Formulation Design of Polyurethane Adhesive

The adhesive is designed for the purpose of achieving end-use performance. Formulations for polyurethane adhesives are designed taking into account the workability, operative conditions, bonding strength, heat resistance, and chemical resistance of the resulting adhesives. Sex, durability and other performance requirements.

1. Polyurethane Molecular Design - Structure and Properties

Polyurethanes can be used to synthesize various properties of polymer materials due to the variety of raw materials and their composition. For example, from their bulk materials (ie, containing no solvent), the appearance of the polyurethanes is strong, and flexible and hard elastomers can be obtained. , Foam materials, polyurethane from its bulk properties (or its cured product), essentially elastomer properties, some of its physical and chemical properties such as adhesive strength, mechanical properties, durability, low temperature resistance, drug resistance Sex mainly depends on the chemical structure of the resulting polyurethane cured product. Therefore, to formulate the polyurethane adhesive, we must first carry out molecular design, that is, from the impact of chemical structure and composition on performance, related to polyurethane raw material varieties and chemistry. The relationship between 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, broadly speaking, are compounds containing active hydrogen, and therefore also include polyamines, water, etc.) and another class of additives such as solvents and catalysts. Formulations for polyurethane adhesives are designed from the viewpoint of raw materials. The methods are as follows: Two kinds.

(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 and additives) are simply mixed and used directly. This method is not often used in the formulation of polyurethane adhesives, because most oligomer polyols have a molecular weight. Lower (usually polyether Mr<6000, polyester Mr<3000), so the prepared adhesive composition viscosity is small, early tack is small, sometimes even if the catalyst is added, the curing speed is still slow, and the strength of the cured material is low, practical The value is not great, and the unmodified TDI has a high vapor pressure, large odor, and high volatility toxicity. However, MDI is solid at room temperature and is inconvenient to use. Only a few commercial polyisocyanates such as PAPlDesmodur RDesmodur RFCoronate L can be used. Isocyanate raw materials.
However, there are several situations in which polyurethane adhesives can be formulated as described above. 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 in the composite layer. Pressure film and other uses, better performance, because its main component high molecular weight polyester itself has a higher initial adhesive force, the composition of the adhesive cohesive strength; (2) from polyether (or polyester) or And water, polyisocyanate, catalysts and other composition of the composition, as foaming polyurethane adhesives, adhesives, adhesive materials used in insulation manufacturing, etc., have 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, the viscosity is shortened to a certain degree. For the time required for strength, a polyether or polyester polyol is usually reacted with a TDI or MDI monomer to form an NCO- or OH-terminated urethane prepolymer for use as an NCO component or an OH component.

3. Design PU adhesive from the requirements of using form

Divided from the use of polyurethane adhesives, there are mainly single-component and two-component components.

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 adjust the adhesive 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. The formation of strong chemical bonds, this type of polyurethane adhesive is generally solvent-free type, due to the ease of sizing, the viscosity can not be too large, one-component moisture-curing polyurethane adhesives are mostly polyether type, that is, the main containing an OH raw material is poly Ether polyols, the extent to which the free NCO content of such glues is appropriate, should be based on the viscosity of the glue (impacting operability), the way of gluing, the thickness of the sizing and the type of adherend, etc., and the glue should be considered. The storage stability (2) One-component solvent-based polyurethane adhesive based on thermoplastic polyurethane elastomer, the main component is a high molecular weight end OH linear polyurethane, the number of hydroxyl groups is very small, the viscosity of the adhesive rapidly increases when the solvent starts to volatilize, The initial tack force is generated. When the solvent is completely completely evaporated, a sufficient adhesive force is generated. After being placed at room temperature, most of the polyurethane elastomer in this type is crystallized in the segment, which can further increase the bonding strength. This type of Component polyurethane adhesive is generally a crystalline polyester as the 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 properties of the cured product can be adjusted. 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 trace amount of NCO groups. The surplus of the group is appropriate, so that the NCO loss caused by possible moisture can be compensated to ensure that the adhesive produces enough cross-linking 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 fields and application environments, there are some special requirements for polyurethane adhesive, such as the use of industrial production lines Polyurethane adhesives require rapid curing. Polyurethane adhesives for composite flexible packaging films require acid and hydrolysis resistance. Among them, adhesives for cooking resistance flexible packaging also require a certain degree of high temperature adhesion, and so on.

A. high temperature
Polyurethane adhesives are generally insufficient in high temperature resistance. If they are to be used in special temperature-resistant applications, polyurethane adhesives can be designed in advance. There are several ways to improve the heat resistance of polyurethane adhesives, such as 1) Polyethers containing benzene rings, poly Ester and isocyanate raw materials; (2) increase the content of isocyanates and chain extenders (their hard segments); (3) increase the amount of curing agent; (4) use of high temperature resistant pyrolyzed polyisocyanates (such as isocyanurates) Cyclic), or produces isocyanurate when cured; (5) Blending with urethanes using resins such as epoxy resins or polysulfone amides that are relatively temperature resistant, while using pN technology to improve polymer compatibility The effective way.

B. Hydrolysis resistance
Polyester polyurethane adhesives have poor hydrolysis resistance, and can be improved by the addition of hydrolysis stabilizers (eg, carbodiimides, epoxy compounds, etc.). In order to increase the hydrolysis resistance of polyesters, long chain dibasic acids can be used. Dihydric alcohols (such as azelaic acid, 1,6-hexanediol, etc.) and branched glycols such as neopentyl glycol can also improve the hydrolysis resistance of polyesters. Yes, polyurethane adhesives can sometimes be used in combination with polyester, and the addition of a small amount of silicone coupling agent to the adhesive formulation 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 is no longer easy to detach after bonding, thus increasing the molecular weight of the main agent. The raw material that can produce crystalline polyurethane is to improve the initial tack and curing. The effective method of speed, sometimes adding a small amount of triethanolamine catalytic cross-linking agent also helps to improve the initial viscosity? Adding a catalyst is also the main method to speed up the curing

Source: 21st Century Fine Chemicals Network