JPH0735094B2 - Film for single-wafer packaging - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to a shrinkable film mainly composed of an ethylene resin. Further, the film of the present invention is useful in the field of shrink wrapping of individual sheets such as envelopes and postcards.

<Prior Art> Conventionally, sheets, such as notebooks, paper plates, and papers, including postal items, have been stacked and then packaged. In particular, postal items such as envelopes and postcards are collected at the central office, then sorted by district, tied with a string, or put in a plastic film bag and transported to the regional offices. It has a system that delivers it to your home. However, the shape of postal items, especially envelopes, is not a fixed form, but various forms are mixed. Due to the sharp corners and the like, the method of using a string or a bag often causes troubles such as loosening of the string or tearing of the bag, which causes the mail to be broken or damaged. In addition, it is difficult to meet the demand to streamline work and distribution by attaching a bar code to the sorted mail and wrapping it.

On the other hand, it has been proposed to cover the sorted mail items with a shrinkable film and heat it to perform a tight packing. For example, JP-A-60-228220 and JP-A-61-1132
The packaging method is as shown in Japanese Patent Publication No. 5 etc. According to this publication, a stack of two cross-linked stretched films made of an ethylene vinyl acetate copolymer having a vinyl acetate content of 5 to 15 wt% is preferable for shrink-wrapping a single sheet such as a mail.

<Problems to be Solved by the Invention> However, the above-described film (a) is insufficient in suitability for a packaging machine, and stacked mail pieces collapse during packaging. Problems such as film wrapping around the machine are likely to occur. (B) Although the strength is increased by stacking two films, the strength is not sufficient and the film is easily broken. To compensate for this, the film becomes thick and the cost becomes high. (C) Not only fixed size mail,
Various shapes are mixed, and the number of packages ranges from ten to more than 100, and it is difficult to package such objects to be packaged without loosening or deformation. As described above, the conventional film does not have sufficient packaging machine suitability, strength, and shrinkage characteristics, and cannot substantially satisfactorily perform packaging of single-piece products represented by mail.

<Means for Solving the Problems> In order to solve the above problems, a shrinkable film mainly composed of an ethylene-based resin, a tensile elastic modulus of 20 kg / mm ² or more,
It is proposed to use a film for packaging single-wafer products, which has a tear strength of 6 g or more and a shrinkage stress of 30 to 80 g / mm ² after 30% shrinkage at 120 ° C in the longitudinal direction.

The present invention will be described in more detail below.

First, the shrinkability of the film of the present invention will be described. As mentioned above, there are various types of postal items such as postal items depending on their shapes and the number of stacked sheets, and in order to wrap them tightly and without deformation, the film requires special shrinkage characteristics. . Generally, the shrinkage characteristics of a film are as follows: heat shrinkage (ASTM-D-2732) and shrinkage stress (ASTM-D-2732).
D-2838), but the special shrinkage properties required for the packaging of single-wafer products are difficult to express with these. As a result of repeated studies on the field of this special shrink wrapping, the present inventors have developed a shrink wrap that is tight and has no deformation by using a film that retains a constant shrinkage stress after high shrinkage. We have found what we can achieve. That is, the film of the present invention has a longitudinal direction of 12
The film has a shrinkage stress of 30 to 80 g / mm ² after 30% shrinkage at 0 ° C. Shrinkage stress under these conditions is difficult to maintain tight packaging with a film of less than 30 g / mm ^2, problems such as loose packaging during mailing tend to occur, while a film of more than 80 g / mm ² is Deformation occurs when the number of sheets is small. It sometimes causes damage to the leaf. The orientation direction of the film may be uniaxial or biaxial, but a biaxially oriented film having a strong longitudinal orientation is preferable. After shrinking by 30% at 120 ° C, the machine direction of the film, that is, the machine direction (hereinafter referred to as MD) and the transverse direction of the film, that is, the width direction of the film (hereinafter referred to as TD).
That is, MD: TD is preferably 1: 1 to 1: 0.1.

Next, the suitability of the film of the present invention for a packaging machine will be described.
FIG. 1 illustrates a packaging machine used for shrink-wrapping single-wafer products. The packaging process will be described with reference to FIG. The upper and lower wrapping films 7 and 8 are in a state in which their leading end portions are sealed by the wrapping operation in the preceding stage. The stacked single-wafer products 9 are extruded there by the charging fork 1 and conveyed to the first conveyor 2. At this time, the upper and lower packaging films 7 and 8 are pulled out to cover the sheet 9 from above and below. Next, the body sealer 3 descends, sandwiches the two films from above and below, and performs sealing and cutting. The package is conveyed to the shrink tunnel 6, and while moving by the shrink conveyor 5, the film is shrunk by the hot air produced by the hot air generator 4, and the packaging in the form shown in FIG. 2 is completed.
The important thing in such a packaging process is to put the sheet 9 first.
There is a place where the film is conveyed to the conveyer 2 and a case where the barrel sealer 3 descends to sandwich the film, and troubles such as the collapse of the sheet and the film winding around the roll or the barrel sealer are likely to occur. In order to prevent such troubles and realize stable running of the film, it is required that the film has a firmness. In order to satisfy this, it is essential that the tensile elastic modulus is 20 kg / mm ² or more. The tensile modulus of elasticity is preferably 25 kg / mm ² or more.

Next, the strength of the film of the present invention will be described. Single leaf,
For example, postal items such as postcards and envelopes are packaged as shown in FIG. 2, placed in hemp bags, and transported by truck, freight train, or the like from the collection station to the delivery post office. During this period, the handling of the postal package is rough, such as throwing, and the postal product is fragile because the corners and edges of the postal item are sharp. In order for the package to withstand such abuse, the film must have a tear strength of 6 g or more. Preferably, the tear strength of the film is desired to be 8 g or more.

As described above, it is necessary for the film for packaging a sheet of the present invention to simultaneously satisfy the three properties of shrinkage stress after 30% shrinkage, tensile elastic modulus, and tear strength. However, a film having excellent shrinkage properties tends to have a low tear strength because of high orientation, a film substrate having high elasticity tends to have a low tear strength because of high crystallinity, and a film substrate having high elasticity tends to have a high tear strength. It is difficult to obtain a film that satisfies all of the above three characteristics, since it is difficult to achieve stable stretching when orientation is stretched. The present inventors have studied the relationship between the controller of the degree of orientation by stretching the ethylene-based resin and the higher-order structure of the film and the characteristic value, and as a result, by using the unique stretching orientation method shown below, the shrinkage characteristics and the tensile elasticity are The present inventors have for the first time found that a film having the three properties of rate and tear strength can be obtained at the same time.

That is, when the shrinkable film is stretched by, for example, a tubular method, first, as a pre-stage of stretching, the resin is melt-extruded from a circular die and cooled and solidified to obtain a tubular unstretched film. Next, generally, while injecting air or the like into the tubular unstretched film, it is heated and simultaneously stretched in both longitudinal and transverse directions to orient. On the other hand, the feature of the unique method found by the present inventors is that preferably first subjected to a cross-linking treatment before stretching, and then while stretching the film while heating the film to MD by the injection pressure of air. Shrink to TD. Further, in the next step, the film is further stretched in the MD from the vicinity of the melting point of the resin used to a temperature of the melting point or more and 30 ° C. and at the same time, is also stretched in the TD. It is presumed that such a sequential stretch orientation method imparts a unique higher-order structure to the film and facilitates orientation control. In this stretching method, the preferred stretching ratio is 3 in MD with respect to the unstretched film.
~ 8 times, TD 1.5 ~ 6 times. For example, when stretching MD7 times and TD3 times, the unstretched film is subjected to a cross-linking treatment with a gel fraction of about 20%, and MD4 times is stretched while heating in the first stage, and at the same time, TD is 0.8 times contracted. At 140 ° C (this is the maximum heating temperature), MD1.75 times and TD3.75 times were stretched in two stages, and MD = 4 × 1.75 = 7 times, TD = 0.8 ×
3.75 = 3 times stretching. However, the specific examples disclosed here are merely examples, and the films according to the present invention are not limited to these examples as long as they are in accordance with the concept of the stretch orientation method.

The film of the present invention obtained by the stretching method as described above,
It was epoch-making that it had excellent shrinkage properties, high tensile modulus, and high tear strength. Not only that, it was surprising that the above-mentioned stretching method could stably produce a film having excellent thickness uniformity.

Further, when producing the film of the present invention, it is a preferable example to carry out a crosslinking treatment in the step before stretching, in such a case, α rays, β rays, γ rays, neutron rays, accelerated electron beams, etc. The film can be crosslinked by irradiating the film with the ionizing radiation. The degree of irradiation is preferably 0.5 to 15 Mrad.

The ethylene-based resin in the present invention means high-density polyethylene, medium-density polyethylene, low-density polyethylene, linear low-density polyethylene, ultra-low-density polyethylene, and a copolymer of ethylene and a vinyl compound that is copolymerizable with ethylene (for example, ethylene acetate). Vinyl copolymer), and one or a mixture of two or more selected from these ethylene-based resins is mainly used in a proportion of at least 50 wt% of the base resin of the film. Among these ethylene resins,
Linear low-density polyethylene and ultra-low-density polyethylene, which are copolymers of ethylene and other α-olefins, are preferably used. Examples of the other α-olefins at that time include butene-1,4-methylpentene. -1, hexene-
1, octene-1 and the like.

The film of the present invention may be used as a monolayer film,
You may use as a multilayer film. It is also a preferable example to use a plurality of single-layer or multi-layer films in close contact with each other.

The film of the present invention is a variety of additives usually used in plastic films within a range that does not impair the object of the present invention, for example,
An antioxidant, an antiblocking agent, a slip agent, an antistatic agent, etc. may be contained. In particular, by adding a nonionic surfactant such as glycerin fatty acid ester, polyglycerin fatty acid ester, and fatty acid diethanolamide to the film, the film is provided with slip property and antistatic property, and the work at the time of packaging and unpacking is performed. It is preferably used because it improves the properties.

The list of resins used in the examples is given below.

LLDPE-a :( linear low density ^{polyethylene, d = 0.925g / cm 3,} MI = 2.0g / 10min, the comonomer = octene -1) LLDPE-b: (d = 0.930g / cm 3, MI = 2.1g / 10min, comonomer = 4-methylpentene -1) VLDPE-a :( ultra low density ^{polyethylene, d = 0.910g / cm 3,} MI = 3.6g / 10min, the comonomer = 4-methylpentene -1) LDPE-a: (D = 0.920g / cm ³ , MI = 0.4g / 10min) LDPE-b: (d = 0.928g / cm ³ , MI = 0.5g / 10min) EVA-a: (Ethylene vinyl acetate copolymer, vinyl acetate Content = 10 wt%, MI = 1.0 g / 10 min) HDPE-a: (d = 0.954 g / cm ³ , MI = 0.8 g / 10 min) [Examples] Hereinafter, the present invention will be specifically described with reference to Examples. By way of explanation, the invention is not limited by these examples.

The measuring method and the evaluating method in the examples are as follows.

(1) Tensile elastic modulus It was measured according to the ASTM-D-882 method.

(2) Tear strength According to JIS-P-8116 method, MD and TD of the film were measured, and the one with lower numerical value was used.

(3) Shrinkage stress after 30% shrinkage Set the reference temperature to 120 ° C, slacken the sample with a length 1.3 times the measurement length (distance between sample chucks), and set A
It was measured according to the STM-D-2838 method.

(4) Packaging machine test Using the packaging machine shown in Fig. 1, the weight of a mixture of postcards and envelopes
A 100 g to 800 g mixture was mixed and a packaging test was performed with n of 100. It was evaluated by the rate of occurrence of troubles in which the postal matter collapsed during packaging and could not be covered with a film.

◎: Postage collapse 0%, excellent continuous wrapping ○: Postage collapse less than 3%, continuous packaging possible △: Postage collapse 3% or more and less than 10%, poor continuous wrapping property Item x: Collapse of postal items occurs more than 10% and continuous packaging is not possible. (5) Drop test of packaged item After the postal item was packaged in the form as shown in Fig. 2 by using the device of Fig. 1, the height was increased. The sample was naturally dropped from 1.5 m and evaluated by the breaking rate at that time.

⊚: The film is a strong packaging without tearing. ○: The film has a part of scratches, but the original packaging form is maintained. Δ: The film is torn, and less than 10% of unfolding defects occur. Item x: Same as above with 10% or more defects (6) Looseness test of packaging After packing the mail with the device shown in Fig. 1, the packaging is randomly placed in a hemp bag and the truck goes back and forth between Tokyo and Osaka. After shipping, the looseness of the package after arrival was observed and evaluated.

⊚: Tight packaging maintained ○: Film slightly loose but holding mail △: Film loosened and mail dropped out less than 10% ×: Same as above 10% or more occurred (7) Deformation test of package The evaluation was made by observing the state of deformation of a postal product when the postal item was packaged under the same conditions without adjusting the tension by the device shown in FIG.

◎: The postal product was not deformed ○: The postal item was slightly distorted with a radius of curvature of 50 cm or more △: The postal item was distorted with a radius of curvature of 5 cm or more and less than 50 cm ×: The postal item was curved Distortion less than 5 cm in radius occurs, and it becomes rounded or bent However, in the tests of (5), (6), and (7), the following three types of items are the items to be packaged. Using.

A: Postcards and postcards mixed with a small number of about 100g B: Postcards and postcards mixed together with an average of about 400g C: Postcards only about 800g and high weight <Example 1> As shown in Table 1 Various polymers were supplied to an extruder, extruded from an annular die and rapidly cooled to obtain an unstretched tubular film having a thickness of 400 to 500 µ. At this time, 0.5 wt% of glycerin monooleate as an antistatic agent was poured into an extruder, kneaded, and contained. Next, in stretching the film, the characteristic stretch orientation method referred to in the present invention as described in the present specification was adopted. That is, the tubular film was folded flat and irradiated with 1.0 to 10.0 Mrad by an electron beam irradiation device to perform a crosslinking treatment. Next, this tube is stretched by a stretching machine while being stretched 2 to 5 times in MD and at the same time contracted to 0.9 to 0.6 times in TD,
Next, at a maximum heating temperature of 100 to 150 ° C., MD was stretched 1.2 to 3 times and simultaneously TD was stretched 1.5 to 5.5 times, cooled and wound to obtain a film. The film thickness was 20μ. This was designated as Sample No. 1 to No. 6.

Table 1 shows the results of evaluation of the characteristic values and practical characteristics of these films.

According to Table 1, the film of sample No. 4 has a tensile elastic modulus of 18 kg.
/ mm ² and the film lacks the rigidity of the film. When the packaging test was carried out by the packaging machine of Fig. 1, the film could not sufficiently follow the intermittent movement of the fork 1 and the sheet 9 and was supplied. The film length becomes short and unstable.
If packaging is continued in this state, if the film length becomes shorter than the specified length, the strongly stretched film collapses the sheet, and if the film is too long, there is a margin between the film and the sheet. Becomes too large, and there is a problem that the single-wafers will collapse during the movement before contraction. Furthermore, in the case of a film having a remarkably lack of rigidity, such as the film of Sample No. 5, not only the length of the above-mentioned supply film becomes remarkable and the collapse of the sheet often occurs, but also the film is wound around the guide roller or the body sealer. However, the packaging work was interrupted, and it was far from smooth packaging. On the other hand, the film of Sample No. 6 was supple, and although the packaging was carried out extremely smoothly, the film had a weak tear strength, and in the drop test, the film was torn, and the phenomenon that the sheet was disassembled was observed. . Mails sorted by district are mixed due to unpacking. This trouble takes a lot of effort for re-sorting and re-packing.
The mail is also damaged and bent, which is a fatal problem.

On the other hand, the films of Sample Nos. 1 to 3 are excellent in both elastic modulus and tear strength of the film, are smoothly and efficiently packaged, and have sufficient strength in the drop test of the package. It was

From these facts, it has been clarified that the packaging film needs to have a tensile elastic modulus of 20 kg / mm ² or more and a tear strength of 6 g or more when packaging the above-mentioned single-wafer products. The film base polymer is ethylene α
It is desirable to contain an olefin copolymer.

<Example 2> Using the raw material of Sample No. 2 of Example 1, an unstretched tubular film was prepared in the same manner as in Example 1. to this,
After irradiating 2-8 Mrad with an electron beam irradiation device and performing a crosslinking treatment, while heating with a stretching device, MD is stretched 3 to 4 times and at the same time TD is contracted 0.8 to 0.7 times, and then the maximum heating temperature is 120. ~ 1
At around 40 ° C., the film was further stretched 1.5 to 2.5 times in MD and simultaneously 2 to 6 times in TD, cooled, and wound to obtain a film. The film thickness was 20μ, and this was designated as sample Nos. 7 and 8. For comparison, using the same raw material, a conventional general stretching method, that is, MD at a stretching temperature of 110 to 140 ° C. is 3 to 10 times,
Samples Nos. 9 to 11 were films made by adopting a method of simultaneously stretching 1 to 5 times in TD and having a thickness of 20 μm. Table 2 shows the characteristic values of each film and the results of the practical tests of loosening, deformation and dropping when the sheet form was changed to A, B and C.

Considering the results in Table 2, the films of sample Nos. 7 and 8
The special orientational state of the film, which is represented by the shrinkage stress after 30% shrinkage, has an appropriate value and the film has excellent strength. In the package looseness test, these films are used even in the most easily loosened packaged object (B) in which large and small sheets such as postcards and envelopes are mixed, and
Tight packaging is maintained under the same conditions for both small and large number of sheets. At the same time, in the package deformation test,
It was possible to package even a small number of sheets (A) to be packaged, which has low strength and is easily deformed, while maintaining the original shape, and it was possible to perform a beautiful finished packaging of a variety of sheets. Furthermore, the films of Sample Nos. 7 and 8 have outstanding strength in the drop test, are heavy objects, and firmly protect even the object to be packaged (C) on which a large number of sharp postcards are stacked, and do not tear. It was a thing.

On the other hand, the comparative examples, that is, the films of Sample Nos. 9, 10, and 11 were obtained by the conventional manufacturing method.

Although the film of Sample No. 9 was not deformed immediately after the packaging of the package, the film tension was weak, and the sheet was dropped from the loosened film after transportation. The strength in the drop test was almost satisfactory except for C. The film of Sample No. 10 had the same degree of orientation as that of the example, but the strength of the film was lowered, and as a result of the drop test, each of the sheets A, B and C was broken. The film of Sample No. 10 was a highly oriented film, and was significantly deformed and bent in the sheet A, and was also inferior in strength. From these things, sample N
The films of o.9 to 11 were inferior in practical tests of looseness, strength, and deformation, and were poor in practicality.

<Effects of the Invention> The film of the present invention is excellent in shrinkability, strength, and packaging machine suitability due to having the above-mentioned constitution, and is extremely high in packaging suitability for packaging single-wafers such as mail. Is to have. That is, the sheets stacked at the time of packaging do not collapse, the film does not wind around a part of the packaging machine, and the film is not easily broken, so that a relatively thin film can be used. Further, it can be easily applied to a sheet having an irregular shape or number of sheets, the sheet is not deformed or broken, and the sheet is not loosened and the sheet is not dropped. Films with such performance can be used in notebooks, magazines,
It is useful because it can be applied to shrink-wrapping of other sheets such as paper plates.

Therefore, the present invention is an excellent invention having a high role to play in the industrial world.

[Brief description of drawings]

FIG. 1 is an explanatory view of an example of an apparatus for packaging single-wafer products according to the present invention. FIG. 2 is a perspective conceptual view of an example of a package obtained by the apparatus of FIG.

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display area B29L 7:00

Claims (1)

[Claims] 1. A shrinkable film mainly composed of an ethylene resin, having a tensile modulus of 20 kg / mm ² or more, a tear strength of 6 g or more, and a shrinkage stress of 30 to 8 after 30% shrinkage at 120 ° C. in the longitudinal direction.
A film for packaging single-wafer products, characterized in that it is 0 g / mm ² .