Is it feasible to heat the hot melt adhesive particles for rework after solidification? Key points for rework of adhesive components

Can the adhesive components be reworked if there is a deviation in their fitting

In the binding and packaging of books and magazines, it is occasional to encounter situations where the cover of the adhesive package deviates from the book core and needs to be reworked. Many people’s first reaction is whether it is possible to open and re stick the hot melt adhesive, but whether it is feasible to heat it again after curing depends on the degree of performance degradation of the adhesive after secondary heating, rather than simply “baking it again”.

Hot melt adhesive particles belong to thermoplastic materials and can theoretically be repeatedly heated and melted – but “being able to rework” and “meeting quality standards after rework” are two different things. Taking the standard EVA type as an example to illustrate typical phenomena (the following values are for illustrative reference, and may vary depending on the type of adhesive, equipment, and process).

What changes will occur during secondary heating (example)

Taking standard EVA hot melt adhesive as an example, heating the same adhesive layer multiple times generally results in the following trends:

• First bondingThe adhesive layer is uniform, without bubbles, and the peel strength is within the normal range.
• Secondary heating reworkAfter the old adhesive layer is heated and melted again, the peel strength usually decreases to a certain extent, and the adhesive layer may show slight yellowing.
• Heating three or more timesThe strength decreases more significantly, the adhesive layer becomes brittle, the amount of carbides increases, and the bonding reliability significantly deteriorates.

Conclusion: Rework with secondary heating is feasible in terms of process, but the quality will deteriorate with the number of heating cycles. It is generally not recommended to rework after three or more cycles. The usual rework method is to heat and soften the spine adhesive layer, then disassemble it, reapply the adhesive, and press it back together.

Key points of rework operation

The secondary heating temperature can be 5-10 ℃ higher than the initial heating temperature to compensate for the decrease in fluidity; After rework, new glue needs to be applied (old glue cannot be used purely); Rework products should be marked for quality tracking purposes. It is generally recommended to control the rework ratio within 5% of the total output to avoid repeated heating affecting batch stability.

Frequently Asked Questions (FAQ)

Q: Can PUR hot melt adhesive particles be reworked?

A: I can’t. PUR is cured by moisture cross-linking and belongs to thermosetting. It will not melt after secondary heating, and can only be reworked by changing the adhesive.

Q: How long can the reworked book last?

A: Secondary reworked products can generally be used normally for 1-2 years in a dry environment at room temperature; But in high temperature and high humidity environments, the lifespan will be significantly shortened, and storage and transportation need to be careful.