I. Density
1. Definition and Units
The mass per unit volume of a substance is called its density. The main unit in the International System of Units (SI) is kilograms per cubic meter (kg/m³), while the commonly used unit for electronic adhesive products is grams per milliliter (g/ml).
2. Measurement Method and Significance
(1) Measurement Method: Test using a 5ml density cup combined with a syringe (this is a non-standard method for reference only).
(2) Core Significance: Density is a basic physical constant of a substance. For electronic adhesives, its value has no special technical implication and mainly serves as a reference for basic physical properties.
II. Viscosity
1. Definition
The property of internal friction between molecules when a liquid flows is called the viscosity of the liquid, and its magnitude is characterized by the viscosity value. When a liquid moves relatively under the action of an external force, resistance is generated between the liquid molecules to hinder the flow, and the magnitude of this resistance is called viscosity. Viscosity is mainly divided into dynamic viscosity and kinematic viscosity.
2. Dynamic Viscosity
Two plates with an area of 1㎡ are immersed in a liquid, with a distance of 1 meter between them. If a shear stress of 1 Newton (N) is applied and the relative speed between the two plates reaches 1 meter per second (m/s), the viscosity of the liquid is 1 Pascal-second (Pa·s). In the International System of Units, the unit of dynamic viscosity is Pa·s, and the "viscosity" commonly referred to usually refers to dynamic viscosity.
3. Kinematic Viscosity
It refers to the ratio of dynamic viscosity to density at the same temperature, which is a physical quantity representing the internal friction of a liquid flowing under the action of gravity. It can also be understood as the ratio of fluid shear stress to shear rate, reflecting the fluid's resistance to flow under the influence of gravity. Its unit in the International System of Units is square meters per second (㎡/s).
III. Curing Reaction
1. Definition and Core Significance
The curing reaction is a process in which the adhesive obtains and improves properties such as bonding strength through chemical reactions (e.g., polymerization, crosslinking, etc.). It is a key link for the adhesive to achieve excellent bonding effects—only when fully cured can the adhesive reach the maximum bonding strength.
2. Division of Curing Stages
The curing process can be divided into three stages: initial curing, basic curing, and post-curing. The characteristics of each stage are as follows:
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Initial Curing/Gelation: Under specific temperature conditions, the adhesive reaches a certain strength after a period of time, the surface hardens and becomes non-tacky, but the curing reaction is not yet complete.
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Basic Curing: After initial curing, most of the reactive groups in the adhesive participate in the reaction to form a certain degree of crosslinking, and the bonding performance tends to be stable.
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Post-Curing: A supplementary treatment performed on the basically cured bonded parts to improve bonding performance or meet process requirements. Usually, maintaining it at a specific temperature for a period of time can supplement curing, further improve the degree of curing, effectively eliminate internal stress, and enhance bonding strength.
3. Key Curing Parameters
(1) Curing Temperature
It refers to the temperature at which the adhesive reacts most violently and is a core influencing factor of the curing reaction. Different types of adhesives have different curing temperatures. Temperature not only determines the completion degree of the curing reaction but also affects the curing rate: appropriately increasing the temperature can accelerate the curing process and help improve the final bonding strength.
(2) Curing Time
It refers to the time required for the adhesive to complete curing at a specific temperature, which is closely related to the curing temperature: increasing the temperature can shorten the curing time, while lowering the temperature requires an appropriate extension of the curing time. Whether it is room temperature curing or heating curing, sufficient curing time must be ensured to achieve complete curing and obtain the maximum bonding strength.
In addition, the gel time in the curing time curve (obtained through DSC testing) is an important reference data for studying the curing conditions of adhesives. It can be used as a simple method for testing the performance of finished adhesives and verifying the correctness of the formula.
(3) Curing Conditions
Standardized curing process parameters (including key conditions such as temperature and time) formulated with the measured curing temperature, curing time, and gel time as the core references, combined with the adhesive type and application scenario.
IV. Storage Life & Pot Life
1. Storage Life
It refers to the maximum storage time that the adhesive can maintain its operational performance and expected physical and chemical properties under specified storage conditions. Generally speaking, if the viscosity of the adhesive does not change significantly during the storage period, its overall performance will not fluctuate significantly.
2. Pot Life
It refers to the maximum working time that the adhesive can maintain its operational performance and expected physical and chemical properties under specified usage conditions. Ideally, the pot life is related to the storage time at room temperature, but affected by the ambient temperature and humidity during operation, as well as other factors caused by the operation itself (such as shear thinning, frictional heating, etc.), the actual pot life is usually shorter than the storage time at room temperature. Among them, red glue (a type of electronic adhesive) is more significantly affected by the above factors, and its specific applicable time needs to be determined by simulating actual customer usage scenarios.
V. Shear Force
1. Core Indicator: Tensile Shear Strength
Tensile shear strength is the core bonding strength indicator of electronic adhesives, and testing is often conducted under high or low temperature conditions. Its specific definition and calculation method are as follows:
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Apply a tensile shear load to the adhesive layer of the bonded joint specimen, and the ratio of the maximum load until the joint fails to the bonding area is the tensile shear strength.
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Essentially, under the load parallel to the adhesive layer, when the adhesive specimen fails, the shear force borne by the unit bonding area is expressed in megapascals (MPa).
VI. Glass Transition Temperature (Tg)
1. Definition
The glass transition is a transition process of amorphous polymer materials between the glassy state and the highly elastic state, and the glass transition temperature (Tg) is the critical temperature for this transition.
2. Influence of Temperature on Polymer Properties
Taking the glass transition temperature as the boundary, polymer materials exhibit significant differences in physical properties:
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When the temperature is below Tg, the polymer material exhibits plastic-like properties (high hardness and brittleness);
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When the temperature is above Tg, the polymer material exhibits rubber-like properties (good flexibility and elasticity).
