Synthesis and thermal characterization of composites based on Epidian 601 with flame retardants compounds

Chemicals

The epoxy resin Epidian^® 601 (EP601) as well as, curing agent PAC and IDA were purchased from CIECH Resin Sp.z.o.o. (Nowa Sarzyna, Poland). Epidian^® 601 is a clear, viscous liquid of light colour, epoxy number: 0.500–0.550 mol/100 g, viscosity at 25 °C: 700–1100 mPas, density at 20 °C: 1.140–1.170 g/cm³. Triphenyl phosphate (TP) was purchased from Merck (Darmstadt, Germany). The triethylenetetramine (TETA) (99 %) was obtained from Avantor Performance Materials Poland S.A. (Gliwice, Poland). Fire Retardant 421 (FR) was from West System (Romsey Hampshire, United Kingdom). The solvents: acetone (99.5 %), methanol (99.8 %), hydrochloric acid (35 %), potassium hydroxide (40 %) came from Merck (Darmstadt, Germany). The distilled water was obtained from Millipore (UMCS Lublin, Poland). The curing agent PAC contains: fatty acids, C18-unsaturated compounds, dimers, polymers, reaction products with triethylenetetramine. The curing agent IDA contains: 4,4′-isopropylidenedophenol, oligomeric reaction product with 1-chloro-2,3-epoxypropane, reaction product with 3-aminomethyl-3,5,5-trimethylcyclohexylamine, isophorone diamine.

Methods/analyses

The attenuated total reflection (ATR) was recorded based on the Fourier transform infrared (ATR/FT-IR) spectroscopy using a TENSOR 27 Bruker (Bruker GmbH, Mannheim, Germany) spectrometer equipped with a diamond crystal (Bruker Optics, Ettlingen, Germany). The spectra were recorded in the range of 4000–600 cm⁻¹ with 64 scans per spectrum at a resolution of 4 cm⁻¹. The analysis was preceded by the background spectrum measurements.

Differential scanning calorimetry (DSC) curves were obtained with the use of a DSC Netzsch 204 calorimeter (Netzsch, Günzbung, Germany). The measurements were made in the aluminum pans with a pierced lid with the sample mass approximately 10 mg in the nitrogen atmosphere (30 mL/min). Dynamic scans were performed at a heating rate of 10 °C/min in the temperature range −20–500 °C. An empty aluminum crucible was used as the reference.

The flammability tests were carried out in the laboratory of the Department of Technology and Polymer Processing, Lublin University of Technology. The flammability testing device was equipped with a combustion chamber, a ventilation system, and a thermal imaging camera. During the test the samples were fixed to a tripod, then the burner was brought closer to the sample for 20 s (and 30 s) at an angle of 45°. After that time, the samples burnt freely for another 30 s. The flammability tests were made in a horizontal burning test according to the PN-EN 60695-11-10 – method A. During the burning process, observations were made using a V-20 thermovision camera model ER005-25 (Vigo System, Ożarów Mazowiecki, Poland) in the temperature measurement range from 0 to 250 °C. During the tests, photos were taken.

The hardness of the materials was measured based on the Shore D method using a 7206/H04 analog hardness testing apparatus (Zwick, Ulm, Germany) at 20 °C. Readings were taken after 15 s.

The swelling coefficients (B) were determined using equilibrium swelling in acetone, methanol, hydrochloric acid, potassium hydroxide and distilled water. B is expressed:

where Vs is the volume of a sample after swelling and Vd is the volume of a dry sample.

The ageing tests were carried out in traditional vials. The cut 10 mm × 10 mm composite pieces were poured over with the solvent (the same as used in swelling tests). Weight changes were recorded using the scales.

Synthesis of composites

All composites were prepared using the cross-linking technique. An appropriate amount of the resin EP601 was added to the curing agent (TETA or PAC or IDA). The chemicals were stirred at room temperature until a homogenous solution was obtained. Then the flame retardant (FR or TP) was added. The whole content was stirred to obtain a homogeneous mixture and next put into the oven (for 10 min at 50 °C). The beaker contents were poured into the glass molds (10 mm × 8 mm × 2 mm) and polymerized for 24 h at room temperature. The samples were heated at 80 °C for 60 min after being tested. The detailed information regarding the reagents and their amounts is presented in Table 1. Figure 1 shows the structure of the monomers and the proposed scheme of the composite structure.

Fig. 1:

Fragment of composite structure.

ATR-FTIR analysis

Characterization of chemical structure by the spectroscopic analysis ATR/FT-IR was made for the all obtained materials. Figure S1(a–c) shows the spectra of the samples (Supplementary material) and Table 2 presents the wavelength values with the attributed intramolecular vibrations. Generally, the spectra of all obtained systems have a similar course. The epoxy resin is the fundamental component of the analyzed samples. Thus, characteristic bands of these reactants are pointed out on the spectra. In the 3332–3293 cm⁻¹ range wide bands derived from the stretching vibrations of –OH groups are found. Moreover, this signal can be more intensive due to the presence of amine (TETA) and strong tendency towards water absorption of these molecules. In the modified composites by IDA and PAC these signals become weaker. Characteristic bands of methyl and methylene groups are visible as two signals, from one 2959 to 2923 cm⁻¹ and the other in the range of 1459–1450 cm⁻¹. These peaks correspond to the symmetrical and asymmetric stretching vibrations of both types of the groups. This effect is visible for each composite. There is a doublet of bands at 1509–1508 cm⁻¹ corresponding to the symmetrical and asymmetric stretching vibrations of aromatic rings. Multiple bands ranging from 1509 cm⁻¹ to 1508 cm⁻¹ can be associated with the vibrations of C–H and C=C bonds related to the benzene rings and aromatic skeletons ones. They come from both the aromatic Epidian 601 part. Additionally, in the range 828–827 cm⁻¹, the signal of deformation vibrations of Ar and Ar-H is notable. In the range from 1295 to 1293 cm⁻¹, a band derived from the stretching vibrations of oxygen atoms associated with aromatic carbon atoms can be seen in the spectra. The bands at 1036–1034 cm⁻¹ are associated with the stretching vibrations of hydroxyl groups. Deformation vibrations of the mentioned groups –CH₃, –CH₂– are visible in the range from 1459 to 1458 cm⁻¹. The presence of the TETA-derived amine groups is confirmed by the valence band of stretching vibrations in the N–H bonds. This signal is observed from 638 to 617 cm⁻¹. In summary, the addition of a flame retardant and triphenyl phosphate does not drastically change the course of the ATR-FTIR curves of the copolymers, however, it does affect the intensity of the signals.

DSC analysis

DSC measurements were carried out in the course of the heating cycle at temperature ranging from 0 to 500 °C. The numerical data of the DSC analysis is presented in Table 3.

The thermal behaviour of the obtained composites was studied by means of the DSC analysis. The DSC curves of these materials are presented in Fig. 3(a–c). The composite consists of an epoxy resin, a crosslinker, and a flame retardant additive. All these reagents influence the thermodynamic phenomena occurring in the sample during the DSC analysis. On the DSC curves, there can be seen several endo and exo effects, their presence indicates the composites heterogeneity. The EP601 + PAC system can be seen in Fig. 1(a). With the addition of flame retardants, the glass transition temperature changed from 57 °C to 35 °C. On the curve of the parent EP601 + PAC system, the exothermic effects (250 °C) associated with the additional crosslinking are visible. For the other samples exothermic effects probably related to the opening of the remaining epoxy rings in the range 327–361 °C are observed. For the systems with the flame retardant additives, two endothermic effects related to the degradation of the sample are observed. For the initial sample, this decomposition is three-stage (278, 358, 445 °C).

Fig. 3:

DSC curves. (a) EP601 + PAC. (b) EP601 + IDA. (c) EP601 + TETA.

In the case of the EP601 + IDA systems, the studied DSC curves have different courses. The glass transition temperature changed from 110 °C to 65 °C with the addition of flame retardants. An exoenergetic effect characteristic of epoxy resins around 321–364 °C is also observed. The effects connected with the degradation of samples occur at about 260–320 and 353–395 °C. The last analyzed system contains TETA as a resin curing agent. This amine is used in the smallest (10 wt%) amount compared to the other commercial hardeners (IDA, PAC). The curve courses are similar to those for the samples with IDA. In general, it can be concluded that the influence of the additive on the temperature effects in the analyzed samples.

As can be seen in each graph, the DSC curves from the heating cycle reveal both endo- and exothermic changes. Endothermic peaks for the EP601 + PAC system (in the range of 35–78 °C) are responsible for the glass transition of the crystalline form. For the second system based on EP601 + IDA, these effects occur in the range of 65–110 °C. For the system, EP601 + TETA glass transition effect is visible only for modifications (50–89 °C). Another endothermic effect occurs in temperature higher than 250 °C. Values are higher for EP601 + PAC + 10 %TP (379 °C), and lower for EP601 + TETA + 20 %TP (250 °C). Next, the endothermic effect above 400 °C is visible for the EP601 + PAC system only. The exothermic peaks for the materials (pure and 10 %/20 %FR) are responsible for the cross-linking of materials.

In Supplementary materials one can see Figure S2 with the DSC curve of pure Fire Retardant.

Flammability tests

The parent samples as well as those with the largest content of FR and TP were selected for the flammability tests. Figure S3 (Supplementary materials) shows the burning compositions. The photos were taken 20 s after the burner had been put off. As follows from the photos the composites without any additives burnt the most intensively. This became apparent in the form of the brightest and greatest flame. The addition of Fire Retardant and Triphenyl Phosphate reduces the flame height and intensity. For the systems with TETA and PAC as curing agents, the greatest reduction in the flame height and intensity was observed for commercial FR. In the case of the IDA curing agent, a better effect was found for triphenyl phosphate. The flame was shorter and less intense. The temperatures of burning from the thermographic camera show that composites based on TETA (22–23, 227–249 °C) and PAC (29–21, 199–249 °C) as curing agents have lower temperatures than composites based on IDA (22–25, 231–249 °C). These values are affected by the structural constructions of curing agents. Amine curing agent IDA is based on modified cycloaliphatic polyamine. It is moisture-resistant. Polyamide curing agent – polyaminoamide C – PAC is produced by polycondensation of polyamine with dimers of unsaturated fatty acid methyl esters. The use of this curing agent results in higher impact resistance and elasticity of adhesive compounds. IDA has an amine number in the range of 200–350 mg KOH/g, although PAC has between 290 and 360 mg KOH/g. This proves that PAC binds the epoxy resin more intensively with the modifier. Therefore, during burning, the flame is smaller [38, 39].

Figure S4 (Supplementary materials) shows the composites after burning. All the tested samples were burnt with a bright flame, after burning the samples were charred with no visible blisters or cavities. Nevertheless, the additives affected the appearance, shape, and size of the flame, the temperature in the centre area of the burning sample, and the rate at which the fire was absorbed by the sample section. From Figure S4 it can be concluded that the sample b (EP601 + PAC + 20 %FR) burnt to the smallest extent, and the sample g (EP601 + IDA) to the largest extent. The addition of flame retardants affected the length of sample charring.

Table 4 includes the temperature data from the thermographic camera. Additionally, in Figure S5 (Supplementary materials) the sample thermograms are presented. The analysis of the temperature field in the area of the burning sample includes the determination of changes in the maximum temperature in the entire recorded thermal image after 20 and 30 s from turning on the ignition source and the temperature recorded along a straight line running through the longitudinal axis of the sample. The data presented in the paper were obtained after 30 s of burning. Analyzing the data in Table 3 and the thermograms in Figure S7 (Supplementary materials), it can be concluded that the highest flame temperatures were achieved by pure composites without additives. Virtually all three systems reached their maxima at 250 °C. The addition of 20 % Fire Retardant reduced the temperature in the EP601 + PAC system most effectively.

In Supplementary materials, there is Figure S6 (it shows the temperature graphs after 20 and 30 s of burning) and Figure S7 (histograms). Figure 4 presents the proposed scheme of decomposition.

Fig. 4:

Proposal scheme of decomposition.

Hardness test

Measurements of hardness consisted in vertical immersion of the indenter into the composite surface. The numerical values of these parameters are expressed in the D scale in Table 5. For the system based on EP601 + PAC, the composites with 20 % Fire Retardant reached the greatest value. The composites without the modifier addition were characterized by the smallest hardness. Analyzing the obtained results one can conclude that the addition of modificatory reduces slightly the composites hardness by about 1.3–1.9 Sh. For the system based on EP601 + TETA the addition of 10 % of Fire Retardant caused the hardness growth. The composites hardness was in the range of 76.7–82.4 units. The smallest hardness was observed for the sample with 20 % of Triphenyl Phosphate. For the latter system based on EP601 + IDA the highest value of hardness was characteristic of the pure composite. The composite with 20 % of Triphenyl Phosphate had the smallest value, it was soft, and its addition prevented complete crosslinking. The range of this parameter for the materials was 39.8–76.2 units in the D scale.

Swelling test

Swellabillity is a factor that defines accessibility of the internal chemical structure of cross-linked polymers to the solvent molecules’ penetration. This factor provides information how the resulting materials will behave under varying environmental conditions (in different solvents). The swellability tests in hydrochloric acid (0.1 M), potassium hydroxide (0.1 M), methanol, water, and acetone were carried out. A 1 cm × 1 cm sample was cut from each composite. Initially, the samples were weighed and placed in solvents. After 24 h they were taken and reweighed. Based on the results presented below, it can be concluded that the greatest weight changes were obtained for acetone. The increase in the FR/TP content in the material during tests caused a slight increase in swelling. The highest swelling coefficient was found in the case of EP601 + TETA in acetone and the lowest EP601 + IDA + 20 %TP in potassium hydroxide. The additives influenced the extent of the swelling. Both Triphenyl phosphate and Fire Retardant increased the factors. Supplementary materials in Figure S8 show the swelling test. Table 6 shows the exact values of the swellability factor.

Ageing test and ATR-FTIR analysis after the ageing

In order to assess the resistance to solvents action to the ageing tests were carried. The measurements were made weekly for 9 weeks in glass tubes just like the swelling tests. Before each weighing, the samples were dried for 1 h at 70 °C to remove the adsorbed solvent. The results are given as the difference between the largest and smallest measurements are summarized in Table 7. The measurements included the determination of the mass after each week that the composite has been flooded with the solvent. Based on the results presented below, one can conclude that the highest resistance to methanol action has EP601 + IDA, acetone action EP601 + PAC + 20 %TP, potassium hydroxide action EP601 + TETA + 20 %FR, hydrochloric acid action EP601 + TETA + 10 %FR. The differences in weight gain are the smallest. Water has the lowest tendency to penetrate composite structures which proves that obtained composites are less resistant to organic solvents than water. The results obtained are a consequence of the chemical nature of the additives and resins used.

Figure 5 shows the exemplary samples for the PAC system. One can see that solvents 1, 2, and 3 did not change at all, i.e., water, hydrochloric acid, and potassium hydroxide did not degrade the composites after 9 weeks. However, such solvents as acetone and methanol caused slight damage to the materials. The greatest damage was observed for acetone and methanol for the compositions with the 20%TP addition as the flame retardant. In summary, the composites after 9 weeks of ageing under light conditions were in good shape and showed adequate resistance to such an environment.

Fig. 5:

Photo of the PAC system after ageing.

The ATR/FT(IR) spectra were made in order to evaluate the changes occurring in the composite structure after degradation in the solvents. Chemical structure characterization was made for the PAC system. In the range of 3372–3293 cm⁻¹, broad bands derived from the stretching vibrations of the residual –OH groups or adsorbed water are present. Their intensity changed only in the 0 % and 10 % systems of the flame retardant after ageing in HCl. In each of the five composites in the solvent acetone, a separate band appeared at about 1700 cm⁻¹. The most intense bands in the data range come from the solvents: methanol, acetone or potassium hydroxide. Figures S9–13 show the spectra after ageing tests (Supplementary materials).