ABSTRACT:The data on the effects of polymerization duration, cocatalyst, and monomer concentrations upon ethylene polymerization in the absence of hydrogen, and the effect of an additional chain transfer agent (hydrogen) on the molecular weight (MW), molecular weight distribution (MWD), and content of vinyl terminal groups for polyethylene (PE) produced over the supported titanium-magnesium catalyst (TMC) are obtained. The effects of these parameters on nonuniformity of active sites for different chain transfer reactions are analyzed by deconvolution of the experimental MWD curves into Flory components. It has been shown that the polymer MW grows, the MWD becomes narrower and the content of vinyl terminal groups in PE increases with increasing polymerization duration. It is assumed to occur due to the reduction of the rate of chain transfer with AlEt 3 with increasing polymerization duration. The polydispersity of PE is found to rise with increasing AlEt 3 concentration and decreasing monomer concentration due to the emergence of additional low molecular weight Flory components. The ratios of the individual rate constants of chain transfer with AlEt 3 , monomer and hydrogen to the propagation rate constant have been calculated.
Electron spin resonance (ESR) studies reveal a high yield of isolated Ti3+ compounds (30–70% of the total titanium content) that form in superactive supported titanium–magnesium catalysts (TMCs) of composition TiCl4/MgCl2 with a low titanium content (≤0.1 wt%) after their interaction with different aluminum trialkyls (AlR3). These catalysts strongly differ from conventional TMCs with titanium contents of 1–3 wt%, which produce a set of isolated or associated Ti(II) and Ti(III) species after the interaction with the AlR3. A correlation is found between the content of the isolated Ti(III) ions and the activity in ethylene polymerization. According to temperature rising elution fractionation (TREF) data, the catalysts with a low titanium content (isolated Ti(III) ions) mainly produce polypropylene with a very high atactic‐fraction content. Organic products formed on the interaction of AlEt3 and AlMe3 with TMC are identified. The data obtained allow a scheme of Ti3+ and Ti2+ compounds formed in these catalysts to be proposed.
Heterogeneity of active centers (AC) of titanium-magnesium catalysts (TMC) and vanadium-magnesium catalyst (VMC) in ethylene-hexene-1 copolymerization has been studied on the base of data of polymer molecular weight distribution (MWD) deconvolution technique and copolymer fractionation onto narrow fractions. It was found that 3 and 4 Flory components (groups of active centers) are required to describe experimental MWD curves of copolymers produced over TMC with different Ti content. In the case of VMC MWD of homopolymer and copolymer are characterized by set of 5 Flory components (5 groups of AC). Different character of inter-relationship between MW and short chain branching (SCB) was found for ethylene-hexene-1 copolymers produced over different catalysts: heterogeneous type in the case of TMC and more uniform for copolymer prepared over VMC. The content of Ti affects on the slope of that profile in copolymers produced over TMC. The results indicated that TMC and VMC are different greatly on the heterogeneity of active centers to the formation of polymers with different molecular weights and to formation of copolymers with different inter-relationship between MW and short chain branching. TMC produces polymers with more narrow MWD but it contains highly heterogeneous centers regarding comonomer reactivity ratios. VMC produces polymers with broad and bimodal MWD but it contains more homogeneous centers regarding comonomer reactivity ratios.
Summary The reaction‐to‐fire properties of coextruded wood–plastic composites containing different fire retardants (melamine, zinc borate, ammonium polyphosphate, aluminium trihydroxide, natural flake graphite and expandable graphite) in the shell layer have been studied with the cone calorimetry technique. The effect of ammonium polyphosphate in combination with graphite has also been studied with a cone calorimeter test. A coextruded composite manufactured without any fire retardant addition has been used as a reference. The fire properties measured in the cone calorimeter are discussed, including the heat release rate, total heat release, smoke production, specific extinction area, CO yield and mass loss rate. The results show that the introduction of fire retardants in the shell layer of coextruded wood–polypropylene composites has a favourable effect on the fire resistance properties of the composite materials. The reaction‐to‐fire properties have been improved according to the fire classification of construction products based on the Euroclass system. Copyright © 2015 John Wiley & Sons, Ltd.
Experimental data on the molecular weight distribution (MWD) of polyethylene (PE) produced over a broad number of Ziegler-Natta catalysts differing in composition and preparation procedure are presented. These catalysts include nonsupported TiCl 3 catalyst, four types of supported titanium-magnesium catalysts (TMC) differing in the content of titanium and the presence of various modifiers in the composition of the support, and a supported catalyst containing VCl 4 as an active component instead of TiCl 4 . The studied catalysts produce PE with different molecular weights within a broad range of polydispersity (M w /M n ¼ 2.8-16) under the same polymerization conditions. The heterogeneity of active sites of these catalysts was studied by deconvolution of experimental MWD curves into Flory components assuming a correlation between the number of Flory components and the number of active site types. Five Flory components were found for PE produced over nonsupported TiCl 3 catalysts (M w /M n ¼ 6.8), and three-four Flory components were found for PE produced over TMC of different composition. A minimal number of Flory components (three) was found for PE samples (M w /M n values from 2.8 to 3.3) produced over TMC with a very low titanium content (0.07 wt %) and TMC modified with dibutylphtalate. It was shown that five Flory components are sufficient to fit the experimental MWD curve for bimodal PE (M w /M n ¼ 16) produced over VMC.
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