All About Adhesives archive

Tuesday 26 January 2010

Steven Russell looks at commonly used adhesives and how they work

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If you take a look around any woodturner's studio, you'll no doubt find a handful of different adhesives such as polyvinyl acetate, polyurethane, cyanoacrylate and epoxy. As much as we depend on adhesives to complete our projects, few of us give much thought to how they work. Unless we experience a failure, we are content that our adhesives will do everything we require and more.

To fully understand adhesives it is handy to know about surface chemistry and fracture mechanics, as well as industrial engineering, polymer rheology and polymeric material science. However, to use adhesives successfully in your workshop you do not need to become a glue guru, but you do need to understand the basics. Here I will profile common adhesives and how they cure.

Adhesives in general

Adhesives can be simply defined as any substance capable of holding two surfaces together by chemical or mechanical interfacial forces, or by combinations thereof. Modern adhesives are manufactured by compounding resinous base materials with additives like pigments, plasticisers, stabilisers and fillers.

The term structural adhesive refers to formulations where the principal component is a thermoset polymeric resin, for example, epoxies, polyurethanes, acrylates and phenolics. Structural adhesives are rated by their shear strength, or the holding power of an overlapped joint, divided by the bond area.

The term non-structural adhesive refers to adhesive formulations where the principal component is a thermoplastic resin-like polyvinyl acetate, acrylic, or cellulosic polymers. Non-structural adhesives are rated by their peel strength, or the stress necessary to cause a separation, divided by the bond width.

Adhesion and cohesion

Adhesives function through a property known as adhesion. This is the attraction of two different substances, resulting from intermolecular forces between the substances. Adhesion is caused by the molecular interactions between the substrate and the adhesive, or by chemical bonding. Cohesion refers to the forces within the molecule itself, or those that connect two or more molecules in a substance. Adhesion and cohesion both contribute to the strength of a bond.

Glue failures

Glued joints typically fail in one of three ways. 1. Adhesive failures are evidenced by an interfacial bond failure between the adhesive and the adherend. (Note: the substrate after bonding is called the adherend). 2. Cohesive failures are evidenced by a failure within the adhesive, or the adherend, or 3. a combination failure.

You may have experienced one of these failures in your workshop. If you have used a glue chuck and had the joint fail, leaving adhesive on both surfaces, you experienced a cohesive failure. If the glue chuck failed leaving one surface free of adhesive, with all of the adhesive left on the other surface, you experienced an adhesive failure. Another way the bond can fail is through adherend failures. In an adherend failure, the adherend fails before the adhesive, leaving the joint intact. This is known as a cohesive failure of the adherend.

The ultimate strength of a glued joint is determined by numerous factors, including the specific type of adhesive, the quality of surface preparation, the design of the joint and the strength of the substrate. Other factors that can impact the bond include moisture, temperature (either in the ambient environment or the temperature generated during sanding), chemicals used in finishing and the internal stresses caused by seasonal movement in the wood, or the stress caused by movement of dissimilar materials like stone and wood.

Common adhesives used by woodturners

Anaerobic adhesives

These contain an acrylic, or methacrylic monomer and cure due to the absence of oxygen. They are excellent for use on grub screws in boring bars to prevent loosening from shock or vibration, or to seal pipe fittings and machine fasteners. Special versions are available for plastic fasteners.

Advantages

1. Rapid curing (~1 hour) at room temperatures

2. Non-toxic

3. No mixing required

4. Good resistance to solvents. Good temperature resistance

5. Up to two-year shelf life

Disadvantages

1. Expensive

2. Will not cure in the air as a wet filet

3. Limited gap curing

4. Standard versions not recommended for permeable surfaces like plastics and rubber

5. Special primers are required with passive metals, large bond gaps, or inert surfaces

Hints and tips

1. Anaerobic adhesives can damage some thermoplastics. Special versions are available for plastics

2. A small paintbrush is helpful for application to larger fasteners

3. Cure speed is dependent on formulation, temperature and material

4. At 20°, 100% cure is achieved in six hours on steel using Loctite No. 242 medium strength thread locker

Cyanoacrylate ester (Super Glue or CA)

Cyanoacrylate adhesives typically contain stabilisers, thickeners, soluble polymers and plasticisers to alter the viscosity, physical characteristics and performance of the formulas. Cyanoacrylate polymers spontaneously form via anionic reaction mechanisms, when their liquid precursors or monomers come into contact with a weak base, forming a high molecular weight thermoplastic material.

CA's are used for bonding dense substrates in the electronics, watch making, engineering, plastics and rubber industries. In woodturning, CA's are used as a binder for crushed stone and other inlays, for filling voids, making quick glue blocks, securing parts in project kits and a thousand other uses.

Advantages

1. One-part, fast curing adhesives that polymerise at room temperature

2. CA's offer excellent adhesion to many substrates including wood, metal, plastics and rubber

3. Good bond strength in tensile and shear mode

4. Accelerators are available to speed curing of viscous CA's

5. One year shelf life, up to two years with some products

Disadvantages

1. Expensive

2. High acid content in some woods inhibits polymerisation

3. Bonds skin rapidly

4. Limited resistance to moisture and solvents

5. Poor peel strength and impact resistance

6. Limited gap cure, with low temperature resistance

7. Produces an exothermic reaction during curing

8. Requires 24 hours for full cure

9. Vapours cause eye and respiratory irritation

Hints and tips

1. Surfaces must be clean, dry and free from contaminates

2. Recommended for use between 13°C and 30°C

3. Over application of accelerators can cause blooming, crazing and frosting

4. Available in thin, medium, thick, super thick and gel viscosities

5. Bond line strength 2,800 PSI - 4,200 PSI depending on substrate

6. Deep inlays or large treated areas with thicker CA's require 24 hours or more to fully cure

7. Spray shellac can be used in and around inlay areas to prevent CA from wicking into surrounding timber cells

8. Clean up uncured adhesive with Acetone

Expoy (polyepoxide)

Epoxy adhesive resins are usually based on the diglycidyl ether of bisphenol-A, or bisphenol-F. Two-part epoxy resins polymerise (cross-link) to form thermoset polymers, when covalent bonds between the resin and the hardener form, through the reaction of the epoxide ring with the ring opening species on the hardener. Different types of hardeners are used, depending on the specific epoxy.

Epoxy adhesives are used to adhere wood, metals, glass, fibreglass, plastics, rubber and ceramics. In woodturning, epoxies are used for segmented constructions, bonding project blanks, as a binder for inlays, for waterproofing vessels and for gluing metal tubes into project blanks.

Advantages

1. Excellent adhesion and cohesive strength on a variety of substrates

2. Excellent chemical resistance

3. Curing can be accelerated with heat

4. Waterproof when fully cured

5. Up to two-year shelf life

Disadvantages

1. Produces an exothermic reaction during curing

2. Exact measuring of two-part systems is critical for optimal structural properties

3. Heat curing is required on one-part systems

4. Structural epoxies require a long cure time

5. Only fair resistance to heat

6. May degrade if exposed to UV

Hints and tips

1. T-88 is a highly viscous structural epoxy formulation that will fully cure in any thickness without shrinking

2. Surfaces must be clean, dry and free from contaminates. If applying on oily exotics, clean surface with acetone

3. T-88 is mixed in a one-to-one ratio by volume, or 100 parts resin to 83 parts hardener by weight. Proper mixing is critical. Thoroughly mix both parts until a uniform colour is achieved

4. Pot life is 45 minutes or less at 21°C

5. On hardwoods, apply to both surfaces and leave open for 20-30 minutes. Re-apply to any visible dull spots and assemble joint with minimal clamping pressure

6. Hardens in 6-9 hours at 25°C, 24 hours for full cure. Tensile strength up to 7,000 PSI

7. Clean up uncured epoxy with white vinegar, and then rinse with water

8. May be used in temperatures down to 1.6°C, but will require one week to fully cure. Best results are achieved above 10°C

Hide glue (scotch glue)

Hide glue is a protein-based adhesive made from the hide or bones of cattle, with collagen being a primary ingredient. Hide glue is popular with musical instrument makers and period furniture builders, as well as some woodturners for built-up constructions and segmented work.

Advantages

1. Non-toxic

2. Hard curing adhesive that can be reversed with heated water, if desired

3. Good resistance to solvents, moisture, shocks and heat

4. Indefinite shelf life for dried glue, one year for liquid glues

Disadvantages

1. Dried glue requires mixing with water in a heated pot for application

2. Continually reheating cooled glue causes a loss of strength

3. One day pot life for heated glue for optimal results, but may be used for a few days

4. May become brittle with age

Hints and tips

1. Surfaces must be clean, dry and free from contaminates

2. Dry pearls require mixing with water (weight of glue to same weight of water) and being heated prior to use. Once water is added, let stand until the water is absorbed. Then, heat in a double boiler (water jacketed), or in a commercial glue pot

3. Glue must be kept at approximately 60°C during use

4. Wetting properties can be increased by adding 1% vinegar to solution

Holt melt adhesives

Hot melt formulations are based on various polymers including ethylene-vinyl acetate - a random copolymer of vinyl acetate. Adhesion is provided by a combination of low glass transition, low to no crystallinity and high polarity of acetate groups.

Hot melts are a 100% solids adhesive that requires heat to raise the temperature to a workable viscosity. Upon cooling, the adhesive re-solidifies, creating a bond. Conventional hot melts do not chemically cross-link when cured. Newer polyurethane reactive hot melts undergo cross-linking, when residual isocyanate groups react with water after application, creating a thermoset adhesive. Used for light bonding of wood, metal, plastics and laminates.

Advantages

1. Super fast application and curing time

2. Easy to store - solid sticks have an indefinite shelf life

3. Various formulations are available to suit specific needs

4. Polyurethane hot melts offer cross-linking and increased strength

Disadvantages

1. Hot melts are unsuitable for structural applications

2. Requires the use of a heating gun to apply the glue to the item you are working on

3. Working time is very short with most formulations

4. Hot adhesive may damage heat sensitive substrates

5. Shelf-life for unopened polyurethane hot melt glue is less

than a year

Hints and tips

1. Surfaces must be clean, dry and free from contaminates

2. Traditional hot melts are suitable for light duty bonding only. Polyurethane hot melts cross-link when cured, do not foam and are waterproof. Full cross-linking may take 48 hours or longer

3. Warming cold substrates with a small hair drier prior to application can improve overall working time

4. Allow glue gun and stick/cartridge to reach working temperature prior to use

5. Arranging pieces to be bonded in order of assembly helps to ensure fast assembly of components

6. When using polyurethane hot melts, unused adhesive may last up to four weeks if air is purged from cartridge by squeezing out hot adhesive and immediately replacing cap

7. Polyurethane hot melts are stronger than traditional hot melts, producing 900 PSI to 1,480 PSI on maple-to-maple glue ups after 24 hours, depending on specific product used

Polyvinyl acetate and aliphatic resins

One-part PVA adhesives typically contain polyvinyl acetate, stabilisers, plasticisers and water. Polyvinyl acetate emulsion is manufactured from vinyl acetate monomer. Polyvinyl acetate is used in white glues and yellow carpenter's glues, which are commonly known as aliphatic resins. PVA cures by the evaporation of water, accompanied by the coalescence of the particles. PVAs are widely used for gluing joints in woodworking and woodturning, for bookbinding, lamination, in paper packaging and for sealing stryofoam materials.

Advantages

1. Inexpensive

2. Non-toxic with easy clean-up

3. Type II PVAs are water resistant, Type I PVAs are waterproof

4. Good adhesion to a variety of substrates

5. Odourless, with good resistance to solvents, acids, grease and oils

Disadvantages

1. Standard PVAs have only fair moisture resistance, with low resistance to acids, solvents and heat

2. Substrates joined with thick glue lines may creep under sustained loads, making them inappropriate for structural applications

3. Short working time with some formulations, making complicated glue-ups a challenge

Hints and tips

1. Surfaces must be clean, dry and free from contaminates

2. Ambient temperature, glue and materials should be above 10°C for Original formula, 13°C for Type II and 7°C for Type III

3. Adhesive should be applied liberally to both mating surfaces with a brush or roller and worked into the surface. Open time is 5-10 minutes

4. Clamping Pressure - 100-150 PSI for softwoods, 125-175 PSI for medium density woods and 175-250 PSI for hardwoods

5. Clamping time - 30-60 minutes, depending on temperature and humidity. Do not stress joint for 24 hours

6. Clean up wet glue with warm water

7. Bond line strength up to 3,600 PSI with Titebond Original, up to 3,750 PSI with Type II (water resistant) and up to 4,000 PSI with Titebond III (waterproof)

Phenol resorcinol formaldehyde

Resorcinol is a thermosetting polymer produced by a condensation polymerisation between formaldehyde (from the oxidation of hydrocarbons in the presence of a catalyst), phenol (from the oxidation of cumene) and resorcinol (from sulfonating benzene with fuming sulphuric acid and fusing the resulting benzenedisulfonic acid with caustic soda).

Two-part resorcinol adhesives (liquid resin, powered catalyst) have a long and successful track record for superior bonding in load bearing timber structures, plywood laminating and for building wooden aircraft and boats. In addition, structural bonds can be achieved on many plastics, leather, natural and synthetic rubbers, primed metals, textiles and fibreboards.

Advantages

1. Completely waterproof and resistant to UV

2. High resistance to heat and cold extremes

3. Good shock resistance

4. Impervious to most chemicals

5. Can withstand continuous immersion in salt or fresh water

6. Some products are also boil proof

7. Up to two-year shelf life when stored below 21°C

Disadvantages

1. The colour of the cured adhesive varies by manufacturer (reddish brown, purple etc.), making it unsuitable for light coloured timbers due to visibility of the glue line

2. Minimum workshop temperature of 21°C recommended for proper curing

3. Contains formaldehyde

Hints and tips

1. Surfaces must be clean, dry and free from oils, or other contaminates

2. Due to hazardous ingredients, special safety precautions must be observed

3. 100% true waterproof adhesive. Extensively used in boat and yacht building

4. Mix at the ratio of 4 parts liquid to 1 part powdered catalyst by weight, or 4 parts liquid to 3 parts powder by volume

5. Add powder to liquid and stir thoroughly for 5-10 minutes until a creamy uniform purple colour and consistency is achieved

6. Recommended applications at ambient temperatures between 21°C-35°C, pot life at 21°C is 3-4 hours

7. Adhesive must be applied to both mating surfaces with brush, roller, or trowel. Leave coated pieces in the air for 5-10 minutes before assembly and clamping

Polyurethane

One-part adhesives consist of isocyanate containing pre-polymers, dissolved in a solvent carrier. Reaction with moisture occurs as the solvent evaporates. Unless the curing takes place inside of pressure devices, bubbles may form during curing. One-part polyurethanes are used extensively in woodworking and in woodturning for bonding wood, wood to metal and are also used for gluing metals, ceramics, stone, glass and most plastics.

Two-part polyurethanes contain a polyol resin and an isocyanate hardener that are mixed by the user prior to application. Pot life varies from a few minutes to a few hours. Two-part polyurethanes are used for larger surface adhesion bonds in automotive, marine and container construction. Polyurethane adhesives are also available in the form of reactive hot-melts. After application, these polyurethanes crosslink with moisture to form a heat, moisture and impact resistant adhesive.

Advantages

1. Extremely tough waterproof adhesives, with good flexibility and resistance to impacts and solvents

2. Good abrasion and creep resistance, with fairly good shear strength

3. Excellent solvent resistance

4. Very good for bonding dissimilar materials

5. One-year shelf life

Disadvantages

1. Moderate cost

2. Foaming during curing may discolour, or limit use with some substrates

3. Sensitive to moisture in uncured state

4. Short shelf life in high humidity environments

5. Use on some substrates may require application of water to surfaces prior to bonding

Hints and tips

1. Surfaces must be clean, dry and free from oils or other contaminates

2. Recommended application temperatures above 10°C. Lower temperature will cause the glue to thicken, making it difficult to dispense

3. When bonding wood, adhesive can usually be applied to one mating surface and then assembled. For extremely porous materials, apply adhesive to both surfaces

4. If using in very low humidity or on very dry substrates, lightly spray water on applied adhesive before assembly to help speed curing

5. Adhesive will expand and foam as it cures. Average cure time, 100% in 4 hours. Bond strength up to 3,500 PSI

6. Clamping Pressure - 30-80 PSI for HPL, 100-150 PSI for softwoods, 125-175 PSI for medium density woods, 175-250 PSI for hardwoods. Clamp time, 1-4 hours

7. Foaming can be reduced if parts are cured inside pressure vessels

8. Clean up wet adhesive with white spirit or acetone

Urea-formaldehyde (plastic resin glue)

Urea-formaldehyde adhesives are thermosetting polymers produced by a condensation polymerisation reaction. This takes place between urea (derived from a reaction between liquid ammonia and liquid carbon dioxide that forms ammonium carbamate, which decomposes at low pressure into urea and water) and formaldehyde (derived through the oxidation of hydrocarbons like methanol, propane, or butane in the presence of a catalyst).

Widely used to make products like plywood and particleboard with heated hydraulic presses, where dimensional uniformity and surface smoothness are important. Plastic resin glues are also used for building fly fishing rods and water skis, furniture construction, millwork, assembling hollow core doors, in aircraft construction and for adhering decorative laminates to wood.

Advantages

1. Easy to apply and low cost

2. Used widely in the manufacture of composite wood products

3. Light colour of resin is suitable for manufacture of decorative products or use with light coloured timbers

4. Excellent resistance to oils, gasoline and solvents

5. Available in liquid and powder forms

6. Up to one-year shelf life when stored below 15°C

Disadvantages

1. Minimum workshop temperature of 21°C recommended for proper curing

2. Contains formaldehyde

Hints and tips

1. Mating surfaces must be clean, dry and free from contaminates

2. Due to hazardous ingredients, special safety precautions must be observed

3. Resin may be mixed by weight at five parts powder to three parts water, or by volume at five parts powder to two parts water, however weight is preferred

4. Water must be above 21°C when mixing powdered resin

5. Requires minimum temperature of 21°C in workshop and on substrates for proper curing, with a 4-5 hour pot life once mixed

6. Add entire volume of powder to two-thirds of water volume and mix thoroughly into a smooth paste, then add remaining water and mix well

7. Application of resin to one surface is usually sufficient. Mixed resin may be brushed, towelled, rolled, or sprayed onto surface

8. Clamping pressure - 50 PSI for softwoods and well machined hardwoods, 175 PSI for rough or poorly machined surfaces on softwoods, 300 PSI on rough hardwoods. Minimum clamping time, 14 hours at 21°C. Requires seven days to develop full strength and moisture resistance

9. Clean up uncured resin with water


Tegan Foley

Tagged In:

steven russell , technique , beginner , Adhesives

Manufacturer Details

Loctite
Gorilla Glue
Tel: 0845 900 0068
Tel: (001) 800 966 3458
Titebond
Tel: 0845 604 0064
Tel: (001) 800 877 4583
Evostick
Behlen
Tel: 0115 973 7288
Tel: (001) 866 785 7781
Humbrol
System Three
Stanley glue gun
Dap Weldwood glue