Thermogravimetric behavior of natural fibers reinforced polymer composites—An overview

Materials Science & Engineering A 557 (2012) 17–28

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Thermogravimetric behavior of natural fibers reinforced polymer composites—An overview Sergio N. Monteiro a,n, Veronica Calado b, Rube´n Jesus S. Rodriguez c, Frederico M. Margem c a

Military Institute of Engineering, IME, Materials Science Department, Prac- a General Tiburcio, 80, Praia Vermelha, Urca, CEP 22290-270, Rio de Janeiro, RJ, Brazil Escola de Quı´mica, Universidade Federal do Rio de Janeiro, UFRJ, Rio de Janeiro, Brazil c State University of the Northern Rio de Janeiro, UENF/LAMAV, Brazil b

a r t i c l e i n f o

abstract

Available online 29 June 2012

Natural fibers obtained from plants, known as lignocellulosic fibers are environmentally friendly alternatives for synthetic fiber, as polymer composite reinforcement. Applications of natural fiber composites are expanding in many engineering areas, from civil construction to automobile manufacturing. In recent years, a considerable number of scientific and technological works, including review papers, were dedicated to the characterization and properties of natural fibers and their composites. The mechanical behavior and the fracture characteristics are usually the most investigated and reviewed themes for the purpose of comparison to corresponding polymer composites reinforced with synthetic fibers, mainly fiberglass. The thermal behavior is also of practical interest for conditions associated with temperatures above the ambient, as in fire damage, curing or process involving heating procedures. In fact, several works also assessed distinct thermal responses, particularly in terms of thermogravimetric properties of natural fiber polymer composites. As no general review was conducted so far on the thermogravimetric (TG) behavior of these materials, this article presents an overview limited to temperature effects related to the loss of mass by means of TG analysis and the related derivative, DTG, for different polymer composites reinforced with the most common and relevant lignocellulosic fibers. & 2012 Elsevier B.V. All rights reserved.

Keywords: Natural fiber Fiber treatment Polymer composite Thermogravimetry TG/DTG

1. Introduction In the past few decades, environmental issues concerning global scale pollution and climate changes renewed the interest in natural materials including cellulose-rich fibers extracted from cultivated plants, also known as lignocellulosic fibers, and their polymer composites [1–11]. Actually, natural fibers are considered environmentally friendly not only for their saving in process energy, which is an unavoidable problem for synthetic fibers, but also for their renewable and biodegradable characteristics. Both natural fibers and their polymer composites, if incorporated with a reasonable amount of fiber, are neutral with respect to CO2 emissions that cause the earth greenhouse effect, a major responsible for global warming [12]. In fact, at the end of these ‘‘green’’ materials life cycle, the release of CO2 due to combustion or atmospheric degradation will be balanced by the content assimilated during the fiber’s plant biological growth [1]. In addition to environmental, economical and social benefits, some physical and mechanical properties found in natural fibers and related composites [1–11] represent important technical advantages over synthetic fibers. This is of special relevance when

n

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0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.05.109

compared to glass fiber polymer composites (fiberglass) today extensively used in a wide variety of engineering systems in most industrial sectors. Fiberglass is heavier, difficult to machine and cannot be recycled even by incineration in thermoelectric plants. It also has potential health hazards posed by fiber particulates [13,14]. An expansion of industrial applications of natural fiber composites is currently taking place aiming to substitute the traditional uses of fiberglass in building construction, packaging, sports, electrical parts, medical prosthesis and automobile components [10]. In particular, the automobile industry is increasingly adopting natural composites in numerous interior and exterior components [15–19]. Among the technical advantages of natural fibers and their composites, it stands the relatively low density, which results in improved specific properties of significant interest to the automobile industry. For instance, the energy consumption of 9.6 MJ/kg to produce a flax fiber mat, including cultivation, harvesting and fiber separation, is significantly lower than the energy of 54.7 MJ/kg to produce a glass fiber mat [17]. Moreover, for composite processing, a non-abrasive natural fiber is associated with less damage to tools and molding equipments as well as a relatively better finishing in comparison to glass fiber. Additionally, owing to an inherent flexibility and easy delamination with respect to polymer matrices, natural fiber composites are also tougher and resist impact loads without shattering [9].

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Comparatively, important drawbacks are exhibited by natural fibers that, to some extent, also affect their polymer composites [1–11]. As biomaterials, natural fibers have dimensions that are limited by anatomical restrictions with accentuated statistical dispersion. Consequently, all lignocellulosic fibers present a technically undesirable feature associated with heterogeneous characteristics in contrast to uniform synthetic fibers fabricated within precise dimensional values. Another shortcoming is the fact that natural fibers are hydrophilic and tend to develop a weak bonding with hydrophobic polymers normally used as composite matrices [1–4,9–11]. Surface modifications [20,21] may improve the fiber adherence to a polymer matrix and thus increase both the composite mechanical and thermal resistance [1–4]. A surface treatment, however, results in additional cost and decreases the economical competitiveness of the natural fiber composites. The thermal stability of any natural fiber composite may also impose limitations in applications at temperatures that cause degradation of the fiber organic structure. In principle, the temperature not only degrades the structure but also affects most properties of the natural fiber composites. A complete understanding of these effects requires a review on the composite basic thermogravimetric, i.e., weight loss with increasing temperature characterization. In spite of many specific works, no overview on thermal behavior has been presented so far and, therefore, this is a motivation for this overview article.

2. Thermal degradation of lignocellulosic fibers Among the review articles and books [1–11] on properties and structural characteristics of natural lignocellulosic fibers and their polymer composites, only that one of Nabi Sahed and Jog [2] presented a section related to general aspects of the thermal stability. Works referred in that section [22–29] served for the brief summary on the state of art discussed by these authors [2]. In two conference articles on thermal analysis of lignocellulosic materials, Nguyen et al. [22,23] reviewed the effect of temperature on cellulose, hemicellulose, lignin, and other carbohydrates as well as different types of wood. It was indicated that the thermal decomposition of cellulose begins to occur at 210–260 1C by dehydration followed by major endothermic reaction of depolymerization with DTG peaks that vary from 310 to around 450 1C. Hemicellulose was found to decompose at a maximum of 290 1C and up to 150 kJ/mol for activation energy while lignin would thermally decompose with peaks from 280 to 520 1C and up to 229 kJ/mol for activation energy. The cellulose, hemicellulose and lignin degradation caused by temperature was considered by Nabi Sahed and Jog [2] a crucial aspect for the thermal stability of natural fiber reinforced polymer composites. In the case of a thermoset matrix, this is a limiting factor for choosing the curing temperature, while in thermoplastic matrix composites, a limitation for extrusion temperature. Thermal protection of the fiber was attempted by grafting of acrylonitrile on jute [25], which increased the degradation temperature to 280 1C, and on sisal [26], by lowering both the rate of degradation as well as the total weight loss. The maximum rate of degradation was also reduced in wood flour by previous treatment with phosphonate [25]. Works on the effect of thermal degradation of wood flour incorporated polymer composites [27,28] showed that mechanical properties deteriorate with increasing temperature. Toughness and bending strength were the most affected. This was attributed to changes in the surface chemistry of the wood flour, which might affect the bonding with the polymeric matrix and impair the composite properties. In another work on wood flour incorporated polypropylene composites, similar loss in properties were reported after extrusion at

250 1C [29]. As a general comment, Nabi Sahed and Jog [2] mentioned that thermal degradation of natural fibers also results in production of volatiles at processing temperatures above 200 1C. This can lead to porous polymer composites with inferior mechanical properties. They stated that the real challenge for the scientists is to improve the thermal stability of these fibers, so that they can be used with engineering polymers for better performance and thus widen the applications of natural fiber composites. As final remarks, the reader should find relevant to know that the thermogravimetric behavior of natural fibers apparently bears a correlation with their chemical constituents. In fact, the TG/DTG curves of common lignocellulosic fibers such as jute, sisal, wood and cotton display similar aspects that could be correlated to the thermal decomposition of their main constituents. Three stages of weight loss are associated with the TG curve. A first stage, up to about 200 1C, corresponds to a maximum weight loss of 10% and is followed by a second stage with more than 70 wt% of loss, up to about 500 1C. The final third stage extends to the usual ending test temperature at about 800 1C in association with a loss that may reach 20 wt%. The corresponding DTG curves display maximum rate of thermal decomposition peaks as well as shoulder and tail peaks that could be ascribed to the fiber’s constituents. Table 1 exemplifies important TG/DTG parameters obtained from figures of quoted review papers [7,22]. The values shown in this table are similar to others reported for distinct natural fibers but may vary depending on treatments performed on the fiber [7]. According to Nguyen et al. [22], the weight loss and DTG peak in first stage can be attributed to water loss. The thermal degradation of the main lignocellulosic constituents of the fiber begin to occur at the onset of the second stage. The main DTG peak in the second stage can be ascribed to the cellulose decomposition, while the shoulder peak to the hemicellulose and the tail peak to the end of lignin decomposition. The weight remaining in the third stage could be assigned to char or other products from decomposition reactions. Another point worth discussing is the influence of different TGA atmospheres on the thermogravimetric results of natural fibers. In principle, two distinct atmospheres, inert (helium and nitrogen) and oxidative (air and oxygen) may be used. Moreover, as gases conduct heat at different rates, thermograms obtained in nitrogen may be significantly different from those obtained in helium. Under inert atmosphere, the thermal degradation of cellulose results in a main DTG peak associated with the formation of macromolecules containing rings bearing double bonds [22]. In an oxidative atmosphere there is partial overlapping of this peak with the exothermic peak corresponding to the oxygen reaction with the cellulose. As a consequence, the main DTG peak is shifted to lower temperatures in oxidative atmosphere as compared to the inert one. For instance, the maximum rate of

Table 1 Thermogravimetric parameters of common natural fibers [7,22]. Natural fiber

1st stage weight loss (%)

1st stage DTG peak (1C)

2nd stage onset T0 (1C)

2nd stage weight loss (%)

2nd stage DTG shoulder (1C)

2nd stage DTG main peak (1C)

2nd stage DTG tail (1C)

3rd stage weight loss (%)

Jute [7] Sisal [7] Wood [22] Cotton [7]

8 9 2

60 52 107

260 250 290

89 76 85

290 275 270

340 345 367

470 465 400

3 15 13

4

55

265

91

280

330

410

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decomposition of wood occurs at 320–330 1C in air and 350– 370 1C in nitrogen [22].

3. Thermal analysis of natural fiber polymer composites 3.1. Preliminary considerations Thermal analysis may be regarded as the experimental procedures and tests used to evaluate chemical, physical and structural changes occurring in a material under an imposed change in temperature. In principle, temperature is a fundamental state variable that affects most chemical reactions, physical properties and structural transformations. Thus, as a general concept, any scientific or technological characterization of a material, in which temperature is varied as an experimental parameter, could be considered as thermal analysis. However, this term has long been limited to specific techniques related to thermogravimetric and calorimetric effects [22]. It is now widely accepted that the main techniques associated with a thermal analysis are the difference in temperature between a sample and a reference (DTA), the loss of weight measured by thermogravimetry (TG), its derivative (DTG), and the determination of heat flow by differential scanning calorimetry (DSC). Other techniques used to calculate the thermal conductivity, specific heat, and thermal diffusivity are, in many instances, embodied as thermal analyses. Among these techniques, TG/DTG analysis has been extensively used to characterize the thermal stability of natural fiber polymer composites. This will be the main subject of the present overview. It is beyond the scope and space of this work to cover the other techniques. Even though the well known and extensively applied dynamic mechanical analysis (DMA) might be understood as a type of thermal analysis, it will also not be part of this overview. TG/DTG results are usually displayed as curves of weight loss variation with temperature and referred as thermograms. As mentioned, lignocellulosic fibers are sensibly affected by temperature and complete thermal degradation is expected to occur above 400 1C [22]. Plants are mainly composed of cellulose, hemicellulose, and lignin that are responsible for their fibers physical properties [1]. Other volatile or partially stable constituents, such as pectin, waxes and water soluble substances, may also exist in lignocellulosic fibers. As the major constituent, cellulose conditions the physical properties of natural fibers and has a significant contribution in their thermal degradation. The linear polymeric chain of the cellulose begins to decompose at relatively lower temperatures and, owing to a catalytic effect of naturally existing inorganic ions, may form a higher amount of char [30,31]. Hemicellulose corresponds to polysaccharides with different sugar units associated with the cellulose. It has comparatively higher degree of chain branching but a much smaller degree of polymerization than the cellulose. Thermal degradation of hemicellulose precedes that of cellulose but its effect is proportionally limited by its content in the fiber. Lignin is a complex hydrocarbon polymer with both aliphatic and aromatic constituents [1]. The thermal decomposition of lignin occurs in a broader range that initiates earlier but extends to higher temperatures than those of hemicellulose and cellulose degradation [31]. However, its effect is also limited by the smaller content in the fiber. 3.2. Relevant and common natural fibers A natural fiber polymer composite is obviously composed of two main parts, the natural fiber, as reinforcing phase, and the polymer matrix. The present overview will focus attention on natural fibers, as their already discussed peculiar characteristics

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provide special properties to the composite material. This does not mean that the polymer matrix lacks importance. However, the reinforcement with synthetic fibers, such as glass, fiber, carbon, aramid and others, places the polymer composite in an entirely different category. In other words, the natural fiber is the factor that differentiates the polymer composite as a more convenient material in terms of specific environmental, economical, technical (lighter, smoother, tougher) and social advantages [1–11]. A considerable number of natural fibers have been investigated as a possible composite reinforcement. It would be a formidable task to cover all existing works that either directly or indirectly investigated the thermogravimetric behavior of polymer composites incorporated with these fibers. In addition to animal fibers (silk, wool, hair, etc.), the reader may be surprised with the reinforcement potential and specific interest of some exotic lignocellulosic fibers, such as abaca, buriti, caroa, sansevieria, palmyrah, kapok, maize, sponge gourd, artichoke, okra, curaua, milkweed, sabei, phragmites, sparto, malva, piassava, date palm, paina, communis, pita floja, fique and many others. Polymer composites with both animal and the aforementioned less common lignocellulosic fibers will not be covered in the present overview. Only composites incorporated with well known natural fibers, such as jute, hemp, flax, sisal, coir, cotton, kenaf, wood, pineapple, bamboo, ramie, banana and bagasse that are commonly investigated and even industrially applied will now be overviewed. These relevant fibers serve as sub-titles for revisiting related works. 3.3. Thermogravimetric analysis of commonly known natural fiber composites The growing interest for using natural fibers as reinforcement of polymer composites has motivated investigations on the thermal behavior of these materials. The reader will find that many works here overviewed for the thermogravimetric behavior of natural fiber composites were also dedicated to the thermal behavior of the pure fiber encompassing, in some cases, not only TG/DTG analysis but also DTA, DSC and DMA results. These works are now revisited but the reader will be spared from the pure fiber TG/DTG results as well as other techniques applied to investigate the thermal behavior of natural fiber composites. 3.3.1. Jute fiber composites Das et al. [32] showed TG/DTG curves obtained at a heating rate of 10 1C/min in nitrogen for 7 wt% phenol formaldehyde matrix composites (resin sprayed) reinforced with, supposedly, 93 wt% of both untreated and steam-stabilized jute fibers for 4 (SB-4) and 8 (SB-8) minutes. An initial DTG peak at 65–68 1C, common to the untreated jute fiber, was attributed to the fiber loss of moisture. The existing hemicellulose decomposition peak for the pure fiber at 282 1C is missing in both composites. For the authors [32], this means that during steam stabilization, hemicellulose may have been modified to some other form. On the other hand, the cellulose decomposition peaks for both composites at 348 1C, are slightly higher than that of jute at 345 1C. Finally, the steam stabilized fiber composites were indicated to yield almost the same amount of char, which is higher than that of the untreated fiber. Dash et al. [33] showed TG/DTG curves of polyester composites reinforced with 60 wt% jute fibers (sliver) modified by carding operation, in condition of both untreated and NaCl bleached treated. The authors [33] indicated the existence of an initial DTG peak at 59 1C for the untreated and 65 1C for the bleached fibers because of the loss of moisture. It was concluded that the bleached fiber is less hydrophilic than the untreated one.

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A second major peak at 337 1C for the untreated and 328 1C for the bleached indicated that the untreated fiber/polyester composite is thermally more stable than the bleached fiber/polyester composite, although the difference is not remarkable. Ray et al. [34] presented TG/DTG of vinyl ester composites reinforced with untreated and alkali treated (5% NaOH for immersion times up to 8 h) jute fibers. Just like the simple untreated fiber (37 1C), all composites display an initial DTG peak around 42–58 1C owing to the evaporation of moisture. The hemicellulose degradation temperature, second shoulder peak, remained the same in all composites, 300 1C. This is very close to the corresponding of untreated fiber, 297 1C, but much lower than that of the neat vinyl ester, 419 1C. By contrast, the cellulose degradation temperature, third peak, decreased from the untreated fiber composite, 364 1C, to the 8 h alkali treated fiber composite, 357 1C. The main degradation peak decreased from 416 to 410 1C, for untreated and 8 h-alkali treated fiber composite, respectively. The authors concluded that the changes occurring in the fiber because of the alkali treatment, such as the fiber splitting into finer filaments, the increase in the crystallinity of the fiber and the improved bonding between the fiber and the resin, had a considerable effect on the thermal degradation behavior of the composites. The incorporation of alkali-treated jute fibers reduced the thermal stability of the composite. Manfredi et al. [35] investigated the thermogravimetric behavior of both unsaturated polyester (UP) and unsaturated polyester with acrylic acid (Modar) as matrices for untreated jute fibers. Although not presenting quantitative parameters for the composites, the authors [35] indicated that the Modar matrix exhibits a higher thermal resistance than the unsaturated polyester. In fact, the TG curve of jute reinforced Modar composite is shifted up to 70 1C to higher temperature, as compared to the jute-UP curve. Both jute composites display much lower thermal stability, by TG curves, than glass fiber composites with same matrices. Mohanty et al. [36] presented TG/DTG curves of both untreated and maleic anhydride grafted polyethylene (MAPE) modified jute fiber as reinforcement for high density polyethylene (HDPE) composites. They found that the decomposition of virgin HPDE started at 430 1C and practically finished 515 1C. This was comparatively higher than that of jute fibers. For the composites, apparently, no

peak below 200 1C could be seen in the DTG curves. The authors [36] indicated that for the untreated jute composite, an initial DTG peak at 352 1C was probably because of dehydration from cellulose unit and thermal cleavage of glycosidic linkage. The second peak at 522 1C was attributed to aromatization, involving dehydration reactions. Above 515 1C, while the virgin HDPE got completely decomposed, in the untreated fiber composite a charred residue of 3.2% was left. By contrast, the treated fiber composite displayed an initial peak of 364 1C and a main decomposition peak at 523 1C, almost the same as in the untreated jute fiber composite. However, the charred residue was about 5.0% above 530 1C. These results ensure, in the opinion of the authors [36], a higher thermal stability of the composites reinforced with MAPE modified jute fibers. Doan et al. [37] investigated the thermogravimetric behavior of polypropylene (PP) matrix composites reinforced with 9.4; 19.8; and 31.3 vol% of jute fiber with or without addition of 2 wt% of maleic anhydride grafted polypropylene (MAHg PP) modifier. The TG/DTG analysis was carried out at a heating rate of 10 1C/min in either nitrogen or air. The authors [37] found that, as compared to pure jute fiber, the thermal degradation of PP is much more influenced by the atmosphere. In nitrogen, a single DTG peak at 431 1C was said to be initiated by thermal scissions of carbon bonds accompanied by transfer of hydrogen. By contrast, in air the peak is prominently reduced to 299 1C, indicating that thermal degradation of PP in air is easier and faster than in nitrogen. Doan et al. [37] stated that the thermal stability of the composites was found to be higher than those of either neat PP or pure jute fiber in both nitrogen and air. Fig. 1, reproduced from [37], shows that the composite thermal resistance decreases with increasing fiber content in nitrogen, but the reverse occurs in air. The authors’ [37] explanation was based on the difference in fraction of PP that decomposes in nitrogen or air. Yu et al. [38] showed TG curves obtained at a heating rate of 20 1C/min in nitrogen, for polylactic acid (PLA) matrix reinforced with up to 50 wt% of untreated jute fibers. According to the authors [38], the thermal degradation of PLA takes place in a single stage with a peak at 356 1C. Composites show lower degradation temperatures than that of PLA. No discussion was offered on possible degradation mechanisms or on the thermal stability of the composites.

Fig. 1. TG/DTG curves of jute/PP þ 2 wt% MAHg PP composites in nitrogen and air with different fiber contents. Reproduced from [37].

S.N. Monteiro et al. / Materials Science & Engineering A 557 (2012) 17–28

Table 2 Degradation temperatures at different levels of TG weight loss for glass and selected natural fibers as well as cured epoxy composites. Adapted from [39]. Weight loss (%)

T (1C) at 5 wt%

T (1C) at 25 wt%

T (1C) at 50 wt%

T (1C) at 75 wt%

Hemp Kenaf Henequen Flax Glass Hemp/epoxy/oven Hemp/epoxy/ microwave Kenaf/epoxy/oven Kenaf/epoxy/ microwave Henequen/epoxy/ oven Henequen/epoxy/ microwave Flax/epoxy/oven Flax/epoxy/ microwave Glass/epoxy/oven Glass/epoxy/ microwave Neat epoxy/oven Neat epoxy/oven

105 64 61 88 586 348 352

329 313 301 323 – 384 390

341 341 337 339 – 392 393

458 371 392 441 – 472 474

321 336

370 382

390 392

479 479

306

375

391

456

355

389

392

483

364 351

389 385

393 391

474 472

400 400

N/A N/A

402 402

539 482

497 403

N/A N/A

398 N/A

490 484

when compared to neat PP, around 240 1C. This indicates that the addition of hemp fiber reduces the thermal stability of the composite. It can also be seen that there is an apparent improvement in thermal stability of the untreated fiber composite in comparison to the alkali treated composite. This is in contrast to what was expected. However, the authors [41] attributed the maleic anhydride grafted polypropylene (MAPP) coupling agent, used in the untreated fiber, as able to bond with the hemicellulose. The authors [41] indicated, quoting [34,36,42], that the poor thermal stability of hemicellulose and pectin in a composite can be negated by the inclusion of a coupling agent. As conclusions, the authors [41] stated that both untreated hemp fiber composites and NaOH/Na2SO3 treated hemp fiber composites, each with a matrix of PPþ 4% anhydride modified polypropylene (MAPP), were less thermally stable than the neat PP. Moreover, the thermal stabilities of composites containing untreated fiber and NaOH/Na2SO3 treated fiber, were found to be similar to each other. Moriana et al. [43] presented TG/DTG results on three lignocellulosic fibers: cotton, kenaf and hemp, 10 wt%, reinforced biocomposites using a commercial starch-based, Mater-Bi KE, thermoplastic biopolymer as the matrix. Fig. 2 reproduces the TG/DTG curves of the neat Mater-Bi KE and lignocellulosic fiber composites. The thermal decomposition of the studied reinforced biocomposites presents two main weight loss regions, similarly to the neat matrix. The thermogravimetric parameters, including the corresponding activation energies, are presented in Table 3, adapted from tables in the work of Moriana et al. [43]. The authors [43] indicated that, when the pure Mater-Bi KE is reinforced, the region associated with the synthetic component of this matrix is not significantly affected. In addition, the peak owing to starch degradation is completely overlapped with hemicellulose/pectin as well as with cellulose degradation of the fiber. This causes a slight displacement to higher temperatures and is an indication of the improved thermal stability of the biocomposites. As for activation energies of hemp fiber composites (Table 3), the values for the first, 175–177 kJ/mol, and second, 219–222 kJ/mol, thermal decomposition processes are comparatively higher than the corresponding ones, 98–102, and 172–175 kJ/mol for the Mater-Bi KE matrix. The second (main) thermal decomposition activation energy is also greater than the corresponding, 201–202 kJ/mol, for the pure hemp fiber. This also corroborated the fact that the biocomposites have a higher thermal stability than either the fiber or the matrix. (1) (2) (3) (4)

(1)

(1)

(2) (3) (4) DTG, (/°C)

3.3.2. Hemp fiber composites Sgriccia and Hawley [39] investigated the effectiveness of oven and microwave curing of diglycidyl ether of bisfenol-A (DGEBA) epoxy composites reinforced with 15 wt% of several natural fibers by means of thermogravimetric analysis conducted at a heating rate of 25 1C/min in nitrogen. Table 2, adapted from the tables shown in Sgriccia and Hawley [39] paper, presents the TG degradation temperatures for 5, 25, 50 and 70 wt% weight loss of fibers and composites. In the particular case of the hemp fiber and epoxy composites, this table shows that the limit values for the degradation temperatures for the pure hemp fiber are 105 1C (5 wt%) and 458 1C (75 wt%). These degradation temperatures increased, respectively, to 348 1C (5 wt%) and 472 1C (75 wt%) for the oven cured and further to 352 1C (5 wt%) and 474 1C (75 wt%) for the microwave cured epoxy composites. As a main conclusion, the authors [39] indicated that natural fiber composites have lower degradation temperature as compared to corresponding glass fiber ones, which affects processing of composites and usage temperatures. Pracella et al. [40] showed TG curves obtained at a heating rate of 10 1C/min in nitrogen for polypropylene (PP) as well as for various compositions of PP/hemp fibers, PP/glycidyl metacrylatte (GMA) modified hemp fibers, and GMA grafted PP/hemp fiber. As a summary of their results, the authors [40] indicated that all composites started degradation earlier than the PP matrix. Moreover, for modified fiber composites, the weight loss at a given temperature was higher than that of untreated fiber composite. In the DTG curves, not shown in the work, the maximum degradation rate was always shifted to higher temperature with respect to the neat PP and cellulose. Composites with GMA modified PP matrix resulted in higher thermal stability. Furthermore, the temperature corresponding to the maximum decomposition rate was found to be almost independent of hemp fiber content. Beckermann and Pickering [41] investigated the thermogravimetric behavior of polypropylene (PP) matrix composites reinforced with up to 40 wt% of both untreated and NaOH/Na2SO3 treated hemp fibers. Results of TG, obtained at a heating rate of 2 1C/min in air, showed that both type of fibers reinforced PP composites start to lose weight at lower temperatures, 214 1C,

21

(4) (3)

(2)

112

187

262

337

412

487

562

637

T(°C) Fig. 2. Thermogravimetric analysis of the biocomposites: DTG curves of (1) MaterBi KE, (2) Mater-Bi KE/cotton, (3) Mater-BI KE/kenaf and (4) Mater-Bi KE/hemp at the heating rate of 20 K/min. Adapted from [43].

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Table 3 Thermogravimetric parameters (a) and activation energies (b) for neat matrix as well as cotton, hemp and kenaf biocomposites. Adapted from [43]. (a) Composite

Mater-Bi KE (neat) Cotton/Mater-Bi KE Hemp/Mater-Bi KE Kenaf/Mater-Bi KE (b) Composite

Mater-Bi KE (neat) Cotton/Mater-Bi KE Hemp/Mater-Bi KE Kenaf/Mater-Bi KE

First weight loss

First weight loss

Residue (%)

Onset (1C)

Peak (1C)

Onset (1C)

Onset (1C)

319 324 327 334

336 344 351 356

413 415 414 416

435 434 432 433

Thermal decomposition process (region)

First Weight Loss Second Weight Loss First Weight Loss Second Weight Loss First Weight Loss Second Weight Loss First Weight Loss Second Weight Loss

3.3.3. Sisal fiber composites Albano et al. [44] investigated the thermogravimetric behavior of untreated and acetylated sisal fibers reinforced neat polypropylene, PP, of polypropylene/high density polyethylene blend (80PP/20 HDPE) matrix composites. The polymer blend was also modified with 5 wt% of ethylene–propylene copolymer (EPR). TG/ DTG analysis was carried out at a heating rate of 10 1C/min in nitrogen. All sisal composites (blends with filler) display three decomposition stages that are attributed to the decomposition of the different components of the blend, i.e., fiber and polymer. The authors [44] presented the values of the starting decomposition temperature (Ti), temperature of the maximum decomposition rate (Tmax), associated with the main DTG peak, and final decomposition temperature (Tf) for the sisal fibers and composites investigated. As relevant examples, for the acetylated sisal fiber, Ti ¼250 1C; Tmax ¼385 1C; and Tf ¼410 1C. For the PP/HDPE/nf-EPR: Ti ¼320 1C; Tmax ¼ 445 1C; and Tf ¼470 1C. For the PP/HDPE/nf-EPR composite reinforced with acetylated fiber: Ti ¼280 1C; Tmax ¼ 448 1C; and Tf ¼470 1C. The authors [44] indicated that when the sisal fiber is added to the matrix, the starting decomposition temperature, Ti, decreases by 40 1C. However, both the maximum, Tmax, and the final, Tf, decomposition temperatures are not much affected. These temperatures represent the long-term stability of the decomposition process. An extensive study on the activation energy led Albano et al. [44] to conclude that when acetylated fiber is mixed with polymers, a greater polymer–filler interaction takes place, which slightly favors the stability of these composite materials as compared to blends of non-acetylated fiber. Nair et al. [45] studied the thermogravimetric of polystyrene (PS) composites reinforced with short sisal fibers. These fibers were used in the condition of both untreated as well as treated with benzoylation, polystyrene maleic anhydride coating and acetylation. TG analysis was carried out at a heating rate of 20 1C/min. It was reported that the treatments improved the fiber/matrix adhesion and the PS/sisal composites are thermally more stable than unreinforced neat PS and pure sisal fiber. In fact, for neat PS decomposition starts at 288 1C and is finished at 435 1C. In the case of the 20 wt% untreated sisal fiber composite, the major degradation temperature begins at 32 1C and is almost complete at 447 1C, which indicates a better thermal stability than neat PS or pure sisal fiber. Moreover, TG curves show that the thermal stability of sisal fiber treated composites is higher than that of the untreated fiber composite. The authors [45]

1.2 2.5 4.0 4.3

Activation energy (kJ/mol) obtained by different methods Friedman

Flynn-Wall-Ozawa

Coats-Redfern-Criado

98 173 112 295 175 219 163 200

102 175 113 297 175 222 159 199

101 172 111 295 177 220 162 201

attributed this higher composite thermal stability not only to the inherent greater thermal stability of the treated fibers but also to the improved fiber/matrix interactions, which produce intermolecular boding between the sisal fiber and PS. Xie et al. [46] investigated the thermogravimetric behavior of composites consisting of polypropylene (PP) and maleic anhydride grafted styrene–ethylene–co-butylene–styrene copolymer (MA-SEBS) reinforced with 20 wt% of untreated sisal fibers. Hybrid PP/MA-SEBS matrices were compounded with 0 wt% (PP-0); 8 wt% (PP-8); and 16 wt% (PP-16) of MA-SEBS. TG/DTG analysis was carried out at a heating rate of 10 1C/min in helium. The neat MA-SEBS presented only one DTG peak at 442 1C while the composites display two peaks. The first small peak occurred, for the different MA-SEBS contents, at approximately the same temperatures: 363 1C (PP-0); 367 1C (PP-8); and 365 1C (PP-16). The existence of this first peak was an indication to the authors [46] that the thermal stability of neat MA-SEBS is better than that for the composites. The second major peak was observed at slightly higher temperatures, when MA-SEBS is introduced in the composite. In fact, this second peak occurred at 451 1C (PP-0); 459 1C (PP-8); and 457 1C (PP-16). Xie et al. [46] indicated that the incorporation of MA-SEBS improves the thermal stability of the composites, which was attributed to MA-SEBS interactions between PP and sisal fibers. No discussion was presented on degradation mechanisms. Joseph et al. [47] studied the thermal behavior by TG analysis, carried out at a heating rate of 10 1C/min in inert atmosphere, of polypropylene (PP) composites reinforced with sisal fibers that were both untreated as well as treated with urethane derivative of PP glycol (PPG/TDi) and maleic anhydride modified PP (MAPP) in order to improve the fiber/matrix interfacial adhesion. The authors [47] found that the treated fiber composites show superior thermal stability comparing with the untreated fiber composites as well as the neat PP and the pure sisal fiber. DTG curves display main decomposition peaks at: 350 1C—pure sisal fiber; 400 1C—neat PP; 476 1C—30 wt% untreated fiber composite; 478 1C—PPG/TDi treated fiber composite; and 785 1C—MAPP treated fiber composite. The authors [47] indicated that, in general, the degradation temperature is shifted to a slightly higher value, in the case of treated fiber composite, than that of the untreated sisal fiber composite. This was attributed to the improved fiber/matrix adhesion due to specific chemical mechanisms discussed by Joseph et al. [47].

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˜ a´n et al. [48] showed TG/DTG curves for epoxy (DGEBA/ Gan TETA) composites reinforced with 30 wt% of sisal fibers that were both untreated or mercerized, silanized, or silanized with previous mercerization. According to the authors [48], the composite present a region at 210–350 1C associated with the sisal fiber constituents decomposition. Treated fiber composites show a slight increase on thermal stability with respect to that of untreated fiber composite. In fact, a shoulder peak around 310 1C for the untreated fiber composites is shifted to about 330 1C for the silanized sisal fiber composite. These results, as indicated by the authors, are possibly related to the treatment. Additionally, for composites whose fibers have been subjected to mercerization, only a single decomposition region, 380–400 1C, is observed owing to the presence of the sisal fiber. Paiva and Frollini [49] presented results of thermogravimetric analyses conducted at a heating rate of 10 1C/min in air of both phenolic and lignophenolic matrices composites reinforced with 70 vol% of sisal fibers that were unmodified as well as modified by mercerization (10% NaOH for 1 h); esterification (mercerizationþsuccinic anhydride in xylene); and ionization (7.5 kV electric discharge in air). From a list of weight losses at certain temperature levels (no TG curves given), the authors [49] indicated that the composites present lower thermal stability than their respective thermosets, as the sisal fibers decompose at lower temperatures. Events responsible for the different levels of weight loss were discussed, and the authors proposed that, over a certain temperature, the polymer suffers continuous weight losses as a consequence of these successive processes. In particular, Paiva and Frollini [49] state that the lignophenolic composites show at 400 1C a larger weight loss than the phenolic sisal fiber composites, probably because of the fragmentation of lignin interunit bonds. Kim and Netravali [50] investigated the thermal degradation profiles of sisal fiber reinforced composites with soy protein concentrate modified by blending with gelatin (SG) resin matrix. Actually, two types of matrices SG-0 (no gelatin) and SG-20 (with 20% gelatin) were used, both containing 20% of sorbitol. The thermogravimetric results were obtained at a heating rate of 20 1C/min in nitrogen. The thermal degradation profiles of the neat matrices, SG-0 and SG-20, were almost identical and show three weight loss steps associated with DTG peaks. The first peak around 110 1C is due to water evaporation. The second, a shoulder peak around 200 1C, is attributed to the degradation of sorbitol, which was added as a plasticizer. The third broad main peak, around 280 1C, is assigned to the degradation of soy protein and gelatin. The authors [49] mentioned that, although the thermal stability of SG-0 and SG-20 resins was identical, the composite with SG-20 matrix displays a major DTG peak around 313 1C, a higher temperature than that of the SG-0 composite, around 304 1C. For Kim and Netravali [50], this result, together with a final char residue at 600 1C for the SG-20 composite, indicates an improvement in thermal stability owing to the better interaction between the sisal fiber and the SG-20 matrix.

3.3.4. Flax fiber composites Sgriccia and Hawley [39] evaluated the effectiveness of oven and microwave curing of epoxy composites reinforced with several lignocellulosic fibers. The degradation temperatures from TG weight losses of 5–75 wt% were determined. The hemp/epoxy composite was already evaluated in Section 3.3.2. In the case of the flax/epoxy composite, Table 2 shows that the limit values for the degradation temperature (weight loss %) for the pure flax fiber, 88 1C (5 wt%) and 441 1C (75 wt%) increased to 351 1C (5 wt%) and 472 1C (75 wt%) for the microwave cured epoxy composite. The authors [39] indicated that the degradation temperatures of the evaluated

23

lignocellulosic fiber/epoxy composites, oven or microwave cured, fall between those for the neat epoxy and the fibers. 3.3.5. Coir fiber composites Rosa et al. [51] presented TG/DTG curves for starch/ethylene vinyl alcohol copolymer (EVOH) matrix composites reinforced with 15 wt% of both untreated as well as washed, alkali and bleached treated coir fibers. Glycerol was added as plasticizer during the composite melting extruded processing. The authors [51] indicated that changes occurring in the coir fibers because of the treatment led to a positive effect on the thermal stability of the composites. For instance, the moisture loss peak at 134 1C for the untreated fiber composite shifted to 142–144 1C for the treated coir fiber/starch EVOH composite. Slight increases were also found in the hemicellulose degradation DTG peak. Although not mentioned by the authors, DTG peaks around 450 1C could probably be associated with the polymeric matrix degradation. From the main DTG peak results, it is possible to conclude that coir fiber treated composites present improving thermal stability. Santafe´ Jr. et al. [52] characterized the thermogravimetric behavior of untreated coir fiber incorporated in different composition (up to 30 vol%) into polyester matrix composites. TG/DTG curves were obtained at a heating rate of 10 1C/min in nitrogen. Fig. 3, reproduced from their work [52], shows the composites as well as the neat polyester TG/DTG curves. Apparently, the neat polyester displays a major DTG peak at 385 1C and a tail peak around 415 1C. The authors [52] attributed these polyester thermal degradation events to the evolution of volatile compounds followed by depolymerization of macromolecular chains, quoting Mark et al. [53]. Similar curves are observed in Fig. 3 for the coir fiber/polyester composites with two additional DTG peaks. An initial peak around 57 1C was assigned to the release of absorbed humidity, mainly from the coir fiber surface as in the other overviewed pure lignocellulosic fibers and related composites. Two shoulder peaks around 285 and 348 1C were clearly detected, as shown in Fig. 3 for the 30% coir fiber composite. The authors attributed these shoulder peaks respectively to the degradation of hemicellulose and cellulose of the coir fiber. 3.3.6. Cotton fiber composites Silva et al. [54] presented TG/DTG curves obtained at a heating rate of 10 1C/min in air for phenolic thermoset (PT) matrix composites reinforced with up to 70 wt% of cotton textile fibers. The composites were prepared with randomly oriented fibers in a mold but cured at higher pressure. The DTG curve for the neat PT displays two peaks at 540 and 700 1C that were not interpreted by

TG curves

DTG curves

Fig. 3. TG/DTG curves for neat polyester and coir fiber composites. Reproduced from [52].

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the authors. The DTG curve for the 50% cotton fiber composite also shows two clear peaks. The first at 349 1C was attributed to the decomposition of cellulose. The main second peak at 445 1C was assigned to events related to the matrix. In fact, the TG/DTG curves of the neat PT reveal a beginning of thermal decomposition around 350 1C. Although not mentioned by Silva et al. [54], a faint peak at about 50 1C in the DTG curve of the composite and not seen in the neat PT, might be ascribed to the release of moisture from the cotton fiber. The authors [54] indicate that all cotton fiber composites have lower thermal stability than the neat phenolic thermoset. 3.3.7. Kenaf fiber composites In an already presented work (see Section 3.3.2 and 3.3.4), Sgriccia and Hawley [39] studied the degradation temperature, associated with TG weight losses of 5–75 wt%, of oven and microwave cured epoxy composites reinforced with hemp, flax, kenaf and henequen fibers. In the particular case of kenaf/epoxy composite, Table 2 shows that the values for the degradation temperature (weight loss %) of the pure kenaf fiber, 64 1C (5 wt%) and 392 1C (75 wt%), increased to 336 1C (5 wt%) and 479 1C (75 wt%) for the microwave cured epoxy composite. The authors [39] mentioned that the degradation temperature for both microwave and convection oven cured fiber composites investigated, including kenaf/epoxy, fall between the degradation temperature of the epoxy matrix and that of the pure kenaf fiber. Julkapli and Akil [55] performed thermogravimetric analysis on chitosan matrix biocomposites reinforced with up to 28 wt% of kenaf fiber. The ground kenaf fiber, 30–40 mm mesh size, was dispersed together with the chitosan powder in acetic acid solution, which renders the molecules more reactive with the presence of NH3þ . Thus, compatibility between kenaf fiber and chitosan matrix is expected to improve. TG/DTG curves were obtained at a heating rate of 20 1C/min in nitrogen. The authors [55] reported two DTG peaks in all curves for both neat chitosan and kenaf fiber composites. The first degradation step, 25–140 1C, with an apparent DTG peak around 120 1C was attributed to evaporation of water or volatile compounds, like solvent. The second main peak around 280 1C and tail peaks around 360 and 500 1C were generally attributed to the major degradation of the polymers including the pyrolysis of chitosan and decomposition of kenaf fiber. Julkapli and Akil [55] concluded that the addition of kenaf fiber (dust) in chitosan matrix (film) does not give any significant change in thermal stability of the biocomposite, except that the TG/DTG decomposition curves were recorded in two stages. For the reader, this exception is rather surprising as a conclusion. In a review of kenaf fiber reinforced composites, Akil et al. [56] confirmed the conclusion expressed in the afore-mentioned work of Julkapli and Akil [55]. The authors [56] concluded on the potential of kenaf fiber as an alternative to replace conventional material or synthetic fibers as composites reinforcement. Although the basic results and discussion of the work of Moriana et al. [43] have been presented in Section 3.3.2, the reader may find worthwhile a comment on the results of kenaf fiber/ Mater-Bi KE biocomposite. As shown in Fig. 2 and Table 3, the first DTG peak of this biocomposite at 356 1C is sensibly higher than that, 336 1C, of the neat Mater-Bi KE. Furthermore, as presented in Table 3, the activation energy 162–163 kJ/mol for this peak in the composites is significantly higher than the corresponding 99–101 kJ/mol of the matrix. This indicates a comparatively higher thermal stability. 3.3.8. Wood fiber composites Coutinho et al. [57] studied the thermal behavior of both untreated and treated (silane and methanol) aspen wood fibers reinforced composites with polypropylene (PP) or maleated

polypropylene (MAPP) matrices. Thermogravimetric results obtained at a heating rate of 10 1C/min in nitrogen were presented in terms of the onset and peak temperatures, although the authors did not show TG curves and characteristics peaks. For instance, the 20 wt% untreated fiber/PP composite is associated with the onset range of 307–453 1C and a peak at 476 1C, while the 20 wt% A1100 silane treated and subjected to propylene polymerization wood fiber/PP composite has onset range of 325–447 1C and peak around 471 1C. By comparing these results to the neat PP, with onset at 459 1C and peak around 473 1C, also presented in the work of Coutinho et al. [57], the reader sees that only minor changes occurred with wood fiber addition to the investigated composites. Nevertheless, the authors stated that thermal parameters could be influenced by the presence of wood fibers. Doh et al. [58] investigated the thermogravimetric behavior of wood fiber in the form of liquefied wood (LW) added in different amounts, up to 40 wt%, into three distinct polymeric matrices: low density (LDPE) and high density polyethylene (HDPE) as well as polypropylene (PP). TG curves were obtained at a heating rate of 5–50 1C/min in nitrogen. According to the authors [58], the addition of 10 wt% LW into LDPE, HDPE and PP showed no significant effect on the polymer matrix thermal decomposition. However, as the LW amount increased, the thermal stability of the investigated polymer composites decreased. This behavior was attributed to the decrease in the compatibility and interfacial bonding between LW/polymer in the composite. After thermal degradation of the composites above 500 1C, the tar and ash content increased with the amount of added LW. As relevant examples of the thermogravimetric parameters obtained by Doh et al. [58], the decomposition temperature was determined at the starting point of severe weight loss along with the activation energy for, HDPE with 472 1C, 240 kJ/mol; and PP with 450 1C, 75 kJ/mol. These values change with the addition of LW into the polymers: LW/HDPE with 474 1C, 223 kJ/mol; and LW/PP with 452 1C, 55 kJ/mol. As main conclusions, the authors [58] indicate that the thermal stability of LW composites decreased with addition of LW. Moreover, higher heating rates provided better thermal stability resulting from the decelerated decomposition rate. Bhardwaj et al. [59] conducted thermogravimetric analysis at a heating rate of 20 1C/min, atmosphere not indicated, of a polyhydroxybutyrate-co-hydroxyvalerate (PHBV) as well as polypropylene (PP) matrices composites reinforced with 40 wt% of recycled (wood) cellulose fiber (RCF) from selected newspapers, magazines or Kraft paper stock. The authors [59] indicated that the onset of thermal degradation in the PHBV-based composite was comparable to that of PP-based composite. However, the degradation rate of the former was more drastic after 250 1C, which was attributed to the degradation (lower thermal stability) of both the fiber in RCF and the PHBV. Lei et al. [60] studied the thermal stability of 30 wt% bagasse or pine wood fiber reinforcing recycled high density polyethylene (RHDPE) by TG analysis, at a heating rate of 10 1C/min in nitrogen. In the case of wood fiber composites, two active coupling agents were used: a maleated polyethylene (MAPE) and a titaniumderived mixture (TDM) of chemical agents. It was reported that the onset degradation for the neat RHDPE occurs at 441 1C and the maximum decomposition rate at about 471 1C. The introduction of 30 wt% wood fiber generates another intermediate decomposition peak. These three peaks, onset, stage I and stage II, were respectively found as: pure fiber/RHDPE with 262, 353 and 469 1C; 1.2% MAPE coupled fiber/RHDPE with 263, 353 and 469 1C; and 0.9% TDM coupled fiber/RHDPE with 260, 349 and 468 1C. As indicated by the authors [60], the coupling agents seem to have little influence on the thermal degradation of the composites. The reader could also conclude that the addition of pine wood fiber to the RHDPE significantly decreased the onset

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degradation temperature and thus the beginning of the composite thermal stability.

3.3.9. Pineapple fiber composites George et al. [61] investigated the thermal behavior of both untreated and treated pineapple leaf fiber (PALF) reinforced polyethylene composites by thermogravimetric analysis. The PALF treatment was carried out using isocyanate, silane and hydrogen peroxide to improve the fiber/matrix interfacial adhesion. The authors [61] indicated that at the high temperatures the PALF degrades before the polyethylene matrix. Moreover, the treated PALF composites impart better properties as compared to untreated PALF polyethylene composites. Luo and Netravali [62] presented thermogravimetric results of poly (hydroxybutyrate-co-valerate) (PHBV) matrix biocomposite reinforced with up to 28 wt% of pineapple leaf fiber. TG curves obtained at a heating rate of 10 1C/min in both nitrogen and air confirmed that the PHBV easily undergo thermal degradation. Moreover, the thermal decomposition of PHBV is very similar, regardless the atmosphere. As indicated by the authors [62], the main decomposition temperatures were observed at 270 1C and 273 1C, in air and nitrogen, respectively, with a final thermal degradation residue, mostly composed of crotonic acid. The ‘‘green’’ biocomposites display, not only the main decomposition temperatures associated with the PHBV matrix, at 268 1C in air, and 273 1C in nitrogen, but also practically the same main decomposition temperatures, as a second peak, associated with the pineapple fiber at 327 1C in air and 334 1C in nitrogen. It was concluded that the presence of pineapple fibers did not affect the thermal decomposition of PHBV. Threepopnatkul et al. [63] showed TG curves (no conditions given) for pineapple leaf fiber (PALF) as well as polycarbonate (PC) and related composites reinforced with up to 20 wt% of both alkali and silane treated PALF. It was commented, without quantitative parameters, that the PALF has the lowest, while the PC the highest thermal stability with all composites lying in between. The authors [63] indicated that the PALF composites showed the onset for thermal degradation at lower temperature, approximately 270 1C. It was then interpreted that composites have lower stability than the neat PC because of the low thermal stability of the PALF.

3.3.10. Bamboo fiber composites Lee and Wang [64] investigated the thermogravimetric behavior of 30 wt% bamboo fibers (BF) reinforcing two distinct biocomposites with polylactic acid (PLA) and polybutylene succinate (PBS) as matrices. TG/DTG analyses were performed at a heating rate of 10 1C/min in nitrogen. The interfacial adhesion of both composites was improved by the addition of different amounts of lysine-based diisocyanate (LDI) as a coupling agent. TG/DTG curves show in either case neat PLA or neat PBS, the onset of thermal degradation and the main (single) DTG peak, 376 1C for PLA and 405 1C for PBS, occurring at higher temperatures than those for pure BF and related composites. In the case of BF/PLA, a two-stage loss of weight was observed with degradation in the range of 280–340 1C attributed to PLA and a small shoulder peak 350 1C, owing to fiber degradation. Thermal degradation was increased with LDI content. The authors [64] indicated that, in general, the increase in the molecular weight by cross-linking reactions between matrix and fiber, or molecular chain extension of the matrix itself, could increase the thermal stability. In the case of BF/PBS composites, an intermediate thermal stability was verified between those of BF and PBS. The addition of LDI also improved the thermal stability of the composite.

25

Shih [65] showed TG curves obtained at a heating rate of 10 1C/ min in nitrogen for composites of both powder and fiber of water bamboo husk incorporated, in formulation up to 10 wt%, into epoxy matrix. The bamboo fibers were either untreated or treated with distinct silane coupling agents. Thermogravimetric parameters were not presented by Shih [65], only mentioning that the char yields of the composites were larger than that of the epoxy. This reveals that the addition of bamboo powder or fiber into epoxy would effectively raises the char yield and thus limits the production of combustible gases, decreasing the exothermicity of the pyrolysis reaction. This, therefore, inhibits the thermal conductivity of the burning material. Singh et al. [66] showed TG/DTG curves obtained at a heating rate of 20 1C/min in nitrogen for pure bamboo fiber as well as 30% bamboo fiber reinforcing poly(hydroxybutyrate-co-valerate) (PHBV) biocomposite. The DTG curve for the neat PHBV presents a major, single, peak, at 316 1C, while the composite has its main peak around 320 1C followed by a small peak at about 375 1C, practically coinciding with the main pure bamboo fiber peak. According to the authors [66], the thermal degradation of the biocomposite appears to be a cumulative phenomenon of matrix and fiber alone. The maximum degradation temperature peak of the composite shifted to a higher temperature from that of PHBV indicating the composite to be slightly more thermally stable than the matrix. Thus, the presence of bamboo fiber did not have any practical degradation effect on PHBV. 3.3.11. Ramie fiber composites In the already reviewed (see Section 3.3.1) work of Yu et al. [38], in addition to jute fiber/polylactic acid (PLA) composite, a TG curve of PLA reinforced with 30 wt% ramie fibers was also presented. From this curve, one may infer that the composite onset thermal degradation temperature, around 220 1C, is significantly lower than that for the neat PLA, at about 310 1C. The authors [38] indicate that the thermal degradation of PLA occurs completely in a single stage at 356 1C, while the ramie fiber composite shows a lower degradation temperature. It was also mentioned that this might be a consequence of the decrease of relative molecular mass of PLA. Moreover, Yu et al. [38] stated that when the composite is mixed by the two rolls operation, the incorporation of ramie fiber into PLA matrix has also an effect on the thermal degradation temperature. 3.3.12. Banana fiber composites Zainudin et al. [67] showed TG/DTG results obtained at a heating rate of 10 1C/min in nitrogen for banana pseudo-stem (BPS), in amounts up to 40 wt%, reinforcing unplastisized polyvinyl chloride (UPVC) composites in both unmodified and acrylic modified condition. For the reader information, the authors [67] always referred to the BPS as a composite filler not a fiber. The physical aspect of the BPS was not revealed, according to the authors, because of the confidential nature of these data. The TG/ DTG curves for the neat UPVC display a main DTG peak at apparently 268 1C, but not mentioned, combined with a shoulder around 300 1C. Another smaller peak was reported at 435 1C. The authors attributed the main peak to the dehydrochlorination of the UPVC with succeeding formation of double bonds and release of HCl as the main volatile product of degradation, quoting Semsarzadeh et al. [68]. The 435 1C peak was assigned to the separation of polyene structure sequence formed during the first main degradation stage. For all composites, an initial DTG peak occurred around 63 1C, owing to the volatilization of moisture, which was correlated to loss of water in BPS. Zainudin et al. [67] indicated that the addition of acrylic increased the thermal stability of the composite. In fact, from the DTG curves, one can

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see that the main peak temperatures (and amount of BPS) for the acrylic modified at 259 1C (40 wt%) and 293 1C (10 wt%) are sensibly higher than the corresponding for the unmodified composites, respectively 256 1C (40 wt%) and 281 1C (10 wt%). The authors [67] also concluded that the thermal stability of the composites decreased with increasing BPS loading because the thermal stability of the filler (fiber) is much lower than that of the matrix.

3.3.13. Bagasse fiber composites Mothe´ et al. [69] investigated the thermogravimetric behavior of polyurethane (PU) matrix composites reinforced with up to 20 wt% of untreated sugarcane bagasse fibers. TG/DTG analysis was carried out at a heating rate of 10 1C/min in nitrogen. DTG curves for the pure bagasse fiber as well as for composites with different (5, 10 and 20 wt%) fiber contents show small initial peaks at a common temperature around 40 1C. These peaks are probably due to fiber moisture release. Three other peaks could also be seen. A clear shoulder peak for the pure bagasse around 310 1C shifts to a faint shoulder peak at about 280 1C for the 20 wt% fiber composite. Although not mentioned, these shoulder peaks could be a result of the hemicellulose degradation. Other clear peaks at about 355–370 1C might be caused by cellulose degradation in the bagasse fiber. A major peak around 420 1C for all composites, but not seen in the pure bagasse fiber, might be attributed to PU degradation. As the main objective of Mothe´ et al. [69] was to investigate the thermal decomposition kinetics, the free interpretation of degradation mechanisms was an additional contribution to the reader. In the already reviewed (see Section 3.3.8) work of Lei et al. [60], in addition to pine wood fiber/RHDPE composites, TG curves were also presented for RHDPE matrix composites reinforced with 30 wt% of sugarcane bagasse fiber. DTG measurements were carried out at a heating rate of 10 1C/min in nitrogen. Three active coupling agents were applied to the fiber: a maleated polyethylene (MAPE), a carboxylated polyethylene (CAPE) and a titanium

derived mixture (TDM). The TG curves revealed that the pure bagasse has a significantly lower onset degradation temperature, 248 1C, than the RHDPE, 441 1C. With the incorporation of 30 wt% of bagasse fiber, this onset temperature becomes 254–256 1C. In addition to the onset degradation temperature, the authors [60] reported two more weight loss stages for the composites. The first with fastest decomposition temperature (TdI) around 341–342 1C, was close to the only corresponding stage for the pure bagasse at 327 1C. The second (TdII) around 469–470 1C, was very close to the only corresponding stage for the neat RHDPE at 470 1C. Based on these results, Lei et al. [60] concluded that the first stage of the composite thermal degradation, at much lower temperature, was related to the bagasse fiber influence, while the second stage to the RHDPE influence. It was also concluded that the applied coupling agents had little effect on the thermal stability of the composites, although SEM fractographs in Fig. 4, reproduced from [60], show that the gap (white arrows) between bagasse fiber and matrix is reduced when the coupling agents were added.

3.4. Remarks and conclusions More than an extensive list of results acknowledging the effort of researchers, this overview aims to bring into attention relevant advances in the comprehension of thermogravimetric behavior of natural fiber composites. As a first remark, the reader should keep in mind that, due to space limitation, a relatively small number (13) of the most commonly known lignocellulosic fibers was selected. Moreover, the published works, at least in easily available and well recognized sources, are also restricted to limited types of polymers as composites matrices. Therefore, a rather fragmented picture exists regarding both fibers and polymers. Consequently, the reader should expect uncertainties not only in a complete view but also in reliable conclusions on the thermogravimetric stability of natural fiber composites. In spite of these restrictions, some general trends appear to hold and are worth discussing.

100µm

100µm

100µm

100µm

Fig. 4. Impact fracture surface of bagasse fiber (30 wt%)/RHDPE composites containing: (a) no coupling agent; (b) 1.5% MAPE; (c) 1.5% CAPE; and (d) 0.9% TDM. Reproduced from [60].

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3.4.1. Initial water loss Normally, an initial weight loss (o10 wt%) is observed in natural fiber polymer composites below 200 1C. This is usually associated with a relatively small DTG peak assigned to evaporation of water from the fiber surface. The peak indicates a maximum rate of released water (moisture, humidity). Common synthetic polymers give only a minor contribution, if any, to this low temperature water loss. However, biocomposites with naturally containing water matrices may also provide a significant amount to this initial DTG peak. In several works [43,48,54,66,69], this initial peak has been overlooked or just ignored. Whenever relevant, its existence in this overview was inferred from presented DTG curves. Although in relatively small amount, the water loss from inside a composite may cause porosity and impair the properties. Actually, its DTG peak temperature may be considered as a first stability limit for the composite. In practice, even if a natural fiber undergoes oven-drying before incorporation into a polymer composite, the total elimination of water is not possible because of the hydrophilic characteristic of the fiber. Fiber treatments such as alkalinization [34,49] and bleaching [33] that partially extract the highly hydrophilic hemicellulose, considered as greatest responsible for water absorption [49], tend to increase the corresponding DTG peak temperature and reduce weight loss. In the present overview, water loss peaks were found at temperatures as low as 37 1C [34] and inferred at about 140–144 1C [48,51].

3.4.2. Onset of degradation From a practical standpoint, the thermal stability of a natural fiber composite is related to the onset of a massive weight loss. This is clearly observed as a sharp downward inclination in the TG curve and has been reported in terms of an onset temperature (T0). The determination of T0, however, can be done by distinct methods, based on transition from an initial baseline [57], tangents intercepts [58] or a percentage of weight loss deviation from baseline [67]. A more visually reliable method is to consider the first decomposition DTG peak, either single or as a shoulder. The works overviewed have shown that this first DTG peak occurred in a range from 250 1C [50] to 365 1C [46], depending on the fiber and polymer matrix. Apparently, the onset of thermogravimetric degradation in a natural fiber composite is a complex process, which depends not only on the comparative thermal stability of fiber vs. matrix but also on the experiment atmosphere. In petroleum-based polymers that are traditionally used as engineering materials and possess a high degree of biodegradability resistance (non-degradable), such as high density polyethylene [36], epoxy [48], polyester [52], and polyurethane [69], a single DTG peak under nitrogen is observed at relatively higher temperatures: 515, 380, 385, and 420 1C, respectively. Comparative decrease may occur in air [37]. The introduction of a lignocellulosic fiber in these polymers normally reduces T0 and in particular the first DTG peak to 352, 310, 285 and 280 1C, respectively. As an exception, the reverse was reported by Doan et al. [37] in jute/polypropylene. It is accepted [41,52] that earlier decomposition of hemicellulose, known to have a maximum rate around 250–290 1C [22,70], is probably responsible for these lower T0 and first DTG peaks of thermal degradation in the composite as compared to neat polymer matrix. Decomposition of other fiber components, such as waxes, glycosidics, and pectin, is also taking place at this lower temperatures, including the beginning of lignin degradation at about 110–220 1C [22,70]. By contrast, environmentally friendly biodegradable polymers, such as starch [43], soy protein [50], chitosan [55], and PHBV [66], with more than one DTG decomposition peak, show the first at 336, 270, 300, and 316 1C, respectively. Introduction of lignocellulosic

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fiber results in composites with very close or higher first DTG peak: 356, 304, 300, and 320 1C, respectively. In this case, the biocomposite thermal stability is limited by the matrix. From all these results, the reader may conclude that applications of natural fiber composites are, in general, thermally restricted to a safe temperature of 250 1C, or to a maximum of 365 1C in case more stable specific composites, such as sisal/polypropylene [46] are selected. 3.4.3. Higher temperatures degradation In addition to a first DTG peak, most natural fiber polymer composites display other higher temperature peaks, the second usually with higher intensity. In common non-degradable polymer composites, this second main DTG peak is related to matrix degradation [33,44,52,60], while biocomposites depict second peaks associated with cellulose/lignin decomposition [50,55,62,64,66]. Other peaks at even higher temperatures are attributed to thermal degradation of substances resulting from earlier decomposition stages. For engineering applications, these higher temperature peaks are not as important as the first. However, they may be used to calculate the activation energy (Table 3) [43], which is another parameter associated with thermal stability [58,70].

Acknowledgments The authors thank the financial support of the Brazilian agencies: CNPq, CAPES and FAPERJ. References [1] A.K. Bledzki, J. Gassan, Prog. Polym. Sci. 4 (1999) 221–274. [2] D. Nabi Sahed, J.P. Jog, Adv. Polym. Technol. 18 (1999) 221–274. [3] A.K. Mohanty, M. Misra, G. Hinrichsen, Macromol. Mater. Eng. 276/277 (2000) 1–24. [4] A.K. Mohanty, M. Misra, L.T. Drzal, J. Polym. Environ. 10 (2002) 19–26. [5] A.N. Netravali, S. Chabba, Mater. Today 6 (2003) 22–29. [6] A.K. Mohanty, M. Mishra, L.T. Drzal (Eds.), Natural Fibers, Biopolymers and Biocomposites, Serial Publications, CRC Press, Boca Raton, USA, 2006 , 2005 and Taylor and Francis, London,. [7] K.G. Satyanarayana, J.L. Guimara~ es, F. Wypych, Compos. Part A 38 (2007) 1694–1709. [8] J. Crocker, Mater. Technol. 2–3 (2008) 174–178. [9] S.N. Monteiro, F.P.D. Lopes, A.S. Ferreira, D.C.O. Nascimento, JOM 61 (2009) 17–22. [10] S. Kalia, B.S. Kaith, I. Kaurs (Eds.), first ed,Springer, New York, 2011. [11] S.N. Monteiro, F.P.D. Lopes, A.P. Barbosa, A.B. Bevitori, I.L.A. Silva, L.L. Costa, Metal. Mater. Trans. A 42 (2011) 2963–2974. [12] A. Gore, An Inconvenient Truth. The Planetary Emergency of Global Warming and What We Can do About it, first ed, Rodale Press, Emmanaus, 2006. [13] P. Wambua, I. Ivens, I. Verpoest, Compos. Sci. Technol. 63 (2003) 1259–1264. [14] S.V. Joshi, L.T. Drzal, A.K. Mohanty, S. Arora, Compos. Part A 35 (2004) 371–376. [15] H. Larbig, H. Scherzer, B. Dahlke, R. Poltrock, J. Cell. Plast. 34 (1998) 361–379. [16] G. Marsh, Mater. Today 6 (2003) 36–43. [17] J. Holbery, D. Houston, JOM 58 (2006) 80–86. [18] R. Zah, R. Hischier, A.L. Lea~ o, I. Brown, J. Clean. Prod. 15 (2007) 1032–1040. [19] C. Alves, P.M.C. Ferra~ o, A.J. Silva, L.G. Reis, M. Freitas, L.B. Rodrigues, J. Clean. Prod. 18 (2010) 313–327. [20] J. George, M.S. Sreekala, S. Thomas, Polym. Eng. Sci. 41 (2001) 1471–1485. [21] S. Kalia, B.S. Kaith, I. Kaurs, Polym. Eng. Sci. 49 (2009) 1253–1272. [22] T. Nguyen, E. Zavarin, E.M. Barral, J. Macromol., Sci.—Rev. Macromol. Chem. C 20 (1981) 1–65. [23] T. Nguyen, E. Zavarin, E.M. Barral, J. Macromol. Sci.—Rev. Macromol. Chem. C 21 (1981) 1–60. [24] A.K. Mohanty, S. Patnaik, B.C. Singh, J. Appl. Polym. Sci. 37 (1989) 1171–1181. [25] M.W. Sabaa, Polym. Degrad. Stabil. 32 (1991) 209–217. [26] M.G.S. Yap, Y.T. Que, L.H.L. Chia, H.S.O. Chan, J. Appl. Polym. Sci. 43 (1991) 2057–2065. [27] C. Gonzalez, G.E. Myers, Int. J. Polym. Mater. 23 (1993) 67–85. [28] G.E. Myers, I.S. Chahyadi, C. Gonzalez, C.A. Coberly, D.S. Ermer, Int. J. Polym. Mater. 15 (1991) 171–186. [29] S. Takase, N.J. Shiraishi, J. Appl. Polym. Sci. 37 (1989) 645–659. [30] C. Di Blasi, C. Branca, G. D’Errico, Thermochim. Acta 364 (2000) 133–142. [31] E. Me´sza´ros, E. Jakab, G. Va´rhegyi, J. Anal., Appl. Pyrol. 79 (2007) 61–70.

28

S.N. Monteiro et al. / Materials Science & Engineering A 557 (2012) 17–28

[32] S. Das, A.K. Saha, P.H. Chourdhury, R.K. Basak, B.C. Mitra, T. Todd, S. Lang, R.M. Rowell, J. Appl. Polym. Sci. 76 (2000) 1652–1661. [33] B.N. Dash, A.K. Rana, H.K. Mishra, S.K. Nayak, S.S. Tripathy, J. Appl. Polym. Sci. 78 (2000) 1671–1679. [34] D. Ray, B.K. Sarkar, R.K. Basak, A.K. Rana, Appl. Polym. Sci. 94 (2004) 123–129. [35] L.B. Manfredi, E.S. Rodriguez, M. Wladyka-Przybylak, A. Vazquez, Polym. Degrad. Stabil. 91 (2006) 255–261. [36] S. Mohanty, S.K. Verma, S.K. Nayak, Compos. Sci. Technol. 66 (2006) 538–547. ¨ [37] T.-T.-L. Doan, H. Brodowsky, E. Mader, Compos. Sci. Technol. 67 (2007) 2707–2714. [38] T. Yu, Y. Li, J. Ren, Trans. Nonferrous Met. Soc. China 19 (2009) s651–s655. [39] N. Sgriccia, M.C. Hawley, Compos. Sci. Technol. 67 (2007) 1986–1991. [40] M. Pracella, D. Chionna, I. Anguillesi, Z. Kulinski, E. Piorkowska, Compos. Sci. Technol. 66 (2006) 2218–2230. [41] G.W. Beckermann, K.L. Pickering, Composites A 39 (2008) 979–988. ˜ a´n, I. Mondragon, J. Therm. Anal. Cal. 73 (2003) 783–795. [42] P. Gan [43] R. Moriana, F. Vilaplana, S. Karlsson, A. Ribes-Greus, Composites A 42 (2011) 30–40. [44] C. Albano, J. Gonzalez, M. Ichazo, D. Kaiser, Polym. Degrad. Stabil. 66 (1999) 179–190. [45] K.C.M. Nair, S. Thomas, G. Groeninckx, Compos. Sci. Technol. 61 (2001) 2519–2529. [46] X.L. Xie, K.L. Fung, R.K.Y. Li, S.C. Tjong, Y.-W. May, J. Polym. Sci. Part B: Polym. Phys. 40 (2002) 1214–1222. [47] P.V. Joseph, K. Joseph, S. Thoas, C.K.S. Pillai, V.S. Prasad, G. Groeninckx, M. Sarkissova, Composites A 34 (2003) 253–266. ˜ a´n, S. Garbizu, R. Llano-Ponte, I. Mondragon, Polym. Compos. 26 (2005) [48] P. Gan 121–124. [49] J.M.F. Paiva, E. Frollini, Macromol. Mater. Eng. 291 (2006) 405–417. [50] J.T. Kim, A.N. Netravali, J. Biobased Mater. Bioenergy 4 (4) (2010) 338–345. [51] M.F. Rosa, B.-S. Chiou, E.S. Medeiros, D.F. Woods, T.G. William, L.H.C. Mattoso, W.J. Orts, S.H. Iman, Bioresource Technol. 100 (2009) 5196–5202.

[52] H.P.G. Santafe´ Jr., R.J.S. Rodriguez, S.N. Monteiro, T.E. Castillo, Characterization of thermogravimetric behavior of polyester composites reinforced with coir fiber. In: Characterization of Minerals, Metals and Materials Symposium—TMS 2011 Annual Conference, San Diego, CA, USA, 2011, pp. 1–6. [53] H.F. Mark, N.,.M. Bikales, C.G. Overberger, G. Menges, Encyclopedia of Polymer Science and Engineering, John Wiley & Sons, New York, 1988. [54] C.G. Silva, D.B. Benaducci, E. Frollini, BioResources 7 (1) (2011) 78–98. [55] N.M. Julkapli, H.M. Akil, Polym.-Plast. Technol. Eng. 49 (2010) 147–153. [56] H.M. Akil, M.F. Omar, A.A.M. Mazuki, S. Safiee, Z.A.M. Ishak, A.A. Bakar, Mater. Des. 32 (2011) 4107–4121. [57] F.M.B. Coutinho, T.H.S. Costa, D.L. Carvalho, M.M. Gorelova, L.C. de Santa Maria, Polym. Test. 17 (1988) 299–310. [58] G.H. Doh, S.-Y. Lee, I.-A. Kang, Y.-T. Kong, Compos. Struct. 68 (2005) 103–108. [59] R. Bhardwaj, A.K. Mohanty, L.T. Drzal, F. Pourboghrat, M. Misra, Biomacromolecules 7 (2006) 2044–2051. [60] Y. Lei, Q. Wu, F. Yao, Y. Xu, Composites A 38 (2007) 1664–1674. [61] J. George, S.S. Bhagawan, S. Thomas, J. Therm. Anal. 47 (1996) 1121–1140. [62] S. Luo, A.N. Netravali, Polym. Compos. 20 (3) (1998) 367–378. [63] P. Threepopnatkul, N. Kaerkitcha, N. Athipongarporn, Composites B 40 (2009) 628–632. [64] S.-H. Lee, S. Wang, Composites A 37 (2006) 80–91. [65] Y.-F. Shih, Mater. Sci. Eng. A 445–446 (2007) 289–295. [66] S. Singh, A.K. Mohanty, T. Sugie, Y. Takai, H. Hamada, Composites A 39 (2008) 875–886. [67] E.S. Zainudin, S.M. Sapuan, K. Abdan, M.T.M. Mohamad, Mater. Des. 30 (2009) 557–562. [68] M.A. Semsarzadeh, M. Mehrabzadeh, S.S. Arabshahi, Eur. Polym. 38 (2002) 351–358. [69] C.G. Mothe´, C.R. de Araujo, M.A. de Oliveira, M.I. Yoshida, J. Therm. Anal. Cal. 67 (2002) 305–312. ´ rfa~ o, F.J.A. Antunes, J.L. Figueiredo, Fuel 78 (1999) 349–358. [70] J.J.M. O