ADVANCED BIO-FRIENDLY POLYMERS
Photo degradation of polymers
Štefan Chmela
One of the disadvantages of using polymers in high temperature
conditions or in outdoor applications – degradation
environment negatively influences the service life.
This process is called weathering - ageing
an irreversible chemical process,
undesired changes of properties of the polymers,
discoloration and loss of mechanical properties.
outdoor applications -reactions of the polymer with and without
oxygen induced by terrestrial sunlight
the UV-radiation is one of the most important factors determining
the polymers lifetime.
Solar irradiance spectrum above atmosphere and at surface.
Sunlight in space at the top of Earth’s atmosphere at a power
of 1366 watts/m2 is composed (by total energy) of about 50%
infrared light, 40% visible light, and 10% ultraviolet light.
Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm.
The term ultraviolet refers to the fact that the radiation is at higher
frequency than violet light (and, hence also invisible to the human
eye). Owing to absorption by the atmosphere very little reaches the
Earth's surface. This spectrum of radiation has antiseptic properties
and is used in germicidal lamp.
Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly
absorbed by the atmosphere, and along with UVC is responsible for
the most of photochemical reactions.
Ultraviolet A or (UVA) spans 315 to 400 nm.
Degradation due to UV-radiation is called photodegradation.
Chemical reactions -chain scissions,
- cross linking
- oxidation
influence the physical properties and thus the article's lifetime
Besides the service environment, other parameters, as the polymer
itself and the use of stabilizers influence the rate of degradation.
The most important polymer-related parameters for degradation
are the type of polymer, (e.g. polyolefins, engineering plastics as
polyamides or polycarbonates), the amount of branching, catalyst
residues, or end groups.
Different techniques to stabilize polymers have been developed,
e.g. adding different types of stabilizers, or applying a protective
coating
light is absorbed by a polymer - photochemical reactions can occur as a
result of activation of a polymer macromolecule to its
excited singlet or triplet states
precursors of all photochemical reactions
The most important mechanisms causing weathering of polymers are
photolysis and photooxidation.
If the absorption of light leads directly to chemical reactions causing
degradation, this is called photolysis.
Photo-oxidation is a result of the absorption of light that leads to the
formation of radicals that induces oxidation of the material.
oxidation products are distributed non-homogeneously in the sample
photo-oxidation
polyolefines (PE, PP) = photo-oxidation is the dominating
mechanism.
These polymers do not have an inherent absorption at wavelengths
present in terrestrial sunlight (>290-400 nm)
photolysis can not play an important role.
Nevertheless, irradiation of these polymers with terrestrial
wavelengths results in accelerated degradation – especially for PP
This can be ascribed to impurities that are formed during storage
and processing.
Due to photolytic reactions of these absorbing species, radicals are
formed that initiate the photo-oxidation reaction.
impurities responsible for the initiation
Hydroperoxide >> carbonyls > unsaturations
> complex [PH...O2], atmospheric impurities (SO2, NO), aromatic
hydrocarbons, singlet oxygen
Main steps of photo-oxidation
Initiation
Propagation
Branching
Termination
Iniciácia:
R
R
.
H
R
R
(1.1)
Propagácia:
.
ROO
+ O2
R
.
.
(1.3)
ROH
+
R
.
(1.4)
+ RH
H2O
+
R
.
(1.5)
R
C.
.
R + C
O
R
.
. + RH
OH
1
(1.2)
ROOH + R
ROO + RH
RO
.
C
.
C R
R
C
2
-štiepenie
(1.6)
O
.
1
R C R2 +
R3
(1.7)
3
fragmentácia
.
R
olefín
+
R
,.
(1.8)
Vetvenie:
. .
ROOH
2 ROOH
RO + OH
.
.
RO + ROO + H2O
(1.9)
ROOR
(1.11)
R
R
(1.12)
R
O R
(1.13)
RH + olefín
(1.14)
(1.10)
Terminácia:
. + ROO .
.
.
R + R
.
.
RO
R
R
+
.
2R
2
disproporcionácia
ROO
.
OH
O
ROO
+
ROO
+ O2 (1.15)
In contrast to polyolefins, the majority of engineering plastics
e.g. aromatic polyesters, polyamids, polyuretanes, polycarbonates, polyketones
etc.)
do have absorptions
at wavelengths being present in terrestrial sunlight, so that for these polymers
photolysis can play an important role too.
For these polymers in principle there are three mechanisms that can describe
their light-induced degradation:
• Photolysis - absorption as a result of the inherent polymeric structure results
in chemistry causing changes in the molecular structure;
• Photo-oxidation initiated by photolysis reactions of the polymer itself;
• Photo-oxidation initiated by impurities not part of the inherent polymer
structure.
Photolysis: Norrish I, Norrish II
When light is absorbed by the polymer, Norrish reactions can occur, which lead to
changes in molecular structure resulting in degradation. The Norrish I reaction
leads to chain cleavage and radicals that might initiate the photo-oxidation.
The Norrish II reaction is a non-radical intramolecular process, in
which hydrogen (hydrogen on gama C), is transferred, leading to
chain cleavage.
For polyamides and polyesters (biodegradable polymers) the most
important photolytic reactions are the Norrish I and II reactions.
photo-Fries reactions
phenyl ester reaction, production of more photoactive ketone
photo-Fries rearrangement may occur naturally, for example when a plastic bottle made of
polyethylene terephthalate (PET) is exposed to the sun, particular to UV light at a
wavelength of about 310 nm.
Radical oxidation process of irradiated PLA samples: hydroperoxide chain propagation
and formation of anhydrides by photolysis of hydroperoxide
General scheme for degradation and stabilization
PH
PH - O2
PH
S/X
MECHANICAL
STRAIN
..
S /X
n+
h, ,M
3
O2
h
PH
LIGHT SCREENING
UV ABSORBERS
S/X
CH
CH *
h
.
PH
P-P
DISPROP
P
.
PO2
PH
3O
QUENCHER
2
.
PO
1O
2
n+1
h,M
PH
PH
POOH
METAL
DEACTIVATOR
HYDROPEROXIDE
DECOMPOSER
RADICAL
SCAVENGER
Testing methods
Natural ageing
Artificial ageing
Changes are followed by
spectral methods (FTIR, UV-Vis, fluorescence)
GPC – molecular mass
mechanical properties
SUNTEST CPS+
The Atlas SUNTEST CPS+ is the small entry model.
CPS+ is the most widely used bench top xenon
instrument in the world. Its compact design, easy
handling and proven reliability make it the ideal
quality control and R&D screening device for a
variety of industries, such as plastics, packaging,
pharmaceuticals, cosmetics, and many more.
1x 1500 W air-cooled Xenon Lamps
560 cm2 exposure area
Direct Setting and Control of Irradiance in the wavelength range 300-800 nm / Lux; or
300-400 nm / 340 nm
Direct Setting and Control of Black Standard Temperature (BST)
Display of Chamber Air Temperature
Display of Test Values and Diagnostic Messages
Parameter Check
Two pre-programmed test methods
Ci5000 Weather-Ometer
The Ci5000 is approved by many automotive, paints &
coatings and plastics industries as the exclusive platform to
deliver accurate, reproducible and repeatable results for
predicting service life.
Unmatched Repeatability and Reproducibility
Design and engineering innovations in the
airflow, irradiance and control systems have
dramatically reduced variability in critical test
parameters. As a result, the Ci5000 achieves
new levels of temperature, humidity and light
exposure uniformity.
12000 W water cooled xenon arc lamp system
Total exposure area: 11,000 cm2 (1,705 in2)
Direct Setting and control of Irradiance: 340nm, 420nm, 300-400nm or Lux
Direct setting and control of Black Panel Temperature;
Direct Setting and control of relative humidity
Direct setting and control of specimen chamber air temperature
Example of testing new stabilizer
Additives: processing
sterically hindered phenols
phosphites
long term stabilizers UV absorbers
Hindered Amine Stabilizer - HAS
Usually the mixture of low molecular weight additives are used.
Advantage
Disadvantages
possible synergistic effect
possible antagonistic effect
physical loss during processing – evaporation (high
temperature)
during long term application - washing out
Solving the problem - synthesis of combined higher molecular
weight additives
Combination of phenol/HAS
Monitoring of photo-oxidation by FTIR spectroscopy
Changes of FTIR spectra of PP film during irradiation
0,6
0,5
600 hrs
pure iPP
0,5
pure iPP
0,4
0,3
600 hrs
CO absorptions
OH absorptions
0,4
535 hrs
0,2
490 hrs
0,1
535 hrs
0,3
490 hrs
0,2
0,1
0 hrs
0 hrs
0,0
0,0
-0,1
3700
3600
3500
, cm
3400
3300
3200
-1
Changes of the –OH vibrational region
1800
1700
, cm
1600
-1
Changes of the carbonyl region
PP powder fresh
0.5
Carbonyl Absorption
0.4
pure PP
TMP-I
TMP-II
TMP-III
TMP-IV
0.3
HO
NH
0.2
Ph
O
0.1
NH
CH3
O
NH
0.0
0
500
1000
1500
2000
2500
3000
3500
4000
X
HAS
O
4500
Irradiation time (h)
OH
CH3 O
OH
OH
OH
0.5
I
II
III
Carbonyl Absorption
0.4
pure PP
PMP-I
PMP-II
PMP-III
PMP-IV
0.3
0.2
HO
0.1
0.0
0
500
1000
1500
2000
2500
Irradiation time (h)
3000
3500
4000
N CH 3
IV
Thank you for your attention