GB2055869A - A polyurethane elastomer and a method of bonding said elastomer to metal - Google Patents

Abstract

A polyurethane elastomer is obtained from a rapid curing mixture comprising specified proportions of (1) a liquid prepolymer formed from specific proportions of (i) diphenylmethanediisocyanate in admixture with carbodiimide-modified diphenylmethanediisocyanate and (ii) a polytetramethylene glycol of specified molecular weight or a mixture thereof with an aliphatic diol and (2) a curing agent comprising said polytetramethylene glycol, a diol and an organometallic catalyst. Rapid curing of the mixture occurs at room temperature. The polyurethane elastomer can be bonded to metal by coating the metal with a primer prepared by adding a strong acid to an organic solvent solution of a polyvinyl acetal resin modified with a formaldehyde condensation thermosetting resin, forming the abovementioned rapid curing mixture and applying it to the primer coated metal while the mixture is curing to bond the resultant polyurethane elastomer to the metal.

Description

SPECIFICATION A polyurethane elastomer formed from a rapid curing mixture and a method of bonding said elastomer to metal This invention relates to a polyurethane elastomer obtained from a rapid curing mixture and to a method of bonding the elastomer to metal. More particularly, this invention provides polyurethane elastomer prepared from a mixture which is curable and releasable at a relatively low temperature and in a short time, and provides a method of bonding the elastomer to metal in which a strong bonding layer of the elastomer to metal is formed, the elastomer being bonded by employing instead of a conventional primer on the metal, a primer containing a curing catalyst.

There are several kinds of known polyurethane elastomers.

One kind are the so called casting polyurethane elastomers, which have been widely used in the manufacture of, for example, rollers, belts and solid tyres, this being because of their excellent mechanical characteristics, wear resistance and resistance to oils. Such casting polyurethane elastomers are of two types, one being the TDI (toluene diisocyanate) prepolymer type which employs MOCA (3,3'-dichloro-4,4'-diaminodiphenylmethane) as curing agent and employs, as polyol, polytetramethylene ether glycol (hereinafter simply referred to as PTMEG), polyester diol, polyoxypropylene glycol (hereinafter simply referred to as PPG) and the like, and the other being the type prepared by the so-called semione-shot process in which a 1 ,5-naphthalenediisocyanate or diphenylmethanediisocyanate/polyester diol system is employed and glycol is employed as curing agent.

However, the processes for preparing both of the two types mentioned above have drawbacks in that curing thereof takes several hours at about 1 000C and that workability is extremely low.

For both types of polyurethane elastomer mentioned above the pot life on mixing the prepolymer with the curing agent can be shortened by use of a catalyst, but it is quite impossible to shorten the curing time to several minutes. Moreover, it is necessary to raise the temperature of the raw material up to around 1 ooac because of the high viscosity of the prepolymer and the high melting-point of MOCA.

However, a high pressure casting machine, which must be employed for the raw material at such a high temperature, is not available at present. Even if such a high pressure casting machine were available, it would still be impossible to shorten the curing time to several minutes.

Another kind of polyurethane elastomer are high density polyurethane foams (also known as micro-cellular elastomers), and these are used in the manufacture of soles and automobile parts such as bumpers. Japanese published examined patent application Nos. 22711/1972,38587/1971 and 1 386/1 972 disclose respectively a process in which curing of such polymers is effected more rapidly and at a lower temperature compared with the so-called casting elastomers mentioned above. All of the processes disclosed above follow the so-called "one-shot" process, in which modified diphenylmethanediisocyanate is used as isocyanate.

Yet another kind of polyurethane elastomer are the rapid curing polyurethane elastomers.

Japanese published unexamined patent application Nos. 118795/1974 and 125199/1976 disclose the use of a prepolymer as isocyanate, and in particular, the latter discloses the use of PTMEG in the prepolymer and as a curing agent.

However, using the process disclosed above for preparing the various kinds of polyurethane elastomer, it is very difficult to satisfy simultaneously both rapid curability and desired physical properties,of the cured product, and no elastomer with high strength physical properties equal to an elastomer obtained from PTMEG/TDI prepolymer represented by Adiprene L-1 00 (produced by E. I. Du Pont de Nemours s Co.), where MOCA is employed as a curing agent, has been obtained.

Polyurethane elastomers generally, which are cured at a temperature in the range of from room temperature to 600 C, have been widely used for various applications including uses for building material such as waterproof materials, floor coverings, and sealing materials, and some such applications require bonding of the polyurethane elastomer to metal. Various primers of the cold curing type have been used for bonding of a cold curing polyurethane elastomer to metal. Examples of such primers generally include an isocyanate series primer, epoxy resin series primer, a wash primer, and silane series primer.

In the case of polyurethane sealing material, it is well known that its adhesive force is retained satisfactorily even after immersion in water when a silane series primer modified with isocyanate or the like is used as disclosed in Japanese published unexamined patent application No. 23335/1 978.

However, in the case of floor coverings and the like where the cohesive force of the polyurethane elastomer is high, although the use of an isocyanate series primer or epoxy series primer gives a high adhesive force, stripping of the primer from the metal surface occurs after immersion in water.

Heat casting type polyurethane elastomers in particular, for example, Adiprene or Vulkollan, which are used as materials for industrial parts such as belts, rollers, solid tyres, and the like, also often require a strong bonding thereof to metal, and primers of polyvinyl acetal resin modified with formaldehyde condensation thermosetting resin such as Conap AD-i 146 (Conap Inc.), Chemlok #218 (Hughson Chemicals, Lord Corp.) are widely used.

These primers are the so-called baking type primers and polyurethane elastomers bonded to metal with these primers have such a high adhesive force that when subjected to an adhesive strength test, as later described, the polyurethane elastomers suffer cohesive failure rather than the bond suffering adhesive failure, this in spite of their much greater cohesive force as compared with polyurethane sealant and the like. It is also known that the adhesive force thereof is only slightly lowered after immersion in water.

However to develop this adhesive force it is necessary to heat at 1 000C for several hours.

Thus as explained above, known polyurethane elastomers suffer from disadvantages associated with their preparation and physical properties and disadvantages are also associated with known methods of bonding them to metals.

We made an extensive study of polyurethane elastomers and found that an elastomer having rapid curability and such high strength physical properties as to be equal to those of a casting type elastomer (such as Adiprene L-1 00 produced by E. I. Du Pont de nemours s Co. which is cured on heating by use of MOCA) can be obtained by satisfactorily selecting the composition of a prepolymer so that the viscosity thereof may be lowered to such an extent as to be applicable to a casting machine at a temperature of from room temperature to a relatively low temperature.

Bonding of such a rapid curing polyurethane to metal may be effected by coating a so-called baking type primer as previously mentioned onto a metal surface, baking for several hours at a temperature of from 100 to 1200 C, and then by curing at a temperature of from room temperature to 600C and applying the rapid curing elastomer to obtain an adhesive force approximately equal to that of heat casting polyurethane elastomer.

However, it is often necessary to effect bonding of rapid curing elastomer to metal while the elastomer is being cured, and this needs a cold curing primer. We studied various cold curing primers which are used for building material and the like but found that an adhesive force as high as that obtained by baking at a temperature of from 1 00 to 1 200C using a baking type primer was not obtained.

Therefore, we made an extensive study of primers and found a cold curing primer which gives a polyurethane to metal bond with an extremely high adhesive force. Indeed when bonding the abovementioned rapid cold curing polyurethane elastomer (which is curable at a temperature of from room temperature to 600C) to metal using this primer, the adhesive force is so high that on subjecting the bond to an adhesion strength test as later described, the polyurethane elastomer suffers cohesive failure and the adhesive bond remains, this despite the polyurethane elastomer having a cohesive force as high as that of a heat casting elastomer such as Adiprene.

Suprisingly, we found that such a desired high adhesive force can be obtained using, as primer, a mixture prepared by adding a strong acid to a conventional baking type primer and by heating at a temperature of from room temperature to 600C, and that lowering of the adhesive force is small even after immersion in water.

According to one aspect, the present invention provides a polyurethane elastomer formed from a curable mixture comprising (1) a prepolymer, which is liquid at room temperature, prepared by reacting (i) 4 to 6 equivalents of a mixture of diphenylmethanediisocyanate (hereinafter referred to as MDI) and a carbodiimide-modified diphenylmethanediisocyanate, which diphenylmethanediisocyanate mixture is liquid at room temperature, with (ii) one equivalent of polytetramethylene glycol having a mean molecular weight of from 650 to 2000 or a mixture thereof with an aliphatic diol having a molecular weight less than 250; and (2) a curing agent comprising a mixture of one equivalent of a polytetramethylene ether glycol as defined above, 1 to 4 equivalents of a diol having a molecular weightless than 250 as chainelongating agent, and an organometallic catalyst; the said liquid prepolymer (1) and curing agent (2) being present in the mixture in a proportion such that the polyurethane elastomer has an OH: NCO equivalent ratio of from 1 :1.00 to 1:1.10.

Rapid curing of the mixture can be achieved by mixing the liquid prepolymer (1), with the curing agent (2) at a temperature of from room temperature to 600C to obtain an elastomer which is hereinafter called a rapid curing polyurethane elastomer.

The rapid curing polyurethane elastomer in accordance with the invention has an OH :NCO equivalent ratio of from 1 :1.00two to 1:1.10, and preferably from 1:1.03 to 1:1.07 to give a slight excess of NCO, and is curable and releasable in a short period of time of from one minute to about a dozen minutes at a relatively low temperature in the region of from room temperature to 600C by controlling the amount of the catalyst so that the pot life on mixing (1) and (2) may be in the range of from about a dozen seconds to several minutes.

The carbodiimide-modified MDI, which is liquid at room temperature, can be prepared according to the processes disclosed in Japanese published Examined patent application Nos. 4576/1 963 and 2908/1977, etc.

Examples of the aliphatic diol having a molecular weight less than 250 include ethylene glycol (hereinafter simply referred to as EG), propylene glycol (hereinafter simply referred to as PG), dipropylene glycol (hereinafter simply referred to as DPG), diethylene glycol (hereinafter simply referred to as DEG), 1 ,4-butane-diol, cyclohexane dimethanol and the like, aromatic diols such as bishydroxyethoxybenzene, p-xylylene-diol and the like.

Examples of the organometallic catalyst used include known catalysts for urethane formation such as dibutyltin dilaurate (hereinafter simply referred to as DBTDL), lead octylate and cobalt naphthenate.

Control of the pot life on mixing a prepolymer and a curing agent can be controlled by selecting the composition of the prepolymer and catalyst, and their amounts.

In particular, the pot life can readily be controlled simply by increasing or decreasing the amount of the catalyst. For example, 0.05 parts by weight (hereinafter all parts referred are by weight), 0.2 parts, and 0.6 parts respectively of DBTDL based on 1 00 parts of a curing agent result 5 minutes, one minute, and about 20 seconds as the pot life respectively. (note) The "pot life" means that period of time from the time at which two solutions are mixed to the time at which the fluidity thereof is almost lost due to a viscosity increase up to 1000 ps.

Mixing of the prepolymer and curing agent may be achieved using, for example, a high pressure casting machine, a low pressure casting machine or a batch type agitator depending upon the pot life of the desired mixture.

The high pressure casting machine is, for example, a casting machine in which two stock solutions are circulated under a circulating pressure of from 150 to 200 kg/cm2 by a high pressure pump respectively such as an axial piston pump or a Bosch pump, and in which these high pressure streams impinge directly on to each other in a small mixing cavity and cause an intimate mixing of liquids, the mixture then being subjected to casting. The low pressure casting machine is a casting machine in which two stock solutions are each sent to a mixing part by a low pressure pump such as a gear pump, and these solutions are mixed with agitation by e.g. a rotor mixer or a static mixer, the mixture then being subjected to casting.The batch type agitator is most preferably of such a type that air bubbles are not produced on agitation, for example, an "AJITER" (trade name of Shimazaki Equipment Co., Ltd.).

In preparing a rapid curing polyurethane elastomer in accordance with the present invention, the prepolymer (1) and the curing agent (2) may be used at room temperature as they are, or they may be warmed up to about 600C in order that the viscosity thereof may be lowered depending on the application required. Examples of the casting process applicable include various casting processes applicable to so-called "casting elastomers", for example, the Reaction Injection Moulding Process (simply referred to as RIM process), in which a high pressure casting machine is used, and a process in which a thick coating is effected using an airless spray machine.

According to another aspect of the present invention there is'a provided a method of bonding a rapid curing polyurethan elastomer described above to metal, which method includes coating the metal with a primer prepared by adding a strong acid as catalyst, to an organic solvent solution of a polyvinylacetal resin modified with a formaldehyde condensation thermo-setting resin, forming a said rapid curing mixture as decribed above and, during rapid curing of the mixture to form the polyurethane elastomer, applying the said rapid curing mixture to the metal to bond the resultant polyurethane elastomer to the metal.

Thus a strong bonding of the polyurethane elastomer to metal is formed while the elastomer is being cured using a primer containing a curing catalyst and which is cold curing instead of a conventional bnaking type baking type primer which has to be cured by heating at 1 000C for several hours to achieve an adequate adhesive force.

The polyvinylacetal resin used in preparing the primer used in the method in accordance with the invention for bonding the rapid curing polyurethane elastomer to metal can be prepared by the acetalization of polyvinyl alcohol obtained by saponifying polyvinyl acetate with formaldehyde, acetaldehyde, butylaldehyde, or the like, and can be any of various types having different degrees of acetalization and average degrees of polymerization.

Examples of the formaldehyde condensation thermosetting resin used in preparing the primer include known resins such as phenol resin, melamine resin, urea resin, guanamine resin and the like.

The polyvinyl acetal resin and formaldehyde condensation thermosetting resin mentioned above can be reacted in the organic solvent by heating to obtain the modified polyvinyl acetal material mentioned above.

Examples of the organic solvent used for dissolving the modified material prepared by modifying polyvinylacetal resin with formaldehyde condensation thermosetting resin of the present invention include ketones such as acetone, methylethylketone, methylisobutylketone and the like, alcohols such as methanol, ethanol, isopropanol, and the like, esters such as ethyl acetate, n-butyl acetate, and the like, others such as toluene, xylene, trichloroethylene, methyl cellosolve, and the like. These organic solvents may be used singly or in combination.

The modified material is used at a concentration of from 3 to 30% by weight (hereinafter % represents % by weight).

Examples of the strong acid added thereto as catalyst in the present invention include phosphoric acid, p-toluene sulfonic acid, and the like.

The strong acid is added thereto in an amount of from 0.1 to 20% preferably 1.0 to 15% of the modified material prepared by modifying polyvinylacetal resin with formaldehyde condensation thermosetting resin.

Examples of the metal of the present invention include iron, aluminium, brass, and the like.

Examples of the application process of the primer of the present invention include conventional processes such as brushing, spray coating, casting, and the like. Prior to the application of the primer, it is preferable to effect the conventional metal surface treatment process such as degreasing, sand paper finishing, various blastings, various chemical treatments, or the like in order to be adhered satisfactorily.

The present invention will be further illustrated by way of the following Examples, Preparation Examples, and Comparative Examples.

EXAMPLE 1 One equivalent of PTMEG having an average molecular weight of 1000 and 5.4 equivalents of polyisocyanate mixtures prepared by mixing MDI and carbodiimide-modified MDI (liquid at room temperature and hereinafter simply referred to as liquid MDI) at the ratio by weight of 7 to 3 are reacted at 800C for 3 hours to obtain prepolymer (C) having a free isocyanate radical of 14.8% and a viscosity of 2000 cps at 250C.

On the other hand, one equivalent of PTMEG having an average molecular weight of 1000, 2 equivalents of 1,4-butanediol, and 0.05 g of DBTDL are mixed to obtain a curing agent (D).

The prepolymer (C) and the curing agent (D) are subjected to vacuum degassing, bth solutions are then mixed throughly for one and half minutes in such a manner that bubbles may not be involved therein by use of an agitator at such a blend ratio that OH :NCO equivalent ratio may be 1:1.05, and the mixture is casted into a mold for a sheet of 2 mm in thickness in an oven maintained at 600C. A period of time from the time at which the mixing of two solutions is started to the time at which the viscosity of the mixture reaches 1000 ps is five minutes, and the cured material has neither bubbles involved therein nor cracks and is releasable in 10 minutes after starting mixing of two solutions.

The physical properties of the product showed values for physical properties equal to those of elastomer prepared by curing Adiprene L-1 00 of E. I. Du Pont de Nemours 8 Co. with MOCA as shown in Table 1.

EXAMPLE 2 A curing agent prepared by mixing one equivalent of PTMEG having an average molecular weight of 1000, 2 equivalents of 1 4-butane-diol, and 0.2 g of DBTDL and the prepolymer (C) obtained in Example 1 are mixed by a low pressure casting machine (AF-206 Type of Toho Machinery Co. Ltd.; Foaming machine for urethane foam in which a gear pump of a maximum discharge amount of 3 1/min is used for both solutions and the solutions are mixed by a mixing rotor.) are mixed with agitation at such a blend ratio that NCO/OH equivalent ratio may be 1.05 to be casted into a mold for a sheet of 2 mm in thickness maintained at 500C at a discharge amount of 2 kg/min.The period of time from starting discharge from the mixing head to the time when a viscosity thereof reaches 1000 ps at 250C is one minute, and the cured product is releasable in 5 minutes and has neither bubbles involved therein nor cracks. The product shows the same values for physical properties as in the case of the batch type agitation in Example 1 as shown in Table 1.

EXAMPLE 3 A curing agent prepared by mixing one equivalent of PTMEG having an average molecular weight of 1000, 2 equivalents of 1,4-butane-diol, and 0.6 g of DBTDL and the prepolymer (C) obtained in Example 1 are subjected to casting under the condition of a OH :NCO equivalent ratio of 1:1.05 and a discharge amount of 7.5 kg/min (125 g/sec) by use of a high pressure casting machine (NR-215 Type of Toho Machinery Co. Ltd.; where an axial piston pump having a maximum discharge amount of 7.5 1/min is used for both solutions.). The period of time from starting discharge from the mixing head to the time when the viscosity thereof reaches 1000 ps is approximately 20 seconds, and the cured product is releasable in one and half minutes.

The cured product thus obtained is free of bubble involved therein, voids, cracks, and the like. The physical properties of the cured product are found not to be greatly different from those in Examples 1 and 2 as shown in Table 1.

COMPARATIVE EXAMPLE 1 One equivalent of PTMEG having an average molecular weight of 1 500 and 7.1 equivalents of polyisocyanate mixtures prepared by blending MDI and the liquid MDI at a ratio by weight of 7 to 3 are reacted at 800C for 3 hours to obtain a prepolymer (E) having a free NCO of 14.8% and a viscosity of 2800 cps at 250C.

The prepolymer (E) thus obtained and the curing agent (D) obtained in Example 1 are subjected to a casting test at a NCO/OH equivalent ratio of 1.05 in the same manner as in Example 1.

The period of time from starting to mix two solutions of (E) and (D) to the time when the viscosity of the mixture reaches 1 000 ps is three and half minutes, and it takes 20 minutes for release.

.Fine cracks was found in a part of the cured product. The physical properties of the portion free of cracks are good as shown in Table 1, but the cured product shows poor moldability such as a prolonged time for release, development of cracks, and the like as described above.

COMPARATIVE EXAMPLE 2 One equivalent of glycol mixtures prepared by blending PTMEG having an average molecular weight of 1000 and DPG to make the average molecular weight of the mixture 500 and 3.6 equivalents of a polyisocyanate mixtures prepared by blending MDI and liquid MDI at a ratio by weight of 7 to 3 are reacted at 800C for 3 hours to obtain a prepolymer (F) having a free NCO of 14.8% and a viscosity of 7500 cps at 250C.

The prepolymer (F) and the curing agent (D) obtained in Example 1 are subjected to a casting test at a NCO/OH equivalent ratio of 1.05 in the same manner as in Example 1.

The period of time from the moment at which mixing of two solutions is started to the time when the viscosity of the mixture reaches 1000 ps is five and half minutes, and it takes 10 minutes for release.

Neither bubbles involved therein nor cracks are found in the cured product, but a high viscosity of the prepolymer requires a prolonged mixing time. Resilience, which is very important as a physical property of elastomer, is greatly lowered.

That is, according to Comparative Example 2, no lowering in moldability such as development of cracks, and the like, took place, but it showed difficulties in handling due to an increase in viscosity of the prepolymer, lowering in physical properties of the elastomer, and the like.

COMPARATIVE EXAMPLE 3 Hiprene L-1 00 (Trade name of Mitsui-Nisso Corporation for prepolymer from PTMEG/TDI having NCO of 4.2% and viscosity of 18,000 cps at 250C, and equal to Adiprene L-1 00) is subjected to thorough degassing at 800 C, and then is thoroughly mixed with fused MOCA at 1 200C in such a manner that bubbles may not be involved therein for one minute at a blend ratio of 100 to 12.5, that is, at a NCO/OH equivalent ratio of 1.07, and then is casted into a mold in an oven maintained at 100 C.

One hour after casting, a cured elastomer is released from the mold, and is subjected to post curing for about 20 hours in an oven at 100 C. The physical properties of an elastomer obtained after post curing are shown in Table 1.

TABLE 1

Example Comparative I Examples Examples Physical Properties 1 2 3 1 2 3 Hardness (JIS-Atype durometer) 88 88 87 88 89 90 Modulus 100% (kg/cm2) 75 78 74 73 83 77 300% 136 144 133 134 200 - 148 Tensile strength (kg/cm2) 350 370 400 - 380 410 350 Elongation at breakage (O/o) 520 500 490 540 410 450 Tear strength (kg/cm) 89 90 90 92 81 89 Resilience (O/o) 58 56 55 59 36 55 Compression set ( /0) 32 35 33 31 37 27 (700C, 22 hours) Preparation of Primer Material Solution (Hereinafter primer material solution means a solution of modifying polyvinyl acetal resin/formaldehyde condensation thermosetting resin to which no strong acid has been added yet).

PREPARATION EXAMPLE 1 To a three-necked flask equipped with an agitator and reflux condensor 140 g of DENKA BUTYRAL #40001 (polyvinyl butyral resin of Denki Kagaku Kogyo Kabushiki Kaisha; having a mean degree of polymerization of about 920 and containing vinylbutyral of 75% or more), 120 g of PLYOPHEN 5010 (resol type phenol resin of DAINIPPON INK AND CHEMICALS, Inc.), and 790 g of a mixed solvent of toluene and isopropanol at a ratio by weight of 1:2 are added and subjected to agitation by heating at 600C for about one hour to obtain a uniform solution having a viscosity of 1350 cps at 250C.

PREPARATION EXAMPLE 2 The procedure of Preparation Example 1 is repeated except that U-VAN 11 HV (urea resin of Mitsui Toatsu Chemicals Incorporated) is used instead of PLYOPHEN 5010 to obtain a uniform solution having a viscosity of 1200 cps at 250C.

Coating Process of Primer (Hereinafter primer is referred to as one prepared by diluting primer material solution with an organic solvent only or by further adding a strong acid thereto.) A primer is brushed on an iron plate surface treated by grit blasting to a degree of roughness of 50 ju, and is air-dried at room temperature for 30 minutes, then a second coating is applied thereto and air-dried for 30 minutes at room temperature. The dry coating weight of the primer is in the range of from 30 to 40 g/m2.

Adhesion Test Procedure (1) Initial Adhesive Strength A bonded specimen is allowed to stand for 7 days at 250C under a relative humidity of 55%, then the elastomer is cut to 25 mm in width and is subjected to 1 80 degree peel test at a speed of pulling of 50 mm/min to obtain the initial adhesive strength.

(2) Adhesive Strength after water immersion A bonded specimen is allowed to stand for 7 days at 250C under a reiative humidity of 55%, then the elastomer is cut to 25 mm in width and immersed into water at 250C or 400C for 7 days, and then is subjected to 1 80 degree peel test at a rate of pulling of 50 mm/min to obtain adhesive strength after water immersion.

EXAMPLE 4 To 100 g of a primer material solution obtained in Preparation Example 1 a solution prepared by diluting 1 g of 88% phosphoric acid with 100 g of methylethylketone (hereinafter referred to as MEK) is added and subjected to uniform agitation to obtain a primer. An iron plate coated with the primer in the same manner as above is preheated at 600C for 1 5 minutes, and then Hiprene Q-583 (polyoxytetramethylene glycol/diphenylmethanediisocyanate prepolymer of NCO 15%, produced by Mitsui-Nisso Corporation) and curing agent, which is obtained by adding 0.03 parts of DBTDL to 100 parts of Hiprene MC-532 (polyoxytetramethylene glycol/diol curing agent produced by Mitsui-Nisso Corporation) (the pot life of these mixture is adjusted to about one minute), are mixed at the ratio of 100 to 66 by weight by use of two pack type polyurethane casting machine (AF-206 Type of Toho Machinery Comp. Ltd.) at a discharge amount of 50 g/sec, and said mixture is poured on the said iron plate to form an elastomer of 5 mm thickness. The results of adhesion test thereof are shown in Table 2.

EXAMPLE 5 To 100 g of a primer material solution obtained in Preparation Example 1 a solution prepared by diluting 0.1 g of p-toluene sulphonic acid with 100 g of MEK is added and subjected to uniform agitation to obtain a primer.

The procedure of Example 4 is repeated by use of the primer for adhesion test.

The results of adhesion test thereof are shown in Table 2.

EXAMPLE 6 The procedure of Example 4 is repeated by use of a primer material solution obtained in Preparation Example 2 for adhesion test.

The results of adhesion test thereof are shown in Table 2.

COMPARATIVE EXAMPLES 4 AND 5 To 100 g of a primer material solution obtained in Preparation Example 1 or 2, 100 g of MEK only is added respectively and diluted therewith, and is subjected to uniform agitation to obtain a primer.

The procedure of Example 4 is repeated by use of the primer for adhesion test. The results thereof are shown in Table 2.

COMPARATIVE EXAMPLE 6 To 100 g of a primer material solution obtained in Preparation Example 1, 100 g of MEK only is added for dilution, and is subjected to uniform agitation to obtain a primer. The primer is coated on an iron plate in the same manner as above, and then is heated at 1 000C for 6 hours for baking. Then the iron plate is cooled down to 600 C, and then an elastomer layer is formed in the same manner as in Example 4. foe results df adhesion test thereof are shown in Table 2.

COMPARATIVE EXAMPLE 7 To 100 g of a primer material solution obtained in Preparation Example 2, 100 g of MEK only is added for dilution and is subjected to uniform agitation to obtain a primer. The primer is coated on an iron plate in the same manner as above, and then heated at 1 000C for 6 hours and further at 1 500C for one hour for baking. The iron plate is cooled down to 600C, and an elastomer layer is formed in the same manner as in Example 4. The results of adhesion test thereof are shown in Table 2.

EXAMPLE 7, COMPARATIVE EXAMPLES 8 AND 9 Procedures of Example 4, Comparative Example 4, and Comparative Example 6 are repeatedly for adhesion test by use of Chemlock #218 (modified polyvinylbutyral primer having a solid content of 20% and equal to the primer material solution of the present invention, produced by Hughson Chemical Co.) instead of a primer material solution obtained in Preparation Example 1 or 2. The results of adhesion test thereof are shown in Table 2.

TABLE 2

Adhesive strength after water immersion kg/cm Initial After immer- After immer Composition Heat treatment adhesive sion into sion into of after primer strength water at 25 C water at 40 C elastomer Primer coating kg/cm for 7 days for 7 days Example 4 Hiprene Q-583 One prepared by adding a None 10.0 6.0 4.9 solution obtained by diluting Hiprene MC-532 1 g of 88% phosphoric acid with 100 g of MEK to 100 g of a primer material solution obtained in Preparation Example 1.

Example 5 the same as One prepared by adding a None 9.5 6.3 5.5 above solution obtained by diluting 0.2 g of p-toluene sulfonic acid with 100 g of MEK to 100 g of a primer material solution obtained in Preparation Example 1 Comparative the same as One prepared by adding a None 6.1 less than less than Example 4 above 100 g of MEK only to 100 g 1.0 1.0 of a primer material solution obtained in Preparation Example 1 Comparative the same as the same as above Heating for 6 12 8.6 7.9 Example 6 above hours at 100 C for baking TABLE 2 (Continued)

Adhesive strength after water immersion kg/cm Initial After immer- After immer Composition Heat treatment adhesive sion into sion into of after primer strength water at 25 C water at 40 C elastomer Primer coating kg/cm for 7 days for 7 days Example 6 Hiprene Q-583 One prepared by adding a None 6.1 5.9 4.8 solution obtained by diluting Hiprene MC-532 1 g of 88% phosphoric acid with 100 g of MEK to 100 g of a primer material solution obtained in Preparation Example 2 Comparative the same as One prepared by adding 100 g 1.7 less than less than Example 5 above of MEK only to 100 g of a 1.0 1.0 primer material solution obtained in Preparation Example 2 Comparative the same as the same as above Heating for 6 7.3 5.7 4.4 Example 7 above hours at 100 C, and further for one hour at 150 C for baking Example 7 the same as One prepared by adding a None 11.0 9.0 6.8 above solution obtained by diluting 1 g of 88% phosphoric acid with 100 g of MEK 100 g of Chemlock #;218 TABLE 2 (Continued)

Adhesive strength after water immersion kg/cm Initial After immer- After immer Composition Heat treatment adhesive sion into sion into of after primer strength water at 25 C water at 40 C elastomer Primer coating kg/cm for 7 days for 7 days Comparative Hiprene Q-583 One prepared by adding 100 g None 8.0 less than less than Example 8 of MEK only to 100 g of 1.0 1.0 Hiprene MC-532 Chemlock #;218 Comparative the same as the same as above The same treat- 13 10 8.9 Example 9 above ment as In Comparative Example 6 Example 8 the same as One prepared by adding a None 14.0 10.0 8.0 above solution obtained by diluting 1 g of phosphoric acid with 100 g of MEK to 100 g of Conap AD- 1146 Comparative the same as One prepared by adding 100 g None 10.0 less than less than Example 10 above of MEK only to 100 g of Conap 1.0 1.0 AD-1146 Comparative the same as the same as above The same treat- 15 11 9.5 Example 11 above ment as in Comparative Example 6 EXAMPLE 8, COMPARATIVE EXAMPLES 10 AND 11 Procedures of Example 4, Comparative Example 4, and Comparative Example 6 are repeated for adhesive test by use of Conap AD-1 146 (modified polyvinyl butyral primer having a solid content of 25% and equal to the primer material solution of the present invention, and produced by Conap Incorp.) instead of a primer obtained in Preparation Example 1 or 2. The results of the adhesion test are shown in Table 2.

As shown in Comparative Examples 6, 7, 9, and 11, of Table 2, a primer is set thoroughly by being heated at a high temperature after coating the primer, so that an adhesive force is retained satisfactorily after water immersion test.

However, as shown in Comparative Examples 4, 6, 8, and 10 of Table 2, when applied in cold curing without heat treatment, an initial adhesive strength shows considerable values, but water immersion test shows values less than 1.0 kg/cm.

Examples 4 to 8 show that in accordance with the present invention, the adhesive force is found to be retained satisfactorily from values in initial adhesive force and after water immersion test, and that the purpose of curing the primer at a temperature of from room temperature to 600C is attained.

Claims (6)

1. A polyurethane elastomer formed from a curable mixture comprising (1) a prepolymer, which is liquid at room temperature, prepared by reacting (i) 4 to 6 equivalents of a mixture of diphenylmethanediisocyanate and a carbodiimide-modified diphenylmethanediisocyanate which diphenylmethanediisocyanate mixture is liquid at room temperature, with (II) one equivalent of polytetramethylene glycol having a mean molecular weight of from 650 to 2000 or a mixture thereof with an aliphatic diol having a molecular weight less than 250; and (2) a curing agent comprising a mixture of one equivalent of a polytetramethylene glycol as defined above, 1 to 4 equivalents of a diol having a molecular weight less than 250 as chain-eiongating agent, and an organometallic catalyst; the said liquid prepolymer (1) and curing agent (2) being present in the mixture in a proportion such that the polyurethane elastomer has an OH: NCO equivalent ratio of from 1:1.00 to 1:1.10.

2. A method of preparing a polyurethane elastomer, which method comprises rapidly curing, at a temperature of from room temperature to 600C, a mixture of (1) a prepolymer, which is liquid at room temperature, prepared by reacting (i) 4 to 6 equivalents of a mixture of diphenylmethanediisocyanate which diphenylmethanediisocyanate mixture is liquid at room temperature with (ii) one equivalent of polytetramethylene glycol having a mean molecular weight of from 650 to 2000 or a mixture thereof with an aliphatic diol having a molecular weight less than 250; and (2) a curing agent comprising a mixture of one equivalent of a polytetramethylene glycol as defined above, 1 to 4 equivalents of a diol having a molecular weight less than 250 as chain elongating agent, and an organometallic catalyst; the said liquid prepolymer (i) and curing agent (2) being present in the mixture in a proportion such that the polyurethane elastomer has an OH:NCO equivalent ratio of from 1 :1.00two to 1:1.10.

3. A method of bonding a polyurethane elastomer given and defined in claim 1 to metal, which method includes coating the metal with a primer prepared by adding a strong acid to an organic solvent solution of a polyvinylacetal resin modified with a formaldehyde condensation thermosetting resin, forming a said rapid curing mixture given and defined in claim 1, and, during rapid curing of the said mixture to form the said polyurethane elastomer, applying the said rapid curing mixture to bond the resultant polyurethane elastomer to the metal.

4. A polyurethane elastomer according to claim 1 substantially as herein described and exemplified.

5. A method according to claim 3 of bonding a polyurethane elastomer to a metal, which method is substantially as herein described and exemplified.

6. A composite whenever produced by bonding a polyurethane elastomer to a metal by a method according to claim 3 or claim 5.