WO2001049802A1

WO2001049802A1 - Polyurethane reactive hot melt adhesive composition

Info

Publication number: WO2001049802A1
Application number: PCT/US2001/000411
Authority: WO
Inventors: Ricardo De Genova; William A. Clay; Paul E. Cranley; Michelle D. Harper
Original assignee: The Dow Chemical Company
Priority date: 2000-01-04
Filing date: 2001-01-04
Publication date: 2001-07-12

Abstract

Prepolymer comprising the reaction product of: a) one or more polyisocyanates; and b) i) one or more copolymer polyols having a solid dispersed in a continuous phase or ii) a mixture of one or more copolymer polyols having a solid dispersed in a continuous phase or iii) one or more alkoxylated bisphenols; or iv) a mixture thereof; wherein the prepolymer has an isocyanate content of 2 to 5 percent by weight. Reactive hot melt adhesive comprising the prepolymer of the invention wherein the prepolymer has an application temperature of from 50 to 140 °C and a viscosity at the application temperature of from 3,000 to 50,000 centipoise. Method of bonding using the adhesive of the invention.

Description

POLYURETHANE REACTIVE HOT MELT ADHESIVE COMPOSITION

Background of the Invention

This invention relates to polyurethane prepolymers and reactive hot melt adhesive compositions containing such polyurethane prepolymers. The invention further relates to a process for bonding two or more substrates together using the reactive hot melt compositions of the invention and to the composite structures bound together using the reactive hot melt adhesives of the invention.

Many commercially available polyurethane reactive hot melt adhesives are methylene diphenyl diisocyanate (MDI) prepolymers of polyester polyols, for example, polyester polyols based on adipic acid. Such prepolymers are crystalline and solid at room temperature and demonstrate low viscosities when heated to about 120°C. After being applied to a substrate in the liquid state, a bond can be formed that offers initial (green) strength due to the prepolymer cooling and returning to solid state. The crystallinity of the polyester polyol is largely responsible for the solid to liquid phase transition, which contributes to the green strength. Polyether polyols on the other hand, are known to have limitations when used in a similar manner since they form amorphous polymers that do not crystallize at room temperature. Recent work however has demonstrated that by combining long and short chain polyether diols at certain ratios, it is possible to obtain polyether based polyurethane reactive hot melts with performance comparable to that obtained with polyester polyols. Even though the performance of the cured polyether polyol based polyurethane hot melt adhesives is very similar to the polyester based ones, the initial adhesive strength (green strength) is not quite the same and this has prevented this technology from being widely accepted. What is needed is a polyether based polyurethane prepolymer which is useful in a reactive hot melt adhesive wherein the hot melt adhesive has a viscosity above its melt temperature which allows it to be used and applied as an adhesive. Furthermore, such a prepolymer and adhesive prepared therefrom should have good adhesive strength after 15 minutes and after 30 minutes and should not require the presence of a solvent.

Summary of Invention

The invention is a prepolymer comprising the reaction product of a) one or more polyisocyanates; and b) i) one or more copolymer polyols having a solid dispersed in a continuous phase wherein the copolymer solids have an average molecular weight of about 50,000 to about 200,000, and the continuous phase is liquid polyether polyols having a number average molecular weight of about 400 to about 7000, or

ii) a mixture of one or more copolymer polyols having a solid dispersed in a continuous phase wherein the copolymer solids have an average molecular weight of about 50,000 to about 200,000, and the continuous phase is liquid polyether polyols having a number average molecular weight of about 400 to about 7000 with one or more diols having a number average molecular weight of about 800 or less wherein at least 50 percent by weight of the mixture of polyols and diols comprises the diols, or iii) one or more alkoxylated bisphenols, iv) or a mixture thereof;

wherein the prepolymer contains from 2 to about 5 percent by weight of isocyanate.

In another embodiment, the invention is a reactive hot melt adhesive composition comprising the prepolymer of the invention wherein the prepolymer has an application temperature of from 50°C to about 140°C and a viscosity at the application temperature of from 3,000 to 50,000 centipoise.

In another embodiment, the invention is a method of bonding two substrates together which comprises: a. heating an adhesive according to the invention to its application temperature; b. applying the adhesive to one or both of the substrates; c. contacting the two substrates such that the adhesive is located between the two substrates; d. allowing the adhesive to cool and cure so as to bond the two substrates together.

In another embodiment the invention is a composite structure comprising two or more substrates and located between the two or more substrates is a cured adhesive according to the invention such that the cured adhesive bonds the substrates together.

The polyurethane prepolymers of the invention enable the preparation of reactive hot melt adhesives with advantageous properties. The reactive hot melt adhesives of the invention are useful at reasonable temperatures and do not require the use of solvents for application. Further, the reactive hot melt adhesives of the invention demonstrate good green strength and set times. Further, the adhesive compositions of the invention exhibit lower viscosity growth in the melt with time than other polyurethane based reactive hot melt adhesives.

Detailed Description of the Invention

Preferably the polyurethane prepolymers for use in preparing the adhesive composition of the invention have an average isocyanate functionality of at least about 2.0 and a number average molecular weight of at least about 400. Preferably the isocyanate functionality is no greater than about 3.0, more preferably, no greater than about 2.5 and most preferably, no greater than about 2.3. Preferably, the weight average molecular weight of the prepolymer excluding the high molecular weight solids, is at least about 400, and is more preferably at least about 2000; and preferably no greater than about 4000.

The prepolymer may be prepared by any suitable method, such as by reacting one or more copolymer polyols, in some embodiments in admixture with one or more diols, one or more adducts of alkylene oxides and bisphenols, or mixtures thereof, optionally in the presence of one or more additional isocyanate-reactive compound containing at least two isocyanate-reactive groups, with an excess over stoichiometry of a polyisocyanate per isocyanate-reactive groups present under reaction conditions sufficient to form the corresponding prepolymer. Such process conditions are disclosed, for example, in US Patent 5,852,137 at column 4, line 65 to column 5, line 66 and column 7, line 3 to 15, incorporated herein by reference.

The endgroup isocyanate content of the prepolymer is preferably about 2 percent or greater, more preferably about 3 percent or greater and most preferably 4 percent or greater. The isocyanate endgroup content of the prepolymer is preferably about 5.0 percent or less and most preferably about 4.5 percent. As used herein isocyanate content refers to the weight percent of isocyante moieties bound to the prepolymer.

Isocyanates useful in preparing the prepolymer of the invention are disclosed in US Patent 5,852,137 at columns 2, line 40 to column 3, line 45 incorporated herein by reference. Preferably the isocyanates used in the invention include methylenediphenyldiisocyanate, oligomers and polymers thereof, and chemical modifications thereof .

As used herein, the additional isocyanate reactive compound is as disclosed in US Patent 5,852,137 at column 3, line 46 to 64, incorporated herein by reference. In general such compounds are those containing at least two active hydrogen moieties. Such active hydrogen moieties include -COOH, -OH, -NH₂, -NH-, -CONH₂, - SH and -CONH .

Copolymer polyols as used herein represent the product of free radical polymerization of styrene and/or acrylonitrile in a polyether polyol continuous phase. A chain transfer agent is used and the copolymerization is carried out in the polyether polyol with a stabilizing material being also fed into the reactor. This stabilizing material may be mono- or di-functional with respect to hydroxyl groups and additionally contains a vinyl reactive group. This provides for grafting of the stabilizing material to the styrene acrylonitrile copolymer particle. The stabilizer allows the stable dispersion of solid styrene acrylonitrile particles in the polyol continuous phase. Such copolymer polyols are further described in US Patent 4,390,645; 4,463,107; 4,148,840; and 4,574,137 all incorporated herein by reference. As used herein with respect to copolymer polyols, solids refer to the styrene and/or acrylonitrile particles dispersed in the liquid polyol carrier. The solid phase is the discontinuous phase. Liquid polyol carrier refers herein to the liquid polyol continuous phase of a liquid polyol into which solids are dispersed. The solid phase of the copolymer polyols used preferably have an average molecular weight of from 50,000 or greater and more preferably about 100,000 or greater, and preferably about 200,000 or less and more preferably from 175,000 or less. The solids content of the copolymer polyol is generally about 5 to 50 weight percent based on the total weight of the dispersion. Preferably the solids content is 25 to 45 weight percent of the total dispersion .

The glass transition temperature of the styrene acrylonitrile copolymer particles can be adjusted by changing the ratio of styrene (T_g of 100°C) to acrylonitrile (T_g of 140°C) . This allows tailoring of the melt application temperature of the adhesive by methods known in the art.

The continuous phase of the copolymer polyol can have any molecular weight which is suitable for preparation of a reactive hot melt adhesive. In one embodiment the one or more copolymer polyols is blended with one or more additional diols. In this embodiment, the amount of additional diol in the blend is at least about 50 percent. It is believed that the presence of the diol increases the physical strength of the adhesive prepared. In one preferred embodiment the additional diol used has a molecular weight of about 800 or less and more preferably about 500 or less. Preferably the diol used has a molecular weight of about 62 or greater and more preferably about 90 or greater. Preferably the amount of additional diol present in this embodiment is about 50 percent by weight or greater of the diol copolymer polyol mixture, and preferably about 90 percent by weight or less and most preferably about 70 percent by weight or less. Preferably the diol is one or more polyalkylene oxide based diols. More preferably the diol is one or more polypropylene oxide diols or a mixture of one or more polypropylene oxide diols and one or more polyethylene oxide diols. In another embodiment the diol can be prepared from a mixture of propylene oxide and ethylene oxide. Preferably the mixed ethylene oxide propylene oxide diols are random diols with the placement of the ethylene oxide and propylene oxide residues found in a random fashion along the backbone of the diol. Among preferred diols are ethylene glycol and higher homologues for example, polyethylene glycols, propylene glycol and higher homologues, for example, polypropylene glycols, 1,4 butane diol and higher homologues, for example, polytetrahydrofuran glycols, 1,2 butane diol and higher homologues, for example, poly 1,2 butylene glycols and random or block copolymers of ethylene oxide, propylene oxide, 1,2 butylene oxide or tetrahydrofuran .

The bisphenol alkylene oxide adduct useful in the invention can be any product of a bisphenol having a bridge between the two aromatic groups and reacted with an alkylene oxide. Representative preferred bisphenol resins are disclosed in US Patent 5,308,895 at column 6, line 3 to Column 7, line 65 and represented by formulas 2 and 4 incorporated herein by reference. The most preferred bisphenols are bisphenol A and bisphenol F. The bisphenols are reacted with an alkylene oxide using methods well known to those skilled in the art. Useful alkylene oxides are preferably butylene oxide, propylene, ethylene oxide, or a mixture thereof with ethylene oxide, propylene oxide, or a mixture thereof being more preferred and propylene oxide being most preferred. The average number of alkylene oxides per bisphenol is preferably about 2.0 or greater, more preferably about 4.0 or greater and most preferably about 6.0 or greater. The average number of alkylene oxide moieties per bisphenol is preferably about 12.0 or less, more preferably about 10.0 or less and most preferably about 8.0 or less. The molecular weight of the bisphenol alkylene oxide adducts is preferably about 288 or greater, more preferably about 316 or greater and most preferably about 344 or greater. The molecular weight of the bisphenol alkylene oxide adducts is preferably about 1000 or less, more preferably about 750 or less and most preferably about 500 or less.

The adhesive composition comprises about 10 percent by weight or greater of prepolymer based on the weight of the adhesive composition, more preferably about 50 percent by weight or greater and most preferably about 60 percent by weight or greater. The adhesive composition comprises about 100 percent by weight or less of the prepolymer based on the weight of the adhesive composition, more preferably about 90 percent by weight of less and most preferably about 80 percent by weight or less .

The adhesive formulation of the invention may further comprise other components which are well known to those skilled in the art in formulating adhesive compositions. The adhesive formulation of the invention may contain a catalyst for the reaction of a polyurethane prepolymer with active hydrogen containing substrates. Preferably the catalyst has good stability in the absence of atmospheric moisture, but has a rapid cure rate in the presence of atmospheric moisture. Catalysts useful are well known to those skilled in the art and many examples may be found for example, in the POLYURETHANE HANDBOOK, Chapter 3, §3.4.1 on pages 90-95; in POLYURETHANE CHEMISTRY AND TECHNOLOGY, in Chapter IV, pages 129-217 and as described in U.S. Patent 5,852,137. Catalysts preferred in polyurethane reactions include, organotin compounds, dimorpholinodialkylethers, organo silicon titinates, alkyl titinates, bis carboxylates, tertiary amines, tin mercaptides, napthenates or alkanoate salts of lead, zinc, cobalt, manganese, bismuth or iron.

The preferred catalysts are the tin carboxylates and the dimorpholinodialkylethers. The dialkyltin dicarboxylates preferably correspond to the formula (R²OC (0) ) ₂-Sn- (R³) _ wherein R² and R³ are independently in each occurrence a Cι_ιo alkyl, preferably a Cχ-₃ alkyl and most preferably a methyl. Dialkyl tin dicarboxylates with lower total carbon atoms are preferred, as they are more active catalysts in the compositions of the invention.

The catalyst may be used preferably in an amount of about 0 percent by weight or greater based on the weight of the adhesive, more preferably about 0.006 percent by weight or greater, even more preferably about 0.05 percent by weight or greater and most preferably 0.2 percent by weight or greater. The catalyst may preferably be used in an amount of about 4.0 percent by weight or less, based on the weight of the adhesive, more preferably 1.0 percent by weight and most preferably 0.4 percent by weight or less.

The dimorpholinodialkyl ether or di ( (dialkylmorpholino) alkyl) ether when employed, are preferably employed in an amount, based on the weight of the adhesive, of about 0.01 percent by weight or greater, more preferably about 0.05 percent by weight or greater, even more preferably about 0.1 percent by weight or greater and most preferably about 0.2 percent by weight or greater and preferably about 2.0 percent by weight or less, more preferably about 1.75 percent by weight or less, even more preferably about 1.0 percent by weight or less and most preferably about 0.4 percent by weight or less. The organo tin catalyst is present in an amount of about 60 parts per million or greater based on the weight of the adhesive, more preferably about 120 parts by million or greater. The organo tin catalyst is present in an amount of about 1.0 percent or less based on the weight of the adhesive, more preferably about 0.5 percent by weight or less and most preferably about 0.1 percent by weight or less.

For formulating adhesive compositions, the prepolymer may be combined with fillers and additives known in the art for use in adhesive compositions. By the addition of such materials, physical properties such as viscosity, flow rate, and sag can be modified. However, to prevent premature hydrolysis of the moisture sensitive groups of the polymer, the fillers and additives are preferably thoroughly dried before admixture therewith. Exemplary filler materials and additives include materials such as carbon black, titanium dioxide, clays, calcium carbonate, surface treated silicas, ultraviolet stabilizers, antioxidants . This list, however, is not comprehensive and is given merely as illustrative. The fillers are preferably present in an amount of about 0 percent by weight or greater based on the amount of the adhesive and more preferably about 15 percent by weight. The fillers are preferably present in an amount of about 70 percent by weight or less based on the adhesive, more preferably about 50 percent by weight or less and even more preferably about 30 percent by weight or less.

The adhesive composition may further contain one or more plasticizers, tackifiers, solvents or a mixture thereof to modify rheological properties to a desired consistency. Such materials should be free of water, inert to isocyanate groups, and compatible with the polymer, except in the case of tackifiers which in some cases may be compatible only at application temperatures and in some cases have reactive groups which can be advantageously reacted with the prepolymer isocyanate end groups. Such material may be added to the reaction mixtures for preparing the prepolymer or the adduct, or to the mixture for preparing the final adhesive composition, but is preferably added to the reaction mixtures for preparing the prepolymer and the adduct, so that such mixtures may be more easily mixed and handled. Preferably plasticizers are well-known in the art and include dioctyl phthalate, dibutyl phthalate, a partially hydrogenated terpene commercially available as "HB-40", trioctyl phosphate, epoxy plasticizers, toluene-sulfamide, chloroparaffins, adipic acid esters, and castor oil.

Although not preferred, a solvent may be added to the adhesive formulation. Such solvents include those inert to isocyanate endgroups and with boiling points above the application temperatures of the hot melt adhesive. Preferably tackifiers include rosin and rosin esters, aromatic resins, aliphatic resins, aliphatic/aromatic resins, terpene resins, terpene phenolic resins, modified terpene resins, phenolic resins, partially hydrogenated aromatic resins, pure monomer resins, hydrocarbon resins, epoxy and phenoxy resins. The amount of plasticizer, tackifier, solvent or a mixture thereof, used is that amount sufficient to give the desired rheological properties and disperse the components in the adhesive composition. Preferably the plasticizer, tackifier, solvent, or a mixture thereof is present in an amount of about 0 percent by weight or greater, more preferably about 5 percent by weight or greater and most preferably about 10 percent by weight or greater. The plasticizers, tackifiers, solvents or mixtures thereof are preferably present in an amount of about 45 percent by weight or less, more preferably about 40 percent by weight or less and most preferably about 20 percent by weight or less .

The adhesive composition of this invention may be formulated by blending the components together using means well known in the art. Generally the components are blended in a suitable mixer or mixing extruder. Such blending is preferably conducted in an inert atmosphere and in the absence of atmospheric moisture to prevent premature reaction. It may be advantageous to add any plasticizers, tackifiers or solvents to the reaction mixture for preparing the isocyanate-containing prepolymer so that such mixture may be easily mixed and handled. Alternatively, the plasticizers, tackifiers or solvents can be added during blending of all the components. Once the adhesive composition is formulated, it is packaged in a suitable container such that it is protected from atmospheric moisture. Contact with atmospheric moisture could result in premature cross-linking of the polyurethane prepolymer-containing isocyanate groups.

The adhesive composition preferably has a viscosity at its application temperature of about 3,000 centipoise (3.00 pascal seconds (pas)) or greater and more preferably about 5,000 centipoise (5.00 pascal seconds) or greater. Preferably the adhesive composition of the invention has a viscosity of about 50,000 (50.00 pascal seconds) centipoise or less and more preferably about 20,000 (20.00 pascal seconds) centipoise or less at its application temperature. Application temperature means the temperature at which the adhesive is applied to a substrate. Typically the application temperature is a temperature at which the reactive hot melt adhesive is a liquid or will flow under the shear stress of the application equipment and a temperature at which initiation of cross-linking of the polyurethane prepolymer occurs. Activation temperatures used herein refers to the temperature at which cross-linking of the polyurethane prepolymer in the reactive hot melt adhesive is initiated. The prepolymer may react with residual moisture in the atmosphere, or in the substrate or with reactive groups on the substrate. Additionally, if there are present any isocyanate reacting (or blocking) groups in the formulation ingredients, for example, phenolic groups, these groups may unzip or de-block under application temperatures and allow the formerly blocked isocyanate to react with atmospheric moisture, substrate moisture or substrate reactive groups.

Preferably the adhesive of the invention has an activation temperature of about 10°C or greater, more preferably about 20°C or greater, most preferably about 30°C or greater. Preferably the adhesive of the invention has an activation temperature of about 140°C or less and more preferably about 120°C or less. Typically the adhesive is applied at or above its activation temperature. Preferably the adhesive of the invention has an application temperature of about 90°C or greater, more preferably about 110°C or greater, most preferably about 120°C or greater. Preferably the adhesive of the invention has an application temperature of about 140°C or less and more preferably about 130°C or less.

The adhesive can be applied by any means known to those skilled in the art. The adhesive can be applied using rollers, extrusion apparatus. Preferably, the adhesive of the invention is heated to a temperature at which the adhesive meets the viscosity requirements discussed hereinbefore. Thereafter the adhesive is applied to one or more of the substrates and the substrates are contacted such that the adhesive is located between the two or more substrates. The adhesive is allowed to cool and cure thereby bonding the substrates together. The adhesive upon cooling forms a solid and crystallizes. The crystallized solid has sufficient green strength to hold the substrate together until the adhesive chemically cures. Curing of the adhesive can take up to about 24 hours. The adhesive of the invention can be used to bond plastics, metals, glass, wood, leather and cloth.

In those embodiments where the adhesive cures at or near room temperature in the presence of atmospheric moisture it is desirable to protect the adhesive composition from premature crosslinking by keeping the adhesive in a sealed container, preferably under an inert atmosphere . The adhesive of the invention demonstrates excellent peel strength after 15 minutes, preferably about 3.0 pli (538 gm/cm) or greater and more preferably about 20.0 pli (3572 gm/cm) or greater. The adhesive of the invention demonstrates excellent peel strength after 30 minutes, about 20.0 pli (3572 gm/cm) or greater and more preferably about 30 pli (5358 gm/cm) or greater. As used herein, pli means pounds per linear inch.

Specific Embodiments

The examples provided hereinafter are provided to illustrate the invention and are not intended to limit the scope of the claims. Unless otherwise stated all parts and percentages are by weight.

Test Procedures

In the following examples the viscosity was determined using a PAAR Physica UM100 rheometer in a cone and plate configuration. The viscosities were evaluated at 120°C under rotational shear while performing a shear sweep of from 1 to 500 1/Df. The values reported are at 155 1/Df. Peel strength data was collected by heating the prepolymer materials to 121°C and applying as a relatively thin film (10 to 30 mil) on a metal substrate (pre roughed steel). Cotton cloth (duck canvas) was then bonded to the metal via the prepolymer and tested at two minutes, fifteen minutes and twenty-four hours after bond assembly. The specimens were conditioned at ASTM D903 temperature and humidity standard conditions during the period between bonding and testing. The peel testing was performed in a 180°-peel mode using an Instrom model 1125 test apparatus. Examples 1 to 3

Several copolymer polyols were prepared with polyether polyol having different molecular weights. The copolymer polyols were made with 425 and 1200 molecular weight difunctional polyols at 43 percent solids (styrene- acrylonitrile) . The solids had a number average molecular weight of 165,000. The 425 molecular weight polyol is a polypropylene oxide diol (VORANOL™ 220-260 polyol available from The Dow Chemical Company) . The 1200 molecular weight polyol is polypropylene oxide diol

(VORANOL™ 220/094 polyol available from The Dow Chemical Company) . Each of the copolymer polyols were then reacted with excess methylenediphenyl diisocyanate (MDI) such that the resulting prepolymer had an NCO content of 2 percent or greater on average. The MDI used contained 50 percent p,p' isomer and 50 percent o,p' isomer by weight (ISONATE™ 50 0, P isocyanate available from The Dow Chemical Company) . The results are compiled in Table 1.

Table 1

*Blend of 425 and 1200 MW polyols

Examples 4-5 and Comparative Example A

A prepolymer was prepared from a copolymer polyol having 43 percent solid content of styrene acrylonitrile (70/30 weight ratio) and a continuous phase of a 3100 molecular weight triol having a backbone of the residue of propylene oxide and ethylene oxide arranged in a random order in a weight ratio of 87 to 13, a 425 molecular weight polypropylene oxide diol, and tripropylene glycol. The weight polyol blend comprised 50 percent by weight of the copolymer polyol, 33 percent by weight of the polypropylene oxide and 17 percent by weight of the tripropylene glycol. The above polyol blend was reacted with sufficient MDI (ISONATE™ 50 O, P isocyanate available from The Dow Chemical Company) such that a 4 percent free isocyanate content prepolymer was prepared. The diol, copolymer polyol and MDI were reacted without catalyst for two hours at 80°C, then the tripropylene glycol was added and reacted for two more hours at the same temperature. This is the prepolymer used for Example 4.

A comparative prepolymer was prepared from the following polyol blend. 50 percent by weight of a 1200 molecular weight polypropylene oxide diol (VORANOL™ 220- 094 polyol available from The Dow Chemical Company) , 25 percent by weight of a 425 molecular weight polypropylene oxide diol (VORANOL™ 220-260 polyol available from The Dow Chemical Company) and 25 weight percent triproplylene glycol. The polyol blend was reacted with excess MDI (ISONATE™ 50 OP polyisocyanate available from The Dow Chemical Company) so that a prepolymer was prepared having 3 percent free NCO content. The two prepolymers were evaluated for viscosity at application temperature, peel strength and melt viscosity growth. The results are compiled in Table 2. In Example 5, the following polyol blend was reacted with excess MDI (ISONATE™ 50 0, P isocyanate available from The Dow Chemical Company) to prepare a 4 percent NCO content prepolymer. The polyol blend comprises a 2,000 molecular weight polypropylene oxide diol (VORANOL™ 220-056N available from The Dow Chemical Company) (50 percent) and a 425 molecular weight polypropylene oxide diol (VORANOL™ 220-260 available fr :com The Dow Chemical) (20 percent) and 30 percent by weight of a propoxylated bisphenol A having an average of 2.05 propylene oxide units per molecule. The results are compiled in Table 2.

Table 2

Claims

WHAT IS CLAIMED IS:

1. A prepolymer comprising the reaction product of a) one or more polyisocyanates; and b) i) one or more copolymer polyols having a solid dispersed in a continuous phase wherein the solid copolymer phase has an average molecular weight of about 50,000 to 200,000 and the continuous phase is liquid polyol having a number average molecular weight of the about 400 to 7000 or ii) a mixture of one or more copolymer polyols having a solid dispersed in a continuous phase wherein the solids have an average molecular weight of about 50,000 to 200,000 MWU and the continuous phase is liquid polyol having a number average molecular weight of about 400 to about 7000, with one or more diols having a number average molecular weight of about 800 or less wherein at least about 50 percent by weight of the mixture comprises diols having a molecular weight of about 800 or less, or iϋ) one or more alkoxylated bisphenols; or iv) a mixture thereof; wherein the prepolymer has an isocyanate content of from 2 to 5 percent by weight.

2. A prepolymer according to Claim 1 wherein the a) polyisocyanate is reacted with b) ii) a mixture of one or more copolymer polyols having a solid dispersed in a continuous phase wherein the solids have an average molecular weight of about 50,000 to 200,000 MWU and the continuous phase is liquid polyol having a number average molecular weight of about 400 to 7000, with one or more diols having a number average molecular weight of about 800 or less wherein at least about 50 percent by weight of the mixture comprises diols having a molecular weight of about 800 or less, or iii) one or more alkoxylated bisphenols; or iv) a mixture thereof.

3. A prepolymer according to Claim 2 wherein the diols have a molecular weight of about 500 or less.

4. A prepolymer according to claim 3 wherein the diol comprises repeating units of ethylene oxide, propylene oxide, 1,2 butylene oxide, tetrahydrofuran or a mixture thereof.

5. A prepolymer according to Claim 2 wherein the isocyanate content is from 3 to 5 percent.

6. A prepolymer according to Claim 5 wherein the polyisocyanate comprises methylenediphenyl diisocyante, oligomers or polymers thereof, or chemical modifications thereof.

7. A prepolymer according to Claim 4 wherein the polyisocyanate is reacted with a mixture of a copolymer polyol and a diol and the amount of diol in the mixture of copolymer polyol and diol is from 50 percent by weight to 90 percent by weight.

8. A reactive hot melt adhesive comprising the prepolymer of Claim 1 wherein the prepolymer has an application temperature of from 90 to 140°C and a viscosity at the application temperature of from 3,000 to 50,000 centipoise.

9. A reactive hot melt according to Claim 8 wherein the prepolymer comprises a) polyisocyanate reacted with b) ii) a mixture of one or more copolymer polyols having a solid dispersed in a continuous phase wherein the solids have an average molecular weight of about 50,000 to about 200,000 MWU and the continuous phase is liquid polyol having a number average molecular weight of about 400 to about 7000, with one or more diols having a number average molecular weight of about 800 or less wherein at least about 50 percent by weight of the mixture comprises diols having a molecular weight of about 800 or less, or iii) one or more alkoxylated bisphenols; or iv) a mixture thereof.

10. An adhesive according to Claim 9 wherein the diols mixed with the copolymer polyols are diols having a molecular weight of about 500 or less.

11. An adhesive according to Claim 10 wherein the diol comprises repeating units of ethylene oxide, propylene oxide, 1, 2 butylene oxide, 1,4 butylene oxide or tetrahydrofuran or a mixture thereof.

12. An adhesive according to Claim 10 wherein the isocyanate content is from 3 to 5 percent.

13. An adhesive according to Claim 9 wherein the polyisocyanate comprises methylenediphenyl diisocyanate, oligomers or polymers thereof, or chemical modifications thereof.

14. An adhesive according to Claim 8 which comprises a) from 10 to 100 percent by weight of prepolymer; from 0 percent by weight to 4 percent by weight of a catalyst for the reaction of an isocyanate with an isocyanate reactive compound; from 0 to 70 percent by weight of fillers; and from 0 to 45 percent by weight of plasticizers;

15. A method of bonding two substrates together which comprises: a. heating an adhesive according to Claim 8 to its application temperature; b. applying the adhesive to one or both of the substrates; c. contacting the two substrates such that the adhesive is located between the two substrates; d. allowing the adhesive to cure so as to bond the two substrates together.

16. The process of Claim 15 wherein the adhesive is heated to a temperature of about 90 to about 140°C.

17. The process of Claim 16 wherein the adhesive is heated to a temperature of about 90 to about 120°C.

18. A composite structure comprising two or more substrates and located between the two or more substrates a cured adhesive according to Claim 8 such that the cured adhesive bonds the substrates together.