The formulation of epoxy-polyester matrix with improved physical and mechanical properties for restoration of means of sea and river transport

Abstract

The physical and mechanical properties of matrix based on epoxy resin ED-20 were investigated by adding unsaturated polyester resin ENYDYNE H 68372 TAE at different concentrations. The contents of polyester resin in the epoxy one by changing its concentration in the range of q = 10–120 mas.fr. were analysed. It was experimentally determined that matrix is characterised with its maximal rates of physical and mechanical properties with q = 10 mas.fr. of polyester resin ENYDYNE H 68372 TAE per 100 mas.fr. of ED-20. The received material is marked by the following indexes of physical and mechanical properties: fracture stresses during the flexion – σ_fl = 56.2 MPa, the modulus of elasticity during the flexion –E = 4.2 GPa, impact toughness –W = 12.8 kJ/m². It was proved that indexes of modulus of elasticity and impact toughness during the flexion of developed composite are higher in 1.5–2 times, in comparison with the indexes of matrix which is based on the epoxy resin ED-20. The images of fracture of the composite materials were analysed through optical microscopy. It was determined that the results of investigation correlate with the obtained indexes of physical and mechanical properties, confirming their authenticity. The clearly defined heterogeneity of given polymers was not observed on the photos of fracture, which indirectly points on the compatibility of the ingredients of the binder, chosen for the study.

KEYWORDS:

• Polymer
• composite material
• epoxy resin
• unsaturated polyester resin
• physical and mechanical properties

1. Introduction

The polymeric composite materials (CM) (Brailo et al. 2018) have gained a wider use today in different industries, in particular sea and river transport. Polymers may differ significantly from the metal-containing materials and constructions by improved operational characteristics, simplicity of formation and economic efficiency (Lubin 1988; Kerber et al. 2008). Materials based on epoxy oligomers are one of these composites. The thermoset polymer is characterised by high indexes of adhesion strength, heat resistance, the impact toughness, insignificant shrinkage during curing, strength and high hardness (Rabek 1983; Lubin 1988; Kerber et al. 2008; Buketov, Sapronov, Brailo, Aleksenko 2014). These characteristics provide extensive use of materials with epoxy matrix as anti-corrosion coatings, sealing pastes, repair materials, including the operation and repair of ships (Buketov, Sapronov, Brailo, Aleksenko 2014; Brailo et al. 2018). At the same time, the continuous development of modern technology puts high demands on the properties of polymeric materials. This is why the receipt of polymers with improved indexes of performance has important scientific, technical and practical meaning (Krishan 1998).

It is known (Shimbo et al. 1989) that the indexes of properties of epoxides are influenced by different factors, including changing the molecular structure due to the impact of energy fields and use of chemical modifiers or plasticizers, properties of fillers and their content, use of multicomponent compositions. It should be noted that epoxy diane resin ED-20 (GOST (state standart) 10587-84) (Shimbo et al. 1989; Stukhlyak 1994; Krishan 1998; Chung Do et al. 2010; Yatsishin et al. 2012; Buketov, Sapronov, Brailo, Aleksensko 2014) is widely used nowadays as bonding agent for epoxy materials. At the same time, by analysing the market of polymer materials, the widespread use and distribution of raw material CM based on unsaturated polyesters should be noted (Baljinder et al. 2015; Frank 2017). Polyester materials differ from epoxy resins, in terms of gel time, molecular weight, operational characteristics and the ability to quick solidification at room temperature. Therefore, the creation of a polymeric material with a combination of two components of different nature is more important. The use of polyester resin (PR) in epoxy binder in the formation of CM will not allow us to create a two-component matrix with predictable indexes of performance characteristics.

2. Experimental

In the formation of polymeric matrix of the CM, the next ingredients were used.

1. The main component of matrix is a low molecular epoxy diane oligomer ED-20. It should be noted that the molecules of epoxy oligomers contain glycidyl and epoxy groups, which are capable of interacting with the hardener, to form a cross-linked structure in the materials in the form of a grid (Kerber et al. 2008; Fabulyak et al. 2010).

2. Ortho-phthalic dicyclopentadiene (DCPD) unsaturated pre-accelerated polyester resin ENYDYNE H 68372 TAE, which has an inhibitor to prevent instant polymerisation (gel time τ = 20–24 min) (Technical data sheet 2015). It should be noted that during the copolymerisation reaction of composition of unsaturated polyesters with non-limiting monomer compounds in the presence of initiators, a significant amount of heat is released; therefore, the reaction is exothermic (Lubin 1988; Kerber et al. 2008).

3. The cold curing hardener polyethylene polyamine (PEPA) (TU 6-05-241-202-78) – for cross-linking of epoxy compositions.

4. The initiator for polyester resins – Butanox-M50 (Fabulyak et al. 2010), which is a peroxide of methyl ethyl ketone (MEKP) and contains a low amount of water with a minimum number of polar compounds in comparison to ethylene glycol (Stukhlyak 1994).

The physical and mechanical properties of the matrix were investigated in order to identify the optimal concentration of unsaturated polyester of brand ENYDYNE H 68372 TAE in combination with low molecular weight epoxy diane oligomer of brand ED-20. The content of unsaturated polyester was changing within the range q = 10–120 mas.fr. per 100 mas.fr. of epoxy oligomer of brand ED-20. The next physical and mechanical properties were investigated in this work: the modulus of elasticity, fracture stresses during the flexion and the impact toughness.

The fracture stresses and the modulus of elasticity were determined according to GOST 4648-71 and GOST 9550-81, respectively. The specimen parameters were: length – l = 120 ± 2 mm, width – b = 15 ± 0.5 mm, height – h = 10 ± 0.5 mm.

The impact toughness was measured by pendulum copra according to Charpy method (GOST 4647-80). We determined the working angle of deviation of the pendulum after the destruction of the specimen at a predefined initial angle of lifting of working body of testing machine. The investigation was conducted under temperature T = 298 ± 2 K and relative humidity of d = 50 ± 5%. We used specimens with the following size: l × b × h = (65 × 12 × 12) ± 0.5 mm.

The structure of fracture of the CM was investigated additionally on a XJL-17AT metallographic microscope, which was equipped with a Levenhuk C310 NG (3.2 MegaPixels) camera. The image enlargement range varied from ×100 to ×1600 times. Directly in the work, the specimens were examined with an increase of ×400 times. For the processing of digital images, Levenhuk ToupView software was used.

The materials were solidified according to the following regimen: forming of the specimens and their holding over time t = 12.0 ± 0.1 h at temperature T = 293 ± 2 K, heating at a speed of υ = 3 K/min to temperature T = 393 ± 2 K, keeping the specimens at a given temperature during the time t = 2.0 ± 0.05 h, and slowly cooling to temperature of t = 293 ± 2 K. In order to stabilise the structural processes in matrix, the specimens were kept in air during time t = 24 h at temperature T = 293 ± 2 K, followed by the conduction of the experimental tests.

All materials were kept in air temperature T = 298 ± 2 K and relative humidity of d = 50 ± 5%.

3. Experimental results

At the initial stage, to create a two-component polymer matrix with improved physical and mechanical properties, the impact of content of PS of ENYDYNE H 68372 TAE brand in epoxy diane oligomer ED-20 was investigated. It is known (Technical data sheet 2015) that the rate of hardening of polyester resin of ENYDYNE H 68372 TAE brand is higher than epoxy binder ED-20, for this reason, Butanox-M50, which initiates a chain reaction of polymerisation at the hardening of unsaturated polyester, was infused after adding all the components. The concentrations of hardeners for epoxy and polyester resins were accepted according to the recommendations of manufacturers and based on the results of previous investigations (Technical data sheet 2015; Buketov, Maruschak, et al. 2016, Buketov, Brailo, et al. 2016). Accordingly, the following sequence of hydrodynamic combination was set:

1. Epoxy diane oligomer of brand ED-20 – q = 100 mas.fr.

2. Polyester resin ENYDYNE H 68372 TAE – q = 10–120 mas.fr. (the content is indicated on 100 mas.fr. of epoxy resin).

3. The hardener of cold hardening polyethylene polya-mine (PEPA) – q = 10 mas.fr. (the content is indicated on 100 mas.fr. of epoxy resin).

4. The initiator for polyester resins Butanox-M50 – q = 1.5 mas.fr. (content is indicated on 100 mas.fr. of polyester resin).

It should be mentioned that the optimal temperature of PCM cross-linking T = 393 K (Technical data sheet 2015) was recommended by the manufacturer of polyester resin ENYDYNE H 68372 TAE, and it coincides with the temperature of cross-linking of CM, based on epoxy oligomer ED-20.

It is known from thermodynamic, kinetic and mechanical theories that it is almost impossible to create homogeneous single-phase polymer blends with different components by nature. Therefore, before the experiment, to determine the degree of compatibility of polymers the following properties of resins and composites on their base were analysed: dynamical viscosity, volume shrinkage, modulus of elasticity and fracture stresses during the flexion, the impact toughness (Table 1). The data in the table are given according to the results of previous investigations (Buketov, Sapronov, Brailo 2014; Buketov, Maruschak, et al. 2016, Buketov, Brailo, et al. 2016) and GOST (state standard) of epoxy diane resin, and also according to the data of the manufacturer (Technical data sheet 2015). Comparing the indexes of the dynamical viscosity of the epoxy resin ED-20 (η = 11–20 Pa s) and the unsaturated polyester ENYDYNE H 68372 TAE (η = 0.4–0.5 Pa s), in accordance with the theory of the degree of heterogeneity of the polymer mixture of two polymers as a relative degree of their interoperability (Rabek 1983; Krishan 1998), an assumption was made that these polymers are compatible. Analysing the indexes of shrinkage of each binder it was determined that its values for CM, based on epoxy resin ED-20, are ΔV = 0.5–2.3%, but for material, based on polyester resin ENYDYNE H 68372 TAE, are ΔV = 6–9%. The analysed data are essential in the formation of multicomponent composite because the significant difference of indexes may cause the additional tension in materials.

Table 1. The characteristics of resins and composites on their base.

Characteristics	ED–20	ENYDYNE H 68372 TAE
Dynamical viscosity of resins, η (Pa s)	11–20	0.4–0.5
Volume shrinkage, $\mathrm{\Delta }V$ (%)	0.5–2.3	6.0–9.0
Modulus of elasticity, E (GPa)	2.8	3.8
Fracture stresses, σfl (MPa)	47.6	29.4
Impact toughness, W (kJ/m^2)	6.6	4.4

At the next stage, the modulus of elasticity and fracture stresses during the flexion, and the impact toughness of multicomponent material, based on epoxy oligomer ED-20, with the addition of polyester ENYDYNE H 68372 TAE were investigated. Analysing the results obtained in the study, shown in Figure 1, it can be stated that matrix is characterized with its maximal indexes of properties with the introduction of the resin ENYDYNE H 68372 TAE in the amount of q = 10 mas.fr. into ED-20. At the same time, the fracture stresses during the flexion are σ_fl = 56.2 MPa, the modulus of elasticity during the flexion is E = 4.2 GPa, the impact toughness is W = 12.8 kJ/m². Comparing the results of the study (Table 1) with the indexes of properties of the matrix based on the epoxy resin ED-20 (σ_fl = 47.6 MPa, E = 2.8 GPa, W = 6.6 kJ/m²), it was determined that indexes of modulus of elasticity and impact toughness during the flexion grew up in 1.5 and 2.0 times, respectively The dynamics of the decrease of physical and mechanical properties of the CM was observed with further increase of content of polyester resin in the range of q = 20–120 mas.fr. In this case, the indexes of fracture stresses during the flexion decrease from σ_fl = 54.4 MPa (with q = 20 mas.fr.) to σ_fl = 13.8 MPa (with q = 120 mas.fr.) (Figure 1, curve 1), the modulus of elasticity decreases from E = 3.9 GPa to E = 2.1 GPa (Figure 1, curve 2), the impact toughness decreases from W = 7.9 kJ/m² to W = 3.9 kJ/m² (Figure 1, curve 3). The results of the study confirm the previously made assumptions about the decrease in indexes of physical and mechanical properties with the increase of content of polyester resin. The increase in the concentration of polyester within the range of q = 20–120 mas.fr. leads to the increase in shrinkage of each component of the composite during the polymerisation. Obviously, the significant difference in the indexes of shrinkage leads to the formation of microcracks and residual tensions in composites, which have a negative influence on their physical and mechanical properties. At the same time, it should be mentioned that the indexes of fracture stresses during the flexion and of impact toughness of the matrix, based on the polyester resin, are much lower than in the composite, based on the epoxy oligomer. Accordingly, it can be stated that the increase in the concentration of polyester in the epoxy matrix within the rate of q = 20–120 mas.fr. leads to the deterioration of indexes of physical and mechanical properties.

Figure 1. The dependence of the physical and mechanical properties of the matrix at the content of polyester resin ENYDYNE H 68372 TAE in epoxy binder ED-20: 1 – fracture stresses during the flexion (σ_fr); 2 – modulus of elasticity during the flexion (E); 3 – impact toughness (W).

The dynamics of these results can be explained by the fact that, as mentioned earlier, the formation of a homogeneous single-phase polymer blend is almost impossible. Therefore, the blend is two-phase. Obviously, the polyester ENYDYNE H 68372 TAE of insignificant concentration disperses in epoxy oligomer ED-20 as an independent phase, and epoxy resin ED-20, as a dispersion medium. The increase of polyester content to q = 30–70 mas.fr. on 100 mas.fr. of ED-20, as a rule tends to invert the phase. Both polymers can exist as two continuous phases within the range of this concentration interval. The interval, where phase inversion of polymers is taking place, depends on the viscosity of the components, which form the blend. As the polyester of this brand is a thixotropic resin with low viscosity and with a gel time of t = 20–24 min, it reaches the peak time at t = 40–60 min and forms a continuous phase, which leads to deterioration of physical and mechanical properties. According to this, the determination of compatibility of components of PCM, which characterise the ability of polymers to form the single-phase blend, is a relative measure of the degree of heterogeneity of composite. It is an important stage at the creation of the CM with the content of components with different properties, as homogeneity and single-phase of a blend allows us to get the optimal characteristics of the final product of polymerisation.

Method of optical microscopy is one of the ways to analyse the compatibility of components of polymer after its formation. This method is a visual investigation of the surface of the CM at the site of destruction after its dynamical test. This makes it possible to study the heterogeneity of phases of polymers. The heterogeneity, which is defined through the methods of optical microscopy, is rather relative but characterises the degree of cross-linking of CM. It may indirectly confirm the increase in indexes of physical and mechanical properties of the CM.

The results of the investigation (through the method of optical microscopy) of the surface of the fracture of PCM at different content (q = 10–120 mas.fr.) of polyester resin ENYDYNE H 68372 TAE in the matrix are shown in Figure 2. The fractogram of fracture microrelief of the composite, shown in Figure 2(a), is without the addition of polyester resin, which is characterised by the significant tension state of the material. The structure of the surface has fibrous needle-shaped formations, directed along the lines, which indicates a relatively high viscosity of material (Table 1). The fracture structure of the composite, with the addition of polyester at the concentration of q = 10 mas.fr., is shown on the fractogram (Figure 2(b)). The arrows show visible insignificant inclusions of polyester resin, gel time of which is much smaller. It is leading to the polymerisation of the composite components with different speed and creation of physical and chemical bonds between them. The lines of tension of investigated material resemble meshed branches. There are no explicit directed lines of cleavage, which could indicate significant residual tensions in the composite. The results of the study are confirmed by high indexes of physical and mechanical properties of the CM. It is notable on fractograms (Figure 2(c,d)) that the inclusions from polymerised polyester resin have sizes ≈15–20 µm. There is chaotic and extensive distribution of lines, which indicate insignificant tense state of the material. Apparently, this is due to incomplete cross-linking, which leads to the destruction of the composite without applying to significant pressures in relation to other CM. A sharp decrease in indexes of physical and mechanical properties is noted Figure 1. The reason for this is the increase of the mass fraction of polyester resin, which leads to a decrease in the gel time of CM. By this, the uncontrolled self-heating of the composition, the formation of temperature tensions, and as a result premature destruction of connections in CM were observed.

Figure 2. The fractogram of fracture of CM with different content of polyester resin ENYDYNE H 68372 TAE on q = 100 mas.fr. of epoxy oligomer ED-20 (with an increase of ×400): (a) epoxy matrix; (b) CM with content of q = 10 mas.fr. of polyester resin; (c) CM with content of q = 10 mas.fr. of polyester resin; (d) CM with content of q = 40 mas.fr. of polyester resin; (e) CM with content of q = 60 mas.fr. of polyester resin; (f) CM with content of q = 80 mas.fr. of polyester resin; (g) CM with content of q = 100 mas.fr. of polyester resin; (h) CM with content of q = 120 mas.fr. of polyester resin.

The analysis of the structure of fracture on the fractograms of matrices at the content of polyester q = 40–120 mas.fr. (Figure 2(e–h)) confirms the dynamics of indexes of physical and mechanical properties. There is no order and branches of cleavage lines on the images. The propagation of lines of cleaving by a gradual increase in the concentration of polyester from q = 60 mas.fr. (Figure 2(e)) to q = 120 mas.fr. Figure 2(h) shows that residual tensions in the material are insignificant. As a result, the indexes of properties of composites significantly deteriorate.

Thus, the pronounced heterogeneity of phases of the polymers is not seen on the received images of a fracture of matrices, which indirectly indicates the compatibility of the selected for the research epoxy and polyester resins. However, taking into account the indexes of volume shrinkage, different gel time of epoxy and polyester components, it was determined that it is impractical to increase the concentration of ENYDYNE H 68372 TAE above q = 10 mas.fr. on 100 mas.fr of epoxy resin ED-20.

4 Conclusion

According to the results of experimental investigations, it was found that the maximal indexes of physical and mechanical properties of the investigated matrices were obtained by the addition of polyester binder ENYDYNE H 68372 TAE at the content of q = 10 mas.fr. into epoxy oligomer ED-20 (100 mas.fr.). The developed matrix was characterised by the following indexes of the physical and mechanical properties: fracture stresses during the flexion – σ_fl = 56.2 MPa, modulus of elasticity during the flexion – E = 4.2 GPa, impact toughness – W = 12.8 kJ/m². The obtained values are higher than the properties of epoxy matrix 1.5–2.0 times.

The fractograms of the fracture of the composite materials at the different content of the polyester resin in epoxy oligomer were analysed. It was determined that the lines of cleavage of investigated materials have branched reticulated nature, which indicated insignificant tensions in composites. It is proved that tense state of materials and character of lines of cleavage correlates with indexes of physical and mechanical properties. However, the pronounced heterogeneity of compound was not noted during the analysis of images of fracture of composites. It indirectly indicates the feasibility of combining the epoxy and polyester components for the formation of the polymer matrix, which can be used for restoration of means of sea and river transport.

Disclosure statement

No potential conflict of interest was reported by the authors.