Terpene-Based Sustainable Elastomers: Vulcanization and

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Applied Chemistry

Terpene Based Sustainable Elastomers: Vulcanization and Reinforcement Characteristics Preetom Sarkar, and Anil K. Bhowmick Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00163 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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Industrial & Engineering Chemistry Research

Terpene Based Sustainable Elastomers: Vulcanization and Reinforcement Characteristics

Preetom Sarkar and Anil K. Bhowmick* Rubber Technology Centre, Indian Institute of Technology Kharagpur Kharagpur- 721302, West Bengal, India

*Corresponding author: Email: [email protected] (AKB) Tel.: +91 (3222) 283180; Fax: +91 (3222) 220312

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Allured by the quest to fabricate sustainable elastomers having tangible applications, the present work reports the study of vulcanization and reinforcement characteristics of several sustainable elastomers derived from β-myrcene - a naturally occurring ‘waste’ monoterpene from plant. The elastomers were synthesized by environmentally benign and industrially robust emulsion polymerization method. Conventional elastomer processing techniques were employed for the compounding and vulcanization of the synthesized elastomers. The vulcanization characteristics of the synthesized sustainable elastomers are similar to natural rubber. Polymer microstructure, molecular weight and type and amount of comonomer influence the curing characteristics and physic-mechanical properties of the elastomer vulcanizates. An attempt has been made to elucidate the plausible network structure of the polymyrcene vulcanizate. Due to wide range of polarity, these sustainable elastomers could also aid in dispersion of functional additives. It is thus envisaged that these terpene-based elastomers could replenish the dearth of sustainable elastomers in near future.

KEYWORDS: Bio-based elastomer, Terpene, β-Myrcene, Vulcanization, Reinforcement 2 ACS Paragon Plus Environment

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1. INTRODUCTION Elastomers are a genre of macromolecular materials that encompasses a panoramic application portfolios – from niche eraser to high performance tyre to critically designed spacecraft seal.1 Other than natural rubber (NR), this monumental product portfolios of elastomers are met with the steady supply of a range of synthetic elastomers, which are in turn being fabricated by the petro-derived synthones.2 Dwindling petroleum reserves along with the harmful environmental side effects associated with it, have however, created a great concern about these petro-derived synthetic elastomers. On top of it, the recent surge of the concept of ‘sustainability’ has further enunciated the need of sustainable alternates for these synthetic elastomers. Thus, of late, there has been a growing trend to prepare elastomeric and allied materials from a variety of renewable resources.3-10 Amidst the vast reserve of natural resources, the term ‘terpene’ refers to a family of naturally occurring ‘waste’ product from plants (particularly conifers like Pine and Spruce), limited number of insects, marine microorganisms and fungi.11 Within the plethora of terpene family, the chemical structure of the acyclic mono-terpene: β-myrcene (7-methyl-3methylene-1,6-octadiene) is similar to many petro-based unsaturated hydrocarbons (such as 1,3 butadiene and isoprene) used for synthetic elastomer preparation.12 β-Myrcene is a component of essential oil of various plants like wild thyme, ylang-ylang, bay, cannabis, parsley, hops etc. and its use as a fragrance and flavour is known from old days.11,12 In recent time, several workers have reported the synthesis of polymer and allied materials utilizing this monomer by following a wide range of polymerization techniques.13-18 Although these reports are interesting, they were not carried out with applications in mind and entail the use of specialized catalysts, solvents and rigorous experimental conditions. On the contrary, in the backdrop of sustainability, a green, robust and industrially viable route is preferred for the synthesis of sustainable elastomers.

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Elastomer science and technology has been the subject of interest in our research group for more than thirty years. Motivated by the urge of sustainability and dearth of sustainable elastomers, we have synthesized polymyrcene elastomer by water borne emulsion polymerization method.19 Emulsion polymerization is not only environmentally benign and robust, but also scalable. Most of the synthetic elastomers, adhesives, coatings etc. are prepared by this polymerization method.20 To modify the property of the as synthesized polymyrcene elastomer and to impart a wide range of functionality, several series of copolymers namely: poly(myrcene-co-styrene), poly(myrcene-co-dibutyl itaconate) and poly(myrcene-co-methacrylate) were also fabricated.21-23 The microstructures of these sustainable elastomers were elucidated and their structure-property relationship was established in details. On the other hand, from an application point of view, elastomers are only useful after its transformation into a cross-linked (vulcanized) state.1,2 Appropriate elastomer additives (such as sulfur, accelerator, activator etc.) in requisite amount are mixed with raw elastomers (known as ‘compounding’) in elastomer processing equipments, followed by crosslinking under the dual effect of temperature and pressure for a particular time (known as ‘vulcanization’), to form elastomer vulcanizate.24 Also, in most of the occasions, particulate filler (mostly carbon black) is mixed with gum elastomers to reinforce the product (known as ‘reinforcement’) and improve the failure properties.25 In these days, carbon black is also being manufactured from natural resources such as methane gas and plant wastes.26,27 In this present work, the vulcanization and reinforcement characteristics of several β-myrcene based sustainable elastomers are presented for the first time. This is in view to explore and affirm the utility of such synthesized sustainable elastomers.


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The comonomers – styrene (ST), dibutyl itaconate (DBI) and butyl methacrylate (BM) were chosen due to their different types and functional nature. For example, ST contains aromatic moiety and is one of the comonomers used in the production of SBR, the largest volume of synthetic elastomer used in rubber industries. DBI is a derivative of itaconic acid (rated as a “top value added chemicals from biomass” by US Department of Energy) and is unique due to its unsaturated diester nature. Apart, as the polymyrcene is fully hydrocarbon and non-polar elastomer, the inclusion of DBI as a comonomer would impart polarity into the structure, which would in turn aid in the dispersion of polar additives. BM on the other hand is a derivative of methacrylic acid - a widely used monomer in polymer synthesis and an altogether different class of monomer than ST and DBI. Although, we have synthesized a series of copolymers along with the polymyrcene homopolymer, the materials are rubbery only when the percentage of the comonomer is lower than that of β-myrcene. Consequently, the 70/30 β-myrcene/co-monomer composition (similar to 1,3 butadiene based commercial elastomers such as SBR, NBR) of the elastomers having sub ambient glass transition temperature was chosen. The main emphasis was given to study the effect of type and amount of comonomer on the vulcanizate properties of the synthesized elastomers and to establish a structure-property relationship. The curing characteristics of the unfilled compounded mix of polymyrcene elastomer were investigated at first. A chemical probe method was used to provide an insight into the network structure and to quantify various types of sulfur crosslinks. A schematic illustration of various probable sulfur crosslinked structures of the polymyrcene elastomer has been presented. Thereafter, various physico-mechanical and dynamic mechanical properties of the carbon black filled elastomer vulcanizates were evaluated.



2.

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EXPERIMENTAL SECTION

2.1 Materials 2.1.1 Monomers. The monomers - β-myrcene (MY, 98%), styrene (ST, ≥ 99%), dibutyl itaconate (DBI, 96%), and butyl methacrylate (BM, 99%) were purchased from Sigma Aldrich chemical company, U.S.A and the inhibitor was removed by shaking the monomer twice with 2(M) NaOH solution followed by washing with deionized water. 2.1.2 Other chemicals. Sodium bicarbonate (NaHCO3, 99%) and ammonium persulphate (APS, 98%) were procured from E. Merck, India. Sodium dodecyl sulphate (SDS, 99%) was obtained from Loba Chemie, India. 1-hexanethiol (97%) and 2-propanethiol (98%) were purchased from Alfa Aesar, U.K. Piperidine (≥ 99 %), petroleum ether (≥ 95 %, also known as diethyl ether) and toluene were obtained from E. Merck Ltd., Mumbai, India. All these chemicals were reagent-grade and used without further purification. Deionized water (DI) was used for all the experiments. 2.1.3 Elastomer compounding ingredients. Zinc oxide (ZnO), stearic acid and sulfur of chemically pure grade were obtained from E. Merck Ltd., Mumbai, India. Carbon black N330 was generously provided by Birla Carbon, Mumbai, India. Process oil was procured from M/s Apar Industries Ltd., Mumbai, India. Antioxidant IPPD (N-isopropyl-N'-phenyl-pphenylenediamine) was obtained from Bayer Chemicals AG, Leverkusen, Germany. Accelerators

CBS

(N-cyclohexyl-2-benzothiazole

sulfenamide)

and

TBzTD

(tetrabenzylthiuram disulfide) were supplied by National Organic Chemical Industries Ltd. (NOCIL), Mumbai, India.


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2.2 Synthesis of elastomers. β-Myrcene based sustainable elastomers were synthesized by persulfate initiated emulsion polymerization method. The recipe (Table S1) and the brief synthesis protocol are provided in the Supporting Information section of this manuscript. The following elastomers were synthesized to evaluate the vulcanization and reinforcement characteristics: Poly(MY100), Poly(MY70ST30), Poly(MY80ST20), Poly(MY70DBI30) and Poly(MY70BM30), where the subscript in the sample designation indicates the weight fraction of the respective monomers in the feed. The ratio of the comonomer was set at 30 weight percentage (ST30, DBI30, BM30) as the comonomer content in the commercial elastomers resides close to 30% (e.g., 23.5% ST in SBR, 33% acrylonitrile in NBR etc.). ST20 was made to study the effect of varying comonomer content within a particular series. Copolymerization type, molecular weight and glass transition temperatures of the synthesized elastomers are collated in Table S2 (Supporting Information). The details of the other physical and chemical properties of the synthesized elastomers, are reported in our earlier publications.19, 21-23

2.3 Compounding and vulcanization of the synthesized elastomers. 2.3.1 Compounding of the synthesized elastomers. In order to understand the vulcanization characteristics of the synthesized elastomers, unfilled gum compounds of polymyrcene were prepared at first with different vulcanization recipes namely: conventional vulcanization (CV) and efficient vulcanization (EV). For further compounding of the synthesized elastomers, and to evaluate the vulcanizate and reinforcement characteristics in details, CV system was adopted along with N330 carbon black, as most of the elastomer formulations are based on such system. The generalized compounding formulation is presented in Table S3 (Supporting Information). The synthesized elastomer(s) and the compounding ingredients were mixed using a two-roll mill manufactured by Schwabenthan, Berlin, Germany. The mill had individual drive



motors and a friction ratio of 1:1.2 (front roll:back roll). It is to be noted that as the elastomers were synthesized in laboratory scale (~2-3 g batch), the compounding batch size was ~15-16 g. The ingredients were added in the nip gap and onto the formed ‘elastomer bank’. The curatives were added at the last stage of the mixing operation. The mixing was carried out as per the standard method (ASTM D-3182) and completed within 12 min. In order to prevent any unwanted heat generation during the mixing operation, the temperature of the rolls was maintained at ~ 45 °C by passing cold water through the roll channels. The compounded mix was then cured for the optimum cure time, as determined from the rheometer studies, and vulcanized sheets were obtained. 2.3.2 Oscillating disk rheometer (ODR) studies. The curing characteristics of the compounded elastomer were evaluated by a Monsanto oscillating disk rheometer R100S machine, U.S.A., operated at 150 °C temperature and 3° arc of oscillation. This is in accord with the ASTM D 2084 specification. The elastomer compounds were conditioned (maturation time) at room temperature for 16 h prior to the testing. Curing parameters such as - minimum and maximum torques (ML and MH), scorch time (ts2), optimum curing time (tc90), reversion time (tc98), cure rate index (CRI) etc. were obtained from the ODR study. 2.3.3 Vulcanization. The compounds were vulcanized in an electrically heated single daylight hydraulic compression-moulding machine from David Bridge Company, England, having 300 mm × 300 mm platens. The curing was done at 150 °C for the optimum cure time, as determined from the ODR experiment, at a moulding pressure of 5 MPa. The vulcanizate samples were stored in a cool and dark place for 24 h before testing.


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2.4 Characterization of the elastomer vulcanizate 2.4.1 Mechanical properties. Dumb-bell shaped specimens were punched (Punch Press, Model P/44, MS Instrument Company Inc., Switzerland) out from the moulded sheets (dimensions: 8.5 cm length × 4.5 mm width × 1.5 mm thick) along the moulding direction with ASTM D 412 Type C die. The tensile strength of the vulcanized elastomer samples was measured according to ASTM D 412 method. The tests were carried out in a Zwick/Roell Z010 Universal Testing Machine at room temperature at a crosshead speed of 500 mm/min. The data were analyzed by testXpert II software of the Zwick/Roell machine and the results presented are average of three measurements. 2.4.2 Density, hardness and thickness. The density of the samples was measured by Wallace High Precision Densimeter X22B equipment. The hardness of the vulcanized elastomer sheets was determined by using a Shore A Durometer from Bowers Metrology, Yorkshire, U.K., following the ASTM D 2240 method. The thickness of the vulcanized elastomer sheets was measured by a Mitutoyo thickness gauge. An average of three measurements is reported for all the cases. 2.4.3 Dynamic mechanical analysis (DMA). Dynamic mechanical analysis of the elastomer vulcanizates was performed using a METRAVIB 50N (France) dynamic mechanical analyzer in tension mode. Rectangular sample (20 mm length × 6 mm width × 1.5 mm thickness, gripto-grip separation: 10 mm) was taken out from the vulcanized sheets and the measurements were done. Temperature sweep experiments were carried out at 1 Hz frequency and 0.1% dynamic strain over a temperature range of −120 to +120 °C. The heating rate was 3 °C/min. 2.4.4 Chemical cross-link density. The chemical crosslink density of the cured samples was measured by equilibrium solvent swelling method. After recording the initial weight of the samples, these were immersed for 72 h at room temperature into a swelling tube containing toluene. After the stipulated period, the specimens were taken out of the solvent and the



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surfaces of the samples were smoothly blotted with a tissue paper to remove the excess solvent. The samples were then immediately weighed in a weighing balance to obtain the swollen weight of the samples after extraction of the soluble materials. Thereafter, the samples were dried at 60 °C for 24 h to get the de-swollen weight of the samples. The overall crosslink density was determined by using the well known Flory-Rehner equation:28,29

 ρ   Mc

  1 / 3 Vr  Vs Vr −  = − (1 − Vr ) + Vr + χVr2 2  

[

]

(1)

Where, Mc = molecular weight between the crosslinks, ρ = density of the vulcanized sample, Vs = molar volume of the swelling liquid, χ = Flory-Huggins solvent-elastomer interaction parameter, which was determined by Hildebrand equation as: 29,30

χ=

Vs (δ s − δ r )2 RT

(2)

Where, R = universal gas constant = 1.987 cal K-1 mol-1, T = experimental temperature in Kelvin, Vs = molar volume of the swelling liquid, δs = solubility parameter of solvent and δr = solubility parameter of elastomer. The volume fraction of elastomer (Vr), as calculated from equilibrium solvent swelling data is given by the following equation: 29,30 Vr =

( D − FH ) / ρ r ( D − FH ) / ρ r +

As

(3)

ρs

Where, D = de-swollen weight of the test specimen, F = the weight of the insoluble fraction, H = weight of the test specimen, ρr = density of the elastomer, ρs = density of the solvent and As = weight of the absorbed solvent.


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2.4.5 Mooney-Rivlin constant. To identify the non-ideal behaviour of the network, MooneyRivlin model31,32 was used. The Mooney-Rivlin equation is given as:

σ 1   2 λ − 2  λ  

= C1 +

C2

λ

(4)

Where, σ is the stress, λ is the extension ratio, C1 and C2 are the empirical constants. The left hand side of equation 4 is popularly known as ‘reduced stress’. From the plot of ‘reduced stress’ versus 1/λ, the constants (C1 and C2) were evaluated.

2.4.6 Thiol-amine chemical probe for network characterization. In order to examine the chemical nature of the sulfur crosslinks (i.e. to determine the proportion of disulfidic and polysulfidic linkages), thiol-amine chemical probe method was used. Unfilled vulcanizate samples (both CV and EV systems) of polymyrcene were used for this test. The preparation of the thiol-amine chemical probe reagents and the treatment procedure were developed by Saville and Watson33 and were followed here. Vulcanized sheets treated with propane-2thiol/piperidine/toluene chemical probe for 2 h cleaves the polysulfide linkages. On the other hand, treatment of the vulcanizate sheets with 1-hexane thiol/piperidine/toluene chemical probe for 24 h cleaves disulfide and polysulfide crosslinks. Nitrogen atmosphere was maintained throughout the treatment process with occasional shaking. Thereafter, the samples were washed thoroughly with petroleum ether several times and dried in a vacuum oven at 70 °C for 24 h. Further, the chemical crosslink density of these dried samples was calculated using equation 1. The percentage of a specific type of sulfur crosslink was then determined from the chemical crosslink densities of the thiol-amine treated specimens and that of the control specimens (without thiol-amine treatment) by simple algebraic calculations.

2.4.7 Fourier-transform infrared (FT-IR) spectroscopy. The FT-IR spectra of the gum polymyrcene elastomer, Poly(MY100)CV and Poly(MY100)EV were recorded by a Perkin Elmer Spectrum 400 machine working within the spectral range of 400 - 4400 cm-1 using an



universal attenuated total reflectance (UATR) mode. The resolution of the instrument was 4 cm-1. Twelve scans were performed on an average for each sample.

2.4.8 Thermogravimetric analysis (TGA). The thermal degradation behaviour of the filled elastomer vulcanizates was studied by a SDT Q600 (TA Instruments) TGA machine under nitrogen flow at a heating rate of 10 °C/min.

2.4.9 Differential scanning calorimetry (DSC). The thermal transitions of the compounded and unvulcanized elastomers were studied in a NETZSCH DSC 200F3 Maia® differential scanning calorimeter machine under nitrogen atmosphere. At first, the samples were equilibrated at -100 °C for 2 min followed by heating from -100 °C to +250 °C at a heating rate of 10 °C/min. The thermal transition was determined from the temperature corresponding to the midpoint of the heat flow change. This was detected from the baseline tangents drawn by using the NETZSCH Proteus - Thermal Analysis – Version 6.1.0 software.

3. RESULTS AND DISCUSSION 3.1 Unfilled compounds 3.1.1 Curing characteristics At first, Poly(MY100) was compounded with CV and EV vulcanization formulations (Table 1) without adding fillers. The sample designations were Poly(MY100)CV and Poly(MY100)EV respectively. As all the synthesized elastomers were new, their milling characteristics were noted. The physical appearance of Poly(MY100) was small rubbery crumb-like19 and thus a non-coherent band was observed around the two-roll mill at the beginning of the mixing. Upon the addition of ZnO, stearic acid and antioxidant, a coherent band was formed. At the end, accelerator(s) and sulfur were added. Finally, the sheet was taken out from the mill. The sheet appeared rough on the surface. The representative photo of the Poly(MY100)CV mix is


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presented in Figure 1. The behaviour is just like the mixing of natural rubber, excepting the final surface was relatively smooth in the case of natural rubber.

Figure 1. Compounded mix of Poly(MY100)CV (1X magnification)

The characteristic rheometer curves of the compounds are presented in Figure 2 and the relevant curing parameters like - minimum torque (ML), maximum torque (MH), scorch time (ts2), optimum cure time (tc90), torque increment (∆M), reversion time (Tc98) and cure rate index (CRI) are given in Table S4 (Supporting Information).

Figure 2. Rheometer curves of the gum elastomer compounds at 150 °C 13 ACS Paragon Plus Environment


It is evident that Poly(MY100)CV showed highest value of MH. The higher dosage of elemental sulfur not only increased the percentage of polysulfidic linkages in the CV system (as determined from the thiol-amine chemical probe method, discussed later), but could also give rise to additional crosslinks, which are manifested in the form of a higher MH value. Correspondingly, the torque increment value of Poly(MY100)CV system is higher. Poly(MY100)EV compound, however, showed higher cure rate index, i.e. faster curing rate and proved to be better reversion resistant as observed from the value of tc98 (Table S4). These observations are similar to common CV and EV vulcanization systems based on natural rubber. Figure 3 presents a representative image of cured Poly(MY100)CV samples (in the form of tensile specimen). It is thus evident that smooth vulcanizate sheets were obtained from the unfilled gum compounds (CV and EV systems) after curing, from which dumb-bell shaped tensile specimens and other samples were prepared for further evaluation.

Figure 3. Gum vulcanizates of Poly(MY100)CV (in the form of tensile specimen)

3.1.2 Physico-mechanical properties Various physical and mechanical properties of the vulcanizates are collated in Table 1 and the tensile stress-strain plots are presented in Figure 4. The density of the vulcanizates was in the range of 939 - 950 kg/m3. The hardness of the vulcanizates was in the lower region (30-32 Shore A) due to their unfilled nature. 14 ACS Paragon Plus Environment

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Poly(MY100)CV registered higher torque increment (∆M) and the difference in ∆M in between the two samples (11.3 dN.m) was significant. The crosslink density of the Poly(MY100)CV system was, however, found to be slightly higher than the Poly(MY100)EV system.

Table 1. Physical and Mechanical Properties of the Gum Elastomer Vulcanizates Samples Properties Poly(MY100)CV

Poly(MY100)EV

Density (kg/m3)

950

939

Hardness (Shore A)

32

30

Crosslink density (mol/cm3)

1.29 × 10-4

1.13 × 10-4

Tensile strength (MPa)

0.60 ± 0.20

0.55 ± 0.10

Elongation at break (%)

140

115

Figure 4. Tensile stress-strain plot of the gum elastomer vulcanizates



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The type and distribution of the crosslinks in a given elastomer vulcanizate depends on the type of cure system used. Table 2 summarizes the amount of various types of crosslinks in the Poly(MY100) vulcanizates, as estimated by thiol-amine chemical probe method. In the case of CV system [Poly(MY100)CV], the crosslink network contains 29% polysulfidic, 21% disulfidic and 50% others sulfidic linkages (mostly monosulfidic crosslinks).

Table 2. Types of Crosslinks in Various Gum Elastomer Vulcanizates Samples

Crosslink types

Poly(MY100)CV

Poly(MY100)EV

Polysulfidic (%)

29

13

Disulfidic (%)

21

17

Other sulfidic crosslinks (%) (Mainly monosulfidic)

50

70

The EV system, where a combination of low concentration of elemental sulfur and high concentration of accelerators was used, registers lower percentage of polysulfidic linkages and higher amount of monosulfidic linkages than the CV system. These observations are similar to other common CV and EV systems in natural rubber.24 It is, however, interesting to note that, both the systems have significantly higher amount of monosulfidic linkages than most natural rubber systems34 (Table 2). One of the plausible explanations is that - the presence of residual unsaturation in the iso-propylidene group of polymyrcene19 may form additional crosslink structure with sulfur. Hence, some amount of the elemental sulfur would have been utilized and redistributed over the side chains (discussed in forthcoming section). By this process, the rank of sulfur crosslinks reduces.


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The tensile strength was found to be 0.60 MPa for the CV system and 0.55 MPa for the EV system. The elongation at break for the CV system was ~140% (Figure 4), which is much better than the EV system. Higher percentage of polysulfidic linkages (as determined from the thiol-amine chemical probe method) increases the tensile properties of the CV system. Thus, from the studies of the unfilled compounds, CV system was chosen as a preferred vulcanization system to study the vulcanization and reinforcement characteristics of the compounds.

3.1.3 Mechanism of vulcanization In this section, an attempt has been made to elucidate the plausible network structure of the polymyrcene vulcanizate. Scheme 1a-b elaborates various steps and conjectures probable crosslinked structures of polymyrcene elastomer vulcanizate. The initial step of the accelerated sulfur vulcanization is the formation of a zinc perthio salt – XSxZnSxX, where ‘X’ is a group derived from the accelerator and ‘x’ in the subscript represents the rank of sulfur.2 This salt reacts with the polymyrcene hydrocarbon (MY-H) to generate a polymyrcene-bound intermediate (XSxMY). Further reaction of this intermediate with another polymyrcene hydrocarbon leads to a crosslinked structure with reduced degree of polysufidity (MYSx-1MY). The hydrogen atom, which is in the α-position with respect to the double bond, is the most labile one.2,24,35 As polymyrcene contains four different microstructures,19 several probable structures could arise due to the attachment of the sulfur atom (Scheme 1a). As the percentage of 1,4 –cis/-trans microstructures is higher (~ 47 %),19 it is anticipated that most of the crosslinking reactions would prevail from these sites. There could be other probable structures for the crosslinked network, other than those depicted in Scheme 1a. Simultaneous reactions on other double bond could make the crosslinking very dense and complicated, as presented in Scheme 1b.



Scheme 1. Schematics for (a) accelerated sulfur vulcanization of polymyrcene elastomer (b) crosslinked structure of polymyrcene vulcanizate 18 ACS Paragon Plus Environment

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Figure 5 represents the FTIR stack plot of unvulcanized polymyrcene, Poly(MY100)CV and Poly(MY100)EV samples. The emergence of a new peak at 1541 cm-1 arises due to N-H vibration of the accelerator moiety. The peaks at 1444 and 1376 cm-1 are due to the CH2 and CH3 bending respectively. After the abstraction of labile protons from these groups, the peak intensity decreases. The peak around 1723 cm-1 in the Poly(MY100)CV and Poly(MY100)EV samples may be ascribed to the carbonyl stretching frequency, arising from the oxidative degradation during the curing operation.

Figure 5. FTIR spectra of unvulcanized and vulcanized (CV and EV system) polymyrcene

In the case of Poly(MY100)CV, more number of polysulfidic linkages makes the crosslinks more prone toward oxidative degradation and hence an increase in the peak intensity than the Poly(MY100)EV sample. Reduced peak intensities at 992 and 889 cm-1 (sp2 C–H bending in the polymyrcene), and 824 cm-1 (C–H bending at isolated double bond in the polymyrcene) are attributed to the formation of crosslink structures at those points. The peaks around 1265 and 729 cm-1 were ascribed to the stretching frequency for C=S and C-S-C units, arising from the crosslinked structure.



3.2 Effect of comonomer on the filled elastomer compounds From the previous sections, it is apparent that the strength of the gum polymyrcene vulcanizates was lower with respect to natural rubber. In order to have comparison of various vulcanizates, fillers were used to increase the strength further.

3.2.1 Curing characteristics For the sake of simplicity, the designation of the compounded samples was kept similar to that of the corresponding raw elastomer as: Poly(MY100)bv, Poly(MY70ST30)bv, Poly(MY80ST20)bv, Poly(MY70DBI30)bv and Poly(MY70BM30)bv, where the subscript ‘bv’ stands for ‘black vulcanizate’. The mixing procedure was similar to the unfilled compounds, excepting the addition of requisite amount of carbon black and process oil. Amongst the compounded mix, Poly(MY70ST30)bv, Poly(MY80ST20)bv and Poly(MY70DBI30)bv appeared bit sticky on the mill surface and hence proper sheeting of these materials was difficult. Poly(MY100)bv and Poly(MY70BM30)bv, however, gave smooth sheets. The representative images of the compounded samples of each type are presented in Figures 6a-d. The sheets were very similar to natural rubber compounds.

Figure 6. Filled compounded mix of (a) Poly(MY100)bv (b) Poly(MY70ST30)bv (c) Poly(MY70DBI30)bv (d) Poly(MY70BM30)bv (1X magnification) 20 ACS Paragon Plus Environment

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The characteristic rheometer curves of the compounds are presented in Figure 7 and the relevant curing parameters like - minimum torque (ML), maximum torque (MH), scorch time (ts2), optimum cure time (tc90), torque increment (∆M)) and cure rate index (CRI) are given in Table S5 (Supporting Information). It was observed that Poly(MY70BM30)bv displayed

highest

value

of

MH,

followed

by

Poly(MY100)bv,

Poly(MY70ST30)bv,

Poly(MY70DBI30)bv and finally Poly(MY80ST20)bv. The ML value of Poly(MY100)bv was, however, found to be highest.

Figure 7. Rheometer curves of various filled elastomer compounds

The torque increment (∆M) value of the compounds was in between 15.6 - 49.4 dN.m [higher value being observed for Poly(MY70BM30)bv] and the scorch time was in between 2 min – 6 min [the scorch time was found to reduce for Poly(MY100)bv compound]. All the compounds, however, showed comparable optimum cure time in the range of 14 min – 15 min. The relatively lower ∆M value for Poly(MY80ST20)bv and Poly(MY70DBI30)bv (Table S5, Supporting Information) could be attributed to the lower molecular weights (36,000 and 29,820 Da)21,22 of these elastomers compared to others. The highest torque increment value of the Poly(MY70BM30)bv compound could be ascribed to its high molecular weight (97,500 Da)23 as well as high crosslink density, discussed later. 21 ACS Paragon Plus Environment


The difference in ML of the filled polymyrcene and the copolymers could be attributed to their difference in molecular weight values. Poly(MY100) and Poly(MY70BM30) possesses relatively higher molecular weight than the other copolymers, which is reflected in the higher ML value of their filled compounds. With comparison to the unfilled compound (c.f. Figure 2), Poly(MY100)bv registered higher ML value due to the presence of carbon black filler. The presence of a higher percentage of 1,2 vinyl and 3,4 addition microstructures make polymyrcene19 more susceptible toward sulfur vulcanization and hence lower scorch safety in comparison to the copolymers. The appearance of the sheets for the filled vulcanizates was similar to that of natural rubber and styrene butadiene rubber based filled vulcanizates.

3.2.2 Physico-mechanical properties Various physical and mechanical properties of the filled elastomer vulcanizates are collated in Table 3 and the tensile stress-strain plots are presented in Figure 8. The density of the filled vulcanizates was in the range of 1029 - 1105 kg/m3. The hardness of the vulcanizates was in the range of 40-58 Shore A. It is apparent that even with 50 phr carbon black, softer vulcanizates were obtained. The calculated crosslink density values are in the same order as that of other synthetic elastomers.29 The highest ∆M was obtained for Poly(MY70BM30)bv, whereas the crosslink density for Poly(MY100)bv was higher than Poly(MY70BM30)bv. Interplay of various factors such as polymer molecular weight and number of double bonds available for crosslinking is anticipated to play roles in such manifestations. The highest molecular weight of Poly(MY70BM30) [97,500 Da vs. 93,200 Da of Poly(MY100)] leads to highest ∆M amongst the samples. On the other hand, the number of double bonds as well as the microstructural defects (such as 1,2 vinyl and 3,4 structure) available for crosslinking is highest in the case of Poly(MY100)bv (as it comprised of


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polymyrcene units only and there is no comonomer moiety), which leads to highest crosslink density. Table 3 also enlists the value of Mooney-Rivlin constant C1 for various samples.31,32 The trend is similar to the crosslink density.

Table 3. Physical and Mechanical Properties of the Filled Elastomer vulcanizates

Samples

Hardness (Shore A)

Crosslink density (mol/cm3)

Mooney-Rivlin constant (C1) (MPa)

Tensile strength (MPa)

Elongation at break (%)

100% Modulus (MPa)

Properties

Density (kg/m3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Poly(MY100)bv

1105

57

2.50 × 10-4

8.6 × 10-3

2.1 ± 0.4

120

1.75

Poly(MY70ST30)bv

1098

53

1.18 × 10-4

4.5 × 10-3

6.4 ± 0.6

395

1.06

Poly(MY80ST20)bv

1029

40

0.56 × 10-4

2.3 × 10-3

0.8 ± 0.2

159

0.58

Poly(MY70DBI30)bv

1103

53

1.16 × 10-4

4.2 × 10-3

1.3 ± 0.5

133

1.05

Poly(MY70BM30)bv

1091

58

2.37 × 10-4

10.4 × 10-3

2.7 ± 0.3

110

2.20

a

a

a

The properties for these samples are adapted from Reference 21

Figure 8. Tensile stress-strain plot of the filled elastomer vulcanizates



The tensile stress-strain plot of the filled elastomer vulcanizates (Figure 8) indicate that the Poly(MY70ST30)bv sample shows highest tensile strength and elongation at break value amongst the systems studied. The maximum tensile strength was 6.4 MPa and the elongation at break was 395%. The lower 100% modulus may arise from lower molecular weight of Poly(MY70ST30) and hence lower entanglement density. Amongst the elastomers studied, Poly(MY70ST30) possesses less amount of microstructural defects (1,2 vinyl and 3,4 microstructures of the polymyrcene unit), which in turn prevents unwanted crosslinks between the macromolecular chains. This is reflected in its higher tensile property values. It is also anticipated that the sequence length distribution in the copolymer microstructure influences the properties of the elastomer vulcanizates. In addition, higher strength of Poly(MY70ST30)bv may be due to the presence of styrene moiety which may act as reinforcing centre on straining the rubber vulcanizate. The tensile strength of Poly(MY100)bv vulcanizate was found to be 2.1 MPa with an elongation at break of 120%, indicating an increase in tensile strength compared to the unfilled vulcanizate (Table 1) due to filler reinforcement. The lower elongation than the styrenic copolymer could be attributed to the presence of additional cross-link points (higher cross-link density), presence of additional pendent iso-propylidene type [–CH=C(CH3)2] unit within the polymyrcene and also the presence of higher percentage of 1,2 vinyl and 3,4 microstructure.19 Although Poly(MY70BM30)bv showed highest torque increment value, its tensile properties (tensile strength = 2.7 MPa, elongation at break = 110%, Table 3) were found to be lower than those of the Poly(MY70ST30)bv sample. With lower amount of styrene content [Poly(MY80ST20)bv], however, the tensile properties decreased significantly, due to an increased microstructural defects, as explained in the earlier section. The tensile properties of Poly(MY70DBI30)bv were lower than Poly(MY70ST30)bv. The 100% modulus value of the filled vulcanizates is also presented in Table 3. In line with the torque increment (∆M) and


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Mooney-Rivlin

constant

C1,

the

100%

modulus

values for Poly(MY100)bv and

Poly(MY70BM30)bv were found to be higher, followed by other elastomer vulcanizates.

3.2.3 Dynamic mechanical properties Storage modulus vs. temperature and Tan δ vs. temperature are plotted for various filled elastomer vulcanizates in Figure 9a-b. The storage modulus in the temperature range of 0 to -100 °C was found to be highest for Poly(MY70ST30)bv. At higher temperature range (0 to 125 °C), Poly(MY70BM30)bv displayed higher storage modulus (Figure 9a). For other vulcanizates, the curves were similar. The temperature corresponding to the peak Tan δ in DMA curve is glass transition temperature (Tg), which is strongly related to the type and amount of the comonomer, stiffness of polymer chains, the morphology of polymer network and its crosslink density.

Figure 9. Plot of (a) Storage modulus vs. temperature and (b) Tan δ vs. temperature for various filled elastomer vulcanizates

Amongst the vulcanizates, Poly(MY100)bv displayed a Tg of -49 °C followed by Poly(MY70DBI30)bv (-47 °C), Poly(MY80ST20)bv (-41 °C), Poly(MY70BM30)bv (-29 °C) and Poly(MY70ST30)bv (-25 °C) (Figure 9b). It is interesting to note that Poly(MY70ST30)bv exhibits the highest Tg despite lower crosslink density than Poly(MY100)bv. It was anticipated 25 ACS Paragon Plus Environment


that the presence of aromatic styrene substituent increases the stiffness of polymer backbone resulting in higher Tg. The Tg value for the gum elastomers from our earlier papers (Table S2, Supporting Information) also reflects similar effect of aromatic styrene substituent. Higher Tan δ value at a particular temperature indicates higher loss. In this case, Poly(MY70ST30)bv displayed highest Tan δ value at -25 as well as at 0 °C.

3.2.4 Thermal properties The thermal degradation pattern of the filled elastomer vulcanizates are presented in Figure 10. Various characteristic degradation parameters such as temperature corresponding to 20% weight loss (T20), and percent residue at 750 °C are collated in Table S6 (Supporting Information).

Figure 10. TGA thermograms of various filled elastomer vulcanizates

At the initial stage of the thermal degradation, the thermograms of Poly(MY100)bv and Poly(MY70ST30)bv were found to be similar. The difference in the degradation pattern (T20) for of Poly(MY70DBI30)bv and Poly(MY70BM30)bv could be attributed to the different thermal stability of the pristine copolymers (T20 as presented in Table S6, Supporting Information). 26 ACS Paragon Plus Environment

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All the elastomer vulcanizates showed distinct two-step degradation pattern. As the experiments were performed in nitrogen atmosphere, the thermograms showed significant amount of residues (varying from 19 – 23 %) in each case, due to the presence of compounding ingredients such as carbon black filler and zinc oxide. The DSC study of the compounded and unvulcanized elastomers was performed to provide an insight into the curing behavior. Only Poly(MY100) and Poly(MY70ST30) samples were studied as they were the base elastomer and copolymer having best set of properties, The corresponding plots are presented in Figure 11.

Figure 11. DSC traces of compounded and unvulcanized elastomers

The DSC traces of the compounded and unvulcanized mixes registered baseline shift at -63 °C for Poly(MY100) and -43 °C for Poly(MY70ST30), indicating the Tg of the mixes (region a). While the endothermic peaks in the range 87-91 °C can be attributed to the melting of I-PPD (antioxidant) and stearic acid (region b), the endothermic peaks around 101104 °C was due to accelerator CBS (region c). The melting of elemental sulfur was



characterized by an endothermic peak around 116 °C (region d). Subsequently, the curing reaction was evident from the exothermic peaks at 181 and 194 °C (region e) for Poly(MY100) and Poly(MY70ST30) respectively. The curing starts at 150 °C and ends around 220 °C, as the samples were under a dynamic heating rate of 10 °C/min during the DSC experiments. Due to lower scorch time of the Poly(MY100) sample (presence of more number of unsaturations and microstructural defects), the exothermic peak was narrower and sharper (8 J/g) in comparison to the Poly(MY70ST30) sample (23 J/g). Poly(MY100) sample, however, registered an endothermic peak immediately after the curing, which could be attributed to possible degradation reaction.

CONCLUSIONS The present work reports the study of vulcanization and reinforcement characteristics of several terpene based sustainable elastomers. The elastomers were synthesized via emulsion polymerization method and could be compounded and vulcanized with ease using conventional rubber processing methods. Poly(MY100)CV was found to possess higher percentage of polysulfidic linkages than Poly(MY100)EV, which is similar to common CV and EV systems in natural rubber compounds. Addition of carbon black reinforces the elastomer matrix, as the tensile strength of Poly(MY100) increases from 0.6 MPa for the unfilled vulcanizate to 2.1 MPa for the carbon black filled vulcanizate. The molecular weight, polymer microstructure, type and amount of the comonomer and the stiffness of polymer chains were found to influence the properties of the vulcanizates to a significant extent. For example, higher ML was observed for elastomers having higher molecular weight. On the other hand, the crosslink density of Poly(MY100) was found to be highest as the number of double bonds and the microstructural defects (such as 1,2 vinyl and 3,4 structure) available for crosslinking were highest in this case. The value of Mooney-Rivlin constant C1 was found


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to be in accord with the trend of torque increment (∆M), as obtained from the rheometric studies. From the DMA experiment, it was evident that the presence of aromatic styrene substituent [(Poly(MY70ST30)] increased the stiffness of polymer backbone, which resulted in a higher Tg. The different microstructures of the pristine copolymers was also found to influence the thermal degradation behaviour of the elastomer vulcanizates. Efforts are underway to increase the molecular weight of these terpene based sustainable elastomers so that further improvement in physico-mechanical properties could be achieved. Additional study is required to understand and elucidate the mechanism of vulcanization and network structure of these completely new sustainable elastomers fully.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr. Recipe for persulfate initiated emulsion polymerization; synthesis of β-myrcene based sustainable elastomers; physical properties of the synthesized elastomers; compounding formulation for the synthesized sustainable elastomers; curing parameters of various gum compounds; curing parameters of various filled compounds; characteristic thermal degradation parameters from TGA (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] (AKB) Tel.: +91 (3222) 283180; Fax: +91 (3222) 220312.

ORCID Anil K. Bhowmick: 0000-0002-8229-5353

Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors would like to thank IIT Kharagpur for providing the facilities. P.S. would like to acknowledge IIT Kharagpur for awarding a research fellowship to carry out this work. A.K.B. would like to thank Indian National Academy of Engineering for providing research grant in the form of a Chair Professorship. P.S. and A.K.B. would also like to acknowledge the partial support of Central Research, Bridgestone Corporation, Japan.

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for Table of Contents use only

Terpene Based Sustainable Elastomers: Vulcanization and Reinforcement Characteristics Preetom Sarkar and Anil K. Bhowmick*


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