How to control the viscosity of polyurethane prepolymers?

1. When purchasing raw materials, choose polyether with low molecular weight and low functional group and low -NCO% content.

2. Strictly control the moisture content of raw materials, and the moisture content should be lower.

3. The preparation of prepolymers should be carried out under acidic conditions.

4. Reduce metal impurities in polyols, and do not contact copper and iron and other product parts.

5. In the production process, the reaction temperature and time are strictly controlled, and the polymerization reaction of mixed polyether and TDI at low temperature (under appropriate temperature conditions) can be prepared with low viscosity, excellent performance and final performance, and low -NCO% content of prepolymers, but under the condition of no catalyst, if the reaction temperature is low, the reaction time is longer, usually 75 85

6. The feeding sequence and mixing should be uniform. Therefore, the polyurethane prepolymer has low viscosity, so it does not need to be diluted with organic solvents such as xylene, toluene, etc., and has good leveling and workability. In this way, there are no adverse factors such as pollution to the environment.

The key is to look at what you're for. For specific questions, you may wish to consult Handan Shiwei New Materials Technology Department.

The smaller the molecular weight, the higher the NCO excess value and the smaller the viscosity. Of course, it also has to do with the materials used. For example, in general, the viscosity of polyether is lower than that of polyester for the same formulation. Hehe, let's learn! Sometimes leave a lot of messages.

Polyurethane is a polymer formed by the reaction of polyisocyanate and polyether or polyester polyol under certain conditions. The prepolymer of polyurethane is simply a reactive semi-finished product obtained by controlling a certain proportion of polyisocyanate and polyol. Due to the wide variety of polyisocyanates and polyols, the reaction ratio is different.

Polyurethane prepolymers are used in a wide range of applications such as polyurethane adhesives, coatings, elastomers, foams and fibers.

Adhesive and coating prepolymer technology has been widely used in the preparation of one- and two-component coatings or adhesives. Polyurethane adhesive not only has good adaptability to the surface of different substrates, but also has large cohesion energy and high strength. Polyurethane prepolymers can be used as the bulk component of adhesives.

According to the reactivity of prepolymers, they can be roughly divided into three categories: one-component reactive adhesives, one-component non-reactive adhesives and two-component reactive adhesives. One-component reactive adhesives typically use prepolymers with active NCO groups or with blocked NCO groups. Prepolymers containing free NCO groups can often be used directly or formulated into room-temperature-curing, one-component, moisture-curing polyurethane adhesives or coatings.

In the preparation of prepolymers for one-component adhesives, whether in bulk or solution form, the molar ratio of NCO to OH groups is usually controlled to be greater than 1 in the chain expansion reaction of polyester polyol with TDI or MDI, so that the free NCO content in the reaction product is controlled at a certain level. End NCO-based prepolymers can be used in many fields such as adhesives, coatings, impregnating agents, etc. Mixing polyurethane prepolymers with waste tire rubber particles to cast elastic pads for use in public places such as kindergartens is a typical example of the application of moisture-curing polyurethane prepolymers as adhesives for rubber particles.

If the bond between the polyurethane glue and the primer is not strong, you can change the primer, which may be caused by the poor adhesion between the primer and the substrate to be bonded, and it is easy to have a weak bond and degumming.

1. What is the function of the primer?

The role of Shizukawa Chemical's primer is differentiated according to the different surface treatment processes, and in the adhesive process, it is mainly to improve the bonding strength of the adhesive and the substrate, solve the problems such as degumming of the difficult-to-stick substrate, and have good wettability and adhesion ability, and promote the combination of the two through the connection channel of the primer. In addition, in the painting process, it is in the form of improving the surface of the substrate and enhancing the adhesion between the layers to promote the adhesion of the paint to the substrate and achieve a firm coating.

2: What substrates are primers suitable for?

Plastics and metals are the most commonly used materials in industrial production, so it is natural that these two types of substrates are used for finishing through surface treatment. The substrates used more in plastics are nylon, PP, etc., and of course, there are other plastic materials, which are not listed one by one, in addition to plastics, there is also the bonding between metal substrates.

3. What is the use of primer?

The use of Shizukawa Chemical primer is actually very simple and easy to operate, spraying a layer of primer on the surface of the substrate and then spraying or applying paint and adhesive. The specific amount needs to be determined according to the actual use, the general painting is to spray 3-m, and then wait for the next step of the process after drying, and part of the primer can also be wet spray.

4. What should be paid attention to in the use of primer?

1.Because the primer is a surface treatment technology developed based on the properties of the substrate, it is necessary to determine the properties of the substrate before use, and the different molecular structures make it necessary to choose different primers for different substrates.

2.The surface of the substrate needs to be cleaned before spraying or applying Shizukawa Chemical primer, so some release agents or detergents remain that make the primer useless.

3.If the primer is baked during use, the increase in temperature can increase the molecular activity and have a beneficial effect on adhesion (it is generally recommended to bake at 60-70 for 3-5 minutes).

This problem can be solved with Hengying's primer.

Polyurethane foam.

Tensile bond strength after molding;

Before polyurethane foaming molding, it is called polyurethane composition, which is divided into polyurethane black material (component A) and polyurethane white material (component B).

The viscosity of polyurethane black material (component A) is about 300 psi, and the viscosity decreases with increasing temperature;

The viscosity of polyurethane white material (component B) is about 200 psi.

Therefore, in practical application, the temperature of the black material should be higher than that of the white material to ensure that the viscosity of the two components can be controlled the same.

According to the reaction characteristics of the terminal groups, polyurethane prepolymers can be divided into: terminal isocyanate-based prepolymers, terminal hydroxyl prepolymers, prepolymers containing occlusive groups, and polyurethane prepolymers containing other groups such as terminal silyl and terminal alkyl acrylates.

Prepolymers with NCO end groups are sometimes referred to as modified polyisocyanates, which have high reactivity characteristics, are susceptible to moisture, etc., and have a short storage period; The prepolymer with the OH end group is called the end hydroxyl prepolymer, which has general reactivity and long storage period, and is usually used as the main agent of the adhesive. A blocking polyurethane prepolymer is prepared by using a sealant containing active hydrogen to react with the NCO group to protect the free NCO group in the prepolymer. The prepolymer formulation of the coating or adhesive is thermally desealed after application to regenerate the NCO group, which participates in the cross-linking reaction to cure the system.