Main Components of Polyurethane Sealant

Main Components of Polyurethane Sealant
The properties of polyurethane sealants (such as elasticity, adhesion, and weather resistance) are determined by their core ingredients. The ratio and quality of these ingredients directly impact the sealant’s ultimate performance. The main components can be divided into five categories: base resin, curing agent, performance modifier, functional additive, and auxiliary ingredients. The functions and common types of each component are as follows:
1. Core Components: Determine the Basic Structure and Elasticity of the Polyurethane Sealant
These components are the core of the “elastic layer” of polyurethane sealants, typically accounting for over 50% and directly determining the adhesive foundation and elastic properties of the sealant.
1. Polyurethane Prepolymer
Function: The “skeleton” of the sealant, serving as the core raw material that determines the sealant’s elasticity, strength, and adhesion. The reaction between the prepolymer and the curing agent ultimately forms an elastic, cross-linked polymer network (the cured sealant layer).
Common Types:
Formed by the reaction of an isocyanate (such as MDI or TDI) with a polyether polyol (or polyester polyol). Polyether prepolymers: Offers improved low-temperature resistance and elasticity (suitable for low-temperature outdoor environments, such as doors and windows in northern China);
Polyester prepolymers: Offers improved hardness and oil resistance (suitable for applications exposed to oil and dirt, such as automotive seals).
Quality Correlation: High-quality adhesives use high-purity isocyanates (such as MDI) and long-chain polyethers, resulting in improved elasticity and aging resistance. Low-quality adhesives may use low-purity isocyanates (containing impurities) or short-chain polyethers, resulting in brittleness and poor weathering resistance.
2. Curing System: Determines Curing Speed ​​and Formability
Sealants must cure from a liquid/paste state to an elastic solid. The curing system is crucial to achieving this process, directly impacting application efficiency and post-cured performance.
1. Curing Agent
Function: Reacts with the isocyanate groups (-NCO) in the polyurethane prepolymer to form a cross-linked structure, enabling the curing and final hardness of the adhesive. It also regulates the curing speed and final hardness. Common Types:
Moisture Curing Agents: The most commonly used (such as dibutyltin dilaurate and stannous octoate), these agents trigger curing by absorbing moisture from the air, eliminating the need for additional curing agents and making application easier (corresponding to “one-component polyurethane sealants”).
Cross-Linking Curing Agents: Used in two-component sealants (requiring mixing of components A and B), such as polyamines and polyols, they cure by directly reacting with the prepolymer, resulting in faster curing and more stable performance (suitable for applications requiring fast curing times, such as large-scale projects).
Quality Relevance: High-quality sealants use highly effective, low-toxic curing agents (such as environmentally friendly organotins), resulting in a uniform curing speed (e.g., tack-free in 4-6 hours at 25°C) and no “cure on the surface but soft inside” effect. Low-quality sealants may use excessive or underactive curing agents, resulting in slow curing (still sticky after 24 hours) or brittleness after curing.
2. Latent Curing Accelerator (Optional)
Purpose: In low-temperature or high-humidity environments, they assist the curing agent in accelerating the reaction, preventing environmental factors from hindering curing (e.g., during winter construction). 3. Performance Modifiers: Optimizing the Physical and Application Properties of Colloids
These ingredients do not directly participate in the curing reaction, but they can adjust the hardness, flexibility, and smooth application of the colloid, making them key to balancing “performance” and “user experience.”
1. Plasticizers
Purpose: Reduces the hardness of the colloid after curing, increases its flexibility and elasticity, and improves its flowability during application (for smoother extrusion).
Common Types:
High-quality adhesives use phthalates (such as DOP and DINP, which must meet environmental standards) or aliphatic polyesters (which offer better migration resistance and prevent precipitation during long-term use).
Inferior-quality adhesives may use cheap “non-environmentally friendly” plasticizers (such as recycled waste oil and low-purity phthalates), which are prone to precipitation (causing the colloid to become hard and sticky) and may contain toxic substances (such as heavy metals).
Quality Relevance: The type and content of plasticizers determine the elasticity of the colloid—too low a content makes the colloid too hard and brittle; too high a content makes the colloid too soft and prone to collapse and dust buildup. 2. Fillers
Purpose:
Reducing costs (replacing some expensive prepolymers);
Improving the hardness, tensile strength, and shrinkage resistance of the colloid (preventing post-curing shrinkage that could lead to leaks);
Improving workability (preventing colloid flow, such as drooping during vertical joint construction).
Common Types:
High-quality colloids use active fillers (such as fumed silica and nano-calcium carbonate), which are compatible with prepolymers and enhance colloid strength without affecting elasticity.
Low-quality colloids use inert fillers (such as talc and ground calcium carbonate), or even sand and mud, which can result in low colloid strength, easy pulverization, and a significant decrease in elasticity.
Additional Ratio: Filler content is typically 20%-40%. A higher ratio (over 50%) can make the colloid brittle, while a lower ratio can lead to excessive cost.