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
2004-09-28
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...
C525S088000
Reexamination Certificate
active
06797775
ABSTRACT:
The present invention relates to a method of improving the melt processing of thermoplastics by forming blends with segmental polymers. The present invention further relates to articles produced thereby.
Compositions of the thermoplastic polymer “poly(vinyl chloride)”, “PVC”, in the absence of a plasticizer or processing aid, are difficult to process into homogeneous, useful articles. The compositions are difficult to flux (i.e., to transition from a solids blend to a fused melt blend), and the resulting melts are heterogeneous and “cheesy” having poor melt strength and low elongation. Plasticizers eliminate many of these processing problems but with a resultant loss of physical properties, particularly rigidity, in the thermoplastic articles produced.
Processing aids for PVC are polymeric additives that allow PVC to be processed to give good physical properties without loss of rigidity. It is known in the art that reducing the glass transition temperature, Tg, of processing aids for PVC allows those processing aids to disperse more completely into PVC (U.S. Pat. No. 3,833,686, EP394659). This can be seen in improved clarity and reduced gel defects in clear PVC sheet and indirectly in how rapidly the PVC fuses from a solids blend into a thermoplastic melt blend during processing. This greater ease of fusion manifests itself in, for example, shorter fusion time at a given temperature, or the achievement of a given fusion time at a lower temperature.
Unfortunately, there are limits to the extent to which the Tg of a processing aid can be lowered in that the processing aid, in the dry state, must be a free flowing solid, typically a powder, and it must remain so under typical storage conditions in order to be blended and formulated with PVC powder prior to melt processing. If the processing aid is an emulsion or suspension polymer it must not fuse or form a film during its isolation which is typically accomplished by such operations as spray drying at elevated temperature. Typically, if the Tg of the processing aid drops below 50° C., a free flowing, storage stable powder can not be obtained, nor can the clumps, or even “bricks” that form during storage be easily broken down during solids blending to form a uniform solids blend.
One approach to reducing overall Tg of a processing aid is to use polymers prepared in two polymerization stages, wherein one component has a reduced Tg. U.S. Pat. No. 3,833,686 discloses two-stage sequentially produced, core-shell particles made by emulsion polymerization, wherein the lower Tg component is the core and a high Tg material, e.g., methyl methacrylate (MMA) is the shell. These core-shell particles contain substantially proportions of low Tg polymers having no high Tg segment, high Tg polymers having no low Tg segments, and polymers having both high and low Tg segments. Further, many of the chains are very long and have networked structure due to adventitious crosslinking. While one achieves a free flowing powder in this way even if the Tg of the core is near to, or below, room temperature, it is an unfortunate reality that such polymer particles are unable to break down fully during melt processing and, therefore, cannot realize their full potential as processing aid.
We have, surprisingly, found that segmental copolymers may be produced as non-tacky powders, yet behave as processing aids for thermoplastics (e.g., PVC), providing short fusion times and low fusion temperatures consistent with those promoted by low Tg processing aids. These segmental copolymers can further be stored under typical conditions without compacting, clumping, or fusing. They can be transported to a processing hall, and combined with thermoplastics to make solids blends, again without clumping. 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. Such improved processing behavior is essential if articles of consistently high quality are to be 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 segmental copolymer; and
(b) mixing and heating said solids blend to form a melt blend;
wherein said melt blend has a melt processing improvement term having a value of at least 10.
A second aspect of the present invention relates to a method comprising the steps of:
(a) forming a solids blend comprising a thermoplastic polymer and a segmental copolymer;
(b) mixing and heating 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 melt blend has a melt processing improvement term having a value of at least 10.
A third aspect of the present invention relates to an article, wherein said article comprises a thermoplastic polymer and a segmental copolymer.
A fourth aspect of the present invention relates to a plastic article produced by the method of the second aspect of the present invention.
In another aspect, the thermoplastic polymer of aspects one through four is a polymer selected from the group consisting of poly(vinyl halide) homopolymer, poly(vinyl halide) copolymer, chlorinated poly(vinyl chloride) “CPVC”, and combinations thereof. A preferred thermoplastic polymer is poly(vinyl chloride). In a still further aspect, the segmental copolymer of aspects one through four is a copolymer selected from the group consisting of comb copolymer, block copolymer, and combinations thereof. It is preferred that the segmental copolymer is a comb copolymer.
Used herein, the following terms have these definitions:
The “backbone” of a polymer chain is a collection of polymerized monomer units attached to one another. The attachment is typically achieved by covalent bonding. “Non-terminal” monomer units are directly attached to at least two other monomer units. A “terminal” monomer unit resides at the end of the polymer chain and is directly attached to one other monomer unit. For example, the polymerized monomer units of the backbone may be derived from ethylenically unsaturated monomers.
A “linear” polymer (homopolymer or copolymer) is a polymer having a backbone that is not branched. As used herein, the term “linear” is also meant to include polymers wherein a minor amount of branching has occurred. For example, hydrogen abstraction may lead to branching during free radical polymerizations.
A “branched” polymer is a polymer having a first “backbone segment” that has other backbone segments (i.e., “branches”) chemically attached to it through a “non-terminal” atom of the first backbone segment. Typically, this first backbone segment and all of the branches have the same, or similar, composition.
A “pendant” group is a group that is attached to the backbone of a polymer. The term pendant may be used to describe a group that is actually part of a polymerized monomer unit. For example, the hydroxyethyl group of a polymerized unit of 2-hydroxyethyl methacrylate may be referred to as a “pendant hydroxyethyl group”, or as “pendant hydroxy functionality”. It is also common to refer to large groups attached to a polymer backbone as “pendant” when those large groups are compositionally distinct from the backbone polymer. These large groups may themselves be polymer chains. For example, when a macromonomer becomes incorporated into a polymer chain by reaction with other monomers, the two carbons of its reactive double bond become part of the backbone, while the polymeric chain originally attached to the double bond of the macromonomer becomes a “pendant group” that may, for example, have a molecular weight of 500 to 100,000. A “pendant” group may further be described as “pendant to” the backbone.
A “terminal” group resides at the end of the polymer chain and is c
Lau Willie
Van Rheenen Paul Ralph
Clikeman Richard R.
Mullis Jeffrey
Rohm and Haas Company
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