I
A. L. MORAN,
R. P. KANE,
and J. F. SMITH
Elastomer Chemicals Department, E. 1. du Pont d e Nemours & Co., Inc., Wilmington, Del.
Safe Processing Curing Systems for luoroelastorners The use of blocked diamine curing agents, alone and in combination with a diimine retarder-activator, permits development of safe processing Viton fluoroelastomer compounds
T H E combination of vinylidene fluoride and hexafluoropropylene represented in Viton has been carefully chosen to give good processing compounds and high quality vulcanizates. This combination of monomers produces an elastomer containing approximately 65Yo fluorine, which is outstanding in fluid resistance and thermal stability (7, 2 ) . However, the proper utilization of any elastomer is dependent, to a large extent, on the development of safe processing curing agents. An ideal curing agent has little or no curing activity at processing temperatures, but produces a maximum number of cross links in a minimum time at curing temperatures; it causes no premature vulcanization while being mixed into a stock, but produces a good, fast cure at curing temperatures. This article describes a number of materials which have been evaluated as curing agents for Viton.
Experimental
Compounding Ingredients. All stocks contain a metallic oxide, a filler, and one or more curing agents. Mixing Procedure. The stocks were mixed on a cold 6 X f 6 inch laboratory mill. T h e metallic oxides wcre added first, followed by the filler, and finally the curing agent. T h e mixed stocks were held overnight a t room temperature and given a 5-minute remill prior to sheeting out for cure. Processing Safety. This was measured on the Mooney shearing disk viscometer (ASTM D 927-55T) operating at 250' F., using the small rotor. T h e scorch values reported are the total times in minutes for the Mooney viscosity to increase 10 points from the minimum value. Curing. Test pieces were formed in a press for 30 minutes a t 300 or 400' F. They were given a post oven cure of 1
of zinc oxide and dibasic lead phosphite, produces a stock with a 36-minute scorch (compared with 2 for triethylenetetramine) having excellent vulcanizate properties. However, the scorch or processing safety of stocks containing hexamethylenediamine carbamate is also dependent on the type of metallic oxide and the filler loading as well as the type of Viton used. Changing the type of metallic oxide from the zinc oxide-dibasic lead phosphite system to magnesium oxide reduces the Mooney scorch by approximately two thirds. Scorch resistance is further reduced by increasing the filler loading and using Viton A-HV, the high viscosity Viton polymer. I n Discussion an attempt to improve the processability The first Viton formulations were of these Viton A and Viton A-HV stocks cured with an aliphatic polyamine such containing magnesium oxide, other as triethylenetetramine or a peroxide blocked diamines were evaluated as such as benzoyl peroxide. Triethylenecuring agents. tetramine produces a much higher state ETHYLENEDIAMINE CARBAMATE. The of cure than benzoyl peroxide, but stocks carbamate of ethylenediamine proved containing either curing agent are exparticularly advantageous. This is a tremely scorchy and very difficult to white, free-flowing powder similar in process. T o utilize these materials in appearance to hexamethylenediamine Viton, it was necessary to find some means carbamate. As indicated by Mooney of retarding their curing action at temscorch, stocks containing ethylenediaperatures ordinarily used to process mine carbamate are three to four times Viton. Because the polyamines appeared safer processing than stocks containing particularly interesting, blocked diaminehexamethylenediamine carbamate. type curing agents were considered. Furthermore, stress-strain tests and comBlocked Diamine Curing Agents. pression set results show that 0.85 part of HEXAMETHYLENEDIAMINE CARBAMATE. ethylenediamine carbamate produces T h e first blocked diamine tested was a state of cure equal to that obtained with hexamethylenediamine carbamate. This 1 part of hexamethylenediamine caris a salt of hexamethylenediamine which bamate. Ethylenediamine carbamate is is unreactive at room temperature, but more easily dispersed in Viton stocks than decomposes rapidly in the range of 130' hexamethylenediamine carbamate. to 170' F. to produce the free reactive OTHERDIAMINE CURING AGENTS.In diamine. Substitution of hexamethyladdition to blocked diamines of the carenediamine carbamate for triethylenebamate class, a number of other hintetramine in a Viton A stock containing dered diamines were considered. Sub20 parts of MT black and 10 parts each stituted piperazines and diamines incorporating the cyclohexyl nucleus Tables supplementary to this article proved the most promising. Although appear in the Journal of Chemical Engipiperazine itself is somewhat slow as a curing agent for Viton polymers, 2,5-di-
hour at 212O, 250', 300', 350' F.; and 24 hours at 400" F. Stress-Strain Tests. Standard ASTM dumbbells (0.25-inch neck with a 1-inch bench mark) were die-cut from 3 X 6 X 0.075 inch test slabs. These were pulled on a Scott tester at 40 inches per minute (ASTM D 412-51T). Heat Aging. Standard dumbbells were aged for 5 days at 500' F. in a circulating air oven (ASTM D 573-53). Compression Set Tests. Pellets, 0.5 X 1.129-inch diameter, were tested for 70 hours at 250' F. (ASTM D 395-55, Method B).
VOL. 51, NO. 7
JULY 1959
831
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(o hydroxybenzylidene) - 1.2-propylenediamine, a well known metal chelating agent, was included in a Viton formulation. Its addition to a Viton stock containing 1 part of hexamethylenediamine carbamate provides improved processing safety, as indicated by Mooney scorch and an increased state of cure as shown by stress-strain properties and compression set results. Formulations possessing various degrees of processing safety and cured physical properties can be obtained by varying the ratio of
methyl piperazine produces well cured vulcanizates from stocks which are much safer processing than those containing hexamethylenediamine carbamate. T h e one disadvantage is that the 2,s-dimethylpiperazine stocks do not rerain as much elongation after aging as the stock cured with hexamethylenediamine carbamate. Two members of the cyclohexyldiamine class which produced Viton vulcanizates with good physical properties were 1,3-diaminocyclohexane and bis(p-aminocyclohexy1)methane. Stocks containing these curing agents are approximately equivalent to those containing hexamethylenediamine carbamate with regard to processing safety. Delayed Action Type Curing Agents. Another phase of the investigation involved a consideration of true delayed action-type materials, SULFENAhlIDE DERIVATIVES. Diamines will form sulfenamides which act as delayed action curing agents for Viton. Examples of these are piperazinebisthiobenzene and piperazinebis2-thiobenzothiazole. At high temperatures, these materials will cross-link Viton. Both of these sulfenamides produce very safe processing stocks. Stressstrain properties and compression set results indicate that piperazinebis-2-thiobenzothiazole is somewhat more active than piperazinebisthiobenzene. T h e main disadvantage of these materials as curing agents for Viton is that stocks containing them require relatively high press cure temperatures (400” F. or higher) to obtain satisfactory molded items. DIIMINEACCELERATORS. Another class of delayed action accelerators for Viton polymers is diimines or the aldehyde derivatives of aliphatic diamines. This was discovered when iV,N’-bis-
liminary results with some other diimines have given more encouraging results. i%-,lV’-Bis(N’N-dimethyl-fi-arninobenzylidene)-l,6-hexylenediamine Ar,:N’-Biscinnamylidene-1 ,6-hexylene-
diamine -V, .V’-Bis (o-methox ybenzylidene) -1,Ghexylenediamine -V,.V’-Bis (p-methoxyhenzylidene)-l,6hex ylenediamine
Stocks containing these diimines are all safer processing than those containing hexamethylenediamine carbamate and N,,Ar’-bis(o-hydroxybenzylidene)-l,2-pro- produce equivalent states of cure as pylenediamine to hexamethylenediajudged by compression set resistance. mine carbamate. A complete study of Standard press cures at 300’ F, produced curing conditions has shown that below satisfactory test pieces. 300’ F., Il‘,LV’-bis(o-hydroxybenzylidene)1,2-propyIenediamine has a retarding Conclusions effect; at higher temperatures, an accelerating effect. Polyfunctional amines such as triethylA number of other diimines have been enetetramine produce Viton vulcanizates with good properties, but these stocks are evaluated in comparison with S,S‘-bisvery scorchy and extremely difficult to (o-hydroxybenzylidene)-l,2- p r o p y l e n e process. diamine as retarder-activators in a standard Viton formulation containing Stocks containing blocked diamines hexame t h y l e n e - d i a m i n e carbamate. such as hexamethylenediamine carbaNone of the diimines evaluated were mate are much safer processing than those superior to N,,\”-bis(o-hydroxybenz)-licontaining triethylenetetramine and produce vulcanizates with very similar dene) - 1,2 - propylenediamine. All the properties. However, the processing materials tested improved compression safety experienced Lvith hexamethyleneset resistance, but only diimines based diamine carbamate is sensitive to the on aromatic aldehydes improved proctype of metallic oxide and the filler essing safety. loading as well as the type of Viton used. T h e diimines have also been evaluated Ethylenediamine carbamate is a safe as curing agents by themselves-Le., in Viton stocks containing no hexamethylprocessing accelerator in all types of Viton formulations and produces vulenediamine carbamate. Materials such canizates with properties identical to as AT,N‘-his (0- hydroxybenzylidene) - 1,2those obtained with hexamethylenedipropylenediamine and 2Y,iV’-bis(o-hyamine carbamate. droxybenzylidene) - 1,6 - hexylenediamine were too slow to be practical. Stocks -4series of delayed action curing agents has been developed for Viton based on containing them require long press cures diimine-type compounds. T h e addition at very high temperatures to produce satisfactory vulcanizates. However, preof A\r,iT’-bis(o-hydroxybenzylidene)-1,2propylenediamine to a Viton stock containing hexamethylenediamine carbamate improves processing safety and state of cure. By varying the ratio of Ethylenediamine Carbamate and Diimines Improve Processing Safety of Viton A-HV this diimine to hexamethylenediamine Compound -4-1 B-1 c-1 carbamate, a series of Viton formulations 100 I!OO Viton A-HV 100 possessing various degrees of processing 15 15 Magnesium oxidea 15 safety as well as a variety of cured physi20 20 MT carbon black 20 1 1 Hexamethylenediamine carbamateb cal properties can be obtained. PrelimiEthylenediamine carbamatec 0.85 nary results with other experimental diiS,N’-Bis(o-hydroxybenzylidene) -1,Zmines are very encouraging. Stocks propylenediamined 1.3 containing them are safe processing and Mooney scorch at 250° F. (MS) 36 7 30 Minutes to IO-point rise produce well cured vulcanizates hy standard press and oven cures, Curing Conditions. Press cure. 30’/300° F. Oven cure. Step cure t o 400’ F. and 24 hours
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at 400’ F. Stress-strain, original Modulus at loo’%, p.s.i. Modulus at 200%, p.s.i. Tensile strength, p.s.i. Elongation at break, % Hardness, Shore A Compression set, Method B 70 hours at 250’ F.
390 1130 2500 320 67
350 1190 2875 350 69
580 2075 2525 240 69
35
34
16
Darlington 601, Darlington Chemical Co., Philadelphia, Pa. Diak No. 1, Du Pont. Diak No. 2, Du Pont. active ingredient of Copper Inhibitor 65, Du Pont.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited (1) Dixon, S., Rexford, D. R., Rugg, J . S., I N D . END.CHEM. 49, 1687 (1957). (2) Gallagher, G. A., Eubank, T. D., Moran,-A. L., Division of Rubber Chemistry, 134th hleeting, ACS, Chicago, Ill.,
September 1958.
RECEIVED for rebiew September 23, 19.58 *ACCEPTED March 9, 1959 Division of Rubber Chemistry, 134th Sleeting, ACS, Chicago, Ill., September 1958.
