Surface free energy analysis of natural and modified natural rubber latex films by contact angle method

The contact angles of soft latex dipped films of high-ammonia natural rubber (HA) latex concentrate, deproteinized natural rubber (DPNR) latex, and oil-incorporated natural rubber latex were determined by the sessile drop method. The surface free energies were calculated using different procedures available in the literature. For each type of latex film, the harmonic mean treatment gives the highest polar component of the surface free energy, while those from the Lifshifz-van der Waals acid-base method were the lowest. This trend is the same for each type of latex films but the magnitude varies. Among the various latex types, the unleached NR latex film surface is the most hydrophilic; the deproteinized latex film, the least. All the NR film surfaces were found to be monopolar basic in nature, with the unleached NR film being the most basic and those of HA the least. The behavior of the various film surfaces is discussed in terms of the presence of non-rubbers and their removal by washing (leaching) of the film or by deproteinisation of the latex. The polar nature of the film surface is correlated well with the presence of the exuded non-rubber materials at the film surface. The surface free energies of the latex films are comparable to that for synthetic cis-polyisoprene latex film but lower than that for chloroprene. These are in turn in reasonable agreement with the surface free energies of some common solid polymer surfaces. Discussion on the possible implications of the present results in the dipped-goods manufacturing process was attempted.