ASTM F1921-98
(Test Method)Standard Test Method for Hot Seal Strength (Hot Tack) of Thermoplastic Polymers and Blends Comprising the Sealing Surfaces of Flexible Webs
Standard Test Method for Hot Seal Strength (Hot Tack) of Thermoplastic Polymers and Blends Comprising the Sealing Surfaces of Flexible Webs
SCOPE
1.1 These test methods cover laboratory measurement of the strength of heatseals formed between thermoplastic surfaces of flexible webs, immediately after a seal has been made and before it cools to ambient temperature.
1.2 These test methods are restricted to instrumented hot tack testing, requiring a testing machine that automatically heatseals a specimen and immediately determines strength of the hot seal at a precisely measured time after conclusion of the sealing cycle. An additional prerequisite is that the operator shall have no influence on the test after the sealing sequence has begun. These test methods do not cover non-instrumented manual procedures employing springs, levers, pulleys and weights, where test results can be influenced by operator technique.
1.3 Two variations of the instrumented hot tack test are described in these test methods, differing primarily in two respects: (a) rate of grip separation during testing of the sealed specimen, and (b) whether the testing machine generates the cooling curve of the material under test, or instead makes a measurement of the maximum force observed following a set delay time. Both test methods may be used to test all materials within the scope of these test methods and within the range and capacity of the machine employed. They are described in Section 4.
1.4 SI units are preferred and shall be used in referee decisions.
1.5 This 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 and health practices and determine the applicability of regulatory limitations prior to use. The operator of the equipment is to be aware of pinch points as the seal jaws come together to make a seal, hot surfaces of the jaws, and sharp instruments used to cut specimens. It is recommended that the operator review safety precautions from the equipment supplier.
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Designation: F 1921 – 98
Standard Test Methods for
Hot Seal Strength (Hot Tack) of Thermoplastic Polymers and
Blends Comprising the Sealing Surfaces of Flexible Webs
This standard is issued under the fixed designation F 1921; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 These test methods cover laboratory measurement of the 2.1 ASTM Standards:
strength of heatseals formed between thermoplastic surfaces of D 882 Test Methods for Tensile Properties of Thin Plastic
flexible webs, immediately after a seal has been made and Sheeting
before it cools to ambient temperature. E 171 Specification for Standard Atmospheres for Condi-
1.2 These test methods are restricted to instrumented hot tioning and Testing Flexible Barrier Materials
tack testing, requiring a testing machine that automatically
3. Terminology
heatseals a specimen and immediately determines strength of
3.1 Definitions:
the hot seal at a precisely measured time after conclusion of the
sealing cycle. An additional prerequisite is that the operator 3.1.1 adhesive failure, n—a failure mode in which the seal
fails at the original interface between the surfaces being sealed.
shall have no influence on the test after the sealing sequence
has begun. These test methods do not cover non-instrumented 3.1.2 burnthrough, n—a state or condition of a heatseal
characterized by melted holes and thermal distortion.
manual procedures employing springs, levers, pulleys and
weights, where test results can be influenced by operator 3.1.2.1 Discussion—Burnthrough indicates that the sealing
conditions (time or temperature, or both) were too high for an
technique.
1.3 Two variations of the instrumented hot tack test are acceptable seal.
3.1.3 cohesive failure, n—a failure mode where either or
described in these test methods, differing primarily in two
respects: (a) rate of grip separation during testing of the sealed both of the sealed webs fails by splitting, approximately
parallel to the seal, and the seal itself remains intact.
specimen, and (b) whether the testing machine generates the
3.1.3.1 Discussion—Refer to Fig. 1. The term may be
cooling curve of the material under test, or instead makes a
measurement of the maximum force observed following a set defined somewhat differently when applied to sealing systems
involving an adhesive material as a separate component.
delay time. Both test methods may be used to test all materials
within the scope of these test methods and within the range and 3.1.4 cooling curve, n—the graphical depiction of the in-
crease in strength of the seal with time, as it cools during the
capacity of the machine employed. They are described in
Section 4. period immediately following conclusion of the sealing cycle
(see Fig. 2).
1.4 SI units are preferred and shall be used in referee
decisions. 3.1.4.1 Discussion—The cooling curve is a plot of hot seal
strength versus cooling time. The portion of the cooling curve
1.5 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the of greatest practical significance is the first 1000 ms following
opening of the heatseal jaws.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- 3.1.5 cooling time, n—the time interval from when the
heatseal jaws open at conclusion of the sealing cycle, to the
bility of regulatory limitations prior to use. The operator of the
equipment is to be aware of pinch points as the seal jaws come point at which the hot-tack force is determined.
3.1.6 delay time, n—the time interval from when the heat-
together to make a seal, hot surfaces of the jaws, and sharp
instruments used to cut specimens. It is recommended that the seal jaws open at conclusion of the sealing cycle, to the point
at which withdrawal of the sample from between the jaws is
operator review safety precautions from the equipment sup-
plier. initiated.
These test methods are under the jurisdiction of ASTM Committee F-2 on
Flexible Barrier Materials and are the direct responsibility of subcommittee F02.30
on Test Methods. Annual Book of ASTM Standards, Vol 08.01.
Current edition approved Oct. 10, 1998. Published February 1999. Annual Book of ASTM Standards, Vol 15.09.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F 1921
FIG. 1 Test Strip Failure Modes
3.1.7 dwell time, n—the time interval during the heatsealing 3.1.9 hot tack, n—strength of a hot seal measured at a
cycle when the sealing jaws are in contact with, and exerting specified time interval after completion of the sealing cycle but
pressure on, the material being sealed. prior to the temperature of the seal reaching ambient.
3.1.8 failure mode, n—a visual determination of the manner 3.1.10 hot-tack curve, n—a plot of hot-tack strength versus
in which the test strip fails during grip separation. sealing temperature (see Fig. 3).
F 1921
FIG. 2 Cooling Curve
FIG. 3 Hot Tack Curve
3.1.10.1 Discussion—This is the basic curve used for com- 4.3 As the grips move apart at a set speed and the sealed
paring materials for their hot tack performance. It shows not sample is elongated to eventual failure, the force required is
only the maximum hot seal strength achievable by each measured by the testing machine.
material and the sealing temperature required, but also the 4.4 In Method A (machines of the Theller type) the machine
breadth of the sealing temperature range at any specified level measures and plots strength versus time after jaw opening,
of hot tack. starting after a withdrawal period of 100 to 150 ms, which is
3.1.11 sealing temperature, n—maximum temperature the cooling curve for the material. The computer then deter-
reached at the interface between the two web surfaces being mines the force coordinates of the curve at various times, and
sealed, during the dwell time of the sealing cycle. reports the values as hot-tack strength at those cooling times.
3.1.11.1 Discussion—Sealing temperature will equal jaw The machine is factory-set to start withdrawal within 10 ms
temperature (both jaws at same temperature) if the dwell time after jaw opening.
is long enough for the interface to reach equilibrium with the 4.5 In Method B (machines of the DTC andJ&B type) the
jaws. At this point, seal strength will no longer rise with computer plots strength versus time after completion of a set
increasing dwell time. delay time. The maximum force encountered during grip travel
3.1.12 withdrawal time, n—the time interval from when is determined from that plot and reported as hot-tack strength
withdrawal is initiated, to the point in time when all slack has for the delay time employed in that test.
been removed from the test strip between the seal and the grips, 4.6 In both methods the operator cannot influence the test
so that measurement of the strength of the seal can commence. once the sealing cycle is initiated.
4.7 Hot-tack strength at various sealing temperatures is
4. Summary of Test Method
plotted as the hot-tack curve of the material tested (see Fig. 3).
4.1 A sample strip is sealed by applying pressure from two
5. Significance and Use
flat heated jaws under defined conditions of temperature,
contact time and pressure. 5.1 In form-fill operations, sealed areas of packages are
4.2 When the jaws of the sealing unit open, the sealed strip frequently subject to disruptive forces while still hot. If the hot
is automatically withdrawn from between the jaws at conclu- seals have inadequate resistance to these forces, breakage can
sion of a set delay time (which may be zero), by retraction of occur during the packaging process. These test methods
the grips. measure hot seal strength and can be used to characterize and
F 1921
rank materials in their ability to perform in commercial maxima or other features of the curve smaller steps may be
applications where this quality is critical. desirable locally. The first temperature point of the curve is
typically at about the seal initiation temperature.
6. Apparatus
8.1.2 Dwell Time—Must be long enough for the sealing
6.1 Specimen Cutter—Sized to cut specimens to a width of interface to come to the known temperature of the jaws, which
either 25 mm (0.984 in.), 15 mm (0.591 in.), or 1.00 in. (25.4 depends on the thickness and construction of the material.
Typical minimum dwell times:
mm). Tolerance shall be 60.5 %. Cutter shall conform to
requirements specified in Test Method D 882.
Films—25 μ 1 (mil) and under: dwell time, 500 ms (0.5 s).
Films—25 μ to 64 μ (1 to 2.5 mil): dwell time, 1000 ms (1 s).
6.2 Testing Machine —Equivalent to those available from
DTC, Theller, andJ&B, with the following minimum
8.1.3 Sealing Pressure—Set pressure in the range of 15 to
2 5
capabilities:
30 N/cm (22 to 44 psi).
6.2.1 Equipped with two heated jaws for making seals,
8.2 Clamp the strip to be tested in the machine grips,
6.2.2 Variable and precise control of jaw temperatures,
observing alignment precautions and proper orientation of the
dwell time and pressure,
heatseal side.
6.2.3 Variab
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