Triethanolamine (TEA) is widely used in industry, especially as base material for pharmaceutical and cosmetic products and surfactants, and must also meet increasingly strict criteria. One of these criteria relates to the colour index of TEA. This is because it is known that TEA has a tendency to become coloured during its manufacture and/or its storage, whereas TEA is normally a colourless product. This phenomenon is described in particular in"SRI International, Process Economics Program Report No. 193" (January 1991), pages 6-9 and 6-10. It appears that the coloration of TEA is a phenomenon which is both difficult to explain and to control, and that the proposed solutions generally consist either in using manufacturing and storage equipment made of special steel or in using an additive which reduces or eliminates the coloration. In the first case, the proposed solution is extremely expensive and, in the second case, the presence of an additive in TEA makes the latter less pure and may raise new toxicological problems.

An extremely simple and inexpensive process for decolourizing TEA during or after its manufacture has been found, this process having the advantage of avoiding the use of an anti-coloration additive. More particularly, the invention relates to a process for decolourizing TEA, characterized in that a coloured TEA is exposed to electromagnetic radiation of wavelength chosen within the range from 100 to 1200 namometres (nm), for a time long enough to reduce the colour of the TEA.

Figure 1 and 2 are, as an illustration, schematic representations of a device for carrying out the continuous manufacture of a purified and colourless TEA using the decoloration treatment according to the present invention.

The invention is particularly suitable for TEA manufactured by the reaction between ethylene oxide and ammonia, for example using one of the processes described in"SRI International, Process Economics Program Report No. 193" (January 1991), pages 6-1 to 6-9. The reaction is generally carried out by bringing ethylene oxide into contact with ammonia, for example in a molar ratio of ammonia to ethylene oxide of 0.5/1 to 40/1, preferably 1/1 to 10/1, in particular 1.5/1 to 6/1. The reaction may take place in aqueous medium so that the weight ratio of ammonia to water may be from 0. 5/1 to 1/1. The reaction may be carried out at a temperature of 0 to 150°C, preferably 20 to 100°C, in particular 40 to 80°C, and at an absolute pressure that can range from 0.1 to 15 MPa, preferably from 0.2 to 5 MPa, in particular from 0.2 to 2 MPa. It is preferred to carry out the reaction in aqueous medium, and especially continuously, so as to form a so-called"crude"TEA, comprising TEA as a mixture with one or more other ethanolamines produced during the reaction, such as monoethanolamine (MEA), diethanolamine (DEA) and possibly ethoxylated triethanolamines (ETEAs) (also called triethanolamine glycol ethers), in an aqueous medium possibly comprising one of the two reactants in excess, especially unreacted ammonia. The manufacturing process may then comprise one or more steps intended to separate the crude TEA from the aqueous medium possibly containing one of the two reactants in excess, especially unreacted ammonia, and then isolate a so-called"purified"TEA from the crude TEA, especially by separating the TEA from the other ethanolamines such as those mentioned above, preferably by distillation. The TEA separating and isolating steps may advantageously be carried out continuously. In a continuous manufacturing process, water and a possible excess of one of the reactants, especially unreacted ammonia, are generally separated from the crude TEA and are advantageously returned to the reaction zone.

The TEA decoloration process is particularly suitable for one of the TEAs manufactured according to one of the processes described above, especially a crude TEA or, preferably, a purified TEA, as mentioned above, and which, during or after its manufacture, is in the form of a coloured TEA. The expression"coloured TEA"is generally understood to mean a TEA having a colour index greater than 40 Pt/Co

according to the ASTM D 1209 standard. Moreover, the expression"purified TEA"is generally understood to mean a TEA having a TEA weight content equal to or greater than 85%, preferably equal to or greater than 99%, and possibly containing, as main impurity, generally DEA with a weight content of less than 15%, preferably less than 1% or even 0.5%.

The process of the invention is especially characterized in that a coloured TEA is exposed to electromagnetic radiation of wavelength chosen within the range from 100 to 1200 nm, preferably from 150 to 1000 nm, in particular from 170 to 800 nm, for example 200 to 780 nm. The radiation may lie completely or partly within the aforementioned wavelength range, preferably in the visible range, for example from 400 to 780 nm, and/or in the ultraviolet range, for example from 200 to less than 400 nm. It is possible to use radiation preferably covering part of the visible range and of the ultraviolet range, especially by using suitable filters or lamps for the desired type of radiation, in particular radiation having a wavelength ranging from 280 to 520 nm, preferably from 290 to 510 nm, in particular from 300 to 510 nm and especially from 300 to 460 nm. In particular, it is possible to use radiation chosen from within one or more of the following wavelength ranges: from 300 to 320 nm, from 350 to 370 nm, from 390 to 410 nm, from 420 to 440 nm, or from 450 to 460 nm, or else possibly from 500 to 510 nm, which wavelength ranges may be selected in a specific manner appropriate to the TEA to be treated.

The radiation used may be delivered by an electromagnetic radiation source, such as a lamp appropriate to the chosen wavelength range, possibly one or more specially selected filters being added to it. It is thus possible to use a xenon lamp, the emission spectrum of which corresponds substantially to that of daylight, that is to say a wavelength spectrum ranging from 200 to 700 nm approximately. It is also possible to use a high-pressure mercury vapour lamp, such as a lamp known by the reference "HBO 500 W"(g) and sold by Osram (Germany), or else a lamp operating in the ultraviolet, known by the reference"UV 125 W" (E) and sold by"Verre et Quartz Flashlamps" (France). One or more filters, such as special glass filters or filtering solutions, may be added to the electromagnetic radiation source. Thus, it is possible to use a filter operating at about 365 nm, such as a filter known by the reference"UG11 Optilas SV781162"fl) and sold by BFI Optilas (France). It is also possible to use a filter

operating at about 430 nm, resulting from the combination of two filtering solutions comprising, in succession after the lamp, firstly a solution containing 25 g/1 of CuS04-5H20 in NH40H (optical path of 1 cm) and secondly a solution containing 75 g of NaN02 in 100 ml of water (optical path of 1 cm). It is also possible to use a filter operating at about 320 nm, resulting from the combination of two filtering solutions comprising, in succession after the lamp, an equivolumic aqueous solution of 500 g/l NiS04-6H20 and of 240 g/1 CoSO4 7H20 (optical path of 1 cm) and an aqueous solution of 7.5 g/1 CuS04-5H20 (optical path of 1 cm).

The coloured TEA is exposed ta electromagnetic radiation for a time long enough to reduce the colour of the TEA. The period of exposure to the radiation may depend from the type of the electromagnetic radiation source used, particularly if the said source radiates the electromagnetic radiation continuously or discontinuously with the time. The period of exposure to the radiation may be from 0.1 second to 50 hours, preferably from 30 seconds to 50 hours, in particular from 1 minute to 40 hours, especially from 0. 1 to 35 hours. The reduction in colour may be assessed by measuring the colour index of the TEA according to the ASTM D 1209 standard before and after exposure to the radiation. It may be considered that the time for which the coloured TEA is exposed is long enough when the colour index of the treated TEA becomes equal to or less than 40 Pt/Co, preferably equal to or less than 30 Pt/Co, especially equal to or less than 25 or even less than 20 Pt/Co (according to the ASTM D 12. 09 standard).

As an illustration, the colour index of TEA (according to the ASTM D 1209 standard) may be measured using a colour meter known by the reference"CT 320"0 and sold by Minolta (Japan).

The electromagnetic radiation may be applied to the TEA with a radiant energy which may cover a wide range and which is generally dependent on the period of exposure to the radiation. The intensity of the radiation may have a mean value (expressed in milliwatts per cm2 of exposure area of the TEA to be treated) ranging from 10 to 2000 mW/cm2, preferably from 20 to 1500 mW/cm2, for example from 30 to 1200 mW/cm2. The intensity of the radiation emitted may be determined and monitored by means of an actinometer, for example a chemical actinometer, such as an actinometer based on potassium ferrate oxalate (K3Fe (C204) 3'3H20). Moreover, it is possible to use a radiation source, like the one mentioned above, to which may be added one or more

selective filters of a wavelength range like those described above.

The electromagnetic radiation may be applied to the coloured TEA with the help of a source radiating the said radiation continuously with the time, in particular with the help of a continuously radiating lamp, such as the previously described lamps, e. g. a xenon lamp or a high pressure mercury vapour lamp. The period of exposure of the coloured TEA to such a radiation may be relatively long, for instance from 10 minutes to 35 hours, preferably from 30 minutes to 30 hours, especially with a radiant energy and/or an intensity of the radiation relatively low, such as the intensity previously described.

Preferably, the electromagnetic radiation may be applied to the coloured TEA with the help of a source radiating the said radiation discontinuously with the time, in particular with the help of a flash lamp. By flash lamp it is generally understood a gaseous-discharge lamp used in a photoflash unit to produce flashes of electromagnetic radiation or light of short duration and high intensity. In the photoflash unit, the flashes are generally produced by a high-current pulse of an electrical discharge with the help of a capacitor-charging power supply. A flash lamp may radiate a continuous emission spectrum, particularly in the visible and/or ultraviolet range, e. g. with wavelengths from 200 to 500 nm about, each flash being produced in a very short time, preferably several microseconds, e. g. from 5 to 100 u. s (microseconds), with a frequency in a range from 0.1 to 100 Hz, preferably from 1 to 60 Hz. The energy of a flash lamp may be from 0.3 to 300 J, preferably from 5 to 150 J. The period of exposure of the coloured TEA to such a radiation may be advantageously relatively short, e. g. from 0.1 second to 10 minutes, preferably from 0.2 second to 5 minutes, in particular from 0.3 second to 1 minute, preferably with an energy radiated by the flash lamp in a range from 100 to 2000 J per cm2 of the coloured TEA exposed to the said radiation. It can be used a flash lamp sold by"Verre et Quartz Flashlamps" (France) under the commercial reference "VQXN 1015 DU 600 WOTS"@ or"VQ 7Q50 AP45"@. With the flash lamp, it can be also used a capacitor-charging power supply sold by"Eurofeedback" (France) under the commercial reference"GLP 3400"@ or"Get 125/4000"(E).

The treatment process of the invention may be carried out at a temperature such that the TEA is in the liquid state, for example at a temperature of 20 to 190°C, preferably 20 to 120°C, especially 25 to 60°C. It may be carried out in an atmosphere

inert with respect to TEA, for example in nitrogen or else in ambient air, and at an absolute pressure ranging from 10-2 to 103 kPa, especially from 5x10-2 to 5xl o2 kPa.

The treatment process of the invention may be particularly advantageous when it is carried out continuously. Thus, for example, the process may comprise continuous exposure of a coloured TEA to the electromagnetic radiation as described above. In this case the coloured TEA may be used continuously in the form of a continuous stream which is exposed to the electromagnetic radiation and which may, if necessary, be returned in order to be again exposed one or more further times to this radiation, especially by means of a loop.

The present invention also relates to a process for the continuous manufacture of a colourless triethanolamine (TEA) which may include the treatment described above.

In particular, the process may comprise, in succession: (i) a step for the continuous synthesis of TEA which comprises bringing ammonia into contact with ethylene oxide in aqueous medium under conditions for fonning a crude TEA comprising TEA, monoethanolamine (MEA) and diethanolamine (DEA), as a mixture with water and unreacted ammonia; (ii) a step of continuously separating the crude TEA from the water/ammonia mixture so as to recover the crude TEA; and (iii) a step of continuously isolating the TEA by distilling the crude TEA, so as substantially to separate the MEA and DEA from the TEA and isolate a purified TEA; which process is characterized in that, after the separating step (ii) and during or after the isolating step (iii), the crude or purified TEA is continuously exposed to electromagnetic radiation of wavelength chosen within the range from 100 to 1200 nm, for a time long enough to obtain a colourless TEA.

The treatment of the TEA by exposure to electromagnetic radiation, carried out in the present manufacturing process, is for example identical to that described in detail above for the decoloration treatment of TEA. The crude TEA and the purified TEA, such as those obtained in steps (ii) and (iii) of the process, are especially those described above in the case of TEA decoloration and may, in particular, be in the form of a coloured TEA, especially having a colour index greater than 40 Pt/Co according to the ASTM D 1209 standard. The continuous manufacturing process of the invention makes

it possible to provide a colourless TEA, especially one having a colour index (according to the ASTM D 1209 standard) equal to or less than 40 Pt/Co, preferably equal to or less than 30 Pt/Co, especially equal to or less than 25 or even less than 20 Pt/Co. In particular, the time for which the TEA is exposed to the electromagnetic radiation in the manufacturing process is such that the purified TEA thus obtained is colourless, especially according to the colour index expressed above in order to define the colourless nature of the TEA.

In the continuous TEA manufacturing process, the step of exposing the TEA to the radiation may take place as soon as possible after the step (ii) of separating the crude TEA. It may therefore be carried out on the crude TEA once the latter has been separated from the water and ammonia, and in particular after the separating step (ii) but before the TEA isolating step (iii).

The step of exposing the TEA to the radiation may also be carried out during the TEA isolating step (iii), that is to say on a TEA in the process of being purified. In particular, exposure to the radiation may be carried out at any moment when the TEA has been substantially separated from the MEA and the DEA, especially between the moment when the TEA is firstly substantially separated from the MEA and the moment when the TEA is then substantially separated from the DEA.

The step of exposing the TEA to the radiation may preferably be carried out after the TEA isolating step (iii), that is to say on the purified TEA. It may be carried out immediately after the isolating step (iii) or also after the purified TEA has been recovered, for example in a zone for storing the latter, especially at a temperature of 25 to 60°C.

The step of exposing the TEA to the radiation may also be carried out at two or more times after the separating step (ii).

The step of exposing the TEA to the radiation may be carried out with the help of a source radiating the said radiation continuously with the time, in particular with a continuously radiating lamp as previously described. Preferably, it may be carried out with the help of a source radiating the radiation discontinuously with the time, in particular with a flash lamp as previously described.

The present invention also relates to a device for manufacturing a purified and colourless triethanolamine (TEA), comprising four successive zones (A), (B), (C) and

(D) successively and respectively connected together by transfer lines: (a) a TEA synthesis zone (A) comprising a reaction chamber in which ethylene oxide is brought into contact with ammonia in the presence of water, so as to form a mixture of water, ammonia and a crude TEA comprising TEA as a mixture with other ethanolamines formed during the contacting process; (b) a zone (B) for separating the crude TEA, comprising at least one, or preferably two, distillation columns placed in series, so as to separate the crude TEA from water and unreacted ammonia; (c) a zone (C) for isolating a purified TEA, comprising at least one or two, or preferably three, distillation columns placed in series, so as to separate a substantially purified TEA from the other ethanolamines; and (d) a zone (D) for storing the purified TEA, comprising at least one storage tank; which device is characterized in that it furthermore includes at least one radiation zone capable of delivering electromagnetic radiation of wavelength chosen within the range from 100 to 1200 nm and of exposing the crude or purified TEA to the said radiation, the radiation zone being placed at any point in the device downstream of the zone (B) so as to obtain in the zone (D) the purified and colourless TEA.

The radiation zone may be placed between the zones (B) and (C), or within the zone (C), or preferably between the zones (C) and (D) or else preferably within the zone (D).

More particularly, the plant comprises, in succession: (a) the zone (A) for synthesizing the TEA, comprising a reaction chamber into which run, directly or indirectly, one or more ethylene oxide, ammonia and water feed lines and a recycling line for an aqueous ammonia mixture, and from which runs a line for withdrawing the crude TEA comprising especially TEA, MEA and DEA which are formed in the zone (A), as a mixture with water and unreacted ammonia; (b) the zone (B) for separating the crude TEA, comprising at least one distillation column which is fed via the crude TEA withdrawing line of the zone (A) and which comprises at least one ammonia/water discharge line which communicates with the recycling line of the zone (A), and, at the bottom, a line for withdrawing the crude TEA;

(c) the zone (C) for isolating the purified TEA, comprising at least one distillation column which is fed via the crude TEA withdrawing line of the zone (B) and which comprises at least two lines for discharging mixture containing essentially and respectively MEA and DEA, and a line for recovering the purified TEA; and (d) the zone (D) for storing the purified TEA, which is fed via the recovery line of the zone (C).

The device according to the invention may include one or more radiation zones placed in parallel or in series, at one or more of the aforementioned points in the device.

The radiation zone may comprise a radiation chamber fitted with a radiation source capable of delivering electromagnetic radiation of wavelength chosen within the range from 100 to 1200 nm, preferably within one of the aforementioned ranges. One or more filters may be placed in the chamber along the path of the radiation, so as to select specific wavelength ranges suitable for the TEA treatment, such as the aforementioned ranges. In the radiation chamber, the radiation may be directed, either directly or indirectly, onto the crude TEA or preferably onto the purified TEA, especially through a window transparent to the said radiation, for example a quartz window, behind which the crude or purified TEA is exposed to the radiation.

The radiation zone may also include at least one radiation source, as mentioned above, which is immersed in the crude or purified TEA, especially within the zone (C) or the zone (D), or else at the point where the TEA passes between these two zones or at the point where the TEA passes to the outlet of the zone (D).

The radiation zone may also comprise at least one radiation source, as mentioned above, past which the crude or purified TEA flows in the form of a stream, preferably continuously, and especially in the form of a thin film.

The radiation zone may comprise at least one source radiating the radiation continuously with the time, in particular at least one continuously radiating lamp as previously described. Preferably, it may comprise at least one source radiating the radiation discontinuously with the time, in particular at least one flash lamp preferably in a photoflash unit wherein flashes are produced by a high-current pulse of an electrical discharge preferably provided with a capacitor-charging power supply, as previously described.

It is preferred to place the radiation zone at the point in the recovery line where

the purified TEA passes between the zones (C) and (D), that is to say after the outlet of the zone (C) for isolating the purified TEA, but before the inlet of the storage zone (D).

Again according to a preferred variant, the radiation zone may be placed in the zone (D) for storing the purified TEA, especially at the inlet of the zone, or within the zone, for example inside a storage tank, or else at the outlet of the zone, that is to say just at the moment when the TEA is delivered to a customer. The radiation zone may also be placed in a recycling loop provided with a pump and placed in parallel with the storage zone (D) so that the recycling loop leaves from the zone (D) and returns thereto.

Figure 1 shows schematically, as an illustration, a device for continuously manufacturing a purified and colourless TEA using especially the decoloration treatment according to the invention. The device comprises a zone (A) for the continuous synthesis of TEA, essentially comprising a reaction chamber (1) into which run, indirectly, an ethylene oxide feed line (2) and an ammonia feed line (3) via a line (4) for recycling a water/ammonia mixture. Running from the chamber (1) is a line (5) for withdrawing a crude TEA, comprising TEA, MEA and DEA which are formed in the chamber (1), as a mixture with water and unreacted ammonia (e. g. in excess). The withdrawal line (5) runs towards a zone (B) for continuously separating the crude TEA, comprising two distillation columns (6) and (7) placed in series and connected together by a transfer line (8). In particular, the withdrawal line (5) runs into the distillation column (6), from the top of which essentially the unreacted ammonia (e. g. in excess) leaves via a discharge line (9). The crude TEA as a mixture with water leaves the bottom of the column (6) via the transfer line (8). The transfer line (8) runs into the distillation column (7), from the top of which essentially water leaves via a discharge line (10). The discharge lines (9) and (10) join together and run into the recycling line (4) which returns the water/ammonia mixture to the reaction chamber (1). The crude TEA, essentially stripped of water and ammonia, leaves the bottom of the column (7) via a withdrawal line (11). The withdrawal line (11) runs towards a zone (C) for continuously isolating a purified TEA, comprising three distillation columns (12), (13) and (14) placed in series and connected together, in succession, by transfer lines (15) and (16) respectively. More particularly, the withdrawal line (11) runs into the column (12), from the top of which essentially MEA leaves via a discharge line (17). A crude TEA essentially stripped of MEA leaves the bottom of the column (12) via the transfer

line (15). The transfer line (15) runs into the column (13), from the top of which essentially DEA leaves via a discharge line (18). A crude TEA, essentially stripped of MEA and DEA, but containing ETEAs, leaves the bottom of the column (13) via the transfer line (16). The transfer line (16) runs into the column (14), from the bottom of which essentially the ETEAs leave via a discharge line (19). The purified TEA leaves the top of the column (14) via a recovery line (20). The recovery line (20) passes through a radiation zone (21) comprising an electromagnetic radiation source capable of emitting radiation of wavelength chosen within the range from 100 to 1200 nm (or another preferred range as mentioned above), optionally at least one filter (as described above) placed in the path of the radiation, the radiation being directed towards a window (transparent to the said radiation) placed in the recovery line (20). After having passed through the zone (21), the recovery line (20) runs towards a zone (D) for storing the purified and colourless TEA. More particularly, the zone (D) comprises a storage tank (22) fitted with a withdrawal line (23) capable of delivering the purified and colourless TEA intended for customers.

According to another variant, Figure 2 shows schematically, as an illustration, a device exactly identical to that shown in Figure 1 except for the fact that the line (20) for recovering the purified TEA does not pass through the radiation zone (21), but runs into the storage tank (22) of the zone (D). Moreover, the radiation zone (21) is identical to that shown in Figure 1, except for the fact that it is provided with a recycling loop (24) provided with a pump (not shown in Figure 2) and placed in parallel with the storage tank (22) so that the recycling loop (24) leaves the tank (22) and returns thereto.

The recycling loop passes through a radiation zone (21) identical to that described in Figure 1.

The following examples illustrate the present invention.

Example 1 Three specimens of purified and coloured TEA were continuously manufactured using a process as shown in Figure 1. The process comprised a step of synthesizing the TEA by reaction between ethylene oxide and excess ammonia in aqueous medium, in a molar ratio of ammonia to ethylene oxide of 2/1 at a temperature of 60°C, the resulting mixture comprising a crude TEA, water and unreacted ammonia, then being subjected to a step of continuously separating the crude TEA by distillation in two distillation

columns in series placed so as to separate the crude TEA from the water and ammonia, the crude TEA then being subjected to a step of isolating a purified TEA by distillation in three distillation columns placed in series so as to separate, in succession, substantially MEA, DEA and the ETEAs from the TEA and form a purified and coloured TEA. The three specimens of purified and coloured TEA had a TEA weight content greater than 99% and contained about 0.2% by weight of DEA. They had colour indices greater than 40 Pt/Co (according to the ASTM D 1209 standard), these colour indices differing from one another without it being possible to explain the reason for these differences.

Each of the three specimens of purified and coloured TEA was subjected to a decoloration treatment under the following conditions. A volume of 125 cm3 of TEA was placed, at atmospheric pressure and at room temperature (20°C), at a distance of 30 cm from a"UV 125W"(g) lamp (a continuously radiating lamp) sold by"Verre et Quartz Flashlamps" (France), which emitted radiation of wavelength in the range from 200 to 500 nm, the area of exposure of the TEA to the radiation being 95 cm2. The colour index of each of the TEA specimens was measured before and after various periods of exposure to the radiation using a colour index meter known by the reference "CT-320"(g) and sold by Minolta (Japan). The treatment conditions and the results are given in Table 1.

Table 1 TEA Initial Colour index (Pt/Co) after a period exposure of specimen colour No. index (Pt/Co) 2h 3h 4h 30h No. 1 50-20 20 20 No. 2 70 50-15- No. 3 100--25- These tests showed that a coloured TEA having an initial colour index (according to the ASTM D 1209 standard) substantially greater than 40 Pt/Co became colourless after having been exposed for various periods of exposure to radiation

according to the invention, so that the colour index of the resulting TEA was especially less than 40 Pt/Co, in particular equal to 25 or 20 or even 15 Pt/Co.

Example 2 A specimen of purified and coloured TEA was continuously manufactured using a process as shown in Figure 1 and comprising steps similar to those described in Example 1. The specimen of purified and coloured TEA had a TEA weight content greater than 99% and contained about 0.2% by weight of DEA. It initially had a colour index of 80 Pt/Co (according to the ASTM D 1209 standard) which differs from those of the specimens described in Example-1 without it being possible to explain the reason for this difference.

The specimen of purified and coloured TEA was subjected to a decoloration treatment under the following conditions. A volume of 60 cm3 of TEA was placed, at atmospheric pressure and at room temperature (20°C), at a distance of 5 cm from a flash lamp sold by"Verre et Quartz Flashlamps" (France) under the commercial reference "VQXN 1015 DU 600 WTS" (R). The flash lamp was provided with a capacitor-charging power supply sold by"EuroFeedBack" (France) under the commercial reference"GPL 3400"oxo. The flash lamp emitted an electromagnetic pulsed radiation of wavelength in the range from 200 to 500 nm, at a pulse-repetition frequency (PRF) of 20 Hz, the area of exposure of the TEA to the pulsed radiation being 20 cm2. The colour index of the TEA specimen was measured before and after various pulses of radiation and various periods of exposure to the pulsed radiation using a colour index meter known by the reference"CT-320"iE) and sold by Minolta (Japan). The treatment conditions and the results are given in Table 2.

Table 2 Number of flashes Period of radiation Colour index (Pt/co) exposure (second) 0 0 80 50 0. 325 75 100 0. 65 65 150 1. 625 55 200 2 40