Construction Adhesives and Bonding
Construction Adhesives and Bonding
Construction and civil engineering are the largest markets for adhesives as they bond large surfaces and thick joints.Earlier bonding and sealing of traditional construction materials was done with cement, plaster and natural adhesives for ceramic tiles, erection of walls and decoration. Now, many new construction materials have been developed, such as:
- Plastics
- Plasterboard panels
- Insulation foams
- Wood-based panels (particle board), and
- Many new synthetic raw materials for decoration
All construction materials (concrete, glass, plastics, etc.) need adhesives in order to display a good adhesion, improved performances and easier application techniques.
The two types of adhesives used in construction are:
- Non-structural adhesives are used to bond decorative materials, such as floor and wall coverings, and other materials which do not need very high strength (panels, fibers and particle boards, trim, some interior doors). These adhesives may be emulsion or dispersion adhesives, neoprene contact adhesives, solvent-based adhesives, natural and renewable adhesives and glues.
- Structural adhesives give at least 5 to 8 MPa tensile shear strength. They are required for structural parts, such as bonding of concrete to concrete, ceramic tiles to concrete, masonry, construction panels, outside doors, sealing metal parts into concrete, civil engineering, composite bonding (FRP panels), metal bonding, structural glass. These adhesives may be epoxy adhesives, structural polyurethanes, silicones and structural acrylics.
All construction adhesives used on site should cure or dry at ambient temperatures i.e. 12 to 35°C according to seasons and countries, because there are no heating devices on sites. But the adhesives used to manufacture prefabricated parts in plants often use heating in order to speed up drying and/or curing.
Let’s discuss the different chemical types of construction adhesives, their formulation, characteristics, performance and applications in detail.
Chemistries of Non-Structural Adhesives
Chemistries of Non-Structural Adhesives
Under non-structural construction adhesive segment, we will be focusing on the following chemistries.- Neoprene Contact Adhesives
- Acrylic Dispersion Adhesives
- VAE Emulsions (Vinyl Acetate-Ethylene Copolymers)
- Rosin Solvent-based Adhesives
- Cellulosic and Starch-based Powder Adhesives
#1 Neoprene Contact Adhesives
These adhesives are based on neoprene or polychloroprene rubber, mostly in solvent solutions but also sometimes in emulsions formulated with the addition of tackifying resins and other additives.
Structure of Neoprene
Neoprene is the trade name of polychloroprene rubber manufactured by DuPont. The main features of neoprene rubber adhesives are listed below.
- It should be applied always on both surfaces to be bonded and after drying the solvents of solution or the water of emulsion.
- It leaves two films which will fuse together by a simple short contact (a few seconds) under 5 bars pressure, hence they are named contact adhesives. This will immediately give a strong bond (a few MPa) and this strength will double or triple within the next 12 to 24 hours.
- It is very convenient for hand jobs on-site because there is no need for clamps during the drying of adhesives, and also for fast production lines.
E.g., the manufacture of prefabricated panels, doors, etc. on fast lines because, after a short drying (1 minute) in force hot air oven at 60 to 90°C, parts may be assembled and given strong enough bonds for further processing, finishing or even shipping sometimes.
These are the reasons why polychloroprene contact adhesives have been so successful since their discovery by DuPont.
Many neoprene rubber suppliers developed neoprene emulsions in order to eliminate the danger of solvents, but these emulsions have the following disadvantages:
- They lack high tack and fast grab of solvent-based adhesives.
- They need 30 minutes waiting and pre-drying time in order to start to develop some tack.
- They do not wet substrates and stick as well as the solvent-based adhesives on impervious surfaces, such as metal, glass, plastics and elastomers.
Formulation of Polychoroprene or Neoprene Solvent-based Adhesives
A classical formulation comprises the component as discussed in the table below.
Constituents | Features |
10-15% Neoprene rubber | Neoprene® AC soft, Neoprene® AF, Neoprene® AG
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Neoprene® AD
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Neoprene® W
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Neoprene® WHV
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Magnesium oxide and zinc oxide |
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Antioxidants |
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Resins and tackifiers (4 to 8% of the total adhesive weight) |
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Solvents | All solid raw materials are dissolved in an appropriate solvent blend, generally based on a mix of:
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Method of Application
![Adhesives for Building and Construction (2) Adhesives for Building and Construction (2)](https://i0.wp.com/adhesives.specialchem.com/_/media/articles/adhesives/neo-import/2003/09/483/fig2.jpg?la=en&h=190&w=150)
- Hand tools, such as brush & trowel
- Spray guns or rollers for prefabricated parts neoprene adhesives
Then the solvents are left for 10 minutes to evaporate. The adhesive is dry to the touch and becomes tacky to the finger, and when the two coated surfaces are assembled the two films of adhesive fuse together immediately by simple contact under a pressure of 5 bars, such as nip rolls.
Parts must be aligned carefully because a strong bond is formed immediately by assembly and if one tries to separate it the adhesive films would be broken and the whole operation should be made again. The open time varies from 30 minutes to 2 hours according to the formulation.
Speed of setting: Immediately after assembly and pressing the shear strength reaches already 1 MPa, approximately and after 12 hours it will reach 2 to 3 MPa i.e. 80% of the final strength.
The advantages and disadvantages of Neoprene Adhesives are discussed below.
Advantages | Disadvantages |
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Uses of Neoprene Contact Adhesives
Neoprene contact adhesives are used to bond materials for several applications in construction, such as:
- Decorative materials, like wood panels, PVC tiles, concrete, wood, metals.
- Rubber sheets & gaskets.
- Sandwich panels made of wood, plastic or steel facings, wood frames and foam insulation boards.
- Impervious materials, such as metals, plastics (PVC, high pressure laminates, rubbers, insulation foams, etc.) & high-pressure laminates.
- Concrete and fibrous materials, such as wood & particle boards.
The are also used in footwear manufacture, furniture, construction and decoration industry.
#2 Acrylic Dispersion Adhesives
These adhesives are water-based adhesives based on acrylic ester (polyacrylates) polymerized in emulsion form. They are widely used in construction to bond decorative floor coverings and ceramic tiles adhesives.
Formulation of Acrylic Floor Covering Adhesives
Polyacrylates are internally plasticized and have low glass transition temperatures, hence they are soft and tacky at room temperature. Polyacrylates show a very good resistance to plasticizers and for this reason have been the best choice for bonding of PVC floor tiles.
Later their ease of application, excellent aging and performances also allowed it to develop into wall-to-wall carpets bonding, wall coverings and ceramic wall tiles. E.g., the features of Acronal® dispersions which may be used for both plastic and textile floor coverings are listed below.
Properties/Grades | Acronal® V 312 | Acronal® A 378 | Acronal® A 200 | Acronal® A 380 | Acronal® A 280 |
pH | 4.5 - 6.0 | 5.7 - 6.7 | 6.5 - 8.0 | 5.5 - 6.5 | 3.5 - 6.0 |
Solids content, % | 55 | 62 | 70 | 62 | 69 |
Viscosity, mPa.s | 30 - 150 | 100 - 250 | 150 - 500 | 50 - 350 | 200 - 1000 |
Glass transition temperature, °C | -21 | -22 | -43 | -22 | -43 |
Uses | Offer broad range of adhesion for low-solvent and solvent-free adhesives. | For low emission adhesives having a broad range of uses. | For weak odor adhesives free of resins; very low emissions as per EC 1 requirements | Used in formulations with soft resin; very low emissions as per EC 1 requirements. | For the production of surface tackifier for carpeting, in particular with felt backing; very low emissions as per EC 1 requirements. |
A typical formulation for floor covering adhesives comprises of the following constituents:
Constituents | Content |
Acrylic Emulsion | From 25 to 35 % by weight in wet form |
Tackifying resin, such as rosin, rosin esters or others | ~10%* |
Fillers such as calcium carbonate or others | Acrylic emulsions accept high filler ratio up to 50 to 100 % based on emulsion weight |
Emulsion stabilizer | As needed |
*If it is a solid form it should be dissolved first in a solvent and then mixed under high speed agitation with the emulsion, but some tackifying resins exist also as dispersion which may be readily mixed with the acrylic emulsion. |
Method of Application
All acrylic emulsion adhesives are used by one-way application i.e. on one substrate, which is generally the solid one (concrete or plaster wall). At least one of the substrates must be able to absorb the water of the adhesive in order to dry the adhesive film, but before assembly of the floor covering the worker should let the adhesive film pre-dry for 10 to 25 minutes at ambient temperature (preferably 15 to 30°C) so that the adhesive develops a high tack and will grab the floor covering immediately when assembling.
Acrylic floor covering adhesives are applied with a notched trowel. The depth of teeth controls the amount of adhesive applied, which should vary from 200 to 350 g/m2 according to roughness and flatness of materials.
After pre-drying, the floor covering is laid on the adhesive and pressed with a suitable hand tool, and the adhesive will finish drying at room temperature after a few hours through the absorption of its water by the substrates or materials. The bond will reach 100% of its final strength after 24 hours.
When these acrylic adhesives are used to bond plasticized PVC floor coverings:
- The peel strength will reach 10 to 20 daN for 5 cm width
- Bonds will resist to 10 or 20 years aging, and
- Frequent cleaning of the floor with water or some humidity in the ground.
Some acrylic emulsion grades have a good resistance to water and humidity.
Formulation Acrylic Emulsion Adhesives for Ceramic Tiles Bonding
The table below lists the Acronal® acrylic adhesives which may be used for the formulation of wall ceramic tiles adhesives. Acronal® S 533 which displays a very low minimum film forming temperature (MFFT) of -1°C allows to use this emulsion on-sites at low temperatures, while the Acronal® 290 D needs the addition of a film forming agent or plasticizer in order to allow using it in European climates.
Properties/Grades | Acronal®290 D | Acronal® S 533 | Acronal® S 710 | Acronal® DS 3534 |
MFFT, °C | 22 | 1 | 24 | 5 |
Solids content, % | 50 | 51 | 50 | 50 |
Standardadhesives | X | X | X | X |
Water-resistantadhesives | X | X | ||
Formulationswithout film forming agents | X | X | ||
Description | Standard | Alternativefor low-emission standard adhesives | Water-resistant | Alternativefor less-emission water resistant adhesives |
Formulationswithout film forming agents |
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List of Acronal® Acrylic Dispersions for Ready-to-use tile Adhesives (Source: BASF)
A typical formulation for ceramic tile includes the following constituents:
Constituents | Content |
Acrylic base emulsion | 20 to 30 parts |
Solvents | 1 to 2 % to speed-up drying |
Mineral Filler | They must fill the gap between a construction surface and the back of the tiles which are never flat (castellated or embattled for the purpose of manufacturing the tiles) |
Plasticizers & Other Additives | As needed |
Method of Application
Here the tile adhesive will be applied on the concrete, plaster or panel substrate with a flat trowel first in order to deposit a thick coat (1 to 2 mm) which will flatten the wall surface. Then, it will be notched with a notched trowel with large teeth.
- The worker will let the adhesive thicken a little bit for a few minutes, and then apply & press the tiles in the adhesive bed.
- The ridges or fillets of adhesive will be squeezed by hand pressure and transfer to the back of the tiles (the transfer should be at least 80% of the surface)
The adhesive coat will slowly dry through absorption of the water by both substrates and through the joints, within 12 to 24 hours, according to the formulation, drying speed, and the bond strength will reach 90 % of its final strength after 24 hours approximately. The joints may be filled with a powder based jointing compound or grout after 6 hours.
Some of the performance features of these type of formulations include:
- Good resistance to water which permits the use in wet rooms, such as kitchens, bathrooms and even shower rooms.
- Very good mechanical resistance up to 2 or 3 MPa of tensile or shear strength.
- Very good resistance to aging up to 20 years in wet rooms.
However, there are certain drawbacks associated with them, which include:
- These adhesives which contain as much as 30 % water may fill gaps only up to 1 to 2 mm thickness because they will shrink when drying. Therefore, they are only used on walls and not on concrete floors because they are usually not flat. They require mortars (cement-based mortars) very often that can harden a thickness up to 5 or even 8 mm.
- Acrylic emulsion adhesives are more expensive than cement-based mortars (0.4 Euros per kg) for large users versus only (0.15 to 0.2 Euros) for cement-based mortars.
Uses of Acrylic Adhesives
Acrylic adhesives in thick form as a mastic or putty are also used for bonding ceiling tiles, insulation boards, panels.
Note: Acrylic emulsions are also used to formulate pressure sensitive adhesives (PSA) in construction.
#3 VAE Emulsions (Vinyl Acetate-Ethylene Copolymers)
These emulsions are also internally plasticized like the acrylic emulsions and find the same applications. In some countries they are used in formulations for floor coverings and ceramic tiles bonding.
These are based on hom*o or copolymers of vinyl acetate in a water emulsion or dispersion. They are white liquids with low viscosities (400 to 5000 MPa.s) and pH from 4 to 5. Base emulsions have generally dry solids from 55 to 60%.
When the emulsion is spread on a substrate, it will start to lose its water and the particles of the dispersed polymer will come in contact and will fuse together if the temperature is higher than the film forming temperature (10 to 30 °C), depending on the particle size and protective colloid. Therefore, it is often necessary to add a coalescing agent.
In order to get a ready-to-use adhesive, these emulsions should be formulated by:
- Adding a plasticizer, such as phthalates (keeping their toxicity legislations in mind) at a ratio of a few percent based on total weight of adhesive.
- A small amount of solvent (1 to 3%) to speed up drying and/or to facilitate the film forming and coalescence at low temperatures (10 to 15°C) which may occur in some construction sites in some countries, although such low temperatures of application are not recommended in Europe or USA, where the work on site should wait till the ambient temperature reaches at least 15°C.
In construction adhesives, these PVAc emulsion adhesives would contain mineral fillers, in order to lower cost, improve the gap filling properties, increase the viscosity in order to get a nice creamy consistence which may be applied with a notched trowel without sagging. Theses fillers are usually calcium carbonate, clay, etc.
When a high degree of water and heat resistance is needed, PVAc emulsions may be compounded with water-soluble thermosetting resins, or a copolymer emulsion having some reactive sites on the molecules may be used and crosslinked by addition of a suitable crosslinking agent or hardener. These are the ways to manufacture the E3 and E4 water resistant vinyl adhesives for wood and construction (according to European standard EN 204).
Method of Application
![Adhesives for Building and Construction (4) Adhesives for Building and Construction (4)](https://i0.wp.com/adhesives.specialchem.com/_/media/articles/adhesives/neo-import/2003/09/483/fig1.jpg?la=en&h=189&w=150)
On construction sites, pressing systems are not available and therefore the PVAc adhesives which are used should have a high viscosity from 5000 to 30000 mPa.s, so that parts will be maintained together by some suction. For instance, a parquet PVAc adhesive will have:
- Viscosity of 20000 mPa.s
- Dry solids of 60%
- Specific gravity of 1.27 because it contains fillers
- Waiting time of 15 to 20 minutes at 20 °C
- Open time of 25 minutes at 20°C.
PVAc adhesives provide fairly high mechanical resistance (e.g. for wood to wood bonds the tensile shear resistance reaches 5 MPa).
- They bond to all porous materials, such as wood, concrete, plasterboard and plaster, textiles, cardboard, etc.
- They do not bond usually to impervious materials, such as metals, glass, plastics or rubber (except some specially modified formulations which may bond, for instance wood to metals).
- They are classified as E2 water resistant according to European standard EN 204, but some 1K formulation may be E3 and some 2K formulations are classified as E4.
Uses of VAE Emulsions
- Bonding of wood to wood, wood to concrete, such as bonding wood parquets to concrete floor
- Joining of prefabricated laminated parquets planks
- Prefabrication of insulating panels, such as bonding of foams to plasterboard
- Prefabrication of doors, wood windows with water and heat-resistant PVAc adhesives
- Bonding some wall coverings (e.g. textile or paper backing)
- Primer and tie coat when applying new concrete to old concrete
- Vinyl copolymers may also be prepared as dry powders which are used in cement-based mortars in order to improve adhesion.
» Select the suitable Vinyl Acetate-Ethylene Copolymers (VAE) Emulsion for your needs
#4 Rosin Solvent-based Adhesives
Natural wood rosin or gum rosin is a cheap and renewable raw material for adhesives and glues. It is very tacky in solution of organic solvents. It is also a hard and brittle resin which cannot be used as it is, but it may be plasticized by addition of some soft polymers, such as rubbers (polyisobutylene), acrylic copolymers, etc.
Such a formulation will include:
Constituents | Percentage (%) |
Rosin such as WW grade | 20 to 25 |
Soft polymer | 15 to 20 |
Mineral Filler | 15 to 20 |
Antioxidants, Stabilizers and Solvent blends | As needed |
Method of Application
The adhesive on the concrete floor or the floor leveling compound is applied:
- At a rate of 350 to 450 g/m2 depending on the roughness of the carpet backing
- 5 minutes waiting time at 20°C as the solvents evaporates quickly
The carpet is laid onto the adhesive coat, pressed by manual tools, such as wood rounded tool. A complete drying of this adhesive takes 6 hours at 20 °C.
Some of the performance features of these type of formulations include:
- Peel strength after 12 hours reaches 15 kg for a 5 cm width.
- Aging resistance is fair because rosin will oxidate and become harder after several years, but it is good enough to last for 10 years which is the service life of the carpets.
Uses of Rosin Solvent-based Adhesives
- They are used mostly to bond wall-to-wall all textile carpets.
- They may or may not have a good adhesion to foam backing and to the Action back carpets where the backing is made of polyester and polypropylene yarns.
- By using an acrylic solvent-based polymer as a modifier the rosin adhesive will bond to these backings.
» View All Gum Rosins Suitable for Solvent-based Adhesives
#5 Cellulosic and Starch-based Powder Adhesives
These adhesives are used to bond wallpapers. The powders are dissolved in water at low solids (4% for starches) and (1.5 to 2% for cellulosics), to obtain viscosities from 3000 to 15000 MPa.s. These adhesives are applied only on the back of the wallpaper, at a rate of 200 g/m2 wet.
The water of the adhesive penetrates the paper and swells it. It will enlarge by 5%, then the wallpaper is applied on the wall and the water will be absorbed by the porous wall. The wallpaper will shrink back during drying of the water so that the air bubbles or wrinkles which may have been formed during application will also shrink and the wallpaper will be perfectly flat after drying.
The large amount of water in the glue must dry for setting of the adhesive so the substrates must quickly absorb this water. These adhesives can be used only on plaster, concrete or wall putties and renderings or wood-based panels. These adhesives can bond only to paper and fibrous materials.
Chemistries of Structural Adhesives
Chemistries of Structural Adhesives
Under structural construction adhesives segment, we will be focusing on the following chemistries.- Epoxy Adhesives
- Polyurethanes (PU) Adhesives
- Acrylic Adhesives
- Silicones Adhesives
- MS Polymers (Hybrid) Adhesives
- Cement-based Mortars
- Plaster-based Putties
- Adhesives Used for Prefabricated Parts
#1 Epoxy Adhesives
Construction and civil engineering may only use on-site adhesives which cure at ambient temperatures from 5 to 40°C. Temperatures below 5°C would not allow a proper cure or drying. Thus, the 2K epoxy adhesives are employed in the construction industry.
Formulation of 2K Epoxy Adhesives
2K epoxy adhesives are made of two components, Part A and Part B. The Part A called the resin, is generally a diglycidylether of bisphenol A. These epoxy resins are difunctional with epoxide groups at both ends of the molecule, and the epoxide equivalent weight is the half of the molecular weight.
Structure of Epoxy Resin
The epoxy groups may react with polyamines, polyamides, anhydrides, and polysulfides. This reaction will give a fully crosslinked product as illustrated below.
Crosslinking of Epoxy Resin by Polyamines
The grades which cure at ambient temperatures are the liquid and semi-solid epoxy resins, with epoxy equivalent weight from 170 to 500. For instance, Araldite® 6010 is a liquid epoxy resin with a viscosity of 13000 MPa.s and epoxide equivalent weight of 190.
Other epoxy adhesives may be formulated with epoxy novolac resins, but they are very brittle.
Part B, also called hardener or curing agent with different chemical types as listed below
Polyamines - Primary amines (RNH2) react with two epoxy groups while secondary amines R-NHR' will react with only one. Tertiary amines cannot react readily with epoxy groups but may act as a catalyst to accelerate the epoxy reaction. In construction adhesives formulations, use of liquid aliphatic polyamines or their adducts provide fast curing at ambient temperatures as well as excellent chemical, water and solvent resistance.
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Cycloaliphatic amines may cure at low temperatures and in humid weather. They givehigh heat resistance and toughness. They have longer pot life than previous amines and allow thicker joints. For these reasons, they are often used to prepare thick mastics or grouts in civil engineering. |
Polyamides are available in a large range of molecular weights, and the hardener to resin ratio is much higher than with the amines (1:1 ratio is possible, and this ratio is also less critical). A deviation of 10 or even 20% is acceptable without degrading the performances of the cured product. Polyamides give higher flexibility, but other properties tend to be inferior, such as water resistance, modulus and slower speed of reaction. |
Polysulfides and polymercaptans may also be used as hardeners. They give non-brittle, more flexible epoxy adhesives, and polymercaptans give fast curing at room temperature (some formulations may harden after 10 minutes only). |
Find Suitable Curing Agent for Your Application » |
Apart from curing agent, epoxy formulations also include these constituents as well.
- Reactive diluents are low viscosity, low molecular weight mono or difunctional epoxies, that will reduce the viscosity of the adhesive.
- Fillers (e.g. talc, silica) are added to:
- Modify viscosity and thixotropy (non-sagging mortars for wall applications)
- Reduce the cost of the adhesive reduce the exotherm.
Fillers allow the adhesive to fill gaps up to 1 or 2 mm thickness (epoxy mortars for civil engineering, bonding of ceramic tiles and stones) . - Flexibilizers, such as plasticizers & polysulfides
- Elastomeric modifiers or Tougheners are used to increase the peel strength or toughness of epoxy adhesives. Unmodified epoxy adhesives may be very rigid and brittle so it is necessary to add tougheners.
E.g., carboxyl terminated butadiene acrylonitrile (CTBN) or amine terminated butadiene acrylonitrile rubbers. CTBN is usually reacted with the epoxy in order to improve compatibility and toughness. The ratio is usually 3 to 25% parts of rubber for 100 parts of epoxy resin. Toughened epoxies provide high peel strength, impact resistance with little loss of modulus and mechanical resistance.
Characteristics of Epoxy Adhesives and Mortars
- Pot life depends on the hardener, formulation, temperature and the quantity mixed. It may vary from 40 minutes for an epoxy-polyamine formulation to 100 minutes for an epoxy-polyamide, when mixing 1 kg of the 2 components at 20°C.
- Hardening time necessary to reach 50% of maximum resistance may vary from 30 minutes up to 15 hours according to formulation, hardener, and temperature.
- Excellent adhesion to many materials, such as concrete, metals, stone, ceramic tiles, glass, plastics, thermosetting composites & wood.
- High mechanical strength (from 15 to 35 MPa) for the tensile shear resistance, according to the materials to be bonded and the formulations.
- Some formulations are brittle so that it may be better to use polyamide or polysulfide hardeners.
- Peel strength are often low (1 to 8 N/mm width according to formulations), thus it may be necessary to use the toughened epoxies which are more flexible.
- Excellent resistance to water, chemicals, and solvents (full crosslinking).
- The figure below provides tensile stress/strain curves for different epoxy adhesives and helps to select the type of adhesive with a modulus adapted to requirements of the bond.
Typical Tensile Stress/Strain Cruves for a range of Epoxy Adhesives used in Construction
- Good heat and cold resistance, from - 100 to +100°C for service temperature, but some formulations start to soften at 50°C (see figure below)
Temperature Dependence of Bulk Epoxy Adhesive Shear Strength - Excellent resistance to fatigue
- 104 cycles at 50 % of the maximum load
- 105 to 106 cycles at 30%, and
- >107 at 20%
- Besides bonding, epoxies are also used for fixing/anchoring steel bars into concrete as shown below.
Fixing of a steel bar in a Tunnel Made of Concrete or Rock with Epoxy Mortar
One of the largest suppliers for ready-to-use formulated epoxy adhesives is Huntsman Chemicals (Araldite®) which is used in aircraft construction.All the major construction adhesive manufacturers that offer epoxy adhesives and mortars include:
- In Europe: SIKA, Henkel and Henkel-Teroson, Bostik Findley, PCI Germany, Mapei Italy, Dyckerhoff Germany, Deitermann Germany and others.
- In the USA: H B Fueller, Vantico, ITW, Stonhard, Master Builders, SIKA, and others.
Learn more about the fundamentals of epoxy resins in detail. Uncover the main chemical classes and selection tips to find the right product for your adhesive or sealant formulation.
#2 Polyurethanes (PU) Adhesives
Polyurethanes are semi-structural that provide a tensile shear resistance of 5 to 12 MPa. The polyurethane chemistry is rich in many polymers, reactions types, and raw materials.
There are many different types of PU formulations for use in construction. The different possibilities are:
- Solvent-less formulations, 1K or 2K – They may be rigid or flexible according to the formulations.
- Solvent-based formulations, 1K or 2K – They are generally flexible.
- Solvent-less formulations – They harden by reaction with the humidity in the ambient air or in the materials to be bonded.
- PU-reactive Hot-melts – They are used in prefabricated parts (not on-site)
Characteristics of Polyurethanes
- Adhesion - PU adhesives adhere to many substrates, such as metals, glass, wood, plastics, rubbers. They use many adhesion systems like hydrogen bonding, covalent chemical bonds or true chemical reaction with the materials to be bonded. They also stick quite well to concrete but the high alkalinity of cement is detrimental to long range adhesion.
- Flexibility - PU adhesives are much more flexible than the epoxies and some of the industry players have developed theories for using PU adhesives in construction, transportation, etc. for semi structural bonds. They explained that the work necessary to break a bonded joint may be higher with a flexible PU (which allows a much higher strain) compared to a rigid epoxy (which displays a high shear resistance but a low strain)
- Crosslinkability - PU adhesives which crosslink through reaction with humidityIn PU adhesives and sealants, the isocyanate groups at the end of the PU polymer react with the water contained in the air or in the materials. They may harden even in thick joints up to 20 mmm thicknesses.
- Extrusion - The adhesive or sealant may be extruded, for instance from a cartridge with a caulking gun and it will cure after 6 to 24 hours according to formulation, humidity, joint thickness, and ambient temperature.
- Formulation - PU may be formulated as foamable adhesives and sealants, which are sold in aerosol cans from 500 g to a few kg sizes. When the valve is opened the product is extruded and reacts quickly with the humidity in the air, materials, and foams (with the CO2 which results from the chemical reaction and with the foaming agent in the formulation). Then it hardens in a few hours to give a foam of specific gravity from 30 to 50 g/cm3. With 1kg of product one can get as much as 20-25 liters of foamed adhesive or sealant. These PU foams are not structural because the foam cells break under low stresses, but they are used to position and bond a door frame into the masonry, with an additional metal fastener.
#3 Acrylic Adhesives
Structural or modified acrylics are thermosetting adhesives that are used in thermoplastic non-structural applications. Modified acrylic adhesives are based on polymethyl methacrylate grafted to either vinyl-terminated rubbers or polyurethane. A small quantity of hardener or catalyst is added to start polymerization. The modified acrylics used in construction may be one part, 2 part, or one part with accelerator, ambient curing systems which harden in a few minutes up to a few hours at 23°C.
According to formulations, the 2 components may be mixed just before application or the hardener may be applied on one surface and the resin or monomer on the other surface. The parts are assembled immediately and pressed together for a given time which depends on formulations.
- They have gap filling properties up to 5 mm thickness.
- They provide high shear resistance of 15 to 35 MPa to metals, plastics, reinforced plastics or composites.
- Some formulations are able to absorb some quantities of rolling oils on steel.
- They display high peel strength and impact resistance, they are much less brittle than epoxies.
- They have good water resistance and weathering properties, and therefore they are widely used now for construction of boats and outside bonds of plastics and composites. But their cold resistance is generally - 40°C.
Get Inspired with Structural Acrylic Starting Point Formulations»
#4 Silicone Adhesives
Silicones are mostly known as sealants, but some grades may be used as adhesives, such as the silicones for structural glazing of glass windows bonded to metal frames in the construction of curtain wall facades.
They have high flexibility combined with high peel and fair shear resistance. They are semi-structural because their shear resistance is only 1 to 3 MPa, and they can hold large glass windows on facades for a long period of time (probably up to 40 years or even more).
For instance, DOWSIL™ 993 Structural Glazing Sealant offers the following properties:
- Dry to the touch: 80 to 100 minutes at 25°C and 50% RH.
- Excellent adhesion to various glasses, anodized aluminum and stainless steel profiles.
- They provide the following mechanical properties after full polymerization of 7 days at 25°C and 50%, such as:
- Tensile resistance: 0.95 MPa
- Tear resistance: 6 kN/m
- Elongation at break: 130%
- Hardness: 40 (Shore A)
- Service temperature: from -50°C to + 150°C
- Long term durability
In order to bear the load of heavy glass panels it is enough to design the bonds so that the bonded surface will be large enough.
#5 MS Polymers
Silyl-terminated polyether adhesives is end capped by methyl dimethoxy silane, which may cure by moisture with a catalyst. Hybrid silane polymers display good adhesion to many materials, such as metals, plastics, composites, wood, ceramics and concrete.
Advantages | Disadvantages |
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Get Inspired with SMP-based Starting Point Construction Adhesives Formulations»
Learn more about silane hybrid prepolymers to achieve a better balance of mechanical properties (strength, elongation, viscosity control…) in your adhesives.
#6 Cement-based Mortars
Cement-based mortars may be considered as adhesives and are structural because they provide tensile shear resistance up to 20 MPa on concrete, ceramic tiles, bricks or stones. Their formulations are based mostly on cement and sand like a cement mortar.
In order to improve ease of use, adhesion to various substrates and other performances formulators have added several types of solid additives as listed below.
- Cellulosic resins
- Acts as a water retaining agent - When the mortar is mixed with water, the cellulosic resin swells and dissolves quickly in a mode similar to the wallpaper adhesives, and retains the water in the mix so that there will be enough water in the mix to react with the cement, even if the weather is hot or the substrates absorbing.
- Acts as a viscosity adjuster - The cellulosic resin will also help to adjust the viscosity of the paste so that it will not sag on vertical surfaces and the ribs resulting from application with a notched trowel will not flatten on horizontal floor surfaces. These ribs are necessary in order to get a good control of joint thickness and a good transfer of the cement adhesive to the back of the tiles or stones.
- Examples of can be found in our Adhesives Ingredients Material Selector Database.
- Adhesion promoters
- They are used to increase adhesion on concrete, ceramic tiles, glass mosaics or stones)
- Vinyl copolymers are solid powders which dissolve quickly when the user adds water to the powder mortar. These vinyl copolymers increase the adhesion of the mortar to concrete and ceramic tiles.Examples of these vinyl powders are VINNAPAS® by WACKER or styrene-acrylate powders such as Acronal® by BASF that act as flexibilizers and adhesion promoters.
- Other can be found in our Adhesives Ingredients Material Selector Database.
The proportions of these additives are small a total of 1 to 3% based on the weight of cement is enough.
Characteristics of Cement-based Mortars
- Water to powder ratio is usually 20 to 30%.
- May fill large gaps up to 20 mm thick.
- After mixing, the mix may be used during several hours.
- Hardening time must be long enough to allow the worker to install several m2 with one mixed bag.
- Open time after spreading the mortar on the substrate is usually half an hour.
- The mortar is spread with a notched trowel on the wall or floor with a coverage of 2 to 7 kg/m2 because it is necessary to fill the gap between the substrates which may be uneven and the back of the tiles which may be grooved or fluted (not flat) and the finished surfaces must be perfectly flat however after tile installation.
- Cement-based mortars stick well to concrete, cement, stones, ceramic tiles, glass mosaics, but they do not stick to metals or plastics.
- Their tensile resistance is quite high, usually around 10 to 20 MPa.
- Water resistance is excellent, similar to the water resistance of cement or concrete.
Uses of Cement-based Mortars
These cement-based mortars are used mainly for the following applications.
- Ceramic tiles and stones bonding onto concrete walls & floors, inside & outside the buildings.
- They are used to bond cellular concrete blocks, insulation hollow bricks, prefabricated building blocks.
- They also for repair of worn concrete.
#7 Plaster-based Putties
These are mixtures of plaster and a few percentages of powder additives which bring improved properties:
- Cellulosic resins control the viscosity and rheology of the paste after mixing it with water.
- An additive slows down the setting (hardening) of plaster.
- Parts to be bonded may be slightly adjusted during the first hour to half hour before the plaster starts to harden.
- Powder vinyl copolymers increase the adhesion of the mortar to the various construction materials (concrete, wood, metals)
Uses of Plaster-based Putties
- Jointing and bonding of plaster blocks & plasterboard
- Repairs of cracks
- Bonding of insulation panels on walls
These plaster "mortars" are applied with a trowel to the required thickness which may be from 1 to 10 mm.
#8 Adhesives Used for Prefabricated Parts
Many parts of buildings are prefabricated in a plant, for example doors and windows, panels, insulated panels or laminated arches. This is done on high speed lines and it requires:
- Fast bonding, fast drying or curing adhesives, such as hot-melts, solvent-based adhesives or
- Heat-cured thermosetting adhesives, such as Urea-formaldehyde (UF) or resorcinol-formaldehyde.
Hot melts are solid at ambient temperature. On heating up to 160 to 200°C they melt and can be applied on the substrates, by cooling back to ambient temperature they become quickly hard again. Their open time is very short (only 1 to 5 seconds), and the hardening time is also very short (from 4 to 10 seconds for most formulations).
Hot melts for prefabricated parts are usually based on a mix of 3 components:
- A plastic polymer which is usually ethylene-vinyl acetate copolymer (EVA)
- A resin which provide adhesion to specific substrates, tack and heat resistance
- A wax that can melt quickly and is useful to control melting time, viscosity and rheology of the adhesive.
EVA hot melts are used for bonding edges to panels on edge banding machines, for profile laminating, cabinets, countertop assembly, and sometimes to bond insulation panels to plasterboard, plywood, fiber cement, etc.
The heat resistance of EVA hot melts is limited to 60-70°C because they start to soften at these temperatures. They are thermoplastic due to the polymer and wax. Their water resistance is only fair thus for these reasons they may be used only inside the buildings.
PU-reactive hot melts provide high cohesion and high heat resistance than EVA. They are used for windows assembly, and bonding parts that must withstand temperatures up to 80°C.
Formaldehyde Resins
The UF resins and adhesives cure by hot pressing at 70 to 110°C. They are widely used for the lamination of decorative veneers or foils and films (PVC, melamine paper) to plywood, particle board or MDF.
They may be applied by roller gluing machines, with a coverage of 80 to 120 g/m2, then the laminate is applied on the glue film and parts are pressed for a few minutes at temperatures ranging from 70 to 110°C.
Resorcinol-formaldehyde are 2 components adhesives that cure at 45 to 60°C. They give a hard and cohesive bond on wood, with long durability, high creep resistance, high heat resistance and for all these reasons they have been widely used to manufacture laminated beams arches for large buildings (stadium, shopping centers, markets, swimming pools, etc.)
Other types of adhesives are used for prefabricated parts, such as:
- Neoprene solvent-based adhesives may be sprayed on automatic lines, dried with forced hot air ovens, then assembled parts are pressed through nip rolls that gives an immediate bond by contact.
- Vinyl emulsions are dried by hot pressing, used for wood bonding, and as primers for bonding new concrete to old one.
- Polyurethanes (solvent-based, solvent less), 2 components are used to provide a quick setting and curing.
Having learnt about the different chemistries of adhesives used in structural and non-structural applications, now let's explore the technical, physical and mechanical characteristics of these adhesives.
Technical Characteristics of Construction Adhesives
Technical Characteristics of Construction Adhesives
Surface Preparation
In order to get a strong and durable bond with construction adhesives, it is always mandatory to prepare the surfaces of the substrates so that they are:
- Clean - The adhesive should not stick to a dirty, dusty oily or polluted surface
- Sound and strong - The adhesives should have no loose or weak particles
- Dry - Any moisture in the substrates would not be acceptable with solvent-based or hot melt adhesives
- Flat - The two substrates will come in intimate contact together and with the adhesive
- Absorb the carrier (water or solvent) - To allow the adhesive to dry
Surfaces Cleaning and Chemical Preparation
- Concrete must be cleaned in order to get rid of dust, loose particles, pollution (such as: oils, loose paints) by:
- Washing
- Wire brushing
- Sand blasting
- Followed by vacuum cleaning or at least a good washing and wiping
- Metals and plastics must be cleaned in order to get rid of rust, dirt, pollution, oils, etc. by wire brushing and solvent wiping. For structural bonding, metals will need adequate surface preparation by chemical etching.
- Glass should be cleaned by solvent wiping, cleaning with water + surfactant, or chemical cleaning and preparation, followed by a thorough water rinsing in order to eliminate all contaminants.For structural bonding, glass will need a chemical treatment with a special primer ( silanes ) as recommended by the adhesives and sealants suppliers.
Flatness of Substrates
Most adhesives require a thin joint (not more than .1 or .2 mm thickness) so that substrates should be flat and should match closely:
- Wood surfaces must be flattened and smoothed
- Concrete floors may need a leveling compound to make it flat enough before sticking a thin floor covering (such as a plastic floor covering)
- The surface of panels to be covered with a thin plastic film should also be perfectly flat so that no defect will be visible after bonding.
However, some adhesives (such as the epoxies) and mortars may be used in thick layers, up to 1 mm for epoxies and up to 5 mm for cement or plaster based mortars. In these cases there is no need for a leveling compound.
Also, on the other hand, some desirable roughness is required in order to provide mechanical adhesion or "keying". Roughness may be measured with SURFTEST roughness meters.
Leveling Compounds
Powder (containing cement, sand and special additives) is mixed with water immediately before spreading on the concrete floor with a special trowel, in order to get a smooth, flat, hard and clean surface having also the required absorption.
On walls it is also necessary to smooth the surface with a filling compound or a smoothing compound, when bonding a thin wall covering (wallpaper, thin wall covering) or else the surface defects would show through.
Surface Treatments - Primers
- If the surface of substrates is too smooth and impervious, and if the adhesive is supposed to stick through mechanical adhesion, it will be necessary to sand or abrade the substrate with a coarse sand paper or an abrasion wheel, to make it rough in order to increase the adhesion.
- If the surface is too smooth or difficult to adhere, some adhesives or sealants will need a primer that should be applied prior to the adhesive, and will provide chemical adhesion.
- Also, when the substrates absorb too much the adhesive, a primer or first coat should be applied before the adhesive, in order to fill the voids.
- For structural bonding of metals or glass, a specific chemical surface treatment should be made before the application of the adhesive or sealant.
- The surface treatment and recommended primer are usually indicated by the adhesive or sealant supplier or manufacturer.
Humidity Content
Most adhesives do not withstand humidity in the substrates. But in construction and civil engineering, moisture frequently exists inside or on the surface.Therefore it may be necessary to measure the water content with a humidity tester, refrain from bonding when the humidity exceeds a given value, and let the substrates dry, according to the type of adhesive.
As a general rule, in construction and decoration bonding, concrete substrates should not contain more than 3% humidity and wood substrates should have a humidity content near to the content it will have in the finished product and in the rooms where it will be used.
Moreover, when the weather is very humid (more than 75% RH) and cool (less than 20 - 25°C), the humidity in the air may condense on the surface of the solvent based adhesives, a blush of water condensation ( matt aspect ) will be noticed, and it will prevent a good bond.
Application Temperature (On site)
- Should not be too high (not more than 35°C): When temperature is too high drying may be too fast for some solvent based adhesives, and open time will be too short.
- Should not be too low (not less than 5°C): It would slow down drying and it would harm some water-based adhesives (for instance, emulsion vinyl adhesives will not give a continuous polymer film if temperature is too low)
Mixing Ratio and Pot Life
These terms are used for two component adhesives and cement-based mortars. Weight of each component should be measured as indicated in the Technical data sheets, within + or -5 %.After mixing, the chemical reaction will begin and the mix should be used within a given time which is called pot life.
Mode of Application or Tools
Adhesives should be applied with the proper tools which include:
- Notched trowel for floor and wall coverings: The amount and thickness of adhesive will depend on the size of the teeth of the trowel, and also of the materials to be bonded. (Strictly follow the indications on the TDS of the material)
- Brush, paint roller or spray gun, according to the job and adhesives.
- Hand gun with a cartridge for sealants and for thick, gap filling mastics.
Types of Hand Spray Guns for Construction Adhesives
Tools for Application
As a general rule, the adhesives are applied on one substrate only (generally on the rigid one i.e. on the floor or on the wall, except for adhesives that are always applied on the wallpaper).
Exception: The neoprene contact adhesives are always applied on both substrates.
Physical and Mechanical Characteristics of Adhesives
Physical and Mechanical Characteristics of Adhesives
Adhesion/Cohesion, Curing and Drying
At time of application, adhesives are liquid or paste products, so that they can be spread evenly on the substrates. They must wet and adhere on these substrates, by some mechanical/physical/chemical adhesion or attraction.
After application, the adhesive must dry or become solid and hard in order to hold strongly together the two substrates. This is called cohesion.
The hardening and drying or curing may result from various physical or chemical processes:
- Evaporation or absorption of the liquid part (water or solvent carrier) leaving a dry film.
- Heat (which can speed up evaporation or induce chemical curing).
- Chemical reactions or polymerization or condensation.
The table shows the seven different ways of drying, curing and hardening for the adhesives used in building / construction and civil engineering.
Types of Adhesives and Sealants | Modes of setting | Setting parameters | Examples |
Water based adhesives, emulsions or dispersions, aqueous solutions | Evaporation of water before assembly, absorption of water by porous substrates | Jointthickness absorption of substrates, binding forcesbetween water and polymers | Vinyl,acrylic dispersions, cellulosic, starch powders forwallpapers |
Solvent based adhesives | Absorption by substrates and evaporation of solvents | Evaporation rate of the solvents, ventilation, absorption of substrates | Neoprene mastic, PVC rosin-alcohol adhesives |
Two component adhesives | Chemical reaction between the two components (with heat or at RT) | Chemical reactivity of the two components, proportions, temperature and time | Epoxies, PU, thiokols, UF, RF, MF and PF |
Thermosetting adhesives one or two components | Chemical reaction (polymerization or condensation) accelerated by heat or by catalyst | Chemical reactivity, proportions, time and temperature | Urea formaldehyde, resorcinol formaldehyde, melamine formaldehyde, onecomponent epoxies |
Hot-melt adhesives prefabricated | Adhesiveis melted, applied and will set quickly by cooling | Coolingspeed, viscosity vs. temperature, joint thickness | EVA andpolyamides HM used only for parts, never on site |
Humidity curing adhesives | Chemical reaction between ambient humidity or moisture in the materials and the isocyanates of the adhesive or sealant | Chemical reactivity of the isocyanate moisture in the air and in the materials, temperature | Polyurethanes, silicones (one component) MS polymers |
Power mortars based on cement or plaster | Hardening of cement or plaster by mixing with water | Chemical reactivity of cement or plaster temperature | Cement and plaster based mortars |
Viscosity
Viscosity gives an indication of the fluidity of the adhesive or sealant. It is measured in MPa.s. The lower the viscosity the more fluid is the adhesive. It is measures with a viscometer, such as Brookfield viscometer.
Several European and American standards may be used for the measurement of viscosity:
- ASTM D 1084-88 standard test method for viscosity of adhesives.
- ASTM D 2256 test method for apparent viscosity of adhesives having shear rate dependent flow properties.
It is adjusted by the adhesive manufacturer in order to get the right spreading i.e. the right amount with the proper tool. The user should not try to modify it (for instance, by diluting the adhesive).
For wall application on vertical surfaces, the viscosity should be higher than for floors in order to avoid sagging.
Examples:
- A trowelable dispersion adhesive used for the bonding of floor coverings will have a viscosity of 20,000 MPa.s.
- A solvent-based adhesive, brushable for bonding of PVC pipes and fittings will require a lower viscosity (2,000 to 3,000 MPa.s) in order to flow in between the two parts and spread evenly.
- A mastic or putty for gap filling , non sagging applications on vertical surfaces, or a mortar may have from 1,00,000 to 2 millions MPa.s.
![Adhesives for Building and Construction (17) Adhesives for Building and Construction (17)](https://i0.wp.com/no-cache.hubspot.com/cta/default/8989367/419d620f-d252-4068-9829-6efc855d6f5f.png)
Minimum and Maximum Thickness
- When both surfaces are flat and smooth (for most decorative materials) the adhesives thickness should be only 0.1 to 0.2 mm after pressing surfaces together.
- But when surfaces are not flat/rough or uneven, it is necessary to fill the joint in order to get at least 90% contact between the adhesive and each substrate.
For instance, for the bonding of ceramic tiles on floors or walls, it might be necessary to apply 1 to 4 mm thickness of the cement-based mortar or adhesive. Such a high thickness is only possible with cement-based mortars because it can harden in thick coats through the chemical reaction of the cement with water, and it is non-sagging on walls.
Sealants and epoxy adhesives can also cure or dry in high thickness because they also cure through a chemical reaction. Solvent-based and water-based adhesives which dry through the evaporation or absorption of the carrier cannot be used in thickness higher than 0.2 mm because they will shrink during drying, as a result of this evaporation it will consequently not fill the gap.
Coverage or Consumption
This is the amount of adhesive which is applied on the surface, in g/m2 (also expressed in ft2/gallon in USA). It depends on:
- The flatness and absorption of substrates
- The specific gravity of the adhesive, and
- Tools which are used to spread the adhesive (depth of the teeth of notched trowel, fine or coarse application with a spray gun)
Less coverage will give a poor bond (not enough contact between surfaces and adhesive). More is not good either because the solvent or the water will be trapped between the two surfaces and will dry too slowly or will not dry if substrates are impervious.
Mechanical Resistance
Following are the mechanical properties important for construction adhesives.
- Peel strength for decorative flexible coverings. It is measured in kg/cm in most countries (pounds per linear inch in USA).
- Shear strength for structural bonds and wood-to-wood assembly. It is measured in MPa (lbs/in2 in USA) with a tensile tester, such as Instron, Zwick, Lloyds or other brands. For instance,
- Epoxy structural adhesives may reach 20 to 35 MPa,
- Neoprene contact adhesives provide only 4 to 10 MPa, and
- Acrylic adhesives for floor coverings provide even lesser values.
There are many standards to measure these mechanical properties. - Creep under a given load at a given temperature is important for hot countries and hot places in the building. For instance, on walls and terraces in hot countries, the surface temperature of panels may reach 80°C. Many classical adhesives, such as neoprene or vinyl adhesives will soften when the temperature will exceed 60 to 100°C. Several standards allow to measure creep, for instance EN 1308: creep of ceramic tiles adhesives.
- Elongation at break is mostly measured for sealants (in %). The elastomeric sealants for building and civil engineering may reach 200 to 300%, but a safety factor is needed so that these sealants are qualified by construction standards only up to 25 to 40% allowable elongations. Some adhesives are also elastomeric and may have up to 20 or 30 % elongation at break.
- Elasticity modulus for sealants and structural adhesives.
- Hardness and Flexibility
- Fatigue resistance is measured by submitting (e.g. the test samples to 1 million cycles at 25% of ultimate bond strength).
Resistance to Environmental Agents
In order to test these characteristics, the mechanical resistance of the bond is measured before and after it was submitted to these agents, and the percentage loss is recorded. Some factors to be tested are:
- Resistance to heat, cold, aging (durability)
- Resistance to water and humidity
- Resistance to acids, alkalis, oils, solvents, chemicals
The specific equipment may be used for these tests:
- Weather-o-meters such as: QUV from Q Panel USA & VOTSCH Germany
- Salt spray units such as those from ASCOTT UK.
Environmental Risk and Safety
Some adhesives and sealants contain some chemicals which may be hazardous for the health:
- Some solvents which are flammable or toxic, or dangerous for the environment (VOC)
- Some hazardous chemicals, such as isocyanates, chlorinated solvents and other solvents, etc.
- Some chemicals may cause dermatitis, for instance epoxies and polyamine hardeners, cement.
All these dangers or risks are indicated clearly on the packaging and also on TDS and safety data sheets in order to warn the end users. There is many European and international regulations for storage, transportation, utilization purposes.
Quality Control, Standards and Specifications
These characteristics are measured by the adhesive manufacturer in his plant in order to check the quality of the product and the correct proportion of the raw materials:
- Dry solids measured according to standard EN 827
- Viscosity/Specific gravity measured according to standard EN 542
- Color and aspect, pH
- Chemical reactivity (may be measured with a gel timer LABOMAT or according to BS 2782-835 C or others)
In many countries, adhesives and bonds must meet national, trade or international specifications and standards, such as International Standards (ISO), CEN European standards, building codes (UEATC and French CSTB, DIN, BS), national standards such as French AFNOR, British standards (BS), German (DIN), American standards (ASTM) and Japanese standards, etc. Production and QC must also follow ISO standards such as ISO 9000 series.
Now, let's move our attention towards bonding characteristics of construction materials and adhesive chemistries used to design a strong bond.
Role of Bonding in Construction
Role of Bonding in Construction
Bonding Construction Materials with Different Adhesives
Construction of buildings uses many different adhesives and raw materials. All these materials may be bonded by careful selection of the proper adhesive, sealant or binder, and it will frequently require an adequate surface preparation.
Construction Material* | Type of Adhesive | Features of Bonding |
Concrete |
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Stones |
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Glass |
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Wood |
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Metals |
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Plastics and composites (reinforced plastics) |
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* Prior to bonding with adhesives, construction materials should be cleaned thoroughly with various techniques, in order to eliminate all contaminations, such as plaster, dirt, dust, etc. |
How to form a Bond?
Waiting Time
After application of the adhesive with the carrier, it is mandatory to wait for a given time in order to let the solvent or the water to evaporate or be absorbed by the substrates. This time is called Waiting time.
- During the waiting time, the adhesive becomes thicker, more sticky, so that it becomes ready for assembly of the two materials. It will be able to hold the two surfaces together, hence it develops a good "tack".
- The waiting time is shorter in hot weather, longer when it is cold.
- The required waiting time may be:
- 2 to 10 minutes with solvent-based adhesives
- 5 to 15 minutes with water-based adhesives
- Only 20 seconds for a solvent-based adhesive used for PVC pipes and fittings because the adhesive must transfer and solvate the other part.
Open Time
If one waits too long before assembly, the adhesive coat becomes too dry, hence it cannot wet and stick to the other material. This maximum time is called open time. Heat, low adhesive consumption and high absorption of substrates will reduce open time.
For instance, an acrylic emulsion adhesive used for PVC floor tiles may have an open time of 50 minutes at 20°C and 320 g/m2, but it will be only 10 to 20 minutes at 250 g/m2 and 35 °C.
Therefore, it is the responsibility of the worker (the end user) to adjust his working speed to these conditions, or to select an adhesive with the proper open time.
For the mortars the open surface should also remain wet in order for the adhesive to transfer to the other materials. Therefore, an additive is included in the formulation in order to slow down the evaporation rate if necessary.
Determination of the Right Time for Assembly
WT = Waiting time and OT = Open time
Tack
The power of holding together the two materials immediately after assembly is called tack. It depends on:
- The type of adhesives(E.g. neoprene contact adhesives used as double application display a very high tack after 10 minutes drying of the two coats of adhesive, and will give a strong bond immediately after contact under 5 bars pressure during 2 seconds)
- The waiting time when the adhesive starts to dry it become thicker and tacky.
- The viscosity - High viscosity adhesives tend to have a higher tack, through some suction effect.
Tack may be measured by tack tester or by a practical bonding test with the materials and the adhesive will be actually used on site.
Setting Time, Setting Speed
The adhesive will reach its full mechanical andchemical strength only after complete drying, curing or hardening(also called setting).
This may take from several hours up to one day, according to the typeof adhesive, the absorption of materials, the temperature, the humidityin the air and the thickness of the joint.
Progress of Bond Strength During First Hours/Days for DifferentTypes of Adhesives
Design of Bonds in Construction Adhesives
- Decorative coverings, such as wall coverings, floor coverings like carpets, PVC tiles or coverings are bonded on the whole surface. Since the mechanical stresses are low, the floor coverings stay on the floors naturally. It is enough to use a regular adhesive, such as acrylic emulsions, contact neoprene adhesive or PVAc adhesives. These will provide a good enough bond which will be able to last 10 to 20 years.
- In case of structural bonding outside, the bond strength and design must be calculated in order to:
- Resist to high stresses (weight of the materials and parts, weight of the building and other stresses which may result from traffic, wind, etc.)
- Resist to shear stresses and other stresses
- Avoid peel and cleavage stresses, and
- Resist for a long period of time to water, weathering, sunlight, outside temperatures and temperatures cycles, alkalis contained in the cement and other chemicals if necessary.
- Horizontal bonds will often work in compression so that the bonds are not submitted to any separation.
- For vertical bonds, bonding may be complemented by some mechanical fasteners if the loads are very high and if there is some risk of heavy parts falling from high level.
It is also very useful to use embedding, rabbeting, grooving, in order to eliminate the cleavage stresses and enlarge the surfaces which will work in shear, because adhesives resist much better in shear (they can reach 30 or more MPa in shear, while an epoxy structural adhesive will only withstand 10 to 50 N/cm width in peel).
For instance, segmented bridges are assembled with metal cables complemented with epoxy adhesives and the segments surfaces to be bonded are grooved in order to spread the loads on a larger area and reduce cleavage stresses.
It is also very useful to design the parts so that a stop or locking system will prevent parts to move in a non-desirable way, so that the adhesive will not be submitted to peel, cleavage or pure tension.
Preventing Peel, cleavage or Pure Tension
Bonding: Advantages and Disadvantages
Advantages | Disadvantages |
Bonding allows the assembly of dissimilar materials, for instance:
| In order to get a good and strong bond, surfaces to be bonded must be clean, dry, fairly flat (unless the adhesive tolerate thick gaps ), and these requirements are frequently difficult to satisfy in construction, where the workers work outside. |
Only adhesive bonding can assemble any combination of concrete, metals, wood, stones, plasterboard, foam materials, glass, ceramics, rubber, plastics and composites with a proper selection of the adhesives. | Bonding requires care, cleanliness from the workers and supervisor, and a proper training, design and adequate knowledge of bonded joints from the architects, designers, craftsmen and construction companies. |
Adhesive bonding is the best method for the assembly of large surfaces that allows to spread the stresses on large areas, thereby reducing stresses concentrations. | Heat resistance of construction adhesives is limited to 100°C for the best epoxy adhesives, and 70-80°C for the thermoplastic adhesives, such as neoprene or vinyl adhesives which is usually enough for outside exposure. The highest temperatures are usually found at the surface of exterior facades panels where it can reach 70 to 80°C on dark surfaces exposed to the south, in hot countries.Sometimes the adhesives will start to soften at 60 or 70°C. Only the cement or plaster-based mortars can withstand high temperatures, up to 300 or 400 °C, because of their mineral nature. |
Bonding is the only way to manufacture sandwich panels, double panel windows (glass to glass bonding), composite boards, and it is also the only method for film laminating on panels, or attaching decorative wall coverings, or waterproofing films on inclined roofs. | Heat resistance of construction adhesives is limited to 100°C for the best epoxy adhesives, and 70-80°C for the thermoplastic adhesives, such as neoprene or vinyl adhesives which is usually enough for outside exposure. Setting time and full cure or hardening may take with some adhesives up to 10 - 15 hours at room temperature when there is a chemical reaction for curing. For instance: with 2 components epoxy adhesives, and this may require some holding fixtures during curing. But one can always find a faster curing adhesive if needed by changing the hardener or the formulation. |
Bonding provides flat and smooth outside surfaces through the elimination of mechanical fasteners, such as screws, rivets, nuts and bolts or nails heads. Thus, the design is greatly improved. For instance, bonded architectural glass allows to build flat and almost seamless and continuous facades. | Bonding chemistry, design and calculation of bonded parts, mechanical and physical properties of adhesives are not well known from the users and are not always clearly stated by the adhesives suppliers. On the contrary mechanical attachments are easier to use, the design and calculation of attachment is well known, and follows rules which are easy to understand. |
There is no weakening of bonded parts, while with riveting or nuts and bolts requires the drilling of holes that weakens the mechanical resistance around the holes. | Durability of bonds when submitted to weathering, water, heat, thermal cycles, may be limited with some standard adhesives to 10 to 15 years. Although epoxy adhesives have shown a 50 years durability in civil engineering works, such as bonded concrete segmented bridges. |
The bonded part may even have a mechanical resistance greater than the sum of individual parts, for instance in sandwich panels where the sandwich displays a much higher resistance and rigidity than the resistance of the skins + that of the core. Figure 2 explains the advantages of sandwich panels. | Parts to be bonded must be specially designed for bonding. For instance the bonds should not be submitted to peeling or cleavage effect or to pure tension on small surfaces, the sides of the joints should preferably be protected from water penetration because water will often degrade the adhesion by migrating at the interface. |
The adhesive joints may be flexible and dampen the vibrations or withstand some differences of dilatation coefficients when the materials are quite different. For e.g. when bonding glass to metal or when the bond is very elastic and fills gaps, the jointing material becomes a sealant. | |
TThe cost of assembly may be lower than the cost of mechanical assembly if there is:
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Some mortars and thick layers of adhesive that do not shrink when curing may fill gaps and provide some leveling effect. For e.g. bonding of ceramic tiles or large concrete parts which are not flat, and for sealing metal parts into concrete. | |
The adhesive layer may protect from corrosion. For e.g:
| |
It may also provide waterproofing, depending on chemical types and formulations. For instance, a coat of epoxy, PU or rubber-based adhesive will provide waterproofing under a floor covering. This is useful for floor waterproofing under ceramic tiles in bathrooms. |
Commercially Available Additives & Polymers for Construction Adhesives
View a wide range of additives or polymers available today for your building and construction adhesives, analyze technical data of each product, get technical assistance or request samples.Sealants for Construction - Know More
Starting Point Adhesive Formulations for Construction