JP2011046907A - Flexible polyurethane foam and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flexible polyurethane foam having low density, strong mechanical properties and durability as a cushion and a seat back for vehicles; and to provide a method for manufacturing the same. <P>SOLUTION: This flexible polyurethane foam is obtained by reaction foaming of a mixed liquid including an organic polyisocyanate (A), a polyol (B), a chain extender (C), a catalyst (D) and a foaming agent (E), where the core density of the flexible polyurethane foam is in a range of 25-35 kg/m<SP>3</SP>, and the organic polyisocyanate (A) comprises 70-90 mass% of diphenylmethane diisocyanate (A-1) and 10-30 mass% of polymethylenepolyphenylene polyisocyanate (A-2) to 100 mass% of the organic polyisocyanate (A), and the (A-1) comprises 50-100% of 2,4-diphenylmethane diisocyanate and 2,2-diphenylmethane diisocyanate. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a flexible polyurethane foam, and more particularly to a flexible polyurethane foam that can be suitably used for a vehicle seat back and a vehicle seat cushion.

    Conventionally, flexible polyurethane foams are widely used for automobile seats as well as cushion members for furniture and the like because of their excellent cushioning properties.

Specifically, for example, a polyhydroxyl compound, an organic polyisocyanate, a catalyst, a foam stabilizer, a foaming agent (water) and an additive can be used as a method for producing a flexible polyurethane foam excellent in foam physical properties, moldability and surface curing properties. A method of using toluene diisocyanate as an isocyanate component has already been widely known as a flexible polyurethane foam (Patent Document 1). However, the freezing point of toluene diisocyanate is as high as about 17 ° C. under normal pressure, and it needs to be kept warm and controlled to solidify in winter. In addition, in the production place, the health condition of workers due to toluene diisocyanate may be a problem.

Therefore, a flexible polyurethane foam using diphenylmethane diisocyanate having a low vapor pressure as a polyisocyanate component has also been proposed (Patent Document 2). However, the freezing point of 4,4'-diphenylmethane diisocyanate, which is the main component of conventional diphenylmethane diisocyanate, is about 38 ° C under atmospheric pressure, which is higher than that of toluene diisocyanate, which requires pretreatment such as prepolymerization. Since the viscosity is very high, there are problems such as poor workability and limitations due to production equipment conditions. Further, all diphenylmethane diisocyanate (2,2′-diphenylmethane diisocyanate (hereinafter abbreviated as 2,2′-MDI), 2,4-diphenylmethane diisocyanate (hereinafter abbreviated as 2,4-MDI) and 4,4′-diphenylmethane diisocyanate ( (Hereinafter abbreviated as 4,4'-MDI), generically referred to as "pure MDI" below), when the content of 2,2'-MDI and 2,4-MDI (hereinafter abbreviated as isomer ratio) is low, storage stability There was a problem such as deterioration of sex.

Therefore, as an MDI isocyanate for flexible polyurethane foam, an organic polyisocyanate modified with a polyol is used which has an isomer ratio and a high amount of pure MDI (hereinafter referred to as di-content) in all isocyanates (pure MDI and polymeric MDI). There exists a thing (patent document 3). However, by modifying the isocyanate component with a polyol, the viscosity is increased and the NCO content is decreased. Therefore, it has been very difficult to obtain a flexible polyurethane foam having a low density and high elastic performance. Furthermore, it did not describe the chlorine ion concentration in the isocyanate composition.

In the previous inventions, when only diphenylmethane diisocyanate was used as the organic polyisocyanate component, it was difficult to mold a low density product such as a vehicle seat back, that is, a flexible urethane foam having a core density of 25-35 kg / m3. It was. In other words, a flexible polyurethane foam that satisfies the mechanical properties capable of withstanding fogging and durability as a cushioning material could not be produced.

JP 2001-172356 A JP 2005-194486 A WO2008 / 136179

  The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to provide a method for producing a flexible polyurethane foam having low density, strong mechanical properties, and durability as a vehicle cushion and seat back.

As a result of intensive studies to achieve the above object, the present inventors contain an organic polyisocyanate (A), a polyol (B), a chain extender (C), a catalyst (D) and a foaming agent (E). The present inventors have found that a flexible polyurethane foam capable of achieving the above-mentioned object can be obtained by reaction foaming of the mixed solution to complete the present invention.

That is, the flexible polyurethane foam of this invention and its manufacturing method are as follows.
(1) A flexible polyurethane foam obtained by reactive foaming of a mixed liquid containing an organic polyisocyanate (A), a polyol (B), a chain extender (C), a catalyst (D) and a foaming agent (E), The core density of the flexible polyurethane foam is in the range of 25 to 35 kg / m ³ , and the organic polyisocyanate (A) is 70 to 90% by mass with respect to 100% by mass of the organic polyisocyanate (A). Diphenylmethane diisocyanate (A-1) and 10 to 30% by mass of polymethylene polyphenylene polyisocyanate (A-2), and (A-1) is 2,4-diphenylmethane diisocyanate and 2 A flexible polyurethane foam characterized by containing 50 to 100% of 2-diphenylmethane diisocyanate.
(2) The diphenylmethane diisocyanate (A-1) has 2,90-90% 2,4-diphenylmethane diisocyanate and 2,2-diphenylmethane diisocyanate, has an NCO content of 32% or more and a chlorine ion concentration of 5 ppm or less. The flexible polyurethane foam as described in (1) above.
(3) The polyol (B) has a nominal average functional group number in the range of 2 to 4 and a number average molecular weight in the range of 3000 to 8000. The flexible polyurethane foam according to (1) or (2).
(4) Core density is 25 to 35 kg / m ³ , wet heat compression set is 25% or less, elongation is 100% or more, tear strength is 4.5 N / cm ² or more, and 25% compression stress is 80 to 120N. The flexible polyurethane foam as described in any one of (1) to (3) above,
(5) The method for producing a flexible polyurethane foam according to any one of (1) to (4).

The flexible polyurethane foam in the present invention uses only an environmentally friendly tail pure MDI / polymeric MDI as an isocyanate raw material, a core density of 25 to 35 kg / m3, a wet heat compression set of 25% or less, an elongation of 100% or more, and tearing. It has physical properties of strength of 4.5 N / cm ² or more and 25% compressive stress of 80 to 120 N. As a result, it is possible to obtain a vehicle seat cushion and a seat back flexible polyurethane foam that have good riding comfort and can withstand fogling.

  When producing the flexible polyurethane foam in the present invention, since it is highly volatile and harmful to the human body, it does not use tolylene diisocyanate, so it is environmentally friendly.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

The flexible polyurethane foam of the present invention is a flexible polyurethane obtained by reaction foaming of a mixed solution containing an organic polyisocyanate (A), a polyol (B), a chain extender (C), a catalyst (D) and a foaming agent (E). A form,
The core density of the flexible polyurethane foam is in the range of 25 to 35 kg / m3, and the organic polyisocyanate (A) is 70 to 90% by mass with respect to 100% by mass of the organic polyisocyanate (A). It contains diphenylmethane diisocyanate (A-1) and 10 to 30% by mass of polymethylene polyphenylene polyisocyanate (A-2). Furthermore, the diphenylmethane diisocyanate (A-1) is characterized by having 50 to 100% of 2,4-diphenylmethane diisocyanate and 2,2-diphenylmethane diisocyanate.

  Moreover, it is preferable that the isocyanate group content (NCO content) in such organic polyisocyanate (A) is in a range of 32% by mass or more. When the NCO content is less than the lower limit, the expansion ratio is lowered, and it tends to be difficult to reduce the density.

  In the pure MDI according to the present invention, the content of the isomer MDI is required to be 50 to 100% by mass, more preferably 55 to 100% by mass, and particularly preferably 55 to 65% by mass. .

  When the ratio of the isomeric MDI in the pure MDI according to the present invention is less than 50% by mass, the resulting flexible urethane foam has a high 25% compressive stress, and the wet heat compressive strain exceeds 25%, making it unsuitable as a flexible polyurethane foam. Further, if the content of the isomer MDI exceeds 90% by mass, the wet heat compression strain exceeds 25%, and therefore it is preferably 90% by mass or less.

  Although such an isomer MDI according to the present invention has been difficult to obtain due to problems in production technology, it can be obtained in recent years, and organic polyisocyanates having various compositions can be obtained.

  The synthesis method for reacting the diphenylmethane diisocyanate and the polyoxyethylene polyoxypropylene polyol is not particularly limited, and is a method of pre-polymerizing MDI and polyol in a total amount, by reacting a part of MDI with the polyol. A method of mixing the remaining MDI can be applied.

  In addition, other isocyanates can also be used in combination for the purpose of flowability, hardness, foaming speed adjustment and the like. Specific examples include aromatic diisocyanates such as o-xylylene diisocyanate, m-xylylene diisocyanate, and p-xylylene diisocyanate, hexamethylene diisocyanate, 2-methyl-1,5-pentane diisocyanate, and 3-methyl-1,5. -Diisocyanates such as aliphatic diisocyanates such as pentane diisocyanate, isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenated toluene diisocyanate, hydrogenated xylylene diisocyanate, cyclohexyl diisocyanate and the like, biuret modified products of these diisocyanates, isocyanurates Examples include modified products, uretonimine modified products, carbodiimide modified products, and polyol modified products.

  The polyol (B) according to the present invention is not particularly limited, but a polyether having a nominal average functional group number of 2 to 6 and a number average molecular weight of 3,000 to 8,000, which easily develops physical properties as a flexible polyurethane foam. Those mainly composed of polyol are preferred.

  As such polyether polyols, known ones can be used. For example, ethylene oxide, propylene oxide using low molecular weight polyols having a number average molecular weight of less than 700, low molecular weight polyamines, low molecular weight amino alcohols and the like as initiators. And those obtained by adding a cyclic ether such as tetrahydrofuran. Such initiators include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5- Pentanediol, 2-methyl-1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, 1,8-octanediol, 1,9-nonanediol, 2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, 2-ethyl-1 , 3-hexanediol, 2-n-hexadecane-1,2-ethylene glycol, 2-n-eicosane-1,2-ethylene glycol, 2-n-o Tacosan-1,2-ethylene glycol, diethylene glycol, dipropylene glycol, 1,4-cyclohexanedimethanol, ethylene oxide or propylene oxide adduct of bisphenol A, hydrogenated bisphenol A, 3-hydroxy-2,2-dimethylpropyl- Low molecular weight polyols such as 3-hydroxy-2,2-dimethylpropionate, trimethylolpropane, glycerin, pentaerythritol; aniline, ethylenediamine, propylenediamine, toluenediamine, metaphenylenediamine, diphenylmethanediamine, xylylenediamine, etc. Low molecular weight amines; low molecular weight amino alcohols such as monoethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine And the like.

  Such polyether polyols are water; polyols such as propylene glycol, dipropylene glycol, glycerin, trimethylolpropane, pentaerythritol, sorbitol; amino alcohols such as diethanolamine, triethanolamine, tripropanolamine; or ethylenediamine, It can also be obtained by subjecting amines such as 1,6-hexanediamine, triethylenetetraamine, aniline, toluylenediamine, and methylenebisaniline to ring-open addition of ethylene oxide, propylene oxide or the like at random.

  For the purpose of adjusting the hardness, a polymer polyol produced by polymerizing a vinyl monomer in a polyol by an ordinary method can be further used in combination. Examples of such a polymer polyol include those obtained by polymerizing a vinyl monomer in the presence of a radical initiator using the same polyether polyol as described above and stably dispersing it. Examples of the vinyl monomer include acrylonitrile, styrene, vinylidene chloride, hydroxyalkyl, methacrylate, and alkyl methacrylate. Among them, acrylonitrile and styrene are preferable. Specific examples of such a polymer polyol include EL-910 and EL-923 manufactured by Asahi Glass Urethane Co., Ltd. and FA-728R manufactured by Sanyo Chemical Industries.

  As the catalyst (C) used in the present invention, various urethanization catalysts and trimerization catalysts known in the art can be used. Representative examples include triethylamine, tripropylamine, tributylamine, N-methylmorpholine, N-ethylmorpholine, dimethylbenzylamine, N, N, N ′, N′-tetramethylhexamethylenediamine, N, N, N ′, Tertiary amines such as N ′, N ″ -pentamethyldiethylenetriamine, triethylenediamine, bis- (2-dimethylaminoethyl) ether, 1,8-diaza-bicyclo (5,4,0) undecene-7, dimethylethanol Reactive tertiary amines such as amines, N-trioxyethylene-N, N-dimethylamine, N, N-dimethyl-N-hexanolamine or organic acid salts thereof, 1-methimidazole, 2-methylimidazole, 1 , 2-dimethylimidazole, 2,4-dimethylimidazole, 1-butyl-2- Imidazole compounds such as tilimidazole, stannous octoate, dibutyltin dilaurate, organometallic compounds such as zinc naphthenate, 2,4,6-tris (dimethylaminomethyl) phenol, 2,4,6-tris (dialkylaminoalkyl) ) Trimerization catalysts such as hexahydro-S-triazine, potassium acetate, potassium 2-ethylhexanoate and the like. It is preferable to use at least an imidazole series and an ether group-containing tertiary amine compound as a catalyst, and particularly preferable catalysts are 1,2-dimethylimidazole and bis- (2-dimethyl), because the reaction is mild and the balance between high curing properties is used. (Aminoethyl) ether.

  The foaming agent (D) used in the present invention is foamed by carbon dioxide gas generated by the reaction between isocyanate groups and water. For the purpose of reducing the density, carbon dioxide is mixed in a liquid state and vaporized and foamed during foaming. A method can also be used together. The amount of water is preferably 2 to 20 parts by mass with respect to 100 parts by mass of the polyisocyanate composition. When liquefied carbon dioxide gas is used in combination, the amount is preferably 0.5 to 6 parts by mass with respect to 100 parts by mass of the polyisocyanate composition.

  The foam stabilizer (E) used in the present invention is an organosilicon surfactant known in the art, for example, SZ-1328E, SZ-1325E, SZ-1336E, SZ- manufactured by Toray Dow Corning. 1346E, Y-10366, L-3430, L-3420, L-3151, L-3415 manufactured by Momentive, B-8737LF2, B-8724LF2, B-8734LF2, B-8715LF2, manufactured by Evonik, Shin-Etsu Chemical Co., Ltd. Examples thereof include F-122 and the like. As for the quantity of these foam stabilizers, 0.1-3 mass parts is preferable with respect to 100 mass parts of polyisocyanate compositions.

  Further, diethanolamine or triethanolamine can be added to the present invention for the purpose of stabilizing the cell. A preferable addition amount is 0.1 to 5 parts by mass with respect to 100 parts by mass of the polyol component.

  If necessary, the present invention further includes trichloroethyl phosphate, trichloropropyl phosphate, phosphorus-halogen liquid flame retardant represented by these condensation types, solid flame retardant represented by melamine powder, and conductivity represented by ketjen black. Plasticizers such as reactive carbon and dioctyl phthalate, antioxidants, ultraviolet absorbers, colorants, internal mold release agents, and other auxiliaries can be used. These auxiliaries are usually used by adding them to polyols. However, auxiliaries having no active hydrogen capable of reacting with isocyanate can be mixed in advance in the polyisocyanate composition.

  The equivalent ratio (NCO / NCO reactive group) of all isocyanate reactive groups in the isocyanate reactive compound containing water and all isocyanate groups in the polyisocyanate composition of the present invention is 0.5 to 1.5 (isocyanate index). (NCO INDEX) = 50 to 150) is preferable, and 0.7 to 1.2 (isocyanate index (NCO INDEX) = 70 to 120) is more preferable.

  The method for producing a flexible polyurethane mold foam according to the present invention includes a method for producing a flexible polyurethane mold foam characterized by injecting the soft polyurethane foam stock solution (A) to (E) above into a mold and then foam-curing it. Can be used.

  The mold temperature when the foaming stock solution is poured into the mold is usually 30 ° C. or higher, preferably 40 ° C. or higher, and the upper limit is usually 70 ° C. or lower, preferably 60 ° C. or lower. When the mold temperature at the time of pouring the foaming stock solution into the mold is less than 30 ° C., the curing property is deteriorated. On the other hand, when the mold temperature is higher than 70 ° C., the foam surface is roughened, and the cost is increased due to an increase in energy cost. Can be a disadvantage.

  The time for curing the foaming stock solution in the mold is usually 3 minutes or longer, preferably 4 minutes or longer, and the upper limit is usually 8 minutes or shorter, preferably 6 minutes or shorter. If the curing time when the foaming stock solution is foamed and cured is shorter than 3 minutes, the curing property may be lowered. On the other hand, if it is longer than 8 minutes, the energy cost is increased and the obtained foam may shrink. is there.

  The liquid mixture obtained by the foaming machine is discharged into a mold, foamed and cured, and then demolded. In order to perform the demolding smoothly, it is also preferable to apply a release agent to the mold in advance. The release agent to be used may be a release agent usually used in the field of molding processing, and is not particularly limited. However, from the viewpoint of suppressing unnecessary foaming of the foaming stock solution, the wax solvent is used in the present invention. It is preferable to use a system release agent. In addition, the product after demolding can be used as it is, but in order to prevent foam shrinkage, the cell is destroyed by compression or decompression by a conventionally known method to stabilize the appearance and dimensions of the product. You can also.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example. Unless otherwise specified, “part” and “%” in the text are based on mass. Moreover, the raw material used in the Example and the comparative example is as follows.

In preparing the organic polyisocyanate, the raw materials used are described below.
MDI-1: dimeric methane diisocyanate containing 94% isomer MDI and 6% 4,4′-MDI, manufactured by Nippon Polyurethane Industry Co., Ltd. MDI-2: diphenylmethane diisocyanate containing 100% 4,4′-MDI, Nippon Polyurethane Industry MDI-3: Diphenylmethane diisocyanate containing 31% of 4,4'-MDI and 69% of polymeric MDI, manufactured by Nippon Polyurethane Industry Co., Ltd.

In preparing the polyol premix, the raw materials used are described below.
<Polyether polyol>
P-1: Polyoxyethylene polyoxypropylene polyol, nominal average functional group number 3, number average molecular weight 6000, product name FA-921 manufactured by Sanyo Chemical Industries
P-2: Polyoxyethylene polyoxypropylene polyol, nominal average functional group number 4, number average molecular weight 8000, Sanyo Chemical Industries, Ltd. Product name KC-285
P-3: Polyoxyethylene polyoxypropylene polyol, nominal average functional group number 3, number average molecular weight 3,500, product name FA-103, manufactured by Sanyo Chemical Industries
<Catalyst>
C-1: Triethylenediamine 33% DPG solution, product name TEDA-L33 manufactured by Tosoh Corporation
C-2: 70% bis (dimethylaminoethyl) ether, 30% dipropylene glycol, manufactured by Tosoh Corporation Product name Toyocat-ET
<Crosslinking material>
DEA: Diethanolamine, manufactured by Mitsui Chemicals Co., Ltd. <Foam stabilizer>
S-1: Silicone foam stabilizer, Momentive product name Y-10366

  In a reactor equipped with a stirrer, 70 kg of P-1, 30 kg of P-2, 5 kg of P-3, 5.8 kg of water, 0.4 kg of C-1, 0.1 kg of C-2, S- A polyol premix could be obtained by charging 1 kg of 1 and 0.5 kg of DEA and mixing uniformly.

Using a high pressure foaming machine manufactured by Klaus, a flexible polyurethane mold foam was produced in the following manner. The polyol premix and the isocyanate component temperature-controlled at a raw material temperature of 23 ± 2 ° C. were mixed at a high pressure under a predetermined ratio and discharged to a mold (400 mm × 400 mm × 70 mm thickness) temperature-controlled at 60 ± 2 ° C. After 5 minutes, the mold was removed from the mold and the foam was crushed. The obtained foam was allowed to stand for 16 hours or more at a temperature of 23 ± 2 ° C. and a relative humidity of 50 ± 5%. Then, it cut | judged to the magnitude | size required for each physical-property measurement, and measured the physical property. The results are shown in Table 1.
The physical properties are measured according to JIS K6400.
Further, in “Hardness Measurement Method A” in the JIS, a load when 25% of the foam thickness is compressed is referred to as 25% compression stress.

As shown in Table 1, flexible polyurethane foams could be obtained in Examples 1 to 3. That is, when the ratio of (A-1) is 70 to 90% in the organic polyisocyanate, the softness that ensures the tear strength is 4.5 N / cm or more, the elongation is 100% or more, and the wet heat compression residual strain is 25% or less. A form is obtained. On the other hand, when the polyisocyanate composition of Comparative Example 1 was used, the foam was depressed and the flexible polyurethane foam could not be satisfactorily filled into the mold. Moreover, when the polyisocyanate composition of Comparative Example 2 was used, the foaming ratio of the foam was low, and sufficient moldability could not be established. This phenomenon is attributed to the NCO content being lower than 32%.

In Table 2, when the ratio of isomer MDI to (A-1) is 50% or more, the tear strength is 4.5 N / cm or more, the elongation is 100% or more, and the wet heat compression residual strain is 25% or less. A flexible foam is obtained. When it is less than 50%, the 25% compressive stress exceeds 120 N, and it becomes difficult to use as a vehicle seat back for this purpose.

In Table 3, when the chlorine ion concentration in the organic polyisocyanate is 5 ppm or more, the curing property deteriorates, so that a flexible polyurethane foam could not be obtained. Generally, when the curing property is poor, the curing property can be improved by increasing the amount of the urethanization catalyst. However, if the catalyst amount is large, a catalyst odor is generated from the obtained flexible polyurethane foam, and the urethane foam. Does not show product value as a product. Further, since the catalyst occupies a large weight economically in the raw material, it is not preferable to increase the amount of the catalyst in order to solve the curing property.
The chlorine ion concentration can be measured by a known measurement method using ICP or the like.

As is clear from the results shown so far, all of the flexible polyurethane foams obtained by the method of the present invention using the polyisocyanate composition of the present invention are excellent in the above-mentioned physical properties (foam physical properties). It was confirmed that there was.

Accordingly, the flexible polyurethane foam obtained by the present invention is excellent in durability, supportability, etc., and is used for furniture cushion materials such as sofas, clothing, cushion materials for automobiles and railway vehicles, automobile interior materials, mattresses, pillows, etc. It is useful for bedding, sound-absorbing materials, sound-insulating materials, household electrical appliances, electronic parts, industrial sealing materials, packing materials, and household goods.

Claims (5)

A flexible polyurethane foam obtained by reactive foaming of a mixed liquid containing an organic polyisocyanate (A), a polyol (B), a chain extender (C), a catalyst (D) and a foaming agent (E), wherein the soft polyurethane The core density of the foam is in the range of 25 to 35 kg / m ³ , and the organic polyisocyanate (A) is 70 to 90% by mass of diphenylmethane diisocyanate with respect to 100% by mass of the organic polyisocyanate (A). (A-1) and 10 to 30% by mass of polymethylene polyphenylene polyisocyanate (A-2), and (A-1) is 2,4-diphenylmethane diisocyanate and 2,2- A flexible polyurethane foam comprising 50 to 100% of diphenylmethane diisocyanate. The diphenylmethane diisocyanate (A-1) has 50 to 90% of 2,4-diphenylmethane diisocyanate and 2,2-diphenylmethane diisocyanate, has an NCO content of 32% or more and a chlorine ion concentration of 5 ppm or less. The flexible polyurethane foam according to claim 1.   The polyol (B) has a nominal average functional group number in the range of 2 to 4 and a number average molecular weight in the range of 3000 to 8000. 3. The flexible polyurethane foam according to 1 or 2. The core density is 25 to 35 kg / m ³ , the wet heat compression set is 25% or less, the elongation is 100% or more, the tear strength is 4.5 N / cm ² or more, and the 25% compression stress is in the range of 80 to 120N. The flexible polyurethane foam according to any one of claims 1 to 3, wherein: The manufacturing method of the flexible polyurethane foam of any one of Claim 1 to 4.