JPH024426B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to thermoplastic, heat-shrinkable packaging films, particularly polypropylene films with improved heat-sealing properties. These films can be widely used for packaging various food and non-food products. Oriented polypropylene films are useful and widely accepted as packaging films because of their good moisture barrier properties, toughness, high strength, and optical properties. However, polypropylene films generally do not exhibit good heat sealing properties, an important consideration in packaging applications. This is due to the fact that polypropylene films have a very narrow sealing temperature range. The sealing temperature range of a thermoplastic film can be thought of as the point at which the surfaces of the film begin to seal or bond together when heat and pressure are applied.
As the temperature increases further, a point is reached where the thermoplastic melts and flows easily, making it difficult to achieve a seal within a given area, which defines the upper end of the sealing temperature range. And also at the upper end of the sealing temperature range the film will tend to char and leave a weak and ugly seal. In the case of propylene homopolymers, the film does not become sealable as the temperature increases, but rather begins to flow rapidly as the melting point is approached, making it quite difficult to obtain a smooth, continuous and satisfactory seal. Such sealing regimes are unacceptable in high speed packaging equipment. Various types of coatings have been applied to films in the past in order to obtain good sealability. Conventionally, coatings have been applied by separate, separate coating operations such as emulsion coating, extrusion coating, etc., but such operations are expensive and require additional handling of the film. Although coextrusion has successfully applied heat-sealable coatings onto polypropylene to produce films with good seal strength, coextruded polypropylene film laminates prior to the present invention had very narrow thermal seals. It had only a sealing area and required high manufacturing costs. It is therefore an object of the present invention to provide a polypropylene film with a wide heat sealing range. Another object of the invention is to provide a polypropylene film having acceptable seal strength for shrink wrapping applications. The major US patents relating to polypropylene film coatings are as follows. U.S. Pat. No. 3,285,766 (November 15, 1966, Edward Barkis et al.) discloses a method for extruding a coating containing at least 65% ethylene onto a polypropylene sheet; U.S. Pat. No. 3,671,383 (1972) June 20th, Rikita
Sakata et al.) at least 75% by weight polypropylene
discloses a biaxially oriented polypropylene film laminated to a uniaxially oriented film of an ethylene-propylene copolymer; U.S. Pat. No. 4,132,050
No. (January 2, 1979, Roger N. Young) describes a film consisting of a substrate and a heat-sealable layer made from a blend of 87.5 to 60 parts of polypropylene and 12.5 to 40 parts of ethylene-propylene loblock copolymer. No. 4,148,972 (April 10, 1979, Toshihiro Yamane et al.) discloses a polypropylene film laminate made by laminating a polypropylene layer having 1 to 8 weight percent ionomer with a layer of ionomer. Disclosing the body. It is now surprising that the primary object of the present invention to increase the sealing temperature range of multilayer polypropylene films and to provide such films with acceptable sealing strength is to provide a polypropylene homopolymer base layer and at least one base layer of said base layer. In a heat-sealable, biaxially oriented, multilayer, optionally perforated, polypropylene packaging film having a skin layer comprising a propylene-ethylene copolymer on one surface, The skin layer consists of a mixture of 60 to 80% by weight propylene/ethylene copolymer and 40 to 20% by weight propylene homopolymer, the copolymer comprising 2 to 6% by weight propylene homopolymer.
% by weight of ethylene and characterized in that the thickness ratio of the base layer to the skin layer is at least 4:1. Ta. In a particularly desirable embodiment, the skin layer consists of a blend of 70% by weight propylene-ethylene copolymer and 30% by weight propylene homopolymer, the copolymer containing 2 to 6% by weight ethylene, more preferably 3 to 6% by weight.
It contains 4% by weight. In another aspect, the present invention is a method for increasing the sealing temperature range of propylene homopolymer films, which comprises providing a base layer of propylene homopolymer; a skin layer consisting of a blend containing % by weight is laminated onto at least one side of the propylene homopolymer film; and the laminated film is biaxially stretched to a final total thickness of 6.35 to 63.5 microns (0.25 to 63.5 microns).
2.5 mils). Such films have a seal range of at least 20°C and exhibit seal strengths of 300 g/in or greater. A preferred method for making this laminated film is to coextrude the skin layer and base layer. In yet another aspect, the present invention relates to a method of producing a well-sealed, shrunken polypropylene film package, which comprises providing a base layer of propylene homopolymer; propylene-ethylene with an ethylene content of 2 to 6% by weight. A skin layer consisting of a blend containing 60 to 80% copolymer by weight is laminated to at least one side of the propylene homopolymer film; the laminated film is biaxially stretched to a final total thickness of 6.35 to 63.5 microns (0.25 to 2.5 microns). preparing the article to be packaged; wrapping the article with the film and applying the skin layer surface at the point to be heat sealed; in contact with each other; applying heat and pressure to the points to be sealed to seal the film at the points, the applied heat being in the range of 135° to 160°C, and the resulting package being heat sealed. Minimum strength 300g/
and applying heat to the package in the range of 110 DEG to 195 DEG C. for a time sufficient to cause the film to shrink around the article. In all aspects of the invention described above, the film can be perforated. A preferred embodiment of the invention is a two-layer film made by blending 70% by weight propylene-ethylene copolymer containing 3.5-4.2% ethylene and 30% by weight propylene homopolymer as the skin or outer layer. This mixture is blended in a Banbury blender and then fed into the hopper of an extruder feeding into an annular die to extrude a tube with two layers. The inner or base layer consists of a polypropylene homopolymer which is fed into the hopper of a second extruder which also feeds the annular die for providing the inner layer. In the annular die, the two tubular extrudates are combined into a multilayer tubular laminate. The tubular laminate is rapidly cooled and then collapsed. The tube is then heated to a polypropylene orientation temperature range of 135 DEG to 150 DEG C. and inflated by gas blowing.
Heating takes place in a furnace, and as the flattened film exits the furnace it is passed through pinch rolls and opened against a valve by the well-known trap-valve technique, where it is heated approximately five times in both the vertical and horizontal directions. The thickness of the stretched film is 0.60~
Reduced to 0.80 mil. The tube before stretching is typically about 18 mils thick, with a polypropylene layer to blend layer thickness ratio of 5:1. One such bubble technique for biaxial stretching of polypropylene is disclosed in U.S. Pat. No. 3,260,776 (July 17, 1966, CA
Lindostrom et al.). After the film has expanded into a bulb and the bulb has cooled, it is crushed, cut open and rolled up. In some cases it is desirable to perforate the film so that there are perforations or arrays of small holes in the film. The number of holes varies depending on the desired application.
They may range from 8-10 to several hundred per square inch, and diameters may range from 2 to 20 mils. A typical apparatus for perforating film uses a cooled grid over which the film is exposed to a jet of hot gas that melts the film in the uncooled areas. Such devices and methods are described in U.S. Pat.
No. 3038198 (June 12, 1952, CHScharr). In order to measure the seal temperature range and seal strength of the film, two sheets of film were stacked so that their outer skins were in contact with each other. The two sheets were pressed together against the back support surface for 0.5 seconds at a pressure of 1 Kg/cm ² using a sealing bar heated by electrical resistance and having a width of 5 mm, and the temperature of the sealing bar was recorded. After the seal is complete, cut 1-inch strips perpendicular to and across the seal formed by the burr and place the end of each strip into the jaws of the Instron tester so that the seal grips and is close to the jaws. It is located at the center point. Force was applied to drive the jaws apart until the seal peeled or broke. A seal with a minimum seal strength of about 300 g/in considered sufficient for packaging applications is achieved by the preferred embodiment at sealing bar temperatures as low as 135°C, and remains satisfactory until the temperature increases to about 160°C. A seal is then carried out, which corresponds to a sealing range of approximately 25°C. (As described below, field tests indicate an operational seal range of 30°C.) A maximum seal strength of approximately 1400 g/inch was achieved. As previously stated, a single layer polypropylene film will have an essentially "zero" wide sealing temperature range. Since the sealing bar is cooled when it comes into contact with the film, in the case of polypropylene it is practically necessary to set the temperature of the sealing bar in the packaging machine somewhat above the melting point, thereby obtaining a sealed package. However, the seals thus formed have poor appearance and burnt areas. With the film of the present invention, there is no risk of melting or scorching the film when the sealing bar is set near the upper end of the sealing temperature range, and in addition, even at the high speeds at which the film attempts to cool the sealing bar. The sealing bar still retains enough heat to seal the film and is not cooled below the sealing temperature range. Various articles can be packaged within the films of the present invention, and various machines use different techniques.
In some cases the article is placed on one sheet of film and then a second sheet is placed over the article and then sealed all around; in other cases the sealing operation is too time consuming. Cut the film to shape the packaging. After sealing, the article passes through a thermal tunnel, where the temperature can be as low as 110°C, and is usually set within the temperature range at which the film was stretched. Within the tunnel, heat releases the stretching tension and causes the film to contract around the article. In another packaging method, the article can be placed on the fold of the film with a center crease and the sealing performed on the remaining three unsealed sides. Many methods of encasing articles in shrink film are well known and understood by those skilled in the art. Articles such as soft baked goods, including rolls, pizza, and the like, can be packaged with the films of the present invention. Additionally, food products that require breathability through the film, such as fruits and vegetables, can be packaged using perforated films. EXAMPLES A number of compositions of blended sealing layers were tested to overcome the problems encountered with polypropylene, which has virtually no heat sealing range. Table 1 below shows two layer films of the indicated seal compositions coextruded with a base layer of propylene homopolymer.
Only those compositions that have been successfully processed into films by the trap-valve technique are shown in Table 1. The footnotes below the table explain the polymer abbreviations.
Seal range and seal strength were measured as previously described. Of course, haze is the average percentage of transmitted light that deviates more than 2.5° from the incident beam as it passes through the sample, which is
Measured by the method of ASTMD1003. The thickness measurements described refer to the total thickness of the film laminate.

【table】

[Table] In Examples 3, 10, and 11, the temperature in the sealing range is expressed with a wide range, but these examples were conducted using actual equipment in the field, and the sealing range was determined in the laboratory. This is because an increase in the sealing temperature range of about 5°C was observed in the field, thus increasing the upper limit of the sealing temperature to about 165°C.
These three examples were operated as perforated films for packaging rolls and crusty breads.
All of these films had good machinability, ie, sufficient toughness to be processed through standard packaging equipment. The film according to Example 11 also performed well in the field as a non-perforated film for crescent roll and pizza packaging. Looking at Table 1, it is clear that Examples 10, 11 and 12 have desirably wide sealing ranges, and that the films all have good machinability and are not problematically cloudy. . However, when the propylene-ethylene copolymer in the blend exceeds approximately 80%, processability problems in the trap-bubble process become significant, and the sealing layer tends to flow and deform at the high temperatures required to stretch and orient the polypropylene base layer. represents. In other words, the propylene-ethylene copolymer begins to solvate at the orientation temperature of the propylene homopolymer and tends to stick to the pinch rolls after the outer sealing layer of the tube is heated and before being blown into the bubble. Thus, the preferred range for the amount of propylene-ethylene copolymer in the seal layer blend is from 60 to 80% by weight, thereby providing a
A sealing temperature range varying in °C is given. Example 11
The composition is the best in all combinations of seal temperature range, seal strength, processability and machinability with modern film manufacturing equipment. Industrial packaging machines are capable of producing 50 to 80 packages per minute, and the indicator is that this production rate can be successfully exceeded. Coextruding the basal layer and skin as a flat sheet through a slot die, rapidly cooling the sheet, and
By stretching this flat sheet using a tenterframe when heated to the orientation temperature, continuous production of films with skin layer blends greater than 80% can be achieved; It is less attractive due to plastic debris and other factors. In any case, the widening of the heat sealing range is due to the fact that the copolymer in the skin layer blend is at least
It should be noted that it starts significantly from 50% and occurs continuously up to 100% copolymer. The propylene homopolymer used in this invention consists of film grade, primarily isotactic polypropylene available from a number of well known resin suppliers. Similarly, propylene-ethylene copolymers are also film grade random copolymers available from well known resin manufacturers, and ethylene contents of 2 to 6% will work satisfactorily. As the ethylene content in the copolymer increases, it is expected that the proportion of copolymer in the blend will increase. The layer thickness ratio in the multilayer film of the present invention is a ratio that provides the overall best conditions for machinability, sealability, and processability. If the thickness of the sealing layer exceeds much more than the 4:1 ratio, it will lead to processing problems at polypropylene orientation temperatures, as discussed above. Thus, the base layer to seal layer ratio should be greater than 4:1, and as mentioned above a 5:1 ratio gives quite satisfactory results. In addition, a total film thickness of 6.35 to 63.5 microns (0.25 to 2.5 mils) produces the most satisfactory combination of film properties. Also within the scope of this invention are three layer films in which the center layer is an oriented propylene homopolymer with sealing layers on either side to provide balance to the oriented homopolymer layer. This is to prevent curling that can occur in structurally unbalanced multilayer films.

Claims (1)

[Scope of Claims] 1. A heat-sealable biaxially oriented polypropylene homopolymer having a base layer of polypropylene homopolymer and a skin layer comprising a propylene-ethylene copolymer disposed on at least one surface of the base layer. a multilayer, optionally perforated, heat-shrinkable polypropylene packaging film, the outer skin layer consisting of a mixture of 60-80% by weight propylene/ethylene copolymer and 40-20% by weight propylene homopolymer; Copolymer is 2-6
% by weight of ethylene and characterized in that the thickness ratio of the base layer to the skin layer is at least 4:1. 2. The packaging film according to claim 1, having a total thickness in the range of 6.35 to 63.5 microns.