The distribution of rubber oil in oil-extended thermoplastic elastomer styrene–butadiene

Abstract

Rubber oil and thermoplastic elastomer styrene–butadiene–styrene (SBS) were selected for this work to study the distribution of rubber oil in SBS. Glass transition temperature (T_g) of different components in rubber oil and rigid or soft phase in oil-extended thermoplastic elastomer SBS were measured by means of rheometer and dynamic mechanical analysis. Results calculated by Fox equation and verification tests proved that rubber oil is not only distributed in soft phase but also in rigid phase of SBS. Saturated hydrocarbon components of rubber oil tend to dissolve in soft phase, while aromatic components tend to dissolve in rigid phase.

Keywords

Rubber oil thermoplastic elastomer distribution styrene–butadiene–styrene glass transition temperature

Introduction

Thermoplastic elastomer styrene–butadiene–styrene (SBS) is the most widely used thermoplastic elastomer in the world.¹ It is a three-block copolymer of SBS in which polystyrene is the rigid phase and polybutadiene is the soft phase. Based on the two phases, normally two glass transition temperatures (T_gs) can be found for thermoplastic elastomer SBS. Polystyrene phase is dispersed in the polybutadiene phase; meanwhile, the polybutadiene phase is sealed by polystyrene chains. The cross-linking network structure is formed by the rigid polystyrene phases as physical junction and the polystyrene also play the role of conventional vulcanized elastomer junction point.² The excellent tensile properties, elasticity, and processing performance of SBS can be attributed to the unique structure by which direct injection molding or extrusion can be easily performed.

To improve the processability, flexibility, and tensile property of thermoplastic elastomer SBS, rubber oil can be filled into thermoplastic elastomer SBS.¹ The filling can reduce the melting viscosity of elastomers, and it is worth to mention that it can also improve the T_g without changing the basic chemical properties of the elastomer.³ It is generally recognized that the process of filling rubber oil into SBS can be divided into four steps.⁴ Firstly, the rubber oil molecules enter the gap of thermoplastic elastomer SBS and adhere to the surface of SBS copolymers. Secondly, the oil molecules dissolve or swell in SBS molecules. In this step, the total volume of rubber oil and SBS decreases; meanwhile, SBS chains (reffered to the polybutadiene phase) are unwrapped and separated. Thirdly, SBS is treated with high energy (generally heated to 160–180°C) or is milled. The structure of SBS is destroyed and turning it elastic state, so that rubber oil will penetrate into the molecular micelle of SBS. Finally, with cooling process, a strong structure of SBS system is formed.

Rubber oil is a complex mixture of different hydrocarbons. Normally the ratio of alkane-carbon, naphthenic-carbon, and aromatic-carbon in rubber oil can be quantitatively described by hydrocarbon analysis. Rubber oil can be divided into naphthenic-base, paraffinic-base, and aromatic-base according to the results of hydrocarbon analysis. The hydrocarbon analysisis is usually performed by carbon nuclear magnetic resonance (NMR)⁵ and Brandes infrared method.⁶ Using column chromatography,⁷ rubber oil can be separated into saturated hydrocarbon, aromatic hydrocarbon, and polar component. According to ASTM D2786 and ASTM D3239 standards, the rubber oil component can be further refined to specific content of different rings, number of naphthenic hydrocarbon, aromatic hydrocarbon and thiophene, and so on.

With more and more strict environmental regulations and the anti-yellowing requirements of SBS, aromatic base rubber oil, especially the rubber oils with more than 3% of polycyclic aromatic hydrocarbons have to be withdrawn from the SBS market. Currently, rubber oil that is allowed to be filled into SBS is mainly eco-friendly naphthenic-based and paraffinic-based oil. For the distribution of rubber oil in SBS, it is generally believed that the rubber oil with high aromatic content should be avoided in order to prevent the plasticizing of polystyrene in micro area. Ohlsson et al.⁸ found that the rubber oil filled into amorphous region of ethylene–butylene and polypropylene (PP) phase by analyzing styrene–ethylene–butylene–styrene/PP/rubber oil blends. Sengers et al.⁹ calculated the distribution of rubber oil in elastic phase and in PP phase using dielectric relaxation spectrum for PP/ethylene–propylene–diene monomer/extender oil blends. Both of the assumptions are that the rubber oil cannot be filled into polystyrene phase. In other words, the work only focused on the distribution of rubber oil in elastic phase and PP phase, without considering the change of the polystyrene phase. It is reported that the solubility parameter of aromatic component in rubber oil is similar to that of polystyrene phase in SBS and the solubility parameter of saturated hydrocarbon component in rubber oil is similar to that of polybutadiene phase in SBS.^10,11 Then the aromatic components in rubber oil may tend to be dissolved in polystyrene phase, while saturated components tend to be dissolved in polybutadiene phase. While so far, the distribution of rubber oil in the two phases of SBS is rarely reported. This work explores the distribution of rubber oil in the two phases of SBS. The results are calculated by Fox equation and examined by the verification tests.

Experimental

Materials

SBS samples with the type of YH-792 are provided by Baling Petrochemical Co. (Yueyang, China) and the ratio of soft phase to rigid phase is 60:40. Two types of naphthenic-based rubber oil (coded KN4006 and KN4010) and two types of paraffinic-based rubber oil (coded 150 N and 500 N) with different viscosity grades were filled into SBS, respectively. The rubber oil are all provided by Karamay Refinery (Karamay, China) and the characteristic data are shown in Table 1.

Table 1.

The characteristic data of the rubber oils.

Rubber oil	Viscosity at 100°C (mm² s⁻¹)	Viscosity– gravity constant	Molecular weight (g mol⁻¹)	Clay–gel absorption chromatographic method (wt%)			Infrared Brandes method (wt%)
Rubber oil	Viscosity at 100°C (mm² s⁻¹)	Viscosity– gravity constant	Molecular weight (g mol⁻¹)	Saturated hydrocarbons	Aromatic hydrocarbons	Polar compounds	C _A	C _N	C _P
150 N	5.321	0.7958	383.5	97.08	2.92	0	1.2	23.23	75.57
500 N	10.40	0.7992	456	94.92	5.08	0	1.2	23.56	75.24
KN4006	5.410	0.8463	321	92.04	7.96	0	1.2	54.31	44.49
KN4010	10.30	0.8398	365	89.96	10.04	0	1.2	53.39	45.41

Equipments

A SK-160 model mill which is provided by Shanghai First Rubber Machinery (China) was used in the experiments. Platen vulcanizing machines are also from the same company with the model of XLB-D400×400×2-Z and QLB-D350×350×2C. Dynamic mechanical analysis (DMA) was performed on a Q800 type dynamic mechanical spectroscopy and an AR-2000 type rheometer was used for the rheological analysis. Both the equipments are manufactured by TA Instruments (New Castle, Delaware, USA).

Preparation of oil-extended thermoplastic elastomer SBS film

As there is no standard method available, the oil-extended thermoplastic elastomer SBS films were prepared as follows: 100 g blank SBS micelle was added in a beaker, followed by addition of 30 g, 50 g, and 80 g naphthenic-base rubber oil KN4006 and KN4010 into the SBS micelle, respectively. In the same way, 30 g and 50 g paraffinic-base rubber oil 150 N or 500 N was filled into 100 g blank SBS micelle as well. Due to poor compatibility, 80 g paraffin-base rubber oil cannot be absorbed in blank SBS micelle. Dry micelle and rubber oil were stirred with glass rod and stand for 24 h to ensure the rubber oil was fully filled into the blank SBS.

Ten of the aforementioned oil-extended SBS micelles were milled and the temperature of the roller is 110 ± 5°C, the distance of the rollers is 0.5 mm. The oil-extended SBS micelles were extruded and pressed for several times. Then, the distance of the rollers was adjusted to 0.8 mm and the oil-extended SBS micelles were pressed for several times. When the films were transparent, the distance of the rollers was adjusted to 2.2 mm, and the films were removed from the roller. Next, an XLB-D400×400×2-Z-type platen vulcanizing machines was used to the vulcanizing process shown as follows. Firstly, the films were preheated for 15 min at 145 ± 5°C and under the contact pressure of 3.5 MPa. Secondly, the pressure was adjusted to 7–8 MPa, and the films were vulcanized for 10 min. Finally, the films were vulcanized for 10 min at 13.5 MPa. The films were vulcanized for 10 min at 13.5 MPa and during the process, the system was exhausted 3 times. After the films were cool pressed for 10 min using the QLB-D350×350×2C-type vulcanizing machine under 14 MPa at room temperature, the oil-extended SBS films were obtained. Dry film could be also obtained for blank SBS by the similar operation.

The separation of saturated hydrocarbon and aromatic hydrocarbons components from rubber oil

Naphthenic-based KN4006 and KN4010 as well as paraffinic-based 150 N and 500 N were separated according to the standard similar to ASTM D2007, which is known as clay–gel absorption chromatographic method. Specifically, a glass column (500 × 40 mm²) was charged with a slurry of 250 g silica gel 40 (70–230 mesh) in n-pentane. The oil sample (10 g) mixed with n-pentane (10 mL) was then added to the column, and n-pentane (500 mL, ≥99%), toluene (500 mL, ≥99%), and ethyl acetate (500 mL, ≥99%) were used as eluents.¹² Ethyl acetate was used instead of acetone due to the lower vapor pressure. The procedure for removing n-pentane insolubles was excluded in this study as the oil did not contain any asphaltenes. In this way, the saturated hydrocarbon and aromatic hydrocarbon components were enriched. The T_g of each component was tested using rheometer; meanwhile, clay–gel absorption chromatographic data are shown in Table 1.

DMA and rheometer measurements

Differential scanning calorimetry (DSC), DMA, and rheometer are general protocols for the T_g analysis.¹³ DSC can be used for the T_g analysis of the rubber oil. For SBS, DSC can be also used for the T_g analysis of the polybutadiene phase, while it cannot be used for the analysis of polystyrene phase. Therefore, it cannot be used for T_g analysis in this work. DMA can be used to analyze the T_g of soft phase and rigid phase in SBS for the the oil-extended SBS film samples as well as the dry film samples, while it cannot be used for the T_g analysis of the liquid sample. In additiion, rheometer can be used to analyze the T_g of the rubber oil. Therefore, DMA and rheometer were chosen for T_g analysis in this work due to the similar principles and the comparable results.

Specificlly, the T_g analyses of the oil-extended SBS film samples and dry film samples were performed on a Q800 DMA at a frequency of 1 Hz in membrane tensile mode. The samples were heated from −150°C to 180°C at a rate of 3°C min⁻¹. The T_g was defined by the peak temperatures in the dielectric loss factor. The T_g of the rubber oils, the saturated hydrocarbon and aromatic hydrocarbon components were determined by the rheometer. The detailed conditions are as follow: the frequency is 1 Hz and the temperature range is −120 to 30°C at a rate of 3°C min⁻¹. The fixture is 40 mm aluminum parallel plate, and the dynamic scanning is under the control of 5% strain. The T_g was expressed by the peak temperature of the loss modulus.

Results and discussion

Results of DMA and rheometer

The T_g of rubber oil, saturated hydrocarbon, and aromatic hydrocarbon components in rubber oil are shown in Figure 1. From the figure, it is shown that for the rubber oil, the T_g of naphthenic-based KN4006 or KN4010 is always higher than that of paraffinic-based 150 N or 500 N. For each rubber oil, the T_g of rubber oil, saturated hydrocarbon, and aromatic hydrocarbon components in rubber oil is ranked as follows: aromatic hydrocarbon components > rubber oil ≈ saturated hydrocarbon component. The T_g of saturated hydrocarbon component and the rubber oil are similar due to the fact that the content of aromatic hydrocarbon component are both lower in naphthenic-based and paraffinic-based rubber oil. This is mainly because of that both of the oils are obtained by high-pressure hydrogenation process.

The DMA curves of 0, 30, 50, and 80% of KN4006 filled in SBS are shown in Figure 2. The results of T_g for the rigid and soft phases in other different oil-extended SBS are shown in Figures 3 and 4.

Compared with the T_g of the rigid phase in the blank SBS film, the T_g of the rigid phase in different oil-extended SBS film samples reduced gradually whenever naphthenic-based rubber oil or paraffinic-based rubber oil was used, which is shown in Figure 3. The increase of the rubber oil results in a decrease of the T_g of rigid phase. From Figure 4, similar trend can be found for the T_g of soft phase filled with 150 N or 500 N. When KN4006 or KN4010 was filled, the T_g of soft phase is higher than that of the blank SBS samples. Also the increase of the rubber oil results in an increase of the T_g of the soft phase. Therefore, the T_g of the rigid phase and the soft phase in SBS are changed due to plasticizing effect of the rubber oil. This can prove that rubber oil is filled in both the soft and rigid phases in SBS.

The distribution of rubber oil in oil-extended SBS

Rubber oil can be fully dissolved in SBS without exudation, therefore the T_g of soft and rigid phases in oil-extended SBS are consistent with Fox equation.¹⁴ Levin and Karlsson also reported that the T_g of rubber oil as well as saturated hydrocarbon component, aromatic component, and polar compound component can be accorded with Fox equation.¹²

If the proportion of the rubber oil (in quantity) filled into the soft phase in the total rubber oil is defined as x, then the proportion of the rubber oil filled into the rigid phase is 1 − x. The ratio of soft phase to rigid phase is 60:40, when 30 phr rubber oil was filled into the oil-extended SBS:

F o r r u b b e r o i l : \frac{1}{T_{g_{r u b b e r o i l}}} = \frac{x}{T_{g_{r u b b e r o i l i n s o f t p h a s e}}} + \frac{1 - x}{T_{g_{r u b b e r o i l i n r i g i d p h a s e}}}

For soft phase in SBS : \frac{1}{T_{g_{soft phase after filling into rubber oil}}} = \frac{60 / 60 + 30 x}{T_{g_{softphase}}} + \frac{30 x / 60 + 30 x}{T_{g_{rubber oil in soft phase}}}

\begin{aligned} For rigid phase in SBS : & \frac{1}{T_{g_{rigid phase after filling into rubber oil}}} \\ = \frac{40 / 40 + 30 (1 - x)}{T_{g_{rigid phase}}} + \frac{30 (1 - x) / 40 + 30 (1 - x)}{T_{g_{rubber oil in rigid}}} \end{aligned}

When 50 phr and 80 phr rubber oil were filled into the oil-extended SBS, similar results can be obtained from the above-mentioned equations.

After the calculation, the comparison of x with the content of saturated hydrocarbon in rubber oil is shown in Figure 5, and the comparison of 1 − x with the content of aromatic hydrocarbon in rubber oil is shown in Figure 6. Figure 7 shows the T_g of saturated hydrocarbon in rubber oil and the T_g of rubber oil in soft phase, and Figure 8 shows the T_g of aromatic hydrocarbon in rubber oil and the T_g of rubber oil in rigid phase.

From the figures, firstly, it can be seen that the fractions of rubber oil filled into soft phase are not the same as those of rubber oil filled into rigid phase. Subsequently, different T_g can be found. Therefore, the rubber oil with two different solubilities could be distributed in two phases in SBS, and the rubber oil is not wholly distributed in SBS. Secondly, for the same kind of rubber oil, the T_g and mass percentage of rubber oil distributed in two phases are basically consistent even when the percentage of rubber oil is different. Thirdly, it is found that the mass percentage of rubber oil distributed in soft phase is related to the saturated hydrocarbon content in rubber oil, and the mass percentage of rubber oil distributed in rigid phase is in accordance with aromatic hydrocarbon content in rubber oil.

In summary, the aromatic component of rubber oil could be filled into rigid phase of SBS, while saturated hydrocarbon component could be filled into soft phase of SBS

Verification test

From the results of Fox equation, it is assumed that aromatic component of rubber oil can be filled into rigid phase in SBS; meanwhile, saturated hydrocarbon component can be filled into soft phase in SBS. In order to verify the assumptions, the T_g of oil-extended SBS, which is filled with saturated hydrocarbon in rubber oil as well as aromatic hydrocarbon in rubber oil was performed by means of DMA. The results of the verification tests are shown in Figures 9 and 10.

It can be seen from the figures that, on the one hand, the T_g of rigid phase in SBS film that is filled with saturated hydrocarbon component is in accordance with the T_g of rigid phase in the blank SBS. Compared with the T_g of soft phase in the blank SBS, T_g of soft phase in oil-extended SBS film, which is filled with saturated hydrocarbon component changed. That is to say, the saturated hydrocarbon component in rubber oil can be dissolved in the soft phase in SBS. On the other hand, the T_g of soft phase in SBS film, which is fillied with aromatic hydrocarbon component is consistent with the T_g of soft phase in the blank SBS. Compared with the T_g of rigid phase in the blank SBS, the T_g of rigid phase in oil-extended SBS film that is filled with aromatic hydrocarbon component also changed. This also means that the aromatic hydrocarbon component in rubber oil can be dissolved in rigid phase in SBS.

Conclusions

The distribution of rubber oil in thermoplastic elastomer SBS was studied in this work, and the conclusions can be drawn as follows:

The results of T_g of oil-extended SBS and the blank SBS demonstrate that rubber oil is not just distributed in soft phase in SBS but distributed in two phases in SBS.

Fox equation results and rubber oil group composition measurement results proved that part of rubber oil filled into soft phase and part of rubber oil filled into rigid phase. Also rubber oil with two different solubilities distribute in the two phases of SBS, that is, the aromatic component of rubber oil dissolve in rigid phase in SBS and saturated hydrocarbon component dissolve in soft phase in SBS.

The verification tests match the above conclusions well.

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) received no financial support for the research, authorship, and/or publication of this article.

References

Holden

Thermoplastic elastomers. In: Kirk-Othmer encyclopedia of chemical technology. Kirk

-Othmer

(ed). New York: John Wiley & Sons, Inc., 2002, p. 1–23.

Legge

. Thermoplastic elastomers. Rubber Chem Technol 1987; 60: 83–117.

Rahman

Brazel

. The plasticizer market: an assessment of traditional plasticizers and research trends to meet new challenges. Prog Polym Sci 2004; 29: 1223–1248.

Bary

Hauser

. Researches on the structure of rubber. Rubber Chem Technol 1928; 1: 401–409.

Van De Ven

Jansen

Heijer

. An infrared analysis method to determine the aromatic carbon content of mineral lubricant base oils based on correlation with carbon-13 NMR aromaticities. Lubric Sci 1995; 7: 197–209.

Brandes

. Dir strukturgruppen analyse mit hilfe der uhrzeit spektroskopie. Brennst Chem 1956: 263–267.

Wanasundara

Shahidi

. Novel separation techniques for isolation and purification of fatty acids and oil by-products. In: Bailey's industrial oil and fat products. Kirk-Othmer (ed). New York: John Wiley & Sons, Inc., 2005, 3:18.

Ohlsson

Hassander

Törnell

. Blends and thermoplastic interpenetrating polymer networks of polypropylene and polystyrene-block-poly(ethylene-stat-butylene)-block-polystyrene triblock copolymer. 1: Morphology and structure-related properties. Polym Eng Sci 1996; 36: 501–510.

Sengers

Wübbenhorst

Picken

. Distribution of oil in olefinic thermoplastic elastomer blends. Polymer 2005; 46: 6391–6401.

10.

Levin

Redelius

. Determination of three-dimensional solubility parameters and solubility spheres for naphthenic mineral oils. Energy Fuel 2008; 22: 3395–3401.

11.

Ovejero

Perez

Romero

. Solubility and Flory Huggins parameters of SBES, poly (styrene-b-butene/ethylene-b-styrene) triblock copolymer, determined by intrinsic viscosity. Eur Polym J 2007; 43: 1444–1449.

12.

Levin

Karlsson

. The effect of molecular composition of naphthenic mineral oil on the glass transition temperature. Thermochim Acta 2010; 499: 171–173.

13.

Prime

Turi

. Thermal characterization of polymeric materials. In: Turi

(ed). New York: Academic Press, 1981, p. 435.

14.

Dean

. Analytical chemistry handbook. New York: McGraw-Hill, 1995.

The distribution of rubber oil in oil-extended thermoplastic elastomer styrene–butadiene–styrene