We’ve all been there. Sitting, waiting for a product to be ready to use. In fact, you’ve probably said it yourself a number of times. It’s so boring it’s like watching paint dry.
In the manufacturing industry though, it can be a massive problem. This is particularly the case when it comes to waiting for adhesives to cure before moving on to the next step in the production line or so that another part of a process can take place. Adhesive cure time will affect the overall production time and throughput.
The time it takes for adhesives to cure is not always consistent either. Sometimes it can take double what it was the month before. Same product, same substrate but vastly longer cure time. It’s a frustrating issue because you just need the glue to cure for the next step to take place.
Here at Forgeway, we are an adhesive manufacturer specialising in adhesive solutions for the manufacturing industry. Manufacturers often ask us what is going on in this situation, what is affecting their adhesive cure time? Normally there is a very good reason. Sometimes it could be a little bit more complicated. But there is always a solution.
This article will go over what could be affecting your adhesive cure times. Then we’ll explore potential solutions for you to evaluate which one sounds good to you.
By the end of this article, you will be able to determine why you have variability in your adhesive cure time. Then you will be able to go figure out how to sort the issue and help you move onto the next production stage quicker.
There is one thing you need to be aware of first. The difference between dry time and cure time.
The difference between dry time and cure time for adhesives
It is often quite confusing to know the difference between these two terms. When we get asked “Why is my adhesive taking so long to dry?”, the first step is to clarify whether you mean dry or cure.
There is lots of technical jargon on the internet around this topic.
In simple terms, dry glue means tack-free. This means there would be no wet transfer to a finger when touched or to the other substrate. The picture below indicates what a wet glue would be. This glue is not yet ‘tack-free’.
However, a dry or ‘tack-free’ glue will not yet have reached its full strength, unlike cured glue.
Cured glue is at full strength and is fully crosslinked. You will really struggle to remove the glue at this point. Curing will always take longer than drying unless the glue is command cured. We will discuss this later in the article.
What does this all mean? It depends. But most of the time, we hear manufacturers saying “the glue needs to cure before it can move onto the next step.” Whilst it’s not a bad thing, this isn’t correct. The glue does not NEED to be fully cured before you can move the structure or product.
The glue needs to reach handling, or working strength (we will call it handling strength) before you can move or work on the product being glued. Handling strength is reached when the adhesive has become strong enough that it can be put into light operation. This is particularly the case if the adhesive is structural.
At this stage the adhesive hasn’t fully completed its chemical reaction and therefore cannot yet be put under its full intended stress or force. So handling strength is somewhere between drying and curing.
For the purpose of this article, we will stick to talking about cure time.
Now we understand what the different terms mean, let’s jump into what actually affects the cure time.
What affects adhesive cure speed? Why do some adhesives take longer to cure?
Maybe your adhesive usually takes one hour to cure but has suddenly changed. Now what you’re finding is that it takes two hours to fully cure. What is going on? We regularly come across this situation and are often asked what is happening. It’s confusing, right?
The first thing we need to determine here is what type of glue you have. One-component or two-component. Depending on the type of glue you’re using, different factors will affect the cure time.
Temperature is the biggest factor in varying cure time
Temperature will influence the cure time of both one-component and two-component glues. The temperature of the surrounding environment, as well as that of the substrate, will play a huge part in the cure time. If it is too cold, the glue will take a lot longer to cure.
The environment’s temperature should be anywhere between 15ºC – 25ºC, but anything below 5ºC would be too cold. The substrate should be as close to the environment temperature as possible. Keeping to these guidelines will speed up the adhesive cure time.
Here at Forgeway, we regularly come across the situation where adhesive cure time randomly slows. In fact, it’s so common that we can predict when we are going to receive these questions. In the Northern Hemisphere around the beginning of April and the end of October are times when we receive these questions.
There’s a good reason for this. In the Northern Hemisphere around the beginning of April, manufacturing companies will usually turn off the winter heating. This makes sense, it’s warm enough in the day to justify no heating. But, there’s a problem. The temperature at night is still cold enough to have a significant impact on the curing time of the glue.
We can say the opposite for the end of October. It’s not quite cold enough to turn on the heating just yet. However, the nights are becoming cold enough to significantly affect the curing process.
Humidity also plays a factor in the cure time
Temperature isn’t the only environmental factor affecting the cure speed. Humidity also has a massive impact on the cure speed of one-component adhesives. Increased humidity equals increased moisture. When there is more moisture, the adhesive cures faster.
Two-component adhesives aren’t as affected by humidity levels. However some one-component adhesives (like MS polymers) require moisture to cure. So ensure there is sufficient humidity if the adhesive required moisture to cure.
Overapplication could be slowing your adhesive cure time
When using one-component adhesives, you should be careful not to use excessive amounts. The larger the volume of adhesive in an area, the slower the cure rate will become. This is because single component products cure from the outside surface inwards with the moisture in the air.
Because two-component adhesives are not as affected by moisture levels, overapplication doesn’t affect their cure times in the same way. However, in view of not causing excess and wastage, we would always recommend staying away from overapplication.
Ultimately, many different factors will affect the cure speed of your adhesive. We haven’t mentioned every factor that might be affecting your cure speed. However, the factors we have mentioned are the most significant and you should look into them first.
However, the single greatest influence on the cure time of your adhesive is temperature. We recommend that you start by looking at this before you look at what else could be affecting it.
What can you do to speed up adhesive cure time?
If I were to hazard a guess, there are two situations you will likely find yourself.
- You have been using a reliable adhesive for a few months. No problems. That is until recently. It has started to take significantly longer to cure which is having a huge impact on the remainder of the manufacturing process.
- You have recently purchased a new adhesive. You purchased that adhesive on the premise that it cured within 20 minutes. Now that you are actually using the product in the production line, it takes more like 40 minutes to cure.
Whatever the situation, it will probably seem like you are stuck. Fear not, there are always ways around this.
In both situations, look at the temperature of the environment around where the adhesive is being applied. In situation one, it’s likely that the temperature of the surrounding environment has changed. In situation two, it’s likely your surrounding environment is cold enough to impact the cure time.
Either way, you likely haven’t thought of the temperature being able to impact the cure time like this. So what can you do?
Natural methods of speeding up adhesive cure
Warm the adhesive. Pre-application, during application and post-application warming of the adhesive is a game-changer.
There is an unwritten rule “for every 8℃ that the ambient temperature increases, the cure time halves.”
We could also say the opposite, for every 8℃ that the ambient temperature decreases, the cure time doubles.
Keep the storage warm
Keep the storage warm, but not too warm. Having the ambient temperature above 40℃ will significantly increase the likelihood of pre polymerisation. 21-23℃ is a good temperature to aim at in order to achieve consistent cure times throughout summer or winter. However, anywhere between 5-25℃ is acceptable for most adhesive systems.
Ensure there are sufficient humidity levels
Make sure there is sufficient humidity and moisture levels in the air if it is a one-component adhesive. Additional moisture may need to be added to achieve consistent curing
Keep it warm post application
Post application is no less important. Ensure the room where the adhesive is curing isn’t significantly colder. Keeping the adhesive warm post-application is much more likely to yield a quicker cure time.
If you have tried these solutions and it still hasn’t increased your cure time to the desired length, don’t worry. There are more potential solutions.
As mentioned earlier, with one component adhesives over-application will significantly slow the cure time. Check the production team aren’t over-applying the adhesive.
Unconventional methods of improving adhesive cure speed
If none of these more ‘natural’ methods have worked, that’s when you should look towards the more unconventional methods.
Banned methods of increasing cure time
Most catalysts and hardeners have long since been banned. Their levels of toxicity have seen them become recognised as too dangerous to use. But command cure has been rising in popularity in recent years.
Command cure is often the last resort to increase the speed of an adhesive. It subjects the adhesive to either an increase in temperature or UV rays. This then allows you to control the speed of the adhesive cure time. It is only suitable for use with certain types of adhesive though. Epoxy and some acrylic-based adhesives can be command cured.
Command cure also isn’t suitable for every type of substrate either. If you cannot heat the substrate or it cannot pass UV rays through, command cure will not work.
What if the adhesive just has a slow cure time?
After all that, you still aren’t at your desired cure time. Now is the time to question whether you have the right adhesive for the job.
There are two options at this stage.
- Keep the adhesive with the same chemistry but change to a faster version.
- Change the adhesive completely.
Option one isn’t likely to cause too much disruption to the production line. You already know the adhesive works with the substrates, you just want a quicker time. Sure there may be other side effects, such as becoming more brittle, or a decrease in durability, but this option will definitely bond and it will have a quicker cure time. Reach out to your adhesive manufacturer and they should be able to assist with this option.
Option two is where it becomes slightly more difficult. Changing adhesives is never an easy process. We know that. But needs must and that adhesive cure time isn’t just magically going to speed up by itself. So what are the adhesive options?
Which adhesive could suit your needs?
If we are talking solely about the quickest cure time, then two-component adhesives would likely be the best option. The two most common two-component adhesives are acrylic-based and epoxy-based.
Acrylic-based adhesives are much less susceptible to variations in temperature. Their cure times are much less likely to change when there is a decrease in ambient temperature. Acrylic-based adhesives will likely take around 24 hours to reach maximum strength. There are a range of times the adhesive could take to reach a strength that most people would consider as ‘handling strength’. However, we would say anywhere between 15 minutes to 4 hours depending on the classification you choose. Methyl methacrylates are an acrylis-based adhesive that can have a cure time as quick as 15 minutes. Methyl methacrylates will offer the quickest cure time out of the structural adhesives.
On the other hand, epoxy-based adhesives are the complete opposite. Ambient temperature has a significant impact on the cure time for epoxy-based adhesives. But epoxy-based adhesives are the same as their acrylic-based alternatives in that they also take one – 24 hours to reach full strength.
After one hour, they also would reach a strength widely considered as ‘handling strength’. Some epoxy-based adhesives have the capability to cure even faster than the times mentioned above. These adhesives can have decreased durability and end-strength though.
So in that regard, whether epoxy or acrylic-based, the cure times are very similar. There are of course other types of glue. But the two mentioned above reach a higher strength in a much shorter time than most other adhesives. And because the cure time of epoxy-based adhesive is more susceptible to temperature change, you will likely be able to make the cure time of epoxy-based adhesive more favourable by changing the temperature.
We should note that this is just a guide. Cure times will change depending on the different specifications.
You must also consider the other factors of your bond too. Will the adhesive be able to bond to a low surface energy plastic substrate (if you are using that type of substrate)? What if the new adhesive isn’t as durable as the previous option? What about price?
- Are all plastics bondable? And why surface energy affects adhesion
- What to consider when analysing adhesive cost
You must consider all these factors – and more – when choosing a new adhesive. It’s not a simple process. But once complete, and you have determined what you need, you will reap the rewards of your hard work. Oh, and you will (hopefully) have an adhesive with a quicker cure time.
Poor Adhesive Vs Poor Environment
There are always options to speed up the adhesive cure time. You won’t have to be stuck watching glue dry any longer. Ultimately the solution comes down to what works for you.
If over 30 years of experience has taught us anything here at Forgeway though, it would be that nine times out of 10 the adhesive cure time is affected by temperature.
As discussed, there are other ways to increase adhesive cure time. However, there is an old wives’ tale in the industry. Whilst it may not be the most scientific expression, it has stood the test of time and can still bear true today.
“The shorter the cure time, the less durable that adhesive is going to be.”
Hopefully this article has given you an overview of why your adhesive might be taking so long to cure. If we were to take a guess, it will either be a poor adhesive, or the surrounding environment isn’t consistent.
You may just want to ask an expert what is going on with your adhesive cure time. There may of course be other factors that we haven’t covered here. The technical expert will be able to walk through your specific problem and help come to the best solution.
If you want to find out more about the technicalities of adhesive cure time, there is more you can read. Chapter 11 and 12 of the ‘Fundamentals of Adhesives’ book covers this topic in even more detail.