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The Theory: Why Does Paper React to Moisture?
The Hygrometry Concept
If you are lucky enough to have wooden doors in your home, you know that when it is humid, they tend to swell and sometimes rub on their frame. On the other hand, during the particularly dry Québec winters, a gap can appear on their outline. The wood is then said to be “working.”
Paper is said to be a hygroscopic material, which means that it absorbs moisture from the air and is therefore sensitive to moisture variations. This constant exchange of moisture with the ambient air works both ways: the paper is either absorbing or desorbing.
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Paper is a material that is mainly made of wood cellulose fibres. There are two types of pulp:
- Chemical pulp, which uses the long fibres of the spruce and is obtained by boiling wood chips;
- Mechanical pulp, which is produced by grinding short fibres from hardwoods, such as birch. Long fibres from chemical pulp are often added to strengthen the paper.
The papermaking trade consists of working the pulp, beating it and then refining it by adding various components to give it its quality and strength. These components are:
- The glue that was formerly a mixture of alum and rosin (pine resin) and is now starch-based. It makes the paper more resistant to water and tearing;
- Minerals, such as calcium carbonate, kaolin or sometimes synthetic latex, are used to obtain a smooth appearance for coated papers;
- Then, if necessary, pigments are added to obtain a better opacity or to colour the paper, as is the case with cream-coloured paper, or brighteners that will increase the whiteness of the paper.
The Evolution of Paper Quality
For the past twenty years, paper manufacturers have been facing decreased volumes and struggling to increase their prices, therefore, they have been forced to find save elsewhere: on energy, labour and the components used.
They have gradually replaced the kaolin used in the composition of coated paper with calcium carbonate, which is much cheaper, but whose properties have degraded the mechanical behaviour of paper for temperature variations. This problem is well known to web offset printers who use ovens to dry the ink and extract water from the paper. The paper will tend to “curl” more when it comes off the press.
Clients often believe that this is due to their printer using poor quality paper, or that their machine is badly set up, which is absolutely incorrect. Hundreds of tests have been carried out with various engineers and laboratories, and the phenomenon cannot be countered. Quality has its price.
A calendering treatment can be applied at the end of the line to give the paper a satin or even a glossy appearance.
The Paper’s Reaction to Changes in Humidity
You may have noticed that paper buckles mostly during the humid summer months or during the warmer months of March and April when the temperature rises above zero. If the paper’s humidity is different from that of the environment, it will tend to “curl” for a few days. If the air in your office is particularly dry, you will see the same phenomenon in the middle of winter.
A book will even tend to retain this moisture gap between the outside and the inside longer than a ream of paper for two reasons:
- The laminated cover prevents moisture from penetrating the paper from the top and the bottom of the book;
- The glue on the spine of the book blocks the passage of moisture and as with all thermodynamic exchange phenomena, the brakes are moisture traps.
In Practice: How Does Paper React to Its Environment?
Arrival in the Printing Workshop
It is generally accepted in the industry that the relative humidity level in the workshop has to be between 35 and 55%. This is the case at Rapido.
Influence of the Seasons and the Winds
As you can see, temperature and humidity exchanges between the inside and the outside are ever-present for buildings but also for paper. This is the law of thermodynamics.
You may not have known it, but Montréal is one of the cities in the world that uses the most salt to de-ice its roads and streets, which produces, even when it is very cold, an increase in the air humidity level during snowy periods. We have to live and work with this additional parameter.
The books’ humidity level will, of course, change during transport. This is what happens when your books leave our workshop to be delivered to your home or to a distributor:
- Books that are well packed in cardboard boxes are protected from shocks but are not really protected from humidity variations because cardboard is also hydrophilic;
- If they spend a night in the back of a truck where it can freeze or in a poorly heated logistics centre, the paper loses its moisture. If it rains, the opposite happens;
- Once on your desk, where the humidity level is likely to be lower than outside, the paper will tend to fall to around 30% relative humidity after a few hours. If your premises are heated by electric heaters, the air is likely to be very dry. This is when you may see buckling on the edges of your books.
Conclusion: What to Do With Books Whose Paper Buckles?
If this happens to you, what should you do? Wait a day or two for the moisture exchange to become more evenly distributed within the book! And if the weather gets better, the buckling will probably disappear even faster. You can say to yourself that you have done the experiment that shows that paper is a hygroscopic material.
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