Influence of phthalocyanine pigments on the properties of flame-retardant elastomeric composites based on styrene-butadiene or acrylonitrile-butadiene rubbers

1 Introduction

Synthetic elastomers are exceptional among the most popular commercially used polymeric materials due to their unique viscoelastic properties. They have found various practical applications, for example, in tires or gaskets production. As they are often used, apart from having specific properties for the function they are supposed to provide, it is important that their usage be accompanied by an appropriate safety. One of the biggest dangers that threaten life is a fire, therefore all of the massively produced materials should be as less flammable as it is possible to make them. General use rubbers, such as styrene-butadiene rubber (SBR) and acrylonitrile-butadiene rubber (NBR), are materials that are neither resistant to high temperatures nor fire. Their thermal decomposition is not only rapid but the amount of toxic gasses emitted to the atmosphere during this process classify them as being toxic (1).

Combustion is a complex process that still has not been fully explained (2). Cullis et al. (3) described two main mechanisms of a polymers’ combustion, namely radical and thermal. Eventually, if the energy contribution from both of them is very similar, a combined mechanism may occur. In the case of elastomeric materials, the main factors influencing behavior during combustion are composition of a material, its physical structure, the occurring chemical reactions and transport processes of both mass and energy in the boundary layer between gaseous and solid phases (4).

In order to limit elastomers’ flammability, one may target increasing the yield of carbonization processes. The emerging char residue works as a barrier phase blocking further transport of combustible substances to the flame. Imparting into structure of polymeric materials substances, which create in the oxidization process non-flammable products, such as water or inert gasses, may cause smothering of a flame, resulting in stopping thermal decomposition and degradation. It is also possible to use fillers acting as radical scavengers, which results in limiting radical combustion mechanism (5, 6). Facilitating one or more of the mentioned effects, a vast amount of substances were used to limit flammability of polymers such as various phosphorus compounds (7), calcium carbonate (8), carbon black (9), montmorillonites (10), aromatic boronic acids (11), functionalized carbon nanotubes (12), silica (13) and hybrid kaolinite/silica filler (14).

Mechanical durability of elastomeric composites is another issue which needs to be taken into account no matter what the main function of the obtained material is since it will undoubtedly be exposed to mechanical stresses. Bearing in mind that over time composites degrade losing initial properties, it is recommended that aging processes are also taken into consideration.

To improve aesthetic properties of obtained vulcanizates, pigments or dyes are being added to rubber mixtures. There are reports available providing informations about pigments influencing other properties of elastomeric composites than color, such as the extensive studies carried out by El-Sabbagh et al. (15, 16) about kaolin-metal oxides core-shell pigments imparted into SBR. Investigated vulcanizates turned out to have good mechanical durability and electrical insulating properties. In the paper provided by El-Nashar et al. (17) it was reported that modified phosphate pigments may act as a strong reinforcement for an elastomeric composite. Ahmed et al. (18) reported that it is economically justified to create a core-shell pigment out of the previously studied phosphate pigments with zinc oxide. Generally, any substance contributing to both changing the absorbance in visible-light spectrum and enhancing other properties of a final composite may find practical application in the industry.

Phthalocyanine derivatives (Figure 1) were reported to exhibit a positive remark on cross-linking degree, thermal stability and flammability of elastomeric composites (19). Further studies were focused on the synergistic effect of flame-retardancy by the simultaneous application of phthalocyanines and antimony trioxide in SBR and NBR (20) or creating hybrid halloysite nanotube-phthalocyanine filler (21) and finding the best proportions between pigments and other compounds of rubber mixture (22). In the paper provided by Pająk et al. (20) it was reported that the presence of an additional flame retardant in a rubber mixture significantly increases thermal stability and reduces flammability of a final product. However, the antimony trioxide used in this study was proven to cause cancer so its application has been strictly limited (23). Mg(OH)₂ and Al(OH)₃ are both non-toxic substances that when exposed to elevated temperatures have a tendency to detach water molecules, greatly enhancing heat capacity of a system resulting in limiting flammability and increasing thermal stability of a material. They have been both successfully used in elastomeric composites by Barta et al. (24), Patil et al. (25), Chen et al. (26) and others. As the cross-linking process determines all of the properties of a vulcanizate including flammability and thermal stability, as it was previously described, for example, in a paper by Ahmed et al. (27) or by Rybiński et al. (28), investigating different cross-linking agents may provide valuable information.

Figure 1:

Structures of zinc (A) and chloroaluminum (B) phthalocyanine.

In this paper the results of cross-linking degree determination, mechanical properties, aging coefficient, thermal analysis (TG, DTG, DTA) and flammability tests (oxygen index, time of combustion in air) of vulcanizates consisting of acrylonitrile-butadiene (NBR) or styrene-butadiene rubber (SBR), crosslinked by sulfur or dicumyl peroxide, filled with non-toxic flame retardant (Mg(OH)₂ or Al(OH)₃) and zinc or chloroaluminum phthalocyanines as a pigment are presented.

2 Materials

Pigments: zinc phthalocyanine (FZ) and chloraluminum phthalocyanine (FC) were synthesized at the Institute of Polymer and Dye Technology of the Lodz University of Technology. Phthalonitrile reacted with metal (zinc) or metal salt (aluminum chloride) according to the dry method described in the literature (29–31). The latter reaction occurred in a presence of sodium sulfate. Products of synthesis were subjected to 5% chloroacid treatment for 2 h while being constantly stirred. Then they were washed with distillated water until neutral pH was achieved, filtered and dried at 80°C for 24 h. Next, they were subjected to 5% sodium hydroxide treatment for 2 h and again washed with water to achieve neutral pH, filtered and dried at 80°C for 24 h. Prior to acidic-basic treatment, zinc phthalocyanine was kept in a solution of boiling 96% ethanol for 2 h. After cooling to 45°C it was filtered. Chloroaluminum phthalocyanine prior to acidic-basic treatment was washed away with hot water. After acidic-basic treatment, it was kept in 70% ethyl acetate at 70°C for 1.5 h. This process was done according to the one performed by A. Pająk (32). Phthalonitrille, zinc, aluminum chloride and sodium sulfate were purchased from Sigma-Aldrich Co. LLC. Chloroacid, sodium hydroxide, ethanol and ethyl acetate were purchased from POCh, Gliwice, Poland.

NBR (Perbunan^® 2255VP produced by Lanxess Deutschland Gmbh (Cologne, Germany), 22% of combined acrylonitrile) or SBR (Ker1500 produced by Synthos (Oświęcim, Poland), 23.5% of combined styrene) were used. They were both cross-linked with either dicumyl peroxide (0.3 phr – parts by weight per hundred parts by weight of rubber, purity 99.5%, produced by Merck-Schuchardt, Germany) or sulfur (1.5 phr, purity 99.5%, density ρ=2.07 g/cm³, S8 extracted from the mine in Tarnobrzeg, Poland), accompanied by activators: zinc oxide (5 phr, purity 99.5%, specific surface area: 5–7 m²/g, particles size 0.1–0.9 μm, produced by Huta Oława cat. 1, Oława, Poland) and stearic acid (1 phr, purity 98%, density ρ=0.86 g/cm³, produced by POCh, Gliwice, Poland), whereas in rubber blends containing sulfur, N-cyklohexyl-2-benzylsulfenamide (1.5 phr, purity 95%, Tioheksam CBS produced by POCh, Gliwice, Poland) was also applied as an accelerator. Although being a cross-linking activator, stearic acid also acted as a dispergator of fillers and softener of a rubber. As a coloring compound, zinc phthalocyanine or chloroaluminum phthalocyanine were applied (5 phr). These pigments impart a blue (chloroaluminum phthalocyanine) or blue-green (zinc phthalocyanine) color to the elastomeric composites. Vulcanizates cross-linked by sulfur were slightly darker than those in which dicumyl peroxide was used (Figure 2). Prior to application into rubber blends, phthalocyanines were grounded in a mortar alongside with zinc oxide. This process was done to ensure optimal dispersion of a pigment in a polymer matrix. According to Pająk (32), particle size of grounded compounds was 4.2 μm for FZ and ZnO or 4.7 μm for FC and ZnO. To maximalize flame retardancy and thermal stability, Mg(OH)₂ (Magnifin H5 produced by Albemarle Europe SPRL (Feluy, Belgium), purity 99.8%, specific surface area 4.0–6.0 m²/g, particle size 2.4–4.4 μm, density ρ=2.4 g/cm³) and Al(OH)₃ (Martinal OL-107 LEO produced by Albemarle Europe SPRL (Feluy, Belgium), purity 99.4%, specific surface area 6–8 m²/g, particle size 1.6–1.9 μm, density ρ=2.4 g/cm³) were applied to rubber mixtures simultaneously with all of the other compounds (20 phr).

Figure 2:

The picture of the obtained vulcanizates. White samples were cross-linked by dicumyl peroxide, light brown were cross-linked by sulfur. Dark blue samples contain chloroaluminum phthalocyanine, blue-green contain zinc phthalocyanine.

3 Methods

Elastomeric mixtures were prepared at room temperature with the use of a laboratory two-roll mill (David Bridge Ltd., Rochdale, UK) (dimensions: D = 200 mm, L = 450 mm). The rotational speed of the front roller was 20 rpm and the posterior roller 22 rpm, i.e. the friction ratio was 1.1. The mixtures were vulcanized in steel molds placed between electrically heated press shelves (home made). The optimal vulcanization time (τ_0.9) at the temperature of 160°C was determined by means of a WG-2 vulcameter (ZaCh Metalchem, Gliwice, Poland) according to PN-ISO 3417:1994, where τ_0.9 is the time to reach 90% of the ultimate torque value.

The equilibrium swelling method was used to determine the degree of cross-linking (α_c) of the vulcanizates. Samples were swollen in toluene (POCh, Gliwice, Poland) at the temperature T = 25°C for 48 h. Afterwards they were removed from the solvent, and toluene present on the surface was quickly blotted off. The samples were immediately weighed using a balance (RADWAG, Radom, Poland) and then dried in a vacuum oven (BINDER GmbH, Tuttlingen, Germany) for 36 h at 80°C to remove all the solvent and reweighed. The value of equilibrium swelling is calculated by the equation 1:

where Q_w is equilibrium swelling, m_sp is a mass of a swollen sample (mg), m_s is a mass of dried sample after swelling (mg), ms∗ is a mass of dried sample after swelling, corrected with the contribution of mineral substances (mg) (equation 2):

where m₀ is an initial mass of a sample (mg), m_n is a mass of mineral substances incorporated in the rubber mixture (mg), m_c is a mass of all rubber mixture components (mg). The degree of cross-linking α_c was calculated by the equation 3:

The result of equilibrium swelling was an arithmetical average of four determinations.

Thermal analysis of vulcanizates was carried out in air by means of Derivatograph MOM (Hungary), using approximately 90 mg samples at a heating rate of 7.9°C min^-1 within the temperature range from 20 to 800°C. Sensitivity of curves: DTA = 1/5, DTG = 1/30, TG = 100. DTA and DTG sensitivity (n^-1) means that the distance between two points on sensitized paper, corresponding to zero and the maximum deflection of the galvanometer mirror, was reduced n-times. Sensitivity TG = 100 means that the distance between two points on sensitized paper, mapping zero, and the maximum deflection of the mirror corresponds to a change of mass amounting to 100 mg.

Mechanical properties of the composites were determined by means of Zwick (Ulm, Germany), model 1435, accordingly to PN-ISO 37:1998, before and after thermooxidative aging. Thermooxidative aging was performed in a laboratory oven at 70°C for 24 h. The error of measuring S_e (stress at elongation) was ±0.01 MPa. The error of measuring T_Sb (tensile strength at break) was ±0.01 MPa. The error of measuring E_b (elongation at break) was ±1%. Aging coefficient S was calculated accordingly to the equation 4:

Where T_sba, T_sbb, are tensile strengths at the break after and before thermooxidative aging whereas E_ba and E_bb are elongations at the break after and before thermooxidative aging, respectively.

The combustibility of the vulcanizates was determined by the OI method, using an apparatus of our own design (33), accordingly to the PN-ISO 4589-2:1998 standard. We also measured the time needed for the whole sample to burn or to self-extuinguish under ambient air condition, using the same apparatus.

In the sample name, the first letter stands for the rubber used: S for styrene-butadiene rubber, N for acrylonitrile-butadiene rubber. The second letter stands for the substance used to cross-link the rubber: N for dicumyl peroxide, S for sulfur. The third and fourth letters stand for the applied flame retardant: Al for Al(OH)₃ and Mg for Mg(OH)₂. The fifth and the sixth letters, if present, stand for the pigment used: FC for chloroaluminum phthalocyanine, FZ for zinc phthalocyanine. Therefore, the sample named SNAlFC was made out of SBR, cross-linked with dicumyl peroxide, filled with Al(OH)₃ and chloroaluminum phthalocyanine.

4 Results and discussion

5 Conclusions

It was proven that the impartment of zinc or chloroaluminum phthalocyanine into SBR or NBR rubbers filled with non-toxic flame retardants, Mg(OH)₂ or Al(OH)₃, enables obtaining self-extinguishing, environmentally friendly rubber products. Investigated materials exhibited enhanced thermal stability. In the case of the SSAlFC sample, there was an improvement in mechanical properties in comparison to the non-colored composite, namely SSAl. Color of obtained vulcanizates varied from blue-green to dark blue. The composite based on acrylonitrile-butadiene rubber, cross linked by dicumyl peroxide, filled with Mg(OH)₂ and chloroaluminum phthalocyanine underwent self-exinguishment in ambient air in 35 s, with 94% of the original length of the sample untouched by the flame. The synergistic effect of simultaneous application of Mg(OH)₂ or Al(OH)₃ with either zinc or chloroaluminum phthalocyanine as a flame-retardant is based on different mechanisms of limiting flammability. Hydroxides undergoing thermal decomposition detach water molecules that dilute gaseous products of pyrolysis and increase the heat capacity of a system. Phthalocyanine pigments, facilitating carbonization processes, influence creating a barrier blocking the flow of mass and energy between flame, where oxidation processes occur, and a solid phase.