Due to cost and processing difficulties, usually large-sized magnets are made up of many small magnets bonded with magnet adhesive.
In addition, as the core component of permanent magnet motors, the fixing method of magnets is crucial during the assembly process of the motor. In surface-mount motors, magnets usually require the use of adhesives to be fixed to the rotor core of the motor. The magnets of in-line motors also require the application of adhesives before being inserted into the slot of the rotor before installation.
Considering the diverse performance requirements and working environments of motors, selecting the appropriate adhesive is particularly critical. Common bonding materials include ferrite magnets and neodymium magnets with different coating treatments (such as zinc plating, epoxy plating, etc). In addition, there are motor components that work with these magnets, such as zinc-plated steel or aluminum alloy motor casings, motor rotors , and so on.
The performance requirements for the adhesive used in magnets are very strict. It must ensure high bonding strength, good toughness, and be able to withstand harsh environments such as high temperature and aging. When selecting a motor’s magnet adhesive, multiple factors need to be considered, including the specific performance requirements of the motor, the design of the bonding structure, the feasibility of the production process, and cost accounting.
It must ensure high bonding strength, good toughness, and be able to withstand harsh environments such as high temperature and aging. When selecting a motor magnet adhesive, multiple factors need to be considered, including the specific performance requirements of the motor, the design of the bonding structure, the feasibility of the production process, and cost accounting.
Effective chemical bonding and mechanical bonding are two different main bonding methods that provide adhesion between the adhesive and the plating surface.
Effective chemical bonds: Chemical bonds are formed through chemical reactions between the adhesive and the surface of the plating. This bonding mechanism usually occurs when there is good chemical compatibility between the adhesive and the plating. For example, when using an epoxy resin adhesive, it can undergo chemical reactions with certain metal surfaces to form covalent or ionic bonds, providing strong adhesion. Effective chemical bonds usually result in durable bonding effects and exhibit high resistance to shear, tensile, or peeling forces.
Mechanical bonding: Mechanical bonding refers to the penetration of adhesive into the microscopic structure of the plating surface, which physically “grabs” the plating to provide adhesive force. This mechanism usually occurs in cases where the plating surface is rough or porous, as the adhesive can fill these gaps to form a structure similar to mechanical interlocking.
When the adhesive cures, these adhesion points can resist external forces and prevent the adhesive from separating from the plating. The effect of mechanical bonding usually depends on the surface treatment of the plating, the viscosity and fluidity of the adhesive, and the strength of the adhesive after curing.
In practical applications, the optimal bonding state of magnets is usually a combination of chemical bonding and mechanical bonding. By appropriately selecting the adhesive and plating surface treatment methods, both bonding methods can be optimized to ensure the strength and durability of the bond.
Two types of commonly used magnet adhesives
In the magnet industry, epoxy resin and acrylate are two widely used adhesive types. Different manufacturers have differences in raw material ratios and additive use, resulting in differences in the characteristics and performance parameters of their respective adhesive products. Therefore, here we only introduce the most common characteristics of these two types of adhesives.
Epoxy Resin Adhesive：
1.Heat resistance: Excellent heat resistance, generally capable of long-term use at temperatures ranging from -60°C to 250°C, and can withstand high temperatures above 280°C for short periods of time. Its working temperature in high-temperature environments is basically the same as that of neodymium magnets (≤230°C), making it the preferred choice for bonding and fixing high-temperature neodymium magnet products.
2.Corrosion resistance: The epoxy resin molecules contain a large number of epoxy groups (-O-CH2-CH-), and are not easily broken in extreme environments such as high temperatures, which makes it resistant to the erosion of various chemical substances, including acids, alkalis, and salt solutions.
Epoxy resin adhesives are commonly used to bond high-performance magnets, such as rare earth magnets (neodymium magnets, samarium cobalt magnets), due to their excellent corrosion resistance. These magnets are commonly used in precision instruments, high-performance motors, and other applications that require high reliability and long-term stability.
The cured product of epoxy resin has good sealing properties and is not easily absorbing water. Water is one of the main causes of corrosion in many materials, and the low water absorption of epoxy resin helps improve its corrosion resistance.
3.Vibration resistance: Epoxy resin is formed by in-situ polymerization of epoxy groups, and the large number of epoxy groups (-O-CH2-CH-) in the molecule can react with other active functional groups (such as amines, phenols, etc.) to form three-dimensional polymers, forming a polymer network structure, which greatly improves its physical and mechanical properties, strength, and toughness.
4.Features: Epoxy resin adhesives are widely favored for their excellent electrical properties, excellent chemical stability, and high mechanical strength. Especially in high-temperature working environments such as high-power motors, it is often used as an adhesive material for high-temperature neodymium magnets.
Although epoxy resin adhesives exhibit good adhesion to materials such as metal, glass, and ceramics, their bonding effect on plastics is relatively weak. In addition, the curing process of epoxy glue usually takes several hours or even days, which reduces production efficiency to some extent.
1.Heat resistance: Acrylate adhesives have poor heat resistance and are generally suitable for working environments with temperatures ranging from -30°C to 120°C.
2.Corrosion resistance: The chemical resistance and corrosion resistance are relatively weak. The molecular structure of acrylate adhesive contains long carbon chains and ester bonds. These ester bonds are relatively easy to hydrolyze, especially in alkaline or acidic environments, resulting in molecular chain breakage, thereby reducing the bonding strength and corrosion resistance.
The structural density after curing is relatively low, which means that there are more permeable spaces in the cured network, making it easier for corrosive substances to penetrate into the bonding layer. Therefore, it is generally suitable for magnets that do not require high corrosion resistance.
3.Vibration resistance: Acrylate adhesives have stronger toughness and better vibration resistance than epoxy resin adhesives.
Acrylate adhesives are composed of long-chain acrylate polymers, which form flexible cross-linked networks during curing, effectively absorbing and dispersing external forces, making them less prone to breaking during impact or vibration.
The three-dimensional network structure of epoxy resin adhesives has high rigidity and strength, but it is relatively lacking in flexibility. Therefore, when subjected to impact or vibration, the rigid structure of epoxy resin adhesives is prone to stress concentration, resulting in the formation and propagation of cracks.
4.Features: Acrylate adhesives can be cured quickly, with a preliminary curing time of only 3-5 minutes. Acrylate adhesives have good bonding properties for plastics and metals, but are relatively poor for glass.
Acrylate adhesives have excellent impact resistance and peel resistance, and are mainly used for assembly and bonding between magnets (ferrite, low-performance rare earth magnets) and motor housings in small and medium-sized DC motors and brushless motors.
In addition, the glass transition temperature (Tg) of acrylate adhesives is usually lower than that of epoxy resin adhesives. The glass transition temperature refers to the temperature at which a material transitions from a soft state to a rigid state. The low Tg of acrylate adhesives allows them to maintain flexibility and elasticity at lower temperatures, while epoxy resin adhesives tend to become hard and brittle at lower temperatures.
Surface treatment before using adhesive
Before using the adhesive, a series of treatments are usually required on the surface of the magnet to ensure effective bonding of the magnet.
1.Cleaning the surface: First, it is necessary to remove the grease, dust, dirt, and other contaminants on the surface of the magnet. This can be done by using organic solvents, detergents, or specialized surface cleaners. Ensuring a clean surface is a prerequisite for achieving good adhesion.
2.Roughening treatment: For some magnets with smooth coatings, surface roughening treatment may be necessary to increase the roughness of the coating. This helps improve the mechanical bite between the adhesive and the magnet surface, thereby enhancing the bonding strength. Roughening treatment can be achieved through sandblasting, grinding, chemical etching, and other methods.
3.Activation treatment: The purpose of activation treatment is to increase the chemical activity of the magnet surface, making it easier for it to react with the adhesive. This can be achieved by using chemical activators, acid washing, or anodic oxidation methods. Activation treatment helps form an active layer on the magnet surface that is easily combined with the adhesive.
4.Drying process: After completing the above processes, it is necessary to ensure that the surface of the magnet is completely dry to remove moisture and other volatile substances that may affect bonding. Methods such as using a heat gun, oven, or natural drying can be used for drying.
Types of magnet plating that need attention
Zinc is the most adherent of the coatings on neodymium magnets and is well suited for the use of adhesives. Zinc is a relatively active metal, ranking ahead of hydrogen in the metal activity sequence. This means that zinc atoms are more likely to react with other atoms or molecules to form stable chemical bonds.
Nickel plating has strong corrosion resistance and is one of the most commonly used coatings for neodymium magnets. However, its disadvantage is that it is difficult to bond with adhesives, which can easily cause the plating to fall off.
Nickel is an inert material with high chemical stability, which makes it difficult for adhesives to form effective chemical bonds with nickel plating, affecting the bonding effect. When the surface roughness of the plating is 15µm, it is most suitable for bonding. However, the surface of the nickel plating is too smooth and the roughness is not enough, so it is also difficult to obtain effective mechanical bonding in the surface area. In addition, nickel plating will react with water vapor. Over time, water vapor penetrates into the interior of the adhesive, resulting in a gradual decrease in the strength of the adhesive.
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