adhesives for rubber compounds

ADHESIVES FOR RUBBER COMPOUNDS

Luis Tormento

July/2017

Introduction

• According to the American Society for Testing Materials (ASTM), the adhesive is defined as a substance that when applied to the surface of other materials can bind them together, offering resistance to separation.

• The term adhesive covers different denominations: glues, pastes, cements and gums. Originally, the term cement was given to the ground or calcined stone with which the mortar was prepared. Today the term refers to adhesive mixtures, or more particularly resin, rubber, dental, etc. cements,

Introduction

• The term glue (from the Latin colla, from the Greek Kolla) was given to the protein extract obtained from skins, bones and fishbone. As this extract was used for a long time in the union of pieces of wood and currently in the manufacture of pieces in plates, the use of the term was generalized for all the adhesives used in the wood industry, thus saying "glue" phenolic, of urea or casein substances which would be classified among the plastic adhesives. Gums are water-soluble, vegetable-based adhesives. Glues called pastes are made of starch, flour or dextrin and are used in mail stamps and paper ribbons. By extension, it is called adhesive to any adhesive that has a pasty or very thick consistency. All the above terms are generally interchangeable: the term "adhesive" is often referred to with some of the properties that characterize them, either by their physical state - liquid adhesive, adhesive film; For its chemical composition: epoxy adhesive, cyanoacrylate; By its specific use - metal-metal adhesive; or conditions of use -cold cure adhesive, dissolving adhesive, etc.

Introduction

• The term adhesion or adherence is used to refer to the attraction between two substances.

• We must differentiate the meaning of this term from the physical-chemical and technological point of view. In physical-chemistry, it is called adhesion to the force of attraction between a solid surface and a second phase, which may consist of individual particles or a continuous film, which may be a liquid or a solid. Adhesion leads to sorption which can be on the surface (adsorption) or inside the surface layer (absorption). Adherence can be given by Van der Waals forces or chemical bonds. In this case we call the chemisorption. In adhesive technology adhesion is only called the bond between a solid surface and a second solid or liquid phase, adhesive bonding is called to the technical process that is supposed to produce adhesion between two solids. The materials to be bonded are called substrates or adhesives, the latter being more appropriate for those materials which form part of the adhesive bond. The adhesive bond depends on the force between the adhesive and the substrate, called adhesion and the internal strength of the adhesive, called cohesion. The effectiveness of a bond through the use of adhesives is analyzed by bond failure.

Historical background

• Existem evidências históricas de que os adesivos foram utilizados pelo homem há milhares de anos. É o caso das esculturas de Tebas (3300 anos), que representam a união de uma peça de chapa fina ao que parece ser uma tabua de madeira.

• Estes adesivos, como os utilizados até 100 anos atrás, eram colas de origem animal ou vegetal, cujo uso mantêm-se até hoje para unir materiais porosos tais como papel, madeira, etc. As soluções de borracha, nitrocelulose, etc. em solventes orgânicos começaram a ser utilizadas no século XIX.

• Os adesivos à base de caseína foram utilizados durante a primeira guerra mundial para construir estruturas de madeira para aviões; porém, foram abandonados em virtude da pouca resistência à umidade e por causa do ataque de fungos.


• This limitation in the use of natural adhesives has been the driving force since in the great expansion of adhesives based on synthetic resins and other materials after 1930.

• The formulation of the present adhesives allowed the development of stronger, more durable and more versatile substances, to unite materials that were previously difficult or impossible to be united; Currently, adhesives are considered materials of interest in engineering.


• In some cases it is not feasible to use conventional mechanical means for joining materials such as nails, rivets, screws, welds, etc., because they can cause changes in materials such as distortion or corrosion. In these cases, the use of adhesives may be the solution to the problem, since they provide the simplest and quickest way to achieve the assembly of the parts, making possible the design of new surfaces.

• Not forgetting the economic factor, which ultimately is decisive for the choice of the method to be used for bonding, generally the cheapest procedure is the use of adhesives.


• In joining different materials, for example with different coefficients of thermal expansion or different modulus of elasticity, the advantage of using an adhesive shows that it can decrease the tensions between the materials in the event of a mechanical shock or a thermal change.

• By using an adhesive, the tensile forces are distributed over the entire bonding area, thus being noticeably reduced, thereby increasing the fatigue strength.

• The high strength-to-weight ratio is a very appreciated feature in lightweight fabrication.

• Without direct contact between metals of different natures, the risk of corrosion is reduced.


• Concerning the disadvantages they present:

– On some occasions, the time is great, slowing down the pace of production.

– The difficulties that appear to inspect the union, since it is necessary to use destructive, are the biggest inconveniences in the union by adhesives.

– By using the materials at elevated temperature, there is a risk of degradation.

Forces of Union

• The union forces are classified in primary and secondary, depending on the value of the bonding energy. The primary forces are true chemical bonds, while the secondary ones correspond to Van der Waals forces. All have the common characteristic of rapidly decrease as the distance increases, so they are only efficient at distances less than 10 Angstroms (1 Á = 10-11 m).

• When two surfaces are formed through a rupture, very few points of these surfaces make contact or remain at distances less than 10 Å so that the bonding forces act. In addition, a newly formed surface is a suitable place for corrosion, deposition of foreign solid particles or moisture adsorption: the surface is covered by a thin film or film of another material.

• It has been found that these foreign deposits remarkably decrease adhesion between solids.

Forces of Union

• Therefore, to achieve an effective bond between solids, there must be contact at the molecular level. This contact occurs by incorporating a substance that eliminates surface roughness defects by filling the surface depressions and in turn eliminating the deposits of foreign substances that have formed on the surface. These objectives are achieved when there is a close proximity at the molecular level between the incorporated substance and the solid surface, which only a liquid can provide. However, even a very viscous liquid has no cohesive force on the surface to withstand the high shear stresses or stress to which it may be subjected to bonding; So it looks like a solid is the best state of an adhesive.

Forces of Union

• To achieve a good bond with a sticker, it must meet certain requirements such as:

• A) It is a liquid (adhesive) that extends completely on the surface of the solid (adherent).

• B) Extend over the surface of the solid at a suitable speed so that air bubbles do not form on the surface and fully occupy its irregularities.

• C) initially it must have a relatively low viscosity, of the order of a few centipoise, which will progressively increase until it becomes a solid.

• D) Having a flexible surface in order to reduce the increase of the elastic forces generated during the formation of the joint.

Forces of Union

• Because it is a surface phenomenon, in order to determine if an adhesive / adherent system is capable of making an adhesive bond on different surfaces, the equilibrium wettability of the assembly, as well as the free surface energy values of the adhesive and adherent and the free Interfacial energy. An adhesive must completely wet the surface on which it will act. For this to be possible, the surface energy of the adhesive should be lower than that of the substrate;

• Otherwise, the adhesive will acquire spherical shape and will not spread over the surface. Metals have a high surface energy, while plastics have a very low value. The metals can be easily bonded with adhesives whereas in plastics this process is more difficult. The surface energy can be modified with appropriate treatments.

Adhesion Mechanisms

• Several mechanisms have been proposed to understand the phenomenon of adhesion, but none of them fully justifies the results. Four main mechanisms contribute to adhesion:– Mechanical interpenetration

– Diffusion

– Electrostatic attraction

– Adsorption

Mechanical Interpenetration

• This theory proposes the accommodation or interpenetration of the adhesive in the surface irregularities of the substrate: it is the main cause of adhesion. Adhesive strengths were determined by destructive tests on polished and rough surfaces of the same material, with the latter having higher adhesive strength values. In principle, this behavior could be attributed to the greater interpenetration capacity of the adhesive in the roughness of the substrate, but also to the surface increase and cleaning of the surface during the treatment.

Diffusion

• This theory establishes that the intrinsic adhesion of polymers to themselves or to others, is due to the mutual diffusion of polymer molecules through the interface.

• This requires that the macromolecules or segments of the polymer chain of both the adhesive and the substrate have sufficient mobility and are mutually soluble, i.e., having similar solubility parameter values.

• The great defender of this theory is Voyustkü, who presents as experimental evidence that the contact time, temperature, type of polymer, molecular weight and viscosity also affect the diffusion processes, whereby adhesion is assumed to be a result of diffusion.

Diffusion

• What is certain and has been proven experimentally is that the interdiffusion is of great importance in polymers which have a glass transition temperature (Tg) below ambient temperature or in those cases where adhesion takes place in the presence of a solvent. In the first case, the chains or segments of both adhesive and adherent have sufficient mobility and the chains interpenetrate each other or through the interface as long as both are mutually soluble.

• The polymer bonding technique; With solvent in the absence of adhesives, or by effect of temperature, is based on the diffusion mechanism. The effect of the solvent is to plasticize the surface of the polymer by swelling it in order to increase its volume and hence the mobility of the polymer chain in the interfacial region, increasing the speed and extent of the polymer chain interpenetration. The effect of temperature is to situate the polymer at a temperature higher than its Tg, facilitating the mobility of the chains.

Electronic Theory

• This theory postulates that adhesion is due to electrostatic forces, where the adhesive joint acts as a condenser, covered by a double electric layer formed by the adhesive-bonding interface. During the rupture of the union it produces a potential difference of discharges when the union breaks. In some cases electrical discharges and electron emissions were observed when the union is broken. This is attributed to the transfer of electrons between adherent and adhered.

Adsorption

• This theory is the most accepted to explain the phenomenon of adhesion. It proposes that, once enough contact has been achieved at the molecular level. An interface is formed. The adhesion happens as a consequence of the secondary surface forces that act between the atoms of its surfaces, being the most common the forces of Van der Waals.

• It happens the phenomenon of chemicalsorption, when they act metallic chemical bonds, ionic or covalent through the interface.

Superficial Treatments

• The surface treatments that apply to the substrates, are fundamentally aimed at increasing their surface energy. In this way, the adhesive is allowed to wet. The treatments used vary with the nature of the adherent, since the treatment that must be given to a polymer is not the same as that which should be given to a metal.

• The treatment to be given to a high surface energy material is of a mechanical or chemical nature. By means of the first, the foreign particles or films deposited on its surface are eliminated and the roughness is increased, favoring the mechanical interpenetration.


• Polymeric materials are usually subjected to treatments which increase their free surface energy, generally with the creation of certain reactive functional groups on which the adhesive can be deposited. Except for polar polymers such as polyesters or cellulose derivatives with which a good adhesive bond can be achieved provided that the surface is perfectly clean and sanded, the remainder of the polymers must undergo a rigorous chemical pretreatment.

• Among the treatments that demonstrated greater effectiveness are:


– Chemical oxidation– Flame treatment– Electric discharge treatment

• All these treatments oxidize the surface of the polymer, leading to the formation of carbonyl groups or other type of oxygenated function, such as ether, hydroxyl and carboxyl.

• In the encounter of the polymers on the surface, when this theory is the most accepted to explain the existence of plasticizers, these should be eliminated by washing with solvents and subjected to the treatment with grit for the application of the adhesive

Adhesives Nature

• The adhesives are generally macromolecular, soluble and flexible compounds; Are thermoplastic compounds which, once bonded by adhesive, can continue to maintain their thermoplastic condition or become thermoset resins by polymerization or crosslinking reactions.

• They can be applied dissolved in solvent to give wettability to the adherent and will harden with its evaporation. They are well presented in solutions or dispersions in the form of latex. In this case, the solvent is eliminated through the adherent pores:

Adhesives Nature

– If it is porous– or evaporated by the action of the heat before the realization

of the union– in the case of non-porous materials

• The adhesives under this presentation have a too slow bonding and, if the solvent to be removed is organic in nature, they present risks of toxicity, flammability, tendency to polymerization, and need adequate storage. Nonetheless, they continue to be used, due to their good wettability characteristics, wide coverage of use and application at low temperatures.

Classification of adhesives

• A wide variety of adhesives are of common use and there is no simple sorting system suitable for all products: the same adhesive can be included in several classifications.

• The adhesive industry generally uses a classification based on the end use, such as metal-metal adhesive, wood adhesives, adhesives for wide use, etc. One limitation that arises with this classification system is that an adhesive stick can be useful in different fields of application.

Classification of adhesives

• For a systematic study the following classification will be followed:

– Its origin

– The method of application and hardening

– Its functionality

– Its form of presentation

– The nature of the component

Source

• They are classified as natural and synthetic. Among the former, animal-based adhesives -albumin, glue, casein, etc. - and those of vegetable origin are considered separately. Among those of synthetic origin the organic ones are differentiated from the inorganic ones.

Method of application and hardening

• Curing temperature– Depending on the temperature required to bond, they can be

classified into:

• Low Temperature Adhesives– Cure below 20°C,

• Ambient temperature stickers– Cure between 20ºC and 30ºC

• Adhesives for temperature, intermediate– Cure between 30ºC and 100ºC

• High Temperature Adhesives– Cure above 100 ° C

Method of application and hardening

• 1-Pressure Sensitive Adhesives– This type of adhesive forms a joint by a slight pressure, simply

by squeezing with one finger.

• 2 - Chemical hardening,– Certain adhesives need to be cured to be able to bond. The

curing process takes place through a chemical reaction, whereby a thermoplastic material is converted into a thermostable.

• 3 - Elimination of the solvent– When it is interesting to increase the fluidity and achieve a

good wettability of the adherent. Adhesives are prepared accompanied by a solvent. Once bonding is done, the solvent should be discarded.

By their functionality (structural or non-structural)

• This classification is totally arbitrary, since a specific definition of structural in terms of union strength is not accepted. It is usually defined as a structural adhesive, one that is used in joints or assemblies that carry loads or that are subjected to great efforts.

• The non-structural adhesives can not withstand heavy loads and are generally used to join the components to make temporary adhesions.

Form of presentation

• They may be present in solution, dispersion, solid state and in films.

Majority Component Properties

• Within this classification, a new subdivision can be made according to its rheological properties. Thus we have thermoplastic adhesives, thermosets and elastomers. The thermoplastic adhesives may be fused, soluble in solvents, heat-softened, and undergo stress under strain. They are used in unions that will not be subjected to stresses nor to elevated temperatures. They join glass, porcelain and porous materials like paper, leather, etc.

• The thermosetting adhesives are based on a low molecular weight resin, which cures and hardens into a highly crosslinked adhesive. They are adhesives of great acceptance because they form essentially infusible and insoluble bonds; Have good creep resistance and are used as structural adhesives for applications that need to withstand great stress.

Majority Component Properties

• They exhibit good behavior in drastic environmental conditions such as heat, cold, humidity and different atmospheric chemical components.

• To this group belong the families of adhesives of great use: phenolic resins and epoxy resins.

• The adhesives based on elastomers are formed by rubber resin mixtures; Can be used both for general purposes and for structural purposes according to their formulations.

• Adhesives made from thermoset resins modified with synthetic rubber are used to make structural bonds of metals and other rigid materials.

Major Adhesive Families

• Thermosetting adhesives– Phenolic resins– ResorcinoI-phenol-formaldehyde– Urea-formaIdehyde– Melamine-formaldehyde– Epoxy– Polyisocyanates

• Thermoplastic Adhesives– Vinyls– Acrylics– Hybrid adhesives

• High Temperature Resistant Adhesives

Contact

LT Químicos

Av. Pedro Severino Jr., 366 Cjto 35

04310-060 – São Paulo – SP – Brasil

Luis Tormento

NPD Director

[email protected]

Tel: +55 (11) 5581-0708

mailto:[email protected]

adhesives for rubber compounds

Engineering