Synthesis and Properties of Organosilicon-Grafted Cardanol Novolac Epoxy Resin as a Novel Biobased Reactive Diluent and Toughening Agent.

The aim of this work is to develop a biobased functional reactive diluent for thermosetting epoxy resins suitable for high-performance applications. An advanced organosilicon-grafted cardanol novolac epoxy resin (SCNER) was synthesized from cardanol novolac epoxy resin and heptamethyltrisiloxane. After the chemical structure of SCNER was identified by Fourier transform infrared, 1H NMR, and 13C NMR, it was used to modify the diglycidyl ether of the bisphenol A (DGEBA)/methylhexahydrophthalic anhydride system. The SCNER showed unique advantages, reducing the viscosity of DGEBA and improving the properties of the cured resin. With 10 wt % SCNER, the cured resin exhibited a higher tensile strength (78.84 MPa) and impact strength (32.36 kJ·m-2). The single glass transition temperature (T g) step proved the homogeneous phase structure of the cured resin. Inevitably, the T g of the cured resin decreased for the addition of SCNER. The dynamic mechanical analysis results indicated that the storage modulus of the cured resin decreased with the increasing content of SCNER. The morphology showing the ductile fracture of the cured resin was testified by scanning electron microscopy. The dilution and toughening properties of SCNER paves the way to a wide range of possible "eco-friendly" applications, especially in the fields of coatings, paintings, and adhesives.

Introduction

Epoxy resins are widely used in coatings, adhesives, castings, reinforced composites, and electronic industries because of their outstanding mechanical and electrical properties, excellent chemical resistance, good adhesion, and low minimal shrinkage.[1−4] However, one of the major disadvantages of epoxy resins is the high viscosity at room temperature that causes poor processability to limit the applications.[5,6] The main approach to overcome the crucial issue of high viscosity is to add diluents into the epoxy resin.[7]

Generally, reactive diluents are low-viscosity compounds with multiple reactive functionalities that have good compatibility with epoxy resins. As it was well known, the reactive diluents could reduce the viscosity of epoxy resin systems markedly to improve the curing reaction process,[8,9] and participate in the curing reaction to improve the mechanical and thermal properties of cured epoxy resins.[10,11] Because of fossil fuel shortage and environmental crisis, the biomass-based chemicals from renewable resources became a global research interesting. Reactive diluents derived from renewable resources would be promised to replace the petrochemicals and fabricate green materials.[12,13] Therefore, efforts have been made to develop biobased reactive diluents in order to obtain easily operated low-toxicity epoxy resins based on the fundamental of “green chemistry”.

Cardanol is a natural phenol compound with an unsaturated C15 alkane side chain containing 1, 2, or 3 nonconjugated C=C bonds at the meta position.[14,15] It could be obtained extensively via the distillation of cashew nut shell liquid and regarded as a valuable renewable resource.[16] Cardanol had been widely applied in many areas, such as coatings,[17−19] adhesives,[20−23] plasticizers,[24] and biocomposites.[25−28] The cardanol-based epoxy resins could be prepared either from epoxidation of unsaturated alkyl groups or phenolic hydroxy groups.[29−33] Moreover, in our previous work, we synthesized a lignin and cardanol-based novolac epoxy resin by reacting epichlorohydrin (ECH) with the mixture of lignin and cardanol novolac resin.[34] However, the organosilicon-containing cardanol epoxide has not been investigated.

In view of the defects of epoxy resin, combined with the unique properties of organosiloxanes, we envisage the synthesis of a low-viscosity, bifunctional biomass-based toughening reactive diluent. Thus, a novel biobased organosilicon-grafted cardanol novolac epoxy resin (SCNER) was prepared and used as reactive diluent and toughening agent to modify the anhydride-cured epoxy resin system. The results showed that SCNER had an outstanding dilution and toughening effect on the anhydride-cured epoxy resin system.

Results and Discussion

Characterization of SCNER

SCNER was prepared by the epoxidation of cardanol-based phenolic resin, and further hydrosilation of double bonds on the cardanol side chain (Scheme ). The chemical structure of SCNER was characterized by Fourier transform infrared (FTIR), 1H NMR, and 13C NMR spectroscopy. The FTIR spectra of SCNER and cardanol are shown in Figure . The absorption bands at 1259 and 1050 cm–1 in the FTIR spectrum of SCNER were the symmetric and asymmetric stretching peaks of C–O–C and O–Si–O. The bands at 1600 and 1591 cm–1 were assigned to the vibration absorption of the aromatic ring. The cluster peak assigned at 2927 and 2854 cm–1 were the stretching peak of C–H bonds. Compared with the FTIR spectrum of cardanol, the characteristic peak of the epoxy group at 912 cm–1 appeared and the peak of the OH group at 3320 cm–1 disappeared, which indicated that the OH group of cardanol reacted with ECH completely.[35,36] The peak of C=C–H at 3015 cm–1 in the FTIR spectrum of cardanol disappeared and a new peak of the Si–O–C group could be found at 834 cm–1, which meant that the double bonds on the side chain of cardanol had been functionalized by heptamethyltrisiloxane (HMTS).

Scheme 1

Synthesis of SCNER and SCNER-Modified Epoxy Thermoset

Figure 1

FTIR spectra of cardanol (a) and SCNER (b).

Figure displays the 1H NMR spectra of cardanol and SCNER. The multiplet peaks in the range of 6.78–7.28 ppm were assigned to the protons in the aromatic ring. Compared to the spectrum of cardanol (Figure a), the peaks at 5.15–5.22 ppm in the 1H NMR spectrum of SCNER (Figure b) almost disappeared. These peaks were attributed to the protons of −CH=CH– and −OH. This variation implied that the phenolic hydroxyl groups and unsaturated double bonds on the side chain were converted. Meanwhile, the peaks at around 3.30 and 2.73 ppm corresponded to methine and methylene protons of the oxirane ring, respectively. The peak at 4.18 ppm was due to the presence of methoxy protons.[37] The multiplet peaks around 0.11 ppm assigned to the Si–CH3 protons supported the occurrence of hydrosilylation reaction.[38] The peaks at 49.81 and 44.24 ppm in the 13C NMR spectrum of SCNER (Figure b) were attributed to the carbons in the epoxy group. The peak at 68.25 ppm was assigned to the −OCH2 carbons on the benzene ring. The aromatic carbons gave peaks at around 120–150 ppm. The NMR data further indicated that the SCNER had been successfully synthesized.

Figure 2

1H NMR spectra of cardanol (a) and SCNER (b) in CDCl3.

Figure 3

13C NMR spectra of cardanol (a) and SCNER (b) in CDCl3.

Effect of SCNER Content on the Viscosity of the Epoxy Resin

The viscosity of the epoxy resin with different contents of SCNER was investigated and the results are displayed in Figure . It was shown that the viscosity of diglycidyl ether of bisphenol A (DGEBA) was rapidly decreased with the increase of SCNER content. The results indicated that SCNER was compatible with DGEBA and could efficiently reduce the viscosity of DGEBA.

Figure 4

Effect of SCNER content on the viscosity of DGEBA at 25 °C.

Mechanical Properties

The reinforcing and toughening effects of the incorporated SCNER into this curing system were investigated by impact, tensile, and bending tests. The results are shown in Table . It can be observed that the impact strength increased first and then decreased with the incorporation of SCNER. A thermoset with 10.0 wt % SCNER loadings shows the best toughening effect and the impact strength of the cured resin reaches 32.36 kJ·m–2, which is almost twice higher than that of neat epoxy (15.43 kJ·m–2). Meanwhile, tensile test results showed that the tensile strength and break elongation also increased after adding SCNER. The tensile strength and break elongation of the cured resin with 10.0 wt % SCNER loadings reach 78.84MPa and 3.52% respectively, which were 26.3% increase in tensile strength and 43.7% increase in break elongation with respect to those of neat epoxy. Such a result shows that SCNER could effectively toughen the DGEBA/methylhexahydrophthalic anhydride (MeHHPA) system. Such an efficient toughening effect could be attributed to the specific structure of SCNER with a rigid aromatic core, flexible aliphatic chain, and organosilane chain. In addition, the flexural strength gradually decreased with increasing SCNER content. This is because the cross-link density of the cured resin was reduced with incorporation of SCNER, which was also reflected in the reduced Tg.

Table 1

Mechanical Properties of Cured Resins with SCNER

no.	samples	impact strength (kJ·m–2)	tensile strength (MPa)	elongation at break (%)	flexural strength (MPa)
1	DGEBA	15.43	62.44	2.45	120.44
2	DGEBA with 5% SCNER	21.85	68.50	2.81	118.56
3	DGEBA with 10% SCNER	32.36	78.84	3.52	115.36
4	DGEBA with 15% SCNER	29.03	71.74	4.31	108.81
5	DGEBA with 20% SCNER	20.23	68.94	4.81	99.53
6	DGEBA with 25% SCNER	14.89	61.28	6.08	85.72

Dynamic Mechanical Analysis of Cured Resins

Dynamic mechanical behaviors of neat DGEBA and DGEBA/SCNER resins were investigated, as shown in Figure . As shown in Figure a, the G′ of the cured resin was affected by the addition of SCNER. The modulus of cured resins was decreased with the addition of SCNER moderately. It is well-known that G′ in the glassy state is affected synthetically by chemical structure and chain packing.[39,40] On one hand, SCNER with soft long alkyl chain and organosiloxane structures could decrease the stiffness of the resins. On the other hand, SCNER could increase the distance of resin chains and make the resin segment easy to move. Therefore, it was also indicated that the toughness of the cured resin was enhanced with the increase of the content of SCNER.

Figure 5

Dynamic mechanical spectra of cured resins with SCNER: (a) G′ and (b) tan δ.

Figure b shows that each sample had one narrow and smooth α-relaxation peak. The peak of the tan δ versus temperature is taken as glass-transition temperature (Tg). As shown in Figure b, the Tg of cured epoxy resins was decreased by the addition of SCNER into the epoxy resin lowered. The SCNER possessed a relatively low epoxy value to reduce the cross-linking density of cured epoxy resins, to reduce the Tg of the cured epoxy resins. The low Tg of DGEBA/SCNER resins might also result from the flexible linkage of the C15 alkyl chain and the Si–O– group in the backbone of the resins. Moreover, for DGEBA/SCNER resins, incorporating SCNER into the cured epoxy resin could result in epoxy compounds with bulk pendants. The bulk pendants might simultaneously increase the free volume and reduce the cross-linking density of the cured resins, to reduce the Tg of the resins.[41,42] The variation of Tg could also be confirmed by the differential scanning calorimetry (DSC) analysis results (Figure S1). However, there was a gap of Tg between dynamic mechanical analysis (DMA) and DSC analysis results because of the difference test principle and mechanism of these two equipment.

Fracture Surface Morphology

The morphology of the fracture surface of neat DGEBA and DGEBA/SCNER resins with different contents of SCNER to DGEBA by weight 0, 5.0, 10.0, and 15.0% was observed by scanning electron microscopy (SEM), as shown in Figure . The cured neat DGEBA exhibited a characteristic smooth fracture surface without deformation, which was consistent with the low impact strength. After the addition of SCNER, the original smooth surfaces of the cured resin became rougher and appeared to have many ridges (referring to the filar strips). This change meant energy dissipation and improved toughness of the epoxy-SCNER resins (Figure b–d). Furthermore, with the increase of content of SCNER, the rough and irregular cracks started to turn into the tear of large areas gradually. The size and depth of these tearing areas were related to the energy required to destroy the cured resin, which corresponded to the impact strength results. As shown in Figure b–d, only few holes could be observed after the addition of SCNER, which indicated no phase separation in the epoxy-SCNER system. It was in good agreement with DMA detection results.

Figure 6

SEM micrographs of cured resins with the content of SCNER of (a) 0, (b) 5, (c) 10, and (d) 15%.

Conclusions

Cardanol is a natural phenol compound, which has an unsaturated C15 alkane side chain with 1–3 double bonds at the meta position. Because of this versatile chemical structure, the manufacturing of novolac epoxy resin from cardanol (CNERs) has received much attention from the researchers in recent years. Usually, the high viscosity of CNERs restricted the application of them as diluents. In this work, we reported a novel low-viscosity modified CNER from the hydrosilation of the double bonds in the side chain of CNER. The obtained SCNER could be utilized as reactive diluent and toughener. Therefore, we further investigated the properties of SCNER in the modification of the epoxy/anhydride system. Owing to the unique structure of SCNER, it could be easily dispersed in epoxy resin and efficiently toughen the cured resin. The results showed the impact strength of the cured resin with 10.0 wt % SCNER reached 32.36 kJ·m–2, which is almost two times higher than that of neat epoxy (15.43 kJ·m–2). Furthermore, the introducing of SCNER into the epoxy matrix could simultaneously enhance the tensile strength and break elongation of the cured resin. The efficient diluent coming from low-cost, renewable cardanol has considerable potential for epoxy thermosets with high performance.

Experimental Procedures

Materials

Cardanol (average molecular weight 300 g/mol) was supplied by Zaozhuang Nate Biological Materials Co. Ltd. (Shandong, China); ECH was purchased from Nanjing Haolifeng Chemical Co. Ltd. (Nanjing, China). DGEBA [epoxy equivalent weight (EEW) = 196 g/equiv] was obtained from Jiaxing Lianxin Chemical New Materials Co. Ltd. (Zhejiang, China), N,N-dimethylbenzylamine (DMBA) was supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China), MeHHPA was provided by Jiaxing Lianxin Chemical New Materials Co. Ltd. (Zhejiang, China). Karstedt catalyst, HMTS, benzyltriethylammonium chloride (BTEAC), and other chemicals were analytical grade and obtained commercially from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).

Synthesis of CNER

CNER was synthesized by cardanol reacting with formaldehyde and ECH according to the previous procedure.[34,43,44] Typically, cardanol (150.0 g, 0.50 mol) and oxalic acid (1.53 g, 0.017 mol) were mixed in a 2000 mL four-necked round bottom flask equipped with a mechanical stirrer, a spherical condenser, a constant pressure dropping funnel, and a thermometer. The system was preheated at 100 °C under the protection of nitrogen. Subsequently, 37% aqueous solution of formaldehyde (20.4 g, 0.25 mol) was added dropwise in 1 h. After reacting for 3 h under nitrogen atmosphere, the traces of water and acid catalyst were removed by raising the temperature to 115 °C. ECH (462.5 g, 5.0 mol) and BTEAC (1.5 g, 6.6 × 10–3 mol) were added into the above product under constant stirring. The system was heated to 100 °C and maintained for 4 h, and then cooled down to 60 °C. NaOH (0.53 mol, 21.2 g) was added five times in 2 h. The mixture was stirred for another 2 h at 60 °C and thereafter washed with hot water several times to remove the byproduct (NaCl). The organic phase was collected and distilled in vacuum to recycle the excess amount of ECH. CNER could be obtained as a reddish-brown viscous liquid with a yield of 87.3%.

Synthesis of SCNER

SCNER was prepared from CNER with HMTS by a method similar to that given in previous literature.[45,46] Into a 250 mL flask were added CNER (71.2, 0.2 mol), HMTS (71.04 g, 0.32 mmol), and Karstedt’s catalyst (40 mg/kg platinum relative to CNER); then the mixture was heated to 110 °C and reacted for 6 h. The mixture was cooled to 80 °C. Activated carbon (2.2 g) was added to the mixture and reacted for another 1 h. The solution was then cooled to room temperature and filtered to remove the solids. The filtrate was distilled under reduced pressure and SCNER was obtained with 67.73% yield. Scheme shows the preparation of SCNER.

Preparation of Cured Epoxy Resins

DGEBA (100 phr), SCNER with varying contents (5–25 phr), and DMBA (2.5 phr) were mixed together, and then MeHHPA with stoichiometric amounts corresponding to epoxy equivalents was added. The above system was fully mixed and then left in vacuum to remove any entrapped air bubbles at 80 °C. The homogeneous mixture was poured into greased poly(tetrafluoroethylene) molds to cure at 100 °C for 3.0 h, followed by post curing reaction at 130 °C for 8 h in a vacuum oven. The mold was cooled and the casting was finally released from the mold for further characterization.

Characterization

The FTIR spectra were obtained from a Nicolet IS10 spectrometer (Nicolet, USA) using attenuated total reflectance mode. All spectra were based on 32 scans with 2 cm–1 resolution across a wavenumber interval between 4000 and 500 cm–1. 1H NMR and 13C NMR spectra were recorded on a 300 MHz NMR spectrometer (Bruker, Germany) using tetramethylsilane as an internal and deuterated chloroform (CDCl3) as solvent.

The epoxy value and viscosity measurements of SCNER were determined according to Chinese National Standards GB/T 1677-2008 and GB/T 22314-2008, respectively. The EEW of SCNER was 1234.5 g/equiv, and the viscosity of SCNER was 83.2 mPa·s at 25 °C.

The impact testing was carried out according to China National Standards GB/T 1043-2008 using a XJJY-5 impact test machine (Chengde, China). The tensile strength and flexural strength were measured by an XJZ-50 electronic universal testing machine (Shenzhen Skyan Power Equipment Co., China) according to China National Standard CB/T 1040.1-2006 and CB/T 9341-2000, respectively.

DMA was carried out with a DMA Q800 dynamic mechanical thermal analyzer (TA Instruments, USA). Specimens were studied with dual cantilever geometry model in a frequency of 1 Hz and heated from 30 to 200 °C at a heating rate of 3 °C·min–1. The S-3400N scanning electron microcopy (Hitachi, Japan) was adopted to observe the fracture surface of specimens after the impact test.