Development of the powder reaction moulding process

BACKGROUND: The powder reaction moulding process uses a reactive monomer as carrier and binder for the moulding of metal or ceramic powders. De‐binding is achieved using thermal depolymerisation which is followed by sintering to give the finished component. Binder can be recovered for re‐use. RESULTS: Moulding compounds, with various powder volume fractions, have been prepared using stainless steel, silicon nitride and alumina with n‐butyl cyanoacrylate as binder, and the stability of the compounds established. Rheological properties of the compounds have been measured using both pressure flow and drag flow methods. Compounds are strongly pseudoplastic. Comparison of experimental results with theoretical models, describing suspension flow behaviour shows that experimental maximum volume fractions are close to the theoretical volume fraction of 0.42 for silicon nitride, 0.68 for alumina and 0.7 for stainless steel. Differential scanning calorimetry and thermogravimetry have been used to simulate de‐binding and show a rapid loss of binder through depolymerisation. Post‐sintering porosity of the ceramic materials is high but this is thought to arise from the low pressure moulding techniques used. Porosity of the stainless steel mouldings is much lower. CONCLUSIONS: The results validate the powder reaction moulding idea and demonstrate applicability to three widely different powder materials. Copyright © 2008 Society of Chemical Industry     

Relationship between mechanical properties of ceramic green body and structures of photo-cured acrylate polymer for ceramic 3D printing based on photo polymerization

3D printing technologies using photo polymerization of photo-curable monomer and oligomers as an organic binder have been studied to fabricate ceramics with a complicated shape. For minimizing distortion of ceramics during the manufacturing process and fast 3D printing of ceramic green body, high mechanical strength of ceramic green body and fast photo polymerization of the photo-curable resin are required, respectively. In this study, the ceramic green bodies with various compositions were fabricated by photo radical polymerization of the slurry-type resin composed of fused silica bead, and tri- or (and) mono-functional acrylate. Photo radical polymerization behaviors of mono-, tri-functional acrylate monomer, and blend of two monomers were analyzed by photo-DSC and FT-IR measurements. The structure analysis of photo-cured polymers made by each monomer was performed by thermo-mechanical analysis. Through mixing of mono- and tri-functional acrylate monomer, we confirmed that polymerization rate more increased compared with those of only mono-, tri-functional monomer. Unreacted vinyl groups in the polymers prepared with blend of two monomers decreased by an addition of mono-functional monomer in tri-functional monomer. The polymers prepared with the blend showed higher storage modulus and broader viscoelastic behavior compared to those fabricated with tri-functional monomer. Thus, to achieve high fracture strength of the green body, we verified that the photo-cured polymer in the green body should have high crosslinking density and low free volume based on reduction of unreacted vinyl groups in the polymer. Additionally, through analysis of cross-sectional SEM of the green body, we confirmed that acrylate monomer should include proper content in the slurry-type resin to maximize the fracture strength of the body regardless of the type of the used acrylate monomer. This was because low content of acrylate monomer in the slurry-type resin makes it difficult to form neckings composed with photo-cured polymers between the silica beads.     

Dewatering of organic slurries using reinforced thermosensitive porous gels

Summary The use of a thermosensitive porous gel for dewatering organic slurries was investigated. The gel was synthesized from N-isopropylacrylamide (NIPAM), the monomer and N,N’-methylenebisacrylamide (MBAA), a crosslinker. A porous gel was produced by simple radical polymerization at higher temperature than the LCST (Lower Critical Solution Temperature) of the N-isopropylacrylamide polymer. At such a high temperature, phase separation occurs as polymerization proceeds, and a porous structure is formed. The thermosensitive porous gel, which was supported by stainless steel wire, was prepared and the effect of preparation conditions such as the concentration of crosslinker and monomer used on the dewatering performance of the gel was examined. The effects of these parameters on gel structure and the relationship between gel structure and absorption rates were also examined.     

Interfacial adhesion between polyurethane binder and poly(tetrafluoroethylene) powder in bonded solid lubricant films

Poly(tetrafluoroethylene) (PTFE) powder was irradiated with ⁶⁰Co γ-rays to improve its dispersing ability in polyurethane (PU) as a binder. The bonded solid lubricant films of the irradiated PTFE were prepared on an AISI 1045 steel block by spraying and curing at ambient temperature, with PU as the binder. The tribological properties of bonded solid lubricant films with the PTFE pigment volume fraction were examined on a ring-on-block friction and wear tester. The interfacial adhesion between the PU binder and PTFE powder was investigated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), immersion heat, and X-ray photoelectron spectroscopy (XPS). It was found that γ-ray irradiation increases the activity of the PTFE powder surface and improves the interfacial adhesion between the PTFE powder and the PU binder, which is helpful for improving the wear resistance of the corresponding bonded solid lubricant films.     

丙烯酸在YSZ颗粒表面原位聚合的性能研究

YSZ粉体经表面聚合后,当温度大于650℃时,表面聚合物中的有机物完全分解;表征结果为表面聚合的厚度大约在5nm左右;经表面聚合的Y SZ粉体颗粒可使其等电点由6.7移动到4.3左右,最大的Zeta电位绝对值达到67m v左右（pH 10～11时）。经表面聚合作用后,对YSZ水基悬浮体的颗粒的分散和稳定起到了非常重要的作用。将经表面聚合的Y SZ粉体颗粒与复合粘结剂可形成稳定、均匀的高固相含量的流延浆料,其流延生坯片的断面上,气孔分布均匀,流延膜平整、光滑。经烧结后的Y SZ陶瓷基片达到了理论密度的98%。     

环形狭缝式透气砖的研制与应用

以板状刚玉为主要原料,纯铝酸钙水泥为结合剂,研究了尖晶石细粉、活性Al2O3微粉、Cr2O3微粉对透气砖性能的影响,并研究了不同材质,狭缝形式、狭缝长度和狭缝厚度对钢水渗入狭缝的影响。结果表明:适量的活性Al2O3微粉可提高透气砖浇注料的流动性和体积密度;尖晶石可显著改善试样的抗热震性能,但加入量过大会影响材料的高温体积稳定性,并恶化抗渣性能;Cr2O3微粉对材料的流动性和抗热震性能影响不显著,但可提高其抗渣性能。透气砖材质、狭缝形式和狭缝长度对狭缝渗钢的影响不明显,但狭缝厚度会显著影响钢水在狭缝中的渗透行为。与条形缝相比,环形缝可改善透气砖的透气性能,提高透气砖的使用寿命,吹通率达到了100%。     

The influence of pitch-binder coke content on the properties and irradiation behavior of molded graphite

A series of needle-coke graphites were prepared with various fractions of pitch-binder coke in the range 5.5–14.2 wt-%. The specimens were irradiated to 9.70 × 10²¹n/cm² (E > 0.18 MeV) at 1225°C. Apparent bulk density and apparent crystallite size L_c were dependent on the binder-coke content. Crystallite orientation and thermal expansivities were not affected. Dimensional and volumetric changes were sensitive to binder-coke content; specimens deficient in binder-coke showed volumetric turnaround at lower fluences and higher expansion rates at high fluences. Specimens of 5.5 wt-% binder coke expanded 20 vol-%, whereas those of 11.7 wt-% binder coke expanded only 4 vol-%. Thermal expansivity increases closely followed the volume changes and were large in specimens deficient in binder coke. Unbonded coke particles expanded at three times the rate of coke particles bonded with pitch. The needle-coke particles were restrained by the binder coke during expansion at high fluences. Large cracks formed between filler particles during expansion and were responsible for the large volume expansions.     

Effect of microcrystalline wax on the solvent debinding of the Sr-ferrite ceramics prepared by powder injection molding

Powder injection molding (PIM) opens new possibilities to process complex Sr-ferrite components for magnetic applications. In the present study, new binder system with the addition of microcrystalline wax (MW) was used for the Sr-ferrite powder injection molding. The optimum binder composition was determined based on rheological measurement, mircrostructure observation by SEM, thermal change by DSC and debinding process. The results indicated that MW with 10 vol.% addition in the binder system containing high density polyethylene (HDPE), paraffin wax (PW) and stearic acid (SA) decreases the defects of the green parts after solvent debinding because the distribution of the binder system between the Sr-ferrite particles becomes more homogeneous. Using the proper binder system containing MW, the defect-free and dense Sr-ferrite ceramics can be prepared after solvent and thermal debinding and sintering. Based on the experimental results, the effects of MW microcrystalline wax on the solvent debinding of the Sr-ferrite ceramics were detailed discussed.     

光致聚合物材料中各组分对光聚合速度的影响及其在全息存储中的应用实验

本文研究了光敏剂、光引发剂、光促进剂、单体及高分子成膜树脂等成份对全息光致聚合物薄膜光存储性能的影响，在一定范围内，材料中光引发剂浓度的提高使光聚合速度显著增加，链转移剂浓度则存在一个最佳的相对浓度，适当地增加液态辅助单体含量及其所含双键官能团的数目，会明显增加光聚合速度，在聚醋酸乙烯酯薄膜中单体聚合速度明显优于在醋酸纤维素中的速度，所制备的光致聚合物材料实现了同一位置的多幅全息图像的存储和再现，表明了该材料有用于大容量全息存储的可能。     

Binder jetting additive manufacturing of silicon carbide ceramics: Development of bimodal powder feedstocks by modeling and experimental methods

A limitation of binder jetting additive manufacturing is the low density of fabricated parts. Mixing powders with different sizes is a promising approach to increase powder bed packing density and, hence, printed part density. However, in previous studies mixed powder feedstock was prepared by trial and error method. In this research, both modeling and experimental methods were used to prepare the bimodal powder feedstocks. Analytical packing model was introduced for irregular powders. A bimodal powder was prepared by mixing two different-sized silicon carbide powders (i.e. coarse and fine) using ball mill, and their tap densities were measured. Silicon carbide plates were printed using the coarse and bimodal powders by a commercial binder jetting system. Results showed that the modeling method could predict the tap density of bimodal powders with high accuracy. The printed parts from bimodal powder achieved higher green densities than those from the unimodal powder.     

Fabrication and characterization of silica ceramic compact prepared using Aloe-Vera mucilage as a binder

In this study, silica compacts were fabricated through a powder processing route at different compaction pressure, using Aloe-Vera (AV) mucilage as a binder. The silica compacts were prepared at 90, 100, and 110 MPa compaction pressure using 0%–16 wt% of AV binder. The optimum amount of AV binder was 14 wt% for both 90 and 100 MPa and 12 wt% for 110 MPa. The maximum achieved green density and green strength of silica compacts at the optimum binder amount were 62.3% and 4 MPa, respectively at 110 MPa compaction pressure. The green silica compacts prepared at 110 MPa compaction pressure exhibited a minimum porosity of 21% and maximum flexural strength of 15 MPa after sintering at 1400°C. The green silica compacts with the optimum amount of binder were strong enough for machining. The Fourier transform infrared spectroscopy analysis revealed the functional groups present in AV mucilage. The binder burnout characteristic of AV mucilage in the silica compact was determined by thermogravimetric analysis and differential thermal analysis. Additionally, AV gel acted as a binder and solvent simultaneously for ceramic compaction.     

Direct Evidence for Low-Density Regions in Compacted Spray-Dried Powders

Pore structure in a ceramic green body was directly observed with a novel characterization tool to examine the detrimental effect of a binder on the microstructure of ceramics for powder compaction processing. The binder segregated on the surface of granules and formed a low-density region at the boundaries of granules in the compact after binder removal. A network of pores with round shapes was produced in the sintering process from the low-density region. This paper also explains a new characterization technique, confocal laser scanning fluorescent microscopy.     

Structure and adhesion properties of acrylic acid grafted high‐density polyethylene powders synthesized by a novel photografting method

A novel photografting, nonvapor, and nonliquid phase living graft polymerization was developed to functionalize high‐density polyethylene (HDPE) powder. The structure and adhesion properties of HDPE powder grafted with acrylic acid (AA) were studied by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), water contact angle, peel strength, and graft degree measurements. The result shows that HDPE powder can be grafted with AA via the method with a short reaction time and a high monomer conversion. The graft degree increases with the reaction time. Then, the hydrophilicity of the grafted HDPE powder increases also. The peel strength of HPDE/steel joint improved significantly when acrylic acid grafted HPDE powder was used as hot melt adhesive in place of ungrafted HDPE powder. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Aspects of the Safe Storage of Acrylic Monomers: Kinetics of the Oxygen Consumption

Safe storage and handling of acrylic monomers assumes detailed knowledge about possible reactions and influence parameters on the radical polymerization. An investigation was made in order to predict the stability of the monomers by quantifying the oxygen consumption due to the inhibition process for different influence parameters. The oxygen consumption was investigated for acrylic and methacrylic acid without inhibitor as well as in the presence of p-methoxyphenol (package inhibitor) and phenothiazine (process inhibitor) in the temperature range from 40 to 80°C. The addition of stabilizer lowers the oxygen consumption in the monomers, which results in a higher stability. The oxygen consumption rates for unstabilized (up to 60°C) and p-methoxyphenol-stabilized monomers fit the Arrhenius equation and can therefore be extrapolated to other temperatures. The stability of the monomers at higher temperatures can be predicted with the help of the oxygen consumption rates (not for phenothiazine). At lower temperatures, slow polymerization rates can extend the stability of the monomer beyond the time of oxygen inhibition.     

Formation of polypyrrole coatings onto low carbon steel by electrochemical process

Thin polypyrrole coatings (∼ 10 μm thick) were formed on low carbon steel by an aqueous constant current electrochemical polymerization using oxalic acid as the electrolyte. The amount of polypyrrole coatings formed on steel increased with the applied current and monomer concentration. No significant change in the electropolymerization of pyrrole occurred as a result of increased electrolyte concentration. The induction time for electropolymerization decreased significantly with current density but was unaffected by the initial monomer and electrolyte concentration. The electropolymerization potential of pyrrole increased with increased current density (Cd), i.e., Ep = 0.62 + 0.41 [Cd], and decreased exponentially with increased monomer and electrolyte concentration, Ep = E₀ exp^-[M]. Scanning electron microscopy (SEM) showed that the microstructure of the polypyrrole coatings formed on steel was dependent on the current density to the extent that smoother and more uniform coatings are formed at low current density. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:417–424, 1997     

Carbide phases formed in WC–M (M = Fe/Ni/Cr) systems

The effect of composition in the formation of carbides in the WC–M (M = Fe/Ni/Cr) system has been studied. Three conventional compositions have been prepared with 10 wt.% of metallic binder. Fe as a metallic binder was first studied and its partial substitution by Ni and Cr was also investigated. A WC powder coated with a similar binder (Fe/Ni/Cr) amount was also investigated. The XRD results, after thermal treatment at 1400 °C, reveal that the substitution of half percent of the iron content by Ni stabilized the austenite Fe(γ) formation. The introduction of Cr in the binder composition induces the formation of Cr₂C carbide. With the decrease of Cr amount in the binder composition of the coated powder, Cr₂C carbide was not formed, but instead other carbide, M₆C, was detected. In order to eliminate this carbide a small excess of carbon (3.6 wt.%) was added to the coated powder. The results are discussed in terms of the phases formed, the binder composition and the final carbon content.     

Experimental study of the spontaneous thermal homopolymerization of methyl and n‐butyl acrylate

This article presents an experimental study of the spontaneous thermal homopolymerization of methyl acrylate (MA) and n‐butyl acrylate (nBA) in the absence of any known added initiators at 120 and 140°C in a batch reactor. The effects of the solvent type, oxygen level, and reaction temperature on the monomer conversion and polymer average molecular weights were investigated. Three solvents, dimethyl sulfoxide (DMSO; polar, aprotic), cyclohexanone (polar, aprotic), and xylene (nonpolar) were used. The spontaneous thermal polymerization of MA and nBA in DMSO resulted in a lower conversion and higher average molecular weights in comparison to polymerization in cyclohexanone and xylene under the same conditions. The highest final conversion of both monomers was obtained in cyclohexanone. The high polymerization rate in cyclohexanone was most likely due to an additional initiation mechanism where cyclohexanone complexed with the monomer to generate free radicals. Bubbling air through the mixture led to a higher monomer conversion during the early stage of the polymerization and a lower polymer average molecular weight in xylene and cyclohexanone; this indicated the existence of a distinct behavior between the air‐ and nitrogen‐purged systems. Matrix‐assisted laser desorption/ionization time‐of‐flight analysis of the polymer samples taken from nitrogen‐bubbled batches did not reveal fragments from initiating impurities. On the basis of the identified families of peaks, monomer self‐initiation is suggested as the principal mode of initiation in the spontaneous thermal polymerization of MA and nBA at temperatures above 100°C. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Tape Casting Slip Preparation by in Situ Polymerization

The present work describes the polymerization of methylmethacrylate monomer into polymethylmethacrylate (PMMA), in the presence of either coarse or ultrafine alumina powder. The flexibility of the material was modified by addition of either butylbenzyl phthalate (S-160), or butylacrylate (BA) monomer as plasticizers. It was found that BA is more compatible than S-160 in the PMMA system. A density of 97%–99% was reached with ultrafine alumina. The maximum tensile strength of the green and sintered tapes was 10 and 140 MPa, respectively. Due to improved homogeneity in the polymerized slip, the sintered tapes prepared by in situ polymerization were stronger and denser than those prepared by conventional blending of commercial PMMA.     

Preparation and characteristics of polyacrylic ester‐based water‐free powder coatings in supercritical carbon dioxide

Powder coating for dry coating technique of paper as the promising method has attracted more and more attentions in recent years due to its advantages in reducing the dosage of water and saving energy compared with conventional coating. This study focused on the in situ polymerization of methyl methacrylate (PMMA) under a water‐free condition in supercritical carbon dioxide in the presence of inorganic kaolin. The effects of varying the concentrations of the monomer, initiator, and stabilizer on the molecular weight and morphology of the resultant PMMA were investigated and discussed. Then the powder coating was systematically evaluated and characterized by gel permeation chromatography, scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Meanwhile, the feasibility of manufacturing PMMA/kaolin powder coatings was explored, and the mechanism of polymerization and the thermal degradation kinetics of powder coating were studied. The experimental results showed that the properties of PMMA as a film former were suitable with the molecular weight and narrow molecular weight distribution close to that in conventional coating when the concentration of monomer was about 10 wt %, concentration of initiator ～1–1.5 wt % and stabilizer about 10 wt % with respect to monomer. Moreover, the interfacial bonds and dispersion situation of polyacrylic ester‐based water‐free powder coating particles were fairly well, the powder coating possesses good film‐forming property combined with outstanding thermal‐stability performance. The combination of these characteristics makes PMMA/kaolin powder coating an excellent candidate for dry coating technique of paper applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42439.     

UV Curable Systems for Tape Casting

The substitution of solvents by photopolymerizable binders in the tape casting process allows to achieve high ceramic loading and to eliminate the drying stage which is a critical step of the tape casting process. After the rapid UV polymerization of the resin, the high strength green tapes can be debinded and sintered. Ceramic suspensions containing alumina or zirconia powder, dispersant, UV curable binder and photoinitiator have been prepared. The use of a low viscosity organic vehicle allows to prepare low viscosity ceramic suspensions, containing about 50 vol% powder, which have a shear thinning behaviour. Because of the rapid attenuation of the incident light in UV curing systems containing ceramic particles it is important to estimate the thickness of the tape that can be treated. The effect of incident energy, of photoinitiator concentration and of powder volume fraction was studied. There is an optimal photoinitiator concentration which maximizes the cured depth and which depends on the volume fraction of solid. A theoretical model based on the Beer–Lambert’s law enables the prediction of cured depth for any volume fraction of solid. To prove the ability to manufacture ceramic sheets by tape casting, some suspensions were tape cast.