ADVANCED BIO-FRIENDLY POLYMERS

Štefan Chmela

Photo degradation of polymers

One of the disadvantages of using polymers in high temperature conditions or in outdoor applications – degradationenvironment 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

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.

photo-oxidation

Hydroperoxide >> carbonyls > unsaturations> complex [PH...O2], atmospheric impurities (SO2, NO), aromatic

hydrocarbons, singlet oxygen

Main steps of photo-oxidation

InitiationPropagationBranchingTermination

impurities responsible for the initiation

Iniciácia:

Propagácia:

Vetvenie:

Terminácia:

R HR R

R.

R.

R.

R.

R.

ROOH

ROH

OH

RH

RH

ROO. R

.+

R.

++

+

RO.

.

+ O2

.ROO

+ RH H2O R.+

+ C C.R C C

R1 C R2

O

R1C

R3

R2

O.

-štiepenie+ R3.

fragmentácia olefín + R, .

2 ROOH

2 R.

RO.

R. R

.R

.

R.

ROOH OH.

RO. +

ROO.

H2ORO. ++

ROO+

+

+

. ROOR

R R

R O R

RH + olefíndisproporcionácia

2 ROO ROO

O

ROO

OH.

+ + O2

(1.1)

(1.2)

(1.3)

(1.4)

(1.5)

(1.6)

(1.7)

(1.8)

(1.9)

(1.10)

(1.11)

(1.12)

(1.13)

(1.14)

(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 describetheir light-induced degradation:

• Photolysis - absorption as a result of the inherent polymeric structure resultsin 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 polymerstructure.

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

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.

phenyl ester reaction, production of more photoactive ketone

Radical oxidation process of irradiated PLA samples: hydroperoxide chain propagation and formation of anhydrides by photolysis of hydroperoxide

General scheme for degradation and stabilization

Testing methods

Natural ageingArtificial ageing

Changes are followed by

spectral methods (FTIR, UV-Vis, fluorescence)GPC – molecular massmechanical properties

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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 possible synergistic effect Disadvantages 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

1800 1700 1600

0,0

0,1

0,2

0,3

0,4

0,5

0,6

600 hrs

535 hrs

490 hrs

0 hrs

CO

abs

orpt

ions

pure iPP

, cm-1

3700 3600 3500 3400 3300 3200-0,1

0,0

0,1

0,2

0,3

0,4

0,5

600 hrs

535 hrs

490 hrs

0 hrs

, cm-1

pure iPP

OH

abs

orpt

ions

Changes of the carbonyl region Changes of the –OH vibrational region

0 500 1000 1500 2000 2500 3000 3500 4000 45000.0

0.1

0.2

0.3

0.4

0.5

pure PP TMP-I TMP-II TMP-III TMP-IV

Car

bony

l Abs

orpt

ion

Irradiation time (h)

0 500 1000 1500 2000 2500 3000 3500 40000.0

0.1

0.2

0.3

0.4

0.5

pure PP PMP-I PMP-II PMP-III PMP-IV

Car

bony

l Abs

orpt

ion

Irradiation time (h)

NHOH

N CHO 3H

CH3NH

NH

O

XO

O

HAS

Ph

PP powder fresh

OHO OH OHOHCH3

I II III IV

Thank you for your attention