Dissect the paper to examine the water content and pH of the paper

The basic raw material for paper is plant fibers, which are generally subdivided into wood and non-wood. Most of the paper is made of wood fibers. This is due to the fact that all parts of the wood have a lot of fiber, and its cost is also relatively cheap. However, in addition to wood, there are also papers that are made from non-wood fibers such as bagasse, reeds, and rags. Different raw materials can affect the paper's chemical properties. The following will discuss about the moisture content and acidity of paper.

Water content of paper

The water content of paper is usually expressed as a percentage of the weight of the water contained in the paper and the weight of the paper. The moisture content of general printing paper is about 4 to 8%. Based on this ratio, about 40 to 80 kilograms of weight is water in one ton of paper.

How much water in the paper is water is a very important issue. Regardless of whether the water content of the paper is too much or too little, the effect on printing will have some influence. With too little moisture, the paper tends to become brittle, and it is very easy to generate excessive static electricity on the printing press. However, too much water on the paper is not a good thing because it will make the printing ink less likely to dry. However, relatively speaking, the amount of water, the impact on printing is still small, it has a greater impact on the paper itself, paper tension, flexibility, bending degree, etc., will change due to the change in water content .

As mentioned earlier, the basic raw material for papermaking is plant fiber. Plant fibers are hydrophilic and are hydrophilic substances. All of the original hydrophilic substances have a characteristic that they should be combined with water as a medium. That is to say, pressure alone to squeeze the fibers of the paper, and lose water to reconcile, the fibers can not be tied together, resulting in paper. However, this does not mean that the more water there is, the tighter the fiber will be. The so-called polarity of the object, the strength of the bond between the fibers, and the amount of water will be a bell curve. Assume that the water increases to a certain amount. The structure will become loose, the fiber's bond strength will begin to decline, affecting the paper's tensile strength.

Conversely, too little water will affect the paper. Because the water content is too little, the paper becomes too rigid, and therefore it is brittle and easily slips between fibers, which greatly reduces the flexibility and folding resistance of the paper. Therefore, for the paper to exert its best performance, it is necessary to properly control the moisture content of the paper.

However, we also need to take into account the humidity in the air. When the fiber in the paper is exposed to the air, it naturally absorbs the moisture in the air, how fast it absorbs, depending on the humidity and temperature at that time; if the air is very dry at the time, then paper in the water, it will in turn lost in the air. Whether paper absorbs moisture or water is lost depends on the relative humidity of paper and air. If the moisture content of the paper is equal to the moisture content in the air, it will reach the equilibrium moisture level, and the structure of the paper will not change due to the humidity of the air. Different papers have correspondingly balanced moisture under different relative humidity, and even under the same humidity, different papers will have different balanced moisture. The more hydrophilic the paper, the higher the paper's balance of water, and vice versa.

The method of determining moisture is not complicated. Simply explain that the sample to be tested is placed in an oven at 100-105°C for [constant weight]. The difference in weight is less than two ten-thousandths, and then calculated according to the following formula:


X : moisture content in sample (%)

M1: mass of sample before drying (g)

M2: mass of sample after drying (g)

pH of paper

The pH is expressed in terms of pH. When the pH is 7, it means that the pH of the paper is neutral. If the pH is lower than 7, it means that the paper is acid. If it is higher than 7, it is alkaline. Of this large number of values, the best of course is the neutral 7, but in fact, there is not much paper with a pH of just 7. The pH of many non-coated papers is between 4.5 and 7.0, with weak acidity; and the acidity of smeared paper is between 7 and 9, with weak alkalinity.

However, even if the pH of the paper is measured, it does not mean that the paper is always at this pH, and the pH will change due to the surrounding environment.  The air contains carbon dioxide. This oxygen will chemically react with water in the paper, causing the pH to gradually decrease and produce weak acidity. Alkaline paper may become gradually acidic as a result, and the acid becomes more acidic.

Why do we have to know the pH of the paper? Because of the pH of the paper, it will directly affect the life of the paper. A piece of paper, the lower its pH, the paper's fading and tension will become worse and worse, it means that this piece of paper will soon die. Under the influence of acidity, the cellulose and hemicellulose of the paper will accelerate decomposition, making the molecular structure of the paper loose. However, increasing the pH does not necessarily increase the lifespan of the paper, because in the over-alkaline environment, the fibers will also accelerate decomposition.

So what would happen if you print on paper with low pH? During printing, the acidity of the paper will act chemically with the desiccant added to the ink, resulting in a substance that is insoluble in oil, causing the drying process of the oxidized conjunctiva to be hindered, slowing down, slowing down the drying speed of the ink; In addition, because the printing lubricating fluid itself has been weakly acidic, while the paper is printing, it continuously exudates acidic substances, which adds to the acidity of the lubricating fluid, and further elongates the drying time, causing the printing product to appear dry and backside. Dirty phenomenon.

Paper with high pH has another problem. Like an overly acidic paper, an overly alkaline paper will decompose the material during printing, but this time the base is disintegrated. These alkaline substances will neutralize the fountain solution and increase the pH of the fountain solution, thus affecting the proper transfer of the ink. What is more, it will reduce the gloss of the printed product and make the ink look less vibrant.

Therefore, as long as we understand the pH of the paper prior to printing, we can use this to adjust the pH of the fountain solution and print it with compatible inks, thus effectively maintaining the quality of the printing.

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