Thermal Post-curing Research Articles

Photopolymerization-based 4D-printing of shape-memory materials containing high-performance polymers

High-performance aromatic heterochain polymers are engineering thermoplastics with exceptional mechanical and thermal properties that have attracted great interest in various areas ranging from aerospace to biomedicine. However, there have been a number of difficulties to 3D-print materials based on such polymers with new promising performance characteristics. Herein, a number of new photosensitive compositions (PSCs) based on high-performance polyetherimide (PEI) or polysulfone (PSU), reactive functional monomer (N,N-dimethylacrylamide) and oligomer (bisphenol A ethoxylate diacrylate) has been developed. It has been shown that the use of the developed PSCs allows the formation of 3D-structures with high printing resolution by LCD 3D-printing. Subsequent thermal post-curing of 3D-printed green-state samples at 250°С for 1 h led to the fabrication of materials with the highest tensile strength (up to 41.9 ± 3.1 MPa), glass transition temperature (141 °C) and thermal stability (above 350 °C). In addition, 3D-printed structures demonstrate high-temperature shape memory effect with shape fixity ratio > 99% and shape recovery ratio up to 97.1%.

4D-printing of high-temperature shape-memory polymers based on polyimide, N,N-dimethylacrylamide and photoactive cross-linkers

Thermally responsive shape memory polyimides (PIs) have attracted significant attention in the high-tech fields like aerospace industry. At the same time creating of materials and approaches to the formation of three-dimensional (3D) shape memory structures containing PIs remains an urgent task. In this work we have developed new photopolymeric compositions (PPCs) based on high-performance readily soluble PI, reactive solvent (N,N-dimethylacrylamide) and photoactive cross-linker (bisphenol A ethoxylate diacrylate (BAEDA) or tris[2-(acryloyloxy)ethyl] isocyanurate (TAI)). The resulting PPCs were used to form 3D-structures using LCD 3D-printing. It has been found that the nature of the cross-linker and post-processing temperature have a significant impact on the thermal and mechanical properties of the materials. The tensile strength has the maximum value after the thermal post-curing at 250°С for 1 h (83.3 ± 7.4 MPa and 60.8 ± 4.8 MPa for PPCs with TAI and BAEDA, respectively). Moreover, 4D-printed PI-containing materials exhibit an excellent shape memory performance at recovery temperatures >150 °C, with shape fixity ratio >99% and shape recovery ratio up to 95.6%.

Impact of Bark-Sourced Building Blocks as Substitutes for Fossil-Derived Polyols on the Structural, Thermal, and Mechanical Properties of Polyurethane Networks.

The hydrophilic extractives isolated from black alder (Alnus glutinosa) bark through hot water extraction were characterized as novel renewable macromonomers capable of forming polyurethane (PU) networks based on a commercial polyisocyanate, with partial or complete replacement of petroleum-derived polyol polyether. The bark-sourced bio-polyol mainly consists of the xyloside form of the diarylheptanoid oregonin, along with oligomeric flavonoids and carbohydrates, resulting in a total OH group content of 15.1 mmol·g-1 and a molecular weight (Mn) of approximately 750 g∙mol-1. The 31P NMR data confirmed a similar proportion of aliphatic OH and phenolic groups. Three-component PU compositions were prepared using polyethylene glycol (Mn = 400 g∙mol-1), bio-polyol (up to 50%), and polymeric diphenylmethane diisocyanate, which were pre-polymerized in tetrahydrofuran (THF) solution with tin organic and tertiary amine catalysts. The resulting mixture was cast and subjected to thermal post-curing. Calculation and experimental data confirmed the crosslinking activity of the bark-sourced bio-polyol in PU, leading to an increase in glass transition temperature (Tg), a decrease in sol fraction yield upon leaching of cured PU networks in THF, a significant increase in Young's modulus and tensile strength. The macromonomers derived from bark promoted char formation under high temperature and oxidative stress conditions, limiting heat release during macromolecular network degradation compared to bio-polyol-free PU. It was observed that amine catalysts, which are active in urethane formation with phenolic groups, promoted the formation of PU with higher Tg and modulus at tensile but with less limitation of heat liberation during PU macromolecular structure degradation. The high functionality of the bark-derived bio-polyol, along with the equal proportion of phenolic and aliphatic OH groups, allows for further optimization of PU characteristics using three variables: increasing the substitution extent of commercial polyethers, decreasing the NCO/OH ratio, and selecting the type of catalyst used.

Simultaneous rheology and cure kinetics dictate thermal post-curing of thermoset composite resins for material extrusion.

Thermoset composites are excellent candidates for material extrusion because they shear thin during extrusion but retain their shape once deposited via a yield stress. However, thermal post-curing is often required to solidify these materials, which can destabilize printed parts. Elevated temperatures can decrease the rheological properties responsible for stabilizing the printed structure before crosslinking solidifies the material. These properties, namely the storage modulus and yield stress, must therefore be characterized as a function of temperature and extent of reaction for various filler loadings. This work utilizes rheo-Raman spectroscopy to measure the storage modulus and dynamic yield stress as a function of temperature and conversion in epoxy-amine resins with fumed silica mass fractions up to 10 %. Both rheological properties are sensitive to conversion and particle loading, but only the dynamic yield stress is reduced by elevated temperatures early in the cure. Notably, the dynamic yield stress increases with conversion well before the chemical gel point. These findings motivate a two-step cure protocol that starts at a low temperature to mitigate the drop in dynamic yield stress, then ramps up to a high temperature when the dynamic yield stress is no longer at risk of decreasing to rapidly drive conversion to near completion. The results suggests that structural stability can be improved without resorting to increasing filler content, which limits control over the final properties, laying the groundwork for future studies to evaluate the stability improvements provided by the multi-step curing schedules.

A photosensitive composition based on an aromatic polyamide for LCD 4D printing of shape memory mechanically robust materials

High-temperature shape memory polymers (SMPs) draw growing interest from researchers due to their potential as materials for application in the field of high-temperature smart devices. However, in the majority of cases the processing of high-temperature SMPs is limited by fabrication of two-dimensional films. Here, we have prepared a photosensitive composition based on high-performance heat resistant poly-N,N′-(m-phenylene)isophthalamide (MPA) and photosensitive components (N,N-dimethylacrylamide and bisphenol A ethoxylate diacrylate (BAEDA)). It has been shown by IR spectroscopy, that LCD 3D printing followed by post-curing at 150 °C results in the formation of semi-interpenetrating polymer networks, while thermal processing at a temperature ≥200 °C leads to the formation of fully crosslinked structures due to the interaction of NH groups from MPA and acrylic groups from BAEDA via the Michael addition. The post-curing temperature has a significant effect on the thermal and mechanical properties of materials. Thermal post-curing of 3D printed samples at 250 °C for 1 h results in the fabrication of materials with the highest tensile strength (92.4 ± 6.1 MPa), glass transition temperature (148 °C) and thermal decomposition temperature (above 350 °C). Remarkably, MPA-based materials exhibit an excellent shape memory performance at temperatures >150 °C, with fixity ratio (Rf) of 99.7 % and recovery ratio (Rr) of 99.9 %. Moreover, we showed that the glass transition temperature decreased by no more than 1 % after γ-radiation of 200 Gy. In addition, the material maintains a high storage modulus and good shape memory performance after being irradiated, and thus has a great potential in the field of aerospace structural materials.

Heat-Resistant Phthalonitrile-Based Resins for 3D Printing via Vat Photopolymerization

In this study, a maleimide/phthalonitrile bifunctional monomer PNB-M was used in vat photopolymerization via the maleimide group to obtain phthalonitrile-containing polymers suitable for further thermal postcuring. A high concentration (40 wt %) and low viscosity (less than 500 mPa·s at 25 °C) photopolymer resin was prepared from the phthalonitrile monomer PNB-M with 4-acryloylmorpholine as comonomer and trimethylolpropane trimethacrylate as cross-linker. Photo-DSC, NMR, and FTIR tests revealed free-radical photoinduced copolymerization of the maleimide with the acrylic groups during 3D printing. Postprinting thermal curing of the phthalonitrile groups was performed with different curing agents as well as via self-initiation by PNB-M. The developed phthalonitrile-containing polymerizable compositions made it possible to increase glass transition temperatures of the 3D printed polymer materials up to 351 °C after additional thermal postcuring. Moreover, mechanical properties of the 3D printed materials have been improved after curing of the phthalonitrile monomer in the maleimide–acrylate copolymer.

Cationic photopolymerizationand thermal post-curing of epoxy/TiO2 and epoxy/BN nanocomposites

Cycloaliphatic epoxy resins containing cationic initiators which are activated by UV radiation give rise to thermoset networks when are exposed to UV light. On the other hand, due to its good mechanical properties, adhesive, thermal stability and electrical insulation epoxy resins are widely used in electrical and electronic applications.The inclusion of ceramic nanoparticles improves the properties of epoxy thermosets, but can influence the curing process. The objective of this work is to study the curing process of nanoparticle dispersions of boron nitride and titanium oxide in a cycloaliphatic epoxy curable epoxy resin. This study has been carried out using photo-calorimetry (UV-DSC) and traditional DSC. The effect of the presence of BN and TiO2 nanoparticles in the ultraviolet curing has been analyzed, finding that the UV curing is not complete, and a subsequent thermal post-curing stage is necessary. It has been found that TiO2 hinders the reaction reaching lower total conversions after both curing stages. Finally, in order to study the thermo-mechanical properties of the nanocomposites, thick specimens were prepared by applying the two stages of UV and thermal curing, the presence of nanoparticles leading to a more heterogeneous specimens. FTIR analysis of curing has shown that the surface of KBr support influences the curing reaction.

Development and Characterization of Field Structured Magnetic Composites

Polymer composites containing ferromagnetic fillers are promising for applications relating to electrical and electronic devices. In this research, the authors modified an ultraviolet light (UV) curable prepolymer to additionally cure upon heating and validated a permanent magnet-based particle alignment system toward fabricating anisotropic magnetic composites. The developed dual-cure acrylate-based resin, reinforced with ferromagnetic fillers, was first tested for its ability to polymerize through UV and heat. Then, the magnetic alignment setup was used to orient magnetic particles in the dual-cure acrylate-based resin and a heat curable epoxy resin system in a polymer casting approach. The alignment setup was subsequently integrated with a material jetting 3D printer, and the dual-cure resin was dispensed and cured in-situ using UV, followed by thermal post-curing. The resulting magnetic composites were tested for their filler loading, microstructural morphology, alignment of the easy axis of magnetization, and degree of monomer conversion. Magnetic characterization was conducted using a vibrating sample magnetometer along the in-plane and out-of-plane directions to study anisotropic properties. This research establishes a methodology to combine magnetic field induced particle alignment along with a dual-cure resin to create anisotropic magnetic composites through polymer casting and additive manufacturing.

PDMS Curing Inhibition on 3D-Printed Molds: Why? Also, How to Avoid It?

Three-dimensional (3D)-printing techniques such as stereolithography (SLA) are currently gaining momentum for the production of miniaturized analytical devices and molds for soft lithography. However, most commercially available SLA resins inhibit polydimethylsiloxane (PDMS) curing, impeding reliable replication of the 3D-printed structures in this elastomeric material. Here, we report a systematic study, using 16 commercial resins, to identify a fast and straightforward treatment of 3D-printed structures and to support accurate PDMS replication using UV and/or thermal post-curing. In-depth analysis using Raman spectroscopy, nuclear magnetic resonance, and high-resolution mass spectrometry revealed that phosphine oxide-based photo-initiators, leaching out of the 3D-printed structures, are poisoning the Pt-based PDMS catalyst. Yet, upon UV and/or thermal treatments, photo-initiators were both eliminated and recombined into high molecular weight species that were sequestered in the molds.

Additive Manufacturing for Automotive Applications: Mechanical and Weathering Durability of Vat Photopolymerization Materials.

In this study, the effect of the post-curing process as well as the long-term weathering behavior were studied for three different three-dimensional printable, pigmented (black), nonstabilized, ultraviolet (UV) cure resin formulations (A and B being thiol-ene chemistry, and C being acrylate chemistry). To study the effect of the post-cure process, the printed parts were post-cured using one of five different processes: no post-cure, UV-only, heat-only, UV+heat, and electron beam (EB) post-curing. Bulk tensile properties and nanohardness were measured for each of the systems and post-cure conditions. For weathering studies, the parts were post-cured using the recommended UV-only process and exposed using the ASTM D7869 exterior weathering protocol. The results show that the post-cure process had a significant effect on the final mechanical properties of the resins and was dependent on the underlying resin chemistry. Thermal post-curing was not as effective as UV curing for Resin C compared with the two other resins, which could both undergo thermal polymerization. In addition, Resin B showed the smallest change in mechanical properties before and after post-curing, regardless of the type of post-curing process. EB post-curing, even at very low dosages, that is, from 0.05 to 1 Mrad, resulted in considerable post-cure cross-linking to the point of embrittlement and a significant drop in percent elongation at break for dosages above 0.5 Mrad. Although Resins A and C outperformed Resin B in photooxidation performance, all three resins demonstrated that promising results considering no hindered amine light stabilizers were used in the formulations.

Highly flexible, thermally stable, and static dissipative nanocomposite with reduced functionalized graphene oxide processed through 3D printing

In this study, we designed a reduced functionalized graphene oxide/acrylate nanocomposite processed using a facile 3D printing technology of stereolithography (SLA), simultaneously achieving flexibility, thermal stability, and static dissipation capacity. A one-step, highly efficient, and environmentally friendly ultrasonication method was developed for preparing functionalized graphene oxide (fGO), which demonstrated an excellent dispersion and stability in an acrylate derived solution, and significantly increasing the solid loading of fGO in suspensions for the SLA 3D printing. The introduction of 5 wt% fGO endowed the nanocomposite with impressive flexibility and mitigated dimensional shrinkage after the thermal post-curing. Additionally, facilitated by the thermal reduction of fGO inside the acrylate-based nanocomposite, the electrical conductivity of the nanocomposite (2.6 × 10−4 S/cm) was up to 108 times higher than that of the neat resin (5 × 10−12 S/cm), and the nanocomposite exhibited less than 0.1 s static decay time from 1000 to 100 V. Therefore, an SLA-printed nanocomposite with highly flexible, thermally stable, and static dissipative properties holds significant potential for smart electronics with sophisticated applications.

Novel Dual-Curing Process for a Stereolithographically Printed Part Triggers a Remarkably Improved Interlayer Adhesion and Excellent Mechanical Properties.

Stereolithography (SL) is widely used because of its numerous advantages over other three-dimensional printing (3DP) techniques. However, SL is a layer-by-layer process, where interlayer adhesion between adjacent layers becomes more brittle than the intralayer adhesion, common to all 3DP process. Here, we report a facile method to strengthen the interlayer adhesion for SL. By the addition of monomers with thermally curable functional groups (epoxy or hydroxyl groups), thermal post-curing induces chemical reactions between them in adjacent layers after the photoinitiated printing process. It leads to fully 3D cured objects with enhanced interlayer bonding and substantially improved mechanical properties, but not a significant change in the dimensional stability. This approach can expand the use of 3DP techniques in load-bearing applications that require mechanically robust printed objects with precise dimensions.

Thermal post-curing as an efficient strategy to eliminate process parameter sensitivity in the mechanical properties of two-photon polymerized materials.

Two-photon polymerization direct laser writing (TPP-DLW) is one of the most versatile technologies to additively manufacture complex parts with nanoscale resolution. However, the wide range of mechanical properties that results from the chosen combination of multiple process parameters imposes an obstacle to its widespread use. Here we introduce a thermal post-curing route as an effective and simple method to increase the mechanical properties of acrylate-based TPP-DLW-derived parts by 20-250% and to largely eliminate the characteristic coupling of processing parameters, material properties and part functionality. We identify the underlying mechanism of the property enhancement as a self-initiated thermal curing reaction, which robustly facilitates the high property reproducibility that is essential for any application of TPP-DLW.

Influence of temperature and BN nanoparticles on UV, thermal and dark curing of a cycloaliphatic epoxy resin

The influence of boron nitride (BN) nanoparticles on cationic photopolymerization of an epoxy resin at different temperatures was investigated employing photo-DSC. A difunctional cycloaliphatic epoxy resin was polymerized in the presence of triarylsulfonium hexafluoroantimonate salts (cationic photoinitiator). BN-epoxy dispersions containing 5 mass% of BN were obtained by sonication. The epoxy conversion during UV irradiation was determined at seven temperatures, from 30 to 90 °C. The conversion reached was strongly dependent on temperature, and lower conversions were obtained in the presence of BN. After the UV curing, the samples were thermally postcured by dynamic heating in the DSC. The thermal postcuring shows two exothermal peaks, being the main one dependent on the temperature of the previous UV curing and on the composition. The total achieved conversion (65–77%) is lower for the samples containing BN compared to neat epoxy. The UV-irradiated samples at 40 °C reached conversions between 24 and 36%; afterward, they were kept under isothermal dark curing (40 °C) up to 1 month leading to significant conversion increases, independently of the composition and reaching a final conversion between 60 and 70%. DMTA measurements of dark-cured specimens evidence a heterogeneous epoxy matrix, thus needing higher postcuring temperatures to get homogeneity. The Tg of the epoxy matrix is reduced in the presence of BN nanoparticles.

Influence of polymer/filler composition and processing on the properties of multifunctional adhesive wood bonds from polyurethane prepolymers I: mechanical and electrical properties

ABSTRACTMultifunctional adhesives out of a polymer matrix and electrically conductive fillers are known to typically decrease in bond strength with increasing filler content, while the electrical resistance drops extensively at the percolation threshold. Three experiments have focused on different aspects of the production of electrically conductive adhesive wood bonds, experiment I with a variation of the adhesive components, experiment II is varying the process parameters in the bonding process, and experiment III is investigating a thermal postcuring effect. Tensile shear strength (EN 302–1, requirement for structural wood bonding dry condition (A1 treatment)) and electrical resistography under direct current (DC measurements) of the bondline was used to identify major influences on tensile shear strength and the formation of the electrically conductive network. The results show high differences due to the adhesive polymer selection, the filler type and some of the process parameters. As an example, it was revealed that the bond strength of multifunctional adhesive wood joints can be increased by integrating increasing contents of carbon nanotubes in polyurethane prepolymers, if the polymers do not contain fibrous fillers before the dispersion.

Influence of polymer/filler composition and processing on the properties of multifunctional adhesive wood bonds from polyurethane prepolymers II: electrical sensitivity in compression

ABSTRACTElectrically conductive fillers enhance the functionality of adhesive bondlines by inducing a strain-dependent electrical resistance. Applied as sensor elements in engineered timber, these multifunctional adhesive bondlines are influenced by a lot of process parameters, including the dispersion process, manufacturing parameters and postcuring conditions. Additionally, the sensitivity of these sensor elements is counteracted by instabilities of the piezoresistivity. A setup and analysing method of the piezoresistive response have been developed, which includes sensitivity as well as instability effects. With this setup, the quality of the piezoresistive bondline has been assessed by three experiments, varying 10 influencing factors. The piezoresistive response was investigated in compression perpendicular to the bondline. It was shown that the electrical sensitivity in compression and stability of the piezoresistive bondline can be influenced by polymer formulation, filler type, filler content as well as the press temperature, glue spread, dispersion technique and thermal postcuring.

Silane Based Redox Initiating Systems: Toward a Safer Amine-Free, Peroxide-Free, and Metal-Free Approach

Room temperature redox initiated free radical polymerization (RFRP) has always attracted high attention in the field of materials due to its advantages of energy saving, high efficiency, and easy operation. However, the current redox initiating systems are based on toxic aromatic amines and hazardous peroxides (e.g., dibenzoylperoxide). In the present paper, the redox two component (2K) initiating performances of silanes (as reducing agents) in combination with a highly stable iodonium salt (as oxidizing agent) were studied for the first time under mild conditions (RT, under air). Optical pyrometry measurements and DSC investigation results showed that the diphenylsilane (DPS) exhibited a unique initiating property for several (meth)acrylate monomers. Remarkably, thermal postcuring (B-stage) is also possible using this system. Based on electron spin resonance (ESR) experiments, the initiating chemical mechanisms of RFRP are established. Importantly, the new proposed initiating systems can be used for the preparation of tack-free glass fibers and carbon fibers composites at room temperature.

Effect of thermal postcuring on the micro- and macromechanical properties of polyurethane for wood bonding

The optimization of mechanical properties of adhesive bonds is of interest especially in structural applications. Besides transferring stresses, bondlines can also provide additional functionality, such as measuring deformations in structural timber applications by electrically conductive adhesives. This study investigates the influence of a thermal postcure treatment of polyurethane bonded wood joints. Bonded beech wooden samples were manufactured with three adhesives—a commercial one-component polyurethane for structural laminated timber and two modified ones, filled with electrically conductive particles. Adhesive bonds were subjected to a subsequent postcuring at 80 and 95 °C for 1 and 48 h, respectively. Mechanical properties of the bonds were studied on the macroscopic level by tensile shear tests and the properties of the cured adhesive on the microscopic level by nanoindentation. As a result, the tensile shear strength slightly dropped with addition of filler, while all specimens still fulfilled the requirement of EN 302-1 in dry condition. Nanoindentation revealed minor decreases in mechanical properties of the cured adhesive with postcuring time for two adhesives and a different reaction of carbon black filled polyurethane, as the creep factor decreases with the thermal postcure.

Bio-based polymer networks by thiol-ene photopolymerization of allylated l-glutamic acids and l-tyrosines

Triallylated l-glutamic acid and l-tyrosine (A3E amd A3Y) and their tetraallyl derivatives (A4E and A4Y) were synthesized by reactions of l-glutamic acid and l-tyrosine with allyl bromide in the presence of potassium carbonate, respectively. Thiol-ene photopolymerizations of the allylated amino acids and a pentaerythritol-based tetrathiol (S4P) at allyl/SH 1/1 in the presence of photo-initiators produced photo-cured products (pA3E-S4P, pA3Y-S4P, pA4E-S4P and pA4Y-S4P). Furthermore, thermal post-curing after photo-curing of allylated amino acid/S4P mixtures containing photo-initiators and a thermal initiator produced photo and thermal cured products (ptA3E-S4P, ptA3Y-S4P, ptA4E-S4P and ptA4Y-S4P). Although the thermally post-cured products became dark in color, all the cured films were flat and had smooth surfaces. The FT-IR analysis revealed that allyl and thiol conversions increased by the thermal post-curing for all the curing systems. The glass transition temperature (Tg), 5% weight loss temperature (Td5), tensile strength and tensile modulus of each thermally post-cured product were higher than those of the corresponding photo-cured product, reflecting the increase of crosslinking density by the thermal post-curing. Also, A4E- and A4Y-based cured products exhibited higher Tgs, Td5s, tensile strengths and moduli than the corresponding A3E- and A3Y-based cured products, respectively. Especially, ptA4Y-S4P exhibited the highest Tg (24.4 °C), Td5 (319 °C), tensile strength (18.2 MPa) and tensile modulus (1037 MPa) among all the cured products.

Mechanism of Particle Formation in Silver/Epoxy Nanocomposites Obtained through a Visible-Light-Assisted in Situ Synthesis.

A detailed understanding of the processes taking place during the in situ synthesis of metal/polymer nanocomposites is crucial to manipulate the shape and size of nanoparticles (NPs) with a high level of control. In this paper, we report an in-depth time-resolved analysis of the particle formation process in silver/epoxy nanocomposites obtained through a visible-light-assisted in situ synthesis. The selected epoxy monomer was based on diglycidyl ether of bisphenol A, which undergoes relatively slow cationic ring-opening polymerization. This feature allowed us to access a full description of the formation process of silver NPs before this was arrested by the curing of the epoxy matrix. In situ time-resolved small-angle X-ray scattering investigation was carried out to follow the evolution of the number and size of the silver NPs as a function of irradiation time, whereas rheological experiments combined with near-infrared and ultraviolet-visible spectroscopies were performed to interpret how changes in the rheological properties of the matrix affect the nucleation and growth of particles. The analysis of the obtained results allowed us to propose consistent mechanisms for the formation of metal/polymer nanocomposites obtained by light-assisted one-pot synthesis. Finally, the effect of a thermal postcuring treatment of the epoxy matrix on the particle size in the nanocomposite was investigated.