JPH0662799B2 - Low temperature shrinkable foam sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a foam shrink wrapping material. More specifically, the present invention relates to a styrene resin foam sheet having heat shrinkability at a low temperature.

[Prior Art] Using a polystyrene foam sheet joined in a cylindrical shape for heat shrinkage, for example, is disclosed in Japanese Patent Publication No. 52-10156.
Etc.

[Problems to be Solved by the Invention] Using the above expanded polystyrene sheet, soft drink water contained in a glass bottle or the like is shrink-wrapped, printed on the sheet to serve as a label, and heat insulating ( Packing with heat and cold insulation) and buffering properties has been widely commercialized. The shrink wrapping of this expanded polystyrene has good performance as such and is useful, but in the case of shrink wrapping of beverage bottles, it is necessary to heat the expanded polystyrene to 120 to 140 ° C. to shrink it due to the thermal property of the polystyrene. There is. The lower the heating temperature, the more useful it is in terms of workability and energy saving. Furthermore, when used partially for packaging natural juice or fermented milk, heating is required to be as low as possible in order to utilize the characteristics of the food and beverage. Some plastic food containers and packaging (eg polystyrene, polyethylene,
Blow-molded containers such as polyvinyl chloride and polyethylene terephthalate, etc.) need to be shrink-wrapped at a low temperature when shrink-wrapped with a foamed sheet in order to impart heat insulating property and cushioning property due to the heat resistance of the plastic container. is there. For example, shrink wrapping a polystyrene container with an expanded polystyrene sheet is actually quite difficult.

An object of the present invention is to provide a styrene resin foam sheet that can shrink at low temperature.

[Means and Actions for Solving Problems] As the above-mentioned low-temperature shrinkable foam sheet, various plastic foam sheets seem to be conceivable, but it is the actual situation that no commercially available one can be found. Is. First, in order to satisfy all of the extrusion foaming moldability, the secondary processability such as printability, the rigidity during packaging, and the dimensional stability, the above-mentioned resin containing styrene as a main component is optimal.

To impart low temperature shrinkability, the glass transition point T _{g of the} resin
Should be reduced. A guide when the T _g of the present invention ASTM D1
The Vicat softening point specified in 525 was adopted. With respect to the Vicat softening point of polystyrene of 100 to 120 ° C., the Vicat softening point of the present invention is 45 ° C. for the purpose of imparting low temperature shrinkability.
A thermoplastic resin containing styrene at a temperature of up to 90 ° C as a main component is preferable. When the Vicat softening point of the thermoplastic resin is 90 ° C. or higher, the low-temperature shrinkability that is the object of the present invention cannot be imparted, and when it is 45 ° C. or lower, in practical use, for example, a foam-shrinked bottle or the like is being transported or in a vending machine. It is difficult to put into practical use because the foamed shrinkable sheet undergoes secondary foaming and the sheets are fused together.

A method of adding a plasticizer such as DOP (dioctyl phthalate) is known as a method of adjusting T _g to 45 to 90 ° C in order to impart low-temperature shrinkability. It is not preferable because it is difficult to control the bubbles, and the plasticizer migrates to another or volatilizes in the air during use.

Therefore, as a monomer that easily copolymerizes with styrene and reduces the T _g while maintaining the characteristics of the styrene-based resin described above, a copolymer of acrylate and methacrylic acid ester and styrene is used to form a foamed sheet. The present invention was completed by discovering that a sheet as an object of the invention was obtained.

That is, according to the present invention, there is provided a foamable shrinkable sheet of a copolymer comprising (A) 70 to 97% by weight of styrene and 30 to 3% by weight of one or both of acrylic acid ester and methacrylic acid ester. Provided.

As acrylic acid ester, n-propyl acrylate, n-butyl acrylate, hexyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate are lower in T _g in a small amount than methyl acrylate, isopropyl acrylate, dodecyl acrylate, etc. N-dodecyl methacrylate is particularly effective as the methacrylic acid ester for the same reason.

Although the reason for these acrylic acid esters and methacrylic acid esters is not clear, they are adhesive to the object to be packaged. In other words, it is also a feature of the foam shrinkable sheet of the present invention that the adherend and the foam shrinkable sheet do not stick to each other and do not rotate around each other, and a solid and reliable covered packaging can be performed.

The copolymer used for the foam shrinkable sheet of the present invention can be obtained by a usual radical polymerization method, but a complete mixed continuous polymerization method in which a uniform composition is easily obtained, for example, the same polymerization as disclosed in U.S. Pat.No. 3,035,033. The one produced by the method has a small compositional distribution and molecular weight distribution of the copolymer, and uniform foaming,
It is particularly effective because a uniform shrinkable sheet is easily obtained, but the sheet is not particularly limited thereto.

Acrylic ester in the composition of the copolymer of the present invention,
If the amount of the methacrylic acid ester exceeds 30% by weight, the above-mentioned characteristics of styrene are reduced, and if it is less than 3% by weight, the effect of lowering the T _g is small and the purpose of the present invention is not met.

The weight average molecular weight M _{w of the} copolymer of the present invention is from 50,000 to 5,000 in view of extrusion foam molding conditions and mechanical performance of the foam sheet.
It is preferably 500,000, more preferably 100,000 to 300,000.

If the M _w is less than 50,000, the range of conditions for foam molding is narrow and the foam sheet tends to be brittle, and if the M _w is 500,000 or more, extrusion moldability tends to be poor.

In addition, for the extrusion foamed sheet molding of the copolymer of the present invention, for adjusting the viscosity of the copolymer, a metal salt of acrylic acid and / or methacrylic acid, for example, sodium salt of acrylic acid is used with respect to 100 parts by weight of the copolymer. Addition of 3 to 0.05 parts by weight is also effective in adjusting the melt viscosity during foam molding. Examples of the metal of the metal salt include Na, Li, K, Ca, Ba, Mg, Zn and Fe.

Further, the copolymer of the present invention has good compatibility with polystyrene, and it is also possible to make a foamed sheet by mixing and using the copolymer of the present invention and polystyrene for fine adjustment of the heat shrinkage temperature. The heat shrinkage temperature has additivity and can be calculated and predicted from the heat shrinkage temperature and the mixing ratio of each of the copolymer and polystyrene.

The foam shrinkable sheet of the present invention can be prepared by a usual extrusion foam molding machine for polystyrene. Either the inflation method or the T-die tenter method may be used. The foaming agent can be arbitrarily selected from the foaming agents used for ordinary foaming. For example, propane, butane, n-
Aliphatic hydrocarbons such as pentane and isopentane, halogenated hydrocarbons such as dichlorodifluoromethane, tetrafluoroethane, trichlorofluoromethane, methyl chloride and ethyl chloride, and ethers such as methyl ether and ethyl ether. . Of these, a mixture of dichlorodifluoromethane and methyl chloride and / or ethyl chloride is preferred for foam molding. Further, from the viewpoint of imparting low-temperature shrinkability, the above-mentioned mixed foaming agent having affinity with the styrene resin is preferable,
Copolymer 10 is a mixture of dichlorodifuromethane and methyl chloride and / or ethyl chloride in a weight ratio of 20:80 to 70:30.
It is preferable to use 1 to 7 parts by weight with respect to 0 parts by weight.

The properties of the foamable shrinkable sheet of the present invention include biaxially stretched and uniaxially stretched sheets, but uniaxially stretched sheets are particularly preferable, but not limited thereto. However, when used for cylindrical food container packaging, a uniaxially stretched foam sheet is preferable. The uniaxially stretched foamed sheet is obtained by re-stretching the foamed sheet produced by the inflation method using a mandrel or the like. Uniaxial stretching means that the heat shrinkage percentage in the direction orthogonal to the main stretch direction is 1/2 or less of the heat shrinkage percentage under the same measurement conditions in the main stretch direction, more preferably 10 to -25%.
A contraction sheet in the range of (negative means expansion). Here, a uniaxially stretched foam sheet will be described as one embodiment thereof.

The low temperature shrinkability of the foamed sheet of the present invention means in the stretching direction.
The heat shrinkage percentage at 80 ° C. is at least 10% or more, preferably 20% or more.

The heat shrinkage is expressed as a percentage ratio (%) of a value obtained by dividing the amount of shrinkage of the original size by the original size when treated for 5 minutes in a hot air circulation type oven. The shrinkage responsiveness is expressed by the same shrinkage factor when the sample is treated in the oven for 10 seconds. This value is at least 20% at 100 ° C. Generally, it is common knowledge that the shrinkage rate of a foamed sheet is represented by the equilibrium shrinkage rate at that temperature, and it differs from that method. For example, when using hot air, when immersing in hot water, when immersing in glycerin bath or silicon bath, there are many differences, and it is necessary to select the same method that suits the purpose. In general, the value when immersed in liquid is usually higher than that when hot air is used. The reason for this is considered to be that the material is a foam and the relaxation speed of the orientation in the thickness direction is different, the thermal conductivity is different depending on the medium, and that the medium affects the foamed sheet. When actually shrinking with hot air for practical packaging, a high value is obtained by the liquid medium immersion method, but hot air often does not give a good packaging finish. It should be noted that this responsiveness differs depending on the composition manufacturing method of the foamed sheet. In this respect, the foamed sheet of the present invention has sufficient responsiveness even with hot air.

The waist hardness (tensile modulus) of the foamed sheet is
Related to packaging speed, scratch resistance, practical shrinkage rate of packaging, etc.
It is one of the important factors, and the foamed sheet of the present invention is generally 500 to 7,000 kg, though it varies depending on its use.
/ cm ² .

The foamed sheet of the present invention has a thickness of 0.1 to 2.5 mm and a density of 0.4
It is preferably ˜0.05 g / cm ³ . If the thickness is 0.1 mm or less, the heat insulating property and the buffer property are inferior, and when the shrinkage is large, the film breaks. If the thickness is 2.5 mm or more, the shrinkage becomes uneven. In addition, it is not preferable because it is too rigid and it is difficult to shrink-wrap a small-diameter product. A foamed sheet having a density of 0.4 g / cm ³ or more has too high rigidity and poor buffering property. On the other hand, if the density is 0.05 g / cm ³ or less, the flexibility is too high and the impact absorption capacity is too low to meet the purpose of the present invention.

EXAMPLES Next, the present invention will be described in more detail with reference to examples.

Examples 1 to 4 Styrene and n-butyl acrylate or 2 ethylhexyl acrylate, ethylbenzene and dimethylformamide as a solution for adjusting the viscosity of the reaction system, and peroxide [1,1-di (t-butylperm) as a polymerization initiator. A predetermined amount of [oxy) cyclohexane] was injected into the reactor to carry out continuous completely mixed solution polymerization. The reaction rate was about 50 wt%, and the reaction product was taken out from the reactor and the unreacted product and the solvent were recovered to obtain a copolymer containing styrene as a main component. The composition of the copolymer,
The Vicat softening point and the molecular weight are as shown in Table 1.

These copolymers shown in Table 1 were mixed with dichlorodifluoromethane and methyl chloride as a foaming agent at a weight ratio of 50:50 in the same manner as in the conventional method for producing a foamed polystyrene sheet, to 100 parts by weight of the copolymer. Pour into the extruder to contain 2.5-4% by weight. The amount of the foaming agent may be appropriately adjusted depending on the density of the foamed sheet to be obtained, but the density of the foamed sheet when the content is about 3% by weight is 0.1 g / cm ³ . Foam extrusion temperature is 150-170 ° C, and extrusion foam molding is performed from a die having an annular slit of 0.5 mm. If necessary, the draw ratio of the extruded foam sheet may be readjusted using a mandrel. This cylindrical sheet was incised to obtain two sheets. The sheet thickness is about 1 mm. Table 1 also shows typical physical properties of the obtained stretched and foamed sheet.

In Table 1, the heat expansion / contraction rate is 80 ° C., the percentage (%) of the value obtained by dividing the length contracted by the linear contraction rate after heating in hot air for 5 minutes by the original dimension; in this case, the stretching direction.

Practical shrinkage is a margin ratio of + 5% for a glass bottle for natural juice (300cc body diameter 68mmφ, mouth 28mmφ, total length 140mm)
Inner diameter (71.4mmφ) cylindrical tube (length: 135mm)
The following shows a state in which the foamed sheet of 1 is prepared by overlapping the ends and heat-sealed, and the film coated on the bottle is tightly adhered to the bottle by a hot tunnel.
However, 15mm is put out on the bottom, and packaging and shrinking are performed in a shrink tunnel on the market with the bottle lying down.

◎: The product was completely tight by passing it once in hot air at 200 ° C for about 10 seconds, and the body, bottom and small diameter parts at the top of the bottle could be wrapped quickly without wrinkles or distortion.

◯: Even if the small diameter part does not shrink enough in one pass, it is completely finished in two passes.

△: Partially wrinkled on the body part even after one or two passes, the small diameter part was loose and no more finishing was done, and the temperature was raised to 300 ° C.
When it does, it contracts a little, but it is at a slightly more complete level.

X: It is difficult to completely package even with the above operation.

Heat sealability is obtained when the temperature and time of each sample are adjusted and the sample is best sealed and then heat-shrink wrapped. ◎: Even if it is pulled, the seal part and the periphery of the seal part become brittle and cannot be easily cut. Those that can withstand, and those that do not immediately break even if pulled by hand after packaging.

Δ: The seal part is temporarily sealed, but after practical packaging,
It will break if pulled hard with your hand.

X: The seal is not well developed, and therefore, after the practical packaging, the seal portion is broken or ruptured simply by holding the bottle strongly, and defects of the seal portion are recognized even during packaging.

As described above, as shown in Table 1, all of Examples 1 to 4 exhibited excellent characteristics. Naturally, the foam shrinkable sheet according to the present invention has the same heat insulating properties and cushioning properties as those of the conventional polystyrene foam shrinkable sheet, and further has good adhesion to a bottle, which is not seen in polystyrene products, and so-called foaming. It was found that the shrinkable sheet was difficult to rotate around the glass bottle, and was a unique packaging material.

Comparative Examples 1 and 2 As a result of evaluating and producing a foamed sheet using a copolymer containing styrene as a main component and having a composition shown in Table 1 as in Examples,
Unlike the case of Examples, the shrinkable foam packaging did not work well as shown in Table 1.

[Effects of the Invention] As described above, the foamable shrinkable sheet of the present invention has a shrinkage temperature of 10 to 10 as compared with the conventional expandable polystyrene shrinkable sheet.
Industrially meaningful and energy-saving that shrinks at temperatures as low as 30 ° C or higher and does not impair the characteristics of conventional expanded polystyrene such as heat insulation, cushioning, rigidity and secondary processability such as printing. It is a type of packaging material.

Claims (3)

[Claims] 1. A foamable shrinkable sheet of a copolymer comprising (A) 70 to 97% by weight of styrene and 30 to 3% by weight of one or both of acrylic acid ester and methacrylic acid ester. 2. A foam shrinkable sheet according to claim 1, wherein n-propyl acrylate, n-butyl acrylate, hexyl acrylate, nonyl acrylate is used as the acrylic acid ester, and n-dodecyl methacrylate is used as the methacrylic acid ester. 3. A sheet having a thickness of 0.1 to 2.5 mm, a density of 0.4 to 0.05 g / cm ³ , and a heat shrinkage ratio at 80 ° C. of 1.
The expandable shrinkable sheet according to claim 1 or 2, which is 0% or more and is uniaxially stretched.