Anchor and bridge functions of APTES layer on interface between hydrophilic starch films and hydrophobic soyabean oil coating

Highlights

• Water resistant starch film coated with AESO-APTES double layers has been developed.

• Interfacial adhesion between starch and AESO coating is significantly improved after surface treatment with APTES.

• The WVP of starch film decreases after coating, especially the coating treated with APTES.

• Mechanical properties of starch films are much more stable under high humidity conditions after double layer coatings.

Abstract

One of the well-recognized weaknesses of starch-based materials is their sensitivity to moisture, which limits their expanding applications. Natural materials, soyabean oils have been used as a coating for starch film, but the poor interface between hydrophilic starch and hydrophobic soyabean oil needs to be improved. In this work, (3-Aminopropyl) triethoxysilane (APTES) was used to reinforce the bonding between starch matrix and the coating of bio-based acrylated epoxidized soyabean oil (AESO). Study results show that APTES interacted effectively with both starch films via hydrogen bonding, and chemical bonds with AESO through the Michael addition reaction. Pull adhesion and cross-cutting tests demonstrated that the interfacial adhesion was significantly improved after treating their surface with APTES. The interfacial adhesion strength increased over 4 times after treating with 1.6 wt% APTES. The starch films treated with APTES and AESO coating were intact after soaking in water for more than 2 h.

Introduction

Due to the pollution problems caused by non-degradable wastes from petroleum-based synthetic polymers (Menzel, 2020), renewable materials, such as proteins, lignin, carbohydrates, natural oils (Hu et al., 2019) have been receiving increased attention for environmental and economic purposes. Starch is one of the most promising and economical bio-resources that can replace petroleum-based polymers for applications in food packaging, electronics, and biomedical composites (Zhu, Chen, Lu, Liu, & Yu, 2020). However, its high hydrophilic and hygroscopic nature largely hampers its application (Olsson, Hedenqvist, Johansson, & Jarnstrom, 2013). The ability of starch materials to absorb water greatly results in their poor mechanical properties under wet or high humidity conditions.

Many works have attempted to improve the water resistance of starch-based materials by chemical modification (Bhure & Mahapatro, 2010; K. Lu et al., 2020; Mehboob, Ali, Sheikh, & Hasnain, 2020; Qiao et al., 2018), blending (Chen et al., 2020; Y. Chen et al., 2019; Oliveira Filho et al., 2020; Xu et al., 2021) or coating (X. Chen, Cui, et al., 2019; Chen et al., 2020; Estevez-Areco, Guz, Candal, & Goyanes, 2020) with hydrophobic materials. Concerning the chemical modification method, the effect of hydrophobic modification is limited due to its strong specificity, strict reaction condition, and low efficiency (Wang et al., 2020). On the other hand, the application of organic solvents such as sodium hypo-chlorite, trimetaphosphate and sodium trimetaphosphate to obtain oxidised, crosslinked or esterified starches can negatively affect the environment. (Sifuentes-Nieves et al., 2020) A blend containing starch cannot become hydrophobic until the starch content is low enough to form a separate domain phase in a hydrophobic polymer matrix. Furthermore, the poor compatibility between two blending phases is another inevitable problem. Therefore, coating is facile (X. Wei et al., 2015) (low capital investment) and efficient (excellent moisture barrier property) compared to blending and chemical modifications. In our previous work, we developed acrylated epoxidised soyabean oil (AESO) which formed a coating when exposed to UV for only several minutes on starch to reduce the moisture sensitivity and permeability of starch films for more than 10 times (Ge et al., 2019). Previous work has shown that the WVP of the 40 wt% shellac coated films could decrease from 2.63 × 10⁻¹¹ to 0.37 × 10⁻¹¹ g/ (m s Pa) (X. Wei et al., 2015).

However, the bonding between hydrophobic oil layer and hydrophilic starch layer is so weak that the coating was easily removed from the starch surface as displayed in SEM images (Fig. 5). Similar poor compatibility was also observed between hydrophobic poly (butylene adipate-co-terephthalate) (PBAT) and hydrophilic starch in starch/PBAT blends (Zhai et al., 2020). In this work, (3-Aminopropyl) triethoxysilane (APTES) was used to improve the interface between hydrophilic starch and hydrophobic AESO. APTES has amphoteric terminals and is widely used in the functionalisation of silica (Shang & Zhang, 2020), graphene oxide (Roushani, Rahmati, Farokhi, Hoseini, & Fath, 2020), carbon nanotubes (Jorge, Coulombe, & Girard-Lauriault, 2019), and metals (Bhure & Mahapatro, 2010; Vuori et al., 2014).

The ethoxy silane end of the molecule can be hydrolysed into silanol, and the OH of the silanol can undergo dehydration condensation reaction or hydrogen-bonding with the hydroxyl groups (Hao, Chen, Xia, & Gao, 2019). On the other hand, the amino end of the molecule can form hydrogen-bond with hydroxyl groups (Yang et al., 2019) (and covalent-bond with CC group in AESO through Michael addition reaction (Meng et al., 2019; Wu et al., 2018; Yi & Shen, 2017). Therefore, it is expected that APTES could effectively improve the adhesion performance of the surface of the starch substrate coated with AESO.

This paper will firstly report the preparation of APTES layer between starch film and AESO coating, and then characterise the effect of APTES on the hydrophilic matrix and hydrophobic coating. The deposition of APTES layer on the starch films was investigated by XPS and SEM-EDS. The reactions of APTES with starch and AESO were characterised by FTIR and ¹H NMR. The interfacial structure between the starch film and AESO coating was characterised by SEM. The adhesive bonding was measured by pull-off adhesion and cross-cutting tests. The water resistance of the modified starch material was evaluated by water soaking and gas permeability test.