LIQUID EBONITE ANTICORROSIVE COATING FOR METALS

Abstract: A mixture comprising epoxidized liquid rubbers, amine hardeners and sulfur applied to a metal and cured produces a coating that is highly durable and provides strong anticorrosive protection to the metal. The amine hardeners (16) serve to activate and dissolve the sulfur, speed up the curing time, and also to link the rubbers (10) to each other, creating a continuous molecular matrix (22). The activated sulfur (12, 20) also links the rubber molecules together, as well as anchoring the polymer matrix (22) to the metal substrate (18) via chemical bonds. In a preferred embodiment, the formulation also includes two types of accelerators which act synergistically to further reduce the curing time to less than one hour. The addition of micronized aluminum oxides provides increased uniformity of heating during curing and results in a more uniform coating. The addition of calcium oxide together with the presence of amines and aluminum oxide provides a complete absorption of acidic gases produced during curing. The addition of high molecular butadiene nitrile rubber increases the coatings impact resistance, low-temperature flexibility, and adhesion.

Description: Liquid Ebonite Anticorrosive Coating for Metals FIELD OF THE INVENTION This invention relates generally to the creation of protective coatings for metals . More particularly, it relates to the use of amines to improve rubber vulcanization, and to anticorrosive rubber coatings designed to protect metals from aggressive action of chemicals such as alkalis, acids, salts, solvents, oils, gasoline and contaminated water. BACKGROUND OF THE INVENTION There is a pervasive and continuing need for protecting metals from corrosive chemical action, such as in metal pipes, stacks, chimneys, bridges, chemical plant constructions, ship hulls, and containers for aggressive chemicals, to name just a few. In addition to having a high resistance to chemical action, an ideal coating has certain other properties: the raw materials required to produce the coating are commercially available, inexpensive and non-hazardous ; the coating has the ability to be easily applied to the metal, e.g. by spraying, spreading, or free casting; the coating has strong adhesion to many different metals; it is strong, hard, abrasive resistant and thermostable; and the hardening process of the coating is capable of being carried out in contact with moisture, does not require extreme or long heating, and does not release toxic fumes . An ideal metal coating may have many additional properties, depending on the particular application or purpose of the coating. The most widespread anticorrosive coatings possessing many of the above properties are polyurethanes and epoxide resins (see for example, Coa ting Sys tems : A guidance Manual For Field Surveyors , American Bureau of Shipping and Affiliated Companies, 1995) . These coatings have good chemical resistance to many substances, have adhesion to metals that is satisfactory for many purposes, and have good mechanical properties. Neither polyurethanes nor epoxide resins, however, satisfy all the criteria for an ideal coating for metal. In particular, although polyurethanes have outstanding oil-gasoline resistance, a unique combination of favorable physical-mechanical properties, and strong adhesion to some metals, they are not stable under elevated temperature changes, alkaline hydrolysis, and persistent tension. Epoxide resins, although they have outstanding adhesion to some metals, do not have a satisfactory resistance to acids, certain solvents, temperature changes, and vibration. One of the most significant problems associated with both epoxide resins and polyurethanes is their susceptibility to underfilm corrosion connected with defects in the coating surface. Because these coatings are bonded to the metal only by adhesive bonding, these bonds can be broken by the introduction of moisture, solvents or other substances. As is known from rubber chemistry (Encyclopedia of Polymer Science & Technology, John Wiley & Sons, N.Y., vol 12, p.161, 1970) , solid ebonite, commonly known as hard rubber, is a polymer material with sulfur content used for vulcanization. Ebonite, like elastomeric or flexible rubber, is made from a combination of sulfur with polydienes (unsaturated rubbers containing double bonds) . The sulfur and polydienes are combined with some auxiliary additives and heated to produce vulcanization. Typical mass ratios of sulfur to rubber are 2:100 for elastomeric rubber and 40:100 for hard rubber. Due to the large degree of sulfide cross-linking formed in the vulcanization process, solid ebonite is a hard, non-flexible, plastic-like material possessed of unique chemical resistance to aggressive substances such as acids, alkalis, salt solutions, oil, and gasoline. In addition, solid ebonite has good mechanical properties. Consequently, these conventional rubbers are commonly used as materials for fuel tanks, containers for aggressive substances, and other applications. 75 In spite of these advantages, however, solid rubbers can not 76 be easily applied to metal surfaces, they release toxic fumes 77 during vulcanization, and they require a long time to harden. 78 79 More than 30 years ago liquid rubbers were synthesized. (See 80 Alan R. uxton, "The Preparation, modification and application 81 of non-functional liquid polybutadienes" , Rubber Chemistry and 82 Technol ogy, 54 (1981) 3, 596-626) Like earlier rubbers, 83 liquid rubbers are formed from compounds such as 84 polybutadiene , polyisoprene, butadiene-styrene, and butadiene - 85 nitrile. In contrast to the hard rubbers, which are made from 86 such compounds having molecular weights on the order of 87 100,000 to 500,000, the liquid rubbers are made from such compounds having molecular weights of only 2,000 to 4,000. 89 Consequently, the low molecular rubbers permit castable 90 processing by pouring, spreading, spraying, or rolling, while 91 providing similar properties as the hard rubbers after curing. 92 Liquid rubber, therefore, may be used to more easily coat 93 metal surfaces . 94 95 A liquid ebonite formulation for coating metals was developed 96 by Ju. N. Pushkarev, V. I. Anosoff, and A. L. Labutin, and 97 described in their article "A vulcanization and the properties 98 of ebonite coatings made on the base of liquid CiS-1.4- 99 polybutadiene" , Caou tchouc and Rezina , N3 , 1979 (Russian). 100 The formulation is prepared by mixing the components as shown 101 in Table 1. 102 103 TABLE 1 105 It is important to recognize that in this coating, the 106 polydiene molecules 10 are only joined together by sulfur 107 "bridges" 12 (See Fig. 1). Hence, the separate cured 108 polydiene molecules do not connect entirely into a united 109 network, but rather form fragmented structures. A fragmented no structure in conventional cured liquid ebonite can cause non- ill uniformities in the final material. Additionally, being a 112 heterogeneous system, this liquid ebonite formulation can not 113 be transformed into a perfect uniform material. The existence 114 of non-uniformities in the conventional cured liquid ebonite 115 is likely a primary contributing factor to its insufficient 116 durability under repeated loading and/or impact, making it 117 unsuitable for certain important applications. 118 119 Another disadvantage of the prior art liquid ebonite is the 120 long time required for curing. It requires 5-6 hours to cure 121 at temperatures of 150 C to 160 C. An invention sharply 122 reducing the curing time was filed by L. Lubomirsky, 123 M. Rozanoff and the present inventor in a Russian patent 124 application N4.813.508 / 05/ 014085 entitled "Method for 125 Producing Ebonite Coating" filed 31 January 1991 and published 126 22 October 1991. The inventors demonstrate that the curing 127 time can be diminished to a few minutes. However, special 128 devices (e.g., high frequency current generators) are required 129 for its realization. 130 131 The curing process of liquid ebonite has also presented 132 additional problems in the past . During the process volatile 133 and toxic products are released , including hydrogen sul f ide . 134 Moreover , the release o f gases during curing produces 135 microcavi t ies which reduce the s trength and protec tive 136 propert ies o f the c oat ing . Furthermore , s inc e many 137 applications require fas ter hardening times , these coatings 138 are of limited use . In addition , such a long contact with 139 heated metal of ten causes local destruction of the polymer 140 coating , again weakening its protective properties . 141 142 Another disadvantage of prior liquid ebonite coatings lies in 143 their suboptimal strength of adhesion to metal. Although the 144 coatings adhere to some degree to the metal, the strength is 145 not outstanding. 146 147 OBJECTS AND ADVANTAGES OF THE INVENTION 148 In summary, although some coatings in the prior art may have 149 some of the properties desired of an ideal coating, none 150 provides all the desired properties. Specifically, the 151 desired conditions needed to improve on the coating properties 152 of prior art liquid ebonite should incorporate the following: 153 154 1. The structure of the cured ebonite would not be 155 fragmented, but rather a united three-dimensional matrix. 156 2. The initial liquid formulation would be more homogeneous . 157 3. The release of gases, most importantly hydrogen sulfide, 158 would be minimized or eliminated entirely. 159 4. The flexibility and impact-resistance of the final product 160 would be improved significantly. 161 5. The time of curing would be reduced from 5-6 hours down to 162 less than 1 hour. 163 6. The bonding of sulfur with metal as a result of the 164 chemical reaction would be more effective. 165 166 There is therefore a current need for a coating for metals 167 that overcomes the disadvantages in the prior art, addressing 168 these above specific points. 169 170 Accordingly, it is a primary object of the present invention 171 to provide an improved method for protectively coating a 172 metal. More specifically, it is an object of the present 173 invention to provide a coating for metals that gives the metal 174 a strong resistance to acids, certain solvents, temperature 175 changes, and vibration. Further objects of the invention are 176 to provide a coating for metals which has a strong resistance 177 to underfilm corrosion, whose raw materials are readily 178 available, inexpensive and non-hazardous, which may be easily 179 applied to the metal, and which has a strong adhesion to many 180 different metals. Additional objects of the present invention 181 are to provide a coating for metals which is strong, hard, 182 abrasive resistant and thermostable, and whose hardening 183 process can be carried out in contact with moisture, does not 184 require extreme or long heating, and does not release toxic 185 fumes. These and other objects and advantages will become 186 apparent from the following description and accompanying 187 figures. 188 189 SUMMARY OF THE INVENTION 190 An aspect of the present invention is based on the surprising 191 and novel combination of two phenomena. On the one hand, 192 liquid rubbers use sulfur in the vulcanization process. On 193 the other hand, sulfur forms bonds with most metals and their 194 oxides, e.g. the ferric oxides, or rust. (See for example, 195 Cotton and Wilkinson, Advanced Inorgani c Chemi s try : A 196 Comprehensive Text , 3rd edition, Interscience Publishers, John 197 Wiley & Sons, Inc., p.427-430, 1972.) The use of liquid 198 rubber to coat metal, therefore, introduces the possibility 199 for a chemical reaction between a sulfur in the coating 200 material and the metal or metal oxide that is being coated. 201 Because chemical bonds are substantially stronger than 202 adhesive bonds, a liquid rubber coating for metal has the 203 potential to provide significantly increased protection. The 204 present inventor appears to be the first to have recognized 205 this possibility. 206 207 Moreover , the inventor has di scovered new and us e f ul 208 techniques that reali ze such a poss ibil ity and attain the 209 above obj ects and advantages . An essential aspect of the 210 invention is the application of the activation of sul fur by 211 amines to the process of producing rubber coatings for metal . 212 The presence of amines breaks the closed-ring structure of 213 virgin sulfur , producing a highly reactive linear sulfur chain 214 dissolved in the amines . Because the sulfur is dissolved, the 215 formulation is very homogeneous. In addition, because the 216 sulfur is activated by the amine, it very easily forms bonds 217 between many of the epoxidized liquid rubber molecules and 218 anchors these clusters to the metal substrate, thereby 219 reducing the curing time. In addition to activating and 220 dissolving the sulfur, the amines also react with the 221 epoxidized rubber, linking together the clusters to form a 222 uniform and continuous matrix. The result is a unified 223 polymer coating that is chemically bonded to the metal 224 substrate. The coating comprises an epoxidized liquid rubber, 225 an amine, and sulfur. 226 227 In a preferred embodiment, the epoxidized liquid rubbers are 228 epoxidized polydienes, and the amines are polyamines. The 229 addition of butadiene-nitrile rubber to the formulation 230 introduces a high-molecular weight polymer "nerve" structure 231 to the coating that increases its impact resistance, low- 232 temperature flexibility, and adhesion to the metal. Since 233 butadiene-nitrile rubber is not compatible with hydroxylated 234 polydienes nor with non-functional polydienes, it was not 235 possible to make this addition to prior art formulations . 236 Butadiene-nitrile rubber is, however, compatible with 237 epoxidized low-molecular weight polydienes, allowing the 238 present invention to enjoy these advantageous properties. 239 240 The addition of micronized aluminum oxide (which has 241 outstanding heat conductivity) improves the temperature 242 uniformity during heating, and also assists in the thorough 243 mixing of the other components of the mixture. Another 244 additive, calcium oxide, absorbs hydrogen sulfide and the 245 sulfur oxides. This action of calcium oxide enhances the 246 similar absorbing properties of the polyamines and aluminum 247 oxide. Two accelerators (2-mercapto benzothiazole and di- 248 phenyl-guanidine) present in the mixture act synergistically 249 to dramatically reduce the curing time. An activator (zinc 250 oxide) helps initiate vulcanization, a thixotropic agent (cab- 251 o-sil) makes the formulation easier to apply to the metal 252 surface, and a thinner (white spirit) is sometimes used, 253 depending on the specific end use of the coating. 254 255 A method for protecting a metal using a coating of the present 256 invention includes dissolving sulfur in a liquid mixture 257 comprising an epoxidized liquid rubber and an amine, applying 258 the mixture to the metal to form a coating, and heating the 259 coating until it hardens. In a preferred embodiment, the 260 mixture contains additional components as described above. 261 The relative proportions of the components in the preferred 262 embodiment are as follows: epoxidized liquid rubber, 100 mass 263 parts; sulfur, 30-35 mass parts; aluminum oxide, 5-10 mass 264 parts; zinc oxide, 5-6 mass parts; cab-o-sil, 2-10 mass parts, 265 polyamine, 2-6 mass parts; di-phenyl-guanidine, 2-3 mass 266 parts; 2-mercapto benzothiazole, 2-3 mass parts; butadiene- 267 nitrile rubber, 1-2 mass parts. In alternate embodiments, the 268 relative proportions of these components are adjusted to suit 269 the particular application. In other alternate embodiments 270 one or more of the additional components are not present in 271 the formulation. 272 273 DESCRIPTION OF THE FIGURES 274 Fig. 1 shows rubber molecules bonded to each other by sulfur, 275 as is the case in conventional rubber vulcanization. 276 Fig. 2 shows a liquid rubber molecule with epoxy groups, as 277 used in the present invention. 278 Fig. 3 shows two liquid rubber molecules bonded to each other 279 by sulfur according to the present invention. 280 Fig. 4 shows a pair of rubber molecules bonded to another pair 281 of rubber molecules by an amine molecule, according to the 282 invention . 283 Fig . 5 shows a polymer matrix of rubber molecules , amines , and 284 sul fur bonded to a metal subs trate according to the 285 invention . 287 DETAILED DESCRIPTION 288 In a preferred embodiment of the present invention, liquid 289 epoxidized rubbers containing the epoxies are used in forming 290 a protective coating. The examples of epoxidized low 291 molecular weight liquid rubbers include Ricopoxy polymers 292 (Ricon) and Poly BD-600 or -605 (Elf Atochem North America ) , 293 which are commonly called the oxiranes (see Wheelock, C. E., 294 Industrial Eng. Chem . vol. 50, N3 , p.299-304, 1958). These 295 rubbers are substitutes for the non-functional or hydroxyl- 296 containing derivatives used in the prior art. The 297 consequences of this substitution are significant and provide 298 many advantages, the most important of which are listed below. 299 300 1. As is well known, the epoxies react with amines (amine 301 hardeners) vigorously (see Epoxy Resins : Chemis try & 302 Technology, eds . Clayton A. May and Yoshio Tanaka, Marcell 303 Dekker, Inc., N.Y., 1973, ch. 3-4). The amines consequently 304 act as links between the molecules of liquid epoxidized 305 rubber, resulting in the formation of a distributed matrix or 306 network from the separate epoxidized rubber molecules and the 307 clusters thereof. The formation of such a network provides 308 the coating with desirable physical, chemical and mechanical 309 properties. In particular, the continuous network has a 310 higher resistance to penetration by aggressive agents. 311 312 2. The epoxidized derivatives of the present invention have 313 an increased polar character. Therefore, they are compatible 314 with more polar substances, for example with butadiene-nitrile 315 rubber (BNR) . Thus, in contrast with the prior art, it is 316 possible to dissolve a small amount (1 % to 5 %) of BNR in the 317 epoxidized liquid rubber, which improves the flexibility, 318 pliability, adhesion to metal, durability, and thixotropy 319 properties, the last of which improves the ease of application 320 to metal surfaces. 321 322 3. Epoxies react with hydrogen sulfide. Consequently, rather 323 than releasing hydrogen sulfide produced by the sulfur 324 vulcanization reaction, the hydrogen sulfide bonds with the 325 epoxies and reduces or eliminates these toxic emissions. 326 327 The present invention is also characterized by the use of 328 amines to activate and dissolve sulfur in a vulcanization 329 process. Although it was previously known that sulfur can be 330 dissolved in certain low molecular weight amines, the inventor 331 has discovered that sulfur may be dissolved in and activated 332 by amine hardeners for epoxies and used to dramatically affect 333 a vulcanization process, providing a metal coating with many 334 important advantages, the most prominent of which are 335 discussed below. Examples of amine hardeners for epoxies are 336 the various types of Unirez (Union Camp Chemical Co), 1,3- 337 meta-xylene diamine (MXDA) , 1, 3-bis (aminomethyl) cyclohexane 338 (Mitsubishi Gas Chemical Companies), and poly (oxypropylene) 339 amines (Jeffamines, Huntsman Corporation). 340 341 1. As a result of the dissolving of sulfur, the formulation 342 becomes much more homogeneous. Consequently, the reaction 343 between the sulfur and liquid rubber is improved. Moreover, 344 the homogeneous distribution of sulfur results in a more 345 uniform and perfect three-dimensional structure after curing. 346 347 2. Significantly, the dissolving is actually not only a 348 physical process, but a chemical reaction as well that serves 349 to activate the sulfur. In the virgin state, sulfur has the 350 form of an 8-membered ring. The amines react with the sulfur 351 by breaking this ring, transforming the sulfur ring into a 352 very active linear chain. Consequently, the dissolving of the 353 sulfur in the amine hardener activates the curing process, as 354 well as activating the reaction of the sulfur with the metal 355 substrate (see point 4 below) . Since the polyamine hardener 356 not only activates the sulfur, but also acts to link the 357 epoxidized rubber fragments into a large matrix, it plays two 358 important roles in producing the coating. 359 360 3. Since amines are good absorbents of acidic gases, such as 361 hydrogen sulfide and sulfur oxides, the use of amines also 362 reduces the amount of toxic fumes which may be released in the 363 vulcanization process, as mentioned before. 364 365 4. Since activated dissolved sulfur is more reactive with 366 metals than its virgin counterpart, it creates a chemical 367 interface bonding the polymer matrix to the metal substrate. 368 This intermediate metal-polymer layer plays an especially 369 important role in preventing chemical corrosion (to be 370 discussed further) . 371 372 The preferred embodiment of the present invention is also 373 distinguished by the use of two different types of 374 accelerators . These accelerators used together result in a 375 synergistic effect which provides significant acceleration of 376 the curing process. 377 378 The inventor has also discovered that the addition of aluminum 379 oxide to the formulation provides notable advantages, namely: 380 381 1. Aluminum oxide increases the heat conductivity and 382 improves the temperature uniformity during the heating 383 process. Consequently, the quality of the coating is 384 improved. 385 386 2. Since Aluminum oxide is an abrasive material, the 387 effectiveness of the mixing process is enhanced by its 388 presence. 389 390 3. Aluminum oxide, together with calcium oxide, is an 391 effective absorbent system for gases released during 392 vulcanization. 393 394 In the preferred embodiment, the coating formulation is 395 produced by mixing the compounds listed in Table 2 in the 396 relative amounts indicated. 397 398 In contrast with Table 1, it is seen that the formulation of 399 the present invention differs in the use of epoxidized rubber 400 (Fig. 1) in place of liquid polydiene rubber with hydroxyls or 401 without functional groups. In addition, the formulation of 402 the invention contains additional components, namely, an amine 403 hardener, micronized aluminum oxide, 2-mercapto benzothiazole, 404 and butadiene-nitrile rubber. This unique formulation leads 405 to important properties of the resulting coating, as will now 406 be further described. 407 408 TABLE 2 409 410 As shown in Figs. 2-4, liquid rubber molecules 10 with epoxy 411 groups 14 (Fig. 2) react with short activated sulfur chains 12 412 to form cross-linked clusters of rubber molecules (Fig. 3). 413 These rubber molecule clusters are further bonded together 414 (Fig. 4) by amines 16 to form a unified matrix. Typically, 415 only part of the total amount of sulfur present in the 416 formulation is used to form links between the rubber 417 molecules. The remaining excess sulfur is dissolved in the 418 sulfidized polymer as a result of the strong chemical affinity 419 between the excess sulfur and the sulfur in the links. These 420 sulfur bonds remain strong at temperatures up to the melting 421 point of sulfur, i.e. up to more than 120 C. 422 423 On the other hand, at elevated temperatures the excess sulfur 424 can react with all metals as well as with metal oxides. 425 Consequently, as shown in Fig. 5, strong chemical bonding can 426 be produced between the polymer matrix and a metal substrate 427 18, with the sulfur acting as an intermediary anchor 20 428 between the polymer matrix 22 and the metal 18. When applied 429 to the metal surface in liquid form, the presence of a 430 homogeneous distribution of dissolved sulfur is capable of 431 producing a large number of anchors 20, resulting in a high 432 density of polymer-metallic bonds. Consequently, the polymer 433 coating is very strongly bonded to the metal substrate via 434 this interface of anchors, resulting in an extremely high 435 degree of durability and chemical resistance. Because the 436 polymer-metal bond is chemical in nature, it is far superior 437 to merely adhesive bonds which can be easily destroyed by 438 temperature changes, vibration, chemicals, etc. 439 440 The metal-polymer interface can play another important role in 441 anticorrosive protection. As it is known, to prevent 442 electrochemical corrosion an electrically conductive primer 443 must be applied to the metal before a separate main polymer 444 coating is spread. For example, the electric semi-conductive 445 polymer polyaniline is used as a primer to prevent the above 446 mentioned type of corrosion ( Chemical & Engineering News, pp. 447 17, 44-45, July 1996). The inventor, on the other hand, has 448 found that the metal-polymer interface formed during the 449 liquid ebonite hardening has semiconductive properties. Thus, 450 in this case, the metal-polymer interface is effectively a 451 semiconductive "primer" that is automatically formed between 452 the metal substrate and polymer coating matrix. The automatic 453 formation of this semiconductive interface is a very important 454 advantage of the invention, effectively promoting its status 455 to a universal protective coating not requiring the use of a 456 separate primer coating. As mentioned before, the 457 introduction of polyamine hardener in the liquid ebonite 458 formulation enhances the semi-conductive layer formation by 459 activating the sulfur and also enhances anticorrosion 460 protection, especially protection from electrochemical 461 corrosion, through the creation of bonds between the ebonite 462 molecules. 463 464 Since the sulfur bonds to both metals and to metal oxides 465 (including ferric oxide, or common rust) , the coating has the 466 unique and highly desirable property that it can be applied to 467 metal surfaces which are partly oxidized. This remarkable 468 feature eliminates the need to thoroughly clean the metal 469 substrate of all oxides prior to application of the coating. 470 Since such cleaning represents a large portion of the expense 471 in traditional methods of coating metals, this feature 472 dramatically reduces the cost and complexity of protecting 473 metal surfaces. 474 475 Moreover, since grease and oil are readily dissolved in the 476 liquid ebonite formulation, these impurities also need not be 477 removed from the metal surface prior to the application of the 478 coating. Because greases and lubricants have a similar 479 chemical nature as the liquid rubbers (i.e. a hydrocarbon 480 structure) , both mentioned components are soluble in the 481 liquid rubber formulation, absorbing and dissolving the 482 remnants of grease from the metal surface. Although this 483 capability is present in the case of non-polar non- functional 484 liquid rubbers, the inventor has discovered that it is 485 enhanced in the case of epoxidized liquid rubbers due to their 486 polar character. Grease spread over a metal surface and 487 exposed to the air may be transformed from a non-polar virgin 488 product into slightly polar produce due to its oxidation. 489 Since epoxidized rubbers, in contrast to the prior art 490 rubbers, are slightly polar products also, the coating of the 491 present invention has an improved capability for absorbing and 492 dissolving surface oils and other similar compounds, thereby 493 significantly increasing the protective properties of the 494 coating. 495 496 Due to the nature of the formulation of the present invention, 497 it is possible to easily vary certain properties of the 498 coating by the simple adjustment of the ratio of components. 499 For example, the flexibility and impact resistance can be 500 enhanced by reducing the amount of sulfur or increasing the 501 amount of butadiene-nitrile rubber. 502 503 Another important notable feature of the present invention is 504 its simplicity of manufacture and application. The components 505 required for the formulation are commercially available and 506 can be simply mixed together without a large temperature rise, 507 thereby preventing a premature vulcanization (so called 508 "scorching") . In a preferred embodiment, an effective method 509 to mix all components is as follows. First, each powder-like 510 component except sulfur is separately mixed with an amount of 511 liquid rubber, creating a few master batches. In another 512 master batch, a portion of the sulfur is dissolved in the 513 amine. Then these separate master batches are mixed together 514 with the remaining amount of liquid rubber which has the 515 remaining portion of sulfur dissolved in it. Obviously, this 516 is one of many possible methods of mixing the components to 517 produce the coating of the invention, and such methods will be 518 evident to those skilled in the art upon reading the present 519 description. 520 521 After all the components of the formulation are mixed, the 522 resulting liquid is applied to a metal surface by any of many 523 common techniques such as spreading, brushing, rolling or 524 spraying. As explained previously, it is not necessary to 525 completely clean the metal or otherwise treat it with any 526 preliminary chemicals or agents. The thickness of the applied 527 layer of the liquid formulation is preferably in the range of 528 approximately 0.1 mm to 0.3 mm. A layer less than 0.1 mm 529 thick can still provide protection but is typically difficult 530 to apply. A layer more than 0.3 mm thick will provide 531 protection but is not necessary and wastes the formulation. 532 533 After the liquid formulation has been applied to the metal, it 534 is heated by any of many common techniques. For example, the 535 coated metal may be convectionally heated, e.g. by baking in 536 an oven, radiatively heated, e.g. by exposure to infrared 537 radiation, or electrically heated, e.g. by passing electric 538 current through it. Depending on the size of the object and 539 the type of heating used, the coated metal is heated at 540 different temperatures for differing periods of time. For 541 example, it is sufficient to heat at 180 C to 190 C for 10 542 min., or at 160 C for 40 min. Typically the coating changes 543 its visible appearance upon hardening, indicating that the 544 curing process is complete. In particular, the formulation 545 initially has a slightly brown color, which changes to an 546 intensive brown and finally to black when completely hardened. 547 In the case where aluminum oxide is replaced by another heat 548 conductive oxide (such as titanium dioxide) the final color of 549 the hardened coating may be different (brown in the case of 550 titanium dioxide) . 551 552 The total curing time for the present formulation is between 553 10 min. and 60 min., as compared with 5 hours to 6 hours for 554 prior art formulations. This substantial reduction in curing 555 time, due to the amine hardeners, the synergistic effect of 556 two accelerators, and the presence of calcium oxide and 557 aluminum oxide, dramatically simplifies and quickens the 558 process of applying the coating to a surface. Moreover, the 559 time for curing is reduced further by the increased efficiency 560 of heating due to the presence of aluminum oxide. This 561 formulation also has the important benefit that the acidic 562 gasses, such as hydrogen sulfide and sulfur oxide, are 563 absorbed by chemical reactions with the epoxy groups of the 564 liquid rubber and the alkaline agents, and are not released 565 during the curing process as in the prior art. 566 567 The coating produced f rom the present f ormulation has very 568 high impac t res i s tance , improved thixotropy , and good 569 pl iabi l i ty due to the presence of high-molecular-weight 570 butadiene-nitrile rubber ( BNR) , which acts as a long backbone 571 to the polymer structure . 572 573 Significantly, the present coating not only provides strong 574 resistance to many different types of corrosion, but it also 575 provides such protection to a wide range of metals. For 576 example, this coating successfully protects many kinds of 577 steels (including stainless steel), bronze, brass, zinc, 578 aluminum, and cast iron. Moreover, because the polymer - 579 metallic interface of the coating provides outstanding bonding 580 strength between these metals and the polymer matrix, this 581 coating can be used as an effective primer for coatings 582 composed of other kinds of polymers, for example, epoxies. 583 584 ILLUSTRATIONS AND EXAMPLES 585 To supplement the foregoing disclosure, the following examples 586 are provided to illustrate specific aspects of the invention 587 and particular techniques useful for making various coatings 588 according to the invention. 589 590 To illustrate the dissolving of sulfur in amines, one may mix 591 3 grams of sulfur powder with 6 grams of the polyamine 592 hardener-amino-amide curing agent UniRez 2810 at ambient 593 temperature. Immediately, the yellow color of the sulfur 594 changes to a bright orange, which gradually changes to brown 595 as the sulfur is activated and dissolves. This exothermic 596 reaction produces a temperature increase from 21 C to 35 C. A 597 viscous and homogeneous brown liquid results. Additional 598 sulfur may be dissolved in the liquid, albeit with increasing 599 difficulty as the liquid becomes highly viscous. If the amine 600 hardener meta-xylene-diamine (MDXA) is used instead of UniRez, 601 however, it is possible to dissolve up to 6 grams of sulfur 602 into 6 grams of hardener while still maintaining relatively 603 low viscosity. In this case the temperature rises to 41 C. 604 Another alternative is the use of the amine hardener 1,3-bis- 605 (aminomethyl ) -cyclohexane (1,3-BAC), which permits the 606 dissolving of up to 8 grams of sulfur in 6 grams of hardener. 607 In this case the temperature of the mixture rises to 43 C. 608 Those skilled in the art will appreciate that other amine 609 hardeners may be used to similarly dissolve and activate 610 sulfur, e.g. Jeffamine T-403 and the polyamine described in 611 U.S. Pat. No. 5,175,231. 612 613 To illustrate the formation of liquid ebonite solution, one 614 may mix 100 grams epoxidized low-molecular-weight 615 polybutadiene (Ricopoxy 30 from Ricon Resin, Inc.) with 35 616 grams sulfur powder. Such polybutadiene preferably has 617 viscosity 4500 cp and 6 % oxirane oxygen content. After 618 mixing for ten minutes, 10 grams of aluminum oxide (air 619 abrasive powder no. 3 from S. S. White Pennwalt Equipment and 620 Chemical Products Corporation) and 2 grams of UniRez are 621 added. After mixing for 40 minutes the formulation should be 622 absent any visible inhomogeneities , even under a microscope. 623 Several components may then be added: 5 grams zinc oxide 624 (obtained from LabPro , Inc.), 3 grams calcium oxide (from 625 American International, Inc.), 3 grams 1 , 3-di-phenylguanidine 626 (Vanax DPG from R. T. Vanderbilt Co., Inc.), 2 grams 2- 627 mercaptobenzothiazole (from Eastman Organic Chemicals), and 628 1.5 grams Butadiene-Nitrile rubber (Chemigum P 612 from 629 Goodyear Chemicals) . Preferably, the rubber is first 630 dissolved in a 15 % ethyl acetate solution. The ethyl acetate 631 can be removed under vacuum after the rubber is mixed 632 thoroughly. In order to achieve complete homogeneity, the 633 solution should be mixed for at least three hours. 634 Preferably, the temperature during mixing is maintained near 635 45 C. The final formulation should be slightly brown with a 636 viscosity near 510 poises. If the formulation is intended for 637 coating a vertical metal surface, 5 grams of amorphous fumed 638 silica (Cab-0-Sil grade M-5 from Cabot Corporation) can be 639 added. 640 641 It may be observed that if the above formulation is prepared 642 without the addition of Unirez , microscopic nonhomogeneities 643 remain present , which would lead to imperfections and reduced 644 protective properties . The amine hardener , therefore , plays a 645 demons trable role in improving the homogenei ty o f the 646 f ormulat ion . It may also be observed that wi thout the 647 addition of aluminum oxide , complete homogeneity is requires 648 20 hours of mixing , which is four times longer than the time 649 required wi th the aluminum oxide . The aluminum oxide , 650 theref ore , also ass is ts in the creation of a homogeneous 651 mixture . 652 653 Rather than mixing the primary ingredients together all at 654 once, as described above, they may alternatively be mixed in 655 preliminary batches that are subsequently mixed. In 656 particular, a first master batch is prepared by mixing 10 657 grams of aluminum oxide in 10 grams of Ricopoxy 30. A second 658 master batch is prepared by mixing 5 grams of zinc oxide in 5 659 grams Ricopoxy 30. A third master batch is prepared by mixing 660 2 grams of mercaptobenzothiazole in 2 grams of Ricopoxy 30. A 661 fourth master batch is prepared by mixing 3 grams of di- 662 phenylguanidine in 2 grams of Unirez, and a fifth master batch 663 is prepared by mixing 35 grams of sulfur in 80 grams of 664 Ricopoxy 30. The master batches are then mixed together for 665 10 minutes, after which a solution of Chemigum and Cab-O-Sil 666 is added and the formulation is mixed for 30 minutes. 667 668 As an alternative, the above formulation may be prepared with 669 epoxidized polybutadiene (Poly BD 600 from Elf Atochem North 670 America) instead of Ricopoxy 30. Another alternative is to 671 substitute titanium oxide for aluminum oxide. Still another 672 alternative is to use 1,3-BAC in place of Unirez, resulting in 673 superior thixotropic properties . Even better thixotropic 674 properties are obtained with the use of MXDA in place of 675 Unirez. These various substitutions may be made separately or 676 together . 677 678 A formulation prepared as originally described above may be 679 spread over a metal surface to form a 0.1 mm thick layer. The 680 coated metal sheets are then placed in a 160 C oven for 45 681 minutes. The cured liquid ebonite coating thus produced has a 682 2000 psi adhesion to black steel, an abrasion resistance 2 683 times better than conventional cured epoxies, a less than 1 % 684 swelling in water, in salty water, in mineral oil, and in 685 toluene, a tensile strength of 4800 psi at the break (nearly 686 twice that of cured ebonite made from non-epoxidized liquid 687 rubber) , an elongation at the break of 6 %, and a Youngs 688 modulus of 220 ksi. The improved mechanical properties of the 689 present coating can be attributed primarily to the formation 690 of the continuous polymer matrix formed from the epoxy-amine 691 bonds. 692 693 It may be observed that without the addition of Chemigum, the 694 resulting coating cracks under slow bending at angles of 40 695 degrees. In contrast, with the presence of Chemigum the 696 coating maintains integrity under slow bending at angles up to 697 90 degrees. It may also be observed that the substitution of 698 MXDA for Unirez decreases the curing time from 40-50 minutes 699 to only 20-25 minutes (at 160 C) . As an alternative or 700 supplement to baking, the coating may be cured by exposure to 701 infrared radiation. Using IR radiation can produce curing in 702 a relatively short time compared with baking. 703 704 The superior durability of the present coating may be 705 demonstrated by placing it in a 70 kHz ultrasonic cleaner. 706 After 10 hours conventional epoxies experience traces of 707 underfilm corrosion and peeling. In contrast, the coating of 708 the present invention experienced no detectable defects, 709 corrosion or peeling after over 100 hours of exposure. 710 Surprisingly and significantly, the same results are obtained 711 in the case where the coating was applied to a metal surface 712 containing grease and rust. To further demonstrate the 713 durability, metal sheets coated according to the present 714 invention were attached to a motor shaft and immersed in 715 contaminated sea water. After two hours of rotation at 3000 716 rpm, conventional coatings demonstrate observable corrosion, 717 while no trace of defects in the invent rs coating was 718 present . 719 720 It will be clear to one skilled in the art that the above 721 embodiments may be altered in many ways without departing from 722 the scope of the invention. Accordingly, the scope of the 723 invention should be determined by the following claims and 724 their legal equivalents.

Claims: 725 CLAIMS 726 727 What is claimed is : 728 1 1. A method for protecting a metal, the method comprising: 2 forming a liquid mixture comprising epoxidized liquid 3 rubber, polyamine and sulfur; 4 applying the mixture to the metal to form a coating; and 5 heating the coating until it hardens. 1 2. The method of claim 1 wherein the step of forming a liquid 2 mixture comprises mixing a combination of the sulfur and 3 the polyamine with a combination of the sulfur and the 4 epoxidized liquid rubber 1 3. The method of claim 1 wherein the step of applying the 2 mixture to the metal is adapted so that the coating thus 3 formed has a thickness between 0.1 and 0.3 mm. 1 4. The method of claim 1 wherein the heating step is 2 completed in less than one hour. 1 5. The method of claim 1 wherein the heating step comprises 2 heating the coating to a temperature in the range 150 C to 3 180 C. 1 6. The method of claim 1 wherein the liquid mixture further 2 comprises a heat conducting agent chosen from the group 3 consisting of micronized aluminum oxide and micronized 4 titanium oxide. 1 7. The method of claim 1 wherein the liquid mixture further 2 comprises an accelerator chosen from the group consisting 3 of 2-mercapto benzothiazole and di-phenyl-guanidine. 1 8. The method of claim 1 wherein the liquid mixture further 2 comprises butadiene-nitrile rubber. 9. A method for vulcanizing liquid ebonite, the method comprising dissolving sulfur in an amine hardener solution, and heating a mixture of the sulfur, amine hardener, and liquid ebonite. 10. A protected article comprising a metal and a coating covering a surface of the metal, wherein the coating is made by applying to the metal a mixture comprising epoxidized liquid rubber, a polyamine and sulfur, and heating the mixture. 11. The article of claim 10 wherein the mixture further comprises a heat conducting agent . 12. The article of claim 11 wherein the heat conducting agent is chosen from the group consisting of micronized aluminum oxide and micronized titanium oxide, and wherein the mass parts of the epoxidized liquid rubber, sulfur, polyamine and heat conducting agent are, approximately and respectively, 100, 30-35, 2-6, and 5-10. 13. The article of claim 10 wherein the mixture further comprises an accelerator chosen from the group consisting of 2-mercapto benzothiazole and di-phenyl-guanidine . 14. The article of claim 13 wherein the mass parts of the epoxidized liquid rubber, sulfur, polyamine and accelerator are, approximately and respectively, 100, 30-35, 2-6, and 2-3. 15. The article of claim 10 wherein the mixture further comprises an elastifier. 16. The article of claim 15 wherein the elastifier is butadiene-nitrile rubber, and wherein the mass parts of the epoxidized liquid rubber, sulfur, polyamine and elastifier are, approximately and respectively, 100, 30-35, 2-6, and 1-2.

Inventors: RAPPOPORT LEONID, RAPPOPORT, LEONID
Applicants: MMR TECHNOLOGIES
Application year: 1997
Main classification: B05D3
IPC codes: B05D3, B05D7, B32B9, B32B15, C08G59, C09D115, C09D163, C23C26