FIELD OF INVENTION:
This invention relates to a process to reduce the dropping Intrinsic Viscosity and
to reduce the physical chemical and toxic contaminants in recycled PET resin
obtained from wasted PET bottle flakes.
BACKGROUND OF THE INVENTION:
In the convensional process the excess of water present in the flakes after the
washing process is removed in a centrifugal dryer. From here the flakes are
pneumatically conveyed by hot air to a storage silo / pelletising machine. The hot
air during conveying slightly reduces the surface moisture. The pelletising
machines are equipped with a high speed agitator, which heats up the flakes by
friction and reduce the moisture content further by evaporation.
It was observed that the above process is unable to reduce the moisture content
sufficiently to prevent hydrolysis, resulting in a drop in the I.V. When the I.V of
the recycled resin is compared to the input flakes, it is observed that there is a
drop of approximately 0.10 dl/g i.e. where typically the I.V. of the input flakes is
in the region of 0.73-0.76 dl/g, the I.V. of Recycled PET Resin is in the region of
0.63-0.65 dl/g. This makes the Recycled PET Resin unsuitable for bottle
application.
Whereas, the instant application arrests the drop of I.V. by bringing down the
moisture level in the flakes to less than 10 ppm and restricting the drop in I.V to
minimum 0.005 to 0.01 dl/g.
The fine melt filtration system used with 28 micron screen delivers the cleanest
possible B2B repellets.
Similarly, as there was no system in the earlier process for forced extraction of
evolved gases when the flakes are heated, the residual level of chemical
contamination in the Recycled PET Resin is higher than the desired levels.
In the instant application, the process ensures that all contaminants are isolated
and the resultant gases evolved are completely removed from the input. As a
result the contamination level in the Recycled PET Resin is at a level which can
be termed "not measurable", i.e. 0.1 to 0.3 ppm.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a process to reduce the dropping of
Intrinsic Viscosity and to reduce the physical chemical and toxic contaminants in
recycled PET resin;
Another object of this invention is to propose a process to reduce the dropping of
Intrinsic Viscosity and to reduce the physical chemical and toxic contaminants in
recycled PET resin to obtain recycle resin having a contaminants level which is
almost immeasurable at the range of about 0.1 to 0.3 ppm;
A further object of this invention is to propose a process to reduce the dropping
of Intrinsic Viscosity and to reduce the physical chemical and toxic contaminants
in recycled PET resin to obtain recycled PET resin having moisture level in the
flakes less than 10 ppm;
Still further object of this invention is to propose a process to reduce the
dropping of Intrinsic Viscosity and to reduce the physical chemical and toxic
contaminants in recycled PET resin to obtain flakes having restricted the drop in
Intrinsic Viscosity to minimum of 0.005 to 0.01 dl/g.
DESCRIPTION OF THE INVENTION:
According to this invention a process to reduce the dropping intrinsic viscosity
and to reduce the chemical toxic contaminants in recycled PET resin comprising:
drying the PET flakes;
subjecting the dried flakes to the hot air treatment in the 1st reactor;
introducing the flakes to the 2nd reactor comprising dehumidified air to reduce
the moisture content;
subjecting the flakes continuously to recirculating air to remove the contaminants
and volatiles and feeding the flakes to the extrusion system.
The process application relates to the reduction in I.V. drop in the recycling of
Ultra Clean PET Resin from PET bottle flakes. When PET is recycled, hydrolysis
occurs, which results in reduction of molecular weight, i.e. decrease in I.V.
Hydrolysis in PET recycling proceeds according to the following structure:
Thus, residual moisture present in PET flakes is sufficient for the above reaction
to occur and cause the resultant drop in I.V / molecular weight. Such molecular
weight reduction is the prime cause in the decrease of impact strength of the
moulded part. Also a decrease in viscosity leads to running from nozzle and
makes it difficult for size enlargement in PET performs, i.e. stretch blowing of
bottles.
When bottle waste is collected from curbside recycling programmes, there is no
surety as to the materials stored in the bottles prior to final disposal. This
unregulated use of the bottles before final disposal may result in toxic and
chemical contaminations, which are not desirable when the recycled resin goes
for manufacture of bottles for packaging. The washing process is also partially
ineffective in removing such physical and chemical contamination.
The instant process ensures that such physical and chemical contaminants are
removed during the preparation and processing of the input itself during the
manufacturing process and the resultant Ultra Clean PET resin is free from such
contaminants, rendering it suitable for manufacture of packaging bottles.
The process requires the input PET flakes to be made bone-dry and all physical,
chemical and toxic contaminants evolved during the drying process to be
extracted out.
For this, the flakes from the washing system, after centrifuge drying and hot air
conveying / drying are fed into a reactor.
Hot air is continuously passed at a temperature of about 110°C to 130°C and
blown through the vessel (pre-treatment). A residence time of 30 mins to 1 hour
ensures that residual surface moisture is completely removed from the flakes.
The flakes are then transferred to the subsequent reactor.
In the second reactor, hot desiccant air is circulated through the flakes. The air
temperature is maintained at 140°C to 160°C. The air is dehumidified to a dew
point of at least (-) 40°C. The flakes are given a residence time of about 2 hours
to 6 hours.
The hot dehumidified air maintained at a temperature of 140°C to 160°C with a
dew point of at least (-) 40°C and a residence time of 2 hours to 6 hours ensures
that the moisture content in the flakes on exit is less than 10 ppm. The air is
continuously passed through a desiccant dryer system to ensure that the re-
circulating air is moisture free. With this, as much as 1.5% inherent moisture and
moisture peaks of up to 3% brought in on the surface of material either due to
condensation problems in silos, or when using post-consumer material, for
example-is evaporated in a flash at the temperatures and pressures inside the
reactor. Residual moisture is therefore very effectively eliminated within the
shortest possible time. This ensures that moisture peaks in the starting material
are effectively eliminated prior to processing in the extruder without having a
detrimental, destabilizing influence on the downstream extrusion process.
The unit is also well insulated, so further external thermal energy supplies (dried,
heated air, for example), are minimal. At around 0.1. kW per Kg of material
processed, the reactor is extremely economical regarding the amount of specific
energy consumed.
The product discharged continuously or semi-continuously through a second
chute is crystallized PET material with uniform residual moisture values of below
10 ppm, which is quite sufficient for most applications; in special cases it is also
possible to achieve lower residual moisture values by modifying the settings of
the unit slightly.
All oligomers, gels, other contaminants and volatiles evolving out of the flakes
and are continuously extracted by the air and is filtered out. This ensures that
the recyclate is chemical contaminant free and is UltraClean.
From here the flakes are fed to the downstream extrusion system for
manufacture of pellets. A moisture content of less than 10 ppm ensures that no
hydrolysis occurs and that there is practically no drop in I.V.
In the downstream High Performance fine filter system with 28 micron screens
are used. This is absolutely necessary to achieve quality levels like new
materials. Visible black spots in new PET bottles are practically eliminated
altogether.
Thus, all physical, chemical and toxic contaminants have been removed from the
material.
EXAMPLE:
The washed PET flakes were charged as under:
a) Reactor-1 (Pre-treatment)
Quantity of material in reactor : 210 Kgs.
Residence time : 35 mins.
Hot air temperature : 126°C ± 2°C
b) Reactor 2
Quantity of material in reactor : 210 Kgs
Residence time : 4 mins
Dehumidified hot air temperature : 152°C ± 2°C
c) Extrusion System - Pellets / Resin manufactured
Final Test & Observation
Starting I.V. of Flakes : 0.72
Resultant I.V. of the pellets : 0.718
Residual volatile contaminant in pellets - not measurable.
We Claim:
1. A process to reduce the dropping intrinsic viscosity and to reduce the chemical
toxic contaminants in recycled PET resin comprising:
drying the PET flakes;
subjecting the dried flakes to the hot air treatment in the 1st reactor;
introducing the flakes to the 2nd reactor comprising dehumidified air at a
temperature in the range of 140°C to 160° C, to reduce the moisture content;
subjecting the flakes continuously to hot dehumidified air to remove the
contaminants and volatiles and feeding the flakes to the extrusion system.
2. The process as claimed in claim 1, wherein the said PET flakes are dried at a
temperature of 110° C to 130° C for 30 min to 1 hr.
3. The process as claimed in claim 1, wherein the second reactor has a temperature
between 140°C to 160° C and the air is dehumidified to a dew point of atleast (-)
40°C and the residence time in 2 to 6 hours.
4. The process as claimed in claim 1, wherein the moisture content of the flakes on
exit is less than 10 ppm.

A process to reduce the dropping intrinsic viscosity and to reduce the chemical
toxic contaminants in recycled PET resin comprising:
drying the PET flakes;
subjecting the dried flakes to the hot air treatment in the 1st reactor;
introducing the flakes to the 2nd reactor comprising dehumidified air at a
temperature in the range of 140°C to 160° C, to reduce the moisture content;
subjecting the flakes continuously to hot dehumidified air to remove the
contaminants and volatiles and feeding the flakes to the extrusion system.