Analysis of the Influence of the Fiber Type in Polymer Matrix/Fiber Bond Using Natural Organic Polymer Stabilizer

Polymers 2014, 6, 977-994; doi:10.3390/polym6040977 polymers ISSN 2073-4360 www.mdpi.com/journal/polymers Article Analysis of the Influence of the Fiber Type in Polymer Matrix/Fiber Bond Using Natural Organic Polymer Stabilizer Carlos Rivera-Gómez 1, Carmen Galán-Marín 1,* and Fiona Bradley 2 1 Department of Building Construction, Escuela Técnica Superior de Arquitectura, University of Seville, Avda. Reina Mercedes, 2, Seville 41012, Spain; E-Mail: [email protected] 2 Department of Architecture, Faculty of Engineering, University of Strathclyde, Level 3, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, UK; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +34-954-556-591; Fax: +30-954-557-018. Received: 8 February 2014; in revised form: 18 March 2014 / Accepted: 20 March 2014 / Published: 31 March 2014 Abstract: This research study compares the effect of polypropylene and wool fibers on the mechanical properties of natural polymer based stabilized soils. Biocomposites are becoming increasingly prevalent and this growth is expected to continue within a number of sectors including building materials. The aim of this study was to investigate the influence of different fiber reinforced natural polymer stabilized soils with regards to mechanical properties and fiber adhesion characteristics. The polymer includes alginate, which is used in a wide range of applications but has not been commonly used within engineering and construction applications. In recent years, natural fibers have started to be used as an ecological friendly alternative for soil reinforcement within a variety of construction applications. Test results in this study have compared the effects of adding natural and synthetic fibers to clay soils and discussed the importance of an optimum soil specification. A correlation between the micro structural analysis using scanning electron microscope (SEM), fiber typology, fiber–matrix bonds and the mechanical properties of the stabilized soils is also discussed. Keywords: natural polymers; polymer matrix/fiber bond; polypropylene fibers; stabilized soil; mechanical properties; SEM OPEN ACCESS Polymers 2014, 6 978 1. Introduction Sustainable and ecologically responsible specification of materials in architectural design has increased in recent years due to concerns about reducing the embodied energy of materials and increasing the opportunities for recycling and/or encouraging opportunities for biodegradability at the end of life of a building. With increasing scrutiny of material compositions, commonly used fibers within composites such as glass and carbon fibers, combined with thermosets such as polyester, polypropylene and polyamide have started to appear less attractive compared with potential natural fiber substitutes such as jute, coir flax and bamboo. Within the scientific community there has therefore recently been a number of research studies which have tested and analyzed natural fiber performance within various polymer matrices, to determine flexural, tension and compression strength characteristics as well as durability performance. With regards to soil reinforcement in particular, sisal, bamboo, hemp, coir and a few other plant fibers have recently been studied but to date, animal fibers, such as wool, have been relatively neglected in the search for improving soil reinforcement properties. Animal fibers exhibit a large surface area in relation to their bulk and have an ability to form linkages with polymers improving shrinkage resistance and their surface properties play a critical role in many wool applications. An understanding of the chemical structure and behavior of wool fiber surfaces is therefore crucial in order to advance wool processing techniques and finishing technologies as well as examine potential applications for wool across a range of industrial sectors. Several authors [1–6] provide a broad review of different research studies on biocomposites, natural fibers and biopolymers. Biocomposites are becoming increasingly prevalent and this growth is expected to continue over the coming decades. Biocomposite research has mostly focused on mechanical testing of short-fiber composites, micromechanical studies such as fiber treatments for improved fiber-matrix interface properties and modeling of biocomposite properties [1]. It is important to find mechanisms that will improve the stability of soils and natural fibers [7–12] (which are usually by-products of mainly plants or animals) are a potentially significant resource. Natural fibers can be used as reinforcement in eco-friendly composites suitable for the building industry and these fibers have been tested as reinforcement in cement [13], polymer matrix composites [14–16] and also, to varying degrees, as reinforcing materials in order to improve the engineering properties of different types of soil [17]. There are a variety of interactions which control the load transfer between fibers and their matrices including chemical bonds, secondary interaction forces (van der Waals, acid/base etc.) and mechanical interlocking. The quality of the fiber/matrix bond significantly affects not only the overall composite properties, but also the water uptake of the composites and authors such as Joseph et al. [18] have showed that improved adherence between sisal fibers and a polypropylene matrix, via chemical treatments, can reduce weight gain due to water sorption, by reducing capillary action. In this research project, a new approach based on analyzing microscopic structural changes in stabilized specimens has been introduced to try to understand the behavior of the swelling and shrinkage phenomena that occur in natural polymer stabilized soils when dried at room temperature. Four different types of soils from the Andalusia region of Spain, with varying clay mineral contents, and different plasticity index values, were combined with two different types of fibers: one was a Polymers 2014, 6 979 synthetic fiber polypropylene (PP) and the other was a natural fiber, wool. These samples were also combined with a form of alginate and then compressed into brick moulds under laboratory conditions at the Universidad de Sevilla. After drying and curing, these samples were then subjected to a series of characterization and mechanical tests and then results compared and analyzed. All the specimens were carefully dried to their initial water content, in an oven for 24 h and then at room temperature; to evaluate partial shrinkage and the relationship of this change was compared for different amounts of natural fiber within different soil samples. Microscopic structural changes in stabilized specimens were then studied with a scanning electron microscope (SEM) and the results from this analysis were compared with the results of mechanical tests. In addition, physical changes were compared with information relating to the plasticity index, Atterberg limits and the chemical composition of each soil. 2. Materials 2.1. Alginate Alginates are naturally occurring polymers that occur as sodium alginate, calcium alginate and magnesium alginate salts within the cell walls and intercellular mucilage of seaweed [19]. They contribute to the flexibility and mechanical strength of algal plants [20] and are therefore comparable to the cellulose and pectin components of land based plants [21]. Moreover, they have interesting properties including a high capacity for holding water, improving viscosity and stabilizing emulsions. They are sold in filamentous, granular or powdered forms and in this study a dry alginate in the form of a powder was used which when mixed with water formed a colloidal sol. The chemical reaction, which occurs in this process, produces fibrils that form a gel, because the alginate has within its composition, the salt of calcium sulfate dehydrate, which releases a calcium ion, facilitating the reaction. Interestingly alginate gels have the ability to replicate the characteristics of a semi-solid when the gelation process concludes but to date, only a few previous research studies by Friedemann et al. [22] and Galán et al. [23,24] have been carried out which incorporate types of alginate into building materials. 2.2. Fibers Natural fibers comprise plant fibers such as jute, coir, sisal, bamboo, wood, palm leaf, coconut leaf, coir dust, cotton, hemp, grass, etc. but current research is focusing on sisal [25,26], bamboo, jute, hemp, coir and a few other natural, plant-derived fibers. To date, animal fibers such as wool have been relatively neglected in the search for improving soil reinforcement properties despite the fact that these fibers exhibit a large surface area in relation to their bulk. Natural fibers, acting as reinforcement within composites, offer many advantages including good strength properties, low cost, low density, high toughness, good thermal properties, biodegradability, non-abrasive behavior and widespread availability. However, organic products containing cellulose fibers have several negative characteristics, such as an incompatibility with hydrophobic polymer matrices [27] and a tendency to show little resistance to prolonged moisture. Finite natural lengths and large diameters also limit their potential applications. Polymers 2014, 6 980 Despite these disadvantages, many previous studies have demonstrated the positive effects of adding natural fibers as composite, mortar and soil reinforcement improving the particular compound’s ability to decrease shrinkage and enhance compressive, flexural and shear strength, if an optimum reinforcement ratio can be found. In many cases today, reinforcing fibers tend to be of a synthetic nature, such as carbon fibers, glass fibre-reinforced plastics and polypropylene, but in recent years, natural fibers have started to be used as an ecological friendly alternative for soil reinforcement within a variety of material applications. Their shrinkage properties are due to the drying effect, but there are innovative chemical treatments now being developed to counteract their absorption characteristics [28]. An understanding of the chemical structure of the fiber surface is therefore crucial in order to advance wool processing and finishing technology as well as examine alternative applications. 2.2.1. Polypropylene Polypropylene (PP) is a thermoplastic polymer used in a wide variety of formats and applications such as plastic food containers, carpets and insulation. It has a variety of advantageous engineering properties such as resistance to fatigue, physical damage and freezing, as well as being unusually resistant to many chemical solvents, bases and acids. Polypropylene fibers are generally superior to polyamide fibers, for example, with regards to elasticity and resiliency properties but they have a lower wear resistance. Their resistance to various external conditions is largely determined by the effectiveness of added stabilizers. PP filaments and monofilaments are used in the manufacture of floating cables, nets, filter fabrics and upholstery whereas PP fibers are used in carpeting, blankets, outerwear fabrics, knitwear, and filter fabrics. PP fibers are cylindrical and usually have a uniform and homogeneous section of around 40 µm (see Figures 1 and 2). They display good heat-insulating properties but are sensitive to heat and ultra-violet radiation. Figure 1. (a) Polypropylene fiber (full size image) and (b) SEM of polypropylene fiber (×950). (a) (b) PP fibers have a low Young’s modulus value, which means that they cannot prevent the formation and propagation of cracks at high stress levels, however they can bridge large cracks in certain circumstances [29,30]. They have also been used to considerably reduce the amount of cracks in various materials and to enhance residual strength [31,32]. There are a few relevant results in natural Polymers 2014, 6 981 polymer-soil reinforcement studies incorporating polypropylene fibers. Most of the existing research is oriented towards soil stabilization with cement or lime, the enhancement of soil properties for roads and pavements or the improvement of the characteristics of expansive soils. In cementitious products, additional porosity caused by the melting of polypropylene fibers when exposed to high temperatures, can lead to a decrease of the residual mechanical performances of concretes [33]. Experimental results from various different studies in academic literature on this subject are contradictory. Several studies carried out by authors such as [34,35] show a decrease in residual strength in agreement with the hypothesis relating to increased porosity caused by chemical reactions, whilst other authors such as [36,37] show improvement in residual strength. Nevertheless, PP’s low moisture absorption rate compared to most natural fibers, makes Polypropylene, more stable volumetrically (see Table 1). Figure 2. (a) Wool fiber (full size image) and (b) SEM of wool fiber (×700). (a) (b) Table 1. Fiber absorption assessment. Synthetic Fibers E-glass Polypropylene Polyester Polyamide Moisture absorption (%) - 0.01 0.4 6 Natural Fibers Hemp Jute Ramie Coir Moisture absorption (%) 8 12 12–17 10 Natural Fibers Sisal Flax Cotton Wool Moisture absorption (%) 11 7 8–25 10–28 2.2.2. Wool Natural protein-based fibers are generally obtained from animal hairs and secretions. These protein fibers generally have a greater resistance to moisture and heat than natural cellulosic and vegetal fibers, however proteins fibers have little resistance to alkalis, so they are not appropriate for use within mixes that contain cement. A small amount of research has been carried out into the use of animal fibers within composites and Barone and Schmidt [38], for instance, reported on the use of keratin feather fibers as short-fiber reinforcement within LDPE composites; this keratin feather fiber they used had been obtained from chicken waste. A very common natural protein fiber containing keratin is wool, which grows outwards from the skin of sheep. Different species of sheep produce different types of wool with varied fiber length, diameter and other differing physical characteristics. Polymers 2014, 6 982 Generally however, fine wool fibers are 40–127 mm in length, 14–45 μm in width, are roughly oval in cross-section and grow in a wavy type of form which gives rise to a degree of twist. To date, wool has not been looked at in great detail as fiber reinforcement. It is a hygroscopic fiber, which takes up moisture in vapor form, and tiny pores in the cuticle make the fiber semi-permeable, allowing vapor to pass through to the heart of the fiber. This means that wool can easily absorb up to 30% of its weight in moisture without feeling damp or clammy, which is obviously a significant advantage to animals trying to keep warm in wet weather. There is generally a two-phase structure for wool fibers, which consists of a water-absorbing matrix, embedded within non-water-absorbing cylinders. The macroscopic appearance and physical structure of the wool fibers is shown in Figures 2 and 3. Figure 3. (a) Different samples of SEM of Polypropylene fiber in the white soil mix (×300); (b) SEM of Polypropylene fiber in the yellow soil mix (×400); (c) SEM of Polypropylene fiber in the black soil mix (×300); and (d) SEM of polypropylene fiber in the red soil mix (×300). (a) (b) (c) (d) 2.3. Soil Four soils were used in this research project and they were all supplied by the Innovarcilla Foundation (Bailén, Spain), which promotes the activities of the Andalusian Ceramics Technology Centre (Bailén, Spain). The Foundation brings together the most important manufacturers of ceramic construction materials in the Andalusia region and it generously supplied the soils utilized in this research study. Polymers 2014, 6 983 The term “clay” refers to a material that occurs in nature composed primarily of fine grained minerals, which are generally plastic at appropriate water contents but will harden during drying or curing. Different mechanical properties such as plasticity and hardening characteristics vary depending upon the mineralogical composition of the clays, which in the main comprise phyllosilicates with variable contents of quartz, feldspar, carbonates, gypsum, etc. The four different types of clay soils utilized in this research study were identified in accordance with their predominant color; namely White, Yellow, Black and Red and in all cases, the soils were sourced from the Southern region of Spain, Andalusia. Some of the parameters considered essential in the understanding of the behavior of clays as components within composite materials are described below. In particular, phyllosilicates, which are fine particles (<2 μm) that contribute to the plasticity of clays in their reactions with water, are identified. Indeed, full characterization spectrums for each one of the clays—including the percentage of phyllosilicates—is shown in Table 2. Table 2. Mineralogical composition of the four types of soil used. Soil Clay minerals (%) Phyllosilicates (<2 µm) (%) Q Fd C D Fl Sm Ill Ka Ch P White 28 6 25 8 33 68 32 Ud. Ud. Ud. Yellow 40 tr. 15 tr. 45 40 60 Ud. tr. Ud. Black 43 3 18 4 32 32 58 10 Ud. Ud. Red 32 5 tr. 10 53 Ud. 66 33 tr. Ud. Notes: Q: Quartz; Fd: Feldspar; C: Calcite; D: Dolomite; Fl: Phyllosilicates; Sm: Smectites; Ill: Illite; Ka: Kaolinite; Ch: Chlorite; P: Pyrophyllite; Ud: undetected; tr: traces; Ill: Illite; Ka: Kaolinite; Ch: Chlorite; P: Pyrophyllite; tr.: Traces (1%–3%); Ud.: Undetected. As can be seen from the table, there are significant variations in the mineral content of the clays and this gives rise to quite different behavior within the polymer matrices. Phyllosilicates consist of sometimes two layers (one tetrahedral “T” and one octahedral “O”) that repeat and in other cases three layers that repeat. The former type (known as “T–O” structures), comprise one tetrahedral sheet of silicon linked through oxygen atoms to one octahedral sheet of aluminum and in the three layer T–O–T type two silicate layers surround an octahedral layer filled with Aluminum or Magnesium. Within the T–O–T type there is a sub-group which exhibits weak bonding between layers thereby facilitating delamination and swelling—this behavior occurs typically in the montmorillonite group of clays. The other group demonstrates that delamination is much more difficult and indeed swelling is not observed in common applications. This group is termed the illite group of clays. The four clay types used for this research study belong to this second group and depending on the type of clay, the surface charges vary, inducing significantly different interactions forces between the clay particles and as a consequence, different rheological properties. Clay’s plasticity is directly related to the size distribution of the clay particles with smaller particles producing greater plasticity. The following Table 3 shows the different percentages of particles smaller than 63 μm in the different clays used in this series of experiments. Polymers 2014, 6 984 Table 3. Percentages of rejections at 63 µm obtained by wet sieving. Clays White Yellow Black Red Rejection at 63 µm 28.1% 23.8% 43.8% 51.1% In addition, chemical compositions of the four soil samples were obtained after naturally drying the samples and sieving them and then measuring their composition by chemical precipitation as illustrated in Table 4. Table 4. Chemical analysis of the four types of soil used. Soil (%) in weight SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P 2O5 L.O.I. White 47.5 8.8 3.3 0.0 1.7 16.6 0.4 1.9 0.5 0.1 17.7 Yellow 56.6 12.1 4.2 0.0 2.2 9.1 1.7 3.1 0.0 0.0 10.8 Black 54.2 12.3 4.3 0.1 2.1 10.4 1.4 2.9 0.7 0.1 12.2 Red 58.5 16.7 7.0 0.1 2.8 2.2 0.2 5.0 0.8 0.2 0.08 Note: L.O.I., Loss on ignition. The water content of each soil type was also determined by using Atterberg Limits [39] which are empirical divisions between the solid, plastic and liquid limits of clay. Different plasticity indexes and different consistencies in mixtures are due to their relative water absorption characteristics and as can be seen below, the four soils showed significant variations in their index of plasticity (Table 5), with the drier White soil having a significantly higher plasticity index. Table 5. Atterberg limits of the four soils used. Physical Characteristics White Yellow Black Red Liquid Limit 45.4% 32.8% 38.8% 25.6% Plastic Limit 15.5% 11.1% 18.6% 14.1% Plasticity Index 29.9% 21.7% 20.2% 11.5% 2.4. Mixes The samples were prepared by mixing four different ingredients (soil, alginate, fiber and water) in a variety of different proportions. The alginate used was dried alginate as soluble salts from alginic acid, with a content of approximately 15% sodium alginate and calcium fillers that improve the resistance and the irreversibility of the polymerization process. Two types of fibers were used: one a man-made fiber, polypropylene, and the other a natural wool fiber. Both were short fibers 10 mm long randomly distributed at a rate of 0.25% by weight. The water used was the optimum proportion related to the alginate content and the samples were manually compacted and cast in steel prismatic molds 160 mm × 40 mm × 40 mm and specimens from the four soils were prepared in the two following dosages (Table 6). In the manufacturing process, different consistencies and workability were observed between the different clay samples and typically the White soil appeared to be drier. The consequences of the drier consistency can be observed within the mechanical properties section. Polymers 2014, 6 985 Table 6. Proportions for the different mixes used (by weight). Proportion Soil Alginate Product * Polypropylene Wool Water Soil_PP 79.5% 1% 0.25% 0.00% 19.25% Soil_W 79.5% 1% 0.00% 0.25% 19.25% Notes: * Dry alginate (natural polymer); PP, (polypropylene fiber) and W, (wool fiber). 3. Experimental Section 3.1. SEM Analysis Scanning electron microscopy (SEM) has been shown to be a useful tool for the direct study of polymer-soil matrix interfaces. In particular, SEM studies have helped to illustrate the spatial relationships between the various components of matrices and reinforcement fibers. The samples in this project were examined by scanning electron microscopy (SEM), using a JEOL JSM-6460LV microscope in CITIUS laboratory of the University of Seville (Seville, Spain). 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