In the use of hot melt adhesive, many customers will react to the phenomenon of non-stick glue.
And most of the actual users do not have a deep understanding of hot melt adhesive, resulting in a variety of troubles in the production process.
The following specific introduction on how to improve the hot melt adhesive bonding strength.
1. Improve the surface roughness
When the adhesive well infiltration is sticky material surface (contact Angle theta < 90 °), the surface roughness is helpful to improve adhesive liquid infiltration degree of surface density increasing adhesive and glued materials contact, thus to improve the bonding strength.
2. Surface treatment
Because glued materials oxide layer (such as rust), chromium plating layer, phosphating, formation "weak boundary layer", such as release agent of glued surface treatment will affect the bonding strength, surface treatment of aluminium and aluminium alloy, hope generated on the surface of alumina, aluminum and nature of the surface of the aluminum oxide is very irregular, quite porous alumina layer, is not conducive to bonding
The adhesive joint, affected by the environmental atmosphere, is often infiltrated into some other low molecules, for example, the joint in a humid environment or underwater, water molecules infiltrate into the adhesive layer;
Polymer adhesive layer in organic solvents, solvent molecules permeate into the polymer, low molecular penetration first makes the adhesive layer deformation, and then into the adhesive layer and the interface, so that the strength of the adhesive layer, resulting in the destruction of adhesion
4, the migration
Plasticizer containing adhesive materials, due to the compatibility of these small molecules and polymer macromolecules is poor, easy to migrate from the surface layer of the polymer or the interface, the migration of small molecules if gathered on the interface will hinder the adhesive and the adhesive material bonding, resulting in bonding failure
5, the pressure,
Adhesive, the adhesive surface pressure, make it easier for adhesive with the stuck holes on the surface of the body surface, even into the deep hole and capillary, reduce the bonding defects, for smaller adhesive viscosity, flow pressure will be excessively, due to lack of glue, therefore, should be large pressure, viscosity has prompted glued on the surface of the body surface gas escape, reduce the bonding area of porosity
6. Thickness of rubber layer
The thicker rubber layer is prone to bubbles, defects and early fracture, so the rubber layer should be made as thin as possible to obtain higher bonding strength. In addition, the thermal stress caused by the thermal expansion of the thick rubber layer in the interface area after heating is also greater, which is more likely to cause joint failure
7. Load stress
The stresses acting on the actual joints are complex, including shear stress, peel stress and alternating stress.
(1) shear stress: due to the eccentric tension, the stress concentration appears at the adhesive end. In addition to the shear force, there is a tensile force consistent with the interface direction and a tearing force perpendicular to the interface direction.
At this point, under the action of shear stress, the greater the thickness of the adhesive, the greater the strength of the joint.
(2) try to avoid the use of joint will produce peeling stress in the design.
(3) alternating stress: the adhesive on the joint gradually becomes fatigue due to the alternating stress, and is destroyed under the condition that the static stress value is much lower.
A strong, elastic adhesive, such as some rubber - like adhesives, has good fatigue resistance.
(1) shrinkage stress: when the adhesive solidifies, the volume shrinks due to volatilization, cooling and chemical reactions, resulting in shrinkage stress.
When the contraction force exceeds the adhesion force, the apparent adhesion strength decreases significantly.
In addition, the stress distribution around the bonding end or the gap of adhesive is not uniform, which also produces stress concentration and increases the possibility of cracks.
Crystallization of the adhesive in the solidification, due to the crystallization of the larger volume shrinkage, but also cause the internal stress of the joint.
If a certain amount of rubbery material which can crystallize or change the size of crystallization is added, the internal stress can be reduced.
Adding toughening agent to the thermosetting resin adhesive is the best illustration. Epoxy resin company's modified epoxy resin A/B adhesive can improve the adhesive strength of the modified epoxy resin from 10-15mp to 25Mp.
(2) thermal stress: at high temperature, when the molten resin is cooled and solidified, volume shrinkage will occur, and internal stress will be generated on the interface due to the constraint of adhesion.
When there is the possibility of slip between the chains, the internal stress produced disappears.
The main factors affecting thermal stress include thermal expansion coefficient, temperature difference between room temperature and Tg, and elastic difference.