The author explains the types of substances and their performances according to the need of the substrate.
When it comes to automotive finishes, the global industry is constantly in demand for excellent finishes and attractive surfaces. Paint manufacturers are constantly improving to meet such demands. As a support, surfactant additives play a crucial role in reaching the strictest standards.
Some features such as: increase in brightness, shortwave or longwave effects, smoothness, good adhesion between layers, bubble release and wetting; they must be considered for excellent performance of the film formed.
The main task of a finishing film is to give appearance and protection to the finished car. To meet these requirements, it must be properly covered and extended over the substrate. This mechanism that seems simple, has its complications since it needs a suitable environment to paint, taking care of the presence of air shots, impurities or suspended dust, and even observing the right temperature to do the paint job. Defects such as craters, pin points (pinoles), orange peel, loss of shine, trapped microbubble and others, can appear if the above is not taken into account.
Types of additives for paints
The additives of Fluidity, Leveling, Deaeration (Degasification) and Wetting of substrates are reinforcers used in the industry with frankly ostensible results. In this instance, we can use purely organic chemical compounds or the organosiliconate type.
The former are characterized by excellent compatibility, can generally be added at any stage of manufacturing and have deaeration, fluidity and leveling properties. They have no negative effects if repainted or retouched.
On the other hand, Organosiliconated additives have properties such as organic, in addition to promoting moisturizing the substrate, anti-scratch and smoothness effects or "slip", and even "antigraffiti" properties.
Small amounts of additives are often sufficient to differentiate the performance and appearance of the coating. Chemical compounds such as organic (Acrylic Polymers, Acetylene or Alkoxylated Alcohols, etc.) or modified siloxanic copolymers (with Polyether, Acrylic, Alkyl, Aril, etc.) are the most used to achieve the tasks mentioned above.
Deaeration
The microbubbles present in the paint or in the film already applied are invariably generated by the inclusion of air inside the paint or during the application of the same, the processes of dispersion, agitation and packaging irremediably generate gas inclusion in the form of microbubbles. The sudden evaporation of solvents by increasing temperature can also be the product, such as in baking curing; or chemical reactions that generate gases as in the case of polyurethane systems.
The microbubble is, in essence, a small part of air in large quantities of liquid or paint. It is a thermodynamically stabilized system by other surfactants present that must be removed to avoid film defects such as pin point, corrosion points, trapped or "boiled" microbubble.
The appropriate additives are surfactants of chemical principles already described that must have a balance between compatibility and instability in the system. It must therefore be considered that deserating agents must be carefully selected to perform both functions as far as possible. There are criteria for the selection of additives, the more accepted is that the more hydrophobic an additive is, this is the greater the amount of Polysiloxane in its molecule, this will be more efficient to attack the bubble but is critical in its compatibility which can lead to turbidity or surface defects, undesirable for the performance and appearance of the coating. On the contrary, an additive that has high compatibility with the system, will not cause defects but could eliminate the bubble more slowly.
{mospagebreak}
Substrate wetting
In this instance, there are some concepts to consider such as surface energies of substrates and surface tension of liquids. These values are calculated in the laboratory with special devices such as the DuNoüy Tensiometer or the Bubble Pressure method for pure liquids. In contrast, the surface energies of a solid or substrate can only be measured with indirect methods, such as measuring the contact angle of several liquids on the substrate in question.
The wetting of solids by liquids is influenced by the surface tension of the compounds involved, hence they have determined values of Surface Energy and Surface Tension for solids and liquids as shown in the following tables:
To obtain the best results in the wetting of a given substrate, the following criteria must be considered, which can be corroborated in practice:
- A substrate with high surface energy is easy to moisturize.
-A liquid that has low surface energy is a good moisturizer.
-The wetting process is ideal if the surface energy of the liquid is significantly lower than the surface energy of the substrate.
There are substrates with low surface energy that includes Polyethylene, Polypropylene, Polyurethane, opposite examples are metals and glass. It must be considered that the removal of fat, oils and certain impurities from the substrate greatly increases its surface energy. Not only the substrate itself, but also its texture and surface treatment or plays an important role in its surface energy.
The additives that best support the task of wetting are the Flurorosiliconados, from Polìeter-Siloxane; or of the organic type such as Sulfosuccinates or Alkyl-Phenol Ethoxylated, these last two tend to stabilize the microfoam, so you should use a desereante agent.
Leveling and Fluency
Some combinations are strongly used to meet the requested standards. Today, Siloxane Polyether compounds in combination with Acrylic Polymers have demonstrated the greatest effectiveness in long and short wave effects, gloss, smoothness in both original type finishes (OEM) and repainting.
The results of combining additives of different nature are represented in web diagrams. A fully colored diagram represents 100% covered characteristics. This is an ideal scenario that in practice cannot be achieved, however, satisfactory optimization can be reached. The results of evaluated additives are represented below.
Another feature to consider is the type of bond in the additive molecule. Previously the available bond was Si-O-C, which due to the presence of oxygen is not stable to hydrolysis. This bond also makes the molecule less hydrophobic which causes it to dissolve in aqueous or acidic media, therefore the efficiency decreases when the paint is subjected to stability tests. In automotive coatings the most accepted entity is the stable Si-C bond.
Using a Si-C type additive is normally more compatible with the system, better results can be expected in terms of brightness and lower susceptibility to turbidity. Another variable is the modification of the additive molecule, an organic modification of the functional hydroxy type results in a good performance in five of the characteristics of the web graph but the adhesion between layers is compromised. This is explained by the fact that if the –OH functional group reacts with the resin, the entire molecule will not be able to migrate to the next applied wet film either due to the lack of Hydrogen Bridges and Van Der Waals Forces.
That is why the additive that yields the same results but without negative influence on the adhesion between layers is the modified non-reactive and encapsulated (endcapped).
{mospagebreak}
Forms of use and exploitation
Silicone oils are purely Polydimethyl Siloxanes (PDMS), their fundamental property lies in their chain that is flexible, torsional capacity and surface activity. While these oils correct defects such as craters, presence of foam, and lack of leveling or fluidity, they also create potential film defects such as "fish eyes," turbidity, adhesion between layers, etc.
That is the reason why they are modified with organic compounds to make them more compatible with the resins most used in the industry and thus avoid collateral defects while taking advantage of the beneficial properties of the fòrmula.
Even when highly compatible compounds have been developed on the market, many of them designed to be post-added, it should not be overlooked that as good surfactants, if they are not properly incorporated they can be associated with each other forming miscelles and potentially a source of contamination that leads to the defects already mentioned.
In some cases the miscelles are not so severe as to cause surface defects, but the formula could demand more additive to achieve the same effect, leading to a higher cost of the formula since, despite its low formula content, the additives are not exactly those of more low cost.
Therefore, some general recommendations for its best use are:
-Perform predilutions with solvents of medium to high polar activity. Ketones, Glycols, Alcohols, Aromatics, etc. are recommended, and are usually already part of the original formula. From 4 to 10 parts of the solvent for each part of the additive is the recommended range. This prediluciòn is generally stable to storage for a few months.
-Addition of the compound manufactured in the earliest phase of paint production, usually in the grinding phase, when it comes to pigmented systems. In the case of lacquers or whites, medium agitation is needed.
-Sometimes it is possible to manufacture a compound with several additives for a formulation. The general characteristic of additives to be chemically stable and harmless allows this. However, you should always consult the respective technical bulletin to verify that the contrary is not indicated. Additional stability tests are highly recommended.
The way of supply of the additives are: in solution, emulsified or 100% active matter. All additives in this last modality should always be prediluted, but in order not to create confusion about which additives to predilute, it is recommended to subject all additives to this process.
The 100% concentrated products stand out because they can be used in different versions, these being water-based, solvent-based, high solid and even cured by energy. The development of new technologies at a global level, have allowed additives to comply with standards in general such as low emission of solvents, zero content of Volatile Organic Compounds (VOC) and compliance with the highest regulations worldwide.
References and Bibliography:
Winfried Heilen y Kirstin Schulz- "Additives to improve flow and leveling properties in OEM Coatings" Tego Chemie Service GmbH.
Tego Journal 2007– Degussa- 3rd Edition – Pages 55-57,70-73.
*Technical Sales Manager, Tego Coating Additives & Specialty Resins.
[email protected]
Leave your comment