Polymerization of melamine and formaldehyde in homogeneous continuous-flow stirred-tank reactors using functional group approach: Part B: Molecular weight distribution

Polymerization of melamine and formaldehyde in homogeneous continuous-flow stirred-tank reactors (HCSTRs) is reversible and leads to formation of branched polymer due to the hexafunctional nature of melamine. The reversed reaction of branched molecules depends upon the chain structure, and herein a simple model is presented to account for this. The functional group analysis of part A of this series has been extended and the molecular weight distribution (MWD) relations for HCSTRs have been solved. The MWD of the polymer is extremely sharp and the molecules are more branched compared to those for batch reactors and can be explained as follows. A given reacted bond can react with water as well as free formaldehyde through the reversed reaction, and the rate constant for the latter step is much larger. Consequently, the chain growth is limited as long as there is free formaldehyde in the reaction mass. Lower conversion of melamine and formaldehyde in HCSTRs is used to explain the behavior observed.     

Modeling of polymerization of melamine and formaldehyde using a functional group approach

A functional group approach that accounts for the formation of higher oligomers was used in modeling the batch polymerization of melamine and formaldehyde. Five rate constants are required instead of 24 as needed in earlier studies to fit experimental data in the entire range. The model presented allows the formation of higher oligomers even though their formation is small. The batch polymerization of melamine and formaldehyde was studied under a wide variation of reaction parameters to find conditions that would lead to formation of higher oligomers preferentially.     

Computational scheme for the calculation of molecular weight distributions for nylon 6 polymerization in homogeneous,continuous-flow stirred-tank reactors with continuous removal of water

An efficient computational scheme has been established for obtaining the molecular-weight distributions (MWDs) for Nylon 6 polymerization in homogeneous, continuous-flow stirred-tank reactors (HCSTRs) with water vaporization. A three-step procedure is used: the moment equations are solved by a sequential application of Brown's algorithm, the moments so calculated are used to give an empirical Flory-Schultz distribution and, finally, this empirical distribution is used as a starting guess in Brown's algorithm applied to species mass-balance equations to give the exact MWD. The moment-closure approximations normally used for hutch reactors have been found to work equally well with HCSTRs for residence times of industrial significance.     

An efficient algorithm of computation of molecular weight distributions and its moments for reversible step growth polymerization in homogeneous continuous flow stirred tank reactors

The reversible step growth polymerization in homogeneous continuous flow stirred tank reactors (HCSTRs), in which the condensation product (W) leaves the reactor through flashing, has been analyzed. The molecular weight distribution (MWD) of the polymer formed is governed by nonlinear coupled algebraic relations to be solved simultaneously. To find the MWD numerically a large number of these are normally solved simultaneously using a suitable iterative procedure. In this paper, these have been decoupled using the technique proposed in our earlier works (1, 2) and the MWD can now be obtained sequentially without any trial and error. This leads to considerable saving in computation time compared to methods currently used. To demonstrate the efficacy of the algorithm, the polycondensation step of the poly(ethylene terephthal-ate) (PET) formed in HCSTRs has been analyzed. The MWD, the average chain length and the polydispersity index of the polymer have been computed and it takes 0.1 CPU seconds on a DEC 1090 as opposed to the earlier method which would take seventy minutes for similar computations. The simple model of the HCSTR for the PET formation gives the effect of reactor temperature and pressure and the quantitative results have been presented in this paper.     

Modeling of melamine formaldehyde polymerization. II. Development of a simpler model

The kinetic modeling of melamine–formaldehyde polymerization presents a relatively formidable mathematical challenge because of the simultaneous presence of several types of deviations from Flory's equal reactivity hypothesis. A molecule of melamine has six reactive amide hydrogens, which can react with the ? CH₂OH groups of formaldehyde in solution. The reactivity of the secondary hydrogens on the melamine is about 61% of that of the primary hydrogens (induced asymmetry). There is a shielding effect present, i. e., the reactivity of the hydrogens near the outside of a multiringed polymer molecule is higher than that of the hydrogens inside the coiled molecules. Two bound ? CH₂OH groups on the polymer molecules can self-condense to give methylene linkages, the reactivity depending upon the location of the two groups. And, to confound modeling effort still further, all these reactions are reversible. An earlier attempt at molding this system considered the reactions between 36 “basic” species. This model was far too detailed and failed to acount for the reverse reactions. In the present study, a simpler model has been proposed which involves fewer “basic” species. An improved model for intramolecular reactions is also developed. Several important characteristics of the polymerization have been obtained as a function of time. Results from this model have been compared with those obtained from the earlier model, and also compared with the short-time experimental results. The present model can be extended to account for the reverse reactions quite easily.     

Modelling of condensation polymerization of novolac-type phenol–formaldehyde in homogeneous,continuous-flow,stirred-tank reactors

A detailed kinetic model for the novolac-type phenol–formaldehyde polymerization has been proposed in terms of five reactive sites. These sites have been shown to have different reactivities. Mass-balance equations for homogeneous, continuous-flow, stirred tank reactors (HCSTR) for each of the species have been written, and these have been solved numerically. A comparison of the results with those for the batch reactor shows that, for a given residence time, smaller but more polydispersed and branched chains are formed.     

Kinetics of the addition stage in the melamine–formaldehyde reaction

The addition reaction between melamine and formaldehyde has been kinetically separated from the subsequent condensation stage by suitable choice of concentration and temperature conditions. The reaction, which is reversible, has been monitored by estimation of the free formaldehyde content of the system. It has been investigated over the range of mean degree of methylolation 1 < R′ < 3.7 of the melamine nuclei, the temperature range 25–55°C., and the pH range 5.7–10.2. The rate data thus obtained have been treated according to the random reversible addition scheme for which reasonable, first approximation, agreement was obtained. Average kinetic and thermodynamic constants have been calculated and are discussed in terms of the present model. The factors which are likely to cause deviations from randomness are described. The addition of formaldehyde to melamine proceeds by superposition of an OH^?-catalyzed step with a minor solvent-catalyzed or uncatalyzed one.     

Modelling of reversible novolac type phenol-formaldehyde polymerization

The reversible polymerization of acid-catalysed phenol and formaldehyde has been modelled through the use of five species A to E. There are two possible mechanisms for the reverse reaction of the polymer formation based upon which two possible kinetic models (1 and 2) have been proposed. These two models arise because during the reverse reaction between these species and water, two possible routes can occur. It has been shown analytically that both these kinetic models would yield identical concentrations of A to E in the reaction mass; only an uncertainity in predicting concentrations of formaldehyde and bound CH₂OH exists. Thus any of these models may be equivalently chosen if the purpose of the simulation is to analyse the product formed. Analysis of the batch reactor under the variation of the ratio of phenol and formaldehyde, [P]₀/[F]₀, in the feed shows that the maximum branching in the polymer occurs at a ratio of 0.5 whereas a ratio 1.60 is found to maximize the formation of linear chains. The application of a vacuum to the reaction mass has been modelled to give a constant but lower water concentration. The effect of the vacuum has been examined and is found to give a longer but more branched polymer compared with that for batch reactors with no vacuum.     

Large scale preparation of melamine-based superplasticizing admixtures. I. Process optimization

The process for preparing novel sulfonated melamine–formaldehyde resins was studied in detail with the objective of optimizing the production cycle time. The effects of minimizing temperature differences between consecutive reaction steps and reducing reaction time of selected steps of the process were assessed. Experimental work focused on increasing the polymerization step temperature and decreasing its time, resulting in considerable savings in the process time and energy consumption. The resin of the optimized cycle was found to have excellent superplasticizing properties when added to concrete mixes.     

An analytical solution of the molecular weight distribution of reversible step growth polymerization in homogeneous continuous flow stirred tank reactors

Mole balance for the molecular weight distribution in homogeneous continuos-flow stirred tank reactors (HCSTRs) for reversible step-growth polymerization has been written. The relation for the moment generating function G is found to be a nonlinear ordinary differential equation and has been solved analytically. The solution of the MWD of the polymer formed is shown to be valid even if the condensation product is removed. At equilibrium, the solution reduces to the Flory distribution. The computations show that the polydispersity of the polymer first increases with the residence time θ of the reactor, but, for large θ, it reduces to the equilibrium value after undergoing a maximum.     

Effect of intramolecular reactions in multifunctional step growth polymerizations in cascades of continuous-flow,stirred-tank reactors

A simple kinetic model describing intramolecular reactions in multifunctional step growth polymerizations of RA_j monomers is presented. The model is far simpler than more detailed kinetic or Monte Carlo models which pose severe numerical difficulties close to gelation. The formulation presented is quite general, but it requires a relationship for the ensemble-cum-time average number of rings, r _n, per molecule having n RA_j units. A very simple yet intuitively correct equation has been chosen for r _n, which involves an empirical parameter ?? Results on cascades of homogeneous continuous-flow stirred-tank reactors (HCSTRs) show trends which are physically expected. The present formulation could possibly be used for scale-up of HCSTRs, with ?? obtained experimentally. It is hoped that future studies using rigorous models would lead to more fundamental and improved relationships for r _n.     

Molecular weight distribution of polyethylene terephthalate in homogeneous,continuous-flow-stirred tank reactors

Poly(ethylene terephthalate) (PET) formation in homogeneous, continuous-flow-stirred tank reactors (HCSTRs) operating at steady state has been simulated. The feed to the reactor is assumed to consist of the monomer bis-(hydroxyethyl) terephthalate and monofunctional compound (MF₁) cetyl alcohol. The overall polymerization is assumed to consist of the polycondensation, reaction with monofunctional compounds, redistribution, and cyclization reactions. At a given time, the reaction mass consists of polyester molecules (P_n), polyester molecules with an ending of molecules of monofunctional compound (MF_n), and cyclic polymers (C_n). A mass balance for each of these species in the reactor gives rise to a set of algebraic equations to be solved simultaneously. The MWD calculations show that the redistribution reaction plays a major role and cannot be ignored, This result is in contrast lo the observation for semi-batch reactors, for which redistribution becomes important when the cyclization reaction is included. For the same residence times of semi-batch and HCSTRs, the latter gives considerably lower-number average molecular weight, N_av, and polydispersity index, ρ. However, for the same conversions, the ρ for CSTR is higher. The concentration of the monofurctional compound, [MF₁]₀, in the feed and the reactor temperature both influence ρ, but the effect is small within the range studied.     

Dynamic optimization of the methylmethacrylate cell‐cast process for plastic sheet production

Traditionally, the methylmethacrylate (MMA) polymerization reaction process for plastic sheet production has been carried out using warming baths. However, it has been observed that the manufactured polymer tends to feature poor homogeneity characteristics measured in terms of properties like molecular weight distribution. Nonhomogeneous polymer properties should be avoided because they give rise to a product with undesired wide quality characteristics. To improve homogeneity properties force‐circulated warm air reactors have been proposed, such reactors are normally operated under isothermal air temperature conditions. However, we demonstrate that dynamic optimal warming temperature profiles lead to a polymer sheet with better homogeneity characteristics, especially when compared against simple isothermal operating policies. In this work, the dynamic optimization of a heating and polymerization reaction process for plastic sheet production in a force‐circulated warm air reactor is addressed. The optimization formulation is based on the dynamic representation of the two‐directional heating and reaction process taking place within the system, and includes kinetic equations for the bulk free radical polymerization reactions of MMA. The mathematical model is cast as a time dependent partial differential equation (PDE) system, the optimal heating profile calculation turns out to be a dynamic optimization problem embedded in a distributed parameter system. A simultaneous optimization approach is selected to solve the dynamic optimization problem. Trough full discretization of all decision variables, a nonlinear programming (NLP) model is obtained and solved by using the IPOPT optimization solver. The results are presented about the dynamic optimization for two plastic sheets of different thickness and compared them against simple operating policies. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

苯乙烯连续搅拌釜式本体聚合工业过程模拟

采用了机理分析建模法对苯乙烯工业本体预聚过程进行了模型化研究，并通过对流体在反应器中混合模式的分析，表明预聚釜可视作全混流（CSRT）反应器模型，此外对热聚合机理中的引发级数和终止反应进行了考察，利用预聚釜的工业数据确立了三分子引发，同时考虑偶合和歧化终止以及向溶剂链转移的反应机理模型，并选取了合适的动力学参数，然后利用工业装置采集的数据进行了模拟，从宏观转化率，平均分子量和微观分子量分布两个方面对模型进行了验证，模拟结果和工业数据符合得较好，该模型方程不仅能模拟稳态，非稳态预聚过程，同时也为装置开发新产品，改造扩建，优化操作方案等提供了理论基础。     

三聚氰胺改性木质素的制备及银离子吸附性能

以酶解木质素、三聚氰胺和甲醛为原料,通过接枝反应制备三聚氰胺改性酶解木质素(MEHL),并研究MEHL对银离子的吸附性能。接枝反应的研究结果表明,三聚氰胺和甲醛的添加量对产率有很大影响。酶解木质素、三聚氰胺、甲醛质量比为10:6:6,于90℃下反应4 h,所制得的MEHL的产率可达83.7%。红外光谱、热重和元素分析的结果表明,三聚氰胺成功接枝到酶解木质素分子上。MEHL的含氮量由酶解木质素的0.81%提高到18.55%。银离子的吸附研究表明,MEHL对银离子表现出很好的吸附性能。在银离子初始浓度为0.16 mol·L^-1,在MEHL用量为50 mg,吸附温度为30℃时吸附36 h,MEHL对银离子的吸附容量为944.1 mg·g^-1。     

Optimization of the polycondensation stage of poly(ethylene terephthalate) reactors

A general kinetic scheme for the polycondensation step of the PET formation has been used to establish the mole balance equations of various functional groups in batch reactors. An objective function has been defined which aims to attain a desired degree of polymerization in the shortest time, has a specified level of diethylene glycol group content, and minimizes the other side products. Using the control vector iteration procedure, an optimum temperature profile has been calculated. The computations suggest that a high temperature should be used initially which must be lowered as the time of reaction increases for limiting the formation of side products.     

Computer-simulation der polykondensation von phenol mit formaldehyd

The polycondensation of phenol with formaldehyde to resole type phenolic resins has been studied using a kinetic model suitable for computer simulation. Important properties like residual amounts of monomers, number average degree of polymerization as well as degrees of branching and methylolation are calculated as functions of time for different values of initial molar ratios of phenol to formaldehyde and various rate constants. In addition the influence of intramolecular curing, due to formation of loops has been discussed in detail.     

Improvement of storage stability and physicochemical properties by addition of benzoguanamine in melamine‐formaldehyde resin synthesis

The improvement of melamine‐formaldehyde resin storage stability was achieved using benzoguanamine as partial replacement of melamine during synthesis (up to 15% substitution of melamine). The results showed that when benzoguanamine is added in the later stages of reaction, the resulting resins have improved storage stability (4 days or more, compared to 1 day in unmodified melamine‐formaldehyde resin). High‐pressure laminates produced with décor paper impregnated with the new developed resins showed surface properties equivalent to those obtained using commercial melamine‐formaldehyde resins. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45185.     

N,N-bis (2-hydroxyethyl) fatty amide as novel environmentally friendly crosslinking agent for acid functional acrylic copolymer resin

N,N-bis (2-hydroxyethyl) fatty amide (HAFA) derived from dehydrated castor oil (DCO) has been employed as a curing agent for a carboxylic acid functional acrylic resin (AAR). Coating compositions with a varying AAR/HAFA ratio have been prepared and their various film properties have been studied and compared with the conventional butylated melamine formaldehyde (MF) based systems. The study reveals that HAFA can be used successfully as a crosslinking agent for AAR in baking systems. In addition to other technical advantages, the use of HAFA is environmentally friendly as no toxic volatiles are evolved during curing reaction, unlike in MF-cured systems. Department of Industrial Chemistry, Vallabh Vidyanagar, Gujarat 388 120 India. Hyderabad, India.