Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2002-01-25
2004-09-07
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S444000, C524S047000
Reexamination Certificate
active
06787613
ABSTRACT:
DESCRIPTION
The present invention relates to mixtures of biodegradable polyesters which include at least three polyesters in proportions whereby it is possible to provide biodegradable film with improved characteristics compared to the individual initial polyesters and demonstrating, in particular, properties of considerable strength, both longitudinally of and transverse the direction of the formation of the film, transparency and rigidity.
Film manufactured from such mixtures will prove particularly useful in food packaging, for mulching, for silage and in various other applications.
THE PRIOR ART
Conventional polymers such as low or high-density polyethylene are characterised not only by excellent flexibility and water resistance, but also by a good level of transparency and by excellent resistance to tearing. These polymers are used, for example, for sacks and bags, as packaging material and as film for agricultural mulching. However, their poor biodegradability has caused a problem of visual pollution which has steadily worsened over the past few decades.
Polymers such as L-polylactic, D,L-polylactic or D-polylactic acid and copolymers thereof are thermoplastic materials which are biodegradable, come from a renewable source, are transparent and have excellent resistance to mould and are thus well suited to packaging food products, contributing to preserving the organoleptic qualities thereof These materials, however, break down only slowly in the soil and, if composted, decompose only at high temperatures. However their main drawback is that the thin film obtained under normal conditions, by either the blown or cast methods, has a low tear resistance. In addition, these films are very stiff and thus unsuitable for mulching, for making food packaging bags, bin liners or other packaging films which do, however, require considerable strength.
The aliphatic polyesters, on the other hand, which are mainly constituted by monomers from renewable sources, based on diacids and diols, such as polymers of sebacic, brassylic or azelaic acid, for example, have the huge disadvantage of being highly anisotropic, with regard to resistance to both longitudinal and transverse tearing, and also show extremely poor resistance to longitudinal tearing. These characteristics also make film produced from these resins unsuitable for use in mulching, in food packaging or for bin liners and the like.
Polyhydroxy-acids, such as poly-&egr;-caprolactone, also have a typical tendency to a transverse orientation.
In order to maintain biodegradable characteristics conforming with the CEN 13432 method, biodegradable aliphatic-aromatic polymers, in particular polymers with the aromatic portion constituted by terephthalic acid and the aliphatic portion constituted by diacid diols, and/or hydroxy acids, with a C2-C20 aliphatic chain, either branched or not (possibly chain extended with isocyanates, anhydrides or epoxides) and, in particular, polymers based on terephthalic acid, adipic acid and butandiol, must contain quantities of terephthalic acid (as moles of the total acid) not exceeding 55% and preferably not exceeding 50%. Examples of this type of material include Ecoflex by BASF or Eastarbio by Eastman, which are strong but with extremely low moduli, of the order of 100 MPa or less.
Binary compounds of polylactic acid and aliphatic polyesters have formed the object of numerous patents. In particular, the Patent EP-0 980894 A1 (Mitsui Chemical) describes a significant improvement in tear resistance and in the balance of mechanical properties in film manufactured from mixtures of polylactic acid and polybutylenesuccinate, with the addition of a plasticizer.
However the films described are not transparent and have fairly low strengths, of the order of 120 g according to the JIS P8116 method. In addition, the presence of a plasticizer limits use of the film in contact with food products and, since it ages rather quickly, for use as an agricultural mulch.
The U.S. Pat. No. 5,883,199 describes binary compounds of polylactic acid and polyesters, with a polylactic acid content of between 10 and 90% and with the polyester forming either a continuous or co-continuous phase. The tear resistance of the compounds described here is very poor, however.
Object, characteristics and advantages of the invention
Starting from the need to find a biodegradable material which combined the two properties of transparency and tear resistance, it was a surprise to find that if the three different types of polyester described (lactic acid polymers, aliphatic polyester derived from diacids/diols and aromatic aliphatic polyester) were combined in specific ratios, there was a critical compositional range in which it was possible to achieve resistance to tearing in both directions, comparable to that of conventional plastics materials such as polyethylene, moduli of elasticity with values found between those of low and high-density polyethylene. It was found, even more surprisingly, that it was possible for the transparency of the ternary mixture of polyesters of the invention to be comparable to that of the individual component materials, even when drawn.
DESCRIPTION OF THE INVENTION
The invention relates to a mixture of biodegradable polyesters which includes:
(A) an aromatic-aliphatic polyester with a melting point of between 50 and 170° C. and preferably of between 80° and 120° C.;
(B) an aliphatic polyester with a molecular weight Mw greater than 40,000, and preferably>60,000 and a melting point of between 50° and 95° C., preferably of between 55° and 85° C., and even more preferably of between 57° and 80° C.;
(C) a polylactic acid polymer containing at least 75% of L-lactic or D-lactic acid or a combination thereof, with a molecular weight Mw greater than 30,000;
in which the concentration of A varies, with respect to (A+B), in the range between 40% and 70% by weight, and the concentration of C with respect to (A+B+C) is of between 6 and 30%, preferably of between 10 and 25% by weight.
More in particular, in the mixture of the invention:
(A), the aromatic-aliphatic polyester, is biodegradable according to the CEN13432 standard, it has (at T=23° and Relative Humidity=55%) a modulus which is less than 150 MPa, lengthens to breaking point by more than 500% for blown film with a thickness of 25-30 &mgr;m, tested within three days from production;
(B), the aliphatic polyester, preferably a diacid/diol type, has (at T=23° C. and Relative Humidity=55%) a modulus of elasticity of between 200 and 900 MPa and lengthens to breaking point by more than 200%, for blown film with a thickness of 25-30 &mgr;m, tested within three days of production;
(C), the polylactic acid polymer, has a modulus of more than 1,500 MPa.
The mixture of biodegradable polyesters of the invention is obtained in a process which is carried out in a two-screw or one-screw extruder at a temperature of between 100 and 200° C., either by a one-step method or a method involving separate steps of mixing and then film forming.
In the event of film forming being separate from the mixing operation, it is carried out by means of conventional machinery for polyethylene extrusion (high or low density), at a heat in the range of 100 to 200° C., preferably of 140 to 197° C. and more preferably of 185 to 195° C., with a blowing ratio normally in the range of 1.5-5 and a drawing ratio of between 3 and 100, preferably 3 and 25 and produces film with a thickness of between 5 and 50 &mgr;m.
Films of the invention, with a thickness of between 25-30 &mgr;m, show a tear resistance in both directions, according to the Elmendorf test, of between 15 and 100 N/mm, preferably of between 20 and 90 N/mm and even more preferably of between 25 and 80 N/mm, with a ratio of transverse to longitudinal Elmendorf values of between 3.5 and 0.4, and preferably of between 2.5 and 0.5. Such films have a modulus of between 150 and 800 MPa, preferably of between 250 and 750 MPa and prove biodegradable both in soil and when composted. Such films are also characte
Bastioli Catia
Del Tredici Gianfranco
Guanella Italo
Ponti Roberto
Connolly Bove & Lodge & Hutz LLP
Novamont S.p.A.
Short Patricia A.
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