What would paper be like without stiffness?
Paper is used widely in our daily world for many different purposes. One of the most important inherent properties of paper is its uniquely high bending stiffness per unit grammage of the material. I am quite sure that the world would have been a different place if paper had lacked this high bending stiffness. Paper used for printing - so-called cultural paper - could not be handled so simply and rationally in printing presses if paper did not have a high stiffness. A uniform tensile stiffness over the width of the paper web is for instance a condition for good register. During post-treatment in the bindery, the folding machines usually require a certain bending stiffness for good convertibility. Another example is in the copying machines by which we are surrounded. Correct bending stiffness is here necessary for the machine to function properly.
Paper products like corrugated fibreboard boxes are manufactured from materials which have a marked stiffness towards tensile and compression forces. Modern liquid packages, cartons etc are further examples of a paper usage which would be impossible without the considerable bending stiffness of the material.
Even when we read our daily newspaper we hold a product where great attention has been given to the bending stiffness of the paper in the choice of pulp and in the papermaking, for who wants the paper to buckle and droop into the morning coffee? I do not exaggerate if I say that there are more than 1000 different areas of use for paper and that the stiffness of the paper, with few exceptions, is essential in all cases. It is not therefore so strange that many scientists have carefully studied the stiffness of different paper grades. Different measurement methods have also been developed. Some methods are good but many methods unfortunately have such flaws that we can easily make mistakes if we use results obtained by them to estimate the true stiffness of a material.
Stiffness measurement - a confused concept
The task of measuring the different properties of paper and board without objection has kept scientists and instrument makers completely busy for many years. The measurement of tensile and bending stiffness is here no exception. The difficulties have been, on the one hand, to define measurement methods and measurement units and, on the other hand, to interpret the measurement results. Various factors have strongly affected the measurement results and have made it difficult to know what we have really measured. One result of these flaws in the measurement technology has been that wrong and expensive measures have often been adopted. Mistakes have been made and are being made because of lack of measurement technology knowledge and because of deficient measurement methods and also - unfortunately too often - because of direct faults in the measurement equipment used.
Part of the responsibility can be placed on the standardization organizations, but we who daily work on the measurement of the physical properties of paper must accept the main responsibility.
The suppliers of measurement instruments must also accept their responsibility. Better product information and products which measure what they are intended to measure and as far as possible eliminate mistakes made by the operator are a good help.
The climate affects the stiffness properties of the paper
Since paper and board are hygroscopic materials, the stiffness properties are affected particularly by the relative humidity and temperature of the ambient air. It is also important to remember that it is the current moisture content of the sample which affects the stiffness. The moisture content of the material depends on the current climate but it also depends on earlier climatic conditions because of the so-called moisture hysteresis effect. We can reach different moisture contents and thus different stiffnesses even in the standardized climate of 50% RH and +23 C, depending on whether the paper has come to this climate from a dry or a humid climate.
To avoid the problem, it is usually required that all testing shall be done in a standardized climate and that the sample shall first be pre-conditioned in a climate about 20% RH and +23 C. In this way, the test material will, from a moisture content viewpoint, always be tested under the same moisture conditions. The stiffness is changed by about 5 - 10% for each unit percentage change in moisture content. This rule of thumb can be considered to be valid in a climate which yields deviations in moisture content of 4% from the moisture content of the standardized climate.
Since the stiffness is a property which is relatively sensitive to moisture content, the paper to be tested must be treated very carefully. It can for example be disastrous for the measurement result if the sample is held between the fingers during handling. The samples must therefore be handled only outside the active measurement region so that involuntary conditioning does not arise.
As a rule of thumb, the stiffness is changed by about 5 -10% for each percentage deviation in moisture content from the moisture content achieved when the material is conditioned in a standard climate.