Thermoplastic hot-melt adhesives such as EVA, PSA, or PO are melted, applied while hot, and solidify again as they cool. Reactive hot-melt adhesives (PUR) are also applied while hot. After application, the PUR adhesive cools and reaches an initial strength. It then reacts with moisture from the environment or the substrate and achieves its final strength through chemical cross-linking. This combination makes PUR particularly high-performance. A thin layer of adhesive is sufficient for strong adhesion. This can be seen, for example, in books: A book spine glued with PUR remains stable yet flexible, even though the adhesive layer is very thin.
PUR is not a one-size-fits-all adhesive for every application. The investment in the adhesive application system is higher than for non-reactive hot-melt adhesives. However, PUR offers clear advantages when the bond must withstand high demands over the long term.
PUR is particularly worthwhile when products:
are exposed to moisture or water
are exposed to high temperatures
are subjected to chemical or mechanical stress
require a thin, clean, and visually high-quality adhesive joint
must adhere reliably to challenging materials
need to remain stable over the long term
One example is edge banding in the wood industry. Furniture components are transported in shipping containers and are exposed to heat, cold, moisture, and extreme temperature fluctuations. Under such conditions, non-reactive hot-melt adhesives quickly reach their limits. PUR can also be a useful option for packaging where tamper-evidence is desired or required by law. The packaging is securely sealed, making any tampering immediately visible.
In short: PUR is always worth it when the bond needs to do more than just hold things together quickly. When quality, durability, safety, and service life are crucial, PUR is often the right solution.
Many problems encountered when working with PUR are not caused by the adhesive itself. The key factor is the interaction between the adhesive, the adhesive application system, process control, and operator skills. A suitable system for reactive adhesives is therefore just as important as proper handling in day-to-day production. Well-trained employees are a key factor for success. In the EU, training is mandatory for certain products containing diisocyanates.
The following tips highlight the problems that can arise in everyday operations and how to prevent or resolve them.
PUR and moisture – that’s a delicate matter. After application, PUR adhesive requires moisture to chemically cure and reach its final strength. Before application, however, moisture is a critical issue. If it enters the container, tank, or adhesive application system too early, the adhesive will cure prematurely. The consequences quickly become apparent: skin formation in the tank, gel particles in the adhesive, clogged filters, blocked nozzles, rising adhesive pressure, irregular application, or poorly bonded initial products after a shutdown. In the worst-case scenario, the adhesive must be disposed of and the system thoroughly cleaned.
Particularly critical factors include open or poorly sealed containers, damaged packaging, tank lids left open for extended periods, humid compressed air, condensation, or open nozzles during downtime. Cold containers should also not be opened directly in a warm production environment, as this can lead to condensation.
The entire adhesive application system must therefore be consistently protected against moisture. This includes a closed adhesive system, a tightly closing tank lid, suitable seals, and, if necessary, a protective atmosphere of dry air or nitrogen.
How to optimize:
Open the adhesive container only immediately before use.
Keep opened containers open for as short a time as possible.
Allow cold containers to acclimate before opening.
Keep the tank lid and adhesive system closed.
Use only dry, clean, oil- and water-free air.
Protect application nozzles when not in use according to the manufacturer’s instructions.
The application temperature affects how well the adhesive flows, how cleanly it is applied, and how well it wets the substrate. Therefore, the temperature recommended by the adhesive manufacturer in the technical data sheet should be followed. Even if the adhesive flow decreases, it is not a good idea to simply increase the temperature. It makes more sense to identify the cause and correct it.
Just as important as the processing temperature is the dwell time. The longer the adhesive remains in the system at processing temperature, the greater the thermal stress. This can lead to higher viscosity, deposits, increased adhesive pressure, and an uneven application pattern.
Production interruptions are particularly critical. During a shutdown, the adhesive remains under thermal stress within the system. At the same time, the risk increases that moisture will enter the adhesive application system via the tank, nozzles, or leaks. Upon restart, the consequences often manifest as rising adhesive pressure, stringing, unstable application, or defective initial products.
How to handle a production standstill depends on several factors:
the adhesive used
the amount of adhesive in the system
the duration of the interruption
and the specifications provided by the adhesive and equipment manufacturers.
Short breaks, shift changes, weekends, and longer production interruptions therefore require different measures. Consult a specialist for advice on this matter. A simple measure for shorter downtime is to lower the temperature to protect the adhesive from unnecessary thermal stress.
When restarting, check the adhesive pressure, the application pattern, and the first products. Depending on the condition of the system, a flushing procedure may be necessary.
How to optimize:
Adhere to the application temperature specified in the technical data sheet.
Do not keep the adhesive heated longer than necessary.
Define clear guidelines for different periods of inactivity.
Lower the temperature during downtime in accordance with the manufacturer’s specifications.
During longer interruptions, provide additional protection against moisture.
Properly seal or protect the nozzles during downtime.
When restarting, check the adhesive pressure, application pattern, and first products.
Flush the system if necessary.
It is not only the adhesive and the application system that determine the quality of the bond. The amount applied, the substrate, cleanliness, and environmental conditions are equally important. Too little adhesive can lead to insufficient wetting or open joints. Too much adhesive prolongs the curing time, compromises the visual appearance, and increases contamination as well as the effort required for cleaning.
The condition of the substrate also plays an important role. Dust, oil, moisture, cold, or significant fluctuations in temperature and humidity can impair the bond. Cold products should therefore be allowed to acclimate before use.
How to optimize:
Follow the application amount and curing time specified in the technical data sheet.
Keep substrates clean and dry.
Allow cold products to acclimate before gluing.
Keep the material and ambient temperatures as constant as possible.
PUR adhesives must not be mixed with other adhesives, cleaners, oils, or foreign substances. Even small amounts of residue can interfere with the process. Such problems are not necessarily immediately apparent; in some cases, they do not occur until weeks or months after the change. It is therefore extremely important to thoroughly clean the entire adhesive application system when changing adhesives. Cleaners, flushing agents, and the new PUR adhesive must be compatible with each other and with the system. After the change, the application pattern and the first products produced should be carefully inspected.
How to optimize:
Change the adhesive only according to the specified procedure.
Use only approved cleaners or rinsing agents.
Completely remove the old adhesive.
Check the application pattern and the first products after the change.
If problems recur, document the process data and contact the service department.
A PUR system should not be designed solely for maximum pump capacity. Actual adhesive consumption is the decisive factor. A pump may need to deliver high output for short periods during application, while average consumption is significantly lower.
An oversized system keeps an unnecessarily large amount of adhesive at processing temperature at all times. This increases dwell time, thermal stress, cleaning effort, cleaner consumption, maintenance costs, and the risk of downtime. Long adhesive delivery paths also increase the amount of adhesive that is continuously heated.
A demand-based system design keeps the amount of heated adhesive as low as possible while ensuring the required peak output. For low or highly fluctuating consumption, a melt-on-demand system may be a good option. With this system, only as much adhesive is melted as is currently needed.
How to optimize:
Size the system according to actual adhesive consumption.
Keep the amount of heated adhesive as small as possible.
Avoid long adhesive flow paths.
Use a melt-on-demand system for low or irregular consumption.
PUR adhesives offer significant advantages. In my experience, they are not complicated adhesives to work with. However, they do require the right interaction between material, equipment, environment and operation. What matters is not one single parameter, but proper handling in day-to-day production.
It is therefore important to follow the relevant guidelines, to ensure that employees are properly trained and to use an adhesive application system that is suited to the process. By taking these points into account, manufacturers can achieve stable processes, reduce unplanned downtime and maintain consistently high bonding quality. Taking the time to handle PUR adhesives correctly is therefore well worth the effort.
Do you have questions about the correct handling of PUR adhesives or the right system design for your application? Your local Robatech representative will be happy to support you with system selection, commissioning, training and the optimization of your PUR application.