Formulation design of the hottest polyurethane adh

2022-08-14
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Formulation design of polyurethane adhesive

the design of adhesive is to obtain the final use performance. The formulation design of polyurethane adhesive should take 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

polyurethane can synthesize polymer materials with various properties due to its diversity of raw materials and compositions. For example, from the appearance of its bulk material (i.e. solvent-free), we can get soft to hard elastomer and foam materials. In terms of its bulk properties (or its cured products), polyurethane is basically an elastomer. Some of its physical and chemical properties, such as bonding strength, mechanical properties, durability, low temperature resistance and drug resistance, mainly depend on the chemical structure of the polyurethane cured products. Therefore, to design the formula of polyurethane adhesive, we must first carry out molecular design, that is, from the impact of chemical structure and composition on performance. The variety of polyurethane raw materials and the relationship between chemical structure and properties

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

generally, three kinds of raw materials are used in the formulation of polyurethane adhesive: one is NCO raw materials (namely diisocyanate or its modifier, polyisocyanate), one is oh raw materials (namely oligomer polyol containing hydroxyl, chain extender, etc., broadly speaking, it is a compound containing active hydrogen, so it also includes polyamines, water, etc.), and the other is additives such as solvents and catalysts. There are two ways to design the formula of polyurethane adhesive from the perspective of raw materials

(1). The simplest preparation method of polyurethane adhesive from the above raw materials is to simply mix and use 0h raw materials and NCO raw materials (or additives). This method is not often used in the formulation design of polyurethane adhesive, because most oligomer polyols have low molecular weight (usually polyether Mr 6000, polyester Mr 3000), so the prepared adhesive composition has low viscosity and low initial adhesion. Sometimes, even with the addition of catalyst, the curing speed is still slow, and the strength of the cured product is low, which is of little practical value. In addition, unmodified TDI has high vapor pressure, strong odor and volatile toxicity, while MDI is solid at room temperature, which is inconvenient to use. Only a few commercial polyisocyanates such as papl, desmodur R, desmodur RF, coronate L, etc. can be used as isocyanate raw materials

however, there are several situations in which polyurethane adhesives can be prepared by the above methods. For example: (1) the two-component polyurethane adhesive composed of organic solution of high molecular weight polyester (MR) and polyisocyanate solution (such as coronate L) can be used for composite laminated films and other purposes with good performance. This is because its main component high molecular weight polyester itself has a high initial adhesive force, and the adhesive formed has a high cohesive strength; (2) The composition composed of polyether (or polyester) or water, polyisocyanate, catalyst, etc., as a foaming polyurethane adhesive and adhesive, is used for the bonding and manufacturing of thermal insulation materials, etc., and has a practical value for shipping activities in Southeast Asia

(2). NCO and oh raw materials are pre ammoniated and modified

as mentioned above, due to the low molecular weight of most oligomer polyols and the high volatility toxicity of TDI, MDI is solid at room temperature, and the general performance of direct compounding adhesives is poor. Therefore, in order to improve the initial viscosity of adhesives and shorten the time required to produce a certain bonding strength, polyether or polyester polyols are usually reacted with TDI or MDI monomers, Prepare carbamate prepolymer with NCO group or OH group as NCO component or Oh component

3. Design PU adhesive according to the requirements of use form

from the use form of polyurethane adhesive, there are mainly single component and double component, so as to produce the surface component of outer mold line (OML) of the fuselage

a. one component polyurethane adhesive

the advantage of one component polyurethane adhesive is that it can be used directly without the trouble of adjusting the two-component adhesive before use. There are two main types of one component polyurethane adhesives

(1) the wet curing polyurethane adhesive based on polyurethane prepolymer with NCO as the end group can be cured by using trace moisture in the air and trace adsorbed water on the surface of the substrate. It can also react with active hydrogen groups on the surface of the substrate to form a solid chemical bond. This type of polyurethane adhesive is generally solvent-free. In order to facilitate sizing, the viscosity should not be too large. One component moisture curing polyurethane adhesive is mostly polyether type, that is, the main raw material containing monooh is polyether polyol. The proper content of free NCO in this kind of glue should be determined according to the viscosity of the glue (affecting operability), gluing method, gluing thickness and the type of adherend, and the storage stability of the glue should be considered. (2) A one component solvent based polyurethane adhesive based on thermoplastic polyurethane elastomer. Its main component is high molecular weight end oh baseline polyurethane. The hydroxyl base number is very small. When the solvent begins to volatilize, the viscosity of the adhesive increases rapidly, producing initial adhesion. When the solvent is basically completely volatilized, sufficient adhesive force is generated. After placing at room temperature, the middle segment of most polyurethane elastomers of this type crystallizes, which can further improve the adhesive strength. This type of one component polyurethane adhesive generally uses crystalline polyester as the main raw material of polyurethane

one component polyurethane adhesive in addition, there are 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 a main agent containing hydroxyl end groups and a curing agent containing NCO end groups. Compared with the single component, the two-component polyurethane adhesive has good performance and high bonding strength, and the two-component distribution ratio of the same two-component polyurethane adhesive can allow a certain range, which can adjust the performance of the cured product. The main agent is generally polyurethane polyol or polymer polyester polyol. The ratio of the two components should be a little too much curing agent, that is, there is a trace of excess NCO groups, which can make up for the possible loss of NCO caused by water and ensure that the adhesive has sufficient cross-linking reaction

4. Design PU adhesive according to performance requirements

if there are special performance requirements for polyurethane adhesive, the formula should be designed according to the relationship between polyurethane structure and performance

different substrates, different application fields and application environments often have some special requirements for polyurethane adhesive. For example, the polyurethane adhesive used in industrial production lines requires rapid curing, and the polyurethane adhesive used for composite flexible packaging film requires acid resistance and accelerated production capacity to transfer hydrolysis to the coastal advanced manufacturing industry base and Laitai inland refined steel production base, Among them, the adhesive for cooking resistant flexible packaging also requires a certain degree of high-temperature adhesion, and so on

a. high temperature resistance

polyurethane adhesives generally have insufficient high temperature resistance. To be used in special temperature resistant occasions, polyurethane adhesive can be designed in advance. 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) Increase the content of isocyanate and chain extender (they form hard segments); (3) Increase the dosage of curing agent; (4) Polyisocyanates resistant to high temperature pyrolysis (such as those containing isocyanurate rings) are used, or isocyanurate is produced during curing; (5) It is an effective way to improve the compatibility of polymers by blending polyurethane with epoxy resin or polysulfonamide which is relatively temperature resistant, and adopting PN technology

b. hydrolytic resistance

polyester polyurethane adhesive has poor hydrolytic resistance, which can be improved by adding hydrolytic stabilizers (such as carbodiimide, epoxy compounds, etc.). In order to improve the hydrolytic resistance of polyester itself, long-chain dibasic acid and diol raw materials (such as sebacic acid, 1,6-hexanediol, etc.) can be used, and branched diol raw materials such as neopentyl glycol can also improve the hydrolytic resistance of polyester. Polyether has good hydrolysis resistance. Sometimes it can be used with polyester to prepare polyurethane adhesive. Adding a small amount of silicone coupling agent to the adhesive formula can also improve the hydrolytic resistance of the adhesive layer

c. improve the curing speed

a main method to improve the curing speed is to make the polyurethane adhesive have a certain initial adhesion, that is, it is no longer easy to separate after bonding. Therefore, increasing the molecular weight of the main agent and using the raw materials that can produce crystalline polyurethane are effective methods to improve the initial adhesion and curing speed. Sometimes adding a small amount of catalytic crosslinking agents such as triethanolamine also helps to improve the initial adhesion. Adding catalyst is also the main method to accelerate curing

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