PROTECTIVE COATING COMPOSITION

FIELD OF THE INVENTION

The invention relates to the field of protective coatings or waxes derived from fats and oils. In particular, the invention relates to the formulation of an improved coating suitable for paper-based containers, foodstuffs and the like. BACKGROUND TO THE INVENTION

Various emulsions are in common use as coatings in a number of industries. Typically the purpose of the coating is to provide or promote one or more of the following properties to the substrate:

• water resistance;

• structural strength;

• chemical inertness.

An application where such properties are desired is the manufacture and use of cellulosic fibre (paper or board) products (e.g. cardboard boxes/shippers) as containers for food produce. Cellulosic fibre is used in the construction of these containers because it is relatively cheap and the manufacturing technology is well developed, making the resulting containers very cost competitive.

However, these paper board containers have particular drawbacks when used in particular applications as these materials tend not to be particularly strong when placed under compression (such as when stacked), and they become very weak when wet, as they have very little natural water resistance.

Accordingly, it has been found that coating of the surface of such paper board can reduce or eliminate these problems. For this purpose, there are two broad groups of wax-based coatings known in the prior art: water-based emulsions and oil-based emulsions.

Water based emulsions, such as those disclosed in US Patent Document No. 20070068642 by Borsinger et al, which include between about 55% - 75% water are well known and can be used for coating paper board products, as well as foodstuffs such as fruit. They are known to provide reasonable water resistance and crush-resistance to paper board. They are also identified as being suitable for paper board which is to be repulped, as this is an important criterion for such products as the rate of paper board recycling increases.

However, one drawback of this type of emulsion is that the process of drying will tend to either increase manufacturing time or require the input of heat energy to speed the drying process. Also, the water may soak into the cardboard or paper substrates making it difficult to remove. If the water is not removed properly during processing it may remain in the substrate: reducing its strength or it may cause defects in the film (such as cracks) if it evaporates after film is fully cured. In addition, water may migrate to the surface during storage causing the film to become wetted.

The water-based emulsions tend to produce softer and less cohesive films and therefore less durable/handleable films. Water-based films tend to be more porous and therefore less water resistant. Also, water-based emulsions will have a higher surface tension and therefore a greater tendency to bead, ciss or pull- away from the packaging creating defects or weak spots in the film. The higher surface tension may lead to poorer adhesion which could lead to delamination or flaking away.

Oil-based emulsions presently known in the art, which include no (or negligible) levels water in their formulation, tend to have very good moisture resistance. However, they are typically less successful in providing other desirable properties, such as good handleability or repulpability. They also tend to be prone to other undesirable effects, such as the formation of fat 'blooms' at the surface of the coating, which is the result of some of the fats coming out of emulsion and crystallising at the surface.

However, fat based emulsions are preferred in some circumstances as they can provide a stronger film, are not subject to water loss from the emulsion and can be applied via a 'curtain coating' process.

'Curtain coating' is understood by those skilled in the art as a process traditionally used to coat flat objects. A reservoir of coating is formed into a 'curtain' of free-falling fluid using either a slot or a slide in the underside of said reservoir, as illustrated schematically in Figure 1. This curtain falls under gravity until it hits the substrate to be coated, which is conveyed underneath said curtain. This coating method tends to ensure that all peaks, valleys, grooves etc of packaging receive same film build. Due to the higher surface tension of typical

water-based systems, the coating may not flow into valleys or grooves in the packaging, leaving uncoated areas.

The above discussion centres on coatings for paper-board containers, however, these coating may be used for other purposes, such as directly coating foodstuffs such as fruit or vegetables, in order to promote aesthetics and improve shelf life.

Accordingly, it is an object of the invention to provide a fully fat- or oil- based coating composition which has fewer of the limitations associated with fat- or oil-based emulsions in the prior art. SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a coating composition incorporating: up to 97% by mass of a hydrogenated triglyceride having a melting point of greater than 23 ⁰ C; up to 10% hardened castor oil by mass; and incorporating less than 1 % water by mass. Hardened castor oil is also known as hydrogenated castor oil or as castor wax.

It has been found by the inventors that a coating composition according to the above has some significant advantages over the prior art. In particular: the incorporation of the hardened castor oil provides the coating composition with: increased durability (related to the higher overall melting point); resistance to the formation of surface blooms (i.e. precipitation of visible fats on the surface of the container); better handleability; and a noticeably better odour than tallow-based emulsions, whereas said tallow-based prior art coatings can impart an undesirable odour to the foodstuffs transported in containers coated with same.

The inventive coating tends to be more resistant to cracking, peeling, scratching, marring, marking, staining, crease/bend marks, stains (food, dust), tearing and deforming. The coating also helps to protect the integrity of any printing on the packaging, and the surface can be wiped down without adversely affecting the gloss level.

Preferably, said hydrogenated triglyceride is selected from the group comprising hardened palm oil, hardened palm kernel oil and hardened coconut oil. These sources have been found to provide the best results with regard to the advantages discussed above.

Advantageously, said hydrogenated triglycerides are incorporated in the coating formulation at a total of approximately 90% - 95% by mass, hardened castor oil is incorporated at approximately 5% - 10% by mass. This formulation has been found by the inventors to produce particularly good results with respect to the advantages described above.

Other additives may be advantageously added to the coating formulation, including wetting agents, flow-promoting agents and slip-promoting agents.

According to another aspect of the invention, there is provided a paperboard container coated with a coating composition as described above. According to another aspect of the invention, there is provided a foodstuff coated with a coating composition as defined in any preceding claim.

According to another aspect of the invention, there is provided the use of a coating composition, as described above, in the manufacture of a paperboard container or in coating of a foodstuff. Now will be described, with reference to specific, non-limiting examples, preferred embodiments of the invention. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a schematic illustration of the 'curtain coating' process known in the prior art. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following table indicates a typical formulation for a coating composition according to the invention. This particular formulation is adapted to be used as a coating for a paper board food carton.

Table 1.

The purpose of the additives is to protect the cartons and maintain their surface finish appearance and functional (protective) properties while the cartons are being repeatedly handled.

Castor oil is a vegetable oil obtained from what is commonly referred to as the castor bean (technically castor seed as the castor plant, Ricinus communis, is not a member of the bean family). Castor oil (CAS number 8001-79-4) is a colorless to very pale yellow liquid with mild or no odor or taste. Its boiling point is 313 ⁰ C (595 ⁰ F) and its density is 961 kg-rrf ³ . It is a triglyceride in which approximately ninety percent of fatty acid chains are ricinoleic acid. Oleic and linoleic acids are the other significant components.

Ricinoleic acid, a monounsaturated, 18-carbon fatty acid, is unusual in that it has a hydroxyl functional group on the twelfth carbon. This functional group causes ricinoleic acid (and castor oil) to be unusually polar, and also allows chemical derivatization that is not practical with most other seed oils. It is the hydroxyl group which makes castor oil and ricinoleic acid valuable as chemical feedstocks. Hardening of the the castor oil occurs where the C=C double bonds on the glyceride chains are hydrogenated, usually in the presence of a nickel catalyst.

In this particular example, the bulk of the coating is made up from hardened palm oil. Palm oil (and the alternative palm kernel oil) is composed of fatty acids, esterified with glycerol just like any ordinary fat. Both are high in saturated fatty acids, about 50% and 80%, respectively. The oil palm gives its name to the 16-carbon saturated fatty acid palmitic acid found in palm oil; monounsaturated oleic acid is also a constituent of palm oil while palm kernel oil contains mainly lauric acid.

Another alternative to hydrogenated palm oil is hydrogenated coconut oil. Coconut oil is a fat consisting of about 90% saturated fat. The oil contains

predominantly medium chain triglycerides, with roughly 92% saturated fatty acids, 6% monounsaturated fatty acids, and 2% polyunsaturated fatty acids. Of the saturated fatty acids, coconut oil is primarily 44.6% lauric acid , 16.8% myristic acid a 8.2% palmitic acid and 8% caprylic acid, although it contains seven different saturated fatty acids in total. Its only monounsaturated fatty acid is oleic acid while its only polyunsaturated fatty acid is linoleic acid.

Unrefined coconut oil melts at 24-25 ⁰ C (76°F) and smokes at 17O ⁰ C (350°F), while refined coconut oil has a higher smoke point of 232°C (450 ⁰ F). Among the most stable of all oils, coconut oil tends to be slow to oxidize and thus tends to be resistant to rancidity, lasting up to two years due to its high saturated fat content.

The additives Byk 306, Byk 307 and Byk 355 are supplied by BYK-Chemie GmbH, P.O. Box 10 0245, 46462 Wesel, Germany. Additives having similar properties can readily be obtained from other sources. A coating formulation according to the above was applied to sheets (200 x

300mm) of cardboard fruit boxes. The performance of this coating was directly compared with the performance of a typical water-based formula and with paraffin wax via a number of tests for coating quality well known in the art. The results are summarised in table 2 below. Table 2.

In summary, the sheets coated with the inventive formulation displayed comparable to superior performance in all test measures.

It will be understood by those skilled in the art that the above example is merely an illustration of one way in which the invention can be put into practical effect. Other embodiments will readily be conceived of which, while different in some details, nevertheless stand within the spirit and scope of the invention.