The present invention relates to a continuous polymerization method of laurolactam (dodecanoic lactam) and the apparatus therefor. More specifically, the present invention relates to a continuous polymerization method of laurolactam (dodecanoic lactam), comprising continuously polymerizing laurolactam as a raw material monomer by using water as a catalyst, thereby producing a desired high quality Nylon 12 with a wide range of molecular weight, and the apparatus therefor.
Nylon 12 has conventionally been produced by heating and polymerizing laurolactam (dodecanoic lactam) or 12-aminododecanoic acid. By the method using 12-aminododecanoic acid as a raw material monomer, 12-aminododecanoic acid is continuously heated and melted under atmospheric pressure for polycondensation. Alternatively, the ring opening reaction of laurolactam progresses very slowly, which rate is 1/10 or less that of the rate of hydrolysis and polymerization of relatively easily ring opening caprolactam under atmospheric pressure as a raw material of Nylon 6. Therefore, the process using laurolactam as a raw material monomer generally comprises effecting a prepolymerization process wherein laurolactam is firstly heated along with water or an acid catalyst or the like under pressure, thereby promoting the ring opening reaction following a high-temperature/high-pressure system, rendering the pressure to atmospheric pressure, and polycondensing the resulting product under atmospheric pressure or reduced pressure to prepare a high molecular weight Nylon 12.
If it is intended to produce Nylon 12 by continuous polymerization using laurolactam as a raw material monomer, a large-scale, complex apparatus for high temperatures and high pressures is therefore inevitable due to the prepolymerization process, causing problems such as high equipment cost. Thus, a batch polymerization system (discontinuous two-step polymerization system) has conventionally been employed primarily for industrial polymerization of laurolactam.
However, such a batch system has the following drawbacks; the operational procedure is complex with a low productivity; when drawing out a generated polymer after the termination of the post polymerization process, the adhesion of the generated polymer onto the polymerization vessel and the retention thereof in the vessel is unavoidable; if the prepolymerization and post polymerization are repeated in the same container, the residual polymer is deteriorated through thermal hysteresis, thereby reducing the quality of the product polymer. In order to prevent these drawbacks, the vessel should necessarily be washed periodically, resulting in a far lower productivity.
In producing Nylon 12 having a high polymerization degree from laurolactam as the raw material monomer, the most serious problem in either one of the continuous polymerization or the batch polymerization is a laurolactam polymerization-conversion rate at the prepolymerization process under pressure. For the ring opening polymerization of laurolactam, the conversion rate obtained by the prepolymerization process influences the final polymerization product. While the prepolymerization process is conducted under pressure, the procedures in the prepolymerization process thereafter are conducted under atmospheric pressure or reduced pressure, and because it cannot be expected to increase the conversion rate at the prepolymerization process by a much greater degree though it is sometimes possible that the rate slightly increase. If the prepolymerization process is insufficient, disadvantages may develop such as the occurrence of contamination of a nozzle or a mold during the molding process, so it is important for laurolactam polymerization that the polymerization-conversion rate (ring opening reaction) at the prepolymerization process under pressure should be facilitated to a nearly equilibrium value (about 99.7%).
In order to promote the conversion rate at the prepolymerization process up to at the equilibrium value in such case, water conventionally employed as a single catalyst is generally added to laurolactam at 1 to 10% by weight under a pressure of about 10 to 50 kg/cm.sup.2 G and a higher temperature above 300.degree. C. But at a higher temperature above 300.degree. C., thermal deterioration, namely, partial gelation or discoloring or the like, is likely to occur due to the occurrence of undesirable secondary reactions, so that a high-quality polymer cannot be obtained. At a temperature below 300.degree. C., alternatively, a prepolymerization time period of at least 10 hours or more is required, resulting also in the deterioration of the quality of a polymer due to the factor of thermal deterioration via a prolonged polymerization at a high temperature and a high pressure.
If the water addition rate is less than 1% by weight, the ring opening polymerization of laurolactam is extremely difficult to occur, whereby the polymerization addition rate is lowered. On the other hand, if above 10% by weight, the dehydration from a reaction product occurs involving bubble, so that the plug flowability of a reaction solution inside a reaction column is lowered, resulting in the variation of the conversion rate, unfavorably.
Furthermore, when intending a high conversion rate by a prepolymerization at a temperature below 300.degree. C. and for 10 hours or less, acid or alkali catalysts are required besides water. When these catalysts are used, the molecular weight is hard to adjust at post polymerization, and hence, it is difficult to obtain a Nylon 12 with a desired molecular weight. Also, the viscosity change and the like at the molding process (at remelting) are remarked as problems, so that the industrial practice thereof is difficult.
As has been described above, the method comprising continuously polymerizing laurolactam employing water as a catalyst thereby industrially producing a high-quality Nylon 12 with a predetermined molecular weight has a variety of problems, so further improvement thereof has been desired.
It is the object of the present invention to overcome the problems in polymerizing laurolactam as a raw material monomer, and to provide a method for continuous polymerization of laurolactam, comprising industrially producing a high-quality Nylon 12 with a desirable molecular weight at a lower polymerization temperature and for an extremely short period of time, and the apparatus therefor.