JPS61254326A - Manufacture of polyester molded part - Google Patents

Abstract

PURPOSE:To enable to easily manufacture a polyester formed part with excellent transparency and strong mechanical strength by a method wherein a substantially un-oriented polyester sheet is firstly biaxially oriented and secondly shrunk thermally and thirdly thermoformed and finally and successively heat-treated at a temperature lower than its melting point. CONSTITUTION:Firstly, an un-oriented polyester sheet is oriented respectively in two axial directions intersecting each other at right angles. Secondly, the polyester sheet is thermally shrunk in two axial directions same as the ones, in which the sheet has been oriented at the stretching process. At this time, the final draw ratio of the sheet is preferably chosen within the range of 100-150% of the length before orientation of the polyester sheet. Thirdly and successively, the polyester sheet is formed into the desired shape through forming. Finally, the formed part is heat-treated at a temperature lower than the melting point of the raw material resin. The heat temperature of the formed part causes to crystallize the polyester. Consequently, the heat transistance of the formed part can be improved and at the same time the transparency and mechanical strength of the formed part can also be maintained.

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial Field of Application" The present invention relates to a method for producing polyester molded articles having excellent heat resistance and transparency.

``Prior art'' Sheets made of polyethylene terephthalate or polyester containing polyethylene terephthalate as a main component have excellent thermoformability, transparency, sparability, mechanical strength, etc., and can be used as a sheet or after secondary molding. It is widely used in packaging containers such as cups and trays after being molded by the process. (7) However, since polyester has a low secondary transition temperature (approximately 70°C in the case of polyethylene terephthalate), molded products obtained by secondary molding from polyester sheets have poor morphological stability at high temperatures. However, there has been a drawback that use at high temperatures must be avoided.Various attempts have been made to overcome this drawback, but the results have not always been satisfactory.

For example, in Japanese Patent Publication No. 44-5108, a polyethylene terephthalate film is heated to 85 to 200°C and then molded, and then the molded product is heat-treated in a mold at a temperature of 140 to 220°C to crystallize the polyethylene terephthalate. A method is disclosed in which the heat resistance of the resulting molded product is maintained close to the melting point of the raw material polyethylene Tele 7 grade. However, when using the method described here,
Since the polyethylene terephthalate crystals become spherulites, the molded product obtained becomes cloudy and opaque. Furthermore, this method has the disadvantage that the resulting molded product has very low mechanical strength because the crystals are polycrystalline spherulites.

Also, JP-A-54-43971, JP-A-54-439
No. 72, JP-A-55-146617, JP-A-55-1
Publications such as No. 7516 disclose a method for preventing clouding of polyester molded products by biaxially stretching and orienting a polyester sheet, then thermoforming, and heat-treating the resulting molded product. . In the methods described in these publications, if there is little orientation of the sheet due to biaxial stretching, it is necessary to use secondary forming methods such as vacuum forming, pressure forming, and plug assist forming to form a molded product at relatively low pressure. However, the resulting molded product will be inferior in terms of heat resistance and transparency. On the other hand, if the orientation of the sheet is increased by biaxial stretching, high pressure must be applied when manufacturing the molded product by the secondary molding method, and conventional thermoforming equipment must be used as is. It is not possible. Therefore, it is necessary to newly prepare a special molding device that can withstand high pressure, and it must be said that the industrial value is extremely low.

Further, Japanese Patent Publication No. 56-7855 discloses a method for thermoforming heat-resistant and transparent polyester molded products at relatively low pressure.

The method described here involves stretching an unstretched polyester sheet uniaxially by 1.5 to 3.0 times in a temperature range above the positive transition point of the raw material resin and below 100°C, and then stretching the raw material resin by 1.5 to 3.0 times. This is a method for producing a polyester molded product, which is characterized in that the polyester molded product is heat-shrinked at a temperature above the lath transition point and at a shrinkage restriction rate of 50% or less, and then a desired molded product is obtained by a thermoforming method. A polyester sheet that has been stretched and heat-shrinked under the above conditions based on this method has relatively good moldability, and its transparency is not impaired. However, when heat-shrinking a uniaxially stretched polyester sheet, it is difficult to shrink uniformly within the sheet surface.
The disadvantage is that the shrinkage rate varies depending on the location.

To explain this phenomenon in more detail,
The polyester sheet obtained by carrying out one pongee stretching and then heat shrinking according to the method described in Publication No. 5 has shrinkage I,
There is a mixture of areas where the stretching has not finished and is oriented, and amorphous areas where shrinkage is almost complete and almost no alignment remains.

When such a polyester sheet is thermoformed using a female mold under constant temperature and constant pressure, the resulting molded product is unlikely to accurately reproduce the shape and size of the female mold. Therefore, even if such a molded product is heat-treated using the same female mold, the heat treatment may be insufficient in areas that are not in close contact with the female mold, resulting in parts that are heat-treated to different degrees within the same molded product. becomes. Therefore, the molded product finally obtained will have variations in heat resistance and transparency among the same molded product, resulting in a low commercial value.

Furthermore, Japanese Patent Publication No. 55-3126 discloses that a polyester sheet is thinned to a maximum of 1/9 of the original thickness by applying tension in the orthogonal direction within the plane of the sheet, and then the sheet is
Retardation at an environmental temperature of 20 to 140℃
A method is disclosed for obtaining a deep drawable polyester sheet by thickening the sheet by up to twice the thickness of the sheet. As stated in the same publication, the polyester sheet obtained according to this method can be deep drawn at temperatures of 120°C or higher, and also exhibits excellent dimensional stability at temperatures up to 120°C. Excellent in

However, in other words, these characteristics mean that in order to subject the polyester sheet to secondary forming such as thermoforming, it is necessary to heat the sheet to a temperature of 120° C. or higher. However, polyester sheets heated to 120°C or higher tend to undergo crystallization, so even though the tensile elongation behavior of polyester sheets is excellent at room temperature as stated in the publication, at temperatures of 120°C or higher, room temperature The elongation is significantly lower than that at . Therefore, in order to deep-draw such a polyester sheet at a temperature of 120° C. or higher, the molding pressure must be extremely high, which has the drawback of severely lacking industrial utility.

"Problems to be Solved by the Invention" The present invention solves the above-mentioned drawbacks that could not be solved with the conventional technology, and the entire molded product has excellent transparency and an extremely high The object of the present invention is to provide a method for easily manufacturing polyester molded products that exhibit excellent heat resistance such as not being deformed up to a temperature range and have strong mechanical strength by applying low pressure.

"Means for Solving the Problem" The gist of the present invention is to stretch a substantially unoriented polyester sheet containing ethylene terephthalate as a main repeating unit in two axial directions perpendicular to each other within the sheet plane. After stretching, the polyester sheet is stretched by 1 of the length before stretching in the two axial directions perpendicular to each other.
．． Each polyester sheet is heat-shrinked to a length of 0 times or more, and then this polyester sheet is thermoformed into a desired shape, and the polyester molded product obtained by this thermoforming is
The method of manufacturing a polyester molded article is characterized in that heat treatment is performed at a temperature lower than the melt temperature of the raw material resin of the polyester sheet.

Hereinafter, the content of the present invention will be explained in more detail. In the present invention, polyester means not only a homopolymer of polyethylene terephthalate but also a substantially linear copolyester containing 65 mol% or more, more preferably 90 mol% or more of ethylene terephthalate units. . Components constituting this copolyester include, for example, isophthalic acid, naphthalene-2,6-dicarboxylic acid, diphenyldicarboxylic acid, diphenoxyethanedicarboxylic acid, diphenyl etherdicarboxylic acid, diphenylsulfonedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid. , aromatic, cycloaliphatic and aliphatic difunctional carboxylic acids such as adipic acid, sepacic acid, azelaic acid, p-β-hydroxyethoxybenzoic acid, ε-oxycaproic acid, trimethylene glycol, tetramethylene glycol, Neopentyl glycol, hexamethylene glycol, decamethylene glycol, diethylene glycol, triethylene glycol, 1,1-cyclohexanetumethylol, 1,4-cyclohexanedimethylol, 2,2-bis(4゛-β-hydroxyethquinphenyl) Glycols such as propane, bi-X (4'-β-hydroxyethoxyphenyl) sulfonic acid, and the like can be mentioned. It may contain one or more of the compounds listed here.

The polyester may contain a small amount of a trifunctional or higher polyfunctional compound as a copolymer component as long as it is substantially linear.

The polyester preferably has an intrinsic viscosity (value measured at 35° C. using orthochlorophenol as a solvent) of 0.2 or more, preferably 0.5 or more, and more preferably 0.6 or more.

Further, additives such as a nucleating agent (preferably fine particles), a lubricant, an ultraviolet absorber, a heat stabilizer, an antioxidant, a coloring agent, an antistatic agent, etc. may be added to the polyester. Furthermore, reinforcing agents such as polymethylpentene, polycarbonate, and polymethyl methacrylate may be added depending on the case. 1. The non-oriented polyester sheet is obtained by melting raw polyester using an extruder, extruding it into a sheet, and then immediately cooling and solidifying it with a cooling roll.The temperature of the cooling roll is below the glass transition temperature of the polyester. Preferably, the temperature is 65° C. or lower, which is suitable for imparting transparency to the sheet.

The polyester sheet used in the present invention refers to a sheet having a thickness suitable for producing a molded article by subjecting it to secondary molding such as thermoforming, and is usually selected to have a thickness of 0.1 m or more.

The thickness of the sheet is appropriately selected depending on the purpose, size, shape, etc. of the molded product manufactured by secondary forming such as thermoforming.

When using the method of the present invention, an unstretched polyester sheet that has been cooled and solidified with a cooling roll is first stretched at a stretching temperature (hereinafter referred to as Td).
), and then stretched in two axial directions perpendicular to each other within the sheet plane. The specific method of the stretching process is: (1) longitudinal stretching in the direction along the flow direction of the sheet, followed by transverse stretching in a direction perpendicular to the longitudinal stretching using a tenter; Any method may be used, such as a method in which the order of stretching is reversed, (1) a method in which longitudinal and transverse stretching is carried out simultaneously, (2) a method in which longitudinal stretching and transverse stretching are divided into several times and sequential stretching is carried out. Other stretching methods that can be used include a tube method, a rolling method, and a hydrostatic extrusion method.

The temperature Td at which the polyester sheet is stretched has a lower limit of the lath transition temperature (hereinafter referred to as Tg) of the polyester, and Td
It is preferable to select the temperature within a temperature range whose upper limit is 30° C. higher than g. If Td is lower than Tg, the force required to stretch the sheet becomes very large, making it difficult to stretch the sheet, which is not preferable.

Furthermore, if Tcl is too high, crystallization will occur in the polyester sheet during the stretching process, which will impede the next heat shrinking process, so it is preferably below the upper limit temperature (Tg+30)'C.

The upper limit temperature (Tg+30)° C. is 100° C. in the case of polyethylene tere-7-talate homopolymer.

The magnification when stretching the polyester sheet in the stretching step according to the method of the present invention is preferably selected within the range of 1.5 to 3.0 times in each of the two axial directions orthogonal to each other within the plane of the sheet. When the stretching ratio is less than 1.5 times,
Since the orientation effect on the sheet is small, the transparency of the sheet is impaired and the sheet becomes cloudy during the thermoforming process, which is not preferable. Moreover, if the stretching ratio exceeds 3.0 times, the polyester sheet will not shrink to a predetermined length in the next heat shrinking step, and the orientation will remain in the sheet, making the thermoforming step difficult, which is not preferable.

When using the method of the present invention, the polyester sheet is stretched in two directions perpendicular to each other within the sheet plane, and then the polyester sheet is heat-shrinked in the same two directions as in the stretching step. The sheet temperature when heat-shrinking is
The Td of the polyester sheet is the lower limit, and it is 20°C above the Td.
It is preferable to select a temperature range having an upper limit of a high temperature (Td+20)°C. If the sheet temperature is lower than Td, thermal contraction will not occur quickly, which is not preferable. The higher the sheet temperature, the faster the heat shrinkage will occur, but if the upper limit temperature (Td+20)°C is exceeded, crystallization will occur in the polyester sheet at the same time as the heat shrinkage, reducing the elongation of the sheet and causing problems during thermoforming. This is not preferable since the molding pressure must be extremely high.

The final stretching ratio of the polyester sheet after heat shrinking is 1.0 to 1.5 of the length of the polyester sheet before stretching in the same two axial directions as the stretching and heat shrinking.
It is preferable to choose within a double range.

In the present invention, the method for heat-shrinking a polyester sheet is to heat-shrink a biaxially stretched sheet in hot air and C//*
Alternatively, the polyester sheet may be heat-shrinked by passing it through the opening of two heated rolls and adjusting the rotational speed of the rolls. The prepared polyester sheet is stored in dry air if necessary, and sealed and packaged if necessary.

When using the method of the present invention, the polyester sheet that has been stretched and heat-shrinked according to the above method is then heated to a temperature selected within a temperature range between the glass transition point of the raw material resin as the lower limit and the melting point of the raw material resin as the upper limit. It is then molded into a desired shape by a molding method. Examples of the secondary forming method include vacuum forming, pressure forming, plug assist forming, and match molding. Air pressure molding is suitable for obtaining molded products that are precisely molded according to the shape of the mold.

When using the method of the present invention, after the molded product is molded into a desired shape by a secondary molding method, the molded product is heat-treated by heating it to a temperature below the melting point of the raw resin.

The heat treatment temperature of the molded product is appropriately selected depending on the composition of the raw polyester, but in the case of a molded product made of polyethylene tere-7-thalerate homopolymer, it is preferably 120°C or higher, preferably in the range of 150 to 250°C, and Preferably, the range of 180 to 240°C is suitable because the heat treatment time can be shortened. For heat treatment of molded products, the mold used for secondary molding may be used as is, or in order to speed up the production of molded products (shorten the molding cycle), a mold that is not heated during molding may be used. The molded product may be transferred to another heated and temperature-controlled mold for heat treatment. This geothermal treatment may be carried out in hot air or hot water, and infrared rays or other heating methods may also be used.

Crystallization of the polyester is induced by heat treating the molded article. This crystallization is different from the crystallization that occurs in unstretched or non-oriented polyester sheets, and since the formation of spherulites is inhibited, whitening does not occur, and the transparency of the molded product can be maintained.

The crystallization that occurs during this heat treatment process improves the heat resistance of the molded article, and the transparency and mechanical strength of the molded article can be maintained up to near the crystalline melting point of polyester.

"Effects of the Invention" The polyester molded article obtained by the method of the present invention exhibits particularly remarkable effects as described below, and its industrial utility value is extremely large.

(1) The polyester molded product obtained by the method of the present invention has the contradictory properties of heat resistance and transparency at high temperatures, which could not be obtained with conventional polyester molded products, and is transparent even at extremely high temperatures. can keep,
Moreover, mechanical strength is not impaired. Therefore, it can also be used as a container for heating or cooking in hot air ovens, microwave ovens, etc., which was not possible with conventional polyester molded products.

(2) The polyester molded article according to the method of the present invention can be molded at the low pressure normally used in the past when secondary molding is performed from the polyester sheet according to the present invention, so a conventional thermoforming machine can be used. can be used.

``Example'' The present invention will now be described in more detail based on examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof.

Note that the evaluation of the characteristic values of the sheets and molded products listed in the following Examples and Comparative Examples was performed by the method described below.

(1) Set value and actual measurement value of final stretch ratio The set value of final stretch ratio is calculated by the following formula.

Setting value of final stretching ratio = (interval of clips after shrinkage)/(interval of clips before stretching) The actual value of the final stretching ratio is calculated according to the following.

1011I perpendicular to the stretching direction on the sheet before stretching.
Mark lines at I11 intervals. After heat shrinking, measure the distance between the lines.

Actual value of final stretching ratio = (line spacing after heat shrinkage/10mm) x 100(2)
Moldability This is to evaluate whether the molded product is accurately deformed along the shape of the molding die. Molding mold (100ff1m
Measure the radius of curvature of part of the corner of the molded product compared to the radius of curvature of part of the corner (0,5+++a+) of the female mold for molding a rectangular dish-shaped molded product of how to. The evaluation criteria were as follows.

A: Radius of partial curvature of molded product corner = O9EzoIll
B: Radius of curvature of part of molded product corner = 0.5 am or more 0
．． Less than 6!11111 C: Radius of partial curvature of molded product corner = 0.6 ma + or more D: Molded product is damaged or the straight part of the molding die cannot be reproduced in the molded product (3) Heat resistant A method in which a molded product is left in an air oven at 220°C for 10 minutes, then taken out and the deformation status observed with the naked eye. The evaluation criteria were as follows.

A: The shape of the molded product is the same as before it was left in the air oven. B: There is some deformation. (4) Transparency Test was conducted at the same time as the heat resistance test, and after the molded product was left in the air oven at 220°C for 10 minutes. , a method of taking out the molded product and observing the transparency of the molded product with the naked eye. The evaluation criteria were as follows.

A: Molded products whose transparency does not change B: Products whose transparency has decreased Examples 1 to 4 Polyethylene terephthalate (intrinsic viscosity 0.7, DSC
(differential heating 1 meter) at a heating rate of 16°C/Tg measured with ain = 70°C) was melted in an extruder, molded by extrusion method, and formed into a material with a thickness of 0.3 to 1.51111 m. An unoriented, amorphous, unstretched sheet was obtained.

This sheet was biaxially stretched in two perpendicular directions for 1.5 to 3.0 times each at a temperature of 75 to 100° C. using a simultaneous biaxial stretching machine that stretches the sheet while holding the edges with clips.

Next, this stretched sheet is left in hot air at a temperature higher than the stretching temperature and lower than 120°C for 1 minute to heat shrink the polyethylene terephthalate sheet in biaxial directions, and the final stretching ratio of the sheet is 1.0 to 1.5 times (details). (see Table 1). The shrinkage rate was controlled by adjusting the clip spacing.

The polyethylene terephthalate sheet thus obtained was heated to 80 to 100°C in an oven, and a female mold with a surface temperature of 180 to 250°C was heated to 3 kg/cm.
A pressure of 2 was applied to leave the dish-shaped molded product. Thereafter, the molded product was heat-treated for 1 minute while the female mold was held as it was.

The obtained molded products were subjected to various evaluation tests. The results are shown in Table 1.

Comparative Examples 1 to 4 An unstretched sheet of the same type of polyethylene terephthalate as used in the above examples was stretched once in the longitudinal direction of the sheet at a temperature of 80 to 85°C by the same stretching machine used on the same side. The pongee was stretched and then heat-shrinked in the same direction as the uniaxial stretching under the same conditions as in the above examples. (
For details, refer to ti'IJ1 table). The obtained sheet was subjected to pressure forming under the same conditions as in the above example to produce a dish fl-d shape, heat treated in the same manner as in the above example, and various evaluations were performed. Ta. The evaluation results are shown in Table 1.

Examples 5 to 17 An unstretched polyethylene terephthalate sheet of the same type as that used in Example 1 was stretched once in two orthogonal axes directions at a temperature of 75 to 85°C using the same stretching machine used on the same side. The pongee was stretched 2 times at .5~3°0. Next, this stretched sheet was left in hot air at a temperature higher than the stretching temperature and lower than 110°C for 1 minute to heat shrink the polyethylene terephthalate sheet in biaxial directions, and the final stretching ratio of the sheet was 1°0.
~1.5 times (see Table 2 for details). The shrinkage rate was controlled in the same manner as in Example 1.

A molded article was produced from the polyethylene terephthalate sheet thus obtained in the same manner as in the previous example, and various evaluations were performed on the obtained molded article. The evaluation results are shown in Table 2.

Comparative Examples 5 to 7 The unstretched sheets of polyethylene terephthalate used in the above examples were biaxially stretched and heat-shrinked in the same manner as in the above examples. At this time, the stretching ratio was set to 3°0 times or more, or the final stretching ratio was set to 1.5 times or more. A plate-shaped molded product was produced from the obtained sheet by air pressure forming under the same conditions as in the above example, and heat treated in the same manner as in the above example.

Details of the sheet manufacturing conditions and various characteristic evaluation results of the molded product are shown in Table 2.

From Tables 1 and 2, it becomes clear about dogs.

■The polyester molded product obtained by the method of the present invention is
It exhibits excellent properties in terms of moldability, heat resistance, and transparency. Even when the molded product is exposed to a high temperature of 220° C., there is no change in its heat resistance or transparency, which is a surprising characteristic not found in American polyester molded products.

(2) When a polyester sheet is biaxially stretched and further heat-shrinked in two axial directions based on the method of the present invention, the heat shrinkage rate hardly changes within the sheet plane, and a homogeneous polyester sheet can be obtained.

■On the other hand, when the stretching direction and heat shrinkage direction of the polyester sheet are set to only one axis as shown in the comparative example, the actual heat shrinkage of the sheet is within the sheet plane relative to the preset final stretch ratio. It is inevitable that a homogeneous polyester sheet will be obtained due to non-uniformity and variations in the actual final stretching ratio.

Claims (5)

[Claims] (1) After stretching a substantially non-oriented polyester sheet containing ethylene terephthalate as a main repeating unit in two axial directions perpendicular to each other within the sheet plane,
The polyester sheet is then heat-shrinked in each of the two axial directions to a length that is at least 1.0 times the length before stretching, and then this polyester sheet is thermoformed into a desired shape, and the polyester molded product obtained by this thermoforming is then carried out. A method for producing a polyester molded article, characterized in that the polyester sheet is heat-treated at a temperature below the melting point of the raw material resin of the polyester sheet. (2) The temperature (Td) of the polyester sheet when stretching the polyester sheet is set from Tg to (Tg+30)°C (
However, Tg means the glass transition temperature of the raw material resin of the polyester sheet. ) The method for producing a polyester molded article according to claim (1), characterized in that the temperature is selected within the temperature range of (1). (3) The stretching ratio when stretching the polyester sheet,
The method for manufacturing a polyester molded article according to claim (1), characterized in that each axial direction is selected within a range of 1.5 to 3.0 times. (4) The temperature of the polyester sheet when heat-shrinking the polyester sheet in each axial direction is Td~(Td+20)℃
A method for producing a polyester molded article according to claim 1, wherein the temperature is selected within the temperature range of . (5) Select the final stretching ratio of the polyester sheet after heat shrinking the polyester sheet in each axial direction within the range of 1.0 to 1.5 times the length before stretching in each axial direction. A method for producing a polyester molded article according to claim (1), characterized in that: