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
1998-05-01
2001-02-13
Mullis, Jeffrey (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...
C525S085000, C525S087000, C525S302000, C525S309000, C525S310000, C525S315000, C525S316000
Reexamination Certificate
active
06187862
ABSTRACT:
The invention concerns a thermoplastic molding composition which contains a grafted rubber, and more particularly a composition where the grafted phase entails grafting, in sequence, a first and subsequent phase.
SUMMARY OF THE INVENTION
A thermoplastic molding composition comprising a grafted rubber and a copolymeric matrix is disclosed. The composition is characterized in that its grafted rubber includes an elastomeric, crosslinked substrate, a first grafted phase and a subsequent grafted phase which are sequentially grafted to said substrate. Preferably, the substrate contains preferably crosslinked, core containing the polymerization product of at least one vinylaromatic monomer, and a shell containing a crosslinked rubber. The first grafted phase and the subsequent grafted phase, both contain copolymers of at least one vinylaromatic monomer and at least one member selected from the group consisting of (meth)acrylonitrile and alkyl (meth)acrylate, yet the relative ratios of the monomers making up the copolymerized phases differ one from the other. The grafted rubber is present in particulate form having a weight average particle size of about 0.1 to 1.0 microns. The composition features improved impact strength and reduced opacity.
BACKGROUND OF THE INVENTION
It has long been known that thermoplastically processable styrene/acrylonitrile copolymers (SAN) may be imparted improved impact properties by the incorporation of rubbers. In ABS polymers, diene polymers are used as the rubbers for toughening, most notably at low temperatures; however, the relatively poor weathering and aging resistance make these materials less favored in more demanding applications. In some such applications, use has successfully been made of crosslinked acrylic acid ester polymers; these are the well known ASA copolymers. Such have been described in U.S. Pat. No. 3,055,859 and in German 1,260,135 and 1,911,882. Accordingly, the preferably cross-linked, rubbery acrylic acid ester polymer which serves as the grafting base (substrate) is first prepared by emulsion polymerization and the latex thus prepared is then grafted, preferably by emulsion, with a mixture of styrene and acrylonitrile. The art thus has long recognized that improved impact strength, notched Izod, greater hardness and reduced shrinkage are associated with such ASA products which have as a grafting base a coarse polyacrylate latex having a mean particle diameter of about 150 to 800 nm and a narrow particle size distribution. Also, noted in the present context is the disclosure in U.S. Pat. No. 4,224,419 which disclosed an ASA based composition which contains two different graft copolymers of SAN onto crosslinked acrylates, and a hard copolymeric SAN component.
The art refers to “hard” and “soft” segments in terms of the relative position of their glass transition temperature, vis a vis, room temperature. “Hard” means T
g
above room temperature and “soft” means below room temperature, as well as to core/shell structures including structures which contain a multiplicity of shells. Multi-phase structured emulsion copolymers, including hard-soft and hard morphologies, have been disclosed in EP 534,212 and in the documents referenced therein. Accordingly, graft copolymers with a hard core of polystyrene, a first butyl acrylate shell and an outer SAN shell have been disclosed in the art. Significantly, the '212 document disclosed a monomodal system wherein particles measure less than 0.2 microns and have a styrene core. In an example, the '212 document disclosed the preparation of a four stage graft copolymerized with a styrene core as a graft base, a first shell of butyl acrylate, a second shell of styrene and a third shell of styrene/acrylonitrile.
Also presently relevant are copending patent applications U.S. Ser. No. 08/974,541 filed Nov. 19, 1997 and Ser. No. 08/955,857 filed Oct. 21, 1997, both assigned to Bayer Corporation and both concern weatherable composition containing grafted rubber components.
DETAILED DESCRIPTION OF THE INVENTION
The invention is directed to a thermoplastic molding composition comprising a grafted rubber and a copolymeric matrix wherein grafted rubber includes a crosslinked elastomeric substrate, a first grafted phase and a subsequent grafted phase which are sequentially grafted to said substrate. The first grafted phase and the subsequent grafted phase are chemically different one from the other. Preferably, the substrate contains an advantageously crosslinked core which contains the polymerization product of at least one vinylaromatic monomer, and a shell containing a crosslinked rubber selected from the group consisting of polyalkyl acrylate, hydrogenated polydiene polydiene, and wherein said first grafted phase contains a copolymer of at least one vinylaromatic monomer and at least one member selected from the group consisting of (meth)acrylonitrile and alkyl (meth)acrylate, where the polymerized product of said monomer is present in an amount of at least 75 percent and the polymerized product of said member is present in an amount of at most 25% relative to the weight of said first grafted phase, and where said subsequent grafted phase contains a copolymer of at least one vinyl-aromatic monomer, and at least one member selected from a group consisting of (meth)acrylonitrile and alkyl(meth)acrylate, where said polymerized product of said vinylaromatic monomer is present in an amount of about 60 to 75 percent and the polymerized product of said member of said group is present in an amount of about 25 to 40 percent relative to the weight of said subsequent grafted phase, and wherein weight ratio of said first grafted phase to said subsequent grafted phase is 1/4 to 4/1, said grafted rubber is present in particulate form having a weight average particle size of about 0.1 to 1.0 microns, and wherein matrix contains the polymerized product of at least one vinylaromatic monomer and at least one monomer selected from the group consisting of (meth)acrylonitrile and alkyl(meth)acrylate.
In a more preferred embodiment, the invention is directed to the above described thermoplastic molding composition comprising (i) 15 to 50 percent by weight (pbw) of the above described grafted rubber and (ii) 85 to 50 pbw of a the above described matrix, the percent being relative to the total of (i) and (ii).
(i) The Grafted Rubber and Its Preparation
The grafted rubber component includes a substrate (S) onto which are grafted: a first grafted phase (P1) and a subsequent grafted phase (P2). The grafted phases are sequentially grafted to the substrate by conventional processes. Essentially, the first grafted phase is polymerized in the presence of the substrate to produce a precursor and the subsequent phase is polymerized and grafted on to the precursor in a subsequent step. The processes for the preparation of the substrate and the grafting processes are conventional and are well known in the art. Relevant information may be found in ABS Resins, Stanford Research Institute, Report No. 20, December 1966, Muller, R. G., Takaoka, S., and in Encyclopedia of Polymer Science and Engineering, 2nd Ed., Vol. 1, pp 400-405, (1985).
The weight ratio of the substrate to the total weight of the grafted phases, the ratio (S)/(P1+P2) is advantageously in the range of 60/40 to 90/10, most preferably the ratio is about 70/80 to 30/20. The first and subsequent phases relate by weight one to the other (P1/P2) in the range of 20/80 to 80/20, preferably 33/67 to 67/33.
The substrate (S) is characterized in that it contains an elastomeric crosslinked rubber. In a more preferred embodiment, the substrate contains a core (C) and a shell (SH). In this more preferred embodiment, the core and shell relate one to the other by weight, that is (C)/(SH), in the range of 1/99 to 25/75.
The substrate is essentially a crosslinked elastomer which contains at least one member selected from the group consisting of polyalkyl acrylate, hydrogenated polydiene and polydiene.
In the preferred embodiment where the substrate contains a core and a shell
Auclair Richard M.
Chang Moh Ching Oliver
Sanocki Robert A.
Bayer Corporation
Gil Joseph C.
Mullis Jeffrey
Preis Aron
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