Sebastião V. Canevarolo

Chain scission distribution function for polypropylene degradation during multiple extrusions

The degradation of polypropylene subjected to multiple extrusions was evaluated by following the changes in the molecular weight distribution curves (MWD) obtained by size exclusion chromatography (SEC). The curves shift to the low molecular weight side and we calculate the chain scission distribution function (CSDF) as a function of the initial molecular weight. Assuming that the SEC curves of both, virgin and degraded PP follow Gaussian behaviour the calculated CSDF data would be a straight-line function of the original molecular weight (MW). A random chain scission process gives a constant and positive CSDF value independent of MW. If the process is no longer random the CSDF value increases as a result of the increase in the chain scission probability. Polypropylene subjected to multiple extrusion shows that the chain scission processes during thermo-mechanical degradation display a CSDF curve with a constant positive value in the low molecular weight region which rises in the higher MW region. This indicates that the probability of chain breaking is dependent on the degradation aggressiveness and the molecular weight of the chain. At low MW, it is independent of it, i.e. the chain scission is random, but at higher MW it becomes dependent, increasing with MW. The behaviour has been measured during multiple extrusions and using different screw configurations. The CSDF values are higher the greater the aggressiveness of the degradation, i.e. number of extrusions and screw configuration: kneading elements with 45° (KB45) degrade more than conveying elements (CON), increasing the overall level of chain scission.

Mechanical and morphological characterization of polypropylene toughened with olefinic elastomer

The effect of incorporating (C2-C8) ethylene-octene elastomer on the mechanical properties and morphology of polypropylene copolymers has been investigated employing two types of PP copolymer, with and without nucleating agent. The results were compared to the ones presented by a commercial PP heterophase (reactor impact modified PP/EPR). The addition of the elastomer increases the toughness of the blends but reduces their stiffness. PP blends in the low elastomer content region (< 20%) show low values of the Izod impact strength and both, elastomer content and impact strength, are directly proportional to the area under the b damping peak or its maximum intensity of the elastomer. The morphology is a continuous pattern of segregate elastomeric particles with average particle size in the range of 0.27 mm to 0.39 mm. The average particle size and particle size distribution plotted in log-normal distribution curves, increases slightly with the increase in the elastomer content. The reactor modified PP heterophase has a broader particle size distribution and an average particle size of 0.56 mm, at the lower limit but inside the range for good impact performance, as observed.

Degree of conversion and molecular weight of one denture base and three reline resins submitted to post-polymerization treatments

The effect of post-polymerization treatments (MW-microwave irradiation and WB-water-bath) on the degree of conversion (DC) of three reline resins (Ufi Gel hard-U, Kooliner-K, and Tokuso Rebase Fast-T) and one denture base resin (Lucitone 550-L), submitted to two polymerization cycles (LS-short and LL-long), was evaluated by using FT-Raman spectroscopy (n = 5). The molecular weight (Mw) of the powder of all materials and of K polymerized specimens (control; MW; and WB; n = 3) was analyzed using GPC. DC data were analyzed using Kruskal-Wallis test (α = .05). For control specimens, there were no significant differences between U (68%) and LL (77%) and among LL, K (81%), and T (84%). LS (92%) had the highest DC (P < 0.05). Only material K exhibited an increased DC after WB (P < 0.05). All powders had Mw from 4.0 x 10 5 to 6.5 x 10 5 and narrow Mw distributions (2.1 to 3.6). Polymerization and post-polymerization produced K specimens with Mw similar to that of K powder.

Cisão de cadeia na degradação termo-mecânica do poliestireno sob múltiplas extrusões

The number of chain scissions during thermo-degradation of polystyrene under multiple extrusions was calculated. The degradation produces changes in the molecular weight distribution MWD curves. Following the physical meaning of the average molecular weights of a MWD curve it was shown that the number average molecular weight Mn is the only average that can be directly related to the number of molecules in the system. From that it was calculated the number of sectioned chains (ns) as a ratio between the number of chains in the degraded and original polymers. The chain scission distribution function (CSDF) shows that the thermo-mechanical degradation process of polystyrene under multiple extrusions at 240 °C is of a random type, independent of the initial molecular weight.

Effect of reline material and denture base surface treatment on the impact strength of a denture base acrylic resin

OBJECTIVE In this study, the effect of relining and surface treatment on the impact strength (IS) of a heat-polymerising denture base acrylic resin (Lucitone 550-L) was evaluated. MATERIALS AND METHODS Rectangular bars of L were made (60 x 6 x 2 mm) and relined (2 mm) with the relining resins Ufi Gel Hard (UH) and Tokuso Rebase Fast (TR). Specimens relined with L and intact L, TR and UH specimens were also made (60 x 6 x 4 mm), for comparison. Before relining, the L surface was left untreated or wetted with methyl methacrylate monomer and/or the bonding agents (BA) supplied by manufacturers of the reline resins. V-notches were machined at the midpoint of the length of all specimens. The notches were made either across the width (Nw) or across the thickness of the specimens (Nth). The Charpy impact test was performed using a 0.5-J pendulum, which had been specially designed and constructed. Data were analysed separately for each notch position using one-way analysis of variance and Tukey honestly significant difference post-hoc test (p = 0.05). RESULTS The IS of L was similar to that of L/L. For the Nw notch, treating the denture base L with TR BA and relining with TR reline material produced the highest IS. CONCLUSION The IS of specimens made from heat polymerising acrylic resin Lucitone 550 was increased after relining using the hard chairside reline resin TR with its proprietary BA.

Correlação entre o índice de fluxo à fusão e a função da distribuição de cisão de cadeia durante a degradação termo-mecânica do polipropileno

Changes in the Melt Flow Index were related to the shift in the Molecular Weight Distribution Curve (MWD) produced by the thermo-mechanical degradation of polypropylene (PP) during multiprocessing. The degradation was modelled using the Chain Scission Distribution Function (CSDF), which quantifies the shift in the molecular weight distribution curve as a function of the number of extrusions. The effect is normally the reduction of the molecular weight, which means positive values for CSDF and reduction of the polymer melt viscosity. The average molecular weight shift (LogMw(0) - LogMw(D)) calculated from the average weight molecular weight before and after the degradation overlaps the CSDF curve and shows a linear relationship with the MFI values. The thermo-mechanical degradation process of polypropylene can be better visualized when CSDF curves are correlated with MFI values.

Temperature calibration of a dynamic-mechanical thermal analyser

This work presents a procedure for the temperature calibration of a Dynamic-Mechanical Thermal Analyser (DMTA) using pure substances with known melting temperatures. The thermocouple was calibrated in conditions either quasi-static or dynamic, the latter being that realised during a normal DMTA run, using the standards placed in tubes of polypropylene or polytetrafluoroethylene. The measured melting temperatures of the standards were taken from the maximum peak in the damping curve (tan δ as function of temperature) and compared with the theoretical values. Such methodology provides a simple and reliable temperature calibration of a DMTA equipment.

Efeito do Tipo de Elemento de Rosca na Degradação de Polipropileno Durante Múltiplas Extrusões

Resumo: A configuracao da rosca de uma extrusora de dupla rosca modular pode ser projetada com varios tipos de elementos de conducao e mistura, produzindo diferentes niveis de degradacao no polimero fundido. Neste trabalho determinou-se o nivel de degradacao do polipropileno durante multiplas extrusoes em varias configuracoes de rosca, atraves de cromatografia por exclusao de tamanho e espectroscopia de infravermelho. Houve decrescimo do peso molecular e estreitamento da sua distribuicao em funcao do numero de extrusoes e do aumento da agressividade da rosca, assim como, aumento da concentracao de carbonilas e insaturacoes na cadeia polimerica. O elemento de mistura EM90 causou o maior nivel de degradacao e a adicao de elementos de conducao esquerdos ECE reduziram o nivel de degradacao. Palavras-chave: Configuracao de rosca, polipropileno, degradacao, peso molecular. Effect of screw element on the polypropylene degradation during multiple extrusions Abstract: The screw configuration of a double screw extruder can be designed to contain kneading and conveying elements inducing different levels of degradation in the polymer melt. In this work the level of degradation in polypropylene have been measured after multiple extrusions for various screw configurations using Size Exclusion Chromatography and Infra-Red Spectroscopy. The average molecular weight and the polydispersity are reduced and the carbonyl and unsaturation indices increase as the number of extrusions and the agressiveness of the screw increase. The kneading element with 90 degrees caused the greater level of degradation and the addition of left hand elements have reduced the level of degradation.

Impact strength of denture base and reline acrylic resins: An in vitro study

This study evaluated the impact strength of a denture base resin (Lucitone 550—L) and four reline resins (Tokuyama Rebase II—T; Ufi Gel Hard—U; New Truliner—NT, and Kooliner—K), both intact and in a reline combination (L/L, L/T, L/U, L/NT, and L/K). For each group (n = 20), half of the specimens were thermocycled before testing. Charpy tests were performed, and the impact strengths were calculated. Data were analyzed by two-way analyses of variance and Tukey’s test (p = 0.05). For the intact groups, mean impact strength values for L (1.65 and 1.50) were significantly higher than those of the reline resins (0.38–1.17). For the relined groups, the highest mean impact strength values were produced by L/T (5.76 and 5.12), L/NT (6.20 and 6.03), and L/K (5.60 and 5.31) and the lowest by L/U (0.76 and 0.78). There were no significant differences between L and L/L. Thermocycling reduced the impact strength of T (from 0.73 to 0.38) and L/L (from 1.82 to 1.56).

Resistance to impact of cross-linked denture base biopolymer materials: Effect of relining, glass flakes reinforcement and cyclic loading

AIMS The effect of reinforcement and cyclic loading on the resistance to impact (RI) of denture base biopolymer materials was evaluated using Charpy (C) and falling-weight (FW) impact tests. METHODS Bar-shaped (60×6×2mm(3)) and denture-shaped specimens (2mm) for the C and FD tests, respectively, were prepared with Lucitone 550 (L) and Vipi Wave (V) and relined (2mm) using the same material or the autopolymerizing relining resins Tokuyama Rebase II (T) and Ufi Gel Hard (U). Bulk specimens (60×6×4mm(3)) of all materials (L, V, T and U) were also prepared and tested. To evaluate the effect of reinforcement, glass flakes were added to the powder of the relining resins T and U (5% by weight). Half of bar-shaped (n=320) and half of the denture-shaped specimens (n=480) were subjected to cyclic loading (10,000 cycles) before the impact tests. RESULTS Data were analyzed by one-way ANOVAs (α=0.05) and revealed that the RI of L and V were comparable and higher than those of U and T. Compared to L and V, the RI was increased by relining with T and decreased by relining with U. When relining was made using the same material (L and V) the RI was maintained. Flexural cyclic loading and the incorporation of glass flakes into the resins T and U did not cause any significant alteration in the RI. A high correlation between results from C and FW tests was observed (r=0.8854). CONCLUSION Relining may exert effects on the RI of L and V denture base resins, which vary according to the relining material used. The high correlation between C and FW, suggests that the Charpy test, using bar-shaped specimens, can be a simple and reliable method for evaluating factors that may influence the RI of denture base polymers.