One-part adhesives

One-part polyurethane adhesives have been formulated to cure at room temperature or at elevated temperatures. With the room temperature systems, cure may be brought about by the reaction of free isocyanate with moisture as discussed in Section II. These materials are typically used as adhesives/sealants for the bonding of porous substrates, for example, wood and concrete. They are also being used for the bonding of automobile windshields. Such bonding incorporates the windshield into the automotive body as an integral structural member which contributes to the 

Heat cure systems are formulated from an isocyanate-terminated prepolymer and a curing agent, in which one of the components is rendered inactive at room temperature. Upon heating, the inactive species becomes active and curing takes place. For energy reasons, it is desirable to formulate such adhesives with a low activation temperature. However, the shelf stability of the formulation will often suffer if this activation temperature is too 

More recently, systems have been reported which thermally generate isocyanates without the evolution of side products. When these thermal reactions are carried out in the presence of polyols, cured polyurethanes are said to be produced. One such system is based on bis(cyclic ureas). 

Upon heating above 150°C, they ring open to give amido isocyanates as illustrated in Eq. (10) for a bis(tetramethyleneurea) derivative (20). 

Similarly, alkylenediazetidinediones (21) are reported to thermally ring open to give diisocyanates as illustrated in Eq. (11). These reactions are claimed 

---

Activation by a metal surface also takes place in the commercially important anaerobic adhesives. These one-part adhesives are stable in the package, but cure quickly in an oxygen-free environment such as a tightly controlled bond line. Important applications include thread-locking, sealing, retaining, and some structural bonding [111]. A representative model formulation has recently been described [112] (Fig. 3). 

There are also one-part adhesives based on low molecular weight epoxy units that contain the unopened cyclic ether end groups. These materials can be cured when heated to about 200°C for 30 0 min. 

ATBN liquid polymers cannot be mixed directly into the epoxy resin component of a two-part epoxy adhesive or in a one-part adhesive since crosslinking and shortened shelf life will result. ATBN adducts are, therefore, mixed with the curing agent component of two-component epoxy adhesives.30... 

Degas, Package, Freeze or Stored ONE-PART ADHESIVE... 

With improved formulations, organic-based conductive adhesives became feasible replacements for eutectic solders. Table 4 shows typical one- and two-part conductive adhesives from the 1970s. The one-part adhesive system employed a latent catalyst for long pot life but required a high-temperature cure cycle. The two-part system, formulated... 

There are three main polymers presently used for structural adhesive bonding and they are phenolics, epoxies and urethanes. We can also include in our classification, the initial physical state of the uncured adhesive since it governs the type of application and curing conditions. Structural adhesives are manufactured in the form of films or pastes. The films are one-part adhesives, i.e., they contain a latent catalyst which requires heat for activation. Structural adhesive films are made in a number of different thicknesses and can either be supported (containing a scrim) or unsupported. In general, film adhesives require the application of pressure during cure in order to obtain ultimate properties. Paste adhesives are either one- or two-part materials. That is, pastes can either contain a latent heat activatable catalyst, or they can be a system which is separated into two parts, one of which contains the cur a t i ve/ca talys t. Two-part pastes cure at room temperature. In this section the chemistry of one-... 

Boron trifluoride hardeners Boron trifluoride monoethylam-ine melts at 95 °C and is used in one-part adhesives. 

Another urethane one-part adhesive (Urethane Bond) developed by Dow Corning is cured by moisture in the air at room temperature. This material requires a thin glue line and clamping to produce the strongest joints. The resultant bonds are moisture resistant and are claimed to work well on polystyrene, PVC, and acrylics, and fair on polyethylene.There are excellent references for additional study of this adhesive family. 

One-part silicone systems are ready to use, require no mixing, present no pot-life problem, and are generally the least expensive. Conventional one-part adhesive/sealants are available with two different types of cure systems acid and non-acid cure. Both require moisture from the atmosphere to cure. The acid-curing type has the greatest unprimed adhesion and the longest shelf life. The non-acid-curing type is effective when the acetic acid released by the cure reaction may cause corrosion, or be otherwise objectionable. 

Cyanoacrylate adhesives are rapid curing one part adhesives based on alkyl-2-cyano-acrylate monomers (I). 

As dicyandiamide is not directly soluble in epoxy resins under a temperature of about 150 °C, it acts as an effective latent hardener, which means that it can be used in one-part adhesives, which are storage stable for about a year to eighteen months. Once it does dissolve, however, it reacts quickly and fairly exothermically. Most unaccelerated dicyandiamide-containing adhesives are cured in the region of 150 to 180 °C. 

Imidazoline (44) derivatives, which are partially saturated derivatives of imidazole, can be used as curatives, although their use is not widely reported. Stable, one-part adhesives prepared from DGEBA resins and imidazoline derivatives, such as 2-phenylimidazoline (45) and 4-methyl-2-phenylimidazoline (46), have been cured at 140°C for 30 min on aluminum sheets to give shear strengths at room temperature of 42.2 kg/cm. ... 

The resistance to hydrolysis of a polycyanoacrylate can also be improved by copolymerization with another monomer. In fact, the cyanopentadienoates cited above should also be considered comonomers. An electron-rich vinyl monomer like styrene and a cyanoacrylate ester will spontaneously copolymerize a short time after mixing. The product is a one-to-one alternating copolymer. This reaction has been used to make a two-part adhesive with better hydrolytic stability than the standard cyanoacrylate. " The carbon dioxide evolved from a suspension of the polymer in boiling water was measured to follow the degradation. After eight hours, polymethyl cyanoacrylate had lost 28% carbon dioxide by hydrolysis and decarboxylation of the polycyanoacrylate ester groups, while the copolymer had lost none. However, this improvement in moisture durability is achieved at the expense of the convenience of a one-part adhesive. 

One-part. Cyanoacrylates are one-part adhesives, eliminating the need for mixing or metering, but retaining long pot lives. Surface activation is needed occasionally, but this is the exception rather than the rule. 

When the production of the adhesively bonded cabin first started, a one-part product that cured on exposure to atmospheric moisture was used, but this was later switched to a faster curing adhesive system. Thanks to the use of Booster technology (one-part adhesive with cure accelerator) such as Sikafiex -254 Booster, the cycle time was cut from 8 h to 2 h. In other words, output was inaeased by a factor of four. 

There are a number of one-part adhesive systems recommended for use with Lexan resin. Eastman 910MHT cures at room temperature to a clear film and has the advantage of a wide use temperature range. Some epoxy-polyamide one-part systems and rubber or neoprene-based systems also produce good adhesion . 

Anerobic adhesives. Acrylate acid diester and cyanoacrylate resins are called anaerobic adhesives because they cure when air is excluded from the resin. Anaerobic resins are noted for being simple to use, one-part adhesives, having fast cure at room temperature and high cost. However, the cost is moderate when considering a bonded-area basis because only a small volume of adhesive is required. Most anaerobic adhesives do not cure when gaps between adherend surfaces are greater than 10 mils, although some monomers have been developed to provide for thicker bond-lines. 

One manufacturer has developed an acrylic adhesive system in which the hardener is applied to the substrate as a primer solution. The substrate can then be dried and stored for up to six months. When the parts are to be bonded, only the acrylic resin need be apphed between the already primed substrates. Cure can occur in minutes at room temperature, depending on the type of acrylic resin used. Thus, this system offers the user a fast-reacting, one-part adhesive (with primer) with long shelf life. 

---