Paper Test Definitions

Bending Resistance – mN
Bending resistance measures the stiffness of the paper or board, more commonly applied to board grades. The higher the value, the stiffer the paper. Bend resistance is higher in the machine direction.

Burst – Kpa
Burst measures the internal strength of the paper. Paper is held against a flexible membrane and pressure is applied to the membrane forcing it to expand against the paper. The pressure required to burst the paper is the Burst figure quoted. The higher the figure, the stronger the paper.

Brightness - %
The reflection of paper when measured under a specially calibrated wave of blue light.

CIE Whiteness - (D65/10°)
CIE Whiteness is an international standard for measuring whiteness. It attempts to match the way humans perceive white by measuring the reflectance of the paper with a bias towards the blue region of the spectrum as a blueish white is generally perceived as being whiter than a neutral white. A perfect white surface reflects all the light which shines on it and has a whiteness of 100. Many paper grades have whiteness well above 100 due to the addition of Fluorescent whitening agents. These absorb UV light and re-emit it as blue light.

Cobb 1min. (multisheet) - Mls/sqm
This is a measure of how much water will absorb into the paper in grams per square metre per minute. If you have an 80gsm sheet of paper and it has a cobb of 30gsm, it means that 30gsm of water have absorbed into the paper over a minute. The cobb test is traditionally used to measure how quickly liquids such as ink and glues will absorb into the paper. Cobb values of 50gsm are considered high.

Colour a*
This is a measure of how Green or Red the sheet of paper is. A value of zero is neither green nor red. The more positive a number goes, the more red the paper becomes. The more negative the number goes, the more green the paper becomes.

Colour b*
This is a measure of how Blue or Yellow the sheet of paper is. A value of zero is neither blue nor yellow. The more positive a number goes the more yellow the paper becomes. The more negative the number goes the more blue the paper becomes.

Colour L*
This is an alternative measure of lightness and varies from 100 for perfect white to 0 for absolute black. Copy paper grades can typically be around 93.

Density - G/cm³
Density is calculated by dividing the grammage of the paper by its thickness. A high density sheet (>0.9) will be relatively thin at a given grammage while a low density sheet will be more bulky.

Droop Rigidity – mm
Droop rigidity measures the stiffness of the paper or board, more often applied to lighter weight grades. Droop rigidity is higher in the machine direction. The higher the value, the stiffer the paper.

Fluorescence
Fluorescence measures the amount of fluorescent whitening agent present in the paper. Optical brightening agent’s absorb UV light and re-emits it as visible blue light. Under lighting with a UV component this makes the paper appear bluer and brighter. All high white grades have high levels of optical brightener. Less than 5 fluorescence indicates very little optical brightener is present.

Gloss
Gloss is a measurement of the mirror like reflection of light which gives the paper a glossy appearance. In high gloss papers light is reflected from the paper at the same angle as it approached. This is most apparent in smooth-coated papers. Uncoated papers tend to have a rougher surface and reflect light in all directions.

Grammage - g/m²
Weight per unit area of paper (ie basis weight) in grams per square metre.

IGT - m/s
IGT is a measurement of the surface strength of the paper. A tacky ink is applied to a sample of the paper at an increasing speed. As the speed increases the peeling force applied to the paper also increases and the speed at which the fibres begin to be pulled from the sheet is recorded as the IGT. A high IGT (>300) indicates a strong surface strength suitable for demanding offset applications.

Moisture - %
Refers to the amount of moisture found in a sheet of paper. Average amounts range from 5 to 8%. This figure varies from sheet to sheet since paper will emit or absorb moisture according to the condition of the surrounding atmosphere. Moisture loss is realized in the form of shrinkage, which begins at the edges of the paper and moves across the grain causing the sheet to tighten and curl.

Opacity - %
Opacity is a measure of how easy it is to see through a piece of paper. Fibre properties, refining, the shade of the paper, the grammage, and the amount of filler used, all contribute to the final opacity developed in a sheet of paper. High opacity is important for most printing papers and office papers to ensure that print from the opposite side of the sheet is not visible and that print from a printed sheet below is not visible.

Porosity (Bendtsen or Sheffield) - Sheffield Units
Porosity can be measured by a number of methods including Sheffield and Bendtsen. These measure the rate at which air will pass through a sheet of paper at a set pressure. A high porosity indicates the paper allows the air to travel through relatively easily. In some applications a low porosity may be required, such as if the paper is processed using a vacuum; a low porosity paper forms a better seal against the vacuum.

Roughness (Bendtsen or Sheffield)
Bendtsen and Sheffield Roughness are similar methods used to measure how smooth the surface is. Roughness is a key property that describes the surface texture of the paper. A low roughness paper (smooth) allows finer details to be printed. A high roughness paper gives the paper a tactile quality. The test unit involves the measurement of airflow under a very uniform metal ring that is placed on the paper's surface. If the surface is rough, more air escapes and you obtain a higher airflow. The higher the number the rougher the sheet.

Stretch
Stretch is a measure of the degree to which a paper will stretch before breaking under a tensile force.

TEA
TEA is the Tensile Energy Absorption, ie the amount of work required to break the sheet under tension.

Tear Resistance
Tear measures the amount of energy required to tear a sheet of paper. Low tear papers have a brittle feel and will snap fairly easily once a tear is started. High tear papers are tougher.

Tensile Strength
Tensile strength measures the amount of force required to break a strip of paper. Tensile strength is important where high stresses are applied to the paper during processing.

Thickness - Microns (µ)
The thickness of a sheet of paper measured in micrometres (or microns for short). These are sometimes referred to as "ums". 0ne thousand microns is one millimetre.

Wax Pick - Wax Pick Number
This is a measure of paper surface strength. A melted wax stick is inverted and pressed against a sheet of paper until the wax cools. The wax stick is pulled away from the sheet of paper. If the papers surface fibres are pulled out of the sheet then the paper has failed the test. The higher the wax pick number, the better the paper's surface strength.

Wet Burst
Wet burst refers to the burst strength when the paper has been saturated. It is normally compared to the normal burst strength to indicate the proportion of the sheet’s strength that is lost when it is wet. Some grades have wet strength agents added to retain more of the sheet’s strength when wet.

Wet Burst Retention
Wet burst retention is the ratio of wet burst to normal burst. It indicates the proportion of the sheet’s strength that is lost when it is wet. Some grades have wet strength agents added to retain more of the sheet’s strength when wet.

Wet Tensile Retention
Wet tensile retention is the ratio of the tensile strength of a wet sheet of paper to normal tensile strength. It indicates the proportion of the sheet’s strength that is lost when it is wet. Some grades have wet strength agents added to retain more of the sheet’s strength when wet.