Method for Making Pipes, Plates or Containers From Plastic Material

The present invention relates generally to plastic pipes, plates and containers, and more specifically, to a process for manufacturing the same.

Pipes and containers made of chemical resisting plastic materials which are defined for use in the chemical industry are usually made in accordance with the so-called winding method by helically winding a mandrel with one or a plurality of layers of a flat band made of plastic material in such a way that the adjacent edges of the flat band or the flat bands which overlap or engage the adjacent edges are welded together to a pipe with a smooth inner surface by pressing the plastic material together in a softened condition, for example, so that the pipe can be removed from the mandrel after cooling.

The winding method may be basically used for making pipes and plates made of polyvinylidene fluoride (PDVF) plastic material. From the economic point of view limits are set for the use of this material, since this material with a density of 1,78.multidot.cm.sup.-3 is very expensive when comparing with other known mass produced plastic materials.

Since the PVDF material has an excellent thermic stability and an extremely remarkable resistancy against the different chemical influences with low temperatures, as well as high temperatures and has an additional high life span resistancy with respect to the known mass produced plastics, there is considerable interest to economically make, in particular large volume pipes and elements for specific uses in the chemical industry from the material PVDF.

It was tried to reinforce a pipe or a plate made from PVDF plastic material with a wall thickness of 4 mm by means of glass fiber reinforced plastic material, thus providing a final product by combining more expensive and less expensive materials. However, such a solution has been shown of not being usable since such structures made of PVDF and GFK plastic material are used in the chemical industry, in particular in temperature ranges of about 343.degree. K. to 413.degree. K. or about 70.degree. C. to 140.degree. C., respectively, and are exposed to partly very high alternating temperatures of 293.degree. K. to about 413.degree. K., that means, from about 20.degree. C. to 140.degree. C. during the operation. It had been shown that the lifespan and the durability of such a material combination are considerably impaired due to the high alternating temperatures. This becomes clear, in particular in that the PVDF plastic material has a mean linear thermic expansion coefficient of 12.multidot.10.sup.-5 K.sup.-1, while this value is at about 3.6 to 2.0.multidot.10.sup.-5 K.sup.-1 with the glass fiber reinforced polyester resin. The expansion and above all the shrinking of the PVDF plastic material may be 3 to 6 times of the corresponding values of the glass fiber reinforced polyester resin, so that the GFK layer is expanding due to the stronger expansion characteristics and shrinking characteristics of the PVDF plastic material and detach from the thermoplastic inner liner made of PVDF plastic material, after a certain time period.

Against the foregoing background, it is an object of the invention to provide a method for making a pipe or a hollow element from thermoplastic plastic materials, for use in the chemical industry, which on the one hand has the required lifespan resistancy with respect to chemical influences and with respect to temperature alternating stresses and on the other hand can be made in a commercially feasable manner.

Due to the fact that the PVDF plastic material is at first wound around a heated mandrel in forms of bands or strips in accordance with the winding method, that the adjacent edges of the PVDF band overlap and combine with each other, due to pressure, one can use a PVDF band of a low thickness, for example, 2 to 5 mm and can make a commercial inner liner on the pipe or container tobe made which is resistant with respect to the most inorganic acids and base, whereby this inner liner is absolutely sealed due to the mutual welding of the adjacent edges of the helical shaped wound PVDF band and due to the subsequent winding with a polyester web and with an outer jacket made from the substantially less expensive polypropylene (PP) material an ideal support jacket is obtained, whereby the inserting of the polyester web as an adhesive takes care in a substantial manner that shearing forces which occur do not only have an effect on the different plastic layers but are absorbed by the total pipe or container wall. Thereby, the linear expansion coefficient of the polypropylene deviates only slightly with a value of from 15.multidot.10.sup.-5 K.sup.31 1 from the median linear thermic expansion coefficient of the PVDF of 12.multidot.10.sup.-5 K.sup.-1 which eliminates the danger of detachment of the PVDF inner liner from the PP-outer jacket, even at very high temperature alternating stresses.

A particular intensive connection between the inner liner made of PVDF plastic material, the polyester web used as an adhesive and the support jacket made of polypropylene plastic material can be surprisingly obtained in accordance with a further method of the invention in that the mandrel is heated to a temperature which is higher than the melting temperature of the PVDF crystals of 451.degree. K. or 178.degree. C., respectively, during the winding operation.

In order to balance eventual differences in the shrinking and expansion characteristics of the combined material at extreme temperature alternating stresses by an additional heating or cooling one can, in accordance with the invention and with an additional operating step wind extruded polypropylene onto the total polypropylene surface of the winding element which can then be wound with a closed outer layer made of a PP band.

The foregoing and still further objects and advantages of the present invention will be more apparent from the following detailed explanation of the invention in connection with the accompanying drawings wherein:

FIG. 1 is a schematic side view of a device for helical like winding of a pipe or container from a plurality of layers of plastic strips or bands which are discharged from jet nozzles of plastic extruders,

FIG. 2 is a plan view of the device of FIG. 1,

FIG. 3 is a partially broken, fragmentary view illustrating a polypropylene pipe wound and welded into a plastic pipe or container formed with the device of FIG. 1;

FIG. 4 is a schematic longitudinal sectional view through the pipe or the container in accordance with sectional line IV--IV of FIG. 3.

Referring now to FIGS. 1-4, which together illustrate schematically one embodiment for carrying out the inventive method, a winding device is shown.

The winding device shown in the drawing essentially consists of a hollow cylinder mandrel 1 which is supported by rollers 2 of which at least one drives the mandrel 1 in the direction of arrow 1a of FIG. 1. Rollers 2 are mounted on a carriage 3 which is moveable in the longitudinal axis of mandrel 1 and may be displaced with a speed which is synchronized to the winding speed and the width of a plastic material strip 4 which consists of polyvinylidene fluoride (PVDF) and is fed from an extruder nozzle 5 of a PVDF extruder 6.

The mandrel 1 which is driven in the direction of the arrow 1a receives the band like strip 4 of PVDF plastic material which discharges from the extruder nozzle 5 of extruder 6, whereby the plastic material strip 4 is pressed against the circumference of mandrel 1 by a pressure device 7.

On the opposite side of the winding mandrel 1 there is one each feeding device 8 for a polyester web band 9 and for a further plastic material band 10 which discharges from nozzle 11 of a second plastic material extruder 12 and consists of polypropylene (PP) plastic material. The polyester web band, as well as the polypropylene band 10 are pressed against mandrel 1 by one each pressure device 13 or 14, respectively.

As can be seen in FIG. 2, the feeding device 8 for the polyester web band 9 is so arranged on the circumference of mandrel 1 that the polyester web band 9 is fed onto the outside of the already mounted PVDF band 4 after about 1.5 rotations of mandrel 1 and is subsequently covered by the subsequent fed polypropylene band 10.

As shown in FIGS. 3 and 4, polypropylene pipes 15 with an outer diameter of about 20 mm to about 50 mm may be wound and welded onto the total polypropylene surface of the wound hollow element made in accordance with the method of FIGS. 1 and 2, in a further operating step. The pipes 15 are open on both ends and are endlessly connected with each other. Suitable armatures are mounted on the two pipe ends, so as to connect cooling or heating cycles. Thereby, it is possible to either cool or heat the pipe or plate which is made from the PVDF band 4 which is reinforced by the polyester web 9 and the outer mounted polypropylene band 10. Thereby, a balanced expanding and shrinking even at high temperature alternating stresses is assured and the deviating expansion coefficient of the polypropylene plastic material with respect to the expansion coefficient of the PVDF can be balanced in a simple manner. Subsequently, pipes 15 are wound with a polypropylene band 16 which forms a closed outer skin and protects the pipes 15 against damages.

While the invention has been illustrated and described as embodied in a method for making pipes, plates or containers from plastic material, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.