Epoxy resin has good comprehensive mechanical properties, high adhesion, low shrinkage, good stability, excellent electrical insulation properties, as coatings, adhesives, composite resin matrix, electronic packaging materials, etc. in the mechanical, electronic, electrical, aerospace, aviation, coatings, bonding and other fields have been widely used. However, due to the high cross-link density and high internal stress of the cured epoxy resin, there are shortcomings such as brittleness, fatigue resistance, heat resistance and poor impact toughness, which make it difficult to meet the requirements of engineering technology and make its application subject to certain restrictions. In particular, epoxy resins cannot be well used for structural materials and other types of composite materials, for this reason, scholars at home and abroad have carried out a lot of modification research on epoxy resins. Among them, the most important one is to improve the brittleness and humidity and heat resistance of epoxy resin.
Epoxy resins can be modified by chemical and physical methods. Chemical modification is mainly the synthesis of a new structure of epoxy resin and a new structure of curing agent; physical modification is mainly through the formation of a blended structure with modifiers to achieve improved performance. When comparing the two methods, the first method is at a disadvantage compared to the second method in terms of process, cost and ease of use. Therefore, the modification of epoxy resins is currently mainly achieved by blending structures.
There are three main ways of toughening epoxy resins:
- Toughening with rigid inorganic fillers, rubber elastomers and thermoplastic polymers to form a two-phase structure.
- Toughening by continuous penetration of thermoplastics into the epoxy resin network to form a semi-interpenetrating network polymer.
- The chemical composition of the cross-linked network is changed (e.g. by introducing “flexible segments” into the cross-linked network) to improve the mobility of the cross-linked network.
The improvement of the hygrothermal resistance of epoxy resins is mainly through the introduction of structural units containing thick rings in the epoxy resin molecules and the synthesis of fluorine-containing epoxy resins, as well as the use of new curing agents instead of traditional DDS.
The modified epoxy resins will further expand the applications of epoxy resins in electrical and electronic products, composite stress members and high-performance structural adhesives due to the improved resistance to damp heat and toughness.
On the other hand, although epoxy resins have good processability, the handling process needs to be improved for different applications. For example, diphenol propane epoxy resins, due to their high viscosity, have poor processability in some operations and require the addition of diluents to the curing system to reduce the viscosity and improve the processability of the operation. Therefore, in order to meet different applications, different additives such as diluent Liu, fillers and enhancers need to be added.
Adjustment of epoxy resin fluidity: The fluidity of the epoxy resin compound is important for coating, lining, casting and other applications. In order to meet these requirements the viscosity must be reduced, or the viscosity increased, or thixotropy given, and the materials used to meet these requirements are called flow adjusters.
Diluent
Thinners are mainly used to reduce the viscosity of the epoxy adhesive system, dissolve, disperse and dilute the paint, improve the spread ability and fluidity of the adhesive. In addition, thinners also play a role in extending the service life. However, the addition of thinners can also reduce the heat deflection temperature, adhesive strength, media resistance and ageing resistance of the resin after curing. However, in order to make it easier for the resin glue to wet the surface of the glue compound, to improve its wetting and soaking ability and to facilitate handling, the right amount of thinner must be added.
There are many ways of classifying diluents, which can be divided into two categories: inactive and active diluents according to their mechanism of use.
Non-reactive diluents
The inactive diluent does not react with the epoxy resin, curing agent etc. and is purely physically mixed into the resin. It is only mixed mechanically with the resin and acts as a diluting and viscosity-reducing liquid. It is mostly evaporated during the curing process of the glue. It leaves pores in the resin curing material and causes a relative increase in shrinkage. Therefore, non-active diluents have a greater adverse effect on the performance of the resin after curing than active diluents, but can improve the toughness of the resin to a lesser extent. When the use of high requirements cannot use inactive diluent, should choose to use active diluent.
Non-active diluents are mostly high boiling point liquids such as dibutyl phthalate, dioctyl benzo dicarboxylate, styrene, diallyl benzo dicarboxylate, toluene, xylene, etc. Dibutyl phthalate at 12% makes the viscosity of the standard epoxy resin drop from 10 Pa-s to 0.5-0.7 Pa-s (25°C). Some industrial epoxy resins (2.0 to 4.0 Pa-s) contain dibutyl phthalate as an inactive diluent. Solvents are also used as non-reactive diluents, but have a negative effect on chemical resistance. When used in large quantities, the performance of the cured product deteriorates while the volatilization of the diluent in the curing process causes an increase in shrinkage.
Reactive diluents
Active diluents are generally low molecular compounds with one or two or more epoxy groups, which can directly participate in the curing reaction of the epoxy resin and become part of the cross-linked network structure of the cured epoxy resin, with little or no effect on the performance of the cured product and sometimes increase the toughness of the curing system. The reactive diluents are divided into single epoxy-based reactive diluents and multi-epoxy-based reactive diluents. Some single epoxy diluents, such as acryl glycidyl ether, butyl glycidyl ether and phenyl glycidyl ether, for amine curing agent reaction activity is larger. Some olefinic or alicyclic mono epoxy diluents are more reactive to anhydride curing agents. Therefore, the use of active diluent, the amount of curing agent, species should be adjusted accordingly.
In solvent-free epoxy coatings, the amount of monofunctional active diluent does not exceed 15% of the epoxy resin, and the amount of multi-gong active diluent can reach 20%-25%. However, the dosage is too much and will reduce the performance of the coating film. For example, bisphenol An epoxy coating without active diluent and bisphenol An epoxy coating with well 501 (epoxy propane butyl ether) active diluent in 10% sulphury acid solution after 30 days, the coating weight gain is 2.11% and 4.18% respectively; in methanol after 30 days, the coating weight gain is 2.67% and 13.5% respectively.
Active diluents are generally toxic and care must be taken in their use. Long-term exposure often causes skin irritation and even ulceration in severe cases.
Monocyclic oxides are more effective and aliphatic types have a better dilution effect than aromatic ones. The curing products with aromatic active diluents have little change in acid and base resistance, but solvent resistance is reduced.
The use of mono-epoxide reactive diluents results in a lower heat deflection temperature, which is due to the fact that its use decreases the crosslink density of the cured product. The use of long carbon chains of reactive diluents leads to an increase in flexural strength and impact toughness. In small quantities it has no effect on the hardness of the cured product, while the coefficient of thermal expansion increases.
The use of di- or tri-epoxides as diluents, in appropriate amounts and curing methods, does not reduce the crosslinking density, resulting in a higher retention of mechanical strength and chemical resistance in the hot state. Compared to mono-epoxides, the dilution effect is less favorable and the amount of addition required to bring the resin viscosity down to the same level is higher.
Short chain and ring structure of two or three epoxides, the heat deflection temperature of the cured material almost no effect, while the impact of long carbon chain diluent is very obvious.
The principles of diluent selection
- Try to use active diluents, in order to improve the process at the same time, to improve its bonding, mechanical ‘performance.
- Choose those diluents with the main resin chemical structure, because they will be in the presence of other additives, together with the main resin to participate in the reaction, and greatly improve the performance of the adhesive layer. For example, in l00 parts of bisphenol An epoxy resin as the main component of the rubber, add 20 parts of butylene glycol double epoxy diluent, and use mixed amine as a curing agent, the elongation of the rubber layer can reach 7.0%, while the tensile strength remains above 29MPa, and the heat deflection temperature reaches 120℃.
- Attention should be paid to the use of small volatility, odor (odor), toxicity as low as possible varieties, in order to reduce the diluent in the dispensing of glue, glue when the human body, because most of the active or inactive diluent have odor and have low toxicity.
- Easy to source, non-flammable and inexpensive, is also an important factor to consider. Therefore, as long as water can be used as a diluent, it should be used as much as possible.
The most suitable amount to be added should be selected through experimentation and theory.
Solvents, thickeners
The main difference between solvents and inactive thinners is that solvents mainly play the role of dissolving the resin system and of course can adjust the viscosity of the adhesive; while the main role of inactive thinners is to adjust the viscosity of the paint, which may or may not be soluble to the resin system.
The addition of solvents makes the adhesive easier to work with and can be cured at room temperature, making the adhesive viscosity low, easy to wet the surface of the sticky object, good workmanship and so on. However, the addition of solvents also cause the adhesive in the curing volume shrinkage rate is large, the solvent will sometimes make the surface of the bonded material swelling, resulting in poor bonding, as well as most of the solvent volatile and flammable, with a certain degree of toxicity and other deficiencies. Therefore, in the configuration of the adhesive should pay attention to the choice of use.
The choice of solvent for adhesives is based on the solubility of the main resin, followed by the evaporation rate, as only the right evaporation rate can be used to produce good adhesives and coatings. Secondly, the viscosity, flash point and flammability of the solvent should be considered. For safety reasons, higher flash points of alcohols, ethers and esters should be used as far as possible, with propylene glycol ethers instead of glycol ethers to reduce toxicity, and finally to consider the smell, toxicity, ease of origin and price.
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 and have good solubility for epoxy resins. Aromatic hydrocarbons and alcohols are not solvents for epoxy resins, but when mixed with aromatic hydrocarbons and alcohols, they can be used as solvents for medium molecular weight resins.
Epoxy resin coatings mostly use mixed solvents, which are composed of solvents and diluents, which can reduce costs, improve the performance of the paint film and construction properties and increase the solvency power of the solvents. Brush construction of the product should use part of the high boiling point solvents, such as ethyl solubilize.
What should also be noted when selecting solvents is that solvents with different structures will play a different role in the curing reaction. For example, amine curing epoxy resin cannot use esters as solvents, because esters and amine curing agent has a reaction, destroy the curing agent, reduce the curing effect; Lewis acid as curing agent, such as the choice of cyclohexanone as solvent, will occur shrink ketone reaction, this reaction will affect the performance of curing coating film, especially in the baking conditions, therefore, the use of ketone and ester solvents should be very careful.
Thickening agent
Thickeners in the composition of the adhesive is a relatively new class with the auxiliary, they are added to the adhesive, can thicken the construction viscosity and make some of the original not sticky or difficult to stick material bonding strength is improved, especially to improve its initial adhesive force, and improve its wettability on the surface of the object to be stuck.
Most of the thickeners are not very large molecular weight resin substances, in the selection of the following principles: ① and epoxy resin has good compatibility, after mixing, can work together with epoxy resin for a long time stable, not precipitation, not stratified. ②It has the best thickening effect and has a fairly high adhesion to the surface of the object to be adhered to. ③ Abundant sources, cheap and preferably non-dangerous, for easy storage and transportation.
The main thickeners currently used in epoxy adhesives are organ silane compounds, alkyl phenolic resins, rosin or modified rosin and modified starch. Considering the addition of viscosity enhancers, both construction process requirements, and bonding performance, gluing joint performance requirements, sometimes a single thickener cannot meet the requirements, in recent years, most of the mixed thickener. For example, in the epoxy glue to add tert-butyl phenol aldehyde, but also to add modified rosin, in order to increase the initial adhesion. The amount of thickening agent is generally 1%-20% of the total amount.
Rheology and rheology agents
The additives that can improve the rheological properties of coatings are called rheology modifiers, also known as rheology agents. Generally speaking, rheology agents can improve the stability and printability of coatings and improve the quality of the coating film. For example, to prevent the precipitation of pigments and fillers in the storage process, to avoid the spattering and hanging of coatings in the painting process, and to improve the leveling performance of the coating film.
From a rheological point of view, rheological agents are also divided into thixotropic rheological agents and pseudoplastic rheological agents, the difference between the two is the speed of recovery of the system structure after the removal of the applied shear force. This property is the main influence on the flow and leveling of coatings. Pseudoplastic rheology agent has a very fast structure recovery speed, almost immediately after the removal of the applied shear to restore the structure of viscosity, and therefore conducive to anti-settling and anti-sagging coatings, but the amount of high will have a negative impact on the flow and leveling, and then affect the quality of the coating film, such as brush marks too heavy, poor atomization when spraying, etc.. Typical pseudoplastic rheology agents are fumed silica, soluble castor oil and polyolefin slurry.
Thixotropic rheology agent in the shear force after the removal of the applied can show real-time related to the structure of the recovery rate, used in coatings that can be satisfactory anti-sagging, and will not lose the flow and leveling, in the application of coatings better than pseudoplastic rheology agent. These rheology agents are mainly organic clays and hydrogenated castor oil-based organic waxes. Thixotropy is related to the shape of the filler, the larger the particle aspect ratio (aspect ratio), the smaller the size, the higher the thixotropic effect.
A few commonly used rheology agents
- Fumed silica Fumed silica is an early rheology agent, but the product now in use has improved in performance. Fumed silica is a solid powder, a collection of spherical particles containing hydroxyl groups on their molecules, which are able to adsorb water molecules and polar liquids. The spherical particles have a silanol group on their surface. When fumed silica is dispersed in a base solution, the silanol groups between adjacent spherical particles create a loose lattice due to hydrogen bonding, forming a three-dimensional network structure that produces a gelation effect and a high structural viscosity. When subjected to shear, the hydrogen bonding is very weak, the network structure is broken, the gelation disappears and the viscosity decreases. After the shear force is removed, the original resting shape is restored.
The amount of fumed silica in the paint depends on the final viscosity requirements and the viscosity of the paint before the fumed silica is not added, generally 0.5% to 3.0% (mass fraction) of the total paint.
Fumed silica is susceptible to the solvent of the coating when used and works best in non-polar solvents. In polar solvents the attraction between the molecules of the liquid and the silica particles increases, making it difficult to form a loose network structure. For this reason. Fumed silica is available abroad specifically for polar solvents, such as the Aerosol range from Degussa in Germany.
In coatings fumed silica can be used in anti-rust coatings, thick paste coatings and decorative coatings to increase viscosity, prevent pigment settling and improve film sagging. The disadvantage of fumed silica is that the viscosity and thixotropy tend to decrease during storage of the paint.
Organic bentonite Organic bentonite rheology agent appearance for the powder material, microscopically is attached to the clay flake pile, clay flakes on both sides are attached to a large number of organic long-chain compounds, after dispersion and activation, adjacent flakes on the edge of the hydroxyl group by water molecules linked, thus forming a thixotropic network structure, the appearance of the gel state. If there are no water molecules, a gel structure cannot be formed.
Organic bentonite is best made into a gel when in use, and the gel is put in during the pigment feeding phase of the paint production process. This means that the solvent or resin solution enters the capillary pores under the action of shear and wets the attached flake pile, causing the attached flake pile to depolymerize, when the viscosity of the system increases significantly. Adding the activator under shear conditions increases the distance between the flakes. Continued shear to fully disperse the flakes, that is, to obtain an activated thixotropic structure, that is, bentonite gel.
The most commonly used activators are alcohols of low relative molecular weight, such as methanol and ethanol. Ketones with low relative molecular weight, especially acetone, can also be used as activators, but their odor is greater and their flash point is lower, limiting their application in industrial coatings.
Domestic organic bentonite production technology is also relatively mature at present, with many models available. Tianjin organic clay factory production of TN064, TF4065 and Zhejiang Lin’an additives factory production of S01, more widely used in the country. The performance of these products is comparable to that of BENT0NE34 of the US NL company. The effect of making pre-gel is higher than the effect of adding dry powder directly.
- Hydrogenated Castor Oil Hydrogenated castor oil is a waxy solid produced by the hydrogenation of castor oil, which is then treated and used as a thixotropic agent in coatings, mainly for thickening, anti-settling and anti-sagging. It is a 12-hydroxystearate triglyceride with hydroxyl groups in the fatty acid chain and therefore shows some degree of polarity. It is able to swell and gel in non-polar solvents, and the weak hydrogen bonding between swollen particles due to the polar groups in the hydrogenated castor oil molecule forms a thixotropic network structure, improves pigment suspension, controls sagging without sacrificing flow and leveling, usually does not react with other components of the paint, has no adverse effect on the durability of the paint, does not yellow in the formulation, and gives storage stability and reproducibility.
Hydrogenated castor oil also requires an activation process when used. In the first stage of activation, the base material solution is used to disperse the hydrogenated castor oil rheological agent powder, the second stage of activation is under stirring conditions, the base material solution – hydrogenated castor oil powder mixture is heated to 43-53°C, the process needs to continue for 20-30min, so that the particles swell sufficiently. Then it is cooled to room temperature with stirring to obtain a rheological structure with stable thixotropic properties. During the activation process, temperature control is the main concern. If the maximum activation temperature is exceeded and insufficient stirring occurs during cooling, the hydrogenated castor oil will not form a thixotropic gel network and will precipitate “grains”. Likewise, low activation temperatures and insufficient activation time will also result in this situation. In the event of poor activation and “crystallization”, reactivation can be carried out according to the correct activation method.
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