Adhesives

Adhesives 

(glues), natural and synthetic substances used to join various materials by forming an adhesion bond between a film of the adhesive and the surfaces of the materials being bonded. The strength of the bonded joint depends on the adhesion of the substance to the surfaces, the cohesion of the film, and the properties of the bonded materials. In bonding, proper impregnation of the surface by the adhesive, a close fit of the surfaces, and maximization of the bonded surface are necessary. These requirements are met by special finishing of the surfaces (mechanical cleaning, degreasing, and roughing) and by designing the bonded parts in such a way as to produce a large bonding surface and to ensure that the glue line exists under a favorable load distribution—that is, shear or uniform separation strain rather than bending or scaling strain. The adhesion of the film to a surface usually exceeds the cohesion within the film; therefore, minimal thickness of the glue line is desirable. Bonding takes place as a result of the setting to the adhesive film caused by the evaporation of a solvent from the adhesive solution, cooling of an adhesive melt below the yield temperature, or chemical transformation of the components of the adhesive.

Adhesives may exist in various physical states, such as liquids of various viscosities (liquid monomers, solutions, suspensions, and emulsions), films, and powders, or sticks that are melted before use or are applied to hot surfaces.

Adhesives are classified as inorganic, organic, or hetero-organic according to their main component. The inorganic adhesives

Table 1. Conditions of bonding and properties of bonded joints using synthetic and natural adhesives
		Conditions of bonding			Properties of adhesive compounds
	Materials bonded	Temperature, °C	Time, (hrs)	Excess pressure, MN/m2 (kgf/cm2)	Shear strength at 20 C for metals, MN/m2 (kgf/cm2)	Heat resistance, °C
aResistance to direct pull of rubber glued to metal
bTests on pine samples
cTests on etrol samples
dTests on samples of unplasticized polyvinyl chloride
SYNTHETIC THERMOSETTING ADHESIVES
Phenol-formaldehyde	Wood, phenol plastics, graphite	20	4–6	0.2–0.4	10–15	75–100
		50–60	0.5–1.5	(2–4)	(100–150)
Phenol-rubber	Metals, thermosetting plastics, lime silicate glasses	150–200	1–2	0.8–2.0	15–25	200–300
				(8–20)	(150–250)
Phenol-polyvinyl acetate	Metals, plastics, ceramics, etc.	140–200	0.5–1.0	0.8–2.0	15–30	200–250
				(8–20)	(150–300)
Epoxy	Metals and nonmetallic materials	20	24	0.03–0.3	10–30	60–125
		120–200	0.5–0.7	(0.3–3.0)	(100–300)
Polyester (based on unsaturated polyester with styrene)	Metals and nonmetallic materials	20	24	contact	7.5–12.5	60–125
		80	0.5		(75–125)
Polyurethane	Metals and nonmetallic materials	20	24	0.05–0.5	10–20	75–125
		100	4	(0.5–5.0)	(100–200)
Rubber (based on poly-chloroprene)	Rubber, nonmetallic materials, metals, and glass	12	24	0.02	1.3a	50–60
				(0.2)	(13)
Carbamide (urea-formaldehyde)	Wood	20	4–6	0.1–0.5	10–13b	75–125
				(1.0–5.0)	(100–130)
Organosllicon	Metals and nonmetallic materials	150–250	1–3	0.3–0.8	10–17.5	350–1,200
				(3–8)	(100–175)
SYNTHETIC THERMOPLASTIC ADHESIVES
Carbinol	Metals, ceramics, plastics	20	24	0.15	10–15	50–60
				(1.5)	(100–150)
Polyacrylic	Nonmetallic materials and metals	20	24	0.01–0.3	15–25	60–100
		80	4–6	(0.1–3.0)	(150–250)
Polyamide	Nonmetallic materials and metals	150	1	0.1–0.5	15–25	50–60
				(1.0–5.0)	(150–250)
Polyvinyl acetate	Paper, leather, fabrics, plastics	20	0.5–1.0	contact	5–12c	60
					(50–120)
Chlorinated polyvinyl chloride	Plasticized and nonplasticized polyvinyl chloride, fabrics, plastics	20	6–24	0.01–0.3	4–8d	60
				(0.1–3.0)	(40–80)
Polybenzimidazole	Metals and glass-fiber-reinforced plastics	150–350	3–5	1.5–4.0 (15–40)	15–30 (150–300)	350–540
Polyimide	Metals and glass-fiber-reinforced plastics	180–315	1.5–8.0	0.14–0.3 (1.4–3.0)	15–30 (150–300)	300–375
NATURAL ADHESIVES
Casein	Wood, paper, leather, fabric	20	48	0.3–1.5	6–8b	50
		60	12	(3–15)	(60–80)
Glutin (joiner’s glue)	Wood	20	48	0.3–1.0	5–8b	50
				(3–10)	(50–80)

include water glass (aqueous solutions of sodium and potassium silicate) and adhesive frits (aqueous suspensions of compositions containing oxides of alkali and alkaline earth metals). Water glasses are used in the bonding of cellulose materials, and adhesive frits are used in the bonding of metals and ceramics.

Among the organic adhesives are compositions based on natural and synthetic polymers. Adhesives based on natural polymers are made from substances of animal origin, such as the byproducts of processed hide, bones, and scales (collagen), blood (albumin), and milk (casein), and substances of plant origin, such as gum, resin, starch, dextrin, natural rubber, gutta-percha, zein, and soy casein. Adhesives based on natural polymers are used to bond wood, paper, leather, and textiles. They have low resistance to microorganisms and water. They are being superseded in large-scale production by synthetic adhesives, which are prepared from most of the synthetic polymers produced industrially. These adhesives provide high-strength bonding of various materials and are resistant to environmental factors. They are extensively used in bonding metals, glass, ceramics, plastics, wood, textiles, and cellulose.

Hetero-organic adhesives are prepared on a base of organosilicon, organic boron, and organometallic polymers. They have very high thermal stability and heat resistance (they ensure a high-strength joint between different materials under short-term heating to temperatures of the order of 1000°C or more and withstand prolonged heating to 400°–600°C). These adhesives are used to bond metals, graphite, and heat-resistant plastics. The organosilicon types are the most widely used.

The properties and technological characteristics of typical adhesive compositions based on natural and synthetic thermo-setting and thermoplastic polymers are given in Table 1. The bonded joints produced by using synthetic adhesives have good resistance to the prolonged action of gasoline, mineral oils, and aliphatic solvents. Adhesives based on thermosetting synthetic polymers are also resistant to aromatic solvents. Bonded joints of this type are also highly waterproof, except for those that are based on urea-formaldehyde, carbinol, and polyvinyl acetate adhesives.

Adhesives are subdivided into structural, nonstructural, and special types according to function. The composition of structural adhesives provides for the transmission of dynamic and static loads from one portion of a part or article to another joined to it by the adhesive film. The basic requirements made of adhesives in this group are adequate strength under various kinds of loading within the service temperature range of the article and the absence of creep under a sustained load. Nonstructural adhesives have compositions suitable for gluing decorative, facing, or insulating materials and coatings, locking threaded joints, and fastening small unloaded parts (sensors for various purposes; the current-conducting elements of electronic instruments). The compositions of special adhesives have additional important functional properties, including current-conducting, optical, and medical properties.

The principal advantages of adhesives are the simplicity of their technology and the small amount of effort required to apply them. The bonded joints are strong, vibration-proof, and pressure-proof and have other valuable features that are responsible for the increasing use of adhesives in various areas of the national economy and daily life. The wide assortment of modern adhesives makes it possible to solve a great many problems, ranging from the construction of reinforced-concrete bridges with bonded structures to the production of miniature electronic instruments, from the manufacture of bonded clothing and shoes to the use of adhesives as sutures in internal surgery, and from gluing toys to bonding modern helicopter rotors and spacecraft parts.

REFERENCE

Kardashov, D. A. Sinteticheskie klei, 2nd ed. Moscow, 1968.
Berlin, A. A., and V. E. Basin. Osnovy adgezii polimerov. Moscow, 1974.
Khrulev, V. M. Sinteticheskie klei i mastiki. Moscow, 1970.
Handbook of Adhesives. Edited by I. Skeist. New York-London, 1962.

A. B. DAVYDOV