JP2005330299A - Styrenic resin biaxially stretched sheet and thermoformed product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prepare a styrenic resin heat-resistant sheet imparting heat resistance to a styrenic resin sheet and having improved thermoformability and strength and to provide a thermoformed product. <P>SOLUTION: The styrenic resin biaxially stretched sheet comprises a styrenic resin which is a copolymer of a styrenic monomer and an unsaturated carboxylic acid monomer and is characterized as follows. The sheet thickness ät(mm)} is 0.16-0.5 mm. The relationship between the sheet thickness ät(mm)} and the Vicat softening point äVSP(°C)} of the resin composing the sheet satisfies the following (A) and the relationship between the sheet thickness ät(mm)} and the orientation relaxation stress äORS(MPa)} in at least one stretch direction satisfies the following (B). (A) VSP(°C)≤-10×t(mm)+130 and VSP(°C)≥-10×t(mm)+117. (B) ORS(MPa)≤-0.3×Logeät(mm)/0.3}+0.95 and ORS(MPa)≥0.3×Logeä0.3/t(mm)}+0.5. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a colorless and transparent styrene resin stretched sheet, which is thermoformed mainly by means of vacuum forming, compressed air vacuum forming, hot plate pressure forming, etc. and used for lightweight food packaging containers and other various containers, and the sheet. The present invention relates to a thermoformed molded product. More specifically, the present invention relates to a styrenic resin biaxially stretched sheet for molding having heat resistance and strength required as a container lid and a molded product thereof.

Polystyrene biaxially stretched sheets take advantage of excellent properties such as transparency, gloss and rigidity (elastic modulus), and are often used as various molded containers for food packaging applications. In recent years, it has been used in many cases as a lid for lunch box combs, hors d'oeuvres, etc. taking advantage of its transparency. Among them, the number of deep-drawn lids and large lids increases, and the amount of relatively thick sheets having a thickness of 0.2 mm or more tends to increase.

On the other hand, since a polystyrene biaxially stretched sheet is inferior in heat resistance, it may be deformed when a container using a molded article made of a polystyrene biaxially stretched sheet as a container body or a lid is heated in a microwave oven.
Therefore, a heat-resistant biaxially stretched sheet using heat-resistant styrene resin properties such as a styrene-methacrylic acid copolymer and a styrene-maleic anhydride copolymer is provided for the purpose of improving the heat resistance of the polystyrene biaxially stretched sheet. It has been proposed (for example, Patent Documents 1 to 5).

However, when the stretched sheets described in Patent Documents 1 to 5 are thermoformed, the intended performance may not be obtained depending on the sheet thickness. That is, in the case of a relatively thin sheet, in a molded container, if the heat conductivity is relatively fast, sufficient heat resistance cannot be obtained, or the strength of the molded product is inferior because it is deoriented by heating during thermoforming. There was a case. For relatively thick sheets, stretching orientation may interfere with thermoforming, resulting in deterioration of mold reproducibility of molded products. There was a case where the appearance of was damaged. For this reason, in particular, a thick sheet deep-drawn molded product or a large molded product used as a lid for a molded product may be inferior in quality.
Japanese Examined Patent Publication No. 3-67608 JP 2000-309645 A JP 2002-36353 A JP 2002-225127 A JP 2003-12734 A

  The present invention has been made for the purpose of supplying a styrenic resin heat-resistant sheet and a molded product having heat resistance imparted to the styrene-based resin sheet and improved thermoformability and strength. More specifically, the object is to supply a styrenic resin heat-resistant sheet that gives a container lid material that is particularly suitable for container lid molding and heat resistance and excellent strength, and a molded product that is thermoformed from the sheet. It was made.

As a result of intensive studies in order to solve the above problems, the present inventors have found that the sheet thickness, the Vicat softening point (hereinafter abbreviated as VSP) of a specific styrene-based heat-resistant resin, and the orientation relaxation stress (hereinafter abbreviated as ORS). It was found that the purpose can be achieved by setting the orientation degree of the styrene-based heat-resistant resin sheet represented to a specific relationship, and the present invention has been achieved based on this finding.
That is, the present invention

(1) In a biaxially stretched sheet made of a styrene resin, the styrene resin is a copolymer of a styrene monomer and an unsaturated carboxylic acid monomer, and the sheet thickness (t (mm)) is 0.16 mm to 0.00. The relationship between the sheet thickness (t (mm)) and the Vicat softening point (VSP (° C.)) of the resin constituting the sheet is A), and the sheet thickness (t (mm)) and at least one stretching direction The styrene resin biaxially stretched sheet is characterized in that the relationship with the orientation relaxation stress (ORS (MPa)) is B) below.
A) VSP (° C.) ≦ −10 × t (mm) +130 and VSP (° C.) ≧ −10 × t (mm) +117 is satisfied.
B) ORS (MPa) ≦ −0.3 × Loge (t (mm) /0.3) +0.95 and ORS (MPa) ≧ 0.3 × Loge (0.3 / t (mm) ) +0.5 is satisfied.
(2) The styrenic resin is at least one copolymer selected from styrene and methacrylic acid, styrene and acrylic acid, or a copolymer of styrene and maleic anhydride. Regarding the sheet.
(3) At least one surface of the styrenic resin biaxially stretched sheet described in (1) or (2) above has a hydrophilic / lipophilic ratio (HLB) of 12 to 19, solid at 40 ° C. or viscosity of 150 mPa · s. The present invention relates to a styrenic resin biaxially stretched sheet characterized in that at least one of the above nonionic surfactants is coated with a film thickness of 8 to 50 mg / m ² .
The present invention also relates to (4) a styrene resin biaxially stretched sheet thermoformed article using the styrene resin biaxially stretched sheet according to any one of (1) to (3) above.
Furthermore, the present invention relates to (5) a styrene resin biaxially stretched sheet thermoformed product, wherein the thermoformed product is a container lid.

  The styrene resin heat-resistant sheet of the present invention has excellent thermoformability, strength as a molded container, and heat resistance, and can be suitably used as a thermoformed container, particularly a molded container lid material.

The present invention is described in detail below.
The styrene resin constituting the styrene resin biaxially stretched sheet of the present invention is a copolymer of a styrene monomer and an unsaturated carboxylic acid monomer. Styrene monomer is a monomer having at least one styrene skeleton selected from styrene, alkyl styrenes such as α-methyl styrene and p-methyl styrene, halogenated styrenes, etc. Styrene and / or α-methylstyrene is preferable, and styrene is more preferable.

  The unsaturated carboxylic acid monomer is at least one selected from aliphatic unsaturated carboxylic acids such as acrylic acid and methacrylic acid and esters thereof, acid anhydrides such as maleic anhydride and maleimide, and modified products thereof. It is a kind of unsaturated carboxylic acid derivative monomer. Among these, from the viewpoints of improvement in sheet heat resistance, sheet extrudability, and stretchability, preferably at least one monomer selected from acrylic acid, methacrylic acid, and maleic anhydride, more preferably methacrylic acid and / or Or maleic anhydride, more preferably methacrylic acid.

  In the resin constituting the sheet of the present invention, the copolymer ratio of the styrene monomer and the unsaturated carboxylic acid monomer is not particularly limited as long as it is within the range of VSP of the resin constituting the sheet described later.

  Among these, a particularly preferable styrene-methacrylic acid copolymer has a weight average molecular weight of preferably 100,000 to 500,000, more preferably 150 to 400,000, and still more preferably 200 to 350,000. The weight average molecular weight is a standard polystyrene equivalent value measured by gel permeation chromatography (GPC). This lower limit depends on the strength of the resin, and is a preferable value for maintaining the impact resistance and bending strength of the sheet, as well as preventing the tearing during extrusion and stretching. The upper limit depends on the thermal stability of the resin and the viscosity at the time of melting, and is a preferable value from the viewpoint of prevention of gelation by heating during extrusion and extrusion stability.

  In addition, the relationship between the methacrylic acid monomer copolymer ratio of the copolymer of styrene and methacrylic acid and VSP is that 1% by mass of methacrylic acid is copolymerized, and the VSP of the resin is about 1.5 to 2 ° C. (The rate of increase in VSP per 1% by weight of methacrylic acid varies depending on the polymerization method and the formulation of the additive during polymerization). One example of the relationship between the copolymer monomer composition and VSP that is more preferable than this relationship is a copolymer of styrene and methacrylic acid. When a 0.3 mm stretched sheet is obtained, methacrylic acid is added in an amount of about 4 to about 11 It is possible to make it the range of the target VSP by making the mass% (however, VSP is affected by impurities and additives in the resin in addition to the monomer composition, and even if it is the same VSP, the monomer composition Are not necessarily the same, and are not limited to this composition).

  Moreover, the sheet | seat of this invention may add said 1 type or 2 or more types of other styrenic resin, as long as VSP is the range of this invention. Examples of other styrenic resins include polystyrene, impact polystyrene, styrene-conjugated diene block copolymers, hydrogenated resins of styrene-conjugated diene block copolymers, and modified resins thereof, styrene-alkyl (meth) acrylates. Examples thereof include copolymers and styrene-alkyl (meth) acrylate-conjugated diene copolymers. These resins are preferably less than 30% by mass, more preferably less than 10% by mass, and still more preferably less than 5% by mass. Furthermore, one kind of known additives such as plasticizers such as mineral oil, petroleum resin, terpene resin, styrene dimer, styrene trimer, slipping agents such as higher fatty acid esters and higher fatty acid salts, heat stabilizers, ultraviolet absorbers, etc. You may add more. A preferable addition amount is less than 3% by mass in total.

  The sheet of the present invention is a sheet having a thickness of 0.16 mm to 0.5 mm obtained by biaxially stretching the above resin. By setting the sheet thickness to 0.16 mm or more, the sheet rigidity during heating can be maintained, and the practicality as a thermoformed container can be maintained. In addition, it is theoretically possible to reduce the thickness of the sheet by using a resin with high heat resistance (a resin with a high amount of unsaturated carboxylic acid monomer or a resin with a high molecular weight) giving priority to rigidity. There is a tendency for the stability and stretchability of the time to decrease. Moreover, the outstanding thermoforming can be hold | maintained by making it a sheet | seat of thickness 0.5mm or less. In particular, when a biaxially stretched sheet is thermoformed and used as a lid, the sheet thickness is preferably 0.18 mm to 0.5 mm, more preferably 0.2 to 0.5 mm from the viewpoint of sheet rigidity and thermoforming cycle. The sheet thickness is 0.45 mm, more preferably 0.23 to 0.4 mm.

  In the sheet of the present invention, the relationship between sheet thickness (t (mm)), Vicat softening point (VSP (° C.)) and orientation relaxation stress (ORS (MPa)) is particularly important as a sheet for thermoforming. In the present invention, VSP is a value measured according to ASTM D1525 (weighted 49 N, temperature rising rate 2 ° C./min). The orientation relaxation stress (ORS) is a peak stress value measured in an oil bath 25 ° C. higher than the VSP of the resin in accordance with ASTM D1504.

  One of the features of the present invention is that it has been found that the practical heat resistance of the thermoformed product and the formability of the sheet depend on the relationship between the sheet VSP and the sheet thickness. That is, the upper limit of the VSP of the sheet is to satisfy VSP (° C.) ≦ −10 × t (mm) +130 in relation to the sheet thickness (t (mm)). This makes it possible to maintain the mold reproducibility during sheet thermoforming and the forming speed at a practical level. Further, the lower limit of the VSP of the sheet is to satisfy VSP (° C.) ≧ −10 × t (mm) +117 in relation to the sheet thickness (t (mm)). This makes it possible to maintain the practical heat resistance of the thermoformed product of the sheet.

  In particular, stretched styrenic resin sheets are often thermoformed by hot plate contact thermoforming, but depending on the molding conditions, the hot plate contact of the sheet reduces the appearance of the thermoformed product such as transparency and clearness. Sometimes. Molding under milder conditions is necessary to suppress the decrease in transparency and clearness. For this reason, especially when using the sheet | seat of this invention for the cover material by which transparency is requested | required, the VSP upper limit of a sheet | seat is VSP (degreeC) <=-10 * t (mm) +128 from a viewpoint of improving a moldability more. It is preferable to satisfy VSP (° C.) ≦ −10 × t (mm) +126. Further, when the sheet is thermoformed into a cover material, particularly a large-sized cover material or a deep-drawn cover material, the rigidity when the cover material is used by heating is practically important. From the viewpoint of highly maintaining this rigidity, the lower limit of the VSP of the sheet is preferably VSP (° C.) ≧ −10 × t (mm) +118, and more preferably VSP (° C.) ≧ −10 × t (mm) +120.

  Conventionally, it has been generally considered that the heat resistance of a polystyrene-based sheet is determined by the VSP of a resin constituting the polystyrene sheet. However, the relationship between these sheet thicknesses and VSPs defined in the present invention is that, even when the same resin (same VSP) is used, characteristics such as practical heat resistance and sheet formability of the sheet and its thermoformed product are the sheet thickness. It shows different things. This relationship is one of the features of the present invention based on an empirical formula derived as a result of examining combinations that optimize sheet characteristics.

  Another feature of the present invention is that the practical heat resistance of the thermoformed product and the formability of the sheet depend on the relationship between the orientation relaxation stress (ORS (MPa)) in one stretch direction and the sheet thickness, and It is the point that the effect is exhibited only by the relationship and combination of VSP. That is, the upper limit value of the orientation relaxation stress in at least one stretching direction of the sheet is ORS (MPa) ≦ −0.3 × Loge (t (mm) /0.3) +0 in relation to the sheet thickness (t (mm)). .95. This makes it difficult to generate lens-like irregularities in the molded product during hot plate molding, which is called raindrop, even at relatively high molding temperatures, and makes mold reproducibility and molding speed when forming sheet heat practical. It is possible to maintain and maintain the practical heat resistance of the thermoformed product of the sheet. The lower limit of the ORS of the sheet is that ORS (MPa) ≧ 0.3 × Loge (0.3 / t (mm)) + 0.5 is satisfied in relation to the sheet thickness (t (mm)). This makes it possible to impart impact strength and rigidity to the sheet and its thermoformed product.

  Further, in the case where the thermoformed shape of the sheet is complicated or in the case of a deep-drawn molded product, there are cases where harsh heating conditions such as increasing the heating time at the time of thermoforming or increasing the heating temperature may be employed. Under such conditions, problems such as deterioration of the transparency of the molded product and a decrease in clearness, cracking of the molded product and cracking of the molded product, and a reduction in impact strength are likely to occur. For this reason, the relationship between the ORS and the sheet thickness is ORS (MPa) ≦ −0.3 × Loge from the viewpoint of maintaining the appearance such as crack prevention during sheet forming, transparency of the molded product, and clearness even under severe heating conditions. t (mm) /0.3) +0.93 is preferable, and ORS (MPa) ≦ −0.3 × Loge (t (mm) /0.3) +0.9 is more preferable. Furthermore, the relationship between ORS and sheet thickness is ORS (MPa) ≧ 0.3 × Loge from the viewpoint of further increasing the strength of the sheet and preventing cracking during sheet molding and maintaining the impact strength of the molded product even under severe heating conditions. (0.3 / t (mm)) + 0.53 is preferable, and more preferably ORS (MPa) ≧ 0.3 × Loge (0.3 / t (mm)) + 0.55.

  Depending on the thermoforming conditions and the shape of the thermoformed product, these ORS conditions may satisfy the one-stretch direction of the biaxially stretched sheet to obtain a sheet that gives the desired thermoformed product. It is more preferable that the ORS condition is satisfied in the two stretching directions from the viewpoint of molding under relatively wide thermoforming conditions.

  It has been conventionally proposed that the sheet strength and the formability of the sheet change depending on the sheet ORS. However, the present invention has found that the moldability and practical strength are not uniquely determined by the ORS, but are determined by the relationship between the sheet thickness and the ORS. That is, it has been found that even in the case of the same ORS, characteristics such as sheet formability and practical strength of the sheet and its thermoformed product differ depending on the sheet thickness. The result of studying the relationship between the two in order to optimize the sheet characteristics is a feature of the present invention.

  In the above relational expression of ORS and thickness, the maximum value of ORS is mainly determined by the formability of the sheet, and in the case of the same ORS, the formability is negatively related to the sheet thickness, so −0.3 is a constant. . On the other hand, the minimum value of the ORS is mainly determined by the strength and rigidity of the sheet. In the case of the same ORS, these strengths and the like have a positive relationship with the sheet thickness, so +0.3 is set as a constant.

The biaxially stretched sheet of the present invention has a hydrophilic / lipophilic ratio (hereinafter abbreviated as HLB) of 12 to 19 on at least one surface of the sheet and is solid at 40 ° C. or nonionic having a viscosity of 150 mPa · s or more. It is preferable that at least one surfactant is coated with a film thickness of 8 to 50 mg / m ² . By coating the nonionic surfactant, the oil resistance of the sheet thermoformed product can be improved.

  By selecting a surfactant having an HLB of 12 or more, oil resistance can be imparted, and the surfactant can be coated on the sheet with an aqueous solution. Further, from the viewpoint of application uniformity to the sheet and from the viewpoint of highly imparting oil resistance to various oils, the HLB is more preferably 13 or more, and further preferably 13.5 or more. Further, by selecting a surfactant having an HLB of 19 or less, for example, when a sheet is used as a molded container, erosion of the surfactant by water vapor generated from the contents can be prevented, and oil resistance can be maintained. Furthermore, in the case of a sheet used in a deep drawn container or a large container in which the sheet is greatly stretched by thermoforming, the HLB is more preferably 18 or less, and further preferably 17.5 or less from the viewpoint of maintaining oil resistance. is there.

  The surfactant is preferably a solid at 40 ° C. or a liquid having a viscosity of 150 mPa · s or more. By satisfying this condition, it is possible to improve the oil resistance with the surfactant and maintain the effect. More preferred is a nonionic surfactant which is solid at 40 ° C. or a liquid having a viscosity of 200 mPa · s or more, more preferably solid at 40 ° C. The viscosity is measured with a Brookfield rotary viscometer.

Moreover, the preferable coating amount of 8 to 50 mg / m ² is an amount for obtaining an oil resistance effect and suppressing stickiness, blocking, and whitening by the coating film, more preferably 9 to 40 mg / m ² , still more preferably. 10-35 mg / m ² .
Nonionic surfactants include sucrose fatty acid esters, polyglycerin fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, polyoxyethylene fatty acid esters, polyoxyethylene fatty alcohol ethers, etc. It is preferable to select one that satisfies the above conditions. Particularly preferred are sucrose fatty acid esters and polyglycerin fatty acid esters recognized as food additives. These surfactants may be used alone or in combination of two or more. When mixing 2 or more types, it is preferable to satisfy | fill the said conditions in the state of a mixture.

  In addition to the above surfactants, polyoxyethylene polyoxypropylene block copolymers having a high melting point, polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose and alkali metal salts thereof, methyl hydroxypropyl cellulose, polyacrylic acid and alkali metal salts thereof, polyvinyl hydride When a water-soluble polymer substance such as phosphorus is added, the viscosity of the coating film is increased and the oil resistance effect may be improved, which may be more preferable. These additions are preferably 1 to 100 parts by weight, more preferably 2 to 65 parts by weight, and still more preferably 5 to 50 parts by weight with respect to 100 parts by weight of the surfactant.

Further, when the surfactant is coated, the sheet may be coated on both sides or only on one side, but in the case of single-sided coating, the surface coated with the surfactant becomes the inside of the container when the sheet is thermoformed. It is preferable to coat the surface.
In addition, a sheet coated with the above surfactant on one side and coated with a known silicone oil (silicone emulsion) on the opposite side is particularly preferable because the peelability of the molded product is improved.

  Thus, the sheet of the present invention is a copolymer of a styrene monomer and an unsaturated carboxylic acid monomer of a specific VSP, preferably a copolymer of styrene and methacrylic acid, styrene and acrylic acid or styrene and maleic anhydride. A resin selected from the union is biaxially stretched so as to have a specific orientational relaxation stress, and is more preferably coated with a high viscosity (or solid), high HLB nonionic surfactant. It is a sheet.

There is no restriction | limiting in particular in the manufacturing method of these sheets, What is necessary is just to carry out by the method conventionally used in manufacture of the conventional stretched sheet.
One example is a method of continuously extruding a resin from a T-die, cooling the raw material with a roll, and then continuously and successively biaxially stretching by a roll stretching and a tenter method. Moreover, you may perform simultaneous biaxial stretching by a tenter method without carrying out roll stretching, or batch compression by creating a compression sheet. The stretching ratio of these sheets is 1.5 to 8 times, preferably 2 to 6 times in one direction. The stretching temperature is VSP of the resin constituting the sheet + 5 ° C. to + 50 ° C., preferably +10 to + 40 ° C. The sheet of the present invention can be obtained by adjusting these conditions to the combined ORS. There is no particular limitation on the ORS adjustment method, but it is general to employ a condition of stretching at a low temperature when creating a sheet with a low stretch ratio and stretching at a high temperature when creating a sheet with a high stretch ratio.

  A preferred example of the stretching conditions is that when roll-stretching 2.5 to 3 times and then tenter stretching 2.5 to 3 times to produce a 0.3 mm sheet, VSP +15 to 30 ° C is adopted as the tenter stretching temperature. Is preferred. Further, when the stretching ratio is changed from 3 times to 3.5 times, it is preferable to raise the stretching temperature by 4 to 7 ° C. as a rough guide for obtaining the same ORS sheet.

  In addition, when coating a surfactant, the water solubility of the surfactant adjusted to an appropriate concentration on at least one surface is squeeze roll coater, air knife coater, knife coater, spray coater, gravure roll coater, bar coater, etc. It can carry out by the method of apply | coating and drying by a well-known method without a restriction | limiting in particular. In particular, from the viewpoint of improving the uniformity of the coating film, it is preferable to apply the surfactant by the above method after corona treatment of the sheet surface. The strength of the corona treatment is preferably adjusted so that the contact angle with water is 80 to 30 °, and more preferably the contact angle is 60 to 35 °. The preferable upper limit of the contact angle with water is a value for improving the uniformity of the coating film, and the preferable lower limit of the contact angle is a value for preventing blocking when the sheet is rolled.

  The above-mentioned styrenic resin biaxially stretched sheet can be made into a styrenic resin biaxially stretched sheet thermoformed product by thermoforming. As the thermoforming method, a hot plate contact heat forming method, a vacuum forming method, a vacuum / pressure forming method, a plug assist forming method and the like are preferable. The hot plate contact heating molding method is particularly preferred from the viewpoint of the thickness uniformity of the molded product and the production efficiency of the molded product, but when transparency is particularly important, the vacuum molding method by indirect heating or the vacuum / pneumatic molding method can be used. When performing deep drawing, a plug assist molding method can be employed. The forming method may be performed continuously using a sheet roll, or may be formed every shot using a cut plate sheet. An example of preferable molded product manufacturing conditions is given below.

  Preferred hot plate temperature conditions when the sheet of the present invention is formed by the hot plate contact heating molding method are the mold reproducibility and molding cycle viewpoint (lower limit) of the molded article, transparency of the molded article and viewpoint of occurrence of raindrops. From the (upper limit), the hot plate temperature is VSP + 10 to 50 ° C. of the sheet resin, more preferably VSP + 20 to 40 ° C., and further preferably VSP + 25 to 35 ° C. In addition, the heating time (the time for which the sheet is brought into contact with the hot plate in vacuum and / or pressure air and the total delay time until the vacuum and / or pressure air enters to complete the sheet and extend the sheet to the mold) is 0. .5 to 15 seconds is preferable.

Further, it is more preferable to determine the heating time from the sheet thickness (t (mm)) and the ORS of the sheet. Specifically, the value of ORS (MPa) was added to 0.8 times the square root of (t (mm)). from the value (0.8t ^1/2 + ORS), by the time that the 15 times the square root of the sheet thickness (t (mm)) to ORS value the value was added ^{(MPa) (15t 1/2 + ORS} ) More preferably, the molding time is from (1.5 t ^1/2 + ORS) to (10 t ^1/2 + ORS). Here, when there is a difference in the ORS in the two stretching directions, it is preferable to adopt a high ORS value. The relationship between these preferred heating time, sheet thickness, and ORS was empirically derived from the results of studying various conditions in order to obtain a molded product with a high balance of transparency and mold reproducibility in a short shot cycle. It has been.

  The shape of the molded product can be used without particular limitation, such as container lids, trays, food packs, blister packs, other packs, cases, etc., but from the viewpoint of taking advantage of the transparency and rigidity of the styrene resin sheet and the sheet and molded product of the present invention From the viewpoints of heat resistance, moldability, transparency of the molded product, and maintenance of the appearance, which are features of the above, the lid of the container is particularly preferable, and the lid of the food packaging container is particularly preferable.

When using a molded article comprising the sheet of the present invention as a lid for a food packaging container, the container body is not only molded from the sheet of the present invention, but also a polypropylene non-foamed sheet, a polypropylene foamed sheet, a polypropylene filled non-filler A foamed sheet, a polystyrene-based foamed sheet, a polystyrene-based non-foamed unstretched sheet, a crystallized polyester-based sheet, various plastic laminated sheets, paper, pulp, aluminum and the like are also preferably used.
Next, the present invention will be described with reference to examples and reference examples.

In the examples, the evaluation methods are as follows.
Orientation relaxation stress (ORS): measured according to ASTM D1504 (peak value of sheet contraction stress in silicon oil adjusted to VSP + 25 ° C. of resin composition: unit MPa)
VSP: Measured according to ASTM D-1525 (weight 49N (5kg), temperature rising rate 2 ° C / min)

Impact strength: Test sheet of thickness tmm is heated plate contact thermoforming method, hot plate temperature is (VSP + 32) ° C, heating time is (3.2t ^1/2 + ORS) second, depth 5mm, opening 150mm A container having a flat top surface portion of × 180 mm was formed. The impact resistance strength of the obtained molded product top surface portion was measured based on the method A of JISK7124-1, and the results were evaluated according to the following criteria.
A: Impact strength (J) is a sheet thickness before molding (mm) x 8.5 or more B: Impact strength (J) is a sheet thickness (mm) before molding x (8 or more and less than 8.5)
Δ: Impact strength (J) is sheet thickness before molding (mm) × (7 or more and less than 8)
X: Impact strength (J) is less than sheet thickness (mm) x 7 before molding

Heat resistance: 0.65 g of 0.8% saline was placed in a polypropylene container. Separately, a lid having a diameter of 90 mm and a depth of 30 mm was molded from a test sheet by a hot plate contact heating molding method. The lid and container were taped and heated in a 1600 w microwave for 120 seconds. The container volume change rate before and after microwave heating ((1-volume after microwave heating / volume before microwave heating) × 100:%) was measured and evaluated according to the following criteria. The volume of the container was converted from the mass of the container filled with water. In the hot plate contact heating molding, the hot plate temperature was (VSP + 28) ° C. and the heating time was (5.5 t ^1/2 + ORS) seconds.
◎: Volume change rate is less than 1% ○: Volume change rate is 1% to less than 3% Δ: Volume change rate is 3% to less than 5% ×: Volume change rate is 5% or more

Mold reproducibility: Evaluated according to the following criteria from R (mm) of 10 corner parts of the same shape molded under the same conditions as the heat resistance evaluation.
◎: R of corner portion is less than 4 mm ○: R of corner portion is 4 mm or more and less than 4.6 mm Δ: R of corner portion is 4.6 mm or more and less than 5.1 mm ×: R of corner portion is 5.1 mm or more Transparency : Measure HAZE according to JISK7105

Moreover, the following were used for the styrene resin.
PS-1: Styrene-methacrylic acid copolymer having VSP of 120 ° C. (7.2% by mass of methacrylic acid, weight average molecular weight 220,000)
PS-2: Styrene-methacrylic acid copolymer having VSP of 116 ° C. (5.1% by mass of methacrylic acid, weight average molecular weight 220,000)
PS-3: Styrene-methacrylic acid copolymer having a VSP of 125 ° C. (9.8% by mass of methacrylic acid, weight average molecular weight 210,000)

[Example 1]
Styrenic resin (PS-1) is supplied to an extruder having a 65 mm diameter screw, melt kneaded, extruded from a T-die, cooled with a roll, reheated, and then the sheet flow direction (vertical direction) due to the speed difference of the roll group And 2.8 times, the film was stretched 3.1 times in the direction perpendicular to the sheet flow direction (referred to as a horizontal direction) with a tenter to obtain a sheet having a thickness of 0.18 mm to 0.4 mm. The stretching temperature (stretching roll temperature, tenter atmosphere temperature) was adjusted in the range of VSP + 18 to 27 degrees of the resin, and sheets with ORS values shown in Table 1 were obtained. Table 1 shows the physical properties of the sheet.

[Examples 2 and 3]
A sheet was obtained in the same manner as in Example 1 except that the styrene resin was changed to PS-2 and PS-3. Table 2 shows the physical properties of the sheet.

[Comparative Example 1]
Using a styrene resin PS-1, a 0.13 mm sheet was stretched 2.8 × 3.1 times at a temperature of VSP + 25 to 29 ° C., and a 0.3 mm sheet was 3.2 × 3. A film was obtained in the same manner as in Example 1 except that the film was stretched 3 times. Table 2 shows the physical properties of the sheet. Further, the container obtained by the mold reproducibility test using the 0.13 mm thick sheet of Comparative Example 1 had low rigidity and poor practicality.

[Reference Example 1]
After subjecting one side of the 0.3 mm thick sheet of Example 1 to a corona treatment so that the contact angle with pure water is 50 to 55 degrees, sucrose fatty acid ester (individual at room temperature mainly containing lauric acid ester) , HLB of 16), and a 0.375% aqueous solution composition was applied by a spray coater (wet coating amount 4 g / m ² ) and dried. The obtained sheet was molded into a lid having a diameter of 90 mm and a depth of 30 mm by a hot plate contact heating molding method so that the sucrose fatty acid ester coating surface was inside. The sample was prepared in the same manner as the heat resistance evaluation method except that a commercially available salad oil was applied to the inside of the lid, and heated in a 1600 w microwave oven for 120 seconds. The change in the HAZE of the molded product before and after heating was 1%. For comparison, the sheet before coating was tested in the same manner, and as a result, the HAZE change was 2.3%.

[Reference Example 2]
In Reference Example 1, instead of sucrose fatty acid ester, a 9: 1 mixture (weight ratio) of polyoxyethylene sorbitan monostearate (liquid with a viscosity of 230 mPa · S, HLB15) and polyvinyl alcohol (degree of saponification 55%), Alternatively, the test was performed in the same manner except that it was changed to polyethylene glycol monooleate (liquid with a viscosity of 60 MPa · S, HLB 11.6). The change in HAZE of the molded product before and after heating was 1.2% for the former and 2% for the latter. there were.
As mentioned above, it turns out that oil resistance improves by application | coating of a specific surfactant.

[Sheet molding example 1]
Using the 0.3 mm thick sheets of Examples 1, 2, 3 and Comparative Example 1, a fitting lid having an opening of 210 mm × 120 mm and a depth of 38 mm was molded by a hot plate contact heating molding method. A resistance force when the fitting lid was fitted into a fitting container (made of a PP sheet containing filler formed separately) was measured with a compression load meter and evaluated as a guideline for mold reproducibility of the fitting lid. Table 3 shows the conditions (heating time, heating temperature) under which a good molded product was obtained. Further, in the condition range of heating temperature and time shown in Table 3, no appearance defects (transparency) deterioration due to the occurrence of raindrops or significant transfer of the hot plate pattern of the molding machine was not observed. The heating pressure air pressure at the time of molding was 0.19 MPa, the molding pressure air pressure was 0.31 MPa, and the vacuum pressure was 0.085 MPa.
The evaluation criteria for the mold reproducibility of the fitting lid are as follows.
A: Resistance is 650 g or more and less than 850 g.
○: Resistance is 550 g or more and less than 650 g.
Δ: Resistance is 400 g or more and less than 550 g.
X: Resistance is less than 400 g or the lid does not fit into the container.
Further, in order to obtain a molded product of level A with the sheet of Comparative Example 1, for example, a heating temperature of VSP + 42 ° C. or higher was necessary (heating time 3.5). However, in this case, raindrops occurred, and the appearance of the molded article began to deteriorate, so the commercial value was inferior.

表３より、実施例１，２、３のシートは、良好な成形品が得られる成形条件範囲が比較例１のシートより広くまた、比較的短い時間で成形できるため、成形容器の生産効率もよいことがわかる。

From Table 3, the sheets of Examples 1, 2, and 3 have a wider range of molding conditions for obtaining good molded products than the sheet of Comparative Example 1 and can be molded in a relatively short time, so the production efficiency of the molded container is also high. I know it ’s good.

  The styrenic resin sheet of the present invention has excellent thermoformability, heat resistance, and strength, and is useful as a styrenic resin biaxially stretched sheet for molding, and is suitably used as a thermoformed container, particularly a molded container lid material. it can.

Claims (5)

In a biaxially stretched sheet made of a styrene resin, the styrene resin is a copolymer of a styrene monomer and an unsaturated carboxylic acid monomer, and the sheet thickness (t (mm)) is 0.16 mm to 0.5 mm. The relationship between the sheet thickness (t (mm)) and the Vicat softening point (VSP (° C.)) of the resin constituting the sheet is A), and the sheet thickness (t (mm)) and orientation relaxation in at least one stretching direction. A styrene-based resin biaxially stretched sheet characterized in that the relationship with stress (ORS (MPa)) is B) below.
A) VSP (° C.) ≦ −10 × t (mm) +130 and VSP (° C.) ≧ −10 × t (mm) +117 is satisfied.
B) ORS (MPa) ≦ −0.3 × Loge (t (mm) /0.3) +0.95 and ORS (MPa) ≧ 0.3 × Loge (0.3 / t (mm) ) +0.5 is satisfied.   2. The styrene resin 2 according to claim 1, wherein the styrene resin is at least one copolymer selected from styrene and methacrylic acid, styrene and acrylic acid, or a copolymer of styrene and maleic anhydride. Axial stretched sheet. A nonionic interface having a hydrophilic / lipophilic ratio (HLB) of 12 to 19 and a solid at 40 ° C or a viscosity of 150 mPa · s or more on at least one surface of the styrenic resin biaxially stretched sheet according to claim 1 or 2 A styrenic resin biaxially stretched sheet in which at least one activator is coated with a film thickness of 8 to 50 mg / m ² .   A styrene resin biaxially stretched sheet thermoformed product using the styrene resin biaxially stretched sheet according to any one of claims 1 to 3.   The styrenic resin biaxially stretched sheet thermoformed product according to claim 4, wherein the thermoformed product is a container lid.