4.1 When a drop of liquid rests on the
surface of a solid, and a gas is in contact with both, the forces
acting at the interfaces must balance. These forces can be
represented by surface energies acting in the direction of the
surfaces and it follows that:
4.1.1 The right side of the above
equation (the difference between the surface energies of the
gas - solid and solid - liquid interfaces) is defined as the
wetting tension of the solid surface. It is not a fundamental
property of the surface but depends on interaction between the
solid and a particular environment.
4.1.2 When the gas is air saturated with
vapors of the liquid, γGL will be the surface tension of the
liquid. If the angle of contact is 0° the liquid is said to just
wet the surface of the solid, and in this particular case (since
cos θ = 1) the wetting tension of the solid will be equal to the
surface tension of the liquid.
4.2 The ability of polyethylene and
polypropylene films to retain inks, coatings, adhesives, etc., is
primarily dependent upon the character of their surfaces, and can
be improved by one of several surface-treating techniques. These
same treating techniques have been found to increase the wetting
tension of a polyethylene or a polypropylene film surface in
contact with mixtures of formamide and ethyl Cellosolve in the
presence of air. It is therefore possible to relate the wetting
tension of a polyethylene or a polypropylene film surface to its
ability to accept and retain inks, coatings, adhesives, etc. The
measured wetting tension of a specific film surface can only be
related to acceptable ink, coating, or adhesive retention through
experience. Wetting tension in itself is not a completely
acceptable measure of ink, coating, or adhesive adhesion.
Note 3:A wetting tension of 35 dynes/cm or higher has been
generally found to reveal a degree of treatment normally regarded
as acceptable for tubular film made from Type 1 polyethylene and
intended for commercial flexographic printing. It is, however,
possible that some other level of wetting tension may be required
to indicate the acceptability of polyethylene films made by other
processes, or from other types of polyethylene, or intended for
other uses.
At the present date, insufficient experience
has been gained to state a generally acceptable level of wetting
tension for polypropylene films for commercial flexographic
printing.
1. Scope
1.1 This test method covers the
measurement of the wetting tension of a polyethylene or
polypropylene film surface in contact with drops of specific test
solutions in the presence of air.
1.2 The values stated in SI units are to
be regarded as standard. The values given in parentheses are
mathematical conversions to inch-pound units that are provided for
information only and are not considered standard.
Note 1:This test method and the specified reagents were
specifically developed for polyethylene and polypropylene films. It
is possible to utilize this test method and the specified reagents
for films composed of other polymers, but this can affect the
surface energies of the gas-liquid and solid-liquid interfaces,
which will affect the contact angle and wetting tension. The
applicability and significance for use of non-polyolefin materials
must be established by the user.
1.3This standard does not purport to
address all of the safety concerns, if any, associated with its
use. It is the responsibility of the user of this standard to
establish appropriate safety, health, and environmental practices
and determine the applicability of regulatory limitations prior to
use. Specific hazards statements are given in Section 8.
Note 2:This test method is equivalent to ISO 8296.
1.4This international standard was
developed in accordance with internationally recognized principles
on standardization established in the Decision on Principles for
the Development of International Standards, Guides and
Recommendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
2. Referenced Documents
ISO 8296
Plastics--Film and
Sheeting--Determination of Wetting Tension Available from American
National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New
York, NY 10036, http://www.ansi.org.
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