JEC Group have brought together the international community of composites leaders and executives in our Composites Circle as an unique networking opportunity to meet with both peers and future partners.
A new approach for enhancing joint properties by simultaneously integrating micro- and nanosized fillers into an epoxy-silicon-organic glue formulation has been proposed. It was shown that the thermoresistance of the glue joint is improved more than threefold.
(1) PROF. SCD. Е.F. VENGER, HEAD OF SENSOR TECHNOLOGIES AND MATERIALSDIVISION, (2) DR. М.М. LOKSHYN, SENIOR SCIENTIFIC RESEARCHER(3) SCD. MASLOV V.P., HEAD OF THE LABORATORYV.E. LASHKARJOV INSTITUTE OF SEMICONDUCTOR PHYSICS OF NAS OF UKRAINE(Published on January-February 2011 – JEC Magazine #62)
The potential for enhancing composite properties by adding various fillers containing nanoparticles is being intensively examined. It is known that replacing microparticle fillers by nanoparticle fillers in a composite formulation results in improved physical properties [1, 2]. Fillers are introduced into the glue formulation to cure shrinkage, decrease internal stresses caused by shrinkage, improve thermal stability, enhance joint toughness and minimize the amount of glue used . When carbon nanoparticles are added to the glue formulation, the initial durability and elasticity of a glutinous seam increase, and adhesion is improved .
The authors provided a new approach to upgrade the properties of a glue formulation. Proceeding from the assumption that particles of a different size should increase the toughness of the glue seam structure, while the presence of microparticles might raise the resistance of a glue joint to a greater degree than with nanoparticles alone, they proposed to integrate micro- and nanosized fillers simultaneously into the formulation and thus improve its properties through selection of the respective filler concentrations. This work attempted to study the thermoresistance of epoxy glue joints when microparticle and nanoparticle fillers are simultaneously integrated. Because epoxy glue formulations demonstrate the highest shortterm and long-term durability, an epoxy glue matrix was selected to formulate a high-thermal-resistance nanocomposition.
Measured at a temperature of 20ºС, the durability of joints based on epoxy glue cured with maleic anhydride following 5 years of storage was 16% for glue stored in warehouse conditions and 27 % for glue stored in the open air. The silicon-resin glue joint’s loss of durability under the influence of climatic factors within 5 years does not exceed 10%.
A three-component epoxy-silicon-organic glue was selected for the glue formulation used in the experiments. The formulation included an epoxy silicon resin, an amine (hardener) curing agent and a TiO2 filling micropowder. The average size of the filling microparticles reached 30-40 μm.
A base glue formulation was supplemented with a nanopowder composed of 3% Y2O3 (ittrium oxide) doped by ZrO (zirconium dioxide). The average size of ZrO nanoparticles was 10-15 nm, and their concentration in the glue formulation varied from 5 to 50 volume %. The durability of glue joints in Zerodur glass ceramic parts with almost zero thermal expansion was studied.
The joined parts were tested after heat treatment in a resistanceheating muffle furnace at 250ºС for 2 hours. When a standard epoxy-silicon-organic glue formulation containing only a micropowder filler was used without a nanopowder filler, the joint durability did not exceed 10 MPa. We managed to improve this parameter by increasing the volume content of nanoparticles from 5% to 20%. After the heat treatment, when the volume of nanoparticles reached 20%, the joint remained resistant to a 35 МПа rupture stress. But when the 20% limit was exceeded, the glue formulation became extremely viscous, so it was difficult to apply an even glue layer on the surfaces and to obtain the thinnest possible glue seams.
Discussion of experimental results
The thermoresistance of the new glue formulation was enhanced through the joint influence of several factors. First, the toughness of the glue seam structure should be higher than with the standard formulation, since nanoparticles tend to fill the free space between microparticles of a typical microfiller.
Basically, a thin glue seam containing filling particles could be described as a disorderly close-packed structure consisting of solid spheres. The free volume in this structure consists of regular tetrahedral and octahedral voids typical for spheres of ideally closepacked structure and vacancy-type voids (absence of spheres). The size of the regular tetrahedral voids is about 0.225R, octahedral ~0.414 R, vacancies ~ R, where R stands for the sphere radius. When ten-micron fillers are used, the size of voids is also measured in microns. As the real size of nanoparticles is three orders lower, nanoparticles – when added in sufficient concentration – will fill all the voids between microparticles.
Figure 1 (a, b) illustrates how micro- and nanoparticles may increase the toughness of a glue seam when simultaneously introduced into a glue formulation.
In the figure, the filling particles are represented in the form of solid spheres of various diameters. If the share of irregular voids is insignificant, the concentration of nanoparticles necessary to fill them should make up for 20- 40 % of the glue seam volume.
The solid-sphere model has been used to describe "metalmetalloid" nuclear structures in amorphous metal alloys . The assumption that metalloid atoms fill the voids between metal atoms bigger in size than those of metalloid has allowed to quantitatively estimate the optimal metalloid concentration necessary to achieve a dense alloy structure at the level about 20 volume %.
Secondly, it was established earlier by IR spectrometry that, during silicon resin glue polymerization, Si-O ion links form between hydroxyls located on a solid surface of glass ceramics, polycrystalline glass or filler particles and silicon which is a part of silicon resin. As a result, a layer of glue chemically connected to the surface of the solid body is formed. The quantity of Si-O ion links and, accordingly, a share of the reinforced connections, is proportional to the total area of the surfaces in contact with the silicon resin, i.e. the durability of the joint shown on Figure 1(b) should be higher than in the case reflected in Figure 1(a).
The experiments showed that if only the nanoparticle filler is integrated into the glue formulation, thermal resistance does not increase by more than 20-30%. This result may be explained by the fact that the durability of a glue joint is defined not only by the density of the joint seam structure, but also by the durability of the filling particles themselves.
For the first time, it has been exemplified by epoxysiliconorganic glue that glue properties can be modified effectively by simultaneously adding different-sized filler particles – nanoparticles and microparticles – to the formulation. The approach applied to the modification of poxy-siliconorganic glue may be recommended for the development of new formulations based on other glues.
1. Hanemann, T.; Boehm, J.; Henzi, P.; Honnef, K.; Litfin, K.; Ritzhaupt-Kleissl, E.; Hausselt, J.: “From micro to nano: properties and potential applications of micro- and nanofilled polymer ceramic composites in microsystem technology”, IEE Proceedings - Nanobiotechnology, 2004, v. 151, Issue 4, pages 167-172
2. Sprenger, S.; Kinloch, A.J.; Tailor, A.C.; Mohammed, R.D.: “Rubber-toughened FRCs optimized by nanoparticles – Part IV”, JEC Composites Magazine, No 38 January- February 2008, pages 60-63 (2008)
3. Sobolev, K.; Flores, I. ; Hermosillo, R.; Torres- Martinez,:”Nanomaterials and nanotechnology for highperformance cement composites”, Proceedings of ASI, session on “Nanotechnology of concrete: Recent Developments and Future Perspectives”, 7.11.2006, Denver, USA, pages 91-116
4. Sudzuki, K., Fujimori, H., Mosumoto, K., “Amorphous Metals” (in Russian), Metallurgiya, Moscow (1987), 353 p.
5. Kumar, K.S.; Van Swygenhoven, H.; Suresh, S. : ”Mechanical behavior of nanocrystalline metals and alloys”, Acta Materialia, 2003, v.51, pages 5743-5774