The meaning of the word adhesion. What is adhesion Adhesion assessment of the adhesion strength of building materials

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This is the adhesion of materials of different composition and structure, due to their physical and chemical properties. The term adhesion comes from the Latin word adhesion - adhesion. In construction, they give a more narrowly focused and specific designation of what adhesion is - the ability of decorative finishing coatings (paintwork materials, plaster), sealing or adhesive mixtures to a strong and reliable connection with the outer surface of the base material.

An impressive demonstration of the adhesion effect of modern adhesives

Important! A distinction should be made between the concepts of adhesion and cohesion. Adhesion connects different types of materials, affecting only the surface layer. For example, paint on a metal surface. Cohesion is a combination of materials of the same type, as a result of which intermolecular interactions are formed.

Adhesion is one of the key material properties in the following areas:

Metallurgy - anti-corrosion coatings.
Mechanics - a layer of lubricant on the surface of the elements of machines and mechanisms.
Medicine - dentistry.
Construction. In this industry, adhesion is one of the main indicators of the quality of work and the reliability of structures.

At almost all stages of construction, adhesion indicators for the following connections are monitored:

paints and varnishes;
plaster mixes, screeds and fillings;
adhesives, masonry mortars, sealants, etc.

An example of chemical adhesion is the reaction of bonding silicone sealant to glass

There are three basic principles for adhesive bonding of materials. In construction and technology, they manifest themselves as follows:

Mechanical- adhesion occurs by adhesion of the applied material to the base. The mechanism of such a connection consists in the penetration of the applied substance into the pores of the outer layer or in connection with a rough surface. An example is painting the surface of concrete or metal.
Chemical- the connection between materials, including those of different densities, occurs at the atomic level. For the formation of such a bond, the presence of a catalyst is required. An example of this type of adhesion is soldering or welding.
Physical- on the mating surfaces there is an electromagnetic intermolecular bond. May be caused by static electricity or permanent magnetic or electromagnetic fields. An example of use in technology is painting various surfaces in an electromagnetic field.

Adhesive properties of building and finishing materials

The adhesion of building and finishing materials is carried out mainly according to the principle of mechanical and chemical bonding. In construction, a large number of different substances are used, the operational characteristics and specificity of the interaction of which are fundamentally different. We divide them into three main groups and characterize them in more detail.

paints and varnishes

The adhesion of paintwork materials to the surface of the base is carried out according to the mechanical principle. At the same time, the maximum strength indicators are achieved if the working surface of the material is rough or porous. In the first case, the contact area increases significantly, in the second, the paint penetrates into the surface layer of the base. In addition, the adhesive properties of paintwork materials are increased due to various modifying additives:

organosilanes and polyorganosiloxanes have an additional hydrophobizing and anticorrosive effect;
polyamide and polyester resins;
organometallic catalysts for chemical processes of paintwork materials hardening;
ballast fine fillers (for example, talc).

Talc-filled paint - non intumescent flame retardant

Construction plasters and dry adhesives

Until recently, construction and finishing work was carried out using various solutions based on gypsum, cement and lime. Often, they were mixed in a certain proportion, which resulted in a limited change in their basic properties. Modern ready-made dry building mixtures: starting, finishing and multi-finishing plasters and putties, have a much more complex composition. Additives of various origins are widely used:

mineral- magnesia catalysts, water glass, alumina, acid-resistant or non-shrinking cement, microsilica, etc.
polymer- dispersible polymers (PVA, polyacrylates, vinyl acetates, etc.).

Such modifiers significantly change the following main characteristics of building mixtures:

plastic;
water retention properties;
thixotropy.

Important! The use of polymer modifiers gives a more pronounced effect of enhancing adhesion. However, the formation of stable compounds of polymer films at the border of different types of materials (base - hardening plaster) is possible only at a certain temperature. This term is called the minimum film formation temperature - MTP. For different plasters, it can be different from + 5 ° C to + 10 ° C. To avoid delamination, the manufacturer's recommendations for temperatures, both ambient and substrate, must be followed closely.

Sealants

There are three different types of sealants used in construction, each of which requires specific conditions for high strength adhesion to the substrate material. Let's consider each type in more detail.

Drying sealants. The composition includes various polymers and organic solvents: styrene-butadiene or nitrile, chloroprene rubber, etc. As a rule, they have a pasty consistency with a viscosity of 300-550 Pa. Depending on the viscosity, they are applied either with a spatula or with a brush. After their application to the surface, a certain time is required for drying (evaporation of the solvent) and the formation of a polymer film.

Non-drying sealants. They usually consist of rubber, bitumen and various plasticizers. They have limited resistance to high temperatures, no more than 70 0 С-80 0 С, after which they begin to deform.

Curing sealants. After their application, under the influence of various factors: moisture, heat, chemical reagents, an irreversible polymerization reaction occurs.

Of all the listed varieties, curing sealants provide maximum adhesion to microroughness of the substrate surface. In addition, they are resistant to high temperatures, mechanical and chemical influences. They have the optimal combination of stiffness and toughness, allowing them to maintain their original shape. However, they are the most expensive and difficult to use.

How is adhesion measured?

Adhesion measurement technology, test methods, as well as all indicators of the strength of the connection of materials are indicated in the following standards:

GOST 31356-2013: Putties and plasters;
GOST 31149-2014 - Paints and varnishes;
GOST 27325 - Paintwork materials for wood, etc.

Information! Adhesion is measured in kgf / cm 2, MPa (megapascals) or kN (kilonewtons) - this is an indicator of the force that must be applied to separate the substrate and coating materials.

Whereas previously the adhesion characteristics of materials could only be measured in a laboratory environment, at the moment there are many devices that can be used directly on the construction site. Most methods for measuring adhesion, both in the field and in the laboratory, involve the destruction of the outer covering layer. But there are several devices based on ultrasound.

Knife adhesion meter. It is used to determine the adhesion parameters by the lattice and or parallel notch method. It is used for paint and varnish and film coatings up to 200 microns thick.

Pulsar 21. The device detects the density of materials. It is used to detect cracks and delamination in concrete, both piece and monolithic. There are special firmwares and subroutines that, according to the adhesion density, allow you to determine the adhesion strength of various types of plasters to concrete surfaces.

SM-1U. It is used to determine the adhesion of polymer and bituminous insulating coatings by the method of partial destruction - shear. The measuring principle is based on the detection of linear deformations of the insulating material. As a rule, it is used to determine the strength of the insulation coating of pipelines. It is allowed to use for quality control the application of bituminous waterproofing to building structures: walls of basements and basements, flat roofs, etc.

Factors that reduce the adhesion of materials

Various physical and chemical factors influence the decrease in adhesion. The physical temperature and humidity refers to the environment at the time of application of decorative and finishing or protective materials. Various contaminants, in particular, dust covering the surface of the base, also reduce the adhesive interactions. During operation, ultraviolet radiation can affect the strength of the connection of paints and varnishes.

Chemical factors that reduce adhesion are represented by various materials that contaminate the surface: gasoline and oils, fats, acid and alkaline solutions, etc.

Also, the adhesion of finishing materials can be reduced by various processes that occur in building structures:

shrinkage;
tensile and compressive stresses.

Information! A substance applied to a surface to increase the adhesion between the substrate and the finishing material is called an adhesive. The substrate on which the adhesive is applied is called the substrate.

Methods for increasing adhesion

In construction, there are several universal ways to increase the adhesion of decorative finishing materials to the base surface:

Mechanical- the surface of the base is roughened to increase the contact area. To do this, it is treated with various abrasive materials, notches are applied, etc.
Chemical- various substances are added to the composition of the applied protective and finishing materials. These are, as a rule, polymers that form stronger bonds and give the material additional elasticity.
Physicochemical- the surface of the base is treated with a primer that changes the basic chemical parameters of the material and affects certain physical properties. For example, a decrease in moisture absorption in porous materials, anchoring of a loose outer layer, etc.

Ways to increase adhesion to various materials

Let's take a closer look at the methods of increasing adhesion for various materials used in construction.

Concrete

Concrete building materials and structures are widely used in construction. Due to the high density and smoothness of the surface, their potential adhesive properties are quite low. To increase the strength of the connection of finishing compounds, the following parameters must be taken into account:

dry or damp surface. Generally, the adhesion to a dry surface is higher. However, many adhesive mixtures have been developed that require pre-wetting of the substrate surface. In this case, you need to pay attention to the manufacturer's requirements;
ambient and substrate temperature. Most of the finishing materials are applied to concrete surfaces at an air temperature of at least + 5 ° C ... + 7 ° C. In this case, the concrete should not be frozen;
primer. It is used without fail. For dense concrete, these are compositions with a filler of quartz sand (concrete contact), for porous concrete (foam, aerated concrete), these are deep penetration primers based on acrylic dispersions;
adding modifiers. Ready-made dry plaster mixes already contain various adhesive additives. If the plaster is mixed independently, then it is recommended to add to it: PVA, acrylic primer, instead of the same amount of water, silicate glue, which gives the finishing material additional moisture-repellent properties.

Metal

The method and quality of surface preparation play a key role in the strength of the connection of paints and varnishes with a metal surface. At home, it is recommended to do the following:

degreasing- metal processing with various solvents: 650, 646, P-4, white spirit, acetone, kerosene. In extreme cases, the surface is wiped with gasoline;
matting- processing of the base with abrasive materials;
padding- the use of special primer paints. They are realized in a set with decorative paints of a certain type.

Important! The adhesion of lead, aluminum and zinc is much lower than that of cast iron and steel. The reason is that these metals form oxide films on their surface. Therefore, the peeling of paint and varnish coatings occurs along the oxide layer. It is recommended to color these materials immediately after removing the film by mechanical or chemical means.

Wood and wood composites

Wood is a porous surface with a lot of unevenness and does not experience any particular problems with the strength of the connection of finishing materials. But there is no limit to perfection, so various technologies have been developed to improve adhesion in combination with the preservation of the protective and decorative properties of the finish itself. Their use, for example, in combination with acrylic paints, significantly improves weather resistance, resistance to ultraviolet fading, and gives biological protection to the material. The surface of the wood is treated with a wide variety of primers, most often, based on boron compounds and nitrocellulose.

Welding adhesion

Welding is one of the most durable methods of joining metal structures. This is the adhesion of molecules of two elements without the use of intermediate or auxiliary substances - glue or solder. This process takes place under the influence of thermal activation. The outer layer of the elements to be joined is heated above the melting point, after which intermolecular convergence and joining of materials occurs.

The following factors can be an obstacle to good adhesion during welding:

the presence of oxide films. They are removed mechanically or chemically during surface preparation or disappear directly during the welding process under the influence of high temperatures or fluxes;
inconsistency in the chemical composition of materials and electrodes. Particular attention should be paid to the presence and amount of silicon and carbon in the parts to be joined. For joining steels of different grades, it is recommended to use electrodes with a low diffusible hydrogen content;
insufficient penetration depth, which directly depends on the strength of the current and the speed of movement of the electrode.

The term "adhesion" is often found in documents from various scientific disciplines. It is used in physics, chemistry, and biology. However, each science has its own approach to what adhesion is, the definition of which, taking into account all facets of the phenomenon, cannot be given by any scientist yet. True, everyone agrees on one thing: it is a compound, the interaction of various particles.

If we consider it as a process, we can say that adhesion is a phenomenon that consists in the appearance of interaction between certain condensed phases. When their molecular contact occurs, this interaction leads to the emergence of a new heterogeneous entity.

If this phenomenon is understood as a property, then adhesion is (in the case of liquids) the interaction between the liquid and solid phases at their interface.

Physics

From the point of view of physics, adhesion is the adhesion of the surfaces of various substances when they come into contact. Moreover, substances can be in the same or in a different state of aggregation. Thus, the effect can relate to two solids, two liquids, or a liquid and a solid.

Substances adhere under the influence of the following factors:

there are chemical bonds between molecules of two substances,
diffusion occurs when the molecules of the first substance penetrate under the boundary of the surface of the second,
the forces of van der Waals act, which arise when the polarization of molecules occurs.

There are also special cases where adhesion can occur. They are often confused. This is autohesion and cohesion.

Autohesion arises as a consequence of the adhesion of homogeneous bodies, but the phase boundary is preserved in this case.

Cohesion can occur when molecules of one body interact.

In natural conditions, there are cases when adhesion becomes cohesion due to various external reasons. This situation arises during diffusion if the phase boundaries become blurred. In some cases, the strength of the adhesive bond between the phases may be greater than the cohesive one. Then, depending on the strength of the substance, when a force is applied to the connection of substances, the interface is preserved or the cohesive bonds are broken.

Chemistry

Chemistry has a vision of the adhesion process similar to physics. Many technological processes in the chemical industry have adopted the practical use of this phenomenon. It is it that underlies the technology of manufacturing composite materials, and the production of paints and varnishes is also based on it. The concept of adhesion in chemical science is used when we are talking about the process of gluing surfaces in a solid state with an adhesive (substrates are glued together with an adhesive).

Biology

In biological science, the term is used not in relation to molecules, but in relation to relatively large biological particles - cells. Adhesion is a connection of cells that allows the formation of histological structures correctly, and the type of these structures is determined by the specifics of the cells involved in the interaction. The result of the interaction depends on the presence of certain proteins on the surface of the connecting cells.

Influence on material properties

Adhesion has the ability to significantly change the characteristics of surfaces that come into contact. It can help surfaces acquire a low coefficient of friction. If the substances have a solid crystalline structure, then their further use as antifriction lubricants becomes possible. Effects such as capillarity and wettability also result from this phenomenon.

unit of measurement

When adhesion occurs, the energy of the body on some part of the surface instantly becomes less. For this reason, it is customary to measure it by the work or force required to tear off surfaces from each other on a certain unit of area.

Application of adhesion in construction

A physical phenomenon such as adhesion has contributed to the improvement of the technological process for the manufacture of steel plates and blocks with thin and thick walls. Knowledge of the mechanisms of the phenomenon made it possible to increase the productivity of lines for the manufacture of these construction products and significantly reduce the weight of structures.

Only this phenomenon makes it possible to paint and varnish the surfaces of building materials, to apply galvanic and anodic coatings. These operations contribute to the creation of anti-corrosion protection of the metal, giving the material a marketable appearance.

Knowledge of the nature of the phenomenon is of significant assistance in the high-quality gluing of various materials and their durable welding. With the participation of adhesion, metals are coated with oxide films that perform protective functions. The effect finds application in the production of concrete works - in situations where it is not possible to immediately achieve complete concrete filling of the object. When re-pouring, two concrete bases form a so-called cold joint between themselves, which negatively affects the strength characteristics of the joint. Adhesion is also recommended for applications where it is necessary to separate the concrete from the steel mold. It is simply impossible to perform this operation in other ways. The use of adhesion makes it possible to successfully deal with surface defects of finished concrete products.

Cement mortars

The division of adhesive mortars with the participation of cement into classes C1 and C2 is based on the assessment of the degree of adhesion of the mortar to the base after hardening. According to the requirements of European quality standards, the adhesion of the C1 class adhesive solution to the base should be more than 0.5 MPa, while for the C2 class cement adhesive mortar its value is not less than 1.0 MPa. Thus, the difference between the two classes of mortars is determined by the adhesion strength.

Methods for determining adhesion

Methods by which adhesion is determined (GOST 15140-78):

flaking;
lattice cuts;
lattice cuts with a reverse impact;
parallel incisions.

Adhesion in metallurgy

During adhesion, the phase boundary between the bodies is preserved. The adhesion of metals is manifested when the coagulation of non-metallic inclusions in the composition of liquid metals and alloys is carried out. Adhesion promotes the enlargement of non-metallic inclusions, which further leads to their removal from the metal into the slag.

The effect of adhesion or wetting of non-metallic inclusions with liquid metal can:

interfere with the extraction of inclusions from the metal in the event that the metal melt wets the non-metallic inclusions well (in this case, good adhesion takes place);
to create conditions for the removal of non-metallic inclusions from the metal in a situation when these inclusions are not sufficiently wetted by the metallic melt (in this case, the value of adhesion is low).

When cold welding is carried out, almost all hard metals in a ductile state are bonded under pressure. Adhesion underlies the adhesion to the metal of galvanic, oxide, sulfide coatings, which are applied to the metal surface in order to protect products from corrosion. The adhesion of the coating ensures its reliable adhesion of such compounds to the surface of metals. It has found its application in powder metallurgy, when products are formed and sintered from metal powders.

The adhesion of materials is widely used in cases where it is necessary to solder, tin, zinc, apply a variety of paint and varnish coatings. Creation of various composite materials cannot do without it. In the manufacture of such materials, particles of any substance come into contact with the alloy base. The effect increases in the presence of an electric charge on the surfaces of bodies, which makes it possible to form a donor-acceptor bond upon connection. Adhesion is also enhanced during chemical cleaning of the surfaces to be joined. For these purposes, degreasing, evacuation, ion bombardment, exposure to electromagnetic radiation are used.

Adhesion promoter

When a car is in operation, the smallest pores on the surface of the paint layer and polymer parts become clogged with the remnants of dust, resins, and auto chemistry. As a consequence, an attempt to glue something to a part often fails due to poor surface adhesion. Degreasing does not remove all contamination. The adhesion promoter is designed for use in the preparation of surfaces before applying decorative films, stickers, nameplates, double-sided tape. The activator significantly increases the adhesion properties of surfaces due to a specially developed composition. Its use makes it possible to guarantee that the gluing will be reliable and will allow the materials to be joined to be used for a long time. The high adhesion provided by the activator is the reason for the high demand for it.

Polar, sometimes - mutual diffusion) in the surface layer and is characterized by the specific work required to separate surfaces. In some cases, adhesion may turn out to be stronger than cohesion, that is, adhesion within a homogeneous material; in such cases, when a breaking force is applied, a cohesive rupture occurs, that is, a rupture in the volume of the less durable of the contacting materials.

Adhesion significantly affects the nature of friction of contacting surfaces: for example, when surfaces with low adhesion interact, friction is minimal. An example is polytetrafluoroethylene (Teflon), which, due to its low adhesion value, in combination with most materials, has a low coefficient of friction. Some substances with a layered crystal lattice (graphite, molybdenum disulfide), characterized by both low adhesion and cohesion values, are used as solid lubricants.

The most famous adhesion effects are capillarity, wettability / non-wettability, surface tension, meniscus of a liquid in a narrow capillary, resting friction of two absolutely smooth surfaces. The adhesion criterion in some cases can be the time of separation of a layer of a material of a certain size from another material in a laminar flow of liquid.

Adhesion takes place in the processes of gluing, soldering, welding, coating. The adhesion of the matrix and filler of composites (composite materials) is also one of the most important factors affecting their strength.

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Adhesion is an extremely complex phenomenon, which explains the existence of many theories that interpret this phenomenon from different positions. The following adhesion theories are currently known:
- Adsorption theory, according to which the phenomenon occurs as a result of the adsorption of the adhesive on the pores and cracks of the substrate surface.
- Mechanical theory considers adhesion as a result of the manifestation of the forces of intermolecular interaction between the contacting molecules of the adhesive and the substrate.
- Electrical theory identifies the "adhesive - substrate" system with a capacitor, and the double electric layer, which appears at the contact of two dissimilar surfaces, with the capacitor plate.
- Electronic theory considers adhesion as a result of molecular interaction of surfaces that are different in nature.
- Diffusion theory reduces the phenomenon to mutual or one-sided diffusion of adhesive molecules and substrate.
- Chemical theory explains adhesion not by physical, but by chemical interaction.
Physical Description

Adhesion is the reversible thermodynamic work of forces aimed at separating two dissimilar (heterogeneous) phases brought into contact. Described by the Dupre equation:
W a = σ 13 + σ 23 - σ 12 (\ displaystyle (Wa = \ sigma _ (13) + \ sigma _ (23) - \ sigma _ (12)))
W a = - Δ G o (\ displaystyle (Wa = - \ Delta G ^ (o)))
A negative ΔG ° value indicates a decrease in the work of adhesion as a result of the formation of interfacial tension.
Changes in the Gibbs energy of the system during the adhesion process:
Δ G 1 o = σ 13 + σ 23 (\ displaystyle (\ Delta G_ (1) ^ (o) = \ sigma _ (13) + \ sigma _ (23)))
Δ G 2 o = σ 12 (\ displaystyle (\ Delta G_ (2) ^ (o) = \ sigma _ (12)))
Δ G o = Δ G 2 o - Δ G 1 o (\ displaystyle (\ Delta G ^ (o) = \ Delta G_ (2) ^ (o) - \ Delta G_ (1) ^ (o)))
σ 12 - σ 13 - σ 23 = Δ G o (\ displaystyle (\ sigma _ (12) - \ sigma _ (13) - \ sigma _ (23) = \ Delta G ^ (o))).
At the interface between two phases (liquid-gas), cosθ is the contact angle, Wa is the reversible work of adhesion.

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First, let's assume that the first condition for adhesion is to maintain close contact at the molecular level between the adhesive and the substrate. Now let's imagine what will happen after the materials come into contact, and how they will interact. An adhesive bond can be mechanical, physical, or chemical, but it is usually a combination of these types of bonds.

Mechanical adhesion

The simplest form of adhesion is the mechanical adhesion of the adhesive components to the substrate surface. This adhesion is formed due to the presence of such surface irregularities as depressions, cracks, cracks, during the development of which microscopic undercuts are formed.

The main condition for the formation of mechanical adhesion is the ability of the adhesive to easily penetrate the depressions on the surface of the substrate, and then harden. This condition depends on the wetting of the substrate surface with the adhesive, which, in turn, is related to the ratio of the surface energies of the materials in contact, which determines the value of the contact wetting angle. The ideal situation is to completely wet the substrate with the adhesive. To improve contact, get rid of any air or steam present in the grooves before applying the adhesive. If the adhesive can fill in the undercuts and then harden, then naturally it will be blocked by the undercuts (Fig. 1.10.7).

Rice. 1.10.7. Microscopic mechanical engagement between adhesive and substrate

The penetration rate of the adhesive into the undercuts depends both on the pressure applied during application and on the properties of the adhesive itself. If an attempt is made to tear the adhesive away from the substrate, this can only be done by breaking it, since the adhesive cannot be removed from the undercuts. The concept of mechanical adhesion does not contradict the conditions for the attachment or retention of fixed dentures used in their fixation, except for those phenomena that occur at the microscopic level. An important difference between these concepts is that good wettability is not a prerequisite for macroretention, while it plays a decisive role in creating mechanical engagement at the microscopic level.

In general, undercuts often increase the mechanical strength of the joint, but this is usually not enough to trigger the (specific) adhesion mechanism itself. There are a number of additional adhesion mechanisms due to physical and chemical reasons. The term true or specific adhesion is usually used to distinguish physical and chemical adhesion from mechanical adhesion, however, such terms are best discarded as they are not entirely accurate.

The concept of true adhesion assumes that there is false adhesion in addition to it, but in reality adhesion either exists or it does not. Physical and chemical adhesion differ from mechanical adhesion in that the former involve the adhesive and the substrate in molecular interaction with each other, while for mechanical such interaction at the interface of the two phases is not required.

Physical adhesion

When two planes are in close contact, secondary bonds are formed due to the dipole-dipole interaction between polarized molecules. The magnitude of the resulting forces of attraction is very small, even if they have a high value of the dipole moment or increased polarity.

The value of the binding energy depends on the relative orientation of the dipoles in two planes, but usually this value is no more than 0.2 electron volts. This value is much less than that of primary bonds, such as ionic or covalent bonds, which typically have binding energies between 2.0 and 6.0 electron volts.

Secondary bonds due to the dipole-dipole interaction arise very quickly (since no activation energy is needed for their formation) and are reversible (since the molecules on the surface of the substance remain chemically unaffected). This weak adsorptive physical attraction is easily destroyed by increasing temperature and is not suitable for applications where a permanent bond is required. However, bonds such as hydrogen bonds can be an essential prerequisite for the formation of a chemical bond.

It follows from this that the connection of non-polar liquids with polar solids is difficult, and vice versa, since there will be no interaction between these two substances at the molecular level, even with their close contact. This behavior is observed in liquid silicone polymers, which are non-polar and therefore do not form secondary bonds with solid surfaces. Connections with them are possible only through the passage of a chemical crosslinking reaction, which will create junctions between a liquid and a solid.

Chemical adhesion

If, after adsorption on the surface, the molecule dissociates, and then its functional groups, each individually, can combine covalently or

ionic bonds with the surface, the result is a strong adhesive bond. This form of adhesion is called chemisorption and can be either ionic or covalent in nature.

A chemical bond differs from a physical bond in that two neighboring atoms share the same electrons together. The surface of the adhesive must be firmly connected to the surface of the substrate through chemical bonds, therefore, the presence of reactive groups on both surfaces is necessary. In particular, this applies to the formation of covalent bonds, which occurs, for example, when binding reactive isocyanates to polymer surfaces containing hydroxyl and amine groups (Fig. 1.10.8).

Rice. 1.10.8. Formation of a covalent bond between isocyanate and hydroxyl and amine groups on the substrate surface

Unlike non-metallic compounds, a metallic bond is easily formed between solid and liquid metals - this mechanism underlies soldering. The metallic bond arises from free electrons and does not depend on the presence of reactive groups. However, this connection is only possible if the metal surfaces are perfectly clean. In practice, this means that fluxes must be used to remove oxide films, otherwise these films will prevent contact between metal atoms.

The only way to separate the adhesive from the substrate is by mechanical breaking of chemical bonds, but this does not mean that these, and not other valence bonds will be broken in the first place. This imposes limits on the strength that can be achieved in the joint. If the strength of the adhesion or adhesive bond is higher than the tensile strength of the materials of the adhesive or substrate, then before the adhesive bond breaks down, the breakdown of the cohesive adhesive or substrate will occur.

Molecular entanglement adhesion (Diffusion adhesion mechanism)

So far, we have assumed that there is a distinct interface between the adhesive and the substrate. Typically the adhesive is adsorbed to the surface of the substrate and can be considered a surfactant that accumulates on the surface but does not penetrate. In some cases, the adhesive or one of its components is able to penetrate into the surface of the substrate, and not accumulate on it. It should be emphasized that absorption of molecules results from good surface wetting, not the cause.

If the absorbed component is a long-chain molecule, or forms a long-chain molecule after being absorbed by the substrate, entanglement or interdiffusion of the adhesive and substrate molecules can result, leading to very high adhesive strength (Figure 1.10.9).

Rice. 1.10.9. Diffusion transition layer formed by the interweaving of molecular fragments of the adhesive and substrate

This equality is called the Dupre equation. It means that the work of adhesion (W) is the sum of the free surface energies of the solid (y) and liquid (y | v) minus the energy at the interface between the liquid and the solid (ysl).

Young's equation implies

Ysv Ysi = Ysi cose

Adhesion will be maximal at full (ideal) wetting, i.e. in the case when cosq = 1, therefore, the energy of the glued surfaces and the energies of each of these surfaces separately (Fig. 1.10.10).

Rice. 1.10.10. Separation of a liquid from a solid surface to form two new surfaces

The surface tension of the liquid hydrocarbon is approximately 30 mJ / m. If we assume that the forces of attraction decrease to zero at a distance of 3 x 10 ~ meters, then the force required to separate the liquid from a solid surface is equal to the work of adhesion divided by the distance and is equal to 200 MPa.

In fact, this figure is much higher.

Thus, adhesives must be chemically strongly attracted to the surface of the substrates in order to provide high adhesive strength.

Clinical significance

The physician needs to know what kind of bond he is seeking, and this requires an understanding of the steps for creating an adhesive bond. This will avoid mistakes in work.

Fundamentals of Dental Materials Science
Richard van Noort

With this adhesion process, different types of substances are attracted at the molecular level. It can affect both solids and liquids.

Determination of adhesion

The word adhesion in translation from Latin means adhesion. It is a process in which two substances are attracted to each other. Their molecules stick together. As a result, in order to separate the two substances, it is necessary to produce an external influence.

This is a surface process that is typical for almost all dispersed type systems. This phenomenon is possible between such combinations of substances:
- liquid + liquid,
- solid + solid,
- liquid body + solid.
All materials that begin to interact with each other during adhesion are called substrates. Substances that provide substrates with tight adhesion are called adhesives. For the most part, all substrates are represented by solid materials, which can be metals, polymeric materials, plastic, ceramic material. Adhesives are predominantly liquid substances. A good example of an adhesive is a liquid such as glue.

This process can be the result of:
- mechanical action on adhesion materials. In this case, in order for the substances to stick together, it is necessary to add certain additional substances and the use of mechanical methods of adhesion.
- the appearance of an interconnection between the molecules of substances.
- Formation of an electrical double layer. This phenomenon occurs when an electrical charge is transferred from one substance to another.
Currently, it is not uncommon for cases when the adhesion process between substances appears as a result of the influence of mixed factors.

Adhesion strength

Adhesion strength is an indicator of how tightly certain substances adhere to each other. To date, the strength of the adhesive interaction of two substances can be determined using three groups of specially developed methods:
1. Tear-off methods. They are subdivided into many more methods for determining adhesion strength. To determine the degree of adhesion of two materials, it is necessary to try, using an external force, to break the bond between the substance. Depending on the bonded materials, the simultaneous pull-off method or the sequential pull-off method can be used here.
2. A method of actual adhesion without interfering with the structure, created by adhesion of two materials.
When using different methods, different indicators can be obtained, which depend largely on the thickness of the two materials. The peeling speed and the angle at which it is necessary to carry out the separation are taken into account.

In the modern world, there are various types of material adhesion. Polymer adhesion is not uncommon today. When mixing different substances, it is very important that their active centers interact with each other. At the interface between two substances, electrically charged particles are formed, which provide a strong connection of materials.

Adhesion of glue is the process of attraction of two substances through mechanical interaction from outside. The glue is used to glue two materials together to create one piece. The strength of the bonding of materials depends on the strength of the glue in contact with certain types of materials. For gluing materials that do not interact well with each other, it is necessary to strengthen the effect of the glue. To do this, you can simply use a special activator. Thanks to it, strong adhesion is formed.

Very often in the modern world, one has to deal with the bonding of materials such as concrete and metals. The adhesion of concrete to metal is not strong enough. More often in construction, special mixtures are used that ensure reliable bonding of these materials. Also, construction foam is often used, which forces metals and concrete to form a stable system.

Adhesion method

Methods for determining adhesion are methods by which it is established how different materials can interact with each other within a certain specificity. Various construction objects and household appliances are created from materials that are bonded together. In order for them to function normally and not cause harm, it is necessary to carefully monitor the level of adhesion between substances.

Measurement of adhesion is carried out using specialized devices that allow at the production stage to determine how firmly the products are attached to each other after using certain bonding methods.

Adhesion of paints and varnishes

Paint adhesion is the adhesion of paint to various materials. The most common is the adhesion of the paint and varnish substance and metal. In order to coat metal products with a layer of paint, tests of the interaction of two materials are initially carried out. It is taken into account what layer the paint and varnish should be applied in in order to determine its degree of adsorption. Subsequently, the level of interaction of the coloring film and the material with which it is covered is determined.