JP2895498B2 - Thermoformed polyester container - Google Patents

Description

The present invention relates to an improved orbable food container, and more particularly to a thermoformed partially crystallized polyester container having excellent moldability and impact resistance.

<Problems to be solved by the prior art and the invention> A crystalline polyester resin represented by polyethylene terephthalate (PET) is widely used as a polymer for sheets and films, including fibers, but has excellent chemical resistance. It has been applied to bottles for beverages such as carbonated beverages, juices and beers, cosmetic containers, trays for foods and the like by utilizing its properties and low gas permeability. Above all, due to the rapid spread of microwave ovens / oven, foods stored in refrigerators or freezers are more often cooked in microwave ovens / oven as they are, and the demand for correspondingly heat-stable containers has been increased. That is, a container is required that can withstand the temperature exceeding 200 ° C. used in a usual microwave oven / oven, and that is not damaged during storage at a low temperature of −20 ° C. or less, and handling during transportation and cooking. I came.

The biaxially stretched film of polyethylene terephthalate is
Although it is excellent in heat resistance and mechanical strength, it cannot be thermoformed such as vacuum forming and cannot be used in the field of blister pack containers and the like. Therefore, a thermoforming method such as injection molding or vacuum molding in which the sheet is once extruded and then preheated to a temperature sufficient to deform the sheet and shaped is employed. However, in order to obtain a thin molded product such as a tray, the latter method that does not generate a flow mark or a weld layer is advantageous, and is used as a general method. The molded article as formed has low crystallinity as it is, and cannot withstand high temperatures of 200 ° C. or more at all. Therefore, the same as the shaping process, or subsequent heat treatment to increase the crystallinity,
It has heat resistance close to the melting point of polyethylene terephthalate, that is, 200 ° C. or higher.

Even with unmodified polyester containing no other components, the crystallinity can be increased by the above method, but it is necessary to increase the heating crystallization temperature or to increase the treatment time, and the productivity is extremely poor. In addition, it tends to stick to the surface of the mold, which induces deformation of the molded product and lowers the quality of the product. In addition, the impact strength of the final product is insufficient for many applications.

Therefore, it is a common method to mix a substance (nucleating agent) which serves as a nucleus of crystal growth for rapidly increasing the degree of crystallinity and a crack preventing agent (preferably polyolefin) having an impact improving effect. Although the crystallization speed and impact resistance of the product are improved by these methods, the nucleating agent for increasing the crystallization speed causes the crystallinity of the final product to be too high, so that the impact strength, especially at low temperatures, The impact strength was not sufficient.

Therefore, a method of blending a high molecular nucleating agent (preferably polyolefin) that shares the crystallization promoting effect and the crack preventing effect has been considered, and the low-temperature impact strength has been greatly improved.

However, in such a method, when producing a thermoformed container at a high speed, the crystallization speed is not sufficient, and when the product is removed from the mold, mold release failure occurs, the mold collapses, and the quality of the product is significantly reduced. There was a problem.

The present inventors have conducted intensive studies to solve such conventional problems, and as a result, by producing a thermoformed polyester container from a sheet having a two-layer structure having a different composition, excellent moldability and impact strength have been obtained. Have been achieved, and the present invention has been achieved.

<Means for solving the problem> That is, the present invention provides an inorganic particle 0.1 having an average particle diameter of 0.1 to 20μ.
Polyethylene terephthalate (layer A) having an intrinsic viscosity of 0.6 to 0.95 containing 10 to 10% by weight, 1 to 10% by weight of a polyolefin having a repeating unit derived from a monomer having 2 to 6 carbon atoms, and a crystal nucleating agent. Polyethylene terephthalate having an intrinsic viscosity of 0.7 to 1.1 containing 0 to 3% by weight (B
Layer A) is laminated to form a sheet, and then the layer A is brought into contact with a mold to form a thermoformed polyester container.

 The constituent features of the present invention will be described below.

Polyethylene terephthalate used for layer A is 20 ° C
The intrinsic viscosity measured in a phenol / tetrachloroethane mixed solvent having a weight ratio of 60/40 is preferably from 0.6 to 0.95, particularly preferably from 0.80 to 0.9. Polyethylene terephthalate having a low intrinsic viscosity is more advantageous in terms of the crystallization rate, but has a lower low-temperature impact.
is necessary. Conversely, those having an intrinsic viscosity of more than 0.95 are advantageous against impact at low temperatures, but are disadvantageous when the crystallization rate during thermoforming becomes slow and the molding cycle is increased.

Examples of the inorganic particles include talc, mica, calcium carbonate, magnesium carbonate, titanium oxide, aluminum oxide, silica, silicate, barium sulfate, wollastonite, kaolin, and carbon black.
Talc, carbon black and zinc oxide are preferred. Of these, talc (mainly 3MgO.4SiO ₂ .nH ₂ O) having a particularly excellent crystallization promoting effect is provided. The average particle diameter of the inorganic particles is preferably 0.1 ~ 20μ, especially 0.5 ~ 10μ
Are preferred. Particles having a small particle diameter, that is, particles having a particle diameter of less than 0.5 μm, and particularly those having a particle diameter of less than 0.1 μm, are liable to cause secondary agglomeration when blended with polyethylene terephthalate as a matrix, and not only impair the effect as a nucleating agent, but also form with aggregates It has the drawback of deteriorating the appearance and quality of the product.

On the other hand, particles having a particle diameter of more than 10 μm, especially those having a particle diameter of more than 20 μm are not suitable because of a low crystallization promoting effect. The compounding amount is required to be 0.1 to 10% by weight, preferably 0.3 to 7% by weight, particularly preferably 0.5 to 5% by weight. If the compounding amount is less than 0.1%, the heat treatment temperature for crystallization must be increased or the treatment time must be increased without changing the crystallization rate of the undeformed polyethylene terephthalate, making it difficult to remove from the mold. Productivity decreases and is unsuitable.
On the other hand, when the blending amount is increased, the ultimate crystallinity during thermoforming tends to increase and the impact resistance tends to decrease. In addition, when the amount exceeds a certain amount, a large improvement in the crystallization rate is not observed. Further, the moldability at the time of thermoforming deteriorates and the range of molding conditions becomes narrow, so that the compounding amount is preferably suppressed to less than 10% by weight.

The polyethylene terephthalate used in the layer B has an intrinsic viscosity of 0.7 to 1.1, preferably 0.85, measured at 20 ° C. in a phenol / tetrachloroethane mixed solvent having a weight ratio of 60/40.
~ 1.05 is required. Polyethylene terephthalate with a low intrinsic viscosity should be at least 0.7
Preferably, 0.85 or more is required. Conversely, the intrinsic viscosity is 1.1
If it exceeds 100, it is advantageous for the impact strength at low temperatures, but the melting temperature when forming a sheet becomes extremely high, and the viscosity is greatly reduced due to this, so that there is no practical advantage.

The polyolefin preferably has a repeating unit derived from a monomer having 2 to 6 carbon atoms, and is preferably a low-density polyethylene, a linear low-density polyethylene,
Examples include high density polyethylene, polypropylene, polybutene, polypentene, polymethylpentene. Among them, polyethylene and polypropylene are preferred. In general, polyolefin has a lower glass transition point than polyethylene terephthalate and has resistance to impact. Therefore, by blending polyolefin,
The impact resistance of polyethylene terephthalate with increased crystallinity can be improved. The compounding amount of the polyolefin is required to be 1 to 10% by weight, and particularly preferably 2 to 8% by weight.

If the content is less than 1% by weight, it is difficult to increase the impact strength of polyethylene terephthalate having increased crystallinity. On the other hand, if the content exceeds 10% by weight, the heat resistance is greatly reduced, and the
Cannot be used sufficiently. Further, in the present invention, the layer B may contain a small amount of inorganic particles such as TiOS ₂ silica, mica, etc., which do not greatly reduce the impact strength. Normally, the scrap after thermoforming, regardless of the number of layers, is pulverized and dried for recycling. Even if a small amount of the inorganic particles contained in the A layer is mixed into the B layer, the impact strength is hardly reduced.

By the way, the crystallinity of thermoformed products is kept frozen,
It is not preferable to increase the impact resistance unnecessarily so as not to reduce the impact resistance during transportation. Generally, the crystallinity of the product is 15
About 30% is considered appropriate. The crystallinity of the product is 15
If it is less than 30%, sufficient dimensional stability cannot be obtained during removal of the molded product during thermoforming, while if the crystallinity of the product exceeds 30%, the impact strength, particularly the impact strength at low temperatures, decreases.

When a thermoformed polyester container is produced from a normal single-layer sheet, those containing an inorganic nucleating agent such as talc tend to have a high degree of crystallinity of the product and have a reduced impact resistance. On the other hand, those containing only a polyolefin-based nucleating agent cannot be said to have a sufficient crystallization rate and are not suitable for high-speed thermoforming.

However, layer A in which inorganic particles (talc) or the like as a crystal nucleating agent is blended with easily crystallizable polyethylene terephthalate having a low intrinsic viscosity and polyethylene terephthalate having a high intrinsic viscosity which is advantageous in impact resistance and polyolefin which is an impact modifier It was found that when the sheet in which the layer B containing the compound was laminated was thermoformed while the layer A was in contact with a mold, excellent high cycle properties and low-temperature impact resistance could be simultaneously achieved. That is, by thermoforming two layers of sheets having different crystallization rates, the degree of crystallinity of the molded article can be freely controlled. The A layer has an extremely high crystallization rate, and the degree of crystallinity is increased by short-time thermoforming. Therefore, poor releasability due to insufficient degree of crystallinity does not occur even in high cycle molding. On the other hand, the B layer has a slightly lower crystallization rate than the A layer, so that the reached crystallinity during thermoforming is suppressed to a low level, and further contains a polyolefin as an impact modifier, so that the impact strength at a low temperature is reduced. Very good. When performing thermoforming from a sheet in which two layers having such characteristics are laminated, the crystallinity of the layer A is 25 to 35% and the crystallinity of the layer B is
Preferably it is 15 to 25%. The crystallinity of each of the A layer and the B layer can be freely controlled by the composition of each layer, the mold temperature during thermoforming, and the molding time. In general, it is possible to control the ultimate crystallinity to be low by blending a polyolefin, particularly preferably polyethylene and / or polypropylene.

In the present invention, the ratio of the thickness occupied by the layer A of the laminated sheet is preferably 30% or less, particularly preferably 10% or less. When the ratio of the layer A is larger than 30%, the effect of improving the impact by the layer B is reduced.

The present invention has an excellent high cycle property and impact strength, but when a product thermoformed at about 140 to 170 ° C. is actually cooked at 200 ° C. or more in a microwave oven / oven, the crystallinity is further increased. Needless to say, it has sufficient heat resistance since it rises to about 40 to 55%.

<Effects of the Invention> The container of the present invention has an improved low-temperature impact strength due to a short thermoforming cycle due to a high crystallization rate and a moderately suppressed crystallinity and excellent heat resistance due to an increased crystallinity when used at a high temperature. It is very suitable for cooking frozen food containers called TV dinners.

<Examples> Examples 1 to 5 and Comparative Examples 1 to 8 A polyethylene terephthalate having an intrinsic viscosity of 0.6 to 1.2 measured in a phenol / tetrachloroethane mixed solvent having a weight ratio of 60/40 at 20 ° C to a water content of 0.01% or less. After drying,
The layer A containing the inorganic particles (talc) and the layer B containing the polyolefin (linear low-density polyethylene) at the ratios shown in Table 1 below were mixed with a layer A occupying the entire thickness of the sheet by a co-extruder. Produce a sheet with a ratio of 10%,
The sheet is extruded directly onto a chilled casting roll and quenched. Next, the sheet was preheated to a soft state, and subjected to thermoforming at a molding cycle shown in Table 1 and a mold temperature of 165 ° C. by a thermoforming machine equipped with a female mold.
A container having a size of 57 mm × width 110 mm × depth 34 mm was prepared, and the following items were evaluated.

1. Impact resistance: Put 200g of water in a container and seal, at -30 ℃
After standing for 24 hours, the container was dropped on a concrete floor surface with the bottom of the container facing down, and the height at the time of 50% destruction was measured. X: less than 60 cm, Δ: 60 to 80 cm, and ○: 80 cm or more.

2. Releasability: Evaluate the shape of the molded product when it is released from the mold. The case where the molded body could be taken out smoothly in the shape according to the mold was evaluated as ○, the case where the molded body was taken out in a shape extending from 0 to 2 mm was rated as Δ, and the case where the molded body was taken out in a shape extending 2 mm or more was considered as ×.

3. Heat resistance: Leave the molded product in a 220 ° C oven for 60 minutes. ○: No deformation at all, ○: Bubble on the side of the container or dent on the bottom within 2 mm, ×: 2 mm or more And However, only a molded article having a good appearance was evaluated.

 Table 1 shows the obtained results.

When the intrinsic viscosity of polyethylene terephthalate is 0.65 or less, crystallization proceeds rapidly and the releasability is excellent. However, even if a polyolefin as an impact modifier is blended, the impact strength is remarkably inferior, which is not preferable. On the other hand, the intrinsic viscosity is 1.
In the case of 1 or more, since the progress of crystallization is slow, the releasability from the mold is deteriorated. In the present invention, it is understood that the intrinsic viscosity of polyethylene terephthalate is preferably 0.7 to 1.0. In general, polyethylene terephthalate has a low crystallization rate, but the one containing a nucleating agent such as polyolefin or inorganic particles promotes crystallization and can be thermoformed in a cycle of 10 seconds or less. However, those containing a polyolefin without inorganic particles as a nucleating agent are excellent in impact resistance, but have insufficient crystallization speed, and are released from the mold when the molding cycle is set to 6 seconds or less. Cause a defect.

Generally, by increasing the amount of the polyolefin, the mold release during thermoforming can be improved, but on the other hand, the heat resistance tends to decrease and those containing 10% or more of the polyolefin are:
Not suitable for use above 220 ° C. Also, relatively low viscosity P
By blending polyolefin with ET, the crystallization rate is improved and mold releasability is improved, but the impact improvement effect of polyolefin is significantly reduced.

On the other hand, when inorganic particles such as talc are blended as a nucleating agent, crystallization progresses rapidly, and although excellent in releasability, impact resistance is poor, and even when polyolefin as an impact modifier is used in combination, the tendency is does not change.

It is understood that the average particle diameter of the inorganic particles is preferably small, but is not preferable to be 0.1 μm or less, at which secondary aggregation occurs in PET as a matrix.

Examples 6 and 7 and Comparative Examples 9 and 10 A layer containing 0.3% of talc (average particle diameter of 3 to 5 μm) in polyethylene terephthalate having an intrinsic viscosity of 0.85 and low density polyethylene (melt index of 1.0) in polyethylene terephthalate having an intrinsic viscosity of 0.95 Was prepared by using a co-extruder to form a sheet having the ratio of the A layer and the B layer as shown in Table 2, and thermoforming was performed in the same manner as in the above example. Evaluation was carried out in the same manner as in the above-mentioned Example, and the obtained results are shown in Table 2 below.

The co-extruded sheet of Example 6 and 7 in which the proportion of the layer occupied by the layer A is 30% or less shows that the tray produced has excellent molding and releasability due to the high crystallization rate of the layer A and the layer B. It had excellent impact resistance characteristics.

In the case of Comparative Example 9, only the layer B had a low crystallization rate, so that when the molding cycle was set to 5 seconds, the moldability was not good. On the other hand, when the ratio of the layer A in Comparative Example 10 exceeds 30%, the impact strength is low and is not suitable.

Continuation of the front page (56) References JP-A-59-14948 (JP, A) JP-A-59-62149 (JP, A) JP-A-2-127374 (JP, A) JP-A 1-272659 (JP) , A) JP-B-62-1894 (JP, B2) JP-B-63-56066 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) B29C 51/00-51/46 B32B 27/36 B65D 1/00

Claims (1)

(57) [Claims] 1. A repeating unit derived from polyethylene terephthalate (A layer) having an intrinsic viscosity of 0.6 to 0.95 containing 0.1 to 10% by weight of inorganic particles having an average particle diameter of 0.1 to 20 μm and a monomer containing 2 to 6 carbon atoms. Viscosity containing 1 to 10% by weight of a polyolefin having olefin and 0 to 3% by weight of a nucleating agent
A polyester container obtained by laminating 0.7 to 1.1 polyethylene terephthalate (layer B) to form a sheet, and then thermoforming the layer A by contacting the layer A side with a mold.