GB2218997A - Plastic film - Google Patents

Abstract

Plastic film, produced substantially from recycled polyethylene, contains at least 10 per cent by weight of linear low- density polyethylene having a density beneath 0.915, thereby improving the welding properties and mechanical properties of the film. The plastic film is particularly suited for the manufacture of sacks, primarily sacks for the collection of rubbish or garbage by local authorities.

Description

Plastic film The present invention relates to recycled polyethylene film of improved mechanical and welding properties.

More particularly, but not exclusively, the invention relates to such film for the manufacture of sacks or bags, primarily for the manufacture of waste-disposal sacks, and particularly for the manufacture of such sacks for use by municipal waste dispdsal authorities.

In recent years, endeavours made to recycle different waste products have increased markedly, in keeping with the increased contamination of our planet and the increased awareness that the natural sources of raw materials are not inexhaustible. Examples of such endeavours are found in the recycling of paper and glass materials.

Attempts have also been made to recycle domestic waste and to separate different items therefrom, such as plastic items. The attempts to recycle plastics have been relatively unsuccessful, however, owing to the fact that the plastic material concerned is often excessively soiled, and primarily because of the mutually different kinds of plastic material involved. Furthermore, such plastic material is often decomposed. The only products which have been produced successfully from recycled plastic material are thick, moulded or extruded products, such as distance blocks for loading pallets. One method employed in this respect involves the collection of plastic material of known composition and the production therefrom of new products, in this case primarily different plastic film and waste-disposal sacks or bags.

Various organizations have been employed in the collection of waste plastic material in this respect. The plastic material worked with is primarily polyethylene, particularly polyethylene film, bags, sacks and like products. This is because polyethylene products are to be found in large quantities and because such products can be reformed into new plastic film, subsequent to beating (grinding), washing and granulating the products. Waste-disposal sacks are particularly suited for manufacture from recycled plastic material, because the sacks are ultimately either burned or dumped on rubbish tips after use. It is difficult to recycle plastic material repeatedly.

Serious problems are encountered, however, when recycling polyethylene in the manufacture of plastic film and sacks. For example, the recycled polyethylene is often decomposed, which makes the new film and sacks extremely brittle and easily torn. Furthermore, the material is often so soiled and contaminated as to prevent it from being washed clean. In addition, small quantities of other plastic materials are often present.

This presence of other plastic materials makes it difficult to obtain a coherent film in the extrusion process, and film produced from such admixtures will often rupture when subjected to low loads in use. Such film is also difficult to weld-seam into sack form. When the sacks are intended as waste-disposal sacks, it is imperative that the weld seams will remain intact and not burst when the sack is used.

These problems are greatly alleviated by the present invention. For instance, the extrusion process is effected more smoothly, in respect of the number of stoppages per unit of time, and the important mechanical properties of the plastic material produced are greatly improved (film toughness is increased) as is also the weldability of said plastic material.

The present invention relates to the manufacture of plastic film having improved welding properties and mechanical properties from recycled polyethylene, which may also contain normal polyethylene additions as filler material, together with pigments and minor quantities of impurities in the form of other plastic types, the film being characterised in that it contains at least 10 per cent by weight of at least 1 linear low density polyethylene having a density beneath 0.915.

The recycled polyethylene may either be polyethylene collected from external sources or internal spillage from different polyethylene converters or manufacturers.

Despite its source of origin, the polyethylene is seldom pure, but will contain various additives such as filler, e.g. chalk or silica, pigment, stabilizers, antistatic agents, release agents, etc. The quantities in which these substances are normally present are well known to the person skilled in this art. The polyethylene itself will often comprise a mixture of different polyethylene grades or types. Most often, the major ingredient of the polyethylene will consist of low-density polyethylene (LDPE) with minor quantities of linear low-density polyethylene (LLDPE) and/or high-density polyethylene (HDPE). The polyethylene will also often include ethylene co-polymers, such as ethylene-vinyl acetate (EVA) and/or ethylene-butyl acrylate co-polymers (EBA). Normally, these materials are also considered to belong to the polyethylene group.Naturally, densities and melting index will vary, partly depending on different starting materials and partly on different stages of decomposition. In addition to polyethyene, the recovered material will often contain various types of contaminants. These contaminants may have the form of "dirt", sand, soil, adhesive tape, fertilizer, etc. The majority of these contaminants are washed away during the recycling process, although a minor quantity will remain. Similarly, the recycled polyethylene will contain a minor quantity of other plastic material. Part of the recycled polyethylene may also be replaced with newly manufactured starting material. For instance, if the material is felt to be too soft, part of the recycled material may be replaced with HDPE.This is not an essential feature of the invention, however, the important criterion being that at least 10 per cent by weight of the plastic film comprises at least one linear low-density polyethylene having a density beneath 0.915.

The linear low-density polyethylene (normally designated VLLDPE) belongs to the relatively new group of materials produced by co-polymerization of ethylene with co-monomers which, for instance, can be butene, hexene, octene, or methylpentene. At present, these materials have a density of between 0.880 and 0.915-0.919. (The classification limits are somewhat obscure .) The invention can also be practised with a material having a density beneath 0.915. It has namely been found that a density of, for instance, 0.918 will not at all have the same good effect as a density beneath 0.915.

The melting index of the linear low-density polyethylene can vary within wide limits, although it will normally lie between 0.1 and 20. In the case of film, there is normally employed a melting index which lies within the range 0.1-10. The linear low-density polyethylene will preferably be present in an amount of at least 10 per cent by weight. This is because smaller quantities will not provide a sufficiently good effect. It is assumed that this is because a coherent phase which binds the recycled material together will not be obtained when less than 10 per cent by weight of linear lop-density polyethylene is used. In this respect, quantities between 10 and 45 per cent by weight have been found particularly suitable. Excessively high quantities can result in slippery and sticky film, particularly when a low density material is used.Furthermore, the film becomes more expensive. It is possible to use larger quantities, however, depending on the other additives present.

For instance, a tack inhibitor can be added to prevent the film from sticking on to itself and to prevent the sack from being difficult to open. Such inhibitors are known to the person skilled in this art, and normally consist of chalk, silica, myanite, talcum or similar material. The inhibitors are normally added in quan tities from 0.5 per cent by weight, although it may at times be necessary to use larger quantities.

The film or sack may be given a coarse or rough finish, by admixing a "roughening" material. This is particular ly beneficial in the case of waste-disposal sacks, since otherwise the sacks may slip from the stands or like supports in which they are held. Such roughening agents - are known to the skilled person, and may comprise, for instance, fine-grain material which will not melt during the extrusion process. Such agents are normally added in quantities of 0-5 per cent by weight, although larger quantities can be used when necessary.

In those cases when the material being recycled is to be admixed with newly manufacturedos-tartlng material, this starting material may comprise low-density polyethylene or linear low-density polyethylene, both having a den sity of between 0.916 and 0.930, or a high density polyethylene having a density of between 0.930 and 0.965, and a melt index between 0.01 and 20. The admixed, newly manufactured starting material may comprise ethylene-vinyl acetate co-polymer (EVA) and/or ethylenebutyl acrylate co-polymer (EBA), both with a density of between 0.920 and 0.950 and a melt index between 0.1 and 20. The quantities in which such newly manufactured starting materials are admixed with the recycled material can be determined by the skilled person on the basis of the special properties required of the finished film.For instance, an addition of high density polyethylene may be used when a more rigid film is required.

The newly manufactured material can replace up to 50% of the recycled plastic material, although this limit is not critical. The inventive effect afforded by the addition of linear low-density polyethylene is less pronounced when smaller quantities of recycled plastic material are used.

The inventive film may also contain conventional additives, such as dyestuffs, stabilizer, antistatic agent, filler and the like, all of which are well known to the skilled person.

The inventive film is produced in a conventional manner, preferably by film blowing techniques although flat extrusion techniques are also possible. As will be understood from the aforegoing, film which is produced in accordance with the invention will have improved mechanical properties.

The thlckness of the film produced in accordance with the invention ill be the same as standard film thicknesses, i.e. thicknesses of between about 15 and about 150 microns. The thinner film is used for the manufacture of smaller bags or for packaging purposes, whereas the thicker film is used in the manufacture of rubbish or garbage disposal sacks.

As before mentioned, the admixture of said linear lowdensity polyethylene having a density beneath 0.915 is beneficial from several aspects. Firstly, the material produced has improved coherency, which results in fewer stoppages during the extrusion process. Blockaging of the filters is also reduced.

Secondly, a number of mechanical properties of particular importance with regard to waste-disposal sacks are also improved. It has often been found difficult to assess the value of these properties with the aid of standard test methods. Consequently, it has been elected to illustrate the results obtained when subjecting inventive material to more realistic tests. One of the most important tests to which such sacks can be subjected is the so-called "bursting-tendency test". The bursting tendency of a sack is measured by making a rupture impression in the rim of the sack and then pulling on the sack. The rupture tendency is said to be high when the sack material ruptures very easily, in straight-line tears, without any noticeable resistance.

It is particularly important to carry out such tests in the vicinity of lateral folds and infolds at which the sack is folded. These folds are often weak points in sacks manufactured from recycled material. It is also important to test such sacks for their puncturing strength. The puncturing strength is a measurement of the ease in which holes can be punched or pierced in the film or sack. The rubbish or garbage disposed of in such sacks will often contain sharp objects liable to make holes in the sack. The puncturing strength of the film is determined by stretching the fil-m over an open cylinder having a diameter of 76 mm, and then pressing a cylinder of diameter 7 mm against the film, in the centre of the open cylinder, until rupture takes place.

Corresponding determination of the puncturing strength can be effected with a conical test body of diameter 3 mm.

The combination of puncturing test and rupture tendency provides a good indication of how easily a hole can be formed in the sack and how readily this hole can be enlarged.

The third important advantage or benefit obtained by the aforesaid admixture of linear low-density polyethylene is that the weldability and welding strength of the recycled polyethylene is greatly enhanced thereby. The best method of assessing the strength of a weld or seam is to attempt to pull the welded surfaces apart manually. This test can be carried out by first splitting the weld or seam and then pulling transversely across said weld.

The invention is illustrated more- clearly in the following working examples, while the results of the various tests carried out are set forth in the following Table.

Example 1 Film according to the invention was produced by film blowing at 180-220"C at a rate of 20 m/min. The blowing ratio was 1:3.0 and the film thickness 55 Wm. The film consisted of 60 per cent by weight recycled polyethy lene, 20 per cent by weight newly-manufactured polyethylene having a density of 0.920 and a melting index of 0.3, and 20 per cent by weight linear low-density polyethylene having a density of 0.900 and a melting index of 1.0. The film was readily extruded and was subsequently used in the manufacture of waste-disposal sacks.

No difficulties were experienced in the manufacture of the sacks.

Example 2 This Example was effected in the same manner as Example 1, although with 50 per cent by weight recycled polyethylene, 20 per cent by weight newly-manufactured polyethylene, and 30 per cent by weight linear low-density polyethylene.

Example 3 For comparison purposes, film was produced under the same conditions as those recited in Example 1, but with 80 per cent by weight recycled polyethylene and 20 per cent by weight low density polyethylene having a density of 0.920 and a melting index of 0.3. It was very difficult to weld the film and the sacks produced therefrom where easily ruptured (see the following table).

Example 4 Inventive plastic film was produced under similar conditions to those recited in Example 1, but with a temperature of 220-240 C and a blowing ratio of 2.7:1. The film had a thickness of 75 microns. The film contained 80 per cent by weight recycled polyethylene and 20 per cent by weight linear low-density polyethylene having a density of 0.900 and a melting index of 1.0. Sacks could be produced readily from the film. It was found that these sacks were somewhat difficult to open.

Example 5 For comparison purposes, film was produced under the same conditions as those recited in Example 4, but with 100% recycled polyethylene. Sacks were manufactured from the film.

Example 6 Inventive plastic film was produced by film blowing at a rate of 25 mm/min. The film had a thickness of 75 microns and the extrusion temperature was 160-1900C. The blowing ratio was 2.6:1. The film contained 65.5 per cent by weight recycled polyethylene, 2.5 per cent by weight finely-divided chalk as tack inhibitor, 2 per cent by weight of a coarse-grain substance to provide a rough surface, and 30 per cent by weight of a linear low-density polyethylene having a density of 0.900 and a melting index of 1.0. The film was made into sacks without difficulty, and the sacks could be easily opened.

Example 7 Inventive plastic film was produced by film blowing at a rate of 18 m/min. The extrusion temperature was 180 215"C and the film had a thickness of 70 microns. The film contained 57 per cent by weight recycled polyethylene, 10 per cent by weight HDPE having a density of 0.944 and a melting index of 0.3, 3 per cent by weight finely-divided chalk as tack inhibitor, and 30 per cent by weight of a linear low-density polyethylene having a density of 0.900 and a melting index of 1.0. The film could be readily made into sacks, which were easily opened.

Example 8 For comparison purposes, film was produced under the same conditions as those recited in Example 7, but with 100% recycled polyethylene. The film was made up into sacks.

Example 9 Inventive film was produced by film blowing at an extrusion temperature of 175-210"C and a blowing ratio of 1:3.5. The film thickness was 65 microns; the rate of manufacture was 19 m/min. The film contained 52.5 per cent by weight recycled polyethylene, 15 per cent by weight LDPE having a melting index of 1.2 and a density of 0.922, 2.5 per cent by weight finely divided chalk, and 30 per cent by weight of a linear low-density polyethylene having a density of 0.900 and a melting index of 1.0. The film could be readily converted into sacks.

Example 10 For comparison purposes, film was produced under the same conditions as those recited in Example 9, with the exception that in this case the film contained 82.5 per cent by weight recycled polyethylene, 15 per cent by weight LDPE having a melting index of 1.2 and a density of 0.922, and 2.5 per cent by weight finely-divided chalk. Sacks could also be readily made from this film.

The following Table discloses the results of the tests carried out on the sacks produced. As will be seen from the Table, wide differences are to be found between the inventive sacks and the reference sacks. None of the inventive sacks exhibited rupture tendencies, whereas all of the reference sacks ruptured to a greater or lesser extent. The welds formed on the inventive sacks were also much more effective than the welds on the reference sacks. The greatest difference is found in the puncturing strengths of the sacks, the puncturing strength of the inventive sacks being twice that of the reference sacks. It is important to compare the correct reference sample with the correct inventive film. The quantity of recycled material used varies greatly and the absolute values will consequently differ to a relatively large extent.Thicknesses and operating conditions will also, of course, be influential.

Table: Film and sack properties Puncture strength in cm Rupture Weld strength Pulling 7mm 3mm straight Example test body test body Fold Material across Splitting 1 2-2.5 2.0 None None Weld holds Breaks, but According to the invention with good resistance 2 3.5 2.5 None None Weld holds Breaks, but According to the invention with good resistance 3 1.0 1.0 Ruptures Ruptures Breaks, but Breaks Comparison sample easily at with good easily all folds resistance 4 3.5 2.0 None None Weld holds Weld holds According to the invention 5 1.5 1.0 Ruptures None Breaks Breaks Comparison sample easily at easily easily all folds 6 4.0 2.5 None None Weld holds Weld holds According to the invention 7 4.0 3.0 None None Weld holds High According to the invention resistance 8 1.5 1.0 Ruptures Ruptures Breaks Breaks Comparison sample easily at easily easily all folds 9 5.0 3.0 None None Weld holds Weld holds According to the invention 10 2.5 1.5 Ruptures Ruptures Breaks, but Breaks Comparison sample at all at cer- with good inner tain resistance folds places

Claims (9)

1. Claims: 1. Plastic film which is weldable and substantially comprises recycled polyethylene, characterised in that the film contains at least 10 per cent by weight of at least one linear low density polyethylene having a density beneath 0.915.
2. 2. Plastic film according to claim 1, in which a part of the recycled polyethylene is replaced with newly-manufactured starting material comprising at least one of the materials low density polyethylene or linear low-density polyethylene having a density between 0.916 and 0.930, high density poly ethylene having a density between 0.930 and 0.965, ethylene vinyl acetate co-polymer, and ethylene butyl acrylate co-polymer.
3. 3. Plastic film according to claim 1 or 2, in which the linear low-density polyethylene whose density is beneath 0.915 is present in an amount of between 15 and 45 percent by weight.
4. 4. Plastic film according to any of claims 1 to 3, in which the linear low-density polyethylene whose density is beneath 0.915 has a density between 0.880 and 0.910.
5. 5. Plastic film according to any of claims 1 to 4, in which the film also contains up to 5 percent by weight, preferably at least 0.5 percent, of tack inhibitor.
6. 6. Plastic film according to any of claims 1 to 5, in which the film also contains up to 5 percent by weight, preferably at least 0.5 percent, of a substance which will impart a rough or coarse surface to the film.
7. 7. Plastic film according to any of claims 1 to 6, made into a sack, particularly a waste-disposal sack.
8. 8. Plastic film substantially as described in Example 1, 2, 4, 6, 7, or
9. 9.