JPH0624758B2 - How to make polyester containers for beverages - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a polyester container for beverages, and more specifically, to a stretcher having a small shrinkage with time and an excellent mechanical strength such as compressive strength. The present invention relates to a method of manufacturing a molded container.

(Prior Art) A biaxially stretch blow-molded container made of a thermoplastic polyester such as polyethylene terephthalate (PET) has excellent transparency and surface gloss, and also has impact resistance required for a bottle.
It has rigidity and gas barrier properties, and is widely used as a bottled container for various beverages.

Generally, in the production of stretch blow molded PET containers,
A PET resin is injection-molded to produce a substantially amorphous bottomed preform, and this bottomed preform is preheated to a stretching temperature, then held by a split mold and axially moved by a stretching rod. Along with the tensile stretching, the fluid is blown to expand and stretch in the circumferential direction. Preheating of the preform is generally performed by supporting the preform with a mandrel or the like and irradiating infrared rays from the outer surface side of the preform, but in this case, the outer surface side of the preform has a higher temperature than the inner surface side. Therefore, in order to prevent this, it is necessary to carry out uniform heating over a significantly long heating time, or to provide a special uniformization region for uniformizing the temperature difference between the inner and outer surfaces.

In order to prevent such a defect, it is already known that the preform is heated from the inner surface side and the outer surface side to make the temperature uniform and shorten the heating time, for example, Japanese Patent Publication No. 62-42852. The publication describes that the preform is heated from the inside by providing a heat insulating material layer that generates heat with far infrared rays on the outer periphery of the core shaft that is inserted inside the preform. Further, Japanese Patent Laid-Open No. 62-77919 describes that a large-diameter portion is provided at the end of a rod-shaped heater inserted inside the parison so that a radiation intensity distribution suitable for parison heating can be obtained. ing. Furthermore, JP-A-61-16382
No. 8 discloses that the preform is heated from the outside in a heating furnace and is also heated from the inside by a heat pipe inserted in the preform.

(Problems to be Solved by the Invention) In the method shown in the above prior art in which the preform is heated from both the outer surface side and the inner surface side, the temperature of the inner and outer surfaces of the preform is made uniform and the heating time is shortened. Although the stretch blow-molded container actually formed by this method has good shrinkability over time, it has a large creep and a considerable decrease in pressure strength. The drawback is that it does.

Therefore, it is an object of the present invention to provide a method for producing a polyester stretch-blow molding container having a combination of a small shrinkage property over time and a large compressive strength.

Another object of the present invention is to provide a method for producing a stretch-blow molded polyester container capable of significantly shortening the time required for preheating a preform and significantly improving the overall productivity.

Still another object of the present invention is to provide a method for producing a stretch-blow molded container made of polyester, which can be advantageously applied to the use of filling beverages, especially carbonated beverages.

(Means for Solving the Problems) According to the present invention, a preform made of a thermoplastic polyester having a temperature (T _I ) of the inner surface and a temperature (T _O ) of the outer surface represented by the formula T _I ≧ 85 ° C. (1) T _The heating preform is heated so as to satisfy _O ≤ 105 ° C (2) and 10 ° C ≥ T _O -T _I > 0 ° C (3), and the heating preform is stretched in the hollow mold at a circumferential stretching speed of 350%. / Sec or more and expression In the formula, D _I is the area stretch ratio of the inner surface of the preform, and D _O is the area stretch ratio of the outer surface of the preform.

There is provided a method for producing a polyester container, characterized in that the axial tensile stretching and the circumferential expansion stretching are performed so that the stretching deviation ratio (D) defined in (3) is 20 to 40%.

In the present invention, it is preferable to heat the preform made of thermoplastic polyester with an internal heater and an external heater.

(Operation) In the present invention, a thermoplastic polyester preform is heated from both sides by an internal heater and an external heater. At this time, the above formulas (1), (2) and (3) are simultaneously satisfied. As described above, the first feature is to heat the outer surface temperature (T _O ) to be higher than the inner surface temperature (T _I ) by a certain range. First, the inner surface temperature (T _I ) is calculated by the formula (1)
The reason for satisfying is that when T _I is lower than 85 ° C., microcracks are generated in the final container, the transparency is lowered, and the strength of the container is lowered. On the other hand, the outer surface temperature (T 2 _O ) is set so as to satisfy the formula (2) because when T 2 _O exceeds 105 ° C., thermal crystallization of the polyester occurs during preheating or during the subsequent stretch molding operation. The transparency is impaired and the molecular orientation is relaxed, so that the strength cannot be expected to be improved by stretching. Next, providing the temperature difference of the above formula (3) between the outer surface temperature and the inner surface temperature is
This is for the purpose of achieving a balance that maximizes both the time-dependent shrinkability and the pressure resistance in connection with the subsequent stretch-molding operation.

Conventionally, in stretch blow molding of plastic preforms, since the stretch ratio on the inner surface side is larger than that on the outer surface side, in order to make the degree of molecular orientation of the both uniform, (T _I ) of the outer surface is generally higher than the temperature (T _O ) of the outer surface (for example, JP-A-49-103956, page 4, lower left column, and JP-A-SHO). No. 58-167127, No. 5
Top right column).

On the other hand, in the present invention, contrary to this general common sense, the preform is provided with a temperature gradient such that the outer surface temperature (T _O ) becomes higher than the inner surface temperature (T _I ). This is because under the stretch-plow molding conditions adopted in the present invention, internal heat is generated on the inner surface side of the preform, and it becomes possible to impart appropriate molecular orientation and crystallization while relaxing strain.

When the outer surface temperature (T _O ) is equal to or lower than the inner surface temperature (T _I ), the molecular orientation of the inner surface side resin is excessively relaxed due to the effect of internal heat generation during stretch blow molding, Creep becomes large and the pressure resistance is significantly reduced.

On the other hand, when the value of T _O -T _I exceeds 10 ° C., the residual strain of the inner surface side resin regardless heat generation during stretching becomes large,
Greater shrinkage over time. In the present _invention, T O -T value of _I is preferably in the range of 0 to 10 ° C..

Next, in the present invention, the stretching strength in the circumferential direction is 350%.
/ Sec or more, particularly 450% / sec or more, and the above formula
The stretching deviation ratio (D) defined by (4) is 20 to 40%,
In particular, the second characteristic is that the axial tensile stretching and the circumferential expansion stretching are performed so as to be 25 to 35%.

In general, between the stretching speed of a thermoplastic polyester and the heat generated by internal friction and crystallization (internal heat generation), when the stretching speed exceeds a certain point, the temperature rise due to the internal heat tends to increase sharply. In the present invention, the reason why the stretching speed is set in the above range is that the temperature rise due to internal heat generation is remarkable in this range and generally reaches 10 to 30 ° C.

The stretch deviation ratio (D) defined by the above formula (4) means the area stretch ratio (times) of the inner surface of the preform and the area stretch ratio (times) of the outer surface of a given preform and a container formed therefrom. Is the percentage normalized by the internal surface area stretching ratio, and generally takes a value larger than zero and smaller than 100.

This stretching deviation rate (D) is related to the internal heat generation of the inner surface side resin as well as the molecular orientation of the inner surface side resin. The larger the value of D, the larger the internal heat generation and the higher the degree of molecular orientation. When the internal heat generation becomes large, the relaxation of the orientation also progresses at the same time, so that there is a certain optimum range with respect to the shrinkage property over time and the pressure resistance.

That is, when the stretching deviation ratio (D) is smaller than the above range, the relaxation of the inner surface side resin is insufficient and the crystallinity is not improved, so that there is a tendency that the shrinkage with time becomes large. If it is too large, the relaxation of the resin on the inner surface side becomes too large, and the pressure resistance of the container tends to be insufficient.

In the present invention, it is also important to perform stretch blow molding so that the obtained stretch blow molded container has orientation and crystallinity that satisfy the following formula.

That is, the thickness direction refractive index (n _O ) on the outer surface side of the body portion and the thickness direction refractive index (n _I ) on the inner surface side of the body portion of the obtained stretch-blow molded container are 1.500 ≧ n _I ≧ 1.492 (5) 0.020 ≧ n _O −n _I ≧ 0.010 (6), and the crystallinity (X _O ) on the outer surface side of the body and the crystallinity (X _I ) on the inner surface side of the body are 35% ≧ X _I ≧ 20% ... (7) 3 % ≧ X I -X O> 0% ... (8) it is important to stretch blow molding to satisfy.

In this specification, the refractive index in the thickness direction and N as the light source are used.
aD line, Abbe's refractometer and a polarizing plate are used as a refractometer, light is made to enter in parallel to the sample surface, and the polarization direction of the polarizing plate is measured in the thickness direction. That is, the smaller the degree of orientation in the plane direction (axial direction and circumferential direction) of the sample, and conversely, the smaller the degree of refraction in the thickness direction, the greater the degree of orientation in the plane direction of the sample.

When the inner surface side refractive index n _I is larger than the upper limit of the formula (5), the degree of in-plane orientation of the container body is small, and the strength of the container is not sufficient. Further, when the inner surface side refractive index n _I is smaller than the lower limit of the expression (5), the residual strain of the container body becomes large and the time-dependent deformation tends to become large.

The difference between the outer surface side refractive index and the inner surface side refractive index (n _O
It has been found that −n _I ) is closely related to the degree of relaxation of the orientation of the resin on the inner surface side. In a container formed from a conventional preform external surface heating type preform, n _O −
The value of n _I is much higher than 0.020, generally 0.030 to
The value is 0.060, and in such a container, the residual strain on the inner surface side is large and shrinkage with time reaching 3% by volume or more occurs. On the other hand, n _O -n in a container formed from the preform of the preform inside surface, an outer surface heating method to the inner surface side and high temperature
The value of _I is less than 0.010, generally 0.005 to 0.000
In such a container, the orientation relaxation on the inner surface side is too large, and the pressure resistance thereof is reduced to 3/4 or less of the former. In the present invention, the value of n _O -n _I is in the above range, the degree of orientation relaxation of the inner surface side resin while suppressing aging shrinkage to 3% by volume or less, yet 16 kg / cm ² or more even It is possible to obtain the pressure strength to reach.

The crystallinity (X) in the present specification is calculated from the measured value of density using a density gradient tube. In the formula, ρ: sample density (g / cm ³ ) ρ _am : amorphous density (PET = 1.335g / cm ³ ) ρ _c : value calculated by crystalline density (PET = 1.455g / cm ³ ). Is.

The crystallinity X _I of the inner surface side resin is closely related to the pressure resistance of the container. That is, when X _I is increased beyond the upper limit of the formula (7), and if smaller beyond its lower limit, pressure resistance decreases as compared with the case of satisfying the formula (7). This is probably because in the former case, the relaxation of the amorphous orientation portion is too large, and in the latter case, the crystal has not developed sufficiently to prevent creep.
Further, when the crystallinity is high enough to generate spherulites, the impact resistance against dropping or the like is also lowered.

The crystallinity X _I of the resin on the inner surface side is the crystallinity X _{O of the} resin on the outer surface side.
Must be higher than the above and satisfy the above formula (8). In an internal pressure container such as a carbonated beverage container, it is the inner surface side resin that helps prevent creep, but according to the present invention, by increasing the crystallinity of the inner surface side resin within a certain range, it is possible to improve the pressure resistance. You can X _I -X value of _O is desirably in the range of 0 to 3%.

(Preferred Embodiment of the Invention) In the present invention, as the thermoplastic polyester, a thermoplastic polyester mainly composed of ethylene terephthalate units, for example, PET or a small amount of other glycols such as hexahedroxylene glycol as a glycol component, or As the dibasic acid component, so-called modified PET containing a small amount of other dibasic acid components such as isophthalic acid and hexahydroterephthalic acid is used. These polyesters may be used alone or in the form of blends with other resins such as nylons, polycarbonates or polyarylates.

The intrinsic viscosity of the thermoplastic polyester used is 0.67 dl / g or more, and the content of diethylene glycol units is 2.0.
It is desirable to be in the range of not more than wt%.

The bottomed preform used for stretch blow molding is manufactured by any method known per se, such as an injection molding method and a pipe extrusion molding method. In the former method, molten polyester is injected to produce a bottomed preform having a mouth and neck corresponding to the final container in an amorphous state. The latter method is an advantageous method for producing a bottomed preform provided with a gas barrier intermediate resin layer such as an ethylene-vinyl alcohol copolymer, which is obtained by cutting an extruded amorphous pipe and compressing it at one end. The mouth and neck are formed by molding, and the other end is closed to form a bottomed preform. In order to maintain good engagement and sealing with the lid at high temperature, only the part that becomes the neck of the container can be preliminarily thermally crystallized. Of course, this thermal crystallization can also be performed at any subsequent stage.

Preheating of the preform can be carried out by using an arbitrary heating mechanism.
The one described in JP-A-3-306023 can be used, and an infrared heater or the like can be used as the external heater.

Stretch blow molding of the preform can be carried out under conditions known per se except for the above-mentioned restrictions. It is desirable that the temperature of the hot air blown into the preform is 10 ° C or more higher than the preform temperature (T), and the axial draw ratio is 1.3 to 3.5 times, especially 1.5 to 3 times, and the circumferential direction It is preferable that the body has a stretching ratio of 2 to 5.5 times, particularly 3 to 5 times.

(Effects of the Invention) According to the present invention, by giving a specific temperature gradient to the inner and outer surfaces of the preform and skillfully and effectively utilizing the internal heat generated during stretch blow molding, the shrinkage with time is small and the pressure resistance is low. It has become possible to manufacture a stretch-blown polyester container having high strength. Further, according to this method, the heating time of the preform is short and the stretch blow molding can be performed at a high speed, so that the productivity of the container can be remarkably improved.

(Example) The present invention will be described in the following examples. Each evaluation item was measured by the following method.

Preform temperature Thermocouples are attached to the inner and outer surfaces of the center of the preform in the height direction, temperature is measured, and the temperature immediately before the heated preform enters the hollow mold is measured. The reform temperature was used.

Circumferential stretching speed by blowing A temperature sensor was attached to the inner surface of the mold, near the center of the bottle body, and the time from the start of blowing until the start of temperature rise was measured. When this time is Δt, the average radius of the preform is r, and the distance from the center line of the bottle to the temperature sensor mounting position is R, the circumferential stretching speed is expressed by the following equation.

Crystallinity The wall surface of the bottle was stripped by a cutter in approximately 1/3 each in the thickness direction, and pieces on the outer surface side and the inner surface side of the bottle were used as samples.

An n-heptane-carbon tetrachloride-based density gradient tube (Rika Ikeda, Inc.) was prepared, and the density of the sample was determined under the condition of 20 ° C. Thus, the crystallinity was calculated according to the following formula.

Crystallinity In the formula, ρ: measured density (g / cm ³ ) ρ _am : amorphous density (PET = 1.355g / cm ³ ) ρ _c : crystalline density (PET = 1.455g / cm ³ ) Refractive index NaD ray as a light source , Abbe's refractometer and a polarizing plate are used as refractometers. J. SAMUELS (Journal) of Applie
d Polymer Science, Vol.26.1383 (1981))
Refractive indexes n _I and n _O in the thickness direction of the inner and outer surfaces of the sample cut out from the bolt were measured.

Burst strength Water was put in a bolt, pressure was applied, and the pressure at the time of bursting was measured.

Shrinkage rate A strain gauge (Kyowa Denki Co., Ltd.) is attached to the outer surface of the bottle in the bottle circumferential direction and axial direction, and then placed in an oven at 60 ° C
The shrinkage rate after 10 minutes of time insertion and removal and cooling was measured with a static strain measuring instrument.

Incidentally, the evaluation bottle was stored for one day in an atmosphere of 30 ° C. and 80% RH after molding. Further, the measurement position of the refractive index, the crystallinity, and the shrinkage ratio was at the height of the bottle of about 3/4.

The present invention can be applied to conventionally known containers having various shapes. For example, it is preferable to use a container having a shape shown in FIG. 1 or a container having a shape shown in FIG. 2 in order to improve pressure resistance and impact resistance.

Example 1 An injection-molded polyethylene terephthalate preform (weight: 49 g) was heated from the inside and the outside and then biaxially stretch blown to a bottle having an internal volume of 1.5 and having a shape shown in FIG. 300 μm) was prepared. The stretch deviation (D) of the preform used was 32.3%, and the circumferential stretching speed was 45 when measured by the above-mentioned method.
It was 0% / sec. Table 1 shows the values of the refractive index in the thickness direction and the crystallinity of the inside and outside of the bottle obtained by adjusting the inside and outside temperatures of the preform by adjusting the inside heater and the outside heater.

In a, the temperature T _I on the inner surface side of the preform was too low, so that microcracking occurred due to overstretching, resulting in a thin pearl color and poor transparency. Therefore, the burst strength also showed a small value.

Under the condition b, since the outer refractive index n _O in the thickness direction is large, the orientation can be said to be small, but the outer crystallinity shows a large value. This means that the outer preform was at a high temperature, so that crystallization proceeded, but the molecular orientation was rather too high and the relaxation was remarkable. Since the strength depends on the molecular orientation, it is influenced by this outer relaxation and has a small value.

At d, the temperature on the inner surface side is higher,
This is generally considered to be an appropriate temperature gradient because the stretching ratio in the circumferential direction on the inner surface side is larger than that on the outer surface side, but the stretching speed is very high as in the main molding conditions. In this case, the material internally heats up due to the stretching, and the temperature rises considerably immediately after the stretching. Therefore, it is expected that the temperature further increases on the inner surface side where the stretching ratio is high. As a result, crystallization is promoted and molecular orientation is relaxed, and the latter in particular becomes unnecessarily large, which is favorable in terms of shrinkage resistance over time, but the strength is reduced. On the other hand, c and e are set to be slightly lower than the preform temperature of the outer surface, even though the inner draw ratio is larger than that of the outer surface, considering that the inner surface temperature is increased by self-heating. The bottle thus obtained, the inner surface side has a smaller thickness direction refractive index than the outer surface side, that is, oriented, while maintaining a state of high crystallinity,
The values are very close. In other words, despite the fact that the stretching ratio in the circumferential direction was very different inside and outside, it was under the same condition, which means that proper stretching was performed. Since it shows a large value, it can be said that the stretching effect is maximized.

Example 2 With respect to the bottles a and e of Example 1, Table 2 shows the measurement results of the shrinkage rate caused by placing them in a 60 ° C. oven for 1 hour.

From the above, it can be seen that the bottle obtained by the production method of e has excellent shrinkage resistance.

Comparative Example 1 Using a preform similar to the preform used in Example 1, the blow pressure was reduced and bottle molding was performed. Table 3 shows each measured value. Preform temperature is outside 102
The inner surface was 99 ° C.

From this, it can be understood that when the stretching speed is slow, the shrinkage ratio increases. If the stretching speed is slow, internal heat generation due to stretching hardly occurs, so that relaxation of molecular orientation due to self-heating and high temperature cannot be expected. Therefore, the residual strain increased and the shrinkage ratio increased.

Comparative Example 2 Table 4 shows the measured values of the bottle in which the preform was heated only by the external heater. When only external heating was used, it was difficult to adjust the inside and outside temperatures of the preform within the range specified in the claims. Therefore, the relationship between the inner and outer preform temperatures and the draw ratio became improper, and the burst strength and shrinkage resistance of g were worse, and the burst strength of h was worse than those of the examples.

[Brief description of drawings]

FIG. 1 is a diagram showing the shape of a bottle prepared in Example 1. FIG. 2 shows the shape of the container applicable to the present invention.

Claims (4)

[Claims] 1. A preform made of a thermoplastic polyester having an inner surface temperature (T _I ) and an outer surface temperature (T _O ) of the formula T _I ≧ 85 ° C. T _O ≦ 105 ° C. and 10 ° C. ≧ T _O −T _I > 0. The heating preform is heated in the hollow mold so that the drawing rate in the circumferential direction is 350% / sec or more and In the formula, D _I is the area stretch ratio of the inner surface of the preform, D _O is the area stretch ratio of the outer surface of the preform, and the axial stretch stretching is performed so that the stretch deviation ratio (D) is 20 to 40%. And a circumferential expansion / stretching process for producing a polyester container. 2. The method according to claim 1, wherein the preform is heated from both sides by an internal heater and an external heater. 3. The thickness direction refractive index (n _O ) on the outer surface side of the body and the thickness direction refractive index (n _I ) on the inner surface side of the body are 1.500 ≧ n _I ≧ 1.492 0.020 ≧ The method according to claim 1, wherein the stretching is performed so that n _O = n _I ≧ 0.010. 4. The crystallinity (X _O ) on the outer surface side of the body and the crystallinity (X _I ) on the inner surface side of the body measured by the density method are 35% ≧ X _I ≧ 20% 3% ≧ X _I The method according to claim 1, wherein the stretching is performed so that -X 2 _O > 0%.