Cyanoacrylates
A Technical Discussion

Modern cyanoacrylates have enjoyed a universal appeal among woodturners. Commonly referred to as alkyl-2-cyanoacrylate, SuperGlue, or CA-Glue, these adhesives have been hailed as miracle products and have significantly impacted the art and craft of woodturning.

 Super Glues and their cronies, the accelerators and debonders

Overview: Monofunctional SG/CA's were first discovered in 1942 during World War II, but were not patented until 1949. In 1951, scientists at Eastman Kodak were working on thermal polymerisation and accidentally discovered the rapid ambient-temperature cure and superior adhesion properties of super-glue. While working on a freshly prepared monomer, the Kodak scientists discovered that the glass prisms of their refractometer had become tightly bonded. Extensive research thereafter found many different types of substrates bonded in a similar tenacious manner. The first viable production process did not evolve until 1954. Subsequently in 1958, Eastman #910 debuted, the first commercially available product.


Properties

2-cyanoacrylate ester monomer bases are all thin, crystal clear liquids with viscosities ranging from 1 - 3 mPa’s (=cP). Because the base monomers are very thin, stabilizers, thickeners and other property-modifying additives (soluble polymers and plasticizers) are used to alter the viscosity, physical characteristics, performance and elastification of the formulations.

CA adhesives are available in numerous viscosities ranging from near water thin wicking grades, to thixotropic (fluids that are gel-like at rest, but fluid when agitated) gels that range from 20,000 to 50,000 mPa’s (=cP). These super thick versions are used with large gaps, or when a longer setting time is required for proper application.

To illustrate the viscosity ranges available, it is useful to compare some common products and their viscosities as measured in centipose (cP).

Water-thin CA's have the ability
to wick into very tight cracks and fissures

Water @ 70.F 1 - 5
Blood 10
SAE 10 Motor Oil 50 - 100
SAE 40 Motor Oil 250 - 500
Honey 2,000 - 3,0000
Hershey's Chocolate Syrup 10,000 - 25,0000
Heinz Ketchup 50,000 - 70,000
Peanut Butter 150,000 - 250,000

Polymerisation of Cyanoacrylates

Cyanoacrylates are one-part adhesives that polymerise in three basic steps: initiation, propagation and termination. The polymerisation reaction continues until all available monomer has reacted, or it is terminated by an acidic species. 2-cyanoacrylate polymers spontaneously form via anionic reaction mechanisms when their liquid precursors, or monomers, come into contact with a weakly basic surface to form high molecular weight thermoplastic material.

Because cyanoacrylates produce a thermoplastic type of material, most CA’s have poor temperature resistance. Specially formulated CA products feature better high temperature performance than traditional CA formulas. Trivial atmospheric or surface moisture is usually sufficient to initiate the curing reaction.

The monomers will also polymerise free-radically, in the presence of prolonged light, or heat. The great utility of these adhesives, comes from the electron-withdrawing character of the groups adjacent to the polymerisable double bond.

The high reactivity (cure rate) of cyanoacrylates and their polar nature, enables the polymers to adhere very tenaciously to numerous substrates. Low ambient humidity and/or acidic groups on the substrate surfaces will slow or inhibit the cure reaction. To extend the shelf life, free-radical stabilizers such as quinines, or hindered phenols are sometimes added.

Thin CA was used on this Water Oak hollow form to stabilize some hairline checks
that radiated through the punky and wormy areas


Types of Cyanoacrylates

Cyanoacrylates are available in Methyl, Ethyl, Isopropyl, Allyl, n-Butyl, Isobutyl, Methoxyethyl and Ethoxyethyl esters, with various setting characteristics and rheological properties. However, the Methyl and Ethyl esters dominate the commercial industrial cyanoacrylate market.


Variables in the Curing Process

The vinyl structure of 2-cyanoacrylates makes them prone to spontaneous polymerisation. The chain propagation can be initiated by ionic, or radical mechanisms. The rate of polymerisation depends on temperature, humidity, light and the presence of accelerators like peroxides, or bases.

In addition to polymerisation, 2-cyanoacrylates undergo reactions typical of vinyl compounds, such as addition. CA polymers are soluble in N,N-dimethylformamide, N-methyl-pyrrolidinone and nitromethane. Typically, surface moisture is sufficient to initiate the curing process.

Thick CA was used on this Ambrosia Maple
hollow form to join the top flared neck portion
to the main base body

Although full functional strength may be developed rather quickly, the curing process continues for at least 24 hours, before full chemical and solvent resistance is achieved. The specific substrate used also affects the overall cure rate. The following examples demonstrate the fixture time (time required to develop a shear strength of 0.1 N/mm² (14.5 psi) according to ASTM D1002) achieved with Loctite #401 at 22°C and 50% relative humidity.


Substrate Fixture Time in Seconds
Balsa Wood 2 to 5
Oak Wood 90 to 180
Chipboard 30 to 90
Paper 1 to 10

The width, or gap of the bond-line also affects the overall cure time. Thin bond-lines cure faster than wide bond-lines. Ambient relative humidity can significantly affect the cure speed. High ambient humidity (60%+) will cause faster curing than lower (20%) humidity levels. If the CA used is of higher viscosity, or the filet gap is large, accelerators will improve the overall cure speed. However, over-application of the accelerator may reduce the ultimate bond strength significantly.


Manufacturing of Cyanoacrylates

2-cyanoacrylates can be manufactured by many different methods. The basic method to manufacture 2-cyanoacrylate esters involves preparation via the Knoevenagel condensation reaction – (the alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst, to form a low molecular weight polymer). The resulting polymer slurry is acidified and water is removed. Next, the polymer is cracked and redistilled at high temperatures onto a suitable stabilizer combination to prevent repolymerisation. Protonic, or Lewis acids are typically used in combination with small amounts of a free-radical stabilizer.

Although the methods and processes have continually changed and evolved over the years, this is the standard method to manufacture these esters. One recent and significant advancement in the manufacturing process is a continuous process where the condensation is carried out in an extruder. By-products are then removed in a degassing zone and the molten polymer (mixed with stabilizers) is cracked to yield a raw monomer.

Water-thin, or wicking grade, CA's can be used on natural edge bowls to stabilize the bark edge and prevent it from separating


Additives and Speciality Formulas

In addition to stabilizers, cyanoacrylates may contain thickeners, colorants, tougheners and other property enhancing additives. Once cured, CA polymers are clear (unless coloured), hard thermoplastic resins with high tensile strengths. However, they tend to be somewhat brittle and typically have low to moderate peel and impact strengths. To reduce brittleness, fillers such as polymethyl methacrylate (PMMA) are sometimes incorporated into the formulas.

To increase the toughness, some manufacturers use various dissolved elastomeric materials. Known as rubber-modified products, these specialty formulas offer improved overall toughness. Recent advances have led to flexible 2-cyanoacrylate formulas, which remain somewhat flexible when cured.

These types of esters are particularly useful to turners when bonding dissimilar materials like stone/metal and wood. The dissimilar expansion and contraction rates of these materials can cause subsequent failure of the bond when using traditional cyanoacrylates that feature less flexible, or brittle bonds.

Starbond offers a unique CA that provides an intense black color in a thin base

Insta-bond's flexible CA product offers improved flexibility when cured


Kickers and Accelerators

Although some water thin CA formulas cure almost instantly, numerous surface conditions (highly acidic substrates, or low ambient humidity) can alter the overall cure time significantly. In addition, thicker CA formulas typically take several minutes to cure. To speed the curing process, accelerators or "kickers", which contain amines as an active ingredient, are available from most manufacturers. These accelerator solutions are typically applied by spraying, dipping, or wiping the accelerator onto an adjoining substrate, or over an exposed CA filet.

The specific base used varies by product and manufacturer, but may include Isopropanol, Heptane, Acetone, Electronic Grade Acetone, Perfluorocarbon, or Propylene based Glycol Ether. In addition to speeding the curing of cyanoacrylates, accelerators also work to effectively clean and degrease the substrate prior to application of the liquid CA.

Foreign debris, excessively oily, or overly wet surfaces can reduce the overall bond strength of the cured CA. Some exotic timbers contain natural accelerators and may prematurely cure the adhesive during assembly of components. Cocobolo (Dalbergia Retusa) is an example of a timber which can cause premature curing of cyanoacrylate adhesives.

Accelerators, or kickers, are available
in pump sprayers, aerosol sprayers or bottles


Blooming and Frosting


If you’ve worked with CA products before, you may have occasionally noticed a white haze, or frosting on your CA during its cure cycle. This phenomenon is known as blooming or frosting.

Blooming is caused by high ambient humidity levels, or improper application of accelerators. Overuse of accelerators (when an excessive amount is used) can cause a violent curing reaction, causing the CA to frost, or bloom when cured.

In addition, placing the item into a closed container, prior to it being fully cured, can cause this problem. Closed containers prevent the vapours from dissipating during the cure cycle and allows them to redeposit on the surface of the CA. To prevent this problem:

  • Use low odour or low bloom products. These are specifically formulated to reduce blooming and frosting.
  • Reduce the ambient studio humidity. Dehumidifiers should be set to the range of 40 – 50% for best results.
  • If you do not have a dehumidifier, choose low humidity days for your CA work.

Bulk accelerators can be applied with inexpensive aerosol spray units, like this Preval disposable pressurized sprayer

  • Cross-ventilate your workspace to dissipate curing vapours, before they can resettle onto the CA area.
  • Reduce the amount of accelerator, or kicker used.

Tip Clogging

It’s quite common for cyanoacrylates to clog the tips of their applicator bottles. One or more of the following procedures can effectively mitigate this.

  • When you have finished using the CA, allow sufficient time for the CA left in the tip to return to the bottle before replacing the cap. Sometimes, a sharp rap on the counter will assist in clearing the tip of the bottle of uncured CA. Always wear face and eye protection when rapping the bottle to clear the tip.

To prevent tip clogging: Use a small jar half filled with acetone. Drop used tips into the jar at the end of the day. Replace with dry, clean tips from the jar. Rotate each time you use the CA and you will NEVER have a clogged tip again!

Replacement nozzle tips are available from most manufacturers and often include needle-point extensions for applying the CA into very tight spaces

  • Wipe the exterior of the tip with kitchen papers before storing the bottle away.
  • Do not touch the tip of the bottle onto a surface that has been sprayed with active accelerator.
  • Switch the used tip with the cured, or uncured CA for a spare clean tip. Place the used tip in a small jar of Acetone until it is needed again.
  • When another fresh tip is needed, use tweezers to swap the clean tip in the Acetone jar with the used tip. Allow the tip to fully dry before replacing the tip on the CA bottle.

Limitations

Although cyanoacrylates can be effectively used in numerous situations and with a wide variety of substrates, they do have their limitations. CA’s should not be used for immersion waterproofing of containment vessels, such as the inside of flower vases. Under prolonged immersion, the bond strength of CA is weakened and may fail.

Because most CA bond-lines are quite thin, there is little allowance for stress to be relieved through the cured adhesive. In particular, differing rates of thermal expansion between two different substrates may cause bond failures when CA’s are used. The ultra fast curing properties of CA’s requires precise positioning of components prior to application. With thinner viscosity grades, or where an accelerator has been used, repositioning a part or component is usually precluded.

When cured, cyanoacrylates form a solid, acrylic polymeric layer between the adjoining substrate surfaces. High temperatures can cause the bond to weaken, or fail completely. If the subject piece will be exposed to elevated temperatures, choose a CA formula that is specifically engineered to perform in elevated temperature environments.

When sanding CA filets, filled voids, or bond-lines on the lathe, be careful of generating excessive surface heat. Excessive surface temperatures may adversely affect the ultimate strength of the cured adhesive, causing weakening or subsequent failure.

To prevent CA's from prematurely curing in the bottle, they must be properly stored. The cut away botte (left) prematurely cured with 1/4 of the CA still left in the bottle. The unopened bottle (right) is kept in the freezer until ready for use.


Storage of Cyanoacrylates

Most CA’s have an average shelf life of 6-12 months once opened. Unopened bottles can be stored in the freezer for an extended period of time. However, once a CA bottle is opened and exposed to atmospheric moisture, different storage procedures are required.

Unopened Containers

  • Should be stored in the freezer for longest shelf life.
  • Before using, allow the bottle to fully come to room temperature. I prefer to allow the frozen bottle to sit overnight on the bench before using it, to insure that it has reached uniform ambient temperature.

Opened Containers

  • Do not store opened bottles without their caps, unless you live in areas that routinely have very low humidity levels year round. Exposure to high humidity can cause premature curing of the CA in the bottle.
  • If you purchase your CA in bulk containers and transfer them into smaller applicator bottles for use, insure that these bottles are manufactured from polyethylene for best storage results.
  • Do not store opened containers near your accelerator. During the summer months, high heat can cause accelerator vapours to leave the pump spray unit, causing premature curing.
  • Do not store opened bottles in the freezer. When removed, condensation may develop inside the bottle causing premature polymerisation.


Other Significant Uses

Vacuum pumps can be used to store opened CA bottles for an extended period of time. The bottles are placed in a mason jar and a vacuum is pulled on the lid. This removes ambient moisture, extending the shelf life.

One of the most important uses of cyanoacrylates worldwide is by law enforcement professionals. Fingerprints are one of the most valuable types of physical evidence that may be found at crime scenes. Cyanoacrylates have revolutionized the field of latent fingerprint recovery by assisting with the preservation and identification of fingerprints.

The super glue fuming method was discovered almost simultaneously in the United Kingdom, Canada and Japan. First employed by the Criminal Investigation Division of the Japanese National Police Agency in 1978, the super glue fuming method of latent fingerprint recovery is now used by nearly every major police and law enforcement agency worldwide.

Commonly referred to as the “cyanoacrylate or super glue fuming method”, small amounts of cyanoacrylate are heated in a sealed chamber. The resulting cyanoacrylate gases attach themselves to the residual components and organic compounds found in fingerprints.

Cyanoacrylate fumes attach to the trace amounts of amino acids, fatty acids and proteins found in latent fingerprints and together with the moisture in the air, react to form a visible, white material on the ridges of the fingerprint. This enhanced latent fingerprint can then be photographed or further studied.

Another significant use of cyanoacrylates is in the medical field. Specially prepared cyanoacrylate formulas are used in numerous medical procedures, including heart surgery, eye surgery, ear surgery, cerebrospinal fluid leak repair, skin closure, emergency wound closure assistance in military battlefield conditions (Vietnam saw widespread use by MASH units) and numerous other areas.

Thin CA, in conjunction with crushed turquoise, was used for the inlay on the side of this hollow form, filling the large void left by a bark inclusion.

However, the common industrial types of cyanoacrylates are not the same formulas used in the medical profession. Typical industrial cyanoacrylates are usually ethyl, or methyl based esters and can produce significant heat during polymerisation. Medical grade cyanoacrylates by contrast, are typically n-butyl-ester, isobutyl ester, or octyl ester based and are specially prepared to cure with as little heat during polymerisation as possible. Excessive heat or rapid cure during polymerisation can lead to tissue necrosis.

The same hollow form, showing additional areas where thin CA was used to stabilize crushed turquoise.

Inside the body, these special medical grade cyanoacrylates are bacteriostatic, painless to apply when used as directed and eventually break down harmlessly in the tissue. Once cured, the cyanoacrylate is essentially inert. N-Butyl ester based surgical cyanoacrylates are rigid when dry, but provide a strong bond.

Octyl products offer increased flexibility when dried, but reduced the bond strength. Medical doctors and researchers are constantly pioneering new ways to utilize cyanoacrylates in delicate surgical procedures.


Safety Concerns When Using Cyanoacrylates

CA’s can immediately bond skin tissue and if present in large enough quantities, may cause severe burns when curing. If using CA’s, protective gloves should be worn, in addition to protective eyewear and a suitable respirator with the appropriate vapour cartridge installed. When using CA’s for gap or void filling, allow sufficient time for the CA to fully polymerise before turning the lathe back on. On deep pockets, this may mean one hour or longer, up to a full 24 hours. If the lathe is switched on too soon, the centrifugal force will cause the uncured CA to spray out, possibly into the turner’s face or eyes.

CA de-bonders are used to remove
cured CA from skin and other surfaces

CA adhesives produce sharply irritating odours to the eyes and mucous membranes. CA’s should only be used in areas that offer excellent cross ventilation. If your studio does not offer good cross ventilation with fresh air, you should wear a good quality respirator with the appropriate vapour cartridge installed. Sensitive individuals should only use low odour, or low bloom products.

If you happen to get a bit of CA on your skin, de-bonders are available from most manufacturers. These contain Acetone, or gamma Butyrolactone typically and will soften, or remove cured CA from skin and other surfaces. De-bonders will damage most finishes and cannot be used to thin CA’s. Although de-bonders can also be used to separate bond-lines, it is a slow process because the de-bonder has to work its way through the entire bond-line. A future article will cover using CA in your studio and the best ways to apply this adhesive in a woodturning environment.


Safety Note: Always follow all manufacturers safety instructions before working with your lathe, or any of the tools or products you may use. If you are unsure about any operation, obtain competent professional instruction before proceeding. Use and wear all necessary safety devices during turning and observe safe woodturning practices to prevent accident or injury.


Steven D. Russell is a professional studio woodturner, teacher and writer. He has written numerous articles for international woodturning magazines, which have been published in more than 78 countries around the world. Steve has demonstrated in numerous cities across the United States. His studio, Eurowood Werks, specializes in bowls, platters and hollow forms with unique visual and tactile treatments.

Steve is also the current and founding President of the Lone Star Woodturners Association, Inc., an AAW member chapter. The LSWA is a 501(c)3 non-profit educational organization dedicated to teaching and demonstrating the art and craft of woodturning.

Steve is also a featured writer for the Guild of Master Craftsman's "Woodturning" magazine, published in London England. Woodturning magazine is the world's leading magazine for woodturners. Look for his articles covering technical topics, or project based articles in an upcoming issue.