The Hows And Whys of Hot Melt Coating
Some of the thinnest, most basic materials we commonly use in daily life may seem like single-layer materials, but they are actually often the combination of multiple layers with various functions. Adhesive tapes, films, paper products, etc. all have functional coatings that have to be applied after the base layer material has been created. These coatings must be applied using specialized processes that preserve the base material while still binding the material and the coating together.
Before a coating can be applied, there has to be a substance to which it can be applied. That base substance is called a substrate, and it can be composed of any number of materials depending on the desired rigidity, durability, appearance, and performance of the finished product. More often than not, though, the substrate is simply a functional vehicle for delivering the properties of the coating. As an example, consider clear, one-sided adhesive tape; the adhesive portion is the coating that’s been applied, while the non-adhesive film underneath is the substrate.
Adhesives possess a certain amount of stickiness and strength necessary to perform the functions for which they are intended. These qualities increase the difficulty of evenly spreading and applying them to the substrate. The resistance of a liquid to being separated, or its strength to remain together, is called viscosity. Viscosity is probably the single biggest challenge that must be overcome in the coating process. Over the years, a number of different processes have been employed to negate viscosity, but the goal has always been to increase efficiency and decrease waste.
ADDING WATER IS A WASTEFUL APPLICATION METHOD
In the past, the primary method for ensuring that an adhesive or film can be evenly spread over a substrate has been to thin the adhesive or film by adding a liquid like water to reduce the consistency and the viscosity. This allows it to be applied smoothly and easily without clumping or clinging too much to the application mechanisms. It also ensures that the substrate is less likely to be damaged during the application process.
According to Thomas Dunn, the water-based liquid adhesive or film solution is generally applied via a “doctored” method. This involves using a chamber doctor blade and sometimes a rod with wire wound around it. An excess of the solution is first applied to a moving web or belt using the rod or another applicator. The surplus is then removed from the web or belt using the doctor blade before it is applied to the substrate by an final applicator.
This method is effective for controlling the volume of the adhesive or film applied to the substrate, says E. Shim, but it also results in the discarding of the excess liquid solution. While that discarded excess is generally returned to and recirculated from a reservoir, the time that passes during the process may allow some of the solution to begin to cure naturally, resulting in a certain amount of undesired waste. This cured waste has the added disadvantage of potentially interfering with the operation of the mechanism and with the eventual uniform application of the solution to the substrate. Modern versions of these machines include systems for monitoring and controlling the viscosity of the liquid adhesive or film solution to be applied, thus reducing the amount of waste produced. Any amount of cured solution waste, though, creates the necessity for a greater ratio of raw materials to finished products.
Once the adhesive or film solution containing the added water has been deposited on the web, the adhesive or film must be cured so that it will bond with the substrate and so that it ultimately takes on the desired properties for which it is being applied. Remember, the water was added to create a liquid solution with reduced stickiness and consistency, so if we want to reintroduce those properties, we need to reverse that liquidation. This is frequently accomplished using ovens that circulate hot air over the material to initiate and accelerate evaporation.
One of the risks of this drying or evaporation process is that the exposed surface of the liquid adhesive “skins over”, which can trap liquid beneath the dried outer layer. For this reason, it is important that the drying process is strictly regulated to achieve even and thorough drying. Historically, the water-based coating operation is halted at some point during the process while the adhesive is still on the web and before it has been applied to the substrate to test coating weight, solvent retention, and drying effectiveness.
Once the adhesive exits the drying ovens on the web, it passes through a system that adheres the adhesive to the substrate using pressure. At this point, the bonding properties and consistency of the adhesive should be substantial enough to permanently attach the adhesive to the substrate while allowing it to be released from the production web. From here, the material is distributed to slitting rewinding machines through which it is divided up and arranged into saleable portions and sizes.
ADDING SOLVENT IS SIMILAR AND IN SOME WAYS WORSE
The general physical process of adding solvents to adhesives or films to reduce consistency and viscosity is essentially the same as adding water for the same purpose, according to Sina Ebnesajjad. The variation essentially lies in the chemical reactions employed. Aside from water, oil and low-boiling gasoline are the most commonly used solvents. The combination of the adhesive material with the effective solvent creates an adhesive polymer solution that can be effectively and evenly applied to a substrate.
Not all polymers are compatible with all solvents, so an effective pairing must be determined based on the desired polymer to be used as the adhesive material. This is most effectively accomplished when using polymers with stable molecular compositions. One of the disadvantages of the solvent-based solutions, though, is that it can take a great deal of time for a stable polymer to dissolve in a given solvent, and attempts to accelerate the process can result in a degradation of the desirable qualities of the adhesive.
Many solvent-based adhesives also require the addition of an inhibitor to keep the adhesive from curing to completion or hardening prior to its intended usage. Finally, some solvents and inhibitors can create hazardous byproducts and exhaust that must be properly disposed of following the production and application process.
THE WONDER OF WATER-LESS, SOLVENT-LESS APPLICATION
A hot melt coating machine, or slot die coating machine, achieves the application of an adhesive, film, or coating without the use of water or solvents. As mentioned above, in alternative methods, the addition of water or solvents is used to reduce consistency and viscosity of adhesives and coatings in order to allow them to be applied evenly and effectively. However, many adhesives can also be thinned by heating them to the point that their consistency and viscosity are temporarily compromised.
In a hot melt coating machine, the adhesive is pre-melted and then applied through the use of specially slot dies to ensure even coating at high speeds and with a high degree of accuracy. This application style negates the need for the removal of excess liquid adhesive solution by properly applying the precise amount needed from the beginning. The reduced number of steps, along with the reduced periods of interaction with and pressure against substrates results in a decrease in disastrous wrinkling and curling.
Arguably the greatest benefit of hot melt or slot die coating over water-based or solution-based applications, according to F. Shepherd, is the lack of need for a drying element. In a hot melt coater, the adhesive simply firms and regains its viscosity as it cools. This negates the need for extraction of excess liquid or condensation, eliminating an entire step of the previously related process, as well as the need to deal with exhaust or liquid by-products.
As explained above, when a water-based or solvent-based solution is applied, it is applied in excess, and the surplus must be removed, often to a reservoir. That excess is necessary in those processes because the concentration of the adhesive has been compromised by mixing in an amount of non-adhesive liquid. With hot melt coating, the original concentration of the adhesive has not been altered, so it is not necessary to pour or spray a greater amount than will be used on the substrate itself. When no excess adhesive solution has to be removed, the need for a large recirculation reservoir becomes unnecessary and the required volume of the initial liquid adhesive source is reduced, meaning that the overall structure of the hot melt coating machine can be smaller.
The size of the overall structure is also reduced because there is no need for a powered drying element in water-less or solvent-less machines. Powered drying creates exhaust and condensation which must be expelled and dealt with as well. Depending on the solvent or adhesive being used, that exhaust may be toxic, which adds an increased complexity to its disposal. Because there is no powered drying element in hot melt coating, as well as no water or solvent needing to be dried out of the adhesive, these considerations are unnecessary. The reduction in the number of necessary powered parts in the hot melt coater also allows for less energy consumption, making the machine more cost-efficient and environmentally friendly.
LESS WASTE, MORE EFFICIENCY
Hot melt or slot die coating machines offer improvement to nearly every aspect of the industrial adhesive or coating application process. These machines take up less space than alternative water-based and solvent-based versions, and produce far less waste. They have fewer dynamic and independent elements and require fewer separate processes, meaning that their operational costs are lower and allow less opportunity for complication or error. Finally, they virtually eliminate the production of unwanted byproducts, thus negating the need for dealing with the exhaust and runoff created via alternative means.