Additives Handbook by Dr

TWO THOUSAND ELEVEN Additives Handbook By Dr. Joseph V. Koleske, Robert Springate and Dr. Darlene Brezinski Additives belong to a broad and diffuse work well in some systems but cannot be particular problem, formulators can find category of key components in a coating used in others. In addition, because of the themselves in trouble if the wrong addi- formulation. They comprise a small proprietary nature of many additives, their tive is initially selected or added to allevi- percentage in that formulation, usually less chemical composition is not disclosed. This ate or correct a problem. Correct additive than 5%, but their impact is significant. can make general recommendations dif- selection is important to success, and such Additive function is almost always very ficult. In addition, this lack of structural selection is made through vendor assis- specific in nature. Some additives are knowledge means that additive substitu- tance or years of experience. multi-purpose; for example, they may be tions cannot be made on the basis of fun- Please note that there are a number of important to the manufacturing process damental structural chemistry. new nano-sized additives on the market as well as to the coating’s performance. In The focus on green technology, lower today that are difficult to categorize. Their recent years, multi-purpose additives have cost and safer products has led to the functions are varied and tend to overlap been developed, thus allowing the use introduction of newer additives and chem- our traditional categories. For this reason of fewer additives in many formulations. istries. The industry demands that green we have included a number of these types Occasionally the use of one additive will additives perform the same or better than under the Nanotechnology section. require the use of another to counter some their traditional counterparts and that they The following is a brief description of undesirable effect of the first. combine performance, sustainability and various coating additives along with some Some additives are proprietary prod- efficiency along with lower cost. With a generic examples. The majority of additive ucts with highly specific functions that large number of additives available for a types are represented. We have chosen to publish a select number of definitions, or partial definitions, found in the Additives Handbook. Full definitions for all of the categories shown are available at www.pcimag.com, and are available on CD. Contact Andrea Kropp at [email protected] for more information. 2011 Additives Handbook ABRASION-RESISTANCE IMPROVERS clear topcoats, ink over-print varnishes and pigmented finishes. The See Slip Aid, Nanotechnology commercial availability of nanoparticles allows formulators to obtain Abrasion is a phenomenon caused by the mechanical action of rubbing, new properties that were unachievable in the past, not only in scratch scraping or erosion. It has two forms, marring or wearing. Mar abrasion is resistance but many other physical performance attributes. the permanent deformation of a surface, but the deformation does not For nanoparticles to be of use in transparent coatings, it is critical break the surface. Wear abrasion is removal of a portion of the surface by that aggregates present in the powder be dispersible to their primary some kind of mechanical action: wind erosion, sliding back and forth of particle size in the coating formulation to avoid rapid settling and exces- an object, wear of tires on traffic paint, and so on. The surface removal sive light scattering. In addition, it is critical that the dispersed primary is gradual and progressive in nature. Abrasion resistance is a combina- particles avoid re-aggregation during the coating curing process. tion of basic factors such as elasticity, hardness, strength (both cohesive, Thousands of scratch-resistant coating applications are present in tensile and shear strength), toughness, and, especially in the case of our everyday lives. Examples of these applications include coatings for wear resistance, thickness. In addition, abrasion resistance is intimately wood floors, safety glasses, electronic displays, automotive finishes related to scratching and slip. Thus, compounds that enhance these and polycarbonate panels. Improving the mar, scratch and/or abrasion properties will improve abrasion resistance. resistance in these transparent coating applications is a major chal- The nature of the polymeric resin and the pigments affect abra- lenge, particularly with regard to not affecting the other performance sion resistance. In the case of the pigments, it should be noted that attributes of the coating. extender pigments are noted for their ability to contribute to a variety of mechanical properties. Examples of compounds that have been used to Inorganic Fillers enhance abrasion resistance include: silica glass spheres, specialty glass Incorporation of inorganic fillers into coatings to improve mechanical spheres such as UVT™ Sunspheres, and similar compounds that improve properties is well known. Drawbacks associated with this approach can hardness. Certain silicones and other oils will decrease surface friction, include loss of transparency, reduced coating flexibility, loss of impact making it easier for objects to slide over the surface and thus reduce resistance, increase in coating viscosity and appearance of defects. To wear abrasion. Increasing crosslink density by use of higher functionality overcome these defects, a filler material should impart improved scratch oligomers and/or larger amounts of crosslinking agents has been used resistance without causing the aforementioned drawbacks. Nanomate- to improve abrasion resistance. rials have the potential to overcome many of these drawbacks because Waxes have also been used to improve slip and thereby abrasion. of their inherent small size and particle morphology. Hard waxes resist abrasion better than soft materials. Both PE and PTFE Maintaining transparency in a coating containing inorganic filler par- waxes function by the ball bearing mechanism, while the softer micro- ticles is a challenge. Four properties dictate the degree of transparency crystalline waxes work via the layer (bloom) mechanism. in a composite material: film thickness, filler concentration, filler particle The use of nano-sized materials in coating formulations can signifi- size, and the difference in refractive index between the bulk coating and cantly improve scratch resistance. These improvements can be used in the filler particle. PAINT & COATINGS INDUSTRY 1 2011 Additives Handbook Silica particles, colloidal or fumed, and clays are among the most free-radical (acrylate) radiation-curable oligomeric composite materials. widely studied inorganic fillers for improving the scratch/abrasion resis- These products are stable, transparent and have low viscosity, even at a tance of transparent coatings. These fillers are attractive from the stand- silica loading of 60%. point that they do not adversely impact the transparency of coatings Nanoscale materials for coatings also include complex silicon oxides due to the fact that the refractive indices of these particles (fumed silica and aluminum silicates. Nanoparticles of these materials have been = 1.46; bentonite clay = 1.54) closely match those of most resin-based incorporated into automotive coating formulations that have good sag coatings. The drawback to silica-based fillers is that high concentrations resistance. The cured coatings have excellent chip and scratch resis- of the particles are generally required to show a significant improvement tance, outstanding appearance, superior sandability, and resistance in the scratch/abrasion resistance of a coating, and these high loadings to water spotting and acid etching. Some properties, such as scratch can lead to various other formulation problems associated with viscosity, resistance, are maintained after accelerated weathering. thixotropy and film formation. Sol-gel Alumina It is also possible to improve the scratch- and wear-resistance properties The use of alumina particles in transparent coatings is much more lim- of a coating as well as its photostability/weatherability by the addition ited even though alumina is significantly harder than silica-based mate- of nanoparticles prepared by sol-gel processing. This method has the rials and, as a scratch- and abrasion-resistant filler, higher performance advantage in that it starts from existing, well-developed formulations at lower loadings is often observed. For alumina particle sizes greater to which a sol containing nanoparticles is added. After curing, the than 100 nm, the high refractive index (1.72) results in significant light modified systems give transparent coatings with high wear and scratch scattering and a hazy appearance in most clear coatings. Currently, only resistance. high-refractive-index coatings, such as the melamine-formaldehyde res- Very often, hybrid (organic-inorganic) materials are produced by ins used in laminate production, can use submicron alumina for scratch sol-gel. The most common way to produce nanocomposites is to form, resistance and maintain transparency. in-situ, an inorganic phase by hydrolysis and condensation of alkoxides To use alumina as a scratch-resistant filler in transparent coatings, the or alkoxysilanes. A further curing results in covalent bonding between particle size must be sufficiently small to overcome its refractive index the organic and inorganic phase. mismatch. A Physical

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