Various Modifications Of Epoxy Resin

Various Modifications Of Epoxy Resin

Epoxy resin is widely used in the aerospace field due to its excellent properties such as high strength, low shrinkage, corrosion resistance, and waterproofing. It can be used for aircraft structural components, spacecraft structural components, and avionics equipment. It can also be used for spacecraft thermal protection coatings and spacecraft chemical test materials. In order to meet the usage requirements of various application products, various modification methods of epoxy resin have emerged, such as filler modification, fiber reinforcement modification, elastomer toughening, nanomaterial modification, heat resistance modification, flame retardant modification, and photosensitive modification, etc. Novices are often confused by these various modifications. Don't worry, let's explore the various modifications of epoxy resin together today.

Modification of Epoxy Resin

Epoxy resin, as a thermosetting resin, has been widely used in fields such as construction, machinery, electronics and electrical, and aerospace due to its excellent electrical insulation, chemical stability, adhesion, and good processability. However, this material contains a large number of epoxy groups. After curing, the cross-linking density is high, which leads to increased internal stress, brittleness, and insufficient impact resistance, crack resistance, weather resistance, and moisture-heat resistance, limiting its further application in engineering technology.

In recent years, with the increasing requirements for the comprehensive performance of epoxy resin materials in fields such as structural bonding, encapsulation, fiber-reinforced materials, laminates, and integrated circuits, such as higher toughness, lower internal stress, and excellent heat resistance, water resistance, and weather resistance, the research on the modification of epoxy resin has become a hot topic in the industry.

I. Toughening Modification of Epoxy Resin

In order to increase the toughness of epoxy resin, the initial method adopted by people was to add some plasticizers and softeners. However, these low-molecular substances would greatly reduce the material's heat resistance, hardness, modulus, and electrical properties. Since the 1960s, research on the toughening modification of epoxy resin has been widely carried out both domestically and internationally, with the aim of improving the toughness of epoxy resin without significantly reducing its thermal properties, modulus, and electrical properties.

1. Rubber Elastomer Toughening of Epoxy Resin

The rubber elastomers used for toughening epoxy resin are generally reactive liquid polymers with a molecular weight of 1000 to 10000, and they have functional groups at the terminal or side groups that can react with epoxy groups.

The main types of reactive rubber elastomers used for toughening epoxy resin include: carboxyl-terminated butadiene acrylonitrile rubber, hydroxyl-terminated butadiene acrylonitrile rubber, polysulfide rubber, liquid random carboxyl butadiene acrylonitrile rubber, butadiene acrylonitrile-isocyanate prepolymer, hydroxyl-terminated polybutadiene, polyether elastomer, polyurethane elastomer, etc.

In the past 10 years, with the application of interpenetrating polymer network technology, there has been new development in the toughening of epoxy resin with rubber elastomers. The interpenetrating polymer network of polybutyl acrylate and epoxy resin synthesized by the synchronous method has achieved satisfactory results in improving the toughness of epoxy resin.

2. Thermoplastic Resin Toughening of Epoxy Resin

The main thermoplastic resins used for toughening modification of epoxy resin are polysulfone, polyethersulfone, polyetherketone, polyetherimide, polyphenylene ether, polycarbonate, etc. These polymers are generally engineering plastics with good heat resistance and mechanical properties, and they are either mixed into epoxy resin by thermal melting or in solution.

3. Hyperbranched Polymer Toughening of Epoxy Resin

Hyperbranched polymers are a new type of polymer material that emerged only in the past 10 to 15 years. They are a series of compounds with continuously increasing molecular weight and similar structures, obtained through stepwise controlled repetitive reactions with low-molecular substances as growth points. Hyperbranched polymers have unique structures and good compatibility, low viscosity, and other characteristics, so they can be used as modifiers for epoxy resin. The application of hyperbranched polymers in the toughening modification of epoxy resin also has the following advantages:

• The spherical three-dimensional structure of hyperbranched polymers can reduce the shrinkage rate of epoxy curing products.

• The active terminal groups of hyperbranched polymers can directly participate in the curing reaction to form a three-dimensional network structure, and the numerous terminal functional groups can accelerate the curing speed.

• The size and spherical structure of hyperbranched polymers eliminate the harmful particle filtration effect observed in other traditional toughening systems, playing an internal toughening role.

4. Core-Shell Structured Polymer Toughening of Epoxy Resin

Core-shell structured polymers refer to a class of polymer composite particles obtained by emulsion polymerization of two or more monomers. The interior and exterior of the particles are enriched with different components, showing a special double-layer or multi-layer structure. The core and shell have different functions. By controlling the particle size and changing the composition of the polymer to modify epoxy resin, internal stress can be reduced, adhesion strength and impact resistance can be improved, and a significant toughening effect can be achieved.

II. Moisture and Heat Resistance Modification

To enhance the moisture and heat resistance of epoxy resin, it is necessary to reduce the number of polar groups in the resin matrix molecular structure, thereby decreasing the interaction between the resin matrix and water, which in turn lowers the water absorption rate of the resin matrix. Additionally, optimizing the molding process of composite materials to minimize the formation of microvoids, microcracks, and free volume during the molding process can also improve its moisture and heat resistance. Increasing the degree of crosslinking and introducing heat-resistant groups, such as imino, isocyanate, and oxazolidinone groups, as well as forming interpenetrating polymer networks, are the most important means of improving heat resistance. Using aniline diphenyl ether resin containing terminal amino groups as a curing agent to modify epoxy resin results in composite materials with high initial decomposition temperatures in air and good moisture and heat resistance.

III. Flame Retardancy Modification

Epoxy resin has poor flame retardancy. To improve its flame retardancy, halogens, nitrogen, phosphorus, boron, and silicon, which are flame-retardant elements, are typically introduced into the epoxy resin. These elements can be introduced by using flame-retardant curing agents, such as those containing halogens, phosphorus, boron, and silicon, to cure the epoxy resin, or by structurally modifying the epoxy resin to incorporate flame-retardant elements within its molecular structure. Brominated phenolic epoxy resin can serve as a reactive flame retardant for epoxy resins used in encapsulation materials.

For example, YINSU Flame Retardant Company's epoxy resin red phosphorus paste RP-EP is a high-performance flame-retardant product. This product features high flame-retardant efficiency, low addition amount, and low cost. Its paste form facilitates processing and operation, eliminates dust pollution, and ensures safe use. With a fine particle size (D50 reaching 2500 mesh), it has minimal impact on the surface smoothness of the finished product. Moreover, it is halogen-free and environmentally friendly, complying with RoHS and REACH regulations.

IV. Fluorination Modification

The fluorination modification of epoxy resin aims to optimize its properties by adding fluorine atoms or fluorinated groups, including enhancing heat resistance, corrosion resistance, hydrophobicity, moisture resistance, dielectric properties, pollution resistance, flame retardancy, and mechanical properties. These modified materials, due to their excellent heat resistance, corrosion resistance, and low friction characteristics, are widely used in special fields such as aerospace solar panels, ship coatings, and optical fiber adhesives.

Since fluorine atoms have a high electronegativity and form strong bonds with carbon atoms, and there is a significant repulsion between fluorine atoms, making the internal rotation of the molecular bonds difficult, fluorinated epoxy resins exhibit excellent corrosion resistance, electrical insulation, hydrophobicity, and anti-pollution properties, and have good wettability to the adherends. 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) as a novel flame-retardant modifying monomer has entered practical application. The reaction of DOPO with bisphenol A epoxy resin produces phosphorus-containing epoxy resin, which is favored for its excellent flame-retardant performance and environmental friendliness .

The synthesis of fluorinated epoxy resins mainly includes four methods:

• Monomer Polymerization Method: This involves the polymerization of fluorinated monomers to prepare fluorinated epoxy resins. For example, Rao Jianbo and others synthesized a new type of fluorinated epoxy resin suitable for coatings, which showed good corrosion resistance and determined the optimal reaction conditions .

• Fluoride Introduction Method: Also known as grafting method, it uses fluorinated modifiers to graft with epoxy resins. Some researchers synthesized a new type of fluorinated epoxy curing agent, which, after reacting with epoxy resin, produced adhesives with high room-temperature bonding strength and low water absorption .

• Physical Blending Method: This involves directly mixing fluorinated polymers or additives into epoxy resins. Some researchers improved the interfacial and mechanical properties of epoxy resins by blending fluorinated epoxy resin with tetraglycidyl-4,4'-diaminodiphenylmethane .

• Direct Fluorination Method: This involves directly introducing fluorine atoms or fluorinated groups onto the epoxy resin molecular chain through chemical means. Some researchers achieved fluorination modification of epoxy resins using low-temperature plasma technology, improving their surface properties .

After fluorination modification, the surface tension of epoxy resins is reduced, enhancing hydrophobicity and pollution resistance. The molecular structure becomes more compact, improving corrosion resistance and heat resistance. The refractive index is adjustable, making it suitable for optical adhesives. the dielectric properties are enhanced, making it suitable for electronic and electrical insulation materials. Although fluorinated epoxy resins are more expensive, their synthesis methods are developing towards environmental friendliness and cost-effectiveness, and they are mainly used in fields with high-performance requirements .

V. Phosphorus Modification

Phosphorus modification, as a mainstream trend in epoxy flame retardancy, endows epoxy resin systems with excellent flame-retardant properties through its flame-retardant and char-increasing characteristics in both the gas phase and condensed phase. Compared to halogenated compounds, phosphorus-modified epoxy resins release significantly less smoke and harmful gases during combustion. Phosphorus elements can be effectively incorporated into epoxy resins through phosphorus-containing epoxy systems.

Some researchers have developed phosphorus-containing difunctional and trifunctional cycloaliphatic epoxy resins. These resins exhibit high glass transition temperatures, excellent reprocessability, high mechanical modulus, and halogen-free flame-retardant properties, making them suitable for environmentally friendly optoelectronic and microelectronic packaging fields. The increase in high glass transition temperature ensures the mechanical properties and stability of the material in high-temperature environments.

A researcher synthesized 10-(2,5-dihydroxyphenyl)-9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DHPDOPO), an organic phosphorus compound, which shows good flame-retardant properties. When phosphorus-containing epoxy resins are used in conjunction with nitrogen-containing curing agents such as Novolac, phenolic melamine, and dicyandiamide, their flame-retardant properties are further enhanced.

A phosphorus-containing silicone epoxy resin has been developed by researchers. This resin maintains excellent thermal stability, high mechanical properties, and flame retardancy at temperatures above 400°C. The resin is prepared using triammonium phosphate, epoxy-functionalized polysiloxane, and bisphenol-F epoxy resin as raw materials, with DDM as the curing agent.

The introduction of phosphorus compounds and their synergistic action with other elements significantly improves the thermal stability, mechanical properties, and flame-retardant properties of epoxy resins, opening up broad prospects for their application in the field of high-performance and environmentally friendly flame-retardant materials.

VI. Silicon Modification

Silicon, as an environmentally friendly flame retardant, can effectively enhance the flame-retardant performance of epoxy resins.

There are two main methods for preparing such resins:

• Esterification and Etherification Reactions: Through the esterification and etherification reactions of alkoxysilanes with glycidyl ethers or the condensation reactions of siloxanes with epichlorohydrin.

• Silane Hydrosilylation: From the reaction mechanism perspective, these two methods can be categorized as physical blending and graft copolymer modification. Resins prepared using these techniques combine the characteristics of both silicone and epoxy resins.

Some researchers used silicon-containing epoxides or prepolymers in conjunction with 4,4'-diaminodiphenylmethane curing agents to prepare epoxy resins with different silicon contents. Compared to traditional epoxy resins, silicon-based compounds show higher reactivity with amine curing agents. As the silicon content increases, the glass transition temperature of the material moderately decreases, and the initial thermal decomposition temperature is reduced, but the proportion of char residue during pyrolysis increases. The addition of silicon significantly enhances the flame retardancy of epoxy resins, as evidenced by their higher limiting oxygen index (LOI value).

Some researchers synthesized silicone-epoxy resin (SiE) through the hydrolysis and condensation of 2-(3,4-epoxy cyclohexylmethyl)ethyltrimethoxysilane (EMDS) and its copolycondensation with dimethyldimethoxysilane. Compared to commercial LED encapsulation materials (CEL-2021P epoxy resin), the cured SiE resin exhibits superior thermal stability and heat and UV resistance. The epoxy value has a significant impact on the thermomechanical properties, thermal aging, and UV aging performance of the cured SiE resin. As the epoxy value decreases, the flexibility of the epoxy resin increases, while the resin with an appropriate epoxy value shows the highest heat and UV resistance.

VII. Chemical Modification

By altering the structure of epoxy resins and introducing certain chemical groups into the epoxy resin molecules, the performance of epoxy resins can be improved and their application range can be broadened. For example, by reacting acrylic or methacrylic acid with some epoxy groups in epoxy resins, carbon-carbon double bonds are introduced while retaining some epoxy groups in the molecule. This modification endows the epoxy resin with both photosensitive characteristics and some of the excellent properties of epoxy resins. Alternatively, by introducing hydrophilic groups into the molecule, epoxy resins can be modified into waterborne epoxy resins, giving the modified epoxy resins water dispersibility.

In conclusion, epoxy resin, a typical thermosetting resin, is widely used in fields such as aerospace, automotive manufacturing, electronics, and construction due to its high thermal stability, good heat resistance, and outstanding mechanical properties. Its main characteristic is that it does not produce free radicals during curing. These modifications aim to enhance the performance of epoxy resin and expand its application range.

In order to meet the flame retardant needs of epoxy resin in the high-performance field, Yinsu Flame Retardant Company has continuously innovated and developed, and launched a number of professional epoxy resin flame retardants, including brominated epoxy, epoxy resin red phosphorus paste and bromine antimony flame retardant. These products can effectively maintain their mechanical properties and stability while improving the flame retardant properties of the material. They are widely used in electronics, aerospace and other high-end industries, becoming an ideal choice in the industry. We are committed to providing customers with safer and more environmentally friendly solutions and promoting the technological progress and application development of epoxy resin materials.