The effect of extrusion processing conditions on EVOH/clay nanocomposites at low organo-clay contents

Abstract

Ethylene vinyl alcohol (EVOH) copolymer is studied as a host for low concentrations, up to 1 wt%, of organically treated clay. The clay develops a high interaction level with EVOH and thus high torque levels accompany the structuring process leading to the formation of nanocomposites. Extrusion residence time, successive extrusion passes, screw rotational speed, and processing temperature were all found to affect the morphology and the thermal and mechanical properties of the resulting composites. The extrusion compounded composites were subsequently injection molded. A subtle balance of processing parameters is required to achieve improved properties. Long extrusion residence times were found important for good clay dispersion in some cases, whereas in other cases an exfoliated structure was obtained already after the first extrusion pass. Two organically treated clay types processed at the same conditions were examined, and found to result in different morphology and mechanical behavior. Compression molding of extrusion compounded materials, under several extrusion conditions, was studied to illustrate the effect of shear level on the resulting morphology. The delamination level was higher after compression molding compared to that after injection molding. EVOH thermal properties and thermal stability of the related composites were also examined using differential scanning calorimetry and thermal gravimetric analysis. Higher extrusion processing temperature (220 compared to 200°C) was found to change the crystallization process of EVOH in the presence of clay, leading to significant decrease in Tm and Tc compared to that of the neat EVOH. POLYM. COMPOS., 26:343–351, 2005. © 2005 Society of Plastics Engineers

Influence of processing on the ethylene-vinyl alcohol (EVOH) properties: Application of the successive self-nucleation and annealing (SSA) technique

“…Such behaviour supports the idea of an increase in molecular weight (M w ) altogether with a slight increase on molecular weight distribution. Artzi et al [19] working with EVOH/nanocomposites pointed out that when EVOH is processed at high temperatures, two competitive mechanisms may take place, namely thermal degradation (with chain scission) and grafting and oxidative crosslinking reactions. Similar results have been reported by Lagaron et al [20] under thermo-oxidative conditions (about 2 h at 215°C) without shearing.…”

Subcritical gas‐assisted processing of ethylene vinyl alcohol + nanoclay composites

“…The competing mechanisms explanation has been attributed to the strong interaction between EVOH and the organomodifiers in NC, which prevents the EVOH chains from participating in the nucleating/crystal growth process. In any event, the reported magnitude of the changes in temperatures and enthalpies for loading levels from 1% to 15% by weight of NC is on the order of 1% of the total values for those measurements . Given the small variations in the DSC data presented in Table , it is likely that the effects of oxidation/crosslinking and nucleation/polymer‐clay interactions cancel each other out.…”

“…In Figures and , the TGA weight loss percent and the derivative weight loss are shown for the samples with and without NC. Several studies show that EVOH is easily degraded during thermal processing, like extrusion . The method of degradation is thermally induced chain‐scission via oxidation.…”

“…This accounts for a change in T onset of about 10°C for N 2 and 14°C for CO 2 , and a difference in T max of about 10°C for N 2 and 8°C for CO 2 . It is likely that both gases act as a plasticizer while displacing environmental oxygen during extrusion in a so‐called blanket effect, thus lowering the chance that a thermo‐oxidative event will occur . The difference in T onset and T max between processing steps is smaller.…”

“…Some researchers have reported the effects of additional thermal treatments, such as isothermal conditioning above the melt temperature or multiple extrusion passes and higher processing temperatures on EVOH's thermal behavior . As noted earlier, there are competing processes that can counteract each other, rendering changes to the crystallization and T g s small even though the polymer structure is changing.…”