Patent GB-690285-A (GB)
Title: Process for the surface treatment of flexible sheets of thermo-plastic materials
IPC: B29C43/22
Filed: 1950-05-01
Abstract: Flexible sheets of thermoplastic materials already produced by rolling and calendering are given a surface treatment by a process wherein the flexible sheets are subjected to a powerful pressure in direct contact with hot rigid engraving plates having the finish it is desired to impart, the pressure at the beginning of the process being kept low only in order to maintain the contact with the surface of the engraving plates, such contact being maintained under a low pressure for a certain space of time during which heating is effected to the temperature required for the flexible sheets to acquire a sufficiently soft condition to allow engraving by pressure, the pressure being then increased and maintained at the said temperature for a determined period and the plates being finally cooled while the high pressure is maintained. The plates may be of metal or of a non-metallic material such as glass or crystal.  Metallic plates may be prepared to give the desired finish to the plastic material by treating them with acids, or by electrolytic treatment, or mechanical treatment such as sand blasting, engraving or polishing.  It is stated that glass or crystal plates may easily be prepared to give the desired finish by various chemical or mechanical processes and that glass plates are available with a great variety of surfaces.  To avoid fracture of glass or crystal plates the pressure should be applied through a layer or layers of flexible cushioning material, e.g. a plastic such as polyvinyl chloride, which is softer when heated than the flexible material under treatment, a higher degree of softness being obtainable by a high proportion of plasticizer. Hydraulic presses are preferred, the pressure plates of which are provided with bores for the circulation of steam and cooling water, for temperature control.  An example is quoted in which flexible sheets of polyvinyl chloride 0.5 mm. thick are treated, first under a low pressure of about 8 kilogrammes per square centimetre maintained for approximately forty-five minutes while the temperature is gradually increased to about 160 DEG  C., and then under a high pressure of 40 Kg. per square centimetre for about five minutes, cooling then being applied, whilst the high pressure is maintained, until the temperature has fallen to about 40 DEG  C. The cushioning material in the example is also polyvinyl chloride, with fifty-five parts of plasticizer instead of forty-five parts as in the sheet under treatment.ALSO:Flexible sheets of thermoplastic materials already produced by rolling and calendering are given a surface treatment by a process wherein the flexible sheets are subjected to a powerful pressure in direct contact with hot rigid engraving plates having the finish it is desired to impart, the pressure at the beginning of the process being kept low only in order to maintain the contact with the surface of the engraving plates, such contact being maintained under a low pressure for a certain space of time during which heating is effected to the temperature required for the flexible sheets to acquire a sufficiently soft condition to allow engraving by pressure, the pressure being then increased and maintained at the said temperature for a determined period and the plates being finally cooled while the high pressure is maintained. The plates may be of metal or of a non-metallic material such as glass or crystal.  Metallic plates may be prepared to give the desired finish to the plastic material by treating them with acids, or by electrolytic treatment, or mechanical treatment such as sand blasting, engraving or polishing.  It is stated that glass or crystal plates may easily be prepared to give the desired finish by various chemical or mechanical processes and that glass plates are available with a great variety of surfaces.  To avoid fracture of glass or crystal plates the pressure should be applied through a layer or layers of flexible cushioning material, e.g. a plastic such as polyvinyl chloride, which is softer when heated than the flexible material under treatment, a higher degree of softness being obtainable by a high proportion of plasticizer.  Hydraulic presses are preferred, the pressure plates of which are provided with bores for the circulation of steam and cooling water, for temperature control.  An example is quoted in which flexible sheets of polyvinyl chloride 0.5 millimetres thick are treated, first under a low pressure of about 8 kilogrammes per square centimetre maintained for approximately forty-five minutes while the temperature is gradually increased to about 160 DEG  C., and then under a high pressure of 40 Kg. per square centimetre for about five minutes, cooling then being applied, whilst the high pressure is maintained, until the temperature has fallen to about 40 DEG  C.  The cushioning material in the example is also polyvinyl chloride, with fifty-five parts of plasticizer instead of forty-five parts as in the sheet under treatment.