Synthesis and research of epoxy resin toughening agent
© The Author(s) 2016
Received: 7 March 2016
Accepted: 17 May 2016
Published: 21 June 2016
In this paper, a synthesis method of epoxy resin toughening agent was presented, then the chemical composition and molecular number were studied, which include the DSC curves analysis, the fracture surface morphology and bonding strength. In addition, the mechanism of epoxy resin toughening agent and the effect of toughening agent’s content to bonding strength were studied. The testing results reveal that this toughening agent can form a micro two-phase structure in epoxy-amine system, which results in the stable chemical properties and excellent physical properties.
KeywordsEpoxy resin Toughening agent Bonding strength Two-phase structure
Due to excellent mechanical, electrical properties and chemical stability, epoxy resin (EP) curing products are widely used in electronics, machinery, construction and other industries (FInk 2013). However, the brittleness of EP affects its further application. To ensure impact resistance and reduce the stress during curing, some measures should be taken to improve the toughening of EP curing products.
Generally, the EP toughening methods mainly include thermoplastic resin toughening, rubber toughening, organic silicon resin toughening, rigid particle toughening and nano-particles toughening, etc. Rubber toughening mechanism is the “silver streaks—nail anchor” mechanism and “silver lines—shear zone” mechanism (Yahyaie and Ebrahimi 2013). After having toughened with rubber, the resistance to impact and bending performances have been significantly improved (Rahman et al. 2013), however, the strength, modulus and heat resistance performance is weakened due to the lower strength and modulus of rubber. Thermoplastic resin modified by rubber can improve the toughness (Giannotti et al. 2003), but the stiffness and heat resistance are reduced. Nano-particles have large surface activity, which is easy to produce physical or chemical combination with the polymer (Al-Turaif 2010), which can resist micro-crack initiation. Compared with EP composite material toughened by rubber, the strength and rigidity are decreased (Créac’hcadec et al. 2014).
To obtain high performance of EP composite material, some new toughening methods and techniques are proposed, such as the macromolecular curing agent toughening, hollow particle toughening (Tagliavia et al. 2011) and resin alloying toughening. This paper describes a new synthesis method of EP toughening agent, which can effectively improve the compatibility between toughening agent and EP. Furthermore, the toughening mechanism is studied, which includes the curing reaction temperature, glass transition temperature, bonding strength, curing heating rate. Particularly, the amount of toughening agent is studied after EP system being toughened by the new synthesis toughening agent.
Synthesis of toughening agent
Epoxy resin E54, Guangzhou Rong Sheng Chemical Co., Ltd.;
Polypropylene glycol (PPG), MW = 2000, Wanhua Chemical Group Co., Ltd.;
4,4′-Diphenylmethane diisocyanate (MDI), MW = 250, Wanhua Chemical Group Co., Ltd.;
Polyethylene glycol (PEG), MW = 400, Wanhua Chemical Group Co., Ltd.;
Diethylenetriamine (DETA), curing agent, Changzhou Deye Chemical Industry Co., Ltd.
Analysis of synthetic product
Ionic concentration of different polymer solution
Solution with 2 mL water
Solution with 3 mL water
Solution with 5 mL water
Solution with 10 mL water
Ionic concentration (10−2 mol/L)
Composition of toughening agent
Infrared absorption spectrum (Qi et al. 1988) generated by continual vibration and rotational motion of the molecules, the molecular vibrations means the relative motion of the atoms near the equilibrium position, polyatomic molecules can composite various vibration graphics. Infrared spectroscopy can be used to study the molecular structure and chemical bonds (Pandita et al. 2012), which can also be used as characteristically approach of chemical species. The functional group structure of the synthetic toughening agent was studied by using infrared absorption spectra.
Isocyanate group content
Performance analysis of epoxy resin adhesive
The performance of synthetic toughening agent should be tested in EP adhesive system. Firstly, the adhesive system can be formed by adding a certain amount of toughening agent and curing agent into EP, and then some tests and experiments were performed to evaluate performance of the new synthetic toughening agent.
Curing reaction temperature, curing heat and glass transition temperature
It can be seen from the second heating curve that there is no obvious crest, which indicates that resin has been cured completely after the first heating. The glass transition temperature (Tg) is regarded as the intersection point of the transition line’s extension line and the baseline’s extension line (Zhang et al. 2011), the glass transition temperature of the polymer is 57.27 °C. This experiment was performed at Physical and Chemical Test Center of the University of Science and Technology of China.
The effects of heating rate on the curing temperature and curing heat
To reveal the effects of heating rate on the curing temperature and curing heat (Jang and Paik 2009), the DSC test of the same EP system in three different heating rates of 5, 10, 20 °C/min were performed respectively. The DSC curves of three different heating rate is drawn.
Microstructure of the specimen port
It can be seen from the SEM pictures that the shear port surface morphology of formulas A and B show two-phase structure, in which the continuous phase likes river and the dispersed phase likes holes. But the shear port surface morphology of formula C is randomly distributed. Formulas A and B both contain toughening agent, the content of toughening agent for formula B is higher, the hole density for formula B is more than formula A. While the EP-curing agent system constitutes the continuous phase, the new synthetic toughening agent distributes in it to form the dispersed phase. As a result, the two-phase structure greatly improves compatibility between the EP and additives.
The bonding strength test
The geometry size is 40 mm × 10 mm × 2 mm;
The overlap area is 40 mm × 10 mm.
Research on the effect of the toughening agent amount to the bonding strength
When the ratio of EP adhesive to toughening agent is 20 %, the bonding strength reaches the peak value, however, if the ratio of toughening agent is too high, which will cause damage to the original curing system, thus reducing the system’s bonding strength (Al-Turaif 2010). If the ratio of toughening agent is <20 %, a perfect two-phase structure was formed by the combination of toughening agent and resin system without affecting the epoxy value of the system, therefore, the bonding strength of the system can be improved.
The compatibility between the synthesized toughening agent and EP E54 is satisfactory.
The epoxy resin E54 system can be cured completely under proper condition, the curing heat reaches the peak value at the heating rate of 11 °C/min, therefore, it is necessary to select the proper heating rate.
In the epoxy resin E54 system toughened by the synthesized toughening agent, toughening agent contributes to form a perfect two-phase structure. When the ratio of curing agent and epoxy resin is fixed, the proportion of toughening agent affects the bonding strength of epoxy resin system greatly.
The synthesis of toughening agent was performed by HM, NH and CW. The performance analysis of epoxy resin adhesive was performed by YC and QC. YZ is responsible for the overall research program of this manuscript. All authors read and approved the final manuscript.
Support by scientific research program of Power Research Institute of Yunnan Power Grid Co., Ltd. (No. K-YN2014-108) and Natural Science Foundation of Anhui Province (No. 1508085ME84).
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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