7 Silica and Silanes Walter Meon Degussa Corporation, Parsippany, New Jersey, U.S.A. Anke Blume and Hans-Detlef Luginsland Degussa AG, Cologne, Germany Stefan Uhrlandt Degussa Corporation, Piscataway, New Jersey, U.S.A. I. INTRODUCTION The invention of synthetic amorphous silica, in the late 1940s, also marked the birth for the development of new rubber compounds with many more options and for creating new improved types of rubber. Silica’s progress from a simple filler used for producing colored rubber compound to an important physical performance additive for substantially improving rubber com- pounds, especially in tire applications, took technically about 20 years. But it did not take long to learn that silica-containing compounds achieve significantly better tensile strength and tear strength. Therefore, silica became an important physical reinforcement ingredient in all kinds of rubber goods in the 1950s and 1960s. Silica manufacturers started to develop and offer specific silicas for different rubber requirements. After the first success with commercially available mercaptopropyltri- methoxysilane in the late 1960s, the polysulfidic silane Si 69 R * from Degussa was introduced in 1972 and set the benchmark for significantly improved silica-containing compounds. Silanes turned silica into an active chemical reactant in the rubber compound. An additional 20 years was necessary to * Si 69 is a registered trade name of Degussa. 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 285 Copyright © 2004 by Taylor & Francis commercially develop the silica and silane success story for the high-volume tire application that began in Europe. The use of silica–silane compounds grew slowly. The consumption of silanes in 1990 was below 3000 metric tons per annum (t/a). The introduc- tion of the ‘‘green tire’’ from Michelin, with a tread based on a silica from Rhodia (formerly Rhone Poulenc) and silane from Degussa, was a real breakthrough in tire innovation. All tire manufacturers had to face this challenge. But also the silica manufacturers were obliged to offer better dispersing silicas. Because silica had become an active, high-performance ingredient for rubber compounds, all major silica manufacturers were devel- oping so-called highly dispersible (HD) silicas, because only homogeneously and finely dispersed silica particles can lead to improved reinforcement in the rubber compound. Because silane’s importance in rubber has been increasing, new com- petitors have appeared as a normal consequence of a product’s life cycle in the market. Competition requires professional research and development to respond to new requirements and set new trends, to investigate and discover the complex chemistry that is involved with silanes in silica- and carbon black–filled rubber compounds. Therefore, this chapter emphasizes the chem- istry in rubber compounding based on Degussa’s long dedication and expe- rience in silica and silane research and development conducted by Dr. Stefan Uhrlandt (Section II) and Dr. Hans-Detlef Luginsland (Sections III and IV). The whole broad field of rubber applications using silica and silane as high-performance reactants in rubber compounds is delineated finally in an extended Section V by Dr. Anke Blume and Dr. Hans-Detlef Luginsland, supported by experts from the Degussa Applied Technology Centers in Kalscheuren and Hanau, Germany. Valued editorial reviewing work was contributed by colleagues from our Technical Center in Akron, Ohio, in conjunction with John Byers’ support. II. SILICAS A. General Considerations and Basic Information A glance at the occurrence of the homologous chemical elements silicon and carbon in a variety of locations (Table 1) reveals that although life is based on carbon, the accessible outer layer of the earth’s lithosphere is primarily of a siliceous nature (1,2). Silicon as a carrier of ‘‘inorganic life’’ occurs in nature almost exclusively in the form of crystalline solids in approximately 800 different siliceous minerals. Even in the evolutionary history of humans there are numerous indications of the omnipresence of silicates in our natural habitat. Siliceous minerals were utilized either by processing natural silicate 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 286 Copyright © 2004 by Taylor & Francis deposits (e.g., clay, kaoline, chalk, or talc) or by means of chemical conversion (silica, silicones, ceramics). The properties of pure elemental silicon are now of pivotal importance in the manufacture of integrated switching circuits and therefore also form the basis for the age of electronics. Degussa began dealing with siliceous chemicals because of the com- pany’s involvement in the use of carbon black. In addition to their classic application as black pigment, carbon blacks were increasingly used as active fillers in the rubber industry, in particular in the manufacture of automobile tires. Because the starting material for carbon black at the time—natural gas—was not available in sufficient quantities in Germany, a substitute was sought that could be prepared from indigenous starting materials. The concept of ‘‘white carbon black’’ was born, and research into the manufacture of siliceous fillers commenced. The idea to apply the manufacturing method for carbon black to volatile silicon compounds (pyrogenic silica, AerosilR) also originates from that time. To cite the then Degussa chemist Harry Kloepfer (3). Because the significance of a white active filler for the rubber industry is very evident, I performed several experiments on the manufacture of a ‘‘white carbon black’’ parallel to the first active carbon black ex- periments. Even during the initial experiments, I endeavored to imitate the manufacturing conditions for gas black. The important aspect in the manufacture of gas black seemed to me to be the direct separation of ultra-fine carbon particles from the gas phase, i.e., the production of aerosols. As described above, fine silicas can be produced by a pyrogenic reaction or by precipitation. Silicates are manufactured by a precipitation process. It is an interesting task to precipitate silicas and silicates from aqueous media. The methods are so variable that various products customized for the respective applications are attainable. Manufacturers and users of fine silicas and silicates require analytical characteristic values in order to compare products (4). Owing to the great differences in chemical composition of silicas, silicates, and other natural substances such as chalk or siliceous minerals, the charac- Table 1 Occurrence of the Elements Silicon and Carbon Location Occurrence of silicon Occurrence of carbon Most common elements Outer space 0.003% 0.005% H + He 99% Earth’s crust 27.7% 0.1% O 51% Human organism 0.01% 9.5% H + O 89% Source: Ref. 2. 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 287 Copyright © 2004 by Taylor & Francis terization usually commences with chemical analysis, which provides impor- tant information on the product composition, the main constituents, and, of particular importance, the secondary constituents. One significant constitu- ent of precipitated silicas and silicates is water, which occurs in various quantities either chemically bonded or adsorbed. The specific surface area, mean particle size, particle size distribution, pH, and absorption of certain oils all play significant roles in the final products. These somewhat ‘‘classic’’ methods of analysis and a number of new methods for characterizing silicas and silicates will be dealt with in greater detail in Section II.C. The versatility of fine silicas and silicates is attributable to a variety of fundamental properties. In chemical terms, they are largely inactive and exhibit high thermostability; they influence the viscosity of liquids and act as antisedimentation agents; they are capable of producing matting effects and of preventing adhesion between foils and the caking of powders. Because of their high absorbing power, they also serve as carriers for feed and pesticides. Targeted organic modification of the silica surface converts ‘‘water-friendly,’’ or hydrophilic, silicas into ‘‘water-repelling,’’ or hydro- phobic, products. The hydrophobic properties have proven to be very useful for certain applications, e.g., in silicone rubber or for defoaming liquids. The oldest application of ‘‘ white’’ silicas and silicates, their use in shoe soles and technical rubber articles, and the use of silicas in modern automobile tires will be discussed later. B. Essentials of Rubber Silicas The oldest use for fine silicas and silicates is in shoe soles. The high demands with regard to wear can be met with ease by such reinforced vulcanized materials. The great advantages of light-colored reinforcements vis-a ` -vis carbon black is the possibility of satisfying practical, not to mention fashion-related, requirements, whether they be that shoe soles be nonmark- ing, transparent, or a particular color, or whether it be necessary to stamp cable sheaths with colored lettering. If the fineness and therefore the high specific surface of a substance is considered to be a main contributing factor to the reinforcing effect in rubber, then fine silicas and silicates must act similarly to rubber black. This is indeed the case, but clear distinctions must be made (5). In simple terms, the cross- linking of rubber during vulcanization proceed in a different manner in the presence of carbon black than in the presence of silica or silicates. The reasons for this are adsorption of cure ingredients and the immiscibility of hydrophilic silica with the hydrophobic rubber. Consequently, silicas are used mostly in conjunction with bifunctional organosilanes, such as Si 69R. Organosilanes undergo chemical bonding to the silica surface, thereby providing the 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 288 Copyright © 2004 by Taylor & Francis potential for cross-linking to the rubber. The behavior of silica and organo- silanes will be described in detail in Section III. C. Characterization of Silicas (Analytical Properties) Despite the fact that synthetic silicas have been known for a long time, the question as to how they are best assigned characterization values has yet to be resolved. In a particular application, the behavior of products frequently differs from that expected on the basis of the characterization values. There is, however, still hope that the accurate characterization of a silica will allow conclusion to be drawn with respect to its mechanism of formation and its behavior in its intended application. Characterizing a silica means describing its (surface) structure, i.e., its morphology, as accurately as possible. This include the ratio between the ‘‘ inner ’’ and ‘‘ outer ’’ surfaces, the size and shape of pores, the absorption capacity, the surface roughness, the primary particle size, the formation of aggregates from these primary particles with the development of siloxane bonds, and the combination of aggregates to form agglomerates held together by van der Waals forces. Other methods attempt to characterize the surface chemistry; i.e., they describe the number of silanol groups and their arrangement, surroundings, and reactivity, and the chemical composition and degradation behavior of the silica surface. Fur- thermore, there are a number of methods that can collectively be termed bulk chemical analysis. This review attempts to summarize physical and chemical analytical methods for characterizing silica (6–8) but does not lay claim to completeness. The more ‘‘ traditional ’’ characterization methods that have been in use for quite some time are only upon briefly because they are dealt with in the relevant literature. The main emphasis here is on the new methods—those that have been specially developed or adapted for the purpose of silica analysis. Moreover, some analytical methods are more of scientific interest and are therefore not described in detail either. An extensive bibliography provides interested parties with the opportunity to pursue the methods in detail. 1. Methods for Characterizing the Morphology of Silica Specific Surface. The specific surface of a silica is generally determined by using the Brunauer–Emmett–Teller (BET) adsorption method (9) or a modification thereof (10,11). For measurement, the sample is cooled to the temperature of liquid nitrogen. At low temperatures, nitrogen is adsorbed on the silica surface. The quantity of adsorbed gas is a measure of the size of the surface. When performed under defined conditions, the BET method yields perfectly reproducible results. The BET method always provides the sum of 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 289 Copyright © 2004 by Taylor & Francis the so-called outer geometrical surface and the inner surface, i.e., the surface within the porous silica structure. If the value obtained for the surface using the BET method is compared to that from electron microscopic images, which indicate only the outer surface, then it is at least possible to estimate the ratio of outer and inner surfaces. Evaluation of other sections of the BET isotherms provides additional information. The so-called C value calculated from the BET isotherms gives a qualitative indication of the magnitude of the interaction between the surface of the silica and the adsorbed material and hence of the chemical reactivity of the surface (12). If it is ensured not only that the surface of a sample is covered with nitrogen but also that the pores are filled, then the distribution of mesopores (pores between 2 and 30 nm in size) can also be determined using the Barrett-Joyner-Halendar method (BJH) method (13). CTAB Surface Area. One method that is known from the area of carbon black technology and is also used in the case of silica is based on the adsorption of surface-active molecules from aqueous solutions. The adsorbed molecule is cetyltrimethylammonium bromide (CTAB) (14). The preferred adsorption site for these large CTAB molecules is the outer, geometrical surface, which correlates quite well with the surface area accessible to the rubber (15,16). Comparison with the BET surface, which is the sum of the outer and inner surfaces of a filler, provides an indication, with a certain margin of error, of the ratio between the inner surface and the total surface. The CTAB surface often coincides very closely with the surface transmission determined from electron microscopic (TEM) images. Sears Determination of the Specific Surface. The Sears number is a measure of the number of silanol groups on the surface and therefore a measure of the specific surface of a silica (17,18). The Sears number is equiv- alent to the quantity of 0.1 N NaOH required to titrate a suspension of silica from pH 6 to pH 9. The acidic silanol groups on the silica surface react with NaOH. The Sears number gives an indication of the number of reactive centers on the surface of a silica. Pore Volume and Pore Distribution. The term ‘‘ pore volume’’ and a specific evaluation method have not been described explicitly (19,20). The pore volume of synthetic silica can be understood as 1) the surface roughness, 2) the micro- or submicropore volume within the particles or aggregates, or 3) the void volume. The most common method of determining the pore volume in a silica is mercury porosimetry (21,22). The measured quantity in mercury porosimetry is the pressure, p, required to force the mercury into the pores of a sample. The necessary pressure is inversely proportional to the pore diameter. If the volume of mercury at this pressure is known, then the pore volume can 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 290 Copyright © 2004 by Taylor & Francis be calculated. Comparison of the measured curves of different silicas reveals distinct differences in intrusion and is indicative of very different structures of the measured products. E STIMATION OF INTRA-AGGREGATE AND INTERAGGREGATE STRUC- TURE (23). The sharp step at the end of the intrusion curve V = f(R) (Fig. 1a) corresponds to the intrusion of mercury inside the pores originating from the open shape ofsilica aggregates. The intrusion volume in this range of pore sizes is characteristic of the intra-aggregate structure of silica aggregates. This intra-aggregate structure can be described quantitatively by use of the struc- ture index (IS), which is the mercury intrusion volume measured between R min Figure 1 (a) Intrusion curve and (b) differential curve determination of R min and R max and the calculation of the structure index 15. 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 291 Copyright © 2004 by Taylor & Francis and R max , pore radius values corresponding respectively to the beginning and the end of the step. R min and R max can be easily determined by considering the derivative curve dV/dR = f(R) (Fig. 1b). R max is chosen to match the following condition on the slope: d dR dV dR          R max V 0:004 dV dR  max     R min is chosen to match the condition dV dR  R min ¼ dV dR          R max The structure index IS depends only on the size and shape of individual aggregates. The global structure of a silica, including both intra-aggregate and inter- aggregate pore volumes, can be estimated using a second structure index called IS2, defined as the total intrusion volume corresponding to pore radius values smaller than 4000 nm. IS2 depends on both aggregate shape and aggregate organization resulting from the drying process. DBP Number. The assessment of the liquid-absorbing capacity of synthetic silicas and silicates may involve the absorption of dibutylphthalate (DBP) (24–26). This largely automated measurement technique provides an indication of the total volume of liquid that can be absorbed by a silica sample. The magnitude of the DBP number (24M4-DBP) gives an initial indication of the interaggregate structure and the processing and dispersion properties of a silica. Owing to the toxicity of DBP oil, attempts are being made to replace it with paraffin oil (27). Considering the polarity of the silica, polar substances such as triethanolamine (TEA) can also be used to determine the interaggregate structure (24M4-TEA), analogous to the DBP measurement. Void Volume. The void volume (28), i.e., the silica structure as a function of the pressure, is considerably more reliable than the DBP number. In the void volume method, the sample is subjected to a defined coarse crushing and then a specified quantity of the sample is transferred to a cylindrical glass chamber with a volume scale. The chamber is sealed by means of a moving piston. For the measurement, a constant pressure is applied to the piston until the volume of the sample in the chamber remains constant. Then the measured value is read off. Subsequently, the pressure exerted on the sample is increased again. Figure 2 shows the decrease in structure (or the void volume) as the pressure increases. This involves comparison of a new silica, 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 292 Copyright © 2004 by Taylor & Francis prepared using a modified manufacturing method, with the reference silicas Zeosil R * 1165 MP, Ultrasil R 7000 GR, and the conventional silica Ultrasil R VN 3 GR with the same CTAB surface area. It is obvious that the new products have a significantly higher void volume even at the start of the measurement. As the pressure is increased the structure decreases yet always remains well above the level of the respective reference. In other words, the higher structure remains intact even when subjected to great stress, e.g., during mixing in a Banbury mixer. The more stable structure can be penetrated more easily by polymers and is indicative of the more advantageous mixing and processing behavior of the silica. WK Coefficient. To determine the particle distribution and dispers- ibility (ease of incorporation) of a silica in a polymer matrix, a new method was developed (29) based on the principle of laser diffraction. The method uses defined ultrasonic treatment of silica and subsequently measures the size distribution of particles between 40 nm and 500 Am in diameter. The latest generation silicas [highly dispersible (HD) silica] exhibit a bimodal distribu- Figure 2 Decrease in structure of silica as pressure increases. * Zeosil (Rhodia SA) and Ultrasil (Degussa) are registered trade names. 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 293 Copyright © 2004 by Taylor & Francis tion. The main peak at approximately 10 Am depicts the original agglomerate structure of the silica. During ultrasonic treatment the silica structure is destroyed; deagglomeration takes place, and silica aggregates are formed. The peak corresponding to the silica aggregates lies at approximately 0.4 Am. The energy input via ultrasound simulates the energy input in mixers typically used in the rubber industry. The silica with the highest fraction of deagglom- erated particles has the best dispersibility. By comparing the peak height of the original agglomerate to that of the degraded agglomerates, the WK coefficient, which is a measure of the dispersibility of a silica, can be determined. There is a close correlation between these results and those of other methods. To a certain extent tire abrasion resistance correlates with the silica dispersion. This has been substantiated by comparing the results of dispersion measurements of various silicas of the same surface area with the results of tire tests on the road (5). Microscopic Methods. The only method allowing direct insight into the dimensions of interest in the case of silicas is electron microscopy, which provides information on the size of primary particles and aggregates or agglomerates and, with certain limitations, on the particle size distribution of an examined sample. ‘‘Electron microscopic surfaces’’ can be calculated from the various particle size distributions, and these can be compared with those from BET measurements or other investigations. T RANSMISSION ELECTRON MICROSCOPY (TEM). Transmission elec- tron microscopy (TEM) (30) works in much the same manner as light microscopy: Electrons are passed through a thin object and, following their interaction with the prepared sample, are used to produce an image. However, the resolution exceeds that of a light microscope by a factor of 1000; for TEM, resolution is 0.2–0.3 nm (for a light microscope it is f 200 nm). TEM images with high resolution provide valuable information on the composition or structure of different silica samples. With a suitable imaging technique, even the crystalline short-range order of silicas and silicates can be detected. The structure of the silicas can also be recognized, as can the manner in which the primary particles unite with one another. S CANNING ELECTRON MICROSCOPY. The scanning electron micro- scope (31) is not actually a microscope, in the sense that it uses electromag- netic lenses in order to magnify images, similar to the case with light optics (this comparison also applies to the transmission electron microscope). It merits the name ‘‘electron microscope’’ only because it produces a strongly magnified image with the help of electrons. As is also the case with TEM, scanning electron microscopy (SEM) initially produces beams of electrons from an electron source. The extremely sharply focused beam of electrons produces very good resolution and depth of focus. It is this great depth of focus in particular that makes SEM superior to TEM for certain applications. 4871-9_Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 294 Copyright © 2004 by Taylor & Francis [...]... diffuse X-ray diffraction bands Silicas from a variety of manufacturing processes differ from one another in terms of their X-ray diffraction bands When the sample is tempered, changes in the short-range order can be detected at temperatures as low as 200jC using X-ray diffraction (19) Nevertheless, due to the noncrystallinity in long-range precipitated silicas are classified as nontoxic and can be handled... microstructure of carbon black and silica surfaces AFM images allow the analysis of filler dispersion in the rubber matrix Depending on the resolution of the images, filler aggregates and agglomerates in the polymer matrix can be identified Small-Angle Scattering The method of small-angle scattering (SAS) is a well-known tool to characterize the structure of fine particles and has been used to investigate... in various media, for example, in rubber (71) 3 Chemical Bulk Analyses X-Ray Diffraction Synthetic silicas and silicates are amorphous solids That is, unlike crystalline solids, they do not possess an infinite three-dimensional long-range order Consequently, use of the classic X-ray diffraction method is not possible Silicas, however, like glass, do have areas of short-range order that can be determined... is reached between the aggregates and agglomerates (Scheme 3) After that, the obtained suspension is filtered and the filter cake washed It can then be resuspended and spray-dried or fed directly to a short-term drying process Depending on the drying technology, the product can be optionally first milled and then granulated or granulated directly to convert it into a low-dust form Scheme 2 Schematic representation... Francis Scheme 3 Particle formation during precipitation 3 Dispersibility and Surface Activity An easy incorporation of the silica into the rubber mixture and its good dispersion are crucial because they have a considerable effect on the processing costs and on the rubber product’s performance By selecting appropriate process parameters, it is possible to influence the surface properties and therefore the... case of silicas is electron microscopy, which provides information on the size of primary particles and aggregates or agglomerates and, with certain limitations, on the particle size distribution of an examined sample ‘‘Electron microscopic surfaces’’ can be calculated from the various particle size distributions, and these can be compared with those from BET measurements or other investigations TRANSMISSION... polymers and is indicative of the more advantageous mixing and processing behavior of the silica WK Coefficient To determine the particle distribution and dispersibility (ease of incorporation) of a silica in a polymer matrix, a new method was developed (29) based on the principle of laser diffraction The method uses defined ultrasonic treatment of silica and subsequently measures the size distribution of particles... electrons produces very good resolution and depth of focus It is this great depth of focus in particular that makes SEM superior to TEM for certain applications Copyright © 2004 by Taylor & Francis 2 Characterization of Surface Chemistry Chemical Reactions of Silanol Groups In the literature, a variety of methods are described that are suitable for determining silanol groups and their chemical reactivity on... are based on applying the experience gathered in small molecule and low molecular weight chemistry to surface chemistry However, the fact that this analogy is flawed is demonstrated by the following simple consideration On the surfaces of solids, particularly in the interior of micropores, spatial inhibition, other equilibrium conditions, and other reaction possibilities may prevail Chemical reactions... is to first supply a particular quantity of water glass and initially dose just the sulfuric acid Normally, this is followed by a second stage in which water glass and sulfuric acid are added simultaneously under defined reaction conditions 2 Influence of Process Parameters on Product Properties During the reaction time, primary particles are first formed in the reactor; later these particles react with . chem- istry in rubber compounding based on Degussa’s long dedication and expe- rience in silica and silane research and development conducted by Dr. Stefan Uhrlandt (Section II) and Dr. Hans-Detlef. the 487 1-9 _Rodgers_Ch07_R2_052404 MD: RODGERS, JOB: 03 286 , PAGE: 288 Copyright © 2004 by Taylor & Francis potential for cross-linking to the rubber. The behavior of silica and organo- silanes. Hi-Sil 170 Ultrasil AS 7 Hubersil 1714 Hi-Sil 185 /195 Ultrasil 88 0 Hubersil 1715 Zeosil 195 Gr Hubersil 1613 Hubersil 1743 Hubersil 1633 Hubersil 1745 Hubersil 1635 Hi-Sil 170 Hi-Sil 315 Hi-Sil