2. Heat Seal Parameters
Heatsealability of the material should be evaluate from 2 aspects including hot tack, which could be tested when the sealing area is not cooled down completely and heat seal strength (ultimate strength) which could be tested when the sealing area is completely cooled down. Generally, the main factors influencing the heat seal property of the packaging materials include heat seal temperature, heat seal pressure and dwell time among which the heat sealing temperature is most critical.
Hot tack is the essential index that is used to evaluate the heat property of the packaging material. Since the interval between the heat sealing package making and product filling in production line is so short that the product is often filled when the temperature of the sealing area has not fallen to the room temperature. If the sealing area could not withstand the impact of the filled products in production line, the packaging bags may fail in the packaging process. Package breakage usually occurs in packaging process with high speed Form-Fill-Seal machine. As well it may occur if the sealing area does not cool down completely in packaging process with low speed packing machine.
Hot tack is tested to evaluate the combination capability of the sealing area when it is impacted by external force when it does not cool down completely within the short period after the sealing process. Technically hot tack is the total sum of the sealing agent’s tack strength under heat sealing temperature and the adhesive intensity to multiplayer structure and other elements. Generally, hot tack of material is much less than its heat seal strength (ultimate strength), which could be obviously seen from Fig.1 (Quoted from ASTM F 2029-00). The curve marked with “Equil. dwell ” on the left is the curve of seal strength and temperature tested after the sealing area has completely cooled down (it is noted in the figure that the balance retention time is 1000ms). The curve marked with “100ms dwell ” on its right is the curve of heat seal strength (ultimate strength) and temperature. The two curves have similar tendency with the change of temperature. The hot tack of material tested after an interval of only 100ms is much lower than that tested after being completely cooled down.
Fig.1 Heatsealability Test
3. Failure Mode of Heat Seal Test Specimen
Heat seal strength (Ultimate strength) of material is the peeling force required by per unit width of heat sealing layer (the failure modes illustrated in Figure 2a). However, in actual testing, failure always occurs at the non-sealing area instead of the heat sealing layer. The actual heat seal strength (ultimate strength) of such material is a little higher than the test result. Failure mode of material in ultimate strength test is important information in evaluation of specimen heatsealability. Operators should carefully observe the material’s condition after the test is finished.
Failure mode refers to the separation mode of the sealing part of the specimen when the sample grips separate from each other in heat seal strength test. Generally, failure mode include peeled sealing surface (Figure 2a), failure of adhesive layer (Figure 2b), failure in seal layer and bottom layer (Figure 2c), failure in sealing edge (Figure 2d), failure in non-sealing part (Figure 2e), elongation of sealing material (Figure 2f), elongation of peeled area (Figure 2g).
Note: Figure is quoted from ASTEM F 1921-98
Fig.2 Failure Mode of Heat Sealing Specimen
The failure mode is closely related to the selection of heat sealing temperature in practical production. Usually the temperature interval of heat sealing curve is within the range of 5-10 °C. Specimen failure mode at each temperature should also be illustrated in detail (usually in the form of graphic illustration, please refer to figure 1). Generally, heat seal temperature should be close to the melting temperature of heat sealing layer, which is in the turning point of the sealing curve where the curve ascends quickly and then becomes flat. If the test material will be used in melt down sealing, heat sealing temperature should be higher than the turning point mentioned above. If specimen is used in strippable sealing, the sealing temperature should be lower than the turning point.
4. Conclusion
The heat seal performance of the packaging material could be optimized by adjusting the heat seal parameters. With modification and improvement of the packaging technology, the materials are required to have better heatsealability so that the product could be sealed in the package and it could be easy to open the package.