The heat sealing of flexible packaging film bags is a key process in the processing of plastic packaging. The quality of heat sealing is directly related to the protection effect of the packaged objects. No matter how good a packaging material is, once the heat sealing quality is not good, everything is empty.
Here mainly discusses the heat sealing characteristics of flexible packaging materials and the factors affecting the heat sealing effect.
(1) Heat seal strength
Heat seal strength refers to the peel strength achieved when two layers of the same or different plastic materials are thermally bonded together after a certain pressure at a certain melting temperature. According to relevant standards, heat sealing is the strength measured after a heat sealing time of 2 ~ 3 seconds at a temperature of 130 ℃ ~ 140 ℃ and a pressure of 1kgf / cm2. There are many factors that affect the heat seal strength, mainly:
(1) Performance of resin;
(2) The composition and amount of additives in the resin;
(3) Processing and molding conditions, especially the degree of surface oxidation during extrusion coating;
(4) When the film is compounded, the properties of other materials compounded with the heat-sealable resin are different, and the heat-sealing strength and temperature are also different;
(5) During compound operation, the degree of surface treatment such as the size of corona treatment, etc.
The heat-sealing strength of polyethylene is related to the thickness of polyethylene. With the increase of the thickness of the heat-sealing resin, such as PE, the heat-sealing strength increases. For example, when the thickness of the LDPE heat-sealing layer is 20 μm, the peel strength, namely: heat-sealing The strength is 1.0kgf / 15mm wide, and 1.5kgf / 15mm when 30μm thick, 2.5kgf / 15mm wide when 40μm thick, and peel strength can be 4.2kgf / 15mm wide when 60μm thick. This is why the inner sealing layer material PE or PP film of the high-temperature cooking bag should be 60-70 μm thick.
The performance of the heat-sealing resin is also closely related to the heat-sealing strength: for a certain thickness of the composite film, under the pressure of 2kgf / cm2 and the heat-sealing conditions of 0.5 seconds, heat sealing has the highest heat sealing strength at 150 ℃. MI = 3.7g / 10min, d = 0.923g / cm3, heat seal strength is 1.25kgf / 15mm wide; MI = 3.0g / 10min, d = 0.921g / cm3, heat seal strength is 1.4kgf / 15mm wide; MI = 7.0g / 10min, d = 0.917g / cm3 heat seal strength is 1.75kgf / 15mm wide; MI = 9.5g / 10min, d = 0.917g / cm3, heat seal strength is 1.9kgf / 15mm wide. It shows that the heat seal strength increases with the increase of MI.
(2) Low temperature heat sealability
The low-temperature heat-sealability is a measure of the ease of heat-sealing, and represents a comprehensive factor of the heat-sealing starting temperature and heat-sealing strength. For LDPE resin, low temperature heat sealability is closely related to MI and density. High MI and low density LDPE, excellent low temperature heat sealability. For ethylene copolymers, the monomer content copolymerized with the ethylene phase and the performance of the comonomer have a great influence on its low temperature heat sealability. For example: EVA, as the VA content increases, the softening temperature decreases, the low temperature heat sealability is good, and as the EVA density increases, the low temperature heat sealability improves and becomes better.
(3) Heat sealability of inclusions
This refers to a property that still has good heat seal strength when the surface of the heat seal layer is contaminated with dust, oil, debris, or packaging contents. Resins with heat sealability of such inclusions have various adhesion Junction resin and LLDPE.
Various heat sealing resins:
(1) LDPE:
This is the most commonly used resin for heat sealing. It can be produced by extrusion blown film or extrusion salivation method and then used as a composite film with other films. It can also be extrusion coated or compounded on other films, aluminum foil, paper The film can also be coextruded with other resins to form a composite film. Its melting point is between 105 ° C and 120 ° C. It has good heat sealability, transparency, and non-toxicity.
(2) MDPE and HDPE:
The melting point is between 120 ℃ and 135 ℃. It also has good heat sealability, transparency is not as good as LDPE, crystallinity is higher than LDPE, film strength is greater than LDPE, heat sealing temperature is higher than LDPE, generally speaking, the best heat of PE The sealing temperature is between 150 ℃ and 160 ℃. At this time, it has the highest heat sealing strength.
(3) PE copolymer:
PE and other α-carbon olefins, such as octene, pentene and other copolymers, metallocene (MPE), LLDPE, etc., as well as ethylene and acrylate, ethylene and vinyl acetate, ethylene and maleic anhydride Copolymers, the common characteristics of these resins are good low temperature heat sealability, high heat seal strength and good thermal adhesion.
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