CA1047188A - Coating compositions based on amine-blocked disocyanate and a polyepoxide - Google Patents

Abstract

ABSTRACT
A coating composition for treating organic polymeric substrates to improve the adhesive bond between the substrate and rubber. The coating composition contains an amine-blocked diphenylmethane-4,4'-diisocyanate and a polyepoxide in a liquid carrier.

Description

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This invention relates to the treatment of shaped structures and more part~cularly to a method for treating shaped structures such as films and fibrous textile materials to improve their adhesion to rubber.
Specifically, the invention pro~ides a new and useful com-position of matter for treating shaped structures of organic polymers to improve their ability to adhere to rubber under severe flexing conditions.
Prior Art -Coatings of the type herein disclosed may be made by use of a phenol-blocked isocyanate and a polyepoxide.
Such coatings are disclosed in U. S. Patent 3,307,966, issued March 7, 1967 to Shoaf. Amine-blocked isocyanates are known, for example, in U. S. Patent 3,692,719, issued September 19, 1972 to Srail.
Statement of Invention In accordance with the present invention, a novel composition of matter, useful as a precoating for improving adherence between rubbers and substrates of organic polymers, is provided comprising a liquid carrier containing from about 0.05% by weight to about 10.0% by weight of solids of (A) a polyepoxide having an average of at least two epoxy groups in each molecule, a melting point below about 150C., an average molecular weight below about 3000 and an epoxide equivalent below about 2500 and (B) an amine-blocked diphenyl-methane-4,47-diisocyanate; the weight ratio of SA)/5B) being within the range of from about 0.01 to about 5.0, the said liquid carrier being chemically inert to each of (A)

- 2 ~

~ 4~
and (B). The liquid carrier may be a solvent Eor one or both of components (A) and (B) or either or both of (A) or (B) may be suspended in the carrier as a dispersed or emulsified phase. This composition, when applied and processed as described below provides a final shaped struc-ture bearing a coating of the reaction prod-uct of (A) with (B) constituting from about 0.2 to about 5% by weight of the coated structure. The composition of the present invention is applied to the polymeric substrate by any conventional means such as dipping, spraying, brushing, padding or the like with the structure relaxed or under tension. The wetted shaped structure is heated at a tem-perature of from about 150C to about 235C. for a per-iod of from 0.5 to about 15 minutes to remove the liquid carrier and to cure the coating. Particularly where the shaped structure is fibrous in nature, it is preferred that the structure be subjected to at least sufficient tensioll to prevent shrinkage during the wetting and subsequent curing operations. Alternatively the reactants (A) and (B) can be applied separately in either order from the same or different liquid carriers to the shaped structure instead of applying them simultaneously from the same carrier as described above.
Following formation of the polyepoxide-polyiso-cyanate reaction product on the shaped structure, rubber coatings may thereafter be applied by conventional techniques, preferably, but not necessarily, first coating with a phenol-formaldehyde rubber latex, preferably an aqueous resorcinol-formaldehyde solution and butadiene-vinyl ~7~38 pyridine latex as taught in U. S. Patent No. 2,990,313 to Knowles et al. (referred to hereinafter as an RFL
coating). Thereafter the rubber is applied. Optionally, after at least partial curing of the polyepoxide and the amine-blocked diphenylmethane-494'-diisocyanate and prior to application of rubber, the organic polymeric structure is given a second coating with a phenol-aldehyde rubber latex.
The product is a reinforced rubber shaped structure In the examples illustrating the manner in which the invention may be carried out and the advantages ob-tained, two methods of determining the strength of the adhesive bond are used:
(1) The "H-pull" test.
(2) The two-ply strip adhesion test.
The "H-pull" test is the well-known test described, for example, in India Rubber World, 114, 213-219 (May 1946), "Study of the 'H' Test for Evaluating the Adhesive Properties of Tire Cord in Natural and GR-S Rubber."
Briefly, a dipped cord is cured across the center of two small rectangles of rubber with a shor~ length of the cord exposed between the rubber pieces forming the cross-bar of the H. The pieces of rubber are gripped in an Instron Tensile Testing machine and stress is applied so that the cord is pulled out from one of the pieces of rubber. The load required is regarded as a measure of the adhesion. In the examples of the present specification, the width of the rubber pieces are reduced to 1/4 inch since greater width samples resulted in cord breaks rather than adhesive failure due to the high level of adhesion experienced with this new adhesive system.

~ ~)47~88 The two-ply strip adhesion test is carried out on a laminated structure containing two plies o parallel dipped tire cords (29 ends per inch) separaled by a thin layer of rubber. The laminar structure is built up in successive layers consisting of a cotton duck reinforcing backing, a 25 mil sheet of un w lcanized rubber, a layer of parallel cords, two layers of 25 mil rubber, another layer of parallel cords, a 25 mil layer of rubber and a final cotton duck backing. A small strip of Holland cloth or other suitable material is inserted between the center two layers of rubber at one end of the structure to prevent bonding and allow a portion of each ply to remain free for clamping when testing. The laminate is then molded, vul-canized under pressure, and cut into l-inch wide strips.
The force required to separate the two fabric plies of a l-inch wide strip is taken as a measure of the adhesion.
If desired, a woven fabric, such as used for conveyor belt reinforcement, may be substituted for the parallel cords when adhesion of fabric to rubber is being tested.
The rubber stock A referred to in the examples is a blended natural rubbsr/styrene-butadiene rubber tire skim stock of approximately 50/50 composition by weight.
In the following examples which illustrate the invention, all parts and percentages are in parts by weight unless otherwise specified.
EXAMPLE I
A solution of ~00 gm. diphenylmethane-4,4'-diiso-cyanate in 1000 ml. dry trichloroethylene is slowly added to a solution of 698 gm. dry dibutylamine in 500 mlO
dry trichloroethylene. The reaction is exothe~ic and is maintained at 70C by external cooling, Af-ter the addition the mixture is heated to 80 90C for one hour.
After cooling to room temperature the precipitated dibutylamine blocked diphenylmethyl~4~4'-diisocyanate is filtered of~, dried at ~0C and micronized.
Polyepoxide A is prepared ~rom glycerol and epichlorohydrin in accordance ~ith the disclosure o~ U.S.
Patent 2 902 398 (column 4~ lines 1-24: "Polyether A").
This material is a pale yellow liquid with an epoxide equivalent o~ about 170, a corrected hydroxyl equivalent of about 300, and a viscosity of about 1.2-2.0 poises at 25C. (This material is also available ~rom Shell Chemical Co. a3 EPON~ Resin 812).
A homogeneous solution o~ 16.0 lbs. water and 0.24 lb, sodium dioctylsul~osuccinate (sold as AEROSOL OT*
by American Cyanamid) is prepared by stirring. Additional water, 6.26 lb., at 90-lOO~F, and 7.5 lb. dibutylamine-blocked diphenylmethane-4~4'-diisocyanate are added and the mixture vigorously stirred until -the blocked isocyanate is dispersed. m e dispersion is ball-milled ~or 72 hrs., pro-ducing a white suspension conta-Lning 25 w-t. ~ blocked isocyanate.
A coating composition (hereina~ter referred -to as the Primary Coating Composition) (4.6~ solids) is prepared by adding 1.29 lb. of the above slurry to 8.37 lbs. o~ wa-ter containing 0.20 lb. 2~ aqueous gum tragacanth solution~ Then 0.14 lb~ polyepoxide A is added and the composition is ready for use.
A resin master is prepared by adding 2024 pounds o~ 1.7~ aqueous sodium hydrox-tde to 27~8 pound~ of water * denotes trade mark .~

at 75-78F~ stirring for one minute, adding l.38 pounds o~ resorcinol flakes slowly to the resulting solution, stirring for five minutes, adding 2.02 pounds of 37~0 aqueous formaldehyde, stirring for two minutes, stopping the mixing and aging the resin master at 75 to 78F ~or six hours, The pH is 7.0 to 7.5.
A latex composition is prepared by adding 7.64 pounds of water at 75-78~ to 30.5 pounds of a ~l wt.
l,3-butadiene/styrene/2-vi~ylpyridine latex (sold as GEN-TAC* by General Tire and Rubber Co.) adjusting the temperature to 45~5OF~ adding l~42 pounds of 28~ aqueous ammonium hydroxide, and stirring slowly for five minutes.
The resin master (33.41~ pound.s) is added to the latex co~position wi-th slow mixing and after the addition mixing is continued for three more minutes. This mixture is aged for 12 hours at 45~50FO Its pH is then lO.0 to 10.4. The composition i8 kept at 45-50F and may be used for 5 days. Thls composition is hereafter referred to as the RFL coating composition.
A commercially available polyethylene terephthal-ate tire cord h~ving a lO00 (denier)/2 (ply-twist)/2 (ply-twist) construction is dipped in Primary Coating Composition and then heated in an oven for one minute at 232C ~450F) ~mder an applied stretch of ~. The percent pick-up (dry solids) is about OD5~
The coated cord is then dipped in RFL Goating Composition and heated in an oven for one minute at 2l8C
(425F) with a relaxation of ~. The percent pick-up of this RFL coating is about 3~6.
A rubber Compound is prepared on a rubber roll mill according to the ~ollowing recipe:

* denotes -trade mark :, ~7~
~ Parts by Wel~
Natural Rubber (smoked sheet No~) 50 S~R-1500a 50 HAF carbon blackb 35 Stearic acid Zinc oxide 3 Petroleum oilC 9.37 Antioxidantd 2,2'-Dithiobisbenzothiazole Sulfure 3-13 A 15-mil 9 in. x 18 in. sheet of rubber stock is placed, backing side up, on a building drumJ the backing is removed, tire cord i5 wound onto the drum across the ~ull width using the maxlmum end count available without over-lapping the cord, the ~reshly exposed side of ~nother 15-mil 9 in. x 18 in. sheet o~ rubber stock is placed against the cords on the drum, and the assembly is stitched a _ Non-pigmented sulfur curable elastomer made b~
copolymerizing 1,3-butadiene and styrene at 6 C
continuously in a continuous water phase using emulsi-fiers, vi~cosity regulators and a redox catalyst system. It contains about 23.5~ ~tyrene by weight and has a Mooney viscosity (ML-l ~ L~/100C) o~ 52.
b _ AS~I Designation N-330.
c _ Naphthenic petroleum oil ASTM D-2226 Type 103 ha~ing Saybolt Universal Viscosity values of 2525 and 87.2 at 100F and 210F respectively, a molecular weight of 395, and a viscosity~gravity constant of 0.889.
(Sun Oills tradename ~or thiæ oil is CIRCOSOL 1~240.) d _ Polymerlzed 2,2,1~-trimethyl-1,2-dihydroquinoline, m1p~ 74C (Monsanto's tradename ~or this materlal is AGE RI~E RESIN D.) e _ Amorphous, insoluble sulfur with 20~ oil. (St&u~er Chemical~s registered trademark ~or this composition is CRYSTEX.) ~ 8 and removed from the drum. Two warps are cut, each 8-7/8 in. long and 9-7/8 in. wide.
Six 8-7/8 in. x 9-7/8 in. pieces of 125-mil rubber stock and two pieces of polyester fabric are cu~.
Two more pieces of 2 in. x lO in. polyester fabric are cut.
A laminated structure is built up in successive layers. First, two of the 125-mil rubber sheets are jo:ined (longer ends parallel). Then, successively, polyester fabric and one more 125-mil rubber sheet are positioned.
One 8-7/8 in. x 9-7/8 in. warp is placed parallel to the 9 in. dimension; the side originally next to the buildlng drum is down. Two 2 in. x 10 ~n. fabric strips are placed at both ends of the mold perpendicular to the cord direction.
The other 8-7/8 in. x 9-7/8 in. warp are placed on top and in line with the first set (the side originally next to the building drum is up). The remaining layers are added to duplicate the first ply, 2 125-mil sheets being the last to be positioned. The 2 in. x 10 in. fabric strips at the center of the pad allow a portion o each ply to remain free for clamping when testing.
The laminate 1s then molded, cured under pressure, and cut into one-inch wide strips. The force required to separate the two fabric plies of a one-inch wide strip is called the two-ply strip adhesion value.
In general the pad is cured at 6 tons to~al pressure for 60 minutes at 150C.
Two-ply strip adhesion test pads are made using the coated polye~ter tire cord and the black loaded natural rubber/SBR~1500 s~ock described above. Some pads are tested at once at 60C. [140F.]~ others are aged at 162.8C

~ 7~88 [325F.] for 8 hours. The unaged pad gave an adhesion value o~ 54 lbs./in. and the aged pad gave an adhesion value of 14 lbs./in.
"H-pull" specimens are made using the coated polyester tire cord and the black loaded natural rubber/
SBR-1500 stock described above. An unaged specimen gave a value of 35 lbs.

Example 1 was repeated, only this time 471 grams of morpholine was used in place of the dibutylamine.
Test samples fabricated using this amine-blocked isocyanate gave an unaged two ply adhesion value of 52 lbs./in., and an aged two ply adhesion value of 18 lbs./in.
Definitions of Components The term "polyepoxide" is used to describe uncured chemical compounds having an average of at least two epoxy groups, i.e., at least two --C~\C--in each molecule, a melting point below 150C., an average molecular weight below 3,000, and an epoxide equivalent below 2,500. The preferred epoxy compounds are those having a melting point below 100C., an average molecular weight below 1,500, and an epoxide squivalent below 850.
They may be saturated or unsaturated, aliphatic, cyclo-aliphatic, aromatic or heterocyclic and may con~ain sub-stituents for hydrogen such as alkyl, halogen, hydroxyl and alkoxy. Such compounds are commonly prepared by the reaction of halohydrins with polyhydric alcohols or with ~ ~ 7 ~ ~ ~
polyhydric phenols; as for example, the reaction of epi-chlorohydrin with glycerol or with bis-phenol Such com-pounds are described in "Epoxy Resins" by ;Lee and Neville, McGraw-Hill Book Company Inc., New York, 1957, pages l 21.
The term "epoxide equivalent" is the weight of resin in grams which contain l gram chemical equivalent of epoxy groups. The value is determined by the method described in Lee and Neville, page 21. Many suitable specific poly-epoxides, all of which are suitable in the present invention are listed in U. S. Patent No. 2,902,398 to Schroeder, dated September l, 1959.
The second essential component of the precoating composition of the present adhesive system is a secondary amine adduct of diphenylmethane-4,4'-diisocyanate. Compounds of this type are relatively inert, being stable in the pres-ence of water at room temperature. However, when heated to temperatures above about 100C., these compounds dissociate, yielding free isocyanate compounds capable of undergoing all the reactions characteristic of polyisocyanate compounds.
Suitable secondary amines for blocking the diphenyl-methane-4,4'-diisocyanate include those having the formula HNRlR2 wherein Rl and R2 are monovalent hydrocarbon radicals, either the same or different containing from l to 30 carbon atoms and optionally also containing here~o atoms such as O, N, and S in functional groups that do not react with isocyana~es or Rl and R2 may also form a bivalent hydrocarbon radical with the same limi~ations as above. Specific examples of sui~able amines are:

~¢~47~8 morpholine ~N
H

2,6-dimethyl morpholine the dibutylamines the dipropylamines the dipentylamines the dihexylamines the diheptylamines diethylamine diphenylamine N-ethylaniline

3-anilinopropionitrile C6H5NH(CH2)2C_ N
anilinoacetonitrile C6H5NHCH2C~ N

4-anilino-2-butanone C6H5NH(CH2)2cOcH3 N-sec-octylaniline N-butenylaniline N-cyclohexylaniline N-phenylbenzylamine N,N'-diphenyl~ormamidine C6H5NHC = NC6H5 tetrahydrocarbazole anilinoacetate 2-phenylindole ~
N ~6H5 H
benzpyrrole (indole) skatole ~ CH3 ~ ~ 7 ~ ~ 8 The polyepoxide-polyisocyanate reactants are applied in a liquid carrier to the shaped structure. The nature of the carrier is not critical. Water is generally the most convenient carrier. Since the carrier acts solely as a vehicle for the reactants, the reactants need not be in solution in the carrier but may be dispe:rsed or emul-sified in it. The concentration of reactants in the carrier may vary widely but generally the use o~ compositions COII-taining from about 0.05% to about 10% by weight of reac-tants wherein the ratio of polyepoxide to amine-blocked polyisocyanate is between about 0.01 to about 5.0 is satis-factory. Pre~erably the ratio o~ reactants is between about 0.05 to about 2 3.
In addition to the polyepoxide and the amine blocked polyisocyanate adduct, the preferred aqueous pre coating composition of this invention may also contain, i~
desired, optional materials such as wetting agents, dis-persing agents, viscosity builders~ and epoxide curing agents and promoters such as a tertiary amine. For example, a dispersing agent, such as an alkyl aryl polyether alco-hol or sodium dioctylsul~osuccinate may be used to disperse a finely-divided solid isocyanate adduct in water ~or the preparation of the precoating composition. The viscosity of the coating composition may be adjusted by the addition of well-known thickening agents such as gum tragacanth, or by a compound such as the acetate salt of polydiethylamino-ethylmethacrylate which ser~es both as a viscosity builder and as an ami~e catalyst non-reactive to epoxides at room temperature but reactive at elevated temperatures.

~g)4~88 As pointed out previously, after applying the liquid carrier containing the polyepoxide-polyisocyanate reactants to the shaped structure it is necessary that reaction be made to occur between the reactants. A
temperature at least about 135C. and preferably at least about 150C. is required to promote reaction. Higher tem-peratures may be employed to hasten the reaction but usually a temperature within the range of from about 150C~ to about 235C. is preferred in the treatment of fibrous structures. The period necessary for this operation wi.ll vary widely depending upon factors such as the nature of the carrier and the temperature employed. The shorter periods are favored by higher temperatures and more volatile solvents. When treating fibrous structwres it is generally preferable to perform this "curing" operation with the structure under at least sufficient tension to prevent shrinkage. At times it is advantageous to apply su~f,icient tension to stretch the structure during this operation.
When the composition is applied as taught herein the shaped structure will pick up a coating of reactants constituting from about 0.2 to about 5% by weight of the coated structure.
Generally it is preferred to adjust concentrations and con-ditions of application to provide a coated structure con-taining reactants constituting about 0.5 to about 2.5V/~ by weight of the coated structure.
The polymeric structures bearing the cured poly-epoxide-polyisocyanate coating may be bonded to rubber in the customary manner by use of heat and pressure ~o o:rm reinforced articles in which both the dry and wet adhesion of the synthetic polymer to rubber is outstanding, and in 1U147~8 which both cold and ~ot adhesion is superior to that hitherto known. The adhesion afforded by the composition of this invention is about e~uivalent on polyester struc-tures over a broader range of application and testing conditions and in a wider variety of rubber stocks, to that obtained with other adhesive systems known. While the composition i9 particularly valuable for polyester structures~ it is to be understood that it is also suitable for other polymeric structures, such as polyamide fibers;
as well as cellulosic structures and the natural ~ibers.
The combination of polyepoxide and amine-blocked diphenylmethane-4,4'-diisocyanate is preferable to the conventional combination of a polyepoxide and a phenol or a beta-naphthol blocked diisoc~anate, in that during the c~ring cycle the amine vaporizes to a lesser extent than the phenol and beta-naphthol do; furthermore, the amines are inherently less toxic and polluting than are these other compounds.
Prior to applying the rubber coating, it is pre-ferred (but not essential) to overlay the cured polyepoxide-polyisocyanate coating with at least about 0.5/O by weight (and preferably from about 2 to about 25% by weight) of adhesive coating of a phenol-aldehyde condensate (~or example as taught in United States Patent 2,330,217 to ~Iunn dated September 28, 1943) and a butadiene-vinyl pyridine latex. Such compositions and their methods of application are well known in the art and taught in detail in United States Patent 2,990,313 to Knowles et al. dated June 27, 1961.

~4718~

The nature of the "rubber" in the final shaped structure is not critical and may be either a natural or a synthetic ~ubber. Furthermore, the technLique of apply-ing the rubber to the reinforcing structure prepared in accordance with the present invention (e.g., filmS fiber or the like~ is accomplished by conventional and well known techniques. It will be apparent to those skilled in the art that thq rubber stock applied may contain additives such as w lcanizers, fillers, pigments, antioxidants and the like.
Compositions produced according to the presen~
invention may be utilized for a wide variety of important industrial applications. They may be used, for example, in the preparation of pneumatic tires for automobiles, buses, tractors and aircraft, in transmission belts, conveyor belts, floor tiles, hoses, raincoats, luggage, and the like.
Especially valuable results are obtained when the coating composition of this invention is applied to polyester fibers and other shaped structures such as those prepared from polyethylene terephthalate. Illustrative of the polyesters useful in preparing shaped structures which may be bonded to rubber by the process of this invantion are those disclosed in United States Patents 2,465,319, 2,965,613 and 2,901,466.

Claims (5)

CLAIMS:

1. A coating composition for use in treating an organic polymeric substrate to improve the adhesive bond between the substrate and rubber, comprising a liquid carrier containing from about 0.05 wt. % to about 10.0 wt. %
of (A) a polyepoxide having an average of at least two epoxy groups in each molecule, a melting point below about 150°C., an average molecular weight below about 3000 and an epoxide equivalent below 2500, and (B) a secondary amine-blocked diphenylmethane-4,4'-diisocyanate, the weight ratio of (A)/(B) being in the range of from about 0.01 to 5.0, the said liquid carrier being chemically inert to each of (A) and (B).

2. The coating composition of Claim 1 in which the amine is selected from the class consisting of those having the formula HNR1R2 wherein R1 and R2 are monovalent hydrocarbon radicals, either the same or different con-taining from 1 to 30 carbon atoms and optionally also containing hetero atoms such as O, N, or S in functional groups that do not react with isocyanates or R1 and R2 form a bivalent hydrocarbon radical with the same limita-tions as above.

3. The coating composition of Claim 1 in which (B) is morpholine-blocked diphenylmethane-4,4'-diisocyanate.

4. The coating composition of Claim 1 in which (B) is dibutylamine-blocked diphenylmethane-4,4'-diiso-cyanate.

5. The coating composition of Claim 1 in which the liquid carrier is water.