Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-12-20
2003-09-23
Harlan, Robert D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C525S083000, C525S232000, C525S902000
Reexamination Certificate
active
06624212
ABSTRACT:
The present invention relates to a plastics additives powder composition, a process for preparing the same, thermoplastic resin blends containing the same, and methods of improving the impact properties and enhancing processability of thermoplastics. These compositions and processes provide plastics additive powders having excellent powder flow properties that provide a combination of superior impact strength and processability to thermoplastic resins, especially polyvinyl chloride.
Thermoplastic resins ordinarily require various additives for modifying their processing and/or property characteristics. Examples of such additives for plastics include: dyes and pigments for altering color; thermal stabilizers and antioxidants for reducing degradation and coloring during processing, high temperature use, and/or long-term weathering; fillers for reducing cost and/or increasing rigidity; lubricants for improving processability and reducing sticking to machine surfaces; antistatic agents for reducing build up of static charge in plastic parts; plasticizers for increasing plasticity and flexibility; impact modifiers (“IM”) for improving impact strength to reduce part breakage; and high polymer processing aids (“processing aids”, “PA”) for controlling the rheological characteristics for optimizing resin processability and increasing process efficiency.
During the preparation of thermoplastic resin blends and plastic part fabrication, the various additives are usually added as separate powdered, pelletized, or liquid components to the thermoplastic resin. Because thermoplastic blend formulation usually requires one to handle many materials having varied physical properties, preparation of these blends is both costly and complex. (Generally, see “
Plastics Additives and Modifiers Handbook
” J. Edenbaum, Ed., Van Nostrand Rein, 1992 for discussion of various additives for plastics.)
It is therefore desirable to obtain compositions of plastics additives that not only reduce cost but also reduce the complexity of preparing fully formulated thermoplastic resin blends. It is also desirable to obtain compositions of plastics additives that provide further improvements to the physical use properties as well as the processability of fully formulated thermoplastic resin blends.
Impact modifiers for thermoplastic resins are rubber-containing particles, typically having diameters in the range of from 50 to 1000 nm, which are dispersed throughout the thermoplastic resin. Conventionally, these impact modifiers include at least one rubbery polymer particle surrounded by at least one hard polymer shell and are prepared using emulsion polymerization techniques. The rubbery polymer portion is believed to enable the thermoplastic matrix resin to absorb physical shocks, prevent crack initiation, and prevent crack propagation in plastic parts, resulting in reduced breakage and increased impact strength. For high impact efficiency the mean particle size of the impact modifier should generally be greater than 100 nm. Such rubbery polymers are conventionally based on units derived from ethylenically unsaturated monomers that provide glass transition temperatures (“Tg”) below 25° C. Examples of monomers that provide rubbery polymers include butadiene, isoprene, C1-C8 alkyl acrylates, alpha-olefins, ethylenically unsaturated siloxanes and ethers, and copolymers of mixtures thereof.
Because polymer particles tend to be sticky and not isolatable as a dry powder, a hard polymer shell is typically added to the exterior of each rubbery “core” particle in order to prepare impact modifiers as dry powders that are easily handled. The hard polymer shells of impact modifiers are ordinarily selected to be compatible with the thermoplastic resin so that the impact modifier (“IM”) disperses readily into the thermoplastic resin during compounding. The hard polymer shells are usually derived from vinyl aromatic (e.g., styrene), methacrylic (e.g., methyl methacrylate), and acrylonitrile monomers. Often, graftlinking agents are added to either the rubbery or hard polymer phases to increase the strength of attachment of the shell to the core.
Generally, as the rubbery weight fraction of an IM increases, the required amount of IM in the thermoplastic formulation decreases. The amount of impact modifiers in a thermoplastic resin formulation varies with the type of resin and application, but is generally between 3 and 30 parts based on 100 parts thermoplastic resin (“phr”). In creating “efficient” impact modifiers, therefore, the weight fraction of the rubbery core in the IM is typically maximized. However, it has been conventionally known that if the rubbery core fraction is too high then the hard shell polymer is not able to completely cover the rubbery core, thereby resulting in poor powder properties and dispersibility. Depending on the monomers used, the maximum core:shell weight ratio in powdered impact modifiers has conventionally been about 88:12. It is therefore desirable to increase the rubbery weight fraction in impact modifiers for plastics which have good powder properties and disperse readily in thermoplastic resins using conventional equipment.
Processing aids for thermoplastic resins are typically polymers and copolymers containing units polymerized from ethylenically unsaturated monomers such as vinyl aromatic, (meth)acrylonitrile, and/or C1-C4 alkyl methacrylate monomers. Processing aids are typically prepared using emulsion polymerization techniques to yield dispersions of 20-500 nm mean diameter hard polymer particles having a molecular weight in the range of from at least 50,000 to greater than 5,000,000 g/mol and a Tg greater than 25° C. The processing aid particle dispersions are typically dried and isolated to form a free-flowing powder, the powder particles having a 50-500 micron mean diameter. This PA powder is subsequently added to thermoplastic resin formulations.
The amount of PA used in a thermoplastic resin formulation varies with the type of resin and application, but is generally between 1 and 15 phr. Processing aids are commonly compatible with the thermoplastic resin. For example, processing aids based on polymers and copolymers prepared with methyl methacrylate (“MMA”) monomer which have a molecular weight greater than 1,000,000 g/mol are commonly added to PVC resin formulations to promote quick fusion (melting), and thereby increasing process efficiency, of the PVC resin. Processing aids are also useful in increasing the melt strength of thermoplastic resins, which is important during certain types of process applications, such as during foaming and thermoforming of molten thermoplastic resin blend formulations.
U.S. Pat. No. 5,442,012 describes encapsulated plastics additives containing both impact modifier and flow improving (processing aid) particles for modifying the impact properties and processing characteristics of PVC and thermoplastic matrix polymers. Both impact modifier and processing aid particles are prepared separately at particle sizes less than 100 nm by emulsion polymerization, co-micro-agglomerated at temperatures above 70° C., and subsequently encapsulated by a final shell polymer. Although the encapsulated shell polymer allows for the isolation of the impact modifier having acceptable flow properties, its presence dilutes the concentration and effectiveness of the impact modifier and processing aid components in the encapsulated plastics additives. Moreover, the impact modifying efficiencies afforded by these plastics additives are limited because the impact modifier particles must have a particle size below 100 nm. As a result, using these encapsulated plastics additive powders in PVC provide similar, but not improved, impact strength and processing characteristics compared to using equal amounts of separate impact modifier and processing aids.
The present inventors have discovered new plastics additives powders and processes for preparing these powders that overcome the shortcomings of U.S. Pat. No. 5,442,012. The present inventors have discovered new plastics additives po
Coyle Robert William
Weier Jane Elizabeth
Wills Morris Christopher
Harlan Robert D.
Rohm and Haas Company
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