VOC Regulations and High Solid Acrylic Coatings
0 Introduction
Currently, there is a great deal of global concern about the earth’s environment and therefore many restrictions have been set to protect the environment, so that many products and raw materials have to face major innovations, the standard of living of human beings continues to improve while the deterioration of the natural world is becoming more and more serious. The main implementation steps to control environmental pollution are 66 rulemaking and CAA. For the paint industry, the main (actually the only) air quality related to volatile organic compounds (vOc). Increased consumption of coatings and solvents has resulted in fairly stringent regulations for vOc control, and environmental, economic, technological, and competitive pressures have created the need to develop less polluting but more economical coating systems.
VOC
In Europe, on March 11, 1999, the Steering Committee (Solvent Emissions Steering Committee SED) 1999/13/EC on “Limits on emissions of volatile organic compounds resulting from the use of organic solvents in certain practices and facilities”, which defines VOC as any organic compound with a vapor pressure of 0.01 Kpa or higher at 293.15 K, or any organic compound with a vapor pressure of 0.01 Kpa or higher at 293.15 K, or any organic compound with a vapor pressure of 0.01 Kpa or higher at 293.15 K. higher, or an organic compound that has such volatility under special conditions of use. Solvents and volatiles are not a permanent part of the coating, they volatilize and lose and pollute the environment due to photochemical reactions with nitrogen oxides in the presence of ultraviolet light (sunlight) to form fumes in the atmosphere that contain ozone as well as aldehydes (mainly), ethyl nitrate peroxide and small particles, collectively referred to as oxidants, which all have a negative effect on health. Ozone damages the lungs, irritates the eyes and may reduce immunity and is a potential greenhouse gas. The resulting smog can last for days and travel long distances. The costs of air pollution due to voc emissions include negative impacts on human health, reduced crop yields, accelerated aging of raw materials, and destruction of ecosystems. The reaction steps are described in more detail below.
It has been reported that the paint industry is responsible for 38% of the VOC emissions in Europe, where in 1995 about 3 million tons of paints were produced for a wide range of applications. This resulted in a staggering 100,000 tons of VOCs being produced in a single year.
Increasingly Stringent Environmental Regulations Promote the Development of Environmentally
Friendly Coatings
In Europe, different restrictive regulations have been established at different times, and environmental requirements are becoming increasingly stringent. The coatings community is facing major challenges. Increased demands for product-specific properties and VOC pressures have accelerated the development of new technologies that can meet customer requirements, and the response to these environmental control regulations has been so strong that resin manufacturers have expended considerable effort to meet the regulatory needs in the coatings industry. A number of low or non-polluting raw materials for coatings have been developed and introduced into the market to maintain the ecological balance. They are
1) High solids solvent based coatings
2)Solvent free or 100% solids based coatings
3)Solvent-free liquid coatings
4)Electrophoretic coatings
5) radiation curing coatings
6)Water-based coatings
7)powder coatings
8) non-aqueous dispersion (NAD) coatings.
They replaced traditional solvent-based coatings to meet increasingly stringent governmental restrictions to avoid pollution of the environment (Fig. 1)
High-solids coatings of the alkyd, vinyl compound, acrylate, polyester, epoxy, or polyurethane type, typically containing more than 70% solids, have been developed to meet pollution restrictions and reduce energy consumption. Despite significant advances in waterborne systems and powder coatings, solvents are still indispensable for many applications. Thermoset acrylic resins are discussed below because the thermoset acrylic resins available today not only have the typical acrylic resin properties, but also have the ability to be used in a wide variety of applications.
Thermoset acrylics are discussed below because they are now available with not only typical acrylic properties, but also higher work solids and better chemical resistance than thermoplastic acrylics.
3 Thermoset Acrylics
Acrylics are commonly used in coatings as a Class II resin due to their unique chemistry, large selection of monomers, and the ability to be designed with a variety of polymerization methods and flexibility.
properties. The ability to obtain different copolymers by using two or more monomers allows for new products with unique and useful properties, which depend on the composition and ratio of the monomers, as well as the method of preparation.
The acquisition of properties depends on the composition and ratio of monomers, as well as on the method of preparation. In the development of high solids and high performance coating systems, it is necessary to reduce the relative molecular mass of the base material and to develop and build a zwitterionic system. Critically, thermoset acrylics should be a heteropolymer in which one component provides stiffness, another provides flexibility, and a third component contains reactive functional groups that can be used for cross-linking. Other monomers can sometimes be introduced into the polymer backbone to enhance certain specific properties. They may be self-reactive (e.g., resins containing glycidyl and hydroxymethyl groups) or potentially reactive (e.g., resins containing carboxyl and hydroxyl groups).
Acrylic polymers can be synthesized by native polymerization, solution polymerization, suspension polymerization and emulsion polymerization. In the suspension polymerization method, water is used as the solvent, and the polymerization temperature should not exceed 100°C. However, the development of low-solvent coatings requires resins with very low viscosities so that the reaction temperature should be above 100°C. Therefore, the production of low-viscosity polymers is carried out by the propriety and solution polymerization methods.
Therefore, both propriety and solution polymerization are used to produce low-viscosity polymers. The main parameters that determine the viscosity of this base are the relative molecular mass and the relative molecular mass distribution. The approximate solids distribution at construction viscosity for different relative molecular masses is shown in Table 1.
3.1 Low Viscosity Acrylic Resins
A narrow relative molecular mass distribution favors low viscosity, however, film properties improve with increasing relative molecular mass. A low molecular base system that crosslinks after application is used in all high solids coatings.
The system is used in high solids coatings that can crosslink after application. During application, the paint contains low viscosity zwitterions that form higher molecular weight polymers after crosslinking and curing.
Further possible measures to reduce viscosity include the selection of low viscosity solvents or solvent mixtures with specific intermolecular reactions between the base materials and no significant interactions with the resin. The choice of solvent is influenced by chemical properties, boiling point, evaporation rate, price, flash point, toxicity, solubility, viscosity and chain transfer efficiency. High-solids coatings can be further improved in solids if the solvent selected solubilizes more of the relatively low molecular weight polymer at the same viscosity. The monomers used in thermoset acrylic copolymers and the co-reactants that can affect cross-linking are shown in Table 2.
Knowledge of the properties of the monomers involved, Tg, functional group concentration, crosslink density, and knowledge gained from studying the composition of a single copolymer allows selection of a formulation that will give the best overall performance. The main factors affecting viscosity (related to the acrylic resin itself) at a given temperature and at the same solids are as follows:
Relative molecular mass
The mass average relative molecular mass (Mw) of the base material is a fundamental parameter in the design of coatings. The viscosity (rl) of the base material solution has been studied,
average relative molecular mass (Ma) and solids (SC).
The relationship between viscosity (rl), average relative molecular mass (Ma) and solids (SC) has been studied Log rl = KMaXSC.
Relative molecular mass distribution
For acrylic resins, the relative molecular mass distribution is highly dependent on the relative molecular mass itself. If the relative molecular mass is increased, the relative molecular mass distribution is also increased, so it is difficult to study the real effect of the relative molecular mass distribution on the viscosity.
Therefore, it is difficult to investigate the real effect of relative molecular mass distribution on viscosity.
Base Functionality and Concentration
The viscosity of the base material increases with the concentration of the functional group. The properties of the functional groups largely determine this increase in viscosity. It is well known that carboxyl and hydroxyl groups increase the viscosity of the base material. This phenomenon is undoubtedly due to hydrogen bonding which improves intermolecular forces and solubility of the base material.
Possible ways to reduce viscosity
In order to reduce the viscosity 3 main base material parameters were studied.
1) Reduction of the relative molecular mass, but only to a limited extent, since too low a relative molecular mass leads to poor drying properties, hardness and chemical resistance of the final coating film. The relative molecular mass distribution must be kept as narrow as possible (a high relative molecular mass fraction increases the viscosity).
2) Base materials for clear coats for automotive paints usually contain hydroxyl groups for crosslinking of the coating film. The concentration of these hydroxyl groups increases the viscosity of the base material, but cannot be reduced because it decreases the cross-linking density and thus affects the film properties.
3) Some other parameters can also affect the acrylic resin viscosity, such as the solvent used. In order to reduce the viscosity, the recommendation should be able to improve the acrylic resin and solvent miscibility between the species
Reduce the viscosity of the method
Introduction of reactive diluents: Reactive diluents are very low viscosity co-reactants specifically designed to reduce the VOC of coating systems. They typically provide similar ability to reduce the viscosity of the polymerized components as solvents, but must also be able to react with the polymer matrix to effectively reduce VOCs. There are a variety of chemistries of low-viscosity co-reactants or reactive diluents, which include
1)Low molecular esters of urethane diols.
2)Ketoimines/aldehydeimines.
3)Azolines.
Small molecule polyols such as trimethylolpropane appear to work as active diluents, but are actually limited in solubility and viscosity; more useful are caprolactone-modified polyols, which reduce VOCs, but at the expense of film hardness. The form of active diluent usually contains closed or site-blocked active hydrogen. Aldehydes and ketoimides are examples of closed amino compounds. Azolines are examples of closed amino alcohols. Aldehyde imines and site-blocking amines typically have very low viscosities (less than 500 mpa-s at 100% solids), however, their reactivity with isocyanates is too high and thus their service life is too short. Even so, they are very attractive where very fast curing is required. Ketimines and azolidines also have very low characteristic viscosities, however, unlike aldimines and site-blocking amines, their reactivity with isocyanates in systems without catalysts is quite low (see Figure 2).
Vijay, Swarup, Albert Yezrielev et al. have developed a reactive diluent, the alcohol phenol ester PHEA, which improves the hardness, chemical resistance, and weatherability of compositions rather than degrading the f’s. These properties are shown in Fig. 3, where PHEA differs from other reactive diluents due to the formation of butyl cyclic cross-links (benzoxazine rings) in the phenol group.
The benzoxazine cross-linking improves the chemical stability of the crosslinked network, weatherability, and stiffness.PHEA can be used in a variety of formulations, including traditional high solids coatings.
(1) A recent successful approach is the use of acetoacetate functional compounds, which have better weathering properties than polyols, lower hydrogen bonding and thus significantly lower viscosity, and coatings that cure quickly at room temperature. The reaction is carried out at room temperature in the presence of an alkali catalyst to obtain mixtures of mono- or di-adducts (e.g., FIG. 4).
(2) Introduction of cyclic monomers into the polymer such as isobornyl methacrylate, cyclohexyl methacrylate, methyl
tert-butyl cyclohexyl methacrylate, etc., the viscosity is reduced at a certain Tg, Mw, functionality, and solid content, as described by Zezza.C.A. et al. Thus, new types of acrylic polyols based on macromonomers with high Tg do offer better performance and low VOC’s
(3) Introduction of “CarduraE10”: Cardura E10 can be introduced into acrylic resins by the reaction of its epoxy group with any monomer containing nucleophilic groups, providing low viscosity, good reactivity, attractive coating film mechanical properties, high coating film gloss, and excellent weathering resistance.Slinckx.M., et al. and Andre. L.P. et al. have elaborated on the use of CarduraE10 for VOC reduction.The structure of CarduraE10 and its preparation is shown in Figure 6.
Low Relative Molecular Mass Acrylic Resins
Low relative molecular mass acrylic resins can be prepared by increasing the concentration of free radicals produced during the preparation/polymerization of solvent-based acrylic resins.
This can be obtained by the following methods:
1) Increasing the polymerization temperature
Increasing the temperature at which the acrylic resin is prepared for a certain monomer composition can reduce the average relative molecular mass, relative molecular mass distribution and viscosity. However, care should be taken not to polymerize at too high a temperature as this may cause yellowing of the resin, which is unacceptable for use as a varnish. The advantage of high processing temperatures is not only due to the accelerated decomposition of the initiator, but also to the increased chain transfer of the solvent.
2) Increasing the amount of initiator utilized (4% or more)
Increasing the initiator concentration is the classical method of reducing the relative molecular mass, but too much dosage leads to a large number of
However, too much can lead to a large number of decomposition products, affecting durability and odor, and increasing the price.
3) Selection of initiators with lower half-life temperatures
Generally, the polymerization of acrylic monomers in solvents uses organic peroxides as free radical initiators. Generally, free radicals can be categorized into long-lived, stable (slightly less reactive) and short-lived, unstable (reactive) free radicals based on their duration of existence. Thus the chemical structure of the peroxide has a great influence on the rate of free radical generation.
The properties of the free radicals themselves affect the linear and relative molecular mass distribution of the polymer. Peroxides that require less decomposition energy can produce more radicals at the same temperature resulting in a lower relative molecular mass. Comparison of different initiators revealed that 2,2-azobis(methyl isobutyrate) is an effective initiator for the preparation of resins for acrylic coatings, yielding polymer solutions with better flexibility, lower relative molecular mass, narrower distribution, and reduced volatiles.
Callais.P.A. et al. described the use of tertiary amyl peroxide and initiator mixtures to obtain high quality acrylic resins as compared to conventional organic peroxides and azo initiators.Ginger G.Myers compared the price/performance ratios of the initiators and found that DTBP (mono-tert-butyl peroxide), TBPB (tert-butyl peroxybenzoate), BPO (dibenzoate peroxybenzoate), BPO (diphenyl peroxybenzoate), and BPO (diphenyl peroxybenzoate) are effective initiators for polymer solutions with better flexibility, lower relative molecular mass, narrower distribution, and reduced volatiles, BPO (benzoyl peroxide), TBPA (tert-butyl peroxyacetate) and TBPEH (tert-butyl peroxy-2-ethylhexyl) were found to be more efficient than tert-amyl. This is due to the fact that the slight improvement in performance with tertiary amyl is not sufficient to offset the price difference compared to its tert-butyl counterpart. It can be said that the choice of initiator depends on the price, the decomposition rate at processing temperature and the ability to capture hydrogen. The less hydrogen capture, the less branched the resin is and the less viscous it is. Therefore, the performance of the initiator can affect the relative molecular weight, viscosity and dispersibility of the final resin.
4) Reduction of monomer drop addition time
The relative molecular weight of the zwitterion can be controlled by reducing the time of monomer dropwise addition to the reactor.
5) Other methods
Other methods include controlled radical polymerization, which is currently undergoing practical research in academia and industry. This work is based on several techniques: nitrogen oxide mediated polymerization, atom transfer radical polymerization, reversible addition cleavage transfer and catalytic chain transfer, with the aim of controlling the polymer structure, relative molecular mass and dispersion. Previously, mercaptans were used to control the relative molecular mass, but high dosages were required to obtain low relative molecular masses, and high dosages of mercaptans resulted in odor problems, low conversion rates, and poor durability of the coatings when exposed to the outdoors. Therefore, it is necessary to use thioglycolic acid, allyl thiols, styrene ethers, etc. and square planar cobalt compounds as effective catalytic chain transfer agents. The use of various cobalt oximes is described below. In the synthesis of acrylates and styrenes, it improves both oxidizing power and hydrolytic stability as well as catalytic chain transfer properties compared to earlier catalysts. The use of cobalt oxime boron fluoride (CoBF) in solution and suspension polymerization has been well documented.
This catalytic chain transfer agent is effective at very low concentrations and can be used to synthesize acrylic polyols with a narrow relative molecular mass distribution. The mechanism of chain transfer activity also facilitates the preparation of macromolecular monomers that can be used in the synthesis of graft copolymers.
The use of ring-opening condensation polymers allows the preparation of zwitterions with narrow relative molecular mass distributions and controlled functionality and compositional distributions. These “star” shaped zwitterions are more efficient than acrylic polyols in terms of dryness and V
