METHOD AND DEVICE FOR THE INVESTIGATION OF THE HUMIDITY-RELATED BEHAVIOUR OF POWDERS

Abstract: The invention relates to a method for studying hygroscopic behaviour of a material wherein a sample of the material to be studied is weighed in the beginning of the test with a balance connected to a programmed processor; the sample is exposed to an atmosphere with known relative humidity; the sample is weighed in the atmosphere at several successive points of time and the weighing results introduced into the processor, and the successive weighings are continued for a desired time. It is characteristic that in the beginning of the test several samples N1, N2, N3, ... are weighed, the samples are simultaneously exposed to the atmosphere with the known relative humidity, the separate samples are repeatedly weighed in the atmosphere and the weight readings of each sample and the point of time of weighing of the sample in question are recorded with the processor.

Description: M etho d and device for the investigation of the humidity-related behaviour of powders The invention relates to a method and apparatus for studying hygroscopic behaviour of a powder material. In the following the term "hygroscopic behaviour" denotes moisture adsorption or desorption of the material in air with known relative humidity. The term also covers the rate of said moisture adsorption or desorption as well as the equilibrium moisture content of the material. In an atmosphere with a specific relative humidity, a material may adsorb moisture i.e. it is hygroscopic, desorb moisture i.e. it is efflorescing or neither of the mentioned phenomena occurs . In the last-mentioned case the material is in an equilibrium state with the surrounding atmosphere and the moisture content of the material is then called the equilibrium moisture content. The equilibrium moisture content of a material depends, besides on the physical and chemical properties of the material itself, on the relative humidity and temperature of the surrounding atmosphere. Adsorption isotherm is a curve that describes the equilibrium moisture content of the material as a function of the relative humidity of the atmosphere at constant temperature (Fig. 1). Adsorption isotherm describes a situation wherein the starting moisture content of the material has been lower than the equilibrium moisture content at that atmosphere to which the material has been exposed. If, on the contrary, the material has been conditioned to equilibrium moisture content in an atmosphere with very high relative humidity and the material is exposed to an atmosphere with a lower relative humidity, the equilibrium moisture content of the material will settle on a value above the adsorption isotherm i.e. on the desorption isotherm (Fig. 1) which describes a situation wherein the starting moisture content of the material has been higher than the equilibrium moisture content at that atmosphere to which the material has been exposed. Due to this hysteresis phenomenon, the "history" of the material i.e. the humidity conditions at the space where it has been stored before being exposed to the new atmosphere, is also an important factor which has an influence on the equilibrium moisture content. From the point of view of storing, handling, and processing of a material it is important to know how the material reacts with the humidity of the surrounding air, in other words, to which moisture content the material settles and how fast moisture adsorption or desorption takes place. Fig. 2 shows weight change of a material as a function of time when dry samples have been exposed to atmospheres with different relative humidities. If two or more powders must be mixed and processed into a final product, it is essential to know how these materials affect the hygroscopic behaviour of one another. For example in pharmaceutical industry, it has been observed that uncontrolled hygroscopic behaviour of powder raw materials cause all kinds of problems in the manufacture of solid drugs such as tablets. Poor tabletting is often corrected by changing the moisture content of the tabletting mass. Mechanical strength of tablets and rheological properties of powders can be controlled by changing the moisture content of the tablet mass. Excessive moisture content is harmful in the sense that it makes microbiological growth possible. If the hygroscopic behaviour of different powder components are known, such materials can be selected to the formulation which in this sense match well together. Most commercial apparatuses in use measure primarily the moisture content of a solid sample. However, moisture content does not provide sufficient information about the hygroscopic behaviour of the material in process or application situations. Commercial instruments do not provide any information about the adsorption or desorption rates of the material. In the publication European Journal of Pharmaceutical Sciences,1(1994) , 203-8, Pirttimaki and Laine describe an apparatus which allows determination of the adsorption or desorption rate and equilibrium moisture content for a small amount of material. The apparatus comprises a microbalance coupled to a computer, a sample container suspended from the balance, a humidity chamber and a lift whereby the humidity chamber can be lowered or lifted so that it surrounds the sample under study. The humidity chamber is a vessel which opens upwards. The relative humidity in the humidity chamber can be controlled with various saturated salt solutions by putting such a saturated salt solution in the vessel that produces the desired humidity. Table 1 presents some suitable salt solutions and the relative humidities they produce. Silica gel provides 0 % and water 100 % relative humidity. A water bath is around the humidity chamber which can also be lowered and lifted. The water bath allows control of the temperature in the humidity chamber. The apparatus is used so that a small amount of sample is placed in the container suspended from the microbalance and initial weighing is performed. Then the humidity chamber is lifted around the sample container. By monitoring and taking the balance readings of the sample to the computer one obtains the change in the moisture content of the sample as a function of time. When there is no more change in the weight of the sample, the equilibrium moisture content of the sample in the relative humidity in question has been reached. By performing a similar test with another sample and a humidity chamber with another relative humidity (i.e. with another salt solution), the equilibrium moisture content of the material in another relative humidity is obtained. The mentioned values of the equilibrium moisture content constitute points on the adsorption/desorption isotherm curve of the material . In studying the hygroscopic behaviour of a material and in constructing its isotherm curve it is reasonable to use several measurements for each point of the curve. Using the apparatus described above this is done so that several successive tests are done in the same relative humidity. This is, however, very time consuming since it takes several hours to reach the equilibrium. Table 1. Some salts and the relative humidities produced by their saturated salt solutions Salt Relative humidity, % 20 °C 25 °C 30 °C LiClxH 2 0 12.4 12.0 11.8 CH 3 COOK 23.3 22.7 22.0 MgCl 2 x6H 2 0 33.6 33.2 32.8 K 2 C0 3 x2H 2 0 44.0 43.8 43.5 KN0 2 49.0 48.1 47.2 Mg(N0 3 ) 2 x6H 2 0 54.9 53.4 52.0 NaN0 2 65.3 64.3 63.3 NaCl 75.5 75.8 75.6 ( NH« ) 2 S0, 80.6 80.3 80.0 KN0 3 93.2 92.0 90.7 K 2 S0 4 97.2 96.9 96.6 A solution has now been found whereby several samples can be put simultaneously into the humidity chamber. In this way, the necessary duplicate tests can be carried out during the same test period, which considerably speeds up the study of the material . The typical features of the invention appear in claims 1 and 6. The invention will be explained in the following referring to the enclosed drawings in which Fig. 1 shows the adsorption and desorption isotherms of a material, Fig. 2 shows the weight change of a material as a function of time when dry samples have been exposed to atmospheres of different relative humidities. Fig. 3 shows an apparatus according to the invention, Fig. 4 shows a sample carousel of the apparatus of Fig. 3 according to one embodiment. Fig. 5 shows another embodiment of the sample carousel of the apparatus of Fig. 3, and Fig. 6 shows a sample container provided with a handle. Fig. 3 shows one solution for the apparatus to realize the method of the invention. The apparatus comprises a balance 1 connected to a programmed processor 15, from which balance hangs a suspension string 3 with a hook to carry a sample container 20. Fig. 4 shows in greater detail how the different sample containers 20a, 20b, 20c ... have been placed on the carousel 4. Fig. 6 shows that the sample container 20 is provided with a handle 25 for lifting the container. The sample container is preferably provided with a wide bottom so that the sample can be spread onto it as a thin layer so that the sample becomes efficiently ventilated. The humidity chamber 2 can be fitted around the carousel 4 by hoisting with a lift 9. A desired relative humidity can be accomplished in the humidity chamber 2 by placing a suitable salt solution 7 onto the bottom of the chamber 2. The rotation of a vertical axle 16 coupled to a motor 5 is transmitted to the carousel 4 which may be rotated in either direction as indicated by the arrows. Temperature in the humidity chamber 2 can be controlled by a water bath 8 placed around it. Fig. 3 shows the humidity chamber 2 together with the surrounding water bath 8 in the uplifted position. Humidity and temperature probes are marked with the reference number 6. Reference number 13 denotes the data transmission cables between the processor and the balance. Reference number 14 denotes the data transmission cables between the processor and the motor 5 driving the carousel 4 and the motor 10 driving the lift 9. Reference number 11 denotes the frame of the apparatus and number 12 the casing protecting the balance. The vertical axle 16 connected to the carousel 4 is arranged movable also in its longitudinal direction as indicated by the arrow. The apparatus is operated in the following way. The lift 9 lowers the humidity chamber 2 with its water bath 8. Samples Nl , N2, N3, .. are placed into tared sample containers 20a, 20b, 20c... The vertical axle is lifted with the motor 5 so that the plate 4 reaches such a height that the hook in the suspension string 3 goes through the handle 25a of the sample container 20a when the plate 4 is rotated by the motor 5 in clockwise direction. When the sample container suspends from the hook, the rotating motion of the plate 4 is halted and the plate 4 is lowered down so that the sample container is left hanging from the suspension string 3 and weighing takes place. The weight reading is transferred to the processor 15. After the weighing, the plate 4 is lifted up so as to leave the sample container resting on the plate, the plate is slightly rotated counterclockwise to release the hook of the suspension string 3, the plate is lowered and it is rotated clockwise so that the sample container 20a passes the hook. Then the plate is lifted to a suitable height for attaching the next sample container 20b with the hook of the suspension string 3 and all the above operations are repeated at each sample container. It is important for successful weighing of the different samples that the containers be situated at equal distance from the centre of the plate and therefore the disc 4 may have marked places for the sample containers. Control is preferably realized so that the rotating motion of the disc is halted when the disc is lifted or lowered. Initial weighing of the different samples should be done in the shortest possible time and therefore the plate is rapidly rotated to the subsequent sample to be weighed. After the initial weighing of each sample Nl, N2, N3 ... has been carried out, the humidity chamber 2 is lifted so that it surrounds the carousel with its samples and the sample containers are weighed each in turn in the manner described above. The computer program notes the weight reading and the point of time of weighing for each sample. The test may be carried out so that immediately after the first weighing round a second weighing round is carried out and so on. In this case the carousel is continuously in motion. Alternatively, a specific pause may follow different weighing rounds. This is a good practice if the weight readings of the samples do not vary very rapidly as a function of time in the atmosphere of the humidity chamber. From the weight readings of samples Nl, N2, N3,... taken from the same material to be studied, the moisture adsorption or desorption curve which fits best to all the weight readings obtained can be calculated with the processor for the material in question. If it is desired to study the hygroscopic behaviour of several different materials, one can in a similar manner take samples from several materials to be studied and study them in parallel during the same test period. From the weight readings of samples Nl, N2, N3,... taken from different materials to be studied the moisture adsorption or desorption curves each of which fits best to all the weight readings of the material in question are calculated with the processor for the different materials. If the moisture content of the material to be studied is determined by a separate test and the value is fed into the processor and if weighing is carried out in the humidity chamber as long as there is no change in the weight of the different samples as a function of time, the equilibrium moisture content of the material in question can be calculated at the temperature and relative humidity of the ambient atmosphere. The carousel 4 may optionally be a circular disc rotating in a vertical plane as shown in Fig. 5. An axle (not shown in the figure) passing through the centre of the disc is at right angles to the vertical axle 16 and transmits its rotating movement to the disc 4. In this solution there are preferably seats for the sample containers (the seats are not shown in the figure). The seats are preferably mounted on bearings to the disc so as to maintain a constant position despite the rotating motion of the disc. This can be simply accomplished e.g. by suspending weights from the seats preventing them from turning upside down. The method according to the invention may of course be carried out by solutions other than the one described above. For example, it is quite possible to have a plate on which the sample containers rest performing a to and fro motion instead of the rotating motion described above. It is also possible that the suspension string connected to the balance is moving and the plate with its sample containers remains stationary. It is obvious to the specialist in the field that different embodiments of the invention may vary within the enclosed claims .

Claims: CLAIMS 1. Method for studying hygroscopic behaviour of a powder material wherein a sample from the material to be studied is weighed in the beginning of the test with a balance connected to a programmed processor; the sample is exposed to an atmosphere with a known relative humidity; the sample is weighed in the atmosphere at several successive points of time and the weighing results are taken into the processor, and the successive weighings are continued for a desired time, characterised in that - in the beginning of the test several samples Nl, N2, N3 ... are weighed, - the samples are simultaneously exposed to the atmosphere with the known relative humidity, - the separate samples are repeatedly weighed in the atmosphere and the weight readings of each sample and the point of time of weighing of the sample in question are recorded with the processor. 2. Method according to claim 1 characterised in that from the weight readings of samples Nl, N2, N3, ... taken from the same material to be studied the moisture adsorption or desorption curve which fits best to all the weight readings is calculated with the processor for the material. 3. Method according to claim 1 characterised in that from the weight readings of samples Nl, N2, N3,... taken from different powder materials to be studied the moisture adsorption or desorption curves each of which fits best to all the weight readings of the material in question are calculated with the processor for the different materials. 4. Method according to claim 1, 2 or 3 characterised in that the moisture contents of the samples are fed into the processor in the beginning of the test and weighings are carried out as long as there is no change in the weight of the different samples as a function of time and equilibrium moisture content of the material in question is calculated at the temperature and relative humidity of the ambient atmosphere. 5. Method according to any of the claims 1-4 characterised in that the atmosphere is adjusted to a desired temperature during the test. 6. Apparatus for studying hygroscopic behaviour of a powder material by the method of claim 1, the apparatus comprising a sample container (20), a balance (1), a programmed processor (15), a humidity chamber (2), and a lift (9) for bringing the humidity chamber around the sample to be studied characterised in that it further comprises - a carousel (4) on which several sample containers can be placed and devices (5,16) for driving and controlling the carousel . 7. Apparatus according to claim 6 characterised in that the carousel is a horizontally rotating, circular disc (4J provided with a vertical axle (16) passing through its centre, the rotating motion of the axle (16) being transmitted to the disc (4). 8. Apparatus according to claim 6 characterised in that the carousel is a vertically rotating, circular disc (4) provided with an axle passing through its centre at right angles to the vertical axle (16) transmitting the rotating motion of it to the disc (4), and that the disc (4) comprises seats for the sample containers mounted on bearings so as to maintain them a constant position despite the rotating motion of the disc. 9. Apparatus according to claim 7 or 8 characterised in that the disc (4) has seats for the sample containers, which are all situated at equal distance from the centre of the disc. 10. Apparatus according to claim 7, 8 or 9 characterised in that the vertical axle (16) is arranged movable also in its longitudinal direction. 11. Apparatus according to any of the claims 6-10 characterised in that each sample container (20) is provided with a handle (25) and that a suspension string (3) provided with a hook is connected to the balance (1). 12. Apparatus according to any of the claims 6-11 characterised in that the temperature of the humidity chamber (2) is adjustable e.g. by means of a water bath (8) around it.

Inventors: AARNIO MIKA, LAINE ENSIO, PIRTTIMAEKI JUKKA, AARNIO, MIKA, LAINE, ENSIO, PIRTTIMAEKI, JUKKA
Applicants: AARNIO MIKA, LAINE ENSIO, PIRTTIMAEKI JUKKA, AARNIO, MIKA, LAINE, ENSIO, PIRTTIMAEKI, JUKKA
Application year: 1996
Main classification: G01N5
IPC codes: G01N5