Sustainable, Biobased Silicone with Layered Double Hydroxide

Research Article pubs.acs.org/journal/ascecg

Sustainable, Biobased Silicone with Layered Double Hydroxide Hybrid and Their Application in Natural-Fiber Reinforced Phenolic Composites with Enhanced Performance Cheng Li,† Jintao Wan,‡ Ye-Tang Pan,‡ Peng-Cheng Zhao,§ Hong Fan,*,† and De-Yi Wang*,‡ †

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China ‡ IMDEA Materials Institute, C/Eric Kandel, 2, Getafe, Madrid 28906, Spain § Leibnitz Institute of Polymer Research Dresden, Hohe Strasse 6, Dresden D-01069, Germany S Supporting Information *

ABSTRACT: With wide application of natural fibers in polymer composites, improvements in their flame retardancy, water absorption, and electrical resistance become an urgent need. To this end, 4.5 wt % of layered double hydroxide (LDH) is introduced into sisal fiber reinforced biobased silicone modified phenolic composites. The modified composites optimally shows 60% reduction in total heat release (20.2 MJ/m2) compared to the composites without LDH. In addition, the biobased silicone modifier TDS is incorporated into phenolic resins (SPF), to further reduce water absorption rate to 6 wt %, and increase volume electrical resistance up to 4.6 × 1016 Ω m. The SPF-SF-SDBSLDH exhibits a high impact strength of 4.2 kJ/m2, over 50% higher than the unmodified PF-SF composites. The SEM observations show that the SPF composites exhibit better interfacial interaction with sisal fiber than normal phenolic (PF) composites. All these flame retardant, impact strength and electrical resistance properties are compatible with the requirement for applications as molding compounds. Our research provides a cost-effective method to improve the performance of this sustainable natural-fiber reinforced composites with novel and low cost biobased silicone modifier and LDHs. These high performance composites are promising for applications in high technology areas such as the microelectric industry and lightweight automotives. KEYWORDS: Biobased silicone, Layered double hydroxide, Sisal fiber, Phenolic composites, Flame retardancy, Electrical resistance, Water absorption rate

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surface of sisal fiber, and further infiltrate into its porous structure by capillary effect. The sisal fiber reinforced composites express high water absorption rate: for natural rubber reinforced by 15 wt % sisal fiber, it reaches 0.3 wt % when immersed in 70 °C water for 15 days;20 for phenolic composites reinforced by 30 wt % sisal fiber, it achieves 7.5 wt % after immersed in 100 °C water for 3 days.21 The wet sisal fiber further lowers the electrical resistance. The phenolic composites reinforced by 30 wt % sisal fiber exhibit a volume electrical resistance of lower than 1013 Ω m,21 while its glass fiber reinforced counterparts exhibit that of over 1014−1015 Ω m.22 In addition, the cellulose, hemicellulose, and lignin contained in sisal fiber are highly flammable, and seriously affect the sisal fiber contained composites’ flame retardancy.5 All of these problems urgently need to be solved. Some researchers tried to improve property of natural-fiber reinforced composites by surface modification of sisal fiber.19

INTRODUCTION Traditionally, in order to improve mechanical properties, polymer composites are reinforced by aramid, glass, and carbon fibers.1 Considering environmental protection, the recent research interests are attracted to substitute the traditional fibers with natural ones, such as sisal fiber, which exhibits advantages mainly on green regeneration, low price (0.36 USD/kg, versus glass fiber’s 3.25 USD/kg), light weight (density: approximately 1.5 g/cm3), and flexibility during processing which results in less abrasion on processing machinery.1−3 A lot of literatures studies mentioned its applications on polypropylene,4 phenolics,5−10 epoxy,11−14 and unsaturated polyester composites.15−17 However, before its further application in composites, a lot of shortcomings that originated from sisal fiber need to be overcome, such as high water absorption and poor electrical resistance. The sisal fiber is constituted by approximately 60 wt % of cellulose, 20 wt % of hemicellulose, and 10 wt % of lignin.2,18,19 The cellulose and hemicellulose contain abundant hydroxyl groups.19,20 Besides, the sisal fiber exhibits a multiporous structure; thus, water can be adsorbed on the © XXXX American Chemical Society

Received: January 20, 2016 Revised: April 5, 2016

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DOI: 10.1021/acssuschemeng.6b00134 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

between their properties and biobased silicone with LDH hybrid are investigated.

The surface treatment by alkali thermal method increases the sisal fiber’s crystallinity from 62.4% to 66.2%, which consequently improves its toughness and modulus according to Yang’s research.23 The treatment of sisal fibers with NaOH solution improves the tensile strength of sisal fiber polyester composites.24 In addition, such alkali thermal treatment can remove their thermally instable components, such as lignin and hemicellulose, thus improve their thermal and mechanical properties.1 Singh treated sisal fibers with different coupling agents, including silane and titanate coupling agents. After treatment, the sisal reinforced composites absorb less water than before, due to formation of hydrophobic matrix interphase.25 The silane coupling agents such as 3-aminopropyltriethoxysilane and N-(2-aminoethyl)-3-aminopropyltriethoxysilane are also effective on modification of sisal fiber, especially reducing the hydroxyl group amounts on the surface, thus making the sisal fiber hydrophobic.26 Megiatto and Frollini modified sisal fiber with biobased furfuryl alcohol and polyfurfuryl alcohols, and found that a thin polyfurfuryl alcohol coating was formed at the sisal fiber surface, decreasing the sisal fiber’s water absorption while improving the interactions between fibers and the phenolic resin matrix.8,19 Filling resin into sisal fiber’s multiporous structure is a clever idea: Li Yan filled epoxy resin into the sisal fiber lumen, and found that the resin inside the lumens strengthens the bonding between the sisal fiber, and hinders the crack propagation, thus increasing the composites’ failure strain.27 Rather than modification on sisal fiber, other researchers tried to enhance composites’ properties by incorporating modifiers, such as flame retardants into matrix. Addition of ammonium polyphosphate, magnesium hydroxide, and zinc borate is effective in improving flame retardant behavior of sisal fiber/polypropylene composites. According to Rachasit Jeencham’s research, the UL-94 vertical burning rating of such modified composites achieves V-0, with a limiting oxygen index of 30%.28 In this paper we try to improve the properties of sisal fiber reinforced composites by modification of phenolic resin matrix. We introduce layered double hydroxide (LDH) into phenolic resins, to study its improvement in composite flame retardancy. The LDH is host−guest clay consisting of positively charged metal hydroxide layer intercalated with anions and water. The molecular structure of intercalated anions and intercalation rate determine the interlayer distance, further affecting the layer exfoliation in the matrix. The typical LDHs include Mg−Al LDH,29 Zn−Al LDH,30 and Co−Al LDH,31 etc. During burning, the metal hydroxide layer can produce water, promote char formation, and prevent further development of flame. It works effectively as flame retardants in polypropylene,32 polyamide,33 polylactic acid,30 polystyrene,34 poly(vinyl alcohol),35 and rubber.29 Introducing LDHs into sisal fiber reinforced phenolic composites might be effective to enhance flame retardancy. However, their electrical resistance and water absorption behavior still need further improvement. So, we introduce a biobased silicone containing bisphenol compound [4,4′-(1,5dipropyl-3,3-diphenyl-1,1,5,5-tetramethyltrisiloxane) bis-2-methoxyphenol, TDS as acronym] into phenolic resin. The TDS is synthesized with eugenol, a sustainable biological resource, extracted from lilac plants. The flame retardancy, mechanical properties, water absorption, and volume electrical resistance of these composites are tested systematically. The morphologies inside composites are observed by SEM. The relationships

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METHODS

Materials. Isopropanol (99%), magnesium(II) nitrate hexahydrate [Mg(NO3)2·6H2O] (99%), aluminum(III) nitrate nonahydrate [Al(NO3)3·9H2O] (99%), sodium dodecyl benzenesulfonate (SDBS) (99%), phenol (99%), eugenol (99%), formaldehyde solution (37 wt %), oxalic acid (99%), sodium hydroxide (98%), 3-aminopropyl triethoxysilane (99%), and ethanol (99%) were purchased from Sinopharm Chemical Reagent Ltd. 1,1,5,5-Tetramethyl-3,3-diphenyltrisiloxane (99%) was kindly provided by Jiaxing United Chemical Co., Ltd. Chloroplatinic acid (99%) was purchased from Shanghai Jiuling Chemical Co., Ltd. Other materials were kindly provided by Jiaming Plastic Ltd., Zhejiang province, China. All the chemicals were used without further purification. Sisal fibers were provided by the Guangxi sisal fiber group, China. In order to improve their interaction with phenolic matrix, their surfaces were treated according to the literature:36 they were soaked in alkali solution (2 wt %, ratio between alkali solution and sisal fiber: 30 L/kg) at room temperature for 2 h, washed with distilled water, dried in oven at 150 °C for 4 h, immersed in the silane coupling agent solution for 1 h (with the same weight volume ratio in alkali solution), washed with distilled water, dried in vacuum at 80 °C overnight, and cut into 2−4 mm in length. The silane coupling agent solution mentioned above was prepared by adding 2 wt % 3-aminopropyl triethoxysilane in 95 vol % alcohol/water solution, adjusting the pH value to be 4−5, and hydrolyzing for 2 h, to turn ethoxy into more reactive hydroxyl groups, leading to good surface modification on sisal fiber. Preparation of LDHs. The unmodified Mg−Al LDH (named as NLDH) and sodium dodecyl benzenesulfonate (SDBS) intercalated Mg−Al LDH (named SDBSLDH) were prepared by a one-step method according to previous literature studies:21,28 To a three-necked 1 L round-bottom flask equipped with a magnetic stirrer, sodium dodecyl benzenesulfonate (SDBS, 13.94 g, 0.04 mol) and distilled water (500 mL) were charged. A mixed metal salt solution [Mg(NO3)2·6H2O: 10.26 g, 0.04 mol; Al(NO3)3·9H2O: 7.50 g, 0.02 mol; total metal ion concentration: 0.3 M] was slowly added to the SDBS solution under continuous stirring at room temperature. The pH value of the solution was kept at 10 ± 0.2 by adding 1 N NaOH solution. The resulting mixture was continuously stirred at room temperature for 30 min, aged at 60 °C for 18 h, filtered, and washed several times with distilled water. The remaining powder was dried in a vacuum oven at 80 and 120 °C for 2 h until constant weight was achieved to acquire SDBSLDH. The NLDH was prepared by the same process, except for no SDBS was charged into distilled water at the initial stage. Their morphological status was characterized by wideangle X-ray scattering spectra (see Figure S1 and Table S1). For the NLDH, the first basal reflection (003) appears at 11.55° (2θ angle), corresponding to interlayer distance of 0.77 nm. For SDBSLDH, the position of peak (003) is at 3.10°, and the corresponding interlayer distance is 2.84 nm. The results are similar to the published literature studies.37−39 The SDBS intercalated LDH is expressed as the following chemical formula:

Mg 2Al · (OH)6 ·SDBSx ·(NO3)1 − x ·0.4H 2O x is the SDBS intercalation degree. On the basis of the elemental analysis results of sulfur (6.15 wt %), the x value can be calculated according to eq 1,29 being 96%. The x value can be calculated according to eq 1,37 being 96%. Mws is the sulfur weight in each mole of SDBSLDH (6.15 wt %); MwSDBSLDH is the molecular weight of SDBSLDH. S(wt %) =

M wsx 32x = M wSDBSLDH 246.2 + 263.5x

(1)

Preparation of Silicone Modified Novolac Type Phenolic Resins. The silicone modified novolac type phenolic resins (SPF) were prepared by two steps: (i) preparation of silicone modifier 4,4′B


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Characterization. 1H NMR spectra of TDS and 13C NMR spectra of SPF were recorded on a Bruker Avance 500 spectrometer at 300 K, with CDCl3 as solvent. The scan numbers are 32 for each sample. GPC Analysis. These studies were carried out using a WATERS 1515 apparatus equipped with WATERS 2489 UV detectors and WATERS Styragel HR0.5, HR1, HR3, and HR4 columns (eluent, tetrahydrofuran; flow rate, 1 mL/min; column temperature, 35 °C). The calibration curves for GPC analysis were obtained by polystyrene standards (Standards’ molar mass: 162, 575, 1260, 2700, 4800, 19 600, 28 000, 43 900, 72 450 g/mol). Wide-Angle X-ray Scattering Spectra (XRD). These tests were performed using a 2-circle diffractometer XRD (XPERT-PRO) with Cu Kα radiation (λ = 0.154 nm) in the range 2θ = 1−30° (step length: 0.033°). The LDH interlayer distance is calculated using the Bragg equation (eq 2) and averaged over the first four orders of diffraction.37,39 Here, d is the plane spacing distance. θ is the Bragg angle, and λ is the wavelength.

(1,5-dipropyl-3,3-diphenyl-1,1,5,5-tetramethyltrisiloxane) bis-2-methoxyphenol (TDS); (ii) in-situ polymerization of SPF. Preparation of TDS. To a three-necked 500 mL round-bottom flask equipped with a magnetic stirrer, a dropping funnel, a thermometer, and a condenser were charged eugenol (16.42 g, 0.1 mol) and chloroplatinic acid solvent (1.44 × 10−2 g chloroplatinic acid dissolved in isopropyl alcohol, Pt content 0.8 wt %). The mixture was heated to 60 °C, and 1,1,5,5-tetramethyl-3,3-diphenyltrisiloxane (16.63 g, 0.05 mol) was dropped slowly in 3 h. The reaction temperature was kept at 60−70 °C for 4 h. The remaining reagents were removed by rotary evaporator under vacuum (80 °C, 0.001 MPa). The acquired product was 4,4′-(1,5-dipropyl-3,3-diphenyl-1,1,5,5-tetramethyltrisiloxane) bis2-methoxyphenol (TDS) (97% yield). Preparation of SPF follows: The TDS (10.4 g), phenol (94 g, 1 mol), and oxalic acid (3 g, 0.02 mol) were added into a three-necked 500 mL round-bottom flask equipped with a mechanical stirrer, dropping funnel, thermometer, and condenser. When the mixture was heated to 100 °C, 37 wt % formaldehyde−water solution (64.93 g, 0.8 mol) was dropped into it in 1 h. The reaction was carried out at 100 °C for additional 3 h. The viscous product was dried under reduced pressure (150 °C, 0.001 MPa) to yield silicone modified novolac type phenolic resin (SPF). The normal phenolic resin (PF) was prepared with the same process, except for no TDS was added. Preparation of Phenolic Composites. The NLDH and SDBSLDH were incorporated into PF/SPF by solvent mixing (acetone as solvent, later removed by rotary evaporation). Then, they were milled on a two-roller miller (temperature: 110 and 140 °C), together with surface treated sisal fibers (SF), inorganic filler, hexamethylenetetramine, and other agents (see Table 1). The

Elemental Analysis. Studies was conducted on a Thermo Electron SPA EA 1112 type instrument. Charpy Impact. These tests were carried out on rectangular specimens (size: 80 × 10× 4 mm3, un-notched) with CEAST impact testing machine, following the ISO 179-1 standard. Each composite is tested with five identical specimens. Scanning Electron Microscopy (SEM). SEM was carried out in a Karl Zeiss Utral 55 apparatus, electron acceleration 20 kV. The samples were covered with a thin layer of gold by a sputter coater SC7620 apparatus. Flame Retardant Properties. The materials were characterized by cone calorimeter tests (CCT, Fire Testing Technology Ltd., U.K.), following the ISO 5660 procedures (without “frame and grid” application). Specimens [dimension: 100 × 100 × 3.0 (±0.1) mm3] were irradiated with a heat flux of 50 kW/m2. Dynamic Mechanical Analysis (DMA). DMA was performed on a TA instrument (TA Q800, single-cantilever mode). The equipment was calibrated with a metallic standard before tests. The dimensions of the samples were 64 × 10 × 1 mm3. Test conditions follow: oscillation amplitude, 10 μm; frequency, 1 Hz; heating rate, 2 °C/min; temperature range, 30−270 °C. The experimental data were analyzed with TA Universal 2000 software. Water Absorption Rate. The samples were tested according to the ASTM D 570-98 standard. The dimensions of the specimens are 100 mm × 10 mm × 4 mm. Three specimens of each composite were dried in an oven at 50 °C for 24 h (weight: W1), and then immersed in distilled water at 100 °C. After certain periods of time, the specimens were taken out from water, wiped, and weighed on a high precision balance (weight: W2). The water absorption rates of specimens were calculated by eq 3:

Table 1. Components of Phenolic Composites sample

PF (g)

PF-SF PF-SF-NLDH PF-SF-SDBSLDH SPF-SF SPF-SF-NLDH SPF-SF-SDBSLDH

90 90 90

SPF (g)

NLDH (g)

SDBSLDH (g)

10 10 90 90 90

(2)

2d sin θ = nλ

10 10

following ingredients were added to every sample: mica powder, 30 g; stearic acid, 1.6 g; stearic acid monoglycerides, 2 g; sisal fibers (SF), 32 g; hexamethylenetetramine (HMTA), 15 g; wood powder, 44 g. The novolac type phenolic resin (PF and SPF) is thermoplastic polymer, and need curing agent (HMTA) to form cross-linked thermoset composites. The samples were shaped and cured by resin transfer molding (RTM) techniques to produce cured phenolic composites (pressure 30 MPa, molding temperature 175 °C, holding time 1 min/mm) (Figure 1).

water absorption rate =

W2 − W1 × 100% W1

(3)

Figure 1. Preparation route of sisal fiber reinforced phenolic composites. C


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Figure 2. Molecular structure characterization: (a) 1H NMR spectrum of TDS, (b) 1H NMR spectrum of SPF, (c) GPC spectra of SPF and PF, and (d) regional enlarged graph of SPF 1H NMR spectra.

Figure 3. Cone calorimeter test results: (a) heat release rate, (b) total smoke production, (c) total heat release, and (d) mass loss. Volume Electrical Resistance. Resistance was tested on a ZC-43 electrical resistance testing device. The specimens were disks with a radius of 50 mm and 3 mm thick. Volume electrical resistance of specimens was calculated following eq 4. ρυ = R x

A h

structure can be identified from the following remarks: The signals in 0.059 ppm are attributed to methyl hydrogen atoms attached in silicon atoms (Si−CH3), and the peaks at around 0.568 ppm are attributed to methylene hydrogen atoms close to silicon (Si−CH2−CH2). The detailed assignment of 1H NMR spectra (500 MHz, CDCl3, δ ppm) are listed as follows: 0.059, s, 12H, Si−CH3; 0.568, t, 4H, Si−CH2−CH2; 1.605−1.639, m, 4H, Si−CH2−CH2; 2.450−2.479, m, 4H, Si−CH2−CH2−CH2; 3.880−3.902, m, 6H, Phenyl-OCH3; 5.453, m, 2H, Phenyl-OH; 6.472−7.687, m, 16H, Phenyl-H.21 The 1H NMR spectra of SPF are presented in Figure 2b, and its regional enlarged spectrum is shown in Figure 2d. According to the spectra, the signals between 3.7 and 4.0 are attributed to the hydrogen in methylene bridges that link phenol rings, and

(4)

ρv is the volume electrical resistance (Ω m). Rx is the electrical resistance (Ω). A is the area of protected electrode (m2), and h is the thickness (m).

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RESULTS AND DISCUSSION Molecular Structure Characterization. The 1H NMR spectra of silicone modifier, TDS, are presented in Figure 2a. Its D


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ACS Sustainable Chemistry & Engineering Table 2. Cone Calorimeter Test Resultsa TTI (s) PHRR (kW/m2) T-PHRR (s) FIGRA (kW/m2 s) THR (MJ/m2) TSP (m2) char residue (%)

PF-SF

PF-SF-NLDH

PF-SF-SDBSLDH

SPF-SF

SPF-SF-NLDH

SPF-SF-SDBSLDH

95 ± 2 559 ± 18 125 ± 3 4.5 ± 0.3 55.9 ± 0.5 8.9 ± 0.2 47.3 ± 1.0

105 ± 3 508 ± 16 120 ± 3 4.2 ± 0.3 26.7 ± 0.2 3.2 ± 0.1 59.6 ± 1.3

115 ± 3 379 ± 10 130 ± 3 2.9 ± 0.2 27.3 ± 0.2 2.9 ± 0.1 58.1 ± 1.2

105 ± 3 574 ± 20 120 ± 2 4.8 ± 0.3 53.8 ± 0.6 8.2 ± 0.2 48.5 ± 0.9

105 ± 3 451 ± 15 120 ± 2 3.8 ± 0.3 25.9 ± 0.2 3.0 ± 0.1 59.7 ± 0.7

130 ± 4 342 ± 8 140 ± 3 2.4 ± 0.2 20.2 ± 0.2 2.3 ± 0.1 63.0 ± 1.1

The T-PHRR means the time from ignition to achieve PHRR; the FIGRA represents fire growth rate, and it is acquired through dividing PHRR by T-PHRR.

a

Figure 4. DMA curves: (a) loss modulus versus temperature, (b) storage modulus versus temperature, and (c) tan δ versus temperature.

heat radiation, and detection of the heat release data during the whole burning stage. According to the CCT results in Figure 3 and Table 2, the PF-SF experiences a long and intensive burning process after ignition, with the peak heat release rate (PHRR) of 559 kW/ m2. In comparison, the LDH containing PF-SF-NLDH and PFSF-SDBSLDH burn relatively slower, with the PHRR value decreasing to 508 and 379 kW/m2, respectively. The silicone containing SPF-SF-NLDH and SPF-SF-SDBSLDH express a further decreased PHRR value of 451 and 342 kW/m2, corresponding to 19% and 39% reduction of the PF-SF’s PHRR value, respectively. The fire growth rate (FIGRA) of PF-SF is 4.47 kW/m2 s, while for PF-SF-NLDH, PF-SF-SDBSLDH, SPF-SF-NLDH, and SPF-SF-SDBSLDH, the rates are 4.23, 2.92, 3.76, and 2.44 kW/m2 s, respectively. The biobased

the signals in 6.84−7.40 are attributed to hydrogen in phenol rings. The TDS signals can be found in the SPF spectra, proving that the TDS is incorporated into the SPF (see Figure 2d).21 As for the molecular weight, the SPF exhibits a little higher number-average molecular weight (Mn) than the PF (1980 versus 1700), but its weight-average molecular weight (Mw) is nearly twice that of the PF (11 400 versus 6990, Figure 2c). The TDS promotes the molecular chain growth of SPF, and largely increases the high molecular weight fraction. Flame Retardant Properties. The composites’ flame retardant behavior is very important in industries such as microelectric insulation, electrical appliances, and automotive industries.4 They were characterized by cone calorimeter tests (CCTs). The test data exhibit samples’ burning behavior in real fire disasters. The process includes ignition of samples under E


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ACS Sustainable Chemistry & Engineering silicone and LDH lead to a slower fire growth speed and lower risk of fire hazard. When the burning ends, the PF-SF-SDBSLDH’s total heat release (THR) reduces to 48.8% the value of the PF-SF (27.3 versus 55.9 MJ/m2). Further, the silicone modified samples express lower THR values than their counterparts: the SPF-SFNLDH’s THR value is 25.9 MJ/m2, versus the PF-SF-NLDH’s 26.7 MJ/m2; the SPF-SF-SDBSLDH’s is 20.2 MJ/m2 versus the PF-SF-SDBSLDH’s 27.3 MJ/m2. The total smoke production (TSP) is very important data in flame retardancy; for during a real fire disaster, the toxic smoke is the main cause of human death.40 The PF-SF exhibits the highest TSP value, being 8.91 m2 (Figure 3b). The PF-SFNLDH and PF-SF-SDBSLDH’s TSP values are 3.22 and 2.91 m2, respectively, being lower than 30% that of PF-SF (8.91 m2). The SPF-SF-SDBSLDH’s TSP value is further reduced to only 2.32 m2. The LDH can largely improve the flame retardancy of phenolic composites. Further, the SDBSLDH containing composites express superior flame retardancy than NLDH containing samples. The SDBSLDH’s larger interlayer distance makes it easier to exfoliate and disperse than NLDH in composite matrix (see Figures S1 and S2 and Table S1 in Supporting Information).37,41 Such sisal fiber composites with improved flame retardant property are suitable for automotive applications, which require low smoke production and low flammability.28,42 From an application point of view, the commercial microelectric insulation thermosets, such as epoxy resins,43 exhibit the THR value of around 80−90 MJ/m2, while their TSP values are over 20 m 2 . 44 After flame retardant modification, the epoxy’s THR can be reduced to lower than 50 MJ/m2, with a TSP value of lower than 10 m2, which is still higher than phenolic composites.41 In consideration of its excellent flame retardant behavior and lower cost than epoxy resin,42,45 the SPF-SF-SDBSLDH samples exhibit great advantages and huge potential in microelectric insulation applications. Mechanical Properties and Morphology. The mechanical properties are tested by dynamic mechanical analysis (DMA), providing key information regarding the molecular mobility, cross-linking density, and interactions between different components. According to Figure 4, the silicone modified phenolic composites, SPF-SF-NLDH, SPF-SFSDBSLDH, and SPF-SF, express higher loss modulus and storage modulus than their PF based counterparts. It resulted from the existence of a soft siloxane chain, which enhances the cross-link density of SPF composites. It would be further studied by curing kinetics in our later research. In the meantime, the LDHs containing composites exhibit enhanced loss and storage modulus than PF-SF and SPF-SF. The high storage modulus implies that the introduction of the rigid Mg/Al nanolayers improves the composites’ stiffness effectively,46 while the high loss modulus resulted from the intensive friction between the Mg/Al nanolayers and the composite matrix, especially for NLDH. A similar phenomenon is also reported in clay/polypropylene47 and LDH/polyimide46 blending systems. According to the tan δ curves in Figure 4c, the composites with a different matrix and LDHs exhibit similar glass transition temperature, being around 230 °C. The impact strength of composites is shown in Figure 5. The SPF composites exhibit much higher impact strength than the

Figure 5. Impact strength of composites.

PF composites. The SPF-SF’s impact strength is 4.5 kJ/m2, while the SPF-SF-SDBSLDH and SPF-SF-NLDH exhibit the value of 4.1 and 3.8 kJ/m2, respectively. In comparison, the PFSF’s impact strength is only 2.7 kJ/m2, while the PF-SFSDBSLDH and the PF-SF-NLDH are even lower, being 2.5 and 2.4 kJ/m2. The SPF composites exhibit over 50% higher impact strength than the PF composites. It means the SPF enhances impact strength to a large extent. In order to further investigate the fracture mechanism and their interactions between matrix and fiber, the composites’ cross sections were scanned by SEM. One can see from Figure 6 that all SPF based composites (SPF-SF-NLDH, SPF-SFSDBSLDH, and SPF-SF) exhibit fiber breakage, while few pull out and debonding is observed. It exhibits remarkable toughness fracture surface morphology.48 The excellent interfacial interaction is beneficial for the energy transfer and dissipation, leading to the SPF composites’ high impact strength. In comparison, the PF based composites (PF-SFNLDH, PF-SF-SDBSLDH, and PF-SF) exhibit a large amount of fiber pull-out and debonding, which implies poor interactions between matrix and fiber, further affecting the energy transfer and dissipation. These two group composites are different only in matrix resin, SPF versus PF, leading to such huge differences in impact strength and fracture surface morphology. The silicone containing SPF promotes the interactions between matrix and sisal fibers to be stronger than the PF composites. Water Absorption Rate. The water absorption rate (WAR) was tested according to the ASTM D 570-98 standard. It is acquired by calculating the water being absorbed by composites in boiling water. According to Figure 7, the PF-SFSDBSLDH expresses the highest WAR value, being over 8 wt %. The PF-SF-NLDH’s WAR value is a little lower, being 7.5 wt %. Both of them are higher than that of PF-SF (7.3 wt %). Fortunately, introducing silicone modifier (TDS) reduces the WAR significantly. The WAR value for the TDS containing SPF-SF is less than 6 wt %, compared with the PF-SF’s 7.3 wt %. The SPF-SF-NLDH’s WAR is a little higher, being 6.8 wt %, but still lower than the PF-SF. Even the WAR of SPF-SFSDBSLDH (containing highly hydrophilic SDBSLDH) is lower than that of PF-SF (7 wt % versus 7.3 wt %). The decreased water absorption is attributed to the hydrophobic behavior of the silicone in TDS, altogether with its capability to wet the fiber and matrix. Electrical Resistance. The LDHs and silicone modifier (TDS) express different impacts on composites’ electrical F


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resistance. In Figure 8, the LDHs containing composites PFSF-SDBSLDH and PF-SF-NLDH express very low volume

Figure 8. Volume electrical resistance.

electrical resistance, being 9.0 × 1014 Ω m and 7.5 × 1014 Ω m, respectively. The LDH layers contain metal hydroxides, which are highly electrically conductive. This problem is solved by the biobased silicone modifier: the TDS containing SPF-SF expresses the highest volume electrical resistance among all the samples, being 4.7 × 1017 Ω m, nearly 102 higher than that of PF-SF (3.6 × 1015 Ω m). Even for the LDH containing samples, the silicone samples containing SPF-SF-NLDH and SPF-SF-SDBSLDH still express 10 times higher electrical resistance than PF-SF (SPF-SF-NLDH, 3.3 × 1016 Ω m; SPFSF-SDBSLDH, 4.6 × 1016 Ω m). The large volume siloxane chain in TDS prevents the transmission of electron. In addition, we find that the NLDH containing samples express lower electrical resistance than SDBSLDH containing counterparts (SPF-SF-NLDH versus SPF-SF-SDBSLDH, and PF-SF-NLDH versus PF-SF-SDBSLDH). The molecular weight of SDBS is higher than that of NO3−. So, with the same weight, there are more weight fractions of electrically conductive metal hydroxide layers contained in NLDH than in SDBSLDH, leading to the former’s lower volume electrical resistance.49

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Figure 6. SEM images of (A) SPF-SF-NLDH, (B) SPF-SFSDBSLDH, (C) SPF-SF, (D) PF-SF-NLDH, (E) PF-SF-SDBSLDH, and (F) PF-SF.

CONCLUSION In this study, the LDHs were introduced into sisal fiber reinforced phenolic composites to improve the flame retardancy. The total heat release (THR) of PF-SF-SDBSLDH was reduced over 50% (27.3 MJ/m2) compared with PF-SF (55.9 MJ/m2). The dynamic mechanical analysis showed that the LDHs work as an excellent functional nanofiller, simultaneously increasing loss and storage modulus of phenolic composites. The biobased silicone modifier, TDS, improved the composites’ impact strength in large extent. It results from the TDS, for according to the SEM images, the TDS containing SPF composites exhibit better interfacial interactions between matrix and sisal fiber than PF composites. In addition, the SPF composites exhibited lower water absorption rate and higher electrical resistance than PF composites: the water absorption rate was reduced from 7.5 wt % (PF-SF) to 6 wt % (SPF-SF); the volume electrical resistance of SPF-SF-SDBSLDH achieved 4.6 × 1016 Ω m, nearly 10 times higher than that of PF-SF. We develop a simple but effective method to prepare sustainable high performance sisal fiber reinforced phenolic

Figure 7. Water absorption rate of composites. G


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ACS Sustainable Chemistry & Engineering composites. The properties exhibited by the sisal fiber reinforced biobased silicone and LDH modified phenolic composites show a cost-effective method to prepare naturalfiber reinforced composites, and push the natural-fiber reinforced polymer composites in a big step toward wide applications. This method is promising for expansion to other natural-fiber reinforced polymer composites, such as epoxy, polyester, polyethylene, and polypropylene, helping them to be applied in cutting edge industries, such as microelectric insulation and lightweight automotives.4

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structions - Simulation of Sustainable Materials for the Automotive Industry. Int. Conf. Eng. Des. 2015, 312. (5) Fung, K. L.; Xing, X. S.; Li, R. K. Y.; Tjong, S. C.; Mai, Y. W. An investigation on the processing of sisal fibre reinforced polypropylene composites. Compos. Sci. Technol. 2003, 63 (9), 1255−1258. (6) Kumar, R. P.; Nair, K. C. M.; Thomas, S.; Schit, S. C.; Ramamurthy, K. Morphology and melt rheological behaviour of shortsisal-fibre-reinforced SBR composites. Compos. Sci. Technol. 2000, 60 (9), 1737−1751. (7) Joseph, K.; Varghese, S.; Kalaprasad, G.; Thomas, S.; Prasannakumari, L.; Koshy, P.; Pavithran, C. Influence of interfacial adhesion on the mechanical properties and fracture behaviour of short sisal fibre reinforced polymer composites. Eur. Polym. J. 1996, 32 (10), 1243−1250. (8) Megiatto, J. D., Jr.; Oliveira, F. B.; Rosa, D. S.; Gardrat, C.; Castellan, A.; Frollini, E. Renewable Resources as Reinforcement of Polymeric Matrices: Composites Based on Phenolic Thermosets and Chemically Modified Sisal Fibers. Macromol. Biosci. 2007, 7 (9−10), 1121−1131. (9) Faulstich de Paiva, J. M.; Frollini, E. Unmodified and Modified Surface Sisal Fibers as Reinforcement of Phenolic and Lignophenolic Matrices Composites: Thermal Analyses of Fibers and Composites. Macromol. Mater. Eng. 2006, 291 (4), 405−417. (10) Megiatto, J. D., Jr; Silva, C. G.; Ramires, E. C.; Frollini, E. Thermoset Matrix Reinforced with Sisal Fibers: Effect of the Cure Cycle on the Properties of the Biobased Composite. Polym. Test. 2009, 28 (8), 793−800. (11) Paluvai, N. R.; Mohanty, S.; Nayak, S. K. Mechanical and thermal properties of sisal fiber reinforced acrylated epoxidized castor oil toughened diglycidyl ether of bisphenol A epoxy nanocomposites. J. Reinf. Plast. Compos. 2015, 34 (18), 1476−1490. (12) Paluvai, N. R.; Mohanty, S.; Nayak, S. K. Studies on thermal degradation and flame retardant behavior of the sisal fiber reinforced unsaturated polyester toughened epoxy nanocomposites. J. Appl. Polym. Sci. 2015, 132 (24), 42068. (13) Fiore, V.; Scalici, T.; Nicoletti, F.; Vitale, G.; Prestipino, M.; Valenza, A. A new eco-friendly chemical treatment of natural fibres: Effect of sodium bicarbonate on properties of sisal fibre and its epoxy composites. Composites, Part B 2016, 85, 150−160. (14) Debnath, K.; Singh, I.; Dvivedi, A. Drilling Characteristics of Sisal Fiber-Reinforced Epoxy and Polypropylene Composites. Mater. Manuf. Processes 2014, 29 (11−12), 1401−1409. (15) Zeng, D. M.; Lv, J.; Wei, C.; Yu, C. B. Dynamic mechanical properties of sisal fiber cellulose microcrystalline/unsaturated polyester in-situ composites. Polym. Adv. Technol. 2015, 26 (11), 1351−1355. (16) Lv, J.; Zeng, D. M.; Wei, C. Mechanical and Wear Properties of Sisal Fiber Cellulose Microcrystal Reinforced Unsaturated Polyester Composites. Adv. Polym. Technol. 2015, 34 (2), 21483. (17) Ramesh, M.; Palanikumar, K.; Reddy, K. H. Mechanical property evaluation of sisal-jute-glass fiber reinforced polyester composites. Composites, Part B 2013, 48, 1−9. (18) Ramires, E. C.; Megiatto, J. D., Jr.; Gardrat, C.; Castellan, A.; Frollini, E. Biobased composites from glyoxal-phenolic resins and sisal fibers. Bioresour. Technol. 2010, 101 (6), 1998−2006. (19) Megiatto, J. J. D.; Hoareau, W.; Gardrat, C.; Frollini, E.; Castellan, A. Sisal Fibers: Surface Chemical Modification Using Reagent Obtained from a Renewable Source; Characterization of Hemicellulose and Lignin as Model Study. J. Agric. Food Chem. 2007, 55 (21), 8576−8584. (20) Jacob, M.; Varughese, K. T.; Thomas, S. Water Sorption Studies of Hybrid Biofiber-Reinforced Natural Rubber Biocomposites. Biomacromolecules 2005, 6 (6), 2969−2979. (21) Li, C.; Fan, H.; Wang, D.-Y.; Hu, J.; Wan, J.; Li, B. Novel siliconmodified phenolic novolacs and their biofiber-reinforced composites: Preparation, characterization and performance. Compos. Sci. Technol. 2013, 87 (0), 189−195. (22) Pilato, L. Phenolic Resins A Century of Progress 2010, 1.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00134. Wide-angle X-ray diffraction graphs for LDHS (Figure S1), WAXS graphs for LDH containing phenolic resins (Figure S2), and WAXS data for LDHs and LDH containing phenolic resins (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected].. *E-mail: [email protected] Author Contributions

The manuscript was written through contributions of all authors. C.L., H.F., and D.-Y.W. contributed equally to the sample preparation and all the data analysis, including data interpretation. H.F. constructed the whole work frame, C.L. prepared all the composite samples, and D.-Y.W. mainly focused on the flame retardant result interpretation. J.W., Y.T.P., and P.-C.Z. carried out the flame retardant measurements. All the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is subsidized by the special funds for key innovation teams of Zhejiang Province (2009R50016) sponsored by the Science and Technology Department of Zhejiang Province, P. R. China, the China Scholarship Council ([2012]2013), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT09412). Pu Qun, Hu Jijiang, and Zhang Huibo (State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University) are highly appreciated for their contribution in characterization for this paper.

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Sustainable, Biobased Silicone with Layered Double Hydroxide

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