Abstract
Abstract The aim of the study was to assess the effect of the addition of citric and malic acid and heat curing on the mechanical, physical and optical properties of poly(vinyl alcohol) (PVOH) films. The addition of the organic acids without successive thermal treatments has a mere plasticising effect, while their application with heat curing has a combined crosslinking and plasticising effect. While conventional plasticizers and crosslinkers improve either extensibility or tensile strength of films, respectively, the addition of citric and malic acid coupled with heat curing determined good tensile strength and extensibility. Hydrophilicity was significantly reduced by thermal curing and even further reduced with the organic acids addition. The high transparency of the PVOH films was not affected either by heat-curing, acid addition and their combination, while the use of high curing temperature coupled with acid addition caused a slight yellowing of the films. The use of citric and malic acid in combination with thermal curing is a viable strategy for tailoring the performances of PVOH films thus broadening their spectrum of application.
Highlights
Poly(vinyl alcohol) (PVOH) is a non-toxic, odourless and tasteless polymer with several applications, such as food packaging material, as a coating agent for food supplements, in the pharmaceutical and cosmetics industries and several medical applications, as well as in the paper and textile industries (Demerlis & Schoneker, 2003; Vieira, Altenhofen da Silva, Oliveira dos Santos, & Beppu, 2011)
Our results suggest that the water absorption (WA) reduction in PVOH films was related with heat treatment or crosslinking which, seems to occur at 100 °C or 125 °C, but it was strongly dependant on the acids concentration
PVOH films commonly stand out for their transparency: our results demonstrated that transparency was maintained after curing, even when the curing conditions determined significant color changes
Summary
Poly(vinyl alcohol) (PVOH) is a non-toxic, odourless and tasteless polymer with several applications, such as food packaging material (as film, layer in composite films, coating for films of different nature), as a coating agent for food supplements, in the pharmaceutical and cosmetics industries and several medical applications, as well as in the paper and textile industries (Demerlis & Schoneker, 2003; Vieira, Altenhofen da Silva, Oliveira dos Santos, & Beppu, 2011). Research aimed at the improvement of PVOH properties is of paramount importance in order to broaden its range of application. Previous works have studied the physical and mechanical response of PVOH films to curing: this can be performed by heat treatment, addition of cross-linking agents or both. Chemical cross-linking agents which have been studied for PVOH are glutaraldehyde, hexamethylene diisocyanate, glyoxal and boric acid (Lim et al, 2015). Multifunctional compounds able to react with the hydroxyl groups present in each repeating unit of PVOH may be used as crosslinkers and yield three-dimensional networks, enhancing resistance to dissolution in water, improving mechanical and thermal properties (Lim et al, 2015): it is the case of dialdehydes, dicarboxylic acids and dianhydrides (Gohil, Bhattacharya, & Ray, 2006)
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