Method of manufacturing polybutylene terephthalate resin films

Abstract: A polybutylene terephthalate resin film is produced by turning into film a polybutylene terephthalate resin having an inherent viscosity of more than 1.0 by the inflation molding technique and under the condition of extrusion resin temperatures within a range defined by the following relation: melting point (°C) < extrusion resin temperature (°C) < melting point - 26 + 53 x inherent viscosity (°C)

Description: (Field of the Invention) This invention relates to a novel method of making polybutylene terephthalate resins film. (Brief Description of the Drawing) Figure 1 is a graph showing extrusion resin temperature ranges applicable to inflation molding according to the method of the invention. (Prior Art) Polybutylene terephthalate resins are widely used as typical engineering plastics. Most of these resins, however, are exclusively used for injection moldings, and are rarely used for extrusion moldings, and films in particular. The reason for this is that molding such resin into films is extremely difficult. Generally, there are two types of methods for production of films, namely, T-die method and inflation method. The T-die method is usually such that a stream of molten plastics introduced through one inlet is broadened to a desired film width and made as uniform in thickness as possible, so that the melt in extruded in a thin film pattern through a slit-shaped nozzle, the extruded resin being then cooled into a film. The inflation method, often employed in molding polyethylene into film, comprises extruding molten plastic through an annular extrusion nozzle to form a tube, the tube being inflated by blowing a fluid, e.g., air, thereinto, whereby the extruded resin is formed into a tubular film. The T-die and inflation methods have their respective advantages and disadvantages. Generally, the inflation method, as compared with the T-die method, provides higher productivity and greater economy and is suitable for production of thin films, but its applicability varies according to the kind of resin material used. As such, it cannot be said that the method is applicable for use with any kind of resin, and naturally the range of resin varieties with which the method can be employed is limited to a few special kinds of resins, such as polyolefins. Generally, polyesters, such as polyethylene terephthalate and polybutylene terephthalate, are film-moldable by the T-die method, but on the other hand, manufacturing of films of these resins involve various technical problems to be solved and, as such, it has not been industrialized to date. In view of this situation, the present inventors conducted a series of studies directed to making it possible to produce films of polybutylene terephthalate by the inflation technique, and as a result, they found that polybutylene terephthalate could be adapted for film extrusion by the inflation technique by selecting a specific variety of polybutylene terephthalate and a specific range of temperature conditions. This discovery led to the present invention. (Summary of the Invention) Accordingly, this invention provides a method of manufacturing a polybutylene terephthalate resin film in which a tubular melt of the resin is continuously extruded through a crosshead die, the tube is held at one end between pinch rolls and air is fed thereinto, characterised in that a polybutylene terephthalate resin having a melting point of MP °C, and an inherent viscosity IV of between 1.2 and 2.5, is extruded at an extrusion temperature T °C, which is greater than or equal to MP + 10, but less than or equal to MP - 36 + 53 IV. The term "inherent viscosity" used herein refers to a value therefor as measured in orthochlorophenol at 25°C. We are aware of European Patent Specfication EP-A-95995 (The Goodyear Tire & Rubber Company) which describes the manufacture of polyester especially polyethylene terephthalate film by an extrusion blowing process in which a polyester bubble is maintained at a temperature of between 0 and 30 degrees Celcius above the glass transition temperature. We are also aware of British Patent Specfication GB 1314070 (Eastman Kodak Company) which describes a polyester film made by an extrusion process in which the quenching temperature employed depends on the inherent viscosity of the polyester. After their series of studies with polyalkylene terephthalate resin, the present inventors found that whereas stable film making of polyethylene terephthalate resin by inflation molding is extremely difficult under any conditions, it is possible to form a film of a polybutylene terephthalate, if its inherent viscosity (hereinafter referred to as IV) is between 1.2 and 2.5 by inflation-molding same in such a way that the extrusion resin temperature comes within the range covered by the foregoing relation. Such polybutylene terephthalate resins from 1.2 to 2.5 in IV are higher in IV and greater in average molecular weight than ordinary-types of polybutylene terephthalates, and those are preferred which may be in the form of either a homopolymer or a copolymer consisting principally of a polybutylene terephthalate. One having an IV of more than 2.5 can hardly be produced as such. One having an IV of less than 1.2 is undesirable because it is difficult to form a film because of drawdown difficulties during inflation molding. However, a mixture of a polybutylene terephthalate having an IV of less than 1.2 and one having an IV of more than 1.2 is formable into film if the resulting IV of the mixture is more than 1.2. The polybutylene terephthalate resin used is preferably of an IV of 1.5 - 2.2. The temperature of the resin is preferably within a range defined by the following relation: melting point + 10 (°C) < extrusion temperature (°C) < melting point - 36 + 53 x inherent viscosity (°C) These ranges of resin temperatures for inflation molding are shown in Figure 1, which is a graph of IV plotted against extrusion temperature. The area shown with single hatching is the range which satisfies the conditions : melting point < extrusion temperature < melting point - 26 + 53 IV, and IV > 1. The area shown with double hatching is the range according to the invention, which satisfies the conditions: melting point + 10 (°C) ≦ extrusion temperature ≦ melting point - 36 + 53 IV (°C), and 1.2 ≦ IV ≦ 2.5. If the resin temperature is too high, the viscosity of the melt becomes so low that making film from the resin is impracticable because of drawdown possibilities. If the temperature is too low, the viscosity of the melt becomes so high that greater pressure loss in the die is involved, the power requirements becoming greater for operation of the extruding machine, which is followed by decreased productivity. Another undesirable effect of such low temperature is increased film thickness irregularity due to non-uniformity of resin flow in the die. According to the invention, film making of a polybutylene terephthalate resin by the inflation technique is possible by maintaining the foregoing essential requirements, and for the other conditions, general conditions of the inflation technique are applicable. By employing a crosshead die, a tubular melt of polybutylene terephthalate resin is extruded upward or downward; the tube is held at one end between the pinch rolls and air is fed thereinto, so that the tube is continuously taken up while being inflated to a predetermined size. Meanwhile, the die is rotated forward or reversed, whereby any possible thickness irregularity can be prevented. The tubular film may be cut at both ends and sealed at one end so that it can be used as a bag. Or, it may be suitably drawn into one continuous film and heat set and/or otherwise treated; thus, as a film having advantageous properties of polybutylene terephthalate resins and having some suitable variations given according to the purpose for which it is used, the product can be offered for various end uses. In the manufacture of films according to the invention, it is possible to adjust film gage in practically same manner as in the conventional inflation process. Generally, film gage is adjustable within the range of 0.005 - 0.1 mm. In combination with the polybutylene terephthalate resin used in the practice of the invention, various known materials which are usually added to ordinary thermoplastic or thermosetting resins may be suitably used depending upon the performance characteristics required of the product. Materials useful for such purposes include plasticizers, stabilizers, such as antioxidant and U.V. light absorber, antistatic agents, surface active agents, colorants, such as dyes and pigments, and lubricants and crystallization accelerators (nucleating agents) useful for fluidity improvement. Also, other thermoplastic resins and/or inorganic fillers may be supplementarily used for addition in small amounts insofar as it is not detrimental to the purpose of the invention. (Advantages of the Invention) As above described, according to the invention, it is possible to manufacture polybutylene terephthalate resin films by the inflation technique. The invention assures improved productivity and economy as compared with the conventional T-die method, and makes it possible to easily produce even thinner gage films through application of known techniques. Films produced according to the invention have excellent properties inherent to polybutylene terephthalate resins, including, for example, good mechanical, physical, chemical and thermal properties. Further they have a special advantage that they are less subject to moisture and gas permeation. Therefore, they can be advantageously used either alone as such or in lamination with other film or metallic foil for packaging and various other purposes. (Examples) The invention will be further illustrated by the following examples. It is understood, however, that the invention is not limited by these examples. Example 1 A polybutylene terephthalate resin P (IV 2.0, melting point 228°C) was used. From a 50 mm⊘ extruding machine, with a die diameter of 120mm and a die lip clearance of 1mm, a tubular film was extruded at a resin temperature of 285°C, and a blow ratio of 1.3, and the film was taken off at a take-up rate of 30 m/min while being cooled by air. Thus, a film of 18 µ gage was obtained. The properties of the film produced were as shown in Table 1. Property evaluation of the resin and of the film was carried out in the following manner: IV: measured in orthochlorophenol at 25°C. Melting point: measured by DSC (heat-up rate 5°C/min) and at a heat absorption peak location. Tensile strength and elongation: measured according to ASTM D 882 Oxygen and nitrogen permeabilities: measured according to ASTM D 1434. Examples 2 - 4; Comparative Examples 1 - 5 Polybutylene terephthalate resin P, and polybutylene terephthalate resin Q (IV 1.63, melting point 228°C), and polybutylene terephthalate resin R (IV 0.76, melting point 228°C) were used. In an apparatus similar to the one used in Example 1, films of various different gages as shown in Table 2 were produced at varied extrusion resin temperatures. Properties of the films were evaluated. The results are shown in Table 2.

Claims: A method of manufacturing a polybutylene terephthalate resin film in which a tubular melt of the resin is continuously extruded through a crosshead die, the tube is held at one end between pinch rolls and air is fed thereinto, characterised in that a polybutylene terephthalate resin having a melting point of MP °C, and an inherent viscosity IV of between 1.2 and 2.5, is extruded at an extrusion temperature T °C, which is greater than or equal to MP + 10, but less than or equal to MP - 36 + 53 IV. A method of manufacturing a polybutylene terephthalate resin film according to claim 1, wherein the inherent viscosity of the polybutylene terephthalate resin is 1.5 - 2.2.

Inventors: NEDZU SHIGERU, SHIMADA TAKEHIKO, TANIGUCHI KOTARO, TANIMOTO SHIGEKAZU, NEDZU, SHIGERU, SHIMADA, TAKEHIKO, TANIGUCHI, KOTARO, TANIMOTO, SHIGEKAZU
Applicants: POLYPLASTICS CO, POLYPLASTICS CO. LTD.
Application year: 1988
Main classification: C08J5
IPC codes: B29C47, C08J5, B29K67