Polymeric compounds based on thermoplastic elastomer styrene-butadiene-styrene block copolymers and siliconic rubber powder

Abstract

This paper presents the obtaining and characterization of polymeric composites based on thermoplastic elastomer type styrene-butadiene-styrene block copolymers and vulcanized rubber powder. The rubber powder used was obtained as waste in the technological processes in the rubber industry. It was analyzed by determining the acetonic extract, ash and FTIR analysis and it was observed that the base elastomer in the rubber waste is silicone rubber, and the amounts of inorganic fillers, plasticizers, antioxidants, lubricants etc. existing in the rubber powder are lower than 19%. The samples were obtained by mixing technique, on Brabender Plasti-Corder internal mixer type. The test specimens necessary to determine the characteristics were obtained on a laboratory electrical press at 170°C, applying a force of 300 kN, and moulding time 6 min. The characteristics of the obtained samples show that the addition of rubber powder improved the hardness and elasticity, and by the applying compatibility method, namely crosslinking and grafting in melt, in dynamic regime, there was a significant increase in hardness, elasticity, modulus 100%, tensile strength, and the material obtained is more compact. The new composites have physical and mechanical properties suitable for producing a wide range of rubber consumer goods.

Keywords

Polymeric composites rubber powder thermoplastic elastomer Fourier transform infrared spectroscopy physico-mechanical characteristics

Introduction

In the rubber industry, one of the main operations in the technological flow of obtaining rubber consumer goods is the vulcanization. This operation consumes a lot of energy (because the process takes place at high temperatures and under high pressing forces), leads to the release of harmful gases and generates large amounts of rubber post-production waste (the waste includes outflows from vulcanization moulds and production shortages). These types of waste are non-biodegradable and can seriously affect the natural environment and human health. For these reasons, the restrictive legislative regulations of the European Union require the superior recovery of these types of waste through recycling, grinding, devulcanization, reuse in obtaining new products, etc.^1–3 In general, these rubber post-production wastes are grounded and sieved when rubber powder is obtained (with rubber particles of a certain size), and then the rubber powder obtained is used in the rubber and plastics industry to obtain composites intended for various applications such as: shoe soles, windshield wipers, housings, trolley wheels for transport, mats for of gyms and playgrounds, bicycle pedals, etc.^3–7

Several studies have been performed on the use of rubber powder as a filler in the production of polymeric composites with thermoplastic matrix, in which the thermoplastic materials used were: polypropylene (PP),⁸ high density polyethylene (HDPE),⁹ ethylene vinyl acetate copolymer (EVA),¹⁰ polyamide, thermoplastic polyurethane⁵, polyvinyl chloride,^11,12 etc. These polymer composites can be processed into finished products by specific methods of plastics, and their mechanical properties depend on the type of polymer matrix used, the nature of the interactions between the thermoplastic matrix and rubber powder, the type and size of rubber granules, the obtaining method of rubber powder (by cryogenic grinding or at room temperature), the amount of rubber powder in composites, etc.^3,13,14 It has been observed that these polymeric composites are generally characterized by a low adhesion between the thermoplastic matrix and the rubber powder and that the mechanical properties decrease with increasing amount of rubber powder and are improved by using a rubber powder with very fine grain sizes, namely 0.25–0.50 mm.^14–16 To improve the composites properties, various methods of compatibility between the thermoplastic matrix and the rubber particles can be used, such as the physical or chemical modification of the rubber powder surface or the use of some compatibilizing agents.^13,17,18

In this paper, new types of polymeric composites are presented in which the matrix consists of a thermoplastic elastomer of the styrene-butadiene-styrene block (SBS) type, and the reinforcing filler of cryogenically grounded rubber powder, with particle sizes below 0.5 mm. In order to improve the properties of the composites, the rubber powder was mixed with polydimethylsiloxane (PDMS) in order to improve its dispersion in the polymer matrix, and di-2-tert-butylisopropyl benzene and polyfunctional monomer - trimethylolpropane trimethacrylate was used in order to initiate grafting and crosslinking reactions in the melt, in dynamic regime, leading to enhanced adhesion between the two phases.

Experimental

Materials

The following materials were used: (1) thermoplastic rubber (TR) SBS K03/65/9000, manufactured by KIK Compounds, Targoviste, Romania. The TR granules used, in addition to SBS rubber, also contain fillers, extenders, additives and other resins; (2) waste in the form of black rubber powder, cryogenically grounded, from RONERA Rubber S.A., Bascov, Romania, with particle sizes smaller than 0.5 mm; (3) linear PDMS fluids - silicone oil, from Sigma-Aldrich, Inc, USA; (4) Luperox F40 di-2-tert-butylisopropyl benzene (40% active substance content) from Alkema; (5) polyfunctional monomer trimethylolpropane trimethacrylate (TMPTMA) Alcanpoudre TMPT MA 70 (70% active substance and 30% precipitated silica) from Safic Alcan.

Obtaining compounds

Formulations are presented in Table 1

Table 1.

Formulations.

Ingredients	Symbol
Ingredients	M	N20	N20+5P	PT/N20	PT/N20+5P
TR KIK compounds K03/65/9000, g	100	80	80	80	80
Black powder, g	—	20	—	20	—
Black powder with 5% PDMS, g	—	—	20	—	20
Luperox F40P, g	—	—	—	1	1
Alcanpandre TMPTMA 70, g	—	—	—	1	1

For the N20+5P and PT/N20+5P blends, the rubber powder was mixed with PDMS in order to avoid the agglomeration of the powder particles in the SBS type thermoplastic elastomer polymer matrix. Thus, for 120g of black powder, 6.3 g of PDMS (representing 5% by mass) were added and were well homogenized and put in a hot air oven at 60°C for 6 h, homogenizing every 30 min. In addition, in order to obtain blends with improved properties, a copolymer of the SBS-PDMS type was aimed for, with the role of a compatibilizing agent, at the interface between the continuous phase of SBS and the dispersed phase of the rubber powder, through grafting and crosslinking reactions in dynamic regime. For this, as Table 1 shows, the PT/N20 and PT/N20+5P blends contain an agent for initiating grafting and crosslinking reactions (di-2-tert-butylisopropyl benzene) and a polyfunctional monomer (trimethylolpropane trimethacrylate) which possessed high reactivity; this can undergo free radical grafting reaction on the polymer chain producing a network structure.^19,20

The compounds were obtained by mixing technique, on Brabender Plasti-Corder internal mixer type, at 150°C for 7 min. The rotor speed was 30 r/min in the first 2 min and increased to 80 r/min for the next 5 min. Homogeneous blends were obtained from which the test pieces were obtained.

Obtaining test specimens

The modelling of the specimens for the determination of the characteristics was carried out using specific moulds and the laboratory electrical press Fortune Presses TP 600 manufactured by Fontijne Grotnes, Vlaardingen, The Netherlands. The processing parameters were: temperature 170°C, pressing force 300 kN and moulding time 6 min, followed by the cooling stage down to 45°C, for 10 min and 300 kN pressing force.

Specimen characterization

Tensile strength, modulus 100%, elongation at break and residual elongation tests were carried out according to the conditions described in ISO 37/2020, on dumb-bell shaped specimens of type 2. Tearing strength tests were carried out using angular test pieces (type II) according to EN 12771/2003. Tests were carried out with a Schopper1445 strength tester with testing speed 500 mm/min.

Hardness was measured in °Sh A (scale specific for low hardness elastomeric materials), by using 6-mm thick samples and a hardness tester according to ISO 48-4/2018.

Resilience was determined according to ISO 4662/2017 with a Schob test machine using 6-mm thick samples.

Determining abrasion resistance was carried out according to ISO 4649/2017 the cylinder method, using a force of 10 N. Abrasion resistance was expressed by relative volume loss in relation to calibrated abrasive paper. The samples used were obtained from rolled blends and pressed, by cutting with a rotating die and have cylindrical shape, with a diameter of 16 mm and height of min. 6 mm.

The densities of samples were measured according to ISO 2781/2018.

Accelerated ageing trial was carried out according to ISO188/2010 using the hot air circulation oven method. Test duration was of 7 days and temperature of 70 ± 1°C. The results were compared with those from samples not subjected to ageing.

In order to establish the composition of the rubber powder, the ash was determined according to ISO 247-1/2018 and the acetonic extract according to ISO 1407/2011 method B.

Fourier Transform Infrared Spectroscopy (FTIR) spectra of rubber powder and samples were obtained using Nicolet iS50 FT-IR spectrophotometer in the wave number ranging from 400 cm⁻¹ to 4000 cm⁻¹.

Results and discussion

Characterization of rubber powder

The rubber powder used to obtain the composites was generated during the cryogenic deburring operation of some rubber consumer goods and has dimensions smaller than 0.5 mm. According to the literature,^1,3 the rubber powder obtained by cryogenic grinding shows a low oxidation level, the particle shape is regular and the surface area is smooth/non-developed, unlike the rubber powder ground at ambient temperature, which has a high level of oxidation, the shape of the particles is irregular and the surface area is spongy/well-developed. Along with these characteristics, in order to obtain polymer composites containing rubber powder, it is important to know the filler content and the type of rubber in the powder.^3,13,14 For these reasons, the rubber powder was tested in order to establish the composition by: determining the acetonic extract, determining the ash and FTIR analysis.

The value obtained from the acetonic extract was 9.87%, representing: the amounts of antioxidants, resins, mineral oil, fatty acids, plasticizers, wax, lubricants existing in the powder. For ash, the determined value was 8.9%, representing: the amounts of inorganic compounds in the powder such as metal oxides, inorganic dyes or inorganic fillers. In conclusion, the powder used has a high elastomer content.

In order to identify the type of rubber in the powder and its chemical composition, FTIR analyses were performed. Figure 1 shows the FTIR spectrum of the black rubber powder and Table 2 the correspondence between the bands in the FTIR spectrum and the chemical groups. FTIR spectra revealed the existence of bands characteristic of silicone rubber, which are associated with (1) the Si-O-Si stretch in the main chain of silicone rubber, which appears at 1081 cm⁻¹, (2) the band from 1009.84 cm⁻¹ and 1014.96 cm⁻¹ due to Si-O-Si and Si-O-CH₃ from main chain, (3) the bands of 2959.81 cm⁻¹, 2916.02 cm⁻¹ and 2848.15 cm⁻¹ which can be attributed to the symmetrical and asymmetrical stretch of C–H in methyl and methylene groups, respectively, (4) the appearance of a band at 1257.14 cm⁻¹ corresponding to the deformation vibration of the CH bond in Si (CH₃)₂, (5) the band from 796.25 cm⁻¹ specific to the Si-C and C-H bond, and the band from 697.89 cm⁻¹ due to the group Si(CH₃)₃.^21–24

Figure 1.

FTIR spectra of vulcanized rubber powder: (a) 2700-3000 cm⁻¹, (b) 600-1300 cm⁻¹.

Table 2.

Correspondence between FTIR spectrum bands and appropriate chemical groups for rubber powder.

Absorption band, cm⁻¹	Appropriate chemical groups
2959.81 cm⁻¹, 2916.02 cm⁻¹ and 2848.15 cm⁻¹	C-H symmetrical and asymmetrical in the -CH₃ and -CH₂ group
1257.14 cm⁻¹	Deformation -C-H from Si-CH₃
1081.47 cm⁻¹	Si-O-Si asymmetric stretching
1009.84 cm⁻¹ and 1014.96 cm⁻¹	Si-O-C and Si-O-Si linear from main chain
1009.84–1014.96 cm⁻¹ and 962.95–907.44 cm⁻¹	CH₂ wagging vibration of Si-CH=CH₂
796.25 cm⁻¹	Si-C and -CH₃ from Si-CH₃
697.44 cm⁻¹and 697.89 cm⁻¹	Si-O from O-Si-(CH₃)₃

Plastograms characteristics

The composites were obtained by mixing technique, on Brabender Plasti-Corder internal mixer type, which records the variation of torque and temperature versus time. Figure 2 shows the variation of the torque in time when obtaining the blends. This show that for all the composites, the maximum torque is obtained when melting the TR granules, after which, it decreases. There is another increase in torque when the rotational speed is increased from 30 r/min to 80 r/min, after which the torque decreases, except for the PT/N20 blend where there is an increase in torque, between minute 3 and minute 4, which is due to crosslinking and grafting of the elastomers in the blend. In the PT/N20+5P blend, containing peroxide and polyfunctional monomer, this increase in torque is not observed due to the presence of PDMS which acts as a plasticizer and reduces the viscosity of the blend, but the final torque values of this blend are higher than those observed in the N20+5P blend (similar composition but does not contain grafting and crosslinking agents) and are due to crosslinking and grafting under the influence of peroxide, leading to a three-dimensional network that increases the viscosity of the blend, in this way the torque recorded on Brabender Plasti-Corder increases.^18,25 The temperature during the obtaining of the blends (Figure 3), decreases initially as a result of the introduction in the mixer of the ingredients at room temperature, after which it increases due to the temperature in the mixing chamber (150°C) as well as the increase of the shear moment.

Figure 2.

Variation of torque versus time when obtaining the mixtures.

Figure 3.

Diagrams showing the variation of the torque and the temperature, when obtaining the mixtures on Plasti-Corder Brabender: (a) N20 sample, (b) N20+5P sample, (c) PT/N20 sample, (d) PT/N20+5P sample.

Table 3 shows the parameters recorded during the process of obtaining the blends. A correspondence can be observed between the temperature variation range and the torque variation range and the fact that the specific energy increases by adding rubber powder or by adding crosslinking and grafting agent, but decreases for blends containing PDMS-functionalized rubber powder. The viscosity of the melted homogeneous blends was calculated with the formula (1)²⁶

η (poise) = \frac{450 M (Nm)}{S (rpm)}

(1)

where M is torque and S is speed of the rotors (in our case it was 80 rpm). Viscosity measurement was appropriate for mixing from 6 to 7 min, because during this period constant torque was obtained (see Figures 2 and 3) and the blends are homogeneous. It is observed that the viscosity of the blends increases by 50% by introducing rubber powder and decreases for blends containing PDMS and rubber powder (by 20.85% and 44.75%, respectively), indicating that PDMS had a plasticizing role that lower the viscosity of melt blends.

Table 3.

The characteristics recorded on Plasti-Corder Brabender during the mixing.

Sample	Specific energy, (kNm/g)	Melting time, (seconds)	Gelling rate, (Nm/min.)	Torque variation range, (Nm)	Temperature variation range, (°C)	Melt viscosity (poise)
Control	0.3	122	243.1	31.1–95.1	97–163	180
N20	0.5	126	346.3	47–102.4	104–172	270
N20+5P	0.3	70	302.3	28.2–59.4	119–165	213.7
PT/N20	0.6	—	—	51.9–102.5	99–186	427.5
PT/N20+5P	0.4	116	281.9	39–56.4	121–163	236.2

The reaction mechanism

By adding a small amount of crosslinking agent di-2-tert-butylisopropyl benzene and a vulcanization coagent like polyfunctional trimethylolpropane trimethacrylate monomer type, the partial grafting and crosslinking of PDMS and SBS was intended with the formation of a copolymer at the interface between the dispersed phase of the rubber powder and the continuous phase of SBS thermoplastic elastomer. Figure 4 shows the structural formulas of SBS thermoplastic elastomer (Figure 4(a)) and polydimethylsiloxane (Figure 4(b)), which are either in the form of fluids - silicone oil, or as the base elastomer of vulcanized rubber powder. It is very likely that the grafting and cross-linking reactions by radical mechanism take place for SBS, especially at the double bonds existing in the aliphatic phase.²⁷ respectively to the C-H bond of the - CH₃ group for PDMS.^28,29

Figure 4.

Structural forms of SBS (a) and PDMS (b).

In Figure 5, a reaction mechanism is proposed that shows some of the reactions that could take place during crosslinking and grafting in a dynamic regime, in the SBS melt. The first stage is the initiation of the reactions, when the initiator decomposes at the high temperature existing in the mixer, and forms free radicals - very reactive species. They extract hydrogen atoms from the polymer materials existing in the system, such as SBS or PDMS, forming larger free radicals - macroradicals (SBS•, PDMS•). By addition reactions, larger macroradicals can be obtained (SBS-SBS•, SBR-PDMS•, PDMS-PDMS•). Obtaining the reaction final products can be achieved both through hydrogen transfer reactions and through reactions combining two macroradicals. It can be observed that through these reactions in a dynamic regime, in the SBS melt, initiated by reactive species (initiators), both crosslinking reactions of the SBS elastomer in the polymer matrix, as well as grafting and crosslinking reactions at the interface between the continuous and the dispersed phase can take place, with the formation of copolymers of the SBS-PDMS, SBS-PDMS-SBS type etc. They can have the role of compatibilizing agents and reduce the interfacial tension coefficient, in order to obtain the desired degree of dispersion, respectively forming and the stabilization of the desired morphology. In this way, the adhesion between the phases is improved and the performance of the obtained materials will be enhanced.^25,30

Figure 5.

Proposed mechanism for peroxide crosslinking and grafting of SBS/PDMS/vulcanized silicone rubber powder blends.

Physico-mechanical characteristics of the samples

The physical-mechanical characteristics of the blends in normal state and after accelerated aging are presented in Table 4.

Table 4.

Physico-mechanical characteristics of the samples.

Characteristics	Symbol
Characteristics	Control	N20	N20 + 5P	PT/N20	PT/N20 + 5P
Normal state
Hardness, °ShA	64 ± 1	74 ± 0.58	73 ± 0.6	81 ± 0.56	78 ± 0.6
Elasticity, %	12 ± 0.14	14 ± 0.35	12 ± 0.17	20 ± 0.4	24 ± 0.14
Modulus 100%, N/mm²	1.08 ± 0.02	1.56 ± 0.03	1.64 ± 0.3	3.6 ± 0.24	3.85 ± 0.15
Modulus 300%, N/mm²	2.16 ± 0.01	2.85 ± 0.09	2.74 ± 0.17	—	—
Tensile strength, N/mm²	4.5 ± 0.09	3.11 ± 0.13	3.1 ± 0.11	3.6 ± 0.43	5.32 ± 0.38
Elongation at break, %	580 ± 0.58	360 ± 23.09	380 ± 0	100 ± 11.55	160 ± 20
Residual elongation, %	36 ± 0	28 ± 0	32 ± 0	20 ± 0	20 ± 1.15
Tear strength, N/mm	25 ± 0.19	25.8 ± 0	26.15 ± 0.21	17.4 ± 1.27	19.1 ± 0
Density, g/cm³	1.03 ± 0.006	1.04 ± 0.005	1.02 ± 0.02	1.07 ± 0.006	1.07 ± 0.006
Abrasion resistance, mm³	179.82 ± 7.96	236.68 ± 1.55	204.26 ± 10.31	312.11 ± 4.27	202.12 ± 4.32
After accelerated aging for 168 h at 70°C
Hardness, °ShA	68 ± 0.57	75 ± 0.58	75 ± 0	82 ± 0.58	80 ± 0.57
Elasticity, %	12 ± 1	14 ± 0.8	14 ± 0.46	22 ± 0.56	22 ± 0.05
Modulus 100%, N/mm²	1.28 ± 0.47	1.75 ± 0.02	1.55 ± 0.03	—	—
Modulus 300%, N/mm²	2.20 ± 0.14	2.97 ± 0.13	2.59 ± 0.04	—	—
Tensile strength, N/mm²	3.99 ± 0.08	3.12 ± 0.04	2.99 ± 0.07	3.33 ± 0.23	5.23 ± 0.16
Elongation at break, %	500 ± 11.55	340 ± 11.55	380 ± 0	100 ± 0	100 ± 0
Residual elongation, %	32 ± 1.15	28 ± 0	24 ± 0	16 ± 0	20 ± 0
Tear strength, N/mm	21 ± 0	24.8 ± 0.85	25.5 ± 2.12	16.3 ± 1.84	16.8 ± 1.7

Analyzing the characteristics of the samples in normal state, the following can be observed:

Hardness increases from 64°ShA (Control test) to 73-74°ShA by reinforcement with simple black powder or black powder with PDMS, respectively. By grafting and crosslinking with peroxide, the hardness increases to 78-81°ShA, showing an increase of the crosslinking degree of samples.^25,30

The elasticity is significantly improved by crosslinking and grafting in dynamic regime, namely from 12% to 20%, respectively 24%, as a result of the formation of a three-dimensional network and the formation of C-C type bonds both between the thermoplastic elastomer chains of TR and between these and vulcanized rubber powder.

The 100% modulus shows an improvement both by adding the filler of black powder (by 44–52%) and especially by grafting and crosslinking with peroxide (increases of 233–256%), because there is an increase in the degree of physical or chemical crosslinking of the samples. There is a slight improvement for the samples in which PDMS was added to the rubber powder, which may be due to a better dispersion of the rubber powder in the polymer matrix.^13,18

The tensile strength decreases (by about 33%) by adding rubber powder to the TR compound, as a result of reinforcing the blends and increasing the rigidity, but for the PT/N20+5P sample there is an improvement of this characteristic by 18% which is due to both adding PDMS in rubber powder, as well as grafting and crosslinking in dynamic regime.^30–32

Elongation at break decreases both by introducing rubber powder, as well as by crosslinking and grafting because both restrict the movement of polymer chains under the action of a force.^30–32

The residual elongation shows small values, of 20–36% (being lower for the samples PT/N20 and PT/N20+5P), indicating a very good return to the initial shape after deformation by applying a force.

Tear strength has values of 17.4–26.15 N/mm, has a slight improvement for samples with rubber powder (by 3–5%) and is reduced due to grafting and crosslinking in dynamic regime by 33%, respectively 27% due to increasing the rigidity of the network by crosslinking.^30–32

The density of the samples shows an increase of 3%, respectively 5% by crosslinking in dynamic regime, indicating the fact that a more compact network has been obtained, there being a good compatibility between the existing polymer phases in the system.

The abrasion resistance has very good values, below 312.11 mm³. It worsens (increases by 12–74%) by adding the reinforcing filler, due to the size of the rubber powder and its degree of dispersion in the thermoplastic matrix.^33,34 There is an improvement in this characteristic for samples containing PDMS (sample N20+5P and sample PT/N20+5P) compared to those which do not contain PDMS (decrease by 14% and 35% respectively), indicating that by adding PDMS, an improvement in the dispersion of the rubber powder in the polymer matrix was achieved.³⁵

After accelerated aging for 168 h at 70°C, the samples had a very good behavior, as follows: the hardness increases by 1–4°ShA, the elasticity varies from −8% to +17%, the values of the Modulus 100% improve with max 19%, tensile strength decreases by 2–11%, elongation at break decreases by max 38%, residual elongation decreases by max 25% (has low values of 16–32%, so the samples have a very good ductility), and tear strength it is over 16.3 N/mm (reduced by max 16%). This very good behavior to accelerated aging makes the new types of materials suitable for a wide range of rubber consumer goods.

FTIR analysis of samples

In order to identify the structure and composition of TR thermoplastic rubber granules and to be able to distinguish the bands due to the SBS polymer matrix from the other bands that appear as a result of the introduction of rubber powder and the use of compatibility methods, the FTIR spectrum of the Control sample was performed (Figure 6). The spectrum of the Control sample shows: the bands from 965.21 cm⁻¹ indicating 1.4 trans polybutadiene and respectively the band from 687.12 cm⁻¹ indicating polystyrene (C-H from the benzene nucleus),³⁶ bands specific to SBS rubber. The correspondence between the absorption bands of the SBS thermoplastic elastomer and the corresponding chemical groups are detailed in Table 5.

Figure 6.

FTIR spectrum of thermoplastic rubber granules TR (Control sample): (a) 2700-3100 cm⁻¹, (b) 500-1500 cm⁻¹.

Table 5.

Correspondence between the bands in the FTIR spectrum of the Control sample and the chemical groups.

Absorption band, cm⁻¹	Appropriate chemical groups
3025.46 3079.56 cm⁻¹	Terminal (vinyl) C-H stretch and aromatic C-H stretch
2953.23 cm⁻¹, 2919.83 cm⁻¹ and 2850.34 cm⁻¹	C-H symmetrical and asymmetrical from -CH₃ or -CH₂ group
1600.73 cm⁻¹	C=C-C conjugated, aromatic ring stretch
1451.45 cm⁻¹ and 1492.49 cm⁻¹	C-H symmetrical and asymmetrical from -CH₃ or -CH₂ or aromatic ring
955.27 cm⁻¹	1,4 trans C-H of polybutadiene
909.18 cm⁻¹	Butadiene
874.78 cm⁻¹	C-H from vinyl
756.58 cm⁻¹	-CH₂ from monosubstituted benzene ring
697.18 cm⁻¹	Monosubstituted benzene - the deformation vibration of (C-H) in the benzene nucleus
538.81 cm⁻¹	O-Si-O from filler

Figure 7 shows the FTIR spectra of the TR samples containing the rubber powder and Table 6 shows the correspondence between the bands in the FTIR spectrum of these samples and the corresponding chemical groups. Analyzing the spectra, it can be observed the presence of specific bands of the SBS thermoplastic rubber matrix, that of silicone rubber powder, and changes in spectrum intensity for grafted and peroxide crosslinked samples.^21–24,36

Figure 7.

FTIR spectra of samples based on thermoplastic elastomer SBS type and silicone rubber powder.

Table 6.

Correspondence between the bands in the FTIR spectra of the samples based on SBS/silicone rubber powder and the corresponding chemical groups.

Absorption band, cm⁻¹	Appropriate chemical groups
3005-3081 cm⁻¹	Terminal (vinyl) C-H stretch and aromatic C-H stretch
2919.86 cm⁻¹ and 2850.61 cm⁻¹	C-H symmetric or asymmetric from -CH₂- or -CH₃
1737.88 cm⁻¹	C=O of esters
1603.8 cm⁻¹	C=C-C conjugated, aromatic ring stretch
1451.32 cm⁻¹and 1492.45 cm⁻¹	C-H symmetrical and asymmetrical from -CH₃ or -CH₂ or aromatic ring
1264.34 cm⁻¹	Deformation -C-H from Si-CH₃
1020 -1154 cm⁻¹	Si-O-C and Si-O-Si linear from main chain
965.11 cm⁻¹	1,4 trans C-H of polybutadiene
909.2 cm⁻¹	Butadiene
909.2–965.11 cm⁻¹	CH₂ wagging vibration of Si-CH=CH₂
874.45 cm⁻¹	C-H from vinyl
756.14 cm⁻¹	-CH₂ from monosubstituted benzene ring
697.9 cm⁻¹	Monosubstituted benzene - the deformation vibration of (C-H) in the benzene nucleus
538.64 and 467.06 cm⁻¹	O-Si-O from precipitated silica (filler in TR)

Conclusions

This paper aimed at the superior recovery of rubber powder obtained as waste in the manufacturing process in the rubber industry. The rubber powder was analyzed and it was observed that the basic elastomer is silicone rubber, and the amounts of fillers and other ingredients in the powder is low (according to the acetone extract and the ash). FTIR analyzes identified the type of elastomer in the rubber powder - namely the silicone elastomer.

The samples were obtained by mixing technique, on Brabender Plasti-Corder internal mixer type, and from the parameters recorded during the process of obtaining the blends it was observed that the specific energy required to obtain the blends increases by adding rubber powder or by adding crosslinking and grafting agent, but decreases for blends in which the rubber powder was mixed with PDMS (in order to better disperse the rubber powder in the TR polymer matrix). The viscosity of homogeneous melted blends increases by 50% by introducing rubber powder and decreases for blends containing rubber powder with PDMS (by 20.85% and 44.75%, respectively), indicating that PDMS played the role of plasticizer which led to a decrease in the viscosity of the blends in the melt. The physical-mechanical characteristics of the samples indicate that the addition of silicone rubber powder has improved the hardness and elasticity and reduced the abrasion resistance, and by applying advanced methods of compatibility, namely the crosslinking and grafting with peroxide, a significant increase in hardness, elasticity, modulus 100%, and tensile strength was observed, also the material obtained is compact.

The main advantages of the new composites are that they represent a method of superior recovery of rubber powder obtained as rubber post-production waste, and the products obtained have a high elasticity specific to elastomers and have adequate physical and mechanical properties to achieve a wide range of rubber consumer goods.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Romanian Ministry of Research, Innovation and Digitalization through Nucleu Program, PN 19 17 01 03/2019 project and The LIFE program in the frame of LIFEGREENSHOES 4 ALL, LIFE17ENV/PT/000337 project.

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