How light impacts recycled
polyethylene terephthalate
(rPET) characteristics
by Dr. Francis M. Schloss and Julie Brown
Plastic Technologies, Inc.
How light impacts recycled polyethylene
terephthalate (rPET) characteristics
One of the problems that limits the amount of recycled PET (rPET) used in rigid packaging
applications is the degree of yellowness that it can cause. The more rPET that is added to virgin
PET during bottle manufacture, the more yellow the resulting bottle tends to become.
The source of the rPET used has a great influence on yellowing. Deposit-grade materials
are in the greatest demand as they result in less yellowing than do curbside-grade material.
Even virgin PET will discolor and yellow with each additional melting cycle.
But there are many other causes that contribute to this problem. For example, it is known that
low levels of nylon coming from multilayer bottles can remain trapped in the PET
flake after washing and cleaning. This residual nylon will cause considerable yellowing when the
rPET is extruded. The presence of additives such as ultraviolet (UV) light blockers, acetaldehyde
and oxygen scavengers, and slip agents can all contribute to yellowing as the rPET is subjected
to additional melt histories.
A study performed by Phoenix Technologies, LLC looked at the effects that adhesives used to
adhere labels to bottles had on yellowing. The results showed that these could be a significant
contributor when not washed away from the PET flake during the cleaning process1.
But one cause of yellowing that has not been widely addressed is the effect that UV radiation
from the sun, as well as that emitted by artificial sources, such as fluorescent lighting, can
have on PET. PET is sensitive to UV light especially at elevated temperatures, under high
humidity, and in the presence of oxygen—all of which are present when PET bottles are
exposed to the weather.
UV radiation is part of the electromagnetic spectrum that occurs from below 100 to 400
nanometers (nm). Long wavelength UV radiation occurs in the UVA part of the spectrum. The
UVA portion of the spectrum represents the greatest amount of UV radiation that penetrates
the atmosphere and reaches the earth. This type of radiation is strong enough that it can cause
chemical reactions to occur and it is responsible for the damage that plastics experience when
exposed to the elements.
Free radicals form within the plastic that cause subsequent degradation. The degradation
effects seen are also compounded by the presence of oxygen in the air. The free radicals
created by the UV radiation react with this oxygen to form hydroperoxides that can result
in polymer chain breakage. Thus the end result in many plastics, including PET exposed
to an outdoor sunny environment, is that they will discolor, embrittle, and crack over time.
While antioxidant and UV absorbers can be added to the plastic to help stabilize and
prolong the material’s useful life, PET bottles normally do not contain significant amounts
of these additives.
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The Association of Postconsumer Plastic Recyclers (APR) notes in their model bale
specification for PET bottles, that bales should not be stored outdoors uncovered for a
period exceeding two weeks to prevent UV degradation2. There have been many papers
published that describe the degradation effects that UV light has on PET3-5. But these
papers have primarily focused on the degradation seen on the actual PET article being
studied and not what happens to the properties of PET when it is recycled.
The life of a PET bottle is one that is not usually exposed to direct sunlight. UV radiation
will begin to pass through PET that does not contain UV absorbers above 320 nm.
Products packaged in PET that need UV protection will either have a UV blocking fullsleeve label covering the bottle or will have UV absorbers added to the PET during the
bottle manufacturing process. However only a small number of PET bottles in the market
contain these UV absorbers. The majority of bottles do not.
While PET bottles and their contents do not experience significant exposure to sunlight,
bottles under storage after manufacture as well as product-filled bottles sitting on a
store shelf can be exposed to artificial light sources that can also emit low levels of
ultraviolet light. But the most probable time that PET bottles might see significant
exposure to the elements occurs when bales of PET bottles are stored outside while
waiting to be brought into a reclamation facility to be ground into flakes, washed and repelletized for sale back into the industry.
This study was undertaken to understand the effects weathering would have on PET
bottles exposed to Toledo, Ohio weather for one year. Two-liter PET bottles produced
using a commercial grade of PET were used for this study. This resin did not contain any
ultraviolet absorbing additives. These virgin bottles were crushed, stacked five to six
deep, and placed in uncovered open sided crates to afford maximum exposure to the
elements. These crates of bottles were then placed on the roof of Plastic Technologies,
Inc. building in early January. Every three months, the bottles in the crates were agitated
so that those on the bottom had a chance over time to move to the top or outside
edges. Another set of bottles was stored indoors approximately 18-inches under a
fluorescent light source. This light source was left on continuously, exposing the bottles
for two months. A third set of bottles was stored and protected from light exposure for
one year for use as a control.
Sunlight exposure effects on the intrinsic viscosity of PET
Intrinsic viscosity measurements are used to relate the measured value to the molecular weight of PET. The
cleaned and washed flake samples were all measured to determine if there was any loss due to the effects
of sunlight. The results showed a definite trend downward with an apparent loss in IV of about 0.04 dL/g
over 12 months. This same molecular weight loss was seen when the weathered flake was injection molded
into the 3mm thick plaques. While the loss due to exposure of the flake to ultraviolet radiation was ~0.04
dL/g, the degree of IV loss increased to ~0.06 dL/g when this flake was melt processed into 3mm plaques.
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Periodically bottles were taken off the roof to study the effects of sunlight and
weathering over the course of 12 months. Each aged set of bottles was ground through
a 3/8” screen into flake. The flake was then aggressively washed to remove accumulated
surface dirt at 88°C for 15 minutes using a wash composition of 0.3% by weight Triton
X-100 and 1.0% caustic (NaOH). Following the hot caustic wash, the flake was rinsed
twice with fresh water, dried to less than 50 ppm moisture and injection molded into
3mm thick plaques.
Samples of flake from each of the exposure time periods was measured for IV to
determine if the ultraviolet light from the sun’s radiation caused any loss in molecular
weight. The plaques molded from this flake were also measured for IV, color and haze.
The IV results are quite interesting but not surprising in that the data clearly shows that
exposure to ultraviolet radiation was very damaging to the PET material.
Sample flake was also measured for color before and after the one year exposure to
determine if any obvious yellowing occurred. Surprisingly there was not a significant
amount of yellowing seen in the color of the bottle flake. The yellowing only became
significant after the weather exposed PET has been subjected to melting required for
molding the plaques.
PET reclaimers will take washed and cleaned flake and extrude it into pellets (not
plaques). Thus if their bottles have been exposed to similar levels of ultraviolet radiation
from being stored outside, there would be an expected increase in yellowness resulting
from the extrusion process. Further yellowing will then result when this extruded
pelletized rPET is blended with virgin PET resin and melted yet again during preform
manufacture. Thus the bottle production process will only further exacerbate the
yellowing seen when this RPET is used.
Effects of sunlight exposure on PET color and haze
L* indicates the lightness of the sample with 100
being very white and 0 being very dark or black.
The injection molded plaques made from these
weathered bottles show a definite trend with a
reduction in L* over time. Typical bottles made from
PET today will have L* values ranging from the high
80s to the low 90s. Recycle content bottles made
with this weather exposed recycled PET would show
a reduction in their L* values resulting in bottles that
are less clear to the eye and more dull in appearance.
The diamonds on the graphs show the effect that
fluorescent light had after the bottles had been
exposed to two months of fluorescent light when
molded into plaques. While there was a measureable
decrease in L* seen on plaques molded from bottles
exposed to the fluorescent light, direct sunlight had a
much more significant and pronounced effect.
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b* denotes the degree of chromaticity from yellow
(+b*) to blue (-b*). The effect that the ultraviolet
radiation exposure had on the molded plaque b*
values is even more noticeable than was L*. The
increase in yellowness on molded plaques was very
significant even after only one month of exposure.
Beyond 4 months, the effects of the ultraviolet
radiation, while still significant, were not as dramatic
as what was seen during the initial months. As was
the case with L*, yellowness development was not
as significant from the fluorescent light exposed PET
compared to the effect seen from direct sunlight.
The effect of the sunlight exposure on the haze of the molded plaques was minimal. The
control plaque showed haze values of 9.9% where the 12 month weathered PET haze dropped
only to 9.5%.
In conclusion, the results of this study demonstrate that storing bottles outside with direct
exposure to sunlight in humid environment can have a severe effect on the L*, b* and IV
properties of recycled PET. The bottles used in this study were not densely packed into large
bales, thus sunlight could easily penetrate through the several layers of bottle sidewalls
reaching even the bottles located at the bottom of the open crate.
What the average effect might be on an entire bale of bottles compared to what was seen
in this study is a matter of debate. However when one considers all the potential causes of
yellowing that can that limit the percentage of use with today’s recycled PET, exposure to
ultraviolet radiation, whether it is from outside bale storage or possibly even exposure to
fluorescent lighting in retail stores should be considered as another contributor to rPET
quality degradation.
This is just one more factor that can be added to the natural tendency of PET to yellow with each
additional melting due to the influences of adhesive label residues, trace amounts of residual
label material, inks and coatings that can bleed from the label onto the PET flake, as well as
additives that cannot be removed by the washing process. All present problems to PET recycling.
Plaque made from warehouse
control, stored for 12 months
L* = 88.8
b* = 3.0
Plaque made from bottles
weathered for 3 months
L* = 85.5
b* = 14.2
Plaque made from outside
weathered bottles after 1 month
L* = 86.0
b* = 9.3
Plaque made from bottles
weathered for 6 months
L* = 82.8
b* = 15.9
Plaque made from bottles
exposed to fluorescent light
for 2 months
Plaque made from bottles
weathered for 12 months
L* = 87.8
b* = 6.1
L* = 82.7
b* = 20.2
About Plastic Technologies
Plastic Technologies, Inc. (PTI) is recognized worldwide as the preferred source for preform and
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Footnotes:
1
D. Hayward, The Effect of Contaminants on Yellowness in Recycled PET, APR presentation, Feb., 16, 2006
2
The Association of Postconsumer Plastic Recyclers, www.plasticsrecycling.org
3
Jovan Radulovic, Degradation of Polyethylene Terephthalate in Natural Conditions, Scientific Technical Review, Vol. LVI. No. 2, 2006
4
S. Venkatachalam, Shilpa G. Nayak, Jayprakash V. Labde, Prashant R. Gharal, Krishna Rao and Anil Kelkar, Degradation and Recyclability
of Poly(Ethylene Terephthalate), Reliance Technology Group, Reliance Industries Limited, B4, MDIC Industrial Area, Patalganga, Raigad
District, Maharashtra State, India
5
Yu. S. Akishev, M.E. Grushin, A. I. Drachev, V.B. Karalnik, A.V. Petryakov and N.I. Truskin, The Open Plasma Physics Journal, 2013, 6,
(suppl 1: M4) 19-29
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