Mechanical properties of a polyurethane hybrid composite with natural lignocellulosic fibers

Abstract

Several low-cost hybrid composites composed of polyurethane and renewable natural fibers were developed and analyzed for their mechanical and physical properties. Composites were fabricated by replacing up to 20% w/w of the polyethylene glycol present in conventional polyurethane foams with one and the mixture of three natural fibers: sugarcane bagasse, sisal or rice husk. Prior to composite production, fibers were mercerized with sodium hydroxide and hydrogen peroxide to remove lignin and hemicellulose. A simplex-centroid mixture design model was used to evaluate the effects of the added fibers on composite properties such as resilience, elastic modulus and deformation under permanent compression. Obtained hybrid composites demonstrated up to 32% of resilience, 0.1 GPa of elastic modulus, and 7.32% of permanent deformation. In order to optimize these properties, fiber amounts were adjusted using a quadratic mathematical model, indicating that formulations containing only the rice husk or an 82/18 (% w/w) rice husk/sugarcane bagasse mixture will perform best. The obtained composite is a unique low cost material because is environmentally friendly and has a high potential for applications in shock absorption and padding materials, due its proven good resilience and elastic modulus.

Introduction

Recently, the use of natural fibers in polymer composites has increased considerably due its low price compared to synthetic fibers [1]. Problems like the necessity to improve the harvesting waste management and the poor mechanical properties of polymers can be solved producing polymer/natural fibers composites with reinforcement efficiency modulated by natural fibers from different origins [2], [3], [4]. The mechanical properties of natural fibers are influenced by the chemical composition related mainly to cellulose and lignin content by the orientation of crystals and by the degree of crystallinity, and these factors are dependent of type of plant, conditions during growth as well as extraction methods used [5]. Some natural fibers can be compared to synthetic ones, such as fiberglass and carbon fiber, in terms of reinforcement properties in composites, besides presenting decreased health hazards, low density and high flexibility [6], [7]. Several natural fiber polymer composites has already developed [8], [9], [10], [11], [12], [13], [14] and the goals were mainly improve mechanical resistance. These natural fibers include fibers obtained from flax, hemp, jute, sisal, kenaf, coir, kapok, banana, henequen and many others plants [15], [16].

Among natural fibers, sisal is one of the most used in the world, and Brazil is one of the biggest producers [17]. Automotive industry and civil construction already use the exceptional mechanical characteristics of sisal in car parts and concrete, respectively [18], [19], [20]. Numerous researches have been recently studied rice husk and sugarcane bagasse fibers due to their large availability, trying to obtain the same success obtained with the use of sisal fibers. In fact, as consequence of the large production of rice, approximately 600 million tons per year [21], there is a large amount of rice husk waste (about 20% w/w) [22]. The production of sugarcane in world is 1.7 billion ton per year [23]. Brazil is the largest producer of sugarcane in the world, and was harvested 658 million tons in 2015/2016 crop [24]. Many studies have investigated possible uses for husk fibers, such as a fuel and concrete production [25], [26], and the use of rice husk as a thermoplastic composite reinforcement [27], [28]. From sugarcane harvesting around 27% are converted into sugarcane bagasse [28]. Due to the low economic value of sugarcane bagasse, it is mainly utilized as fuel to produce energy to sugarcane industry, contributing to the greenhouse effect. Recently, studies have reported the use of sugarcane bagasse fibers as filler in thermoplastic composites. These works developed composites with distinct mechanical properties [29], [30], [31].

The mechanical characteristics of a polymer composite reinforced by natural fibers are mainly result of the quantity and fibers type, besides the interfacial strength between reinforcement and matrix. An alternative to modulate the composite mechanical performance is develop hybrid composites, a material produced by combination of two or more types of reinforcement [32], [33].

Flexible polyurethane foams (PUs) are three-dimensional polymers with open cells which are permeable to air and return to their original shape after compression. Specific properties such as elastic modulus, hardness, resilience and thermal conductivity may be prioritized during its production [34], distinguishing PUs from all other materials. Because of this permeability and flexibility PUs are heavily utilized as architectural and structural materials, and in the automotive and refrigeration industries [35], [36]. PUs hold the sixth position worldwide for plastic sales with a production of 17.0 billion tons per year [37].

Although PUs have many appealing qualities, their high chemical stability, a lack of waste management infrastructure and a high demand are combining to amass a growing environmental hazard. Traditionally, manufactured PUs persist in the environment for 30 years before decomposition begins [38]. This is where this investigation aims to intervene. The presented research introduces a novel production method which replaces 20% (w/w) of the synthetic material present in traditionally manufactured PU by natural fibers, in order to reduce the impact of post-consumption PUs, increasing their biodegradability.

Silva et al. [39] studied the influence of Eucalyptus grandis fibers on rigid PUs and found that the addition of 16% (w/w) natural fiber drastically increased their mechanical strength and thermal conductivity. Javni et al. described that the addition of up to 20% (w/w) silica fibers increased both hardness and elastic modulus with only a small reduction in resilience. Since then, the hybrid composite elastic modulus has been correlated with the number of cross-links between PU chains and hydroxyl groups exposed on fiber's surface [40], [41].

In our study, a factorial experimental design was used to evaluate and statistically compare potential hybrid composites (rice husk, sugarcane bagasse and sisal fiber with PU matrix) based on their resilience, elastic modulus, permanent compressive deformation, porosity, density, and degree of swelling. This prescribed combination of materials meets the project goals of enhancing the decomposition process while increasing material strength. It also reduces production costs.