There are many misconceptions about paper, its manufacture, its environmental impact and its properties. We sometimes receive comments and questions from our clients about the waviness of the books we print, as if it were something abnormal, evidence of poor-quality paper or even a manufacturing defect.
Although, according to Albert Einstein, it is “easier to smash an atom than a prejudice,” we will try to explain in this article how paper reacts when it encounters variations in humidity, and we will demonstrate how this support, which is mainly composed of vegetable fibres (wood cellulose), is an organic material that remains in constant relationship with its environment, just like wood.

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.”

The same thing happens with paper: it contains mainly wood cellulose fibres, to which certain minerals and chemicals are added to make it more tear-resistant. Like wood, paper is a porous body that contains water, both in gaseous form in its pores and in liquid form in its capillary networks.

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.

Source : Richard Wheeler (Zephyris)
It is during this exchange that structural changes occur. One could say that paper has a great affinity with water and is even in love with it because it is hydrophilic!

Paper Composition

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.
These are very general information, as paper pulp results from complex engineering blends to meet the changing needs of the market.

Source : Salamandre

Source : Salamandre

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.

Source : Paprec

Finally, the paper is cut into sheets or cut into reels and packed. In the first case, the paper has a relative humidity of about 50% when it is packaged, whereas in the reel it is around 40%. To ensure that the paper retains the same humidity level until it is printed, the packaging contains a special liner. You can spill water on a well-packed reel or a ream of paper without affecting its quality. Prolonged bathing is, of course, not recommended.
To finish with the composition of paper, note that the cellulose molecule is also present in cotton, linen and hemp, which are still used for art papers.

To learn more, read our blog post: “Which Paper to Choose When Printing a Colour Photo Book

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.

The reason for this is that moisture affects the edges of the paper first. Since the centre of the sheet remains drier or wetter than the outside, it creates a tension in the sheet and manifests itself in the form of a ripple.
If the paper fibre absorbs more moisture, the diameter of the fibres will increase. A difference of 10% in relative humidity can cause a change of 0.1% in the height direction of the book and 0.2% in the width direction.
The transfer of moisture takes place exclusively at the edges, this is where you will see the deformation.

Source : Rotronic

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:

  1. The laminated cover prevents moisture from penetrating the paper from the top and the bottom of the book;
  2. 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.
As the Québec’s ministère de la Culture website says about paper fibres, “In the presence of moisture, they expand and buckle. When the environment becomes dry, they contract and shrink. This phenomenon can cause tension in the paper, especially when these changes are sudden, repeated and of large amplitude.”
The same site adds, “In North America, and particularly in Canada, it is extremely difficult to maintain a relative humidity of 50% in winter without creating condensation.”

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.

Packing the reams of paper limits the impact of humidity on the paper, especially during the transport from the mill to our workshop. We then have to wait for about 24 hours before printing so that the paper’s temperature gradually becomes the same as the workshop’s. When it is -20 °C outside, it obviously takes a little longer. We are used to manage these parameters.
The paper is then taken out of its packaging a few hours before use according to well-established rules, which alone guarantee both press productivity and print quality.
If the level were to fall below 40%, it would change the shape (buckling), the size and also the physical properties of the paper, such as the electrostatic charge that can cause the sheets to stick together. For this reason, the humidity level in all the printing plants in Montréal is controlled and regulated.

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.

In Eastern Canada, the air is less humid during the winter. At this time of year, the prevailing westerly and northwesterly winds bring us drier continental air. Precipitations are also less frequent.

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 weather in March, April and November is particularly unstable. The weather alternates between cold periods and milder ones. With negative temperatures at night and positive temperatures during the day, the variations in temperature lead to sudden changes in the air humidity. It is usually during this period that we receive the most messages from our clients that say that the book edges are buckling.

Source : Radio Canada

Books Transport

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?

Buckled paper is not a problem that stems from the quality of paper, nor is it the result of misuse by your favourite printer. As frustrating as this answer is, you will have realized that it is a natural phenomenon, intrinsic to the organic nature of paper. It is a fact of life.

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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