Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10002-10010
Properties Enhancement of Room Temperature Vulcanized Silicone Rubber by Rosin Modified Aminopropyltriethoxysilane as a Crosslinking Agent Qiaoguang Li,†,∥ Xujuan Huang,†,∥ He Liu,*,† Shibin Shang,*,†,‡ Zhanqian Song,*,†,‡ and Jie Song§
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†
Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Key Laboratory of Biomass Energy and Material, National Engineering Laboratory for Biomass Chemical Utilization, Key and Laboratory on Forest Chemical Engineering, State Forestry Administration, Nanjing, Jiangsu Province 210042, China ‡ Institute of New Technology of Forestry, Chinese Academy of Forestry, Beijing 100091, China § Department of Chemistry and Biochemistry, University of MichiganFlint, Flint, Michigan 48502, United States S Supporting Information *
ABSTRACT: Rosin modified aminopropyltriethoxysilane (RA) was prepared via an epoxide ring opening reaction of rosin based glycidyl ester with aminopropyltriethoxysilane. The structure of RA was confirmed by Fourier transform infrared spectroscopy (FT-IR), 1H NMR, and 13C NMR. RA was used as a cross-linking agent to prepare room temperature vulcanized (RTV) silicone rubber with hydroxy terminated polydimethylsiloxane (PDMS) matrix in the presence of an organotin catalyst. Morphological, thermal, and mechanical properties of the rosin modified RTV silicone rubbers were characterized by scanning electron microscope (SEM), thermal gravimetric analysis (TG), universal testing machine, and dynamic mechanical analysis (DMA), respectively. Compared to the silicone rubber using tetraethoxysilane (TEOS) as the cross-linking agent, the RA modified RTV silicone rubber exhibited a significant enhancement in thermal stabilities and mechanical properties due to the strong rigidity and polar hydrogenated phenanthrene ring structure of rosin and the uniform distribution of RA in the RTV silicone rubber. KEYWORDS: Cross-linking agent, Rosin, RTV silicone rubber
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synthesized α,ω-bis(trimethoxysilane)-polydimethylsiloxane as a cross-linking agent which induced the elevated thermal properties and improved the hardness of silicone rubber by the incrementing cross-linking density.12 In addition, the rigid and polar inorganic materials were also used as cross-linking agents to improve the thermal and mechanical properties of silicone rubber. Chen et al. investigated the effects of two types of polyhedral oligomeric silsesquioxanes (POSS) cross-linking agents on the RTV silicone rubber. The remarkable improvement in thermal and mechanical properties of silicone rubber is attributed to the effective three-dimensional network structures and the uniform distribution of POSS.13−15 Shi et al. prepared a series of silicone rubbers using tetra-silanol-phenyl-polyhedral oligomeric silsesquioxane (TOPO) as a cross-linking agent, and the thermal stability was enhanced because of the nanoreinforcement effect of the POSS cage.9 These studies indicate that thermal stabilities and mechanical properties of silicone
INTRODUCTION Silicone rubber has characteristics of both inorganic and organic materials and offers many advantages not found in other organic rubbers.1 It has better heat resistance, ozone resistance, weather resistance, chemical stability, electrical insulation, and low surface tension compared to organic rubber.2,3 Because of these unique properties, silicone rubber has been widely applied in the aerospace industry, munitions industry, construction, electrical industry, medical industry, and so on.4,5 However, the low intermolecular force results in lower mechanical properties and limits its practical applications.2 Therefore, improving the mechanical properties of silicone rubber has attracted more attention. Incorporation of fillers and modification of the polydimethylsiloxane (PDMS) matrix are two effective procedures to reinforce the silicone rubber.6−8 In addition, using new types of cross-linking agents to improve the properties of silicone rubber is effective as well.9,10 Han et al. found that the thermal and mechanical properties of silicone rubber were improved significantly by using a novel polymethylmethoxysiloxane (PMOS) as the cross-linking agent because of the improvement effect of PMOS phases in situ at the microscale.11 Ramli et al. © 2017 American Chemical Society
Received: June 15, 2017 Revised: September 21, 2017 Published: September 25, 2017 10002
DOI: 10.1021/acssuschemeng.7b01943 ACS Sustainable Chem. Eng. 2017, 5, 10002−10010
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Synthetic route of RTV silicone rubber. Rosin (RO), glycidyl ester of rosin acid (ER), and rosin modified aminopropyltriethoxysilane (RA).
thylsiloxane (PDMS) matrix (Figure 1). The effects of RA on the morphology, thermal properties, mechanical properties, and dynamic mechanical properties of RTV silicone rubber were explored.
rubber are improved significantly by the chemical incorporation of rigid and polar inorganic materials as cross-linking agents. In recent years more attention has been paid to research about polymer materials from biomass.16−21 In nature some renewable biomass resources have strong rigidity and polar rings as well, which are good alternatives for the preparation of polymers. Rosin is a solid form of resin obtained directly from the exudates of pines and conifers or as a byproduct from the pulping process.22 It is a mixture of natural products which is composed of 40−60 wt % of mainly isomeric abietic-type acids, 9−27 wt % of pimaric-type acids, and 10 wt % of neutral compounds.23−25 Rosin, which is similar to cyclic aliphatic or aromatic compounds in molecule rigidity, has a large hydrogenated phenanthrene ring structure, a carboxylic acid group, and a conjugated carbon−carbon double bond of active functional groups.22,26 Therefore, rosin derivatives can be easily prepared via esterification, D−A addition, acylation, disproportionation, and amination reactions.22,26−28 Rosin derivatives may serve as alternatives to the petroleum based cyclic aliphatic or aromatic monomers used in polymers including polyesters, epoxy resin, silicone rubber, adhesives, coating, and biobased plastics to obtain better mechanical and thermal stabilities.22,25−27,29−32 Deng et al. prepared a series of novel rosin based siloxane epoxy resins using ethylene glycol diglycidyl ether modified acrylpimaric acid (AP-EGDE) with poly(methylphenylsiloxane).32 In our previous study rosin and its derivatives were grafted with vinyl polysiloxane via the reaction of carboxyl with the primary amino at high temperature.29,33,34 Then, these rosin modified polysiloxanes were used as raw materials to prepare silicone rubber via the free radical vulcanization reaction. Rigid and polar hydrogenated phenanthrene rings of rosin on the side chain of polysiloxane have remarkable effects on the mechanical and thermal stabilities of silicone rubber. Although rosin derivatives have attracted more attention for the synthesis of polymers in recent years, they have rarely been employed as cross-linking agents of silicone rubber.29 In this work, a novel cross-linking agent was synthesized from glycidyl ester of rosin acid and aminopropyltriethoxysilane. RTV silicone rubbers were prepared using different amounts of rosin modified aminopropyltriethoxysilane (RA) as a cross-linking agent with the hydroxy terminated polydime-
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MATERIALS AND METHODS
Materials. Purified rosin (RO) composed of rosin acids (ca. ≥95 wt %) was purchased from Hunan Pine Forest Technologies Co., Ltd. Aminopropyltriethoxysilane was purchased from Wanda Chemical Co., Ltd. Hydroxy terminated polydimethylsiloxane (PDMS, 5000 mpa s) was obtained from Hubei New Universal Chemical Co., Ltd. Sodium hydroxide (NaOH), calcium oxide (CaO), Celite, tetraethoxysilane (TEOS), epichlorohydrin, toluene, benzyltriethylammonium chloride, and dibutyltin dilaurate were obtained from Nanjing Chemical Reagent Co., Ltd. Deionized water was obtained at a conductivity of 18.3 MΩ cm on a Hitech-Sciencetool Master-Q (Shanghai Hetai Reagent Co., Ltd.). All chemicals were used without further purification. Synthesis of Glycidyl Ester of Rosin Acid (ER). A 500 mL flask equipped with a stirrer, a thermometer, and a condenser pipe was filled with 50.0 g of rosin, 157.3 g of epichlorohydrin, and 0.381 g of benzyltriethylammonium chloride under the protection of a nitrogen atmosphere.26,35 The mixture was held around 117 °C for 2 h and then cooled to 60 °C. The two materials, 6.62 g sodium hydroxide and 9.272 g calcium oxide, were added to the flask, and the mixture was kept at 60 °C for another 3 h. The mixture was filtered by Celite and filter paper, and then the filtrate was distilled under vacuum at 100 °C to remove excess epichlorohydrin. Finally, 54.5 g (92 wt % yield, Figure 1) of brown and viscous glycidyl ester of rosin acid with an epoxide equivalent weight of 352 g/mol (theory gives 358 g/mol) was obtained without further purification. Synthesis of Rosin Modified Aminopropyltriethoxysilane (RA). A flask equipped with a stirrer, an inert gas inlet, a thermometer, and a reflux condenser was filled with 48 g of ER and 29.6 g of aminopropyltriethoxysilane, and the mixture was kept at 80 °C for about 1 h at which time the appearance of the mixture changed from turbid to transparent. A 77.6 g portion of brown and viscous rosin modified aminopropyltriethoxysilane (100 wt % yield, Figure 1) was obtained. The RA products were directly applied in the following experiments without further purification. Preparation of Modified RTV Silicone Rubber. PDMS and cross-linking agents (TEOS and RA) were added into a three-necked flask and stirred by mechanical stirrer for 15 min at room temperature under dry nitrogen protection. Then, a dibutyltin dilaurate catalyst was added into the flask and vigorously stirred for 15 min. Finally, the mixture was quickly poured into a mold after the air bubbles were 10003
DOI: 10.1021/acssuschemeng.7b01943 ACS Sustainable Chem. Eng. 2017, 5, 10002−10010
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Components of RTV Silicone Rubbers sample
PDMS (g)
RA (g)
TEOS (g)
catalyst (μL)
RA (wt %)
oxethyl of RA (mmol)
oxethyl of TEOS (mmol)
total oxethyl (mmol)
SRTE-1 SRRA-1 SRRA-2 SRRA-3 SRRA-4
30 30 30 30 30
0 1.70 3.53 5.51 7.66
2.70 2.24 1.75 1.21 0.64
100 100 100 100 100
0 5 10 15 20
0 8.8 18.3 28.5 39.7
51.9 43.1 33.6 23.4 12.2
51.9 51.9 51.9 51.9 51.9
Here, φ is the volume fraction. wo is the weight of the original sample. ρ is the density of the RTV silicone rubber before swelling. ws is the swollen weight of the RTV silicone rubber. ρ1 is the density of the toluene, and the value is 0.87 g/cm3. γe is the cross-linking density. MC is the average molecular weight between cross-linking point. χ1 is the interaction parameter of polymer and solvent, and the value is 0.465. vo is the molar volume of toluene, and the value is 106.54 cm3/mol.
removed. The mixture was cured for 7 days at room temperature to obtain an RTV silicone rubber sheet with smooth surface. The RTV silicone rubber sheets cured with different amounts of RA are presented in Table 1. PDMS (30 g) and dibutyltin dilaurate (100 μL) catalyst were used to prepare the RTV silicone rubber in all samples. RA was used as the cross-linking agent to replace TEOS partially, and the total oxyethyl group amount of RA and TEOS was kept constant at 51.9 mmol. With the increase of RA from 0 to 7.66 g, the mass of TEOS decreased from 2.70 to 0.64 g. With the increase from 0 to 20 wt % in the mass fraction of RA, the silicone rubber allowed us to study the effects of RA on the morphological, thermal, and mechanical properties of RTV silicone rubber. Characterizations and Measurements. FT-IR. Fourier transform infrared (FT-IR) spectra were carried out on a Thermo Scientific Nicolet IS10 spectrometer (Nicolet) by ATR (attenuated total reflectance). The spectra were recorded over the range 4000−600 cm−1 at 4 cm−1 resolution and averaged over 16 scans per sample. NMR. 1H NMR and 13C NMR spectra for RA were recorded at 40 °C on a AV400 spectrometer (Bruker, Germany) with frequency of 400.13 and 100.61 MHz, respectively. Deuterochloroform (CDCl3) was used as the solvent, with tetramethylsilane (TMS) as an internal standard. The chemical shift values were referenced to the signals of CDCl3 and TMS. Density. The density of samples was taken using a pycnometer. Hardness. The hardness of samples was measured using an LX-A durometer (Eide fort, China) at 23 °C and around 50% relative humidity (RH). TG. The thermal stability analysis of samples was carried out on a TG209F1 (Netzsch, Germany). About 10 mg of the sample was heated from 25 to 800 °C at a heating rate of 10 °C/min in an Al2O3 crucible in a nitrogen atmosphere. Mechanical Properties. Five dumbbell-shaped specimens were prepared and performed using a capacity 500 N with the UTM6502 universal testing machine (Suns). The samples were characterized at 23 °C and around 50% RH using a cross-head speed of 500 mm/min, and an average value of five time measurements was reported. SEM. New sections of the silicone rubber were coated with gold. Then morphology studies were performed using a QUANTA 200 (FEI, Holland) scanning electron microscope at a voltage of 10 kV. EDS. The element composition of silicone rubber was performed using X-ACT energy dispersive X-ray spectroscopy (OXFORD, England). DMA. The dynamic mechanical analysis of the samples was performed using a DMA Q800 (TA). The DMA characterized the samples at a frequency of 1 Hz from −135 to −75 °C with a heating rate of 3 °C/min by stretching mode. Average Cross-Linking Density. The cross-linking density of RTV silicone rubber was recalculated by using the equilibrium swelling method. RTV silicone rubber (about 0.2 g) and toluene (25 mL) were put into a sealed vessel at 25 °C. After being immersed in toluene for 48 h, the samples were weighed after being blotted with filter paper to remove the excess toluene, and then immersed into toluene again. The above step was repeated every 3 h until the swelling equilibrium was obtained. The cross-linking density is calculated using the following eqs 1 and 2.37,38
φ = (wo/ρ)/[(ws − wo)/ρ1 + wo/ρ]
(1)
ye = ρ /MC = − [In(1 − φ) + φ + χ1 φ2]/(Voφ1/3)
(2)
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RESULTS AND DISCUSSION Characterization of RA. FT-IR Analysis. The FT-IR spectra of RO, ER, aminopropyltriethoxysilane, and RA are shown in Figure 2. From spectra a and b in Figure 2, we note the
Figure 2. FT-IR characterization: (a) RO, (b) ER, (c) aminopropyltriethoxysilane, and (d) RA.
disappearance of the CO peak at 1688 cm−1 and the new characteristic peak of the carbonyl groups at 1726 cm−1 which indicate that the ring opening reaction of the epoxy groups with carboxyl groups was completely accomplished. In addition, a new characteristic peak of the epoxide group at 910 cm−1 can be preliminary evidence for the appearance of epoxide groups in ER. In Figure 2d the absorption peak of epoxide groups at 910 cm−1 disappeared because of the epoxy ring opening reaction of ER with aminopropyltriethoxysilane. From spectrum c in Figure 2 we note the characteristic peak for the stretching vibration of NH, bending vibration of NH, and SiOC of aminopropyltriethoxysilane which are displayed at 3364, 1593, and 957 cm−1, respectively. From spectra c and d in Figure 2 we can see a wide characteristic peak for the NH and OH functional group at 3319 cm−1, and the characteristic peak for NH at 1593 cm−1 moved to 1624 cm−1 which provided further evidence for the reaction of ER with aminopropyltriethoxysilane. NMR Analysis. The structure of RA was further confirmed by the 1H NMR and 13C NMR spectra in CDCl3 (see Supporting Information Figure S1). In the 1H NMR spectrum, clusters of signal peaks at 5.7 and 5.3 ppm are related to the protons of the carbon−carbon double bond of rosin. Clusters of signal peaks at 1.2 and 3.8 ppm are attributed to the chemical shifts of the Si−O−CH2−CH3 protons, respectively. In the 13C NMR spectrum the peaks at 120 and 123 ppm that are observed correspond to the carbon−carbon double bond of rosin. The characteristic peak at 178 ppm belongs to the carbonyl carbon. 10004
DOI: 10.1021/acssuschemeng.7b01943 ACS Sustainable Chem. Eng. 2017, 5, 10002−10010
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Figure 3. Morphology and microstructure of RTV silicone rubber. (a) Photo image of SRTE-1. (b) Photo image of SRRA-3. (c) Photo image of SRRA-4. (d) SEM images of SRTE-1. (e) SEM images of SRRA-3. (f) SEM images of SRRA-4. (g) EDS spectra and element composition of point A in part h. (h) EDS detection domain of SRRA-3. (i) EDS spectra and element composition of point B in part h.
Figure 4. Thermal stabilities of RTV silicone rubber. (a) TG curves of RTV silicone rubber. (b) DTG curves of RTV silicone rubber. (c) 10% mass loss temperature of RTV silicone rubber. (d) The greatest rate of mass loss temperature of RTV silicone rubber. (e) Residual yield at 800 °C of RTV silicone rubber.
The two peaks at 58 and 18 ppm are due to the chemical shifts of Si−O−C−C carbons. Characterization of RTV Silicone Rubber. RTV silicone rubber was synthesized via a condensation reaction between PDMS and cross-linking agents with an organotin catalyst in a moist environment at room temperature. The cross-linking process was affected by cured catalysts, cross-linking agents, PDMS, moisture, and temperature under ambient conditions.
In our experiments a certain amount of dibutyltin dilaurate catalyst and 5000 mpa s PDMS were used in all samples, and the RTV silicone rubber showed better mechanical performances when the total oxyethyl group content of TEOS was 51.9 mmol. Hence, in order to study the effect of RA on the thermal and mechanical properties of RTV silicone rubber, RA was used as the cross-linking agent to replace TEOS partially and the 10005
DOI: 10.1021/acssuschemeng.7b01943 ACS Sustainable Chem. Eng. 2017, 5, 10002−10010
Research Article
ACS Sustainable Chemistry & Engineering
Figure 5. Mechanical properties of RTV silicone rubber with different RA content. (a) Photos of mechanical properties test of SRTE-1 and SRRA-3. (b) Stress−strain curves of RTV silicone rubber. (c) Tensile strength of RTV silicone rubber. (d) Elongation at break of RTV silicone rubber. (e) Tensile modulus at 100% elongation of RTV silicone rubber. (f) Density of RTV silicone rubber. (g) Cross-linking density of RTV silicone rubber. (h) Shore hardness of RTV silicone rubber.
element content.39 The hard segment domains of rosin are well-dispersed in the soft segment of polysiloxane. Thermal Properties. The effects of the RA cross-linking agent on the thermal degradation behavior of RTV silicone rubber were characterized by TGA. The TG and DTG curves of RTV silicone rubber are presented in Figure 4a,b. The temperature of the 10% mass loss is delayed from 363 °C (SRTE-1) to 439 °C (SRRA-1), and it begins to decrease with increasing RA content (Figure 4c). Furthermore, the temperatures of the 10% mass loss of all SRRA silicone rubbers are higher than that of SRTE-1 due to the introduction of RA with a phenanthrene ring structure. It is known that the polysiloxanes exhibit dissociation routes with low energy in the presence of hydroxyl-containing impurities.36 The hydroxyl groups can participate in a “backbiting” reaction to form volatile cyclic products at elevated temperatures.9,36 The stable phenanthrene ring structure of RA increases the chain entanglements which restricted the molecular motion of polysiloxane chain.3,40 These may result in preventing the rearrangement of Si−O bonds of polysiloxane to degrade the formation of cyclic oligomers and to decrease the rate of thermal degradation.8 However, the C−N bond energy of
total oxyethyl group amount of RA and TEOS was kept constant at 51.9 mmol. Morphologies. The RTV silicone rubber with TEOS as a single cross-link agent is clear and colorless (Figure 3a). However, when RA partially replaces TEOS, the silicone rubber is opaque, and the color becomes yellow (Figure 3b,c), which is caused by the change of microstructure. Morphologies of the RTV silicone rubbers were investigated by scanning electron microscopy. The SEM images of SRTE-1, SRRA-3, and SRRA4 RTV silicone rubber are presented in Figure 3d−f. The surfaces of the silicone rubber become rough with the increase of RA. This can be attributed to the stronger polarity and rigid fused ring structures of RA, which increase the molecular chain rigidity of the polymer matrix.26 The rigid fused ring groups of rosin tend to aggregate by molecular polarity and rigidity which result in a microphase separation in silicone rubber.29,39 The EDS spectra and elemental composition of SRRA-3 are presented in Figure 3g−i. The C element content (61.87%) of point A is higher than that of point B (28.94%). Point A represents the hard segment which is formed by the aggregation of rosin groups from RA. Point B represents the soft segment which is composed of polysiloxane with a low C 10006
DOI: 10.1021/acssuschemeng.7b01943 ACS Sustainable Chem. Eng. 2017, 5, 10002−10010
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ACS Sustainable Chemistry & Engineering
Figure 6. Dynamic mechanical properties of silicone rubber. (a) Storage modulus (E′) curves of RTV silicone rubber. (b) Tan δ curves of RTV silicone rubber. (c) Tg of the RTV silicone rubber.
amine groups is relatively lower so it breaks first at a given temperature.41 Therefore, the thermal stability of SRRA silicone rubbers decreases with increasing RA content because the amine groups increase as well. The greatest rate of mass loss temperature is also delayed from 400 °C (SRTE-1) to 653 °C (SRRA-1), which is an enhancement of 253 °C (Figure 4d). Because of the weak intermolecular force, the trace of residual hydroxyl groups in SRTE-1 silicone rubber could accelerate decomposition of PDMS, leading to rapid degradation at 400 °C.13,14 The incorporation of RA in silicone rubber results in restricting the mobility of the PDMS chains and preventing the rearrangement of Si−O bonds in polysiloxane. Therefore, polysiloxane is inhibited from degrading into forming cyclic oligomers.36 As a result, the decomposition temperature of RA modified silicone rubber increases to 653 °C. Note that these higher temperatures might facilitate random degradation because the chain mobility and molecular motion are relatively enhanced.13,14 Ultimately the SRTE-1 cured without RA completely degraded into cyclic oligomers with no residue left at 800 °C. However, the silicone rubber cured with RA has more residual yield than that of the SRTE-1 (Figure 4e).32 Mechanical Properties. The effects of RA on the mechanical properties of RTV silicone rubber were evaluated as illustrated in Figure 5. The test for mechanical properties of SRTE-1 and SRRA-3 (Figure 5a) and the stress−strain curves of RTV silicone rubber (Figure 5b) directly show that RTV silicone rubber using RA as the cross-linking agents possesses better mechanical properties. The tensile strength and the elongation at break of the rosin modified RTV silicone rubber are improved significantly (Figure 5c,d). The tensile strength and the elongation at the break of SRRA-3 are 1.25 MPa and 355%, respectively. Both of them increase more than double compared with those of the SRTE-1 sample. The tensile modulus at 100% elongation of RTV silicone rubber improves from 0.352 to 0.454 MPa (Figure 5e). These increases in tensile strength, elongation at break, and tensile modulus of the silicone rubber could be due to the positive role of hydrogen bonds between the NH and CO groups and the hydrogenated phenanthrene ring of RA which increases the amounts of formed rigid segments and promotes the chain entanglements.22,25,42 Although all the samples have the same total amount of oxyethyl groups, the cross-linking densities increase from 1.07 × 10−4 (SRTE-1) to 1.28 × 10−4 mol/cm3 (SRRA-2) when the RA cross-linking agent content increases from 0 to 10 wt %. In addition, the cross-linking densities of SRRA-3 and SRRA-4 decrease to 1.25 × 10−4 and 1.18 × 10−4 mol/cm3 with increasing the RA cross-linking
agent content to 15 and 20 wt %, respectively. This indicates that the RA cross-linking agent can promote the cross-linking degree of silicone rubber. This should be attributed to the enhancement of the chain entanglement by the rigid and polar hydrogenated phenanthrene rings of rosin on the side chain of polysiloxane.22 Butane modified aminopropyltriethoxysilan (BA) cross-linking agent was synthesized to prepare BA cured silicone rubber (SRBA-1) to confirm this speculation (see Supporting Information). The cross-linking densities of the SRBA-1 and SRRA-3 silicone rubber are calculated by using the equilibrium swelling method and are shown in Figure S3. The cross-linking density of SRRA-3 is 1.25 × 10−4 mol/cm3, which is higher than that of SRBA-1. Compared to the SRBA-1 silicone rubber, SRRA silicone rubbers were cured by the RA cross-linking agent with hydrogenated phenanthrene ring. The ethyl group (red color in Figure S2a) of the BA cross-linking agent is smaller and more flexible than the hydrogenated phenanthrene ring group (blue color in Figure S2b) of the RA cross-linking agent. Therefore, the improved cross-linking density of SRRA is attributed to the enhancement of chain entanglement as the rigid and polar hydrogenated phenanthrene ring of the RA cross-linking agent. These results indicate that the incorporation of hydrogenated phenanthrene ring into the silicone rubber via the cross-linking reaction is an effective approach to enhance chain entanglement and increase the cross-linking density of the silicone rubber. Since cross-linking density is an important factor to influence the mechanical properties of silicone rubber, the effect of hydrogenated phenanthrene ring on the cross-linking density must be reflected in the mechanical properties of silicone rubber. The mechanical properties of SRBA-1 and SRRA-3 RTV silicone rubber were evaluated as illustrated in Figure S4. The tensile strength and the elongation at break of SRRA-3 are increased significantly to 1.25 MPa and 355%, respectively. The incorporation of RA as a cross-linking agent caused the increase of cross-linking density of silicone rubber, while the tensile strength and the elongation at the break increase significantly. This indicates that the RA cross-linking agent possesses both reinforcing and toughening effects on the silicone rubber, which cause the tensile modules of all SRRA samples are higher than that of the SRTE-1. However, when the toughening effect plays a primary role for the silicone rubber, the increment of strain is more significant than the stress, which results in the decrease of the modulus (SRRA-2) while the cross-linking density and tensile strength both increase. These characterizations prove that the incorporation of hydrogenated phenanthrene ring into the silicone rubber result in the increase of cross-linking density and the improvement of mechanical properties as well. 10007
DOI: 10.1021/acssuschemeng.7b01943 ACS Sustainable Chem. Eng. 2017, 5, 10002−10010
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respectively. In addition, the tensile strength and the elongation at the break of SRRA-3 increase significantly. These phenomena are attributable to the synergistic effect of the hydrogenated phenanthrene ring structure of rosin, increment of cross-linking density, and uniform distribution of RA in the RTV silicone rubber. This study may facilitate an increase in the use of renewable bioresource rosin to improve the properties of RTV silicone rubber.
When the silicone rubbers are cured with lower RA crosslinking agent content (SRRA-1, SRRA-2 and SRRA-3), RA is found to be uniformly dispersed in the matrix, even though microphase separation occurs (from part b to part d of Figure S5). The mechanical properties of SRRA-1, SRRA-2, and SRRA-3 are improved with the increase of RA and higher than that of SRTE-1. However, the cross-linking density, the tensile strength, and the elongation at the break of the silicone rubber start to decrease when the RA cross-linking agent content exceeds 15 wt % (Figure 5b−d,g). This can be due to the agglomeration and self-cross-linking from an overdose of RA cross-linking agent, which led to inhomogeneous RA-rich domains and serious microphase separation in silicone rubber supported by SEM (Figure 3f).14,29 Meanwhile, the agglomeration and self-cross-linking of RA cross-linking agent also induce the loss of many active sites, which results in the decrease of the cross-linking density. This indicates that uniform distribution of RA in the silicone rubber is beneficial for the improvement of mechanical properties. In addition, the density and the Shore hardness increase from 0.984 to 1.027 g/ cm3 and from 18 to 26 A, respectively (Figure 5f,h). Therefore, the RA can effectively improve the mechanical properties of RTV silicone rubber which can be attributed to the strong rigidity of the phenanthrene ring structure of rosin, the increment of cross-linking density, and the uniform distribution of RA in silicone rubber as well. Dynamic Mechanical Properties. Figure 6a shows the E′ curves for RTV silicone rubbers. The cross-linking density and the chemical structure play a key role in the E′ of silicone rubber. The E′ values of RTV silicone rubbers increase from 4050 MPa (SRTE-1) to 4577 MPa (SRRA-2) with the increase of the RA cross-linking agent. This is because the increment of the chain entanglements and the molecular chain rigidity impose considerable restrictions on the polysiloxane chain mobility of silicone rubber.22 When the RA cross-linking agent content reaches more than 15 wt %, the agglomeration and selfcross-linking from an overdose of RA cross-linking agent can lead to inhomogeneous RA-rich domains and serious microphase separation in silicone. Meanwhile, the agglomeration and self-cross-linking of the RA cross-linking agent also induce the loss of many active sites, which results in the decrease of the cross-linking density. The inhomogeneous RA-rich domains, serious microphase separation, and decrease of the cross-linking density cause the decrease of E′. The peak temperature of tan δ corresponds to the glass transition temperature (Tg). The tan δ curves show that each sample has only one clearly single Tg from −135 to −75 °C (Figure 6b). The Tg of RTV silicone rubbers increased from −116.6 °C (SRTE-1) to −113.5 °C (SRRA-4) with the increase of the RA cross-linking agent because of the more rigid hydrogenated phenanthrene ring structure and correspondingly less flexible polysiloxane chains that impose restriction on the polysiloxane chain mobility (Figure 6c).20,40,43
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01943. 1 H NMR and 13C NMR spectra of rosin modified aminopropyltriethoxysilane (RA), synthesis of butane modified aminopropyltriethoxysilane cross-linking agent (BA), preparation of BA cured silicone rubber (SRBA-1), cross-linking densities of SRBA-1 and SRRA-3, mechanical properties of SRBA-1and SRRA-3, and morphology of SRRA RTV silicone rubbers (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.L.). *E-mail:
[email protected] (S.S.). *E-mail:
[email protected] (Z.S.). ORCID
He Liu: 0000-0001-9177-9459 Author Contributions ∥
Q.L. and X.H. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (31570562), Key Laboratory of Biomass Energy and Materials of Jiangsu Province of China (JSBEM-S201504), and Discipline Group Construction Project of CAFICIFP (LHSXKQ1).
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REFERENCES
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CONCLUSION In summary, a novel cross-linking agent to RTV silicone rubber was prepared via an epoxide ring opening reaction of rosin based glycidyl ester with aminopropyltriethoxysilane. The thermal and mechanical properties of RTV silicone rubber are improved significantly by using RA as a cross-linking agent. Compared with the properties of SRTE-1 samples, the 10% mass loss temperature and the greatest rate of mass loss temperature of SRRA are delayed by 76 and 253 °C, 10008
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