When the pages of a book are detached by themselves when opening it 180º or by pulling them, one tends to put the blame on the adhesive. However, it is very important the quality of the paper, especially its absorption.
Why does a binding come off?
A paper with a medium-high absorption does not lead to problems of penetration of the adhesive and almost any adhesive can be used for this type of paper. On the other hand, when the paper shows a poor absorption, high quality adhesives containing highly adhesive resins are required. Moreover, the adhesive should be applied at higher temperature to favour the penetration of the adhesive into the paper fibres.
A low/poor absorption can be due to the use of varnishes, which are normally employed to make the ink brighter and give the paper a better appearance. In this case, it is very important that the varnish is completely dry and penetrated in the paper before the bookbinding process: it should start 48 hours after the varnish application, 24 hours being the lower limit. Otherwise, the varnish will not be dry enough and will not only act as a non-stick material repelling the adhesive but it will also be able to soften it due to a chemical reaction between the varnish and the adhesive components.
How can we know if the paper is suitable for our application?
A quick and easy way to check if the paper shows high or low absorption is to apply an aqueous fluorescent marker (one without alcohol in order to have high surface tension). As it can be seen in Figure 1a, a paper with poor absorption (top) shows an irregular application and even some holes are observed. This effect is not seen when the paper shows good absorption (bottom). If both applications are checked under the microscope (Figure 1b), irregularities and disordered fibers in the paper with low absorption are clearly observed.
How can we measure the absorption of paper?
The absorption of sized paper or cardboard is determined as water absorptiveness according to ISO 535:1991, which is also known as Cobb method. It consists on a gravimetric analysis in which the paper is weighed before and after its exposure to water in a given time. Figure 2 shows how this determination is carried out:
- The weighed paper is placed in the holder and fastened firmly to prevent any leakage
- A specific amount of water, which depends on the internal section the holder (100 mL of water for 100 cm2), is poured inside the holder and kept there a certain time according to the test
- Then the water is removed, the paper is taken out from the holder and the surplus water is eliminated by placing the sample between two sheets of blotting paper with a grammage around 250 g/m2 and moving a hand metal roller (stainless steel, 200 mm long, 90 mm wide, 10 kg) once back and once forward over the pad without exerting any additional pressure on the roller
- The wet paper is reweighed. The results are given in en g/m2 (water absorbed in grams per square meter)
This method is not suitable for porous paper such as newsprint or unseized papers such as blotting paper, and other papers having a relatively high water absorptiveness. It may not be suitable for paper of grammage less than 50 g/m2 or embossed paper.
Figure 3 shows the different water absorptiveness of two pieces of paper right after the above-mentioned procedure. At a glance, it can be seen that the paper on the left shows an irregular and poor absorption.
It should be noted that, just as a poor absorption of the paper causes problems in the correct application of the adhesive, a paper showing an excessive absorption would also be problematic, especially for the adhesive applied on the sides of the book. In this case, the paper would absorb too much adhesive, leaving very few for the correct bonding. The paper used for offset printing usually has a Cobb value between 20 and 35 g/m2. A blotting paper shows values higher than 150 g/m2 while the paper used for packaging would be lower than 20 g/m2.
In summary, we should remember that the bookbinding process involves two parts: the (hotmelt) adhesive and paper. When the pages detach from the spine of the book, one tends to blame the adhesive but often the cause of the problem is the paper. The presence of poorly applied/dried varnishes results in a poor absorption of the adhesive, preventing its correct penetration and therefore its proper performance.