Testing                                                                                                                                                    STRENGTH

Strength

General 

Paper is not uniform in structure. Its properties vary from sheet to sheet and also from place to place in the same sheet. To get a true estimate of a particular strength property, a number of tests has to be made. The test pieces should be cut in direction parallel to that of the machine and cross direction of the paper. The determination of strength may relate to (a) tensile strength and stretch (or elongation), (b) breaking length, (c) bursting strength, (d) folding endurance, and (e) tearing resistance.

Determination of Machine Direction

Any one of the following three methods shall be employed for determining the machine direction of the paper.

Method 1 - For the purpose of identification, draw a straight line on the paper under test. Cut out a circular test piece, about 50 mm in diameter, taking care to see that the test piece and the adjacent parts of the paper from which the test piece is cut, carry the line. Float the test piece on water and note the direction of curl. The axis of the curl will be found parallel to the machine direction of the paper.

Method 2 - Burst a test piece in a manner similar to that employed while testing for bursting strength. The chief line of rupture is at right angles to the machine direction of the paper.

Method 3 - Cut two test pieces, 15 x 150 mm each in directions at right angles to each other. Superimpose and hold the pieces together at one end so that the edge is in the horizontal. The free ends of both the pieces will bend over towards the vertical. Observe the angle between the pieces. Place the bottom piece on top and note the new angle. The cross-direction strip is the one which is at the bottom when the angle is greater.

DETERMINATION OF TOP-SIDE AND WIRE-SIDE

Any one of the following two methods shall be employed for determining top­side and wire-side of the paper.

Method 1 - Examine the paper under oblique illumination, if necessary, using a low power lens. The side which shows a mesh like structure is the wire side. If the mesh structure cannot be ascertained in this way, moisten the paper with water or dilute caustic soda solution. This causes the fibres to swell and usually makes the mesh structure more clearly visible.

Method 2 - Cut several test pieces 2 to 3 cm wide and 5 to 10 cm long so that the cross direction of the paper runs with the length of the test piece. Mark the test pieces on the same side. Clamp two of them breadth wise in the middle in a narrow clamp so that when the test pieces are viewed from either side, only one identification mark is visible. Immerse the test pieces in water. Observe the direction in which the test pieces curl in the water. The convex side is the wire side.

Tensile Strength and Stretch (or Elongation)

General - This test is performed to determine the resistance to pull of paper and the percentage elongation the paper undergoes before fracture. Tensile strength is greater in the machine direction than in the cross direction. Elongation is usually less in the machine direction than in the cross direction. Tensile strength and stretch is measured by the tensile pull necessary to break a strip of paper and allied products.

Apparatus - Any apparatus which is capable of acting on the test piece at the defined rate and permitting the tensile force at the moment of failure to be read to an accuracy of 1 percent may be used.

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