Polymer stabilizers examples

Formaldehyde homopolymer is composed exclusively of repeating oxymethylene units and is described by the term poly oxymethylene (POM) [9002-81-7]. Commercially significant copolymers, for example [95327-43-8] have a minor fraction (typically less than 5 mol %) of alkyUdene or other units, derived from cycHc ethers or cycHc formals, distributed along the polymer chain. The occasional break in the oxymethylene sequences has significant ramifications for polymer stabilization. 

Although examples in the Kureha patent Hterature indicate latitude in selecting hold times for the low and high temperature polymerization periods, the highest molecular weight polymers seem to be obtained for long polymerization times. The addition of water to PPS polymerizations has been reported to effect polymer stabilization (49), to improve molecular weight (50,51), to cause or enhance the formation of a second Hquid phase in the reaction mixture (52), and to help reprecipitate PPS from NMP solution (51). It has also been reported that water can be added under pressure in the form of steam (53). 

Many antioxidants ia these classes are volatile to some extent at elevated temperatures and almost all antioxidants are readily extracted from their vulcanizates by the proper solvent. These disadvantages have become more pronounced as performance requirements for mbber products have been iacreased. Higher operating temperatures and the need for improved oxidation resistance under conditions of repeated extraction have accelerated the search for new techniques for polymer stabilization. Carpet backiag, seals, gaskets, and hose are some examples where high temperatures and/or solvent extraction can combine to deplete a mbber product of its antioxidant and thus lead to its oxidative deterioration faster (38,40). 

In most cases, these active defoaming components are insoluble in the defoamer formulation as weU as in the foaming media, but there are cases which function by the inverted cloud-point mechanism (3). These products are soluble at low temperature and precipitate when the temperature is raised. When precipitated, these defoamer—surfactants function as defoamers when dissolved, they may act as foam stabilizers. Examples of this type are the block polymers of poly(ethylene oxide) and poly(propylene oxide) and other low HLB (hydrophilic—lipophilic balance) nonionic surfactants. 

Polymer structure and formulation. As an example, Woo et al. [7] measured OIT values for series of commercial PVC resins and polyester thermoplastic elastomers (TPEs). The researchers used the ASTM D3895-80 procedure, but substituted air as the oxidising gas instead of pure oxygen. A dependency on thermal processing history of the TPE film samples appeared to influence the measured OIT in the PVC study, chemically different chain ends affected polymer stability and hence OIT values. 

One important point should be stressed here the efficiency of any stabilizing system depends very much on the removal or depletion of the defect structures in the polymer. An example is shown in Figure 1 where the case 5 mechanism vide supra) of a synergistic mixture is theoretically depicted [7]. The line 2 shows that the effect of the synergistic mixture of two antioxidants on thermo-oxidation stability is negligible, if there occurs a relatively significant initiation of oxidation reaction in a way independent from the route taking place via hydroperoxides. 

Nafion, a perfluorinated sulfonated polymer, is a typical example of an ion-exchangeable resin with high promise as a catalyst support. Its properties are significantly different from those of common polymers (stability towards strong bases, and strong oxidizing and reducing acids and thermal stability up to at least 120 °C if the counter ion is a proton, and up to 200-235 °C if it is a... 

It should be pointed out that the addition of substances, which could improve the biocompatibility of sol-gel processing and the functional characteristics of the silica matrix, is practiced rather widely. Polyethylene glycol) is one of such additives [110— 113]. Enzyme stabilization was favored by formation of polyelectrolyte complexes with polymers. For example, an increase in the lactate oxidase and glycolate oxidase activity and lifetime took place when they were combined with poly(N-vinylimida-zole) and poly(ethyleneimine), respectively, prior to their immobilization [87,114]. To improve the functional efficiency of entrapped horseradish peroxidase, a graft copolymer of polyvinylimidazole and polyvinylpyridine was added [115,116]. As shown in Refs. [117,118], the denaturation of calcium-binding proteins, cod III parvalbumin and oncomodulin, in the course of sol-gel processing could be decreased by complexation with calcium cations. 

The original method of polymers stabilization was invented by Gladyshev and coworkers [14-18]. They proposed to introduce in polymer a metal compound inert toward dioxygen. This compound is decomposed at elevated temperatures with production of a thin metal powder. Formates, oxalates, and carbonyls of metals were suggested as predecessors of an active metal powder. For example, ferrous oxalate decomposes at 600-630 K with the formation of pyrofore iron and ferrous oxide... 

As a final example of catalytic hydrogenation activity with polymer-stabilized colloids, the studies of Cohen et al. should be mentioned [53]. Palladium nanoclusters were synthesized within microphase-separated diblock copolymer films. The organometallic repeat-units contained in the polymer were reduced by exposing the films to hydrogen at 100 °C, leading to the formation of nearly monodisperse Pd nanoclusters that were active in the gas phase hydrogenation of butadiene. 

The use of oleochemicals in polymers has a long tradition. One can differentiate between the use as polymer materials, such as linseed oil and soybean oil as drying oils, polymer stabilizers and additives, such as epoxidized soybean oil as plasticizer, and building blocks for polymers, such as dicarboxylic acids for polyesters or polyamides (Table 4.2) [7]. Considering the total market for polymers of ca. 150 million tonnes in 1997 the share of oleochemical based products is relatively small - or, in other terms, the potential for these products is very high. Without doubt there is still a trend in the use of naturally derived materials for polymer applications, especially in niche markets. As an example, the demand for linseed oil for the production of linoleum has increased from 10000 tonnes in 1975 to 50 000 tonnes in 1998 (coming from 120000 tonnes in 1960 ) [8a]. Epoxidized soybean oil (ESO) as a plastic additive has a relatively stable market of ca. 100000 tonnes year-1 [8b]. 

The thermal stability of hyperbranched polymers is related to the chemical structure in the same manner as for linear polymers for example, aromatic esters are more stable than aliphatic ones. In one case, the addition of a small amount of a hyperbranched polyphenylene to polystyrene was found to improve the thermal stability of the blend as compared to the pure polystyrene [31]. 

Polymers, for example, polypropylene or polymethylmethacrylate (PMMA, Plexiglas transparent) and many others, may be interesting materials of easy machinability. But their chemical stability, especially against organic solvents, has to be tested individually. 

The behaviour of ternary systems consisting of two polymers and a solvent depends largely on the nature of interactions between components (1-4). Two types of limiting behaviour can be observed. The first one occurs in non-polar systems, where polymer-polymer interactions are very low. In such systems a liquid-liquid phase separation is usually observed each liquid phase contains almost the total quantity of one polymer species. The second type of behaviour often occurs in aqueous polymer solutions. The polar or ionic water-soluble polymers can interact to form macromolecular aggregates, occasionally insoluble, called "polymer complexes". Examples are polyanion-polycation couples stabilized through electrostatic interactions, or polyacid-polybase couples stabilized through hydrogen bonds. 

An alcohol reduction method has been applied to the synthesis of polymer-stabilized bimetallic nanoparticles. They have been prepared by simultaneous reduction of the two corresponding metal ions with refluxing alcohol. For example, colloidal dispersions of Pd/Pt bimetallic nanoparticles can be prepared by refluxing the alcohol-water (1 1 v/v) mixed solution of palladium(II) chloride and hexachloro-platinic(IV) acid in the presence of poly(/V-vinyl-2-pyrrolidone) (PVP) at about 90-95°C for 1 h (Scheme 9.1.5) (25). The resulting brownish colloidal dispersions are stable and neither precipitate nor flocculate over a period of several years. Pd/ Pt bimetallic nanoparticles thus obtained have a so-called core/shell structure, which is proved by an EXAFS technique (described in Section 9.1.3.3). 

Polymer-stabilized bimetallic nanoparticles containing both a light transition metal element and a precious metal element can also be prepared by a modified alcohol reduction method. For example, Cu/Pd bimetallic nanoparticles were successfully prepared with various Cu Pd ratios by refluxing a glycol solution of the hydroxides of Cu and Pd in the presence of PVP or by thermal decomposition of metal acetates. 

X-Ray Photoelectron Spectrometry. X-ray photoelectron spectrometry (XPS) was applied to analyses of the surface composition of polymer-stabilized metal nanoparticles, which was mentioned in the previous section. This is true in the case of bimetallic nanoparticles as well. In addition, the XPS data can support the structural analyses proposed by EXAFS, which often have considerably wide errors. Quantitative XPS data analyses can be carried out by using an intensity factor of each element. Since the photoelectron emitted by x-ray irradiation is measured in XPS, elements located near the surface can preferentially be detected. The quantitative analysis data of PVP-stabilized bimetallic nanoparticles at a 1/1 (mol/mol) ratio are collected in Table 9.1.1. For example, the composition of Pd and Pt near the surface of PVP-stabilized Pd/Pt bimetallic nanoparticles is calculated to be Pd/Pt = 2.06/1 (mol/ mol) by XPS as shown in Table 9.1.1, while the metal composition charged for the preparation is 1/1. Thus, Pd is preferentially detected, suggesting the Pd-shell structure. This result supports the Pt-core/Pd-shell structure. The similar consideration results in the Au-core/Pd-shell and Au-core/Pt-shell structure for PVP-stabilized Au/Pd and Au/Pt bimetallic nanoparticles, respectively (53). 

Chemical Stability. Chemical stability is just as important as the physical stability just discussed. In general, chemical deterioration of the polymers is no problem, and they can be stored at room temperature for years. However, the polymeric surfaces are subjected to an extreme variety of chemicals during the accumulation process. Some of these may react with the polymer. For example, reactions of styrene-divinylbenzene polymers and Tenax with the components of air and stack gases have been documented (336, 344, 540). The uptake of residual chlorine from water solutions has also been observed in my laboratory and elsewhere (110, 271, 287). Although the homogeneous nature of synthetic polymers should tend to reduce the number of these reactions relative to those that occur on heterogeneous surfaces of activated carbons, the chemical reaction possibility is real. In the development of methods for specific chemicals, the polymer stability should always be checked. On occasion, these checks may lead to... 

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instability in both the neutral and doped forms precludes any useful application. In contrast to polyacetylene, both polyaniline and polypyrrole are significandy more stable as electrical conductors. When addressing polymer stability it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode applications require long-term chemical and electrochemical stability at room temperature while the polymer is immersed in electrolyte. Aerospace applications, on the other hand, can have quite severe stability restrictions with testing carried out at elevated temperatures and humidities. 

The stabilization of dispersed species induced by the interaction (steric stabilization) of adsorbed polymer chains. Example adsorbed proteins stabilize the emulsified oil (fat) droplets in milk by steric stabilization. Also termed depletion stabilization. See also Protection. 

TGA is the most widely used tool in analyzing the thermal performance of the PNs. A nanodis-persed nanoadditve, as seen in Figure 11.20, usually brings a significant improvement of the thermal stability to the matrix polymer. More examples are given in Figure 11.21 in which the TGA curves... 

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