| 摘要 |
<p>1. Process for the manufacture of bioriented hollow bodies made of thermoplastic, comprising firstly the manufacture of a tubular blank, a parison or a preform, covered with a barrier material based on polyvinyl alcohol, and then the biaxial stretching-blowing of the said tubular blank, parison or preform under the customary temperature conditions for biorientation, the said process being characterised in that the manufacture of the covered tubular blank, parison or preform involves the following series of steps : (a) the introduction, into an extruder, of flakes consisting of polyvinyl alcohol having a high degree of purity, and of water, the water content of the said flakes being between 20% and 50%, (b) the fluxing, by shearing, and the melting of the flakes, at a temperature at least 100 degrees C above the equilibrium melting point of the polyvinyl alcohol/water mixture constituting the flakes so as to avoid any phase heterogeneity in the molten mass, (c) the extrusion of the molten material through a covering die suitable for coating a tube, a parison or a preform made of thermoplastic, care being taken to ensure, by cooling, that the temperature of the molten extrudate in the die has a value which is at least equal to the equilibrium melting point of the polyvinyl alcohol/water mixture in question, but is below the temperature at which the vapour pressure of the water is sufficient to cause the expansion of the intended covering, in the open air, by the formation of bubbles, the said cooling being carried out homogeneously, before the molten extrudate passes through the die, by means of a device consisting of a cooler-homogeniser interposed between the end of the extruder barrel and the covering die, (d) the taking-up of the tube, parison or preform covered in this way, followed, in the case of a tube, by its conversion to tubular blanks, parisons or sections of tube which are precursors of preforms, by cutting to the desired length.</p> |
