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Stabilizing agricultural films: a question of balance
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Stabilizing agricultural films: a question of balance UV stabilizers are essential for protecting agricultural films from degradation, but balancing UV stabilization with other properties such as chemical resistance is a challenge. Katherine Simpson reviews some of the current products, together with their benefits and drawbacks. Polymer greenhouse films are widely used around the world for crop production and most of these are based on low density polyethylene (LDPE), linear low density polyethylene (LLDPE) or ethyl vinyl acetate (EVA) with a thickness of 80220 µm. Generally, films are required to last for periods of one to five years, during which they need to retain flexibility, toughness, and in some cases, translucency. But exposure to ultraviolet (UV) radiation degrades these properties and without protection, polymer films can be made useless within the space of a few months. Therefore UV stabilizing packages are vital to ensuring a polymer film will live up to these requirements. In
many cases films come into contact with agricultural chemicals such as pesticides, in which case they must be able to resist attack. Ideally, UV stabilizers used in agricultural film need to have a high inherent light stability, low volatility and should not interact with other additives such as antifogging agents or thermal insulation compounds, says UV stabilizer manufacturer Cytec Specialty Chemicals. The lifetime of a stabilization package can be shortened if the additives are too volatile, are insoluble or exhibit excessive migration in the polyolefin matrix, and to ensure success, the stabilization package must be resistant to pesticides.
UV stabilizers play an essential role in the cultivation of crops and flowers.
Plastics Additives & Compounding July/August 2003
20
According to Cytec, there are two main challenges to overcome when producing a greenhouse film for three or more seasons. First, is that some commonly used agricultural chemicals severely impair the performance of many of the most effective light stabilizers. Second, is that the greenhouse films' UV screening effect, which enhances the growth of many crops, often diminishes over time. This means crop yields may drop over time, cancelling out the labour and waste disposal savings made by using such a film.
Stabilizer types The main UV stabilizer packages currently used in agricultural films are UV absorbers (UVA), nickel quenchers (NiQ), hindered amine light stabilizers (HALS) and aminoxyamine hindered amine light stabilizers (NOR-HALS). All have advantages and disadvantages. UV absorbers work by absorbing UV radiation and dissipating the resulting energy in the form of heat. These include benzophenones (Ciba's Chimassorb™ 81, Cytec's Cyasorb® UV-531 and Clariant’s Hostavin® ARO 8) and benzotriazoles (Ciba's Tinuvin™ 326 and Ampacet 100977). To make them effective, UV absorbers require a greater film thickness compared with other stabilizers such as HALS, so they are generally used in combination with other products to achieve the desired balance of properties. Cytec says benzophenones and benzotriazoles are unsuitable for films required for two or more years of service because their ability to absorb UV radiation diminishes over time. It claims that triazine UV absorbers, such as its Cyasorb
ISSN1464-391X/03 © 2003 Elsevier Ltd.All rights reserved.
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UV Stabilizers UV-1164 last much longer with the rate of UV absorption independent of exposure to pesticides.
UVA/NiQ blends Before the relatively recent introduction of HALS packages, UVA/NiQ combinations were the main choice of stabilizer. Their main advantage is an extreme resistance to pesticides and they are still used in warmer regions such as southern Europe, the Middle East, Africa and Central and South America, where strong pesticides need to be used to protect crops from insects. The drawbacks with nickel quenchers are that they contain heavy metals, which are restricted by government regulations in some countries, and they are always green in colour. This reduces the light transmitted through the film and therefore limits the concentration that can be used. Their melting point, generally 285°C, also makes it difficult to achieve an even distribution as polymer films are usually processed at lower temperatures than this (LLPE, for example is processed at around 200°C). Nickel quenchers generally provide less thermal protection than
Nickel quencher pellets from Great Lakes.
hindered amines, and because of this, polymer degradation can occur at points where the film is attached to the greenhouse structure. One company developing NiQ products with the aim of overcoming some of these problems is Great Lakes Chemical Corporation. Its UVA/NiQ products are marketed under the Lowilite® brand and include Lowilite Q84 additive and
100 90
UVA-1164 (0.33%)
Transmittance %
80
UVA-531 (0.33%)
70 60 50 40 30 20 10 0 0
3
6
9
12
15
18
21
24
Relative loss of UVAs from exposed 150 µm LDPE film in the presence of pesticides. Source: Cytec Industries.
21
Lowilite 22. The products are designed for use in greenhouse and tunnel films from 150-200 µm thick, and mulch films from 10-80 µm, made of LDPE, LLDPE and EVA blends. Lowilite NDB Q21 incorporates the company's No Dust Blends (NDB)™ technology to eliminate the risk to exposure to nickel powder during processing. Nickel quencher additives are dusty by nature, but NDB Q21 is a non-dusting product form, containing a precise ratio of two parts nickel quencher to one part UV absorber. As an additional benefit to the user, Great Lakes has also succeeded in making nickel quenchers easier to process by drastically reducing the melting point. Rather than melting at 285°C, the typical melting point for nickel quenchers, NDB Q21 melts at a temperature of 55-60°C, which the company says makes it much easier to achieve an even distribution of stabilizer within the polymer film. Other additives can be combined with NDB Q21 to alter its properties and increase the lifetime of the film. For example, HALS can be added to increase the thermal stabilization. Yulia Meszaros, technical service manager for Europe, Middle East and Africa at Great Lakes told Plastics Additives & Compounding it is possible to produce films that last for five seasons (five winters, four summers) using NDB Q21.
Plastics Additives & Compounding July/August 2003
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UV Stabilizers
Applications where polymer films need protection against the weather include greenhouses and silage films.
Other companies producing nickel quenchers include Clariant, which offers masterbatches under the CESA name, Ampacet, and Cabot, which produces the Plasadd® PE9136 masterbatch. PE9136 is a UVA/NiQ designed for films with a thickness of more than 100 µm and contains 10% nickel quencher with 10% benzophenone in an LDPE carrier. Cabot says PE9136 dilutes easily for homogeneous mixing and is suitable for direct addition using automatic dosing units or by preblending. HALS HALS are extremely efficient at light and thermal stabilization and convey little or no colour to the film. The biggest downside to HALS is that they are basic in nature so acidic pesticides and by-products from decomposed agrochemicals destroy their stabilizing action. The mechanism by which HALS work makes them effective even in thin films.
Rather than absorbing UV radiation, HALS trap the free radicals produced during the oxidation process, which stops the polymer from degrading. It is possible to achieve a high level of stabilization with a relatively low concentration of additive because HALS work in a cyclic process where they are regenerated and reused during stabilization, rather than being consumed. Several different techniques have been used to improve the resistance of HALS to pesticides. One approach is to reduce the basicity of HALS through methylation, as it is thought the lower basicity will minimize any acid/base reactions occurring between the HALS and decomposed acricultural chemicals. An example of this is Cytec's Cyasorb UV-3529 light stabilizer, a methylated, high molecular weight HALS, which is approved by the US Food and Drug Administration (FDA). Clariant also produces a low-basicity product in its Hostavin® range – Hostavin N30. N30 is an oligomeric
Plastics Additives & Compounding July/August 2003
22
HALS and is said to combine excellent resistance to extraction with a very low volatility and resistance to agricultural chemicals and sulphur acids. Hostavin N30 is also approved for use in polyolefins that come into contact with food. Another approach is to use HALS along with co-additives that neutralize acids, commonly known as 'acid scavengers'. Ciba Specialty Chemicals' Tinuvin 494 and Clariant's Hostavin N391 are examples of these, with both containing zinc oxide and calcium stearate. NOR-HALS Most recently, the agricultural film market has seen the introduction of NOR-HALS. These have a very low basicity to limit their interaction with acid chemicals. According to Cytec, these are more effective than using HALS together with acid scavengers, but less effective than NiQ/UVA blends under rigorous chemical exposure conditions. It adds that NOR-HALS tend to be significantly more expensive than NiQ/UVA systems. Ciba's Tinuvin NOR 371 is based on NOR-HALS chemistry and is claimed to show excellent chemical resistance together with a high degree of light and thermal stability. It can be used in agricultural films made of LDPE, LLDPE and high density polyethylene (HDPE) as well as EVA and ethyl vinyl benzene (EVB) blends. Ciba says Tinuvin NOR 371 can be used in thin pigmented mulch films and greenhouse films exposed to elemental sulphur, such as flower greenhouses.
UV Stabilizers HALS and NOR-HALS systems are transparent in the UV range, so UV absorbers are often added to improve weatherability and filter out some light from the UV range to enhance crop yields and growth rates.
Masterbatches Several companies produce HALS and NOR-HALS masterbatches. Clariant offers the CESA range of HALS based masterbatches, and both Ampacet and Cabot produce HALS and NOR-HALS products. Cabot recently introduced four new products under the Plasadd brand name. All use a LDPE carrier and are compatible with LDPE, LLDPE, HDPE, EVA and other ethylene copolymers. Among the new additions is Plasadd PE8740 for environments requiring strong resistance to chemicals, and for critical mulch film applications. Based on 20% non-interacting HALS combined with a UV absorber, PE8740 is claimed to resist most agrochemicals including burnt sulphur. For polyolefin films that are used for a period of 1-2 years, Cabot launched the Plasadd PE8830 masterbatch. PE8830 is highly concentrated and uses a 20% HALS based stabilizer system to protect against pesticides, although unlike PE8740, it is not resistant to burnt sulphur or sulphur powder. Plasadd PE8860 has similar properties to PE8830 but provides protection for up to 3 years.
2000
0 (h)
4000
With HALS/UVA Stabilizer
6000 Without stabilizer
UV stability of LDPE film measured as time to 50% elongation at break.The treated LDPE contains 0.25% Hostavin N30 and 0.25% Hostavin ARO 8. Source: Clariant.
To conclude Where pesticides are not used, HALS based systems offer the most effective UV stabilization with the added benefits of thermal stability and lack of colour. However, this performance can be greatly diminished once acidic chemicals are introduced. Modified HALS can go some way to protecting against acid attack, and NOR-HALS further still, though these are expensive. Nickel quencher/UV absorbers, the oldest of these technologies, may have the downside of being less a less efficient UV stabilizer than HALS, but they still provide
23
the most effective resistance to chemical attack in very warm countries where strong pesticides are in use. Contacts: Ampacet Website: www.ampacet.com Cabot Website: www.cabot-corp.com Ciba Specialty Products Website: www.cibasc.com Clariant Website: www.clariant.com Cytec Industries Website: www.cytec.com Great Lakes Chemical Corporation Wesbsite: www.greatlakes.com.
Plastics Additives & Compounding July/August 2003
11/07/2003
14:40
Page 20
Stabilizing agricultural films: a question of balance UV stabilizers are essential for protecting agricultural films from degradation, but balancing UV stabilization with other properties such as chemical resistance is a challenge. Katherine Simpson reviews some of the current products, together with their benefits and drawbacks. Polymer greenhouse films are widely used around the world for crop production and most of these are based on low density polyethylene (LDPE), linear low density polyethylene (LLDPE) or ethyl vinyl acetate (EVA) with a thickness of 80220 µm. Generally, films are required to last for periods of one to five years, during which they need to retain flexibility, toughness, and in some cases, translucency. But exposure to ultraviolet (UV) radiation degrades these properties and without protection, polymer films can be made useless within the space of a few months. Therefore UV stabilizing packages are vital to ensuring a polymer film will live up to these requirements. In
many cases films come into contact with agricultural chemicals such as pesticides, in which case they must be able to resist attack. Ideally, UV stabilizers used in agricultural film need to have a high inherent light stability, low volatility and should not interact with other additives such as antifogging agents or thermal insulation compounds, says UV stabilizer manufacturer Cytec Specialty Chemicals. The lifetime of a stabilization package can be shortened if the additives are too volatile, are insoluble or exhibit excessive migration in the polyolefin matrix, and to ensure success, the stabilization package must be resistant to pesticides.
UV stabilizers play an essential role in the cultivation of crops and flowers.
Plastics Additives & Compounding July/August 2003
20
According to Cytec, there are two main challenges to overcome when producing a greenhouse film for three or more seasons. First, is that some commonly used agricultural chemicals severely impair the performance of many of the most effective light stabilizers. Second, is that the greenhouse films' UV screening effect, which enhances the growth of many crops, often diminishes over time. This means crop yields may drop over time, cancelling out the labour and waste disposal savings made by using such a film.
Stabilizer types The main UV stabilizer packages currently used in agricultural films are UV absorbers (UVA), nickel quenchers (NiQ), hindered amine light stabilizers (HALS) and aminoxyamine hindered amine light stabilizers (NOR-HALS). All have advantages and disadvantages. UV absorbers work by absorbing UV radiation and dissipating the resulting energy in the form of heat. These include benzophenones (Ciba's Chimassorb™ 81, Cytec's Cyasorb® UV-531 and Clariant’s Hostavin® ARO 8) and benzotriazoles (Ciba's Tinuvin™ 326 and Ampacet 100977). To make them effective, UV absorbers require a greater film thickness compared with other stabilizers such as HALS, so they are generally used in combination with other products to achieve the desired balance of properties. Cytec says benzophenones and benzotriazoles are unsuitable for films required for two or more years of service because their ability to absorb UV radiation diminishes over time. It claims that triazine UV absorbers, such as its Cyasorb
ISSN1464-391X/03 © 2003 Elsevier Ltd.All rights reserved.
p20-23.qxd
11/07/2003
14:40
Page 21
UV Stabilizers UV-1164 last much longer with the rate of UV absorption independent of exposure to pesticides.
UVA/NiQ blends Before the relatively recent introduction of HALS packages, UVA/NiQ combinations were the main choice of stabilizer. Their main advantage is an extreme resistance to pesticides and they are still used in warmer regions such as southern Europe, the Middle East, Africa and Central and South America, where strong pesticides need to be used to protect crops from insects. The drawbacks with nickel quenchers are that they contain heavy metals, which are restricted by government regulations in some countries, and they are always green in colour. This reduces the light transmitted through the film and therefore limits the concentration that can be used. Their melting point, generally 285°C, also makes it difficult to achieve an even distribution as polymer films are usually processed at lower temperatures than this (LLPE, for example is processed at around 200°C). Nickel quenchers generally provide less thermal protection than
Nickel quencher pellets from Great Lakes.
hindered amines, and because of this, polymer degradation can occur at points where the film is attached to the greenhouse structure. One company developing NiQ products with the aim of overcoming some of these problems is Great Lakes Chemical Corporation. Its UVA/NiQ products are marketed under the Lowilite® brand and include Lowilite Q84 additive and
100 90
UVA-1164 (0.33%)
Transmittance %
80
UVA-531 (0.33%)
70 60 50 40 30 20 10 0 0
3
6
9
12
15
18
21
24
Relative loss of UVAs from exposed 150 µm LDPE film in the presence of pesticides. Source: Cytec Industries.
21
Lowilite 22. The products are designed for use in greenhouse and tunnel films from 150-200 µm thick, and mulch films from 10-80 µm, made of LDPE, LLDPE and EVA blends. Lowilite NDB Q21 incorporates the company's No Dust Blends (NDB)™ technology to eliminate the risk to exposure to nickel powder during processing. Nickel quencher additives are dusty by nature, but NDB Q21 is a non-dusting product form, containing a precise ratio of two parts nickel quencher to one part UV absorber. As an additional benefit to the user, Great Lakes has also succeeded in making nickel quenchers easier to process by drastically reducing the melting point. Rather than melting at 285°C, the typical melting point for nickel quenchers, NDB Q21 melts at a temperature of 55-60°C, which the company says makes it much easier to achieve an even distribution of stabilizer within the polymer film. Other additives can be combined with NDB Q21 to alter its properties and increase the lifetime of the film. For example, HALS can be added to increase the thermal stabilization. Yulia Meszaros, technical service manager for Europe, Middle East and Africa at Great Lakes told Plastics Additives & Compounding it is possible to produce films that last for five seasons (five winters, four summers) using NDB Q21.
Plastics Additives & Compounding July/August 2003
p20-23.qxd
11/07/2003
15:07
Page 22
UV Stabilizers
Applications where polymer films need protection against the weather include greenhouses and silage films.
Other companies producing nickel quenchers include Clariant, which offers masterbatches under the CESA name, Ampacet, and Cabot, which produces the Plasadd® PE9136 masterbatch. PE9136 is a UVA/NiQ designed for films with a thickness of more than 100 µm and contains 10% nickel quencher with 10% benzophenone in an LDPE carrier. Cabot says PE9136 dilutes easily for homogeneous mixing and is suitable for direct addition using automatic dosing units or by preblending. HALS HALS are extremely efficient at light and thermal stabilization and convey little or no colour to the film. The biggest downside to HALS is that they are basic in nature so acidic pesticides and by-products from decomposed agrochemicals destroy their stabilizing action. The mechanism by which HALS work makes them effective even in thin films.
Rather than absorbing UV radiation, HALS trap the free radicals produced during the oxidation process, which stops the polymer from degrading. It is possible to achieve a high level of stabilization with a relatively low concentration of additive because HALS work in a cyclic process where they are regenerated and reused during stabilization, rather than being consumed. Several different techniques have been used to improve the resistance of HALS to pesticides. One approach is to reduce the basicity of HALS through methylation, as it is thought the lower basicity will minimize any acid/base reactions occurring between the HALS and decomposed acricultural chemicals. An example of this is Cytec's Cyasorb UV-3529 light stabilizer, a methylated, high molecular weight HALS, which is approved by the US Food and Drug Administration (FDA). Clariant also produces a low-basicity product in its Hostavin® range – Hostavin N30. N30 is an oligomeric
Plastics Additives & Compounding July/August 2003
22
HALS and is said to combine excellent resistance to extraction with a very low volatility and resistance to agricultural chemicals and sulphur acids. Hostavin N30 is also approved for use in polyolefins that come into contact with food. Another approach is to use HALS along with co-additives that neutralize acids, commonly known as 'acid scavengers'. Ciba Specialty Chemicals' Tinuvin 494 and Clariant's Hostavin N391 are examples of these, with both containing zinc oxide and calcium stearate. NOR-HALS Most recently, the agricultural film market has seen the introduction of NOR-HALS. These have a very low basicity to limit their interaction with acid chemicals. According to Cytec, these are more effective than using HALS together with acid scavengers, but less effective than NiQ/UVA blends under rigorous chemical exposure conditions. It adds that NOR-HALS tend to be significantly more expensive than NiQ/UVA systems. Ciba's Tinuvin NOR 371 is based on NOR-HALS chemistry and is claimed to show excellent chemical resistance together with a high degree of light and thermal stability. It can be used in agricultural films made of LDPE, LLDPE and high density polyethylene (HDPE) as well as EVA and ethyl vinyl benzene (EVB) blends. Ciba says Tinuvin NOR 371 can be used in thin pigmented mulch films and greenhouse films exposed to elemental sulphur, such as flower greenhouses.
UV Stabilizers HALS and NOR-HALS systems are transparent in the UV range, so UV absorbers are often added to improve weatherability and filter out some light from the UV range to enhance crop yields and growth rates.
Masterbatches Several companies produce HALS and NOR-HALS masterbatches. Clariant offers the CESA range of HALS based masterbatches, and both Ampacet and Cabot produce HALS and NOR-HALS products. Cabot recently introduced four new products under the Plasadd brand name. All use a LDPE carrier and are compatible with LDPE, LLDPE, HDPE, EVA and other ethylene copolymers. Among the new additions is Plasadd PE8740 for environments requiring strong resistance to chemicals, and for critical mulch film applications. Based on 20% non-interacting HALS combined with a UV absorber, PE8740 is claimed to resist most agrochemicals including burnt sulphur. For polyolefin films that are used for a period of 1-2 years, Cabot launched the Plasadd PE8830 masterbatch. PE8830 is highly concentrated and uses a 20% HALS based stabilizer system to protect against pesticides, although unlike PE8740, it is not resistant to burnt sulphur or sulphur powder. Plasadd PE8860 has similar properties to PE8830 but provides protection for up to 3 years.
2000
0 (h)
4000
With HALS/UVA Stabilizer
6000 Without stabilizer
UV stability of LDPE film measured as time to 50% elongation at break.The treated LDPE contains 0.25% Hostavin N30 and 0.25% Hostavin ARO 8. Source: Clariant.
To conclude Where pesticides are not used, HALS based systems offer the most effective UV stabilization with the added benefits of thermal stability and lack of colour. However, this performance can be greatly diminished once acidic chemicals are introduced. Modified HALS can go some way to protecting against acid attack, and NOR-HALS further still, though these are expensive. Nickel quencher/UV absorbers, the oldest of these technologies, may have the downside of being less a less efficient UV stabilizer than HALS, but they still provide
23
the most effective resistance to chemical attack in very warm countries where strong pesticides are in use. Contacts: Ampacet Website: www.ampacet.com Cabot Website: www.cabot-corp.com Ciba Specialty Products Website: www.cibasc.com Clariant Website: www.clariant.com Cytec Industries Website: www.cytec.com Great Lakes Chemical Corporation Wesbsite: www.greatlakes.com.
Plastics Additives & Compounding July/August 2003