BLENDS OF POLYETHYLENE AND POLYBUTENES

Abstract: Blends, and films made therefrom, are disclosed such blends being made from 80-99 wt. % of a polyethylene, 0.5 to 10.0 wt. % of a polybutene having a number average molecular weight of from 500 to 1500 and 0.5 to 10.0 wt. % of a polybutene having a number average molecular weight of from 1700 to 10 000. The films are useful in stretch wrap­ ping applications.

Description: BLENDS OF POLYETHYLENE AND POLYBUTENES The present invention relates to polymer blends and self-sealing stretch wrapping films. In particular the invention relates to such films made from blends of polyethylenes with polybutenes. Blends of polyethylenes and polybutenes are known. Self-sealing films made from blends of polyethylenes and polybutenes are also known. West German Patent 1 288 293 to BASF published 1969 January 30 discloses a two step process for mixing polyethylene with a polyisobutylene having a molecular weight between 20 000 and 300 000. The method for determining molecular weight is not disclosed. A bulletin, published about December 1970 by Amoco Chemicals Corporation, relating to Indopol* polybutenes indicates that Indopol H-100 polybutene is compatible or partially compatible in concentrations of 10% and 50% in certain polyethylenes manufactured by Allied Chemical Corporation or Union Carbide Corporation. It is further indicated that U.S. Food and Drug Administration had approved the use of Indopol polybutenes as additives in polyethylene film for food packaging applications in accordance with FDA Regulation 121.2610. U.S. FDA Regulation 21 CFR 121.2610 revised as of 1976 April 1 permitted the presence of isobutylene-butene copolymers, of average molecular weight in tile range of 3005000, as a plasticizer in polyethylene. Average molecular weight is determined by the procedure of ASTM-D2503. U.S. patent 3 821 332 to E. Solop, which issued 1974 June 28 discloses blending 70-85 wt. % polyethylene having a molecular weight between 250 000 and 1 500 000 with 30-15 wt. % polyisobutylene having an average Staudinger molecular weight of 80 000 to 120 000. Such blends are disclosed as being useful for a variety of end *denotes trade mark. uses e.g. blow moulded and injection moulded parts, and film. Films made therefrom are described as being clear, tough and strong. U.S. Patent 4 022 850, to Booth et al, which issued 1977 May 10 discloses a transparent self-sealing film made from a composition comprising 90-99.9 wt. % of a copolymer of ethylene and a vinyl ester of a C2-C30 monocarboxylic acid containing at least 89 wt. % of ethylene, and 0.05 to 10 wt. % of a liquid polyisobutylene or polybutylene. A preferred ester is vinyl acetate having a melt index of 0.2 to 20. The liquid polyisobutylene or polybutylene preferably has a viscosity average molecular weight (Florymethod) of 30 000 to 50 000. West German patent 2 821 733 A49, to British Cellophane Limited, published 1978 November 30 discloses a method of manufacturing a wrapping film suitable for use in spin wrapping applications, comprising extruding a blend of polyethylene and 0.5 to 10 wt. % polyisobutylene to form a film, and treating one surface of the film with corona discharge. Low density polyethylenes are exemplified, but the required molecular weight of the polyisobutylene is not indicated or suggested. Canadian patent 1 106 520, to Doomernik et al, which issued 1981 August 04 discloses a blend and a selfsealing stretch wrapping film made therefrom of 90-98 wt. % of polyethylene or ethylene/vinyl acetate (EVA) copolymer having a melt index of 0.3 to 7.0 and a density 0.917 to 0.933 g|cm3, and 10 to 2 wt. % of polybutene having a molecular weight of 700 to 2000. The method of determining molecular weight is not disclosed. One of the polybutenes disclosed is derived from isobutene. Said Canadian patent refers to Japanese pre-published application No. 49-54472, published 1974 May 27, in which there is proposed a film, suitable for stretch wrapping food products, made from EVA copolymer, containing not less than 708 ethylene, or from low density polyethylene, modified with a mixture of polybutenes having molecular weights in the ranges of 11001500 and 100-500. With respect to molecular weight determination of polymers, solution viscosity methods for determining molecular weight e.g. Staudinger and Flory methods, relate to weight average molecular weight (Mw), and are used generally for determining weight average molecular weights of about 30 000 and above. Osmometric methods for determining molecular weight relate to number average molecular weight (Mn) and are generally used for determining number average molecular weights of 20 000 and below. Molecular weights referred to herein, unless otherwise specified, are number average molecular weights determined using ASTM Procedure D-2503-67. A blend of polyethylene and two polybutenes, from which a film exhibiting better stretch-cling properties than film from prior blends containing the same concentration of a single polybutene, has now been found. Accordingly the present invention provides a blend comprising 80 to 99% by weight of polyethylene, 0.5 to 10.0% by weight of a polybutene having a number average molecular weight of from 500 to 1500 and 0.5 to 10.0% by weight of a polybutene having a number average molecular weight of from 1700 to 10 000. In a preferred embodiment one of the polybutenes has a number average molecular weight of from 700 to 1400, and especially from 900 to 1300. In another embodiment one of the polybutenes has a number average molecular weight of from 1800 to 5000 and especially from 2100 to 3000. In yet another embodiment the blend has from 2 to 12 wt. % of the polybutenes, especially from 3 to 10 wt. % of the polybutenes. Preferably the higher molecular weight polybutene is at a concentration about the same as or more than that of the lower molecular weight polybutene. More preferably the blend contains from 1 to 5 wt. % of each of the polybutenes. In a further embodiment the polyethylene comprises 70-100 wt. % of a linear ethylene-C4 to C8-a-olefin copolymer and 30 to 0 wt. % of a polymer selected from high pressure polyethylenes, ethylene/vinyl acetate copolymers and mixtures thereof. Preferably the olefin copolymer has a density from Q.916 to 0.930 g/cm 3, and more preferably from 0.916 to 0.925 g/cm3. The term '±polybutene1, as used herein in relation to the present invention, refers to polymers having a backbone predominantly based on n-butene or isobutylene. Polybutenes known in the trade as polyisobutylenes may be made by catalytically polymerizing an isobutylene-rich butene mixture, with more I- and 2-butenes being incorporated in the lower molecular weight polyisobutylenes than in the higher molecular weight polyisobutylenes. Polybutenes may also be synthesized, by a low temperature catlytic process, from a refinery gas stream containing polymerizable olefins, particularly isobutylene. Th present invention also provides film made from blends of the present invention. In a preferred embodiment such films have a thickness of from 12 to 76, especially from 20 to 5QJ m. The constituents of the composition of the invention may be preblended using a high shear mixer e.g. a twin screw extruder. The blend may be directly extruded into film form using the flat film or blown film methods. Alternatively the preblend may be extruded into pellets and stored before further extrusion, at an appropriate time and place, into film form. A concentrate of 10 to 40 wt. % of the combined polybutenes in a polyethylene may also be prepared in pellet form, prior to blending with the same or a different polyethylene or with an EVA copolymer, in order to lower the concentration of polybutenes in the final mixture. Blending of the concentrate and the additional polymer may be accom plished using the known Zsalt and pepper" blending technique prior to extrusion, or by known melt blending techniques in an extruder. The blown film method is the preferred method for forming the film of the present invention. In the blown film method the polyethylene/polybutene blend of the present invention is extruded through a circular die. The resulting tubular film is pulled from the die by cooperating nip rollers which collapse the tubular film at the nip. The tubular film, between the circular die and the nip rollers is sometimes referred to as a bubble. While in the molten or plastic formative state the tubular film is expanded by air or inert gas admitted into the bubble through the die land in the centre of the circular die. The tubular film, which is expanded by introduction of air into the bubble, may be cooled by directing air onto the exterior of the film while in the plastic formative state. The film is cooled until it is in the solid state. Cooling to the solid state is often referred to as quenching. It has been found that the faster the film is quenched the higher the cling level of the film. Faster quenching, in the blown film method, may be accomplished by passing the expanding film about a cooled mandrel which is situated within the bubble. One such method, using a cooled mandrel, is disclosed in Canadian patent 893 216 which issued 1972 February 15 to M. Bunga and C.V. Thomas It has been found that use of the two polybutenes, of differing number average molecular weights, gives a film which not only has cling levels higher than that obtained by using a single polybutene but which produces a film in which cling blooms more quickly. The method for measuring cling is as follows: The composition, for which cling is to be determined, is first formed into film having a thickness of from 12 to 38,am. Test results herein are shown for films made by a blown film method, such films having a thickness of 15 or 25,am. Strips of the film, 25.4mm in width, are then cut from the film under study. Pairs of like strips are taken and are placed in face to face contact such that 25.4mm of their lengths overlap. The overlapping area is subjected to a pressure of 6.89 kPa for a period of one minute. The pair of strips are then mounted in an Instron* testing machine and pulled apart at a rate of 12.27 mm/min. *denotes trade mark. The cling value is the maximum force, in Newtons per centimetre width of film, required to pull the strips apart in a shearing mode. It is to be understood that the blends and films of the present invention may contain ultra-violet light stabilizers, pigments and other modifiers known in the art. The following examples illustrate the)invention. Example I This example demonstrates the apparently synergistic effect of using a combination of two polybutene when compared to using the same concentrations of the two polybutenes when used separately. In test No. 1, five parts by weight of a polybutene having a Mn of 920, blended with 95 parts by weight of a linear ethylene-butene-l copolymer having a density of 0.919g/cm3 and a melt index of 0.6g/10 min., was extruded into film ftrm using a blown film method. The film was 25m in thickness. In test No. 2, the polybutene of test No. 1 was replaced by 5 parts by weight of a polybutene having an Mn of 2300. In test No. 3, 2.5 parts by weight of a polybutene having an Mn of 920 blended with 97.5 parts by weight of the linear ethylene-butene-l copolymer of test No. 1, was extruded into film form using a blown film method. The film was 25Jxm in thickness. In test No. 4, the polybutene of test No. 3 was replaced by a polybutene having an Mn of 2300. In test No. 5, the polybutene of test No. 1 was replaced by 1.8 parts by weight of a polybutene having an Mn of 920 and 3.2 parts by weight of a polybutene having an Mn of 2300. In test No. 6, the polybutene of test No. 5 having the Mn of 920 was replaced by a polybutene having an Mn of 1290. Tests 1, 2, 3 and 4 are outside the scope of the present invention while tests 5 and 6 fall within the scope of the present invention. Cling levels for all films were determined 24 hours and 48 hours after extrusion into film form. Table I summarizes the results obtained. Table I Cling(N/cm) Test Parts(PB)* Mn 24h 48h 1 5 920 3.05 3.75 2 5 2300 1.94 3.34 3 2.5 920 0.74 0.84 4 2.5 2300 0.19 0.28 5 1.8 920 4.08 5.24 3.2 2300 6 1.8 1290 4.78 4.82 3.2 2300 *PB=polybutene Example II A number of compositions coming within the scope of the present invention were extruded into film form using the blown film method. Cling levels of the films, each having a thickness of 25vim, were measured after 24 and 48 hours after extrusion into film. The polyethylene of the composition comprised was a linear low density polyethylene having a density of 0.919 g/cm3 and a melt index of 0.6 g/l0min. The results are shown in Table II. Table II PB* Cling(N/cm) Parts Mn 24h 48h Film A 2.O 920 3.28 4.08 3.0 2300 Film B 2.5 920 3.66 4.20 2.5 2300 Film C 2.0 920 1.52 4.97 4.0 2300 Film D 2.5 1290 4.61 4.71 2.5 2300 Film E 2.0 1290 4.99 3.96 4.0 2300 * PB=polybutene Example III A further composition falling within the scope of the invention, was extruded into film form using a blown film method. The polyethylene of the composition comprised a blend of 85 parts by weight of a linear polyethylene having a density of 0.919 g/cm3 and a melt index of 0.6 g/lOmin, and 15 parts by weight of a high pressure polyethylene having a density of 0.917 g/cm3, and a melt index of 6,5 g/10 min. The film was 25 > =m thick and had the following cling level several days after production. Parts(PB)* Mn Cling(N/cm) 1.6 1290 4.17 2.9 2300 *PB=polybutene

Claims: CLAIMS 1. A blend comprising 80 to 99% by weight of polyethylene, 0.5 to 10.0% by weight of a polybutene having a number average molecular weight of from 500 to 1500 and 0.5 to 10% by weight of a polybutene having a number average molecular weight of from 1700 to 10 000. 2. A blend according to Claim 1, wherein one of the polybutenes has a number average molecular weight of from 700 to 1400. 3. A blend according to Claim 1 or Claim 2, wherein one of the polybutenes has a number average molecular weight of from 1800 to 5000. 4. A blend according to any preceding claim, wherein the blend has from 2 to 12 wt. % of the polybutenes. 5. A blend according to any one of Claims 1 to 3 wherein the blend contains fro 1 to 5 wt. 5 cf each of the polybutenet. 6. A blend according to any preceding claim wherein the polyethylene is made from 70 to 100 wt. % of a linear ethylene - Cq to C8- d-olefin copolymer and 30 to 0 wt. % of a polymer selected from high pressure polyethylenes, ethylene/vinyl acetate copolymers and mixtures thereof. 7. A blend comprising (a) 80 to 99% by weight of a polyethylene, said polyethylene comprising 70 to 100 wt. % of a linear ethylene - Cd to C C8--olefin copolymer having a density of from 0.916 to 0.930 g/cm3 and 30 to 0 wt. % of a polymer selected from high pressure polyethylene, ethylene/vinyl acetate copolymers and mixtures thereof, (b) 0.5 to 10.0% by weioht of a polybutene having a number average molecular weight of from 700 to 1400, and (c) C.5 to 10.0% by weight of a polybutene having a number average molecular weight of from 1800 to 5000. 8. A blend according to Claim 7, wherein one of the polybutenes has a number average molecular weight of from 2000 to 3000 9. A film, having a thickness of from 12 to 50pm, made from the blend of any preceding claim. 10. A film, having a thickness of from 12 to 76cm, made from the blend of any of Claims 6 to 8.

Inventors: BRECK, ALAN KEITH, MOLLISON, ALISTAIR NORMAN
Applicants: DU PONT CANADA INC.
Application year: 1984
Main classification: C08J5
IPC codes: C08L23