UV Weathering and Related Test Methods - Cabot Corporation

UV Weathering and Related Test Methods

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UV Weathering and Related Test Methods
Table of Contents
Page
I. Introduction to Weathering 3
1. UV light spectrum and solar radiation 3
2. Radiation energy – definitions 4
3. Average solar radiation by country 5
4. Light stabilizers for plastic materials 6
a) UV light absorbers 6
b) Quenchers 7
c) Hindered Amine Light Stabilizers (HALS) 7
II. Weathering Test Methods 8
1. Natural weathering 8
2. Artificial weathering chambers 9
a) QUV fluorescent light source 9
b) Xenon lamp weathering (Ci65A chamber) 10
c) QUV versus Xenon lamp testing 11

Introduction to Weathering
Long term exposure to sunlight leads to the degradation of plastic
materials. In particular, the non-visible UV radiation characterized by
short wavelengths is responsible for photo-degradation, a process
that generally results in breaking down the polymer chains. This
frequently results in a deterioration of the physical properties,
changes in color or chalking of the part surface. As an example, films
lose their flexibility and disintegrate, garden furniture becomes brittle
or stadium seats become chalky.
In order to limit or postpone the onset of degradation, several types
of UV light stabilizers can be added to the polymer. The most
important stabilizer types work by screening out the harmful
ultraviolet light - for instance UV absorbers such as benzophenones
or small dispersed particles such as carbon black or inorganic
pigments. Other very effective UV stabilizers are UV quenchers and
HALS (Hindered Amine Light Stabilizers). A brief description of these
stabilizers is given in the next pages.
One important factor, when the light stability of a given material or the
performance of a UV stabilizer needs to be assessed, is the selection
of a suitable test method. Besides outdoor exposure that closely
reflects natural weathering conditions but requires long exposure
times, artificial weathering tests have been developed using light
sources such as Xenon arcs or fluorescent lamps under controlled
temperature and humidity conditions.
Ultraviolet light spectrum and solar radiation
UVB VISIBLE INFRA RED
280 nm 315 nm 385 nm 780 nm 3 µm
SOLAR SPECTRUM
The solar spectrum covers a broad range of radiation including short
wavelength UV's, visible light, as well as infrared radiation.
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Radiation components
UV radiation represents only 4.6% of the solar spectrum, but causes
the most important damage to the polymeric materials. The complete
solar UV spectrum ranges between 280 and 400 nanometers, but the
most aggressive part is the UVB range with very short wavelengths
between 280 and 315 nanometers.
visible light
45%
UV
5%
Radiation energy: definitions
The irradiation is the radiation energy incident over a specific area
for a given period of time. It is expressed either in W*s /m 2 , Joule/m 2
or very often in Langley (Ly).
1 Ly = 1 cal/cm2 = 4.184 E 4 Joule/m 2
infrared
50%
Example : what is the total irradiation for a 3-year outdoor exposure
in Belgium?
➢ annual sunlight radiation in Belgium : 80 kLy (see table below)
➢ total irradiation after 3 years : 80 x 3 = 240 kLy
The global annual sunlight radiation level (kLy/year) for various
countries is given in the table below. It corresponds to the radiation
energy that can be transmitted to a plastic part in one year of
continuous outdoor exposure.
Note: values in this table are only indicative. Within certain (larger)
countries, radiation levels can vary significantly from one area
to another.

Average solar radiation per country in kLy (kcal/cm 2 /year)
COUNTRY kLy
Austria 80
Afghanistan 180
Alaska 70
Algeria 160
Angola 120
Argentina 160
Australia 180
Bahamas 140
Bahrain 200
Belgium 80
Burma 120
Bolivia 140
Brazil 120
Bulgaria 100
Canada 100
Chad 200
Chile 140
China 140
Columbia 100
Costa Rica 140
Cuba 140
Cyprus 140
Denmark 70
Egypt 200
Ecuador 120
El Salvador 140
Ethiopia 140
Finland 70
France 120
COUNTRY kLy
Germany 80
Great Britain 70
Greece 120
Guatemala 140
Guyana 120
Haiti 160
Hong Kong 140
Honduras 140
Hungary 80
India 180
Indonesia 140
Iraq 180
Iran 180
Israel 180
Italy 120
Jamaica 160
Japan 100
Jordan 180
Kenya 140
Kuwait 180
Korea 120
Lebanon 180
Luxembourg 80
Libya 180
Madagascar 140
Mali 200
Malta 160
Malaysia 140
COUNTRY kLy
Morocco 160
Mauritania 180
Mexico 160
Mozambique 160
Nepal 160
Netherlands 80
Nicaragua 140
Niger 200
Norway 70
New Zealand 120
Oman 160
Pakistan 180
Panama 40
Paraguay 160
Peru 140
Philippines 140
Poland 80
Portugal 40
Rumania 100
Russia (North) 70
Russia (South) 140
Sardinia 20
Saudi Arabia 200
Senegal 180
Sicily 140
Singapore 140
South Africa 160
Spain 140
Sudan 220
COUNTRY kLy
Suriname 120
Sweden 70
Switzerland 80
Taiwan 140
Thailand 140
Tunisia 160
Turkey 140
Uruguay 160
USA
North 100
Arizona 180
Florida 140
Uganda 140
Vietnam 140
Venezuela 160
Zambia 180
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Light stabilizers for plastic materials
To provide an appropriate protection against UV radiation, several stabilizing systems can be utilized in plastic materials.
The most important types of light stabilizers are Ultraviolet Light Absorbers, Energy Transfer Agents or Quenchers, as
well as Hindered Amine Light Stabilizers. A brief description of these different light stabilizers is given below.
a) UV light absorbers
Absorbers convert harmful ultraviolet radiation to harmless infrared
radiation or thermal energy, which is dissipated through the polymer
matrix. They can be either transparent as hydroxybenzophenone or
opaque like carbon black.
Carbon black
Carbon black is one the most efficient and widespread light
absorbers. Its efficiency as a UV absorber depends primarily on the
primary particle size and structure. At the same loading, carbon black
aggregates based on fine prime particles will present more surface to
incident light - and hence a larger ultraviolet light absorbing efficiency
- than a coarser grade.
Primary particle
Carbon black aggregate
Primary particle size
(typically 15 to 60 nanometers)
Effect of primary particle size on weathering performance
%RE at break
125
100
75
50
25
0
Accelerated Weathering – ATLAS
55 µm LDPE films with 2.5% CB
0 250 500 750 1000 1250
Exposure time (hours)
The appropriate loading level depends on the part thickness, exposure
conditions and type of carbon black. Usual loadings to impart
optimum UV protection vary between 2 and 3% (it should be noted
that these carbon black levels correspond to 4 to 7% masterbatch,
depending on their loading).
< 25 nm particle size
60 nm particle size

Absorption Coefficient
(Kilo Abs. Units/meter)
Effect of loading levels on ultraviolet light absorption
600
500
400
300
200
100
0
The strong absorption characteristics and high opacity of carbon
black make it the most cost effective UV absorber. Typical applications
for carbon black as a UV stabilizer in plastics are exterior pipe,
polyolefin agricultural film, pond linings, automotive parts and exterior
cable jacketing (PVC, PE, etc…).
Titanium dioxide
Certain pigments such as rutile titanium dioxide absorb in the 300 to
400 nm range. Hence, they contribute to the protection of the polymer,
provided that the pigment has a suitable coating to prevent the
photo-degradation processes usually observed with TiO 2 .
Hydroxybenzophenone and hydroxyphenylbenzotriazole
These well-known UV absorber types offer the advantage of being
suitable for natural or transparent applications. To provide a good
protection to the plastic material, a certain absorption depth is needed
(part thickness) which makes these absorbers inefficient in thin
items such as films (below 100 micron), fibers or tapes.
b) Quenchers
1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5
% Carbon Black in LDPE films
This type of light stabilizer functions by bringing ‘excited’ state
polymer molecules (chromophores) back to their stable state,
preventing bond cleavage and finally formation of free radicals. Nickel
stabilizers are typical quenchers used, for instance, in agricultural film
applications.
c) Hindered Amine Light Stabilizers (HALS)
These very efficient light stabilizers do not modify the color of the
plastic material and are suitable for both thin and thick cross-section.
HALS are not active by absorbing the UV radiation but rather by
reacting with the radicals which are formed and thus by limiting the
degradation reactions in presence of some chemicals (pesticides,
insecticides, acidic chemicals, etc…).
< 25 nm particle size
60 nm particle size
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Weathering Test Methods
Besides natural weathering, several test methods have been
developed using artificial light sources to provide accelerated test
procedures. All methods are based on the regular observation of
characteristics reflecting an ageing process such as mechanical
properties (elongation at break, tensile properties or impact strength)
or visible characteristics, such as crack formation, chalking, changes
in color or gloss.
The main testing methods for plastic material ageing are the following:
a) natural weathering stations
b) artificial accelerated weathering chambers:
• QUV fluorescent light source
• ATLAS Xenon lamp
• SEPAP chamber (medium pressure mercury arcs)
Outdoor testing racks are used by Cabot in Dukinfield (United Kingdom),
Grigno (Italy), Berre (France), Hong Kong and Altona (Australia)
representing a broad range of climate and irradiance levels. QUV
fluorescent light chambers as well as Atlas Weatherometers (Xenon lamp)
are located in the Cabot Weathering Center in Loncin (Belgium).
Natural weathering
Outdoor exposure is performed on samples mounted on testing racks
oriented under standard conditions (typically facing South in the
northern hemisphere, 45° vertical inclination). In this way, the material
is exposed to the full radiation spectrum from the infrared to the
ultraviolet ranges. Of course radiation, but also temperature, and
relative humidity levels strongly depend on the location, the seasons
and can show some fluctuation from year to year.
The average solar radiation levels for the different Cabot testing sites
are the following:
Cabot Natural / Outdoor Facilities
Exposure site Climate Type Yearly Solar
Dukinfield (UK) Oceanic temperate 65 kLy
Grigno (I) Mediterranean 100 kLy
Berre (F) Mediterranean 110 kLy
Hong Kong Tropical 120 kLy
Altona (Aus) Tropical 135 kLy
1 kLy = 1 cal/cm 2 / 1 kLy = 41.84 MJ/m 2
Natural weathering tests provide the most accurate and reproducible
data. However, their duration can be very long, which frequently leads
to the selection of an artificial weathering method.

Artificial weathering chambers
a) QUV fluorescent light source
Weathering chambers have been developed to provide a QUV
weathering.
The QUV simulates the effect of sunlight with fluorescent ultraviolet
(UV) lamps, while rain and dew are simulated by the condensation of
humidity. As stated previously, the UV light only represents roughly 5%
of the sunlight but it is responsible for most of the polymer
degradation. Also, materials are often tested with equipment, which
simulate only the shortest wavelengths (UV).
The UV-B range includes the shortest wavelengths found in sunlight.
Therefore, for many applications, it is a fast and efficient method.
QUV equipment uses two main types of lamps: UVA-340 and UVB-
313.
As shown in Figures 1 and 2, while these lamps have different light
emission spectrum, they are both characterized by a maximum of
emission in the UV range.
UVA provides a reasonable match of the UV region of the solar
spectrum, but this match is no longer valid for the long wavelengths
(visible, IR).
UVB lamps also emit UV light, but the maximum of the emission
spectrum is shifted towards short wavelengths compared to the UVA
lamps. The UVB-313 lamp is a widely used type of fluorescent UV
lamp that provides fast test results. However, as shown in Figure 2,
the spectrum contains short wavelengths, which are not present in
the solar radiation.
Irradiance (W/2/nm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
270
Figure 1: UVA-340 spectrum Figure 2: UVB-313 spectrum
UVA-340 versus Sunlight
UVA-340
290 310 330 350 370 390
Wavelength (nm)
Sunlight
By courtesy of QPanel
Irradiance (W/m2/nm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
UVB Lamps versus Sunlight
UVB-313
QFS-40
270 290 310 330 350 370 390
Wavelength (Nm)
Sunlight
By courtesy of QPanel
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QUV chambers, Cabot Weathering Center
For customer support, Cabot typically uses UVB lamps characterized
by their short wavelength spectrum in order to provide fast test
results. Although these data might not always perfectly correlate with
outdoor exposure results, QUV-B is very useful for preliminary or comparative
testing, as well as for very durable applications. For more
realistic exposure conditions, ATLAS Weathering Chambers are generally
preferred.
b) ATLAS xenon arc weathering chambers
Among all artificial UV sources, Xenon lights provide the best simulation
of natural sunlight. As shown below, with an appropriate filter
combination, their irradiance spectrum can be adapted to match
closely the natural sunlight over a broad range of wavelengths.
Irradiance (W/m 2/nm)
2.0
1.6
1.2
0.8
0.4
0.0
Figure 3: Xenon arc versus Miami sunlight
spectrum
Xenon Arc With Boro/Boro Filter Combination
”Average”
Mia
Xenon
25 35 45 55 65 75
Wavelength
By courtesy of ATLAS

The Xenon arc weathering chambers have automatic control of light
intensity, temperature and humidity. Specific programs allow the samples
to be sprayed with water or exposed to alternating cycles of dark
and light periods.
ATLAS Ci65A Weathering Chamber, Cabot Weathering Center
c) QUV versus Xenon lamp testing
Key characteristics Usual Standards at Cabot
QUV • Only match the short UV part of solar spectrum ISO 4892/3
Fluorescent • Faster comparative results Light source : UV B (313nm)
UV B lamps • Features : Light/ dark/ condensation/ Irradiance : 0.63 W/m2 at 313 nm
no humidity control Cycle : 8 hours light at 60°C,
4 hours condensation
ATLAS • Emission spectrum closer to the entire solar ISO 4892/2
Ci65A spectrum Referenced in the CEN norm for
Xenon lamps • More "absolute" comparative results agricultural films
• Features : Light/dark/spray/ Light source : xenon burner with
condensation/humidity control 2 borosilicate filters
Irradiance : 0.35 W/m2 at 340 nm
Cycle: 102 min light (65°C,65%RH),
18min light & spray
Cabot Technical Support
Our technical support team is ready to help you for any further questions
you might have and can assist you in finding the appropriate
solution to your problems and projects.
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North America:
Cabot Corporation
Business and Technical Center
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Billerica, MA 01821-7001
USA
Tel: (978) 663-3455
Tel: (800) 462-2313 (Technical Service)
Fax: (978) 670-7035 (Technical Service)
Tel: (800) 526-7591 (North America Customer
Service)
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Av. Joao Castaldi 88
04517-900 Sao Paulo, SP
BRAZIL
Tel: +55 11 5536 0388
Fax: +55 11 5542 6037
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Cabot Specialty Chem. Inc.
Jebel Ali Free Zone
LOB 15, Office 424
Dubai
UNITED ARAB EMIRATES
Tel: +971 4 8871 1800
Fax: +971 4 8871 1801
Europe:
Cabot
Interleuvenlaan, 5
B - 3001 Leuven
BELGIUM
Tel: +32 16 39 24 00
Fax: +32 16 39 24 44
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Level 14, MNI Tower 2
11, Jalan Pinang
50450 Kuala Lumpur
MALAYSIA
Tel: +60 3 2164-8352
Fax: +60 3 2162-0253
Notice and Disclaimer. The data and conclusions contained herein are based on work believed to be
reliable; however, Cabot cannot and does not guarantee that similar results and/or conclusions will be
obtained by others. This information is provided as a convenience and for informational purposes only.
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(c) Cabot Corporation, M.A.-U.S.A. All rights reserved
www.cabot-corp.com/plastics
UV.TEST/02.02/E