A Guide to Silane Solutions: Plastics, Polymerization and Rubber

A Guide to Silane Solutions

Plastics, Polymerization and Rubber Plastics, Polymerization and Rubber

The unique properties of silanes are Figure 1. Grafting of VTMOS to polyethylene – Sioplas® process. used to enhance performance and improve processes in the plastics and rubber industries. Silanes function as coupling and dispers- ing agents for fillers in rubber and OMe plastics formulations, as polymer- ROOR OMe ization modifiers in the synthesis of + Si Heat OMe Si polypropylene, and as crosslinking OMe OMe agents for polyethylene homopoly- OMe VTMS mers and copolymers. Silane-Grafted Polyethylene

Plastics Compounding Vinyl silanes have been used Polyethylene commercially since the 1970s to crosslink polyethylene homopolymer Figure 2. Crosslinking of polyethylene in the presence of and its copolymers. Vinyltrimeth- moisture – Sioplas® process. oxysilane and vinyltriethoxysilane are the most common silanes used in the process. In an extruder in the presence of peroxide and heat, Si OMe MeO the vinyl group will graft to the Si OMe polyethylene backbone, yielding a MeO DBTDL MeO silane-modified polyethylene that O MeO +H2O/-MeOH contains pendant trialkoxysilyl MeO MeO Si OMe functionality. The grafted polyethyl- Si OMe ene can then be immediately crosslinked in the presence of a tin catalyst, moisture and heat to DBTDL = Dibutyltindilaurate create a silane-crosslinked product. Diagrams of the grafting of vinyltrimethoxysilane (VTMOS) to polyethylene and the moisture transformed into its final product Silane-crosslinked polyethylene is crosslinking process are shown in configuration. Using the same used for electrical wire and cable Figures 1 and 2. The ease of silanes, it is also possible to insulation and jacketing where ease processing and the simple equip- copolymerize the vinyl silane with of processing, increased tempera- ment required make this the ethylene monomer to make ture resistance, abrasion resistance, preferred method of producing trialkoxysilyl-functionalized polyeth- stress-crack resistance, improved crosslinked ethylene polymers and ylene. This then can be crosslinked low-temperature properties and copolymers. The process also in the same manner as the graft retention of electrical properties are allows crosslinking to be delayed version. until after the grafted product is needed. Other applications for this alkoxy groups are either methoxy technology include: or ethoxy with one, two, or three alkoxy groups on the silane • Cold- and hot-water pipe molecule. Two of the more common where resistance to long- silane donors are Donor C, term pressure at elevated cyclohexylmethyldimethoxysilane temperatures is essential (XIAMETER® OFS-6187 Silane); • Natural gas pipe with good and Donor D, dicypentyldimeth resistance to stress cracking oxysilane (XIAMETER® OFS-6228 • Foam for insulation and Silane). packaging with greater Reactive silicone polymers have resiliency and heat resistance also been used to produce ther- • Other product and process moplastic vulcanizates (TPVs). types, such as film, blow- TPVs are prepared by chemically molded articles, sheeting crosslinking a rubbery phase in and thermoforming a thermoplastic matrix. TPVs are produced by dynamic vulcanization, Polymer Manufacturing and silane chemistry allows new Selected silanes, known as and unique crosslinking chemistries “external donors,” or electron to be used in the manufacturing donors, are used in conjunction process. with Ziegler-Natta catalysts in the manufacture of polypropylene. Rubber Compounding Ziegler-Natta catalysts are A major use for silanes has organometallic compounds. developed in the organic rubber Organoalkoxysilanes can industry as a result of the benefits chemically coordinate with the that can be obtained from the use of organometallic catalyst to modify inorganic filler in place of carbon the course of the polymerization. black in the reinforcement of rubber. Specific variations in the tacticity of Silica and other inorganic filler the propylene polymer are possible reinforcements for rubber provide by optimizing the use of a silane unique physical properties and donor in the process. Different performance properties versus silane donors with differing organo- carbon black reinforcement; alkoxy structures are used depend- however, silane coupling agents ing on the exact nature of the are necessary for the non-black catalyst and the type of polypropyl- reinforcing fillers to be effective. ene being manufactured. Organic substituents, such as cyclohexyl, Silanes are the key to providing a cyclopentyl, methyl, isobutyl, and method of effectively bonding phenyl, are some of the organic inorganic fillers to organic elasto- groups attached to silicon. The mers. Silane-coupled, mineral-filled rubber products are used for automotive Figure 3. Bonding organic rubber to silica with sulfur silanes. and off-road tires, shoe soles, belts, (EtO)3Si(CH2)3-Sx-(CH2)3Si(OEt)3 hoses, and mechanical goods.

EtO OEt Methoxy- or ethoxy-silanes will bond Si OEt tenaciously to the silica or clay surface; OEt OEt s then the organic portion of an organo- Si O Si Sx Si O Si S O OEt O OEt s Si OH Si OH functional silane will bond to the rubber OEt s O OEt O polymer. See Figure 3. Si O Si Si O Si O O Sx O O Sx s Silica Si O Si Silica Si O Si Rubber The silane is usually added during the O O O O compounding process to treat the filler Si O Si Sx Si O Si S O OEt O OEt in situ. It must have the proper rate of Si O Si O s EtO Si s reactivity to spread and react over the filler EtO OEt s surface and still be able to react with the The silane can react in the elastomer at a rate that allows processing sulfur vulcanization. of the rubber to be completed.

This can be done with silane coupling Figure 4. Structure of sulfidosilanes used in rubber compounds. agents that have triethoxysilyl groups at EtO H2 H2 OEt both ends of a polysulfido (tetrasulfide, C Sx C disulfide or mixture thereof) organic Si C C C C Si group. See Figure 4. H2 H2 H2 H2 EtO OEt These coupling agents are supplied as OEt OEt neat liquids or as blends with a carrier such as carbon black. Even though silica x ranges from 2 to 10 can be used as the only filler, rubber tires These are termed S2, S3, etc., monomers incorporate small levels of carbon black Bis-TriEthoxy Silyl Propyl Polysulfide - TESPX to give consumers the uniform black color they expect. Without carbon black in the rubber compound, it is possible to make tires in a variety of colors.

A specific example of this application (as listed above). They also use a min- In addition to silanes, the XIAMETER® is the silica/silane technology used in eral-derived filler rather than one derived brand product line includes silicone rubber “green” tires to impart: from a fossil fuel (natural gas or oil). This compounds and bases. Silicone rubber is is currently the largest market for silane made from silicone polymers compound- • Increased abrasion resistance coupling agents. ed with non-black fillers, usually fumed or • Reduced rolling resistance precipitated silica. These compounds The use of vinyl silanes as a coupling and improved fuel economy require silanes and functional silicone agent in kaolin clay reinforced EPDM of tires fluids. Silanol-functional silicone fluids and wire and cable coatings is another • Better grip on wet and snow/ vinyl-functional silanes are available for important rubber application. The vinyl ice surfaces silicone rubber compounding. silane improves the electrical properties Silica-reinforced tires are known of the reinforced rubber so a stringent as “green” tires because they provide im- power-factor electrical test can be proved fuel economy while maintaining passed, but only when optimum silane or improving other tire properties coupling agent technology is used. Product Information A complete list of XIAMETER® brand silanes for plastics compounding, polymer manufacturing, and rubber compounding is available at xiameter.com.

Dow Corning Corporation also offers a wide variety of Dow Corning® brand specialty silicone material and service options as well as other silicon-based materials to help you keep your innovative edge in the marketplace. Visit dowcorning.com to learn more about the many additional silicone and silicon-based options available to you from Dow Corning.

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