Can You Make Rubber Pliable Again

Please notation before you start reading this older post that I have long since included a version in themouldmaking section of myMaterials pages, under silicone condom, which can be accessed to a higher place. It is that version that may have been updated or expanded since.

At that place'due south nix better than silicone rubber if one wants to brand reliable and long-lasting moulds. Information technology is really second-to-none for reproducing detail, in addition to having the ideal combination of flexibility and force. Just silicone condom isn't inexpensive, averaging about £22 per kg and unlike many other materials in that location are no 'cut-price' versions. It'south rare to find one for under £xx per kg (in the United kingdom www.tomps.com may offer the all-time deals) and even the reduction one would commonly expect when buying in bulk isn't really that meaning in this instance. Moreover, unless one definitely needs to use a substantial corporeality all at once, buying silicone in bulk can frequently lead to wastage because its shelf life is comparatively short.

The mutual practice of building up a silicone 'pare' or layer over a form (rather than pouring a block) helps to reduce the amount and cost. Encounter the series of posts Making a hollow 2-slice bandage in fibreglassin the Methods section under Mouldmaking and casting for an account of this method. Simply at that place are additional, perchance lesser known, ways of reducing the cost which at the same time modify or extend the capabilities of the material. Once cured, silicone cannot be simply re-melted (every bit vinyl can) merely hardened leftovers and moulds which are no longer needed tin be put to good utilise. In short, if these tin can be granulated they can exist added as a filler to newly-mixed silicone.

In the photo below I am using a traditional-style kitchen mincer to plough silicone leftovers into granules. I'd heard almost this being done before but didn't really make much notation of it until I came across this YouTube video below. I suppose I just hadn't believed before that silicone could be granulated and so easily! The commentary is in Portuguese, just it is fairly self-axiomatic.

http://www.youtube.com/watch?v=-t6T92DFya4

I then discovered that the exercise is featured in a book that I've really got! .. good sometime Thurston James' The Prop Builder'south Molding & Casting Handbook 1989. I must confess that I read this book thoroughly a long time ago and probably still owe a lot to it without existence conscious of that anymore. But I call back that afterwards showtime reading I stowed it away with a little frustration, purely considering many of the materials he features are either a niggling out-of-date now or only available in the US. I'chiliad at present planning to read it again with a bit more respect!

I bought my mincer ( a KitchenCraft brand 'bandage iron mincer') at John Lewis in London for £18 concluding year, but Robert Dyas likewise has them and there are many online for betwixt £17-£25.

I've plant that it works all-time if the silicone is cut (with pocketknife or scissors) into strips rather than chunks and that the receiving funnel is fairly continuously supplied while turning the handle. At times this is tough to turn, but 'back pedalling' a piddling before continuing will usually make it easier. These cast fe mincers usually come up with a few extrusion plates with differently sized holes. I assume these will affect the size of the granules but I oasis't tested them all the same. According to the YouTube demonstration the granulated silicone can be put through the mincer again to brand information technology effectively if needed. Another point to make hither is that, although I cleaned the metallic parts of the mincer as best I could before using it the activeness of the mincer appears to darken the silicone granules a piddling. I think it'due south something in the metal itself merely, equally far as I can tell, information technology doesn't touch on the bonding of the silicone.

What I found surprising was that this doesn't just work with the standard softer types (Shore A 20-25) but equally with a harder i. As long as it's cut into manageable strips, as below, fifty-fifty a tougher Shore A 55 silicone (this is RTV-101 from Tiranti) will comminute without too much effort. Perhaps this is because the harder ones often have a lower tear resistance.

The sample to the left (above) includes a few different brands of silicone which take the same or similar Shore A value, while the pile to the right is just the ane harder blazon. And so far I've plant that granules of dissimilar brands or even different hardnesses can be mixed into fresh silicone and the but thing I'm assuming will not work (and oasis't bothered to try) is mixing condensation cure silicones with add-on cure (run into Dictionary). I didn't launder or wipe the old moulds or leftovers first. Some might have been a little dusty and I don't think that would make much difference in the end, but every bit a rule it's probably better at least to wipe them first.

But to find out more than nearly what could exist done I made the following exam pieces.

For some of these small blocks I catalyzed an amount of new silicone first (roughly half the book to be filled)  and poured or brushed a footling of this every bit an initial layer in the Lego containment earlier mixing the residual with its equal weight in granules, then adding it on top. The silicone should always exist catalyzed before any other additions to it because otherwise these could affect the fifty-fifty spread of the catalyst. If a proper mould were being fabricated this first 'pure' layer or detail glaze would be essential, to keep any granules at a picayune altitude from the mould surface. With some of the other blocks I waited until this commencement 'pure' layer was cured (i.e. added the recycling mix the next day) merely to run across if this might make a difference to the bonding of the layers.

Above are the test pieces made mixing 'like for like' granules and new silicone together (i.e. recycled and new silicone of the same blazon; Lukasil 429 Shore A 20), the one poured in ii stages (with a solar day betwixt) and the other all at the same time. There was no departure in feel between the cured blocks and certainly nodelamination (i.due east. separation of the layers) or cracks appearing when strongly flexed.

Only the recycled granules do seem to have quite an impact in terms of flexibility! I made a test cake of the same size using the Lukasil 429 pure to compare this, and the beginning photo below shows how this can be flexed with ease compared to exerting the aforementioned amount of force per unit area on the recycled block below it. Although the recycled granules had the same Shore A value the cake was much tougher.

The samples were fabricated on a perfectly shine ceramic tile surface and the blocks match this without a blemish when relaxed, but I did detect that the underlying granular structure is noticeable when flexed (if this comes across at all in the photograph below). I would assume however that this wouldn't touch on the surface during casting.

Beneath are similar flexing tests on samples fabricated using granules of the harder silicone (Tiranti RTV-101 Shore A 55) in which the layers can be better seen. Although I expected more problems since the silicones are not 'similar for like', over again there was no fissuring or delamination fifty-fifty nether strong flexing. Besides as expected the block was tougher than the previous ane but, interestingly, not by that much considering the RTV-101 is so much harder.

While I was about it I thought I might equally well meet what happened mixing other materials with the silicone instead of the recycled granules. It provided an interesting comparing and led to a surprise discovery. The sample cake below contains cork granules and, as expected, the silicone doesn't bond with them. The cake started to fracture when flexed.

But then I tried Fillite and the results were dissimilar. Fillite is an industrially produced ash usually used as a filler for resins and plasters. I use it a groovy deal for extending polyurethane resin.

As before I made ane block laying a thick detail coat of pure silicone first which was left to cure and for the other I just poured in the mix. I mixed nigh the same weight of Fillite to silicone (actually 10g of Fillite to 12g of silicone because it already seemed quite a lot by book). Information technology mixed in very well and the mixture was still pourable though very thick. As with all the other tests, there was no effect on the normal curing fourth dimension.

On curing the silicone appeared to take bonded firmly with the Fillite, including the Fillite-filled layer with the pure silicone layer, with no sign of fracturing when flexed. In the procedure the composite had become substantially tougher, changing the Shore A 20 silicone into, at a guess, a Shore A seventy or more. The block equanimous wholly of silicone/Fillite mix above was very resilient, very slightly flexible merely quite difficult to bend.

The Fillite and silicone had fused then well in fact that I suspected that some form of bonding was taking place at a molecular level i.east. not just 'mechanical gripping' but rather that the silicone polymer chains were linking direct with the Fillite. I accept to say that earlier this I had been largely ignorant of what silicone rubber (i.e. what nosotros buy off the shelf) actually is! I know at to the lowest degree a little more now. For example .. yes, 'silicone' is related to 'silica'. Silicone safety is derived from quartz sand and one reliable scientific source I looked at defined silicone safe equally 'made from crosslinked silicon-based polymer reinforced with filler'. So information technology makes perfect sense that the long chain molecules of silicone rubber .. the flexible version of silica .. volition attach themselves naturally to any less flexible version of silica, but because they are long will still go along much of their flexibility. Incidentally, this is also the reason why silicone rubber, which owes its popularity to the fact that it will not stick to well-nigh things, will stick fast to clean glass! Fillite is, as I at present know, alumino-silicate, and the silicone rubber nosotros buy off the shelf contains similar fillers as standard. Cabosil for example, which Thurston James besides mentions every bit something which can be used to extend or thicken silicone, is so-chosen 'fumed silica'. The silicone condom manufacturers I looked at list fumed silica, precipitated silica, aluminum silicate or alumina trihydrate as their standard fillers.

What I'm suggesting this means is that 1 can modify whatever standard silicone rubber oneself, without fright of 'weird' consequences, whether y'all want to economise by extending the volume or whether you only want to increase the hardness. Information technology's what the manufacturers do anyway!

To sum up

My tests were not 100% scientific i.e. not rigorously controlled and I only tried a few variations, merely at least they gave me some indications of possibilities and drawbacks. For me they definitely suggested that;

– addition of silicone safety granules will toughen the resultant composite fifty-fifty if both the recycled and the fresh silicones used are relatively soft and of the same type. I would assume that when a fresh silicone with a low Shore A value (soft) is being used this will exist toughened up in proportion to the amount of recycled silicone added, although I haven't tested the range specifically.

– recycled silicone and fresh silicone don't necessarily need to be 'like for like' or the aforementioned hardness to bond together

– the quondam silicone moulds or leftovers don't need to be rigorously cleaned if they're going through the mincer anyway.

– there was no noticeable effect on the normal cure time of the fresh silicone. There was no obvious result on the general force of the cured composite but the effect on tear strength in practice and in the long term would have to be further examined.

– if other fillers are used they should exist 'silicate' and there is a range of these available. Fillite is 1 of the cheapest, will bail firmly with fresh silicone and, dependant on the ratio, tin can exist used to adjust the flexibility/hardness of the composite from just a trivial tougher to almost plastic hardness. I'm assuming that fillers will ever alter the flexibility of the cured silicone in some measure out.

– according to all my tests so far 'silicone will bond with silicone', whether the showtime layer is however fresh or tacky or whether it's been left to fully cure (as long as the surface has been kept clean and free from grit!) and this applies as between different silicone brands, different hardness, filled or unfilled. It is however unlikely to utilise betwixt condensation cure and improver cure silicones!

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Source: https://davidneat.wordpress.com/tag/making-silicone-rubber-harder/