How reliable are the glue joints?

The field of application of glue is diverse, it is used in industrial production, construction, repair and modeling. Adhesive joints are used in assembly processes, at the stage of installation of structures and materials. Most often, glue is used in the paper industry and in wood processing.

During its history, the adhesives and mixtures were modified. Thanks to modern technology, synthetic resin adhesives have appeared that can bond any materials and surfaces. Synthetic adhesives have increased elasticity, waterproof and seal joints, and also ensure their long life. An example of the effective use of synthetic adhesives can be considered the installation of PVC pipes for adhesive bonding or the use of synthetic glue mixtures for repair and adjustment of machines (with the help of glue adjust the landing surfaces of the buildings and buildings).

The main advantage of the adhesive is the high reliability of the connection of various surfaces and parts, achieved without undue influence on the structural elements (drilling, screeding, welding).

What is glue and glue joints?

The glue is a mixture or a multicomponent composition used to join various structural elements and materials. The glued joint is an integral, strong bond formed by the adhesion of surfaces with the adhesive layer. Adhesives are different in composition. Compositions consist of organic and inorganic polymers. The basis of the adhesive composition is classified:

• Hot cured polymers;
• Polymers of cold curing;
• Heat-resistant polymers;
• Thermosetting and thermoplastic polymers;
• Polymers used in the usual temperature range.

The development of production technologies and chemistry of polymers made it possible to create compositions that provide strong adhesive joints of materials whose surfaces previously did not yield to a strong coupling: metal, ceramic or glass. In addition to the polymer base, the composition of the glue includes various additives to give the adhesive layer the desired properties and characteristics (heat resistance, moisture resistance, strength, plasticity, etc.). Additives for the intended purpose can be: hardeners, solvents, dyes, plasticizers, as well as other fillers.

The qualitative characteristics and properties of adhesive joints depend on the choice of glue and the precise adherence of the gluing technology.

Glue: conditions that determine the choice

In the selection of glue, the main role is played by the properties and characteristics of materials intended for bonding, the conditions of work (humidity, temperature regime), the estimated life of the adhesive joint and the load on it. When choosing an adhesive, you should pay attention to the cost, hygienic parameters, flammability. A feature of all glue joints is the degradation of the adhesion characteristics over time, therefore, during the entire service life, adhesion bonds are destroyed.

Decrease in the index of strength and elasticity of the adhesive layer occurs due to the influence of temperature fluctuations, humidity and other conditions. So, before gluing the building structure, technological product or PVC pipe, the adhesive joint is intensively tested for strength under the influence of the forecasted operating conditions.

General technology for bonding surfaces

The technology of bonding involves the preparation of surfaces, the mixing of components of the adhesive composition (if required), applying a thin layer of glue on the surface and waiting until a rigid coupling is obtained . One of the most important stages is the preparation of surfaces before gluing, this provides an optimal coupling. When applying the adhesive to the surface, attention should be paid to the adequacy of the thickness of the film. The assembly of adhesive joints is completed by curing the adhesive composition, which must take place in the optimal technological mode.

At the preparatory stage of surface treatment, any impurities are removed, with special solutions, the surfaces are degreased, they are roughened. The best variant of preparation of surfaces is the one in which there is noticeable cohesive destruction (destruction along the binding layer) of adhesive joints. To handle the surfaces to be bonded, both chemical and mechanical methods for improving the strength of the joint are used.

The quality of the adhesive seam depends on the way the adhesive layer is applied to the surface. The adhesive should be evenly distributed over the surface, the film thickness should be uniform and be in the range of 0.1 to 0.2 mm. The method of applying the adhesive depends on the viscosity of the composition.

For example, consider the features of the choice of glue and the technology of gluing installation of a water supply system consisting of PVC pipes.

Selection of glue for PVC pipes

The pipe for glue joint of PVC is used in engineering communications for the construction of water supply systems, it is used in heating systems and sewage systems. There are several technologies for joining PVC pipes and fittings. Most often, diffusion welding is used, however, the technology of gluing pipes with fittings is also very popular, which also achieves high strength of the glued joint.

Used for gluing PVC pipes glutinous formulations contain the main component of polyvinyl chloride, which is diluted with a solvent to obtain the desired consistency. The adhesive composition can additionally contain various additives and fillers that increase the waterproofing and sealing properties of the adhesive. The physical process of gluing is to apply glue composition on the surface of the products, then they are mixed, the solvent evaporates over time, a strong adhesive bond is formed. The use of polyvinyl chloride in the glue ensures reliable adhesion of surfaces at the molecular level.

To choose the optimal adhesive composition for PVC pipes, the following should be considered:

• Clearly know for what purposes the glue is purchased;
• It is important to get acquainted with the technology of preparation of adhesive composition, glues based on polyvinyl chloride can consist of two components, directly affect the strength of the adhesive bond;
• Setting speed, which in most compositions does not exceed 1-2 minutes (depending on humidity and air temperature);
• Adhesive compounds for PVC pipes have a different color;
• The viscosity of the ready-to-use composition.

The technology of installing PVC pipes glued

The adhesive solution is thoroughly mixed. If the main components are to be connected independently, it is important to observe the correct proportions. The pipe for the adhesive joint is prepared at the bonding site, the surface is cleaned and degreased. Installation begins with the application of adhesive composition on the surfaces to be bonded. The glue is applied with a brush with a uniform distribution over the surface, the intermittence of the film and the omission of individual areas are not allowed. After the procedure for wetting the surfaces with adhesive solution, the pipe section is combined with the fitting.

After aligning the fitting and the pipe, the adhesive bonding must be firmly fixed for 15-20 seconds, after which the initial adhesive coupling is formed and the glue thickens. It is important to make sure that the structure is fixed in the correct position before gluing, as cranking or other impact is not allowed. Adhesive bonding should be at rest for 15 minutes until the solvent evaporates completely.

Sequential gluing of all sections of the water supply system consisting of fittings and PVC pipes is carried out. Adhesive joint after 24 hours from the moment of completion of installation is checked for leaks and strength under the load of water pressure.

Reliability of glued joints

Requirements for the reliability of adhesive joints are required depending on the characteristics of the materials to be bonded and the operating conditions of the joint. However, there are a number of universal parameters that any connection must match.

Basic reliability characteristics for all types of adhesive joints:

• The strength of the bonded material with the glue joint;
• Monolithic glue seam with glued elements;
• The stability of adhesion bonds and the quality of surface treatment determine the indices of durability and reliability.

The main feature of the reliability of the adhesive seam is to ensure the strength of a joint equal to or superior to the strength of the monolithic part of the material being glued. Criteria of reliability of the glued joint, proceeding from the operation characteristics, can be:

• Water resistance: the ability of the adhesive layer to resist the damaging effects of water;
• Heat resistance: the ability of the glue joint to retain strength, monolithicity and elasticity under the influence of high temperatures;
• Biostability: the ability of the adhesive layer and the entire compound to retain its original properties under the destructive effect of biological processes.

Advantages of adhesive joints

The use of adhesive mixtures based on polymers and synthetic melts has significantly expanded the field of use of plastics. Synthetic glue, which is a solution (melt) of the polymer composition, is a product of plastic processing. The main advantage of synthetic adhesive bonding is the preservation of physical and aesthetic parameters of the materials to be glued. A reliable connection of planes on large areas is provided by polymeric adhesives. Glutinous compounds from polymer compositions are successfully used for both coupling large areas and for point bonding of small structural elements.

9 reasons to choose gluing

1. Adhesives can form strong bonds between materials that have different physical properties and sizes. The technology of bonding surpasses a number of other technologies, including welding, because it is able to connect thin sheet elements and brittle materials.
2. Gluing allows you to evenly distribute the stress across the entire plane, which is difficult to achieve using welded or riveted joints. The use of welded and riveted joints is characterized by a high concentration of point stresses only at the coupling points.
3. Adhesive joints provide a high speed assembly of structures, while characterized by efficiency. The gluing technology is universal and can replace several ways of joining elements.
4. The developed production technology and the variety of glutinous mixtures make it possible to use them at various stages of the production process.
5. The cost of adhesive bonding and its strength often exceeds the corresponding indicators of alternative designs for joining structures and elements. Connections made using glue have less weight compared to welded or riveted joints.
6. Adhesive materials have a unique ability to absorb, distribute and transfer stresses across the entire plane between the structural elements. Elasticity and deformation capacity of the adhesive layer increases the reliability of the coupling.
7. The glue can successfully connect materials and elements that are sensitive to heat, which are capable of breaking down or deforming under the influence of high temperature.
8. The glue can act as a sealing material, providing not only a reliable connection of the structure, but also an effective resistance to the destructive effect of moisture and chemical reagents. Adhesive seams well isolate heat and sound, are excellent dielectrics.
9. There are a lot of adhesive compositions developed for difficult climatic conditions, with their help, especially reliable adhesive joints are obtained. The use of special compounds makes it possible to glue structures and elements without heating equipment in conditions of low temperatures and high humidity.

Disadvantages of glue joints

The main disadvantages of adhesive compounds, in particular, obtained with the use of synthetic adhesives, is the inability to use them in harsh and particularly severe operating conditions.

Connections of structures with glue require high accuracy and a phased control of their performance. Specialists need to monitor the parameters and modes of gluing, compliance with the technology of curing the glue, as well as resting the structure during the formation of the hitch. Checking the obtained connections, the specialist is not always able to determine possible defects of gluing.

For many industrial adhesives, the curing process involves the use of heating elements. Hot-melt bonding is often impossible to perform when connecting to dimensional complex structures.

What is important to know when considering glue joints?

1. All the technologies of gluing presuppose a thorough multi-operation surface treatment, which significantly complicates the process. The process of curing the glue joint must take place under conditions of a certain humidity, maintaining a constant pressure and temperature regimes. In addition, the period of full curing can take a long time. For some types of industrial adhesives, the adhesive bonding process is performed during the gluing phase.
2. Adhesive joints are carefully designed, specialists perform the analysis of tensile and peeling loads, calculate the stresses resulting from the difference in the coefficients of thermal expansion of the bonded plane and the glue seam.
3. Adhesive seams, despite their positive properties, have a significant disadvantage - low heat resistance. The use of adhesives for assembly of structures is limited in the case of operating the structure at elevated temperatures. In addition, many types of adhesive joints can be destroyed under the influence of mechanical shocks.
4. Features of gluing elements do not give the opportunity to fully control the quality of the connection.
5. The joints obtained by gluing are not collapsible, for this reason the use of the adhesive considerably complicates the process of disassembling or dismantling the structures. In conditions of intensive operation in an aggressive environment or hard operation it is not possible to calculate the durability of the adhesive joint.