Text: http://www.sinrubtech.com/short%20notes\Short%20Notes%205.1.htm Silane Coupling Agents By Sin Siew Weng, FIM, FPRIM, AMIC, ANCRT, FIManf, F.Prof BTM. Sin Siew Mun, BSc, ANCRT, APRIM Sin Yoong Cheong, MEng Sin Yoong Leong, BSc  Lee Aik Kwong, PhD, FPRIM, FMIC, ANCRT First Published: 1999 Revised :          31 May 2000 2nd Revision      9 November 2000 1.Introduction Silanes have been around for over 40 years but their early applications are more in adhesives and coatings. In rubber compounding, the first silane coupling agent was commercialised in 1971, the well known Si-69 by Degussa. This was to rectify the short comings of precipitated silica, introduced only about 1951, mainly poorer processing/ dispersion, poorer abrasion resistance compared to carbon blacks and is still expensive to use and until 1992, found only niche applications such as in high end transparent coloured shoe solings. With the introduction of the ŒGreen Tyre¹ concept by Michelin in 1992, silane coupling agents became hot chemicals and we now have several tailor-made silane coupling agents to select from. The chemistry of silane coupling agents, its practical applications and its selection for each formulation are still not easy to comprehend. Silane coupling agents are bifunctional organosilanes which can be represented viz:- X-R-Si (OR¹)3-n where X = organo functional groups which can form strong covalent crosslink with polymers. Examples:-         Polysulphides         Disulphides         Amino         Mercapto         Vinyl                                 Epoxy                                 Methacryl                                 Others n = 0,1 or 2             OR¹ = hydrolysable group capable of forming strong covalent bonds with the hydroxyl groups on silica surfaces such as:-                                 CL                                 OCH3                                 OC2H5                                 OC2H4OCH3                                 Others R = methylene group Bifunctional organosilanes therefore permit chemical cross linking between polymer and filler such as silica, silicate, talc, mica, clay an whiting which contains surface hydroxyl groups ­ hence their appropriate name silane coupling agents. The basic chemistry seems simple enough, but the selection of each silane coupling agent to suit the polymer and/ or its curing system, the effects of other additions that can compete with silanes on their reactivity with the silanol groups on the silica surfaces, the different rates of silanes reactions and their effect of curing characteristics etc make their application a bit more complicated. 2. Commercial Silane Coupling Agents At least nine classes of silane coupling agents are known. Class Commercial Names Abbreviations 1. Polysulfide Class Bis-[3-(triethoxysilyl) propyl ] - tetrasulfide Degussa Si-69 TESPT UC Silane A1289 Witco A1289 Uniroyal/ Witco RC2 Behn Meyer Couplink 89 Hung Pai HP 669 SRT TESPT TESPT-50G Bis-[3-(triethoxysilyl) propyl ] ­ disulfide Degussa VP Si-75 TESPD Witco 1589 Hung Pai  HP 1589 2. Mercapto Class 3-mercaptopropyltrimethoxysilane UC A189 MPTMS 3-mercaptopropyltriethoxysilane UC A1891 MPTES Hung Pai HP 1891 3. Amino Class 3-aminopropyltriethoxysilane UC A1100 APTES Witco A1100 N-2-(aminoethyl)-3-amino propyltrimethoxysilane UC A1120 AEAPTMS Witco A1120 4. Chloro Class Degussa Si 230 5. Vinyl Class Vinyltrimethoxysilane UC A171 Witco A171 Vinyl-tris(2-methoxyethoxy) silane UC A172 Degussa Si-225 Witco A172 6. Methacrylate Class 3-methacryloxypropyltrimethoxy silane UC A174 Witco A174 7. Epoxy Class 2-(3,4-epoxycyclohexy)-ethyl trimethoxysilane UC A186 Witco A186 3-glycidoxy ­ propyltriethoxysilane UC A187 Witco A187 8. Isocyanato Class 3-isocyanatopropyltriethoxysilane UC A1310 Witco A1310 9. Thiocyanato Class 3-cyanatopropyltriethoxysilane Degussa Si 264 TCPTES Hung Pai HP 264 3. General Guidelines On Selection Based On Reactivity Of X Polymers Reactivity Of X Silanes Recommended 1. Unsaturated Polymers.     E.g. NR, BR, SBR,                NBR, EPDM, etc Free Radical/ Ionic Polysulfides, Mercapto, Thiocynato, Amino. 2. Peroxide Cured Free Radical Vinyl & Methacrylate 3. Halogenated Polymers.     E.g. CR, CPE, CSPE Ionic Chloro or Mercapto 4. PU¹s Condensation Epoxy, Isocyanato 5. Unsaturated Polymers Free Radical Vinyl, Methacrylate, Epoxy 4. Chemistry Silica, silicates, talc, mica, clay and whiting contain adsorbed water and hydroxyl groups. The surface chemistry of silica has been well studied and can be taken as representation. Silica surfaces contains siloxane, isolated hydroxyls and hydrogen bonded hydroxyls. Silica contains 110OC, the release of water creates more siloxane groups on the silica particle surface whilst at 110OC to enable the adsorbed moisture on the silica surfaces to be liberated. This reaction is fast at > 110OC. The second Eqn. (2) reaction is relatively slow and since water is liberated, this reaction has to be completed and the liberated water flashed off. For TESPT, a second remill mixing step is often recommended whereby the masterbatch temperature is 160OC before it is dumped. Higher temperatures can cause TESPT to liberate some of its sulphur (TESPT is a sulphur donor as well) and cause scorch and premature vulcanisation. The TESPD, MPTMS, MPTES, TCPTES and the amino class do not have above problem and 1st Step masterbatch can even be mixed up to 180OC to flash of the liberated water. All the 5 sulphur containing silanes TESPD, TESPT, MPMTS, MPTES & TCPTES can then react during sulphur curing/ vulcanisation after Eqn. (2) with the polymer viz: Example for MPTES: For peroxide cured vinyl silane treated silica filled NR formulation, the reaction of the silane vinyl groups with NR is viz:- Readers are advised to consult our Short Notes #11 for further information on peroxide curing. 5. Theoretical Considerations In The Use OF Silane Coupling Agents. Although the basic chemistry of silanes has been described, there are many theoretical and practical areas which are still not clear or pose problems. Take the more studied TESPT with sulphur cured unsaturated polymers in silica filled formulations. The functionalities of the (OR¹) & X groups are accepted as illustrated in Equations 1 ­ 4. Questions can be asked. 1.It is not confirmed whether TESPT is Eqn. (1) reacts with the free water or more probable from water liberated by heat > 110OC and/ or mechanical shear from the silica hydroxyl groups. 2.It is idealistic that each molecule of TESPT can form a cross-link between one particle of silica and one polymer chain. Statistically, one particle of silica can react with X molecules of TESPT and 1 or more of these can cross-link with polymer chain or chains. 3.The (OR¹)3 group on the TESPT has to compete with other formulation additives such as glycols, amines and accelerator containing amino, bivalent metallic soaps such as Zn or Mg stearates and other silanol hydroxyable functional compounds. 4.The very reinforcing nature of silica depends on its hydroxyl group which can form hydrogen bonding between silica particles. Removal of all these hydroxyl groups by treating silica with e.g. dimethyldichlorosilane or hexamethyldisilizane renders the silica hydrophobic and such silicas do not show reinforcing properties but behave more like inert fillers with poor abrasion resistance etc.  5.The ideal solution seems to be just enough TESPT to react with each silica particle surface but leaving other hydroxyl groups on its surface to form hydrogen bonding with an adjacent silica particle. 6.From above, it seems a fruitless exercise to attempt to solve the high viscosity or Œcrepe hardening¹ problem even in silane treated silica formulations. One can however attempt to strike a compromise ie. after the silanization reaction, to allow sufficient additives like glycol or amines, to mop up sufficient remaining silanol groups on the silica to further reduce viscosity but yet maintaining the silica reinforcing characteristics. 7.That TESPT can split off free elemental sulphur at > 180OC and can cause scorch is well understood, but that TESPT does not accelerate cure as compared example with MPTMS or MPTES which can reduce t2 & t90 considerably, has not been fully elucidated. 8.Whether the reaction in Eqn. (1) can proceed at typical open mill temperatures 160OC. The improved disulfide grade TESPD is expected to replace TESPT in time ­ but tyre compounders are known to take a long time to change a formulation. So far TESPT is found to give the lowest tan ? at 60OC compared to other silanes. 15.Because of the known chemistry of silanes, it is understood that silica + silane should be mixed first before the addition of competing chemicals like glycols, amines, zinc oxide and even some antidegradants. 16.In tyre manufacturing, the silanization and hydrophobation reactions need to be taken to near completion by a 2nd Step hot remilling/ mixing, as released alcohol and/ or water can create problems of porosity in extrusion calendaring and even final shaping/ curing of the tyres. 17.In footwear manufacture the above is not so critical as in subsequent moulding of soles, the alcohol and/ or water can easily be bumped off. 18.In footwear manufacturers, the scorchy silane like MPTES is sometimes preferred to give faster curing cycles with less dosage of TMTM needed as compared with TESPT. This is only true if scorch is well under control. 19.It is well accepted that even silane treated silicas filled formulations crepe harden and in remilling of matured storage hardened masterbatches, a lot of frictional heat is generated leading to scorch problems. To avoid this, such stocks should not be matured too long, 4-16 hours should be sufficient, and we recommend warm to hot rolls to start with in final mill mixing. 20.Some reduction in stock viscosity can be achieved with slightly higher dosages of zinc oxide/ stearic acid or PEG4000 which has less tendency to bloom. 21.All silanes are liquid form and prone to hydrolysis. Since compounders loathe to handle unstable liquids some suppliers of silanes offer heat-sealed silane + N330 in EVA bags, wax bound silane in pellet form and even thermoplastic resin bound silanes in pellet form mainly for plastic compounding. 22.We are probably the first to attempt a polymer bound predispersed silane in granular form and the storage stability of our TESPT-50G even after 3 months open ambient storage is still good. (See rheometric comparison in Appendix 1) 23.The different effects of silanes on cure behaviour in a typical transparent soling are illustrated in Appendix1. Appendix 1 Rheometric Study of Silanes in a Typical Silica Filled Transparent Outsole Formulation. Date Tested: 30 May 2000 Rheometer @ 180OC. All silanes added at 2PHR active. Control HP 669 TESPT-50G1 A 1289 HP 1891 SI 264 Tmin (N.m) 1.427 1.003 1.045 1.065 1.231 1.051 t2  (min.) 1.300 1.30 1.300 1.300 0.820 1.480 t90 (min.) 2.380 2.740 2.680 2.680 2.780 2.560 Tmax (N.m) 5.828 5.953 6.098 5.911 4.597 5.460 Tmax ­ Tmin (N.m) 4.401 4.950 5.080 4.846 3.366 4.409 Notes: 1. Our Polymer bound TESPT-50G was produced 18 Feb. 2000 i.e. more than 3 months ambient storage. Comments 1.Both sources of TESPT from Hung Pai HP 669 and Witco A1289 give nearly similar rheometric data. 2.The scorch safety of the silanes are in the order TCPTES > TESPT > MPTES. Our own evaluation confirms that use of Mercapto class silanes may give scorch problems. 3.Higher modulus as expected are given by TESPT as compared to TCPTES and MPTES. 4.The highest Tmax ­ Tmin is given by our TESPT-50G indicating the least loss of active ingredient. For this parameter our TESPT-50G is 3-5% more efficient than the liquid form. 5.The similarity of 3 months old TESPT-50G with fresh liquid active TESPT illustrates that its storage stability is good up to at least 3 months ambient storage. 6.The better scorch safety and yet faster cure time t90 of TCPTES vs. TESPT is noted. Table 1. Rheometric Comparison Of  3 Months Old TESPT-50G1 With Fresh Active TESPT In A Typical Transparent Soling Formulation. Formulation Control Fresh TESPT 3 months old TESPT-50G SMRL2 10.00 10.00 10.00 NBR3 10.00 10.00 10.00 BR4 80.00 80.00 80.00 Silica 48.00 48.00 48.00 ZnO Active 1.50 1.50 1.50 Stearic Acid 1.00 1.00 1.00 Antioxidant BHT 1.00 1.00 1.00 Processing Aid 2.00 2.00 2.00 PEG 40005 3.00 3.00 3.00 Naph. Oil6 14.00 14.00 14.00 ISE-75G7 3.00 3.00 3.00 MBTS-80G8 1.00 1.00 1.00 TMTM-80G9 0.20 0.20 0.20 TESPT -- 2 -- TESPT-50G -- -- 4 Rheometer@180OC Control Fresh TESPT 3 months old TESPT-50G Tmin (N.m) 1.427 1.003 1.045 t2 (min) 1.300 1.300 1.300 t90 (min) 2.380 2.740 2.680 Tmax (N.m) 5.828 5.953 6.098 Tmax ­ Tmin (N.m) 4.401 4.950 5.080 Notes:   1.Tespt-50G denotes 50% active TESPT polymer bound in granular form. 2.Standard Malaysian Rubber Grade 3.Butadiene Acrylonitrile Copolymer 4.Polybutadiene rubber 5.Polyethylene glycol 6.Naphthenic Oil 7.Sin Rubtech insoluble sulphur in PBPC form 8.Sin Rubtech dibenzothiazole disulfide in PBPC form 9.Sin Rubtech tetramethylthiuram monosulfide in PBPC form.

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