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Here we present the spatially resolved study of six Galactic planetarynebulae (PNe), namely IC 4593, Hen 2-186, Hen 2-429, NGC 3918, NGC 6543 and NGC6905, from intermediate-resolution spectra of the 2.5 m Isaac Newton Telescopeand the 1.54 m Danish telescope. The physical conditions (electron densities,N$_{e}$, and temperatures, T$_{e}$), chemical compositions and dominantexcitation mechanisms for the different regions of these objects are derived,in an attempt to go deeper on the knowledge of the low-ionization structures(LISs) hosted by these PNe. We reinforce the previous conclusions that LISs arecharacterized by lower (or at most equal) N$_{e}$ than their associated rimsand shells. As for the T$_{e}$, we point out a \textit{possible} differenttrend between the N and O diagnostics. T$_e$[NII] does not show significantvariations throughout the nebular components, whereas T$_e$[OIII] appears to beslightly higher for LISs. The much larger uncertainties associated with theT$_e$[OIII] of LISs do not allow robust conclusions. Moreover, the chemicalabundances show no variation from one to another PN components, not evencontrasting LISs with rims and shells, as also found in a number of otherworks. By discussing the ionization photon flux due to shocks and stellarradiation, we explore the possible mechanisms responsible for the excitation ofLISs. We argue that the presence of shocks in LISs is not negligible, althoughthere is a strong dependence on the orientation of the host PNe and LISs.

Low-ionization structures in planetary nebulae -- II. Densities, temperatures, abundances and excitation of 6 PNe