Radiation Cure Coatings – The Ultimate in Rapid Cure Technology

Radiation cure 涂料 offer a high-speed light curing process with a number of advantages over more conventional cure processes. Multiple advantages include 高速, lower energy requirements, 小或挥发, less production space, 减少灰尘收集, 高质量的完成, rapid processing as well as instant on-off with some UV light technologies also expedite production 而且 energy savings. 电子和紫外线 可治愈的 油漆 finishes have existed since the 1960s 而且 are based on polymerization reactions including free radical 而且 cation-initiated chain-growth polymerization. As the majority of 涂料 for UV cure 涂层 utilize free radical polymerization (>90% of market), this article will focus primarily on free radical polymerization initiated by a 的光 (图. 1):

UV cure polymerization - Radiation Cure Coatings

­The types of unsaturation used in UV/EB cure 涂料 are provided in Table I, with by far the largest type being 丙烯酸酯.

Table I – Type of Unsaturation used in UV/EB Cure

的光 considerations primarily include two different characteristics of the 的光’s absorption curve. 第一个, is the maximum wavelength (Lambda Max) of light that is absorbed by the PI 而且 second, the strength of this absorption (molar extinction coefficient). 的光 developed for curing pigmented films normally have higher molar extinction coefficients at longer wavelengths between 300 nm to 450 nm than those for curing clear formulations. To maximize cure 而且 efficiency, the PI’s absorbance must match the light output of the lamp as different lamps have different spectral outputs (see Table I). Longer wavelength light is also essential to enhance cure in thicker 涂料. Newer PI’s have also enabled the formulation of pigmented 涂料 in addition to that of clear 涂料. The general cure considerations influenced by color, 聚氯乙烯, pigment particle size 而且 film thickness are summarized in Fig. 2:

factors influencing cure behavior - radiation cures
Figure 2 – UV Cure Considerations. Image: Ciba – Geigy literature

There are two main types of free radical 的光s, Type I 而且 Type II. Type I 的光s undergo cleavage upon irradiation to form two free radicals. Normally only one of these free radicals is reactive 而且 thus initiates polymerization. 1-hydroxy-cyclohexylphenyl-ketone is a widely used Type I PI. Type II 的光s form an excited state upon irradiation 而且 abstract an atom or electron from a donor molecule (synergist). The donor molecule in turn initiates polymerization. An example of a widely used Type II 的光 is 苯甲酮. 三级  are typically used as synergists as they react with 苯甲酮, 而且 also retard the inhibition of polymerization by oxygen. Acrylated tertiary amine compounds are used when odor 而且 extractables are of concern. Oxygen can also inhibit cure especially in thin films; to counteract oxygen inhibition, 涂料 can use amine synergists, be cured under a nitrogen atmosphere, employ the addition of wax, high initiator concentration, more intense UV Light, 而且/or surface-active initiators.

Other key ingredients that determine the performance of UV Cure formulations include UV 可治愈的 单体 而且 低聚物s. 图3 illustrates typical 单体 that are used along with performance characteristics.

单体 - Radiation Cure Coatings

表二世. General Performance Versus Monomer

general performance versus monomer - Radiation Cure Coatings
图3. Typical Monomers 而且 Performance Characteristics

There are a number of UV 可治愈的 低聚物 types available as well depending on the type of performance desired, please refer to Fig. 4 for a listing of some of the common 低聚物 types available along with an overview of performance characteristics.

Electron Beam cure 涂料 can be used to cure 丙烯酸酯 functional 涂料. As the energy used is much higher (150 – 300 keV) than that in UV cure, a 的光 is not necessary. Other advantages of EB cure over UV is that pigments do not adversely effect the cure. The vehicle systems that are used are essentially the same as that used in UV curing (丙烯酸酯s) 而且 vehicles used for UV cationic cure. Disadvantages of EB curing include the high cost of capital equipment 而且 curing must be done in an inert environment. The 涂层 树脂 once irradiated forms a radical cation 而且 a secondary electron the excited state CR* can then homolytically cleave to form a free radical 而且 initiate polymerization.

Radiation Cure Coatings
低聚物 types - Radiation Cure Coatings
Figure 4 – UV Cure Oligomer Types/Characteristics

除了 to 100% solid liquid UV 涂料, other UV types include 水传播的 UV 而且 powder UV. Waterborne UV 可治愈的s have advantages over conventional UV cure as no reactive diluent is necessary to control viscosity. 也, as opposed to conventional UV cure formulations, the viscosity of the 涂层 is independent of the molecular weight of the 树脂 而且 for spray application viscosity, solids are adjusted by adding water rather than low viscosity reactive monomer. 除了, since there are fewer double bonds to cure, shrinkage is lower 而且 can thus improve adhesion. The main disadvantage is that the water needs to be removed by passing through an oven at about 80°C prior to UV curing. In powder UV cure 涂料, the part is sprayed electrostatically. Automatic guns are recommended over manual application to ensure an even, consistent film thickness is applied. Next, the applied 涂层 is baked in a convection, IR or oven to melt 而且 flow the powder. This step is at a much lower temperature 而且 less time (175-280 °F for a few seconds instead of 320-390°F for 5 to 20 minutes) for conventional powder 涂层. Once the powder is melt flowed, the parts enter a UV cure chamber that cure the 涂层 in seconds instead of minutes, as with traditional thermal powder.

Cationic UV cure 涂料 have the following advantages:

  • 低收缩
  • 优异的附着力
  • Not inhibited by oxygen
  • Dark cure permits a high level of conversion
  • Improved physical properties

Typical 的光s for cationic cure 涂料 are typically onium salts of strong acids such as 碘鎓 而且 锍盐 of hexafluoroantimonic 而且 hexafluorophosphoric acids. Once exposed to suitable irradiance in the range of 200 – 360nm a strong Bronsted acid is activated which acts a catalyst for the homopolymerization of oxirane functional groups on the reactants. Normally cycloaliphatic epoxies are used as they react faster that aromatic based epoxies such as

reactivity increases - Radiation Cure Coatings

Tetrakis pentafluorophenyl borate anion is fastest in the above group (lowest nucleophilicity). The addition of photosensitizers such as thioxanthones苯甲酮 而且 anthracenes can enhance a spectral response into the mid-visible energy range to improve reaction efficiency. From a vehicle st而且point, cationic UV cure 涂料 use cycloaliphatic epoxies as they are faster reacting than those utilizing aromatic epoxies based on BPA. 络合阳离子的盐 can also be used as phtoinitiators for hybrid free radical-cationic polymerization. Hybrid radical-cationic 涂料 use cycloaliphatic epoxies in addition to reactants such as 乙烯基醚苯乙烯, 而且 4-alkoxy苯乙烯. As moisture acts as chain transfer agent in cationic cure, cure speeds decrease dramatically above 50% R.H. After activation of the cationic 的光, the polymerization reactions are thermally driven. This accounts for a high conversion rate especially in the presence of a thermal bump.

Any discussion of UV-LED cure 涂料 is remiss without at least a short overview of UV-LED bulbs as well as the characteristics of each type. As illustrated in 表二世I.

Types of UV LED bulbs - Radiation Cure Coatings
表二世I – Types of UV-LED Bulbs 而且 their Characteristics

A final consideration of UV cure 涂料 is that they are normally line of sight. 换句话说, for complex three-dimensional surfaces, where the light does not shine, the 涂层 will not cure. 也, most UV cure technologies provide optimum uniform cure on a two-dimensional surface using focused light. LED curing has multiple advantages over more traditional UV cure technology such as low heat generation. This is ideal for curing heat sensitive substrates. 除了, LED offers an ozone free environment, 能源效率, 而且 ultralong bulb life 而且 the stable spectral output means consistent quality.