Studies on the Effect of Accelerated Ageing on Structural, Physico-Mechanical and Ballistic Properties of Hybrid Silicone-Ceramics Composites

To produce the hybrid silicone-ceramic (HSC) composites, the materials listed below were used: –

hexagonal ceramics tiles made from aluminum oxide (Al₂O₃ content 98%, CeramTec, Plochingen, Germany) with a tile thickness equal to 3.0±0.2 mm,

Method of preparing hybrid silicone-ceramic composites was presented in detail in a previous work [18].

Accelerated ageing studies were performed for:

HSC composites used in combination with a soft ballistic insert with a surface mass of 5.5±0.5 kg/m², consisting of sheets of Twaron® CT612 p-aramid fabric,

The results of the aging tests for the Twaron® CT612 material are presented in article [28].

The accelerated aging process was performed by incubating the samples in a KMF 240 thermostatic chamber (BINDER GmbH, Germany) at a set temperature of (60±2)°C and humidity of (50±2)% over a time interval of: 63, 129 and 194 days, which equals the time of use of inserts in real conditions of 2, 4, and 6 years, respectively.

The simulation of the accelerated aging process was performed using the Arrhenius formula (1), according to ASTM 1980F - 2021 ed. “Standard Guide for Accelerated Aging of Sterile Barrier Systems for Medical Devices”: (1) AAF=Q10 TAA−TRT/10, {\rm{AAF}} = {{\rm{Q}}_{10}}^\left[ {{\rm{TAA}} - {\rm{TRT}}/10} \right], where: AAF – accelerated aging factor; TAA – accelerated aging temperature [°C]; TRT – storage temperature in the real-time aging of the sample [°C], Q₁₀ – aging factor, determined using the kinetics of changes in the selected property/parameter of the material, for temperature changes of 10°C.

The following equation was used to calculate the actual aging time: (2) ATT=365 days/AAF, {\rm{ATT}} = \left( {365\;{\rm{days}}} \right)/{\rm{AAF}}, where: ATT – the time of accelerated aging, which is equivalent to the real-time aging (corresponding to 2, 4 or 6 years of use under real conditions). AAF – accelerated aging factor;

The Q₁₀ aging factor, which defines the aging function curve, was set at the level of 2, and the value of the TRT parameter was set at 25°C. In turn, the accelerated aging temperature TAA was set at (60±2)°C. According to the guidelines, its value should be within in the range from 50°C to 60°C, which is related to the fact that higher temperature significantly reduces the process time, however, it may adversely affect the structural properties of the tested materials, and consequently, the tested materials may show results different from normal.

The fatigue tests simulating the mechanical loads of HSC composites involved bending both ends of the samples in an S 625 device, produced by PPU STOGUM (Poland), according to parameters determined on the basis of the following formula [28, 33]: (3) N=n×x×k, {\rm{N}} = {\rm{n}} \times {\rm{x}} \times {\rm{k}}, where: N – the number of deformation cycles on the S 625 device; n – the number of days of effective insert exploitation per year (values of n were assumed to be equal to 52, 156 or 260); x – simulated time of use (values from 2 to 6 years were assumed); k – the number of daily deformations (the value of 30 was taken for investigations).

Therefore, the research defined the number of deformation cycles as: –

9360 cycles, which corresponds to the estimated use of the insert once a week over 6 years;

The research also assumed a testing angle equal to 30° and a duration of a single cycle of 4 s.

The physical and mechanical parameters of silicone elastomer and PU foam, such as: density, hardness, and tensile strength were tested according to the methodology described below.

Thermal analyses were performed using a TGA/DSC3+ thermogravimetric analyzer (Mettler Toledo, Switzerland). The measurement was carried out in an inert gas (nitrogen) atmosphere using an alumina crucible with a capacity of 70 μl. The analysis was performed using the following measurement program: heating 25°C to 900°C, heating rate 5°C/minute and/or 10°C/minute. The measurement results are presented in the form of thermograms.

FT-IR analysis was performed on both samples for accelerated aging and not, using a NICOLET iS10 spectrophotometer (Thermo Scientific, USA) within the wavelength range from 400 to 4000 cm⁻¹. Measurements were carried out using a DTGS KBr detector, with a resolution of 4. For FTIR-ATR testing, the material was cut to dimensions of 20×20 mm and applied to the crystal by pushing it down in a suitable manner. Both the background spectrum (i.e., the spectrum of the crystal) and the spectrum of the crystal with the sample were measured. The background measurement was saved in the internal memory of the spectrophotometer and automatically subtracted from the sample measurement as a correction for the external conditions of the tests. The FT-IR analysis was repeated 3 times for each sample; if the standard deviation was within 5%, no further repetition of the study was performed.

Measurements were made using an HR-SEM high-resolution electron microscope (High resolution FEI Nova Nano SEM 450 electron microscope with EDS, Thermo Fisher Scientific Inc., USA) with the use of a highly sensitive CBS (Circular Backscatter) detector with 4 concentrically arranged sectors, enabling the detection of backscattered electrons (BSE) and cooperating with the electron energy deceleration mode. The measurements were carried out at accelerating voltage values of 3 and 5 kV and under magnifications of 500× and 1500×.

The samples intended for the assessment of fragment resistance were HSC composites, consisting of Al2O3 ceramics, Poron® XRDMA and Twaron®CT612 embedded in Za Mould 22 elastomer, used in combination with a soft ballistic insert with a surface mass of 5.5±0.5 kg/m², consisting of sheets of Twaron® CT612 p-aramid fabric.

The resistance to fragments was determined on the basis of the ballistic limited velocity parameter (V50). V50 was measured at ambient temperature according to STANAG 2920 “Ballistic test method for personal armor materials and combat clothing”. The limit of ballistic protection V50 was determined as the average of the equal number of the highest measured speeds of the fragment causing only partial puncture and the lowest measured speeds causing total puncture within the velocity spread Δ≤40 m/s. An FSP.22 standard fragment with a mass of (1.10÷0.03) g, diameter of (5.38±0.02) mm and length of 6.35 mm, made of steel with a hardness of (30±2) HRC was used for the fragment resistance tests.

In the first part of this work, FT-IR structural studies were carried out to determine possible changes at the molecular level after accelerated aging processes in the silicone structure and PU foams. The FT-IR spectra obtained were summarized and presented in Figure 2.

Fig. 1.

Fig. 2.

The spectrums obtained for silicone (Fig. 2a) are characterized by the occurrence of bands at wavelengths of 660–690 cm⁻¹, corresponding to the bending oscillation of single carbon bonds. The presence of a low intensity band in the wavelength range 761 cm⁻¹ corresponding to bending vibrations of C–H bonds was also observed. Moreover, the presence of bands at wavelengths of 801 cm⁻¹ and 865 cm⁻¹ was observed from the deformed pendulous oscillation of the CH₃–Si–CH₃ group, and 1021 cm⁻¹ and 1093–1094 cm⁻¹ - characteristic for the asymmetric tensile oscillation of the Si–O–Si group. Based on the analysis of the FT-IR spectrums obtained, the presence of bands characteristic for symmetrical and asymmetrical deformation oscillations of Si–CH₃ groups can also be noted, with wavelengths of 1261 cm⁻¹ and 1411 cm⁻¹, respectively. The bands in the range of wave numbers 2905 cm⁻¹ and 2963 cm⁻¹ characteristic for the tensile oscillations of C–H bonds, indicate the presence of methyl groups in the silicone molecule. However, no bands in the range of 2160–2170 cm⁻¹ were recorded, which indicates the absence of –SiH groups in molecules of the compounds used. The FT-IR spectra also indicate that after a period of accelerated aging simulating a 6-year period of silicone use, there is a decrease in the intensity of the bands coming from the vibrations of the CH₃–Si–CH₃, Si–CH₃ groups in the wavenumber range of 801 and 1261 cm⁻¹ and in the intensity of the band located at a wave number of 1094 cm⁻¹ originating from the vibration of Si–O–Si bonds, which may result from the breaking of Si–C and Si–O bonds and the formation of trace amounts of cyclic oligomers [34, 35].

In the PU foam sample (Fig. 2b), the following characteristic absorption bands were found: 3315 cm⁻¹ - attributed to the stretching vibration of N−H bonds that form intermolecular hydrogen bonding, 3000–2850 cm⁻¹, indicating the asymmetric and symmetrical stretching vibrations of the methyl groups. The band located at the wavenumber value of 1533 cm⁻¹ corresponds to the N−H deformation, and the bands present at the wavelengths in the range 1728–1602 cm⁻¹ are attributed to the stretching vibration of C=O bonds. The absorption band at 1224 cm⁻¹ is attributed to the asymmetric stretching vibration of C−O−C bonds of ester groups. The bands located in the range of 1453-1316 cm⁻¹ correspond to deformation vibrations of the –CH₂–, –CH₃ and/or C–N bands, and the band with the wavenumber value equal to 1108 cm⁻¹ corresponds to the vibration stretching of C–N or C–O bonds or vibrations of the Si–O–Si group derived from possible additive/fillers contained in the PU foam [19]. The results above indicate that polyurethane is the main component of the foam. For PU foam samples subjected to accelerated aging corresponding to a period of 6 years of use, the following were found: an increase in the intensity of the bands located at wavelengths of 3315 cm⁻¹ and 1602 cm⁻¹, corresponding to the stretching vibrations of the N–H groups and the vibrations of the carbonyl group, respectively; a decrease in the intensity of the 1728 cm⁻¹ band, corresponding to the stretching vibrations of the C=O bonds, and in the 1533 cm⁻¹ band, which corresponds to the N−H deformation. In addition, the intensity of the band assigned to the vibrations of the C–N/C–O and/or Si–O–Si group was reduced (1108 cm⁻¹). The reduction in the intensity of the above-mentioned bands indicates a slow degradation of the urethane bond under the influence of environmental factors affecting the polyurethane as part of the accelerated aging process.

For the self-adhesive film (Fig. 2c), the following bands were noted: 2968 cm⁻¹, 1732 cm⁻¹, 1718 cm⁻¹, 1505 cm⁻¹, 1471 cm⁻¹, 1409 cm⁻¹, 1237 cm⁻¹, 1120-1105 cm⁻¹, 1020 cm⁻¹, 970 cm⁻¹, 794 cm⁻¹, assigned to vibrations of –C–H bonds of methylene and methyl groups and C=O, –C–O–bonds of ester groups coming from the polyester of which the mesh is composed. In addition, the studies showed the presence of bands located in the wavenumber range 2926–2867 cm⁻¹, 1454 cm⁻¹, 1401 cm⁻¹ or 700 cm⁻¹, which indicate the presence of an adhesive layer, probably based on polyterpenes. The accelerated aging process caused a slight decrease in the intensity of all spectral bands, especially the bands located at wavenumber values of 1732 cm⁻¹ and 1718 cm⁻¹ and for the range of 1237-1020 cm⁻¹, as a result of changes in the structure of polyester mesh fibers. The intensity of the 2926 cm⁻¹ band originating from the vibrations stretching the C–H bonds of the methylene group present in the adhesive resin also decreased, indicating structural changes in the self-adhesive film.

In order to determine whether the FT-IR results obtained are statistically significant, an ANOVA statistical analysis in accordance with Tukey HSD/Tukey Kramer for silicone, PU foam and self-adhesive film samples before and after accelerated ageing was performed. The significance level (a) was assumed as 0.05. For p-values <α, H₀ is rejected, and the difference between the sample averages of some groups is enough to be statistically significant. This means the ageing process statistically affects structural properties of specified materials used in HSC composite.

The physico-mechanical properties of impact protection materials strongly depend on the internal cellular structures of these materials, such as cell type, cell fraction, cell size and size distribution, etc. Therefore, the surfaces and internal cellular structures of PU foam and silicone were studied using an SEM microscope. The results obtained are presented in Figure 3.

Fig. 3.

Images taken using SEM microscopy, presenting the morphology of silicone, did not show any structural changes after the accelerated aging process. In both cases (before and after accelerated aging processes), the silicone samples were characterized by the presence of a significant number of globules with sizes ranging from several dozen nanometers to approximately 7 μm.

SEM images of the PU foam surface indicate that the polymer material used in its construction has a porous structure with open pores with a diameter of 40 to 100 μm. Comparison of images obtained for the material surface before and after the accelerated aging processes did not reveal any significant changes in the surface structure of the material.

The HSC composite materials were also analyzed thermogravimetrically in a nitrogen atmosphere, in the temperature range from 25 to 900°C, in order to assess the impact of their chemical structure on thermal stability. Figure 4 shows thermograms of the initial samples and those subjected to accelerated aging processes, simulating use in real conditions, for up to 6 years. Additionally, Table 1 presents the numerical values of the thermogravimetric analysis of the tested materials.

Fig. 4.

The results of thermogravimetric (TG) analysis (Fig. 4a) indicate a relatively high thermal resistance of the silicone obtained compared with different types available on the market that can be used in composites, especially high temperature vulcanizing (HTV) silicones. The thermogram is characteristic of siloxanes. The TG curve obtained for silicone shows two stages of decomposition: the first one in the temperature range of 103–410°C, in which the mass loss is up to 10%, and the second one in the temperature range of 410–785°C (mass loss of 40%). The value of the T5 parameter (the temperature at which 5% mass loss occurred) increases with an increase in the length of accelerated aging cycles. This increase is approximately 8% for the accelerated aged silicone, reflecting 6 years of use, and is likely related to the structural changes in the silicone described above, based on the analysis of the FT-IR spectra.

In the case of both unaged silicone and silicone subjected to accelerated aging processes, the thermograms did not show a characteristic, small increase in the sample mass in relation to the formation of new Si–O–Si bonds and the oxidation of Si–H bonds, which most often takes place after the first stage of decomposition of the tested materials. It is related to with two aspects, firstly, the lack of Si–H groups in the structure of silicone elastomers, which is confirmed by FT-IR spectra (Fig. 2a). This is also due to the breaking of Si–O–Si bonds and the simultaneous loss of mass resulting from the degradation of methyl groups present in the structure of the tested material. The results of thermo-gravimetric tests obtained are consistent with the literature data obtained for other polydimethyl-siloxanes, i.e. Sylgard 184 (Dow Corning) [34].

Thermogravimetric analysis of the PU foam (Fig. 4b) indicates that thermal degradation under a nitrogen atmosphere is consistent with that of polyurethane, and very possibly polyurethane/SiO₂ composite [36,37,38]. PU foam shows a multiple-step decomposition. The temperature value at which there is a 5% loss in the mass of PU foam does not change significantly after accelerated aging processes (differences within the range of 6°C, i.e. approximately 2% for the unaged and aged samples). However, in the case of a temperature value for 50% mass loss (T50), thermogravimetric data indicate a slight increase in the value of the T50 parameter with an increase in the number of accelerated aging cycles (a difference of approximately 20°C, which is a change of over 6%).

In the case of thermogravimetric tests of the adhesive film (Fig. 4c), there were no changes in the values of the T5 and T50 parameters as a result of accelerated aging processes, so it should be assumed that this film does not significantly change its thermal properties as a result of aging processes. Thermogravimetric curves indicated the presence of a two-stage decomposition process of the self-adhesive film under the influence of temperature, which is consistent with the composition of the film. The first stage involves the decomposition of glue based on synthetic resins, while in the second stage, the polyester mesh decomposes. In the case of polyester mesh, the mass loss in the samples is due to thermal decomposition into CO and carbonaceous char.

Research was also carried out to assess the physico-mechanical properties of materials used in the construction of the HSC composite. The results of physical and mechanical tests for the initial samples and those subjected to accelerated aging processes are presented in Table 2.

The density and hardness values obtained for silicone and PU foam practically do not change for the initial samples and samples subjected to accelerated aging processes. Minor deviations are within the limits of measurement errors. Also, in the case of tensile strength, no significant changes in this parameter were observed for PU foam and silicone samples subjected to accelerated aging compared to unaged samples. In the case of silicone, an increase in tensile strength of approximately 14% was observed, and in the case of PU foam, a decrease in the value of this parameter of approximately 9% after accelerated aging cycles corresponding to a 6-year period of use. These data indicate that structural changes occurring as a result of aging processes only slightly affect the tensile strength of silicone and PU foam.

An increase in the values for the parameters of self-adhesive film such as: the maximum stretching force, relative elongation at maximum force and the adhesion force was obtained for samples subjected to accelerated aging processes as compared to unaged samples. In the case of self-adhesive film subjected to a period of 194 days of accelerated aging corresponding to 6 years of usage in real conditions, an increase in the maximum stretching force of 19%, an increase in the relative elongation at a maximum force of 36% and an increase in the adhesion force of 59% were achieved as compared to the samples not subjected to accelerated aging processes.

The improvement in the physical and mechanical parameters and in the strength of the connection is the result of the continuation of the adhesive hardening processes, which strengthen the adhesive connections [39].

Adhesive joints usually undergo thermal degradation processes under the influence of increased thermal energy, which causes a decrease in strength [40]. The reason for the loss of strength is the deterioration of both the cohesive parameters of the adhesive film and the adhesive parameters on the phase contact surface. Adhesive subjected to long-term exposure to increased thermal energy oxidizes and thermally decomposes. This increases its brittleness and thus reduces its cohesive strength. On the other hand, at the phase boundary, as a result of adhesive shrinkage, shrinkage stresses and chemical reactions occur on the glued surface [39, 41]. However, due to the fact that the nature of aging depends on the type of glue, joined materials and environmental conditions, the changes in the strength of the adhesive joint during aging does not always have to be negative from the beginning of exposure to environmental factors affecting the adhesive, which was probably the case with the research results presented in this publication.

The samples intended to assess resistance to fragments were HSC composites combined with “soft” ballistic inserts sewn into the cover. The samples were tested to determine the fragment resistance parameter (V50 parameter) after: –

cycles of accelerated aging simulating the usage life of ballistic composites ranging from 2 to 6 years;

The test results (Table 3) were summarized and compared with the data obtained for HSC composites not subjected to accelerated aging cycles.

The HSC composite does not show any significant changes in its fragment resistant properties under the influence of accelerated aging processes corresponding to a period of 2, 4 or 6 years of use, and as a result of fatigue tests which correspond to a period of use of up to 3 times a week over 6 years.

In order to determine whether the resistance to fragmentation results obtained are statistically significant an ANOVA statistical analysis was performed in accordance with Tukey HSD/Tukey Kramer for initial HSC composites, and HSC after accelerated ageing cycles and after fatigue tests. The significance level (a) was assumed as 0.05. For HSC composite p-values >α, the averages of all groups were assumed to be equal. This means the ageing and fatigue processes statistically do not affect fragment resistant properties of the HSC composite.

The value of the V50 parameter of resistance to fragmentation was not determined for the use of the developed HSC composite for periods with a higher frequency than a use of >3× per week during 6 years, because the outer silicone layer began to show signs of degradation consisting in the appearance of kinks and creases on the silicone surface. Therefore, it should be considered that a frequency of use of 3 times a week for a period of 6 years is the limit frequency ensuring the stability of ballistic protection parameters and the safety of potential users of the ballistic composite.

The research showed that the ballistic resistance of the HSC composite is the result of the destruction of ceramic elements by the fragment and the pulling, rupture, decrumbing and extension of yarns and fibers occurring in the soft ballistic insert. The outer silicone layer makes a slight contribution to the fragments’ stopping mechanism [18]. It acts as a matrix in which ceramic elements are placed and allows the composite system to adjust to the user’s body, which increases the ergonomic properties of the armor. The research conducted allowed to determine that reinforced layers (PU foam) and self-adhesive film also do not affect the fragment-resistant properties of the HSC composite. They are intended for the “stabilization” of the ceramic layer and limit the generation of secondary fragments from the ceramic fragments formed during the impact of a fragment or bullet on the sample [18].