Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-06-17
2002-02-19
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S052000, C524S734000
Reexamination Certificate
active
06348524
ABSTRACT:
DESCRIPTION
The present invention relates to heterophasic polymeric compositions having a high resistance to ageing, even under conditions of low temperature and humidity, comprising thermoplastic starch and a thermoplastic polymer incompatible with starch, in which the starch constitutes the dispersed phase and the polymer the continuous phase.
The invention relates particularly to manufactured products which maintain high impact strength and tear strength in low humidity conditions.
It is known that products (in particular films) manufactured from compositions containing thermoplastic starch and a thermoplastic polymer incompatible with starch, in which the starch constitutes the dispersed phase, show a significant deterioration in their mechanical properties, in particular, their impact strength and tear strength, due to the fact that the starch gives up or absorbs water until it reaches equilibrium with the ambient humidity at its interface.
In relatively low humidity conditions, the material tends to become brittle, as the dispersed phase becomes insufficiently plasticised due to the loss of water which takes the glass transition temperature above ambient temperature.
This phenomenon can damage the interface with the matrix when the interface is not sufficiently bonded.
Under these conditions, when the starch particles constituting the dispersed phase are subjected to stress, they are unable to deform and absorb the stress, but instead remain rigid, thus initiating a tear.
Italian patent application No. T096A000890 filed by the Applicant describes compositions comprising thermoplastic starch and a thermoplastic polymer incompatible with the starch, having improved characteristics of resistance to ageing under conditions of relatively low humidity, obtained by introducing an agent having an interfacing action during the mixing of the components. This compatibility-inducing action improves the adhesion between the matrix and the dispersed particles.
Reducing the interface tension also enables the dimensions of the particles to be reduced to submicronic values, whereby the materials have the characteristics of a polymeric alloy. Compositions comprising starch, a thermoplastic polymer and a plasticiser are widely described in patent literature.
However, the concentrations of these plasticisers at which the mechanical properties of the compositions are greatest are never taught, nor suggested, in the prior art.
EP-A-0 327 505 describes compositions in which the plasticiser is used in a quantity of from 0.5 to 15%, preferably between 0.5 and 5% by weight, together with such quantities of water that the sum of the plasticiser and the water does not exceed 25% by weight of the compositions (the quantitative minimum of water in these compositions is 10% by weight).
WO92/19680 describes compositions comprising starch, a polyester of a hydroxyacid or the corresponding lactone such as, for example, polycaprolactone, and a plasticiser usable in a quantity of from 1 to 50% by weight, preferably 1-40%, and more preferably 5-25% by weight of the composition.
The compositions preferably have a final water content of between 1.5 and 5% by weight (measured on leaving the extrusion press, before conditioning).
In the aforementioned document, there is no use of nor any indication of the existence of a possible critical range of the concentration of the plasticiser corresponding to that for obtaining very high mechanical properties, nor is there any indication of which plasticisers are suitable for this purpose.
The quantity of plasticiser used in the examples is always greater than 10% by weight of the composition.
U.S. Pat. No. 5,334,634 describes compositions comprising starch, an ethylene-vinyl alcohol copolymer and a plasticiser usable in a quantity of from 0.5 to 100% by weight of the starch.
In this case also, the quantity of plasticiser effectively used is always greater than 10% by weight of the composition.
It is known that starch, in particular, its amylose fraction, forms “V”-type complexes with synthetic polymers such as polyethylene vinyl alcohol or polyethylene-acid acrylate (C.Bastioli and others in “Biodegradable Plastics and Polymers”, pages 200-213; 1994, Elsevier Science). In such multiphase systems in which the synthetic polymer comprises the continuous phase and the starch the dispersed phase, the complex acts as a compatibility-inducer or phasing agent.
Similar complexes can form between starch and aliphatic polyesters or aliphatic/aromatic copolyesters. However, if, in the preparation of the compositions comprising starch and the aforementioned polyesters, relatively high quantities of the starch plasticisers are used to ensure the plasticity of the material under the conditions of use of the manufactured product and low specific energy for destructurization and complexation is used, the quality of the interface is insufficient to ensure the toughness of the material at low temperatures and humidity in the presence of the plasticiser itself.
Furthermore, if plasticisers which are solid at room temperature are used in relatively high concentrations, at which the complex between starch and incompatible polymer can form in a quantity sufficient to ensure an effective compatibility-inducing action, these plasticisers cause, in conditions changing from high to low relative humidity, brittleness in the material.
It has unexpectedly been found that it is possible to prepare heterophasic compositions comprising starch and a thermoplastic polymer incompatible with starch, in which the starch constitutes the dispersed phase and the thermoplastic polymer the continuous matrix, which compositions have characteristics of high impact strength even when passing from conditions of high to low relative humidity if they are prepared using a quantity of plasticiser that is liquid at room temperature comprised within a critical range wherein the concentration of the complex between starch and the incompatible polymer reaches a maximum, and a specific energy of destructurization of starch higher than a certain value.
The critical quantity of plasticiser, which is preferably glycerin, is generally from 2 to 8% and preferably from 3 to 7% by weight of the starch and the thermoplastic polymer. Quantities outside this range are, however, possible, depending on the type of plasticiser and its efficacy.
The specific energy for the destructurization of the starch and its complexation are comprised from 0,1 to 0.5 Kw.h/Kg, preferably from 0,15 to 0,4 Kw.h/Kg and most preferably from 0,2 to 0,35 Kw.h/Kg.
For specific energy for the destructurization and complexation of the starch it is to meant the energy supplied by an extruder the screw or screws of which are capable of developing a specific energy of at least 0,1 Kw.h/Kg at the extrusion temperature of 120-210° C.
The specific energy is determined according to the formula: A×B×C/D×E×F wherein
A=engine power
B=RPM
C=energy absorption
D=RPM max
E=energy absorption max
F=flow rate
Until now, critical values as indicated above had never been used nor suggested in prior art compositions.
It has been discovered, and this constitutes a characterising aspect of the invention, that the complex of starch and incompatible polymer reaches maximum concentration values within the aforesaid critical range.
The presence of the complexes of starch and incompatible polymer can be demonstrated by the presence in the second derivative FTIR spectra of a band at a wavelength of 947 cm-1 (specific to the complex) and in the X-ray diffraction spectra of a peak in the range of 13-140 on the 2 theta scale (with Cu K
alfa
radiation of 1.5418 A°). In both cases, the position of the band or the peak of the complex remains unchanged, even on changing the nature of the complexed polymer.
FIGS. 1 and 2
show the X-ray and second derivative FTIR spectra, and are typical of the formulations based on starch and aliphatic polyesters (PCL in particular).
It has been found that in the X-ray spectra of the compositio
Bastioli Catia
Bellotti Vittorio
Del Tredici Gianfranco
Guanella Italo
Lombi Roberto
Bryan Cave LLP
Cain Edward J.
LeeKat Wynozebshi
Novamont S.p.A.
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