Mechanical and water barrier properties of hydroxypropyl methylcellulose films
Abstract
In line with the increasing demand for sustainable packaging materials, this contribution aimed to investigate the film-forming properties of hydroxypropyl methylcellulose (HPMC) to correlate its chemical structure with film properties. The roles played by substitution degree (SD) and molecular weight (Mw) on the mechanical and water barrier properties of HPMC films were elucidated. Rheological, thermal, and structural experiments supported such correlations. SD was shown to markedly affect film affinity and barrier to moisture, glass transition, resistance, and extensibility, as hydroxyl substitution lessens the occurrence of polar groups. Mw affected mostly the rheological and mechanical properties of HPMC-based materials. Methocel® E4 M led to films featuring the greatest tensile strength (ca., 67 MPa), stiffness (ca., 1.8 GPa), and extensibility (ca., 17%) and the lowest permeability to water vapor (ca., 0.9 g mm kPa−1 h−1 m−2). These properties, which arise from its longer and less polar chains, are desirable for food packaging materials.
Conclusions
In summary, we confirmed our hypothesis that both chain length and backbone pendant group affected the mechanical and water barrier properties of HPMC films. Thermal and rheological properties also showed influence of HPMC chemical structure. SD had a pronounced effect on the affinity and barrier to moisture, glass transition, tensile resistance, and extensibility of HPMC films. This effect has been attributed to the reduced polarity provided by methoxyl substitution. Molecular weight, in turn, affected mostly the rheological behavior of HPMC solutions as well as the mechanical properties of its films. This outcome arises from the higher level of physical entanglement and reduced free volume of longer chains. Considering food packaging applications, the trend towards more mechanically resistant and less permeable films guide the choice of HPMC Methocel® E4M as the optimum film-forming matrix among the studied HPMCgrades. Further studies are suggested to investigate the role played by other substitution degrees as well as hydroxypropyl substitution on the physical-mechanical properties of HPMC-based films.
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