Enclosed laser processing of laminated metallised polymer films for security marking

A Q-switched Nd:YAG laser was used to selectively transparentise areas of a substrate consisting of a 40μm layer of oriented polypropylene (OPP) upon which a layer of aluminium had been vacuum coated and subsequently laminated with a second, 40μm thick, OPP film. The process, known as enclosed laser processing, occurs via an entirely new mechanism. Instead of allowing material to escape as in conventional ablation processes, enclosed laser processing melts or vaporises the metal layer and redistributes it as particles of between 0.1 and 2 μm in diameter which are still entrapped inside the polymer films. This process produced results which varied substantially with laser parameters, modelling of the thermal behaviour of the films was carried out to gain an understanding of why this might happen. Twenty-one different microscopic phenomena were identified and four distinct mechanisms describing behaviour at different parameters were identified. In addition to this, studies were made of the transmission behaviour, the chemical behaviour and process maximisation via the processing of a stack of ten substrates simultaneously. This technique allows the user to process a hermetically sealed surface for the creation of durable, individual marks in a single step.