Surface modification and lamination of polymers by photografting

Abstract

This paper describes experimental processes for UV initiated “surface photografting” of polymers: the batch process with vapor-phase transfer of initiator and monomer and the continuous process with liquid-phase transfer of initiator and monomer in a thin layer of solution on the surface of the substrate. The grafting effiiency is about 80% and the resulting grafted layers are very thin (less than 10 nm). The mechanism of “surface photografting” is discussed and proposed to be based on excitation of benzophenone (BP) as initiator to an S 2 state by absorption of far UV irradiation (≈250 nm) and transition to a “superheated” “BPS 1” state, which transfers its excess energy to CH groups at the substrate surface and forms BP triplet states (T 1 and T 2). The excited CH groups and the BPT 1 and BPT 2 states react by hydrogen abstraction and form radicals at the substrate surface. These radicals add monomer to grafted chains. “ Surface photografting” is developed as a method for “ photoinitiated lamination” of plastic films. A thin layer of initiator and monomer between two films is grafted to the two film surfaces by UV irradiation. Short linear grafted chains are formed which by continued UV-irradiation grow to branched chains which eventually fill the space between the two substrate films. The chain ends are terminated by addition of ketyl radicals from the benzophenone. Some growing chain ends may terminate by radical combination. With addition of a small amount of a multifunctional monomer, a network is formed which is grafted to the two film surfaces by main valence bonds. The “ bulk surface photografting” gives laminate of high mechanical strength. When a functional film is grafted between two carrier films, laminates of selected and high barrier properties are formed which are of interest for application as packageing materials.