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
2001-09-13
2003-12-30
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...
C525S066000, C525S067000, C525S068000, C525S069000, C525S076000, C525S080000, C525S087000
Reexamination Certificate
active
06670419
ABSTRACT:
The present invention relates to a method of toughening thermoplastic polymers through incorporation of comb copolymers. The present invention further relates to compositions and articles produced thereby.
It is well known in the art that thermoplastic polymers are often brittle, in and of themselves, when melt processed into articles. These thermoplastic polymers may, however, be toughened through incorporation of toughening agents commonly known in the art as impact modifiers. An impact modifier may, typically, be combined with a thermoplastic polymer as a solids blend, or dry blend. The solids blend may then by subjected to melt processing (i.e., mixing at elevated temperature) and shaping to form an article. Through this process of dry blending, followed by melt processing, the impact modifier particles may become dispersed throughout the melt blend and, hence, throughout the article that is ultimately formed.
Typically, an impact modifier is a particulate polymer having one or more phases, wherein at least one of those polymeric phases is characterized by a glass transition temperature, Tg, below the envisioned use temperature of the article into which it is to be incorporated. Although not wishing to be bound by theory, it is thought that the dispersed low Tg impact modifier phases serve to distribute and dissipate the energy associated with a narrowly focused impact event.
Core-shell impact modifiers are used commercially to toughen poly(vinyl chloride) and other thermoplastic polymers. These core-shell polymers exist as particles having a diameter (i.e., particle size) of characteristically from 0.1 to one micron. The core is made of a low Tg polymer (e.g., less than 0° C.) which is crosslinked. The core polymer is further not miscible with the thermoplastic polymer due to its composition and its state of crosslinking. As a result, it is necessary to provide the core with a shell made of a polymer that is miscible with the thermoplastic polymer. During melt processing, the miscible shell polymer, which may be grafted to the core polymer to some extent, interacts with the thermoplastic polymer to speed the break up of the larger powder particles (typically a few hundred microns in diameter) into much smaller core-shell particles (0.1-1 micron), and to enhance dispersion of the individual core-shell particles into the thermoplastic melt. In this way, thermoplastic articles are made that contain uniformly distributed core-shell particles, and hence rubbery core domains, throughout. While these core-shell polymers perform as impact modifiers for many thermoplastics, they are limited in that their crosslinked cores have only a single size and not all of the compatibilizing chains of shell polymer are attached to the core network. Thus, the shell polymer never fully realizes its potential to compatibilize the core polymer (Plastic Additives and Modifiers Handbook, Ed. J. Edenbaum, pp 559-589, Chapman and Hall NY, 1996).
U.S. Pat. No. 5,120,790 discloses comb copolymers bearing polylactone side chains. These polylactone comb copolymers, having number average molecular weights of at least 10,000, are disclosed as forming compatible blends with thermoplastic resins. The extent of compatibility of these comb copolymers, the backbones of which are not miscible in the thermoplastic resins, is limited by the extent to which the polylactone side-chains are miscible in any given thermoplastic resin. Unfortunately, the solubility characteristics of thermoplastic resins vary greatly, yet the solubility characteristics of the side chains of the U.S. Pat. No. 5,120,790 disclosure are confined to the narrow range of values possible for polylactones. As such, there will be many thermoplastic resins in which the polylactone side chains will not be soluble, and therefore will not be effective to compatibilize the comb copolymer backbone. Without such compatibilization, the comb copolymer is unable to bring about improvement in features of the thermoplastic polymer, such as toughening, or the ability to form compatible blends with other thermoplastic polymers. Moreover, it is well known in the art that polyesters like the polylactones of U.S. Pat. No. 5,120,790 are easily hydrolyzed by water at elevated temperature or by basic pH conditions (Encyclopedia of Polymer Science and Engineering, Ed. H. F. Mark, volume 12, p. 5, John Wiley and Sons, NY (1988)). Further, when processing at melt temperatures, polyester-containing materials (here, polylactone-containing) must be carefully dried to avoid polymer decomposition, and under use conditions (weathering, exposure, etc.) contact with alkaline substrates and exposure to pH conditions deviating significantly from neutral must be avoided.
We have, surprisingly, found that comb copolymers bearing graft segments other than polylactone may be blended with thermoplastic polymers and the melt blend processed into articles that are toughened when compared with similar articles prepared from the thermoplastic polymer absent the comb copolymer. The comb copolymers of the present invention are particularly resistant to degradation under conditions of prolonged weathering. Because the compositions of the graft segments of the present invention may be varied over a wide range, compatibilization of the comb copolymer of the present invention with a wide variety of thermoplastic resins is possible. The comb copolymers of the present invention, preferably, produced by an aqueous emulsion process that results in the preparation of a plurality of comb copolymer particles having uniform size well defined molecular weight. The synthetic method further makes possible preparation of compositionally diverse backbones and graft segments, so that a comb copolymer well suited to toughen a particular thermoplastic polymer may be readily prepared. The comb copolymers typically can be isolated and stored as powders without compacting, clumping, or fusing. Heating and mixing of these solids blends produces melt blends, having improved melt processing behavior, that can be shaped and cooled to produce homogeneous, useful thermoplastic articles. The improved melt processing behavior imparted by the comb copolymer assures production of homogeneous articles having uniform distribution of domains of immiscible rubbery polymer throughout. Articles toughened in this way are thus produced at high output rates with minimal downtime in such processing operations as, for example, calendering, extrusion, blow molding, injection molding, expansion into foam, and making of bi-oriented materials.
One aspect of the present invention relates to a method comprising the steps of:
(a) forming a solids blend comprising a thermoplastic polymer and a comb copolymer;
(b) heating and mixing said solids blend to form a melt blend;
(c) shaping said melt blend to form an article; and
(d) cooling said article to room temperature;
wherein said comb copolymer comprises a backbone and at least one graft segment;
wherein said graft segment does not contain, as polymerized units, lactone monomer; and
wherein said comb copolymer is present in an amount sufficient that said article has an impact resistance energy increased at least 15 percent compared to a second article formed identically, absent said comb copolymer.
A second aspect of the present invention relates to a toughened article:
wherein said article comprises a thermoplastic polymer and a comb copolymer;
wherein said comb copolymer comprises a backbone and at least one graft segment;
wherein said graft segment of said comb copolymer does not contain, as polymerized units, lactones; and
wherein said comb copolymer is present in an amount sufficient that said article has an impact resistance energy increased at least 15 percent compared to a second article formed identically, absent said comb copolymer.
A third aspect of the present invention relates to an article produced by the method of the first aspect of the present invention.
In yet another aspect of the present invention, the thermoplastic polymer is a polymer selected from the group consistin
Lau Willie
Van Rheenen Paul Ralph
Clikeman Richard R.
Mullis Jeffrey
Rohm and Haas Company
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