Introduction

We often hear from packaging technologists who are holding their products in storage for up to 6 months to test the fibre tear that the adhesive provides. They ask us if that is necessary, how long it should take a hot melt sealed pack to have reached its full bond strength, and what can have an impact on this?

In this technical whitepaper, with the support of data from our testing, we’ll go through exactly how long it takes a hot melt bond to achieve its maximum strength. We’ll also discuss a few things that can affect the time, and what you can do to speed up the process.

The 3 Phases of a Hot Melt

To look at this in depth it is necessary to examine the 3 phases of a conventional hot melt bond forming.

1. Liquid phase. This is the initial stage of the bond and where the hot melt adhesive is at its hottest to enable it to be jetted out of the nozzle cleanly and then wet out onto the substrate. This is the upper grey band in the diagram below.
2. Plastic phase. As soon as the adhesive leaves the nozzle it starts cooling down. Immediately on contact with the first substrate, it is necessary to bring the second substrate together to wet out on both surfaces and then hold them in compression throughout this phase (shown by the green band). This is the most vulnerable part of the bond formation as any movement of the two substrates relative to each other during this time can cause failed or weak bonds. The time necessary to be held in compression from the substrates being brought together is the set speed and this will vary depending on the adhesive grade used.
3. Solid phase. The temperature has been dropping steadily throughout the plastic phase and with this, the viscosity has been increasing exponentially until the adhesive has solidified enough to enable the bond to hold the two substrates together as the product being bonded emerges from compression. This is the solid phase and without mishap, will last throughout the lifecycle of the product.

On a modern packaging line, the whole process is complete in seconds and even with slow setting grades, 95% of the final bond strength will be attained in under 2 minutes with the remaining 5% coming over the next 24 hours.

How Long Does It Take a Hot Melt to Reach Full Bond Strength?

To measure the exact amount of time it takes for the glue to reach a full bond strength, we tested the levels of fibre tear shown when bonds are pulled after increasing lengths of time. The more fibre tear seen, the stronger the bond.

We bonded up substrates and pulled them after 10 seconds, 30 seconds, 45 seconds, 1 minute, 5 minutes, 15 minutes, 1 hour, 6 hours, 1 day, 7 days and 14 days. The table and graph below show the results we saw from this test:

Time before pulled	Level of fibre tear
10 seconds	0%
30 seconds	5%
45 seconds	20%
1 minute	50%
5 minutes	75%
15 minutes	100%
1 hour	100%
6 hours	100%
1 day	100%
7 days	100%
14 days	100%

 

Overview of Results

From our testing, it’s clear that the strength of the bond takes time to reach its maximum, with the 15-minute mark a good benchmark to see full fibre tear. So, if you’re wondering why your packages are showing little to no fibre tear if you’re testing them right after coming off the line, you may want to consider waiting 15 minutes to an hour to see a real indication of the adhesive performance. However, if you’re also waiting a long time to test the results, you may consider lowering this time.

What Can Impact This Amount of Time and How Can You Reduce That Impact?

A number of factors can have an impact on the amount of time a hot melt takes to reach its maximum strength as follows:

• The set speed of the adhesive you are using – if you are using a hot melt that is not the most suitable for your production line, it can impact and delay the time taken to reach maximum bond strength. It may be worth a review of your adhesives if you are having issues with bond strength being reached too slowly.

• The climatic conditions that your product is stored in – if you are packing off a hot product such as tomato ketchup, this retains its heat for a long time and therefore can slow down the setting of the glue, potentially causing it to take longer to reach full adhesion.

• Applying too much glue – the more glue you apply, the slower it will set. Whilst this will only impact the initial setting speed, it may mean that you reach full fibre tear slightly longer if more glue is applied. Your adhesive provider can carry out a health check on your line to check that the amount of glue being applied is correct, as well as providing other benefits too.

• The substrate you are applying glue onto – if you apply onto a product which has excellent thermal properties, the glue will retain its heat for longer and therefore take more time to reach full fibre tear. This can be tested in the lab of your supplier to ensure the adhesive you are using is relevant to the material you are bonding.

Summary

So, to answer the question of ‘how long does a hot melt adhesive take to reach maximum bond strength?’, we can see from our testing that the hot melt has reached full bond strength after just 15 minutes and there is little benefit in holding the products in storage beyond 6 hours.

If you are seeing any issues with the bond strength of your hot melt glue and want a second opinion, call in the experts by emailing marketing@alphabond.com, or just book straight into our diary through the booking form below, and we’ll offer a 30 minute free consultation to help fix your problem.