It is known that good adhesion of melt-coated polyethylene to metals occurs when the polyethylene can oxidise. The present work confirms that adhesion to copper with a copper (I) oxide film is poor because the copper inhibits polyethylene oxidation. The adhesion may be improved if the copper is chemically oxidised to produce a copper (II) oxide and previous work claims that this oxide is able to oxidise the polyethylene and become reduced to copper (I) oxide. Using a coulometric reduction technique of oxide assay it is shown that the copper (II) oxide does not behave in this way. However during heating in the presence or absence of polyethylene some of the copper (II) oxide is converted to copper (I) oxide. Infra-red spectroscopy shows that very little polyethylene oxidation occurs for coating tines at 200°C which are needed to provide good adhesion measured by a 180° peel test. Further, it is shown that good adhesion is still obtained when polyethylene oxidation is inhibited. No serious conflict with previous observations for this adhesd.ve system are encountered, but the previous observations are re-interpreted by showing that it is the ability of copper (II) oxide to adopt a whisker growth mode which accounts for the good adhesion. Pretreatments are devised for steel and for zinc which produce a fibrous surface comparable to the oxidised copper, and it is found that these afford good adhesion of polyethylene even when polymer oxidation is inhibited. For the oxidised copper it was found that failure was always cohesive in the polyethylene and that the measured peel strength was associated with the amount of polyethylene which underwent deformation. If the mechanical properties of the interfacial polymer were altered by quenching or by introducing voids the peel strength was increased. Purifying the polyethylene or contaminating it with low molecular weight material has only a small effect on adhesion. This importance of fibrous or porous surfaces in polymer-metal adhesion may encourage the development of pretreatments for other metals. They have the advantage that not only is adhesion improved but heavily stabilised polymers may be successfully melt coated directly to the substrates.
|Date of Award||1977|