Hot Curing Research Articles

Empirical model for the effect of air humidity during manufacturing on the glass transition temperature of carbon fiber reinforced hot curing epoxy – a case study

Carbon fiber-epoxy laminates are used in aerospace manufacturing, e.g. as substrates for solar cells of satellites. Commonly, fibers or fibermats are impregnated with epoxy resin and placed in the required orientation. During subsequent curing, the resin molecules are crosslinked. Cured parts are characterized by their glass transition temperature (Tg). It has been observed that Tg of epoxy matrix resin vary with recorded absolute air humidity during wet fiber placement manufacturing. Based on the production data of a series production of 203 carbon fiber laminates for space application, an empirical linear relationship between the absolute air humidity at the beginning of each production day and the observed glass transition temperature of the fully cured laminate is formulated and validated. The empirical equation describes a linear decrease of achievable glass transition temperature with increasing absolute air humidity. The quantitative nature of the results encourages straightforward practical application to determine the maximum achievable Tg for given production conditions.

Development and sustainability assessment of red mud-based green bricks: Techno-economic and environmental performance

Red mud (RM) is an alkaline discarded hazardous waste for mining alumina from bauxite ore and thus has the potential to be used for geopolymerization. This work has been designed to couple RM with by-product waste to develop a green structural block, an alternative to fired clay or cement brick. Specimens were initially prepared at a lab scale for optimization and thus verified at an industrial scale. At the lab scale, the proportion of RM was optimized by using it in the range of 0–50 % at 10 % intervals and activated by the combination of NaOH and Na2SiO3. The physico-mechanical performance was evaluated, and the specimen with 30 % RM performed best at ambient and hot curing conditions. Upon transitioning to an industrial scale, structural block/brick measuring 225 × 113 × 75 mm was manufactured using uniaxial compression, resulting in a notable 202 % increase in strength compared to laboratory-scale specimens. Furthermore, blocks/bricks fulfilled the structural requirements outlined by ASTM and ACI, validating their suitability for industrial applications. Economic and environmental analyses of developed bricks revealed fewer costs and significantly lower CO2 emissions than conventional bricks and geopolymer-based bricks. The economic and environmental indices also made the RM the appropriate choice. Furthermore, the RM-based brick is sustainable and feasible based on the sustainability matrix.

Static Compaction on Coupled Precursors and Optimizing Molarity for Enhanced Strength and Durability of Geopolymer.

The static compaction technique emphasizes the reduced activator dosage required to develop geopolymers. Therefore, it is crucial to comprehend the optimal alkaline activator concentration for blending low-calcium precursor (fly ash) with high-calcium precursor (GGBS) to produce geopolymer blocks. This work was designed to optimize structural blocks' compressive strength and durability. In experimentation, fly ash (FA) and slag (GGBS) proportions were initially investigated under NaOH solution with varying molarity (8-12) and curing conditions to develop a load-bearing structural block. Subsequently, the durability of the optimized block was evaluated over 56 days through subjection to sulfate and acidic solutions, with efflorescence monitored over the same period. The results reveal that the structural block comprised of 100% FA exhibits the highest compressive strength and lowest bulk density. Conversely, the block incorporating 25% slag that underwent hot curing demonstrates a remarkable 305% strength increase compared to ambient curing. Considering the physico-mechanical performance, the 100% FA block was chosen for durability investigation. The findings indicate a substantial strength loss exceeding 40% after exposure to sulfate and acidic environments over 56 days, coupled with pronounced efflorescence. Catastrophic failure occurs in all cases due to significant strength deterioration. The FTIR spectrum revealed the shifting of the wavenumber to a higher value and verified the depolymerization and leaching of alumina under acidic exposure. However, the developed geopolymer blocks demonstrate superior sustainability and feasibility compared to conventional fired clay bricks and cement-based FA bricks. Despite slightly higher costs, these blocks exhibit greater strength than their counterparts after enduring severe exposures.

Hot curing and lime boiling of high-arsenic copper concentrate pressure oxidation product and their effect on precious metals recovery during subsequent cyanidation

Hot curing and lime boiling of high-arsenic copper concentrate pressure oxidation product and their effect on precious metals recovery during subsequent cyanidation

Enhancing Strength and Quantifying Sustainability of Building Blocks Manufactured by Geopolymerization.

Enhancing the strength of fly ash (FA)-based geopolymer by increasing the alkaline activator content is a costly and unsustainable technique. Therefore, this work was designed to reduce the activator by employing the pressured catalysis (PC) technique, coupled with the use of minerals that have filler and occupying effects. The main objective was to enhance the strength of the mix with a lower alkaline-to-precursor (A/P) ratio and create a sustainable, load-bearing building block from it. Initially, the compressive strength of the FA-based geopolymer was investigated experimentally by varying sodium silicate to sodium hydroxide and A/P ratios with ambient and hot curing. Afterward, PC was applied to the optimized proportion of constituents, and a significant increase in strength (9.6 to 20.0 Mpa) was observed at a 0.25 A/P ratio. By adding clay and dune sand (DS), the compressive strength was 19.5 and 40.4 Mpa at an A/P of 0.25 and 0.16, respectively. The strength gain mechanism was evaluated at the molecular and micro levels by conducting FTIR and SEM analyses. The environmental and economic indices and strength indicated the high sustainability of DS-based geopolymers compared to analogous blocks. The environmental and economic benefits of 23.9% reduced CO2 emissions and 24.2% less cost were provided by the DS-based block compared to the FA-clay-based block. A DS-based geopolymer obtains strength at a low A/P due to its occupying effect and results in sustainable building blocks.

Surface modification strategies of 304 austenitic stainless steel plate with different micromorphology and composition for improved mechanical interlocking and chemical bonding in metal–epoxy joints

304 austenitic stainless-steel (SS) strip-based carbon fiber metal laminates (CFMLs) have raised significant concerns, in terms of their mechanical performance. On top of that, the hybrid laminates’ interlamination performance is vital in regulating their failure patterns. Along these lines, in this work, SS strips with regulatable micromorphology and composition were obtained by using physical and chemical surface modification strategies, including polishing, acid etching, anodization, and thermal treatment. The shear strength of the treated-SS/epoxy single lap joints (two treated-SS strips bonded by the epoxy resin via hot curing process) was systematically explored, and the origins of the enhancement mechanism were attributed to the resulting surface microstructure, as well as the possible chemical reactions between the treated surface and the curing agent. The etching-anodizing-thermally treated-SS (EAT-SS)/epoxy joint exhibited the maximum shear strength (16.04 MPa) and work of fracture (5.85 kJ·m−2) due to the substantial cohesive failure pattern of the epoxy. The encouraging enhancement was mainly attributed to the synergistic effect of the micro-annular pit and the nanoporous array structure of the EAT-SS surface. These effects induced a significant mechanical interlock and a plausible chemical reaction at the interface, which led to improved bonding strength between the epoxy and treated-SS surface. Overall, in this work, it was demonstrated that the combination of the anodization and thermal treatment can effectively improve the interlamination bonding properties between the SS strip and the epoxy resin due to the combination effect of mechanical interlocking and chemical bonding.

Multicriteria analysis for quantifying sustainability of developed load bearing lightweight geopolymer

In geopolymers, several factors such as the limited availability of amorphous aluminosilicate material, the need for a high dosage of activator, the requirement for hot curing conditions, and the activation by pressure pose challenges in considering geopolymer as a sustainable load-bearing material. Therefore, this experimental and analytical work aimed to address the aforementioned issues. The experimental work was carried out in four distinct series, where fly ash (FA) was combined with sugarcane bagasse ash (SCBA), slag (GGBS), and lime. The alkaline to precursor (A/P) ratio and curing conditions were varied. Initially, the compression casting approach was employed to reduce the dosage of activator. But later on, lime was added to prevent such a technique, and the alkaline dosage was reduced due to occupying effect. Thereafter, three key performance indices of sustainability were quantified: load-bearing lightweight (Dp), environment (Ep), and economic (Cp). These indices were determined by normalizing density, CO2 emissions, and cost by compressive strength, respectively. It was observed that FA-lime-based mortar is a sustainable geopolymer. It is ambiently cured and has the least alkaline activator, high strength, minimal CO2 emissions, and low cost. It was also compared to conventional construction materials such as fired clay brick and concrete, and promising results were obtained. Sustainable quantification was conducted analytically by extracting seven alternatives from four series and analyzing a 7 × 7 decision matrix using the multicriteria decision-making technique. The FA-lime-based specimen was selected experimentally and verified analytically, along with a prioritized list of all specimens.

Cold pressing and hot curing process for assembly of hexagonal pre-shaped bamboo culm sections

Bamboo is widely used in the construction industry due to its excellent mechanical properties, short production cycle, and environmental concerns. Reconstructing and processing bamboo into biomass-based composite materials that can replace wood alleviates the imbalance between wood supply and demand. This article uses cold press setting and hot press curing molding processes to prepare small samples of Hexagonal Recombinant Bamboo Material (glued HexBam). Through orthogonal analysis, the preparation process parameters, adhesive types, adhesive application amounts, and the relationship between the temperature and time of hot pressing solidification molding and the mechanical properties of glued HexBam were determined. A phenolic adhesive (PF) was used with a dosage of 200 g/m2, a heat setting and molding temperature of 130 °C, and a heat setting molding time of 10 minutes. Under these process conditions, the prepared glued HexBam had a longitudinal compressive strength of 73.8 MPa and a specific strength of 0.0535 N•m/kg, which is superior to ordinary steel.

The use of NDT to evaluate maturity for taking off formwork under hot curing

The maturity concept assumes that the properties of concrete are directly related to its age and its previous temperature history. Irrespective of curing history, engineers use the maturity concept to stop curing or take off (remove) formwork of site concrete once strength is reached. Previous research concentrated on the use of maturity concept for strength evaluation but neglected other concrete properties. Although the maturity method is good indication for the strength of concrete, it may not be good indication for the durability and internal properties. The research investigates the applicability of the maturity of concrete to attain a reasonable level of both the strength, durability and internal properties of concrete, and hence recommends, for the first time, threshold limits for effective curing periods that attain both strength and durability. For this purpose, concrete samples were prepared and tested in the lab at three different temperatures, 20℃(68°F), 40℃(104°F) and 65℃(149°F). Several destructive and nondestructive tests have been implemented for this purpose. The tests included strength, pulse velocity, density, voids ratio, sorptivity and conductivity. Results show that if maturity-strength only is used as a threshold to stop curing, then durability and internal properties of concrete may not be attained. This is obvious for samples cured at high temperatures (>20 °C). Also, curing history is an important factor for achieving the required age to stop curing. Hence, further curing is necessary under hot weather conditions. Minimum curing periods to attain both strength and durability are suggested. The use of NDT is quite helpful in this respect. The results show that to attain the same durability (conductivity), a minimum maturity of 600℃.days is required irrespective of the strength attained. Higher maturities are required to attain other properties depending on the curing temperature. In practical applications, these findings are important for concrete under hot weather and steam curing.

Influence of Upcycled Post-Treatment Bark Biomass Addition to the Binder on Produced Plywood Properties

The valorization of tree bark through chemical treatment into valuable products, such as bark acid, leads to the formation of process residues with a high solids content. Since they are of natural origin and are able to be suspended in water and acid, research was carried out on the recycling of suberic acid residues (SAR) as a bi-functional component of binder mixtures in the production of plywood. The 5%–20% (5%–30% for curing time) mass content of SAR has been investigated with urea-formaldehyde (UF) resin of about 66% of dry content. The results show that the curing time of the bonding mixture can be reduced to about 38% and 10%, respectively, for hot and cold curing, of the initial curing time for the lowest SAR content. The decreasing curing time of the tested binder mixtures with the increase in SAR content was caused by the increasing amount of acidic filler, since amine resins as UF require acidification hardening, and the curing dynamics are strongly dependent, among others, on the content of the acid medium (curing agent). In the case of hot curing, a SAR content of about 20% allowed us to achieve the curing time of bonding mass with an industrial hardener. Investigations into the mechanical properties of examined panels showed a significant modulus of elasticity (MOE) increase with filler content increase. Similar conclusions can be drawn when analyzing the results of the modulus of rupture (MOR) investigations; however, these were only significant regarding hot-pressed samples. The shear strength of the plywood samples increased with the SAR rise for both cold- and hot-pressed panels. The in-wood damage of samples with SAR filler, hot-pressed, rose up to about 30% for the highest SAR filler content. For cold-pressed samples, no in-wood damage was found. The positive effect of veneer impregnation limiter by resin was identified for SAR acting as a filler. Moreover, a higher density of SAR-containing bonding lines was reached for hot-pressed panels. Therefore, the results confirmed the ability to use the SAR as an upcycled component of the bonding mixture for plywood production.

О возможности создания крупногабаритных конструкций в условиях открытого космоса

The paper gives a brief overview of the history of the development of inflatable space struc-tures. Further development should be the use of inflatable shells that harden in space. The possibility of using this technology in the framework of the use of simple tasks is considered in the article. On the ex-ample of thick-walled spherical and cylindrical shells, using Lame's analytical solutions, it is shown that the thickness of the shell can be thin. The thin shell design is much easier to inflate. No high pressure is required for this. Using the classical heat conduction equation, the temperature distribution inside a cy-lindrical shell is determined. When setting the boundary conditions, the flux of rays incident both from the side of the Sun and from the side of the Earth was taken into account. The radiation on the surface of the cylinder was calculated according to the Stefan-Boltzmann law. It has been established that a tem-perature occurs in the shell, the maximum value of which is in the range from 150 to 200 degrees Celsi-us. Prepreg casings require the use of a high temperature resin suitable for hot curing.

Study of the influence of rubber on stremgth properties of carbon plastic

The use of elastomers allows us to purposefully change the structure of epoxy compositions, which largely determines the characteristics. Of particular interest are elastomers, such as silicone and polyurethane rubbers. In experimental works for the manufacture of carbon fiber plates, we used the method of manual molding with mechanical pressing. This technology is economical, does not require large labor costs, and less material consumption than when using other methods of formation. The effect of polyurethane and silicone rubbers as modifiers of increasing the impact strength of epoxy resin Etal-Inject-T hot curing has been studied. To determine the compressive strength of carbon fiber, the samples were tested on a universal test machine MUP200. The impact strength of carbon fiber was determined by the Charpy method. It is established that polyurethane and silicone rubbers have the same qualitative effect on the properties of epoxy resins. When modifying carbon fiber with polyurethane rubber with an increase in the content of up to 10%, the impact strength of carbon fiber increases to a maximum of 215 kJ/m2, with a significant increase in compressive strength up to 495 MPa. Optimal results were obtained with polyurethane rubber at 10% content of the substance. The main role of rubber in increasing the strength and elasticity of the material is that when the impact in a small amount of carbon fiber concentrates a lot of mechanical energy, the rubbers are stress concentrators, so the crack arises in the matrix adjacent to the elastomer (rubber). The increase in the toughness of the material can be associated with the formation of a dispersed phase of rubber, which loosens the structure of the cured epoxy resin.

A temperature- and conversion-dependent cohesive zone approach for the calculation of process-induced damage in adhesive bondlines of multi-material structures

Adhesive bondlines in multi-material car bodies underlie various influences from the production process. Especially the mismatch of the coefficient of thermal expansion during the oven process of cathodic dip coating can cause damage and weaken the joint strength. Temperature-dependent mechanical properties and the increasing conversion that describes the degree of cure of the adhesive are important parameters affecting the adhesive bondlines during production. To take these effects into account in numerical simulations, a cohesive zone approach, which is commonly used to model adhesive bondlines in structural simulations, is adapted to a one-component, hot curing, structural epoxy adhesive for body-in-white manufacturing and is extended with a reaction kinetic model of the curing reaction, conversion-dependent material properties, and temperature dependency. The investigation of the material properties using a rheometer and differential scanning calorimetry experiments is described. Furthermore, the cohesive zone model is presented and an experimental validation of the model using customized tapered-double-cantilever-beam specimens to investigate the crack opening behavior of incompletely cured adhesives shows a good agreement with the numerical data.

Decomposition Products of the Initiator Bis(2,4-dichlorobenzoyl)peroxide in the Silicone Industry: Human Biomonitoring in Plasma and Urine of Workers.

Bis(2,4-dichlorobenzoyl)peroxide (2,4-DCBP) is used as an initiator for silicone rubber production. During hot curing, 2,4-DCBP decomposes into 2,4-dichlorobenzoic acid, 1,3-dichlorobenzene, and the polychlorinated biphenyl (PCB) congeners PCB-47, PCB-51, and PCB-68. The extent of occupational exposure to these decomposition products has not been investigated yet. We determined for the first time the corresponding internal exposure of employees (n = 104) of a German silicone rubber facility by human biomonitoring in plasma and urine. Collected samples were investigated by gas chromatography/mass spectrometry for levels of PCBs in plasma and by liquid chromatography/tandem mass spectrometry for urinary post-shift levels of 2,4-dichlorobenzoic acid (2,4-DCBA) and the metabolites 3,5-dichlorocatechol (3,5-DCK), 2,4-dichlorophenol (2,4-DCP), and 3,5-dichlorophenol (3,5-DCP). PCB-47 and PCB-68 levels correlated significantly and were found in >97% of all samples with maximum values of 4.43 and 0.77 μg/L, respectively. 2,4-DCBA, 3,5-DCK, 2,4-DCP, and 3,5-DCP were quantified in >80% of all urine samples with maximum levels of 1.46; 26.92; 7.68; and 0.39 mg/L, respectively. There is a considerable uptake of decomposition products of 2,4-DCBP in workers of a silicone rubber facility, affecting employees in all work areas. Individual levels depended on the work task. Considering the carcinogenic potential of PCBs, the workers' additional exposure to PCB-47 and PCB-68 might be of concern.

The Novelty of Using Glass Powder and Lime Powder for Producing UHPSCC

In recent years, UHP self-compacted concrete is an innovative category of concrete that has attached a lot of attention because of its higher durability and compressive strength than conventional concrete. So, to overcome the cost of preparation of UHPC and preservation of high-strength deformation and rheological characteristic of self-compacting concrete when replacing a part of expensive cement with three types of production waste. In addition, the problem of reducing environmental pollution is solved. In this study. recycled glass (GP) and lime (LP) powder were used as substitution materials in the manufacture of the UHPSCC. The flowability of UHPSCC was measured by slump flow, T50, V-funnel tests as an indication for the capability of filling and J-ring tests as an indication for the capability of passing. Furthermore, durability and mechanical properties were investigated. The elevated temperature effect was investigated on several UHPCSCC samples with glass (GP) and lime (LP) powder. The test results showed that the incorporation of GP and LP partially replaced cement improved the flowability of UHPSCC. The compressive, tensile, and flexural strength were enhanced by using GP till 20% replacement of cement also, the compression strength values were highly improved by using LP replacement of cement at different ages for (hot and normal curing). The highly compressive strength values for UHPSCC mixes with a 20% replacement ratio of GP and LP as cement replacement materials were 119.0 and 128.8 MPa under hot curing regimes and increased by 6.25% and 9.62%, respectively, than that of similar mixes under normal curing regimes at 90 days. The highly splitting and flexural strength values for UHPSCC 7 mix with 20% replacement level of LP and UHPSCC 9 mix with 20% replacement level of LP and GP were reported at 11.80 and 17.85 MPa which increased by 24.20% and 58.60%, respectively, compared to the control mix.

Semi-Reliability Probability Damage Assessment of GFRP Bars Embedded in Steam-Curing Concrete Beams Based on the Multiple Factors Related Moisture Absorption Model.

GFRP bars will be damaged due to a series of irreversible hygroscopic chemical reactions under humid and hot curing environmental conditions. The multiple factors related to the moisture absorption model were established through the moisture absorption test of GFRP bars embedded in steam-curing concrete, which considered different curing temperatures, different thicknesses of the protective layer, and different diameters of GFRP bars. Semi-reliability probability damage assessment of GFRP bars embedded in steam-curing concrete was described by introducing the reliability and stochastic theory. Subsequently, the tensile test of GFRP bars was carried out to verify the feasibility of the damage assessment. The results showed that the moisture absorption curves of GFRP bars were basically in line with Fick’s law. In addition, the influences of the curing temperature, the thickness of the protective layer, and the diameter on moisture absorption performance were presented. The semi-reliability probability damage assessment model of GFRP bars embedded in steam-curing concrete beams adequately considered the multiple factors related to moisture absorption and the uncertainty and randomness of the influencing factors during the process of moisture absorption.

Chipboards as raw material of furniture

The problem of using wood materials in residential premises is closely related to the issues of forestry. This is due to the fact that the wood subsequently processed into materials and products emits a gas harmful to humans - formaldehyde. In Russia, there are strict requirements for the permissible level of formaldehyde in the air (0.01 mg/m3) both in residential premises and in the atmosphere. The environmentally friendly wood emits formaldehyde. The emission of formaldehyde may exceed the permissible level up to 10 times when obtaining chipboards using urea-formaldehyde and phenol-formaldehyde resins. Such an excess of the permissible level of formaldehyde is especially characteristic for chipboards. Moreover, a much larger amount of formaldehyde is emitted from urea-formaldehyde and phenol-formaldehyde resins during their hot curing than it is in the liquid resin in its free form. Therefore, much attention in this work is paid to the curing process of urea-formaldehyde and phenol-formaldehyde resins in relation to the manufacturing conditions of chipboards. The study of this process was based on the models related to the manufacturing conditions of chipboards using the microscopy method. It was found that some formaldehyde is preserved in vapor-gas bubbles and eventually moves out the chipboards.

Robustness to water and temperature, and activation energies of metakaolin-based geopolymer and alkali-activated slag binders

The aim of this paper is to evaluate the properties of four alkali-activated materials subjected to precasting and in situ concreting conditions: fluctuations in water content (±5%) and in temperature (10 °C, 20 °C, 30 °C). The initial slump and compressive strengths at 1, 2, 7, 28 and 90 days of a metakaolin-based geopolymer, and of sodium silicate-activated slag, sodium metasilicate-activated slag and sodium carbonate-activated slag mortars were determined, then compared to the properties of an ordinary Portland cement mortar (CEM I). A total of 50 mortars were cast for the five formulations studied and the different conditions. This paper reports that alkali-activated materials are as robust as CEM I in precasting and in situ concreting conditions. However, some adjustments should be made to offset the low reactivity in cold weather, such as hot water or hot curing, or reducing the water/binder ratio. The apparent activation energies were determined for the five pastes to explain these observations.

Effect of Accelerated Curing on Strength of Quaternary Blended Cement Concrete with Recycled Aggregate

This paper presents the results of an experimental investigation carried out on accelerated curing of Quaternary Blended Cement (QBC) Concrete with and without recycled aggregate. Cement is partially replaced with fly ash, micro silica and nanosilica to produce QBC Concrete. The variables of study include the grade of concrete, powder content and percentage of recycled aggregate. Two grades of concrete M-40 and M-60 were used in this investigation. Based on the earlier studies, fly ash and micro silica are kept constant as 20% and 10% respectively while nanosilica is varied as 2% and 3%. Three percentages of recycled aggregate as partial replacement of natural aggregate (0%, 50% and 75%) were used. Two methods of curing were employed; boiling water method and hot air curing. The test results are encouraging and it is observed that 90% of the design strength could be achieved in one day for both the grades of concrete by curing at 100°C for a period of 3 hours. Keywords: Quaternary Blended Cement Concrete (QBCC), Fly ash (FA), Micro silica (SF), nanosilica (NSF), Recycled Concrete Aggregate (RCA), Accelerated Curing.

Study of thermal resistance of cold mixtures with clay addition

The article analyzes the results of heat resistance of cold-harmful mixtures with the addition of clay. The article presents the results of an experimental laboratory study of the properties of samples from CTS, obtained by various methods. The scientific novelty of the technology lies in the fact that the technology of casting in cold-rolled steel ensures high quality of the casting surface, the absence of gas defects and blockages in the casting. The selection of the composition and technological parameters for the manufacture of the mold, contributing to the production of high-quality defect-free castings, will significantly reduce their cost, which will have a positive effect on the economy of our country. The study of the change in density over time depending on the applied load was studied in a series of experiments. The density of the mixture was determined every thirty seconds during the entire time of the formation of the dispersed medium. These experiments showed that at the beginning of the formation of the shell, its density increases significantly. In works on the thermal stability of polymers, the general laws of thermal decomposition and carbonization of cyclic, spatially crosslinked hot curing polymers were investigated and it was shown that the kinetics of destruction, the composition of gaseous products, the amount of coke formed and its properties depend on the properties of the initial polymers - the degree of crosslinking of molecular chains, chemical structure, the presence of impurities of catalysts and other curing conditions, as well as the heating conditions - speed and temperature. The article summarizes some of the results of the study, which is most appropriate for thin-walled steel castings to use the following mixture composition: filler - quartz sand; binder - clay of the Kazakhstani deposit, epoxy resin, hardener.