DE2015548A1 - Polypropylene sheet for projection screens - Google Patents

Abstract

Sheet is produced by pouring or extruding a polypropylene composition of thickness 0.5 to 2 mm. on a heat transfer surface for sufficient time for the sheet to solidify, the surface being maintained between 85 degrees and 170 degrees C., and then maintaining the sheet surfaces at 85 degrees to 160 degrees C. until three quarters at least of the sheet has become crystalline. The composition should have a flow index (A.S.T.M. D 1238) of at least 90, and the sheet should contain large spherulites in its central core of 15 - 45 micron size, with smaller spherulites in the outer layers. Additives may be included. The screen is of an improved transparent diffracting type which is robust and reduces scintillation.

Description

Projection screen The invention concerns a projection screen, in which the image is projected onto the side facing away from the viewer, d. H. a so-called "rear projection screen" and a method for its production.

There are already projection screens and devices of the most varied Type known, with the help of a microfilm image in which Usually through several lifts and after reflection through one or more mirrors, is projected onto the string of the projection screen facing away from the viewer. Typical projectors based on the principle given are e.g. B. the under the projectors known as "Dodestar" and "Starlet".

There are already a wide variety of manufacturing processes such projection screens made of materials of the most varied of types are known z, B, for years projection screens made of etched glass, if necessary, to this To make screens opaque, coat them with suitable materials became, manufactured. However, a disadvantage of such glass screens is their fragility. In recent times, methods have therefore been known by means of which projection screens made of plastics or waxy substances that are not as fragile as glass, can be produced. E.g.

sur production of such projection screens microcrystalline wax used. Typical projection screens that can be produced with the aid of such known methods made of plastics or polymers are e.g. made up of laminates, for example in the way that a. Surface of a clear, rigid plastic sheet with a dark, opaque, dye-containing layer, or single-layer solid, uniform polymer films of various types, for example made of polyethylene foils.

The projection screens that can be produced using conventionally known methods de. However, the specified type have numerous disadvantages, the main of which are Permeability properties and / or optical criteria that are important to the viewer of images projected on such screens are of concern.

The assessment and evaluation of projection screens where the Image is projected onto the side facing away from the viewer, is expedient according to four very specific criteria that are important for your optical behavior, specifically according to (1) "contrast", (2) "brightness", (3) "flicker effect" and (4) "light spot effect".

For example, Bo has an ideal projection screen of the specified type. practically no "light spots", i.e. no areas with significantly greater brightness than that of the rest of the screen area, furthermore practically no "flickering", i.e. none tiny glittering light spots appearing on the surface of the screen, furthermore a maximum contrast and a high brightness. From the specified The criteria are "Light spot effect" and the "flicker effect" are evident of crucial importance and the elimination of the occurrence of these effects is of the utmost importance.

To determine the relative advantages and relative merits one. Projection screen of the specified type, it has proven to be expedient to use the specified four criteria, albeit necessarily in a somewhat subjective way, individually to judge. This assessment is expediently carried out in the manner that each. the Criteria are rated with increasing numbers, e.g. ranging from O bia 9 where O denotes the worst quality and 9 the best possible quality. The individual evaluation numbers determined for each of the four criteria in the specified manner can then be added and the received, the quality assessment of the tested The total permitting projection screen can be referred to as the so-called "Q-value" will. The specified evaluation is carried out under image projection conditions, i.e. in which an image is directed away from the viewer performing the assessment Beite of the projection screen to be tested is projected in the usual known manner will.

According to the specified determination method is a projection screen of perfect perfection with a "Q-value" of 36 su, whereas one Projection screen of inferior quality to be assigned a correspondingly lower Q value " is. The specified method of determination is of course not perfect, as it is based on the individual visual analysis of a test person and consequently usually varies from test person to test person. Even so, the specified Method of determination a satisfactory evaluation of the characteristics and criteria the projection screens tested with their help, so that such screen at least are subjectively comparable with each other. One that is particularly useful in practice Determination of the Q value "can be achieved by the fact that one and the same evaluation of different test subjects performed and from the test results obtained the average value is calculated, bin according to the specified method of determination evaluated, known projection screen, hereinafter referred to as A, consisting of a glass plate, which is provided with a diffusion layer made of wax is, s.B. a Q value "of 24, which is derived from the following evaluation numbers made up of: screen luminous spot effect flicker effect contrast A 9 2 brightness Q value 5 24 The values listed show that this well-known projection sohiri has a significant flicker effect and that the projected image does not is particularly bright. A projection screen evaluated according to the specified method of determination may have a slightly higher rating for one assessment and a higher rating for another Assessment a slightly lower "Q-value", so that from the individual assessments a mean value can be formed. The average values obtained in this way gain the more meaningfulness, the more assessments of the projection screen to be tested can be carried out by different test persons.

other known, ii Commercially available projection screens, hereinafter referred to as B and C are rated. The results obtained are shown in the table below.

Umbrella luminous spot effect, liner effect, contrast B 7 8 9 C 9 2 9 brightness Q value .4 28 4 24 From the evaluations for the individual criteria and the resulting calculated "Q-values" it can be seen that all known projection screens tested are to be assessed badly with respect to at least one criterion, so that their application is noticeably impaired than projection screens of the specified type. Both tested known projection screens are commonly used such screens and there are currently no projection screens of the specified Type traded, which is characterized by a better, with a higher "Q-value" to be assessed Distinguish overall behavior.

The object of the invention is to provide a projection screen with evaluation grades for each of the four specified criteria of at least 7 as well as a so-called Q value "of at least 28 as well as a process for its production to be specified.

The invention relates to a projection screen in which the image is projected onto the side facing away from the viewer, characterized in that that the screen made of polypropylene was determined according to the ASTM test method D 1238 Melt flow rate of 90 or less and a thickness of about 500 to about 2000 ji and that the screen also consists of at least about 60% by weight crystalline Spherolites with an average diameter of about 5 to about 20 microns and has a core layer that is at least 3/4 the thickness of the screen and in comparison to the two surface layers of the screen comparatively large crystalline spherulites from-about 15 to about 45 microns in diameter contains.

The invention also relates to a method for transferring of polymer films based on polypropylene for use as projection screens, in which the image is projected onto the side facing away from the viewer, in particular advantageously suitable foils, which is characterized in that nan a) first of all an approximately 500 to 2000 µ thick, melted film made of a polypropylene with a melt flow rate determined according to ASTM test method D 1238 of at most about 90 bits at least one at a temperature of about 85 to 170 ° C Brings the surface of a heat exchanger into contact until the surfaces the film are made and b) then the solidified surfaces of the Foil heated to a temperature of about 85 to 160 ° C until at least three quarters of the core zone of the film, based on the total thickness of the film, practically are crystalline.

To carry out the process of finding the melted Polypropylene in the form of a forgotten or extruded film of the specified thickness on a heat exchanger, the surface of which has the specified temperature, i.e. whose temperature is low enough to solidify the film surfaces, but is not so low that the foil solidifies, applied, whereupon the solidified surfaces of the foil are heated to the specified temperature in this way the speed of heat dissipation from the core piece to keep the foil comparatively low and the crystal growth in the core stimulate the film so that the specified high crystallinity is achieved. In the surface of the heat exchanger is usually directly in process stage a) before being brought into contact with the melted film at a temperature in specified range, preferably kept at a temperature of about 90 to 150 ° C.

According to a preferred embodiment of the method of the invention one of the two surfaces of the film will have a uniform roughness in porm from Bumps of about 25 to 76 x 10-6 cm (10 to 30 microinches) in statistical terms awarded randomly. In this way the light diffusion becomes even wider and a light reflection from the room or background light on the viewed screen surface switched off. The film obtained is directly as Projection screen of the specified type can be used.

Crystalline structures or on those made by the method of the invention produced foils present areas of the same optical property or Characteristics, e.g. so-called spherulites, are sometimes referred to as "crystal areas" in the following describe. For example, a single spherulite, which is easy to detect, e.g. when looking at a cross-section of the polymer film achievable according to the invention under polarized Iltoht, in the sense of the definition given, a "crystal area" the, Obviously, the advantages to be achieved according to the invention wind on the unusual advantageous combination of the films which can be produced according to the invention to trace back crystal areas consisting of crystal grains of various sizes, those in their composition of comparatively few and small crystals in the relatively uncrystallized outer surface areas of the film up to comparatively large crystals in the strongly crystallized inner core zone reaching the film, which in turn leads to the obtained according to the invention Projection screens the most advantageous degree and type of light diffusion imaginable as well have very high "Q values". The exact quantitative below the limit of the ratio from comparatively less crystalline "outer" areas to highly crystalline The core zone of the foli is obviously not critical in terms of being successful Use of such a film as a projection.

shield, provided, as already mentioned, the highly crystalline core zone of the film is at least about 3/4 of the total thickness of the film.

The relative thickness of this highly crystalline core zone is customary known methods can be easily determined, see B. when looking at a cross section of a such slide under a microscope. So s.B. Under the microscope at 150x magnification that the largest polypropylene crystal grains in the indicated core zone has a diameter of about 15 to 45 microns, in particular advantageously from about 20 to 40 microns measured between the furthest points apart. In contrast, those in the surface districts Comparatively small crystals present in the film usually have a diameter of about 10 microns or less. It should be noted, however, that both the core zone as well as the outer districts of the polymer films obtained according to the invention Crystals of the most varied sizes and doses the indicated Yees values refer only to the comparatively large crystals.

Obviously, the advantageous optical properties are the projection screens obtained according to the invention both on the unusually advantageous Arrangement of the crystalline and comparatively amorphous polypropylene districts in the Films as well as their relative ratio to each other, as well as on the size and attributed to the type of crystals present in these neighbors. So there is s.B. the core zone of this polypropylene is made entirely of crystallized polypropylene and on both sides of this core zone there is an eandwichartiter arrangement each a layer of comparatively little crystalline-based polypropylene, whereby these little crystalline layers on the surface of the films, which, as already mentioned, about 500 to 2000 µ (20 to 80 nils), preferably about 635 to 1525 p (25 to 60 mils) thick.

In the projection screens obtained according to the invention there are a total of at least about EO *, preferably about 70 to 90%, crystal grains with a average diameter of about 5 to 20 microns, measured at the widest diverging points. The comparatively large crystal grains have in the core zone of such projection screens made of polypropylene a diameter of about 15 to 45 microns, preferably about 20 to 40 microns, measured at the most distant points.

In contrast, the largest crystals wind in the outer, less crystalline ones Layers of the surface areas of the foils are almost without exception smaller than 10 microns In diameter.

To achieve the unusually advantageous projection screens of the it has been found to be necessary.

to strictly adhere to the specified features of the method of the invention. If, for example, in process step a) the melted polypropylene film is covered with a Heat exchanger surface brought into contact, which irt too cold or which is on Foil solidified on the surface for a long time on a relatively cold heat exchanger surface are left, so are the areas having the comparatively small crystals the film is too large and may extend over the entire film, which is disadvantageous for the formation of a projection screen with insufficient Light diffusion leads so that the projector lamp in the form of a "light spot" through the screen is visible. On the other hand, the melted polymer film has been in contact with or is in contact with the heat exchanger surface for too short a time the heat exchanger surface solidifies at too high a temperature the outer surface areas of the film are not sewn, which is disadvantageous Way leads to bulging and deformation of the film, and also often one The result is brittleness of the cooled film, since the comparatively large ones Spherolites, i.e. spherical crystal grains, which are used for the sake of simplicity are referred to as "crystal grains" containing highly crystalline grain zones percentage is very much larger than that obtained in a according to the invention than Projection screen is present in a particularly advantageous manner usable film. It thus follows that both rapid cooling and too slow cooling leads to unsatisfactory results, since too rapid cooling the formation of small Crystals in too large areas of the film UM, on the other hand, cooling down too slowly causes the formation of too large of the crystals in too large a "core zone" of the film.

The projection screens obtained by the method of the invention are characterized by a. heterogeneous structure of a very specific type individual areas with crystals of different sizes, being in the core zone comparatively large crystals with a diameter of about 15 to 45 p and in the outer surface layers have significantly smaller crystals.

When carrying out process stage b) of the process of the invention, within a few seconds, preferably within about 10 seconds Start bringing the melted film into contact with the heat exchanger surface must, the aim is to bring about the cooling of the core zone of the film so slowly, that the formation of crystals of the specified size in one of the interior of the Foil comprehensive section, the three quarters, preferably 9/10 of the total thickness of the slide is made. Although there is complete crystallization in this core zone is probably not to be expected, detailed examinations of the film revealed that the core zone is practically completely crystalline. This aspired goal can be achieved by passing the film through a warm section, in which their surfaces as long as at a temperature of about 90 to 16,000, preferably from about 90 to 125 ° C until the indicated highly crystalline core zone formed with crystals of suitable size.

The carefully controlled process stage b), in which the crystallization occurs in the core zone of the film is usually within about 2 to 10 Minutes feasible. Regarding the maximum response time, the requirement is decide the crystals located in the outer surface areas for comparison to keep small and in this outskirts a less dense distribution of To effect crystals than in the core zone.

In terms of apparatus, the method of finding in various ways. 80 can e.g. the cooling of the film surface caused by the molten polymer film between two. Comparatively cold rollers, the distance from each other to the required layer thickness is set, is passed through. Furthermore, the cooling can be effected by that the film is extruded onto a comparatively cold roller and at the same time Relatively cold air is directed onto the outer surface of the film, thereby strengthening the film To facilitate the outer part of the film, the melted Poymerizat wipe two metal plates. their surface is kept at the specified temperature will be passed through.

Practically immediately after the specified surface cooling the film obtained is passed into a section in which both you the film surrounding gas as well as the surfaces of the foil at a temperature of about 90 to 1900B, which can be effected in a section included in the Is designed so that the transported over a series of agitator rollers, slowly cooling foil either on one or on both surfaces using is heated continuously or intermittently by electrical heaters.

Should one of the surfaces that can be used as a projection screen Polie can be given a roughness, so this can be practical to take place at any point in time. For example, such a roughness of the film surface with the help of one or more rollers with which the relatively soft film during or even before the process stage b) which brings about the crystallization in contact is brought to be sufgepreeet. Furthermore, the film surface can have such a Roughness, if necessary, in an additional step following process step b) Process stage in which the film has the required roughness in a press. under Application of pressure imprinted to be awarded.

The degree of roughness that one of the surfaces of the invention achievable projection screen is compared, has, as already mentioned, the purpose of to increase the light diffusion as well as the surface reflections, which or other background light. Therefore becomes a If such a roughened projection screen is used, the rough surface should expediently be facing the viewer. Furthermore, the surface roughness should be distributed fairly evenly over the entire surface of the screen and it should also be measured with the aid of a commonly known surface roughness analyzer, s.B. with the help of a so-called "Berdix Proficorder" determined value of about 25 up to 76 x 10-6 cm. The surface is smoother than a roughness of 25 x 10-6 cm, the light reflection is not separated and shows it has a roughness of about 76 x 10 cm, it is too rough and will be used by a Viewers sitting right in front of the screen are perceived as annoying. The image is viewed from a greater distance, there is a surface with ei-over roughness which is greater than about 76 x 10 6 cm can be used.

The roughness of the screen surface is expediently statistical random distribution as it is s.B. by sandblasting, etching or plating processes is achievable. If a uniform roughness pattern is applied to the screen surface, z.3. by embossing a pattern, 80 this often leads to a disadvantageous one Flicker effect.

To carry out the method of the invention, the Use of polymer films of the specified thickness, i.e. a thickness of approx 500 to 2000, proved very important. Thinner polymer films of the specified Type are obviously either not so quiet that all projection screens of the specified type usable films can be achieved, or they have properties that do not enable the high "Q values" to be achieved. on the other hand Thicker foils with a thickness of more than about 2000 μ are also unsuitable because they evidently just as little to projection screens with the desired high ones "Q-values" lead.

It thus follows that the advantages to be achieved according to the invention result from an unusually advantageous interaction of all specified features, namely the thickness of the foil, which depends on the crystal shape, crystal number and crystal size due to the opacity of the screen as well as the special big shaft of polypropylene films of the specified.

Strength to let through clear, in no way discolored images. Another surprising advantage of the projection screens obtained according to the invention Usable foils also result from the fact that color images with an extraordinary a high degree of color fastness.

In the case of carrying out the method of the invention in more advantageous Way usable foils are those made of a polypropylene with a soil melt rate of less than about 90, preferably from about 1.5 to 10, are constructed.

The values given relate to measurements made according to the ASTM test method 1238 in dg / min, at; 23,000 were performed under a load of 2.16 kg. The foils used can also contain additives that improve the distribution and size of the crystallites formed do not significantly affect them, if necessary have been caused by such additives, e.g. finely divided starch or calcium carbonate, the "Q-values" of the projection screens obtained even improved.

Verden the projection screens obtained according to the invention are used to Reading negative pamphlets for a long time has proven to be particularly beneficial proved such negatives by a comparatively dark layer of neutral color Consider density. This can be done, for example, in such a way that dyes of neutral density or dye combinations either the sur making the film added polypropylene used or on the projection screens obtained according to the invention be applied in the form of a layer, the dyes in such amount can be used that a considerable increase in diffusion density, i.e. one Density increase of 0.1 or mohr is achieved. If necessary, sum can read such Negatives also have a thin transparent polymer film with a dye neutral density is colored, in combination with that obtained according to the invention Projection screens are used. When using such a polymer film does not need the surface of the projection screen facing the viewer to be roughened to seim, but the polymer film used can have a roughened surface.

The invention is illustrated in more detail by the accompanying drawing, 1 shows the arrangement of the projection screen obtained according to the invention with regard to the prejector and the viewer in sight and FIG. 2 a greatly enlarged view vertical cross-section through a obtained according to the invention, for use as Projection screens suitable polypropylene film, the projection screen shown in Fig. T is between the projector 11 the viewer or the audience i2 in the way arranged that the image is projected onto the side facing away from the viewer. The image-like light projected onto one side of the screen passes through the Projection screen through and'can be on the other side of the Screen can be absorbed by the eye of the beholder.

In the cross section shown in FIG. 2 through a device according to the invention The film obtained is the distribution of the polypropylene crystals of different sizes and number, -as it has been found to be essential to the invention and above was discussed in detail, recognizable.

The following example is intended to explain the invention in more detail, B e is p i e l Using well known devices and tools, a Polypropylene film with a melt viscosity of 2.5 and a density of 0.90 g / cm3 at a temperature of 22,000 directly onto the surface of a steel roller.

whose surface temperature was kept at 9000, extruded. the The film used had a thickness of 762μ.

The temperature of the air surrounding the roller was kept at 930C. The diameter of the chill roll used was 6.35 cm and the melted one Polypropylene was fed at a speed of 0.9 m / min. continuously to one Deformed foil. The extruded sheet was made with the surface of the steel roller as a whole Brought into contact for 9 seconds. It turned out that the specified time was sufficient to solidify the surface of the oxime with the formation of thin layers made of low crystallinity polypropylene on both surfaces of the film.

The resulting film was then on the surface of a second Roller of the same type and there under the same conditions as on the first roller held. After leaving the surface of this second, comparatively cold roll, the film was passed into a vertical chamber in which the air temperature was held at 9300. The temperature of the film surface and that of the film surrounding air was total in that way for about 90 seconds kept at 93 0C that the slide is attached to a number of electrical heating devices, which were mounted along the rollers arranged in the chamber, was passed by. As the film passed through the vertical chamber, the film became strong translucent, which was due to the fact that it was in the core zone of the film a large number of comparatively large crystals with a diameter in the specified Area trained.

The resulting film was then peeled from the chamber and under cooled to room temperature under normal conditions. The cooled slide was then cut into individual pieces of the desired size of the projection screens, whereupon these were provided on one side with a roughness pattern that them in a conventional known hydraulic press under a pressure of 9.84 kg / cm2 between a very smooth steel plate and a roughness pattern of 63.5 x 10 6 cm (25 microinches) plate were pressed.

The projection screens obtained in this way had a "Q value" of 31 based on the following evaluation grades for, the specified four Criteria composed: light spot, flicker, contrast, brightness, Q-value effect effect ~~~~~~~~ ~~~~~~~~~~~ 7 8 9 7 31 The results show that the after Method of the invention obtained film represents a projection screen, which according to none of the four specified criteria is to be assessed worse than with the assessment grade 7 and therefore in a particularly advantageous manner by a very balanced Range of advantageous properties.

In further experiments that obtained by the method of the invention was obtained Proection screen in a usual known, to project the Bild'auf the side facing away from the viewer intended for the projection apparatus Projection of 35 mm photocopies; recorded on a transparent color film were used. It was found that the screen obtained according to the invention Passing through a very sharp image that is not only free of undesirable things Veil, but also a very faithful color reproduction of the one on the original located-color image is guaranteed.

Claims (3)

P A T E N T A N S P R Ü C H E Wl projection screen in which the image is facing away from the viewer Side is projected, characterized in that the screen made of polypropylene with a melt flow rate determined according to the ASTtl test method D 1238 of at most 90 is constructed and has a thickness of about 500 to about 2000 and that the screen furthermore to at least about 60 wt .-% of crystalline spherulites with an average Is about 5 to about 20 microns in diameter and has a core layer, which is at least 3/4 the thickness of the screen and compared to the two Surface layers of the screen are comparatively large crystalline spherulites about 15 to about 45 microns in diameter. 2. Projection screen according to claim 1, characterized in that ar a rough surface with bumps of about 25 to 76 x 10 6 cm (10 to 30 microinches). 3. Verrahren for the production of projection screens according to claim 1, characterized in that a) initially an approximately 500 to 2000 p strong, Melted film made of a polypropylene with one according to the ASTM test method D 1238 determined melt flow rate of at most about 90 with at least one a temperature of about 85 to 170 ° C located surface of a heat exchanger brought into contact until the surfaces of the film are solidified, and b) then the solidified surfaces of the film for so long at one temperature heated from about 85 to 1600C, to at least three quarters of the core zone of the film, based on the total thickness of the film, are practically crystalline. 4, method according to claim 3, characterized in that one Foil with a thickness of about 635 to 1525 used and this in the process stage a) with a surface located at a 'temperature of about 93 to 150 ° C a heat exchanger in contact as well as the solidified surfaces of the obtained In process step b), the film is heated to a temperature of about 93 ° to 1200C. 3. The method according to claims 3 and 4, characterized in that one of the two surfaces of the polymer film is also uniform Roughness in the form of bumps of about 25 to 76 x 10-6 cm (10 to 30 microinches) confers in a statistically random distribution. u. Method according to claim 5, characterized in that one A roughness pattern is imprinted on the two surfaces after the temperature of this Surface is lowered to below about 8500. L e r s e i t e