Silicone vs Polyurethane

Silicone vs polyurethane is a type of thermosetting polymer. They are commonly used in liquid form during processing and undergo internal cross-linking as they cool down within a mold. This process ultimately leads to the hardening of the materials. Despite being thermosetting polymers, silicone, and polyurethane differ significantly in their chemical composition and characteristics.

Silicone is an inorganic polymer that has a backbone made of silicon and oxygen atoms. It possesses excellent resistance to heat and UV radiation. On the other hand, polyurethane is an organic polymer composed of a polyol and an isocyanate, and it can have a wide range of chemical structures. Polyurethane exhibits exceptional tensile strength and resistance to abrasion.

Both silicone and polyurethane come in various formulations, resulting in a wide range of physical and mechanical properties. This variability allows design engineers to select the most suitable grade for a specific application.

1. What is Silicone?

Silicone is a type of polymer composed of repeating molecular units that combine to form long chains. These chains consist of alternating silicon and oxygen atoms, creating a structure that allows for the attachment of hydrocarbons or other chemical groups, resulting in a complete silicone polymer molecule.

As an inorganic thermosetting polymer, silicone is commonly mixed in liquid form with a base polymer and a catalyst. Upon mixing, it initiates an exothermic chemical reaction, leading to the curing process.

Silicone can be categorized into three main subcategories: Liquid Silicone Rubber (LSR), Heat-Cured Rubber (HCR), and Room Temperature Vulcanizing (RTV) silicone. RTV silicone further branches into RTV-1, which cures upon exposure to air, and RTV-2, a combination of a pre-polymer and a catalyst that cures at room temperature.

Different classes of silicone may undergo various processing methods. RTV-1 is typically dispensed from a tube and cures in the presence of air, while the two liquid components of RTV-2 are mixed immediately before use. Generally, silicone is processed through techniques such as open mold casting, compression molding, or injection molding.

Silicones exhibit a wide range of physical and mechanical properties, varying from a Shore A hardness of 10 to 90. They possess excellent resistance to high temperatures, ultraviolet (UV) radiation, and ozone. However, they tend to have poor resistance to cutting and tearing, low tensile strength, and limited abrasion resistance.

2. What is Polyurethane?

Polyurethanes are organic polymers that exhibit thermosetting properties. They consist of intricate molecular chains composed of both hard and soft segments. The flexibility of polyurethanes is attributed to the presence of soft segments, while the strength is provided by the hard segments. The term “polyurethane” encompasses a wide range of material formulations, typically classified as either polyesters or polyethers.

Polyurethanes can be processed through various methods such as open mold casting, rotational casting, or reaction injection molding. As thermosetting materials, polyurethanes are commonly mixed in liquid form. A prepolymer is typically blended with a catalyst, either at room temperature for cold curing or at elevated temperatures for hot curing. Once mixed, the polyurethane undergoes a curing process that releases heat in an exothermic reaction. Hot-cure polyurethane generally exhibits superior mechanical properties compared to cold-cure polyurethane.

Polyurethanes possess exceptional toughness and resistance to abrasion. They can be manufactured with a broad range of hardness levels, ranging from Shore 35 A to Shore 70 D. However, the maximum recommended operating temperature for polyurethane is 80°C, as exposure to higher temperatures can lead to chemical degradation. Furthermore, elevated temperatures also cause a decline in the material’s mechanical properties, including hardness and abrasion resistance.

Silicone vs Polyurethane Overview Table

3. Silicone vs Polyurethane: Recyclability and Sustainability

Both silicone and polyurethane are classified as thermosetting polymers, which means that they undergo chemical breakdown and cannot be reshaped into useful forms like thermoplastics when exposed to temperatures beyond their maximum operating limits. Consequently, recycling silicone and polyurethane poses challenges, and most recycling facilities lack the necessary capabilities to economically recycle these materials. One potential recycling method involves mechanical granulation, where the material is ground into small particles that can be utilized as fillers.

It is important to note that silicone and polyurethane are not considered sustainable materials since they are not derived from renewable resources.

4. Silicone vs Polyurethane: Applications and Uses

Polyurethane finds extensive use in various applications that benefit from its enhanced toughness and resistance to wear. Here are some typical examples:

• Forklift wheels, skateboard wheels, and shoe soles.
• Impact arrestors for railways and elevators.
• Pipe linings, pump impellers, casings, seals, molds, and conveyor belts.
• Chute linings, vibratory screens, and cyclones.
• Electrical potting or encapsulation.

Silicone, on the other hand, is available in diverse forms, each with different levels of mechanical strength, thermal resistance, and biocompatibility. It finds application in the following areas:

• High-temperature cookware, such as utensils and bowls.
• Molds are sometimes used for casting polyurethane.
• Industrial sealing and bonding.
• Catheters, hoses, and implants.
• Seals and gaskets.
• Automotive Industry
• Medical Industry
• Aerospace

5. Silicone vs Polyurethane: Cost

Cost plays a significant role in the decision-making process for any project. When considering whether to choose polyurethane or silicone for a replacement project, it is crucial to consider several factors.

Firstly, the cost of materials should be taken into account. Silicone can be up to twice as expensive as polyurethane, which can impact the overall project budget.

Secondly, the cost of labor should be considered. Although the initial application process takes a similar amount of time for both materials, the frequency of repairs and replacements required for polyurethane can be considerably higher. This means that additional labor costs may arise from the need for more frequent maintenance or replacement of polyurethane material.

Taking these factors into consideration is important in order to make an informed decision that aligns with both budgetary constraints and long-term project requirements.

6. Alternative Materials to Silicone and Polyurethane

While silicone and polyurethane are thermosetting elastomers with limited alternatives, there are situations where thermoplastic elastomers (TPEs) can be used as viable substitutes. Thermoplastic rubber (TPR) and thermoplastic polyurethane (TPU) are two examples of TPEs that can serve as suitable alternatives to thermosetting silicone and polyurethane. These TPEs can be processed using standard thermoforming techniques such as injection molding, roto molding, and blow molding. This provides flexibility in manufacturing and expands the options available for applications requiring elastomeric properties.

This article provided an overview of silicone and polyurethane, explaining their characteristics and applications in manufacturing. To learn more about silicone and polyurethane, contact an A1 Silicones