Method Of Attaching Polyvinyl Base To Pressure Vessel

Abstract

A primer is applied to the exterior surface of the bottom of a metal vessel. The vessel and primer are heated above the plastic temperature of a base made of polyvinyl chloride and, while above that temperature, are pressed into engagement with the polyvinyl chloride base. The primer preferably is partially thermosetting and partially thermoplastic. After a sufficient amount of time to achieve surface fusion between the primer and the base, but before heat can permeate the base too deeply, cooling water is poured into the vessel to rapidly cool the vessel bottom and the fusion interface.

Description

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to the attachment of bases to containers and, more specifically, to the attachment of nonmetallic bases made of, for example, polyvinyl chloride to pressure vessels, or vessels in general.

2. Description of Prior Art

Various proposals have been made for the attachment of bases of synthetic material, such as polyvinyl chloride, to metallic containers. Chief among these have been the use of adhesives. Due to base separation from the container, adhesives have not proven to be entirely satisfactory. U.S. Pat. No. 3,165,228,issued to Milton F. Kroesch and Willert O. Kugler, and assigned to the assignee of this application, discloses a solution to this problem of base attachment and this application is concerned with yet another solution to that problem.

SUMMARY OF INVENTION

An object of this invention is to provide an improved method of securing a synthetic attachment to a metallic container, more specifically, the attachment of a base made of polyvinyl chloride to metallic containers such as pressure vessels. For the achievement of these and other objects, this invention proposes that the area to which the particular attachment is to be secured be primed with a primer having both thermosetting and thermoplastic properties. The attachment is heated and heating is sufficient so that when the primed area and attachment are in engagement surface fusion with the attachment occurs. Heating is preferably achieved by heating the area to be engaged by the attachment to the desired temperature. Pressure can be applied to the contacting areas as fusion is taking place and the area of the attachment is rapidly cooled before excessive heat penetration of the attachment occurs. but after the heat has been held a sufficient length of time to achieve surface fusion.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a base and vessel at an intermediate stage in the process; and

FIG. 2 illustrates the base and vessel during the rapid cooling stage of the process.

DESCRIPTION OF PREFERRED EMBODIMENT

The invention will find particular application in attaching bases to pressure vessels and, therefore, will be discussed in that environment but the invention has wider application.

Pressure vessel 10 includes elongated sidewalls 12 and an ellipsoidal bottom 14. Vessel 10 is generally intended to stand upright and this requires a support base. One form of base has been an integral bumped-back bottom which consists of, for example, a concave central portion with an annular supporting ridge or an undulating-type bottom having a plurality of support feet arranged generally in a cloverleaf design. These bases are effective in standing the vessel upright, but they are not the most efficient from a pressure standpoint and require a heavier gauge bottom, as compared to the walls of the vessel, and are also more costly to fabricate. The ellipsoidal bottom provides a most efficient pressure configuration and allows a uniform wall thickness throughout the vessel. A separate, attached base permits the use of the ellipsoidal bottom. In one aspect this invention is directed to this problem of attaching a separate base and thereby permitting use of the ellipsoidal base. Over and above this problem, which is peculiar to pressure vessels, many other vessels and containers require secure attachment of a support base and one which will remain securely attached over extended use and in environments generally considered deteriorating to the base and a secure bond between base and vessel. This invention also offers a solution to those more general problems.

Base 16 is made of polyvinyl chloride (PVC) and includes an outer skirt 18, a central semispherical web 20 and a foot 22 which extends axially beyond web 20. Foot 22 can be either a continuous annular ring as illustrated or a plurality of individual, radially extending foot sections arranged in a circle.

To attach base 16 to vessel bottom 14 the vessel bottom is coated with a primer 24. The primer can be coated on, sprayed on, hot dipped or applied in anyone of a number of other forms of application.

The PVC base exbibits a temperature i.e. a plastic temperature at which the base becomes somewhat plastic an d can be made to fuse with another element. In the case of PVC this temperature is approximately 325.degree.-350.degree. F. After having applied the primer, the vessel is heated above the plastic temperature. The amount of the overheat (temperature above the plastic temperature) will depend to a large measure on the thickness of the vessel walls which is a major factor in the heat retention characteristic of the vessel; that is, the thicker the vessel wall the lower the overheat and vise versa.

This preheating of the vessel brakes the primer and prepares the vessel for fusion to the PVC base. The heated vessel, while still above the plastic temperature, is engaged with the base, the upper end of the skirt engaging the primed sidewall and the web engaging the primed vessel bottom as illustrated in FIG. 2. Heat from the vessel initially renders the area of the base engaging the vessel plastic and a fusion of the primer and PVC base occurs providing a fusion interface or primer and PVC of the base. After a predetermined time the vessel bottom and the fusion interface of primer and base is cooled rapidly to terminate penetration of heat into the base. The time between initial engagement and cooling is selected to allow sufficient time for adequate fusion but not so long as to result in excessive heat penetration which might be detrimental to the base. This time can be determined by experience. As stated above, the temperature to which the vessel bottom should be heated will depend on the thickness of the bottom wall. For a vessel having a wall thickness of approximately 0.200 inches, heating the vessel to approximately 450.degree.-500.degree. F. has provided satisfactory results.

During the fusion step it is desirable to apply a uniform pressure over the abutting surfaces of the base and vessel bottom. This pressure can be applied in any number of ways. An effective way is illustrated for use where the base 16 is hollow. A vacuum is drawn within the base vacuum apparatus 26 so that atmospheric pressure presses uniformly on the outer walls of the base and results in uniform pressure on the engaging surfaces.

cooling can be achieved effectively by pouring a cooling medium into the vessel, for example water can be used.

A number of suitable primers can be used. A primer which has provided particularly satisfactory results is one which is partially thermosetting and partially thermoplastic. This type of primer produces an effective fusion, and a primer which possesses these characteristics of being partially thermosetting and partially thermoplastic is available commercially from Michigan Chrome and Chemical Co. and is identified as C1445. This primer is a blend of thermoplastic and thermosetting resins in a mixture of solvents.

Fusion may also be further enhanced by sand blasting the vessel bottom prior to applying the primer and base. Sandblasting cleans the surface and also pits the surface which results in an anchor lock of the primer and base material with the vessel bottom. Sandblasting also cleans the attachment area and, as an alternative, the attachment area can be cleaned chemically.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

Claims

I claim:

1. A method for securing a base made from a thermoplastic material to the bottom of a generally hollow, metallic vessel comprising the steps:

coating the exterior surface of the bottom of said vessel to be secured to said base with a primer comprised of a blend of thermoplastic and thermosetting materials;

heating the bottom of said vessel to a temperature above that at which said base material plasticizers, said primer coating being baked onto said vessel during the heating step;

pressing said heated vessel, while still heated above said temperature, into engagement with said base so that the heat from said heated vessel plasticizers the surfaces of said base engaged by said heated vessel; and

rapidly cooling said vessel, after said heated vessel and said base have been engaged a sufficient time to produce a fusion interface between the surface of said base and said primer coating, to prevent the further heating of said base beyond said surface of said base.

2. The method of claim 1, wherein said base is made of polyvinyl chloride.

3. The method of claim 1 wherein said base is hollow and said uniform pressure is applied by drawing a vacuum within said base when said base is engaged with said vessel bottom and said base is pressed into engagement with said vessel bottom by atmospheric pressure on said base.

4. The method of claim 1 wherein a cooling medium is introduced into vessel to cool said vessel bottom and said interface.

5. The method of claim 2 wherein said coated vessel is heated to a temperature in the range of about 450.degree. to about 500.degree. F. and the plastic temperature of the material of said base is in the range of about 325.degree. to about 350.degree. F.

6. The method of claim 2 wherein said base is hollow and a vacuum is drawn within said base when said base is engaged with said vessel and said base is pressed into engagement with said vessel by atmospheric pressure on said base.

7. The method of claim 2 wherein said vessel bottom is treated for receipt of said primer prior to application of said primer thereto.

8. The method of claim 7 wherein said treatment is sand blasting.

9. The method of claim 1 wherein a uniform pressure is applied over the abutting surfaces of said vessel bottom and said base.