BONDING, SEALING, POTTING/ ENCAPSULATION AND COATING WITH RTV COMPOUNDS

CREATING TOMORROW’S SOLUTIONs Where to Find What Introduction 3 Properties 4 Bonding, Sealing 8 Potting/Encapsulation, Coating 10 Silicone Systems 12 RTV-1 Systems 14 RTV-2 Systems 16 RTV-2 Processing Information 18 WACKER Worldwide 20

WHOEVER IS FAMILIAR WITH WACKER RTV SILICONE RUBBER COMPOUNDS KNOWS THEIR STRENGTHS

 LOOK FOR A PRODUCT AND FIND IT!

Room-temperature-vulcanizing silicone With more than 50 years’ experience in rubber compounds are a traditional silicone technology and an above- mainstay of WACKER SILICONES’ average commitment to R&D, WACKER product portfolio. Our range of ready- SILICONES has long since become to-use products for the numerous indispensable to its customers for their applications in the fields of bonding, technical progress. This would not sealing, potting/encapsulation, coating be possible without ongoing, confidential and mold-making is accordingly dialog – without the trading of knowledge, impressive. Equally impressive, however, experience and ideas that powers a is WACKER SILICONES’ experience in shared undertaking. Our joint undertaking the processing techniques and material right here and now is to find the ideal requirements specific to these industries: product for your application. from DIY-scale right up to industrial production lines.

You can always rely on WACKER’s technical support service.

 BENEFIT FROM QUALITY

Thermal Resistance Bonding Properties Mechanical Properties Thermal resistance is just one of the Silicone rubber grades adjusted to be WACKER supplies RTV silicone rubber outstanding material properties that self-adhesive are suitable for many compounds to meet a wide range of characterize silicones. For applications substrates. The bond quality will depend mechanical requirements. Compounds involving temperatures above 150 °C, on the nature of the material to be with a processing viscosity in excess silicones are the only elastomer bonded, the stress, the kind of bond of 10,000 mPa s yield elastomers with candidate. Unlike other elastomers, and possibly the surface treatment. excellent tensile strength, elongation at they can withstand temperatures up The most superior bonds are obtained break and tear strength. Low-viscosity to 180 °C permanently; special heat- on oxidic and siliceous surfaces. products (less than 10,000 mPa s) yield stabilized grades will even withstand Good adhesive strength is also obtained elastomers with mechanical properties brief exposure to temperatures of with a number of plastics. that are generally not as strong. up to 300 °C. For maximum thermal stability, it is important that the silicone In other instances, it may be necessary RTV silicone foams constitute a special rubber cures, or vulcanizes, completely. or advisable to prime the substrate or class of RTV silicones. Although their Thermal stability can be enhanced subject it to flame or plasma treatment mechanical properties are less good on still further by subjecting the silicone prior to bonding. Surfaces must always account of their porous structure, these compound to subsequent, well- be clean and free of grease. Low-boiling foams offer excellent damping and ventilated post-curing, during which solvents such as acetone1 or aliphatic insulation. Their compressibility makes the temperature is raised slowly. hydrocarbons are suitable cleaning them ideal for use in seals. At the other end of the scale, silicones agents. remain flexible down to -50 °C, with Silicone gels are another special class of specialty grades resisting temperatures Release Properties RTV silicones. These very soft materials as low as -110 °C. The surfaces of silicone elastomers (Shore A < 0) have an extremely low show a pronounced release effect when elastic modulus, so that even large Expansion used with inorganic and organic temperature differences cause only little Silicone rubber elastomers have a materials such as gypsum, concrete, thermomechanical stress. Silicone coefficient of linear expansion that is a , epoxide, , gels are thus ideal for protecting delicate direct function of filler loading and polyamide, polystyrene, PVC, wax and electronic components. hence of specific gravity. The higher the metal alloys. This effect is exploited density of a rubber, therefore, the lower in the use of silicone rubber compounds is its coefficient of linear expansion. to make molded parts and reproductions. Typical values are in the range of 2·10-4 WACKER supplies pourable, spreadable m/mK and are thus substantially higher and kneadable silicone rubber grades to than for other potting/encapsulation suit a wide variety of applications. compounds. Since silicone rubber undergoes relatively pronounced expan- sion during the curing process, the shrinkage encountered on cooling is likewise pronounced. This “apparent” shrinkage must be taken into account in advance.

1 Please pay attention to the material’s flammability.

 Permeability to Gas and Water-Vapor Chemical Resistance Radiation Resistance At room temperature, the gas Silicone rubber is resistant to aqueous Silicone rubber can be exposed to high permeability of silicone rubber is about solutions, diluted acids and bases doses of electromagnetic radiation – ten times that of natural rubber. Not until and polar solvents. Solvents such as from the microwave to the UV range – 100–150 °C do the permeability values ketones, esters and hydrocarbons without any noticeable effects. In converge. Under normal conditions, may cause reversible swelling of the terms of radiation resistance and hot- silicone rubber may contain some silicone rubber but do not destroy its air resistance, it is superior to all other 15–20 vol % of dissolved air. Reducing chemical structure. Products with a organic elastomers. This explains the or increasing the gas pressure causes high filler loading and/or that are very use of silicone rubber in such a wide a decrease or increase in the quantity of hard tend to swell less than softer variety of applications, ranging from dissolved gas. Consequently, a sudden compounds. The only way to remove microwave cookers to solar generators. pressure drop will make dissolved air cured silicone rubber from a surface become visible in the form of bubbles. is by destroying the rubber completely Flame Resistance Special care must be taken when with concentrated sulfuric acid, The auto-ignition temperature of cured metering equipment is used, as the use alcoholic alkali solution or the like. silicone rubber is about 430 °C. of compressed air to convey the The resultant flame burns at 750 °C. compound can also cause bubbling. In addition to the usual combustion Similar effects can occur on rapid products, namely carbon dioxide and heating, as the gas solubility decreases water, silicon dioxide is produced with a rise in temperature. The water- and forms an insulating layer of ash vapor permeability, as determined in on the surface. Silicones are flame- accordance with DIN 53122 under resistant – even without the addition climatic conditions D and a film of halogen compounds, which can thickness of 2 mm, is about 20 g/m2 d. generate corrosive and toxic gases in a fire. Generally speaking, their fire performance is better than that of many organic elastomers and resins.

 Environmental Compatibility Electrical Properties Optical Properties Silicones have the same basic chemical The electrical properties of silicone The color and appearance of silicone structure as quartz. The cured rubber rubber remain almost constant over the rubber is determined by the fillers therefore poses no known ecological or temperature range from -45 °C to selected for the compound in question. physiological risks. +180 °C. At room temperature they The light permeability of thin layers of compare well with those of other unfilled materials is almost 100 % in the Thermal Conductivity insulating materials. What makes silicone visible range from 400 to 760 nm. These The thermal conductivity of most rubber elastomers far superior to materials only become opaque in the silicone elastomers is in the range from other materials, however, is the fact that UV range below 200 nm. Their refractive

0.15–0.25 W/(Km) at room temperature. elevated temperatures have no appreci- index nD25 is between 1.410 and 1.404. However, values up to 2.5 W/(Km) may able effect on their insulation resistance, be obtained with specialty grades that dielectric strength or dissipation factor. Storage Stability have a high filler loading. These properties even remain constant SEMICOSIL®, ELASTOSIL® and when the compounds are immersed in WACKER SilGel® silicone rubber grades water. have a shelf life of up to 24 months (the exact time will depend on the grade in Normally, silicone elastomers are question) when stored in the original, electrical insulators. However, they can sealed containers at between 5 °C be rendered electrically conductive by and 30 °C. Storage beyond the date adding specialty fillers. specified does not necessarily mean that the material is unusable, but a Unlike organic elastomers and casting quality check should be performed on resins, RTV silicone rubber shows the most important properties. excellent arc and tracking resistance. Specialty grades can attain values > 3.5 kV (IEC 587) for arc resistance and > 300 s for tracking resistance.

 DISCOVER DIVERSITY

By virtue of their diverse and excellent Property Overview • Superlative dielectric properties that properties, RTV silicone rubber • Outstanding thermal resistance from remain almost constant over a wide compounds from WACKER have a wide -50 °C to +180 °C; specialty grades temperature and frequency range variety of bonding, sealing, potting/ can withstand temperatures up to • Excellent environmental compatibility encapsulation, coating and mold-making +250 °C and, at the other end of the and no known harmful effects applications2. scale, down to -100 °C • Water-repellent surface and low • Very good bonding to a variety of moisture uptake substrates • Low elasticity modulus • Excellent weathering and radiation • High chemical purity resistance • Very good chemical resistance

2 You will find more on the subject of mold-making in our brochure “ELASTOSIL® M Mold-Making Compounds for Maximum Precision.” Please contact your WACKER sales office.

 SUPERIOR BONDING

Mechanical assembly techniques such • Ability to bond dissimilar materials Products for Adhesive Bonding as screwing and bolting, riveting, welding • Thanks to the good insulating WACKER offers products with widely and are being increasingly properties of silicones, metals with differing property profiles for the supplanted by modern adhesive bonding different electrochemical properties adhesive bonding of materials: methods. The advantages are convincing: can be bonded without risk of • From flowable to non-sag galvanic corrosion. The dimensions • Hardness levels from Shore A 10 Advantages of Adhesive Bonding of the substrate remain practically to Shore A 80 • More uniform stress distribution, as unchanged • From transparent to black adhesives distribute stresses over the • An adhesive bond affords good • Electrically insulating or electrically entire bonding area and thus minimize vibration damping, as the silicones conductive high localized stress concentrations have a much lower elasticity modulus • Thermally conductive up to W/(Km) • An adhesive bond is simultaneously than the substrate (on request), but electrically insulating a seal and thus prevents the corrosion • Cost savings at various manufacturing that so frequently occurs with levels: reduced warehousing, reduced mechanical fasteners labor costs thanks to automated • Thanks to the high flexibility and low adhesive application, less critical elasticity modulus of adhesive bonds, machining tolerances for the parts cyclic temperature loading causes to be bonded only small thermomechanical stresses in bonded substrates with different coefficients of thermal expansion

 FULLY AUTOMATED SEALING

Sealing technologies Preformed gasket FIPG LIS CIPG

Application Injection molded separately Pasty to flowable, Pasty to flowable, Pasty to flowable, fully automated application fully automated injection fully automated application Adhesion None Two-sided None One-sided Assembly After curing Prior to curing Prior to injection and After curing curing Disassembly Possible Impossible Possible Possible Sealing function Due to adhesive bonding Due to expansion Due to compression Silicone sealants ELASTOSIL® R or LR ELASTOSIL® RTV-1 and ELASTOSIL® silicone ELASTOSIL® RTV-1, RTV-2 RTV-2 sealant LIS (RTV-2) and ELASTOSIL® SC 3 Fig. 1

Seals are frequently exposed to extreme In wet assembly (FIPG), the parts to be This fully automated process using conditions. As the interface between sealed are assembled before the WACKER silicones offers a number of inside and outside, hot and cold or wet silicone rubber cures; accordingly, the benefits: and dry, they have to be able to with- accuracy of the part geometry is less • Less material input stand all of these opposites. critical. The adhesion of the silicone • Simplified warehousing rubber to the components enhances the • Fully automated application Silicone seals are either manufactured reliability of the seal. • Flange surface needs no special post in a separate injection-molding process operations and subsequently incorporated into the In dry assembly (CIPG), the seal adheres • Simple groove design multi-component part, or else they are to the part to be applied and thus stays • Seals stay in place until time of cured “in place”. Curing can take place in place without being secured in any assembly before or after the components have way. The seal can be dimensioned such been assembled: that the joined components are mutually noise and vibration-damped.

3 Foams as dust and splash protection; large tolerances and low contact pressure

 HIGH-VOLUME POTTING AND ENCAPSULATION

Potting and encapsulation – as methods available for sensitive electronic devices Material Property Options of partially or completely covering micro- such as bonded ICs. These gels ensure • Low viscosity or shear diluting chips, hybrid circuits and semiconductor that the device will work properly even • Variable processing and curing times power modules – have proved extremely under conditions of extreme tempera- • Soft to hard successful: they are particularly effective ture fluctuations or strong vibration. For • Transparent to opaque ways of protecting such components sensor applications, gels characterized • High flame resistance from external influences. by particularly low volatility and low • High thermal conductivity bleed are becoming increasingly impor- • Fuel resistance, e.g. fluorosilicones WACKER offers a wide range of one and tant. With its unique process for • Remarkable low-temperature flexibility two-part products for potting/encapsu- manufacturing polydimethylsiloxane, (down to -100 °C) lation applications. Extra-soft gels are WACKER SILICONES is excellently • High thermal shock resistance positioned for this market. • Low shrinkage • Good adhesion to polymer housings • Low outgassing • Low uncured-silicone bleed • Pronounced damping property • Specified low ion content

10 EFFICIENT COATING

Coatings or protective lacquers for Various methods are available for Each of these techniques puts different PCBs or hybrid devices are also known applying conformal coatings in volume demands on the protective lacquer as “conformal coatings.” They afford production: in terms of its rheological, pot-life and protection against external influences • Spray application curing characteristics. WACKER such as dust, light, aggressive media, • Dip coating SILICONES offers customized products temperature fluctuations and mechan- • Flow coating for each technique: both solvent- ical stresses. They insulate electronic • Selective coating (partial cover) based and solvent-free. SEMICOSIL® circuits from the surrounding environ- and ELASTOSIL® silicone rubber grades ment or enhance the dielectric strength meet the challenging functional, quality, of highly complex modules. reliability and cost-efficiency require- ments of conformal coating production.

11 SYSTEMATIC SELECTION

WACKER offers different silicone rubber Comparative curing of a 10-mm silicone coating systems marketed under the brand names [%] SEMICOSIL®, ELASTOSIL® and WACKER 100 SilGel®. The processing properties differ 90 too, sometimes substantially. It is therefore 80 essential to select the silicone system that 70 is best suited to your individual production 60 requirements. Discuss the issue with 50 your WACKER technical service manager. 40 He will be glad to assist you. 30 20 Set Priorities 10 The end result is often the same. The 0 various silicone rubber systems offered 1 2 3 4 5 6 7 8 9 10 Time [d] by WACKER merely constitute different RTV-2 RTV-1 ways of getting there (see Fig. 3). Fig. 2

ELASTOSIL®, SEMICOSIL® and WACKER SilGel® are registered trademarks of Wacker Chemie AG.

12 Silicone systems and product requirements

RTV-2 Silicones One-Part Systems >>

• Addition-curing and condensation-curing 2-part • One-part systems that cure exclusively at high systems that vulcanize at room temperature temperatures

Fast curing • Dual-component metering equipment is needed to • Simple metering equipment suffices to process them process them • Curing is fast, sometimes taking only minutes • Rapid curing in the order of minutes is achieved by working at elevated temperatures or selecting a suitable curing agent

RTV-1 Silicones

• One-part systems that cure at room temperature • Simple metering equipment suffices to process them; these rubber grades can even be applied manually • Atmospheric moisture is needed for curing

Easy processing >>

Fig. 3

13 APPRECIATING ONE-PART SYSTEMS

Condensation-curing RTV-1 silicones ELASTOSIL® RTV-1 Silicone Rubber are ready-to-use, free-flowing to pasty 1-part products that cure at room tem- R R X R R X perature. Their popularity is explained by HO Si O Si OH 2R SiX R Si O Si O Si O Si R 2HX the ease with which they are processed + 3 + R R X R R X (and the minimum of investment that is required) and by the outstanding prop- X X erties of the cured products. By virtue of R R Si O Si R the wide variety of properties they = offer, they are ideal for numerous bond- O Si X X X X R H ing, sealing and coating applications. O R Si O Si R H X X X X X X X O X R Si O Si R R Si O Si R R Si Si O Si O Si R + 4HX X X H O X H X X R R X

R = = = < = X N R Amine X O C CH3 Acetate X O N = C Oxime X O R Alcohol R H O O

Fig. 4

The Advantages at a Glance Acidic systems release small amounts of • Extremely easy processing acetic acid while curing (ELASTOSIL® E • Low capital investment for equipment grades). • Very good bonding to a variety of substrates Neutral systems release small amounts either of an oxime or of an alcohol while Chemistry curing (ELASTOSIL® N grades). RTV-1 silicone rubber compounds from WACKER consist of polydimethylsilox- The crosslinking, i.e. curing or vulcani- anes, curing agents, fillers and, in some zation, of RTV-1 silicone rubber grades cases, solvents and additives. During takes place on exposure to atmospheric the compounding process, terminal OH moisture. It starts with the formation groups on the polysiloxane react with of a skin on the surface of the silicone the crosslinking agent to form curable rubber and gradually works its way into products. The various systems available the compound. The higher the relative differ in the type of curing agent employed: humidity, the faster is the curing rate. Curing is not possible in closed systems Alkaline systems release small amounts without air access. In such cases, we of amine while curing (ELASTOSIL® A recommend the use of RTV-2 silicone grades). rubber grades.

14 Film thickness4 Vulcanized film thickness5 Uncured RTV-1 silicone rubber may be removed with a spatula, paper or a rag, [mm] [mm] and any residues washed off with a 11 15 solvent such as petroleum ether, toluene 10 14 or trichloroethylene. All tools and pro- 9 13 cessing equipment should likewise be 8 12 cleaned immediately before the rubber 7 11 has a chance to vulcanize. Once it has 6 10 vulcanized, silicone rubber can only be 5 9 removed mechanically, if necessary after 4 8 using the previously mentioned solvents 3 7 to make it swell. 2 6 1 5 Safety Precautions 0 4 The vapors comprising volatile by-prod- 1 2 3 4 5 Time [d] 3 ucts released during Fig. 5 2 and possibly also evaporated solvents 1 should not be inhaled for any length If air access is from one side only, a film 0 of time or in high concentrations. The thickness of 10 mm should not be 1 2 3 5 10 12 Time [d] workplace should therefore be well exceeded. Curing times can be greatly Relative humidity 5 % ventilated. Uncured silicone rubber must reduced by the application of heat, Relative humidity 45 % not be allowed to get into contact with especially if the relative humidity is high Relative humidity 75 % the eyes or mucous membranes, as it and the rubber has been applied thinly. Relative humidity 85 % can cause irritation and acid burns. The temperature should be raised slowly Fig. 6 in order to allow any solvents or hydrol- ysis products to evaporate. Final Processing Guidelines temperatures may be as high as 200 °C RTV-1 silicone rubber compounds provided that the films are thin (less may be applied manually or by machine. than 0.5 mm thick). If the films are any In the case of cartridges, either thicker, blistering is likely to occur. hand-operated or pneumatic guns are Curing is still possible at temperatures used. Automatic discharge or dis- down to approx. -15 °C. pensing equipment is generally adopted for the larger containers. After dilution with anhydrous solvents, RTV-1 com- pounds can also be applied by airless spray guns. Optimum adhesion is achieved by pressing RTV-1 silicone rubber firmly onto the substrate surfaces. Parts to be bonded must be assembled immediately after the rubber has been 4 Curing rates of various RTV-1 silicone rubbers at 50 % applied, i.e. before a skin can form, and relative humidity and 23 °C in aluminum pans measuring 24 may need to be clamped. mm in diameter and 18 mm in depth 5 Curing rate of ELASTOSIL® A 33 as a function of relative humidity at room temperature

15 UNDERSTANDING TWO-PART SYSTEMS

There are two essentially different curing Structural formulae illustrating condensation curing systems for RTV-2 silicone rubber:

CH CH O CH CH Condensation-Curing Systems 3 3 3 3 ~ ~ Condensation-curing silicone rubber O Si O Si OH + RO Si OR + HO Si O Si O CH O CH CH compounds consist of a primary com- 3 CH3 3 3 ponent and a curing agent that contains End of polymer chain Crosslinker End of polymer chain the crosslinker (a catalyst). The primary component and the curing agent are Sn catalyst typically mixed in a ratio of 8:1 to 12:1.

After mixing, curing takes place with the CH3 O CH3 elimination of alcohol. ~O Si O Si O Si O Si O Si O~

CH3 O CH3 Typical processing times for condensa- tion-curing RTV-2 silicone rubber ROH + ROH grades are about 10 min pot life and 70 min until the onset of curing (see Fig. 8). Fig. 7

Ultimate mechanical strength is reached Typical pot lives as a function of mixing ratio after about 6 hours. These times can be varied within limits by adjusting the ratio [min] of main component to curing agent. 30 To ensure reliable processing, however, 25 the pot life must not be less than 2 min. 20 It is not advisable to speed up the 15 reaction by increasing the temperature. 10 The temperature must not exceed 90 °C 5 before curing is complete, otherwise the 0 8 10 12 14 Parts RT 77x to silicone rubber could be destroyed. one part curing agent

RT 771 RT 772 RT 778

Fig. 8

The Advantages at a Glance • The curing rate may be boosted by • Fast curing at room temperature, even heating to a maximum of 80 °C; of thick sections heating above 90 °C is apt to cause • Very good thermal resistance depolymerization (reversion) • No inhibition-related curing problems • Only minor weight loss and shrinkage • Curing rates can be adjusted within (0.2–2 % linear) occur on curing given limits by selecting “slow” or “fast” curing agents and by varying the quantities in which these are added

16 Addition-Curing Systems Structural formulae illustrating addition curing Addition-curing silicone rubber com- pounds consist of two components, one ~ of which contains polymer and crosslink- O CH CH CH3 CH3 3 3 er, the other, polymer and a ~ O Si O Si CH = CH H Si CH H C = CH Si O Si O ~ 2 + 3 + 2 catalyst. When the two components are ~ CH3 CH3 O CH3 CH3 mixed, they cure to form the elastomer CH Si H product. During this reaction, the poly- End of polymer chain 3 End of polymer chain mer chains are linked through hydro- O silylation of the vinyl groups by means ~ Crosslinker of the hydrogen-containing crosslinker. The curing rate is controlled here by Pt catalyst the temperature, and not by the mixing ratio as in the case of condensation- ~ CH CH H O curing RTV-2 silicone rubber grades. 3 3

No by-products are formed on curing ~O Si O Si CH CH2 Si CH3

(see Fig. 9). ~ CH3 CH3 O H CH3 CH3 CH Si CH CH Si O Si O~ In addition to the 2-part systems, we 3 2 O CH CH also offer 1-part, addition-type, heat- 3 3 ~ curing silicone rubber grades. These are preferred whenever the procure- ment of a 2-part metering system is not Fig. 9 practicable for technical or economic reasons.

The Advantages at a Glance • Short curing times possible, even for grades with a long pot life • Pot life can be adjusted with additives • Curing can be accelerated by heating • Irreversible cross-linking • No weight loss on curing, and practically no shrinkage (< 0.1 %) • No by-products • Self-adhesive grades show excellent adhesion to a wide variety of substrates

17 ACCOMPLISHED PROCESSING

You will find detailed processing instruc- Viscosity vacuum is applied. Metering equipment tions for WACKER RTV-2 silicone rubber involving the use of compressed air may compounds in the supplementary [mPa s] be problematic due to possible bubble sheet that accompanies this brochure 150 000 formation. The reason for this is the high (Room-Temperature-Vulcanizing Silicone gas solubility of silicones. In the case Rubber Compounds). Please ask your of pasty/thixotropic RTV-2 grades, any WACKER sales office. 100 000 dissolved air will have already been re- moved during filling in the plant. In such Viscosity cases, metering equipment fitted with WACKER RTV-2 silicone rubber com- 50 000 follower plates is used to dispense the pounds lend themselves to a large product directly from the original con- number of processing techniques, 15 000 tainers. As for silicone foams, selective including spraying, dipping, potting/en- admixing of gases into the components capsulating, silk-screen printing and can serve to enhance the structure of automated application. The suitability of 1 2 3 [min] the foam. a compound for a particular processing Prior to processing method is dictated by its rheological During processing Curing properties, for example, its viscosity. After application The pot life and curing time of conden- This is measured in mPa s. RTV-2 Fig. 10 sation-curing silicone rubber com- silicone rubber compounds usually have pounds may be influenced, for one, by a viscosity between 500 and 2,000,000 Mixing and Applying selection of an appropriate curing agent mPa s. Many RTV-2 silicone rubbers Flowable RTV-2 silicone rubber com- (slow or fast curing agent) and, for show non-Newtonian behavior, and so pounds are noted for their ease of the other, by the amount of curing agent it is always necessary to quote the processing in both small-scale and large- used. shear rate at which the viscosity was scale use. The two components are measured. The viscosity, for its part, first mixed in the prescribed ratio to form While heat is not necessary for the vul- depends on the preliminary treatment a homogeneous compound. This may canization of condensation-curing (e.g. stirring) and on the temperature. be done manually or by machine. The silicone rubber grades, it will substantially Precise temperature control is therefore equipment employed must always be shorten the time needed for addition- essential, as also the fulfillment of scrupulously clean. If the components curing silicones to vulcanize. Exactly certain requirements (stirrer speed and have different colors, they should be how much time is required will depend time) during preliminary treatment. mixed until the compound is totally streak- on the grade in question and on the free. For economical, large-scale pro- heat transfer rate. Suitable heat sources cessing, especially of pasty/thixotropic include heated metal molds, circulating- grades, we recommend the use of air ovens and IR tunnels. Several metering equipment6. Air entrained addition-curing silicones have been during mixing must be removed from developed that have extended pot lives flowable products by evacuation at ranging from one day to several months. approx. 20 mbar before they are pro- Curing is effected in this case at high cessed further. temperatures of up to 200 °C. Please consult the respective product leaflets The container should be four times the for details. volume of the product, as the silicone 6 Please ask us for the addresses of manufacturers. rubber will foam extensively when the

18 Vulcanization of addition-curing silicone Adjusting the pot life by varying the mixing ratio rubber compounds may be substantially [min] impaired by contact with substances containing amino, or compounds. 25 This is usually indicated by tackiness, 20 and can be avoided by appropriate 15 preliminary tests. 10 5 Pot Life 0 The pot life of RTV-2 silicones is heavily 8 : 1 9 : 1 10 : 1 13 : 1 temperature-dependent, especially that ELASTOSIL® RT 772 : T77 of addition-curing products: a rise or fall Fig.11 in the temperature shortens or prolongs the pot life accordingly. The average pot Shortening the pot life by adding Catalyst EP life of RTV-2 silicone rubber compounds [mPa s] varies between 30 minutes and 6 hours, 120 000 while for some grades it is of the order 100 000 of a few minutes. It can also be adjusted 80 000 over a wide range – in the case of con- 60 000 densation-curing systems by means of 40 000 special curing agents and in the case of 20 000 addition-curing systems by means 0 of additives – to suit the application in 10 20 30 40 50 60 70 80 [min] question (see Fig. 11). ELASTOSIL® RT 622 + 0.10 % Cat. EP + 0.05 % Cat. EP + 0.20 % Cat. EP A number of addition-curing grades Fig.12 offer a pot life of up to several days at room temperature, and certain com- Extending the pot life by adding Inhibitor PT 88 pounds are even supplied as one-part [mPa s] products with an appropriately long shelf life. These specialty products do 50 000 not allow the fine adjustments (see 40 000 graphs) with Inhibitor PT 88 (longer pot 30 000 life) or Catalyst EP (shorter pot life). 20 000 10 000 0 5 10 15 20 25 30 [h]

ELASTOSIL® RT 601 + 0.10 % Inhibitor PT 88 + 0.05 % Inhibitor PT 88 + 0.20 % Inhibitor PT 88

Fig.13

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WACKER AT A GLANCE

WACKER WACKER SILICONES With its comprehensive expertise, is a technological leader in the chemi- is a leading supplier of complete silicone- WACKER FINE CHEMICALS is a preferred cal and electrochemical industries and based solutions that comprise products, partner for highly challenging custom- a worldwide innovation partner to cus- services and conceptual approaches. As manufacturing projects in the fields of tomers in many key global sectors. a provider of solutions, the business divi- chemistry and biotechnology. With around 14,400 employees, sion helps customers press ahead with WACKER generated sales of EUR 2.76 innovations, exploit global markets fully, WACKER POLYSILICON billion in 2005. Germany accounted and optimize business processes to has been producing hyperpure silicon for for 21% of sales, Europe (excluding reduce overall costs and boost productiv- the semiconductor and photovoltaics Germany) for 31%, the Americas for ity. Silicones are the basis for products industries for over 50 years. As one of the 22% and Asia-Pacific, including offering highly diverse properties for vir- largest global manufacturers of polycrys- the rest of the world, for 26%. tually unlimited fields of application, rang- talline silicon, WACKER POLYSILICON Headquartered in Munich, Germany, ing from the automotive, construction, supplies leading wafer and solar-cell WACKER has some 20 production chemical, electrical engineering and elec- manufacturers. sites worldwide and a global network tronics industries, through pulp and of over 100 sales offices. paper, cosmetics, consumer care and Siltronic With R&D spending at 5.3% of sales textiles, to mechanical engineering and is one of the world’s leading producers of in 2005, WACKER is among the world’s metal processing. hyperpure silicon wafers, supplying many most research-intensive chemical major chip manufacturers. Siltronic devel- companies. WACKER POLYMERS ops and produces wafers up to 300 mm is the global leader for high-quality bind- in diameter at facilities in Europe, the ers and polymer additives. This business USA, Asia and Japan. Silicon wafers form division’s activities encompass construc- the basis of state-of-the-art micro and tion chemicals and functional polymers nanoelectronics used, for example, in for lacquers, surface coatings and other computers, telecommunications, motor industrial applications, as well as basic vehicles, medical technology, consumer chemicals, i.e. acetyls. Products such electronics and control systems. as dispersible polymer powders, disper- sions, solid resins, powder binders and surface coating resins from WACKER POLYMERS are used in the construction, automotive, paper and adhesives industries, as well as by manufacturers of printing inks and industrial coatings.

WACKER FINE CHEMICALS is an expert in organic synthesis, silane chemistry and biotechnology, providing tailored solutions for its customers in the life sciences and consumer care industries. The range of innovative products includes complex organic intermediates, organosilanes, chiral products, cyclodextrins and amino acids. 06.06

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CREATING TOMORROW’S SOLUTIONS www.wacker.com info.silicones Germany 81737München, 4 Hanns-Seidel-Platz AG Chemie Wacker @ wacker.com

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