3.1. Cross-Linking Results of Unfilled and Filled CR/BR/Zn Blends
The cross-linking process is connected with two phenomena: physical heat transfer and chemical reaction [
30,
31]. The rubber vulcanization creates cross-links between the polymer chains, generally with the release of energy that changes the properties of the final material. Rheologically, as new cross-links are formed, the shear modulus of the elastomer increases. Therefore, the rheology of rubbers provides information on the degree of cross-linking and on optimal cross-linking parameters, such as cure temperature and cure time, as these are influential variables in the vulcanization processes. Therefore, the determination of cross-linking characteristics is very important. In terms of vulcametric results, the CR/BR composites cured with zinc were characterized by a long scorch time (t
90 = 5.54 min). The incorporation of sillitin or chalcedonite significantly shortened the t
90 parameter (to values of 4.38 min and 4.42 min, respectively). The shortest scorch time (t
02 = 1.10 min) and the shortest cure time (29.18 min) were found for the CR/BR/Zn blend filled with aerosil. The longest cure time (t
90 = 31.36 min) was observed for the unfilled mix. It is worth noting that the presence of fillers in the CR/BR/Zn blends has little effect on the vulcanization time (
Table 2).
The presence of fillers clearly changed the minimal vulcametric torque. The Mmin value of the unfilled CR/BR/Zn blend was 0.55 dN·m. The application of sillitin or chalcedonite as a filler led to an increase in minimal torque to values of 0.74 dN·m and 0.70 dN·m. The highest minimal torque was recorded for the blend filled with aerosil (4.81 dN·m). Such a large increase in the Mmin parameter results from the high activity of aerosil and the large specific surface area of this filler (BET of aerosil = 380 m2/g). Less pronounced differences in torque increment were observed after 30 min of heating (∆M30) between unfilled and filled blends. For blends without a filler and filled with chalcedonite, the ∆M30 parameter was comparable (~6.48 dN·m). The filling tested blend with sillitin led to less of a torque increment (5.86 dN·m), but the addition of aerosil caused the highest (above 40%) torque increment, equal to 8.79 dN·m. Such a significant increase in the torque increment for aerosil-containing blends compared to the other samples filled with sillitine or chalcedonite may result from the formation of additional bonds between the chloroprene rubber and aerosil. As a result of the chemical reaction between the silanol groups on the surface of this filler and the allyl chlorine atom in the chloroprene rubber, new bonds can be formed, leading to an increase in the degree of cross-linking. When analyzing the obtained vulcametric results, 160 °C was selected as the cure temperature and 30 min was selected as the cure time for all CR/BR/Zn composites.
Swelling studies were conducted to further analyze the degree of cross-linking of CR/BR/Zn compounds. The degree of swelling is one of the basic parameters determining the resistance of vulcanizates to solvents (in this case: toluene). The results showed that the presence of fillers did not affect the degree of swelling (
Table 2). The swelling of both unfilled and filled CR/BR/Zn vulcanizates was comparable (from 8.15 mL/mL to 9.21 mL/mL). The degree of cross-linking calculated on the basis of the equilibrium swelling results was in the range of 0.11 (for sillitin application) to 0.13 (for aerosil application). The obtained swelling results were reflected in the mechanical properties: among the filled CR/BR/Zn variants, the product with the lowest degree of swelling and thus the highest degree of cross-linking showed the highest tensile strength (
Table 3).
3.2. Mechanical and Dynamic Properties of Unfilled and Filled CR/BR/Zn Vulcanizates
The mechanical and dynamic properties of rubber materials are directly related to the cross-linking parameters [
31,
32]. The unfilled and filled CR/BR/Zn vulcanizates were tested for hardness, damping, stress at 100, 200, and 300% strains, tensile strength, elongation at break, storage modulus and loss modulus. Test results are presented in
Table 3. The high tensile strength is required for rubber products applied and used in various areas.
The application of different filling methods of the CR/BR/Zn composites highlighted the influence of the filler type (sillitin, chalcedonite, or aerosil) on their mechanical properties. Unfilled vulcanizates were characterized by the worst mechanical properties, especially with the stress at the 100% strain (0.66 ± 0.01 MPa) and tensile strength (8.56 ± 0.41 MPa). The incorporation of sillitin or chalcedonite improved the tensile strength to equal to ~8.97 MPa. The highest TS
b (10.97 ± 0.72 MPa) was achieved after the filling of the CR/BR/Zn vulcanizate with aerosil. This result is quite obvious as aerosil is the most active filler among those used in these studies. The surface area for aerosil is several times higher than BET for other fillers and amounts to 380 m
2/g. Surface area is probably the most important morphological characterization of reinforcing fillers because it corresponds to the extension of the interface, i.e., the interaction between elastomer and filler surface. Compared to other used fillers, aerosil gives a higher elastomer–filler interaction. The result is higher tensile strength. Elastomer compounds’ reinforcement by aerosil can be generally considered as the consequence of the adsorption of rubber chains onto the surface of this filler. In addition, we have observed that both unfilled vulcanizate and vulcanizate filled with sillitin and chalcedonite had the comparable elongation at break (~755%), whereas the samples containing aerosil showed the E
b value equal to 647% (
Table 3). This fact may indicate lower elasticity and greater hardness of aerosil-containing vulcanizates.
Shore hardness (HA) depends on the elastic properties of the tested material (Young’s modulus). The hardness of materials is a property that allows one to determine changes which occur from the surface to the depth of the material. By analyzing the hardness of the obtained compositions, it was found that the incorporation of aerosil into the CR/BR/Zn samples led to the production of vulcanizates with the highest hardness (65.3 °ShA). Such a high hardness of samples containing aerosil was due to the highest degree of cross-linking achieved by them (
Table 3). The well-developed surface area of aerosil and the possibility of chemical reactions occurring at elevated temperatures between the silanol groups present on the aerosil surface and the chlorine atom in CR macromolecules contribute to the increase in the cross-linking degree and thus to a greater hardness of the CR/BR vulcanizates.
The above-mentioned dependence is also the cause of the highest damping relative (T
τw) recorded for the sample filled with aerosil. The T
τw value of the CR/BR/Zn vulcanizate was 10.25%, but the application of the filler improved the damping properties (the T
τw values were 19.88%, 21.27% and 29.40% for sillitin, chalcedonite and aerosil, respectively). It is worth noting that the higher the damping relative, the more the vulcanizate is able to minimize vibrations (
Table 3).
Table 3 also presents the results of testing the dynamic properties. Dynamic studies of CR/BR/Zn vulcanizates were carried out at constant temperature, with constant deformation frequency and changing deformation amplitude. Under these conditions, the storage modulus decreases as the deformation amplitude increases because the interactions, e.g., rubber–filler or filler–filler, are destroyed. This phenomenon is called the Payne effect. The aerosil-filled vulcanizate obtained the highest Payne effect (ΔG’ = 0.392 MPa), which most likely results from the formation of an extra network by aerosil particles and their tendency to form agglomerates or aggregates. The size distribution of aggregates is a very important structural feature of active fillers. In addition, the quality of the molecular filler dispersion in the rubber is a factor that strongly affects the mechanical and dynamic properties of the final products. Dispersibility is important for obtaining aerosil-reinforced vulcanizates. Obviously, the dispersibility of any filler depends mainly on the interactions between aggregates and/or agglomerates. In the case of aerosil, these interactions are mainly due to the hydrogen bonds that exist between the silica grains. This is indicated by the maximum storage modulus, which is higher the greater the specific surface area of the filler (BET of aerosil = 380 m
2/g). For the samples containing chalcedonite or sillitin, the Payne effect was clearly lower (0.193 and 0.185 MPa, respectively) and comparable to the ΔG’ value of the unfilled compound. The slight Payne effect in these products may indicate weak filler–filler or filler–rubber interactions, which may be due to the small specific surface area (the BET of chalcedonite and sillitin is equal to 10 and 12 m
2/g, respectively).
The loss modulus (G”) informs one about the amount of energy dissipated during the dynamic deformation of the rubber product and its transformation into heat. Its value depends on the destruction and rebuilding of the network structure that the filler can create. The G” modulus is higher the more the filler networks are destroyed and rebuilt during one deformation cycle. The highest value of the loss modulus (G”max = 0.114 MPa) was obtained for the CR/BR/Zn/aerosil, which confirms the possibility of the formation of the filler network in the elastomeric matrix. The smallest value of the maximum loss modulus (G”max~0.044 MPa) was obtained for the unfilled sample and the vulcanizate containing sillitin. Extra network formation by the filler in the elastomers or strong filler–filler or filler–rubber interactions determine to a large extent the morphology, degree of cross-linking and mechanical–dynamic properties of CR/BR/Zn vulcanizates.
3.3. SEM Analysis of CR/BR/Zn Surface Morphology
The aim of SEM tests was to determine the effect of applied fillers on the morphology of the obtained CR/BR/Zn compounds. The surface morphology of composites containing sillitin, chalcedonite or aerosil at a 8 k magnification are shown in
Figure 2.
In the SEM image of the CR/BR/Zn vulcanizate, two mutually interpenetrating phases were visible (
Figure 2a). It was a rather parallel arrangement of phases. The SEM image of the CR/BR/Zn sample filled with sillitin showed brighter areas in the elastomeric matrix, indicating small aggregates of this filler (
Figure 2b). In the structure of the CR/BR/Zn/chalcedonite vulcanizate, both individual filler particles as well as their aggregates and agglomerates (especially one agglomerate in the central part of the SEM image) were visible (
Figure 2c). The sillitin and the chalcedonite have similar specific surface areas (BET of sillitin = 12 m
2/g, BET of chalcedonite = 10 m
2/g) and a similar particle size equal to 9 and 13 μm, respectively, whereas in the SEM image of the CR/BR/Zn/aerosil composition, large flat agglomerates of silica forming a layered system of the elastomer-filler composition were observed (
Figure 2d). This is probably connected with the most developed specific surface area of this filler (BET of aerosil = 380 m
2/g) and the smallest size of grains prone to forming large clusters in the elastomeric matrix. The presence of large agglomerates in this vulcanizate confirm the highest Payne effect, as mentioned earlier (
Table 3,
Section 3.2). It is worth noting that the dispersion of the filler is primarily influenced by the strength of interactions between aggregates and/or agglomerates. This magnitude of interactions results directly from the surface energy of the selected filler. Another factor determining the correct or incorrect dispersion of the filler is its morphology and surface area. The smaller the surface area, the better the dispersion.
The cross-section surface morphologies of the obtained vulcanizates are shown in
Figure 3. The cross-section morphology of the unfilled vulcanizate (
Figure 3a) contained large empty spaces, which proves unsatisfactory processing of these composites. The application of the filler changed the cross-section of the resulting vulcanizates. The presence of sillitin in the CR/BR vulcanizate resulted in a rough morphology with frequent grooves (
Figure 3b).
Figure 3c,d shows the smooth cross-section of vulcanizates filled with chalcedonite or aerosil, although the morphology of the CR/BR/Zn/aerosil was cracked.
3.4. Flammability, Fire Hazard and Toxicity of Unfilled and Filled CR/BR/Zn Vulcanizates
Insufficient thermal stability during ordinary use, undesirable flammability and too-high fire hazard are very important problems for rubber products. The thermal properties of elastomeric materials depend on their chemical structure, the degree of cross-linking and their compounds. Standard cross-linked butadiene rubber (i.e., with sulfur in the presence of accelerators and activators) is a flammable product, as determined by the oxygen index (OI) method. The OI value of BR vulcanizates is only 17%. On the other hand, chloroprene rubber cross-linked with zinc oxide and magnesium oxide has a much higher oxygen index (OI = 26%), which makes it a flame-retardant product. It was found that the combination of both elastomers and their cross-linking with zinc led to the materials with lower flammability (OI = 30.1%). The filling of the CR/BR/Zn blends containing sillitin, chalcedonite or aerosil caused the oxygen index to be 37.5%, which classifies them as non-flammable rubber products (
Table 4). The reason for such a high value of the oxygen index in the tested CR/BR vulcanizates is probably due to the inter-elastomeric reactions that take place during the unconventional cross-linking of such compositions with zinc.
The oxygen index method is not a sufficient test to assess the flammability of elastomers, as it can only be used to compare this phenomenon. Thus, cone calorimetry was performed for a detailed analysis of flammability parameters. On the basis of this method, the basic properties determining the fire hazard were determined, namely, time to ignition (TTI), total heat release (THR), heat release rate (HRR), time to maximum heat release rate (tHRR
max), total heat release (THR), effective heat of combustion (EHC), mass loss rate (MLR) and average mass loss rate (AMLR) [
26]. The data contained in
Table 4 clearly indicate that CR/BR/Zn vulcanizates are materials with high resistance to burning. The average maximum heat release (HRR
max) of the unfilled vulcanizates was only 256.79 kW/m
2. For the sample filled with aerosil, the HRR
max value decreased to 166.46 kW/m
2. The presence of sillitin or chalcedonite in the tested compounds led to the HRR
max values being equal to 212.89 or 200.20 kW/m
2, respectively (
Figure 4). Sillitin, similarly to chalcedonite as well as aerosil, decrease the fire hazard of the studied composites. In the case of composites containing sillitin, the HRR parameter is a little higher than in the case of composites containing chalcedonite or silica. In our opinion, the boundary layer created during the combustion of composites containing chalcedonite or silica has better isolating properties and is more homogenous than in the case of composites containing sillitin. The homogenous structure and isolating properties of the boundary layer are directly connected with the size of the filler. The average size of sillitin grain is 12.7 um, whereas for chalcedonite and silica it is, respectively, 9 and 0.07 um. It is worth noting that the heat release rate (HRR) during the combustion of all CR/BR/Zn vulcanizates was incomparably low compared to other rubber materials. For comparison, cross-linked acrylonitrile butadiene rubber (NBR18, where the number 18 indicates the weight percentage of bound acrylonitrile) was characterized by the HRR value being equal to 3569.23 kW/m
2 [
33]. The average mass loss rate (AMLR) of the tested compounds, which proves the dynamics of material combustion dynamics, reached the highest value (53.90 g/m
2·s) in the case of the unfilled vulcanizate, but the presence of aerosil resulted in the reduction in the AMLR value to 32.57 g/m
2·s.
Figure 5 shows the mass loss rate (MLR) versus the incineration time of unfilled and filled tested vulcanization. It is clearly visible that the mass loss rate was much smaller for the filled samples, which indicates the positive effect of all the fillers used.
The cone calorimetry results allowed one to calculate the fire hazard related to the fire propagation rate (i.e., 1/t
flashover). This is defined as the time inverse to the flashover effect [
34]. The least fire risk was found for the composites filled with aerosil (1/t
flashover equal to 2.73 kW/m
2·s) or chalcedonite (1/t
flashover equal to 3.85 kW/m
2·s). The vulcanizate filled with sillitin had the greatest fire risk (1/t
flashover equal to 5.46 kW/m
2·s) (
Table 4). It should be stressed that the fire hazard determined for the tested CR/BR vulcanizates was clearly lower than the fire risk of products made of acrylonitrile butadiene rubber, styrene-butadiene rubber and butadiene rubber, whose corresponding values amounted to 66.06, 31.68 and 56.37 kW/m
2·s, respectively [
25]. In addition, the tested CR/BR/Zn vulcanizates pose a significantly lower fire hazard than that of the popularly used polyethylene or polypropylene (1/t
flashover equal to 20.15 and 44.22 kW/m
2·s, respectively). Thus, the obtained results confirm that the all zinc-cured CR/BR products are non-flammable and pose a low fire hazard. The FIGRA parameter showing the ratio of maximum heat release rate to time to maximum heat release rate was similar. The highest FIGRA index was achieved for the sample without fillers (2.85 kW/m
2·s), and the lowest was for the vulcanizate filled with aerosil (1.75 kW/m
2·s). The maximum average heat release rate (MARHE) of the CR/BR/Zn/aerosil vulcanizate was 37% lower (MARHE = 67.1 kW/m
2) than for the unfilled sample (MARHE = 106.1 kW/m
2). The analysis of the obtained results confirms that the use of aerosil as a filler has the greatest impact on reducing the fire hazard and favors the formation of a thermally stable boundary layer that hinders the mass and energy flow between the flame and the sample.
In addition to the resistance to burning, the toxicity of substances released as a result of the combustion of elastomeric materials is also important. The emissions of carbon(IV) oxide (CO
2), carbon(II) oxide (CO) and other gaseous substances such as nitrogen(IV) oxide (NO
2), sulfur(IV) oxide (SO
2), hydrogen chloride (HCl) and hydrogen cyanide (HCN) are the main causes of product toxicity resulting from thermal decomposition and the combustion of tested vulcanizates. The parameter that takes into account the concentrations of all six gases emitted at temperatures T = 450, 550 and 750 °C is known as the toxicometric index (W
LC50SM), described by Formula (10) [
25].
Table 5 shows the specific emission of selected gaseous products formed during the combustion and decomposition of tested vulcanizates at the temperatures of 450, 550 and 750 °C. The presented data show that the highest emission values were observed, regardless of the decomposition temperature and the sample type, for carbon(IV) oxide and carbon(II) oxide. The carbon(IV) oxide emission for filled vulcanizates was noticeably lower than in the case of the unfilled CR/BR/Zn vulcanizate. Undoubtedly, the value of the toxicometric index was influenced by the emission of SO
2 (0.01 g/g for all samples) and HCl (the highest emission for an unfilled sample: 0.15–0.20 g/g). A potential source of HCl was chlorine bound to macromolecules of the chloroprene rubber.
The results presented in
Figure 6 show that the volume of gases generated during the thermal decomposition of CR/BR/Zn vulcanizates, in the temperature range of 450, 550 and 750 °C, was significantly higher for the unfilled product and was the lowest for the sample containing chalcedonite. For example, at 450 °C, the volume of gases emitted during the decomposition of CR/BR/Zn/chalcedonite was 42% lower than for the unfilled vulcanizate and ~20% lower than for the vulcanizate filled with aerosil or sillitin.
The basis for the classification of thermal decomposition and combustion products is the parameter W
LC50SM. According to PN-B-02855:1988, the products should be considered very toxic if W
LC50SM ≤ 15, the products are toxic if 15 ≤ W
LC50SM ≤ 40 and the products are moderately toxic if W
LC50SM > 40. The WLC
50SM values show that gaseous products generated during the decomposition of CR/BR vulcanizates should be considered as very toxic (
Table 6). The applied fillers slightly increased the toxicity of gaseous products formed as a result of thermal decomposition and combustion of the tested elastomeric materials, determined by the W
LC50SM index. It should be remembered that the reactions taking place on the surface of the combusted materials play an important role in the combustion process of elastomers, because they affect both the processes occurring in the flame and the processes of thermal decomposition of the elastomer in the solid phase. The formation of gaseous products, accelerating combustion processes, occurs mainly as the result of the polymer decomposition. In addition to the conduction, convection and radiation, strongly exothermic oxidation reactions in the surface layer between the solid and gas phases are the significant sources of thermal energy necessary to support the processes of elastomer decomposition. Limiting the oxidation processes in the surface layer of the tested vulcanizates may contribute to the extinction of the flame.