胶黏剂基础之11——氰基丙烯酸酯胶黏剂(英文版)(常用的502,504系列胶)CyanoacrylateAdhesives

1 Cyanoacrylate Adhesives

SpecialChem - Oct 8, 2003 Edward M. Petrie, Member of SpecialChem Technical Expert Team. Introduction Cyanoacrylate adhesives became well known after their commercial introduction in the early 1970s in the consumer market. These unique products were initially referred to as "superglue". The name is well deserved since cyanoacrylate adhesives have certain characteristics like no other adhesive. These adhesives are solvent-free, one-part formulations that cure rapidly when pressed into a thin film between two substrates. Perhaps no other adhesive bonds so rapidly and so easily to such a variety of substrates as does a cyanoacrylate. Early generations of cyanoacrylate adhesives had significant performance limitations. Since they were essentially thermoplastic in nature, cyanoacrylate adhesives exhibited poor thermal and chemical resistance. Since they were hard and brittle when cured, they exhibited poor impact and peel properties. These drawbacks limited the application of cyanoacrylate adhesives to high volume assembly operations with minimal performance requirements. Newer formulations, however, have greatly improved the performance properties of these "instant" adhesives. New products continue to offer the ease of use of a cyanoacrylate but now with the added performance properties of a truly structural adhesive. Table 1 summarizes the general benefits and limitations of today's cyanoacrylate adhesive systems. Benefits Limitations

Excellent Adhesion to a wide variety of substrates Blooming/frosting

Substrate variety Difficult to cure fillet or exposed liquid adhesive without activator

Simple cure mechanism Limited gap cure

Rapid strength development Stress cracking could occur to some plastics

High strenght possible on polyolefins and fluorocarbons using primers Soluble in polar solvents

Available in USP Class VI compliant formulations Thermal and chemical stability not as good as with certain other structural adhesives

High shear strength Unmodified formulations have low peel and impact strength

No measuring or mixing required Relatively high materials cost

Pungent odor associated with early formulations Table 1: Benefits and Limitations of Cyanoacrylate Adhesives [1]

Curing Mechanism When confined in a thin film between two surfaces or sprayed with a chemical activator, cyanoacrylate adhesives cure rapidly at room temperature to form rigid thermoplastics with excellent adhesion to most substrates. Cyanoacrylates typically reach handling strength within one minute at room temperature and achieve full strength in 24 hours. Cyanoacrylate adhesives undergo anionic polymerization in the presence of a weak base, such as 2 water, and are stabilized through the addition of a weak acid. The stabilizer is usually in the form of a weak acidic gas such as SO2, NO, or BF3. An essential function of the stabilizer is to prevent polymerization in the container, which is usually made of polyethylene. When the adhesive contacts a slightly alkaline surface, trace amounts of adsorbed water or hydroxide ions (OH-) that are present on the substrate's surface neutralize the acidic stabilizer in the adhesive, resulting in rapid polymerization as shown in Figure 1.

Figure 1: Chemical reaction of cyanoacrylate adhesives. [2]

In general, ambient humidity in the air and on the bonding surface is sufficient to initiate curing within a few seconds. Therefore, parts must be joined quickly. The open time is dependent on the grade of adhesive, the ambient temperature and relative humidity, and the nature of the substrate surface (pH and amount of adsorbed water). Optimal bonding conditions are when the ambient relative humidity is between 40% and 60%. Lower humidity slows curing, high humidity accelerates it, but could lead to lower bond strengths. Figure 2 illustrates the effect of relative humidity on curing time for cyanoacrylate adhesives.

Figure 2: Curing of cyanoacrylate adhesives as a function of relative humidity. [3] To achieve the fastest cure, a very thin bond line is desirable. Since the curing mechanism is water initiated, lack of complete moisture penetration in thick bond lines can prevent curing of the center section of the bond. Therefore, only enough adhesive should be applied as to fill the joint gap. The cyanoacrylate adhesive need only be applied to one surface. 3 Without the application of a primer, acidic surfaces may delay or even prevent curing, whereas more alkaline or basic surfaces accelerate curing. Thus, human tissue can be bonded very quickly with cyanoacrylate adhesive, not only due to the water content, but also to the amino acids present, which are quite basic to the adhesive. This has encouraged the use of cyanoacrylates in many surgical and dental areas.