DE4019202A1 - Low-density extruded thermoplastic foam - is mfd. by melt-extruding mixt. of plastic and pore regulator, using nitrogen gas and liq. halohydrocarbon as prim. and sec. blowing agents - Google Patents

Abstract

Prodn. of foam from a thermoplastic (I) comprises (a) mixing (I) with a pore-regulator (II),(b) putting the mixt. in an extruder (c) plastifying, mixing, and homogenising, (d) injecting 0.2-0.5 wt. % (w.r.t. I + II) nitrogen gas as a first blowing agent, (e) homogenising the gas-contg. mixt. (f) injecting 0.5-2 wt. % liq. partly-halogenated cpd. (III) as a sec. blowing agent, (g) mixing and homogenising the mixt. (h) extruding the mixt. to produce foam. The pressure of the mixt. in the extruder head is at least 55 bar, and its temp. is above a previously determined lower limit which is dependent on the material (I). Also claimed is an extruder system for the above process, comprising a hollow cylindrical housing with 2 injection zones, one (B) downstream from the other (A), and a rotating motor-driven homogeniser screw. Zones (A) and (B) fitted with injectors for gaseous and liq. blowing agents, respectively, and the zones are sepd. by a distance of at least 2D (with D = screw dia). Pref. (III) is CHClF2 (R22) CHCl2CF3 (R123) CH2FCF3 (R134a), CH3CCl2F, (R141b) or CH3CClF2 (R142b) (II) is talcum, (I) is polystyrene, (PS) in which case the temp. of the mixt. in the extrusion head is at least 130 deg.C or polyethylene (PE), in which case the temp. is at least 128 deg.C, extruder head pressure is adjusted by means of an axial pin, the extruder head is fitted with pressure sensors, and the head and slit are fitted with channels for temp. controls. USE/ADVANTAGE - The process enables the prodn. of low-density extruded plastic foam with a combination of blowing agents which presents no fire hazard and is less damaging to the ozone layer than prior-art systems. (6pp Dwg.No.0/2)

Description

The invention relates to a method for producing a foam fes from a thermoplastic in an extruder, wherein the foam is formed by a mixture of the plastic, a pore regulating substance and two blowing agents contained in the Extruder mixed, plasticized and homogenized who the.

The market launch of the extruded foam products was significantly promoted in the mid-1960s by the use of chlorofluorocarbons (CFCs) as a substitute for the flammable blowing agents pentane from the hydrocarbon family. With the start of using CFC types R 11 , R 12 , R 113 and R 114 for foam extrusion, semi-finished products and products could be manufactured that met the market demands in terms of both quality and handling.

Based on scientific knowledge, it was found that the CFC gases, which are also used in aerosol and refrigeration technology as spray and  Refrigerant and in the electronics industry as a cleaning agent as well are used as blowing agents in polyurethane foaming, the depletion of the ozone layer in the upper atmosphere is essential accelerate.

It has been very recently inflamed to the use of the very easily pentans originally used, but not Ozone belt is harmful.

The systems used for this do not differ significantly from the conventional systems. However, there are dosing systems for the Use propellant addition to ensure that the propellant do not escape due to wear in the pump's pressure build-up system ten, which would significantly increase the risk of fire. Furthermore it is recommended to separate the propellant dosing systems from the Installation. Other additional security measures that are absolutely necessary, the intensive ventilation in the form of Air renewal at the installation site of the foam extrusion system, in Interim film storage, in the deep-drawing area and in the area of re-extrusion sion of the foam waste and the additional attachment of equipment the static charge of the semi-finished foam products and the pro derive products.

For the extrusion foaming of thermoplastics therefore make certain demands on the propellants.

The blowing agent must be soluble in the plastic melt, but may do not change its viscosity and glass point too much. The Blowing agent must have a high evaporation rate during expansion have so that the residual gas concentration remains low. It Permeation must not occur, since it causes the cell to collapse len before solidification.  

Physical blowing agents that have these properties and in applied in practice are pentane, trichlorofluoromethane, Dichlorodifluoromethane and, in exceptional cases, nitrogen.

Nitrogen can be used successfully as a blowing agent if a ge sufficiently high solution concentration can be achieved, which so far only with a very high and unacceptable machine dosage effort is feasible. The low affinity of nitrogen for Plastic melt requires a very high solution pressure for this, which is several times higher than that of dichlorodifluoromethane lies.

From DE-OS 19 14 584 it is known to form a core former from solid Use substances in the production of foam polystyrene. Prev The disadvantage of solid core formers should be eliminated having to mix melted granules because they are very expensive Mixing plants are required for this. Because of this, it is featured beat liquid core formers, e.g. B. nitrogen, in which a propellant tel, e.g. B. a halogenated hydrocarbon having melt to inject. This measure is a targeted influencing the cell formation or cell size is aimed for.

The teaching disclosed in DE-OS 17 78 104 also assumes to produce a very fine cell, low density foam. To the For this purpose, a low-boiling blowing agent is used together with a Inert gas injected into the plastic melt. Through the common Injection of the blowing agent and the inert gas becomes the fine cell Get foam. Very environmentally harmful propellants are used medium, such as trifluorotrichloroethane.

It is the object of the invention to propose a propellant system gene, which has good foaming properties, without risk of fire  can be processed and the ozone belt damage when used is further reduced.

The blowing agent itself should still have very low foam densities lead to represent little weight in the packaging area and around to have to use as little of the relatively expensive plastic.

The task is characterized by the characterizing features of the patent solved.

Surprisingly and unexpectedly, it was found that the injection of a blowing agent system consisting of two different blowing agents, namely 0.2 to 0.5% by weight of N ₂ and 0.5 to 2% by weight of partially halogenated compounds ( see claim 1), based on a thermoplastic mixed with talc, then leads to extremely low densities of the foam when a gaseous nitrogen is first incorporated into a plastic melt and a liquid, partially halogenated compound is injected at a predetermined distance from the first injection point, the gün actual density can only be obtained if a material blowing agent mixture pressure of at least 55 bar is also maintained in the area of the extrusion head, ie until shortly before the outlet. In addition, a temperature in the head area of at least 130 ° C when using polystyrene and at least 128 ° C when using polyethylene had to be maintained.

The extruder system for carrying out the method shows in in some cases downstream an injection device for a gaseous blowing agent and at a distance of at least 2 D (D = screw diameter) an injection device for a river sig propellant. This distance is very advantageous to one to achieve good and homogeneous mixing of the gaseous blowing agent  chen. In the art that already contains the gaseous propellant melt can then be expected in an unexpected way achieve further blowing agent enrichment when a liquid blowing agent medium is used without it when mixing the second river propellant to adverse effects (segregation, etc.) is coming.

In the drawing, an extruder is shown, which is used for the experimental implementation was used.

Fig. 1 shows a perspective view of the extruder,

Fig. 2 shows a schematic longitudinal section of the ex truder.

In the extruder 1 , a screw 2 is arranged, which is set in rotation by a Ge gear 3 and the material filled through the hopper 4 promotes in the direction of the head 5 .

Gaseous N _{2 is} injected through the propellant injection nozzle 6 and liquid R 22 is injected into the extrusion chamber B through the injection nozzle 7 at a pressure exceeding the pressure in chamber 8 .

Known metering devices 9 are used both for the nitrogen (N ₂ ) and for the chlorodifluoromethane (R 22 ).

Temperature channels 10 are arranged in the head and are connected to a temperature control device, not shown.

By means of pressure sensors 11 , the pressure in the space 12 of the head 5 is measured ge. The pressure in space 12 can be influenced by a corresponding design of the flow channels and by an axial adjustment of the mandrel 13 , according to the arrow.

The pressure in the extrusion space 8 is detected by pressure sensors 14 and passed on.

When carrying out the search described in Examples 3-12, there was a pressure of 150 to 350 bar in the extrusion chamber 8 in the area of the injection nozzles 6 and 7 . At the end of screw 2 , the pressure was set to approximately 100-200 bar. Material blowing agent mixture pressure was set in the head at 80-180 bar. If this pressure range is left in the head, the foam results, ie the degree of foaming of the film produced, are insufficient.

The temperature of the mixture in the extrusion room B was in the range of injectors max. 240 ° C. Then the temperature became Screw tip set to 130 ° C. If this temperature was reduced, the quality of the foam achieved was no more justifiable.

Comparative examples are listed below.

Example 1 (prior art)

99.5 kg of polystyrene mixed with 0.5 kg of talc as a pore regulator substance were metered into an extruder, as shown in FIG. 1, and melted.

0.3% by weight of nitrogen (N ₂ ) in gaseous form (gas volume 0.327 m ³ ) was metered into the polystyrene melt through the injection nozzle 6 and mixed homogeneously therewith.

Then the mixture was extruded into a tube.

The density was 350 kg / m ³ with a hose wall thickness of 1 mm. The density is too large, so that the foam can only be used to a limited extent.

Example 2 (prior art)

In a comparative experiment with 0.5% by weight of nitrogen (N ₂ ) (gas volume 0.545 m ³ ), a density of 200 kg / m ³ , with a thickness of the hose wall of 2 mm, was achieved under otherwise identical process conditions.

This value is also only acceptable to a limited extent.

If the nitrogen content was further increased, it deteriorated the density of the space is increasing rapidly, presumably because of a propellant is present (i.e. the plastic no longer took the blowing agent on).

Example 3 (according to the invention)

99.5 kg of polystyrene in granular form were again in an extruder which entered together with 0.5 kg talc as pore regulator substance and melted.

Were in the melt,
0.5% by weight of gaseous nitrogen (N 2 ), which gives a gas volume of approx. 0.545 m ³ at 373 K and which makes up 61% of the total gas volume of the total amount of blowing agent (1.5% by weight),
dosed.
Then 1% by weight of liquid chlorodifluoromethane (R 22 ), which gives a gas volume of 0.355 m ³ at 373 K and accounts for 39% of the total gas volume of the total amount of blowing agent,
metered in and mixed homogeneously with it in the following part of the extruder.

A material blowing agent pressure of 60 bar was set in the extruder head by an axial adjustment of the mandrel 13 and by the design of the flow channels.

At the same time, the head was tempered by means of a Oil temperature control unit a temperature of the material blowing agent kept at 163 ° C.

The mixture was then extruded into a tube that was cut open and laid flat.

The tube thickness was 1.5 mm. A density of 161 kg / m ^{3 was} achieved, which can be regarded as an acceptable value for many applications.

Through further experiments it was found that when using a other thermoplastic, namely polyethylene, the head temperature could be reduced to just below 130 ° C without the Er influence the results noticeably.  

Example 4 (according to the invention)

The experiment as described in Example 3 was repeated, but with the difference that
1.5% by weight of liquid chlorodifluoromethane (R 22 ) (gas volume 0.532 m ³ at 373 K) and 49% of the total gas volume of the total amount of blowing agent of 2% by weight,
was used.

The thickness of the foam wall was 1.7 mm, but the spatial weight was 132 kg / m ³ , which must be regarded as a good value.

Example 5 (according to the invention) both blowing agents max. Encore

Another experiment, as described in Example 3, was carried out, but the proportion of R 22 was increased.

There were 2 wt .-% liquid chlorodifluoromethane (R 22 ) (gas volume 0.710 m ³ at 373 K) gas volume fraction 57% of the total amount of blowing agent of 2.5 wt .-%,
dosed.

The wall thickness of the foam sheet was 2 mm.

The density achieved was
90 kg / m ³ ,
which is an absolutely good value.

Example 6 (maximum N ₂ addition, R 22 minimum)

The experiment as described in Example 5 was repeated, but with the difference that
0.5% by weight of CHClF ₂ (R 22 ), which gives a gas volume of approximately 0.177 m ³ at 373 K and which accounts for 25% of the total gas volume of the total amount of blowing agent of 1.0% by weight,
was dosed.

The foam film thickness was 1.5 mm and the density was 145 kg / m ³ .

Example 7 (minimum N ₂ addition, maximum R 22 )

The experiment as described in Example 5 was repeated, but with the difference that
0.2% by weight of nitrogen (N ₂ ), gas volume 0.218 m ³ at 373 K, which corresponds to 24% of the total gas volume of the total amount of blowing agent of 2.2% by weight,
was dosed.

The values for the foam thickness were 1.4 mm and for the density 180 kg / m ³ .

Example 8 (minimal N ₂ addition, minimal R 22 )

A further attempt was carried out as described in Example 7, but the proportion R 22 was reduced. 0.5% by weight of CHClF ₂ (R 22 ), gas volume 0.177 m ³ at 373 K, which corresponds to 45% of the total gas volume of the total amount of blowing agent of 0.7% by weight,
dosed.

The result was a very heavy foam which was hard and brittle and had a foam thickness of 0.7 mm and a density of approx. 500 kg / m ³ .

Example 9 (according to the invention)

The experiment as described in Example 3 was repeated, but with the difference that
1.0 kg of liquid CHCl ₂ -CF ₃ (R 123 ) (1% by weight), which gives a gas volume of approx. 0.200 m ³ at 373 K and which represents 27% of the total gas volume of the total amount of blowing agent of 1.5% by weight. % matters
was dosed.

The thickness of the foam wall was 1.3 mm and the density was 160 kg / m ³ . Overall, the foam was somewhat harder and more brittle than in Example 3.

Example 10 (according to the invention)

The experiment as described in Example 3, instead of with R 22,
1.0 kg of liquid CH ₂ F-CF ₃ (R134a) (1% by weight), which gives a gas volume of approximately 0.301 m ³ at 373 K and which is 35% of the total gas volume of the total amount of blowing agent of 1.5% by weight. % matters
repeated.

With a foam thickness of 1.5 mm, a density of 148 kg / m ^{3 was} achieved, which can be regarded as a good value.

Example 11 (according to the invention)

The experiment as described in Example 3 was repeated with the difference that for the R 22 ,
1.0 kg of liquid CH ₃ -CCl ₂ F (R 141 b) (1% by weight), which gives a gas volume of approx. 0.263 m ³ at 373 K and which represents 33% of the total gas volume of the total amount of blowing agent (1.5% by weight) .-%)
was dosed.

With a foam thickness of 1.4 mm, a density of 155 kg / m ^{3 was} obtained. The foam quality is similar to that of Example 3 with R 22 , but with improved spatial density.

Example 12 (according to the invention)

A further experiment was carried out as described in Example 3, but with the difference that instead of R 22 ,
1.0 kg of liquid CH ₃ CClF ₂ (R 142 b) (1% by weight), which gives a gas volume of approx. 0.304 m ³ at 373 K and which accounts for 36% of the total gas volume of the total amount of blowing agent (1.5% by weight). %)
was dosed.

A foam wall of 1.6 mm was obtained with a density of 155 kg / m ³ , which can be regarded as a good value.

Reference symbol list

1 extruder
2 snail
3 gears
4 funnels
5 extruder head
6 propellant injector
7 injector
8 extrusion space
9 pressure build-up device
10 temperature channel
11 pressure sensor
12 room
13 thorn
14 pressure sensor

Claims (9)

1. A process for producing a foam from a thermoplastic, comprising the following process steps:

• - Mixing the plastic with a pore regulation substance;
• - filling this mixture into an extruder;
• - Plasticizing, mixing and homogenizing this mixture in the extruder;
• - Injection of a first blowing agent in the form of gaseous nitrogen in the homogenized mixture in the extruder, the amount of nitrogen supplied corresponds to 0.2% to 0.5% of the weight of the mixture of the plastic and the pore regulating substance;
• - Mixing and homogenizing the mixture with the nitrogen to form a homogenized nitrogenous mixture;
• - Injecting a second blowing agent in the form of a liquid, partially halogenated compound into the homogenized, nitrogen-containing mixture, the second amount of blowing agent supplied corresponding to 0.5% to 2% of the weight of the mixture of the plastic and the pore regulation substance;
• - Mixing and homogenizing the mixture of the plastic, the pore regulating substance and the blowing agents in the extruder;
• Extruding this mixture through an extrusion head of the extruder, the foam being formed,
• - The pressure of this mixture in the extrusion head is at least 55 bar, and
• - The temperature of this mixture in the extrusion head is kept above a previously determined and dependent on the thermoplastic plastic lower limit temperature.

2. The method according to claim 1, characterized in that liquid, partially halogenated compounds of the group CHClF ₂ (R 22 ), CHCl ₂ -CF ₃ (R 123 ), CH ₂ F-CF ₃ (R 134 a), CH as the second blowing agent ₃ -CCl ₂ F (R 141 b) or CH ₃ -CClF ₂ (R 142 b) can be used. 3. The method according to claim 1, characterized, that talc is used as the pore-regulating substance. 4. The method according to claim 1, characterized, that use polystyrene as a thermoplastic det, and that the temperature of the mixture in the extrusion head is higher or is equal to 130 ° C.   5. The method according to claim 1, characterized, that use polyethylene as a thermoplastic det, and that the temperature of the mixture in the extrusion head is higher or equal to 128 ° C. 6. extruder system for performing the method according to claim 1, consisting of a hollow cylindrical, two downstream successively arranged injection areas having housing in which a rotating, drivable homogenizing screw is arranged, characterized in that the first injection area, with an injection device ( 6 ) for a gaseous propellant and the second injection area is designed with an injection device ( 7 ) for a liquid propellant, and that a distance of at least 2 D (D = screw diameter) is maintained between the two areas. 7. Extruder system according to claim 6, characterized in that for a pressure adjustment in the head ( 5 ) the mandrel ( 13 ) is axially adjustable. 8. Extruder system according to claim 6, characterized in that the extrusion head has pressure sensors ( 11 ). 9. Extruder system according to claim 6, characterized in that the head ( 5 ) and the outlet nozzle gap by means of Tem perierkanäle ( 10 ) is formed temperature controlled.