Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Infrared radiation
Patent
1998-03-20
2000-08-22
Berman, Susan W.
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Infrared radiation
522 24, 522 28, 522 60, 522 62, 522157, 522113, 523130, 264493, 264496, 264481, 264476, 2504921, 2504931, H05B 600, B29C 3508, B29C 6346, C08F 246, C08J 328
Patent
active
06106761&
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
The invention in general relates to a method for the uniform and fast heating of polymers or mixtures of polymers, inter alia to a method for cross-linking polymers, which are cross-linkable by means of heat, by means of infrared radiation, particularly to a method for the contactless cross-linking of mouldings of a cross-linkable polymer material in connection with the manufacture of the mouldings, especially in the manufacture of extruded tubes of polyethylene. The invention also relates to an apparatus for carrying out the method.
BACKGROUND OF THE INVENTION
In many applications involving treatment of polymer material there is a need of a fast and uniform, simultaneous heating of the entire polymer material. There are many ways of heating the polymer material, see the discussion below regarding cross-linking, but most of them have the common feature that they involve a heating of the surface of the material and conduction through the material. The heating thus will be slow and non-uniform since polymer materials generally are poor conductors in respect of heat.
Cross-linking, that is, the formation of spatial networks or cross-links in polymers such as polyethylene is nowadays generally carried out in accordance with two main principles.
According to a first main principle the formation of the spatial network is achieved chemically by means of functional groups, that is, chemically reactive side groups on the polyethylene chain take part in a condensation process which causes cross-links. Trismethoxy-vinylsilane is used in this process and for this reason the process is called the silane method.
According to another main principle the cross-linking is achieved by means of radicals which either may be formed by direct action of radiation energy on the polymer chain, for instance by means of irradiation with an electron beam, or with UV-radiation, or which may be formed by means of additives, cross-linking agents, such as organic peroxides or azo-compounds. In the latter case the cross-linking reaction normally is triggered by means of heat. The invention inter alia relates to this latter type of cross-linking by means of heat.
SE-B-324 450 discloses a process in which the polymer which is to be cross-linked is mixed with a cross-linking agent such as an organic peroxide before the extrusion and during or after the extrusion is heated so as to initiate the formation of a spatial network. This process has disadvantages because of the specific design thereof, primarily in the continuous manufacture of products like tubes, particularly in that the speed of manufacture will be very low.
Methods based on the transfer of heat from the tool generally have the advantage that all the peroxide will be used since the peroxide does not have time to evaporate. However, very long tools are then needed in order to allow the temperature to be raised in a degree which is sufficient to trigger the cross-linking. A consequence of this is that there is a high pressure drop in the tool, resulting in the necessity of high extrusion pressures, which in turn entails that the extruder must meet high standards--normally a so-called ram-extruder must be used. A result of long channels also is frictional problems in the tool. Attempts to solve the frictional problems have involved coating the tools with teflon and combining screw extruders with tools for instance provided with coextrusion, see e.g. WO 94/21441. In these processes, however, the coextruded surface layers may have to be removed after the extrusion.
In one process (called PEXEP) described in the Finnish laid-open specification 94106, which is substantially faster than the preceding process, an extruded tube of polyethylene is heated by direct contact with heated wheels in order to initiate the cross-linking. The heating here is mainly carried out by heating the surface and consequently involves conduction through the material. It should be noted that polymers like polyethylene are poor conductors in respect of heat. The disadvantages
REFERENCES:
patent: 4144111 (1979-03-01), Schaerer
patent: 4419304 (1983-12-01), Ficke et al.
patent: 5019417 (1991-05-01), Northcutt
Jarvenkyla Jyri
Rydberg Jan
Sjoberg Michael
Berman Susan W.
Wirsbo Bruks AB
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