Adhesives and sealants are polymeric materials that are used to bond surfaces together or join them. They are widely used in construction, crafts, industrial assembly, packaging and other applications. Adhesives are used to bond materials in a permanent manner while sealants are used to fill gaps and prevent leakage in a semi-permanent manner.

Types of Adhesives

There are various types of adhesives used depending on the materials to be bonded and end-use applications. Some of the major types are:

- Acrylic adhesives: These are based on synthetic polymers like polymethyl methacrylate (PMMA). They have excellent adhesion to a variety of surfaces and are moisture and heat resistant. Common commercial acrylic adhesives are used for wood, fabric, paper and ceramics.

- Polyurethane adhesives: Polyurethane adhesives give very high bond strength and are used for structural and load-bearing applications like vehicle manufacturing. They work well on metals, composite materials and are heat, chemical and weather resistant. Two-part urethane adhesives give highest bond strength.

- Epoxy adhesives: Epoxies are thermosetting adhesives that form very strong bonds with many materials like concrete, metals, ceramics, wood etc. Adhesives And Sealants offer high strength, rigidity and resistance to solvents, acids and bases. Common applications include metal bonding, electrical encapsulation and fiber reinforced plastic joints.

- Silicone adhesives: These sealants are made from silicone polymers that are heat resistant and elastic. They are widely used for insulation, gasketing, weather sealing applications. Silicone rubbers can withstand operating temperatures from -100°C to 300°C.

- Rubber and latex adhesives: Synthetic and natural rubber based adhesives are flexible, durable and damp resistant. Common products include contact adhesives, vinyl flooring adhesives and ceramic tile adhesives. Natural rubber latex is suitable for paper, cloth and wood bonding.

Properties and Testing of Adhesives

An ideal adhesive should possess certain important properties:

- High bond strength - Ability to form strong joints that do not fail easily under stresses.

- Good adhesion - Ability to wet and adhere well to different surfaces like metals, plastics, wood etc without a primer.

- Impact and shock resistance - Capability to withstand stresses from impact blows without failing.

- Heat resistance - Ability to maintain adhesion within a specified temperature range.

- Weatherability - Resistance to degradation from exposure to sunlight, moisture and environmental stresses over long periods.

- Chemical resistance - Non-reactivity towards chemicals, solvents, acids, alkalis that may come in contact.

- Sandability and paintability - Capability to receive finish coatings like paint after bonding.

- Adjustable setting time - Ability to set or cure within a controllable time frame as per need.

Manufacturers test adhesives through standardized tests to determine properties like lap shear strength, peel strength, water resistance, viscosity, drying time, temperature resistance, aging properties, sandability etc. Adhesion tests aid in development and quality control.

Applications of Adhesives And Sealants

Adhesives have a wide range of applications across industries due to their unique bonding capabilities:

- Construction - Used for flooring, roofing, glazing, laying of ceramic tiles and boards. Epoxy and polyurethane structural adhesives replace mechanical fasteners.

- Packaging - Box assembly, carton closing, case sealing in corrugated packaging. Hot melt and rubber based adhesives commonly used.

- Furniture - Binding and laminating of particle boards, plywoods, solid woods. Urea-formaldehyde resins give excellent water resistance.

- Paper products - Binding of books, magazines, folders using starch, dextrin and vinyl adhesives. Wallpaper pastes use acrylic and rubber lattices.

- Automotive - Assembly of vehicle bodies, parts, fixtures using urethane, epoxy, acrylic adhesives. Replacement of welding reduces weight.

- Footwear - Sole bonding, heel-base attachment, lining applications use rubber, polyurethane adhesives.

- Electronics - Circuit board and display panel assembly, flexible circuits, potting and casting requires thermally conductive epoxies, silicones.

- Woodworking - Plywood, blockboard and laminate manufacturing uses urea-formaldehyde resins. Furniture craft uses contact cements.

Thus adhesives have become indispensable in manufacturing across industries by providing cost-effective, reliable joining solutions. Their formulation and applications continue to evolve with emerging technologies.

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