JPH0129693B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing deep-drawn polyester containers having high impact strength.

Sheets of polyethylene terephthalate are
Conventionally, biaxially stretched films have been widely used in large quantities in fields such as electricity, printing, agricultural materials, and packaging.
Very recently, trays made of polyethylene terephthalate or cups, which are deep-drawn products, have begun to be used for food packaging purposes.

Although polyethylene terephthalate is a crystalline polymer, its crystal specific velocity is lower than that of polyethylene.
Since it is slower than polypropylene, nylon, etc., an amorphous sheet can be obtained by rapidly cooling it from a molten state. For this reason, polyethylene,
A highly transparent sheet that is not found in polypropylene, nylon, etc. can be obtained.

Moreover, since polyethylene terephthalate is a crystalline polymer, when the stretching ratio exceeds about 2 times, the toughness increases with the stretching ratio due to the effect of molecular orientation and the effect of promoting crystallization. Therefore, compared to amorphous polymers such as polyvinyl chloride and polystyrene, it is easier to make the wall thickness of a deep-drawn container uniform. Furthermore, when an amorphous transparent sheet is stretched and crystallized, the sheet does not whiten even if the degree of crystallinity is high. These reasons combine to make polyethylene terephthalate thermoformed containers highly transparent and strong.

On the other hand, polyethylene terephthalate does not contain any additives such as stabilizers that can be eluted into the contents, making it extremely hygienic, having high gas barrier properties, and high fragrance retention. Recently, there has been a great deal of interest in packaging containers made from polyethylene terephthalate because they do not have the odor found in olefins and the like.

However, the side walls of deep-drawn containers made of polyethylene terephthalate are almost uniaxially stretched, and although they are strong in the stretching direction, that is, the longitudinal direction, they are extremely weak in the transverse direction; Cracks along the line are likely to occur. These drawbacks are not a problem in normal packaging applications, but under harsh conditions, such as testing containers for Japanese sake, at 70°C.
Under severe test conditions, such as filling at near-freezing temperatures and dropping from a height of 150 cm at sub-zero temperatures, vertical cracks may occur in the molded containers.

As a result of intensive research into deep-drawn containers free of such drawbacks, the inventors of the present invention found that the side walls of deep-drawn containers made of polyethylene terephthalate are also stretched in the lateral direction. If the birefringence on the cut plane parallel to the circumferential direction and perpendicular to the side wall surface is above a specific value,
It has been discovered that vertical cracking when dropped under the above-mentioned severe conditions can be prevented, and the present invention has been achieved.

That is, the present invention provides a method for manufacturing a deep-drawn container biaxially stretched from a polyethylene terephthalate sheet by the plug assist method, in which the diameter or width of the plug is set to 80% or less of the inner diameter of the mold, and the upper part of the side wall expands in the lateral direction. The refractive index in the circumferential direction n ₁ in a cut plane parallel to the circumferential direction and perpendicular to the side wall surface of the upper side wall of the side wall of a deep drawn container characterized by molding using a mold with a diameter This is a method for producing a deep-drawn polyester container with excellent impact resistance, in which the birefringence Δn=n ₁ −n ₂ , which is the difference between the refractive index n ₂ in the thickness direction and the refractive index n 2 in the thickness direction, is at least 0.05.

In the present invention, polyethylene terephthalate refers to homopolymers of ethylene terephthalate, copolymers in which 80% or more of the repeating units are ethylene terephthalate units and 20% or less are other copolymerizable components, and polyethylene terephthalate containing 80% by weight % or more and the other polymer is 20% or less by weight. Furthermore, there are no restrictions on the addition of low-molecular substances to polyethylene terephthalate for the purpose of preventing blocking, promoting crystallization, improving processability, etc., as long as there is no food hygiene problem.

An unstretched sheet made of polyethylene terephthalate is obtained by discharging molten polyethylene terephthalate in the form of a sheet from the mouthpiece of an extruder, and then cooling and solidifying it on the surface of a rotating quenching body. The cooling temperature is preferably 40 to 60°C.

If the temperature exceeds 60°C, cooling will be insufficient and the thermoformability of the sheet will deteriorate. Furthermore, if the surface temperature of the rotary quenching body is too low than 40°C, wrinkles will occur on the sheet surface, which will impair the appearance of the molded product.

A plug assist forming method is generally used to perform deep drawing using this unstretched sheet. In this case, the size of the plug generally used is one whose width or diameter is approximately 85 to 90% of the mold size. However, molded containers manufactured by the conventional plug-assisted molding method have disadvantages in that they are prone to vertical cracking and are weak against impact. On the other hand, the deep drawn container obtained by the manufacturing method of the present invention is obtained by reducing the size of the plug to 80% or less of the mold size, unlike the conventional plug assist molding method. For this reason, in the present invention, a mold as shown in FIG. 1 is used in which the upper part of the side wall is expanded in diameter in the lateral direction. FIG. 1 is a sectional view of the mold, and numeral 1 in the figure represents an unstretched sheet. Further, 2 is a mold, 2A is a mold mouth, 2B is a mold step, and 2C is a vacuum hole. The stepped portion of the mold corresponds to the mouth of the deep-drawn container. In other words, the size of the plug should be 80% or less of the mold dimension, and when the unstretched sheet is pushed into the mold with the plug shown in Figs. A mold is used that has been modified so that the unstretched sheet is set away from the mold mouth so that it does not touch the stepped portion. In the present invention, deep drawing is performed in the steps shown in 7-a to 7-d. Figure 7-a is a sectional view of the process before deep drawing. FIG. 7-b is a process sectional view of the initial stage of deep drawing. FIG. 7-c is a sectional view of the process immediately before the deep drawing is completed by the plug and the next step is the compressed air/vacuum process. FIG. 7-d is a process sectional view after deep drawing is completed. If such a mold is used, the bottom of the resulting molded product can be stretched in the lateral direction, and the top of the molded product can also be stretched in the lateral direction. By the way, if the size of the plug is reduced in this way, the difficulty of molding increases, so the plug temperature must be set within the temperature range of the glass transition temperature Tg of the unstretched sheet to Tg + 90°C. In producing the deep-drawn container of the present invention, it is preferable to use compressed air as an auxiliary since it is difficult to obtain the desired mold shape by forming only by vacuum.

The deep-drawn container obtained by the manufacturing method of the present invention is the first to be obtained by the plug-assisted molding method using the above-mentioned special mold. In conventional deep-drawn containers, the side walls have molecular orientation only in the vertical direction, making it easy for vertical cracks to occur, whereas in the deep-drawn containers obtained by the manufacturing method of the present invention, the uppermost part of the side walls Because it has a biaxially oriented structure with molecules oriented both in the horizontal direction and in the horizontal direction, it does not suffer from vertical cracking and has excellent impact resistance.

The present invention will be explained below with reference to Examples. In addition, each characteristic value in an Example was calculated|required by the following measuring method.

Intrinsic viscosity [η]: Measured at 25°C using a mixed solvent of phenol/tetrachloroethane (50/50).

Birefringence △n: Cut out a 5 mm square specimen with sides parallel and perpendicular to the circumferential direction from the side wall of the molded container, and measure the cross section parallel to the circumferential direction and perpendicular to the side wall using an Atsube refractometer. Refractive index in the circumferential direction measured with polarized light parallel and perpendicular to the side wall surface n ₁
The difference between the refractive index n ₂ and the thickness direction is defined as the birefringence Δn of the cross section.

Example 1 Pellets of polyethylene terephthalate with an intrinsic viscosity [η] = 0.73 were discharged from a 40 mmφ extruder at a resin temperature of 290°C, cooled and solidified on the surface of a rotating quenching body at 53°C, and were made into a pellet with a thickness of 0.45 mm and a width of 300 mm. An unstretched sheet was obtained. This unstretched sheet was deep-drawn using a hand-made deep-drawing machine to obtain a molded container 3 shown in FIGS. 2 and 3. Figure 2 is a longitudinal cross-sectional view of the molded container;
FIG. 3 is a perspective view thereof. At this time, a mold was used in which the unstretched sheet could be set at a position away from the mouth of the mold, as shown in FIG.

The width of the plug was 65% of the mold dimension, and the sheet surface temperature immediately before molding was approximately 120°C as measured with an infrared thermometer. At the same time as the vacuum was drawn, compressed air was applied using dry nitrogen at a pressure of approximately 9 kg/cm ² .

The obtained molded container 3 was fitted with a paper cup 4 as shown in Fig. 4, and 80% of the total content was filled with hot water at 67°C, and the opening was immediately sealed with aluminum foil coated with hot melt adhesive. After that, it was left in a low temperature bath at 3°C overnight.

After standing overnight, a drop test was performed from a height of 150 cm, and 0 out of 50 polyethylene terephthalate molded containers were damaged. The birefringence Δn at the top of the side wall of this molded container was 0.06. Further, the birefringence Δn at the lowest part of the side wall was 0.09.

Comparative Example 1 The same sheet as in Example 1 was used, and the mold shown in Figure 1 was used, and deep drawing was performed in the same manner as in Example 1, except that the width of the plug was set to 95% of the mold dimension. A molded container was obtained. This molded container was fitted with a paper cup in the same manner as in Example 1, and a drop test was carried out in the same manner as in Example 1, and 35 out of 50 polyethylene terephthalate molded containers were damaged. The birefringence Δn at the top of the side wall of this molded container is
It was 0.035. Moreover, the birefringence Δn at the lowest part of the side wall was 0.045.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view of a mold used in manufacturing a molded container obtained by the manufacturing method of the present invention, and FIG.
The figure is a longitudinal sectional view of the molded container of the present invention, FIG. 3 is a perspective view thereof, and FIG. 4 is a longitudinal sectional view of the molded container 3 and paper cup 4 fitted together obtained by the manufacturing method of the invention be. 5 and 6 are longitudinal sectional views of the plug. Figures 7-a to 7-d are cross-sectional views of the deep drawing process. 1 is an unstretched sheet, 2 is a mold, and 3 is a plug.

Claims (1)

[Claims] 1. A method for manufacturing a deep-drawn container biaxially stretched from a sheet made of polyethylene terephthalate by the plug assist method, in which the diameter or width of the plug is set to 80% or less of the inner diameter of the mold, and the upper part of the side wall expands in diameter in the lateral direction. In a cut plane parallel to the circumferential direction of the upper side wall of the side wall and perpendicular to the side wall surface, the refractive index n ₁ in the circumferential direction is A method for producing a deep-drawn polyester container having excellent impact resistance and having a birefringence Δn=n ₁ −n ₂ expressed by the difference in refractive index n ₂ in the thickness direction of at least 0.05.