Characterization of Wood Plastic Composite Based
on HDPE and Cashew Nutshells Processed in a
Thermokinetic Mixer
Victor N.C. Gomes,1Amanda G. Carvalho,2Marciano Furukava,3Eliton S. Medeiros,2 Ciliana R. Colombo,4Tomas J.A. Melo,5Edcleide M. Araujo,5Dayanne D.S. Morais,5 Marcelo M. Ueki,6Carlos A. Paskocimas,3Amelia S.F. Santos2
1
Department of Mechanical Engineering, Federal University of Rio Grande do Norte—UFRN, Natal, RN 59072-970, Brazil
2
Department of Materials Engineering—DEMaT, Federal University of Paraıba—UFPB, Cidade Universitaria, s/n, Castelo Branco, Jo~ao Pessoa, PB 58051-900, Brazil
3
Department of Materials Engineering—DEMat, Federal University of Rio Grande do Norte—UFRN, Natal, RN 59072-970, Brazil
4
Department of Production Engineering, Federal University of Rio Grande do Norte—UFRN, Natal, RN 59072-970, Brazil
5
Department of Materials Engineering, UFCG, Rua Aprıgio Veloso, 882—Universitario, Campina Grande, PB 58429-900, Brazil
6
Department of Materials Engineering, Federal University of Sergipe—UFS, Av. Marechal Rondon, s/n - Jardim Rosa Elze, S~ao Crist ov~ao, SE 49100-000, Brazil
Wood plastic composites (WPC), which are used in high-value markets, contribute to solve some of the problems associated with municipal solid waste accu-mulation. In this study, the effect of cashew nutshell powder (CNSP) content on properties of formulations with recycled high-density polyethylene, 5 wt% of maleic anhydride grafted polypropylene and 5 wt% of struktol TPW 113, a blend of complex, modified fatty acid ester, was evaluated. Mixtures containing from 20 to 60 wt% CNSP were melt-processed, using a ther-mokinetic mixer. WPC’s were characterized by differen-tial scanning calorimetry, Fourier transform infrared spectroscopy, and melt flow index (MFI) measure-ments. Composite morphology and mechanical proper-ties were also accessed, respectively, by scanning electron microscopy and tensile tests. Results show
that the tensile strength of composites decreased as the content of CNSP increased due to poor reinforce-ment–matrix interface and voids formed during residu-al cashew nutshell liquid (CNSL) vaporization. On the other hand, elastic modulus decreased and elongation at break increased, both indicating a plasticizing effect of the residual CNSL, confirmed by crystallinity and MFI data. Differences in the thermal stability of compo-sites were restricted to thermal behavior of main components. POLYM. COMPOS., 00:000–000, 2016. VC 2016
Society of Plastics Engineers
INTRODUCTION
There has been an increasing concern about the envi-ronment leading to research and development of eco-friendly materials that cause less environmental impacts. As an example, natural fillers such as rice husk and straw, bagasse and coconut shells have been successfully used in thermoplastic polymer composites [1, 2]. The use of natu-ral fillers has many advantages due to their light weight,
Correspondence to: A.S.F. Santos; e-mail: amelia@ct.ufpb.br
Contract grant sponsor: National Foundation for Science and Technology Development, CNPq; contract grant numbers: 476549/2012-4 and 564913/2010; contract grant sponsor: Ministry of Education, MEC/SESu (AFOTEC).
DOI 10.1002/pc.24257
Published online in Wiley Online Library (wileyonlinelibrary.com).
VC2016 Society of Plastics Engineers
components. POLYM. COMPOS., 39:2662–2673, 2018. © 2016 Society of Plastics Engineers
low cost, reduced abrasion of machinery, nontoxicity, bio-degradability and recyclability [3, 4].
One of the products obtained from the incorporation of natural fillers in plastic residues is known as wood plastic composites (WPC), which is a value-added recycled prod-uct with many potential and consolidated applications. Since WPC can use plastic residues and reinforcement based on renewable materials, they can bring economic, social and environmental benefits strengthening the plas-tic recycling chain in many aspects.
Despite the abovementioned benefits of natural fillers, their relatively low thermal stability (2008C) to be incorporated in polymers by melt processing, low compat-ibility with many polymer matrices and low tensile strength are drawbacks that limit their extensive use in commercial polymer composites. Moreover, these fillers are usually polar and hydrophilic, mainly as a conse-quence of their chemical structure, which leads to poor adhesion and wettability in nonpolar matrices contributing to matrix–fiber interface debonding [5].
Tensile and flexural properties of WPC formulations can be enhanced with the addition of coupling agents, such as the ones containing anhydride groups [6–17], which can promote ester bonds formation with hydroxyl groups avail-able on filler surface. In general, fillers with high cellulose contents give yield to composites with better adhesion and improved properties. Thus, surface chemistry is of key importance to control fiber/matrix adhesion and produce WPC with improved properties [6, 18–20]. Good interfacial adhesion between the matrix and the fibers is important to transfer stress from the matrix to the fibers and thus, pro-duce composites with enhanced properties [21–24]. More-over, treatments that promote disruption of hydrogen bonding in natural fillers and remove lignin, wax and oils that cover their external surface cell wall can also contribute to improve mechanical properties of WPC [6, 8, 25–27]. Besides the abovementioned factors, properties of WPC are also dependent on fiber length, morphology and mechanical interlocking [6, 10, 20].
On the other hand, many recycled polymers have a non-polar and hydrophobic character, such as polyethylene. This results in inefficient compounding with natural fillers [28, 29] because of their difference in polarity, which is one of the factors that leads to decrease mechanical properties, as mentioned earlier. Maleic anhydride-grafted polypropyl-ene (MAPP) is one of the most used coupling agents used to improve interfacial adhesion between natural fillers and nonpolar matrices [21, 22, 28], since it forms entanglements with polymers chains and acts as a bridge between the non-polar polyethylene matrix and the non-polar fibers, by chemical-ly bonding with the cellulose fibers through the maleic anhydride groups. MAPP has been used to improve the rigidity and tensile strength of high-density polyethylene wood flour composite [13, 30, 31].
High-density polyethylene (HDPE) is one of the most used polymers in bio-based composites due to its excellent chemical and environmental resistance, and processability
[32]. Cashew nutshells (CNS), a residue from the production of cashew nuts and from extraction of cashew nutshell liquid (CNSL), is among the lignocellulosic residues of technologi-cal interest. CNS residue has about 31–33 wt% of CNSL and corresponds to 50 wt% of the whole weight of cashew nut [33–36]. In industrial processing, the shells become brittle during CNSL extraction in hot-oil, which facilitate the automatic extraction of cashew kernels [37, 38].
Cashew (Anacardium occidentale L.) is one of the most known species of the Anacardiaceae family. Vietnam, India, Brazil, Nigeria, and Tanzania account for 80% of global production [39], corresponding to 3.4 million tons of cashew nuts and a market of about US$2 billion. In other words, about 1.7 million tons of shell residues are generated every year, and their use in polymer composites, not only reduces waste accumulation, but also brings benefits owing to CNS lower specific gravity relatively to mineral fillers, sustainability, renewability and biodegradability. CNS is a highly available residue in Brazil, but still of limited explo-ration worldwide.
The use of nutshells (NS) as fillers to low-density poly-ethylene has been reported to cause a reduction in mechani-cal properties to varying extents, but thermal properties are only slightly affected [40]. For pecan shells [41] with a minimum fiber size of 100 meshes, the tensile strength decreased steadily as fiber concentration increased, but significant improvement in tensile strength was achieved with an isocyanate coupling agent. Shells caused no effect on modulus or impact strength of the composites.
As far as the authors are concerned, there is no study that evaluated the use of CNS residues as lignocellulosic fillers in polymer composites in the literature, indicating the innovative nature of this study. Furthermore, studies of WPC using recycled plastics are limited and most indi-cate that these composites, when using a single type of plastic waste, have mechanical properties comparable to, or better than composites using nonrecycled matrices [30, 42–47]. Thus, in this study, thermal, mechanical and mor-phological properties of mixtures of recycled high density polyethylene (HDPE) with varying contents of cashew nutshell powder (CNSP) residue prepared using a thermo-kinetic mixer were evaluated. Additives such as compati-bilizer (maleic anhydride grafted polypropylene, MAPP) and lubricant (struktol TPW 113, a blend of complex, modified fatty acid ester) were also added in order to improve CNSP-polymer interaction and processing prop-erties of composite (fluidity and superficial finishing), respectively.
EXPERIMENTAL Materials
HDPE flakes were donated by a local recycling company (Natal-RN, Brazil). The residue of CNS kindly supplied by Usibras S.A. (Mossoro-RN, Brazil), one of the largest cashew nut (CN) processors in the world.
Lignin and holocellulose content, determined according to TAPPI T13m-54 and TAPPI T19m-54, respectively, were 41.8 and 28.6 wt%. The extract content of CNS deter-mined by soxhlet extraction in cyclohexane followed by ethanol is 29.6 6 0.7 wt%.
A maleic anhydride grafted polypropylene coupling agent (MAPP), PolybondVR 3200, with a melt flow index
(MFI) of 115 g/10 min (1908C, 2.16 kg) and density of 0.91 g cm23 (238C), was purchased from Chemtura Industria Quımica do Brasil Ltda. (S~ao Paulo-SP, Brazil). A modified fatty acid ester lubricant (Struktol TPW 113), with a specific gravity of 1.005 g cm23 (238C), was pro-vided by Parabor Ltda. (S~ao Paulo, Brazil).
Preparation of CNSP and Composite Formulations CNSs were manually classified to remove impurities and processed by a rotary grinder to pass through a 6-mesh sieve. The particle size and distribution, determined using a CILAS 1180 laser diffraction particle size analyz-er, found that most of the CNSP fall in the corresponding diameter ranging between 0.05 and 1.96 mm, with aver-age diameter of 0.95 mm (Fig. 1).
Before melt mixing, the CNSP was dried in a forced air circulation oven for 24 h at 608C to remove the humidity and excess of residual oil. HDPE flakes, CNSP, coupling agent and lubricant were mixed in a thermoki-netic mixer, model MH-50-H (MH Equipamentos Ltda., Guarulhos, SP, Brazil) at 5,250 rpm, according to propor-tions shown in Table 1, which are based on the works of Adhikary et al. [30], de Santi [48], and Razzino [49]. Mixing time was 60–75 s.
Afterwards, composite mixtures were dried in a forced air circulation oven for 12 h at 608C, melted at 2008C for 3 min and pressed to 5 ton for 2 min followed by instanta-neously depressurizing to release the gas due to vaporiza-tion of residual oil from CNSP, and pressed again to 8 ton for 3 min in a hot hydraulic press to prepare sheets of about 200 mm 3 200 mm 3 2 mm, followed by quenching in an
ice bath. All characterizations were carried out on samples taken from these molded sheets.
Thermal Characterization
Differential scanning calorimetry (DSC) experiments were carried out on a DSC 60 Shimadzu calorimeter. Approximately 6–10 mg samples were heated from 30 to 2008C C at 108C min21 under nitrogen atmosphere at 50 mL min21. After a 2 min isotherm at 2008C, samples were cooled to room temperature at 108C min21 in the same atmosphere conditions. Melting enthalpy was deter-mined using constant integration limits. The degree of crystallinity (Xc) was determined, as shown below:
Xcð Þ5%
DHm
W 3 DH100% 3 100 (1)
where DHm is the melting enthalpy per unit of weight of WPC samples,W is the weight fraction of polymer matrix in the composite, and DH100% denotes enthalpy per unit weight of the 100% crystalline polyethylene, which is assumed to be 286.18 J g21 [50]. DSC samples were taken from compression-molded sheets in order to best represent the material undergoing mechanical tests [51].
Thermogravimetric analyses (TGAs) were conducted under argon atmosphere at 50 mL min 21 low, using a TGA 60 Shimadzu thermogravimetric analyzer. The CNSP, HDPEf, and composites were heated from 30 to 5508C at 208C min21. The characteristic degradation tem-peratures Tmax and Tx%, which are respectively, the tem-perature at the maximum rate of weight loss of the DTG curve and the temperature at which the sample loss x% of its initial weight, were determined.
In order to better evidence the difference in thermal behavior of CNSP with and without CNSL (CNSP-e), thermogravimetric analysis was conducted from 30 to 2008C at 508C min21under 50 mL min21of argon flow for CNSP, as received and after soxhlet extraction for 12 h in cyclohexane followed by 12 h in ethanol (CNSP-e).
Morphological Studies
A Zeiss LEO 1430 Scanning Electron Microscope (SEM) operating at acceleration voltage of 10 kV was used to analyze interfacial adhesion and CNSP dispersion in HPDE matrix using cryogenically fractured surfaces coated
FIG. 1. Particle size distribution of CNSP.
TABLE 1. Composition of HDPEf and WPC formulations.
Sample MAPP (wt%) Struktol TPW 113 (wt%) CNSP (wt%) HDPE (wt%) HDPEf 5 5 – 90 20CNSP 5 5 20 70 30CNSP 5 5 30 60 40CNSP 5 5 40 50 60CNSP 5 5 60 30
with Au by sputtering. Prior to the analysis, samples were fractured in liquid nitrogen and were extracted for 12 h in cyclohexane, followed by 12 h in ethanol, using a soxhlet apparatus to remove residual CNSL from samples. This procedure was adopted to avoid the condensation of vapors on the microscope lens. In order to check the effect of soxh-let extraction in morphology of HDPEf and 40CNSP, these samples, with and without soxhlet extraction, were ana-lyzed using a FEI, QUANTA 3D FEG at an accelerating voltage of 20 kV and using an Olympus optical microscope coupled with digital camera, respectively. For optical microscopy (OM), the analysis was performed on the outer surface of 40CNSP test specimens before and after soxhlet extraction.
Fourier Transform Infrared Spectroscopy (FTIR)
Composite samples were placed inside a cotton pouch and its open side was sewed. The closed pouch with the composite sample attached to a small wire was suspended in boiling xylene for 2 h under reflux and magnetic stir-ring. This procedure was repeated four times by using fresh xylene, in order to remove all components that were not covalently bonded to CNSP. For comparison pur-poses, a CNSP sample were xylene extracted at the same conditions (CNSP-xylene). After extraction, the residual CNSP from each composite formulation and CNSP-xylene were dried in an oven at 608C for at least 12 h and analyzed in a Nexus 470 Nicolet FTIR spectropho-tometer, using 32 scans with a resolution of 4 cm21 and an interval of 2 cm21. Analyses were performed in the attenuated total reflectance mode (ATR) by direct analy-sis of CNSP on ZnSe crystal. The FTIR spectra were baseline corrected and ATR corrected.
FTIR was also used to characterize the surface chemistry of CNSL and CNSP, before and after soxhlet (CNSP-e) extraction. About 1wt.% of samples was pelletized with potassium bromide (KBr) at 10 MPa for 5 min under 700 mm Hg vacuum and the infrared spectrum recorded in transmission mode with 32 scans, resolution of 4 cm21and interval of 2 cm21from 4,000 to 675 cm21.
Tensile Tests
The tensile tests were carried out using a Shimadzu AG-X universal testing machine at a constant crosshead speed of 5 mm min21. Type V specimens were prepared by machining the pressed sheets, according to ASTM D 638. For each com-posite formulation, at least five test specimens were tested.
MFI
MFI, in g 10 min21, was determined at 2008C using a load of 2.5 kg, according to the ASTM D 1238. Measure-ments were carried out in triplicate on CEAST melt flow modular line equipment.
RESULTS AND DISCUSSION TGA
TGA and derivative thermogravimetric (DTG) curves of all formulations and CNSP are shown in Fig. 2, respectively. Table 2 shows the temperature where the rate of decomposi-tion is maximum (Tmax) and the temperature at which the sam-ple loses 10% (T10%) and 80% (T80%) of its initial weight.
According to previous studies with different biomass materials, the shoulder on left hand side corresponds to the hemicellulose decomposition while the higher temperature peak on the right hand side represents the degradation of cellulose and lignin [52, 53]. The second peak is overlapped by the main decomposition path of HDPE, as evidenced by the TGA curve of HDPEf (Fig. 2a). According to Heikkinen et al. [53], HDPE decomposes in a narrow temperature range between 300 and 5008C.
It can be noticed from DTG curves (Fig. 2b) that the degradation of CNSP composites follows the same mechanism, since their degradation occurs according to a two-step weight loss. The first step with Tmax around 3008C can be related to the weight loss of CNSP [54]. The majority of biomass thermogravimetric studies pre-sent a DTG curve with a shoulder at lower temperatures and an outstanding point at higher temperatures [52–54].
However, the DTG curve of CNSP (Fig. 2b) shows two distinct DTG peaks [53].
At 5508C, HDPEf was completely decomposed (Fig. 2a), while the CNSP composites still contained residue of carbo-naceous products [55]. These residues increased with the increment in CNSP content (Table 2) and were related to car-bonaceous residue of CNSP in inert atmosphere at that temperature.
Since the first step of weight loss of composites was attributed to CNSP, and its thermal stability was lower than that of HDPE, the temperature at which the compos-ite loss 10% (T10%) of its initial weight decreases as the CNSP content increases (Table 2). On the other hand, the other thermal events of composites remained almost con-stant independent of CNSP content. According to these results (Table 2), the thermal behavior of HDPEf had dominated at high temperatures, since HDPE has higher thermal stability than the second peak of CNSP [53].
Furthermore, the weight loss up to 2008C is ascribed to volatile matter such as humidity and residual CNSL, since HDPEf did not present any weight loss under this temperature range [53]. Figure 3 shows a clear difference in thermal stability of CNSP before and after (CNSP-e) soxhlet extraction in cyclohexane followed by ethanol, confirming the contribution of CNSL vaporization in the weight loss of CNSP and its composites up to 2008C.
DSC
The results of crystallinity degree (Xc), melting temperature (Tm) and crystallization temperature (Tc) are
summarized in Table 3 and the corresponding curves are shown in Fig. 4.
According to these results, the main thermal transitions of HDPE remained almost constant with increasing CNSP concentration, that is, no significant changes were observed. On the other hand, at high CNSP content, the particles inhibited HDPE crystallization, decreasing the degree of crystallinity as filler content increased up to 40 wt%, indi-cating a possible plasticizing effect of residual CNSL.
DSC curve of CNSP showed that the boiling tempera-ture of residual CNSL was about 1728C (Fig. 4), confirm-ing the contribution of CNSL vaporization in the first mass loss of CNSP, as determined by TGA. Nevertheless, CNSL vaporization was not detected in the DSC curves of composite formulations (Fig. 4).
Morphology of Composites
Figure 5a and b shows the fracture surface of HDPEf before and after extraction procedure, respectively. No holes were observed in matrix even after soxhlet extrac-tion, confirming that there is no volatile additive used in matrix formulation, or even soluble in extraction proce-dure. Solubility tests, for example, indicate that struktol TPW 113 was not soluble in cyclohexane or ethanol at room temperature.
Contours of globular particles remaining on the surface are sharp, and adhesion seems to be poor, as shown in Fig. 5c–f. Also, some small holes in polymer matrix of compo-sites can be observed. Probably, these holes in the matrix
TABLE 2. Temperature data from TGA and DTG curves of CNSP, HDPEf, and CNSP composites.
Parameter HDPEf 20CNSP 30CNSP 40CNSP 60CNSP CNSP Tmax1(8C) – 306 302 302 303 278 Tmax2(8C) 480 483 476 481 483 355 T10%(8C) 436 313 283 268 248 239 T80%(8C) 485 489 482 487 493 – Residue (%) 0.8 5.1 5.2 9.2 14.6 29
FIG. 3. TGA curves for CNSP with and without CNSL (CNSP-e). FIG. 4. DSC curves for CNSP, HDPEf, and WPC compositions. TABLE 3. Thermal properties of HDPEf and WPC formulations. Formulation Xc(%) Tm(8C) Tc(8C) HDPEf 53.2 128.0 116.8 20CNSP 53.6 128.7 116.5 30CNSP 55.9 127.9 116.2 40CNSP 42.2 127.5 115.8 60CNSP 45.2 127.0 115.3
and around the CNSP particles evidence that part of residu-al CNSL present in the composite was etched away by the solvent extraction, leaving holes in the matrix and at parti-cle/matrix interface. One hypothesis is that part of residual CNSL migrated from CNSP and stayed trapped in the poly-mer matrix and at the particle/matrix interface.
Since the CNSL vaporize at processing conditions, it is par-tially eliminated during molding, and parpar-tially trapped inside these holes, due to their relative high boiling point and low dif-fusion rate because they have relatively high molecular weight. A similar behavior is observed when water is retained in the inter cell wall of cellulosic structures [56, 57]. Confirm-ing this hypothesis, optical micrographs taken from the exter-nal surface of 40CNSP test specimens before and after extraction procedure (Fig. 6) clearly shows the CNSL con-densed inside the matrix holes and filler particles, as well as around the filler particles before extraction procedure. Another parameter that also contributed to this morphology is the rela-tive low pressures used in compression molding that allows expansion of CNSL vapor inside the polymeric matrix.
Figure 5f shows some aggregates and some dispersed particles in polymer matrix, as expected, for high filler con-tents due to geometrical reasons [58]. Also some holes related to debonding or particle pull out could be observed. Additionally, the poor interfacial adhesion and presence of voids would result in poor mechanical properties, since the stress transfer between the phases was compromised. In fact, particle/matrix interface is a crucial factor in mechani-cal performance of polymeric composites.
FTIR
The FTIR spectrum of CNSL (Fig. 7a) shows absorp-tions bands at 3,030, 1,600, 1,490, 750, and 690 cm21
that are characteristic of the various vibration modes, respectively, of the 5C–H (sp2), C5C, C–H, and C–C bonds of the aromatic ring [59, 60]. This confirms the aromatic nature of CNSL, which major component (approximately 90%) is anacardic acid [61, 62]. Peaks at 2,910 and 2,850 cm21are related, respectively, to stretch-ing of CH2and CH3groups.
In the FTIR spectra of CNSP and CNSP-e (Fig. 7b and c), a strong absorption band at 3,500–3,200 cm21is observed, which is characteristic of OH groups of the fiber constituents. Next to the hydroxyl bands for the CNSP spectrum (Fig. 7b), there are bands at 3,013, 2,924, and 2,855 cm21, which result from the C–H stretching of unsaturated (3,013 cm21) and saturated (2,924 and 2,855 cm21) hydrocarbons, CH2 and CH3groups. The band at 1,462 cm21results from the bend-ing vibrations of methylene groups. The peak at 1,640 cm21 is attributed to C5O stretching vibration of the alpha-keto carbonyl groups [63, 64] and C5C groups stretching vibra-tion of lignin [65, 66]. Also, the intense band at 1,640 cm21 is assigned to H–O–H bending of absorbed water [64, 65]. The absence of any detectable absorption bands in the 1,750–1,700 cm21region confirms the low cellulose content of CNSP [66]. The sharp and strong band at 1,030 cm21is attributed to C–O stretching in cellulose, hemicelluloses, and lignin. A small sharp band at 903 cm21arises from glucosid-ic linkages between the sugar units in hemglucosid-icelluloses and celluloses [67].
The FTIR spectrum of CNSP-e (Fig. 7c) is similar to a mathematical subtraction of the CNSP spectrum from that CNSL one. Only the main absorption bands of lignocellu-losic components are maintained, that is, the strong absorption bands at 3,500–3,200, 1,635, and 1,030 cm21.
Figure 8a and b shows the infrared absorption spectra of CNSPs extracted from composite formulations with
FIG. 5. SEM micrographs of cryogenic fracture surface of HDPEf before (a) and after (b) extraction procedure, 20CNSP (c), 30CNSP (d), 40CNSP (e), and 60CNSP (f) at different magnifications.
different CNSP content by using hot xylene and neat CNSP extracted at the same conditions (CNSP-xylene) in the 1,800–1,500 and 1,400–1,300 cm21 regions, respec-tively. The carbonyl absorption band between 1,700 and 1,750 cm21, which is associated to ester links between hydroxyl groups of CNSP and anhydride groups of MAPP, cannot be evidenced. Only a weak shoulder on the high wavenumber side of the 1,620 cm21 absorption band can be observed (Fig. 8a). Nevertheless, any absorp-tion band from 1,750 to 1,700 cm21 was confirmed by subtracting the spectra of CNSP extracted from composite formulations by using hot xylene from the spectrum of
extracted CNSP in hot xylene (Fig. 8c). Also, no shift on absorption band at 1,620 cm21 was observed. These results imply that the esterification reaction between hydroxyl groups of filler and anhydride groups of MAPP may have not occurred. The absence of esterification between CNSP and MAPP may be due to the presence of CNSL on the fiber surface, as shown in morphological analyses.
Another evidence that there is no MAPP chemically bonded to CNSP extracted from composites is the absence of the strong absorption band at 1,380 cm21, which is characteristic of methyl groups of MAPP, or in digital subtraction of the spectra of CNSP extracted from composites in hot xylene by the spectrum of CNSP-xylene (Fig. 8b and d). Due to the low sensitivity of FTIR analysis and the overlapping nature of the strong and wide absorption band at 1,620 cm21 of CNSP in the 1,700–1,750 cm21 region, these data at least mean that the efficiency of MAPP in bonding CNSP surface was low. Therefore, interface bonding strength at composite interface was probably low.
According to the literature, the nonpolar extractives reduce surface wettability of the reinforcement [68–71] and availability of –OH groups to react with coupling agent [68, 72]. Therefore, the residual CNSL probably inhibited or delayed the formation of ester bonds between the anhydride carbonyl groups of MAPP and hydroxyl groups of the CNSP.
Tensile Properties
The results of tensile strength, elongation at break and elastic modulus of HDPE composites are shown in Table 4 and Figs. 9–11.
Figure 9 shows the evolution of the tensile strength as a function of lignocellulosic filler content. Composites with 20 wt% of CNSP exhibited values of tensile strength, about 42% lower than the HDPEf. The reduction factor of tensile strength in this work is in the same order of magnitude of the values achieved by Sutivisedsak et al. [40] for low density polyethylene (LDPE) and
FIG. 6. Optical micrographs of outer surface of 40CNSP before (a) and after (b) extraction procedure. [Col-or figure can be viewed at wileyonlinelibrary.com]
pistachio, almond and walnut shells mixtures without any coupling agent.
This phenomenon is due to crazing or dewetting effect in which the adhesion between the filler and the matrix is destroyed, leading to a weakening of the interface strength, therefore, in agreement with morphological observations by SEM. The reduced interfacial adhesion to the polymer matrix may explain this reduction, because the effective transfer of stress between matrix and filler requires an adequate interfacial bonding [73, 74]. The lack of intimate adhesion between both components leads to numerous irregularly shaped microvoids or microflaws in the composite structure. Because of these microflaws,
stress transfer from the matrix to the filler is poor, and mechanical properties of filler are not fully exploited. Furthermore, at concentrations of 40 wt % or higher, aggregation may occur purely from geometrical reasons, that is, the maximum packing fraction of the filler (the volume fraction of particles in a packed bed) is smaller than the actual filler content [58]. Indeed, the properties often deteriorate at high filler contents [58, 75, 76].
FIG. 8. FTIR spectra of CNSPs extracted from composite formulations using hot xylene and neat CNSP extracted at the same conditions (CNSP-xylene) in the 1,800–1,500 cm21(a) and 1,400–1,300 cm21regions (b); and the difference spectra of CNSPs extracted from composite formulations by using hot xylene from CNSP-xylene in the 1,800–1,500 cm21(c) and 1,400–1,300 cm21regions (d).
FIG. 9. Tensile strength of HDPEf and composite formulations. TABLE 4. Tensile properties of HDPEf and WPC formulations.a.
Formulation Tensile strength (MPa) Elastic modulus (GPa) Elongation at break (%) HDPEf 15.80 6 1.22 0.88 6 0.05 19.70 6 4.63 20CNSP 9.17 6 0.38 0.48 6 0.07 39.43 6 5.39 30CNSP 4.22 6 0.64 0.26 6 0.06 44.38 6 3.66 40CNSP 2.85 6 0.20 0.24 6 0.09 43.60 6 3.27 60CNSP 1.03 6 0.08 0.08 6 0.02 25.53 6 2.49
Conversely, elongation at break anomalously increased with filler content (Fig. 10). In general, fillers cause a dramatic decrease in elongation at break [8, 77, 78].
In the present case, however, the residual CNSL in the filler acted as a plasticizer, increasing polymer chain mobility and consequently, polymer elongation at break. Greco et al. [79] recently evaluated the plasticizing effect of derivatives from cardanol, a monophenolic component of technical CNSL, in poly(vinyl chloride) (PVC). In their work, the cardanol acetate, obtained from the esterifica-tion of the cardanol hydroxyl group, showed a behavior comparable to that of soft PVC plasticized with di-ethyl-hexyl-phthalate (DEHP). Since the chemical nature of polyethylene is distinct from PVC, the crude oil acted as a plasticizer for PE.
It is noteworthy that the extract content of CNSP is a high value, but less than the extract content found in raw CNS, using to the same extraction methodology, that was 38.6 6 1.1 wt%. According to the manufacturer, the extraction method of CNSL is the hot oil bath that gives a CNSL yield of around 7–12% [37, 38].
Young’s modulus decreased significantly with CNSP addition, as depicted in Fig. 11. That decrease in mea-sured stiffness is thus associated with the abovementioned plasticizing effect of residual CNSL in nutshells, as previ-ously observed in elongation at break data. Furthermore, these results corroborate with the reduced HDPE crystal-linity (Table 3) and even to the porous nature of the pro-duced composites.
Although an intermediate depressurization procedure was done during the hot pressing to reduce porosity, this defect was not totally eliminated due to the relative high residual amount of oil present in the CNSP and relative low molding pressures of compression molding process used to produce the sheets from where test specimens were machined. This porous characteristic of formulations with increasing CNSP content was determinant for the observed mechanical behavior.
On the other hand, according to Danyadi et al. [58], the considerable decrease in modulus (Fig. 11) cannot be
explained only by changes in chain mobility, or low inter-face adhesion between CNSP and HDPE matrix. The larger particles debond very easily even at very small deformations of modulus measurement, leading to a decrease in stiffness. As a consequence, smaller particles can also be debonded, contributing to elastic modulus decrement. Since, the used CNSP particles have a broad particle size distribution, the larger particles probably are among the main factors that contributed to the observed behavior. The presence of coupling agent may improve interfacial adhesion and prevent the debonding of even very large particles, which result in no change or in a moderate increment in modulus [58]. Nevertheless, in this study, the efficiency of compatibility of maleic anhydride grafted PP in HDPE with CNSP could not be noticed and the effect of larger particle in decreasing composite stiff-ness prevailed, together with plasticizing effect of residual CNSL in nutshells and the increased porosity with increased filler content.
MFI
The MFI values (Fig. 12) show that the fluidity of the thermoplastic matrix was considerably affected by the amount of CNSP added. Surprisingly, the greatest resis-tance to flow (lower melt index) was found for HDPEf sample. In a general way, composite formulations decreased their resistance to flow as the amount of filler increases. This behavior can be attributed to presence of residual CNSL that probably has improved the composite flowability by plasticizing the matrix, since the larger the CNSP content, the higher the CNSL content in formula-tions. These data corroborate the results of the elongation at break (Fig. 10) and degree of crystallinity (Table 3), in which residual CNSL acted as a plasticizer. The low MFI value of recycled HDPE, may also have contributed to the poor wetting of filler by the matrix, as observed by Balasuriya et al. [80].
The addition of MAPP usually also plasticizes the polymer matrix and reduces its viscosity, as a result of its
FIG. 10. Elongation at break of HDPEf and composite formulations.
lower MFI [81–83]. This plasticizing effect means that part of MAPP chains remain in composite matrix and part at the matrix–filler interface [81–84]. Nevertheless, as the filler content increases, more MAPP molecules go to the matrix–filler interface and fewer molecules still around the polymer matrix, reducing its plasticizing effect [82]. Therefore, the plasticizing effect depends on filler content and, at high filler content, can be counterbalanced by the reinforcing effect of filler [82]. Since all composite for-mulations had the same MAPP content, the decrease in flow resistance of composite with filler content was also influenced by MAPP added to their composition, predom-inantly at low filler content [82].
CONCLUSIONS
The addition of cashew nutshell powder decreased the tensile strength of HDPEf matrix, as the amount of filler increased, which is attributed to poor adhesion between the CNSP and the matrix. The residual CNSL in filler promoted void formation in polymer matrix and around the CNSP particles. Conversely, the elongation at break increased, indicating a plasticizing effect of residual CNSL, which was also confirmed by a reduction in the crystallinity and an increase in the MFI. Elastic modulus decreased due to plasticizing effect of residual oil and the presence of larger particles in composites. The thermal degradation of CNSP composites occurred in two steps and started at an onset temperature lower than HDPEf matrix. No significant changes on the other thermal prop-erties of HDPEf were observed with increasing CNSP concentration. Due to their flexible behavior, applications other than structural composites such as wooden venetian blinds, interior decoration objects and landscaping timbers can be feasible for WPC with CNSP. On the other hand, the use of alternative processing technologies capable of removing the residual CNSL from CNSP, like degassing extruder, could improve mechanical properties and broad-en the range of applications for these composites.
ACKNOWLEDGMENT
The authors thank to the Rapid Solidification Laboratory (LSR) and Biofuels Laboratory (LACOM) of UFPB for the thermal and FTIR analyses, respectively, and also UFCG for the use of thermokinetic mixer.
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