Study on Improvement of Properties of Epoxy Resin Adhesive with Nano-SiO2

Abstract : The theoretical basis for the modification of epoxy resin adhesives with inorganic nanoparticles and the dispersion principle of inorganic nanoparticles were analyzed and described. Based on this theory, the nano-SiO2 epoxy resin adhesive was modified with a hybrid method, and good results were obtained in improving the performance of the epoxy resin adhesive. Description of inorganic nanoparticle modified epoxy adhesive is a proven method.

Keywords: nanoparticles; epoxy adhesives; dispersants

In the 1980s, Kurauchi et al. first proposed the use of inorganic rigid particles to toughen polymers. After decades of development, this method has become one of the most effective ways to modify materials. The modification of organic polymers with inorganic rigid particles can increase the rigidity, hardness, and wear resistance of the polymer, while reducing the strength and toughness of the polymer. After studying the nanometer-scale materials, the scientific community pointed out four major effects of nanomaterial characteristics: quantum size effect, small size effect, surface effect and quantum tunneling effect.

1 The theoretical basis of modification

Inorganic fillers increase the toughness of the material while increasing its rigidity. However, nanomaterials can give both considerations. What is the reason? Some people attributed to the silver texture theory that nanoparticles are uniformly dispersed in the matrix and the matrix is ​​impacted. At the same time, micro-cracks between the particles and the matrix—the silver grains—are formed. At the same time, the matrix between the particles also undergoes plastic deformation and absorbs impact energy, thereby achieving the effect of toughening. As the size of the nanoparticles decreases, the specific surface area of ​​the particles increases, and the interface between the particles and the matrix increases, resulting in more microcracks and greater plastic deformation, thereby absorbing more impact energy and increasing the toughening effect.

Wang Hongxuan thinks that according to the traditional view, after nano-SiO2 and Other nanoparticles are uniformly dispersed in epoxy resin, when the matrix resin is impacted by an external force, silver lines will occur between the two. The matrix resin between the nanoparticles also undergoes plastic deformation and absorbs a certain impact energy. With the refinement of the particles, the specific surface area will further expand, and the contact area between the nanoparticles and the matrix resin will increase. When the material is impacted by external force, it will produce more silver lines and plastic deformation, and absorb more impact energy to achieve the effect of toughening the material. The presence of rigid nanoparticles tends to produce stress concentration effects and cause silvering of the surrounding matrix resin, which absorbs a certain amount of deformation work. At the same time, the presence of rigid nano-particles causes the matrix resin silver pattern to be blocked and passivated, eventually stopping and not developing to destruction. Cracking, resulting in toughening effect.

2 Adhesive formulations and properties without nanoparticles

Adhesive formula: E51 epoxy resin 100, 651 polyamide 35; test piece bonding process: test piece is 45# steel, the processing method is mechanical derusting → acetone cleaning → chemical treatment → water, blow dry → Coupling agent treatment → bonding.

The curing process of the bonded specimens was: 100°C for 3 hours, followed by furnace cooling.

The measured initial tensile strength of the adhesive was 70.4 MPa, the tensile strength after damp heat aging was 53.7 MPa, the strength retention was 67.4%, the initial impact strength was 3.99 kJ·m-2, and the impact strength after hygrothermal aging was 3.66. kJ·m-2, the strength retention rate was 91.7%.

3 Determination of nanoparticle dispersion methods in adhesives

Although the nano-composite in the polymer nanocomposite has a small amount (5% to 10%), due to the small particle size, large specific surface area, and high surface energy, it is easy to form aggregates with large particle diameters. It is difficult for the nano-components to exert their unique effects. Once the nano-particles are phase-separated in the composite material, their properties will be reduced to the same extent as conventional composites. Therefore, the dispersion of the nano-components in the polymer (or its precursor) is the key to the nano-composite material preparation process. When considering the dispersion scheme, first consider the wettability of the nanoparticles.

The wetting process of nanoparticles is actually a process in which the solid-air interface disappears and the solid-liquid interface forms. According to the change of free energy of the system, under the constant temperature and constant pressure, the energy change formula of nanoparticle agglomerates is:

△G=σSL-σSV

Because the surface energy σSV of the nanoparticle is larger than the free energy σSL of the solid-liquid interface, ΔG<0. The wetting process of this system is a thermodynamic spontaneous process. This spontaneous wetting process is effective for primary dispersion of the nanoparticles. Therefore, in the selection of dispersants, dispersants with a large degree of spontaneous wetting should be selected (at the same time, the compatibility with the matrix resin must also be taken into consideration) as the surface treatment agent of the nanoparticles so that they can achieve the best primary dispersion.

The deep dispersion of nanoparticles should consider the balance of microscopic particle dispersion and agglomeration. According to H. Wang et al.'s point of view, nanoparticles are very easy to form agglomerates because of their special surface structure. There are interactions between particles that are different from those of conventional particles. For the time being, they are called nanoeffect energy (Fn). This kind of nano-action energy is the intrinsic property of nano-particles due to the lack of adjacent coordination atoms, high activity and aggregation of nano-particles, and its physical meaning should be the adsorption force between nano-particles per unit specific surface area. It is the sum of several adsorption forces of nanoparticles: the adsorption of hydrogen bonds and electrostatic interactions between nanoparticles; the quantum tunneling effect between nanoparticles; the adsorption due to charge transfer and local coupling of interface atoms; the adsorption of a large specific surface of nanoparticles . Nanoparticles can be an intrinsic factor in the ease of nanoparticle agglomeration.

To obtain nanoparticles with good dispersibility, small particle size, and narrow particle size distribution, the nano-effect energy must be weakened or reduced. When a proper method is adopted to disperse the nanoparticles, the surface of the nanoparticles will produce solvated film energy (Fs), double-layer electrostatic interaction energy (Fr), and polymer adsorption layer space protection (Fp). In a certain system, nanoparticles should be in equilibrium with these kinds of interactions.

FnFs+Fr+Fp

When Fn>Fs+Fr+Fp, nanoparticles are easy to agglomerate;

When Fn<Fs+Fr+Fp, the nanoparticles are easily dispersed.

For the ready-to-use adhesives, the mixing method is more suitable. The commercially available nanopowder has been subjected to primary dispersion in a production plant. According to the characteristics of nanomaterials, further dispersion must be done before use. We determined the following methods for the dispersion treatment of nanopowders before adding them to the adhesive.

(1) with xylene as a solvent, titanate NDZ311 as a dispersant dubbed mixed solution, the ratio of m (xylene): m (titanate) = 200: 5;

(2) Disperse nano-SiO20.7g in 2.05g of the above mixture;

(3) Take 14g of E51 epoxy resin, add the treated SiO2 powder, and mix well.

Properties of 4 nm SiO2 modified epoxy resin adhesive

Nano-particles were added with epoxy resin adhesive to improve the tensile strength, bending impact strength, and hygrothermal aging properties of the epoxy resin. The initial tensile strength of the tested adhesive was 84.7 MPa, and the tensile strength after damp heat aging was 82.1 MPa. The retention rate was 96.7%; the initial impact strength was 4.64 kJ·m-2, the impact strength after hygrothermal aging was 4.28 kJ·m-2, and the strength retention rate was 92.2%.

From the above, we can see that the modification effect of nano-SiO2, the initial tensile strength increased by 6.7%, 52.9% increase in tensile strength after damp heat aging. The retention rate of tensile strength after wet-heat aging was 96.9% after adding nano-powder, and only 67.4% when nano-SiO2 was not added. From the maintenance of the strength after heat and humidity aging, the nano-SiO2 modified adhesive was also better than the unmodified ones. It can also be seen from the comparison that with the epoxy modified with nano-SiO2, the initial impact strength was increased by 16.3%; the impact strength after damp-heat aging was increased by 16.9%; the strength retention rate after moist heat was also greatly increased.

5 Conclusion

The nano-SiO2 modified epoxy resin polyamide adhesive has obvious improvement in the mechanical properties and heat and moisture aging resistance of the adhesive. The research in this field has broad prospects.