JPH0354048B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to shrink films based on selected linear low-density copolymers of ethylene and certain alpha-olefins, which films exhibit significant optical and other physical properties and shrinkage. It has a good balance of properties. Shrink films of oriented polyethylene and various ethylene copolymers are known; for example, Golike, US Pat. No. 3,299,194.
No. 3,663,662 to Golike et al. Polyolefin shrink films, which are primarily used to wrap food and consumer goods, should have good optical clarity; otherwise the appearance of the packaged product in the wrapping to the consumer will be reduced. will be destroyed or damaged. For practical applications, this film has approximately 100
At least in the direction of orientation within the temperature range of ~120℃
The shrinkage should be on the order of 15% and with sufficient force to provide a tightly adherent film around the product wrapped within the wrap. The film should also have good mechanical properties, such as tensile strength and modulus, so that it can be stretched and then shrunk without tearing.
It always maintains good physical contact with the packaged product and will not be easily damaged during handling. Some conventional methods of making ethylene polymer shrink films required crosslinking of the polymer prior to stretching to impart greater mechanical strength to the film. This crosslinking was usually achieved by irradiation with high energy particles or gamma rays. In order to obtain resin compositions that yield films with satisfactory properties for shrink films without crosslinking prior to stretching, it is generally necessary to formulate ethylene polymers of low density and high density. It was hot. Naturally, it would be desirable to be able to manufacture the containment film from a single low density ethylene polymer resin. In this regard, the term "low density" is 0.940
g/cm ³ or less, and "high density" means greater than 0.940 g/cm ³ . DOWLEX low density "polyethylene" resin, a recent commercial product from the Dow Chemical Company, is indicated in the Dow Bulletin as providing blown films with excellent optical properties and excellent tensile properties. However, the bulletin states that these resins are not suitable for producing shrink films because their shrinkage rate is lower than that of ordinary low-density polyethylene films, and they shrink within a narrow temperature range. It has been shown that DOWLEX resin is actually a copolymer of ethylene and 1-octene. According to the present invention, a shrink film with high optical clarity, good shrink properties and good mechanical properties is now provided. The film comprises: (1) at least one linear copolymer of at least one C ₈ -C ₁₈ α-olefin and ethylene;
~100% by weight, provided that the copolymer contains the following (a) to
(d) have the following characteristics: (a) a melt index of 0.1 to 4.0 g/10 minutes; (b) a density of 0.900 to 0.940 g/ ^cm3 ; (c) a stress index of greater than 1.3.
(d) two different temperatures below 128°C as measured by differential scanning calorimetry (DSC);
There are distinct) crystallite melting regions, and the temperature difference between these regions is at least
15°C; and (2) 0 to 95% by weight of at least one polymer selected from the group consisting of ethylene homopolymers and copolymers of ethylenically unsaturated comonomers and ethylene, provided that the polymer having only one microcrystalline melting point below 128°C; It is obtained by stretching, however, the stretching is not performed in accordance with the above item (1)
The temperature range determined by the two crystallite melting points of the copolymer with C ₈ -C ₁₈ α-olefin shall be used. The drawing shows DSC for three different resins.
(differential scanning calorimeter) plot. Figure 1 is a plot for polyethylene, and Figure 2 is a plot for commercially available linear ethylene/1
- for an octene copolymer and Figure 3 for a blend of high density and low density ethylene polymers. The main resin used in the composition of the present invention is a linear copolymer of ethylene and alpha-olefin. Representative alpha-olefins that can be copolymerized with ethylene include 1-octene, 1-decene, 1-undecene, 1-dodecene, and 1-hexadecene. This copolymer is produced at low to moderate pressure (approximately
29.4MPa). Typical catalysts are various organoaluminum, organotitanium, and organovanadium compounds, especially organoaluminum compounds modified with titanium. The preparation of copolymers of alpha-olefins and ethylene is shown, for example, in Anderson et al., US Pat. No. 4,076,698, and Morita et al., US Pat. No. 4,205,021. Suitable commercially available copolymers of ethylene and higher α-olefins include the DOWLEX resins described above, and a preferred copolymer is with 1-octene. As the proportion of α-olefin in the copolymer or the molecular weight of the α-olefin increases, the density of the copolymer decreases. Based on 1-octene, the amount of alpha-olefin in the copolymer is usually between about 3 and 16 percent by weight. However, the amounts of each such comonomer are selected to provide suitable values for the melt index, density, and stress index of the copolymer. These quantitative proportions are easily determined from known relationships,
and can be verified experimentally by standard methods. At this time, the melt index is
Measured by ASTM method D1238 (condition E) and density by ASTM D1505. The stress exponent is the slope of the plot of log flow rate versus log extrusion force. Since this plot is not a straight line, the slope is measured by ASTM D1238 using 2160g and 640g loads, both at 190°C. The copolymer should give two distinct crystallite melting peaks, meaning that the copolymer has two distinct groups of crystallites;
Each has its own distinct melting area. For ethylene/1-octene copolymers, such regions are about 107°C and 125°C. Figure 1 is 0.917
△H for normal polyethylene with a density of
This is a typical DSC plot of (milliwatts) versus temperature (°C). This copolymer has only one peak at about 107°C. The DSC plot for DOWLEX 2045 ethylene/1-octene copolymer (d=0.920) is shown in FIG. The higher temperature peak is actually a doublet;
The higher melting temperature of this doublet is considered characteristic of this peak. FIG. 3 is a DSC plot for a blend of linear high density ethylene/1-octene copolymer and regular polyethylene. The density of this formulation is 0.926. It can be seen that the peaks of this formulation correspond to the peaks of DOWLEX resin shown in FIG. DSC is a well-known method for measuring the crystallite melting temperature of polymers. Ethylene and 1-octene or other α-olefin comonomers are present in such small amounts that a second DSC peak is not seen.
Linear copolymers with olefins are not suitable for the present invention. The presence of two microcrystalline melting regions in copolymers of ethylene and alpha-olefins is the most prominent feature of these, since films made from these copolymers are This is because it can be oriented between two temperatures. Shrink films made from these copolymers have excellent properties, and shrink films made from blends of low density and high density ethylene polymers, such as those shown in U.S. Pat. No. 3,299,194 comparable to However, only 5% by weight of this group of ethylene/α-olefin copolymers is present in blends with conventional ethylene homopolymers or copolymers that have only one type of microcrystalline melting region below 128°C. It has been shown that the properties of the former copolymers can often be greatly improved by allowing improved shrinkage films to be produced therefrom with desired physical properties, including high optical clarity. discovered. Such conventional homopolymers or copolymers can be both high and low density, linear and branched, produced at high or low pressure. This copolymer is, for example, α
- Can be a copolymer with any comonomer, including olefins, vinyl esters, alkyl acrylates, also called methacrylates, and acrylonitrile. Many such polymers are commercially available from a variety of suppliers. The formulation can be prepared by any conventional method capable of producing a uniform, homogeneous material. Films are prepared from the above copolymers or blends by suitable melt extrusion techniques. The film is either tubular or flat. This is preferably stretched in two axes to a length of at least 3 times, preferably at least 5 times, in each direction in the plane of the film. A convenient method of combining extrusion and orientation of polymeric films is shown in US Pat. No. 3,141,912 to Goldman et al. Upon processing at temperatures of about 100-120°C, the oriented, unconstrained film will shrink by at least about 15%, and this shrinkage will be accompanied by significant forces, typically at least 1400 kPa. A suitable shrink film has a shrinkage of at least 30% at a temperature just below the upper crystallite melting peak and at least 30% at a temperature just below the upper crystallite melting peak.
It will probably shrink by 15%. The contraction force at 100°C should be about 350 kPa or higher. Haze should be less than 4%, especially less than 2%. The gloss should be above 90, preferably above 110. If desired, a limited amount of crosslinking can be introduced after stretching and before shrinkage. This crosslinking is accomplished by irradiation with a minimum amount of high energy radiation, usually less than 8 megarads, as shown, for example, in US Pat. No. 3,663,662 to Golike et al. The irradiated oriented film has improved melt strength and shrink tunneling (shrink tunneling).
tunnel) becomes less sensitive to temperature differences within the tunnel. The invention is illustrated in the following representative examples, in which all parts and proportions are by weight. The shrink film thickness was approximately 0.025 mm in all cases. All data obtained in units other than SI were converted to SI units. on a strip of unrestrained film, usually
The shrinkage rate of the oriented film was measured by marking a predetermined length of 100 mm, heating it for 10 seconds in a bath at a temperature of 100°C, and calculating the shrinkage rate using the change in length as a percentage. Contraction force was measured according to ASTM2838. ASTM modulus, tensile strength, and elongation at break
Measured according to D412. The ethylene resins used in the examples are shown in the table below.

【table】
copolymer

【table】
copolymer
Example 1 An oriented tubular film was prepared by the method of Goldman, US Pat. No. 3,141,912.
A 5 cm extruder operated at 230°C and fed with ethylene polymer resin at a rate of 0.9 kg per hour.
Films were produced at a speed of 2.7 m/min. The hot tubular film was quenched, reheated to 115-120°C, and blown at an internal pressure of 2 kPa. Blowing was controlled using a quench ring and stretched 5 times in the transverse direction. A take-up roll was operated to stretch the film 5 times in the machine direction. A shrink film made from Resin A according to the invention was prepared using Resins B and C (26:74, respectively) according to the method shown in Goreik, US Pat. No. 3,299,194.
compared to a conventional shrink film produced from a formulation with a ratio of . The film was wrapped around the object, sealed with a hot wire, and shrunk in a tunnel held at 167°C. The appearance of the packaging in each case was similar. The properties of these two shrink films are compared and shown in the table below. All properties except haze and gloss are given as machine direction/cross direction ratios.

【table】

[Table] *See table for resin types.
Example 2 A resin formulation was prepared as shown in the table below,
Melt compounded in a standard single screw mixing extruder and melt formed into 5 x 5-cm films. These things are manufactured using a laboratory-scale drawing machine (TM
Long Co., Inc., Somerville, NJ) 120
It was stretched 5 times in each direction at °C. Films of the invention (A/B and A/D blends)
The physical properties of the ethylene polymer blend (B/C
The physical properties of conventional films made from C/D and C/D formulations are compared in Table 1. In the film of the present invention, the physical properties, especially the optical properties, are significantly improved.

【table】

Table: Example 3 An oriented film was prepared from a blend of resins A and C (see table). Stretching was carried out at 110-112°C using the same method and equipment as in Example 2. The physical properties of the stretched film are shown in the table below.
It can be seen that all properties change as the proportion of ordinary low density polyethylene (resin C) increases. The most noticeable change is that the contraction force is greatly reduced while the contraction rate remains at a high level.

【table】

【table】 [Brief explanation of drawings]

Figure 1 represents a DSC plot for polyethylene. Figure 2 shows commercially available linear ethylene/1-
Figure 3 represents a DSC plot for an octene copolymer. FIG. 3 represents DSC plots for blends of high density and low density ethylene polymers.

Claims (1)

[Claims] 1 (1) Ethylene and at least one type of C ₈ to C ₁₈ α
- 5 to 100% by weight of at least one linear copolymer with olefin, provided that the copolymer has the following characteristics (a) to (d): (a) 0.1 to 4.0 g/10 (b) density between 0.900 and 0.940 g/ ^cm3 ; (c) stress index greater than 1.3; and (d) below 128°C as measured by differential scanning calorimetry (DSC). (2) selected from the group consisting of ethylene homopolymers and copolymers of ethylene and ethylenically unsaturated comonomers; from 0 to 95% by weight of at least one polymer, provided that the polymer has a crystallite melting point of only one below 128°C; A shrink film produced by stretching the original linear dimension at least three times in at least one direction, wherein the stretching is performed by stretching the ethylene and C ₈ to C ₁₈ α-olefin of item (1) above. Shrinkage film produced within a temperature range determined by the two crystallite melting points of the copolymer. 2. The film according to claim 1, which is produced from a copolymer of ethylene and 1-octene. 3. The film of claim 2, wherein the proportion of 1-octene is about 3 to 16% by weight. 4 Produced from a blend of a copolymer of ethylene and 1-octene with two microcrystalline melting points and a copolymer of ethylene and 1-octene with only one microcrystalline melting point by differential scanning calorimetry. The film according to claim 1. 5. The film according to claim 1, which is biaxially stretched to an extent of at least 5 times in each direction. 6. The film of claim 5, wherein the film is irradiated with high energy radiation in an amount less than about 8 megarads after stretching but before shrinkage. 7. A method of wrapping an article in an oriented polyolefin film and heat-shrinking the film to provide a tightly fixed coating around the article, comprising: (1) ethylene and at least one C ₈ -C ₁₈ α- At least one linear copolymer with olefin 5
~100% by weight, provided that the copolymer contains the following (a) to (d)
(a) a melt index of 0.1 to 4.0 g/10 minutes; (b) a density of 0.900 to 0.940 g/ ^cm3 ; (c) a stress index of greater than 1.3; and (d) (2) an ethylene homopolymer and 0 to 95% by weight of at least one polymer selected from the group consisting of copolymers of ethylene and ethylenically unsaturated comonomers, provided that the polymer has only one microcrystalline melting point below 128°C; A shrink film produced by stretching a film produced from a homogeneous polymer composition of; in at least one direction to at least three times its original linear dimension; A method using a shrink film carried out within a temperature range determined by the two microcrystalline melting points of the copolymer of ethylene and C ₈ -C ₁₈ α-olefin according to item (1). 8. The method of claim 7, wherein the film is a copolymer of ethylene and 1-octene.