What Is the Preparation Process of Prepolymers?

Viscosity Control in Prepolymer Preparation

The viscosity of prepolymers is a key parameter in their preparation. Due to the high viscosity of polyester itself, polyester-type prepolymers are typically prepared using a one-step process. All required polyester and isocyanate components are added simultaneously, allowing the reaction to reach the desired prepolymer viscosity.

For polyether-type prepolymers, a two-step synthesis is commonly used:

1. In the first step, polyether reacts with part of the isocyanate at a molar ratio of NCO to OH typically ranging from 1:1 to 1:5. This results in a high-molecular-weight prepolymer with terminal isocyanate groups.

2. In the second step, the remaining isocyanate is added to dilute the intermediate product, adjusting the final isocyanate content to approximately 8%–10%. Compared to the one-step method, this approach produces prepolymers with a broader molecular weight distribution. The presence of higher molecular weight components increases the viscosity of the prepolymer, generally reaching 10–50 Pa·s at 25°C.

Control of Branching in Prepolymer Preparation

Branching significantly impacts gel strength during foaming and the final properties of the product. Branching can arise from the following conditions:

1. Reaction of trace water in polyols with isocyanate, forming urea groups that subsequently react with isocyanate to create biuret structures.

2. Reaction of excess isocyanate with carbamate groups under conditions like elevated temperatures or prolonged reaction times, forming allophanate linkages.

3. Incorporation of triols or tetrols into diol-based polyether or polyester formulations during prepolymer synthesis, creating branched chains.

In industrial production, prepolymers with three distinct branching types are utilized.

During preparation, it is crucial to ensure terminal isocyanate groups, achieve appropriate viscosity, and avoid excessive cross-linking that leads to gelation. Factors affecting viscosity and gelation, such as raw material quality, acidity, NCO/OH ratio, water content, branching degree, reaction temperature, and time, must be carefully controlled.

Key Factors in Prepolymer Preparation

1. Raw Material Control

To ensure reproducibility, strict control over raw material specifications is essential. For polyols, factors such as water content, hydroxyl value, pH, and metal ion content must be monitored. Acid values for polyester polyols and unsaturated end-group content in polyether polyols also require attention. Antioxidants like 2,6-di-tert-butyl-p-cresol are often added to polyether polyols to prevent oxidation during storage. The total acidity of isocyanates significantly impacts the prepolymer reaction.

2. NCO/OH Ratio

The molecular weight and viscosity of the polymer are influenced by the equivalent ratio of hydroxyl to isocyanate groups. Theoretical maxima for both molecular weight and viscosity occur at an NCO/OH ratio of 1:1.

• Polyester-Type Prepolymers: Due to the high intrinsic viscosity of polyester, all isocyanate is added in a single step to maintain appropriate prepolymer viscosity.
• Polyether-Type Prepolymers: With lower intrinsic viscosity, a two-step synthesis is typically used, with a controlled NCO/OH ratio in the range of 1.05:1 to 1.5:1 in the first step. The resulting intermediate is diluted with excess isocyanate, ensuring terminal isocyanate groups and suitable viscosity for foaming.

3. Effect of Different Polyols

The choice of polyols, such as polyether or polyester diols, or mixtures with triols or tetrols, significantly impacts prepolymer viscosity and foam gel strength. Excess triol or tetrol can lead to cross-linking and gelation.

4. Reaction Temperature and Time

Temperature and time affect both viscosity and chemical structure:

• Below 100°C and without catalysts, urethane linkages dominate.
• At higher temperatures, biuret and allophanate linkages form, leading to branching.
• Lower reaction temperatures result in foams with higher strength, while temperatures above 100°C increase gel strength during foaming.

5. pH Impact

• Acidic Conditions: Favor chain extension reactions, producing lower-viscosity prepolymers.
• Basic Conditions: Promote branching reactions, potentially leading to gelation.

Residual basic metal ions from polyether synthesis and acidic by-products in isocyanates must be balanced. Industrial processes may add acid regulators, such as benzoyl chloride, to adjust system acidity.

6. Process Variables

Prepolymer viscosity is also influenced by reactor size, design, and stirring efficiency. Efficient stirring promotes uniform reactions, reducing viscosity.