An adhesive, it is first of all a basic polymer which gives the cohesion of the unit. It is then the addition of elements intended to increase the sticking capacity, to increase the flexibility in certain cases.
Let us start with the structural adhesives
The concept of structural adhesive appeared when one started to carry out adhesive bonded joints being able to support constraints as important as the assemblies mechanic - welded. The adhesive bonded joint then has a resistance equivalent to that of materials constituting the structure.
The adhesive is integral part of the whole of the structure to the difference of the products of coatings or cements of sealing, from where the name of structural adhesives.
They are conceived to resist constraints higher than 7 MPA (it is not an insuperable border! …) at ambient temperature. They can be requested without failure with a high percentage of their maximum resistance for a long period, this under hostile conditions (low temperatures, heat, chemical agents,…).
POLYURETHANS
It is a very vast class of products of which some can be classified among the structural adhesives. They are then reactive polymers without solvents or with very high dry extract. They result from condensation between a monomer isocyanate and a monomer hydroxide or polyol. They are appeared as Bi-components but the evolution tends to support the development of the adhesives polyurethanes mono-components because of their simplicity of implementation. They have an excellent behaviour with peeling, with the tear. The polyurethanes Bi-components are regarded as a semi-structural adhesive. They are appropriate perfectly for the mixed assemblies plastic metal/. Certain manufacturers develop polyurethane mono-component ranges. These adhesives are presented under a pasty state then become after reticulation with the ambient moisture of the elastic masses.
The excellent properties of adherence of the polyurethanes found applications in the adhesives to means and high module in all the industrial branches of industry, in particular each time the assemblies are subjected to dilation, of the shocks or the consequent vibrations. They find many uses in fields as different as the means of transport, the shipbuilding, the electric components,… The polyurethanes are slower than epoxies but on the other hand they are much more flexible. Moreover one can stick them on practically all surfaces without using primary educations of adhesion. The latter are necessary only on Teflon,…, i.e. of the materials which are supposed incollables.
One of the large advantages of the polyurethanes currently on the market are that they stick glass without primary education. There is thus a use much simpler than with epoxy. On the other hand if one wants a rapid hardening one frequently uses mixed joinings: polyurethane and adhesive tape. One limits the use of the adhesive tapes because they are of a rather high cost. There are polyurethanes Bi-components. The latter have the advantage of showing higher mechanical characteristics but their implementation is much more complex. They are hard to spread out and the investments are heavy.
The manufacturers now develop mono-components with characteristics which tend to approach those of the Bi-components. One of the principal applications of the polyurethanes is the joining of glass. On the other hand they have a bad behaviour with UV. Many research is undertaken in order to reduce this problem (problem of the joining of avoid-break cars). Another application of these adhesives is the realization of groundsheet, of, assembly seals of elements of boats,… One exploits much the fact of the great flexibility of these adhesives. One speaks besides rather often about flexible joining for the polyurethanes mono-components.
The principal characteristics of the polyurethanes are :
- Important Elasticity and flexibility even at low temperature - Good adherence to the majority of materials - Good resistance to the ageing
- Resistance to the not diluted acids - Good behaviour with moisture - Behaviour at the average temperatures (80-90 °C)
- Hardening hot fast (mono-component) - Speed of reticulation from 3 to 5 MM per day for the mono-components - Implementation technical for the Bi-components
- Electrical insulator Property, property of sealing
- Absence of toxicity in the majority of the cases - Need for drying ovens for the mono-components - Shear strength less low than the other structural adhesives
- Very good Hardness At present, of the manufacturers polyurethanes mono-components market which do not have any more their principal defect i.e. slowness of polymerization. This new adhesive is obtained by the presence of an activator. One thus has the advantages of a polyurethane mono-component (ease of use) and those of Bi-components (speed of catch).
EPOXIES
They are the thermohardening adhesives among most current and most powerful. Their denomination comes from the presence of a characteristic grouping called epoxy grouping. The formulations are presented in the form of liquid or pasty products according to case's.
They are adhesives without solvents, mono or Bi-components. For the adhesives Bi-components, the mixture hardener basic resin is made at the time of joining, by respecting the quantities strictly. An exothermic reaction takes place then, allowing the reticulation. Generally, joining is made at ambient temperature but it can be accelerated by a contribution of heat. For the adhesives mono-components, the adhesive is ready with employment i.e. that the basic resin and the hardener cohabit without the reaction taking place. It is necessary to have a contribution of heat so that the reaction occurs.
The conditions of storage are important: the temperature should not exceed 20 to 30 °C, with the risk to cause a beginning of reaction. The epoxies are frequently used for joinings metal/metal or plastic metal/having to resist very high mechanical constraints. They offer a very good adhesion on the majority of materials and an excellent resistance to the ageing. For epoxies being able to tolerate high temperatures, one inserts ceramics particles.
The problem is then the handling time which reaches 24 H. research is centered on the behaviour in temperature of these adhesives like on the increasingly current realization of epoxy mono-component to high mechanical characteristic. One markets epoxies with rapid hardening, semi-rapid, normal. But resistance drops as the time of catch shortens. It is thus necessary to find a compromise between the requirements of speed of industrial realization and the behaviour of joining.
Let us quote like characteristics for the mono-components:
- Excellent resistance in temperature being able to go from -55 to +200 °C - Excellent shear strength: up to 40 MPa with 20°C - Excellent behaviour with the environments
- Excellent resistance to oils, solvents
- Storage with +4 or -18 °C - High Price - Harmfulness - Unflexibility
For the epoxies Bi-components, one can quote:
- Excellent resistance in temperature
- Excellent shear strength
- Good behaviour with the environments
- Good resistance to oils and solvents
- Possibility of reducing the duration of polymerization by increasing the temperature
- Storage at ambient temperature
ACRYLIC RESINS
There are several types of adhesives at acrylic base.
CYANOACRYLATES
In fact rather recent adhesives make rather important great strides because of some of their characteristics. The reaction is a simple reaction of polymerization of anion type, caused by the presence of a weak base on surface considered. At the beginning these adhesives were intended only for the joining of metals/rubber. Now, one also uses them for porous materials. The principal characteristics of these adhesives are:
- A speed of polymerization low in general but it strongly depends on the nature of the supports - Moisture is a good catalyst
- Polymerization takes place by compression of fine film of monomers - This polymerization takes place on a nonacid polar surface
- The mechanical resistances are high - Their resistance to solvents is good - They have a strong covering capacity
- Their behaviour at the temperature is low - The film is colourless In fact rather specific adhesives are in general intended for the precision tool industries. One uses them on small parts, for the joining of rubber seals,… They resist the shocks generally rather badly, at the temperature (100 °C).
But now certain manufacturers market cyanoacrylates which resist 150 °C, with sets of 0.25 mm, shocks, moisture. One thus eliminates the principal defects from this type of adhesive. One of the great advantages of these adhesives is to be able to adhere on famous surfaces incollables thanks to the use of primary educations of adhesion. One avoids thus on polypropylene, Teflon,…, of the surface treatments such as a plasma treatment followed by CORONA effects.
ANAEROBES
| It is thermohardening adhesives for which polymerization can take place only in the absence of oxygen or of air. Metal surfaces have the property to catalyse the reaction but this one also requires the action of an activator or electromagnetic radiations such as the ultraviolet rays. |
pplication of brake net to block the threaded assembly. |
The principal characteristics are:
- Their time of weak catch
- An excellent shear strength but a very low tensile strength
- Their thermohardening character confers excellent characteristics according to the temperature to them. Some can hold up to 180 °C.
- Their chemical inertia is very good.
- They are unsuited to porous materials. Their fluidity is more or less important to supplement the plays. One uses them for the braking of nets, mechanical joining for the bearings.
The play must remain weak and lower than 0.25 Misters.
The principal parameters on the condition of uses of these adhesives are :
The nature of the substrates: metals are reactive to differing degree, the other materials require the use of the activators.
The play interfaces: the time of polymerization increases quickly with the play.
The choice of the product: with origin, various products can be equipped with different sensitivities to the anaerobic effect, with metals, even with radiation UV.
The cleanliness of surface: dirty surfaces delay polymerization and reduce the performances of the product. The surface treatments of ask for the use of more reactive products or a preparation of the activator. Activators: they are solvents containing of the active ingredients which remain on surfaces after evaporation. Their action is essential with certain adhesives or on nonreactive substrates.
ANAEROBES REACTIVABLES WITH UV
There is a reactivity for lack of oxygen and the presence of U.V.
This adhesive is used for the joining of glass on any support. Polymerization takes place into 20 S if there is a lamp UV, and of 24 H with the action of the sun.
This adhesive is important in optics, the construction of mirrors, decoration,…
They are very interesting since they have the same refractory index as glass. There is not thus a problem for industries of optics. It is possible to clean the surface which was exposed to the action of the air. This type of adhesive is used much in the sector of decoration.
ACRYLIC RESINS MODIFIED
There is, compared to the traditional acrylic resins, the appearance of a radical urethan.
This modification involves completely particular characteristics. Hardening is made possible by the addition of two different parts: an activator and adhesive.
The adhesive is consisted of the basic resin whereas the activator consists of peroxide. The principal characteristics of the acrylic resins modified are:
- A very fast catch, a few minutes for the catch in hand, a few hours for final resistance. - Adherence to badly prepared surfaces.
- A good behaviour at the temperature. - A very good shear strength. - The strength to the chemical agents and moisture is very honest.
- The thickness of the joint must be low. They are used for manufacture in chain because the time of catch is rather weak contrary to epoxies. On the other hand this type of adhesive cannot be used in all the cases of figure. For example the following assembly cannot be carried out with adhesive of an acrylic type modified. This type of joining is impossible to carry out with an acrylic resin modified. Indeed the adhesive is liquid whereas the activator is viscous. The adhesive is thus driven out at the time of the hafting. If the detail of joining observes, we have :
THERMOFUSIBLE THE ONES GOLD HOT-MELTS
At the beginning they were used for joining paperboard/paperboard and of Wood. One quickly realized that they were whitebait to stick much of other things drank. Their principal immediate advantage is the wrestling. In fact adhesive cuts the effect of having to Be heated in order to Be fluxed, the wrestling taking place with cooling. They cuts has problem: during the application one cold materials, there edge Be the formation of has thin to bush-hammer (glacis has) which prevents joining. They consist of has BASIC polymer, has resin and has wax to which one adds additive stabilizing gold plasticizing. The BASIC polymer is intended to give cohesion, the behaviour thermal At the temperature and stability while the waxes cuts the aim of controlling viscosity. The resin tacktifiante is intended to increase the hot sticking capacity. The BASIC polymers are primarily copolymers of ethylene such have the EVA. One also finds polyesters and polyamides. From to their rather easy handling and to their time of weak wrestling, thesis adhesive make rather fast great strides. One principal edge give the advantages of this adhesive: - Setting misses fast -100% of dry extracts from where has facility of handling - Adherence to many supports - Absence of solvents - Réactivable hot permanently - Facility of uses - Machines adapted to evolutionary spleens (folding camera machine-splicing units) Goal one the other hand there are some defects: - Insufficient Creep strength - Speed of sensitive solidification to the temperature of the support - Delicate Compromised between speed of time of wrestling and the time of opening - Sensitivity to heat At the beginning the BASIC component was the acrylic resin. At present drank, the manufacturers uses the thermofusible ones containing mono-component polyurethane. They are packed in hermetic cartridges and in solid form. There are the advantages of has traditional HOT-MELT. Joining is done by heating. Adhesive Then the plays the share of has traditional polyurethane. There is thus has polymerization. The advantage is that the behaviour is fixed. It will not move any more. Moreover the wrestling is fast. Adhesive It is year miracle drank its implementation is complex. Indeed, ounce the cartridge is open to use must uses it until the end. It is necessary to uses has special and expensive gun. One must moreover had has heater of cartridge. Finally the gun must Be imperatively cleaned with has special solvent after each uses. Yew not, the adhesive polymerizes and stops the gun. One thus has in maintenance and has material rather heavy and expensive. |
ADHESIVES IN AQUEOUS DISPERSION
All the products of this family are adapted perfectly to answer a great number of industrial requests. Their use is advised where the products in phase solvent are prohibited. Indeed the adhesives in aqueous dispersion have the advantage of being much less toxic than an adhesive solvent and to be flammable in the liquid state (interest on the level of storage and implementation).
These products give excellent results on a great number of supports such wood, the plastics, the fabrics, etc As example, these adhesives are used today for the fabric fixing on wood in the manufacture of speakers like for the joining of wall linings in particular in public transport.
These products also are very much used in the manufacture of industrial filters, of light materials, like for the assembly of products of insulation. The adhesives in aqueous dispersion result in general from two technologies:
- The adhesives contacts: a double gluing is carried out, one waits the time of evaporation necessary and one posts the two surfaces pasted by maintaining the pressure
- Self-adhesive adhesives: After complete evaporation of solvent (in fact of water), these products remain self-adhesive very a long time. That gets a great flexibility on the level of the process of implementation of the product. A contribution of heat can decrease in a considerable way time of drying of the adhesive. The adhesives in aqueous dispersion have a rate of rather important dry extracts in general. One can often stick with one liter of adhesive in dispersion a surface much more important than with one liter of solvent adhesive. For the implementation of these products, one can apply them without problem to the brush, the roller or the gun. ADHESIVE TAPES One distinguishes several kinds of structural adhesive tapes: supported films, the not supported films époxys, and finally acrylic films. The supported films have been developed for several years but are used little. The structural films mono-components have characteristics higher than the traditional epoxy adhesives. The joints can reach more than 30 to 35 MPA in shearing and from approximately 12 to 13 N/MM in peeling. They make it possible to fill rather important plays. A contribution of heat is necessary to ensure a good polymerization. There are many applications of the adhesive tapes:
- Traditional Joining - Ribbons for the electrical engineering
- Ribbons of protection for plastic condensers - Ribbon of masking (gilding of the contacts of printed circuits) - Adhesive tapes of saving for the surface treatments
- Working Ribbons of assemblies and refermables
- Antiblocking industrial Adhesive tapes Foams and the transfers very high adhesion are acrylic adhesives comparable with adhesives doubles faces. There are various types of adhesive masses on the market: - Rubber
- Natural rubber - Synthetic rubber - Nonthermohardening Rubber - Thermohardening Rubber - Réticulé thermohardening Rubber
- Acrylic resin
- Thermohardening Acrylic resin
- Réticulée Acrylic resin
- Silicone
- Thermohardening Silicone The most effective adhesive is that containing acrylic resin. It is the only one which can be regarded as structural. This type of joining has been known for more than 20 years. At the beginning they were primarily epoxy resins, D-activables hot more pressing. For 15 years the most powerful ribbons have been containing acrylic resin. There is a support, foam, and the adhesive.
One can fill rather important defects with the thickness. On the other hand one realized that when one made work this type of assembly, there was risk of rupture on the level of foam. The manufacturers currently market adhesives containing acrylic resin pure. The problem of foam thus is avoided.
There is according to needs' a more or less great thickness for the ribbon. For the low thicknesses there is acrylic resin pure. On the other hand for the ribbons stronger thicknesses there is acrylic resin emulsified. These products are rather interesting bus in addition to exceptional performances with wrenching and with shearing, they have a viscoelastic memory.
One can thus stick two materials with different dilation coefficients. The ribbon lengthens but does not break. One can also quote aptitudes of impact resistance, vibrations, corrosion, of electric insulation,…
They are moreover ungluable. Like example of use, one can quote the manufacture of the electrical equipment boxes. In the past, one placed a reinforcement in the shape of omega. It was then necessary to weld it then to sandpaper it for the application of painting. Maintaining with the presence of ribbons with very high properties of adhesion, the operation of sandpapering is not effective any more. This saving of time was not possible that because the new ribbons resist strong temperatures. One can quote, for example, with range VHB from 3M: 250 °C during 4 H, 150 °C uninterrupted.
Research is currently centered on the development of ribbons supporting still better heat and with the mechanical characteristics definitely more powerful.
PRIMARY EDUCATIONS Of ADHESION: THEIR FUNCTIONS
ls consists of a resin and often of polyurethane. This is why they are manufactured by the industrialists of the adhesive. The function of the primary educations is very vast. They have been developed for a few years for quite particular employment. Let us quote for example: - Concrete Accelerator of the instantaneous adhesives - Preparation of surface - Degreasing of surfaces - Increase in adhesion on the plastics, glass - Agent of polymerization - Gelation of the visible adhesive - They are used also as barriers with plasticizers, the gasolines, etc |
PRESENTATION OF SOME OTHER EXISTING ADHESIVES
The adhesives from the basic nature of polymer and that of the additives, represent a very vast family which it would be tiresome to detail. We will thus limit ourselves to a fast and brief presentation of the other adhesives.
EMULSIONS AND SOLUTIONS
They are adhesives in aqueous or organic medium whose hardening results from the evaporation of solvent or its absorption by one or the other of the substrates. The natural adhesives of vegetable origin do not use an organic solvent.
The adhesives obtained starting from resins
thermoplastics, of thermohardening elastomer adhesives even, can according to case's being obtained either in aqueous medium, or in solvent medium. $ The natural adhesives cannot be qualified the structural ones but they however are very much used in the industry of wood, paper, delivers,…
THE ADHESIVES CONTACTS
The setting in contact of surfaces to be assembled takes place only after the evaporation of solvents, the assembly requiring a pressure to weld two films of adhesive. The typical example is the Néoprène adhesive.
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