DE4313136A1 - Polyactide coated with metals or silicon oxide as packaging material - Google Patents

Description

Polylactides are produced by ring-opening polymerization of the cycli Lactids produced. Starting from L-lactide, D-lactide or DL-lactide gives poly-L-lactide, poly-D-lactide or poly-DL- Lactide. Inverters do not occur during the polymerization sion on the optically active carbon atom, which maintains the tacticity remains. Poly-L- and poly-D-lactide can be found under corresponding Processing conditions, especially by holding briefly in the Temperature range of 100-120 ° C as partially crystalline polymers with a glass softening temperature of 50-55 ° C and one Crystallite melting point around 175 ° C can be obtained. By adding from DL- or DD-lactide to L-lactide or from DL- or LL-lactide to D-lactide becomes a copolymer through ring-opening polymerization with reduced crystallization rate and reduced obtained crystalline portion. The melting point drops, the gla However, the softening temperature is retained. However, one would like lower the glass softening temperature, so you do a copoly merization with the cyclic glycolide. The homopolymer Polyglycolide has a glass softening temperature of 20-25 ° C on. By copolymerizing appropriate proportions of lactide and Glycolide can therefore be the glass transition temperature of the copolymer Set between 20-25 ° C to 50-55 ° C.

The glass softening temperature of the polylactide plays a special role their role in the composting of the polymer or in its Absorption in body tissues. The material is dismantled Lich in the first step by unspecific hydrolysis of the Po polyester chains to lactic acid. In the second step, this is done Lactic acid formed by microorganisms or enzymatically builds. The rate-specific is the non-specific Esterhy drolysis. The speed of ester hydrolysis depends extremely strongly depends on the glass softening temperature of the polymer. This takes place 5-10 ° C above the glass softening temperature by approx Factor 100 faster than 5-10 ° C below the glass softening temperature.

Polylactides, especially homopolymeric polylactides, are becoming increasingly popular Interest as packaging materials that are natural without development foreign degradation materials are compostable. It is essential that polylactides are stable at normal storage temperatures up to 40 ° C are and only above or in the area of the glass softening temperature dismantling takes place. In quick composting plants  temperatures usually above 50 ° C for a long time, where by rapid ester hydrolysis. pH values above 7 accelerate the hydrolysis considerably. For example White bottles made of poly-L-lactide including the 2-3 mm thick Screw threads removed at pH 8 to 10 within 2 weeks, at pH 3-7 within approx. 8 weeks, the temperature being higher must be below 50 ° C.

For the production of a wide variety of moldings or packaging agents such as foils, bottles, deep-drawn cups or injection molding the solid polymers are melted and the Melt is pressed into molds through nozzles or films are produced.

It is already known (DE-42 30 097.5) that polylactides are then particularly good properties such as strength, rigidity, heat dimensional stability, gas tightness or solvent resistance obtained when multi-axis at a temperature between glass softening temperature and melting temperature based on the cupid phen stretched, stretched or stretched. Doing so the material is oriented, whereby it simultaneously to crystalline part len crystallized up to 80%. Foils, bottles, deep-drawn cups and Foams thus have stiffnesses of 3000 to 6000 N / mm², measured as a tensile modulus at room temperature.

This means that polylactides are characterized by high strength and Rigidity with low gas permeability and good resistance to solvents. Because of the high steep ability, molded articles or foils can advantageously be used with less Wall thicknesses as the corresponding parts or foils on the ba sis made of polyolefins, which saves material becomes.

With oriented poly-L-lactide, the oxygen is permeable of a 100 µm thick film around 200 cm³ / m² × bar and what Vapor permeability of a 100 µm thick film at 23 ° C and 0 up to 85% relative humidity approx. 30 g / m² × day.

These inherently low gas permeabilities are critical Applications, such as in the packaging of coffee, spices or Milk still too big.

So it is common in the packaging sector, plastic films for derar metallize packaging.

The food industry demands packaging in addition to attrak tive exterior and mechanical protection of the packaged goods also a ge know barrier to gaseous media, fats and Ge  flavors. Most packaging meets these requirements no problem. Only when the pack has a particularly long shelf life guts, possibly even without cooling, must be guaranteed or the product contains particularly sensitive flavors special protective measures such as metallizing are necessary. A An overview is given in Kunststoffe 82 (1992) 9. So lose For example, ground coffee has an aroma under ambient conditions relatively quickly. The aromatic oils react with that Atmospheric oxygen and become "rancid". Because trade and consumers but a shelf life of up to 18 months with pre-ground coffee fee, the packaging material must have a special acid offer fabric barrier. The traditional coffee packaging included Therefore, a 9 µm thick aluminum foil that is laminated is embedded between plastic films. So fulfills the orien polyester film in this composite the demand for mecha strength, printability and high surface gloss, while a PE film on the inside of the packaging you Guaranteed gelability. Alternatively, metallization is also possible Polyester film is used or an attempt is made to use Al as a bar to do without the media. Today's metallized 12 µm polyethylene terephthalate (PET) films reached O₂ barrier is less than 1 cm³ / m² × d × bar. Around this value when waiving To achieve aluminum, ethylene vinyl alcohol Copolymers EVOH can be used as a barrier plastic. Which he Required layer thickness under standard ambient conditions (23 ° C, 50% RH is <10 µm.

In order to save thick aluminum foils, is in the packaging area Aluminum is now often applied to foils in a high vacuum vapors in order to achieve good barrier properties. As a sub strate are suitable for almost all films made of polyethylene (PE) up to cellophane, in practice, however, are primarily sol used film types that are inherently good barriers have properties against certain media. This is how biorien becomes PET film metallized when it comes to particularly low sow permeability. With bio-oriented polypropylene fo lie reduces steaming with aluminum which is already very low Water vapor permeability continues. Typical permeability values for PET (12 µm) are less than 1 cm³ / (m² × d × bar) for O₂ or below 0.25 g / (m² × d) with water vapor for PP (20 µm).

A disadvantage for many packaging applications is the loss of transparency in metallized films. Metallized foils are also only of limited suitability for use in packaging for microwave dishes, since the microwaves are reflected by the aluminum layer. The evaporation of films with SiO _{x was} therefore developed. As with aluminum, the coating is carried out in a high vacuum. The thin layer forms an effective barrier against oxygen and water vapor, but does not impair the transparency of the film and allows microwaves to pass through. The barrier properties that can be achieved are comparable to those of metallized films.

However, the more complex process technology currently makes SiO _x -steamed films even more expensive. Nevertheless, the SiO _x vapor deposition is already carried out on oriented PET films.

The process technology for coating films in a vacuum is State of the art; it is used, for example, in vacuum technology, 34th year, issue 7, page 201. The use in Le The food sector is described in "New packaging" 9/83, page 1042 wrote.

In research report No. 01-ZV 890V by the German Federal Minister rium for research and technology is therefore on the possibility pointed out, biodegradable plastics to reduce Coating the permeability with aluminum. So that leaves the above-mentioned oxygen permeability to 2.5 cm³ / m² × bar and the water vapor permeability according to our own measurements Reduce to approx. 1 g / m² × day.

However, this report states that during 30 days on such a piece of film in an aquatic environment no degradation showed, but degradation was not completely impossible is held. Afterwards it appeared to the expert as not at tractive, aluminum-coated polylactide films in the Verpac sector.

This prejudice is supported by the teaching of DOS 2 239 277, with which a water-soluble container and method be claimed for its manufacture. After that, the core this composite container made of a water-soluble "polylactic Acid ", the cover foils consist of water-insoluble films high molecular weight, film-forming polylactides are water-insoluble, it can only be very low molecular weight lactyl milk acids with degrees of condensation from 2 to 5 act as Kle coated and no properties of freestanding Feature films.

So there was the task of one for permeation-proof goods to find dense plastic film, which is easy to practice composting plants is degradable.  

It has now been found that one-sided with 0.01 to 1 microns thick Aluminum coated films made of polylactide under the conditions against conventional municipal composting plants with wall thicknesses 0.1 to 0.5 mm completely within two to four weeks be built, leaving no metallic aluminum.

It shows that contrary to a prejudice with aluminum polylactide coated on one side very well compostable and can therefore advantageously be used as packaging materials.

The aluminum coatings are made according to the state of the art usual processes such as vacuum evaporation of aluminum and Precipitation of the steam applied to the film. The layer thicknesses are 0.01 to 1 µm, preferably 0.05 to 0.1 µm.

It is understood that metals other than aluminum can be used as barrier layers. It is also possible to apply non-metallic barrier layers, such as SiO _x, via the decomposition of siloxanes in a plasma.

The films coated on one side are used to manufacture Packaging used, with the barrier layer on the inside side is facing the product. Gas and liquid density Seams are there by gluing, flanging or a combination of manufactured according to the prior art.

However, it is also possible to use the poly which has already been vapor-coated on one side lactide film on the side coated with the barrier layer to laminate another polylactide film, so that one Composite polylactide / barrier layer / polylactide obtained.

A polylac formed by slot extrusion is used for this tid film after exiting the nozzle on the chill roll over the coated cold polylactide film.

The polylactides used are preferably Poly-L-lactide or poly-D-lactide. Copolymers lactides are only in of interest if they still have melting points above 150 ° C have, that is to say only up to about 5 mol% of comonomer units sen. Comonomers are 1,3-dioxan-2-ones of the structure

1,4-dioxan-2-one of the structure

further lactides of the structure

or lactones of the structure

In the structural formulas mentioned, the radicals R¹ to R⁶ be the same or different and hydrogen, a branched or unbranched alkyl, alkylene or alkyne group with 1 to 12, preferably contain 1 to 4 carbon atoms, optionally by halogens, hydroxyl groups, alkoxy groups, formyl groups, Acrylic groups, amino, alkylamino, dialkylamino or cycloalkyl groups are substituted.

The homopolymers or copolymers lactides are produced according to the prior art in a ring-opening manner starting from the monomer melt at temperatures of 180 to 230 ° C. BF₃ etherate, titanium alcoholates and other manganese, zinc, tin, lead, antimony or aluminum compounds can be used as polymerization catalysts. Tin-II compounds are most commonly used as alkoxides or carboxylic acid salts. Tin-II-octoate or tin-II-ethyl-2-hexanoate is preferably used in concentrations of 10 ^-6 to 10 ^-3 moles per mole of monomer mixture.

example

Homopolymeric poly-L-lactide with an inherent viscosity of 100 ml / g, measured on a 0.1% solution in chloroform 25 ° C, was dried for 20 h at 120 ° C and a pressure of 1-3 mbar  net. The granulate was then flooded with argon Funnel of a film extruder and the material at 200 ° C. melted and through a slot die to films of approx. 20 cm wide extruded. The film was removed from the Wide slot nozzle by means of a cooling roller cooled to 25 ° C. amorphous poly-L-lactide quenched. In the further course of Zugs it was heated to 90-100 ° C and infrared then deducted faster by a factor of five and thereby unidirek tionally stretched and crystallized. So became semi-crystalline Films with a thickness of approx. 30, 60 and 100 µm are produced.

Pieces of these foils with approximately 20 × 20 cm were steamed in a laboratory vaporization system with an approximately 0.03 μm thick aluminum layer. Additional pieces of film were provided with a 0.04 to 0.06 µm thick SiO _x layer.

The coated films as well as an uncoated control Foil were placed in wire baskets in a municipal composting plant buried. After 6 weeks - during this time it was Temperature 70 to 80 ° C - the baskets were dug out. There were no film remains found.

Claims (3)

1. Packing material made with metals or silicon oxide steamed polylactide. 2. Packaging material according to claim 1, characterized in that the layer thickness of the evaporation 0.001 to 1 micron, preferred example is 0.01 to 0.1 microns. 3. Packaging material according to claim 1 and 2, characterized records that the vaporized polylactide side with a white tere polylactide layer is laminated.