Maria Giurginca

Radiochemical oxidation of ethylene–propylene elastomers in the presence of some phenolic antioxidants

Abstract The oxidative degradation of γ -irradiated ethylene–propylene elastomers (EPR and EPDM) in the presence of some phenolic antioxidants (Topanol OC, Cyanox 2246, Santonox R, Irganox 1010 and Irganox 1076) was studied. The stabilization activity of these antioxidants was related to the gel content of polymer matrix and the G values for radiochemically formed ketones, aldehydes, acids, esters and alcohols. The oxidation rates, obtained by IR measurements of CO concentrations, provide an order of antirad efficiency. Some considerations concerning the influence of the chemical structure of antioxidants on the stabilization of elastomers and a radiochemical oxidation mechanism are presented.

Effects of some antioxidants on the stability of ethylene-propylene elastomers

Abstract The thermal-oxidative behaviour of ethylene-propylene elastomers EPDM and EPR, with the same ethylene/propylene ratio has been studied in experiments performed in the temperature range 140–180 °C. The antioxidants tested were sterically hindered phenols, bis-phenols containing either methylenic or sulphur bridges and triazines. Some main kinetic characteristics: induction time, relative activity and activation energy for degradation, were used to establish increasing orders of effect in relation to thermal ageing. IR spectroscopy and oxygen uptake were chosen for determination of chemical changes of the elastomers during their isothermal degradation. A discrepancy between EPR and EPDM was identified because of the presence of ethylidene-2-norbornene in the terpolymer structure, which decreases its thermal stability.

Thermo-oxidative behaviour of halobutyl and butyl elastomers

The thermo-oxidative degradation of butyl and halobutyl elastomers has been studied by both IR spectroscopy and chemiluminescence methods. The kinetic parameters of degradation are similar for both methods and show that the chlorinated elastomer is less stable than the parent hydrocarbon, whereas the brominated elastomer is more stable. Degradation of the butyl rubber is mainly by chain scission, whereas the halobutyl elastomers undergo extensive cross-linking.

Ozone destruction of some trans-polydienes

The most important natural and synthetic elastomers have a polydiene structure and, consequently, are susceptible to attack of ozone, a natural component of the atmosphere. A natural polymer (guttapercha) and synthetic polymers of commercial interest (trans-polyisoprene, trans-polybutadiene, trans-polypentenamer, trans-polyoctenamer, polychloroprene) have been exposed to a mixture of air and ozone in various concentrations, and at temperatures of 30 and 50°C. Infrared spectroscopy was used to follow the structural modifications consisting of formation of oxygen-bearing groups, as well as to reveal some kinetic aspects of the destruction process. The comparison of the results obtained in investigation of the reaction of ozone with trans-polydienes and cis-polydienes allow some comments on the influence of the double bond structure on the mechanism involved and on the resulting products.

Grafted mercapto-1,3,5-triazinic antioxidants for elastomers

Abstract We have prepared 10 new derivatives of 2-amino-4,6-dimercapto-1,3,5-triazine and 2,4-diamino-6-mercapto-1,3,5-triazine. The compounds have a very high antioxidant activity in cast films of natural rubber. They also have a favourable effect on the retention of viscosity after ageing and on the destruction index. Their grafting capacity was appreciable, measured by the extractable sulphur content and the protection of the vulcanizates after acetone extraction. Their extraordinary efficiency is due to the internal synergism, and their grafting capacity is due to the presence of SH or SR groups.

Doxycycline Delivery From Collagen Matrices Crosslinked With Tannic Acid

Collagen-doxycycline matrices crosslinked with a natural polyphenol – tannic acid for the treatment of infected wounds were obtained by the freeze-drying of the corresponding gels. FT-IR spectra show that the triple helical structure of collagen is preserved in all the matrices, doxycycline produce collagen crosslinking and the degree of crosslinking increases with tannic acid concentration. Digestion of matrices using collagenase confirms the crosslinking effect of doxycycline and tannic acid and the increasing of the crosslinking degree with the amount of acid. The release of doxycycline from the matrices crosslinked with tannic acid is slower than that from the uncrossliked one and decreases with increasing of acid concentration, according to FT-IR and digestion results, and follows the power law model, with a release exponent of about 0.4, which indicates an anomalous transport. The matrices containing doxycycline, tannic acid and their combination do not develop gram-positive (Staphylococcus aureus) or gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria, fungi or leavens.

Thermo-oxidative degradation and radio-processing of ethylene vinyl acetate elastomers

Abstract Thermal and radiation degradation of three ethylene vinyl acetate elastomers containing 28 and 33% VA were studied by IR spectroscopy and thermal analysis. Kinetic parameters: induction time, oxidation rate, reaction order and activation energy of oxidation were calculated on the basis of the modification in the carbonyl band absorbance during thermal degradation. Temperatures that characterize various processes occurring during degradation are pointed out. The progress in radiochemical degradation was assessed by changes in the 1735 cm −1 band relative to a standard (1475 cm −1 ). A mechanism of degradation is proposed.

The structural changes in butyl and halogenated butyl elastomers during gamma irradiation

Ionizing radiation induces various changes in the molecular structure of elastomers ; consequently, the macroscopic properties of the irradiated elastomers will be modified. The newly formed products and the alteration of the initial chemical structure are the result of free-radical reactions which provide the crosslinking and/or the depolymerization of the tested materials and destructive simultaneous processes involving diffused aggressive agents. This paper describes the behavior of butyl and chlorinated and brominated butyl elastomers under the action of gamma radiation for total doses up to 0.5 MGy. The experimental data provided by several procedures emphasize a complex chemistry circumstance consisting of the simultaneous crosslinking and scission of the macromolecular chains and consecutive processes involving free radicals. Evaluation of the studied changes was via gel content, oxidation level, unsaturation, and halogen distribution as dependent on the total radiation dose. The mechanism of the degradation and a relationship between the crosslinking/scission ratio and the absorbed dose are discussed.

Grafting antioxidants. V Phenols with mercaptoheterocyclic substituents as antioxidants for dienic rubbers

Abstract Four new antioxidants containing mercaptoheterocyclic groups linked to a phenolic ring were synthesized and tested in dienic rubbers as grafting antioxidants and rubber vulcanizate stabilizers. Their antioxidant activity is lower than that of sulfur containing hindered phenols and they interfere in cross-linking reactions.

NIR Study of Chemically Modified Cellulosic Biopolymers

ABSTRACT Near-infrared spectral analysis was useful to quantify the ester content of chemically modified cellulose and lignocellulosics. Two kinds of samples were studied, as long aliphatic-chain cellulose esters and wood sawdust chemically-modified either by anhydrides or by ethylene carbonate. It was possible to determine the degree of substitution (DS) of such samples through a correlation by partial least square (PLS) of second-order derivative of NIR spectra. This technique was efficient even when DS values were low, which is difficult to do by using FTIR. It was also possible to distinguish reagent molecules that were attached to the cellulosic substrate by hydrogen bonding from those linked by covalent bonding.