Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Utility Patent
1998-12-04
2001-01-02
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S479000, C524S837000
Utility Patent
active
06169149
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an emulsion polymerization method for making silicone rubber and polyacrylate rubber hybrids having a volume average particle size in the range of about 400 nm to about 2 microns, which include monomodal and bimodal size distributions. More particularly, the present invention relates to grafts of such emulsion polymerized silicone rubber and polyacrylate rubber hybrids and to the use of such grafts as impact modifiers in thermoplastic resins. As used hereinafter, the expression “silicone rubber and polyacrylate rubber hybrid” means an interpenetrating composite of silicone rubber and polyacrylate rubber, where the silicone rubber and polyacrylate rubber are entangled in an inseparable fashion at the molecular level.
In Craig, U.S. Pat. No. 5,726,270, which is incorporated herein by reference, a process is shown for making monomodal dispersions of polysiloxanes having a volume average particle size in the range of about 50 nm to about 2 microns. The process of Craig is based on the use of low shear and substantially non-homogenizing polymerization conditions. It would be desirable therefor to be able to use this low shear procedure of Craig to make monomodal dispersions, of emulsion polymerized silicone rubber particles having a volume average particle size in the range of about 400 nm to about 2 microns. It also would be desirable to make multi-modal mixtures, such as bimodal mixtures of such aqueous silicone rubber particles, where at least one of the members of the mixture has a volume average particle size of at least 400 nm.
The semi-continuous emulsion polymerization conditions for making organopolysiloxane rubber having a predetermined volume average particle size of at least 400 nm, for example, can combine dropwise addition of polymerizable monomers, with mild agitation, such as stirring, as contrasted to the high shear homogenization conditions typically employed in polysiloxane emulsion production.
While acrylate rubbers are widely used for impact modification of thermoplastic materials where weathering is a concern, impact strength of acrylate rubber modified thermoplastic materials at low temperatures, such as 0° C., or below are substantially reduced, as compared to thermoplastic materials containing other organic blends, such as polybutadiene rubber.
Efforts to improve the performance of impact modifiers for use in thermoplastic polymer blends by combining the low temperature advantages of silicone rubber while maintaining good weatherability characteristics of acrylate rubber often fail. As shown, for example, in U.S. Pat. Nos. 4,994,522 and 5,132,359, emulsion polymerized silicone and polyacrylate rubber hybrids as impact modifiers in thermoplastic polymer blends, did not provide maximum impact strength, because of particle size limitations (less than 300nm) in the emulsion polymerized silicone rubber.
It would be desirable therefor to provide additional methods for making rubber impact modifiers for thermoplastic resins, which combined the low temperature advantages of silicone rubbers, with the weatherability benefits of acrylate rubbers.
BRIEF SUMMARY OF THE INVENTION
The present invention is based on the discovery that the polymerization techniques as shown by Craig, in U.S. Pat. No. 5,726,270 can be applied to improving the impact performance of thermoplastic compositions containing grafts of silicone rubber and polyacrylate rubber hybrids by achieving improved particle size control in making emulsion polymerized silicone rubber.
It has been found that emulsion polymerized silicone rubber and polyacrylate rubber hybrids produced with an aqueous dispersion of silicone rubber particles having a volume average size of between about 400nm to about 2 micron, are useful weatherable impact modifiers when grafted with an alkenyl monomer, to form a vinyl polymer shell, and incorporated into a thermoplastic resin.
In addition, an enhancement in impact properties in thermoplastic compositions also can be achieved with a mixture of grafts of emulsion polymerized silicone rubber and polyacrylate rubber hybrids, to produce a graft having multi-modal, such as bimodal particle size distribution. It is preferred, moreover, to use in making such grafts having multi-modal particle size distribution, at least one graft member of an emulsion polymerized silicone rubber and polyacrylate rubber hybrid which has a volume average particle size of at least 400 nm, and is present in the graft mixture in at least a major amount by weight.
The incorporation of grafts of silicone rubber and polyacrylate rubber hybrids into a thermoplastic polymer material has been found to enhance the impact strength of such materials. It has been found that the saturated nature of acrylate-derived rubber prevents embrittlement of the thermoplastic over time. Such impact modified materials are used in many applications requiring good weatherability and low temperature impact strength, including automotive, building, and construction applications, and in articles of manufacture such as garden furniture, boats, signs, outdoor enclosures for electrical or telecommunications interface devices such as smart network interface devices (SNID), and the like.
There is provided by the present invention, a method for making an impact modifier for enhancing the weather resistance and low temperature impact strength of organic thermoplastic polymers, which method comprises,
(1) effecting the polymerization of silicone rubber monomers under semi-continuous emulsion polymerization conditions at a temperature of about 30° C. to about 110° C., to provide for the formation of a silicone rubber latex having a volume average particle size in the range of from about 400 nm to about 2 microns,
(2) adding batchwise to the resulting silicone rubber latex of (1), while providing agitation at a pH of about 4 to about 9.5, and a temperature of about 20° C. to about 90° C., a mixture comprising polymerizable acrylate rubber monomers to provide a latex comprising an emulsion polymerized silicone rubber-polyacrylate rubber hybrid having a weight proportion of from about 95 parts to about 5 parts of silicone rubber, and from about 5 parts to about 95 parts of polyacrylate rubber, and where the addition of the polymerizable acrylate rubber monomers into the silicone rubber latex occurs before, or concurrently with sufficient polymerization catalyst to effect acrylate rubber formation,
(3) grafting the emulsion polymerized silicone rubber-acrylate rubber hybrid latex of (2), with a sufficient amount of polymerizable alkenyl monomer to form a graft of the emulsion polymerized silicone rubber-polyacrylate rubber hybrid latex, and,
(4) coagulating, washing and drying the grafted emulsion polymerized latex of (3).
A further aspect of the present invention is directed to impact modifiers comprising melt extrudable polymerized alkenyl monomer grafts of an emulsion polymerized silicone rubber-polyacrylate rubber hybrid arising from the initial use of an emulsion polymerized silicone rubber latex having a volume average particle size in the range of from about 400 nm to about 2 micron.
An additional aspect of the present invention is directed to a moldable thermoplastic organic polymeric material having improved low temperature weather resistance and impact strength comprising an organic thermoplastic polymeric material, and an effective amount of a silicone-acrylate impact modifier comprising the polymerized alkenyl monomer graft of an emulsion polymerized silicone rubber-polyacrylate rubber hybrid arising from the initial use of an emulsion polymerized silicone rubber latex having a volume average particle size in the range of from about 400 nm to about 2 micron.
Still another aspect of the present invention is directed to a method for preparing a moldable thermoplastic organic polymeric material having improved weather resistance and low temperature impact strength, which comprises mixing an organic thermoplastic polymeric material and an effective amount of a silicone-acrylate i
Craig Daniel Horace
Hu Rong
Bennett Bernadette M.
Dawson Robert
General Electric Company
Ingraham Donald S.
Robertson Jeffrey B.
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