Natural fibers are gaining wide attention due to their much lower carbon footprint and economic factors compared to synthetic fibers. The moisture affinity of these lignocellulosic fibres, however, is still one of the main challenges when using them, e.g., for outdoor applications, leading to fast degradation rates. Plastination is a technique originally used for the preservation of human and animal body organs for many years, by replacing the water and fat present in the tissues with a polymer. This article investigates the feasibility of adapting such plastination to bamboo natural fibres using the S-10 room-temperature technique in order to hinder their moisture absorption ability. The effect of plastination on the mechanical properties and residual moisture content of the bamboo natural fibre samples was evaluated. Energy dispersive x-ray spectroscopy (EDS) and X-ray micro-computed tomography (Micro-CT) were employed to characterize the chemical composition and 3-dimensional morphology of the plastinated specimens. The results clearly show that, as plastination lessens the hydrophilic tendency of the bamboo fibres, it also decreases the residual moisture content and increases the tensile strength and stiffness of the fibers.
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A major drawback of using natural fibres is their durability. Lignocellulosic fibers tend to have hydrophilic groups attached, due to which they absorb a lot of moisture. This moisture weakens the cellulose structure and the cell wall of these natural fibers, which results in the degradation of their properties. Within the current trend of research reviewed above, the authors believe chemically altering the material composition of these fibres could be one of the solutions to overcome the above environmental performance limitation. In pursuit of that goal, this article provides novel insights on the application of the S-10 plastination technique [ 14 , 15 , 16 ] on bamboo natural fibres. It includes a detailed procedure developed for the plastination of bamboo, as well as techniques for characterizing the mechanical, physical, and chemical properties of plastinated and virgin bamboo (see Section 3 for methodological aspects of the experiments). Section 2 presents the main findings of the conducted experiments, focusing on two key themes: successfully applying plastination to bamboo and characterizing it, and assessing the environmental durability of the plastinated bamboo. Section 3 outlines the main conclusions of the work and suggests pertinent future work.
Silicone plastination (S-10 technique) is one of the simplest and most versatile types of plastination involving silicone polymers as impregnation mixtures and hardeners. Among different silicone polymers used, S-10 (polydimethylsiloxane) is the most popular and widely used polymer and results in more natural-looking, opaque specimens [ 17 , 18 , 19 ]. The S-10 technique is comprised of four plastination steps, namely fixation, dehydration, forced impregnation, and curing. In this technique, S-3 (dibutyltindilaurate) and S-6 (tetraethoxysilane) are used as a catalytic agent and hardener, respectively. Besides the wide range of applications of plastination techniques among human, animal and yeast tissues, there is no report on whether plastination could be possibly applied to preserve natural fibres as a way to enhance the strength and durability of these fibres and their composites like bamboo fibre reinforced plastic.
In anatomical sciences, the long-term preservation of animal and human tissues has been carried out for decades, and among the various preservation techniques, &#;plastination&#; is a well-established method. First applied in by Dr. Gunther Von Hagens, plastination, also called forced polymerization, has been extensively employed for preserving different bodies and body parts of living organisms [ 14 , 15 , 16 ]. In essence, plastination replaces the water and lipids in the tissue with curable polymers. It involves preserving perishable biological specimens using a series of procedures that replaces tissue water and part of tissue fat with curable polymers, mostly silicone, and epoxy that hinder the decay of body tissue.
Among various natural fibres, bamboo is one of the most desirable options due to its low density and high strength properties. Furthermore, bamboo has over species, and worldwide bamboo production is approximately 30 million tons/year, the highest among all-natural fibres [ 1 , 2 , 3 ]. Bamboo also has a small harvesting cycle due to its very high growth rate [ 11 ]. Given its rapid growth rate and low carbon footprint during the life cycle, it is also referred to as &#;CO 2 better&#;, i.e., net CO 2 negative [ 12 ]. As an example, due to its superior strength and fast growth rate, the use of bamboo fibers in composites as reinforcement of concrete offers an enhanced technique to increase the dependence of the construction industry on renewable resources [ 13 ]. Bamboo fibres can even exceed the strength of, e.g., glass fibres during bending [ 2 ]. Despite the aforementioned desirable attributes of such natural fibre, it is still known to be prone to degradation due to the effect of moisture and weathering through a highly heterogamous and porous material microstructure [ 11 ].
A vast bulk of natural fibre processing literature has also focused on the chemical treatment of natural fibres that have shown improved physical strengths and interfacial strength properties [ 3 , 6 ]. Chemically, such treatments often involve the use of alkali, acetyl, silane, benzyl, acrylic, permanganate, peroxide, isocyanate, titanate, zirconate, and acrylonitrile treatments, along with a maleic anhydride graft interfacial agent [ 9 ]. The alkaline treatment removes fibrous constituents such as lignin, hemicellulose, wax, and pectin, thereby exposing the cellulose and increasing the roughness/surface area, thus improving interfacial bonding [ 10 ]. However, much of the research involving the processing/pre-treatment of natural fibres has been carried out to enhance their mechanical and interfacial properties when used in a composite. These techniques, however, do not address the problem of the hydrophobic nature of these fibres, which requires adequate attention for their durable long-term usage.
Existing research efforts recognize various physical and chemical processes to enhance the durability and strength of natural fibres [ 3 ]. Sinha and Panigrahi [ 5 ] found that plasma-treated fibres demonstrate superior surface hydrophobicity and improved their shear strength. Similarly, research on the treatment of natural fibers has involved heating the fibers to temperatures close to those that can cause their degradation and affect their properties [ 6 ]. Huber et al. [ 7 ] found that electron radiation can improve the interfacial bond between natural fibres and polypropylene (PP) from 21% to 53% due to the generation of free radicals that promote fibre/matrix cross-linking. Further, the beating of the fibres resulted in a 10% increase in kraft fibre/polypropylene strength due to fiber defibrillation and the corresponding increase in the contact area and mechanical interlocking [ 8 ].
Due to growing concerns for environmental issues such as increasing manufacturing pollution, increasing carbon footprint, and increased awareness about the environmental hazards caused by human-made materials like synthetic fibre composites, great demand, and interest for sustainable natural fibre composites have developed over recent years, particularly owing to their low density, low cost, high stiffness, strength-to-weight ratios, and improved sustainability. Examples of such natural fibres include bamboo, hemp, flax, and wool [ 1 , 2 , 3 ]. There is a progression in the industry to use natural fibres, including for structural applications, in the form of reinforcing fibres and/or fillers, e.g., within polymer/biopolymer matrices, in the place of synthetic fibres. The global market for natural fiber-based materials is expected to reach $10.89 billion by [ 4 ]. Moreover, the natural fiber composites market grew at an annual compound growth rate of 8.2% between and [ 4 ].
The specific bamboo species that was primarily used for our experiments was Inversa Bambusoide, also called &#;Yellow Strip Timber&#;. It is a medium-sized Asian timber bamboo that often has a culm diameter of 0.076 m and a height of around 11&#;14 m [20]. It establishes quickly, forming a large grove in only four to five years [20]. The two-bamboo species were derived from a partnering supplier (Canada&#;s Bamboo World, Chilliwack, Canada), in the form of raw bamboo culms and fibre bundles. Single bamboo fibres were technically made up of multiple elementary fibres [21].
The initial attempts for the plastination of bamboo were made using the raw bamboo culms and the fibre bundles using the room temperature S-10 procedure. As the feasibility of the process proved to be successful, some experiments were conducted to optimize the time duration of this natural fibres&#; plastination process. The non-plastinated bamboo is referred to as virgin bamboo hereafter. As mentioned in Section 1.3, the first step of the S-10 standard procedure involves degreasing to remove lipids. However, in the present case, natural fibres do not possess much lipids so this step was skipped, and the process started with the second step, i.e., acetone dehydration. Details of the experimental procedures can be found in Section 3.
As previously mentioned, plastinated samples should ideally not have any residual moisture content. Contrary to expectations, plastinated bamboo samples showed some moisture levels that were however lower than what was observed in the virgin bamboo, per Figure 5. It was also apparent from this figure that the slope of water desorption, i.e., the rate of moisture diffusivity constant is lower for plastinated bamboo. On average, plastinated bamboo has relatively about 13% lower moisture content than virgin bamboo with a lower moisture diffusivity constant, denoted by the slope of water desorption during the initial three hours of testing.
Moisture desorption curves for virgin and plastinated bamboo. Notice the lower residual moisture in plastinated bamboo with reduced moisture diffusivity constant as compared to virgin bamboo.
Figure 6 shows the experimental results on the moisture content of conditioned plastinated and conditioned virgin bamboo, with an average moisture content of 109% for a virgin bamboo case. Surprisingly, plastinated bamboo seems to absorb moisture during the water bath with an average moisture content of 95%. It is interesting that the difference between the latter two values (i.e., ~14%) is well within the earlier relative difference seen from unconditioned samples, pointing to the imperfect (local) silicone impregnation as a source, as was visualized through the micro-CT images. This condition may have led to an increase in the overall bound (trapped) water and/or increased transport of free water in bamboo cells during conditioning (note that the water absorption of silicone itself is known to be very low, about 1%) [18]. Further, a smaller desorption slope for plastinated bamboo denotes a lower permeability rate of water absorbed and trapped within the cellular bamboo and silicone. A closer inspection of this figure also reveals that the moisture desorption of plastinated bamboo may be a two-step process where more than half of the water is lost within the first three hours, followed by a gradual loss until equilibrium.
Weight fraction of water desorbed by conditioned virgin and conditioned plastinated bamboo vs. drying time in the oven, showing lower moisture absorbed by plastinated bamboo.
As can be seen in Table 2, moisture conditioning led to a 51.3% and 50.7% loss in the tensile strength of virgin and plastinated bamboo, respectively. These changes denote that both plastinated and virgin bamboo are affected by moisture conditioning. Such a reduction in the strength of plastinated bamboo could be attributed to water transport and degrading of the bamboo cell wall, as water acts as a plasticizer to cellulose. However, Table 2 also shows that plastination not only increases the tensile strength of unconditioned bamboo, but even after moisture conditioning, the tensile strength of plastinated bamboo is still 73% higher than that of virgin bamboo. Thus, it can be concluded that, although moisture conditioning decreases the tensile strength of plastinated bamboo, plastinated bamboo still seems to be a much more durable option overall as it can support greater loads under moisture.
Tensile strength comparison of different bamboo specimens tested.
Virgin Bamboo (MPa) Plastinated Bamboo (MPa) % Relative Change Unconditioned Bamboo 128.5 ± 24.5 219.5 ± 18 70.8% (increase) Conditioned Bamboo 62.5 ± 2.21 108.3 ± 14 73.3% (increase) % Relative Change 51.3% (decrease) 50.7% (decrease) Open in a new tabFurther, the results from two-way ANOVA in Figure 7 suggest that both of the study factors along with their interaction are statistically significant at a 5% significance level. Plastination significantly increases the strength of bamboo, and moisture conditioning significantly decreases the strength of plastinated and virgin bamboo. Interestingly, the interaction of these factors suggests that the strength of moist virgin bamboo was significantly lower than dry plastinated bamboo, which could explain the plasticization of cellulose in moist bamboo and the absence of silicone present to uniformly distribute the load. Therefore, following plastination, a significant increase in the tensile strength and moisture durability of bamboo can be observed.
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Tensile strength of virgin and plastinated bamboo, before and after moisture conditioning. * indicates a significance level with p-value < 0.05.
Although the decrease in the tensile strength of conditioned virgin and plastinated bamboo is similar, a cross-comparison study from Table 2 illustrates that the final strength of conditioned plastinated bamboo is about 82% of the strength of virgin unconditioned bamboo. Meanwhile, the tensile strength of virgin conditioned bamboo is only 28.4% of the tensile strength of unconditioned plastinated bamboo. Thus, even after moisture conditioning at elevated temperature, plastinated bamboo retains 82% of the tensile strength of original unconditioned bamboo.
In terms of modulus, however, the fall in stiffness of conditioned plastinated bamboo is much lower at 38%, as compared to 51% for virgin bamboo as shown in Table 3, as a result of plasticization. Moreover, the observation that conditioned plastinated bamboo provides a 55% higher modulus than conditioned virgin bamboo definitely deems plastination suitable for bamboo processing to enhance its strength and modulus in both moisture conditioned and unconditioned environments. The results obtained for a two-way ANOVA on the tensile modulus datasets were very similar to what was observed for tensile strength, as shown in Figure 8. Thus, it can be seen that moisture conditioning significantly affects the tensile modulus of plastinated and virgin bamboo and that the modulus of moist plastinated bamboo is significantly different from moist virgin bamboo at the 5% significance level.
Tensile stiffness comparison of different bamboo specimens tested.
Virgin Bamboo (GPa) Plastinated Bamboo (GPa) % Relative Change Unconditioned Bamboo 9.6 ± 1.01 11.8 ± 0.17 22.9% (increase) Conditioned Bamboo 4.7 ± 1.3 7.3 ± 0.46 55.3% (increase) % Relative Change 51% (decrease) 38.1% (decrease) Open in a new tabTensile modulus of virgin and plastinated bamboo before and after moisture conditioning. * indicates a significance level with p-value < 0.05.
Further, a cross-comparison of modulus from Table 3 reveals that moistened plastinated bamboo has about 23% lower modulus than unconditioned virgin bamboo, while this percentage is 60% in the case of virgin conditioned and unconditioned plastinated bamboo. Conditioned plastinated bamboo retains 77% of the stiffness of original bamboo as opposed to only 49% for conditioned virgin bamboo.
The fall in the strength and modulus of bamboo after conditioning may be explained by the fact that water presence dramatically softens the cell walls. During the plasticization, cellulose present in bamboo is affected by water, and the hydrogen bonds between different polymer chains in cellulose can break. Subsequently, hydrogen bonds form with water instead, as they are small, polar molecules and hence can penetrate into the polymer chains. These water molecules form hydrogen bonds with cellulose, stronger than those existing between cellulose and cellulose. This softens the cellulose micro-fibrils as they are no longer so strongly bonded to each other, which results in the decreased stiffness of bamboo. Further, as the water expands the cell wall, there are also fewer cellulose micro-fibrils per unit area. Hence, the strength of the bamboo decreases [19].
Title: Durability of Fiber Reinforced Mortar
Author(s): K. Kosa, A. E. Naaman, and W. Hansen
Publication: Materials Journal
Volume: 88
Issue: 3
Appears on pages(s): 310-319
Keywords: deterioration; durability; fiber reinforced concrete; slurries; flexural strength; glass fibers; metal fibers; mortars (materials); tests; Materials Research
DOI: 10./
Date:
Abstract:
The durability properties of four types of fiber reinforced cement composites are compared. The four composites are conventional steel (SFRM), polypropylene, glass fiber reinforced mortar (GFRM), and slurry infiltrated fiber concrete (SIFCON). To accelerate deterioration, the test program consisted of exposure to intermittent drying and wetting conditions in a 3.5 percent sodium chloride solution (simulating seawater), maintained at 50 C over periods of 2 to 10 months. After exposure, flexural tests were performed to evaluate the effect of deterioration on the flexural strength and toughness of the composites. Results indicate that polypropylene reinforced mortar has the best overall durability, while glass fiber reinforced mortar shows the poorest overall performance. Steel fiber reinforced mortar showed noticeable reduction in flexural strength and a dramatic reduction in toughness. For SIFCON, the reduction in strength and toughness were both moderate. A prediction model for the long-term deterioration of steel fiber reinforced mortar is proposed and a sample analysis is performed. The analysis indicates that corrosion can be very critical for thin panel structures of the order of 0.5 in. (12.5 mm) in depth, but diminishes substantially for structures with depths about 4 in. (100 mm) or larger. While cement mortar was used in this study because of the nature of the thin specimens tested, the conclusions should generally apply to fiber reinforced concrete where the coarse aggregate is of sufficient quality not to contribute to corrosion.
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