My main objective for this profile paper is to find out which glue type is best to be used for electric motors. In this paper I shall try to look at most adhesives and at their properties, to match out and see which one would fit best in an electric motor. A practical experiment is also planned to try and produce some glue out of milk using its main protein - Casein. Different varieties shall be made of this glue and their strength will be compared with hobby glue, to see how strong this homemade glue actually is.
What is an adhesive?
An adhesive is a chemical compound that bonds two items together. Adhesives are unhardened solution of polymers. Adhesive sources may vary from either natural or synthetic sources. A natural adhesive would be the milk protein casein, whilst epoxy reactions account for the synthetic source. Adhesives have been used since the 1800's in Switzerland, when farmers found out that a sticky substance (casein) could be extracted from milk. Adhesives can be very strong, and are becoming increasingly important in modern construction and industry.
Adhesive bonding is a relatively new assembly technique where two materials are joint by an adhesive. The right adhesive must be chosen for the surfaces. Adhesives can be applied to any surface, even surfaces which differ in texture and material.
Adhesives are nowadays often used to replace traditional assembly techniques. Bolting and screwing would be examples of those, bolting and screwing may falter after a certain amount of time and using adhesives also removes the chance of metal fatigue.
Adhesives also carry less weight than the traditional bolts and screws, the joints also seem cleaner and adhesives also lets materials of different thickness to be assembled with each one another.
Though it brings a lot of positives with it, adhesives do have limitations, one of the limitations would be that the surface usually need to be treated beforehand before the glue can be applied. Also it is hard to inspect the quality of the bonded joints. The resistance of adhesives ( although improved significantly ) are still inferior when compared with bolts. Disassembly of parts which have been adheseved is more difficult, and one also has to keep tabs on the temperature and humidity when being applied.
Although having quite some negative aspects, specialists still prefer to use adhesives for specialized appliances which is attended with professional and precise vigilance.
What are the different types of adhesives?
Adhesives are usually organised on the way they stick to the substrate, weather they give a rigid or a flexible bond.  They are then split into two groups; the reactive and non-reactive groups. Reactive adhesives need to undergo a chemical reaction (with an activator or another adhesive) before they harden. Non-reactive adhesives do not rely on other chemical substances to harden.
The main adhesives are:
- epoxy adhesives
- acrylic adhesives
- UV cure adhesives
- Modified Silane (MS) Polymer adhesives
The most widely used structural adhesives currently are the epoxies. Epoxies are copolymers, which means that they are formed from two different chemicals. The two chemicals are referred to as the "resin" and the "hardener". The resin is usually made up of monomers, or short chain polymers which have an "epoxy" group at either end. 
The two main types of epoxies are the Two and One part epoxies. In the Two part epoxy, a chemical A is mixed with a chemical B before it is used. In a one part epoxy the two substances A&B are already pre-mixed. One part epoxies are usually activated by heat. Both one and two part epoxies have their positive and negatives. Usually it depends on the project and the materials involved. Two part epoxies have a longer shelf life than the one part version because it is only activated after the parts A&B are mixed. The picture on the right shows how a two part epoxy usually works. When working with two part epoxies, the user really has to pay attention to the ratio in which the parts A&B are applied, the parts should also be homogeneous to deliver the best result.
This picture here on the left is an example of a one part epoxy. The one part component epoxies usually rely on heat, or on a hardener for the reaction to start. One part adhesives only reach their full strength if they stay heated until they are fully cured. The negative part on one part adhesives is that they need to be stored in a cool place, because they will dry out and harden if not.
Epoxies adhere best with metals, glass and ceramics. Only two part adhesives are used to bond wood, because a one part might put the structure to flame. Though they are poor bonders of rubbers and plastics, epoxies are often used to bond FRP- Thermosets, which are polymers (epoxy or vinyl ester) reinforced with fiberglass or carbon.
Epoxy adhesives have both negative and positive properties, the positives usually outweighing the negatives. A definite positive is that it is very strong when compared to the other adhesives.  It also has a high elastic modulus, which means although it can take a lot of stretch it will still revert back to its original position. Epoxies also have a low shrinkage and they also emit little gasses when being cured, this might be important when working in controlled circumstances. Epoxies are renowned for their chemical resistance, and they also have a good heat ageing resistance. Not only can epoxies be used to bond surfaces, it can also be used as a gap filler.
Next on the list of most used adhesives are the Acrylics. Acrylics are mainly used because they have better elongation than the epoxies.  Acrylics do not need any lengthy preparation and that is also why some people favour it above the adhesives. Acrylics have a wider range of substrates to which they adhere than epoxies which is also a plus point. Bonding with acrylics is also currently the only way to bond with polyolefins; polyolefins are very hard to adhere because they have excellent chemical resistance and a low surface energy level. Acrylics are also well known for their heat and oil resistance.
There are two types of acrylic adhesives, pre- and no-mix acrylics. No-mix adhesives rely on the adhesive on one and the activator on the other. If the two do not come into contact with one another, it will not cure. This makes no-mix acrylics very practical, the activator and adhesives can be left there undeterred and still work perfectly after being left there for a period of time.
The pre-mix variation of the acrylics consist of two different components in one valve. Pre-mix acrylics, have a nozzle which spread the two parts evenly. As with the no-mix adhesive, the reaction only starts when A and B are mixed.
The main advantage of the acrylics is that they react at room temperature and that they react faster than other adhesives. They have a good thermal resistance which is why specialists like to work with them. Surface preparation is minimal when using acrylics which saves time and effort.
Next on the list is an adhesive which is quite new and has yet reached its maximum capacity, polyurethane. Polyurethane is an adhesive which can be best placed in between liquids and pasts, it isn't as liquid as water and not as thick as paste. Polyurethanes are renowned for their wide range of viscosity, which means it has a high resistance from a force trying to reshape it. Just to be clear, water has a low viscosity whilst honey has a high one. There are polyurethanes which are as thin as water, whilst others are as thick as honey. The wide range of viscosity means users can choose polyurethanes which different viscosity which fits best on their substrate.
Another important property of polyurethane is that it is solvent free. Which means there is nothing which can dissolve it, some glues may wear off when exposed to sunlight and water etc., but polyurethanes are free of this risk. This is why airplane technicians use them on airplane wings.
Polyurethanes can be applied either manually or automatically (via machines). It can easily be applied on small and large surfaces, something which is not possible with either epoxies or acrylics. The only negative point of polyurethanes is that it has a relatively low operating temperature of 80 degrees centigrade, which means if the polyurethane is exposed to a temperature of 80 Â°C it can hold on for an hour before it degrades. Developers are already improving this low temperature and the new generation polyurethanes will be most likely to handle temperatures above 140 degrees centigrade.
Polyurethane has a low exothermic temperature meaning, it doesn't turn warm when the reaction takes place. Epoxies on the other hand turn hot which accelerates the reaction but may destroy the bond line, the substrate or even the epoxy itself, that is why people prefer using polyurethanes for delicate operations.
Polyurethanes have high flexibility at low temperatures which makes them a favoured adhesive. They can also withstand a high amount of tensile stress  , from 6 up to 20MPa ( Mega Pascal). Though epoxies have higher stress resistance, polyurethanes can withstand high static loads which epoxies cant. Polyurethanes also have a variable adhesive layer thickness which can range from 100 Î¼m up to 20mm.
Polyurethane has a high Tg ( glass transition temperature), which means a high temperature is needed to let it turn back into a molten/ rubber state ( after it has hardened). Epoxies Tg can be increased by adding heat but this costs extra energy which can be saved while using Polyurethanes.
Ultraviolet (UV) cure adhesives
Ultraviolet cure adhesives are one of the more specialised adhesives. The most important property of ultraviolet cure adhesives is that the bonding of it does not require and heating.
The cure time of ultraviolet cure adhesives is dependent on 2 factors. The intensity and wavelength of the ultraviolet light. Polymerisation initiated by light always requires exact coordination of the product and the ultraviolet rays. The photo initiators are split by the UV radiation. The free radicals formed in turn start the polymerization  . Each UV adhesive has its own system, which emits the best wavelength for the given product. When curing UV adhesives, it is crucial that the whole spread of adhesive is exposed to the ultraviolet radiation.
UV curing is usually divided into 3 different steps, the first being "depth curing" followed by "Surface Curing" and finally "Secondary Cure". Ultraviolet systems which emit high-intensity light are used for depth curing. Wavelengths in the band from 300 to 400 nm are the best with depth curing. This is then followed by surface curing. Surface curing is especially important when potting or bonding with UV materials. If inadequate UV systems are used, the surface may remain sticky. Thus is high intensity radiation with a wave band below 300nm important. This prevents the adhesive from reacting with the surface oxygen. Secondary curing is usually done to reinsure the adhesive is stuck and hardened to prevent any inconsistencies.
One of the advantages of UV curing is that it the cure times are significantly less than other adhesives. Some UV cure adhesives are activated instantly after being exposed to ultraviolet radiation. UV technology is also easy to automate in factories and is ideal for mass production. A picture of a worker working with a UV cure machine can be seen on the right. Whilst the picture on the left displays a UV curing machine while operating.
Modified Silane (MS) Polymer adhesives
MS polymers are a relative new string of adhesives and information on them is scarce. Modified silane is an improved version of the normal silicone adhesives. It is well known for its good adhesion to common substrates like metal, glass and plastics. Modified silane doesn't really have a high shear strength but it is extremely flexible when compared to the other adhesives. It has a good ultraviolet and weather resistance which makes it a good adhesive for outside use. Not like the other adhesives, modified silane has a good paintability. MS polymers have very little cross links which enhances stability and strength. This also causes it to have excellent properties even at low temperatures. Modified Silane is solvent-, silicone- and isocyanate  free. Isocyanate is considered hazardous and the absence of it makes modified silane a good substance for items used in our daily lives.
Modified Silane doesn't need to be tempered with high temperatures, it works just as well with low temperatures. Modified Silane is usually found in either one or two part components, just like its acrylic counterpart. The one part is already pre-mixed, with a short exposure time, whilst the two component can stay exposed longer before being bonded. Two part modified silane is also less influenced by the atmospheric conditions.
The chains of MS polymers are long and free moving and this gives modified silane its elasticity. MS polymers are usually the elastic part of the joint. This can come handy when bonding two materials with different thermal expansion, which means the materials expand at a different rate at the same temperature, modified silane can compensate for these movements with its elasticity. Its elasticity also helps a lot when working with dynamic loads, (loads which move), vibrations and shock loads can be absorbed by modified silane due to its dampening properties. Modified silane is easier to apply than the more technical adhesives (UV & acrylics), that is why this adhesive is preferred when working with non specialised people, or for home use. The curing of Modified Silane is initiated by air humidity and the skin formation starts from outside to inside.
Together with modified silane, they form the two basic flexible adhesives. Silicone is has a lower strength and less flexibility, and is the old version of modified silane.  Silicone has good adhesion with common substrates like metal and glass, and is usually coloured white. Silicone has a high temperature resistance, and also a good chemical resistance due to its structured form, with a lot of steady cross links. Silicone has a good UV resistance. Natural rubber has cis-1,4-polyisoprene which gives it its elastic properties, cis-1,4-polyisoprene degrades under UV radiation because the Oxide (O2) turns into a hydroxide group (OH). Silicones don't have this cis-1,4-polyisoprene, which means it is
Silicone adhesives are made from Si and O2. The (simplified) chemical reaction in which Silicone is formed can be seen below. H2O is extracted from the surrounding air.
"SiO2(s) + 2 H2O(l) <-> H4SiO4(s)" 
Silicone adhesives come in two forms, a one and two part component. The one part component silicone relies on the air humidity to dry, and when the layer of adhesive is too thick, two component glues are used. Two component silicone adhesives do not rely on the air humidity to dry and thus are perfect for thicker layers of glue.
Silane is also used as a crosslinking agent.
"Crosslinking occurs in the presence of ambient moisture. The crosslinking rate can be varied by the selected silane itself or by adjusting the pH of the system. The silane type does also have a crucial effect on the degree of crosslinking." 
The resulting Si-O-Si connection is very strong and gives the polymer a better mechanical resistance with a higher tear strength. The polymer also has a better chemical resistance, because there are less groups which are willing to swap electrons. This also makes the polymer resistant to Chemical polarity, and thus resistant to water.
Note: This is not the chemically known Silane "SiH4". This silane is just a silicone with four similar groups connected.
What factors influence adhesion and how?
Adhesion with adhesives can be a very precarious process. There are many external influences which can affect the strength of the adhesive on the substrate. Adhesion with an adhesive has two different surfaces, of the two substrates, as seen in the picture below.
The main thing we have to differentiate when working with adhesives is the nature of the interaction, once that is sorted out, then the appropriate pre-treatment can be chosen. There are two nature's of interaction, it is either: Mechanical or Chemical.
Degreasing is one of the ways to prepare surface for adhesion. Grease contains fatty acids which can undergo a reaction with the adhesive which weakens the adhesion.
Some adhesives are hyper sensitive to oxygen and react with it the second it is present. The adhesion between Polyimide films and metals is an example of a reaction which is influenced by oxygen. Polyimide ( structure seen below ) is sensitive to oxygen, the amounts of C=O in the polyimide decreased and this weakens the adhesion. (Read quote below)
"The effect of oxygen plasma etching on adhesion between polyimide thin films and metal has been investigated, it was found that the amide ring in the polyimide was opened to make additional C-O and C-OH bondings so that C=O bonding in the oxygen plasma sample and the sample dipped in water decreased."
Department of Material and Science Engineering Japan 
Metals often have an oxide outer layer (rust), this cannot be removed by degreasing, this needs mechanical pre-treatment. Grit blasting, grinding or wire brushing are some examples of mechanical pre-treatment, but it often results in changing roughness of the surface, which in turn also affects the strength of the bond.
Improving the substrate is not the only way adhesion can be improved, an activator can be applied to the substrate which initiates adhesion when contact is made. This is usually done with larger areas and with less sophisticated items. This way of adhesive activation is only used with two part adhesives ( either epoxies or acrylics ).
The main way to test whether the cleaning process was done correctly is by using the "Water Break Test". Distilled H2O is applied to the cleaned surface, and if the surface is not clean enough the circular form is retained. If the surface is cleaned properly the water droplet will be totally flat and will run on the surface. This method is used often because it is easy to get the test fluid, which is just water.
Water Break test:
What are the positives of working with adhesives?
As with everything else in life, items have their positives and negatives, here I will mainly focus on the positive part of working with adhesives as these are the one's which really defy why they are so popular. Adhesive bonding is a relatively modern way of connecting materials and thus offers new possibilities for optimal use of substrates.
When using bolts and screws, the stress will be focused on a smaller area. The use of adhesives spreads same amount of pressure on a bigger area, ( See picture below) thus giving the substrate more stability. Using adhesives also help remove the stress spikes which are retained when drilling holes, because there are no edges in the centre of the material, there are no weaker points where eventual lost power might cause damage. Adhesives also have an edge on welding, because welding often causes the structural property of the material to change, this in turn affects the fatigue strength of the substrate under dynamic loads.
Another advantage is that there is either no or little heating takes place when adhesives are used. Parts are not heated ( which happens with welding ), and this enables substrates with different masses and dimensions to be easily assembled. The manufacturers don't need to take the different expansion and deflation rates of the substrates into account.
An example: Iron and Copper have different density at the same mass, this indirectly causes them to expand at different speeds when heated. So when we try to weld Iron with Copper, the Iron will expand faster and the Copper will be slower, or even melt. With the use of adhesives this factor is taken out of proportions.
A more practical advantage of using adhesives would be that it gives designers new possibilities which wouldn't have been available with bolting. Adhesives allow designers to select and combine materials so that the properties of each are used to best effect. Adhesives are lighter than bolts and screws and this might come handy in the aero-sector where technicians try their best to create light and yet durable planes, or spaceships.
Adhesives can also be used as seals, if one chooses to bolt or to rivet, he will have to consider sealing as a separate job. Adhesives do both jobs in one go. Which also makes maintenance easier.
What factors are taken into account when using adhesives in electric motors? ( Which forces)
How strong is the casein glue compared to household glue?
The point of this experiment is to test the sheer strength  of the glue which is based on casein. I will also test the strength of hobby glue from the HEMA. These results will be used as the blanco, and then the scores of the casein glue will shown relative to these.
I will use two ice-lolly sticks for each test. The glue will then be spread on the sticks and set to dry, after dried I will pull on the glued area with an angle of 180 degrees.
One side of the stick will be attached to a weight meter which will then show how much force is needed to make the glue fail.
The formula for stress is: Stress (Pa) = Force (N) / area (mm2)
Because I am using a weight measure, everything is given in kg, to convert this to Newton it will have to be multiplied by g, the gravitational constant (9,81)
Force (N) = mass (kg) * g (m/s2)
Combine the two formulas and this new formula is given:
Stress (Pa) = Force(N) / area (mm2) = mass (kg) * g (m/s2) / area (mm2)
Area = 20mm * 9 mm = 180mm2
The glue will be spread over an area of 180mm2 , a weight spring will be attached to one side of the sticks and it will be pulled. As shown in the illustration below.
5 sets of sticks are set to dry:
The 1st set uses the hobby glue from the HEMA. The second
Calculation of the amount of force which acts on a magnet in an electromotor .