Patent Application
20110283918
The present invention relates to coatings basically from natural macromolecules, with gas barrier properties that may be tailored in situ, intended as coatings for plastic substrates, especially films, sheets, and tri-dimensional objects, for many different applications, in particular the food and biomedical fields. More specifically, the invention concerns natural matrices (gelatin, chitosan, chitin, pectin, gluten, casein, zein, whey protein, carrageenan, guar gum, xanthan gum, alginate, starch, cellulose in its different forms) that can be blended with inorganic substances (metal alkoxides) and synthetic polymers (polyvinyl alcohol, polyvinyl acetate, ethylenvinyl alcohol, ethylenvinyl acetate) to generate, through a well-defined and controlled process, hybrid structures as transparent coatings, characterized by very high barrier property against gases (oxygen in particular) and strong adhesion to the plastic substrate.
Safety and security are nowadays two major forefront topics, especially within those fields linked to the human health, such as the food and medical ones. In this direction, enormous efforts are continuously being performed in order to guarantee the best conditions in terms of hygiene and security of foodstuffs and medical devices. It is therefore easy to understand the pivotal role of the primary package in protecting the contained item against detrimental external factors like the microbiological contamination, the permeation of gases and vapors, which may contribute to accelerate some adverse reactions. For this reason, packaging materials are requested to possess the best performance in terms of barrier properties. This is especially true for the plastic packaging, since they exhibit poor general barrier properties when compared to glass and metal materials. Among the different routes pursued to enhance the barrier properties of plastics, depositing thin layers is one of the most valuable. This is because such layers, called coatings, are capable of improving the barrier properties of the plastic substrate even though applied as very thin layers (i.e., with thickness ranging from some hundred nanometers to few microns). At present, the most widely coatings used are those based on polyvinyl alcohol (PVOH), ethylenvinyl alcohol (EVOH), polyvinylidene chloride (PVDC), and acrylates. For all these solutions (marketed by specific trade marks) exist many references within the patent literature, and citing all of them would be a hard task. It is however worth noting that in all cases such coatings are obtained by a synthetic process on monomers of fossil origin. Only recently have been reported new solutions where the hydrocarbon structure is blended to an inorganic component.
PCT Publication (WO/2007/042993) describes the method to obtain a laminate structure based on a plastic film and a hybrid coating, i.e. made of an organic component (polyvinyl alcohol) and an inorganic matrix (metal alkoxide). The final structure is characterized by very high barrier properties against oxygen even at high external moisture conditions. Similarly, EP Patent 1 348 747 A1 discloses a method to obtain barrier coatings at high and low hygrometric conditions. In this case, the coating is made of polyvinyl alcohol, a metal alcoholate and a organosilane. US Patent 2002/0197480 describes instead a laminate structure including a gas barrier plastic film and a hybrid coating made of a polyvinylic resin, a Si-alkoxide hydrolyzed, and polyethylene oxide. Such a structure is claimed to have excellent gas barrier properties even at high humidity conditions. JP Patent 7126419 provides details on the obtainment of a laminate structure with excellent barrier properties against gases and water vapour, together with high resistance to humidity and heat. Such a structure is composed by three layers: a plastic polymer as a substrate, a metal alkoxide (or its hydrolyzed), and tin chloride. Also this structure is defined as hybrid due to the simultaneous presence of organic and inorganic matrices. JP Patent 62295931 describes materials and procedures to produce an inorganic coating having excellent gas barrier properties, obtained from an aqueous or either alcoholic solution of a metal alkoxide deposited onto thermoplastic resins such as PVC, PS, PP o PVA. PCT Publication (WO/2005/053954) describes a hybrid coating obtained by using a metal alkoxide and a synthetic polymer containing carboxylate groups (—COO−) partially neutralized. Such a structure exhibits very low gas permeability values regardless the external relative humidity conditions, even after retort process. Even with many merits, barrier coatings described in the literature are not exempt from some negative aspects, which dictate, especially for some specific applications, lack of universality. In particular:
First aim of this invention is to provide coatings having gas barrier properties with special features unknown to the prior art, thus able to afford some advantages in respect to the solutions nowadays available, overcoming in this way the aforementioned lacks of the prior art.
Other aim is to provide specific preparation methods, involving specific manufacturing and economic advantages.
Further goal is to provide specific applications, which would allow exploiting efficiently the functional properties of the invention, with special emphasis to the biomedical/pharmaceutical and food packaging fields.
The most important features of the invention are recited in the claims which are considered herein incorporated. Aspects and advantages of the invention will more clearly appear from the following description of the preferred embodiments which refer to
In particular,
Finally, in
This invention provides methods for obtaining coatings of natural origin with innovative properties and their applications on substrates. According to a feature of the invention, an optimal adhesion to the substrates is reached thanks to the selection of specific natural macromolecules belonging to the categories of proteins and polysaccharides. Among proteins, this invention foresees the following molecules: gelatin (porcine, bovine, fish), gluten, casein, whey protein. The use of pigskin gelatin as a protein is more favourable. Among polysaccharides, this invention envisages the use of the following molecules: chitosan, chitin, pectin, carrageenan, guar gum, xanthan gum, alginates, starch, and cellulose in its different forms. Preferably, pectin and chitosan are used as polysaccharides. Possibly, to obtain the coating subject of this invention, molecule of not natural origin may be used, such as ethylenvinyl alcohol (EVOH) and polyvinyl alcohol (PVOH) as structuring agent, preferably the first one, and a metal alkoxide as reinforcing agent. In this last case, the tetraethyl orthosilicate (TEOS) is preferred. The solvent will be always represented by water.
Among the many benefits arising from the new matrices, we only refer to those related to their characteristics and properties which are better than those of films without the invention (uncoated) and also existing synthetic coatings.
Indeed, when applied as a coating, the matrix of the invention:
According to a first feature of the invention, said matrices comprise:
According to another aspect of the invention, the natural component (protein or polysaccharide) content will be in the range 0.1% -20% (w/w) when used alone. If both bio-macromolecules are used, the specific amounts will be defined according to the following relationship:
y=f(Xprot) (1)
Equation 1 indicates that the amount of polysaccharide (y) to be used will always depend on the amount of protein (x). In one preferred embodiment the amount of polysaccharide is:
The other components would be used according to the specific quantities reported hereinafter:
The complement to 100% will be given by the solvent (H2O).
The easier and rapider method, thereby the preferred one, according to the invention to obtain the starting matrix (in the form of slurry) is as follows:
The treatment sub 1) assures:
The treatment sub 2) assures the total dissolution of the structuring agent (ethylenvinyl alcohol or polyvinyl alcohol), through the breakage of the hydrogen bridges between different —OH groups along the skeleton of the molecule.
The treatment sub 3) assures the partial hydrolysis of the reinforcing component, making it possible the replacement of part of the alkoxid groups with the alcoholic groups to form the silanol (Si—OH) species.
The step sub 4) guarantees the interaction between the different matrices blended together. Such an interaction is characterized by intermolecular forces, especially hydrogen bonds. The so obtained starting matrix (in the form of a sol), has to be obtained onto the plastic substrate, according to the invention. The method of deposition strictly relies on the specific substrate. For example, if the sol has to be deposited onto plastic films, the preferred way of deposition is the coating technique, which allows applying very thin layers in dry form. In this case, the sol will be placed in a large tank at controlled temperature (25° C.) and continuously transferred into a smaller basin (60 litres) by means of a dedicated pumps system. In this place, the sol is in turn moved to a rectangular basin of approximately 20 liters, where a metal engraved roll picks the sol up and spreads it on the plastic film, according to a well defined amount fixed by the number of engraves on the roll. The wet-coated plastic film undergoes at this point the drying effect of an array of infra-red lamps, immediately followed by a long slit where hot hair (90° C.) is fluxed through, so that the residual humidity within the coating is pushed out. In this way, any potential phenomena of blocking in the rolls are avoided. The operation speed has to be ≧150 m min−1, which is always higher than those reported in the prior art. Nevertheless, the coating according to this invention will result perfectly dried even at this speed.
In all those cases where the coating has to be applied onto surfaces different from flexible plastic films, one preferred method is the nebulization. For other items, the dipping method may represent the best way for the deposition of the coating.
The bio-polymeric matrix as described within this invention can find a wide variety of applications. Among them, those preferred are: 1) packaging of food items; 2) packaging of bio-medical devices. In both cases, the coating of this invention is intended as a replacement of the coatings commonly used so far.
In particular, the most characterizing feature of this invention relates to the possibility of modulate in situ the barrier property against gases of the coating once it has been deposited onto the plastic substrate, thanks to external triggers that modify the structure of the coating enabling to change the rate of permeation of gases and vapors. More specifically, in this invention the selected trigger is represented by the humidity of the surroundings. According to the procedure described previously, it is possible to develop coatings able to ‘sense’ more or less intensively the change in the values of the external relative humidity. The basic principle is illustrated in
The examples described below emphasize the surprising characteristics and properties of the edible matrix according to the invention. In all the examples, reference is made to six different coating formulations, as shown in Table 1, deposited onto different plastic substrates commonly used in the food and biomedical packaging fields.
The following analyses have subsequently been performed:
The thickness of the coating has been drawn by the mathematical equation:
G(gm−2)=d(gcm−3)−L (μm)
which relates with each other three different parameters: grammage (G), density (d) and thickness (L).
From the above relationship it is easy to obtain the thickness as follows:
L=G·d
−1
In practice, it is done by determining G by difference (the weight of 1 dm2 of coated film—the weight of 1 dm2 of uncoated film) and knowing that the density of the coating is equal to 1.05 gcm3. When possible, the thickness has been measured by a micrometer (Dialmatic Digital Indicator, Maplewood, N.J., 07040) to the nearest 0.001 mm at 10 different random locations.
Such analysis has been performed according to the Standard Method ASTM D1434-88, by using a permeabilimeter OPT 5000 (Dansensor, Denmark). Measurements have been done at controlled thermo-hygrometric conditions: (T=23° C.; RH gradient=0%, 50%, and 80%). Results from 10 replicates are expressed in ml m−2 24 h−1.
Transparency was determined according to ASTM D 1746-88 by using a spectrophotometer (Lambda 650-High performance-Perkin Elmer). In particular, the transparency of both uncoated and coated films was measured in terms of specular transmittance, i.e. the transmittance value obtained when the transmitted radiant flux includes only that transmitted in the same direction as that of the incident flux in the range 540-560 nm. The final values are expressed as a percentage (%) of the total incident radiation.
Table 2 summarizes the results obtained from the oxygen transmission rate and transparency analyses.
It is herein described the oxygen barrier property of a 2.0 μm thick coating totally obtained from a bio-macromolecule, namely gelatin from pig skin. As it can be see from
It illustrates the oxygen barrier property of a 2.0 μm thick coating totally obtained from pectin as a bio-macromolecule. As depicted in
This example aims at highlighting the characteristics of another bio-macromolecule, chitosan, in terms of barrier property against oxygen. As reported in
It is here highlighted the effect arising from the addition of a small amount (0.1 wt %) of ethylenvinyl alcohol as a structuring agent within the formulation according to the Example 2, where the bio-macromolecule used is pectin. As displayed in
This last example contains the relevant aspects arising from the simultaneous use of the natural, synthetic organic and inorganic components to produce the high gas barrier coatings. The composition of such a solution is reported in Table 1 (last row).
Even though this invention has been described with reference to specific and preferred examples, it has to be intended as not limited to the above examples, rather as a body that can be changed and improved through modifications that to be handled by the mean technician in the field, fall within the domain of this invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2009A000126 | Feb 2009 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IT2010/000016 | 1/25/2010 | WO | 00 | 8/1/2011 |
