Study of Adhesion Between Two Layers in Multilayer Rotomolded Products

Abstract

This communication describes the characterization of the interface between two polymeric layers, grafted polyethylene and poly(ethylene vinyl alcohol), which were thermally welded during rotomolding. Welding was performed at 160±220 C for 20±30 min. Various techniques were used to characterize the welding zone. Using optical and scanning electron microscopy (SEM), it was shown that this zone is relatively large with a thickness of 20±40 lm. This was confirmed by infrared (IR) microspectrometry. X-ray diffraction was also performed, showing that there was no formation of new crystals in the welding zone. IR microspectrometry mapping suggested that the interfacial zone was formed by interdiffusion of molecular chains.The manufacture of multilayer objects by rotomolding is a new and relatively difficult technology, which is expected to have several industrial applications in future. One of the difficulties with multilayer items is ensuring good adhesion between the layers. It is important to know whether the layers are adequately welded, to predict if there is a risk of delamination in service.There have been several studies of the welding of polymers. [1±3] These reports show that the adhesion between layers depends on various physical and chemical parameters, and models have been proposed to explain the nature of the interface between the layers.The chemical model relates the adhesion to the creation of covalent bonds between the two polymers. This model explains adhesion in terms of an interaction between an electron donor group (base) and an electron acceptor group (acid). This type of model was studied in detail by Creton. [4] In the mechanical model, the adhesion between two polymers is the result of anchorage of the molecular chains of one in the surface pores of the other. [5] In this type of adhesion, the quality and the condition of the contact surface (roughness, porosity) and also the hardness of the polymers are the influential parameters.Deryagin developed a model based on the molecular polarity of polymeric chains. [6] In this model, adhesion is ensured by the electrostatic force between opposite charges on the surfaces of the two polymers. According to this model, for good adhesion, the polymers must have almost the same polarity level; so adhesion is practically impossible when one of the polymers is polar and the other non-polar.For semicrystalline polymers, it was shown that adhesion is influenced by the degree of crystallization of the two polymers. Two models are proposed: the first is based on the formation of co-crystals in the interface [3] and the second on mixed-crystals. [7] Another model that describes the welding between two polymers was developed by de Gennes. [2] His model explains that adhesion results from migration of free or hanging polymer chains in a zone situated on either side of the interface, as shown in Figure 1. After cooling, the interpenetrated chains remain in the welding zone, forming a physical connection between the two polymers. This adhesion is enhanced by entangle...

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