Uv Resistant Materials Coursework
The Effects of Ultraviolet Light on Polymeric Materials
UV light contributes to the development of skin cancer, the aging of plastics, and the discolouration of dyes and artwork. Typical property changes in a material include reduced ductility (increased brittleness), chalking, colour changes, a reduction in toughness, and cracking. Recently, polymers have been developed that are designed to degrade under light, and are used in products like Biodegradable? plastic bags.
The effect of UV light on a polymeric material depends on the flux (power) of the light, its wavelength, and the chemical structure of the material. Ultraviolet light interacts primarily with a structure’s pi electrons, meaning that double bonds and aromatic groups in a structure interact most strongly with it. For example, nylon 6 absorbs UV light in its amide bonds:
Structure of nylon 6
Polymers containing no pi electron clouds, e.g. polyethylene, are relatively unaffected by UV light.
The effect of UV light on a material can be determined using a number of analytical methods; typically:-
• Chemical structure analyses by employing UVand infrared spectroscopy and/or NMR (nuclear magnetic resonance);
• Surface analyses by scanning electron microscopy (SEM),
• Detection of free radicals by EPR (Electron Paramagnetic Resonance);
• measurement of molecular weight by viscosity measurements or end group analysis; or by
• changes in mechanical properties.
A major factor for UV resistance is the bulk or thickness of the material: the thicker the material, the more UV resistant it is because less UV will penetrate through to the centre of the material.
The UV resistance of materials may be determined by outdoor or laboratory testing methods. Outdoor testing is more expensive and time consuming than laboratory testing, but it remains the most appropriate by which other methods are compared. Laboratory test methods are not easily correlated with outdoor exposures because the wavelengths of the light sources do not always match those of sunlight.
Sunlight intensity varies considerably according to location, time of day, season, and atmospheric conditions. In the UK the light flux reaching sea level is greatest in July at noon. However, the overall flux is far less than that in, for example, a desert at the Equator, or elsewhere at high altitude. Thus, the testing of a material’s resistance to UV degradation must be carefully considered according to location of use. Common testing sites are Arizona, Florida and Japan. These areas have high ambient temperatures and levels of ultraviolet radiation.
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Ultraviolet Resistance of Engineering Plastics
There are some polymers that are susceptible to attack form sunlight, in particular, the ultraviolet segment of the spectrum. When ultraviolet attack occurs the material may have a color shift, become chalky on its surface, and/or crack. There are a number of methods to reduce this problem. The addition of carbon black to the polymer will usually absorb most UV radiation. Chemical inhibitors are available for certain plastics, which improve the UV resistance. Paint and silicone coatings can also be used to completely cover exposed surfaces to sunlight (UV radiation). Additionally, there are several polymers on the market that are inherently UV resistant as well.
Acetal Homopolymer, Copolymer (polyozymethylene, POM)
Unmodified acetal resin will degrade over time upon exposure to sunlight. The material can crack, embrittle, and develop a chalky appearance. Pigmented and chemically modified formulations are available including Delrin® 107 acetal and Delrin® 507. DuPont has developed 20 year exposure data on these materials.
Nylon (all types)
Unpigmented resins will degrade upon exposure to sunlight evidenced by discoloration and embrittlement. Formulations containing carbon black particles provide the best UV stability.
Thermoplastic polyurethane exhibits good weathering characteristics. Upon exposure to UV radiation it does experience a color shift, however there is a minimal change in mechanical properties.
Unmodified polycarbonate resins (Hydex® 4301, Lexan®, Makrolon®) will degrade upon exposure to sunlight. Polycarbonate will yellow and become hazy after 1 year of exposure. UV resistant grades are available.
Polybutylene terephthalate (PBT) (also known as Hydex® 4101, Valox®, Celanex®, Ultraform®) is “inherently” UV resistant. Supplier data indicates that there is little degradation in mechanical properties after several years of exposure. Black pigmented resins have better property retention.
Polyetherimide (Ultem®) is inherently resistant to ultraviolet radiation. After 1000 hrs. of exposure, no measurable change with tensile strength.
ABS is not suitable for outdoor applications because of its poor UV resistance. Current UV stabilized grades refer only to color fastness not mechanical property retention.
Polysufone (Udel®) will experience some degradation upon exposure to sunlight. Black pigmented formulations are recommended for improved performance.
UV RESISTANCE OF ENGINEERING PLASTICS
UV stable grades available offer excellent performance.
Black pigmented grades have better UV stability.
Hydex® 4101 (PBT)
Inherently UV stable, black grades provide additional protection.
Hydex® 4301 (PC)
UV stable grades available.
Ultem® 1000 (PEI)
Inherently UV resistant.
Hydex® 202 (RTPU)
Pigmented versions will mask color shift.
Not recommended for outdoor applications.
Black pigmented versions available for improved UV resistance.
E = Excellent UV resistance
F = Fair UV resistance
U = Unacceptable UV resistance
Delrin® is a registered trademark of E. I. DuPont de Nemours Co., Inc.
Celanex and Celcon® is a registered trademark of Ticona – Division of Hoechst Group.
Ultraform® is a registered trademark of BASF Corporation.
Lexan® and Ultem® are registered trademarks of General Electric Company.
Udel® is a registered trademark of the Amoco Chemicals.
Hydex® is a registered trademark of the A. L. Hyde Co.
We believe this information is the best currently available on the subject. It is subject to revision as additional knowledge and experience is gained. The A. L. Hyde Company makes no guarantee of results and assumes no obligation of liability whatsoever in connection with this information. Anyone intending to use recommendations contained in this publication should first satisfy himself that the recommendations are suitable for his use and meet all appropriate safety and health standards. This publication is not a license to operate under, or intended to suggest infringement of any existing patents. References to products not of A. L. Hyde manufacture do not indicate endorsement of named products or unsuitability of other similar products.
Copyright © 1999 A. L. HYDE Company
Last Modified: November 2, 1999