Production of polyurethane foam

Flame-retardant polyurethane foam having a very good skin weldability and a very low level of skin staining is produced from a polyhydroxy compound, organic polyisocyanate, blowing agent, catalyst, foam stabilizer, and flame retardant, and other additives, wherein the flame retardant contains a triaryl phosphate ester and the polyhydroxy compound is a mixture composed of 100 parts by weight of high-molecular-weight polyhydroxy compound having a hydroxyl number lower than 100 and 0.5 to 10 parts by weight of low-molecular-weight polyhydroxy compound having a hydroxyl number higher than 560 and having 2 to 4 hydroxyl groups in one molecule.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing a flexible 
polyurethane foam. More particularly, it relates to a flame-retardant 
weldable polyurethane foam having a skin improved in stain resistance. 
2. Description of the Prior Art 
Polyurethane foam is in general use as cushioning materials for vehicles, 
furniture, bedding and the like because of its superior elastic 
properties. It also finds use as building materials and heat insulation 
materials. Polyurethane foam used as cushioning materials is usually 
covered with a skin material such as cloth or leather which is welded to 
the surface thereof. 
Since polyurethane foam is rather combustible, studies have long been made 
to render it flame retardant. One way of achieving this object is to 
incorporate a proper flame retardant into the raw materials of 
polyurethane foam. Flame retardants in common use are as follows: 
(a) Liquid flame retardants based on halogen-containing phosphate ester or 
phosphite ester; e.g., tris-2,3-dichloropropyl phosphate. 
(b) Powder flame retardants composed of antimony trioxide and 
halogen-containing compound; e.g., a composition of antimony trioxide and 
vinyl chloride. 
(c) Phosphorus-containing compounds or halogen-containing compounds used 
alone; e.g., chlorinated paraffin. 
The flame retardant belonging to the above category (a) is good in flame 
retardance and has only a small effect on the welding of skin materials; 
however, it has a disadvantage of staining the skin material. In other 
words, it migrates to the skin material at the time of welding and stains 
it gradually with time. The flame retardant belonging to the above 
category (b) is good in flame retardance and has only a slight tendency 
toward skin staining; however, it impairs the weldability completely. The 
flame retardant belonging to the above category (c) is less effective than 
others in flame retardance and is required to be added in large amounts to 
polyurethane foam. Moreover, it has a disadvantage of markedly impairing 
the weldability. 
With the conventional flame retardants, it was difficult to improve the 
flame retardance of polyurethane foam while keeping the weldability and 
avoiding the skin staining. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a process for 
producing a flexible polyurethane foam having an excellent flame 
resistance as well as very good skin weldability and very low level of 
skin staining. 
In order to attain the above object, the present inventors carried out a 
series of studies, which led to the discovery that a specific triaryl 
phosphate ester is effective as a flame retardant for polyurethane foam. 
More specifically, the present inventors investigated the skin staining 
caused by a flame retardant and found the following. 
(i) Skin staining is less likely to occur where the flame retardant is a 
hydrocarbon compound containing phosphorus alone in addition to C, H, and 
O. 
(ii) Skin staining occurs where the flame retardant is a hydrocarbon 
compound containing both phosphorus and halogen in addition to C, H, and 
O. 
(iii) Skin staining is liable to occur where the flame retardant is a 
liquid halogen-containing phosphate ester. This compound is easy to 
hydrolyze on heating or after use for a long time to give hydrogen 
chloride which in turn decomposes the phosphate ester and forms phosphorus 
chlorides which causes skin staining. Powder flame retardants are less 
liable to cause skin staining than liquid flame retardants. 
The present inventors continued their studies and found that the triaryl 
phosphate ester represented by the formula: 
##STR1## 
[where R.sub.1 to R.sub.6 are a substituent group selected from H and 
C.sub.n H.sub.2n+1 (where n is an integer of 1 to 9), with the exception 
of an instance where R.sub.1 =R.sub.3 =R.sub.5 =H and R.sub.2 =R.sub.4 
=R.sub.6 =CH.sub.3 ] is a good flame retardant with no tendency toward 
skin staining. The triaryl phosphate ester excludes the tricresyl 
phosphate represented by the following formula: 
##STR2## 
because its flame-resistan is little. 
Therefore, according to the present invention, there is provided a process 
for producing a flexible polyurethane foam from a polyhydroxy compound, 
organic polyisocyanate, blowing agent, catalyst, foam stabilizer, and 
flame retardant, and other additives, wherein the flame retardant contains 
a triaryl phosphate ester represented by the following formula; 
##STR3## 
[where R.sub.1 to R.sub.6 are a substituent group selected from H and 
C.sub.n H.sub.2n+1 (where n is an integer of 1 to 9), with the exception 
of an instance where R.sub.1 =R.sub.3 =R.sub.5 =H and R.sub.2 =R.sub.4 
=R.sub.6 =CH.sub.3 ], and the polyhydroxy compound is a mixture composed 
of 100 parts by weight of high-molecular-weight polyhydroxy compound 
having a hydroxyl number of lower than 100 and 0.5 to 10 parts by weight 
of low-molecular-weight polyhydroxy compound having a hydroxyl number of 
higher than 560 and having 2 to 4 hydroxyl groups in one molecule. 
The process of this invention has the effect of producing the 
flame-retardant flexible polyurethane foam having very good skin 
weldability and very low level of skin staining in an industrially 
advantageous way. 
The above and other objects, features and advantages of the present 
invention will be more apparent from the following description. 
DETAILED DESCRIPTION OF THE INVENTION 
The polyhydroxy compound used in this invention is a mixture of a 
high-mocular-weight polyhydroxy compound having terminal hydroxyl groups 
(which is commonly called polyether polyol) and a low-molecular-weight 
polyhydroxy compound having terminal hydroxyl groups. 
The polyether polyol includes, for example, high-molecular-weight 
polyhydroxy compounds having a hydroxyl number of lower than 100, which 
are produced by ring-opening addition polymerization of a polyhydroxy 
compound (such as glycerol, trimethylolpropane, or pentaerythritol) and an 
alkylene oxide such as propylene oxide or ethylene oxide by using an 
alkali catalyst. Those commonly used for the production of flexible 
polyurethane foam are suitable. A preferred example is poly(oxypropylene) 
triol which is produced by reacting glycerol with propylene oxide. 
The low-molecular-weight polyhydroxy compound may be an aliphatic alcohol 
which has a hydroxyl number of greater than 560 and has 2 to 4 hydroxy 
groups in one molecule. Examples of such a compound include diols such as 
ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, 
1,4-butanediol, 1,3-propanediol, and triethylene glycol; and triols such 
as glycerol, trimethylolpropane, triethylolpropane, trimethylolethane, 
triethylolethane, pentaerythritol, and 1,2,6-hexanetriol. 
In the case where a high-molecular-weight polyhydroxy compound and a 
low-molecular-weight polyhydroxy compound are used in combination as the 
polyhydroxy compound, the resulting flexible polyurethane foam has 
improved flame-retardance and weldability. One hundred parts by weight of 
high-molecular-weight polyhydroxy compound is mixed with 0.5 to 10 parts 
by weight, preferably 0.5 to 5 parts by weight, of low-molecular-weight 
polyhydroxy compound. If the amount of the latter compound is less than 
the lower limit specified above, the resulting polyurethane foam is not 
sufficiently improved in flame retardance and weldability. Conversely, if 
it is in excess of the upper limit, the resulting polyurethane foam is of 
closed cell type which shrinks during the cooling process in production. 
The organic polyisocyanate used in this invention is one which is commonly 
used for the production of polyurethane foam. It is an aliphatic 
polyisocyanate or aromatic polyisocyanate or a modified product thereof 
containing two or more isocyanate groups in one molecule. Examples of 
aliphatic polyisocyanate include hexamethylene diisocyanate, isophorone 
diisocyanate, and methylcyclohexane diisocyanate; and examples of aromatic 
polyisocyanate include tolylene diisocyanate(2,4-isomer and/or 
2,6-isomer), diphenyl diisocyanate, triphenyl diisocyanate, 
chlorophenyl-2,4-diisocyanate, p-phenylene diisocyanate, xylylene 
diisocyanate, diphenylmethane diisocyanate, naphthylene diisocyanate, and 
polyisocyanates which are obtained by the reaction of a low-molecular 
polycondensate of aniline and formaldehyde with phosgene. Among them, the 
one in which the ratio of 2,4-isomer to 2,6-isomer is 80:20 to 65:35 is 
economically preferable because of its availability at a low price. 
The isocyanate index may preferably be from 80 to 130, which is the ratio 
of the amount of polyisocyanate used to the total amount of polyhydroxy 
compounds and other compounds having active hydrogen atoms. The preferred 
isocyanate index is 100 to 115 if the resulting polyurethane foam is to 
have improved flame retardance and a low level of smoking. The more 
desirable isocyanate index is 103 to 112 if the resulting urethane foam is 
to have improved physical properties. 
The blowing agent used in this invention is water or a low-boiling volatile 
liquid. Examples of low-boiling volatile liquids are halogenated 
hydrocarbons which include trichloromonofluoromethane, 
dibromodifluoromethane, dichlorodifluoromethane, 
dichlorotetrafluoroethane, monochlorodifluoromethane, 
trifluoroethylbromide, dichloromethane, and methylene chloride. They may 
be used alone or in combination with one another. 
The catalyst used in this invention is an amine compound such as 
triethylenediamine, N-methylmorpholine, tetramethyl-1,4-butanediamine, 
N-methylpiperazine, dimethylethanolamine, diethylethanolamine, and 
triethylamine; and an organometallic compound such as stannous octoate, 
dibutyltin dilaurate, and dibytyltin di-2-ethylhexoate. They may be used 
alone or in combination with one another. The catalyst is used in a broad 
range of amount; usually 0.005 to 2.0 parts by weight for 100 parts by 
weight of polyhydroxy compound. 
The foam stabilizer used in this invention is a silicone-based foam 
stabilizer which is commonly used in the production of polyurethane foam. 
Examples of foam stabilizers are organosilicon surfactants which include 
organosiloxane-polyoxyalkylene copolymer and polyalkenyl siloxane having 
polyoxyalkylene side chains. It is used in an amount of 0.1 to 3.0 parts 
by weight, preferably 0.5 to 2.0 parts by weight, for 100 parts by weight 
of polyhydroxy compound. 
According to this invention, a triaryl phosphate ester represented by the 
following formula is used as a flame retardant. 
##STR4## 
[where R.sub.1 to R.sub.6 are a substituent group selected from H and 
C.sub.n H.sub.2n+1 (where n is an integer of 1 to 9), with the exception 
of an instance where R.sub.1 =R.sub.3 =R.sub.5 =H and R.sub.2 =R.sub.4 
=R.sub.6 =CH.sub.3 ]. This compound is explained below. 
The preferred triaryl phosphate ester contains 6 to 9.5% by weight of 
phosphorus. The typical four examples are given below. 
##STR5## 
The above listed triaryl phosphate esters may be used individually or in 
combination with one another. In addition, the triaryl phosphate ester may 
be used in combination with other flame retardants, in which case the 
content of triaryl phosphate ester should Preferably be more than 50% by 
weight; otherwise, there would be more chances of skin staining. 
The triaryl phosphate ester should be used in an amount of 5 to 30 parts by 
weight, preferably 7 to 20 parts by weight, for 100 parts by weight of the 
above-mentioned high-molecular-weight polyhydroxy compound. Any amount 
below this limit is not effective in improving flame retardance, and any 
amount over this limit causes skin staining and aggravates weldability. 
It is within the scope of this invention to add additives such as 
cross-linking agent, anti-oxidant, and pigment, as required, to the 
above-mentioned components. 
According to this invention, polyurethane foam is produced in the commonly 
practiced manner, for example, the so-called one-shot process in which the 
polyhydroxy compounds, water, catalyst, flame retardant, foam stabilizer, 
and polyisocyanate are mixed all together at a time and are allowed to 
react to make foam, or the prepolymer process in which a portion of the 
polyhydroxy compounds is previously reacted with all the polyisocyanate 
and the thus prepared prepolymer is subsequently mixed with the remainder 
of the polyhydroxy compounds to make foam. In the foaming process, a 
predetermined amount of catalyst is uniformly mixed with the polyhydroxy 
compounds. 
The polyurethane foam produced according to the process of this invention 
is useful as a wadding material, celling panel, and vehicle foam which is 
used at comparatively high temperatures. 
According to this invention, it is possible to improve the flame retardance 
of polyurethane foam while keeping good weldability, without causing skin 
staining, by using the above-mentioned specific triaryl phosphate ester as 
a flame retardance and a mixture composed of a high-molecular-weight 
polyhydroxy compound having a hydroxyl number of lower than 100 and a 
low-molecular-weight polyhydroxy compound having a hydroxyl number of 
higher than 560 and having 2 to 4 hydroxyl groups in one molecule.

The invention is now described in more detail with reference to the 
following examples, which should not be construed to restrict the scope of 
this invention. 
EXAMPLES 
Different kinds of flexible polyurethane foam were produced in the usual 
one-shot process. The formulation of raw materials is shown in Tables 1 
and 2. The resulting polyurethane foam had the physical properties, flame 
retardance, level of surface staining, and peel strength for welded nylon 
cloth as shown in Table 2. 
In the examples, the flame retardance was measured according to FMVSS-302 
(burning test for vehicle interior materials). The burning rate was 
measured for foam test specimens 6.0 mm in thickness. The level of skin 
staining was determined by visually inspecting the discoloration of a 
nylon cloth fixed by stapling in close contact with a foam specimen 
measuring 100.times.50.times.10 mm. The foamcloth assembly was enclosed in 
a 500-cc wide-mouthed bottle, and the bottle was placed in a 
constant-temperature bath at 100.degree. C. for 200 hours prior to 
inspection. 
TABLE 1 
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Formulation (parts by weight) 
______________________________________ 
Triol (Molecular Weight 3000) *1 
100 
Diethylene glycol *2 1.5 
Tolylene diisocyanate *3 
44.2 
Water 3.0 
Triethylenediamine 0.15 
F-258 *4 1.2 
Stannous octoate 0.25 
Trichloromonofluoromethane 
3.0 
______________________________________ 
*1 Highmolecular-weight polyol. 
*2 Lowmolecular-weight polyol. 
*3 Ratio of 2,4/2,6-isomers = 80/20 
*4 Polyalkylsiloxanepolyoxyethylene-polyoxypropylne copolymer made by 
ShinEtsu Chemical Co., Ltd. 
TABLE 2 
__________________________________________________________________________ 
Invention Comparison 
1 2 3 4 5 
__________________________________________________________________________ 
Flame retardant, Product 
Mixture of triaryl phosphates* 
I** II*** 
Amount (parts by weight) 
3 10 15 10 10 
Physical properties 
Density (g/cm.sup.3) 
0.0293 
0.0304 
0.0312 
0.0305 
0.0301 
25% hardness (kgf) 
10.8 9.6 9.2 9.5 9.2 
Tensile strength (kg/cm.sup.2) 
1.13 1.09 1.10 1.08 1.06 
Elongation (%) 
185 195 203 200 197 
Compression set (%) 
2.4 2.6 2.6 2.7 3.1 
under heating 
Flame retardance 
Burning rate (mm/min) 
124 S.E. S.E. S.E. S.E. 
Judgment Fail Pass Pass Pass Pass 
Level of skin staining 
Low Low Low High Very high 
Peel strength (g/inch) 
450 430 400 420 440 
__________________________________________________________________________ 
*A mixture of the abovementioned compounds (1) to (4) 
**Tris(2,3dichloropropyl)phosphate 
***Polychlorophosphonate 
S.E. stands for selfextinguishing. 
It is to be noted for Table 2 that the flame-retardant polyurethane foam 
produced according to the process of this invention has good skin 
weldability and very low level of skin staining. Moreover, the 
polyurethane foam is not impaired in physical properties. It is also noted 
that the flame retardance of polyurethane foam is greatly improved when 
more than 5 parts by weight of triaryl phosphate ester is added for 100 
parts by weight of high-molecular-weight polyhydroxy compound.