The selection, application and use skills and development trend of solvents in coatings

Introduction
The role of solvents in coatings is often not taken seriously, think it is volatile, and finally always evaporated without remaining in the coating film, so the quality of the coating film has little impact. In fact, the solubility of various solvents, volatile rate and other factors will greatly affect the oil coating production, storage, construction and coating film gloss, adhesion, surface status and other aspects of performance.
solvents account for about 30-50% of the volume in conventional liquid coatings,
in
. The main role of solvents in coatings:
1, dissolving, dispersing the resin in the coating, and regulating its viscosity and fluidity,
making it easy to apply;
3, improve the appearance of coating film, such as gloss, plumpness, etc.;
4, increase the wetting of the coating substrate, improve adhesion;
5, the composition of a reasonable volatilization rate to give the paint the best fluidity and leveling;
different resin series can only be dissolved in different active solvents, in the same oil coating formula, often using a variety of resins, so a variety of active solvents with better results.
principle: the most stable and optimal balance of performance between active solvents, while adding as many filled solvents as possible to optimize costs and regulate construction viscosity.
Although low VOC latex coatings, water-based coatings, UV photo-cured coatings and powder coatings have developed rapidly to meet increasingly demanding environmental requirements, solvent-based coatings still occupy a very important position in the field of coatings because of their performance and construction advantages. In order to use various types of solvents more rationally in order to achieve cost-performance balance, it is recommended that paint plants pay sufficient attention to the following points:
1, reasonable use of formulation principles
solvent-to-resin solubility, can be dissolved into a certain concentration of solution velocity, viscosity and true solvent tolerance to fill solvents (dilution ratio) and so on.
A, "similar solubility" principle
"similar solubility" principle is the classic theory to judge the solvent to the size of material solubility. That is, the solubility parameters are similar to the dissolving.
the dissolution process of polymers usually begins with the slow penetration of smaller solvent molecules into the polymer chain, and the rate at which polymer molecules dissolve into solvents is slower, so the dissolution of polymers is first manifested in the expansion of volume, i.e. the process of dissolution. Of course, the dissolution of polymers in coatings is a fairly complex process, because other ingredients in the coating, such as fillers, pigments and additives, affect the degree of polymer dissolution. In selecting the main active solvent, we should try to consider that the solubility parameters of the polymer and solvent solubility parameters are less than 3, and the strength of the hydrogen bond force is similar to the solvent.
B, fully study the volatile properties of solvents
in the coating film process, solvents from the coating film constantly waved away. The volatile properties of solvents have a great impact on the final properties of coatings, and inappropriate volatility rates can lead to coating defects such as flow hooks, pinholes, shrink holes and shrink edges.
temperature is an important factor affecting the rate of solvent volatilization, which accelerates as the temperature increases. This is also the reason for the summer more choice of volatile rate of slow solvents, such as hot-day coating plants will generally replace part of toluene with xylene.
the boiling point of the same type of solvent corresponds to its relative volatility rate, but the boiling point of different types of solvent has no correspondence with its relative volatility rate. For example, the boiling point of ethanol (79 degrees C) is slightly lower than that of benzene (80 degrees C), but the relative volatility rate of ethanol (1.6) is much lower than that of benzene (6.3); (117 degrees C) is slightly lower than the boiling point of butyl acetate (127 degrees C), but the relative volatility rate (0.4) of butanol is much lower than that of butyl acetate (1.0). This has nothing to do with strong hydrogen bonds in alcohol solvents.
the air flow of the applied surface is another important factor affecting the rate of solvent volatility. For specific construction conditions, the solvents in the coating must be selected. If sprayed with air, solvent volatility is significantly faster than airless spraying. In practice, a single solvent is rarely used to prepare coatings. In order to better control the volatile rate of solvents, improve the ability to dissolve and balance costs, more mixed solvent systems are used. When a mixed solvent system is used, the relative proportion of different solvents evaporated into the atmosphere is not the same as that of the original solvent, and the composition of the remaining solvents is constantly changing as the solvent continues to evaporate. This change has a great impact on the performance of the coating, such as the polymer's high solubility capacity parts (active solvent) relative to thinners, such as evaporation too fast may lead to the final dissolution capacity of the solvent is insufficient, so that some membrane diseases such as contraction, pinholes, whitening and other phenomena occur.
C, solvent viscosity
solvent on the polymer's solubility and solvent's own viscosity on the final viscosity of the coating has a great impact. In general, the stronger the solvent's solubility to the resin, the lower the viscosity of the resin solution formed.
in coating systems, the viscosity of the filling solvent is higher than that of the active solvent. Because in the active solvent, resin molecules can be fully extended in the solution, although the molecules are more entangled, but the molecular chain is more flexible; In the filling solvent, the polymer chain is less flexible, not easy to solve tangles, the performance of a significant increase in viscosity. In addition, many of the resins used in coatings are polar and contain groups of hydrogen bonds such as hydroxyl or ammonia. The presence of these groups makes resin molecules tend to bind to each other, increasing the viscosity of the solution. To reduce this tendency, adding hydrogen bond-receiving solvents such as ketones, ethers and alcohols can effectively reduce the viscosity of the system. Adding pigments and fillers also increases the viscosity of the coating to a considerable extent.
in the preparation of coatings, in order to meet the viscosity of the product construction requirements, should take into account the solvent viscosity and solubility, in order to achieve the best balance.
2. Reasonable selection of primary active and filler solvents
not all solvents are soluble polymers:
We refer to organic solvents that have the ability to dissolve polymers as active solvents (medium and strong hydrogen-bond solvents) and organic solvents that do not have or have only a weak solubility of polymers are called fill solvents.
characteristics of active solvents: most with polar groups, the general price is higher. There are mainly the following categories:
1. Ketones: acetone, butylone, methamphetamine, methyl isoclode and cyclosterone, etc.
2. Esters: ethyl acetate, butyl acetate and isopropyl acetate, etc
3. Alcohols: Ethanol, butanol, isopropyl alcohol, etc
4. Solvents with more than two erythropoies: ethylene glycol butyl ether (BCs) and propylene glycol methyl ether acetate (PMA).
, ketones, esters and alcohol ethers are also commonly referred to as oxygenated solvents. The so-called oxygenated solvent is a solvent containing oxygen atoms in a molecule. They provide a wide range of solubility and volatility, and many resins are not soluble in hydrocarbon solvents, but can be dissolved in oxygenated solvents.
solvents: because alcohols are hydroxyl, high polarity and strong hydrogen bonding. The relationship between nonpolar hydrocarbon chains and hydroxyl determines the solubility of alcohols. Low-grade alcohols have a strong solubility against strong polar resins such as aliclicylic resins, arbondehyde resins, nitride cotton, etc. The solubility of alcohol-to-polar resin decreases with the increase of hydrocarbon chain length, so advanced alcohol is not a solvent of polar resins such as nitrosized cotton, but the solubility performance of advanced alcohol is mild, very suitable for use as a solvent for surface coatings, because it does not soften the bottom coating and bite bottom phenomenon occurs. Especially in plastic coatings, alcohol solvents only dissolve the plastic surface, not soften the plastic. Enterprises that produce marine coatings and heavy anti-corrosion coatings use dissolving energy, with a relatively low volatility rate (0). 44-strong hydrogen bond action) of butanol as the main active solvent, ester solvents are currently mainly used in nitro coatings, such as nitrowood coatings, orthotinate acetate is widely used as an active solvent.
In fact, by its very nature, ester solvents can be used in more types of coatings, such as the main active solvents as acrylic-type (especially hydroxypropyl acrylic-type) plastic coatings more choice of esters (ethyl acetate, orthobutyl acetate). Ester solvents are less polar than alcohols, but have a very good solubility to polar resins. As the carbon chain in the alcohol and acid group in the ester increases, the solubility of the polar resin decreases, but the solubility of the low polar resin increases. For example, acetic butyl acetate: nitrite cotton, acrylic resin and acetic acid resin, etc. have a good solubility, it belongs to the medium volatile speed solvent, its volatility is high enough to evaporate rapidly from the coating film, low enough to prevent shrinking holes, whitening and disorderly flow. In the formulation, acetate is often used with aromatic solvents, because it has a lower viscosity, especially suitable for high-solid content coatings, it is also the most widely used solvent in polyurethane coatings; For example, butyl ketones are most commonly used: slightly less volatile than acetone, are wood coatings (nitro and polyurethane), acrylic coatings and vinyl coatings commonly used solvents. Because the volatility rate is too fast, acetone and butyl ketones are rarely used in marine coatings and heavy anti-corrosion coatings. The selection of diacerone alcohol as the main active solvent can make the coating film have good leveling on the basis of fully dissolved resin.
in the manufacture of oil coatings, in addition to active solvents, but also appropriately mixed with some filling solvents. Filled solvent characteristics:
most of the oil is non-polar alkanes and aromatic solvents (weak hydrogen bonds). There are mainly alkane solvents: such as de-aromatic hydrocarbon D-series, 120 s solvent oil and 200 s solvent oil; and aromatic solvents: e.g. toluene, xylene, mixed aromatic S-100, S-150 and S-200, etc. Alkane solvents are not soluble with many active solvents.
are generally insoluble in filling solvents, filling solvents are mainly used to reduce costs, adjust the viscosity of oil coatings for easy construction.
Such as xylene and mixed aromatic S-100 in marine coatings and heavy anti-corrosion coatings, as fill solvents can be fully mutually soluble with butanol and effectively reduce costs, of course, toluene and xylene remain the main filling solvents, complemented by a mixture of aromatic S-100 and S-150 to regulate viscosity and volatility Velocity, while in polyester resin-based coil coatings, ethylene glycol butyl ether with good comprehensive dissolving properties has been used as the main active solvent, due to toxicity and other reasons, is now partially replaced by diacetonol, such coatings generally choose mixed aromatic s-100 and S-150 as filling solvents.
As a filling solvent, we can not ignore the fatty hydrocarbon solvent, which represents the solvent is 200 s solvent oil, they are basically chemical inert, they are used in aolic acid resin coating can be effectively filled to reduce costs, but also aolic acid resin has a certain solubility. Solvent oils are widely used in wood coatings based on alicic acid resins. In order to make refined oil coatings have a lighter odor, some manufacturers take hydrogenation to remove some unsaturated hydrocarbons, in the country, hydrogenation 200, also known as 3 solvent oil.
2. Increasingly, compound solvents - solvents
for specific application requirements, different coatings must have different properties, such as automotive coatings, wood coatings. Causes different oil coating systems to use different polymers (resins). Such as acetic acid resin, acrylic resin, polyester resin, epoxy tree ester and polyurethane resin. Even aolic acid resins can be divided into coconut oil modified, flax oil modified and styrene modified and so on. Newly developed special resins often come with more than one characteristic group, such as polyurethane modified acrylic resin; A growing number of coatings are developing new performance coatings in formulations that use different tree esters in search of optimal performance balance. 1-2 main active solvents alone cannot effectively dissolve tree esters in the formulation. This is also the main reason why many solvents are required in coating manufacturing.
such as marine coatings and heavy anti-corrosion coatings will use some propylene glycol methyl ether and isopropyl alcohol as solvents;
in nitrowood coatings, propylene glycol methyl ether acetate, ketones (butylone, methyl isoclode) and some alcohols (positive) Iso butanol, isopropyl alcohol) is more used as a solvent in nitro coatings, we often add alcohols as a solvent or subsolution: alcohols can not dissolve nitrogen content of 11.7% - 12.2% of nitrosium cotton alone, but they have potential solubility. "Mixed solvents" have the same or even greater solubility when combined with active solvents such as esters and ketones in a certain proportion range. For example, the dilution value of ethyl acetate is 3.4, while the dilution ratio of half ethyl acetate plus half ethanol is also 3.4. The dilution value of toluene of butyl acetate is 2.7, while the dilution ratio of half butyl acetate plus half ethanol is as high as 2.95, and in most cases the price of ethanol is much lower than that of butyl acetate. Adding a small amount of butanol to aolic acid resin coating can significantly reduce viscosity; Adding a small amount of iso butanol to the coating effectively prevents the coating film from whitening, and also improves fluidity and gloss.
methyl isoclode (MIBK) as a medium boiling point solvent, can give nitro coating good fluidity and gloss, and can effectively reduce the viscosity of aolic acid resin coating, its addition will increase the amount of filled solvent, in addition, MIBK with alcohol and aromatic solvents can more effectively dissolve epoxy resin.
When the solvent varieties and dosing are selected properly, not only good quality, but also greatly reduce costs, in polyester resin-based coil coatings, solvents to choose more certain esters (butyl acetate) and alcohols (butanol) and certain ketones (butylone), as acrylic resin-type plastic coatings to help solvents with alcohol (butanol, isopropyl alcohol, positive propylene alcohol) and certain alcohols (ethyl ether), propylene glycol methyl ether), also due to toxicity and other reasons, the positive part of glycol butyl ether is replaced by diacetone alcohol and so on.
3. More consideration is given to the ratio balance between the active solvent and the filler solvent
using only the active solvent will result in a high cost and "too low" the viscosity of the oil coating due to the "too full" dissolved resin;
the dilution ratio---- an active solvent can tolerate the maximum number of filled solvents. Beyond this value, the solubility will be completely lost. When formulating a formula, full consideration must be given to the temperature of the construction environment, the effects of fillers, pigments and additives on solvent solubility, and the excess active solvent must be retained to ensure solubility. As the temperature increases, the dilution ratio decreases. This is also a reason why nitro coatings use more active solvents on hot days.
take nitro coatings as an example: < br