First, what is the heat sealability of plastic? What are the characteristics of various heat sealing resins?
A: The heat sealability of plastics refers to the thermal bonding properties of plastics with their own or other types of plastics after they are heated to the molten state. This thermal adhesive property is of great importance in the welding and bag making of plastic packaging materials. With the development of high-speed automation of production, the production speed of the original automatic filling and sealing packaging machine was only 20 to 30 pieces/min. At present, it can reach 150 to 200 pcs/min, and the tightness of plastics, that is, the requirement of thermal adhesiveness is greatly improved. The heat sealability of plastics cannot meet the requirements of this high-speed automatic Filling Machine, and high quality cannot be produced. Commodity packaging comes.
The heat sealability of plastics includes at least the following:
(A) Heat Sealing Strength Heat sealing strength refers to the peel strength achieved after thermal bonding together after two layers of the same or different materials are subjected to a certain pressure at a certain melting temperature. According to the JISZ1526-1976 standard, heat sealing is a strength measured at a temperature of 130° C. to 140° C. and a pressure of 1 kgf/cm 2 after a heat sealing time of 2 to 3 seconds. There are many factors affecting the heat seal strength, and the main ones are: (1) the properties of the resin; (2) the components of the additive and the amount of the additive in the resin; (3) the conditions for processing and molding, especially during the extrusion coating process. The degree of oxidation; (4) When the composite film, the performance of other materials combined with the heat-sealable resin is different, the heat-sealing strength and the heat-sealing temperature are also different; (5) In the composite operation, the degree of surface treatment is as follows: Halo size and so on.
In the high-speed automatic packaging, the required heat-sealing strength also requires low-temperature heat-sealability, a wide range of heat-sealing temperatures, and a heat-to-hot peel strength when heat-sealed, that is, the thermal adhesiveness of the material, and must also be heat sealed. There are so-called inclusion heat seals on the interface that can contaminate and heat seal.
The heat-sealing strength of polyethylene is also related to the thickness of polyethylene. With the increase in the thickness of heat-sealing resin, such as PE, the heat-sealing strength is increased. For example, when the thickness of LDPE heat-seal layer is 20 μm, the peel strength is: The heat seal strength is 1.0kgf/15mm wide and 1.5kgf/15mm for 30μm thick, 2.5kgf/15mm wide for 40μm thick, and the peel strength can reach 4.2kgf/15μm wide at 60μm thick. This is why the PE or PP film of the inner seal layer of the high-temperature cooking bag should be at a thickness of 60 to 70 μm.
The properties of the heat-sealing resin are also closely related to the heat-sealing strength: in the case of a PT300#/LDPE 20μm-thick composite film, the highest heat is heat-sealed at 150°C at a pressure of 2kgf/cm2 and a heat-seal condition of 0.5 seconds. Seal strength, MI = 3.7g/10min, d = 0.923g/cm3, heat seal strength is 1.25kgf/15mm width; MI = 3.0g/10min d = 0.921g/cm3, heat seal strength is 1.4kgf/15mm width; MI = 7.0 g/10 min, heat seal strength of d=0.917 g/cm3 is 1.75 kgf/15 mm width; MI = 9.5 g/10 min, d = 0.917 g/cm3, and heat seal strength is 1.9 kgf/15 mm width. It shows that the heat seal strength increases with the increase of MI.
(B) Low-temperature heat sealability Low-temperature heat sealability is a measure of the ease of heat sealing and represents a combination of the starting temperature and the heat seal strength of the heat seal. For LDPE resins, low temperature heat sealability is closely related to MI and density. High MI and low density LDPE with excellent low temperature heat sealability. For the ethylene copolymers, the content of the monomers co-polymerized with ethylene and the properties of the comonomer have a great influence on the low-temperature heat-sealability thereof. For example: EVA, as the VA content increases, the softening temperature decreases, and the low temperature heat sealability is good. With the increase of the EVA density, the low temperature heat sealability is improved.
(C) peel strength between hot hot peel strength refers to the heat sealing pressure of 2kgf/cm2, 0.5 seconds heat sealing time, peel angle of 22.5 degrees, peeling load of 90g/10mm width, heat sealing temperature of 90 °C ~ 180 °C Heat seal the melt strength. In Japan, the heat-sealing heat peel distance is generally used. The peeling distance between the hot strips is proportional to the degree of oxidation on the surface of the extruded coating film, and is inversely proportional to the melting strength of the resin under heat-sealing conditions and inversely proportional to the amount of high-melting point components in the resin and the high melting temperature. The smaller the distance between hot stripping, the better the heat sealability of the resin, indicating the possibility of displacement of the heat-sealed part under the mechanical force of the heat-sealed bag making, that is, the occurrence of heat-sealing The smaller the releasability, the better the initial tack force of the heat sealing resin is melted.
(4) Inclusion heat-sealing property This refers to the property of having a good heat-sealing strength when the surface of the heat-seal layer is covered with dust, grease, sundries or package contents, and has such inclusion heat. Sealing resins include surlyn resins, various adhesive resins, and LLDPE.
Among the various heat sealing resins are the following:
(1) LDPE: This is the most commonly used resin for heat sealing. It can be produced as a film by extrusion blown film or extruding flow enthalpy and then used in combination with other films to form a composite film. It can also be extruded on other films, aluminum foils, and paper. The coated or composite film can also be co-extruded with other resins to form a composite film. Its melting point is between 105°C and 120°C, and its heat sealability, transparency and non-toxicity are good.
(B) MDPE and HDPE: Melting point between 120 °C ~ 135 °C, also has a good heat sealability, transparency is not as good as LDPE, crystallinity is higher than LDPE, film strength is greater than LDPE, heat seal temperature is higher than LDPE, generally come In fact, the best heat-sealing temperature of PE is from 150°C to 160°C, which has the highest heat-sealing strength.
(3) Copolymers of PE, such as: PE and other alpha-high carbon atoms of olefins, such as: octene, pentene copolymers, metallocene PE (MPE), LLDPE, VLDPE, etc., as well as ethylene with acrylate , Ethylene with vinyl acetate, ethylene with maleic anhydride and other copolymers, these resins are characterized by good low temperature heat sealing, high heat seal strength and good thermal adhesiveness.
Second, tofu used PET12/LDPE30 composite film packaging, vacuum packaging, placed 3 months later, the composite bag expansion, all deterioration, and why?
Answer: Tofu is rich in protein, and it is easy to produce bacteria by spoilage. Vacuum packaging using PET12/LDPE30 structure can not meet the packaging requirements, but should be retort sterilization packaging at high temperature. After being sterilized for 30 minutes at 100°C, it can be stored for more than one year. In order to increase the tightness of the cooking bag, the thickness of the inner layer LDPE should be increased to 60 to 70 μm. (Zhou Xiangxing)
A: The heat sealability of plastics refers to the thermal bonding properties of plastics with their own or other types of plastics after they are heated to the molten state. This thermal adhesive property is of great importance in the welding and bag making of plastic packaging materials. With the development of high-speed automation of production, the production speed of the original automatic filling and sealing packaging machine was only 20 to 30 pieces/min. At present, it can reach 150 to 200 pcs/min, and the tightness of plastics, that is, the requirement of thermal adhesiveness is greatly improved. The heat sealability of plastics cannot meet the requirements of this high-speed automatic Filling Machine, and high quality cannot be produced. Commodity packaging comes.
The heat sealability of plastics includes at least the following:
(A) Heat Sealing Strength Heat sealing strength refers to the peel strength achieved after thermal bonding together after two layers of the same or different materials are subjected to a certain pressure at a certain melting temperature. According to the JISZ1526-1976 standard, heat sealing is a strength measured at a temperature of 130° C. to 140° C. and a pressure of 1 kgf/cm 2 after a heat sealing time of 2 to 3 seconds. There are many factors affecting the heat seal strength, and the main ones are: (1) the properties of the resin; (2) the components of the additive and the amount of the additive in the resin; (3) the conditions for processing and molding, especially during the extrusion coating process. The degree of oxidation; (4) When the composite film, the performance of other materials combined with the heat-sealable resin is different, the heat-sealing strength and the heat-sealing temperature are also different; (5) In the composite operation, the degree of surface treatment is as follows: Halo size and so on.
In the high-speed automatic packaging, the required heat-sealing strength also requires low-temperature heat-sealability, a wide range of heat-sealing temperatures, and a heat-to-hot peel strength when heat-sealed, that is, the thermal adhesiveness of the material, and must also be heat sealed. There are so-called inclusion heat seals on the interface that can contaminate and heat seal.
The heat-sealing strength of polyethylene is also related to the thickness of polyethylene. With the increase in the thickness of heat-sealing resin, such as PE, the heat-sealing strength is increased. For example, when the thickness of LDPE heat-seal layer is 20 μm, the peel strength is: The heat seal strength is 1.0kgf/15mm wide and 1.5kgf/15mm for 30μm thick, 2.5kgf/15mm wide for 40μm thick, and the peel strength can reach 4.2kgf/15μm wide at 60μm thick. This is why the PE or PP film of the inner seal layer of the high-temperature cooking bag should be at a thickness of 60 to 70 μm.
The properties of the heat-sealing resin are also closely related to the heat-sealing strength: in the case of a PT300#/LDPE 20μm-thick composite film, the highest heat is heat-sealed at 150°C at a pressure of 2kgf/cm2 and a heat-seal condition of 0.5 seconds. Seal strength, MI = 3.7g/10min, d = 0.923g/cm3, heat seal strength is 1.25kgf/15mm width; MI = 3.0g/10min d = 0.921g/cm3, heat seal strength is 1.4kgf/15mm width; MI = 7.0 g/10 min, heat seal strength of d=0.917 g/cm3 is 1.75 kgf/15 mm width; MI = 9.5 g/10 min, d = 0.917 g/cm3, and heat seal strength is 1.9 kgf/15 mm width. It shows that the heat seal strength increases with the increase of MI.
(B) Low-temperature heat sealability Low-temperature heat sealability is a measure of the ease of heat sealing and represents a combination of the starting temperature and the heat seal strength of the heat seal. For LDPE resins, low temperature heat sealability is closely related to MI and density. High MI and low density LDPE with excellent low temperature heat sealability. For the ethylene copolymers, the content of the monomers co-polymerized with ethylene and the properties of the comonomer have a great influence on the low-temperature heat-sealability thereof. For example: EVA, as the VA content increases, the softening temperature decreases, and the low temperature heat sealability is good. With the increase of the EVA density, the low temperature heat sealability is improved.
(C) peel strength between hot hot peel strength refers to the heat sealing pressure of 2kgf/cm2, 0.5 seconds heat sealing time, peel angle of 22.5 degrees, peeling load of 90g/10mm width, heat sealing temperature of 90 °C ~ 180 °C Heat seal the melt strength. In Japan, the heat-sealing heat peel distance is generally used. The peeling distance between the hot strips is proportional to the degree of oxidation on the surface of the extruded coating film, and is inversely proportional to the melting strength of the resin under heat-sealing conditions and inversely proportional to the amount of high-melting point components in the resin and the high melting temperature. The smaller the distance between hot stripping, the better the heat sealability of the resin, indicating the possibility of displacement of the heat-sealed part under the mechanical force of the heat-sealed bag making, that is, the occurrence of heat-sealing The smaller the releasability, the better the initial tack force of the heat sealing resin is melted.
(4) Inclusion heat-sealing property This refers to the property of having a good heat-sealing strength when the surface of the heat-seal layer is covered with dust, grease, sundries or package contents, and has such inclusion heat. Sealing resins include surlyn resins, various adhesive resins, and LLDPE.
Among the various heat sealing resins are the following:
(1) LDPE: This is the most commonly used resin for heat sealing. It can be produced as a film by extrusion blown film or extruding flow enthalpy and then used in combination with other films to form a composite film. It can also be extruded on other films, aluminum foils, and paper. The coated or composite film can also be co-extruded with other resins to form a composite film. Its melting point is between 105°C and 120°C, and its heat sealability, transparency and non-toxicity are good.
(B) MDPE and HDPE: Melting point between 120 °C ~ 135 °C, also has a good heat sealability, transparency is not as good as LDPE, crystallinity is higher than LDPE, film strength is greater than LDPE, heat seal temperature is higher than LDPE, generally come In fact, the best heat-sealing temperature of PE is from 150°C to 160°C, which has the highest heat-sealing strength.
(3) Copolymers of PE, such as: PE and other alpha-high carbon atoms of olefins, such as: octene, pentene copolymers, metallocene PE (MPE), LLDPE, VLDPE, etc., as well as ethylene with acrylate , Ethylene with vinyl acetate, ethylene with maleic anhydride and other copolymers, these resins are characterized by good low temperature heat sealing, high heat seal strength and good thermal adhesiveness.
Second, tofu used PET12/LDPE30 composite film packaging, vacuum packaging, placed 3 months later, the composite bag expansion, all deterioration, and why?
Answer: Tofu is rich in protein, and it is easy to produce bacteria by spoilage. Vacuum packaging using PET12/LDPE30 structure can not meet the packaging requirements, but should be retort sterilization packaging at high temperature. After being sterilized for 30 minutes at 100°C, it can be stored for more than one year. In order to increase the tightness of the cooking bag, the thickness of the inner layer LDPE should be increased to 60 to 70 μm. (Zhou Xiangxing)
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