Application of α-cyanoacrylate Adhesives (2)

5. Application process

When using α-cyanoacrylate for bonding, the material gap is required to be between 0.01-0.08 mm. If it is 0.2 mm or more. Need to add tackifier, otherwise it will extend the curing time and reduce the bonding strength.
The surface of the adherend is generally wiped with an organic solvent such as acetone to remove grease or release agent. It should be noted that the presence of a salt-based substance on the treated surface accelerates the curing of the adhesive, and the presence of an acidic substance can cause a slow curing rate and must be thoroughly cleaned.

The higher the polarity of α-cyanoacrylate, the higher the bonding strength of the material. In general, the strength of bonded metal is highest; neutral glass and rubber tend to damage the material itself, while the glue layer remains intact; the glued plastic is inferior to metal and rubber. The bonding strength of dissimilar materials is greater than that of the same material. Based on this, in order to solve the problem of instant dry rubber problems, metal and rubber are often glued or intermetallic lining with a rubber layer.

For difficult-to-stick materials such as polyethylene, polypropylene, Polyethylene, etc. must be pre-treated with special treatment. When coating, if the shape and material of the two adherends are the same, only one adhesive surface needs to be coated. If they are different, the adhesive surface on the upward, the surface with the smaller surface, the surface with slow curing, or the difficulty Penetration of one side of the coating, coating amount of 5mg/cm is appropriate.

After the glue is applied, the two adhesive joints are accurately attached, and no pressurization is required. Only the two adhesive joints can be in close contact with each other. Although α-cyanoacrylate can reach its maximum strength within 30 minutes, it will take several hours for the reaction to end or the cementing and anchoring to be completely completed. Therefore, the adhesive joints need to be maintained for several hours to one day after being attached. When the wood is glued, the maximum bonding strength can be achieved within 2 to 8 minutes.

After the glue is applied, the two glue joints should be attached immediately, and the tensile strength of the glue is drastically decreased over 30 seconds ^-50 seconds. Operating environment: The humidity is 50-60% and the temperature is 15-350C. If the temperature is 70-100°C, the strength can be increased by 15%.

In general, the cure of alpha-cyanoacrylates is sensitive to various substrates, particularly to acidic surfaces that retard or slow negative ion cure. Surfaces are often treated with various alkaline surfactants such as dimethylformamide to overcome this.

In general, glass and rubber have the fastest adhesion speed, followed by plastics and metals. However, the adhesion speed to the living body, especially human skin, is faster. This is because the skin contains amino acids that promote the curing of the adhesive. Be careful not to let the glue come into contact with the skin during operation. Commonly used surface treatments (or primers) improve cure speed.

The special function of cyanoacrylate reacts rapidly with amino acids, enabling people to use their volatility to track the fingerprints of the perpetrators to help the reconnaissance officers detect and detect crimes.

6. Product examples

Alpha-cyanoacrylate monomers cannot be used directly as an adhesive. Plasticizers, tackifiers, stabilizers, etc. must be added to improve its performance. Take Eastman910 as an example, its approximate formula is as follows:
a methyl cyanoacrylate 90.7%
Quinol. 0.01%
Sulfur dioxide 0.01 to 0.10%
Polymethyl methacrylate 6.0%
Dimethyl sebacate 3.3%

(1) Alpha-cyanoacrylate high alkyl esters

As the number of carbon atoms increases, the bond strength decreases. The tensile bonding strength of n-butyl α-cyanoacrylate was 49% and 64%, respectively. As the number of carbon atoms of the alkyl ester increases, the volatility of its monomer gradually decreases, the whitening phenomenon is also suppressed, and the irritating taste is also greatly reduced.

(2) Alkoxy α-cyanoacrylates

The higher vapor pressure of the alkoxy cyanoacrylate allows the monomer to have an odor of acrylic type; it can be vaporized from an uncured adhesive layer, condensing in a white mist near the surface of the adhesive, contaminating the adherend, and Call this the "whitening phenomenon." Good ventilation has some improvement but it is not ideal. Therefore, a-oxyalkyl α-cyanoacrylates were developed and put on the market.

This type of monomer is essentially odorless, has a much lower vapor pressure, and the whitening phenomenon is greatly reduced or even eliminated. Curing quickly on metal or rubber and slightly slower on plastic. Modified varieties with faster curing speeds have been developed, and costs have also been reduced.

(3) Thixotropic cyanoacrylate.
The fumed silica treated with polymethyl methacrylate, silicone or other hydrophobic agent into α-cyanoacrylate is uniformly dispersed in the ball mill (pre-hydroquinone, sulfur dioxide and hydrazine II are added. Dioctyl acid) can produce a colorless, transparent, thixotropic cyanoacrylate. The adhesive can be used not only on the vertical surface, but also because it reduces the spatter and run-off of the glue, avoids contaminating the adherend and splashing the skin, and enhances the safety of use. It is very popular with users. , its impact strength and heat resistance

Also improved. This is used for assembly of components such as printed circuit boards and automotive industries.

(4) Medical cyanoacrylate.
The toxicity test showed that. A butyl cyanoacrylate and octyl ester are harmless to the human body and can be used for the adhesive bonding of soft tissues such as skin, organs, nerves, muscles, blood vessels and mucous membranes, and play a role in hemostasis, anastomosis and embolization. The United States, Japan, Western Europe, the Soviet Union, and countries and regions all have clinical experience. The number of carbon atoms in the ester group increases, the toughness increases, and the polymerization heat during polymerization decreases significantly, which is advantageous to the application of the eyes and liver.

The disadvantage is that the degradation rate in the body is slow and it is used to prevent tissue healing between the two tissues.

China has carried out a lot of research, development, clinical trials and application work on medical cyanoacrylates. Most of them are n-butyl α-cyanoacrylate and mono-n-octyl ester. 10-15 seconds after solidification of blood tissue fluid.

The α-cyanoacrylic acid 1.2 isopropylidene glyceride developed by the Department of Chemistry of Southwest Normal University is a rapid biodegradable human tissue adhesive and hemostatic agent. The compounded isoamyl α-cyanoacrylate was used in 194 traumatic surgical adhesive hemostasis or in place of surgical suture. The efficiency and wound healing rate were both 100%. The adhesive hemostasis was within 60 seconds. Less, the wound will heal faster.

(5) Ultra-fast cyanoacrylate.
The curing speed of cyanoacrylate on different adherent substrates varies, which is related to the surface finish, polarity or solubility parameters of the substrate and the catalytic effect on monomer polymerization. The solubility parameters of methyl and ethyl α-cyanoacrylates were 11.6 and 10.2, respectively. The polarity of cyanoacrylates is rather high. For low polarity adherends, the number of carbon atoms in the monomeric ester groups tends to change and its polarity decreases to increase the bondability.
The acid and alkali of the adherend influence the curing rate. Because the anion curing of cyanoacrylate is the result of the basic catalysis, the acidic substance can play a slow or inhibitory role, so the presence of acid and alkaline substances can adjust the curing speed. In order to speed up the curing speed of the adhesive, primers (alkaline) can be used when applied on acidic surfaces. Currently available cyanoacrylate products that are essentially cured in 1 to 3 seconds. It must be noted that curing too quickly can result in brittle coatings. Ultra-fast curing cyanoacrylates are only used in special applications.

The use of primer Jing lost the significance of the single-component adhesive. It has been reported that the phase transfer catalysis of crown ethers, silane crown ethers, cupromatic ethers, and certain linear polyvinyl hydrazines can also accelerate the curing rate of cyanoacrylates, especially when wood or porous surfaces are bonded. More effective.

(6) Heat-resistant cyanoacrylate.
The allyl ester-based cyanoacrylate adhesive, first made by Eastman in the 1970s, is a heat-resistant product, and is produced by Permabond under the trade name Powerbond. The curing rate and bonding strength of these adhesives are similar to that of ethyl and propyl esters. The side-chain allyl group undergoes a second-stage radical crosslinking reaction at 177°C. The second cross-linked cyanoacrylate adhesive layer can withstand 120-1500C high temperature, short-term resistance to 200 °C.
It must be noted that during the second stage of curing, the linear first stage polymer will soften and the bonded parts must be supported. Addition of polyfunctional carboxylic acid anhydrides and borides to human cyanoacrylates may also improve the heat resistance and impact resistance of the adhesive layer.

(7) High-viscosity cyanoacrylate.
The low viscosity of cyanoacrylates is a big advantage, but they cannot be bonded to porous materials. Thickening it can overcome this problem. For example, the adhesive is prepared by mixing 7% acetylated hydroxypropyl cellulose with 93% alpha-cyanoacrylic acid ethyl ester at 40-60° C. and mixing for 2-4 hours. The viscosity is 37 Pa·s, the initial curing time is 10 seconds, and the curing time after storage for 6 months is 15 to 20 seconds. Adhesive strength of 15. 3MPa,

(8) High-strength cyanoacrylates.
In order to change the chemical binding force of the cyanoacrylate to the adherend, an isocyanate may be added thereto. For example, methyl α-cyanoacrylate, nitrile rubber, and polyisocyanate are prepared in a ratio of 100:100:10 to ZO (mass ratio) and dissolved in nitromethane solvent to form a 6-7% solution. It is sold in ampoules. Its Japanese brand is Biobond, which has been applied in medicine.

(9) Surface treatment agent.
The use of different surface treatment agents can speed up the curing speed of the adhesive, improve the initial tack force and the final adhesive strength, and even make difficult-to-stick materials such as polyethylene, polypropylene, etc. can be directly glued. Can also improve the durability of the glue, moisture, heat and other properties. It has been reported that the use of a 1,1,1-trichloroethane solution of a metal chelate compound phosphite as a surface treatment agent improves the adhesion of cyanoacrylates to non-polar materials such as polypropylene or polyethylene.


Source: 21st Century Fine Chemicals Network