Soft polyurethane foam, method of producing the same and vehicle interior member using the same

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The present application is based on Japanese Patent Applications Nos. 9-295713 filed Oct. 28, 1997, 10-224655 filed Aug. 7, 1998 and 10-286825 filed Oct. 8, 1998, the content of which is incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a soft polyurethane foam suitably used as a material of interior members of automotive vehicles, and a method of production thereof. In particular, the invention is concerned with such a soft polyurethane foam used in the vehicle interior members such as an energy absorbing headrest capable of protecting a passenger of the vehicle from suffering from a whiplash injury, and a seat back of a vehicle seat exhibiting good energy absorbing properties. Further, the invention is concerned with methods of producing the soft polyurethane foam.

2. Discussion of the Related Art

A soft polyurethane foam has been widely used in vehicle interior members such as a seat cushion, a headrest, an instrument panel and a steering wheel, as well as furniture and bedding, taking advantage of its high rebound resilience property. The soft polyurethane foam is required to exhibit desired properties depending upon its applications. In particular, when the soft polyurethane foam is used in the vehicle headrest, it is required to exhibit high rebound resilience property and suitable hardness, so as to give a good feel to the vehicle passenger, in other words, good cushioning characteristics.

In recent years, the vehicle is equipped with various kinds of energy absorbing devices in an attempt to protect a passenger of the vehicle upon a vehicle collision accident. For instance, an air bag is installed in the steering wheel, instrument panel or doors. As disclosed in JP-A-8-193118, a rigid polyurethane foam is provided in the inside of the door, or in the side portion of the seat back adjacent to the door, for the purpose of absorbing the energy applied to the vehicle upon its collision, to thereby protect the loins, shoulders and abdomen of the passenger upon the collision.

In general, the rigid polyurethane foam does not restore its original shape once it is deformed by stresses applied thereto. Accordingly, the rigid polyurethane foam is used in limited portions of the vehicle. That is, the rigid polyurethane foam cannot be used as a material of the vehicle interior members such as the headrest, seat back, instrument panel, steering wheel, door trim and pillar, which members are frequently contacted by the vehicle passenger and subject to stresses due to the frequent contact.

JP-A-2-52607 and JP-A-1-280413 disclose a seat cushion having an improved comfort as felt by the passenger. The disclosed seat cushion is formed of a combination of a soft polyurethane foam of low rebound resilience whose percentage is not higher than 25% and a soft polyurethane foam of high rebound resilience whose percentage is not lower than 55%. JP-A-7-67757 discloses a pillow which uses as a core material a soft polyurethane foam having a rebound resilience percentage of not higher than 5%, to thereby keep the head of the user in a stable state while the user is sleeping. JP-A-2-175713 discloses a method of producing a soft polyurethane foam of low rebound resilience whose percentage is not higher than 25% by using a polyether polyol having an average molecular weight of 400-2000 and an average functionality of 2-3.5.

The vehicle interior members which use the soft polyurethane foam having the low rebound resilience property assure good energy absorbing characteristics as well as a good feel as given to the passengers. Since the soft polyurethane foam with the low rebound resilience property can restore its original shape after it has been subjected to compression deformation, the soft polyurethane foam can be suitably used in the interior members for the energy absorbing purpose, unlike the rigid polyurethane foam described above.

In the conventionally proposed soft polyurethane foam having the low rebound resilience property, however, the hardness of the resin which gives the soft polyurethane foam rapidly increases with a decrease in the temperature of the soft polyurethane foam. In this case, the soft polyurethane foam is too hard to exhibit its excellent properties. For instance, when the soft polyurethane foam with the low rebound resilience property is used as a pad material for the vehicle headrest, it is soft enough to assure a good feel at the room temperature of 10.about.30.degree. C. In a cold condition at the temperature of 0.about.-20.degree. C., however, the resin hardness of the soft polyurethane foam undesirably rises to a value as high as that of the rigid polyurethane foam, deteriorating the energy absorbing characteristics of the soft polyurethane foam. In this case, the soft polyurethane foam is not likely to restore its original shape immediately after it has been subjected to compression deformation, and the feel as given to the vehicle passenger is considerably deteriorated.

In an attempt to lower the rebound resilience percentage of the soft polyurethane foam for improving its energy absorbing characteristics, the soft polyurethane foam used in the vehicle interior member is formed by using a polyol material whose average molecular weight is relatively low. However, when the polyol material with the low average molecular weight is used in forming the soft polyurethane foam, it undesirably takes a lot of time for the reaction with a polyisocyanate component for forming the polyurethane. In this case, when the soft polyurethane foam is obtained by a foaming operation using a suitable mold, it requires a relatively long period of time before the formed polyurethane foam can be removed from the mold. In other words, it requires a relatively long period of time to completely cure the polyurethane foam, so that its production efficiency is deteriorated. Thus, the use of the polyol material with the low average molecular weight is not practically desirable.

It is well known to use a mono-ol or a monohydric alcohol in producing the polyurethane foam. For instance, JP-A-52-21097, JP-A-2-163112, JP-A-3-109412, JP-A-4-146916, JP-A-5-202161 and JP-A-5-287047 teach the use of the monohydric alcohol in the reaction system for forming the soft polyurethane foam, whereby the obtained soft polyurethane foam has a desired softness value and a reduced weight, without suffering from scorching. JP-A-60-177035, JP-A-3-185018 and JP-A-7-18045 disclose that the use of the monohydric alcohol prevents an increase of the viscosity of materials used for producing the polyurethane, so that the materials can be sufficiently blended. JP-A-56-36514, JP-A-60-219220, JP-A-62-238256, JP-A-2-199111 and JP-A-5-84858 disclose the use of the monohydric alcohol in an attempt to prevent shrinkage of the obtained polyurethane foam upon its removal from the mold, and improve its mechanical strength, storage stability and soundproof characteristics in a lower frequency range. None of those publications, however, teach a soft polyurethane foam which exhibits low rebound resilience characteristics both at the room temperature and under the cold condition while assuring an excellent feel as given to the user. Moreover, none of the publications suggest the use of such a soft polyurethane foam as an energy absorbing member.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to provide a soft polyurethane foam which has a high degree of energy absorbing characteristics and assures a good feel as given to the user even when the temperature of the resin which gives the soft polyurethane foam is relatively low at the relatively low ambient temperature, and which soft polyurethane foam can be easily removed from a mold when it is formed by molding.

It is a second object of the invention to provide a method of producing such a soft polyurethane foam.

It is a third object of the invention to provide, by using such a soft polyurethane foam, a soft interior member for the vehicle, especially a vehicle headrest capable of protecting the passenger from suffering from a whiplash injury, and a seat back of a vehicle seat capable of absorbing energy so as to effectively support the back of the passenger.

As a result of an extensive study of the soft polyurethane foam by the inventors of the present invention, it was found that the feel or the cushioning characteristics of the polyurethane foam is affected by its temperature dependency of its hardness. It was further found that the temperature dependency of the hardness of the soft polyurethane foam relates to its temperature dependency of the compression force deflection when the polyurethane foam is compressed by 25%.

The above first object of the present invention may be attained according to a first aspect of the invention which provides a soft polyurethane foam which has a rebound resilience percentage of not higher than 30% and whose temperature dependency of compression force deflection represented by a difference between a 25%-compression force deflection value at -20.degree. C. and a 25%-compression force deflection value at +20.degree. C. is not larger than 0.030 kg/cm.sup.2.

The soft polyurethane foam constructed according to the above first aspect of the present invention exhibits a high degree of energy absorbing characteristics owing to its rebound resilience percentage of not higher than 30%. In the present soft polyurethane foam wherein the temperature dependency of the compression force deflection is not larger than 0.030 kg/cm.sup.2, the dependency of its hardness on a change of the ambient temperature is significantly reduced, whereby a difference between the hardness values of the resin of the soft polyurethane foam at the room temperature and under the cold condition is accordingly reduced. According to this arrangement, the soft polyurethane foam exhibits its excellent properties without suffering from deterioration of its feel as given to the user even when the temperature of the resin is relatively low.

In a first preferred form of the above first aspect of the invention, both of the 25%-compression force deflection value at -20.degree. C. and the 25%-compression force deflection value at +20.degree. C. are not larger than 0.110 kg/cm.sup.2. According to this arrangement, the soft polyurethane foam advantageously assures excellent feel under the cold condition as well as at the room temperature.

In a second preferred form of the above first aspect of the invention, the soft polyurethane foam is produced by reaction of a polyol component having an average molecular weight of 2000-8000 and an organic polyisocyanate, in the presence of a monohydric alcohol having a molecular weight of not larger than 100.

The inventors of the present invention found that the presence of the selected monohydric alcohol in the reaction system of the polyol component and the organic polyisocyanate is effective to lower the rebound resilience percentage of the obtained soft polyurethane foam. Owing to the inclusion of the monohydric alcohol, the obtained polyurethane foam has the rebound resilience percentage of not higher than 30% even when the polyol component having a high average molecular weight is used.

According to the above second preferred form of the first aspect of the present invention, the soft polyurethane foam has the rebound resilience percentage of not higher than 30%, and exhibits a high degree of energy absorbing characteristics. In this arrangement, the soft polyurethane foam has such a low rebound resilience percentage owing to the presence of the monohydric alcohol in the polyurethane-forming reaction which uses the polyol material (component) having a high average molecular weight. Accordingly, the hardness of the resin that gives the soft polyurethane foam is not largely dependent on a change of the ambient temperature, so that a difference between the resin hardness values at the room temperature and under the cold condition is reduced. In this case, the soft polyurethane foam is capable of exhibiting its excellent properties without suffering from deterioration of its feel as given to the user even when the resin temperature is lowered with a decrease in the ambient temperature. In addition, the use of the polyol component having a high average molecular weight is effective to reduce a time required time for curing the polyurethane foam, and to assure easy removal of the obtained polyurethane foam from the mold, resulting in the improvement of its productivity.

In a third preferred form of the above first aspect of the present invention, the soft polyurethane foam is produced by reaction of a polyol component and an organic polyisocyanate, wherein the -polyol component comprises polyoxyalkylene polyol having an average functionality of 2-8 and a hydroxyl equivalent of 900-3300, and the reaction is effected in the presence of a compound represented by the following formula (1), in an amount of 0.005-0.1 mol per 100 g of the polyol component, ##STR1## wherein, n is an integer of 0-20, x and y represent respective different ones of integers 0-20, and a sum of x and y is in a range of 0-20.

The inventors of the present invention found that the feel of the soft polyurethane foam varies depending upon the temperature dependency of its hardness, and that the temperature dependency of the hardness relates to the temperature dependency of the compression force deflection obtained on the basis of the 25%-compression force deflection values. Further, it was found that the desired soft polyurethane foam is obtained by the

polyurethane-forming reaction which uses the predetermined polyol component in the presence of the selected mono-ol compound.

The soft polyurethane foam produced according to the above third preferred form of the first aspect of the present invention has the rebound resilience percentage of not higher than 30% owing to the combined use of the polyol component and the mono-ol compound, whereby the obtained soft polyurethane foam exhibits a high degree of energy absorbing characteristics. Further, since the present soft polyurethane foam has the temperature dependency of the compression force deflection of not larger than 0.030 kg/cm.sup.2, its hardness is not siginificantly dependent on a change of the ambient temperature, whereby a difference between the resin hardness values at the room temperature and under the cold condition is appreciably reduced. Accordingly, the present soft polyurethane foam does not suffer from deterioration of its feel even when the resin temperature is lowered as the ambient temperature is lowered.

In a fourth preferred form of the above first aspect of the present invention, n is an integer of 0-9, x and y represent respective different ones of integers 0-10, and a sum of x and y is in a range of 0-10.

In a fifth preferred form of the above first aspect of the present invention, the compound represented by the above formula (1) is nonyl phenol or a substance obtained by addition polymerization of 1-10 mol of ethylene oxide to the nonyl phenol.

The above second object of the present invention may be attained according to a second aspect of the invention, which provides a method of producing a soft polyurethane foam by reacting an active hydrogen compound with an organic polyisocyanate using water, wherein the active hydrogen compound is a mixture of polyether triol having a hydroxyl equivalent of 900-1500 and containing 5-30 wt. % of terminal polyoxy ethylene units, and polyether diol having a hydroxyl equivalent of 1010-1500 and containing 5-30 wt. % of terminal polyoxy ethylene units, a weight ratio of the polyether triol to the polyether diol being in a range of 80/20.about.0/100, and the water is used in an amount of 2.0-6.0 parts by weight per 100 parts by weight of the active hydrogen compound.

In a first preferred form of the above second aspect of the present invention, the organic polyisocyanate is a mixture of polymethylene polyphenylisocyanate and 2,4-tolylenediisocyanate and/or 2,6-tolylenediisocyanate, a weight ratio of the polymethylene polyphenylisocyanate to the 2,4-tolylenediisocyanate and/or 2,6-tolylenediisocyanate being in a range of 100/0.about.70/30.

In a second preferred form of the above second aspect of the present invention, the organic polyisocyanate is a prepolymer having terminal isocyanate groups, the prepolymer being obtained by reacting polymethylene polyphenylisocyanate with at least one polyether polyol which has an average functionality of 1-4 and a hydroxyl equivalent of 800-2000 and which is obtained by addition polymerization of alkylene oxide having three or more carbon atoms, and ethylene oxide, in a molar ratio of 20/80.about.100/0.

In a third preferred form of the above second aspect of the present invention, the polyether triol and the polyether diol are mixed in a weight ratio of 70/30.about.40/60.

The above second object of the present invention may be also attained according to a third aspect of the invention, which provides a method of producing a soft polyurethane foam having a rebound resilience percentage of not higher than 30%, by reaction of a polyol component having an average molecular weight of 2000-8000 with an organic polyisocyanate in the presence of water, wherein the reaction is effected in the presence of a monohydric alcohol having a molecular weight of not larger than 100.

In a first preferred form of the above third aspect of the invention, the monohydric alcohol is used in an amount of 0.1-15 parts by weight per 100 parts by weight of the polyol component.

In a second preferred form of the above third aspect of the invention, the monohydric alcohol is used in an amount of 0.5-5 parts by weight per 100 parts by weight of the polyol component.

In a third preferred form of the above third aspect of the invention, the polyol component contains 30-70 wt. % of a polymer polyol. Alternatively, a polyether polyol and/or a polyester polyol, and the polymer polyol are mixed in a weight ratio of 30/70.about.70/30, so as to provide the polyol component.

In a fourth preferred form of the above third aspect of the invention, the polyol component has an average molecular weight of 3000-6000. According to this arrangement, the soft polyurethane foam to be obtained has the desired low rebound resilience percentage and the desired temperature dependency of its hardness while assuring easy removal thereof from the mold.

The above second object of the present invention may be also attained according to a fourth aspect of the invention, which provides a method of producing a soft polyurethane foam by reaction of an organic polyisocyanate and a mixture of a polyol component and water, the soft polyurethane foam having a rebound resilience percentage of not higher than 30%, and temperature dependency of compression force deflection represented by a difference between a 25%-compression force deflection value at -20.degree. C. and a 25%-compression force deflection value at +20.degree. C. being not larger than 0.030 kg/cm.sup.2, wherein the polyol component comprises polyoxyalkylene polyol having an average functionality of 2-8 and a hydroxyl equivalent of 900-3300, and the reaction is effected in the presence of a compound represented by the following formula (1), in an amount of 0.005-0.1 mol per 100 g of the polyol component, ##STR2## wherein, n is an integer of 0-20, x and y represent respective different ones of integers 0-20, and a sum of x and y is in a range of 0-20.

In a first preferred form of the above fourth aspect of the present invention, n is an integer of 0-9, x and y represent respective different ones of integers 0-10, and a sum of x and y is in a range of 0-10.

In a second preferred form of the above fourth aspect of the present invention, the compound represented by the above formula is nonyl phenol or a substance obtained by addition polymerization of 1-10 mol of ethylene oxide to the nonyl phenol.

In a fourth preferred form of the above fourth aspect of the present invention, both of the 25%-compression force deflection value at -20.degree. C. and the 25%-compression force deflection value at +20.degree. C. are not larger than 0.110 kg/cm.sup.2.

The above third object of the present invention may be attained according to a fifth aspect of the invention, which provides a vehicle interior member comprising a skin layer, and a pad layer covered by the skin layer so as to absorb external energy applied to the vehicle interior member, wherein at least a portion of the pad layer is formed of a soft polyurethane foam which has a rebound resilience percentage of not higher than 30% and whose temperature dependency of compression force deflection represented by a difference between a 25%-compression force deflection value at -20.degree. C. and a 25%-compression force deflection value at +20.degree. C. is not larger than 0.030 kg/cm.sup.2.

The vehicle interior member constructed according to the above fifth aspect of the present invention exhibits a high degree of energy absorbing characteristics while assuring a good feel as given to the user.

In a first preferred form of the above fifth aspect of the present invention, both of the 25%-compression force deflection value at -20.degree. C. and the 25%-compression force deflection value at +20.degree. C. are not larger than 0.110 kg/cm.sup.2.

In a second preferred form of the above fifth aspect of the present invention, the vehicle interior member is a headrest comprising a bag-like skin member as the skin layer, the skin member having an inside space, a pad material as the pad layer, the pad material filling the inside space of the skin member, and a core member having an inner portion fixed within the pad material and an outer portion projecting from a surface of the skin member, wherein at least a portion of the pad material is formed of the soft polyurethane foam.

The headrest constructed according to the above second preferred form effectively absorbs a shock when the passenger hits the head against the headrest in the event of a rear-end collision of the vehicle, so as to protect the passenger from a whiplash injury.

In a third preferred form of the above fifth aspect of the present invention, the vehicle interior member is a seat back comprising a seat back cover as the skin layer, a seat back frame, and a seat back pad as the pad layer, the seat back pad being interposed between the seat back cover and the seat back frame, wherein at least a portion of the seat back pad is formed of the soft polyurethane foam.

In a fourth preferred form of the above fifth aspect of the present invention, the soft polyurethane foam is produced by reaction of a polyol component having an average molecular weight of 2000-8000 and an organic polyisocyanate, in the presence of a monohydric alcohol having a molecular weight of not larger than 100.

In a fifth preferred form of the above fifth aspect of the present invention, the polyurethane foam is produced by reaction of a polyol component with an organic polyisocyanate, wherein the polyol component comprises polyoxyalkylene polyol having an average functionality of 2-8 and a hydroxyl equivalent of 900-3300, and the reaction is effected in the presence of a compound represented by the following formula (1), in an amount of 0.005-0.1 mol per 100 g of the polyol component, ##STR3## wherein, n is an integer of 0-20, x and y represent respective different ones of integers 0-20, and a sum of x and y is in a range of 0-20.

In a sixth preferred form of the above fifth aspect of the present invention, n is an integer of 0-9, x and y represent respective different ones of integers 0-10, and a sum of x and y is in a range of 0-10.

In a seventh preferred form of the above fifth aspect of the present invention, the compound represented by the above formula is nonyl phenol or a substance obtained by addition polymerization of 1-10 mol of ethylene oxide to the nonyl phenol.

DESCRIPTION OF THE DRAWINGS

The above and optional objects, features, advantages and technical significance of the present invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in conjunction of the accompanying drawings, in which:

FIG. 1 is a longitudinal cross sectional view of a headrest as one example of a vehicle interior member constructed according to the present invention;

FIG. 2 is an elevational view, partly in section of a seat back as another example of the vehicle interior member constructed according to the present invention;

FIG. 3 is a view schematically explaining an experiment using headrests according to the present invention and according to comparative examples;

FIG. 4 is a graph showing a time-displacement curve upon application of an impact to the headrest of Example 4 according to the first embodiment and the headrest of Example 23 according to the third embodiment of the present invention: and

FIG. 5 is a graph showing a time-displacement curve upon application of an impact to the headrest of Comparative Example 4 according to the first embodiment and the headrest of Comparative Example 11 according to the third embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The soft polyurethane foam according to the present invention has flexibility that permits free deformation thereof upon receiving a load, and restores its original shape upon removal of the load therefrom. The present soft polyurethane foam has a rebound resilience percentage of not higher than 30%, preferably not higher than 25%, so that the soft polyurethane foam exhibits a high degree of energy absorbing characteristics. The rebound resilience percentage is measured in accordance with a method A specified in section "9. Rebound resilience" of JIS-K 6400 (1997). If the rebound resilience percentage exceeds 30%, the soft polyurethane foam does not exhibit sufficient energy absorbing characteristics.

The soft polyurethane foam according to the present invention has temperature dependency of compression force deflection of not larger than 0.030 kg/cm.sup.2. Described in detail, the temperature dependency of the compression force deflection is represented by a difference between a 25%-compression force deflection value at -20.degree. C. and a 25%-compression force deflection value at +20.degree. .C [.DELTA.25%CFD=(25%-compression force deflection value at -20.degree. C.)-(25%-compression force deflection value at +20.degree. C.)]. According to this arrangement, the hardness of the soft polyurethane foam is not significantly dependent on a change of the ambient temperature. Accordingly, even when the temperature of the resin that gives the soft polyurethane foam is comparatively low at a comparatively low ambient temperature, the soft polyurethane foam does not suffer from deterioration of its feel as given to the user.

The above-described feel of the polyurethane foam is evaluated by a sensory test, in which the polyurethane foam is examined of its shape restoring characteristic after it is compressed with a predetermined force. Namely, the polyurethane foam has a good feel if it restores its original shape immediately after removal of the compressive force, while the polyurethane foam has a bad feel if it restores its original shape a long time after the removal of the compressive force. The above-described 25%-compression force deflection values of the polyurethane foam are measured according to ASTM-D-3574 at predetermined temperature values, i.e., at -20.degree. C. and +20.degree. C., respectively. In general, it takes a relatively long period of time for the polyurethane foam to restore its original shape when it receives a load at a relatively low temperature. The polyurethane foam is compressed by 25% at -20.degree. C. and at +20.degree. C., respectively. Namely, the 25%-compression force deflection values are obtained by compression to reduce the initial height of the polyurethane foam by 25%. On the basis of the 25%-compression force deflection values obtained as described above, the temperature dependency of the compression force deflection (.DELTA.25%CFD) is obtained. If the temperature dependency of the compression force deflection exceeds 0.030 kg/cm.sup.2, the feel of the polyurethane foam deteriorates due to a rapid increase of its hardness when the resin temperature of the polyurethane foam is relatively low. In view of this, the temperature dependency of the compression force deflection (.DELTA.25%CFD) needs to be not larger than 0.030 kg/cm.sup.2, preferably not larger than 0.020 kg/cm.sup.2.

In the present soft polyurethane foam, both of the 25%-compression force deflection value at -20.degree. C. and the 25%-compression force deflection value at +25.degree. C. are not larger than 0.110 kg/cm.sup.2, preferably in the range of 0.020-0.080 kg/cm.sup.2, more preferably in the range of 0.030-0.070 kg/cm.sup.2, still more preferably in the range of 0.040-0.070 kg/cm.sup.2, whereby the soft polyurethane foam can exhibit its excellent properties to a satisfactory extent. If the 25%-compression force deflection values at -20.degree. C. and +20.degree. C. exceed 0.110 kg/cm.sup.2, the hardness of the soft polyurethane foam increases to a great extent, whereby the feel of the cushioning characteristics of the soft polyurethane foam is considerably deteriorated.

The soft polyurethane foam having the rebound resilience percentage and temperature dependency of the compression force deflection (.DELTA.25%CFD) as specified above is used as a material for vehicle interior members such as a steering wheel, an instrument panel, a seat cushion, a seat back, a door trim, a pillar, a knee pad and a headrest. The vehicle interior members formed of the present soft polyurethane foam exhibit a high degree of energy absorbing characteristics and assure a good feel as felt by the vehicle passenger. Described in detail, the vehicle interior members

generally include a skin layer, a frame member or a core member, and a pad layer which is interposed therebetween and which absorbs external energy applied to the interior members. In the present invention, at least a portion of the pad layer is formed of the soft polyurethane foam having the rebound resilience percentage and temperature dependency of the compression force deflection (.DELTA.25%CFD) as specified above.

For instance, the present soft polyurethane foam is used in a headrest as one example of the vehicle interior members. Referring to FIG. 1, there is shown such a headrest including a bag-like skin member 2 (as the skin layer) having an inside space, a pad material 10 (as the pad layer) filling the inside space of the skin member 2, and a core member 8 having an insert 4 fixed within the pad material 10 and a stay 6 fixed to the insert 4 and projecting from the surface of the skin member 2. In this headrest, at least a portion of the pad material 10 is formed of the soft polyurethane foam according to the present invention. The headrest is attached to a seat back of a vehicle seat such that the stay 6 is fixed to the upper portion of the seat back, so that the headrest absorbs a shock generated upon hitting of the head of the vehicle passenger against the headrest in a direction indicated by an arrow in FIG. 1. Accordingly, in a collision or crash accident of the vehicle, the shock generated upon hitting of the head against the headrest in the direction indicated by the arrow is effectively absorbed by the soft polyurethane foam which at least partially forms the pad material 10 of the headrest. Accordingly, the vehicle passenger is protected from suffering from a whiplash injury.

Referring next to FIG. 2, there is shown a seat back 12 of a vehicle seat as another example of the vehicle interior members, in which the present soft polyurethane foam is used. Described in detail, the vehicle seat shown in FIG. 2 consists of the seat back 12 which supports the back of the passenger, and a seat cushion 14 which supports the buttocks. The seat back 12 includes a seat back frame 16, a seat back pad 18 as the pad layer, and a seat back cover 20 (as the skin layer) covering the seat back pad 18. In the present invention, at least a portion of the seat back pad 18 is formed of the soft polyurethane foam having the rebound resilience percentage and temperature dependency of the compression force deflection as specified above, whereby the seat back 12 exhibits excellent energy absorbing characteristics, and assures a good feel or good cushioning characteristics.

According to a first embodiment of the present invention, the soft polyurethane foam having the excellent properties as described above is formed by reaction of an active hydrogen compound and an organic polyisocyanate, in the presence of water. As the active hydrogen compound, a polyether triol having a hydroxyl equivalent of 900-1500 and containing 5-30 wt. % of terminal polyoxyethylene units connected to its ends by addition polymerization of ethylene oxide, and a polyether diol having a hydroxyl equivalent of 1010-1500 and containing 5-30 wt. % of terminal polyoxyethylene units connected to its ends by addition polymerization of ethylene oxide are mixed in a weight ratio of 80/20.about.0/100.

Described more specifically, the polyether triol is obtained by addition polymerization of at least one kind of alkylene oxide to a starting material having a functionality of 3, such that the polyether triol to be obtained has the hydroxyl equivalent of 900-1500, and such that the content of the terminal polyoxyethylene units connected to the molecule ends of the polyether triol among all alkylene oxide units connected to the starter by the addition polymerization is 5-30 wt. %. The polyether diol is obtained by addition polymerization of at least one kind of alkylene oxide to a starting material having a functionality of 2 and, such that the polyether diol to be obtained has a hydroxyl equivalent of 1010-1500, and such that the content of the terminal polyoxyethylene units connected to the molecule ends of the polyether diol among all alkylene oxide units connected to the starting material by the addition polymerization is 5-30 wt. %. The hydroxyl equivalent is obtained by dividing the molecular weight of the active hydrogen compound by the functionality of the starting material.

As the starting material having the functionality of 3 that gives the polyether triol described above, trihydroxy compounds such as glycerin and trimethylolpropane or alcanol amines such as triethanol amine and diethanol amine are used. As the starting material having the functionality 2 th