Resin Viscosity Research Articles

Photocurable elastomers with tunable mechanical properties for 3D digital light processing printing

Digital light processing (DLP), a subclass of vat photopolymerization, is one of the most efficient additive manufacturing (AM) techniques that can be used to print complex objects with high precision at a greater speed than those of the other AM processes. However, printed objects are typically brittle because networks of cured resins are highly crosslinked, thereby limiting their range of possible applications. Currently, only a few commercial photocurable elastomers used for DLP can produce materials with moderate tensile strain, poor strength, and low modulus. Herein, we present a simple method for preparing photocurable resins that not only exhibit a low viscosity at room temperature, but also demonstrate tunable elasticity and strength. These novel resins are prepared from commercially available acrylate-based oligomers and monomers, which are appropriately selected to achieve a variety of mechanical properties. Subsequenlty, key parameters are investigated, including the viscosity of the uncured resin, critical energy at which the resin begins to solidify (Ec), depth of penetration of the curing light (Dp), and degree of curing. The viscosities of the developed resins range between 0.85 and 3.10 Pa s, and their curing and mechanical properties can be adjusted by varying the oligomer-to-monomer weight ratio. The Ec (6.9–9.2 mJ cm−2) and Dp (192–271 μm) of all the uncured resins are obtained from the working curves. The developed photoelastomers exhibit a wide range of tensile properties (tensile strain of 110–365%, tensile strength of 0.8–10 MPa, and modulus of 0.5–10 MPa), thereby facilitating their use in various applications.

Formulation of a Model Resin System for Benchmarking Processing-Property Relationships in High-Performance Photo 3D Printing Applications.

A well-defined resin system is needed to serve as a benchmark for 3D printing of high-performance composites. This work describes the design and characterization of such a system that takes into account processability and performance considerations. The Grunberg–Nissan model for resin viscosity and the Fox equation for polymer Tg were used to determine proper monomer ratios. The target viscosity of the resin was below 500 cP, and the target final Tg of the cured polymer was 150 °C based on tan-δ peak from dynamic mechanical analysis. A tri-component model resin system, termed DA-2 resin, was determined and fully characterized. The printed polymer exhibited good thermal properties and high mechanical strength after post-cure, but has a comparatively low fracture toughness. The model resin will be used in additive manufacturing of fiber reinforced composite materials as well as for understanding the fundamental processing–property relationships in light-based 3D printing.

Investigation of the effects of silica aerogel particles on thermal and mechanical properties of epoxy composites

Aerogel particle filled epoxy was fabricated in this work by investigating the effect of resin viscosity on pore infiltration and density of resulting composites. Aerogel porous structure was successfully preserved by carefully selecting epoxy viscosity for mixing procedure. The effect of aerogel content and particle size was subsequently investigated on thermal conductivity and compressive properties of epoxy composites. The results showed that incorporating aerogel particles into the resin could lead to over 40% reduction in both density and thermal conductivity. However, there was a linear trade-off between compressive properties and thermal conductivity of silica aerogel/epoxy composites. A wide range of aerogel particle sizes was also studied and it was found that the size effect was dependent on the aerogel content. Overall, the results of the presented work allow conclusions to be drawn regarding the usefulness of delayed wet mixing technique and the impact of particle size and loading on aerogel filled resin composites.

Analysis of the non-isothermal resin impregnation process of carbon fibre reinforced composite wing spar structure

This paper presents non-isothermal resin impregnation process of high-performance light weight composite wing spar structure for trainer aircrafts. The manufacturing process of the spar is simulated in PAM-RTM software. In this research developed a reliable model of resin impregnation process simulation to determine the exact infusion time and optimize the resin inlet ports for large scale aircraft structure without using the expensive trial and error experimental approches. The work reported the use of different heating temperature processes to manufacture wing spar from carbon fibre-reinforced epoxy resin matrix composite laminates by using VARI. The influence of heating temperature during the infusion stage on the resin viscosity and impregnation time was analyzed. These results indicate that a proper heating temperature during the infusion process is necessary for quality control and manufacturing time reduction. The error between theoretical simulation and experimental studies is no more than 10%.

Effective cure cycle development via flow optimization and advanced cure environments

Out-time and moisture absorption in prepregs are generally unavoidable in an industrial setting, where prepreg layup can take weeks. The resin cross-linking and viscosity increase that occurs during out-time can reduce resin flow during a specified cure cycle, causing porosity. In this study, a flow-optimized cure cycle was developed, leveraging both flow level and time during out-of-autoclave (OoA) processing. First, predictive cure kinetics and viscosity models were used to model viscosity evolution under selected cure conditions, accounting for initial out-times and humidity conditioning. To quantify resin flow, an ‘effective flow number’ metric was defined as the integration of inverse viscosity evolution until the resin gelation point. The method described revealed that a rapid heating rate achieved by use of advanced tooling was essential to achieve a high effective flow number. The experimental results showed that the effective flow number is a useful criterion to limit flow-induced defects. The method presented also extends the boundary (by 175%) of the manufacturer’s specified out-life for OoA prepreg materials.

가정간편식 용기용 바이오매스 기반 발포구조체의 특성에 관한 연구

A series of foamed plastic sheets containing biomass (as HMR container) were developed via different foaming process temperatures, and their density, porosity, WVTR, and pore morphology were evaluated. Thermal stability of samples during re-heating the food in oven, change in morphology, density, porosity, and WVTR were investigated using a simulated thermal shock process according to MIL-STD-883E assay. As such, the pore size of samples was generally increased with increasing temperature of the foaming process. It can be explained that as foaming temperature increased, the viscosity of molten resins and the repulsive force against pore expansion decreased. In addition, an increase in the thermal shock cycle reduced the pore size and WVTR, while density increased because high temperature treatment that softened the sheet matrix was followed by a low temperature incubation, which contracted the matrix, thereby changing the physical and morphological properties of samples. However, an insignificant change in density was observed and WVTR tended to be decreased, indicating that as-prepared foamed plastic sheets could be used as a high thermal stable container for HMR application. Therefore, it found that the properties of newly developed HMR containers containing biomass were dependent on the foaming process temperature. Moreover, to better understanding of these newly developed containers, further investigations dealing with foaming process temperature based on various food items and cooking conditions are needed.

Improving glass transition temperature of unsaturated polyester thermosets: Conventional unsaturated polyester resins

AbstractHighly crosslinkable unsaturated polyester resins (UPR) have attracted many interests in the application as reinforced matrix materials. Here, we present a systematical study of the influence of different curing conditions and styrene concentrations on resin viscosity and dynamic‐mechanical properties of the thermoset. The pure maleic Palapreg® P18‐03 was selected as model UPR because of its broad industrial use. By applying newly developed thermal curing profiles (without thermal initiators) and by raising the styrene content, the Tg of the network could be increased up to 206/215°C (1/10 Hz). For the first time, a fast curable UPR based on propylene glycol and neopentyl glycol with a Tg of up to 215°C is described. A partial substitution of problematic styrene with methylmethacrylate, tert‐butylacrylate, and maleic anhydride (MA) was studied as well. MA leads to significantly improved resin reactivity. A resin containing 42 wt% styrene and 8 wt% MA yields thermosets with remarkably improved mechanical properties and with a narrower glass transition range compared to the original P18‐03.

Mechanical and thermal properties of microchannel insulating foams comprising a multifunctional epoxy/polyhedral oligomeric silsesquioxane nanocomposite

AbstractPolymer matrix composites are well suited for applications in space environments due to their excellent specific strengths and moduli, thermal properties, and tailorability. In efforts to further improve their performance, additives based on functionalized polyhedral oligomeric silsesquioxanes (POSS) have been successfully shown to increase long‐term durability by improving Atomic Oxygen (AO) and UV radiation resistance, as well as enhance both the thermal and mechanical properties of the polymer‐based structure. In the present work, an epoxy‐functionalized POSS has been used as a low‐percentage additive in the epoxy matrix structure of microchannel foams, resulting in improved mechanical properties and thermal stability without significantly increasing the resin viscosity, final bulk density, or thermal conductivity of the final foam structures. These nanocomposite POSS resins are also expected to display increased AO and UV longevity in space applications, allowing these multifunctional materials to not only achieve high‐performance capabilities but also be economically suitable for devices used in space environments.

Preparation of rapid (chain-stopped) alkyds by incorporation of gum rosin and investigation of coating properties.

In this study, the synthesis, characterization, and properties of a short oil length chain–stopped(rapid) alkyd resin is investigated. Gum rosin modified alkyd resin (RA-GR) was prepared using soybean oil, phthalic anhydride, glycerin and gum rosin acid. An alkyd modified with benzoic acid (RA-BA) was also prepared for comparison. FTIR analyses and GPC measurements of the alkyds were used for characterization. Other properties such as the viscosity, acid value, and solid content of the final resins were determined. Separately, the synthesized resins were used in paint formulations without any changes in other parameters such as filler, airdrying agents, solvents, etc. Paints were applied to metal and glass surfaces and the effect of gum rosin was investigated by looking at touch and hard drying times, adhesion to metals and gloss changes. Compared to the benzoic acid modified resin (RA-BA), gum rosin modified resin (RA-GR) exhibited remarkable positive effects on the paint with a better adhesion to the metals, and short drying times without any loses in the glosses.

A Bayesian belief approach to quality control of resin transfer molding process

In recent years, there has been a significant global shift towards use of polymer matrix composite materials in a wide range of industries, including aerospace, automotive, marine, and sports, among others. Despite the rapid uptake and widespread adoption of this material technology, there are still technical challenges faced daily by manufactures due to the inherent complexities of different composite processing techniques. This paper aims at establishing a new scheme for better quality control management of resin transfer molding (RTM) processing for thermosetting composites, using a Bayesian belief network (BBN). The data were collected through knowledge engineering among the manufacturing experts with the help of real-life application history. A total of 13 governing factors for the RTM process, which would theoretically have a role on the final product quality, were identified. The sensitivity analysis of the BBN showed that the major contributing factors of the quality are the resin viscosity profile, part design features, resin cure peak temperature, and reinforcement permeability. The conditional probability tables were constructed using a quality index from industrial experts, and the causal relationships captured by the BBN were built using knowledge engineering. It is also shown how the basic BBN model can be further updated by integrating the interaction weights between the attributes that define the product quality.

Improving the properties of urea-lignin-glyoxal resin as a wood adhesive by small addition of epoxy

The aim of this work was to improve the properties of plywood panels bonded with urea-lignin-glyoxal (ULG) resin by small addition of epoxy resin. For this reason, various contents (1, 3 and 5% on resin solids) of an epoxy resin were added to the prepared ULG resin. The structural, thermal and physicochemical properties of the resins prepared as well as the water absorption and wet and dry shear strength of the plywood panels bonded with them were measured according to standard methods. The results showed that the addition of an epoxy resin significantly accelerated the gel time and increased the ULG resin viscosity, density and solids content. The cross-linking structure in the cured ULG resin with the epoxy resin was confirmed by FTIR analysis. The curing temperature of the ULG resin also decreases by addition of the epoxy resin as shown by differential scanning calorimetry (DSC). As a consequence, the plywood panels bonded with ULG resins containing any amount of the epoxy resin presented a better dimensional stability than those bonded with the control resin. The dry and wet tensile shear strength of plywood panels bonded with ULG resins indicate that they could be significantly improved by increasing the percentage of the epoxy resin added.

Impedance spectroscopy study of anticorrosive properties of epoxy compositions

Although metals and their alloys are important structural materials, electrochemical or chemical interactions between metal structures and the environment leads to their spontaneous destruction. In order to protect metal products from corrosion, epoxy resins and polymer composite materials can be applied. However, polymer-coated metals may degrade under operating conditions due to electrochemical reactions at the polymer-metal interface caused by water absorption and diffusion in epoxy coatings. The present study is aimed at an investigation of the anticorrosive behaviour characteristic of epoxy compositions following exposure to sulphuric acid. The method of impedance spectroscopy was applied to evaluate epoxy coatings on a steel base. The composition of the epoxy binder included bisphenol A resins cured with various amine curing agents. In order to identify structural changes in materials, as well as the changes in their protective properties, the effects of an aggressive environment were simulated by means of exposure to concentrated sulphuric acid over a 30-day period. Impedance hodographs of the studied systems were obtained and equivalent schemes for the approximation of experimental data proposed. The high porosity of the NPEK-114L based epoxy system was established to result in an increase in the corrosion rate. Dissolution of corrosion products over a longer test period – and consequent higher diffusion of corrosive ions in the coating – was determined to cause a decrease in corrosion resistance. Epoxy compositions based on NPEL-128 resin (oligomeric product based on diphenylolpropane diglycidyl ether) demonstrated improved anticorrosion characteristics. The addition of alkyl glycidyl ether for NPEK-114L resin viscosity reduction was shown to affect the protective properties of the composite under acid exposure.

A preform deformation and resin flow coupled model including the cure kinetics and chemo-rheology for the VARTM process

The present paper deals with preform deformation and resin flow coupled to cure kinetics and chemo-rheology for the VARTM process. By monitoring the coupled resin infusion and curing steps through temperature control, our primary aim is to reduce the cycle time of the process. The analysis is based on the two-phase porous media flow and the preform deformation extended with cure kinetics and heat transfer. A novel feature is the consideration of temperature and preform deformation coupled to resin viscosity and permeability in the VARTM process. To tackle this problem, we extend the porous media framework with the heat transfer and chemical reaction, involving additional convection terms to describe the proper interactions with the resin flow. Shell kinematics is applied to thin-walled preforms, which significantly reduces the problem size. The proposed finite element discretized system of coupled models is solved in a staggered way to handle the partially saturated flow front under non-isothermal conditions efficiently. From the numerical example, we conclude that the cycle time of the VARTM infusion process can be shortened over 68% with the proper temperature control. Moreover, the proposed framework can be applied to optimize the processing parameters and check the compatibility of a resin system for a given infusion task.

Low‐Viscosity Limonene Dimethacrylate as a Bio‐Based Alternative to Bisphenol A‐Based Acrylic Monomers for Photocurable Thermosets and 3D Printing

AbstractBisphenol A glycidyl methacrylate (BisGMA) is well established as photocurable resin in dental restoratives and 3D printing. At present there are raising concerns regarding the estrogen‐mimicking bisphenol A (BPA) contamination of health care and consumer products. It is an important challenge to substitute BPA‐based resins for bio‐based cycloaliphatic monomers while lowering resin viscosity without sacrificing high stiffness and glass temperature. Particularly high viscosity is critical for 3D printing by photopolymerization. Unlike BPA the cyclic monoterpene limonene, extracted from citrus fruit peels, is safe in human uses. Herein it is reported on limonene‐based dimethacrylate (LDMA) tailored for 3D printing application and derived from limonene oxide (LO) and methacrylic acid (MA). Residual MA is converted into glycerol dimethacrylate (GDMA) serving as an in situ reactive diluent. The influences of temperature, catalysts, MA/LO stoichiometry, and the addition of glycidyl methacrylate (GMA) and magnesium oxide on the LDMA‐based resin performance are elucidated. As compared to BisGMA (560 Pa s) LDMA‐based resins exhibit significantly lower viscosity (5–117 Pa s) governed by the MA/LDMA molar ratio and the GMA addition. At 30 wt% LDMA content photocured resin yields thermosets having high Young’s Modulus (3.4–3.7 GPa), tensile strength (88–98 MPa), and glass transition temperature (119–135 °C), surpassing the performance of the corresponding BisGMA‐based resins.

Tuning the architecture and performance of multifarious benzoxazine resin based on guaiacol and polyethylenimine

Molecular designing flexibility offered by benzoxazine chemistry, has been utilized to synthesize side-chain benzoxazine functionalized resins, in one-step approach via reaction of polyethylenimine (PEI) and guaiacol. The functionalization of polyethylenimine bearing amine functionalities, with benzoxazine groups as side-chains, is expected to confer a densely cross-linked network in the resulting thermoset. In the present work, guaiacol has been chosen as bio-based phenol, and polyethylenimine enriched with primary, secondary, tertiary amine groups, has been employed as an amine co-reactant. The reactants stoichiometry (phenol: amine: paraformaldehyde) has been varied from 2:1:4 to 20:1:40 molar ratio, resulting in a series of side-chain type guaiacol based benzoxazine-functionalized polyethylenimine resins (G-pei), which differ in percentage of oxazine functionalization. All the resins prepared have been characterized using FTIR, 1H, 13C NMR spectroscopies, and the evolution of oxazine functionalization in the resins has been monitored via 1H NMR spectroscopy. The polymerization behaviour in resins has been evidenced using non-isothermal DSC studies, which reveals that G-pei resins exhibit peak polymerization temperature (Tpeak) below 200 °C. Temperature sweep rheological experiments have been performed to obtain flow behaviour and processing window of the developed resins, where an evident reduction in the viscosity of resins has been observed around 100 °C, thereby making it an appropriate temperature for processing. The influence of polar groups on the surface property of thermosets has been investigated by performing contact angle measurements, which reveals the hydrophilic nature of prepared thermosets. Thermal degradation behaviour of the multifarious bio-based thermosets, has been studied using thermogravimetric analysis, where maximum degradation temperature (Tmax) for the cured specimens has been found to be above 300 °C, with a high char yield (30–50%). The influence of dense network on the glass transition temperature of thermosets, has been additionally studied by DSC experiments. The adhesive performance of side-chain type benzoxazine-functionalized polyethylenimine resins, enriched with oxazine functionalities as well as polar groups, has been evaluated by lap shear strength values, which have been observed in the range of 22 to 49 Kg/cm2.

Improved Fracture Toughness and Conversion Degree of Resin-Based Dental Composites after Modification with Liquid Rubber.

There are many methods widely applied in the engineering of biomaterials to improve the mechanical properties of the dental composites. The aim of this study was to assess the effect of modification of dental composites with liquid rubber on their mechanical properties, degree of conversion, viscosity, and cytotoxicity. Both flow and packable composite consisted of a mixture of Bis-GMA, TEGDMA, UDMA, and EBADMA resins reinforced with 60 and 78 wt.% ceramic filler, respectively. It was demonstrated that liquid rubber addition significantly increased the fracture toughness by 9% for flow type and 8% for condensable composite. The influence of liquid rubber on flexural strength was not statistically significant. The addition of the toughening agent significantly reduced Young’s modulus by 7% and 9%, respectively, while increasing deformation at breakage. Scanning electron microscopy (SEM) observations allowed to determine the mechanisms of toughening the composites reinforced with ceramic particles. These mechanisms included bridging the crack edges, blocking the crack tip by particles and dissipation of fracture energy by deflection of the cracks on larger particles. The degree of conversion increased after modification, mainly due to a decrease in the matrix resin viscosity. It was also shown that all dental materials were nontoxic according to ISO 10993-5, indicating that modified materials have great potential for commercialization and clinical applications.

Electrical Resistance Sensing of Epoxy Curing Using an Embedded Carbon Nanotube Yarn.

Curing effects were investigated by using the electrical response of a single carbon nanotube yarn (CNTY) embedded in an epoxy resin during the polymerization process. Two epoxy resins of different viscosities and curing temperatures were investigated, varying also the concentration of the curing agent. It is shown that the kinetics of resin curing can be followed by using the electrical response of an individual CNTY embedded in the resin. The electrical resistance of an embedded CNTY increased (~9%) after resin curing for an epoxy resin cured at 130 °C with viscosity of ~59 cP at the pouring/curing temperature (“Epon 862”), while it decreased (~ −9%) for a different epoxy cured at 60 °C, whose viscosity is about double at the corresponding curing temperature. Lowering the curing temperature from 60 °C to room temperature caused slower and smoother changes of electrical resistance over time and smaller (positive) residual resistance. Increasing the concentration of the curing agent caused a faster curing kinetics and, consequently, more abrupt changes of electrical resistance over time, with negative residual electrical resistance. Therefore, the resin viscosity and curing kinetics play a paramount role in the CNTY wicking, wetting and resin infiltration processes, which ultimately govern the electrical response of the CNTY immersed into epoxy.

Rheological Study of Phenol Formaldehyde Resole Resin Synthesized for Laminate Application.

Heat explosions are sometimes observed during the synthesis of phenol formaldehyde (PF) resin. This scenario can be attributed to the high latent heat that was released and not dissipated leading to the occurrence of a runaway reaction. The synthesis temperature and time played important roles in controlling the heat release, hence preventing the resin from hardening during the synthesis process. This study aims to assess the rheological and viscoelasticity behaviors of the PF resin prepared using paraformaldehyde. The prepared PF resin was designed for laminate applications. The rheological behavior of the PF resin was assessed based on the different molar ratios of phenol to paraformaldehyde (P:F) mixed in the formulation. The molar ratios were set at 1.00:1.25, 1.00:1.50 and 1.00:1.75 of P to F, respectively. The rheological study was focused at specific synthesis temperatures, namely 40, 60, 80 and 100 °C. The synthesis time was observed for 240 min; changes in physical structure and viscosity of the PF resins were noted. It was observed that the viscosity values of the PF resins prepared were directly proportional to the synthesis temperature and the formaldehyde content. The PF resin also exhibited shear thickening behavior for all samples synthesized at 60 °C and above. For all PF resin samples synthesized at 60 °C and above, their viscoelasticity results indicated that the storage modulus (G′), loss modulus(G″) and tan δ are proportionally dependent on both the synthesis temperature and the formaldehyde content. Heat explosions were observed during the synthesis of PF resin at the synthesis temperature of 100 °C. This scenario can lead to possible runaway reaction which can also compromise the safety of the operators.

Flow modeling and simulation study of vacuum assisted resin transfer molding (VARTM) process: A review

In recent times, vacuum-assisted resin transfer molding (VARTM) has become a promising technique for the processing of large composite structures. On the contrary, there are few issues related to VARTM processing as many process parameters are involved and required to optimize them to achieve high fiber volume fraction containing composite material. These issues can be addressed and resolved by an understanding of various flow modeling techniques and simulation methods. In this present study, a review of different approaches in flow modeling and simulation of the VARTM process is presented. The processing technology of VARTM along with the fundamental and constitutive models for mold filling stage as well as curing stage such as permeability, compaction, resin viscosity and cure kinetics are presented. The numerical simulation methods adopted by various researchers used to simulate the filling processes with their specific applications and simulation software are also reported and reviewed. As an outcome, identification of permeability of various fibers and simulation processing time are more predominant factors to perform accurate VARTM simulation.

Hot Mixing Behavior and Curing Process of Epoxy Asphalt

A new hot mixed epoxy asphalt system was developed. The reaction process of epoxy resin was characterized by Fourier transform infrared spectroscopy (FTIR). The viscosity was investigated by Brinell viscometer when epoxy resin was mixed with asphalt. The glass transition temperature (Tg), homogeneity, thermal stability and viscoelasticity of epoxy asphalt were analyzed by differential scanning calorimetry (DSC), metallographic microscope, thermogravimetric (TG) and dynamic mechanical analysis (DMA). The results showed that the viscosity of epoxy resin modified asphalt reached 1 000 mPa·s for more than 180 minutes at 180 °C, while the best construction process is mixing at 180 °C for 2 hours and curing at 60 °C for 4 days. The particle size of asphalt is less than 50 µm. In addition, the mechanical properties of the materials are uniform within the specified pavement thickness.