Shoe Material Chemical Knowledge: The Formula Of Polyurethane Adhesive

The design of the adhesive is aimed at achieving the ultimate performance. The formulation of polyurethane adhesive is designed, taking into account the constructability (operability), curing conditions, bonding strength, heat resistance, chemical resistance, durability and other performance requirements of the adhesive.

1. polyurethane molecular design - structure and properties

Because of its variety and composition, polyurethane can synthesize various kinds of high-performance polymer materials.

For example, from the appearance of the body material (that is, without solvents), it can be obtained from soft to rigid elastomers and foam materials.

For polyurethane, its physical properties, such as bonding strength, mechanical properties, durability, low temperature resistance and drug resistance, depend mainly on the chemical structure of the polyurethane cured material.

Therefore, for the formulation design of polyurethane adhesives, first of all, molecular design should be carried out, that is, the effect of chemical structure and composition on the properties.

The relationship between polyurethane raw materials and their chemical structure and properties is discussed.

2. design the preparation of PU adhesive from the perspective of raw materials.

Polyurethane adhesives generally use three types of raw materials: one is NCO type material (that is, diisocyanate or its modifier, polyisocyanate), one is oH type material (that is, oligomer, polyol and chain extender containing hydroxyl group), broadly speaking, it is a compound containing active hydrogen, so it includes polyamine, water and so on, and another kind is additive such as solvent and catalyst.

The formulation of polyurethane adhesives is designed from the perspective of raw materials. The following two methods are used.

(1) directly prepared from the above raw materials.

The simplest method for preparing polyurethane adhesives is simply mixing and directly using 0H type raw materials and NCO materials (or additives).

This method is not often used in the formulation design of polyurethane adhesives. The reason is that most oligomeric polyols have low molecular weight (usually polyether Mr<6000, polyester Mr<3000), so the adhesive composition is small in viscosity and small in initial adhesion.

Sometimes even if the catalyst is added, the curing rate is still slow, and the solidified material has low strength and little practical value.

Moreover, the unmodified TDI has higher vapor pressure, large odor and high volatility toxicity, while MDI is not solid enough to use at room temperature. Only a few commercialized polyisocyanates such as PAPl, DesmodurR, DesmodurRF and CoronateL can be used as isocyanate raw materials.

However, there are several cases that can be used as polyurethane adhesives.

For example: (1) two component polyurethane adhesives made up of high molecular weight polyester (Mr5000-50000) organic solution and polyisocyanate solution (such as CoronateL) can be used for laminated laminates and other properties, and the performance is better.

This is because its main component high molecular weight polyester itself has a higher initial adhesive force, and the adhesive strength of the adhesive is large. (2) the composition composed of polyether (or polyester) or water, polyisocyanate, catalyst and so on can be used as foaming polyurethane adhesives and adhesives for the bonding and manufacturing of thermal insulation materials and so on, so it has certain practical value.

(2) pre ammoniate modification of.NCO and OH raw materials.

As mentioned above, as most of the oligomeric polyols have low molecular weight and high TDI volatilization toxicity, MDI is usually solid at room temperature and has poor general properties. Therefore, in order to improve the initial viscosity of adhesives and shorten the time required to produce certain bonding strength, polyether or polyester polyols are usually reacted with TDI or MDI monomers to form urethane prepolymer with end NCO or OH base, which is used as NCO component or OH component.

3. design PU adhesive from the requirement of use form.

There are mainly one component and two components from the use form of polyurethane adhesives.

A. single component polyurethane adhesive

The advantage of one component polyurethane adhesive is that it can be used directly.

One component polyurethane adhesives are mainly of two types.

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(1) a moisture curable polyurethane adhesive based on a polyurethane prepolymer with NCO terminal as the main body. The synthesis reaction is solidified by using trace moisture in the air and trace adsorption water on the substrate surface, and can react with the active hydrogen group on the substrate to form a strong chemical bond.

This type of polyurethane adhesive is generally solvent free, because the viscosity can not be too large for the sake of easy sizing. The single component moisture curing polyurethane adhesive is polyether type, that is, the main raw material containing OH is polyether polyol.

The degree of free NCO content in this kind of adhesive is appropriate. It should be determined according to the viscosity of the adhesive, including the influence of operability, the way of coating, the thickness of the glue and the type of the sticky substance, and the storage stability of the adhesive should be considered.

(2) a single component solvent based polyurethane adhesive based on thermoplastic polyurethane elastomers. The main component is high molecular weight end OH base polyurethane. The hydroxyl number is very small. When the solvent starts to volatilize, the viscosity of the adhesive increases rapidly and produces initial adhesive force.

When the solvent is completely evaporated, enough adhesive force is generated. After room temperature placement, most of the polyurethane elastomers in the chain segment crystallize, which can further improve the bonding strength.

This type of one component polyurethane adhesive generally uses crystalline polyester as the main raw material of polyurethane.

One component polyurethane adhesive includes polyurethane hot melt adhesive, one component waterborne polyurethane adhesive and other types.

B. two component polyurethane adhesive

The two-component polyurethane adhesive is composed of the main agent containing the hydroxyl terminate and the curing agent containing the NCO group. Compared with the single component, the two-component polyurethane adhesive has good properties and high adhesive strength compared with the single component, and the two component distribution ratio of the two component polyurethane adhesive can allow a certain range, which can adjust the properties of the solid compound.

The main agents are polyurethane polyols or macromolecule polyester polyols.

The ratio of the two components is slightly higher than that of the curing agent, that is to say, there is a surplus of trace NCO group, which can make up for the NCO loss caused by the possible moisture and ensure that the adhesive produces enough crosslinking reaction.

4. design PU glue according to the performance requirements

If the polyurethane adhesive has special performance requirements, the formulation design should be based on the relationship between polyurethane structure and properties.

Different substrates, different application fields and application environments often have some special requirements for polyurethane adhesives. For example, polyurethane adhesives used in industrial production lines require fast curing. Polyurethane adhesives for composite soft packaging films require acid and hydrolysis resistance.

A. high temperature resistance

Polyurethane adhesives generally lack high temperature resistance.

Polyurethane adhesives can be designed in advance if they are to be used in special temperature resistant occasions.

There are several ways to improve the heat resistance of polyurethane adhesive, such as: (1) using polyether, polyester and isocyanate raw materials containing benzene ring; (2) increasing the content of isocyanate and chain extender (their hard segment); (3) increasing the amount of curing agent; (4) using polyisocyanate bearing high temperature pyrolysis (such as containing isocyanuric acid ring) or producing isocyanuric acid when curing; (5) blending and modifying the resin with polyurethane, such as temperature resistant epoxy resin or polysulfone amide, and adopting pN technology is an effective way to improve polymer compatibility.

B. water resistance

Polyester polyurethane adhesive has poor hydrolysis resistance and can be improved by adding hydrolytic stabilizers such as carbonized two imines and epoxides.

In order to improve the water resistance of polyester, long chain dicarboxylic acid and diols (such as sebacic acid, 1, 6 - hexamethylene glycol, etc.) can be used. Branched chain diols such as NPG can also improve the water solubility of polyester.

Polyether has good hydrolysis resistance, and sometimes it can be used to prepare polyurethane adhesive with polyester.

Adding a small amount of silicone coupling agent to the adhesive formula can also improve the water solubility of the adhesive layer.