Instant, Epoxy and Dispersion Adhesives
Text: Josef Blažek
To continue our discussion on modelling chemistry and tips for beginners (and maybe advanced modellers will find out something new, too), we will look at more types of adhesives that are commonly used in the hobby.
We will then focus on cyanoacrylate (CA) glues, their accessories, epoxy adhesives and dispersive type glues. We will discuss their differences and their composition, how they work, and their properties that make them suitable for different situations. We’ll demonstrate specific glue types, with products from Bolt due to their wide range of products and popularity among modellers. However, the characteristics we will be discussing are applicable to products from other manufacturers as well.
These are also known as instant glue and often shortened to CA, and as the name implies, these are rapid curing systems that harden on contact with the bonding surfaces. Practical applications of these types of adhesives are far and wide, to include such materials as plastics, glass, metal, fabric, paper and wood. Bonded materials can be dissimilar. They can easily be obtained from hardware and hobby stores and pretty much any place that caters to the handyman. To begin, we’ll discuss the principle on which these glues work. They are based on an unstable compound, which in this case is ethyl cyanoacrylate. The compound reacts with water (or moisture in the air) to quickly harden. To get a little more technical, the chemical reaction with water leads to reactive sites at the end of the molecular chains. These reactive sites then connect to each other under the influence of the ongoing reaction, which leads to the formation of a dense network of interconnected molecules. This process is actually visible to the naked eye. If there is a sufficient quantity of the glue applied, the gradual hardening can be observed until the adhesives has hardened as a whole. On the contrary, if we spread a small amount of glue, the whole process of gradual solidification is extremely dynamic, which is caused by the rapid penetration of moisture from the air (that is, water) into the glue. This penetration of moisture into the glue fundamentally affects the speed of setting, which, from a modeler’s perspective, explains why open containers of glue gradually set (moisture penetrates the glue after initial opening and consequent initiation of the reaction).
Storage and Shelf Life of CA Glues
In the first two parts of our series focusing on modelling chemistry and tips for beginners and advanced modellers alike, we presented the basic, readily available and commonly used types of glues that can be used for plastic modelling. All the mentioned types described in the first and second parts of our series have their strengths and weaknesses, and the use of all the mentioned types of glues gives the plastic modeller the ability to deal with constructing all conceivable assemblies from various materials that they might come up against. In the next part, we will focus on tools and aids suitable for applying and working with the types of glue presented thus far.There are a lot of myths in the modelling community about how to store and how not to store instant glues in order to extend their life. Unopened adhesives have an extended shelf life due to the protective gas filling, but this does not mean that their shelf life is infinite. In general, higher storage temperature, higher exposure to UV radiation (light) and higher humidity always have negative effects on the overall life of cyanoacrylate adhesives. To give a specific example illustrating the idea of the dynamics of the shortening of the shelf life, when storing instant glues at an ambient temperature of about 30 degrees Celsius, the shelf life will be shortened by some 60 to 70%, which is a relevant decrease of the overall usability of the product. It follows that the ideal storage environment is dry, cool and dark. You have a refrigerator or a freezer, as does every household, and this is ideal for storing CA. To the contrary, these recommendations do not apply after opening the glue container and exposing the contents to the local environment. Storing opened adhesives in the refrigerator or freezer in order to extend their life is a pure myth. The aforementioned locations provide high levels of moisture, relatively speaking, which within the conditions of the way the glues work, impact the shelf life of the material negatively. On opening, the container with the glue loses its protective gas layer, and the moisture can now directly activate the chemical reaction of the adhesive. One way to combat this might be to store the containers in plastic Tupperware with sealable lids that will restrict the amount of moisture getting to the glue, and combine this with storage in a cool, dry dark place. But in the case of storing the container in a sealable containers, it is necessary to consider the unavoidable rise in the glues temperature to that of the local environment. The shelf life of the glue can be extended, but the gradual thickening of the liquid is unstoppable. With this in mind, it makes sense that the size of the package and quantity of glue that it contains, are a factor corresponding to shelf life. The modeller should acquire bottles of CA that they can use in the course of a couple of months after first opening it. It is also important to store the glue away from other materials, especially those that act as activators. The glues are generally compatible with one another, so that if one bottle of CA becomes thick, it can be thinned with thinner stuff to achieve a compromised viscosity that is acceptable to the user.
After opening the adhesive, its lifespan in the
above-mentioned storage in the freezer or refrigerator will be quite variable,
depending on the quality of the seal and exposure to moisture. However, if after
opening, the glue is stored in a typical modelling room at a temperature
between 18 and 20 degrees Celsius and always closed tight after use, a shelf
life of about three months can be expected.
Cyanoacrylate Glue Vapors
After the application of cyanoacrylate adhesives and their subsequent hardening, effects of the inevitable vapors can often be seen, which resemble frost. Such evaporation has negative effects when it is released into difficult to reach places on the model, such as the inner parts of the canopy, especially when they are displayed closed. Why does this phenomenon arise and can it be eliminated or reduced? During the hardening of the glue, the natural and gradual evaporation of ethyl cyanoacrylate and its reaction with humidity occur. These vapors thus harden as a result of the reaction, which results in the formation of micro particles of hardened instant glue. A larger amount of these hardened micro particles settle in the vicinity of the applied glue and the frosting effect becomes obvious. There are some options to minimize or optically eliminate this phenomenon. It is easiest to use only a small amount of glue to minimize the fumes. Another option is the use of special alkoxycyanoacrylates, which have a reduced evaporation rate. In general, it is good to avoid closing the area where we apply the CA adhesives early, so that there is room for evaporation. When drying, it is a good idea to rotate the parts resulting with the glue at the top, to allow vapors to escape into space and prevent them from spreading over the surface of the model, which would happen when they are turned over and the fumes have access to them as they escape. Since the release of vapors can occur even after some time, the entire glued joint can be sealed against vapor release by applying an overcoat, which ensures coverage of the release adhesive even after it has hardened and thus minimizes negative effects on the model. Despite all our efforts, however, a situation may arise when unwanted vapor escape occurs and we are forced to remove these negative consequences of CA use. In most cases, it is sufficient to clean the affected areas with a dry brush or cotton swab (on clear parts). Alternatively, you can use the thinner embedded in the surface, which, however, must not dilute the base color of the given parts.
Basic Tips for Cyanoacrylate Glues and their Use According to Composition and Viscosity
A large number of cyanoacrylate adhesives from various manufacturers are available on the market today. As was mentioned at the beginning of today's chemistry class, for the sake of clarity between the basic types, we will focus on Bolt brand products. At the same time, the described properties and use can be freely applied to various products of other manufacturers with the same or similar consistency and composition.
BOLT! CA BOLT THIN
The classic representative among cyanoacrylate adhesives are the thin variety, which are the most accessible type of CA in terms of their consistency. Their use is very universal and they represent the ideal ‘entry’ type for working with instant glues. They can be applied in the usual way, i.e. by applying them to one or both glued parts, which are then pressed together. Alternatively, the procedure where the parts are pressed together and the glue is applied along the joint can also be used. Here, however, it is necessary to maintain a gap between the parts to provide the glue someplace to wick into. The minimum size of such a gap depends on the viscosity of the thin instant glue, but in general it needs to be at least around 0.03 mm, which in modelling practice is quite small. Thin instant adhesives are especially suitable for gluing non-porous materials, and wood and paper should generally be avoided, where the glue will quickly soak into the material, causing insufficient amounts of CA in the joint its subsequent instability and fragility. The reaction time of thin second glues is generally very short and varies between 1-3 seconds, which restricts the possibility moving and manipulating glued parts.
CA HOT BOLT ULTRA THIN
The follow-up type to thin instant adhesives is the so-called ‘super fast’ adhesives with an accelerated reaction time. Their application differs from regular thin glues in that it becomes impossible to apply them to the abutting surfaces of the parts and then pressing them together, because during the necessary handling, the glue reacts with the surrounding environment and hardens virtually immediately. So, super fast instant glues need to be used by applying drops at points of contact of the target components, which can be used to advantage, for example, in the joints between main parts of the model. Complete curing of the applied glue is generally within one second of application. Thanks to the speed of curing, the disadvantage of using the adhesive in porous materials is eliminated, since the adhesive sets up faster, before it is absorbed into the surface and thus remains in the glued joint, which ensures the required strength of the joint. For the sake of completeness, it is good to mention that the presented representative of super fast instant glues is not interchangeable with other types of cyanoacrylate glues.
CA BOLT MEDIUM
Secondary adhesives of medium viscosity are very versatile in their use. This type of glue is suitable for most materials (even porous ones) and can be used for most common situations encountered when assembling plastic models and their accessory, dissimilar items. Their application is possible in one of two ways, as in the case of thin instant adhesives. They can be applied to one or both bonding surfaces, which are then pressed together. The reaction time is longer compared to thin glues and varies between 5 and 15 seconds. This extended curing interval gives us the possibility of subsequent manipulation and correction of part positions. This can be used to great advantage when gluing more complex or larger units, where it may be necessary to achieve optimal alignment. The second typical method of application is the drip method, which, however, due to the thicker consistency of the glue compared to thin glues, requires a gap of increased size to a minimum of 0.1 mm. However, its viscosity is of a universal consistency that can be mixed with both thin and thick types of instant glues.
CA BOLT THICK
Thick cyanoacrylate adhesives (often described as gel types) are characterized by having the highest viscosity of the discussed basic types of CA adhesives. This property allows them to be used effectively in the case of gluing porous materials and at the same time the possibility of joining glued surfaces with a greater clearance of up to 0.5 mm. When gluing parts with greater clearance, the curing time of the glue is significantly extended and can be sped up using accelerators, which will be discussed in the next part of this article. Thick CA adhesives are best applied in the classic way, where they are applied to one or both mating surfaces, and then the components are pressed together. As already indicated, the total reaction time is significantly longer, which carries with it the added bonus of manipulation time and fine adjustments. The use of thick instant adhesives is very suitable for small parts, when after applying the glue to point(s) of contact, time is allowed for a clean and precise connection. Due to their composition, this class of adhesive is normally mixable with thinner types (medium and thin adhesives). A longer curing time is associated with the negative impact stemming from a greater amount of fumes visible emanating from the area of glue application. These vapors, which we have already described, can be produced for a long time with this type of glue, and it is advisable to take this manifestation into account during future assembly steps, especially with regard to clear parts of the model.
CA BLACK BOLT
It's been a few years since the modelling world was hit by the phenomenon of the so-called ‘black instant glues’, which sand very well. In most cases, it is not an admixture of a special miracle ingredient or black pigment, but the addition of rubber (if its presence is desired) to the glue. This special type of cyanoacrylate adhesives stands out, in addition to its visibility on most common surfaces, due to its elasticity. It is therefore particularly suitable for flexible materials, which are very rare in plastic modelling. Examples of such materials include the aforementioned rubber parts, softened and foamed plastics or composite materials, and several others. In general, these are joints where it is claimed that the flexibility of the glued materials and the durability of the glued joint under stress are maintained. Such connections are not typical for static plastic modelling and the use of such special glues is not generally of any great advantage to plastic modellers.
Specialized Accessory Items for CA Adhesives
Since the mere application of cyanoacrylate adhesives and subsequent gluing is often not enough for the ideal desired results, there are special accessories for working with instant adhesives. The basic products are accelerators and, conversely, debonders of already hardened glue The following takes a closer look at these.
BOLT UP Accelerator
Accelerators are used for instant or very fast curing of CA adhesives, even in thick layers. They have a thin consistency and their application is via a swab, wire tip, dispersal sprayer or a brush. The reaction speed on contact with the glue is almost immediate, which is appreciated when there is a need to quickly cure a large amount of glue between parts with visible clearance or when the glued surfaces are compressed under ambient stress. Immediate bonding of the contact surfaces between the parts is achieved, and the need to further manually press the parts together becomes no longer necessary. When choosing an activator, it is good to pay attention to the suitability of its use on plastic parts.
BOLT OFF Debonder
In modelling practice, it is almost impossible to always achieve gluing of parts without unwanted and visible traces of glue around the application. Furthermore, incorrect positioning of the parts can lead to a need to separate the attached parts and start over. All these cases can be solved with the help of cyanoacrylate adhesive removers. Simply applying this liquid to the glued joint will, over a few dozen seconds, help to separate wrongly adhered parts, but it is always better to take an active approach to cleaning and ungluing parts and removing the glue using a debonder. Probably the most suitable tool for cleaning parts from cyanoacrylate glue is the use of cotton swabs dipped in the remover. Traces of the glue are removed by gently rubbing the area with a moistened swab until we are satisfied with the appearance of the fixed area. Since the glue dissolves under the cotton swab and clogs the cotton tip, depending on the extent and amount of glue to be removed, it is advisable to use several applications with clean swabs, as required. For areas that are difficult to access or areas with a broken surface, it is appropriate to use fine brushes, that have been relegated specifically for this purpose after serving out their usefulness as paint applicators. Inexpensive brushes from art supply stores or hobby shops will suffice for these needs. When choosing a cyanoacrylate adhesive remover, or debonder, it is good to check to make sure that the type doesn’t etch the plastic of the model, as some will.
Epoxy Type Adhesives
Epoxy glues, which consist of two components, are also included among the glues that are commonly found in plastic modelling. The principle and application of this class of adhesive is very simple. After mixing both components in the specified ratio, usually one to one, a chemical reaction occurs, the observable start of which begins after a few dozen seconds. Subsequently, the mixed components harden very quickly. Epoxy glues are especially suitable for hard materials, which underlines the suitability of their use in plastic modelling. Their biggest advantage is that after curing, they do not leave visible traces in the form of fume production in their local vicinity, as in the case of instant glues, and are thus a big help when gluing larger models, especially if they are made of a polyurethane or laminate. Such types of models cannot be glued with solvent type glues, which we detailed in the first part of this article, and epoxy two-part glues are a suitable alternative for gluing together large surfaces and eliminating the shortcomings of cyanoacrylate glues. Another good use of these types of adhesives is joint reinforcement along invisible join lines, allowing any required thinning of plastic parts in preparation for the installation of aftermarket accessory sets. A general shortcoming is their apparent softness compared to hard cyanoacrylate joints, and therefore they are sand poorly. Currently, there are already special two-component epoxy adhesives with additional additives on the market that can minimize these shortcomings.
Dispersion Type Adhesives
As in the previous cases, we will explain what dispersion adhesives are and what they are suitable for in plastic modelling. A common mistake in the definition of dispersion adhesives is that they are a solution consisting of a polymer in water. In fact, it is a mixture of polymer with water, whereby the aforementioned polymer is not dissolved, but only perfectly dispersed in the resulting emulsion. Dispersion adhesives have a milky white color that gradually fades to clear as it hardens. The white color is therefore not caused by any pigment, but by small polymer particles that scatter light and create that ‘milky effect’. Many dispersion adhesives can be diluted as needed with water (non-waterproof dispersion adhesives such as those produced by Herkules) and thus create a liquid that can be embedded into porous surfaces to attach individual details. A typical example is the simulation of natural surfaces in the creation of dioramas. The opposite is the case of dispersion water-resistant adhesives, which have a substantially higher dry matter content of the polymer and are therefore not as thin as adhesives that can be diluted with water. Dilution of these adhesives is only possible to a small amount, to some 5% and always only with distilled cold water; otherwise precipitates will form. At the same time, if we let such diluted glue stand, the water will begin to separate. A higher dry matter content of the polymer then means a higher strength of the joints, and the adhesives thus have up to 3 times greater strength (as in the case of, for example, Herkules vs. PERFECT G Express). Another difference compared to glues with a lower dry matter content is their hardness and the possibility of sanding the hardened glue. If we use glues with a lower content of polymer solids, when trying to sand, the glue starts to chew up because it is soft. On the other hand, dispersion adhesives with a higher proportion of polymer solids are significantly harder after curing and can be sanded. Here, however, it is important to note that it is better to use coarser/sharper sandpapers and to minimize friction, because high friction creates a higher temperature, which causes the glue to soften after setting, despite its high percentage of polymer solids (i.e. D3/D4-based glue).
PERFECT G Express glue
An example of dispersion PVAC glues is Bolt Perfect G Express. The specific properties of this glue make it suitable for porous materials such as wood or paper, and it is therefore useful mainly for the needs of creating dioramas or for attaching accessories to models of combat equipment. The benefit of this type of glue is mainly the shortened curing time compared to classic dispersion glues (for example Herkules, Tamiya Craft Bond, etc.), which enables accelerated creation and thus saves invested time. Specifically, the complete drying time is between 5-15 minutes, depending on the amount of glue and the extent of the glued joint. As with other dispersions, the glue is white in its liquid state and clear after curing. The high dry matter polymer content of the in this type of glue means above all the ability of the glued joint to be stronger. Such a property is suitable for the use of gluing clear parts on aircraft models. The glue is thus a good compromise for those who are afraid of damaging clear parts when gluing them with solvent glues or, on the other hand, are afraid of the fumes that are released from cyanoacrylate glues. Gluing with a dispersion adhesive is not as strong as compared to the mentioned types of adhesives, but if we choose a suitable dispersion adhesive with a higher polymer dry matter content, we will also achieve good bond strength.
In the first two parts of our series focusing on modelling chemistry and tips for beginners and advanced modellers alike, we presented the basic, readily available and commonly used types of glues that can be used for plastic modelling. All the mentioned types described in the first and second parts of our series have their strengths and weaknesses, and the use of all the mentioned types of glues gives the plastic modeller the ability to deal with constructing all conceivable assemblies from various materials that they might come up against. In the next part, we will focus on tools and aids suitable for applying and working with the types of glue presented thus far.
Winner of international IPMS competitions in Master categories and multiple Czech Republic IPMS champion. Author of articles in international magazines with worldwide reach (Military Illustrated Modeller, Cocardes International, ModellFan, Modelář). Co-author of modeling startup Plastic Invasion.