Edina Epacher

Two-step degradation of high-density polyethylene during multiple extrusion

Ensuing our recent studies on polyethylene stabilization,1,2 further multiple extrusion experiments were carried out with a high-density polyethylene (HDPE) polymer containing various amounts of a sterically hindered phenolic antioxidant. Discoloration, thermoxidative stability, and melt flow index (MFI) retention were measured by standard techniques; the functional group content of the polymer was determined by Fourier transform infrared (FTIR) spectroscopy, and rheological, as well as mechanical properties, were also measured. The results indicated that degradation and stabilization reactions take place according to two different mechanisms in the first and subsequent processing steps, respectively. Color development could be described well by the simple first-order overall reaction kinetics proposed earlier, and color change could be related to the stability of the polymer. The existence of general correlations among the properties proved that all chemical reactions are interrelated. The reactions of the stabilizer lead to color development, while those of the polymer to a modification of its molecular architecture, which determines the rheological and mechanical characteristics of the product.

Chemical reactions during the processing of stabilized PE: 2. Structure/property correlations

Interesting correlations were observed in an optimization project directed towards the development of a recipe for blown film production. Analysis of the data and additional experiments indicated that some reactions take place during the processing of stabilized PE which also change the structure of the polymer. Modification of the rheological properties of the polymer and the mechanical characteristics of blown films can be related to these chemical processes. A close correlation was found among basically all rheological and mechanical properties. A tentative explanation was given which relates chemistry to the structure of the polymer and the properties of the product. According to the hypothesis long chain branches are formed during processing which decrease MFI, change the orientation of the film in the perpendicular direction and result in a considerable decrease in the strength of the blown films.

Chemical reactions during the processing of stabilized PE: 1. Discolouration and stabilizer consumption

A series of experiments was carried out to develop an additive package for HDPE blown films. The evaluation of the results yielded interesting correlations relating the colour of the polymer to its chain structure and to the properties of the films. Additional experiments proved that all reactions are related to each other. The discolouration of the polymer could be described by simple first order reaction kinetics and a linear correlation was found between stability and a quantity calculated from the kinetic model. The reaction of the vinyl functionality of the polymer seems to be related to the changes in its rheological properties. In spite of the good general correlations found, some details remained unexplained and need further investigation.

Interactions of Pesticides and Stabilizers in PE Films for Agricultural Use

Publisher Summary This chapter deals with the interactions of pesticides and stabilizers in PE films for agricultural use. In recent years the use of PE films for greenhouses has become widespread, and the production of such films has increased significantly. Proper stabilization of films used for such a purpose is an important financial issue, a more efficient stabilizer package extends the lifetime of the film, but increases its price. The extensive study of the effect of pesticides on the stability and lifetime of agricultural films showed that the combined influence of some active compounds, stabilizers, UV irradiation, and oxygen leads to the rapid deterioration of film properties. Some sulfur containing compounds and organic halogenides initiate the oxidation of PE and lead to the fast deterioration of its mechanical properties. The extent of this negative effect depends on the molecular weight, dispersion, and allotrope modification of elementary sulfur. Finally, according to the results and in agreement with the above presented explanation, the introduction of a UV absorber into the package considerably improves the light stability and lifetime of the film.