An Overview Of Adhesives Biology Essay

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Adhesives are defined as a nonmetallic substance capable of joining materials by surface bonding (adhesion),the bond itself processing adequate internal strength (cohesion).adhesive is a generic term and covers other common terms such as glue, paste, gums, adhesive cement and bonding agent.

Adhesives are an advantageous for joining thin or dissimilar material, minimizing weight, and when a vibrational damping joint is needed. one such recipe shows that people in the middle ages had an appreciation for the generation of composition materials as well as the use of drying oils.

'' very strong ,very good glue ''

History-

Adhesive joint is not a new joints method .use of adhesives is

describe in ancient (1).Egypt and (2) in the Bible (3)Bommorito, describe recipes for adhesives that were formulated during the middle ages.

Adhesive is among the oldest technologies of mankind. as early of the stone age ,our ancestors made stone axes and other tools using mineral pitch or wood rosin .in the middle ages, the first glue-bonding plants come into being and produced protein glues from animal raw material or styerch paste from plants 6000-year-old ceramics show evidence of adhesives based upon animal glues made by rendering animal products such as horse teeth. During the times of Babylonia, tar-like glue was used for gluing statues. The Egyptians made much use of animal glues to adhere furniture, ivory and papyrus The Mongols also used adhesives to make their short bows, and the native Americans of the eastern United States used a mixture of spruce gum and fat as adhesives to fashion waterproof seams in their birch bark canoes

Theories of adhesives-

Conditions for Satisfactory Bonding-

There are accepted conditions which result in higher adhesive bond strengths as listed below

Cleanliness of surfaces.. The bond surface is ideally cleaned of loose matter and also cleaned of surface oxides and adsorbed gases.

The choice of adhesive should be such that it wets the adherent surface and also solidifies under an acceptable regime of time, temperature and pressure.

The adhesive should be selected to suit the service conditions of environment and temperature. It should be noted that the difference in coefficient of thermal expansion between the adhesive and adherent can have an important effect on the joint design working over a significant temperature range.

Bonding Theories-

There are a number of adhesive theories contributing to the overall study of bonding as listed below:

The mechanical interlock theory

The adsorption theory

The chemisorptions theory

The electrostatic theory

The diffusion theory

The weak boundary layer theory-

The mechanical interlock theory

This is the simplest theory and is based on the factor that, at the microscopic level all surfaces are very rough consisting of crevices, cracks and pores. The adhesive penetrates these features and hardens such that it keys into the surfaces and forms a strong surface bond ( this is probably similar to Velcro ). The adhesive thus is able to bond two surfaces together and ideally the only weakest part of the bonded joint is the adhesive strength.

The adsorption theory-

This theory is based on the assumption that the adhesive "wets" the surface of the adherent surface (meaning that the adhesive applied to the adherent spreads spontaneously when the join is formed ).. This theory has resulted in adhesive materials being developed which have a lower surface tension than the adherent surfaces. Examples supporting this theory include epoxy resins which wet steel and result in a good bond - these resins do not wet PE or PTFE and result in a poor bond.

According to this theory, in the event of intimate contact between the adhesive and the adherent, the adhesive strength arises as a result of secondary intermolecular forces at the interface.   These may include Vander Waals forces (dipole-dipole, dipole-induced dipole interactions and hydrogen bonds ).

The chemisorptions theory

This is a variation on the adsorption theory in that stronger chemical bonds (ionic, covalent metallic ) from across the joint interface. Ref molecular bond In this regard, introduction of molecular bonding between the adhesive and the adherent will obviously improve the adhesive bond strength. This can be attained by reactions at the surfaces, using proper surface treatments, or by using additional coupling agents.

The electrostatic theory

This theory states that an electrostatic ally charged double bond develops at the bond interface as a result of the interaction of the adhesive and adherent which contributes significantly to the bond strength.

The diffusion theory

When an adhesive contains an adherent solvent the adhesive can diffuse into the adherent surface (substrate) with an interchange of molecules. The theory is is only really applicable to polymers where a movement and entanglement of long molecules can occur.

This can be viewed as a molecular interlock enabled adhesion. For plastics, the theory includes for effects of contact time, influence of time and temperature on bonding rate, and the influences of polymer molecular weight and polymer structure.

While the diffusion theory applies well for cases of self-adhesion or auto-adhesion, it does not fit well in providing an explanation for polymer-polymer adhesion. High molecular weight thermoplastic polymers often display very high melt viscosity and will not diffuse easily within the time scale of most bonding operations.

The weak boundary layer theory

For most metals there is a surface layer such as a scaly oxide layer. For a successful bond this layer is ideally removed by surface treatments before a strong adhesive bond can be achieved.Aluminium has a strong coherent oxide layer which is suitable for bonding.

Mechanical theory

The oldest theory of adhesion is definitely the mechanical theroy.It's based on mechanical anchorage of the adhesive in pores and irregularities in the adherent and is discussed primary in reference to wood and similar porous material.

Classification of adhesives-

Adhesives are typically organized by the method of adhesion. These are then organized into reactive and non-reactive adhesives, which refers to if the adhesive chemically reacts to harden.

This classification based on whether the adhesive is produced from natural source or synthesized from basic hydrocarbon

1.Natural adhesives-

This term is used to include vegetable and animal based adhesives and natural gums. these include organic materials such as casing ,blood, albumin ,hide ,bone,fish,starch,resin,shallac ,asphaltend inorganic adhesives. their use ,except for the inorganic adhesives, is mostly limited to paper,paperboard,soil and light wood. they are inexpensible,easy to apply and have a long shelf life .these adhesives develop tack quickly, but have low strength properties. most are water soluble and use where as a solvent.

2. Synthetic adhesives-

The term usually used to apply to all adhesives other then natural adhesives(i.e,thermosetting,thermoplastic,elastomeric and alloys) .all structural adhesives are synthetic adhesives

Classification by chemical composition-

This classification describes synthetic adhesives as a thermoplastic ,thermosetting ,elastomeric and alloys of these types-

1.Thermosetting adhesives-

These are material that can't be heated and melted after the initial cure. Curing takes place by chemical reaction at room temperature or at an elevated temp.,depending on the type of adhesives. some thermosetting adhesives required considerable pressure, while other require only contact pressure.

Thermosetting adhesives are provided as one- and two- part system. the one-part system usually require elevated temp. Cure and have a limited shelf life. the two-part system have longer shelf lives and can usually be cured solely at room temprature.once the adhesives mixed ,the useful life is limited.

2.Thermoplastic adhesives-

These materials don't cross-link melted without significant change in their properties .they are single-component system that harden upon cooling from a melt state or by evaporation of a solvent or water vehicle. Wood glues are thermoplastic emulsion that are commonly household items. they harden by evaporation of water from an emulsion.

Thermoplastic adhesives are not ordinarily recommended for use at above 66'c although they can be used up -99'c in some applications.

3. Elastomeric adhesives-

These materials are based in synthetic or naturally occurring polymers. they have superior toughness and elongation. Elastomeric adhesives may solvents, latex cements,dispersions,pressure sensetive tapes and single-or multi-part solvents ,liquid or pastes.

Classification by physical form-

adhesives can be subdivided by their form such as liquid ,powder, silica or paste. the physical state of the adhesives generally determines how it is applied.

1 .Liquid adhesives-

These adhesives are easily applied by means of mechanical spreaders such as rolls, by spraying or bruising.

2. Paste adhesives-

paste adhesives have high viscosities to allow application on virtual surface with little tendency to say .these adhesives can serve as gap fillers and sealants.

3.Tape and silica adhesives-

These adhesives provide a band line with conform thickness and metering and ease of dispersing. Adhesive films are available as pure sheets of adhesives or with filter paper reinforcement.

Classification by functions-

1. Structural adhesives-

These are materials of high strength and performance. their primary functions is to hold structures together and to be capable of resizing high loads.

2. Non-reactive adhesives-

These adhesives are not required to withstand substantial loads, but merely hold materials in place. this group is sometimes called ''holding adhesives'' ,example include adhesives/sealants that are primary intended to fill gaps and rubber cements that are used to adhere paper to paper in office applications.

Basic properties of adhesives-

what is an adhesives and what are its basic properties? A definition is a material which when applied to the surface of materials can join terms adhered and subtract are used for a body or material to bee bonded by an adhesives .other basic terms are ''self -life'' for the time and adhesives can be started before use and ,''pot life''; the maximum time between final mixing and application.

Basically an adhesive must do two things--

1.It must used the surface ,that if it must spread and make a contact angle approaching zero, as illustrated in figure .Intimate contact is required b/w the molecules of the adhesives and the atoms and molecule in the surface, when applied the adhesives will be a liquid of relatively low viscosity .

2.the adhesive must then harden to a cohesively strong solid .thus can by chemical reaction .loss of solvent or water ,or by cooling in the case of hot melt adhesives, there is an exception to this, and that pressure-sensitive adhesives which remain permanently stickly,these are adhesive used in sticky tapes and labels

Mechanisms of adhesion

Adhesion, the attachment between adhesive and substrate may occur either by mechanical means, in which the adhesive works its way into small pores of the substrate, or by one of several chemical mechanisms. The strength of adhesion depends on many factors, including the means by which it occurs.

In some cases an actual chemical bond occurs between adhesive and substrate. In others electrostatic forces, as in static electricity, hold the substances together. A third mechanism involves the Vander Waals forces that develop between molecules. A fourth means involves the moisture-aided diffusion of the glue into the substrate, followed by hardening.

Composition- an adhesive is composed of basic raw material ,which are called binders and which determine its adhesiveness (adhesion)and its internal strength(cohesion) and of frequent necessary auxiliries,which establish particular end use and processing characteristics.

Failure of the adhesive joint

Failure of the adhesive joint can occur in different locations

There are several factors that could contribute to the failure of two adhered surfaces. Sunlight and heat may weaken the adhesive. Solvents can deteriorate or dissolve adhesive. Physical stresses may also cause the separation of surfaces. When subjected to loading, deboning may occur at different locations in the adhesive joint. The major fracture types are the following:

Cohesive fracture

Cohesive fracture is obtained if a crack propagates in the bulk polymer which constitutes the adhesive. In this case the surfaces of both adherents after deboning will be covered by fractured adhesive. The crack may propagate in the centre of the layer or near an interface. For this last case, the cohesive fracture can be said to be "cohesive near the interface". Most quality control standards consider a good adhesive bond to be cohesive.

Interfacial fracture

The fracture is adhesive or interfacial when deboning occurs between the adhesive and the adherent. In most cases, the occurrence of interfacial fracture for a given adhesive goes along with a smaller fracture toughness. The interfacial character of a fracture surface is usually to identify the precise location of the crack path in the interphone.

Adhesives coating,general rules-

1.Diffrent methods-

The choice of the method of coating depends on several factors-

Size and shape of the parts to be coated.

Accessibility of the places to be coated.

Shape of adhesives or sealants joints.

Production output(m2 or parts to be assembled per hour)

Viscosity and rhcology of the products.

Types of adhesives and sealents,pot life when it's a two component products.

Types of solvents ,flammability,noxiousness of solvents.

Manual or automatic application,degree of automation required .

Labour and equipment cost

Applications of adhesives-

1.Bookbindings-

Bookbindings with adhesives has played an important roll since the bigning of the production of graphical products .Originally,vegetables and animal raw materials were the main ingrdients of adhesives,but a steady development of the adhesives took place to fulfill manufacturing requirements.

Four types of adhesives are used for perfect bindings-

Glutin glues

Emulsion adhesives

Hot-melts adhesives

reactive polyurethane hot-melt adhesives.

2.Case making-

In case making ,when the covering material consist of textils plastics film,or paper ,as in the backlining of books,hot animal glues are used because they set rapidly by gelatination and rapidly bond the narrow overlap of the coating materials around the board.

3.Envelops-

In general,the top-flap gamming of envelops is made with modified conventional

dextrin and dextrin-emulsion mixed glues,which dry more quickly and show better ability to stay flat for automatic envelopping to an increase extent.

4.adhesives for automatic packing in machine-

In automatic packing ,package production,filling and closing are often carried out,

for this reason ,very fast setting adhesives are uses.

5.Cigrette manufaturing-

For gluing cigrette seams ,depending in the application system,hightly viscosity or few-flowing starch glues or emulsion based adhesives are used.

6.adhesive for laminated films-

Laminated films are currently produced by two techniques,lammination anextrus-

ion .In lamination ,the low viscosity adhesives solution is applied by rolls to one of the films in a thickness of a few micrometers and dried.

7.Construction adhesives-

APAO -based adhesives are mainly applied by slot nozzles on both the polyethylene

backsheets and nonwovents top sheets.

Advantages and disadvantages of adhesives-

Adhesives are very helpful and important in many cases but they have some disadvantages also.In this we will see both factors--

1.Advantages of adhesive-

a. Uniform disturbution of stress and larger stress,bearing area.

b. Join thin or thick materials of any shape.

c. Join similar or dissimilar materials,minimize or prevent electrochemical

corrosion between dissimilar materials.

d. Desist fatigue and cyclic loads.

e. Provide joints with smooth contours.

f. Seal joints against a variety of enviroments.

g. Provide an attractive strenth/weigh ratio

2.disadvantages of adhesives-.

a. The bond doesn't permit visual exammination of the bond area.

b. Careful surface preparation is require to obtain dirable bonds,offten with

corrosion chemicals.

c. Long cure times may be needed ,particular where high cure temp.are not used.

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