PUR adhesive has solved a lot of problems in the bindery. The standard EVA hot-melt adhesive used for most soft-cover binding ran into it’s technological limits some years ago. The photo book segment wanted better “lay open” for photo album books. Digitally-printed stocks contained silicone-based oils which would not allow hot-melts to gain a hold. Many customers wanted a replacement for book sewing that was less costly.
PUR (polyurethane reactive) adhesive solved most of these problems because of it’s curing process. PUR reacts with moisture in the air and paper, and forms a strong chemical bond that is a permanent “thermoset” once cured. This strength works particularly well for oblong- or landscape-format books with a high clay-coated content. Page-pull strength is usually superior to books bound with hot melt, and useful book life is extended. A critical item when producing a product with a lifetime warranty.
But, for all it’s technical prowess, successful PUR binding requires more of a learning curve than hot-melt. PUR can be applied with both a glue pot, and with extrusion-head, or nozzle systems. Glue pots can be tricky, since PUR begins drawing moisture as soon as it’s exposed to air. Glue pots used for PUR are typically teflon-coated, and need a thorough cleanup after each use. Temperature of both the pre-melting system, and the glue pot must be carefully monitored. Extrusion systems are far more expensive, but they can apply a carefully calibrated bead of the adhesive to the book block. Their other advantage is that they are sealed systems. Once the job is done, a “plug,” or cap material can be applied to the extrusion head, or nozzle to keep air out and the adhesive usable.
There are lots of additional techniques that can be used to enhance the PUR process. Good spine preparation is vital. Some binders use an emulsion coating station (prior to PUR application) to apply a water-loaded coating to the book block. This gives the PUR more water to react with. One new technique is a plasma-discharge station (similar to the electrical corona discharge used to treat many flexible films). This is thought to change the surface tension and polarity of the book block, enhancing the bind strength.