Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2001-09-28
2003-05-06
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S170000
Reexamination Certificate
active
06559273
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing polyamide, and more particularly to a process for producing polyamide in which industrial wastes resulting from the production process of polyamide (hereinafter occasionally referred to merely as “recovered polyamide”) such as shaped articles rejected for imperfection, scraps from molding or fabricating process, and products discarded because of lack of demand, are recycled to melt polymerization process and reused as a part of raw materials.
2. Brief Description of the Prior Art
Polyamides have been extensively used in various applications such as injection-molding materials for electrical appliances, automobile parts, and computer housings; industrial or leisure filament materials for yarns, clothing such as knitted or woven fabrics, tire cords, fishing nets and fishing lines; food-packaging films; sheets for various containers; and materials for bottles, because of excellent chemical and mechanical properties such as high tenacity, high wear resistance, high fatigue resistance, good dye affinity and high gas-barrier properties. In particular, polyamides, such as polyamide MXD6, having repeating amide linkages formed from xylylenediamine and aliphatic dicarboxylic acid exhibit a high tenacity, a high Young's modulus and a low water absorption as well as excellent gas-barrier properties as compared to polyamide 6, polyamide 66, etc. Therefore, the polyamides of this type are particularly useful as various industrial materials and materials for food-packaging films, sheets and bottles.
Conventionally, polyamides have been produced from diamine and dicarboxylic acid by a so-called pressure process in which an aqueous solution of a diamine-dicarboxylic acid equimolar salt is heated under pressure, or a so-called atmospheric process in which diamine is added dropwise into molten dicarboxylic acid under a temperature rise condition. The atmospheric process is useful when the melting points of the dicarboxylic acid and the polyamide being produced are lower than the boiling point of the diamine, as in the case of producing polyamide MXD6 from m-xylylenediamine and adipic acid. The atmospheric process is superior to the pressure process because of a short reaction time, a large yield per one batch, and simplified steps and apparatuses.
In general, after completion of the polycondensation, the resultant polyamide is extruded into so-called strands through a plurality of orifices by pressurizing an interior of polymerization vessel with inert gas in a batch process, or by applying a mechanical force from a screw, etc. in a continuous process. Then, the extruded strands are cooled, solidified, and then cut into pellets. During the extrusion, if the flow of strands are disturbed by an unexpected change in the pressure condition within the polymerization vessel or in the state of the molten resin, and strands are pelletized together while contacting each other, odd-shaped pellets, e.g., chain-bonded pellets, will be produced. Since such odd-shaped pellets inhibit smooth feeding and cause malfunction of machines during injection molding process and extruding process, the odd-shaped pellets are usually removed by screening.
Since used in various applications as described above, polyamides are required to have properties, such as melt viscosity, which are suitable for a molding method according to their applications. For example, polyamides for injection molding are required to have a low melt viscosity to enhance the fluidity in a mold. Polyamides for extrusion are required to have a high melt viscosity because an adequate melt strength is required during the extrusion process. When a plurality of resins are used in combination to produce polymer alloys, polymer blends or multi-layer sheets or films, the respective resins are occasionally required to have different melt viscosities depending upon their compatibility, mixing ratios or amounts used. For this reason, material makers must usually supply several grade of polymers having different melt viscosities to meet the market's demand. However, some grades become in surplus because of change in the market's demand and demanded amounts.
Hitherto, polyamides recovered as the rejected products, surplus products and scraps have been merely discarded as industrial wastes, making the production process less profitable because of additional costs required for the disposal and transportation. In addition, it is required to minimize the industrial wastes in view of recently increasing demand for protecting the environment. Further, for the purpose of increasing the yield, the recycling and reuse of the recovered polyamide have been required.
The recovered polyamide from the industrial polyamide production are usually recycled and reused by re-melting in an extruder, etc., and then re-granulating. This method is costly advantageous, but the reclaimed polyamide are poor in quality such as color tone, and cannot be used equally to the products before re-melting.
Also, there is known a so-called thermal recycle method in which wastes are recovered in the form of heat. However, this is a method to be finally selected when wastes are not suitable for reclaiming.
In addition, there is also known a so-called chemical recycle method in which waste polymers are decomposed into reusable monomers. This method has been now most prevailing, and studied and researched by many material makers. For example, WO97/00846 discloses a process for recovering diamine and dicarboxylic acid from polyamides. In this process, the polyamides are hydrolyzed by adding nitric acid to recover monomers. This process, however, requires a special apparatus as well as large apparatus and maintenance costs to avoid the corrosion of the apparatus due to the use of nitric acid as a strong acid.
Japanese Patent Application Laid-Open No. 9-12711 discloses a process for producing polyamide in which odd-shaped pellets are made into a slurry in a liquid medium such as water, and then mixed with raw materials. This process is cost-advantageous since substantially no change is required in the apparatus, but, disadvantageous in view of saving energy because a large amount of the liquid medium such as water is used.
Under these circumstances, it has been strongly required to develop a process for recycling the recovered polyamide, which is capable of producing a polyamide having a quality not deteriorated as compared when produced without using the recovered polyamide, with minimum cost for additional apparatus and without any significant change in the production process.
As described above, the polyamide for injection molding is required to have a low melt viscosity to ensure a good fluidity in a mold. In addition, the polyamide is required to have a high crystallization speed to shorten the molding cycle. In general, the crystallization speed of polyamide is increased by adding various inorganic materials or polymers having a high crystallization speed and a good compatibility with the polyamide. However, since the amount of addition is limited because of deterioration in properties, the crystallization speed can be increased only to a limited extent. Therefore, to further increase the crystallization speed, it is necessary to increase the crystallization speed of the polyamide itself. Thus, it has also been strongly required to develop a method for increasing the crystallization speed of the polyamide itself.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for producing a polyamide by reusing a recovered polyamide, which has been conventionally disposed as wastes, as a part of raw materials, the process being capable of producing the polyamide without causing deterioration of quality at minimum costs for additional apparatus without needing the change of the production process. Another object is to provide a method for increasing the crystallization speed of a polyamide which is produced from a raw material containing the recovered p
Shida Takatoshi
Tanaka Kazumi
Hampton-Hightower P.
Mitsubishi Gas Chemical Company Ltd.
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