Polymer alloys, characteristics

In the area of specialty polymers, we are seeing an explosion of new polymer blends, alloys, and composites. The properties of novel polymer alloys, for example, are significantly better than those of the materials from which they are blended, but many aspects of these alloys are not well understood. Most of the materials consist of multiple polymer phases. But there is still uncertainty as to the desired characteristics and size of the polymer domains and the mechanisms by which forces are transferred through the material. All of these questions will benefit from the chemical engineering approach. 

Polymer melt flow properties determine to a large extent the characteristics of the forming process. The detailed discussion on rheology is provided in Chapter 7 Rheology of Polymer Alloys and Blends. Rheological behavior will only be summarized here in order to provide sufficient information to understand and discuss polymer forming. 

The automotive industry started to use plastics in 1946. Since then, its content steadily increases (see Figure 16.4). Today, the plastics for automotive apphcations are dominated by polymer alloys, blends, and composites. The reason for it is the need for automotive parts to show a wide range of performance characteristics that are virtually impossible to meet using a single-component polymer. Thus, for example, in Saturn, front fenders and rear quarter panels are from PA/PPE, door outer skins are from PC/ABS, the bumper fascias are TPO, etc. 

During the last years, a number of products consisting of a mixture of different plastics have made their appearance they are usually called polymer blends or polymer alloys. Their identification using simple methods presents considerable difficulties because flame tests and pyrolysis tests are usually not unambiguous. Also a separation into different groups according to the pH-value of the pyrolysates does not permit a definite conclusion. In some cases, however, it is possible to separate polymer mixtures into their components if these have different solubility characteristics and then to identify the components (see Section 6.3). 

The combination of lithography, electroforming, and plastic molding termed LIGA was devised by Ehrfeld s group for the mass production, at low cost, of microsized components made from polymers, metals, and alloys. Characteristics include structures of any lateral geometry. 

The cost of polymer alloys is mainly determined by the composition. By contrast, the profit that is based on the alloys performance is controlled by the way the material is processed, i.e., by morphology and stability. The compounding process must ascertain that the alloy has the desired spectrum of the performance characteristics. 

Block copolymers are useful in many applications where a number of different polymers are connected together to yield a material with hybrid properties. For example, thermoplastic elastomers are block copolymers containing a rubbery matrix (polybutadiene or polyisoprene) containing glassy hard domains (often polystyrene). The block copolymer, a kind of polymer alloy, behaves as a rubber at ambient conditions, but can be molded at high temperatures because of the presence of the glassy domains that act as physical cross-links. In solution, attachment of a water-soluble polymer to an insoluble polymer leads to the formation of micelles in amphiphilic block copolymers. The presence of micelles leads to structural and flow characteristics of the polymer in solution, that differ from either parent polymer. 

Polymer alloys are a commercial polymer blend with improvement in property balance with the use of compatibilizers. They exhibit an interface and show varied physical characteristics. Sometimes they have excellent physical properties in one area but possess poor physical properties in others. For example, silicone rubber has poor oil and abrasion resistance but possess excellent heat resistance. A product solution in this regard would be to obtain a polymer blend with constituents possessing physical properties that complement each other such that the resultant polymer blend would exhibit superior physical properties compared with the components of the blend. 

They used amphiphilic methacrylate polymer with a ternary amino group, poly(A,A-diisopropylaminoethyl methacrylate (DlPAM)-co-n-decyl methacrylate (DMA)) (PDD), composed of 25 unitmol% of DMA in the polymer as a polymeric additive to SPU. The content of the PDD in the SPU was in the range of 1-5 wt%. The surface characteristics of the SPU/PDD polymer alloy were examined by XPS and dynamic contact angle measurements with water. These evaluations revealed that the PDD was located on the surface of the polymer alloy. This induced a more hydrophilic and movable surface compared with untreated SPU. Protein adsorption from human plasma was also reduced on the surface of the SPU/PDD polymer alloy. During the blending process, the solubility of the polymer added to the SPU is important. The solubility parameter of the polymer is one of the factors used to estimate the blending state of both polymers. 

The nickel coupling of aryl chlorides was studied in-depth in our laboratories. It was demonstrated that the corresponding biaryls can be produced in quantitative yields. Hence, it was felt that the method should be appropriate for the preparation of high polymers. Our studies, as herein reported, do indeed show that materials of excellent quality can be prepared via this novel route. Also, proper choice of the starting monomers allows for the synthesis of an almost infinite variety of polymers. In general, these polymers contain biphenyl moieties which contribute excellent thermal, mechanical, impact and alloying characteristics. 

Polyamides and polyolefins are two important classes of commercial polymers. The former are frequently blended with lower-modulus polyolefins to prepare polymer alloys of enhanced characteristics. Usually, in a blend, polyolefins offer melt strength, flexibility, lubricity, impact strength, electrical resistance, low dielectric constant and dielectric loss, water resistance and low price. Polyamides offer melt fluidity, rigidity, strength, high-temperature performance and solvent resistance. 

In designing an alloy, polymer chemists choose candidate resins according to the properties, cost, and/or processing characteristics required in the end product. Next, compatibility of the constituents is studied, tested, and either optimised or accommodated. 

Certain polymers have come to be considered standard building blocks of the polyblends. For example, impact strength may be improved by using polycarbonate, ABS and polyurethanes. Heat resistance is improved by using polyphenylene oxide, polysulphone, PVC, polyester (PET and PBT) and acrylic. Barrier properties are improved by using plastics such as ethylene vinyl alchol (EVA). Some modem plastic alloys and their main characteristics are given in Table 1.2. 

Alloys are combinations of polymers that are mechanically blended. They do not depend on chemical bonds, but do often require special compatibilizers. Plastic alloys are usually designed to retain the best characteristics of each constituent. Most often, property improvements are in such areas... 

In a study of thermal stability and hydrogen sorption characteristics of a series of sorbent tablets composed of hydride-forming metals dispersed in polymers under a 50% hydrogen in argon atmosphere, it was found that tablets of 80% palladium in PTFE, and 80% of 1 5 atom lanthanum-nickel alloy in PTFE could not be used above 247° C because of explosive decomposition of the PTFE. 

Polyolefins are well adapted to the mono-material concept talc-filled polypropylene and LFRT for structural parts, foamed polyethylene and polypropylene for damping, polypropy-lene/EPDM alloys or copolymers for skins. Some other functions need incompatible polymers with specific characteristics such as optical properties. Without claiming to be exhaustive, the other thermoplastic materials are ... 

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