1,720,977 research outputs found
The role of silica in radiation induced grafting and crosslinking of silica/elastomer blends
No full textWhen silica/polybutadiene and silica/styrene-butadiene are submitted to g irradiation at 77 K a scavenging
of the silica radiolytic species by absorbed vinyl units takes place at the surface with concomitant
grafting and crosslinking of the polymers. Key intermediate in such reactions are SiO2-bonded radicals of
structure SiO2eC(CH3)CH(R)(R0). The role of silica is rationalized in terms of the following steps: a)
absorption of the radiation energy in the bulk of the silica particles followed by exciton migration at the
surface; b) reactions of excited groups (silanols) with absorbed vinyl units giving SiO2-bonded radicals; c)
crosslinking of the polymers initiated by grafted radicals
Chemical reaction networks as a model to describe UVC- and radiolytically-induced reactions of simple compounds
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Mechanical degradation of elastomers in the presence of silica and inhibitors using a new design of mechano reactor
No full textA newly designed mechanochemical reactor has been home built and used to investigate mechanical degradation of SBR rubber and SBR/SiO 2 blends under N 2 and O 2 atmosphere and in the presence of aromatic amines (DTPD) and tetramethyl piperidine (HALS) radical inhibitors. Continuous on line determination of torque, temperature and O 2 uptake have been made together with solution viscosity and mechano-radical yields at definite time intervals. The fraction of mechanical energy resulting in chemical effects is determined to be less than 0.01%. Chain scission and crosslinking are found to be major mechanochemical reactions with crosslinking acquiring importance with increasing the processing time. According to the results of COGEF type DFT UB3LYP 6-311++G (d,p) relaxed scan computations, the favoured chain scissions sites are predicted to be the ones leading to allylic (prominent) and benzylic radicals. The kinetics of mechano-radicals formation is coherent with torque and viscosity decay kinetics being expressed by the equation [R •] (mol / kg) = - 0.0775 × 10 - 2 t 2 + 0.6755 × 10 - 2 t. The mechano-radical yield is greater than expected from the chain scission yield; furthermore, oxygen and radical inhibitors have minor effects on torque and viscosity decay kinetics in the first part of the process where chain scissions are dominant, whilst they do inhibit crosslinking. It is inferred from these results that a significant part of permanent chain scissions takes place via caged-radical disproportionation and that only scavengeable mechano-radicals escaping cage reactions are responsible for the initiation of crosslinking. From free radical chemistry analysis supported by DFT calculations it is inferred that H abstraction and 1-3 double bond addition by chain scission allyl radicals can be major paths for crosslinking initiation, the addition mechanism acquiring importance with increasing reaction time. The oxygen uptake is approximately linear with process time and consequently does not fit the formation and decay kinetics of scavengeable mechano-radicals; it is suggested that the contribution by other mechanisms involving mechanical excitation coupled with thermal activation may be important. The addition of SiO 2 filler causes an increase of total absorbed mechanical energy and a decrease of the equilibrium molecular weight limit (enhanced chain scissions). In contrast, the scavengeable mechano-radicals yield is found to be less than 50% of that determined in the absence of SiO 2. These observations are related to the mechanical degradation of bound rubber in conditions of enhanced cage effect and build up of frictional energy by silica-rubber Van der Waals interactio
Formation of Hexamethylenetetramine (HMT) from HCHO and NH3 – Relevance to Prebiotic Chemistry and B3LYP Consideration
No full textExperimental characterization of oil-colophony varnishes: A preliminary study
No full textHistorically, the varnishes had the aim to protect the bowed musical instruments by the
external agents and to confer them an aesthetic value. During the 17th and 18th century, in
Italy, the bowed instruments, especially violins, were generally covered by a layer of varnish
made with several natural materials such resins, oil or hide glue: i.e., instruments by the great
violin maker Antonio Stradivari were covered often with a layer of varnish made of linseed oil
and colophony in the ratio 3:1, respectively. The main aim of this work was to study the
modifications that occur in those kinds of varnishes, after exposing them to some factors of
degradation. In order to study the different properties of organic coatings and their suitable
compositions, different mixtures of linseed oil and colophony were recreated in the laboratory
following an ancient recipe: linseed oil and colophony were mixed together with different
ratios (50/50 and 75/25, respectively) and then, they were applied on Maple wood samples and
on glass slides for experimental purposes. In order to investigate the different external factors
which cause the varnish layer degradation, samples were analyzed by different techniques
before and after different ageing processes (thermo-hygrometric cycles, exposition to UV lamp
and to acid vapors). Out of strong experimental evaluation, all the results suggested that the
composition of 75/25 (oil: colophony) is much better as a varnish for musical instruments
The mechanisms of the sulphur-only and catalytic vulcanization of polybutadiene: An EPR and DFT study
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