ABSTRACT:The development of the interfacial bond strength as a function of bonding conditions has been investigated in two representative semicrystalline thermoplastics, isotactic polypropylene and polyamide 12. If one side of the interface is well above the melting point immediately before contact, more rapid effective bonding is obtained for a given estimated interface temperature than under isothermal conditions. This is discussed in terms of a simple two-parameter model for the critical strain energy release rate associated with crack propagation along the interface, which incorporates the rate of establishment of intimate contact at the interface. The model provides a self-consistent phenomenological description of the time and temperature dependence of the bonding kinetics in polyamide 12 joints, although questions remain regarding the detailed mechanisms of bonding.
The strength of fusion bonds between polyamide 12 plaques has been investigated as a function of processing conditions, using the double cantilever beam geometry to measure the mode I critical strain energy release rate, Gc. Bonding was carried out using an instrumented press, which allowed systematic variation of the bonding time, the holding pressure, the interface temperature and the initial difference in temperature between the plaques. Gc was generally found to increase with hold time and interface temperature. However, for a given hold time, hold pressure and estimated interface temperature, the highest Gc were obtained under nonisothermal conditions, that is, when the initial temperatures of the plaques were different. Based on fractographic analysis and observations of the effect of the hold pressure, it is concluded that enhanced wetting is an important factor in the improved efficiency of non‐isothermal bonding.
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