rus
Issue #4/2016
A.Sapronov, N.Buketova, A.Leshchenko
Study of thermal properties of epoxy composites filled with nanoparticles
Study of thermal properties of epoxy composites filled with nanoparticles
Composites on the basis of bisphenol epoxy resin with the low molecular weight polyethylene polyamine curing agent were studied. As an additive the nano-filler C60 was used. Thermal properties of epoxy composites were investigated.
The modern industry needs to develop effective new materials with improved properties. The use of composite materials (CM) based on the epoxy matrix reduces the specific content of metal of structures and protect the surface from external influences (aggressive media, temperature variations etc.). Thereby the performance properties of structures largely depend on the heat resistance of the new materials. It is possible to improve the properties of the final composite material by the introduction in its composition of nanoparticles of various physical and chemical nature [1–4]. Therefore, modification of the thermo-physical properties as a result of the introduction of nanoparticles is an actual problem of the modern polymer materials science.
An analysis of studies [1, 2, 4–9] suggests that one of the effective ways to improve the thermal properties of epoxy composites is to introduce fillers of various degree of dispersion as well as physical and chemical nature. The combination of these materials allows to abandon the use of traditional painting materials and use CM as protective coating. To assess the operating temperature range of the new CM, a study of their properties during heating is conducted [9–11].
The purpose of this project is to investigate the effect of the amount of С60 on the heat resistance and coefficient of thermal linear expansion of nanocomposites.
Materials and methods
of research
As the main component of the binder in the creation of nano-composite materials (NCM) selected was a diane epoxy oligomer of the ED-20 brand (GOST 10587-84). The use of the low-molecular polyethylenepolyamine hardener (PEPA) [-CH2-CH2-NH-]n (TU 6-05-241-202-78) made it possible to crosslink the epoxy composition at room temperature. The hardener was administered at a stoichiometric ratio of the components (wt.%) 10 (ED-20) to 1 (PEPA).
The particles of the fullerene С60 with the dispersion of 5 nm are used as filler. Epoxy NCM filled with С60 were created with the use of ultrasonic dispersion compositions in the following modes:
•pre-dosing of ED-20, resin heating to T = 353 ± 2 K and its holding at this temperature for the time τ = 20 ± 0.1 min;
•dosing of nanofiller and the further introduction of it into the epoxy oligomer;
•hydrodynamic alignment of the oligomer ED-20 and nanofiller for τ = 1 ± 0.1 min;
•ultrasonic treatment (UST) of the composition for τ = 1.5 ± 0.1 min;
•cooling the composition to room temperature over a period of τ = 60 ± 5 min;
•introduction of PEPA and mixing the composition for τ = 5 ± 0.1 min.
•at a later stage NCM was hardened in the experimentally established mode:
•creation of the samples and holding them for τ = 12.0 ± 0.1 h at T = 293 ± 2 K;
•heating at a rate of υ = 3 K/min up to T = 393 ± 2 K;
•holding NCM for τ = 2.0 ± 0.05 h;
•slow cooling to T = 293 ± 2 K.
In order to stabilise the structural processes in NCM the samples were held for τ = 24 h in the air at T = 293 ± 2 K.
In the project the following thermal characteristics of NCM are studied: thermal resistance (Martens), the thermal linear expansion factor and shrinkage.
The Martens heat resistance for NCM was determined according to GOST 21341-75 by heating the sample at a rate of υ = 3 K/min subject to the constant bending load F = 5 ± 0.5 MPa until it is deformed by a predetermined amount (h = 6 mm).
The thermal linear expansion factor of materials was calculated by the curve of the relative deformation and temperature by approximating the dependence of the exponential function. The relative deformation was determined by the change in the sample length with increasing the temperature under steady state conditions (GOST 15173-70). Samples for the research with the size of 65 × 7 × 7 mm had non-parallelism of the polished ends of not more than 0.02 mm. The length of the sample was measured with an accuracy of
± 0.01 mm. The heating rate was υ = 2 K/min.
The deviation of values in the studies of the thermal performance properties of NCM (Martens heat resistance, thermal linear expansion factor) is 4–6% of the nominal value.
Results and discussion
To determine the behaviour of composites under the influence of the thermal field, their thermal linear expansion factor was studied. Taking into account the operating conditions of epoxy composites, the temperature range of
ΔТ = 303–473 К was chosen.
The results of calculation of the thermal linear expansion factor of nanocomposites at various temperature ranges are shown in Table 1. It was found that with increasing temperature, the thermal linear expansion factor increases, which indicates an increase in the internal energy of the NCM under research due to the mobility of macrochains and epoxy binder segments.
During the experimental research it was found that in the ranges Т = 303–323 К, ∆Т = 303–373 К and ∆Т = 303–423 К the lowest thermal linear expansion factor is characterised by NCM with the content of the fullerene of С60 q = 0.010 wt.%, and in the area of ΔТ = 303–473 К a composite with a maximum content of С60 (q = 0,100 wt.%) is characterised by the lowest value of the thermal linear expansion factor. The obtained results indicate a decrease in the fluctuation of relations and mobility of macrochains and segments of the epoxy binder as a result of the increased density of the spatial grid. It is worth noting that the value of the thermal linear expansion factor in the range of higher temperatures does not differ much for composites with different filler content (Δα = ± 0.09 ∙ 10–4 К–1) indicating that the uniform flow of physical and chemical processes in cross-linking the NCM and the practicability of using a smaller filler content.
Especially important is the analysis of the relaxation transitions in epoxycomposites, particularly the glass transition temperature (Tg) which characterises the mobility of independent kinetic elements (segments and macromolecules) of the epoxy binder. It was found that the glass transition temperature of epoxycomposites with fullerene particles С60 (q = 0.010–0.100 wt.%) is in the range T = 330–337 K. A slight difference in ∆Тс = 7 К values indicates a uniform flow of relaxation processes in all the studied materials.
The results of the NCM heat resistance (Martens) study are presented in Table 2. Preliminary tests have shown [6–9] that the heat resistance of the epoxy matrix modified by UST is T = 339 K. The introduction of С60 (q = 0.010–0.100 wt.%) enhances heat resistance of NCM to T = 341–342 K.
No less important is to analyse the shrinkage values of nanocomposites. Shrinkage of less than 1% (Table 2) is indicative of isotropic materials and durability when applied to the surface of a complex profile.
Consistency of the dynamics of the thermal linear expansion factor and heat resistance (Martens) of NCM depending on the content of the filler C60 indicates the reliability of the experimental data. An analysis of the study results allows to ascertain the usefulness of the developed nanocomposites and coatings based on them for the protection of equipment operated at temperatures T = 303–342 K. At the same time, it is needed to do more research into the possibility of using NCM developed at temperatures above T = 342 K.
Conclusion
Based on the studies it can be stated that for the creation of nanocomposite materials or protective coating with higher thermal-physical properties it is expedient to add to the epoxy binder the nanosized filler С60 in the amount of q = 0.01–0.05 wt.% per 100 wt.% of oligomer ED-20 and 10 wt.% of the hardener PEPA. Heat resistance (Martens) of the nanocomposite material is T = 342 K.
The research into the behaviour of the developed nanocomposites under the influence of the thermal field has shown that in the temperature range ∆Т = 303–473 К it is advisable to use NCM with the content of particles С60 q = 0.01–0.05 wt.% since the difference in thermal linear expansion factor values for the entire spectrum of the studied composites is in the range of experimental error and is Δα = (1.10–1.14) ∙ 10–4 К-1.
An analysis of studies [1, 2, 4–9] suggests that one of the effective ways to improve the thermal properties of epoxy composites is to introduce fillers of various degree of dispersion as well as physical and chemical nature. The combination of these materials allows to abandon the use of traditional painting materials and use CM as protective coating. To assess the operating temperature range of the new CM, a study of their properties during heating is conducted [9–11].
The purpose of this project is to investigate the effect of the amount of С60 on the heat resistance and coefficient of thermal linear expansion of nanocomposites.
Materials and methods
of research
As the main component of the binder in the creation of nano-composite materials (NCM) selected was a diane epoxy oligomer of the ED-20 brand (GOST 10587-84). The use of the low-molecular polyethylenepolyamine hardener (PEPA) [-CH2-CH2-NH-]n (TU 6-05-241-202-78) made it possible to crosslink the epoxy composition at room temperature. The hardener was administered at a stoichiometric ratio of the components (wt.%) 10 (ED-20) to 1 (PEPA).
The particles of the fullerene С60 with the dispersion of 5 nm are used as filler. Epoxy NCM filled with С60 were created with the use of ultrasonic dispersion compositions in the following modes:
•pre-dosing of ED-20, resin heating to T = 353 ± 2 K and its holding at this temperature for the time τ = 20 ± 0.1 min;
•dosing of nanofiller and the further introduction of it into the epoxy oligomer;
•hydrodynamic alignment of the oligomer ED-20 and nanofiller for τ = 1 ± 0.1 min;
•ultrasonic treatment (UST) of the composition for τ = 1.5 ± 0.1 min;
•cooling the composition to room temperature over a period of τ = 60 ± 5 min;
•introduction of PEPA and mixing the composition for τ = 5 ± 0.1 min.
•at a later stage NCM was hardened in the experimentally established mode:
•creation of the samples and holding them for τ = 12.0 ± 0.1 h at T = 293 ± 2 K;
•heating at a rate of υ = 3 K/min up to T = 393 ± 2 K;
•holding NCM for τ = 2.0 ± 0.05 h;
•slow cooling to T = 293 ± 2 K.
In order to stabilise the structural processes in NCM the samples were held for τ = 24 h in the air at T = 293 ± 2 K.
In the project the following thermal characteristics of NCM are studied: thermal resistance (Martens), the thermal linear expansion factor and shrinkage.
The Martens heat resistance for NCM was determined according to GOST 21341-75 by heating the sample at a rate of υ = 3 K/min subject to the constant bending load F = 5 ± 0.5 MPa until it is deformed by a predetermined amount (h = 6 mm).
The thermal linear expansion factor of materials was calculated by the curve of the relative deformation and temperature by approximating the dependence of the exponential function. The relative deformation was determined by the change in the sample length with increasing the temperature under steady state conditions (GOST 15173-70). Samples for the research with the size of 65 × 7 × 7 mm had non-parallelism of the polished ends of not more than 0.02 mm. The length of the sample was measured with an accuracy of
± 0.01 mm. The heating rate was υ = 2 K/min.
The deviation of values in the studies of the thermal performance properties of NCM (Martens heat resistance, thermal linear expansion factor) is 4–6% of the nominal value.
Results and discussion
To determine the behaviour of composites under the influence of the thermal field, their thermal linear expansion factor was studied. Taking into account the operating conditions of epoxy composites, the temperature range of
ΔТ = 303–473 К was chosen.
The results of calculation of the thermal linear expansion factor of nanocomposites at various temperature ranges are shown in Table 1. It was found that with increasing temperature, the thermal linear expansion factor increases, which indicates an increase in the internal energy of the NCM under research due to the mobility of macrochains and epoxy binder segments.
During the experimental research it was found that in the ranges Т = 303–323 К, ∆Т = 303–373 К and ∆Т = 303–423 К the lowest thermal linear expansion factor is characterised by NCM with the content of the fullerene of С60 q = 0.010 wt.%, and in the area of ΔТ = 303–473 К a composite with a maximum content of С60 (q = 0,100 wt.%) is characterised by the lowest value of the thermal linear expansion factor. The obtained results indicate a decrease in the fluctuation of relations and mobility of macrochains and segments of the epoxy binder as a result of the increased density of the spatial grid. It is worth noting that the value of the thermal linear expansion factor in the range of higher temperatures does not differ much for composites with different filler content (Δα = ± 0.09 ∙ 10–4 К–1) indicating that the uniform flow of physical and chemical processes in cross-linking the NCM and the practicability of using a smaller filler content.
Especially important is the analysis of the relaxation transitions in epoxycomposites, particularly the glass transition temperature (Tg) which characterises the mobility of independent kinetic elements (segments and macromolecules) of the epoxy binder. It was found that the glass transition temperature of epoxycomposites with fullerene particles С60 (q = 0.010–0.100 wt.%) is in the range T = 330–337 K. A slight difference in ∆Тс = 7 К values indicates a uniform flow of relaxation processes in all the studied materials.
The results of the NCM heat resistance (Martens) study are presented in Table 2. Preliminary tests have shown [6–9] that the heat resistance of the epoxy matrix modified by UST is T = 339 K. The introduction of С60 (q = 0.010–0.100 wt.%) enhances heat resistance of NCM to T = 341–342 K.
No less important is to analyse the shrinkage values of nanocomposites. Shrinkage of less than 1% (Table 2) is indicative of isotropic materials and durability when applied to the surface of a complex profile.
Consistency of the dynamics of the thermal linear expansion factor and heat resistance (Martens) of NCM depending on the content of the filler C60 indicates the reliability of the experimental data. An analysis of the study results allows to ascertain the usefulness of the developed nanocomposites and coatings based on them for the protection of equipment operated at temperatures T = 303–342 K. At the same time, it is needed to do more research into the possibility of using NCM developed at temperatures above T = 342 K.
Conclusion
Based on the studies it can be stated that for the creation of nanocomposite materials or protective coating with higher thermal-physical properties it is expedient to add to the epoxy binder the nanosized filler С60 in the amount of q = 0.01–0.05 wt.% per 100 wt.% of oligomer ED-20 and 10 wt.% of the hardener PEPA. Heat resistance (Martens) of the nanocomposite material is T = 342 K.
The research into the behaviour of the developed nanocomposites under the influence of the thermal field has shown that in the temperature range ∆Т = 303–473 К it is advisable to use NCM with the content of particles С60 q = 0.01–0.05 wt.% since the difference in thermal linear expansion factor values for the entire spectrum of the studied composites is in the range of experimental error and is Δα = (1.10–1.14) ∙ 10–4 К-1.
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