“Thermoplastic” and “thermoset” may sound similar, but they represent distinct categories of materials that provide suitable options for complex applications across various markets. However, the key factors that determine the best choice for your project are the properties and processing behaviors of the materials within these categories.
To gain a clearer understanding of the similarities and differences, let’s compare a plastic from each group: thermoplastic elastomer (TPE) and liquid silicone rubber (LSR) thermoset”
What Is LSR?
Silicones are made from quartz sand, a raw material that is practically unlimited in quantity, in some cases is used to drive out volatiles.
Opposite of thermoplastic elastomer (TPE), LSR hardens when subjected to heat. Initially during the molding process and secondarily during the post-cure process, heating LSR causes vulcanization, which locks in its physical properties such as tensile strength, flexural strength, and heat distortion temperature. Post-curing, which involves heating the molded parts in an oven, serves the purpose of accelerating vulcanization, thereby maximizing certain physical properties of the material and, in some cases, removing volatile compounds.
Platinum-cured LSR is a two-part formula that is mixed prior to injection into the mold. One component acts as a catalyst to initiate the cross-linking process. The rubber undergoes chemical cross-linking during the molding process, known as curing or vulcanization. The duration of the vulcanization process can range from a few seconds to several hours, depending on the specific circumstances.
Silicone possesses a structure with alternating oxygen and silicon atoms, and it can exist in various forms, including LSR, heat-cured rubber, and room-temperature vulcanized rubber.
What Is TPE (Thermoplastic Elastomer)?
Thermoplastic Elastomer (TPE), also known as thermoplastic rubber, is a compound or blend of polymers that can be melted and shaped into plastic when heated and hardened when cooled. Its chemical composition remains unchanged during this process. TPE belongs to a class of copolymers that combines plastic and rubber components.
TPE can be molded and fabricated once it reaches its melting point temperature. While it exhibits similar elasticity to cross-linked materials like LSR, its elastomeric properties are not reliant on cross-linking. Thermoplastic Elastomer does not undergo chemical bonding in the molten and solidification process. The plastic pellets liquefy when heated, allowing them to be pressure-molded into various components, which retain their shapes during the curing process. Some common types of TPE solutions include:
- Olefinic thermoplastic elastomer
- Polyester thermoplastic elastomer
- Amide thermoplastic elastomer
- Polyurethane thermoplastic elastomer
- Thermoplastic vulcanization
- Styrenic block copolymer
- Molten processable rubber
When Is LSR The Best Choice?
When choosing between silicone and thermoplastic elastomers, there are several critical factors to consider, including temperature, compression set, and heat resistance. Conventional rubbers such as natural rubber, nitrile rubber, ethylene polypropylene diene rubber (EPDM), and poly-acrylic elastomer have a limited working temperature range. For instance, natural rubber typically operates between -50°C and 100°C, while nitrile rubber can handle temperatures ranging from -25°C to 150°C. When comparing thermoplastic elastomers vs. silicone elastomers, LSR provides consistent performance in a wider temperature range than TPE, both on high and low scales.
Wide Operation Temperature
LSR can operate at 350 to 400 degrees F (175 – 205 degrees C) without altering its shape. It maintains its properties reliably in both hot and cold environments. For example, LSR remains flexible and resistant to compression set even in temperatures as low as -40°C. These characteristics make LSR highly desirable in the automotive industry, particularly for exterior components such as sensors, connectors, seals, and gaskets.
Additionally, LSR can be applied to interior automobile parts like gaskets and keypads in intelligent keys, as well as vibration-dampening cushions in air conditioning systems. In contrast, thermoplastic elastomers do not perform as effectively as LSR within these temperature ranges.
High Flexibility
LSR has low viscosity, ease of the process, and availability in a range of durometers, this offers designers and engineers the added design flexibility to achieve complex features’ requirements: tight dimensions, thick and thin details, and consistency from soft to firm. Because of its ability to fill thin walls and stability in a wide range of temperatures, LSR is the perfect material for pressure-sensitive applications.
For example, gaskets and membranes in gas appliance applications, pups and surgical deviance in ophthalmology surgeries, and thin membranes and gaskets in medical fluid management devices. LSR is available in different durometers from soft to firm, its most common stiffness is between 20 and 80 Shore A.
Stability
Silicone elastomer is UV – stable making them highly resistant to weathering. This makes it an excellent choice for products with long-term exposure to UV rays. They can endure harsh outdoor conditions for extended periods without deterioration and without the need for additional additives. Furthermore, silicone elastomers exhibit ozone stability, providing superior resistance against humidity and moisture.
Bio-Compatibility
LSR (Liquid Silicone Rubber) demonstrates outstanding chemical resistance to various solvents and cleaning agents. This attribute makes it an ideal material for medical and healthcare applications, particularly those that encounter ambient contaminants and necessitate regular cleaning with harsh agents. LSR is the preferred choice for products that come into contact with the body, thanks to its biocompatibility and hypoallergenic properties. It finds widespread use in respiratory devices and wearable medical devices without causing skin irritation.
See more: Medical Grade Liquid Silicone Rubber
LSR Advantages
- LSR also has other advantages, including:
- Ease and clean processing,
- High clarity and light transmittance
- Odor-free and tasteless,
- Radiation, UV light, and bacteria resistance
- Resistant to other light energy, including VIS, IR, and microwave radiation
- Strong dielectric property and superior insulation
- Minimal water uptake and water resistance
- Extensive pigment colors.
- Minimal flammability
Applications Of LSR
LSR also has thousands of applications in various industries such as::
- Connector seals in all industries
- Duckbill valves, membranes, and diaphragms
- Bellows
- Membranes, diaphragms, and vales
- Gaskets and hardware in appliances
- Septa
- Electronic sealed housing
- Plunger plugs/tips
- Pump housings
- Hearing aid parts/ear tips-buds
- Respiratory masks
- Baby nipples and pacifiers.
Other industries LSR products can serve to include:
- Food & Beverage
- Appliances
- Consumer Electronics
- Sanitary
- Mother & Child Care
- Building Technology
When Is TPE The Best Choice?
From a processing perspective, thermoplastic elastomers offer cost savings to processors and are priced less than silicone elastomers which is also attractive to their customers. TPEs consume less energy during production because the process avoids cross-linking. TPE melts when exposed to higher temperatures, thus has recyclability properties, permitting the reprocessing, if not contaminated.
TPEs offer some of the same benefits as silicone elastomers including vibration resistance, hypoallergenic, and compression set, however with limitations in certain temperature ranges. For this reason, they are not as well-suited for many automotive under-the-hood applications or exterior parts with these operating temperatures.
For some applications, TPE can offer advantages over LSRs and other silicone elastomers. For seals and other permeability-related applications, TPEs provide a strong barrier against oxidation, and moisture, and retain pressure and vacuum. The material can also be formulated to be non-tacky, which helps repel dirt and other contaminants making them a good choice for consumer products and other products where aesthetics and surface finish are important.
TPE Advantages
- TPE advantages include:
- Excellent colorability
- Heat-sealable
- Abrasion resistance
- Good electrical properties
- High elasticity
- High fatigue resistance and impact strength
- Low-density and compression set
- Low specific gravity
- Resilient to chemicals and weathering
- Strong and flexible at room temperature.
The primary disadvantage of TPEs is their diminished dimensional stability, which can occur due to incomplete crosslinking. Furthermore, the inclusion of additives like plasticizers, anti-oxidants, and processing promoters can lead to higher levels of leachable and extractable volatiles. When TPE components are intended for use in medical devices and life science applications, these levels are typically closely monitored to ensure compliance with safety standards.
Applications Of TPE
- Depending on your requirements and environment uses of TPEs include items such as:
- Bottle caps,
- Baby bottles
- Impact-resistant components and devices housing
- Seal rings
- Closure liners
- Seals
A thermoplastic elastomer is an alternative to latex, PVC, or rubber, especially in medical and healthcare applications.
Comparison of LSR and TPE
| Silicone | TPE | |
| Materials | LSR (liquid silicone rubber) possesses the ability to withstand sterilization and demonstrates excellent compatibility with miniaturization, which has fueled its rapid expansion in medical and healthcare applications. As a result, LSR is well-suited for products that come into contact with the human body. LSR exhibits a hardness range of 3 to 80 Shore A, with 50 Shore A being the most common and offering the highest tensile and tear strength. A hardness of 30 Shore A provides optimal sealing properties. On the other hand, low-durometer silicone elastomers, such as gels, can achieve hardness levels below 10 Shore A. | The reliability and moldability of thermoplastic elastomers (TPEs) allow for repeated reuse of the resin without significantly compromising the material’s integrity and performance. By re-heating, softening, and re-hardening the TPE, it can be reprocessed and remolded multiple times while still maintaining its compression set properties.
Elastomers undergo a transition from a liquid to a solid state and are measured based on their softness and hardness. The Shore Durometer scale is used to determine the value. For instance, soft gel TPE materials have a range of 20 Shore OO to 90 Shore A. As the hardness of a product increases, so does its durometer value. When hardness values reach up to 85 Shore D, the material is considered very hard, reaching the Shore D level. Regarding more elastic materials, their hardness typically falls within the range of 20 to 95 durometer A. A typical hardness measurement for thermoplastic elastomer products is approximately 70 durometer A. |
| Recyclability | Once solidified, LSR does not return to a liquid state, making it impossible to recycle. | Just like rigid plastics, solid TPEs can return to liquid form upon heating. Thus, TPEs can also be recycled in general. |
| Production Method | LSR (liquid silicone rubber) can be injection molded for large-scale production. The temperature requirements for LSR injection molding are the reverse of TPE injection molding. In this process, two materials are continuously mixed at a cooled temperature and then heated within the mold to solidify into the desired form. The use of a specialized injection molding machine is necessary to accommodate different materials and ensure optimal results. | Injection molding involves heating pellets of raw material until they melt, then injecting them into a mold cavity. The material is subsequently cooled down to solidify and acquire the desired shape. This process utilizes the same equipment and techniques as standard thermoplastic injection molding, although additional expertise is required to produce TPEs effectively. Injection molding machines are available in various tonnages, indicating the amount of clamping pressure they can apply to a mold. These machines can be used for molding both hard plastics and TPE materials interchangeably. |
| Cycle Time | Compared to molding TPE, injection molding of LSR typically requires more time due to additional steps involved in LSR injection molding, stemming from the different chemical reactions that occur. For instance, in LSR injection molding, the initial cooling process involves mixing two materials for the catalyst to cure the material. This chemical reaction is more intricate than the initial heating process for TPE, which simply involves melting pellets. As a result of this complexity, the cycle time for LSR injection molding can extend to minutes. However, LSRs can still be processed using multi-cavity molds, allowing for the simultaneous production of multiple parts. | “Cycle time” refers to the duration it takes for an injection molding machine to complete one full cycle of part production, starting from melting the raw materials to cooling them into a solid part. The actual cycle time can vary depending on factors such as the size, material, and complexity of each part. However, compared to LSR injection molding, the injection molding process is generally simpler and typically falls within the lower tens of seconds per cycle. By utilizing “multi-cavity” molds, which can range from two to more than sixteen cavities, multiple copies of parts can be produced simultaneously. This allows for more parts to be manufactured within the same cycle time. |
| Assembly Process (Overmolding) | There are only a few silicone materials formulated to adhere to plastic substrates. While over-molding with mechanical gripping features is still feasible, it’s important to note that the silicone molding process involves high temperatures. Therefore, if the other plastic component has low heat resistance, it may be negatively affected during the molding process. | Specific TPE formulations are designed to create a chemical bond with rigid plastic substrates, making them well-suited for over-molding or two-shot molding processes. The combination of chemical and mechanical bonding properties can significantly enhance the strength of the seal between the TPE and the substrate. |
| Material Storage | The raw material of the LSR requires refrigeration while in storage. | The raw materials of the TPE are stored at room temperature. |
| Production Cost | The overall production cost is typically higher for LSR injection molding compared to TPE due to various factors. These include the need for customized equipment, longer cycle time, the requirement of refrigeration facilities, higher density or specific gravity per cm3 compared to TPE, and, in certain cases, the need for additional assembly processes instead of over-molding. | In addition to the production method, cycle time, and material storage requirements mentioned earlier, TPEs have a lower density or specific gravity per cm3 compared to LSR. This means that more parts can be produced using the same amount of material in comparison to silicone. These factors contribute to lower production costs for TPEs in injection molding when compared to LSR. |
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