TAPPI T 549_{Coefficients of static and kinetic friction of uncoated writing and printing paper by use of the horizontal plane method}

Test summary

Test standard TAPPI T549 enables the calculation of the COF, µ, between two surfaces. The static, or starting, coefficient of friction, µs, and the kinetic coefficient of friction, µk are determined.

This test standard is applicable uncoated writing and printing paper, where frictional effects between successive sheets may cause double-feeding

Test apparatus

• 
  Sled, of mass 200 g ± 5 g, 63.5 mm (2.5 in.) square, with neoprene cellular rubber backing of between 1.5 mm and 3.0 mm thickness.


• 
  A horizontal test plane surface of a smooth, incompressible material and a width at least 25 mm wider than the sled.


• 
  A connection between the sled and the loadcell which should ideally be rigid for the kinetic friction measurement part to be free from stick-slip.


• 
  Test system, with loadcell, capable of applying a vibration-free increasing force and the capability to measure the force against time.


• 
  Cutting tools to precisely cut the paper test specimens without damaging the edges

Preparation

To test a specific lot of paper product, the sample should be selected according to TAPPI T 400—in the form of a finished ream or a ‘lift’ from a roll.

Precondition in accordance with TAPPI T 402. Note the machine direction and/or the cross (or across) machine direction plus the felt (or top) side.

Prepare and cut test pieces in pairs, to the required sizes: lower: 100 x 215 mm (4 x 8.5 in.); upper: 75 x 130 mm (3 x 5 in.).

Testing of a finished ream:

1. 
   Take a stack of 6 consecutive sheets,cut the specimens, discard the largest from the top sheet.


2. 
   Test the larger test piece from the second sheet against the smaller from the top sheet. Each pair of test pieces follows the same pattern of testing the larger against a smaller from the sheet above. Longer dimension parallel to the relevant machine direction.


3. 
   Discard the left-over smaller sized test piece from the sixth, lowest, sheet.

Testing of a ‘lift’:

1. 
   Take a stack of at least 7 consecutive sheets, discarding the top sheet.


2. 
   Test five pairs of specimens, each cut from the same consecutive sheet.


3. 
   Discard the remaining bottom sheets.

Test method

To test for the static and kinetic coefficients of friction:

1. 
   Place the COF tester on a solid, vibration-free table and zero the force reading.


2. 
   Select the first pair of test pieces and clamp the longer to the horizontal plane.


3. 
   Place the smaller specimen on top of the larger and the sled on top of that, or securely attach the smaller specimen to the sled itself and carefully place on top of the lower test piece.


4. 
   Attach the linkage from the crosshead to the sled, ensuring no relative motion.


5. 
   Start the drive mechanism of the COF tester and record the initial maximum force, FS, required to start the motion.


6. 
   The dwell time (between placing the sled and initiating the motion) should be consistent in all tests.


7. 
   Once sliding commences, the motion should continue at a constant 150 ± 30 mm/min (2.5 ± 0.5 mm/s) for a distance of 130 mm (5 in.).


8. 
   Record the force exerted during the sliding and calculate the average, FK.


9. 
   Stop the motion, discard the samples—only one test per pair is performed—and repeat the test procedure until five valid results have been obtained.

Calculate, for each specimen pair the coefficient of friction from the formula: µ = F/mg, using the appropriate force reading.

The test report should indicate the average coefficient of friction, the standard deviation of the five replications, clearly indicate the machine orientations applicable to the calculations and whether the sample is taken from a ream or a lift