Process for bonding polyester reinforcement elements to rubber

Polyester reinforcement fibers are treated to improve adhesion to rubber by applying a dissolved aromatic urethane coating to the fibers followed by coating the fibers with a resorcinol-formaldehyde latex and then heating the fibers sufficiently to convert the aromatic urethane to an aromatic isocyanate and applying to and curing a layer of rubber on said treated polyester fibers.

BACKGROUND OF THE INVENTION 
The invention relates to a process for bonding polyester reinforcement 
elements to rubber. 
Polyester fibers, cords and fabrics are used extensively for reinforcing 
rubber articles such as tires, belts, hoses and the like. But, polyester 
textile elements generally adhere poorly to rubber unless first treated to 
improve adhesion, as by coating. Because of the exceptional strength and 
reinforcing ability of polyester fibers there has been a continuous effort 
to improve the adhesion of polyesters to rubber. 
One widely used process for adhering polyester to rubber involves coating 
polyester with a resorcinol-formaldehyde latex followed by heat treating 
the coated polyester which is then incorporated into rubber. But, this 
system can be unsatisfactory depending on factors such as the degree of 
polyester heat treatment; the composition of the polyester substrate; the 
amount of stress and heat the reinforced rubber article undergoes; and the 
like. 
Active isocyanates, particularly the polyisocyanates, have been added to 
resorcinol-formaldehyde latexes to further improve polyester to rubber 
adhesion. But, the rapid reaction of isocyanates in water and the toxicity 
of isocyanate compounds are shortcomings to this system. To overcome these 
problems, the isocyanates have been added in "blocked" form to latexes. 
Thus, isocyanates are reacted with blocking agents, most notably phenol, 
to provide compounds which are stable at normal temperatures but which, 
upon heat treatment at above about 100.degree. C., disassociate to form a 
free isocyanate and phenol. A polyester substrate is coated with the 
mixture of resorcinol-formaldehyde and blocked isocyanate. Upon subsequent 
heat treatment of the coated polyester, the isocyanate is formed, in situ. 
This improves bonding between the latex and polyester. 
Blocked isocynates, such as alcohol or phenol blocked isocyanates, i.e. 
urethanes, are however, typically water-insoluble and must be added to the 
latex as an aqueous emulsion or as a solids dispersion. It is difficult to 
achieve and maintain a uniform dispersion of the urethane throughout the 
resorcinol-formaldehyde latex. This can lead to non-uniform adhesion of 
the polyester substrate to the rubber. 
Polyepoxides have also been added to the latex-urethane mixtures, see U.S. 
Pat. Nos. 3,234,067 to Krysiak and 3,933,677 to Auftermarsh. Since these 
systems involve emulsions or dispersions of the urethane, as above, 
uniformity of adhesion can be subject to question. 
It has been suggested to precoat polyester fibers prior to latex dipping, 
with an aqueous emulsion containing a dispersion of blocked isocyanate 
solids. (TBI 29; FORTREL.RTM. TYPE 900 POLYESTER INDUSTRIAL YARN; Celanese 
Fibers Marketing Company; October 1977, page 9). This predip does result 
in increased adhesion. But as with previous systems which employ solids 
dispersions, coating uniformity is difficult to control, and the 
availability of reaction sites is lessened with solid reactants. 
It has also been proposed to coat polyester fibrous substrates with the 
combination of a polyepoxide compound and a blocked isocyanate compound 
which is then heated causing unblocking of the polyisocyanate and curing 
of the epoxide/isocyanate coating. The coated polyester is then treated in 
a resorcinol-formaldehyde latex which may contain other materials, 
followed by a second heat treatment and bonding to rubber. Such processes 
are disclosed in U.S. Pat. Nos. 3,272,676 to Kigane et al; 3,307,966 to 
Schoaf; 3,503,845 to Hollatz et al; and Japanese Patent Publication No. 
62,269/1980 to Takada. The dual heat treatments required by these 
processes involve increased energy usage with attendant increased cost. 
Additionally, unblocking the isocyanate prior to exposing the polyester to 
resorcinol-formaldehyde bath renders the isocyanate unavailable for 
reaction with the resorcinol-formaldehyde. 
Numerous other treatments for adhering polyester to rubber have been 
proposed, but none has been completely successful in overcoming adhesion 
difficulties. There is thus a continuing need for improving processes for 
adhering polyester to rubber. 
SUMMARY OF THE INVENTION 
Following extensive experimentation, an improved process for bonding 
polyester reinforcement textiles to rubber has been found. Polyester 
fibrous substrates, such as polyester fibers, cords, fabrics and the like 
are coated with a dissolved aromatic urethane which undergoes heat 
disassociation at a temperature in the range of between about 220.degree. 
F. (104.degree. C.) and about 465.degree. F. (241.degree. C.) to form an 
aromatic polyisocyanate and which has the formula: Ar(NHCOX).sub.n ; 
wherein Ar is an organic residue containing at least one aromatic nucleus; 
X is a moiety selected from the group consisting of alkoxy, aryloxy and 
alkaryloxy and n is greater than about 2. Preferably Ar contains two or 
more aromatic nuclei. The coated polyester substrate is thereafter coated 
with a resorcinol-formaldehyde containing latex. The latex coated 
substrate is heated to a temperature in the range of between about 
220.degree. F. (104.degree. C.) and 480.degree. F. (249.degree. C.) and 
above the thermal disassociation temperature of the aromatic urethane. 
Thereupon, a layer of rubber is applied to and cured on the treated 
polyester material. 
In advantageous embodiments of the invention, the polyester substrate is 
treated, as by epoxy coating, to activate its surface prior to coating 
with the dissolved urethane. It is also preferred to provide the dissolved 
urethane in a lubricant vehicle. When the resorcinol-formaldehyde coated 
polyester is to be heat treated at a temperature of above about 
440.degree. F. (227.degree. C.), a flexibilizing agent, e.g. raw castor 
oil, is preferably included with the dissolved urethane coating. 
Polyester reinforced rubber articles provided according to the invention 
exhibit increased and more uniform polyester to rubber adhesion. The 
dissolved urethane coating is stable for long periods of time, e.g., 
several months. Thus, polyester reinforcing elements can be precoated in 
accordance with the invention; stored and then used when needed. 
Precoating can thus be preformed in a large scale operation, not limited 
to quantities which can be immediately used. This improves economics of 
the system and uniformity of bonding from batch to batch. 
DETAILED DESCRIPTION OF THE INVENTION 
The invention is applicable to any of various polyester textile 
reinforcement elements such as fibers, yarns, cords, fabrics and the like. 
The term "polyester" is used herein to mean any high polymeric linear 
ester obtained by heating one or more glycols of the series 
HO(CH.sub.2).sub.n OH wherein n is greater than 1 but not exceeding 10 
with a dicarboxylic and preferably terephthalic acid or an ester forming 
derivative thereof. The phrase "high polymeric linear esters" is used 
herein to mean polyesters which are capable of molecular orientation as 
shown by characteristic x-ray patterns, by drawing or rolling. Examples of 
ester forming derivatives of terephthalic acid are its aliphathic 
(including cycloaliphatic) and aryl esters and half-esters, its acid 
halides and its ammonium and amine salts. Examples of such glycols are 
ethylene, trimethylene, tetramethylene, hexamethylene and decamethylene 
glycols. The preferred polymer for purposes of this invention is 
polyethylene terephthalate. The improved adhesion obtained according to 
the teachings of this invention may be obtained with all polyethylene 
terephthalate polymers regardless of their carboxyl end group contents or 
diethylene glycol content. But, the invention is especially useful in 
conjunction with polyesters having a low carboxylic end group content. 
These are typically more difficult to bond to rubber. 
Both polyester reinforcing elements which have and have not been previously 
treated or coated can be treated according to the invention. 
Advantageously, the polyester element will have been previously treated to 
increase surface energy and/or to render hydrophilic the polyester 
surface. Treatments to increase surface energy involve applying a liquid 
crystalline agent to the surface of polyester fibers, preferably during 
spinning, and then drawing the fibers under conditions which inhibit 
chemical reaction between the substrate and the crystalline agent. Such 
crystalline agents include polyepoxides, polyvinylalcohols, 
polyvinylacetates and the like. Suitable processes for increasing surface 
energy are disclosed in U.S. Pat. No. 3,793,425 to Arrowsmith; U.S. Pat. 
No. 4,044,189 to Arrowsmith and U.S. Pat. No. 4,247,658 to Arrowsmith. The 
disclosures of these patents are hereby incorporated herein by reference. 
A preferred method of increasing surface energy involves applying to 
polyester filaments a spin finish which comprises an epoxide such as 
dimethylol-bisphenol-A-diepoxide, butanediol diglycidic ether, glycerol 
epoxide or sorbitol epoxide, together with a suitable basic catalyst. The 
epoxide is applied in an amount of from 0.02% to 0.5% based on the fiber 
weight followed by drawing. 
The polyester textile element can also be treated to render its surface 
hydrophilic, for example as by the application of a curable polyepoxide 
coating followed by curing or by other suitable chemical or physical 
treatments as will be known to those skilled in the art. 
The aromatic urethanes used in the invention are provided by reacting an 
active aromatic isocyanate which preferably has a functionality of between 
2.3 and 3, with an alcohol, phenol or alkylphenol. Such isocyanates will 
be known to those skilled in the art and include toluene diisocyantes, 
methylene diphenyl diisocyanates, naphthalene diisocyanates, 
polymethylenepolyphenylisocyanates, and the like. Superior results have 
been obtained using polyphenylenepolyisocyanates having an average 
functionality of 2.7 (PAPI No. 135, commercially available from Upjohn 
Polymer Chemicals) (MONDUR MRS-10, commercially available from Mobay 
Chemical Corporation). 
The blocking agent can be a lower alkyl alcohol such as propanol, butanol, 
pentanol and the like; an aromatic alcohol such as phenol; para- or 
ortho-alkyl phenols such as decylphenol, nonylphenol, octylphenol and the 
like. The para-alkyl phenols are especially preferred blocking agents 
because the monomeric urethanes formed therefrom unblock at lower 
temperatures than other blocked isocyanates. Blocked isocyanates are 
discussed in ORGANIC ISOCYANATES AND BONDING POLYMERS TO TEXTILES by H. 
Foulkes and D. L. Loan, RAPRA Technical Review No. 40 (1967) and in 
BLOCKED ISOCYANATES by Z. W. Wickes, Jr., Progress in Organic Coating, 
Volume 3, Pages 73-79 (1973). The disclosures of these are hereby 
incorporated herein by reference. 
The urethanes are applied to the polyester textile elements in an amount 
between about 0.05% to about 1.0% by weight, preferably 0.1 to 0.3% by 
weight, based on the weight of the textile, as a solution in any suitable 
organic solvent. Preferably, the urethanes are dissolved in an oily, 
non-volatile lubricating finish and applied as a topcoat. Advantageously 
the urethane will consititue from about 20% to about 40% by weight of the 
lubricating topcoat. When so applied, the total lubricating topcoat with 
urethane advantageously will be used in an amount of from between about 
0.2% by weight to about 2.0% by weight, preferably 0.4 to 1.0% by weight 
based on the weight of the yarn. Suitable lubricating agents are known to 
those skilled in the art and include palm oil, coconut oil, cottonseed 
oil, mineral oil and esters of the predominant fatty acids of said oils; 
glycerides, polyglycol mono-and di-fatty acid esters, butyl stearate, 
octyl stearate, esters of oleic acid, trimethylol propane/caprylic acid 
esters, 2-methyl-2-propyl-propane diol-1,3-dilaurate and 2-ethyl-2 
-butyl-propane diallyl-1, 3, dilauraute and the like. 
Additionally, it is preferred to include a conventional antioxidant 
compound, for example, an arylamine, an alkylene-bis-phenol, a 
thio-bis-phenol or the like, in an amount of up to about 5% by weight, 
based on the weight of the topcoat. The coating can also contain a 
catalytic amount of a conventional urethane unblocking catalyst as are 
disclosed in the previously mentioned Wickes, Jr. publication at page 79. 
Especially preferred are the tin catalysts such as dibutyltindilaurate and 
the like. 
In accordance with another aspect of the invention, the coating containing 
the dissolved urethane will also contain a flexibilizing agent. 
Flexibilizing agents are especially advantageous where the coated 
polyester element is heat treated prior to incorporation into rubber, at a 
temperature greater than about 440.degree. F. (227.degree. C.). Such high 
temperature heat treatments are typically conducted on polyester elements 
that are incorporated into tires and especially truck tires. Flexibilizing 
agents which can be used in accordance with this aspect of the invention 
include internal and external plasticizers for polyurethane resins. These 
materials are known to those skilled in the art and include raw castor 
oil, polyether polyols such as polypropylene glycol and polyethylene 
glycols, polyester polyols and hydroxy terminated polyester polyols such 
as those based on the phthalic esters, carpolactone, and the like. 
Inclusion of flexibilizing agents can overcome increase in stiffness and 
strength loss which can be encountered when polyester fibrous elements 
treated in accordance with the invention, are heat treated at temperatures 
above 440.degree. F. (227.degree. C.) prior to incorporation into rubber. 
The flexibilizing agents are advantageously included in an amount of 
between about 5 and about 50% by weight, based on the weight of the 
urethane, preferably between about 10 and 40% by weight, most preferably 
about 30 to 40% by weight of the urethane. 
Coating can be conducted in a conventional manner, as by spraying, dipping, 
padding, kiss roll application or the like. If the coating containing the 
dissolved urethane includes a volatile or otherwise undesirable solvent, 
the coated polyester can be treated, as by heating, to remove such 
solvents. But care must be taken that the polyester not be heated 
sufficiently to convert the urethane to a free isocyanate. Advantageously, 
the coating containing the dissolved urethane will not contain any such 
volatile solvent so that intermediate heating is unnecessary. 
The polyester bearing the urethane coating is stable and can be stored for 
extended periods of time. When it is desired to incorporate the same into 
rubber, it is simply treated in any conventional resorcinol-formaldehyde 
latex by conventional procedures as are known to those skilled in the art. 
The resorcinol-formaldehyde latex can contain vinyl pyridine latexes, 
styrene butadiene latexes, waxes, fillers and other additives. 
The latex coated fiber element is thereupon heated to a temperature in the 
range of between about 220.degree. F. (104.degree. C.) and 480.degree. F. 
(249.degree. C.) and above the thermal disassociation temperature of the 
aromatic urethane. The particular heating temperature will depend upon 
considerations known to those skilled in the art, especially including the 
contemplated end use of the polyester reinforced rubber article. Thus, for 
example, polyester tire reinforcing elements are typically heated to about 
460.degree. F. (238.degree. C.) or higher; polyester elements used to 
reinforce V-belts and hose are typically heated to about 440.degree. F. 
(227.degree. C.) while polyester elements used to reinforce articles such 
as conveyor belts and similar articles are typically heated at lower 
temperatures. The thus treated polyester fiber reinforcing element is then 
applied to and cured in a layer of rubber by means known to those skilled 
in the art.

The following examples illustrate the best mode contemplated for carrying 
out the invention. 
EXAMPLE 1 
A topcoat composition was prepared from the following materials (all 
percentages by weight): 
30% nonylphenol blocked polymeric methylene diphenyl diisocyanate having an 
isocyanate functionality of 2.4 (commercially available as MONDUR MRS-10 
from MOBAY Chemical Corporation). 
66.5%, dicocoate ester of tetraethyleneglycol, and 
3.5% by weight of a dialkyl phenol sulfide antioxidant. 
The topcoat was applied in an amount of about 0.6% by weight, based on the 
weight of the yarn, to polyester yarns A and B which had been treated 
during spinning, prior to drawing, with a spin finish containing a 
glycerol epoxide as the active ingredient. The epoxy composition was 
applied to the yarns in amount of about 0.15% by weight based on the 
weight of the yarn. Polyester yarn A is a polyester industrial yarn and 
polyester yarn B is a similar industrial yarn having a low carboxylic acid 
end group content. 
The topcoated yarns were dipped in a conventional RFL latex adhesive, 
treated at the temperatures set forth below and incorporated into 
conventional rubbers by conventional procedures. For comparison the same 
yarns were topcoated with a conventional butyl stearate lubricant topcoat, 
treated and tested in the same manner. The resultant rubber samples were 
tested for adhesion. Results are set forth below: The rating "initial 
adhesion" represents the average of ratings from 1/4 inch H-block adhesion 
tests on two different rubbers and a 250.degree. F. peel adhesion test 
force rating and visual rating. The "aged adhesion" value set forth below 
represents the average of visual and peel force ratings after treatment of 
the sample for two hours in steam under high temperature. All results are 
expressed as percentage values based on Polyester Yarn A when treated at 
400.degree. F. 
______________________________________ 
Polyester 
Polyester 
Yarn A Yarn B Urethane Urethane 
No No Topcoated Topcoated 
Urethane 
Urethane Polyester Polyester 
Topcoat 
Topcoat Yarn A Yarn B 
______________________________________ 
Treating Temp. 
350.degree. F. 
Initial Adhesion 
100 88 163 121 
Aged Adhesion 
100 89 183 126 
% Strength 
99 98 98 98 
Conversion 
Treating Temp. 
400.degree. F. 
Initial Adhesion 
100 77 145 115 
Aged Adhesion 
100 100 184 164 
% Strength 
100 97 96 96 
Conversion 
Treating Temp. 
465.degree. F. 
Initial Adhesion 
100 97 98 95 
Aged Adhesion 
100 92 208 150 
% Strength 
93 94 90 90 
Conversion 
______________________________________ 
As can be seen from the above, adhesion values were significantly improved 
with the topcoated yarns. At the same time, strength loss was minimal. 
Especially to be noted are the aged adhesion increases. It is believed 
that aged adhesion tests are a more accurate predictor of performance in 
rubber articles such as tires, conveyor belts and the like. 
EXAMPLE 2 
A topcoat was prepared identically to Example 1 except that the blocked 
isocyanate used was n-butanol blocked polymeric methylene diphenyl 
diisocyanate having a functionality of 2.7 (MONDUR MR available from Mobay 
Chemical Company). The topcoat was applied in an amount of 0.6% by weight, 
based on the weight of the yarn to polyester yarn B as in Example 1. The 
yarn was dipped in a conventional RFL latex, heated at the temperatures 
set forth below, incorporated into rubber samples which were then tested 
for adhesion. Results are set forth below; 
______________________________________ 
Polyester Yarn B 
Topcoated Polyester 
No Topcoat Yarn B 
______________________________________ 
Treating Temp. 
350.degree. F. 
Initial Adhesion 
100 107 
Aged Adhesion 100 100 
% Strength Conversion 
99 98 
Treating Temp. 
400.degree. F. 
Initial Adhesion 
100 100 
Aged Adhesion 100 100 
% Strength Conversion 
98 96 
Treating Temp. 
465.degree. F. 
Initial Adhesion 
100 107 
Aged Adhesion 100 118 
% Strength Conversion 
95 93 
______________________________________ 
EXAMPLE 3 
Example 2 was repeated except that the blocked isocyanate used in the 
topcoat was n-butanol blocked polymeric methylene diphenyl diisocyanate 
having a functionality of 2.4 (MONDUR MRS-10, commercially from Mobay 
Chemical) results are set forth below: 
______________________________________ 
Polyester Yarn B 
Topcoated Polyester 
No Topcoat Yarn B 
______________________________________ 
Treating Temp. 
350.degree. F. 
Initial Adhesion 
100 100 
Aged Adhesion 100 100 
% Strength Conversion 
99 96 
Treating Temp. 
400.degree. F. 
Initial Adhesion 
100 106 
Aged Adhesion 100 117 
% Strength Conversion 
98 100 
Treating Temp. 
465.degree. F. 
Initial Adhesion 
100 78 
Aged Adhesion 100 129 
% Strength Conversion 
95 91 
______________________________________ 
EXAMPLE 4 
This example demonstrates the effect of flexibilizing agents. Topcoat 
compositions were prepared wherein the blocked isocyanate, i.e., urethane, 
used was nonylphenol blocked polymeric methylene diphenyl diisocyanate, as 
in Example 1. The topcoats were formulated to contain the same amounts of 
the same lubricant and antioxidant as in Example 1. The remaining 30% of 
the topcoat consisted of the combination of the nonylphenol blocked 
isocyanate and the flexibilizing agent, in the amounts shown below. 
The topcoat was applied in an amount of 0.6% by weight, based on the weight 
of the yarn to a polyester industrial yarn having a low carboxylic end 
group content. The yarn had been treated during spinning, prior to drawing 
with a spin finish containing a glycerol epoxide as the active ingredient. 
The epoxy composition had been applied to the yarns in an amount of about 
0.15% by weight, based on the weight of the yarn. 
The yarns were dipped in a conventional RFL latex adhesive, treated first 
at a temperature of 350.degree. F., followed by tretment at 455.degree. F. 
and then incorporated into conventional rubbers by conventional 
procedures. Adhesion was tested in the same manner and using the same 
tests as in Example 1. Results are set forth below wherein the values 
represent percentages based on the same yarn treated with the same spin 
finish but topcoated with a conventional butyl stearate based topcoat. 
______________________________________ 
Nonylphenol Blocked 
Isocyanate 
Wt. % 
25.7 20 25.7 20 
Raw Raw Polypro- 
Polypro- 
30 Castor Castor 
pylene pylene 
Flexibilizer 
None Oil Oil Glycol Glycol 
______________________________________ 
Flexibilizer 
0 4.3 10 4.3 10 
Wt. % 
% Strength 
96 97 97 96 95 
Conversion 
Stiffness 141 153 116 134 110 
Initial 105 111 112 107 103 
Adhesion 
Aged Adhesion 
112 118 114 118 112 
______________________________________ 
It can be seen that flexibilizing agents can reduce stiffness without 
affecting adhesion especially when used in high concentrations. Strength 
of the treated cord was not severely affected by use of the flexibilizing 
agent. 
The invention has been described in considerable detail with reference to 
perferred embodiments. But variations and modifications can be made 
without departing from the invention as described in the foregoing 
specification and defined in the appended claims.