Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
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
Mixing of two or more solid polymers; mixing of solid...
C528S373000, C528S374000, C528S378000
Reexamination Certificate
active
06486268
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to sealants and, in particular, to sealants containing multimodal polymer blends operative in a variety of environments including aerospace.
BACKGROUND OF THE INVENTION
Aviation fuel resistant sealants are widely used by the aircraft industry for many purposes. Principal among these uses are the sealing of integral fuel tanks and cavities, the sealing of the passenger cabin to maintain pressurization at high altitude, and the aerodynamic smoothing of the aircraft's outer surfaces. Today's aircraft design criteria specify that sealants must be light in weight but still maintain the strength and toughness of older high-density sealants. Sealant manufacturers have responded by producing lower density polymers, e.g., the polythioether polymers described in U.S. Pat. Nos. 4,366,307 and 5,912,319, or by the incorporation of lightweight fillers as described in U.S. Pat. No. 5,663,219. Unfortunately, utilizing either of these approaches to obtain lightweight sealants results in compromised properties.
Substituting polythioether polymers (typical specific gravity (s.g.) of about 1.16) for the heavier polysulfide polymers (typically s.g. of about 1.27) results only in a modest reduction in sealant weight. Incorporating lightweight organic or inorganic fillers into a sealant can lower the sealant's specific gravity to near 1.0. Unfortunately, the addition of only a few percent of these fillers dramatically reduces the strength and toughness of the sealant, especially after immersion in aviation fuel.
U.S. Pat. No. 5,985,997 discloses the utility of bimodal polymer distributions in obtaining high impact strength and good gloss in polystyrene polymers. U.S. Pat. No. 5,905,131 discloses multimodal siloxane based formulations having improved strength without added fillers. There are also numerous references to the use of biomodal polymer blends to achieve enhanced emulsification properties in waterborne latexes. U.S. Pat. No. 5,670,601 discloses polyol dispersity in forming polyurethanes.
Unfilled systems suggested by Madkour and Mark in
Polymer Bulletin
31, 615-621, 1994 and in
Macromolecular Reports
A31, 153-160, 1994 include what are referred to as bimodal systems, in which a mixture of functionally-terminated polydimethyl siloxanes were used and end-linked. Trimodal systems were also prepared and tested (using three specified different molecular weight siloxane polymers), but the authors found that although changing from a unimodal distribution to a bimodal distribution significantly improves mechanical properties, changing from a bimodal distribution to a trimodal distribution does not give a further improvement in properties and may actually be detrimental.
In aerospace sealant applications, it is important that polythioether materials are provided which have a good mechanical strength, but which do not have a high viscosity prior to curing. Lightweight systems have been provided which have a relatively low viscosity before curing, but they tend to use organic solvents to reduce the inherent viscosity of lightweight filler-containing compositions. Solvent evaporation during curing leads to materials that dimensionally shrink after application, a highly undesirable property in aerospace sealants. There is a need to provide lightweight compositions that achieve a low viscosity prior to curing to an elastomer form, without the use of organic solvents, yet retain good mechanical properties.
SUMMARY OF THE INVENTION
A composition includes a multimodal polymer blend containing sulfur or ether linkages therein. The multimodal polymer blend has a ratio of between 2 and 4.5 for a maximal polymer mode average molecular weight and a minimal polymer mode average molecular weight, where the minimum polymer mode average molecular weight is less than 1000 Daltons.
In another embodiment, a composition includes a multimodal sulfur containing polymer blend having an average molecular weight modal ratio between maximum and minimum modes of greater than 1.5.
A process for preparing multimodal polymer composition blends of sulfur or ether linkage containing polymers includes blending a plurality of such polymer constituents, each constituent having a modal distribution of average molecular weights and curing the plurality of polymers. The ratio of maximal average molecular weight mode to minimal average molecular weight mode of greater than 1.5. The product obtained from such a process affords superior physical properties. A commercial package is contemplated having a premixed blend of multimodal sulfur or ether linkage containing polymer constituents.
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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
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