In a recent blog, we discussed hot melt adhesive application methods and variables to consider. Now, let’s look at troubleshooting two common hot melt adhesive problems: charring and stringing.
Hot Melt Adhesive Charring in Reservoirs
Char is comprised of burned particles that result from the thermal degradation of organic substances. This can include:
- Certain adhesives
- Paper materials
- Other contaminants
When manufacturers have to deal with char, valuable productivity time is lost. As a result, it’s important to understand the causes of char and how to prevent it from happening as best as possible.
How a Hot Melt Adhesive Can Cause Charring
Charring can be a common problem with hot melts used for packaging applications. These applications typically require the adhesive to be in large melt tanks. This means the adhesive sits stagnant in the tank and “cooks”. Over time, this causes char. Certain hot melts have the potential to char at a slower rate than others, yet all types can be susceptible to this problem over time.
How the Reservoir Can Cause Charring
If improperly maintained, the reservoir can contribute to charring and cause damage to the equipment overall. This can occur if the melt tank lid is left open for too long, and contaminants fall into it.
Additionally, the reservoir may not be sized properly for the given application and throughput of the line. This impacts how much adhesive is used and how much remains within the tank. Furthermore, if the reservoir temperature is too high, it increases the likelihood for the adhesive to get “cooked” more quickly and cause char.
Solutions to Reduce Hot Melt Adhesive Charring
Bostik recommends ensuring that the hot melt adhesive used is the correct one for your application. In some cases, a low application temperature adhesive or a metallocene will do the job and is much less prone to char. However, some adhesives are more prone to char than others, yet are still the right one for a given application. Board stock or process may require an adhesive that is more likely to char in order to get the required performance.
Tips for proper maintenance of the reservoir include:
- Keep the level of adhesive in the reservoir 70% or more of the tank capacity at all times.
- If the adhesive level gets too low and cold adhesive is added to fill the tank, the overall temperature of the adhesive will drop (think of ice cubes in a drink) and higher viscosity and stringing can result.
- Another reason to keep the tank full is that char forms in the presence of high heat and oxygen. If the adhesive level in the tank is allowed to get low, the adhesive residue that clings to the sides of the melt tank is exposed to the air and heat on the side of the melter. These are perfect conditions for char to form.
- Ensure the reservoir is sized properly for the throughput of the line. The goal is to have the adhesive turn over at least once a day so that fresh material is added at regular intervals. In most applications, a smaller melt tank is deemed better, since it reduces the amount of time the adhesive is sitting in it.
- Keep everything clean around the hot melt tank, and maintain the lid properly.
Hot Melt Adhesive Stringing
Another common hot melt adhesive problem during the manufacturing process is called “stringing”. Stringing occurs when some of the adhesive material is left behind on the nozzle and gets pulled down the line. This is especially an issue for food packaging applications.
Causes of stringing include:
- Too low of an application temperature
- Low application temperature increases the viscosity of the adhesive, which is strongly correlated to stringing.
- Substrate located too far away from the applicator tip
- Poor nozzle shut off
- Excessive airflow across the equipment from fans or HVAC systems.
To reduce stringing, Bostik recommends the following solutions:
- Raise the application temperature. For example, start with +5°F on the hoses and +10°F on the nozzles. This will decrease the viscosity of the adhesive as it exits the nozzle and will decrease the amount of stringing.
- Position the substrate closer to applicator tip.
- Redirect the airflow away from the applicator head.
- Clean or replace the applicator.
For more information on Bostik’s adhesives and troubleshooting hot melt adhesive problems, call 800-7-BOSTIK.