KR20110067738A - A polyurethane foam with low thermal conductivity and a manufacturing method thereof - Google Patents

Abstract

PURPOSE: Polyurethane foam for heat insulation is provided to ensure excellent physical properties and low thermal conductivity and to reduce the power consumption of electronic products. CONSTITUTION: Soft polyurethane foam for heat insulation is prepared by catalyst-reacting polyol and polyisocyanate with foaming agents and additives, wherein the foaming agent is hydrocarbon and the additive comprises an alkylformate compound. The alkyl of the alkylformate is C1-C8 alkyl and the alkylformate is methylformate.

Description

A polyurethane foam with low thermal conductivity and a manufacturing method

The present invention relates to a rigid polyurethane foam for thermal insulation, and more particularly, to a rigid polyurethane foam for thermal insulation with low thermal conductivity and a method of manufacturing the same.

Attempts have been made to reduce the power consumption of refrigerated appliances by improving the thermal conductivity of rigid PU foams.

PU foam is prepared by foaming polyol and polyisocyanate together with foam stabilizers, catalysts, blowing agents, etc., mainly chlorinated fluorocarbons (CFCs), hydrochloride fluorocarbons (HCFCs), etc. have been used on a large scale. In addition to improving the thermal conductivity of these PU foams, in addition to trichlorofluoromethane, other physical blowing agents have been used to prepare rigid PU foams. Particular examples given in US 3391053 are gaseous hydrocarbons having up to 3 carbon atoms, such as methane, ethane, ethylene, propane and propylene and hydrocarbons, for example chloromethane, dichlorodifluoromethane, dichlorofuluromethane, Chlorodifluoromethane, chloroethane, dichlorotetrafluoroethane, octafluorocyclobutane and hexafluoropropane, and examples described in the publication of Belgian Patent No. 596608 include haloalkanes such as 1,1-difluoro Rho-2, 2-dichloroethane, 1,2-difluoro-1, 2-dichloroethane, 1,1-dichloroethane, 1-fluoro-1, 2-dichloroethane, 1-fluoro-2, 2-dichloroethane, 1,2-dichloroethane, trichloroethane, tetrachloroethane, 1-fluoro-1,2,2-trichloroethane, 1-bromoethane and 1,1,2-trifluoro R-2--2-ethane and 1,1,1-trichloroethane as described in PCT Application No. 89/00594, It is used as a mixture with other blowing agents.

The blowing agents exemplified to improve the thermal conductivity are at least in some cases toxic, and their boiling points result in lower gas yields in the development of PU foams compared to false chlorofluoromethane, and in rigid PU foams. Has the disadvantage of providing low insulation and causing shrinkage of the foam and even forming cavities in the foam core or partially collapse the foam during deployment.

In addition, chlorinated carbon chloride (CFC) and hydrochlorinated carbon (HCFC) have recently been strongly regulated in order to protect the atmosphere of the earth and prevent carbon dioxide generation.

On the other hand, a method of using a low boiling point hydrocarbon such as cyclopentane or pentane as a foaming agent has been adopted as an alternative foaming agent of polyurethane.However, a polyol for preparing polyurethane foam and a compatibility with these cyclopentane or pentane foaming agents are inferior to the polyurethane foam premix ( preservability) is reduced, there is a problem that has a relatively low thermal conductivity compared to the CFC and HCFC foam. In order to solve this problem, a nucleator such as perfluroroalkane, which reduces the cell size, has been proposed, but it is incompatible with the polyol for preparing polyurethane, and thus, the storage stability and compatibility of the polyol premix are inferior. There is a problem that can not be.

Accordingly, there is a continuing need for new polyurethane foams capable of lowering thermal conductivity while using cyclopentane.

The problem to be solved in the present invention is to provide a polyurethane foam having a low thermal conductivity, for example, a polyurethane foam having a conductivity level of 17.5 * 10 ^-4 mW / mk while using a low boiling point hydrocarbon blowing agent.

The problem to be solved in the present invention is to provide a method for producing a polyurethane foam with a low thermal conductivity while using a low boiling point hydrocarbon blowing agent.

 The problem to be solved in the present invention is to provide a resin premix excellent in storage stability while using a low boiling point hydrocarbon blowing agent.

In order to achieve the above object, the insulating rigid polyurethane foam is prepared by catalyzing a polyol and a polyisocyanate together with a blowing agent and an additive, and the blowing agent is hydrocarbon, and the additive is an alkyl formate compound. It is characterized by including.

In the present invention, the alkyl is C1-C8 alkyl, preferably C1-C4 alkyl, for example methyl formate, ethyl formate, propyl formate, or butyl formate.

In the present invention, the polyol includes a polyether polyol, a polyester polyol or a mixture thereof, preferably a mixed polyol of a polyether polyol and a polyester polyol. Although not theoretically limited, the polyurethane foam of the present invention uses a polyol including a polyether polyol and a polyester polyol to reduce thermal conductivity, while adding a methylal compound to the miscibility between the polyester polyol and the polyether polyol, By improving the miscibility between the blowing agents, it is possible to improve the overall stability of the system, thereby improving the shrinkage and thermal conductivity due to the reaction with the polyisocyanate (MDI), which is a curing agent, and the strong bonding force with the polyisocyanate during molding of the PU foam. Increasing the adhesion to maximize product stability.

In the present invention, the polyol may be used by mixing the polyether polyol and the polyester polyol in any ratio so long as the stability of the reaction is not impaired, and preferably included in a weight ratio of 10:90 -90: 10. .

In a preferred embodiment of the present invention, the mixed polyol composition comprises: (a) 0 to 50% by weight of a polyether polyol obtained by adding propylene oxide to sorbitol, and (b) polyether polyol obtained by adding propylene oxide to glyserine. To 50% by weight, (c) 0 to 50% by weight of polyether polyol obtained by adding propylene oxide to sucrose and glycerine, and (d) ester polyol to be obtained by adding diethylene glycol or propylene glycol to phthalic anhydride. 50% by weight, (e) linear or powdered with short term ester, by transesterification reaction of monocarboxylic or polycarboxylic ester with ether polyol obtained by adding propylene oxide or ethylene oxide to diol or triol Poly (ether-ester) with topographic, non-crosslinked polyfunctional poly (ether-ester) structure A mixed polyol composition comprising a rheology 0 to 50% by weight.

In the present invention, the average OH value of the mixed polyol composition is preferably 300 to 500. When the average OH value of the mixed polyol composition is 300 or less, the mechanical strength and low temperature dimensional stability of the product is inferior, and when the average OH value exceeds 500, thermal conductivity and post-foaming are likely to occur. That is, the average OH value out of the above-described proper range becomes a cause of product defects and the productivity decreases. Therefore, the average OH value of 300 to 500 is preferable for producing a stable PU foam.

It is preferable that the organic polyisocyanate which is one of the components of the internal heat insulating material of this invention contains an isocyanate group, such as aliphatic type, aromatic type, and mixtures thereof. In more detail, the aliphatic polyisocyanate is diisocyanate or 4,4 ', 4-triphenylmethane triisocyanate such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, methylene diphenyl diisocyanate, Aromatic triisocyanates such as 2,4,6-tolylene triisocyanate.

As the isocyanate component to be reacted with the mixed polyol composition, polyisocyanate (MDI) having 2.1 to 3.0 functional groups was used. The isocyanate component preferably has an average NCO% of 29 to 32%. At this time, fluidity | liquidity falls that the average NCO% of the said isocyanate component is 29% or less, and low temperature dimensional stability falls when it is 32% or more. Therefore, the average NCO% of the isocyanate component is preferably 29 to 32 for producing a stable PU foam.

The reaction ratio of the polyisocyanate component and the polyol component should have an equivalent ratio of polyisocyanate and polyol of 1: 1.0 to 1: 3.0. The equivalent ratio of 1: 1.0 means that the NCO of the polyisocyanate (MDI) is 100% of the OH component in the polyol system. Therefore, as the average NCO% is lower, the adhesion and the compressive strength increase to a certain range, but the polyisocyanurate The flame retardancy peculiar to the foam is reduced, and the higher the average NCO%, the higher the polyisocyanurate synthesis, the more the foam is brittle, the low temperature dimensional stability is lowered, and the adhesion to the iron plate is reduced. It is preferable to adjust to 1: 1.0 to 1: 3.0.

In addition, the reaction catalyst used in the present invention is a typical catalyst that can be used to obtain a polyisocyanurate foam, for example triethylamine, tripropylamine, triisopropanolamine, tributylamine, trioctylamine, hexadecyl Dimethylamine, N-methylmorpholine, N-ethylmorpholine, N-octadecylmorpholine, monoethanolamine, diethanolamine, dimethylethanolamine, triethanolamine, N-methyldiethanolamine, N, N-dimethylethanol Amine, diethylenetriamine, N, N, N ', N'-tetramethylbutanediamine, N, N, N', N'-tetramethyl-1.3-butanediamine, N, N, N ', N'- Tetraethylhexamethylenediamine, bis [2- (N, N-dimethylamino) ethyl] ether, N, N-dimethylbenzylamine, N, N-dimethylcyclohexylamine, N, N, N ', N', N -Pentamethyldiethylenetriamine, triethylenediamine, formic acid and other salts of triethylenediamine, amino groups and oxyalkylene adducts of the first and second amines, Amine-type urethane-forming catalysts such as azacyclic compounds such as N, N-dialkyl piperazines and aminocarbonyl catalysts of various N, N ', N' '-trialkylaminoalkylhexahydrotriazines .

In particular, the isocyanate trimerization catalyst used in the present invention is a hydroxy alkyl ammonium compound, 1,3,5-tris (N, N-dimethylaminopropyl) -S-triazine, 2,4,6-tris (dimethyl amino Amines such as methyl) phenol, and aliphatic monocarboxylic acid alkali metal salts.

Tertiary amine catalysts include dimethylcyclohexylamine (DMCHA), dimethylethanol amine (DMEA), pentamethyldiethylenetriamine (PMDETA), tetramethylhexamethylenediamine (TMHMDA), and the like. The catalyst controls the tertiary primary and secondary reactions and affects the productivity, processability, and polyisocyanate conversion of the polyisocyanurate, and in particular, the use of a mixture of the trimerization catalyst and the tertiary amine catalyst desirable.

These catalysts are used individually or in mixture, The usage-amount is 0.5-10 weight% with respect to 100 weight% of polyol components, More preferably, it is 0.5-5 weight%. When the total weight of the tertiary amine catalyst, trimerization catalyst or mixture of tertiary amine catalyst and trimerization catalyst is less than 0.5% by weight, there is a problem that the rate of the trimerization reaction is slowed, and when the content exceeds 5% by weight, the addition of a catalyst Accelerated effect of the reaction rate can not be seen, because the cost burden problem is preferably 0.5 to 5% by weight.

In the present invention, the blowing agent uses hydrocarbon as a volatile alternative blowing agent which is CFC and HCFC, preferably cyclopentane or pentane, and water may be mixed. In the practice of the present invention, the blowing agent also affects the density of the blowing agent and the density should be 35kg / ㎥ or more according to the KMS 3809 regulation, the total amount of blowing agent is 0 to 30% by weight based on 100% by weight of the polyol component Is preferable, More preferably, it is 5-20 weight%. If the weight is less than 5% by weight, the thermal conductivity and other physical properties tend to be inferior due to the high density, and if the weight is more than 30% by weight, the density is decreased, which adversely affects the properties of the foam such as thickness strain and compressive strength. , 5 to 20% by weight is preferred.

In the present invention, as the additive added to the PU foam, a miscibility improving agent which improves the miscibility of the blowing agent and the polyol or the miscibility of the polyester polyol and the polyether polyol in the polyol is used.

In the present invention, the alkyl formate compound in the PU foam is preferably used in an amount of 0.01 to 10 parts by weight, preferably 0.1 to 5 parts by weight with respect to 100 parts by weight of the polyol, mixed by the use of the alkyl formate compound Miscibility with a polyol is improved, the cell structure is finely formed by inflow into the foam, and the internal density is increased. On the other hand, when the amount is small, it is not effective, and when the content is excessive, layer separation occurs in the raw liquid phase, and the product formation time is long due to the increase in the foam pressure inside the foam.

In the present invention, a nucleating agent may be used in the additive to make the size of the cell fine for the purpose of improving the thermal conductivity of the PU foam. Nucleating agents used in the present invention include perfluorocarbons, titanium oxide, titanium oxide compounds, carbon black and the like. The amount of the nucleating agent used is preferably 0 to 10% by weight, more preferably 0 to 5% by weight based on 100% by weight of the polyol component. Here, the amount of the nucleating agent is 0% by weight based on 100% by weight of the polyol component indicates a case in which the nucleating agent is not used, and more than 10% by weight may cause unfavorable effects on manufacturing cost and deforming and other physical properties.

In the present invention, the additive uses a polyalkylene glycol silicone copolymer as the organic silicone-based compound generally used for producing polyurethane foam as the surfactant. The amount of surfactant used is preferably 1.0 to 4.0% by weight, more preferably 1.5 to 3.0% by weight based on 100% by weight of the polyol component.

In the present invention, the polyurethane foam has a density of 28 ~ 38kg / ㎥, thermal conductivity of 17.5 * 10 ^-4 mW / mk or less, independent foaming rate of 90% or more, low temperature dimensional stability -0.1% or less, Adhesion strength at room temperature shows excellent mechanical properties of more than 4kg / ㎠.

According to an aspect of the present invention, a rigid polyurethane foam for thermal insulation is formed between plates, wherein the rigid polyurethane foam is prepared by catalyzing a polyol and a polyisocyanate together with a blowing agent and an additive, wherein the polyol is poly A mixed polyol comprising an ether polyol and a polyester polyol, the blowing agent is hydrocarbon, and the additive is characterized in that it contains a methylal compound.

In the present invention, the inner layer and the outer layer of the polyurethane foam include ABS resin injection product, iron sheet, zinc iron sheet, aluminum sheet, colored zinc iron sheet, color aluminum zinc alloy plated steel sheet, stainless steel sheet, titanium steel sheet and the like. The thickness of the inner / outer layer will be readily determined by one skilled in the art and will vary depending on the application of refrigeration and freezing, but 0.4 to 0.8 mm is preferred.

In another aspect, the present invention, in order to improve the thermal conductivity of the rigid polyurethane foam for thermal insulation, the polyol and polyisocyanate are foamed by catalytic reaction with a blowing agent and additives, the polyol is polyether polyol and polyester polyol Mixed polyol comprising a, the blowing agent is a hydrocarbon, the additive provides a method for producing a rigid polyurethane foam for thermal insulation, characterized in that it comprises a methylal compound.

Polyurethane foam produced by the present invention is excellent in physical properties and low thermal conductivity. As a result, power consumption of electronic products using the same as a heat insulating material can be significantly reduced.

Through the following examples, the present invention will be described in more detail. The following examples illustrate the invention in detail, but are intended to illustrate the invention and in no case to be construed as limiting the invention.

Example

Example 1

40 parts by weight of a polyol having 1,2-propylene oxide added to sorbitol having an average OH value of 450 to 550, and 1,2-propylene after being mixed with sucrose having a mean OH value of 380 to 440. Silicone-based foam stabilizer in a polyol consisting of 40 parts by weight of polyol added with ethylene oxide and ethylene oxide, and 20 parts by weight of polyol with 1,2-propylene oxide and ethylene oxide added to gryserine having an average OH value of 200 to 300. 2.0 parts by weight, 2 parts by weight of water as blowing agent, 10 parts by weight of cyclopentane, 5 parts by weight of methyl formate, 1.2 parts by weight of N, N-dimethylcyclohexylamine as catalyst and N, N, N'N ', N'- 1.0 parts by weight of pentamethyldiethylenecramine, 0.5 parts by weight of N, N ', N'-dimethylaminopropylhexahydrotriene as trimerization catalyst, 1 part by weight of perfluorocarbon, 1 part by weight of titanium oxide compound, In addition, 149 parts by weight of polymeric MDI with an average NCO% of 29 to 32% is used. Was discharged to the vertical / horizontal jig of PU foam, a predetermined time after the internal density of 30kg / manage the ㎥ then attached to the thermal conductivity of the product and 0.5mm ABS resin was carried out the cold resistance test and adhesion test.

[Example 2]

30 parts by weight of polyol added with 1,2-propylene oxide and ethylene oxide after mixing sucrose with sucrose having an average OH value of 380 to 480, and 1,2- to sorbitol having an average OH value of 440 to 500 20 parts by weight of polyol added with propylene oxide, 40 parts by weight of polyester polyol having an average OH value of 100 to 250, and an average OH value of 50 to 150 to 10 parts by weight of polyol having 1,2-propylene oxide added to glyserine 2.5 parts by weight of a foam stabilizer based on silicone in a polyol resin, 10 parts by weight of cyclopentane as a blowing agent, 5 parts by weight of methyl formate, 1.0 part by weight of N, N-dimethylcyclohexylamine as a catalyst, and N, N, N ' N ', N'- pentamethyl diethylene Cri amine 0.5 parts by weight of fluorinated mono-methane 1.5 parts by weight of water, 3.0 parts by weight of trichloroethane, ammonium salts 0.15 parts by weight as a trimerization catalyst and a tertiary amine catalyst 0.15 parts by weight of this average in the 160 parts by weight of polymeric MDI with NCO% of 29-32 Type was discharged into the jig, and this raw liquid surface PU foam of the present invention produce a filled jig, this time to be generated with the shape of the conical inner guhyeongwa cell density of the resulting foam with 32kg / ㎥ product. In order to measure the appearance and the adhesive strength and dimensional stability of 0.5mm steel during compression, the front and rear plates were manufactured using the above materials.

Example 3

15 parts by weight of polyol having 1,2-propylene oxide added to pentaerythritol having an average OH value of 380 to 450, and 40 parts of polyol having 1,2-propylene oxide added to sorbitol having an average OH value of 440 to 500 Part 20, 20 parts by weight of a polyol containing 1,2-propylene oxide and ethylene oxide mixed with sucrose in a sucrose having an average OH value of 380 to 480, a polyester polyol having an average OH value of 250 to 350 10 parts by weight of polyol obtained by adding 1,2-propylene oxide to ethylenediamine and a mixed polyol resin component and 1.0 parts by weight of water as a blowing agent and dimethoxy based as an auxiliary blowing agent. And 3 parts by weight of an alkane mixture, 10 parts by weight of cyclopentane and / or methyl formate as main blowing agent, 3.0 parts by weight of foam stabilizer based on silicone, 1.25 parts by weight of N, N-dimethylcyclohexylamine as catalyst, N , N, N'N ', N'-pentamethyldiethylenecrea 0.3 parts by weight, 0.45 parts by weight of a tertiary amine catalyst as trimerization catalyst, 3 parts by weight of a mixture of perfluorocarbon and carbon black as a nucleating agent, and molded using 178 parts by weight of polymeric MDI having an average NCO% of 29 to 31 parts by weight. PU foam was produced by discharging in the jig, the inner density of the foam to 34kg / ㎥ should be produced as a product having a spherical and conical form of the cell. In order to measure the appearance and the adhesive strength and dimensional stability of 0.5mm steel, the product was manufactured using the above-mentioned materials on the front and back plates.

Comparative Example 1

20 parts by weight of a polyol obtained by adding 1,2-propylene oxide and ethylene oxide to gryserine having an average OH value of 250 to 330, and a polyol having a 1,2-propylene oxide added to sorbitol having an average OH value of 440 to 500. 60 parts by weight of a mixed polyol resin component consisting of 20 parts by weight of polyol with 1,2-propylene oxide mixed with glycerine in a sucrose having an average OH value of 380 to 480 and 2.0 parts by weight of water and a nucleus as a blowing agent. Zero by weight 0.5 parts by weight of perfluorocarbon, 17 parts by weight of a cyclopentane mixture as a blowing agent, 1.5 parts by weight of a foam stabilizer based on silicone, 1.85 parts by weight of N, N-dimethylcyclohexylamine as a catalyst, N, N, N 0.8 parts by weight of 'N', N'-pentamethyldiethylenecramine, 0.15 parts by weight of an amine catalyst as the trimerization catalyst, and molded using 149 parts by weight of polymeric MDI having an average NCO% of 29 to 31% by weight. PU foam was produced by discharging in a jig. When the internal density of the foam was compressed to 32 kg / m 3, the external appearance of the foam was measured to measure adhesion and dimensional stability with 0.5 mm steel.

Comparative Example 2

20 parts by weight of polyol containing 1,2-propylene oxide mixed with sucrose having an average OH value of 350 to 450, and 1,2-propylene oxide to sorbitol having an average OH value of 440 to 500 40 parts by weight of a polyol added thereto, 10 parts by weight of a polyol obtained by adding 1,2-propylene oxide to an ethylenediamine having an average OH value of 600 to 700, and 1,2-to gryserine having an average OH value of 250 to 350. 20 parts by weight of polyol added with propylene oxide, 1.85 parts by weight of mixed polyol resin component and 10 parts by weight of polyester polyol having an average of 250 to 350, and 17 parts by weight of a cyclopentane and isocyclopentane mixture as a blowing agent. 2.0 parts by weight of foam stabilizer, 1.55 parts by weight of N, N-dimethylcyclohexylamine, 1.0 part by weight of N, N, N ', N', N'-pentamethyldiethylenecramine, and a trimerization catalyst 0.5 parts by weight of N, N ', N'-dimethylaminopropylhexahydrotriene The PU foam was produced by the mouth to the average NCO% is ejected into the molding jig by using 29-31% by weight of polymeric MDI 149 parts by weight here. When the internal density of the foam was compressed to 32 kg / m 3, the external appearance of the foam was measured to measure adhesion and dimensional stability with 0.5 mm steel.

Comparative Example 3

1,2-propylene oxide in sorbitol having an average OH value of 440 to 500, 25 parts by weight of a polyol containing 1,2-propylene oxide mixed with sucrose in a sucrose having an average OH value of 350 to 450. 30 parts by weight of a polyol added thereto, 20 parts by weight of a polyester polyol having an average OH value of 100 to 250, 5 parts by weight of a polyol having 1,2-propylene oxide added to ethylenediamine having an average OH value of 600 to 700, 10 parts by weight of a polyol having 1,2-propylene oxide added to gryserine having an average OH value of 250 to 350, a mixed polyol resin component consisting of 10 parts by weight of a polyester polyol having an average of 250 to 350, water of 1.55 parts by weight, and a foaming agent 14 parts by weight of a mixture of cyclopentane and isocyclopentane, 1.0 part by weight of perfluorocarbon as a nucleating agent, 2.2 parts by weight of a foam stabilizer based on silicone, 1.35 parts by weight of N, N-dimethylcyclohexylamine as a catalyst, N, N, N ', N', N'-pentamethyldiethylenecrea 1.5 parts by weight, 0.5 parts by weight of N, N ', N'-dimethylaminopropylhexahydrotriene was added as a trimerization catalyst, and molded using 147 parts by weight of polymeric MDI having an average NCO% of 29 to 31% by weight. PU foam was produced by discharging in a jig. When the internal density of the foam was compressed to 32 kg / m 3, the external appearance of the foam was measured to measure adhesion and dimensional stability with 0.5 mm steel.

Test Methods:

[Thermal conductivity test method] KS L 9016

[Compression test method] KS M 3809

[Dimensional Stability Test Method] ASTM D2126-99

[Adhesion test method] KS M 3718

TABLE 1

Property comparison unit Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 density kg / ㎥ 30 32 34 32 32 32 Thermal conductivity mW / mk 16.85 16.75 16.55 20.8 21.21 21.55 compression Kg / ㎡ 1.45 1.55 1.62 1.21 1.28 1.36 Adhesion kg _f 8 7 9 5 7 9 Dimensional stability vol.% 0.85 0.78 0.71 1.57 1.52 1.42 Compatibility ¹ % 40 45 65 75 85 90

1: The value which measured the compatibility of an additive and a foaming agent in polyol resin. (1-100)

Meaning of the measured value

As seen above, the PU foam of Examples 1 to 3 of the present invention was excellent in compatibility after the addition of the additive and the mixed blowing agent in the polyol resin, it was possible to derive the results of improved thermal conductivity. In addition, when the internal density increases and the work cost (increased polymer MDI content) tends to decrease in thermal conductivity, in the case of the embodiment, the thermal conductivity is significantly improved in all aspects. Comparative test results of the relatively similar results as shown in Comparative Examples 1 to 3 were obtained.

Claims (8)

In a rigid polyurethane foam for thermal insulation, a polyol and a polyisocyanate are prepared by catalytic reaction with a foaming agent and an additive, wherein the foaming agent is hydrocarbon, and the additive is a polyurethane foam comprising an alkylformate compound. . The polyurethane foam according to claim 1, wherein the alkyl of the alkylformate is C1-C8 alkyl. The polyurethane foam according to claim 1, wherein the alkyl formate is methyl formate. The polyurethane foam according to claim 3, wherein the polyol is a mixture of polyether polyol and polyester polyol in a weight ratio of 1: 9-9: 1. The polyurethane foam of claim 1 wherein the blowing agent comprises cyclopentane. A method for producing a polyurethane foam, characterized in that a polyol and a polyisocyanate are reacted in the presence of a blowing agent comprising cyclopentane and an alkylformate compound to produce a low thermally conductive foam. 7. The method of claim 6, wherein said alkyl formate is methyl formate. Refrigeration apparatus containing the polyurethane foam according to claim 1.