Log In Sign Up. Radiation curing: coatings and composites. Berejka, Daniel Montoney, and composites Marshall R. Recently, the binder systems used for EB curable coatings have also been successfully used without pigments as the matrices for EB and X-ray cured fiber composites. Insights gained from the development of coatings were translated into desirable properties for ma- trix materials.
For example, understanding the surface wetting characteristics of a coating facilitated the development of a matrix that would wet fibers; the development of coatings that would adhere to rigid substrates as metal while being bent, as for coil coatings, and which would exhibit impact resistance when cured on a metal also imparted impact resistance to cured composite materials.
Thermal analyses conducted on the coating binder cured at low energies were consistent with analyses performed on thick cross-sections as used for matrices. The configuration of the final product then dictated the modality of curing, be it low-energy EB for coatings or higher energy EB or X-ray curing for composites.
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Thus, one can approach the development of coating binders or matrix systems as one would approach the synthesis of organic polymers. The desired final material is a fully cured and cross-linked polymer. When synthesiz- ing a radiation curable coating or matrix system, greater attention is given to microphase compatibility as reflected in the microhomogeneity of the entire material.
Montoney in this estimate. The pie chart of Fig. Since accelerator energy governs beam Syracuse, New York , USA penetration, different end-use applications have found different beam energies more suitable for their needs. Loiseau on exit from the material in unit density materials. IBA Industrial, Industrial accelerators are limited to a maximum energy Chemin du Cyclotron, 3, of 10 MeV so as to preclude inducing any radioactivity Louvain-la-Neuve, Belgium in the target material .
Of these market segments, the fastest growing area Received: 10 June over the past decade has been in the use for surface Accepted: 18 August curing. Berejka et al. Table 2. Industrial electron beam end-use markets. Electron beam energy by industrial market segment systems. Toughness was enhanced by using low dose Surface curing 80— keV 0. Coatings development to composite matrix materials Electron beam curing of composites — historical background In , Strathmore Products, Inc. The demands to the accelerator manufacturer, Radiation Dynamics, placed on the coil coating material for excellent adhesion Inc.
Brenner was to metal, for coating flexibility, for durability in environ- working on developing high gloss, pigmented coatings mental tests and for curing at low doses were shown to be that could be applied to bricks and, when EB cured, beneficial in taking essentially the same formulation, but would give the bricks the appearance of higher priced without pigments, and using it as a matrix for EB cured glazed ceramics.
Removing the pigment from his coat- composites.
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A free radical curing metal coating based ings, he prepared three ply, 3 mm thick wet lay-ups using on an epoxy diacrylate was demonstrated to cure at low different then available fiberglass cloth and mat with doses at speeds up to m per min the maximum speed unsaturated polyesters as the matrix material. The objective was to demonstrate that immersion in hot water and hot acid solutions for up to X-rays, with their far greater penetration than electrons a month. These EB and thermally cured systems were from industrial EB accelerators, could cure materials also comparable in flexural modulus at room tempera- while in a mold.
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A coating applications were used as matrix materials . Goniometer pictures of surface wetting on steel. Figure 2 shows readily wet and saturate carbon fiber twill being used pictures for a control of a high surface tension drop of for composites development. The advantage of having water and of an EB curable formulation tailored to wet a coating system that could be sprayable, requiring a the metal substrate.
It was found that between the platens . When using a mold with a clear a coating binder when used as a matrix binder could polycarbonate PC upper platen, this matrix material was observed to flow up and into the carbon fibers by capillary action even after the vacuum had been turned off, as shown in Fig. The higher gloss on the carbon fiber twill indicates where the fibers had been wet.
From the understanding of surface wetting charac- teristics and how different ingredients in a formulation affect wetting, problems reported with the adhesion to carbon fibers, albeit they are sized for thermally cured matrix materials, can be minimized. The carbon fiber materials produced within these thick plastic molds were cured using X-rays derived from a high-current, 3.
The X-rays penetrated the Fig. Relationship between contact angle and surface ten- sion. Wetting carbon fibers. Gel content vs. The methylene chloride evaporated quickly so the weights of the im- mold walls and carbon fibers and then cured the matrix mersed materials could also be quickly determined. From the results, as shown in Fig. At all data points, the X-ray cured material exhibited A material is deemed to be cross-linked if it is in- slightly higher gel formation than the EB cured mate- soluble in solvents that would dissolve its precursors.
If no coating dissolved onto the cloth, the coating was deemed cured. This substrate is then put in the EBC machine together with a protective foil. In the machine this sheet is then shot with electrons at such high velocity that the color impregnated paper hardens almost instantly. After being stored in a temperature-controlled room for a short duration the sheets are ready to be adhered to unfinished HPL boards in a process called dry forming.
High-energy ionizing radiation can modify the macroscopic properties and molecular structure of the irradiated polymers [ 2 ]. During electron beam irradiation EB , the energy of accelerated electrons is injected directly into a material, in which it subsequently induces chemical reactions, often without the need for any catalyst.
The processes initiated by EB radiation in the polymer are very complex, as they are accompanied by several simultaneously running, competitive reactions, such as cross-linking, degradation, oxidation, fragmentation, grafting, and others [ 3 ]. However, the most useful reaction occurring during polymer irradiation is a radiation-induced cross-linking of polymer chains, which is capable of improving some physical and chemical properties, such as hardness, resilience, thermal resistance, solubility, and several others [ 4 ]. For a relatively large group of polymers modified by high energy EB radiation, the improvement of these properties is even more substantially pronounced than that achievable by conventional cross-linking [ 5 ].
In general, EB radiation causes the excitation of polymeric macromolecules occurring adjacent to the incident electrons. The energies associated with the excitation of these molecules are proportional to the electron velocities and the radiation dose [ 6 ]. The interaction of incident electrons with molecules leads to ionization in the polymer and numerous highly-reactive forms are produced such as free neutral radicals, radical cations and anions, low energy electrons, and singlet and triplet states of molecules excited by electrons in a whole series of processes such as fragmentation to carbocations and free radicals, capture of electrons by polymeric and oxygen molecules, dissociative capture of electrons, and others [ 7 ].
Free radicals formed by dissociation of the molecules in their excited state or by the interaction of molecular ions, as well as molecular ions, can react by linking the polymer chains directly to the 3D network structure or by initiating grafting reactions. Due to the full depth of penetration of the incident electrons, EB radiation results in uniformly-cured polymers [ 8 ]. Chain branching and cross-linking increase the molecular weight of the polymer, while degradation or scission causes a reduction of the initial molecular weight.
During irradiation, both these phenomena coexist, and their prevalence depends on several factors, such as the initial molecular structure and morphology of the polymer, and the radiation conditions. The EB radiation-induced cross-linking will result in an increase in physical and chemical properties until chain scission and breaking of intermolecular bonds reduces them; therefore, finding an optimal radiation dose is always necessary in order to prevent polymer degradation caused by excessive irradiation [ 8 , 9 ].
At present, a number of research papers are dedicated to the modification of polymers by EB radiation. The fundamental principles of all applications of radiation treatment of polymers are evaluated, for example, in [ 10 ]. In general, it is well known that some polymers can be cross-linked with EB radiation, while others tend to degrade. Some of them are capable of self-cross-linking, while some have to be first mixed with a cross-linking agent, and EB modification is applied only during their polymerization process, which is presented in a number of cases in [ 11 ].
The aim of the paper [ 13 ] is to balance the applications of radiation cross-linking of polymers in chemically-aggressive and high-temperature conditions. The result of radiation curing is compared with the result of the process of heat curing that is used in a standard manner for this type of polymers. The samples of the investigated material were irradiated by EB irradiation for several minutes with a total dose of kGy. The heat curing process was carried out for 20 h in total.
The glass transition temperatures, T g , determined by the DMA dynamic mechanical analysis technology, achieved values higher even by 17 per cent in irradiated samples compared to heat-cured samples, due to the higher density of the formed polymeric 3D network resulting in higher stiffness of the material cured by irradiation.
The authors of this paper concluded that curing of epoxy resin by EB irradiation may be particularly suitable for applications in which high thermal stability and heat resistance are required. The effects of individual radiation doses were investigated using standard static tensile tests, as well as with the use of TGA thermogravimetric analysis and DMA methods.
It was found that the thermal properties of the investigated material slightly increased after EB irradiation. Mechanical properties increased after irradiation by doses of 30 kGy and kGy, while at a dose of kGy, they significantly decreased. The impact of EB irradiation on the mechanical and dynamic mechanical properties of cross-linked fluorocarbon rubber, natural rubber, ethylene-propylene-diene monomer of rubber, and nitrile rubber was investigated [ 16 ].
It was found that the modulus of elasticity, gel portion, the temperature of glass transition, and dynamic elastic modulus of the investigated vulcanizates increase with the increasing irradiation dose, while the elongation at break and the loss factor decrease. The authors of the paper [ 17 ] studied the mechanism of interaction between carbon fibers and phenol epoxy matrixes of polymeric composites cured by EB radiation, as well as improving their interfacial shear strength.
Promising results were obtained by electrochemical processing of carbon fibers followed by subsequent application of a reagent compatible with the applied EB. The morphology of both modified and unmodified carbon fibers was characterized by the use of SEM scanning electron microscopy , AFM atomic force microscopy , and XPS X-ray photoelectron spectroscopy. It was proved that acidic electrolytes are particularly well-suited to improving interfacial adhesion in the EB curing process. Alkaline groups on carbon fibers prevent cationic polymerization and improve the shear strength of EB-cured composites.
The majority of relevant papers publish the results of research on the impact of high-energy EB irradiation on the micromechanical, thermo-mechanical, visco-elastic, and rheological properties of various types of polymeric materials, their mixtures and composites, nanocomposites based on polymeric matrixes, as well as the influence of irradiation on changes in their structure, degree of cross-linking, crystallinity, and gel content under various conditions of radiation exposure, typically up to kGy, using the most advanced analytical tools for modern, standard mechanical tests: TGA, DMA, and DSC differential scanning calorimetry techniques, FT-IR Fourier-transform infrared spectroscopy and Raman spectroscopy, SEM, AFM, or XPS.
However, to the best of our knowledge, no previous studies have been reported describing the effect of EB irradiation on the polymer blend of melamine resin, phenol-formaldehyde resin, and nitrile rubber, representing a specific three-component mixture of two reactoplastics and one elastomer, as well as its individual components.
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The models of the results of an interaction of ionizing radiation with polymeric systems are virtually completely missing. For this reason, the aim of the present paper is to investigate and model the influence of EB irradiation on the mechanical properties of the aforementioned polymeric composition which are commonly used as a polymeric matrix of friction composite systems in a number of practical applications, particularly in the automotive industry [ 18 ].
The PMX3 polymer system PS was purchased from a professional material manufacturer for polymer matrices of friction composite systems [ 19 ]. PS PMX3 consists of a mixture of melamine resin, phenol-formaldehyde resin, and nitrile rubber, which is distributed by the manufacturer Ahshenhui, Jinhu, China in the form of granules as a polymer blend with a proportion of ingredients adapted for wider use in the automotive industry.
The source of electrons in the accelerator is an indirectly-heated, pressed Ba-Ni cathode with a diameter of 5 mm. The emitted electrons are formed by the electron-optical system designed by J. The increase in the kinetic energy of the electrons is ensured by the electric field of the stationary electromagnetic wave produced by a magnetron operating in the pulse mode at a frequency of MHz.
The focusing of the beam is realized by the high-frequency field. The output device scanning chamber provides the ebeam with a band of lengths mm, mm and mm. The dispatch of the beam is realized in a vacuum chamber by a scanning electromagnet. Homogeneous irradiation of the samples of the investigated PS PMX3 was ensured in a dynamic manner, i.
The experiments were carried out in the air, at normal pressure, and at room temperature.
Electron Beam Curing of Composites
Excessive overheating due to higher doses of irradiation was prevented by the placement of the samples on heat-insulating materials and by air flowing from the ventilation system. The ventilation system also provided an extraction of the ozone generated during irradiation. The samples were irradiated with a wide span of target doses, namely: 77, , , , , and kGy. The doses above kGy, which were above the maximum limit of dosimetric systems, were executed by multiple irradiations.
The break between two exposures did not exceed 20 min. The total irradiation time of the samples at individual radiation doses did not exceed 54 min. Due to the required high radiation doses, the highest accelerator outputs corresponding to pulse frequencies of ebeam the Hz and Hz bands were used. The dose depends on the speed of the conveyor section below the accelerator window, and this dependence is non-linear and is determined experimentally. The foils cut to circles with a diameter of 1 cm react to radiation by showing a change of color.
The radiation dose is calculated from the experimentally-determined dependencies of the dose on absorbance. The mean value of at least five dumbbell specimens of each sample, prepared in accordance with standards ISO 37 or ASTM D [ 20 , 21 ], was taken, although specimens that broke in an unusual manner were disregarded. The average engineering stress-strain curves were constructed from the obtained experimental average force vs. As expected, the effect of EB irradiation on the shape of the stress-strain curves, obtained under the given conditions of tensile tests, is evident. It is apparent already at a first glance that all curves exhibit a relatively short linear elastic region with a low proportionality ratio, a relatively short linear visco-elastic region with low value of limit of elasticity, a relatively low value of tensile modulus and ultimate strength, as well as tear stress, but that their strain at break achieves an extremely wide range of values.
A detailed view of the linear region of the stress-strain curves is shown in Figure 1 b. The shape of the stress-strain curve of the virgin sample resembles the stress-strain curve of the linear amorphous reactoplastics non-cross-linked resins with low strength, without an ultimate strength, but with ductility of non-cross-linked, non-crystallizing elastomers in a rubbery state near viscous flow temperature [ 22 ].
Once the strength limit has been reached, the stress decreases with the increasing strain in a non-linear manner, albeit continuously. The stress-strain curves after irradiation of material with radiation doses of 77 kGy and kGy show the characteristics of partially cross-linked non-crystallising elastomers in a rubbery state high above the temperature of their glass transition T g [ 23 ], with much higher strength and lower ductility than in the non-irradiated sample.