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
2001-05-09
2002-12-10
Nutter, Nathan M. (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...
C525S071000
Reexamination Certificate
active
06492467
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for preparing blends of ABS polymers; more particularly, the process relates to the continuous mass polymerization of ABS polymers.
SUMMARY OF THE INVENTION
A process for the preparation of a blend of ABS polymers is disclosed. The invention comprises introducing a core/shell grafted rubber in the form of powder (crumb) to at least one stage of the continuous mass polymerization process for making an ABS polymer. Accordingly, the process comprises, in sequence, steps (i) through (iii) wherein
(i) refers to charging into a first reactor a material system containing primarily monoalkenylaromatic monomer and an ethylenically unsaturated nitrile monomer having dissolved therein diene rubber under conditions to promote at least partial polymerization to form a partially polymerized first mixture and where
(ii) refers to polymerizing the first mixture in a subsequent reactor to form a subsequent mixture and where
(iii) refers to devolatilizing the subsequent mixture,
and introducing said core/shell grafted rubber into at least one of step (i), (ii) and (iii) to yield a blend having a dispersed grafted rubber phase with a bimodal or multimodal particle size distribution.
BACKGROUND OF THE INVENTION
Polyblends, that is blends of rubber grafted with styrene/acrylonitrile polymers represent a family of thermoplastic molding compositions having excellent mechanical properties, most notably, resistance to impact. The conventional process to make polyblends of ABS (acrylonitrile-butadiene-styrene) may entail compounding of an emulsion-made intermediate having high rubber content (50-60%) with pellets of styrene/acrylonitrile copolymer (SAN) and/or with continuous mass-made intermediates having a lower rubber content (less than 15%). The continuous mass (CM) process is economical, yet, the product thus produced is limited in terms of its properties due to limitations on the rubber content. For example, low gloss extruded products have large rubber particle size and relatively low rubber content resulting in relatively lower impact strength. Compounding of CM-product with small amounts of emulsion-produced intermediate increases the impact strength but entails an additional process step, which makes the process less desirable.
Various processes for the manufacture of such polyblends, including emulsion., suspension and mass polymerization techniques, and combinations thereof have been reported. Such polyblends of monoalkenyl aromatic and ethylenically unsaturated nitrile monomers and grafted rubber that are prepared in the mass polymerization process exhibit desirable properties, yet, this technique has a practical limitation upon the maximum degree of conversion of monomers to polymer. Techniques have, therefore, been adopted where initial mass polymerization is carried out to a point of conversion beyond phase inversion where the viscosity is still of practical magnitudes, and the resulting pre-polymerized syrup is suspended in water or other inert liquid and polymerization of the monomers is continued. U.S. Pat. No. 3,509,237 disclosed a mass/suspension method of polymerization styrene/acrylo-nitrile having diene rubbers dissolved therein with the rubber being grafted, inverted and dispersed as rubber particles under agitation. After phase inversion, the viscous mixture is suspended in water and polymerization is completed producing a polyblend in the form of beads. Such mass/suspension processes have the shortcomings of batch operations. U.S. Pat. No. 3,243,481 disclosed a process in which diene rubbers are dissolved in predominantly monovinylidene aromatic monomers and polymerized in four reaction zones. Such processes require physically separated reactors providing different reacting conditions for each step of polymerization involving costly multiple reactors and specialized equipment. U.S. Pat. No. 3,658,946 disclosed a process wherein the prepolymerization is run to a solids content of no more than 16% to provide a rubber particle having a particular structure. U.S. Pat. No. 3,751,526 disclosed a process for producing rubber modified thermoplastic resins by extracting a grafted diene rubber from a latex into a styrene-acrylonitrile monomer phase, separating the monomer-rubber phase from the water phase and mass polymerizing the monomer-rubber phase to provide a polyblend of styrene/acrylonitrile polymer and a single grafted rubber phase. The process disclosed in U.S. Pat. No. 3,509,237 refers to the preparation of styrene/acrylonitrile/rubber polyblends having a first and second grafted rubber phase wherein the first grafted rubber has a large particle size and the second grafted rubber has a smaller particle size. Such polyblends have superior properties if the smaller particle size rubber phase constitutes the largest proportion of the total rubber phase. A process for making such polyblends is disclosed wherein the two grafted rubber polyblends are prepared in batch processes separately and, thereafter, melt blended mechanically to form a polyblend having a first and second grafted rubber phase. U.S. Pat. No. 3,903,199 disclosed a relevant continuous mass polymerization process for preparing a polymeric polyblend. The process entails polymerizing monomers containing 0.1 to 15% by weight of a soluble diene rubber so as to convert 10 to 40% of the monomers to a polymer. A portion of the polymer is grafted on the diene rubber and dispersed as a first grafted diene rubber in a monomer-polymer phase to provide a partially polymerized mixture. This grafted diene rubber is dispersed as rubber-monomer particles having an average particle size of 0.7 to 10 microns and having present occluded and grafted polymer. The process further entails continuous withdrawal of this partially polymerized mixture and continuous polymerizing of this mixture until the polymerization has reached a conversion of about 25 to 80%. A second grafted crosslinked diene rubber is continuously being dispersing in the polymerized mixture.
The present invention is predicated on the findings that a particularly structured, emulsion-produced grafted rubber, namely a core/shell-structured grafted ABS may be added directly to the CM process resulting in a polyblend having improved properties.
DETAILED DESCRIPTION OF THE INVENTION
The invention is predicated on the finding that a polyblend having desirable mechanical properties may be prepared by a hybrid process that incorporates a particular product that is produced by emulsion polymerization (the products is herein sometimes referred to as “crumb”) as a component in the continuous mass polymerization process. The morphology of the crumb, the core/shell structure of the rubber particles was found to be a key parameter for making the necessary stable dispersion of rubber in a system containing monomers and/or solvent. The rubber content of the crumb needs to be sufficiently high and the CM product needs to be the dominant component in the resulting polyblend.
Accordingly, the process comprises introducing a emulsion polymerization produced core/shell grafted diene rubber to at least one stage of the continuous mass polymerization process for making an ABS polymer. Comprising in sequence, steps (i) through (iii) wherein
(i) refers to charging into a first reactor a material system containing primarily monoalkenylaromatic monomer and an ethylenically unsaturated nitrile monomer having dissolved therein diene rubber under conditions to promote at least partial polymerization to form a partially polymerized first mixture and where
(ii) refers to polymerizing the first mixture in a subsequent reactor to form a subsequent mixture and where
(iii) refers to devolatilizing the subsequent mixture,
and introducing said core/shell grafted rubber into at least one of step (i), (ii) and (iii) to yield a blend having a dispersed grafted rubber phase with a bimodal or multimodal particle size distribution.
Specifically, in step (i) a continuous mass polymerization process of monomers containing 5 to 20% by weight of a soluble d
Kim Joong-In
Wu Wan Chu
Asinovsky Olga
Bayer Corporation
Gil Joseph C.
Nutter Nathan M.
Preis Aron
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