The Diversity of Styrene-Butadiene Rubber Latex

Dr. Christoph Scholten, PolymerLatex GmbH

Abstract

Styrene-butadiene rubber (SBR) latices are copolymers of styrene and butadiene obtained in an emulsion polymerization process. Mostly, they are agglomerated, degassed and concentrated. Those latices are delivered to the processing industry at a solid content of more than 60 % (HS- SBR). After processing products with highly elastic and other properties are obtained. They may be used in combination with natural rubber latex.

The production and processing of styrene-butadiene rubber latex are well established processes. This paper briefly reviews the different possibilities of the manufacture, the characteristics and the applications of the latices. Moreover, it will depict parameters which influence the properties of the end products. These factors are used in the design of the latices for various fields of application and the optimum performance therein. The focus will be on molded foam applications and the recently developed product that shows viscoelastic properties.

Introduction

Concerning the emulsion polymerization of butadiene and styrene the first laboratory experiments have been performed in 1920s. In the following decade the production of styrene butadiene rubber (SBR), named GR-S or Buna S, was established mainly for the production of tyres. The use of the synthetic latex started immediately after the Second World War. In the following the production and processing have been improved continuously. Nowadays, many companies switch from the processing of rubber using organic solvent to the processing of the aqueous latex to avoid hazardous chemicals. For the manufacture of latex foam a dispersion with a solid content of above 60 % and a viscosity of below 2500 mPas is required. To realize this total solid viscosity ratio a relatively big particle size has to be achieved reproducibly. A photo and a particle size distribution (PSD) of an exemplified sample obtained by transmission electron microscope (TEM) and capillary hydrodynamic fractionation (CHDF), respectively, are depicted in figures 1 and 2.

6

4

2 VolumeFraction [%]

0 0 300 600 900 1200 Particle Diameter [nm]

Figure 1 Photo obtained by TEM Figure 2 PSD obtained by CHDF (Measurement by AQURA) (Measurement by PL)

- 1 - Polymerization and Agglomeration

The principles of emulsion polymerization techniques have been studied and described by M. S. El-Aasser 1, R. G. Gilbert 1,2 , E. D. Sudol 1 and A. van Herk 2 and others. In case of standard SBR recipes an emulsion of monomers in water is formed by vigorous stirring and stabilised by soap molecules (Emulsification, Figure 2). The monomer is partitioned in monomer droplets, micelles and in the water phase. The initiator generates radicals which react with monomers in the water phase and these oligomer radicals diffuse into the micelles subsequently (Nucleation). In the micelles the radical consumes the present monomer. New monomer diffuses out of the water phase into the micelles. According to the developed monomer concentration gradient monomers diffuses from the droplets via the aqueous phase into the growing latex particles (Particle Growth). The polymerization slows down when the droplets have vanished.

Monomer Growing Droplet Particles Dissolved Soap

Oligomer Final Radical Particle

Dissolved Monomer Swollen Micelle

Initiator Initiator Growing Micelle Particle

Figure 3 Schematic description of the polymerisation in emulsion (Micelle Model)

The various possibilities of manufacturing HS-SBR latices were described by A. C. Blackley 3. Today the first polymerisation recipes of the 1930s are called hot rubber recipes because of their relatively high polymerization temperature of 50 °C (Table 1). It was found that the rubber obtained from a polymerization at a lower temperature yield better properties, e.g. tread wear. As the decomposition rate of potassium persulfate at 5 °C is insufficient to result in adequate polmyerization time another initiator system had to be chosen. A redox system based on an organic peroxide was found to be suitable. Moreover, the conversion was lowered to prevent gel formation. The analytical data was summarized by R. G. Bauer 4, D. C. Blackley 5, M. M. Morton 6 and R. P. Quirk 6 showing that the cold in comparison with the hot rubber recipes yield a polymer with higher molecular weight, less crosslinking and more trans-1,4 units at the expense of cis-1,4 units. Those latices with a solid content of 27 - 29 % at the end of the polymerization have to be concentrated. As the average particle size of the monodisperse latex will be approximately 60 nm an increase and a polydispersity has to be obtained to realize the above mentioned viscosity to total solid ratio. The most important methods in this respect are the agglomeration by mechanical means, by the use of chemicals and by freezing. It is said that the freeze-agglomeration is less economical as refrigeration costs are involved.

- 2 - A polymerization directly to a solids content of about 50 % has been developed as well. But this procedure is disadvantageous due to long reaction times and bad heat transfer.

Table 1 Typical SBR emulsion polymerization recipes 6

HOT RUBBER COLD RUBBER Polymerization temperature [°C] 50 5 Time [h] 12 12 Conversion [%] 72 60 – 65 Ingredients [phm] Styrene 71 71 Butadiene 29 29 Water 190 190 Fatty or rosin acid 5.0 4.5 – 5.0 n-Dodecanethiol 0.5 - tert -Dodecanethiol - 0.2 Potassium persulfate 0.3 - p-Menthane hydroperoxide - 0.08 FeSO 4 * 7 H 2O - 0.04 EDTA - 0.06 Sodium formaldehyde sulfoxylate - 0.10 Na 3PO 4 * 12H 2O - 0.5

Production

The polymerization of cold rubber is either performed in a batch or a continuous process. Agglomeration may be performed batchwise or continuous as well. Due to the conversion of only 60 – 65 % the monomer recovery is an important step. In the following not only the concentration but also a further reduction of unreacted monomers is performed. A laboratory setup used by the R&D Department of PolymerLatex is shown in figure 4.

Figure 4 Batch Polymerization (Laboratory Setup)

Processing and Application

The application fields and the relevant processing technologies were summarized by W. von Langenthal and J. Schnetger 7. Some examples for the processing of styrene butadiene rubber latex without vulcanization are tyre cord impregnation (low solid SBR latex), asphalt modification,

- 3 - clutch linings and can coating and sealing. The processing of the latex with vulcanization includes the manufacture of carpet backing, shoe insoles, upholstery, pillows and mattresses.

Latex foam

The manufacture of latex foam based on synthetic latex has been described in detail by E. W. Madge 8, J.C. Fallows 9, H. P. Schwenzfeier 10 and R. Joseph 11 . An orientation for a standard compound recipe is given in Table 2.

Table 2 Standard Recipe Latex Foam

Portion Solid Content Compound Recipe Ingredients [Parts] [%] Latex HS-SBR 100 67 Emulsifier Potassium Oleate 2.9 17 Thickener Carboxymethylcellulose 6.7 2.5 Zink Oxide, Colloidal Sulphur, Vulcanization Paste 9.5 63 Accelerators*, Antioxidants * e.g. Zinc diethyldithiocarbamate ZDEC, Zinc mercaptobenzothiazol ZMBT

The soap is added to adjust the foamability. The thickener serves to set the viscosity to the desired level. And the right choice of the vulcanization paste makes a vulcanization at 100 °C feasible. This compound mixture has to be foamed by mechanical mixing. The resulting foam is then ready to be filled into the molds. Before vulcanization starts the foam structure has to be stabilized. This is done by lowering the pH in order to gel the latex foam. The different gelling procedures are combined with special techniques. The mostly applied Dunlop-process makes use of sodium silicofluoride (SSF) as a gelling agent which is added in the last step of foaming the compound. The laboratory procedure performed by the Application Technology Department of PolymerLatex is depicted in Figure 5. The compound is foamed until a wet foam density of ca. 112 g/l is achieved. Thereafter SSF dispersion (25 %) as gelling agent is added and the mixing is continued for two minutes. The latex compound is filled into a mold. The mold is sealed and placed in an oven at 100 °C with saturated steam for 20 minutes. The vulcanized foam is removed from the mold, washed and dried for 90 minutes at 120 °C in a forced circulation oven. The density of the dry foam is about 75 g/l.

Figure 5 Dunlop Process (Laboratory Procedure)

In case of the Talalay-process the molds are partially filled with foam and closed. The foam is then expanded to the desired foam density by applying a vacuum. The mold is then cooled to - 30 °C, purged with carbon dioxide as gelling agent and finally heated to start the vulcanization process. Thereafter, the vulcanized molded foam has to be washed and dried.

- 4 - Testing

For quality checks, optimization and further development purposes molded foams are characterized mainly by hardness, hysteresis, tear strength, elongation at break and compression set (Figure 6). The hardness is defined as the force per area at a compression of 40 %. The hysteresis refers to the relative energy absorption of the difference of the loading and the unloading cycle in case of a 40 % compression. The tear strength complies with the maximum force divided by the diameter of the sample. The elongation at break is the length of the sample at break divided by the original size. The compression set corresponds to the missing recovery after storage in a compressed form (50 %) at different temperatures.

Figure 6 Selective test methods - Zwick Reißmaschine Z010 (a) hardness, b) hysteresis, c) elongation at break and tear strength)

Hardness and hysteresis refer to the comfort in using a mattress. The elasticity could be judged by elongation and tear strength. The compression set accounts for comfort properties and at elevated temperature for handling during production.

Vulcanizate Properties

According to A. Y. Coran 12 and J. A. Brydson 13 it is of major practical interest to obtain information on the macrostructure of the vulcanizate (Figure 7) and its effect on the use-related properties.

Cross-links

Entanglements

Chain ends

Cross-link cluster

Figure 7 Macrostructural network

The total crosslink density is the sum of the chemical and physical crosslink density. The chemical crosslink density comprises carbon-carbon-bonds and junctures due to sulphur bridges. The physical crosslink density is based on entanglements. Various correlations of use-related properties and crosslink density for rubber vulcanizates have been described. E.g. hysteresis is reduced with increasing crosslink density.

- 5 - Moreover, it is known from the literature 5,9 that a polymerization at 5 °C (Cold Rubber) instead of 50 °C (Hot Rubber) gives improved strength properties. Two major reasons seem to be relevant. Less branched and crosslinked polymer molecules are present when the polymerization temperature is decreased and in case of a “cold rubber” polymerization a smaller portion of polymer of low molecular mass is being formed. Other effects of the polymerization and processing parameters on the product properties of vulcanized rubber have been studied intensely and outlined by D. C. Blackley 5. E.g. a drop of the elongation at break is associated with the reduction of the bound styrene content.

New Ideas

A few years ago the question arose in the molded foam industry whether viscoelastic properties can be provided by latex foam. Molded foams based on standard grades combine a low hardness with a low hysteresis or a high hardness with a high hysteresis. In contrast viscoelastic molded foams should show the combination of a low hardness (< 150 N) and a high hysteresis (> 50 %). By altering some of the variables in the HS-SBR system it was possible to provide a new latex that shows the required characteristics in its foamed and vulcanized form (Figure 8). At the same time the other relevant properties remained in the target range of the standard products, e.g. elongation at break and tear strength.

100 100 300 0,15

75 75 200 0,1

50 50

Hardness[N]

Hysteresis [%] Hysteresis 100 0,05 25 25 Tear Strength [N/mm2] Strength Tear [%] break at Elongation

0 0 0 0 Standard Viscoelastic Standard Viscoelastic

Figure 8 Mechanical data of a standard and a viscoelastic latex foam

The major difference is the change in the hysteresis while keeping the hardness constant. As outlined above the hysteresis is a value for the relative energy absorption concerning loading and unloading of the molded foam. In case of standard products a low hysteresis means that after compression the recovery force is high and the related time is short (below 3 seconds). The new product with the high hysteresis shows a low recovery force and longer recovery time (e.g. 30 seconds). The effect is visualized in figure 9.

Figure 9 Recovery of a standard (blue) and a viscoelastic (white) latex foam (a) 0 seconds, b) 3 seconds, c) 30 seconds)

- 6 - Summary

The manufacture of the various end products has been going through a continuous optimization. Many aspects contribute to the improvements; Colloid and polymer science, production and engineering expertise, latex specific handling and application technology experience.

The new viscoelastic latex foam shows a property profile that cannot be achieved with standard grades. The main innovation comprises the combination of slow recovery and low hardness and, thereby, a change from a local elasticity to an area elasticity. This behaviour is advantageous in many fields of application. It, thereby, opens up new potentials and complements the product portfolio of the molded foam industry.

Acknowledgments

The author thanks Dr. Klaus Dören, Dr. Sabine Hahn, Dr. Gunther Müller, Dr. Hans-Peter Schwenzfeier, Dr. Katja Siepen for their contribution to this paper.

Literature

[1] M. S. El-Aasser, E. D. Sudol, in “Emulsion Polymerization and Emulsion Polymers” (P. A. Lovell, M. S. El-Aasser), John Wiley & Sons Ltd., 1997, Chapter 2 [2] A. van Herk, R. G. Gilbert, in “Chemistry and Technology of Emulsion Polymerisation” (A. van Herk), Blackwell Publishing Ltd., 2005, Chapter 3 [3] D. C. Blackley, “High Polymer Latices”, McLaren & Sons Ltd., 1966, Volume 1, Chapter 5 [4] R. G. Bauer, in “Kirk-Othmer Encyclopedia of Chemical Technology”, 3 rd edition, Wiley, New York, Vol. 8, pp. 608 - 625 [5] D. C. Blackley, in “Emulsion Polymerization and Emulsion Polymers” (P. A. Lovell, M. S. El- Aasser), John Wiley & Sons Ltd., 1997, Chapter 15 [6] R. P. Quirk, M. M. Morton, in “Science and Technology of Rubber” (J. E. Mark, B. Erman, F. R. Eirich), 2 nd Edition, Academic Press Limited, 1994, Chapter 2 [7] W. von Langenthal, J. Schnetger, in “Ullmann´s Encyclopedia of Industrial Chemistry”, VCH Publishers, Inc., 1993, Vol. A 23, pp. 421 - 435 [8] E. W. Madge, “Latex Foam Rubber”, McLaren & Sons Ltd. London, 1962, Part II [9] J. C. Fallows, in “Polymer Latices and their Applications” (K.O. Clavert), Applied Science Publishers Ltd. London, 1982, Part II [10] H. P. Schwenzfeier, in “Wäßrige Polymerdispersionen” (D. Diestler), Wiley VCH Weinheim, 1999, Chapter 12 [11] R. Joseph, in “Handbook of Polymer Foams” (D. Eaves), Rapra Technology Limited, 2004, Chapter 9 [12] A.Y. Coran, in “Science and Technology of Rubber“ (I. E. Mark, B. Erman, F. R. Eirich), Acad. Press (1994) Chapter 7 [13] J. A. Brydson, "Rubber Chemistry“, Applied Science Publishers Ltd. London, 1978

- 7 -