DE19531631A1 - Prodn. of thermoplastic foam with syntactic foam structure - Google Patents

Abstract

A process for the prodn. of thermoplastic foam with a syntactic foam structure on extruders or injection moulding machines from commercial bulk plastics with melting or softening temps. of 375-450 K (I), such as LDPE, HDPE, high-mol. wt. EVA copolymers with vinyl acetate (VAC) contents of less than 30%, PP or PS, using gas- or liq.-filled microspheres which expand on heating. The novelty is that 5-50 wt% of soft and/or low- melting plastic or unvulcanised rubber (II) is added to (I) before extrusion or injection moulding.

Description

The invention relates to a method for producing thermoplastic foam with syntactic foam structure using extrusion or injection molding machines commercially available bulk plastics with melting or expansion temperatures between 375 K and 450 K, such as LD-PE, HD-PE, high molecular EVA copolymers with vinyl acetate content below 30%, PP or PS, using gas or liquid filled, thermoplastic and expandable under the effect of heat rer microballoons.

To achieve foam structures in thermoplastic Plastic parts or profiles are two basic Procedures have long been common. The first is pressurized low-boiling liquids or gases to be pressed into the viscous melting phase of the matrix plastic and distribute. The second approach is in gases inside the matrix materials (plastics, To produce rubbers) in that thermally unstable Solids are incorporated under the influence of heat during processing at least in part by chemical The reaction breaks down into gases. Received in both procedures the foam structure in that the pressurized enclosed gases relax and the matrix material inflate. The result is binary foam structures that  from the plastic matrix and the enclosed gases consist.

In order to achieve certain foam properties, you must technological and material conditions are created, under which the propellant gases in the molten, possibly also in Stage of crosslinking or vulcanization, Matrix materials kept to form a cell structure will. This is usually very technical wall connected because the propellant gases tend to come out of the matrix materials to escape or become energetically more favorable unite large cells and the most desired high Leaving degree of distribution with many small single cells.

To avoid these disadvantages, one has tried instead of additives mentioned above or in addition to these additives Add so-called microballoons to the starting material. This Microballoons made of thermoplastic are included low-boiling liquids or pressurized Filled with gases, e.g. B. pentane or butane (DE-OS 14 95 485). The Envelopes of this hollow plastic body consist of special diffusion resistant thermoplastic polymers such as Polyvi nylidene chloride, polyacrylonitrile or combinations thereof diffusion resistant polymers.

These gas-filled microballoons are used to manufacture Fabrics with a syntactic foam structure applied (plumbers, Frisch, Handbook of Polymeric Foam Technology, Chapt. 16, Carl Hanser Verlag 1991). Such foams consist of Matrix plastic, the gas trapped in the microballoons and the diffusion-resistant, deformable and stretchable Envelopes that enclose the gas.

To produce the foams with such syntactic, structures consisting of at least three components  both pre-expanded microballoons as well Microballoons used in the unexpanded state.

In the former case, the unexpanded microbal lons expanded under certain conditions (e.g. US 4,826,094) and then into liquid reactive resins (unsaturated Polyester, epoxy resins, polyurethanes, e.g. B. US 4 843 104) or liquid coating compositions, e.g. B. for underbody protection of motor vehicles (de Walque, expanded polymer beads, rubber, Fibers, plastics 11/94 - Year 47, pp. 736-37) incorporated tet. The resins or coating compositions harden as in the usual applications due to chemical reaction or evaporation from solvents or dispersants. Solids are obtained with a syntactic foam structure that matches the microballoon Show volume proportion corresponding reduction in mass.

When using unexpanded microballoons, e.g. B. in aqueous dispersions (ethylene-vinyl acetate copolymers or Polyacrylates), PVC plastisols or printing inks Incorporation and distribution in the liquid phase and the Expansion of the microballoons by exposure to heat Coatings, whereby gelation, drying and Film formation run (documentation "Unexpanded EXPANCEL Microspheres ", Expancel Nobel Industries Sweden, 3.6.1991). This procedure is limited to such applications where a quick heat input can take place from the outside, like this with wallpaper coatings or the drying of Printing inks is the case or where due to exothermic chemical Reactions when curing reactive resins the necessary Mold heat to expand the microballoons inside the foam parts are created.

To achieve syntactic foam structures further the possibility of hollow bodies with rigid shells, for. B. Micro hollow glass spheres to be incorporated into resins.  

While using microballoons in liquid matrix material If there are no major problems, use will occur of gas-filled hollow bodies in thermoplastic materials considerable difficulties in extrusion or injection molding machines come on.

For pre-expanded microballoons, such as Belly for hollow glass microspheres that they in the extruder or in the Injection molding machine due to the high shear forces before the melt zen of the plastic will be destroyed and that the trapped then gases without the formation of the desired foam structure door escape.

Because temperatures to melt plastic always above half the expansion temperatures of the diffusion resistant thermoplastic microballoons are necessary, it was close lying, the use of unexpanded microballoons for Propose foaming process (DE-OS 34 36 592). Doing so assumed that the expansion of the microballoons through the high pressure in the plastic processing machines is suppressed and only after leaving the extrusion or spray nozzle. Contrary to these representations is the Pressure build-up during processing in the injection molding machine or Extruder is not guaranteed in all phases in which it Expanding the microballoons can come.

The material feed and melting phases are in extruders and Injection molding machines largely analog. Pressure can be in the cylinder only arise in the process sections in which a conclusion against the environmental conditions. To the exit end this is due to the extrusion tool or the spray nozzle ensures that with their relatively small opening cross cut in proportion to the amount of the snail derten melt and its viscosity to backflow the mass and thus lead to pressure build-up.  

The hermetic seal of the extruder or injection molding machine cylinder towards the feed opening is full constant filling of the volume between the screw core and Cylinder wall reached. As long as this volume still exceeded with solid, unmelted and moving against each other Plastic particles are filled, can not be complete Completion and therefore no pressure build up. First one relatively high melt content between the solid particles ensures the flameproof closure.

So that the mass melts, you have to use the cylinders from the outside wall heat can be supplied. The temperatures in this Cylinder section must be well above the melting temperature of the respective plastic (usually at least 25 K) in the short time available to achieve sufficient heat transfer to the plastic.

The lower limit for melting temperatures for molded parts The plastic used is 375 K (e.g. EVA, LD-PE).

The expansion of the microballoons begins depending on the type in the area between 350 K and 395 K. This inevitably results an overlap of cylinder temperatures in the up melting zone with the temperatures of the beginning of expansion of the Microballoons.

The heat capacity of the approximately 10 µm large microballoons is still orders of magnitude smaller than that of the approx. 0.5 mm (in the case of plastic powder) up to 5 mm plastic par article. Accordingly, they will be much faster than that Plastic particles over their entire cross section through the heated extruder barrel heated. The microballoons begin consequently, while the plastic particles are still expanding not melted at all or only superficially. As a result of Screw movement occurs between the plastic particles large shear forces on the expanding microballoons  destroy so that they are no longer for training a syntactic foam structure are available.

The previously known method (DE-OS 34 36 592) is consequently only on low-melting plastics with melting or softening ranges below 375 K applicable for normal Use as foam plastic parts are hardly relevant. For Plastics with melting temperatures above 380 K, as in Form of LD-PE, HD-PE, high molecular EVA copolymers with Vinyl acetate fractions below 30%, PP or PS generally used Lich, this procedure is not applicable.

The object of the invention is to create a Process for the production of thermoplastic foaming with syntactic foam structure using extrusion or injection molding machines using microballoons, at which can also be used in bulk plastics Melting or softening temperatures in the area of the expansion temperature of the microballoons or above this range lie.

The solution according to the invention is that soft and / or low-melting plastics or unvulcanized rubbers with proportions of 5 to 50% of the bulk plastic before the Extrusion or injection molding processing are added.

The invention provides a method with which thermoplastic, unexpanded gas filled microballoons in commonly used bulk plastics such as LD-PE, HD-PE, high molecular EVA copolymers with vinyl acetate content below 30%, PP or PS for extrusion foaming and for foam injection molding can be incorporated. The effectiveness of the foam the theoretically possible expansion gets through the microballoons close, and the structure of the formed Foam is even and across the cross section of the parts corresponds almost freely in its cell size to the dimensions expanded microballoons. When extruding the  Foam can be largely independent of the pressure in the extruder, independent of the nozzle cross-section, without additional rain temperature and without the foam extrusion usual technical measures (long screws, melt conditioning, special tool designs etc.) be worked. The extruded foam melt is free malleable and has no tendency to change shape or to Sink marks during cooling and solidification.

The method of the invention is characterized in that the to process matrix plastics, their softening or Melting temperatures can be between 375 K and 450 K, Components are mixed in before they are reached the temperatures at which the expansion of the microballoons would start under normal conditions, form melt portions, So at temperatures below 375 K. Such components can low melting, very soft plastics or uncrosslinked Be rubbers. Due to the early formation of melt components building pressure even before melting the matrix art enables the expansion of the microballoons in the fabric Extruder or the injection molding machine during existence firm, higher melting matrix plastic parts effective prevented. It also acts on shear forces the microballoons largely avoided. Farther hold the unexpanded microballoons acting shear forces far better than expanded or those who are currently in the state of expansion.

The expansion capacity of the microballoons is so for that non-shear stage after leaving the extruder or spray nozzle preserved and to train the desired Almost completely usable syntactic foam structures made.

In an advantageous embodiment of ground Matrix plastics with which the non-expand  dated microballoons and the additional components before the extru be mixed. This creates a high degree of distribution of the microballoons in the plastic before processing reached over the melt.

Further advantageous embodiments are in the Unteran sayings.

The design of the process flows after the melting phase is then largely analogous to the procedure at Processing of non-foamed plastics. When extrusion foaming can be done without the usual additional precautions of special melt conditioning (melt pressure, temperature and influencing viscosity) and also without complex consequences devices for stabilizing the foam structure after the Extrusion process. It is possible with great out tread sections of the extrusion dies to work only slight pressure build-up in the extruder, e.g. B. 10 bar. For comparison, e.g. B. for profile foam extrusion Polypropylene with chemical blowing agents approx. 180 bar required Lich, for PVC hard foam profiles approx. 300 bar.

The melt containing the microballoons expands after leaving the tool opening until the inside of the balloons Corresponding pressures prevail. The melt is constantly deformable in volume and can either be used for filling used by injection molding tools or in extrusion foaming with the relatively simple means of shaping the Profile extrusion without foam structure or using a profile Rolls are formed without subsequent deformation of the Profile cross-sections during cooling and solidification to enter through solidification.

example 1

Cylinder granules made of a polyethylene with a density of  0.918 g / cm³, a melting point of about 380 K and one Melt index at 563 K from 21 to 23 g / 600 s is 0.5% Mineral oil wetted. Then 3% are not expanded Microballoons mixed in, made from a polyacrylonitrile envelope le and about 18 wt .-% included isopentane. These microballoons start to expand at around 400 K. The Mixture is to be extruded into a syntactic foam.

If one assumes a possible degree of expansion of the microballoons from 40, a procedure according to DE-OS 34 36 592 and with full use of the expansion volume expanded microballoons in the plastic melt a foam extrudate with a density of about 0.3 g / cm³.

The mixture is in a laboratory extruder with 28 mm Screw diameter, water-cooled feed zone and uniform temperatures of 403 K in all cylinder zones and processed on the extrusion tool. The extrusion takes place through a 5 mm round nozzle.

The extrudate emerging from the nozzle has an irregular, torn surface. It swells after leaving the Shortly strongly nozzle, afterwards completely collapse irregularly. The extruded strand points insufficient strength to work with a device to be deducted continuously. He has one inside coarse-celled. Irregular and wrinkled structure.

Example 2

A powder is made from the same PE type as in Example 1 a grain size distribution of less than 315 µm. In this PE powders become 10% of a non-cross-linked ground under 2 mm ethylene-propylene-diene polymer rubber (EPDM, Shore A hardness 35) mixed in, also 2% microballoons of the same Chen type as under 1.  

This premix is made under the same conditions as processed under 1. on the laboratory extruder.

A very uniform, white strand is extruded, the yourself easily at different speeds pull off and pull out to different diameters. over the cross section has a very uniform foam structure on. The individual foam cells on a smooth cut surface are so fine that they cannot be seen with the naked eye. The density of the foam strand is 0.46 g / cm³. That means tet that the maximum possible expansion volume of the microbal lons almost completely to form a foam with syntactic structure was used.

Example 3

The same experiment as in 2. is repeated, but with 3% PAN-type microballoons, analogous to example 1.

It becomes an even and removable foam strand extruded, its homogeneous and fine cell structure with bare Eye is not recognizable. The density measurement of the foam profile yields 0.295 g / cm³, which makes it almost completely usable the expansion volume of the microballoons to form the syntactic foam structure.

Example 4

A granular ethylene-vinyl acetate copolymer with 16% VAc content, a melt index of 1.5 g / 600 s, a melt point of approx. 365 K, a specific density of 0.936 g / cm³ and a Shore A hardness of 92 comes with 0.5% mineral oil wetted. There are 3% microballoons with an envelope Acrylonitrile-vinylidene chloride copolymer mixed, the Expansion begins at around 378 K.  

This mixture is analogous to the mixture under 1., but at 400 K. Cylinder and tool temperature with the laboratory extruder processed.

The emerging foam strand is not so irregular and torn like that of Example 1. but evenly and smooth. However, the plastic is barely up at 0.85 g / cm³ foams. The microballoons are evident during the Much of the EVA granulate is melted in the extruder have been destroyed and failed to form a syntacti foam structure.

Example 5

Ethylene-vinyl acetate copolymer as in Example 4. is 30% ground EPDM rubber as in Example 2. mixed and wetted with 0.5% mineral oil. There will be 3% microballoons analogous to Example 4. with PAN / PVDC casing mixed in. Then Processing is carried out on the laboratory extruder, too analogous to example 4.

It becomes a foam strand with a very fine even Cell structure extruded, which has a density of 0.285 g / cm³ has and the rest of the properties as in Examples 2. and 3. owns.

Example 6

PE powder analogous to Example 2. with 40% of a granulate mixed, the equal parts from an LLD-PE and high molecular weight polyisobutylene. The latter Component is already highly viscous under normal conditions flowable. 2.5% of PAN-type microballoons become analog Example 2. homogeneously mixed, and there is again Processing on the laboratory extruder under the described at 1. conditions.  

Again, a strand of foam with very fine cells extruded syntactic foam structure that has a density of 0.43 g / cm³.

Example 7

Powder under 500 im with a polypropylene homopolymer a melt index of 12 g / 600 s and a melting temperature of 435 K is made with 30% EPDM rubber analogous to 2nd and 3% Mixed PAN type microballoons. The subsequent one Extrusion on the laboratory extruder for cylinder and tool parts temperatures of 455 K result in a uniform and fine lige foam strand, the outside of the foam profiles of the previous examples are the same.

The mechanical behavior after solidification is determined by the higher hardness and rigidity of the PP. The density of the Foam is 0.35 g / cm³, which is again a high grade Use of the microballoons to close the foam can. This is particularly important because of this Purpose of the invention, since the melting temperature of the PP with 435 K is far above the temperature range of approx. 400 K, in where the expansion of the microballoons begins.

The applicability also follows from this exemplary embodiment of the invention on plastics, their melting ranges clearly are above the expansion temperatures of the microballoons.

Example 8

Powder with a grain size distribution below 1000 µm from one Polystyrene with a melt index of 10 g / 600 s and one Softening range from 395 K to 405 K becomes one with 20% granular polybutadiene rubber with 30% crystalline Proportions and a melting range of approx. 355 K and 3% micro PAN-type balloons mixed according to Example 2. The subsequent extrusion on the laboratory extruder with cylinder and  Tool temperatures of 445 K lead to a uniform one Foam strand, the outside and in its cell structure Foam profiles are the same as in the previous examples.

The mechanical behavior after solidification is determined by the higher hardness and rigidity of the PS. The density of the Foam is 0.38 g / cm³, which is again a high grade Use of the microballoons for foam formation follows. So that will the applicability of the invention again for such art substances with a melting range above 380 K lies.

Example 9

On a 90 mm single screw extruder with one Length / diameter ratio of 20 will be a mix of both LD-PE powder and 15% EPDM rubber and 2% microballoons from PAN type analog 2. processed.

The speed of the screw is 120 min ^-1 , the cylinder and tool temperatures are 395 K to 405 K with a water-cooled feed zone. A constant melt temperature of 401 K occurs in front of the screw tip, the melt pressure is constantly low at 10 bar, which is unusually low for foam extrusion processes. The extruder nozzle consists of a rectangular gap 150 mm wide and 7 mm wide. The throughput is approx. 180 kg / h.

The foam extrudate emerging from the nozzle is cooled t, slow-running pair of rollers on which there is a Bead forms. There is a profile between the rollers Rectangular cross-section cut out as a gap, reflecting the dimensions 180 mm × 12 mm. After passing the profile rollers the foam profile is cooled in a water bath and with common technique cut to length.

The foam sheets produced have the same homogeneous,  fine-celled, syntactic foam structure as in the examples len 2nd, 3rd, 5th, and 6th, are completely plane-parallel and dimensionally stable. The density is 0.49 g / cm³.

This is the portability of the results from the previous examples given on a laboratory scale on technical products systems confirmed.

Claims (7)

1. Process for the production of thermoplastic foams with syntactic foam structure by means of extrusion or injection molding machines from commercially available bulk plastics with melting or softening temperatures between 375 K and 450 K, such as LD-PE, HD-PE, high molecular EVA copolymers with vinyl acetate Proportions below 30%, PP or PS, using gas or liquid filled ter, thermoplastic and thermally expandable micro balloons, characterized in that soft and / or low melting plastics or unvulcanized rubbers with proportions of 5 to 50% of the mass plastic the extrusion or injection molding processing are mixed. 2. The method according to claim 1, characterized in that the soft and / or low melting plastics or unvulcanized rubbers melting or softening own rich under 375 K. 3. The method according to claim 1 or 2, characterized in that the soft and / or low melting plastics or unvulcanized rubbers ethylene vinyl acetate Copolymers with vinyl acetate proportions equal or larger 30%, ethylene-propylene-diene polymer rubbers, Polybu tadiene, polyisobutylene or ethylene-acrylate copolymers are. 4. The method according to any one of claims 1 to 3, characterized characterized in that 0.2 to 10 wt .-% expandable Microballoons can be added.   5. The method according to any one of claims 1 to 4, characterized characterized in that 1 to 4 wt .-% expandable micro balloons are added. 6. The method according to any one of claims 1 to 5, characterized characterized by powdered or ground Bulk plastics with which they are not expanded microballoons and the additional components the extrusion are mixed. 7. The method according to any one of claims 1 to 6, characterized characterized in that in the mixtures to be processed before reaching the temperatures at which the expansion start on the microballoons under normal conditions would, fused portions are generated that have the effect prevent large shear forces on the microballoons and the allow the build up of pressures that the emergence prevent shear-sensitive expanded microballoons.