800-541-0559 Unitedchem Formulating Silicone Adhesives
2731 Bartram Road, Bristol, PA cone AdhesivesGels and SealantsJohn H. MacMillan Ph.D.Ph.D.1
Silicone Technology is a wide and mature science spanning the gamut ofmaterials from commodity lubricants to high performance defense coatings. This paperwill give an overview of the basic technology and classes of silicone fluids. Curetypes,basic formulations,applications,catalyst types and blending methods will all bediscussed. United Chemical Technologies (UCT) is a major manufacturer of highquality silicones. During the discussions the catalog numbers for the correspondingUCT product will be introduced when appropriate. In many chemical reactions thereacting groups are color coded for clarity.For those wanting additional detail on industrial manufacturing methods forbulk silicones, the Kirk Othmer Encyclopedia (Ref 1) is an excellent guide. More recentadvances in high performance silicone materials are reviewed by Brook (Ref 2) in anexcellent text.Before we discuss basic silicone structures and curing mechanisms,it isnecessary that the reader be familiar with simple silicone structural terminology andother common industrial terms. Figure 1 and Table 1 review the structural featuresand industrial terms commonly encountered.2
Figure 1SiliconesINTRODUCTIONCH3CH3SiCH3( )CH3OSiCH3OnCH3SiMeSiD unitOMeM unit(Me 3 SiO)CH3CH3OMeSiOT unitOQ unitOOOSiO3
Table 1 Common TermsBasic TerminologyCure: Crosslink a liquid polymer system to where it forms an elastic gel (sets up and solidifies).Addition Cure: Platinum Catalyzed,Examples:PC072, PC075, PC085Condensation Cure: Tin, Zinc Catalyzed,Examples:PC040, PC050Peroxide Cure: Peroxide Catalyzed,Example: PC010RTV : Room Temperature Vulcanizabe (curable) system,Usually an addition or condensation cure resin.4
The most basic class of silicones is the conventional inertpolydimethylsiloxane series. They are trimethylsiloxy terminated and possess noreactive chemical functionality. These fluids find utility as plasticisers, lubricants,defoamers, in high temperature baths, and cosmetic products. Typical servicetemperatures are to 250 celsius. UCT offers the full line of these materials from verylow to gum like viscosities. Table 2 is a compilation of this series.5
Table ashPointPourPointPolydimethylsiloxanes Trimethylsiloxy Terminated CAS No. ed Polydimethylsiloxane CAS No. (68037-74-1)PS05250 cSt.0.971.4030-1 38 63 79 100 135 150 163 232 285 315 315 315 315 315 315 315 315 315 315 321 321 321 321 321 -68 -85 -75 -80 -70 -65 -65 -65 -65 -65 -65 -60 -60 -55 -50 -48 -48 -46 -43 -42 -41 -41 -41 -39 -38 279 -85 Polydimethylsiloxane EmulsionPS053.530% silicone antifoam in water.Used to defoam biological mixtures by adding 2-50 mg/liter.6
Although conventional fluids are stable thermally to 250 celsius, many highperformance coating applications require higher temperature stability. Introduction ofaromatic groups (phenyl rings) raises thermal stability to greater than 300 celsius. Theindices of refraction and the rigidity of cured resins are also increased overconventional fluids. Figure 2 describes these materials in more detail. They areavailable with curable vinyl terminals at the polymer terminals and containingvariable viscosities and mole percentages of phenyl groups. The most utilized UCTproduct line of curable aromatic silicones is the PS732-PS793 series of vinyl terminatedfluids.7
Figure 2 Thermal SiliconesCH3CH3SiCH3( )( )CH3OSiSi - OmCH3OnCH3CH3SiCH 3CH3SiCH3CH3O( )Si - OCH3nCH3SiCH3The most commonly used high temperature silicone fluids are the phenyl containing siloxanes. The phenylgroup is usually incorporated in two ways. It may be introduced as a phenylmethylsiloxane or adiphenylsiloxane. As phenyl groups replace methyl groups in a polysiloxane, several changes occur.Lubricity, oxidation resistance, thermal stability and shear resistance are enhanced. For polymethylphenylsiloxane the service temperature is -55º to 290º C.In a closed oxygen-free system the polymethylphenylsiloxanes are stable for thousands of hours at 230ºC. Thematerials are used in heating baths. The tetrachlorophenyl-dimethylsiloxane copolymer is well-suited for metalto metal lubrication. They are also used as base oils for high temperature lubrication.The phenyl group also introduces rigidity in the silicone chain. When substitution exceeds 75 mole percent thepolymers are solid. Diphenylhomopolymers demonstrate liquid crystal behavior between 250ºC and 500ºC.Silanol terminated polydiphenylsiloxane is a glassy solid useful as a resin intermediate. The refractive indexalso increases with phenyl concentration. At 15-20 mole percent phenyl concentrations the refractive indexmatches that of amorphous silica and transparent compounds may be prepared.8CH3
Figure 2 Thermal Silicones (Contd)At low phenyl concentrations these fluids are dielectric coolants. They also function in place ofstandard dimethylfluids where extended service temperature is necessary. One special class of phenylfluids are the dimer, trimer and tetramer fluids (see section on diffusion pump fluids). Because theintroduction of a moderate concentration of phenyl groups also improves lubricity, members of thefamily have found utilization as lubricants for instruments and timing devices. High phenyl contentfluids are also used as heat transfer media and chromatographic stationary phases.Thepolymethylphenylsiloxanes exhibit good radiation resistance by remaining serviceable up to 200megarads exposure.The compressibility of phenyl containing siloxanes is reduced in comparison to dimethyl fluids.Polymethylphenylsiloxane has a compressibility of 5.5% at 20,000 psi. Dimethyl phenylmethylsiloxanecopolymer has a compressibility of 6.5% at 20,000 psi.9
Silicone fluids typically show low solubility in petroleum oils. Addition oflong hydrocarbon chains on the backbone increases solubility in these oils and also thelubricity. The fluids are then of utility as oil well defoamers and cosmetic lubricants.These fluids are described in detail in Figure 3. The UCT product line is shown inTable 3.10
Figure 3 PolymethylalkylsiloxanesCH3CH 3SiCH3( )(CH2 )mCH3OSiOCH3nCH3SiCH3CH3Silicones can be modified to impart organic characteristics to their inorganic structure, which makes them more compatiblewith organic materials such as petroleum oils and synthetic resins.Replacement of a methyl group with longer chain aliphatic moieties produces silicones with properties that more closelyresemble hydrocarbons. When compared to polydimethylsiloxanes, methylalkylsiloxanes have greatly improved lubricationcharacteristics and greater compatibility with organic materials. The fluids have higher viscosity-temperature coefficients,lower compressibility and decreased oxidation stability. This last characteristic has been substantially overcome bycompounding the fluids with stabilizers such as BHT or DSTDP or copolymerizing them with aromatic siloxanes as internalstabilizers.Lubrication properties are optimized when the alkyl unit is at least eight carbons long. Polymethyloctylsiloxane is useful as alubricant for soft metals such as aluminum, zinc and copper. It is also useful as a rubber or plastic lubricant especially whenmated against steel or aluminum. Polymethyloctylsiloxane is also employed in aluminum machining operations.With increasing length of the alkyl substituent the melting point increases. As the pour point of the alkyl modified siloxanesincreases, the resemblance and compatibility with hydrocarbon oils increase. Polymethyltetradecylsiloxane has a highdegree of hydrocarbon compatibility and maintains liquid behavior at room temperature. Polymethyloctadecylsiloxane is acreamy solid with a melting point just above room temperature. It is compatible with paraffin wax. It is used as a componentin thread and fiber lubricant formulations and as a process aid in melt spinning. Methylalkylsiloxane reduces the surfacetension of many non-aqueous solvents allowing them to act as wetting and leveling agents in coating and ink formations.11
Table 3 PolymethylalkylsiloxanesProd#Description loxane [68607-75-0]250-3000.891.44350 39.5PS130.5(70%) Dimethyl (30%)Methyloctadecyl tradecylsiloxane [76684-67-8]1500-30000.891.455PS134.5(70%) Dimethyl (15%)Dodecyl (15%) TetradecylSiloxane Terpolymer150-2000.9161.43PS134.8(50%) Dimethyl (25%)Dodecyl (25%) TetradecylSiloxane xanePS136.5(35-40%)Methyloctyl- (3-4%)Vinylmethyl (56-64%)Dimethylsiloxane l HomopolymerVinyldimethyl Terminated2001.431235 -50 30.4
The most outstanding thermal stability and solvent resistance is imparted bythe inert fluorosilicones (see description in Figure 4). Indices of refraction areconsiderably lower than for conventional fluids. See the UCT product catalog for moreinformation. Curable versions of this series are UCT catalog number PS184.5, a silanolfluid, and PS185, a vinyl fluid. The curing chemistries for these classes of silicone willbe described later.13
Figure 4FFluorosiliconesFFCH 2CH 3SiCH 3CCH 3( )CH 2OSiCH 3OnCH 3SiCH 3CH 3Many advantages of fluorocarbons and silicones are combined in fluorosilicones. Fluorination of compounds usuallyenhances their thermal stability. In the case of the polysiloxanes, fluorination usually begins at the gamma position of analkyl chain. Due to the electropositive nature of silicon, fluorination at the alpha and beta positions generally results in poorthermal stability. As a consequence, commercial fluorosilicones are currently limited to trifluoropropyl substituted methylfluids. The materials are useful from -40 to 285 C in a wide range of aggressive service environments. The fluids areexcellent lubricants under extreme pressure conditions. Trifluoropropylmethylsiloxane fluids have a compressibility of7.45% at 20,000 psi. They are not miscible with fuel or oils.Trifluoropropylsilicone fluids have achieved a number of unique applications due to their chemical and solvent resistance,lubricity and thermal stability. They have been employed in mechanical vacuum pumps where moisture and hightemperature oxygen exposure is encountered. They are used as defoamers in processes involving solvent-based washsolutions. They have been employed as grease bases when formulated with thickeners such as PTFE. Their high densityhas led to their use as flotation media for inertial guidance systems. Longer chain fluorinated silicone fluids demonstrateincreased solvent resistance and have been employed as partitioning phases in gas chromatography. Acoustic velocitiesin fluorosilicones are lower than in conventional silicones, allowing sonar lens development.Copolymers of fluoroalkylsiloxanes with dimethylsiloxanes exhibit improved boundary lubrication properties whilemaintaining excellent solvent and chemical resistance. They also form high performance greases when thickened withfluoropolymer powders. Such greases have been used in sealed transmissions and other extreme pressure applications.Other fluorocopolymers have been employed as lubricants for electrical contacts and precision timing devices.14
Silicone fluids typically show limited water solubility and must beemulsified. To impart such solubility long chain polyethylene oxide or polyethyleneoxide/polypropylene oxide chains are grafted on to the silicone backbone. Figure 5 andaccompanying text describes this series. See the UCT product catalog for moreinformation. Available materials are non curable and are typically used as surfactantsor defoamers.15
Figure 5 Hydrophilic SiliconesHydrophilic silicones are non-reactive fluids that have been modified to give them slight to complete solubility in water.Their chemical description is polyalkylene oxide modified polydimethylsiloxanes. The typical structure of a hydrophilicsilicone is:(CH2)3(OCH2CH2)XOCH3( )( )CH3(CH3)3SiOSi - OCH3SiCH3mOSi(CH3)3nPolydimethylsiloxanes prepared as copolymers with alkene oxides are widely used as surfactants. By altering theamounts of alkene oxide (hydrophile) and dimethylsiloxane (lipophile) the desired surfactant properties may be balanced.The higher the alkene oxide content the higher the hydrophilicity. Materials with ethylene oxide contents 75% and higherare freely soluble in water, as well as other commonly used organic solvents such as methanol, isopropanol, acetone,xylene, and methylene chloride. PS071 is a low molecular weight water soluble fluid that is used in lithographic andphotographic plates to facilitate wetting and spreading of developers. It is also used as an anti-fogging treatment for glassand plastic optical surfaces. PS073 is used as an anti-foam in water-based coatings. PS072 is a water solublecopolymer employed as a lubricant for fibers and plastics. It also imparts anti-tack and mar resistant qualities coatings.PS071, PS072 and PS073 all have excellent long term hydrolytic stability. PS071 and PS072 provide slip in flexographicand gravure inks. PS073.5 reduces the static charge generation of fiber substrates and has been incorporated into rollingoil formulations for metal drawing and stamping. The coefficient of thermal expansion is 8 x 10 C.16
The silicone fluids described to this point are non reactive, that is, they arenot readily cured to solid elastomers. Table 4 summarizes many of the uses of thesematerials. Usually applications involve adding the liquid to a formulation as aplasticiser or stabilizer. Uses also include lubricants in machinery and hightemperature bath oils.17
Table 4 Silicone Fluid Selection GuideFunctionApplicationFluid ional (Low Viscosity)Hydrophilic orFluorosiliconeHydrocarbon CompatibilityOrganic CompatibleFlow ControlConventional (Low Viscosity)WettingHydrophilicRadiation ResistanceThermalAcousticalSonobuoySound CouplingConventional (Low Viscosity)FluorosiliconeOpticalOptical Coupling FluidAnti-fog AgentThermalHydrophilicHeat TransferHeat Treatment BathThermalConstant Temperature BathConventional (Intermediate Viscosity)or ThermalTemperature MeasurementDeviceConventional (Intermediate Viscosity),Thermal orFluorosiliconeClosed Loop HeatingThermalRefrigerated SystemsLow Temperature18
Silicone Fluid Selection Guide (contd)FunctionApplicationFluid ClassWorking MediaFluid ClutchConventional orThermalConventional orOrganic CompatibleLow Temperature,Conventional orThermalSmart FluidsHydraulic FluidBrake FluidConventional (Intermediate Viscosity)Shock AbsorberConventional,ThermalGeneral DampingConventional,Thermal orFluorosiliconeMeter DampingConventionalTiming DevicesConventional orThermalMagnetic AmplifierThermalDiffusion PumpThermal (Oligomeric)19
Silicone Fluid Selection Guide (contd)FunctionApplicationFluid ClassLubricationMold ReleaseConventionalOrganic CompatibleEmulsionAluminum Machiningand ExtrudingOrganic CompatibleDie CastingOrganic CompatibleBall Bearing and GearLubricationOrganic CompatibleThermal orFluorosiliconeAirborne RadarLow TemperatureRubber/Plastic ContactConventional orOrganic CompatibleFiber/Plastic ContactHydrophilicMetal/Plastic ContactOrganic CompatibleThermal orFluorosiliconeMetal/Metal ContactOrganic CompatibleThermal (Chlorophenyl)GreaseConventional,Thermal orFluorosilicone20
Silicone Fluid Selection Guide (contd)FunctionApplicationFluid ClassDielectricCoolant/FluidTransformers, nalThermalDielectric Impregnationof Porous SubstrateConventional21
The basic feed stocks employed in homogeneous platinum catalyzed siliconeresin systems are vinyl fluids, whose structural types are outlined in Figure 6, andhydrosilicones, outlined in Figure 8. Vinyl fluids are typically blended in the “ base” or“part A” side while hydrosilicones are put into the “crosslinker” or “part B” side. TheA side and B side convention is sometimes violated and inverted by major turn keyformulation manufacturers. Technical datasheets and MSDS sheets should be reviewedfor unequivocal determination of each sides identity.United Chemical Technologies is also a major manufacturer of these silicone resins.Table 5 and Table 6 list these materials and their most common applications.22
Figure 6 Vinyl FluidsMeCH 2CHSiMe( )MeOSiMeOnMeSiCHCH 2MeVinyl Terminated SiloxanesCH 2MeMeSi( )( )MeOMeSiMeCHSiOmMeOnMeSiMeVinyl Functional Copolymers23Me
Vinyl functional silicones are commonly cured by two routes. The “additioncure” route involves adding a silicon hydride functional group to a terminal “vinyl”double bond. The catalyst is a homogeneous platinum catalyst of the Karstedt (Ref 4)type. These cures can be either at room or high temperature, depending on the activityof the catalyst. We will describe these platinum catalysts in more detail later (seeFigure 15).The second route involves cures catalyzed by a high temperature radicleinduced polymerization , initiated via decomposition of an organic peroxide. Both ofthese routes are illustrated in Figure 7. In a later section we will discuss in detail theadvantages of the platinum catalyzed route over the peroxide route for curing siliconeresins.24
Figure 7 Cure of Vinyl SiliconesSi-CH CH2VINYL FUNCTIONAL SILICONESOOSiH H2C CHSiCH3CH3OCH3PtOOSiCH3SiCH2 CH2CH3OCH3addition cureOOSiCH3CH3 H2C CHSiCH3OCH3ORO OSi- ROHCH3CH3peroxide activated cure25 CHCH2 CH2SiCH3O
The complete product line of United Chemical Technologies vinyl silicones islisted in Table 5. PS437 through PS449.5 are di-functional at the the polymerterminals ( the ends). These fluids are the base resins incorporated into the “Part A” oftwo part silicones employing platinum catalysis. These formulations will be describedand illustrated in later sections. PS422 through PS488 are poly ( 2) functional invinyl, either in the backbone or at the terminals. These resins are additives to the di functional fluids in Part “A” to impart greater hardness and tear strength.“Q” resins (see Figure 1 nomenclature) are also used as additives toimprove tear strength and abrasion resistance. PS496 and PS498 are representativemembers of this class offered by UCT.PS437 and PS925 are efficient cure retarders for the platinum basedaddition cure silicone curing systems.26
Table 5 Vinyl FluidsDescription (CAS #)ViscosityWeight % VinylSGPrimary Use in RTVAddition Cure SystemsPolydimethylsiloxane, Vinyldimethyl Terminated CAS No. 10012-140.97PS441.22000.97Base polymer or part ofPS4425000.97Polymer blend of liquid 449600,0000.98Main polymer for HighPS449.51,000,0000.98Consistency RTV systems0.18-0.260.03-0.0710,000High strength RTV systems0.970.01-0.02Polydimethylsiloxane, Monovinyl, Mono n-Butyldimethyl TerminatedPS491Cure moderator or as an inhibitor0.05-0.10.9827
Table 5 Vinyl Fluids(CONTD)Description (CAS #)ViscosityWeight % VinylSGPrimary Use in RTVAddition Cure SystemsVinyldimethyl, Dimethylsiloxane Copolymer, Trimethylsiloxy Terminated CAS No. oxide or
Formulating Silicone Adhesives Gels and Sealants John H. MacMillan Ph.D. Gels and Sealants John H. John H. MacMillan Ph.D.D. 2 Silicone Technology is a wide and mature science spanning the gamut of materials from commodity lubricants to high performance defense coatings. This paper
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