Study on curing shrinkage and mechanism of DHOM-modified epoxy-acrylate-based UV-curing 3D printing materials

UV-curing 3D printing technology is one of the technologies with the highest molding efficiency and accuracy in the field of 3D printing. At present, the main problems of UV-curing 3D printing materials include the high curing shrinkage and the low toughness. In this work, free radical/cation hybrid curing system was developed, the effects of expansion monomer (DHOM, 3,9-diethyl-3′,9′-dihydro-xymethyl-1,5,7,11-tetraoxaspiro[5,5]undecanone) on the curing shrinkage, toughness, and other properties of the epoxy acrylate-based UV-curing 3D printing materials were investigated. The results showed that when the added amount of DHOM was 15 wt%, the best comprehensive performance of UV-curing 3D printing materials was obtained. At this time, the curing shrinkage rate decreased from 6.13% to 3.47%. The main reason was that DHOM undergoes ring-opening polymerization under the action of cationic photoinitiator, resulted in volume expansion. The impact strength increased from 9.61 to 12.13 KJ/m², which was because that the ring-opening reaction of DHOM reduced the curing shrinkage of the resin system and released the internal stress between the resin molecules.     

Apparent volumetric shrinkage study of RTM6 resin during the curing process and its effect on the residual stresses in a composite

A comprehensive characterization of the volumetric shrinkage of a commercially important aerospace resin (RTM6) during the various stages of the curing process was studied. The apparent volumetric shrinkage, evaluated from density measurements at room temperature, was correlated with the progress of epoxide conversion. During the entire curing process, the apparent volume shrinkage was found to be less than 3% and occurred before vitrification. A slight re‐expansion of the resin, attributed to self‐antiplasticization effects, was observed during postcuring at 180°C. It was concluded that residual stresses were not generated due to chemical cross‐linking during curing but rather from thermal contraction occurring during the cooling stage after cure. A photo‐elastic method was used to characterize residual stresses during cooling in a deliberately engineered resin rich hole of a carbon fiber/RTM6 composite. The residual stress was found to reach approximately 28 MPa, which is in good agreement with the value calculated from the shrinkage and elastic moduli. It is proposed that this simple method can be provide insights useful to the design and materials selection processes by measuring and localizing residual stresses from resin during curing and or thermal cycling. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

基于热芯缠绕工艺的缠绕张力研究

本文提出了一种适用于热芯缠绕工艺的缠绕张力制度设计方法.与传统缠绕工艺不同的是,热芯缠绕工艺使复合材料在缠绕同时加热固化.该方法在考虑外部纤维张力对内部环向应力影响的基础上,考虑了缠绕过程中温度引起的热应力、固化收缩应力以及各参数变化,通过对各缠绕层环向应力的分析,推导出考虑温度影响的缠绕张力制度,使各缠绕层纤维受力均匀,从而提高复合材料壳体的强度和抗疲劳性能.     

高绝缘,耐低温浇注胶的防开裂

从树脂、增韧剂、填料及固化剂的种类和用量等方面讨论了如何降低固化收缩应力及低温收缩应力,从而减少因固化收缩和冷冻收缩引起的开裂现象,以保持所合成浇注胶固化物在常温和低温下的高绝缘性能。     

Water absorption in internally cured mortar made with water-filled lightweight aggregate

The increased propensity for shrinkage cracking in low water-to-cement ratio (w/c) concrete has inspired the development of new technologies that can reduce the risk of early-age cracking. One of these is internal curing. Internal curing uses saturated lightweight aggregate to supply ‘curing water’ to low w/c paste as it hydrates. Significant research has been performed to determine the effects of internal curing on shrinkage and stress development; however, relatively little detailed information exists about the effects of internal curing on fluid transport properties such as water absorption or diffusivity. This study examines the absorption of water into mortar specimens made with saturated lightweight aggregates (SLWA). These results indicate that the inclusion of SLWA can reduce the water absorption of mortar specimens. This observation was reinforced with electrical conductivity measurements that exhibited similar reductions.     

Internal curing of high-performance concrete with pre-soaked fine lightweight aggregate for prevention of autogenous shrinkage cracking

The effectiveness of internal curing (IC) to reduce autogenous shrinkage cracking in high-performance concrete (HPC) was investigated using different levels of internal curing on four pairs of large-size prismatic HPC specimens tested simultaneously under free and restrained shrinkage. Internal curing was supplied by pre-soaked fine lightweight aggregate (LWA) as a partial replacement to regular sand. It was found that the use of 178 kg/m³ of saturated LWA in HPC, providing 27 kg/m³ of IC water, eliminated the tensile stress due to restrained autogenous shrinkage without compromising the early-age strength and elastic modulus of HPC. It was shown that the risk of concrete cracking could be conservatively estimated from the extent of free shrinkage strain occurring after the peak expansion strain that may develop at very early ages. Autogenous expansion, observed during the first day for high levels of internal curing, can significantly reduce the risk of cracking in concrete structures, as both the elastic and creep strains develop initially in compression, enabling the tensile strength to increase further before tensile stresses start to initiate later.     

环氧浇铸料及其玻璃钢热性能与内应力

研究了塑料/铸铝复合模具用环氧浇铸料及其玻璃钢热性能及内应力,探讨了塑料配方对浇铸料固化收缩率、力学性能、热膨胀性能及塑料/铝复合板经热处理后的内应力的影响。     

Shrinkage and internal stress during curing of epoxide resins

The shrinkage and internal stress of bisphenol-type epoxide resins cured with aliphatic α,ω-diamines, H₂N? (CH₂)_m′? NH₂(m′ = 2, 4, 6, and 12), were investigated by measuring the change of density and the strain of the steel ring embedded in the cured resins. Internal stress was found to be induced by the shrinkage occurring in the cooling process from the glass transition temperature (T_g) to room temperature. Shrinkage and internal stress increased with increase in the concentration of network chains and T_g of the cured systems, and then with a decrease in m′ of the curing agents. It appears that the reductions in the concentration of network chains and T_g were necessary to reduce the shrinkage and internal stress caused by the curing.     

钢板混凝土组合剪力墙早期裂缝试验研究

针对钢板混凝土组合剪力墙结构早期开裂问题,设计了混凝土中掺加膨胀剂、内养护剂、钢纤维、膨胀剂与内养护剂复掺4种情况下的早期收缩及早期平板约束开裂试验,以此来研究钢板混凝土组合剪力墙中高强高性能混凝土的抗裂减缩效果.结果表明:膨胀剂、内养护剂、钢纤维、膨胀剂与内养护剂复掺都可以降低混凝土的早期收缩;膨胀剂与内养护剂复掺明显改善了膨胀剂的膨胀效果,使混凝土的膨胀应变及膨胀时间得到显著提升,降低28 d收缩达87.6%,且混凝土早期开裂时间推迟,裂缝宽度和数量都明显降低.本文的研究结果为钢板混凝土组合剪力墙早期裂缝的控制提供了参考.     

Internal stress of epoxy resin modified with acrylic core-shell particles containing functional groups prepared by seeded emulsion polymerization

In order to reduce the internal stress in a cured epoxy resin, submicrometer-sized poly(butyl acrylate) (PBA)/poly(methyl methacrylate) (PMMA) core-shell particles having cross-links were dispersed in the resin prior to curing. For the introduction of cross-links, monoethylene glycol dimethacrylate or glycidyl methacrylate monomer was copolymerized. Cross-links in the PBA core reduced the shrinkage of the cured epoxy resin, and cross-links at the PMMA shell produced a strong interaction with the epoxy matrix. The internal stress was reduced effectively by the introduction of cross-links.     

Compliant phase reduces polymerization shrinkage in dental composite resin

Commercially available dental composite resins undergo polymerization shrinkage during curing, setting up internal stress that can result in debonding at the tooth‐composite interface, tooth fracture and composite failure. Debonding can also lead to marginal leakage and recurrent decay. The presence of a compliant phase, in this case polybutadiene in aggregate form with fumed silica, is shown to reduce polymerization shrinkage in the bis‐GMA/TEGDMA resin system. The addition of a small amount of the plasticizer triphenylethylene promoted additional reduction in cure shrinkage by reducing the glass transition temperature, T_g. A trend toward reduced cure shrinkage was noted as the polybutadiene : silica ratio decreased within each molecular weight of polybutadiene tested. The optimal formulation tested was determined to be 0.25 wt. % triphenylethylene in bis‐GMA with aggregates formed from polybutadiene of molecular weight 2–3 million, in a polybutadiene : silica ratio of 0.005. Shrinkage reduction in this system was ～ 42% without loss of Knoop hardness. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2306–2310, 2005     

Integrated shrinkage,relative humidity,strength development,and cracking potential of internally cured concrete exposed to different drying conditions

Internal curing (IC) technology using prewetted lightweight fine aggregates (LWFAs) as additives has been proved an effective means for mitigating both autogenous shrinkage and early-age cracking under sealed curing conditions. However, for practical structures in the field, concrete experiences more complex environmental conditions compared to under sealed curing. To better utilize internal curing technology for durable concretes, this study conducts comprehensive investigations on both control and internally cured concretes at water/cement (w/c) ratio of 0.3 and 0.4 in terms of the influence of age when concrete is exposed to drying and the drying duration on the developments of integrated shrinkage, internal relative humidity, compressive and flexural strengths, mass loss, and cracking potential. The results are beneficial for producing internally cured concrete with increased resistance to early-age cracking and enhanced durability for infrastructure applications.     

一维固化收缩应力的研究

本文利用自制的测试固化应力的仪器测试研究了树脂固化时一维固化收缩应力的产生情况,然后通过低温固化低轮廓添加剂控制收缩,进一步测试研究其产生情况,观察了低收缩对固化应力的影响.     

纳米SiO_2对环氧树脂胶粘剂的改性机制及应用研究

环氧树脂(EP)具有粘接力强、电绝缘性好、稳定性高和固化收缩率小等优点,但由于纯EP固化后呈三维交联网状结构,导致其内应力大、质脆和抗冲击韧性较差。采用共混法将纳米SiO2(nano-SiO2)加入到EP基体树脂中,制备nano-SiO2/EP复合材料。结果表明:复合材料的剪切强度由16.66 MPa升至18.01 MPa,冲击强度从15.40 kJ/m2升至33.68 kJ/m2,弯曲强度从70.50 MPa升至85.94 MPa,最终nano-SiO2/EP复合材料体系的韧性比不含nano-SiO2体系提高了82.8%。     

Build-up of structure and viscoelastic properties in epoxy and acrylate resins cured below their ultimate glass transition temperature

The build-up of structure and viscoelastic properties with conversion during cure below the ultimate glass transition temperature of epoxy and acrylate resins has been investigated. Using a torsional dynamic mechanical analyser, dynamic shear modulus and change in sample thickness was monitored simultaneously, thus giving information on both the physical properties (stiffness) and the progress of the reaction (shrinbdkage) in one experiment. Two step-wise curing epoxy systems and two chain-wise curing acrylate systems with different crosslink densities were studied and compared. The results showed that in the epoxies vitrification was a distinct event, occurring separately from gelation and ending with the end of the cure reaction. In the acrylates vitrification began immediately after gelation, the two events being indistinguishable, and lasted until the end of the reaction, leaving the sample in its transition zone. Scaling of modulus—cure time data obtained at different frequencies showed that the data for each system followed one single curve, independent of frequency over five decades. This made it possible to estimate the modulus development at low frequencies early in the reaction, which is difficult to measure directly. From the shrinkage and storage moduli approximate values of the relaxation modulus as a function of chemical conversion were calculated. The relaxation modulus curves at different conversions were then shifted along the time axis to provide a relaxation master curve. The data and understanding gained in this work provide the basis for analysing the time-dependent mechanical behaviour during cure, e.g. build-up and relaxation of residual stresses.     

混凝土内部湿度场与自约束应力场的研究

为防治混凝土自约束作用导致的开裂现象,对混凝土内部湿度场和自约束应力场开展了研究。通过位移传感器、温湿度传感器对不同强度、养护条件下的混凝土收缩、内部温度和湿度进行测试。之后,通过理论推导,建立混凝土内部湿度、应变与内部湿度关系的理论计算模型。研究表明,混凝土内部湿度随着水泥水化作用和干燥作用的增强而降低。混凝土湿度扩散系数是表征其内部湿度的函数,随着与干燥面的间距增加,混凝土内部湿度扩散系数增大。混凝土的收缩变形与内部湿度之间存在显著的相关性,计算模型与试验结果拟合良好。混凝土内部湿度场的存在导致了应变梯度的存在,进而使混凝土内部产生自约束应力。相同环境条件下,高强混凝土内部自约束应力高于普通强度混凝土。     

DETERMINING THE SOURCE OF CHANGING SHRINKAGE RATES DURING KILN DRYING

The conclusions drawn by researchers on the source of changing shrinkage rates are discussed. The objective of this study was to determine the source of changing shrinkage rates early in drying and whether shrinkage would make a useful parameter for an automated kiln control system. In this study, four loads of red oak and one of maple were kiln dried using established schedules Shrinkage of the lumber was continuously recorded, and moisture content and released elastic strain were periodically recorded. The moisture content and strain data showed that the changing shrinkage rates early in drying boards were caused by reduced internal stress levels, not the occurrence of fiber saturation point or temperature as reported by some. Results of this study indicate it may be possible to develop an automated kiln system using shrinkage as the controlling parameter. This would reduce the drying time while avoiding additional drying defects.     

Chemorheology of cyanate ester—organically layered silicate nanocomposites

The effect of nanoparticle addition on the flow and curing behavior of a phenolic triazine cyanate ester resin system has been studied using chemorheological, thermal and spectroscopic techniques. While the neat system exhibited Newtonian flow, the nanodispersed prepolymer exhibited pseudoplastic flow behavior, typical of polymeric fluids such as gels and pastes. Evolution of the morphology during curing has been found to be dependent on the rate of intergallery diffusion of the prepolymer and subsequent gelation and vitrification, as well as the intra and extragallery cure kinetics. Curing reactions of the cyanate ester nanocomposite system consisting of a di-functional phenol, a halogen cyanate and organically layered silicates were studied. Gel times were measured as a function of temperature by time sweeps on a controlled stress rheometer. Gelation and vitrification times and activation energies for the nanocomposite systems were lower than that of the neat resins, indicating a catalytic effect of the clays on the curing reaction. Curing kinetics experiments performed on DSC and FTIR confirmed this phenomenon. Based on above experiments, time-temperature-transformation diagrams for the different systems were constructed.     

Stress‐initiated void formation during cure of a three‐dimensionally constrained thermoset resin

During cure of epoxy resins, polymerization induces an increase in mechanical properties, which is accompanied by a volumetric shrinkage. When the resin is cured in a constrained mold to which it adheres, tensile stresses will hence develop, which may exceed the stremgth of the resin at a given curing stage. Voids will then form. The origin and governing parameters of void formation are studied using an epoxy resin cured in a three‐dimensionally constrained glass mold following isothermal cure cycles. Two types of voids are shown to appear during cure, one early in the process and a second around the gelation point. A viscoelastic analysis of the material stress state over the whole range of cure is performed. Both the viscoelastic modulus obtained from a time‐cure‐temperature superposition and the volumetric shrinkage, which was continuously measured by density change, are taken into account. A value for the critical internal stress at void initiation is thus proposed. This criterion can be used to provide guidelines for tailoring the material properties toward an increase of the critical stress for void initiation. Also, since during theprocessing of composite materials, cases may arise where the resin cures within the interstices left between consolidated fibres that do not move, this critical stress failure criterion can be of use in the eastablishment of a process window providing guidelines for the production of void free composites.     

Analytical estimate for curing‐induced stress and warpage in coating layers

A thermoset coating that is applied to an elastic substrate will develop residual stresses during curing because of polymerization shrinkage of the resin. This shrinkage only partly contributes to the residual stresses because, before gelation, the stresses relax completely. In this study, we developed explicit analytical expressions for the curing efficiency factor, the residual stresses, and the resulting warpage. We did this by assuming that after gelation, the material was in its rubbery state and that viscoelastic effects were absent. A difference between the free and constrained warpages during curing was made. The analytical warpage models were shown to give results comparable to those of the numerical calculations with a fully curing‐dependent viscoelastic material model. Furthermore, for the first time, accurate analytical expressions for the stress‐free temperature and stress‐free strain were obtained. With these expressions, the effect of curing shrinkage on the residual stresses could easily be incorporated into existing (numerical) stress analysis without the need for extensive curing‐dependent viscoelastic material models. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012