DE69316785T2 - Foamed polyester moldings and process for their production - Google Patents

Description

Background of the invention Field of the invention

The present invention relates to polyester foamed articles produced using aliphatic polyesters having biodegradability and having sufficiently high molecular weights and specific melting properties for practical use. In particular, the present invention relates to a process for producing foamed articles by extrusion or impression molding using expandable particles (beads) and foamed articles produced thereby.

Discussion of the state of the art

Although recently plastic foamed articles characterized by their lightness, elasticity, moldability, etc. are mainly used as packaging materials, cushioning materials, etc., there is a major social problem in that the resulting waste from the large amounts of plastic used in these materials is difficult to dispose of and may pollute rivers, oceans, and soil. To prevent such pollution, the development of biodegradable plastics is desired; for example, poly(3-hydroxybutylate) produced by fermentation processes using microorganisms and blends of multipurpose plastics and starch, a naturally occurring polymer, are already known. The former polymer has a disadvantage in that it has poor moldability properties because the polymer has a heat decomposition temperature close to its melting point and that a raw material utility value is very poor because it is produced by microorganisms. On the other hand, since the naturally occurring polymer of the latter does not inherently exhibit thermoplasticity, the polymer has defects in moldability properties and is greatly limited in its application range.

On the other hand, aliphatic polyesters, although known to be biodegradable, have been rarely used because polymeric materials sufficient to obtain practical molded products cannot be obtained. (See "Design of Biodegradable Polymers", Yutaka Tokiwa, Polymer, 33, 378-381 [1984]; "Hydrolysis of Copolyesters Containing Aromatic and Aliphatic Ester Blocks by Lipase", Yutaka Tokiwa and Tomoo Susuki, Journal of Applied Polymer Science, 26, 441-448 [1981]; and "Synthesis of Copolyamide-Esters and Some Aspects Involved in Their Hydrolysis by Lipase", Y. Tokiwa, T. Suzuki and T. Ando, Journal of Applied Polymer Science, 24, 1701-1711 [1979]). Recently, it has been discovered that ring-opening polymerization of 8-caprolactone produces a polymer with a higher molecular weight, and it has been proposed to use the polymer as a biodegradable resin. However, the resulting polymer is limited to special applications only because of a low melting point of 62°C and high cost. Furthermore, although glycolic acid, lactic acid and the like are polymerized with ring-opening polymerization of glycolides and lactides thereof to obtain polymers with higher molecular weights, such as are sometimes used as medical fibers, the polymers are not used in large quantities as packaging materials and cushioning materials. because their decomposition temperatures are close to their melting point and they have defects in their formability properties.

Materials typically used for foam molded items are polystyrene or polyethylene. These materials are mixed with foaming agents and then formed into foamed items by extrusion or molding in a mold.

A material such as a styrene polymer containing 0.5 to 40 parts by weight of a volatile foaming agent such as propane, butane, pentane, methyl chloride or dichlorofluoromethane is known in the form of expandable beads. When these expandable beads are heated to a temperature not lower than their softening point, they become pre-expanded beads in which a large number of small cells have been formed. A method of producing foamed articles is known, according to which a closed but not airtight mold having a large number of small holes in its walls and a configuration which is the same as that of the final product is filled with the above pre-expanded beads, and a heating medium such as steam is sprayed onto the beads through the above-mentioned small holes to heat them to a temperature not lower than their softening point, thereby causing the beads to expand and fuse together, so that a porous styrene polymer foam having exactly the same shape as the above mold is produced.

On the other hand, the expanded polystyrene objects generate a high heat of combustion of the order of 9 500 kcal/kg, so that there is a risk of damaging a waste incinerator.

Polystyrene is usually used in the manufacture of such expandable beads, and even polyethylene terephthalate, which is a condensation product of ethylene glycol and a terephthalic acid (including dimethyl terephthalate) and is a polyester having a high molecular weight (here meaning a number average molecular weight of at least 10,000) used for conventional packaging purposes, has not been applied in this field, and no attempts have been reported to impart biodegradability to such materials.

As a result, there has been absolutely no technical concept to attempt to put into practical use expandable beads formed from a biodegradable aliphatic polyester based on an aliphatic dicarboxylic acid.

One of the reasons for the lack of a concept of bringing such foamed articles to practical use is that most of such aliphatic polyesters, even in a crystalline state, have a melting point not higher than 100°C, although special production conditions and physical properties are required with respect to the above expandable beads. Furthermore, they have poor stability in a molten state. More importantly, the properties of these aliphatic polyesters, particularly the mechanical properties thereof such as tensile strength, are markedly poor even with a number average molecular weight of the same magnitude as that of the above-mentioned polyethylene terephthalate, so that it was difficult to even imagine the possibility of obtaining a molded article from such materials which must have high strength.

It must also be assumed that this is partly attributable to the fact that studies into the possibility of obtaining an improvement in physical properties by increasing the number average molecular weight of such an aliphatic polyester have not yet been carried forward to a satisfactory extent because of the poor thermal stability of such polyesters.

It is an object of the invention to provide polyester foamed articles containing an aliphatic polyester as mentioned above, which has a sufficiently high molecular weight for practical use, which is superior in mechanical properties such as thermal stability and tensile strength, and which, as a disposable material, exhibits biodegradability, that is, which can be degraded by microorganisms or the like, thereby facilitating disposal thereof after use.

Summary of the invention

After examining various reaction conditions in order to obtain a polyester that provides foamed articles having a high molecular weight and a sufficient degree of utility, the inventors have found that foamed articles using a specific aliphatic polyester having a sufficiently high molecular weight for practical use while retaining its biodegradability offer excellent thermal stability and mechanical strength, not to mention the above-mentioned biodegradability, so that the object of the present invention is achieved.

The scope of the invention is defined in the appended claims.

Detailed description of the invention

The present invention will be described in more detail below.

The aliphatic polyester of the present invention consists mainly of a polyester obtained by reacting two components, namely glycols and dicarboxylic acid (or acid anhydrides thereof) and, if necessary, reacting with a third component with at least one polyfunctional component selected from the group consisting of trifunctional or tetrafunctional polyols, oxycarboxylic acids and polybasic carboxylic acids (or acid anhydrides thereof). The aliphatic polyesters are prepared by reacting polyester prepolymers with a relatively high molecular weight having hydroxyl groups at the terminals with a coupling agent so as to make them into a polymer with a still higher molecular weight.

It is known to obtain polyurethane by reacting a low molecular weight polyester prepolymer with a number average molecular weight of 2,000 to 2,500 and having hydroxyl groups as end groups with diisocyanate as a coupling agent in the manufacture of rubbers, foams, coatings and adhesives.

However, the polyester prepolymers used in these polyurethane foams, coatings and adhesives are low molecular weight prepolymers with a number average molecular weight of 2,000 to 2,500, which is the maximum that can be produced by a non-catalytic reaction. In order to obtain practically useful physical properties of polyurethane, it is necessary that the diisocyanate content be so high that it is 10 to 20 parts by weight per 100 parts by weight of this low molecular weight prepolymer. When such a large amount of diisocyanate is added to the low molecular weight polyester melted at 150° or above, gelation occurs so that a normal resin capable of being molded in the form of a melt cannot be obtained.

Therefore, polyesters obtained by reacting a large amount of diisocyanate with a low molecular weight polyester prepolymer as a raw material cannot be used as a plastic raw material for the process for producing a foamed article according to the present invention.

However, as has been shown in the case of polyurethane rubbers, a process is conceivable in which hydroxyl groups are converted into isocyanate groups by the addition of diisocyanate and then the number average molecular weight thereof is further increased using glycols, whereby the same problem as already mentioned above occurs because 10 parts by weight of diisocyanate should be used per 100 parts by weight of the prepolymer in order to obtain practically useful physical properties.

When a polyester prepolymer having a relatively high molecular weight is used, heavy metal catalysts required to prepare the prepolymer promote the reactivity of the above-mentioned isocyanate groups to undesirably cause poor preservability, generation of cross-linking and branching; thus, a number average molecular weight of not more than about 2,500 of the polyester prepolymer would be the limit if they were prepared without catalysts.

The polyester prepolymers used in the present invention to obtain the aliphatic polyesters are saturated aliphatic polyesters having a relatively high molecular weight having substantially hydroxyl groups at the terminals thereof, number average molecular weights of at least 5,000, preferably at least 10,000, and a melting point of 60°C or higher, obtained by reacting glycols and dibasic carboxylic acids (or acid anhydrides thereof) in the presence of catalysts. When a prepolymer having a number average molecular weight of less than 5,000 is used, small amounts of 0.1 to 5 parts by weight of coupling agents used in the present invention cannot provide polyesters for blow molding having good physical properties. When polyester prepolymers having a number average molecular weight of 5,000 or higher with hydroxyl values of 30 or less are used, the use of a small amount of coupling agents can produce polyesters having a high molecular weight even under severe conditions such as in a molten state or the like without gelation if the reaction is not influenced by a catalyst residue.

Therefore, the polymer for the foamed articles of the present invention has a repeating chain structure in which a polyester prepolymer having a number average molecular weight of 5,000 or more, preferably 10,000 or more, consisting of an aliphatic glycol and an aliphatic dicarboxylic acid is combined through the urethane bonds derived from, for example, diisocyanate or a coupling agent.

Furthermore, the polymer for the foamed articles of the present invention has a repeating chain structure in which the above-mentioned polyester prepolymer provided with branched long chains derived from polyfunctional components is repeatedly combined by the urethane bonds derived from, for example, diisocyanate derived from a coupling agent. When oxazoline, epoxy compounds and acid anhydrides are used as coupling agents, the polyester prepolymer has a repeating chain structure due to the ester bonds.

The foamed articles of the present invention consisting of an aliphatic polyester have a melt viscosity of 1.0 x 10³ to 1.0 x 10⁶ poise at a shear rate of 100 sec⁻¹ at a temperature of 190°C and a melting point of 70°C to 190°C, particularly the foamed articles of the present invention consisting of an aliphatic polyester obtained by reacting 0.1 to 5 parts by weight of diisocyanate and 100 parts by weight of an aliphatic polyester prepolymer having a number average molecular weight of 5,000 or higher and a melting point of 60°C or higher have biodegradability when buried in the soil; and are excellent in heat insulation and cushioning properties.

Examples of glycols that can be used as reaction components include aliphatic glycols. Among them, those having a straight-chain alkylene group with an even number of carbon atoms of 2, 4, 6, 8 and 10, such as ethylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol and mixtures thereof, are preferred.

Of these glycols, those having a smaller number of carbon atoms such as ethylene glycol, 1,4-butanediol and 1,6-hexanediol are preferred because they produce an aliphatic polyester having a high crystallinity and a high melting point. In particular, ethylene glycol and 1,4-butanediol are most preferred because they produce good results.

Examples of aliphatic dicarboxylic acids or anhydrides thereof which give aliphatic polyesters by reacting with glycols include aliphatic dicarboxylic acids. Among them, those having a straight-chain alkylene group with an even number of carbon atoms of 2, 4, 6, 8 and 10, such as succinic acid, adipic acid, suberic acid, sebacic acid, 1,10-decanedicarboxylic acid, succinic anhydride and mixtures thereof, are preferred. Of these dicarboxylic acids, those having a smaller number of carbon atoms, such as succinic acid, adipic acid and succinic anhydride, are preferred because they can give an aliphatic polyester having high crystallinity and high melting points. In particular, succinic acid, succinic anhydride and an acid mixture of succinic acid or succinic anhydride and other dicarboxylic acids, such as adipic acid, suberic acid, sebacic acid or 1,10-decanedicarboxylic acid are preferred.

In the system of an acid mixture containing two or more acid components, for example, succinic acid and other dicarboxylic acids, the mixing ratio of succinic acid is at least 70 mol%, preferably at least 90 mol%, and the mixing ratio of the other carboxylic acids is 30 mol% or less, preferably 10 mol% or less.

A combination of 1,4-butanediol and succinic acid or succinic anhydride and a combination of ethylene glycol and succinic acid or succinic anhydride are particularly preferred for the present invention since the combinations exhibit melting points close to that of polyethylene.

Third component

To these glycols and dicarboxylic acids, if necessary, there may be added as a third component an at least trifunctional component selected from the group consisting of trifunctional or tetrafunctional polyols, oxycarboxylic acids and polybasic carboxylic acids (or acid anhydrides thereof). The addition of this third component, which causes long chain branching, can impart desired properties to the polyester prepolymer in the molten state because the ratio of weight average molecular weight (MW) / number average molecular weight (Mn), i.e. the molecular weight distribution, increases with the increase in its molecular weight.

With respect to the amount of polyfunctional components added without the risk of gelation, a trifunctional component is added at 0.1 to 5 mol% or a tetrafunctional component is added at 0.1 to 3 mol% relative to 100 mol% of the total aliphatic dicarboxylic acid (or acid anhydride thereof) components.

Polyfunctional compounds

Examples of polyfunctional compounds as the third component include trifunctional or tetrafunctional polyols, oxycarboxylic acids and polybasic carboxylic acids

The trifunctional polyols typically include trimethylolpropane, glycerin or anhydrides thereof. The tetrafunctional polyols typically include pentaerythritol.

The trifunctional oxycarboxylic acid components are divided into two types:

(i) a moiety having two carboxyl groups and one hydroxyl group in one molecule, and

(ii) another component having one carboxyl group and two hydroxyl groups in one molecule.

Malic acid, which has two carboxyl groups and one hydroxyl group in one molecule, is practically applicable in view of commercial availability at low cost and is sufficient for the purpose of the present invention.

The tetrafunctional oxycarboxylic acid components are the following three component types:

(i) a moiety having three carboxyl groups and one hydroxyl group in one molecule;

(ii) another component having two carboxyl groups and two hydroxyl groups in one molecule; and

(iii) the remaining component having three hydroxyl groups and one carboxyl group in one molecule.

Any type may be used, although in view of commercial availability at low cost, citric acid and tartaric acid are practically applicable and sufficient for the purpose of the present invention.

As the trifunctional polybasic carboxylic acid (or acid anhydride thereof) component, for example, trimesic acid and propanetricarboxylic acid can be used, with trimesic anhydride being practically applicable for the purpose of the present invention.

As the tetrafunctional polybasic carboxylic acid (or anhydride thereof), various types of aliphatic compounds, cycloaliphatic compounds and aromatic compounds described in the relevant literature can be used. In view of commercial availability, for example, pyromellitic anhydride, benzophenonetetracarboxylic anhydride and cyclopentanetetracarboxylic anhydride are practically applicable and sufficient for the purpose of the present invention.

These glycols and dibasic acids are mainly composed of aliphatic groups, while small amounts of other components, for example aromatic groups, may be used simultaneously. These other components may be present in amounts up to 20% by weight, preferably up to 10% by weight. and particularly preferably up to 5 wt.% are added or copolymerized, since the use of these components reduces biodegradability.

The polyester prepolymer for aliphatic polyesters used in the present invention has hydroxyl groups at the terminals. In order to introduce the hydroxyl groups, it is necessary that glycols be used in some excess.

For producing the polyester prepolymer having a relatively high molecular weight, it is necessary to use deglycol reaction catalysts in the deglycol reaction following esterification. Examples of the deglycol reaction catalysts include titanium compounds such as acetoacetoyl type titanium chelate compounds and organic alkoxy titanium compounds. These titanium compounds may be used in combination. Examples of compounds used in combination include diacetoacetoxy titanium oxide (Nippon Chemical Industry Co. Ltd.; Nursem Titanium), tetraethoxy titanium, tetrapropoxy titanium and tetrabutoxy titanium. The amount of the titanium compound used is 0.001 to 1 part by weight, and preferably 0.01 to 0.1 part by weight, with respect to 100 parts by weight of the polyester prepolymer. These titanium compounds may be mixed before esterification, or they may be mixed immediately before the deglycol reaction. To the polyester prepolymer, which has a number average molecular weight of at least 5,000, preferably at least 10,000 and whose end groups are essentially hydroxyl groups, diisocyanate is added to increase the number average molecular weight.

Examples of a diisocyanate, but not limited to, include 2,4-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, diphenylmethane diisocyanate, 1,5-naphthylene diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate.

In particular, hexamethylene diisocyanate is preferably used in view of the color tone of the prepared resins and the reactivity at the time of blending the polyesters.

The addition amount of the diisocyanate is 5 parts by weight, and preferably 0.5 to 3 parts by weight, with respect to 100 parts by weight of the polyester prepolymer. Addition of less than 0.1 parts by weight causes insufficient coupling reaction, whereas more than 5 parts by weight causes gelation to occur.

The addition is preferably carried out when the polyester is in a uniformly molten state under easily stirred conditions. Although it is not impossible to add the coupling agent to the polyester prepolymer in a solid state, melt it and mix it by an extruder, adding the agents into a polyester preparation unit or adding them to the polyester prepolymer in a molten state (for example, in a kneader) is more practical.

An aliphatic polyester used in the present invention requires a specific melting characteristic in order to be molded into foamed articles using a foaming agent. Accordingly, the melt viscosity thereof at a temperature of 190°C and a shear rate of 100 sec-1 should have a value of

1.0 x 10³ - 1.0 x 10&sup6; poise, preferably

5.0 x 10³ - 5.0 x 10&sup5; Poise and more preferably

7.0 x - 1.0 x 10&sup5; poise.

A melt viscosity of less than 1.0 x 10³ poise results in too low a viscosity to form cells, whereas a melt viscosity of more than 1.0 x 10⁶ poise results in a viscosity too high for cell growth, so that a foam suitable for practical use cannot be obtained.

The melt viscosity at a shear rate of 100 sec-1 was calculated from a curve showing the relationship between the apparent viscosities and the shear rates determined with a capillary flow meter using a nozzle of 1.0 mm diameter and an L/D ratio of 10 at a resin temperature of 190°C.

The melting point of the aliphatic polyester used in the present invention must be 70°C to 190°C, preferably 70°C to 150°C, and more preferably 80°C to 135°C. A melting point lower than 70°C gives the foamed articles poor heat resistance so that they warp, whereas foaming is difficult to carry out with a melting point higher than 190°C.

To obtain a melting point of more than 70ºC, the polyester prepolymer must have a melting temperature of at least 60ºC.

In the case where urethane bonds are contained in the aliphatic polyester used in the present invention, the amount of urethane bonds is 0.03 to 3.0 wt%, preferably 0.05 to 2.0 wt%, and more preferably 0.1 to 1.0 wt%.

The amount of urethane bonds is measured by ¹³C-NMR, which shows a good correlation with the loading amount.

Less than 0.03 wt% urethane bonds have little effect on polymerization and result in poor molding properties, whereas more than 3 wt% causes gelation.

It goes without saying that when the above-mentioned aliphatic polyester is used to obtain the foamed articles according to the invention, if necessary, lubricants, waxes, colorants and crystallization promoters as well as antioxidants, heat stabilizers, UV absorbers and reinforcing fibers can be used simultaneously.

Antioxidants include hindered phenol antioxidants such as p-tert-butylhydroxytoluene and p-tert-butylhydroxyanisole, sulfur antioxidants such as distearyl thiodipropionate and dilauryl thiodipropionate;

Heat stabilizers include triphenyl phosphite, trilauryl phosphite and trisnonylphenyl phosphite;

UV absorbers include p-tert-butylphenyl salicylate, 2-hydroxy-4- methoxybenzophenone, 2-hydroxy-4-methoxy-2'-carboxybenzophenone and 2,4,5-trihydroxybutylphenone;

Lubricants include calcium stearate, zinc stearate, barium stearate and sodium palmitate;

Antistatic agents include N,N-bis(hydroxyethyl)alkylamine, alkylamine, alkylallylsulfonate and alkylsulfonate;

Flame retardants include hexabromocyclododecane, tris-(2,3-dichloropropyl) phosphate and pentabromophenylallylethane;

inorganic fillers include calcium carbonate, silicon dioxide, titanium oxide, talc, mica, barium sulfate and alumina; crystallization promoters include polyethylene terephthalate and poly-trans-cyclohexanedimethanol terephthalate;

Reinforcing fibers include inorganic fibers such as glass fibers, carbon fibers, boron fibers, silicon carbide fibers, Graphite fibers, aluminum oxide fibers and amorphous fibers, and organic fibers such as aramid fibers.

Foam Foaming with volatile foaming agents

Examples of the volatile foaming agents used in the present invention include aliphatic hydrocarbons such as propane, butane, pentane, isobutane, neopentane, isopentane, hexane and butadiene, and further halogenated hydrocarbons such as methyl chloride, methylene chloride, dichlorofluoromethane, chlorotrifluoromethane, dichlorodifluoromethane, chlorodifluoromethane and trichlorofluoromethane.

In order to produce a foam using such an aliphatic polyester and the above-mentioned volatile foaming agent as main components, a material containing the aliphatic polyester as a main component is melted and kneaded by an extruder while adding the volatile foaming agent thereto, or the granules are impregnated with the volatile foaming agent (pre-expanded beads) and then extruded into the atmosphere by the extruder. In this way, it is possible to produce a foam having an expansion ratio of 1.02 to 50.

An expansion ratio less than 1.02 does not provide the required properties for a foam, whereas an expansion ratio greater than that causes the cells to become continuous, resulting in excessive irregularities of the surface so that a foam suitable for practical use cannot be obtained.

The kneading temperature must not be lower than the melting point of the aliphatic polyester. Kneading temperature is preferably 100°C to 270°C, and more preferably 100°C to 250°C. A kneading temperature lower than 100°C results in too high a viscosity so that kneading is difficult to carry out, whereas a kneading temperature higher than 270°C results in degeneration of the resin.

Furthermore, the expandable beads can be pre-expanded and foamed in a mold to produce foamed articles of the desired shape.

Foamed articles generate heat of 7,000 kcal/kg or less. This can minimize the erosion of a combustion furnace. The heat of combustion is measured using the calorimeter method of JIS M-8814.

Foaming with a heat-decomposable agent

The heat-decomposable foaming agent used in the present invention is selected from various organic and inorganic agents. Examples of the organic foaming agent include: azodicarbonamide, N,N'-dinitrosopentamethylenetetramine and P-P'-oxybisbenzenesulfonylhydrazide. Examples of the inorganic foaming agent include: sodium hydrogen carbonate, ammonium carbonate, ammonium hydrogen carbonate and calcium azide.

The expansion ratio is normally 1.02 to 30.00.

A foam consisting of such an aliphatic polyester and having an expansion ratio of 1.02 to 30.00 can be prepared by mixing 100 parts by weight of the aliphatic polyester with 0.5 to 30 parts by weight of the heat-decomposable foaming agent and heating the mixture during or after mixing to a temperature not lower than the decomposition temperature of the foaming agent.

An expansion ratio lower than 1.02 does not provide the required properties for a foam, whereas an expansion ratio higher than 30 results in the bubbles becoming continuous, causing excessive irregularities of the surface, so that a foam suitable for practical use cannot be obtained. The kneading for dispersing the foaming agent can be carried out by any known method. For this purpose, a kneader, a roll or an extruder is preferably used. It is necessary for the kneading temperature to be not lower than the melting point of the aliphatic polyester. The kneading temperature is preferably 100°C to 270°C, and more preferably 150°C to 250°C. When the kneading is carried out at a temperature not lower than the decomposition temperature of the foaming agent, it is necessary to carry out the kneading under pressure. A kneading temperature lower than 100ºC results in too high a viscosity, making kneading difficult to perform, whereas a kneading temperature higher than 270ºC results in deterioration of the resin.

Heating to decompose the foaming agent can be carried out using a heating device such as an extruder, a press, a blast furnace, an infrared heating furnace or a molten salt bath furnace.

In particular, when heated under pressure by means of an extruder, a press or the like, a foam can be obtained by reducing the pressure to normal pressure.

Foaming with simultaneous use of a cross-linking agent

The crosslinking agent used in the present invention is an inorganic peroxide whose decomposition temperature is not lower than the melting point of the resin, for example dicumyl peroxide, cumene hydroperoxide, methyl ethyl ketone peroxide or t-butyl peroxyisopropyl carbonate.

A foam consisting of such an aliphatic polyester and having an expansion ratio of 2 or more can be prepared by mixing 100 parts by weight of the aliphatic polyester with 0.5 to 10 parts by weight of the heat-decomposable foaming agent and 0.01 to 30 parts by weight of the crosslinking agent and heating the mixture during or after mixing to a temperature not lower than the decomposition temperature of the decomposing agent and the foaming agent.

The expansion ratio is not less than 1.02 and not greater than 50.

The kneading for dispersing the foaming agent can be carried out by any known method. The kneading temperature is required to be not lower than the melting point of the aliphatic polyester and not higher than the decomposition temperature of the crosslinking agent. The kneading temperature is preferably 150°C to 270°C, and more preferably 100°C to 250°C.

Furthermore, it is also possible in the present invention to effect cross-linking by means of ionizing radiation, such as electron beams, beta rays or gamma rays.

If crosslinking is effected by radiation, 1 Mrad to 50 Mrad of radiation is applied in an N₂ atmosphere so as to avoid degeneration of the resin. An amount of radiation less than 1 Mrad does not cause crosslinking, whereas an amount of radiation more than 50 Mrad causes significant degeneration of the resin. The thickness of the resin is preferably 1 mm or less, and more preferably 0.5 mm or less. A thickness of more than 1 mm does not allow the radiation to reach the inner region, which causes the inner bubbles to become bulky and uneven at the time of expansion.

Expandable beads, pre-expansion, foaming in a mold

The average radius of the aliphatic polyester particles for forming the expandable beads of this invention is preferably 0.05 to 10 mm, more preferably 0.2 to 6 mm, and most preferably 0.3 to 3 mm.

A particle diameter of less than 0.05 mm results in the mold being filled too densely with the beads, so that the inner portion thereof is insufficiently heated. A particle diameter of more than 10 mm results in the mold being insufficiently filled with the beads, resulting in deterioration of the appearance.

It is necessary for the volatile foaming agent used in the present invention to have a boiling point not higher than the melting point of the aliphatic polyester and which does not cause the aliphatic polyester to dissolve or only causes it to float slightly.

If the volatile foaming agent has a boiling point higher than the melting point of the resin or causes it to decompose, evaporation occurs after the resin is completely melted or dissolved, so that it is impossible to obtain uniform fine cells. The expandable beads can be prepared by suspending the aliphatic polyester particles in water, forcing the volatile foaming agent into the water while stirring and heating the mixture.

When the expandable beads of the present invention are molded after being subjected to pre-expansion with the expansion ratio of 5 or more, they enable foam products with a high expansion ratio to be produced.

After the expandable beads of the present invention are expanded by steam heating at an expansion ratio of 5 or more, the expanded particles are dried and deposited, the thus obtained particles are filled into a mold and further heated with steam, a foamed product having a high expansion ratio and having exactly the same shape as the mold can be obtained.

The mold used must have an internal cavity of the desired shape provided with a large number of holes or slits through which a heating medium such as steam can be sprayed. In short, the mold should be "closed but not airtight." The foam product is useful as a material for food containers such as dried noodle soup, as an insulating material for use in transporting televisions, stereos, etc., as a material for fish boxes, thermal insulation boxes, and refrigeration boxes.

The term "insulated box" in this patent specification is used to refer to a box used for transporting and storing food, drinks or medicines, especially fish, meat and vegetables in a chilled or frozen state. Consequently, the box is made of a material with a high thermal insulation effect to prevent the temperature inside the box from rising easily. The insulated box consists of a box body alone or of a box body and a Cover that closes an open upper area of the box body. One of the most important requirements for the insulated box is that the box material has a high heat insulating effect. The insulated box in this patent description should also be understood as a generic term for so-called "warming boxes" that work to prevent the temperature of the contents from falling.

The insulation box according to the present invention can be easily degraded by microorganisms and exhibits superior heat insulation effects while having sufficient mechanical strength for practical use.

The thermal conductivity as a practical characteristic of the insulation box is measured in accordance with JIS A-1412 and should be 0.1 (kcal/mhºC) or less, preferably 0.08 (kcal/mhºC) or less, and suitably 0.06 (kcal/mhºC). A thermal conductivity exceeding 0.1 (kcal/mhºC) is not preferable because such a thermal conductivity value improperly affects the important function of the insulation box, for example, preserving the freshness of fish.

The term "damping medium" in this patent specification is a material that reduces an external impact force to prevent damage or breakage of objects such as electrical equipment, e.g. televisions and radios, precision instruments, e.g. computers and watches, optical instruments, e.g. glasses, microscopes, and other objects such as glassware and ceramics. The damping medium encloses such objects per se or is arranged between the object and a packaging container such as a cardboard box.

The damping medium according to the present invention exhibits a combustion calorific value smaller than that of polystyrene. It can be easily degraded by microorganisms and exhibits superior creep rupture behavior while having sufficient mechanical strength for practical use.

The creep rupture behaviour as a practical property of the damping medium is expressed in values of gradual compression deformation measured in accordance with the JIS K-6767 standard after 24 hours with a load of 0.1 kg/cm².

The gradual compression deformation exhibited by the damping medium of the present invention should be 15% or less, preferably 12% or less, and most suitably 10% or less. If the gradual deformation exceeds 15%, the damping medium will be undesirably deformed too much under the action of an external force, with only a small proportion being recovered. This means that the rate of breakage of electronic devices during transportation would increase.

EXAMPLES

Methods of the present invention are illustrated with reference to the following examples, but it is not intended to limit the invention only to these.

example 1

A 700-liter reactor is purged with nitrogen, after which it is charged with 183 kg of 1,4-butanediol and 224 kg of succinic acid. After the temperature was raised under a nitrogen flow, an esterification was carried out by means of a dehydration condensation for 3.5 hours at 192ºC to 220ºC and after the nitrogen loading was released, the reaction was continued for an additional 3.5 hours under a reduced pressure of 20 to 2 mmHg. A collected sample had an acid value of 9.2 mg/g, a number average molecular weight (Mn) of 5,160 and a weight average molecular weight (Mw) of 10,670. Then, 34 g of tetraisopropoxytitanium, a catalyst, was added at normal pressure under a nitrogen stream. The temperature was increased to carry out a deglycol reaction at temperatures of 215ºC to 220ºC and under reduced pressures of 15 to 0.2 mmHg for 5.5 hours. A collected sample had a number average molecular weight (Mn) of 16,800 and a mass average molecular weight (Mw) of 43,600. The yield of the resulting polyester prepolymer (A1) was 339 kg without condensation water.

5.42 kg of hexamethylene diisocyanate was added to the reactor containing 339 kg of the polyester prepolymer (A1) to conduct a coupling reaction at 180°C to 200°C for 1 hour. The viscosity increased rapidly, but no gelation occurred. Then, 1.70 kg of Irganox 1010 (Ciba-Geigy) as an antioxidant and 1.70 kg of calcium stearate as a lubricant were added, and the mixture was further stirred for 30 minutes. The resulting product was extruded into water and cut into pellets by a cutting knife. The aliphatic polyester (B1) obtained after drying in vacuum at 90°C for 6 hours gave a yield of 300 kg.

The obtained polyester (B1) was a light ivory-colored white wax-like crystal and had a melting point of 110°C, a number average molecular weight (Mn) of 35,500, a weight average molecular weight (Mw) of 170,000, an MFR (190°C) of 1.0 g per 10 minutes, a viscosity of 230 poise in a 10% ortho-chlorophenol solution and a melt viscosity of 1.5 x 10⁴ poise at a temperature of 190°C at a shear rate of 100 sec-1 The average molecular weight was measured with a Shodex GPC System-11 (Showa Denko gel permeation chromatography) using an HFIPA solution containing 5 mmol of CF3COONA (concentration of 0.1 wt%) as a medium. A calibration curve was drawn using a PMMA standard sample (Shodex Standard M-75, Showa Denko).

100 parts by weight of the polyester (B1) and 0.5 parts by weight of lime were kneaded by means of a tandem extruder composed of a 40 mm= extruder and a 50 mm= extruder at a temperature of 160°C. Then, 24 parts by weight of dichlorodifluoromethane per 100 parts by weight of the resin mixture was injected into the extruder halfway along the barrel of the extruder and extruded into the atmosphere through a circular die having a bore diameter of 60 mm= and an outlet gap of 0.6 mm at a die temperature of 110°C, thereby producing a foam.

In this way, a foam with an expansion ratio of 40 and a cell diameter of 0.4 to 0.6 mm∅ was obtained.

The foam was buried in the ground and after remaining there for 5 months it had decomposed into small pieces.

Example 2

A 700-liter reactor was purged with nitrogen, after which it was charged with 177 kg of 1,4-butanediol, 119 kg of succinic acid and 25 kg of adipic acid. After the temperature was increased under a nitrogen flow, esterification was carried out by means of dehydration condensation for 3.5 hours at 190ºC to 210ºC and, after the nitrogen loading had decreased, for a further 3.5 hours under a reduced pressure of 20 to 2 mmHg. A collected sample had an acid value of 9.6 mg/g, a number average molecular weight (Mn) of 6,100 and a weight average molecular weight (Mw) of 12,200. Then, 20 g of tetraisopropoxytitanium, a catalyst, was added at normal pressure under a nitrogen stream. The temperature was raised to carry out a deglycol reaction at temperatures of 210°C to 220°C and under a reduced pressure of 15 to 0.2 mmHg for 7.5 hours. A collected sample had a number average molecular weight (Mn) of 17,300 and a weight average molecular weight (Mw) of 46,400. The yield of the resulting polyester prepolymer (A2) was 337 kg without condensation water.

4.66 kg of hexamethylene diisocyanate was added to the reactor containing 337 kg of the polyester prepolymer (A2) to conduct a coupling reaction at 180°C to 200°C for 1 hour. The viscosity increased rapidly, but no creep occurred. Then, 1.70 kg of Irganox 1010 (Ciba-Geigy) as an antioxidant and 1.70 kg of calcium stearate as a lubricant were added, and the mixture was further stirred for 30 minutes. The resulting reaction product was extruded into water with an extruder and cut into pellets with a cutting knife. The aliphatic polyester (B2) obtained after drying in vacuum at 90°C for 6 hours gave a yield of 300 kg.

The obtained polyester (B2) was a light ivory-colored, white, wax-like crystal and had a melting point of 113°C, a number average molecular weight (Mn) of 36,000, a weight average molecular weight (Mw) of 200,900, an MFR (190°C) of 0.52 g per 10 minutes, a viscosity of 680 poise in a 10% ortho-chlorophenol solution and a melt viscosity of 2.2 x 10⁴ poise at a temperature of 190°C at a shear rate of 100 sec⁻¹.

100 parts by weight of the polyester (B2) and 0.5 parts by weight of talc were kneaded at a temperature of 160°C by means of a tandem extruder composed of a 40 mmØ extruder and a 50Ø-5 extruder. Then, 18 parts by weight of dichlorodifluoromethane per 100 parts by weight of the resin mixture was injected into the extruder halfway along the barrel of the extruder and extruded into the atmosphere through a circular die having a bore diameter of 60 mmØ and an outlet gap of 0.6 mm at a die temperature of 110°C, thereby producing a foam.

In this way, a foam with an expansion ratio of 30 and a cell diameter of 0.4 to 0.6 mm∅ was obtained.

The foam was buried in the ground and after remaining there for 5 months it had decomposed into small pieces.

Example 3

A 700-liter reactor was purged with nitrogen, followed by charging with 200 kg of 1,4-butanediol, 250 kg of succinic acid, and 2.8 kg of trimethylolpropane. After the temperature was raised under a nitrogen stream, esterification was carried out by dehydration condensation for 4.5 hours at 192°C to 220°C and, after the nitrogen loading was decreased, for a further 5.5 hours under a reduced pressure of 20 to 2 mmHg. A collected sample had an acid value of 10.4 mg/g, a number average molecular weight (Mn) of 4,900, and a weight average molecular weight (Mw) of 10,000. Then, 37 g of tetraisopropoxytitanium, a catalyst, was added at normal pressure under a nitrogen stream. The temperature was increased to temperatures of 210ºC to 220ºC and under a reduced pressure of 15 to 1.0 mmHg for 8 hours to conduct a deglycol reaction. A collected sample had a number average molecular weight (Mn) of 16,900 and a weight average molecular weight (Mw) of 90,300 (Mw/Mn=5.4). The yield of the resulting polyester prepolymer (A3) was 367 kg excluding the condensation water of 76 kg.

3.67 kg of hexamethylene diisocyanate was added to the reactor containing 367 kg of the polyester prepolymer (A3) to conduct a coupling reaction at 160°C to 200°C for 1 hour. The viscosity increased rapidly, but no creep occurred. Then, 367 g of Irganox 1010 (Ciba-Geigy) as an antioxidant and 367 g of calcium stearate as a lubricant were added, and the mixture was further stirred for 30 minutes. The resulting reaction product was extruded into water with an extruder and cut into pellets with a cutting knife. The aliphatic polyester (B3) obtained after drying in vacuum at 90°C for 6 hours gave a yield of 350 kg.

The obtained polyester (B3) was a light ivory-colored, white, wax-like crystal and had a melting point of 110°C, a number average molecular weight (Mn) of 17,900, a weight average molecular weight (Mw) of 161,500 (Mw/Mn-9.0), an MFR (190°C) of 0.21 g/10 minutes, and a melt viscosity of 2.0 x 10⁴ poise at a temperature of 190°C at a shear rate of 100 sec⁻¹.

The polyester (B3) was prepared and foamed as in Example 1 to produce a foamed article having an expansion ratio of 40 and a cell diameter of 0.3 to 0.5 mm∅.

The foam was buried in the ground and after remaining there for 5 months it had decomposed into small pieces.

Comparison example 1

The polyester (A1) was molded under the same conditions as in Example 1 without subsequently obtaining bubbles having a stable composition, so that a foam capable of being put to practical use could not be obtained.

Comparison example 2

An attempt was made to mold a commercial polyester (Mn=40 0001 polyethylene terephthalate) without subsequently obtaining a foam. A biodegradability test was then carried out using a polyethylene terephthalate film, which is a condensation product obtained from 13 µmol of a commercial terephthalic acid and ethylene glycol. The film did not show any significant changes in its appearance and consequently did not show any biodegradability.

Example 4

100 parts by weight of the polyester (B1) obtained in Example 1, 4 parts by weight of azodicarbonamide, 0.5 part by weight of zinc oxide and 0.5 part by weight of talc were kneaded at a temperature of 180°C with a tandem extruder composed of a 40 mmo extruder and a 50 mm extruder and then extruded into the atmosphere through a circular die having a width of 300 mm at a die temperature of 110°C, thereby producing a foam having an expansion ratio of 1.8 and a cell diameter of 0.4 to 0.6 mm=.

The foam thus obtained was buried in the ground, where, after remaining there for 5 months, the foam had decomposed to such an extent that its strength had decreased that it was of no practical use.

Example 5

100 parts by weight of the polyester (B2), 16 parts by weight of azodicarbonamide, 0.5 part by weight of zinc oxide and 0.5 part by weight of talc were kneaded at a temperature of 180°C with a tandem extruder composed of a 40 mm= extruder and a 50 mm= extruder, and then extruded into the atmosphere through a circular die having a bore diameter of 60 mm and an outlet gap of 0.6 mm at a die temperature of 90°C, thereby producing a foam having an expansion ratio of 10 and a cell diameter of 0.4 to 0.6 mm=.

The foam thus obtained was buried in the ground, where, after remaining there for 5 months, the foam decomposed to such an extent that its strength was reduced that it was of no practical use.

Example 6

A 700-liter reactor was purged with nitrogen, followed by charging with 145 kg of ethylene glycol, 251 kg of succinic acid and 4.1 kg of citric acid. After the temperature was raised under a nitrogen stream, esterification was carried out by dehydration condensation for 3.5 hours at 190ºC to 210ºC and, after the nitrogen loading was reduced, for a further 5.5 hours under a reduced pressure of 20 to 2 mmHg. A collected sample had an acid value of 8.8 mg/g, a number average molecular weight (Mn) of 6,800 and a weight average molecular weight (Mw) of 13,500. Then, 20 g of tetraisopropoxytitanium, a catalyst was added at normal pressure under a nitrogen stream. The temperature was raised to conduct a deglycol reaction at a temperature of 210°C to 220°C and under a reduced pressure of 15 to 0.2 mmHg for 4.5 hours. A collected sample had a number average molecular weight (Mn) of 33,400 and a weight average molecular weight (Mw) of 137,000. The yield of the resulting polyester (A4) was 323 kg without condensation water.

3.23 kg of hexamethylene diisocyanate was added to the reactor containing 323 kg of the polyester (A4) to conduct a coupling reaction at 18°C to 200°C for 1 hour. The viscosity increased rapidly, but no gelation occurred. Then, 1.62 kg of Irganox 1010 (Ciba-Geigy) as an antioxidant and 1.62 kg of calcium stearate as a lubricant were added, and the mixture was further stirred for 30 minutes. The resulting reaction product was extruded into water with an extruder and cut into pellets with a cutting knife. The polyester (B4) obtained after drying in vacuum at 90°C for 6 hours gave a yield of 300 kg.

The obtained polyester (B4) was a light ivory-white wax-like crystal and had a melting point of 96°C, a number average molecular weight (Mn) of 54,000, a weight average molecular weight (Mw) of 324,000, an MFR (190°C) of 1.1 minutes, a viscosity of 96 poise in a 10% ortho-chlorophenol solution and a melt viscosity of 1.6 x 10⁴ poise at a temperature of 190°C at a shear rate of 100 sec⁻¹.

100 parts by weight of the polyester (B4), 5 parts by weight of azodicarbonamide and 0.5 part by weight of zinc oxide were kneaded to a sufficient extent with a roller maintained at 110°C to obtain a tabular sheet having a size of 2 mm (thickness) x 200 mm x 200 mm, and the sheet was heated by a hot blast stove in an atmosphere at 160ºC for 6 minutes, thereby producing a foam. In this way, a foam having an expansion ratio of 8 and a cell diameter of 0.4 to 0.6 mm∅ was produced.

The foam was buried in the soil and after remaining there for 5 months, the same results as in Example 4 were obtained.

Example 7

The polyester (B3) used in Example 3 was prepared and extruded as in Example 4 to produce a molded article having an expansion ratio of 2 and a cell diameter of 0.4 to 0.6 mm∅.

The foam thus obtained was buried in the ground, where the foam, after being left there for 5 months, deformed significantly and showed signs of degradation.

Comparison example 3

The polyester (A1) was molded under the same conditions as in Example 4. The molded article obtained showed excessive irregularities of the surfaces and bulky, uneven cells (having a diameter of 1 mm and more), so that a foam capable of being put to practical use could not be obtained.

Comparison example 4

An attempt was made to mold a commercial polyester (Mn=40,000 polyethylene terephthalate consisting of terephthalic acid and ethylene glycol) under the same conditions as in Example 4, without achieving the following to obtain a foam. Subsequently, a biodegradability test was carried out using a polyethylene terephthalate film, which is a condensation product obtained from 13 u*mol of a commercial terephthalic acid and ethylene glycol. In the test, the film showed no significant change in its appearance and consequently did not show biodegradability.

Example 8

100 parts by weight of the polyester (B1) obtained in Example 1, 5 parts by weight of azodicarbonamide and 0.8 part by weight of dicumyl peroxide were kneaded with an extruder at a temperature of 120°C and then extruded through a T-die, thereby preparing a sheet having a width of 300 mm and a thickness of 2 mm. Further, this sheet was allowed to stand for 5 minutes in a hot blast stove, thereby preparing a foam.

In this way, a foam with an expansion ratio of 10 and a cell diameter of 0.4 to 0.6 mm∅ was obtained.

The foam thus obtained was buried in the ground, and after being left there for 5 months, the foam decomposed to such an extent that it showed noticeable deformation.

Example 9

100 parts by weight of the polyester (B2) obtained in Example 2 and 5 parts by weight of azodicarbonamide were kneaded with an extruder at a temperature of 150°C and then extruded through a T-die, thereby producing a sheet having a width of 300 mm and a thickness of 0.5 mm. Both sides of this sheet were irradiated with an electron beam of 10 Mrad in an N₂ gas atmosphere using an electron beam irradiation machine. (Curetron (EBC-200-AA2), surface irradiation type electron irradiation machine, manufactured by Nisshin High Voltage Kabushiki Kaisha) and were left in a hot blast stove set at 200ºC for 5 minutes, thereby producing a foam.

In this way, a foam with an expansion ratio of 10 and a cell diameter of 0.3 to 0.5 mm∅ was obtained.

After the foam had been buried in the ground and left there for 5 months, the foam thus obtained decomposed and showed such a degree of reduction in its strength that it was of no practical use.

Example 10

From the polyester (B3) used in Example 3, foamed articles having an expansion ratio of 10 and a cell diameter of 0.4 to 0.6 mm∅ were obtained.

The foam thus obtained was buried in the ground, and after remaining there for 5 months, the foam decomposed to such an extent that it showed noticeable deformation.

Comparison example 5

The polyester (A1) obtained in Example 1 was molded under the same conditions as in Example 8. The molded article obtained showed excessive surface irregularities and bulky, non-uniform bubbles (having a diameter of 1 mm or more), so that a foam capable of practical use could not be obtained.

Example 11

2,200 g of pure water was charged into a 5.6-liter autoclave equipped with a stirrer. 2,200 g of the polyester (B1) particles obtained in Example 1 having a size in the range of 28 mesh to 35 mesh in Tyler standard, 7 g of magnesium oxide and 0.22 g of dilauryl thiodipropionate were added to the water. Then, with stirring, 44 g of propane and 186 g of pentane were forced into the autoclave and after raising the temperature to 100°C, the mixture was left to stand for 1.5 hours. Next, after lowering the temperature within the above system to 30°C, expandable beads of the polyester (B1) were extracted from the system. The thus obtained expandable beads were expanded at an expansion ratio of 10, thereby preparing pre-expanded beads. After drying and ripening the pre-expanded beads, a bowl-shaped "closed but not airtight" mold having a wall thickness of 3.0 mm was filled with these pre-expanded beads, and heat molding was carried out by steam. When the expandable beads of polyester (B1) were expanded at an expansion ratio of 50, the cell size distribution range was 0.08 to 0.30 mm. The heat of combustion was 5,720 kcal/kg.

After being buried in the ground for 5 months, the expandable beads had decomposed so that they had lost their bead shape, not to mention their expandability.

Example 12

2,200 g of pure water were placed in a 5.6-liter autoclave equipped with a stirrer. 2,200 g of the polyester (B2) particles obtained in Example 2 with a size range of 9 mesh to 12 mesh in Tyler standard and 0.6 g of dodecyl sodium benzenesulfonate were added to the water. was added. Then, with stirring, 44 g of propane and 186 g of pentane were injected into the autoclave, and after raising the temperature to 100°C, the mixture was allowed to stand for 6 hours. Next, after lowering the temperature within the above system to 30°C, expandable beads of polyester (B3) were obtained and dried by dehydration. The obtained expandable beads were kept at 10°C for 2 weeks and then expanded by steam at an expansion ratio of 10, thereby preparing pre-expanded beads. After allowing these pre-expanded beads to stand for 24 hours, a bowl-shaped mold having a size of 300 mm x 400 mm x 100 mm was filled with these pre-expanded beads, and heat molding was carried out by steam. When the expandable beads of polyester (B3) were expanded at an expansion ratio of 50, the cell size distribution range was 0.1 to 0.35 mm. The average cell size was 0.2 mm.

After these expandable beads were buried in the soil for 5 months, the same results as those in Example 11 were obtained.

Example 13

2,200 g of pure water was put into a 5.6-liter autoclave equipped with a stirrer, and 2,200 g of the polyester (B3) particles obtained in Example 3 having a size range of 9 mesh to 12 mesh in Tyler standard and 0.6 g of dodecyl sodium benzenesulfonate were added to the water. Then, with stirring, 44 g of propane and 186 g of pentane were forced into the autoclave, and after raising the temperature to 100°C, the mixture was reacted for 6 hours. Next, after lowering the temperature within the above system to 30°C, expandable beads of polyester 12 were obtained and prepared by dehydration. The obtained expandable beads were kept at 10°C for 2 weeks and then expanded by steam at an expansion ratio of 10, thereby preparing pre-expanded beads. After allowing these pre-expanded beads to stand for 24 hours, a bowl-shaped mold having a size of 300 mm x 400 mm x 100 mm was filled with these pre-expanded beads, and thermoforming was carried out by steam. When the expandable beads of polyester 12 were expanded at an expansion ratio of 50, the cell size distribution range was 0.7 to 0.30 mm. The average cell size was 0.2 mm.

After the expandable beads were buried in the soil for 5 months, the same results as those in Example 11 were obtained.

Comparison example 6

The polyester (Al) obtained in Example 1 was molded under the same conditions as in Example 11 without subsequently maintaining the cell diameter. Thus, a foam capable of being put to practical use could not be obtained.

Example 14

An autoclave with an internal volume of 5.6 liters equipped with a stirrer was charged with 2,200 g of purified water, 2,200 g of pellets of polyester (B1) obtained in Example 1 with a size range of 28 mesh to 35 mesh in Tyler standard units, 7 g of magnesium oxide and 0.22 g of dilauryl thiodipropionate. Then, while stirring the mixture, 44 g of propane and 186 g of pentane were added under pressure, the mixture was heated to 100°C and kept at this temperature for 1.5 hours. The temperature of the system described above was then cooled to 30°C, and the reaction product was taken out of the system to obtain foamable pellets. The obtained pellets were pre-expanded at a foaming ratio of 10 by heating them with steam, thereby obtaining pre-expanded beads. The pre-expanded beads were then loaded into a rectangular mold of 3 mm thick and heated with steam so that an insulating box of 80 cm in length, 30 cm in width and 30 cm in height was obtained. The insulating box 1 thus formed was foamed at a molding ratio of 50 and had a main wall thickness of 1.5 cm. The thermal conductivity of this insulating box was measured in accordance with JIS A-1412 and was 0.02 (kcal/mhºC)

A test piece of this cooler was obtained and buried in the ground for 5 months. The test piece was degraded and so altered that its original shape could not be recognized.

Example 15

As in Example 14, polyester pellets (B2) obtained in Example 2 were pre-expanded at a molding ratio of 10 and subsequently foamed at a foaming ratio of 50, thus forming an insulating box. The thermal conductivity of this insulating box was measured in the same manner as in Example 14. The thermal conductivity was 0.03 (kcal/mhºC). The heat of combustion was 5,850 kcal/kg.

The resulting cooler was buried and left in the ground for 5 months. It was confirmed that the box changed into a box-shaped substance that had no practical strength.

Example 16

As in Example 14, polyester pellets (B4) obtained in Example 4 were pre-expanded at a foaming ratio of 10 and subsequently foamed at a foaming ratio of 50, thus forming an insulating box. The thermal conductivity of this insulating box was measured in the same manner as in Example 14. The thermal conductivity was 0.05 (kcal/mhºC). The heat of combustion was 5,850 kcal/kg.

The resulting cooler was buried and left in the ground for 5 months. The result was similar to that of Example 15.

Example 17

As in Example 14, polyester pellets (B3) obtained in Example 3 were pre-expanded at a foaming ratio of 10 and subsequently foamed at a foaming ratio of 50, thus forming an insulating box. The thermal conductivity of this insulating box was measured in the same manner as in Example 14. The thermal conductivity was 0.04 (kcal/mhºC). The heat of combustion was 5,750 kcal/kg.

The resulting cooler was buried and left in the ground for 5 months. The result was similar to that of Example 14.

Comparison example 7

An article was molded from the polyester (A1) under the same conditions as in Example 14. The article had a large surface roughness and coarse voids of irregular size (> mm). Consequently, the obtained foamed article was practically unusable.

Example 18

An autoclave having an internal volume of 5.6 liters and equipped with a stirrer was charged with 2,200 g of purified water, 2,200 g of pellets of polyester (B1) obtained in Example 1 having a size range of 28 mesh to 35 mesh in Tyler standard units, 7 g of magnesium oxide and 0.22 g of dilauryl thiodipropionate. Then, while stirring the mixture, 44 g of propane and 186 g of pentane were added under pressure and the mixture was heated to 100°C and kept at that temperature for 1.5 hours. The temperature of the system described above was then cooled to 30°C and the reaction product was taken out of the system, thereby obtaining expandable pellets of polyester (B1). The thus-obtained expandable pellets were pre-expanded at a foaming ratio of 10 by heating them with steam, thereby obtaining pre-expanded pellets. The pre-expanded expandable pellets were then placed in a rectangular mold to form a transport box for a cassette recorder, and heated with steam to obtain a damping box of 20 cm in length, 15 cm in width and 8 cm in height. The thus-formed damping box was foamed at a foaming ratio of 50 and had a main wall thickness of 1.5 cm. The gradual compression set of this box was measured in accordance with JIS K-6767 and was found to be 13%, while the heat of combustion was 5,750 kcal/kg.

A tabular test piece obtained from this damping box was buried in the ground for 5 months. The test piece was changed into a tabular part which had no practical strength and consequently showed decomposition.

Example 19

As in Example 18, a polyester (B2) obtained in Example 2 was pre-expanded at a foaming ratio of 10 and subsequently foamed at a foaming ratio of 50 to form a sheet-like member of a damping medium. The gradual compression set of this box was measured in accordance with JIS K-6767 and was 12%, while the heat of combustion was 5,850 kcal/kg.

A tabular test piece obtained from this damping box was buried in the ground for 5 months. The test piece was decomposed to such an extent that it could not maintain its original shape.

Example 20

As in Example 18, a polyester (B4) obtained in Example 6 was pre-expanded at a foaming ratio of 10 and subsequently foamed at a foaming ratio of 50 to form a sheet-like member of a damping medium. The gradual compression set of the sheet-like member thus obtained was measured by the same method as before and was found to be 11%.

The resulting insulation box was buried in the ground for 5 months. The result was similar to that of Example 18.

Example 21

As in Example 18, a polyester (B3) obtained in Example 3 was pre-expanded at a foaming ratio of 10 and subsequently foamed at a foaming ratio of 50 to form a sheet-like member of a damping medium. The gradual compression set of the sheet-like member thus obtained was 13% as measured by the same method as above.

The resulting insulation box was buried and left in the ground for 5 months. The result was similar to that of Example 18.

Comparison example 8

An article was formed from the polyester (A1) obtained in Example 1 under the same conditions as in Example 18. The article had a large surface roughness as well as large voids of irregular size (> 1 mm). Consequently, the obtained foamed article was not suitable for practical use.

Claims (18)

1. A method for producing polyester foamed articles comprising the following steps: a) Adding a foaming agent to an aliphatic polyester having a melt viscosity of 1.0 x to 1.0 x 10⁶ poise at a temperature of 190°C and a shear rate of 100 sec-1 and having a melting point of 70°C to 190°C, the aliphatic polyester having a number average molecular weight of at least 10,000 and containing 0.03 to 3.0% by weight urethane bonds and which is obtained by reacting 0.1 to 5 parts by weight of diisocyanate with 100 parts by weight of a saturated aliphatic polyester prepolymer having essentially hydroxyl groups at its ends, having a number average molecular weight of at least 5,000 and a melting point of at least 60°C; then b) Heating and foaming. 2. A process for producing polyester foamed articles according to claim 1, wherein the foaming agent is a volatile foaming agent. 3. A process for producing polyester foamed articles according to claim 1, wherein the foaming agent is a heat-decomposable foaming agent. 4. A process for producing polyester foamed articles according to claim 1, wherein a heat-decomposable foaming agent and a crosslinking agent are added to the polyester. 5. A method for producing polyester foamed articles according to any one of the preceding claims 1 to 4, wherein the expansion ratio is 1.02 to 50. 6. A process for producing polyester foamed articles according to any one of the preceding claims 1 to 5, wherein the aliphatic polyester contains 0.05 to 2.0 wt.% urethane bonds. 7. A process for producing polyester foamed articles according to claim 6, wherein the aliphatic polyester contains 0.11 to 1.0 wt% urethane bonds. 8. A process for producing polyester foamed articles according to any one of the preceding claims, wherein the aliphatic polyester is obtained by reacting 0.1 to 5 parts by weight of diisocyanate with 100 parts by weight of an aliphatic polyester prepolymer. 9. A method for producing polyester foamed articles according to any preceding claim, wherein the aliphatic polyester has a repeating chain structure in which the polyester prepolymer having a number average molecular weight (Mn) of 5,000 or more consists of an aliphatic glycol and an aliphatic dicarboxylic acid and is linked by the urethane bond. 10. A process for producing polyester foamed articles according to any one of the preceding claims, wherein the aliphatic polyester has a repeating chain structure in which the polyester prepolymer having a number average molecular weight (Mn) of 5,000 or more is obtained by reacting an aliphatic glycol, an aliphatic dicarboxylic acid and a third component, wherein the third component contains at least a polyfunctional component selected from the group consisting of trifunctional or tetrafunctional polyols, oxycarboxylic acids, and polybasic carboxylic acids or acid anhydrides thereof, and wherein the polyester prepolymer is linked by the urethane bond. 11. A method for producing polyester foamed articles according to claim 9 or 10, wherein the polyester prepolymer has a unit selected from the group consisting of ethylene glycol, 1,4-butanediol, 1,6-hexanediol, decamethylene glycol, neopentyl glycol and 1,4-cyclohexanedimethanol as the glycol unit and has a unit selected from the group consisting of succinic acid, adipic acid, suberic acid, sebacic acid, dodecanoic acid, succinic anhydride and adipic anhydride as the carboxylic acid unit. 12. A method for producing polyester foamed articles according to claim 10, wherein the polyester prepolymer comprises one or more compounds selected from the group consisting of trimethylolpropane, glycerin and pentaerythritol as the trifunctional or tetrafunctional polyol of the third component. 13. A method for producing polyester foamed articles according to claim 10, wherein the polyester prepolymer comprises one or more compounds selected from the group consisting of malic acid, citric acid and tartaric acid as the trifunctional or tetrafunctional oxycarboxylic acid of the third component. 14. A method for producing polyester foamed articles according to claim 10, wherein the polyester prepolymer comprises one or more compounds selected from the group consisting of trimesic acid? Propanetricarboxylic acid, trimellitic anhydride, pyromellitic anhydride, benzophenonetetracarboxylic anhydride and cyclopentanetetracarboxylic anhydride as the trifunctional or tetrafunctional polybasic carboxylic acid of the third component. 15. Expandable polyester beads1 having: a) 100 parts by weight of an aliphatic polyester having a melt viscosity of 1.0 x 10³ to 1.0 x 10⁶ poise at a temperature of 190°C and a shear rate of 100 sec⁻¹ and having a melting point of 70°C to 190°C, said aliphatic polyester having a number average molecular weight of at least 10,000 and containing 0.03 to 3.0% by weight of urethane bonds and being obtained by reacting diisocyanate with a saturated aliphatic polyester prepolymer having essentially hydroxyl groups at its ends, having a number average molecular weight of at least 5,000 and a melting point of at least 60°C, and b) 0.5 to 40 parts by weight of a volatile foaming agent. 16. Expandable polyester beads according to claim 15, in which the aliphatic polyester is obtained by reacting 0.1 to 5 parts by weight of diisocyanate with 100 parts by weight of the aliphatic polyester prepolymer. 17. An insulated packing or cushioning agent formed by heating and foaming a resin composition consisting of a foaming agent and an aliphatic polyester having a melt viscosity of 1.0 x 10³ to 1.0 x 106 poise at a temperature of 190 ºC and a shear rate of 100 sec-1, and having a melting point of 70 ºC to 190 ºC, wherein the aliphatic polyester has a number average molecular weight of at least 10,000 and contains 0.03 to 3% by weight of urethane bonds, and is obtained by the reaction of diisocyanate with a saturated aliphatic polyester prepolymer having substantially hydroxyl groups at its terminals and having a number average molecular weight of at least 5,000 and a melting point of at least 60°C. 18. A method of making an insulating packing or a cushioning means, comprising the steps of: pre-expanding expandable beads according to claim 15 or 16, placing the pre-expanded beads in a closed but not airtight mold and heating the mold.