Flexible polyurethane foam based on MDI

A flexible, hydrophobic polyurethane foam is produced by mixing together an aqueous phase which can optionally contain reinforcing fibers and surfactants and a resin phase comprising a prepolymer derived from a poly (oxy C.sub.2-4 alkylene) diol having a molecular weight of at least about 1100, an MDI containing isocyanate product having a functionality greater than 2.0 made of a mixture of MDI and isocyanate containing derivatives of MDI, and a monomeric polyol crosslinking agent having 3 or 4 hydroxyl equivalents per mole. The preferred polyol is trimethylolpropane and the diol is an equimolar mixture of Carbowax 1000 and Carbowax 1450. The diol and the polyol, such as trimethylolpropane, are present in a mole ratio in the range of 4:1 to 8:1 while the ratio of the isocyanate equivalents to the total hydroxyl equivalents is in the range of 2.5:1 to 3.5:1. The isocyanate containing product comprises less than 50% by weight of the prepolymer and Isonate 143L is the preferred isocyanate product. Flexible foams are obtained which are water-absorbing for medical or personal care applications.

CROSS-REFERENCE TO RELATED APPLICATION 
U.S. Application Ser. No. 314,555 filed Oct. 26, 1981 discloses flexible 
MDI based polyurethane foams made with a polymeric polyol cross-linking 
agent having 3 or 4 hydroxyl equivalents per mole and a molecular weight 
of at least 500. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a polyurethane prepolymer and the resulting 
flexible foam that can be made from the prepolymer when it is mixed with 
water. 
2. Description of Previously Published Art 
Flexible polyurethane foams made with TDI (toluene diisocyanate) have been 
manufactured for many years, especially for cushion and mattress 
applications. However, for hydrophilic foams used in medical or personal 
care applications it is desirable to replace the TDI in the foams with MDI 
(methylene diphenyl isocyanate) because of the high vapor pressure and 
relatively high toxicity of TDI which requires special precautionary 
measures during processing and use. Furthermore, TDI-based foams can be 
weakened by hydrolysis during sterilization or storage in a wet package. 
For example, TDI-based hydrophilic foams can liquify after a few cycles in 
a steam autoclave at 120.degree. C. TDI-based hydrophilic foams also swell 
excessively when wet such as on the order of more than 100% by volume. 
Conventional polyurethane foams have been made from MDI. These foams are 
rigid or semi-rigid because MDI imparts crystallinity. In British Pat. No. 
874,430, flexible polyurethane foams are produced by reaction of polyether 
polyols with at least two hydroxyl groups and a polyisocyanate mixture 
consisting of diarylmethane diisocyanates and 5 to 10 percent by weight of 
a polyisocyanate having a functionality greater than 2 in the presence of 
a small amount of water. A catalyst can be used in optional embodiments. 
These foams have the disadvantages that they are not hydrophilic and are 
not made with sufficient quantities of water to allow transport of large 
amounts of fibers, fillers, antiseptics, or other water-dispersible 
components into foams used in medical or personal care applications. The 
term hydrophilic as used herein means that the foam product is able to 
absorb 15-20 times its weight of water. A further disadvantage is that in 
the case of the optional catalyst there can remain catalyst residue which 
is not desirable. 
In U.S. Pat. No. 4,237,240, flexible MDI-based foams with high load-bearing 
and high energy-absorption capacity are made by reaction of 
diphenylmethane diisocyanates with polyester polyols or mixtures of 
polyester polyols and polyether polyols with a polyester polyol content of 
at least 60 percent by weight of the polyol mixture, and small amounts of 
water. As set forth in the claims, a catalyst is employed. These foams 
have the same drawbacks as those of the above-described British Pat. No. 
874,430 including the undesirable catalyst residues in the foam and in 
addition they require the use of the more expensive polyester polyols. 
In British Pat. No. 1,209,058, flexible hydrophilic polyurethane foams can 
be made by reacting a polyisocyanate with polyether polyols which contain 
at least 10% by weight of a block copolymer of ethylene oxide capped with 
propylene oxide to obtain hydrolytic stability. The method requires using 
at least one divalent tin salt of a fatty acid and/or at least one 
tertiary amine as a catalyst. The foam products made by this method, while 
being hydrophilic, have the drawback of being made with only small amounts 
of water as well as requiring the use of the block copolymers. Moreover, 
there is no teaching of the use of MDI, which is hydrophobic to make 
hydrophilic foam products and the resulting foam will contain undesirable 
catalyst residues. 
3. Objects of the Invention 
It is an object of this invention to provide an improved, flexible 
polyurethane foam which is made from a prepolymer system containing MDI as 
the only isocyanate source. 
It is a further object of this invention to provide an improved, flexible 
polyurethane foam which can contain large amounts of fibers, fillers, 
antiseptics or other water-dispersible components. 
It is a further object to produce a flexible, hydrophilic foam which is 
white in color so that it can be used in medical and health care 
applications. 
It is a further object to produce a polyurethane prepolymer having MDI and 
its derivative as the only isocyanate source which can be stored for an 
indefinite period of time and yet when mixed with an approximately equal 
amount of water, which can be used as a carrier for water-dispersible 
ingredients, will form a hydrophilic foam product. 
It is a further object to produce a polyurethane prepolymer having MDI and 
is derivative as the only isocyanate source which has viscosity stability 
over time. 
It is also an object to produce a polyurethane prepolymer having MDI and 
its derivatives as the only isocyanate source which has less than 50% by 
weight of the isocyanate and which will produce a flexible foam. 
These and further objects will become apparent as the description of the 
invention proceeds. 
SUMMARY OF THE INVENTION 
Flexible foams which are water-absorbing for medical or personal care 
applications can be made by using an MDI derived isocyanate as the sole 
isocyanate. The foams are made from a prepolymer having at least one 
poly(oxy C.sub.2-4 alkylene) diol having a molecular weight of at least 
about 1100 and having at least 50% by weight oxyethylene groups such as a 
Carbowax made by Union Carbide, a monomeric polyol crosslinker having 3 or 
4 hydroxyl equivalents per mole such as the triol trimethylolpropane and a 
methylene-bis (phenyl isocyanate), hereinafter MDI, based isocyanate 
product having a functionality greater than 2.0 such as Isonate 143-L made 
by Upjohn Polymer Chemicals which has a functionality of approximately 2.1 
and which is made of a mixture of MDI and isocyanate containing 
derivatives of MDI. 
One of the key features of the prepolymer formulation is restricting the 
amount of the isocyanate such as Isonate 143-L so that it is less than 50% 
and typically 38-46% of the weight of the prepolymer. The requirement for 
the isocyanate component is reduced by increasing the average molecular 
weight of a diol component so it is greater than 1100. When using 
trimethylolpropane as the monomeric crosslinking agent, the preferred diol 
molecular weight levels are at 1200-1400 and the diol contains at least 
80% by weight of oxyethylene groups. The diol used in a preferred 
embodiment is a mixture of two diols having different molecular weights. 
For example one diol can have a molecular weight of 1000 while the other 
can have a molecular weight of 1450. 
The actual content of free MDI can be adjusted up or down in this 
isocyanate containing product so long as the functionality remains greater 
than 2.0. For example, additional pure MDI could be added. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prepolymers with moderate viscosity and with good viscosity stability can 
be made with polyether polyols, an MDI containing isocyanate product such 
as Isonate 143-L and TMOP as a monomeric crosslinking agent. However, when 
this prepolymer is mixed with water the foam obtained is typically a 
white, semi-rigid foam which, while suitable for use as an abrasive 
sponge, is not suitable for use as a cushion or as a flexible foam. See, 
for example, U.S. application Ser. No. 314,537, filed Oct. 26, 1981 which 
is a continuation-in-part of U.S. application Ser. No. 220,562 filed Dec. 
29, 1980, now abandoned, where MDI containing foams were made with Isonate 
143-L, Carbowax 1000, and trimethylolpropane (TMOP) as the crosslinking 
agent to impart strength to the foam. Since the TMOP has such a low 
equivalent weight of 44.7, the prepolymer requires large amounts of the 
isocyanate component, Isonate 143-L, to cap all of the hydroxy groups. The 
amount of isocyanate is on the order of 50-55% by weight and this makes 
the foams relatively stiff and hydrophobic. 
It has now been discovered that in MDI containing systems containing a 
poly(oxy C.sub.2-4 alkylene) diol and TMOP, for example, as the monomeric 
crosslinking agent the isocyanate component can be advantageously reduced 
by increasing the average molecular weight of the poly (oxy C.sub.2-4 
alkylene) diol component to a molecular weight value of greater than 1100. 
These diols preferably have at least 80% by weight of ethyleneoxy groups. 
The resulting flexible foams retain their physical properties even when 
subjected to steam in an autoclave at 120.degree. C. for 5 hours and the 
foams have a significant decrease in their swell property as compared to 
TDI foams. TDI based hydrophilic foams swell more than 100% by volume when 
wet while the present foams swell only 30-60% when wet. 
The reduced percentage of the isocyanate such as Isonate 143-L allows the 
resulting foams to be flexible and resilient, compared to the foams having 
an Isonate 143-L content of greater than 50% which are semi-rigid and not 
resilient. Increases in the molecular weights of the diol allows for a 
decrease in the required amount of Isonate 143-L to such an extent that 
the ratio of isocyanate functional groups to the hydroxyl functional 
groups (known as the isocyanate index) can be reduced from 3.5/1 to about 
3/1, a change which contributes to the flexibility of the foam product. 
Although flexibility is best characterized by the flexural modulus, the 
more readily available tensile modulus at 1% elongation has been used as 
an approximate classification of these foams into soft and flexible 
(modulus under 20 psi), firm but flexible (modulus between 20 and 40 psi), 
and the semi-rigid or rigid (modulus above 40 psi). 
The preferred isocyanate containing product having a functionality greater 
than 2.0, is a mixture of diphenylmethane diisocyanate, abbreviated MDI, 
and isocyanate containing derivatives of MDI. One commercial product 
meeting this requirement is Isonate 143L which is produced by reacting MDI 
to form a carbodiimide and this material in turn then reacts to form a 
tri-functional cycloadduct. The mixture of MDI, the carbodiimide and the 
cycloadduct are in equilibrium. The mixture contains a major amount of 
pure diphenylmethane diisocyanate and minor amounts of carbodiimides and 
trifunctional cycloadducts of diphenylmethane diisocyanate. A mixture of 
the A and B components below constitute the 143L system. 
##STR1## 
As used herein the term derivatives of diphenylmethane diisocyanate means 
products that have been made from MDI as a starting material. It would 
include adducts, dimers and trimers. It would not include materials such 
as polymethylene polyphenylisocyanates which are not made from MDI. 
The prepolymer technology for making foams requires mixing with 
approximately an equal volume of water. This requires the prepolymer to be 
hydrophilic and it is the reason that at least some poly(oxyethylene) 
containing diol must be used in making the prepolymer made from the 
Isonate 143-L. The desirable viscosity for good mixing and foaming with 
water is in the range of about 10,000 - 35,000 cp at 25.degree. C., and 
preferably about 20,000 cp. Both the viscosity and the hydrophilicity of 
the prepolymers are controlled by the proper choice of polyol type and 
molecular weight. The prepolymers must not thicken significantly during 
storage. It has been found that there will be sufficient storage stability 
if the viscosity does not rise above 100,000 cp (measured at 25.degree. 
C.) after accelerated storage testing for two weeks at 80.degree. C. 
For a flexible foam to be useful in most applications, it should have 
tensile strength of at least 20 psi, a foam density of about 3-6 lb/cu. 
ft., and an elongation at failure of at least 100% with higher values 
being preferred. In one embodiment an elongation at failure of 200% has 
been obtained. 
The diol used is a poly(oxy C.sub.2-4 alkylene) diol containing at least 
50% by weight of oxyethylene groups. Thus when the diol contains 
oxypropylene or oxybutylene or mixtures thereof, there must be also 
present this minimum amount of oxyethylene. The preferred diols have at 
least 80% by weight of oxyethylene groups. 
In one of the preferred embodiments of this invention, it has been found 
satisfactory to use as the diol, a mixture of Carbowax 1000 and Carbowax 
1450 manufactured by Union Carbide, although it is within the scope of the 
invention to use any combination of the more preferred poly(oxyethylene) 
diols with a number average molecular weight of 1000-2000, but preferably 
in the range of 1200-1400. In these instances the average of the molecular 
weights of all the diols will be of about at least 1100. By using a blend 
of these two diols in about equal molar amounts the resulting foams 
addvantageously have low volume swell and low density. 
Although it is within the scope of this invention to use any monomeric 
polyol as the crosslinking agent, the more preferred are those polyol 
crosslinking agents having 3 or 4 hydroxyl equivalents per mole. These 
include trimethyolpropane, trimethylolethane, glycerol, triethanolamine, 
pentaerythritol, or mixtures of these polyols. The most preferred is 
trimethylolpropane, TMOP, which has the formula 
##STR2## 
The density and physical properties of the foam appear to be sensitive to 
the polyol content. If it is desirable to have a foam wth a lower density, 
but which is firm and less rubbery then the amount of the polyol is 
increased. Similarly, if it is desirable to have higher density and more 
elasticity, then the amount of polyol is decreased. The operable amount of 
polyol crosslinking agent having a hydroxy functionality of 3 or 4 is such 
that 2% to 35% of the hydroxyl content of the diol and polyol mixture 
comes from the polyol. The preferred range for a good flexible foam is 10% 
to 30% hydroxyl from the polyol. When the amount of polyol crosslinking 
agent is expressed with respect to the amount of diol present, the diol 
and polyol crosslinking agents are present in a mole ratio of about 4:1 to 
8:1. 
Carbowax 1000 has an equivalent weight of 500 per hydroxyl group while TMOP 
with a molecular weight of 134 has an equivalent weight of 45 per hydroxyl 
group. Since the Isonate 143L is used to combine with the hydroxyl groups, 
the amount of the isocyanate required will be very sensitive to the amount 
of TMOP and thus the amount of TMOP is relatively carefully controlled. 
The operable amount of Isonate 143-L is such that the isocyanate index is 
2.5-3.5, but preferably 2.8 to 3.2 and with very good results at 3.1. 
Higher ratios allow lower polymer viscosity, lower foam density and less 
swelling, but they also add cost and reduce softness and elongation. When 
the prepolymer is made, one isocyanate group of the polyisocyanate 
component reacts with a hydroxyl group to leave the remaining isocyanate 
group unreacted. These free isocyanate groups on the prepolymer then react 
with water to form polyurea linkages with simultaneous increase in 
molecular weight and the release of CO.sub.2 which aids in forming the 
foamed product. 
When reacting the components to form the prepolymer, it has been helpful to 
measure the isocyanate level by titration after the reaction has taken 
place for about one hour. From this reading and subsequent titrations one 
can determine the additional reaction time required to reduce the 
isocyanate level down to about the level which is the theoretical point at 
which all of the hydroxyl groups will have reacted with the isocyanate. If 
the reaction is permitted to continue so the isocyanate level is further 
reduced, then the prepolymer viscosity increases, making it more difficult 
to subsequently mix the prepolymer with water. Over reacting the 
prepolymer components will also cause the foam density to increase as well 
as to decrease the water absorptive property of the resulting foam. 
Surfactants are chosen to give a foam with a good appearance that has the 
correct cell size, shape and lack of collapse or splits. Surfactants which 
are known to be useful in polyurethane foams may be employed here. 
Examples of preferred surfactants are the block copolymers of oxyethylene 
and oxypropylene such as the Pluronic Polyol surfactants manufactured by 
BASF Wyandotte Corp. of Wyandote, Michigan. A preferred surfactant is 
Pluronic L-62. 
In making the polyurethane foam the preferred procedure is to add about an 
equal amount of the aqueous suspension with the prepolymer mixture and to 
then mix the two together. The composition of the aqueous suspension can 
be also expressed on the basis of 100 parts of prepolymer resin. Thus, 100 
parts of water per 100 parts of resin is written 100 phr water. The ratio 
of the amount of the prepolymer mixture to the aqueous suspension can vary 
over a wide range. However, if the amount of the aqueous suspension is too 
large, then the strength of the resulting foam drops. On the other hand if 
the amount of the aqueous suspension is decreased too much, then it will 
not be possible to add enough fibers and fillers which are supplied via 
the aqueous suspension. 
The preferred method of foaming the prepolymer is to heat the aqueous 
suspension, for example, a 2% solution of Pluronic L-62 surfactant, as 
well as the prepolymer, to a temperature of approximately 35.degree. C. 
These are poured or pumped together in a ratio of about 100 parts by 
weight of aqueous suspension to about 80 parts by weight of the prepolymer 
and stirred immediately for up to 30 seconds by a mechanical stirrer such 
as a blade attached to a drill motor. This length of time allows complete 
mixing but does not allow an appreciable degree of chemical reaction to 
occur. This mixture is poured immediately into a mold, where the rising 
and curing of the foam product takes place. 
Fibers can be added to the aqueous phase for insertion into the foam 
composition to provide additional structural rigidity. Polyester fibers 
are especially advantageous and they can be cut up into lengths of about 
1/2 inch or less. With respect to the aqueous suspension the fibers can be 
added in various amounts although they preferably do not constitute more 
than about 10% by weight because above that level the suspension becomes 
too difficult to pump. 
A high molecular weight suspending or thickening agent can also be added to 
serve two functions. First it keeps the ingredients suspended so the water 
does not drain out of the fibers and so that the fibers do not float. 
Secondly, the thickening agent acts as a lubricant for the fibers so they 
do not tangle, dewater and jamp up when going through the mixing pump. 
Examples of thickening agents are Polyox WSR, Natroso, Xanthan gums, and 
polyacrylamides such as Dow's Separan AP 30 which have high molecular 
weights of about one million or so. 
Preferred suspending or thickening agents are the Carbopol resins made by 
the B. F. Goodrich Chemical Co. such as Carbopol 934, Carbopol 940 and 
especially Carbopol 941. Since the Carbopol resins are acrylic acid 
polymers with an acid moiety, a neutralizing agent such as sodium or 
ammonium hydroxide can be added. When adding ammonium hydroxide as the 
neutralizing agent to the Carbopol, there is an advantageous increase in 
viscosity of the aqueous phase. Also since ammonium hydroxide is less 
expensive then Carbopol, a more economical formulation having the same 
amount of thickening can be made which utilizes less Carbopol. 
Depending on optional fibers, fillers, antiseptics, pigments, thickeners, 
or other water soluble or water dispersible components which might be 
incorporated, the best surfactant might vary from a highly hydrophobic 
silicone type such as L-520 (Union Carbide) or other silicone surfactants 
to a very hydrophilic type such as Brij-58 (ICI-America) or the other 
Brij, Span, or Tween products from ICI. For general use, we prefer the 
nonionic surfactants such as the Pluronics, especially L-62, L-72, L-92, 
P-75 or P-85 (BASF-Wyandotte). The use of these surfactants would be 
familiar to one skilled in the formulation of polyurethane foam products. 
The water adsorption tests is conducted by cutting out a rectangular foam 
sample of dimensions 1.times.3.times.5 inches. It is soaked in water and 
held up until it is no longer dripping. The ratio of the wet weight to the 
dry weight is the water adsorption ratio. 
Having described the basic aspects of our invention, the following examples 
are given to illustrate specific embodiments thereof.

EXAMPLE 1 
A mixture of Carbowax 1000 (174 g, 0.174 mole),Carbowax 1450 (249 g, 0.172 
mole) and trimethylolpropane (6.1 g, 0.045 mole) was dried by heating for 
two hours at 70.degree. C. under a reduced pressure of 2 Torr. To the 
dried and degassed polyol mixture was added 365 g (2.56 equivalents of 
isocyanate) of Isonate-143L. The temperature was maintained at 70.degree. 
C. for 70 minutes to complete the reaction. The product was a light yellow 
liquid with isocyanate content of 2.18 meq/g and viscosity at 25.degree. 
C. of 23,000 cp. After storage of a sample for two weeks at 80.degree. C., 
the product had thickened only to 40,000 cp at 25.degree. C. When 80 g of 
the product heated to 35.degree. was stirred with 100 ml of a 2% solution 
of Pluronic L-62, a nonionic surfactant from Wyandotte, there resulted a 
foam with a density of 4.1 lb/ft.sup.3 which was soft, flexible, 
hydrophilic, and which exhibited the properties summarized in Table II. 
TABLE I 
______________________________________ 
Prepolymer Composition 
Examples 
1 2 3 4 
______________________________________ 
Composition of Diol 
Carbowax 1000 g 
174 83 90 -- 
Carbowax 1450 g 
249 357 387 456 
Avg. Mol. Wt. 
1265 1335 1365 1450 
Triol Compound 
TMOP g 6.1 7.3 1.6 7.0 
% Hydroxyl 16 20 5 20 
Contributed 
Mole Ratio 
Diol/Triol 7.6 6.0 30.0 6.0 
Isonate 143 L g 
365 352 320 337 
Isocyanate Index 
3.1 3.0 3.4 3.0 
Isocyanate Content 
2.18 2.07 2.51 1.97 
meq./g 
Viscosity at 25.degree. C. 
Initial cp .times. 10.sup.-3 
23 24 18 19 
Aged - 2 weeks at 
40 80 54 43 
80.degree. C. cp .times. 10.sup.-3 
______________________________________ 
TABLE II 
______________________________________ 
Physical Properties of Foamed Products 
Examples 
1 2 3 4 
______________________________________ 
Density lb/ft.sup.3 
4.1 5.1 4.1 5.5 
Tensile psi 31 24 38 24 
Elongation % 200 170 130 170 
1% Modulus psi 16 18 40 17 
% Volume Swell Wet/Dry 
47 53 32 58 
Compression Set % after 
20 19 14 12 
5 hrs. in steam 
autoclave at 120.degree. C. 
Water adsorption ratio 
16 20 -- 21 
______________________________________ 
EXAMPLES 2-4 
These examples illustrate the foams obtained using different prepolymer 
compositions. The compositions are listed in Table I and the foam 
properties are given in Table II. 
In Example 2, the average molecular weight of the diol was increased, 
allowing the use of less isocyanate and resulting in a foam with higher 
density and more capacity for water. 
In Example 3, the amount of TMOP crosslinker was reduced and the isocyanate 
index was increased to 3.4 to maintain strength. However, the ratio of 
diol to triol was outside the range of this invention and thus this is a 
comparison example. Although a stiff foam was obtained, it would not wick 
water and become wet so it was not possible to obtain the water adsorption 
ratio. 
In Example 4, a single diol, Carbowax 1450, was employed with TMOP as the 
crosslinking agent. A good, soft hydrophilic foam with high elongation, 
yet good water-holding capacity and steam resistance was obtained. 
It is understood that the foregoing detailed description is given merely by 
way of illustration and than many variations may be made therein without 
departing from the spirit of this invention.