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Polymer Degradation and Stability 108 (2014) 23e34

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

The role of lignin in polypropylene composites with semi-bleachedcellulose fibers: Mechanical properties and its activity as antioxidant

Renan Gadioli a, Jaqueline A. Morais a, Walter R. Waldman b, Marco-A. De Paoli a, *

a Laborat�orio de Processamento de Polímeros, Instituto de Química, Unicamp, C. Postal 6154, 13083-970 Campinas, SP, Brazilb Departamento de Física, Química e Matem�atica, UFSCar, 18052-780, Sorocaba, SP, Brazil

a r t i c l e i n f o

Article history:Received 7 April 2014Received in revised form30 May 2014Accepted 4 June 2014Available online 12 June 2014

Keywords:Cellulose fiber compositesSemi-bleached celluloseLigninPolypropyleneMechanical properties and stabilization

* Corresponding author.E-mail address: mdepaoli@iqm.unicamp.br (M.-A.

http://dx.doi.org/10.1016/j.polymdegradstab.2014.06.00141-3910/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Reports on the use of bleached Eucalyptus cellulose as a reinforcing agent for polypropylene and on theuse of lignin, from Eucalyptus and other vegetal species, as a stabilizer for different polymers can befound in the scientific literature. This work focuses on polypropylene composites using cellulose fiberswith different bleaching levels, i.e. containing controlled lignin contents. We compared the propertiesand stability of composites with bleached and with semi-bleached fibers. These were prepared by twin-screw extrusion and injection molding and characterized by their tensile and flexural mechanicalproperties, thermogravimetry, oxidation induction time, reflectance infrared spectrophotometry andscanning electron microscopy. The kinetic evaluation of the accelerated and environmental aging of theinjection molded test samples was followed by mechanical properties variation and reflectance infraredspectrophotometry. An increase in the reinforcing effect of the fibers and on the stability of the com-posites was observed for composites containing semi-bleached Eucalyptus fibers, with different lignincontents, compared to composites with the bleached cellulose fibers. The lignin present in these semi-bleached fibers positively affected the mechanical properties of the composites and the hindered phe-nols in the lignin structure acted as a primary anti-oxidant, indicating their advantageous use in com-parison to the bleached fibers. Scanning electron micrographs showed that the excellent bulk fiber/matrix adhesion was not affected by accelerated or environmental aging.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The aim of making polymeric composites is to reinforce athermoplastic polymer matrix, thus improving its tensile strengthand modulus. Glass fibers are the most frequently used materials increating these composites, even though these fibers have a highdensity and their production is energy consuming. In addition, theadhesion of the glass fibers to the thermoplastic diminishes uponaging [1]. The use of vegetal fibers can reduce the weight of thecomposites and allows the use of raw materials from renewableresources [2]. The vegetal fibers also reduce the thermoplasticenvironmental impact while improving its tensile and flexuralmechanical properties. These lignocellulosic vegetal fibers can beobtained from different sources, depending on climate, localizationand availability. However, working with vegetal fibers is chal-lenging because their mechanical and chemical properties dependon variables: such as species, climate, soil feedstocks, crop, fiber

De Paoli).

05

extraction methods, etc. [3]. These conditions change for differentcultivation sites, causing poor reproducibility of fiber propertiesand of the resulting composites.

Lignocellulosic fibers extracted from Eucalyptus have a distinctadvantage in comparison to other fibers. They have reproducibleproperties because they are produced on very large scale fromcloned Eucalyptus species and cultivated under controlled condi-tions. Like other vegetal fibers, the raw fibers extracted fromEucalyptus are a lignocellulosic material, comprised of cellulose,hemicellulose and lignin. Pulp and paper companies treat thesefibers by the Kraft Process to extract lignin and other impurities,thus producing the “bleached cellulose fibers”, used to make highquality paper, with lignin contents lower than 0.1 wt % [4]. TheBrazilian annual production of cellulose is ca. 1.5 � 107 kg [5].

Lignin is, after cellulose, the second-most abundant polymer onthe planet. It has a cross-linked structure with many differentchemical groups. Its intricate chemical structure has been studiedusing fragmentation methods. In general, it contains severalphenolic groups with different substituents in the positions 2-, 4-and 6- [6,7]. However, the most important chemical groups, for ourpurposes, are the phenols from the guaiacyl and syringyl groups,

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e3424

Fig. 1, derived from the conipherylic and sinapylic acids, respec-tively, with methoxy substituents in the 2- and 6- positions [8].These phenolic groups have a chemical structure similar to themost commonly used primary anti-oxidants for polymers producedby the chemical industries, such as 2,6-di-t-butyl-p-hydrox-ytoluene, commercialized under different trade names. Thesecommercial primary anti-oxidants are known as “hindered phe-nols” due to the steric hindrance of the substituent in the 2,6-positions, that impede reactions of the free radicals formed duringthe polymer stabilization process [9]. These primary anti-oxidantsact as free radical scavengers during the thermo or photo-oxidation of the polymers [10].

A previous study determined, by different methods, thesyringyl/guaiacyl (S/G) rate of six clones of Eucalyptus samplescollected from different cultivation sites in Brazil [11]. The S/G rateaverage determined by pyrolysis-gas chromatography-massspectra varies from 2.61 to 2.97, with an overall predominance ofthe 2,6- substituted syringyl structure. In another work, an S/G ratioranging from 2.1 to 2.5 for the lignin present in the crude fibersextracted from one of the eucalyptus species used to produce cel-lulose and paper in Brazil was determined through pyrolysis-gaschromatography coupled to mass spectrometry [6]. This ratio canreach 3.0, depending on the extraction method. The higher con-centration of syringyl groups in relation to guaiacyl indicates thatthe lignin present in these Eucalyptus species have good perspec-tives for use as primary anti-oxidants.

In previous studies we demonstrated that the lignin extractedfrom sugar cane bagasse had a significant effect as both a thermoand photo-stabilizer for polybutadiene [12,13], poly(styrene-co-butadiene) and poly(cis-isoprene) or natural rubber [14]. Its usein rubbers is favored because these polymers are vulcanized withcarbon black, thus masking the natural dark color of lignin. Otherauthors demonstrated the free radical scavenging capacity ofBeachwood lignin and its stabilizing effect towards polypropylenecompared with a commercial hindered phenol antioxidant [15].Lignins extracted from 15 different plants were also studied as anti-oxidant for polypropylene [16]. However, these authors did not takeinto account that lignins extracted from different plants havedifferent relative concentrations of syringyl and guaiacyl groups, i.e.S/G ratio [17], and concluded that the solubility of lignin in PP wasthe most important factor in determining stabilization.

Composites of bleached cellulose fibers and polypropylene havebeen prepared with different fibers concentrations by means of atwin-screw extruder, thus achieving a substantial reinforcing effect[18,19]. The formulation included additives and a lubricant as aprocessing aid. These fibers exhibited excellent reinforcing prop-erties and produced a composite with lower density in comparisonto PP reinforced with glass fiber.

At different steps of a large-scale bleaching process, it is possibleto obtain cellulose fibers with controlled concentrations of lignin,which are identified here as semi-bleached cellulose fibers. Weused these semi-bleached Eucalyptus cellulose fibers in this studyto produce composites with polypropylene and to study the asso-ciation of two effects; the reinforcing effect of these cellulose fibersand the stabilizing effect of the lignin present in the fibers. The

Fig. 1. Guaiacyl (left) and syringyl (right) structures present in lignin.

composites were prepared in an intermeshing co-rotating twin-screw extruder and test samples were injection molded. Thecomposites were characterized and test samples were exposed toaccelerated and environmental aging. They were analyzed at reg-ular time intervals to track properties variations and to study thestabilization effect of the lignin present in the fibers on the ther-moplastic matrix.

2. Experimental part

2.1. Materials

Braskem (Triunfo, Brazil) supplied polypropylene (PP) with amelt flow index of 10 g 10 min�1. This was mixed with 6 wt % ofMaleic Anhydride-Modified Polypropylene (PPMA), Bondyram1001 from Polyram, Israel), containing 1 wt % MA and melt flowindex of 100 g 10min�1. For purposes of this study this mixturewascalled pure PP and used to compare properties with the composites.

The Eucalyptus cellulose fibers were supplied in the form ofpellets by Suzano Pulp and Paper (Suzano, Brazil) in three grades:bleached (B), semi-bleached 1 (SB1) and semi-bleached 2 (SB2).The semi-bleached fibers were collected in the bleaching plant atdifferent steps of the industrial process. The lignin contents were:B ¼ 0%, SB1 ¼ 1.5 wt % and SB2 ¼ 2.4 wt %, as determined by thesupplier. The same company supplied pure lignin, extracted fromthe same species of Eucalyptus, in the powder form.

2.2. Processing

The composites were processed by extrusion in a CoperionWerner-Pfleiderer ZSK-26, intermeshing, co-rotating twin-screwextruder (L/D ¼ 44 and D ¼ 26 mm), equipped with a side-feederand two Brabender gravimetric feeders.

Composites were prepared with 30 wt % of fibers in relation toPP. Previous experiments with 10, 20, 30 and 40 wt % showed that30 wt % of fibers provided the best reinforcing effect. PP was fed atzone 1 of the extruder and the cellulose fibers were fed through aside feeder at zone 4. Both materials were gravimetrically dosedusing Brabender feeders controlled by the extruder computer. Thetemperature profile used was 160, 160, 165, 165, 170, 170, 170, 170,180, 175, 170 �C, from feed to die, with main screws rotation speedof 300 rpm and side feeder screws rotation of 265 rpm. After pro-cessing, the pelletized composites were dried in an air-circulatingoven at 100 �C for 1 h and then injection molded, with di-mensions according to the ASTMD638 standard, with an Arburg AllRounder 221K 250-75 equipment. The temperature profile usedwas 150e205 �C from feed to nozzle and themold temperature was25 �C.

Pure PP test samples were injection molded from pellets pro-cessed under the same conditions to make blank control samples,identified here as PP.

Composites were denominated as PP30B, PP30SB1 andPP30SB2, where 30 represents the fiber content and B, SB1 and SB2correspond to the fiber type. They were characterized by tensileand flexural mechanical tests, oxidation induction time (OIT)determination by differential scanning calorimetry (DSC), ther-mogravimetry (TGA), Scanning Electron Microscopy (SEM), andreflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR).

2.3. Samples aging

2.3.1. Accelerated agingThe injection molded tensile test samples were aged in a

photochemical aging chamber constructed according to the ASTMG-154 standard [20]. The radiation sources were 80 W Philips

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e34 25

lamps (model Cleo PerformanceWR 80) with a spectral distributionin the UVA region (315e400 nm). In addition to UV, the sampleswere subjected to water condensation cycles generated by a heat-ing system in a water tank located under the specimens. Theaccelerated aging had the following cycle: 22 h of irradiation in atemperature range of 25e35 �C, controlled by a ventilation system,followed by a condensation cycle of 2 h. The temperatures moni-tored were 40 �C in the water bath and 50 �C in the region of thesamples during the condensation cycle. Six groups of five testsamples were removed for characterization after 0, 100, 200, 300,400e500 h. Another group of five test samples, tc in Table 2, wasreserved as a control and kept in the dark at 25 �C and 50% relativehumidity during 500 h. Freshly prepared samples, time zero, arenamed “non-aged samples”.

The accelerated degradationwas followed by tensile mechanicaltests and ATR-FTIR of the exposed surface. SEM was done for non-aged and for samples aged for 500 h.

2.3.2. Environmental agingEnvironmental aging was done according to ASTM D1435-99

standard, with injection molded tensile and flexural test samplesplaced on a rack at an inclination of 23� (corresponding to the locallatitude), facing north (southern hemisphere) and in an openlocation at Campinas, Brazil (latitude 22� 49' 08” S, longitude 47�

030 5700 W, as determined with a Garmin Montana 600 GPS). Fivetest samples of each set were removed for characterization after 0,60, 120, 180, 240 and 300 days. Control samples, tc in Table 3 andTable 4, consist of one set of each type of test samples stored in thedark at 25 �C and 50% relative humidity during 300 days. Freshlyprepared samples, time zero, are named “non-aged samples”. Afterremoval, the surfaces of the test samples were carefully cleanedwith cotton bloated in distilled water, to remove dust and otherimpurities, and dried at room temperature.

The environmental aging was followed by tensile and flexuralmechanical tests and ATR-FTIR of the exposed surface. SEM wasdone for non-aged and for samples aged for 300 days.

2.4. Characterization methods

Tensile and flexural mechanical properties were determinedaccording to ASTM D638 and D790 standards using EMIC DL 2000equipment. All samples were pre-conditioned for 48 h at 25 �C and50% relative humidity.

Oxidation induction time (OIT) was determined using a differ-ential scanning calorimeter (Du Pont Instruments 910) with thefollowing procedure: heating to 200 �C at 20 �C min�1 under inertatmosphere, isotherm for 3 min, change of atmosphere to syntheticair and isotherm for the time necessary to observe an exothermicprocess. Synthetic air was used, in substitution of pure oxygen, topermit a clearer distinction of the induction times.

Thermogravimetry was performed using a TA Instruments 2950under oxidative atmosphere of synthetic air at 100 mL min�1 and aheating rate of 10 �C min�1. Samples were cut at approximately thesame size and weight to reduce discrepancies.

Table 1Tensile and flexural mechanical properties of the composites (s ¼ tensile strength,М ¼ elastic modulus) and oxidation induction time, OIT.

Sample Tensile Flexural OIT

s/MPa М/MPa s/MPa М/MPa minutes

PP 29.5 ± 0.2 897 ± 247 41.0 ± 0.3 1409 ± 22 10.1PP30B 49 ± 2 1663 ± 136 67.0 ± 0.3 3091 ± 68 6.1PP30SB1 50 ± 1 1662 ± 422 66 ± 3 3150 ± 42 5.9PP30SB2 54 ± 1 1824 ± 86 71 ± 1 3464 ± 33 8.8

ATR-FTIR spectra of the exposed face of the test samples weremeasured using an Infrared Microprobe for Optical MicroscopyIlluminatIR II, Smiths, coupled to an Olympus BX51 optic micro-scope, with a fixed resolution of 4 cm�1. Spectra were measuredafter careful cleaning of the samples surface with cotton anddistilled water. The spectra, recalculated to the absorbancemode bythe equipment software, were normalized in relation to the band at1376 cm�1. To calculate the carbonyl index, the band at 1725 cm�1

was taken as carbonyl absorbance and the band at 1166 cm�1 wasused as reference [21].

Micrographs were obtained by SEM (FEI Quanta 650 FEG) byselecting the tensile specimens with tensile strength values closerto the average. These were transversely cryofractured after im-mersion for 3 min in liquid N2. The fractured surface was coatedwith a thin layer of gold with a SCD 050 sputter coater (Oerlikon-Balzers, Balzers, Lichtenstein), with 40 mA for 60 s. The experi-ments were performed in the range of 10e20 kV acceleration. Allmeasurements were executed at the Brazilian NanotechnologyNational Laboratory (LME/LNnano/CNPEM e Campinas, Brazil).

3. Results and discussion

3.1. Preparation and characterization of the composites

Fig. 2 shows micrographs of the bleached fibers at differentmagnifications. All samples (B, SB1 and SB2) present similarmorphology (all micrographs are supplied as Supplementarymaterial).The cellulose fibers, in the form of pellets, are in anaggregated form (Fig. 2a) with a collapsed appearance (Fig 2b).There are some foliar formations at the surface of the fiber aggre-gates (Fig 2c).

Fig. 3 shows micrographs of the cryogenic fracture of aninjection-molded sample of the polypropylene composite with30 wt % of SB1 fibers (PP30SB1) at different magnifications. Non-aged composites predominantly show a homogeneous distribu-tion of the fibers in the polymer matrix (Fig. 3a and b). We alsoobserve some fiber agglomerates in a significant portion of thecryogenically fractured samples, independent of the nature of thecomposites investigated. Poor fiber dispersion in the polymericmatrix during processing was the likely cause of these agglomer-ates; however, increasing the shear by using more kneading blocksin the screw design would cause matrix degradation, as demon-strated by Canevarolo [22]. Thus, it was necessary to compromisebetween fibers dispersion and matrix stability. Fiber/matrix adhe-sion is very important for achieving energy transfer in the com-posites. At higher magnifications (Fig. 3b) we observe an excellentfiber/matrix adhesion for PP30SB1, which reproduces for the othercomposites (micrographs supplied as Supplementarymaterial). It isalso important to note that; fiber breakage rather than fiber pull outis observed in the samples submitted to cryogenic fracture. Thisfurther corroborates the excellent fiber/matrix adhesion in thesecomposites, which is likely caused by the presence of the PPMAcoupling agent. All composite samples showed similar morphology,as observed by SEM (all micrographs in the Supplementarymaterial).

3.1.1. Mechanical propertiesThe primary goal of making composites is improving the tensile

and flexural strength andmodulus of a polymer matrix. To evaluatethis we compared the mechanical properties of the compositesprepared in this work with those of pure PP, as shown in Table 1. Incomparing the tensile and flexural strength andmodulus of pure PPand the composite samples we note that there was an improve-ment in the mechanical properties resulting from adding 30 wt % ofthe bleached fibers, as well as a further improvement with the

Table 2Tensile mechanical properties of composites submitted to accelerated aging and control sample (tc), s tensile strength, M modulus.

Sampling time/h t1/100 t2/200 t3/300 t4/400 t5/500 tc

PPs/MPa 29.9 ± 0.3 13.7 ± 0.6 24.1 ± 0.2 23.3 ± 0.2 22.3 ± 0.2 28.8 ± 0.1M/Mpa 877 ± 122 924 ± 95 907 ± 130 855 ± 88 884 ± 85 983 ± 78PP30Bs/MPa 50 ± 1 49 ± 1 45 ± 0.8 45 ± 0.9 42 ± 2 49 ± 2M/MPa 1941 ± 51 2270 ± 139 2517 ± 213 2111 ± 135 2194 ± 175 2426 ± 71PP30SB1s/MPa 51 ± 2 50 ± 0.4 47 ± 0.3 47 ± 0.6 45 ± 0.6 50 ± 1M/MPa 1887 ± 267 2070 ± 112 2347 ± 301 2161 ± 447 2171 ± 37 2271 ± 159PP30SB2s/MPa 54 ± 1 54 ± 0.5 52 ± 1 51 ± 1 51 ± 1 55 ± 0.3M/MPa 2180 ± 155 2428 ± 169 2344 ± 359 2392 ± 90 2402 ± 148 2430 ± 77

Table 3Tensile mechanical properties of composites submitted to environmental aging and control sample, tc, (s tensile strength, M elastic modulus).

Sampling time/days t1/60 t2/120 t3/180 t4/240 t5/300 tc

PPs/MPa 26 ± 5 22 ± 1 19 ± 1 17 ± 1 15 ± 1 30 ± 0.2M/MPa 1022 ± 122 982 ± 99 833 ± 91 823 ± 68 594 ± 53 885 ± 11PP30Bs/MPa 42 ± 1 39 ± 2 35 ± 1 32 ± 2 30 ± 2 49 ± 1M/MPa 1999 ± 137 1978 ± 154 1691 ± 109 1629 ± 200 1314 ± 220 1702 ± 69PP30SB1s/MPa 45 ± 1 48 ± 1 41 ± 1 40 ± 1 37 ± 1 42 ± 6M/MPa 1961 ± 200 2006 ± 230 1907 ± 135 1554 ± 236 1719 ± 199 1494 ± 57PP30SB2s/MPa 52 ± 1 43 ± 1 46.0 ± 0.4 45 ± 1 42 ± 2 56 ± 3M/MPa 2331 ± 356 1615 ± 313 1572 ± 288 2025 ± 225 1949 ± 78 1960 ± 135

Table 4Flexural mechanical properties of composites submitted to environmental aging and control sample, tc (s tensile strength, M elastic modulus).

Sampling time/days t1/60 t2/120 t3/180 t4/240 t5/300 tc

PPs/MPa 21 ± 2 23 ± 1 21 ± 2 16 ± 1 16 ± 1 41 ± 1M/MPa 1474 ± 23 1442 ± 96 1278 ± 51 941 ± 26 898 ± 64 1361 ± 50PP30Bs/MPa 54 ± 1 48 ± 1 47 ± 1 46 ± 3 53 ± 1 59 ± 2M/MPa 2906 ± 50 2878 ± 84 2625 ± 137 2210 ± 123 2063 ± 118 2936 ± 155PP30SB1s/MPa 55 ± 1 53 ± 2 48 ± 2 52 ± 1 54 ± 1 60 ± 1M/MPa 2851 ± 52 3114 ± 85 2624 ± 92 2937 ± 41 2220 ± 89 3096 ± 28PP30SB2s/MPa 65 ± 1 51 ± 1 52 ± 1 51 ± 1 57 ± 1 63 ± 2M/MPa 3143 ± 86 2781 ± 21 2987 ± 46 2834 ± 90 2770 ± 70 3524 ± 100

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semi-bleached fibers with different contents of lignin. The tensileand flexural strengths increased 70% and 63%, respectively, in thecomposites with the bleached fibers in comparison to pure PP,demonstrating the reinforcing effect of the fibers. For the compositewith the semi-bleached fibers with the highest lignin content (SB2)this increase was 83% and 73%, respectively. Similar improvementsoccurred for the tensile and flexural moduli. These were propor-tional to the concentration of lignin in the fibers, indicating that thiscomponent might be responsible for the improvement. This couldbe caused either by a better fiber/matrix adhesion in the presenceof lignin or by the higher rigidity of the lignin containing fibers.Previous work by Wood et al. reported an increase in flexural andtensile properties in hemp-epoxy composites by using 2.5 wt % oflignin as coupling agent [23]. However, as indicated in the SEManalysis, good fiber/matrix adhesion occurs for composites withbleached as well as with semi-bleached fibers, thus the higher ri-gidity of the lignin containing fibers is the most likely cause for theimproved mechanical properties.

Pickering and Beg also studied PP composites with 40 wt % ofbleached or unbleached fibers of pinus radiate and observed almostno difference in the mechanical properties of these composites[24]. Despite the different fiber concentrations, this conflictingresult could be a consequence of the different vegetal species fromwhich the fibers where extracted, producing fibers with differentintrinsic mechanical properties [25].

3.1.2. ThermogravimetryTGA is used to study the thermal degradation of polymers under

inert or oxidizing atmospheres. When using an oxidizing atmo-sphere of synthetic air, the onset mass loss temperature (Tonset)indicates oxidation reactions with the production of volatileproducts and the maximum temperature for composite processing.Fig. 4a presents the TGA results for the fibers and Fig. 4b showstheir first derivatives as a function of temperature. We also presentin the same Figure the TGA results of pure lignin for comparison.The three weight loss stages were similar to those observed for

Fig. 2. SEM of the bleached fibers showing: (a) a pellet, b) collapsed fibers structure and c) foliar formation at the fiber surface.

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e34 27

other lignocellulosic fibers [26]. In both figures, we observe that theweight loss is influenced by the concentration of lignin in the fi-bers: the higher the lignin content, the lower the Tonset and Tmax(temperature of higher rate of mass loss, determined by the peak onthe first derivative curve). Conversely, despite the higher thermalstability of pure lignin, its increased concentration did not improvethe stability of the semi-bleached fibers. Tonset is above 200 �C for allfibers, which is the usual processing temperature for PP, indicatingthe feasibility of composites production through extrusion andinjection molding.

Fig. 5a shows the TGA curve for pure PP and the composites andFig. 5b shows their first derivative curves. Two evident mass lossprocesses are observed for all of the composites in the derivativecurves, the first due to PP degradation at 262e267 �C and thesecond (seen as a shoulder) at 280e306 �C due to fiber degradation.The TGA curves of the composites present lower residue at 350 �Cin comparison to pure PP, indicating a higher oxidation rate for the

Fig. 3. SEM of the cryogenic fractures of as prepared PP30SB1 sample, at

fibers and the polymer. The second mass loss process became lessevident with the increase in the lignin content of the fibers. Thefirst derivatives of the TGA curves for the composites also indicate athird mass loss process at a higher temperature, probably due tocarbonization of the fiber residue. The first onset temperatures andthe mass loss rate, in the TGA curves of the composites wereaffected by the amount of lignin present in the fibers, showing aclear interaction of the thermo-oxidative degradation processes ofthe fibers and the polymer. The other inflections in the curves arerelated to thermo-oxidation of the cellulose and lignin present inthe fibers, as observed for other composites with vegetal fibers [27].

To evaluate the contribution of the fibers to the stability of thecomposites we compare the first derivative of the TGA curves forthe main processes of mass loss of each fiber and their compositesin Fig. 6. The curve for pure PP was included for comparison. Thecomposites with the semi-bleached fibers show a Tmax shift tohigher temperatures in relation to the composite with the bleached

two different magnifications; scale in a) is 200 and in b) is 20 mm.

Fig. 4. TGA curves (a) and their first derivative (b) for fibers (SB1, SB2 and B) and for pure lignin. Heating rate of 10 �C min�1 and synthetic air atmosphere.

Fig. 5. TGA (a) and first derivative curves (b) for pure PP, and composites PP30SB1, PP30SB2 and PP30B. Heating rate of 10 �C min�1 and synthetic air atmosphere.

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e3428

fiber, indicating the stabilizing effect of the lignin present in thefibers. In comparing the two composites with semi-bleached fibers,we observe a higher shift for SB2 (27 �C), than for SB1 (18 �C).

3.1.3. Oxidation induction timeOIT measurements are used to compare the induction time

necessary to observe an exothermic event associated to thethermo-oxidation reactions of a polymer at a certain temperatureand in the presence of oxygen. The OIT of the pure PP and thecomposites were determined to compare the thermal stability ofthe samples and the effect of the lignin present in the semi-bleached fibers in comparison to the bleached fibers. The OITvalues are depicted in Table 1 and the heat evolution curves as afunction of time at 200 �C in the presence of synthetic air are shownin Fig. 7. The OIT was shorter for the composites with bleached orsemi-bleached fibers in comparison to the pure PP. This was ex-pected because the cellulose fibers in the composite increase oxy-gen diffusion throughout the material, along with the thermo-oxidative degradation of the fibers. However, when comparingthe composites with bleached and semi-bleached fibers, there wasan increase in OIT only for the composite with the higher ligninconcentration in the fibers, SB2. This may be a further indication ofthe stabilization effect of the lignin present in the fibers. Theseresults contradict the TGA results, where the pure PP was the lessstable material. One hypothesis that could explain these apparentlycontradictory results is that the fibers influenced the initial steps ofthe exothermic reactions in the oxidative degradation of the com-posites (OIT results), however, the kinetic of formation of volatiles,or even their diffusion are slower for composites (TGA results).

3.2. Accelerated aging tests

3.2.1. Mechanical propertiesThe tensile mechanical properties variations measured during

the accelerated aging experiment and of the control samples areshown in Table 2 (values corresponding to non-aged samples, t0,are shown in Table 1). After aging, all of the composites showedhigher mechanical properties retention (calculated in relation tothe control samples) in comparison to pure PP, indicating thatthe sole presence of the fibers reduced the degradation of themechanical properties. This could be due either UV shielding orcrack propagation reduction by the fibers. Other authors haveobserved this behavior in sisal/PP composites [28]. The percentretention of the mechanical properties was even greater forcomposites with the lignin containing fibers; the tensile strengthretention was 90 and 93% for PP30SB1 and PP30SB2, respectively,in comparison with 86% of retention for PP30B. A similar trendwas observed for the elastic modulus, with percent retentions asfollows; pure PP 89%, PP30B 92%, PP30SB1 98% and PP30SB2100%.

An interesting trend was observed for the modulus, there wasan increase in its value during aging for all composite samples, incontrast to a decrease for pure PP. The control samples also had ahigher elastic modulus in comparison to the non-aged (t0) samples.One possible reason could be an increase in the crystallinity degreeof the polymer matrix, improved by the nucleation effect of thefibers. However, this property was not measured in this work.

Because the mechanical properties depend on the bulk of thetest samples, we can rule out the protection effect by absorption

Fig. 6. First derivative of the TGA curves in the 200e400 �C range, comparing the main process of mass loss for fibers, its composites and pure PP. Heating rate of 10 �C min�1 andsynthetic air atmosphere: a) PP30SB1, b) PP30SB2 and c) PP30B.

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e34 29

of UV light by the lignin dispersed within the polymer. Addi-tionally, the depth of penetration of light in the polymer is veryshort. Thus, the stabilization effect is probably due to a mecha-nism similar to the hindered phenol primary stabilizers. Thisindicates that the lignin present in the fibers is responsible forthe low loss of mechanical properties in the composites uponaccelerated aging.

Fig. 7. OIT curves (exothermic heat flow as a function of time of exposure to syntheticair at 200 �C) for PP and PP composites with 30 wt % cellulose fibers: bleached, B, andsemi-bleached, SB1 and SB2.

3.2.2. Infrared spectrophotometryThe ATR-FTIR spectra of the exposed surface of the aged test

samples were measured at the aging time intervals and the spec-trum of the corresponding surface of the control test sample wasalso measured after the total aging time. Fig. 8 shows the spectralvariation in the ranges of interest. The formation of oxidationproducts on the surface of the samples, as evidenced by the FTIRspectra, contrasts with the low variation of the mechanical prop-erties, thus confirming that the degradation process was restrictedto the irradiated surface.

The most prominent changes observed in the spectra of Fig. 8were a broad band at 3600 e 3200 cm�1, assigned to OeHstretching, and a less broad band at 1800 e 1650 cm�1, assigned toC]O stretching. An absorption observed at ca. 1600 cm�1 can beassigned to the C]C bonds formed by the beta-scission mechanism[29]. As for the composites, an intense and broad band at 1100 cm�1

was also observed, and was primarily assigned to CeOeC bonds. Inthe spectra of the composite with the semi-bleached fiber con-taining the higher lignin concentration, shown in Fig. 8d, anotherband was observed at 1650e1600 cm�1, which can be assigned tothe photodegradation products of the lignin [30]. This wasconfirmed by the discoloration of the samples during the agingexperiment; the samples loose the characteristic color of ligninafter the second exposure period.

An important feature observed in these normalized spectra,Fig. 8, was the inconstant formation rate of hydroxyl containinggroups, evidenced by the broad absorption band at3600e3200 cm�1. There was a growth followed by a reduction inintensity. Parallel to this we also observe in all groups of spectra ashift of theC]Ostretchingbandsat 1750e1650 cm�1. Theoxidativeprocesses of the polyolefins lead to the formation of hydroperoxide,

Fig. 8. ATR-FTIR spectra of samples during the accelerated aging experiments. All spectra were normalized in relation to the band at 1376 cm�1.a) PP, b) PP30B, c) PP30SB1 and d)PP30SB2.

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e3430

hydroxylic, carboxylic, ketone, aldehyde and ester functional groups[22]. In addition, we must account for the oxidation products of thefiber itself and the carbonyl groups of the coupling agent used tomake the composites, PPMA. Thus, it is difficult to assign each in-dividual band to a specific oxidation product.

To minimize errors, we used the normalized ATR-FTIR spectra ofthe samples to calculate the carbonyl index of the samples as afunction of aging time. This was calculated as the ratio of the ab-sorbances at 1720 and 1166 cm�1 (determined as reference in aprevious work [19]). Its variation, shown in Fig. 9, indicates a ten-dency towards a lower formation of carbonyl compounds in thecomposites with the bleached fibers in comparison to pure PP. Thismight be caused by UV radiation shielding by the fibers, protectingthe polymer from photodegradation, as observed with other fibers[21]. We also observed a further lowering of the carbonyl index forthe composites with the lignin containing fibers, in comparison tothose with the bleached fibers, indicating that the hindered phe-nols present in lignin can act as primary stabilizers, as observed forpure lignin in previous works [12e16].

Fig. 9. Variation of the carbonyl index as a function of accelerated aging time forsamples indicated in the insert. The lines indicate the variation tendency.

3.2.3. Scanning electron microscopyFig. 10 presents the SEM images of the cryogenic fracture of the

PP30SB1 sample aged for 500 h, with different magnifications. Themicrographs did not show any significant variation on compositebulk in comparison to the non-aged samples (see Fig. 3) withmaintenance of a good fiber/matrix adhesion in the bulk. No fiberpull out was observed, confirming that there was no degradationlocalized at the fiber/matrix interface. The micrographs of the othercomposites presented the same behavior, and are available in theSupplementary material.

3.3. Environmental aging tests

3.3.1. Mechanical propertiesEnvironmental aging is a time consuming experiment, but it

causes more severe damages to the materials in comparison to theaccelerated tests and provides results that are closer to reality. Thephotochemical reactions initiated during the day period propagateduring the night, producing more chromophoric groups, and

Fig. 10. SEM micrographs of the cryogenic fracture of the PP30SB1 composite aged for 500 h, with different magnifications.

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e34 31

accelerating the process in an autocatalytic manner [12]. In addi-tion, the longer exposure time and higher environmental temper-atures for the composites increase the diffusion of the degradationproducts into the bulk of the samples, where they can react withthe fibers or within the fiber/matrix interface. Thus, the determi-nation of mechanical properties retention is more accurate incomparison to the accelerated test.

The results for the tensile and flexural mechanical propertiesvariations for pure PP and the composites, determined during theenvironmental aging, are presented in Tables 3 and 4, respec-tively (values corresponding to non-aged samples, t0, are shownin Table 1). There was an overall reduction of tensile strength, as

Fig. 11. ATR-FTIR spectra of samples during the environmental aging experiments. All spectrd) PP30SB2.

measured in the tensile mode, after the environmental aging.However, the composite with the bleached fibers showed highermechanical properties retention in comparison to pure PP (50%and 61% for tensile strength, respectively) and the compositeswith the semi-bleached fibers showed a higher retention inrelation to the bleached fibers (88 and 75% for composites withSB1 and SB2, respectively). In contrast to the accelerated tests,the composite using the fiber with the higher lignin contentexhibited lower retention. A similar retention trend wasobserved in the elastic modulus. However, there was an overalldecrease of the modulus, contrary to what was observed in theaccelerated tests.

a were normalized in relation to the band at 1376 cm�1.a) PP, b) PP30B, c) PP30SB1 and

Fig. 12. Variation of the carbonyl index as a function of environmental aging time forsamples indicated in the insert. The lines indicate the variation tendency.

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e3432

The pure PP showed a lower retention of flexural tensilestrength in relation to the tensile (39 and 50%, respectively) and thecomposites showed a greater retention of flexural properties inrelation to the tensile, however, all composites showed the sameretention (ca. 90%) independently on the lignin content of the fiber.This similarity in behavior is attributed to degradation mainlyoccurring on the surface of the composites, while the flexuralstrength mostly depends on the polymer/fiber adhesion in the bulkof the samples. The polymer/fiber adhesion was maintained afteraging, as observed in micrographs of Fig. 14c and d.

An initial discoloration of the samples was also observed duringthe environmental degradation experiment. This is probablycaused by photochemical reactions of the chromophores present inthe lignin [23]. The discoloration persists for the entire agingperiod. In this case, the retention of mechanical properties can beassigned either to light shielding by the fibers or to the stabilizationeffect of the lignin present in the semi-bleached fibers.

3.3.2. Infrared spectrophotometryThe ATR-FTIR spectra of the exposed surfaces of the environ-

mentally aged test samples weremeasured at regular time intervals

Fig. 13. SEM of the cross section of cryogenically fractured samples submitted to enviro

and the spectrum of the corresponding surface of the control testsample was measured after the total aging time. These spectra,Fig. 11, were normalized for better comparison. Similarly to theaccelerated aging experiment, the most prominent changesobserved in these spectra were a broad band at 3600 e 3200 cm�1,assigned to the OeH stretching of hydroperoxide, alcohol andcarboxylic acid functions, and a less broad band at 1800 e

1650 cm�1, assigned to the C]O stretching of ketone, aldehyde,carboxylic acid and ester functions. An intense and broad band at1100 cm�1 was also observed, and is assigned to the CeOeC bonds.

Depending on the aging time (e.g. t2 in Fig. 11b, c and d), a bandwas observed at ca.1650 cm�1 in the spectra of the composites. Thisband was less evident in the accelerated aging experiment. Itsappearancewas associated with an increase in intensity of the OeHstretching band at 3500 cm�1. One probable cause of this secondband was the formation of carboxylic groups as a secondary poly-propylene oxidation product or as a product of the reaction of theoxidized polyolefin and the fibers.

Using the same procedure as the accelerated aging, we calcu-lated the variation of the carbonyl index as a function of theexposure time for all of the samples, Fig. 12. Unlike the acceleratedaging, the carbonyl index increased up to 120 days of exposure andthen fell to a lower value until 300 days. This does not means thatthe photodegradation reactions ceased; rather, it indicates that theprimary effect of the degradationwas not the formation of carbonylcontaining groups. The major chromophores responsible for thephotochemical reactions occurring in polyolefins are the ketonegroups, due to the n / p* transitions localized at the C]O bond.These form a reactive diradical in the excited state. The drop in thecarbonyl index can be attributed to these photochemical reactions,producing e.g. other oxidation products absorbing at 1650 cm�1.However, the number of components and their oxidation productsin this heterophasic material precludes a precise assignment of thespectra. The curves in Fig. 12, up to 120 days, indicate that althoughall of the composites had a lower carbonyl index in comparison tothe pure PP, they behaved in approximately the same manner.

3.3.3. Scanning electron microscopyThe micrographs of the control and aged samples are shown in

Fig. 13. It is clear that aging produced a layer of oxidizedmaterial onthe exposed surface of the samples. It is interesting to note that, thisoxidized layer had low adhesion to the bulk of the sample and mayhave acted as a barrier to the penetration of light. This layer was notproduced nor observed in the pure PP samples subjected to UV

nmental aging. FE are the exposed surfaces and FNE are the non-exposed surfaces.

Fig. 14. Micrographs of a) oxidized layer on the surface of PP30SB1 that have undergone environmental degradation; b) higher magnification micrograph of the oxidized layer,showing lignin residues; c) and d) two magnifications of the bulk of the cryogenically fractured sample of PP30SB1 submitted to environmental aging. All other compositespresented similar morphology for both, the oxidized layer and the bulk.

R. Gadioli et al. / Polymer Degradation and Stability 108 (2014) 23e34 33

radiation [31], strongly indicating that its formation was caused bythe presence of the fibers. This brittle layer formed on the exposedsurface was easily detached and examined by SEM at higher mag-nifications, Fig. 14a and b. Surprisingly, even in the composite withthe bleached fiber we observed spherical structures similar tolignin [32,33].

The micrographs of the bulk of the cryogenically fractureddegraded composite PP30SB1, Fig. 14c and d, confirm that the fibermatrix adhesion in the bulk of the composites was not affected bythe environmental aging. The fibers remained covered by thepolymer matrix after 300 days of exposure.

4. Conclusions

Two important conclusions can be drawn from our results. First,the presence of lignin in the cellulose fibers leads to composites withimproved mechanical properties in comparison to those withbleached fibers. This is probably caused by the higher rigidity of thelignin containing fibers and by improved fiber/matrix interaction.Second, composites of polypropylene with cellulose fibers containingligninpresent higher stability, compared to compositeswith bleachedfibers, after accelerated and environmental degradation. This con-firms the anti-oxidant activity of the lignin present in the fibers to-wards the free-radical oxidationmechanism of the polyolefinmatrix.

These results are important to the field of cellulose fibers rein-forced thermoplastics because it indicates that shortening thebleaching process of the fibers, improves the mechanical propertiesof the composites and permits reduction of the concentration ofprimary antioxidants in the formulation. These features willcertainly have a positive effect on the cost/benefit ratio for makingthese composites.

Acknowledgments

The authors thank Fundaç~ao de Amparo a Pesquisa do Estado deS~ao Paulo for a grant (2010/17804-7), Suzano Pulp and Paper forsupplying the fiber pellets and Braskem for the polypropylenesample. The authors are indebted to the Brazilian NanotechnologyNational Laboratory (LNNano) for use of the Scanning ElectronicMicroscope Quanta 650-FEG and the facilities of Electronic Micro-scopy Laboratory (LME/LNNano/CNPEM). JAM also thanks SAE-Unicamp for a fellowship.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymdegradstab.2014.06.005.

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