Silicone Sheeting in Laboratory Research Settings
Best Practices for Common Silicone Sheeting Challenges
This whitepaper addresses the use and challenges of silicone sheeting in labs, focusing on its curing process, especially platinum-based curing. It highlights issues like catalyst inactivation and excess silicon hydride leading to performance inconsistencies. Solutions include handling best practices, post-curing methods, and customization options for specific research requirements.
In addition to various industrial uses, silicones are deployed in various laboratory experiments, particularly in silicone sheets, which are often used as a substrate for cell culture growth. However, processing techniques can lead to variability that might affect the performance of silicone sheeting in research applications. Careful planning can address these issues to ensure consistent and reliable substrate performance.
Cure Chemistry Can Affect Processing
The silicone elastomers used in the production of the silicone sheeting are made from polymers consisting of repeating siloxane units (-Si (CH3)2-O-) wherein, along the chain, a methyl group (-CH3) is replaced with a vinyl group (-CH=CH2) on occasion, typically at defined intervals. Before these elastomers are cured, silicones tend to be liquids or gels. However, depending on the viscosity of the chain lengths of the base polymers, they can be paste-like to semi-solid as well. The curing process bonds, or cross-links, the silicone polymer chains together, creating a more structured, elastomeric material. Silicone elastomers have a wide range of applications because they exhibit high gas and drug permeabilities, excellent temperature performance capabilities (-40ºC to 200ºC), and are formulated without plasticizers, leading to low levels of extractable in the cured sheeting.
Curing, also known as vulcanizing, is the process by which silicone polymers are cross-linked to create a more structured three-dimensional system, or the silicone elastomer. There are three primary methods of curing: peroxide-based curing, condensation curing, and platinum-based curing. At Specialty Manufacturing, Inc. (SMI), we only use platinum-based curing systems and have identified best practices to address common issues with this curing method.
With platinum-based curing systems, silicon hydride (-Si-H) present in the cross-linker in the elastomer reacts with the double bond of vinyl groups (-CH=CH2) on the siloxane polymers to form ethylene linkages. The full completion of this addition reaction can be slow because the double-bonds of the vinyl groups can be difficult for the silicon hydride to reach since some of the vinyl groups are sterically hindered because of polymer chain overlap and entanglement. To ensure the completion of the reaction in a reasonable amount of time, an excess of silicon hydride can be added to the reaction, which makes it more likely that silicon hydride will react with all the vinyl groups in the material. This reaction, which cross-links the siloxane polymers, ultimately produces a silicone elastomer without producing the corrosive or volatile by-products other curing methods can generate.
There can be, however, a few drawbacks associated with this process. First, the platinum catalyst readily binds to amines, such as those in proteins or other electron-donating groups. This inactivates the catalyst and consequently hinders the curing process. Manufacturers of the sheeting avoid contact with such electron-donating functional groups during platinum-curing by performing the reaction in controlled and somewhat isolated conditions. In addition, because excess silicone hydride may be added to the material, the curing reaction will continue over time until all the vinyl groups have been reacted. At this point, excess silicon hydride eventually leaves the system. Although most of the curing process is carried out during the manufacture of the sheeting, the silicone elastomer continues to cure somewhat as it ages, increasing in strength over time.
Raw Material Chemistry
Silicone manufacturers typically develop and manufacture their materials with validated recipes to ensure the manufacturability of the product and provide a consistent, controlled range of physical properties from batch to batch. To achieve a state of cure that would make the molded article usable in an “as molded” state, manufacturers may formulate an excess amount of silicon hydride or cross-linker. If that is the case in the material used to make silicone sheeting, it may interfere with laboratory experiments. For example, a researcher who used SMI silicone sheeting performs experiments in which she studies how the adhesive properties of small glass spheres to polydimethylsiloxane (PDMS) change with stretching. The PDMS is expected to be soft, sticky (tacky?), and pliable.
During the experiment, a curable silicone fluid is coated and then cured in a top layer over an SMI silicone sheet, which is expected to provide a sturdy backbone and prevent tearing of the cured top layer of silicone. However, this researcher (and others) noticed that it becomes stiff and rigid when the top layer is coated over an SMI silicone sheet manufactured from a specific lot of raw silicone material. This means researchers were not able to stretch the top layer properly for the purposes of the experiment. However, an SMI silicone sheet manufactured several years prior from a different lot of raw silicone material yielded satisfactory results by not reacting with the top layer.
These observations would be consistent with the second drawback of platinum curing mentioned earlier. Excess silicon hydride cross-linker still potentially present in the silicone sheeting could be causing additional curing in the top layer of the construction. This additional curing would change the desired properties of the subject material. In the study mentioned above, the top layer (a curable silicone polymer) came into contact with the sheeting and, quite possibly, excess silicon hydride. In the experiment, the top layer underwent what can be described as additional curing with the material increasing in hardness. The SMI silicone sheets manufactured from a raw material lot several years prior did not present the same problem, possibly because all the excess silicone hydride polymers had time to exit the cured silicone sheeting after the sheeting was completely cured.
In other applications, researchers who use platinum-cured silicone sheets for cell culture growth can also run into situations where excess silicon hydride interferes with the performance of the silicone substrate in their experiments.
Partnering with your sheeting manufacturer on a solution
SMI understands that consistency and repeatability are essential to successful research applications and has partnered with research customers and raw silicone material suppliers to identify solutions and best practices to address this challenge. SMI is in the early stages of formulating proactive testing that may be conducted on silicone lots before sheeting is manufactured and tests to detect potential issues with the manufactured sheeting. In the meantime, we are encouraging all research customers to identify the best post-curing methodology to ensure consistent and repeatable performance of the sheeting material regardless of any excess silicone hydride present in the silicone supply.
To remove excess silicone hydride and accelerate the curing process, one can post-cure the silicone sheets. Post-curing involves placing the sheets in the oven at a specific temperature for a certain amount of time. The most common post-curing temperatures and times are 6 hours at 121℃, 4 hours at 177℃, and 2 hours at 200℃. A few researchers have noticed that the sheets wrinkle at higher temperatures, so a low-temperature, longer post-curing cycle is preferred by most.
Recommended methods for cleaning and handling
The best practice for handling the silicone sheeting for these types of experiments is to wear vinyl gloves to prevent contamination of the surface of the silicone sheeting. Latex and other types of gloves have been shown to inhibit platinum cure systems.
Second, when cleaning silicone sheeting, use a non-detergent soap (such as ivory flakes) in a 5% non-detergent soap solution with 95% warm water. Rinse sheets thoroughly and let dry overnight.
Additional options for sheeting in research applications
Silicone sheeting is available in a variety of finishes and sizes. This specific issue has only been identified in recent batches of the gloss sheeting product line, which is often selected for cell culture growth applications because the gloss finish gives the sheeting some opacity, and researchers can see through the material as needed. Sheeting can also be purchased in matte non-reinforced, matte reinforced, and firm matte finishes, durometers of 40 or 60, and thicknesses ranging from 0.002” to 0.120” to fit your specific research applications.
If having a uniform flat sheet is vital to your application, SMI can provide sheeting with a polycarbonate backing that can withstand oven temps up to 121℃, allowing the sheet to be post-cured with the backing attached to avoid the wrinkling that has been observed at higher temperature post-cure processes.
If desired, SMI can also create custom silicone sheet sizes, shapes, colors, durometer blends, and thicknesses, eliminating some handling requirements within the laboratory setting.
If you are experiencing issues with silicone sheeting in research applications, please contact SMI at 1-989-790-9011