The present invention relates to a press for pressing thin films of thermoplastic sample materials, with two opposite mould plates between which the sample material to be pressed is placed and means for pressing the mould plates against each other. The sample materials may be, for example, PE or PVC plastic. Samples in the form of a plastic film are suitable, for example, in infrared and thermoanalysers.

It is well known that the preparation of a sample, for example, for an infrared analysis is often the most critical stage in an analytical method. In the prepara¬ tion of thermoplastic materials for an infrared analysis the following methods will come into question:

1. The potassium bromide powder method

2. The microtome method

3. The extraction method

4. The hot pressing method. Of these methods the last-mentioned is in general the quickest one which in general results in a reasonably high-quality prepared sample. However, the preparation of thin sample films of constant thickness has been difficult . The object of the invention is to provide a new plastic film press by means of which very high-quality sample films can be prepared in a simple manner.

The press according to the invention is mainly characterized in that a peripheral groove for receiving excess sample material is formed in the press surface of one of the mould plates.

By means of the apparatus according to the inven¬ tion a thin plastic film having a constant thickness, for example, 20 urn or 50 n is pressed of the samples

to be examined. For example, an infrared spectrum is run of the film. When using samples having a thickness of 20 urn the important infrared absorption bands are sharp enough for qualitative purposes so that they can be used for a quick identification of the plastic. When aiming at more exact, quantitative results thicker films, for example, 50 or 100 urn should be used. The diameter of the film is about 13 mm due to which it fits into a conventional potassium bromide tablet holder. When the film to be pressed has a thickness of

20 yum, a suitable sample quantity is more than 3 mm , preferably 10 to 20 mm . By means of the beam condenser technique a spectrum can be easily run also of a film pressed of a smaller sample. It is also possible to run a spectrum of a thinner sample in which case the ordinate spreading technique is preferably used.

The sample may be in the form of granules or a small plastic bit. Fine powders, on the contrary, should first be sintered because they easily leave the mould of the press when the pressure is increased.

It is known to prepare plastic films by pressing between plates in which case the thickness of the film is adjusted by means of a polytetrafluorethylene plastic film. However, in this case films of an even thickness will not be obtained but the thickness will depend on the pressing force and the pressing temperature (see, for example, Philips Pye Unicam Infrared Sampling Accessories, p. 12).

By means of the new plastic film press films of an extremely even thickness are obtained in addition to which the thickness of the films each time is precise¬ ly the same, for example, 20 urn. This permits a standard treatment of thermoplastic samples while obtaining a very good comparability.

The quality of the primary plastisizer of soft polyvinylchloride plastic pressed by means of the press according to the invention can in general be directly determined by means of the infrared spectrum. During thermal treatment some additives of the plastic sample, such as peroxides, antioxidants and foaming agents may react. If, at the same time, data of the quality of these agents are required, the lowest possible pressing temperature should be used. In order to accelerate heating and to prevent disintegration of the agents a protective gas, for example, nitrogen may be fed to the press.

In the following, the invention will be described in more detail with reference to the accompanying drawing.

Figure 1 is a longitudinal section of the apparatus.

Figure 2 is a top view of the apparatus. Figure 3 is an exploded view of the apparatus. Figures 4 and 5 illustrate the upper mould plate in side view, partly in section, and in bottom view, respectively.

Figures 6,7 and 8 illustrate one embodiment of the lower mould plate in bottom and top view and in longitu- dinal section, respectively.

Figures 9 and 10 illustrate the housing of the apparatus in longitudinal section and in top view, respectively.

Figures 11 and 12 illustrate the counterpiece of the housing in longitudinal section and bottom view, respectively.

Figure 13 is a longitudinal section of a second embodiment of the lower mould plate.

Figure 14 is a top view of a ring to be fitted in the peripheral groove of the lower mould plate, and Figure 15 is a side view of a part of the ring. The housing of the apparatus is denoted by refe- rence numeral 1. To the housing is connected a hollow shaft 2 which terminates in a likewise hollow handle 3. The thermoplastic sample materials are pressed into a film between mould plates 4 and 5 abutting on the bottom of the housing 1. The required pressing is produced by means of a screw 7 turning in the counterpiece 6 of the housing 1, through a spring 8. The screw 7 acts on the spring 8 through a support piece 9 and a bearing 10; by means of the bearing 10 a direct and even transmission of the pressing force to the upper mould plate 4 is ensured so that said plate has.no tendency to rotate. As appears from Figures 1 and 3, the upper mould plate 4, the spring 8, the supporting piece 9, the bearing 10 and the screw 7 are joined together by means of a*5retaining screw 11 and a pin 12 which is formed in the mould plate 4 and termi- nates in a thread 13 extending into a thread bore 14 in the retaining screw 11. The screw 7 abuts, on one hand, on the counterpiece 6 through threads 15 and 16, and the counterpiece is fastened to the housing 1 by means of fastening elements 17 and 18, preferably in a manner similar to, for example, bayonet joints for pipes (Figures 3 and 9 to 12).

The housing 1 of the press together with the mould plates 4 and 5 and clamping elements is placed in a heating chamber 19 surrounded by a heating resistor 20, preferably a so-called band resistor. 21 denotes a connection piece for an electric cable, 22 denotes a protective casing, 23 a bottom plate and 24 a fixing screw by means of which the heating chamber and the bottom plate are fixed to the protective casing 22. For

tightening the band resistor 20 around the heating cham¬ ber 19, for example, a screw-nut joint 25 may be used.

The upper mould plate 4 is, as appears e.g., from Figures 4 and 5, very simple and provided with a smooth pressing surface which can preferably be nitrated to a depth of about 0.1 ^m.

The lower mould plate 5 again comprises an outer ring 26 and a plate 27 arranged within said ring; the ring 26 is preferably shrunk onto the plate 27 and pro- jects from the surface of the plate 27 while forming a stop surface 29 abutting on the upper mould plate 4. The stop surface 29 and the front surface of the plate 27 are also preferably nitrated. The gap left between the plates 4 and 27 determines the thickness of the film to be pressed of the sample material; a suitable thickness for existing infrared analysers is about 20 ^m. In the embodiment according to the Figures 6 to 8, the inner edge of the ring 26 and the outer edge of the plate 27 are bevelled so as to form a groove 28 around the plate 27. Figure 13 illustrates an alternative embodiment in which a corresponding groove 28a is made in the plate 27a surrounded by the ring 26a.

The quantity of sample material required for the film proper is quite small while at the same time remain- ing within strict limits, and a sufficiently precise dosage of sample material for the film only is extremely difficult. For this reason, by means of the presses used hitherto, in which the mould space has been dimensioned to an even thickness according to the thickness of the film to be pressed, is has nαt been possible to obtain films of a precisely equal thickness, which again is a desicive factor in-order to be able to compare the analysis results with each other. In the apparatus according to the invention, on the contrary, excess

sample material is collected in the groove 28 (or 28a) due to which the film proper will always have an exactly equal thickness. The dosage of sample material need not be very precise because the volume of the groove 28 is rather large. In addition to these advantages, the handleability of the pressed plastic film is improved because of the stiffening rim formed around the film. The rigidity of this rim can be further considerably improved by placing in the groove 28 a corrugated ring 37, Figures 14 and 15, which may be e.g., of bronze and which remains in the rim of the sample film.

Preferably the apparatus is dimensioned according to existing analysers; a very common diameter dimension of sample films used in these analysers is 13 mm. The small press plates 4 and 5 will be quickly heated to the required temperature, usually the user of the apparatus will himself be able to estimate the required time on the basis of some experiments. Sometimes, however, it will be necessary to follow the temperature more precisely wherefore preferably a bore 35 coinciding . with a bore 26 made in the shaft is made in the lower mould plate, for a thermometer. In order to align the bores 35 and 36, openings 31 and 32 for a retaining pin (not shown in the drawings) are made in the plate 5 and the housing 1, respectively. For cooling the apparatus, an opening 33 is made in the bottom of the housing 1. Cooling gas, for example, air flows from said opening through grooves 30 on the underside of the mould plate 5 and further between the plates 4, 5 and the housing 1 out through openings 34 made in the wall of the housing.

If no advance information is available of the melting properties of the sample to be pressed, the final pressing temperature can be determined by tighte¬ ning the screw 7 at suitable intervals as the temperature

increases. When the temperature is reached at which the thermoplas ic sample material starts to melt, this will be noted from the fact that the screw is easily tighten- able, at which moment heating can be stopped. Example 1

Preparation of a polyvinylchloride sample The lower mould plate 5 is placed in the mould housing or the housing 1, and the sample, about 10 mm of polyvinylchloride plastic, is placed in the central part of the plate 5. The corrugated bronze ring 37 is placed in the groove 28 in the plate. The counter-piece of the press carrying the upper mould plate 4 is posi¬ tioned in place and locked, and the sample is lightly locked in place by means of the clamping screw 7. The press is transferred to the heating means 19, and the sensor of a thermometer is pushed through the channel 36 in the handle of the press into the bore 35 in the lower counter plate 5, and heating is connected. When the required temperature of 160 C is reached in the press, i.e. within about 3 to 6 minutes, The press screw is turned to a fully closed position, the press is lifted from the heating device, transferred to a cooling bed and cooled with pressure air for about 1 minute whereupon the sample is removed. The result is a 20 ^m thick plastic film having a diameter of 13 mm and surrounded by a plastic ring reinforced with a bronze ring and supporting said film. The sample is easy to transfer to an infrared spectrometer for analysis.

The invention is not limited to the embodiment according to the drawing only. The mould plate provided with a peripheral groove may be in one piece and may alternatively serve as the upper plate. Instead of the heating means 19 to 22 and the clamping means 6 to 11 also other solutions may naturally be used.