packaging materials 7. metal packaging for foodstuffs - Europa

Pa c k a g i n g ma t e r i a l s: 7. m e t a l pa c k a g i n g f o r f o o d s t u f f sClosuresClosures can be divided into many groups and the internal coating requirements will depend on thesubstrate, closure design and pack. Depending upon the type of closure and product, jars may be steamevacuatedand the closure twisted on. Processed foodstuffs, be they in cans or jars, normally will be undervacuum once they have cooled down. This helps in the preservation of the product by limiting the amountof oxygen in the headspace. Whilst it is difficult to give specific examples the following acts as a generalguide.Vacuum closuresLug Caps (Twist-Off® type) normally contain a plasticised PVC gasket and may be used with almost anyfood pack ranging from jams to pickles to vegetables in oil. For jams and preserves, a single coat of anepoxy phenolic lacquer or an organosol is adequate, while pickles demand at least a two-coat system. Inthis instance, an epoxy phenolic basecoat is normally used followed by an organosol topcoat to give PVCgasket adhesion. Plasticised PVC gaskets need additives in order to prevent their degradation, ensurehermetic seals and aid the application. There have been some issues in Europe concerning migrants fromPVC gaskets as regards the plasticisers, the blowing agent and other additives used.Baby-food closures can be sealed with either a PVC gasket (PT, push-twist type) or a rubber ring,depending on the closure design. Full retorting (sterilisation) properties are required in addition to productresistance. Normally an epoxy phenolic basecoat is followed by one or two coats of an organosol.Push-fit closures for preserves normally have a single coat of vinyl in order to provide the required gasketadhesion and resistance properties. Where high SO 2content (>20 mg/l) packs are being used, either adouble coat of vinyl solution or a single coat of organosol could be used.For screw closures for dry food, gasket compounds are not generally used and are usually replaced by acardboard or composite wad. Protection from microbiological contamination by the use of a hermetic seal(needed during processing of the contents of wet foods) is unnecessary for most dry powders; they onlyrequire an airtight seal. An internal coating is often present to prevent tool wear. A typical example in thisclass is coffee closures, which are normally coated with an epoxy phenolic gold lacquer.Aluminium closuresRoll-on closures (RO) are the most important group of aluminium closures and can be further subdividedinto pilfer-proof and non-pilfer-proof groups. A typical example of a roll-on closure is that used onbottles of spirits. The choice of coating will obviously depend upon the product being packed. A phenolicacrylic lacquer is used with a full gasket (covering the whole food contact surface) for carbonated drinks,whilst for high acid resistance either a two-coat system (epoxy phenolic + vinyl) or a three-coat system(two coats thermosetting vinyl + one unmodified vinyl) is normally used. In the above examples, thelacquers are not in direct contact with the beverages.The majority of pilfer-proof closures are used on spirit bottles and have a wad in place of a PVCcompound. Their size can vary from those used on miniature bottles to those on deep-skirted ones.Generally, unmodified vinyl lacquers are successfully used either pigmented with titanium dioxide or asuitable dyestuff. Vinyl coatings of this type have no water resistance (i.e. they will not pasteurise). Whenpasteurising properties are required, for example with beer, a PVC liner would be used in conjunctionwith a phenolic/acrylic lacquer. This combination of properties is never required on deep drawn roll-onpilfer-proof (ROPP) closures.Pre-threaded closures are often used on pharmaceutical packs and may require sterilisation (they are notnormally lined with a gasket). A thermosetting vinyl system is commonly used for this application.22