Exploring The Chemistry Of Adhesives Chemistry Essay

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An adhesive is a substance that sticks to the surface of an object such that two surfaces become bonded. A typical home improvement store carries many different adhesives for many different applications..The interaction of molecules is known as intermolecular bonding, or secondary bonding. Primary bonding, also known as intramolecular bonding, is the interaction of atoms within a molecule and includes covalent and polar covalent bonding. Secondary bonding includes dipole-dipole bonding (the interaction of molecules that have a permanent net dipole moment) and hydrogen bonding (an interaction that occurs when a hydrogen atom is bonded to an N, O, or F atom in a molecule).

Adhesives cure when the small resin molecules join together to form extremely large molecules known as polymers. For example, one of

Some adhesives, such as this wood adhesive, are used only with specific substrates because of the cure time needed to allow for good bonding.

The simplest polymer is polyethylene. The mer (basic building block of the polymer) is ethylene, H2 C=CH2. The addition of an initiator (R) causes the formation of the radical RCH2 CH2 . A radical is a species that has an unpaired electron and is very reactive because it seeks the source of electrons. This radical will attach the ethylene mer (the double bond in ethylene is rich in electrons) to start a chain reaction that continues until very large polymer molecules form. This and other forms of polymerization processes are the basis for the formulation of polymers. This process is known as curing when dealing with adhesives.

Two criteria must be met in order for a molecule to possess a permanent net dipole moment: (1) an unequal sharing of electrons within the molecule such that one or more intramolecular bonds has a partial positive end and a partial negative end, and (2) a geometry such that the vector sum of the individual dipole moments does not equal zero.

The ability of an atom within a molecule to attract electrons is known as electronegativity, a concept proposed by Linus Pauling who established a table of relative electronegativities. In Pauling's table, fluorine is the most electronegative element and is given the value of 4.0. The greater the difference in electronegativity between two atoms within a molecule, the larger is the dipole moment in that bond. Because the bond between two atoms having unequal electronegativities has a partial positive end and a partial negative end, it is said to be a polar bond. If the geometry of the molecule is such that the vector sum of all of the dipole moments does not equal zero, then the molecule is polar.

The electronegativities for carbon and oxygen are 2.5 and 3.5, respectively; therefore, the carbon-oxygen bond is a polar bond. A carbon dioxide molecule has two carbon-oxygen bonds; however, its geometry is such that the vector sum of the two dipole moments equals zero, and thus carbon dioxide is a nonpolar molecule. The electronegativity of hydrogen is 2.1, thus a hydrogen-oxygen bond would be polar. A water molecule has two hydrogen-oxygen bonds. The geometry of a water molecule (the H-O-H bond angle is 104.5°) is nonsymmetrical, hence the vector sum of the dipole moments is not equal to zero and water is a polar molecule.

Polar molecules will attract other polar molecules because of their net dipole moments. Water molecules, however, have an additional attraction for one another, based on hydrogen bonding. This attraction is so strong that, although water is a small molecule and small molecules tend to be gases, water is a liquid at room temperature. This aspect of the chemistry of water demonstrates that hydrogen bonding is a relatively strong force that can hold molecules together.

Two surfaces there must be several types of interaction between the adhesive and both substrates. The first type of interaction is that the adhesive must wet the substrate, that the adhesive must spread itself out into a film that covers the substrate surface. In order for this to happen, the adhesive must have a low enough viscosity so that it will flow. Viscosity is the resistance of a liquid to flow. Water has a low viscosity whereas honey has a high viscosity. Because viscosity is temperature dependent, the application of a cold adhesive to a substrate, or the application of an adhesive to a cold substrate, may result in poor wetting. Another factor that affects wetting is the relative strengths of cohesive forces, and those of adhesive forces. If the cohesive forces among adhesive molecules are weaker than the adhesive forces between the adhesive molecules and the substrate surface, then the adhesive molecules will spread out over the substrate and wet its surface. An adhesive that has a relatively low viscosity and is able to wet the substrate surface will flow into any tiny cracks or pores on the substrate surface.

Mechanical bonding is one of several ways that an adhesive bonds substrates. All surfaces, except those that are highly polished, have pores. If the adhesive flows into these pores and then polymerizes, a mechanical bond is formed.

The interactions of adhesive molecules with substrates are so critical, it makes sense that some adhesives would be more appropriate for a specific substrate than others. Adhesives are designed for specific applications.

For example, adhesives known as "super glues" (cyanoacrylates) are useful around the home in the bonding of common substrates (e.g., dishes, toys, etc.), which can take place in a matter of seconds. Cyanocrylates tend to be brittle thus they are vulnerable to impact and dramatic changes in temperature. To reduce these shortcomings, small amounts of finely ground rubber has been used as filler. The rubber introduces flexibility thus reducing brittleness. In addition, cyanocrylates are attacked by polar solvents. Polar solvents will weaken cured cyanocrylate bonds over time. Therefore, applications involving water, alcohols, or other polar solvents should be avoided.

Cyanoacrylates are not appropriate for the bonding of the steel parts of an automobile, because of the environments that the car will be exposed to. Those environments include such things as rain, variations in temperature, exposure to solvents (such as gasoline, oil, and windshield washer solution), ozone, acid rain, salt spray, and ultraviolet light from the Sun.

Another example of a "special" adhesive would be the one used to attach a new rearview mirror in an automobile. Because the cured adhesive in this case will be exposed to wide variations in temperature and to an extremely large amount of ultraviolet light from the Sun for prolonged periods of time, an adhesive formulated specifically for these conditions should be used.

Finally, the strength and permanence of the bond formed between adhesive and substrate must be considered when one is selecting an adhesive. Most of the time it is desirable to have maximum strength and permanence; the very common Post-it note, however, is a counterexample. Its adhesive is neither strong nor permanent.

Lists of some common types of adhesives and their uses. Because of the different possible substrates and combinations of substrates, and because adhesives are subject to such a range of environmental conditions, it is no wonder that there are so many types of adhesives on the market. However, if one has some knowledge of how adhesives bond to substrates and the types of substrates being bonded, the task of selecting adhesives will not be overwhelming.

TYPES OF ADHESIVE

NATURAL ADHESIVE

Animal glue

Casein glue

Blood albumen glue

Starch and dextrin

Natural gums

SYNTHETIC ADHESIVE

Contact cements

Structural adhesives

Hot-melt adhesives

Pressure-sensitive adhesives

Ultraviolet-cured adhesives

NATURAL ADHESIVE: -

Natural adhesives are primarily of animal or vegetable origin. Though the demand for natural products has declined since the mid-20th century, certain of them continue to be used with wood and paper products, particularly in corrugated board, envelopes, bottle labels, book bindings, cartons, furniture, and laminated film and foils. In addition, owing to various environmental regulations, natural adhesives derived from renewable resources are receiving renewed attention.

ANIMAL GLUE

The term animal glue usually is confined to glues prepared from mammalian collagen, the principal protein constituent of skin, bone, and muscle. When treated with acids, alkalies, or hot water, the normally insoluble collagen slowly becomes soluble. If the original protein is pure and the conversion process is mild, the high-molecular-weight product is called gelatin and may be used for food or photographic products. The lower-molecular-weight material produced by more vigorous processing is normally less pure and darker in colour and is called animal glue.

CASEIN GLUE

This product is made by dissolving casein, a protein obtained from milk, in an aqueous alkaline solvent. The degree and type of alkali influences product behaviour. In wood bonding, casein glues generally are superior to true animal glues in moisture resistance and aging characteristics. Casein also is used to improve the adhering characteristics of paints and coatings.

BLOOD ALBUMEN GLUE

Glue of this type is made from serum albumen, a blood component obtainable from either fresh animal blood or dried soluble blood powder to which water has been added. Addition of alkali to albumen-water mixtures improves adhesive properties. A considerable quantity of glue products from blood is used in the plywood industry.

STARCH AND DEXTRIN

Starch and dextrin are extracted from corn, wheat, potatoes, or rice. They constitute the principal types of vegetable adhesives, which are soluble or dispersible in water and are obtained from plant sources throughout the world. Starch and dextrin glues are used in corrugated board and packaging and as a wallpaper adhesive.

NATURAL GUMS

Substances known as natural gums, which are extracted from their natural sources, also are used as adhesives. Agar, a marine-plant colloid is extracted by hot water and subsequently frozen for purification. Algin is obtained by digesting seaweed in alkali and precipitating either the calcium salt or alginic acid. Gum arabic is harvested from acacia trees that are artificially wounded to cause the gum to exude. Another exudate is natural rubber latex, which is harvested from Hevea trees. Most gums are used chiefly in water-remoistenable products.

SYNTHETIC ADHESIVE:-

Although natural adhesives are less expensive to produce, most important adhesives are synthetic. Adhesives based on synthetic resins and rubbers excel in versatility and performance. Synthetics can be produced in a constant supply and at constantly uniform properties. In addition, they can be modified in many ways and are often combined to obtain the best characteristics for a particular application.

The polymers used in synthetic adhesives fall into two general categories-thermoplastics and thermosets. Thermoplastics provide strong, durable adhesion at normal temperatures, and they can be softened for application by heating without undergoing degradation. Thermoplastic resins employed in adhesives include nitrocellulose, polyvinyl acetate, vinyl acetate-ethylene copolymer, polyethylene, polypropylene, polyamides, polyesters, acrylics, and cyanoacrylics.

CONTACT ADHESIVE

Contact adhesives or cements are usually based on solvent solutions of neoprene. They are so named because they are usually applied to both surfaces to be bonded. Following evaporation of the solvent, the two surfaces may be joined to form a strong bond with high resistance to shearing forces. Contact cements are used extensively in the assembly of automotive parts, furniture, leather goods, and decorative laminates. They are effective in the bonding of plastics.

STRUCTURAL ADHESIVE

Structural adhesives are adhesives that generally exhibit good load-carrying capability, long-term durability, and resistance to heat, solvents, and fatigue. Ninety-five percent of all structural adhesives employed in original equipment manufacture fall into six structural-adhesive families: (1) epoxies, which exhibit high strength and good temperature and solvent resistance, (2) polyurethanes, which are flexible, have good peeling characteristics, and are resistant to shock and fatigue, (3) acrylics, a versatile adhesive family that bonds to oily parts, cures quickly, and has good overall properties, (4) anaerobics, or surface-activated acrylics, which are good for bonding threaded metal parts and cylindrical shapes, (5) cyanoacrylates, which bond quickly to plastic and rubber but have limited temperature and moisture resistance, and (6) silicones, which are flexible, weather well out-of-doors, and provide good sealing properties. Each of these families can be modified to provide adhesives that have a range of physical and mechanical properties, cure systems, and application techniques.

HOT-MELT ADHESIVE

Hot-melt adhesives are employed in many nonstructural applications. Based on thermoplastic resins, which melt at elevated temperatures without degrading, these adhesives are applied as hot liquids to the adherend. Commonly used polymers include polyamides, polyesters, ethylene-vinyl acetate, polyurethanes, and a variety of block copolymers and elastomers such as butyl rubber, ethylene-propylene copolymer, and styrene-butadiene rubber.

PRESSURE SENSETIVE ADHESIVE

Pressure-sensitive adhesives, or PSAs, represent a large industrial and commercial market in the form of adhesive tapes and films directed toward packaging, mounting and fastening, masking, and electrical and surgical applications. PSAs are capable of holding adherends together when the surfaces are mated under briefly applied pressure at room temperature

TYPES OF ADHESIVES

Adhesive Type

Sources/Properties

Common Uses

Animal glue

Obtained from animal byproducts such as bones, blood, and hooves

Binding of abrasives in sandpaper and other grinding materials

Casein

Main protein in milk

Labels on beer bottles that do not come off in ice water, yet are recyclable

Starch

From corn and maize

Corrugated cardboard bonding

Natural rubber

Not "sticky enough"by itself but is used as an additive in other adhesives

Self-adhesive envelopes and other pressure-sensitive adhesives; adhesives that bond to substrates on contact (like tapes)

Butyl rubber/isobutylene

It is elastomeric-it stretches

Additive for hot-melt adhesives, window sealants, and pressure-sensitive adhesives

Amino resins

Water-soluble adhesives

Bonding of layers in plywood and the bonding of particles in particle board

Polyurethane

A flexible adhesive

Bonding soles to the bodies of shoes; also used in food packaging

Polyvinyl acetate

Common "white"glue

Book bindings and labels

Polyolefin/ethylene copolymer

No solvents involved

Hot melts

Acrylates or anaerobic adhesives

Cure when air is removed

Adhesive used to keep nuts tight on bolts, such as those within ATMs and heavy machinery

Silicone

Both an adhesive and a sealant and only common adhesive that is based on silicon rather than carbon

Bathtub and shower sealants; also many car applications, such as oil pans and head gaskets

Uses of Adhesives:-

The exact combination of reasons will vary from case to case, but advantages should be sought in the following areas:

·         Increased production speed. In comparison with other fabrication methods, adhesive assembly is essentially fast. Even if curing is required, this can often be accommodated "off line", or combined with other processing stages such as paint curing.

·         Wider material selection. In contrast to welding, adhesives allow a wide freedom of choice during material specification. It is possible to mix and match material combinations to suit product function and save production costs in ways which have been impossible in the past. Traditional materials may be combined with new metal alloys, plastics, composites and ceramics to give distinct product advantages. Absorbing the full potential of this new freedom is perhaps one of the biggest challenges in finding significant market opportunities.

·         Design for manufacture. Adhesive assembly offers significant cost savings if material costs can be reduced and production operations can be simplified. There is sample evidence that this can be the case if the design and manufacturing functions co-operate to design or redesign the product with manufacturing in mind. New approaches can be taken to the manufacture of sub components, and castings may be combined with extrusions, sheet components and parts produced in a variety of other ways.

·         Better production sequences. Traditional assembly methods such as welding impose fairly rigid sequences during production, and frequently demand intermediate processing to remove contamination or rectify distortion. Bottlenecks can be removed, unnecessary operations can be eliminated, and work in progress can be reduced.

·         Low capital costs. Many adhesive operations involve manual application and the use of adhesive packages which have built in applicators. Even when mechanised or automated application is justified for high volume work, the equipment is usually lighter than would be used for welding.

·         Low production costs. This is a source of significant confusion Costs "per tube" of some adhesives may be high, but this should not be confused with all in production costs or cost per product. Cost comparisons should be based on the costs of the whole joining process, including plant, preparation and other pre-assembly costs, production and rectification expenses. All in cost assessments of this type provide a basis for accurate comparisons, and adhesive assembly may often give significant benefits.

Advantages of Using Adhesives

·         Dissimilar materials can be joined.

·         The bond is continuous.

·         Stronger and stiffer structures can be designed.

·         On loading there is a more uniform stress distribution (Figure 2).

·         Local stress concentrations are avoided.

·         Porous materials can be bonded.

·         Adhesives prevent catalytic corrosion.

·         Adhesives seal and join in one process.

·         No finishing costs.

·         Improved fatigue resistance.

·         Vibration damping.

·         Reduced weight and part count.

·         Large areas can be bonded.

·         Small areas can be bonded accurately.

·         Fast or slow curing systems available.

·         Easy to combine with other fastening methods.

·         Easily automated/mechanised.

Figure 2. Stress distributions in a riveted joint and an adhesively bonded joint. Note areas of high stress concentration in the riveted joint.

All these advantages may be translated into economic advantages: improved design, easier assembly, lighter weight (inertia overcome at lower energy expenditure) and longer life in service.

Limitations of Using Adhesives

·         Not as strong as metals.

·         Increasing the service temperature decreases the bond strength.

·         Short term handle ability is poor.

·         Bonded structures are usually difficult to dismantle for in service repair.

·         Need to prepare the surface.

·         Environmental resistance depends on the integrity of the adhesive.

·         Need to ensure wetting.

·         Un-familiar process controls.

·         Health and safety responsibility.

Manufacture of Adhesive

CONSTRUCTION CHEMICALS:-

Nelion Exports has recently forayed into the supplies of construction chemicals from one of the most reputed company with global acquaintance.

The products under this category include applications for Waterproofing, Repairing, Grouting, Gunniting, Bonding, Plastering, Flooring, Curing, Concreting & Expansion joints. 

Of the 188 products developed, to name a few, the exclusive range of products include Sacrifical Anodes, Heat Insulating Coating, Fire Retardants, Anti Freezing Admixtures, Antiwashout Admixtures, Corrosion Inhibitors, PU Coatings, Hydrophilic Sealants, Auto Suction Crack Fillers, Aqua Reactive Leakage Sealing Liquid, Vapour barrier coating for the it industry, Cemetetious Floor Hardner. 

Tailor made construction chemicals are developed bearing in mind the application desired. 

CYNOCRYLATE ADHESIVE

POLYFIX ® Cyanocrylate Adhesive are single component "instant curing" solvent free adhesives. They are quick and easy to apply and do not require any mixing or heating before the use. In order to achieve higher performance, only application of contact pressure is required. The Cyanocrylate Adhesive delivers best performance when applied as a thin film between two surfaces.

TILE ADHESIVE

The tile adhesive is mainly comprised of cement making it ideal for the fixing of tiles on the external floor and internal walls. Also well suited for fixing most of the natural stones, the ceramic tile adhesive is available in HDPE packets containing 20 kg of tile adhesive. These should be used within 30 minutes after being mixed with water so as for better result.

MALTO DEXTRIN

Our optimum quality of Malto Dextrin is formulated in the hydrolysis of starch. This intermediate of starch and glucose, is hydrolyzed by Bacterial alpha Amylase and further conversation to get the desired DE which ranges 4 to 30. This can be further refined by means of clarification, carbon treatment and ion exchange followed by spray drying to a moisture level of 3% to 5%.

LIQUID SEALANT

This single component system is available in packings of different sizes such as 50gm, 100gm, 200gm and 500gm. These V-TITE Strong Liquid Sealants are totally non toxic and are resistant to corrosion and water. These sealants prevent corrosion of the surface and increase the life of the material. They provide strong adhesion to the surface and can also work on alloys and uneven surfaces. 

MALTODEXTRIN

Maltodextrin is a starch hydrolysis product ranging up to 20 DE. The main characteristic of this carbohydrate is that it provides body to a food product without appreciably increasing the sweetness while retaining the calorific value. Another characteristic of Maltodextrin subject to medical confirmation is that this carbohydrate is more easily digested and more easily tolerated by the diabetic persons. It is aloes used as base in preparation of food products such as health drinking, soups Milk shakes etc.

 YELLOW DEXTRIN POWDER

We present to our clients, quality assured yellow dextrin powder that is widely applicable in the production of adhesives, emulsions, firecrackers and other industrial supply products. Their inherent quality of being easily soluble in water and their binding qualities makes them extremely popular for the aforementioned.

Yellow dextrine provided by us possesses the following features:

• Low viscosity

• Fast drying

• It is converted form of the treated starches

• Completely soluble in cold water

PASTING GUM

The range of pasting gum manufactured by us is widely catered in many industries fir various purposes. It enhances an enhanced penetration for deeper fiber tearing bonds which have the least stack cure time. These are also known to increase the dry strength measured as per the pin adhesion test, edge crush test and box crush test. If the gum is dried within the expected time it leads to great speeds and higher production output.

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CRUSHER BACKING COMPOUNDS

We offer a wide range of crusher backing compounds that are 100% solid epoxy compound, used for backing wear liners in cone and gyratory crushers. These are highly compressive and impact strength in nature that facilitates heavy duty crushing. In addition, our range of crusher backing compound is in great demand because of following features.

COLD VULCANISING ADHESIVE

Our cold vulcanizing adhesives are manufactured based on the latest German Know how and using imported raw materials. These adhesives are used in jointing conveyor belts and bonding rubber to metal surfaces. They are available in fire retardant and heat resistant grades also. 

HIGHLY EFFECTIVE TILE ADHESIVE POWDER

Conpro TA-1 is a polymer modified cement based dry power. This construction chemical is mixed with water to make workable mortar which is highly suitable for fixing of Tiles over cementitious surfaces.

SILICONE ADHESIVE

Pressure sensitive adhesives: Dow Corning Q2-7406 Adhesives (Silicone Adhesive) Features:

• Excellent high-temperature properties

• Adhesion to 288°C (550°F)

• Food-contact capability

Composition:

• Polydimethylsiloxane gum & resin dispersion; high-viscosity liquid 

Applications:

• General purpose adhesive 

• Splicing & plating tapes 

Typical Properties:

Specification Writers: These values are not intended for use in preparing specifications. Please contact your local Dow Corning sales representative prior to writing specifications on this product.

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