In situ cure monitoring of adhesively bonded joints by dielectrometry

Since the reliability of adhesively bonded joints is much dependent on the curing status of thermosetting adhesives, the in situ cure monitoring during the cure of adhesive joints could improve the quality of adhesively bonded joints as it enables one to control the cure parameters. In this work, a dielectric method which measures the dissipation factor of the adhesive during the cure of joints and converts it into the degree of cure of the adhesive was devised. Steel adherends were used for the adhesively bonded joints because the steel adherends worked as the electrodes for the measurement of dissipation factor without additional electrodes. The relation between the dissipation factor and the degree of cure of adhesive was investigated, which could eliminate the temperature effect on the dissipation factor that is largely affected by the degree of cure and temperature of adhesive. Comparing the results obtained by the method developed with those by DSC (differential scanning calorimetry), it was found that the dissipation factor showed a trend similar to the cure rate of the adhesive.     

Variation in dielectric properties of an epoxy-novolac molding compound during dynamic cure

The variation in dielectric characteristics of an epoxy-novolac molding compound during dynamic cure was studied by means of dielectrometry (DE). Dielectric parameters such as permittivity (?′), loss factor (?″), and dissipation (tan δ) were observed to depend on various factors including temperature, frequency, the extent of cure (α, measured previously by using differential scanning calorimetry, DSC), as well as contributions from ionic conductivity (σ) and electrode polarization. The characteristic relaxation time (τ) and the relaxed permittivity (?_r), suggested in the literature as possible parameters for cure monitoring purposes, was found difficult to determine because of interfernce from ionic conduction and electrode polarization. In comparison, σ could be measured throughout the entire cure process and was observed to depend only on temperature and α. In combination with our previous DSC results, an empirical function relating α to temperature and α was constructed. Our analysis also indicated that the strong maximum in the ?′ and the ?″ curves during the course of dynamic cure was a direct result of the α-and the temperature-dependence of σ; the strong maximum of ?″ is directly related neither to gelation nor to vitrification.     

Dielectric relaxation and crosslinking in unsaturated polyester resins

The properties of crosslinked thermosetting resins depend markedly on the completeness of the crosslinking process. Determination of the degree of cure of an unsaturated polyester resin has been studied previously by mechanical, spectroscopic and volume resistivity methods. In this respect the effect of cure time and temperature on the ac dielectric constant and dissipation factor at 1 kc/s and 10 kc/s is considered. The dissipation factor appears to be a most useful parameter for detecting changes in the degree of cure in the later stages of reaction. The electrical properties of the cured resin are discussed, and values for the energy of activation for electrical conduction are compared with literature reports on similar materials.     

Modeling the dielectric response of unsaturated polyester resin during cure

The dielectric relaxation of unsaturated polyester resin during cure was modeled using a modified Williams–Watts decay function. The dielectric response was studied using dielectric measurements coupled with two dynamic mechanical measurement methods. It was found that the variation of the relaxation time during cure is a WLF process using T_g (α) (α-conversion) as the varied temperature. It was shown that this process fits the Williams-Watts decay function Φ(t)=exp(-(t/τ)^β) where τ-relaxation time and β-empirical parameter are time dependent. It was found that τ obeys a time dependent power law rule and β depends linearly on log(time). Using this modified decay function, it was shown that the experimental dielectric response measured during cure agrees well with the computed response. Relaxation times above and below the dielectric measurement system capability were computed thus, demonstrating the capability of yielding the entire relaxation times spectrum during cure, out of a single limited frequency dielectric measurement.     

Evolution of mechanical properties during cure for out‐of‐autoclave carbon‐epoxy prepregs

Extent of cure and rheological properties were obtained for out‐of‐autoclave materials, Cycom 5320‐8HS and Cycom 5320‐PW, for the manufacturer recommended cure cycle using differential scanning calorimeter and encapsulated sample rheometer (ESR), respectively. Rheological properties from ESR were further used in designing the cure cycles to study the evolution of mechanical properties. Five panels were cured at different cure stages using the designed cure cycles and coupons were tested for short beam shear and combined loading compression properties at different cure stages. To correlate the mechanical properties with its respective glass transition temperature, dynamic mechanical analyzer was used to obtain the glass transition temperature for the coupons obtained from the respective panels. Statistical results showed significant difference in short beam shear and combined loading compression properties up to vitrification, however, no significant difference was observed on these mechanical properties after vitrification. The observed linear trend between degree of cure (DOC) and glass transition temperature (T_g) was validated using DiBenedetto relation. Linearly increasing trend between DOC and glass transition temperature (T_g) for different cure states suggests that both DOC and T_g can be used interchangeably to define the state of material. A good correlation was observed between material cure state and the mechanical properties. A mathematical model was also proposed to determine the short beam shear and combined loading compression properties based on material cure state. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41548.     

The effect of curing of unsaturated polyester resin on the dynamic relaxation time

The effect of hot curing of unsaturated polyester resin on the dynamic relaxation time was studied using dielectric measurements along with two dynamic mechanical measurement methods. It was found that the dynamic response during cure was a material frequency dependent property and did not depend on the measurement method. All relaxation times, measured during cure, by all three measurement methods used, converged to a single equation: τ(t)_av=at^b where t= curing time, a, b=constants. The increase of the relaxation time during cure followed the same trend as a friction factor, which was found to increase with conversion. The crosslinking density was found to increase slowly with conversion, while the relaxation time increased exponentially. These two different modes of behavior during cure explain the high resolution of dynamic measurements as a cure monitoring tool, which can easily detect small curing changes. This behavior of the relaxation time was explained by the sharp rise of activation energy due to a parallel decrease of free volume at high conversion.     

Use of the frequency dependence of the impedance to monitor viscosity during cure

The ability to conveniently and continuously measure the processing properties of polymer resins is important both to the resin supplier and to the fabricator. Frequency dependent electromagnetic sensors (FDEMS) provide an in-situ technique for continuous measurement of the resin's rheological changes both in a laboratory press and in manufacturing tools in an autoclave. In this paper the frequency dependence of ?*(w) is used to quantitatively monitor the viscosity for a tetraglycidyl 4,4′-diaminodiphenyl methane (TGDDM) amine epoxy, to quantitatively monitor the viscosity during processing In a styrene-polyester resin, and to monitor the cure process in an autoclave during cure of a high temperature polyimide-graphite prepreg. In addition, the technique is used to measure the viscosity at various ply positions in a thick TGDDM graphite epoxy laminate during processing in an autoclave.     

Cure characterization of Cycom 977‐2A carbon/epoxy composites for quickstep processing

In this effort, Quickstep, a relatively a new technique, have been employed for manufacturing of composite materials. The cure schedule provided by a prepreg manufacturer is usually designed for autoclave or other traditional processing techniques and thermosetting resin systems are formulated for ramp rate curing 2–3 K min^?1. While in case of Quickstep processing, ramp rates of 15 K min^?1 can be achieved, thus changing the chemorheology of resin. The cure process of 977‐2A carbon/epoxy composites was evaluated for Quickstep processing using differential scanning calorimetry (DSC), dynamic mechanical and thermal analysis, and Fourier transformed infrared and results were compared with cure cycle employed for autoclave curing. Optimum hold time for Quickstep processing at upper curing temperature (180°C) was determined using DSC. The hold time of 120 min at 180°C was found to be suitable for Quickstep cure cycle, producing a panel of similar degree of cure to that achieved through autoclave processing schedule. Final degree of cure was dependent on time spent at upper cure temperature and slightly on initial steps of the cure cycle which was used to control the resin flow, fiber wetting, and void removal. Quickstep processed samples exhibited higher T_g and crosslink density and similar molecular network structure to the autoclave cured samples. POLYM. ENG. SCI., 54:887–898, 2014. © 2013 Society of Plastics Engineers     

The devolatilization of polyimide fiber composites: Model and experimental verification

A one-dimensional devolatilization model is developed for autoclave processing of themroplastic polyimide, graphite-reinforced, composites. The model rests on the assumption that continous gas passages are formed during processing due to partical crystallization of the polymer in the course of polymerization. Both DSC and optica microscopy indicate that crystallinity develops in the Avimid-III polyimide resin during polymerization, which in turn facilitates formation of cracks. These cracks heal upon removal of volatiles and polymer melting. Model-Calculated temperature profiles in the laminate during polymerization, and the predicted removal rates of volatile species (water, ethanol, NMP) are compared to experimental miniautoclave data for the DuPont Avimid K-III/IM6 system. Good agreement is obtained when the volumetric mass transfer coefficient required by the model is assumed to vary proportionally to the inverse of the changing resin viscosity. The model allows the investigation of desired cure cycle parameters for different laminate thicknesses.     

Influences of the grafting percentage of natural rubber‐graft‐poly(2‐hydroxyethyl acrylate) on properties of its vulcanizates

The graft copolymerization of 2‐hydroxyethyl acrylate (HEA) monomer onto natural rubber (NR) latex was successful using cumene hydroperoxide and tetraethylene pentamine as redox initiators. The grafting of poly(2‐hydroxyethyl acrylate) (PHEA) on the NR particles was confirmed by Fourier transform infrared spectroscopy, ¹H NMR spectroscopy and TEM. The NR‐g‐PHEA with various grafting percentages (0%, 8.7%, 14.3% and 18.7%) was compounded on a two‐roll mill with a sulfur vulcanization system. The effects of grafting percentage on the cure characteristics, dielectric properties, thermal properties and physical properties of NR‐g‐PHEA vulcanizates were investigated. It was found that increased grafting caused NR‐g‐PHEA vulcanizates to have reduced water contact angle, scorch time and cure time, while the dielectric constant and dissipation factor increased. The NR‐g‐PHEA vulcanizate with 8.7% grafting exhibited the highest delta torque (M_H ? M_L), crosslink density, tensile strength, moduli at 100%, 200% and 300% strains, and hardness, with insignificant loss of elongation at break in comparison to the other cases. © 2018 Society of Chemical Industry     

Effect of double vacuum bagging (DVB) in quickstep processing on the properties of 977‐2A carbon/epoxy composites

An aerospace grade prepreg 977‐2A carbon/epoxy composite was processed using traditional oven/autoclave and relatively new technique, Quickstep (QS). The manufacturer's recommended autoclave cure cycle was employed for oven/autoclave processing while due to greater flexibility to control the cure cycle temperature in order to manipulate the resin viscosity, several cure cycles were employed for Quickstep processing. Optimization methodology and effect of these cure schedules and their comparison with traditional oven/autoclave curing is presented in previously published article. However, in order to reduce the total cycle time of QS_AD (QS‐60) cure schedule, an attempt was made to facilitate the excess resin to squeeze out during QS‐35 and QS‐45 cure. This attempt was based on the incorporation of double vacuum bagging (DVB) technique into Quickstep processing. Statistical analysis was performed to investigate any effect of DVB in Quickstep processing and it was observed that incorporation of DVB in Quickstep processing is not found to be significantly effective, however, it is believed that small resin rich patches of QS‐45 cured panels can be removed with controlled application of external pressure using DVB. POLYM. COMPOS., 34:942–952, 2013. © 2013 Society of Plastics Engineers     

Dielectric analysis of epoxy/amine resins using microwave cavity technique

Dielectric properties (permittivity and dielectric loss factor) of stoichiometric mixtures of DGEBA (diglycidylether of bisphenol A) epoxy (DER 332) and amine (diaminodiphenyl sulfone; DDS) as a function of temperature and extent of cure have been measured at a microwave frequency of 2.45 GHz. Permittivity and dielectric loss factor of this resin increase with increasing temperature and decrease with increasing extent of cure. Dielectric loss factor is more dependent on temperature in the early stages of cure but becomes less dependent on temperature as the cure proceeds. Dielectric loss data can be related to extent of cure in order to monitor the cure process. Online measurements of temperature- and cure-dependent dielectric loss factor show three material structure stages and significant changes in dielectric loss factor during the microwave cure process. Dielectric Joss factor is also found to be the same for both thermal and microwave cured samples.     

Dielectric properties of cyanoethylated bagasse composites

Bagasse was converted into a thermo-moldable material by cyanoethylation. The effect of reaction conditions employed during the preparation of cyanoethylated bagasse (CE-B) fibers on dielectric properties of hot-pressed composites was studied. Increase in the nitrogen content of the cyanoethylated fiber, i.e., the nitrile groups resulted in an increase in the dielectric constant and a decrease in the dissipation factor (tan δ) peak of the composites. Increase in the reaction temperature and the alkali concentration resulted in a decrease in the dielectric constant and tan δof the composites. Thickness swelling (TS) and equilibrium moisture content of composites conditioned at different relative humidities (RHs) were studied and the extent of the effect of the absorbed moisture on the dielectric properties was also studied. Increase in the nitrogen content, the alkali concentration, and the reaction temperature during the preparation of cyanoethylated fibers resulted in a decrease in TS and moisture absorption of the composites formed. The dielectric properties of the composites conditioned at 60 or 90% RH deteriorated severely. The effect of temperature on the dielectric constant and tan δof a selected CE-B composite was studied. The dielectric constant and tan δincreased as the temperature increased.     

Sulfonyl‐Containing Polyimide Dielectrics with Advanced Heat Resistance and Dielectric Properties for High‐Temperature Capacitor Applications

Polymer dielectrics, with advanced dielectric properties and heat resistance, are critical for high‐temperature capacitors in various applications. However, the high performance of heat resistance and dielectric properties are quite difficult to achieve all together due to their mutual implication. Here, by intensively investigating the correlation between molecular structure and properties, polyimide dielectrics with i) enhanced dielectric constant by introducing sulfonyl group, ii) low dissipation factor by introducing flexible linkage, and iii) high T_g (glass transition temperature) by retaining an aromatic structure, are obtained. The sulfonyl‐containing polyimides with different flexible linkages exhibit simultaneously a high dielectric constant (4.50–5.98), low dissipation factor (0.00298–0.00426), and outstanding breakdown strength (most above 500 MV m^?1), as well as superior heat resistance (T_g : 244–304 °C). Specifically, the polyimide (SPI‐1) with sulfonyl group in diamine moiety and para‐para linkage shows stable dielectric properties up to 150 °C, and the discharged energy density and charge–discharge efficiency can be as high as 7.04 J cm^?3 and 91.3% at 500 MV m^?1, respectively.     

Dielectric determination of cure state during non-isothermal cure

As a result of increased interest from industry in using dielectric cure monitoring, a need has arisen for simplifying frequency, cure, and temperature dependent data so that control decisions can be readily made. Techniques utilizing data covering several decades of frequency now exist for separating ionic conduction levels from dipole and electrode polarization responses. Ionic conduction levels are particularly useful since they can be correlated to both viscosity and extent of cure. In addition to being a function of extent of cure, dielectric properties are also influenced by temperature. This dependence often makes the dielectric response more difficult to interpret. This paper investigates two methods for overcoming the temperature dependence of the dielectric response during nonisothermal cure. The first method utilizes recent WLF modeling techniques and extends them with the end result of extracting T_g in real time during cure. The second technique involves measuring the temperature dependence of uncured and cured material. Utilizing the correlation between log ionic conductivity and extent of cure, which has been noted by previous researchers, the normalized conductivity can be converted to a cure index. Several examples including epoxy, polyurethane, and a UV cured photoresist are presented, showing data before normalization and after both T_g and cure index determination.     

Studies on electrical properties of nanoclay filled thermoplastic polyurethane/polypropylene blends

The electrical properties of ester/ether‐based thermoplastic polyurethane (TPU) and polypropylene (PP) blends are presented in this article. Special attention has been paid to analyze the effect of blend ratio, compatibilization, and effect of nanoclay on the electrical properties of TPU/PP blends. The electrical properties measured were dielectric constant (ε′), volume resistivity (ρ_υ), loss factor (ε″), and dissipation factor (tan δ). Addition of PP into TPU increases the volume resistivity and reduces the dissipation and loss factor due to the decrease in the overall polarity of the system. Further addition of compatibilizer and nanoclay to this system reduced the dissipation factor and loss factor with increased volume resistivity. Compared with the ether‐TPU based blend nanocomposites, the ester‐TPU blends show better compatibility as confirmed by analysis. POLYM. COMPOS., 35:1671–1682, 2014. © 2013 Society of Plastics Engineers     

Monitoring the composite curing process with a fluorescence-based fiber-optic sensor

A novel cure sensor, based on the combination of fiber-optic fluorometry and the viscosity/degree-of-cure dependence of the epoxy resin fluorescence, was developed to provide a reliable low-cost cure-monitoring sensor for control of composite manufacturing. The capability of the first-generation sensor to monitor chemorheological changes that occur during the autoclave cure of carbon-epoxy laminates was successfully demonstrated. An improved second-generation sensor, which simultaneously monitors the changes in the epoxy fluorescence intensity and the wavelength of maximum emission, was also developed. The changes in the sensor intensity and wavelength signals, as a function of cure time, provide characteristic profiles that reveal the main chemorheological events of the cure cycle; these signals follow the changes in the degree-of-cure to completion. The configuration of the optrode-laminate interface strongly affects the signal profiles and their reproducibility. A “resin cavity” optrode-laminate interface, which improves reproducibility, was developed and tested.     

In-process control of epoxy composite by microdielectrometric analysis. Part II: On-line real-time dielectric measurements during a compression molding process

The curing of a fiberglass epoxy composite based on diglycidyl ether of bisphenol A (DGEBA) with dicyanodiamide (DDA) as the hardener and imidazole as the catalyst agent was analyzed using microdielectrometry. The curing behavior of thick epoxy composite parts was examined in a production environment for compression molding process. The particular focus of this paper is to present the method used to collect on-line real-time conversion measurements during an epoxy/fiberglass composite cure. For this purpose, temperature and ionic conductivity profiles during industrial moldings of a thick epoxy part were recorded. Corresponding conversion profiles were deducted from a previous empirically established correlation and discussed in terms of cure gradients as a function of the through-the-thickness location and of the cure cycle time.     

Comparison of microwave and thermal cure of an epoxy/amine matrix

An investigation was carried out into the effect ofa microwave cure on an epoxy prepolymer with a cycloaliphatic diamine mixture, as compared to a standard thermal cure. The microwave waveguide and process (propagation mode TE₀₁) were adjusted to obtain large homogeneous samples. The extent of reaction, x, was measured during the microwave processing by size exclusion chromatography and differential scanning calorimetry. A good estimate of x was found using a modified DiBenedetto equation correlating x and the glass transition temperature T_g. The homogeneity of the samples was checked during the last steps of cure, showing the efficiency of the microwave processing and waveguide. The influence of the nature of the mold (metallic or dielectric) on the reaction kinetic was also investigated. Samples cured by both thermal and microwave processing were characterized by dynamic and static mechanical properties and then compared with those of fully crosslinked networks, i.e., postcured at a high temperature.     

In‐line monitoring of the injection molding process by dielectric spectroscopy

In recent years, electrical techniques like microdielectrometry have increasingly been utilized for their ability to continuously monitor, in a nondestructive way, the advancement of the reaction of thermoset resins under cure. This paper discusses an extension of this technique for the “in‐situ” monitoring of the crystallization of thermoplastics applied during an injection molding process. Electric sensors were positioned at the walls of the mold cavity so that an analysis of the volume dielectric properties of material during the filling, the post‐filling, and the cooling steps could be carried out. Poly(vinylidene fluoride) was chosen for this study. A correlation between the evolution of the dielectric parameters and the succession of the steps in this process was undertaken. The dielectric response was sufficiently sensitive to identify the steps of the closing of the mold, filling, post‐filling, cooling, and ejection of the part. In addition, information concerning the crystallization phenomenon near the wall or in the middle of the sample was collected. The gradual filling of the cavity of the mold was also identified by dielectric measurements. The temperature dependence of dielectric properties of the sample was beneficial in evaluating the increase of the temperature of the mold with the succession of injection cycles. The influence of the packing pressure has been clearly identified and confirms the usefulness of the dielectric method as a probe for detecting the shrinkage of the part during the optimization phase of the machine parameters. The dielectric method detailed herein provides a new non‐invasive technique and could be applied to a closed‐loop control of the injection molding process.