Stretch is the amount of distortion which paper undergoes under tensile stress. Stretchlelongation is usually expressed, as percent stretch to rupture. Stretch can be related to the paper’s ability to conform and maintain conformance to a particular contour, e.g. Copier paper, multicolor offset printing papers, liquids packing cartons base papers etc. It is an important property in sack kraft papers which are used for cement bags etc. Stretch is higher in cross direction than machine direction.
Tearing resistance indicates the behaviour of paper in various end use situations; such as evaluating web runability, controlling the quality of newsprint and characterizing the toughness of packaging papers where the ability to absorb shocks is essential. fibre length and interfibre bonding are both important factors in tearing strength. The fact that longer fibres improve tear strength is well recognised. The explanation is straight forward; longer fibres tend to distribute the stress over a greater area, over more fibres and more bonds, while short fibres allow the stress to be concentrated in a smaller area.
Temperature and Humidity: Conditioning of Paper
Conditioning of paper is also of importance in many printing and converting operations. In addition to the effect of moisture content on physical properties, it also determines the build up of static of the paper sheet subjected to pressure and to friction. The tendency for paper to develop static becomes greater with increasing dryness. Cellulosic fibres are hygroscopic i.e. they are capable of absorbing water from the surrounding atmosphere. The amount of absorbed water depends on the humidity and the temperature of the air in contact with the paper. Hence, changes in temperature and humidity, even slight changes, can often affect the test results. So, it is necessary to maintain standard conditions of humidity and temperature for conditioning.
Thickness or Caliper of paper is measured with a micrometre as the perpendicular distance between two circular, plane, parallel surfaces under a pressure of 1 kg./ CM2. Caliper is a critical measurement of uniformity. Variations in caliper, can affect several basic properties including strength, optical and roll quality. Thickness is important in filling cards, printing papers, condenser paper, saturating papers etc.
The moisture content of paper and paperboard is the quantity of water present and measurable in paper. This will vary according to the environment and the moisture added during manufacturing and conversion processes. Uniform moisture content is critical for paper to be free from distortions such as curl, twist and waviness which cause printers and copiers to jam. The moisture content of paper can vary at any given relative humidity depending on whether the moisture was desorbed (brought into equilibrium from a higher relative humidity) or adsorbed (brought into equilibrium from a lower relative humidity) – the hysteresis effect. The moisture content can be determined by the oven-drying technique. The paper is weighed before and after oven drying at 105°C. Results are quoted as a percentage (%) moisture content of the original or dry sample weight.
Wax Pick No. (Surface Strength)
This indicates the surface strength of the paper. This test is important for all uncoated printing papers.
Wire side and Felt side
Also referred as wire side and top side. The side which is in contact with the paper machine wire during paper manufacture is called the wire side. The other side is top side. Certain properties differ between wire and felt side and it is customary to measure these properties on both the sides. In case of paper to be printed on one side only, best results are obtained by printing on felt side. Postage stamps are printed on wire side and then gummed on felt side, where the smoothness is helpful for attaining an even application.