JPH0314813A - Sponge-forming prepolymers - Google Patents

Abstract

PURPOSE: To provide a sponge forming prepolymer imparting sponge rapidly dried, reduced in shrinkage and generating no curl by reacting a specific diol component, a diphenylmethanediisocyanate-containing isocyanate product and a crosslinking agent.

CONSTITUTION: A poly (oxy 2-4C alkylene) dial (A) having 50 wt.% or more of an oxyethylene group, a nominal average mol.wt. of 1,100 or less and two hydroxyl equivalents per nominal mol, a diphenylmethanediisocyanate-containing isocyanate product (B) composed of a mixture of diphenylmethanediisocyanate and isocyanate containing a derivative thereof and having two or more functionalities and a monomeric polyol crosslinking agent (C) having three or four hydroxyl equivalents per mole are prepared. The components A, B, C are reacted so that the component B becomes 50 wt.% or more and the mol ratio of the component A/the component C becomes 4/1-8/1 and the ratio of isocyanate equivalent/total hydroxyl equivalent becomes 3/1-4/1 to produce a prepolymer for producing foamed sponge.

COPYRIGHT: (C)1991,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to polyurethane prepolymers from which sponges can be made that have the general appearance and wiping properties of cellulose sponges with minimal swelling when wet and non-curling properties during the drying period.

Composites based on sponges, such as wiping sponges or cleaning pads or bathroom mats.
It has been the ultimate goal of the sponge industry to replace the cellulose foam used in polyurethane foam with polyurethane foam. In general, prior art polyurethanes 7 ohms that are sufficiently hydrophilic to be useful tend to swell significantly when they are wetted by more than 75-100% by volume, for example. They dry slowly and unevenly resulting in curling or wrinkling as they dry. This characteristic is not desirable for household sponge applications.

7-locked wall cover
It has been proposed to use polyurethane foam as an alternative to coverings. However, these forms and the resulting buckling (bu
Due to their curling and wrinkling properties, these foams are not suitable for use as replacements for flocked wall coverings.

In U.S. Pat. No. 3,806,474, hydrophilic polyester urethane foams are disclosed. However, this foam rapidly becomes wet
Wipe (wipe) to obtain a clean surface without
ping).

In US Pat. No. 4,160,076 a pseudo-natural sponge based on hydrophilic polyurethane is disclosed. However, even though this simulates a natural sponge, polyurethane sponges still have the problem of swelling extensively when they get wet.

A commonly used polyurethane is made from MDI, which is methylene bis(phenylisocyanate). These foams are rigid or semi-rigid because the MDI imparts crystallinity.

They are generally hydrophobic since MDI itself is hydrophobic. In British Patent No. 874,430, a flexible polyurethane foam consists of a polyether polyol having at least two hydroxyl groups, a diarylmethane diisocyanate and 5 to 10% by weight of a polyisocyanate with a functionality greater than 2. Produced by reaction with a polyisocyanate mixture in the presence of a small amount of water. Catalysts can be used in any embodiment. These 7-year-olds are not hydrophilic and are manufactured using sufficient water to transport large amounts of fibers, fillers, preservatives, or other water-soluble or water-dispersible ingredients into the foam. It has the disadvantage that undesirable catalyst residues may remain in the case of any catalyst.

In U.S. Pat. No. 4,237,240, a flexible MDI-based foam with high load bearing and high energy absorption capacity is made of a polyester polyol or a polyester polyol whose polyester polyol content is at least 60% by weight of the polyol mixture. It is made by reacting diphenylenemethane diisocyanate with a mixture of polyols and polyether polyols and a small amount of water.

As claimed, catalysts are used; these foams have the same disadvantages as that of GB 874,430, including undesirable catalyst residues in the foam, and furthermore they are more expensive. Requires the use of a polyester polyol.

In British Patent No. 1.209.058, a flexible hydrophilic polyurethane foam contains at least 10% by weight of a block copolymer of ethylene oxide capped with propylene oxide to provide hydrolytic stability. It can be produced by reacting polyether polyol and polyisocyanate. This process requires the use of at least one divalent tin salt of a fatty acid and/or at least one tertiary amine as a catalyst.

The 7 ohm raw wood produced by this method is hydrophilic, but has the disadvantage that it can only be made using small amounts of water. Furthermore, there is no teaching of using MDI, which is hydrophobic, to produce hydrophilic 7-ohm raw materials, and the resulting 7-ohm materials would contain undesirable catalyst residues.

The ultimate objective of the present invention is to provide a material with good water retention and wiping stability, shrinkage less than 15% by volume, no curling and quick drying. /m” (2
To provide an improved polyurethane 7 ohm having a density of ~41 b/ft3).

Another ultimate object of the present invention is to provide an improved polyurethane foam having good dimensional stability that can be used as a substrate for seven-walled wall coverings or various other foam applications as well as for sponge applications.

The basic object of the present invention is to provide a prepolymer which, when mixed with water, can form a polyurethane 7 ohm having properties as described above similar to cellulose-based foams.

These and other objects will become apparent as the description of the invention proceeds.

The novel polyurethane prepolymers are poly(oxyC2~,
alkylene) diols, di7enylmethane diincyanate-containing incyanates with a functionality greater than 2.0 emitted by a mixture of isocyanates containing MDI and derivatives of MDI, and with a hydroxy functionality of 3 or 4. Derived from monomeric polyol crosslinkers. This prepolymer can be reacted with water in the absence of a catalyst to produce a durable polyurethane sponge that has many of the properties of cellulose sponge.

These 7 ohms are white and therefore aesthetically pleasing and suitable for medical and health supervision applications.
and health care applic
ations).

A preferred component is Carbowax 1000 (Carbow
Diols having more than 80% by weight of oxyethylene groups such as ax l 0 0 0 ), trimethylolpropane (TMOP) as a crosslinking agent and an abjone polymer chemical ( U ) with a functionality of about 2.1 as incyanate pjohnP polymer C
Isonate 1 4 3 which is methylene bis(phenylisocyanate), hereinafter referred to as MDI, based on the incyanate product of
L (isocyanate I son)
atal 4 3 L).

In a preferred formulation, union carbide poly(
Carbowax, an oxyethylene) diol raw material, is used to impart water absorption properties to the final raw material. The preferred TMOP is added to impart strength to the foam product through its action as a crosslinking agent. Diol (like carpo wax)
It is important that the molar ratio of monomeric polyol crosslinking agent (such as TMOP) is in the range of about 4:1 to 8:1, preferably about 6:1. Similarly, the ratio of incyanate equivalents to hydroxyl equivalents in the prepolymer is about 3 to
4:l, preferably in the range 3.3 to 3.7:1.

The isocyanate-containing raw material accounts for 50% of the total prepolymer.
% by weight and it should have a functionality greater than 2.0. This would exclude pure MDI with a functionality of only 2.0 as only a single incyanate. The use of isocyanate 143L as the preferred incyanate source is desirable. This is because it contains a dimer trimer and a carposiimide component that increases functionality to levels greater than 2.0 and is believed to contribute to the storage stability of the prepolymer and the strength and dimensional stability of the foam.

The actual content of free MDI can be adjusted up or down in this incyanate-containing product as long as the functionality is greater than 2.0. For example, additional pure MDI can be added.

To further improve the durability of the sponge, about 10% or less of a relatively short 7'8 inch or less 7-Iva is added to the aqueous suspension. Preferred fibers are polyester fibers. Thickening or suspending agents can be added to the aqueous phase to allow the mixture to be pumped through a foam Ffca machine. For example, Carpopol 941, a high molecular weight acrylic acid polymer, can be added, which can be neutralized to a pH level of 7 with a base such as ammonium hydroxide.

Sponge 7 ohm is made by reacting water and Brepolymer, preferably while heating both. Surfactants can be added to the water to control foam cells. A preferred incyanate-containing herb with a functionality greater than 2.0 is diphenylmethane diisocyanate, abbreviated MDI and a mixture of incyanate-containing derivatives of MDI. One commercial product that meets this requirement is incyane}143L, which is produced by reacting MDI to form a carbodiimide, which material is then reacted to form a trifunctional cyclic adduct. MD
The mixture of I, carbodiimide and cyclic adduct is in equilibrium. The mixture contains a predominant amount of pure diphenylmethane diisocyanate and a minor amount of the diphenylmethane diisocyanate compound and trifunctional cyclic adduct. The mixture of A and BfR components below constitutes a 143L system.

A OCN-R-NCO→ [OCN-R-N = C
= N-R-NCO]B N-R-NGO/OR-NGO As used herein, the term derivative of diphenylmethane diicyanate refers to a derivative made from MDI as a starting material. It will include adducts, dimers and trimers.

It will not contain materials such as polymethylene polyphenylisocyanate that is not made from MDI.

The diol used to make the prepolymer preferably contains at least 50% by weight oxyethylene groups, and preferably at least 80%! It is a poly(oxyC, -, anolekylene) diol having i% of oxyethylene groups. Thus, if the diol contains oxypropylene or oxybutylene or mixtures thereof, this minimum amount of soxyethylene must also be present. Preferred diols are hydrophilic poly(oxyethylene) diols having at least 80% by weight oxyethylene groups and made by Union Carbide under the product name Carpowax. Isocyanate 143 as a single isocyanate source
When L is used, it is preferable to use a carbowax having a molecular weight of about 1000. 600 to 1 too
The most preferred form has a molecular weight of 950 to 1050, although other carpowax treatments can be used. It is this range of products that is produced when producing the commercial grade Carpowax I 000. If a molecular weight type lower than about 600 is used, the resulting sponge formulation made from the prepolymer will also require large amounts of relatively hydrophobic isonate.

Similarly, if a molecular weight form of carbowax higher than 1100 is used, the resulting sponge will lose significantly its crosslinking density and its dimensional stability and stiffness.

Relatively short, low molecular weight monomeric polyols having 3 or 4 hydroxyl equivalents per mole are added to provide crosslinking strength to the final 7 ohm blank to be made from the prepolymer. Examples are trimethylolethane, trimethylolpropane, chloride, triethanololeamine, pentaerythritol or mixtures thereof. A preferred polyol is trimethylolpropane, TMOP, having the formula C H zO H /HsC CH2 C CH2 0H\CH,01-1.

It is also desirable to control the ratio of diols, such as carpowax, to short crosslinked polyols in making prebolimers. Generally, the molar ratio should be in the range of about 4:l to 8:l, with a preferred ratio of about 6:l. If the proportion is too low such that too much polyol is present, the resulting sponge made from the grepolymer will lose its hydrophilicity and the foam will also have a poor foam structure. On the other hand, if the proportion is TM
If the polyols such as OP are increased to be less present, then a weak, brittle 7 ohm is obtained, which is characterized by fine bubbles and partially closed cells. The molar ratio expressed for these two materials defines the relative number of hydroxy equivalents present from each material. Thus, at a ratio of 1 mole of preferred diol carpowax to 1 mole of preferred polyol TMOP, the carpowax contributes 12 equivalents of hydroxyl to 3 equivalents from TMOP.

Carpowax l 000 is 50 per hydroxyl group
TMOP with an equivalent weight of 0 but a molecular weight of 134
has an equivalent weight of 45 per hydroxyl group. Since isonate 143L is used in combination with the hydroxyl group, the amount of isonate required is very sensitive to the amount of TMOP, and therefore the amount of TMOP is carefully controlled relative to the hydroxyl group.

The ratio of incyanate to hydroxyl groups in the reactants should be in the range of 3 to 4, more preferably about 3
．． Should be between 3 and 3.7.

When the probolymer is prepared, one incyanate group of the polyisocyanate fraction reacts with the hydroxyl group leaving residual incyanate groups unreacted. These free isocyanate groups of the prepolymer react with water to form polyurea linkages, with a concomitant increase in molecular weight and release of CO, which facilitates the formation of the foamed sponge.

The incyanate-containing biomaterial should be more than 50% by weight of the brepolymer to provide the necessary stiffness to the sponge.

When reacting fractions to form brevolimers, it is useful to measure incyanate levels by titration after the reaction has run for about one hour.

From this reading and subsequent titration, it is possible to determine the additional reaction time needed to reduce the incyanate level below or near the theoretical point at which all of the hydroxyl groups would react with the incyanate. can. If the reaction is allowed to continue for too long, the incyanate level is further reduced, in which case the prepolymer viscosity increases, making subsequent mixing of the prepolymer with water more difficult. Overreacting the prepolymer component is a problem for 7 year olds.
increases the density of the membrane and reduces the water absorption of the resulting membrane.

The fibers are preferably added to the aqueous phase for insertion into a 7 ohm set to provide sufficient structural rigidity. Polyester fibers are particularly advantageous and they can be cut to lengths of about 1.27 cm ('8 inches) or less. For aqueous suspensions, the fibers can be added in varying amounts, although preferably greater than about 10% by weight. This is because above that level the suspension is too difficult to pump.

High molecular weight suspending or thickening agents are added to serve two functions. Firstly, it keeps the pores in suspension so water does not drain out of the fibers and the 7-eyes do not float. Second, the thickener acts as a lubricant for the 7-aivers, so they do not tangle, dehydrate and jam as they pass through the mixing pump.
up). An example of a thickener is Polyox WSR (Pol
yox WSR), natrosol (Natroso
l), xanthan gum
Dot'sSeparan AP30 (Dot'sSeparan) having a high molecular weight such as approximately
AP30).

A preferred suspending agent or thickening agent is Carpopol 9
3 4 (Carbopol 9 3 4), Carbopol 9 4 0 (Carbopol 9 4
0) and especially Carpopol 941 (Carbo
B. pol 9 4 1). F. Goodri
ch C chemical C o. Carpopol resin (Carbopol resin) manufactured by
esins). Carbobol resin has an acid moiety (ac
id moiety), neutralizing agents such as sodium or ammonium hydroxide can be added. There is an advantageous increase in the viscosity of the aqueous phase when adding ammonium hydroxide as a neutralizing agent for carpopol. Since ammonium hydroxide is more stable than carpopol, a more economical formulation with the same amount of thickening can be created, which uses less carpopol.

Some of the suspending agents or thickeners may have surface-active properties, so they may also be part of a surfactant system that controls the cell size of the resulting sponge.

The surfactant should have the correct cell size, shape, and collapse (c
It is chosen to give 7 ohms with a good appearance without any ollapses or splits. Surfactants known to be useful with polyurethane 7 ohms can be used in this case. Examples of preferred surfactants include Wyandotte, Michigan.
BASF Wyandott, Michigan
A block copolymer of oxyethylene and oxypropylene such as Pluronic Polyol surfactant manufactured by eCorp. A preferred surfactant is Pluronic L-62 (Pluronic L-62).
6 2).

In making the polyurethane 7-year-old, the preferred process is to add approximately equal amounts of the aqueous suspension with the prebolymer mixture and then mix the two together. The composition of the aqueous suspension can also be expressed based on 100 parts of Brepolymer resin. Thus, lOO parts of water per 100 parts of resin is written as 100 phr water. The ratio of amounts of prepolymer mixture to aqueous suspension can vary over a wide range. However, if the amount of aqueous suspension is too large, the strength of the resulting foam will be reduced. On the other hand, if the amount of aqueous suspension is reduced too much, it will not be possible to add enough fiber and filler to be delivered via the aqueous suspension.

The preferred method of foaming the brepolymer is an aqueous suspension;
For example, heating a 2% solution of Pluronic L-62 surfactant and the prepolymer to a temperature of about 35°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 prepolymer and immediately stirred by a mechanical stirrer, such as a blade attached to a drill motor, for up to 15 to 30 seconds. Stir.

This length of time allows complete mixing but does not allow any appreciable chemical reaction to occur. This mixture is immediately poured into the mold, where the foam material bulges (r).
ising) and curing occur.

Depending on the fibers, pigments and thickeners added, the best surfactant is L-520 (UnionC
Brij-58 (IC1 -
America) and other ICI B rij, S
They can vary up to very hydrophilic types such as pan or T vieen products. Pluronics for general use, especially L-62, L-72, L-92, P-75
or P-85 (BASF-Wya
Non-ionic surfactants such as Ndotte are preferred. The use of these surfactants will be familiar to those skilled in the art of formulating polyurethane foam products.

Depending on the conditions of foaming, these foams may be manufactured by SEASPONGE, as disclosed in U.S. Pat.
The sponge has the appearance and wiping properties of a cellulosic sponge. It has large cells (pores) that are several millimeters in diameter, but the walls of these cells are The fine bubbles (finel - Unlike other flexible urethane foams, which are graded as celled (smooth) or reticulated, reticular features have the majority of the cell membrane removed so that they have a fibrous appearance. They are used as filters in furnaces and air conditioners.SEA SPON
The GE foam and the foam of the present invention, in contrast, have an intact but perforated cell membrane. This is different from the familiar polyurethane sponges on the market. This is because they absorb water into the polymer itself and leave it dry.

Having described the basic aspects of the invention, the following examples illustrate specific aspects thereof.

Tests Used to Evaluate Sponges The following test methods are used. All foam samples to be tested were placed in a dry uniform 2-54 cm x 7.62 cm x 2.7 cm
(1'X3#X5"), cut to size 105±5
Dry to constant weight without moisture at °C. The weight of the sample is approximately 0. ! Gram. A group of 10 representative samples were used for testing.

A. Wet-Out time
me) In a pan filled with water, float the thoroughly dry foam to the surface. The whole sample becomes wet, i.e. water appears,
The time required to wet the entire top surface of the sample in seconds is then recorded.

B. Full wet time foam from wet state is completely wetted and washed with a washing wringer (was
machine wrapper) to obtain uniform moisture. The moist 7 ohms float to the surface of the pan filled with water. The time required for complete wetting of all samples is recorded as 0.1 seconds.

C. Determination of washing machine durability2.54cmX7.62cmXl2.7cm (1”
x3#x5'') Sample is dried at 105°C ± 5°C for 1 hour and then weighed to the nearest ±0.02 grams.

In the Maytag type, 48 samples are placed in a vertical washer filled with water and stirred for 72 hours. Dry the sample and reweigh it.

This test is very demanding and should be satisfactory for use as a sponge or for other 7 ohm applications, even though the initial dry weight will not last in one bead for a period of 72 hours. It could be a kimono. If the sponge does not survive the 72 hour period, the results record the observation period in hours during which visible deterioration is observed.

D, Determination of tear strength2.54cmX7.62cmXl2.7cm (1"×
6.3" x 5") sample. 3 5 cm (2”
/z”) distance of 2.54cmX7.62cm (1#
Tear on the X3” plane. Scott Tester, Model J
-1 (Scott Tester, Model
J-1) (or equivalent) Set the jaws with a 1 inch opening between the upper and lower jaws. Clamp one of the tear areas to each of the jaws of the Scott tester. Operate the tester and set the tear resistance to lbs. Record with .

E. Determination of breaking stress Instron tester with jaws for testing sponges or similar materials
ster) is used to determine tenacity.

Set the jaws of the Instron tester with a 1 inch opening between the upper and lower jaws. Clamp the 3 inch dimension in each show and activate the tester.

Record toughness with a bong. This value is converted to kg/cm'' (pounds per square inch) by dividing the toughness by the width of the tail show in contact with the sample.

F. Determination of Air Permeability The test equipment used consists of a fan connected to the tube by a sample section located on the end of the tube. Measure the air pressure upstream of the sponge. The air gauge pressure is configured to read 0.60 if the material is impermeable. If the reading is zero, it indicates very high porosity and a very open cell structure with very little pressure drop across the sponge.

G. elastic modulus This is measured from the stress-strain curve at 1% elongation.

Example 1 This example describes the preparation of a prepolymer used in accordance with the invention.

Carpowax 1000 (412 g, 0.4 mol) and trimethylolpropane (9.05 g, 0.06
A mixture of 75 mol) was dried by heating at 70° C. for 2 hours under a vacuum of 0.003 atm (2 Torr). 143 L of incyanate (3.55 equivalents of isocyanate) 507.
Added 5g.

The temperature was maintained at 70 DEG C. for 2-18 hours and after this time the incyanate content of the green wood was found to be 2.67 milliequivalents per duram. It is 19 at 25℃
．． It had an initial viscosity of 0.000 cp. 2 at 80°C
After a week of storage testing, the viscosity increased to only 30,000 cp.

Example 2 (Reference Example) This example describes the preparation of a foam using the prepolymer of Example 1.

To a beaker was added 700 g of prepolymer heated to 35°C and 686 g of an aqueous solution heated to 35°C and containing 14 g of Pluronic L-62, a Wyandotte nonionic surfactant.

They were stirred together in a drill motor for 15 seconds and then poured into molds. The dried foam blank had a density of 34.05 kg per cubic meter (2.1 pounds per cubic foot).

Approximately 2.54cmX7.62cmXl3.97Cm (1
A sponge having dimensions of "X3" x 5-% 8#) was cut from a 7 ohm block. When saturated with water, it holds 8 to 9 times its weight in water without dripping, and has a capacity of 275.35 cm3 to 308.13 cm3.
6.8 cubic inches to 18.8 cubic inches) (12% increase). When squeezed by hand, all but about 90% of its dry weight was released. It had a similar appearance and wiping properties to a cellulose sponge of the same dimensions.

Example 3 This grepolymer was prepared using the same reactants and conditions as Example 1, except that the reaction was stopped after the raw wood had an incyanate content of 2.61 meq/Guam. .

Example 4 This prepolymer was prepared using the same reactants and conditions as Example 1, except that the reaction was stopped after the raw material had an isocyanate content of 2.70 meq per duram. t;.

Examples 5 and 6 (Reference Examples) These examples illustrate the effect of using the prebolimers of Examples 3 and 4 and adding reinforcing fibers.

In Example 5, 700 g of the prebolimer of Example 3 was mixed with 686 g of water and the surfactant Pluroni 7k L-62.
An aqueous solution containing 14 g was mixed.

In Example 6, 700 g of Grepolymer of Example 4 was mixed with 683.2 g of water and the surfactant Pluronic L-62.
14g, Weber City, Virginia (Weber C
35 g of polyester fiber 2, a 1-27 cm ('/z inch) fiber manufactured by Mini Fiber, Inc. of Virginia, 33% solution of ammonium hydroxide 7 The mixture was mixed with an aqueous solution containing 2.8 g of the thickener Carbopol 941 neutralized at .0 g. The results are listed in Table 1.

Dimensions e e 0) The formulation of Example 5 contained only water and surfactant from the aqueous phase. These sponges had unusual properties. They are bright white and well developed (deve l
opened) has a uniform SEA SPONGE'3 type structure. The density is 34.54 kg/m' (2.1
It was an extremely low value of 3 lb/ft'). When this sponge is dry, it may be rigid depending on the surrounding conditions, but when wet it is soft. Although the initial drying time is relatively long, it is highly absorbent when wet. Sponges hold large amounts of water, clean well, wipe well, and have reasonable machine durability values. Air permeability is moderate, but volume swelling is very low. This sponge has a good appearance and surface feel.

The sample from Example 6 shows that Carpobol 941 and 51)h in the aqueous suspension used to make the sponge
7 ohm fiber reinforcement of Example 5 containing r7 ivar. This sponge was able to pass a very aggressive 72 hour washer test with 0% weight loss. Tear strength for wet and dry conditions was high. Very good air permeability, exhibiting an open cell structure. Although the cellular structure may not be considered aesthetically pleasing as the foam in Example 5, this 7 ohm cleans well and wipes away satisfactorily.

This is very durable and has a large sponge.

Example 7 (Reference Example) This example describes the preparation of a sponge without any surfactant added to the water.

A foam was made according to the method in Example 2, except that no surfactant was present in the water. The resulting hydrophilic foam has a density of 48.64 kg/m3 (3 lb/ft3), and it has a fine texture (fi) due to the small, partially closed cell structure.
It was similar to the 7-year-old boy in Example 2, except that it had a ner texture.

Claims (1)

Claims: 1. A prepolymer for use in making foamed sponges, comprising: a) at least 50% by weight of oxyethylene groups and a nominal number average molecular weight of at most 1100; , 2- per nominal mole
Poly(oxyC_2_~_4) with hydroxyl equivalents
b) a diphenylmethane diisocyanate-containing isocyanate product having a functionality greater than 2.0 consisting of a mixture of diphenylmethane diisocyanate and an isocyanate containing a derivative of diphenylmethane diisocyanate; and c) 3 or 4 hydroxyl equivalents per mole. The isocyanate product consists of the reaction product with a monomeric polyol crosslinking agent having 50% of the prepolymer.
% by weight, the diol and polyol crosslinking agents are present in a molar ratio ranging from 4:1 to 8:1, and the ratio of isocyanate equivalents to total hydroxyl equivalents is 3:
Prepolymers ranging from 1 to 4:1. 2. The prepolymer of claim 1, wherein the poly(oxyC_2_-_4 alkylene) diol has a molecular weight of 600 to 1100, and the diol has at least 80% by weight of oxyethylene groups. 3. The prepolymer of claim 1, wherein the polyol crosslinker is selected from the group consisting of trimethylolpropane, trimethylolethane, glycerol, triethanolamine, pentaerythritol, and mixtures thereof. 4. The prepolymer of claim 1, wherein the isocyanate product is a modified diphenylmethane diisocyanate containing a high percentage of pure diphenylmethane diisocyanate and lesser amounts of carpodiimide and carpodiimide cyclic adducts.