Hot melt adhesive: FAQs
- What are the conceivable causes of cobwebbing and the measures against it?
- (1) If the viscosity of the hot melt adhesive at that temperature is high, then raise the melting temperature.
(2) If the viscosity of the hot melt adhesive is too high, then use a lower viscosity item.
(3) If the ambient temperature is too low, then raise and stabilize the ambient temperature.
(4) If the distance between the adherend and the nozzle is too large, then bring them closer together.
(5) If the nozzle selection is not appropriate, then change to an appropriate nozzle.
- At an easily understandable level, what are the differences in performance between EVA group hot melts and rubber group hot melts for sealing?
- The elastic behavior is different. Rubber group hot melts have higher elasticity.
EVA group hot melts are applied for sealing after forming.
Rubber group hot melts are used when the sealant is applied and then the forming is performed.
- What is hot melt adhesive?
- Hot melt adhesive is an adhesive that is mainly composed of thermoplastic resin and contains absolutely no water or solvent. It is solid at room temperature. In general, hot melt adhesive is melted using a heating, melting and applying device called an applicator. A gun is used to apply it to the adherend as a liquid and then it is crimped and cooled for adhesion. The time required for the adhesion is only the time required for the cooling and solidification, so it can be completed in between a few seconds and a few minutes.
- What are the open time and setting time?
- When a hot melt adhesive that has been melted at a high temperature is applied, the surfaces must be bonded together before the adhesive cools and loses its adhesive properties. The open time is the period of time when adhesion is possible, between when the adhesive is applied to the adherend and when it cools and loses its adhesive properties. This is normally somewhere between several seconds and several minutes and it is determined by the conditions of use, ambient environment and hot melt adhesive blending.
The setting time is the time from when the hot melt adhesive has been applied and the surfaces have been adhered, up to when the adhesive cools, solidifies and exhibits its initial adhesive strength.
- What is the adhesive strength?
- In general, the evaluation of the adhesive strength of hot melt adhesive is performed using the tensile adhesive strength, tensile shear adhesive strength, compressive shear adhesive strength and peeling strength.
(1) For the tensile adhesive strength (N/cm2), the force when pulling in the direction perpendicular to the adhered surface is measured and the strength is expressed as the destructive force per unit of area.
(2) For the tensile shear adhesive strength (N/cm2), a shear force acts on the adhesive layer and destroys it when the bonded items are pulled in opposite directions parallel to the adhered surface, so this is expressed as the destructive force divided by the adhered surface area.
(3) Compressive shear adhesive strength (N/cm2) is used for materials such as wood, metal, glass and hard plastic.
In the same way as for tensile shear adhesive strength, a force is applied parallel to the adhered surface.
(4) Peeling strength (N/25 mm) is used in adhesion testing on flexible materials such as metal foil, rubber, cloth and plastic films. The test specimen has a width of 25 mm and generally the measurement is of the force required for peeling at an angle of either 180° or 90°.
- What is the heat resistance?
- For hot melt adhesive, an upper limit is indicated for the usage temperature of end products produced using that adhesive.
In general, the softening point and peeling temperature are measured to indicate the heat resistance.
- What is the softening point?
- This is the borderline temperature where a solid changes into a liquid. Hot melt adhesives with a low softening point start to melt at a lower temperature than those with a higher softening point. This indicator must therefore be considered when selecting an applicator. The heat resistance temperature of an end product bonded with a hot melt adhesive is at least 20°C below the softening point. The softening point is measured using the ring and ball method in accordance with JIS K 2351.
- What is the thermal stability?
- Hot melt adhesive is heated for use, so it is required that its properties do not change as a result of continuous heating.
The general testing method is to place hot melt adhesive in an oven at 180°C and to measure the melt viscosity and hue changes over time.
- What is the melt viscosity?
- This is the viscosity when a hot melt adhesive has melted. For the melt viscosity, the viscosity at the optimal usage temperature is measured using a B-type rotational viscometer (made by Brookfield). In general, it indicates the viscosity between 140°C and 160°C for low viscosity products and between 160°C and 180°C for high viscosity products.