Low resilience polyurethane foam

Abstract

The low-resilience polyurethane foam of the present invention is obtained by reacting a raw material composition containing a polyisocyanate component and a polyol component. The polyisocyanate component comprises: an n-functional isocyanate (n≧3) and a difunctional isocyanate prepolymer. Preferably, the polyisocyanate component has an average functional group number of 2.05 or higher. Preferably, the polyol component contains one or more high molecular weight polyols and one or more low molecular weight polyols.

Description

Low resilience polyurethane foam The present invention relates to a low resilience polyurethane foam. The so-called "polyurethane" refers to a polymer compound having a urethane bond (-NH-C(O)O-). Polyurethane is generally obtained by reacting the hydroxyl group (-OH) of a polyol with the isocyanate group (-NCO) of a polyisocyanate. Polyurethane can exhibit diverse properties by optimizing the types of polyol and / or polyisocyanate. Therefore, polyurethane is applied to various automotive parts, synthetic leather, paints, adhesives, etc. In addition, polyurethane foam obtained by foaming polyurethane is applied to heat insulating materials, cushioning materials, etc. Polyurethane foams are roughly classified into: (a) a soft polyurethane foam having interconnected cells, low compression hardness and flexibility; (b) a rigid polyurethane foam having closed cells, highly crosslinked in molecular structure and not having the high elasticity of a soft polyurethane foam; and (c) a semi-rigid polyurethane foam having intermediate properties between rigid and soft. Among these, the soft polyurethane foam exhibits viscoelasticity. In addition, a soft polyurethane foam with reduced elasticity and increased viscosity is particularly also called a "low resilience (SR: Slow Recovery) polyurethane foam". Because the low resilience polyurethane foam has excellent shock absorption properties, it is used for, for example, shock absorption materials, cushions, cushioning materials, shock absorbing materials, shoe insoles, sole cushioning materials, pillow cushioning materials, seat cushioning materials, chair cushioning materials, bedding cushioning materials, etc. Regarding such low resilience polyurethane foams, various proposals have been known. For example, the low resilience foamed polyurethane resin disclosed in Patent Document 1 is obtained by reacting a composition containing: a polyol having an average functionality of 2 to 3 and a hydroxyl value of 20 to 200 mgKOH / g, an isocyanate, resin microspheres not containing chlorine atoms, and a catalyst. The above-mentioned document also describes: (A) Low hardness polyurethane has adhesiveness and adheres tightly to the hand and is not easy to use; (B) If microspheres are added to low hardness polyurethane, although it is not easy to adhere tightly to the hand, since the microspheres float during the hardening process of the urethane resin, a density difference appears between the upper and lower parts of the molded product; and (C) If the polyol is a mixture containing 10% by weight or more of an alkylene oxide adduct of a polyphenol having a functionality of 2 to 3, the density difference between the upper and lower parts of the molded product can be reduced due to the floating of the microspheres. The sheet disclosed in Patent Document 2 is not a low-resilience polyurethane foam, but has an epidermal layer composed of a polyurethane film and a foam layer composed of a polyurethane foam, and the average pore diameter of the polyurethane foam is 50 μm or more and 300 μm or less. The above document also describes that a sheet having an epidermal layer and a foam layer is excellent in both abrasion resistance and impact absorption. The low-resilience polyurethane foam can be used as a cushioning material for smartphones, game consoles, etc. For the cushioning material, an extremely thin thickness of about 0.2 to 1.0 mm is required, and in addition to the SR property, physical properties such as small compression residual stress and high tensile strength are also required. However, there has been no case of a low-resilience polyurethane foam that can fully meet these requirements. In particular, the SR property and the tensile strength have a seesaw relationship. That is, if the SR property is to be exhibited, the tensile strength will decrease, and if the tensile strength is increased, the SR property will not be exhibited. Therefore, it is generally difficult to balance the SR property and the tensile strength. [Prior Art Documents] [Patent Documents] Patent Document 1: Japanese Patent Laid-Open No. 2016-113537 Patent Document 2: Japanese Patent Laid-Open No. 2022-100615 (Problems to be Solved by the Invention) The problems to be solved by the present invention are to provide: a low-resilience polyurethane foam having excellent low resilience. Another problem to be solved by the present invention is to provide: a low-resilience polyurethane foam having high tensile strength in addition to excellent low resilience. Further, another problem to be solved by the present invention is to provide: a low-resilience polyurethane foam having a small compression residual stress in addition to excellent low resilience. (Technical Means for Solving the Problems) To solve the above problems, the low-resilience polyurethane foam of the present invention is obtained by reacting a raw material composition containing a polyisocyanate component and a polyol component; the above polyisocyanate component contains: an n-functional isocyanate (n≧3) and a bifunctional isocyanate prepolymer. (Effects Compared with the Prior Art) When manufacturing a low-resilience polyurethane foam, if the polyisocyanate component is a mixture containing an n-functional isocyanate and a bifunctional isocyanate prepolymer, excellent SR properties will be exhibited. This phenomenon can be considered that by using a bifunctional isocyanate prepolymer with a longer molecular length as one of the polyisocyanate components, the rigidity of the chain structure of the polyurethane is reduced. Furthermore, when manufacturing a low resilience polyurethane foam using a raw material mixture containing an n-functional isocyanate and a bifunctional isocyanate prepolymer, if the isocyanate index is relatively large and / or the number of branches of the raw material composition is optimized, the tensile strength can be increased and / or the compression residual stress can be reduced while maintaining excellent SR properties. The reason for the increase in tensile strength is considered to be that the number of crosslinking points is maintained at an appropriate value by optimizing the isocyanate index and / or the number of branches. The reason for the reduction in compression residual stress is considered to be: (a) by relatively increasing the isocyanate index, the reactivity is increased, the residual polyol component is reduced, and the viscosity is lowered; and (b) by optimizing the number of branches, the necessary minimum limit of elasticity can be ensured. Hereinafter, a detailed description will be given of an embodiment of the present invention. [1. Low resilience polyurethane foam] The low resilience polyurethane foam of the present invention can be obtained by reacting a raw material composition containing a polyisocyanate component and a polyol component that satisfy predetermined conditions. [1.1. Raw material composition] [1.1.1. Polyisocyanate component] The "polyisocyanate component" is one of the main raw materials for manufacturing the low resilience polyurethane foam of the present invention and is a mixture of two or more polyisocyanates. In the present invention, the polyisocyanate component contains: an n-functional isocyanate (n ≧ 3) and a bifunctional isocyanate prepolymer. The polyisocyanate component may consist only of an n-functional isocyanate and a bifunctional isocyanate prepolymer, or may further contain a bifunctional isocyanate in addition to these. [A. n-functional isocyanate] The "n-functional isocyanate" refers to a polyisocyanate having three or more isocyanate groups. If the raw material composition contains an n-functional isocyanate, the number of branches of the raw material composition becomes an appropriate value, and the polymer chains are moderately crosslinked. As a result, it is considered that the tensile strength of the low resilience polyurethane foam is increased or the compression residual stress is reduced. Examples of the n-functional isocyanate include: polynuclear bodies of 4,4'-diphenylmethane diisocyanate (4,4'-MDI), 1-methylbenzene-2,4,6-triisocyanate, 1,3,5-trimethylbenzene-2,4,6-triisocyanate, biphenyl-2,4,4'-triisocyanate, diphenylmethane-2,4,4'-triisocyanate, methyldiphenylmethane-4,6,4'-triisocyanate, 4,4'-dimethyldiphenylmethane-2,2',5,5'-tetraisocyanate, triphenylmethane-4,4',4"-triisocyanate, etc. In the raw material composition, any one of these n-functional isocyanates may be contained, or two or more may be contained. [B. Bifunctional Isocyanate Prepolymer] The so-called "isocyanate prepolymer" refers to a compound having an isocyanate group at its terminal obtained by reacting a polyol with a polyisocyanate. The so-called "bifunctional isocyanate prepolymer" refers to a compound having two isocyanate groups in the isocyanate prepolymer. In other words, the so-called "bifunctional isocyanate prepolymer" refers to a linear compound (OCN-R'-NH-C(O)O-R-O(O)C-NH-R'-NCO) obtained by reacting one molecule of diol (HO-R-OH) with two molecules of bifunctional isocyanate (OCN-R'-NCO). Since the molecular length of the bifunctional isocyanate prepolymer is relatively long, when using it to manufacture a low resilience polyurethane foam, the rigidity of the polyurethane chain structure is reduced. As a result, it is considered that the SR property of the low resilience polyurethane foam is further improved. In the present invention, the type of the bifunctional isocyanate prepolymer is not particularly limited, and the best one can be selected according to the purpose. As the bifunctional isocyanate prepolymer, for example: (a) urethane-modified MDI, allophanate-modified MDI, biuret-modified MDI, isocyanurate-modified MDI, urea-modified MDI, carbodiimide-modified MDI, (b) urethane-modified TDI, allophanate-modified TDI, biuret-modified TDI, isocyanurate-modified TDI, urea-modified TDI, carbodiimide-modified TDI, etc. Any one of these bifunctional isocyanate prepolymers can be contained in the raw material composition, or two or more kinds can also be contained. [C. Bifunctional Isocyanate] The so-called "bifunctional isocyanate" refers to a compound having two isocyanate groups and a compound other than the bifunctional isocyanate prepolymer. For example, commercially available polymeric MDI contains not only polynuclear bodies of 4,4'-MDI but also 4,4'-MDI. Also, commercially available MDI prepolymer contains not only a linear compound (urethane-modified MDI) obtained by reacting 4,4'-MDI with a low molecular weight diol but also unreacted 4,4'-MDI. In the raw material composition, in addition to the above-mentioned n-functional isocyanate and bifunctional isocyanate prepolymer, one or two or more kinds of such bifunctional isocyanates can also be contained. The specific bifunctional isocyanates are as described below. (a) Bifunctional aromatic isocyanates may include, for example: 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenylmethane diisocyanate (4,4'-MDI), 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate, diisocyanatophthalate, 3,3'-dimethyl-4,4'-biphenyl diisocyanate, 3,3'-dimethoxy-4,4'-biphenyl diisocyanate, and the like. (b) Bifunctional alicyclic isocyanates may include, for example: cyclohexane-1,4-diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexane diisocyanate, and the like. (c) Bifunctional aliphatic isocyanates may include, for example: butane-1,4-diisocyanate, hexamethylene diisocyanate, isopropylidene diisocyanate, methylene diisocyanate, lysine isocyanate, and the like. [D. Average number of functional groups of the polyisocyanate component] The so-called "average number of functional groups of the polyisocyanate component" refers to the average value of the number of functional groups per molecule of the polyisocyanate. The average number of functional groups of the polyisocyanate component affects the SR property, tensile strength, and / or compression residual stress. Therefore, the average number of functional groups of the polyisocyanate component is preferably selected as the optimal value according to the purpose. Generally, the larger the average number of functional groups of the polyisocyanate component, the higher the tensile strength and / or the smaller the compression residual stress. To achieve this effect, the average number of functional groups of the polyisocyanate component is preferably 2.05 or more. More preferably, the average number of functional groups is 2.07 or more, and particularly preferably 2.10 or more. On the other hand, if the average number of functional groups of the polyisocyanate component is too large, the SR property will decrease. Therefore, the average number of functional groups of the polyisocyanate component is preferably 3.00 or less. More preferably, the average number of functional groups is 2.90 or less, 2.80 or less, 2.70 or less, 2.60 or less, 2.50 or less, or 2.40 or less. [E. Isocyanate index] The so-called "isocyanate index" refers to the value obtained by multiplying the ratio of the isocyanate group equivalent of the polyisocyanate in the raw material composition to the active hydrogen group equivalent in the raw material composition by 100. Generally, although the higher the isocyanate index, the higher the tensile strength, the SR property will decrease. However, in the low resilience polyurethane foam of the present invention, because polyisocyanates with different numbers of functional groups are used and the molecular structure of the polyisocyanate is optimized, compared with the conventional situation, excellent SR property is still exhibited despite the relatively high isocyanate index. In particular, if the average number of functional groups of the polyisocyanate component is optimized, excellent SR property, high tensile strength, and low compression residual stress can be achieved at a high level. To obtain high tensile strength, the isocyanate index is preferably 80 or more. The isocyanate index is more preferably 85 or more, 90 or more, or 95 or more. On the other hand, if the isocyanate index is too high, the number of crosslinking points is excessive, which may lead to a decrease in SR property. Therefore, the isocyanate index is preferably 130 or less. The isocyanate index is more preferably 125 or less, 120 or less, or 115 or less. [1.1.2. Polyol component] The so-called "polyol component" refers to another main raw material for manufacturing the low resilience polyurethane foam of the present invention. The raw material composition may contain one kind of polyol, or may also contain two or more kinds. [A. Materials] The types of polyols contained in the polyol component are not particularly limited, and the most suitable materials can be selected according to the purpose. The polyol can be any one of ether-based polyols, ester-based polyols, ether-ester-based polyols, and polymer polyols. The polyols are specifically as follows. Examples of ether-based polyols include: (a) polyols such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, butylene glycol, neopentyl glycol, glycerin, pentaerythritol, trimethylolpropane, sorbitol, and sucrose; (b) polyether polyols obtained by adding alkylene oxides such as ethylene oxide and propylene oxide to polyols. Examples of ester-based polyols include: (a) polyester polyols obtained by polycondensing aliphatic carboxylic acids such as malonic acid, succinic acid, and adipic acid, or aromatic carboxylic acids such as phthalic acid, with aliphatic diols such as ethylene glycol, diethylene glycol, and propylene glycol; (b) phthalate polyols. The polymer polyol can be, for example: (a) a polyol such as a polyether polyol in which polymer particles obtained by polymerizing ethylenically unsaturated monomers such as acrylonitrile and styrene are dispersed; (b) a core-shell rubber (CSR) dispersed polyol, etc. The so-called "CSR dispersed polyol" means that core-shell rubber (CSR) particles are dispersed in a polyol. The so-called "core-shell rubber particles" specifically refers to rubber particles in which a polymer different from the above-mentioned core component is graft-polymerized on the surface of a particulate core component mainly composed of a crosslinked rubber-like polymer, and a part or all of the surface of the particulate core component is covered with a shell component. The core component can be, for example, crosslinked rubber particles. The rubber type of the above-mentioned crosslinked rubber particles is not limited, and can be, for example: butadiene rubber, acrylic rubber, polysiloxane rubber, butyl rubber, nitrile rubber, styrene rubber, synthetic natural rubber, ethylene-propylene rubber, etc. The shell component can be, for example, a polymer obtained by polymerizing one or more monomers selected from the group consisting of acrylate, methacrylate, and aromatic vinyl compounds. In addition, the above-mentioned shell component is graft-polymerized on the above-mentioned core component, and preferably forms a chemical bond with the polymer constituting the above-mentioned core component. Also, considering the dispersibility with the polyol, the core-shell rubber particles preferably contain an MBS (methyl methacrylate-butadiene-ethylene copolymer) based polymer. [B. Number average molecular weight, molecular weight] The low resilience polyurethane foam can be produced using a single type of polyol, or can also be produced using two or more types of polyols. When the low resilience polyurethane foam is produced using two or more types of polyols, the polyol component can be: (a) a mixture of two or more polyols having the same number average molecular weight or molecular weight; or (b) a mixture of two or more polyols having different number average molecular weights or molecular weights. In order to obtain a low resilience polyurethane foam with excellent SR properties, the polyol component preferably contains: one or two or more high molecular weight polyols, and one or two or more low molecular weight polyols. Here, the so-called "high molecular weight polyol" refers to a polyol having a number average molecular weight of 1000 or more, and the so-called "low molecular weight polyol" refers to a polyol having a number average molecular weight or molecular weight of less than 1000. The so-called "molecular weight" refers to the formula weight according to the chemical formula. The number average molecular weight (Mni) of the i-th (i≧1) high molecular weight polyol is preferably 1500 or more, more preferably 2000 or more. The number average molecular weight or molecular weight (Mnj) of the j-th (j≧1) low molecular weight polyol is preferably 800 or less, more preferably 600 or less. [C. Number-average molecular weight ratio of polyol component] The so-called "number-average molecular weight ratio of polyol component" refers to, when the polyol component is a mixture of high-molecular-weight polyol and low-molecular-weight polyol, the total number-average molecular weight (Mn Htotal ) of the high-molecular-weight polyol relative to the total number-average molecular weight (Mn Ltotal ) of the low-molecular-weight polyol, and the ratio is (=Mn Htotal / Mn Ltotal ). If the number-average molecular weight of the i-th (i≥1) high-molecular-weight polyol is set as Mni, and the number ratio of the i-th high-molecular-weight polyol in the total high-molecular-weight polyol is set as ni, it can be expressed as: Mn Htotal =Σni×Mni Similarly, if the number-average molecular weight of the j-th (j≥1) low-molecular-weight polyol is set as Mnj, and the number ratio of the j-th low-molecular-weight polyol in the total low-molecular-weight polyol is set as nj, it can be expressed as: Mn Ltotal =Σnj×Mnj The number-average molecular weight ratio of the polyol component mainly affects the SR property of the low-resilience polyurethane foam. When manufacturing a low-resilience polyurethane foam using two or more polyols with different molecular weights, generally, the larger the number-average molecular weight ratio, the more the SR property is improved. To obtain such an effect, the number-average molecular weight ratio of the polyol component is preferably 2.0 or more. More preferably, the number-average molecular weight ratio is 2.5 or more, 3.0 or more, 3.5 or more, or 4.0 or more. On the other hand, if the number-average molecular weight ratio of the polyol component is too large, there will be problems such as: (a) due to the difference in glass transition points of each polyol, the temperature dependence becomes larger; (b) the resin backbone separates into hard and soft segments, resulting in damage to the SR property; (c) the compression residual stress deteriorates. Therefore, the number-average molecular weight ratio is preferably 10 or less. [D. Content of low-molecular-weight polyol] The so-called "content of low-molecular-weight polyol", when the polyol component is a mixture of high-molecular-weight polyol and low-molecular-weight polyol, refers to: the total weight (W L ) of the low-molecular-weight polyol relative to the total weight (W T ) of the polyol component, and the proportion is (=W L ×100 / W H )。 The content of the low-molecular-weight polyol mainly affects the SR property of the low-resilience polyurethane foam. If the content of the low-molecular-weight polyol is too small, the SR property may be reduced. Therefore, the content of the low-molecular-weight polyol is preferably 40.0 mass% or more. More preferably, the content is 45.0 mass% or more, or 50 mass% or more. On the other hand, if the content of the low-molecular-weight polyol is excessive, the SR property may be reduced instead. Therefore, the content of the low-molecular-weight polyol is preferably 75.0 mass% or less. More preferably, the content is 70.0 mass% or less, or 65.0 mass% or less. [1.1.3. Branching number] The so-called "branching number" refers to the number of branches per 1 mol molecule contained in the raw material composition, and is expressed by the following formula: Branching number (pieces / mol) = Σ (number of functional groups - 2) × (number of added parts / molecular weight) The branching number of the raw material composition affects the SR property, tensile strength, and compressive residual stress. Generally, the smaller the branching number, the easier it is for the SR property to appear. However, if the branching number is too small, the tensile strength may be reduced or the compressive residual stress may become smaller. Therefore, the branching number is preferably 0.010 or more. More preferably, the branching number is 0.012 or more, 0.014 or more, or 0.016 or more. On the other hand, if the branching number is too large, the SR property may be reduced. Therefore, the branching number is preferably 0.050 or less. More preferably, the branching number is 0.048, 0.046 or less, or 0.044 or less. [1.1.4. Other components] In the raw material composition for manufacturing the low-resilience polyurethane foam, in addition to the above-mentioned polyisocyanate component and polyol component, the following components may also be contained. The addition amount of each component is not particularly limited, and the best addition amount is preferably selected for the purpose of compounding. [A. Resinification catalyst] The raw material composition may also contain a resinification catalyst. The resinification catalyst is a catalyst that promotes the reaction between the OH group of the polyol and the NCO group of the polyisocyanate. In the present invention, the type of the resinification catalyst is not particularly limited. The resinification catalyst can be, for example, an amine-based catalyst, a metal catalyst, etc. The amine-based catalyst can be, for example: 1,2-dimethylimidazole, 1-methylimidazole, N・(N',N'-dimethylaminoethyl)- Lins, tetramethylguanidine, dimethylaminoethanol, triethylenediamine, N-methyl-N'-(2-hydroxyethyl)-piperazine, N,N,N',N'-tetramethylpropane-1,3-diamine, N,N,N',N'-tetramethylethylenediamine, N,N,N',N",N"-pentamethyl-(3-aminopropyl)ethylenediamine, N,N'-dimethylpiperazine, N,N,N',N'-tetramethylhexane-1,6-diamine, N,N,N',N",N"-pentamethyldipropyltriamine, N-(2-hydroxyethyl) Lins, ethylene glycol bis(3-dimethyl)aminopropyl ether, N,N-dimethylcyclohexylamine, N-methyl-N'-(2-dimethylamino)ethylpiperazine, etc. The metal catalyst can be, for example: (a) tin catalysts such as stannous octoate and dibutyltin dilaurate; (b) phenylmercury propionate; (c) lead octenoate, etc. [B. Foam stabilizer] The raw material composition may also contain a foam stabilizer. The foam stabilizer makes the gas involved during the mechanical foaming of the polyurethane easily dispersed and stabilizes the bubbles, so as to adjust the bubble structure. In the present invention, the type of the foam stabilizer is not particularly limited. The foam stabilizer can be, for example: (a) polysiloxane-based foam stabilizers; (b) fluorine-containing compound-based foam stabilizers; (c) anionic surfactants such as sodium dodecylbenzenesulfonate and sodium lauryl sulfate; (d) phenolic compounds, etc. [C. Filler] The raw material composition may also contain a filler. The filler increases the volume of the polyurethane foam and reduces the amount of polyurethane raw material used per unit volume, so as to reduce the cost of the polyurethane foam. In the present invention, the type of the filler is not particularly limited. The filler can be, for example: aluminum hydroxide, calcium carbonate, talc, clay, etc. [D. Moisture absorbent] The raw material composition may also contain a moisture absorbent. The moisture absorbent is to remove the moisture contained in the composition and inhibit the reaction between the polyisocyanate and moisture. If the polyisocyanate reacts with moisture, CO 2 gas will be generated, resulting in a more difficult situation to control the appearance of bubbles. In the present invention, the type of the moisture absorbent is not particularly limited. The moisture absorbent can be, for example: molecular sieve, synthetic zeolite, silica powder, alumina powder, lithium hydroxide powder, barium hydroxide powder, etc. [E. Antioxidant] The raw material composition may also contain an antioxidant. The antioxidant is to inhibit the deterioration caused by the oxidation of the polyurethane. In the present invention, the type of the antioxidant is not particularly limited. The antioxidant can be, for example: hindered phenol-based antioxidants, amine-based antioxidants, sulfur-based antioxidants, phosphorus-based antioxidants, etc. [1.2. Reaction of the raw material composition] The low resilience polyurethane foam of the present invention is manufactured using the Mechanical Froth Method. The so-called "Mechanical Froth Method" refers to a method including the following steps: (a) Using a high-shear stirrer, while blowing an inert gas, mixing the raw material composition to form a foamed raw material composition containing fine bubbles; (b) Coating the foamed raw material composition on the surface of a substrate (such as a PET film); (c) Heating the coated film to a predetermined temperature to cause hardening. In the present invention, the reaction conditions of the raw material composition are not particularly limited, and the best conditions can be selected according to the purpose. [1.3. Characteristics] [1.3.1. Thickness] In the present invention, the thickness of the low resilience polyurethane foam is not particularly limited, and the most suitable thickness can be selected according to the purpose. When using the low resilience polyurethane foam as a cushioning material for electronic / electrical appliances, the thinner the better. If the low resilience polyurethane foam of the present invention is used, a sheet excellent in SR property and having a thickness of 2.0 mm or less can be manufactured. If the manufacturing conditions are optimized, the thickness can be made 1.5 mm or less, or 1.0 mm or less. [1.3.2. Recovery speed] The so-called "recovery speed" refers to the time from when a load of 1 kg is applied to a compression surface with a diameter of 15 mm for 5 seconds using a constant pressure load device and then the load is released until the shape of the sample is restored. The greater the recovery speed (the longer the recovery time), the more excellent the SR property. For the low resilience polyurethane foam of the present invention, if the molecular structure of the polyisocyanate used as the raw material, the average number of functional groups of the polyisocyanate, the isocyanate index, the number of branches, etc. are optimized, the recovery speed can be increased. If the manufacturing conditions are optimized, the recovery speed reaches 1.5 seconds or more. If the manufacturing conditions are further optimized, the recovery speed can be made 3.0 seconds or more, 6.0 seconds or more, or 10 seconds or more. [1.3.3. Compression residual stress] The so-called "compression residual stress" refers to the value measured based on JIS K6401:2011. For the low resilience polyurethane foam of the present invention, if the molecular structure of the polyisocyanate used as the raw material, the average number of functional groups of the polyisocyanate, the isocyanate index, the number of branches, etc. are optimized, the compression residual stress can be reduced. If the manufacturing conditions are optimized, the compression residual stress is 20% or less. If the manufacturing conditions are further optimized, the compression residual stress can be made 10% or less, or 5% or less. [1.3.4. Tensile Strength] The so-called "tensile strength" refers to the value measured based on JIS K 6251:2010. For the low-resilience polyurethane foam of the present invention, if the molecular structure of the polyisocyanate used as a raw material, the average functionality of the polyisocyanate, the isocyanate index, the number of branches, etc. are optimized, the tensile strength can be increased. If the manufacturing conditions are optimized, the tensile strength can reach 0.3 MPa or more. If the manufacturing conditions are further optimized, the tensile strength can reach 0.4 MPa or more, or 0.5 MPa or more. [1.3.5. Elongation] The so-called "elongation" refers to the value measured based on JIS K 6251:2010. For the low-resilience polyurethane foam of the present invention, if the molecular structure of the polyisocyanate used as a raw material, the average functionality of the polyisocyanate, the isocyanate index, the number of branches, etc. are optimized, the elongation can be increased. If the manufacturing conditions are optimized, the elongation can reach 200% or more. If the manufacturing conditions are further optimized, the elongation can reach 250% or more, or 300% or more. [1.3.6. Density] The so-called "density" refers to the value measured based on JIS K 6401:2011. Since the low-resilience polyurethane foam of the present invention is manufactured by the mechanical foaming method, the density is relatively low. If the manufacturing conditions are optimized, the density can be made to be 600 kg / m 3 or less. If the manufacturing conditions are further optimized, the density can be made to be 550 kg / m 3 or less, 450 kg / m 3 or less, 250 kg / cm 3 or less, 200 kg / m 3 or less, or 150 kg / m 3 or less. [1.3.7. Average Pore Diameter] The so-called "average pore diameter" refers to the average of the equivalent circle diameters of the pores appearing in the cross-section of the polyurethane foam. Since the low-resilience polyurethane foam of the present invention is manufactured by the mechanical foaming method, the interior of the polyurethane foam is in a state of uniformly dispersed fine pores. If the manufacturing conditions are optimized, the average pore diameter can be made to be 50 μm or more and 300 μm or less. If the manufacturing conditions are further optimized, the average pore diameter is preferably 50 μm or more and 250 μm or less, more preferably 50 μm or more and 200 μm or less. [1.4. Use] The low resilience polyurethane foam of the present invention can be used for various purposes. The uses of the low resilience polyurethane foam of the present invention can be, for example: shock absorbers, cushions, buffers, shock absorbers, shoe insoles, sole buffers, pillow buffers, seat cushions, chair buffers, bedding buffers, etc. The low resilience polyurethane foam of the present invention is particularly suitable as a buffer material for electronic / electrical appliances because it not only has excellent low resilience, but also has high tensile strength even though it is relatively thin. The buffer materials for electronic / electrical appliances can be, for example: (a) a buffer material disposed on the back of various image display devices such as liquid crystal displays to absorb the impact received by the display device; (b) a buffer material for display components such as touch panels, cameras, and lenses used in mobile communications such as mobile phones, smartphones, and mobile information terminals; etc. Furthermore, in addition to being used as a buffer material, the low resilience polyurethane foam of the present invention can also be used as a base material for adhesive tapes, as well as cushions and sealing materials. [2. Function] Conventionally, low resilience polyurethane foams are generally manufactured using a raw material composition with an excess of polyol (a raw material composition with an isocyanate index of less than 80). The low resilience polyurethane foam obtained in this way has a high SR property because it retains a large amount of unreacted OH groups, but has a low tensile strength and a relatively large compression residual stress. On the other hand, if only the isocyanate index is simply increased, although the tensile strength is increased and the compression residual stress is reduced, the SR property is decreased. In contrast, when manufacturing a low resilience polyurethane foam, if the polyisocyanate component uses a mixture containing an n-functional isocyanate and a bifunctional isocyanate prepolymer, excellent SR properties are exhibited. The reason can be considered that by using a bifunctional isocyanate prepolymer with a longer molecular length as one of the polyisocyanate components, the rigidity of the polyurethane chain structure can be reduced. Furthermore, when manufacturing a low resilience polyurethane foam using a raw material mixture containing an n-functional isocyanate and a bifunctional isocyanate prepolymer, if the isocyanate index is relatively increased and / or the branching number of the raw material composition is optimized, the tensile strength can be increased and / or the compression residual stress can be reduced while maintaining excellent SR properties. The reason for the increase in tensile strength can be considered that by optimizing the isocyanate index and / or the branching number, the number of crosslinking points can be maintained at an appropriate value. The reason for the reduction in compression residual stress can be considered: (a) by relatively increasing the isocyanate index, the reactivity is increased, the residual polyol component is reduced, and the viscosity is decreased; and (b) by optimizing the branching number, the necessary minimum limit of elasticity can be ensured. [Examples] (Examples 1 to 29, Comparative Examples 1 to 7) [1. Preparation of Samples] Table 1 shows a list of raw materials used. The raw materials shown in Table 1 were blended at a predetermined ratio. The raw material composition was put into a mixing head and stirred and mixed while introducing an inert gas (nitrogen) until it became homogeneous, obtaining a foamed raw material composition containing fine bubbles. The foamed raw material composition was coated on a PET film, and the coated film was then heat-cured at 200°C. In addition, the number average molecular weight was calculated by the following formula. Number average molecular weight = (56100 × number of functional groups) / hydroxyl value In Table 1, the number average molecular weight of the CSR-dispersed polyol represents the number average molecular weight of the dispersion medium PPG. Similarly, the number average molecular weight of the high molecular polyol represents the number average molecular weight of the dispersion medium PPG. Also, the CSR particles contained in the CSR-dispersed polyol are MBS (methyl methacrylate-butadiene-styrene copolymer) -based polymers. [Table 1] [2. Test Methods] [2.1. Recovery Rate] A 1 kg load (compression surface: 15 mm) was applied to each sample using a constant pressure load cell (manufactured by ASKER, CL-150) for 5 seconds. Then the load was released and the recovery rate was measured. [2.2. Compression Residual Stress] The compression residual stress was measured based on JIS K6401:2011. [2.3. Density] The density is measured based on JIS K 6401:2011. [2.4. Tensile Strength] The tensile strength is measured based on JIS K 6251:2010. [2.5. Elongation] The elongation is measured based on JIS K 6251:2010. [2.6. 180° Peel] On the surface of the reinforcing plate made of ABS resin, a sample with a width of 30 mm × a length of 125 mm is pasted via double-sided tape. The size of the double-sided tape is set to be the same as the sample size. Then, a PET film with a width of 24 mm × a length of 130 mm is pasted on the sample surface via double-sided tape. Next, the PET film is pressed against the sample surface. During pressing, a 2 kg roller is moved back and forth twice on the surface of the PET film. After pressing, the sample is left for 24 hours. Then, the PET film is torn in the 180° direction of the joint surface. The test speed is set to 300 mm / min. For the 50 mm interval in the center of the sample, the force (N / 24 mm) when the PET film is peeled is measured. [2.7. Shear Strength] On both sides of a 25 mm × 25 mm sample, SUS plates are pasted via double-sided tape respectively. The size of the double-sided tape is set to be the same as the sample size. The SUS plates are stretched up and down, and the force (N) at the shear failure of the sample is measured. [2.8. 25% CLD Hardness] The 25% CLD hardness is measured based on JIS K6254:2010. [3. Results] The results are shown in Tables 2 to 4. In addition, the raw material formulations of each sample are combined and labeled in Tables 2 to 4 respectively. The following matters are known from Tables 2 to 4. (1) In Comparative Examples 1 to 5, it was found that the smaller the isocyanate index, the greater the recovery speed and the greater the increase in elongation. On the other hand, there was a tendency for the tensile strength to decrease. This is considered to be because the smaller the isocyanate index, the fewer the number of crosslinking points. (2) Although the isocyanate index of Examples 1 to 7 is the same as that of Comparative Example 1, compared with Comparative Example 1, the recovery speed of these is greater, and the tensile strength is also higher. This is considered to be because the use of bifunctional isocyanate prepolymer reduces the rigidity of the chain structure of polyurethane. (3) In Examples 1 to 7, it was found that the smaller the average number of functional groups of the polyisocyanate component, the greater the recovery speed. However, when the average number of functional groups of the polyisocyanate component is 2.132 (Example 6), the tensile strength shows a maximum value. This is considered to be because if the average number of functional groups is too small, the number of crosslinking points is excessively reduced. (4) Compared with Examples 1 to 7 with the same average number of functional groups of the polyisocyanate component, although Examples 8 to 14 have higher tensile strengths respectively, there is a tendency for the recovery speed to decrease slightly. This is considered to be because the isocyanate index of Examples 8 to 14 is larger than that of Examples 1 to 7. (5) Compared with Examples 1 to 5 where the average functionality of the polyisocyanate component is the same, although the recovery speed increases respectively in Examples 15 to 19, there is a tendency for the tensile strength to decrease. This is considered to be due to the fact that the isocyanate index in Examples 15 to 19 is smaller than that in Examples 1 to 5. (6) The 180° peel force of Comparative Examples 6 to 7 is slightly lower. On the other hand, Examples 20 to 29 exhibit a higher 180° peel force and a higher shear strength while maintaining a relatively high recovery speed. This is considered to be due to the further addition of a low molecular weight polyol containing core-shell rubber (CSR) particles and a low molecular weight polyol with a functionality of 3.0 in the raw materials. [Table 2] [Table 3] [Table 4] As described above, the embodiments of the present invention have been described in detail. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention. (Industrial Applicability) The low resilience polyurethane foam of the present invention can be used, for example, as impact absorbers, cushions, buffers, shock absorbers, shoe insoles, sole buffers, pillow buffers, seat cushions, chair cushions, bedding cushions, etc. Furthermore, the low resilience polyurethane foam of the present invention can be used, for example, as: (a) a buffer material disposed on the back of various image display devices such as liquid crystal displays to absorb the impact received by the display device; (b) a buffer material for electronic / electrical devices such as display components such as touch panels, cameras, and lenses used in mobile communications such as mobile phones, smartphones, and mobile information terminals. Furthermore, the low resilience polyurethane foam of the present invention can also be used as a base material, cushion, and sealing material for adhesive tapes.

Claims

1. A low-resilience polyurethane foam, obtained by reacting a raw material composition containing a polyisocyanate component and a polyol component; wherein the polyisocyanate component contains: an n-functional isocyanate (n≧3) and a difunctional isocyanate prepolymer; wherein the average number of functional groups in the polyisocyanate component is 2.05 or more.

2. The low-resilience polyurethane foam as claimed in claim 1, wherein, The above-mentioned polyol components contain: one or more high molecular weight polyols and one or more low molecular weight polyols; the above-mentioned "high molecular weight polyols" refer to polyols with a molecular weight average molecular weight of 1000 or more, and the above-mentioned "low molecular weight polyols" refer to polyols with a molecular weight average molecular weight or a molecular weight of less than 1000.

3. The low-resilience polyurethane foam as claimed in claim 2, wherein, The number average molecular weight ratio of the above-mentioned polyol components is 2.0 or higher; the above-mentioned "number average molecular weight ratio of polyol components" refers to the ratio of the total number average molecular weight (MnHtotal) of the above-mentioned high molecular weight polyols to the total number average molecular weight (MnLtotal) of the above-mentioned low molecular weight polyols (=MnHtotal / MnLtotal).

4. The low-resilience polyurethane foam as claimed in claim 2, wherein, The total content of the aforementioned low molecular weight polyols is 40.0 mass% or more and 75.0 mass% or less; the aforementioned "total content of low molecular weight polyols" refers to the ratio of the total weight of the aforementioned low molecular weight polyols (WL) to the total weight of the aforementioned polyol components (WT) (=WL×100 / WT).

5. The low-resilience polyurethane foam as claimed in claim 1, wherein, The thickness is less than 2.0mm.

6. The low-resilience polyurethane foam of claim 1 is used as a cushioning material for electronic / electrical machinery.