Process and a dryer for drying polychloroprene sheets

Object of the invention is a process for drying polychloroprene wherein hot air is blown onto both sides of the moist polychloroprene sheet. Optionally the moist sheet is exposed to infrared rays also. A further object of the invention is the dryer consisting of a warm air section and an air cooling section and optionally an infrared section.

This invention relates to a process for drying polychloroprene sheets 
obtained by low-temperature coagulation on a rotating drum and to a dryer 
therefor. 
The low-temperature coagulation of polychloroprene sheets and the following 
working-up stages to form a solid polyschloroprene rubber, have long been 
known, and are described in Chem. Eng. Progr. 43 (1947), pages 391 to 398. 
In this known process, coagulation takes place on a cooled drum rotating in 
a latex bath. The band of rubber formed on the surface of the drum is 
lifted off and passed through a washing bath where it is sprayed with 
water in order to remove impurities. The wet band then travels through 
squeezing rollers where most of the water present is removed and enters a 
dryer. The moisture content of the sheets before entering the dryer 
amounts to about 30% by weight. After leaving the dryer, the sheet is 
condensed into a strand which is then cut into pieces of the required 
length and stored. 
In conventional processes, considerable difficulties are involved in 
carrying out the drying process in the dryer because, due to adverse flow 
conditions, it is only possible to obtain relatively low heat and mass 
transfer values. It is desirable that the sheet should have a residual 
moisture content of about .ltoreq. 0.4% by weight on leaving the dryer. In 
principle, this value is obtained by two different process variants: 
(1) In a shelf dryer, the polychloroprene band travels by way of guide 
rollers over a plurality of horizontally adjusted shelves. Ievitably most 
of the band adheres to the lower side of the conveyor belt, rather than 
lying on it. If the tackiness of the sheet is inadequate, a large amount 
of waste is produced. In addition to this design-related disadvantage, the 
shelf dryers used for this purpose have inter alia the following further 
disadvantages: 
(a) poor thermal efficiency, due inter alia to the constant cooling and 
heating of the conveyor belt; 
(b) a large radiation surface; and 
(c) poor distribution of air inside the dryer attributable to the fact that 
the air is blown in a parallel stream on to one side of the sheet at a 
very low rate of flow (less than 1 m/second). 
In dryers of this type, repair and maintenance costs are extremely high on 
account of the large number of movable parts under temperature stress. 
The need to lubricate the moving parts of shelf dryers, for example the 
rollers of the conveyor belt, involves an increased risk of fire on 
account of lubricant deposition onto the belt. 
The large amount of space taken up by a shelf dryer and also the small 
number of warm air nozzles mean that an extremely long period of time is 
required to reach the operational equilibrium. For these reasons, 
precision control of the dryer, necessary for example in the event of 
sudden changes in the operating conditions, is extremely difficult to 
obtain. 
(2) In the other variant, the so-called loop dryer, the polychloroprene 
band is no longer conducted horizontally through the dryer, instead guide 
rollers are arranged in such a way that the band can be guided 
substantially vertically. 
Loop dryers have inter alia the following further disadvantages in relation 
to shelf dryers: 
In a loop dryer, the conveyor belt consists of rods surrounded by cotton 
sleeves which are fixed between lateral guide chains. 
The sheet to be dried is placed on these sleeves. On completion of the 
drying process, the dry sheet can be removed more easily from this surface 
than from the uncovered metal rods. When, after about 4 to 6 weeks in use, 
the sleeves have become solid with product, they have to be removed and 
replaced manually. In view of the fact that a loop dryer normally contains 
about 2000 rods, the labour which this involves represents a considerable 
cost factor. 
In addition, problems are involved in starting up loop dryers, because the 
sheet to be dried is in danger of tearing in view of the particular type 
of guide system used. 
Factors common to both types of dryer are their limited drying capacity, 
their long drying time and, from the mechanical point of view, their 
complicated structure. 
Thus, conventional shelf dryers have a length, width and height of 
approximately 50 meters, 5 meters and 9 meters, respectively, for a drying 
capacity of about 1 t of polychloroprene per hour. The drying belt is 
approximately 400 meters long, and the drying time ranges from 20 to 30 
minutes. Drying is generally obtained by a combined arrangement of 
electrical infrared heaters and a few hot-air nozzles arranged parallel to 
the sheet which blow hot air only on to the upper surface of the sheet, 
the electrical infrared heaters being arranged in most cases at the inlet 
end of the dryer where the polychloroprene band enters. 
Conventional loop dryers have a length of 20 meters, a width of 6 meters 
and a height of 4 meters for a drying capacity of 1 t per hour. The drying 
belt is approximately 200 meters long and the drying time approximately 15 
minutes. Drying is obtained by means of hot air which is directed onto the 
sheet from nozzles at a rate of flow of approximately 0.6 to 1.0 meter per 
hour. 
The object of the present invention is to replace the complicated dryers 
which have been used in the past by more simple and more effective dryers 
and also to develop a drying process which provides for considerably 
quicker and, hence, more gentle drying and, optionally, for a considerable 
increase in capacity. 
The drying time in the dryer may be shortened by drying the polychloroprene 
sheet thoroughly on both sides with hot air. Optionally the moist 
polychloroprene sheet can be exposed to infrared heaters and, at the same 
time, blowing air onto the upper and lower surfaces of the sheet. When 
carrying out the process, it is important to ensure that the polymer sheet 
is not heated beyond about 350.degree. C. It has proved to be best to heat 
the polymer sheet to a temperature in the range from 60.degree. to 
200.degree. C., even more preferably from 65.degree. to 150.degree. C. It 
is of particular advantage to use infrared heaters with an emission 
maximum of from about 1.5 to 3.mu., if used at all. In this range, the 
polymer only absorbs to a very limited extent and, as a result, is not 
chemically changed, even when exposed to high energy radiation. 
In the infrared section of the dryer, the radiation intensities applied are 
advantageously in the range from about 5 to 30 watts per cm.sup.2 and the 
air flow rates in the range from about 5 to 30 m per second. 
In the second drying section, the energy required for evaporating the water 
is supplied in the form of hot air. The air is blown at a high rate onto 
both sides of the polymer and through nozzles. 
The drying process is governed primarily by the radiation intensity, by the 
temperature of the hot air, by the rate of travel of the polymer band 
through the dryer, by the rate of flow of the hot air onto the polymer 
band and also by the thickness of the polymer band. 
For a polymer band of given thickness, these parameters may be adapted 
during the drying process to the requirements subsequently imposed on the 
sheet of polychloroprene. 
The hot-air temperature is best in the range from 100.degree. to 
400.degree. C. and preferably in the range from 100.degree. to 250.degree. 
C., so that the polymer band is heated to a temperature of no more than 
about 350.degree. C. and preferably to a temperature in the range from 
60.degree. to 200.degree. C., particularly between 65.degree. and 
150.degree. C. 
The hot air can be produced by the usual methods, for example by the 
combustion of gas and oil, or by employing moisture or heat transmitters. 
The hot air can be produced either outside or inside the dryer. 
The rate at which the polymer band is conveyed through the dryer best 
amounts to between 1 and 100 meters per minute and preferably to between 
20 and 30 meters per minute. The optimum rate of travel of the polymer 
band is determined on the one hand by the working speeds of the 
installations preceding the dryer in the production cycle and, on the 
other hand, by the residence time required to obtain the final moisture 
content of 0.4% by weight. 
The rate at which the hot air flows onto the polymer band best amounts to 
between 1 and 50 and preferably to between 10 and 35 m/second. 
The thickness of the polymer band is normally between 0.2 and 0.7 mm, 
depending upon the type of chloroprene to be dried. The thickness of the 
band is essentially determined by the mode of operation of the coagulation 
drum and of the squeezing rolls by which most of the water originally 
present in the sheet is removed.

A continuous conveyor belt 4, for example in the form of a non-porous or 
porous belt, is guided through an infrared section 1, a following warm-air 
section 2 and an air cooling section 3, being supported by supporting 
rollers 5 and held in the required operational state by special tension 
and control means 6. 
The infrared section 1 consists of infrared heaters 7, air nozzles 8 and 9 
arranged above and below a belt, a fresh air fan for IR-heaters 16 (FIG. 
5) which is connected to air inlets 11; of an exhaust fan 12, which is 
connected to exhaust outlets 13, of a flame monitor (FIG. 5) and of an 
ignition device 15 (FIG. 5). The infrared section is an optional feature. 
The infrared section of the dryer is followed by the warm-air section 2 
which has substantially the following structure: 
Air nozzles 17, 18 are arranged both above and below the belt 4. 
Recirculating air fans 19 circulate the air. Hot air is produced by means 
of gas burners 20 and introduced into the drying chamber through the hot 
air inlet 21. Exhaust outlets 22 are connected to an exhaust fan 23. 
The third section of the dryer is the air cooling section 3 which follows 
the warm air section 2. It also comprises air nozzles 26, 27 above and 
below the belt. In this section, too, the air is circulated by 
recirculating air fans 19. Cold air is introduced through cold air inlets 
24, whilst used air is discharged through air outlets 25. Instead of the 
air cooling section described, cooling the polymer sheet by means of 
contact cooling, as for example with cooled metallic surfaces, is also 
possible. 
By comparison with conventional dryers, it is possible for the first time 
with the dryer according to the invention considerably to reduce the 
length of the drying belt and greatly to reduce the drying time whilst at 
the same time doubling the drying capacity. A dryer of the type 
illustrated in the accompanying drawings has a length of 40 meters, a 
width of 5 meters and a height of 2 meters for a drying capacity of 
approximately 2.5 t per hour (1.8 t/h in case of a dryer without an 
infrared section), a drying belt length of approximately 35 meters and a 
drying time of approximately 1 minute. 
In cases where the process and dryer according to the invention are used, 
efficiency, based on the heat of evaporation of the water, amounts to 
approximately 40% as against 20% for example in cases where conventional 
shelf dryers are used. 
As already mentioned the dryer does not necessarily contain an infrared 
section. Such a dryer is described in the following Figures: 
FIG. 6 is a side elevation of the dryer. 
FIG. 7 is a plan view of the dryer. 
FIG. 8 is a section through the dryer on the line A-B of FIG. 6. 
FIG. 9 is a section through the dryer on the line C-D of FIG. 6. 
A continuous conveyor belt, for example in the form of a non-porous or 
porous belt, is guided through a warm-air section and an air-cooling 
section, being supported by supporting rollers and held in the required 
operational state by special tension and control means. 
The warm-air section of the dryer has substantially the following 
structure: 
Air nozzles 17A, 18A are arranged both above and below the belt 4A. 
Recirculating air fans 19A circulate the air. Hot air is produced by means 
of gas burners 20A and introduced into the drying chamber through the hot 
air inlet 21A. Exhaust outlets 22A are connected to an exhaust fan 23A. 
The section of the dryer which follows the warm-air section is the air 
cooling section. It also comprises air nozzles 17A, 18A above and below 
the belt 4A. In this section, too, the air is circulated by recirculating 
fans 19A. Cold air is introduced through cold air inlets 24A, while used 
air is discharged through air outles 25A. 
The invention is illustrated by the following Examples: 
EXAMPLE 1 
In Example 1, the polymer sheet is dried by means of a conventional dryer. 
3.35 t/h of a polychloroprene latex with a polymer content of 
approximately 30% are coagulated on a cooled rotating drum, the polymer 
sheet thus formed is lifted off, washed and squeezed out. The polymer 
sheet is delivered to a conventional dryer with the following parameters: 
EXAMPLE 2 
In Example 2, the polymer sheet is dried by a dryer according to the 
invention. 8.35 t/h of a polychloroprene latex with a polymer content of 
approximately 30% are coagulated on a cooled rotating drum, the polymer 
sheet thus formed is lifted off, washed and squeezed out. The polymer 
sheet is passed through a dryer according to the invention, as shown in 
FIGS. 1-5, with the following parameters: 
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Examples 2 
Example 1 Dryer according to 
Conventional dryer 
the invention 
(shelf dryer) 
(jet dryer of Fig 1-5) 
external dimensions 
length: 51000 mm 
40 000 mm 
width: 4880 
5 000 mm 
height: 8790 mm 
2 000 mm 
Drying 
drying principle 
drying oven 
impact stream 
temperature 
IR-zone approx.300.degree. C 
300.degree. C 
recirculating air zones 
120.degree. C and 80.degree. C 
250.degree. C and 160.degree. C 
drying time approx.18 minutes 
approx. 1 minute 
drying length 
approx. 340 000 
approx. 35 000 
IR-heaters(number) 
approx. 200 
six 
power source (for IR) 
electricity 
gas 
power density 
1.1 W/cm.sup.2 to 
8 to 12 W/cm.sup.2 
2.2 W/cm.sup.2 
position of radiation 
max. 1.3 to 3.5 .mu.m 
1.3 to 3.5 .mu.m 
nozzles 
type perforated nozzles 
perforated nozzles 
and protective 
nozzles 
distance from sheet 
200 mm 35 mm 
number 24 190 
opening 2 mm .phi. 5 mm .phi. 
outflow rate 1 m/s 10 and 35 m/s 
outflow direction 
parallel to sheet 
perpendicular to 
sheet 
outflow plane 
above above and below 
throughflow of 
recirculating air 
approx.15 000 m.sup.3 /h 
approx. 380 000m.sup.3 /h 
exhaust output 
3000-10000 m.sup.3 /h 
approx.3000-10000m.sup.3 /h 
exhaust-air temperature 
approx. 130.degree. C 
approx. 120.degree. C 
efficiency(%) 
20 to 24 approx. 40 
heating medium 
current/steam 
natural gas 
__________________________________________________________________________ 
In Example 1, approximately 1 t/h of a dried polychloroprene with a 
residual moisture content of approx. .ltoreq. 0.4% by weight is obtained 
using the dryer described above. 
In Example 2, approximately 2.5 t/h of a dried polychloroprene with a 
residual moisture content of approx. .ltoreq. 0.4% by weight is obtained 
using the dryer described above.