DE3409319C1 - Process for producing foamed plastics mouldings - Google Patents

Abstract

Rigid and dimensionally stable foamed plastics mouldings having an outer zone of relatively high density and a core of relatively low density are obtained if a liquefied gas is introduced into a prefoamed preform.

Description

H, it must be ensured that the first Injection a gas phase arises, i.e. the Injection needle must be a have sufficiently high heat storage capacity so that the liquefied gas is verdainpfl. If the heat storage capacity of the needle is too low, the injection needle can also be preheated accordingly.

I) ic number of injection needles depends on the form and the size of the molded body to be produced.

Advantageously, as many needles are provided as appears necessary, since they are spaced 5 to 10 ciii apart. Also received the needles advantageously have multiple bores to match the number of outflow points to increase.

The main liquefied gases are nitrogen and argon into consideration, but other gases, such as low-boiling fluorocarbonslolfc, be used. Preferably nitrogen is used.

It is particularly advantageous for the method according to the invention especially for the production of plates, rods and the like, a so-called Use "breathing form". "Breathing form" is understood to mean a form, which yields under the pressure of the expanding preform and its void volume enlarged to the volume of the molding. By means of such a breathing form moldings with an extremely open-cell core are obtained. The method according to the invention can be carried out not only discontinuously, but also continuously, the strand emerging from the extruder successively of concomitant molds is enclosed, while also wandering hypodermic needles the cryogenic Bring gas into the strand.

The shapes and needles then return to the starting point.

The moldings produced by the process according to the invention are particularly suitable for insulation technology, where they can be used, for example, as plates, Rods, pipe jackets can be used.

Example 1 An initially pre-foamed, thick-walled solid rod was produced extruded. A commercial polypropylene with a density of 0.905 g / ciii3 and an MFI 230/5 of 9.0 / 10 min Usual chemical blowing agent in an amount of 0.3 to 0.8%, calculated as an active substance, mixed in. Melting and homogenizing the mixture took place in an extruder (screw diameter 45 mm / 20D) using a Short compression screw, where-the temperatures of the cylinder zones of the feed hopper up to the nozzle were 220 ° / 225 ° / 200 ° / 200 ° / 180 ° / 180 ° / 180 ° C.

This resulted in a melt temperature of 1970 C.

The extruded fine-pored preform was in portions into the In the Fig. 1 shown shape (1) introduced, which to 60 to 70% of the cavity volume was filled by the pre-expanded thermoplastic mass (2). The mold temperature was 35 ° C.

After the mold (1) was closed, an injection needle (3) was inserted into the thermoplastic mass (2) introduced. C. Flows through the injection needle (3) cryogenic stickstolf (pressure 10 bar) from a - not shown - cold failure in the thermoplastic mass (2) and inflate them, so that molded body (4) according to F i g. 2 (longitudinal section) and 3 (cross section 111-111) with a dense outer skin (5) and a core (6) with a kind of lattice structure.

The molded bodies had a density of 0.45 to 0.55 in the core area g / cm3, in the edge zone area approx. 0.80 to 0.90 g / cm3 and in the area in between one Density from 0.65 to 0.75 g / cm3. The moldings had an average density of 0.65 up to 0.70 g / cin3.

Example 2 The procedure described in Example I was followed.

A commercially available propylene copolymer was used as the starting material with a density of 0.905 g / cm3 and an MFI 230/5 and 60 g / 10 min.

As a propellant, a commercially available product was used in an amount from 0.3 to 0.8%, calculated as active substance, added. Thermoplastic and blowing agent were digested as in Example 1. The temperatures along the extruder barrel were 200/200/200/200/190/180 ° C. The melt temperature adjusts to 185 ° C.

With this blowing agent, the foamed preform had a coarser one Cell structure as in example 1.

The result was as in example 1.

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Claims (2)

Claims: 1. Process for the production of foamed plastic moldings with an outer zone of higher density and a core of lower density by extrusion a foamable thermoplastic, foaming in a mold to form a macroporous one Core and a microporous outer zone by introducing an additional pressure medium in the form of an evaporating liquid into the core through at least one piercing Needle. characterized in that one pre-expanded preform egg into one There is form and that the additional pressure medium is a liquefied gas. 2. The method according to claim 1, characterized in that one gckenn breathing form used. The invention relates to the production of foamed plastic moldings with a higher density outer zone and a lower density core. It is known to produce molded bodies of this type by a type of injection molding produce, being in an injection mold with the plastic moldings defining Mold inner wall of the plastic in the form of a stream of core material and surrounding Skin material is introduced via an injection channel with an injection nozzle connected to it is - and wherein the core material inflates the skin material in the mold space and against the inner wall of the mold presses and a bubble former is introduced into the core material, that which is macroscopic in the core material under the thermodynamic conditions in the mold space Forms bubbles, and the core material provided with the bubble former in a to Injection nozzle for the skin material and injection nozzle concentric to the injection channel is guided into the area of the junction of the injection channel in the mold space, namely under thermodynamic conditions, which the bubble formation up to the injection nozzle and practically prevent it in the injection nozzle (cf. German Offenlegungsschrift 3442 136). In addition, a compressed gas or an evaporating gas can be used in the core material Liquid to be introduced through a hypodermic needle. However, it has been found that this method does not produce large can produce foamed molded body with sufficiently low weight. It is also known to use hollow plastic bodies by a blow molding process produce, in which the molding is aofge by a cryogenic liquefied gas expands and is cooled (see. German Offenlegungsschrift 22 23 580). Plastic hollow body with very thin walls, however, are not rigid and dimensionally stable, but flexible. It has now been found that rigid and dimensionally stable foams can be used Plastic molded body with an outer zone of higher density and a core of lower density Can produce density if you put a liquefied preform in a pre-expanded preform Introduces gas. The subject matter of the invention is thus what is described in the claims Procedure. For the production of the foamed plastic moldings according to the Processes according to the invention are suitable for all thermoplasia that are caused by propellants foam, for example LD-, MD- and HD-oil, polypropylene, I> olystyrene, polyvinyl chloride, polyacetal. LD, MD and HD polyethylene are preferred and also poly (propylene) used, in particular polypropylene. Suitable blowing agents for the pre-expansion are those for the aforementioned Plastics commonly used, for example those that are at temperatures decompose from about 120 to 180 "C. These include the aromatic sulfohydrazides, the aliphatic sulfohydrazides and the aralkyl sulfohydrazides, such as. B. benzenesulfohydrazide. Toluene-bis-sulfohydrazide, xylene-bis-sulfohydrazide, biphenyl-bis-sulfohydrazide, p, p'-oxi-bis-benzenesulfohydrazide, Methanesulfohydrazide, isobutanesulfohydrazide, octanesulfohydrazide, α-toluenesulfohydrazide, Xylene-bis-sulfohydrazide and the like, the aromatic sulfosemicarbazides, the aliphatic Sulfosemicarbazides and the aralkylsulfosemicarbazides, such as benzenesulfosemicarbazides, Toluene-bis-sulfosemicarbazide, xylene-bis-sulfosemicarbazide, biphenyl-bis-sulfosemicarbazide, p, p'-Oxibis-benzenesulfosemicarbazide, methanesulfosemicarbazide, isobutanesulfosemicarbazide, Octanesulfose micarbazide, toluoisulfose micarbazide, xylene-bis-sulfose semicarbazide, and the like; N-nitroso compounds, such as. B. N, N'-dimethyl-N, N'-dinitrosoterephthalamide and N, N'-Dinitroso-penta-methylenetetramine and azo compounds, such as azodicarbonamide, also systems based on citric acid / bicarbonate. As propellants are special p, p'-Oxi-bis-benzene-sulfohydrazide, azodicarbonamide and mixtures thereof as well as citric acid / bicarbonate month usable. The amount of the propellant is 0.1 to 1.0, preferably 0.3 to 0.80 / 0, calculated as the active substance. In addition to the propellant, the plastics also contain the common additives such as non and heat stabilizers, pigments, lubricants, antistatic agents, Flame retardants and the like. In addition, the plastics to be used still contain fillers in the usual and required amounts, for example Calcium carbonate, chalk, talc, glass spheres, silicates of magnesium and / or aluminum, Clay, soot, Iiolzniehl. The one provided with the propellant and any other additives Thermoplastic is melted in an extruder in such a way that the thermoplastic containing propellant decomposes and the gas that forms in the melt remains dissolved under high pressure, so that the thermoplastic only outside of the extruder schiiumt. The mass strand is cut to length and the preforms in forms, their # Cavity corresponds to the shape of the intended molded body, filled. The pre-expanded mass only fills part of the mold cavity, namely approx. 50 to 80%. Immediately after closing the mold, at least an injection needle inserted into the thermoplastic mass and a liquefied one Injected gas. The liquefied gas heats up inside the injection needle and goes into the required gas phase. The gas volume seeks its way through uncontrolled Tearing open the fine cell structure in the pre-expanded thermoplastic mass Path. The result is a cracked grid-cell structure, which is influenced by p, v and t will. During this process, the pressure in the mass increases and thus pushes the thernioplast on the forin wall. The cooling process begins.