The advantages and disadvantages of polyurethane-based adhesives

December 8, 2020 - Dr. Keith Berry, Technical Services Manager, Chemique Adhesives and Sealants Co., Ltd., describes in detail the advantages and disadvantages of polyurethane-based adhesives.
the purpose of this white paper is to provide background on the advantages and disadvantages of parts 1 (1K) and 2 (2K) polyurethane-based adhesives. Polyurethane-based adhesives are designed to give them a wide range of properties that allow them to adhere to many substrates, and there are many advantages to using them, and some limitations must be considered

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polyurethane-based adhesives are versatile and have good performance characteristics and can be made to provide a variety of physical properties, such as viscosity and construction properties, as well as application periods. Polyurethane systems for adhesives can be divided into two main categories: 1K (one part) and 2K (two parts) systems, each with some similarities and its own strengths and weaknesses.
the advantages and disadvantages of all polyurethane-based adhesives use the same chemical reaction as alkali, which is the reaction between isocyanate and polyols (Figure 1). For 2K systems, isocyanate and polyols will be produced and supplied as separate components. Before use, the two components need to be mixed to produce a chemical reaction and fully interlink the system. To ensure proper conduct, it is important to ensure an accurate scale of the ingredients and a sufficient mix.
, when using a 1K system during manufacturing, polyols react with excess isocyanate so that the polyol chain is sealed by isocyanate-based groups (Figure 2). The ratio of isocyanate to polyol will help determine the chain length of the sealed polyurethane pre-polymer. The polyurethane pre-polymer becomes a reproceive part of the 1K system.
in order to complete the reaction, the 1K system will need to interact with water to fully interlink. Figure 3 shows a chemical reaction between isocyanate and water at the end of the pre-polymer chain of a 1K system that causes the polycyclic bonding. This means that the 1K system requires water to be cross-linked, which may come from moisture in the atmosphere, but for some adhesives, this is due to the application of water mist bonding before adding a second substrate. Alternatively, the prejudes can be dissolved in a solvent carrier and will react with moisture as the solvent evaporates.
advantages and limitations
both types (1K and 2K) of polyurethane adhesives have some advantages and limitations. These advantages include curing time and the ability to change the application period by changing the formulation, as well as good solvent resistance after curing. Depending on the formulation, the cost of polyurethane-based adhesives is also moderate, and coatings can be done in a variety of ways, such as roller coating, brushing, spraying, manual or automatic.
polyurethane-based adhesives are also non-flammable and have excellent adhesion to a wide range of substrates. Although they can be solventized, polyurethane-based adhesives may not provide solvents. Another advantage is that good flexibility can be obtained in curing products and does not require the heat required for curing, but once two substrates are present, they can be used to speed up curing. They can also remain bonded over a wide operating temperature range.
these advantages, polyurethane-based adhesives have some limitations. These include un cured materials that are moisture sensitive and have a short shelf life - typically 3 to 12 months for 1K and 2K adhesives. Although they adhere to most substrates, some substrates may require primers to adhere to polyurethane-based adhesives. Since the formation of carbon dioxide causes foam, components made with polyurethane adhesives may also need to be kept under sufficient pressure in the press while the adhesive cures to prevent this from happening.
1K adhesive has some other advantages due to the way the adhesive is provided and the formation of urea bonds when cured. These include non-mixing, ease of use, excellent chemical resistance, and the use of solvents or solvents for as needed purposes.
they will also have some other limitations, as the curing time and application period will depend very much on environmental conditions. Due to the wet curing properties of the material, once the container is opened, the container begins to react with moisture in the air, reducing its shelf life and increasing its viscosity. In addition to requiring sufficient moisture to fully cure, this may also mean that a certain amount of water mist treatment is required before a second substrate can be applied.
2K adhesives are also suitable because of the advantages and disadvantages of 1K polyurethane-based adhesives and other advantages and limitations. The advantages of 2K adhesives typically include longer shelf life compared to the advantages and disadvantages of 1K polyurethane-based adhesives, curing times are more dependent on formulation than environmental conditions, and no additional processes (e.g. water mist) are required after adhesive application.
limitations of 2K polyurethane-based adhesives include the need for the correct proportion of the ingredients to properly cure the adhesive and the full mixing of the two ingredients before coating.
most common problems with polyurethane adhesives are usually due to one of the following reasons. First, use adhesives with a short application period, as this condition results in the substrate not being assembled and under pressure during the application period. Similarly, when not enough adhesive is applied to the substrate, the coverage is not sufficient to provide complete bonding. If one of the substrates is porous, it must be taken into account and the uniformity of the surface taken into account. Finally, problems can occur if there is insufficient internal pressure in the press. The total surface area of the substrate needs to be taken into account, so the pressure is about 10 Pa.
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