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Ultraviolet/Antioxidant Stabilizer System Advances—Serve & Protect!

Ultraviolet Light Stabilizer Update

Ultraviolet Light Degradation

Ultraviolet (UV) light is electromagnetic radiation with shorter wavelengths than visible light, but longer than x-rays, in the range of 10 to 400 nm (nanometers). This radiation has considerable damaging effect on plastics, particularly UVB radiation, the portion of the solar energy spectrum with wavelengths in the range between 315 and 280 nm. (UV radiation is subdivided into UVA or long 400-315 nm; UVB or medium 315-280 nm; and UVC or short 280-100 nm.) This UV radiation breaks down chemical bonds in a polymer through photodegradation, which ultimately causes changes in polymer appearance and deterioration in the material's physical properties. The most apparent result is surface cracking and discoloration, particularly yellowing or whitening ('chalking') which is usually accompanied by degradation of physical properties such as impact strength, tensile strength and elongation. UV radiation attacks all types of polymers, especially polyolefins, styrenics (PS), polyvinyl chloride (PVC), polycarbonate (PC) and polyurethane (PUR), with a few, such as acrylonitrile (AN) and methyl methacrylate (acrylic), having better UV resistance than most.

Figure 1: Electromagnetic Radiation and Ultraviolet Light (Source: Purdue University College of Science)

All plastics but especially those used in outdoors applications such as roofing, window frames and vehicles or applications where plastic parts will be exposed to fluorescent and filtered daylight are at risk of photodegradation from UV radiation. Various factors influence the rate of this degradation including part thickness, opacity and the use of stabilizers. Counter-measures which can be taken to prevent UV radiation from attacking a plastic product include application of a coating, introduction of pigments "UV screeners' to effectively screen out the UV rays, or the use of additives ('UV absorbers' and 'UV stabilizers') to neutralize the UV energy within the compound and dissipate it harmlessly.

UV Absorbers

These additives protect polymers by inhibiting initiation of the degradation process via preferentially absorbing incident UV radiation. While receptive to UV radiation UV absorbers do not themselves degrade rapidly, but convert UV energy into a nondestructive form of infrared energy which is dissipated throughout the polymer matrix as heat. Although effective, UV absorbers are restricted by the physical limitations of the absorption process, adequate additive concentration and polymer thickness is needed before sufficient absorption is possible to effectively retard photodegradation. UV absorbers are therefore more effective in thicker cross-sections than in thin ones such as film, sheet and fibers. Benzophenones are good general-purpose UV absorbers for clear polyolefin systems, and can also be used in pigmented compounds. Benzotriazole UV absorbers are used mainly in polystyrene and polyvinyl chloride (PVC), and are also highly effective in high temperature resins such as acrylics, and polycarbonates.

Chemtura's Innovative Stabilizer Solutions!

Chemtura is a leading global supplier of UV stabilizers, antioxidants, intermediates and inhibitors, and polymer modifiers solutions. As the only manufacturer to market all major families of stabilizers, they offer the widest range of performance-based polymer additives in the industry. Their goal is to be the most innovative supplier of applied stabilizer solutions worldwide. Their UV stabilizers play an important role in polymers by counteracting the deteriorating effects of the sun and its UV radiation on many organic materials and plastics. Chemtura is committed to maintaining the highest standards of excellence in product quality, productivity, and customer satisfaction.

Chemtura HALS (Hindered Amine Light Stabilizer) solutions allow fine tuning of UV stabilizer properties with synergistic blend effects. LOWILITE® 7794 light stabilizer for example, guarantees optimum protection for the polymer against UV degradation and long term heat exposure. LOWILITE® 7794 light stabilizer is a blend of LOWILITE® 77 and LOWILITE® 94 light stabilizers offering excellent performance, particularly in the surface protection of thick PP (polypropylene) sections and in PP tapes. It is also suitable for PP/EPDM (ethylene propylene diamine), PP/PE (polyethylene)/EPDM blends, TPO (thermoplastic olefin), and PA (polyamide, nylon) material systems.

Chemtura's LOWILITE® 6294 light stabilizer is a blend of LOWILITE® 62 and LOWILITE® 94 high molecular weight light stabilizers. It is a very effective UV stabilizer for LDPE (low density polyethylene), LLDPE (linear low density polyethylene), HDPE (high density polyethylene) films, tapes, and thick sections, and for PP in fibers and films. It is also recommended for use in thick sections where indirect food contact approval is required; in fact, where it is unique in that it incorporates two food contact approved HALS solutions yielding a higher value added concentration limit than competitive individual UV formulated components. LOWILITE® 6294 light stabilizer is also suitable for polyolefins (PP, PE), olefin copolymers such as EVA (ethylene vinyl acetate) important in high growth solar energy encapsulant applications, blends of polypropylene with TPO elastomers, and PA resins.

Figure 2: UV Light Stabilizer Mechanism (Source: Chemtura)

Chemtura's LOWILITE® UV absorbers function by preferentially absorbing harmful ultraviolet radiation and dissipating it as thermal energy. Such stabilizers function according to the Beer Lambert law, which specifies that the amount of UV radiation absorbed is a function of both sample thickness and stabilizer concentration. In practice, high concentrations of UV absorbers and sufficient thickness of the polymer are required before enough absorption takes place to effectively retard photodegradation. Chemtura's benzophenone and benzotriazole LOWILITE® UV absorbers are two classes of stabilizer in broad industrial use.

Chemtura's new, liquid LOWILITE® UV B1260 light stabilizer confers outstanding stability to flexible foams for polyurethane applications that have been exposed to light and heat, while also outperforming conventional light stabilizer packages in fogging tests. At the recent April 2012 UTECH Europe conference Dr Andrea Landuzzi presented a particularly appropriate technical paper entitled "Latest Innovations in Post-Treatment Foam Stabilizers with Improved Environmental Characteristics."

Solasorb Inorganic UV Light Absorber

Solasorb, a novel type of inorganic UV light absorber from Croda Polymer Additives is based on ultra-fine metal oxides (TiO₂ and ZnO) that provide stable dispersion, low migration, and long-term protection. In comparison to traditional nanoparticle additives, this UV protection material is said to deliver:

Careful particle size control yields good UV absorbance coupled with significantly improved transparency compared to other metal oxide powders. Its use in plastics for packaging applications reduces color change in cosmetics and personal care products and prevents vitamin loss, development of off-tastes and odors in beverages.

Figure 3: Comparison of UV Absorbers (Source: Croda Polymer Additives)

Chromophore Based UV Absorber for Durable Goods

CGX UVA 006 developed by BASF is based on highly stable chromophore of the triazine family and has outstanding absorption capacity. Having a high degree of absorption and a broad absorption curve of 290 to 350 nanometers, its absorption performance exceeds all other UV absorbers currently in use. CGX UVA 006 also has excellent light stability. In a 20 micron thick cast film of PMMA, CGX UVA 006 performance is more than 80% of its original absorption value after 20,000 hours exposure to xenon light versus less than 50% of the original absorption performance after 5,000 hours light exposure for benzotriazole. This tremendously high UV absorption performance and light-stability makes CGX UVA 006 well suited for long-term outdoor use of up to 20 years under very strong UV light exposure.

Figure 4: UV Absorber Absorption Capacity (Source: BASF AG)

With its very broad absorption curve, CGX UVA 006 can provide good UV protection for the various transparent polymers, such as polycarbonate (PC), polyethylene terephthalate (PET) and polymethyl methacrylate (PMMA, acrylic), which vary greatly in terms of wavelength ranges at which they are most sensitive to UV radiation.

Figure 5: Transparent Greenhouse, Winter Garden, and Roofing UV Absorber Applications (Source: BASF AG)

What's more, it operates better than other UV absorber additives in preserving optical properties of transparency, and color. CGX UVA 006 also demonstrates very little volatility relative to other UV absorbers.

At 320°C the relative weight loss in TGA (thermogravimetric analysis) is less than 1%, an important factor given that PC sheets are typically extruded at between 250 and 290°C. The high thermal stability as well as low outgassing and low plate-out during processing are important to manufacturing productivity and working condition considerations.

The outstanding absorption capabilities of CGX UVA 006 together with its good compatibility with PC, PET, and PMMA make it an ideal UV absorber additive for transparent durable outdoor applications.

NanoArc ATO Infrared Protection

NanoArc Antimony Tin Oxide (ATO) nanoparticle additives by Nanophase Technologies Corporation can be incorporated into clear film and sheet to absorb IR but not visible light. It provides this benefit by absorbing energy in the near-IR region and reflecting energy at longer IR wavelengths, while maintaining excellent transparency in the visible region. In products like skylights it prevents heating of interior space. The incorporation of NanoArc ATO into surface coatings is an effective means of managing radiant heat without adversely impacting the optical clarity or other desirable physical properties of the article on which it is applied.

Figure 6: Nanophase ATO Absorption Properties (Source: Nanophase Technologies Corporation)

UV Stabilizers in Fiber Applications

Chemtura's LOWILITE® 6294 HALS light stabilizer is a synergistic blend of oligomeric LOWILITE® 62and LOWILITE® 94 HALS, both of which show good compatibility with polyolefins and excellent extraction resistance. LOWILITE® 6294 HALS is an excellent light stabilizer, particularly effective for polypropylene (PP) fibers and tapes containing organic pigments.

Figure 7: Polypropylene Geotextiles (Source: Fibertex)

PP fiber has excellent chemical resistance to acids and alkalis, high abrasion resistance, and resistance to insects and pests. PP fiber is easy to process and very inexpensive compared to other fibers. However its Achilles heel is poor UV resistance. A major PP fiber enduse is geotextiles where they are used in building and construction applications such as construction, ground systems, drainage/filtration, hydraulic works and waste disposal. Geotextiles are made of virgin polypropylene fibers with the key added Chemtura HALS UV stabilizer according to the EN12224 regulation. The basic strength of geotextiles is obtained by needle-punching the PP-fibers which gives strong elastic bonding between the fibers.

Elsewhere, Americhem has commercialized the nDuramax family of advanced high performance UV stabilizers that includes a stabilizer system for PET fiber. PET fibers are used in automotive interiors such as seat belts and outdoor applications like awnings and boat covers where it is important for the fibers to remain strong and light fast while under constant UV exposure. Two years of work in development of nDuramax accomplished significant performance advances in UV protection for PET fibers with color, physical/mechanical properties retention and service life all impressively improved. Americhem nDuramax line of stabilizers are also provided as customizable high-performance UV stabilizer concentrates for use in flexible and rigid PET packaging. These stabilizer concentrates are designed to lengthen shelf life of product contents and improve package color consistency.

Figure 8: Polyester Fiber UV Stabilization with nDuramax (Source: Americhem Inc.)

UV Pigment Screeners

Figure 9: Black Pearls Carbon Black (Source: Cabot Corporation)

UV screeners, pigments that render the polymer translucent or opaque and absorb or reflect UV light, can be used to help photostabilize polymers. Although its application is somewhat limited by its color, carbon black is a very effective UV screener promoting UV energy absorption to protect polymers from harmful UV radiation. It absorbs over the entire UV and visible light range, can be used at low loadings of 1-2 percent and may also serve as a free radical scavenger. Cabot Corporation's Black Pearls 800 is an excellent example of a carbon black having a particle size and shape that makes it particularly efficient for use as a UV screener to help photostabilize polymers from the harm of UV radiation.

White pigments are also used as UV screeners. Calcium carbonate can have a screening effect but usually at high loadings which can impair mechanical properties. Zinc oxide in particular has been used extensively to photo-protect polypropylene (PP). Titanium dioxide (TiO₂) is also used but can be expensive.

DuPont's Light Stabilizer (LS) 210 is an ultrafine/nano TiO₂ pigment that provides a low cost, high performance UV screening alternative. The product uses extremely small particles of TiO₂ to deliver superior broad spectrum UV blocking performance against UVA and UVB rays. Additionally the small size TiO₂ does not significantly pigment the plastic allowing UV stabilized parts to be produced in a wide variety of colors.

Unlike many organic light stabilizers that migrate from the plastics over time, reducing their effectiveness, LS 210 does not migrate out of the plastic under normal conditions thereby providing products with a much longer useful life. It features low chemical interaction with other compound ingredients both initially and upon UV exposure making it suitable for a range of plastics applications such as films, packaging, outdoor furniture or virtually any other product exposed to sunlight.

Figure 10: Comparison of D (DuPont) LS 210 with Other Light Stabilizers (Source: DuPont)

Antioxidant Stabilizer Update

Polymer Oxidation Cycle

Once oxidation starts, as it invariably will, it sets off a chain reaction which accelerates degradation unless antioxidants/stabilizers are used to interrupt the oxidation cycle. Polymer oxidation sometimes termed 'autoxidation' proceeds starting from generation of a free radical. These react rapidly with oxygen to form peroxy radicals which in turn further react with the polymer chains to form hydroperoxides (ROOH). Cleavage of these hydroperoxides, for example on exposure to heat, produces additional free radicals that reinitiate the process to continuously fuel the degradation cycle. The key to effective protection against oxidation is the use of different types of antioxidants/stabilizers that can intercept radicals and degradation products at different stages of the chain reaction in order to bring the degradation cycle to a halt.

Figure 11: The Autoxidation Cycle (Source: BASF AG)

Antioxidants are used to provide polymer protection both against oxidation during melt processing and through the product's life cycle as a 'long term thermal stabilizer.' Such long term thermal stabilizers differ from melt processing stabilizers in that they must function at temperatures considerably below the polymer melting point.

Figure 12: Effective Temperatures for Stabilizers (Source: BASF AG)

Primary Antioxidant Type Recommendations

'Primary antioxidants,' sometimes referred to as free radical scavenging antioxidants, retard oxidation through chain terminating reactions. They are predominantly hindered phenols, and, in certain applications (elastomers, polyols) secondary aromatic amines. These antioxidants with reactive OH or NH groups are hydrogen donors that provide antioxidant protection by transferring protons to the oxygen based free radical species to transform them into hydroperoxides. Without their use, the oxygen-based radicals would attack the polymer, resulting in further degradation. Phenolic stabilizers are offered in a wide range of molecular weights, product forms, and functionalities.

Some of the most frequently used stabilizers in this family are sterically hindered phenols which are effective during both melt processing and long term thermal aging. Secondary aromatic amines are also excellent hydrogen donors, often more active than hindered phenols however they tend to be more discoloring particularly on exposure to light or combustion gases.

Hindered phenols are active over a broad temperature range from ambient up to approximately 300°C. As a result they can be used to provide antioxidant protection during polymer melt processing (high temperatures for short duration), while also conferring long-term thermal stability (moderate temperatures over extended periods), as well as aging resistance over the lifetime of the plastic material. These antioxidants have proved to be of great importance in making polypropylene (PP) a significant commercial success in such higher heat applications as automotive components. Hindered phenolics are also widely used to protect engineering plastics that have high processing temperatures.

Secondary Antioxidant Type Recommendations

'Secondary antioxidants' are typically used in combination with primary antioxidants to realize synergistic stabilization effects. Hydroperoxides, the by-products formed by the reaction of hydrogen donors of primary antioxidants with oxygen-based radicals are inherently unstable and can also decay into free radicals. Secondary antioxidants, commonly referred to as hydroperoxide decomposers, are needed to transform these materials into thermally stable non-radical products. These antioxidants, typically organophosphorous compounds (phosphites/phosphonites), and thiosynergists, prevent the split of hydroperoxides into extremely reactive alkoxy and hydroxy radicals.

Phosphites/phosphonites are extremely effective at elevated temperatures (150-300°C), and serve as processing stabilizers during compounding, and melt processing (i.e., injection molding, extrusion). Some organophosphorous compounds are sensitive to water and can hydrolyze to form acidic species so hydrolysis resistant compounds are primarily used. Esters of 3,3-thiodipropionic acid are the most frequently used antioxidants of the sulfur-derived hydroperoxide decomposers. The most common of these are based on lauric or stearic acid. As thiosynergists are only effective below 150°C they can only provide long-term thermal and aging stability.

Antioxidant Selection Characteristics

Intrinsic sensitivity to oxidation varies greatly from one thermoplastic to another. While polystyrene and polymethyl methacrylate are quite stable, even at processing temperatures, polypropylene (PP) exhibits oxidative sensitivity at room temperature. Furthermore highly unsaturated polymers such as copolymers derived from butadiene or isoprene are particularly sensitive to oxidation. At the same time, antioxidants differ widely from each other in terms of reactivity/effectiveness and are selected based on the antioxidant needs of specific plastics in a given application. However, other factors are equally important in selecting the most suitable antioxidant.

These include antioxidant volatility, compatibility with the matrix polymer (impacting migration/blooming behavior) color stability, physical format (e.g. liquid, powder), transformation products (taste/odor considerations), food contact application regulations, and performance versus cost considerations. Synergistic or additive combinations of two or more antioxidants are increasingly used particularly in high-performance applications where a single antioxidant can only rarely provide complete polymer stability. However care must be taken when combining mixtures of antioxidants and other stabilizers as antagonistic effects can also result such as that which may be encountered in combining hindered amines with thiosynergists.

Commercial Multifunctional Antioxidant Advances

Irgatec NC 66 is proprietary antioxidant system developed by BASF to address the unique stabilizing requirements of nanoclay-based nanocomposites. The material is specifically developed to enhance the heat stability of nanoclay composites while processing, to permit faster, hotter processing conditions. The additive package also reduces odor upon processing, retards yellowing during aging, and improves long-term heat and light stability.

Figure 13: Multifunctional Antioxidant (Source: BASF AG)

This patented blend of phenolic and other antioxidant functionalities, plus calcium organic salts, and metal oxides works on the different elements of a nanocomposite that are known to increase risk of degradation, including the nanoclay, compatibilizer, and impact modifier.

Multifunctional antioxidants are a relatively recent advance that beneficially combines both primary and secondary antioxidant functions in one compound. Combining multiple stabilizing functions in one molecule eliminates the requirement for co-stabilizers to be used (e.g. phosphites, thioesters) greatly simplifying antioxidant storage, handling, and formulation. Irganox 1520 and Irganox 1726 from BASF are two principal antioxidant products in this new class of stabilizers.

The broad range of Chemtura's antioxidants for the plastics and elastomers industry plays an important role in counteracting the effects of degradation and oxidation of material when exposed to ambient air during processing and in the end product. For example, Chemtura is a leader in delivering its materials in non-dust forms, including its proprietary ANOX® NDB® stabilizer blend technology that enables up to eight components to be delivered in a single granular blend and a variety of liquid antioxidants.

Chemtura is showcasing plastic additives that offer a greener profile such as its Weston NPF 705, a nonyl phenol free phosphate stabilizer. Weston NPF 705 is a drop-in, liquid replacement for TNPP (tris [nonylphenyl] phosphate), an industry-standard secondary antioxidant which is used in linear-low density polyethylene (LLDPE), high-density polyethylene (HDPE), polyvinyl chloride (PVC), and styrene-butadiene rubber (SBR).

Ultraviolet and Antioxidant Stabilizer Technology Sources

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