Process for separating polyethylene terephthalate from polyvinyl chloride

Today there is a growing demand to recycle plastics including polyethylene terephthalate. Sources of polyethylene terephthalate for recycling often contain polyvinyl chloride. It is necessary to separate polyvinyl chloride from the polyethylene terephthalate before it can be recycled into useful products. This invention reveals a process for recovering polyethylene terephthalate flakes from a recycle stream containing polyethylene terephthalate flakes and polyvinyl chloride flakes which comprises: (1) treating the recycle stream with (a) at least one inorganic base selected from the group consisting of sodium hydroxide and potassium hydroxide and (b) at least one nonionic surfactant under conditions and for a time sufficient to decrease the contact angle of the polyethylene terephthalate flakes with water below a value of 25.degree. while maintaining the contact angle of the polyvinyl chloride flakes above a value of about 45.degree.; (2) adding the treated recycle stream to water, to form an aqueous mixture containing the treated polyethylene terephthalate flakes and treated polyvinyl chloride flakes; (3) agitating the aqueous mixture to allow the polyvinyl chloride flakes to come in contact with gas bubbles; (4) allowing the polyvinyl chloride flakes to float to the surface of the aqueous mixture and allowing the polyethylene terephthalate flakes to sink to the bottom of the aqueous mixture; and (5) removing the polyvinyl chloride flakes from the surface of the aqueous mixture and removing the polyethylene terephthalate flakes from the bottom of the aqueous mixture.

BACKGROUND OF THE INVENTION 
In 1989 approximately 700 million pounds of polyethylene terephthalate 
(PET) were consumed to produce soft drink bottles. Presently about 28% of 
this material is recycled with the remainder being placed in landfills or 
incinerated. Due to problems with overloaded landfills and the negative 
environmental image plastics has received recently, bills have been 
introduced aimed at establishing bottle deposits and the number of 
curbside recycling programs has increased tremendously. The collection of 
PET bottles through these programs has resulted in a source of 
post-consumer PET which has been used for a number of applications. 
Generally these applications involve the conversion and fabrication of the 
post-consumer PET into materials of lower value. Examples of such 
applications are polyols for unsaturated polyesters or polyurethanes, 
fiberfill, carpet fibers, and strapping. Recycled PET is also blended with 
other materials such as polybutylene terephthalate, polycarbonate, or 
glass fibers, etc., for automotive as well as other engineering 
applications. 
Post-consumer PET can also be recycled into resin which can be used in 
manufacturing containers for foods and beverages, such as carbonated 
beverage bottles. In such a procedure, the post-consumer PET is 
depolymerized to oligomers which are subsequently utilized as a raw 
material in the preparation of the recycled PET resin. Such a procedure 
for depolymerizing PET is described in U.S. Pat. No. 3,703,488. 
It is, of course, necessary to separate the post-consumer PET from other 
plastics in the recycling procedure. For instance, polyvinyl chloride 
(PVC) is often present in sources of post-consumer PET. PVC presents a 
particular problem in that it is a clear plastic material which is 
sometimes misidentified as PET and in conventional processes the 
separation of PVC from PET is difficult, if not impossible. In any case, 
sources of post-consumer PET are often contaminated with PVC. 
PET articles which are being recycled are typically ground into flakes for 
further processing. For instance, PET beverage bottles are typically 
ground into flakes which have a cross-sectional area of from about 4 
mm.sup.2 to about 100 mm.sup.2. It is more typical for such flakes to have 
an area of about 15 mm.sup.2 to about 65 mm.sup.2. The thickness of the 
flakes varies with the wall thickness of the bottles which are being 
recycled. 
Because post-consumer PET sources are often contaminated with PVC, recycle 
streams of PET flakes also generally contain PVC flakes. It is important 
to remove these PVC flakes from the PET flakes in the recycle stream to 
allow for further steps in the recycling operation. However, the removal 
of the PVC flakes from the PET flakes in the recycle stream has not proven 
to be straight forward. For instance, PVC generally has a density which is 
close to the density of PET. This renders flotation separation techniques 
which are based upon density differences of the flakes virtually 
impossible. 
U.S. Pat. No. 4,064,677 discloses a technique for separating various 
plastics by an aqueous flotation procedure. In the procedure described in 
U.S. Pat. No. 4,064,677, the less wettable plastic floats to the surface 
of the aqueous medium by virtue of the fact that a greater number of gas 
bubbles adhere to its surface. However, PET and PVC have contact angles 
which are relatively close to one another. This problem is compounded by 
the fact that the contact angle of PVC can vary considerably based upon 
the manner in which it was compounded. Because the contact angle of 
compounded PVC is close to the contact angle of PET, separation based upon 
a differential density and wettability is extremely difficult without 
conditioning the PVC and PET in accordance with this invention. 
SUMMARY OF THE INVENTION 
This invention is based upon the unexpected finding that a recycle stream 
containing PET flakes and PVC flakes can be conditioned to reduce the 
contact angle of the PET flakes to a value of less than 25.degree. while 
maintaining the contact angle of the PVC flakes above a value of about 
45.degree.. Such a differential in contact angles makes it possible to 
separate the PVC flakes from the PET flakes by selective adherence of gas 
bubbles to the PVC which is based upon the differential wettability 
between the PET and PVC. This requires prior conditioning of the recycle 
stream containing the PET flakes and the PVC flakes with (a) at least one 
inorganic base selected from the group consisting of sodium hydroxide and 
potassium hydroxide and (b) at least one nonionic surfactant. 
The subject invention more specifically relates to a process for recovering 
polyethylene terephthalate flakes from a recycle stream containing 
polyethylene terephthalate flakes and polyvinyl chloride flakes which 
comprises: (1) treating the recycle stream with (a) at least one inorganic 
base selected from the group consisting of sodium hydroxide and potassium 
hydroxide and (b) at least one nonionic surfactant under conditions and 
for a time sufficient to decrease the contact angle of the polyethylene 
terephthalate flakes below a value of 25.degree. while maintaining the 
contact angle of the polyvinyl chloride flakes above a value of about 
45.degree.; (2) adding the treated recycle stream to water, to form an 
aqueous mixture containing the treated polyethylene terephthalate flakes 
and treated polyvinyl chloride flakes: (3) agitating the aqueous mixture 
to allow the polyvinyl chloride flakes to come in contact with gas 
bubbles; (4) allowing the polyvinyl chloride flakes to float to the 
surface of the aqueous mixture and allowing the polyethylene terephthalate 
flakes to sink to the bottom of the aqueous mixture; and (5) removing the 
polyvinyl chloride flakes from the surface of the aqueous mixture and 
removing the polyethylene terephthalate flakes from the bottom of the 
aqueous mixture. 
DETAILED DESCRIPTION OF THE INVENTION 
The recycle streams utilized in practicing this invention contain PET 
flakes and PVC flakes. However, other plastics such as high density 
polyethylene, low density polyethylene, polypropylene, and ethylene vinyl 
acetates can also be present in the recycle stream. The PET in the recycle 
stream is typically comprised of repeat units which are derived from 
terephthalic acid or a diester thereof and ethylene glycol. However, it is 
understood that the PET can also be modified with small amounts of other 
monomers. Such modified PET can contain small amounts of repeat units 
which are derived from diacids other than terephthalic acid and/or glycols 
in addition to ethylene glycol. For instance, small amounts of isophthalic 
acid or a naphthalene dicarboxylic acid can be used in the diacid 
component utilized in preparing the PET. PET which has been modified with 
a small amount of a diol containing from 3 to 8 carbon atoms is also 
representative of such a modified PET. For instance, a small amount of 
1,4-butane diol can be utilized in the glycol component used in preparing 
the modified PET. Normally, no more than about 5 weight percent of the 
repeat units in such modified PET will be comprised of diacids or diols 
other than a terephthalic acid and ethylene glycol. It is, of course, 
contemplated that diesters of such dicarboxylic acids and diols can also 
be used. In most cases, such modified PET will contain less than about 3% 
diacids other than terephthalic acid and less than 3% diols other than 
ethylene glycol. Such modified polyesters most typically contain only 
about 1% dicarboxylic acids other than terephthalic acid and/or less than 
1% glycols other than ethylene glycol. Polyethylene isophthalate and 
copolymers thereof will also normally be in the recycle feed stream and 
are considered to be PET for the purposes of this invention. 
The PVC in the recycle feed stream is primarily derived from PVC compounded 
into bottles and containers. The PVC can contain thermal stabilizers such 
as organo metallic salts, epoxides, and phosphite chelators and other 
additives such as lubricants (waxes, fatty esters and fatty soaps of 
metallic acids), plasticizers (phthalate esters, adipates, azelates and 
organic phosphate esters and epoxidized oils), impact modifiers 
(elastomeric polymers), process aids (methyl methacrylate copolymers or 
styrene-acrylonitrile copolymers), fillers and primary pigments (organic 
and inorganic). Other additives may include ultra-violet light 
stabilizers, antidegradants, fungicides, anti-static agents and 
anti-blocking agents. 
The PET and PVC flakes in the recycle stream will normally have a 
cross-sectional area which is within the range of about 4 mm.sup.2 to 
about 100 mm.sup.2. It is more typical for such flakes to have an area of 
about 15 mm.sup.2 to about 65 mm.sup.2. The thickness of the flakes will, 
of course, vary with the thickness of the article which was ground for 
recycling. In most cases, the flake will be from ground bottles or 
containers. The PET flakes in the recycle stream will come primarily from 
PET beverage bottles which have a standard wall thickness of about 0.028 
inches (0.711 mm). Accordingly, the flake will generally have a thickness 
which is on the order of 0.025 to 0.030 inches (0.64 mm to 0.76 mm). 
However, the neck and finish (threads) regions of the bottle will 
typically be of substantially greater thickness. Accordingly, flakes which 
are derived from the neck and finish regions of bottles will be 
considerably thicker. 
The first step in the practice of this invention involves treating the 
recycle stream containing the PET flakes and PVC flakes in a conditioning 
step. In this conditioning step, the recycle stream is treated in an 
aqueous medium with (a) at least one inorganic base selected from the 
group consisting of sodium hydroxide and potassium hydroxide and (b) at 
least one nonionic surfactant under conditions and for a time sufficient 
to decrease the contact angle of the polyethylene terephthalate flakes 
below a value of 25.degree. while maintaining the contact angle of the 
polyvinyl chloride flakes above a value of about 45.degree.. Preferably, 
this conditioning step will result in the contact angle of the PET flakes 
being reduced to a level of less than about 15.degree. with the contact 
angle of the PVC flakes being increased to a value of greater than about 
50.degree.. It is most preferred for the conditioning step to result in 
the contact angle of the PET flakes being reduced to a value of less than 
about 5.degree. with the contact angle of the PVC flakes being increased 
to a value of greater than about 55.degree.. 
The contact angle is the measurement of the angle formed between a solid 
substrate and a drop of liquid on its surface. The contact angles referred 
to herein were measured with a rame-hart Model 100 Goniometer System. 
Any type of nonionic surfactant can be employed in the conditioning step. 
For instance, the nonionic surfactant can be a polyol which is a copolymer 
of ethylene oxide and propylene oxide. Such polyols have the structural 
formula: 
##STR1## 
wherein n and m are integers, wherein the ratio of m to n is at least 
about 5:1, and wherein indicates that the distribution of monomeric 
units can be random. The polyols which can be used also have molecular 
weights of at least about 1500. The polyols which are preferred contain 
less than about 10% bound ethylene oxide (have a ratio of m to n of at 
least about 10:1). 
Fatty alcohols are a preferred class of nonionic surfactant which can be 
utilized in the conditioning step. The fatty alcohol utilized will 
typically be of the structural formula R-OH wherein R represents an alkyl 
group containing from 5 to 22 carbon atoms. In most cases, R will be an 
alkyl group containing from 10 to 18 carbon atoms. It is generally 
preferred for the fatty alcohol to contain from 12 to 14 carbon atoms. For 
instance, lauryl alcohol is a particularly preferred fatty alcohol. Such 
fatty alcohols are available commercially from Shell Chemical Company 
under the tradename NEODOL.RTM.. NEODOL.RTM. 91-6 which has a 
hydrophile/lipophile balance (HLB) number of 12.5 has been found to be 
particularly useful. 
The nonionic surfactant will typically have a HLB number which is within 
the range of about 4 to about 18. It is generally preferred for the 
nonionic surfactant to have a HLB number which is within the range of 
about 8 to about 16 with HLB numbers within the range of about 10 to about 
14 being most preferred. HLB numbers are indicative of a surfactant's 
emulsification behavior and relate to the balance between the hydrophilic 
and lipophilic (hydrophobic) portions of the molecule. HLB numbers are 
further described in Griffin, W. C., J. Soc. Cosmet. Chem. 1, 311 (1949) 
which is incorporated herein by reference. The HLB number of a given 
surfactant generally decreases with increasing temperatures. 
Nonionic surfactants which have the structural formula: 
##STR2## 
wherein n is an integer from 1 to about 40 and wherein m is an integer 
from about 6 to about 12 have also been found to be extremely useful. It 
is normally preferred for m to be 8 or 9. It is most preferred for n to be 
an integer which is within the range of about 6 to 10. The HLB number of 
such compounds increases with increasing levels of ethylene oxide 
incorporation. The HLB number of such compounds increases as a function of 
n as follows: 
______________________________________ 
n HLB Number 
______________________________________ 
1 3.6 
3 7.8 
4 10.4 
10 13.5 
16 15.8 
30 17.3 
40 17.9 
______________________________________ 
Such nonionic surfactants are sold by Rohm and Haas under the tradename 
TRITON. For example, TRITON X-45, TRITON X-114, and TRITON X-100 are 
useful. TRITON X-114 has been found to be extremely useful in the 
conditioning step. 
The aqueous solution utilized in the conditioning step will typically 
contain from about 1 weight percent to about 20 weight percent of the 
inorganic base. It will preferably contain from about 2 weight percent to 
about 15 weight percent of the inorganic base. It is normally more 
preferred for the conditioning medium to contain from about 5 weight 
percent to about 10 weight percent of the inorganic base. 
The aqueous conditioning medium will also contain from about 0.001 weight 
percent to about 0.5 weight percent of the nonionic surfactant. It is 
preferred for the aqueous conditioning medium to contain from about 0.005 
weight percent to about 0.1 weight percent of the nonionic surfactant. It 
is most preferred for the aqueous conditioning medium to contain from 
about 0.01 weight percent to about 0.05 weight percent of the nonionic 
surfactant. 
The time required in the conditioning step is dependent upon the 
concentration of the inorganic base in the conditioning medium. As a 
general rule, the time required for conditioning will be within the range 
of about 1 to 15 minutes. It is preferred for the recycle stream to be 
conditioned for about 3 to 11 minutes. It is most preferred for the 
recycle stream to be conditioned for about 5 to 8 minutes. The 
conditioning step will typically be carried out at a temperature which is 
at a range of about 5.degree. C. to about 100.degree. C. It is generally 
convenient for the conditioning step to be conducted at ambient 
temperature which will normally be at a range of about 15.degree. C. to 
about 30.degree. C. To properly expose the surfaces of the PET flakes and 
PVC flakes to the conditioning solution, agitation is normally employed. 
After the recycle stream has been conditioned, the mixture of PET flakes 
and PVC flakes are removed from the aqueous conditioning medium. Then, the 
treated recycle stream which is comprised of the conditioned PET flakes 
and conditioned PVC flakes is added to water. It is often desirable for 
the water to contain a foaming agent. The foaming agent is preferably the 
nonionic surfactant used in the conditioning step. In practice, the 
residual nonionic surfactant left on the flakes from the conditioning step 
could serve this purpose. However, in some cases it will be desirable to 
add additional foaming agent. The foaming agent will be added in 
sufficient quantities to maintain a foam on the surface of the water 
during agitation. 
The water is agitated in such a manner that the PVC flakes and PET flakes 
come in contact with gas bubbles. The gas can be air, nitrogen, oxygen, 
carbon dioxide, helium, hydrogen, argon, krypton, neon or the like. For 
economic reasons air is normally utilized as the gas. 
The agitation causes gas bubbles to selectively adhere to the PVC flakes 
without adhering appreciably to the PET flakes. This preference for the 
gas to adhere to the PVC flakes and not to the PET flakes is a result of 
the surface modification accomplished in the conditioning step. The 
attached gas bubbles cause the PVC flakes to rise to the surface of the 
water with the PET flakes sinking in the water rather than floating. 
Without the conditioning step, most of the PET flakes and the PVC flakes 
would float to the surface of the water. 
The PVC flakes floating on the water surface can be removed by any number 
of commercially feasible operations. The PET flakes are, of course, 
recovered from the bottom of the aqueous mixture. The PET flakes can then 
be further treated to remove additional contaminants and/or recycled as 
desired. 
This invention is further illustrated by the following examples which are 
merely for the purpose of illustration and not intended to limit the scope 
of the present invention or the manner in which it can be practiced. 
Unless otherwise indicated herein all parts and percentages are given by 
weight.

EXAMPLE 1 
In this experiment PVC flakes were separated from PET flakes by the 
technique of this invention. In the procedure utilized, a 5 weight percent 
sodium hydroxide solution was made by mixing 581 grams of water with 30.6 
grams of sodium hydroxide. Fifteen drops (0.30 grams) of NEODOL.RTM. 91-6 
was added to this solution to make a 0.05 weight percent concentration. 
The solution was agitated using a standard axial flow turbine agitation 
system which was run at 1100 rpm until all the sodium hydroxide was 
dissolved. 
A nominal 50/50 PVC/PET flake mixture was added to the conditioning 
solution. The PVC flakes were obtained by cutting commercially obtained 
Evian water bottles. The PET flakes were obtained by cutting commercially 
obtained green beverage bottles made from Goodyear Cleartuf.RTM. 84EG. 
After 5 minutes of agitated conditioning, the agitator was stopped and the 
solution was allowed to sit for approximately 2 minutes. The conditioning 
solution was then decanted from the mixture of the PET flakes and PVC 
flakes. The PVC/PET flake mixture was rinsed with three nominal 500 ml 
volumes of water. 
This rinsing was accomplished by pouring the first 500 ml volume into the 
conditioning vessel containing the decanted PET/PVC flakes. The water was 
then decanted and the procedure repeated for the second 500 ml volume. The 
third 500 ml volume was used to transfer the PET/PVC flakes from the 
conditioning vessel to the flotation vessel. 
The flotation vessel contained 5,000 ml of water to which 0.5 drops (0.01 
grams) of NEODOL.RTM. 91-6 had been added. This was accomplished by adding 
one drop of NEODOL.RTM. 91-6 to 5,000 ml of water and removing 2,500 ml of 
water and then adding back 2,500 ml of clean tap water. 
The third 500 ml volume of rinse water and the conditioned PET/PVC flakes 
were added to the flotation vessel. This vessel was a Denver Equipment 
DR-1 laboratory flotation device. The agitator, which is provided by 
Denver Equipment, is specially designed to introduce and disperse air or 
gas bubbles into the aqueous mixture. There are many commercial agitators 
that accomplish the same objective. In addition, the gas could be added 
directly such as via a feed line from a compressed gas source. 
After the PET/PVC flakes were added to the flotation vessel, the agitator 
was operated at 1100 rpm. The PVC instantly rose to the top. The PVC was 
removed by adding a sufficient amount of water to cause the foam with the 
trapped PVC to overflow the flotation vessel. It was observed at this 
point that as the water was added, diluting the surfactant concentration 
in the flotation vessel, the amount of gas bubbles (foam) on the water 
surface diminished. This can be overcome by recirculating the overflowing 
water and achieving a steady state surfactant concentration in a 
continuous process. 
The results of the separation are shown below: 
______________________________________ 
Amount Added Float Residue* 
______________________________________ 
PET 7.92 g 0.53 g 7.39 g 
PVC 7.78 g 7.59 g 0.19 g 
______________________________________ 
*Sunk to the bottom 
The PVC removal efficiency was 97% and the PET recovery was 93%. It should 
be noted that the PVC flakes which remained in the residue were all larger 
than 100 mm.sup.2 in surface area. Thus, PVC removal efficiencies could be 
improved by better control of flake size. 
EXAMPLE 2 
In this experiment, the procedure utilized in Example 1 was repeated except 
that 69.14 grams of sodium hydroxide and 0.13 grams of the TRITON X-114 
was added to 691.4 grams of water to make a conditioning solution which 
contained 10 weight percent sodium hydroxide and 0.25 weight percent 
TRITON X-114. The conditioning time was 6 minutes at 900 rpm. 
The water in the flotation vessel did not contain the half drop of TRITON 
X-114 utilized in Example 1. Agitation of the flotation vessel was 
provided as described in Example 1. Most of the PVC separation occurred 
instantly. However, after adding 4,500 ml of water to the float vessel and 
subsequently collecting 4,500 ml of overflow, nitrogen was added to the 
agitator via the special Denver Equipment DR-1 attachment. Even more PVC 
rose to the surface with gas bubbles attached. Only seven PVC pieces were 
left in the residue and only two PET pieces were floated off with the PVC. 
The results of the separation are shown below: 
______________________________________ 
Amount Added Float Residue* 
______________________________________ 
PET 5.10 g 0.05 g 5.05 g 
PVC 5.23 g 4.51 g 0.72 g 
______________________________________ 
*Sunk to the bottom 
It should be noted that of the seven PVC flakes in the residue only one was 
less than 140 mm.sup.2 in surface area. 
EXAMPLE 3 
In this experiment the procedure described in Example 1 was repeated except 
that 29 grams of potassium hydroxide was added to 551 grams of water and 
2.5 drops of TRITON X-114 to make a 5 weight percent potassium hydroxide 
and 0.01 weight percent TRITON X-114 solution. The PET/PVC flakes were 
conditioned at 900 rpm for 5 minutes before the agitator was stopped. The 
conditioned PVC/PET flakes were decanted and rinsed with three 500 ml 
volumes of water with the third volume being added to the flotation 
vessel. 
The PVC floated immediately when the agitator was started. While the 
flotation vessel did not have any TRITON X-114 intentionally added, it was 
apparent from the amount and size of the bubbles that there was a high 
level of residual nonionic surfactant remaining on the flakes. This 
confirmed the preference of having some level of foaming agent in the 
flotation vessel. 
The results of the separation are shown below: 
______________________________________ 
Amount Added Float Residue* 
______________________________________ 
PET 4.33 g 0.04 g 4.29 g 
PVC 3.72 g 3.42 g 0.30 g 
______________________________________ 
*Sunk to the bottom 
EXAMPLE 4 
In this example, the conditioning solution preparation was the same as 
described in Example 1, except that 29 grams of potassium hydroxide and 3 
drops of TRITON X-114 was added to 546 grams of water to make a 5 weight 
percent potassium hydroxide and 0.013 weight percent TRITON X-114 
solution. 
The flotation water was prepared as described in Example 1. The difference 
between this example and the previous examples is that the PVC/PET which 
was carried over in the float was subjected to a separate separation using 
the overflowed water. 
The PET/PVC flakes were conditioned for 5 minutes at 1100 rpm. The agitator 
was stopped and the PVC/PET flake mixture decanted and rinsed with 3 
volumes of 500 ml of water as described in Example 1. 
The results of the separation are shown below: 
______________________________________ 
Amount Added Float Residue* 
______________________________________ 
Float Separation Number 1 
PET 12.96 g 5.20 g 7.76 g 
PVC 12.80 g 12.24 g 0.56 g 
Float Separation Number 2 
PET 5.20 g 0.42 g 4.78 g 
PVC 12.24 g 11.98 g 0.26 g 
Combined Results of the Two Stage Flotations 
PET 12.96 g 0.42 g 12.54 g 
PVC 12.80 g 11.98 g 0.82 g 
______________________________________ 
*Sunk to the bottom 
This two stage system had a 97% PET recovery and a 94% PVC removal 
efficiency. It is, of course, possible to attain even higher levels of 
efficiency by utilizing multiple stage flotation procedures which entail 
three or more flotation steps. 
Variations in the present invention are possible in light of the 
descriptions of it provided herein. It is, therefore, to be understood 
that changes can be made in the particular embodiments described which 
will be within the full intended scope of the invention as defined by the 
following appended claims.