POLYOLEFIN COMPOSITIONS FOR EXTRUSION COATING This invention relates to polyolef n compositions containing at least one thermally degraded crystalline propylene polymer such as thermally degraded crystalline polypropylene and at least one low density polyethylene. The compositions are formulated using polymers having specific properties that are essential to provide extrusion coatings exhibiting an outstanding combination of coatability, adhesion and heat seal strength.

Extrusion coating of a single polyolefin or blend of polyolefins onto a substrate such as paper or aluminum foil, to form a coated substrate is known in the art. Various polyethylenes and blends of polyethylenes with other polyolefins have been widely used as extrusion coating compositions. However, crystalline polypropylene, regardless of its melt flow rate, is not a satisfactory extrusion coating material since it normally does not form acceptable coatings at high speeds, nor can it be coated over a wide range of coating weights. Therefore, many of its excellent physical properties cannot be utilized in extrusion coating applications.

To improve the coating properties of polypropylene, blends of polypropylene with polyethylene have been used to form extrusion coatings. For example, U.S. Patent 3,418,396, issued December 24, 1968, discloses blends of polypropylene with polyethylene that have excellent extrusion coating properties.- Unfortunately, extrusion coatings formed from such blends fail to provide the heat seal strength needed in many packaging materials, particularly those employed to form containers used to both store and cook foodstuffs.

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Also, presently available polyolefin compositions cannot be extruded at commercially acceptable coating speeds to form thin, good quality coatings having heat seal strengths greater than about 22.2 N per linear 2.54 cm. Accordingly, it would be an advance in the state of the art to provide coating compositions that could be applied to substrates by extrusion coating at commercially acceptable coating speeds, e.g. coating speeds of 48 meters per minute

10 or more, to form coatings having good adhesion to the substrates and exhibiting heat seal strengths much greater than 22.2 N per linear 2.54 cm., e.g. heat seal strengths of at least 44 N per linear 2.54 cm. This invention provides polyolefin

15 compositions that are particularly useful in forming coatings by extrusion. Such polyolefin compositions have a melt flow rate of 5 to 55 dg. per minute at 230°C and comprise (1) 75 to 95, often 80 to 90, percent, by weight, thermally degraded crystalline

20 propylene polymer having a melt- flow rate of 5 to 55 dg. per minute at 230°C and (2) 5 to 25, often 10 to 20, percent, by weight, polyethylene having a density of 0.916 to 0.925, a melt index at 190°C of 0.5 to 4.5 and a melt index recovery greater than 40. The

^• --5 proper selection of components according to this invention, provides polyolefin compositions that form coatings by extrusion having thicknesses of less than 0-03 mm. and heat seal strengths of at least 44 N per 2.54 cm., determined using a tensile tester at a jaw

30 separation rate of 25.4 cm. per minute after heat sealing at a temperature of 260°C using a bar sealing device.

The compositions of this invention are particularly useful for making extrusion coated 5

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substrates that can be formed into so-called "retortable pouches or packages." Such pouches or packages are used as containers for foodstuffs such as vegetables or sauces which are stored under refrigeration and then cooked, e.g. in boiling water, without being removed from the container. For this use, it is very desirable, and in some instances required, that the coating have a high heat sealing strength. The compositions of this invention provide excellent coatings on substrates such as paper stock and primed aluminum foil which are particularly useful for fabricating the retortable food pouches or packages. Such use requires the coating to substrate bond strength to be high for fabricating food packages where handling or flexibility is encountered. Depending upon the end use, such coated substrates should also have good adhesion, flexibility, barrier properties and heat resistance. For example, retortable food storage pouches need sufficient adhesion strength to be handled during filling of the pouch, during preparation and storage and subsequent heat seal resistance during immersion in boiling water and subsequent handling.

It is quite surprising that the polyolefin compositions of this invention for thin coatings exhibiting high seal strengths, e.g. at least 44 N per 2.54 cm., often at least 66 N per 2.54 cm. Thus, a comparable blend formed from crystalline polypropylene or propylene copolymer which has not been thermally degraded, with low density polyethylene provides a coating that exhibits a heat seal strength of only about 22 N per cm.

The polyolefin compositions of this invention have a melt flow rate of from 5 to 55 dg.

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per minute at 230°C. At commercially acceptable coating speeds such as 48 m. per minute, a composition having a melt flow rate less than 5 dg. per minute exhibits "surging", an uneven flow resulting in a rough or streaked surface, or tearing of the freshly extruded polyolefin web. At melt flow rates in excess of 55 dg. per minute, there is a tendency for the polyolefin web to streak or "neck-in", i.e. reduce in width between the point of extrusion and the point of contact with the substrate. This results in wasteful edge beads of polyolefin which must be trimmed from the product. The melt flow rate (sometimes simply referred to as flow rate) for the polyolefin composition, as well as for the thermally degraded propylene polymer, can be determined using any convenient method. One such method is to determine melt flow rate by Plastics Laboratory Test Procedure No. 270-204 in conformance with ASTM designation D 1238.

The polyolefin compositions of this invention contain, as one component, at least one thermally degraded crystalline propylene polymer having a melt flow rate of 5 to 55 dg. per minute at 230°C. Examples of such polymers are crystalline polypropylene and crystalline copolymers of propylene with other alpha olefins, such as propylene/alpha olefin copolymers in which the alpha olefins contain 2 to 10 carbon atoms, as exemplified by ethylene, butene-1, 3-methyl-1-butene, hexene-1 and decene-1. Preferred propylene polymers are crystalline polypropylene and crystalline propylene/ethylene copolymers containing up to 5 weight percent, ethylene. The propylene polymer component is obtained by thermally degrading a crystalline

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propylene polymer having a lower melt flow rate until the flow rate is increased such that it is in the required range of 5 to 55 dg. per minute at 230°C. For example, a crystalline polypropylene having a melt flow rate of 1 to 2 dg. per minute at 230°C is thermally degraded until its melt flow rate is 10 to 40 while a crystalline propylene/ethylene copolymer having a melt flow rate of 3 or less dg. per minute at 230°C is thermally degraded until its melt flow rate is 5 to 55.

The crystalline propylene polymers ' (thermally undegraded) used to prepare the crystalline thermally degraded crystalline propylene polymers used in practicing this invention are well known in the art. Such polymers can be prepared using so-called stereospecific catalyst systems. A method of preparing the aforementioned crystalline polypropylene is described in U.S. Patent 3,679,775, issued July 25, 1972. This patent pertains to a process for polymerizing alpha olefins such as propylene employing a catalyst containing an organopolylithiumaluminum compound, an alkyl compound and the alpha form of titanium trichloride. Crystalline propylene/alpha olefin copolymers that are particularly useful are described in U.S. Patent 3,529,037, issued September 15, 1970. This patent pertains to so-called "polyallomers" which are crystalline copolymers comprising major segments of polymerized propylene and minor segments of polymerized alpha monoolefins. The crystallinity of the lower melt flow rate crystalline propylene polymers, as well as that of the thermally degraded higher melt flow rate crystalline propylene polymers

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can be determined using any convenient method. For example, insolubility in an organic solvent such as hexane is a well known indication of crystallinity. The crystalline propylene polymers are preferably highly crystalline, i.e. they are preferably at least 88 percent hexane insoluble.

Methods for thermally degrading crystalline propylene polymers having a lower melt flow rate to form the crystalline propylene polymers having the higher melt flow rate described herein are well known in the art. For example, crystalline propylene polymer of high melt flow rate can be thermally degraded simply by extruding it through a conventional screw type extrusion apparatus, usually in the substanial absence of oxygen, at elevated temperatures for an appropriate time to achieve polymer chain scission and the desired increase in melt flow rate. It is often advantageous to conduct the degradation in the presence of a free-radical initiator such as an organic peroxide as exemplefied by benzoyl peroxide, di-tert. amyl peroxide or t-butyl hydroperoxide. Thermal degradation reactions of this type are described in detail in U.S. Patent 3,144,436, issued August 11, 1964 and U.S. Patent 3,849,241, issued November 19, 1974.

The compositions of this invention contain at least one low density polyethylene. Such polyethylene has a melt index at 190°C. of 0.5 to 4.5, preferably 3.5, a density of 0.916 to 0.925 and a melt index recovery greater than 40, preferably 50 or greater, most preferably about 70. Such low density polyethylene can be prepared by methods known in the art. Typical low density polyethylene useful

in practicing this invention is described in U.S. Patent 3,418,396, issued December 24, 1968. The melt index, density and melt index recovery values of the low density polyethylene can be determined using conventional procedures. For example, melt index can be determined in accordance with ASTM D-1238 and density can be determined in accordance with ASTM D-1505. Details for measuring melt index recovery (MIR or plastic recovery) are set forth in the prior art., e.g. U.S. Patent 3,418,396, at column 5, lines 59-73. Melt index recovery is the increase in the diameter of an extrudate over the diameter of the orifice of the extrusion plastometer in ASTM designation D-1238-62T. The diameter of the sample is measured in the area between 1.5 mm and 10 mm of the initial portion of the sample as it emerges from the extrusion plastometer. Measurements are made by standard methods according to ASTM designation D-374.

As previously indicated herein, the polyethylene component used in practicing this invention is present in the compositions in an amount of 5 to 25 percent, by weight, based on the weight of the composition. However, depending upon the type of thermally degraded crystalline propylene polymer used, and the particular combination of properties desired, the specific amount of polyethylene will vary within the broad range. For example, as the amount of polyethylene in a composition containing thermally degraded crystalline polypropylene increases beyond

15 percent, by weight, there is some sacrifice in heat seal strength of the resultant coating.

Likewise, compositions containing thermally degraded crystalline propylene copolymers such as propylene/ethylene copolymer generally exhibit the best quality thin coatings at higher coating speeds

such as 213 m. per minute when the amount of polyethylene in the composition is 15 to 25, percent, by weight.

The polyolefin compositions of this invention can be prepared in various ways. The components can be dry-blended and then passed through a compounding extruder, milling roll or Banbury mixer. Pellets of each polymeric component can be blended mechanically and the blend then fed to an extruder where it is fused and extruded. Any method whereby the components are uniformly blended will produce the desired composition.

Additives, stabilizers, pigments, and fillers which are commonly employed in polyolefin compositions can be added to the compositions of this invention. Preferably, these coating compositions are stabilized against thermal degradation since coatings formed therefrom are generally applied at elevated temperatures. Such materials can be present in the components forming the polyolefin composition or they can be added when the polymers are blended to form the extrusion coating composition.

The polyolefin compositions can be coated on a wide variety of substrates including paper, metallic sheet and foil. Due to their excellent adhesion and flexability they can be used to coat substrates employed as photographic paper supports. Of course, such use does not require the superior heat sealing characteristics which are. needed in applications such as the retortable pouches or packages discussed herein. Typical useful photographic paper supports are those which are partially acetylated or coated with baryta.

This invention can be further illustrated by the following examples.

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Example 1

A polyolefin composition having a flow rate of 30 at 230°C (ASTM D-1238) was prepared by blending 90 percent, by weight, thermally degraded crystalline polypropylene with 10 percent, by weight, polyethylene.

The polypropylene component was obtained by thermal degradation in the presence of an organic peroixde and had a melt flow rate of 35 at 130°C (ASTM D-1238).

The polyethylene component had a melt index of 3.5 dg. per minute at 190°C (ASTM D-1238), density of 0.917 g. per cc. (ASTM D-1505) and melt index recovery of 70. As indicated previously, melt index recovery is the increase in the diameter of the extrudate over the diameter of the orifice of the extrusion plastometer in ASTM designation D-1238-62T. It was measured as described in detail hereinbefore. To demonstrate the extrusion coating characteristics and the properties of the resulting coatings, the polymeric components of the composition were blended by feeding them to an 8.89 cm. Egan extruder having a barrel length to diameter ratio of 24:1. The four zones of the extruder were maintained, from back to front, at 204°C, 260°C. , 3l6°C.,-and 338°C. A metering type screw having six compression flights, and 12 metering flights was used. Prior to entering the die the melt passed through a 9 x 9 strand per square cm. mesh screen. The die was an Egan die, center-fed with 1.27 cm. long lands, with an opening of 40.64 cm. x 0.05 cm. The temperature of the die was held at 316°C. The extrusion rate was held constant at 59 kg. per hour.

The resulting film extrudate was passed through an 11.4 cm. air gap into the nip formed by a rubber-covered pressure roll and a chill roll. At the same time, 18.14 kg. basis weight Kraft paper stock 40.64 cm. wide was fed into the nip with the pressure roll in contact with the foil. The nip pressure applied was 445 N per linear 25.4 mm. The ^' chill roll was a 60-96 cm. diameter matte finish steel roll, water cooled to maintain a temperature of 15.5°C. on the roll. The coated paper was taken off the chill roll at a point 180° from the nip formed by the pressure roll and chill roll. The chill roll was operated at linear speeds of 48.8 m. to greater than 213.4 m. per minute which is an accepted target range for commercial extrusion coatings.

At a coating speed of 213.4 m. per minute the coating on the paper stock was 0.03 mm. thick and had excellent adhesion to the paper stock. The coated paper had a heat seal strength of 80 N per linear 2.54 cm. using a tensile tester at a jaw separation rate of 25.4 cm. per minute after heat sealing at a temperature of 260°C. using a conventional bar sealing device. EXAMPLE 2 An extrusion coating composition was prepared and tested according to Example 1 except that 20 "percent, by weight, polyethylene, and 80 percent, by weight, of the thermally degraded crystalline polypropylene were used. The heat seal strength of the coated paper was 53-38 N per linear 2.54 cm. The composition extrusion coated sat sfactorily. However, a comparison of the heat seal strength obtained in Example 1 (80 N) with that obtained in this Example (53.38 N) shows that

increasing the polyethylene level to 20 percent, by weight, provided a useful extrusion coating composition, but sacrificed some heat seal strength. The use of more than the 25 percent, by weight, of polyethylene, as described and claimed herein, has a substantial deleterious effect upon the heat sealability of the coated substrate. To illustrate, an extrusion coating composition was prepared and evaluated according to Example 1 except that 30 percent, by weight, polyethylene and 70 percent, by weight, thermally degraded crystalline polypropylene were used. The composition extrusion coated onto Kraft paper satisfactorily. However, the heat sealability of extrusion coating was only 7 pounds per linear 2.54 cm. . EXAMPLE 3

A polyolefin composition was prepared and evaluated according to the procedure of Example 1 except that 20 percent, by weight, polyethylene was blended with 80 percent, by weight, of thermally degraded crystalline propylene/ethylene copolymer having a melt flow rate of 30, and an ethylene content of 1.2 percent, by weight. The composition extrusion coated onto primed aluminum foil satisfactorily. Good quality coatings less than 0.01 mm. thick were obtained at coating speeds greater than 396.24 m. per minute. The heat seal strengths of such coatings were at least 84.5 N -per linear 2.54 cm. Similar ^' results can be obtained when thermally degraded crystalline propylene/alpha olefin copolymers such as propylene/butene-1 or propylene/- decene-1 copolymers are used in place of the thermally degraded crystalline propylene/ethylene copolymer.

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The use of more than the 25 percent, by weight, polyethylene, as described and claimed herein, with thermally degraded crystalline propylene/ethylene copolymer has a substantial deleterious effect upon heat seal strength. To illustrate, a polyolefin composition was prepared and evaluated according to Example 1 except that 30 percent, by weight, polyethylene and 70 percent, by weight, thermally degraded ethylene/propylene copolymer were used. The composition coated satisfactorily on Kraft paper at 365.76 m. per minute to form a coating 0.01 mm. thick, but the heat seal strength of the coating was only 26.69 N per linear 2.54 cm. The use of less than the 5 percent, by weight, of polyethylene, as described and claimed herein, fails to meet coatability requirements. To illustrate, a polyolefin composition was prepared and evaluated according to Example 1 except that 3 percent, by weight, polyethylene and 97 percent, by weight, thermally degraded polypropylene were used. The composition failed to form acceptable quality extrusion coating at speeds in excess of 30.5 m. per minute. In another run, a polyolefin composition was prepared and evaluated according to this Example 3 except that 3 percent, by weight, polyethylene and 97 percent, by weight, thermally degraded, ethylene/- propylene copolymer were used. As in the run described in the ^' previous paragraph, this composition coated satisfactorily at only 30.5 m. per minute.

EXAMPLE 4

A polyolefin composition was prepared and evaluated according to Example 1 except that crystalline polypropylene which had been thermally degraded in the absence of organic peroxide to provide a melt flow rate of 11 was used in place of the thermally degraded crystalline polypropylene of Example 1. The composition had a melt flow rate of 10 and coated satisfactorily to provide a thin coating having a heat seal strength of 115.65 N per linear 2.54 cm..

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