JPH0356570B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a laminate for retort food packaging bags. More specifically, regarding a laminate using a polypropylene film mainly composed of ethylene propylene copolymer as a heat-sealing layer of a retort packaging bag, it has particularly excellent heat resistance, as well as excellent impact resistance and flexibility at low temperatures. , relates to a thermoplastic film for use in retort packaging bags that has stable heat-sealing properties and is excellent in terms of food hygiene. BACKGROUND ART Food packaging bags for retort pouches made of ethylene propylene copolymer laminated with a heat-resistant film and aluminum foil have been well known. recent years,
As retort food products are showing remarkable growth in the market, various combinations of laminated films or sheets for retort food packaging bags are being considered for this field. However, the melting point of polyethylene resin used as the innermost heat sealant layer is 130.
℃ or below, 130℃ is said to be particularly effective in retort processing without damaging the flavor of filled foods.
High-temperature, short-time retort sterilization (high retort)
In this case, the films are fused together and deformed, which impairs the appearance and at the same time makes it difficult to take out the filling. However, it has the disadvantage that a large amount of extraction is transferred to the filled food, and it cannot be used for high-temperature, short-time retort sterilization. On the other hand, films that place emphasis on heat resistance and use so-called ethylene propylene block copolymer as the innermost heat-sealing layer have a melting point of around 160°C, which causes the films to fuse together and deform during high-temperature, short-time retort sterilization. Since it is difficult for the filling to occur, it is easy to take out the filled food without damaging the appearance. Furthermore, the amount of extraction transferred to the filled food is also small and good. However, the disadvantage is that the practical heat sealing temperature range is narrow and it is difficult to control the temperature to obtain stable heat sealing. As mentioned earlier, the so-called high retort is currently in practical use as a high-temperature, short-time sterilizer, and its use is increasing because it shortens time and improves productivity. Since the general sterilization conditions for this high retort are 130°C to 135°C for 10 to 20 minutes, the melting point of the composition needs to be 135°C or higher. In general, thermoplastic films for retorts on the market are molded films made by blending ethylene propylene block copolymer with a melting point around 160°C, ethylene propylene random copolymer, and rubber-based polymer such as ethylene propylene rubber (EPR). It can be roughly divided into two types. However, in the former case, as mentioned above, it is difficult to obtain stable heat sealing. The latter can provide sufficient stable heat sealing, but at low temperatures (-10 to -40℃)
If a retort pouch made of this film is used in a cold region, there is a high risk that the bag will break due to the bag being dropped. Furthermore, the latter method cannot be said to be perfect in terms of food hygiene because the amount of n-hexane or xylene extracted from the film itself is large. Furthermore, a serious defect of retort pouches using these films is that their flexibility is greatly reduced at low temperatures, and pinholes occur due to bending during transportation or rough handling, causing the contents to rot. It reaches a fatal state. As will be discussed later, films made from either ethylene propylene block copolymer, ethylene propylene random copolymer, or any resin in which rubber is added have the same rigidity as polypropylene homopolymer at temperatures below -10°C. There is no known retort film that has both heat resistance and flexibility at low temperatures, and its development is strongly desired. The present inventors have found that it has heat resistance that can withstand high retort processing, excellent impact resistance at low temperatures, flexibility at low temperatures, and stable heat sealing properties. to have,
Considering the characteristics required for retort films, such as excellent food hygiene, especially for use in cold regions, we investigated changes in flexibility at low temperatures (-10° to -30°C) for various polyolefin films. was evaluated using dynamic storage modulus. As a result, both copolymer films based on polypropylene and films made from blends of rubber and other materials lose flexibility in exactly the same way, but only films with a specific ethylene content and melting point can be used at very low temperatures. It was discovered that the flexibility of retort film at low temperatures, which is shown by the dynamic storage modulus, is effective in suppressing the formation of pinholes caused by repeated bending. The invention has been achieved. That is, in the first stage of polymerization, the ethylene propylene random copolymer is polymerized in an amount of 70 to 90% by weight of the total polymerization amount so that the ethylene content is 0.7 to 5.3% by weight, and in the second and subsequent stages, the ethylene concentration is reduced. Raise the remaining 10-30% by weight
A bag laminate for retort food packaging containing as a heat seal layer a polypropylene film having the following characteristics, which is mainly composed of an ethylene propylene copolymer obtained by a method for producing a propylene ethylene block copolymer by polymerizing a polypropylene block copolymer. exists in (1) Ethylene content 5.5 to 17.0% by weight (2) Melting point 135 to 155°C (3) E ₁ ′/E ₂ ′<0.7 However, E ₁ ′ is the value at a frequency of 110 Hz of the polypropylene film used in the present invention. Measured -30
The dynamic storage modulus at °C, E ₂ ′, was measured under a frequency of 110 Hz for a film consisting of a homopolymer of propylene −
This is the dynamic storage modulus at 30℃. The ethylene propylene copolymer constituting the polypropylene film used in the present invention is a copolymer mainly containing ethylene as a comonomer, and contains other α-olefins such as butene-1, hexene, etc., which have an ethylene content of less than 30% by weight. -1, 4-
Including those containing methylpentene-1, octene-1, etc. The MFI of the resin is not particularly limited as long as it does not interfere with molding, but is preferably 0.5 to 15.0.
g/10 minutes. This is determined from both film strength and film formability, and film forming becomes particularly difficult below the lower limit and above the upper limit. The ethylene content of the film of the present invention is 5.5% by weight
~17.0% by weight, more preferably 7% to 15% by weight
It is. Below this range, the impact resistance at low temperatures will be insufficient and the bag will break when dropped at low temperatures.If it exceeds the upper limit, an uneven condition on the film surface known as "yuzu skin" will occur during retort processing, and the retort The appearance of the packaging bag deteriorates and the product value is lost. The melting point of the polypropylene film used in the present invention is preferably 135° to 155°C, more preferably 140° to 150°C. If the temperature is below this range, the film will be deformed or fused during high-temperature short-time retort sterilization, and if the temperature is above this range, the heat-sealing properties at low temperatures will deteriorate and stable heat-sealing strength cannot be ensured. Furthermore, the dynamic storage modulus at low temperatures is large, which will be discussed later.
It cannot exhibit flexibility at low temperatures.
Especially in the low temperature range as mentioned above (below 0°C, especially -10°C)
-40°C), the flexibility of the film at that temperature has a large effect on impact resistance. - of the polypropylene film used in the present invention
The ratio of the dynamic storage modulus (E ₁ ') at 30°C to that of the homopolymer (E ₂ '), E ₁ '/E ₂ ', is required to be 0.7 or less. Generally, when the dynamic storage modulus of a film whose main component is polypropylene is measured by changing the temperature, the first
As shown in the figure, it increases as the temperature decreases, and becomes a nearly constant value around -10 to -30°C, which is sufficiently lower than the glass transition point. For example, when measuring the dynamic storage modulus of a polymer film containing polypropylene homopolymer, propylene ethylene random copolymer, block copolymer, and ethylene propylene copolymer rubber, there is a slight difference in the temperature at which it converges to a certain value. Although there are differences, the converged values are almost the same. That is, in general, the correlation curve with temperature converges to approximately one curve in the low temperature range for homopolymerized polypropylene polymers, random copolymers, and block polymers whose dynamic storage modulus is measured by forming polypropylene resin into a film. The dynamic storage modulus represents the stiffness and rigidity of the film at that temperature. Therefore, even if impact resistance at low temperatures can be improved by methods such as random copolymerization, block copolymerization, or addition of rubber components, flexibility and suppleness will be lost. On the other hand, the polypropylene film of the present invention having a specific ethylene content and melting point has a low dynamic storage modulus, still shows a clear difference even at low temperatures, and maintains flexibility in the low temperature range. , the dynamic storage modulus (E′) shows independent different correlations. The correlation is also much lower than in the former case, and becomes E ₁ '/E ₂ '<0.7 at -30°C. However, E ₁ ' is the value of the dynamic storage modulus of the film molded from the polypropylene of the present invention, and E ₂ ' is the value of the dynamic storage modulus of the film molded under the same conditions from the known homopolymerized polypropylene. be. Regarding the phenomenon of bag breakage when a bag is dropped, a small break called a "pinhole" often occurs, and this is caused by stress concentration caused by the packaging bag folding when dropped, and a large stress being applied to a very small part. This is thought to be the cause. That is, packaging bags for this purpose are required to be resistant to folding. However, as mentioned above, in conventional polypropylene films, whether they are random copolymers or block copolymers, the dynamic storage modulus converges to approximately the same value below -20°C, and the flexibility is the same for both films. However, as mentioned above, the polypropylene film of the present invention has a low dynamic storage modulus and still shows a clear difference even at low temperatures and maintains flexibility in the low temperature range. The polypropylene film used in the present invention has a low-temperature impact of 50 kg cm/mm or more at -5°C, so even if the bag is dropped at low temperatures, it will hold up even when the bag is broken, and the incidence of pinholes that affect pliability is small. , shows good content preservation. This phenomenon is presumed to be due to good compatibility between the rubber layer and the polypropylene layer in the ethylene propylene copolymer used in the present invention, but it is not clear. The ethylene propylene copolymer constituting the polypropylene film used in the present invention can be produced, for example, by the following method. In the first stage of polymerization, an ethylene propylene random copolymer with a melting point of 135 to 155°C is added to
In this method, 90 wt% is polymerized, and then in the second and subsequent stages, the ethylene concentration is increased and the remaining 10 to 30 wt% is polymerized to produce a so-called propylene ethylene block copolymer. In order to obtain a random copolymer with a melting point of 135-155℃ in the first step, the ethylene content is 0.7-
The melting point of the polypropylene film formed from the copolymer thus obtained is almost the same as the melting point of the first stage copolymer, which accounts for 70 to 90 wt%. Become. The ethylene content of the copolymer part after the second stage is, for example, 40 to 80 wt%, and the molecular weight of this part is adjusted by the amount of hydrogen added to the polymerization zone so that the solution viscosity (decalin at 135°C) is 3 or more. Amount of 10~30wt
% polymerized. When producing the polypropylene film used in the present invention, additives such as antioxidants and lubricants, and other resins may be added to the ethylene propylene copolymer. In the present invention, in order to make a retort food packaging bag using the film, for example, the CPP film (ethylene propylene block copolymer cast film) made of the above-mentioned ethylene propylene copolymer is used as the innermost layer, and aluminum foil is used as the innermost layer. and a polyester film are laminated to form the laminate of the present invention, which is then heat-sealed to produce a retort food packaging bag made of these laminates. In this case, each layer of foil and film does not need to have a specific thickness, but the polypropylene film used in the present invention usually has a thickness of 30 to 100 μm, and the aluminum foil has a thickness of 1 to 1 μm.
30μ, polyester film ranges from 5 to 30μ. Furthermore, instead of aluminum foil, it is also possible to use film materials such as EVAL and nylon that have barrier properties, and on the other hand, instead of polyester film, it is also possible to use films that have heat resistance and scratch resistance such as stretched nylon. It is. Further, each layer may be laminated using an adhesive made of a thermosetting resin such as a polyester isocyanate adhesive, or may be laminated using a heat-resistant adhesive resin. A retort food packaging bag is obtained by heating the peripheral edge of the thus obtained laminate so that the polypropylene films of the ethylene propylene copolymer face each other to a certain size. For heat sealing, known methods such as heating bar method, impulse method, ultrasonic sealing, and induction heating can be used. In this heat sealing process, for example, if a heating bar method is used, the temperature can be lowered by 15°C compared to the commonly used ethylene propylene block copolymer, which has a melting point of around 160°C. At the same time, it has become possible to increase the speed of bag making by approximately 30%. In addition, the variation in heat seal strength, which had been affected by temperature changes in the heat seal bar and caused trouble, has been reduced, the sealing performance of the heat seal has been improved, and the reliability of heat seal has been greatly improved. Fill the retort food packaging bag obtained by the above method with the contents of curry, stew, etc., and heat it at 135℃.
When retorted under high-temperature heat sterilization conditions for 10 minutes, so-called high retort conditions, there was no fusion between the inner surfaces of the packaging bag, and the contents were easy to remove, and the appearance remained good with no change. Ta. It goes without saying that the retort food packaging bag using the laminate of the present invention is suitable not only for high retort conditions but also for retort sterilization conditions at lower temperatures. The present invention will be further explained below with reference to Examples and Comparative Examples. The method for measuring each characteristic value of the film and resin used in the following examples is as follows. (1) Ethylene content (wt%) Measured according to _C13- NMR method. JEOL Ltd.
Measurements were made using a FT nuclear magnetic resonance absorption spectrometer (FX-100) under the following conditions in a mixed solvent of 1,2,4 trichlorobenzene and deuterated benzene (C ₆ D ₆ ), and the area of each peak was determined. Calculated. Observation width 180Hz Pulse width 6μs (45° pulse) Pulse interval 3s Number of integrations 10,000 or more Measurement temperature 100℃ (2) Melting point (℃) Differential scanning calorimeter (manufactured by PerkinElmer)
DSC type), the sample amount was 4 mg,
Measure at room temperature. Measurement conditions are 503〓5
After holding for 5 minutes, the temperature is lowered at a rate of 20°C/min, held at 313〓 for 5 minutes, and then raised at a rate of 20°C/min, and the peak temperature of the maximum peak is taken. (3) Dynamic storage modulus (dyne/cm ² ) Using a dynamic viscoelasticity measuring device according to a known method, the heating rate ΔT was measured from −100°C at a frequency of 110 Hz.
Measure continuously under the condition of = +10°C/min.
E ₁ ′/E ₂ ′ was set to the value at −30°C.
The samples used in the Examples and Comparative Examples described later had a length of 25 mm, a thickness of 70 μm, and a width of 5 mm.
The reference E ₂ ′ measurement sample is
Polypropylene homopolymer Showaromer FA130 (manufactured by Showa Denko KK) with MFI = 1.72 was used. (4) Impact resistance strength Measured using a pendulum impact tester manufactured by Toyo Seiki Seisakusho. A 1 inch hemisphere is used for the impact section. (5) Measure according to MAI JIS K6758-1968. Example 1 A 70μ polypropylene film mainly made of ethylene propylene was used as the heat seal layer.
A 9μ aluminum foil was used as the intermediate layer, and a 12μ polyester film was used as the heat-resistant film. The adhesive for each layer is a heat-resistant polyurethane adhesive (Adcoat manufactured by Toyo Ink Co., Ltd.).
AD1050), the above films and foils were laminated together. A polypropylene film mainly composed of the ethylene propylene copolymer was obtained by the following method. First, in the polymerization and first stage, a polymer with an ethylene content of 2.4% by weight was polymerized as an ethylene propylene random copolymer, and then in the second stage, ethylene propylene with an ethylene content of 55.2% by weight was further polymerized, and then in the first stage and the second stage. Polymerization ratio of stages is 85:15
An ethylene propylene block copolymer was synthesized. This is supplemented with BHT (2,
0.1% of 6-di-tert-butyl-p-cresol) and 0.1% of calcium stearate as an internal lubricant were added and melt extruded to form pellets.
The melting point of this resin is 14.7℃, MFI = 2.0g/10
The unit was minutes (hereinafter, this unit will be omitted, but it is the same for all). Die temperature of this pellet-like resin
A flat film with a thickness of 70 μm was obtained by melt extrusion from a T-die at 250° C. to a chill roll at 40° C. The ethylene content of this polypropylene film is 10.3%, the melting point (Tm) is 147.2℃, the MFI is 2.5,
E ₁ '/E ₂ ' was 0.52, and the film impact at -5°C was 92 Kg·cm/mm. FIG. 1 shows the measurement results of the dynamic storage modulus (E') in a graph including this example and the following examples. The middle line A in FIG.
Line B shows the dynamic storage modulus of the polypropylene film of Example 1 of the present invention, and line C shows the dynamic storage modulus of the film of Comparative Example 1, which will be described later. Next, three-way heat sealing was performed using the polypropylene film obtained above as the inner surface at 150° C. for 1 second to produce a retort food packaging bag of 130 mm x 170 mm. This heat-sealing temperature is about 100% higher than that of conventional high-retort thermoplastic film, which is mainly made of ethylene propylene and has a melting point of around 160°C.
The temperature has been lowered by 20℃. This bag was filled with cooked curry, heat-sealed, and then heat sterilized at 135°C for 10 minutes under high retort conditions. The heat-sterilized finished product had no internal fusion and had a smooth exterior with no problems.
Also, vertically from 1.2m in a low temperature atmosphere of -5℃,
Due to horizontal drops, there is a high probability of surface tears, pinhole tears, and bag tears at the heat-sealed area.
It was an excellent retort food packaging bag with a content of 0.1% or less. Table 1 shows the physical property measurement results of the retort food packaging bag along with the following examples. Example 2 A resin in the form of a polypropylene film based on a 70μ ethylene-propylene copolymer was polymerized in the first stage, and a polymer with an ethylene content of 3.5% by weight was polymerized as an ethylene-propylene random copolymer, followed by a second stage. The ethylene content is
40.2% by weight of ethylene propylene copolymer was further polymerized, and the polymerization ratio between the first and second stages was 85.
An ethylene propylene block copolymer with a ratio of 15 to 15 was synthesized. This was made into a film in the same manner as in Example 1 and evaluated in the same manner. Comparative Example 1 As a heat-sealing layer, homopolypropylene was polymerized in the first step, and an ethylene propylene copolymer with an ethylene content of 55.6% by weight was polymerized in the second step as a resin for a polypropylene film mainly composed of a 70μ ethylene propylene copolymer. Polymerization was carried out so that the ratio of homopolymer to block copolymer was 85:15. The melting point of this polymer is 159.6℃, MFI=
It was 2.1. The rest was the same as in Example 1. Comparative Example 2 An ethylene propylene copolymer was polymerized in the same manner as in Example 1, except that the ethylene content in the copolymer was polymerized at 18.5% by weight in the second step of Example 1, and the same procedure was carried out. Molded. Melting point is 147.2
℃, MFI was 2.6. Comparative Example 3 15% ethylene propylene rubber (EPR) was blended with ethylene propylene lambdam copolymer (melting point 147.0°C) with an ethylene content of 2.5% by weight,
A resin with MFI=2.5 was obtained, and a film was formed and used as a polypropylene film. The E ₁ '/E ₂ ' of this film was 0.99. Comparative Example 4 An ethylene propylene random copolymer with an ethylene content of 6.0% by weight was polymerized, with a melting point of 133.1°C and an MFI of 6.0% by weight.
=2.6 resin was obtained. A film was then formed. The E ₁ '/E ₂ ' of this film was 0.98. This film was used as a polypropylene film. Comparative Example 5 In the first step, an ethylene propylene random copolymer with an ethylene content of 6.3% by weight was polymerized, and in the second step, an ethylene propylene copolymer with an ethylene content of 51.3% by weight was polymerized, and the total ethylene content
13.05% by weight of ethylene propylene block copolymer was polymerized. MFI of the resin at this time = 2.7, melting point is 132.4℃
It was hot. This was made into a film in the same manner as in Example 1. 【table】

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between dynamic storage modulus (E') and temperature. In the figure, line A indicates a reference, line B indicates an example of the present invention, and line C indicates a comparative example.

Claims (1)

[Claims] 1. In the first stage of polymerization, an ethylene propylene random copolymer is polymerized in an amount of 70 to 90% by weight of the total polymerization amount so that the ethylene content is 0.7 to 5.3% by weight, and in the second stage and thereafter, ethylene is Heat-seal a polypropylene film with the following characteristics, which is mainly composed of an ethylene-propylene copolymer obtained by a method of producing a propylene-ethylene block copolymer by increasing the concentration and polymerizing the remaining 10 to 30% by weight. A laminate for retort food packaging bags containing layers. (1) Ethylene content 5.5-17.0% by weight (2) Melting point 135-155℃ (3) E ₁ ′/E ₂ ′<0.7 However, E ₁ ′ is the value of the polypropylene film used in the present invention at a frequency of 110 Hz. Measured -30
The dynamic storage modulus at -30° C., E ₂ ′, is the dynamic storage modulus at −30° C. of a film made of a homopolymer of propylene, measured under a frequency of 110 Hz.