Highly dispersible silica in non-tire formulations.The typical process (ref. 1) for the production of a standard or conventional silica silica or silicon dioxide, chemical compound, SiO2. It is insoluble in water, slightly soluble in alkalies, and soluble in dilute hydrofluoric acid. Pure silica is colorless to white. is illustrated in figure 1. The initial raw materials are a sodium silicate sodium silicate, any one of several compounds containing sodium oxide, Na2O, and silica, Si2O, or a mixture of sodium silicates. Sodium orthosilicate is Na4SiO4 (or 2Na2O·SiO2); sodium solution, commonly called "water glass," and sulfuric acid sulfuric acid, chemical compound, H2SO4, colorless, odorless, extremely corrosive, oily liquid. It is sometimes called oil of vitriol. Concentrated Sulfuric Acid . The acid is added to the silicate silicate, chemical compound containing silicon, oxygen, and one or more metals, e.g., aluminum, barium, beryllium, calcium, iron, magnesium, manganese, potassium, sodium, or zirconium. Silicates may be considered chemically as salts of the various silicic acids. solution in a vessel or tank under controlled conditions to form a precipitate precipitate /pre·cip·i·tate/ (-sip´i-tat) 1. to cause settling in solid particles of substance in solution. 2. a deposit of solid particles settled out of a solution. 3. occurring with undue rapidity. of silica particles and aggregates. Sodium sulfate sodium sulfate, chemical compound, Na2SO4. It is a white, orthorhombic crystalline compound at ordinary temperatures; above 100°C; it assumes a monoclinic structure, and above about 250°C; it assumes a hexagonal structure. salt is formed as a by-product by·prod·uct or by-prod·uct n. 1. Something produced in the making of something else. 2. A secondary result; a side effect. by-product Noun 1. of the precipitation precipitation, in chemistry precipitation, in chemistry, a process in which a solid is separated from a suspension, sol, or solution. In a suspension such as sand in water the solid spontaneously precipitates (settles out) on standing. reaction. The product stream is then filtered and the excess salt is washed out in filter presses. The resulting filter cake is then dried, typically in a rotary drum dryer, milled to a fine powder and pneumatically pneu·mat·ic also pneu·mat·i·cal adj. 1. Of or relating to air or other gases. 2. Of or relating to pneumatics. 3. a. Run by or using compressed air: a pneumatic drill. conveyed to a storage tank. The silica powder can be packaged in bags or bulk containers for sale in powder form, or it can be further processed in a granulator to produce the low-dust, higher bulk density form. [ILLUSTRATION OMITTED] The critical steps of the manufacturing process are described in more detail in the following discussion to illustrate some of the areas where the typical processes were modified to produce the new highly dispersible silica, Ultrasil 7000 GR (ref. 1). The precipitation is the primary reaction step in the process that is critical for producing silica particles, and aggregates of particles, with the desired properties. The critical parameters for the reaction and its kinetics kinetics: see dynamics. Kinetics (classical mechanics) That part of classical mechanics which deals with the relation between the motions of material bodies and the forces acting upon them. include the pH, temperature, concentration of ingredients, reaction time, dosing and mixing parameters. The proper control of these parameters will influence the surface area, structure, dispersibility, silanol group density (the surface chemistry or surface activity) and the particle size distribution The particle size distribution[1] ("PSD") of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amounts of particles present, sorted according to size. of the final silica product. When acid is added to the silicate solution, small particles of silica start to form and grow into discrete primary particles as the reaction proceeds. The size of these primary particles determines the surface area of the final silica product (small particles give a high surface area per gram of silica). As these primary particles collide col·lide intr.v. col·lid·ed, col·lid·ing, col·lides 1. To come together with violent, direct impact. 2. and bond to each other in the reaction vessel, structural aggregates of the particles are formed. This secondary phase of the precipitation gives the persistent or permanent structure of the silica aggregates that is a key factor in reinforcement. Later in the process when the small silica aggregates come in contact with neighboring neigh·bor n. 1. One who lives near or next to another. 2. A person, place, or thing adjacent to or located near another. 3. A fellow human. 4. Used as a form of familiar address. v. aggregates they will bond to each other to form larger clusters of aggregates that are called agglomerates. The agglomerates are bound together by Van der Waals forces van der Waals forces: see intermolecular forces. van der Waals forces Relatively weak electrical forces that attract neutral (uncharged) molecules to each other in gases, liquefied and solidified gases, and almost all organic liquids and solids. or hydrogen bonding hydrogen bonding Interaction involving a hydrogen atom located between a pair of other atoms having a high affinity for electrons; such a bond is weaker than an ionic bond or covalent bond but stronger than van der Waals forces. . These forces are not as strong as the chemical bonds that form the permanent structure of the aggregates, but they can be a significant force that must be overcome by shear in the mixing of a rubber compound in order to achieve a uniform dispersion dispersion, in chemistry dispersion, in chemistry, mixture in which fine particles of one substance are scattered throughout another substance. A dispersion is classed as a suspension, colloid, or solution. of the aggregates of silica. Larger agglomerates can be broken up by mechanical treatment, such as milling, but some reagglomeration will occur. The filtration and washing step can influence structure and agglomeration ag·glom·er·a·tion n. 1. The act or process of gathering into a mass. 2. A confused or jumbled mass: tendencies of the silica, as well as pH and conductivity conductivity /con·duc·tiv·i·ty/ (kon?duk-tiv´i-te) the capacity of a body to transmit a flow of electricity or heat; the conductance per unit area of the body. con·duc·tiv·i·ty n. 1. from the sodium salts. The drying process is another critical step in achieving the desired properties of the silica. The important parameters include, the type of dryer that is used, the temperature and time of drying, and the solids content of the filter cake introduced to the dryer. These can influence water content, the structure and agglomeration, dispersibility, surface area and particle size distribution of the final product. The final process step in producing the 7000 GR is a new granulation granulation /gran·u·la·tion/ (-shun) 1. the division of a hard substance into small particles. 2. the formation in wounds of small, rounded masses of tissue during healing; also the mass so formed. technique to produce a low dust product that will break up and incorporate readily during rubber mixing. The granulation process is a controlled agglomeration of the silica to increase the bulk density and reduce its dustiness for easier handling. Besides influencing the sieve residues and dust content, the granulation technique determines the structure and particle size distribution of the agglomerates and their effect on the dispersibility of the final product. Although all of the process steps are important factors in developing a more dispersible silica, variations in precipitation and drying processes were found to have significant influences, as shown in table 1.
Table 1 - process variations - changes in
properties
Precipitation Drying Dispersibility Product
High pH Long time Poor Conventional
silica
Lower pH Long time Medium Semi-HD silica
High pH Short time Good HD silica
Optimized Short time Excellent HD silica
precipitation (different type)
Altering the pH and other parameters during the precipitation can lead to improved dispersibility of the silica, even when using a conventional drum-drying technique (long time or slow drying). A new quick drying technique (short time) showed even more potential for improvement (ref. 1). The new 7000 GR highly dispersible silica incorporates both an optimized precipitation method and a new drying process, as well as a new granulation technique. Void volume testing Volume Testing belongs to the group of non-functional tests, which are often misunderstood and/or used interchangeably. Volume testing refers to testing a software application for a certain data volume. (figure 2) illustrates the higher and less-fragile structure of 7000 GR compared to a conventional silica, such as Ultrasil VN 3, and a semi-HD silica. The increased void volume throughout the range of test pressures indicates that the structure is maintained, which enables better interaction with polymer chains during mixing and processing of rubber compounds. The pore pore (por) a small opening or empty space. alveolar pores openings between adjacent pulmonary alveoli that permit passage of air from one to another. cavities of the structure allow penetration by the polymer chains, which hastens the break-up of silica agglomerates. [ILLUSTRATION OMITTED] The increased interaction with polymer leads to improved dispersibility, as shown in the photographs in figure 3, from optical microscopy microscopy /mi·cros·co·py/ (mi-kros´kah-pe) examination under or observation by means of the microscope. mi·cros·co·py n. 1. The study of microscopes. 2. . The photographs show cross-sections from silica-filled tire tread compounds. The compound containing the conventional silica shows many more dark spots of undispersed silica aggregates. [ILLUSTRATION OMITTED] A typical silica tire tread formulation is shown in table 2. It features a high level of silica and TESPT silane silane or silicon hydride Any of a series of inorganic compounds of silicon and hydrogen with covalent bonds and the general chemical formula SinH(2n + 2). (Si 69) coupling agent replacing carbon black for lower rolling resistance Rolling resistance, sometimes called rolling friction or rolling drag, is the resistance that occurs when an object such as a ball or tire rolls. It is caused by the deformation of the wheel or tire or the deformation of the ground. and improved traction. Table 2 - silica tire tread formulation Stage 1 Phr S-SBR styrene 25% 95 vinyl 73%, oil 27% BR 98%, 1,4-cis 30 Silica 80 TESPT/N 330 (50:50) 12.8 Zinc oxide 3 Stearic acid 2 Aromatic oil 10 Antioxidant wax 1 6 PPD 1.5 Stage 2 Stage 3 CBS 1.5 DPG 2 Sulfur 1.5 The improved dispersion behavior is further illustrated in figure 4 using another optical method. The dispersion coefficient is calculated from optical photos (ref. 3) and reflects the area of the undispersed agglomerates that are shown in the micrographs. [ILLUSTRATION OMITTED] A special testing technique has been adopted as a quality control method to measure the ease of de-agglomeration of highly dispersible silicas (figure 5). A suspension of the silica is subjected to an ultrasonic ultrasonic /ul·tra·son·ic/ (-son´ik) beyond the upper limit of perception by the human ear; relating to sound waves having a frequency of more than 20,000 Hz. ul·tra·son·ic adj. 1. treatment and the resultant particle size distribution is measured using laser diffraction (ref. 4). The ultrasonic treatment simulates the energy input during the mixing process of a rubber compound. Larger agglomerates (roughly 10 microns in diameter) are shown as peak II in the illustration. 7000 GR shows a much higher peak I of smaller agglomerates (roughly 0.5 microns) that have resulted from the ultrasonic treatment. The WK-coefficient relates the break-down of the agglomerates as a ratio of the relative heights of the two peaks. [ILLUSTRATION OMITTED] Figure 6 illustrates the excellent correlation of the WK-coefficient of silica samples with the in-rubber dispersion coefficient measurements. This gives a relatively quick means to rate the dispersibility of a silica without having to mix it into a rubber compound, and has been adopted as a quality control method for production of 7000 GR. [ILLUSTRATION OMITTED] The improved dispersion characteristics of 7000 GR in the silica tire tread recipe can also be demonstrated using surface roughness measurement techniques (ref. 8). The topography topography (təpŏg`rəfē), description or representation of the features and configuration of land surfaces. Topographic maps use symbols and coloring, with particular attention given to the shape and elevations of terrain. profile of the compound cross-sections also illustrates fewer undispersed agglomerates of silica, and thus more potential for reinforcement of the robber compound. The silica tire experience has shown that tire treads filled with conventional silicas give advantages in reducing fuel consumption (low rolling resistance) and in improving traction. However, there was an accompanying loss of abrasion abrasion /abra·sion/ (ah-bra´zhun) 1. a rubbing or scraping off through unusual or abnormal action; see also planing. 2. a rubbed or scraped area on skin or mucous membrane. resistance (tread wear) that shortened the usable life of the fire compared to carbon black filled treads. Highly dispersible silicas have improved the abrasion resistance and eliminated that shortfall (refs. 1 and 2). Improving the dispersion of the silica results in a more highly reinforced tread compound with better abrasion resistance. The excellent correlation between dispersion and tire treadwear index is illustrated in the graphs (refs. 1 and 2) shown in figure 7. [ILLUSTRATION OMITTED] Objectives of the current study The improved dispersibility of 7000 GR silica, with its accompanying improvement in abrasion resistance compared to conventional silicas, has been demonstrated in the silica tire tread application. The following comparisons in some mechanical rubber goods formulations were initiated to determine whether the advantages that have been shown in tires would also translate to non-tire compound applications. 7000 GR was compared to two conventional silicas in general MRG MRG Merge MRG Minority Rights Group International MRG Mad River Glen (Vermont) MRG Mouvement des Radicaux de Gauche (French: Left Radical Movement) MRG Manyetik Rezonans Görüntüleme formulations based on four different polymers. The MRG formulations were taken from the R.T. Vanderbilt Handbook. The two conventional silicas were milled powders, but 7000 GR was the granulated gran·u·late v. gran·u·lat·ed, gran·u·lat·ing, gran·u·lates v.tr. 1. To form into grains or granules. 2. To make rough and grainy. v.intr. , lower dust form. VN 3 has about the same surface area as 7000 GR, while HiSil 233 is a lower surface area silica. The silicas were compared in each of the four types of compounds, both with and without a silane coupling agent. The variations that were compared in the NBR NBR Number NBR Nightly Business Report (PBS show) NBR National Business Review (New Zealand weekly business newspaper) NBR National Bureau of Asian Research NBR National Board of Review (nitrile rubber Nitrile rubber, or Buna-N,is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Some trade names are: Nipol, Krynac and Europrene. ) formulation are shown in table 3. The silicas were compared at the 50 phr level with no silane and with 3 phr silane coupling agent. The comparisons in the NR, SBR SBR - Spectral Band Replication and CR (Neoprene neoprene: see rubber. neoprene Any of a class of elastomers (rubberlike synthetic organic compounds of high molecular weight) made by polymerization of the monomer 2-chloro-1,3-butadiene and vulcanized (cross-linked, like rubber), by sulfur, W) formulations were similar to this - with the silicas at 50 phr in each formulation. Two stage internal mixing was used in each case, and the first stage was dumped at about 310 [degrees] F (155 [degrees] C) to facilitate the reaction of the silane with the silica.
Table 3 - R-9733 U700 GR in NBR
Min. Mixer 1: 1 2 3
0' 80RPM NBR N6393 100.0 100.0 100.0
0.5 HiSil 233 50.0 50.0
" Ultrasil VNS 50.0
" Ultrasil 7000 GR
" Si264 silane 3.0
2.0 Stearic acid 1.5 1.5 1.5
" Vanfre AP-2 2.0 2.0 2.0
" Agerite stalite S 1.5 1.5 1.5
" Dibutyl phthalate 12.5 12.5 12.5
" Cumar R-13 resin 12.5 12.5 12.5
4.0 Zinc oxide 50.0 6.0 5.0
Total phr 185.0 188.0 185.0
Dump 6' OR 310
[degrees] F
Min Mixer 2:
0' 60 RPM MB 185.0 188.0 155.0
" Sulfur 1.8 1.8 1.8
" MBTS 1.5 1.5 1.5
" TMTD 0.5 5.0 0.5
Total phr: 188.8 191.8 188.8
Min. Mixer 1: 4 5 6
0' 80RPM NBR N6393 100.0 100.0 100.0
0.5 HiSil 233
" Ultrasil VNS 50.0
" Ultrasil 7000 GR 50.0 50.0
" Si264 silane 3.0 3.0
2.0 Stearic acid 1.5 1.5 15.0
" Vanfre AP-2 2.0 2.0 2.0
" Agerite stalite S 1.5 1.5 1.5
" Dibutyl phthalate 12.5 12.5 12.8
" Cumar R-13 resin 12.5 12.5 12.8
4.0 Zinc oxide 5.0 5.0 5.0
Total phr 188.0 185.0 188.0
Dump
Min Mixer 2:
0' 60 RPM MB 188.0 185.0 188.0
" Sulfur 1.8 1.8 1.8
" MBTS 1.5 15.0 1.5
" TMTD 0.5 0.5 0.5
Total phr: 191.8 188.8 191.8
Test results Each group of three bar graphs in figure 8 shows the results with the three silicas at the 50 phr level; giving eight silica comparisons. The second group for each formulation illustrates the effect of adding the silane coupling agent (at 3 phr, or 6% by weight of silica). The Si 230 silane used with the CR (Neoprene) formulation has a chlorine functional group, while Si 264 silane used in the other three has a sulfur functionality (blocked mercapto). Some of the test results include: * Silica comparison - effect on compound viscosity. The comparisons of HiSil 233 and VN 3 in figure 8 illustrate the normal trend expected for conventional silicas - the lower surface area HiSil 233 showed a lower compound viscosity in seven of the eight comparisons. However, 7000 GR gave equal or lower viscosity than HiSil 233 in half of the comparisons, despite its higher surface area. This is believed to be due to the increased polymer-filler interaction with 7000 GR and the resultant reduced tendency to re-agglomerate and form filler-filler networks. [ILLUSTRATION OMITTED] * Silane coupling agent effect on compound viscosity. The silane coupling agent produced a lower compound viscosity in three of the four formulations, with no significant effect shown in the SBR formulation. The chemical modification In biochemistry, chemical modification is the technique of chemically reacting a protein or nucleic acid with chemical reagents. Chemical modification can have several goals, such as
* Silica comparison - effect on cure rate. All three silicas showed similar results overall for time to 90% cure. * Silane coupling agent effect on cure rate. The Si264 silane coupling agent produced a faster cure rate, but no significant effect was shown with Si230 silane in the CR (Neoprene) formulation. * Silica comparison - effect on 300% modulus See modulo. . The three silicas gave essentially equal modulus overall. * Silane coupling agent effect on 300% modulus. Adding a silane coupling agent resulted in higher modulus in all four formulations. * Silica comparison - effect on hardness. Overall, the three silicas again showed similar results, although 7000 GR produced somewhat lower hardness in the SBR formulation and slightly higher hardness in the CR formulation (figure 9). [ILLUSTRATION OMITTED] * Silane coupling agent effect on hardness. Addition of silane coupling agent did not produce significant changes in hardness. * Silica comparison - effect on rebound. The three silicas gave similar results for rebound resilience measured at room temperature. * Silane coupling agent effect on rebound. No significant trend is shown as a result of adding silane coupling agent. * Silica comparison - effect on tensile strength tensile strength Ratio of the maximum load a material can support without fracture when being stretched to the original area of a cross section of the material. When stresses less than the tensile strength are removed, a material completely or partially returns to its . Overall, the three silicas gave similar results for tensile strength. The expected trend - slightly higher tensile strength with the higher surface area silicas - was not exhibited. However, these formulations were not optimized and 50 phr of silica may be more than is needed to reach the maximum strength. * Silane coupling agent effect on tensile strength. Addition of silane coupling agent resulted in slightly higher tensile strength in three of the four formulations, while the NBR formulation showed the opposite effect. * Silica comparison - effect on tear strength. No significant trend in tear strength was shown for the silica comparison. * Silane coupling agent effect on tear strength. Silane coupling appeared to enhance tear strength in the SBR and NR formulations, but did not show a significant effect in the NBR or CR formulations. * Silica comparison - effect on compression set. The three silicas did not show any significant trends in the SBR, NR and NBR formulations. HiSil 233 gave lower compression set in the CR formulation. * Silane coupling agent effect on compression set. The silane coupling agent gave lower compression set results in all four formulations. * Silica comparison - effect on dispersion. Surface roughness measurements (figure 10) using a Surfanalyzer (stylus stylus: see pen. (1) A pen-shaped instrument that is used to "draw" images or select from menus. Styli (the plural of stylus, pronounced "sty-lye") come with handheld devices that have touch screens, such as PDAs and video games. method) showed significant improvements in dispersion with 7000 GR in SBR, NR and NBR, and equal or improved dispersion compared to HiSil 233 in the CR formulation. This demonstrates that the Ultrasil 7000 GR incorporates and disperses in a rubber compound more readily than the milled conventional silica powders that are more difficult to handle and generate more dust. [ILLUSTRATION OMITTED] Although the preceding property comparisons showed essentially equal results, the improved dispersibility should produce better batch to batch uniformity in a production mixing environment. * Silane coupling agent effect on dispersion. The addition of silane coupling agent generally produced slightly improved dispersion results. * Silica comparison - effect on angle abrasion. Results from angle abrasion testing showed improved abrasion resistance with 7000 GR (less abrasion loss), which confirms the advantage that has been shown in tire treadwear performance. In the angle abrasion test, a small solid rubber disk is abraded by a rotating ro·tate v. ro·tat·ed, ro·tat·ing, ro·tates v.intr. 1. To turn around on an axis or center. 2. grinding wheel. * Silane coupling agent effect on angle abrasion. Addition of silane coupling agent resulted in significant improvements in abrasion resistance. * Silica comparison - effect on Pico abrasion. Pico abrasion loss measurements did not show any significant differences between the silicas. In the Pico abrasion method, a pair of rotating knives abrade a·brade v. 1. To wear away by mechanical action. 2. To scrape away the surface layer from a part. abrade ( the surface of a robber test block. * Silane coupling agent effect Pico abrasion. Addition of a silane coupling agent showed a general trend of improved Pico abrasion resistance, with a significant improvement shown in the NR formulation. Compared to HiSil 233 powder, the new 7000 GR silica showed approximately equal performance in the four MRG formulations for most of the physical properties that were tested, despite the differences in surface area between the silicas. 7000 GR did show significantly improved dispersion compared to conventional silica milled powders, offering the advantages of easier handling, lower dust and the potential for improved uniformity. Plant mixing trials are needed to confirm the improved mixing behavior and the potential for improved batch to batch uniformity. Shortened mixing cycles may be feasible. The enhanced dispersion that was achieved with 7000 GR should contribute improved abrasion resistance in most applications. The initial tests suggest that lab abrasion testing responses may depend on the mode of abrading the test samples. The MRG compounding study confirmed that silane coupling agents can improve the performance of silica filled non-tire compounds by reducing compound viscosity, tightening the cure (may shorten scorch delay, however), increasing modulus and lowering elongation elongation, in astronomy, the angular distance between two points in the sky as measured from a third point. The elongation of a planet is usually measured as the angular distance from the sun to the planet as measured from the earth. , increasing tensile tensile, adj having a degree of elasticity; having the ability to be extended or stretched. and tear strength, reducing the compression set and improving abrasion resistance. For potential applications, the improved performance of the final rubber product should be weighed against the increased material cost of adding a silane coupling agent. Finally, it does appear that the improved dispersion and abrasion resistance noted with 7000 GR in fires does translate to similar results in MRG compounds. Future work is planned to evaluate the other advantages. References (1.) S. Uhrlandt, A. Blume, "Development of HD silicas for tires - processes, properties, performance," paper No. 32 presented at the 157th Meeting of the Rubber Division, ACS (Asynchronous Communications Server) See network access server. , Dallas, TX, Apr. 4-6, 2000. (2.) B. Schwaiger, A. Blume, "Silica/silane - a winning formula in reinforcement," Rubber World, April 2000. (3.) R.H. Schuster, H. Geisler and D. Buflmann, 2nd Conference on Carbon Black, Mulhouse (France), Sept. 1992. (4.) A. Blume and S. Uhrlandt, GAK GAK Gesellschaft für Aktuelle Kunst (German) GAK Gemeenschappelijk Administratiekantoor GAK Grazer Athletikklub (German: Graz, Austria soccer club) GAK Göteborgs Astronomiska Klubb 2, 116 (1999). (5.) A. Blume, paper No. 73 presented at a meeting of the Rubber Division, ACS, Chicago, IL, Apr. 13-16, 1999. Patrick DiMauro is technical marketing manager for the mechanical rubber goods market segment for Degussa-Huls' Advanced Fillers & Pigments Division, responsible for technical support for carbon black, silica and organosilanes sold to the NAFTA NAFTA in full North American Free Trade Agreement Trade pact signed by Canada, the U.S., and Mexico in 1992, which took effect in 1994. Inspired by the success of the European Community in reducing trade barriers among its members, NAFTA created the world's market. John Byers is a technical consultant with 36 years of experience. He has held technical positions with Firestone fire·stone n. 1. A flint or pyrite used to strike a fire. 2. A fire-resistant stone, such as certain sandstones. Noun 1. , Ashland Chemical, Wyrough & Loser, PPG Industries PPG Industries (NYSE: PPG) was founded in 1883 as the Pittsburgh Plate Glass Company. PPG is an American manufacturer of glass and chemical products, including automotive safety glass. and Degussa-Huls. Stefan Uhrlandt is pilot plant manager for precipitated silicas and matting agents for Degussa-Huls Advanced Filler fill·er 1 n. One that fills, as: a. Something added to augment weight or size or fill space. b. A composition, especially a semisolid that hardens on drying, used to fill pores, cracks, or holes in wood, plaster, and Pigments Division. He is responsible for silica process technology and for the global development of precipitated silicas for the rubber industry. |
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