Adhesion, adhesion. Adhesion is an important property of solids and liquids in industrial applications Mortar adhesion

Definition of the concept of adhesion. Classification of adhesive compounds in dentistry. Mechanisms for the formation of adhesive joints. Conditions for the formation and nature of destruction of adhesive joints.

Adhesion- This is a phenomenon that occurs when dissimilar materials brought into close contact are joined together and force must be applied to separate them. When two materials are brought into such close contact with each other that their surface monomolecular layers can interact, the molecules of one substance interact in a certain way with the molecules of the other, experiencing mutual attraction. The forces of this attraction are called adhesion forces or adhesive forces. Unlike cohesive forces(cohesion forces), which determine the mutual attraction of molecules of the same substance in its volume.

The material or layer that is applied to form an adhesive bond is called an adhesive. The material to which the adhesive is applied is called a substrate.

Adhesion occurs in many dental restorative applications. For example, when connecting a filling with the walls of a tooth cavity, sealant and varnish with tooth enamel. When fixing fixed dentures with cements. In orthodontics, braces are attached to the surface of the teeth based on the principles of adhesion. Adhesion is also present in combined prostheses, in which they seek to impart aesthetic and functional properties to restoration, namely, when using porcelain and metal in metal-ceramic prostheses, plastic and metal in metal-plastic ones.

Figure 3.1 shows the classification of adhesive compounds used in dentistry.

Scheme 3.1. Classification of types of adhesive compounds in dentistry

It should be emphasized that there is a significant difference between the adhesive bonds of restorative materials with the tissues of a living organism and the bonds of dissimilar materials that are used in dentures.

There are several mechanisms for the formation of an adhesive bond due to different types of adhesive bonds (the classification of types of adhesive bonds is given in Scheme 3.2).

Mechanical adhesion is the seizure of the adhesive in the pores or uneven surfaces of the substrate. It can occur at the microscopic level, as in the case of polymer bonding with the etched enamel of the tooth, or at the macro level, when a plastic veneer is applied to the surface of a metal frame with special grips. A good example of mechanical adhesion is the fixation of fixed dentures with inorganic cement, such as zinc phosphate cement.

A stronger and more reliable bond can be achieved with chemical adhesion. It is based on the chemical interaction of two materials or phases that make up an adhesive bond. This type of adhesion is inherent in aqueous cements on polyacrylic

Scheme 3.2. Types of adhesive bonds *

acid, in which there are functional groups capable of forming a chemical compound with hard tissues of the tooth, primarily with calcium hydroxylapatite.

A diffusion bond is formed as a result of the penetration of a structural phase or components of one material into the surface of another with the formation of a "hybrid" layer, which contains both phases.

In practice, it is difficult to find a case of an adhesive joint in which any of the listed adhesion mechanisms would be presented in its pure form. In most cases, when using materials of various chemical nature for tooth restoration, an adhesive interaction of a mechanical, diffusion, and chemical nature takes place.

Conditions for creating a strong adhesive bond:

1. The cleanliness of the surface to which the adhesive is applied. The surface of the substrate should be free of dust, foreign particles, adsorbed monolayers of moisture and other contaminants.

2. Penetration (penetration) of the liquid adhesive into the surface of the substrate. Penetration depends on the ability of the adhesive to wet the surface of the substrate.

Wetting characterizes the ability of a liquid droplet to spread on a solid surface. The measure of wetting is the contact wetting angle (Θ), which is formed between the surfaces of a liquid and a solid at their interface (Fig. 3.1).

* Based on WJ classification. O "Brien" Dental Materials and Their Selection ", Quintessence Publ. Co., Inc, 3rd ed., P. 66.

Rice. 3.1. Contact wetting angle

When fully wetted, the contact angle is 0 °. Small values ​​of the contact angle characterize good wetting. With poor wetting, the contact angle is greater than 90 °. Good wetting promotes capillary penetration and indicates a strong mutual attraction of molecules on the surfaces of the liquid adhesive and the solid substrate.

The formation of strong chemical bonds at the interface will significantly increase the number of attachment points for one material to another. This is believed to be the case between the porcelain veneer and the tin oxide deposited on the surface of alloys with a high content of noble metals.

3. Minimum shrinkage and minimal internal stress during hardening (curing) of the adhesive on the substrate surface.

4. The lowest possible thermal stresses. If the adhesive and the substrate have different coefficients of thermal expansion, then when this joint is heated, the glue line will experience stress. For example, a porcelain veneer was applied to a metal frame in the process of firing the porcelain at a high temperature, and then the cermet prosthesis was cooled to room temperature. If materials with close coefficients of thermal expansion are selected for this pair, then the resulting stresses in the porcelain layer will be minimal.

5. Possible influence of corrosive environment. The presence of water, corrosive liquids or vapors often results in poor adhesion. The oral environment with its high humidity, presence of saliva, food, variable pH, inconsistent temperature and presence of microflora is recognized as aggressive. This has a significant impact on the reliability and durability of the adhesive joints of the restorative materials in the oral cavity.

Adhesion is usually judged by the value of the adhesive strength, i.e. on the resistance to destruction of the adhesive joint. As follows from the definition of adhesion, it is sufficient to measure the applied force to separate the materials making up the adhesion pair in order to determine the strength of the given bond. However, it is not so easy to achieve that the measured force of separation of the glued pair numerically corresponds exactly to the adhesive strength. Therefore, so many methods have been proposed for measuring the various adhesive compounds used in dentistry. With all the variety of options, there are only three mechanisms of destruction in them: during stretching, shear, and uneven separation.

When testing an adhesive joint, be sure to pay attention to the nature of the destruction. Distinguish between adhesive (adhesive detachment) and cohesive destruction. It is obvious that the fracture surface runs along the weakest link of the joint.

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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:

1. Metallurgy - anti-corrosion coatings.
2. Mechanics - a layer of lubricant on the surface of the elements of machines and mechanisms.
3. Medicine - dentistry.
4. Building. 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:

1. 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.
2. 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.
3. 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 adhesion 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 varieties listed, 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 an 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 be measured only in laboratory conditions, at the moment there are many instruments 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, 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:

1. 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.
2. 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.
3. 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, it is necessary to pay attention to the requirements of the manufacturer;
• ambient and base 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 on its own, 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- 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, 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 current strength and the speed of movement of the electrode.

Dictionary of Medical Terms

adhesion (lat.adhaesio adhesion, adhesion; sip.adhesive process) in morphology

fusion of serous membranes as a result of inflammation.

New explanatory and derivational dictionary of the Russian language, T. F. Efremova.

adhesion

f. The adhesion of the surfaces of two contacting dissimilar solids or liquids (in physics).

Encyclopedic Dictionary, 1998

adhesion

ADHESION (from Lat. Adhaesio - adhesion) adhesion of surfaces of dissimilar bodies. Due to adhesion, it is possible to apply electroplating and paint and varnish coatings, gluing, welding, etc., as well as the formation of surface films (for example, oxide).

Adhesion

(from Lat. adhaesio - adhesion), adhesion of the surfaces of two dissimilar solid or liquid bodies. Example A. - Adhesion of water droplets to glass. A. is due to the same reasons as adsorption. Quantitatively, A. is characterized by the specific work expended on the separation of bodies. This work is calculated per unit area of ​​the contacting surfaces and depends on how they are separated: by a shift along the interface or by separation in a direction perpendicular to the surface. A. sometimes turns out to be more than cohesion, which characterizes the cohesion force of particles inside a given body. In this case, the rupture occurs cohesively - inside the least strong of the contacting bodies.

The A. of solids with an uneven surface is usually small, since they actually touch only separate protruding sections of their surfaces. A. of a liquid and a solid and two immiscible liquids reaches an extremely high value due to full contact over the entire area of ​​contact. When a solid body is coated with a polymer in a fluid state, the latter penetrates into depressions and pores. After the polymer hardens, a bond is formed, sometimes called mechanical A. In this case, in order to tear off the polymer film, it is necessary to overcome the cohesion in the hardened polymer. To achieve the limiting adhesion, solids are joined together in a plastic or elastic state under pressure, for example, when glued with rubber glue or when cold welding metals. Durable aluminum is also achieved when a new solid phase forms at the interface, for example, in the case of galvanic coatings, or when surface chemical compounds (oxide, sulfide, and other films) arise.

A. polymers are best when the macromolecules are polar and have a large number of reactive functional groups. To improve adhesion, active additives are introduced into the composition of the glue or film-forming polymer, the molecules of which are firmly bound with one end to the film, and the other with the substrate, forming an oriented adsorption layer. When two volumes of the same polymer come into contact, autohesion (self-adhesion) can occur, when there is a diffusion of macromolecules or their sections from one volume to another. In this case, the bond strength increases with time, tending to the limit ≈ cohesive strength.

The A. phenomenon occurs during welding, soldering, tinning, gluing, in the manufacture of photographic materials, as well as in the application of paint-and-lacquer polymer coatings that protect metal parts from corrosion; In the latter case, the reasons for the violation of A. are the stresses arising from the shrinkage of the film, as well as the difference in the coefficients of thermal expansion of the film and metal.

A. is not only a condition for the formation of a high-quality coating, a bonding weld or glue seam, but also causes increased wear of rubbing parts. To eliminate A., a layer of lubricant is introduced that prevents the contact of surfaces.

Lit .: Krotova N.A., About gluing and sticking, M., 1956; Voyutsky S. S., Autohesion and adhesion of high polymers, M., 1960; Deryagin B.V., Krotova N.A., Adhesion, M. - L., 1949.

V.I.Shimulis.

Wikipedia

Adhesion

Adhesion in physics - adhesion of surfaces of dissimilar solid and / or liquid bodies. Adhesion is due to intermolecular interactions (van der Waals, 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 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 is also one of the most important factors affecting their strength.

In biology, cell adhesion is not just the connection of cells with each other, but their connection that leads to the formation of certain correct types of histological structures specific to these types of cells. The specificity of cell adhesion is determined by the presence on the cell surface of cell adhesion proteins - integrins, cadherins, etc. For example, platelet adhesion to the basement membrane and collagen fibers of the damaged vascular wall.

In anticorrosion protection, the adhesion of the paint and varnish material to the surface is the most important parameter affecting the durability of the coating. Adhesion - adhesion of paint and varnish material to the painted surface, one of the main characteristics of industrial paintwork materials. The adhesion of paints and varnishes can be of a mechanical, chemical or electromagnetic nature and is measured by the force of peeling off the paint and varnish coating per unit area of ​​the substrate. Good adhesion of the paint and varnish material to the surface to be painted can only be ensured by thorough cleaning of the surface from dirt, grease, rust and other contaminants. Also, to ensure adhesion, it is necessary to achieve the specified coating thickness, for which wet layer thickness gauges are used. Criteria adopted and approved for the assessment of adhesion / cohesion

Examples of the use of the word adhesion in the literature.

Negative ions, accelerating in the cyclotron, acquire a centripetal tendency, that is, they tend more toward adhesion than dissipation.

Initially, blue plays the role of a passive center adhesion, and as a result, an agglomerate is formed that does not have the properties of a codon, but actively collects those fragments of information agents, which we conditionally called dirt.

Cured epoxy resins are characterized by low shrinkage, high adhesion, mechanical strength, moisture resistance, good electrical insulating properties.

15927 0

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, crevices, 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 angle of wetting. 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 avoided 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, in which the bond energy typically ranges from 2.0 to 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 bonded 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, 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 the 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 clinician needs to know what type of bond he is seeking, and this requires an understanding of the steps in creating an adhesive bond. This will avoid mistakes in work.

Fundamentals of Dental Materials Science
Richard van Noort

During large-scale or repair concrete works, situations very often arise when it is not possible to simultaneously fill the entire concrete structure.

As a result, cold seams appear at the point of contact between the concrete layers, which lead to a loss of strength, a violation of waterproofing, delamination and other "troubles".

In this regard, when repairing concrete and reinforced concrete structures, as well as during the construction of screeds, it is necessary that concrete-to-concrete adhesion was as deep and reliable as possible.

The main reason for poor adhesion of concrete to concrete and, accordingly, the reason for the formation of cold joints and delamination is the natural process of concrete carbonization.

Free lime, as the main source of functional interaction of concrete layers, is practically absent on the surface of "old" concrete. " Under the influence of CO2 from the ambient air, active lime is converted into calcium carbonate, which is an inert substance that reacts only with acidic compounds.

Therefore, fresh concrete, which has an alkaline reaction, very poorly "adheres" to the old carbonized surface, and if adequate measures are not taken, over time it forms cold joints or "comes off".

A general case of a set of measures to ensure high-quality adhesion of concrete to concrete

• Mechanical preparation of the old surface: sanding, dust removal, removal of grease stains, etc.;
• Coating with a special primer;
• Surface treatment with special compositions "related" to each other chemists;
• Surface treatment with compositions with a high degree of "adhesion";
• The use of compounds that are not "related" to each other in chemical composition.

An example of a set of measures to ensure high adhesion of concrete to concrete

• Application of intermediate adhesive composition ASOCRET-KS / HB to the pre-treated surface. Provides the required level of adhesion to old concrete;
• Application of a non-shrinking repair compound with a high rate of curing: ASOCRET-RN - up to 20 mm of adhesion, ASOCRET-GM100 - up to 100 mm of adhesion depth;
• Application of finishing mortar ASOCRET-BS2.

The above materials have a cement-sand base modified with appropriate additives. The so-called "dry polymers" are used as additives, which are powdered high-molecular compounds.

When such mixtures are mixed with water, a full-fledged liquid polymer is formed, which gives the composition the required functional property - ensuring reliable adhesion (adhesion) of concrete to concrete.

Adhesion is the bond between dissimilar surfaces brought into contact. The reasons for the appearance of an adhesive bond are the action of intermolecular forces or forces of chemical interaction. Adhesion determines the bonding of solids - substrates - with the help of an adhesive - an adhesive, as well as the bond of a protective or decorative paint and varnish coating with a base. Adhesion also plays an important role in the dry friction process. In the case of the same nature of the contacting surfaces, we should talk about autohesion (autohesion), which is the basis of many processes for the processing of polymeric materials. With prolonged contact of identical surfaces and the establishment in the contact zone of a structure characteristic of any point in the volume of the body, the strength of the autohesion joint approaches the cohesive strength of the material (see cohesion).

On the interface between two liquids or a liquid and a solid, adhesion can reach an extremely high value, since the contact between the surfaces is complete in this case. The adhesion of two solids, due to surface irregularities and contact only at separate points, is usually small.

What is surface adhesion?

However, high adhesion can also be achieved in this case if the surface layers of the contacting bodies are in a plastic or highly elastic state and are pressed against each other with sufficient force.

Liquid adhesion

Adhesion of a liquid to a liquid or a liquid to a solid. From the point of view of thermodynamics, the cause of adhesion is a decrease in free energy per unit surface of the adhesive joint in an isothermally reversible process. The work of reversible adhesive separation Wa is determined from the equation:> Wa = σ1 + σ2 - σ12

where σ1 and σ2 are the surface tension at the interface between phases 1 and 2, respectively, with the environment (air), and σ12 is the surface tension at the interface between phases 1 and 2, between which adhesion takes place.

The value of the adhesion of two immiscible liquids can be found from the equation indicated above, from the easily determined values ​​of σ1, σ2 and σ12. On the contrary, the adhesion of a liquid to the surface of a solid, due to the impossibility of directly determining σ1 of a solid, can be calculated only indirectly using the formula:> Wa = σ2 (1 + cos ϴ)

where σ2 and ϴ are the measured values, respectively, of the surface tension of the liquid and the equilibrium contact angle of wetting formed by the liquid with the surface of the solid. Due to the wetting hysteresis, which does not allow an accurate determination of the contact angle, only very approximate values ​​are usually obtained from this equation. In addition, this equation cannot be used in the case of complete wetting, when cos ϴ = 1.

Both equations, applicable in the case when at least one phase is liquid, are completely inapplicable for assessing the strength of the adhesive bond between two solids, since in the latter case the destruction of the adhesive joint is accompanied by various irreversible phenomena due to various reasons: inelastic deformations of the adhesive and substrate, the formation of a double electric layer in the zone of the adhesive seam, rupture of macromolecules, "pulling out" of the diffused ends of macromolecules of one polymer from the layer of another, etc.

Polymer adhesion

Almost all adhesives used in practice are polymer systems or form a polymer as a result of chemical transformations that occur after the adhesive is applied to the surfaces to be glued. Non-polymer adhesives include only inorganic substances such as cements and solders.

Methods for determining adhesion

1. The method of simultaneous tearing of one part of the adhesive joint from another over the entire contact area;
2. The method of gradual delamination of the adhesive joint.

Pull-off method - adhesion

In the first method, the breaking load can be applied in a direction perpendicular to the plane of contact of the surfaces (pull test) or parallel to it (shear test). The ratio of the force overcome with simultaneous separation over the entire contact area to the area is called adhesive pressure, sticking pressure or adhesive bond strength (N / m2, dyn / cm2, kgf / cm2). The pull-off method gives the most direct and accurate characteristic of the strength of the adhesive joint, however, its use is associated with some experimental difficulties, in particular, with the need for a strictly centered load application to the test specimen and ensuring a uniform distribution of stresses along the adhesive joint.

The ratio of the forces overcome during the gradual delamination of the sample to the width of the sample is called the resistance to delamination or resistance to delamination (N / m, dyn / cm, gf / cm); often, peeling adhesion is characterized by the work that must be spent on separating the adhesive from the substrate (j / m2, erg / cm2) (1 j / m2 = 1 n / m, 1 erg / cm2 = 1 dyn / cm).

Delamination method - adhesion

Determination of peel adhesion is more appropriate in the case of measuring the bond strength between a thin flexible film and a solid substrate, when, under operating conditions, peeling of the film proceeds, as a rule, from the edges by slowly deepening the crack. With the adhesion of two rigid solids, the separation method is more indicative, since in this case, with the application of sufficient force, an almost simultaneous separation over the entire contact area can occur.

Adhesion Test Methods

Peel, shear and delamination adhesion and autohesion tests can be determined using conventional dynamometers or special adhesion testers. To ensure complete contact between the adhesive and the substrate, the adhesive is used in the form of a melt, a solution in a volatile solvent, or a monomer, which polymerizes upon formation of an adhesive bond.

However, during curing, drying and polymerization, the adhesive usually shrinks, as a result of which tangential stresses arise at the interface, weakening the adhesive bond.

These stresses can be largely eliminated by introducing fillers, plasticizers into the glue, and in some cases by heat treatment of the adhesive joint.

The strength of the adhesive bond determined during the test can be significantly influenced by the size and design of the test specimen (as a result of the so-called edge effect), the thickness of the adhesive layer, the history of the adhesive bond, and other factors. The values ​​of adhesion strength or autohesion can, of course, be discussed only in the case when destruction occurs along the interface (adhesion) or in the plane of initial contact (autohesion). When the sample is destroyed by the adhesive, the obtained values ​​characterize the cohesive strength of the polymer.

Some scientists believe, however, that only cohesive destruction of the adhesive bond is possible. The observed adhesive nature of destruction, in their opinion, is only apparent, since visual observation or even observation with an optical microscope does not allow detecting the thinnest adhesive layer remaining on the substrate surface. However, recently, both theoretically and experimentally, it has been shown that the destruction of an adhesive bond can be of the most varied character - adhesive, cohesive, mixed, and micromosaic.

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.

What is adhesion? Adhesion: definition

This phenomenon is possible between the following 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 bond, it is necessary to add certain additional substances and use 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.

Material adhesion

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 the 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 it is necessary to apply the paint and varnish substance 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.

Adhesion property

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The adhesive properties are characterized by the normal pull-off stress p of two interacting solid surfaces. An increase in the strength of adhesion increases the intensity of granule formation, but makes it difficult to work with the material due to its adhesion to the walls of the apparatus. All other things being equal, / ad significantly depends on the concentration of the binder, and this dependence is of an extreme nature.

The adhesive properties of glues of plant and animal origin are inextricably linked with their chemical nature. However, in some cases, it is difficult to reveal a direct connection between the chemical nature of the adhesive and the substrate when gluing wood, not only because of the complexity of the chemical nature of wood, but also because it is subject to more significant changes than the adhesive layer. For example, in conditions of high humidity and high temperatures, wood deforms due to swelling and shrinkage. In addition, wooden structures and products illuminated by sunlight absorb radiant energy and heat up to temperatures well above the ambient air temperature. The temperature in the plywood sheathing of an aircraft, for example, can reach 90 C.

The adhesive properties play an important role in the functioning of the dressings.

On the one hand, the lower layer of the dressing should be easily wetted, ensuring a tight fit of the dressing to the wound, on the other hand, the surface energy at the dressing-wound interface should be minimal in order to ensure the least trauma when it is removed from the wound.

Adhesion properties sometimes have a decisive influence on the choice of the method and conditions for the manufacture, storage, use and transportation of powdered materials.

The adhesion properties of various high-strength and heat-resistant enamels are approximately the same and significantly higher than that of PEL and PELU wires. When testing by twisting, samples with a length of 50 mm in accordance with GOST 7262 - 54 must withstand, depending on their dimensions, at least 7 - 17 torsions. In fact, these tests often produce better results. So, wires of the PELR-2 brand with a diameter of 0 55 - 1 20 mm often withstand up to 30 - 24 torsions.

The adhesive properties (stickiness) of synthetic adhesives have not yet been studied enough, but scientists suggest that they depend on at least two main factors: the flexibility of the macromolecule links and the presence of polar groups in it.

The adhesion properties of various high-strength enamels are approximately the same and significantly higher than those of PEL and PELU wires. When tested by twisting, samples with a length of 50 mm in accordance with the standard must withstand, depending on their dimensions, at least 7 - 17 torsions. In fact, these tests often produce better results. So, when testing PELR-2 wires with a diameter of 0 55 - 1 20 mm, the samples often withstand up to 30 - 24 torsions.

The adhesive properties of some film-forming materials depend on their plastic properties. Since the shrinkage of the film-forming materials occurs during hardening, the stresses developing between the film and the wood can lead to a significant weakening of the bond between the coating and the wood - their lagging, and in brittle coatings - to cracking. Therefore, plasticizers are introduced into many paints and varnishes, which increase the plastic properties of the coating. An increase in the thickness of the lacquer film negatively affects the adhesion properties of the coatings due to an increase in shrinkage stresses.

Adhesion properties can manifest themselves only in a monolayer of particles that have settled on the walls or filter surfaces of gas cleaning devices, and due to the very small thickness of such a layer, as a rule, they do not affect the operation of dust and ash collection systems.

Concrete-to-concrete adhesion: how, what and why?

The adhesion properties of paraffin are most strongly enhanced by atactic polypropylene and oxidized petrolatum, while their combined presence gives a synergistic effect.

Dust adhesion characterizes the tendency of dust particles to stickiness, which affects the performance of dust collectors.

The adhesion properties of substrates can be changed by grafting. Vaccination is carried out using high energy sources or in an electric field.

The adhesive properties of bitumen make it a valuable material for the production or fastening of many products.

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There are many types of fastening: welding, rivets, connection using fasteners, and so on. However, the use of an adhesive remains one of the most demanded, as it allows you to connect surfaces of very different materials and without mechanical impact on objects.

Glue laying

One of the fundamental selection factors in this case is the high adhesion of the adhesive.

What it is

Gluing is a method of permanently connecting any elements by forming an adhesive bond between the surfaces to be glued. The composition used for this is called glue. The substance can be of natural or artificial origin, but in any case it must have certain properties.

Adhesion is a property that ensures the strength of the connection of materials. After the adhesive layer has hardened, the objects should form, as it were, a single whole. If the connection cannot be separated, we can talk about the high adhesion properties of the substance.

Adhesive preparation

This quality indicates the ability of the adhesive to adhere to the surface. So, the metal is a low-porosity substance, which indicates its low adhesive properties. Ordinary glue, for example, on the surface of metal or glass, simply does not hold.

Adhesion - what is it in construction

The highly adhesive adhesive forms a bond strong enough to bond smooth surfaces.

What is cohesion? The strength that the glue itself provides when solidifying. For example, plasticine can temporarily fix two objects, but under the influence of the weight of one of them, the material is easily destroyed. The adhesive composition with good cohesion ensures bond strength.

This value is relative, since it depends on the nature and weight of the glued objects. So, the label attached to the bottle has a minimum weight, and in order to keep it, a mixture with rather low cohesive qualities is enough. But tile glue with adhesion to concrete should have increased cohesion, since tiles are a heavy product.

Mixing tile mortar

Another important composition parameter is the ability to maintain bond strength at different temperatures. In everyday life, mixtures are used that ensure setting at normal temperature, that is, about 20-30 C. However, already in construction work, when fixing stone and ceramics, when fixing metal panels and bricks, this is not enough. They produce different types of products designed for operation at different temperatures.

Adhesion, cohesion, temperature operating range of the product is regulated by GOST.

The essence of gluing

Regardless of the nature of the adhesive mixture, its mechanism of action is the same and is determined by 2 main factors.

Adhesive with good adhesion - tile, for metal surfaces and so on, is supplied to the consumer in a semi-finished form. Its components are mixed, but not fully reacted. When preparing the composition - mixing and mixing dry components with water, a chemical reaction occurs, and the substance begins to polymerize. In this case, the pasty product slowly or quickly turns into a solid state.

In everyday life, this process is called grasping or hardening. It is known that it is possible to glue materials only while the mixture is in a semi-liquid state.

Glue application

Affinity of materials - it is clear that substances similar in nature have high adhesion to each other, the only exceptions are metals. And a ceramic product - tile, porcelain stoneware, and concrete are complex compounds, they include quite a lot of various components. If the solution connecting them has a similar composition, its adhesion properties in relation to these materials will be increased. So, for laying tiles on concrete and brick bases, compounds that include cement are most often used.

How to choose a glue of increased adhesion for tiles

There is a fairly decent list of factors to take into account:

• Operating conditions - if we are talking about external finishing, then it is clear that ceramics will be exposed to low temperatures, which means that it makes sense to use only a good special composition that is resistant to frost. When it comes to facing a fireplace, the situation is the opposite - you need a material that can withstand very high temperatures.
• In addition, humidity must be taken into account. For a damp room, you will need glue that is elastic. The photo shows examples of good adhesive mixtures.
• Affinity to the base - concrete, brick, cement-sand bonds are considered a simple base for finishing with ceramics, since, firstly, they themselves are rather porous materials, and, secondly, they include many components such as cement, mineral filler, and so on. For bonding to metal or glass surfaces, mixtures are used only with specialized ones, with increased adhesion in relation to low-porosity materials.

Cement Tile Adhesive

Tile adhesive adhesion is regulated by GOST. If we are talking about a porous version, then conventional mixtures are used, even cement ones. When it comes to low porosity materials, a special solution is required. For example, porcelain stoneware and clinker, for example, fall into this category, since their porosity is very low and the usual cement tile composition does not hold the product on the wall.

GOST 31357-2007

It is used for laying heavy large-format slabs and slabs of medium size and weight made of marble, natural and artificial stone for interior and exterior works. The maximum weight of the boards to be glued is not more than 100 kg / m2 of the surface.

ADHESIVE is recommended for external cladding of bases subject to increased operational loads: plinths, columns, external stairs, basements, in internal rooms with normal and high humidity: for bathrooms, balconies and terraces.

Coating adhesion

Ideal for tiling difficult substrates such as old tiles, heated surfaces, etc.

• For interior and exterior use
• For children's and medical institutions
• Impact and crack resistance
• Application when facing "complex" substrates
• Laying slabs using the "top-down" method
• Use in the "Warm floor" system

Characteristics

Work temperature
Amount of water for 25 kg. dry mix
Layer thickness
Consumption when working with a spatula 6X6
Pot life of the solution
Tile laying time
Time to correct tile position
Hardening time
Adhesion to the substrate
Supported tile weight
Frost resistance	not less than 35 cycles
Operating temperature	from -50 to + 70 ° С
Package

The GLUE has increased strength characteristics, which allows it to be used when laying heavy boards and to operate in harsh conditions. The high adhesive capacity allows for top-down veneering.

GLUE is used on heated surfaces (up to + 70C), including in the "Heat-insulated floor" system.

The plasticity of the finished solution makes the glue easy to use. After curing, the glue retains its properties in direct contact with water and when exposed to negative temperatures.

GLUE is an environmentally friendly material because does not emit substances hazardous to human health and the environment during work and operation.