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Eliminating the Char Enemy

Thursday, May 14, 2015
Best Practices and Procedures for Char-Proof Hot Melt Systems.

If you are the hot melt superhero and coating and laminating is your mission, then you have one evil villain lurking at around every corner: char. Okay, you might not consider yourself a hot melt superhero, but make no mistake; there is no greater villain in a hot melt operation than char. But while char terrorizes many hot melt operations all over the world, the truth is you have the power right now to eliminate char and char-related problems. Before we can delve into how to defeat this malicious adversary, though, first it’s crucial that you understand your enemy.

What is char? Char is adhesive that has been blackened or burned, and it is a particular challenge to hot melt operations because of the unique process required. Put simply, hot melt has to be heated and melted. Apply too much heat or heat it for too much time, and you will adversely affect the formulation, which can result in char.
What creates char? Char can result from:

• Heating hot melt adhesive at a temperature that is too high
• Exposing hot melt adhesive to heat for too long
• Exposing hot melt to heat and oxygen

When the effects of heat, time and oxidation begin to attack the hot melt, the adhesive begins to break down as the adhesive’s polymer chains begin to form active sites. These sites can then combine to form gels. Gels stick to the walls of hoses and crevices in melt tanks, forming an anchor. Anchored hot melt does not effectively flow through hoses and tanks, which is a big problem. Without the benefit of movement, these anchored pieces of hot melt are exposed to too much heat, and that’s when they begin to form char. Char is burnt adhesive. In almost all cases, char turns black, and char wreaks havoc on your coating and laminating process.

Important note: Char is a tricky enemy, and it does not always come from adhesive. In some cases char is actually burnt particulate that gets into the adhesive. This is almost always the result of poor housekeeping. If the lid of the melter or adhesive container is left open, exposing the contents to particulates, then you are inviting char into your operation.

What are the effects of char? Once formed, char will bake onto the heated grids. Once this happens it will break off in pieces and...

  1. Clog filters
  2. Stop up spray nozzles
  3. Clog bead nozzles
  4. Work its way onto the substrate leaving marks, streaks, and uneven surfaces
  5. Work its way, in the worst cases, into pumps breaking seals, scoring and damaging the pump walls

​One of the most problematic issues with char is that once it gets into your systems, it is impossible simply to flush out. There will always be dead areas (corners) that can’t be flushed. The char will harden on the surface of the burn and continue to collect in those places. Those ever-growing char spots will periodically break off and cause ongoing problems. That means that once you get a char problem, it will cause ongoing product quality problems, extensive maintenance issues and work stoppages. You will be fighting a never-ending char battle until you take extreme measures.

How can char be defeated? The best way to overcome the problem of char is to prevent it from ever forming. Once char has formed, however, there is only one way to remove it from a hot melt system. The entire system needs to be taken apart and the components need to be burned out in a burn out oven, completely removing the char from the components. Normally this will also require replacing seals and re-calibrating the hot melt system.  This is a time-consuming and very expensive process with no shortcut: the only other option is a total rebuild.

Now the good news. You don’t need to fight this battle. With proper procedures, char can be prevented.

Introducing char-proof melt systems. Hot melt systems can actually be char-proof, but the key is adhering to the correct procedures required by these systems as well as developing a thorough understanding of the adhesive. Good communication is your secret (or not so secret) weapon. Effective and consistent communication between the production and maintenance departments is like kryptonite to char. 

Remember, it is far easier and significantly less expensive to prevent char than it is to eliminate it once it gets into your system. The only good way to deal with char is to ensure that it never has the chance to infiltrate your system.

Know your adhesive.   It is very important to know the details of the adhesive. You first must understand the softening point of the adhesive and know the temperature versus viscosity curve. The adhesive manufacturer should state the maximum temperature for the intended adhesive, but that does not mean you should heat the adhesive to that maximum temperature. Companies have a tendency to really turn the temperature up to lower the viscosity, but the opposite should be the case. Keep the temperature at the lowest temperature possible to still apply the adhesive.

As mentioned earlier, excessive heat breaks down the adhesive formulation, accelerating the gelling, which leads to char formation. Some adhesives are much more susceptible to heat breakdown than others, so it is critical to get familiar with your adhesive and know how susceptible it is to breakdown. This is sometimes referred to as “pot life,” or the recommended amount of time that a particular adhesive can be under heat in a static state before it starts to break down. Consult the adhesive manufacturer for this information, and communicate it to everyone who works with the adhesive. You might also consider consulting your adhesive manufacturers to be sure you are working with an adhesive that is less susceptible to charring.

Wide applicators and narrow coating.  Narrow coating with a wide applicator head is a common source of char creation in hot melt systems. This setup creates dead zones where the adhesive cannot flow, exposing it to constant heat for days or weeks at a time, leading to adhesive breakdown, gelling, and the dreaded char. Further complicating matters, when you adjust the applicator a wider width, char ends up clogging up the system and/or depositing onto the substrate, diminishing the product’s quality.

Fiberized spray systems.  When purchasing a fiberized spray applicator, companies will buy a wide-width applicator to handle the widest width product they have. They often have a variety of narrower products as well, but they tend to choose the widest possible applicator in hopes that it will work for their full range of products. For example, a company might purchase a 72-inch-wide applicator, but also run products that are as narrow as 48 inches. The natural tendency is to turn off the nozzles not being used when running a narrow product, but that creates a problem. Just because those nozzles are closed does not mean there is no adhesive behind them. In fact, the adhesive is blocked up behind the closed nozzles, breaking down and eventually forming char. When you open these nozzles back up for the wider application, they are often clogged with char and can spray char onto the substrate.

Once again, though, you can prevent this problem. To avoid char-clogged applicators, design the applicator head in section sizes that can be separated and completely turned off so that there are no dead zones or heated areas. The smaller sections would not only be turned off but slid out of the way to prevent transfer of heat to the sections that have been turned off.

Slot.  When dealing with slot dies, the tendency is to buy a die for the widest part and shim it down for narrower widths. Once again, the adhesive that sits behind the shim does not flow, and it therefore cooks, breaks down, and eventually becomes char. Afterward, anytime a wider shim is inserted for wider-width coating, the char either creates streaks in the coating or puts char onto the substrate.  The proper way to handle this:  Simple slot dies. For narrow widths in simple slot dies, you can close down some of the modules to the die. Another option is to insert pieces of O-ring cord to block off the dead areas inside the die.

Precision contoured manifold type dies. When using this type of slot die system, have the company that manufacturer the die make sets of filler blocks that match the contour of the internal manifold.

Adhesive level in the tank. One error is the cause of the greatest amount of char creation: neglecting the proper adhesive level. When the adhesive level gets too low, the heater grids become exposed, which creates a hot spot that can be oxidized. The result is char. Typically this error occurs when you manually load pellets of adhesive into the melter. With manual loading, it’s common for people to get busy and forget to keep the tank full. Fortunately, this is one of the simplest fixes. There are two items that can help solve this issue.

  1. Automated filling system:  This device is basically a suction wand that communicates with a level control device on the melt tank and automatically feeds the pellets into the melt tank on demand.
  2. Level control system:  The level control system automatically monitors the adhesive level in the melt tank and ensures that the level will never get too low and the grids will never be uncovered. This is the single most important thing a company can do to help prevent char. There are many different types of level control systems, but one of the best types is a three-point level control system with adjustable set points. The top set point says it is full—no need to feed any adhesive to the tank. The next set point says it is low on adhesive and requires feeding the melt tank either manually or automatically. The third set point is critical: if the low level reaches this point it means that the required adhesive feeding did not take place. When the system reaches this set point, it is triggered to shut down to prevent charring.

Adhesive melt tanks.  It is very important to size the melt tanks properly. If a melt tank is too large for the application, adhesive will not turn over frequently enough. The adhesive that is retained in the tank for long periods of time will begin to break down and eventually char.

Adhesive temperature set points.  As discussed under “Know your adhesive,” one of the most imperative steps in char prevention is to set the temperature as low as possible while still accomplishing optimum coating or laminating. A good hot melt system will have a temperature adjustment screen that allows you to adjust temperatures by zone. These temperature settings should also allow you to set an acceptability range or band on either side of the set point.
For example, let’s say that the intended application temperature is 350◦F. Assuming that there are four zones in the melter, one zone for the hose, and four zones in the applicator head. You may set the top two zones in the melter to 325◦F and then increase the temperature to 350◦F as you get closer to the applicator head. You can also set the bandwidth of acceptability to ±10◦F. Should the temperature drift outside the band of acceptability, the system will set off a temperature alarm and shut down the system. Another device that can enable proper temperature maintenance is an in-flow temperature probe with PID loop capabilities.
Additionally if the system will sit idle for any length of time, the melt system should have a set-back temperature to drop temperature significantly— to just above the softening point —during idle times. This will help minimize the possibility of adhesive breakdown.

Nitrogen blankets.  Some adhesive are very sensitive and therefore prone to charring. In these situations, the melter needs a nitrogen blanket. Remember that oxygen is a component that causes the charring process, so to help prevent charring, eliminate oxygen. You can achieve this by placing a nitrogen port on the side of the melt tank. A nitrogen bottle can be connected to this port and slowly feed nitrogen, under very low pressure, into the melt tank. Melt tanks are not pressure vessels and so the nitrogen will leak out and be replenished by the nitrogen bottle. This forms a blanket and does not allow oxygen in the air to contact the adhesive.

Adhesive hoses.  If heated hoses sit with adhesive in them for long periods of time, with no adhesive movement, a film of adhesive can form on the Inside diameter of the hose. The film that does not move will break down and eventually char. After, anytime the hoses are moved or bumped, char will break free, which can clog filters or, worse, come out on the substrate.

Poorly designed melt tank systems.  The ideal melter is one that has a melt on demand system. This means that heat is not applied to the adhesive until there is a demand for it. This type of melt system typically involves a non-heated tank with heater grids inside. This style melter uses surface area and wattage to generate the heat to bring the adhesive to temperature.

Some melters on the market do not have heater grids but rather band heaters around the perimeter of the tank. They draw the adhesive from the bottom of the tank. In order for the adhesive in the middle of the tank to reach 350◦F, for example, the outer perimeter of the tank might need to be heated to 500◦F to get the temperature in the middle of the tank to 350◦F. By now it’s clear why that is a problem: overheated adhesive equals breakdown and char.

Char is the number one challenge when coating with hot melt adhesive, but it is also entirely preventable. With the right melt equipment, the right set up, the right maintenance, and a thorough knowledge of the adhesive being used, you can conquer your biggest hot melt enemy for now and for good.”

Written by: Michael Budai, Coating & Laminating Manager for ITW Dynatec

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