The combination of carbon nanotube buckypaper and insulating adhesive for lightning strike protection of the carbon fiber/epoxy laminates

Carbon fiber reinforced polymers (CFRP) have been increasingly used in aircraft structures. However, their relatively low electrical conductivity leads to the vulnerability to lightning strike. Herein, the carbon nanotube buckypaper-based coatings composed of conductive buckypaper and insulating adhesives were developed to protect the CFRP laminates. Their influence on the lightning strike protection (LSP) effectiveness was systematically studied and the possible mechanisms were discussed. It was demonstrated that the conductive layer of buckypaper could facilitate the lightning current to the ground and dissipate the energy. Moreover, a relatively thick insulating adhesive could hinder the transfer of the lightning current through the thickness direction to CFRP laminates, thus further enhance the LSP effectiveness. An optimized LSP coating developed in this work was composed of a ∼70 μm thick buckypaper and a ∼200 μm thick boron nitride modified epoxy insulating adhesive, which resulted in a weight reduction up to 30% compared to the commercial Cu LSP coating, and could sustain the simulated lightning strike with peak current up to 100 kA.     

Corrosion protection by epoxy/poly(aniline-co-pyrrole)/ZnO nanocomposite coating

Anticorrosion behavior of epoxy/poly(aniline-co-pyrrole)/ZnO (EPAPZ) coating on stainless steel 304 alloys is investigated using the electrochemical impedance spectroscopy (EIS) method, and the coating is compared with epoxy/polyaniline/ZnO (EPAZ) and pure epoxy (EP) coatings. Scanning electron microscopy images are used for structural characterization and to compare the particle size of nanoparticles. EIS result showed that coating resistance for EPAPZ, EPAZ, and EP coatings after 90 days of immersion in 3.5% NaCl was 1.18 × 10⁷, 1.08 × 10⁶, and 4.28 × 10⁴ Ω cm⁻², respectively. In addition, the volume percentage of water absorbed by the coating, which could be obtained by coating capacitance, is 2.81, 4.21, and 9.11, respectively. Immersion tests showed 0.063, 0.194, and 0.752% of weight loss in the metals under EPAPZ, EPAZ, and EP coatings, respectively. These results show that the EPAPZ coating has superior anticorrosive performance compared with EPAZ and EP coatings. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48265.     

The use of noncovalently modified carbon nanotubes for preparation of hybrid polymeric composite materials with electrically conductive and lightning resistant properties

In this work, approach to use of noncovalently modified carbon nanotubes is given for preparation of functional hybrid polymeric composite materials (HPCM) based on epoxy resin. Conductive glass‐fiber plastics with resistivity in transverse and lengthwise direction 9.0·× 10² and 30–50 Ohm cm, respectively, were obtained. The tetrafluoroethylene telomer and fluorocontaining organosilicon copolymer with amino groups were used as modifiers for carbon nanotubes. Thermal, electrical, and mechanical properties of the obtained materials were studied. The mechanism of the effect of noncovalent modification of carbon nanotubes on functional properties of HPCM was discussed. It was found, that type of modifier significantly affects the level of functional properties. The use of fluorocontaining organosilicon copolymer is more optimal in comparison with tetrafluoroethylene telomer. Thus, HPCM with carbon‐fiber filler and this modifier has higher electrical conductivity and lightning strike resistance in comparison with nonmodified HPCM. This approach is promising to impart antistatic properties for glass‐fiber plastics and increase lightning resistance of carbon‐fiber plastics. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46108.     

Bio-based epoxy/polyaniline nanofiber-carbon dot nanocomposites as advanced anticorrosive materials

Application of anticorrosive coating on metal surface enhances the durability of the metal. Anticorrosion can be achieved by the incorporation of conducting nanomaterials like polyaniline or its nanohybrid to a coating material. Thus, bio-based epoxy nanocomposites were fabricated by the in situ method using polyaniline nanofiber-carbon dot nanohybrid (0.50 and 1 wt % with respect to epoxy), as the anticorrosive material. The epoxy resin was obtained by polycondensation of bisphenol-A, sorbitol, and monoglyceride of castor oil (mole ratio of 16:3:1), as hydroxyl compounds with epichlorohydrin (1:3 equivalent, hydroxyl:epichlorohydrin). The morphological analyses of the nanocomposites revealed the uniform dispersion and good compatibility of the nanohybrid in the epoxy matrix. The thermosets demonstrated good tensile strength (30 MPa), elongation at break (45%), scratch resistance (>10 kg) and impact resistance (14.75 kJ/m), good thermal stability (above 250°C), and chemical resistance. The anticorrosion study of the nanocomposites showed excellent corrosion protection efficiency (corrosion rate: 5.68 × 10⁻³ mils per year) in 3.5 wt % NaCl compared to the pristine epoxy system. Therefore, this bio-based thermosetting epoxy nanocomposite was demonstrated as efficient anticorrosive material. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47744.     

PANI-chitosan-TiO2 ternary nanocomposite and its effectiveness on antibacterial and antistatic behavior of epoxy coating

A straightforward approach has been developed for fabricating antibacterial and antistatic epoxy coatings by using polyaniline-chitosan modified TiO₂ ternary nanocomposite. This nanocomposite was synthesized through the following steps. First, chitosan was grafted onto the TiO₂ nanoparticles and then final nanocomposite was prepared via solution polymerization of aniline. Electrical conductivity measurement revealed that nanocomposite with 7.5 wt % of the modified TiO₂ nanoparticles has noticeably higher conductivity compared to polyaniline. Evaluating the coatings' antibacterial property indicated epoxy coatings with the content of ternary nanocomposite show significant bactericidal activity against Gram-positive bacteria and have acceptable antibacterial action against Gram-negative ones. Also, obtained results showed that the ternary nanocomposite would greatly decrease coatings' surface resistivity and when nanocomposite content is about 2 wt % surface resistivity is about 3 × 10⁷ Ω sq⁻¹. On the contrary, the coating with nanocomposite loading exhibits improved thermal and mechanical performance compared to the coating made of neat epoxy. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47629.     

Synthesis of a novel biphenyl epoxy resin and its hybrid composite with high thermal conductivity

A novel biphenyl epoxy monomer of p-methyl phenylhydroquinone epoxy resin (p-MEP) was synthesized and characterized. We researched its potential in the area of thermal conduction application and prepared a series of hybrid composites based on it with different mass ratios of sphere Al₂O₃ filler. From the good mobility and low viscosity of p-MEP, it allowed mixing with more Al₂O₃ fillers. The hybrid epoxy resins owned the advantages of traditional epoxy resins as well as quite considerable thermal conductivity. Therefore, the hybrid composite at the maximum mass fraction of 70% possess the highest thermal conductivity of 5.6 W mK⁻¹, which is 5.6 times higher than that of pristine p-MEP (0.1 W mK⁻¹). © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47078.     

Strain rate-dependent deformation behaviors of multi-layer carbon fiber reinforced polymer laminates

This paper focuses on the contributions of diversities of strain rate and orientations for aggravating the diversities of micro failure behaviors on carbon fiber reinforced polymer (CFRP) laminates. A miniature horizontal type tensile tester is employed to conduct experiments with strain rate ranging from 2.6 × 10⁻⁶ s⁻¹ to 2.6 × 10⁻³ s⁻¹. The CFRP laminates are obtained based upon a thermoset toughened epoxy matrix (termed CF/Epoxy) with ply orientations of (0°/0°) and (0°/90°). Significant differences in deformation behaviors of CFRP laminates are determined through tests. The study clearly reveals the strain rate-dependent deformation modes of CFRP laminates, involving pure fiber fracture, epoxy crack with stepped surface and interface failure with residual voids, determines the “low-high-low” variation tendency of Young's modulus and strength as a function of strain rate. Ply orientation-dependent differences in deformation behaviors are also investigated via severe interfacial shearing effect. A unified model consisted of four deformation modes to is clarified to analyze the complexity of CFRP laminates failure mechanism.     

Electrical conductivity behavior of epoxy matrix nanocomposites with simultaneous dispersion of carbon nanotubes and clays

In this work, nanocomposites with simultaneous dispersion of multiwalled carbon nanotubes (MWCNT) and montmorillonite clays in an epoxy matrix were prepared by in situ polymerization. A high energy sonication was employed as the dispersion method, without the aid of solvents in the process. The simultaneous dispersion of clays with carbon nanotubes (CNT) in different polymeric matrices has shown a synergic potential of increasing mechanical properties and electrical conductivity. Two different montmorillonite clays were used: a natural (MMT‐Na⁺) and an organoclay (MMT‐30B). The nanocomposites had their electrical conductivity (σ) and dielectric constant (ε_r) measured by impedance spectroscopy. The sharp increase in electrical conductivity was found between 0.10 and 0.25 wt% of the MWCNTs. Transmission electron microscopy (TEM) of the samples showed a lower tendency of MWCNT segregation on the MMT‐30B clay surface, which is connected to intercalation/exfoliation in the matrix, that generates less free volume available for MWCNTs in the epoxy matrix. Data from electrical measurement showed that simultaneously adding organoclay reduces the electrical conduction in the nanocomposite. Moreover, conductivity and permittivity dispersion in low frequency suggest agglomeration of nanotubes surrounding the natural clay (MMT‐Na⁺) particles, which is confirmed by TEM. POLYM. COMPOS., 37:1603–1611, 2016. © 2014 Society of Plastics Engineers     

Shape memory epoxy nanocomposites with carbonaceous fillers and in‐situ generated silver nanoparticles

Aim of this work is to develop a novel epoxy based nanocomposite and to analyse its shape memory behavior. In particular, silver nanoparticles are in‐situ generated within an epoxy resin subsequently filled with both carbon black (CB) and carbon nanofibers (NFs) at different ratios and at a total amount of 4 wt%. Differential scanning calorimetry shows how the introduction of both CB and NF induces a slight decrease of the glass transition temperature (T_g) of the samples. The T_g drop due to nanofiller addition determines a decrease of both flexural modulus and stress at yield with respect to the neat resin, especially at elevated CB concentrations, while the presence of Ag nanoparticles plays a positive effect on the flexural properties. The best mechanical properties can be detected at a CF/NF ratio of 50%, coupled with a noticeable decrease of the electrical resistivity down to 10² Ω·cm and an interesting heating capability through Joule effect. The electro‐mechanical shape‐memory characterization under bending configuration demonstrates how it is possible to obtain an almost complete shape recovery after 60 s under an applied voltage of 220 V. POLYM. ENG. SCI., 59:694–703, 2019. © 2018 Society of Plastics Engineers     

Reinforcing effect of amine-functionalized and carboxylated porous graphene on toughness,thermal stability,and electrical conductivity of epoxy-based nanocomposites

Epoxy-based nanocomposites reinforced with nonfunctionalized porous graphene (NPG), carboxylated porous graphene (CNPG), and amine-functionalized porous graphene (ANPG) were investigated with regard to mechanical properties, thermal stability, and electrical conductivity. Nanomaterials were added to the epoxy matrix in varying contents of 0.5, 1, and 2 wt %. Generally, mechanical properties were improved as a result of introducing nanomaterials into the epoxy resin. However, the amelioration of toughness was only observed in functionalized NPGs/epoxy nanocomposites. Field emission scanning electron microscopy images showed that functionalized nanomaterials induced a rougher fracture surface compared to the neat epoxy. Dynamic mechanical analysis along with differential scanning calorimetry confirmed an increment in the glass-transition temperature (T_g) of the reinforced nanocomposites. Also, they proved that functionalization made the epoxy network tougher and more flexible. The electrical conductivity and thermal stability of the epoxy resin were also improved when loaded with nanomaterials. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47475.     

Investigation of the Thermal and Dielectric Behavior of Epoxy Nano-Hybrids by using Silane Modified Nano-ZnO

The present work focuses on a comparative study of the thermal and electrical behavior of diglycidyl ethers of bisphenol-A (DGEBA) to uncover the suitability for its use in high performance applications. An epoxy nanohybrids coating was developed using aminosilane functionalized ZnO (1, 3, 5 and 7 wt%) as the dispersed phase and commercially available DGEBA as the matrix phase, with curing using triethylenetetramine (TETA). The structural features of these materials were ascertained by FTIR spectral studies, SEM and AFM analyses. The peak shift in all the samples at ～ 1032 cm^?1 explains the etheric linkage of ZnO-APTES core shell nanoparticles with the DGEBA virgin epoxy resin. The thermal behavior of the diglycidyl resins and their corresponding nano-hybrids was studied by TGA and DSC. The first decomposition stage of DGEBA neat epoxy resin starts at 325 °C and the second stage at 513.2 °C which varied in all epoxy nanocomposites. Further thermodynamic parameters are calculated using the Coats-Redfern method from TGA results to examine the thermal stability. The sample with 3% ZnO-APTES-DGEBA film exhibits the highest activation energy of 26.20 kj/mol. The dielectric permittivity, dielectric loss and AC conductivity variation with frequency, temperature and filler concentration were studied using an impedance analyzer. The variation in electrical behavior is more pronounced in 1 and 7% ZnO-APTES-DGEBA epoxy nanocomposites.     

Influences of copper roughness on the electrical and mechanical performances of embedded capacitance materials

In this letter, the influences of copper roughness on the electrical and mechanical properties of embedded capacitance materials (ECMs) are investigated. The very low-profile ECM (VLP-ECM) is found having a capacitance of 1.24 nF/cm² with the lowest tolerance of 5.5% (1 × 1 mm). In contrast, the rolled anneal ECM has a higher tolerance, and invalid embedded capacitors are even found in the electrodeposited ECM. The VLP-ECM is also found having the highest peel strength of 10.2 N/cm among the three ECM. The appropriate copper roughness (R_z ≤ 3.5 μm) is beneficial for getting a uniform coating for the restriction of the flowing of BaTiO₃/epoxy composite. And the hook functioned copper bulges can increase the contacting area against stripping. Therefore, the ECM fabricated with VLP copper foil has the best electrical and mechanical performances, which is favored in the application of printed circuit boards manufacturing.     

High-toughness,environment-friendly solid epoxy resins: Preparation,mechanical performance,curing behavior,and thermal properties

The development of a facile and efficient approach to prepare high-toughness epoxy resin is vital but has remained an enormous challenge. Herein, we have developed a high-performance environment-friendly solid epoxy resin modified with epoxidized hydroxyl-terminated polybutadiene (EHTPB) via one-step melt blending. The characterization, mechanical performance, curing behavior, and thermal properties of EHTPB-modified epoxy resin were investigated. EHTPB-modified epoxy resin exhibited excellent toughness with a 100% increase in elongation at break of tensile than that of neat epoxy resin. The transfer stress and dissipated energy in the rubber phase were predominant mechanisms of toughening. The toughening effect of EHTPB on solid epoxy resin was better than that of some of the previously reported liquid epoxy resins. Meanwhile, at 10 wt % of EHTPB loading, the EHTPB-modified epoxy resin displayed high strength and 22 and 101% improvement of flexural strength and impact strength, respectively. Moreover, at 10 wt % of EHTPB loading, the activation energy of EHTPB-modified epoxy resin for curing reaction decreased from 73.89 to 65.12 kJ·mol⁻¹, which is beneficial for the curing reaction. Furthermore, EHTPB-modified epoxy resin had a good thermal stability and the initial degradation temperature increased from 249 to 313 °C at 10 wt % of EHTPB loading. This work provides a simple-preparation and highly efficient and large-scale approach for the production of high-toughness environment-friendly solid epoxy resins. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48596.     

Investigation of covalently grafted polyacrylate chains onto graphene oxide for epoxy composites with reinforced mechanical performance

To enhance the dispersion and interfacial interaction of graphene–epoxy matrix, polyacrylate chains grafted graphene oxide (PA-GO) was manufactured with A-174 functionalized GO (A-GO), methyl acrylate, and glycidyl methacrylate via free-radical random copolymerization technique. Fourier transform infrared, thermogravimetric analysis, X-ray photoelectron spectrum, Raman spectroscopy, X-ray diffraction, transmission electron microscopy, and nuclear magnetic resonance were performed to investigate the structure of A-GO and PA-GO. Then, the PA-GO was incorporated into epoxy resin via in situ solution intercalation dispersion method in order to form an interpenetrating network structure with epoxy resin. Field emission scanning electron microscope results indicate that the PA-GO exhibits excellent dispersion and interfacial compatibility in the epoxy matrix. In compared with pure epoxy, the tensile strength and impact strength of the epoxy composite with 1 wt % PA-GO were shifted from 62.78 ± 2.54 to 70.68 ± 2.02 MPa (about 12.6%) and 3.55 ± 0.41 to 4.98 ± 0.33 kJ m⁻² (about 40.3%), respectively. Moreover, increased storage modulus is also observed in the dynamic mechanical analysis measurements compared with that of neat epoxy resin. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47842.     

Effect of surface modified WO3 nanoparticle on the epoxy coatings for the adhesive and anticorrosion properties of mild steel

The effect of introducing WO₃ (tungsten oxide) nanoparticle in the epoxy coating was analyzed by electrochemical impedance spectroscopy and scanning electrochemical microscopy (SECM) methods in 3.5% NaCl. The (3-glycidyloxypropyl)trimethoxysilane was treated with the nanoparticle for the proper dispersion and chemical interaction of nanoparticle with the epoxy resin. The introduction of WO₃ nanoparticle in the epoxy coating enhances the charge transfer resistance (R_ct) as well as the film resistance (R_f). The observation of iron dissolution and oxygen consumption was done by applying the appropriate SECM tip potential in the WO₃-modified nanocomposite coated steel. The epoxy and epoxy–WO₃ nanocomposite-coated samples were used to study the adhesion and anticorrosion properties. The analysis by SEM/EDX displayed that the enriched W was detected in the nanocomposite coating of steel. The presence of the nano level corrosion product containing W was confirmed by focused ion beam-transmission electron microscope analysis. The high corrosion protection properties of the epoxy-based nanocomposite coating was due to the complex nanoscale layer formed and chemical interactions of epoxy resin with surface-modified nanoparticle in nanocomposites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48323.     

Investigation on surface treatments of CFRP and aluminum to improve fracture toughness of adhesively-bonded CFRP–aluminum joints

In this study, the role of surface treatments of CFRP (graphite/epoxy composite) and aluminum (7075-T6) on the adhesively-bonded CFRP-aluminum joints has been investigated. The CFRP was surface-treated by Ar⁺ ion irradiation in an oxygen environment and the aluminum was surface-treated using a DC plasma. Ar⁺ ion irradiation treatment was carried out at Ar⁺ ion dose of 10¹⁶ ions/cm². Plasma treatment was carried out at a volume ratio of acetylene gas to nitrogen gas of 5:5 and the treatment time was 30 s. The effect of surface treatments on the fracture behavior CFRP-aluminum joints was determined from fracture tests using three different CLS (cracked lap shear) specimens: (1) untreated CFRP/untreated aluminum, (2) ion-irradiated CFRP/untreated aluminum and (3) untreated CFRP/plasma-treated aluminum. Fracture behaviors (fracture load, fracture toughness, fracture surfaces) of these three different specimens were compared. The results showed that both fracture load and fracture toughness of CFRP-aluminum joints were in the following order: ion-irradiated CFRP/untreated aluminum specimen > untreated CFRP/plasmatreated aluminum specimen > untreated CFRP/untreated aluminum specimen. SEM examination of fracture surfaces showed that fracture occurred as an interfacial failure for untreated specimens. On the other hand, a cohesive failure in the adhesive was the primary fracture mode for specimens surface-treated by ion irradiation or plasma.     

Surface treatment of CFRP composites by Ar+ irradiation to improve bonding strength between aluminum and CFRP composites

The effect of surface treatment of carbon fiber reinforced plastic (CFRP) composites on the T-peel strength and the shear strength between CFRP and aluminum panels was studied. The surface of the composite panel was treated with Ar⁺ irradiation under oxygen environment. The optimal Ar⁺ ion dose was determined by measuring the changes of contact angle and surface energy as a function of ion dose. T-peel tests and SLS tests were performed using irradiated CFRP/aluminum specimens and unirradiated CFRP/aluminum specimens to determine the treatment effect by Ar⁺ irradiation under oxygen environment on the T-peel strength and shear strength of CFRP/aluminum composites. The results showed that contact angle on the surface of the composite panel was reduced from ∼80° to ∼8° and the surface energy increased from 31 ergs/cm² to 72.4 ergs/cm² with an ion dose of 10¹⁷ ions/cm². T-peel strength and shear strength are significantly affected by the surface treatment of composite panel. T-peel strength and shear strength improved 650% and 56%, respectively, when the treatment was made with an ion dose of 10¹⁶ ions/cm². SEM examination showed that the improvement of bonding strength was attributed to the uniform spread and fracture of epoxy adhesive.     

DGEBA-polyaminoamide as effective anti-corrosive material for 15CDV6 steel in NaCl medium: Computational and experimental studies

The present study is focusing on evaluating theoretically and experimentally stability and type of interactions between the epoxy resin bisphenol A diglycidyl ether (DGEBA)-polyaminoamide anticorrosive coating and high strength low alloy steel surface 15CDV6. The coated steel samples were subjected to a harsh environment of an electrolyte solution of 3 wt % NaCl to simulate the corrosive marine environment. The performance of the epoxy coating was investigated using electrochemical impedance spectroscopy (EIS). The EIS results revealed the occurrence of some deterioration in the film after subjecting it to a harsh environment for 4392 h, because the impedance of the coating dropped by about 1.4 kΩ.cm². Surface morphological study of metallic specimens before and after exposing to the simulated marine environment (3 wt % NaCl) was carried out using scanning electron microscopy, energy dispersive spectroscopy (EDS), and optical microscope (OM) methods. The interactions between DGEBA-polyaminoamide and the metallic surface were further carried out using computation modeling such as density functional theory (DFT)-based quantum chemical calculations, Monte Carlo (MC), and molecular dynamics (MD) simulations. Results showed that DGEBA-polyaminoamide possesses a strong tendency to adhere and inhibits the corrosive dissolution of 15CDV6 steel surface in the stimulated marine environment. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48402.     

Fibre reinforced composites of multifunctional epoxy resins

Glass and carbon fibre reinforced epoxy composites were fabricated for N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenyl methane (TGDDM) and its formulated systems with tri- and di-functional reactive epoxy diluents using 30% diaminodiphenyl sulphone (DDS) as a curing agent. The epoxy laminates were evaluated for their physical, chemical and mechanical properties [at room (26°C) and high (100°C) temperatures]. A marginal increase (<20%) in the mechanical properties of CFRP was found compared with GFRP laminates. Incorporation of epoxy diluents altered the mechanical properties of the composites significantly. The incorporation of triglycidyl-4-aminophenol diluent to TGDDM systems resulted in an improvement in mechanical properties of about 2–6%.     

Epoxy‐Polypyrrole‐Cotton Tertiary Composite with Electro‐Tunable Stiffness

In this work, a tertiary epoxy composite reinforced with polypyrrole (PPy) coated cotton fabric layers exhibiting electrical voltage induced thermally tunable stiffness is reported. The thin coating of PPy over cotton fabric is accomplished via oxidative vapor phase polymerization that allows creation of an active thin layer over the fibers without affecting their mechanical properties. Six such functional layers are stacked within an epoxy matrix to prepare the composite that shows in‐plane electrical insulator behavior (volume resistivity > 10⁹ Ω cm) but considerably reduced resistivity by an order of 10³ across the cross‐sections. The presence of conductive layers enables the composite to heat via Joule's effect when an electrical voltage is applied across two ends. This causes softening of matrix near the matrix‐reinforcement interface and thereby changing composite's stiffness. On application of variable voltage, a non‐linear decrease of 91% in composite stiffness is observed (6371.2 N m^?1 at 0 V to 566.4 N m^?1 at 63 V). A stable and tunable mechanical performance of the composite is further demonstrated by cyclic changes in stiffness due to voltage change with recovery up to 95% of original stiffness after 14 continuous cycles.