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(11) | EP 0 035 980 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method and means for pyrolytic destruction of documents |
| (57) O Secret or valuable documents (3) are placed in contact with sheets (2) containing
a base material and an oxidizing agent in an amount sufficient for sustaining a glowfront
inthe sheets (2) and for pyrolyzing the documents (3), whereafter the sheets (2) which,
preferably, are pyropapers are ignited. Complete and reliable destruction is achieved
by employing sheets (2) which present a rougher surface than the documents (3), for
example obtained by embossing channels (4) upon the sheets' surfaces. |
Technical field
[0001] The present invention relates to a method and means for pyrolytic destruction of documents by placing the documents in contact with sheets containing oxidizing agent in an amount sufficient for sustaining after ignition a glow front under heat generation in the sheet for pyrolysis of the documents.
Background of the invention
[0002] In order to avoid unauthorized access to secret or valuable documents a number of methods have been suggested for rapid destruction of the documents in case the owner looses the control over them. Mechanical devices for disintegration tend to be heavy, complicated, expensive and less suitable for portable use. Spraying of colored ink on for example bills has been used to indicate stolen money but can of course not be used where information on the documents must be protected. Corroding substances like strong acids are hard to safely trigger and evenly distribute to obtain complete destruction and are furthermore dangerous to use. It has been suggested to obtain destruction by combustion by pumping large amounts of oxygen or oxidizing agent into α-chamber containing the documents. These systems are by nature complicated and static. It has also been suggested to arrange thick plates or panels containing oxidizing agent between bundles of documents. One reason for making the panels thick is that the propagating burning front tend to be extinguished in a thin plate due to a too strong thermal coupling to the surrounding documents, which cools the plates. Instead the thick panels has made it impossible to use a large enough number of panels which has resulted in a incomplete destruction of especially the interior of larger bundles of documents due to their insulating nature. The thick panels are also inflexible and hard to use in portab- . le systems.
The invention in general
[0003] The main object of the present invention is to provide method an means for destruction of documents which avoids the problems discussed above. Especially the invention has for object to provide method of this kind in which a thin sheet containing oxidizing agent can be used and to provide a suitable thin sheet. A further object is to provide such method and means which are especially suitable for use in portable systems.
[0004] These objects are reached by the steps and constructive features disclosed in the attached claims.
[0005] According to the invention the document to be destroyed are placed in contact with sheets containing oxidizing agent, whereby the sheets are given a rough surface. Hereby are formed channels and voids between the documents and the sheets which results in an isolating effect between these parts and thereby a thermal de-coupling so that the heat generated in the sheet after ignition is better maintained in the sheet to secure further propagation of the glow front. By making-the surface more rough than the document the greater part of the surface limiting the channels and voids between the document and the sheet will belong to the sheet and the lesser part of the channel limiting surface will belong to the document. This means that the evolved hot gases streaming towards unreacted parts of the sheet delivers a greater part of its heat content to the sheet than to the document, which further improves ignition security in the unreacted part of the sheet and burning stability. After ignition, which is a microsecond event, the burning transforms the materials so that the isolating effect is no longer of interest and the heat generated is transferred in a much slower process to the document to pyrolize it. By in this way improving the burning stability and ignition safety in the sheets it is no longer necessary to have thick sheets but thin and light sheets can be used. This means economic advantages as well as weight advantages of special importance for portable use. The thin nature of the sheets means that the documents need not be rearranged for security reasons but can normally be maintained in their original files, binders or folders together with the interleaved pyrolizing sheets. The thin and cheap sheets makes it possible to surround each document to be destroyed with sheets on both sides which significantly increases completeness of destruction. The thin but rough sheets also reduces packing density for the sheets themselves which means a better handling and storing safety for the sheets when not used. Finally the rough surface means less risk of unwanted adhesion between document and sheet.
Detailed description of the invention
[0007] The pyrolytic sheet of the invention includes a base material which preferably is a combustible material and especially such a material with low ash content. Paper is a preferred material. A cheap paper of low density and without much inorganic additives can be used, e.g. craft paper or toweling paper. Paper weights in the range of 10 to 200 g/m2, preferably 30 to 80 g/m2, may be used.
[0008] The sheet also includes an oxidizing agent. which can be included in several ways, e.g. by a nitridating procedure for the material itself if this is of a suitable material for example cellulosic material. A simpler and for slow burning material of the present kind more suitable method is to impregnate the base material with an oxidizing agent in solution and then dry the impregnated material. Suitable pyrotechnical agents usable as oxidizing agents are chlorates, perchlorates or gun powders. Nitrates are especially suitable and potassium nitrate is preferred among others due to low hygroscopic effect.
[0009] It is preferred to include a glueing component of some kind in the sheet together with the oxidizing agent in order to better retain the agent in the base material. Hereby dusting is avoided, which is both a hygienic advantage and a security advantage since pure oxidizing agent should not be allowed to accumulate anywhere. By including a glue it is also possible to reuse unignited sheets over and over again without risk of deteriorated burning properties. Many different kinds of non-dusting glues can be considered for use in the present connection but gelatine has proved to give flexible and good bonds against the oxidizing agents used. If necessary other additives may be included, for example dyes, softening agents or emulsifiers.
[0010] Impregnation can be obtained by dipping, spraying or painting a solution containing the oxidizing agent and possible other additives. It is in general advantageous to use rather concentrated solutions, for example close to the sa-turation limit, which for potassium nitrate means about 20 to 30 percent by weight.
[0011] After impregnation excess amounts of solution can be removed and the sheet be left to dry, possibly under slight pressure. The final amount of oxidizing agent in the base material should be sufficient for complete combustion of the base material and preferably also for combustion of at least a part of the material in the combustible documents. All this without presence of much air. For safety reasons it is preferred to keep the amount of oxidizing agent below 50 percent by weight of the dry sheet. Preferably the amount is kept over 25 percent. The sheet shall normally burn without an open flame but as a propagating glow front. A suitable speed of the front is between 1 and 1000 cm/min. and preferably between 10 and 100 cm/min.
[0012] Although the interior of the sheet may well be of normal density the rough surface will give the sheet an apparent density, when placed between flat smooth surfaces, which is rather low. Hence the apparent density, when the sheets are placed between flat surfaces or when placed together in a bundle with contact between the surfaces but without high pressure, may be between 200 and 1000 g/dm and preferably between 400 to 800 g/dm3. The apparent thickness, similarily measured, may be in the range between 0.05 and 0.5 mm and preferably between 0.1 and 0.3 mm. The average distances between crests an troughs of the rough surface will vary with the surface pattern but may be in the range between 0.05 and 1 mm or preferably between 0.1 and 0.5 mm.
[0013] Although the surface shall be rough from a heat transfer point of view the actual surface pattern can vary. The simplest way to obtain the rough surface is to select a base material with the desired surface characteristics naturally. It is, however, also possible to provide a special surface pattern for the purpose of the invention, for example by embossing the surface with elevations in the form of dots or lines over the surface. If the surface pattern can be selected freely it can preferably be made to facilitate the flow of hot gases form an ignition point at the edge towards the middle of the sheet, for example by providing from the center of the sheet radiating lines. Other shapes of the sheets than described may be used. It may for example be suitable to build the sheet from two or more layers.
[0014] When using the described sheets the documents are placed in contact with the sheet. It is preferred that a sheet is arranged on each side of a document and especially so that both sides of the document to be destroyed is in contact with a sheet surface. If only one document or single spaced apart documents shall be destroyed, it is prefered that at least two sheets of the described type are placed on each side of the document. If several documents in a budle shall be destroyed it is normally sufficient to interleave one sheet for each document and arrange one sheet at the top and one at the bottom, so that each document is on both sides in contact with a sheet. Heavy pressure should not, be applied in order not to smooth out the irre- gulatities in the rough surface of the sheets but light pressure is of no harm.
[0015] Since the sheets are used as a security system, an ignition of the sheets and actual destruction of the documents will only occasionally be necessary. If ignition is desired it can be achieved by an open flame. Normally, however, some kind of automatic ignition is to prefer, which can be done by use of any automatically ignitable pyrotechnical device able to form sparks or flames. Gun powder in connection with an electrically activated igniter has been used with success. Although the sheets can be ignited anywhere over their surfaces, an ignition at the edge is to prefer. In order to facilitate this, the sheets can be made slightly larger than the documents or interleaved so as to expose a free edge in relation to document, where ignition can easily occur. To have an efficient ignition and fast burning it is preferred to ignite each sheets at more than one point.
[0016] A suitable way to provide a complete saftey system is to contain the documents with the interleaved sheets in a container, which can be a box for a permanent arrangement or a suitcase for a portable arrangement, and to provide several ignition devices within the container which can be activated by one or more of several indications of encroachments, for example after a certain delay of time, use of force against the container or use of false keys. Means for direct manual activations may also be provided. The container can be equipped with isolating and filtering means to limit the effects of the generated heat and gases on the surroundings.
[0017] Any combustible or pyrolizable document can be destroyed by use of the present invention. The documents can have any surface characteristics as long as the pyrolytic sheets are made with a more rough surface than the documents.
Description of the drawing
[0018] Figure la and lb illustrates the temperature conditions at a section through a sheet, according to the invention, placed between two documents: Figure la illustrates the conditions for a sheet with smooth surface, while figure lb illustrates the conditions for a sheet with rough surface.
[0019] The figure illustrates the theoretical background of the invention. The heat generated at the reaction in the sheet, in the following also called pyropaper, shall 1) be conducted from the reaction zone into the unreacted part of the pyropaper so that the temperature is raised there above the ignition temperature and the reaction zone is propagated and 2) be conducted from the reaction zone into the neighbouring document to pyrolize it. If the heat transfer according to phase 2 takes place too easily the heat transfer according to phase 1 will be reduced and the reaction will stop when the temperature in the pyropaper in front of the zone (section A-A in fig. 1) drops below the ignition temperature. The same amount of heat is developed in both cases, but the temperature in section A-A is higher in case lb, which gives a more stable reaction zone.
[0020] A further advantage with the rough surface appears when studying the hot gases evolved in the reaction zone. If the surface of the pyropaper is rough, channels are formed between the solid part of the pyropaper and the surrounding smoother documents. In the reaction zone hot gases are evolved. The hot gases can now flow in the channels formed from the reaction zone towards the unreacted part of the pyropaper. Of the surface limiting the channels the greater part is constituted of pyropaper while a lesser part is constituted of the more smooth document. This means an easier transfer of the heat in the hot streaming gases to the pyropaper (in section B-B fig. 1) than to the'document and the result will be that the pyropaper more easily reaches its ignition temperature in front of the reaction zone.
[0021] In the figure the sections through the documents and the pyropapers are shown. The profile of the temperature in section A-A is indicated with the hight of thin arrow and a curved dotted line. A thick arrow indicates the direction of movement for the reaction zone.
1 = pyropaper with smooth surface
2 = pyropaper with rough surface
3 = document
4 = channel
5 reaction zone
6 = reacted material
T = temperature axis
T. = ignition temperature
T = original temperature in unaffected material
Example
[0022] A pyropaper was prepared from a low density-craft toweling paper with rough surface which was dipped for about 15 seconds in a solution containing in percent by weight:
[0023] The paper was removed and allowed to drain for about 30 seconds before drying. During the final part of the drying a slight pressure was applied to give a more flat paper.
[0024] The paper absorbed about 175 grams of the solution and contained about 39 grams of potassium nitrate and about 2.8 grams of gelatine. It had an ignition temperature of about 310°C and a heat generation capacity of about 175 kJ/m2.
[0025] A box was charged with 1 photografy (A6), 1 original (A4), 1 drawing on parchment (A4) and 36 xerografic copies with text (A4). The documents was surrounded by 40 sheets (27 x 34 cm) of the pyropapers prepared. The size of the box was 43 x 29 x 9 cm and made of enameled sheet metal. The bundle of documents and pyropapers was ignited at one corner. The burning time was about 20 seconds.
[0026] The result showed a non-burned enamel of the metal box. The content in the box was completely carbonized. The photografy and the drawings could not be traced. The high ash content of the xerografic copies resulted in single rather rigid flakes on which single letters could be detected in suitable illumination. The result is better than what can be obtained with other methods for destruction of documents.
1. A method for security protection of documents by placing at least one side of at
least one document in contact with a sheet containing oxidizing agent in an amount
sufficient for sustaining a glow front in the sheet and a pyrolysis of the document
and whereafter the sheet is possibly ignited, characterized in that a contact between
document and sheet is provided which includes channels between document and sheet
and that the channels have a larger surface area towards the sheet than towards the
document, by using a sheet with a more rough surface than the document.
2. A method according to claim 1 in which both sides of the document are placed in
contact with a sheet.
3. A method according to claim 2 in which two sheets are placed in contact with each
side of the document.
4. A method according to claim 1 in which the sheet is positioned so as to expose
a free part in relation to the document.
5. A method according to claim 1 in which the sheet is ignited at more than one position.
6. A sheet containing a base material and oxidizing agent in an amount sufficient
for sustaining a glow front in the sheet under heat generation, characterized in,
that the sheet has a rough surface.
7. A sheet according to claim 6 in which the apparent density of the sheet is between
200 and 1000 grams/dm2.
8. A sheet according to claim 6 in which the base material is combustible.
9. A sheet according to claim 8 in which the base material is paper.
10. A sheet according to claim 6 in which the surface is embossed with channels simplyfiing
gas transport towards the center of the sheet.