Introduction

Elastomers and materials made of them are commonly used in various spheres of life. Despite numerous ecological restrictions and considerable amounts of process and postconsumer wastes produced, often being difficult to manage, the world production and consumption of elastomers and related products have been systematically increasing since 2002 [1]. In this connection, studies concerning the effects of the composition of polymeric blends, mostly multicomponent products, on the properties of materials made of them are continually up to date and very important issue in both cognitive and practical terms. In recent years, one has observed a greatly increased interest in elastomeric materials with special properties, resistant to the action of both considerably lowered and elevated temperatures, flame-retardant, and characterized by appropriate mechanical strength. Of particular importance is a reduction in the flammability of polymeric materials because of serious health and live hazards posed by products emitted during fires as well as repeatedly huge material losses. Therefore, much attention has been paid to processes proceeding at elevated temperature and phenomena accompanying combustion, such as flame and glow propagation, heat emission, toxic substance formation, and smoke emission. Recently, a rapid development in the manufacturing field of new polymeric materials such as polymer nanocomposites has been observed [2–8]. The reduction achieved in the size of compound particles incorporated into a polymeric matrix from micrometers to nanometers enables us to obtain composites with better, often new properties [9, 10]. Commonly used nanofillers included laminar aluminosilicates modified with organic compounds. From a literature review, it follows that nanomaterials containing intercalated montmorillonite show better thermal and strength properties than those filled with conventional fillers. Nanometric dimensions are also shown by the macromolecules of keratin that, according to the scientific literature, can be used as filler of elastomers [11, 12]. Polymeric materials filled with keratin show good functional properties and after their exploitation period, they are bio-decomposed, which is of great importance from the ecological point of view.

This study presents the findings concerning the effect of keratin, derived from environmentally arduous waste from the tanning industry wastes, on the thermal properties and flammability of butadiene–acrylonitrile rubber (NBR) crosslinked by means of dicumyl peroxide with zinc oxide or nano-zinc oxide as crosslinking activators. Elastomeric-protein materials containing a modified montmorillonite were also examined.

Experimental

Under investigation was a butadiene–acrilonitrile rubber, NBR, (Perbunan 2255VP from Lanxess Deutschland Gmbh, containing 22 % of combined acrilonitrile, average mass particle viscosity M v = 268.900 g/mol, elements content: C = 82.93 %, H = 9.38 %, N = 5.59 %, S = 1.57 %, O = 0.53 %) crosslinked with the use of dicumyl peroxide, DCP (product of Merc-Schuhardt, 0.3 parts by wt.) in the presence of

  • zinc oxide, ZnO (5 phr)––sample denoted with N, and

  • nanometric zinc oxide, nZnO [13–15] (1 phr)––sample denoted with nN, as vulcanization activator and technical stearine (1 phr).

ZnO is a product from Huta Oława, first class, with surface area 5–7 m2 g−1 and particle size of 0.1–0.9 μm, and nanometric ZnO (nZnO)––from Nanostructured & Amorphous Materials Inc. (USA), with surface area 50 m2 g−1 and particle size of 130–165 nm, and spherical morphology.

As a filler, we used keratin (K) recovered from Kalisz Tannery Plant (Poland) with average particle size of about 165 nm and the main elements' contents: N = 15.4 % and S = 2.26 % [16]. Keratin was incorporated into elastomer in a quantity of 5, 10, and 30 phr. The composites containing keratin were, respectively, designated with the following symbols: N5 K, N10 K (with ZnO), and nN5 K, nN10 K (with nZnO).

Keratin was also incorporated into polymeric blends in the form of a mixture with ZnO prepared by careful rubbing of both components in a mortar. Composites containing the rubbed keratin–ZnO mixture are designated as N5 Kr, N10 Kr, and N30 Kr, respectively.

In addition, montmorillonite NanoBent ZS-1 (ZS) [7], made in Poland, modified with an ammonium salt containing OH groups, with an average particle size of 20–60 μm and layer displacement of 3.8–3.9 nm, was incorporated into N5 K blend in a quantity of 5 phr (sample designated as N5 KZS).

Rubber mixtures prepared with the use of a laboratory rolling mill at room temperature were vulcanized in an electrically heated press shelves at 160 °C for optimal vulcanization time τ 90, determined according to the standard: PN-ISO 3417:1994.

The crosslinking degree of composites, \( \alpha_{\text{c}} \)= 1/Q w, was determined according to equilibrium swelling (Q w) in toluene (PN-ISO 817:2001/ap1:2002).

Swelling measurements of vulcanizates in water (\( Q_{{{\text{H}}_{2} {\text{O}}}} \)) lasting for 48 h were carried out in the same way as in the case of swelling in toluene. In addition, water content in the swollen vulcanizates (\( Z_{{{\text{H}}_{2} {\text{O}}}} \)) was determined.

The mechanical properties of the tested composites were tested according to standard, PN ISO 37:1998, by means of Zwick tensile testing machine model 1435. Dumbbells with a measurement section width of 4 mm were used to determine tensile strength and elongation at break before (TSb1 and E b1) and after thermal-oxidative aging (TSb2 and E b2).

The thermal-oxidative aging resistance was investigated according to the standard, PN-88/C-04207. The aging coefficient, S, was determined from the following relationship:

$$ S = \frac{{{\text{TS}}_{\text{b1}} \cdot E_{\text{b1}} }}{{{\text{TS}}_{\text{b2}} \cdot E_{\text{b2}} }} $$

The measurements of broad-angle X-ray scattering angle (WAXS) in nanocomposites were carried out at room temperature by means of a Philips X-ray apparatus at the Lodz CMMS (Polish Academy of Sciences). Diffraction patterns were recorded within the angle range of 2Θ = 1–7°; measurement step 0.05°; measurement time 25 s; radiation Cu Kα (λ = 0.15418 nm); operation in the transmission mode.

The thermal analysis was preformed as under:

  • air atmosphere––90 mg samples were heated in a derivatograph furnace (MOM Budapest) at a heating rate of 7.9 °C min−1 within the temperature range from 20 to 800 °C using Al2O3 as a reference substance. Thermal curves' sensitivities were as follows: TG = 100, DTA = 1/5, and DTG = 1/20.

  • inert atmosphere––by means of differential scanning calorimetry [17] (DSC-204 Netzsh), using portions of about 5 mg at a heating rate of 10 °C min−1 within temperature ranges from 20 to −100 and −100 to 500 °C.

The flammability of vulcanizates was determined by the method of oxygen index using an in-house designed apparatus [18]. The flammability was also assessed on the basis of sample combustion time under air, or the time after which the sample was extinguished. The sample's shape and dimensions, its arrangement, and flame treatment time were the same as in the case of the oxygen index method (PN-ISO 4589-2).

Results and discussion

The composition of elastomeric blends exerts a small influence on the duration of their vulcanization time (τ 90) at T = 160 °C; nevertheless, under the influence of zinc oxide with nanometric dimensions, the crosslinking of NBR proceeds at slightly slower rate (Table 1). Regardless of the type of zinc oxide used, because of the influence of keratin, the degree of elastomer crosslinking (α c) increases as well as the swelling of vulcanizate in a hydrophilic solvent such as water (Table 1: \( Q_{{{\text{H}}_{2} {\text{O}}}} \), \( Z_{{{\text{H}}_{2} {\text{O}}}} \)). The increased water absorption of NBR vulcanizates filled with keratin indicates their increased susceptibility to bio-decomposition resulting from facilitated transport of enzymes (Table 1). The elastomer–protein composites show proper mechanical properties; although the increased keratin content causes their deterioration, it exerts no significant effect on the resistance of the materials to thermo-oxidative aging determined by coefficient S (Table 1). A clear improvement in the mechanical properties of the polymeric composites investigated occurs under the influence of the modified montmorillonite incorporated, N5 KZS (Table 1). The results of our study show that the presence of keratin does not impede the intercalation of the nanofiller ZS used by NBR as shown by the weakly marked, single peak in the X-ray curves of N5 KZS and NZS (vulcanizate N containing 5 phr of ZS) (Fig. 1).

Table 1 Influence of keratin on properties of NBR composites and nanocomposites
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Fig. 1
figure 1

XRD curves of NanoBent ZS, nanocomposites NZS (vulcanizate N containing 5 phr of ZS), and N5 KZS

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The results of derivatographic analysis under air show that the keratin used contains 4.4 % of adsorbed water and is protein resistant to the action of heat at elevated temperature. The indices of its thermal stability, as temperatures of sample 5 and 50 % mass loss, are T 5 = 210 °C; and T 50 = 410 °C [16].

Under neutral gas, the evaporation of water uncombined with keratin becomes visible in the high endothermic peak in the DSC curve at T = 112 °C (Fig. 2). Successive endothermic processes at T = 259 and T = 306 °C are connected with the thermal decomposition of the protein. The DSC curve shows no symptom connected with the glass transition of keratin. From the literature, it follows that the glass transition temperature (T g) of this biopolymer is affected by both the water contained in it and the content of α and β keratin [16, 19]. In this connection, T g is at ∆T = 30–130 °C, i.e., within the range of temperature, at which water evaporates (Fig. 2). Considering the considerable thermal effect caused by the evaporation of water physically combined with the polymer (change in enthalpy ∆H = 166 J g−1), the glass transition process resulting from its thermal capacity, shown by the changes in the position of thermal curve base line, is “masked”.

Fig. 2
figure 2

DSC curve of keratin

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NBR crosslinked with dicumyl peroxide under the influence of heating undergoes thermal crosslinking processes at two temperatures: ∆T 1 = 180–270 °C and ∆T 2 = 300–370 °C (Fig. 3). The thermal crosslinking within the lower temperature range proceeds due to the decomposition of hydroxyl groups contained in the elastomer and formed in it during heating. At ∆T 2 = 300–370 °C crosslinking proceeds as a result of crosspolymerization of butadiene mers contained in the macromolecules of the elastomer, which is accompanied by chain fragmentation and formation of small quantities of volatile products of their thermal decomposition (Fig. 3) [20]. At T ~ 400 °C, an intensive thermal decomposition of the elastomer begins, while the resultant residue combusts as shown by the exothermic peak recorded in the DTA curve at T = 545 °C. The use of nZnO in the vulcanization of elastomer does not change the character of its thermal transitions, but it increases the maximum rate temperature of its first stage of thermal crosslinking (Fig. 4). The increase in the temperature of the first thermal crosslinking stage of NBR also occurs under the influence of keratin incorporated into the elastomeric blend (Fig. 5). It indicates that both nZnO and keratin impede the formation of hydroperoxides, decomposition of which initiating the first thermal crosslinking stage of NBR occurs at a higher temperature.

Fig. 3
figure 3

DTA, TG, and DTG curves of N

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Fig. 4
figure 4

DTA, TG, and DTG curves of nN

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Fig. 5
figure 5

DTA, TG and DTG curves of N 5Kr

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The comparative analysis of the test results listed in Table 2 leads to a conclusion that the thermal stability of the crosslinked NBR, determined with index T 5, decreases under the influence of keratin. On the other hand, the presence of keratin does not change the elastomer thermal stability determined with index T 50. It should be, however, underlined that under the influence of the biopolymer used, regardless of the type of zinc oxide, the thermal decomposition rate of the elastomer–protein composite (dm/dt) obtained is considerably reduced, and the resultant residue after this process (P w) is increased (Table 2). It is very important from the point of view of reduction in the flammability of the composites investigated, as the change in both the parameters mentioned indicates that lower quantity of volatile and flammable thermal decomposition products passes to flame.

Table 2 Influence of keratin on thermal properties in air of NBR composites and nanocomposites
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Keratin contained in the peroxide vulcanizate of NBR does not change the character of the elastomer thermal transitions (Fig. 6; Table 3). A low content of protein (5 phr) decreases the glass transition temperature (T g) of the rubber, and consequently, slightly increases its resistance to the action of low temperatures. Such a quantity of keratin also facilitates the crosslinking of the elastomer tested under nitrogen beginning at T ~ 290 °C (Table 3; Fig. 6).

Fig. 6
figure 6

DSC curve of N5Kr

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Table 3 Influence of keratin on thermal properties in neutral gas of NBR composites
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The conclusions resulting from testing the thermal properties of the elastomer–protein composites investigated are confirmed by the measurements of their flammability. Regardless of the type of zinc oxide, under the influence of keratin contained in the peroxide vulcanizates of NBR, their flammability has been decreased as shown by both the tests of combustion in air (t s) and oxygen index (OI) (Table 4). The decreased flammability of NBR results, first of all, from a considerable reduction in its thermal decomposition rate (dm/dt) under the influence of keratin (Tables 2, 4) as well as the emission of nitrogen during vulcanizate destruction. Nitrogen as nonflammable gas considerably decreases the free-radical reactions of combustion in air and also reduces the diffusion of oxygen to the flame zone. In our opinion, the reduction in the flammability of the composites obtained is connected with nanometric size of keratin particles [21], but it can also result from the chemical structure of peptide chains twisted in helixes that are combined by mono- and disulfide cystine bridges [22]. The method of incorporating the biopolymer into the elastomeric matrix (separately or in the form of a mixture with zinc oxide) does not exert any significant influence on the flammability of the composites obtained.

Table 4 Flammability of NBR composites and nanocomposites containing keratin
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Conclusions

The composite thermal stability and flammability characteristics depend on the method of composite preparation and the quantity of added keratin.

The waste keratin used reduces the flammability of crosslinked nitrile rubber.

Regardless of the type of zinc oxide, the addition of waste keratin improves the crosslinking of NBR rubber by means of dicumyl peroxide.

The elastomer–protein composites show appropriate mechanical properties. However, a clear improvement in the composite strength parameters occurs under the influence of the modified montmorillonite incorporated.

The materials obtained are characterized by increased water absorption. The water content in swollen samples proportionally increases with the keratin content in the composites. This suggests an increased susceptibility of these materials to biodegradation in the environment after their exploitation period.