Stock material or miscellaneous articles – Composite – Of polyester
Patent
1999-01-20
2000-11-28
Thibodeau, Paul
Stock material or miscellaneous articles
Composite
Of polyester
428483, 4285375, 264 3732, 26417113, B32B 2710
Patent
active
061533068
DESCRIPTION:
BRIEF SUMMARY
The invention relates to paper and board products which are coated with polylactide and to a method for the manufacture of such products.
The coating of paper and board with various polymers or waxes is a commonly used method. The purpose of the coating is to improve resistance to water and other barrier properties, sealability, strength properties and gloss. The coating is typically done with polyolefins, in particular polyethylene, or by using multilayer structures.
One problem involved with coated paper and board is their poor recyclability. Polyethylene, which is commonly used, does not degrade in the conditions used in the repulping of paper. Also, polyethylene-coated paper does not break down completely in nature and does not compost. When the aim is biodegradable products or recycling, the paper must be coated with a biodegradable plastic, such as polylactide.
The extrusion coating technique is typically used in the coating of paper; high temperatures and high run speeds are usually employed in this technique, and additionally a thin but tightly adhering plastic layer is desired on the paper or board.
The use of polylactide for the coating of paper has been described previously in patent publication WO094/08090. The said publication discloses the coating of paper with a chloroform solution which contains 20% of polylactide. Alternatively with molten polylactide, which is added by means of a nozzle very close to the paper surface. By this method it is possible to prepare only a coating which is relatively thick, more than 25 .mu.m. If the distance of the nozzle from the paper is increased, the molten polylactide is not spread evenly. This is due to the fact that the melt strength of the polymer is not sufficient for the formation of a proper film.
A typical method for the coating of paper is to extrude molten polymer through a nozzle onto a moving paper web. The paper may be treated (activated) by a separate corona or plasma treatment, or even by heating. The distance of the nozzle from the paper is regulated, for example, according to the polymer. The distance affects, for example, neck-in. However, neck-in is also affected by the stabilization of the polymer and by additives; neck-in is greater for an unstabilized polymer than for a stabilized polymer.
The biggest problem in coating with polylactide is the rapid cooling of the polylactide film after it emerges from the nozzle before it ends up on the paper, even if the nozzle is kept as close to the paper as possible. This results in poor mechanical adhesion.
However, it has now been observed, surprisingly, that too rapid cooling of the polylactide can be inhibited by coextrusion, wherein the polylactide is extruded together with some polymer conventionally used for coating, such as polyolefin, in such a manner that the polylactide layer will be against the paper and the polyolefin layer will be topmost. It is preferable to use, for example, polyethylene having a melt index of approx. 5-20 g/10 min. An excellent adhesion of the polylactide film to paper is achieved by this method. After the extrusion, the polyolefin film, which detaches easily from the polylactide coating, is removed before the reeling of the coated paper and is recycled. Thus a compostable, completely biodegradable or easily recyclable paper or board product is obtained.
The difference of polylactide from polyethylene, which is normally used for coating, is its narrower processing window, i.e. lower resistance to high temperatures and more rapid cooling. Characteristics typical of polylactide, on the basis of which adhesion would be very good, are its lower viscosity and high polarity. Low viscosity will cause good mechanical adhesion through spreading onto the paper and into its pores. Polarity causes dipole--dipole interactions with normally polar paper fibers. The oxygen and gas barrier properties of polylactide are better than those of polyethylene or polypropylene, but its water vapor barrier properties are poorer. The sealability of polylactide is very good.
Viscosity can be
REFERENCES:
patent: 5380813 (1995-01-01), Seppala et al.
patent: 5498385 (1996-03-01), Yabusa et al.
patent: 5594095 (1997-01-01), Gruber et al.
patent: 5852166 (1998-12-01), Gruber et al.
WPI, Derwent accession no. 94-329584 TOYOBO KK.
Selin Johan-Fredrik
Skog Maria
Tuhkanen Elisa
Neste Oy
Thibodeau Paul
Zacharia Ramsey
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