Influence of peroxide modifications on the properties of cereal straw and natural rubber composites

Influence of peroxide modifications on the properties of cereal straw and natural rubber composites Keywords Peroxide modification  Natural rubber  a linear chain of 1,4-b-bonded anhydroglucose units Alkalization  Straw  Biocomposites which contains alcoholic hydroxyl groups; hemicel- lulose—heterogeneous polysaccharides, located between the lignin and cellulose fibres; lignin— phenolic polymeric material, formed from phenolic Introduction precursors such as p-hydroxycinnamyl alcohols, p-coumaryl alcohol, coniferyl alcohol and sinapyl Natural straw fibers can be considered as an environ- alcohol; pectin, waxes and water soluble substances. mentally friendly reinforcement for composite mate- Natural fibers being hydrophilic are incompatible with rials, having potential of use in various applications by the hydrophobic polymer matrix. Fiber–matrix inter- their economic, technical and environmental concerns face is affected by hydrophilic nature of cellulosic (Masłowski et al. 2017). Nowadays different cellulose fibers (Saheb and Jog 2015). In addition to this, waxy fibers such as flax, hemp, sisal, cotton, kenaf, jute, and pectin substances cap the reactive functional bamboo, coconut and date palm, which have devel- groups of the fiber and act as a hindrance to polymer- oped over the last decades, offer a number of filler interactions. Insufficient adhesion between advantages over synthetic fibers (mainly glass, carbon, hydrophobic polymers and hydrophilic fibers result plastic) due to their renewable nature (Mwaikambo in poor mechanical properties of the natural fiber and Ansell 2002; Bernard et al. 2011). Major unique reinforced polymer composites. As the natural fibers properties (but not limited) of natural fibers include contain hydroxyl groups from cellulose and lignin, among others: low cost, biodegradability, recyclabil- their surface needs to be modified with different ity, low density, good thermal properties, reduced tool chemical treatments to enhance the effectiveness of wear, non-irritation to the skin, and enhanced energy interfacial bonding (Ali et al. 2016b; Militky ´ and recovery (Thakur et al. 2014; Azam et al. 2016). Jabbar 2015). Recently, mercerization, acrylation, The components of natural straw fibers include: acetylation as well as isocyanate, permanganate, cellulose—a hydrophilic glucan polymer consisting of silane and peroxide treatments of natural fibers have 123 Cellulose been adopted as successful improvement of fiber Unfortunately, the problem of utilization of straw is strength and reported in the literature (Li et al. 2007; still not fully resolved and requires search for new Kaushik et al. 2013; Valadez-Gonzalez et al. 1999; opportunities and technologies of its use. Ray et al. 2002; Herrera-Franco and Valadez-Gonza ´- The objective of this study was to investigate the lez 2005; Wong et al. 2010). influence of peroxide modification on the properties of Sodium hydroxide (NaOH) is the most commonly cereal straw and to developed and characterized a new used chemical for bleaching and/or cleaning the type of material: straw treated natural rubber com- surface of plant fibers. This treatment removes lignin, posites. Effect of these treatments on morphology and pectin, waxy substances, and natural oils covering the thermal stability of natural lignocellulose fibers is external surface of the fiber cell wall. It also changes meagerly reported in literature as well as the properties the fine structure of cellulose I to cellulose II, of bio-composites filled with them. Achieving optimal depolymerizing the native cellulose structure and mechanical and utility properties was the primary exposes short length crystallites (Fernandes et al. purpose of the researches. Which depends on the 2011; Collett et al. 2015; Andersson et al. 2003). reinforcement of the surface area of the fibers/matrix. Based on the acquired knowledge, NaOH treatment is In polymer technology, the use of an appropriate currently used as a pre-modification before the main medium is a key issue and forms the basis of its modifications of cellulosic fibers. subsequent, relevant properties. In comparison to thermoplastic biocomposites, widely described in the Peroxide treatments have attracted the attention of the majority of researchers in the field of cellulose literature, the use of new, elastomeric matrix is fiber processing, due to the ease of their implemen- undoubtedly a scientific novelty, both from a cognitive tation with the simultaneous provision of good and application point of view. Furthermore, the aim of mechanical properties. Organic peroxides are easily this work was to investigate peroxide modifications of broken down into free radicals, which then react with cereal straw using dicumyl and benzoyl peroxide with cellulose fiber and also with a hydrogen group matrix previous sodium hydroxide pre-modification on both (Sreekala et al. 2000). During peroxide treatment, fillers and vulcanizates characteristics. fibers are treated with 6% dicumyl peroxide or benzoyl peroxide in acetone solution after previous alkali pre- modification. Fibers treated with dicumyl peroxide Experimental first soak in the solution, then are wash with distilled water, and finally place in oven to improve their Materials hydrophobic properties(Ali et al. 2016a). Nowadays there is a big interest in research Natural rubber (Torimex Chemicals) cis-1,4-polyiso- concentrated on polymer composites of natural bio- prene, density 0.93–0.98 g/cm . materials, including straw. Agricultural crop residues, Sulfur cure system: such as straws represent an important resources, with a • Sulfur (S ) great potential as raw materials for polymers compos- • Zinc oxide (ZnO) ites (Matsumura et al. 2005; Talebnia et al. 2010). The • 2-mercaptobenzothiazole (MBT) structural advantage of straw as compared to other • Stearic acid (SA) annual plant materials is its resemblance to wood composition (Kijen ´ ski et al. 2016). As a result, Biofillers: pure straw (PS), dicumyl peroxide composites filled with straw are expected to have modified straw (DCPS), benzoyl peroxide modified good mechanical properties (Scarlat et al. 2015). straw (BPS). Utilization of wheat straw for industrial application Cereal straws (wheat, rye, triticale, barley and oat) leads to substantial consumption of the straw, as well were obtained from local farms (Poland). as reducing the cost of the product as it is available at The straw was prepared as follows: low cost compared to other bast fibers such as juta • drying at 70 C, (Zou et al. 2010). Researches on the use of straw in • cut into 10 mm fiber biocomposites focus mainly on thermoplastic poly- mers consisting of rapeseed and rice straw as a fibres. 123 Cellulose • grinding using a ball mill for 0.5 h (SPEX nitrogen flow - 60 mL/min, heating rate - 10 C/ SamplePrep 8000D Mixer/Mill) min. • modified as follows: Rubber compounds were prepared using a Braben- der measuring mixer N50 at 50–60 C. The mixing Pre-modification First straws were soaked in 5% time equaled 12 min which included 4 min of masti- (by weight) solution of sodium hydroxide for 2 h at cation of natural rubber, 4 min of mixing NR with the room temperature. After treatment, materials were biofillers (PS, DCPS, DBS) and 4 min of mixing the thoroughly washed with water several times in order to blend with the sulfur curing system. Next, two–roll remove NaOH residual, until a pH of 7 was reached. mill was used to obtain rubber sheets. Next the straws were dried in hot air oven at 70 Cto The rheometric properties and kinetics of elastomer constant weight. Reaction is shown in Fig. 1. mixtures vulcanization were tested using MonTech Main modifications The peroxide treatment was DRPA 300 Rheometer at 160 C. The measurements carried out on an alkali pretreated straw using benzoyl were studied according to ISO 3417. Next, the samples and dicumyl peroxide, reaction shown in Fig. 2. were cured at 15 MPa, until they developed a 90% Pretreated lignocellulosic materials were modified increase in torque, according to rheometric with: measurements. • 6% benzoyl peroxide in acetone for 120 min at The tensile properties and tear strength tests of the 25 C, vulcanizates were measured according to ISO-37 and • 6% dicumyl peroxide in acetone for 120 min at ISO 34 standard, respectively using a universal 25 C, machine (Zwick, Ulm, Germany). The strain rate of tensile tests was 500 mm/min. The elongation at break Next the straws were rinsed with distilled water and measurements were examined using an extensometer then dried in an oven at 70 C. with sensor arms. The tensile strength carried out for The compositions of the tested elastomer mixture five standard dumbbell-shaped samples. The stress– are presented in Table 1. strain curves was plotted. Tear strength tests were performed at a cross-head speed of 50 mm/min for Methods three ‘‘trousers’’ shape of the samples. Dimensions of the samples: length 100 mm, width 15 mm, and Fourier transform infrared spectra was studied in the thickness 1 mm with a precut of 40 mm at the center. -1 range of 4000–400 cm using an Fourier-transform DMA analysis were determined by means of DMA/ infrared spectroscopy (FTIR) Nicolet 6700 spec- SDTA861e analyzer (Mettler Toledo). Test parame- trophotometer. Tools and process parameters: a single ters: temperature range - 150–60 C, heating rate reflection diamond ATR crystal on ZnSe plate, DTGS/ - 3 C/min, frequency - 1 Hz, strain amplitude -1 KBr detector, 128 scans; resolution—8 cm . - 0.05%. The thermal stability of the straws was examined Hardness of biocomposites was studied using a using a TGA/DSC1 (Mettler Toledo) analyzer. Sam- Shore type A Durometer (Zwick/Roell). The mea- ples (total weight in the 8–10 mg range) were placed surements were determined according to ISO 868 into alumina crucibles and heated from 25 to 600 Cin standard. At least 10 points were tested for each a nitrogen atmosphere. Measurement parameters: sample. Fig. 1 Reaction of cellulose fiber with NaOH 123 Cellulose Fig. 2 Reaction of lignocellulose fiber with benzoyl (a)/dicumyl peroxide (b) Table 1 The compositions Straw NR SA ZnO MBT Sulfur of biocomposites and (phr) (phr) (phr) (phr) (phr) (phr) reference sample Ref. sample (NR) 0 100 1 5 2 2 Untreated straw 10 100 1 5 2 2 Benzoyl peroxide straw 10 100 1 5 2 2 Dicumyl peroxide straw 10 100 1 5 2 2 Phr parts per houndred parts of rubber To analyze the crosslinking density (c ), samples of V —the volume fraction of elastomer in the swollen 20–50 mg were swollen to equilibrium in toluene at gel. room temperature. Next, the samples were dried in air The thermo-oxidative ageing was performed at a oven at 50 C to constant weight. The (c ) value was temperature of 70 C for 14 days in a dryer with calculated on basis of the Flory-Rehner (Flory and thermo-circulation. The ageing coefficient (K) was Rehner 1943) (Eq. 1): determined as the numerical change in the static mechanical properties of the samples upon degrada- lnðÞ 1  V þ V þ lV r r c ¼ ð1Þ e 1 tion process (Eq. 3): 3 r V V 0 r K ¼ðÞ TS  E =ðÞ TS  E ð3Þ b b after aging before aging where l—the Huggins parameter of the NR-solvent where E —elongation at break, TS—tensile strength. interaction, was calculated from the Eq. 2: The relative damping of vulcanizes were investi- l ¼ 0:478 þ 0:228  V ð2Þ r gated under the influence of compressive stress using a 123 Cellulose ZWICK 1435 (Zwick, Ulm, Germany) universal the production process (cross-linking, extrusion or machine. Dimensions of the sample (disc shape): injection molding) as well as in the subsequent use of diameter—35 mm, height—17.8 mm. During the test end products. The components of lignocellulosic samples were stressed from 0 to 0,7 MPa and then fibers (cellulose, hemicellulose, lignin, etc.) are sen- stress was reduced. Hysteresis loops were recorded sitive to different temperature ranges. Lignin decom- and the relative damping values were determined poses slower and has a wider range of decomposition according to the Eq. 4: temperatures (200–500 C) than cellulose, hemicellu- lose and other biomass components. The initial DW T ¼  100% ð4Þ decomposition temperatures of lignin and hemicellu- sw DW ibel lose are similar (around 200 C), but the weight loss is where: T —relative damping, DW —the difference greater for lignin. Cellulose degradation occurs sw i between the compression work and the work during between 315 and 390 C, and the maximum degrada- reducing the compressive stresses, W —compres- tion rate is observed at 355 C (Yang et al. 2007). The ibel sion work. thermal stability of the fiber can be increased by The morphology of composites samples (10 phr removing some of the hemicellulose and lignin by straw) and the dispersion of the filler were evaluated various chemical methods. by means of Scanning Electron Microscopy with field Thermogravimetric tests performed for fillers used emission Hitachi TM-1000 (Japan). at work revealed large changes in thermal properties. Thermal analysis (DTG) showed an endothermic peak in the range of 40–120 C, associated with the removal Results and discussion of moisture contained in the fibers. The mass loss for unmodified fibers was 6.1% and was much higher than Characteristic of fillers for peroxide-modified straw, where water loss was 4.3%. This could be the result of the increased Thermogravimetric analysis hydrophobicity of the fibers through chemical treat- ment, making the material less hygroscopic. The Thermogravimetric analysis (TGA) is a very useful results showed that the thermal degradation of the thermal analysis technique to investigate the thermal unmodified straw began at around 180 C and became stability of a material. Figures 3 and 4 show the results fast at around 260 C. The maximum filler weight loss of the TGA performed on the pure and modified straw. rate was reached at 330 C on the DTG curve, this The thermal stability of the filler is an important phenomenon is the result of degradation of the issue in the development of elastomeric composites in lignocellulosic material. The applied modifications Fig. 3 TGA thermograms of modified and pure straw showing mass loss 123 Cellulose Fig. 4 DTG curves determined for straw -1 significantly improved the thermal resistance of straw, The vibration peak at 1500 cm , assigned to the the initial temperature of fiber distribution increased, benzene ring vibration of lignin. CH , CH symmetric -1 thermal stability extended to a temperature of about bending peaks at 1425 and 1360 cm are also 220 C, for straw modified with benzoyl peroxide and present. Vibrations of C–O groups in ring -1 dicumyl. A narrower distribution of the peak related to 1320 cm are characteristic for cellulose. -1 the degradation of the lignocellulosic material and the The peak at 1230 cm , which is C=O stretch of shift of the peak (maximal decomposition rate) to a acetyl group of lignin was reduced after modification. -1 temperature of 360 C was observed. This is probably The intense peak near 1030 cm correlated with C– the reason for the partial removal of lignin and O–C bonds. Pre-alkali treatment is expected to reduce hemicellulose from the straw, which have lower lignin and hemicellulose content in straw. It was noted thermal stability than cellulose. a decreasing of the peak relative to C=O stretching and -1 C–O–C asymmetrical stretching (1030 cm ) charac- Fourier transform infrared (FTIR) analysis teristic for hemicelluloses and C=C aromatic -1 (1500 cm ) and C–O aryl group vibrations -1 In this study, the effects of peroxide treatment on straw (1230 cm ) typical for lignin. These results indicate fibers structure were investigated. Changes regarding that modification leads to the partial removal of lignin treated and untreated materials by FTIR measurement, and hemicellulose. were presented. The FTIR spectra of pure and modified straws are showed in Fig. 5. Characteristic of biocomposites The broad absorption band observed in the -1 3300 cm was related to the hydrogen bonding Rheometric properties (OH) stretching vibration. The intensity of this peak is weaker in the spectrum after modification. Vibration The influence of straw filler type and content on curing -1 peaks at 2900 and 2870 cm , corresponded to the properties of biocomposites is presented in Table 2 stretching of the CH and CH aliphatic group, and Fig. 6. The vulcanization characteristics -1 respectively. The band for 1720 cm is attributed to expressed in terms of the minimum torque value the C=O stretching of the acetyl groups of hemicel- (M ), torque increase (DM), scorch time (t ) and min S2 lulose and lignin [16]. It is assumed that the removal of optimum cure time (t ). hemicellulose from the straw surfaces makes this peak The minimum torque and torque gain increased -1 disappear. The peaks detected at 1650 and 1620 cm with a higher content of lignocellulosic material, is correlated with the carbonyl group of the acetyl ester regardless of its type. The increase in these values in hemicellulose and the carbonyl aldehyde in lignin. resulting from the limited mobility of polymer chains 123 Cellulose Fig. 5 FTIR spectra of pure 0.4 and treated (benzoyl peroxide and dicumyl 0.3 peroxide) straw Straw_Untreated 0.3 Straw_Dicumyl 0.2 peroxide 0.2 Straw_Benzoyl peroxide 0.1 0.1 0.0 0.0 400 900 1,400 1,900 2,400 2,900 3,400 3,900 -1 Wave number [cm ] Table 2 Rheometric Concentration of straw (phr) M (dNm) DM (dNm) min characteristics of rubber mixtures filled with straw Ref. sample (NR) 0 0.6 4.95 Untreated straw 10 0.65 5.61 20 0.72 6.50 30 0.79 7.42 Benzoyl peroxide straw 10 0.70 5.68 20 0.75 6.92 30 0.82 8.15 Dicumyl peroxide straw 10 0.71 5.68 20 0.78 6.87 30 0.89 8.05 Fig. 6 The influence of 0.49 2.13 straw filler type and content on scorch time (t ) and 0.49 2.08 S2 optimum cure time (t ) 10 0.49 2.06 30 0.52 2.29 t2s t90 20 0.52 2.21 0.49 2.10 0.49 2.23 0.46 2.06 10 0.47 1.97 Ref. sample (NR) 0.51 2.87 00.5 11.5 22.5 33.5 4 t[min] is related to the rubber-filler interactions and cross- has a significant impact on the viscosity, stiffness and linking density of biocomposites. This phenomenon shear modulus of vulcanizates. The lowest values of Absorbance [-] Benzoyl Dicumyl Untreated peroxide peroxide straw straw straw Cellulose the minimum torque were characterized by NR/ density of biocomposites, measurements of equilib- untreated straw biocomposites. Surface modification rium swelling were carried out. The results are given presumably increased rubber–filler interfaces and in Table 3. filler–filler interactions influencing the rheological Crosslinking density of vulcanizates increased with characteristics of the composites. the degree of filling, regardless of the type of straw Scorch time, which is the measure of premature used. The effect of straw modification on the m value vulcanization and optimum cure of NR vulcanizates was varied. The addition of 10 phr of modified filler decreased for filled composites. With the increase in increased the concentration of network nodes in the the filler content and regardless of the type of straw system. However, the cross-linking density of com- used, a slight increase in the vulcanization time was posites filled with 20, 30 phr of modified straw observed. decreased compared to vulcanizates containing pure From the economical point of view of the process- straw. ing of polymer products, the kinetics of cross-linking, The increase in the overall cross-linking density of including the vulcanization time is of great impor- the system results in improved stiffness and strength of tance. It should be emphasized that the addition of the composites as the filler–polymer interaction straw filler allows obtaining composites with a increases. In order to examine the impact of straw vulcanization time reduced by 1/3 in relation to the modifications and the fillers content on mechanical reference sample. properties of biocomposites a number of strength parameters were determined. Mechanical properties and crosslinking density Static mechanical properties The tensile strength of Mechanical properties of fiber-reinforced composites biocomposites containing pure and peroxides are influenced by the strong adhesion of fiber and modified straw in different contents is shown in polymer matrix. The filler–polymer interactions Fig. 7, stress–strain curves are also included (Figs. 8, depends on the amount and type of filler and its 9, 10). characteristic features. The properties of biocompos- The tensile strength of vulcanizates containing ites with natural fibers are determined by the diameter unmodified straw increased with higher filler content and length of the fiber, as well as its content and up to 20 phr. The addition of 30 phr resulted in a distribution. The shape and quality of fiber surface also decrease in TS to a value comparable to an unfilled influence the composite’s strength. Due to the large system. For composites containing 10 phr of the filler, diversity in the structure and morphology of bio- the beneficial effects of the modifications applied were fillers, the functional properties of composites, includ- visible. Treatment of straw with peroxides resulted in ing mechanical ones, can undergo significant changes. an increase in tensile strength of 1.7 MPa for NR/ benzoyl peroxide straw and of 3.9 MPa for NR/ Crosslinking density The type of rubber and the dicumyl peroxide straw, compared to reference degree of crosslinking as well as the properties of the sample. medium in which the filler is dispersed are equally The results indicated that the incorporation of fibers important for strengthening effect. The interactions at into the rubber medium increased the stiffness of the the rubber-filler interfaces play a significant role in the composites. Modification of the filler influenced the reinforcing effect, since the mechanism of transferring mechanical properties increment probably as a result stresses during deformation depends on them. The of a better interactions between the fiber and the interactions between the elastomer and the filler act as polymer. However, the use of higher content of physical network nodes and constitute additional modified straw contributed to the reduction of tensile elements of the spatial network of vulcanizates. strength values. This may be due to an increase in the Accordingly, the addition of the filler to the polymer filler activity with respect to intra-molecular interac- matrix affects the overall crosslinking density tions leading to agglomeration of the fiber particles characteristic. and deterioration of the degree of dispersion of the In order to examine the impact of straw modifica- filler in the rubber matrix. Reflecting the obtained tions as well as the fillers content on the crosslinking results are measurements of equilibrium swelling, in 123 Cellulose Table 3 The influence of straw filler type and content on crosslinking density 5 3 Content of straw (phr) c 10 (cm /mol) Reference sample (NR) Untreated straw Benzoyl Peroxide straw Dicumyl Peroxide straw 0 1.30 – – – 10 – 1.91 1.93 2.13 20 – 2.57 2.19 2.33 30 – 2.90 2.83 2.73 20.0 16.0 12.0 8.0 4.0 0.0 Ref. sample 10 [phr] 20 [phr] 30 [phr] (NR) Ref. sample (NR) 13.5 Untreated straw 15.3 17.5 14.3 Benzoyl peroxide straw 17.0 14.1 11.4 Dicumyl peroxide straw 19.2 14.1 10.6 Fig. 7 Effects of straw filler type and content on tensile strength Fig. 8 Stress–strain curves of natural rubber composites filled with 10 phr straw TS [MPa] Cellulose Fig. 9 Stress–strain curves of natural rubber composites filled with 20 phr straw Fig. 10 Stress–strain curves of natural rubber composites filled with 30 phr straw which an increase in cross-linking density was also of 47Sh A was obtained for 30 phr filler amount, and observed at the content of 10 phr of modified straw. this value is higher than in case of composites with 10 Biocomposites of natural rubber were characterized phr fibres (34Sh A) and unfilled system (26Sh A). by high strain, Eb value was in the range of 450–615%. Incorporation of straw into natural rubber reduces The elongation at break of the vulcanizates decreased elasticity of the rubber chains, which leads to more with the amount of filler used. rigid rubber composites than in case of unfilled vulcanized rubber. Hardness measurement Hardness (Fig. 11)isa measure of resistance to applied deformation. In Tear strength The tear resistance (or tear strength) is rubber it is related to degree of crosslinking and resistance to the growth of a cut or nick in a vulcanized amount of filler. Addition of filler and higher (cured) rubber specimen when tension is applied. Tear crosslinking density increased hardness of resistance is an important consideration, both as the composites, because the straw fibres lead to finished material is being removed from the mold and reinforcement of the samples. The maximum value as it performs in actual service. 123 Cellulose 50.0 40.0 30.0 20.0 10.0 0.0 Ref. sample 10 [phr] 20 [phr] 30 [phr] (NR) Ref. sample (NR) 26.2 Untreated straw 34.3 38.8 45.3 Benzoyl peroxide straw 34.8 38.9 43.8 Dicumyl peroxide straw 35.1 39.1 47.0 Fig. 11 Influence of different straw fillers content on the hardness of the vulcanizates Fig. 12 The influence of Ref. sample (NR) Untreated straw straw filler type and content Benzoyl peroxide straw Dicumyl peroxide straw 6.0 on tear strength 4.9 4.6 4.6 4.6 4.6 5.0 4.3 3.9 3.4 4.0 3.6 3.0 3.0 2.0 1.0 0.0 Ref. sample (NR) 10 20 30 Concentration of straw [phr] The value of tear strength for composites contain- Damping properties The relative damping test ing cereal straw depends on the modification method consists in cyclically compressing the sample from 0 (Fig. 12). The vulcanizates containing the pure filler to a specified stress value. The recorded hysteresis exhibited a decrease in tear strength compared to the loop is a manifestation of internal friction and energy dissipation due to compression of an elastic solid. unfilled system. However, the decrease was the smaller, the higher the filler content. In contrast, the During the cyclic loading, part of the energy is dispersed, which makes it possible to determine the use of modified straw has positively influenced the tearing resistance of biocomposites. The treated filler attenuation coefficient. The research aims to achieve elastomeric composites that strongly dissipate energy may be characterized by a lower activity, and also show a greater tendency to accumulate in larger thus exhibit better damping properties. clusters (aggregates, agglomerates), which results a The conducted research shows that polymer bio- lack of material homogeneity caused by poor disper- composites filled with straw showed a higher relative sion. The aggregates or agglomerates formed by the damping factor compared to the unfilled system filler cause concentration of stresses. As a result of the (Fig. 13). This increase was clearly visible with the applied force during the test, the measured Fmit value increased amount of straw added. For 30 phr of pure straw, the value of Ttw was more than doubled. A increases. The observable effect is the improvement of the tearing resistance of the vulcanizates produced. further improvement in damping properties over the Hardness [°Sh A] F mit [N/mm] Cellulose 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 Ref. sample 10 [phr] 20 [phr] 30 [phr] (NR) Ref. sample (NR) 3.2 Untreated straw 4.1 4.8 6.6 Benzoyl peroxide straw 4.5 6.3 8.3 Dicumyl peroxide straw 6.7 6.6 8.3 Fig. 13 Influence of different straw fillers content on the damping of the biocomposites unmodified system can be observed both for the BPS resistance of vulcanizates to degradation under the and DCPS containing vulcanizates. influence of thermo-oxidative factors. Thermooxidative aging process Dynamic mechanical analysis (DMA) During exploitation, rubber products are exposed to The dynamic parameters such as storage modulus (E ), various environmental factors that lead to their loss modulus (E ), and loss factor (tan d) are degradation. Increased temperature and oxygen con- temperature dependent and provide information about tained in the air initiate the aging process of the interfacial bonding between the filler and polymer material. An important aspect is to examine how the matrix of composite material. The influence of differ- 0 00 changes in the composition of rubber mixtures can ent type of straw on the values of E ,E , tan d as a affect the material’s resistance to this type of process. function of temperature are given in Fig. 15, 16 and This effect was determined on the basis of changes in 17. 0 00 mechanical properties of vulcanizates after thermo- The lowest values of E ,E in the glassy region oxidative aging. The numerical measure of degrada- were observed for unfilled NR, the addition of straw to tion processes simulation is the coefficient K. The NR and treatment of the filler increased both loss obtained results are presented in Fig. 14. modulus and storage modulus of composites. The Natural rubber as a compound containing unsatu- highest values of storage and loss modulus were rated bonds in its structure and is characterized by poor measured for samples filled with benzoyl peroxide resistance to polymer chain degradation processes. straw and dicumyl peroxide straw. These results Thermo-oxidative aging caused unfavorable changes correspond correctly to the previous results of the in the properties of NR vulcanizates, as evidenced by crosslinking density and mechanical properties of values of the K coefficient of less than unity. This is obtained biocomposites. most likely the effect of changes in the polymer By definition tan d is the ratio of dissipated energy structure, leading to degradation of the material. The (loss modulus E ) to stored energy (storage modulus introduction of a natural filler did not cause further E ). One of the most important parameters affecting aging of the material. In contrast, the addition of tan d is the adhesion between the matrix and the filler. modified straw caused a slight improvement in the High interfacial energy limits the mobility of the polymer chains, thus reducing the tan d values, while Tτw [%] Cellulose 1.00 0.80 0.60 0.40 0.20 0.00 Ref. sample 10 [phr] 20 [phr] 30 [phr] (NR) Ref. sample (NR) 0.69 Untreated straw 0.70 0.70 0.77 Benzoyl peroxide straw 0.77 0.83 0.84 Dicumyl peroxide straw 0.74 0.84 0.85 Fig. 14 Thermal aging factor of composites filled with straw (0–30 phr) Ref. Sample (NR) NR+ 10 phr Straw_Untreated NR+ 10 phr Straw_Benzoyl peroxide 400 NR+ 10 phr Straw_Dicumyl peroxide -120 -110 -100 -90 -80 -70 -60 -50 -40 T[°C] Fig. 15 Loss modulus of studied biocomposites as a function of temperature the weak one leads to higher tan d values. (Krishna and matrix. For better understanding the interfacial inter- Kanny 2016). actions between NR and used fillers, the adhesion The tan d values of composites filled with straw factor (A) was calculated from DMA data according to max are lower than the reference sample. The lowest peak literature (Wei et al. 2013; Formela et al. 2016; values were observed for vulcanizates containing Akindoyo et al. 2017) using Eq. (5): modified cereal straw. There are a number of factors 1 tan d that may affect tan d of composites including: fiber A ¼   1 ð5Þ max 1  V tan d f r distribution, adhesion between fibers and polymer matrix, shear stress concentration and dissipation of where V was the volume fraction of the fiber (0.1), tan viscoelastic energy. (Pothan et al. 2003) (Shanmugam d was maximum tan d peak of the NR biocomposites and Thiruchitrambalam 2013). Therefore, the maxi- and tan d was maximum tan d peak of the unfilled NR mum of the tan d peak height can be used to determine (reference sample). interfacial adhesion between the fiber and the polymer logE''[MPa] K[ -] Cellulose Fig. 16 Storage modulus of biocomposites filled with 10 phr straw as a function of temperature Fig. 17 Temperature 2.5 Ref. Sample (NR) dependence of tan d for reference sample and biocomposites NR+ 10 phr Straw_Untreated NR+ 10 phr Straw_Benzoyl peroxide 1.5 NR+ 10 phr Straw_Dicumyl peroxide 0.5 -150 -125 -100 -75 -50 -25 0 25 Temperature [°C] Low value of the coefficient A (see Table 4) means creation of additional network nodes, which resulted high level of interface adhesion and enhanced inter- in a more efficient transfer of loads. As a result, an actions between the elastomer and filler particles. The increase in the crosslinking density of vulcanizates used straw modification contributed to the increase in filled with a modified filler was observed, resulting in the interaction between the polymer–filler. improved mechanical properties of DCPS/NR and The increase in straw adhesion to the elastomeric BPS/NR composites. matrix may be due to better dispersion and higher Glass transition temperature (Tg) of prepared surface activity of the filler. The improvement of vulcanizates was determined as the position of the interfacial interactions in the composite led to the maximum on the loss tangent curve (tan d) versus tan δ [-] Cellulose Table 4 Glass transition temperature (Tg), tan d and adhesion factor for reference sample (NR) and untreated or treated straw max composites Ref. sample NR ? 10 phr NR ? 10 phr Benzoyl NR ? 10 phr Dicumyl (NR) Untreated straw peroxide straw peroxide straw Tg (with respect to - 69.40 - 68.76 - 68.70 - 68.45 tan d )(C) max tan d (-) 2.23 2.41 2.30 2.27 max A(-)– - 0.03 - 0.07 - 0.08 Fig. 18 SEM images of NR composites containing 10 phr a untreated straw, b straw modified with dicumyl peroxide, c straw modified with benzoyl peroxide temperature. The values of Tg are presented in temperatures can be associated with the decreased Table 4. The shift in Tg can be observed for compos- mobility of NR chains by the addition of fillers. ites filled with straw compared to unfilled systems. The shifting of glass transition temperature to higher 123 Cellulose Scanning electron microscopy the presence of untreated straw and can be further improved with treated fillers. The influence of the pure and treated filler on Application of cereal straw as biofillers for NR morphology of biocomposites (10 phr of straw) is vulcanizates, is highly promising in terms of their presented in Fig. 18. subsequent commercial use. Moreover significant The purpose of the used filler modifications was to multifunctional properties of the materials can be increase interfacial filler–polymer interactions. Ana- reached by peroxide modifications. In summary, lyzing the obtained scanning electron microscope solving typical agricultural problems related to the (SEM) images of selected composites, the influence of use and management of straw is an extremely impor- applied straw treatments on the improvement of its tant challenge. By combining knowledge in the field of dispersion degree in natural rubber was observed. This chemistry and technology of polymeric materials, it is factor affects the filler activity, the more homogeneous possible to obtain valuable ecological and economic the distribution of the straw particles in the polymer, benefits. the larger the contact area with the elastomeric matrix. Open Access This article is distributed under the terms of the The degree of disperse of the filler particles has a Creative Commons Attribution 4.0 International License (http:// significant impact on its strengthening effect, and thus creativecommons.org/licenses/by/4.0/), which permits unrest- the mechanical properties of the vulcanizates. ricted use, distribution, and reproduction in any medium, pro- Figure 18a represents a SEM picture of a vulcan- vided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and izate containing pure cereal straw, where the particles indicate if changes were made. tend to agglomerate. Straw agglomerates with a size of several lm are clearly visible. Figure 18b, c shows SEM photographs of vulcanizates containing a mod- References ified filler. Its addition resulted in better dispersion of the straw particles in the polymer matrix. As a result, Akindoyo JO, Beg MDH, Ghazali S et al (2017) Effects of vulcanizates with improved tensile strength and surface modification on dispersion, mechanical, thermal and dynamic mechanical properties of injection molded crosslinking density have been obtained. PLA-hydroxyapatite composites. Compos Part A Appl Sci Manuf 103:96–105. https://doi.org/10.1016/j.compositesa. 2017.09.013 Conclusions Ali A, Shaker K, Nawab Y et al (2016a) Hydrophobic treatment of natural fibers and their composites—a review. J Ind Text. https://doi.org/10.1177/1528083716654468 Straw is a lignocellulosic material with huge applica- Ali A, Shaker K, Nawab Y et al (2016b) Hydrophobic treatment tion potential, not yet covered in applications in of natural fibers and their composites—a review. J Ind elastomeric composites. 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Influence of peroxide modifications on the properties of cereal straw and natural rubber composites

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Chemistry; Bioorganic Chemistry; Physical Chemistry; Organic Chemistry; Polymer Sciences; Ceramics, Glass, Composites, Natural Materials; Sustainable Development
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10.1007/s10570-018-1880-6
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Abstract

Keywords Peroxide modification  Natural rubber  a linear chain of 1,4-b-bonded anhydroglucose units Alkalization  Straw  Biocomposites which contains alcoholic hydroxyl groups; hemicel- lulose—heterogeneous polysaccharides, located between the lignin and cellulose fibres; lignin— phenolic polymeric material, formed from phenolic Introduction precursors such as p-hydroxycinnamyl alcohols, p-coumaryl alcohol, coniferyl alcohol and sinapyl Natural straw fibers can be considered as an environ- alcohol; pectin, waxes and water soluble substances. mentally friendly reinforcement for composite mate- Natural fibers being hydrophilic are incompatible with rials, having potential of use in various applications by the hydrophobic polymer matrix. Fiber–matrix inter- their economic, technical and environmental concerns face is affected by hydrophilic nature of cellulosic (Masłowski et al. 2017). Nowadays different cellulose fibers (Saheb and Jog 2015). In addition to this, waxy fibers such as flax, hemp, sisal, cotton, kenaf, jute, and pectin substances cap the reactive functional bamboo, coconut and date palm, which have devel- groups of the fiber and act as a hindrance to polymer- oped over the last decades, offer a number of filler interactions. Insufficient adhesion between advantages over synthetic fibers (mainly glass, carbon, hydrophobic polymers and hydrophilic fibers result plastic) due to their renewable nature (Mwaikambo in poor mechanical properties of the natural fiber and Ansell 2002; Bernard et al. 2011). Major unique reinforced polymer composites. As the natural fibers properties (but not limited) of natural fibers include contain hydroxyl groups from cellulose and lignin, among others: low cost, biodegradability, recyclabil- their surface needs to be modified with different ity, low density, good thermal properties, reduced tool chemical treatments to enhance the effectiveness of wear, non-irritation to the skin, and enhanced energy interfacial bonding (Ali et al. 2016b; Militky ´ and recovery (Thakur et al. 2014; Azam et al. 2016). Jabbar 2015). Recently, mercerization, acrylation, The components of natural straw fibers include: acetylation as well as isocyanate, permanganate, cellulose—a hydrophilic glucan polymer consisting of silane and peroxide treatments of natural fibers have 123 Cellulose been adopted as successful improvement of fiber Unfortunately, the problem of utilization of straw is strength and reported in the literature (Li et al. 2007; still not fully resolved and requires search for new Kaushik et al. 2013; Valadez-Gonzalez et al. 1999; opportunities and technologies of its use. Ray et al. 2002; Herrera-Franco and Valadez-Gonza ´- The objective of this study was to investigate the lez 2005; Wong et al. 2010). influence of peroxide modification on the properties of Sodium hydroxide (NaOH) is the most commonly cereal straw and to developed and characterized a new used chemical for bleaching and/or cleaning the type of material: straw treated natural rubber com- surface of plant fibers. This treatment removes lignin, posites. Effect of these treatments on morphology and pectin, waxy substances, and natural oils covering the thermal stability of natural lignocellulose fibers is external surface of the fiber cell wall. It also changes meagerly reported in literature as well as the properties the fine structure of cellulose I to cellulose II, of bio-composites filled with them. Achieving optimal depolymerizing the native cellulose structure and mechanical and utility properties was the primary exposes short length crystallites (Fernandes et al. purpose of the researches. Which depends on the 2011; Collett et al. 2015; Andersson et al. 2003). reinforcement of the surface area of the fibers/matrix. Based on the acquired knowledge, NaOH treatment is In polymer technology, the use of an appropriate currently used as a pre-modification before the main medium is a key issue and forms the basis of its modifications of cellulosic fibers. subsequent, relevant properties. In comparison to thermoplastic biocomposites, widely described in the Peroxide treatments have attracted the attention of the majority of researchers in the field of cellulose literature, the use of new, elastomeric matrix is fiber processing, due to the ease of their implemen- undoubtedly a scientific novelty, both from a cognitive tation with the simultaneous provision of good and application point of view. Furthermore, the aim of mechanical properties. Organic peroxides are easily this work was to investigate peroxide modifications of broken down into free radicals, which then react with cereal straw using dicumyl and benzoyl peroxide with cellulose fiber and also with a hydrogen group matrix previous sodium hydroxide pre-modification on both (Sreekala et al. 2000). During peroxide treatment, fillers and vulcanizates characteristics. fibers are treated with 6% dicumyl peroxide or benzoyl peroxide in acetone solution after previous alkali pre- modification. Fibers treated with dicumyl peroxide Experimental first soak in the solution, then are wash with distilled water, and finally place in oven to improve their Materials hydrophobic properties(Ali et al. 2016a). Nowadays there is a big interest in research Natural rubber (Torimex Chemicals) cis-1,4-polyiso- concentrated on polymer composites of natural bio- prene, density 0.93–0.98 g/cm . materials, including straw. Agricultural crop residues, Sulfur cure system: such as straws represent an important resources, with a • Sulfur (S ) great potential as raw materials for polymers compos- • Zinc oxide (ZnO) ites (Matsumura et al. 2005; Talebnia et al. 2010). The • 2-mercaptobenzothiazole (MBT) structural advantage of straw as compared to other • Stearic acid (SA) annual plant materials is its resemblance to wood composition (Kijen ´ ski et al. 2016). As a result, Biofillers: pure straw (PS), dicumyl peroxide composites filled with straw are expected to have modified straw (DCPS), benzoyl peroxide modified good mechanical properties (Scarlat et al. 2015). straw (BPS). Utilization of wheat straw for industrial application Cereal straws (wheat, rye, triticale, barley and oat) leads to substantial consumption of the straw, as well were obtained from local farms (Poland). as reducing the cost of the product as it is available at The straw was prepared as follows: low cost compared to other bast fibers such as juta • drying at 70 C, (Zou et al. 2010). Researches on the use of straw in • cut into 10 mm fiber biocomposites focus mainly on thermoplastic poly- mers consisting of rapeseed and rice straw as a fibres. 123 Cellulose • grinding using a ball mill for 0.5 h (SPEX nitrogen flow - 60 mL/min, heating rate - 10 C/ SamplePrep 8000D Mixer/Mill) min. • modified as follows: Rubber compounds were prepared using a Braben- der measuring mixer N50 at 50–60 C. The mixing Pre-modification First straws were soaked in 5% time equaled 12 min which included 4 min of masti- (by weight) solution of sodium hydroxide for 2 h at cation of natural rubber, 4 min of mixing NR with the room temperature. After treatment, materials were biofillers (PS, DCPS, DBS) and 4 min of mixing the thoroughly washed with water several times in order to blend with the sulfur curing system. Next, two–roll remove NaOH residual, until a pH of 7 was reached. mill was used to obtain rubber sheets. Next the straws were dried in hot air oven at 70 Cto The rheometric properties and kinetics of elastomer constant weight. Reaction is shown in Fig. 1. mixtures vulcanization were tested using MonTech Main modifications The peroxide treatment was DRPA 300 Rheometer at 160 C. The measurements carried out on an alkali pretreated straw using benzoyl were studied according to ISO 3417. Next, the samples and dicumyl peroxide, reaction shown in Fig. 2. were cured at 15 MPa, until they developed a 90% Pretreated lignocellulosic materials were modified increase in torque, according to rheometric with: measurements. • 6% benzoyl peroxide in acetone for 120 min at The tensile properties and tear strength tests of the 25 C, vulcanizates were measured according to ISO-37 and • 6% dicumyl peroxide in acetone for 120 min at ISO 34 standard, respectively using a universal 25 C, machine (Zwick, Ulm, Germany). The strain rate of tensile tests was 500 mm/min. The elongation at break Next the straws were rinsed with distilled water and measurements were examined using an extensometer then dried in an oven at 70 C. with sensor arms. The tensile strength carried out for The compositions of the tested elastomer mixture five standard dumbbell-shaped samples. The stress– are presented in Table 1. strain curves was plotted. Tear strength tests were performed at a cross-head speed of 50 mm/min for Methods three ‘‘trousers’’ shape of the samples. Dimensions of the samples: length 100 mm, width 15 mm, and Fourier transform infrared spectra was studied in the thickness 1 mm with a precut of 40 mm at the center. -1 range of 4000–400 cm using an Fourier-transform DMA analysis were determined by means of DMA/ infrared spectroscopy (FTIR) Nicolet 6700 spec- SDTA861e analyzer (Mettler Toledo). Test parame- trophotometer. Tools and process parameters: a single ters: temperature range - 150–60 C, heating rate reflection diamond ATR crystal on ZnSe plate, DTGS/ - 3 C/min, frequency - 1 Hz, strain amplitude -1 KBr detector, 128 scans; resolution—8 cm . - 0.05%. The thermal stability of the straws was examined Hardness of biocomposites was studied using a using a TGA/DSC1 (Mettler Toledo) analyzer. Sam- Shore type A Durometer (Zwick/Roell). The mea- ples (total weight in the 8–10 mg range) were placed surements were determined according to ISO 868 into alumina crucibles and heated from 25 to 600 Cin standard. At least 10 points were tested for each a nitrogen atmosphere. Measurement parameters: sample. Fig. 1 Reaction of cellulose fiber with NaOH 123 Cellulose Fig. 2 Reaction of lignocellulose fiber with benzoyl (a)/dicumyl peroxide (b) Table 1 The compositions Straw NR SA ZnO MBT Sulfur of biocomposites and (phr) (phr) (phr) (phr) (phr) (phr) reference sample Ref. sample (NR) 0 100 1 5 2 2 Untreated straw 10 100 1 5 2 2 Benzoyl peroxide straw 10 100 1 5 2 2 Dicumyl peroxide straw 10 100 1 5 2 2 Phr parts per houndred parts of rubber To analyze the crosslinking density (c ), samples of V —the volume fraction of elastomer in the swollen 20–50 mg were swollen to equilibrium in toluene at gel. room temperature. Next, the samples were dried in air The thermo-oxidative ageing was performed at a oven at 50 C to constant weight. The (c ) value was temperature of 70 C for 14 days in a dryer with calculated on basis of the Flory-Rehner (Flory and thermo-circulation. The ageing coefficient (K) was Rehner 1943) (Eq. 1): determined as the numerical change in the static mechanical properties of the samples upon degrada- lnðÞ 1  V þ V þ lV r r c ¼ ð1Þ e 1 tion process (Eq. 3): 3 r V V 0 r K ¼ðÞ TS  E =ðÞ TS  E ð3Þ b b after aging before aging where l—the Huggins parameter of the NR-solvent where E —elongation at break, TS—tensile strength. interaction, was calculated from the Eq. 2: The relative damping of vulcanizes were investi- l ¼ 0:478 þ 0:228  V ð2Þ r gated under the influence of compressive stress using a 123 Cellulose ZWICK 1435 (Zwick, Ulm, Germany) universal the production process (cross-linking, extrusion or machine. Dimensions of the sample (disc shape): injection molding) as well as in the subsequent use of diameter—35 mm, height—17.8 mm. During the test end products. The components of lignocellulosic samples were stressed from 0 to 0,7 MPa and then fibers (cellulose, hemicellulose, lignin, etc.) are sen- stress was reduced. Hysteresis loops were recorded sitive to different temperature ranges. Lignin decom- and the relative damping values were determined poses slower and has a wider range of decomposition according to the Eq. 4: temperatures (200–500 C) than cellulose, hemicellu- lose and other biomass components. The initial DW T ¼  100% ð4Þ decomposition temperatures of lignin and hemicellu- sw DW ibel lose are similar (around 200 C), but the weight loss is where: T —relative damping, DW —the difference greater for lignin. Cellulose degradation occurs sw i between the compression work and the work during between 315 and 390 C, and the maximum degrada- reducing the compressive stresses, W —compres- tion rate is observed at 355 C (Yang et al. 2007). The ibel sion work. thermal stability of the fiber can be increased by The morphology of composites samples (10 phr removing some of the hemicellulose and lignin by straw) and the dispersion of the filler were evaluated various chemical methods. by means of Scanning Electron Microscopy with field Thermogravimetric tests performed for fillers used emission Hitachi TM-1000 (Japan). at work revealed large changes in thermal properties. Thermal analysis (DTG) showed an endothermic peak in the range of 40–120 C, associated with the removal Results and discussion of moisture contained in the fibers. The mass loss for unmodified fibers was 6.1% and was much higher than Characteristic of fillers for peroxide-modified straw, where water loss was 4.3%. This could be the result of the increased Thermogravimetric analysis hydrophobicity of the fibers through chemical treat- ment, making the material less hygroscopic. The Thermogravimetric analysis (TGA) is a very useful results showed that the thermal degradation of the thermal analysis technique to investigate the thermal unmodified straw began at around 180 C and became stability of a material. Figures 3 and 4 show the results fast at around 260 C. The maximum filler weight loss of the TGA performed on the pure and modified straw. rate was reached at 330 C on the DTG curve, this The thermal stability of the filler is an important phenomenon is the result of degradation of the issue in the development of elastomeric composites in lignocellulosic material. The applied modifications Fig. 3 TGA thermograms of modified and pure straw showing mass loss 123 Cellulose Fig. 4 DTG curves determined for straw -1 significantly improved the thermal resistance of straw, The vibration peak at 1500 cm , assigned to the the initial temperature of fiber distribution increased, benzene ring vibration of lignin. CH , CH symmetric -1 thermal stability extended to a temperature of about bending peaks at 1425 and 1360 cm are also 220 C, for straw modified with benzoyl peroxide and present. Vibrations of C–O groups in ring -1 dicumyl. A narrower distribution of the peak related to 1320 cm are characteristic for cellulose. -1 the degradation of the lignocellulosic material and the The peak at 1230 cm , which is C=O stretch of shift of the peak (maximal decomposition rate) to a acetyl group of lignin was reduced after modification. -1 temperature of 360 C was observed. This is probably The intense peak near 1030 cm correlated with C– the reason for the partial removal of lignin and O–C bonds. Pre-alkali treatment is expected to reduce hemicellulose from the straw, which have lower lignin and hemicellulose content in straw. It was noted thermal stability than cellulose. a decreasing of the peak relative to C=O stretching and -1 C–O–C asymmetrical stretching (1030 cm ) charac- Fourier transform infrared (FTIR) analysis teristic for hemicelluloses and C=C aromatic -1 (1500 cm ) and C–O aryl group vibrations -1 In this study, the effects of peroxide treatment on straw (1230 cm ) typical for lignin. These results indicate fibers structure were investigated. Changes regarding that modification leads to the partial removal of lignin treated and untreated materials by FTIR measurement, and hemicellulose. were presented. The FTIR spectra of pure and modified straws are showed in Fig. 5. Characteristic of biocomposites The broad absorption band observed in the -1 3300 cm was related to the hydrogen bonding Rheometric properties (OH) stretching vibration. The intensity of this peak is weaker in the spectrum after modification. Vibration The influence of straw filler type and content on curing -1 peaks at 2900 and 2870 cm , corresponded to the properties of biocomposites is presented in Table 2 stretching of the CH and CH aliphatic group, and Fig. 6. The vulcanization characteristics -1 respectively. The band for 1720 cm is attributed to expressed in terms of the minimum torque value the C=O stretching of the acetyl groups of hemicel- (M ), torque increase (DM), scorch time (t ) and min S2 lulose and lignin [16]. It is assumed that the removal of optimum cure time (t ). hemicellulose from the straw surfaces makes this peak The minimum torque and torque gain increased -1 disappear. The peaks detected at 1650 and 1620 cm with a higher content of lignocellulosic material, is correlated with the carbonyl group of the acetyl ester regardless of its type. The increase in these values in hemicellulose and the carbonyl aldehyde in lignin. resulting from the limited mobility of polymer chains 123 Cellulose Fig. 5 FTIR spectra of pure 0.4 and treated (benzoyl peroxide and dicumyl 0.3 peroxide) straw Straw_Untreated 0.3 Straw_Dicumyl 0.2 peroxide 0.2 Straw_Benzoyl peroxide 0.1 0.1 0.0 0.0 400 900 1,400 1,900 2,400 2,900 3,400 3,900 -1 Wave number [cm ] Table 2 Rheometric Concentration of straw (phr) M (dNm) DM (dNm) min characteristics of rubber mixtures filled with straw Ref. sample (NR) 0 0.6 4.95 Untreated straw 10 0.65 5.61 20 0.72 6.50 30 0.79 7.42 Benzoyl peroxide straw 10 0.70 5.68 20 0.75 6.92 30 0.82 8.15 Dicumyl peroxide straw 10 0.71 5.68 20 0.78 6.87 30 0.89 8.05 Fig. 6 The influence of 0.49 2.13 straw filler type and content on scorch time (t ) and 0.49 2.08 S2 optimum cure time (t ) 10 0.49 2.06 30 0.52 2.29 t2s t90 20 0.52 2.21 0.49 2.10 0.49 2.23 0.46 2.06 10 0.47 1.97 Ref. sample (NR) 0.51 2.87 00.5 11.5 22.5 33.5 4 t[min] is related to the rubber-filler interactions and cross- has a significant impact on the viscosity, stiffness and linking density of biocomposites. This phenomenon shear modulus of vulcanizates. The lowest values of Absorbance [-] Benzoyl Dicumyl Untreated peroxide peroxide straw straw straw Cellulose the minimum torque were characterized by NR/ density of biocomposites, measurements of equilib- untreated straw biocomposites. Surface modification rium swelling were carried out. The results are given presumably increased rubber–filler interfaces and in Table 3. filler–filler interactions influencing the rheological Crosslinking density of vulcanizates increased with characteristics of the composites. the degree of filling, regardless of the type of straw Scorch time, which is the measure of premature used. The effect of straw modification on the m value vulcanization and optimum cure of NR vulcanizates was varied. The addition of 10 phr of modified filler decreased for filled composites. With the increase in increased the concentration of network nodes in the the filler content and regardless of the type of straw system. However, the cross-linking density of com- used, a slight increase in the vulcanization time was posites filled with 20, 30 phr of modified straw observed. decreased compared to vulcanizates containing pure From the economical point of view of the process- straw. ing of polymer products, the kinetics of cross-linking, The increase in the overall cross-linking density of including the vulcanization time is of great impor- the system results in improved stiffness and strength of tance. It should be emphasized that the addition of the composites as the filler–polymer interaction straw filler allows obtaining composites with a increases. In order to examine the impact of straw vulcanization time reduced by 1/3 in relation to the modifications and the fillers content on mechanical reference sample. properties of biocomposites a number of strength parameters were determined. Mechanical properties and crosslinking density Static mechanical properties The tensile strength of Mechanical properties of fiber-reinforced composites biocomposites containing pure and peroxides are influenced by the strong adhesion of fiber and modified straw in different contents is shown in polymer matrix. The filler–polymer interactions Fig. 7, stress–strain curves are also included (Figs. 8, depends on the amount and type of filler and its 9, 10). characteristic features. The properties of biocompos- The tensile strength of vulcanizates containing ites with natural fibers are determined by the diameter unmodified straw increased with higher filler content and length of the fiber, as well as its content and up to 20 phr. The addition of 30 phr resulted in a distribution. The shape and quality of fiber surface also decrease in TS to a value comparable to an unfilled influence the composite’s strength. Due to the large system. For composites containing 10 phr of the filler, diversity in the structure and morphology of bio- the beneficial effects of the modifications applied were fillers, the functional properties of composites, includ- visible. Treatment of straw with peroxides resulted in ing mechanical ones, can undergo significant changes. an increase in tensile strength of 1.7 MPa for NR/ benzoyl peroxide straw and of 3.9 MPa for NR/ Crosslinking density The type of rubber and the dicumyl peroxide straw, compared to reference degree of crosslinking as well as the properties of the sample. medium in which the filler is dispersed are equally The results indicated that the incorporation of fibers important for strengthening effect. The interactions at into the rubber medium increased the stiffness of the the rubber-filler interfaces play a significant role in the composites. Modification of the filler influenced the reinforcing effect, since the mechanism of transferring mechanical properties increment probably as a result stresses during deformation depends on them. The of a better interactions between the fiber and the interactions between the elastomer and the filler act as polymer. However, the use of higher content of physical network nodes and constitute additional modified straw contributed to the reduction of tensile elements of the spatial network of vulcanizates. strength values. This may be due to an increase in the Accordingly, the addition of the filler to the polymer filler activity with respect to intra-molecular interac- matrix affects the overall crosslinking density tions leading to agglomeration of the fiber particles characteristic. and deterioration of the degree of dispersion of the In order to examine the impact of straw modifica- filler in the rubber matrix. Reflecting the obtained tions as well as the fillers content on the crosslinking results are measurements of equilibrium swelling, in 123 Cellulose Table 3 The influence of straw filler type and content on crosslinking density 5 3 Content of straw (phr) c 10 (cm /mol) Reference sample (NR) Untreated straw Benzoyl Peroxide straw Dicumyl Peroxide straw 0 1.30 – – – 10 – 1.91 1.93 2.13 20 – 2.57 2.19 2.33 30 – 2.90 2.83 2.73 20.0 16.0 12.0 8.0 4.0 0.0 Ref. sample 10 [phr] 20 [phr] 30 [phr] (NR) Ref. sample (NR) 13.5 Untreated straw 15.3 17.5 14.3 Benzoyl peroxide straw 17.0 14.1 11.4 Dicumyl peroxide straw 19.2 14.1 10.6 Fig. 7 Effects of straw filler type and content on tensile strength Fig. 8 Stress–strain curves of natural rubber composites filled with 10 phr straw TS [MPa] Cellulose Fig. 9 Stress–strain curves of natural rubber composites filled with 20 phr straw Fig. 10 Stress–strain curves of natural rubber composites filled with 30 phr straw which an increase in cross-linking density was also of 47Sh A was obtained for 30 phr filler amount, and observed at the content of 10 phr of modified straw. this value is higher than in case of composites with 10 Biocomposites of natural rubber were characterized phr fibres (34Sh A) and unfilled system (26Sh A). by high strain, Eb value was in the range of 450–615%. Incorporation of straw into natural rubber reduces The elongation at break of the vulcanizates decreased elasticity of the rubber chains, which leads to more with the amount of filler used. rigid rubber composites than in case of unfilled vulcanized rubber. Hardness measurement Hardness (Fig. 11)isa measure of resistance to applied deformation. In Tear strength The tear resistance (or tear strength) is rubber it is related to degree of crosslinking and resistance to the growth of a cut or nick in a vulcanized amount of filler. Addition of filler and higher (cured) rubber specimen when tension is applied. Tear crosslinking density increased hardness of resistance is an important consideration, both as the composites, because the straw fibres lead to finished material is being removed from the mold and reinforcement of the samples. The maximum value as it performs in actual service. 123 Cellulose 50.0 40.0 30.0 20.0 10.0 0.0 Ref. sample 10 [phr] 20 [phr] 30 [phr] (NR) Ref. sample (NR) 26.2 Untreated straw 34.3 38.8 45.3 Benzoyl peroxide straw 34.8 38.9 43.8 Dicumyl peroxide straw 35.1 39.1 47.0 Fig. 11 Influence of different straw fillers content on the hardness of the vulcanizates Fig. 12 The influence of Ref. sample (NR) Untreated straw straw filler type and content Benzoyl peroxide straw Dicumyl peroxide straw 6.0 on tear strength 4.9 4.6 4.6 4.6 4.6 5.0 4.3 3.9 3.4 4.0 3.6 3.0 3.0 2.0 1.0 0.0 Ref. sample (NR) 10 20 30 Concentration of straw [phr] The value of tear strength for composites contain- Damping properties The relative damping test ing cereal straw depends on the modification method consists in cyclically compressing the sample from 0 (Fig. 12). The vulcanizates containing the pure filler to a specified stress value. The recorded hysteresis exhibited a decrease in tear strength compared to the loop is a manifestation of internal friction and energy dissipation due to compression of an elastic solid. unfilled system. However, the decrease was the smaller, the higher the filler content. In contrast, the During the cyclic loading, part of the energy is dispersed, which makes it possible to determine the use of modified straw has positively influenced the tearing resistance of biocomposites. The treated filler attenuation coefficient. The research aims to achieve elastomeric composites that strongly dissipate energy may be characterized by a lower activity, and also show a greater tendency to accumulate in larger thus exhibit better damping properties. clusters (aggregates, agglomerates), which results a The conducted research shows that polymer bio- lack of material homogeneity caused by poor disper- composites filled with straw showed a higher relative sion. The aggregates or agglomerates formed by the damping factor compared to the unfilled system filler cause concentration of stresses. As a result of the (Fig. 13). This increase was clearly visible with the applied force during the test, the measured Fmit value increased amount of straw added. For 30 phr of pure straw, the value of Ttw was more than doubled. A increases. The observable effect is the improvement of the tearing resistance of the vulcanizates produced. further improvement in damping properties over the Hardness [°Sh A] F mit [N/mm] Cellulose 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 Ref. sample 10 [phr] 20 [phr] 30 [phr] (NR) Ref. sample (NR) 3.2 Untreated straw 4.1 4.8 6.6 Benzoyl peroxide straw 4.5 6.3 8.3 Dicumyl peroxide straw 6.7 6.6 8.3 Fig. 13 Influence of different straw fillers content on the damping of the biocomposites unmodified system can be observed both for the BPS resistance of vulcanizates to degradation under the and DCPS containing vulcanizates. influence of thermo-oxidative factors. Thermooxidative aging process Dynamic mechanical analysis (DMA) During exploitation, rubber products are exposed to The dynamic parameters such as storage modulus (E ), various environmental factors that lead to their loss modulus (E ), and loss factor (tan d) are degradation. Increased temperature and oxygen con- temperature dependent and provide information about tained in the air initiate the aging process of the interfacial bonding between the filler and polymer material. An important aspect is to examine how the matrix of composite material. The influence of differ- 0 00 changes in the composition of rubber mixtures can ent type of straw on the values of E ,E , tan d as a affect the material’s resistance to this type of process. function of temperature are given in Fig. 15, 16 and This effect was determined on the basis of changes in 17. 0 00 mechanical properties of vulcanizates after thermo- The lowest values of E ,E in the glassy region oxidative aging. The numerical measure of degrada- were observed for unfilled NR, the addition of straw to tion processes simulation is the coefficient K. The NR and treatment of the filler increased both loss obtained results are presented in Fig. 14. modulus and storage modulus of composites. The Natural rubber as a compound containing unsatu- highest values of storage and loss modulus were rated bonds in its structure and is characterized by poor measured for samples filled with benzoyl peroxide resistance to polymer chain degradation processes. straw and dicumyl peroxide straw. These results Thermo-oxidative aging caused unfavorable changes correspond correctly to the previous results of the in the properties of NR vulcanizates, as evidenced by crosslinking density and mechanical properties of values of the K coefficient of less than unity. This is obtained biocomposites. most likely the effect of changes in the polymer By definition tan d is the ratio of dissipated energy structure, leading to degradation of the material. The (loss modulus E ) to stored energy (storage modulus introduction of a natural filler did not cause further E ). One of the most important parameters affecting aging of the material. In contrast, the addition of tan d is the adhesion between the matrix and the filler. modified straw caused a slight improvement in the High interfacial energy limits the mobility of the polymer chains, thus reducing the tan d values, while Tτw [%] Cellulose 1.00 0.80 0.60 0.40 0.20 0.00 Ref. sample 10 [phr] 20 [phr] 30 [phr] (NR) Ref. sample (NR) 0.69 Untreated straw 0.70 0.70 0.77 Benzoyl peroxide straw 0.77 0.83 0.84 Dicumyl peroxide straw 0.74 0.84 0.85 Fig. 14 Thermal aging factor of composites filled with straw (0–30 phr) Ref. Sample (NR) NR+ 10 phr Straw_Untreated NR+ 10 phr Straw_Benzoyl peroxide 400 NR+ 10 phr Straw_Dicumyl peroxide -120 -110 -100 -90 -80 -70 -60 -50 -40 T[°C] Fig. 15 Loss modulus of studied biocomposites as a function of temperature the weak one leads to higher tan d values. (Krishna and matrix. For better understanding the interfacial inter- Kanny 2016). actions between NR and used fillers, the adhesion The tan d values of composites filled with straw factor (A) was calculated from DMA data according to max are lower than the reference sample. The lowest peak literature (Wei et al. 2013; Formela et al. 2016; values were observed for vulcanizates containing Akindoyo et al. 2017) using Eq. (5): modified cereal straw. There are a number of factors 1 tan d that may affect tan d of composites including: fiber A ¼   1 ð5Þ max 1  V tan d f r distribution, adhesion between fibers and polymer matrix, shear stress concentration and dissipation of where V was the volume fraction of the fiber (0.1), tan viscoelastic energy. (Pothan et al. 2003) (Shanmugam d was maximum tan d peak of the NR biocomposites and Thiruchitrambalam 2013). Therefore, the maxi- and tan d was maximum tan d peak of the unfilled NR mum of the tan d peak height can be used to determine (reference sample). interfacial adhesion between the fiber and the polymer logE''[MPa] K[ -] Cellulose Fig. 16 Storage modulus of biocomposites filled with 10 phr straw as a function of temperature Fig. 17 Temperature 2.5 Ref. Sample (NR) dependence of tan d for reference sample and biocomposites NR+ 10 phr Straw_Untreated NR+ 10 phr Straw_Benzoyl peroxide 1.5 NR+ 10 phr Straw_Dicumyl peroxide 0.5 -150 -125 -100 -75 -50 -25 0 25 Temperature [°C] Low value of the coefficient A (see Table 4) means creation of additional network nodes, which resulted high level of interface adhesion and enhanced inter- in a more efficient transfer of loads. As a result, an actions between the elastomer and filler particles. The increase in the crosslinking density of vulcanizates used straw modification contributed to the increase in filled with a modified filler was observed, resulting in the interaction between the polymer–filler. improved mechanical properties of DCPS/NR and The increase in straw adhesion to the elastomeric BPS/NR composites. matrix may be due to better dispersion and higher Glass transition temperature (Tg) of prepared surface activity of the filler. The improvement of vulcanizates was determined as the position of the interfacial interactions in the composite led to the maximum on the loss tangent curve (tan d) versus tan δ [-] Cellulose Table 4 Glass transition temperature (Tg), tan d and adhesion factor for reference sample (NR) and untreated or treated straw max composites Ref. sample NR ? 10 phr NR ? 10 phr Benzoyl NR ? 10 phr Dicumyl (NR) Untreated straw peroxide straw peroxide straw Tg (with respect to - 69.40 - 68.76 - 68.70 - 68.45 tan d )(C) max tan d (-) 2.23 2.41 2.30 2.27 max A(-)– - 0.03 - 0.07 - 0.08 Fig. 18 SEM images of NR composites containing 10 phr a untreated straw, b straw modified with dicumyl peroxide, c straw modified with benzoyl peroxide temperature. The values of Tg are presented in temperatures can be associated with the decreased Table 4. The shift in Tg can be observed for compos- mobility of NR chains by the addition of fillers. ites filled with straw compared to unfilled systems. The shifting of glass transition temperature to higher 123 Cellulose Scanning electron microscopy the presence of untreated straw and can be further improved with treated fillers. The influence of the pure and treated filler on Application of cereal straw as biofillers for NR morphology of biocomposites (10 phr of straw) is vulcanizates, is highly promising in terms of their presented in Fig. 18. subsequent commercial use. Moreover significant The purpose of the used filler modifications was to multifunctional properties of the materials can be increase interfacial filler–polymer interactions. Ana- reached by peroxide modifications. In summary, lyzing the obtained scanning electron microscope solving typical agricultural problems related to the (SEM) images of selected composites, the influence of use and management of straw is an extremely impor- applied straw treatments on the improvement of its tant challenge. By combining knowledge in the field of dispersion degree in natural rubber was observed. This chemistry and technology of polymeric materials, it is factor affects the filler activity, the more homogeneous possible to obtain valuable ecological and economic the distribution of the straw particles in the polymer, benefits. the larger the contact area with the elastomeric matrix. Open Access This article is distributed under the terms of the The degree of disperse of the filler particles has a Creative Commons Attribution 4.0 International License (http:// significant impact on its strengthening effect, and thus creativecommons.org/licenses/by/4.0/), which permits unrest- the mechanical properties of the vulcanizates. ricted use, distribution, and reproduction in any medium, pro- Figure 18a represents a SEM picture of a vulcan- vided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and izate containing pure cereal straw, where the particles indicate if changes were made. tend to agglomerate. Straw agglomerates with a size of several lm are clearly visible. Figure 18b, c shows SEM photographs of vulcanizates containing a mod- References ified filler. Its addition resulted in better dispersion of the straw particles in the polymer matrix. 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CelluloseSpringer Journals

Published: Jun 2, 2018

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