GB2060655A

GB2060655A - Latex of vinylidene chloride copolymers, its preparation and use

Info

Publication number: GB2060655A
Application number: GB8032617A
Authority: GB
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1979-10-12
Filing date: 1980-10-09
Publication date: 1981-05-07
Also published as: FR2467226A1; GB2060655B; FR2467226B1; JPS587664B2; NL8005567A; JPS5655448A; DE3038311A1

Abstract

Latex compositions for coating films to give heat sealable, anti- blocking coatings comprise 0.5-10 parts of particles of a vinylidene chloride copolymer (I) containing more than 94% of units derived from vinylidene chloride and 100 parts of particles of vinylidene chloride copolymer (II) containing 70 to 85% of units derived from vinylidene chloride, 10 to 28% of units derived from vinyl chloride and 2 to 20% of units derived from at least one other monomer. Plastics films coated with such a latex can be used to package foods.

Description

SPECIFICATION Latex of vinylidene chloride copolymers, its preparation and use The present invention relates to a latex comprising a mixture of two kinds of vinylidene chloride copolymers.

Owing to the excellent gas-barrier property of the films and papers obtained by coating or painting plastic films made of polypropylene, polyethylene terephthalate, polyamide and cellulose derivatives, and papers with a latex of vinylidene chloride copolymer, such plastic films and papers coated or painted with the above-mentioned latex are widely used for packing foods. These films coated with a latex of vinylidene chloride copolymer is applied for automatic bag making machines or automatic filling and packing machines. And so, an excellent adaptability comprising heat-sea lability, anti-blocking property and slipping property, to the automatic packing machine is required to such a film.

For the high-speed packing with automatic packing machines, a biaxially oriented polypropylene film of a relatively high rigidity is favorably suitable. However, since the biaxially oriented polypropylene film suddenly contracts at a relatively lower temperature of about 1 200C, it is necessary that the film of vinylidene chloride copolymer prepared by coating the latex onto the surface of the plastic film has an excellent heat-sealability at a temperature lower than the temperature at which the biaxially oriented polypropylene film begins to contract.

Further, it is required that the blocking does not occur at the time of coating the latex of vinylidene chloride copolymer or at the time of storing the coated film. The film coated with a latex of vinylidene chloride copolymer is excellent in heat-sealing at a low temperature, but it has a defect of causing blocking at the time of coating and at the time of storing the coated film. In order to prevent the blocking, a method of adding an anti-blocking agent such as powdery silica or wax into the latex in a large amount or a method of using a latex of copolymer of highly rich in vinylide chloride units for obtaining a hard membrane with a high crystallinity has been known. However, the above-mentioned methods have fatal defects of impairing the transparency of the coated film and deteriorating the heat sea lability at a low temperature.That is, it has been extremely difficult to obtain a latex of vinylidene chloride copolymer which gives a coated film satisfying both the heat-sealability at a low temperature and the anti-blocking property.

The present invention provides a latex of vinylidene chloride copolymer, from which a coated film excellent in both the heat-sealability at a low temperature and the anti-blocking property as well as in the gas-barrier property is obtainable.

The latex according the present invention comprises a mixture of vinylidene chloride copolymer (I) containing more than 94% by weight of vinylidene chloride with vinylidene chloride copolymer (II) containing 70 to 85% by weight of vinylidene chloride, 10 to 28% by weight of vinyl chloride and 2 to 20% by weight of at least one comonomer in an amount of 0.5 to 10 parts by weight of the vinylidene chloride copolymer (I) to 100 parts by weight of the vinylidene chloride copolymer (III).

The latex according to the present invention can be obtained by mixing a latex A of the vinylidene chloride copolymer (I) and a latex B of the vinylidene chloride copolymer (II).

The vinylidene chloride copolymer (II) which constitutes the base of the latex according the present invention and contributes to the heat-sealability of the coated film at a favorably low temperature is produced by copolymerizing a monomer mixture consisting of 70 to 85% by weight of vinylidene chloride, 10 to 28% by weight of vinyl chloride and 2 to 20% by weight of at least one monomer copolymerizable with the above-mentioned two monomers in an emulsified state. In the above-mentioned copolymerization, in the case where the amount of vinylidene chloride is less than 70% by weight of total amount of the copolymer (11), the copolymer of the film obtained from the latex does not attain the predetermined crystallinity resulting in the poor gas-barrier property and the poor anti-blocking property of the coated film.On the other hand, in the case where the amount of vinylidene chloride is more than 85% by weight of total amount of the copolymer (II), the crystallinity of the copolymer of the coated film is so high that the temperature at which the coated film can be heatsealed is too much raised.

The meaning of the use of vinyl chloride in an amount of 10 to 28% by weight of the total amount of the copolymer (II) resides in (1) reducing the melting point of the copolymer to reduce the temperature at which heat-sealing is carried out most favorably and (2) retaining the temperature at which film-formation of the latex is favorably carried out as low as possible. In the case where the amount of vinyl chloride is less than 1 0% by weight of the total amount of the copolymer (II), the heatsealability of the film formed from the latex is impaired. On the other hand, in the case where it is larger than 28% hy weight, the anti-blocking property of the film formed from the latex is impaired.In addition, in order to prevent the change of both the heat-sealability at a low temperature and the crystallinity of the copolymer of the coated film formed from the latex with the passage of time, the use of at least one monomer polymerizable with the two monomers in an amount of 2 to 20% by weight of the total amount of the copolymer (II) is indispensable. For that purpose, for instance, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, octyl acrylate, vinyl acetate, acrylonitrile, acrylic acid and itaconic acid are used. Preferably, methyl acrylate, acrylonitrile and acrylic acid are used. Of course, the crystallinity of the copolymer (II) of the coated film prepared from the latex B depends upon the kind and amount of the monomer copolymerizable with the two monomers, vinylidene chloride and vinyl chloride.

As a criterion of the crystallinity of the copolymer of the coated film, the ratio of infrared absorption coefficient of the copolymer of the film is utilized. That is, the ratio of an absorption coefficient of an absorption band at 884 cm-l (the absorption band due to the crystals of vinylidene chloride copolymer) to the absorption coefficient at 1407 cm-l (the absorption band due to CH2 of the copolymer and used for correction due to the thickness of the film of the copolymer), D88JD,407, is used as a criterion of the crystallinity.

As has been described, in order to produce a latex of copolymer from which a coated film capable of heat-sealing at a low temperature is obtained, the above-mentioned composition of the copolymer (II) is suitable.

Further, it is necessary to crystallize the copolymer of the film formed from the latex to improve the anti-blocking property of the film, however, the temperature at which the film is heat-sealable is the higher as the crystallinity of the copolymer of the film formed from the latex is higher. In such a circumstance, it is desirable to obtain a copolymer of the latex which shows the predetermined crystallinity after aging the coated film for 24 hours at a temperature of 400C and retains the crystallinity without any change by the further aging time. It is particularly desirable in the present invention that the above-mentioned ratio D884/D,407 shows in the range of 0.05 to 0.12 after aging the coated film of the copolymer (II) for more than 24 hours at 400C.The copolymer (II) showing the ratio D884/D1407 of between 0.05 to 0.12 after heating for 24 hours at 400C is easily crystallizable by the influence of a crystallization inducer. The copolymer (II) showing the ratio of less than 0.05 has a small potential crystallinity and so, even when fully crystallized, the film of the copolymer (II) is still poor in anti-blocking property. On the other hand, the copolymer (II) showing the ratio of larger than 0.12 has a too large crystallinity and so, the heat-sealability at a low temperature of its film is poor. In addition, the copolymer (II) which shows the ratio not attaining 0.05 even after heating for 24 hours at 400C crystallizes extremely slowly even under the action of a crystallization inducer.Such a film of the copolymer (II) is not desirable. However, the film of the copolymer (II) which the ratio D884/D,407 suddenly shows a rapid increase within 48 hours of aging is poor in the heat-sealability at a low temperature.

The latex A contains the vinylidene chloride copolymer (I) used as a crystallization inducer. The copolymer (I) consists of 94 to 100% by weight of vinylidene chloride and 0 to 6% by weight of at least one monomer copolymerizable with vinylidene chloride. This copolymer (I) consists of preferably more than 96% by weight of vinylidene chloride. It has begun its crystallization already in the stage of polymerization, and in the case where the copolymer (I) is present in the prepared film from the latex, it has an extremely large effect of inducing and accelerating the crystallization of potentially crystallizable copolymers of vinylidene chloride.

As the comonomer used in copolymerization for obtaining the above-mentioned copolymer (i), for instance, vinyl chloride, methyl-, ethyl-, propyl-, butyl and octyl acrylates, vinyl acetate, acrylonitrile, acrylic acid and itaconic acid are used. Preferably, vinyl chloride, methyl acrylate, acrylonitrile and acrylic acid are used.

The size of copolymer tl) particles in the latex A is preferably smaller in order to exhibit the accelerating effect of crystallization of the vinylidene chloride copolymer (II) as crystal nucleus. It is less than 1000 x 10-8 cm, particularly preferably less than 500 x 10-8 cm in average diameter.

In addition, as a crystal nucleus, talc and organic pigments are also effective to the vinylidene chloride copolymer (II), however, since they impair the transparency of the film coated with the latex or they colour the film, it is far better to utilize the copolymer of vinylidene chloride relatively richer in vinylidene chloride for the crystal nucleus.

The latices A and B are prepared by emulsion polymerization of monomer mixtures, however, it is preferable to add the emulsifier and the polymerization catalyst in the polymerization. And the above mentioned latices are used to obtain the latex of the present invention by mixing them.

Furthermore, the latex according the present invention may be coated by the usual technique of coating after admixing an additive such as silica, waxes and the like therewith for improving the anti blocking property.

The latex according the present invention is excellent in film-forming ability, and the film coated with the latex has a favorable heat-sealing property at low temperature and favorable anti-blocking property as well as favorable gas-barrier property.

The following description is the more detailed explanation of the present invention while referring to Examples and Comparative Examples.

For reference, percentages and parts appearing hereinafter mean percentages by weight and parts by weight, respectively.

EXAMPLE 1 After sufficiently substituting the interior in a glass ampoule with nitrogen, the following mixture was charged into the ampoule and the ampoule was shaken for 40 hours at a temperature of 45 or, after sealing, to carry out polymerization: 80 parts of vinylidene chloride 14 parts of vinyl chloride 2 parts of methyl acrylate 4 parts of acrylonitrile 11 5 parts of de-ionized water 0.5 parts of sodium lauryl sulfate 0.04 parts of potassium persulfate, and 0.02 parts of sodium hydrogen sulfite.

Then, after cooling and opening the ampoule, the following mixture was additionally charged into the ampoule and the polymerization was continued for 20 hours at 450C: 0.02 parts of potassium persulfate 0.01 part of sodium hydrogen sulfite, and 5 parts of de-ionized water.

As the result of polymerization, a latex was obtained at a yield of 98.8% and named as Latex B,.

A coated film was produced by coating Latex B, on a biaxially oriented polypropylene film and drying for 30 sec at 100 C, with a coated layer of 5 to 6y in thickness. The ratios of infrared absorption coefficients, D884/D,407, of the copolymer of the coated film after respectively 24 and 48 hours of aging at 400C were 0.084 and 0.090.

Separately, another latex named as Latex A, was produced by charging the following mixture into a glass ampoule having the interior substituted with nitrogen and shaking for 30 hours at 450C after sealing: 97 parts of vinylidene chloride 2 parts of methyl acrylate 1 part of acrylic acid 300 parts of de-ionized water 3 parts of sodium dodecyibenzenesulfonate 0.1 part of potassium persulfate, and 0.05 parts of sodium hydrogen sulfite.

The yield of Latex A1 was 98,8 zó and this latex was to be used for the crystal nucleus, having copolymer particles of 450 x 10-8 cm in mean diameter.

In the next place, Latex B and Latex A, were mixed at a ratio of 223 parts to 4.0 parts containing solid matter of 100 parts and 0.98 parts, respectively.

Into the thus prepared mixture of Latices B, and A,, each appropriate amount of silica and wax was mixed, and the mixture was coated on the biaxially oriented polypropylene film with a coated layer of copolymer of 5 to 6 y in thickness. The properties of the thus obtained coated film were evaluated by the methods described later. The results were shown in Table 2.

EXAMPLES 2 AND 3 Mixtures of Latex B, and Latex A were prepared at the following ratios, and the same amounts of silica and wax were mixed with the mixtures as in Example 1. The mixtures were respectively coated on the biaxially oriented polypropylene film as in Example 1. The results are shown in Table 2.

Example Latex B, Latex A, 2 223 parts 20.4 parts (solid matter: 100 parts) (solid matter: 5.0 parts) 3 223 parts 37 parts (solid matter: 100 parts) (solid matter: 9.1 parts) EXAMPLE 4 Latex B2 was produced by the same procedures for producing Latex B1 in Example 1, except for the charge of monomers and for the emulsifier shown below.

MONOMERS AND AN EMULSIFIER: 75 parts of vinylidene chloride (instead of 80 parts) 20 parts of vinyl chloride (instead of 14 parts) 4.5 parts of acrylonitrile (instead of 4 parts) 0.5 parts of acrylic acid (instead of 2 parts of methyl acrylate) 0.7 parts of spdium dodecylbenzenesulfonate (instead of 0.5 parts of sodium lauryl sulfate) The yield of Latex B2 was 98.5%. The ratios of infrared absorption coefficients, D884/Dr407, of the copolymer of the coated film prepared as in Example 1 by coating Latex B2 on the biaxially oriented polypropylene film after aging for 24 and 48 hours at 400C were respectively 0.061 and 0.070.

The change of the ratio D884/D,407 with the passage of time for aging at 400C of the copolymer of the coated film obtained from the latex of the present invention is shown in Fig. 1. Curve 1 shows the case where Latex B2 obtained in Example 4 was used; Curve 2 shows the case where Latex B2 added with an emulsified wax at a ratio of 2 parts to 100 parts of solid matter in Latex B2 was used; Curve 3 shows the case where the mixtures of Latices B2 and A, obtained in Example 4 after adding 0.2 parts of silica per 100 parts of solid matter of the mixture of Latices was used; Curve 4 shows the case where the mixture of Latices B2 and A, after adding 2 parts of an emulsifier wax per 100 parts of solid matter of the mixture of Latices was used.By comparing the two Curves 1 and 3, it will be understood that the mixture of the latices according to the present invention is excellent in high speed crystallization. As is seen in Curve 3, crystallization of copolymer in the coated layer begins immediately after coating with a remarkably large velocity thereafter.

Fig. 2 shows the relationship between the temperature of heat-sealing of the coated film prepared by coating the mixture of Latices B2 and A, after adding 0.2 parts of silica and 2.0 parts of the emulsified wax per 100 parts of solid matter of the mixture of Latices onto the both sides of the same polypropylene film in an amount of 2.5 g of solid matter on one side and the tensile strength of the heatsealed part of the coated film. Curve 1, Curve 2 and Curve 3 repectively show the relationship just after coating, after 24 hours of aging at 4O0C and after 7 days of aging at 400C. As is seen in Fig. 2, the temperature of the heat-sealing in Curve 1 is low and the temperature follows Curve 2 after aging for 24 hours at 400C to be higher.After aging for 7 days at 4O0C, however, the temperature of the heatsealing does not show any more change. In other words, the coated film becomes stable by aging for 24 hours at 400C to show a favourable property.

EXAMPLE 5 Latex B3 was produced by the same procedures for producing Latex B, in Example 1, except for the charge of monomers and for the emulsifier shown below.

MONOMERS AND AN EMULSIFIER 85 parts of vinylidene chloride (instead of 80 parts) 10 parts of vinyl chloride (instead of 14 parts) 1.2 parts of methyl acrylate (instead of 2 parts) 3.5 parts of acrylonitrile (instead of 4.0 parts) 0.3 parts of acrylic acid (not used in Example 1) 0.7 parts of sodium alkane (cut4 to C18) sulfonate (instead of 0.5 parts of sodium lauryl sulfate) The yield of Latex B3 was 99.0%. The ratios of D884/D1407 of the copolymer of the coated film of Latex B3 on the biaxially oriented polypropylene film were 0.104 and 0.110, respectively after aging for 24 and 48 hours at 400C.

Separately another Latex A2 was produced following the procedures in producing Latex A1 in Example 1, except for the charge of monomers and the emulsifier shown below.

MONOMERS AND THE EMULSIFIER 95 parts of vinylidene chloride (instead of 97 parts) 3 parts of vinyl chloride (instead of 2 parts) 2 parts of acrylonitrile (not used in Example 1) (methyl acrylate and acrylic acid were not used in Example 5) 4 parts of sodium dodecylbenzenesulfonate (instead of 3.0 parts).

The yield of Latex A2 was 98.6%.

A mixture of Latices B3 and A2 was prepared by mixing 223 parts (containing 100 parts of solid matter) of Latex B3 and 8.2 parts (containing 2 parts of solid matter) of Latex A2, and then adding 0.2 parts of silica and 2 parts of the emulsified wax to 100 parts of solid matter of the mixture of Latices B3 and A2.

In order to more comparatively explain the effects of the present invention, Comparative Examples 1 to 6 were prepared as are shown below: COMPARATIVE EXAMPLE 1 A mixture of Latices B2 and A1 was prepared by mixing 223 parts (containing 100 parts of solid matter) of Latex B2 and 1.6 parts (containing 0.39 parts of solid matter) of Latex A1 and adding 0.2 parts of silica and 2.0 parts of the emulsified wax to 1 00 parts of solid matter of the mixture of Latices.

COMPARATIVE EXAMPLE 2 A mixture of Latices B1 and A1 was prepared by mixing 223 parts (containing 100 parts of solid matter) of Latex B1 and 55 parts (containing 13.5 parts of solid matter) of Latex A1 and adding 0.2 parts of silica and 2.0 parts of the emulsified wax to 1 00 parts of solid matter of the mixture of Latices.

COMPARATIVE EXAMPLE 3 In this Comparative Example 3, Latex B2 prepared in Example 4 was utilized after adding 0.2 parts of silica and 2.0 parts of the emulsified wax into Latex B2.

COMPARATIVE EXAMPLE 4 In this Comparative Example 4, Latex B2 prepared in Example 4 was also utilized, however by adding 2 times by weight of wax of the usual use, that is, 4.0 parts and 0.2 parts of silica.

COMPARATIVE EXAMPLE 5 Latex B4 was produced following the same procedures in Example 4, however, at the following monomeric charge: 65 parts of vinylidene chloride 29.5 parts of vinyl chloride 2.0 parts of methyl acrylate 3.0 parts of acrylonitrile and 0.5 parts of acrylic acid.

The yield of Latex B4 was 98.3%.

The crystallinity of the copolymer of Latex B4, that is, the ratio of D884/D1407, was determined as in Example 1. It was 0.011 after aging for 24 hours and 0.035 after aging for 48 hours at 400C.

A mixture of Latices B4 and A1 (obtained in Example 1) was prepared by mixing 224 parts (containing 100 parts of solid matter) of Latex B4 and 40 parts (containing 9.8 parts of solid matter) of Latex A, and adding 0.2 parts of silica and 2.0 parts of the emulsified wax per 100 parts of solid matter in the mixture of Latices B4 and A1.

COMPARATIVE EXAMPLE 6 Latex B5 was produced by following the same procedures in producing Latex B1, of Example 1, however, with the different monomeric charge shownbelow: 90 parts of vinylidene chloride 5.2 parts of vinyl chloride 4.5 parts of acrylonitrile and 0.3 parts of acrylic acid.

The yield of Latex B5 was 99.0%.

The crystallinity of copolymer of Latex B5, that is, the ratio of D884/D1407 was determined as a coated membrane onto the polypropylene film as in Example 1. It was 0.11 5 after 24 hours of aging at 4O0C, and 0.154 after 48 hours of aging at400C.

The data of the crystallinity of the copolymer of Latices B1, B2, B3, B4 and B5 prepared in Examples 1,4 and 5 and Comparative Examples 5 and 6 were summarized in Table 1.

The anti-blocking property of the copolymer of the above-mentioned mixture of both Latex Bn (n = 1 to 5) and Latex Am (m = 1 and 2) during the preparation of coated film onto the biaxially oriented polypropylene film, the state of the coated film, the lowest temperature at which the coated film is possibly heat-sealed and the permeability of the coated film against gaseous oxygen are determined by the following methods and shown in Table 2.

(1) Crystallinity of the copolymer of Latex Bn: Specimen of a Latex Bn (n = 1 to 5) is coated with a Mayer-rod onto the oxidation-treated surface of a biaxially oriented polypropylene film in an amount of giving a thickness of the copolymer layer of about 581 on the surface of the film.After drying the treated film for 30 sec at 1 000C and cooling for 30 sec at room temperature, the copolymer layer of the latex is exfoliated from the polypropylene film and kept at 4O0C. After 24 and 48 hours of aging, the infrared absorption coefficients of the copolymer layer are determined at 884 and 1407 cm-7, respectively, and their ratio is calculated as D884/D1407. The ratio is taken as the index of the crystallinity of the copolymer of the latex.

(2) Processability of the mixed latex: Processability of the mixed latex is determined by observing the coated surface of the copolymer of the latex on the biaxially oriented polypropylene film obtained by the following method: Preliminarily, a solution of a polyurethane adhesive in ethyl acetate is coated on one side of the above-mentioned polypropylene film in an amount of about 0.3 g (as solid matter)/m2, and the film is dried at 1 000C for 30 sec. After cooling at room temperature, a specimen of the mixed latex or the singie latex added with an anti-biocking agent and adjusted to be 40 dyn/cm (at 200C) in its surface tension is coated with a Mayer-rod in an amount of 2.5 g (as solid matter)/m2, onto the thus treated polypropyrene film, and then the coated film is dried at 100 C for 30 sec.After cooling, the other remaining side of the polypropylene film is coated with the polyurethane adhesive and then with the same specimen of the latex.

The state of the surface of copolymer layer of the latex specimen -on the film to be investigated comprises the roughness, the presence or absence of minute unevenness, the transparency and the presence or absence of distorted vision when looking articles through the layer.

(3) Anti-blocking property during the coating of the mixed latex: Anti-blocking property is determined as follows: Specimen of the latex added with 0.2 parts of silica and 2.0 parts of wax (both per 1 00 parts of solid matter of the latex) is coated onto the above-mentioned polypropylene film of 1 5 cm in width and 50 cm in length with the same procedures as in (2). However, in this case, two kinds of coated films are prepared, one of which is coated on only one side of the film and the other of which is coated on both sides.

Each of the coated film is wound on a pipe of one inch in diameter made of polyvinyl chloride having a smooth surface at a tension of 2.5 + 0.2 kg, and the end of the coated film is fixed with a cellophane adhesive tape. After keeping the wound film for 4 hours at 400 C, the wound film is slowly' unwound to observe the state of adhesion of the coated surface to the coated surface or to the uncoated surface of the film. In addition, the presence or absence of the transcription of the unevenness of the polypropylene film onto the surface of the copolymer layer of the latex is observed. The latex which does not give such adhesion nor is suffered from such a transcription is evaluated as good in antiblocking property.

(4) Temperature at which heat-sealing of the coated film is possible: Following the same procedures shown above, a coated film is prepared by coating the both sides of a biaxially oriented polypropylene film with the specimen of latex. After aging the coated film for 48 hours at 400C, the copolymer layer formed on the polypropylene film is heat-sealed to the other copolymer layer on the polypropylene film at a pressure of 1 kg/cm2 for one sec at a temperature of 90, 95, 100, 105, 110, 11 5, 120, 125, 130, 135 and 140"C, respectively. The heat-sealed part of the coated film is subjected to a tensile tester, after 5 min of the heat-sealing, at 230C and RH of 50% under a tensile velocity of 300 mm/min.The lowest temperature at which the heat-sealing is carried out to give a tensile strength of 50 g/l 5 mm of width of the coated film is recorded as the temperature of heat-sealing.

(5) Permeability of gaseous oxygen through the coated film: A coated film is prepared by coating the both sides of a biaxially oriented polypropylene film in the same procedures as in (2). After aging the thus prepared coated film for 48 hours at 400C, its moisture content was adjusted by keeping in an atmosphere of 200C and RH of 90% for two days. The permeability of gaseous oxygen through the thus pretreated coated film was determined by an oxygen permeability tester under the conditions of a temperature of 200C and RH of 90%.

As is seen in Table 2, the latex according to the present invention showed an excellent antiblocking property after having been added with small amounts of two kinds of anti-blocking agents, silica and wax. The coated film was obtained by coating the latex onto the biaxially oriented polypropylene film, which was smooth and excellent in transparency.

Further, the thus obtained coated film could be heat-sealed at a sufficiently low temperature and the coated film showed a practically favorable low permeability top gaseous oxygen.

On the other hand, in Comparative Example 1, the mixture of Latices B1 and A1 was too low in the content of Latex A1 which serves the role of a crystallization introducer, and accordingly, the antiblocking property was poor even after addition of the anti-blocking agents, 0.2 parts of silica and 2.0 parts of wax.

In Comparative Example 2, the mixture of Latices B1 and A1 was favorable in anti-blocking property due to the higher ratio of mixing of Latex A1, however, the temperature of heat-sealing was too high at 1 250C not to be practicized industrially.

In Comparative Example 3, the latex coated onto the polypropylene film was Latex B2 after having been added only with 0.2 parts of silica and 2.0 parts of wax, and accordingly, the anti-blocking property was poorer than in the cases of Examples 1 to 3.

In addition, as has been seen in Curve 2 of Fig. 1, the copolymer layer of the coated film on the polypropylene film with Latex B2 after addition of only 2.0 parts of wax has not begun the crystallization just after the film-formation by coating and drying.

In Comparative Example 4, the latex coated onto the polypropylene film was Latex B2 after having been added with 0.2 parts of silica and 4.0 parts of wax. Owing to the addition of large amount of wax, the anti-blocking property during the coating process was improved, however, the smoothness of the surface of coated film was poor with ruggedness and the gas-barrier property was also poor, thus poor in practicizability.

In Comparative Example 5, the latex B4 was poor in content of vinylidene chloride units in its copolymer, and accordingly,the velocity of crystallization was too slow resulting in poor anti-blocking property as well as poor gas-barrier property.

Whereas, in Comparative Example 6, the copolymer in Latex B5 was too high in vinylidene chloride content, and accordingly, the heat-sealing temperature was as high as 1 350C, poor in practicizability.

TABLE1 Ratio of an infrared absorption coefficient at 884 cm ' (D884) to that at 1407 cm @ (D1407) of copolymer of Latex 8n (n = 1 to 5)

Copolymer of Latex Bn Content of Ratio: D884/D1407 vinylidene Example* n chloride (%) After 24 hours After 24 hours 1 B1 80 0.084 0.090 4 B2 75 0.061 0.070 5 B3 85 0.104 0.110 Comparative Example 5 5 B4 65 0.011 0.035 6 B5 90 0.115 0.154 Note: *shows the number of Example or Comparative Example wherein the latex Bn was produced. TABLE 2 Ratio of mixing Latex Bn (n = 1 to 5) and Latex Am (m = 1 and 2), Properties of the mixed Latex and Totallized evaluation (TE)

Anti-1) Temperature3) Permeability Mixing ratio of Latex Bn and Latex Am blocking Process-2) of heat- (ml/m.

Example B A property ability sealing ( C) 24 hours.atm) TE 1 B1 solid matter: 100 parts A1 solid matter: 0.98 parts A to B A 110 A 18 A 2 B1 solid matter: 100 parts A1 solid matter: 5.0 parts A A 110 A 19 A 3 B1 solid matter: 100 parts A1 solid matter: 9.1 parts A A 115 A 20 A 4 B2 solid matter: 100 parts A1 solid matter: 3.0 parts A to B A 105 A 21 A 5 B3 solid matter: 100 parts A2 solid matter: 2.0 parts A A 115 A 16 A Comparative Examples 1 B1 solid matter: 100 parts A1 solid matter: 0.39 parts B to C A 110 A - C 2 B1 solid matter: 100 parts A1 solid matter: 13.9 parts A A 125 C - C 3 B2 solid matter: 100 parts without any Am C A 105 A 21 C 4* B2 solid matter: 100 parts without any Am A C 110 A 32 C 5 B4 solid matter: 100 parts A1 solid matter: C A 95 A 44 C 6 B5 solid matter: 100 parts without any Am A A 135 C 14 C 1) Anti-blocking property: 2) Procesbiity: A means no blocking (adhesion or transcription), A means the surface of smooth without unevenness, B means somewhat blocking and B means the presence of somewhat unevenness and C means remarkable blocking. C means the surface with remarkable unevenness and non-transparent.

3) Temperature of heat-sealing: 4) Totallized evaluation (TE): A means lower than 120 C, A industrially practicizable, C means higher than 120 C, C not practicizable.

*of Comparative Example 4: In all the Latices tested herein shown in Table 2 contained 0.2 parts of silica powder and 2.0 parts of emulsified wax per 100 parts of solid matter in the latex, however, only in Comparative Example 4, wax was added in an amount of 4.0 parts instead of 2.0 parts in other latex.

Claims

1. A latex of vinylidene chloride copolymers, said latex comprising a mixture of particles of a vinylidene chloride copolymer (I) containing more than 94% by weight of units derived from vinylidene chloride and particles of a vinylidene chloride copolymer (II) containing 70 to 85% by weight of units derived from vinylidene chloride, 10 to 28% by weight of units derived from vinyl chloride and 2 to 20% by weight of units derived from at least one other monomer copolymerisable with vinylidene chloride and vinyl chloride, said latex containing from 0.5 to 10 parts by weight of the vinylidene chloride copolymer (I) per 100 parts by weight of the vinylidene chloride copolymer (II).

2. A latex according to claim 1 wherein the ratio of the infrared absorption coefficient of the vinylidene chloride copolymer (II) at 884 cm1 to that of said copolymer (II) at 1407 cm-' is from 0.05 to 0.12:1 after a film coated with said copolymer (II) has been aged for more than 24 hours at 4O0C.

3. A latex according to claim 1 or 2 wherein the average diameter of the particles of the vinylidene chloride copolymer (I) is less than 1000 x 1 of8 cm.

4. A latex according to claim 3 wherein said average diameter is less than 500 x 10-8cm.

5. A latex according to any one of the preceding claims wherein the vinylidene chloride copolymer (I) is a copolymer of vinylidene chloride and at least one comonomer selected from vinyl chloride, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, octyl acrylate, vinyl acetate, acrylonitrile, acrylic acid and itaconic acid.

6. A latex according to any one of the preceding claims wherein the vinylidene chloride copolymer (II) is a copolymer of vinylidene chloride, vinyl chloride and at least one comonomer selected from methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, octyl acrylate, vinyl acetate, acrylonitrile, acrylic acid and itaconic acid.

7. A latex according to claim 1 substantially as hereinbefore described in any one of Examples 1 to 5.

8. A process for the preparation of a latex as claimed in any one of the preceding claims, which process comprises mixing a latex of particles of said copolymer (I) and a latex of particles of said copolymer (II) in an amount of from 0.5 to 10 parts by weight of said copolymer (I) per 100 parts by weight of said copolymer (II).

9. A process according to claim 8 substantially as hereinbefore described in any one of Examples 1 to 5.

10. A process for the preparation of a plastics film or paper coated on at least one face by a layer of two vinylidene chloride copolymers, which process comprises coating said face with a latex as claimed in any one of claims 1 to 7 or which has been produced by a process as claimed in claim 8 or 9.

11. A food package comprising a food sealed within a coated plastics film which has been prepared by a process as claimed in claim 1 0.