Polyurethane reactive hotmelts are synthetic polymers which are therrmoset in nature. When heated to their application temperature they react with the moisture and bond with the surface. After solidification they cannot become liquid again i.e. they are irreversible as compared to conventional hotmelts.

Advantages of PUR HMA over conventional HMA

  1. Application temperature for PUR ranges from 85–140°c which is lower as compared to conventional HMA that ranges between 160°-180°c. This means that energy is saved, set-up time is less and wrinkling of spine is reduced since less moisture is absorbed from the book.

  2. The unique property that gives PUR hotmelt adhesives their performance advantage is that they cure to thermoset material that resist melting. This curing process is the reaction with moisture found in the air or in the substrate to produce a strong tough temperature resistant adhesive.

  3. PUR HMA possess very high degree of cold crack and heat resistance

  4. PUR HMA provides high degree of flexibility. Therefore one can produce lay flat books with thicker substrates, as well as books in which the grain direction is not parallel to the spine.

  5. PUR is resistant to solvent therefore you can use them with books which have inks or coatings that bleed into the spine

  6. PUR HMA bonding is a chemical reaction and not a purely surface phenomenon therefore it does not rely on fibre content of the substrate. PUR can be used on substrates with high filler content and thicker coatings

Applications of PUR

  1. Moisture is an important component for use of PUR HMA. Therefore maintaining atmospheric conditions around the perfect binder is essential for good binding. Also contrary to conventional HMA, PUR cures faster on humid days.

  2. It is important to know what press was used to print the book forms. If heatset web offset processed printed jobs arrive at the bindery they should be unpacked and left for sometime to acclimatise and retain moisture.

  3. PUR bound books used to have a longer curing time of around 24 hours , so they had to be left exposed to natural air. This has improved greatly with the new generation PURs that can cure under 4 hours, subject to sufficient moisture content during curing.

  4. PUR HMA cannot be left exposed to moisture for longer time. Therefore they are applied through an enclosed system with the help of slot coaters the slots can be opened or closed according to the width of the spine. It can be used to apply PUR on the spine of the book block or cover material.

Henkel has formulated a grade of PUR hotmelt which is in granule form and can be used in a conventional glue tank with a few modifications to the tank and rollers. The glue tank and rollers are Teflon coated for easy cleaning. The glue tank is made smaller since the adhesives are put in to the tank has to be completely consumed and the remnants had to be thrown away. 

By using the system one can get used to PUR technology and application before buying expensive equipment .

Typical application of PUR include museum quality art books, coffee table books and frequently used library reference material. This is because PUR gives stronger binding than traditionally smyth sewn books.

In continents like Asia and Africa, where the temperatures can soar, PUR is a definite insurance against books falling apart. 

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