Method For Producing Hollow Articles In The Blow Molding Process With Reduced Cycle Time

Abstract

Provided is a method for producing hollow articles in a blow molding process with reduced cycle time and using a nucleation agent to foam a thermoplastic plastic with gas bubbles prior to blow forming. A nucleation agent is provided, which has substances that emit gas bubbles when a decomposition temperature is exceeded. When a mixture of thermoplastic plastic and the nucleation agent are heated, the decomposition temperature is exceeded. A supercritical fluid state of the gas bubbles emitted from the substances is maintained by having supercritical pressure values elevated relative to ambient pressure while continuously heating the mixture. Blow molding of a hollow article is carried out in a blow molding apparatus at the supercritical pressure values of the thermoplastic plastic pre-foamed with homogeneously distributed gas bubbles, at a softening or melting temperature of the thermoplastic plastic below a plastic-specific softening or melting temperature of the thermoplastic plastic.

Claims

1. A method for producing hollow articles in the blow molding process with reduced cycle time and using a nucleation agent that foams a thermoplastic plastic with homogeneously distributed gas bubbles prior to blow forming, wherein the method has the following steps: providing the nucleation agent, the nucleation agent having substances that emit gas bubbles when a decomposition temperature is exceeded; and heating a mixture consisting of thermoplastic plastic and nucleation agent to a temperature exceeding the decomposition temperature of the nucleation agent; characterized in that a supercritical fluid state of the gas bubbles that have formed is maintained by means of supercritical pressure values elevated relative to ambient pressure while heating of the mixture is continued and a blow molding into a hollow article is carried out in a blow molding apparatus at the supercritical pressure values of the thermoplastic plastic that has been foamed with homogeneously distributed gas bubbles, at an ambient temperature of the thermoplastic plastic that is reduced to below a plastic-specific softening or melting temperature of the thermoplastic plastic.

2. The method according to claim 1, wherein the method has a reduced cycle time as compared to methods working with pressure values below the supercritical pressure values.

3. The method according to claim 1, wherein as the nucleation agent for generating homogeneously distributed gas bubbles, the nucleation agent used to generate gas bubbles of carbon dioxide (CO.sub.2) when the decomposition temperature is exceeded comprises constituents of Na.sub.2CO.sub.3 (sodium carbonate) and monosodium citrates (C.sub.6H.sub.7NaO.sub.7).

4. The method according to claim 1, wherein as the nucleation agent for generating homogeneously distributed gas bubbles, the nucleation agent used to generate gas bubbles when the decomposition temperature is exceeded comprises sodium hydrogen carbonate, potassium carbonate, or ammonium carbonate.

5. The method according to claim 1, wherein the nucleation agent comprises as acid carrier, and wherein the acid carrier is selected from the group consisting of monosodium citrate (C.sub.6H.sub.7NaO.sub.7), sodium acid pyrophosphate (Na.sub.2H.sub.2P.sub.2O.sub.7), citric acid (C.sub.6H.sub.8O.sub.7), potassium hydrogen tartrate (KC.sub.4H.sub.5O.sub.6), and calcium dihydrogen phosphate (CaHPO.sub.4).

6. The method according to claim 1, wherein, as the nucleation agent for generating homogeneously distributed gas bubbles, organic substances are used to generate gas bubbles when the decomposition temperature is exceeded.

7. The method according to claim 6, wherein the organic substances comprise azodicarbonamides.

8. The method according to claim 6, wherein the decomposition temperature of the organic substances is lowered by accelerators.

9. The method according to claim 1, wherein the nucleation agent is admixed with a foaming agent for producing foamed hollow articles in the blow molding process, and wherein the foaming agent comprises a fatty acid ester.

10. The method according to claim 9, wherein the fatty acid ester is a glycerol monostearate or a glycerol stearate.

11. The method according to claim 10, wherein the fatty acid ester is a glycerol monostearate.

12. The method according to claim 1, wherein the nucleation agent is admixed with a foaming agent for producing foamed hollow articles in the blow molding process, wherein the foaming agent is also an antistatic agent comprising ethoxylated amines and/or alkyl sulfonates.

13. The method according to claim 12, wherein the ethoxylated amine has the following formula: ##STR00002## wherein R represents an alkyl radical with 10 to 18 carbon atoms and n represents the total number of moles of ethylene oxide, where n corresponds to 2 to 15 moles.

14. The method according to claim 1, wherein the blow molding apparatus, including an extruder, a melt pump, and supply hoses at the mold, is kept at a supercritical pressure for the gas bubbles of greater than or equal to 74 bar or steadily increasing pressure of greater than or equal to 74 bar and at supercritical temperatures greater than or equal to 31 C.

15. The method according to claim 1, wherein an average particle size of a nucleating agent is below 0.05 m, and wherein an average particle size of active components is between 15 mk45 m.

16. The method according to claim 1, wherein a quantity of gas per plastic mass of 0.1 ml/g to 0.2 ml/g is provided for an optimum and homogeneous dispersion of the gas bubbles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 schematically shows a diagram of the physical states of the CO.sub.2 gas bubbles that are produced when the decomposition temperature of the nucleating substance is exceeded.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The invention is now explained in detail with reference to a single attached FIGURE. The sole appended FIG. 1 schematically shows a diagram of the physical states of the CO.sub.2 gas bubbles that are produced when the decomposition temperature of the nucleating substance is exceeded. To this end, the temperature T is plotted on the abscissa in K from 0 K to approximately 370 K, and additionally plotted in C. at characteristic points. The ordinate shows the pressure P in bar from 1.013 bar to approximately 100 bar. A first characteristic point at which all three classical physical states, gaseous, liquid, and solid, of the CO.sub.2 gas bubbles can occur in the physical state diagram is reached at minus 56.6 C. and a pressure of 5.2 bar.

[0032] A second characteristic point at which three additional physical states meet, and which the invention utilizes to reduce the cycle time of the blow molding process according to the invention, is at 31 C. and a critical pressure value of 73.8 bar, rounded up to 74 bar. At this characteristic point, a liquid and a gaseous physical state occur that transition into a supercritical physical state of a supercritical fluid when the supercritical pressure value of 74 bar is exceeded and when the supercritical temperature value of 31 C. is exceeded. This supercritical physical state is employed according to the invention to verify the surprising effectsunforeseeable for the person skilled in the arton the cycle time with unchanged compositions of the substances of the blow molding process.