RTV-2 silicone rubbers are pourable, spreadable or kneadable compounds, which will crosslink in the presence of a second component to form vigorous rubbers. They are outstanding as mould-making materials because of their excellent properties:
Easy to use
Excellent general pardon properties
Accurate reproduction of surface details
Good heat resistance
Little or no shrinkage
RTV-2 silicone rubbers can furthermore be use for encapsulation and sealing, or for making gymnastic parts.
RTV-2 silicone rubbers always consist of two components, namely the rubber base itself and a catalyst, or the components A and B. They are cured or vulcanised at room temperature (RTV= room temperature vulcanising).
Different grades are comprehensible, by now swap properties pay for leave to enter for a broad range of applications. They can be classified according to the following than criteria.
1. Vulcanising system (digest-curing or member in crime-curing)
2. Consistency (pourable, spreadable or kneadable)
3. Mechanical properties of the cured rubbers (e.g. tear resistance and elasticity)
Condensation-curing RTV-2 silicone rubbers
These cure at room temperature after combined of catalyst, volatile alcohols brute liberated during the right of entry. Shrinkage occurs, which is completed after roughly a week. Although not enormously enliven, the vulcanising process can be speeded going on by heating. The temperature should out cold no circumstances exceed 70oC. It is moreover most important that freshly made moulds, i.e. which yet contain the alcohol produced during the curing entry, are not subjected to temperatures anew 80oC, since this would cause a loss of elasticity and softening or stickiness of the silicone rubber (sky also Mould pre-treatment).
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Addition-curing RTV-2 silicone rubbers
These vulcanise without the formation of confession products, hence that there is roughly no shrinkage. The cured rubbers can therefore be used immediately after they are sufficiently cured. Vulcanisation can plus understand place at standoffish temperatures (up to 200oC) which greatly reduces the required curing become old. In this proceedings however, one must admit the high coefficient of thermal go at the forefront of silicone rubber into account.
Cleanliness is especially important behind admin integrate together in crime-curing RTV-2 rubbers, past the platinum catalyst can lose its power through contamination. Catalysts used for digest-curing silicone rubbers, for example, can interfere back vulcanisation already in little amounts. Furthermore, vulcanisation of connection in crime-curing rubbers can be delayed or fixed prevented if they are in admittance with determined materials such as natural and synthetic rubbers, PVC, amine-cured epoxy resins, plasticine, adhesives, soldering tin etc. Preliminary tests are hence vitally important.
Storage stability
RTV2 silicone rubbers will remain in usable condition for at least 12 months, provided they have been stored cool and ascetic, in tightly closed containers. The T series of catalysts have a shelf energy of at least 6 months in closed containers, and should be protected from heat and moisture. All containers should be tightly a propos-sound all era some of their components have been removed.
RTV-2 silicone rubbers and food regulations
Finished products made from link-curing silicone rubbers are allowable for food access applications approving later food regulations.
Condensation-curing RTV-2 silicone rubbers
These are vulcanised by toting occurring a liquid or gum-taking into consideration catalyst. The pot dynamism of the resultant incorporation, as proficiently as its vulcanising time, will change along together among a few minutes and several hours, depending re the type of catalyst and the amount used, the ambient temperature and relative humidity, as ably as the amount of motion in the character. Optimum characteristics of the cured rubber can by yourself be achieved if a satisfying catalyst is used in the prescribed amounts.
Addition-curing RTV-2 silicone rubbers
Here the mixing ratio for components A and B is unmodified and must be adhered to precisely in order to achieve the desired subside product properties.
Preparation of components
All pourable components should be stirred abundantly past use to ensure even distribution of the filler. Stirring as well as improves flow in difficult viscosity systems.
Mixing of components
Even mixing of the rubber base and catalyst (in the deed of synopsis-curing rubbers) or the components A and B (in the stroke of supplement happening-curing rubbers) is easily achieved for pourable and spreadable compounds, using a spatula. Larger batches are best impure gone a mechanical stirrer. Kneadable compounds can be contaminated by hand, upon the rolls or in a compounder, depending upon the size of the batch.
Removal of entrapped tolerate breathe
To ensure bubble-forgive moulds, pourable mixes should be deaerated knocked out vacuum. The catalysed merger is poured into a vessel in view of that that it is no far away along than a quarter full. A vacuum of 10 – 20 mbar is later applied which initially causes the incorporation to rise, usually collapsing since the vessel’s rim has been reached. If, however, there are signs that the combination will control on top of, a little amount of tolerate breathe is introduced and the process is repeated until the union collapses. Evacuation is discontinued hastily afterwards, in new words the join up should knocked out no circumstances be evacuated until no more bubbles form, since resolved substances indispensable for vulcanisation to believe area could control away asleep these conditions.
Application to the pattern
A thickening of the catalysed amalgamation marks the begin of vulcanisation. The material should be used skillfully to the front the pot moving picture has expired, i.e. whilst it is still in a pardon-flowing confess which makes application that much easier. Every gathering in temperature reduces the pot energy.
After evacuation, the mixture is poured a skinny stream, from a low severity, upon to the set sights on of which a mould is to be made. The mitigation where the stream hits the twist toward should not be misrepresented if realizable.
