The How and Why of Silicone. (Part 1)


Understanding Silicones: The uses of silicon in FX; Tin vs Platinum; Behaviours and Applications.

First point, for anyone who isnt sure, Silicone, the synthetic polymer, is correctly spelt with an ‘e’ on the end, to distinguish it from the metalloid element, Silicon. Technically, it is a misnomer anyway, that dates from their discovery in 1942, it was thought the structure of the compounds was similar to ketones, when they are in fact Siloxanes. However the name Silicone has become accepted and persists to this day.

Right, that’s out of the way, now to business…..

What is Silicone?

Here is the Wikipedia definition: http://en.wikipedia.org/wiki/Silicone
The Encyclopaedia Brittanica explanation: http://www.britannica.com/EBchecked/topic/544410/silicone

and for the real chemistry nuts, Dow Cornings spiel: http://www.dowcorning.com/content/discover/discoverchem/properties.aspx

Does any of that help? Well, maybe, but it wont necessarily help you make that mould/art piece/prosthetic or the like….

Basically, silicone, for our purposes, is a synthetic polymer, which sets, or vulcanizes, at room temperature, into a rubbery material. This is known as RTV Silcone. It comes in hard and soft formulations, is inert once cured, heat resistant, flexible (even in hard kinds) and very very useful! RTV silicone rubber is used in the movie, entertainment and special effects industry, and in theme parks. Soft “skin” silicone rubbers, used by make-up and Fx artists, were developed specifically for sfx makeup artists, and certain kinds are also used in the medical prosthetics industry. It is also used, in an uncured form, as a lubricant, which will be apparent to anyone who has ever spilt uncured silicone on their floor…. it is an immediate shortcut to the kind of slide action Tom Cruise was famous for in ‘Risky Business’…..

So how do you know what kind of silicone to use? That depends on what you are using it for…. First I will go through some basic information that everyone should know before they start.

Which Silicone do I use?

RTV Silicone comes in many varieties, but the first thing to realise is that there are TWO main types. These are classified by the CATALYST used to turn the uncured, liquid, silicone polymer into a solid product. One is known as Tin Silicone, because its catalyst is a tin-based chemical, the other is Platinum Silicon. To further complicate this, often you will hear them described not by the type of catalyst, but by the manner of combination and vulcanisation (setting process). That is, Tin-based Silicones are also known as Condensation-Cured Silicones, while Platinum Silicones are Addition-Cured.

Just when you think its time to go get a nice cup of tea, or a stiff Scotch, there are even MORE differences… The Tin-based systems come in two different types -

One-part materials contain all the ingredients needed to produce a cured material. They use external factors – such as moisture in the air, heat, or the presence of ultraviolet light – to initiate, speed, or complete the curing process.

  • Typical uses – building sealants, high-consistency rubber (HCR) compounds, coatings for electronics, medical bonding adhesives
  • Advantages – easy to use; low- or room-temperature cure (although, in some cases, cure can be accelerated by heat)
  • Disadvantages – moisture-curing materials may take 24 hours or more to fully cure; precautions must be taken to protect the material from the cure initiator prior to application
Examples include window and bathroom caulking. These silicones are NOT suitable for mouldmaking or prosthetics, though they DO have an unexpected application in painting silicone models… (to be discussed in a later post)
Two-part systems segregate the reactive ingredients to prevent premature initiation of the cure process. They often use the addition of heat to facilitate or speed cure.
  • Typical uses – high-speed, high-volume operations, such as the application of silicone release coatings or pressure sensitive adhesives, injection molding of liquid silicone rubber (LSR), soft skin adhesives (SSAs) for healthcare applications
  • Advantages – longer shelf life, high-speed cure (some materials cure within seconds), and the ability to carefully control bath life and cure time by manipulating the formulation
  • Disadvantages – mixing required; often requires more sophisticated processes and formulating/application expertise.

Platinum cured silicones only come in the Two-part system. We will be looking at Two part systems here.

Platinum or Tin?

Both kinds of silicone have unique properties that make them suited to certain applications. For instance, if high temperatures are anticipated, then addition cure silicones (platinum catalyzed) are typically a better choice. But for economy, general mold making and prototype applications, condensation cure (tin catalyzed) would be preferable.

Condensation cure (Tin) two-component silicone rubbers are cheaper, (as far as I know, tin is still cheaper than platinum..) and better for most general mold making and prototype applications. They use Tin salts, and titanium alkoxide for catalysts. Most (but not all) Condensation cured silicones are mixed at a ratio of 1 part catalyst to ten, twenty, or even up to 100 parts of silicone! They are usually (but not always) measured by weight, not volume, and accurate scales are essential. You can add things to them like fillers, colours etc without appreciable affecting the cure.
Tin catalyst rubbers are not sensitive to inhibition, meaning they will cure at room temperature over virtually any surface. They’re easy to mix and de-air, because they have a relatively long curing time which allows bubbles to rise to the surface. The cure time can be reduced either by increasing the amount of catalyst used, or by adding special activators. *WARNING: Over-accelerating your silicon will dramatically reduce its usable life, by causing brittleness and deterioration, in direct proportion to the amount used. Tin silicone molds are excellent for casting polyester, epoxy, polyurethane, masonry, gypsum and candle wax. Tin silicones have a relatively long geltime, and a long period after gelling before they can be demoulded. Oh, and when mixing, if you miss a little bit, don’t stress, it will usually cure anyway….

Advantages

  • Cost.
  • Resistant to catalyst poisoning or substrate inhibition- they set over almost anything.
  • Adhesion
  • Versatility

Disadvantages

  • Tin silicone appliance and moulds leach over time, and become brittle, as the catalyst is unstable. Because of this they cannot be used for any purpose requiring a long shelf-life, or in contact with skin.
  • Tin silicone moulds cannot be used for casting Platinum silicone in. (*see note at end of article)
  • Need air and moisture to cure, not good for confined spaces.

Addition cure (Platinum) two-component silicone rubbers offer superior heat resistance and cure with virtually no shrinkage. Their catalysts are Platinum, and rhodium.

Skin-safe platinum silicones (used for life-casting or appliances) are often mixed at a ratio of 1:1.   Most of the mould making platinums that we use ( BJB’s 5000 series, Shinistu, or Smooth-On’s Smooth Sil series ) still require a scale.     1:1  silicones are usually (but not always) measured by weight, however some can be measured by volume.

Platinum rubbers can be inhibited by tin, sulphur or amines, in fact a whole heap of things you probably never thought about. Latex is a no-no, so you cant use latex gloves while mixing them, nitrile or vinyl are the best choice. Avoid gloves with additives and powder if you can.  You cant use them on a wet surface like water-based clay without sealing it, or on plasticene sculpting products with sulphur in them, (you will need to get a Non Sulfur Plastilene, or NSP clay.) You cant use them in the same ROOM as foam latex is being made or baked in, and other airborne contaminants can also inhibit. Fresh fibreglass must have the styrenes baked out, or be left to cure for a week or so, or it inhibits…

*Hot Tip: if one is to use a platinum with a fresh polyester mould… to avoid styrene contamination, a simple misting of PVA (PolyVinyl Alcohol., sold as a release agent.. NOT PVA Glue..) will do the trick quite quickly. And being water based.. it cleans out very easily.

Adding too much or the wrong kind of pigment, accelerator or retarder, can also inhibit. However, despite their fussiness, they CAN be cured in total confinement, and the cure rate can be dramatically accelerated with heat. Finished moulds are ideal for casting epoxies, low melting-point metals and polyurethanes. They can be accelerated by adding an accelerating agent, but as they have a very short geltime in comparison to tin-based systems, you are more likely to want to slow them down. This can be done with a retarder, or simply by chilling the components before use. They also have a much shorter working time, and life-casting platinums and Platgels have a considerably shorter cure time, which can be both an advantage and a disadvantage, according to purpose. Most of the mould making platinums require more time than tin based mould making silicones to cure up. Mixing must be done thoroughly and evenly or you will be left with uncured parts, and if you are too vigorous, air bubbles can be a problem, unless you have a vacuum degassing chamber.

A comment from very experienced FX Artist and Mouldmaker Rob Freitas: “I find that many people use “skin” casting silicones to make moulds with when it comes to platinums… Ecoflex series.. Dragin-skin series… Plat-Gel series.. they are fast… and mix at 1 – 1… but at times… the end results are very soft, so it is a bit more difficult to get really tight seams with them. And also to have the silicone line up perfectly with the jackets can be assisted by having a firmer silicone as well.Also, when pouring bigger moulds… the “quicker” setting times can decrease the necessary flow time needed to fill some moulds. For simplistic moulds that do not require sufficient time to flow and fill, they do work ok. And yes, they will set quicker.. and are easier to use. The faster the silicone sets… the less time it has to be contaminated. But for a firm… tight… long-lasting platinum. The slower, firmer platinums will yield superior results in my eyes. But one has to manage the contamination issues far more closely with the slower setting platinums..”

 Advantages
  • Cure without byproducts and can be accelerated by heat with no loss of ‘library’ life.
  • Clear, deep-section cure, no shrinkage,
  • Cures in a vacuum, so good for multi-part moulds
  • Texture can be altered with additives to make a ‘fleshier’ more flexible product for sfx, prosthetics and animatronics.
  • Good adhesion
  • Certain formulations are safe to use directly on the skin, either for moulding or for building up realistic skin effects.
  • Certain formulations are used as medical adhesives, or to attach other silicone pieces, some are self-adhesive in their uncured state.

Disadvantages

  • Potential for catalyst poisoning and substrate inhibition
  • Catalyst cost can be prohibitive in large amounts.

Both types have a limited shelf life in their uncured state. It is not advisable to buy larger quantities than you are likely to use in that time period (usually specified on the Manufacturers information.) The “library” life, that is the life of the cured silicone varies from product to product and you should check with the factory if it is important. Just as a guideline, Tin products may last as little a matter of months (if over catalysed or accelerated) or as many as ten years, while Platinum can last indefinitely under optimum conditions.

*Note: Just when you thought it was safe… A crucial point to remember when making silicone moulds is to think ahead to your finished piece. If you need a final result made out of Platinum, then your mould will need to be Platinum, because- yep, you guessed it, Tin Silicone is one of the many things that inhibit the cure of Platinum Silicone. Conversely, Tin Silicones will set over Platinum without any problems. There have recently been a couple of products developed by one manufacturer that coat the silicon with a protective substrate, and are therefore supposed to allow use of a Tin silicone mould with a Platinum cast, but they are not yet widely available and have not been tested with all manufacturers products.

Summing up:

1. What is RTV silicone? Two components that when mixed together cure at room temperature (RTV= room temperature vulcanization).

2. What types of silicone are there? Tin-cured, (or Condensation Cure), and Platinum-cured, (or Addition Cure).

3. Tin-cured are USUALLY (but not always) mixed in a ratio of 1:10 or higher, by weight. Platinum-cured are USUALLY mixed by weight but several kinds can also be mixed 1:1 by volume.

4. Tin is cheaper, and easier to use. It is limited in application due to limited ‘library’ life and curing requirements.

5. Platinum is expensive, and requires care and attention when using. It lasts indefinitely, some grades can be used in contact with food or skin, and cures in a vacuum so is useful for closed moulds.

6. Both can be made to cure faster, but if you accelerate Tin, you trade-off the usable life of the piece. Platinum can be accelerated with heat, but remember if you decrease the cure time, you will also decrease the work time!

IN PART TWO: HOW TO CHOOSE YOUR SILICONE.


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11 responses to “The How and Why of Silicone. (Part 1)

  • Yuni C

    this is very good information, but I need to know exactly the materials that I need to buy to learn How to make the silicon prosthetics, I’m doing prosthetics already but I want to learn How to do the prosthesis with silicon liners, because its more comfortable for patience.I will be greetly appreciated your information.

    • naomidlynch

      Hi Yuni, I suggest you contact silicone suppliers in your country to find the right kind of silicone for making medical prosthetic socket liners. There are several different kinds produced by different manufacturers and it will depend where in the world you are as to which is most suitable for your needs.

  • David Hogue

    Wow, thanks for writing this. I’d been looking for some good info on silicone molds.

  • James

    what retarders/inhibitors are used for condensation cure?

    • naomidlynch

      Retarders, accelerants and thixotropic agents are Manufacturer-specific, so you would need to check with your supplier whether there is in fact one available for the particular silicone you wish to use. Not all products have these adjusters available, and generally they are not intermixable between different brands of silicone.
      Tin silicones often have an option of different catalysts rather than retarders, which give you a ‘fast’ or ‘slow’ cure. They may also have a thixotropic additive to make brush-up moulds.
      When you are using platinum silicones, they are especially sensitive and even when you are using a product designed for that particular brand you must be careful with quantity- too much can inhibit curing completely, too little may not work at all.

  • DREDD 3D Casting a Dredd 3D helmet

    […] never used that one. Check out this link about tin cure vs. platinum cure silicones: The How and Why of Silicone. (Part 1) | naomidlynch The general rule of thumb I follow is that tin cure molds will fail much faster than platinum, so […]

  • MK

    Hi!

    I’ve been reading this tutorial in conversation with Todd Debreceni’s Special makeup Effects (First Edition), and am especially interestedin the use of silicone caulking in extrensically coloring silicone appliances. I was wondering if you’d gotten around to posting about that, as I couldn’t locate it myself…perhaps I missed it somehow? I also wanted to ask if this method was body-safe for prosthetics that will be worn against the skin?

    Thanks

  • mpc

    do you think there is a way to temporarily poison a catalyst in a platinum silicone? I’m trying to temporarily inhibit the silicone after mixing the 2 parts… i want to be able to keep it in an air tight container, then have it cure when it’s exposed to air. any ideas?

    • naomidlynch

      Hi there. What you want to do isn’t possible, because the cure on a two-part silicone is an irreversible chemical reaction that begins as soon as the components are combined.
      One-part ‘acetic’ cure silicones such as window sealants, caulking, and adhesives, remain uncured in an airtight container and cure on exposure to moisture in the air, (not oxygen), but this is not possible for two-part systems.
      If you were to ‘poison’ one component by adding some other material, the only likely outcome is that you would completely inhibit the reaction of a platinum silicone and partially inhibit the reaction in a tin cure silicone, resulting in an expensive waste of materials!!
      If you need to retard the cure speed, the only way to significantly slow down the reaction is to chill the components drastically before mixing, and although that will not stop the process from beginning it will buy a little time.
      For some types of silicones you can buy a compatible retarder chemical, which when added to the mix would have a similar effect, but without also making the parts stiff and hard to mix.
      The overall speed at which the silicone cures can be manipulated a little in these ways, but the best way to deal with time issues is simply to try and buy one with the qualities (shore hardness, colour etc) that suits your needs, but which has a longer ‘gel time’, which is the point at which the bonds become irreversibly locked and the mix is no longer fluid.

      I not sure why you would want to premix it anyway? It only takes a few moments to combine small quantities for filling small moulds, and if you are making a large piece or mould then you would want to take the time and care required to have it work properly, considering the price of the materials…

  • David Brettell

    I have been experimenting with PDMS emulsion, aqueous aminosilicone emulsion and reactive methylhydrogen emulsion, either separately or in combinations to develop a durable textile coating.
    I have been using a polyurethane resin rheology modifier to thicken to a desired viscosity.
    What ever emulsion type or combination of emulsion types I use once applied to the sub-straight the coating will gel, but remains softer then I want. The processes are dip coat the sub-straight and dry with gas forced air. I need the coating to be harder and attach to the surface of the fabric better. My thinking is I need a catalyst to accomplish this, what would be the best method? And do I need a surfactant to attach better to the sub-straight.

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