DEFINITION

The Hot Melt Adhesive (HMA) is an adhesive classified in the group of thermoplastic solid adhesives (manufactured by means of a melting - solidification process).

CHARACTERISTICS

This type of adhesive is found in 100% solid state at room temperature and hence does not contain water or solvents. When this adhesive heats above its melting point (temperature ranging between 120 °C and 215 °C depending on the composition) it turns into liquid making its application to certain materials possible. It hardens when cooled giving rise to an almost instantaneous joint of high cohesion and excellent elasticity.

COMPONENTS

In general, the hot melt adhesive comprises three elements: polymers, resins or plasticizers, and waxes. Each one of these elements is an essential part of the structure and properties of the adhesive:

• › The polymers provide the backbone of the adhesive providing it with cohesion and flexibility.
• › The resins provide and improve the moisturization of the substrate.
• › The waxes decrease viscosity and improve the application.

PRESENTATION

For domestic use, the hot melt adhesive is presented in sticks of 100, 200 or 300 mm in length and in diameters ranging from 8 to 18 mm.

For industrial use the hot melt adhesive is dispensed in the form of slugs of 43 mm in diameter, cartridges, pellets, buckets or slugs, and sacks or pillows.

ADVANTAGES

Hot melt adhesive has a long useful life and can be disposed of easily.

In the industrial area, hot melt adhesives provide several advantages over solvent based adhesives. Volatile organic components are reduced or removed and the drying or curing phase is removed. Solvent based adhesives can loose up to 50-70% in layer thickness during drying. As well as joining two surfaces, hot melt adhesives can be used to fill empty spaces unlike other solvent based adhesives. Some HMAs can be resistant to chemical attacks and the weather.

DISADVANTAGES

Some of the disadvantages include the thermal load of the substrate which limits its use to substrates that are not sensitive to high temperatures, as well as the loss of adherence at higher temperatures or those capable of melting the adhesive. This can be reduced by using a reactive adhesive that is chemically cured after solidifying for example due to humidity (e.g. reactive urethanes and silicons) or that is cured by ultraviolet radiation.

OPEN TIME

The open temperature is the period of time available to create a satisfactory joint after having applied the hot melt adhesive. With HMA the time can vary in seconds to infinity for pressure sensitive adhesives.

The maximum open time for hot melt adhesive depends on three factors:

• › The chosen formula.
• › The application temperature.
• › The application method.

FIXING TIME

The fixing time is the time it takes to form an acceptable resistant joint.

JOINT FORMATION TEMPERATURE

The minimum temperature under which sufficient moistening of the substrate is not produced.

VISCOSITY

The ease at which the adhesive flows when it is in liquid form. This property depends on the application temperature, the higher the temperature the less the viscosity. The unit of measurement is the centipoise: Cps.

The viscosity level can be classified as:

• › Low: 500 - 3.000 cps.
• › Medium: 3.000 - 6.000 cps.
• › High: 6.000 - 15.000 cps.

Low viscosity increases the flow rate of the adhesive and reduces the adhesive thickness. However, its application to very porous surfaces such as foams and fabrics is not advisable.

High viscosity is ideal for gluing rough or irregular surfaces, as well as applications in which any holes require filling with the adhesive. Despite this fact, a viscous adhesive reduces its output and the use of stronger applicators is required.

Hot melt adhesives are generally based on one or more base materials combined with several additives.

EVA (Ethylene Vinyl Acetate)

The rubber, EVA foam is a thermoplastic polymer comprising repeated units of ethylene and vinyl acetate. It is one of the most used base polymers in the creation of adhesives for the assembly and packaging of products. If the useful temperature oscillates between -30 °C and 80 °C.

Example of an EVA based hot melt adhesive composition:

• › 30% - 40% EVA copolymer (provides strength and resistance).
• › 30% - 40% tackifier resin (improves moisture and bonding).
• › 20% - 30% wax (for general paraffin based, reduces viscosity and alters the fixing speed).
• › 0,5% - 1% stabilizers.

The copolymer composition influences its properties:

• › High ethylene content promotes adhesion of non-polar substrates (e.g. polyethylene), increases mechanical resistance, blocking resistance and solubility of paraffin.
• › High vinyl acetate content promotes the adhesion of polar substrates (e.g. to paper), greater flexibility, adhesion, hot sealing, and better performance at low temperatures.

A broad range of HMA can be created using EVA, from smooth pressure sensitive adhesives to rigid structural adhesives for construction furniture.

Conclusions:

• › Medium manipulation times.
• › Excellent adhesion to a multitude of substrates.
• › Economic price.
• › Uses: Glues for wood, assemblies, fixings at low temperature.

Polyamide (PA)

It is a type of polymer that contains amide type links. Polyamides can be found in nature, such as wool or silk but can also be synthetic such as Nylon or Kevlar.

Its considered high performance for severe environments, adhesives for high temperatures, usually applied at more than 200 °C but can be degraded or burn during the process. In melted state, it can be partly degraded by atmospheric oxygen.

It is resistant to plasticizers (therefore, it is suitable for polyvinyl chloride), oils and gasoline.

Displays good adhesion to many substrates, such as metal, wood, vinyl, ABS and treated polyethylene or polypropylene.

Properties according to molecular weight (MW):

• › Low MW: low melting temperature and easy to apply but with less resistance to traction and cutting force, and low elongation.
• › High MW: requires sophisticated extruders for its application and is used like adhesives with high structural performance.

Conclusions:

• › Very short manipulation times.
• › Resistant to elevated temperatures (up to 180 °C).
• › Good cohesion.
• › Moderate hardness.
• › Electrical insulation.
• › Uses: Molding of connectors, wall bushings and PCBs, fixings subjected to elevated temperatures.

Polyester (PE)

Are similar to those used for synthetic fibers. Are synthesized from a diol and a dicarboxylic acid.

If the diol chain length is increased:

• › Melting point is increased.
• › Crystallization speed is increased.
• › Crystallization grade is decreased.

Due to the absence of hydrogen bonds in polyesters:

• › Less resistance.
• › Lower melting point.
• › Greater resistance to humidity.

Are used when necessary:

• › Resistance to traction.
• › Resistance to high temperatures.

Conclusions:

• › High softening points.
• › Extrudable.
• › Resistant to abrasion and chemical agents such as fuels, alcohols, oils, etc.
• › Uses: Automobile filters, over-molding of PCBs, etc.

Polyolefin (PO)

Is a thermoplastic polymer obtained from the polymerization of olefins.

Advantages provided by polyolefins:

• › Good adherence to Poliproplylene (PP).
• › Good barrier against humidity.
• › Chemical resistance against polar solvents and acid, base and alcohol solutions.
• › Long open time.
• › Good wetting on materials and polymers.

Conclusions:

• › Medium manipulation times.
• › Moderate temperature resistance (up to 90 - 100 °C).
• › Good native adherence to non-polar materials (PP, PE, PET, etc.).
• › Uses: Diverse gluing, fixing of components at moderate temperatures.

Polyethylene (PE)

Belongs to the polyolefin group.

Tend to be opaque due to the crystallinity and are white or yellowish depending on additives.

The Polyethylene provides:

• › High stability.
• › Does not become degraded or carbonized.
• › Cab be used on non-porous flexible substrates.
• › Good wetting on materials and polymers.

Polyethylene is usually used alone or with a small amount of:

• › Tackifiers or adhesives (usually hydrocarbons).
• › Waxes (usually paraffins or micro-crystalline waxes, improves the anti-block and alters the softening temperature and open time).

According to molecular weight (MW):

• › Low MW: provides better performance at low temperature and greater flexibility.
• › High MW: increases sealing strength, hot adhesion and viscosity.

Polypropylene (PP)

It is a thermoplastic polymer obtained from the polymerization of propylene (or propene) belonging to a group of polyolefins.

Polyethylene is usually used alone or with a small amount of:

• › Tackifiers or adhesives (usually hydrocarbons).
• › Waxes (usually paraffins or micro-crystalline waxes, improves the anti-block and alters the softening temperature and open time).

It is used to create adhesives with specific adhesive properties and large open time.

The useful temperature ranges between -30 °C and 110 °C depending on the formulation.

Usually presented in adhesive spray formulations.

Polyurethane (PUR)

Is a reactive polymer that needs a small amount of humidity provided by the materials surrounding it or by the environment itself, to provide a chemical reaction via which the PUR merges with the physical structure of the products to be bonded.

Provides excellent resistance to solvents and chemical products, and low application temperature, suitable for heat sensitive substrates.

In general, has heat resistance after curing with service temperatures from -30 °C to +150 °C.

Conclusions:

• › They convert into thermoplastics on reticulation, and therefore cured.
• › Excellent adhesion on a multitude of substrates.
• › Excellent mechanical characteristics (cohesives, resistant to forces and bending, etc.).
• › Uses: Engineering of plastics, automotive assembly, wood, etc.

The reliability of the final adhesion will vary according to three essential factors:

SERVICE TEMPERATURE

Hot melt adhesives are thermoplastics and as a result become brittle at low temperatures and ductile at high. Due to the nature of hot melt, the adhesive reliability varies according to the application temperature.

AMOUNT OF APPLIED ADHESIVE

If a greater amount of adhesive is used, the surface area of the product covered by the same is greater, thus increasing the adhesion´s reliability and resistance. Furthermore, a greater quantity of adhesive retains heat for longer, which means that adhesive tends to be at a higher temperature when the materials are joined.

TIME TAKEN TO ADHERE

If the joining of two surfaces to be adhered is not done quickly, the resistance and effectiveness of the adhesion will be lessened, given that the adhesive will have cooled below a level that guarantees good bonding. This error is know as cold bonding, and although its initial results are acceptable, the bond could deteriorate to an unacceptable level within the following 24 hours.

Advice for good operation of a thermofusable glue gun:

1. Always let the glue gun heat to operating temperature and don´t press the trigger until it is ready. The heating period can vary between 5 and 7 minutes. A new glue gun should be loaded with hot melt adhesive before it is connected and the glue gradually pushed through the gun while it heats by pressing the trigger lightly. This will expel the air from the chamber and prevent the melting of cold glue directly in contact with the box of the heater.

2. Always use the glue gun support supplied inside the box. The glue gun should never be laid on its side, as this could produce a "pool" of adhesive on the heater and would melt on the coldest part of the gun. This could result in internal damage of the glue gun.

3. Always disconnect the glue gun if it is not going to be used for 40 minutes or more. To cool the gun faster and facilitate its next start-up, disconnect the power supply when finished working, and while the gun is cooling press the trigger to expel from it approximately half of the melted glue bar.

4. At the end of each work session, always clean any excess of melted glue from around the nozzle using a cloth, to prevent the accumulation of glue residue that could obstruct the output of adhesive. This should be done while the glue gun is still hot. Alway use protective gloves.

5. Never remove a glue bar from the back end of the gun.

6. Never remove the glue gun nozzle when cold, as this could damage the threads.

GENERAL PROTECTION

Hot melt adhesive displays no particular health risk when used in normal industrial practice. However, due to the fact that the adhesives are used at high temperatures in a melted state there is a risk of thermal burns. For this reason, the use safety goggles, gloves and safety clothing is recommended to minimize the risk of burns.

SKIN CONTACT

The affected skin should be immediately submerged in cold water until the sensation of burning subsides. If you have not got access to a tap, make sure you have a bucket of clean cold water nearby. If fingers are covered with hot melt adhesive, move them to prevent the adhesive from oppressing them on cooling.

Do not remove the adhesive while in liquid state as this could remove the skin an leave an open wound. Even when the adhesive has hardened, it must be removed carefully. If it cannot be easily removed, under medical approval, soak a piece of cotton wool in olive oil or paraffin and place it on the effected area. Slowly soften the adhesive with the cotton wool. When the remains of the adhesive have been removed from the affected area, treat it as you would normal burn.

EYE CONTACT

If the adhesive is in a solid state rinse the eyes thoroughly with clean cool water. If the adhesive is in a liquid state rinse the eyes thoroughly with cold water and seek immediate medical attention.

INHALATION

The vapors emitted during the gluing process are not considered toxic. However, if the hot melt adhesive reaches a temperature above that recommended, chemical decomposition could occur, and the mixture of organic materials released could contain irritating or toxic components. Ensure that hot melt adhesive is used within the recommended operating temperatures and in a well ventilated work area.

FIRE

The HMA is only flammable in the case of a fire. To extinguish use dry powder or a CO2 extinguisher. Do not use water.

RESIDUE REMOVAL

According to the base of the adhesive:

• › EVA and PP: turpentine.
• › PA: acetone.
• › PUR: Before reticulation, turpentine or acetone. Once reticulated its removal is very difficult.