JPS6050574B2 - Crosslinking foam molding method and device for high molecular weight polymers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to an improvement in the method of forming crosslinked foams from high molecular weight polymers.

Depending on the desired product, foam molding of high molecular weight polymers can be performed by molding foams with an open cell structure in which foam cells communicate with each other, or molding foams with a cell structure in which each cell is independent. There are two methods, and molded products with different feel, mechanical strength, and electrical properties can be obtained depending on the shape and size of the cells.

It is known that a method for molding a foam having an open cell structure generally uses an inorganic solid foaming agent, an organic solid foaming agent, a volatile foaming agent, etc., and allows a variety of molded products to be freely selected.

Methods for molding foams having a closed cell structure include: 1. A method in which a blowing agent is added to a high molecular weight polymer (hereinafter referred to as a plastic material) in advance and then heated and melted, or a method in which only the plastic material is first heated and melted. In either method of adding a blowing agent from the outside, a method of adjusting the melting temperature so that the melt viscosity of the plastic material is appropriate in balance with the blowing agent gas pressure, a method of adding a thickening agent, 3. There are methods that utilize the increase in viscosity caused by the crosslinking reaction.

Method 1 of these methods is suitable for plastics whose melt viscosity is suitable for foaming, such as polyvinyl chloride, acrylic-butadiene/styrene copolymers, polystyrene, etc.; It is necessary to mold the product as close to the melting point as possible, and this is not preferable from the viewpoint of uniformity of foaming due to the relationship with the foaming temperature of the foaming agent. In method 2, the viscosity of the plastic when melted is supplemented with a thickener, and there is a large degree of freedom in moldability, but the selection of materials depends on the compatibility and dispersibility of the blowing agent and the base plastic. is limited.

Regarding method 3, any plastic material may be used as long as it is a polymeric material for the crosslinking agent, but there was a problem in the moldability of the crosslinked plastic material in relation to the crosslinking temperature and degree of crosslinking. In other words, an attempt is made to achieve independent foaming by utilizing the thickening effect caused by the crosslinking reaction and the three-dimensional molecular structure, but once the temperature at which crosslinking progresses is reached, the degree of crosslinking becomes a function of time. If crosslinking progresses too much, fluidity will be lost and molding will become difficult, and it is not good if crosslinking progresses too much or foaming progresses too much, and foaming should proceed in balance with the change in viscosity that accompanies the progress of crosslinking. This is desirable. From these points of view, conventional methods have been to extrude the uncrosslinked and unfoamed product from a tie, and in the subsequent process, proceed with crosslinking first in a discontinuous press or open state, and then proceed with foaming, or proceed with crosslinking and foaming at the same time. I was doing a lot of things. In recent years, crosslinking molding technology has changed from the conventional extrusion method of extruding at a temperature below the crosslinking initiation temperature to a technology of extruding after raising the temperature to a temperature at which crosslinking substantially progresses. A method in which the temperature is raised uniformly and in a short period of time, and the plastic is extruded from the tie while crosslinking is progressing but the fluidity is not lost yet, such as JP-A-50-40667, JP-A-Sho. 50 -1
54360, JP-A-51-96858, JP-A-51-9
7668 etc. have been proposed.

The present invention is a method for continuously molding a crosslinked foam from a mixture of a plastic material and a crosslinking agent and a blowing agent, which is a further improvement on the above method.

In order to continuously form a predetermined crosslinked foam, it is necessary that substantial crosslinking has already progressed at the exit of the tie nozzle, or the temperature is such that crosslinking will rapidly proceed, and that the desired final crosslinking is achieved after extrusion from the die nozzle. (Ts) and the time to reach the desired final foaming degree (Ts).
Tr), it is necessary to obtain the relationship Tr>Ts.

In addition, in order to obtain such a relationship, the temperature must be raised in a short period of time to a temperature above which the crosslinking agent substantially decomposes and above a temperature at which the blowing agent substantially decomposes or boils. It has been found that once the temperature is raised to a temperature that satisfies the conditions, it is necessary to extrude the crosslinking agent from the die nozzle within the half-life of the crosslinking agent at that temperature. In addition,
The half-life here refers to the time required to decompose 112 at the initial concentration of crosslinking agent when thermally decomposed at that temperature. That is, if the appropriate time (temperature increase time) from the start of temperature increase to the exit of the die nozzle after temperature increase is Tp, then Tp=f
(Q) and Tp can be seen as functions of the extrusion amount Q. Further, if Tc is the limit time possible for extrusion within the time period within the half-life, the transit time (
It is necessary to adjust the heating time (heating time). Therefore,
In order to be able to freely adjust the extrusion amount (time) and calorific value (temperature) each independently, molded products with various foaming states are continuously produced at the outlet of the tie nozzle with a degree of crosslinking that gives the appropriate viscosity for foaming. I found out that it can be obtained by This is based on the knowledge that if the time is longer than the half-life, crosslinking will proceed too much and foaming will be suppressed. The above-mentioned molding conditions according to the present invention are collectively expressed by the following equation.

(Tc is the decomposition temperature of the crosslinking agent, Tf is the decomposition temperature or boiling temperature of the blowing agent, Tp is the predetermined temperature at the end of heating the molten resin, and Tc is the half-life of the crosslinking agent at Tp.
time), Tp is the time from the heating time to reaching Tp and extrusion from the tie nozzle, Q is the extrusion amount) The crosslinking agent used in this invention is t-butyl hydroperoxide, p-
Hydroperoxides such as menthane hydroperoxide and cumene hydroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane,
Dialkyl peroxides such as t-butyl peroxide and cumin peroxide, diacyl peroxides such as acetyl peroxide, propionyl peroxide, isobutyryl peroxide, and benzoyl peroxide, t-butyl peracetate, and t-2-ethylhexanoate.
Peracid esters such as butyl, t-butyl isopropyl percarbonate, ketone peroxides such as methyl ethyl ketone peroxide, cyclohexanone peroxide, 1,1''
- Peroxyketanols such as di(t-butylperoxy)cyclohexane, whose decomposition temperature is higher than the melting point of the plastic, and should be selected in consideration of reactivity with the plastic and stability with the blowing agent. good.

Blowing agents include inorganic blowing agents such as ammonia carbonate, sodium bicarbonate, a mixture of sodium nitrite and ammonium chloride, and organic blowing agents such as dinitrosopentamethylenetetramine, benzenesulfonyl hydrazide, azodicarbonamide, diethyl azodicarboxylate, etc. are used, and a urea-based auxiliary agent, an organic acid-based agent, a metal salt-based auxiliary agent, etc. for adjusting the decomposition temperature may be used in combination with these.

It is also possible to use organic solvents as blowing agents. The foaming agent may be selected in consideration of the melting point of the plastic material, the decomposition temperature of the crosslinking agent, the expansion ratio and quality of the molded product. Inorganic blowing agents and organic blowing agents are not particularly limited, but those with a high decomposition rate are preferred, and molding is easier if the temperature at which the blowing agent significantly decomposes is equal to or lower than that of the crosslinking agent. Become. The amount of foaming agent added is mainly determined by the desired foaming density and decomposition efficiency. Generally, when adding a blowing agent to a plastic material to foam it, only about 15 to 30% of the blowing agent added at the beginning acts effectively, and the remaining 70% acts effectively.
~85% often remains in the molded product, and in order to effectively foam this and obtain the desired foam density, a crosslinking rate of 20 to 30% (regardless of the amount of crosslinking agent added or the type of plastic material) It is preferable to carry out foaming when the insoluble gel content reaches 10% (insoluble gel content when expanded and extracted with an organic solvent). As a result of various tests, the above crosslinking rate can be obtained regardless of the type of crosslinking agent and plasticizer, and the time required for the decomposition amount of the crosslinking agent to reach the initial concentration of 112 at each temperature, that is, within the half-life period. I found that.

This is because the decomposition of the crosslinking agent is an exponential function with respect to time, and decomposition is extremely fast when it reaches about 50%, and decomposition is slow beyond that, so in reality, it decomposes by about 50%. This is presumably because a sufficient crosslinking rate can be obtained for the desired final crosslinking rate. On the other hand, since the reaction with plastics is generally a first-order reaction, crosslinking progresses more slowly than the decomposition of the crosslinking agent, and the crosslinking rate can be freely controlled by time within the half-life. Seem. Figure 1 shows the amount of decomposition of a crosslinking agent, which is generally said to be based on the half-life. Figure 2 shows an example of half-life plotted against temperature, where a is dicumyl peroxide and b is dicumyl peroxide.
,5-dimethyl-2,5-di(t-butylperoxy)
It was calculated by thermally decomposing a 0.2 mol/'benzene solution of hexyne-3 and measuring the decrease in the amount of active oxygen. The above active oxygen amount refers to the amount of oxygen that is highly chemically reactive, and refers to the amount of oxygen whose atomic state is in a metastable state. FIG. 3 shows the change in the crosslinking rate with respect to time at each temperature of a high molecular weight polymer mixed with a crosslinking agent, the details of which are as follows. The experiment was conducted after melting each sample in an uncrosslinked state. A sample piece of the same thickness was pressed, immediately immersed in silicone oil kept at the above temperature for a certain period of time, and then rapidly cooled with water at 18°C, and the crosslinking rate was measured.

Note that the thickness of the sample piece of 1 Tmm is determined by the thermal conductivity of the plastic material (
The thickness is determined to be such that the temperature can be raised in a short time in the thickness direction so that the specimen (generally small) can be ignored. Forced stirring was performed to make the volume as large as possible. When the half-life of each crosslinking agent at each temperature is determined from FIG. 2 and applied to FIG. 3, the crosslinking rate at the half-life is 35 to 40% or less (shaded area).

Here, as a precondition for determining these relationships, when crosslinking a plastic material, it is sufficient that there is an optimal amount of crosslinking agent necessary to obtain the desired final crosslinking rate, and any amount added beyond that amount remains as a residue and is meaningless. I am of the opinion that On the contrary, D in Figure 3 contains less crosslinking agent than B, and the crosslinking rate during the half-life is about 17-18%. It is necessary to raise it.
Therefore, the time within the half-life period can impart an appropriate viscosity to cross-linked foam regardless of the type and temperature of the cross-linking agent or plasticizer, and in this case, temperature and time can each be controlled independently. is important. Next, the present invention will be explained in detail by way of examples. Example polymer: 100 parts by weight of low-density polyethylene Crosslinking agent: 1 part by weight of dicumyl peroxide (decomposition temperature (Te) approximately 166°C) Blowing agent: 20 parts by weight of azodicano Uυ amide Lead stearate was added to the above mixture as a foaming aid. The material 21, in which the decomposition temperature (Tf) of the blowing agent was adjusted to about 185° C. by adding a small amount of 12, the screw 3 is rotated, and the nose cone 4 at the tip of the screw
and the temperature (<Tf
, Tc) to uniformly generate heat and raise the temperature above the decomposition starting temperature of the crosslinking agent and the blowing agent. A plasticized melt 22 is fed into the container.

Inside the cylinder 5, there is a threaded part 6 for conveyance, a rotary cutting tool 7 and a cap 8 for applying shear heat by rotation in the gap between the cylinder 5 and the tip of the cylinder 5, and the variable speed drive source 11 is Drive source 14 and gear 1 that can change speed independently
A rotor 16, which is rotated at a speed of 3, is installed. 1st
The melt 22 fed from the cylinder 2 of the stage is heated by the rotating orifice 7 in a short time to a temperature (Tp) higher than the decomposition temperature (Te, Tl) of the crosslinking agent and the blowing agent,
Further, the heat medium liquid (hot oil) whose temperature is controlled to a predetermined temperature (Tp) is extruded from a die nozzle 9 provided with a jacket 10 for refluxing.

The extruded melt 23 has an extrusion amount due to the rotation of the screw of the first stage extruder, and a shear calorific value due to the second stage rotating rotor 16 whose rotation speed is independently adjusted regardless of changes in the extrusion amount. By selecting and adjusting the passage time (Tp) and temperature (Tp) from the start of temperature rise to the exit of the die nozzle 9 after the end of the temperature rise, foaming is started immediately after exiting the die nozzle 9, and the sizing die ( A desired cross-linked foam molded product of good quality was obtained by adjusting the shape using a method (not shown). The conditions at this time are as follows. Melt temperature at die nozzle exit (Tp)
= 190°C Extrusion amount (Q) = 35k91hr Temperature of melt before heating = 1377C Passage time from the start of heating to the Guin nozzle exit (Tp) = 1 Crosslinking rate at the forging die nozzle exit = Approximately 25% after sizing Foaming ratio of molded product = 28 times Crosslinking ratio of molded product after sizing = 68
．． 3% Comparative Example Using the same materials and the same equipment as in the example, crosslinking foam molding was carried out under the following conditions.

Melt temperature at die nozzle exit (Tp) = 190°C Extrusion amount (Q) = 15k91hr Temperature of melt before heating = 1
Passage time (Tp) from the start of temperature rise to the exit of the die nozzle at 28° C. = approximately 3.2°C The rotation of the rotary orifice 7 was varied under the above conditions, and the crosslinking rate at the exit of the die nozzle at various resin temperatures was measured.

The results are shown in FIG. At the above extrusion rate, a good cross-linked foamed molded product could be obtained, and the expansion ratio after sizing at a temperature above the decomposition temperature of the blowing agent was 1 or less. This means that the temperature required to obtain a half-life of 3 and 2 is the second temperature.
From the figure, the temperature is about 182'C, so if the temperature exceeds 182'C, crosslinking will proceed too much and foaming will be suppressed.

[Brief explanation of drawings]

Figure 1 is a curve diagram showing the relationship between the amount of decomposition of the crosslinking agent and time;
Figure 2 is a diagram showing the half-life of the crosslinking agent versus temperature;
The figure is a curve diagram showing the relationship between crosslinking rate and time at various temperatures of a polymer containing a crosslinking agent, Figure 4 is a cross-sectional view of the apparatus implementing the present invention, and Figure 5 is a resin temperature and crosslinking rate in a comparative example. It is a curve diagram showing the relationship. 2...1st stage cylinder, 3...screw, 5...
・Second stage cylinder, 6... Threaded part, 7... Rotating orifice, 8... Cap, 9... Guin nozzle.

Claims (1)

[Claims] 1. A high molecular weight polymer mixed with a crosslinking agent and a foaming agent is melted and kneaded in a screw extruder at a temperature and pressure that does not cause crosslinking reaction and foaming, and then this melt is mixed with molecules of the melt. In the step of extruding after raising the temperature to a predetermined temperature above the decomposition temperature of the crosslinking agent and above the decomposition temperature or boiling temperature of the blowing agent by shearing, the predetermined temperature is reached after the temperature of the melt starts to rise. The time required for extrusion from the die nozzle is shorter than the half-life of the crosslinking agent at the predetermined temperature, and the extrusion time is adjusted by changing the amount of extrusion of the melt. A method for crosslinking and foaming molding of high molecular weight polymers. 2. A first unit equipped with a single or twin screw capable of rotating at variable speed, which melts and kneads a high molecular weight polymer mixed with a crosslinking agent and a foaming agent at a temperature and pressure that does not cause crosslinking reaction and foaming.
a stage extruder, a threaded part that is connected to the extrusion port of the first stage extruder and conveys the melted polymer; and a threaded part that is integrally formed at the tip of the threaded part and cooperates with the inner peripheral surface of the cylinder. The second stage extruder is equipped with a rotor provided with a rotating orifice so that the rotation speed can be changed independently of the screw of the first stage extruder, and the melt is controlled only at the intermolecular shear rate at the rotating orifice. Therefore, the temperature is exothermically raised to a predetermined temperature that is higher than the decomposition temperature of the crosslinking agent and the decomposition temperature or boiling temperature of the blowing agent, and after the melt starts to exothermicly increase in temperature in the rotating orifice, it reaches the predetermined temperature and is extruded from the die nozzle. A crosslinking and foaming molding apparatus for a high-molecular polymer, characterized in that the time to which the crosslinking agent lasts can be adjusted to be shorter than the half-life of the crosslinking agent at the predetermined temperature only by the amount of supply from the first stage extruder. .