Process for protecting flora using stabilised polymer films

Abstract: A composition wnich comprises a polymer and a nickel dialkyldithiocarbamate of the formula (R¹R²NCSS)₂Ni, wherein R¹ and R² are the same or different branched-chain C₅₋₁₈ alkyl groups. Such a composition, which preferably additionally comprises a zinc dialkyldithiocarbamate, can be used, e.g. in film or sheet form, to protect flora at an agricultural or horticultural locus.

Description: This invention relates to a method for stabilising polymer film or sheet for outdoor use. More specifically, it relates to low density polyethylene (LDPE) film used in agriculture and horticulture for protecting crops or for maintaining artificial environments more suited, for example by consistently higher humidity and/or temperature, to crop growth than the natural environment would be. It is well known that LDPE and similar polymers, e.g. polyolefines, in such applications, are subject to degradation under the influence of energetic solar radiation, oxygen in the air, and thermal stresses. This degradation leads to loss of strength in the film, so that it readily tears or cracks and thus has a limited service life. Many systems have been proposed for stabilising polyolefines such as LDPE against weathering. These systems generally comprise combinations of one or more ultra-violet (UV) stabilisers with other technical adjuvants such as heat stabilisers. For example, GB-A-1163875 discloses combinations of nickel dialkyldithiocarbamates, in which the alkyl groups have 4 to 6 carbon atoms, with 2-hydroxy-4-alkoxybenzophenones. GB-A-684976 discloses the use of nickel dialkyldithiocarbamates of the formula (RR'NCSS)₂Ni, in which the total number of carbon atoms in the two alkyl groups R and R' is 8 or more. US-A-3843595 describes polymers stabilised with a nickel dialkyldithiocarbamate, a phenyl hydroxybenzoate and a zinc dialkyldithiocarbamate, wherein the alkyl groups of the dithiocarbamates have up to 20 carbon atoms. Such systems perform well in laboratory tests and in simple outdoor conditions but, surprisingly, they do not perform as well as expected when used under conditions prevailing in agriculture and horticulture. It has not hitherto been recognised that agricultural and horticultural film is additionally exposed to pesticidal chemicals in a manner that adversely affects its weathering resistance. Chakraborty et al , Polymer Degradation and Stability 8 (1984) 1-11, report that the higher molecular weight homologues (butyl, hexyl and octyl) in the zinc dialkylthiocarbamate series have been found to be much more effective UV stabilisers in polypropylene than the methyl and ethyl analogues, and show that this effect is due to the increased equilibrium solubilities of the former in the polymer. At a given molar concentration, the UV stabilities and UV screening and peroxidolytic activities of the various members of the series are very similar, but the relationship between their UV stabilising activities and concentration is non-linear. Polymer, e.g. of LDPE, compositions, containing stabilisers, have now been found which can be resistant to weathering, even in the presence of fumigant chemicals or when exposed accidentally to sprays of other pesticidal chemicals, and which can be innocuous per se , during processing and in the final film which may be produced. According to the present invention, there is provided a method for protecting flora at an agricultural locus, which comprises using a polymer film or sheet comprising a UV absorber which is a nickel dithiocarbamate of the formula (R¹R²NCSS)₂Ni, wherein R¹ and R² are the same or different branched-chain C₅₋₁₈ alkyl groups. The polymer is preferably of the thermoplastic (as opposed to rubbery) type, e.g. a polyolefine such as polyethylene or polypropylene, and most preferably LDPE. The composition is preferably in the form of a film or sheet. Such films or sheets can be used to protect flora at an agricultural or horticultural locus. Examples of R¹ and R² are isoamyl (3-methylbutyl), 2-ethylhexyl and the higher isoalkyl groups which are usually obtained as complex mixtures of alkyl groups branched at various points in the main chain, e.g. in which R¹ and R² each have at least 7 carbon atoms (and an aggregate carbon atom number of, for example, 14 to 30). R¹ and R² are often the same. The effectiveness of corresponding dithiocarbamates in which R¹ and R² are alkyl groups having less than 5 or more than 18 carbon atoms is apparently less than those used in the present invention. Particularly preferred nickel dithiocarbamates are those containing isononyl groups which have sufficient chain length and branching to give good solubility in polymers such as polyethylene while at the same time not diluting excessively the UV stabilising portion of the molecule, i.e. the (partial) structure N-CS-S-Ni-S-CS-N. A preferred nickel diisononyldithiocarbamate is one in which R¹=R²=C₉H₁₉ and the C₉H₁₉ groups contain a major amount of 3,5,5-trimethylhexyl isomer. This material has an acute oral LD₅₀ greater than 16 g/kg in the rat and caused no mortality to rainbow trout at a concentration in water of 10 mg/litre. The nickel dithiocarbamates of the invention can be used advantageously in combination with other stabilising additives, especially the synergic UV stabilisers such as the hydroxybenzophenones, particularly 2-hydroxy-4-alkoxybenzophenones, e.g. 2-hydroxy-4-octyloxybenzophenone (Cyasorb UV 531) and Hostavin ARO 8. Another preferred additive is a zinc dialkyldithiocarbamate of the formula (R³R⁴NCSS)₂Zn, wherein R³ and R⁴ are the same or different branched-chain C₅₋₁₈ alkyl groups (which may be the same as R¹ and R²). The zinc dithiocarbamates function as thermal anti-oxidants and have environmental safety and stability to pesticidal formulations of the same order as have the nickel dithiocarbamates. R¹, R², R³ and R⁴ are of course each branched-chain alkyl. Corresponding straight-chain alkyl dithiocarbamates are generally less soluble in the polymer. This can be observed by incorporating respective dithiocarbamates in, say, polypropylene at various concentrations, and measuring the UV absorbence immediately after compression-moulding into films of standard thickness. The UV absorbence of the additive in a polymer is directly proportional to the true solubility of the additive in the polymer. For example, the initial UV absorbences (at 285 nm) of zinc dihexyldithiocarbamate and zinc dioctylthiocarbamate (both straight-chain alkyl) in polypropylene, at a concentration of 10⁻³ mole/100 g, are about 0.7; the corresponding values for zinc di(2-ethylhexyl)dithiocarbamate and zinc diisononyldithiocarbamate are between 0.8 and 0.9. The observation can be confirmed by measuring the solubility of the respective dithiocarbamates in a hydrocarbon solvent. Zinc dibutyl-, dihexyl- and dioctyldithiocarbamates (straight-chain alkyl) have solubility values of 116, 246 and 245 mg/100 g cyclohexane at 30 C; the corresponding values for zinc di(2-ethylhexyl)dithiocarbamate and zinc diisononyldithiocarbamate are 302 and 311 mg/100 g. The amount of the nickel dialkyldithiocarbamate in a composition of the invention may be from 0.01 to 5, more preferably 0.1 to 1, and often no more than 0.4, part by weight per 100 parts by weight polymer. The composition may comprise an amount of zinc dialkyldithiocarbamate in the same range, if that is included. The invention is illustrated in more detail by the following Examples. All percentages are by weight, unless otherwise indicated. The following abbreviations are used: NiDNC nickel dialkyldithiocarbamate (predominantly diisononyldithiocarbamate) ZnDNC zinc dialkyldithiocarbamate (predominantly diisononyldithiocarbamate) NiBuD nickel dibutyldithiocarbamate ZnBuD zinc dibutyldithiocarbamate HOBP 2-hydroxy-4-octyloxybenzophenone DTPS di(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (a hindered amine) TBNi 2 , 2'-thiobis(4- tert -octylphenolato)-n-butylamine nickel (a nickel chelate) LDPE low density polyethylene Example 1 LDPE was mixed in a Brabender Plasticoder with 0.2% HOBP and 0.2% of a UV absorber, and extruded into film at various temperatures. The % loss of UV absorber was determined spectrophotometrically. The particular UV absorbers and extrusion temperatures, and the results, are shown in Table 1. Percentage loss of UV absorber during extrusion UV absorber Extrusion Temperature (C) 180 200 220 250 NiDNC 0 2.5 5.0 8.5 ZnDNC 0 0 0 0 DTPS 1.0 3.0 6.5 12.0 TBNi 0 3.5 9.5 19.0 It was observed that films containing nickel dithiocarbamate were clear and tinged with green, and this appearance was unaffected by extrusion temperature. The films containing zinc dithiocarbamate were clear, colourless and unaffected by extrusion temperature. Films containing hindered amine were clear and colourless but tended to develop surface bloom when extruded at the higher temperatures. Films containing nickel chelate were clear and faint green but gave off an odour of amine during extrusion and developed bloom and a deeper green at the higher extrusion temperatures. Thus, the nickel and zinc dithiocarbamates did not release decomposition products during processing and did not leave residues in the form of surface bloom. The compression-moulded films were exposed to 4% aqueous solutions of soap (Lux flakes) or methyl isothiocyanate (CH₃NCS) to simulate exposure to chemicals encountered in agriculture. Methyl isothiocyanate is the volatile decomposition product of the soil sterilising agent Metham-Sodium. Soaps and surfactants are commonly used in spray formulations. LDPE films without additives were used as controls. Films were immersed in soap solution for 3 hours at 70 C before irradiation in a UV cabinet. Films were immersed in methyl isothiocyanate solution for 24 hours at 20 C before each period of 175 hours irradiation in the UV cabinet. The extent of degradation of the films was monitored by measuring the carbonyl index. A carbonyl index value of 20 in LDPE corresponds approximately to a 50% reduction in tensile strength after which LDPE film would be unsuitable for further use. The results are shown in Table 2. Carbonyl index values after UV irradiation UV absorber Soap treated CH₃NCS treated 600 900 1200 600 900 1200 (hrs) (hrs) None (control) 5 10 16 5 10 16 NiDNC 1 1 1 0.5 0.5 0.5 DTPS 3 5 8 1 1 2 TBNi 1.5 2 2 1 4 8 It can be seen that the nickel dithiocarbamate was resistant to both treatments whereas the hindered amine was affected by soap extraction and the nickel chelate by reaction with methyl isothiocyanate. Example 2 Compression-moulded films were prepared from LDPE blended with equal weights of HOBP and a UV absorber. In Series A, the concentrations were 0.2% HOBP and 0.2% absorber; films containing 0.2% HOBP, 0.2% NiDNC and 0.1% ZnDNC were also tested. In Series B, the concentrations were 0.4% HOBP and 0.4% UV absorber, and also 0.1% ZnDNC in combination with 0.4% NiDNC and 0.4% HOBP. The films were immersed in a 4% aqueous suspension of cypermethrin formulation (Ambush C) for 24 hours at 20 C before a period of 175 hours irradiation in a UV cabinet. LDPE films containing no additive and either untreated or treated with cypermethrin were used as controls. The extent of degradation was monitored by measuring the carbonyl index. The UV absorbers and the results are shown in Table 3. Carbonyl index values after UV irradiation UV absorber 450 750 1050 450 750 1050 (hrs) (hrs) Untreated Treated with cypermethrin None (control) 3.5 7 13.5 7.5 13 20 Series A (treated) Series B (treated) NiDNC 2 5 8 1 1 2 DTPS 6 11 19 2 5 9 TBNi 3 6.5 12 1 2 4 NiDNC + 0.1% ZnDNC 1.8 2.6 4 0.7 0.7 0.7 It can be seen from the results on the controls that cypermethrin has a prodegradant effect. The films in Series A were all affected by the treatment but the films containing nickel dithiocarbamate, especially in combination with zinc dithiocarbamate, gave the best protection. At the higher stabiliser concentrations in Series B, the duration of protection is prolonged. This is confirmed by the irradiation times needed to reach a carbonyl index value of 20, which corresponds to a reduction of 50% in the tensile strength of LDPE (after which the LDPE is unsuitable for practical use). The irradiation times are given in Table 4. Time (hours) to reach carbonyl index 20 Series A Series B NiDNC 1500 2040 DTPS 1125 1450 TBNi 1380 1700 NiDNC + 0.1% ZnDNC 1750 2440 Example 3 Polypropylene was processed for 10 minutes at 180 C in a RAPRA Torque Rheometer in five series of experiments, three of which were controls using either no additive or a (C₄ alkyl)dithiocarbamate. The amount of any additive was 0.2%. In each case, samples were subjected to oven-ageing at 140 C, to determine thermal oxidative stability, and to irradiation in a UV cabinet to determine the UV stability. The results are given in Table 5. Additive Embrittlement Times (hours) Stability Thermal UV Stability Additive Alone Additive + 0.2% HOBP None 0.5 70 400 ZnBuD 32 170 945 ZnDNC 92 385 1410 NiBuD 47 796 1740 NiDNC 52 1160 2250

Claims: A method for protecting flora at an agricultural or horticultural locus, which comprises using a film or sheet of a composition comprising a polymer and a nickel dialkyldithiocarbamate of the formula (R¹R²NCSS)₂Ni, wherein R¹ and R² are the same or different branched-chain C₅₋₁₈ alkyl groups. A method according to claim 1, in which the composition comprises from 0.01 to 5 parts by weight nickel dialkyldithiocarbamate per 100 parts by weight polymer. A method according to claim 1, in which the composition comprises from 0.1 to 1 part by weight nickel dialkyldithiocarbamate per 100 parts by weight polymer. A method according to claim 2 or claim 3, in which the composition additionally comprises 0.01 to 5 parts by weight of a zinc dialkyldithiocarbamate of the formula (R³R⁴NCSS)₂Zn, wherein R³ and R⁴ are the same or different branched-chain C₅₋₁₈ alkyl groups. A method according to claim 4, in which the composition comprises from 0.1 to 1 part by weight zinc dialkyldithiocarbamate per 100 parts by weight polymer. A method according to any preceding claim, in which R¹ = R² = C₉H₁₉. A method according to any preceding claim, in which the major proportion of the alkyl groups is comprised by the 3,5,5-trimethylhexyl isomer. A method according to any preceding claim, in which the polymer is polyethylene or polypropylene. A method according to any preceding claim, in which the composition additionally comprises a benzophenone. A method according to claim 9, in which the benzophenone is a 2-hydroxy-4-alkoxybenzophenone. A method according to claim 9, in which the benzophenone is 2-hydroxy-4-octyloxybenzophenone.

Inventors: CHAKRABORTY KHIRUD BEHARI, POYNER WILLIAM RAYMOND, CHAKRABORTY, KHIRUD BEHARI, POYNER, WILLIAM RAYMOND
Applicants: ROBINSON BROS LTD, ROBINSON BROTHERS LIMITED
Application year: 1985
Main classification: C08K5
IPC codes: A01G13, C09K17, A01G9, C08K5, C08L101