Properties of economical and eco-friendly polybutylene adipate terephthalate composites loaded with surface treated coffee husk

Highlights

• Biodegradable poly(butylene adipate terephthalate) (PBAT) was reinforced with coffee husk (CH).

• The PBAT and/or CH were successfully treated with different techniques.

• The composite filled with silane treated CH exhibited superior thermal and mechanical property.

• The ductility of the composite showed 53.8% improvement with the %EB reaching up to 500%.

• The composite is 32% less expensive than the neat PBAT.

Abstract

In this study, we have fabricated ecofriendly and economical polybutylene adipate terephthalate (PBAT) composites loaded with various concentrations of coffee husks (CH) to investigate the effects on the PBAT and CH interfaces as well as the physical properties of the composites. Contact angle (CA) measurements revealed that the silane-treated CH (T-CH) is more hydrophobic than the raw CH, which improved its compatibility with PBAT. Morphological analysis of the composites revealed that the matrix exhibited greater interfacial interactions with T-CH compared to other derivatized PBATs. The elongation at break revealed a unique increase from 325% (neat PBAT) to 500%. The tensile strength, and Young's and storage moduli of the 40 wt% T-CH is greater than that of neat PBAT, indicating its superiority. Furthermore, the addition of 40 wt% CH to PBAT reduced the cost of the composite by 32%, making it economically competitive with other commercial polymers.

Introduction

The generation huge amounts of non-recyclable and non-renewable plastic wastes associated with utilization of petrochemical based plastics are creating immense environmental concerns [1], [2], [3]. As a result, many countries are now forcing the inclusion of biodegradable polymers into plastic commodities to substitute for conventional neat polymers [4], [5]. Some of the most popular biodegradable polymers include polylactic acid (PLA), polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS), and polyhydroxy-alkanoates [6], [7], [8], [9]. However, the cost of these biodegradable polymers is about 2.5–7.5 times higher than conventional polymers like polyethylene, polypropylene [10], [11]. A widely accepted method to lower the cost of biodegradable polymers is to incorporate lignocellulose materials that do not significantly affect the properties of the biodegradable polymers [12], [13], [14].

Lignocellulose materials are any materials that contain a large amount of cellulose, hemicellulose, and lignin in their structure [15], [16]. Many forestry or agricultural wastes like rice straw, wheat straw, bamboo fibers, and coffee waste are considered as lignocellulose materials [17], [18], [19], [20], [21]. Coffee, as one of the most consumed commodities, its processing generates huge amount of waste material with more than 50% of it is disposed as waste. Consequently, many researchers have been incorporating various types of coffee waste to make ecofriendly biodegradable composites with improved mechanical properties [22], [23], [24], [25], [26]. PLA and PBS composites fabricated by Totaro et al. [27] containing up to 30 wt% coffee silver skin (CSS) show an increase in Young’s modulus without affecting the tensile strength. In another study, PLA composite with increased water resistance and biodegradability, compared to the neat PLA, was synthesized by incorporating surface treated coffee spent ground (CSG) [28]. In addition, other researchers stated that the right amounts of extracts have a positive effect on the antiaging properties of PLA without adversely affecting properties of the polymer [29], [30].

Among the biodegradable polymers, PBAT is getting attention because of its unique properties, including high flexibility, ductility, and toughness. However, its low stiffness limits its commercial applications [31], [32], [33], [34]. For almost two decades, PBAT has been widely used in the packaging industry, biomedical sector, and agriculture. However, its high production cost limits its large-scale production for use in alternative fields of applications [35], [36], [37], [38]. Many researches have incorporated lignocellulose materials into PBAT with the aim of lowering its cost by keeping useful properties of the polymer. An excellent enhancement of the thermomechanical properties of PBAT was obtained by incorporating torrified CSG for food packaging applications [39], [40]. A PBAT/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) blend was reinforced with CSS to fabricate a composite with an increased Young’s modulus and degree of crystallinity without altering its tensile strength [41].

In this work, we fabricated ecofriendly PBAT composite loaded with micro-sized coffee husk (CH) fibers by using a melt extrusion technique. CHs are fibrous waste products obtained after dehulling the coffee cherries to get the beans [42], [43]. Like other lignocellulose materials, coffee husks exhibit a hydrophilic nature due to the presence of many hydroxyl groups within its cellulose and hemicellulose. Consequently, the CH material does not have high interfacial interactions and compatibility with the hydrophobic PBAT [44], [45]. We made two modifications to change the interactions of CH and PBAT: silane treatment of CH to increase its hydrophobicity and maleic anhydride (MA) compatibilization of PBAT to increase its hydrophilicity. The physical and thermomechanical properties of the PBAT/CH composites were investigated using a contact angle goniometer, a thermogravimeter, a universal testing machine, and a dynamic mechanical analyzer.