JPS6031919A - Drying method of thermoplastic resin - Google Patents

Abstract

PURPOSE:To improve drying efficiency and to reduce the drying time of material resin by conducting heat treatment due to electromagnetic induction to thermoplastic resin such as polyethylene terephthalate etc. to remove the moisture in the resin. CONSTITUTION:A microwave dielectric heater 6 is provided with a power source 7, a magnetron oscillator 8, an isolator 9, a matching device 10, etc. and equipped on the top of the material tank 3. This microwave dielectric heater 6 dielectrically heats material resin 5 in the material tank by the microwave from the magnetron oscillator 8. The temperature of the material resin 5 rises by the dielectric heating and this rise of temperature is based on heating due to molecular motion in the material resin. Therefore, the rise of temperature as resin particle is equal inside and outside the particle but since heat is radiated actually from the surface of the particle, the rise of temperature is larger inside the particle and the moisture in the resin particle is removed from inside the particle.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for drying a thermoplastic resin used as a molding material.

When injection molding or blow molding industrial parts, general miscellaneous goods, various bottles, etc. using a thermoplastic resin such as polyethylene terephthalate resin (hereinafter referred to as PE1T resin), be sure to use the material resin for at least 3 hours before molding. Dehumidification and drying was performed for 6 hours.

This is because when PET resin is heated and melted, if moisture is present in it, the ester bonds in the main chain easily undergo hydrolysis, leading to a drastic drop in molecular weight (lower intrinsic viscosity) and physical properties. This causes a decrease in strength. Furthermore, at high temperatures, ethylene glycol is separated from the terephthalic acid half ester of ethylene glycol produced at the cutting point, rearranged, and liberated as acetaldehyde, with a portion remaining in the resin.

Carbonated beverage bottles made of PET resin vary depending on the contents to be filled, but for example, colas require strict control over flavor changes, such as setting the amount of acetaldehyde that precipitates in the bottle headspace at 3 micrograms/liter. Therefore, it is also important to minimize the generation of acetaldehyde when using PET.
This is a major purpose of resin drying, and it is generally said that the moisture content of PET resin before melting is preferably 0.003 to 0.005%.

However, it is not easy to reduce the amount of water contained in PET resin to the above value. Most of the methods currently used are as follows.

That is, the air sent from the blower is heated in the drying heater to 160 to 180°C, and then sent to the drying hotber. Moist material is put into the drying hopper, and dry hot air is spread throughout the hopper from the bottom of the hopper.

The dry hot air that came into contact with the surface of the moistened PET resin
The surface of the T-resin is gradually heated and the water contained inside the resin begins to be removed. The dry air is gradually moistened by this moisture and returns to the dehumidifying chamber from the upper part of the hollow chamber.

The dehumidification chamber is filled with dehumidifying molecular sieves, and the moisture in the humid air adheres to and is absorbed by the molecular sieves. Then, it is heated again by the drying heater and sent to the drying hopper. During this time, the molecular sieves in the dehumidifying room cannot use their capacity indefinitely, and at a certain stage, their capacity becomes saturated. At this time, the above cycle must be stopped, and in order to regenerate the molecular sieve itself, it must be heated and water-separated in heated air.

In addition, in order to obtain the moisture content of 0.003 to 0.005% as described above, continuous dehumidification and drying for 4 to 6 hours is necessary, and therefore the drying material becomes very large. For example, 1 hour 7
A blow molding machine that melts 0 K9 PET resin generally requires a drying hopper of 300 to 350 liters.

Furthermore, as mentioned above, a drying heater, regeneration heater, blower, etc. are required, and their electric capacity is extremely large. In addition, the air piping between the drying hopper and the unit that includes the heater, molecular sieves, blower, etc. is also complicated and requires a large installation space. It was never satisfying.

The present invention was conceived in view of the above circumstances, and its purpose is to apply external heat to PET resin, heat the resin by transferring heat from the outside to the inside, and remove the expanded moisture. It is an object of the present invention to provide a new method for drying thermoplastic resins that eliminates the drawbacks of conventional methods such as low efficiency, high power consumption, and large installation space requirements.

The present invention for the above purpose is characterized in that moisture in a thermoplastic resin such as polyethylene terephthalate is removed by dielectric heating using electromagnetic induction.

Dielectric heating in this invention refers to dielectric materials (plastic,
This is due to the dielectric loss (resistance of intermolecular bonding force) of nonmetallic solids such as glass, water, oil, etc., and heat is generated by the movement of the molecules of the material itself, so conventional heating by heat conduction from the surface of the material This is significantly different from the law.

The present invention will be explained in detail below using the illustrated apparatus.

In the figure, 1 is an injection device having a material mixing screw 2 inside;
3 is a material tank attached to the injection device 1, and the material drop port 4
The structure is such that granular material resin 5 is supplied into the injection device.

A microwave dielectric heating device 6 is attached to the top of the material tank 3. This microwave dielectric heating device 6
is a power supply 7, a magnetron oscillator 8, an isolator 9,
Equipped with a matching device 10, etc., the material resin 5 in the material tank is
Dielectric heating is performed by microwaves from the magnetron oscillator 8.

The temperature of the resin material 5 rises due to the dielectric heating between the material and the resin, but since this is heat generation due to molecular motion in the material resin, the temperature rise of the resin particles is the same both inside and outside, but in reality, the temperature rise from the surface increases. Due to heat dissipation, the temperature rises larger inside the particles, and the moisture in the resin particles is removed from the inside.

Next, the results of drying using the above device will be shown.

Microwave dielectric heating device Rated voltage 100■, Rated frequency 5Q'Hz, Rated high frequency output 500W, Oscillation 1 frequency 2450±5Q MH2 heating time 15 minutes, Additional material resin PET resin for carbonated drinks Intrinsic viscosity 0.73 Collection amount approx. 20f Example 1 Heating time 15 minutes Example 2 Heating time Additional Note that the moisture content of 0.4% is due to the fact that the moisture content of the PET resin in the standing state is 0.4%.

Comparative Example The moisture content of PET resin under the same conditions using a hot air dryer with a dehumidifying device was as follows.

7- Before heating After additional heating Moisture content after 60 minutes of heating (%)
0.401 0.294 0.0813 As is clear from the above results, the method of the present invention not only has a much better drying effect than the method using a conventional hot air dryer, but also can significantly shorten the drying time of the material resin. , the drying equipment can be simplified.

[Brief explanation of the drawing]

The drawing shows a partially longitudinal sectional side view of an injection device equipped with an apparatus capable of carrying out the method of drying thermoplastic resin according to the present invention.

Claims (1)

[Claims] A method for drying thermoplastic resins, which includes removing moisture in thermoplastic resins such as polyethylene terephthalate by dielectric heating using electromagnetic induction.