Élida Beatriz Hermida - Academia.edu (original) (raw)

Papers by Élida Beatriz Hermida

The dynamical properties of linear viscoelastic materials are related to the strength of relaxati... more The dynamical properties of linear viscoelastic materials are related to the strength of relaxation as well as to the statistical parameters of the corresponding distribution function. Nowick and Berry established numerically the relationships between the real and imaginary moduli or compliances, the strength of relaxation, and the parameters of a widely used distribution function: the log-normal one. These relationships are modified, applying a perturbation theory, in order to get the parameters from an internal friction peak measured as a function of temperature. Recently, a modified anelastic element (MAE) has been introduced for describing the mechanical properties, particularly the internal friction, of linear viscoelastic solids. It has been shown that the distribution function associated to the MAE is quite similar to a log-normal distribution. Therefore, in the present paper the perturbation theory developed by Nowick and Berry is applied to the internal friction peak for the MAE. It is found that the approximate values calculated for the MAE are in excellent agreement with the analytical parameters associated to this element and also with the approximate expressions derived for the log-normal distribution. However, this treatment is not enough to determine the temperature dependence of the parameters except if very accurate frequency data are measured. Consequently, an alternative procedure is proposed.

The introduction of the so called oxo-biodegradable additives in the Argentine market motivated t... more The introduction of the so called oxo-biodegradable additives in the Argentine market motivated the assessment of the effects of abiotic and biotic factors on the structure and mechanical behavior of polyethylene (PE) with oxo-degradable additives (PE+AD). Samples of oxo-degradable packaging films found in local shops together with polyethylene films with and without d 2 w ® additive were annealed at different temperatures between 50 and 110ºC and submitted to ultraviolet radiation at different irradiances (0,35; 0,45; 0,89 and 1,20 W/m 2). Furthermore, aged oxo-degradable films were set in a controlled compost bioreactor in order to evaluate their biodegradation ability. Experimental results showed that elongation at break was the mechanical property more sensitive to the polymeric degradation. The structural changes determined by FT-IR remarked the importance of the UV degradation time over the irradiation rate; the carbonyl index of the degraded samples pointed out that chain scission was a thermally activated process. Regarding degradation due to UV radiation, at the same dose, the elongation at break is lower at lower irradiance both in PE and PE+AD samples. On the other hand, thermal degradation of PE without additive is more susceptible to degradation than PE + AD. At the beginning of the biodegradation tests, PE + AD showed a higher CO 2 production rate with respect to PE; however, this rate reduced along the first 30 days, reaching the CO 2 production of PE without additive. The maximum biodegradation observed for both PE and PE+AD samples was 24 % after 90 days of incubation.

A procedure t o determine the characteristic parameters of a log-normal distribution of relaxatio... more A procedure t o determine the characteristic parameters of a log-normal distribution of relaxation times, both for stress relaxation and creep tests, is presented. The method is based on the use of either the normalized stress relaxation or creep functions and their first derivatives. Furthermore, the limiting stress or strain can be obtained directly from the experimental data. Finally, the procedure developed is applied to actual stress relaxation curves reported for aluminum and polysulphide rubber and the results are compared with those obtained by approximate methods, based on the use of the error function. Es wird ein Verfahren zur Bestimmung der charakteristischen Parameter einer Log-Pu'ormalver-teilung der Relaxationszeiten sowohl fiir Spannungsrelaxation als auch fiir Kriechtests angegeben. Die Methode beruht auf der Verwendung entweder der normierten Spannungsrelaxations-oder Kriechfunktionen und ihren ersten Ableitungen. Daruber hinaus kann die Grenzspannung oder-dehnung direkt aus den experiment ellen Daten erhalten werden. SchlieBlich wird das entwickelte Verfahren auf aktuelle Spannungsrelaxationskurven fur Aluminium und Polysulfid-Gummi angewendet und die Ergebnisse mit denen verglichen, die durch Niiherungsmethoden auf der Grundlage von Fehlerfunktionen erhalten werden.

The dynamical properties of linear viscoelastic materials are related to the strength of relaxati... more The dynamical properties of linear viscoelastic materials are related to the strength of relaxation as well as to the statistical parameters of the corresponding distribution function. Nowick and Berry established numerically the relationships between the real and imaginary moduli or compliances, the strength of relaxation, and the parameters of a widely used distribution function: the log-normal one. These relationships are modified, applying a perturbation theory, in order to get the parameters from an internal friction peak measured as a function of temperature. Recently, a modified anelastic element (MAE) has been introduced for describing the mechanical properties, particularly the internal friction, of linear viscoelastic solids. It has been shown that the distribution function associated to the MAE is quite similar to a log-normal distribution. Therefore, in the present paper the perturbation theory developed by Nowick and Berry is applied to the internal friction peak for the MAE. It is found that the approximate values calculated for the MAE are in excellent agreement with the analytical parameters associated to this element and also with the approximate expressions derived for the log-normal distribution. However, this treatment is not enough to determine the temperature dependence of the parameters except if very accurate frequency data are measured. Consequently, an alternative procedure is proposed.

The introduction of the so called oxo-biodegradable additives in the Argentine market motivated t... more The introduction of the so called oxo-biodegradable additives in the Argentine market motivated the assessment of the effects of abiotic and biotic factors on the structure and mechanical behavior of polyethylene (PE) with oxo-degradable additives (PE+AD). Samples of oxo-degradable packaging films found in local shops together with polyethylene films with and without d 2 w ® additive were annealed at different temperatures between 50 and 110ºC and submitted to ultraviolet radiation at different irradiances (0,35; 0,45; 0,89 and 1,20 W/m 2). Furthermore, aged oxo-degradable films were set in a controlled compost bioreactor in order to evaluate their biodegradation ability. Experimental results showed that elongation at break was the mechanical property more sensitive to the polymeric degradation. The structural changes determined by FT-IR remarked the importance of the UV degradation time over the irradiation rate; the carbonyl index of the degraded samples pointed out that chain scission was a thermally activated process. Regarding degradation due to UV radiation, at the same dose, the elongation at break is lower at lower irradiance both in PE and PE+AD samples. On the other hand, thermal degradation of PE without additive is more susceptible to degradation than PE + AD. At the beginning of the biodegradation tests, PE + AD showed a higher CO 2 production rate with respect to PE; however, this rate reduced along the first 30 days, reaching the CO 2 production of PE without additive. The maximum biodegradation observed for both PE and PE+AD samples was 24 % after 90 days of incubation.

A procedure t o determine the characteristic parameters of a log-normal distribution of relaxatio... more A procedure t o determine the characteristic parameters of a log-normal distribution of relaxation times, both for stress relaxation and creep tests, is presented. The method is based on the use of either the normalized stress relaxation or creep functions and their first derivatives. Furthermore, the limiting stress or strain can be obtained directly from the experimental data. Finally, the procedure developed is applied to actual stress relaxation curves reported for aluminum and polysulphide rubber and the results are compared with those obtained by approximate methods, based on the use of the error function. Es wird ein Verfahren zur Bestimmung der charakteristischen Parameter einer Log-Pu'ormalver-teilung der Relaxationszeiten sowohl fiir Spannungsrelaxation als auch fiir Kriechtests angegeben. Die Methode beruht auf der Verwendung entweder der normierten Spannungsrelaxations-oder Kriechfunktionen und ihren ersten Ableitungen. Daruber hinaus kann die Grenzspannung oder-dehnung direkt aus den experiment ellen Daten erhalten werden. SchlieBlich wird das entwickelte Verfahren auf aktuelle Spannungsrelaxationskurven fur Aluminium und Polysulfid-Gummi angewendet und die Ergebnisse mit denen verglichen, die durch Niiherungsmethoden auf der Grundlage von Fehlerfunktionen erhalten werden.