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  • Author or Editor: B Howell x
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Abstract  

General purpose poly(styrene) prepared by conventional radical techniques contains a head-to-head unit as a consequence of polymerization termination by radical coupling. As has been previously demonstrated, thermal stress promotes homolysis of the bond linking the head-to-head components. The macroradicals generated depolymerize rapidly to generate styrene monomer. This decomposition during processing can lead to finished articles containing objectionable levels of styrene monomer, particularly for food packaging applications in which even low levels of monomer can promote objectionable taste and aroma. Polymer containing no head-to-head units should not be prone to this facile decomposition. In this instance, poly(styrene) has been prepared by nitroxyl-mediated polymerization of styrene monomer followed by reductive removal of nitroxyl end groups. Polymer prepared in this manner contains no head-to-head units and displays thermal stability much greater than that observed for conventional poly(styrene). A direct comparison of the stability for the two polymers is readily available by thermogravimetric techniques. A quantitative reflection of the difference in stability is available from the rate constants for the respective decomposition.

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Abstract  

Vinylidene chloride polymers are prominent in the barrier plastics packaging industry. They display good barrier to the transport of oxygen (to prevent spoilage of food items) and flavor and aroma constituents (to prevent 'scalping' on the supermarket shelf). However, these polymers undergo thermal dehydrochlorination during processing. This can lead to a variety of problems including the evolution of hydrogen chloride which must be scavenged to prevent its interaction with the metallic walls of process equipment. Such interaction leads to the formation of metal halides which act as Lewis acids to facilitate the degradation. A potentially effective means to capture hydrogen chloride generated might be to incorporate into the polymer a mild organic base. Accordingly, copolymers of vinylidene chloride and 4-vinylpyridine have been prepared and subjected to thermal aging. Results suggest that the pyridine moiety is sufficiently basic to actively promote dehydrochlorination in the vinylidene chloride segments of the polymer.

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Abstract

Two series of styrene monomers, one with phosphorus-containing moieties as substituents and the other with substituents containing both phosphorus and nitrogen, have been prepared, characterized, and converted to oligomers. The oligomers contain, in the one case, phosphorus and, in the other, phosphorus and nitrogen. This provides the opportunity to not only assess the impact of the presence of phosphorus on the combustion characteristics of the oligomers but to determine whether or not this impact is enhanced by the presence of nitrogen. The level of residue from thermogravimetry and heat release rate during combustion suggest that the presence of nitrogen may have a small positive impact on the effectiveness of phosphorus flame retardants.

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Abstract  

General purpose poly(styrene) is a large volume commodity polymer widely used in a range of applications. For many of these the presence of an additive to impart some flammability resistance is required. Most commonly, brominated aromatics are used for this purpose. As the polymer undergoes combustion these compounds decompose to generate bromine atoms and/or hydrogen bromide which escape to the gas phase and trap flame propagating radicals. While these species are effective in inhibiting flame propagation they present the opportunity for loss of halogen to the atmosphere. For this reason, the use of these compounds is being limited in some parts of the world. Phosphorus compounds, on the other had, impart a flame retarding influence by promoting char formation at the surface of the burning polymer. This prevents heat feedback to the polymer and consequent pyrolysis to generate fuel fragments. The combination of both bromine and phosphorus present in a single compound might generate a superior flame-retarding additive in that both modes of retardancy might be promoted simultaneously. Should this be the case smaller amounts of additive might be necessary to achieve a satisfactory level of flame retardancy. A series of such additives, brominated aryl phosphates, has been synthesized and fully characterized spectroscopically. Blends of these additives, at various levels, with poly(styrene) have been examined by DSC, TG and in the UL-94 flame test. The flammability of the polymer is dramatically diminished by the presence of the additive.

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Abstract  

Certain five-membered dioxaheterocyclic compounds (hetero atoms may be P, Si, S, etc.) contain a strained carbon–carbon bond which may undergo homolytic thermolysis at modest temperatures to generate a diradical capable of initiating vinyl polymerization. If substituents contain flame-retarding moieties this represents a convenient method for imparting flame retrdancy to a polymeric material. Of particular interest has been 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane. The thermal degradation of this compound has been studied using 13C NMR spectroscopy. This may conveniently be done by monitoring the intensity of the signal for the benzylic carbon atom as a function of time and temperature. A simple transformation is the conversion of the cyclic compound to the linear polymer.

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Abstract  

The thermal decomposition characteristics of representatives of three classes of organoplatinum compounds have been examined by thermogravimetry. Substituted salicylato(1,2-diaminocyclohexane)platinum(II) compounds undergo thermal decomposition by sequential loss of first the salicylato ligand and then the amine ligand to afford a residue corresponding to the platinum content of the compound. The thermal decomposition of N-arylsalicylaldimino(1,2-diaminocyclohexane)platinum(II) nitrate is more complex, but is also characterized by two major weight losses. Thermal decomposition ofbis-(2-thiophenecarboxylato)platinum(II) is characterized by ligand fragmentation to generate a residual mass corresponding to the platinum content of the compound.

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Abstract

Fully substituted 1,3-dioxa-2-siloles contain a strained carbon–carbon bond that will undergo thermolysis at modest temperatures to generate a diradical capable of initiating vinyl polymerization. If the substituents contain flame-retarding moieties, e.g., halogen or phophorus-containing groups, the use of such compounds as initiators serves to incorporate a flame-retarding unit into the polymer mainchain. Both 2,2-dialkyl- and 2,2-diaryl-4,4,5,5-tetra(3,5-dibromophenyl)-1,3-dioxa-2-siloles may be prepared from the appropriate tetra(bromoaryl)-1,2-ethanediol and are obtained as white solids. Thermal decomosition (thermogravimetry) of these materials occurs in two stages. Initial decomposition is observed at about 250 °C and corresponds to the loss of nearly half of the initial sample mass.

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Abstract

The thermal stability of a commercial triaryl phosphate hydraulic fluid has been assessed using thermogravimetry and pyrolysis. This material is a mixture of triaryl phosphates containing a predominance of triphenyl phosphate. It is volatile at higher temperatures. At temperatures below its boiling point, in the presence of air, it slowly decomposes to evolve phenolic fragments.

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Abstract  

Vinylidene chloride polymers containing comonomer units capable of consuming evolved hydrogen chloride to expose good radical-scavenging sites might be expected to display greater thermal stability than similar polymers containing simple alkyl acrylates as comonomer. Incorporation of a comonomer containing the phenyl t-butyl carbonate moiety into a vinylidene chloride polymer has the potential to afford a polymer with pendant groups which might interact with hydrogen chloride to expose phenolic groups. Copolymers of vinylidene chloride with [4-(t-butoxycarbonyloxy)phenyl]methyl acrylate have been prepared, characterized, and subjected to thermal degradation. The degradation has been characterized by thermal and spectroscopic techniques. The degradation of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl acrylate copolymers is much more facile than the same process for similar copolymers containing either [4-(isobutoxycarbonyloxy)phenyl]methyl acrylate or methyl acrylate, a simple alkyl acrylate, as comonomer. During copolymer degradation, [4-(t-butoxycarbonyloxy) phenylmethyl acrylate units are apparently converted to acrylic acid units by extensive fragmentation of the sidechain. Thus, the phenyl t-butyl carbonate moiety does function as a labile acid-sensitive pendant group but its decomposition in this instance leads to the generation of a phenoxybenzyl carboxylate capable of further fragmentation.

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Abstract  

The thermal polymerization of styrene is a long-known and well-practiced phenomena. While the mechanism of the thermal initiation event has been the subject of several investigations, it is not yet well understood. In an attempt to gain further insight as to the details of possible initiation from styrene dimer, analogous stable cycloadducts (maleic anhydride, tetracyanoethylene) of 1- and 2-vinylnaphthalene have been synthesized, fully characterized spectroscopically, and subjected to thermal decomposition. In the main, the major thermal event observed for these styrene dimer mimics is retro cycloaddition. This process is characterized by an activation enthalpy of approximately 30 kcal mol–1. Aminor process which accompanies the major reaction is the homolysis of a carbon–hydrogen bond to generate a carbon radical which may be trapped as a stable adduct of the 2,2,6,6-tetramethylpiperinyloxy (TEMPO) radical.

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