneous conditions at the surface flotation effects can occur if pigments of different colours and different weight are present. A change in the tone occurs because the heavy pigments sedimentate in the drying period due to currents and turbulences. The lighter pigments swim up in the coating or form borders. This separation of pigments leads to the formation of cells in which the colour of the border is different from that in the including areas (figure 3). In unpigmented systems or coatings with only one type of pigment currents are also caused which leave a structure on the surface. Mainly orange peel is observed as consequence. Under extreme drying or evaporation conditions the surface of the drying film can even crack. 1.6 Prevention of surface defects Levelling agents are able to cure or to avoid surface defects by taking an influence on the surface tension of the system. Generally, surface tension of the coating is lowered. This results in a better wetting of the substrate but levelling and surface agents are also able to keep the tension constant during drying. Due to this no differences in surface tension by the evaporation of the solvents results. Other types of levelling agents does not influence the surface tension but just control the evaporation of solvents due to the fact that they have a controlled incompatibility due to which they rise to the surface. These kinds of additives block the coating surface against a too fast evaporation of volatile compounds but do not change (lower) the surface tension by themselves. Wetting and dispersing agents can prevent surface defects in pigmented systems by avoiding inhomogeneous pigment distribution and stabilise pigments and protect them from flocculation ( EDAPLAN ® - Polymeric Dispersing Agents). Rheological additives can prevent surface defects in that way, that such thickeners are chosen, which do not rise viscosity in the region of shear forces where levelling occurs and where a build up of structure is not allowed ( TAFIGEL ® PUR – Rheology modifiers for aqueous systems). 1.7 Surface modification: Smoothness, Scratch resistance, Water repellence Regarding the film of a coating in first glance the binder is responsible for the properties and is chosen due to the desired properties for the final requirements. Especially hardness is determined by the binder type. Other properties like smoothness, scratch resistance, gloss, or water repellence can be influenced by additives. Smoothness of the surface can be influenced by additives by which improvements regarding scratch and scrub resistance can be obtained. A smooth surface is not harder but gives less frictional resistance. That leads to a better gliding on the surface and less mechanical injuries (figure 4). Figure 4: Movement of a rough body relative to a surface PCA 09/2006 6 Distributed by Lawrence Industries in the UK & Ireland | +44 (0) 1827 31 41 51 | www.l-i.co.uk
Additives for Surface and Substrate If a rough structured body is moved over a rough surface slip resistance has to be overcome which is a force that is opposite orientated to the movement. The friction force is independent from the contact surface and only dependent from the structure of the surface. A smooth surface which shows macroscopically no waves or scars, shows in a microscopically scale up a certain roughness or surface structure (lotus effect, nano-particles). In order to increase slip of the surface special additives can be used which form a film in the surface which lets bodies glide on the surface. Especially modified polysiloxanes and waxes are used for this application. Polysiloxanes are surface-active products which orientate on the surface of the film. With their hydrophobic ends they create a layer on the surface of the lacquer which equals irregularities. Polysiloxanes are active even in very low dosages. At higher rates they worsen adhesion and tend to stabilise foam. ( EDAPLAN ® LA 411, EDAPLAN ® LA 413) Waxes (natural and synthetic) also form a layer on the lacquer surface which reduces gliding friction. They are more persistent as silicones and do not create problems at recoating. Waxes are applied as emulsions and the particle size determines the application range. A big particle size might reduce gloss. Waxes have to be applied in higher concentrations than polysiloxanes. Due to these quantities they increase water repellence of the film. ( OMBRELUB ® 533) 1.8 Substrate wetting As discussed already previously surface tension of an aqueous coating often has to be reduced in order to achieve spreading and a homogeneous surface of the coating. Coating Substrate σ C ≤ σ S A good substrate wetting of the coating is the requirement for a sufficient adhesion of the dry film. Generally, many wetting agents and solvents are able to reduce the surface tension of an aqueous coating under the value, which is relevant for a certain substrate. With this lower tension the coating spreads on the substrate. Often, the consistency of the substrate is inhomogeneous and results in the formation of craters, which are formed due to local pollutions. Additionally, the evaporation of solvents can cause punctual changes in the surface tension, by which craters are formed. Other formulation ingredients, like defoamers for example, which are too incompatible in the medium, can also change the surface tension of the coating punctually and lead to craters in the applied film. The defoamer has to be chosen carefully and has to be adapted to the special requirements ( AGITAN ® - Defoamers). Surfactants based on polysiloxanes and especially fluoro based surfactants and special wetting agents remove local differences in surface tension or wet the substrate surface by which the adhesion is improved. (METOLAT ® 285, METOLAT ® 288) PCA 09/2006 7 Distributed by Lawrence Industries in the UK & Ireland | +44 (0) 1827 31 41 51 | www.l-i.co.uk
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