Information provided by Duplo USA Corporation and BC Adhesives
It can be tough for binderies to decide which adhesive to use in their manufacturing process. The industry seems to be very comfortable with ethylene-vinyl acetate (EVA) since it’s been around for so long, but as polyurethane reactive (PUR) becomes more popular, it is important to know the differences between the two.
Each has different qualities that might make it the correct option for different types of books.
PUR adhesive has many advantages and is becoming more common among customers as a requirement to be used on their books. PUR can bind varnished, UV-coated, carton, mylar and other difficult stocks, where EVA adhesive may struggle with certain coatings or substrates. PUR also is a great choice for digital printing because this type of adhesive isn’t affected by ink migration. It is also a great option for tough papers, cross-grained stocks and higher basis weight recycled papers.
Difficult substrates aren’t the only reason to use PUR: It makes for a higher quality book with excellent flexibility and lay-flat qualities, especially for larger books. Page pulls are usually 40 to 60% higher than when using EVA. And, for most applications, less PUR adhesive is needed, which helps decrease the possibility for adhesive to creep out during the nipping process.
Be cautious when opting for the PUR route, because equipment is going to be more costly and require more maintenance. Since PUR cures when it comes into contact with air, the system needs to be sealed every night. That can be done by applying grease to the nozzle and pot. Failure to complete this step could lead to curing throughout the system.
Another concern with PUR is the MDI (diphenylmethane diisocyanate) emitted during the curing process. This can be emitted as vapor or aerosol and can cause allergic reactions to the skin or respiratory issues. MDI levels drop drastically once substrates are joined, so this is mainly a concern during preventive maintenance. There are tools to test the MDI levels in a facility. These levels can be lowered with venting and other techniques. Finally, the curing time for PUR is much longer than for EVA; it is common for binderies to wait a full 24 hours before shipping finished books.
EVA is a great option for more standard operations. The handling of this adhesive, along with maintaining the system, is much easier than with PUR. Binderies can move much faster when using this product due to the fast curing time. EVA has very quick set points that allow for a short cure time, meaning faster turnaround for trimming and the finished piece.
EVA also is a much more cost-effective option for both the equipment and the adhesive itself. Milling is still extremely important when using EVA, but it’s not as much of a factor as when using PUR. Binders using EVA can notch bind their books. This means that small notches are cut into the spine and filled with glue before the cover is applied. This is not recommended for PUR applications. PUR needs the peaks and valleys of the fibers across the entire spine to succeed. Overall, EVA hotmelt glues still have their place in book-binding – especially when binding thread-sewn books – as the thicker glue allows for greater penetration into the spine area of the folded sections.
In summary, both processes are still widely used within print finishing and bindery operations. The question of “which is best?” needs to be replaced with the question “which is most suitable?” The decision to use one over the other is job-specific and involves numerous considerations – from substrate used and thickness of the book, to the intended use of the product and financial constraints to produce.
Which One is Most Suitable
Advantages of PUR adhesives
- Aggressive adhesive for coatings, digitally printed stocks, other tough surfaces
- Isn’t affected by ink migration
- Excellent flexibility and better lay-flat qualities
- Use less adhesive – less likely for adhesive to “creep”
Advantages of EVA adhesives
- Runs on less costly machinery and less maintenance on machinery
- No concerns with the MDI (diphenylmethane diisocyanate) emittance
- Much shorter curing time for faster turnarounds
- Less costly adhesive and best choice for many standard applications