The present invention relates to a vinylidene chloride interpolymer possessing an improved combination of properties. Specifically, this invention relates to an improved barrier resin comprising a mixture of vinylidene chloride interpolymer and magnesium hydroxide, which resin has good clarity and, after being subject to elevated processing temperatures, good color characteristics.

Resins of vinylidene chloride interpolymers are well-known in the prior art. In the past, vinylidene chloride interpolymers have been produced by an emulsion or suspension polymerization process. Both emulsion and suspension polymerization processes typically produce an aqueous dispersion of polymer particles having a relatively small particle diameter. The polymer particles are recovered from the aqueous dispersion by drying or other means for removing a majority of the aqueous phase. In the past, the practice has been to extrude the vinylidene chloride interpolymer directly from the form in which it is recovered.

In an effort to improve the extrudability of compounds comprising vinylidene chloride interpolymers,

such compounds are fabricated mainly from vinylidene chloride interpolymers and an adequate amount of modifiers such as stabilizers, plasticizers, etc. Unless modifiers are used with the resin, the melt viscosity of the resin is so high that the load on the extruder screw is too large and the extruded compound is subject to thermal decomposition and discoloration due to the close proximation of the compound's thermal decomposition point and melting point. Moreover, the decomposed interpolymer may generate an undesirable level of carbon contamination in the extrudate, which could have an effect upon the gas barrier of the extrudate. In other words, unless appropriate modifiers are used, such interpolymers typically have extremely poor extrudability.

In order to industrially extrude and process the compound of vinylidene chloride resin by using a conventional screw-type extruder without thermal decomposition and discoloration of the product, a relatively large amount of a stabilizer and plasticizer would inevitably have to be incorporated in the resin. The larger amount of stabilizer and plasticizer would lower the melting point of the vinylidene chloride resin, with an accompanied reduction of melt viscosity and improvement of thermal stability of the compound, but with a decrease in barrier to atmospheric gases.

In some instances, it is desirable to form the vinylidene chloride interpolymer into pellets prior to final extrusion. With the increased demand for pellets, the processing conditions in which pellets are exposed has become more demanding. Although satisfactorily extrudable for a period, it has been found that attempts to extrude vinylidene chloride interpolymer pellets over

long periods on certain extrusion equipment have also proven unsatisfactory due to an undesirable level of carbon contamination in the extrudate, and discoloration of the extrudate.

5

It is desirable to produce a vinylidene chloride interpolymer which interpolymer, in either powder or pellet form, is capable of being extruded without having an unacceptable level of carbon

10 contamination, as well as having good color and clarity characteristics.

In a first broad aspect, the present is a process for improving the clarity of a thermally

15 sensitive resin comprising the step of blending into a generally homogeneous mixture, a vinylidene chloride interpolymer and a quantity of a magnesium hydroxide in an amount of from 0.01 to 5 weight percent of the total monomer mixture, excluding a process which comprises the

20 step of blending into a generally homogeneous mixture a vinylidene chloride interpolymer and a combination of additives which comprises an extrusion aid selected from the group consisting of oxidized polyethylene; oxidized _p .- polypropylene; or mixtures thereof, in an amount of from about 0.01 to about 0.05 weight percent; an ethylene- vinyl acetate copolymer present in an amount of from about 0.01 to about 2 weight percent; a paraffin wax present in an amount of from about 0.005 to about 1

30 weight percent; and an epoxidized oil or resin present in an amount of from about 0.1 to about 3 weight percent, all weight percentages being based on the total weight of the mixture.

35 In a second broad aspect, the present invention is a composition comprising a generally homogeneous

mixture of a vinylidene chloride interpolymer and magnesium hydroxide in an amount of from 0.01 to 5 weight percent, based on the total weight of the mixture.

Vinylidene chloride interpolymers suitable for use in the present invention are those vinylidene chloride interpolymers formed from vinylidene chloride and an amount of one or more monoethylenically unsaturated monomer copolymerizable with vinylidene chloride.

The vinylidene chloride interpolymers have polymerized therein vinylidene chloride-in an amount of from 40 to 98 weight percent, beneficially from 50 to 96 weight percent, and desirably from 60 to 94 weight percent, based on total weight of the vinylidene chloride interpolymer.

The vinylidene chloride interpolymer comprises one or more monoethylenically unsaturated monomer copolymerizable with vinylidene chloride. The amount of monoethylenically unsaturated monomer is suitably from 60 to 2 weight percent, beneficially from 50 to 4 weight percent, and desirably from 40 to 6 weight percent, based on total weight of the vinylidene chloride interpolymer.

Monoethylenically unsaturated monomers suitable for use in the present invention include vinyl chloride, alkyl acrylates, alkyl methacrylates, acrylic acid, methacrylic acid, itaconic acid, acrylonitrile, and methacrylonitrile, and the like. The ethylenically unsaturated monomers are desirably selected from the group consisting of vinyl chloride, alkyl acrylates, and

alkyl methacrylates, the alkyl acrylates and alkyl methacrylates having from 1 to 8 carbon atoms per alkyl group. The alkyl acrylates and alkyl methacrylates preferably have from 1 to 4 carbon atoms per alkyl group. The alkyl acrylates and alkyl methacrylates are most preferably selected from the group consisting of methylacrylates, ethylacrylates, and methyl methacrylates.

Methods of forming the vinylidene chloride interpolymers suitable for use in the present invention are well-known in the prior art. The vinylidene chloride interpolymer is generally formed through an emulsion or suspension polymerization process. Exemplary of such processes are U.S. Patents 2,558,728; 3,007,903; 3,642,743; and 3,879,359; and the methods described by R. A. Wessling, in Polvvinylidene Chloride, Gordon and Breach Science Publishers, New York, 1977, Chapter 3. Typically, the monomeric materials are emulsified or suspended in an aqueous phase. The aqueous phase contains a polymerization initiator and a surface active agent capable of emulsifying or suspending the monomeric materials in the aqueous phase. The polymerization of the monomeric materials is usually carried out with heating and agitation.

After polymerization is complete, the aqueous phase is the majority phase in the resulting suspension or emulsion of vinylidene chloride interpolymer. The resultant polymeric material is vacuum stripped. Thereafter, the slurry is cooled down, unloaded and dewatered, and the resin is collected and further dried.

One aspect of the present invention concerns a process for improving the clarity of the above described

vinylidene chloride interpolymer. The process comprises blending the vinylidene chloride interpolymer with magnesium hydroxide.

j- Magnesium hydroxide is suitably blended with the vinylidene chloride interpolymer in an amount of from 0.01 to 5 weight percent of stabilizer. Beneficially, the stabilizer is blended with the vinylidene chloride interpolymer in an amount of from

10 0.1 to 4 weight percent of stabilizer. Preferably, the stabilizer is blended with the vinylidene chloride interpolymer in an amount of from 0.5 to 2 weight percent of stabilizer.

15 A variety of additives may be added to the compatibilized blends of the present invention. Additive type and amount thereof will depend upon several factors. One factor is the intended use of the blends. A second factor is tolerance of the blends for

20 the additives. That is, how much additive can be added before physical properties of the blends are adversely affected to an unacceptable level. Other factors are apparent to those skilled in the art of polymer 2_- formulation and compounding.

Additives which may be incorporated into the mixture of the present invention are selected from the group consisting of plasticizers, heat stabilizers,

30 light stabilizers, pigments, processing aids, lubricants and the like. Each of these additives is known and several types of each are commercially available.

A preferred combination of additives has been -,j- found to provide uniquely beneficial results when the mixture of vinylidene chloride interpolymer and

magnesium hydroxide is formulated into a pellet. Generally, the preferred combination of additives comprises the following components. A first additive is selected from the group consisting of oxidized polyethylene; oxidized polypropylene; or mixtures thereof. A preferred first additive is Allied 629A, an oxidized polyethylene commercially available from Allied Corp. The first additive is present in an amount between 0.01 to 0.5 weight percent, based on the total

10 weight of the mixture. A second additive is ethylene- vinyl acetate copolymer. A preferred second additive is EVA 3180, an ethylene-vinyl acetate copolymer commericially available from E. I. DuPont Demours Co. - The second additive is present in an amount between about 0.01 to about 2 weight percent, based on the total weight of the mixture. A third additive is paraffin wax. A preferred third additive is Bohler 1421, a paraffin wax commercially available from Bohler 0 Industries. The third additive is present in an amount between about 0.005 and about 1, based on the total weight of the mixture. A fourth additive is epoxidized oils or resins. A preferred fourth additive is Vikoflex 7177, an epoxidized oil commercially available from the 5 Viking Chemical Company. The fourth additive is present in an amount of between 0.1 to 3 weight percent, based on the total weight of the mixture.

Blending of the vinylidene chloride 0 interpolymer, the magnesium hydroxide, and any other additives can be accomplished by using conventional melt processing, as well as dry blending techniques.

For melt blending, two conditions must be met. 5 First, melt processing must be accomplished at a temperature below that at which decomposition of the

vinylidene chloride interpolymer becomes significant. Second, sufficient shear must be generated during melt processing to provide a generally homogenous extrudate within a reasonable mixing time. Conventional melt processing equipment which may be used includes heated two-roll compounding mills, Brabender mixers, Banbury mixers, single screw extruders, twin screw extruders, and the like. Desirable results are obtained when an extruder, either single screw or twin screw, is used for melt blending the vinylidene chloride interpolymer and the magnesium hydroxide.

When dry blending, the components should form a visually uniform admixture. Suitable dry blending equipment includes Hobart mixers, Welex mixers, Henschel High Intensity mixers, and the like.

After blending, the mixture of vinylidene chloride interpolymer and magnesium hydroxide is then extruded. In one embodiment, the mixture is physically blended and then melt processed into any suitable final product.

In one embodiment, the mixture of vinylidene chloride interpolymer and .magnesium hydroxide may suitably be pelletized. Methods of forming the mixture into pellets are well-known to those skilled in the art. Any method capable of forming the mixture into pellets is suitable for use in the present invention. For the purposes of this application, the terms "pellet" or "pellets" refer to particles having a minimum cross- sectional dimension of at least 1/32 inch, preferably of at least 1/16 inch, and most preferably of at least 1/8 inch, said pellets suitably have a maximum cross- sectional dimension of at least 1/2 inch, preferably of

at least 3/8 inch, and most preferably of at least 1/4 inch. Exemplary of a method suitable for use in forming the pellets of the mixture are extrusion through a strand die and pelletization by chopping the extruded strand into pellets.

Applicant has discovered that the process and composition according to the present invention improves the clarity of the vinylidene chloride interpolymer and allows for the satisfactory extrusion of vinylidene chloride interpolymer pellets formed therefrom. The pellets are considered to possess "improved extrudability" when the mixture of vinylidene chloride interpolymer and magnesium hydroxide can be formed into a pellet which possesses less carbon contamination and less discoloration than a pellet formed from the vinylidene chloride interpolymer alone.

The process of the present invention can be used to form a variety of films or other articles. As is well known in the art, the films and articles are fabricated with conventional coextrusion, e.g, feedblock coextrusion, multimanifold die coextrusion, or combinations of the two; injection molding; extrusion molding; and lamination techniques. Articles formed therefrom include blown and cast, mono and multilayer, films; rigid and foam sheet; tubes; pipes; rods; fibers; and various profiles. Lamination techniques are particularly suited to produce multi-ply sheets. As is known in the art, specific laminating techniques include fusion, i.e., whereby self-sustaining lamina are bonded together by applications of heat and pressure; wet combining, i.e., whereby two or more plies are laminated using a tie coat adhesive, which is applied wet, the liquid driven off, and combining by subsequent pressure

laminating in one continuous process; or by heat reactivation, i.e., combining a precoated film with another film by heating and reactivating the precoat adhesive so that it becomes receptive to bonding after subsequent pressure laminating.

Exemplary articles include rigid containers used for the preservation of food, drink, medicine and other perishables. Such containers should have good mechanical properties, as well as low gas permeabilities to, for example, oxygen, carbon dioxide, water vapor, odor bodies or flavor bodies, hydrocarbons or agricultural chemicals. Most organic polymers such as the polyolefins, styrene polymers and the like, by themselves, do not possess sufficient resistance to transmission of atmospheric gases and vapors. Consequently, multilayer sheet structures employed in packaging materials have organic polymer skin layers laminated on each side of a vinylidene chloride interpolymer barrier layer, generally with glue layers used to promote adhesion between the barrier layer and dissimilar material layers.

The present invention is illustrated in further detail by the following examples. All parts and percentages are by weight unless otherwise specifically noted.

Comparative Example 1 and Examples 2-7

A vinylidene chloride interpolymer is formed through a suspension polymerization process. The vinylidene chloride interpolymer is formed from a monomer mixture comprising about 80 weight percent

vinylidene chloride and about 20 weight percent vinyl chloride, based on total monomer mixture weight.

The vinylidene chloride interpolymer is dry _c blended into a generally homogeneous mixture with various quantities of magnesium hydroxide as shown in Table 1 below. The magnesium hydroxide is commercially available from Kyowa Chemical Company, under the trade designation Kisuma 5B. The powdery mixtures are formed 10 by dry blending the two components in a Hobart mixer, for a period of approximately one hour. The mixtures are extruded through a 3/4" extruder having a length to diameter ratio of 20/1. The extruder has the following set temperatures: (a) hopper temperature of 160°C; (b)

15 melt zone temperature of 170°C; and (c) die temperature of 175°C. From the extruder, the blends are passed to a slit-type die and extruded into a tape. Total dwell time in the extruder is about 20 minutes. 0 The extrudate tape is visually inspected to determine its "color" and "clarity" by the following tests. "Color" is qualitatively rated on a scale of 1 to 5 over a continuous range of discoloration, wherein 1 »- represents a creamy color and 5 a rather dark brown.

"Clarity" is qualitatively rated on a scale of 1 to 5 over a continuous range of transparency, wherein 1 represents a transparent sample and 5 an opaque sample.

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5

TABLEI

not an example of the present invention.

All examples are carried out with a vinylidene chloride copolymer of 80 weight percent vinylidene chloride and 20 weight percent vinyl chloride, having a weight average molecular weight of 90,000.

Mg(0H) ₂ = magnesium hydroxide, commercially available from Kyowa Chemical Company, under the trade designation Kisuma 5B.

color = according to visual inspection

clarity = according to visual inspection.

As can be seen from the above table, the compositions of the present invention possess good color and clarity. Examples 2-6 also exhibited relatively low level of carbon contamination as compared with Comparative Example 1.

Comparative Example 8 and Examples 9-14

Examples 1-7 are repeated with the following exception: the mixture of vinylidene chloride interpolymer and magnesium hydroxide is extruded through a single strand die, passed through a water bath, and then pelletized. The strand die had an internal diameter of 0.32 centimeter. Pelletizing was accomplished using a Model 304, 15.24 centimeter strand cutter commercially available from Conair Incorporated. The pellets exhibited similarly good color and clarity, and reduced carbon contamination.

Comparative Example 15 and Examples 16-23

Examples 1-7 are repeated with the following exception: the vinylidene chloride interpolymer is formed from a monomer mixture comprising about 94 weight percent vinylidene chloride and about 6 weight percent methyl acrylate, based on total monomer mixture weight. The copolymer has a weight average molecular weight of 100,000. The extrudate tape exhibited similarly good color and clarity, and reduced carbon contamination.

Comparative Example 24 and Examples 25-32

Examples 15-23 are repeated with the following exception: the mixture of vinylidene chloride interpolymer and magnesium hydroxide is extruded through a single strand die, passed through a water bath, and

then pelletized. The strand die had an internal diameter of 0.32 centimeter. Pelletizing was accomplished using a Model 304, 15.24 centimeter strand cutter commercially available from Conair Incorporated. The pellets exhibited similarly good color and clarity, and reduced carbon contamination.

Although the invention has been described in considerable detail, with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be affected within the spirit and scope of the invention as described above and as defined in the appended claims.