Adhesives urethane

Urethane acrylates Urethane adhesives Urethane alkyds Urethane-based resins... 

The vast majority of reactive hot melts are moisture-curing urethane adhesives. Radiation (UV/EB) curable adhesives have been explored in the laboratory since the mid-1970s, but are only recently beginning to gain significant market penetration, particularly for PSA applications. The formulation and properties of these two classes of adhesives are discussed below. 

Pot life is several hours versus several days for conventional non-reactive hot melts. A good reactive urethane is one which exhibits a viscosity rise of less than 10%/h. The slow increase in viscosity with urethane adhesives is due to chain extension via the slow reaction of the active hydrogen of the urethane groups with... 

In order to address these issues, a brief discussion of thermal, oxidative, and hydrolytic stability of urethanes will be offered, so as to aid the adhesion scientist in designing a urethane adhesive with the desired durability. 

Urethane adhesives are classified as one-component or two-component adhesives. Each category includes several different types of adhesives. 

The reaction of water with isocyanate is shown in the third item of Fig. 1 [5]. The water/isocyanate reaction is the major curing mechanism for the one-component urethane adhesives. Most one-component urethanes are based on an isocyanate-terminated prepolymer (I). Usually, the moisture in the air is used to cure the adhesive, but in some instances, a fine mist of water may be introduced on top of the adhesive before the bond is closed, in order to facilitate cure ... 

In most cases, the allophanate reaction is an undesirable side reaction that can cause problems, such as high-viscosity urethane prepolymers, lower pot lives of curing hot-melt adhesives, or poor shelf lives of certain urethane adhesives. The allophanate reaction may, however, produce some benefits in urethane structural adhesives, e.g., additional crosslinking, additional modulus, and resistance to creep. The same may be said about the biuret reaction, i.e., the reaction product of a substituted urea linkage with isocyanate. The allophanate and biuret linkages are not usually as thermally stable as urethane linkages [8]. 

Three classes of urethane raw materials will be described isocyanates, polyols, and chain extenders. The intent is to describe the major types of urethane raw materials used in urethane adhesives, rather than to provide an exhaustive review. The concepts behind the raw materials will be discussed. Once the concepts are understood, it is hoped that the adhesion scientist will then apply the concepts when choosing raw materials for urethane adhesives. 

The major isocyanates used in urethane adhesives are aromatic isocyanates, i.e., methylene diphenylisocyanate (MDI) and toluene diisocyanate (TDI). 

The most commonly used isocyanate in urethane adhesives is MDI. The pure material methylene diphenyl-isocyanate is a solid that melts around 37°C. Many variations of MDI are commercially available, and these variations fall into three major classes monomeric MDI, modified MDI s, and polymeric MDI s. 

Crystalline polyesters are highly important as adhesive raw materials. They are normally crystalline waxes and are highly symmetrical in nature, which can aid the crystallization process [26]. Poly(hexamethylene adipate) and poly(caprolactone), shown in Table 2, are only two of the many crystallizable backbones. Poly(ethylene adipate) and poly(letramethylene adipate) are also commonly used in urethane adhesives. The crystalline polyesters are used in curing hot melts, waterborne polyurethanes, thermoplastic polyurethanes, and solvent-borne urethane adhesives. The adipates are available mostly as diols. The poly(caprolactones) are available as diols and triols. 

The specialty class of polyols includes poly(butadiene) and polycarbonate polyols. The poly(butadiene) polyols most commonly used in urethane adhesives have functionalities from 1.8 to 2.3 and contain the three isomers (x, y and z) shown in Table 2. Newer variants of poly(butadiene) polyols include a 90% 1,2 product, as well as hydrogenated versions, which produce a saturated hydrocarbon chain [28]. Poly(butadiene) polyols have an all-hydrocarbon backbone, producing a relatively low surface energy material, outstanding moisture resistance, and low vapor transmission values. Aromatic polycarbonate polyols are solids at room temperature. Aliphatic polycarbonate polyols are viscous liquids and are used to obtain adhesion to polar substrates, yet these polyols have better hydrolysis properties than do most polyesters. 

The most common catalyst used in urethane adhesives is a tin(lV) salt, dibutyltin dilaurate. Tin(IV) salts are known to catalyze degradation reactions at high temperatures [30J. Tin(II) salts, such as stannous octoate, are excellent urethane catalysts but can hydrolyze easily in the presence of water and deactivate. More recently, bismuth carboxylates, such as bismuth neodecanoate, have been found to be active urethane catalysts with good selectivity toward the hydroxyl/isocyanate reaction, as opposed to catalyzing the water/isocyanate reaction, which, in turn, could cause foaming in an adhesive bond line [31]. 

In order to understand the widely varying physical properties and adhesion properties of urethanes, it is important to understand the different classes of urethane raw materials and their functions in urethane adhesives. In order to... 

It is difficult to generalize regarding adhesion properties and physical characteristics of urethanes, because both the adhesion properties and physical properties can vary widely. One two-part urethane adhesive may be an outstanding adhesive for bonding SMC/SMC. Another urethane adhesive may fail miserably using the same SMC, due to delamination at the interface or adhesive failure. 

The morphology of a typical urethane adhesive was previously shown in Fig. 3. The continuous phase usually comprises the largest part of the adhesive, and the adhesion characteristics of the urethane are usually controlled by this phase. From a chemical standpoint, this continuous phase is usually comprised of the polyol and the small amount of isocyanate needed to react the polyol chain ends. A wide variety of polyols is commercially available. A few of the polyols most commonly used in urethane adhesives are shown in Table 2. As a first approximation, assuming a properly prepared bonding surface, it is wise to try to match the solubility parameters of the continuous phase with that of the substrate to be bonded. The adhesion properties of the urethane are controlled to a great extent by the continuous phase. Adhesion to medium polarity plastics, such as... 

The role of crystallizable soft segments in urethane adhesives... 

The most common non-crystallizing soft segment in urethane adhesives is based on poly(oxypropylene) polyols, shown in Table 2. Most non-crystalline soft... 

Several large applications for one-component moisture-cure urethane adhesives are available. Polymeric MDI is an exceptional binder for wood products, such as oriented strand board and particleboard. One-component urethane windshield adhesives are used almost exclusively in both the OEM and automotive aftermarket. One-part urethane adhesives are used to assemble the sidewalls for recreational vehicles (RV s), manufactured housing, and mobile homes. In construction applications, one-part urethanes are used to bond metal doors, hardwood flooring, panels, and partitions. 

Windshield sealants comprise some of the most significant applications of one-component urethane adhesives. Several years ago, the requirements for these sealants were increased. In an accidental rollover, in certain automobiles, the windshield becomes a structural part of the car. Therefore, the windshield adhesive, which holds the windshield in place, must have structural strength. An example of a one-component windshield sealant is shown below [48] ... 

Moisture-curing hot melts are a small but fast growing segment of the urethane adhesive market. They are used mostly in construction and furniture assembly applications. Recent applications include RV sidewall assembly and other OEM automotive applications. Smaller applications include bookbinding and footwear. A typical adhesive is shown below ... 

Blocked urethane adhesives are termed blocked by virtue of the fact that one of the key reactants is chemically blocked to prevent reaction. The concept of a blocked isocyanate has already been discussed. 

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