Water absorbency, capacity and rate Water absorbency is a measure of the amount of water absorbed by the wetted surface of paper and board materials. Cobb test (Sized papers and paperboards) The Cobb test determines the amount of water absorbed into the surface by a sized (non-bibulous) paper, paperboard, and corrugated fibreboard paper or paperboard sample in a set period of time, usually 60 or 180 seconds (Cobb60 or Cobb180). Water absorbency is quoted in g/m2. The water absorbency of a material can have strong influence on printability and the setting rate of water based adhesives. Klemm method (Unsized papers and paperboards) For unsized papers such as blotting papers and other papers having relatively high water absorbency the Klemm method for determining water absorbency is used. Test pieces are immersed to a set depth in water and the height of the water rise is measure after a set time. Smithers Pira test in accordance with BS 6504. Cage method (Tissue paper and tissue products) The water absorbency time and water absorbance capacity of tissue paper and tissue products is determined in accordance with DD ENV 12625-8. Water absorbency rate This test determines the liquid sorption rate of bibulous paper, paperboard, and paper products using gravimetric principles. A set volume of water is dropped onto the paper surface and the time for the drop to be absorbed is measured. The water absorbency rate is used as a measure of the acceptability of tissue, towelling and blotter papers in sorptive tasks. Other test liquids such as ink, oil and milk are used to assess the comparative performance of papers for specific applications. Wet expansion Wet expansion, also known as hydroexpansion, is determined as the percentage change in length of a test piece when soaked in water. Smithers Pira testing is made in accordance with British Standard BS5879.

Water Absorbency

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Water absorbency, capacity and rateWater absorbency is a measure of the amount of water absorbed by the wetted surface of paper and board materials.

Cobb test (Sized papers and paperboards)

The Cobb test determines the amount of water absorbed into the surface by a sized (non-bibulous) paper, paperboard, and corrugated fibreboard paper or paperboard sample in a set period of time, usually 60 or 180 seconds (Cobb60 or Cobb180). Water absorbency is quoted in g/m2.

The water absorbency of a material can have strong influence on printability and the setting rate of water based adhesives.

Klemm method (Unsized papers and paperboards)

For unsized papers such as blotting papers and other papers having relatively high water absorbency the Klemm method for determining water absorbency is used. Test pieces are immersed to a set depth in water and the height of the water rise is measure after a set time. Smithers Pira test in accordance with BS 6504.

Cage method (Tissue paper and tissue products)

The water absorbency time and water absorbance capacity of tissue paper and tissue products is determined in accordance with DD ENV 12625-8.

Water absorbency rate

This test determines the liquid sorption rate of bibulous paper, paperboard, and paper products using gravimetric principles. A set volume of water is dropped onto the paper surface and the time for the drop to be absorbed is measured. The water absorbency rate is used as a measure of the acceptability of tissue, towelling and blotter papers in sorptive tasks.

Other test liquids such as ink, oil and milk are used to assess the comparative performance of papers for specific applications.

Wet expansionWet expansion, also known as hydroexpansion, is determined as the percentage change in length of a test piece when soaked in water. Smithers Pira testing is made in accordance with British Standard BS5879.

Differences in wet expansion across and along the length of a sheet are the result of the alignment of the paper fibres in the grain direction. The wet expansion properties of paper are critical in applications such as books and outdoor posters.

Moisture content - oven drying methodThe 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.

Smithers Pira determines the moisture content of paper and paperboard 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.

Building Products Bending strength Capillary absorption test

Density

Drying shrinkage

Flexural strength

Water absorption

Moisture movement

Reaction to fire

Voids

Thermal properties

Vapour permeability

Bond strength

Compressive strength