- Overview
Epoxy resins have good overall mechanical properties, high adhesion, low shrinkage, good stability, and excellent electrical insulation properties. They are widely used as coatings, adhesives, composite resin matrix, electronic encapsulation materials, etc. in mechanical, electronic, electrical, aerospace, aviation, coating, bonding and other fields. However, due to the high cross-link density and high internal stress of the cured epoxy resin, there are disadvantages such as brittle quality, fatigue resistance, heat resistance and poor impact toughness, which make it difficult to meet the requirements of engineering technology, so that its application is somewhat limited, especially restricting the epoxy resin can not be well used for structural materials and other types of composite materials. For this reason, scholars at home and abroad have conducted a lot of modification research on epoxy resins, the most important of which is to improve the brittleness and humidity and heat resistance of epoxy resins.
Epoxy resins can be modified by chemical methods and physical methods. Chemical modification is mainly to synthesize new structure of epoxy resin and new structure of curing agent; and physical modification is mainly to improve the performance by forming blended structure with modifier. Comparing the two methods, the first method is inferior to the second method in terms of process, cost and difficulty. Therefore, the modification of epoxy resins is mainly achieved through the blending structure.
There are three main ways to toughen epoxy resins: (1) Toughening by forming two-phase structures with rigid inorganic fillers, rubber elastomers and thermoplastic polymers. (2) Toughening modification by continuous penetration of thermoplastics into the epoxy resin network to form semi-interpenetrating network type polymers. (3) The chemical composition of the cross-linked network is changed (e.g., the introduction of “flexible segments” in the cross-linked network) to improve the activity of the cross-linked network.
The improvement of the humidity and heat resistance of epoxy resins is mainly through the introduction of structural units containing thick rings in epoxy resin molecules and the synthesis of fluorine-containing epoxy resins, as well as the adoption of new curing agents instead of traditional DDS.
The modified epoxy resins will further expand the applications of epoxy resins in electrical products, composite stress members and high performance structural adhesives due to the improvement of humidity and heat resistance and toughness.
On the other hand, although epoxy resins have good processability, the operation process needs to be improved appropriately for different applications. For example, the viscosity of diphenol propane epoxy resin is large, which leads to poor processability in some operations. So it is necessary to add diluent to the curing system to reduce the viscosity and improve the operating process performance. Therefore, in order to meet different applications, different additives such as diluents, 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 it is necessary to reduce the viscosity, or increase the viscosity, or give thixotropy, the matching material that can meet these requirements is called liquidity adjuster.
- Thinners
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 adhesive. In addition, the thinner also plays a role in extending the service life. But the addition of thinners will also reduce the heat deflection temperature, bonding strength, media resistance and aging resistance of the cured resin. However, in order to make the resin glue easy to infiltrate the surface of the adhesive, improve its infiltration ability and wetting ability, and facilitate the operation, it is necessary to add the right amount of diluent.
There are many ways to classify the diluent, according to its use mechanism, can be divided into two categories of inactive diluent and active diluent.
(1) Non-active diluent
Non-active diluent does not react with epoxy resin, curing agent, etc., and is purely physically mixed into the resin. It is only a mechanical mixture with resin, dilute and reduce the viscosity of the liquid. It is mostly evaporated during the curing process of the glue. And it will leave pores to the resin curing material, so that the shrinkage relatively increased. Therefore, the adverse effect of non-active diluent on the cured resin performance is greater than the effect of active diluent, but it can improve the toughness of the resin a little. When the use of high requirements can not use inactive diluent, should be selected to use active diluent.
Non-active diluents are mostly high-boiling point liquids, such as dibutyl phthalate, dioctyl benzodicarboxylate, styrene, diallyl benzodicarboxylate, toluene, xylene, etc.. The amount of 5% to 20% is appropriate. About 12% of dibutyl phthalate makes the viscosity of standard epoxy resin drop from 10Pa-s to 0.5~0.7Pa-s (25°C). Some industrial epoxy resins (2.0~4.0Pa-s) contain dibutyl phthalate as an inactive diluent. Solvent is also used as a non-reactive diluent, but it has a negative effect on chemical resistance. In large quantities, the performance of the cured material deteriorates while the shrinkage increases due to the volatilization of the diluent in the curing process.
(2) Active diluents
Active diluents generally refer to low molecular compounds with one or more epoxy groups. They can directly participate in the curing reaction of epoxy resin and become part of the cross-linked network structure of epoxy resin curing products, which has almost no effect on the performance of curing products and sometimes can increase the toughness of curing system. The reactive diluents are divided into single epoxy reactive diluents and multi-epoxy reactive diluents. Some single epoxy diluent, such as acryl glycidyl ether, butyl glycidyl ether and phenyl glycidyl ether, for amine curing agent reaction activity is greater. Some olefins or alicyclic single epoxy diluent for anhydride curing agent reaction activity is greater. Therefore, when using active diluent, the amount and variety of curing agent should be adjusted accordingly.
In solvent-free epoxy coatings, the amount of single functional active diluent does not exceed 15% of the epoxy resin, and the amount of multi-gong active diluent can reach 20%-25%. But the dosage is too much, it will reduce the performance of the coating film. For example, the weight gain of bisphenol A epoxy coating without active diluent and bisphenol A epoxy coating with well 501 (propylene oxide butyl ether) active diluent is 2.11% and 4.18% respectively after 30 days of soaking in 10% sulfuric acid aqueous solution, and 2.67% and 13.5% respectively after 30 days of soaking in methanol.
Active diluents are generally toxic, and care must be taken in the process of use. Long-term contact often causes skin allergy and even ulceration in serious cases.
The dilution effect of mono-epoxide is better, and aliphatic type has better dilution effect than aromatic type. The use of aromatic type active diluent curing products of acid and alkali resistance does not change much, but solvent resistance is reduced.
The use of mono-epoxide reactive diluent will reduce the heat deflection temperature, which is due to its use will make the cured product crosslink density decreases. The use of long carbon chain active diluent can make the flexural strength, impact toughness can be improved. No effect on the hardness of the cured product when used in small amounts, while the coefficient of thermal expansion increases.
The use of two or three epoxides as diluent, the amount and curing method is appropriate, it will not reduce the crosslink density, so the thermal state of the mechanical strength and chemical resistance retention rate is higher. Compared with monocyclic oxides, the dilution effect is worse, and the amount of addition required to reduce the resin viscosity to the same level is larger.
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.
(3) The choice of diluent principles
1) Try to use active diluents to improve the processability and improve the adhesive and mechanical properties.
2) Choose those diluents with similar chemical structure to the main resin, because they will participate in the reaction with the main resin in the presence of other additives, and greatly improve the performance of the adhesive layer. Such as in l00 parts of bisphenol A epoxy resin as the main component of the rubber, adding 20 parts of butylene glycol double epoxy diluent, and mixed amine as a curing agent, can make its rubber layer elongation of 7.0%, while the tensile strength is still maintained at more than 29MPa, and heat deflection temperature of 120 °C.
3) Attention should be paid to the use of small volatile, small odor (odor), toxicity as low as possible, in order to reduce the diluent in the dispensing of glue, glue on the human body. Because most active or inactive diluents have odor and have low toxicity.
4) Many important factors should be considered, such as easy to source, non-flammable and non-explosive, low price, etc.. Therefore, as long as the water for the diluent should be used as much as possible.
- The most suitable amount of addition should be selected through experiment and theory.
- Solvent and Thickener
Solvent
The main difference between solvents and inactive thinners is that solvents mainly play the role of dissolving resin system, of course, they can also adjust the viscosity of the glue. But the main role of non-active diluent is to adjust the viscosity of the coating, it may have solubility to the resin system or may not have solubility.
The addition of solvents makes the adhesive more convenient for construction, and can be cured at room temperature, so that the adhesive viscosity is low, and easy to wet the surface of the object to be stuck, good workmanship, etc.. However, the addition of solvents also cause the adhesive in the curing volume shrinkage rate, solvents sometimes make the surface of the object to be swollen, resulting in poor bonding, and most of the solvent volatile and flammable, with a certain degree of toxicity and other deficiencies. Therefore, in the configuration of adhesives should pay attention to the choice of use.
Adhesive solvent selection, first of all, consider its dissolution performance of the main resin, followed by its evaporation rate, because only the appropriate evaporation rate to match the good performance of adhesives and coatings. Secondly, we should consider the viscosity, flash point and flammability of the solvent. For safety considerations, should use higher flash point of alcohols, ether alcohols and esters, with propylene glycol ether instead of glycol ether to reduce toxicity, and finally to consider the smell, toxicity, ease of source and price.
Epoxy resin can be dissolved in some organic solvents, and the solubility of resin decreases with the increase of 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 aromatic hydrocarbons and alcohols are mixed, 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 the cost, improve the performance of paint film and construction performance, and increase the solvency power of solvents. Brush construction of the product should use part of the high boiling point solvent, such as ethyl solubilizer.
What should also be noted when choosing solvents is that solvents of different structures will play different roles in the curing reaction. For example, amine curing epoxy resin can not use ester as solvent, because the ester and amine curing agent has a reaction, destroy the curing agent, reduce the curing effect. When Louis acid is used as curing agent, if cyclohexanone is used as solvent, ketone shrinkage will occur, and this reaction will affect the performance of the cured coating film, especially under baking conditions. Therefore, the use of ketones and ester solvents should be very careful.
Thickeners
Thickeners in the composition of adhesives is a relatively new class of matching additives. After 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 adhesion, and improve its wettability of the surface of the object to be stuck.
Most of the thickeners are not very large molecular weight of resin substances, in the selection of the following principles:
(1) Have good compatibility with epoxy resin, after mixing, can work together with epoxy resin stably for a long time, no precipitation, no delamination. (2) It has the best thickening effect, and has a fairly high adhesion force to the surface of the adhered object. (3) The source is abundant, cheap and preferably non-dangerous to facilitate storage and transportation.
At present, the thickeners used in epoxy adhesives are mainly organosilane compounds, alkylphenolic 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 can not meet the requirements, most of the use of mixed thickeners in recent years. For example, in the epoxy glue to add tert-butyl phenol aldehyde, but also add modified rosin, in order to increase the initial adhesion. The amount of thickener is generally 1%-20% of the total.
- Rheology and rheological agents
(1)Introduction
The additives that can improve the rheological properties of coatings are called rheology modifiers, also known as rheology agents. Generally speaking, rheological agents can improve the stability and paintability of coatings and improve the quality of the coating film. For example, to prevent the precipitation of color and filler during the storage of paint, to avoid the spattering and hanging of paint during the coating process, and to improve the leveling performance of the coating film.
From the 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 characteristic is the main influence of the flow and leveling of the coating. Pseudoplastic rheology agent has a very fast structure recovery speed, almost immediately after the removal of the applied shear force to restore the structure of viscosity, and is therefore conducive to anti-settling and anti-sagging coatings. However, high dosage will have a negative impact on flow and leveling, and then affect the quality of the coating film, such as excessive brush marks, poor atomization when spraying, etc.. Typical pseudoplastic rheology agents are fumed silica, soluble castor oil and polyolefin slurry, etc.
Thixotropic rheology agents can show real-time relevant structural recovery speed after the removal of the applied shear force, which can be used in coatings that can get satisfactory anti-sagging, and will not lose flow and leveling, the application of coatings better than pseudoplastic rheology agents. These rheology agents are mainly organic clay and hydrogenated castor oil-based organic waxes. Thixotropy is related to the shape of the filler, the larger the particle aspect ratio (aspectratio), the smaller the size, the higher the thixotropic effect.
(2) Several commonly used rheological agents
1) Fumed silica is an earlier used rheology agent, but the product used now has improved in performance. Fumed silica is a solid powder, is a collection of spherical particles, its molecules contain hydroxyl groups, able to adsorb water molecules and polar liquids. When fumed silica is dispersed in the base material solution, the silanol groups between adjacent spherical particles produce a loose lattice due to hydrogen bonding, forming a three-dimensional network structure that produces a gel effect and a high structural viscosity. When subjected to shear force, the network structure is broken due to the weak hydrogen bonding, 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 addition of fumed silica, generally 0.5% to 3.0% (mass fraction) of the total paint.
Fumed silica is susceptible to the influence of coating solvents 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.
In coatings, fumed silica can be used in antirust 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 in the storage of coatings.
2) Organic bentonite rheology agent appearance for the powder material, microscopically is attached to the clay flake pile. Both sides of the clay flakes are attached to a large number of organic long-chain compounds, after dispersion and activation, the hydroxyl groups on the edges of adjacent flakes are linked by water molecules. Thus, a thixotropic network structure is formed, and the appearance becomes a gel state. If there is no water molecule, the gel structure cannot be formed.
Organic bentonite is best to be made into a gel when it is used, and the gel is put into the pigment feeding stage in the production process of coating. In other words, the solvent or resin solution enters the capillary pores under the action of shear force and wets the attached flake pile, so that the attached flake pile is depolymerized, and then the viscosity of the system increases significantly. Under shear conditions, the activator is added to increase the distance between the flakes. Continue to shear the flakes fully dispersed, that is, to obtain the activated thixotropic structure, that is, bentonite gel.
The most commonly used activators are alcohols with low relative molecular weights, such as methanol and ethanol. Low relative molecular weight of ketones, especially acetone, can also be used as an activator, but its odor is larger, the flash point is lower, limiting its application in industrial coatings.
3) Hydrogenated castor oil is a waxy solid made by hydrogenation of castor oil, which can be used as thixotropic agent for coatings after treatment, mainly for thickening, anti-sinking and anti-sagging. It is a 12-hydroxystearate triglyceride with hydroxyl groups in the fatty acid chain, thus showing some degree of polarity. It is able to swell and gel in non-polar solvents, and weak hydrogen bonding between swollen particles due to the polar groups in the hydrogenated castor oil molecule, forming a thixotropic network structure. It improves pigment suspension and controls sagging without sacrificing flow and leveling properties. It usually does not react with other components of the paint, has no adverse effect on the durability of the paint, does not yellow, and gives storage stability and reproducibility.
Hydrogenated castor oil also requires activation treatment when used. In the first stage of activation, the hydrogenated castor oil rheological agent powder is first dispersed with the base material solution. Activation of the second stage is in the condition of stirring, the base material solution – hydrogenated castor oil powder mixture to 43 ~ 53 °C, and the process needs to continue for 20 ~ 30min, so that the particles swell fully. Then it is cooled to room temperature under stirring condition to obtain a rheological structure with stable thixotropic properties. During the activation process, the temperature control is the main thing. If the maximum activation temperature is exceeded and insufficient stirring is done during cooling, the hydrogenated castor oil will not form a thixotropic gel network and “grains” will be precipitated. Likewise, low activation temperatures and insufficient activation time will also result in this situation. In case of poor activation and “grain”, reactivation can be done according to the correct activation method.
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