{"id":6458,"date":"2017-05-12T08:15:27","date_gmt":"2017-05-12T14:15:27","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=6458"},"modified":"2023-06-06T12:50:21","modified_gmt":"2023-06-06T18:50:21","slug":"pc-settle-down-factors-that-influence-pigment-settling-and-stability","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/6458\/pc-settle-down-factors-that-influence-pigment-settling-and-stability\/","title":{"rendered":"Settle Down: Factors that Influence Pigment Settling and Stability"},"content":{"rendered":"

Introduction<\/em><\/strong> -The first steps in the pigment dispersion process are wetting and separation of the pigment. However, if the pigment dispersion is not properly stabilized, flocculation1<\/sup><\/em><\/strong> (fig. 1, 2) will result. Flocculation is a result of pigment particles being attracted to each other to form loose aggregates that can be redispersed under mild shear.<\/p>\n

When pigment particles are strongly attracted to one other a cementing or agglomeration<\/em><\/strong> of the particles can occur. Agglomerates<\/em> (chemically bound pigment aggregates that are encapsulated by resin or wetting agent) cannot be readily redispersed. Flocculation<\/em> can be reversed by the application of low shear to the paint. Flocculation<\/em> can have an adverse effect on color development, gloss and hiding.<\/p>\n

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Fig. 3: Relationship between Primary Pigment Particles, Flocculation and Agglomeration2 SOURCE: MDPI.com<\/a><\/figcaption><\/figure>\n

The two main mechanisms to obtain pigment stabilization are steric<\/em> and charge<\/em>. In charge repulsion, particle surfaces with like charges repel each other (more applicable to waterborne systems, Fig. 4) whereas steric stabilization is a more common mechanism in solvent born paints (Fig. 5). Properly stabilized pigment dispersions prevent flocculation and agglomeration.<\/p>\n

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Fig. 4: Charge Repulsion Stabilization Mechanism3 SOURCE: Dow Coatings.com<\/a><\/figcaption><\/figure>\n
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Fig. 5: Steric Stabilization Mechanism4 SOURCE: Inkline.gr<\/a><\/figcaption><\/figure>\n

Pigment dispersion in aqueous media uses the same principles as inorganic solvent media: that is, proper wetting, pigment dispersion and stabilization. However, the surface tension of water and high polarity makes it more problematic in wetting low polarity pigments. In many cases, water interacts aggressively with the surface of the pigment, destabilizing the dispersant on the pigment surface. Ensure that the pigment dispersion is uniform and stabilized (elimination of pigment flocculation<\/em> of one pigment with the exclusion of other pigments). Thirdly, the use of suitable wetting agents\/surfactants help to ameliorate differences in polarity and surface tension<\/em> between pigments that contribute to pigment destabilization.<\/em><\/p>\n

Inorganic pigments such as iron oxides, titanium dioxide, calcium carbonate, zinc oxide, and silicon dioxide, calcium carbonate and barium sulfate and many other filler pigments have a very polar surface. However, water alone normally does not adequately wet the pigment surface. Accordingly, they require a surfactant <\/em>to wet and stabilize the dispersion.<\/p>\n

Also, many pigment manufacturers supply surface-treated pigments<\/em> to help pigment stabilization. Many manufacturers modify the surface of organic pigment to increase polarity by adding a layer of inorganic oxide to improve pigment wetting.<\/p>\n

No discussion on pigment stabilization is complete without considering the effect of pigment settling<\/em> with time.\u00a0 These factors all influence the degree of pigment settling and resistance to hard settling:<\/p>\n