Photopolymerization Rate Research Articles

Preparation and properties of polymerizable silica hybrid nanoparticles with tertiary amine structure

To increase the photopolymerization rate and improve the properties of UV coatings, polymerizable silica hybrid nanoparticles with tertiary amine structure were prepared. Organic compound with isocyanate group was first grafted onto the surface of nanosilica by reaction of nanosilica with isophorone diisocyanate, then the nanosilica bearing isocyanate group reacted with N,N-di(3-propionic acid, 1,4,7-trimethyl-3,6-dioxaoctane-8-yl acrylate, ester) ethanolamine synthesized from tripropylene glycol diacrylate and ethanolamine. The preparation was characterized by 1H nuclear magnetic resonance (NMR) and Fourier transform infrared spectrometry (FT-IR). Thermogravimetric analysis (TGA) showed that the organic compounds grafted onto the silica decomposed from 256 °C to 650 °C and the grafting percentage based on nanosilica was 105%. The morphology analysis of nanosilica and modified silica by field-emission scanning electron microscopy (FE-SEM) indicated that the silica kept nanosized scale after modification, while the nanosilica dispersion was improved and formation of agglomerates unlikely. Determination of viscosities of coatings with modified nanosilica, it was found that viscosities of the coatings decreased in comparison with the viscosities of coatings with unmodified nanosilica. Compared with pure organic coating, the photopolymerization rate of coatings were faster when modified nanosilica was used from 1 wt% to 5 wt%, but slower when the loadings of modified nanosilica was 7 wt% because co-initiating effects of tertiary amine compound grafted on nanosilica counterbalanced the effects of UV scattering by silica on photopolymerization rate. The hardness and abrasive resistance of cured films also increased and improvement degree was different when the various amounts of modified nanosilica were used.

Visible light curing of bisphenol-A epoxides and acrylates photoinitiated by (η6-benzophenone)(η5-cyclopentadienyl) iron hexafluorophosphate

Visible light curing of diglycidyl ether of bisphenol-A (DGEBA) epoxy oligomer and acrylate monomers photoinitiated by (η6-benzophenone)(η5-cyclopentadienyl) iron hexafluorophosphate (Fc-BP) under a halogen lamp were studied by near infrared spectroscopy. Fc-BP exhibited high efficiency in the radical photopolymerization of acrylate monomers, even without the presence of tertiary amines. Under the same light source, however, benzophenone did not show any photoinitiating ability. Fc-BP could also be used to photoinitiate the cationic polymerization of DGEBA. There was an obvious increase in the photopolymerization rate of DGEBA and a decrease in the induction period when benzoyl peroxide was used as a photosensitizer. The induction period at the beginning of DGEBA cationic polymerization was eliminated by introducing a certain amount of cycloaliphatic epoxy monomer ERL4221 as an active diluent. However, the final epoxy conversion was decreased when ERL4221 was used.

Synthesis and photopolymerization kinetics of linear alicyclic urethane acrylate macromonomer in presence of reactive diluents

This study deals with the photopolymerization of a macromonomer in the presence of reactive diluents using Photo Differential Scanning Calorimetry or Photo DSC. The kinetic profiles of these systems showed that the rate of photopolymerization rapidly increases at very early stages of the reaction. The rate of reaction was further found to increase with the addition of crosslinking agents. The addition of trifunctional crosslinking agent to the macromonomeric formulation resulted in a higher polymerization rate and conversion than that of a difunctional crosslinking agent. From the heat flow profiles, the kinetic parameters such as induction time, time to attain peak maximum, rate of maximum polymerization, and final conversion were noted for all the formulations. The initiation of photopolymerization was found to depend on the functionality while the in situ viscosity controlled the time scale for reaction diffusion which resulted in varying levels of conversions. Other parameters such as effect of temperature and concentration of photoinitiator on photocuring kinetics are also discussed.

Photopolymerization behavior in nanocomposites formed with thiol–acrylate and polymerizable organoclays

AbstractBecause of the inherent characteristics of the thiol–ene step growth mechanism in preparation of thiol–ene photopolymer clay nanocomposites, the ratio between thiol and ene functional groups at and near the organoclay surfaces may have a significant effect on the polymerization behavior. This study investigates the influence of monomer composition and the type of polymerizable organoclay on thiol–acrylate photopolymerization behavior in preparation of photocurable clay nanocomposite systems. To this end, two types of polymerizable organoclays with acrylate or thiol functional group on the clay surfaces were compared in monomer compositions with different polarity and functionality. Real‐time infrared spectroscopy was used to characterize polymerization behavior in conjunction with photo‐DSC. The degree of clay exfoliation was evaluated using small angle X‐ray scattering and correlated with photopolymerization behavior. Higher chemical compatibility of components induced enhanced clay exfoliation resulting in increase in photopolymerization rate. By affecting the stoichiometric ratio of functional groups in the clay gallery, thiolated organoclays enhance thiol–ene reaction, whereas acrylated organoclays encourage acrylate homopolymerization. In addition, inducing more propagating thiyl radicals on the organoclay surfaces by increasing functionality of thiol monomer also facilitates thiol–ene copolymerization, whereas the increase of acrylate functionality reduces final thiol conversion. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Photoinitiated alternating copolymerization of dialkyl maleates and fumarates with vinyl ethers

AbstractA systematic study of the free radical photoinitiated alternating crosslinking copolymerizations of dialkyl maleates and furmarates with multifunctional vinyl ethers was carried out. The photocopolymerizations were fast and highly efficient when carried out using a variety of α‐cleavage photoinitiators. The effects of the structures of the both the unsaturated esters and the vinyl ether monomers were examined. Dialkyl maleates were observed to be more reactive than the corresponding fumarate esters. The photopolymerization rates of several of these comonomer pairs were compared with standard diacrylate and dimethacrylate monomers. A range of different physical properties can be obtained by varying the length of the alkyl chain on alcohol portion of the unsaturated ester monomer. Prospects for the use of these comonomer systems in UV curable coatings, adhesives, printing inks, and composites are discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Studies on an argon laser‐induced photopolymerization employing both mono‐ and bischromophoric hemicyanine dye—Borate complex as a photoinitiator. Part III

AbstractTwo series of homodimeric hemicyanine dyes based on 4‐(p‐N,N‐dialkylaminostyryl)pyridinium and 4‐(p‐N,N‐dialkylaminostyryl)quinolinium residues have been evaluated as novel photoinitiators for radical polymerization induced with an argon ion laser visible emission. In the tested photoredox pairs, hemicyanine dye cation acts as an electron acceptor and it is coupled with n‐butyltriphenyl borate anion being an electron donor. The photochemistry of the series of bichromophoric stilbazolium borates, 1,3‐, 1,5‐, and 1,10‐bis‐[4‐(p‐N,N‐dialkylaminostyryl)pyridinyl]alkane di‐n‐butyltriphenylborates and 1,3‐, 1,5‐, and 1,10‐bis‐[4‐(p‐N,N‐dialkylaminostyryl)quinolinyl]alkane di‐n‐butyltriphenylborates, was compared with the photochemistry of the structurally related, monochromophoric styrylpyridinium and the styrylquinolinium borates. The experimental results indicated that the rate of photopolymerization depends on ΔGel of the electron transfer between borate anion and hemicyanine cation. The relationship between the rate of polymerization and the free energy of activation shows the dependence predicted by the classical theory of the electron transfer. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

UV-curing behavior and properties of tri/di(acryloyloxyethyloxy) phenyl silane used for flame-retardant coatings

The silicon-containing multifunctional acrylates, tri(acryloyloxyethyloxy) phenyl silane (TAEPS) and di(acryloyloxyethyloxy) methyl phenyl silane (DAEMPS) were prepared by using the transetherification of phenyltrimethoxyl silane and dimethoxymethylphenyl silane with 2-hydroxylacrylate, respectively. The molecular structures were confirmed by FTIR, 1H NMR, 13C NMR and 29Si NMR spectroscopic analysis. The obtained TAEPS and DAEMPS as tri- and difunctional monomers were blended with a commercial oligomer epoxy acrylate (EB600) in different ratios to formulate a series of silicon-containing UV-curable resins. The viscosity of the resins reduced obviously while the photopolymerization rates according to the photo-DSC analysis greatly increased by the addition of monomers. The thermal degradation and flame-retardant behaviors of the cured films were evaluated by the thermogravimetric analysis and the limiting oxygen index (LOI) measurements, respectively. The char yields measured under air atmospheres increased extraordinarily with increasing the silicon content. The LOI values increased from 21 for EB600 to over 30 at 70 wt% monomer loading, demonstrating that the improved flame retardancy was obtained. The data from dynamic mechanical thermal analysis showed that TAEPS and DAEMPS have good miscibility with EB600. The crosslink density of UV-cured EB600/TAEPS film showed a greater increase along with the monomer content, compared with that of EB600/DAEMPS. The mechanical property measurement indicated that the addition of TAEPS led to an increase in elongation-at-break and a first increase and then decrease in tensile strength.

Preparation of a crosslinked sucrose polymer by thiol–ene photopolymerization using dithiothreitol as comonomer

The thiol–ene photopolymerization technique was used to prepare a crosslinked sucrose polymer. Sucrose was chemically modified to introduce two allyl groups, however, in the synthesis were also produced mono and triallyl derivatives. After purification by column chromatography, a fraction (F2A2S) with 94% diallyl sucrose (A2S), 4% of triallyl sucrose (A3S) and 2% of monoallyl sucrose (A1S) was obtained. This fraction was subsequently photopolymerized with dithiothreitol (DTT) which is a difunctional thiol. The presence of small concentration of A3S in F2A2S helped to crosslink in situ the obtained polythioether. Photopolymerization kinetics were determined using the Real-Time Infrared technique. It was found that photopolymerization can proceed in the absence of photoinitiators but the rate of photopolymerization and conversion increased considerably when different photoinitiators at 1 mol% were used and when the intensity of UV light was increased. The analysis by DMA of probes of the obtained sucrose polythioether revealed that the difunctionality of both comonomers, A2S and DTT, resulted in flexible materials with a T g of 30 °C and high storage modulus values. The sucrose polythioether was completely degraded in 2 h in strongly acidic medium. Additionally, the ability of water to absorb the prepared material was tested. We found that it was pH dependent with the maximum absorption at pH = 14. The biopolymer displayed high thermal stability up to 230 °C.

Spectroscopic and photopolymerization studies of benzyl methacrylate/poly(benzyl methacrylate) two-component system

The influence of the viscosity of a two-component system on its molecular dynamics (on the basis of hypersonic wave velocity and attenuation coefficient) and photopolymeriza- tion kinetics was studied. The system investigated represented the solution of poly(benzyl methacrylate), PBzMA (MW ¼ 70000) in its monomer, benzyl methacrylate (BzMA). The viscosity of the system was varied by adding various amounts of the poly- mer to the monomer (10-50 wt %). The molecular dynamics in the neat BzMA was studied by the proton Nuclear Magnetic Resonance (NMR) spin-lattice relaxation time measurements and the wide-line 1 H NMR spectroscopy in a wide range of tem- perature. Information on the local dynamics in liquid BzMA above its melting temperature was gained from the high-resolu- tion 1 H and 13 C NMR spectra. The hypersonic wave velocity and the attenuation coefficient were investigated in the appropriate temperature range related to a viscoelastic relaxation process by the Brillouin light scattering method. The kinetic measure- ments have demonstrated that the photopolymerization rate rapidly increases and the monomer conversion decreases with increasing polymer-to-monomer ratio; this effect has been noted in the whole range of polymer concentration and reaction temperature studied. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1336-1348, 2010

PHOTOINITIATED RAFT CROSSLINKING POLYMERIZATION OF POLYETHYLENE GLYCOL DIMETHACRYLATES

A reversible addition-fragmentation chain transfer (RAFT) agent,S-1-dodecyl-S'-(α,α'-dimethyl-α″-aceticacid) trithioncarbonate was synthesized. The UV-initiated RAFT polymerization of polyethylene glycol dimethacrylates ( PEGDMA ) at room temperature was investigated by using differential scanning photocalorimetry (DPC) and dynamic mechanical analysis (DMA). The photopolymerization rate of the RAFT system was much lower than that in conventional free radical polymerization,a mild autoacceleration occurred as the addition reaction became diffusion controlled. In addition the influence of different spacer lengths between the methacrylate groups on RAFT polymerization rate and final conversion was also explored. The results show that the final conversion increases slowly and the polymerization rate decreases at first and then increases as the number of ethylene glycol units between the methacrylate groups increaseing. Comparison with three different PEGDMAs,PEGDMA with longer middle segments yields more homogeneous networks and lower crosslinking density.

Synthesis and properties of hyperbranched polyurethane acrylate used for UV curing coatings

The hyperbranched polyurethane acrylate (HPUA) was synthesized through the addition of hyperbranched polyurethane endcapped by hydroxyl groups (HPU-OH), with the semiadduct urethane monoacrylate (IPDI-HEA). The HPU-OH was prepared by the amidation reaction of diethanolamine with isophorone diisocyanate. The molecular structure of HPUA was characterized by FTIR and 1H NMR analyses. The number average molecular weight and its polydispersity index were measured by GPC to be 7714 g mol −1 and 1.24, respectively. The HPUA was blended with epoxy acrylate EB600 and difunctional monomer TPGDA in different ratios, and exposed to a UV lamp for photopolymerization in the presence of Runtecure 1104 as a photoinitiator at room temperature. The photopolymerization rate and final unsaturation conversion reached to the highest values with only 5 wt% HPUA addition, whereas decreased as further added. The tensile strength of UV-cured films was improved by adding less than 10 wt% HPUA without damaging the modulus, having the value of 62.56 MPa for EB 90HPUA 10 film. Besides, the elongation at break increased continuously with the addition of HPUA, reaching to 130% for EB 70HPUA 30 film. Moreover, the impact strength was greatly enhanced by the addition of HPUA, possessing nearly two times high for EB 70HPUA 30 film compared with pure EB600 film. However, the T g decreased as HPUA was added from the DMTA measurements. According to the ratios of T s/ T g the HPUA has good compatibility with EB600/TPGDA resin.

Crystal packing and reactivity of di(meth)acrylates of some derivatives of hydroquinone and pyrocatechol in melts

The X-ray diffraction study of 2,2′-(1,2-phenylene-bis(oxy)diethanol and 2,2′-(1,4-phenylenebis(oxy)diethanol dimethacrylates and 2,2′-(1,4-phenylenebis(oxy)diethanol diacrylate (T m = 40–42, 68–70, and 62–64°C, respectively) indicates that oligomer molecules are packed in crystals as stacks in which methacrylate fragments of adjacent molecules are parallel to each other. The minimum distances between the centers of C=C double bonds of adjacent methacrylate fragments in crystals of di(meth)acrylates are 4.373, 4.215, and 3.996 respectively. The conversion dependences of the reduced rates of photopolymerization of melted oligomers (9,10-phenanthrenequinone as a photoinitiator) pass through maxima at conversions of 40, 11, and 2%, while the ultimate conversions are 85, 33, and 73%, respectively. The addition of ionic liquids based on phosphonium and imidazolium cations to dimethacrylates of 2,2′-(1,2-phenylenebis(oxy)diethanol and triethylene glycol increases the maximum reduced rate of photopolymerization.

A novel ferrocenium salt as visible light photoinitiator for cationic and radical photopolymerization

A novel hybrid photoinitiator for visible light photopolymerization, (η 6-3-benzoyl-4-chlorodiphenylamine) (η 5-cyclopentadienyl) iron hexafluorophosphate (Fc-NBP), was synthesized and studied. Its absorption in the UV and visible light regions showed much stronger activity than those of either the commercialized cationic photoinitiator I-261 or the conventional free radical photoinitiator benzophenone, especially above wavelengths of 350 nm. When exposed to visible light, the photoinitiator under study initiates both cationic polymerization and radical polymerization. The photoinitiator's abilities in the photopolymerization of acrylates and epoxides were evaluated by near infrared (NIR) spectroscopy. The results from NIR clearly indicate that Fc-NBP exhibited high efficiency in photopolymerizing acrylate monomers. In the same lamp, however, benzophenone showed no photoinitiating ability. The photopolymerization rate of the diglycidyl ether of the bisphenol-A epoxy (DGEBA) oligomer was found to be slower than that of acrylates when using Fc-NBP as the photoinitiator. This study shows that the polymerization of epoxide DGEBA can be speeded up by adding a photosensitizer benzoyl peroxide (BPO), but BPO and tertiary amines do not affect the free radical photopolymerization of tripropylene glycol diacrylate (TPGDA). We conclude by providing a possible photoinitiation mechanism.

Synthesis and photoinitiating behavior of hyperbranched polymeric photoinitiators bearing coinitiator amine

A series of benzophenone (BP)-terminated hyperbranched polyester (BoltornTM P1000), bearing amine moieties as synergists by reacting with piperidine, were synthesized as yellowish liquids with low viscosity, and used as polymeric photoinitiators (HPPIs). For comparison, acrylate groups were introduced to the terminals of hyperbranched polyester for obtaining a polymerizable photoinitiator. The chemical structures were characterized by FTIR and 1H NMR spectroscopy. HPPIs and BP exhibited the similar absorptions by UV–vis spectroscopy. The photoinitiating behavior of HPPIs with trimethylolpropane triacrylate (TMPTA) as a trifunctional monomer was investigated by using photo-DSC analysis. The results indicated that the maximum photopolymerization rate and unsaturation conversion of TMPTA initiated by HPPIs were both lower than that by BP. Among them, the HPPI with double tertiary amine moiety of BP moiety was found to be the most efficient photoinitiator. Additionally, the films cured with bisphenol A epoxy acrylate EB605 initiated by HPPIs were uniform and possessed high Tg from DMTA. Copyright © 2010 John Wiley & Sons, Ltd.

Visible Light Photoinitiating Systems: Toward a Good Control of the Photopolymerization Efficiency

This paper discusses the photochemistry of three-component photoinitiating systems (3K-PIS) for free radical photopolymerization and the efforts made during the last decades to propose efficient systems applied to the laser imaging area. A special focus is devoted to a new 3K-PIS working in the green region. It is based on a pyrromethene dye which is reduced or oxidized by a coinitiator. A third redox component is used that leads to the recovery of the initial dye and the formation of additional initiating species, preventing a fast photobleaching of the dye. The beneficial effect on the photopolymerization rates and the final monomer conversion is clearly noticed. Laser flash photolysis was used to understand the reaction mechanisms, and detailed photopolymerization kinetics allow the study of polymeric network formation.

Synthesis and characterization of hyperbranched photosensitive polysiloxane urethane acrylate

A novel hyperbranched photosensitive polysiloxane urethane acrylate (HBPSUA) based on hyperbranched polyesters (HBP-OH) has been synthesized. HBP-OH was synthesized from N,N-diethylol-3-amine methylpropionate as AB 2 monomer and trimethylolpropane (TMP) as a core molecule. The structure of the oligomer was characterized by FTIR, GPC and 1H NMR. The molecular weight of the oligomer is about 10,000 and the viscosity is 4446 cps at room temperature. The HBPSUA possesses good compatibility with most of acrylate monomers. The effect of photoinitiators, monomers and light intensity on the photopolymerization kinetics of the oligomer HBPSUA was investigated by real-time infrared spectroscopy (RT-IR). The results show that HBPSUA can well photopolymerize under UV-irradiation in the presence of photoinitiators. Irgacure 1700 showed the highest initiating efficiency among those tested photoinitiators. The optimal concentration of photoinitiator (Darocur 1173) is determined as 0.05 wt.%. The system of HBPSUA/TPGDA has the high conversion of double bond and polymerization rate, and they are 92.38% and 22.25 s −1, respectively. Compared with linear systems (PSUA), HBPSUA has the higher photopolymerization rate and lower viscosity. The cured film of HBPSUA possesses good flexibility.

Photopolymerization of vinyl ether networks using an iodonium initiator—The role of photosensitizers

AbstractPhoto‐differential scanning calorimetry (photo‐DSC) and UV‐visible spectrometry were used to investigate the photocuring kinetics of visible light initiated cationic photopolymerization of triethyleneglycol divinyl ether with a diphenyl iodonium salt and three photosensitizers, 1‐chloro‐4‐propoxy‐9H‐thioxanthen‐9‐one (CPTXO), acridine orange (AO), and camphorquinone (CQ). Although all photosensitizers were effective in causing photopolymerization, CPTXO and AO photo‐reacted during the irradiation; whereas CQ was not significantly consumed during the timescale of the photo‐DSC experiment. This difference in photo‐reactivity has not been reported previously, and indicates that the reaction of the iodonium ion with CPTXO and AO results in the formation of the photosensitizer radical cation, whereas the CQ mechanism involves the reduction of the CQ excited state to a ketyl radical by a H‐donor (monomer) followed by the oxidation of the ketyl radical by the iodonium salt and thus regeneration of the CQ. For the CPTXO and AO systems, the photopolymerization rate was retarded by a radical inhibitor but the CQ system was unaffected, which confirms that different mechanisms are involved. The cure rate was found to be proportional to the concentrations of CPTXO and CQ but appeared to follow an approximately square root dependence on the AO concentration. Mechanisms to explain these differences were presented. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5474–5487, 2009

Molecular and crystalline structure of 2,2-di(phenyl-4-ol)propane dimethacrylate, 2,2-di(phenyl-4-ol)propane diacrylate, pyrocatechol diacrylate, and hydroquinone diacrylate: Reactivity in melts

The X-ray diffraction analysis of 2,2-di(phenyl-4-ol)propane dimethacrylate, 2,2-di(phenyl-4-ol)propane diacrylate, pyrocatechol diacrylate, and hydroquinone diacrylate has shown that oligomer molecules within crystals are packed in stacks, where (meth)acrylate fragments of neighboring molecules are parallel to each other. The minimum distances between the centers of double bonds C=C of (meth)acrylate fragments in 2,2-di(phenyl-4-ol)propane dimethacrylate, 2,2-di(phenyl-4-ol)propane diacrylate, pyrocatechol diacrylate, and hydroquinone diacrylate are 4.208, 4.012, 3.621, and 3.739 describing the reduced rate of photopolymerization of molten monomers (with 9,10-phenanthrenequinone used as a photoinitiator) versus conversion show maxima at degrees of polymerization of 8, 16, 22, and 38%; the limiting conversions are 29, 36, 44, and 86%, respectively. The maximum reduced rates of photopolymerization of 2,2-di(phenyl-4-ol)propane dimethacrylate and diacrylate are nearly the same, whereas the rates of photopolymerization of hydroquinone diacrylate and pyrocatechol diacrylate are higher by a factor of 4 than those of the corresponding dimethacrylates.

Photopolymerization property of a novel benzophenone derivative photoinitiator containing Si

The photopolymerzation property of a novel benzophenone derivative photoinitiator containing Si (HBSiBH) was investigated by real-time FTIR (RTIR) and X-ray photoelectron spectroscopy (XPS). It is noted that both of the polymerization rate and final conversion of the system increased with increase concentration of HBSiBH. Addition of small amount of HBSiBH can greatly improved the photopolymerization rate of radical photopolymerization. XPS experiment showed that the new photoinitiator containing Si had ability to float up to the surface of UV-curable system. It could more effectively mitigate inhibition of oxygen in radical photopolymerization and provide a new approach to improving surface performance of UV-curable materials.

Photopolymerization of Vinyl Ether Networks Using an Iodonium Initiator: Effect of Radiation Intensity and Iodonium Concentration

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the photocuring kinetics of visible light initiated cationic photopolymerization of triethylene glycol divinyl ether (TEGDVE) with a diphenyl iodonium salt initiator and the three photosensitizers: 1-chloro-4-propoxy-9H-thioxanthen-9-one (CPTXO), camphorquinone (CQ), and acridine orange (AO). The effects of water, a tertiary amine, and a radical scavenger were used to probe the photocuring mechanism. The rate of photopolymerization was approximately proportional to the incident light intensity, and in each system, the final conversions, as measured by photo-DSC, also increased with raised incident light intensity. The power-law dependence of the reaction rate on the concentration of iodonium salt with CPTXO and AO was ∼1.5, while the order is nearly unity with CQ as the photosensitizer, suggesting that these photosensitizers do not follow the same sensitization mechanism. For the three photosensitizers, the final conversions all increased with higher iodonium salt concentrations. After cessation of irradiation, a dark reaction was observed—although the rate of this process decreased rapidly with time (as shown by photo-DSC), the polymerization continued very slowly over a long period of time.