JP2005008869A - Sheet-shaped polylactic acid crosslinked foam and preparation process therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sheet-shaped polyactic acid crosslinked foam which has compatibly a heat resistance and a plasticity, and is easy in its secondary molding such as vacuum molding, compression molding, and to provide an optimum preparation process therefor. <P>SOLUTION: The crosslinked foam is obtained by using a biodegradable resin comprising 90-10 wt% of a polyactic acid whose melt viscosity at the temperature of 170°C and the sheer rate of 100 sec<SP>-1</SP>is 1.0×10<SP>3</SP>-1.0×10<SP>4</SP>Pa s and whose mol ratio of the (d) body and the (l) body is d/l=98/2-70/30 or d/l=2/98-30/70 and 10-90 wt% of a biodegradable polyester which is not a polyactic acid and whose melt viscosity at the temperature of 170°C and the sheer rate of 100 sec<SP>-1</SP>is 1.0×10<SP>3</SP>-1.0×10<SP>5</SP>Pa s, a heat decompositon foaming agent and a closslinking aid. The sheet-shaped polyactic acid crosslinked foam has a gel fraction of not less than 10%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a polylactic acid cross-linked foam having both heat resistance and flexibility, which can be easily subjected to secondary molding such as vacuum molding and compression molding, and a method for producing the same.

  Conventionally, resin foams such as polyolefin resin foams and polyurethane resin foams are excellent in lightness, heat insulation, moldability, buffering properties, etc., so they are industrially manufactured and used in a wide range of applications. It has been. However, although these resin foams are lightweight, they are bulky when discarded and difficult to reuse. In particular, in the case of a crosslinked resin foam obtained by crosslinking a resin, there is a drawback that material recycling is virtually impossible. Moreover, these resin foams remain semi-permanently even if they are buried in the soil, and often contaminate the global environment by securing a waste disposal site by incineration or landfilling, thereby damaging the natural landscape.

  On the other hand, from the viewpoint of disposal problems, biodegradable resins that are decomposed by microorganisms and the like in the natural environment have been researched and developed, and products using biodegradable resins for films and fibers have already been commercialized. In addition, an extruded foam using a biodegradable resin has been developed as a foam. Among these biodegradable resins, polylactic acid, in particular, has been attracting attention as a plant-derived clean biodegradable resin because lactic acid, the main raw material, can be produced by fermenting corn starch or corn syrup. Research for utilization is actively conducted.

  However, aliphatic polyester resins such as polylactic acid are difficult to increase in molecular weight due to water generated at the time of polycondensation, and it is difficult to provide the resin with the melt viscosity necessary for producing a foam. there were.

  In order to solve this, there is a method of improving the melt viscosity by blending polyisocyanate with polylactic acid, chain-linking, increasing the weight average molecular weight and number average molecular weight, and reducing the polydispersity to 6 or less. It has been proposed (see Patent Document 1). In addition, a method of improving the melt viscosity by branching using a polyfunctional glycol, carboxylic acid or the like has been proposed (see Patent Document 2). Although it is possible to obtain a polylactic acid foam having a relatively high expansion ratio by improving the melt viscosity in this manner, the obtained foam or foamed particles are non-crosslinked, and thus the obtained foam When heat treatment was performed and vacuum forming or the like was attempted to form into a desired shape, it was difficult to process into a complicated shape due to insufficient heat resistance, and the use application was limited. .

  As a method for solving this problem, use of a method of crosslinking a polyester resin with a peroxide (see Patent Document 3) can be considered. However, polylactic acid is unstable in radicals generated on tertiary carbon due to the electron donating property of the α-position methyl group. Is unlikely to occur. In fact, Patent Document 3 describes an example in which butylene succinate is crosslinked, but does not describe an example in which polylactic acid is crosslinked.

  In addition, polylactic acid has other disadvantages such as inferior impact resistance and flexibility. However, polylactic acid and dicarboxylic acid can be used as a method for solving the disadvantage and obtaining a foam having good elasticity and buffering property at room temperature. Expandable particles using a lactic acid-based polyester copolymer containing an acid component and a diol component as structural units have been proposed (see Patent Document 4). According to this technology, it is possible to control flexibility and mechanical strength. However, this technology discloses foamed particles obtained by absorbing volatile compounds by using the gas permeability of the polymer and foaming them. It is not disclosed that foaming is performed using a gas generated by decomposing a pyrolytic foaming agent. In addition, since the polymerization method of the polymer becomes complicated, the price of the biodegradable resin, which is originally expensive compared with general-purpose plastics such as polyethylene, is further increased, and there is a concern that the use is limited. There was a point.

  In addition, a foam using a resin composition obtained by melting and mixing polylactic acid and aliphatic polycarbonate in the presence of a radical reaction initiator in a nitrogen atmosphere has been proposed (see Patent Document 5). According to this method, it is possible to control the flexibility and mechanical strength by changing the mixing ratio of the polymer. However, when the crosslinking is performed to give sufficient viscosity to hold the decomposition gas of the blowing agent, Since the viscosity becomes too high to be molded into a sheet, it is impossible to obtain a sheet-like crosslinked foam.

JP 2001-98044 A JP 2000-169546 A Japanese Patent Laid-Open No. 2002-60501 JP-A-8-253617 JP 2002-371172 A

  Therefore, in view of the above-described conventional technology, the present invention provides a sheet-like polylactic acid crosslinked foam that has both good heat resistance and flexibility and is easy to perform secondary molding such as vacuum molding, compression molding, and the like. An object is to provide an optimum manufacturing method.

  In order to achieve this object, the present invention employs the following means, and the sheet-like polylactic acid crosslinked foam and the production method thereof of the present invention preferably have the following modes.

(1) The melt viscosity at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ is 1.0 × 10 ^{2 to} 1.0 × 10 ⁴ Pa · s, and the molar ratio of d-form to l-form is d / l = 98/2. From 90 to 10% by weight of polylactic acid having 70/30 or d / l = 2/98 to 30/70, and from 10 to 90% by weight of biodegradable polyester resin other than polylactic acid having a temperature of 170 ⁵ Pa · s. A sheet-like polylactic acid cross-linked foam using a biodegradable resin, a thermally decomposable foaming agent, and a cross-linking aid, and having a gel fraction of 10% or more.
(2) The sheet form according to (1), wherein the biodegradable polyester resin is at least one selected from a lactone resin, an aliphatic polyester, an aromatic copolymer polyester, and a natural linear polyester resin. It is a polylactic acid cross-linked foam.

(3) The sheet-like polylactic acid crosslinked foam according to (1) or (2), wherein the non-biodegradable thermoplastic resin is blended in an amount of 50% by weight or less of the total resin.
(4) The sheet-like polylactic acid crosslinked foam according to any one of (1) to (3), wherein the crosslinking assistant is a polyfunctional methacrylic acid compound.
(5) The crosslinking aid is at least one selected from ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylolpropane trimethacrylate, 1,6-hexanediol dimethacrylate, and 1,9-nonane dimethacrylate. The sheet-like polylactic acid crosslinked foam according to any one of the above (1) to (4).
(6) The sheet-like polylactic acid crosslinked foam according to any one of (1) to (5), wherein a dimensional change rate after standing for 1 hour at a temperature of 120 ° C. is within 5%.

(7) The melt viscosity at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ is 1.0 × 10 ^{2 to} 1.0 × 10 ⁴ Pa · s, and the molar ratio of d-form to l-form is d / l = 98/2. Polylactic acid 90 to 10% by weight with 70/30 or d / l = 2/98 to 30/70, and a melt viscosity at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ is 1.0 × 10 ^{2 to} 1.0 A sheet formed by molding a resin composition containing a biodegradable resin composed of 10 to 90% by weight of a biodegradable polyester resin other than polylactic acid of 10 ⁵ Pa · s, a thermally decomposable foaming agent, and a crosslinking aid. A step of irradiating the sheet with ionizing radiation to crosslink the resin composition to obtain a cross-linked sheet, and further heat-treating the cross-linked sheet at a temperature equal to or higher than a decomposition temperature of the pyrolyzable foaming agent to form a sheet-like cross-linked foam Of a sheet-like polylactic acid cross-linked foam including a step of forming a body It is a production method.

  Thus, the present invention provides a sheet-like polylactic acid cross-linked foam that achieves the above-described purpose, that is, heat resistance and flexibility, and is easy to perform secondary molding such as vacuum molding and compression molding, and an optimal manufacturing method thereof. As a result of earnest study on obtaining, a sheet-like cross-linked foam using a specific polylactic acid, a specific biodegradable polyester resin (other than polylactic acid), a thermal decomposable foaming agent, and a crosslinking aid. And by determining that the gel fraction is 10% or more, the object can be achieved.

  According to the present invention, a sheet-like polylactic acid cross-linked foam having good heat resistance and flexibility and easy secondary forming such as vacuum forming and compression forming can be produced with good reproducibility.

The present invention is described in detail below. First, the polylactic acid used in the present invention has a melt viscosity of 1.0 × 10 ^{2 to} 1.0 × 10 ⁴ Pa · s at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ , and a molar ratio of d-form to l-form. It is necessary that d / l = 98/2 to 70/30 or d / l = 2/98 to 30/70.

When the melt viscosity is less than 1.0 × 10 ² Pa · s, not only the compatibility with biodegradable polyester resins other than polylactic acid is remarkably lowered, but also the heat resistance of the resulting foam is lowered. On the other hand, even if the melt viscosity exceeds 1.0 × 10 ⁴ Pa · s, the compatibility with biodegradable polyester resins other than polylactic acid is lowered, and the productivity is also lowered. In addition, the resin composition comprising polylactic acid and a biodegradable polyester resin other than polylactic acid, a thermally decomposable foaming agent, and a crosslinking aid has a melting point higher than that of polylactic acid and a biodegradable polyester resin other than polylactic acid, and It is necessary to melt and knead the mixture at a temperature lower than the decomposition temperature of the heat decomposable foaming agent to form a sheet. Considering these, it is preferable that the temperature at the time of melt-kneading is in the range of about 130 ° C. to 190 ° C. Therefore, it is very important to grasp the melting characteristics at 170 ° C.

  The polylactic acid used in the present invention is required to have a molar ratio of d-form to l-form of d / l = 98/2 to 70/30 or d / l = 2/98 to 30/70. . In order to improve the heat resistance, it is preferable that the optical activity of lactic acid is high. However, in the case of a polylactic acid resin that is almost 100% of a l-form that is usually used for fibers, the crystallinity is high, and the cross-linked foam and In doing so, it is difficult for the crosslinking reaction to proceed even if electron beam irradiation or the like is applied. If the optical activity of lactic acid is lowered, the crystallinity and melting point of polylactic acid are lowered, so that the heat resistance of the resulting foam is undesirably lowered. Taking the above into consideration, it is preferable that the molar ratio of d-form to l-form is d / l = 98/2 to 70/30 or d / l = 2/98 to 30/70, and more preferably The molar ratio of l-form is d / l = 97 / 3-80 / 20 or d / l = 3 / 97-20 / 80. Furthermore, from a commercial perspective, since naturally occurring l-lactic acid is more easily available, the molar ratio of d-form to l-form is substantially d / l = 2/98 to 30/70. Some are more preferable.

  As a method for producing the above polylactic acid, a conventionally known method may be used. Examples thereof include a method of condensation polymerization of lactic acid and a method of ring-opening polymerization of a cyclic dimer (lactide). In addition, lactic acid of d and l uses corn or the like as a main raw material, starch is decomposed into glucose and fermented by lactic acid bacteria, ethylene is oxidized and a racemic body of d and l lactic acid is synthesized via lactonitrile, Can be synthesized by a method of optically splitting.

  Further, a resin other than polylactic acid may be copolymerized as long as the object of the present invention is not impaired. Moreover, it is a preferable embodiment to adjust the melt viscosity by adding a chain extender such as polyfunctional isocyanate or polyfunctional glycidyl ester as necessary.

Next, biodegradable polyester resins other than polylactic acid used in the present invention will be described. The biodegradable polyester resin other than polylactic acid needs to have a melt viscosity of 1.0 × 10 ^{2 to} 1.0 × 10 ⁵ Pa · s at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ .

When the melt viscosity is less than 1.0 × 10 ² Pa · s, not only the compatibility with polylactic acid is remarkably lowered, but also the heat resistance of the resulting foam is lowered, and 1.0 × 10 Even if it exceeds ⁵ Pa · s, the compatibility with polylactic acid is lowered and the productivity is also lowered. This is the same as in the case of polylactic acid described above.

  Examples of biodegradable polyester resins other than the above polylactic acid include synthetic polymers such as the following lactone resins such as ε-caprolactone, β-propiolactone, γ-butyrolactone, δ-valerolactone, enanthlactone and 4 -Homopolymers or copolymers of various methylated lactones such as methyl caprolactone and 2,2,4-trimethylcaprolactone, and mixtures thereof, aliphatic polyesters represented by the following, for example, polyethylene succinate, polybutylene succi Nylon, polybutylene succinate / adipate, polybutylene succinate / carbonate diol and dicarboxylic acid or aliphatic polyester, polyethylene terephthalate / succinate, polyethylene terephthalate / Adipate, polybutylene terephthalate / succinate, polybutylene terephthalate / adipate and aromatic copolyester of natural linear polyester-based resins such as polyhydroxybutyrate-Barireto the like. These biodegradable polyester resins other than polylactic acid may be a copolymer, may be used alone, or may be used in combination of two or more. Among these, polyethylene succinate, polybutene terephthalate / adipate, polybutylene are good in that they have good compatibility with polylactic acid, have similar crosslinkability to electron beams, and have excellent flexibility that polylactic acid does not have. Succinate / carbonate and the like are particularly preferable, and polybutylene terephthalate / adipate is most preferable.

  Also in these biodegradable polyester resins, melt viscosity can be obtained by a conventionally known method such as branching with a polyfunctional carboxylic acid or glycol, or using a chain extender such as polyfunctional isocyanate or polyfunctional glycidyl ether. Can be adjusted.

  In addition, resins other than those described above may be used, for example, biodegradable cellulose esters such as cellulose acetate, cellulose butyrate, cellulose propionate, cellulose nitrate, cellulose sulfate, cellulose acetate butyrate, cellulose nitrate acetate, etc. , Polyglutamic acid, polyaspartic acid, polyleucine and other polypeptides, polyvinyl alcohol, etc., starch, corn starch, wheat starch, rice starch, raw starch, acetate esterified starch, methyl etherified starch, amylose, etc. Biodegradable resins such as starch, natural polymers such as cellulose, carrageenan and chitin / chitosan may be added.

  Furthermore, the cross-linked foam of the present invention may be one in which a non-biodegradable thermoplastic resin is blended in a range not impairing the object of the present invention in addition to the above-described biodegradable resin. Examples of the non-biodegradable resin include ultra low density polyethylene, low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultra high molecular weight polyethylene, polypropylene, ethylene-propylene rubber, polyvinyl acetate. And general-purpose resins such as polybutene, polyacetal, polycarbonate, polyethylene terephthalate, polyester elastomer, and polyolefin elastomer. The blending amount of this non-biodegradable thermoplastic resin is preferably 0 to 50% by weight of the total resin. If it exceeds 50% by weight, the biodegradability may be impaired. These non-biodegradable thermoplastic resins can be preferably used for imparting a function that is not expressed only by polylactic acid and biodegradable resins other than polylactic acid.

The melt viscosity in the present invention is determined by using a “Capillograph 1C-PMD-C” manufactured by Toyo Seiki Seisakusho, using a nozzle having a nozzle diameter of 1.0 mm and L / D = 10 at a temperature of 170 ° C. It is a value of melt viscosity measured at 100 sec ⁻¹ .

  In the present invention, in order to achieve both heat resistance and flexibility, the mixing ratio (weight ratio) of the polylactic acid and the biodegradable polyester resin other than polylactic acid needs to be 10/90 to 90/10. Yes, preferably 15/85 to 85/15, more preferably 20/80 to 80/20. As the mixing ratio of polylactic acid increases, the heat resistance and rigidity improve, but the flexibility decreases, so that it cannot be wound up as a sheet-like foam, and the buffering property tends to decrease. When the content of polylactic acid is large, for example, it is suitable for automobile ceiling materials and containers that require rigidity and heat resistance. Conversely, the greater the biodegradable polyester resin other than polylactic acid, the more the flexibility is improved, but the heat resistance tends to decrease, and this is suitable for use as a buffer material.

  The foaming agent used in the present invention is a compound that is a liquid or solid compound at room temperature and is a compound that decomposes or vaporizes when heated above the melting point of polylactic acid and biodegradable polyester resins other than polylactic acid. Any material can be used as long as it does not substantially interfere with the reaction. Among them, a thermal decomposition type blowing agent and a decomposition temperature in the range of 120 ° C. to 270 ° C. are preferable. Specific examples thereof include azo compounds such as azodicarbonamide and metal salts of azodicarboxylic acid, hydrazine derivatives such as hydrazodicarbonamide, 4,4′-oxybis (benzenesulfonylhydrazide), and toluenesulfonylhydrazide, N, N Examples thereof include nitroso compounds such as' -dinitrosopentamethylenetetramine, semicarbazide compounds such as toluenesulfonyl semicarbazide, and bicarbonates such as sodium bicarbonate. Of these, azodicarbonamide, 4,4'-oxybis (benzenesulfonylhydrazide), and sodium hydrogen carbonate are particularly preferably used. As azodicarbonamide, a neutral one having a pH of around 7 is preferable.

  In order to adjust the decomposition temperature of the foaming agent, for example, those to which a decomposition temperature adjusting agent such as zinc oxide, zinc stearate, urea or the like is added in combination can be preferably used.

  These foaming agents are used in an amount within the range of 0.1 to 40 parts by weight with respect to 100 parts by weight of the resin mixture composed of polylactic acid and a biodegradable polyester resin other than polylactic acid. The mixing amount can be arbitrarily changed depending on the magnification.

  The crosslinking aid used in the present invention is used to efficiently introduce a crosslinked structure into a resin mixture composed of polylactic acid and a biodegradable polyester resin other than polylactic acid. It can be used as an agent. When a crosslinking aid is used in combination, a crosslinked structure can be efficiently introduced with a small dose of radiation, so that productivity is improved and deterioration of the resin can be prevented, which is preferably used.

  These crosslinking aids are not particularly limited, and conventionally known polyfunctional monomers such as 1,6-hexanediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, tetramethylolmethane triacrylate Acrylate- or methacrylate-based compounds such as 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate; allyl carboxylic acids such as trimellitic acid triallyl ester, pyromellitic acid triallyl ester, diallyl oxalate Esters; cyanuric acids such as triallyl cyanurate and triallyl isocyanurate or allyl esters of isocyanuric acid; N-phenylmaleimide, N, N'-m-phenylenebismer Maleimide compounds such as imides; compounds having two or more triple bonds such as dipropargyl phthalate and dipropargyl maleate; polyfunctional monomers such as divinylbenzene can be used from the viewpoint of ease of handling and versatility , Polyfunctional methacrylic acid compounds are preferable, and among these, ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylolpropane trimethacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol di An ester-based polyfunctional monomer such as methacrylate is particularly preferably used. These polyfunctional monomers can be used alone or in combination of two or more.

  Since polylactic acid is a resin that is difficult to crosslink, if the content of polylactic acid is high, it is possible to use a tri- or higher functional crosslinking aid and a bifunctional crosslinking aid in combination with a small amount of addition. Since a body can be obtained, it is preferable. Among these, trimethylolpropane trimethacrylate and either 1,6-hexanediol dimethacrylate or 1,9-nonanediol dimethacrylate mixed at a weight ratio of 1: 3 to 3: 1 When used, it is particularly preferable because a good crosslinked foam can be obtained.

  If the addition amount of these polyfunctional monomers is too small, a good crosslinked foam cannot be obtained, and if it is too much, the moldability of the obtained foam is lowered, the kneadability is lowered during molding, coarse bubbles, etc. Is preferably 0.5 to 10 parts by weight, more preferably 1 to 7 parts by weight with respect to 100 parts by weight of a resin mixture composed of polylactic acid and a biodegradable polyester resin other than polylactic acid. is there.

  Moreover, you may add various additive components in the range which does not inhibit the effect of this invention. For example, as additives, crosslinking agents, antioxidants, lubricants, heat stabilizers, pigments, flame retardants, antistatic agents, nucleating agents, plasticizers, antibacterial agents, biodegradation accelerators, foaming agent decomposition accelerators, light stabilizers UV absorbers, anti-blocking agents, fillers, deodorants, thickeners, foaming aids, foam stabilizers, metal damage inhibitors, hydrolysis rate modifiers, etc. These may be used alone or in combination of two or more. May be added. In particular, polylactic acid is a polymer that easily undergoes oxidative degradation, and preferably contains an antioxidant in order to obtain a foam having a beautiful appearance.

  The polylactic acid crosslinked foam has a gel fraction indicating the degree of crosslinking of 10% or more, preferably 15% or more, and most preferably 20% or more. By forming a crosslinked foam having such a crosslinking degree, the sheet-like crosslinked foam can be reheated and processed into a desired shape by performing vacuum forming, compression molding, stamping molding, etc. Since the heat resistance of the foam is increased, the moldability is greatly improved, and the foam can be easily molded in a wide molding temperature range from a low temperature to a high temperature.

  In the present invention, by introducing a crosslinked structure into the resin in this way, both flexibility and heat resistance are achieved, and moldability is improved. Therefore, it is used by making a crosslinked foam rather than using an uncrosslinked foam. Uses will expand.

  The gel fraction as used in the field of this invention is the value computed with the following method. That is, 50 mg of a biodegradable resin foam sample is accurately weighed, immersed in 25 ml of chloroform at 25 ° C. for 3 hours, filtered through a 200 mesh stainless steel wire mesh, and the insoluble matter on the wire mesh is vacuum dried. Next, the weight of this insoluble matter is precisely weighed, and the gel fraction is calculated as a percentage according to the following formula.

  Gel fraction (%) = {weight of insoluble matter (mg) / weight of weighed biodegradable resin crosslinked foam (mg)} × 100

  In the present invention, the method for introducing a crosslinked structure into the biodegradable resin foam is not particularly limited, and examples thereof include a method of irradiating a predetermined dose of ionizing radiation, crosslinking with a peroxide, and silane crosslinking. .

  The polylactic acid cross-linked foam of the present invention preferably has a dimensional change rate after standing for 1 hour at a temperature of 120 ° C. within 5% in any of vertical, horizontal and thickness directions. That is, since the polylactic acid crosslinked foam of the present invention has good heat resistance, the heating dimensional change rate is small. On the other hand, in the case of a foam comprising a biodegradable aliphatic polyester resin alone other than polylactic acid, the heat resistance is insufficient and the heating dimensional change rate is large.

  The dimensional change rate referred to in the present invention is measured by the following method in accordance with JIS K6767: 1999. The sheet-like polylactic acid cross-linked foam is accurately cut into a 15 cm square and is left in a hot air circulating drier set at 120 ° C. for 1 hour. After 1 hour, it is removed from the oven and cooled in an environment of temperature 23 ± 2 ° C. and relative humidity 50 ± 5% for about 1 hour. After 1 hour, the vertical, horizontal, and thickness dimensions of the sample are measured, and the dimensional change rate in each direction is calculated as a percentage based on the following formula.

  Heating dimensional change rate (%) = [{sample length before putting into oven−sample length after taking out from oven} / sample length before putting into oven] × 100

  The form of the biodegradable resin crosslinked foam of the present invention is a sheet. By making it into a sheet form, not only the productivity is excellent, but also the biodegradation rate can be increased. The thickness of the biodegradable resin crosslinked foam sheet is preferably 0.1 to 100 mm. These sheets can be easily processed to a desired thickness by once subjected to foaming and then subjected to secondary processing such as slicing and fusion processing.

  The expansion ratio of the foam of the present invention is preferably 1.5 to 50 times. If the expansion ratio is less than 1.5 times, the lightness and flexibility tend to decrease, and if the expansion ratio exceeds 50 times, the mechanical properties and moldability tend to decrease.

Next, the manufacturing method of the sheet-like polylactic acid crosslinked foam of this invention is demonstrated.
The polylactic acid crosslinked foam of the present invention is obtained by molding a foamable sheet using a mixture of polylactic acid, a biodegradable polyester resin other than polylactic acid, a thermal decomposable foaming agent, and a crosslinking aid, The sheet can be produced by irradiating the sheet with ionizing radiation to crosslink the resin, and then heated to a temperature equal to or higher than the decomposition temperature of the thermally decomposable foaming agent to cause foaming. Preferred specific examples thereof are shown below.

  In the method of the present invention, polylactic acid, a biodegradable polyester resin other than polylactic acid, and a thermally decomposable foaming agent or a crosslinking aid can be mixed by a conventionally known mixing method. For example, mixing by Henschel mixer, mixing by Banbury mixer, mixing by mixing roll, mixing by kneading extruder, method of immersing resin in solution dissolving foaming agent and crosslinking accelerator, etc. are used alone or in combination . In particular, when the resin is in powder form, powder mixing with a Henschel mixer is convenient. Since polylactic acid used in the present invention and biodegradable polyester resins other than polylactic acid are susceptible to hydrolysis under heating, the water content should be reduced to 0.1% by weight or less in advance using a vacuum dryer. Is preferred.

  Next, the above mixture is preferably formed into a sheet using a single-screw melt extruder equipped with a vacuum vent, or preferably a twin-screw melt extruder. Even if it is not previously mixed with a Henschel mixer or the like, it can also be directly mixed using a tandem melt extruder or the like. In this case, it is preferable to install drying equipment such as a hopper dryer at the top of the extruder. In the case of producing a continuous sheet-like foam, it is preferably formed into a sheet by extrusion molding at a temperature lower than the decomposition temperature of the foaming agent. The melt kneading temperature at the time of extrusion is preferably 10 ° C. or more lower than the decomposition start temperature of the foaming agent. Specifically, it is in the range of 120 ° C to 240 ° C, preferably 130 ° C to 190 ° C. At this time, the thickness of the sheet is preferably 0.1 to 50 mm. When the thickness of the sheet is less than 0.1 mm, there is a lot of gas escape from the surface of the sheet during foam molding, making it difficult to form a uniform foam. May cause trouble.

  Next, crosslinking of the foamable sheet obtained by extrusion is performed by irradiating with ionizing radiation. As the ionizing radiation, an electron beam from an electron beam accelerator is preferable because the energy is uniform, and the irradiation dose varies depending on the type of crosslinking aid and the target crosslinking rate, but generally 10 to 10%. 300 kGy, preferably 20 to 200 kGy, more preferably 40 to 150 kGy. If it is less than 10 kGy, it is difficult to uniformly crosslink the resin, and it is difficult to obtain a melt viscosity for sufficiently retaining the decomposition gas of the foaming agent, and if it exceeds 300 kGy, the resin deteriorates severely. Difficult to get. Moreover, since the crosslinkability decreases as the content of polylactic acid increases, the resin can be efficiently cross-linked by irradiation with the sheet preheated. Although the preheating temperature of a sheet | seat changes with content of polylactic acid, it is preferable that the surface temperature of a sheet | seat is the range of 30-60 degreeC. If the surface temperature of the sheet is lower than 30 ° C., it is necessary to increase the irradiation dose in order to crosslink, leading to deterioration of the resin and reduction of productivity. If it exceeds 60 ° C., dirt or dust tends to adhere to the surface of the sheet, and it becomes difficult to obtain a foam with a beautiful appearance, which is not preferable.

  In producing the crosslinked polylactic acid resin foam of the present invention, in order to uniformly crosslink the polylactic acid, it is effective to control the temperature of the irradiated sheet after irradiation. When irradiated with ionizing radiation, the irradiated sheet absorbs the energy of the ionizing radiation and generates heat. At this time, the irradiation dose of ionizing radiation is controlled so that the surface temperature of the irradiated sheet after irradiation is in the range above the glass transition temperature of polylactic acid (usually around 57 ° C) and below the Vicat softening temperature of the irradiated sheet. Then, it is possible to obtain a good foam that is particularly beautiful in appearance and excellent in secondary workability. If the sheet temperature after irradiation does not reach the glass transition temperature of polylactic acid, it will be difficult for crosslinking to occur in the crystal part. If the temperature reaches the softening temperature or higher, the sheet is deformed by the tension when the sheet is wound up, which is not preferable. In order to control the sheet temperature after irradiation of the irradiated sheet as described above, the preheating temperature during irradiation and the irradiation dose of ionizing radiation may be adjusted as appropriate.

  The Vicat softening temperature is a temperature measured for a sheet before irradiation with ionizing radiation in accordance with JIS K7206.

  In addition, by controlling the acceleration voltage of the electron beam, it is possible to uniformly crosslink the irradiated object having various thicknesses. The acceleration voltage is preferably 200 to 1500 kV, more preferably 300 to 1200 kV. If the acceleration voltage is less than 200 kV, the whole cannot be uniformly crosslinked, or the electron beam range is shortened and discharge holes are generated, resulting in a poor appearance. If the acceleration voltage exceeds 1500 kV, This is not preferable because the difference in gel fraction at the inner layer portion is large and the production stability is lowered.

  Thereafter, the cross-linked sheet is heat-treated at a temperature equal to or higher than the decomposition temperature of the thermally decomposable foaming agent and foamed. For the heat treatment for foam molding, a conventionally known method may be used. For example, the heat treatment can be performed in vertical and horizontal hot-air foaming furnaces or chemical bath foaming furnaces. Since polylactic acid and biodegradable polyester resin are easily hydrolyzed, foams with better surface condition can be obtained by foaming in vertical and horizontal hot air foaming furnaces than foaming in a chemical bath. can get. Further, if foaming is preheated as necessary and the resin is softened, a stable foam can be obtained with a small amount of heat.

EXAMPLES Next, although this invention is demonstrated based on an Example, this invention is not limited to this. Hereinafter, “part” means “part by weight”.
In the examples and comparative examples, the foams were evaluated based on the following criteria.

[Dimensional change]
In all the vertical, horizontal and thickness directions of the sheet-like foam,
Dimensional change: ◯: The dimensional change rate is within 5%.
Dimensional change: x ... The dimensional change rate is over 5%.

[Compression recovery]
The compression recovery property was measured according to the compression set measurement method of JISK6767, and evaluated according to the following criteria.
Compression recovery: ○: Compression set is less than 15% Compression recovery: X: Compression set is 15% or more

[appearance]
The appearance was evaluated by measuring the Ra75 value of the sheet surface roughness of the sheet-like foam using a surface roughness measuring device SURFCORDER SE-2300 manufactured by Kosaka Laboratory Ltd., and evaluating it according to the following criteria.
Appearance: ○ ・ ・ ・ Ra75 value is less than 20 μm Appearance: × ・ ・ ・ Ra75 value is 20 μm or more

[Secondary workability]
When a sheet-like foam is heated to a surface temperature of about 160 ° C. on a vertical cylindrical female mold having a diameter D and a depth H, the foam breaks when straight molding is performed using a vacuum molding machine. The value of the molding drawing ratio H / D at the limit where it was developed and stretched into a cylindrical shape without being measured was measured and evaluated according to the following criteria.
Secondary workability: ○ ... Molding draw ratio 0.50 or more Secondary workability: × ... Molding draw ratio less than 0.50

[Example 1]
30 parts by weight of polylactic acid having a melt viscosity of 5.2 × 10 ³ Pa · s at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ and a molar ratio of d-form to l-form of d / l = 3/97; 70 parts by weight of polybutylene terephthalate / adipate having a melt viscosity of 0.9 × 10 ³ Pa · s at 100 ° C. and a shear rate of 100 sec ⁻¹ (the ratio of terephthalic acid to adipic acid is 1: 1 by molar ratio) and thermal decomposition 4.5 kg of azodicarbonamide as mold blowing agent, 2 kg of 1,6-hexanediol dimethacrylate as crosslinking aid, 2 kg of trimethylolpropane trimethacrylate, “Irganox 1010” as an antioxidant (manufactured by Ciba Specialty Chemicals Co., Ltd.) ) 0.3kg, "AO-412S" (Asahi Denka Co., Ltd.) 0.3kg is prepared, the temperature of the first half of the cylinder Was set to 175 ° C. and the temperature of the latter half portion was set to 160 ° C., introduced into a twin screw extruder with a vent of L / D = 21, extruded from a T-die, and formed into a crosslinked foamed sheet having a thickness of 1.5 mm. This sheet is preheated to 40 ° C in advance, irradiated with an electron beam of 95 kGy at an acceleration voltage of 800 kV and crosslinked, and then continuously introduced into a vertical hot air foaming apparatus and heated at 240 ° C for 3 to 4 minutes. It foamed and wound up as a continuous sheet-like cross-linked foam.

The foam thus obtained had a thickness of 2.7 mm, a gel fraction of 30%, and an apparent density of 75 kg / m ³ .

[Examples 2 to 4 and Comparative Examples 1 to 4]
A sheet-like cross-linked foam was prepared and evaluated in the same manner as in Example 1 except that the polymers shown in Table 1 were used.

[Example 5]
In addition to polylactic acid and a biodegradable polyester resin, a sheet was prepared in the same manner as in Example 1 except that a resin having a composition shown in Table 1 blended with a polyester elastomer was used as a non-biodegradable resin. A sheet-like cross-linked foam was produced in the same manner as in Example 1 except that the irradiation dose was set to 120 kGy. The obtained foam was a foam excellent in flexibility.

[Example 6]
Other than polylactic acid and biodegradable polyester resin, as the non-biodegradable resin, a resin having a composition shown in Table 1 blended with an ethylene / propylene random copolymer was used, and the temperature in the first half of the cylinder was set to 185 ° C. Produced a sheet in the same manner as in Example 1, and produced a sheet-like crosslinked foam by the same method as in Example 1 except that the electron beam irradiation dose was 80 kGy. The obtained foam was a foam having high rigidity.

[Comparative Example 5]
30 parts by weight of polylactic acid having a melt viscosity of 5.2 × 10 ³ Pa · s at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ and a molar ratio of d-form to l-form of d / l = 3/97; 70 parts by weight of polybutylene terephthalate / adipate having a melt viscosity of 1.2 × 10 ³ Pa · s at 100 ° C. and a shear rate of 100 sec ⁻¹ and “Irganox 1010” as an antioxidant (manufactured by Ciba Specialty Chemicals) ) 0.3 kg, “AO-412S” (manufactured by Asahi Denka Co., Ltd.) 0.3 kg are prepared, the temperature of the first half of the cylinder is set to 175 ° C., the temperature of the second half is set to 160 ° C., and L / D = Introduced into a 21-vented twin screw extruder, about 4.1 wt% of carbon dioxide gas was injected from the middle of the extruder as a foaming agent, extruded from a circular die, and 0.9 mm thick sheet-like uncrosslinked foam The body was made. The obtained foam had a gel fraction of 0% and an apparent density of 110 kg / m ³ .

  The sheet-like polylactic acid crosslinked foam of the present invention comprises mats such as desk mats and floor mats, building materials such as floor materials, wall materials and ceiling materials, heat insulating materials such as pipe covers, air conditioners and long roofs, and adhesives. Tape base materials, cushioning materials such as paper tube cores, door base materials for vehicles and railways, pillars, seat backs, ceiling base materials, vehicle interior materials such as insulators such as dash insulators and floor insulators, packing materials It can be used for a wide range of applications.

Claims (7)

The melt viscosity at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ is 1.0 × 10 ^{2 to} 1.0 × 10 ⁴ Pa · s, and the molar ratio of d-form to l-form is d / l = 98/2 to 70/30. Or 90 to 10% by weight of polylactic acid with d / l = 2/98 to 30/70, and a melt viscosity at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ is 1.0 × 10 ^{2 to} 1.0 × 10 ^5. A cross-linked foam comprising a biodegradable resin comprising 10 to 90% by weight of a biodegradable polyester resin other than polylactic acid which is Pa · s, a thermal decomposable foaming agent, and a cross-linking aid, and A sheet-like polylactic acid crosslinked foam having a gel fraction of 10% or more. The sheet-like polylactic acid crosslinked foam according to claim 1, wherein the biodegradable polyester resin is at least one selected from a lactone resin, an aliphatic polyester, an aromatic copolymer polyester, and a natural linear polyester resin. body. The sheet-like polylactic acid cross-linked foam according to claim 1 or 2, wherein the non-biodegradable thermoplastic resin is blended in an amount of 50% by weight or less of the whole resin. The sheet-like polylactic acid crosslinked foam according to any one of claims 1 to 3, wherein the crosslinking aid is a polyfunctional methacrylic acid compound. The crosslinking aid is at least one selected from ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylolpropane trimethacrylate, 1,6-hexanediol dimethacrylate, and 1,9-nonane dimethacrylate. Item 5. The sheet-like polylactic acid crosslinked foam according to any one of Items 1 to 4. The sheet-like polylactic acid cross-linked foam according to any one of claims 1 to 5, having a dimensional change rate of 5% or less after standing at a temperature of 120 ° C for 1 hour. The melt viscosity at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ is 1.0 × 10 ^{2 to} 1.0 × 10 ⁴ Pa · s, and the molar ratio of d-form to l-form is d / l = 98/2 to 70/30. Or 90 to 10% by weight of polylactic acid with d / l = 2/98 to 30/70, and a melt viscosity at a temperature of 170 ° C. and a shear rate of 100 sec ⁻¹ is 1.0 × 10 ^{2 to} 1.0 × 10 ^5. A step of obtaining a sheet by molding a resin composition containing 10 to 90% by weight of a biodegradable polyester resin other than polylactic acid that is Pa · s, a thermal decomposable foaming agent, and a crosslinking aid. A step of irradiating the sheet with ionizing radiation to crosslink the resin composition to obtain a cross-linked sheet, and further heat-treating the cross-linked sheet at a temperature equal to or higher than a decomposition temperature of the thermally decomposable foaming agent to obtain a sheet-like cross-linked foam. Method for producing a sheet-like polylactic acid crosslinked foam including a process .