NZ241753A - Flexible, gas-permeable polymer films incorporating an ethylene-reactive - Google Patents

Description

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NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION FLEXIBLE FILMS FOR MODIFIED ATMOSPHERE PACKAGING N.Z. PATENT OFFICE 26 FEB 1992 RECEIVED We, AUSTRALIAN CHALLENGE (OPERATIONS) PTY LIMITED, a Company incorporated under the laws of the State of Victoria, Australia, of 103 Bernard Street, Cheltenham, Victoria 3192, Australia do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - FLEXIBLE FILMS FOR MODIFIED ATMOSPHERE PACKAGING This invention relates to flexible polymeric film useful in the modified atmosphere packaging of fresh produce such as fresh fruits, vegetables and flowers.

In a general aspect the invention provides polymeric packaging films Incorporating agents which modify the atmosphere surrounding the fresh produce by reacting with ethylene from that atmosphere. Suitable polymeric films include polyethylene, polypropylene, and polyvinyl chloride and films of other well-known polymers and copolymers. The preferred ethylene-reactive agents incorporated into the films include permanganate salts such as potassium permanganate or sodium, calcium, magnesium or barium permanganate, preferably in association with metal oxide carriers such as magnesium oxide, silica, alumina, titanium oxide, ferricoxide, calcium oxide or synthetic zeolite. The preferred carrier is silica or pumice, and the preferred film is a linear, low density polyethylene (LLDPE). Most preferably, the weight ratio of sodium permanganate to Si02 and/or pumice is from 0.01 to 4.0. The films according to the present invention can also have ethylene absorptive or adsorptive capacity as would be understood by the skilled addressee. 6 DEC 1993 Deterioration of produce after harvesting involves many complex factors, among which oxygen, carbon dioxide, water and ethylene have been identified as playing important and interrelated roles in the ripening and decomposition process. When produce is enclosed in plastic film, respiration continues, consuming oxygen and producing carbon dioxide.

In general, produce is adversely affected by low concentrations of oxygen. The relative concentration of oxygen and carbon dioxide is also important. The concentration of these gases may be passively controlled by selectively varying the permeability of the polymer film. Ethylene is also known to advance the onset of an irreversible rise in respiration and ripening.

The action of low levels of gaseous ethylene as maturation hormone is well established. In particular, addition of low levels (ca 1-10 ppm) of ethylene to produce, stored under controlled atmosphere conditions is an established process to induce the onset of ripening. The reverse procedure, that of removal of ethylene from stored produce, has also been commonly practiced over many years. Methods for removal of ethylene have generally centred around scrubbing processes, in which the gas was reactively scrubbed, out of a recirculating atmosphere, for example by bubbling through potassium permanganate solutions.

In developing the present invention we have taken into account the following theoretical considerations. 1. The chemical basis for the reaction of permanganate with ethylene to form acetaldehyde (CH^CHO) depends upon several parameters. Under acidic conditions, the overall oxidation of ethylene with permanganate may be represented by the following REDOX equations: Mn04 + 8 H+ + 5 e c2h4 + h2o Mn2+ + 4 H20 E° + = 1.51V ch3cho + 2 H+ + 2 e~ 2 Mn04~ + 6 H+ + 5 C2H4 - 2 Mn2+ + 5 ch3cho + 3 H20 The presence of water is essential for this reaction to occur; the "initial" organic product involves incorporation of water and oxygen into the ethylene to afford a putative ethene-1,2-diol, which in turn rearranges to afford the observed product, acetaldehyde.

The overall reaction sequence is. complicated by Mn04~ readily oxidising the Mn2+ produced, 2 Mn04~ + 3 Mn2+ + 2 H20 - 5 Mn02(S) + 4 H+ The observed product in the presence of an excess of permanganate is actually the brown, Mn02• . The latter situation would generally be found within an additive particle, where there is, at least initially, an excess of permanganate. The consequence of this subsequent reaction is to reduce the overall efficiency of the permanganate oxidation reaction of ethylene.- The overall reaction sequence is, 2 Mn04~ + 3 C2H4 + 2 H+ - 2 Mn02 + 3 ch3cho + 1 H20 Mn02 is not able to react with ethylene under the reaction conditions encountered.

If permanganate oxidations are carried out under neutral or slightly alkaline conditions, Mn02 is again the preferred product, Mn04~ + 2 H20 + 3 e~ - Mn02(S) + 4 0H~ E° = +1.23V and in very strong base, the green nvanganate ion is produced: Mn04 ~ + e_ - Mn042~ E° = 0.4 6V For each of these reactions of permanganate, the amount of ethylene which can be oxidised is different. If Mn2+ is formed from Mn04_, then the reaction ratio of Mn04_:C2H4 is 1:2.5. If Mn02 is formed, it is 1:1.5 and if Mn042- is formed as product, it is only 1:0.5. Moreover, the oxidising ability of permanganate, for each of these reactions is reflected in the EO value, which falls in the order Mn2+ (1.51) > Mn02 (1.23) > Mn042- (0.46).

In the presence of moisture the permanganate ion is inherently unstable and decomposes, liberating oxygen, according to the following equation: 4 Mn04_ + 4 H+ - . 3 02 <g) + 2 H20 + 4 Mn02(S) The above reaction is found to very slow in neutral or basic solutions. It is, however, both autocatalytic (catalysed by the Mn02 produced) and catalysed by light.

This decomposition reaction could be quite rapid under the temperature conditions encountered in polymer processing. In consequence, acid conditions are most effective for the oxidation of ethylene by permanganate. However, acid conditions also promote decomposition of permanganate. 2. The thermal stability of the permanganate in the solid state depends upon the counterion. Thus, Tdec f°r KMn04 is <240° C, whereas for NaMn04 it is only 170° C. Not only do the various permanganate salts differ in thermal stability, but also in the amount water of crystallisation; thus the K-salt is anhydrous, but the Na- and Ca- salts exist as hydrates when crystallised from aqueous solution. The presence of water in the crystalline lattice ought to facilitate the solid state reaction with ethylene, as water is required to enable the oxidation of ethylene to proceed, as was indicated above.

It is an object of this invention to incorporate an ethylene reactive agent into the packaging film. 3. If permanganate is to effectively react with ethylene within a polymer film, several chemical requirements need to be accommodated. These include, ,/^EN ^ r 'IN i i ^ \. - A ncri Ci i 6 - i) sufficiently mild processing conditions to avoid excessive thermal decomposition, ii) the presence of water in close association with the permanganate, iii) control of pH to ^ 7 within the particle, as under acid conditions the autocatalytic decomposition of permanganate is accelerated, iv) the finished film should be kept away from strong light, and reducing atmospheres. 4. In addition to the above chemical requirements, certain physical conditions need to be satisfied, for the production of an effective polymer additive. These involve problems associated with mass transfer of reactants and include: i) the ability of ethylene to reach the reactive sites; this will be a function of the film thickness, its inherent permeability towards ethylene and the distribution of additive particles within the film. ii) the ability of water vapour to reach the reactive sites; iii) the dispersion of the additive; mass transfer limitations will be minimised by having an additive particle surface-to-volume ratio as high as possible. High ratios will be achieved by employing small and porous particles. iv) good structural integrity of the particles and their respective sites. In particular, good resistance to leaching of the additives by liquid water.

. The following criteria may be considered in selection of suitable polymer films. i) as low a processing temperature as possible, to minimise permanganate decomposition, 9 < i ii) chemical resistance to the permanganate oxidant and any other additives, iii) good mechanical properties, compatible with end use. This sets a minimum film thickness, iv) high permeability to ethylene. This sets a maximum film thickness. v) ease of processing, costs etc.

Examples Experiments were carried out using different films and additives. The results are summarised in Tables 1 and 2 below.

TABLE 1 Ethylene Uptake of Single Layer Polymeric Films* Column 1 gives the additive.

Column 2 gives the wt% ethylene removed while the gas consisting of 1-5 ppm C2H4 in air, is allowed to permeate through a sealed bag equilibrated with 100 % R.H. for 3 days (the sealed bags have an area of 25 cm and weigh 0.1 to 0.05g).

Column 3 describes the observed leaching that occurred after 3 days of equilibration with 100% R.ft. 1 . 2 3 a) 1% (KMnC>4 75%, silica 25%) 0. 5g 19 slight b) 1% (KMn04 73%, NaOH 2%, silica 25%) 0.5g 62 unacceptable c) 1% <KMn04 95%, NaOH 5%) 0.5g unacceptable d) 1% (NaMnC>4 25%, silica 75%) 0. 5g 12 nil e) 1% (NaMn04 75%, silica 25%) 0.5g 44 nil f ) 1% (NaMn04 71%, NaOH 4%, silica 25%) 0.5g 49 nil g> 0.

% (NaHn04 75%, silica 25%) 0.5% (pumice) nil 0. 5g * Single layer films consisting of linear low density polyethylene were processed at temperatures between 210° C and 220° C.

Table 1 shows three effects: i) the importance of using NaMn04 rather than KMn04 - leaching by water is minimised in (e) vs (a), ii) using the correct permanganate/silica ratios - activity is improved in (e) vs (d) and iii) using a diluent improves the unit activity - although the permanganate loading in (g) is only half of that in (f), the total activity is decreased by only 29%.

Table 2 below shows that choice of polymer is important in design of the ethylene sorbing film. Both film thickness and the nature of the polymer affects removal of ethylene by permanganate.

TABLE 2 Permeability of Polymer Films to Ethylene * Scavenger Powder Gauge (micron) Ethylene removed P 4202 processed at 150°C 50 •'32% 1600 162 processed at 160°C 50 16% 800 4203 processed at 160°C 50 42% 2100 1641 processed at 160°C 50 39% 1900 LLDPE 65% 1850 * Several films were tested for ethylene permeability by heat sealing some active scavenger in a sachet of area 70 cm . The relative permeability, P is indicated by the percent ethylene removed by the standard test multiplied by the film gauge.

Produce trials were carried out as follows: An additive, consisting of NaMnO^SiC^ (75: 25) particles of 10-15^ average size, was prepared and masterbatched with LLDPE to give a 4wt% additive loading. A second masterbatch contained 20wt% modified pumice. A combination of these two batches afforded an active LLDPE film, blown at 220°C using a hexane co-polymer, containing the permanganate additive leaching loading of 0.5wt% and a pumice loading of 0.5wt%.

Films of between 15-50p, with the composition described above, were tested for effectiveness of controlling the ethylene atmosphere above broccoli. The films were also tested as sealed box liners for pre-cooled, correctly handled broccoli under typical storage conditions. They were found to be effective in bringing ethylene concentrations to between 10 and 120 ppb after 28 days cold storage.

In this example the additives were made to the following specifications. 1. Permanganate Additive.

Chemical Composition; NaMnO^iF^O - 75 parts by wt Fumed silica - 25 parts by wt Materials NaMnC^iI^O to be ^ 9 9% pure.

Silica - Aerosil 200 Moisture Level Less than 5% wt loss on heating sample in air from RT to 100°C for 24h.

Physical properties Particle size - 99% to pass 43^ sieve 50% < 15n Method of Preparation By dissolution of NaMn04 in water, addition of silica, evaporating water at temperature <100°C and sizing. 2. Pumice Additive.

Moisture Level Less than 5% wt loss on heating sample in air from RT to 100° C for 24h.

Physical properties Particle Size - 99% to pass 43^ sieve EXAMPLE 1 A 50 kg batch of additive is prepared as follows: Fumed silica (Aerosil 200, 12.5 Kg) is added in one portion to 150L of a rapidly stirred 1.76M solution containing 37.5 Kg of NaMn04 in deionised water contained in a 250-300L tank reactor at 20±5°C. After the slurry is thoroughly mixed it is transferred onto trays and dried in an air oven 7 at 85±5° until solid. The dried powder is then crushed and ground to <10^ particle size in a ball or rod mill or similar and finally sieved.

EXAMPLE 2 To fumed silica (Aerosil 200, 260g) in a suitable vessel (20 dm^) is added deionised water (6-8 dm^) in which NaOH(9g) was previously dissolved. After stirring the mixture a 40% sodium permanganate solution (1574g, 4.43 mole NaMnO^) is added and the stirring continued until the permanganate is thoroughly dispersed. The slurry is transferred to trays, dried at 70-80°C in an air oven until solid. The dried lumps are then crushed and milled to a particle size of between 10-60 microns.

The fine powder obtained after grinding is very hygroscopic and so must be sealed into air-tight containers immediately after sizing. This minimises secondary agglomeration of the particles and water adsorption, both of which are deleterious to the quality of the blown film. 2. Pumice The pumice is modified by boiling in n-butanol at 117°C for 3 hours in a closed flask fitted with a condenser. The powder is then dried at 80° C and stored in a sealed plastic bag.

A polymeric masterbatch is prepared as follows: Well dried permanganate (4 Kg) is mixed with 9 6 Kg of LLDPE in a twin screw extruder at 180°C to form pellets. The modified pumice (20 Kg) is mixed with 80 Kg of LLDPE in a twin screw extruder to form pellets. The two master batches are blended with LLDPE at let downs required for particular applications. ' <1 *7;) b;- 12 -;When sodium permanganate is used together with silica and/or pumice, the weight ratio of NaMn04.H20 to Si02 and/or pumice is not critical but may for example be from 0.01 to 4.0;It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinbefore.;tX.-TT* /

Claims (15)

WHAT WE CLAIM IS:

1. A flexible gas-permeable polymeric film useful in the modified atmosphere packaging of fresh produce, comprising a polymer and an ethylene reactive agent that is incorporated in the film in such a way that it remains capable of reacting with free ethylene.

2. A film according to claim 1 in which the ethylene reactive agent is a permanganate salt.

3. A film according to claim 2 in which the salt is at least one of the permanganate salts of lithium, sodium, potassium, calcium, magnesium and barium. '

4. A film according to claim 1 in which the ethylene reactive agent is sodium permanganate or potassium permanganate.

5. A film according to claim 1 comprising at least one polyolefin.

6. A film according to claim 1 or claim 2 comprising at least one of polyethylene, polypropylene and polyvinyl chloride.

7 . A film according to claim 1 or claim 2 also containing a metal oxide carrier.

8. A film according to claim 7 in which the carrier is at least one of Si02, Ti02, Al203, Fe203, CaO, MgO and mixtures thereof.

9- A film according to claim 7 in which the metal oxide carrier is at least one of magnesium oxide, silica, alumina and synthetic zeolite.

10. A film according to claim 7 in which the carrier is silica and/or pumice. N ?• A O DECS993 " - 14 -

11- A film according to claim 10 in which the ethylene reactive agent is sodium permanganate.

12. A film according to any one of the preceding claims which is a linear low density polyethylene film.

13. A flexible gas-permeable polymeric film useful in the modified atmosphere packaging of fresh produce, characterised in that the film is a linear low density polyethylene film incorporating sodium permanganate as an ethylene reactive agent, and a metal oxide carrier which is a silica and/or pumice.

14. A film according to claim 13 in which the weight ratio of NaM.nO4.H2O to Si02 and/or pumice is from 0.01 to 4.0.

15. A film according to claim 1 or 13 substantially as herein described or exmeplified. AUSTRALIAN CHALLENGE (OPERATIONS) PTY LIMITED By Their Attorneys HENRY HUGHES LTD