Green composites based on polypropylene matrix and hydrophobized spend coffee ground (SCG) powder

Abstract

Green composites were prepared with polypropylene matrix and 20 wt.% spent coffee ground (SCG) powder for uses as a wood plastic composite (WPC). The effects of hydrophobic treatment with palmitoyl chloride on SCG powder is compared with conventional surface treatment based on silanization with (3-glycidyloxypropyl) trimethoxysilane and the use of a maleated copolymer compatibilizer (polypropylene-graft-maleic anhydride, PP-g-MA) in terms of mechanical properties, morphology, thermal properties and water uptake. Composites were previously mixed in a twin-screw co-rotating extruder and subsequently subjected to injection moulding. The comparative effect of the different surface treatments and or compatibilizers on mechanical performance was studied by flexural, impact tests and dynamic mechanical thermal analysis (DMTA-torsion); in addition, the stabilizing effect of SCG was revealed by differential scanning calorimetry (DSC) and thermogravymetric analysis (TGA). As one of the main drawbacks of wood plastic composites and natural fibre reinforced plastics is the moisture gain, water uptake tests were carried out in order to quantify the effectiveness of the hydrophobization process with palmitoyl chloride. Results show a slight increase in flexural modulus for composites with both untreated and treated/compatibilized SCG powder (20 wt.%). As expected, thermal stability is improved as indicated by an increase of more than 8% in the onset degradation temperature by DSC if compared to unfilled polypropylene. Fracture analysis by scanning electron microscopy (SEM) shows better particle dispersion for PP-SCG composites with hydrophobized SCG with palmitoyl chloride treatment; in addition a remarkable decrease in water uptake is observed for composites with hydrophobized SCG.

Introduction

Today, ecological concerns and issues such as recycling and environmental care are increasingly important. As a consequence of such environmental awareness we are witnessing a great interest in the research on more environmentally friendly materials as it is the case of polymer composites reinforced with natural fibres (natural fibre reinforced plastics-NFRP and wood plastic composites-WPC) [1], [2], [3]. Natural fillers are acquiring increasing importance as reinforcing materials in composites due to some advantages they provide such as low cost, low density, no toxicity, balanced mechanical properties and a clear lower environmental impact [3], [4]. The main problem related to the use of natural fillers is their low compatibility with most polymer matrices. Most natural fillers are lignocellulosic-based materials and, consequently, highly hydrophilic, while most polymer matrices are intrinsically hydrophobic. This fact leads to low polymer-filler interactions which lead to poor mechanical properties thus making necessary the use of surface treatments on fillers (i.e. silanization) or addition of compatibilizer agents (maleated copolymers). In addition to the lack of compatibility between the two main components, moisture gain is another big drawback when using natural fibre reinforcements. Hydrophilicity in natural fibre reinforcements is provided by the high amount of cellulose and hemicelluloses with hydroxyl groups which are also responsible for high water absorption capacity. Moisture gain in NFRPs and WPCs is a critical issue as the water uptake leads to dimensional instability [4], [5], [6]. With the aim of improving polymer-filler interactions and reduce the water uptake, different physical or chemical modifications have been proposed (silanization, esterification, etherification, benzylation, etc.) as well as the use of compatibilizer agents (mainly maleated copolymers) [6], [7], [8], [9], [10].

Coffee is one of the most consumed beverages and the second most traded product in the world after petroleum [11], so that the coffee industry generates a lot of waste. A major waste generated by this industry is spent coffee ground (SCG), which is obtained from the treatment of the coffee powder with hot water to prepare instant coffee. About 6 million tons of SCG are generated annually worldwide [11], [12]. In general terms, 650 kg of SCG are obtained during processing one ton green coffee and 2 kg of wet SCG are obtained from one kg soluble coffee [11], [13]. Nowadays most of the SCG wastes are poured into the environment or burned in order to remove them, being these techniques highly disrespectful with the environment [13]. In the literature there are different researches focused on providing an alternative to the residue of SCG to reduce its environmental impact [11]. Some of these proposals are the use of SCG for animal feed [14], for producing organic compost [15], as fuel pellets [16] or the production of active carbon [17]. In addition, the potential of SCG as raw material for the extraction of natural antioxidants [18] the production of ethanol [19], biodiesel [20] or polyhydroxyalkanoates (PHA) [21] have been investigated in the last years. The novelty of this work is the use of SCG as functional reinforcement in polypropylene for wood plastic composites (WPCs); SCG provides a typical dark brown colour. It could be possible to bleach SCG by environmentally friendly processes using enzymes due to its lignocellulosic nature but its natural colour could be attracting as it is similar to some dark or black woods such as: Gaboon Ebony, Ziricote, African Blackwood, Wenge, Panga–Panga, among others.

The main objective of this study is to investigate the effect of a hydrophobic surface treatment of SCG powder with palmitoyl chloride and compare it with conventional treatments on SCG such as silanization with ((3-Glycidyloxypropyl) trimethoxysilane, GLYMO) and/or use of compatibilizers (polypropylene-graft-maleic copolymer, PP-g-MA) in terms of mechanical, morphological and thermal properties as well as the effects on water absorption of PP/SCG composites. Manufacturing of PP/SCG composites was performed using a twin screw extruder and subsequent injection moulding. PP/SCG composites containing 20 wt.% SCG with the different treatments and formulations were compared in terms of mechanical properties (flexural an impact tests) as well as dynamic mechanical thermal analysis (DMTA) in torsion mode. Particle dispersion and particle-matrix were qualitatively assessed by scanning electron microscopy (SEM) and the effect of SCG on thermal stability of composites was evaluated with differential scanning calorimetry (DSC) and thermogravymetric analysis (TGA). The effect of the different treatments on the chemical composition of SCG was analysed by FTIR spectroscopy. Finally, the effect of the hydrophobic treatment with palmitoyl chloride is compared with conventional treatments/compatibilizers in terms of the water uptake and dynamic contact angle measurements.