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Glues And Adhesives

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.

Glue or adhesives are designed to stick two things together. Technically speaking, glues are natural and adhesives are synthetic. Because there is no "universal" glue that can stick anything to anything in any conditions, there are several "general purpose" glues (they stick a lot of things to a lot of things) and a whole bunch of "special purpose" glues (they stick certain things to certain things).

The best first step when picking a glue is to understand:

What you are sticking to what;
Under what conditions do you need it to stick;
How quickly must it reach full strength;
How strong must the glue be;
What properties (clear, sandable, etc) must the glue have when it is dry or cured

Common Glues

Use the following as a guide, but read the manufacturer's directions to make sure the glue meets your requirements. Many times, companies will offer variations of a particular glue in order to bind different types of materials.

Animal glue

Sources/Properties: Made from the protein extracted from the bones, hide, hoofs and horns of animals by boiling.
The extract is cooked to form a gelatin material.
The gelatin can then be reliquified with heat, which gives it quick setting properties.
Animal by-products from meat processing have been the source of supply for this type of glue.

Common Uses: Binding of abrasives in sandpaper and other grinding materials.
It´s major use has been in the wood and furniture industry.
If you have seen a heated glue pot with a brush in it, it was probably an animal glue.

Fish glue

Sources/Properties:
Fish glue is a similar protein-based glue made from the skins and bones of fish.

Common Uses:
An exceptionally clear adhesive can be made from fish and was the first adhesive used for photographic emulsions for photo film and photo resist coatings for photo-engraving processes.

Casein

Sources/Properties: Made from a protein isolated from milk.

Common Uses:The extraction process creates an adhesive that is waterproof.
Used for labels on beer bottles that do not come off in ice water, yet are recyclable.
Its first use was in bonding the seam of cigarette paper.
It provides a fast setting bond that requires very little adhesive, one gram of adhesive can bond 2000 cigarettes.

Starch

Probably better known as paste.

Sources/Properties:A carbohydrate extracted from vegetable plants such as corn, rice, wheat, and potatoes.

Common Uses: Major use area is in bonding paper and paper products such as bookbinding, corrugated boxes, paper bags, wallpaper paste (non-removable), also used as a sizing in textiles.

Cellulose adhesive

Sources/Properties:
Cellulose adhesive is made from a natural polymer found in trees and woody plants.

Common Uses:
Used on the cellophane wrapper on cigarette packs.
Adhesive on window decals.
The adhesive used on easily strippable wallpaper.

White Glue

Sources/Properties:
Non-toxic, odorless, nonflammable and dries clear in under an hour.

Common Uses:
Good for paper, wood, cloth, pottery and more.

Yellow Glue

Sources/Properties:
A higher quality derivative of white glue that dries stronger and is more resistant to moisture.

Plastic Cement

Sources/Properties: Works by dissolving the areas it contacts on the two parts of polystyrene being joined together, and in these dissolved areas the molecules from the two parts mix together.

Common Uses: Used to join polystyrene plastic.

Butyl rubber/isobutylene

Sources/Properties:
It is elastomeric—it stretches

Common Uses: An additive for hot-melt adhesives, window sealants, and pressure-sensitive adhesives

Amino resins

Sources/Properties:
Water-soluble adhesives

Common Uses:
Bonding of layers in plywood and the bonding of particles in particle board

Polyurethane (1)

Sources/Properties:
A flexible adhesive

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

Acrylates or anaerobic adhesives

Based on synthetic acrylic resins, anaerobic adhesives cure when in contact with metal, and the air is excluded, e.g. when a bolt is home in a thread.
They are often known as "locking compounds", being used to secure, seal and retain turned, threaded, or similarly close fitting parts.

Sources/Properties: Anaerobic adhesives are derived from methacrylates, a monomer related to acrylic or more commonly known as Plexiglas.
They become active and cure or polymerize in the absence of oxygen.

Common Uses: Anaerobic adhesives are very versatile and are used in a wide variety of applications.
Adhesive used to keep nuts tight on bolts, such as those within ATMs and heavy machinery
They are used to seal pipe fittings, retain bearings, pulleys, and gears to shafts as well as sealing flanged surfaces to replacing cut gaskets.
The major users are the automotive, truck, construction and farming equipment companies.
The actual market for these adhesives is anywhere that fasteners, gaskets, bearings or any mechanical device that needs to be secured or sealed are used in both OEM or MRO areas.

Cyanoacrylates

Sources/Properties:
Ethyl Cyanoacrylate adhesives cure through reaction with moisture held on the surface to be bonded.
They need close fitting joints and usually solidify in seconds.
Cyanoacrylates are suited to small plastic parts and to rubber.
They are a special type of acrylic resin.
It works best on smaller surfaces, using a very small amount of glue.
It bonds instantly with a colourless and transparent bond that is very strong (except for shear forces). Originally for non-porous surfaces, gel versions are now available for porous surfaces.

Common Uses:
Cyanoacrylates are very rapid curing and provide high bond strengths on plastic and rubber materials. The versatility of these adhesives make them highly useful in all industries.
Some of the larger application areas are in electronics for printed circuit board wires and components, and in medical technology for disposable plastic medical devices.
Other OEM applications exist in the toy, small and large appliance, automotive, and cosmetic packaging. MRO applications exist in all industries for repair of all rubber and plastic parts as well as some metal parts.
The consumer market is a large volume user of cyanoacrylate adhesives for repairing everything in the home from wallpaper tears to broken toys to torn and false fingernails and of course the famous 'Super Glue'.

Toughened Acrylics

Sources/Properties:
Toughened acrylics are fast curing and offer high strength and toughness.
Both one and two part systems are available.
In two part systems, no mixing is required because the adhesive is applied to one substrate, the activator to the second substrate, and the substrates joined.
They tolerate minimal surface preparation and bond well to a wide range of materials.

Epoxies

Extremely tough and durable synthetic resin that is comprised of two parts that when mixed together bond a wide variety of materials in relatively harsh conditions.

Sources/Properties:
Epoxy adhesives are available in one part, two part and film form and produce extremely strong durable bonds with most materials.
They allow great versatility in formulation since there are many resins and many different hardeners.
Epoxy adhesives can be used to join most materials.
These materials have good strength, do not produce volatiles during curing, and have low shrinkage. However, epoxies can have low peel strength and flexibility and can be brittle.

Common Uses:
Epoxy adhesives can bond a wide variety of substrates with high strength particularly metals.
They have been used to replace some traditional metalworking methods of joining like nuts and bolts, rivets, welding, crimping, brazing and soldering.
High strength epoxies are used to construct rotor blades of helicopters, attach aluminum skins to the struts of aircraft wings and tail sections.
Skis are laminates of plastics, wood, and metal joined with an epoxy.
The heads of golfclubs are bonded with an epoxy.

Polyurethanes (2)

Sources/Properties:
Polyurethane adhesives are chemically reactive formulations that may be one or two part systems and are usually fast curing.
They provide strong resilient joints which are impact resistant and have better low temperature strength than any other adhesive.

Common Uses:
Polyurethanes are useful for bonding glass fibre reinforced plastics (GRP).
The fast cure usually necessitates applying the adhesives by machine.
They are often used with primers.

Silicones

Sources/Properties:
Both an adhesive and a sealant and only common adhesive that is based on silicon rather than carbon.
Silicones are not very strong adhesives, but are known for their flexibility and high temperature resistance.

They are available in single or two part forms. The latter function like the two part epoxies, the former like the single part polyurethanes.

When the single part adhesives cure they liberate either alcohol or acetic acid (the familiar smell of vinegar). They are often used as bath and shower sealants. Their adhesion to surfaces is only fair but like their flexibility, their durability is excellent.

The two part versions need a hardening agent to be mixed into the resin. Two forms are available, those which liberate acid on curing and those that do not. As might be anticipated the two part adhesive systems give a better cure in thick sections than do the single part types.

RTV Silicone adhesives are a rubber like polymer called polydimethsiloxanes. RTV stands for room temperature vulcanizing, or simply a rubber which cures at room temperature.

Silicone rubber adhesives are made from a complicated process that turns elemental silicon metal made from sand (silica) into a rubbery polymer.

Common Uses:
Bathtub and shower sealants; also many car applications, such as oil pans and head gaskets. When cured, silicone rubber adhesives/sealants have excellent resistance to heat (500-600ºF) and moisture which makes them exceptionally suited for outdoor weathering applications, such as sealant and caulking compounds in the construction industry.

Because of its exceptional properties, silicone adhesive has been used in some exotic applications such as the soles of the boots worn by the first astronauts to walk on the moon. Silicone adhesive/sealants are used to seal windows, doors and portholes on the space shuttle and many satellite missiles. A special silicone adhesive is used to bond the heat shield tiles on the space shuttle.

Phenolics

Sources/Properties:
Phenolics were the first adhesives for metals and have a long history of successful use for joining metal to metal and metal to wood.
They require heat and pressure for the curing process.

Polyimides

Sources/Properties:
Polyimide adhesives are based on synthetic organic chains.
They are available as liquids or films, but are expensive and difficult to handle.
Polyimides are superior to most other adhesive types with regard to long term strength retention at elevated temperatures.

The following adhesives undergo a physical change and are less effective at forming the adhesive bond.

Hot Melts

Polyolefin/ethylene copolymer

The origin of hot melts probably started with the use of sealing wax used to seal documents and letters with a signature ring or stamp, but the art of hot melts was not pursued until the 1960´s.

Sources/Properties:
No solvents involved, hot melt adhesives are thermoplastic polymers that are tough and solid at room temperature, but are very liquid at elevated temperatures.

Common Uses:
Hot melts are used for fast assembly of structures designed to be only lightly loaded.

Plastisols

Plastisols are modified PVC dispersions that require heat to harden.
The resultant joints are often resilient and tough.

Rubber Adhesives:

Sources/Properties:
Rubber-based solvent cements are adhesives made by combining one or more rubbers or elastomers in a solvent and solidify through the loss of the medium.
These solutions are further modified with additives to improve the tack or stickiness, the degree of peel strength, flexibility, the viscosity or body.
They are not generally suitable for sustained loadings.

Common Uses:
Rubber Cement, litreally, rubber dissolved in a solvent. The bond develops as the cement dries.
Rubber-based adhesives are used in a wide variety of applications such as: contact adhesive from plastic laminates like counter tops, cabinets, desks and tables.
It is adhesive on pressure-sensitive tapes used as floor tile adhesive and carpeting adhesive.
Shipping containers use rubber cements.
Solvent based rubber adhesives have been the mainstay of the shoe and leather industry.
Self-adhesive envelopes and other pressure-sensitive adhesives; adhesives that bond to substrates on contact (like tapes)

Polyvinyl Acetate (PVA’s)

Sources/Properties:
Vinyl acetate is the principal constituent of the PVA emulsion adhesive.
Common "white" glue

Common Uses:
They are suited to the bonding of porous materials, such as paper or wood, and to general packaging work, book bindings and labels

Pressure-Sensitive Adhesives

Sources/Properties:
This term is applied to adhesives that bond on initial contact to most surfaces with only a little pressure and without any drying or curing time.
The strength of the bond varies with the formulation.

Common Uses:
Pressure sensitive adhesives are suited for use as tapes and labels and although they do not solidify they are often able to withstand adverse environments. No-lick stamps and envelopes, various tapes, and Glue Dots (TM) use pressure sensitive adhesives.
This type of adhesive is not suitable for sustained loadings.

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