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BY-NC-ND 3.0 license Open Access Published by De Gruyter December 6, 2013

Preparation and mechanical properties of nano-silica/UPR polymer composite

  • Yan-qing Wang EMAIL logo , Yu Guo , Rong-xin Cui , Zhong-ming Wang and You-liang Wu

Abstract

This article mainly aims at experiments used for modifying unsaturated polyester resin (UPR), and trying to enhance the strengthening and toughening performance of nano-silica/UPR polymer composite in order to improve its integrated mechanical properties and to expand its field of application. Experiments were initially used to get high dispersibility nano-silica powder by adding silane coupling agent KH570 as a dispersant into commonly and commercially available nano-silica powder. Then, high dispersibility nano-silica powder was added as a filling with different mass fraction ratios into UPR and nano-silica/UPR polymer composite samples were fabricated. Infrared spectroscopy analysis, X-ray diffraction analysis, and scanning electron microscopy (SEM) analysis were conducted for high dispersibility nano-silica powder. Mechanical properties test and SEM observation of fracture morphology were investigated for nano-silica/UPR polymer composite samples. The results revealed that adding silane coupling agent KH570 with a mass fraction ratio 3% into nano-silica powder made nano-silica best dispersed after the modification reaction at a temperature of 80°C for 2 h. When the high dispersibility nano-silica powder was added into UPR with a mass fraction ratio 1.5%, the impact strength of nano-silica/UPR polymer composite improved greatly by 9.6%. However, when the high dispersibility nano-silica powder was added into UPR with a mass fraction ratio 2%, the tensile strength, bending strength and extension rate of nano-silica/UPR polymer composite improved greatly by 152%, 102%, and 167%, respectively.

1 Introduction

As one kind of the available thermosetting resins, with the role of a heating or curing agent, unsaturated polyester resin (UPR) forms an insoluble and infusible network polymer which cannot meet most of the requirements because of its poor mechanical properties. With the development of strengthening and toughening of the polymer composite, particle reinforcement by nano-mineral materials has become a new field of development and has got widespread attention from scholars at home and abroad. Wang et al. [1] researched on glass fiber reinforced plastics (GFRP)-anchor’s torsional strength enhanced by adding nano-mullite powder into the matrix phase. Shi et al. [2] investigated the improved tribological properties of UPR composites reinforced by nano-SiO2. Xu et al. [3] found the new bond structure in UPR composites modified by nano-TiO2. Wu et al. [4] indicated that nano-ZnO surface modified by oleic acid improved the bending strength of UPR composites. Wang et al. [5] put forward that the dispersion of nano-MgO affected the dynamic thermodynamic properties, heat resistance, and hardness of UPR composites. Sharma et al. [6] concluded that the silica particles are the best choice to use as filler in UPR matrix for UPR composite used in electrical equipments such as medium-voltage inductive transformers. However, this article mainly aims at comprehensive consideration about the dispersion modification of reinforcing phase, the strengthening and toughening modification of matrix phase, and the functional bond which connects inorganic nano-particles and organic macromolecules of UPR that were designed. Then, because of an increase in the intermolecular binding force, the enhancement of the strengthening and toughening performance of nano-silica/UPR polymer composite is ensured from the viewpoint of microcosmic interface mechanics, and it will finally improve the macroscopic mechanical properties of the nano-silica/UPR polymer composite. Accordingly, the applications of nano-silica/UPR polymer composite should be enlarged. Firstly, high dispersibility nano-silica powder was prepared by adding silane coupling agent KH570 as a dispersant into the commonly and commercially available nano-silica powder with appropriate test parameters such as mass fraction ratio, modification reaction temperature, and time; secondly, the dispersion effect of high dispersibility nano-silica powder was characterized by infrared spectroscopy (IR) analysis, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM) analysis; thirdly, high dispersibility nano-silica powder was added with different mass ratios into UPR and nano-silica/UPR polymer composite samples were fabricated; finally, mechanical properties test and SEM observation of fracture morphology were conducted for nano-silica/UPR polymer composite samples.

2 Materials and methods

2.1 Materials

UPR, the type of 192 (produced by Tianma Company, Changzhou, Jiangsu Province, China) was used as matrix phase, and nano-silica powder, the type of SG-200 (produced by Guangzhou Shinshi Metallurgical Chemical Co., Ltd., Guangzhou, Guangdong Province, China), was used as reinforced phase. The parameters of them are shown in Tables 1 and 2, respectively. Other more experimental, materials included coupling agent KH570 (produced by Nanjing United Silicon Chemical Co. Ltd., Nanjing, Jiangsu Province, China), curing agent Methyl ethyl ketone peroxide (produced by Changzhou Hualike New Material Co., Ltd., Changzhou, Jiangsu Province, China) and promoter agent cobalt napthenate (Produced by Yaodong Chemical Co., Ltd., Zibo, Shandong Province, China) etc. All the above experimental materials were available in the market, and they were all in accordance with green environmental protection requirements.

2.2 Methods

2.2.1 Preparation and characterization of high dispersibility nano-silica powder

Dispersion modification of commonly and commercially available nano-silica powder and preparation of high dispersibility nano-silica powder were operated with the following processes: firstly, the common nano-silica powder was dried at 120°C for 4 h in drying oven; secondly, ethanol/water solution (volume ratio, 11:1 – total volume not more than two-thirds capacity of the flask) was blended in the three-necked flask with reflux condenser and magnetic stirring; thirdly, appropriately dried nano-silica powder and silane coupling agent KH570 (mass fraction ratio 3% of nano-silica powder) was added into ethanol/water solution, and the pH value of the mixture was adjusted to about 4–5 by adding acetic acid solution; fourthly, the three-necked flask was placed in the water bath, and the modification reaction of the above mixing solution was sustained for 2 h at a temperature of 80°C with magnetic stirring; fifthly, separation of the above solution mixture was carried out using a centrifuge (9.8 g) – a precipitate was obtained at the bottom of the flask and the supernatant liquid was poured out; sixthly, the obtained precipitate was dried at 120°C for 48 h in drying oven after it was washed with acetone several times, and the high dispersibility nano-silica powder was prepared; finally, the dispersion effect of high dispersibility nano-silica powder was characterized by IR, XRD, and SEM analysis, respectively, for obtaining different views.

2.2.2 Preparation and characterization of nano-silica/UPR polymer composite

Preparation and characterization of nano-silica/UPR polymer composite were investigated with the following processes: firstly, all the components of the matrix glue including UPR, curing agent methyl ethyl ketone peroxide, and promoter agent cobalt napthenate were weighed accurately in accordance with the established ratio – after being fully stirred in a container, the matrix glue was prepared; secondly, the high dispersibility nano-silica powder with the following different amounts: 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 wt% was added into the matrix glue – after being fully stirred and being poured into the sample mold for solidification, six groups of different nano-silica/UPR polymer composite samples were obtained. While, as samples used for comparision, the seventh group of samples was made of the normal matrix glue without the high dispersibility nano-silica powder. Three samples used for tensile test, bending test, and impact test, respectively, were included in each group; thirdly, mechanical properties test and SEM observation of fracture morphology were conducted for the seven groups of samples, and repeated confirmatory test for each of the above tests was carried out, and the tensile strength, extension rate, bending strength, and impact strength were, respectively, the average values.

2.2.3 Specimen shape and test methods

According to standard GB/T 2567-2008, tensile test specimen, bending test specimen, and impact test specimen were fabricated into 60×10×4 mm3, 120×15×5 mm3, and 80×10×4 mm3, respectively, as shown in Figure 1. Tensile test and bending test were conducted on a WDW-20 type electronic universal testing machine produced by Changchun Tester Institute; applied deformation rate was 2 mm/min and applied load was 2 kN. However, impact test was carried out on a XJJ-5 type impact testing machine, and velocity of the pendulum impacting the specimen was 2.9 m/s. In order to investigate the fracture morphology, SEM observation was conducted with the help of a S-3000N scanner produced by Hitachi, Japan.

Figure 1 Pictures of specimen shapes: (A) tensile test specimen; (B) bending test specimen; and (C) impact test specimen.
Figure 1

Pictures of specimen shapes: (A) tensile test specimen; (B) bending test specimen; and (C) impact test specimen.

3 Results and discussion

3.1 Influence of silane coupling agent KH570 on nano-silica powder’s dispersibility

From the physical point of view, due to its large specific surface area, high surface energy, high ratio of the surface atoms and total number of atoms, nano-silica powder’s spontaneous reunion on the atomic level is inevitable, and it is difficult to be eliminated either by dissolution or vibration [7]. From the chemical point of view, internal polysiloxanes’ structure and external active silanols that are dangling bonds around surface atoms make the dispersibility of nano-silica powder difficult in the UPR. Silane coupling agent KH570 was proposed to modify the nano-silica powder in this article. The basic mechanism was as follows: first, silane coupling agent was hydrolyzed into silanol bonds, as shown in Equation (1); then, dehydration reaction occurred between silanol bonds of silane coupling agent and hydroxyl of nano-silica, as shown in Equation (2).

The IR spectra of unmodified and modified nano-silica were shown in Figures 2 and 3, respectively. Normal absorption peaks existed in IR spectrum of unmodified nano-silica, including the stretching vibration absorption peak of Si-O at 808 and 1106 cm-1, bending vibration absorption peak of O-H at 1640 cm-1, stretching vibration absorption peak of Si-O-H at 475 cm-1 and stretching vibration absorption peak of O-H at 3412 cm-1, as shown in Figure 2. However, there were new characteristic absorption peaks existing in the IR spectrum of modified nano-silica by silane coupling agent KH570. They were vibration absorption peak of C-C at 1505 cm-1, vibration absorption peak of -CH3 and -CH2 groups at 2928 cm-1 which fully proved the presence of organic key group in the modified nano-silica by silane coupling agent KH570.

Figure 2 IR spectrum of unmodified nano-silica.
Figure 2

IR spectrum of unmodified nano-silica.

Figure 3 IR spectrum of modified nano-silica.
Figure 3

IR spectrum of modified nano-silica.

The most apparent and intuitive evidence for the excellent dispersion effect was from SEM observation. The different SEM morphologies of particle distribution between unmodified and modified nano-silica are shown in Figures 4 and 5.

Figure 4 SEM morphology of particle distribution of unmodified nano-silica powder.
Figure 4

SEM morphology of particle distribution of unmodified nano-silica powder.

Figure 5 SEM morphology of particle distribution of modified nano-silica powder.
Figure 5

SEM morphology of particle distribution of modified nano-silica powder.

Moreover, the fact that no apparent changes took place in the XRD patterns of modified nano-silica powder proved that the crystal structure of nano-silica powder remained amorphous after modification of silane coupling agent KH570, as shown in Figures 6 and 7, respectively.

Figure 6 XRD pattern of unmodified nano-silica powder.
Figure 6

XRD pattern of unmodified nano-silica powder.

Figure 7 XRD pattern of modified nano-silica powder.
Figure 7

XRD pattern of modified nano-silica powder.

3.2 Influence of high dispersibility nano-silica powder on UPR matrix’s enhancement of mechanical performance

Due to the gradually increasing mass fraction ratio of high dispersibility nano-silica powder in the UPR matrix, the values of the impact strength, tensile strength, bending strength, and extension rate increased sharply first, reached a maximum, and then declined gradually. When the high dispersibility nano-silica powder was added into UPR matrix with a mass fraction ratio 1.5%, the impact strength of nano-silica/UPR polymer composite improved greatly by 9.6% and reached a maximum of 2.06 J/m2. And when the high dispersibility nano-silica powder was added into UPR matrix with a mass fraction ratio 2%, the tensile strength, bending strength and extension rate of nano silica/UPR polymer composite improved greatly by 152%, 102% and 167%, and then reached a maximum of 63 MPa, 93 MPa and 2% as shown in Figures 8 and 9, respectively.

Figure 8 Contrast curve of the tensile strength and bending strength as a function of the high dispersibility nano-silica powder content.
Figure 8

Contrast curve of the tensile strength and bending strength as a function of the high dispersibility nano-silica powder content.

Figure 9 Contrast curve of the extension rate and impact strength as a function of the high dispersibility nano-silica powder content.
Figure 9

Contrast curve of the extension rate and impact strength as a function of the high dispersibility nano-silica powder content.

The contrastive SEM morphology of impact fracture between pure UPR sample and 1.5% nano-silica/UPR sample was shown in Figure 10. The contrastive SEM morphology of tensile fracture between pure UPR sample and 2% nano-silica/UPR sample was shown in Figure 11. No matter whether it is impact fracture or tensile fracture, the fracture morphology of pure UPR samples appeared obvious and typical brittle fracture characteristics appeared because of the many more smooth platforms and sharp edges on the smooth surface, as shown in Figures 10A and 11A. However, stepwise tear stripes and micro-pits and a large number of fine micro-cracks resulted because the plastic deformation around nano-silica particles appeared in the impact fracture morphology of 1.5% nano-silica/UPR sample, as shown in Figure 10B. The tensile fracture morphology of 2% nano-silica/UPR sample was coarse and lacerated with more uniform dimple distribution characterizing the ductile fracture, as shown in Figure 11B.

Figure 10 Contrastive SEM morphology of impact fracture: (A) UPR sample and (B) 1.5% nano-silica/UPR sample.
Figure 10

Contrastive SEM morphology of impact fracture: (A) UPR sample and (B) 1.5% nano-silica/UPR sample.

Figure 11 Contrastive SEM morphology of tensile fracture: (A) Pure UPR sample and (B) 2% nano-sillica/UPR sample.
Figure 11

Contrastive SEM morphology of tensile fracture: (A) Pure UPR sample and (B) 2% nano-sillica/UPR sample.

Many ductile or plastic fracture characteristics that appeared in the impact fracture morphology of 1.5% nano-silica/UPR sample and in the tensile fracture morphology of 2% nano-silica/UPR sample made the enhancement of the strengthening and toughening performance of nano-silica/UPR polymer composite inevitable. It could be interpreted from the following three views. Firstly, from the chemical point of view, functional bond owned by high dispersibility nano-silica powder modified by silane coupling agent KH570 connected inorganic nano-silica particles and organic macromolecules of UPR. It resulted in an increase of the intermolecular binding force [8, 9]; secondly, from the microscopic interface mechanics point of view, there existed flexible interface layer between two-phase interface which transferred the stress effectively and enabled propagation of the blocked micro-crack [10]; thirdly, from the crystallography point of view, high dispersibility nano-silica powder played the fine grain strengthening role in the UPR matrix because the nano-silica powder resulted in improving the crystal nucleation rate and reducing the UPR matrix crystallinity and grain size [11]. At the base of the above three reasons, the enhancement of the strengthening and toughening performance of nano-silica/UPR polymer composite was apparent.

However, when the mass fraction ratio of high dispersibility nano-silica powder in the UPR matrix was too high and more than 1.5% or 2%, the impact strength, tensile strength, bending strength, and extension rate of nano-silica/UPR polymer composite gradually reduced. As hard and fine phase particles, excessive nano-silica powder in the UPR matrix easily gathered at the phase boundary, and even defects such as particle segregation and fragmentation easily occurred in the UPR matrix [12, 13], so a drop in the above mechanical properties of nano-silica/UPR polymer composite was inevitable.

4 Conclusions

  1. Adding silane coupling agent KH570 with mass fraction ratio 3% into nano-silica powder made nano-silica best dispersed after the modification reaction at a temperature of 80°C for 2 h.

  2. Adding high dispersibility nano-silica powder modified by silane coupling agent KH570 into UPR matrix with appropriate mass fraction ratio improved the mechanical properties of nano-silica/UPR polymer composite greatly. The enhancement of the strengthening and toughening performance resulted from the strong intermolecular binding force, the flexible interface layer between two-phase interface, and the fine grain strengthening role played by the high dispersibility nano-silica powder.

  3. Because of the particle segregation and fragmentation, excessive high dispersibility nano-silica powder in the UPR matrix resulted in the drop of mechanical properties of nano-silica/UPR polymer composite including the impact strength, tensile strength, bending strength, and extension rate.


Corresponding author: Yan-qing Wang, School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China, Phone: +86 151 5001 8363, Fax: +86 516 8359 1877, e-mail:

Acknowledgments

This work was supported by the Fundamental Research Funds for the Central Universities (2012QNB05) and China Postdoctoral Science Foundation funded project (2013M531422). We gratefully acknowledge Jiangsu Province Post-doctorate Scientific Research Program, State Key Laboratory of Tribology of Tsing Hua University, and General manager Wu You-liang from Xuzhou Baoding Supporting Technology Co., Ltd. for fruitful discussion and experimental assistance.

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Received: 2013-3-5
Accepted: 2013-10-5
Published Online: 2013-12-6
Published in Print: 2014-9-1

©2014 by De Gruyter

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