Hot melt adhesives FAQ

What are the conceivable causes and counter-measures of hot melt adhesive stringiness?

(1) The viscosity is high at the hot melt adhesive's temperature, so increase the melting temperature.
(2) The hot melt adhesive's viscosity is too high, so use an adhesive that has a lower viscosity.
(3) The ambient temperature is too low, so raise and stabilize the ambient temperature.
(4) The distance between the adherend and the nozzle is too large, so bring these objects closer together.
(5) An inappropriate nozzle has been selected, so change it to an appropriate nozzle.

Can you explain the easily understandable differences between the performance of EVA group hot melts and rubber group hot melts for seals?

The elasticity of these hot melts is different. Rubber group hot melts have higher elasticity.
EVA group hot melts are applied to items that are sealed after they are formed.
Rubber group hot melts are used with items to which the sealant is applied and then the items are formed.

What are hot melt adhesives?

Hot melt adhesives are solid adhesives at room temperature, contain absolutely no water and solvents, and are mainly composed of thermoplastic resin. Generally speaking, hot melt coaters, known as applicators, are used to melt hot melt adhesives. Guns are used to apply these adhesives in their liquid state to the adherend, which is then crimped, cooled, and adhered. The time required to complete the adhering process is only the time required to cool and solidify the adhesive, which makes it possible to adhere objects to each other in a few seconds to a few minutes.

What can you tell me about the open time and setting time?

Hot melt adhesives melt under high temperatures. You have to apply these adhesives to an object and adhere another object to it before these adhesives cool and lose their adhesive properties. The open time is the period of time from when the adhesive is applied to the adherend to when the adhesive cools to the point that it loses its adhesive properties. The open time is normally a length of time from a few seconds to a few minutes and is determined on the basis of the usage conditions, ambient environment, and hot melt adhesive mixture.
The setting time is the period of time until the hot melt adhesive applied to the adherend cools, solidifies, and exhibits its initial adhesive strength.

What can you tell me about the adhesive strength?

Generally speaking, the tensile strength of the adhesive bond, the tensile shear strength of the adhesive bond, the compression shear strength of the adhesive bond, and the separation strength of the adhesive bond are used to evaluate the adhesive strength of a hot melt adhesive.

(1) The tensile strength of the adhesive bond (N/cm2) is found by measuring the force applied when pulling in a direction perpendicular from the adherend surface and is indicated as the destructive force per unit area.

(2) The tensile shear strength of the adhesive bond (N/cm2) is indicated as the destructive force divided by the adherend surface area in which the destructive force is the shear force that works on and destroys the adhesive layers when the adherend and the adhered material are both pulled in a parallel manner away from each other.

(3) The compression shear strength of the adhesive bond (N/cm2) is used with materials such as wood, metal, glass, and hard plastic.
The same as the tensile shear strength of the adhesive bond, this indicates forces being applied to the adherend surface in a parallel manner.

(4) The separation strength of the adhesive bond (N/25 mm) is used in the separation tests of flexible materials such as metallic foil, rubber, cloth, and plastic films. The test specimen has a width of 25 mm, and the force applied to tear the specimen at an angle of, generally, 180° or 90° is measured.

What can you tell me about the heat resistance?

The heat resistance indicates the upper limit of the usage temperature of finished products in which the hot melt adhesive is actually used.
Generally speaking, the heat resistance is indicated by measuring the softening point and the separation temperature.

What can you tell me about the softening point?

The softening point is the boundary temperature at which a solid changes to a liquid. A hot melt adhesive that has a low softening point will start melting at a temperature that is lower than that of a hot melt adhesive that has a high softening point. This is an indicator that must be taken into consideration when selecting an applicator. The heat resistance value of a final product in which a hot melt adhesive has been used is 20°C or more lower than the softening point. The softening point is measured according to the ring and ball method defined in JIS K 2531.

What can you tell me about the heat stability?

Heat must be applied to hot melt adhesives to use them, so the properties of these adhesives must not be deformed by continual application of heat.
The general test method is to insert a hot melt adhesive into an oven at 180°C and then measure the melted viscosity and the color changes over time.

What can you tell me about the melted viscosity?

The melted viscosity is the viscosity of a hot melt adhesive that has been melted. The melted viscosity is measured with a B-type rotational viscometer (made by Brookfield Engineering Laboratories) by measuring the viscosity at the optimum usage temperature. Generally speaking, this indicates the viscosity between 140 and 160°C for a product with a low viscosity and the viscosity between 160 and 180°C for a product with high viscosity.