The effects of sodium chloride (NaCl) and residues of cellulosic fibres derived from sodium carboxymethylcellulose (NaCMC) synthesis on thermal and mechanical properties of CMC films

• The properties of NaCMC films prepared by casting are studied.
• Both the tested grades (crude and analytical) contain fragments of cellulosic fibres.
• CMC films made from crude grade show a NaCl layer.
• NaCl has a considerable impact on the properties of the films close to the Tg.
• NaCl also influences the E' modulus of NaCMC films.

The main aim of this work is to provide an accurate determination of the morphological, thermal and mechanical properties of NaCMC films. Indeed, their main applications require good thermal stability and mechanical strength and the literature lacks a good description of these properties. We analyse two types of NaCMC, an analytical (Fluka) and a crude (Niklacell) grade. For both grades, optical micrographs of NaCMC solutions show partially soluble cellulosic fragments. As evidenced by SEM, films prepared with Niklacell display the formation of opaque (CMC-fNo) and semi-transparent (CMC-fNt) regions due to the combined effect of the formation of a NaCl layer and the presence of fragments of cellulosic fibres. After the elimination of NaCl by precipitation of the acid form of the crude grade of CMC, the films (CMC-fNp) are slightly opaque compared to films prepared with analytical NaCMC grade (CMC-fF). This is due to the residues of cellulosic fibres not eliminated during the precipitation and filtration procedures. The TGA curves of the NaCMC films display a three-step degradation. The results of DSC analyses show that NaCl has an impact on the water content of films due to its hygroscopic behaviour. As evidenced by DSC and DMA, NaCl does not influence the Tm (which is determined at ca. 240 °C) of NaCMC films. However, NaCl has a considerable impact on the thermal and mechanical properties close to the Tg. CMC-fF (analytical) and CMC-fNp (purified grade) exhibit similar Tg values at ca. 40 °C and CMC-fNt and CMC-fNo (crude grade samples) show the highest temperature values for Tg and a remarkable broadening of this transition. NaCl also influences the E' modulus of NaCMC films in the temperature range studied as observed from DMA. The CMC-fF has the highest E' values.