Additives Reference Guide by Dr
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TWO THOUSAND THIRTEEN Additives RefeRence Guide By Dr. Joseph V. Koleske, Robert Springate and Dr. Darlene Brezinski Additives belong to a broad and diffuse addition, because of the proprietary nature of chemical composition of many additives category of key components in a coating many additives, their chemical composition is is proprietary, a formulator may run into formulation. They comprise a small per- not disclosed. This can make general recom- problems with chemical side reactions. It centage of the coating formulation – their mendations difficult as the lack of structural is important to consider the total additive use level rarely exceeds 1 or 2%, and the knowledge means that additive substitutions package and to always seek recommenda- total level of all additives in a formulation cannot be made on the basis of fundamental tions from the manufacturer. Because of the seldom exceeds 5% of the total product. structural chemistry. changing technology within the industry Their impact, however, is significant as The focus on green technology, sustain- driven by cost, VOC and other legislation, they contribute to the ease of manufac- ability, nanotechnology, lower cost and eco-friendly products and sustainability, ture, the stability of the coating in the safer products has led to the introduction of many of today’s additives are multi-func- package, ease of application, quality and newer additives and chemistries. The indus- tional; some are also incorporated into appearance of the final film. try demands that green additives perform polymeric backbones. These factors, in addi- Additive function is almost always very the same or better than their traditional tion to nanotechnology, make it much more specific in nature. Some additives are multi- counterparts and that they combine perfor- difficult to categorize additives by function. purpose; for example, they may be impor- mance, sustainability and efficiency along Please note that there are a number of tant to the manufacturing process as well with lower cost. With a larger number of new nano-sized additives and biobased as to the coating’s performance. In recent additives available for a particular problem, additives in the market today; their func- years, more multi-purpose additives have formulators can find themselves in trouble tions are varied and they tend to overlap been developed, thus allowing the use of if the wrong additive is initially selected the past traditional categories. For this fewer additives in many formulations. Occa- or added to alleviate or correct a problem. reason we have included a number of sionally the use of one additive will require Correct additive selection is important to these types under the Nanotechnology the use of another to counter some undesir- success, and such selection is made through and Biobased sections. able effect of the first. vendor assistance or years of experience. The following is a brief description of Some additives are proprietary products Additives are generally categorized by various coating additives, along with some with highly specific functions that work well in function rather than by chemical composi- generic examples. The majority of additive some systems but cannot be used in others. In tion or physical type. Because the actual types are represented. ABRASION-RESISTANCE IMPROVERS Waxes have also been used to improve slip and thereby abrasion. See Slip Aid, Nanotechnology, Burnish-Resistant Hard waxes resist abrasion better than soft materials. Both PE and PTFE Abrasion is a phenomenon caused by the mechanical action of rubbing, waxes function by the ball bearing mechanism, while the softer micro- scraping or erosion. It has two forms, marring or wearing. Mar abrasion is crystalline waxes work via the layer (bloom) mechanism. the permanent deformation of a surface, but the deformation does not The use of nano-sized materials in coating formulations can signifi- break the surface. Wear abrasion is removal of a portion of the surface cantly improve scratch resistance. These improvements can be used in by some kind of mechanical action: wind erosion, sliding back and forth clear topcoats, ink over-print varnishes and pigmented finishes. The of an object, wear of tires on traffic paint, etc. The surface removal is commercial availability of nanoparticles allows formulators to obtain gradual and progressive in nature. new properties that were unachievable in the past, not only in scratch Abrasion resistance is a combination of basic factors such as elas- resistance but many other physical performance attributes. ticity, hardness, strength (both cohesive, tensile and shear strength), For nanoparticles to be of use in transparent coatings, it is critical toughness, and, especially in the case of wear resistance, thickness. that aggregates present in the powder be dispersible to their primary In addition, abrasion resistance is intimately related to scratching and particle size in the coating formulation to avoid rapid settling and exces- slip. Thus, compounds that enhance these properties will improve sive light scattering. In addition, it is critical that the dispersed primary abrasion resistance. particles avoid re-aggregation during the coating curing process. The nature of the polymeric resin and the pigments affect abrasion There is available in the marketplace a VOC-free, aqueous dispersion resistance. In the case of pigments, it should be noted that extender pig- of nanohybrid particles to protect glossy, aqueous, pigmented and ments are noted for their ability to contribute to a variety of mechanical unpigmented topcoats from scratching and abrasion. It is recommend- properties. Examples of compounds that have been used to enhance ed for aqueous high-gloss systems and has no affect on transparency. abrasion resistance include: silica glass spheres, specialty glass spheres Thousands of scratch-resistant coating applications are present in such as UVT™ Sunspheres, and similar compounds that improve hard- our everyday lives. Examples of these applications include coatings for ness (see Microspheres). Certain silicones and other oils will decrease wood flooring, safety glasses, electronic displays, automotive finishes surface friction, making it easier for objects to slide over the surface and and polycarbonate panels. Improving the mar, scratch and/or abrasion thus reduce wear abrasion. Increasing crosslink density by use of higher resistance in these transparent coating applications is a major chal- functionality oligomers and/or larger amounts of crosslinking agents lenge, particularly with regard to not affecting the other performance has been used to improve abrasion resistance. attributes of the coating. 22 JUNE 2013 | WWW.PCIMAG.COM Inorganic Fillers curing, such as UV-curable, 2K polyurethane, and melamine-based coat- Incorporation of inorganic fillers into coatings to improve mechanical ings, show greater improvement in their scratch resistance upon alumina properties is well known. Drawbacks associated with this approach can nanoparticle incorporation compared to transparent coatings that do not include loss of transparency, reduced coating flexibility, loss of impact crosslink but rather coalesce, such as emulsion-based coatings. resistance, increase in coating viscosity and defect appearances. To For OPV coating formulations, the combination of sub-micron size overcome these defects, a filler material should impart improved scratch alumina particles with nano-size wax particles provides a level of wear resistance without causing the aforementioned drawbacks. Nanomateri- protection that exceeds that possible using either of the additives alone. als have the potential to overcome many of these drawbacks because of The behavior has been observed in different OPV formulations and with their inherent small size and particle morphology. different wax compositions. The mechanism that drives the synergistic Maintaining transparency in a coating containing inorganic filler behavior is still under investigation. particles is a challenge. Four properties dictate the degree of transpar- ency in a composite material: film thickness, filler concentration, filler SNC particle size, and the difference in refractive index between the bulk SNC is an abbreviation for silica nanocomposites that are composed coating and the filler particle. of colloidal silica particles with an organic surface modification. These Silica particles, colloidal or fumed, and clays are among the most particles, which improve the scratch and abrasion resistance of a vari- widely studied inorganic fillers for improving the scratch/abrasion ety of coatings including radiation-curable formulations, are produced resistance of transparent coatings. These fillers are attractive from the by a unique process that results in monodispersed, non-agglomerating standpoint that they do not adversely impact the transparency of coat- spheres with a diameter of about 20 nm. The flexible manufacturing ings due to the fact that the refractive indices of these particles (fumed process is also capable of producing a broad range of cationic (epox- silica = 1.46; bentonite clay = 1.54) closely match those of most resin- ide) and free-radical (acrylate) radiation-curable oligomeric composite based coatings. The drawback to silica-based fillers is that high con- materials. These products are stable, transparent and have low viscos- centrations of the particles are generally required to show a significant ity, even at a silica loading of 60%. improvement