The Science of Art: Silicone Rubbers
We recently launched a new podcast called, "The Science of Art". We started this podcast with the desire to share information about the science behind how certain materials work to create different types of art. You can find this podcast on all the usual podcast platforms, and we have the audio on our YouTube channel as well. We look forward to sharing more topics with you in the future and if there is anything you'd like to learn more about, please share it with us in the comments. Our first episode was all about silicone rubbers and how they work, so we are sharing the transcript with you here in case you'd prefer to read about this topic. Read on about the science of silicone rubbers!
THE SCIENCE OF SILICONE RUBBERS
Silicone rubbers are a commonly used mold-making, prototyping and industrial application material of today and are grouped into two categories: tin-cure and platinum-cure. For the purpose of this blog, we will be talking about silicones with respect to mold-making and casting. While both of these types of silicones can be used to make molds, their application and ease of use depends on a few different factors. You might hear another term used to describe these types of silicones and that is RTV, which stands for room temperature vulcanizing, meaning these silicones cure at room temperature. More on that later.
TIN CURE SILICONES
Let’s dive in to tin-cure silicones first. Tin cure, also known as tin-based or condensation cure silicones, are a type of silicone rubber that cure through a chemical reaction involving tin catalysts. These silicones are made up of two components: Part A and Part B. Usually, the liquid silicone polymer base makes up Part A, and the liquid tin-based catalyst makes up Part B. These components need to be mixed together in the correct ratio to initiate the curing process. The ratio is usually specified by the manufacturer and must be followed precisely to ensure proper curing, but this is usually 10 parts A to 1 part B by weight.
A little bit about this process: the chemical process that causes these materials to cure into a solid rubber is due to a crosslinking reaction which the tin-based catalyst (part B) initiates, with our help, via a mixing process with the silicone polymer base (part A). Yes, we are an additional “catalyst” if you will, in this scenario, and our role in mixing the tin-based catalyst with the silicone polymer base should not be overlooked as it is very important! Once mixed, the tin-based catalyst reacts with certain functional groups present in the silicone polymer, causing them to link together and form a three-dimensional network. As the crosslinking progresses, the silicone polymer base gradually transforms from a liquid to a solid state.
When mixing these components together, pot life, or the time you have to pour the mixed materials before the curing process begins, is also an important factor, as some reactions are quicker than others. This is usually specified on the product packaging. Sometimes a silicone’s pot life can be as quick as 15 minutes, and sometimes you may have an hour! This has a lot to do with the amount of catalyst used, the strength of the catalyst, ambient room temperature and humidity. Because RTV (room temperature vulcanizing) silicones cure at room temperature, a colder environment will slow down the pot life, giving you more working time, and a warmer environment will speed it up. This is especially important to note when working in a studio that may not be temperature controlled, as you may have to adjust your working time accordingly.
The curing process of tin cure silicone is what is called "condensation curing." During this process, byproducts such as alcohol or other compounds may be released as the chemical reaction takes place. These byproducts can be detected by their odor. Just like pot life, the curing time can also vary depending on formulation, ambient temperature, and humidity. Because byproducts are produced through the curing process, tin-cure silicones are not considered skin-safe and should not be used for body casting projects. The tin catalyst can also be highly sensitizing and cause dermatitis, so use proper PPE, especially gloves, when working with these materials.
Once the silicone is fully cured, it develops its desired properties, such as flexibility, resilience, and heat resistance. Tin cure silicones are known for their good tear strength and dimensional stability. Some examples of tin cure silicones we carry are some of the Smooth-On Mold Max series, our own SR-1600 series, and our purple-stuf, but there are many others in the market as well. A quick note here about shore A hardness as well. This measurement is used across many mold making materials to describe a product’s firmness once cured. The higher the shore A hardness, the more firm it will be. For example, the shore A hardness for a Mold Max 15 silicone rubber will be similar to a rubber band, while the shore A hardness for Mold Max 30 will be similar to a soft pencil eraser.
PLATINUM CURE SILICONES
Now onto platinum cure silicones. also known as addition cure or platinum-based silicones, these materials cure through a platinum-based catalyst. These silicones are also made up of two components: Part A and Part B. Usually, the liquid silicone polymer base being Part A, and the liquid platinum-based catalyst being Part B. These components also need to be mixed together in the correct ratio to initiate the curing process, and for platinum silicones this is usually 1 part A to 1 part B by weight.
When mixing these components, the pot life can also vary in time and will be affected by the strength and concentration of the catalyst, ambient room temperature and humidity.
Once the two parts of the silicone base are mixed together, the platinum catalyst starts the crosslinking process. Unlike tin cure silicones, platinum cure silicones undergo an “addition cure” mechanism, meaning that no byproducts are released during the curing process. for this reason, many platinum silicones are considered skin safe and food safe, however you should always check the instructions from the manufacturer before using these types of silicones for body casting purposes or making food-safe molds.
Platinum cure silicones are generally considered to have a fast and efficient curing process. The curing time can range from a few minutes to several hours, depending on factors such as the silicone formulation, ambient temperature, and catalyst concentration. Heat can be used to accelerate the curing process and this can usually be achieved using a blow dryer, heat gun, or even placing a space heater in your workspace. Examples of platinum cure silicones we carry for mold making are some of Smooth-On’s Mold Star series, body double series, EZ brush and Rebound 25 brush-on silicones, as well as our own SR-2000 series, Dermasil, and Silputty.
Once fully cured, platinum cure silicones have high tear strength, good elongation, and excellent dimensional stability. They are known for their low shrinkage, which is an advantage in applications that require precise replication. Platinum cure silicones also have excellent resistance to heat, chemicals, and aging.
PROS AND CONS OF BOTH SILICONE TYPES
Now onto some pros and cons of tin-cure silicones vs platinum cure silicones: tin-cure silicones are a great material to use where cure inhibition might otherwise occur in your mold making projects. For example: sulphur, latex, ammonia derivatives, certain oil coats and even some resins may prevent platinum silicones from curing properly on your models, so tin-cure silicones in these circumstances are a must! When in doubt, it is best to do a small test on your model before using platinum cure silicones, to make sure they will cure properly to create your molds. Heck, it’s best to do a test in either case, and we always advise it if this is your first time working with these materials. Tin-cure silicones are usually less expensive than their platinum cure counterparts, but it is important to note that because of the greater difference in A to B ratios for tin cure silicones, (remember, these are usually 10 parts A to 1 part B) sometimes you may be paying less because there is actually less material in these kits than a 1:1 platinum cure system. The shelf life of a tin-cure silicone mold, when kept clean and stored correctly in a dark, room-temperature environment is usually around 5 years, but this will depend greatly on how often the mold is used, the use of a mold release and the silicone itself. Everyone has their own opinions, so if you are looking for a good tin-cure silicone, ask us or test some out. Most manufacturers make trial kits for this very reason!
Platinum cure silicone molds are archival and most platinum cure molds, when cared for and stored properly, will last a lifetime. The use of a mold release is often not as important with platinum cure silicones, but we always advise that one be used to keep the mold well lubricated during use and prevent tearing, especially if the mold has a lot of detail.
We often get asked about silicone additives so I want to touch on those briefly here too. Often times, you may want to save materials used to make your molds and prefer to brush your silicone solutions on instead of pouring them onto your models. In this case, a silicone thickener such as viscosil, or as Smooth-On calls it, Thivex works great. This solution will thicken the silicone mixture slightly and make it brushable. Alternately, if someone wants to thin their silicone to make it more pourable, they can add a little silicone fluid to their mixture. The ratio of a silicone thickener, and silicone fluid is usually 1-10% of the total mixture, but most people do this by feel and lots of testing. Just as some users may want to change the physical properties of their silicones, others may want to change the chemical ones too and create a silicone that cures slightly faster. In this case, an additive called Rapid Set is often used for tin-cure silicones, and smooth-on’s Plat Cat is a good additive for their platinum cure silicones. Again, the ratio is usually 1-10% of the total mixture and it is best to do a small batch test to see how it performs. There are also products out there that can slow the curing process.
Most silicone catalysts (platinum and tin) are colored, and the purpose of their coloring is to indicate when Parts A and B have been thoroughly mixed. Sometimes silicones will be clear or semi clear so a user can either see inside their molds, or color them themselves. If you fall into the latter category, silicone pigments work great here. These are silicone-based pigments that can be added in small ratios to achieve different desired colors. The bottom line is, it’s all chemistry here and there are always ways you can alter your recipes to get your desired results.
Lastly, you may be wondering about silicones you can use to create castings from, and yes they are a wonderful material too! These types of silicones can be used to make medical models, skincare tools, toys, and prosthetics and are almost always platinum-based. The same chemistry applies to these silicones, but they are almost always colorless so the user can create a variety of effects and colors in their final pieces. Some examples of casting silicones we carry are Smooth-on’s Dragon Skin series and Ecoflex series.
There you have it! Hopefully you got some insight on how mold making silicone rubbers work and can approach your future projects with this knowledge. You may have had different experiences with these products than us, so I feel the need to mention the opinions expressed here are our own and based on our context as a sculptural supply retailer with 75+ years in the biz. We appreciate you reading this far and as always, if you have any questions, please feel free to ask!
Go forth and create and we look forward to talking more about the science of art with you soon!