Process for producing NDI prepolvmers

The present invention relates to a process for producing naphthalene diisocyanate (NDI) prepolymer and the production of polyurethane polymers from such prepolymers.

BACKGROUND OF THE INVENTION

Polyurethane prepolymers are NCO-terminated polymers that are obtained by reacting a polyol with a polyisocyanate in molar excess, based on functional groups, at a temperature of from room temperature to about 100 ^S C until a constant NCO value is reached. An important application for such NCO-terminated prepolymers is the production of polyurethane elastomers by the casting process.

In the production of such polyurethane elastomers, the prepolymer either undergoes chain extension immediately after production (i.e. reaction, with a short-chain polyol (e.g. 1 ,4- butanediol), or the NCO prepolymer is cooled (to the extent that it is advantageous and possible) to a lower temperature (storage temperature) for the purpose of a subsequent chain extension and stored.

Cast elastomers can be obtained by the prepolymer process or by the one-shot process in which a mixture of long-chain and short-chain polyols is reacted with one or more polyisocyanates. The disadvantage of the one-shot process, however, is that only low-grade polyurethane elastomers are obtained, especially if high-melting polyisocyanates are used, because intermediates formed by short-chain polyol (chain extender) and high-melting polyisocyanate are precipitated out of the reaction melt in some cases and therefore undergo a further reaction, preventing further ordered molecular weight development. This is one reason why the prepolymer process normally leads to better products.

Polyurethane polymers which are produced with prepolymers on the basis of naphthalene- 1 , 5-diisocyanate (NDI) typically exhibit better dynamic properties, higher tear and wear resistance, lower compression set and better oil and chemical resistance than those which are based on other widely used conventional polyisocyanates, such as toluene diisocyanate (TDI), isophorone diisocyanate (IPDI) or diphenylmethane diisocyanate (MDI), for example.

Prepolymers based on TDI or MDI are typically prepared by initially charging the entire amount of liquid or molten polyisocyanate and metering in the polyol under temperature control. This ensures that an excess of NCO groups is present over the entire course of the prepolymer preparation reaction, which substantially prevents premature extension of the polyol with a corresponding increase in molar mass and viscosity. However, this process cannot be employed in the case of solid NDI which has a high melting point (127 ^S C), which is much higher than the melting point of other conventional diisocyanates such as MDI (40 ^S C) or TDI (20 ^S C). Therefore, the processing conditions are quite different from MDI or TDI based materials, as the NDI monomer has to be substantially heated to more than 127 ^S C during the prepolymer production process in order to achieve a suitable prepolymer. Unfortunately, side reactions take place to a considerable degree at that temperature and lead to an increase in molar mass and viscosity. It is well known in the art that NDI prepolymers are, compared to other prepolymers based on diisocyanates with a lower melting point, very instable which means that they typically cannot be stored and, thus, it is recommended in“Prolingheuer et al., Proc. Of the SPI31 st Annual Polyurethane Technical/Marketing Conference 1988, p.394- 402” and“Solid Polyurethane Elastomers, P. Wright and A. P. C. Cummings, Maclaren and Sons, London 1969, pp. 104 ff. in Chapter 6.2.” that these NDI prepolymers are used within 30 minutes after their production. A root cause is the high temperature which has to be applied during the prepolymer production process as more side reactions seem to occur.

Furthermore, the storage stability of the prepolymer and the viscosity of the prepolymer are important for the usability of the prepolymer. The measures to achieve these goals must not adversely affect the polyurethane polymer properties in the particular applications to any great extent.

Several approaches have been tested in the past to provide prepolymer production processes which lead to increased stability of the produced NDI prepolymers.

DE-A-10 2007 054003 discloses a process to prepare NDI prepolymer. The polyol was first heated to 135 ^S C. Then, solid NDI was added while stirring, whereas the temperature fell under 100 ^S C. Due to the reaction, the temperature increased again up to 124 ^S C. 1 ,4-butane diol was added as a chain extender to form a hot curable prepolymer composition.

US-A-2008/108776 discloses a stable prepolymer produced from isocyanate having a melting point higher than 70 ^S C, such as naphthalene-1 ,5-diisocyanate (NDI), via the process of rapid cooling immediately after the reaction has ended, to suppress the side reactions which cause prepolymer instability. However, the rapid cooling is hard to apply especially for larger amounts of prepolymers on an industrial scale as the thermal exchange is difficult. Furthermore, the viscosity of the prepolymer is high and, thus, the handling and processing of the prepolymer is difficult. After some storage at room temperature, NDI will crystallize out.

US-A-2009/127921 discloses a process for producing of cellular polyurethane (PER) casting elastomers and molded articles based on naphthalene-1 ,5-diisocyanate prepolymers. The document further describes the preparation of storage stable NDI prepolymers, whereas NDI prepolymer is rapidly cooled immediately after its preparation to a certain temperature in a specific cooling scheme so that in each case the residence time i) in the temperature range from the end of the reaction to 130 ^S C does not exceed 0.5 hour; ii) in the temperature range from the end of the reaction to 1 10 ^S C does not exceed 1 .5 hours; iii) in the temperature range from the end of the reaction to 90 ^S C does not exceed 7.5 hours; and iv) in the temperature range from the end of the reaction to 70 ^S C does not exceed 72 hours. The storage-stable NDI prepolymers comprise the unreacted NDI after reaction in amounts of more than 0.3 wt% and less than 5 wt%, referred to the prepolymer. The storage stable NDI prepolymer can be used up to 6 months after its production.

US-A-2014/142243 discloses a process for producing a low-viscosity NDI prepolymer by adding naphthyl-1 -isocyanate at small quantity up to 0.7 wt% to the reaction of NDI with a polyol. However, the small quantity of naphtyl-1 -isocyanate is hard to be uniformly mixed into the prepolymer. Furthermore, the viscosity of the prepolymer is still high for a practical use on industrial scale. The document is silent about the use of solvents during the prepolymer production process.

US-A-2017/0152342 discloses a method for continuous production of stable prepolymers based on high-melting diisocyanates, in particular, naphthalene-1 ,5-diisocyanate, and the use thereof for producing polyurethane elastomers, in particular casting elastomers. The NDI is reacted in a tubular reactor with one or more polyols having mean molar masses of 1000 to 3000 g/mol, viscosities of <700 mPas/75 ^s C and a functionality of 1 .95 to 2.15, selected from the group consisting of polyether polyols, polycarbonate polyols and polyester polyols, at temperatures of 80 to 175 ^S C, the diisocyanate already having been in the liquid form prior to contact with the polyol(s), and the maximum reaction temperature being not higher than 60 ^S C above the melting temperature of the diisocyanate and the reaction mixture subsequently being cooled down to <100 ^S C within a period of up to 10 min. However, the process needs laborious heating and rapid cooling techniques in specific tubular reactors to perform the process, which is challenging when performed on industrial scale.

It is still highly desirable for NDI prepolymers to be stable at the storage temperature, i.e. to prevent secondary reactions to take place, and that the viscosity of the NDI prepolymers remain low or change only slightly over time.

An object of the present invention is to provide a process for producing NDI prepolymers which does not exhibit the disadvantages of the known NDI prepolymer production processes (such as need for temperature control) and provide NDI prepolymers which have high storage stability (e.g. no NDI crystallization or precipitation at lower temperatures) and have suitable low viscosities at the processing temperature. A further object of the present invention was to provide a process for producing polyurethanes from such prepolymers, wherein the properties of these polyurethanes are similar to the properties of those cast elastomers obtained with the known processes.

SUMMARY OF THE INVENTION

Surprisingly, these objects are achieved by a process for producing of a prepolymer composition, comprising the steps of i) providing a solution comprising

3 to 60 wt% naphthalene diisocyanate monomer (NDI) and

40 to 97 wt% of one or more inert organic solvents, based on the total weight of the solution, ii) combining the solution of step i) with one or more polyols to form a reaction mixture, wherein said reaction mixture comprises a stoichiometric ratio of isocyanate groups to hydroxyl groups of from 1 .1 :1 to 15:1 , iii) reacting said naphthalene diisocyanate and said one or more polyols of the reaction mixture of step ii) to form a prepolymer composition comprising NDI prepolymer, unreacted NDI monomer and one or more inert organic solvents, and iv) removing at least parts of the one or more inert organic solvents of the prepolymer composition of step iii).

After the NDI is dissolved in the one or more inert organic solvents in step i), the reaction in step iii) may be carried out at much lower temperature (e.g. <100 ^S C) with better temperature control, compared to known processes.

When NDI is dissolved in an inert organic solvent, side reactions are effectively suppressed, which results in stable NDI prepolymers. A high stoichiometric ratio of isocyanate groups to hydroxyl groups (NCO:OH ratio) significantly lowers the viscosity of the resulting prepolymer.

The one or more inert organic solvents are removed at least partially in step iv).

In some embodiments, unreacted NDI (=free NDI) is removed at least partially too to avoid crystallizing out of the prepolymer. If the amount of unreacted free polyisocyanate is too low and the viscosity of the prepolymer composition is too high, one or more polyisocyanate monomers such as NDI or p-phenylene diisocyanate (pPDI) may optionally be added to the prepolymer composition in a step v) if needed. This invention further relates to curable prepolymer compositions and cured polyurethane polymers based on NDI prepolymer compositions obtained by the inventive process.

For a better understanding of the present invention, together with other and further features and advantages thereof, reference is made to the following detailed description. The scope of the invention will be pointed out in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

As used in this description and the appended claims, the singular forms“a”,“an” and“the” include plural referents unless the content clearly dictates otherwise.

When an amount, concentration, value or parameter is given as either a range or a list of upper values and lower values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit and any lower range limit, regardless of whether ranges are separately disclosed. For example, when a range of "1 to 5" is recited, the recited range should be construed as including any single value within the range or as any values encompassed between the ranges, for example, "1 to 4", "1 to 3", "1 to 2", "1 to 2 & 4 to 5", "1 to 3 & 5". Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

It is well understood in the art that a polyurethane prepolymer, such as the NDI prepolymer of the invention, generally contains, in addition to any particular prepolymer compound, other compounds, typically in small amounts. Thus, there should be no confusion when a "prepolymer" is said to contain more than a single prepolymer molecule, such as other analogous prepolymers, unreacted starting materials, side products, solvents, etc.

As used herein, the term "embodiment" or "disclosure" is not meant to be limiting, but applies generally to any of the embodiments defined in the claims or described herein. These terms are used interchangeably herein.

The terms "percent by weight", "weight percentage (wt%)" and "weight-weight percentage (% w/w)" are used interchangeably herein. Percent by weight refers to the percentage of a material on a mass basis as it is comprised in a composition, mixture or solution.

Abbreviations: bp = boiling point mp = melting point mw molecular weight NCO = isocyanate group

NDI = naphthalene diisocyanate

PCL = polycaprolactone

PEAG = polyethylene adipate glycol

RT = room temperature (23±2 ^S C)

Step P

In step i) of the inventive process, a solution comprising 3 to 60 wt% naphthalene diisocyanate monomer and 40 to 97 wt% inert organic solvent, based on the total weight of the solution, is provided.

In one embodiment, the solution provided in step i) comprises 5 to 50 wt% naphthalene diisocyanate monomer and 50 to 95 wt% inert organic solvent, based on the total weight of the solution.

Naphthalene diisocvanate (NDh

The term naphthalene diisocyanates (NDI) of the present invention comprises naphthalene diisocyanates such as 1 ,5-naphthalene diisocyanate or 1 ,4-naphthalene diisocyanate. In one embodiment of the present invention, naphthalene diisocyanate is naphthalene-1 ,5- diisocyanate (CAS Number 3173-72-6).

Inert organic solvent

The solvent used in the process of the present invention is an inert organic solvent, i.e., it has no active hydrogen and does not react with the NDI monomer or the polyol under the conditions of the process.

In one embodiment, a suitable inert organic solvent, which can be used in step i) of the process of the present invention, has a boiling point at atmospheric pressure (101 ,325 Pa) of 200 ^S C or above. An inert organic solvent with a boiling point at atmospheric pressure of 200 ^S C or above has the advantage, that it is easily removed via distillation. In some embodiments, the inert organic solvent is removed together with unreacted NDI (bp ~ 350 ^S C) within one step in step iv).

In one example, the inert organic solvent of the invention, having a boiling point at atmospheric pressure of 200 ^S C or above, is non-toxic and not flammable. In one example, the one or more inert organic solvents of the invention are selected from a group consisting of 1 ,2,3-trichlorobenzene (bp 218 ^S C), 1 ,2,4-trichlorobenzene (bp 213 ^S C), dimethyl glutarate (bp 210-215 ^S C), dimethyl adipate (bp 225-230 ^s C), diethyl adipate (bp 251 ^S C), dimethyl succinate (bp 200 ^S C), dimethyl phthalate (bp 282 ^S C), dibutyl phthalate (bp 340 ^S C), gamma-butyrolactone (bp 204-205 ^s C), delta-valerolactone (bp 208 ^S C), propylene carbonate (bp 240 ^S C), 1 ,3-dimethyl-3,4,5,6-tetrahydro-2(1 H)-pyrimidinone (bp 247 ^S C) and N- methylpyrollidone (bp 204 ^S C).

In one embodiment the inert organic solvent is selected from the group consisting of 1 ,2,3- trichlorobenzene, 1 ,2,4-trichlorobenzene, dimethyl glutarate, dimethyl adipate, diethyl adipate, dimethyl succinate, dimethyl phthalate, dibutyl phthalate, gamma-butyrolactone, delta-valerolactone, propylene carbonate, 1 ,3-dimethyl-3,4,5,6-tetrahydro-2(1 H)- pyrimidinone, N-methylpyrollidone and combinations thereof.

In some embodiments, solvents boiling above 260 ^S C, such as phthalates, are used.

The inert organic solvent of i) may comprise a single inert organic solvent or a mixture of more than one inert organic solvents. Blends or combinations of inert organic solvents can be used in the process of this invention, provided they are miscible with the NDI prepolymer, and do not decompose or react with one another.

In one embodiment, step i) is performed at 23 ^S C or above. In another embodiment, step i) is performed at a temperature of 60°C to 100°C.

Step ii)

In step ii) of the process of the present invention, the solution of step i) is combined with one or more polyols to form a reaction mixture, wherein said reaction mixture comprises a stoichiometric ratio of isocyanate groups to hydroxyl groups of from 1.1 :1 to 15:1 .

Polyol

Polyols used in step ii) of the process for preparing NDI prepolymers may be selected from any polyol known in the art. In one embodiment, suitable polyols are polyether polyols, polyester polyols, polycaprolactone polyols, poly-s-caprolactone polyols, polycarbonate polyols, co-polyester polyols, alkane polyols, or combinations thereof.

In many embodiments the polyol will have a number average molecular weight from 200 g/mol, 250 g/mol or 400 g/mol to 6,000 g/mol or 10,000 g/mol, in some embodiments a lower molecular weight polyol may also be present. In many embodiments, diols are preferred over triols and polyols having a larger number of hydroxyl groups. Despite "ester" being a general term often used to encompass acyclic and cyclic esters, and sometimes even "carbonates", one skilled in the art recognizes that materials sold as polyester polyols, polycaprolactone polyols, and polycarbonate polyols have, and generally impart to the prepolymer and polyurethane, different characteristics, and are typically marketed as different materials. In the present application the terms "polyester polyol", “polycaprolactone polyol", and“polycarbonate polyol” are used to refer to three separate materials. "Polyester polyol” as used herein refers to a polyol having a backbone derived mainly from a polycarboxylate and a poly alcohol, e.g. a majority of the ester linkages in the backbone are derived from a polycarboxylate and a polyol, such as found in polyethylene adipate) glycol:

"Polylactone polyol" as used herein refers to a polyol having a backbone derived mainly from a hydroxycarboxylic acid or lactone, as opposed to being derived from a polycarboxylate and a polyol, as found in poly caprolactone:

"Polycarbonate polyol" as used herein refers to a polyol having a backbone comprising mainly carbonate linkages, -0(C0)-0-, as opposed to carboxylate linkages, -0(C0)-R, wherein R is an organic radical bound to the carbonyl by a C-C bond.

"Co-polyester polyols", as used herein refers to a polyol wherein a portion of the backbone is derived from a polycarboxylate and a poly alcohol as described above, and a portion of the backbone is derived from a hydroxyacid or lactone, or which also incorporates carbonate linkages.

For example, useful polyols may include polyesters of adipic acid or other dicarboxylic acids; polyethers of ethylene oxide, propylene oxide, 1 ,3-propanediol, tetrahydrofuran, etc.; polycaprolactone (PCL), polycarbonate, and copolymers and terpolymers formed from the above, and mixtures thereof. In various optional embodiments, the polyol comprises glycols or triols having molecular weights ranging, for example, from 60 to 400 g/mol, e.g. from 80 g/mol to 300 g/mol or from 100 g/mol to 200 g/mol, for example, such glycols or triols may include ethylene glycol, isomers of propylene glycol, isomers of butane diol, isomers of pentanediol, isomers of hexanediol, trimethylolpropane, pentaerythritol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, etc., and mixtures thereof.

Often, the polyether polyol is a polyalkylene ether polyol represented by the general formula HO(RO) _n H, wherein R is an alkylene radical and n is an integer large enough that the polyether polyol has a number average molecular weight of at least 250 g/mol. These polyalkylene ether polyols are well-known components of polyurethane products and can be prepared by the polymerization of cyclic ethers such as alkylene oxides and glycols, dihydroxyethers, and the like by known methods. Representative polyols include polyethylene glycols, polypropylene glycols (PPG), copolymers from propylene oxide and ethylene oxide (PPG-EO glycol) or poly(tetramethylene ether) glycol (PTMEG or PTMG).

The polyester polyols are typically prepared by reaction of dibasic acids, e.g. adipic, glutaric, succinic, azelaic, sebacic, or phthalic acid or derivatives thereof, with diols such as ethylene glycol, 1 ,2-propylene glycol, 1 ,4-butylene glycol, 1 ,6-hexylene glycol, and alkylene ether polyols such as diethylene glycol, polyethylene glycol, polypropylene glycols, polytetramethylene ether glycol and the like. Polyols such as glycerol, trimethylol propane, pentaerthythritol, sorbitol, and the like may be used if chain branching or ultimate cross- linking is sought. Examples of polyester polyols include poly(adipate) glycol, poly(hexamethylene adipate) glycol, polyethylene adipate) glycol, poly(diethylene adipate) glycol, poly(ethylene/propylene adipate) glycol, poly(trimethylolpropane/hexamethylene adipate) glycol, poly(ethylene/butylene adipate) glycol, poly(butylene adipate) glycol, poly(hexamethylene/neopentyl adipate) glycol, poly(butylene/hexamethylene adipate) glycol (PBHAG), poly(neopentyl adipate) glycol, and the like including copolymers and terpolymers thereof.

Polylactone polyols include those made by polycondensation of, e.g. a caprolactone such as e-caprolactone, and the like, often initiated by a small polyol such as ethylene glycol.

Hydrocarbon polyols can be prepared from ethylenically unsaturated monomers such ethylene, isobutylene, and 1 ,3-butadiene, e.g. polybutadiene polyols and the like.

Polycarbonate polyols can also be used in forming the prepolymers of the invention and can be prepared by reaction of glycols, e.g. 1 ,6-hexylene glycol and the like, with organic carbonates, e.g. diphenyl carbonate, diethyl carbonate, or ethylene carbonate and the like.

Co-polyester polyols of the invention include those wherein the backbone comprises polyester portions and portions comprising caprolactone or polycaprolactone. In many embodiments of the invention, the polyol used in forming the prepolymer comprises a diol, and in some embodiments, the majority or all of the polyols used in to form the prepolymer are diols.

In one embodiment, the polyol is polycaprolactone or polyethylene adipate glycol.

A stoichiometric excess of NDI monomer is provided in the reaction mixture to form the prepolymer, for example, a 1 .1 :1 to 15:1 excess of polyisocyanate monomer relative to polyol may be used, such as ratios ranging from 1 .5 or 2:1 to 8:1 , 10:1 or 12:1 NDI monomer to polyol. In particular embodiments, the ratio is at least 3:1 , at least 4:1 or at least 5:1 of NDI monomer to polyol.

Auxiliary substances and additives

Auxiliary substances and additives common in the art may also be present in the prepolymer composition, curing composition, and polyurethane polymer of the invention including catalysts, dispersants, colorants, fillers, reinforcing agents, solvents, plasticizers, antioxidants, LIVAs, light stabilizers, lubricants, processing aids, anti-stats, flame retardants, and the like.

Step iii)

In step iii) of the process of the present invention, said NDI and said one or more polyols of the reaction mixture of step ii) are reacted to form a prepolymer composition comprising NDI prepolymer, unreacted NDI monomer and one or more inert organic solvents.

Reaction condition

The reaction in step iii) of the process of the present invention is performed at temperatures of from 35 ^S C to 100 ^S C to form a prepolymer composition. In one embodiment, the reaction in step iii) is performed at a temperature of from 70 ^S C to 100 ^S C.

Catalysts or other components common in the art may also be added.

Prepolvmer

The NDI prepolymer, obtained by the reaction of step iii) has typically a viscosity (measured at 100 ^S C) of <1000 mPas. The NDI prepolymer has typically a free NCO content (%NCO) of 1 to 6.

Prepolvmer composition

The present invention further relates to a prepolymer composition, comprising NDI prepolymer, unreacted NDI monomer and one or more inert organic solvents. Amount of unreacted NDI monomer

The amount of unreacted NDI (=free NDI) monomer in the prepolymer composition of step iii) is typically in the range from 1.0 wt% to 10 wt%, based on the total weight of the prepolymer.

Amount of inert organic solvent

The amount of inert organic solvent in the prepolymer composition of step iii) is typically in the range from 30 wt% to 80 wt%, based on the total weight of the prepolymer composition.

Step iv)

In Step iv) of the inventive process, at least parts of the one or more inert organic solvents of the prepolymer composition of step iii) are removed.

Solvent removal

Any process useful in reducing the inert organic solvent content in the prepolymer composition may be employed in step iv) of the process of the invention such as filtration or distillation. In general, the removal of at least parts of the one or more inert organic solvents according to this invention is carried out in a conventional manner employed for purification, for example by distillation. Typically, distillation is used, e.g. thin film or agitated film evaporation under vacuum has been used with good success. Use of distillation equipment, such as wiped film evaporation and vacuum distillation are familiar to those skilled in the art.

In one embodiment, the removal of at least parts of the one or more inert organic solvents of step iv) is carried out by distillation, preferably by distillation under reduced pressure.

In one embodiment, the removal of at least parts of the one or more inert organic solvents of step iv) comprises distillation conducted in a falling film evaporator, wiped film evaporator and/or short-path distiller.

In one embodiment, the prepolymer composition obtained after performing step iv) comprises less than 5.0 wt% inert organic solvents, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step iv) comprises 0.01 wt% to 5.0 wt% inert organic solvents, based on the total weight of the prepolymer composition.

In one embodiment, the prepolymer composition obtained after performing step iv) comprises less than 1 .0 wt% inert organic solvents, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step iv) comprises 0.01 wt% to 1 .0 wt% inert organic solvents, based on the total weight of the prepolymer composition. In one embodiment, the prepolymer composition obtained after performing step iv) comprises less than 0.5 wt% inert organic solvents, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step iv) comprises 0.01 wt% to 0.5 wt% inert organic solvents, based on the total weight of the prepolymer composition.

In one embodiment, the prepolymer composition obtained after performing step iv) comprises less than 0.1 wt% inert organic solvents, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step iv) comprises 0.01 wt% to 0.1 wt% inert organic solvents, based on the total weight of the prepolymer composition.

In some embodiments, unreacted NDI (=free NDI) is removed at least partially too to avoid crystallizing out of the prepolymer.

In one embodiment, the prepolymer composition obtained after performing step iv) comprises less than 5.0 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step iv) comprises 0.01 wt% to 5.0 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition.

In one embodiment, the prepolymer composition obtained after performing step iv) comprises less than 1 .0 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step iv) comprises 0.01 wt% to 1 .0 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition.

In one embodiment, the prepolymer composition obtained after performing step iv) comprises less than 0.5 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step iv) comprises 0.01 wt% to 0.5 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition.

In one embodiment, the prepolymer composition obtained after performing step iv) comprises less than 0.1 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step iv) comprises 0.01 wt% to 0.1 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition. The presence of low amounts of NDI of 0.01 wt% or more reduce the viscosity of the prepolymer composition compared to NDI prepolymers without any free NDI monomer. Low viscosity is important for the usability and processability of the NDI prepolymer.

Step v)

In one embodiment of the inventive process, one or more polyisocyanate monomers are added to the prepolymer composition of step iv) the optional step v).

In one embodiment of the invention, NDI or p-phenylene diisocyanate (pPDI) are added to the prepolymer composition of step iv) in a step v).

In another embodiment of the invention, NDI is added to the prepolymer composition of step iv) in a step v).

In one embodiment, the prepolymer composition obtained after performing step v) comprises more than 5.0 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition. In another embodiment, the prepolymer composition obtained after performing step v) comprises 1 .0 wt% to 5.0 wt% unreacted NDI monomer, based on the total weight of the prepolymer composition.

Addition of free polyisocvanate monomers

Optionally, it is suitable to add free polyisocyanate monomer, preferably free diisocyanate monomer, to the NDI prepolymer composition of step iv).

In one embodiment of the process of the present invention, p-phenylene diisocyanate (pPDI) and/or naphthalene diisocyanate is added to the NDI prepolymer composition provided in step iv).

In one embodiment of the process of the present invention naphthalene diisocyanate is added to the NDI prepolymer composition of step iv).

The addition of free diisocyanate monomer reduces the viscosity of the NDI prepolymer composition.

Process for preparing a curable prepolymer composition

The present invention further relates to a process for preparing a curable prepolymer composition comprising adding one or more curatives to the prepolymer composition provided by the process of the present invention.

Curative Curatives, also called curing agents, coupling agents, cross linking agents or chain extenders, are well known in the art and include various polyols such as diols, triols, tetrols or mixtures thereof.

Common curatives include C _M2 alkylene diols, preferably C _2-6 diols, such as ethylene glycol, 1 ,3-propanediol, 1 ,4-butanediol, 1 ,5-pentanediol, 1 ,6-hexanediol, neopentyl glycol, trimethylol propane, 1 ,10-decanediol, 1 ,1 -cyclohexanedimethanol or 1 ,4- cyclohexanedimethanol; hydroquinone-bis-hydroxyalkyl ethers such as hydroquinone-bis- hydroxyethyl ether; ether diols such as diethylene glycol, dipropylene glycol, dibutylene glycol or triethylene glycol. The curing agent may also comprise alkylene polyols, polyether polyols such as PTMG, polyester polyols, polycaprolactone polyols or polycarbonate polyols, such as those described above as starting materials for the prepolymers, typically as a blend with a diol or triol.

In select embodiments, the curative comprises ethylene glycol, 1 ,3-propanediol, 1 ,4- butanediol, 1 ,5-pentanediol, 1 ,6-hexanediol, neopentyl glycol, trimethylol propane, 1 ,10- decanediol, 1 ,1 -cyclohexanedimethanol, 1 ,4-cyclohexanedimethanol, cyclohexane diol; hydroquinone-bis-hydroxyethyl ether, diethylene glycol, dipropylene glycol, dibutylene glycol or triethylene glycol.

Curable prepolymer composition

The present invention further relates to a curable prepolymer composition comprising the prepolymer composition provided by the process of the present invention and one or more curatives.

The molar ratio of prepolymer to curing composition, for example, may be in the range of from 0.5:1 to 1.5:1 , e.g. from 0.7:1 to 1 .2:1 or from 1 .1 :1 to 0.95:1 . The amount of curing composition to be added may also be determined by methods well known to one of ordinary skill in the art and will depend on the desired characteristics of the resin being formed. In some embodiments catalysts may be used in conjunction with the curative.

Process for preparing polyurethane

Any process known to one of ordinary skill in the art may be used to combine the polyisocyanate, polyol and curative of the present invention (curing process).

Curing of the curable polyurethane prepolymer composition with the curative imparts a network structure to the polyurethanes.

Polyurethane The invention further comprises a polyurethane obtained by reacting the prepolymer composition of the present invention with a curative.

Post treatment

Elastomer parts cured from prepolymers of this invention have excellent properties, even if the curing process is not followed by a treatment in a humidity camber at elevated temperatures. This is an enormous processing advantage over the polyurethane polymers made in accordance with processes of the prior art. For the polyurethanes made from those prepolymers, such kind of treatment is necessary for a good performance, which is resource and energy consuming.

The polyurethane of the invention can be formed into numerous useful articles by various means.

Many of the polyols, solvents and curing agents useful in the present invention are commercially available or prepared according to known methods, as are pPDI and other polyisocyanates that may be employed.

Free NCO content (%NCO) can be determined by a procedure similar to that described in ASTM D1638-70, but employing tetrahydrofuran as the solvent.

The content of inert organic solvents in prepolymer compositions can be determined in accordance with common measurement methods such as GC (gas chromatography) measurement.

The content of unreacted NDI monomer in prepolymer compositions can be determined in accordance with common measurement methods such as HPLC (high performance liquid chromatography) measurement.

EXAMPLES

Materials:

Polvisocvanate:

Naphthalene-1 ,5-diisocyanate NDI; C ₁₂ H ₆ N ₂ 0 ₂ ; CAS-Number: 3173-72-6; (mp

127 ^S C)

Solvents:

Dimethyl adipate Dimethyl hexanedioate; C ₈ H ₁₄ 0 ₄ ; CAS-Number:

627-93-0; (bp 225-230 ^s C)

Dimethyl phthalate Dimethyl benzene-1 ,2-dicarboxylate; C ₁₀ H ₁₀ O ₄ ;

CAS-Number: 131 -11 -3; (bp 282 ^S C)

1 ,3-dimethyl-3,4,5,6-tetrahydro-2(1 H)-pyrimidinone C ₆ H ₁₂ N ₂ 0; CAS-Number: 7226-23-

5; (bp 247 ^S C)

Polyol:

Polyethylene adipate glycol 1000 PE AG 1000 (mw = 1000 g/mol)

Polyethylene adipate glycol 2000 PE AG 2000 (mw = 2000 g/mol)

Polycaprolactone 1000 PCL 1000 (mw = 1000 g/mol)

Polycaprolactone 2000 PCL 2000 (mw = 2000 g/mol)

Curative:

1 ,4 butanediol CAS Number: 1 10-63-4 (commercially available at BASF)

HQEE Hydroquinone bis (2-hydroxyethyl) ether; CAS

Number: 104-38-1

Methods:

Solubility test

To a flask with N ₂ supply and agitation, a certain amount of solid NDI monomer was added before the solvent was added. The solid NDI monomer was fully dissolved under heat and agitation and specific temperature was recorded as the solubility.

Storage test

Prepolymer with a measured %NCO was added to a glass bottle. Then, the bottle was N ₂ purged and sealed. This bottle was stored at specified temperature and the samples were taken periodically for %NCO measurement. Results were compared to the original to see the drop of %NCO, which determines the stability of the prepolymer at that storage condition.

%NCO measurement Free NCO content was measured according to ASTM D1638-70, but employing tetrahydrofuran as the solvent.

Measurement of residual solvent

Residual solvent content of prepolymer compositions is measured by GC.

Measurement of unreacted NDI monomer

Unreacted NDI monomer content of prepolymer compositions is measured by HPLC. Measurement of dynamic properties

Dynamic properties were measured in torsion mode using TA Instruments ARES-G2 Rotational Rheometer Rectangular test specimens were used (approximately 50 mm x 1 1 .8 mm x 3.2 mm). Dynamic temperature step experiments were performed by heating the sample from 30°C to 180°C with 10°C step under an oscillatory strain of 1 % and oscillation frequency of 10 Hz.

EXAMPLE I

80 parts NDI and 920 parts dimethyl adipate are mixed and heated to 70 ^S C, and form a clear solution.

EXAMPLE II

120 parts NDI and 880 parts dimethyl phthalate are mixed and heated to 90 ^S C, and form a clear solution.

EXAMPLE III

250 parts NDI and 750 parts 1 ,3-dimethyl-3,4,5,6-tetrahydro-2(1 H)-pyrimidinone are mixed and heated to 70 ^S C, and form a yellow solution.

Table 1 demonstrates the solubility of NDI in a variety of solvents.

Table 1 . Solubility of NDI

EXAMPLE IV

To a solution of 120 parts NDI and 880 parts of dimethyl adipate in a batch reaction flask equipped with nitrogen sweep, agitator, thermometer, heating mantle, and a vacuum source, was added 380 parts PEAG 2000. The molar ratio of NDI to PEAG (hence the equivalent ratio of NCO groups to OH groups) was 3:1 . The reaction mixture was heated for 4 hours at 90 ^S C followed by 4 hours at 100 ^S C. The crude reaction mixture was then processed through a wiped film evaporator to remove solvent dimethyl adipate to yield a stripped prepolymer having 2.4% available isocyanate groups, and containing 0.3 wt% free NDI monomer.

EXAMPLE V

To a solution of 120 parts NDI and 880 parts of dimethyl phthalate in a batch reaction flask equipped with nitrogen sweep, agitator, thermometer, heating mantle, and a vacuum source, was added 265 parts of the blend of PCL 2000 and PCL 1000 at 70/30 weight ratio. The molar ratio of NDI to PCL (hence the equivalent ratio of NCO groups to OH groups) was 3:1 . The reaction mixture was heated for 5 hours at 100 ^S C, followed by 16 hours at 70 ^S C. The crude reaction mixture was then processed through a wiped film evaporator to remove solvent dimethyl phthalate to yield a stripped prepolymer having 3.3% available isocyanate groups, and containing 0.107 wt% free NDI monomer.

EXAMPLE VI

To a solution of 120 parts NDI and 880 parts of dimethyl phthalate in a batch reaction flask equipped with nitrogen sweep, agitator, thermometer, heating mantle, and a vacuum source, was added 293 parts of the blend of PEAG 2000 and PEAG 1000 at 70/30 weight ratio. The molar ratio of NDI to PEAG (hence the equivalent ratio of NCO groups to OH groups) was 3:1 . The reaction mixture was heated for 5 hours at 100 ^S C, followed by 16 hours at 70 ^S C. The crude reaction mixture was then processed through a wiped film evaporator to remove solvent dimethyl phthalate to yield a stripped prepolymer having 3.2% available isocyanate groups, and containing less than 0.1 wt% free NDI monomer.

Table 2 demonstrates the prepolymer characteristics. The inert organic solvent is effectively removed. Residual NDI monomer is very low without crystallization concern comparing prepolymer composition prepared in accordance with known processes. Viscosity is much lower comparing to prepolymer composition prepared in accordance with known processes with similar %NCO, and even lower than that of using viscosity reducer additive in the prior art. Especially comparing Example V to 3.3 and 3.4 in US-A-2008/108776, those have the same backbone of polycaprolactone, this invention provides the prepolymer having significantly lower viscosity. Table 2. Characteristics of prepolymer compositions

^* inventive examples; ^** based on the total weight of prepolymer; NA = not available

EXAMPLE VII

To 96 parts of prepolymer made from Example VI type, was added 4 parts NDI. This composition was heated at 120 ^S C and mixed, to yield a prepolymer having 4.4% available isocyanate groups, and containing ~4 wt% free NDI monomer.

EXAMPLE VIII

To 96 parts of prepolymer made from Example VI type, was added 4 parts pPDI. This composition was heated at 100 ^S C and mixed, to yield a prepolymer having 4.8% available isocyanate groups, and containing ~4 wt% free pPDI monomer. Tables 3 illustrates the prepolymer storage stability. Prepolymers from this invention are more stable than the ones listed in the prior art, as they show less tendency of NCO drop during storage time.

Table 3. Storage stability of NDI prepolymers

^* US81 10704; NA = not available

The NDI prepolymers of the present invention showed no change in the NCO content at room temperature (23 ^S C) for 90 days, while the prepolymers of the prior art have NCO content drops in 45 days. The inventive NDI prepolymer show a high storage stability at typical storage temperatures over a long time period.

EXAMPLE IX

90 g of prepolymer EXAMPLE V type was mixed with 2.8 g 1 ,4 butanediol and the resulting curing composition was poured into molds and cured/post cured at 120 ^S C for 16 hours. Tough parts were obtained after demolding after curing cycle.

EXAMPLE X

90 g of prepolymer EXAMPLE V type was mixed with 6.4 g molten HQEE and the resulting curing composition was poured into molds and cured/post cured at 120 ^S C for 16 hours. Tough parts were obtained after demolding after curing cycle. EXAMPLE XI

90 g of prepolymer EXAMPLE VI type was mixed with 2.8 g 1 ,4 butanediol and the resulting curing composition was poured into molds and cured/post cured at 120 ^S C for 16 hours. Tough parts were obtained after demolding after curing cycle.

EXAMPLE XII

90 g of prepolymer EXAMPLE VI type was mixed with 6.2 g molten HQEE and the resulting curing composition was poured into molds and cured/post cured at 120 ^S C for 16 hours. Tough parts were obtained after demolding after curing cycle.

Table 4 shows the excellent physical properties of the polyurethane polymers prepared by curing NDI prepolymers with variety of curatives.

Table 4. Physical properties of polyurethane polymers based on NDI prepolymers

Excellent dynamic properties of the polyurethane polymers prepared by curing stable NDI prepolymers with variety of curatives are shown in Table 5.

Table 5. Dynamic properties of polyurethane polymers based on stable NDI prepolymers

EXAMPLE XIV

1000 g of EXAMPLE V type was mixed with 31 .8 g 1 ,4-butanediol and the resulting curing composition was poured into molds and cured/post cured at 120 ^S C for 16 hours. Tough parts with thickness 60 mm were obtained after demolding after curing cycle.

After curing, parts were treated as follows:

XIV.1 Room temperature conditioned for four weeks, then the specimen were cut from for physical and dynamic properties testing

XIV.2 Room temperature and in 50% humidity chamber conditioned for 5 days, then 1 10 ^S C for 16 hours. Repeated above until a total of four weeks, then the specimen were cut from for physical and dynamic properties testing.

EXAMPLE XV

1000 g of EXAMPLE VI type was mixed with 30.9 g 1 ,4-butanediol and the resulting mixture was poured into molds and cured/post cured at 120 ^S C for 16 hours. Tough parts with thickness 60 mm were obtained after demolding after curing cycle.

Parts were treated as follows:

XV.1 Room temperature conditioned for four weeks, then the specimen were cut from for physical and dynamic properties testing

XV.2 Room temperature and in 50% humidity chamber conditioned for 5 days, then 1 10 ^S C for 16 hours. Repeated above until to a total of four weeks, then the specimen were cut from for physical and dynamic properties testing.

Table 6. Dynamic properties of large parts (60 mm thickness)

Table 6 demonstrated that parts cured from this invention have no need to be treated repeatedly in humidity camber and in oven after post cure to have comparable dynamic properties.

EXAMPLE XVI

90 g of prepolymer EXAMPLE VII type was aged for two month and was than mixed with 2.8 g 1 ,4 butanediol and the resulting curing composition was poured into molds and cured/post cured at 120 ^S C for 16 hours. Tough parts were obtained after demolding after curing cycle.

Table 7. Physical properties of aged polyurethane prepolymers based on stable NDI prepolymers

Table 7 demonstrates that the aging of the prepolymer for 2 month has no significant influence on the physical properties of the cured polyurethane.