【Die Bond】The curing mechanism of epoxy resin adhesive

We often use adhesives in Die attach, most typically epoxy-based adhesives, which are a very common thermoset material that can be used in a variety of applications, from simple low-cost adhesives to conductive adhesives for microelectronics assembly to structural composites with extremely high strength. The epoxy molecules are shown in Figure 1:

Figure 1 Epoxy molecules

The biggest feature of epoxy molecules is that they contain highly reactive epoxy groups, epoxy group is a three-membered ring composed of two carbon atoms and one oxygen atom, epoxy group (epoxy group) is sometimes called ethylene oxide group (oxirane).

The ternary ring structure of the epoxy group refers to the cyclic structure formed by the connection of oxygen atoms in the epoxy group (C-O-C) to two carbon atoms, as shown in Figure 2. This structure plays an important role in epoxy resins.

Figure 2 Epoxy group

The ternary ring structure of the epoxy group has the following properties:

1. High reactivity: The oxygen atoms in the epoxy group have high electron affinity and are easy to attract surrounding nucleophiles to react with it. Therefore, the epoxy group has high reactivity and can undergo ring-opening reactions with a variety of compounds to form a cross-linked structure.

2. Strong curing ability: when the epoxy group reacts with the curing agent, a ring-opening reaction can occur to generate a cross-linked structure. This cross-linked structure gives the epoxy resin a strong curing ability to form a hard solid.

3. Good stability: the bond strength between oxygen atoms and carbon atoms in the epoxy group is high, so that the epoxy group has good chemical stability. Epoxy resins maintain long stability under normal environmental conditions.

4. Strong tunability: Due to the reactivity and diversity of epoxy groups, the performance and application range of epoxy resin can be adjusted by selecting different curing agents and additives. Therefore, the epoxy group is highly adaptable and tunable.

Note that there is an epoxy reactive site at each end of the molecule, as shown in Figure 1. "Epoxy reactive site" refers to a reactive site or location in an epoxy resin. In epoxy resins, epoxy groups are functional groups with high reactivity that can chemically react with other compounds. Therefore, epoxy groups are called reactive sites or reaction sites in epoxy resins.

Although the epoxy group has extremely high reactivity, it is also extremely stable, without the participation of curing agents, catalysts, etc., generally no ring-opening polymerization occurs. Therefore, when using epoxy resin as a binder, a curing agent is required to participate in it. In order to improve certain properties, it is sometimes necessary to add auxiliary materials such as flexibilizers, diluents, accelerators, fillers, coupling agents, etc.

Hardeners selection

In the epoxy-based ring-opening polymerization process, the selection of hardener is crucial. A typical hardener has a functional degree equal to greater than 3. A degree of 3 functionality is required to obtain a crosslinked network. High functionality means that hardener molecules can crosslink or react with a large number of other molecules, creating a broader and interconnected network of chemical bonds. In Figure 3, we can see that if both epoxy and curing agents are bifunctional, a linear polymer is formed. The use of a curing agent of f=3 creates a crosslinking and typical thermoset network.

Fig. 3 Curing agent bifunctional and 3 functional response

There are many types of epoxy resins, and according to different requirements, such as bond strength, temperature resistance, curing process, etc., the appropriate epoxy resin type can be selected. The most common epoxy resin is bisphenol A type epoxy, which is currently the most produced and used species.

Bisphenol A type epoxy resin is obtained by polycondensation of bisphenol and epichlorohydrin catalyzed by sodium hydroxide:

Figure 4 Preparation of bisphenol A epoxy resin

It has two functionalities, which are phenolic hydroxyl group (-OH) and epoxy group (-O-). Therefore, when crosslinked with a multifunctional curing agent (f greater than or equal to three), a crosslinking network is formed.

Curing mechanism

Diamine curing agent is the most commonly used curing agent or hardener. The internal reaction of the curing process is mainly completed in two parts.

(1) The first part of curing linear polymerization

In most cases, diamine has a functional degree of 4 due to the four active hydrogens at the diamine terminal. The initial stage of epoxy curing is mainly linear polymerization, as shown in the following figure:

Figure 5 Linear aggregation

The first reaction of the amine with the epoxy group causes the chain to elongate, but the growing chain continues to have active substance at both ends. In the figure above, the –NH2 group (two active hydrogens) is located at each end of the growing chain. As more amines react with epoxy resins, chain expansion is the main reaction pathway.

(2) The second part of the cured crosslinking polymerization

As it continues to cure, the linear chain formed in front reacts with the new epoxy-amine to cause crosslinking, as shown in the figure below:

Figure 6 Crosslinking aggregation

As the growing straight strands begin to crosslink, the molecular weight increases very quickly, because each crosslinking reaction not only brings more small molecules (epoxy and amine monomers) together, but also crosslinks longer segments. As the reaction continues, an infinite network of polymer chains is formed. It is crosslinking that gives thermoset materials their unique thermal stability. When a fully cured thermosetting material is heated above the glass transition temperature (Tg), the material softens but does not flow.

A chemical reaction occurs when the epoxy resin is heated with a curing agent, and the conversion period from liquid components to solids is the curing time. During curing, the epoxy adhesive transforms from a liquid to a gel state and finally to a solid state. When the epoxy resin is hot, the curing time is shorter; When the epoxy resin is less heated, the curing time is longer, as shown in Figure 7:

图7 Cure time

According to the above figure, we can think that the curing time can be divided into three segments:

①Liquid-Open time

This time period can be thought of as the working time or placement time phase of the adhesive. This is the time when the adhesive remains liquid for easy dispensing. All assembly and dispensing is carried out within Open time to ensure reliable bonding.

To ensure adequate open time, a slower curing agent can be used to increase or maintain a longer open time. Or dispense into smaller tubes to ensure dispensing can be completed in a short enough open time. As can be seen from the figure above, the temperature also has a great influence on Open time, the higher the temperature, the shorter the Open time, therefore, it is often necessary to store at low temperatures to extend Open time.

②Gel-Initial cure phase

When the epoxy begins to transform into a gelling state, it enters the initial curing phase. Epoxy resins are no longer available and will evolve from a viscous gel consistency to the hardness of hard rubber.

Gelatinization/gel transition is the formation of junctions between chains of branched polymers, resulting in the polymer becoming progressively larger. At that point in the reaction (defined as the gel spot), the adhesive loses fluidity and becomes very viscous, as shown in the figure below.

Fig. 8 Change of viscosity with curing time

③Solid—Final cure phase

The epoxy resin and curing agent have been cured to a solid state, and at this stage, the crosslinking reaction between the cured products has been completed to form a network structure.

summary

The curing mechanism of epoxy resin is actually to convert linear polymerization into cross-linked polymerization to form a network structure, as shown in the following figure:

Figure 9 Three-dimensional network structure after crosslinking

Figure A is an epoxy resin, B is a curing agent, and C is a cross-linked three-dimensional polymer network formed after curing.

The curing process can be divided into three stages, as shown in Figure 10. Liquid-Open time, which is considered to be stage A, when the epoxy resin has not reacted with the curing agent and exists as a liquid mixture; Gel-Initial cure phase, which is considered to be stage B, at which the epoxy resin and the curing agent gradually react to form a gel state, and the adhesive gradually loses its fluidity and becomes highly viscous; Solid-Final cure phase, considered to be the C stage, at which time it has been fully cured and crosslinked to form a three-dimensional network structure, as shown in Figure 9, thereby increasing its hardness and mechanical strength.

Figure 10 Curing process

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