Contract-coating manufacturers possess a wide range of expertise and tools to conduct the complex processes required for roll-to-roll coating. Among the specialties involved in roll-to-roll coating, solvent screening studies require a skill set and enhancement by experience across a number of markets and solvents, plus a deep knowledge about its impact on coating and drying. Whether a customer presents a polymer for advice on how to dissolve and coat, or provides a target solvent or solvents for the contract manufacturer to dilute into a better fit for a coating model, there are several variables to determine how it works.

Three critical forces to determine approximate solubility include hydrogen bonding, polarity and dispersion forces (also commonly referred to as London forces or induced dipole). Knowledge of these forces can enable the contract coating manufacturer to systematically select a solvent or solvent blend system that best fits its coating methods and drying capabilities. Once a solvent is found in the interaction radius for the solute, coating manufacturers can determine alternate soluble solvent blends.

There are several critical factors to determining the appropriate solvent, including surface tension, adhesion, boiling point and rheology as outlined below:

Surface Tension: Surface tension involves the wettability of the solvent. The lower the surface tension of the solvent, the more likely it is to wet out on a surface and the less likely it is to have repellencies or voids. To determine if something is wettable, the surface tension of the solvent can be compared with the surface energy of the substrate. A typical goal is for the surface tension to be 10 dynes/cm less than the surface energy of the substrate. A tensiometer is used to measure the surface tension of dilution, while both goniometers and dyne pens can be used to determine surface energy. Adding surfactants to a system is often an easy solution if surface tension is a problem; however, it is not always a viable option. Sometimes surfactants can inhibit the functionality of the products. They may disrupt the electrical or thermal conductivity of the coating, adhesion, hardness, color, haze, etc.

• Adhesion: The better a solvent wets out in general, the better adhesion it will have to the substrate. Depending on the application of the product, adhesion requirements will vary. For some products the coating must permanently affix to the substrate, while others use the substrate as a casting medium to create a free-form film. A cross-hatch adhesion test may be used to measure adhesion to the substrate.

• Boiling Point/Vapor Pressure: The physical property of the solvent mix has a great effect on the conditions of the coater after the coating station. A wet coating must be dried gently to avoid defects such as bubbles, blisters, haze, mottle, etc. In contrast, if the coating is not cured quickly enough, it will either track off on face-side idlers or be wound up in a roll and block. It is essential to create a list of solvents that a coater is capable of drying, based upon coating thickness, oven length, and efficiency.

• Rheology: A solution’s viscosity is another critical factor that affects the coating process. In general, the percent solids can be altered to change the viscosity of the solution. Different coating methods have limits as to which viscosity ranges are ideal. For example, if a coater only has spray coating capabilities, the ideal viscosity range is limited to 10-300 cP.  Slot die methods range from 10-20,000cp and gravure methods range from 10-200cP. Other rheological factors to consider include whether the solution is thixotropic, shear thinning and the yield point. Defects that can be caused by improper viscosity consist of drip marks, streaks, voids, etc.

• Manufacturability and Cost: Some solvents are very costly, while others are small volume to an existing plant environment, and other solvents require extensive safety reviews. Working with a coating partner that has knowledge of these factors can dramatically cut the time required to conduct projects.

In conclusion, finding the appropriate solvent for your solute and coater results in many benefits. By working with a coating partner with a wide range of expertise and tools for conducting solvent screening studies, conflicts with regulations and costs regarding environmental concerns from hazardous solvents can possibly be avoided. Additionally, an experienced high-precision, quality coating partner saves a great amount of time in product scale-up, ensuring defect-free coating with a large, stable coating window.