23.04.2016
Conventional coatings are typically formulated using a polymeric coating resin dissolved in an organic solvent, which forms a film on the substrate by evaporation of the solvent after application. All of these technologies allow coatings to be cured in place using specialty acrylics esters — acrylate and methacrylate monomers and oligomers. The variety of available specialty acrylic esters enables formulators to improve the right performance properties for almost any application. Formulators of conventional coatings have been under continuous pressure from the EPA to decrease VOCs in their formulations.
The alkoxylation of monomers has brought a substantial improvement in safety and handling properties and has broadened the markets for radiation-curable coatings.
Monomer chemistry affects many other coating properties, such as resistance to abrasion, adhesion and heat. Oligomers Oligomers are higher-molecular-weight (1,000–30,000) cure-in-place coating components used as the base material that determine the major physical properties. Oligomers are based on a variety of chemistries including acrylated urethanes, epoxies, polyesters, and acrylics.
In EB curing, sufficient energy is produced from electron bombardment of the coating to generate free radicals without an initiator. The reaction mechanism of the free radical formation by photoinitiators is shown in Figure 9. The selection of the peroxide is dependent on the temperature at which the coating will be cured and the required shelf life of the coating.
An important consideration when formulating peroxide-cure coatings is that two competing cure mechanisms occur. Peroxide cure-in-place coatings using acrylic esters are currently used in high-performance automotive primers and topcoats, coatings for concrete bridge decks and industrial flooring, and specialty coatings for wood and paper.
Acrylate esters can be reacted with a number of amine curing agents such as aliphatic amines, cycloaliphatic amines, amidoamines and polyamides. Our September issue focuses on sustainable solutions, specialty chemicals, architectural coatings and the latest pigment technology. A highly corrosion and solvent resistant Vinyl Ester Epoxy Resin with heat and scratch-resistant microscopic quartz particles dispersed.
As the name implies, cure-in-place coatings are applied and immediately cured, or polymerized, on the substrate. Coatings varying from hard- and scratch-resistant to being flexible and exhibiting high elongation can be developed with today’s selection of specialty acrylic esters. Cure-in-place coatings containing acrylic esters not only offer decreased cure times, but also can be formulated to 100%-solid systems, eliminating the need for solvents altogether. One measure of a coating’s ability to withstand heat is its glass-transition temperature (Tg).


Figure 6 depicts the general structure of the two major types used in cure-in-place coatings — epoxy acrylates and urethane acrylates. In these systems, radicals are generated from decomposition of the initiator by heat or by reducing agents like metal driers at ambient temperature, as shown in Figure 10.
The key property is its half-life temperature or the temperature at which half of the radicals are generated in a certain period of time.
At the coating surface, where oxygen is present, aerobic cure takes place; below the surface, where no oxygen is present, anaerobic cure occurs.
Michael Addition Cure-In-Place Technology Acrylic esters can also be used as reactive diluents and modifiers in other cure-in-place coating technologies, such as two-component epoxy systems cured with polyamines.
The ultimate matte finish has outstanding scuff and scratch resistance and vastly improved heat deflection characteristics as well as UV inhibitors.
Methods of curing in place include free radical-initiated curing, such as in ultraviolet (UV) light or electron beam (EB) radiation systems, peroxide or azonitrile thermal systems, and Michael Addition polymerization reactions with polyamines. High Performance Cure-in-place technology generally uses acrylate- and methacrylate-terminated monomers and oligomers. Low VOCs Cure-in-place technology can include a variety of low-VOC materials, in addition to acrylate and methacrylate esters. Elimination of solvents is just one way these cure-in-place systems reduce VOCs; the very nature of the components used is another. Free-Radical Cure-In-Place Technology In free-radical cure-in-place technology, as shown in Figure 7, some type of initiator is required for monomer combination or addition to take place and for polymerization to occur.
Commonly used peroxides are methylethylketone peroxide, cumene hydroperoxide, t-butyl perbenzoate, and 1,1-di-(t-butylperoxy)-3,3,5-trimethylcyclohexane.
The anaerobic cure below the surface is much faster than the aerobic cure since oxygen is a good free-radical scavenger. In these systems, the amine-curing agent is more of a co-reactant than an initiator or catalyst.
The most effective amine curing agents are based on diethylenetriamine, triethylenetetraamine and tetraethylenepentaamine. Allyl derivatives, N-vinyl derivatives, low-molecular-weight epoxies, and acetoacetate derivatives are some examples.
The specialty acrylic monomers and especially higher-molecular-weight acrylic oligomers are already inherently low in VOCs.
The selection of peroxide determines whether the coating will be a single-component or a plural-component system. Therefore, the key to achieving uniform and complete cure is to eliminate oxygen at the surface either by mechanical means such as inerting the atmosphere with nitrogen, or by chemical means through the use of metal driers and monomers that are a good oxygen scavenger, such as allylic functional monomers. Acrylate esters can react with an amine through a Michael Addition reaction, as illustrated in Figure 11a.


Addition of an acrylate ester to an epoxy cure-in-place coating offers the following benefits.
The basic formulating differences between conventional coatings and coatings developed for cure-in-place technology are shown in Table 1.
Table 2 depicts the low volatility of various specialty monomers used in cure-in-place systems compared to other reactive diluents such as styrene and methyl methacrylate.Acrylates and methacrylates differ only slightly in structure, but the variation is enough to yield different performance properties. Monomers and oligomers with functionality greater than one will result in crosslink formation. The resulting secondary amine-acrylate adduct can then react with another acrylate ester or preferably react with the epoxy resin, ultimately forming a highly crosslinked polymer (see Figure 11b).
Polymerization takes place through addition across the C=C unsaturation in the acrylate or methacrylate group.
A comparison of the differences between TMPTA and TMPTMA, two of the most widely used monomers in UV curing and peroxide curing, respectively, is provided in Figure 2. Figure 8 depicts a more specific reaction using a difunctional acrylate, which results in a crosslinked polymer. These basic differences generally hold true for most other acrylate and methacrylate counterparts. Radiation Curing In UV-cured coating systems, a free radical-generating photoinitiator is required. Monomers Monomers are generally low-viscosity acrylates and methacrylates that function as reactive diluents, crosslinkers, and performance-property enhancers. Common photoinitiators include benzophenone, benzil dimethyl ketal and 2-hydroxy-2methyl-1-phenyl-1-propanone.
Mono-, di- and tri-functional monomers, shown schematically in Figure 3, are the most common, but functionalities as high as five are available to formulators.Figure 4 presents a few of the performance properties that can be obtained due to functionality differences. Low Skin Irritation First-generation monomers developed for UV and EB curing often were skin-irritants and not well suited for spray applications.
However, specialty acrylates have skin irritation levels of less than one on the Draize scale. In addition to lower skin irritation, the ethoxylation or propoxylation of acrylates also imparts other advantages, such as faster cure speed, better flexibility and higher impact resistance.



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