MXPA99008000A - Method for the preparation of polyurethane elastomers - Google Patents

Abstract

A method for the production of polyurethane elastomers by reacting an isocyanate prepolymer having an isocyanate content of from 5 to 10%by weight which is the reaction product of an excessive amount of a polyisocyanate containing at least 85%by weight of 4,4'-diphenylmethane diisocyanate or a variant thereof and a polyether polyol having an average nominal hydroxyl functionality of from 2 to 4, an average hydroxyl equivalent weight of from 500 to 3000, and an oxyethylene content of from 50 to 85%by weight, with an aqueous latex to form a foam, and compressing said foam.

Description

METHOD FOR THE PREPARATION OF POLYURETHANE ELASTOMERS DESCRIPTION This invention relates to a method for the preparation of polyurethane elastomers and to the elastomers obtained therefrom. Elastomers made on the basis of polyisocyanate polymers are well known. In general, they are prepared by reacting diisocyanates with isocyanate-reactive compounds of high, and optionally low, molecular weight having a functionality of about 2. The processing of said elastomers can be carried out by various methods, for example, casting, molding or (for elastomers having thermoplastic properties) thermoplastic techniques such as injection molding, extrusion and satin. Compression molding is a known processing technique for working with gums and can be used with vulcanizate polyurethane materials such as so-called 'grinding gums'. It is also known to use aqueous latex compounds in combination with polyurethane-forming materials for the preparation of PU foams. For example, in US-A 4,158,087 a hydrophilic prepolymer is reacted with water and a synthetic polymer latex to produce flexible hydrophilic PU foams and US Pat. No. 5,312,847 describes polyurethane foams, especially foams. flexible, prepared by reacting a polyisocyanate with a polyol in the presence of a blowing agent and a certain particulate organic polymer, which is preferably introduced by means of an aqueous latex.

The hydrophilic isocyanate prepolymers are known and have been used in the production of flexible foams, see for example, publication W094 / 29361. In US-A-5,571,529, polyurethane foams are made for use in bandages by mixing a hydrophilic isocyanate-protected prepolymer with water in the presence of a monohydric alcohol and drying the product. Small amounts of a gum can be added to the reaction mixture, preferably an acrylic-based gum in the form of a latex.

It has now unexpectedly been found that elastomers possessing valuable properties can be successfully produced by compression molding of certain flexible polyurethane foams. The invention therefore relates to a method for the production of polyurethane elastomers comprising the reaction of a isocyanate prepolymer, which has an isocyanate content of 5 to 10% by weight and which is the reaction product of a excess amount of a polyisocyanate containing at least 85% by weight of 4,4'-diphenylmethane diisocyanate or a variant thereof and a polyether polyol having an average nominal hydroxyl functionality of 2 to 4, an equivalent weight of average hydroxyl of 500 to 3000 and an oxyethylene content of 50 to 85% by weight, with an aqueous latex to form a foam and compression of said foam. The term "average nominal hydroxyl functionality" is used in this documentation to indicate the average functionality (amount of hydroxyl groups per molecule) of the polyether polyols assuming that the average functionality of the polyether polyols is identical to the average functionality (number of atoms of active hydrogen per molecule) of the initiator (s) used in the preparation thereof, although in practice it will be somewhat lower due to a certain terminal unsaturation. The polyisocyanate used in the preparation of the isocyanate prepoiimer preferably contains 90, and more preferably 95% by weight, of 4,4'-diphenylmethane diisocyanate. It can consist essentially of 4,4'-diphenylmethane diisocyanate or mixtures of said isocyanate with one or more different organic polyisocyanates, in particular other isomers of diphenylmethane diisocyanate, for example the 2,4'- isomers, optionally together with the isomer 2,2'-. The polyisocyanate can also be a variant of diphenylmethane diisocyanate (MDI) derived from a polyisocyanate composition containing at least 85% by weight of 4,4'-diphenylmethane diisocyanate. MDI variants are well known in the art and, for use according to the invention, include particularly liquid products obtained by the introduction of uretonimine and / or carbodiimide groups in said polyisocyanate composition and / or by reacting said composition with one or more polyols. The polyisocyanate preferably has a functionality of 2.05 or less and more preferably 2.00 and is preferred to be 4,4'-MDI. The polyether polyols used in the preparation of the isocyanate prepolymers are preferably poly (oxyethylene-oxypropylene) polyols having the average nominal hydroxyl functionality, the average hydroxyl equivalent weight and the oxyethylene content required. Suitable polyether polyols can be obtained in conventional manner by reacting ethylene and propylene oxides simultaneously and / or sequentially in any order with an initiator such as water, a polyol, a hydroxylamine, a polyamine and the like having from 2 to 6. active hydrogen atoms or mixtures of said initiators. In order to obtain an average nominal hydroxyl functionality of 2 to 4, a polyether polyol having an average nominal hydroxyl functionality or a mixture of polyether polyols having an average nominal hydroxyl functionality of 2 to 6 may be used, provided that the mixture is in the range of above average nominal hydroxyl functionality from 2 to 4. If desired, the polyether polyol (or one or more of the polyether polyols when a mixture thereof is used) may contain dispersed polymer particles. Said polyols modified with polymers are completely described in the prior art and include the products obtained by the in situ polymerization of one or more vinyl monomers, for example acrylonitrile and styrene, in polyether polyols or by the in situ reaction between a polyisocyanate and an amino or hydroxyl functional compound, for example triethanolamine in the polyether polyol. Preferred polyether polyols possess a nominal hydroxyl functionality of 2 to 3 and an average hydroxyl equivalent weight of 1000 to 2000. It is further preferred that the polyether polyols possess a nominal hydroxyl functionality of 2 and an average hydroxyl equivalent weight of 1200 to 1600. The oxyethylene content is preferably 60 to 80% by weight. The isocyanate prepolymers are conventionally prepared by the reaction of polyisocyanates and the polyether polyols in relative amounts to obtain an isocyanate content of 5 to 10% by weight, preferably at a temperature between 40 and 80EC. The prepolymers prepared in this way are liquid at ambient conditions. To the prepolymers thus prepared, low amounts (up to 30% by weight) of MDI or a variant thereof can be added as long as the content of 4,4'-MDI remains above 85% by weight, calculated on the basis of to all the polyisocyanate used and provided that the average isocyanate functionality of the added polyisocyanate is less than 2.2. The prepolymer preferably has a viscosity of at most 10,000 mPa at 25 ° C. At least 20 parts by weight of aqueous latex are reacted with 100 parts by weight of isocyanate prepolymer. It is preferred to react 20-500, and more preferably, 75-150 parts by weight of aqueous latex with 100 parts by weight of the isocyanate prepolymer. Suitable aqueous latex compounds include natural gum, isoprene gum, neoprene gum, polybutadiene gum, styrene-butadiene gum (SBR), nitrile gum (NBR), isobutylene-isoprene gum (IIR), copolymers of acrylonitrile, methacrylonitrile, acrylates, methacrylates, vinylpyridine with butadiene or 2-chloro-1, 3-butadiene and chlorinated polyethylene or mixtures of any of them. Preferred are aqueous rubbery latex compounds, styrene-butadiene rubber, polybutadiene rubber, isoprene rubber or copolymers comprising acrylates. The solids content of the appropriate aqueous latex compounds is from 20 to 70% by weight. Preferred latexes have a solids content of 30 to 60% by weight.

In addition to the isocyanate prepolymer and the aqueous latex, the formulations may further comprise the additives that are commonly used in the manufacture of elastomers. Such additives include catalysts, organic or inorganic fillers, pigments, desiccators, foam stabilizers, chain extenders, crosslinking agents, internal demolding agents, surfactants and flame retardants. The choice of said additives and the amounts used will depend on the desired properties of the final product. If said additives are used, they are preferably mixed with the aqueous latex. However, it is possible to obtain valuable elastomers without any of these additives. It is preferred not to employ additives, except up to 5 parts by weight per 100 parts by weight of isocyanate prepolymer of foam stabilizer. But it is even more preferred not to use any additives. It is preferred to dry the foams before compressing them. Usually the compression is carried out in a conventional compression mold at a pressure between 50 and 200 bar at a temperature between 100 and 250 EC. The compression time depends on the temperature and the required geometry of the part. The elastomers obtained in this way generally have an average density of 200 to 1200 kg / m3, preferably 300 to 1000 kg m3 and more preferably 400 to 700 kg / m3. The elastomers of the present invention are useful in a wide variety of applications, such as footwear, acoustic insulation, vibration dampers or under carpets. The following examples are illustrations, without limitations for the invention. All relationships are weight ratios unless otherwise indicated. Examples 1-3 The elastomers were obtained by compression of molding foams which were made by mixing an isocyanate prepolymer and a latex in different ratios, as indicated in the Table. The isocyanate prepolymer and the latex were mixed in a conventional manner in a low pressure PU supply equipment. The temperature of the isocyanate prepolymer was 45 ° C and that of the latex was 20 ° C. The obtained foams were dried in an oven at 80 ° C until no further weight loss was observed and they were compressed at 150 ° C in a hydraulic press under a pressure of 100 bar in the separators. The properties measured and the results obtained are shown in the following Table. Table (*): No foam was obtained Prepolymer 1: 'Suprasec' 1002 (commercially available from ICI; Suprasec is a trademark of Imperial Chemical Industries Ltd.) Prepolymer 2: Reaction product of 4,4'-MDI / polyether polyol (weight ratio 75/25) (fn = 3, Ohv = 28.14% tip EO ) Latex: Acrylic latex paint based on water (black color.

Estimated water content: 60% by weight) The molded parts made from the elastomers according to the invention showed a very high definition and a "rubber-like" feeling to the touch, while no foam could be made from the comparative formulation.

Claims (14)

1. CLAIMS 1. A method for the production of polyurethane elastomers, CHARACTERIZED BECAUSE it comprises the reaction of an isocyanate prepolymer having an isocyanate content of 5 to 10% by weight and which is the reaction product of an excess amount of a polyisocyanate containing at least 85% by weight of 4,4'-diphenylmethane diisocyanate or a variant thereof and a polyether polyol having an average nominal hydroxyl functionality of 2 to 4, an average hydroxyl equivalent weight of 500 to 3000 and an oxyethylene content of 50 to 85% by weight, with an aqueous latex to form a foam and compression of said foam.
2. 2. The method according to clause 1, CHARACTERIZED BECAUSE the polyether polyol is a poly (oxyethyleneoxypropylene) polyol.
3. 3. The method according to clause 1 or 2, CHARACTERIZED BECAUSE the polyether polyol has a nominal hydroxyl functionality of 2 to 3 and an average hydroxyl equivalent weight of 1000 to 2000.
4. 4. The method according to clause 3, CHARACTERIZED BECAUSE the polyether polyol has a nominal hydroxyl functionality of 2 to 3, an average hydroxyl equivalent weight of 1200 to 1600.
5. 5. The method according to any of the preceding clauses, CHARACTERIZED BECAUSE the polyether polyol has an oxyethylene content of 60 to 80% by weight.
6. 6. The method according to any of the preceding clauses, CHARACTERIZED BECAUSE the polyisocyanate contains at least 90% by weight of 4,4'-diphenylmethane diisocyanate.
7. 7. The method according to clause 6, CHARACTERIZED BECAUSE the polyisocyanate contains at least 95% by weight of 4,4'-diphenylmethane diisocyanate.
8. 8. The method according to any of the preceding clauses, CHARACTERIZED BECAUSE the solids content of the aqueous latex is 30 to 60% by weight.
9. 9. The method according to clause 8, CHARACTERIZED BECAUSE the aqueous latex is selected from the group formed by aqueous latex compounds of natural rubber, styrene-butadiene rubber, polybutadiene rubber, isoprene rubber and copolymers comprising acrylates.
10. 10. The method according to any of the preceding clauses, CHARACTERIZED BECAUSE 20-500 parts by weight of aqueous latex are reacted with 100 parts by weight of the isocyanate prepolymer.
11. 11. The method according to clause 10, CHARACTERIZED BECAUSE 75-150 parts by weight of aqueous latex are reacted with 100 parts by weight of the isocyanate prepolymer.
12. 12. The method according to any of the preceding clauses, CHARACTERIZED BECAUSE in this documentation the foam is dried before compression.
13. 13. The elastomer obtained by the method according to any of the preceding clauses, CHARACTERIZED BECAUSE it has an average density of 300 to 1000 kg / m3.
14. 14. The elastomer obtained by the method according to clause 12, CHARACTERIZED BECAUSE it has a density of 400 to 700 kg / m3.