WO1994010239A1

WO1994010239A1 - Food wrap film

Info

Publication number: WO1994010239A1
Application number: PCT/EP1993/002909
Authority: WO
Inventors: Frank Louis Maria Vermeiren
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1992-10-23
Filing date: 1993-10-21
Publication date: 1994-05-11
Also published as: GB9222356D0

Abstract

The invention provides a food wrap film in the form of or containing a layer of a composition comprising 20 to 50 wt % of LLDPE; 1 to 10 wt.% of a high molecular weight; thermoplastic olefinic polymer containing units derived from 1-butene; and 40 to 79 wt.% of a copolymer of ethylene and an ethylenically unsaturated ester, the weight percentages being based on the total polymer weight.

Description

FOOD WRAP FILM

The present invention relates to an improved food wrap film with cling characteristics, generally of the type referred to in the USA as PVC food wrap replacement film, and in Europe as "cling film", and to a composition therefor. Such film may be used for wrapping, for example, fresh food; whilst being suitable for manual wrapping it is especially suited for automated food wrapping.

Fresh food is wrapped automatically on processing lines with cling wrap film. Such a procedure involves intermittently pulling off at high speed a film from a supply roll, pushing a tray filled with food upward until it impacts against the film, stretching the film over the sides of the tray to the lower side thereof, heat sealing (without melting) the edges of the film, and allowing the film to be pulled taut by elastic contraction. In use the film should be transparent, puncture resistant and should snap-back when deformed. This procedure has been performed in the art using PVC. However, such PVC based films are environmentally undesirable since chlorine containing compounds are difficult to process as waste, and since the plasticisers used may migrate and pose a health hazard.

It has been difficult to formulate composition based on olefinic polymers for making cling wrap film . Any such composition must not only possess characteristics suitable for processing in the manner described above, but should also be approved under food law.

EP-A-333508 describes an approach to solving this problem and discloses a food wrap film containing a layer of a composition comprising 20 to 50 wt% LLDPE (linear low density polyethylene) , 1 to 10 wt% of an elastomeric polymer and 40 to 79 wt% of a copolymer of ethylene and an ethylenically unsaturated ester (particularly EVA [ethylene vinyl acetate copolymer]). The elastomeric polymer which is preferred and exemplified is poly-isobutene. It has now been found, in accordance with the present invention, rhat an improved cling wrap film based on olefinic polymers can be produced by utilising a composition comprising a high molecular weight thermoplastic olefinic polymer containing units derived from 1-butene, in particular a ho opolymer of 1-butene, as well as LLDPE and a copolymer of ethylene and an ethylenically unsaturated ester, such as EVA.

According to one aspect, the present invention provides a food wrap film in the form of or containing a layer of a composition comprising:

20 to 50 wt% of LLDPE; 1 to 10 wt% of a high molecular weight thermoplastic olefinic polymer containing units derived from 1- butene; and

40 to 79 wt% of a copolymer of ethylene and an ethylenically unsaturated ester,

the weight percentages being based on the total polymer weight.

There is also provided, according to a further aspect, a composition for use in manufacturing a food wrap film, said composition comprising:

20 to 50 wt% of LLDPE;

1 to 10 wt% of a high molecular weight thermoplastic olefinic polymer containing units derived from 1- butene; and

40 to 79 wt% of a copolymer of ethylene and an ethylenically unsaturated ester, the weight percentages being based on the total polymer weight.

A cling wrap film produced in accordance with the present invention possesses easier cutability on an automatic wrapping machine as well as enhanced heat resistance over the prior art, resulting in a broader sealing range. The film also possesses improved overall wrapping machinability with fewer final packaging failures, tending towards zero, and improved package appearance, when compared with the non-PVC prior art.

The use of thermoplastic poly-1-butene is particularly advantageous since it is generally approved for use in foodstuffs and in particular has USA FDA food approval.

The film of the present invention also provides lower energy consumption (use of a lower current) , and easier processing leading to increased output (as evidenced by lower back pressure) .

Suitably the film has a thickness of from 10 to 30 microns, a 1% secant modulus not exceeding 500 MPa, preferably 300 MPa, and/or tensile strength at break not exceeding 75 MPa, preferably less than 50 MPa. Advantageously the film has a haze percentage of from 0.1 to 5 and/or a snap-back of from 20 to 70% recovery after elongation by 100%.

The olefinic polymer is a high molecular weight thermoplastic polymer containing units derived from 1- butene. The thermoplasticity should be sufficiently high and the elastomeric nature should be sufficiently low that the olefinic polymer can be formed to give non-sticky pellets, which can be melt blended, use of bales being avoided. The thermoplasticity should facilitate blending and subsequent extrusion as part of a homogeneous blend with the other components. It is particularly preferred to utilise thermoplastic poly-1-butene homopolymer or a copolymer with 1-butene, e.g. a copolymer of ethylene and 1-butene containing at least 70%, preferably 80% and especially 90 wt% of 1-butene derived units. With regard to compatibility, the amount to the olefinic polymer should be such that there are no problems. Thus no more than 10 wt% is used. It is preferred to use 5-7 wt% of the olefinic polymer. The olefinic polymer, e.g. poly-I-outene, snould desirably have a weight average molecular weight of 50,000 to 1,000,000 preferably 75,000 to 250,000, more preferably from 100,000 to 300,000. The molecular weight is determined as the weight average molecular weight or the number average molecular weight preferably by gel permeation chromatography. For these GPC measurements a Waters 150 TCB can be used with 10 micrometer PL gel using 1,2,4 trichlorobenzene at 135°C and a polystyrene standard. It is preferred that the olefinic polymer have a melt index (MI) of 0.5 to 40, such as 1.8, preferably less than 10 or 20. It is preferred that the Mw/Mn is from 2.5 to 4.

Suitable olefinic polymers will generally be of high (in comparison with elastomeric polymers) crystallinity, and suitable commercially available olefinic polymers, e.g. poly-1-butene, may typically have crystallinities of 46-52% and densities of 0.89 to 0.92 g/cm³ , e.g. 0.90 to 0.92 g/cm³- Examples of suitable poly-1-butene homopolymers are Shell Polybutylene Grade 0200, Shell Polybutylene Grade 0300 and Shell Polybutylene Grade 0400. Shell Polybutylene Grade 0400 is particularly preferred. Typical molecular weights as determined by GPC are: Mw= 94000, Mn 31000, and Mw/Mn of 3.

Suitably the LLDPE is present at 25 to 40 wt%, the melt index (MI) is preferably 0.4 to 4, more preferably o.5 to 0.8, and the density is preferably 0.90 to 0.93. The LLDPE is preferably a polymer containing up to 10 wt% of comonomer derived units to give a density of 0.915 to 0.925. The comonomer derived units may be obtained from a C.-CO„ alpha-olefin, preferablv a CO„ alpha-olefin. With less than 20 wt% LLDPE the material has insufficient strength. With more than 50 wt% LLDPE the ability of the film to stretch will be compromised and an insufficient amount of the copolymer of ethylene and an ethylenically unsaturated ester can be incorporated to provide the desired cling properties. Suitably the ethylenically unsaturated ester is vinyl acetate (VA) and the copolymer contains from 5 to 15 % of the ester units, preferably from 8 to 14% of the ester units and optionally more than 10 % of the ester units and/or the MI is from 0.3 to 7 and preferably from 0.5 to 5 and especially from 2 to 4. The ethylene vinyl acetate

(EVA) may form substantially the balance of the polymer component. In combination with LLDPE and the olefinic polymer, it contributes to a homogenous compatibilised mixture with good properties for a cling film.

Generally speaking the film should conveniently contain a minor amount of additives to provide, in combination with the polymer components, an antifogging and antiblocking effect such that the film is substantially transparent and has a controlled amount of residual cling. In selecting additives for antiblocking and anti- fogging effects, one should take into account that the products are °f low molecular weight and should have a sufficient compatibility to avoid undesired side-effects. The low molecular weight may lead desirably to a degree of added plasticity. Excessive amounts of agents having antiblocking effects should not be used as these materials may give an oil-like feel. The tackiness provided by the copolymer of ethylene and ethylenically unsaturated ester and the olefinic polymer should only be reduced to the extent necessary to allow a thin film to be pulled off at high speed from a supply roll on an automatic processing line without undue difficulty.

The film should, particularly in its surface areas, be substantially devoid of hydrocarbon oils (i.e. parrafinic or naphthenic oils) and waxes. Desirably amounts greater than 1% of such materials are to be avoided.

Suitable the film has, coextruded with a first polymeric composition according to the invention as defined above, a second layer of a polymer or polymer composition having a higher crystallinity for improved film behaviour under automatic processing conditions, the additives for antiblocking and antifogging effects being provided at least in said second layer. The layer of the invention may be a middle layer in a 3-layer structure for example or may form a pair of outer layers in a 3-layer structure.

In selecting the polymer to be used in the optional coextruded outer layers, regard should be had to stretchability and tear strength as well as the added crystallinity which is desirable. The additives may be formulated to be incorporated principally in such outer layers which may have a thickness of from 2 to 10 microns each.

In one possible form of the invention the film consists of single layer as described previously. The preferred form of the invention however is the film obtained by coextruding a layer according to the invention with one or more others layers. Suitably such other layer(s) are of an EVA/LLDPE/LDPE (low density polyethylene) blend or EVA/VLDPE/LDPE (VLDPE= very low density polyethylene) , formulated to provide low haze, good dispersion of the blend components, and good melt flow characteristics. Conveniently such layers contain from 20 to 50% of LLDPE, from 4 to 16% of LDPE and the balance of EVA. Preferably the EVA and/or the LLDPE have the preferred characteristics set out previously in the context of the film according to the invention. The LDPE component preferably has an MI of from 4 to 16 and a density of from 0.910 to 0.930.

By appropriate combination of coextruded layers a product with low haze and unexpectedly high puncture resistance can be obtained. By suitable selection of the components and using appropriate proportions of such components a film is obtained which is similar in its properties to PVC cling film. Transparency and snap-back are both obtained. Whilst the invention involves several components and does not have a strong tackifying additive, all components can be selected so that the necessary clinging properties are provided, together with the desirable mechanical properties for cling film, such as stretch and snap-back to allow automatic processing.

In another facet, when the olefinic polymer is poly-l- butene the present invention allows improved melt-blending production rate of compositions (generally in the form of pellets) via a single step mixing process rather than a two-step process according to the prior art (in particular the PIB component utilised in EP-A-333508) . The present invention thus allows the easier production of a food wrap film, for example from the pelletized composition, at an improved production rate.

Thus, in a further aspect, the present invention provides a process for producing a composition for use in manufacturing a food wrap film, said composition comprising:

(a) 20 to 50 wt% of LLDPE;

(b) 1 to 10 wt% high molecular weight thermoplastic poly-1-butene; and

(c) 40 to 79 wt% of a copolymer of ethylene and ethylenically unsaturated ester, the weight percentages being based on the total polymer weight, said process comprising blending components (a) , (b) and (c) together in a single step. The composition may then be formed into pellets.

There is also provided a method of forming a food wrap film comprising components (a) , (b) and (c) above, which method comprises blending said components using a one-step process as defined in the previous paragraph followed by extrusion of the blend, preferably in pellet form, into a film.

Thus, in the case of the use of poly-1-butene, there is a simple one-step blending procedure using, for example, poly-l-butene pellets. This contrasts with the use of elastomeric poly-isooutene, wnicn is supplied in ne form of bales, which have to be unwrapped, followed by a first step of preparing a 10% FIB concentrate m LLDPE, and a second step of blending the thus produced concentrate with the remaining components to produce the final wrap film blend, ready for extrusion.

The present invention will now be illustrated by means of the following non-limiting Examples.

Example 1

The following components where blended starting from pelletised materials at a temperature of 170-200°C in a twin screw extruder:

65 wt% EVA (MI= 2.1/VA= 9.4%)

30 wt% LLDPE (MI= 0.5/density= 0.918) 5 wt% poly-l-butene (MI= 20/density = 0.915, Shell Polybutylene Grade 0400)

to form a blend (Blend 3) . Corresponding blends with respectively 0% (Blend 1 - Comparative), 3% (Blend 2), 10% (Blend 4) and 15% (Blend 5 - Comparative) by weight of poly-l-butene were also formed in the same way. All blends were formed into pellets and extruded on a blown extrusion machine at temperatures of 105 to 200°C at a blow up ratio of 3.1. The following extrusion properties were obtained:

Table 1

The thus blown films were tested for their optical properties (haze, clarity and gloss) and the following results obtained:

^~ Table 2

Table 2 (continued)

Example 2

The following components were blended at a temperature of 170 to 200°C in a twin screw extruder: 65 wt% EVA (MI= 2.1/VA=9.4%)

28.5 wt% LLDPE (MI= 0.5/density=0.918) 5 wt% poly-l-butene (MI=20/density=0.915, Shell

Polybutylene Grade 0400) 1 wt% GMO (glycerine-mono-oleate) [anti-fogging agent] 0.5 wt% castor oil [anti-blocking agent] to form a blend [blend 6] which was formed into pellets.

The pellets were coextruded on a cast film extrusion line at temperatures from 180°C to 230°C, to produce a 3-layer structure A/B/A (where A is VLDPE (MI=4/density=0.905 and B is Blend 6 above) . The total tickness of the structure was 16 (+/-0.5) microns, where A= 4 microns (x2) and B is 8-9 microns.

This film structure according to the invention (Film 1) was tested for optical and mechanical properties as well as machinability on an automatic wrapping line. Mechanical and optical properties and machine behaviour were measured and compared against a standard PVC food wrap monolayer (Film 3) and a PIB - containing A/B/A type structure according to EP-A-333508 (Film 2) , with A having the same composition as above and B comprising the following composition:

65 wt% EVA 28.3 wt% LLDPE

5 wt% poly-isobutene

1 wt% GMO

0.7 wt% castor oil

The mechanical and optical properties were as follows:

Table 3

Table 3 ( continued)

The machine behaviour, on an automatic wrapping line was as follows:

Table 4: Judgement of different film types (from 0

(unacceptable) to 5 (excellent) at the most critical points

[running on Automac 55 wrapping-line]

Example 3 : Comparison of Production output

A blend in accordance with the invention was made in accordance with Blend 6 of Example 2, on a Banbury compounding line in a single step at a blending temperature of 160^βC. Due to the fact that poly-l-butene is in pellet form, dosing of he composite is easy and accurate. There is also a low risk of contamination resulting in improved film quality and consistency of quality. Due to the single step process the production output rate was high at 1.6 ton/hr.

A blend based on PIB, according to EP-A-333508 and based on the composite of Film 2 of Example 2 was prepared by first producing a 10% PIB concentrate in the LLDPE; in practice this involves unwrapping a polyamide liner from a PIB bale, with a high risk of contaminating the blend given poor film quality. It should also be added that handling of the extremely sticky PIB is difficult. This is then followed by a second step of blending the concentrate on a Banbury compounding line at a temperature of 160°C with the remainder of the components. This procedure has a significantly inferior final production output rate of 1.0 ton/hr.

Claims

1. The composition provides a food wrap film in the form of or containing a layer of a composition comprising:

20 to 50 wt% of LLDPE;

1 to 10 wt% of a high molecular weight thermoplastic olefinic polymer containing units derived form l- butene; and 40 to 79 wt% of a copolymer of ethylene and an ethylenically unsaturated ester, the weight percentage being based on the total polymer weight.

2. Film according to claim 1 in which the layer has a thickness of from 10 to 30 microns, a 1% secant modulus not exceeding 500 MPa, preferably 300 MPa, and/or a tensile strength at break not exceeding 75 MPa, preferably less than 50 MPa.

Film according to claim 1 or claim 2 in which the layer has a haze percentage of from 0.1 to 5 and/or a snap-back of from 20 to 70% recovery after elongation by 100%.

4. Film according to any of the preceding claims in which the layer is substantially devoid of hydrocarbon oils (i.e. paraffinic or naphthenic oils) and waxes, preferably a haze percentage of from 0.1 to 5 and/or a snap-back of from 20 to 70% recovery after elongation by 100%.

5. Film according to any of the preceding claims in which the layer has coextruded with it a second layer of a polymer or polymer composition having a higher crystallinity for improved film behaviour under automatic processing conditions, the additives for antiblocking and anitfogging effects being provided at least in said second layer.

6. A composition for use in manufacturing a food wrap film, said composition comprising:

(a) 20 to 50 wt% of LLDPE; (b) 1 to 10 wt% of a high molecular weight thermoplastic olefinic polymer containing units derived from 1-butene; and (c) 40 to 79 wt% of a copolymer of ethylene and ethylenically unsaturated ester, the weight percentages being based on the total polymer weight.

7. Composition of film according to any of the preceeding claims in which the olefinic polymer containing units derived from 1-butene is poly-l-butene homopolymer.

8. Composition of film according to any of the preceding claims in the thermoplastic olefinic polymer containing units derived from 1-butene has a molecular weight of 50,000 to 1,000,000.

9. Composition or film according to any of the preceding claims in which the ethylenically unsaturated ester is vinyl acetate (VA) and the copolymer contains from 5 to 15% of the ester units, preferably from 8 to 14% of the ester units and optionally more than 10% of the ester units; the MI is from 0.3 to 7 and preferably from 0.5 to 5% and especially from 2 to 4.

10. A process for making a composition according to any of claims 6 to 9 including blending components (a) , (b) and (c) together in a single step and optionally forming the blended composition into pellets.