TIP FROM TECHNOLOGYUV Effect on Polyethylene

UV light characteristics

All polyethylene (PE) is susceptible to degradation upon long-term exposure to sunlight. This degradation is brought about by

physical changes, which occur in the polyethylene as a result of exposure to the ultraviolet (UV) portion of sunlight. Figure 1

shows that UV light contains shorter wavelengths than visible light. The shorter the wavelength, the more energy it contains

and thus, the more damage it does. This is explained in the next section in more detail. Fluorescent lighting also contains a

band of UV light, but only at an intensity of around 15% of normal sunlight.

Why does PE change when exposed to UV light?

Table 1 shows why UV light alters the physical characteristics of polyethylene (PE). It does this by breaking the carbon and

hydrogen bonds, creating free radicals, which, in turn, break the PE into shorter molecules and thus, a more brittle polymer.

Effectively, UV light creates a higher melt index polyethylene, especially on the exposed surface area. This shows up as a

reduction in break elongation and impact properties, typical of higher melt index PE. The subsequent attachment of oxygen

to these broken sites leads to further accelerated degradation and the formation of oxidized species such as carbonyl and car-

boxyl structures, which are often used as analytical indicators of UV degradation. Table 1 shows the importance of choosing

the correct wavelengths for exposure testing that realistically simulates the weathering effects on PE.

Energy containedWavelength of at this wavelength Type of bond or Energy to break

light, nm KJ/m2 structure in PE bond, KJ/mol.

189 647 Carbon-carbon (C-C) 347

253 473 Carbon-hydrogen (C-H) 413

315 228 Double bond 607carbon-carbon (C=C)

Table 1: Why UV light changes PE

60 50 40 30 20 10 0

extremeultraviolet

micro-waves

radiobroadcast

octaves

Wavelengths

gamma rays X-rays ultra-violet

Visible

infrared heatwireless

Angstroms

10-14

1022 1020 1018 1016 1014 1012 1010 108 106 vib/s

10-12 10-10

10 102

m

103 104 105 106 107 1081

10-8 10-6 10-4 10-2 1 102 104

Figure 1: UV light and its position in the Electromagnetic spectrum

1

The shorter the wavelength, the more energy and thus, destructive power it has. Actual UV light is composed of a range of

wavelengths as shown in Table 2. Thus, it is important when doing indoor, accelerated weathering exposure to use a lamp or

light source that best matches natural sunlight.

Test protocol for UV exposure

We use both indoor, accelerated weathering testing as well as outdoor exposure testing in Florida. Indoor, accelerated testing

can be done in shorter time periods as the exposure is continuous and irradiation more intense.

The instruments we use for accelerated testing are Xenon Ci-65 and Ci-5000 Weather-O-Meters. Test conditions follow the

guidelines outlined in ASTM G155 cycle 1*. It uses a xenon arc lamp with a borosilicate inner and outer filter, which best

simulates the UV band of natural sunlight. Carbon arc lights are unrealistic as they produce too much of the wavelengths

between 325 to 425 nm. QUV-B testers also are not as good for non-crosslinked polyethylene as they reduce the higher

wavelengths of light.

UV light intensity is measured by irradiance and is usually expressed in watts per square meter at a given wavelength. The

weather-o-meters are run at 0.35 watts/m2 and 340 nanometers (nm), an industry accepted protocol, which matches one of

the conditions in Florida, as illustrated in Figure 2.

*ASTM test procedures may be modified to accommodate operating conditions or facility limitations.

Type of UV light Wavelength, nanometer (nm)

UV-A 320-380

UV-B 280-320

UV-C 180-280

Natural Sunlight 280-800+

Table 2: UV light has a range of wavelengths

290

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0300 310 320 330 340 350 360 370 380 390 400

Wavelength (nm)

Irra

dian

ce (

W/m

2 /nm

)

calculated summer noonmeasured summer noon

calculated annual averagemeasured "average optimal"

+

x

Figure 2: Irradiance level vs. Miami, Florida conditions

Calculated and measured sunlight in Miami. Summer solstice and“average” annual sunlight are shown.

Source: J.E. Pickett, K.K. Webb, GE Electric Co., Die Angewandte Makromoldekulare Chemie 252, (1997) 217-236 (4524)

2

The temperature of exposure is also important, as the higher the temperature, the more degradation will occur. A black panel

in the machine measures this temperature and is set at 63°C. The backing on the sample may change the actual sample

temperature. For this reason, our samples are unbacked both in the Ci-65 and in Florida.

Figure 3 shows a correlation of Xenon testing

with Florida exposure. It shows that 2000

hours in the weather-o-meter is approxi-

mately 140 kilolangleys (kly) per year, which

is typical of one year exposure in Miami,

Florida. A kilolangley is a measure of UV

exposure per square meter of surface area.

Figure 3 was based on PE film samples. We

have found that using thicker molded sam-

ples and running the weather-o-meter on a

wet and dry cycle still gives a correlation of

approximately 2000 hours, being one year

in Miami. Most of our Florida data is done

at a 45 degree south exposure angle. We

have over 62 data points done over 15 years

showing good agreement between our accel-

erated and outdoor test results in developing

this correlation.

UV performance depends on geographic location

Geographic location, along with changes in climate and elevation, affect actual UV performance. Figure 4 shows an updated

chart outlining isolines of equal global UV radiation. This figure has been updated to reflect the increased severity in higher

elevation areas. In fact, for every 0.5 km increase in elevation, UV exposure increases by 3.5%. This chart also reflects changes

in ozone layers in the world. While well publicized, the changes have been relatively minor when considering exposure to

polymers. Sunshine variations remain the key variable when correlating accelerated and outdoor exposure data.

Another question to consider is the UV performance over water relative to land. While definitive studies on PE are not

available to date, the cooler temperature and generally increased cloudiness near coastal areas tend to offset the increased

reflectance off the water surface.

Below is an example on how to use Figure 4 to relate years of exposure to weather-o-meter indoor exposure, knowing the

UV rating of the resin.

Years = (325/your location’s isoline) x UV rating of resin

Example: Resin with UV-8 rating (See definition in next section) in Southern Florida.

Southern Florida = 660 KJ/cm2 per year (from Figure 4)

Years of exposure = (325/660) x 8 = 3.9 years

10,000

8000

6000

4000

2000

200 400 600

Comparison Weather-O-Meter/Florida exposure200 µm LDPE blown films, started March 1979T50 (E50): Time (energy) to 50% retained elongation

E50 plexiglass backing. kly Florida

T50

(h)

Wea

ther

-O-M

eter

Figure 3: Correlation between Xenon Ci-65 exposure and Florida

3

Source: Dr. Gugumus, Ciba Specialty Chemicals, Polymer Stabilization and Degradation Symposium, Manchester, September 18-20, 1985

Fig

ure

4:

Iso

lines

of

UV

per

form

ance

Ann

ual G

loba

l Rad

iati

onun

its:

KJ/

cm2

per

year

(to

get

MJ/

m2 ,m

ulti

ply

by 1

0)

Ref

eren

ce:

B.

de J

ong,

Net

rad

iatio

n re

ceiv

ed b

y a

horiz

onta

l sur

face

at

the

eart

h, D

elft

Uni

vers

ity P

ress

, 19

73.

Bro

ad B

and

UV

Exp

osur

e (2

95-3

85 n

m)

4

How are our resins rated for UV?

An often-used industry standard is UV-X, where X is a multiple of 1000 hours of weather-o-meter exposure. For instance,

UV-8 means 8000 hours of weather-o-meter exposure. UV-2 is 2000 hours of exposure. At this point, 50% of the original

break elongation is left. The higher the number, the longer the UV exposure and the better the polymer UV protection.

Figure 5 is a representation of this data.

Some suppliers use qualitative ratings, such as long-term or short-term UV protection, but this tends to be vague. Generally,

most of the industry would currently think of short-term as UV-4 with UV-8 being long-term. Refer to our data sheets to

determine the UV performance of each of our grades.

*See text for further details. ASTM test procedures may be modified to accommodate operating conditions or facility limitations.

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000 15,000

100

90

80

70

60

50

40

Xenon Weather-O-Metertest, ASTM D-2565*

Failure Criteria

2000 hrs. is approximately 1 year Florida1250 hrs. is approximately 1 year Southern Canada

UV-4 UV-8

UV Stability

UV-15

Hours of Xenon Exposure

Ret

aine

d E

long

atio

n (%

)

Figure 5: What does UV-8* mean?

5

How is degradation determined?

UV attack on polyethylene results in loss of physical properties. The property of greatest concern is embrittlement. The trend

towards embrittlement may be tested in many ways. In one study, tensile break elongation decline was more severe than ten-

sile impact decline. It also was found that tensile break elongation decline was more sensitive than tensile strength decline or

the formation of oxidized species at the exposed surface of the part, which may provide evidence of polymer degradation.

Thus, our testing tends to be based on break elongation decline. By convention, we take a 50% decline from the original break

value to be a “failure point” to establish the end of test. It is important to determine your own end point, as this will vary

depending on the applications of your products. At the 50% point, surface crazing (a crosshatched pattern caused when the

sample is bent) is not often evident on our polymers and often field use goes well beyond 50%. An example of this surface

crazing is shown in Figure 6 below.

Due to the variability of the molding process, our samples are currently prepared by compression molding to minimize data

scatter. Some people feel that using rotomolded samples and allowing a higher level of break elongational decline is preferred.

Studies conducted at our labs have shown that overcuring of rotationally molded samples can reduce UV life from 25% to

70% of the rated performance. This introduces much variability into reporting UV performance based on rotational molded

samples. We will continue to apply best control capabilities and to try to improve repeatability in rotomolded samples to

some day have confidence in reporting data on this basis.

Figure 6: Surface crazing as aresult of UV damage onthe polyethylene surface

6

How do pigments affect UV performance?

Figure 7 shows the effect of color pigments on UV stability. As can be seen, the proper choice of pigment is important as it

can have a detrimental or beneficial effect depending on the choice of pigment. All pigments should be melt compounded to

obtain the proper dispersion necessary for UV protection. The amount, particle size and chemical type of pigment, such as its

organic or inorganic nature, affects the UV performance. Generally, carbon black tends to be the best UV performer due to its

high absorption of UV light. Dry blending pigment has minimal effect as the dispersion and thus, absorption characteristics

are not sufficient to protect the base polymer.

It is important to not confuse UV performance with color-fading problems when dealing with pigments. Sometimes, a pigment

may fade while the base polymer remains unaffected by true UV degradation. Thus, impact and tensile properties are unaffected

while the part appearance has changed. Ensure your pigment supplier understands that you want not only a pigment that will

enhance or not hinder UV performance, but also one that will be color-stable when exposed to weathering. These pigments

are generally referred to as UV grade pigments.

0 100

330

230

250

570

210

500

460

230

175

>700*0.25%

0.25%

5000

4000

3000

2000

1000

0

0.25%

0.25%

0.5%

0.5%

0.5%

0.5%

0.5%

200 300 400 500 600

0 6 12 18 24 30 36 40

700 800

1% Carbon Black

1% Iron Oxide

1% Phthalocyanine Green

1% Cadmium Red1% Cadmium Yellow

1% Phthalocyanine Blue1% TiO2 (Rutile)

1% TiO2 (Anatase)

Natural

*no sample leftkly Arizona

Months exposed in Arizona

Effect of pigments on UV stabilization of.96 density unstabilized polyethylene resins

Tens

ile S

tren

gth,

psi

Polymer – HDPE (Ziegler)Basic Stabilization: 0.03% IRGANOX 1076 + 0.05% Ca-stearateLight Stabilization: 0.15% TINUVIN 770

Failure Criteria: 50% retained tensile impact strength.

Arizona exposure of pigmented HDPE plaques (1mm)

Phthalocyanine Green

Phthalocyanine Blue

Azo-Red

Organic Yellow(tetrachloroisoindolinone)

TiO2 (stabilized, coated Rutile)

Ultramarine Blue

Iron Oxide

Cd-Red

Cd-Yellow

Unpigmented

Figure 7: Color pigment affect of UV performance

Source: Left figure:Ciba: Stabilization of Polyolefins – Part 2.

Polyolefin Thick Sections,Arizona 45° south (start November)

Source: Right figure:Marlex HDPE Product Brochure,Phillips Petroleum,R.J. Martinovich for Plastic TechnologyNovember 63.

7

What other factors affect UV performance?

Table 3 outlines qualitatively other factors which may affect UV performance.

Resin formulation is important. A resin must be properly stabilized to minimize degradation during the rotational molding

cycle. Sufficient UV stabilizers must be present throughout the product life cycle to protect the part. These stabilizers should

not migrate out of the part, which could reduce protection as well as potentially affect the contents of the material being

stored in the parts.

As discussed before, the most important parameters which the molder may control is part time and temperature history.

Overheating the part tends to shorten UV performance as it provides the same mechanisms that UV light promotes, and thus,

provides it with a faster starting point for degradation when the part is exposed. The extent of this effect is difficult to quantify

due to the variations of part thickness, and variations in heat history which different melt indexes of polyethylene may require

to produce initial acceptable part properties. A more comprehensive understanding of these effects is the subject of continuing

research.

Thicker parts generally would have better UV performance due to reduced UV penetration as it travels through the part.

Once the part is sufficiently thick, no further benefit would be expected.

Generally, higher density resins, which provide a larger crystal structure and less potential for entrapping oxygen, provide better

UV stability. Melt index (MI) does not directly affect UV stability, though the secondary effect of a lower MI providing a tougher

part and thus, longer life to obtain the same absolute break point, is a factor. Thus, all things being equal, higher density and lower

MI PE’s enhance UV performance. Again these factors are generally small.

Other variables include higher particulate levels in the atmosphere such as found in industrial areas. These may result in dark

material being deposited on the polymer surface, which may shorten UV performance by providing elevated temperatures.

Also, some exposure conditions may interfere with UV wavelengths, thus extending service life of the product.

Factor Directional affect on UV performance

Molding cycle Negative(higher temperature or longer time)

Thicker part Positive to neutral

Higher density Positive

Lower melt index • Negative due to need for higher time ortemperatures to properly process part

• Positive in the fact that the resin has inherentlyhigher toughness

Environmental conditions which may Negativepromote particulates depositing on parts

Higher mechanical demands Negativeon the molded part

Higher resin stabilization Positive

Table 3: Other factors which affect UV performance

8

Further references

Many excellent articles exist on this topic. Below are some listings that the reader may find helpful in further understanding

this topic.

• George Wypych, “Handbook of Material Weathering, 2nd Edition,” ChemTec Publishing, 1995.

• F. Gugumus, Ciba Specialty Chemicals, “Examples of use and misuse of accelerated testing of plastics,” Symposium; Polymer

Stabilization and Degradation: Problems, Techniques and Applications, Manchester, September 18-20, 1985.

• K.P. Scott, “Narrow Band vs. Wide Band Control in Accelerated Weathering Tests,” 1st International Symposium on

Weatherability, Tokyo, May 12-13, 1992.

• J.L. Martin, “Test Methods for Determining the Outdoor Durability of Coatings,” DSET Laboratories, Miami, Florida.

• L. Crewdson, “Correlation of Outdoor and Laboratory Accelerated Weathering Tests at Currently Used and Higher IrradianceLevels – Part 1,” J.B. Atlas Co. Ltd.

9

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