Epoxy resins have good comprehensive mechanical properties, high adhesion, small shrinkage, good stability, excellent electrical insulation properties, as coatings, adhesives, composite resin substrates, electronic packaging materials, etc. in machinery, electronics, electrical appliances ､Aerospace, aviation, coatings, adhesives and other fields have been widely used. However, due to the high cross-linking density and high internal stress of the cured epoxy resin, there is brittleness, fatigue resistance, heat resistance and impact resistance. The shortcomings such as poor toughness make it difficult to meet the requirements of engineering technology, which limits its application. Especially, it restricts that epoxy resins cannot be used well for composite materials such as structural materials. The resin has been extensively modified. Among them, the most important thing is to improve the brittleness of the epoxy resin. Moisture resistance.
Epoxy resins can be modified by chemical and physical methods. Chemical method modification is mainly to synthesize new structure epoxy resin and new structure curing agent; physical method modification is mainly to form a blend with the modifier Structure to achieve the purpose of improving performance. Compared with the two methods, the first method is inferior to the second method in terms of process, cost and difficulty. Therefore, the current modification of epoxy resin is mainly through Realized by blending structure.
There are three main ways to toughen epoxy resins: ① Rigid inorganic fillers, rubber elastomers and thermoplastic polymers form a two-phase structure for toughening. ② Continuously penetrate the epoxy resin network with thermoplastics to form a semi-interpenetrating network Type polymer to toughen and modify. ③ By changing the chemical structure of the cross-linked network (such as the introduction of "flexible segments" in the cross-linked network) to improve the mobility of the cross-linked network.
The improvement of the heat and moisture resistance of epoxy resins is mainly through the introduction of structural units containing fused rings and the synthesis of fluorine-containing epoxy resins in epoxy resin molecules, and the replacement of traditional DDS with new curing agents.
The modified epoxy resin will further expand the application of epoxy resin in electronic and electrical products, composite materials and high-performance structural adhesives due to the improvement of moisture resistance and toughness.
On the other hand, although epoxy resins have good processability, for different applications, the operation process needs to be appropriately improved. For example, diphenol-based propane type epoxy resins, due to the larger viscosity, in certain operations If the process is poor, it is necessary to add a diluent to the curing system to reduce the viscosity and improve the performance of the operation process. Therefore, in order to meet different applications, it is necessary to add different additives such as dilution Liu, filler, reinforcing agent, etc.
Adjusting the fluidity of epoxy resin: The fluidity of epoxy resin complex is very important for coating, lining, casting and other uses. To meet these requirements, it is necessary to reduce the viscosity, or increase the viscosity, or impart thixotropy, which can achieve these The required compounding material is called fluidity regulator.
The thinner is mainly used to reduce the viscosity of the epoxy adhesive system, dissolve, disperse and dilute the coating, improve the spreadability and fluidity of the glue. In addition, the thinner also plays a role in prolonging the service life. But adding the thinner will also Reduce the heat distortion temperature of the resin after curing, adhesive strength, resistance to media and aging resistance. However, in order to make the resin glue easy to wet the surface of the cement, improve its wetting ability and wetting ability, which is conducive to operation, it must be added in an appropriate amount Of thinner.
There are many classification methods for thinners. According to the mechanism of use, they can be divided into two categories: inactive diluents and active diluents.
(1) Inactive thinner
The non-reactive diluent does not react with epoxy resins, curing agents, etc. It is purely physically blended into the resin. It is only mechanically mixed with the resin, which is a liquid that functions as a dilution and viscosity reduction. It is cured in the glue Most of it is volatilized during the process. It will leave pores in the cured resin and increase the shrinkage rate accordingly. Therefore, the adverse effect of the non-reactive diluent on the performance of the cured resin is greater than that of the reactive diluent, but Can slightly improve the toughness of the resin. When the use requirements are higher, non-reactive diluent cannot be used, and reactive diluent should be used.
Inactive diluents are mostly high-boiling liquids, such as dibutyl phthalate, dioctyl phthalate, styrene, diallyl phthalate, toluene, xylene, etc. The dosage is 5%~20% It is suitable. About 12% dibutyl phthalate reduces the viscosity of standard epoxy resin from 10Pa·s to 0.5~0.7Pa·s (25℃). Some industrial epoxy resins (2.0~4.0Pa·s ) Contains dibutyl ester as an inactive diluent. The solvent also acts as an inactive diluent, but it has an adverse effect on chemical resistance. When the amount is large, the performance of the cured product deteriorates and shrinkage due to the volatilization of the diluent during the curing process will cause shrinkage. Increased rate.
(2) Active thinner
Reactive diluents generally refer to low-molecular compounds with one or more epoxy groups. They can directly participate in the curing reaction of epoxy resins and become part of the cross-linked network structure of the cured epoxy resin. Almost no effect, sometimes it can increase the toughness of the curing system. Reactive diluents are divided into two types of single epoxy-based reactive diluents and polyepoxy-based reactive diluents. Some single epoxy-based diluents, such as propylene-based shrinkage Glycerol ether, butyl glycidyl ether and phenyl glycidyl ether are more reactive towards amine curing agents. Some olefins or alicyclic monoepoxy diluents are more reactive towards anhydride curing agents. Therefore, use When reactive diluent is used, the amount of curing agent and variety should be adjusted accordingly.
In solvent-free epoxy coatings, the amount of monofunctional reactive diluent does not exceed 15% of the epoxy resin, and the amount of multi-functional reactive diluent can reach 20% to 25%. But too much dosage will reduce the performance of the coating film. For example, After the bisphenol A epoxy coating without active diluent and the bisphenol A epoxy coating with well 501 (propylene oxide butyl ether) active diluent were immersed in 10% sulfuric acid aqueous solution for 30 days, the coating The weight gain is 2.11% and 4.18% respectively; after soaking in methanol for 30 days, the coating weight gain is 2.67% and 13.5% respectively.
Active diluents are generally toxic. Care must be taken during use. Long-term exposure can often cause skin irritation, and even ulcers can occur in serious cases.
The dilution effect of the monoepoxide is better, and the aliphatic type has better dilution effect than the aromatic type. The cured product using the aromatic active diluent has little change in acid and alkali resistance, but the solvent resistance is somewhat decline.
The use of monoepoxide reactive diluent will reduce the heat distortion temperature, which is because its use will reduce the cross-linking density of the cured product. The reactive diluent with a long carbon chain can make the bending strength and impact after use. Toughness is improved. When the amount is not large, it has no effect on the hardness of the cured product, and the thermal expansion coefficient is increased.
Use di-or tri-epoxide as the diluent. The appropriate amount and curing method will not reduce the cross-linking density. Therefore, the mechanical strength and chemical resistance retention rate in hot state are higher. Compared with the single epoxide, in The dilution effect is poor, and the amount of resin needed to reduce the viscosity of the resin to the same level is larger.
Short-chain and cyclic two or three epoxides have almost no effect on the thermal deformation temperature of the cured product, while the effect of long carbon chain thinner is very obvious.
(3) Principles for selection of thinner
1) Try to use reactive diluent as much as possible, in order to improve the manufacturability at the same time, improve its bonding, mechanical ‘performance’.
2) Choose diluents that are similar to the main resin chemical structure, because they will participate in the reaction with the main resin in the presence of other additives, which greatly improves the performance of the adhesive layer. For example, in 100 parts of bisphenol A epoxy resin as the main body When 20 parts of butanediol diepoxy thinner is added to the component's glue, and mixed amine is used as the curing agent, the elongation of the glue layer can reach 7.0%, and the tensile strength is still maintained above 29MPa, and the heat deformation temperature Up to 120℃.
3) Attention should be paid to selecting varieties with low volatility, small odor (odor) and low toxicity as much as possible to reduce the harm of the thinner to the human body when mixing and sizing, because most active or inactive diluents have odor and Has low toxicity.
4) The source is easy, non-flammable and non-explosive, and the price is low. It is also an important factor to be considered. Therefore, as long as the water can be used as the diluent, it should be used as much as possible.
5) The most suitable dosage should be selected through experiment and theory.
The main difference between a solvent and a non-reactive diluent is that the solvent mainly functions to dissolve the resin system, and of course it can also adjust the viscosity of the glue; the main function of the non-reactive diluent is to adjust the viscosity of the coating, which may be dissolved in the resin system Sex may not be soluble.
The addition of solvent makes the adhesive more convenient for construction, and can be cured at room temperature, so that the viscosity of the glue is low, and it is easy to wet the surface of the adherend. The processability is good. However, the addition of the solvent also causes the volume shrinkage of the adhesive to be large during curing. The solvent sometimes swells the surface of the adherend, causing poor adhesion, and most of the solvent is volatile and flammable, with certain toxicity and other shortcomings. Therefore, you should choose to use when configuring the adhesive.
In the selection of adhesive solvents, first consider their solubility in the main resin, and secondly consider their volatilization rate, because only suitable volatilization rates can prepare good performance adhesives and coatings. Secondly, consider the viscosity of the solvent, flash point and easy Flammability. For safety considerations, alcohols with higher flash points, ether alcohols and esters should be used as much as possible. Propylene glycol ethers should be used instead of glycol ethers to reduce toxicity. Finally, odor, toxicity, source of difficulty and price should be considered. Wait.
Epoxy resins can be dissolved in certain organic solvents, and the solubility of the resin decreases with increasing molecular weight. Ketones, esters, ether alcohols, and chlorinated hydrocarbons are solvents for epoxy resins, which are very useful for epoxy resins. Good solubility. Aromatics and alcohols are not solvents for epoxy resins, but aromatics and alcohols can be used as solvents for medium molecular weight resins when mixed.
Most of epoxy resin coatings use mixed solvents, which are composed of solvents and thinners, which can reduce costs, improve paint film performance and construction performance, and improve the solubility of solvents. Products with brush application should use some high-boiling solvents, such as B Base cellosolve, etc. When choosing a solvent, it should also be noted that solvents with different structures will have different effects on the curing reaction. For example, amine-cured epoxy resins cannot use esters as solvents, because esters and amines cure The agent reacts to destroy the curing agent and reduce the curing effect; when Lewis acid is used as the curing agent, if cyclohexanone is used as the solvent, a ketalization reaction will occur, and this reaction will affect the performance of the cured coating film, especially during baking. Under the conditions of, so you should be very careful when using ketone and ester solvents.
Thickener is a relatively new type of compounding aid in the composition of adhesives.After they are added to the adhesive, they can thicken the construction viscosity and increase the bonding strength of some non-sticky or difficult-to-stick materials, especially It is to improve its initial adhesion and improve its wettability to the surface of the adherend.
Most of the thickeners are resins with small molecular weights. When choosing, they mainly follow the following principles: ① They have good compatibility with epoxy resins.After mixing, they can work together with epoxy resins for a long time and stable. No precipitation, no delamination. ②It has the best thickening effect, and has a very high adhesion to the surface of the adherend. ③Various sources, cheap price, preferably non-dangerous goods, to facilitate storage and transportation.
The thickeners currently used for epoxy adhesives are mainly organic silane compounds, alkyl phenolic resins, rosin or modified rosin and modified starch. Considering the addition of the thickener, there are both construction process requirements and Adhesion performance, performance requirements of adhesive joints, sometimes a single thickener can not meet the requirements, in recent years, most of them have used mixed thickeners. For example, in addition to tert-butyl phenolic epoxy resin, but also modified Rosin to increase the initial viscosity. The amount of thickener added is generally 1%~20% of the total amount.
4､Rheology and Rheological Agent
Auxiliary agents that can improve the rheological properties of paints are called rheology modifiers, also known as rheology agents. Generally speaking, rheology agents can improve the stability and paintability of paints and improve the quality of coating films. For example, prevent Precipitation of pigments and fillers during the storage of paint to avoid splashing of paint during the coating process, sag, and improve the leveling performance of the coating film, etc.
From a rheological point of view, rheological agents are also divided into thixotropic rheological agents and pseudoplastic rheological agents. The difference between the two lies in the speed at which the system structure recovers after the applied shear force is removed. This characteristic is a coating The main influencing factors of flow and leveling. Due to the extremely fast structural recovery speed, the pseudo-plastic rheological agent recovers the structural viscosity almost immediately after the applied shear force is removed, which is beneficial to the anti-settling and anti-sagging of the coating, but the amount High temperature will adversely affect the flow and leveling, and further affect the quality of the coating film, such as excessive brush marks, poor atomization when spraying, etc. The typical pseudoplastic rheological agent is fumed silica, soluble castor oil and Polyolefin pulp, etc.
The thixotropic rheological agent can display the real-time related structural recovery speed after the applied shear force is removed.It can be used in coatings to obtain satisfactory sag resistance without losing flow and leveling properties. The application effect is superior to the pseudoplastic rheological agent. Such rheological agents mainly include organic clay and hydrogenated castor oil-based organic wax. The thixotropy is related to the shape of the filler. The larger the aspect ratio of the particles, the smaller the size, The higher the denaturation effect.
(2) Several commonly used rheological agents
1) Fumed silica Fumed silica is a rheological agent used earlier, but the product used now has a great improvement in performance. Fumed silica is a solid powder, which is a collection of spherical particles. The molecule contains hydroxyl groups, which can adsorb water molecules and polar liquids. The spherical particles have silanol groups on their surfaces. When fumed silica is dispersed in the base material solution, the silanol groups between adjacent spherical particles are affected by hydrogen. Bonding produces a loose lattice, forming a three-dimensional network structure, resulting in gelation and high structural viscosity. When subjected to shear forces, the hydrogen bonding force is weak, the network structure is destroyed, and the gelation disappears. Viscosity decreases. After the shear force is removed, it can return to its original shape at rest.
The amount of fumed silica in the coating depends on the final viscosity requirements and the viscosity of the coating before fumed silica is added, generally 0.5%~3.0% of the total coating (mass fraction).
Fumed silica is susceptible to the influence of coating solvents when used, and it works best in non-polar solvents. In polar solvents, the attraction between liquid molecules and silica particles increases, making it difficult to form a loose network Structure. For this purpose. There are fumed silicas specially used for polar solvents abroad, such as Aerosil series products of German Degussa company.