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A comprehensive study on lignin as a green alternative of silica in natural rubber composites
Accepted Manuscript A comprehensive study on lignin as a green alternative of silica in natural rubber composites Peng Yu, Hui He, Yunchao Jia, Tian Shenghui, Chen Jian, Demin Jia, Yuanfang Luo PII:
S0142-9418(16)30338-5
DOI:
10.1016/j.polymertesting.2016.07.014
Reference:
POTE 4714
To appear in:
Polymer Testing
Received Date: 13 April 2016 Revised Date:
21 June 2016
Accepted Date: 13 July 2016
Please cite this article as: P. Yu, H. He, Y. Jia, T. Shenghui, C. Jian, D. Jia, Y. Luo, A comprehensive study on lignin as a green alternative of silica in natural rubber composites, Polymer Testing (2016), doi: 10.1016/j.polymertesting.2016.07.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A comprehensive study on lignin as a green alternative of silica in natural rubber composites Peng Yu, Hui He*, Yunchao Jia, Tian shenghui, Chen Jian, Demin Jia, Yuanfang Luo
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School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510641, People's Republic of China
* Corresponding author: Hui He, E-mail address: [email protected] Abstract
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Using waste materials in polymeric products has drawn great attention over the recent years for economic and environmental concerns. Lignin, which is byproducts
of paper-making industry, can be used as a valuable rubber filler. Silica is one of main
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fillers in rubber industry. Since using two different filler may exhibit some synergistic effects in polymers by making full use of each filler's advantage and character. In this research, we investigated the influence of lignin/silica hybrid filler towards natural rubber. The results revealed that the partial replacement of silica by lignin in the blends did not severely deteriorate mechanical properties of the composites. Besides,
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the inclusion of lignin into the rubber could not only weaken the Payne effect but also improve the processability, anti-aging resistance and anti-flex cracking of composites. The vulcanizate containing 20phr lignin and 30phr of silica in hybrid filler exhibited the optimal overall mechanical properties.
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Keywords: lignin, rubber, silica, hybrid filler, composites 1. Introductions
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Natural rubber is an important industrial raw material which is used extensively in many applications, although NR is known to exhibit numerous outstanding properties, reinforcing fillers are often required in this matrix to improve the modulus, hardness, wear resistance and reduce the material cost. Lignin, as the second most abundant renewable and biodegradable natural resource next to cellulose, is a complex macromolecule which is based on the repetition of three different phenylpropane units. In the past, lignin was mainly burned to produce energy[1], and this was a low-valued utilization. The research of high-value-added application of lignin has considerable significance for both utilization of renewable resources and
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environmental protection. The impressive properties of lignin, such as its low cost, high abundance, low density, environmentally friendly, and bio-renewable [2] make it even more attractive as a reinforcing filler for the rubber. Recently, various academic
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papers regarding preparations of rubber/lignin compounds have been published [3-5]. Silica, which can increase the wet grip and reduce the rolling resistance when it is used in tires, is widely used as an indispensable rubber reinforcing filler that not relay on the fossil fuel.
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Inclusion of hybrid filler into the elastomers has drawn the attentions of many researchers. The most successful commercial hybrid filler is probably the carbon black–silica dual phase filler (CSDPF) introduced by Cabot Corporation[6], the
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CSDPF has a high polymer-filler interaction and low filler-filler interaction, it has been shown to give better overall mechanical properties in comparison with the conventional carbon black and silica fillers. Recently, N. Rattanasom et al [7], reinforced the natural rubber with silica/carbon black hybrid filler. Tang et al.[8] used the hybrid consisting of halloysite and graphene as a reinforcement for elastomers.
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Amit Das et al. [9] used the combination of expanded graphite with multiwalled carbon nanotubes to reinforce the rubber. Many researchers hybrid lignin with other filler,
such as carbon
black[5], montmorillonite[10],
and layered
double
hydroxides[11] to improve the mechanical or dynamic mechanical performance of
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rubber/lignin composites.
To the best of our knowledge, there is not much systematic scientific report on
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the effect of lignin/silica hybrid filler on the cure, mechanical, dynamic mechanic and anti-ageing properties of NR vulcanizates. Since each filler possesses its own advantages, the use of lignin/silica blends should enhance the overall performance of NR vulcanizates. Usually, lignin is incorporated into the rubber matrix by two different methods, i.e. lignin-rubber latex co-precipitate, or incorporation of lignin as dry powder into rubber. It should be noted that lignin as dry powder straightforward milled into rubber shows almost no reinforcing effect due to the strong intermolecular hydrogen bonding[12] and serious agglomeration of the lignin. In this article, NR/lignin compounds were firstly co-coagulated by latex compounding method to
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achieve the fine dispersion of lignin in NR matrix. After that, the silica, interfacial modifier, and rubber additives were added in the NR/lignin compounds by dry-milling method. The NR/silica/lignin composites with various silica/lignin ratio are evaluated
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by: cure properties, crosslink density, stress–strain behavior, abrasion resistance, ageing-resistant, anti-flex cracking resistance and so on. The effects of silica/lignin ratio on various mechanical properties of NR vulcanizates were systematically studied
and reported. The optimum lignin/silica ratio giving rise to the optimum overall
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properties was also reported. 2. EXPERIMENTAL 2.1 Materials
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NR latex with a solid content of 60wt% was purchased from China Hainan Rubber Industry Group Co., Ltd. Precipitated silica (type FINE-SIL518, median diameter (d50) is 5 µm, the pH value is 6.5~7.5, specific surface area is 200~220 m2/g ) was produced by Wanzai Huiming Chemical industry Co., Ltd. (Jiangxi, China). Alkali lignin (type LN-100, bagasse source, lignin content in LN-100 is
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92wt%, the average molecular weight of lignin is 4235g/mol and the polydispersity index is 2.4) was kindly provided by Linge Polymer Material Co., Ltd. (China). Si69, a renowned and widely utilized coupling as an interfacial modifier to boost the compatibility between hydroxyl-containing filler and rubber matrix, was kindly
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provided from Longsunchem Co., Ltd. (China). All the rubber additives such as zinc oxide (ZnO), stearic acid (SA), N-tertbutyl-2-benzothiazole sulfonamide (CBS),
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2,2’-dibenzothiazoledisulfde (DM), tetramethylthiuram disulfide (TMTD), sulfur (S) and process oil were industrial grade and used as received. Other ingredients were
bought from local sources. 2.2 Samples preparation In order to achieve the fine dispersion of lignin in rubber matrix, we prepared
NR/lignin/ by latex compounding method. Lignin was dissolved in deionized water at a mass concentration of 10% and the pH was adjusted to about 12. The NR latex and lignin aqueous suspension were first mixed, followed by vigorously stirring via mechanical agitation for over 30 min. After that, the mixed solution was coagulated
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by the diluted acid. The coagulated mixtures were filtered and washed with water. Lastly, the compounds were vacuum dried. The NR/lignin compounds were compounded with precipitated silica filler and rubber additives according to the Table
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1 by two-roll mixing. (NR/lignin, silica powder, process oil and Si69 was firstly compounded on the X(S)K-160 two-roll mill (Zhanjiang, China). Then the
vulcanizing ingredients were mixed in conventional order (ZnO and Stearic acid were
added firstly, followed by CZ, DM, TT. The Sulfur was added lastly). The rolls run at
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a speed ratio of 1 (front roll):1.2 (back roll). The mixing time and temperature are
about 12 min and room temperature, respectively.) After that, the rubber compounds
U-CAN UR-2030 vulcameter.
for the optimum curing time determined by the
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were compression molded at 145
Table 1. Compounding formulation (phr)* Sample code
NR/Lignin
Silica powder
100/0
50
100/10
40
NR/30silica/20lignin
100/20
30
NR/20silica/30lignin
100/30
20
NR/10silica/40lignin
100/40
10
NR/50lignin
100/50
0
NR/50silica
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NR/40silica/10lignin
*the rubber ingredients of composites were fixed as: ZnO 5, Stearic acid 2, Process oil 5,
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Si69 4, accelerator CZ 1.5, accelerator DM 0.5, accelerator TT 0.3, Sulfur 2.
2.3 Characterizations Pyrolysis Gas Chromatography-Mass Spectrometry. Lignin LN-100 was subject to Pyrolysis Gas Chromatography-Mass Spectrometry (Py–GC/MS) test (Agilent Technologies, GC-7890B, MS-5977A. USA). Chromatographic separation was achieved by using a column which held at 50 °C for 2 min followed by a ramped temperature increase to 250 °C at a rate of 5 °C/min. Finally a 10 °C/min ramp applied to the temperature of 320 °C. 1 mg of lignin was used. The sample was pyrolysed rapidly at 800 °C. The residue of the pyrolysis was not detected. The ion
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source’s temperature was 230 °C. The helium was used as carrier gas at a flow rate of 1 mL/min. A split ratio of 100:1 was used. The chromatograms gathered were analyzed using NIST 011 library. ‐by U-CAN
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The curing characteristics of the compounds were determined at 145 UR-2030 vulcameter (U-CAN Dynatex Inc., Taiwan).
Prediction of uncured samples’ bound rubber was done as follows [13], the uncured rubber composite was divided into scraps and packaged with 200 mesh metal net, then
And the bound rubber was calculated (eqn (1)): ( )
∗ 100%
(1)
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bound rubber content =
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immersed in toluene for 3 days. Then the swollen sample was dried in vacuum oven.
where W1 is the rubber mass in the sample, W2 is the initial sample quality including metal net, and W3 is the sample quality including metal net after swelling and drying. The volume fraction of rubber in the swollen gel (Vr), which was used to represent the apparent crosslinking density of the vulcanizates, was done according to equilibrium
(eqn (2)):
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swelling experiments as reported in related referrence [14]. And it was calculated.
(∗)/ρ
= (∗)/ρ
A! /ρ"
(2)
where H is the weight of the test specimen, D is weight of the de-swollen test
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specimen, F is weight fraction of the insoluble components, Ao is weight of the absorbed solvent, ρs and ρr is the density of solvent and vulcanizate, respectively.
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Strain sweep of rubber composites was carried out by a Rubber Process Analyzer (RPA2000, Alpha technologies Co.) at 60°C and a frequency of 1 Hz. Tensile tests were performed following ISO 37-2005 at room temperature using U-CAN UT-2060 tensile instrument (Taiwan). The heat build-up the vulcanizates were examined using a Gotech RH-2000N Machine (Gotech Testing Machines Inc., Taiwan). Cylindrical rubber specimens (25 mm in height and 17.5 mm in diameter) were subjected to repeated compression according to ISO4666/3-1982. Scanning electron micrographs (SEM) of the composites were conducted by a Nova
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NanoSEM 430 instrument (FEI, the Netherlands). The tensile fracture surfaces were directly obtained from the tensile tests. The quenched fracture surfaces were obtained by splitting bulk sample being quenched in liquid nitrogen. After that, all the surfaces
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were sputter coated with gold before examining under the SEM. Transmission electron microscope (TEM). The rubber samples were firstly sliced with
a diamond knife with an ultramicrotome (Leica Microsystems). And the specimens were observed by TEM (H-7650, Hitachi, Japan) with an accelerating voltage of 80
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kV.
Heat/oxygen aging was carried out in an age-circulating oven (U-CAN UA-2071B) at 70
for 3 or 6 days according to GB/T3512-2001 standard.
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The oxidation induction time (OIT) measurement was measured by TA Q20 DSC instrument according to ISO 11357-6-2008. The sample was firstly equilibrate at 30 , then the sample was heated with 20 kept at 60 170
/min to 60
. Subsequently, the sample was
for another 3 min with a nitrogen flow of 50 mL/min and then heated to
at a rate of 20 /min, and the gas was switched to oxygen at a flow rate of
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50mL/min. The oxidation of the sample was observed until a sharp increase in heat flow due to the exothermic reaction of the oxidation reaction. The OIT was obtained from the software of the TA Q20.
Thermal gravimetric analysis was carried out in a TA Q5000 thermogravimetric
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analyzer over a temperature range from 30 to 700
at a heating rate of 10
/min
under the nitrogen.
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DIN abrasion tests were conducted using a GT-7012-DDIN abrasion tester (Gotech, Taiwan) following ISO standard 4649:2002. The flex-fatigue life was measured using a De Mattia GT-7011-D flexing machine (Taipei, China) according to ASTM D430. The flex-fatigue life was defined by the cycles at which a visible crack, that is, a grade-one crack, appeared. Dynamic mechanical analysis was conducted on a TA DMA Q800 equipment (TA, USA) in the tensile mode. The measurements were carried out at a frequency of 2 Hz, a heating rate of 5
/min over a temperature range of -90 to 90 .
3. Results and Discussion
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3.1 Py–GC/MS of lignin
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Figure 1. Py–GC/MS chromatogram of lignin (the main components are present in the figure).
The Py–GC/MS can be a useful technique to detect the structure of lignin. The results of the Py–GC/MS clearly demonstrate that the lignin contains many phenolic hydroxyl groups (including hindered phenol), which are correlated to the antioxidant activity[15]. And the hindered phenol is widely known as an effective
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antioxidant which could intercept and stabilize free radicals [16]. What is more, researchers also demonstrated that the hindered phenol could delay the vulcanization of rubber [17].
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3.2 Cure characteristics
Figure 2. Curing curves of the NR/silica/lignin compounds at 145°C Table 2. Curing parameters of the rubber compounds* Samples
TS2
T90
ML
MH
ΔM
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(min)
(dN.m)
(dN.m)
(dN.m)
NR/50silica
2.4
8.5
3.3
20.0
16.7
NR/40silica/10lignin
4.3
21.3
3.2
19.3
16.1
NR/30silica/20lignin
4.5
26.3
2.6
17.9
15.3
NR/20silica/30lignin
5.1
29.1
1.9
16.4
14.5
NR/10silica/40lignin
6.4
29.0
2.0
14.3
12.3
NR/50lignin
6.6
32.5
2.0
13.3
11.3
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(min)
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* TS2:scorch time; T90: optimum cure time; ML: the minimum torque; MH: the maximum torque; ΔM: the difference between maximum torque and minimum torque.
The cure-curves and curing characteristics of NR/silica/lignin composites are
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shown in Fig 2 and Table 2. The T90 and TS2 value, on the whole, increases with lignin loading, indicating that the vulcanization were delayed with the increase of lignin/silica ratio. Compared with silica, probably the OH-containing lignin can adsorb more rubber addictives. Researchers also report other OH-containing filler, such as montmorillonite[18], could absorb the rubber additives and prolong the
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vulcanization. What is more, lignin could delay vulcanization of rubber for its radical scavenging effect because the lignin contains the hindered phenol. The MH (the maximum torque) of the NR/silica/lignin composites are decreased with lignin loading, this may have been due to the drop of crosslink density with the addition of
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lignin. The ML(the minimum torque) of the NR/silica/lignin composites is lower than that of the NR/50silica, as it is known, a relatively low ML suggested superior
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flowability of the rubber composites, indicating the replacement of silica by lignin could lead to a better processability. 3.3 Calculation of bound rubber and crosslink density from swelling behavior
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Figure 3. Bound rubber and crosslink density of the NR/silica/lignin compounds
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When rubber and a filler are mixed, they interact in such a way that even a good
solvent can only partially dissolve the rubber which originally is completely soluble in
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the solvent. The insoluble rubber is often called as the bound rubber. For a given elastomer, the amount of bound rubber at a fixed filler content depends on a number of factors[19], such as the surface area, structure, surface activity and dispersion state of the filler. It should be noted that the lignin and silica were incorporated into the rubber matrix by different methods in this paper. (i.e., the lignin was incorporated into the rubber by latex compounding while the silica were incorporated to the matrix by
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two-roll mill). It is widely acknowledged that the latex-compounding methods could
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achieve a better filler dispersion than the dry mixing.
Figure 4. TEM images of (a) NR/50silica , (b) NR/30silica/20lignin, (c) NR/50lignin. (scale bar is 2μm)
To demonstrate that the filler dispersion state is improved with the increase of lignin, we conducted the TEM test and the results is shown in Fig.4. The TEM results clearly showed that filler dispersion improved with the increase of lignin. Although a worse interfacial adhesion between the filler and rubber matrix is associated with the addition of lignin (the interfacial adhesion will be discussed in the
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part of mechanical properties, filler dispersion and interfacial bonding analysis). As shown in Fig.3, the increase of bound rubber is attributed to the significant improvement of filler dispersion with the increase of lignin, what is more, the density
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of lignin(1.3 g/cm3)[20] is much lower than that of silica(2.2 g/cm3)[21], and the filler volume is increasing with the lignin content, which means that more rubber will be trapped or adsorbed, thus more bound rubber would be generated with the increase of lignin.
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Fig.3 clearly showed that the crosslink density decreased with the increase of lignin, this is in accordance with the curing characteristic.
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3.4 Dependence of shear modulus (G’) for the rubber compounds on strain
Figure 5. Shear modulus (G') of the strain for uncured(a) and cured(b) compounds
The strain sweep was carried out by rubber processibility analyzer (RPA) to
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identify the network condition. As shown in Fig.5(a), when the strain amplitude is above certain value, the shear modulus (G') of all rubber compounds decreases rapidly.
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This scenario can be explained by the Payne effect [22], which is associated with destruction of the filler networks and the deformation-induced changes in the microstructure. The value of G' for NR/50silica compound at low strains is high,
indicating the formation of strong silica network in the rubber matrix. The value of G'
for NR/50lignin is much lower than that of NR/50silica, and it is clearly that the value of G' of the NR/silica/lignin composites decrease with the increase of lignin. It is well known that silica has strong filler–filler interaction[7]. Nevertheless, lignin networks were relatively weaker than those of silica. The substitution of silica by lignin diluted
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the silica network and lead to a decreases in the silica–silica interaction. Thus the replacement of silica by lignin leads to a decrease in Payne effect. Other researchers using other bio-based filler, such as cellulose[23], to replace the silica in NR
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composites, and observe the similar result that the Payne effect is weakened. As shown in Fig.5(b) The G' of the cured compounds decreased with the increase of
lignin, this can also be ascribed to the drop of crosslink density of the rubber composites with the increase of lignin.
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3.5 mechanical properties, filler dispersion and interfacial bonding analysis
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Figure 6. Typical tensile stress–strain curves for NR/lignin/silica
Figure 7. Tensile modulus and strength for NR/lignin/silica
Figure 8. Hardness, permanent set and elongation for NR/lignin/silica
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As shown in figure 6~8, it is evident that the hardness, 300% and 500% modulus of the samples tend to decrease with the increase of lignin, one of the reasons for this is that
the lignin is a soft filler and silica is a hard filler (the modulus of lignin has been
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estimated to 2~6.3 GPa[20, 24], while the modulus of the silica is about 70 GPa[25]). As indicated by the Halpin–Tsai model, the modulus of the composites is closely with
the modulus of the filler[26]. We can image that the rigid filler has a higher
reinforcement efficiency than the soft filler when the other condition are same. As noted
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before, the composites with higher lignin content have a smaller crosslinking density. As
a consequence, a rise in elongation at break and permanent set are observed with the increase of lignin. However, it seems that the replacement of silica by lignin in the blends
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does not severely deteriorate tensile strength of composites when the lignin is no more
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than 20phr.
Figure 9. SEM photos of the quenched fracture surface. (a, a': NR/50silica. b, b': NR/30silica/20lignin. c, c': NR/50lignin) Fig. 9 shows the quenched fracture surfaces of NR/50silica, NR/30silica/20lignin
and NR/50lignin composites. In the sample of NR/50silica (Fig 9a'), there are small amounts of silica tended to agglomerate. However, in the sample of NR/50lignin (Fig 9c'), the lignin particles are embedded in the natural rubber matrix, and there are few lignin
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agglomerates or clusters, indicating the lignin is dispersed better than silica. As previously mentioned, this is because that the lignin were incorporated into the rubber by latex compounding while the silica were incorporated to the matrix by two-roll mill. In
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the NR/30silica/20lignin system, we can conclude that some lignin particles might be inserted between silica particles, which would reduce the re-aggregation of silica particles, and the overall filler dispersion state is improved with the increase of lignin.
The SEM results further demonstrated the filler dispersion is improved with the
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increase of lignin, and this is agree with the TEM results.
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Figure. 10 SEM photos of the tensile fracture surfaces. a, a': NR/50silica. b, b': NR/30silica/20lignin. c, c': NR/50lignin(red circles indicate the debonding of lignin)
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As shown in Fig.10, the tensile fracture surface of NR/50silica (Fig.10 (a, a') is
rugged, indicating the matrix could transfer the applied stress to the silica during the tensile test. However, the tensile fracture of NR/50lignin is relatively smooth, and there are many holes in the tensile fracture surface, indicating the lignin filler could debond from matrix during the tensile test, thus we infer that the interfacial bonding between filler and matrix in NR/silica/lignin system is decreasing with the increase of lignin. To further evaluate the interaction between filler and rubber matrix, the tensile stress–strain curves was analyzed by the Mooney–Rivlin equation.[27] As written in
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eqn (3). δ*=δ/2(λ−λ−2)= C1+ C2/λ
eqn (3)
whereδ* is the reduced stress, and δ is the stress applied, λ is the extension ratio
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of deformed and undeformed length, and C1 and C2 are constants which are independent of λ. All the curves exhibit a “U” shape, the upturn of the Mooney– Rivlin plot could indicate the interaction between the rubber and filler, and it could be
used to evaluate the limited extensibility of the polymer chains[28] and the interfacial
adherence between filler and rubber matrix[29]. Usually, the upturn occurs at a lower
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λ (corresponding to higher λ -1) in the curves indicates a stronger interfacial
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adherence between the filler and rubber matrix[30, 31].
Figure. 11 plots ofδ* versus λ−1 for the rubber composites.
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As shown in Fig.11, the value of the λ−1 at which the upturn occurs decreases with the increase of lignin/silica ratio, indicating the finite chain extensibility would
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appear at a larger deformation with the increase of lignin/silica ratio. It is logical to infer that the interfacial adhesion of rubber and filler becomes weaker with the increase of lignin/silica ratio. 3.6 The Influence of lignin/silica ratio to abrasion resistance, crack resistance and Heat build-up properties.
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Figure 12. Effect of the silica/lignin ratio on the DIN abrasion loss, flex-fatigue life and Heat build-up of NR/silica/lignin composites
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As is well known, NR is widely used as the tread rubber, abrasion resistance is one of the most important properties. However, to the best of our knowledge, there
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are not much reports about the silica/lignin hybrid filler towards abrasion resistance of NR rubber. The abrasion resistance of vulcanizate, expressed as volume loss. The lower volume loss means the higher abrasion resistance of the vulcanizate. As shown in Fig 12, the abrasion resistance of compounds is initially rised when the lignin is no more than 20phr, this is because that the filler dispersion state is improved with the increase of lignin. The filler dispersion plays an important role towards the abrasion
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resistance. For instance, N. Rattanasom [7] studied the influence of dispersion of the carbon black/silica hybrid filler on abrasion property and reported that a vulcanizate with a good filler dispersion had a better wear property than that with poor filler dispersion. However, when the lignin content is more than 20phr, the drop of
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crosslink density, hardness, modulus plays a dominant role[32], thus the abrasion resistance starts to decrease. The vulcanizate containing 20phr lignin and 30phr of
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silica exhibited the highest anti-abrasion resistance. It is interesting that the crack-resistance is enhanced with the increase of lignin,
because there are three factors regulating the crack-resistance of the rubber compounds. Firstly, the filler dispersion is increased with loading of lignin. In general, the crack-resistance property is improved by improving the dispersion[33]. Secondly, the crosslink density is decreased with the loading of lignin, and a lower crosslink density could result in higher fatigue life in an intermediate range of crosslink concentration.[34] Thirdly, the flex process is a complicated process which also
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involves the mechano-oxidative aging process[35, 36], and the lignin could act as antioxidant which could partially avoid the deleterious effects. In the heat build-up test, the friction among filler particles as well as the friction
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between filler and rubber matrix under repeated compression, causing energy dissipation as heat. As shown in Fig 12, the heat build-up of the samples increase with the loading of lignin. Generally, filler dispersion and interfacial interaction are the two
critical factors, which determine the heat build-up of rubber composites. In some
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cases, the filler dispersion and interfacial interaction of the rubber composite might not improve simultaneously[37]. Though the filler dispersion is improved and the
filler networks is depressed with the increase of the lignin. However, the rubber-filler
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adhesion is not improved with the increase of lignin. Gusev[38] indicated that energy dissipation was mainly completed by the interfacial phenomena rather than the filler network, thus there is a serious friction between rubber matrix and filler when the lignin loading is high, which cause the energy dissipation as heat. In addition, the crosslink density decreased upon addition of lignin. It has been reported in other
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experiments that the heat build-up of the vulcanizate increases with decreasing crosslink density[39]. The decreased crosslink density of the rubber also leads severer chain friction during cyclic deformation. The combination of inferior interface and the drop of crosslink density resulted in a deterioration in the heat-build up property of
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the rubber composites with higher loading of lignin. However, the results show that the partial replacement of silica by lignin does not seriously deteriorate the heat-build
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up property of the rubber composites when the lignin is no more than 20phr.
3.7 The Influence of lignin/silica ratio to the ageing-resistant performance of the composites
Table 3. The Change of the mechanical properties after aging*
silica/lignin ratio
50/0
40/10
30/20
20/30
40/10
50/0
Change of the properties after ageing for 3 days Tensile strength (%)
-15.5 +7.4
+12.5
+10.4
+10.1
+15.6
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elongation at break (%)
-39.4 -15.1
-13.8
-18.8
-16
-10.1
Shore A hardness
+3
+2
+1
+2
+1
+3
Change of the properties after ageing for 6 days -49.5 +4.8
+7.3
+3.6
elongation at break (%)
-56.1 -26.1
-20.8
-23
Shore A hardness
+5
+4
+3
+4
+3.3
+4.3
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Tensile strength (%)
-21.4
-19.9
+2
+2
*all the change of the mechanical properties are compared with unaged samples
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Thermal oxidative aging is the most common form of rubber aging, diene-based
rubbers with unsaturated carbon-carbon bonds are vulnerable to thermal/oxygen ageing due to the oxidative scission of unsaturated chains and sulphidic crosslinks[40],
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which would lead to reduction in the mechanical properties of composites[41]. Therefore, the development of a natural rubber with high ageing resistance is highly desired. Table 3. shows the changes of the mechanical properties after ageing for 3 or 6 days for the rubber composites. The elongation at break for the NR/50silica composite decreased drastically with increasing ageing time, and the NR/50silica
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sample lost half of its original tensile strength and became brittle after 6 days. In contrast, the lignin-containing vulcanizates exhibited substantially increased anti-ageing performance. Many researcher find that the lignin can act as a potential antioxidant,[42] this excellent anti-ageing performance was attributed to lignin's
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hindered phenolic hydroxyl groups which can act as stabilizer of reactions induced by oxygen and its radical species.[43, 44] Even after being aged for 6 days, the samples
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containing lignin were still flexible and elastic. The tensile strength and modulus even shown an increase, this is because that large number of pendant groups in the lignin-filled vulcanizates take part in further crosslinking during aging.[45] In a word, the sample that containing lignin have an excellent anti-aging properties.
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Figure 13. Determination of OIT
To further study the anti-aging properties of the NR/silica/lignin composites. The
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rubber composites are subject to the oxidation induction time (OIT) test. Usually, the more resistant the sample is to oxidative degradation, the longer the OIT value is. [46, 47] As shown in Fig.13, The OIT values of the samples increases with the loading of
lignin. Meanwhile, it is clear that the oxidation exothermic peak of the NR/50lignin composites is gentle and smooth while the peak of NR/50silica is sharp, indicating
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that the oxidation reaction of NR/50lignin is not severe as that of NR/50silica. The OIT test is agreed with mechanical performance after aging. The OIT test could further demonstrate that the lignin could improve the anti-oxidative property of the rubber composites.
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3.8 TGA analysis
Fig. 14 shows the TG and DTG curves of NR/silica/lignin composites with
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various ratio of silica/lignin. Table 4 shows the thermal degradation characteristics of the composites. The thermal degradation behavior of all NR/silica/lignin composites with one main mass loss step are similar, no separate degradation stage can be found in any TG curves of the composites. A shift of 5, 10% weight loss to lower temperature can be observed with the increase of lignin. For instance, the T5% and T10% of NR/50lignin is 276.9 and 329.4 , which are about 40 and 20
lower than those
of NR/50silica. The results shows that the thermostability of the NR/silica/lignin composites become increasingly worse with the increase of lignin, this result may be originated from the reason that the silica is inorganic mater with excellent thermal
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stability while the lignin is organic biomaterial with poor thermal stability.
Table 4. Thermal degradation characteristics of the rubber compounds* T5%( )
T10%( ) T50%( )
NR/50silica
316.8
349.2
399.9
NR/40silica/10lignin
306.5
345.2
396.6
NR/30silica/20lignin
296.2
341.2
393.8
NR/20silica/30lignin
288.9
337.9
393.2
NR/10silica/40lignin
281.5
333.3
391.5
NR/50lignin
276.9
329.4
388.1
Tmax( ) 376.8
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Samples
378.0
377.9 377.4 376.8 376.3
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*T5% 5% weight loss temperature. T10% 10% weight loss temperature. Tmax peak degradation temperature.
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3.9 DMA analysis
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Figure 14. TG (a) and TGA (b) curves of NR/silica/lignin
Figure 15. Storage modulus E' and loss modulus E'' of NR/50silica, NR/30silica/20lignin and NR/50lignin composites
the
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Figure 16. Tanδ of NR/50silica, NR/30silica/20lignin and NR/50lignin composites
As shown in Fig 15. The E' values of NR/50silica and NR/30silica/20lignin
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composites show an improvement compared with that of NR/50lignin, indicating that the elastic response of NR/50silica and NR/30silica/20lignin towards deformation is superior to that of NR/50lignin. this is probably attributed to the drop of crosslinking density after the replacement of the silica by the lignin.
The tan δ of rubber composites, representing the ratio of the energy loss/energy stored (E''/E'), the value of Tanδ at 0
and 60
could represent the wet grip
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property and rolling resistance, respectively. Using the hybrid filler is an effective strategy to counterpoise the rolling resistance, wet skid resistance[5, 7]. As shown in Fig 16, the tanδ(60
) of NR/50silica, NR/30silica/20lignin, NR/50lignin is 0.056,
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0.057, 0.096, respectively. It is well know that the silica filler could achieve a low rolling resistance in green tires[48]. Compared with NR/50silica, the tan δ of ) of NR/50silica,
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NR/30silica/20lignin increased slightly. The tan δ (0
NR/30silica/20lignin, NR/50lignin is 0.096, 0.114, 0.124, respectively. Indicating the wet grip property of the NR/50lignin and NR/30silica/20lignin is better than that of NR/50silica. This is probably because that lignin and silica are two different kinds of fillers that shows different viscoelastic property. In a word, the DMA tests showed that the replacement of silica by 20phr lignin in the blends did not deteriorate rolling resistance property while increase the wet grip property of the rubber composites. Further work will focus on the mechanism of the rolling resistance, wet skid resistance of lignin and lignin/silica hybrid filler.
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4. Conclusions It can be concluded that using silica and lignin as a hybrid filler could combine both advantages of the two fillers. Partial replacement of silica by lignin (no more
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than 20phr) in the blends did not severely deteriorate mechanical properties of composites, and the inclusion of lignin into the rubber could not only weaken the Payne
effect
but
also
improve
processability,
anti-aging
resistance
and
anti-flex cracking of composites. The vulcanizate containing 20phr lignin and 30phr
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of silica in hybrid filler exhibited the optimal overall mechanical properties. The DMA results indicated that NR/30silica/20lignin synchronously possesses the high
wet grip property and low rolling resistance, which makes it promising for the green
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tire products. What is more, it is much easier to incorporate 30phr silica into NR/20lignin matrix than incorporate 50phr silica into NR matrix by two-roll mixing. In summary, lignin partially replacing silica, through stated method, lead to well performance of the rubber according to investigated aspects, along with environmental benefits and economic interests.
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Acknowledgements
All the authors acknowledge financial support from Science and Technology Project of Guangdong Province(2015B010122002), Science and Technology Project of Guangzhou(201508020090), and National Natural Science Foundation of China
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S0142-9418(16)30338-5
DOI:
10.1016/j.polymertesting.2016.07.014
Reference:
POTE 4714
To appear in:
Polymer Testing
Received Date: 13 April 2016 Revised Date:
21 June 2016
Accepted Date: 13 July 2016
Please cite this article as: P. Yu, H. He, Y. Jia, T. Shenghui, C. Jian, D. Jia, Y. Luo, A comprehensive study on lignin as a green alternative of silica in natural rubber composites, Polymer Testing (2016), doi: 10.1016/j.polymertesting.2016.07.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A comprehensive study on lignin as a green alternative of silica in natural rubber composites Peng Yu, Hui He*, Yunchao Jia, Tian shenghui, Chen Jian, Demin Jia, Yuanfang Luo
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School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510641, People's Republic of China
* Corresponding author: Hui He, E-mail address: [email protected] Abstract
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Using waste materials in polymeric products has drawn great attention over the recent years for economic and environmental concerns. Lignin, which is byproducts
of paper-making industry, can be used as a valuable rubber filler. Silica is one of main
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fillers in rubber industry. Since using two different filler may exhibit some synergistic effects in polymers by making full use of each filler's advantage and character. In this research, we investigated the influence of lignin/silica hybrid filler towards natural rubber. The results revealed that the partial replacement of silica by lignin in the blends did not severely deteriorate mechanical properties of the composites. Besides,
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the inclusion of lignin into the rubber could not only weaken the Payne effect but also improve the processability, anti-aging resistance and anti-flex cracking of composites. The vulcanizate containing 20phr lignin and 30phr of silica in hybrid filler exhibited the optimal overall mechanical properties.
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Keywords: lignin, rubber, silica, hybrid filler, composites 1. Introductions
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Natural rubber is an important industrial raw material which is used extensively in many applications, although NR is known to exhibit numerous outstanding properties, reinforcing fillers are often required in this matrix to improve the modulus, hardness, wear resistance and reduce the material cost. Lignin, as the second most abundant renewable and biodegradable natural resource next to cellulose, is a complex macromolecule which is based on the repetition of three different phenylpropane units. In the past, lignin was mainly burned to produce energy[1], and this was a low-valued utilization. The research of high-value-added application of lignin has considerable significance for both utilization of renewable resources and
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environmental protection. The impressive properties of lignin, such as its low cost, high abundance, low density, environmentally friendly, and bio-renewable [2] make it even more attractive as a reinforcing filler for the rubber. Recently, various academic
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papers regarding preparations of rubber/lignin compounds have been published [3-5]. Silica, which can increase the wet grip and reduce the rolling resistance when it is used in tires, is widely used as an indispensable rubber reinforcing filler that not relay on the fossil fuel.
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Inclusion of hybrid filler into the elastomers has drawn the attentions of many researchers. The most successful commercial hybrid filler is probably the carbon black–silica dual phase filler (CSDPF) introduced by Cabot Corporation[6], the
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CSDPF has a high polymer-filler interaction and low filler-filler interaction, it has been shown to give better overall mechanical properties in comparison with the conventional carbon black and silica fillers. Recently, N. Rattanasom et al [7], reinforced the natural rubber with silica/carbon black hybrid filler. Tang et al.[8] used the hybrid consisting of halloysite and graphene as a reinforcement for elastomers.
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Amit Das et al. [9] used the combination of expanded graphite with multiwalled carbon nanotubes to reinforce the rubber. Many researchers hybrid lignin with other filler,
such as carbon
black[5], montmorillonite[10],
and layered
double
hydroxides[11] to improve the mechanical or dynamic mechanical performance of
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rubber/lignin composites.
To the best of our knowledge, there is not much systematic scientific report on
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the effect of lignin/silica hybrid filler on the cure, mechanical, dynamic mechanic and anti-ageing properties of NR vulcanizates. Since each filler possesses its own advantages, the use of lignin/silica blends should enhance the overall performance of NR vulcanizates. Usually, lignin is incorporated into the rubber matrix by two different methods, i.e. lignin-rubber latex co-precipitate, or incorporation of lignin as dry powder into rubber. It should be noted that lignin as dry powder straightforward milled into rubber shows almost no reinforcing effect due to the strong intermolecular hydrogen bonding[12] and serious agglomeration of the lignin. In this article, NR/lignin compounds were firstly co-coagulated by latex compounding method to
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achieve the fine dispersion of lignin in NR matrix. After that, the silica, interfacial modifier, and rubber additives were added in the NR/lignin compounds by dry-milling method. The NR/silica/lignin composites with various silica/lignin ratio are evaluated
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by: cure properties, crosslink density, stress–strain behavior, abrasion resistance, ageing-resistant, anti-flex cracking resistance and so on. The effects of silica/lignin ratio on various mechanical properties of NR vulcanizates were systematically studied
and reported. The optimum lignin/silica ratio giving rise to the optimum overall
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properties was also reported. 2. EXPERIMENTAL 2.1 Materials
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NR latex with a solid content of 60wt% was purchased from China Hainan Rubber Industry Group Co., Ltd. Precipitated silica (type FINE-SIL518, median diameter (d50) is 5 µm, the pH value is 6.5~7.5, specific surface area is 200~220 m2/g ) was produced by Wanzai Huiming Chemical industry Co., Ltd. (Jiangxi, China). Alkali lignin (type LN-100, bagasse source, lignin content in LN-100 is
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92wt%, the average molecular weight of lignin is 4235g/mol and the polydispersity index is 2.4) was kindly provided by Linge Polymer Material Co., Ltd. (China). Si69, a renowned and widely utilized coupling as an interfacial modifier to boost the compatibility between hydroxyl-containing filler and rubber matrix, was kindly
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provided from Longsunchem Co., Ltd. (China). All the rubber additives such as zinc oxide (ZnO), stearic acid (SA), N-tertbutyl-2-benzothiazole sulfonamide (CBS),
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2,2’-dibenzothiazoledisulfde (DM), tetramethylthiuram disulfide (TMTD), sulfur (S) and process oil were industrial grade and used as received. Other ingredients were
bought from local sources. 2.2 Samples preparation In order to achieve the fine dispersion of lignin in rubber matrix, we prepared
NR/lignin/ by latex compounding method. Lignin was dissolved in deionized water at a mass concentration of 10% and the pH was adjusted to about 12. The NR latex and lignin aqueous suspension were first mixed, followed by vigorously stirring via mechanical agitation for over 30 min. After that, the mixed solution was coagulated
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by the diluted acid. The coagulated mixtures were filtered and washed with water. Lastly, the compounds were vacuum dried. The NR/lignin compounds were compounded with precipitated silica filler and rubber additives according to the Table
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1 by two-roll mixing. (NR/lignin, silica powder, process oil and Si69 was firstly compounded on the X(S)K-160 two-roll mill (Zhanjiang, China). Then the
vulcanizing ingredients were mixed in conventional order (ZnO and Stearic acid were
added firstly, followed by CZ, DM, TT. The Sulfur was added lastly). The rolls run at
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a speed ratio of 1 (front roll):1.2 (back roll). The mixing time and temperature are
about 12 min and room temperature, respectively.) After that, the rubber compounds
U-CAN UR-2030 vulcameter.
for the optimum curing time determined by the
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were compression molded at 145
Table 1. Compounding formulation (phr)* Sample code
NR/Lignin
Silica powder
100/0
50
100/10
40
NR/30silica/20lignin
100/20
30
NR/20silica/30lignin
100/30
20
NR/10silica/40lignin
100/40
10
NR/50lignin
100/50
0
NR/50silica
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NR/40silica/10lignin
*the rubber ingredients of composites were fixed as: ZnO 5, Stearic acid 2, Process oil 5,
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Si69 4, accelerator CZ 1.5, accelerator DM 0.5, accelerator TT 0.3, Sulfur 2.
2.3 Characterizations Pyrolysis Gas Chromatography-Mass Spectrometry. Lignin LN-100 was subject to Pyrolysis Gas Chromatography-Mass Spectrometry (Py–GC/MS) test (Agilent Technologies, GC-7890B, MS-5977A. USA). Chromatographic separation was achieved by using a column which held at 50 °C for 2 min followed by a ramped temperature increase to 250 °C at a rate of 5 °C/min. Finally a 10 °C/min ramp applied to the temperature of 320 °C. 1 mg of lignin was used. The sample was pyrolysed rapidly at 800 °C. The residue of the pyrolysis was not detected. The ion
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source’s temperature was 230 °C. The helium was used as carrier gas at a flow rate of 1 mL/min. A split ratio of 100:1 was used. The chromatograms gathered were analyzed using NIST 011 library. ‐by U-CAN
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The curing characteristics of the compounds were determined at 145 UR-2030 vulcameter (U-CAN Dynatex Inc., Taiwan).
Prediction of uncured samples’ bound rubber was done as follows [13], the uncured rubber composite was divided into scraps and packaged with 200 mesh metal net, then
And the bound rubber was calculated (eqn (1)): ( )
∗ 100%
(1)
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bound rubber content =
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immersed in toluene for 3 days. Then the swollen sample was dried in vacuum oven.
where W1 is the rubber mass in the sample, W2 is the initial sample quality including metal net, and W3 is the sample quality including metal net after swelling and drying. The volume fraction of rubber in the swollen gel (Vr), which was used to represent the apparent crosslinking density of the vulcanizates, was done according to equilibrium
(eqn (2)):
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swelling experiments as reported in related referrence [14]. And it was calculated.
(∗)/ρ
= (∗)/ρ
A! /ρ"
(2)
where H is the weight of the test specimen, D is weight of the de-swollen test
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specimen, F is weight fraction of the insoluble components, Ao is weight of the absorbed solvent, ρs and ρr is the density of solvent and vulcanizate, respectively.
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Strain sweep of rubber composites was carried out by a Rubber Process Analyzer (RPA2000, Alpha technologies Co.) at 60°C and a frequency of 1 Hz. Tensile tests were performed following ISO 37-2005 at room temperature using U-CAN UT-2060 tensile instrument (Taiwan). The heat build-up the vulcanizates were examined using a Gotech RH-2000N Machine (Gotech Testing Machines Inc., Taiwan). Cylindrical rubber specimens (25 mm in height and 17.5 mm in diameter) were subjected to repeated compression according to ISO4666/3-1982. Scanning electron micrographs (SEM) of the composites were conducted by a Nova
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NanoSEM 430 instrument (FEI, the Netherlands). The tensile fracture surfaces were directly obtained from the tensile tests. The quenched fracture surfaces were obtained by splitting bulk sample being quenched in liquid nitrogen. After that, all the surfaces
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were sputter coated with gold before examining under the SEM. Transmission electron microscope (TEM). The rubber samples were firstly sliced with
a diamond knife with an ultramicrotome (Leica Microsystems). And the specimens were observed by TEM (H-7650, Hitachi, Japan) with an accelerating voltage of 80
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kV.
Heat/oxygen aging was carried out in an age-circulating oven (U-CAN UA-2071B) at 70
for 3 or 6 days according to GB/T3512-2001 standard.
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The oxidation induction time (OIT) measurement was measured by TA Q20 DSC instrument according to ISO 11357-6-2008. The sample was firstly equilibrate at 30 , then the sample was heated with 20 kept at 60 170
/min to 60
. Subsequently, the sample was
for another 3 min with a nitrogen flow of 50 mL/min and then heated to
at a rate of 20 /min, and the gas was switched to oxygen at a flow rate of
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50mL/min. The oxidation of the sample was observed until a sharp increase in heat flow due to the exothermic reaction of the oxidation reaction. The OIT was obtained from the software of the TA Q20.
Thermal gravimetric analysis was carried out in a TA Q5000 thermogravimetric
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analyzer over a temperature range from 30 to 700
at a heating rate of 10
/min
under the nitrogen.
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DIN abrasion tests were conducted using a GT-7012-DDIN abrasion tester (Gotech, Taiwan) following ISO standard 4649:2002. The flex-fatigue life was measured using a De Mattia GT-7011-D flexing machine (Taipei, China) according to ASTM D430. The flex-fatigue life was defined by the cycles at which a visible crack, that is, a grade-one crack, appeared. Dynamic mechanical analysis was conducted on a TA DMA Q800 equipment (TA, USA) in the tensile mode. The measurements were carried out at a frequency of 2 Hz, a heating rate of 5
/min over a temperature range of -90 to 90 .
3. Results and Discussion
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3.1 Py–GC/MS of lignin
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Figure 1. Py–GC/MS chromatogram of lignin (the main components are present in the figure).
The Py–GC/MS can be a useful technique to detect the structure of lignin. The results of the Py–GC/MS clearly demonstrate that the lignin contains many phenolic hydroxyl groups (including hindered phenol), which are correlated to the antioxidant activity[15]. And the hindered phenol is widely known as an effective
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antioxidant which could intercept and stabilize free radicals [16]. What is more, researchers also demonstrated that the hindered phenol could delay the vulcanization of rubber [17].
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3.2 Cure characteristics
Figure 2. Curing curves of the NR/silica/lignin compounds at 145°C Table 2. Curing parameters of the rubber compounds* Samples
TS2
T90
ML
MH
ΔM
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(min)
(dN.m)
(dN.m)
(dN.m)
NR/50silica
2.4
8.5
3.3
20.0
16.7
NR/40silica/10lignin
4.3
21.3
3.2
19.3
16.1
NR/30silica/20lignin
4.5
26.3
2.6
17.9
15.3
NR/20silica/30lignin
5.1
29.1
1.9
16.4
14.5
NR/10silica/40lignin
6.4
29.0
2.0
14.3
12.3
NR/50lignin
6.6
32.5
2.0
13.3
11.3
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(min)
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* TS2:scorch time; T90: optimum cure time; ML: the minimum torque; MH: the maximum torque; ΔM: the difference between maximum torque and minimum torque.
The cure-curves and curing characteristics of NR/silica/lignin composites are
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shown in Fig 2 and Table 2. The T90 and TS2 value, on the whole, increases with lignin loading, indicating that the vulcanization were delayed with the increase of lignin/silica ratio. Compared with silica, probably the OH-containing lignin can adsorb more rubber addictives. Researchers also report other OH-containing filler, such as montmorillonite[18], could absorb the rubber additives and prolong the
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vulcanization. What is more, lignin could delay vulcanization of rubber for its radical scavenging effect because the lignin contains the hindered phenol. The MH (the maximum torque) of the NR/silica/lignin composites are decreased with lignin loading, this may have been due to the drop of crosslink density with the addition of
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lignin. The ML(the minimum torque) of the NR/silica/lignin composites is lower than that of the NR/50silica, as it is known, a relatively low ML suggested superior
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flowability of the rubber composites, indicating the replacement of silica by lignin could lead to a better processability. 3.3 Calculation of bound rubber and crosslink density from swelling behavior
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Figure 3. Bound rubber and crosslink density of the NR/silica/lignin compounds
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When rubber and a filler are mixed, they interact in such a way that even a good
solvent can only partially dissolve the rubber which originally is completely soluble in
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the solvent. The insoluble rubber is often called as the bound rubber. For a given elastomer, the amount of bound rubber at a fixed filler content depends on a number of factors[19], such as the surface area, structure, surface activity and dispersion state of the filler. It should be noted that the lignin and silica were incorporated into the rubber matrix by different methods in this paper. (i.e., the lignin was incorporated into the rubber by latex compounding while the silica were incorporated to the matrix by
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two-roll mill). It is widely acknowledged that the latex-compounding methods could
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achieve a better filler dispersion than the dry mixing.
Figure 4. TEM images of (a) NR/50silica , (b) NR/30silica/20lignin, (c) NR/50lignin. (scale bar is 2μm)
To demonstrate that the filler dispersion state is improved with the increase of lignin, we conducted the TEM test and the results is shown in Fig.4. The TEM results clearly showed that filler dispersion improved with the increase of lignin. Although a worse interfacial adhesion between the filler and rubber matrix is associated with the addition of lignin (the interfacial adhesion will be discussed in the
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part of mechanical properties, filler dispersion and interfacial bonding analysis). As shown in Fig.3, the increase of bound rubber is attributed to the significant improvement of filler dispersion with the increase of lignin, what is more, the density
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of lignin(1.3 g/cm3)[20] is much lower than that of silica(2.2 g/cm3)[21], and the filler volume is increasing with the lignin content, which means that more rubber will be trapped or adsorbed, thus more bound rubber would be generated with the increase of lignin.
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Fig.3 clearly showed that the crosslink density decreased with the increase of lignin, this is in accordance with the curing characteristic.
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3.4 Dependence of shear modulus (G’) for the rubber compounds on strain
Figure 5. Shear modulus (G') of the strain for uncured(a) and cured(b) compounds
The strain sweep was carried out by rubber processibility analyzer (RPA) to
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identify the network condition. As shown in Fig.5(a), when the strain amplitude is above certain value, the shear modulus (G') of all rubber compounds decreases rapidly.
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This scenario can be explained by the Payne effect [22], which is associated with destruction of the filler networks and the deformation-induced changes in the microstructure. The value of G' for NR/50silica compound at low strains is high,
indicating the formation of strong silica network in the rubber matrix. The value of G'
for NR/50lignin is much lower than that of NR/50silica, and it is clearly that the value of G' of the NR/silica/lignin composites decrease with the increase of lignin. It is well known that silica has strong filler–filler interaction[7]. Nevertheless, lignin networks were relatively weaker than those of silica. The substitution of silica by lignin diluted
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the silica network and lead to a decreases in the silica–silica interaction. Thus the replacement of silica by lignin leads to a decrease in Payne effect. Other researchers using other bio-based filler, such as cellulose[23], to replace the silica in NR
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composites, and observe the similar result that the Payne effect is weakened. As shown in Fig.5(b) The G' of the cured compounds decreased with the increase of
lignin, this can also be ascribed to the drop of crosslink density of the rubber composites with the increase of lignin.
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3.5 mechanical properties, filler dispersion and interfacial bonding analysis
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Figure 6. Typical tensile stress–strain curves for NR/lignin/silica
Figure 7. Tensile modulus and strength for NR/lignin/silica
Figure 8. Hardness, permanent set and elongation for NR/lignin/silica
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As shown in figure 6~8, it is evident that the hardness, 300% and 500% modulus of the samples tend to decrease with the increase of lignin, one of the reasons for this is that
the lignin is a soft filler and silica is a hard filler (the modulus of lignin has been
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estimated to 2~6.3 GPa[20, 24], while the modulus of the silica is about 70 GPa[25]). As indicated by the Halpin–Tsai model, the modulus of the composites is closely with
the modulus of the filler[26]. We can image that the rigid filler has a higher
reinforcement efficiency than the soft filler when the other condition are same. As noted
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before, the composites with higher lignin content have a smaller crosslinking density. As
a consequence, a rise in elongation at break and permanent set are observed with the increase of lignin. However, it seems that the replacement of silica by lignin in the blends
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does not severely deteriorate tensile strength of composites when the lignin is no more
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than 20phr.
Figure 9. SEM photos of the quenched fracture surface. (a, a': NR/50silica. b, b': NR/30silica/20lignin. c, c': NR/50lignin) Fig. 9 shows the quenched fracture surfaces of NR/50silica, NR/30silica/20lignin
and NR/50lignin composites. In the sample of NR/50silica (Fig 9a'), there are small amounts of silica tended to agglomerate. However, in the sample of NR/50lignin (Fig 9c'), the lignin particles are embedded in the natural rubber matrix, and there are few lignin
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agglomerates or clusters, indicating the lignin is dispersed better than silica. As previously mentioned, this is because that the lignin were incorporated into the rubber by latex compounding while the silica were incorporated to the matrix by two-roll mill. In
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the NR/30silica/20lignin system, we can conclude that some lignin particles might be inserted between silica particles, which would reduce the re-aggregation of silica particles, and the overall filler dispersion state is improved with the increase of lignin.
The SEM results further demonstrated the filler dispersion is improved with the
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increase of lignin, and this is agree with the TEM results.
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Figure. 10 SEM photos of the tensile fracture surfaces. a, a': NR/50silica. b, b': NR/30silica/20lignin. c, c': NR/50lignin(red circles indicate the debonding of lignin)
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As shown in Fig.10, the tensile fracture surface of NR/50silica (Fig.10 (a, a') is
rugged, indicating the matrix could transfer the applied stress to the silica during the tensile test. However, the tensile fracture of NR/50lignin is relatively smooth, and there are many holes in the tensile fracture surface, indicating the lignin filler could debond from matrix during the tensile test, thus we infer that the interfacial bonding between filler and matrix in NR/silica/lignin system is decreasing with the increase of lignin. To further evaluate the interaction between filler and rubber matrix, the tensile stress–strain curves was analyzed by the Mooney–Rivlin equation.[27] As written in
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eqn (3). δ*=δ/2(λ−λ−2)= C1+ C2/λ
eqn (3)
whereδ* is the reduced stress, and δ is the stress applied, λ is the extension ratio
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of deformed and undeformed length, and C1 and C2 are constants which are independent of λ. All the curves exhibit a “U” shape, the upturn of the Mooney– Rivlin plot could indicate the interaction between the rubber and filler, and it could be
used to evaluate the limited extensibility of the polymer chains[28] and the interfacial
adherence between filler and rubber matrix[29]. Usually, the upturn occurs at a lower
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λ (corresponding to higher λ -1) in the curves indicates a stronger interfacial
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adherence between the filler and rubber matrix[30, 31].
Figure. 11 plots ofδ* versus λ−1 for the rubber composites.
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As shown in Fig.11, the value of the λ−1 at which the upturn occurs decreases with the increase of lignin/silica ratio, indicating the finite chain extensibility would
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appear at a larger deformation with the increase of lignin/silica ratio. It is logical to infer that the interfacial adhesion of rubber and filler becomes weaker with the increase of lignin/silica ratio. 3.6 The Influence of lignin/silica ratio to abrasion resistance, crack resistance and Heat build-up properties.
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Figure 12. Effect of the silica/lignin ratio on the DIN abrasion loss, flex-fatigue life and Heat build-up of NR/silica/lignin composites
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As is well known, NR is widely used as the tread rubber, abrasion resistance is one of the most important properties. However, to the best of our knowledge, there
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are not much reports about the silica/lignin hybrid filler towards abrasion resistance of NR rubber. The abrasion resistance of vulcanizate, expressed as volume loss. The lower volume loss means the higher abrasion resistance of the vulcanizate. As shown in Fig 12, the abrasion resistance of compounds is initially rised when the lignin is no more than 20phr, this is because that the filler dispersion state is improved with the increase of lignin. The filler dispersion plays an important role towards the abrasion
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resistance. For instance, N. Rattanasom [7] studied the influence of dispersion of the carbon black/silica hybrid filler on abrasion property and reported that a vulcanizate with a good filler dispersion had a better wear property than that with poor filler dispersion. However, when the lignin content is more than 20phr, the drop of
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crosslink density, hardness, modulus plays a dominant role[32], thus the abrasion resistance starts to decrease. The vulcanizate containing 20phr lignin and 30phr of
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silica exhibited the highest anti-abrasion resistance. It is interesting that the crack-resistance is enhanced with the increase of lignin,
because there are three factors regulating the crack-resistance of the rubber compounds. Firstly, the filler dispersion is increased with loading of lignin. In general, the crack-resistance property is improved by improving the dispersion[33]. Secondly, the crosslink density is decreased with the loading of lignin, and a lower crosslink density could result in higher fatigue life in an intermediate range of crosslink concentration.[34] Thirdly, the flex process is a complicated process which also
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involves the mechano-oxidative aging process[35, 36], and the lignin could act as antioxidant which could partially avoid the deleterious effects. In the heat build-up test, the friction among filler particles as well as the friction
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between filler and rubber matrix under repeated compression, causing energy dissipation as heat. As shown in Fig 12, the heat build-up of the samples increase with the loading of lignin. Generally, filler dispersion and interfacial interaction are the two
critical factors, which determine the heat build-up of rubber composites. In some
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cases, the filler dispersion and interfacial interaction of the rubber composite might not improve simultaneously[37]. Though the filler dispersion is improved and the
filler networks is depressed with the increase of the lignin. However, the rubber-filler
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adhesion is not improved with the increase of lignin. Gusev[38] indicated that energy dissipation was mainly completed by the interfacial phenomena rather than the filler network, thus there is a serious friction between rubber matrix and filler when the lignin loading is high, which cause the energy dissipation as heat. In addition, the crosslink density decreased upon addition of lignin. It has been reported in other
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experiments that the heat build-up of the vulcanizate increases with decreasing crosslink density[39]. The decreased crosslink density of the rubber also leads severer chain friction during cyclic deformation. The combination of inferior interface and the drop of crosslink density resulted in a deterioration in the heat-build up property of
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the rubber composites with higher loading of lignin. However, the results show that the partial replacement of silica by lignin does not seriously deteriorate the heat-build
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up property of the rubber composites when the lignin is no more than 20phr.
3.7 The Influence of lignin/silica ratio to the ageing-resistant performance of the composites
Table 3. The Change of the mechanical properties after aging*
silica/lignin ratio
50/0
40/10
30/20
20/30
40/10
50/0
Change of the properties after ageing for 3 days Tensile strength (%)
-15.5 +7.4
+12.5
+10.4
+10.1
+15.6
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elongation at break (%)
-39.4 -15.1
-13.8
-18.8
-16
-10.1
Shore A hardness
+3
+2
+1
+2
+1
+3
Change of the properties after ageing for 6 days -49.5 +4.8
+7.3
+3.6
elongation at break (%)
-56.1 -26.1
-20.8
-23
Shore A hardness
+5
+4
+3
+4
+3.3
+4.3
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Tensile strength (%)
-21.4
-19.9
+2
+2
*all the change of the mechanical properties are compared with unaged samples
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Thermal oxidative aging is the most common form of rubber aging, diene-based
rubbers with unsaturated carbon-carbon bonds are vulnerable to thermal/oxygen ageing due to the oxidative scission of unsaturated chains and sulphidic crosslinks[40],
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which would lead to reduction in the mechanical properties of composites[41]. Therefore, the development of a natural rubber with high ageing resistance is highly desired. Table 3. shows the changes of the mechanical properties after ageing for 3 or 6 days for the rubber composites. The elongation at break for the NR/50silica composite decreased drastically with increasing ageing time, and the NR/50silica
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sample lost half of its original tensile strength and became brittle after 6 days. In contrast, the lignin-containing vulcanizates exhibited substantially increased anti-ageing performance. Many researcher find that the lignin can act as a potential antioxidant,[42] this excellent anti-ageing performance was attributed to lignin's
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hindered phenolic hydroxyl groups which can act as stabilizer of reactions induced by oxygen and its radical species.[43, 44] Even after being aged for 6 days, the samples
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containing lignin were still flexible and elastic. The tensile strength and modulus even shown an increase, this is because that large number of pendant groups in the lignin-filled vulcanizates take part in further crosslinking during aging.[45] In a word, the sample that containing lignin have an excellent anti-aging properties.
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Figure 13. Determination of OIT
To further study the anti-aging properties of the NR/silica/lignin composites. The
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rubber composites are subject to the oxidation induction time (OIT) test. Usually, the more resistant the sample is to oxidative degradation, the longer the OIT value is. [46, 47] As shown in Fig.13, The OIT values of the samples increases with the loading of
lignin. Meanwhile, it is clear that the oxidation exothermic peak of the NR/50lignin composites is gentle and smooth while the peak of NR/50silica is sharp, indicating
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that the oxidation reaction of NR/50lignin is not severe as that of NR/50silica. The OIT test is agreed with mechanical performance after aging. The OIT test could further demonstrate that the lignin could improve the anti-oxidative property of the rubber composites.
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3.8 TGA analysis
Fig. 14 shows the TG and DTG curves of NR/silica/lignin composites with
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various ratio of silica/lignin. Table 4 shows the thermal degradation characteristics of the composites. The thermal degradation behavior of all NR/silica/lignin composites with one main mass loss step are similar, no separate degradation stage can be found in any TG curves of the composites. A shift of 5, 10% weight loss to lower temperature can be observed with the increase of lignin. For instance, the T5% and T10% of NR/50lignin is 276.9 and 329.4 , which are about 40 and 20
lower than those
of NR/50silica. The results shows that the thermostability of the NR/silica/lignin composites become increasingly worse with the increase of lignin, this result may be originated from the reason that the silica is inorganic mater with excellent thermal
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stability while the lignin is organic biomaterial with poor thermal stability.
Table 4. Thermal degradation characteristics of the rubber compounds* T5%( )
T10%( ) T50%( )
NR/50silica
316.8
349.2
399.9
NR/40silica/10lignin
306.5
345.2
396.6
NR/30silica/20lignin
296.2
341.2
393.8
NR/20silica/30lignin
288.9
337.9
393.2
NR/10silica/40lignin
281.5
333.3
391.5
NR/50lignin
276.9
329.4
388.1
Tmax( ) 376.8
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Samples
378.0
377.9 377.4 376.8 376.3
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*T5% 5% weight loss temperature. T10% 10% weight loss temperature. Tmax peak degradation temperature.
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3.9 DMA analysis
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Figure 14. TG (a) and TGA (b) curves of NR/silica/lignin
Figure 15. Storage modulus E' and loss modulus E'' of NR/50silica, NR/30silica/20lignin and NR/50lignin composites
the
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Figure 16. Tanδ of NR/50silica, NR/30silica/20lignin and NR/50lignin composites
As shown in Fig 15. The E' values of NR/50silica and NR/30silica/20lignin
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composites show an improvement compared with that of NR/50lignin, indicating that the elastic response of NR/50silica and NR/30silica/20lignin towards deformation is superior to that of NR/50lignin. this is probably attributed to the drop of crosslinking density after the replacement of the silica by the lignin.
The tan δ of rubber composites, representing the ratio of the energy loss/energy stored (E''/E'), the value of Tanδ at 0
and 60
could represent the wet grip
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property and rolling resistance, respectively. Using the hybrid filler is an effective strategy to counterpoise the rolling resistance, wet skid resistance[5, 7]. As shown in Fig 16, the tanδ(60
) of NR/50silica, NR/30silica/20lignin, NR/50lignin is 0.056,
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0.057, 0.096, respectively. It is well know that the silica filler could achieve a low rolling resistance in green tires[48]. Compared with NR/50silica, the tan δ of ) of NR/50silica,
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NR/30silica/20lignin increased slightly. The tan δ (0
NR/30silica/20lignin, NR/50lignin is 0.096, 0.114, 0.124, respectively. Indicating the wet grip property of the NR/50lignin and NR/30silica/20lignin is better than that of NR/50silica. This is probably because that lignin and silica are two different kinds of fillers that shows different viscoelastic property. In a word, the DMA tests showed that the replacement of silica by 20phr lignin in the blends did not deteriorate rolling resistance property while increase the wet grip property of the rubber composites. Further work will focus on the mechanism of the rolling resistance, wet skid resistance of lignin and lignin/silica hybrid filler.
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4. Conclusions It can be concluded that using silica and lignin as a hybrid filler could combine both advantages of the two fillers. Partial replacement of silica by lignin (no more
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than 20phr) in the blends did not severely deteriorate mechanical properties of composites, and the inclusion of lignin into the rubber could not only weaken the Payne
effect
but
also
improve
processability,
anti-aging
resistance
and
anti-flex cracking of composites. The vulcanizate containing 20phr lignin and 30phr
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of silica in hybrid filler exhibited the optimal overall mechanical properties. The DMA results indicated that NR/30silica/20lignin synchronously possesses the high
wet grip property and low rolling resistance, which makes it promising for the green
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tire products. What is more, it is much easier to incorporate 30phr silica into NR/20lignin matrix than incorporate 50phr silica into NR matrix by two-roll mixing. In summary, lignin partially replacing silica, through stated method, lead to well performance of the rubber according to investigated aspects, along with environmental benefits and economic interests.
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Acknowledgements
All the authors acknowledge financial support from Science and Technology Project of Guangdong Province(2015B010122002), Science and Technology Project of Guangzhou(201508020090), and National Natural Science Foundation of China
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