NBR foam (nitrile rubber foam) and silicone foam (silicone rubber foam), as two widely used rubber-based foam materials, exhibit fundamentally different core foaming principles and material properties due to variations in their molecular structures and chemical reaction mechanisms. These differences directly determine the selection criteria for each material in industrial applications.
From the perspective of its core foaming mechanism, NBR foaming relies on the synergistic action of high-temperature decomposition chemical foaming and sulfur vulcanization cross-linking. The molecular backbone of NBR consists of carbon-carbon bonds (C–C), with side groups containing acrylonitrile (-CN) and butadiene double bonds (C=C). Its foaming process falls under the category of heterogeneous nucleation thermal decomposition foaming. In industrial production, azodicarbonamide (AC), sodium bicarbonate, or N,N'-dinitrosopyramidetetramine (DPT) are commonly used as blowing agents. When the temperature rises to 150–180°C, these blowing agents undergo thermal decomposition, releasing gases such as N₂, CO₂, and NH₃. These gases expand within the NBR melt to form pores. Simultaneously, sulfur vulcanization reactions occur concurrently, leading to cross-linking of the butadiene double bonds and the formation of C–C and C–S–C network structures, ultimately stabilizing the pore morphology. The essence of this cross-linked network lies in the bridging interactions between carbon-carbon bonds via sulfur bridges, with cross-linking bonds predominantly consisting of C–S–C and single/double sulfur bonds. Due to their relatively low bond energy, these networks exhibit limited thermal stability.
In contrast, the foaming mechanism of foam silicone involves a synchronous reaction of platinum-catalyzed cross-linking and in situ hydrogen evolution during foaming, constituting a homogeneous nucleation-based reactive foaming system. The matrix of foam silicone is polydimethylsiloxane (PDMS), with a molecular backbone composed of silicon-oxygen bonds (Si–O–Si) and methyl side groups (–CH₃). Foaming and curing are typically achieved using a two-component system: Component A consists of vinyl silicone oil containing Si–CH=CH₂, while Component B comprises hydrogen-containing silicone oil with Si–H bonds and a platinum catalyst. During the reaction, two critical processes occur simultaneously: first, the platinum-catalyzed cross-linking reaction where Si–CH=CH₂ bonds with Si–H to form Si–C bonds, enabling colloid curing; second, the hydrogen evolution foaming reaction where Si–H reacts with Si–OH to form Si–O–Si bonds and release H₂. The hydrogen nucleates and expands within the polymer matrix, ultimately forming either closed-cell or open-cell structures. The cross-linked network centers around Si–O–Si main chains and Si–C cross-links, with Si–O bonds exhibiting an energy of up to 452 kJ/mol-significantly higher than the 347 kJ/mol of C–C bonds in NBR-thereby ensuring the exceptional thermal stability of foam silicone (PMC, 2025; MDPI, 2024).
In terms of material properties, the differences between the two materials are primarily reflected in key indicators such as temperature resistance, chemical resistance, mechanical properties, and environmental stability. From a fundamental physical perspective, NBR foam has a density of 0.3–0.8 g/cm³, a Shore A hardness of 20–70°, a predominantly closed-cell structure with pore sizes ranging from 50–300 μm and a porosity of 40–70%. In contrast, foamed silicone exhibits a lower density (0.2–1.1 g/cm³), softer hardness (Shore A 1–50°), more uniform pore sizes (20–100 μm), and higher porosity (50–80%). Experimental data indicate that its pore size consistency is 40% superior to that of NBR foam (MDPI, 2024; PMC, 2025).
Thermal resistance is one of the most fundamental differences between the two materials. NBR foam has a long-term service temperature range of –30°C to +120°C, with a heat loss weight of 5% occurring at approximately 280°C. After aging at 150°C for 72 hours, it exhibits hardening, shrinkage of 10–15%, and loss of elasticity exceeding 50%. In contrast, foam silicone can withstand long-term use temperatures ranging from –60°C to +230°C and tolerate short-term exposure to 280°C, with a heat loss weight of 5% occurring at around 480°C. Even after aging at 200°C for 72 hours, its permanent compression deformation remains ≤30%, and its elastic retention exceeds 70% (data from Solid Rocket Technology, 2019). The fundamental reason for this difference lies in the disparity in chemical bond strength: the high stability of Si–O bonds enables silicone to resist degradation under higher-temperature conditions.
Regarding temperature resistance, a Chinese enterprise has developed a foam silicone that can withstand a temperature of 315°C for a short period.
In terms of chemical resistance, NBR foam exhibits remarkable oil resistance, while foamed silicone demonstrates outstanding performance against acids, alkalis, ozone, and ultraviolet radiation. Experimental data from Wiley (2019) show that after immersion in #3 engine oil at 100°C for 72 hours, the volume change of NBR foam was only 2.1%, indicating excellent tolerance to mineral oils, engine oils, and diesel fuels, as well as good resistance to aromatics and esters; however, its performance against acids, alkalis, and water is average. In contrast, foamed silicone exhibited a volume expansion rate of up to 42.7% under identical conditions, showing poor oil resistance but exceptional tolerance to acids, alkalis, water, ozone, and UV radiation, with an outdoor service life of 20–30 years without aging. In comparison, NBR foam has an outdoor lifespan of only 3–5 years and develops cracking after ozone aging (50 ppm, 40°C, 72 h), while UV aging for 300 hours leads to hardening, brittleness, and surface powdering (data cited from Journal of Wuhan University, 2022).
In terms of mechanical properties, NBR foam exhibits higher tensile strength (0.8–2.5 MPa), permanent compression deformation (25%,72 h, 70°C) ≤25%, and moderate resilience (50–60%). Foamed silicone demonstrates slightly lower tensile strength (0.5–0.8 MPa) but higher elongation at break (≥300%) and superior resilience (70–85%), with significantly better mechanical stability under high-temperature conditions-its permanent compression deformation remains below 30% at 200°C (data from Journal of Wuhan University, 2022). Regarding flame retardancy and safety, NBR foam requires flame retardants to achieve flame resistance and self-extinguishing properties upon removal from flame; it produces dense smoke during combustion and releases toxic gases such as CN⁻ and SO₂. In contrast, foamed silicone possesses intrinsic flame retardancy, meeting UL94 V-0 standards, is non-flammable, smoke-free, and non-toxic/odorless, complying with food-grade requirements (FDA 21 CFR 177.2600).
In summary, the differences between NBR foam and foamed silicone stem from their fundamentally distinct molecular structures and foaming/cross-linking mechanisms: NBR foam is carbon chain-based, achieving its shape through sulfur vulcanization and thermal decomposition, offering strong oil resistance and lower cost, but with limited temperature resistance and environmental stability; foamed silicone centers on a silicon-oxygen backbone, where cross-linking catalyzed by platinum occurs simultaneously with hydrogen evolution during foaming, exhibiting exceptional temperature resistance, aging resistance, and safety, though it has poorer oil resistance and higher cost. These characteristic differences clearly delineate the application scenarios for each material and provide a solid basis for industrial selection.
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