Temperature polymerisation processes

Polymer Production. Three processes are used to produce nylon-6,6. Two of these start with nylon-6,6 salt, a combination of adipic acid and hexamethylenediamine in water they are the batch or autoclave process and the continuous polymerisation process. The third, the soHd-phase polymerisation process, starts with low molecular weight pellets usually made via the autoclave process, and continues to build the molecular weight of the polymer in a heated inert gas, the temperature of which never reaches the melting point of the polymer. 

High molecular weight polymers are produced by an adiabatic bulk polymerisation process ° using di-tert-butyl peroxide (0.02%) and 2,2 -azo-bisdi-isobutyronitrile (0.01%) as initiators and pressurised with N2. Heating to 80-90°C causes an onset of polymerisation and a rapid increase in temperature. After the maximum temperature has been reached the mass is allowed to cool under pressure. A typical current commercial material (Luvican M.170) has a A -value of about 70 (as assessed in a 1% tetrahydrofuran solution). 

It is commonly found that polymers are less stable particularly to molecular breakdown at elevated temperatures than low molecular weight materials containing similar groupings. In part this may be due to the constant repetition of groups along a chain as discussed above, but more frequently it is due to the presence of weak links along the chain. These may be at the end of the chain (terminal) arising from specific mechanisms of chain initiation and/or termination, or non-terminal and due to such factors as impurities which becomes built into the chain, a momentary aberration in the modus operandi of the polymerisation process, or perhaps, to branch points. 

Dartnell, R. C. et al., Loss Prev., 1971, 5, 53-56 MCA Case History No. 1649 A batch of 8 t of material accumulated in storage at 154°C during 72 h decomposed explosively. Stability tests showed that thermal instability developed when 3-methyl-4-nitrophenol is stored molten at temperatures above 140°C. Decomposition set in after 14 h at 185° or 45 h at 165°, with peak temperatures of 593 and 521°C, respectively. In a closed vessel, a peak pressure of 750 bar was attained, with a maximum rate of increase of 40 kbar/s. Thermal degradation involves an initially slow exothermic free radical polymerisation process, followed by a rapid and violently exothermic decomposition at take-off. 

The above picture points to the very interesting possibility of selectively inducing or enhancing the polymerisation process, at a temperature where this is unlikely, by resonantly driving with an intense laser beam in the infrared the vibrational modes and wc that are involved in the polymerisation. As a consequence of their anharmonicity (45) these modes, when driven near resonance by an electromagnetic field, beyond a certain critical value of the later, can reach amplitudes comparable to the critical ones required for the polymerisation to be initiated or proceed the anharmonicity in the presence of the intense laser beam acts as a defect and localizes the phonons creating thus a critical distorsion. 

A temperature-controlled polymerisation process is estimated to have a transfer function of ... 

Solid phase polymerisation is used for chain polymerisation processes which are carried out at low temperatures. In such processes the thermal activation is difficult and so for activation of such processes radiation-activation technique is used. These processes are very slow. An example of such a solid phase polymerisation is the preparation of Polyformaldehyde by the radiation polymerisation of solid trioxane. 

In a first full scale attempt at a new polymerisation process, the thermally unstable initiator was charged and heated to reaction temperature, but there was then an unforeseen delay of an horn before monomer addition was started. The rate of polymerisation effected by the depleted initiator was lower than the addition rate of the monomer, and the concentration of the latter reached a level at which an uncontrollable polymerisation set in which eventually led to pressure-failure of the vessel seals. Precautions to prevent such occurrences are detailed. In another incident, operator error led to catalyst, condensing styrene and acrylonitrile being ducted into an unstirred weighing tank instead of a reactor. When the error was recognised, the reacting mixture was dropped into drums containing inhibitor. One of the sealed drums had insufficient inhibitor to stop the reaction, and it slowly heated and eventually burst [1], The features and use of... 

Until the work by Kruus [76], few workers had systematically investigated the effects of varying such parameters as intensity, frequency, temperature and the nature of the gas on the polymerisation process. 

The in situ polymerisation consists of filling a capillary or a column with the prepolymerisation mixture containing the template, the functional monomer, the crosslinker, the initiator and the porogenic solvent (Fig. 11). Then the column is heated or submitted to UV radiation for polymerisation. In the in situ thermally initiated polymerisation process, the tube with the pre-polymerisation mixture is submerged in a controlled-temperature water bath, whereas for in situ photoinitiated polymerisation, a UV-transparent capillary or column is needed. The resulting continuous rod of polymer is washed with an appropriate solvent to remove the template and the excess of monomer. 

U.S. Chemical Safety and Hazard Investigation Board, 2001, Report 2001-03-I-GA In a first full scale attempt at a new polymerisation process, the thermally unstable initiator was charged and heated to reaction temperature, but there was then an... 

In the next step, an excess of cross-linking monomer (e.g. trimethylol-propane trimethacrylate or ethylene glycol dimethacrylate) is added together with an initiator (e.g. 2,2 -azoisobutyronitrile), which induces the polymerisation process. Under nitrogen and high temperature, the polymerisation process results in the formation of a rigid mass of polymer. 

Although the polymerisation rate increases with increasing temperature, ethylene and 7-olefin polymerisations in the presence of most Ziegler-Natta catalysts are carried out at moderately elevated temperature, usually not exceeding 100 °C. This is due to destabilisation of the system, which occurs when temperature is raised beyond a certain critical value. There are, however, few catalysts that operate in industrial polymerisation processes at temperatures above 200 °C [51,240]. 

Figure 3.57 Flow scheme of polypropylene production using the high-temperature solution polymerisation process...

Industrial polymerisation processes with the use of titanium-, cobalt- and nickel-based aluminium alkyl-activated Ziegler-Natta catalysts, which are employed for the manufacture of cis- 1,4-poly butadiene, involve a solution polymerisation in low-boiling aromatic hydrocarbons such as toluene or in a mixture of aromatic and aliphatic hydrocarbons such as n-heptane or cyclohexane. The polymerisation is carried out in an anhydrous hydrocarbon solvent system. The proper ratio of butadiene monomer and solvent is blended and then completely dried in the tower, followed by molecular sieves. The alkyla-luminium activator is added, the mixture is agitated and then the transition metal precatalyst is introduced. This blend then passes through a series of reactors in a cascade system in which highly exothermic polymerisation occurs. Therefore, the reaction vessels are cooled to slightly below room temperature. 

The development of Nd-based catalysts has permitted the industrial realisation of a polymerisation process for cis- 1,4-polybutadiene in which temperature varies in the range 20-50 °C. In such a process, the polymerisation proceeds relatively fast the average residence time of the monomer varies in the range 0.5-4 h to achieve 90% butadiene conversion. 

In SAS, a compressed gas is added to a polymer solution. The upper critical solution temperature (UCST) and lower critical solution temperature (LCST) of that solution are shifted to higher and lower temperatures respectively until they finally merge to one region of immiscibilty over the whole temperature range. This process can be used for solvent recovery in solution polymerisation processes as well as for molecular weight fractionation of polymers. 

The low temperature polymerisation of isobutene by SnCl4 in ethyl chloride is one of the classical studies of the golden era of cationic polymerisation. Norrish and Russell " found that with no added water an extremely slow reaction period was followed by a sudden acceleration. A similar phenomentm was later reported by Polton and Sigwalt for the polymerisation of indene in a dry system. It seems reasonable to suppose that the slow initial process reflects direct initiation in both systems, and that the sudden accelemtion arises from the internal production of a cocatalyst, probably hydrogen chloride formed from the dehydrochlorination of active species. 

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