Adhesives Setting by Polycondensation

Polyhydroxymethyl Compounds. Hydroxymethyl compounds that release water during condensation are used primarily for polycondensation adhesives. Accordingly, the high-quality bonding of impermeable substrates requires relatively high pressures to compensate for the water vapor pressure developed during hot setting. Polycon­densation adhesives are used both in pure form and as solutions in water or organic solvents. Hardening can be initiated by the application of heat or by the addition of hardeners.

Polycondensation adhesives include phenolic and resorcinol resins and also urea — and melamine-formaldehyde resins. Adhesives of particularly high quality are ob­tained by combining phenolic resins with poly( vinyl formal) resins, nitrile rubber, or epoxy resins. Combinations such as these are often marketed as adhesive film.

Trade Names. In Germany: Redux, Tegofilm, Kaurit, Pressal.

Phenolics or phenol-formaldehyde structural adhesives are chemically reactive systems that cure to form thermosets. In one-component systems, meltable powders (resols) are used as binders for particle board or as alloys (including nitrile-phenolics, vinyl — phenolics, and epoxy-phenolics), which are used in the structural bonding of metals. In two-component systems, the resin and catalyst are mixed and then heated to initiate curing. Both systems cure by a condensation reaction that produces a byprod­uct.

In general, phenolics are low-cost adhesives with good strength and resistance to biodegradation, hot water, and weathering. Elevated-temperature resistance is also good. Limitations include low impact strength and high shrinkage stresses, which lead to brittleness. Shelf life is limited, the adhesives are dark colored, and they can be corrosive.

Phenolics dominate the wood adhesives market, especially for plywood. The struc­tural adhesive bonding of metals, particularly with phenolic alloys, is another appli­cation.

Nitrile-phenolic alloys are composed of nitrile rubber and phenolic with additives. These systems are available in liquid or film form. The advantages include a maximum service temperature of 140°C, low cost, high bond strength, and excellent resistance to water, oil, biodegradation, and salt. Disadvantages include poor to moderately poor low-temperature resistance and high-pressure, high-temperature curing. Typical appli­
cations include the bonding of metals, plastics, rubber, wood, glass, and ceramics. This structural adhesive is used in automobile brake-shoes and clutch disks and in aircraft applications.

Vinyl — phenolics are alloys composed of polyvinyl formal (PVF) — phenolics or poly­vinyl butyral (PVB) — phenolics. They are available as liquids and films. In general, vinyl-phenolics have a maximum service temperature of 80°C and are equal to phenolic — nitriles in strength. These adhesives are better than epoxies in sandwich structures that require high strength.

PVF-phenolics have good resistance to fatigue, weathering, fungi, salt, humidity, water, and oil. Creep resistance is good in some formulations, but is poor in others above 90°C. These adhesives are used in metal-honeycomb and wood-metal applica­tions.

PVB — phenolics also have good resistance to weathering, fungi, salt, humidity, water, and oil, and creep and fatigue resistance are even better than those of PVF — phenolics. These adhesives are used to bond metal or reinforced plastic facings to paper honey­comb, cork to rubber, and steel to rubber.

Epoxy-phenolic adhesives are also available as liquids or films. They are among the best adhesives for long-term use at 150-250 °С. In general, epoxy — phenolics are relatively expensive. They have fairly good shear and tensile strengths over a wide temperature range, but peel strength is poor. Impact resistance is also poor, whereas resistance to weathering, aging, aromatic fuels, glycols, hydrocarbon solvents, and water is good. Typical applications include the bonding of metals, glass, ceramics, phenolics, and honeycomb sandwich structures in aircraft applications.

Silicone Adhesives. Cold-cross-linking one-pack silicone adhesives also harden by a polycondensation mechanism. They are silanols whose OH groups have reacted with acetic acid or amines. Treatment of these compounds with water leads to the elimi­nation of acetic acid or amines and generates the reactive silanols, which cross-link with elimination of water. These products are particularly important as sealing compounds in the building industry.

Usually silicones are available as both one — and two-component systems that cure to thermoset solids. A one-component room-temperature-vulcanizing silicone cures at room temperature upon exposure to atmospheric moisture. Curing is either acidic or nonacidic in the presence of moisture. Adhesives that have an acidic cure have greater unprimed adhesion and a longer shelf life. However, the corrosion of metals by the acid is a potential problem. Thin films of approximately 0.6 mm cure within 90 min., whereas 15 mm films of require ca. 7 d to achieve full cure.

Two-component silicones do not require moisture to cure. During the addition polymerization of silicones, curing is achieved by catalytic action. Pot life, setting time, and cure time are dependent on the catalyst concentration. A system with 5 % catalyst will typically have a 3 h pot life, a 22 h setting time, and a 7 d cure time at room temperature. Increasing the temperature accelerates curing. Addition-polymerized sili­cones exhibit little shrinkage upon curing and have good high-temperature resistance.

In the condensation polymerization of two-component silicones, byproducts are re­leased. These systems are less likely to be inhibited and can be used on a greater variety of substrates. However, reversion of polymerization is a potential problem.

In general, silicone adhesives have good peel strength over a service temperature range of — 60 to 250 °С. Some have limited service to 370°C. Flexibility and impact resistance are good, as are resistance to moisture, hot water, oxidation, and weathering. Typical lap-shear strengths are low in metal-to-metal bonds, with values ranging between 1 and 3.5 MPa. The cost of these adhesives is high.

Silicone adhesives are useful in bonding metals, glass, paper, plastics, and rubbers, including silicone, butyl rubber, and fluoroelastomers.

MS Polymers. A relatively new class of adhesives are the two-component MS polymer adhesives, known as moisture-cross-linkable sealants up to know. Due to their excellent elastic properties they are used for elastic bonding. Chemically they are based on silanealkoxy-modified prepolymers such as polypropylene glycol). The end groups of these so-called MS polymers are silanealkoxyl groups which are hydrolyzed by water and then cross-link. In comparison to silicone adhesives, they adhere to all kinds of surfaces without any primer and are paintable in the freshly cured state.

Polyimides and Polybenzimidazoles [53]. The highly temperature resistant poly­aromatic adhesive resins of the polyimide and polybenzimidazole type are applied as precondensates in the form of solutions or films and are condensed to completion at 230 — 250°C under a relatively high pressure of 8 -10 bar. The precondensates remain stable for only a few hours at room temperature and have to be stored at -18 °С.

These structural adhesives with high-temperature resistance are based on synthetic organics with aromatic (benzene) and/or heterocyclic rings in the main structure. These chemical groups often include imidazoles and substituted imidazoles. These prepol­ymers have open-ring structures that close upon application of heat. The condensation reaction leads to a highly cross-linked structure. The adhesives are available as liquids and films. High-temperature adhesives, including polyimides and polybenzimidazoles, are expensive materials that are difficult to handle and require long curing times during which volatile substances are released. Polyimides are superior with regard to long-term strength retention at elevated temperatures. At 260 °С in air, polyimides have higher bond strength than epoxy-phenolics. Polybenzimidazoles are stable in air for short­term exposures up to 290 °С. Both polyimides and polybenzimidazoles are moisture — sensitive. Applications of high-temperature adhesives are primarily in the aircraft and aerospace industries for bonding metals.

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