CA2247377A1

CA2247377A1 - Process for monitoring a disperse system for non-dispersed impurities and device for carrying out said process

Info

Publication number: CA2247377A1
Application number: CA002247377A
Authority: CA
Inventors: Fredi Stucki
Original assignee: Borealis A/S; Fredi Stucki
Current assignee: Borealis AS
Priority date: 1996-02-29
Filing date: 1997-02-21
Publication date: 1997-09-04
Also published as: WO1997032200A1; DE19607582A1; AU1715697A; JP2000505552A; EP0883800A1; KR19990087358A

Abstract

The invention relates to a process for monitoring a disperse system for nondispersed impurities (16) before processing of said system into an end product. During said process, part of the disperse system is extruded before processing to form a strip (7) and the impurities in said strip are detected. To this end, weak X-rays are beamed through small areas of the strip (7) continuously or gradually and at right angles to the strip plane. The X-rays emerging from the areas of the strip through which they have passed are subsequently detected in areas. The type, number, size, shape and/ or distribution of the impurities (16) are determined by the detected rays. It is particularly advantageous that impurities inside the strip and also on the surface thereof are detected as a result of X-rays passing through the strip (7). If the impurities (16) exceed a predetermined threshold, the disperse system does not undergo further processing.

Description

Ka 02 . 29 . 96 96t021 TI~LE; O~ T~E; IN~E~T~ON

Method for monitorlng a disp~rse 5yste~n for undlspe;c~ed ~npurities and d~ice for ~arr~ing ollt this metho~
BACK6ROU~D OF T~ v~TIo~

~ield o~ the in~antion . .
~O ~ho ~tarting poin~ ~or the in~ention is a me~hod ~or monito~ing a disperse syst~n for undi~persed impuri~ies in accordance with the pzearnble c~f pate~t cl~ im 1 . The inven~on al o relates ~o a de~ice fo~ carrying out the method .
Ir~ electric:al er~g~n~ering, such ~15 in p~ticula:c in the cable industry, poly~ners filled uith electrically ~onductive or el~ctrically sem~conduc~ 3 po~der, ~uch as ~or exam~le conductive car~or~ ~lac~c, are

2 0 increasingl~r Peing used ~s a subs~anee ~or semicond-lc~ ma~ceri~l, such Z~5 ~0~ example scree~ing electrodes ant con~rol electrodes. ~n the case of high or rredium hig~ voltage cables, layers -or~sj sting of such a semiconductive Inaterial a~e ar~anged bet~een the 25 cusrent-carrying corld-uctor and the ca~le insulation for screening and cont~ol pl~rposes. The ope~a~i~ ity and servic:e li~e of the cable depend ~e~Sr decisi~ely on the q~ality of this semiconductive la~e~. On t~e one ~Land, undispersed impurities uhich - such as for exa~nple sant 30 - are incorpor~ted ~nto ~he di~p~xse system together h ~he carbon black ~h~n mixlng c~rbon blaek and polyme~ fo~ inhomogeneit~ es in the semiconductive ~natesial. If the~e inhomogenei~i~s become deposited as i mper~ectiotls at the ~ouzldary laye~ ~o the d~ electric 35 vhen psoducing the ca~les, the~ ~nay locally d~ stort the electric f~ el d of the ca~le ~ery consi.derably . ~owever, on the othe~ hand. these ~pu~it~es may also contain uater-sOluble io~s. ~he~e~o~e, i~ unavoidable lesi~ual 68/b ~S lN~ltldt1M~ 0Z :b~ 86, sn~ 6 9 6~021 ~oisture is still present in the cable, ~ese i ons may di~se i~to th~ cable insulation and i~pair the proper~ies of the ca~les ~ery considerably.

5 Discussion of Bac~qro~nd Th~ i~e~on ~efers to ~ p io~ a~t as laid ou~ in an a~ti~le by A~A. Farkas et al. '~igh performance sem~conduc~ ve cc~ o~ ds tes'c~ny, p~oduction and experience", ~icable S1, 3rt Inter~ational con~erence on Polymer I~ulated Powe~ C~bl-s, page-s B~ e 24-2B 1991, Vess2illes, France. ~n a method described ~ n this prior art for mor~itoring a disperse syste2n o~
conduc~ive carbor~ blac3c and polyethyle~e which ~.s ~ d 15 in a ca~le as a se~co~t~ctive laye~ ~os undispersed impurities, p2~t 0~ ~he disp~s~ system is extrud~d to ~orm a s~rip, the strip is rY~mirled ~Eo~ impur~ ties, and only ~hen ~ s the d~ s~e~se syQ'cem proc~:~sed further. The stxip ~ontains ~ery corlsiderable amounts of carbon ZO black and is t)~ere~ore opaque. In the known snethod, only the sur~ace o~ th~ st~ip is exam~ned ~or ~ undispersed~purities. ~rnpurities situated in the interior ofthe strip cannot be detec~ed in this me~hod .
ZS
SUM~RY OF ~Hi; I~vENTIO~

Ac~:ordlngly, one o~j ect of the irl~rention, as it is de3~ined in the pa~en~ claims, is t~ develop the method :i30 in accordance with the p~io~ art in such a manner that it i5 also reliably able to detec~ undispersed imDuri~ies ~ hich are presen~ in the interior of rhe st~ip and, at t~e same ti~e, ~o p~vide a r~o-rel device ~ hlch is s~litable for car yi~ out thi~ method in a ~ si~nple tnanner 68~5'5 lN31~d~ IZ:bl 86, sn~ 61 , _.. ., ,.. . , .. , . .~ . ..

96/~21 -- 3 -- .

The methot acc~ding to the i~e~io~ is distingulshed pr~marily ~y r~e ~act ~hat ~t is a~e to co~tinuously detect and d~splay the imp~ities distri~uted o~er ~he su~ace and i~ the intexior o~ the scrip. It is particularly advan~ageou9 ~n th~s.con~ec~on thst the method opera~es ~depe~de~tl~ of ~e optical pr~per~les of the s~p and is able to analyze even opaque s~rips ~or incl ~ded impurities ~ thou~ ~roblems . Selec~ion crit~t a which may ~e us~d ~o~ the extent o~ t~e 1~ ~purities may ~e b~th the t~pe, nur~er, size, ~orm and~or ~he d~ stri~ution o~ 'che includ~d impuri~ies. Ihe ~nethod acco~ding to ~he i~verl~ion can readily pick ~E~
undispersed impurities o~ less ~han one ppm. In additior~, lt is p~ssible to distinguish ex~remely 15 p~ecisely bet~een organic and inor~anic impurities.
Since ~he strip is representati~re o~ the enti~e tisperse sys~e~, the quality of the disperse ~y~tem is continuously monitored be~ore it is proces3ed ~urthe~, ~d in the e~ent of impermLssib~y hl~h le~els o~
im~urities occur~ing the ~urther processlng operation can be interr~pted i~mediately. Since ~he method accord~ng to ~he inventlon ~an be used to determine both the type, number, size, for~ andJor ~he dist~i~u~ion o~ the imp~rities, it is poscible, in a Z5 particularly simple ~anner, ~o drav concl~sions about the ~au~t so~rce~ ~hich are responsible for the i~purities.

~ he device for carrying ou~ the ~etho~ according to the 30 in~ention allows re~ab~e, continuous and/o~ step~ise ~onitoring of ~he strip and, at the ~me time, is a~le to control processes in ~hich ~he dis~e~se sys~e~ ~hich is ~e~rese~ed by ~he strlp ~ proc~ssed ~ur~er, ~or example ~y ex~rus~on in the production o~ cab~es.

6~~9-S lN31~d~M~ ~Z:~ 86, 9n~ 61 BRIEF DESCRIPTION OF THE DRAWINGS
A more completed appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Fig. 1 diagrammatically illustrates a preferred embodiment of a device according to the invention for monitoring a disperse system for undispersed impurities, having a signal output which acts of the material flow path of the disperse system, the disperse system, during the subsequent production of a high or medium high voltage cable, being extruded together with insulating material and then forming a semiconductive screening and/or control layer which encloses the table conductor and is arranged between cable conductor and cable insulation, Fig. 2 shows an enlargement of a region which, in the device in accordance with Fig. 1, is shown outlined, and Fig. 3 shows two specimen pictures which were detected by the device in accordance with Fig. 1, of which the upper picture relates to a specimen which is free of impurities and the lower picture relates to a specimen in which impurities are included.

DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in Fig. 1, 1 denotes a stock boiler which serves for the temporary storage of a disperse system. The disperse system has been produced by mixing from 10 to 50 percent by volume of an electrically conductive or electrically semiconductive powder, such as in particular a conductive carbon black with particle sizes of less than 0.1 µm, with a polymer, such as in particular polyethylene. The stock boiler may be connected, via a controllable changeover valve 2, to an extruder 3 which is used to produce a high or medium high voltage cable.
The stock boiler 1 is fed by a conveyor line 4. A small part of the disperse system guided through the conveyor line 4 is branched off continuously through a bypass line 5 and is fed to a test extruder 6. From the branched-off part of the disperse system, the test extruder 6 produces a test strip 7 with a thickness of, typically, 0.2 to 1 mm and strip width of typically 10 mm. This test strip is opaque, since it contains a high concentration of finely divided conductive carbon black. The test strip 7 is fed to a measuring head 8 of a device 9 for monitoring the test strip 7 (and hence also the disperse system) for undispersed impurities.
The output of the measured value pickup 8 is connected to an evaluation and control device 10. A signal output 11 of the evaluation and control device 10 acts on the controllable changeover valve 2.
As can be seen from Figures 1 and 2, the measured value pickup 8 has a radiation-protected casing 12 with two narrow openings, which are arranged on opposite sides of the casing, for guiding the strip 7 through. A tube 13 for generating soft X-rays (operating voltage of the X-ray tube 13 between 1 and 50 kv) and an X-ray-sensitive imaging device 14 with a grid 15, a computer-controlled (CCD) camera or a film are arranged in the casing 12 on respectively opposite sides of the strip.
Information relating to the type, number, size, form and/or the distribution of undesired impurities in the interior and on the surface of the test strip 7, and therefore also in the disperse system which is situated in the stock boiler 1 for further processing, is determined by the measured value pickup 8 and this information is passed on to the evaluation and control device 10.
As can be seen from Fig. 2, soft X-radiation is irradiated through regions of the test strip 7 of, for example 1 cm2, transversely to the plane of the strip.
The radiation coming out of the strip may be detected in a stepwise manner, for example using a film, or continuously, for example using the grid 15 or a CCD
camera. The radiation coming out of the regions through which radiation has passed is absorbed to a greater extent by an impurity 16 which typically stems from a grain of sand or some other inorganic particle than by the remaining, impurity-free part of the region of the strip through which radiation has passed, which predominantly contains organic substances. A silhouette 17 is therefore produced on the grid 15. The silhouette 17 is detected by adjacent grid points - as indicated in the figure, for example, by 2 grid points - and this information is transmitted to the evaluation and control device 10 in the form of electrical signals.
Depending on the quality of the grid 15, it is thus possible, with a resolution of higher than 10 µm, to record all the impurities present on the surface and in the interior of the test strip 7.

The electrical signals supplied may be compared with reference signals in the evaluation and control device 10. In the event of a threshold value being exceeded, the evaluation and control device 10 then transmits a negative status signal which characterizes the impurity state of the strip 7. This negative status signal is fed, via the signal output 11, to a control unit of a device which processes the disperse system further, such as in particular the changeover valve 2, and then actuated the latter, such as in particular by closing the connection to the extruder 3.
Before the comparison with the reference signals in the control and evaluation device 10 output signals are formed which are proportional to the type, number, size, form and/or the distribution of the impurities.
These signals may be extremely valuable when looking for error sources and, at the same time, allow a particularly simple input of the reference signals which are required for the threshold comparison.
Two typical specimen pictures detected by the imaging device 14 are illustrated in Fig. 3. The upper picture relates to a region of the test strip 7 which is free of impurities. The diffuse spots of low contrast and a diameter of, typically, 60 µm which can be seen from the picture are caused by agglomerated of carbon back.
The lower picture relates to a region of the test strip 7 in which coarse impurities are included. These impurities are grains of sand or other inorganic particles, such as metal chips, and entered the disperse system via the starting components, when the starting components were mixed or during some other production step. The impurities lead to a considerably greater contrast than the diffuse spots and have diameters in the millimeter range. Contrast and diameter size may therefore be used as particularly simple criteria for the comparison between impurity-free (upper picture) and impurity-afflicted regions of the test strip 7.
obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

WHAT IS CLAIMED

1. A method for monitoring a disperse system, which can be used in an end product and is designed as a polymer film filled with a finely divided, electrically conductive or electrically semiconductive powder, for undispersed impurities (16), in which method part of the disperse system is extruded into a strip (7) before the system is used further, and in this strip (7) the impurities (16) are detected, wherein soft X-radiation is radiated through regions of the strip (7) which are recorded continuously or in a stepwise manner, transversely to the plane of the strip, wherein the radiation passing out of the regions of the strip (7) through which radiation has passed is detected in regions, and wherein the type, number, size, form and/or the distribution of the impurities (16) is determined from the radiation detected.

2. The method as claimed in claim 1, wherein the soft X-radiation is generated by means of an X-ray tube (13) to which an operating voltage of between 1 and 50 kv is applied.

3. The method as claimed in one of claims 1 or 2, wherein the soft X-radiation passing out of the irradiated strip (7) is detected using an imaging device (14).

4. The method as claimed in claim 3, wherein the impurities (16) are recorded in the imaging device (14) as silhouettes (17).

5. The method as claimed in one of claims 3 or 4, wherein electrical signals are generated in the imaging device (14) from the radiation detected, wherein the electrical signals are then fed to an evaluation and control device (10) and are compared with reference signals, and wherein in the event of a threshold value being exceeded the evaluation and control device (10) transmits a status signal which characterizes the impurity state of the strip (7).

6. The method as claimed in claim 5, wherein the status signal is fed to a control unit (Z) of a device which processes the disperse system further.

7. The method as claimed in one of claims 5 or 6, wherein before the comparison with the reference signals in the control and evaluation device (10) output signals are formed which are proportional to the type, number, size, form and/or the distribution of the impurities (16),

8. A device for carrying out the method as claimed in one of claims 1 to 7, having a measured value pickup (8), which records the impurities (16) in the strip (7), and an evaluation and control device (10) which is connected downstream of the measured value pickup (8), wherein the measured value pickup (8) has a radiation-protected casing (12), which encloses the regions of the strip (7) through which radiation has passed, with two narrow openings, which are arranged on opposite sides of the casing (12), for guiding the strip (7) through, as well as, respectively arranged on opposite sides of the strip (7) in the casing (12), a tube (13) for generating soft X-rays and an imaging device (14).

9. The device as claimed in claim 8, wherein the imaging device (14) contains a film, a grid (15) and/or a computer-controlled camera.

10. The device as claimed in one of claims 8 or 9, wherein the evaluation and control unit (10) has a signal output (11) which can be connected to a control unit (2) of a device for processing the disperse system.