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

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.

Table 1: Benefits and Limitations of Cyanoacrylate Adhesives

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

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.

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.

Table 2: Performance of Cyanoacrylate Adhesives on Various Substrates [4]

The cure of cyanoacrylate can be accelerated by adding crown ethers, oligomers of poly(ethylene oxide) or podants.[2]Among the earliest accelerators was common chalk, a natural carbonate. Modern accelerators include a number of organic and inorganic bases, usually supplied in a solvent carrier. Generally, the accelerator is applied to one surface while the adhesive is applied to the mating surface. This prevents cure of the adhesive by the accelerator before the parts can be mated. Exposed liquid adhesive (e.g., excess adhesive at a filet or an adhesive used for wire tacking) can generally be cured within seconds with activators. In these cases the activator is sprayed directly on the liquid cyanoacrylate.

Primer / accelerators are generally used on acidic surfaces or on surfaces where there is no

adsorbed water or hydroxide ions (e.g., many plastic or elastomeric substrates). When applied to one or more of the surfaces to be bonded, these will accelerate the cure of cyanoacrylate.

Primers are generally applied via a solvent solution in heptane, acetone, or isopropanol. Most manufacturers supply suitable accelerators for their adhesives and can recommend specific combinations for most bonding applications.

Basics Chemistry and Formulation

Cyanoacrylate adhesives are generally methyl or ethyl cyanoacrylate-base, single component liquids. Cyanoacrylate adhesives bond well to many substrates as shown in Table 2. When bonding metals and other rigid surfaces, methyl cyanoacrylate bonds are stronger and more impact resistant than ethyl cyanoacrylate bonds. However, on rubber or plastic surfaces, ethyl cyanoacrylate is preferred.

Other cyanoacrylates of commercial importance include 2-propyl, n-butyl, and allyl esters. All of these monomers are clear, colorless, low viscosity liquids with pungent odors. The relative benefits and limitations of various types of cyanoacrylate monomers make them appropriate for various different applications.

Formulation of cyanoacrylate adhesives is typically difficult because of the sensitivity of cyanoacrylates to contaminants and the extreme reactivity of the cyanoacrylate curing mechanism. Thus, much development work regarding new formulations has been on changes in the chemical structure of the cyanoacrylate monomer itself. The final physical properties will depend on the alkyl group. Table 3 shows typical properties of some of the newer monomers in addition to conventional ethyl and methyl monomers.

Table 3: Relationship of Cyanoacrylate Monomer to Basic Adhesive Properties Cyanoacrylate adhesives based on the ethyl derivative have the deficiencies of a pungent odor and blooming (whitening or frosting of substrates that are in contact with the liquid adhesive). Blooming produces a white, chalky deposit around the bond line that does not affect the physical properties of the joint, but it does affect the aesthetic quality. Products generated from adhesives based on beta-alkoxy derivatives have generally solved these problems while preserving the advantageous properties of cyanoacrylate adhesives in general.

Free radical stabilizers such as hydroquinone are added to improve storage stability. Weak acidic gases, as mentioned above, are also added as stabilizers to reduce the possibility of anionic polymerization.

Thickeners are typically high molecular weight acrylates soluble in the cyanoacrylate monomer. They are generally employed to prevent starved joints. Hydrophobic fumed silica can also be used to formulate gel-like adhesives. A typical formulation for a cyanoacrylate adhesive is shown in Table 4.

Table 4: Formulation of Cyanoacrylate Adhesive

Cyanoacrylates do exhibit several disadvantages in addition to their odor and blooming tendency. They also have limited cure depth (bond line thickness) of only about 0.025 cm, poor moisture resistance on glass, and the potential for stress cracking of plastic substrates. Annealing the plastic and / or using an accelerator to reduce the time that the liquid adhesive contacts the plastic can minimize the potential for stress cracking. Other improvements in formulation are discussed below. Properties and Recent Improved Formulations

Early generations of cyanoacrylate adhesives have been associated with poor heat and moisture resistance due to their thermoplastic nature. Peel and impact strengths were also low because of their brittleness. They found use as adhesives generally where there was minimal environmental stress and great value placed on fast setting times. Cyanoacrylate adhesives also have had and still do have relatively high material cost compared to other adhesives. However, on a cost per bonded part basis, cyanoacrylate adhesives are competitive with most other joining operations.

New generations of cyanoacrylates, based on monomers other than methyl or ethyl cyanoacrylate, have improved toughness and high shear strength (3,500 psi). New cyanoacrylate resin monomers have also been introduced to provide faster cures, higher strength with some plastics, and greater thermal and impact resistance.

The use of phthalic anhydride has been reported to improve both the moisture resistance and heat resistance of the cyanoacrylates.[7] Rubber toughened cyanoacrylates generally also show the best performance in water and humid environments. When moisture or solvent resistance is a concern the cyanoacrylate bond thickness should be kept as thin as possible. The more polar solvents, such as nitromethane and acetone, will rapidly dissolve cyanoacrylate adhesives.

Whereas unmodified cyanoacrylates normally have a maximum operating temperature of about 82°C, new thermally resistant formulations offer continuous service at temperatures as high as 120°C. Improved heat resistance is addressed by the addition of crosslinking agents, heat resistant modifying monomers, or both. Crosslinking monomers such as biscyanoacrylates or alkenyl cyanoacrylates can be used to generate some amount of crosslinking in the adhesive.[8]Several formulations are ommercially available that claim service temperatures up to 150°C. These high temperature formulations are very useful in electrical / electronic applications such as wire tacking and component bonding to printed circuit boards.

Typically unmodified cyanoacrylate adhesives have a high lap shear strength, on the order of 2000-3000 psi, on most substrates. Lap shear strength can be further improved by 10-30% and peel and impact resistance can be greatly improved by incorporation of tougheners or plasticizers to the unmodified cyanoacrylate adhesive formulation.[5, 8] Products have been developed which have 5-10 times higher peel strength than conventional compositions.

Plasticizers such as aliphatic esters, aromatic phosphates, and phthalates can also be added to the cyanoacrylate formulations for flexibilizing the bond line. Plasticizers help improve the peel strength and shock resistance of the cured cyanoacrylate adhesive.

Significantly improved impact strength has been accomplished using various elastomeric tougheners.[9]Modern cyanoacrylate adhesives include toughening agents such as acrylonitrile-butadiene-styrene, ethylene-methyl acrylate copolymers, styrene butadiene rubber grafted with styrene and methyl methacrylate as well as other copolymers and grafted polymers. The toughening agent is usually added at a concentration of 15-20 percent by weight of the cyanoacrylate monomer. The toughening echanism is primarily one of phase separation. Cyanoacrylates are now available in gel form in addition to the conventional low viscosity liquids. These systems provide the ability to bond to substrates such as wood, leather, and fabrics, which have been notoriously difficult substrates for bonding with cyanoacrylate adhesives. Fumed silica and high molecular weight acrylates are generally employed in these formulations as mentioned above.

Unmodified cyanoacrylate adhesives do not polymerize readily on acidic surfaces such as wood or dichromated metals. The incorporation of poly-n-vinyl pyridine or polyethyleneimine or even simple amines presumably serves the dual purpose of thickening the liquid (for better bonding to the porous wood surface) and increasing the pH to speed cure. Surface insensitive cyanoacrylates have been developed by adding agents such as silacrowns, crown ethers, and calixarenes to ethyl cyanoacrylates. These products generally offer the most rapid fixture times of all the cyanoacrylates. Recently light curing cyanoacrylate adhesives have been developed that offer the rapid light cure properties of a thermosetting acrylic adhesive coupled with the ease and speed of a secondary cyanoacrylate cure.[1] Light curing cyanoacrylates are ethyl based products that have photoinitiators added to the formulation, allowing them toixture rapidly on exposure to low intensity light, and to cure in shadowed areas.

A major benefit that light curing cyanoacrylate adhesives offer is that open liquid adhesive can be cured to a tack free surface in less than three seconds through exposure to a low intensity light. This prevents blooming and stress cracking of plastics, and greater gap thicknesses can be cured. Light curing cyanoacrylate adhesive can be cured to depths in excess of 0.64 cm within 15 secs. Since light cured cyanoacrylate adhesives are thermosetting, they provide improved creep resistance and high temperature strength.

Table 5: Bondability of Low Energy Plastics by Cyanoacrylate Adhesives

Bond Strength to Polyolefin

Cyanoacrylate adhesives (or super-glues) do not wet or adhere well to polyolefins. The surface tension of the adhesive is much higher than that of the substrate. However, polyolefins can be primed for adhesion with cyanoacrylates by certain chemical compounds normally considered to be activators for cyanoacrylate polymerization. Itappears that one of the main reasons for improved polyolefin adhesion is that the solvents in the primer system wet-out and swell the polyolefin. This

then facilitates interpenetration of the low viscosity cyanoacrylate resin into the bulk of the substrate. Table 5 shows the significant strength improvements that can be realized with cyanoacylates on primed low energy plastic substrates.

Materials such as long chain amines, quaternary ammonium salts, and phosphine can be applied in either pure form or in solution to the surface of the polyolefin. These primers are simply sprayed or brushed onto the substrate. After drying of the primer, the cyanoacrylate adhesive is conventionally applied and bonds extremely well to the substrate.

Several companies have discovered new primers that effectively interact with the cyanoacrylates. Triphenylphosphine or cobalt acetylacetonate primers used with cyanoacrylate adhesives produce adhesive bonds with polypropylene and low density polyethylene that are sufficiently strong to exceed the bulk shear strength of the substrate. They are also sufficiently durable as to withstand immersion in boiling water for long periods of time.

References:

1. Courtney, P.J., "Light Curing Cyanoacrylates", Adhesives Age, May 2001.

2. Comyn, J., "Moisture Cure of Adhesives and Sealants", International Journal of Adhesion and Adhesives, Vol. 18, 1998, pp. 247-25

3.

3. Loctite Worldwide Design Handbook, Loctite Corporation, 1996-97.

4. Eastman Chemical Company, Leaflet R-206A.

5. Melody, D.P., "Advances in Room Temperature Curing Adhesives and Sealants - A Review", British Polymer Journal, Vol. 21, 1989, pp. 175-179.

6. Millet, G.H., Adhesive Age, Vol. 24, 1981, p. 27, also in reference 8.

7. Harris, S.J., U.S. Patent 4,450,465, 1984.

8. Pocius, A.V., Adhesion and Adhesives Technology, Hanser Publications, 1997, p. 211.

9. Courtney, P.J. and Verosky, C., "Advances in Cyanoacrylate Technology for Device Assembly", Medical Device & Diagnostic Industry, September 1999.

10. Serenson, J.A., "Resin-Adhesive Bond Strength Data Eases Choice When Joining Plastics, Modern Plastics, Mid-November 1997, p. F4.