Silicone vs. TPE: How to make the right choice.
Silicones and TPEs are elastomeric materials that offer an alternative to natural or synthetic rubber. This article will aid product designers and engineers in material choice for elastomeric applications. The material properties of silicones and TPEs will be addressed. Processing methods with emphasis on injection molding will be covered. Typical products for each material family will be given, including justification of why silicone or TPE was chosen.What is silicone?
Silicones are thermoset materials that have a chemical structure based on chains of alternate silicone and oxygen atoms with organic groups attached to the silicone atoms
---Si---O-----Si----O---
Silicones come in many processable forms, including liquid silicone rubber (LIM or LSR), heat cured rubber (HCE or HCR) and room temperature vulcanized rubber (RTV). Liquid silicone rubber that is platinum cured has become the material of choice for most domestic molders, given its ease of processing. It comes in five gallon pails or 55 gallon drums and does not require any roll milling or compounding. Liquid silicone rubber is fed to molding machines using self-contained metering pumps, minimizing any opportunities for contamination or non-deliberate off ratio delivery. In some cases, heat cured rubber peroxide cured silicones are still used due to properties achievable with customized compounding. This is particularly the case in automotive and electrical applications.
Once a silicone material is molded, it forms a permanent shape and it cannot be broken down into a reusable form for molding again, hence a thermoset designation. This feature, as well as a relatively high material cost, makes it particularly important to get things right the first time for the processor. Cured silicone waste is commonly landfilled due to the lack of economically feasible uses.
What is TPE?
TPE stands for thermoplastic elastomer, which is also called thermoplastic rubber. TPEs are a class of copolymers that are a mix of polymers; most commonly a plastic and a rubber. These copolymers allow for the TPE to have a blend of desirable physical and appearance properties. Common types of TPEs are TPO, TPV, SBC, TPU, COPE and COPA (ref. 1).
In terms of perfomiance, TPEs have three characteristics:
* ability to be stretched to moderate elongations and, when the stress is removed, to return to comparable shape;
* ability to be processed when melted at an elevated temperature; and
* lack of significant creep.
TPEs offer the advantage that they can be repeatedly melt processed. No matter how many times you expose TPEs to sufficient heat, they will become soft and re-harden when cooled (ref. 2).
TPEs come in pellet form in bags or gaylords, and are fed into injection molding machines via hoppers. The TPE pellets typically feel softer and more rubbery than rigid thermoplastic resins.
Silicone property overview
Silicone is a very popular material in body contact applications where its hypoallergenic nature is important. Many healthcare products which were once made from latex or natural rubber have been replaced with silicone for this reason.
Silicone offers material stability over a very broad temperature range. It is able to be used continuously at 350-400[degrees]F with minimal change in physical or appearance properties. It remains elastomeric and flexible at low temperatures, typically down to -100[degrees]F.
Silicone offers good to excellent chemical resistance to oils, solvents and other chemicals, making it a very popular choice in automotive applications. The chemical resistance of silicone to various substances is readily available online in the Chemical Compatibility Chart. Silicone is also naturally UV resistant, making it a good choice for applications where long term weatherability is important.
Silicone is resistant to taking a permanent set, and will return to its original shape over and over when stretched. This ability to restore shape corresponds to a physical property referred to as compression set. Silicone typically has a low compression set relative to other elastomeric materials.
Silicone is also an excellent vibration dampening material, making it a good choice in audio products. It is naturally a good electrical insulator, and it is a common choice in wire and cable applications. Metallic additives may be used to create conductive silicones, if desired.
TPE property overview
TPEs are soft rubbery materials that have elastomeric properties. At room temperature, they are strong, flexible materials. TPEs do not maintain their room temperature properties at high or low temperatures. The recommended continuous use temperature range varies among TPEs based on their copolymer composition, and should be considered in product design. TPEs possess moderate UV resistance, but in outdoor applications, it is best to consider a UV stabilizer additive which is often compounded into the pigment. TPEs can be formulated to provide decent chemical resistance at room temperature. With elevated temperatures, TPEs normally do not have good chemical resistance. TPEs offer good compression set at room temperature, but not at high or low temperatures.
Silicones vs. TPEs
In a nutshell, silicones and TPEs often have very similar properties at room temperature. In fact, it can be difficult to distinguish whether products are made from silicone or TPE from an appearance standpoint. A trick of the trade is to put the material in question in direct contact with an open flame for a couple minutes. TPEs will start to deform and possibly melt, whereas silicones will remain unaffected.
Another key difference between TPEs and silicones is the ability to recycle TPEs. Silicones, by their thermoset nature, cannot be melted and reprocessed once molded.
TPEs and silicones are both soft flexible elastomers. Because TPEs are copolymers, a wider array of surface characteristics and properties are achievable. For instance, all silicones exhibit surface tack and tend to attract dirt and lint. Some TPEs can be formulated to be non-tacky and repel dirt and lint, such as TPUs. TPEs generally have better abrasion resistance properties than silicone.
The hardness range of silicones is typically 3-80 durometer A. It is difficult to achieve silicone hardness greater than 70 durometer A without post-baking. The most common durometer for silicone is 50 durometer A. At 50 durometer A, silicones normally possess the best blend of tensile and tear strength properties. The best sealing properties are achieved with 30 durometer A silicones. Higher durometer materials are created either through polymer crosslinking or filler addition, or a combination of both. Very low durometer silicones less than 10 durometer A are often referred to as gels. These materials are lightly crosslinked and are typically measured and reported on the durometer 00 hardness scale.
TPEs commonly span the 20-95 durometer A range, with a property sweet spot a bit higher than the 50 durometer of silicone, typically 70 durometer A. TPEs also come in gel form on the durometer 00 scale, but they have the undesirable property of surface bleed, typically mineral oil. Silicone gels can also bleed, but the effect is not as pronounced. For sure, any molded TPE or silicone gel that is left overnight on a sheet of white paper, with some weight on the part, will leave a trace of oil on the paper. It is much easier to achieve high durometers in the 70-90 durometer A range with TPEs without post-baking than with silicones. Early generation TPEs were usually in the 50-90 durometer A range, and over time as application needs required lower durometer materials, these grades were developed and added to the commercial offerings.
Silicones used to be significantly higher than TPEs in cost per pound. Over time, increased silicone supply has turned this specialized material into more of a commodity, making it an affordable choice in larger parts and in room temperature elastomeric applications (figure 1). Silicone offers the advantage of being odor-free and hypoallergenic, making it the material of choice in body contact and food use applications, as well as in some high end consumer products. TPEs are still slightly cheaper for applications that do not require the clear benefits of silicone.
Often, the driving factor in material choice between silicones and TPEs has become market acceptability and processing advantages.
TPE injection molding
TPEs are injection molded using the same equipment as rigid thermoplastics (figure 2).
TPEs are supplied in pellet form and are added to the injection molding machine via a hopper. Colorant is either compounded in by the material supplier, or blended in automatically or manually at the machine. Typically, it only makes sense economically to have colorants and other additives compounded in at the supplier when volumes are high, i.e., >10,000 pounds.
TPEs are transported along a hot barrel and are injected into a room temperature range mold between 70-120[degrees]F. These materials mold at low injection pressures, typically <10,000 psi. TPEs, like most thermoplastics, shrink when they cure, which causes sink. Sink is more of an issue with thick cross-sectional parts, making part thickness a critical design parameter. The elastomeric nature of TPEs makes it often possible to pull parts with undercuts out of the mold without mechanical slides. Lower durometer grades usually have higher elongation, and are therefore favorable for demolding with undercuts.
TPEs are recyclable. They can be re-melted and re-molded with ease multiple times.
Silicone injection molding
Silicones require customized injection molding equipment to process. Most silicone injection molding machines start out as thermoplastic machines and are converted to silicones by the machine manufacturer prior to shipping to the molder. In order to mold silicones, a chilled versus a heated barrel is required. Silicone is supplied in pail or drum kits with equal volumes of A and B components. The catalyst normally is in the A side and the crosslinker is in the B side. Liquid silicone rubber has a honeylike consistency and is fed into the barrel using highly accurate 1:1 ratio metering pumps. The A + B gets mixed in a static mixer prior to traveling along the chilled barrel and being injected into the mold. Liquid colorant is added as a third stream prior to the static mixer (figure 3).
The need for customized silicone injection molding equipment adds both lead time and cost to the base plastics machines. Typically, there is a 20-30% premium for silicone.
As mentioned previously, silicone is kept chilled during transport through the barrel, and it is injected into a hot mold (270-450[degrees]F). Similar to TPE, silicone molds at low injection pressures, typically <10,000 psi. Silicone flashes very easily at 0.0002", making mold design more complicated and more expensive than with TPEs. Silicone mold makers must be able to make molds with tight tolerances and vacuum for venting. It is also important to have uniformly heated molds with silicone.
Silicones shrink more than most TPEs, with an average value of 2.3% vs. 0.5-1.5%. Silicone initially expands in the mold, which can cause a condition known as back grind, which results in a ragged over-filled appearance at the parting line. A limited pot life of 1-3 days at room temperature with mixed liquid silicone rubber makes it critical to purge machines that are not operating. Purge instructions vary by material supplier. Pot life is extended by keeping the mixed material under chilled conditions. Pot life varies with supplier and the silicone chemistiy they use.
Silicone parts with undercuts can often be pulled out of the mold without the need for mechanical slides, particularly when parts are on the lower end of the durometer range, <50 durometer A. Once molded, silicone is a thermoset, and as such, it cannot be reprocessed.
TPE vs. silicone overmolding
These days it is very common for a product to be composed of two elastomeric materials, or an elastomer and a rigid plastic, which are co-molded or overmolded together. Overmolding allows the product designer to combine the best of two diverse materials and achieve variation in durometer, color, rigidity, etc.
Generally speaking, TPEs are much easier to work with in overmolding scenarios than silicones. TPEs readily bond to many common plastics, including PC, PP and ABS. Silicone has trouble bonding to anything but another silicone. Silicone bonding to plastic substrates is most reliably achieved by creating mechanical undercuts between the two components of the overmolded assembly.
With silicone overmolding, first shots must often be preheated if there is a delay between the processes. An advantage of co-molding machines and processes is everything is done in one machine in a continuous process. The downside is that the machines and molds are quite expensive, making small volume overmolding more practical in separate machines. TPEs do not require preheating of substrates with overmolding.
The high mold temperatures required to cure silicone will often make them incompatible with low deformation/low melting temperature plastics as first shot substrates. The choice of rigid thermoplastics for silicone overmolding processes is quite limited for this reason. Acceptable substrate materials include PC, and co-polyesters including Tritan and polysulfones.
Summary: How to choose between TPE and silicone
Given that pricing for FDA and Class VI compliant TPEs and silicones has become very comparable, the decision on which material family to use is not typically economic.
In many cases, market acceptance is a large factor. Silicones are still the gold standard material in health care and consumer body contact applications. In some cases, the decision is driven by material properties. Clearly, in high heat applications such as baking and with repeated industrial dishwasher use, silicones are the best choice. Processing advantages must also weigh in on the decision to use TPE or silicone. In ovennolding scenarios, when plastic is the substrate, TPE is often a much better choice, offering ease of processing (table 1).
There will be many more molders capable of processing TPE than silicone due to the fact that TPEs can be molded with standard plastics injection molding equipment. However, do not be fooled. TPE molding is a sub-specialty of plastics molding that requires some added expertise. Make sure to have silicone molds built by a mold maker who has experience with these materials.
The bottom line is that, as a consumer, you may not be able to tell if a product is made from silicone or TPE, but there are distinct differences in performance and processing characteristics.
This article is based on a presentation made at the Smithers Rapra Thermoplastic Elastomers U.S. Summit, June 2017.
by Lynn E. Momrow-Zielinski, Extreme Molding
References
(1.) Just What Are TPEs, Frank Esposito, Rubber and Plastics News, Akron, OH, August 7, 2017.
(2.) What's The Difference--TPEs and Silicones, PolyOne, 2017.
Caption: Figure 1--price comparison for TPEs vs. silicones
Caption: Figure 2--TPEs are injection molded using the same equipment I as rigid thermoplastics
Caption: Figure 3--silicone injection molding system
Table 1--typical TPE and silicone products
Product Silicone TPE
Wearable Rare Common--molding
electronic devices temperatures
compatible with
electronics and
easier to overmold
and keep clean
Spatulas, baking Yes No--high
liners, pot holders temperature
resistance lacking
Infant pacifiers Yes--wide market Yes--but less
acceptance common
Soft touch grips Yes--limited Yes--easy to
substrate materials overmold most
because of high substrates
mold temperatures
required
Toys Rare--expensive FDA Yes--soft and less
approved material expensive FDA
choice approved materials,
easier to overmold
Infant dinnerware Yes--market Rare--repeated
acceptance with dishwasher exposure
baby care items and not as much
market acceptance
in baby care
Phone and Yes--strong, can Yes--cheaper
tablet covers be exposed to the solution, nice feel
elements
Footwear No--expensive and Yes--competes
poor abrasion with natural and
resistance synthetic rubber
Automotive Yes--excellent Yes--but less
under-the-hood temperature and common, lower
applications oil resistance temperatures
Automotive Yes--but not that Yes--easier
accessories common--expensive to overmold
Medical Yes--wide market Yes--growing
tubing/catheters acceptance market acceptance
Sporting goods Yes--but expensive Yes--common
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| Author: | Momrow-Zielinski, Lynn E. |
|---|---|
| Publication: | Rubber World |
| Date: | Oct 1, 2017 |
| Words: | 2575 |
| Previous Article: | Surface-modified elastic body. |
| Next Article: | Choosing the right TPE: The seven essential considerations. |
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