Biodegradable hydrophilic foams and method

The invention disclosed is directed to biodegradable hydrophilic polyurethane structures prepared by using an isocyanate capped hydroxyester polyol reactant with large amounts of aqueous reactant. The resultant foam has economy in handling, and upon contact with liquids or body fluids, the foam is highly absorptive while being readily biodegradable after use.

BACKGROUND OF THE INVENTION 
This invention relates to a method for preparing new biodegradable foams. 
More particularly, the present invention provides new dental and 
biomedical foams using a hydrophilic polyurethane foam having a 
biodegradable moiety. 
Numerous devices have been proposed in the prior art for use as dental and 
biomedical foams for absorbing or removing body fluids. Typically, the 
prior art approaches have relied upon natural materials such as cotton, 
which is now becoming relatively expensive while providing a resultant 
structure which is generally fragile in use. Also, the amount of 
absorption by natural materials is relatively low. 
Various polyurethanes have been used as dental and biomedical foams but 
suffer a disadvantage in that such foams are not readily biodegradable. It 
has now been found, however, that by practice of the present invention, 
there is provided a method for preparing new simple and highly efficient 
dental and biomedical foams which are readily biodegradable after use, and 
which are characterized by high absorptive ability of body fluids in use. 
Various attempts have also been made in the prior art to prepare foams of 
organic substances for use in cavities of the human body. However, such 
organic substances typically require, for example, catalysts or the like 
during the foaming reaction. These additives remain in the foam after 
foaming and are readily leached into the human body when in contact with 
body fluids. Thus, although artificial foams, especially those of 
polyurethane, of the prior art possess the capacity of high absorptivity 
of body fluids, usage within the human body typically invites 
disadvantages beyond advantages realized by low cost and high 
absorptivity. Thus, artificial foams such as polyurethanes of the prior 
art have received limited practical acceptance by the medical, dental and 
government regulatory agencies when proposed for internal usage in the 
human body. There is especially a disadvantage of such foams. 
DESCRIPTION OF THE INVENTION 
By the present method, new biodegradable foams may be prepared having 
utility in dental and biomedical applications wherein hydrophilic 
crosslinked polyurethane foams are employed by reacting a particular 
isocyanate capped polyhydroxyester polyol with large amounts of an aqueous 
reactant. The thus generated foams may be formed in handy sizes as 
desired. Such structures may be readily used in the oral cavity and while 
in the oral cavity, the structure absorbs oral fluids, and thereafter may 
be discarded since it is biodegradable. 
The novel biodegradable foams are prepared by reacting an isocyanate-capped 
hydroxyester polyether polyol having a reaction (i.e. isocyanate) 
functionality of at least 2 with sufficient water to provide an H.sub.2 0 
Index Value (as defined below) of from about 1300 to about 78,000. The 
polyol is further characterized in that the hydroxyester linkages are 
formed by condensation of an aliphatic hydroxy carboxylic acid with the 
hydroxyl groups of a) an essentially linear polyether, or b) a monomeric 
low molecular weight aliphatic alcohol containing from 3 to 8 hydroxyl 
groups per mole. 
Suitable acids are the monobasic aliphatic carboxylic acids having the 
structure: 
##STR1## 
wherein n is an integer and 0.ltoreq.n.ltoreq.20 and preferably 
0.ltoreq.n.ltoreq.5; R.sub.1, R.sub.2, R.sub.3, R.sub.4, and R.sub.5 
independently are hydrogen, alkyl, alkylene, aryl, aralkyl, alkoxy, 
carboalkoxy, acyl, acyloxy, and carboxyl with one of the groups R.sub.1, 
R.sub.2 or R.sub.5 being carboxyl. Suitable alkyl groups can be straight 
or branched-chain having a total of from 1 to 20 carbon atoms and 
preferably being a lower aliphatic moiety having from 1 to 5 carbons. The 
alkylene groups are of the same size as the alkyl groups and contain one 
or more unsaturated linkages, e.g. olefinic or alkynyl and would include 
residues from the naturally-occurring fatty acids. Suitable aryl groups 
are monocyclic and may be substituted with halogen, alkoxy or alkyl groups 
having less than 4 carbons. Suitable aralkyl groups are benzyl and similar 
groups corresponding to the formula: where A is methylene, ethylene, 
isopropylene, or propylene. Alkoxy groups include both aryloxy (e.g. 
phenoxy) and lower aliphatic alkoxy groups having from 1 to 5 carbon 
atoms. Carboalkoxy groups are those of the formula 
##STR2## 
where B is alkoxy as defined above. Acyl groups are those of the formula 
##STR3## 
where F is alkyl or alkenyl as defined above, and preferably contains 5 
carbons or less. Acyloxy groups are those of formula 0-G, where G is acyl 
as defined above. Hydroxy acids that may be used in this application 
include but are not limited to glycolic (hydroxy acetic) acid, lactic 
acid, .alpha.-hydroxybutyric acid, .alpha.-hydroxyisobutyric acid, 
.alpha.-hydroxyvaleric acid, .alpha.-hydroxyisovaleric acid, 
.beta.-hydroxy propionic acid, .beta.-hydroxy butyric acid, 
.beta.-hydroxyisobutyric acid, .beta.-hydroxy-n-valeric acid, 
.beta.-hydroxyisovaleric acid, .gamma.-hydroxybutyric acid, 
.gamma.-hydroxy-n-valeric acid, .delta.-hydroxyvaleric acid, 
.epsilon.-hydroxy caproic acid, 9-hydroxystearic acid, 10-hydroxystearic 
acid, 11-hydroxystearic acid, 12-hydroxystearic acid, 
11-hydroxyhexadecanoic acid, 12-hydroxydodecanoic acid, and 
16-hydroxyhexadecanoic acid. Also included are polyhydroxymonocarboxylic 
acids such as glyceric acid, 3,12-dihydroxypalmitic acid, the erythronic 
and threonic acids, trihydroxyisobutyric acid, 9,10,16-trihydroxypalmitic 
acid. Also included are hydroxy unsaturated acids such as 
.alpha.-hydroxyvinylacetic acid, 16-hydroxy-7-hexadecenoic acid, and 
ricinoleic acid (12-hydroxy-9-octadecenoic acid). From the above 
description it is apparent that the hydroxy acids are not limited to 
hydroxyacetic acid but include the straight-chain and omega hydroxy acids 
having 10 carbons or less. 
Suitable aliphatic polyhydroxy alcohols have a molecular weight of less 
than about 1000 and preferably 500 or less and include glycerol, 
1,2,3-butanetriol, 1,2,4-butanetriol, trimethylolethane, trimethylol 
propane, erythritol, pentaerythritol, adonitol, arabitol, mannitol, 
sorbitol, iditol, dulcitol, sucrose, dipentaerythritol, triethanolamine 
and condensation products of ethylene and propylene oxides with ethylene 
diamine, diethylene triamine, and triethylene tetramine. 
The essentially linear polyethers have a molecular weight not exceeding 
about 4000, and preferably not exceeding about 2000, and are prepared by 
homopolymerization of ethylene oxide, propylene oxide, and include block 
copolymers such as polyoxyethylene diol capped with polyoxypropylene 
chains and polyoxypropylene diols capped with polyoxyethylene. Suitable 
linear polyethers may also be prepared by condensing an alkylene oxide of 
4 carbons or less (e.g. ethylene, propylene or tetramethylene oxide) with 
a polyhydroxylic alcohol such as those described above. In such 
condensation products the polyether chains are essentially linear and have 
an average molecular weight of from 50 up to about 4000. The polyether 
chains should not contain more than 50% by weight of alkylene oxide 
condensation units larger than ethylene oxide (e.g. propylene glycol 
units) and should not contain more than 15% by weight of tetramethylene 
oxide units. 
Suitable isocyanate-capped polyols or prepolymers are exemplified by the 
following systems: 
A. Polyether (e.g. polyoxyethylene glycol) blended with a hydroxyacid 
ester, e.g. the condensation product of a polyhydroxy alcohol with 
sufficient hydroxyacid to completely esterify the alcohol. The hydroxyacid 
ester serves essentially as a crosslinking agent in addition to imparting 
biodegradability and is employed in amounts sufficient to provide the 
desired properties, i.e. if it is desired to increase rigidity, solvent 
resistance and other properties associated with crosslink density, the 
amount of crosslinking agent is increased. Sufficient isocyanate is added 
to completely cap all the hydroxyl groups. A specific preferred system is 
the blend of polyoxyethylene diol with the condensation product of 
trimethylol propane or ethane with lactic or glycollic acids. 
B. Essentially linear polyether completely esterified with hydroxy acid 
(preferably lactic acid) and blended with a polyhydroxy alcohol. 
Sufficient isocyanate is added to completely cap all the hydroxyl groups. 
C. An ester is formed as in A above and the ester is condensed with 
ethylene or propylene oxides to form essentially linear polyether chains 
originating with the hydorxyl groups of the ester. Such chains have the 
molecular weight distribution as described above. Sufficient isocyanate is 
added to completely cap all the hydroxyl groups. This system may be 
exemplified by the trimethylol propane (or ethane) ester formed by 
condensation with lactic acid followed by further condensation of the 
hydroxyl groups of the ester (3 per mole) with ethylene oxide to provide 
polyols having three essentially linear polyether chains per mole. 
Sufficient isocyanate is added to completely cap the hydroxyl groups 
terminating the polyether chains. 
The present foams have utility as handy expandable sponges for personal 
use. The sponges are easily carried and may be readily prepared with 
detergents, lotions, perfumes, biostats and the like and upon contact with 
water, the sponges are found to be very soft, very hydrophilic and 
biodegradable. The sponges may be used for washing, wiping, cleaning, etc. 
for external body cleaning; or alternatively for internal body usage such 
as is necessary in dental and medical applications. The present sponges 
also have utility as intimate absorptive products, such as diapers, 
sanitary napkins, incontinent pads and the like. 
Polyurethane foam structures prepared herein with hydroxyester 
polyisocyanates, water and certain surfactants, have an exceptionally 
fine, uniform, soft, hydrophilic cell structure. 
The following conditions seem to be important to obtaining foams of the 
above-mentioned desirable properties. The polyether (e.g. polyoxyethylene 
diol) should have a molecular weight not exceeding about 4000. In forming 
the polyol (prior to capping with isocyanate) for every mole of polyether 
from about 0.1 to about 4.0 moles, and preferably from about 0.2 to about 
2.5 moles of the monomeric aliphatic alcohol should be employed. The 
necessary hydroxyester linkange is provided by condensing the carboxylic 
acid with either the hydroxyl groups on the polyether or on the monomeric 
aliphatic alcohol, as described above. Preferably the carboxylic acid is 
condensed with the monomeric alcohol to provide a hydroxyester 
crosslinking agent having from 3 to 8 hydroxyl groups per mole, e.g. 
trimethylolpropane trilactate, trimethylolethane trilactate or 
trimethylolpropane triglycolate. 
The polyol is next capped with a polyisocyanate (e.g. TDI). The useful 
range of polyisocyanates is about 0.60 to about 1.3 moles of diisocyanate 
per equivalent group in the polyol mixture. The preferred range of 
diisocyanate is about 0.95 to about 1.15 moles of diisocyanate per 
equivalent of the polyol mixture. 
The resultant polyester polyisocyanate prepolymers are foamed by reacting 
with about 10 to about 200 parts of water, preferred range of about 50 to 
about 160 parts of water, to 100 parts of prepolymer in the presence of 
about 0.05 to about 30 parts surfactant, preferred range of about 0.1 to 
about 15 parts surfactant, per 100 parts of prepolymer. The surfactants 
can be added either to the prepolymer or the water. Surfactants which are 
soluble in water and/or in their own right are hydrophilic, are preferred. 
The polyurethane foams made in the manner described above are exceptionally 
soft, hydrophilic and biodegradable. 
Polyoxyethylene polyol used as a reactant in preparing the capped product 
to be foamed may have a weight average molecular weight of about 200 to 
about 4,000, and preferably between about 250 to 3,500, with a hydroxyl 
functionality of 2 or greater, preferably from about 3 to about 8. 
The polyoxyethylene polyol is combined with a crosslinking agent for the 
prepolymer. 
Trimethylolpropane trilactate or the like can be used in combination with 
other polyols or trimethylolpropane trilactate can be oxyethylated or 
oxpropylated to yield the appropriate polyol. Other hydroxy acids may be 
used for the crosslinking agent esterification. These include but are not 
limited to hydroxyacetic acid and other .alpha., .beta., .gamma., .OMEGA., 
etc., hydroxy acids. 
The polyoxyethylene polyol ester mixture is terminated or capped by 
reaction with a polyisocyanate. The reaction may be carried out according 
to conventional practice. The polyisocyanates used for capping the 
polyoxyethylene polyol include polyisocyanates such as PAPI (the brand of 
polyaryl polyisocyanate manufactured by the Upjohn Co. and defined in U.S. 
Pat. No. 2,683,730), tolylene diisocyanate, triphenylmethane-4,4', 4", 
-triisocyanate, benzene-1,3,5-triisocyanate, toluene-2,4,6-triisocyanate, 
diphenyl-2,4,4'-triisoayanate, hexamethylene diisocyanate, xylene 
diisocyanate, chlorophenylene diisocyanate, 
diphenylmethane-4,4'-diisocyanate, naphthalene-1, 5-diisocyanate, 
xylene-alpha, alpha'- diisothiocyanate, 3,3'-dimethyl-4,4'-biphenylene 
diisocyanate, 2,2',5,5'-tetramethyl-4,4'-biphenylene diisocyanate, 
4,4'-methylenebis (phenylisocyanate), 4,4'-sulfonylbis (phenylisocyanate, 
4,4'-methylenebis (orthotolylisocyanate, ethylene diisocyanate, ethylene 
diisothiocyanate, trimethylenediisocyanate and the like. Mixtures of any 
one or more of the above-mentioned organic isothiocyanates or isocyanates 
may be used as desired. The polyisocyanates or mixtures thereof which are 
especially suitable are those which are readily commercially available, 
have a high degree of reactivity and a relatively low cost, e.g. TDI. the 
aromatic isocyanates are preferred. 
Capping of the polyoxyethylene polyol may be effected using stoichiometric 
amounts of reactants. Desirably, however, an excess of isocyanate is used 
to insure complete capping of the polyol. Thus, the ratio of isocyanate 
groups to the hydroxyl groups used for capping is between about 1 to about 
4 isocyanate to hydroxyl, and preferably about 2 to about 3 isocyanate to 
hydroxyl molar ratio. In order to achieve an infinite crosslinked network 
formation on foaming, the reactive components may be formulated in one of 
the following by way of example. First, when water is the sole reactant 
with the isocyanate groups leading to chain growth during the foaming 
process, the isocyanate-capped reaction product must have an average 
isocyanate functionality greater than 2 and up to about 6 or more 
depending upon the composition of the polyol and capping agent components. 
Secondly, when the isocyanate capped reaction product has an isocyanate 
functionality of only about two, then the aqueous reactant, may contain a 
dissolved or dispersed isocyanate-reactive crosslinking agent having an 
effective functionality greater than two. In this case, the reactive 
crosslinking agent is reacted with the capped resin when admixed during 
and after the foaming process has been initiated. Thirdly, when the 
isocyanate capped resin has an isocyanate functionality of only about two, 
then a polyisocyanate crosslinking agent having an isocyanate 
functionality greater than two may be incorporated therein, either 
preformed or formed in situ, and the resultant mixture may then be reacted 
with the aqueous reactant, optionally containing dissolved or dispersed 
reactive isocyanate-reactive crosslinking agent, leading to a crosslinked, 
infinite network hydrophilic polyurethane foam. 
Water soluble or water dispersible crosslinking agents operable in this 
invention desirably should be polyfunctional and reactive with isocyanate 
groups and include but are not limited to materials such as 
diethylenetriamine, triethylenetetramine, tetraethylenepentamine, 
polyethyleneimine, glycerol, trimethylolpropane, pentaerythritol, 
tolylene-2,4,6-triamine, ethylenediamine, aminoethanol, 
trimethylenediamine, tetramethylenediamine, pentamethylenediamine, 
hexamethylenediamine, ethanolamine, diethanolamine, hydrazine, 
triethanolamine, benzene-1,2,4-tricarboxylic acid, nitrilotriacetic acid, 
citric acid, 4,4'-methylenebis (o-chloroaniline), and the like. The water 
soluble or water dispersible crosslinking agents chosen are those which 
cause a crosslinked network to form during or after the foaming process 
begins to take place. 
It has also been found that the capped resin having an isocyanate 
functionality greater than two used to prepare a three dimensional network 
polymer must be present in an amount sufficient to insure formation of the 
dimensional network. Thus, amounts of the capped polyoxyethylene polyol 
having an isocyanate functionality greater than 2 in the component to be 
foamed range from about 3% by weight of this component up to 100% by 
weight. Thus, it is possible to include a capped polyoxyethylene polyol 
having a terminal member with an isocyanate functionality of two i.e., a 
diisocyanate in an amount from 0% by weight up to about 97% by weight of 
the component to be foamed. The maximum amounts of diisocyanate used are 
limited to that necessary to permit crosslinking to take place during 
foaming, as contrasted to formation of a linear polymeric structure, and 
the properties desired in the finally prepared foam. 
To effect foaming and preparation of the crosslinked network polymer, the 
component including the isocyanate capped polyoxyethylene polyol having a 
functionality about 2 or greater is simply combined with the aqueous 
component. For simplicity, this isocyanate capped reaction component will 
herein be referred to as resin reactant. 
The aqueous component, i.e., water slurry or suspension, may include 
various additives such as detergents, biostats, perfumes or the like as 
desired for use in a given product. Obviously, additives are avoided or 
carefully selected for specific purposes for foam structures intended for 
internal body usage. 
The dramatic way in which additions of water influences practice of the 
present invention is by consideration of the following water index value; 
##EQU1## 
Thus, because one-half mole of water is equal to one equivalent of 
isocyanate, where 0.5 m H.sub.2 0 is used with 1 eq. NCO, the water index 
value is 100. 
An Index of 100 indicates that both equivalents are equal or "balanced". An 
Index of 95 indicates that there is a 5% shortage of water while an Index 
of 105 indicates a 5% surplus of water. A slight theoretical excess of 
isocyanate, usually 3-5%, is common practice, in the prior art 
particularly with flexible foams. 
Using the present resin reactant and water in amounts from about H.sub.2 O 
Index Value of 100 up to about H.sub.2 O Index Value of 200, poor foaming 
results unless materials such as surfactants or the like are included. 
Amounts up to about H.sub.2 O Index Value of 200 require a catalyst. When 
using about H.sub.2 O Index Value 78,000, surprisingly good foams result 
which improve in characteristics with added amounts of molar water. Thus, 
the available water content in the aqueous reactant is from about an 
H.sub.2 O Index Value of about 1300 to about 78,000 and desirably from 
about 4,000 to about 40,000. 
"Available water" in the aqueous reactant is that water accessible for 
reaction with the resin reactant, and which is exclusive of water which 
may layer during reaction, or supplemental water which may be necessary 
because of additives present in the forming the aqueous reactant. 
Because large amounts of water are in the aqueous reactant during reaction, 
i.e., the present system is not dependent upon a molar NCO-water type 
reaction, it is possible to combine a great variety of materials in the 
aqueous reactant which are otherwise not possible with limited water 
reacting systems. 
The aqueous reactant may be used at temperatures from about 2.degree. C. to 
about 100.degree. C. as desired. 
Although foaming of the present resin reactant, i.e., prepolymer, is 
effected simply, it is also possible to add, although not necessary, 
supplemental foaming materials such as those well known to the artificial 
sponge foaming art. 
After foaming has been effected, the foam may be dried, if desired, under 
vacuum from 1 to 760 Torr at a temperature of about 0.degree. to about 
150.degree. C. When used internally, the foams may be heat or chemically 
sterilized prior to use.

The following examples will aid in explaining, but should not be deemed as 
limiting, practice of the present invention. In all cases, unless 
otherwise noted, all parts and percentages are by weight. 
EXAMPLE 1 
Polyethyleneglycol PEG 1000 (actual M.W. 1064) and trimethylolpropane 
trilactate (361g and 60g respectively) were dried for 2.5 hours at 
103.degree. C. and 4 Torr. This mixture was added to 225g of 
toluenediisocyanate and 0.2g of Metal and Thermit T-9 catalyst, a catalyst 
containing stannous octoate, over a period of 80 minutes at a temperature 
of 60.degree. C. After completion of addition, the reaction mixture was 
maintained at 60.degree. C. for an additional hour. To the reaction 
mixture there was then added an additional 12g of tolylenediisocyanate and 
heating continued for another hour at 60.degree. C. The final viscosity 
was 24,500 cp at 25.degree. C. and the NCO was 2.38 meq/g (theory 2.33 
meq/g). 
From the above reaction mixture, a foam was prepared using 100 g of 
prepolymer, 10g of Union Carbide Silicone surfactant L-520 and 100 g of 
water. Aqueous solutions of enzymes, 1%, were prepared and tested on this 
foam. Maxatase, H.T. proteolytic concentrate, and protease amylase, gave 
essentially complete degradation after seven days at 25.degree. C. Several 
others, such as mucinase, trypsin and some experimental enzyme broths, 
showed some evidence of degradation. A standard polyurethane foam prepared 
from 100 pts of prepolymer, isocyanate capped polyoxyethylene polyol, 1 pt 
1-520 and 100 pts of water gave no change over the same period of time 
with these enzymes. 
The foam was buried in a compost heap for three months. On removal from the 
compost heap, the foam had started to fragment and could not be washed 
without falling apart. 
The foam was then compared with a foam made in a similar manner with 
trimethylolpropane instead of the trimethylolpropane trilactate. The 
trimethylolpropane lactate based foam in ten minutes at 250.degree. F. 
turned tacky and began to degrade. The trimethylolpropane based foam 
showed no change in 20 minutes at 250.degree. F. Conventional 
polyoxypropylene based polyurethanes shown no change in three hours. 
A similar comparison at 100.degree. C. in boiling water gave breakdown into 
viscous lumps in 240 minutes after becoming tacky in 30 minutes for the 
trimethylolpropane trilactate biodegradable polymeric foam. The 
trimethylolpropane based foam showed no change in 240 minutes at 
100.degree. C. 
EXAMPLE 2 
The procedure of Example 1 was repeated for foam generation. Next a 
synthetic sewage sludge was allowed to react with the foam for one week. 
The trimethylolpropane trilactate based foam had completely disintegrated. 
The standard polyurethane foam was intact. 
EXAMPLE 3 
The trimethylolpropane trilactate that had been prepared was used with a 
different polyol. A mixture of one mole each of trimethylolpropane 
trilactate and Pluronic 10-R-5 (a Wyandotte polyol with a molecular weight 
of 1970, an equivalent weight of 985 and end-capped with oxypropylene on 
an oxyethylene backbone with approximately 50% by weight of oxypropylene 
and 50% by weight of oxyethylene) was dried at 110.degree. C. and 3 torr 
for three hours. The mixture was then added to 4.75 moles of the standard 
80-20 mixture of 2,4 and 2,6-tolylenediisocyanate over a period of 1 hour 
maintaining the temperature at 60.degree. C. The reaction was completed by 
heating for an additional three hours at 60.degree. C. with the addition 
of Metal and Thermits' T-9 stannous octoate catalyst (5 drops). To the 
final prepolymer was added 0.75 moles of tolylenediisocyanate to give a 
prepolymer with a viscosity of 17,250 cp. The prepolymer was converted to 
a foam by the addition of 1 part of silicone surfactant L-520 to the 
prepolymer (100 parts) and then adding 100 parts water to the prepolymer 
phase. The polymeric foam when placed in a synthetic sewer sludge 
disintegrated within one week. EXAMPLE 4 
The trimethylolpropane trilactate was prepared as above in Example 3. A 
mixture was prepared from one mole of timethylolpropane lactate and 0.5 
mole Pluronic P-65 supplied by Wyandotte (this polyol has a molecular 
weight of 3500 and has a polyoxypropylene base, end-capped with 
oxyethylene units, being 50% oxyethylene and 50% oxypropylene by weight) 
and was dried by heating for a period of 3 hours at 115.degree. C. and 5 
torr. This mixture was added to 3.8 moles of commercial TDI over a period 
of 1 hour with the temperature maintained at 60.degree. C. After 
completion of the addition, 10 drops of Metal and Thermit catalyst T-9 was 
added and the reaction mixture was heated for an additional 3 hours at 
65.degree. C. to force the prepolymer formation. Then 0.6 mole of TDI was 
added to the reaction mixture and the reaction heated for an additional 2 
hours to give a prepolymer with a viscosity of 16,000 cp at 25.degree. C. 
A foam was prepared from this prepolymer using 100 parts of prepolymer, 
1.0 part of Plurafac B-26, 1.7 parts a tertiary amine of Thancat DD 
catalyst by Jefferson Chemicals, and 50 parts of water (the latter three 
all being in the aqueous phase). The foam that was formed decomposed in a 
compost heap in two months. The conventional polyoxypropylene polyurethane 
foam showed no change. 
EXAMPLE 5 
Trimethylolpropane hydroxyacetate (glycolate) is prepared by simple 
esterification using one mole trimethylolpropane and 3.12 moles glycolic 
acid (hydroxyacetic acid). the mixture was heated for four hours at reflux 
and then stripped at 125.degree. C. and 12 Torr. 
The prepolymer was prepared by adding a dried mixture of 2.0 moles of 
PEG-1000 (molecular weight 1064) and 1.0 mole of trimethylolpropane 
triglycolate to 6.7 moles of tolylenediisocyanate over a period of one 
hour at a reaction flask temperature of 60.degree. C. The reaction was 
continued at 60.degree. C. for an additional 3 hours and the measured NCO 
content in meq/g. was 1.82 meq/g(theory 1.78 meq/g.). To the reaction 
mixture was then added an additional 1.0 mole of TDI with heating and 
stirring continued for an additional two hours at 60.degree. C. The NCO 
was 2.18 meq/g(theory 2.23 meq/g.), viscosity at 25.degree. C. 19,000 cp. 
A foam was prepared from this prepolymer by using 100 g. of prepolymer and 
100 parts of 5% solution of Plurafac B-26 (By Wyandotte) in water. The 
generated foam was found to degrade with Mexatase enzyme in six days at 
25.degree. C. while the foam prepared from trimethylolpropane was intact. 
In boiling water, decomposition occurred in 200 minutes with the 
trimethylolpropane glycolate based material. With the trimethylolpropane 
based prepolymer, no change was noted in the same time period. 
EXAMPLE 6 
Instead of preparing the hydroxyacid ester of the crosslinking agent as in 
the previous example, the hydroxyacid ester of polyoxyalkylene glycol was 
prepared. To 1 mole of polyethylene glycol having a molecular weight of 
1064 (PEG-1000) there was added 2.22 moles of 88% lactic acid. The mixture 
was refluxed for 4 hours after which the residual lactic acid was stripped 
at 130.degree. C. and 4 Torr. 
The product weighing 1215 g., mainly polyethylene glycol dilactate, was 
combined with 67 g. of trimethylolpropane and stripped of residual water 
at 105.degree. C. and 3 Torr for four hours. This mixture was then added 
at 60.degree. C. flask temperature to 574 g. of tolylenediisocyanate over 
a period of 2 hours. The reaction mixture was then heated for an 
additional 4 hours at 60.degree. C. To the reaction mixture was then added 
an additional 87 g. of tolylenediisocyanate in thirty minutes at 
60.degree. C. and the reaction mixture heated for an additional two hours 
at 60.degree. C. The NCO content was 2.06 meq/g.(theory 2.11 meq/g.); the 
viscosity was 18,000 cp at 25.degree. C. 
A foam was prepared from the prepolymer of this example using 100 parts of 
prepolymer and 100 parts of 5% solution of Pluronic P-75 in water. The 
final foam was dried and on treatment with a synthetic sewer sludge, 
disintegrated within ten days. A similar sample was prepared from a 
prepolymer containing polyethylene glycol 1000 and trimethylolpropane 
showed no signs of decomposition in the same period of time. 
It is understood that the foregoing detailed description is given merely by 
way of illustration and that many variations may be made therein without 
departing from the spirit of this invention.