R-D > PAPERTERMINOLOGY

PAPERTERMINOLOGY

PHYSICAL PROPERTIES OF PAPER
Either in manufacturing or during the printing, there are some physical properties that the paper manufacturers and paper users have to know or consider. Physical properties such as weight in grams, thickness, humidity, Cobb value, resistance, surface smoothness and porosity values, optical properties, surface pH value, water way of paper are very important for the compatibility of paper – ink – printing machine at printing stage.
WEIGHT IN GRAMS
Weight in grams is the weight of 1 m2 of the paper in terms of grams. Some physical properties of the paper differ according to the weight in gram. Therefore, in order to prevent it to influence the printing and usage quality, it is required to check in direction of the machine width and machine size during manufacturing. The weight in grams is measured with accurate electronic scales at laboratories.
THICKNESS
It is the result of the measurement in terms of micron of the distance between the lower and upper surface of a single paper. The thickness of the paper has to be homogeneous both on the same layer and consecutive layers. Because, the differences that may occur between the thickness values may cause wrinkles during the printing and different taking of ink and consequently tone differences. The thickness value is also important for blanket adjustment and for the adjustment of the pressure of the ink to be given.
HUMIDITY
It is the water amount that the paper keeps within its structure. The highness or lowness of the paper humidity causes negative results in terms of quality. As the humidity of the paper lowers down, the paper becomes breakable. The highness of the paper humidity influences the printing quality as it will delay the physical and chemical drying of the ink on the paper.
For the paper, the humidity value in storage conditions is as important as the humidity when coming out from the manufacturing. As the paper is hydrophilic, in other words in a structure liking water, it is inclined to take humidity when it enters into an atmosphere with a humidity higher than itself and to give humidity in opposite situation. If this balance is not available, then there is a humidity exchange between the paper and the atmosphere until the humidity is balanced and this situation influences the smooth posture in paper and may cause ondulation and curling problems influencing the printability and processing of the paper. The most ideal relative humidity under storing conditions is 55%.
COBB VALUE
It is the measurement of the resistance of the surface of paper against water or water acceptance. It is the amount of water absorbed by paper surface of 1 m2 within a certain time in terms of g/m2 and this time is 120 sec on world standard label and cardboard type papers and 60 sec on offset, photocopy and glossy papers.
The Cobb value is required to be kept at an optimum range. In case the Cobb value is low or high, then it significantly influences the printing quality of the paper during the contact of the ink to the paper. In offset printing technique, the water used with the ink contacts to the paper surface through printing rubber during the printing. If this used water can not be received by the paper surface in a controlled manner, then this would cause various printing problems.
RESISTANCE
Resistance is the highest breaking resistance per unit width that the paper resists before breaking. It is controlled in two directions as machine direction (length) and machine reverse direction (width). The resistance after printing is as important as the resistance before the printing. Minimum 28% of increase is expected in the resistance value after four-color printing. If this increase is not observed, then it means that a paper-ink imbalance presents. This increase in the resistance value can increase up to 100% levels.
SURFACE SMOOTHNESS
The surface smoothness is known as finish in the printing industry. As the finish improves, the requirement for required printing ink for sufficient covering decreases and the printing quality and the printing brightness increases. The surface smoothness can be measured with two different methods. In Bendtsen Method, the air speed passing between the paper layer and a smooth surface is measured. Ml/min is used as unit and it is understood that is smoother as the numerical value decreases. In measurement with Bekk Method, the passage time of the air between the paper layer and the smooth surface is measured. Sec is used as the unit and it is understood that the surface is smoother as the numerical value increases.
There are optimum surface smoothness values according to the type and usage area of the paper. For example in photocopy papers, very high surface smoothness is not requested as it causes problems during copying. The surface smoothness is requested to be good in glossy papers.
POROSITY
It is the average amount of air flow passing from a unit area under unit pressure difference at a unit time. The air permeability is expressed as ml/min. It is understood that the air permeability of the paper is high as the porosity value increases numerically. Porosity value also has to be at optimum level according to the type and usage area of the paper. As the highness of the porosity value in printing papers causes problems for the ink to penetrate to the paper, it negatively influences the printing quality.

OPTICAL PROPERTIES:

a- WHITENESS
Whiteness is defined as the reflection rate of the light reflecting from a substance on a substance fully reflecting the light. It is a value measured at R457 wavelength. Whiteness value may differ numerically depending on whether the UV filter of the measuring device is opened or closed. For example, the whiteness value measured as 86.0 as result of the measurement performed with device with closed UV filter can be measured as 93.0 with the same device when UV filter is opened.
b- CIE
The values obtained under D65 sun light source corresponds to the visible whiteness of the paper containing or not containing fluorescent whitening substances. The numerical increase in the value is expressed as a visible increase of whiteness on the paper.
c- OPACITY
It is the light permeability of the paper. It is used for the determination of the displaying degree of a paper of the same type of papers under it and it is expressed as percentage. It is a very important property for printing papers and envelope papers. It is mostly in the first place in low weight papers. As the thinness of the paper increases the visibility risk of printing on the back side, the opacity has to be high.
SURFACE pH value
pH value is the measurement of the acid or base concentration in water. It can be measured with pH meter or turnsole paper. It is valued between 1 – 14 numerically. pH value to be less than 7 shows that the acid concentration of the water is high and to be 7 shows that the water is neutral.
It is required that the pH values of the ink to be selected and the paper surface to be printed to comply with each other. The manufacturing environment of the paper and whether the ink to be used is acidic or basic must be compared. During the printing, it is required to keep the pH value of the container water under control to not experience the problems such as the ink to cause decay, to not see tones in the printing and drying problems.
WATER WAY
The order of the fibers constituting the paper within the lamina constitutes the water way of the paper. During the manufacturing process, the flow direction of the paper in the paper machine is called Water Way and the direction in the machine width is called Water Way Reverse. As the changes in physical properties of the paper such as even posture, resistance and tearing are mostly observed in the direction of the water way which is the arrangement direction of the fibers, Water Way concept is a property that has to be considered in issues such as pilyaj, curling etc.
As a general rule in our country, the second number in paper dimension shows the water way. For example, the water way of a paper with a dimension of 64*90 is parallel to the edge having a length of 90 cm. The water way of a paper with a dimension of 100*70 is parallel to the edge having a length of 70 cm.