Spatial and temporal evolution of the photoinitiation rate for thick polymer systems illuminated by polychromatic light: selection of efficient photoinitiators for LED or mercury lamps

BACKGROUND: Four common free radical photoinitiators were evaluated for use in thick photopolymerizations illuminated with a medium‐pressure 200 W mercury–xenon arc lamp and a high‐intensity 400 nm light‐emitting diode (LED) lamp. For each photoinitiator/lamp combination, the spatial and temporal evolution of the photoinitiation rate profile was analyzed by solving the set of differential equations that govern the light intensity gradient and initiator concentration gradient for polychromatic illumination. RESULTS: The simulation results revealed that two of the four photoinitiators evaluated were ineffective for photoinitiating thick polymer systems. The photoinitiator bis(2,4,6‐trimethylbenzoyl)‐phenylphosphine oxide, in combination with the 400 nm LED lamp, was shown to be the most efficient photoinitiator/light source combination for photoinitiation of thick systems. CONCLUSION: The results show that some photoinitiators commonly used for photopolymerization of thin coatings are ineffective for curing thick systems. LED light sources provide advantages over traditional mercury lamps, and may have tremendous potential in the effective photoinitiation of thick polymer systems. Copyright © 2008 Society of Chemical Industry     

Efficiency of 2,2‐dimethoxy‐2‐phenylacetophenone for the photopolymerization of methacrylate monomers in thick sections

The efficiency of 2,2‐Dimethoxy‐2‐phenylacetophenone (DMPA) for the photopolymerization of methacrylate monomers in thick sections was assessed. DMPA is an efficient photoinitiator for thick sections (≈2 mm) because a fast reaction and high conversions are obtained with concentrations as low as 0.25 wt % DMPA. The polymerization rate increased when the DMPA content increased from 0.125 wt % to 0.25 wt %. However, the conversion versus irradiation time profiles in resins containing 0.25 wt % or 0.5 wt % DMPA were similar. This is attributed to the screening effect caused by excessive levels of DMPA. In addition, the consumption of DMPA under UV irradiation was accompanied by the appearance of light absorbing photoproducts. Because the absorbing species nearest to the light source absorb part of it, the light fails to reach the deeper layers of the sample. The overall effect of light screening is a reduced photoinitiation rate and double bond conversion along the irradiation path. This effect was compensated by the use of irradiation sources of higher intensity; which increased the initiation rate by increasing the production of primary radicals. DMPA is colorless and it does not require the presence of amine as coinitiator. These properties make DMPA attractive as photoinitiator of dental composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Efficiency of 4,4′‐bis(N,N‐diethylamino) benzophenone for the polymerization of dimethacrylate resins in thick sections

The efficiency of 4,4′‐bis(N,N‐diethylamino)benzophenone (DEABP) for the polymerization of dimethacrylate monomers in thick sections ( 1 – 2 mm) was studied. DEABP (λ_max = 365 nm) represents a complete initiating system as it contains both ketone and amine functional groups. During irradiation, DEABP photobleaches at a fast rate causing deeper penetration of light through the underlying layers, but the photoinitiation efficiency (rate of polymerization per photon absorption rate) is relatively poor. As a result, irradiation of methacrylate monomers at 365 nm results in a slow average polymerization rate and a reduced monomer conversion for thick sections due to the light attenuation caused by the high absorptivity of DEABP and photolysis products. These results highlight the inherent interlinking of light attenuation and photobleaching rate in polymerization of thick sections. Copyright © 2011 Society of Chemical Industry     

Impact of radiation attenuation and temperature evolution on monomer conversion of dimethacrylate‐based resins with a photobleaching photoinitiator

The photopolymerization process of a dimethacrylate copolymer system activated by the camphorquinone (CQ)/amine photoinitiator system (1 wt%), was experimentally studied under nonisothermal conditions in 1‐ and 2‐mm thick samples by measuring double bond conversion, temperature rise and radiation attenuation through the sample during polymerization. The peak temperature in 1‐ or 2‐mm thick samples irradiated at 5 mW/cm² was 29 and 38°C, respectively. The temperature evolution during polymerization was also predicted by solving the energy balance coupled with the kinetic expressions for the reaction rate. Radiation attenuation as a function of depth by the photobleachable CQ results in spatial and temporal variation in the local rates of the kinetic steps involved. General relationships for spatiotemporal variations in concentration of a photobleaching initiator, in systems where attenuation and initiator consumption are taken into account, were used to compute local polymerization rates. The effects of radiation attenuation, photobleaching of the photoinitiator and variation of cure temperature at different depths into the resin, all compete to determine the double bond consumption. The increased radiation attenuation in the 2‐mm thick sample was accompanied by a higher cure temperature compared with the 1‐mm thick sample, and as a result, the monomer conversion averaged over the sample thickness in the 1‐ and 2‐mm thick samples was the same. Results obtained in this research highlight the inherent interlinking of thermal and radiation attenuation effects in bulk photopolymerizing systems. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Dyes based on a 1,4‐naphthoquinone skeleton as new type II photoinitiators for radical polymerisation

Photoredox pairs consisting of 1,4‐naphthoquinone dyes and commercially available hydrogen donors (2‐mercaptobenzoxazole, 2‐mercaptobenzothiazole, 2‐mercaptobenzimidazole and 2,5‐dimercapto‐1,3,4‐thiadiazole) are found to be effective initiator systems for the radical polymerisation of trimethylolpropane triacrylate under visible light. The efficiency of these initiator systems is discussed in terms of the free energy change for the electron transfer process from the dye to the hydrogen donor. The results show that the photoinitiation ability of tested photoredox pairs depends on the structure of both the dye and the hydrogen donor.     

Study on spatial–temporal kinetics of photo‐initiated frontal polymerization in stacked reaction cells

The spatial–temporal kinetics for photo‐initiated frontal polymerization(PFP) of isobornyl acrylate with 2,4,6‐trimethylbenzoyldiphenyl phosphine oxide (TPO) as photobleaching initiator was studied experimentally in stacked reaction cells. FTIR and NMR spectroscopy were employed to measure the polymerization conversion, which is dependent on the exposure time, sample depth, light intensity and photo‐initiator concentration. The experimental results are consistent with the theoretical model prediction and show that prolonged irradiation time, higher light intensity and lower photo‐initiator concentration are favorable in enhancing the advance of the polymerization front. The depth‐resolved GPC analysis shows that the average molecular weight of the PFP product dramatically increases with sample depth, while the molecular weight polydispersity reduces steadily with increase in sample depth. Copyright © 2006 Society of Chemical Industry     

基于点击化学和可逆加成-断裂链转移自由基聚合合成极性链接枝聚丁二烯

以1-硫代甘油作为改性剂、聚丁二烯为主链,通过热引发和光引发合成了侧基含有羟基的聚丁二烯,以S-1-十二烷基-S′（α-α′-二甲基-α′′乙酸）-三硫代碳酸酯作为可逆加成-断裂链转移自由基聚合试剂、偶氮二异丁腈为引发剂,合成了端羧基聚N,N-二甲基丙烯酰胺,然后将两种官能化的聚合物进行酯化反应制得聚N,N-二甲基丙烯酰胺接枝聚丁二烯。结果表明,采用光引发可以显著提高点击化学的反应效率。用核磁共振氢谱证实了产物含羟基聚丁二烯和端羧基聚N,N-二甲基丙烯酰胺以及接枝聚合物的结构,并显示通过这种方法所制备接枝聚合物的接枝链数目和长度均可控。     

Novel N‐ethyl‐2‐styrylquinolinum iodides as sensitizers in photoinitiated free radical polymerization of trimethylolopropane triacrylate (TMPTA), part 3

Three‐component systems, which contain a light‐absorbing species (dye), an electron donor (n‐butyltriphenylborate salt), and a third component (N‐alkoxypyridinum salt or 1,3,5‐triazine derivative), have emerged as efficient, visible‐light sensitive photoinitiators of free radical polymerization. It was found that three‐component systems are more efficient than their two‐component counterparts. Kinetic studies based on microcalorimetry revealed a significant increase in polymerization rate with increasing concentration of N‐alkoxypyridinum salt. Such results were not obtained for photoinitiating systems possessing 1,3,5‐triazine derivative as a second coinitiator. Based on the experimental results we concluded that the primary photochemical reaction involves electron transfer from the borate anion to the excited dye followed by the reaction of resulting dye‐based radical with second coinitiator that regenerates the original dye and simultaneously produces the alkoxy radical or triazynyl radical which could start the polymerization chain reaction. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Continuous production of solid polystyrene in back‐mixed and linear‐flow reactors

A mathematical model has been developed to predict the steady state performance of a continuous bulk styrene polymerization process with catalytic initiation for solid polystyrene. The polymerization section contains one boiling CSTR, followed by multiple linear‐flow reactors. The devolatilization section consists of two polymer pre‐heaters and two high‐solids flashes. The polymer moment equations were solved simultaneously with the reactor modeling equations. The non‐linear algebraic equations were solved by a Newton‐Raphson iteration technique to give the steady‐state styrene monomer weight fraction in a CSTR. The coupled, non‐linear ordinary differential equations were numerically integrated using a single‐step, 4th‐order Runge‐Kutta technique, followed by a multi‐step Adams‐Moulton technique. The resulting computer simulation model is capable of evaluating how the production rate and product quality are affected by feed composition, temperature, initiator type, initiator concentration, and residence time. Several case studies were given for commercially important crystal‐clear and impact‐resistant resins. A binary initiation system gives a good balance of monomer conversion, polymer molecular weights, and rubber grafting compared to a single initiation system. The styrene dimer/trimer occur in low concentrations but can be substantially reduced with a low temperature initiator. The ideal mean residence time is approximately one minute or less in a shell‐and‐tube devolatilization pre‐heater. Low flash chamber vacuum is more effective than high polystyrene melt temperature to reduce the volatile content of the final product. The water injected to the low volatile melt shows promising improvement in the second‐stage polystyrene devolatilization.     

Electron‐beam curing of bismaleimide‐reactive diluent resins

Electron‐beam (E‐beam) curing of 4,4′‐bismaleimidodiphenylmethane (BMPM)/BMI‐1,3‐tolyl/o,o′‐diallylbisphenol A (DABPA)–based bismaleimide (BMI) systems and their mixing with various reactive diluents, such as N‐vinylpyrrolidone (NVP) and styrene, were investigated to elucidate how temperature, electron‐beam dosage, and diluent concentration affect the cure extent. The effect of free‐radical initiator on the cure reactions was also studied. It was found that low‐intensity E‐beam exposures cannot cause the polymerization of BMI. High‐intensity E‐beam exposures give high reaction conversion attributed to a high temperature increase, which induced thermal curing. It was shown that the dilution and activation of NVP in BMI cause a more complete BMI cure reaction under E‐beam radiation. BMI/NVP can be initiated easily by low‐intensity E‐beam without thermal curing. FTIR studies indicate that about 70% of the reaction is complete for BMI/NVP with 200 kGy dosage exposure at 10 kGy per pass. The sample temperature only reaches about 75°C. The free‐radical initiator, dicumyl peroxide, can accelerate the reaction rate at the beginning of E‐beam exposure, but does not affect the final reaction conversion. The increase of the concentration of NVP in the BMI/NVP systems increases the reactive conversions almost linearly. © 2004 Wiley Periodicals Inc. J Appl Polym Sci 94: 2407‐2416, 2004     

Analytic formulas and numerical simulations for the dynamics of thick and non-uniform polymerization by a UV light

The kinetics of UV light photoinitiated polymerization system (PPS) with non-uniform initiator concentration is theoretically presented. Analytic formulas are derived for the cross linking time (T*), defined by the depletion level of the initiator, which is an exponentially increasing function of the thickness, whereas it is inverse proportional to the UV light intensity. Typical T* (on the surface, at z?=?0) is about 2 min for a UV light intensity of 20 (mW/cm²) and the range of T*(at z?>?0) is about 3 to 5 min, for a polymer thickness of 1.0 cm. Optimal photoinitiation rate is found to be the result of the competing parameter between the UV light intensity and the initiator concentration. The roles of each of the key parameters in PPS are numerically shown including the initiator concentration and its distribution, the UV light intensity and the three absorption coefficients. Our analytic formulas and numerical results provide useful guidance for the parameters selection and optimalization in PPS with the prediction of the cross linking time in various system thickness.     

Crosslinking of mixtures of diglycidylether of bisphenol‐A with 1,6‐dioxaspiro[4.4] nonan‐2,7‐dione initiated by tertiary amines: III. Effect of hydroxyl groups on network formation

BACKGROUND: Blends of epoxy resin oligomers, diglycidylether of bisphenol‐A (DGEBA), and a bislactone, 1,6‐dioxaspiro[4.4]nonan‐2,7‐dione (s(γBL)), were anionically copolymerized using two tertiary amines as anionic initiators. Their curing rheology and gelation behaviour were studied to provide a more comprehensive knowledge of the curing of these previously studied systems. RESULTS: The activation energy for gelation was found to be similar to that previously measured using differential scanning calorimetry and appeared to increase in the presence of the bislactone. The reaction rate during copolymerization of DGEBA with s(γBL) was slower than DGEBA homopolymerization alone because the alkoxide attack on the epoxide is faster than the reaction of the carboxylate ion and the epoxy group. The effect of the initiator type on the gel conversion was small and was presumably due to differences in the kinetic chain length caused by amine regeneration from the quaternary amine. For the same initiator and at a constant ratio of DGEBA/s(γBL), an increase in the hydroxyl concentration of the DGEBA oligomer raised the gel conversion. For a DGEBA oligomer with low hydroxyl levels, an increase in the concentration of s(γBL) increased the gel conversion; however, for a DGEBA oligomer with high hydroxyl levels, increasing s(γBL) concentration decreased the gel conversion. CONCLUSION: These results are interpreted in terms of the effect of initiation rate and chain transfer rate on the kinetic chain length. The glass transition temperature of the gel was found to be controlled by the fraction of the aliphatic s(γBL) and the amount of plasticizing sol in the matrix. Copyright © 2009 Society of Chemical Industry     

Polymerization of methyl acrylate in the presence of ultrasound and peroxodisulfate

A systematic polymerization kinetic study of methyl acrylate with an added initiator, peroxodisulfate, in the presence of low‐power ultrasound was done. The polymerization experiments were conducted at various concentrations of monomer and initiator at different temperatures (303–323 K). The polymerization was found to proceed without an induction period, and the steady state was attained in a fairly short time. The rate of monomer disappearance showed a second‐order dependence on monomer concentration. The chain lengths of the polymer were calculated, and we found that the chain length increased with increasing monomer concentration and decreased with increasing initiator concentration. The reaction scheme proposed is based on the kinetic studies that indicated linear termination by sulfate‐ion‐radical‐incorporating direct reaction between the monomer and the initiator. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Development of a comprehensive free radical photopolymerization model incorporating heat and mass transfer effects in thick films

A comprehensive kinetic model describing photopolymerization is developed which allows variation of temperature, species concentrations, and light intensity through the thickness of a photopolymerized film. Heat and mass transfer effects are included, as is the generation of heat by both reaction and light absorption. In addition to initiation, propagation, and termination mechanisms, both primary radical termination and inhibition are incorporated into the model. The possible presence and diffusion of an inert solvent are also accounted for. Thus, the model is useful for examining complex polymerization kinetics and behavior in industrially and commercially important thick film photopolymerizations, such as the curing of contact lenses, dental restorative materials, photolithographic resists, and optoelectronic coatings. The comprehensive model is used to predict polymerization rate, temperature, and conversion profiles in a variety of systems. The effects of heat generation and the thermal boundary conditions are explored, with the result that heat generation in thick samples leads to greatly increased conversions approaching 100 percent. Increased temperature in these samples also may lead to the appearance of two rate maxima, with the first due to the temperature increase and the second caused by the autoacceleration process. The magnitude of the temperature increase, along with the resultant effects, is more pronounced in insulated systems.     

Effect of water concentration on photopolymerized acrylate/epoxide hybrid polymer coatings as demonstrated by Raman spectroscopy

Photopolymerization systems based on hybrid monomer 3,4-epoxy-cyclohexylmethyl methacrylate (METHB) were studied to investigate water effects on conversion and polymer coating properties. METHB contains epoxide and methacrylate moieties, which undergo cationic and free-radical photopolymerization, respectively. The conversion of both groups was obtained by Raman spectroscopy in real time and depth. Water concentration and initiator system compositions were varied and shown to affect reaction kinetics and depth profile. With increasing water concentration, the epoxide induction period increased when only cationic initiator was present; however, the induction period disappeared when using the dual-initiator system. In addition, epoxide groups continued reacting after light was shuttered and reached a higher and more homogeneous conversion. Hybrid systems were shown to be less sensitive to water at low or intermediate concentrations. With high water concentrations, hybrid monomer systems manifested increased ring opening during illumination due to chain transfer and decreased physical properties due to loss of cross-linking.     

Photoinitiated polymerization of 1-vinylimidazole

The polymerization of 1-vinylimidazole differs from that of 1-vinyl-2-methylimidazole in that degradative chain addition to the monomer is an important factor, as previously indicated by Bamford and Schofield. The consequence of this is a dependence of rate on monomer concentration, which starts with the first power but levels off to a constant rate or even decreases slightly as the initial monomer concentration rises to 1–4 mol/L. Thus, the mechanism proposed by Bamford and Schofield appears to be confirmed. Initiator dependence of rate is best correlated by using the light absorbed by the initiator rather than the initiator concentration as the independent variable. The 0.72 power dependence found may be higher than the expected 0.5 power because the rates were measured at a monomer concentration (0.25 mol/L) just beyond the linear region. Low values of activation energy are expected, since photoinitiation rate is independent of temperature. The measured value reflects only the propagation and termination steps. Even then, the value found of 1.5 kcal/mol is low compared to the value of 3.9 kcal/mol for 1-vinyl-2-methylimidazole.     

Experimental validation of the optimal trajectory of initiator concentration in a batch MMA polymerization reactor

This article presents a method to determine the trajectory of initiator concentration that will produce polymer with desired number‐ and weight‐average molecular weights at a prespecified level of monomer conversion. The optimal control theory is applied to the mathematical model for a batch methymethacrylate (MMA) solution polymerization reactor system. By imposing the constraint that initiator concentration should decrease within the range of self‐consumption by the initiation reaction, one can obtain the initiator concentration trajectory that can be tracked by feeding the initiator alone. A control scheme is constructed with a cascade proportional‐integral‐derivative (PID) control algorithm for temperature control and a micropump is installed to manipulate the initiator feed rate. The experimental results show satisfactory tracking control performance despite the nonlinear features of the polymerization reactor system. Also, the monomer conversion and the average molecular weights measured are found to be in fairly good agreement with those of model prediction, respectively. In conclusion, the polymer having desired molecular weight distribution can be produced by operating the batch reactor with the initiator supplement policy calculated from the model. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1256–1266, 2000     

Synthesis and properties of natural rubber modified with stearyl methacrylate and divinylbenzene by graft polymerization

The graft polymerizations of stearyl methacrylate (SMA) and divinylbenzene (DVB) onto natural rubber (NR) were carried out in a solution process using benzoyl peroxide (BPO) as an initiator in toluene or chloroform. The main products of the grafted NR include an uncrosslinked (sol) part [sol(SMA–NR–DVB): s‐SNRD] and a crosslinked (gel) part [gel(SMA–NR–DVB), g‐SNRD]. s‐SNRD was obtained by extraction using tetrahydrofuran. It was identified by IR and ¹H‐NMR spectroscopies. The glass transition temperature (T_g) and thermal properties of s‐SNRD and g‐SNRD were studied by DSC and TGA. The glass transition temperature and thermal decomposition temperature of s‐SNRD and g‐SNRD were higher than were those of NR. The light resistance and weatherability of s‐SNRD were measured with a Weather‐o‐Meter. The light resistance and weatherability of s‐SNRD are better than are those of NR. The effects of the initiator concentration, mol ratio of SMA to DVB, reaction time, temperature on grafting ratio, and crosslinking ratio were investigated. The highest grafting ratio and crosslinking ratio in the graft polymerization of SMA and DVB onto NR were obtained when the mol ratio of SMA to DVB and BPO were 4.0 and 2 wt %, at 80°C for 48 h, respectively. Following several studies on oil‐absorptive polymers in our laboratory, 9 the oil absorptivity of g‐SNRD was examined using crude oil. The oil absorptivity of g‐SNRD was 600% when the immersion time was 10 min. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2464–2470, 2001     

Decomposition of peroxide on carboxymethyl cellulose and its ability to initiate graft copolymerization

The decomposition of the peroxide group on fibrous carboxymethyl cellulose (CMC) and its ability to initiate graft copolymerization were investigated. The peroxide on CMC liberated hydrogen peroxide when the sample was heated in an aqueous medium. The decomposition of the peroxide was markedly increased by the use of ferrous salt and the irradiation with light of λ > 300 nm. The grafting of methyl methacrylate on CMC peroxide was initiated by heating or irradiating with light, where the rate of grafting and the reciprocal of the average molecular weight of grafts in general were proportional to the square root of the peroxide content of the sample. The peroxide content of the sample was related closely to the number of grafts in initiation. The number was estimated as 17 times for thermal initiation and 6 times for photoinitiation at the maximum values. The CMC peroxide showed a marked activity toward photografting of acrylamide, acrylic acid, acrylonitrile, and vinyl acetate on the substrate.