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Keeping oxidation at bay

Polymer oxidation is inevitable, but a mixture of antioxidants cannot only improve long-term stability but also improve processing.

Virtually all polymeric materials undergo oxidation reactions, and the process can occur at any stage in the life cycle of a polymer, through manufacturing and storage of the material to its processing and end use. Typical manifestations of oxidation in polyolefins can be appearance changes or a loss of mechanical properties. Antioxidants protect polyolefins against those problems by controlling molecular weight changes, which can lead to loss of physical and mechanical properties.

Long-term heat stabilization

At elevated temperatures and in the presence of oxygen, polymers undergo degradation. Antioxidants effectively stop those chemical reactions by deactivating the formed radicals. Otherwise, those radicals would start chain reactions, degrading the polymer increasingly faster and ending the finished article''s life. Depending on the polymer type, those degradation mechanisms lead to different results, all causing a loss of mechanical and optical properties.

In polypropylene (PP) chain scission can be observed, disintegrating the material to an oxidized powder. In polyethylene (PE), chain scissions are accompanied by partial crosslinking (a decrease in melt-flow index), resulting in specks, gel formation, and variation in wall thickness. Processors of all types employ antioxidant masterbatches to help prevent these effects.

Test methods

The effectiveness of antioxidants under conditions of actual use-as long-term heat stabilizers, for example-is determined almost exclusively by means of oven-aging tests at temperatures below the polymer''s melting point. Optimally, oven-aging tests should be carried out at temperatures as close to the end-use temperature as is practical. Higher temperatures yield rapid results, but these are generally not relevant to the conditions in actual practice. The test criteria most often used for polyolefins is the time it takes disintegration to initiate. Discoloration during oven aging is also significant. Changes in mechanical properties, such as tensile strength, elongation, or impact strength as a function of oven-aging time are often used as test criteria. Antioxidant effectiveness is also tested in that manner, where stabilizer systems and concentrations can be evaluated.

Commercial products

Gabriel-Chemie has introduced Maxithen HP 78100 AO for a wide range of applications that require an antioxidant. The product contains processing and long-term stabilizers in a balanced ratio, and it prevents polymer damage from heat during processing and application. It is reported to have universal application for polyolefins, as far as the PE-carrier is accepted. Gabriel-Chemie can supply other carriers on request. The recommended dosage rate is generally between 1% and 3%, depending on the intended application. Maxithen has food-contact approval and can be used for various packaging applications that come in contact with food stuffs.

The effect of antioxidant masterbatches, and generally for all stabilizer masterbatches, is dependent on premixing and creation of a homogeneous distribution. Since different polymers have basic stabilizer systems, which generally are not disclosed, trials should be carried out to evaluate any antioxidant''s final effect on product.

Inge Hartner, customer care, Gabriel-Chemie GmbH

TAGS: Materials
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