The design of the adhesive is for the purpose of achieving the end-use performance. Formulation of the Polyurethane adhesive is designed considering the workability (operability) of the prepared adhesive, the curing conditions and the bonding strength, the heat resistance, and the chemical resistance durability. And other performance requirements
1. Polyurethane Molecular Design--Structure and Performance
Due to the variety of raw materials and composition of polyurethanes, polyurethanes can be used to synthesize various properties of polymer materials. For example, from the standpoint of their bulk materials (ie without solvents), they can be obtained from soft to hard elastomer foams. The material polyurethane has substantially elastomeric properties in terms of its bulk properties (or its cured product), some of its physicochemical properties such as adhesion strength, mechanical properties, durability, low temperature resistance, and chemical resistance, depending mainly on the generation of The chemical structure of the polyurethane curing material, therefore, to formulate the Polyurethane Adhesive, we must first carry out molecular design, that is, from the chemical structure and composition of the performance of the impact to understand the relevant raw material varieties of polyurethane and chemical structure and performance of the relationship
2. Design of PU adhesive preparation from raw material point of view
Three types of raw materials are generally used in polyurethane adhesive formulations: one is NCO-based material (ie, diisocyanate or modified polyisocyanate), and the other is oH-based material (ie, hydroxyl-containing oligomer polyol chain extender, etc.) Broadly speaking, it is a compound containing active hydrogen, so it also includes polyamine water, etc.), and another class of solvents and catalysts and other additives from the raw material point of view of polyurethane adhesive formulation, there are the following two methods
(1). Prepared directly from the above raw materials
The simplest method of polyurethane adhesive preparation is that 0H raw materials and NCO raw materials (or additives) are simply mixed and used directly. This method is not often used in the formulation of polyurethane adhesives due to the low molecular weight of most oligomer polyols. (usually Polyether Mr<6000, Polyester Mr<3000), so the prepared adhesive composition viscosity is small, and the initial tack is small. Even if the catalyst is added, the curing speed is still slow, and the cured product has low strength and practical value. The unmodified TDI has a high vapor pressure, a large odor and a large volatile toxicity, while MDI is a solid at room temperature and is inconvenient to use. Only a few commercial polyisocyanates such as PAPlDesmodur RDesmodur RFCoronate L, etc. can be used as isocyanate raw materials.
However, there are several situations in which polyurethane adhesives can be formulated as described above. For example: (1) A two-component polyurethane adhesive consisting of an organic solution of a high molecular weight polyester (Mr5000-50000) and a polyisocyanate solution (such as Coronate L). Composite laminating film and other applications, the performance is better because the main component of high molecular weight polyester itself has a higher initial adhesion, the composition of the adhesive cohesion strength; (2) from the polyether (or polyester) The composition formulated with a water-soluble polyisocyanate catalyst, etc., is used as a foamable polyurethane adhesive adhesive, and is used in bonding manufacturing of heat-insulating materials, etc., and has a certain practical value.
(2) Pre-amino esterification modification of NCO and OH raw materials
As described above, since most of the oligomer polyols have a low molecular weight and a large volatile toxicity of TDI, MDI is a solid at room temperature, and the direct formulating rubber generally has poor performance. Therefore, in order to increase the initial viscosity of the adhesive, a certain degree of adhesion occurs. The time required for the strength, usually the reaction of a polyether or polyester polyol with a TDI or MDI monomer to produce an NCO- or OH-terminated urethane prepolymer for use as an NCO component or OH component
3. Design PU adhesive from the requirements of using form
Divided from the use of polyurethane adhesive, mainly single-component and two-component
A. One-component polyurethane adhesive
The advantage of one-component polyurethane adhesives is that they can be used directly. There is no need for two-component adhesives to be glued before use. One-component polyurethane adhesives have the following two main types.
(1) A moisture-curing polyurethane adhesive based on an NCO-terminated polyurethane prepolymer. The synthesis reaction utilizes a trace amount of water in the air and a trace amount of water adsorbed on the surface of the substrate to cure, and also reacts with active hydrogen groups on the surface of the substrate. The formation of a strong chemical bond This type of polyurethane adhesive is generally solvent-free, due to the sizing, the viscosity can not be too large, one-component moisture curing polyurethane adhesive are mostly polyether type, that is, the main material containing a OH polyether The extent of the free NCO content of polyols in this kind of glue is appropriate, and it should be determined according to the viscosity of the glue (influence of operability) and the thickness of the glue and the type of the adherend, etc., and the storage stability of the glue should be considered. (2) A one-component solvent-based polyurethane adhesive based on Thermoplastic Polyurethane Elastomer. The main component is a high molecular weight OH-based linear polyurethane. The number of hydroxyl groups is very small. When the solvent starts to volatilize, the viscosity of the rubber rapidly increases, resulting in initial tack. When the solvent is substantially completely volatilized, sufficient adhesive force is generated, and after room temperature placement, most of the Polyurethane Elastomer in this type is crystallized in the segment, which can further increase the bonding strength of this type of single-component poly. Ester gum generally crystalline polyester as a main raw material of polyurethane
One-component polyurethane adhesives, in addition to polyurethane hot-melt adhesives, one-component waterborne polyurethane adhesives and other types
B. Two-component polyurethane adhesive
The two-component polyurethane adhesive consists of a terminal hydroxyl-containing main agent and a curing agent containing a terminal NCO group. Compared with the single component, the two-component performance is good and the adhesive strength is high, and the same two-component polyurethane adhesive is used. The distribution ratio of the two groups can be allowed to a certain range, and the performance of the cured product can be adjusted by this. The main agent is generally a polyurethane polyol or a high-molecular polyester polyol. The ratio of the two components is slightly excessive with a curing agent, ie, there is a surplus of trace NCO groups. It is advisable to compensate for possible NCO losses caused by moisture and to ensure that the adhesive produces sufficient cross-linking reactions
4. Design PU adhesive according to performance requirements
If the polyurethane adhesive has special performance requirements, formula design should be based on the relationship between polyurethane structure and performance.
Different substrates, different application areas and application environments, there are often some special requirements for polyurethane adhesive, such as polyurethane glue used in industrial production lines require fast curing, polyurethane adhesive for composite flexible packaging film requires acid and hydrolysis resistance, which is resistant to cooking Adhesives for flexible packaging also require a certain degree of high temperature adhesion, etc.
A. high temperature
Polyurethane adhesives generally lack high temperature performance If you want to use in special temperature conditions, you can design polyurethane adhesives in advance. There are several ways to improve the heat resistance of polyurethane adhesives, such as: (1) using polyether polyether containing benzene rings And isocyanate raw materials; (2) to increase the content of isocyanate and chain extender (their hard segments); (3) to increase the amount of curing agent; (4) to use high temperature pyrolysis of polyisocyanates (such as containing isocyanurate ring ), or produces isocyanurate when cured; (5) blending with polyurethanes with resins such as epoxy resins or polysulfone amides that are relatively temperature resistant, and using pN technology to improve polymer compatibility Effective Ways
B. Hydrolysis resistance
Polyester polyurethane adhesives have poor hydrolysis resistance and can be modified by the addition of hydrolysis stabilizers (such as carbodiimide epoxy compounds). In order to improve the hydrolysis resistance of polyesters, long-chain dibasic acids and binary Alcohol raw materials (such as sebacic acid 1,6-hexanediol, etc.) and branched glycols such as neopentyl glycol can also improve the hydrolysis resistance of polyesters. The hydrolysis resistance of polyethers is better, and sometimes it can be related to poly. Adding a small amount of organosilicon coupling agent to the adhesive formulation with an ester-based polyurethane adhesive can also improve the hydrolysis resistance of the adhesive layer
C. Increase the curing speed
One of the main methods for increasing the curing speed is to make the polyurethane adhesive have a certain initial tack, ie it can no longer be easily detached after bonding, thus increasing the molecular weight of the main agent. The raw material that can produce crystalline polyurethane is to increase the initial tack and the curing speed. Effective methods Sometimes adding a small amount of triethanolamine, such as a catalytic crosslinker, also helps to increase the initial tack. The addition of a catalyst is also the main method to accelerate the curing.
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