BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION This invention relates to curative systems for polyurethane. More specifically, the invention relates to a method of curing a polyurethane, a curative composition useful in such method, and a cured polyurethane produced by such method. In particular, polyether-TDI prepolymers are cured with a dihydroxy substituted C _r C _1β alkyl, C ₄ -C ₁₀ cycloalkyl, or aryl tertiary amine, preferably phenyldiethanolamine (PDEA). The PDEA may be optionally combined with one or more short chain diol (or diols), and one or more short chain triols to give desirable low Shore A durometer hardness, high tear strength, and good compression set properties with excellent hydrolysis resistance. BACKGROUND AND RELATED ART

Polyurethane prepolymers are made from the reaction of an organic isocyanate compound with a hydroxy containing material, such as a polyol. The prepolymers are further cured to yield a polyurethane. In some processes, prepolymers of isocyanates and polyois are prepared and further reacted with chain extenders, or curatives. On cross-linking, a variety of polymers can be produced, depending on the selection of materials and processes.

Certain applications of polyurethane elastomers require the use of low Shore A durometer materials, specifically, with Shore A hardness values of less than 60A. Examples of such applications include, but are not limited to graphic arts coating rollers, couch rolls for the paper industry, cardboard processing rolls, baseball wheel machines, mixing screens, and various industrial wheels and trim.

In conventionally known polyether polyurethanes, a hardness which is below 60 Shore A causes a loss of some mechanical properties,

especially tear strength and compression set. Loss of tear strength render these polyether polyurethane materials impractical for use in many industrial applications, including the specific applications mentioned above. Certain polyester polyol based TDI prepolymers give some of the improved physical properties but suffer from poor hydrolysis resistance.

MDI based prepolymers do not readily yield low hardness polyurethanes with conventional curatives.

Polyether polyurethanes offer advantages in terms of good hydrolysis and fungal-growth resistance of the end product. Polγethers are more resistant to fungal resistance and mildews which are waterborne. These resistance qualities are becoming increasingly important as more industrial and chemical processes and systems are being re-engineered toward environmentally friendly waterborne systems. This trend toward water based or water-borne systems eliminates the use of traditional solvents in many industries and answers many environmental and safety concerns.

Unfortunately, until this invention there has not been a way to produce polyether polyurethane elastomers with low hardness (as measured by Shore A durometer) which have good hydrolysis resistance and high tear strength as well as good compression set properties. The curing agents which are usually used for polyether urethanes, such as 4,4'-methγlenebis(2-chloroaniline) [MBOCA] have disadvantages such as toxicity and long curing cycles and they do not produce polyether polyurethanes with the desired combination of low Shore A hardness, high tear strength, and good compression set properties.

US Patent No. 3,892,713 to Burkus, et al. teaches the use of phenyldiethanolamine (PDEA) in very small quantities as a catalyst with triisopropanol amine to speed up the demold time only when used with primary curatives 1 ,4-butanediol (BD) or hγdroquinone bis(beta- hγdroxyethyl) ether. All the elastomers exemplified in Burkus, et al. had

Shore A durometer hardness of 72 to 90. All the prepolymers utilized were all based on MDI, (p, p'-diphenylmethane diisocyanate).

There has been a long felt need in the art for a curative or curative system which would allow the use of polyether-TDI based polyurethanes with their excellent hydrolysis resistance in applications requiring the combination of low Shore A durometer hardness, high tear strength, and good compression set properties.

It is an object of this invention to demonstrate curatives capable of curing a polyether-TDI prepolymers. It is a further object of the invention to produce a polyurethane elastomer with low Shore A hardness, high tear strength, and good compression set properties. A still further object would be to produce a polyether-TDI polyurethane elastomer with low

Shore A hardness, high tear strength, and good compression set properties. The curative blends of this invention optionally use other known polyurethane curatives, such as 1 ,4-butanediol, trimethylolpropane, and triisopropanolamine. The unique properties of the curative blends are optimally produced by the use of these diols and triols with phenyldiethanolamine (PDEA) in certain proportions, as will be disclosed shortly. SUMMARY OF THE INVENTION

In accordance with this invention, it has now been discovered that by curing the polyether polyol-TDI prepolymer with a particular curative system yields the desired combination of low Shore A durometer hardness, high tear strength, and good compression set properties with excellent hydrolysis resistance. The curative mixture comprises a dihydroxy substituted C,-C _1B alkyl, C ₄ -C ₁₀ cycloalkyl, or aryl tertiary amine, preferably phenyldiethanolamine (PDEA). The PDEA may be optionally combined with one or more short chain diol (or diols), and one or more short chain triols. Other additives, such as piasticizer, may optionally be added as well as catalysts, colorants, stabilizers, and the like. DETAILED DESCRIPTION OF THE INVENTION

Polyurethanes are diverse specialty elastomers with many forms and uses. Both thermoset and thermoplastic types are available and they

may be produced by solution, suspension, or melt polymerization from a variety of monomers, polyois, and curatives.

The poiγethers used in preparing the polyurethane elastomer of this invention are polyethers having a terminal functional group containing active hydrogen capable of reacting with an isocyanate group. The functional group is selected from the group consisting of hydroxyl group, mercapto group, amino group and carboxyl group. Suitable polyethers include the polyether polyois and, in particular, poly(oxyalkylene) ether glycols such as the poly(oxγethylene) ether glycols, the poly(oxγpropylene) ether glycols, the polytetramethylene ether glycols, polyoxybutγlene ether glycols, poly(oxγalkylene) ether triols such as the polγ(oxypropylene) ether triols, and the like, poly(oxγalkγlene) ether dicarboxylic acids, the poly(oxγalkylene) ether dithiols, the poly(oxyalkylene) ether diamines and their pre-extended polymers, and so forth. The preferred polyethers are the poly(oxγalkylehe) ether glycols, the more preferred polyethers being the polytetramethylene ether glycols (PTMEGs).

The polyether prepolymer is formed by reacting an excess of organic diisocyanate with a long-chain glycol. In the instant invention, the glycol is a polyether which is then reacted with a multifunotional isocyanate. The molar ratio of diisocyanate to glycol should be greater than one.

Polγfunctional isocyanates used in this invention are preferably aromatic diisocyanates. The most preferred aromatic diisocγanates are the TDIs which include, for example tolylene-2,4-diisocyanate and tolylene-2,6-diisocyanate and any isomer blend thereof. The commonly available isomer blends of 2,4-/2, 6-TD I include 100/0, 80/20, 65/35 and 0/100. Any isomer blend of the foregoing isomers benefits from the unique curatives of this invention. For many solid castable applications, including those of this invention, the toluene diisocyanate (TDI) is reacted with a polyol, such as polytetramethylene ether glycol, to form a prepolymer. The prepolymer is then cured with the curing compositions

and methods of this invention.

The curatives that have been found to generate elastomers with such properties are phenyldiethanolamine (PDEA), combinations of PDEA with a short chain diol such as 1 ,4-butanediol (BD), ethyiene glycol, and combinations of PDEA, a diol or combination of diols, and various short chain triols. The amount of PDEA used in the curative of this invention may range from about 10 to 100 weight percent of the curing components, discounting optional materials such as plasticizer, colorants, catalysts, stabilizers and the like. A more preferred amount ranges from about 40 to 100 percent. A still more preferred amount ranges from about 50 to 100%.

The diol, or bifunctional, portion of the curative may be present in an amount that ranges from 0 to about 50 weight percent. The short chain diols are represented by such materials as 1 ,4-butanediol, ethyiene glycol, cyclohexanedimethylol, and 1 ,6-hexanediol and other conventionally known diols in the urethane art.

The triol, or trifunctional curative component, such as trimethylolpropane or triisopropanoi amine may be present in an amount which ranges from 0 to about 25 weight percent. The short chain triols are those known in use in the polyurethane art represented by such materials as trimethylolpropane, triisopropanolamine, triethanoiamine, glycerol, ethyiene oxide adduct of trimethylolpropane, trimethylolethane and the ethyiene oxide adduct of trimethylolethane.

For curing these prepolymers, the number of -OH groups in the curative package should be approximately equal to the number of -NCO groups in the prepolymer. Another expression of this equivalent ratio concept is the percentage of equivalents of reactive hydrogen in the curative compounds to active isocyanate sites. The amount of curative that is reacted with the above described polyether prepolymer would optimally be calculated on a molar equivalent or stoichiometric basis.

Variation is permissible but in general this molar equivalent relationship will range from about 70 to about 1 10%. A more preferable equivalents

ratio will range from about 85 to 100 percent. A still more preferable equivalents ranges from 90 to 98 percent.

Other ingredients may comprise the polyurethane elastomer product of this invention, including but not limited to plasticizer, colorants, stabilizers, and catalysts.

Typical curing conditions as useful in this invention are from 1 to 36 hours at from 180°F to 300°F (82 to 150°C). Preferably from 4 to 24 hours at from 200°F to 290°F{94 to 143°C) and most preferable from 8 to 16 hours at 212 to 270° F (100 to 132°C). When conventional curatives are used by those skilled in the art to obtain low durometer polyether elastomers, low tear strengths are also obtained. Commonly used polyurethane curatives include a wide variety of conventional and well known organic diamines, polyol materials, or alkyl thio aromatic amines. The latter type of curative have been used in the comparative examples contained herein.

Representative of these types of conventional curative materials include 4,4'-methylene-bis(3-chloro)aniline (MBCA), 4,4'- methylene-bis(3-chloro-2,6-diethyl)aniline (MCDEA), diethyl toluene diamine (DETDA), tertiary butyl toluene diamine (TBTDA), dimethylthio-toluene diamine (Ethacure ^1- 300) from Ethyl Corporation, trimethylene glycol di-p-amino- benzoate (Polacure" ¹ 740) from Polaroid Corporation, and 1 ,2-bis(2-aminophenγlthio)ethane (Cyanacure from American Cγanamid Company).

In the polyurethane art, properties such as tensile strength, tear strength, hardness, and others can be designed through the selection of various polyol, prepolymer, isocyanate, and curative types. It is known that the use of polyester polyois leads to better oil resistance and poorer water resistance due to hydrolysis of the polyester. In contrast, polyether polyois give products that are more sensitive to oil but less susceptible to hydrolysis.

Polyesters are also known to retain tear strength and compression set properties at low hardness levels. For the purposes of this invention

'low hardness' will mean a Shore A Durometer values ranging from about 20 to about 60. Shore Hardness is defined as a value of specific degree of hardness as obtained from the Shore hardness tester called a durometer. This tester is a small pocket-type instrument which gives a measure of the resistance of an elastomer surface to penetration of a blunt point pressed onto the surface. (As defined in ASTM Special Technical Publication No. 184, Glossary of Terms relating to Rubber and Rubber-Like Materials, 1956.)

Common ASTM D 1938 tear strength values are usually well below 20 for most polyether polyurethane elastomers cured with conventional systems. They are usually about half that of the polyester polyurethanes. In the examples of this invention, trouser tear values ranging from 34 to 66 were measured. For the purposes of this invention, 'high tear strength' will include those values for trouser tear greater than or equal to 30. The combination of low hardness and high tear strength will encompass a trouser tear value of 30 or higher and a Shore A hardness value ranging from about 20 to about 60, preferably 30 to 50.

In such uses as printing rolls or graphic arts coating rollers, the polyurethane elastomer used needs to have a hardness measurement of less than 60 Shore A. A polyester polyurethane with a Shore A harness of less than or about 60 retains its compression set, abrasion resistance and tear strength properties. It is, however, subject to degradation due to the hydrolysis of the polyester in the presence of water. In today's industrial environment the use of aqueous-based systems is favored over solvents whenever possible due to the desire to minimize pollution and worker exposure to harmful materials. The increasing use of water-based systems in the printing and graphic arts industries is not amenable to the use of polyester polyurethanes used in rollers. These materials are subject to failure in use and require the expense and nuisance of frequent repair and/or replacement.

Polyether polyurethanes do not have the same performance

problems in aqueous systems since they do not readily hydrolyze. However, use of polyether polyurethanes have not been feasible due to low tear strength at low durometer values, which may cause tearing of the rollers while in use in the stress of the application. In accordance with this invention, curatives are disclosed which, when used to cure polyether isocyanate-terminated prepolymers, produce polyether polyurethane elastomers having the unique combination of Shore A durometer measurements of less than 60 A with significantly improved tear strength. Thus, polyether polyurethanes cured in this way will be able to enjoy use in the printing and graphic arts industry and possess tear strength and other desired physical properties at low Shore A durometer.

- When conventional curatives are used to obtain low durometer values for polyether polyurethane elastomers, low tear strengths result. These conventional curatives may be aromatic diamines used in combination with plasticizer (in order to reduce the hardness). They may be diols, triols, or polyois or combinations and may be used with or without plasticizer. When used with polyether polyurethane prepolymers, the elastomers so generated have low tear strength. These observations may be seen in Comparative Examples A-C which are presented in the

Experimental section following this one.

Plasticizers are substances that soften another substance through solvent, lubricating, or chemical catalytic action. Among the plasticizer in common use and which have been used in the instant invention are dipropylene glycol dibenzoate, commercially available as Benzoflex™,

9-88, Velsicol Chemical Company. Other plasticizer in common use include such materials as tributoxyethγlphosphate.

The examples of the invention and comparative examples were tested for Shore A hardness and tear strength. These values are presented in Table 1. As has been mentioned, the Shore A hardness is measured by the Shore hardness tester called a durometer. This tester is a small pocket-type instrument which gives a measure of the resistance

of an elastomer surface to penetration of a blunt point pressed onto the surface. The method used to determine durometer hardness is referenced in ASTM Method D2240-91. The tear strength is measured by ASTM method D- 1938-92, Standard Test Method for Tear Propagation Resistance of Plastic Film and Thin Sheeting by a Single Test Method.

The TDI prepolymers are prepared by reacting any of the toluene diisocyanates isomers used alone or in combination with any other conventional diisocyanates optionally used, adding the polyol or polyol blend, and maintaining the temperature from room temperature to temperatures as high as 150°C for times necessary to react all the available hydroxyl groups. Preferred reaction temperatures are 30°C to 100°C; more preferred are 50°C to 85oC.

US Patent No. 3,892,713 to Burkus, et al. taught the use of phenyldiethanolamine (PDEA) in very small quantities as a catalyst with triisopropanol amine to speed up the demold time only when used with primary curatives 1 ,4-butanediol (BD) or hγdroquinone bis(beta- hydroxyethyl) ether. The equivalent ratios described, exemplified and claimed in Burkus, et al. utilized Diol : (PDEA plus Triol) equivalent ratios of from 17:1 to 2.5: 1. This invention went against this conventional wisdom by recognizing the value of utilizing large amounts of PDEA with small amounts of short chain Triols and Diols in TDI based polyether polyois. In this invention, by utilizing the same equivalent ratio calculation as used in Burkus (Diol : PDEA plus Triols), the equivalents ratios would range from 0:1 to 2:1 , preferably 0.2: 1 to 1.75:1 , and most preferably 0.4:1 to 1 :1. These ranges give very desirable Shore A durometer hardnesses of about 20 to 60, with excellent tear strengths.

The following examples are illustrative of the present invention and are therefore not intended as a limitation of the scope thereof.

EXAMPLES Comparative Example A: Curing a Polyether Prepolymer with a

Combination of an Aromatic Diamine and a Triol

A toluenediisocyante polytetramethylene ether prepolymer (Adiprene LF 800, supplied by Uniroyal Chemical Company, Inc.), with an isocyanate content of 2.85% was cured as follows: to 100 parts prepolymer was added 1 part dimethylthiotoluene diamine, and 6.17 parts ethoxylated trimethylolpropane of equivalent weight 91. Three different weights of dipropylene glycol dibenzoate, commercially available as Benzoflex ¹ " 9-88, Velsicol Chemical Company were added: 35 parts, 20 parts, and 10 parts. These materials are listed in Table 1 as Al , A2, and A3, respectively with values for Shore A durometer and trouser tear strength.

Comparative Example B: Curing a Polyether Prepolymer with Conventional Triols

A toluenediisocyante polytetramethylene ether prepolymer (Adiprene LF 900, supplied by Uniroyal Chemical Company, Inc.), with an isocyanate content of 3.80% was cured as follows: to 100 parts prepolymer was added each of the following conventional curatives: 1 ) 5.2 parts triisopropanolamine, 2) 3.6 parts trimethylolpropane, 3) 1 1.9 parts propoxylated trimethylolpropane of equivalent weight 146, 4) 166 parts ethoxylated propoxylated trimethylolpropane of equivalent weight 2000. (All parts by weight). The values for Shore A durometer and trouser tear are presented in Table 1 and labelled B1-B4. Comparative Example C: Curing a Polyether Prepolymer with a Combination of a Conventional Diol and Triol

A toluenediisocyante polytetramethylene ether prepolymer (Adiprene L 83, supplied by Uniroyal Chemical Company, Inc.), with an isocyanate content of 3.25% was cured as follows: to 100 parts prepolymer was added 3.8 parts diethoxγlated hydroquinone and 2 parts triisopropanolamine. In this example, 20 parts of tributoxyethylphosphate was also added as a plasticizer. The values for Shore A and Trouser Tear are presented in Table 1.

Example 1 : Curing a Polyether Prepolymer with Phenyldiethanolamine One-hundred parts of the prepolymer of Comparative Example B

was cured with 7.8 parts phenyldiethanolamine, available commercially from BASF or Henkel. The Shore A durometer and Trouser Tear results are reported in Table 1.

Example 2: Curing a Polyether Prepolymer with Phenyldiethanolamine and a triol

One-hundred parts of the prepolymer of Comparative Example B was cured with 6.5 parts phenyldiethanolamine and 0.6 parts triisopropanolamine. Eleven parts of dipropylenegiycol dibenzoate plasticizer was added. The Shore A durometer and Trouser Tear results are reported in Table 1.

Example 3: Curing a Polyether Prepolymer with Phenyldiethanolamine and a Triol

One-hundred parts of the prepolymer of Comparative Example C was cured with 4.9 parts phenyldiethanolamine and 1.0 parts triisopropanolamine. Ten parts of tributoxyethyl phosphate plasticizer was added. The Shore A durometer and Trouser Tear results are reported in Table 1.

Example 4 a,b: Curing a Polyether Prepolymer with Phenyldiethanolamine, a Diol and a Triol One-hundred parts of the prepolymer of Comparative Example C was cured with 2.1 parts phenyldiethanolamine, 1.7 parts 1 ,4-butanediol and 1.0 parts triisopropanolamine. Ten parts and twenty parts of tributoxyethyl phosphate plasticizer was added to Examples 4a and 4b, respectively. The Shore A durometer and Trouser Tear results are reported in Table 1.

TABLE 1 TROUSER TEAR AND SHORE A HARDNESS VALUES FOR VARIOUS POLYURETHANE ELASTOMERS

ExamDle Number Shore A Durometer Trouser Tear

A-1 30 14

A-2 38 15

A-3 46 14

B-1 50 7.9

B-2 59 7.6

B-3 59 6.2

B-4 32 10

C 45 19

1 47 66

2 46 40

3 49 60

4a 46 51 4b 30 34

The above data may be used to verify that fact that the materials tested possess good tear strength as well as low Shore A durometer values.

Table 2 summarizes the formulations of the invention and highlight the relative amounts of curatives utilized.

TABLE 2

PREPOLYMER CURATIVE SYSTEM _{TfM DTMEP} PARTS BY WEIGHT ^TD,: P™ ¹ (WEIGHT RATIO)

Plasticizer Parts by (Parts by Example NC0% Weight Weight) PDEA Triol Diol

1 3.80 100 0 7.8

(100) (0) (0)

2 3.80 100 1 1 6.5 0.6 (92) (8) (0)

3 3.25 100 10 4.9 1.0 (83) (17) 0

4a 3.25 100 10 2.1 1 .0 1 .7 (44) (21 ) (35)

4b 3.25 100 20 2.1 1 .0 1 .7 (44) (21 ) (35)

This unique combination of properties, as provided by the curative systems of the instant invention, is of great economic importance to the providers of products using the polyurethane compositions of the instant invention.

The embodiments of the invention shown are meant to be illustrative and not limitative. Modifications of the examples and description are contemplated. So long as they are encompassed by the appended claims. Such modifications are part of the invention.