IMPROVED PACKAGING FILM

Abstract: Propylene polymer heat shrinkable film which contains slip and antiblocking additives nevertheless fails to be non-sticking with respect to itself immediately after being subjected to the elevated temperature of heat shrinking. Incorporation of wax such as candelilla wax into such film remedies this problem.

Description: TITLE IMPROVED PACKAGING FILM Field of the Invention This invention relates to propylene polymer composition which can be made into a film for packaging articles at elevated temperatures without the packaged articles sticking to one another after exposure to such elevated temperatures. Backσroμnd of the Invention U. S. patent 2,499,486 discloses the incorporation of di-2 ethyl hexyl phthalate and carnauba wax into polyethylene in order to obtain polyethylene film of improved clarity. The amount of these additives used is one-half to two and three parts, respectively, per 100 parts of polyethylene. U. S. patent 2,525,671 discloses the use of 0.1 to 3.0% of an ester wax, such as carnauba wax, as a slip additive for vinylidene chloride/acrylonitrile copoly er film. U. S. patent 2,711,996 discloses an additive system to film of vinylidene chloride copolymer which provides both slip and freedom from hot stick, so that the film can be used in heat sealing machines. The additive system is 0.2 to 1.0 weight percent of a long chain aliphatic polar compound, such as stearic acid, stearyl stearamide, and ontan wax, 0.2 to 1.0 weight percent of ester wax, natural or synthetic, such as carnauba wax and stearyl stearate, and 0.05 to 2.0 weight percent calcium carbonate. U. S. patent 4,590,125 discloses a multilayer film of a base layer of polypropylene of high stereoregularity surrounded by heat sealable skins of polyolefin of low stereoregularity, examples of such polyolefin being propylene polymers such as propylene/ethylene copolymer and propylene/ethylene/l- butene copolymer. The base layer contains a slip additive which "blooms" through the heat sealing layers, and the heat sealing layer contains 5 to 15 weight percent of microcrystalline wax and 0.05 to 0.5 weight percent of anti-blocking additive. The resultant film is disclosed to be useful in high speed packaging machines wherein heat sealing is used. ϋ. S. patent 4,325,859 discloses an improvement to polyolefin film, wherein combination of an anti-block agent, a slip agent and a plasticizing oil exhibits a synergistic effect in enabling the anti-block agent and the slip agent to be used without adversely affecting the optical characteristics of the film In the field of heat shrinkable film, propylene polymer can either be used as heat shrinkable film or as the outer layer of a multilayer heat shrinkable film. In either case, the propylene polymer can contain slip and anti-blocking additives such as eruca ide and finely divided silica, respectively. The anti-blocking additive enables the film to be unrolled after it has been rolled-up for storage and handling. The slip additive enables the film to be handled in the packaging operation so as not to stick to itself when wrapped around the article to be packaged and not to stick to machinery surfaces. After wrapping the film around the article, the wrapped package is subjected to heat to shrink the film onto the package. While the * slip and anti-blocking additives have been successful up to this point, unfortunately, while the film wrapping is cooling down from the elevated heat shrinking temperature, when the wrapped articles come into contact with one another, they tend to stick together, interfering with smooth machine handling of the packages. When the film cools for a sufficient period of time, the wrapped articles no longer stick together. So the problem has arisen as to how to prevent this sticking from occurring in the few seconds after the wrapped articles exit the heat shrinking section of the machine. Summary of the Invention The present invention substantially solves the problem described above. This solution is applied in the context of packaging articles at an elevated temperature with film of propylene polymer which contains slip and anti-blocking additive, wherein the packaged articles nevertheless tend to stick together while at the elevated temperature. The solution to this problem is the improvement to this packaging process which comprises incorporating into the film a small amount of wax. This small amount of wax will be less than about 0.5% based on the weight of the film. At this low concentration, if the slip additive were omitted from the film, the amount of wax would not be effective as a slip additive. This small amount of wax, however, tends to keep the wrapped articles from sticking to one another during the period of cool down from the exposure to elevated temperature. The amount of wax used at the low level indicated is an effective amount to provide this non-stick tendency during this cool down period. This effect is described in terms of non-stick tendency because wax provides the non-stick surface very quickly but not necessarily instantaneously as the wrapped articled begins its cool-down and the non-stick behavior exists during the remainder of the cool-down. Another embodiment of the present invention resides in the composition for accomplishing this improvement. . Detailed Description of the Invention The composition of the present invention contains propylene copolymer, slip additive, anti¬ blocking additive, and wax. These are the essential components of the composition. The propylene polymer component is propylene homopolymer or copolymer thereof with a minor amount, e.g. , less that about 20 weight percent, of at least one other ethylenically unsaturated copolymerizable monomer. A preferred copolymer is propylene/ethylene wherein about 2 to 6 weight percent, and more preferably about 3 to 5 weight percent, of the copolymer is ethylene. The propylene copolymer will have sufficiently high molecular weight to form a film which has toughness so that it can be used in the packaging context. Typically the melt flow index of the propylene polymer will be about 1.5 to 10 g/10 min. The slip additive can be selected from a wide variety of additives known for producing this effect in propylene copolymer. One group of such additives is the amides of a a carboxylic acid having from 8 to 24 carbon atoms and the bisamides thereof. The typical such amides will be water insoluble and except in the case of bisamides, amides of onocarboxylic acids. Examples of such amides are erucamide, olea ide, stearamide, behenamide, and N,N'-ethylene bis-stearamide. Additional examples of slip additive include sorbitol monostearate and glycerol monostea ' rate. The amount of slip additive in the composition will generally be about 0.02 to 0.3% based on the weight of the composition. Greater amounts of the slip additive tend to detract from the properties or appearance of the film made from the composition. The anti-blocking additive is any finely divided inert inorganic material, preferably having a mean particle size of about 0.5 to 5 microns. Examples of such additives include silica, glasses, and clay. The amount of this additive in the composition will generally be about 0.05 to 0.5 weight percent. The formation of the composition of propylene copolymer and slip and anti-blocking additives can be done by conventional means, such as by melt blending these components in the extruder which forms the film used for the packaging. Prior to melt blending these components, they may be dry blended. The film can be formed from the molten composition by conventional means, such as the "trapped bubble" technique, wherein the film is extruded in the form of a tube, followed by expanding the tube radially to orient the film in the transverse direction and winding up the expanded film at a faster rate than the extrusion rate to orient the film in the machine direction. The drawing of the film in the transverse and machine directions is done at a temperature at which orientation occurs. Typically, the film is drawn at least 3X in both the transverse and the machine directions. The film is wound up in the drawn state (after the expanded tubing is longitudinally slit so as to yield flat film) so that it is shrinkable upon heating. In use the film is wrapped around the article and the resultant package is passed through a "shrink tunnel", wherein heating is applied to shrink the film tightly around the article. Typically the temperature within the shrink tunnel will be about 170 to 190°C applied for a period of about 1 to 3 seconds, and the cool-down period for the film wrapping after exiting the shrink tunnel is about 2 to 20 seconds to reach the condition when the film will no longer stick to itself. Preferably, the film can shrink at least about 20% in both the transverse and machine directions. The film can be a monolayer film, i.e., unsupported prior to wrapping around the article, or the film can be supported, i.e., the outer layer(s) of a multilayer film, with the inner layer(s) contributing to the heat shrinkage property and such other properties as oxygen or moisture barrier of the overall film. Typically, the film of the composition of the present invention will have a thickness of about 0.1 to 2 mils, and any multilayer film in which this film is present will have an overall thickness of about 0.5 to 4 mils. The multilayer film can be made by the same extrusion process described above wherein, however, the layers forming the multilayer film are coextruded together. The slip additive can be melt blended with the propylene polymer, especially if it will be a monolayer film, or to an interior layer of a multilayer film wherein the propylene polymer will form the surface layer(s) . When added to the interior layer, some of the slip additive will "bloom" into and through the propylene polymer layer and be present at its outer surface to provide the slip behavior. Often the polymer present in the interior layer will already contain both slip and anti-blocking additive so that such polymer is capable of forming into a monolayer film. In addition, small amounts of these additives may be present in or added to the propylene polymer so as to complement the amount of these additives which bloom from the interior layer to the outer surface of the propylene polymer layer. In either case, the total amount of these additives present in the overall film is small so as not to adversely affect the properties thereof, and the amount of these additives actually present in the propylene polymer layer(s) will generally fall within the compositional ranges set forth above. 7 Whether the film of the composition of the present invention is an unsupported or a supported film, the wrapped articles during the short period of cool- down after exiting the heat shrinkage tunnel tend to stick together if these packages come into contact with one another unless the wax component is present in the film in accordance with the present invention. The slip and anti-blocking additives work fine to prevent the film from sticking to itself up to the time of heat shrinking. During cool-down from heat shrinking, if the packages come into contact with one another, they tend to stick together, film-to-film, interfering with the. smooth operation of the packaging operation. It is to be understood that the cool-down period includes the time for the heated film to reach room temperature and also any residence time at room temperature before the slip additive begins functioning as such again. Thus, the slip additive having lost its ability to confer slip behavior to the film during exposure to heat shrinking temperature returns to provide this behavior shortly after exposure to such temperature, e.g., generally within 20 seconds. The wax additive functions in accordance with the present invention to confer slip behavior to the film during this cool-down period. Typically, the slip effect of the wax becomes evident almost immediately after the cool-down begins, e.g., within one-half to eight seconds. Thus, delay for the film to exhibit slip behavior is greatly reduced by using the wax additive in accordance with the present invention. How fast this reduction will occur will depend on such factors as the particular propylene polymer being used and identity and amount of wax additive. In any event, the reduction in this delay is usually at least 50%, meaning that packaging equipment can be operated faster and more 21719 8 easily in handling the wrapped articles after heat shrinking. According to the improvement of the present invention, wax is incorporated into the propylene copolymer film along with the slip and anti-blocking additives (or the slip additive will become present in the film by blooming from an interior layer of a multilayer film) . Alternatively, the wax can be incorporated into an interior layer of a multilayer film and the wax will become incorporated in the outer propylene polymer layer by migration (blooming) from the interior layer to the outer layer. The amount of wax used in either case is very small, viz. less than about 0.5 weight percent. At least about 0.001 weight percent is required for appreciable non-stick effect. The amount of wax will depend on the particular wax used and will be an amount within this range which is effective to reduce the tendency of the articles wrapped with the film from sticking together at elevated temperatures used. The heat shrinking of the film will generally be carried out at about 160 to 190°C and more often about 170 to 180°C. It is during the cool-down from these temperatures and this heat shrinking operation that the movement of the wrapped articles can cause them to contact one another. The danger of the wrapped articles contacting one another arises after the wrapped articles exit the heat shrinking zone (tunnel) and the cool-down period has begun. Typically, if the wrapped articles can be kept apart for about the first one-half second after this exit, the improvement of the present invention will tend to prevent them from sticking together during the remainder of the cool-down period. How soon the "slip" time occurs for a given composition. however, will depend on such factors as the temperature and heat shrinking time used and the presence or absence of any forced cooling of the film as it exits the shrink tunnel. Examples of wax include the natural and synthetic waxes, preferably those which have a melting point of at least about 45°C and more preferably at least about 65°C and in which most of any acid groups present in the wax are esterified, typically called ester waxes. Such waxes include carnauba and candelilla waxes and mineral waxes such as montan wax, petroleum waxes such as paraffin and macrocrystalline waxes, and Fischer-Tropsch waxes. Generally, the amount of wax that will be effective will be about 0.002 to 0.05 weight percent. The amount of wax used will generally be insufficient to provide the needed slip property of the film prior to reaching the shrink tunnel. This has been demonstrated by using freshly formed film, wherein slip additive has not yet bloomed to the surface of the propylene polymer film to give the film slip behavior even when the wax additive is present. The fresh film exhibits slip during cool-down from heat shrinking but not room temperature slip required for the handling of the film up to heat shrinking. Given sufficient time, e.g., two to three weeks of storage, the blooming of the slip additive occurs and the film then has the desirable slip behavior, except for the period of cool-down from heat shrinking, with or without the wax additive. The wax component can be incorporated into the propylene copolymer composition by grinding up the wax and dry mixing it with the copolymer which will generally be in molding granule form. Alternatively, the wax can be melted and thereby mixed as a liquid with the copolymer and other additives, which upon cooling. produces a dry blend to be fed to the extruder. Within the extruder further mixing and dispersion of the wax within the melt occurs. The wax should be sufficiently thermally stable that it does not decompose to cause spots or discoloration of the film. By way of specific example of the present invention, a heat shrinkable film was made by the "trapped bubble" technique. The film was a monolayer film of ethylene/propylene copolymer in which the ethylene content was 3.5 weight percent and the melt flow index (at 230°C) of the copolymer was 4 g/10 min. Melt blended with the copolymer layer was 0.15 weight percent erucamide slip additive, 0.25 weight percent finely divided silica anti-blocking additive, and 0.005 weight percent of carnauba wax. The resultant film was 1 mil (0.025 mm) thick and had in excess of 20% heat shrinkage in the machine and transverse directions. Two packages were wrapped in the film just described and passed through a shrink tunnel heated at 350°F (177 β C) for two seconds. Upon exiting the shrink tunnel the articles were grasped by hand and rubbed together, this taking about one-half second and the packages slid easily over one another. This was repeated a number of times and the result was the same; the packages did not stick together. In contrast when this experiment was repeated except that the wax was not present in the outer layers, the packages stuck to one another. In repeated experiments, it was found that this sticking persisted during the cool-down period for about two seconds after exiting the tunnel. When candelilla wax at a concentration of 0.01 weight percent was substituted for the carnauba wax, i.e., the packages exiting the shrink tunnel exhibited satisfactory slip after the passage of about 1.5 seconds after exiting the shrink tunnel. A greater amount of this wax will shorten the time before the packages do not stick to one another. The following example is of a multilayer heat shrinkable film: The film was a symmetrical five-layer film. The core layer consisted of (a) 10 weight percent of ethylene propylene copolymer having 3.5 weight percent ethylene and melt flow index at 230°C of 3.5, (b) 67 weight percent of ultra-low density polyethylene (density of 0.912) and melt index (190°C) of 1.0, and (c) 23 weight percent of low density polyethylene (highly branched by virtue of being made in a tubular reactor) . Two tie layers were present, one on each side of the core layer, and each tie layer consisted of the same polymers as the core layer but in the following concentrations: 47 weight percent of the ethylene/propylene copolymer, 40 weight percent of the ULDPE, and 13 weight percent of the LDPE. Two outer layers were present, one on each surface of the film and adhered to the core layer by a tie layer. Each outer layer was comprised of the ethylene/propylene copolymer present in the core layer. Each outer layer constituted 15% of the overall thickness (0.015 mm) of the film. Each tie layer constituted 11% of the thickness, and the core layer constituted 48% of the overall film thickness. One multilayer film as just described had 0.02 weight percent of candelilla wax and 0.02 weight percent carnauba wax added to each in each outer layer composition (film A) , while another multilayer film as just described had no wax additive to the outer layers (film B) . Each of these films contain^ 0.21 weight percent of erucamide and 0.13 weight percent of silica distributed (initially) between the core, tie, and outer 12 layers. These films were aged for eight days after manufacture and then processed through an article wrapping machine, in which the temperature of heat shrinkage in the shrinkage tunnel was 177°C, and the heat shrinkage time was about one second. Packages wrapped with film (A) did not stick to one another as soon as 2.8 seconds passed after exiting the heat shrinkage tunnel. Packages wrapped with film (B) required at least 6.3 seconds to pass before the packages did not stick to one another. When films (A) and (B) were aged for forty additional days, the slip time for film (A) dropped to 2.3 seconds, while the slip time for film (B) was 4.2 seconds. The weight percent of slip, anti-blocking and wax additives described herein is based on the total weight of the layer (or monolayer film) composition, i.e., weight of the copolymer plus additive. As many widely different embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims.

Claims: 13 CLAIMS 1. In the process of packaging articles in film at elevated temperature, which film of propylene polymer which contains slip and anti-blocking additive, wherein the packaged articles nevertheless tend to stick together while at said elevated temperature, the improvement comprising incorporating into said film an effective amount less than about 0.5% based on the weight of the film of wax to reduce the tendency of said packaged articles to stick together, which amount of said wax is ineffective to prevent said film from sticking to itself prior to use in said packaging. 2. In the process of claim 1 wherein said propylene polymer is ethylene/propylene copolymer wherein the ethylene content is about 2 to 6% based on the weight of the copolymer. 3. In the process of claim 1 wherein said propylene is polypropylene. 4. In the process of claim 1 wherein at least about 0.001% by weight of said wax is incorporated into said film. 5. In the process of claim 1 wherein the amount of said wax is about 0.002 to 0.05 weight percent. 6. In the process of claim 1 wherein said wax has a melting point of about 60 to 100°C. 7. In the process of claim 1 wherein said wax is a natural or synthetic wax. 8. In the process of claim 1 wherein said natural or synthetic wax is an ester wax. 9. In the process of claim 1 wherein said wax is carnauba or candelilla wax. 10. In the process of claim 1 wherein said slip additive is an amide of a a carboxylic acid containing from 8 to 20 carbon atoms and said anti¬ blocking additive is finely divided inorganic material. 11. In the process of claim 1 wherein the film is heat shrinkable film and the elevated temperature is from the heating used to shrink the film after wrapping around each said articles to effect packaging thereof. 12. Composition of propylene copolymer containing slip and anti-blocking additive and in addition, about 0.001 to 0.05% of wax based on the weight of the composition. 13. Composition of claim 12 in the form of a film which is supported or unsupported. 14. Composition of 13 wherein said film is heat shrinkable.

Inventors: KNEALE TIMOTHY MICHAEL, KNEALE, TIMOTHY, MICHAEL
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1992
Main classification: C08L23
IPC codes: C08J5, C08L23