Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From sulfur-containing reactant
Reexamination Certificate
2001-03-09
2002-11-26
Truong, Duc (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From sulfur-containing reactant
C528S374000, C528S376000, C525S330900, C525S535000
Reexamination Certificate
active
06486297
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a polymer and process for making the same, more particularly to a polythioether formed by a process of combining a polythiol, polyepoxide and a polyvinyl ether.
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® 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 only 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.
Air frame manufacturers are actively striving to increase productivity by using faster curing aerospace sealants. Hand mixing and application are still common in the aerospace industry owing to high production standards, and as such, an aerospace sealant formulation requires a relatively long application life. For example, prior art manganese dioxide cured polysulfide sealants had a two hour application life and required about a two day cure to attain a Shore A hardness of 30. Current aerospace sealants are expected to have an application life of two to three hours and obtain a Shore A hardness of 30 in less than eight hours.
Until now, aerospace sealants based upon these sulfur-containing polymers have been crosslinked by oxidation of the thiol terminal groups with metal oxides or peroxides. Optimum properties with regard to elasticity are obtained by curing with manganese dioxide. Unfortunately, such sealants, when continuously exposed to modem mercaptan containing aviation fuels, e.g., as in integral fuel tank sealing applications, exhibit polymer chain degradation as evidenced by a phenomena known as “chalking.”
Another disadvantage of these systems has only recently come to light. Aircraft manufacturers, in an effort to increase the fuel economy of their airplanes, have an active program in place to reduce the weight of components they use. Sealant manufacturers have responded to this request for lower density sealants by incorporating lightweight fillers. The disadvantage with this approach is that only a relatively small quantity of these fillers can be used without dramatically decreasing the sealant's physical strength.
Only recently, the specific gravity of aerospace sealants has been reduced from the 1.6-1.8 range down to a minimum of approximately 1.0. Those skilled in the art have attained this specific gravity by incorporating fine hollow spheres and compensating for the loss in physical strength by additions of more highly reinforcing fillers and pigments such as precipitated calcium carbonate or fuimed silica. The flaw with this approach is that the more highly reinforcing fillers have higher surface areas and in most cases higher oil absorptions. This higher surface area results in increased pigment-polymer interactions resulting in a dramatically increased viscosity. These higher viscosities negatively impact application properties and adhesion.
SUMMARY OF THE INVENTION
A polythioether is provided of the formula
wherein R
1
is H, C
1-6
alkyl, C
1-6
alkyl alcohol, C
1-6
alkyl substituted with at least one of:
—NHR
5
wherein R
5
is a C
1-6
alkyl, R
2
is C
2-6
alkyl, C
6-8
cycloalkyl, C
6-10
alkylcycloalkyl, —[—(CH
2
)
r
—Q—]
s
—(CH
2
)
t
—, or C
1-2
alkyl substituted forms thereof, wherein r is an integer value from 2 to 8, Q is a heteroatom selected from the group consisting of: O, S, —NH— and —NCH
3
—, S is an integer value from 1 to 5, and t is an integer value from 2 to 10, R
3
is H or C
1-4
alkyl, R
4
is —CH
2
— or R
2
, M is a C
1-10
alkyl, C
6-20
aryl, C
6-20
aryl substituted with at least one C
1-8
alkyl group, or a N or O heteroatom, Y is C
2-20
alkyl, C
6-20
cycloalkyl, C
6-10
alkylcycloalkyl, or —[—(CH
2
)
r
—Q—]
s
—(—CH
2
—)
t
—, n is an integer value from 1 to 60, m is an integer value from 1 to 60, and p is an integer value from 1 to 10, wherein the polythioether has a viscosity at 20° C. of less than 1000 poise. Preferably, the polythioether has a number average molecular weight between about 1000 and about 10,000 Daltons. A polyfunctionalizing agent is optionally provided in order to increase the functionality of a polythioether from 2 to 5 with the most preferred range being 2 to 3.
A process for forming such a polythioether includes the steps of reacting a polythiol with either an aromatic polyepoxide or a polyvinyl ether to form a prepolymer. The prepolymer is then combined with the other of the aromatic polyepoxide or the polyvinyl ether. The use of such a polythioether is contemplated as an aerospace sealant.
DETAILED DESCRIPTION OF THE INVENTION
It has surprisingly been discovered that the combination of certain polythiols with polyepoxides and oxygenated dienes according to the invention results in polythioether polymers that are liquids at room temperature and pressure. Further, these new polythioether polymers are higher in strength than either conventional polysulfide or polythioether polymers. Despite the increase in physical strength, polymer formulations consistent with this invention do not sacrifice other desirable physical and chemical properties inherent with polythioether polymers. The polythioether polymers of the present invention are substantially free of both malodorous cyclic byproducts and deleterious catalyst residues, and hence have superior thermal resistance properties. The inventive polythioethers have utility as aerospace sealants.
According to the invention, the combination of polythiols with polyepoxides and oxygenated dienes may be represented as follows:
Suitable polythiols include the dithiols wherein R
2
is a C
1-10
alkyl or aryl and may or may not contain a heteroatom. Substituents on R
2
are those which do not interfere with the reaction of the polythiol with either a polyepoxide or polydiene. Thus, R
2
is free of reactive unsaturated carbon to carbon bonds, as well as highly water sensitive species. Preferred heteroatoms are S and O. Preferred dithiols include straight chain aliphatic dithiols with a chain length of two to six carbon equivalents, dimercaptodiethylene (DMDE), dimercaptodiethylsulfide (DMDS), dimercaptodioxaoctane (DMDO), dipentene dimercaptan, and vinylcyclohexane dimercaptan. Additional operative dithiols are recited in U.S. Pat. No. 5,912,319.
Suitable polyepoxides include those wherein group M is a C
2-10
alkyl, C
6-20
aryl, C
6-20
aryl substituted with at least one C
1-8
alkyl group, or a N or O heteroatom. Suitable polyepoxide compounds operative herein are recited in U.S. Pat. No. 4,136,086. Preferably, polyepoxides according to the present invention contain an aryl group within M. It has been discovered that an aryl backbone component synergistically operates with the hydroxyl groups associated with thiol epoxide reaction to form a superior strength and handling property polythioether polymer. R
3
of an inventive polyepoxide reagent is hydrogen or C
1
-C
4
alkyl. Preferably, R
3
is hydrogen or a methyl group. R
4
is methylene or any of the substituents denoted as the polythiol R
2
. P is an integer value ranging from 1 to 10. Additionally operative polyepoxides include the diglycidylether of bisphenol A (such as EPON 828®, Shell Chemicals Ltd.), diglycidylether of bisphenol F (such as ERISYS™, CVC), any of the lower functionality Novolaks (such as DEN 431™, The Dow Chemical Co.), as well as butane- and hexane-diol di
Cosman Michael
Jones George
Jordan David W.
Willard Dean M.
Zook Jonathan D.
Gifford, Krass, Groh Sprinkle, Anderson & Citkowski, P.C.
PBT Brands, Inc.
Truong Duc
LandOfFree
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