Tech gyaan on PUR

Sustaining the traction, Henkel with the support of Welbound Worldwide had organised a seminar to impart training about PUR technology. This closed-door workshop in Mumbai was attended by representatives of 15 print firms.

So what is PUR?

A book can be bound in number of ways which includes ethylene vinyl acetate (EVA) hotmelt, polyvinyl acetate (PVA) cold emulsion adhesives, or Smyth sewing. Since the past three years, polyurethane reactive (PUR) has made an impact in the market.

PUR is a synthetic polymer which once cured cannot be softened again due to its thermosetting nature unlike hotmelt adhesives which are thermoplastic polymers that can be heated to melt and cooled to solidify again and again.

PUR, which is solid at room temperature, is liquefied by heat (130oC) for application. When applied it starts cooling and becomes tacky and finally sets to solid. Now, permanent curing starts. In this process PUR undergoes cross linking reaction by absorption of moisture from the atmosphere, book pages and cover. Curing time is longer and can vary from six hours to 48 hours depending on the availability of moisture.

PUR can adhere to a wide range of substrates except polyethylene (PE) and polypropylene (PP). It is flexible and can sustain temperatures as high as 120oC and as low as -40oC. This means a PUR bound book will remain flexible over wide range of temperatures. Having said that, the flexibility of PUR always depends upon the film properties like the quantity of adhesive applied, back preparation, paper to be bound and cover to be used. Also, PUR can sustain the oils and solvents that migrate into the paper during the printing process.

Normally, for a perfect bound book the grain-direction of the paper should be parallel to the spine of the book. But when using PUR the grain direction does not matter. PUR can bind cross grained papers.

PUR bound books provide lay-flat performance even though thicker substrates are used. Again this depends upon various factors like the thickness of glue film and back preparation (notching). Ideally, 0.4-0.5mm thick PUR film should be applied. Also, notching has to about 0.2 mm so that physical stresses are not induced into the spine. Deeper notches lead to physical stresses and thus the book does not lay-flat.

Also, PUR bound books can be recycled easily. In a case study done by Henkel it was found that when a cold glue bound book is recycled 50% adhesive can be filtered out while the other 50% remains in the pulp. In case of hotmelt bound books, 80-90% of adhesive can be filtered. However when a PUR bound book is recycled almost 100% adhesive is filtered. So, the recycled paper pulp is free from adhesive content.

Though PUR provides many advantages, it has several limitations. Viscosity of PUR increases over the time by temperature and humidity so the pot stability of the adhesive is critical and needs to be controlled properly. PUR has low initial tack as compared to EVA-hotmelt. Also, there are health and safety issues associated with adhesive as it releases free isocyanide during the curing process. So, proper handling is very important while using PUR. Quality control is also an issue with PUR systems as for testing the PUR bound book for page pull or page flex, it has to be completely cured which takes a long time.

In India, key printers have adopted PUR technology. These include: Watchtower, Repro India, Lovely Offset, Shree Printwell, Sanat Printers, HT Burda, Thomson Press, Avantika Printers, Pragati Offset, Replika Press, Parksons Graphics and Sahaya Print Services.