TEARING STRENGTH

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It is essential that the folding endurance test is carried out in a constant humidity
room, since this test is very susceptible to change in atmospheric conditions.
A folding endurance carried out at 70 per cent relative humidity may be double
the value obtained on the same sample conditioned and tested at 60 per cent
relative humidity.

Tearing Strength.—It has been claimed that the tearing strength is a better
criterion of paper strength value for practical purposes than either the tensile
or bursting strength. A popular instrument for the measurement of tearing
strength is the Marx-Elmendorf (Fig. 146). A specimen of paper is accurately cut
to a template in such a manner that two parallel slits form a centre tongue, the
equivalent of the start of a double tear. The outer tongues produced by this
cutting are held in a fixed clamp, while the centre tongue is subjected to the
load of a pendulum to continue the tear.

The load necessarv to continue the tear is
/

recorded through a spring-loaded friction
pointer on the scale forming the pendulum.
Thus the instrument records the work
done in continuing a tear in a given
length of paper, and not the resistance of
the paper to the starting of a tear. The
magnitude of the tearing force as deter-
mined above is very small compared with
the breaking load under tensile stress for
the same sheet of paper. Like the tensile
strength, it is possible to determine the rela-
tive tearing strength in both 'machine' and
'cross' directions, a factor offering some
indication of fibre orientation in the sheet.

Thickness.—The thickness or 'bulk' of paper is measured by small instru-
ments known as micrometers (Fig. 147). These are very delicate instruments
graduated to thousandths of an inch, and they work either by spring pressure,
which is always constant, or by means of a fine screw, the head of which is
fitted with a friction arrangement, so that when the face of the screw presses
the paper against the fixed face the screw cannot be turned further, and so the
paper is not squeezed.

The spring micrometers have a dial, divided into thousandths of an inch,
and a pointer shows the thickness of the paper under test.

It is usual to place four or more pieces of paper in the tester at a
time, for in this way the ever-present irregularities in the surface of
the paper are taken into account, and it is obvious that this is necessary

[Renott Marsh
FIG. 146.—EIMENDORF TEASING TESTER