Stock material or miscellaneous articles – Hollow or container type article – Paper containing
Reexamination Certificate
2001-02-01
2003-11-11
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Hollow or container type article
Paper containing
C428S035700, C428S036600, C428S219000, C428S340000, C428S341000, C428S342000, C428S347000, C428S349000, C428S480000, C428S481000, C428S537500, C264S172190, C264S173160, C264S173110, C264S173120, C156S244110
Reexamination Certificate
active
06645584
ABSTRACT:
The object of this invention is a compostable polymer coated paper or paperboard, in which the coating includes as one component polylactide. A further object of the invention is a method for producing the paper or paperboard in question as well as a number of products produced from the paper or paperboard.
Water-resistant polymer coated paper and paperboard are commonly used as packaging material for food stuffs and as disposable containers. Coating polymers and multilamellar coating structures including the same have been developed that give to the package a good oxygen, water vapour and aroma tightness in order to ascertain good preservation of the packaged product. Another demand of lately increasing importance set on polymer coated packaging materials in their biodegradability. However, these two objects cannot be reached with the same polymers because the efficient block polymers in use such as EVOH or polyamide are practically unbiodegradable where as in commercial biodegradable polymers the blocking properties have more or less been omitted. Examples of biodegradable polymers on the market are e.g. polyhydroxy butyrate, plastics based on starch and polylactide (PLA) the latter of which is preferred for its simple manufacture and for its relatively good blocking properties.
Use of polylactide as packaging material coating is known e.g. from FI Patent Application 951 637 that describes a grease resistant paper intended for food stuff packaging where polylactide is mentioned as one possible material for the biodegradable polymer coating. According to this publication, the coating on the paper may consist of one biodegradable polymer layer or of a layered structure of two, even three layers on top of one another where the different layers have their own functions. The object was to produce a packaging paper which were at the same time grease, aroma, gas and water vapour proof and biodegradable. However, the publication does not include practical examples of application with measuring data, also, the publication does not divulge the role of the biodegradable polylactide might have in a multilamellar coating.
A more specific description of the use of polylactide in a packaging material may be found in EP Application 0 514 137 which describes a polylactide coated biodegradable packaging paper or paperboard for packaging liquids. The paper or paperboard can first be coated with a biodegradable adhesion producing binder layer that may be glue, gelatine, casein or starch after which the polylactide layer is applied on top of the binder layer. However according to this publication, all paper types do not require use of binder. In the examples given in the publication the thickness of the polylactide layer is 25-30 &mgr;m, and the coating is described as transparent, glossy, moisture resistant and as having good mechanical strength. In the examples of application in this publication the biodegradability of the coating has been tested but not its barrier properties.
In the brochure called “Poly(Lactic Acid) Polymers” (1977) published by Neste Oy, the manufacturer of polylactide, polylactide in itself is brittle and stiff if plasticizers are not included. The polyractide film forms a good gas barrier for air but only a fair one for water vapour. The flexibility and strength improving plasticizers added may disturb the barrier properties of the polylactide. According to this information, polylactide is no comparison for the best biodegradable blocking polymers (EVOH) but however, as to its gas barrier properties, essentially better than e.g. the polyethylene commonly used and satisfactory for numerous container and packaging applications. According to this brochure polylactide may be extruded and heat sealed at 180-250° C.
Accruing to observations and experience of the inventors, when extruding polylactide directly on paper or paperboard one comes up against the problem that in order to attain sufficient adhesion, one must operate at the utmost allowed temperature, i.e. at about 250° C., where furthermore the layer applied must be relatively thick. In the temperature mentioned the melt strength of polylactide is weak and pin holes remain easily in the coating layer obtained. Furthermore, because of the high temperature the polylactide is at risk of degradation such that its average molecular weight and viscosity decrease which further increases the fragility of the layer and promotes pin hole formation. Due to the brittleness of polylactide the products produced of a paperboard coated therewith show leaks and cracks whereby the coating does not withstand the creasing or bending and extending according to form inherent in producing plate or mould form products. Even in heat sealing polylactide layers cracking diminishing the tightness of seals is encountered.
Even dilution of coatings is included in developing trends of polymer coated packaging material, whereby e.g. LD polyethylene has been used with good results. As already mentioned, polylactide can be made to adhere to paper or paperboard only as a relatively thick layer, corresponding to what is described in EP Patent Application 0 514 137. With thinner layers the adhesion to the paper or paperboard surface is insufficient where in addition hindering of pin hole formation becomes all the more difficult.
However, decreasing the polylactide content would be desirable even because of the costs involved in using this material.
Still a further significant problem encountered with polylactide coatings is tearing of the edges of the coated film and polymer flakes loosening therefrom which when spilling to the newly applied polymer coating cause irregularities of the coating and clogging of the process. The high processing temperatures of polylactide promote “bearding” of the mouth piece of the extruder due to which the former must be cleansed and production ceased.
The purpose of the invention at hand is to come up with a solution by which the polylactide content in the paper or paperboard polymer coating can be diminished when, at the same time, the other inconveniences encountered with polylactide coatings described above can essentially be avoided. The polymer coated paper or paperboard according to invention is characterized in that the coating comprises an outer layer containing polylactide which weights at most about 20 g/cm
3
plus an adhesive layer binding the outer layer to the paper or paperboard which consists of a polylactide co-extrudable biodegradable polymer material.
With the inventive coextrusion of polylactide and the biodegradable polymer functioning as adhesion material on paper or paperboard are avoided the tearing of film occurring when extruding polylactide alone and the concomitant problems as the adhesion polymer acts as compounding element of the film. As the adhesive layer takes care of the problem presented by polylactide adherence the extrusion temperature may be lowered which in turn enhances the quality of the polylactide layer formed and diminishes the susceptibility of the material to tear and form pin holes when at the same time the polylactide layer may be made thinner than before. As a whole, the bilayered biodegradable polymer coating according to invention is more flexible and tighter than previous unilamellar polylactide coatings, in addition to which the amount of polymer in the polylactide layer or even in the coating as a whole can be diminished where by cost savings are produced.
It is preferred that the materials in the adhesive layer according to invention are biodegradable polyesters such as cellulose esters, aliphatic or aliphatic-aromatic co-polyesters or mixtures thereof. As cellulose esters may be mentioned cellulose acetate, propionate, butyrate, acetate-propionate, acetate-butyrate and propionate-butyrate. As copolyesters may be mentioned polyhydroxy alcanolates such as polyhydroxy butyrate, polyhydroxy valerate and polyhydroxy butyrate-polyhydroxy valerate copolymer. The mixtures mentioned are in particular binary or tertiary mixtures of cellulose esters and the copolyesters mentione
Kuusipalo Jurkka
Nevalainen Kimmo
Penttinen Tapani
Chen Vivian
ENSO Oyj
Rothwell, Figg Ernst & Manbeck, PC
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