When it comes to the application of silicone products, you might encounter questions like these: Why do some silicon rubbers cure without any odor and don't release small molecules, while others emit an alcohol-like smell? Why can some cure rapidly at room temperature, whereas others require heating to take shape? The core reason behind these differences lies in the two fundamental curing mechanisms of silicones—addition-cure and condensation-cure. These two mechanisms differ dramatically in their reaction principles, directly influencing the product’s performance, application scenarios, and usage methods. Today, we’ll thoroughly understand the distinctions between these two types of silicones and learn how to make the right selection for your specific needs.

Let me start with a simple summary: Addition-type silicone is “directly stitched together at the molecular level,” resulting in a clean and efficient reaction; condensation-type silicone, on the other hand, “expels small molecular byproducts during molecular stitching,” offering more flexible reaction conditions. This fundamental difference runs through all the properties and application scenarios of both types.

I. Core Decomposition: Differences in the Reaction Mechanisms of Two Types of Organosilicon Compounds

The curing process of silicone is essentially a process in which linear siloxane molecules undergo chemical reactions to form a three-dimensional network structure—much like weaving individual “ropes” into a “net.” The fundamental difference between addition-type and condensation-type silicones lies in the distinct methods and processes used to “weave” these molecules together.

1. Addition-curing silicone: “Precise splicing, zero byproduct”

The curing reaction of addition-type silicone is centered on the “silicone-hydrogen addition reaction.” We can think of it as a precise interlocking mechanism—much like the mortise-and-tenon joint.

Reaction Principle: Siloxanes bearing vinyl groups (-CH=CH₂) on their molecular chains, in the presence of a crosslinking agent containing silicon-hydrogen bonds (-Si-H), undergo an addition reaction catalyzed by a platinum catalyst. In this reaction, the vinyl groups and silicon-hydrogen bonds directly bond together to form new carbon-silicon bonds (-CH₂-CH₂-Si-). Throughout the entire process, only two molecules are “spliced” together, with no small-molecule byproducts generated.

Key reaction conditions:

- Catalyst: A platinum catalyst is essential; the catalyst’s activity directly determines the reaction rate.

- Temperature: The reaction is relatively slow at room temperature and typically requires heating (80℃–150℃) to accelerate curing. Some highly active formulations can achieve room-temperature curing, though the curing time will be longer.

- Environmental requirements: The catalyst is sensitive to impurities—particularly substances containing elements such as nitrogen, sulfur, and tin, which can “poison” the platinum catalyst and cause the reaction to terminate (i.e., “polymerization inhibition”). Therefore, it must be used in a clean, impurity-free environment.

Typical characteristics: The curing process does not release any small molecules, and the volume shrinkage rate is extremely low (typically <0.1%). After curing, the product exhibits high precision, stable performance, minimal odor, and superior environmental friendliness.

2. Condensation-type silicone: “Jointly produced as a byproduct, with flexible conditions.”

The curing reaction of condensation-type silicone is centered on the “silanol condensation reaction,” which can be understood as “squeezing out excess moisture during assembly.”

Reaction Principle: Siloxanes with silanol groups (-Si-OH) on their molecular chains, in the presence of a crosslinking agent (such as alkoxysilanes) and a catalyst (typically a tin-based catalyst), undergo a condensation reaction between the silanol groups and functional groups in the crosslinking agent, forming siloxane bonds (-Si-O-Si-). In the process, small-molecule byproducts—depending on the formulation—may include water, alcohols (such as methanol or ethanol), acetone, and others—are released. These byproducts volatilize from the curing system, thereby completing the entire curing process.

Key reaction conditions:

- Catalyst: Commonly used tin-based catalysts (such as dibutyl dilauryl thioglycolate) offer high catalytic efficiency and greater tolerance to environmental impurities.

- Temperature: It can rapidly cure at room temperature, and heating can further accelerate the curing process, offering broader temperature adaptability.

- Environmental requirements: A certain level of humidity is needed (moisture in the air can aid curing), but the volatilization of byproducts may lead to bubble formation in enclosed environments. Therefore, this product is more suitable for use in open or well-ventilated areas.

Typical characteristics: The curing conditions are flexible, and it can be used at room temperature with a low operational threshold. However, since it releases small-molecule byproducts, it has a relatively high volume shrinkage rate (typically 1%–3%), which may result in odor emissions. After curing, the product exhibits relatively lower precision.

Understand the core mechanism differences at a glance.

II. Key Selection Points: Match precisely to your needs—avoid pitfalls.

Now that you understand the differences in reaction mechanisms, selecting the right option becomes straightforward—after all, the key is to “match the core needs of your use case.” Here are five critical selection dimensions to help you quickly pinpoint the ideal type:

1. Check the “Product Precision Requirements”: Choose the addition-type for high precision, and the condensation-type for ordinary precision.

Addition-cure silicone exhibits no release of small molecules during curing and has an extremely low volume shrinkage rate. It can precisely replicate the details of molds or products, making it ideal for applications that demand high dimensional accuracy—such as encapsulation of electronic components (e.g., chips, sensors), precision mold making (e.g., jewelry molds, micro-part molds), and sealing of optical devices.

Condensation-type silicone, due to its relatively high volume shrinkage rate and slightly weaker ability to replicate fine details, is more suitable for applications that do not demand high precision—such as sealing building doors and windows, encapsulating ordinary industrial components, and repairing everyday silicone products.

2. Consider the “curing environment conditions”: Choose addition-type for enclosed/high-temperature environments, and choose condensation-type for room-temperature/open environments.

If used in a closed environment (such as inside electronic device enclosures), the small-molecule byproducts released by condensation cannot volatilize, which may lead to bubble formation or residue buildup and consequently degrade product performance. In such cases, addition-type formulations are preferred (as they produce no byproducts and do not generate bubbles).

If rapid curing at room temperature is required and the application environment is open and well-ventilated (such as for building sealing or on-site repairs), a condensation-type formulation is more suitable—no heating equipment is needed, it’s easier to handle, and byproducts can evaporate quickly.

3. Refer to “Environmental Protection and Odor Requirements”: For low-odor/food-grade addition-type formulations, condensation-type formulations are suitable for general applications.

Addition-cure silicone is virtually odorless, leaves no residue after curing, and has passed food-contact safety certification, making it ideal for food-grade applications (such as silicone molds and food seals), medical applications (such as medical catheters and sealing around implantable devices), and indoor decoration—any scenario that is sensitive to environmental concerns and odors.

The small molecules released by condensation (such as alcohol) have a certain odor and may leave trace residues after curing. Therefore, they are not suitable for food contact or enclosed indoor environments, but are more appropriate for outdoor industrial applications.

4. Consider “Cost and Operational Threshold”: Choose the condensation type for low cost/easy operation, and choose the addition type for high budget/high precision.

Condensation-type silicone catalysts (tin-based) are low-cost, do not require heating equipment, and have a low operational threshold, making them suitable for both mass production and on-site construction in common scenarios and effectively helping to control costs.

Platinum catalysts used in addition-cure silicone formulations are relatively expensive, and some applications require heating equipment for curing. During operation, it’s crucial to avoid contamination by impurities, which further increases both costs and operational barriers. However, these catalysts can meet the demands for high precision and environmental friendliness, making them ideal for high-end applications.

5. Consider “Base Material and Environmental Impurities”: Choose the condensation type if the base material contains impurities; choose the addition type if the environment is clean.

If the substrate itself contains elements such as nitrogen, sulfur, or tin (e.g., certain rubber products or sulfur-containing metals), it can cause the platinum catalyst used in addition-curing silicone to lose its effectiveness and prevent curing from occurring. In such cases, it is essential to choose a condensation-type silicone (which exhibits strong tolerance to impurities).

If the substrate is clean and free of impurities (such as glass, metal, or pure plastic) and the operating environment can be kept free of contaminants, addition-curing formulations offer more stable performance and are the better choice.

III. Matching Table for Common Application Scenarios

IV. Summary: Memorize the selection guidelines—avoid pitfalls!

“High precision, odorless, closed-system addition-type at high temperatures; fast curing at room temperature, low cost, open-system condensation-type; substrate selection: choose condensation for general applications, and addition for food and medical uses.”

In fact, there’s no absolute superiority or inferiority between addition-cure and condensation-cure silicones—the key lies in matching them to your specific application needs. By thoroughly understanding the differences in their reaction mechanisms and taking into account core factors such as precision, environmental conditions, and cost, you can precisely select the right product for your needs.