Blending of immiscible polymers offers attractive opportunities for developing new materials with useful combinations of properties. However, simple blends often have poor mechanical properties and unstable morphologies. Compatibilization of such blends is necessary. Preformed graft or block copolymers have been traditionally added to act as compatibilizers. Another route, however, is to generate these copolymer compatibilizers in situ during melt blending using functionalized polymers. In this review, a variety of reactive polymers that have been utilized in the reactive compatibilization of polymer blends is examined. They are classified into six major categories according to the types of reactive groups they have, namely, maleic anhydride, carboxylic acids, carboxylic acid derivatives, primary and secondary amines, hydroxyl and epoxide, and reactive groups capable of ionic bonding. Their preparation methods and applications and the chemical reactions they undergo during melt blending are presented.
Polymer blends with enhanced properties and processability for specific applications are modified through the incorporation of a variety of additives. The performance of these additives is often difficult to predict from considerations of their known behavior in the separate blend constituents. This overview article discusses additives and modifiers in terms of types and specific functions (e.g., impact modifiers, compatibilizers, flame retardants, stabilizers) and provides an analysis of the possible effects of their repartition in the constituent phases and at the interface.
Blends of high viscosity polypropylene (PP) and low viscosity low density polyethylene (LDPE) were prepared in the Brabender batch intensive mixer. The rheological mismatch of these immiscible blend components resulted in coarse blend morphologies and poor mechanical properties. By reacting these blends with 1800 ppm Lupersol 101 peroxide at 180 degree C, the morphology became much finer, and at 50/50 weight ratio, assumed co-continuous phase structure. As a result, the mechanical properties improved significantly. The above observations are attributable to the fact that the peroxide reaction at this level of concentration removes the rheological disparity of the blend components, a condition that is more favorable for efficient dispersive mixing. It has been frequently stated that the scale and uniformity of dispersive mixing affects the uniformity and controllability of reactions occurring in molten blends. This work shows, for the first time, that reactions with peroxides alter the blend morphology in a beneficial manner. The two phenomena are, therefore, mutually dependent.
The author presents a newly discovered polymerization process, called the free-radical retrograde-precipitation polymerization (FRRPP) process, that is shown to have certain desirable features not normally found in free-radical polymerization processes. Here, free radical polymerization occurs with an unusual but widely applicable polymer precipitation phenomenon. Local heating is observed in the vicinity of the radical sites, which results in a controlled rate of propagation with a decrease in the rate of radical-radical termination reactions. In experiments performed, the existence of hot spots in the reactor fluid is shown to occur. There is also evidence of a slowdown in the growth of polymer radicals after initiator decomposition. The polymer radicals, shown to have a relatively narrow molecular weight distribution, continue to grow at a controlled rate long after all the initiator molecules have decomposed. The addition of a second monomer results in the formation of a thermoplastic elastomer triblock copolymer.
Simple blends of poly(2,6 dimethyl 1,4-phenylene ether) (PPE) and polypropylene (PP) are highly incompatible and consequently quite brittle regardless of the molecular weights and ductility of the individual components. Hydrogenated, styrene-diene block copolymers, viz., S-EB-S triblock and S-EP diblock copolymers, can be used to compatibilize and toughen the blends. The ductility and impact toughness of the PPE-PP blends depend not only upon the nature of styrene block copolymers but also on the nature of PPE and PP used. It was found that lower-molecular-weight PPE was actually better in achieving a blend with desired combination of high-impact toughness and high heat distortion temperature. Compatibilization efficiency and morphological considerations influencing the blend's properties are discussed.
This is the first part of a survey of developments reported in the open literature during 1991 on the subject of injection molding of fiber-reinforced thermoplastics. Topics discussed include materials, polymers, reinforcing fibers and composite properties, advances in the injection molding process, fiber breakage, shrinkage, and warpage, as well as alternatives to the injection molding process.
Reactive compounding utilizing extruders has been employed in the industry for quite some time. With better understanding of chemistry combined with more in-depth knowledge of extrusion systems, the field of reactive compounding now stands as a front-runner of new materials and alloys being developed. Better understanding and the availability of more sophisticated compounding equipment, such as twin-screw extruders and kneaders, have provided some unique opportunities to the users as well as producers. The purpose of this article is to discuss the essential elements of the continuous kneader when used as a compounding reactor, particularly when the process is used in contract manufacturing facilities. Two case histories of application of reactive compounding processes are cited, one involving silane grafting, while the other involves crosslinking plus reactive compounding.