Waxes occur naturally or are synthetic compounds made from polymers or high fatty acid esters.
For a compound to be regarded as wax, it should:
- Have a molecular weight of between 200 to 10,000 μm
- Have a melting point of at least 40 ºC
- Be solid at 20 ºC
- Exhibit comparatively low viscosity and non-stringing when it is slightly above its melting point
Currently, a wide range of waxes are available, usually classified in accordance with their origin. A summary of the waxes used extensively for industrial applications is shown in Figure 1.
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Figure 1. Classifications of commonly used industrial waxes.
Why Waxes?
Waxes are environmentally friendly and functionally versatile, and they can be employed in various industries on virtually any surface. Waxes impact and enhance several properties of a surface, some of which include: slip control, anti-blocking, water repellency, and abrasion resistance.
Anti-blocking refers to the resistance to adhesion between two surfaces under the effect of relative humidity, temperature, or even pressure. Hardness, glass transition temperature (Tg) of the polymer, the topography of the coatings, and the coating surface free energy are the factors that affect blocking. Furthermore, carnauba, HDPE, and paraffin waxes are often used to counteract blocking. Anti-blocking agents are widely used for items that are dried, coated, and instantly rolled-up or stacked for shipment or storage. One eminent example of a blocking condition is when a newly painted window frame is closed too quickly. At times, it can be quite hard to open the window after the paint has dried.
Slip enables two surfaces to glide over one another without inducing any mechanical damage. To realize good slip properties, the slip additive should concentrate at the surface during and instantly after application and curing. The solid-state wax crystals are responsible for affecting the property of slip. Harder waxes enable better slip properties; slip can be controlled to a desired level through the selection of a suitable wax.
Harder waxes like PTFE resist liquefying and exhibit a comparatively higher proportion of crystals in the solid state, leading to higher slip levels. On the other hand, softer waxes such as paraffin tend to be easily liquefied and, consequently, have a lower proportion of solid crystals, and thus lower slip.
Abrasion resistance decreases the extent of damage promoted by the mechanical action of erosion, rubbing, or scraping. Since it is closely associated with slip and scratching, it is no wonder that the majority of slip aids also function as mar and abrasion resistance additives. Waxes help in creating abrasion resistance via a combination of basic properties like strength, hardness, elasticity, lubricity, particle size, toughness and, in certain cases, thickness.
Water resistance or water repellency refers to the protection of a surface against water penetration. Based on the type of water exposure, the protection can be either temporary or last for a number of years. In general, water resistance means resistance to water in the liquid state, while moisture resistance means protection against water in a vapor or gaseous state. Paraffin waxes, including scale waxes (a lower refined paraffin grade containing around 5% oil), perform extremely well, especially on porous surfaces. The oil penetrates quickly and easily into the substrate’s pores and fissures, giving a highly hydrophobic property to the treated surface.
To sum up, waxes are employed in formulations because of the advantages they provide in terms of surface enhancement and properties.
Mechanisms of Action for Surface Modification with Waxes
Waxes function as a physical barrier on surfaces to prevent damages caused through external contact. They can also impart tactile properties to substrates by penetrating the coating layer and producing an uneven and rough surface. To have a considerable effect on the ink or coating properties, the wax should migrate to the surface and be present in an adequate amount to impart the required characteristic. Molten blooming and the ball bearing mechanisms are two migration mechanisms that are generally proposed.
Molten Blooming
In molten blooming, molten wax particles bloom or float to the surface. As the coating cools and the re-crystallization of wax particles occurs, a thin but continuous wax-enriched surface layer forms. It must be noted that the blooming mechanism is more predominant in softer waxes along with those having lower densities and lower melting points. The wax migration rate is determined by the compatibility between the wax emulsion and other formulation components (See Figure 2).
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Figure 2. Compatibility determines wax migration rate.
The Ball Bearing Mechanism
In the ball bearing mechanism, solid wax particles migrate separately and protrude via the surface. By slightly projecting above the coating, these wax particles behave as a physical spacer and inhibit another surface from coming into close contact. HDPE is an example of a hard and high melting point wax that behaves according to this mechanism under specific conditions. The density as well as the particle size affects the extent of protrusion and thus the magnitude of the impact (See Figure 3).
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Figure 3. Particle density and extent of protrusion influence the magnitude of the effect.
Establishing the most optimized way to apply a wax on a surface is critical to the performance of the wax. The density, melting point, and molecular weight of a wax are significant criteria in establishing whether a wax will work well under certain conditions. In addition, an understanding of a wax’s melting point can protect the surface from future harm because the wax will begin to lose its surface homogeneity and longevity when it is heated beyond its melting point.
How to Stabilize a Wax Emulsion
Either an Electrostatic Mechanism using ionic emulsifiers or a Steric Mechanism using nonionic emulsifiers are used to stabilize wax emulsions. Yet, integrating both nonionic and anionic emulsifiers offers optimum stability to wax emulsions. This is called the Electro-Steric Stabilization Mechanism.
Each stabilization mechanism has its pros and cons. A comparison of both methods is shown in Figure 4.
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Figure 4. Comparison of steric vs. electrostatic stabilization.
Each mechanism affords varied levels of flexibility in formulating that can provide considerable advantage and impact to the overall formulation. Finally, the emulsion stability impacts the wax’s shelf life and its conditions under which it should be stored.
Waxes Provide High Degrees of Formulation Design Freedom
Waxes can work well on the majority of surfaces and they are capable of providing certain properties based on the application requirements. Waxes possess a broad range of functionalities that enable them to solve a wide range of performance problems, and also enhance the overall aesthetic of surfaces. Waxes are not only environmentally friendly but also offer unparalleled value in surface modification through superior appearance and better performance.
Selecting a Wax for Specific Applications
Several factors have to be assessed when establishing which wax will be most effective. Particle size, melting point, order of component addition, surfactant type, pH, and regulatory requirements are the properties that have the most significant effect on formulations. Each of these factors should be considered before making a choice. When curing is required, the waxes’ melting point should be lower than the curing temperature. This enables the wax time to melt and shift to the surface to ultimately produce a continuous film that promotes blooming.
Particle size as well as particle size distribution should be taken into account when assessing which wax is the most suitable for the preferred surface attributes. This is significant to consider for hard waxes, particularly where the ball bearing mechanism is at work. For instance, larger particle sizes will impart matting characteristics to a substrate. During the emulsification process, Michelman can control the particle size range to satisfy the customer’s exact specifications.
It is essential for the pH of a wax emulsion to stay within +/- 1 unit of the system to which it is added. If required, Michelman can typically modify the emulsion’s pH using a base or an acid to meet the formulator’s requirements.
The type of surfactant utilized can affect compatibility with other components, and also the formula’s overall stability. Stability is improved by matching the emulsion charge with the coating charge.
In water-based formulations, the order of component addition can be a key factor in preserving emulsion stability. The order in which the wax is integrated can inhibit agglomeration and increase the emulsion’s overall stability. Shock can also be reduced by additionally diluting the emulsion with demineralized or soft water before incorporation.
Regulatory compliance of waxes is significant, particularly if emulsions are meant for food contact use. They must conform to applicable regulations for each country (FDA, European Directives, BfR, Inventory Lists; TSCA, and so on). The manufacturer or supplier should provide a confirmation of a wax’s regulatory status, because waxes come in many different grades, may contain any number of varied chemical compounds, or may be chemically modified.
Consideration of all these variables when selecting a wax ensures maximum efficiency and performance.
The Power of Michelman Wax Dispersion and Wax Emulsion Brands
Waxes can be a major additive to any formulation as they work well on almost every surface and possess a wide range of properties that render them appropriate for virtually all environments. When choosing a wax for a particular formulation, it is important to have a foundational understanding of how waxes work. A lack of complete consideration of the effects of a wax on various properties can result in an unstable and potentially unusable product surface. Wax emulsion experts at Michelman can help customers in understanding how the properties of wax can improve product performance and also help them in choosing the best wax for their specific applications. Some popular brands from Michelman include:
This information has been sourced, reviewed and adapted from materials provided by Michelman.
For more information on this source, please visit Michelman.