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Transcript of Adhesives
•Adhesives that are thermosetting are materials which solidify when heated. •Thermosetting adhesives are essentially infusible, insoluble and show good creep resistance. They are used for high load assemblies and severe service conditions such as heat, cold, radiation etc.•Thermoset adhesives set as a result of the build up of molecular chains to produce a rigid crosslinked structure. They include epoxy resins, which are some of the most widely used adhesives. •Thermoset adhesives are generally more expensive,to purchase and to use, than the other adhesive types. •They are the adhesives most used for structural, load bearing applications.
•Some of the toughened variants provide exceptional properties directly comparable with welded and riveted alternatives.
•There are many different thermoset adhesives available including phenolic formaldehyde (PF) resins, phenolic neoprene, resorcinol formaldehydes (RF), polyesters, polyimides and epoxy resins.•They are formed by:a.Condensation-polymerization reactionsb.The presence of two or more groups, each capable of additional polymerizationc.Presence of trifunctional or difunctional molecules TYPES OF
SYNTHETIC ADHESIVES NATURAL ADHESIVES
These adhesives have been replaced in many uses by synthetics., but animal glues, cellulose, starches, gums, bitumens, and natural rubber cements continue to be used in large volumes. Adhesives is a substance that is used to bond two or more surfaces together. The effectiveness of an adhesive depends on several factors including resistance to slippage and shrinkage, malleability, cohesive strength, and surface tension, which determines how far the adhesive penetrates the tiny depressions in the bonding surfaces. INTRODUCTION Most adhesives form a bond by filling in the minutes pits and fissures normally present even in very smooth surfaces. Adhesive bonds are economical, distribute the stress at the bonding point, resist moisture and corrosion, and eliminate the need for rivets and bolts. Adhesives vary with the purpose for which they are intended. Such purposes now include the increasing use of adhesives in surgery. Tape Runners Adhesive Tapes Tile Adhesives Construction Adhesives Animal Glue Cellulose Tape Rubber Cement Starch Resin Gums Bitumen ORGANIC ADHESIVES
These are derived from animal protein include glues made from collagen, a constituent of the new connective tissues and bones of mammals and fish; glue made from casein, a protein constituent of milk, employed in wood bonding in paint; and blood albumen glue, used in the plywood industry. VEGETABLE ADHESIVES
starches and dextrins (derived from corn, wheat, potatoes, and rice) used for bonding paper, wood, and textiles
gums (such as gum Arabic, algin and agar) which when moistened provide adhesion in such products as stamps and gummed envelopes
cellulose adhesives used to bond leather, cloth, and paper
resins such as tree pitch and mastic SYNTHETIC ADHESIVES
These are used both alone or as a modifiers of natural adhesives, perform better or have a greater range of application than the natural products.
Most of them form a polymers, huge molecules incorporating large numbers of simple molecules to form strong chains and nets that link surfaces in a firm bond. ADVANTAGES AND DISADVANTAGES OF
ADHESIVE BONDING ADVANTAGES
Good fatigue resistance
Excellent fatigue strength
Uniform stress distribution over the entire bonded area
Provide large stress-bearing area
Dampen vibration and absorb shock
Good sealing and insulating properties
Join all shape and thickness
Join any combination of similar or dissimilar materials
Ability to bond aluminium to non-metallic materials
Weight reduction compared to bolt and rivet joints
Ability to anodise with no risk of staining
Fewer post bonding operations
Heat, if required, is too low to affect metals parts
Provide attractive strength-to-weight ration DISADVANTAGES
Joints are almost permanent
Surface must be carefully cleaned
Longer cure times may be needed
Useful life depends on environment
Decreased strength at elevated temperatures, especially under long time loading
Heat and Pressure may be required
Possible degradation of bond by environmental influence
Complicated bonding process. Stronger joints demand proper surface preparation and may also require jigs, presses and heating equipment
Inspection of finished joint is difficult
Difficult and expensive to provide for adequate inspection
Special Training if often needed
Additional exhaust and protective clothing might be necessary for reasons of health RAW MATERIALS Aerosol Propellants
for aerosol adhesives, pressurized gases are used to propel the adhesive out of the container. Thickeners
substances added to an adhesive to regulate the visosity of the adhesives. Extenders/Fillers
substances added to an adhesive to reduce the amount of the primary binder required per unit area and to working properties, permanence, strength, or other qualities. Starch
-may come from processing potatoes or corn processing. Solvents/Diluents
an ingredient or mixture of ingredients used to disperse the adhesive constituents to a spreadable consistency, which makes a uniformly thin adhesive coating possible. Binder
a component of adhesives that is primarily responsible for the adhesive forces which hold two bodies together. (cc) photo by theaucitron on Flickr (cc) photo by theaucitron on Flickr Alcohols
used as solvents in adhesives, and they are also intermediates for a number of glycol ethers. Plasticizers
are added to an adhesive to increase softness, flexibility, and extensibility. QUALITY CONTROL Shear Strength
Shear strength is defined as a material's ability to resist forces that can cause the internal structure of the material to slide against itself. In general, adhesives have high shear strength. Single-Lap Joint: Despite all its obvious weaknesses, the lap-shear test is the most widely used method for producing in-situ shear strength data of an adhesively bonded joint. The test consists essentially of two rectangular sections, typically 25 mm wide, 100 mm long and 1.6 mm thick, bonded together, with an overlap length of 25 mm. Variations of this test method are included in both national and international standards. The single-lap specimen is easy to prepare and test. A fixture is used to ensure correct overlap and accurate alignment of the adherend. This may include control of the fillet. Testing can be conducted using standard tension/compression mechanical test equipment. Tensile Adhesion
A material's ability to resist forces that attempt to stretch it or pull it apart. In general, adhesives have high tensile strength.
Transfer of load to an adhesively bonded structure by tension, either directly or indirectly (peel), represents the most severe form of loading, since the strength of the joint relies on the tensile strength of the adhesive, which is low. It is good design practice, therefore, to ensure that the load is transferred by shear or compression and that direct or induced tensile stresses are minimised. Tensile stresses are virtually impossible to avoid, however there is a need for reliable test methods to measure the tensile properties of the adhesive. Tensile Butt Joint
- is used to evaluate the tensile properties of adhesive joints Wedge Cleavage Test
The test involves forcing a wedge into the bondline of a flat-bonded specimen, thereby creating cleavage stresses at the crack tip. The wedge imposes a fixed displacement to the adherends and the energy stored in bending the adherends (i.e. arms) provides the driving force for crack growth. The stressed specimen is exposed to an aqueous environment, usually at an elevated temperature, or an appropriate environment representative of service condition. The resulting crack length is monitored with time. Cleavage Strength
A material's ability to resist forces that can pull it apart by separating two rigid surfaces. In general, adhesives have low cleavage strength. T-Peel Test
Testing is straightforward and requires no special fixture. The specimen can be readily loaded using standard tension/compression mechanical test equipment. It is important to ensure that the bonded portion of the specimen remains perpendicular to the applied load. The specimen is bent backwards 180° and peeled. Tests are normally conducted at displacement rates of 100 mm/min for metals and 10 mm/min for other adherends. The average peeling load per unit length/width is used to define the peel strength. Peel Adhesion
A material's ability to resist forces that can pull it apart by separating a flexible surface from a rigid surface. In general, adhesives have low peel strength. Stress Rupture
Stress rupture testing is similar to creep testing except that the stresses are higher than those used in a creep testing. Stress rupture tests are used to determine the time necessary to produce failure so stress rupture testing is always done until failure. Creep
Creep is the increase in strain or deformation of a material (or structure) with time when the material is subjected to a constant load for an extended period of time (i.e. time-dependent deformation). The change of strain at any time increases with load, temperature, relative humidity and time. Thermoplastic Adhesives :
•Thermoplastic adhesives are fusible, soluble and poor heat and creep resistant.
•They are normally used for low/medium load assemblies under reasonable service conditions In general, thermoplastic adhesives have low/medium shear strength and suffer from creep at high loading.
•They have good resistance to oils but poor resistance to water.
•Over the last 35 years, properties of thermoplastic adhesives have been enhanced by toughening such that variants are available suitable for structural duties.
•There is widespread use of thermoplastic adhesives (anaerobic) adhesives for radial fit assemblies, and screw locking assemblies.
•Thermoplastic adhesives include polyvinyl acetate (PVA), polyvinyl alcohol (PVA), polyacrylates, polyester acrylics, acrylic solvent cement, cyanoacrylates (superglue), silicone resins, polyamides and acrylic acid diesters. Classification of these adhesives are:
•Thermosetting Synthetic Resin Adhesives
•Cellulosic and Starch Derivatives
include various types of products that have such unique characteristics as stability in fluid paste, gelatinization property, excellent fluid viscosity, chemical resisting, electrical property, and have various uses according to their own characteristics.
produced by the chemical reaction of hydroxyl groups of raw starches with various functional groups for the purpose of obviating various defects of the raw starches and giving useful properties to meet the requirements of each industry.
•Water-Soluble Gums in Adhesives
substances known as natural gums, which are extracted from their natural sources, also are used as adhesives. Agar, a marine-plant colloid (suspension of extremely minute particles), 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.
•Adhesives of Animal or Fish Origin and Organic Adhesives
generally set by solvent evaporation. They are generally of low strength and are susceptible to moisture and mold. Their use is restricted to the joining of low strength materials. •Adhesives from Rubber or Synthetic Rubber
Based on natural and synthetic rubbers set by solvent evaporation or heat curing.
They have relatively low shear strength and suffer from creep and are therefore used for unstressed joints.
They are useful for flexible bonds with plastics and rubbers. Contact adhesives’ use rubber in a solvent and will join many materials.
Elastomer adhesives include natural rubbers, polychloroprenes (neoprene), acrylonitride butadiene (nitrile), butyl rubber adhesives, styrene butadiene rubber adhesives, polyurethane adhesives, polysulphide rubber adhesives and silicone rubber adhesives.
Rubber adhesives are not generally suitable for loaded structures or adverse environments . INDUSTRY
UPDATES Flexible Adhesive serves high-temperature applications (August 15, 2012) •Aremco-Bond(TM) 820, a high temperature, flexible epoxy system developed by Aremco Products, Inc., is now used for general purpose bonding of ceramics, glass, plastics, metals and other substrates for applications to 400 °F (204 °C).
•A clear, unfilled, 100% solids, 1-to-1 mix ratio, flexible epoxy system which can be used in applications to 400 °F (204 °C). Flexural and tensile shear strengths are 8,000 and 1,200 psi, respectively.
•The volume resistivity is 2.0 x 1014 ohm-cm; dielectric strength is 860 volts per mil; and the dielectric constant is 6.0 at 1.0 kHz.
•Exhibits exceptional adhesion to ceramics, glass, metals and plastics substrates, and demonstrates outstanding chemical resistance to acids, alkalis, organic fluids and salts. Harry Coover, Super Glue’s Inventor, Dies at 94 (March 27, 2011) •Harry Wesley Coover Jr., the man who invented Super Glue, died on Saturday night at his home in Kingsport, Tenn.
•The cause was congestive heart failure.
•Dr. Coover was born in Newark, Del., on March 6, 1917.
•In 2010, President Obama awarded him the National Medal of Technology and Innovation Electrically Conductive Silver Adhesive cures in 45 seconds (June 8, 2012 ) •A new, ultra-fast curing electrically conductive silver adhesive for cold soldering and bonding electrical and electronic components where soldering is impractical is being introduced by MERECO of West Warwick, Rhode Island.
•MERECO Metaduct(TM) 1245 Silver Adhesive is an electrically conductive two part epoxy that cures in 45 seconds at room temperature and provides over 600 psi tensile shear strength.
•Suitable for a wide variety of cold solder applications, this 100% solid silver adhesive is solvent-free, non-bleeding, thixotropic, and non-flammable to replace leaded and lead-free hot solder. Flexible Conductive Adhesive suits photovoltaic applications (July 16, 2012) •DELAWARE, OH - Engineered Conductive Materials, a leading global supplier of conductive interconnect materials for photovoltaic applications, introduces the new 530-121 low-cost conductive adhesive designed for ribbon stringing in thin-film solar modules.
•The 530-121 has a dispensing work life greater than 48 hours (measured as a 25 percent increase in viscosity), while maintaining optimized rheology for dispensing and excellent damp heat resistance.
•530-121 is the latest addition to Engineered Conductive Materials' full line of conductive stringer attach adhesives, conductive adhesives for back contact crystalline silicon, thin-film and via fill applications, as well as conductive grid inks for photovoltaic applications. Thin Absorbent Skin Adhesive is suited for medical applications (November 28, 2011) •A new medical adhesive formulation from Avery Dennison Medical Solutions, a business unit of Avery Dennison Corporation (NYSE: AVY), provides manufacturers of temporary adherent medical devices with improved performance over conventional acrylic adhesives and rubber-based hydrocolloid dressings.
•The Avery Dennison® Thin Absorbent Skin Adhesive product platform leverages a patented adhesive formulation that can be used for a broad range of applications, including wound dressings, backing material for electrodes, ostomy flanges and products for direct wound contact.
•The platform has adhesion properties and a moisture vapor transmission rate comparable to traditional acrylic adhesives, but our formulation delivers superior FHC that supports extended wear time.
•This also helps prevent moisture build-up to minimize skin maceration, which can make skin more susceptible to damage and can delay healing." TRIVIA 1.When you are sucking in all the toxins from your cigarette, you can rest assured that the glue used to hold it together is completely non-toxic. It is made from a combination of casein (milk) and wax (to increase moisture resistance), and is absolutely harmless. 2.Glues and other adhesives have been around for at least 6000 years. Thousands of years ago people repaired broken pottery with sticky resins from tree sap. Adhesives were made from egg whites. Beeswax and tar were used to caulk the spaces between the planks of ships. 3. A very simple glue can be made by mixing flour and water (it won't be very strong, but it will hold two pieces of paper together). 4. Super Glue can be removed with acetone (fingernail polish remover). 5. Glue doesn't stick inside the bottle because the chemicals within the glue is triggered by oxygen so when the bottle is opened it triggers with chemicals in the air which then make it sticky when use it. 6. The famous cow used as the trademark on all Elmer’s glue products is actually named Elsie, and she is the spouse of Elmer, the bull (male cow) who the company is named after. 7. Alfred Nobel used a cellulose adhesive (nitrocellulose) as the chemical binder for nitroglycerin, which he used in his invention of dynamite. 8. Duct tape dates back to the 1940s and was invented by the Johnson and Johnson Permacel Division. Duct tape was originally used to prevent moisture from creeping into ammunition cases. As the tape was waterproof, it was often called "duck tape". Military employees realized that the tape could be used for other purposes, such as fixing guns and other equipment. After World War II, the adhesive tape was commonly used in construction to join heating and air conditioning ducts. END Thank you for listening!