Except carbon black, silica is one of the most important filler used in the rubber industry. Fillers impart appropriate processing capabilities to elastomers and defined usability to end products. They exert a significant influence on the thermal stability and flammability of elastomers. Active silica is commonly used to prepare rubber mixes and finally rubber goods.

The properties of elastomeric mixes and end products depends on the polymeric matrices, content of a filler, size and a shape of its primary particles, aggregates and agglomerates, their distribution in a matrix, a content of loosely and tightly bounded rubber and structure formed from interacting fillers particles [1].

It is assumed, that big agglomerates (above 20 μm), composed of loosely packed aggregates exert particularly detrimental effect on properties of vulcanizates. Under external stresses they break, initiating formation of microcracks, whose propagation in matrix leads finally to a rubber rupture.

Problem agglomerates of silica has been minimized by specific additives, mixing procedures, or by means of a new cryogenic dezaggregation method modification of mineral fillers and by using silane-coupling agents [2, 3].

The present article presents the results of studies concerning the thermal stability and flammability of cross-linked butadiene–acrylonitrile rubber containing untreated silica, and silica modified by cryogenic method.

This article shows also a fragment of research concerning the flammability and thermal stability of vulcanizates containing silica formed in situ from silica precursors.

Our investigation was focused on cross-linked butadiene–acrylonitrile rubber, Perbunan 1845 (NBR 18) containing 18% acrylonitrile units. The rubber was cross-linked by means of dicumyl peroxide from Merck-Schuhardt, and sulphur with accelerating of curing (MBT). Two types of silica with different specific surfaces were used as fillers: Ultrasil VN-3 (234 m2 g−1) and Zeosil 175 C (175 m2 g−1).

The cryogenic modification of silica (Ultrasil VN-3 and Zeosil 175C) consisted in its out-gassing and saturation with aqueous solution of dispersing agent and next freezing all system by the use of liquid nitrogen. Then, the modified fillers were dried to a constant mass.

The silica filler was synthesised from tetraethoxysilane (TEOS, from Aldrich) in medium of NBR 18 rubber cross-linked with dicumyl peroxide.

Methods

Rubber mixes were prepared with the use of a laboratory two roll mill at room temperature and were vulcanized in an electrical press at 160 °C for a time τ0.9, which was determined by means of WG2 vulcameter according to PN-ISO-3417:1994.

The modification with teraethoxysilane was carried out by swelling the peroxide vulcanizates of NBR 18 in TEOS for 48 h at a temperature of 30 °C. To perform the sol–gel process, the swollen shaped samples were immersed in a 10% aqueous solution of ethanoldiamine at T = 30 °C. After 24 h the vulcanizates were thoroughly rinsed with water and dried to a constant weight at T = 60 °C.

SEM examinations of the modified fillers, and dispersion of the filler synthesized in the medium of elastomers were carried out by means of Jeol scanning microscope. The sample surfaces were dusted with gold.

The surface of vulcanizates containing silica was assessed from photographs taken by means of Metrology Series 2000 camera from Molecular Imaging (USA) in contact mode at a scanning frequency of 1–4 Hz. Samples for measurements were vulcanized in a steel mould. A glass plate was inserted into the mould cavity to obtain vulcanizate surfaces with a low roughness. Prior to insertion, the surface of glass plate was washed into acetone and dried with stream of air to degrease and remove impurities. AMF photographs were analysed with the use of the WSxM program from Nanoyec Electronica S.L. (Spain).

Thermal analysis was performed in air by means Paulik, Paulik, Erdey derivatograph at the temperature ranging from 20 to 800 °C using weighted portions 90 mg and a heating rate of 7.9 °C min−1.

The flammability of vulcanizates was determined by the method of oxygen index according to PN-EN-ISO 4589-2:2006. The flammability in air was also tested, using the same specimens as in the case of oxygen index. In both cases, the same samples with dimensional 100 × 10 × 4 mm, were used in a vertical position. They were ignited for 15 s. with the use burner supplied with propane–butane mixture. In measurements in air, the time of sample combustion or time, after which samples were self-extinguished, was measured.

Experimental

The presence of silica does not change the character of thermal transformations of nitrile rubber cross-linked with dicumyl peroxide (Figs. 1, 2). The first two exothermic processes recorded in the DTA curves at about 200 and 335 °C are connected with the thermal cross-linking of elastomers.

Fig. 1
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Thermal curves of peroxide vulcanizate of NBR 18, 18N

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Fig. 2
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Thermal curves of peroxide vulcanizate of NBR 18 containing 30 phr of Ultrasil VN3 30

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The thermal decomposition of vulcanizates begin at T > 390 °C for 18N as shown by the endothermic peaks at T = 420 and 465 °C (Fig. 1) and at T = 435 and 470 °C (Fig. 2). These processes proceed without the contribution of oxygen since the rate of formation of volatile destruction products is higher than that of oxygen diffusion to the reaction zone. The exothermic peak recorded at T = 535 °C (Fig. 1) and T = 525 °C (Fig. 2) is connected with the combustion of residue after the decomposition of cross-linked rubber (Tables 1, 2).

Table 1 Content of components in elastomer matrix in phr
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The results of derivatographic analysis show that the addition of silica clearly increases the thermal stability of silica-containing vulcanizates determined with T 50 as well as with the temperature of initial decomposition, T R, and temperature of the maximal rate of this process, T Rmax (Fig. 2; Table 2). It has been found that the maximal decomposition rate of the cross-linked elastomer, dm/dt, is also decreased under the influence of silica. In our opinion, this results from the polymer–filler interactions, whose mechanism is very complex and has not been sufficiently explained as yet. The immobilization of elastomer macromolecules adsorbed on the surface of solid phase decreases the amplitude of their thermal vibration and consequently also the probability of degradation and destruction [4].

Table 2 The results of thermal analysis of vulcanizates containing silica
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From the point of view of the reduction in vulcanizate flammability, a very important role is played not only by the mentioned decrease in the value of dm/dt, but also by the increase in the residue, Pw, after the thermal decomposition of vulcanizate, especially in the residue resulting from the destruction of cross-linked elastomer, thus Pe (Table 2). If the increase in the value of Pw under the influence of silica results from the decreased content of elastomer in the sample under analysis, the increase in Pe is due to the polymer–filler interaction.

The decrease in the flammability of cross-linked vulcanizate in the presence of silica is brought about by the considerable reduction in the thermal decomposition rate of polymer. Thus, smaller quantities of volatile and flammable products of destruction pass to flame, the more so as part of them is adsorbed on the filler surface.

During combustion of the vulcanizates filled with silica, non-flammable gaseous products can be formed, first of all, in the form of water vapour. From the thermal analysis it follows that the examined silica samples contained 5.6% of water. Water is adsorbed on the functional groups on the silica surface. These groups include single ≡Si(OH) and twin =Si(OH)2 silane groups and siloxane bridges =Si(OH)–O–Si(OH)=. During combustion, a gradual desorption of water takes place, the evaporation of water decreases the energetic balance of the system and water vapour dilutes the flammable gaseous product passing to flame.

From the point of view of the reduction in vulcanizate flammability, a very important role is played also by the structure of the boundary layer being formed during combustion of the filler-containing vulcanizate [5, 6]. Under the influence of silica, it becomes more thermally stable and impedes the flow of mass and energy between flame and sample.

In the case of NBR vulcanizates, the boundary layer is thermally stable because is formed from partially carbonized elastomer. Inert and insulating char locks up the fuel for further combustion and also provides insulation to inner layers from further heating [7].

The boundary layer is particularly efficiency in reduction of vulcanizate flammability if it’s formed by the use of nano-particles of filler (Table 3).

Table 3 The results of flammability measurements of vulcanizates containing unmodified and cryogenic modified silica
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In our laboratory a cryogenic method of hydrophilic mineral fillers dezaggregation has been elaborated. It is based on the following assumptions. During aggregation of primary particles of fillers empty voids, pores, inside aggregates are formed. Typically pores have size in the order of a few nanometers. They are accessible to water vapour particles. The water introduction into the nanopores of silica and freezing of the system bring about shattering of aggregates structure as a results of an increase of water volume during freezing. However, from our studies it follows that reaggregation of filler primary particles during evaporation of adsorbed water at elevated temperature take place. To avoid reaggregation we have applied an appropriate intercalating agent [8].

Our method of a filler modification and application of adequately chosen dispersing agent enables decrease in size of its particles which greater part is contained in nano range (Fig. 3).

Fig. 3
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SEM photo a unmodified Zeosil 175 C, b modified Zeosil 175 C, c unmodified Ultrasil VN-3, d modified Ultrasil VN-3

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The expected reduced flammability of vulcanizates containing silica, has been confirmed by the measurements of flammability by the method of oxygen index and in air. With the increase in silica content, the value of OI increases and the duration of samples combustion grows longer, but the time, after which samples extinguish in air is shortened (Table 3).

The vulcanizates containing modified silica show the smallest OI value and the time after which samples extinguish in air is smaller for vulcanizates containing modified silica in comparison with samples containing untreated filler (Table 3).

The results obtained by means of AFM show that the modified silica is considerably better dispergated in polymer matrix than unmodified filler (Fig. 4).

Fig. 4
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AFM photographs of vulcanizate containing Ultrasil VN-3: a silica unmodified, b silica modified by cryogenic method

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The conventional untreated silica, e.g. Ultrasil VN-3 or Zeosil 175 C due to its significant polarity, large specific surface and present silanol groups on its surface, is highly aggregated in the rubbery matrix.

This problem has been minimized by specific additives or mixing procedures [1], and by using silane-coupling agents [2].

A alternative method of silica incorporation into rubbers is the “in situ” sol–gel process. Two reactions are generally used to describe the hydrolysis and condensation of silica precursors.

  • Hydrolysis:

    $$ \equiv {\text{Si}}{-}{\text{OR}} + {\text{OH}}_{2} \rightleftharpoons \equiv {\text{Si}}{-}{\text{OH}} + {\text{ROH}} $$

  • Condensation:

    $$ \equiv {\text{Si}}{-}{\text{OH}}\,+ \equiv {\text{Si}}{-}{\text{OH}} \rightleftharpoons \equiv {\text{Si}}{-}{\text{O}}{-}{\text{Si}} \equiv +\,{\text{OH}}_{2} $$

This simple process involves the swelling of a raw rubber [9] or rubber vulcanizates [2, 10] in a silica precursor, e.g. tetraethoxysilane or tetramethoxysilane.

From the analysis of SEM images of the NBR 18 composites it follows that the silica obtained in the sol–gel process possesses nano-metric dimensions (Fig. 5).

Fig. 5
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SEM image of peroxide vulcanizates of NBR 18 containing silica formed “in situ”

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Based on the derivatographic analysis, it has been found that the silica obtained neither alters the character of thermal changes in the cross-linked NBR 18 nor exerts any significant influence on the thermal stability of vulcanizates specified with parameters T 5, T 50 and T R. It seems to result from the small specific surface of silica formed “in situ”, and consequently from slight polymer–filler interactions (Figs. 1, 6).

Fig. 6
figure 6

Thermal curves of peroxide vulcanizate of NBR 18, 18N–SiO2, containing silica formed in situ

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It should be, however, clearly stressed that the presence of the filler formed “in situ” considerably decreases the rate of the thermal decomposition of cross-linked nitrile rubber, dm/dt, which indicates its positive influence on flame retardation in relation to unmodified samples, as a lower quantity of volatile destruction products, including flammable compounds, passes to the gas phase of combustion, i.e. to flame.

Vulcanizate 18NS contains 5 parts by weight of silicone oil/100 parts, added during the mix preparation in order to reduce the polarity of the polymeric matrix. Our test results show that this vulcanizate contains the highest quantity of silica formed “in situ” (Table 4).

Table 4 Influence of silica formed “in situ” on thermal properties of vulcanizates of NBR 18 rubber
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The results of flammability tests show that vulcanizate 18NS–SiO2 containing the highest quantity of silica among the sample tested is characterized by the highest value of oxygen index (OI). We think that the significant reduction in flammability determined by both the OI and combustion time in air of the vulcanizate tested, in which the content of silica formed in situ is relatively low, results from barrier effect (Table 5). The isolating boundary layer formed by silica considerably limits the flow of mass and energy between the solid and gas phase of the burning sample.

Table 5 Influence of silica formed “in situ” on the flammability of vulcanizates
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It has been found that the network structure of nitrile rubber exerts a clear influence on the flammability of its vulcanizates containing silica formed “in situ”. The presence of filler manifests itself especially clearly in the case of peroxide vulcanizates, whose samples are self-extinguishing in air. On the other hand, the combustion time in air of sulphur vulcanizates is clearly prolonged. The smaller influence of the “in situ” silica on the flammability of sulphur vulcanizates in comparison with peroxide vulcanizates results from the lower content of filler (Table 5).

Conclusions

The polymer–filler interactions increase the thermal stability of nitrile rubbers cross-linked in the presence of silica and reduce their flammability.

A considerable reduction in the flammability of nitrile rubber vulcanizates can be obtained by the use of modified silica.

All the vulcanizates containing cryogenic modified silica are self-extinguishing.

The modification of the vulcanizates with tetraethoxysilane that makes it possible to form silica “in situ” considerably reduces the flammability of cross-linked rubbers as a result of the formation of a thermally stable, isolating boundary layer, which impedes the flow of matter and energy between the solid and gas phase of the sample under burning.