The design of the adhesive is for the purpose of achieving end-use performance. Formulation of the Polyurethane adhesive should take into consideration the workability, the curing conditions, and the bonding strength, heat resistance, and chemical resistance of the resulting adhesive. Performance requirements such as durability and durability. 1. Polyurethane Molecular Design - Structure and Properties
Polyurethanes can be used to synthesize a variety of polymer materials due to the variety of raw materials and their composition. For example, from the standpoint of the appearance of the bulk material (ie, containing no solvent), a soft and hard elastomer or foam material can be obtained. Polyurethanes have essentially elastomeric properties in terms of their bulk properties (or their cured products), and some of their physical and chemical properties such as adhesive strength, mechanical properties, durability, low temperature resistance, and chemical resistance depend The chemical structure of the resulting polyurethane cured product. Therefore, to formulate polyurethane adhesives, we must first carry out molecular design, that is, from the chemical structure and composition of the impact on performance to recognize. The relationship between polyurethane raw material varieties and chemical structure and performance.
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 its modified, polyisocyanate), and the other is oH-based material (ie hydroxyl-containing oligomer polyol, chain extension). Agents and the like are, in a broad sense, compounds containing active hydrogen, and therefore also include polyamines, water, and the like, and other additives such as solvents and catalysts. Formulations of polyurethane adhesives are designed from the perspective of raw materials. The methods are as follows.
(1). The simplest formulation of polyurethane adhesives directly prepared from the above raw materials is that 0H raw materials and NCO raw materials (or additives) are simply mixed and used directly. This method is not commonly used in formula design of polyurethane adhesives, because most of the oligomer polyols are of lower molecular weight (usually Polyether Mr<6000, Polyester Mr<3000), and thus the viscosity of the formulated adhesive composition is small, The initial tack is small. Sometimes, even if the catalyst is added, the curing speed is still slow, and the strength of the cured product is low, which is not practical. In addition, the unmodified TDI has a high vapor pressure, large odor, and high volatility toxicity. The MDI is solid at room temperature and is inconvenient to use. Only a few commercially available polyisocyanates such as PAP1, Desmodur R, Desmodur RF, and Coronate L are available. Used as isocyanate raw materials.
However, there are several situations in which polyurethane adhesives can be formulated using the above method. 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) can be used for composite laminated films and the like, and the performance is good. This is because the main component high-molecular-weight polyester itself has a high initial adhesive strength, and the adhesive composition consists of a large cohesive strength; (2) Polyether (or polyester) or water, polyisocyanate, catalyst, etc. The resulting composition has certain practical value as a foamable polyurethane adhesive and adhesive for bonding and manufacturing of heat-insulating materials and the like.
(2). Pre-amino esterification modification of NCO and OH raw materials As mentioned above, most of the oligomer polyols have lower molecular weight and TDI is more volatile and toxic. MDI is solid at room temperature, and the direct matching rubber generally has poor performance. Therefore, in order to increase the initial viscosity of the adhesive and shorten the time required for producing a certain adhesive strength, a polyether or polyester polyol is usually reacted with a TDI or MDI monomer to produce a NCO-based or OH-based urethane preload. The polymer is used as an NCO component or an OH component.
3. From the use of the requirements of the design of PU glue from the use of polyurethane adhesives for the form of points, mainly single-component and two-component.
A. One-Component Polyurethane Adhesives One-component polyurethane adhesives have the advantage of being ready for use. No adhesives need to be adjusted before use. One-component polyurethane adhesives are mainly of the following two 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. Form a strong chemical bond. This type of polyurethane adhesive is generally a solvent-free type, because the viscosity can not be too large for easy sizing, and the one-component moisture-curing polyurethane adhesive is mostly a polyether type, that is, the main raw material containing an OH is a polyether polyol. The extent to which the free NCO content of this type of glue is appropriate should be determined according to the viscosity of the glue (impacting operability), the way of gluing, the thickness of the gluing, and the type of 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 elastomers of this type are crystallized in the segment, which can further increase the bonding strength. This type of one-component polyurethane adhesive generally uses crystalline polyester as the main raw material for the polyurethane.
One-component polyurethane adhesives also include polyurethane hot-melt adhesives and one-component waterborne polyurethane adhesives.
B. Two-Component Polyurethane Adhesive Two-component polyurethane adhesive consists of a terminal hydroxyl-containing main agent and a curing agent containing a terminal NCO group. Compared with a single component, the two-component performance is good, and the adhesive strength is high, and the same double The two sets of partition ratios of the component polyurethane adhesives allow a certain range, and the properties of the cured product can be adjusted accordingly. The main agent is generally a polyurethane polyol or a high molecular weight polyester polyol. The proportion of the two components is slightly excessive with a curing agent, that is, an excess of NCO groups is appropriate, so that the NCO loss caused by possible moisture can be compensated to ensure that the adhesive produces a sufficient crosslinking reaction.
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, and so on.
A. Heat-resistant polyurethane adhesives generally lack high temperature performance. For use in special temperature-resistant applications, polyurethane adhesives can be designed in advance. There are several ways to increase the heat resistance of polyurethane adhesives, such as: (1) the use of polyether-, polyester- and isocyanate-containing raw materials containing benzene rings; (2) increasing the content of isocyanates and chain extenders (which form hard segments); (3) increase the amount of curing agent; (4) use high temperature pyrolysis of polyisocyanates (such as containing isocyanurate ring), or produce isocyanurate during curing; (5) with more temperature resistant Epoxy resin or polysulfone amide and other resins and polyurethanes are blended and modified, and the use of pN technology is an effective way to improve polymer compatibility.
B. Hydrolysis-resistant polyester polyurethane adhesives have poor hydrolysis resistance, and hydrolysis stabilizers (eg, carbodiimides, epoxy compounds, etc.) can be added for improvement. In order to increase the hydrolysis resistance of the polyester itself, a long chain dibasic acid and a dihydric alcohol raw material (e.g. sebacic acid, 1,6-hexanediol, etc.) may be used, and a branched dihydric alcohol such as neopentyl glycol may be used. Raw materials can also improve the hydrolysis resistance of polyesters. Polyethers have good hydrolysis resistance and can sometimes be used together with polyesters to make polyurethane adhesives. Adding a small amount of silicone coupling agent in the adhesive formulation can also increase the hydrolysis resistance of the adhesive layer.
C. One of the main methods to increase the curing speed and increase the curing speed is to make the polyurethane adhesive have a certain initial tack, ie it can no longer be easily detached after bonding. Therefore, increasing the molecular weight of the main agent and using a raw material that can produce crystalline polyurethane are effective means for increasing the initial tack and curing speed. Sometimes the addition of a small amount of triethanolamine, a catalytic cross-linking agent, also helps to increase the initial tack. The addition of a catalyst is also the main method for accelerating the curing.
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