FIELD OF THE INVENTION

The present invention relates generally to polythioethers and, more particularly, to polythioethers formed by addition of a polythiol and a cyclopolyene.

BACKGROUND OF THE INVENTION

Thiol-terminated sulfur-containing polymers have a long history of use in aerospace sealants because of their fuel resistant nature upon cross-linking. Among the commercially available polymeric compounds having sufficient sulfur content to exhibit this desirable property are the polysulfide polymers described, e.g., in U.S. Pat. No. 2,466,963 and sold under the trade name LP.RTM. polysulfide by Morton International of Chicago, Ill. and the alkyl side chain containing polythioether polymers described, e.g., in U.S. Pat. No. 4,366,307 that are sold in complete sealant formulations by PRC-DeSoto International, Inc. of Glendale, Calif. In addition to fuel resistance, polymers useful in this context must also have the desirable properties of low temperature flexibility, liquidity at room temperature, high temperature resistance, a reasonable cost of manufacture, and not be so malodorous as to prevent commercial acceptance of compositions that contain the subject polymers.

U.S. Pat. No. 4,366,307 teaches the use of hydroxyl-functional thioethers having pendent alkylene groups to obtain polymers having good flexibility and liquidity. However, the disclosed condensation reaction has a maximum yield of about 75% of the desired condensation product. Furthermore, the acid-catalyzed reaction of beta-hydroxysulfide monomers, such as thiodiglycol, yields significant quantities (typically not less than about 25%) of an aqueous solution of thermally stable and highly malodorous cyclic byproducts, such as 1-thia-4-oxa-cyclohexane. As a result, the commercial viability of the disclosed polymers is limited. Further, pendent alkylene chains increase the carbon content of the polymer necessitating a high sulfur content to achieve sufficient chemical resistance properties.

U.S. Pat. No. 5,959,071 teaches the use of pendant alkylene chains and high sulfur content to achieve the chemical resistance and room temperature liquidity required for aerospace sealant formulations.

Certain prior art work has developed hydroxyl-terminated polythioethers by condensing thiodiglycol in the presence of certain etherifying catalysts as, for example, shown in U.S. Pat. Nos. 3,312,743 and 3,335,189. Compounds produced by these patents give semi-crystalline waxy solids, gums or low molecular weight liquids that have limited commercial utility.

Another desirable feature in polymers suitable for use in aerospace sealants is high temperature resistance. Inclusion of covalently-bonded sulfur atoms in organic polymers has been shown to enhance high temperature performance. However, in the polysulfide polymers disclosed in U.S. Pat. No. 2,466,963, the multiple --S--S--linkages in the polymer backbones result in compromised thermal resistance. In the polymers disclosed in U.S. Pat. No. 5 4,366,307, enhanced thermal stability is achieved through replacement of polysulfide linkages with polythioether (--S--) linkages. In practice, however, the disclosed materials have compromised thermal resistance due to traces of the residual acid condensation catalyst.

U.S. Pat. No. 5,912,319 teaches the use of combinations of certain polythiols with oxygenated dienes resulting in polythioether polymers that are liquids at room temperature and pressure and have desirable physical properties. Further, these combinations are substantially free of residual catalysts and malodorous cyclic byproducts. Unfortunately, the oxygenated dienes described are very difficult to prepare and only a limited number of commercial compounds are known to exist.

In addition to the foregoing deficiencies with the previously known polythioethers, the prior art polythioethers are typically also crystallizing products which, even if liquid or semi-liquid at ambient temperatures, when cooled sufficiently to solidify will not return to their previous liquid state even when the temperature is raised to ambient.

SUMMARY OF THE INVENTION

A curable sealant compound comprising: a polythiol, a cyclopolyene having a formula: ##STR3##

where X is C.sub.1-5 aliphatic, Y is a C.sub.1-6 aliphatic, R.sup.2 is in each occurrence H, C-C.sub.10 alkyl, C.sub.2 -C.sub.10 alkenyl, ##STR4##

Q--R.sup.3 where Q is O or S and R.sup.3 is C.sub.2-10 alkenyl, such that the cyclopolyene has non-conjugated carbon--carbon unsaturated bonds, and in formula (VII) there is one carbon--carbon unsaturated bond within the ring that is non-conjugated to other carbon--carbon unsaturated bonds; and an effective amount of a free radical catalyst.

A process for forming a polythioether includes the step of mixing a polythiol and an inventive cyclopolyene as detailed above in the presence of an effective amount of a catalyst promoting addition therebetween. A sealant formed by curing such a polythioether in the presence of a filler and a curing agent is also contemplated.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes a polythioether polymer made by the reaction of a thiol (dithiol) and an aliphatic, ring-containing non-conjugated diene in the presence of a catalyst. The reaction is generally represented as follows: ##STR5##

In this reaction, "n" is an integer such that the desired molecular weight is obtained, typically 1000-10,000 Daltons. Preferably, a curable polythioether according to the present invention is a liquid at room temperature to facilitate substrate application.

Polythiols operative in the present invention illustratively include

R.sup.1.paren open-st.SH) (IV)

R.sup.1.paren open-st.Q R.sup.1).sub.n --SH.paren close-st..sub.m (V)

##STR6##

where R.sup.1 is a linear or branched alkyl C.sub.1 -C.sub.20, or a C.sub.6 -C.sub.8 aryl, R.sup.1 lacking groups reactive under addition reaction conditions with an unsaturated carbon--carbon bond; m is an integer of 2 to 6, inclusive; n and p are each independently integers of 0 to 3, inclusive; and Q is a heteroatom of either S or O in each occurrence. Preferably, m is a mixture of integer values to result in an average polythioether functionality of greater than 2. More preferably, the average functionality is less than 3. Still more preferably, the average polythioether functionality is about 2.01 to 2.10. Preferably, a polythiol has a molecular weight of more than 70 and less than 500 Daltons.

Y in equation (III) is an aliphatic ring containing non-conjugated polyene with at least one carbon--carbon unsaturated bond with the ring. A compound Y is defined herein to be a cyclopolyene. A cyclopolyene operable herein illustratively includes ##STR7##

where X is C.sub.1-5 aliphatic, Y is a C.sub.1-6 aliphatic, R.sup.2 is in each occurrence H, C.sub.1 -C.sub.10 alkyl, C.sub.2 -C.sub.10 alkenyl, ##STR8##

Q--R.sup.3 where Q is O or S and R.sup.3 is C.sub.2-10 alkenyl, such that the cyclopolyene has non-conjugated carbon--carbon unsaturated bonds, and in formula (VII) there is one non-conjugated carbon--carbon unsaturated bond within the ring that is non-conjugated to other carbon--carbon unsaturated bonds. Preferably, a cyclopolyene is an unconjugated cyclodiene or a cycloalkene having an unconjugated double bond containing substituent. More preferably, a cyclopolyene is a 5, 6, 8 or 12 member ring or dimer of two such rings. It is appreciated that the functionality of an inventive polythioether can likewise be increased beyond two per polymer chain as detailed with regard to the polythiol compound hereof by the addition of some amount of a cyclopolyene having three or more non-conjugated unsaturated carbon--carbon bonds per molecule.

Suitable thiols include lower alkylene thiols such as ethanedithiol, vinylcyclohexyldithiol, dicyclopentadienedithiol, dipentene dimercaptan, and hexanedithiol; aryl thiols such as benzene dithiol; polyol esters of thioglycolic acid and thiopropionic acid. Preferred thiol components contain heteroatoms with examples being dimercaptodiethyl sulfide (DMDS) with a group of R.sup.1 of HSCH.sub.2 CH.sub.2 SCH.sub.2 CH.sub.2 SH and dimercaptodioxaoctane (DMDO) with a group R.sup.1 of HSCH.sub.2 OCH.sub.2 OCH.sub.2 CH.sub.2 SH.

Suitable cyclodienes illustratively include vinylcyclohexene, dipentene, dicyclopentadiene, cyclododecadiene, cyclooctadiene, 2-cyclopenten-1-yl-ether, s-vinyl-2-norborene and norbomadiene. One preferred diene is vinylcyclohexene. A preferred cyclotriene is cyclododecatriene.

Suitable catalysts for the reaction of the polythiol and the cyclopolyene include any of a number of azo or peroxide free radical initiators such as the azobisalkalenenitrile sold by DuPont under the trade name VAZOTM.TM.. Most preferred is VAZO 67.TM. because of the desirable activation temperature and its low level of residual odor.

In aerospace sealants, it is desirable to use polymers that have greater than two reactive terminals per polymer molecule. In practice, adding small quantities of either a polythiol or a polyene increases the functionality of these materials. As polythiols are often more expensive than cyclopolyenes, generally cyclopolyenes are chosen to achieve this end.

In another embodiment, the inventive polythioether is a capped polymer in which the terminal-SH groups are replaced with alkyl or heteroatom subtituted alkyl substituents, where the heteroatom includes N, O, Si or S. Capping moieties are produced by the reaction of a terminal thiol with a monounsaturated compound such as, for example, vinyl pyridine, triethoxyvinylsilane and other compounds detailed in U.S. Pat. No. 5,912,319.

Table 1 illustrates the results of reacting various thiols (dithiols) and dienes in the presence of a suitable catalyst invention and are examples of the inventive compositions:

                                       TABLE I
                                 Polymer Properties
                                           %          Carbon  Sulfur  Oxygen
     Physical
     Example   Thiol  Diene             Yield*  Odor     %       %       %
     State
        1      DMDS   Vinylcyclohexene,   99    None    55      37       0
     Liquid
                      C.sub.8 H.sub.12
        2      DMDS   Dipentene, C.sub.10 H.sub.16    99    None    58      33
          0    Liquid
        3      DMDS   Dicyclopentadiene,   98     V.     59      36       0
     Liquid
                      C.sub.10 H.sub.12          Slight
        4      DMDS   Cyclododecadiene,   95     V.     60      30       0
     Liquid
                      C.sub.12 H.sub.20          Slight
        5      DMDO   Vinylcyclohexene,   99    None    58      22      11
     Liquid
                      C.sub.8 H.sub.12
        6      DMDO   Dipentene, C.sub.10 H.sub.16    98    None    60      20
         10    Liquid
    *Expressed as percent of theoretical

            Ingredient                Parts by Weight Used
            Dow Epoxy Novolac 431               75
            Shell Epon 828                     25
            Carbon black                       5

                                   TABLE II
                        Formulated Sealant Properties
                        Peel strength Peel strength Adhesion to
                          before       after    test panel
                        immersion,  immersion,  before and  Tensile
               Shore A   lbs/inch    lbs/inch      after    strength Elongation
    Polymer   hardness     width       width     immersion  at break at break
    Example 1    65         85          80       100%/100%    625      400
    Example 2    63         80          72       100%/100%    600      350
    Example 3    62         80          71       100%/100%    610      375
    Example 4    60         75          50       100%/100%    550      400
    Example 5    63         65          45       100%/100%    550      400
    Example 6    60         60          35       100%/100%    500      425
    Polymer      45         35          20       100%/100%    290      300
    from
    Example
    16 of U.S.
    Pat. No.
    4,366,307
    Polymer      52         50          35       100%/100%    550      450
    from
    Example
    16 of U.S.
    Pat. No.
    4,366,307