A substance which increases the rate of a chemical reaction, even when only small amounts of accelerator are present. Accelerators are used in conjunction with chemically curing adhesives to shorten the curing time (see → activator, → catalyst).
Acidic refers to the ability of a substance to release protons (here: hydrogen nuclei) and hence initiate certain reactions which other substances which do not have this property cannot undergo. Aqueous acidic solutions (= acids) have pH values < 7. See also: → alkaline.
Additives in, for example, a cleaning agent which are → acidic.
Auxiliary substances in adhesives or plastics to improve the properties. examples: Additives to improve the long-term stability (=stabilisers), to reduce electrostatic charge (=antistatic agents), to provide fire retarding properties (= fire retardants), to improve the deformation properties (= plasticisers) and to improve the removal of injection-moulded plastic components from moulds (= release agents). Additives in plastic substrates can sometimes adversely affect the adhesion.
Adhesion (surface adhesion)
Forces between the → molecules / → atoms of two different substances or the bonding of two materials or bodies to one another. EXAMPLE: Bonding of an adhesive to a substrate or the adhering of a water droplet to a pane of glass.
Evaporation of volatile substances. EXAMPLE: → Solvents in solvent-containing adhesives or primers; water in dispersion adhesives.
1. Modification of the properties of a material or composite during storage or usage, caused by chemical and physical processes due to the penetration of moisture, oxygen, other media, heat, irradiation, mechanical effects, etc.
2. Storage of lab test specimens under special ambient conditions in order to mimic, in an accelerated way, the effects which the adhesives normally experience during their period of use (= exposure test).
The term alkaline/basic describes the ability of a substance to accept protons and hence trigger certain reactions which other substances cannot trigger. Aqueous alkaline solutions (so-called caustic solutions or bases) have pH values > 7. See also: → acidic.
(from the Greek: atomos = undividable) The smallest uncharged particle of a chemical → element. Atoms consist of a positively charged nucleus, made up of positively charged protons and uncharged neutrons. The nucleus is surrounded by a cloud of negatively charged electrons (number of electrons = number of protons). A piece of a pure → element (e.g. a gold bar) is a collection of atoms having the same number of protons.
Base joining technique
A joining technique which can be used alone to join two substrates (EXAMPLES: soldering/brazing, welding, adhesive bonding, clinching, punch-riveting). Base joining techniques can also be combined with each other (see → hybrid joining).
A chemical bond is a relatively strong bond between two → atoms, with the two atoms sharing one, two or three pairs of electrons (single, double, triple bonds). Chemical bonds are 10 to 100 times stronger than → physical interactions.
Substances which promote, accelerate or steer chemical reactions (see → activator, → accelerator). They are used in certain adhesives to accelerate the curing reaction. EXAMPLE: metal ions are used as the catalyst for anaerobically curing adhesives.
Fracture of the adhesive film. Adhesive still adheres to the two substrates. In general this is the desired fracture pattern when the theoretical strength of the adhesive is reached.
Collective term for materials which comprise a combination of different materials and whose most important properties generally supersede those of the individual components. The most common components of composite materials are metals, wood, glass, polymers and ceramic materials. These are processed into fibre reinforced composite materials, tapes, laminates and particle composites (e.g. ceramics).
→ Tensile load
Refers to the angle which the tangent to the liquid surface makes with the wetted substrate surface. A small contact angle (< 30°) indicates good wetting of the surface by liquids whose surface tension is the same as or lower than that of the test liquid.
(Latin: corrodere = eat away) Surface-initiated chemical or electrochemical attack causing adverse quality changes to materials. EXAMPLE: Rusting of steel due to the effect of moisture and atmospheric oxygen (in some cases accelerated by salts or acids).
Chemical process. Solidification via the formation of chemical bonds, physical interactions and mechanical entanglements. Chemical reactions of the → monomers (or → prepolymers) present in the liquid adhesive result in the formation of polymers. Curing mechanisms are so-called → polyreactions.
(Latin: dispersio = finely distribute) Heterogeneous liquid mixture of at least two substances which are only sparingly soluble or totally insoluble in each other. A fine distribution of one substance in another substance. At least 2 → phases.
Method for applying water-based coatings to electrically conducting surfaces. The workpiece is immersed in a coating bath and connected to the plus pole of a direct current source. The negatively charged polymer molecules migrate to the positively charged workpiece and deposit on it. The coating layer is then baked/annealed at 150 – 180 °C.
A type of plastic with polymer chains crosslinked to form a wide mesh. Elastomers generally possess the following features: rubber-elastic at room temperature, poor creep resistance, cannot be melted, cannot be welded, insoluble but can be swollen (compare with → thermoset, → thermoplastic). Elastomers become brittle at low temperatures.
(Latin: elementum = base material) A substance, which can be isolated from compounds by chemical processes, but which cannot be split further. The smallest uncharged particles of an element are called → atoms. EXAMPLES: carbon, hydrogen, aluminium, silicon.
Wet chemical surface pretreatment method for removing surface layers. Usually acidic solutions (hydrochloric acid, sulphuric acid or phosphoric acid). Normally carried out at elevated temperatures. Depending on the particular formulation, there may also be formation of special reactive layers on the treated surfaces having a corrosion-protection function.
Conversion of a dispersion from the liquid state to the solid state via escape of the dispersing agent and interdiffusion (cold fusion) of the solid phase. For this, the repulsion of the → stabiliser first of all has to be overcome.
Coating technique. Electrochemical surface treatment of materials in which thin metallic (also sometimes non-metallic) layers are deposited on the surface of the material to protect it from corrosion, to give a decorative appearance, to produce composite materials with improved properties, etc.
1) For fibre reinforced plastics: Outermost resin layer of a fibre reinforced laminate. Stops the fibre structure being visible and provides protection for the actual → laminate against weathering and the effect of chemicals. Also called gel coat or fine layer.
2) For glasses: Moisture induced → alkaline layer on the surface of highly alkaline sodium, calcium or potassium containing glass. The layer has low mechanical resistance.
Hand strength (handling strength)
State of a bonded joint which allows further processing of the bonded components without additional securing being required. The adhesive is still not fully cured but is already strong enough for further work to be carried out, provided the bonded joint is not subjected to excessive stress. Adhesive manufacturers often equate hand strength with attainment of a certain strength (e.g. 1 N/mm2).
Adhesive component which induces chemical curing. For 2-component adhesives (used in German-speaking countries), the hardener is generally the B-component (the A-component is the resin). For adhesives made by British or American manufacturers, the situation is often the reverse of this, namely the hardener (accelerator) is the A-component and the resin is the B-component (base).
= thick-bodied. Honey is an example of a liquid which is highly viscous at room temperature. In contrast, water is a liquid of low viscosity. (see also → Viscosity) Viscosity)
Combination of at least two → base joining techniques whereby the joining operations on the same region of the workpiece are carried out in parallel or staggered. The substrates are joined at several positions. When correctly applied, the advantages of the different elementary joining techniques are combined in the hybrid joint. EXAMPLES: adhesive bonding + clinching; adhesive bonding + spot welding.
1. General: The combining of parts to form a whole. Adhesive bonding is a joining technique. Other techniques which can be used to join parts are for example welding, soldering/brazing and riveting.
2. A step of the bonding process which involves the bringing together of the two substrates.
A layer-structured → composite material
Lap shear specimen
Test specimen manufactured according to standard specifications for the → lap shear test.
Lap shear test
The most important destructive test method for high-strength bonded joints. See → shear strength.
Time-dependent → resistance
Matrix (Matrix resin)
(Latin: mater = mother) Rigid or highly viscous bedding material in which another material is embedded. EXAMPLE: A polymer, for example in a fibre reinforced plastic, in which the fibres are embedded.
On a → technical data sheet the mixing ratio describes the relative weight or volume ratio in which the A and B components of a 2-component adhesive must be mixed in order to achieve optimum bonding results.
Chemical compound containing at least two → atoms
A term for a special way of processing polymerisation adhesives in which the → hardener is diluted in a → solvent and applied to one of the surfaces to be joined, whilst the other surface is coated with the → resin. The → curing only starts on bringing the substrates together (after complete evaporation of the solvent).
→ Tensile stress
The period of time when applying a physically hardening adhesive which may elapse between application of the adhesive and joining the substrates without adversely affecting the subsequent properties of the bonded joint.
→ Peel stress
A special case of tensile stress. A type of stress (load) in which the tensile stress acts on a small part of a joint (e.g. bonded joint) and so quickly results in the strength limits being exceeded.
States of a substance, demarcated by interfaces, which are homogeneous, optically uniform and which can be mechanically separated from each other. EXAMPLES: Ice cube in water: 2 phases (one liquid, one solid) of the same substance; Salt solution: 1 phase (2 substances, but these cannot be separated from each other by mechanical means, e.g. filtration); Milk: 2 phases (fat droplets in water); … and in each case the air above (another phase).
Wet chemical method for surface pretreatment, whereby components/substrates are coated with phosphate compounds. EXAMPLE: Method for manufacturing a protective layer of iron phosphate on a steel surface.
Weak attractive forces (only ca. 1/10 to 1/100 of the strength of chemical → bonds) between different molecules (or regions of molecules) in a material or between two surfaces. In adhesive bonding technology the attraction of dipoles plays the biggest role.
Chemical processes. Reactions for forming polymers which proceed via a chain growth mechanism. A distinction is made between 3 different reaction mechanisms: polyaddition, polymerisation, polycondensation.
The period of time during which a reactive adhesive must be applied (period of use). It depends on the rate (curing rate) of the chemical reactions responsible for polymer formation and on the external boundary conditions (temperature, batch quantity) and must be strictly adhered to in order to obtain optimal bonded joints.
Measures to protect against hazardous substances (e.g. disposal, ventilation, personal protection equipment) in the context of health and safety at work and environmental protection.
Property of a → fluid when subjected to shear forces (e.g. on stirring or being pressed through a nozzle) to lower the → viscosity. If the viscosity only increases again slowly after reducing the shear force, then this property is called thixotropy.
Resins are the starting materials for plastics and also used as binders for coating materials, printing inks and adhesives. For 2-component adhesives (used in German-speaking countries), the resin is generally the A-component (the B-component is the hardener). For adhesives made by British or American manufacturers, the situation is often the reverse of this, namely the resin (base) is the B-component and the hardener is the A-component (accelerator).
Resistance of a material or component to the effects of chemicals (e.g. cleaning agents, salts, gases), mechanical loads (static and dynamic forces, intrinsic stresses) or physical influences (temperature, radiation).
Property of a component: resistance of a component to elastic deformation. It is influenced by the component geometry, direction of the load and the modulus (→ modulus of elasticity or → shear modulus).
Safety data sheet (SDS)
Product-specific sheet providing key safety information. An SDS must be provided free of charge to the customer by the manufacturer or supplier when hazardous substances and preparations which contain hazardous substances are introduced into the commercial marketplace. The drawing up of SDSs is internationally regulated (EU directive 1907/2006).
→ Shear load
Shear modulus (G-modulus)
A constant for a given material. It describes the ratio of the → shear stress to → strain for the case of ideal elastic deformation under shear stress (i.e. applicability of Hooke’s Law).
Mechanical parameter. Resistance of a joint (e.g. bonded joint) to shear stress. Can only be determined by a destructive test. There are a variety of standardised tests (e.g. DIN EN 1465 (lap hear test); DIN 54452 (compression shear test); DIN 54455 (torsion shear test)).
Type of stress whereby the stress/load acts parallel to the plane of the joint (compressive shear stress towards the joint, lap shear stress away from the joint). Shear stress in adhesive bonds results in strain in the adhesive film.
Processing time for moisture curing adhesives within which complete wetting of the bonding area is still guaranteed after joining the substrates. It is dependent on the temperature and relative air humidity.
A fluid, homogenous, single-phase mixture of at least two components in which the dissolved substances are distributed as individual molecules in a → solvent (= molecular dispersion).
Liquids which are able to “solve“ other substances without chemical changes taking place, such that the components of the → solutioncan be recovered again by physical methods (e.g. distillation, crystallisation, adsorption, etc.).
Special processes (DIN EN ISO 9000 ff.)
Processes which, via subsequent monitoring and measurement or non-destructive test methods on the product, do not allow the result to be fully verified (tested). Any defects only become evident during usage.
→ List of requirements
Safety phrases. Coded recommendations for protecting people and the environment when working with hazardous substances. They form part of a standardised system for labelling hazardous substances which is prescribed within the EU.
In chemistry, “stabilisation” means the suppressing of chemical reactions (e.g. decomposition, oxidation, etc.) or physical processes (e.g. interdiffusion of particles in dispersions) in non-stable preparations. Substances added to preparations to purposefully fulfil this function are called stabilisers (see → additives).
Storage samples (reference samples)
A sample of a material/component which is stored to allow later determination of features/properties. Storage samples, up to the point of removal from the production process, have been subjected to all prior process steps and therefore reflect the properties of other samples that are produced up to that same point.
Relative deformation as a result of a → tensile load or a temperature increase.
Material parameter representing the maximum mechanical load which a material can withstand for a short time. A distinction is normally made between tensile strength, compressive strength, torsional strength, shear strength, peel strength and bending peel strength. There are standardised tests for determining these strengths.
This refers to the different types of external influences which can act on a bonded joint, for example heat, moisture, mechanical stress or chemical effects (water, solvents, etc.). To create a bonded joint having long-term stability, it is vital to have knowledge of these effects in order to select a suitable adhesive.
Cracking in plastics as a result of the simultaneous action of chemicals (e.g. solvents) and mechanical stress. The intrinsic stress often suffices as the mechanical stress here. EXAMPLE: A butter tub made of polycarbonate shows an increasing number of small cracks on being washed repeatedly in the dish-washer.
Distribution of the → stress in a bonded joint.
A physical phenomenon which occurs at the surface of a liquid and which, by virtue of the cohesive forces in the bulk liquid phase, causes the surface of the liquid to become as small as possible (for example a water droplet under gravity-free conditions takes on the form of a sphere). The surface behaves like a taut, apparently elastic skin. Units: mN/m (millinewton per metre).
The treatment of the surfaces of materials in order to optimise the wetting properties and adhesion of the adhesive and in order to produce reproducible bonded joints having long-term stability.
Surface-active substances. Surfactants lower the surface tension of liquids to which they are added and improve the wetting of surfaces with these liquids. Their special structure also means that fats and oils become soluble in aqueous solutions.
Tensile load / compressive load
Loads which acts perpendicular to the plane of the joint (e.g. bonding area) (compressive load towards the object, tensile load away from the object). Tensile loads lead to elongation; compressive loads lead to contraction.
A type of plastic which can be deformed as desired within a certain temperature range without undergoing chemical change. Its ability to deform is due to chain molecules which are not chemically crosslinked. Thermoplastics possess the following features: tough-hard to brittle at room temperature, can be melted, can be swollen, soluble, tend to undergo creep (see → thermoset, → elastomer).
Type of plastic with polymer chains crosslinked to form a narrow mesh. Thermosets generally possess the following features: hard and brittle at room temperature, thermal stability under loads, cannot be melted or welded, insoluble, creep resistant and poor swelling properties (compare with → thermoplatsics, → elastomers). Depending on the degree of crosslinking, the thermoset is tough-elastic to a greater or lesser extent.
Special case of → pseudoplasticity
Cleaning technique in which a solvent in the sump of a closed container is brought to the boil and condensed on the components to be cleaned. The contaminants dissolve and return with the solvent to the solvent sump.
Viscosity (in fact: dynamic viscosity)
Resistance of fluids to flow. It arises due to the intermolecular forces in a fluid, and is therefore dependent on the cohesion. This is also termed internal friction. The lower the viscosity, the greater the mobility of the molecules of this substance. The viscosity affect the flow behaviour (rheology) and is hence an important property for processing adhesives. Units: mPa⋅s (millipascal seconds).
Property of a liquid to spread out on or cling to a solid surface, whereby the distances involved are of atomic magnitudes (below 1 nm). Wetting plays an important role in adhesive bonding because only an adhesive which has good contact to the surface can develop adhesion forces with the surface. Even if the → adhesion is dependent on additional factors, wetting remains a necessary prerequisite for adhesion.