PETpla.net Insider 03 / 2016

CAPPING / CLOSURES 40 PET planet insider Vol. 17 No. 03/16 www.petpla.net enger is used. These are often made from cobalt salts, a material that is predisposed to bond with oxygen it finds in its environment. It is triggered by the high humidity during filling so that these closures can be stored for a reasonable amount of time (up to 12 months at moderate temperature/ humidity levels) without the scavenger being exhausted before it comes into play in the bottle. Oxygen may also be dissolved in the product as a result of the product coming in contact with it during transfer and filling, and even the PET bottle wall itself gives off oxygen (“matrix oxygen”) that may dissolve in the product. Properly working scav- engers almost immediately absorb oxygen from all these sources and so contribute significantly to maintain the product’s sensory properties. They continue absorbing oxygen for several months and therefore extend the shelf life of the product considerably. Material O 2 permeability coef- ficient (x10^6) [cm 3 cm/cm 3 dy bar] EVA 1,990 - 2,780 EVOH (100% RH) 2.16 - 4.33 EVOH (dry) 0.0276 - 0.187 HDPE 238 - 1110 LDPE 1030 - 1910 Nylon 6 7.87 - 11.8 Nylon MXD6 0.59 PET 22 PP 596 - 1030 Note: Units are (amount of permeant * wall thickness) /area * time * pressure difference Figure 4.19 Permeability factor for vari- ous materials. Note the dependence of EVOH on humidity. This is one reason why Nylon MXD6 has become so popular. A combination of pulp/polysan/ wax has proven successful in limiting oxygen ingress and is used for jars containing mayonnaise, salad dress- ing or horse radish. 4.4 Tamper-evident bands A tamper-evident band (also called pilfer-proof ring) is a short skirt of plastic underneath the thread that will disengage from the main body of the closure once the consumer unthreads the cap for use. The loose band is a sign that the bottle has been opened. There are many reasons why com- panies have increasingly resorted to using these bands. It discourages consumers from sampling the product; prevents criminals to insert nonprod- uct content into the bottle (and later suing the product company for dam- ages); and makes it harder, though not impossible, to replace the brand- name product with a generic one with- out the consumer noticing. The last practice had been such a problem in the alcoholics busi- ness that brand owners went to great lengths to get dispensing closures manufactured that were impossible to take off the bottle without destroying them. In the beverage business, caps have to be taken off, but at least there is some safeguarding against malicious action. All bands consist of a series of nubs oriented on a ring that itself has an interrupted connection to the cap above. When the cap is unthreaded, these nubs hold the band motionless against protuberances moulded into the bottle neck finish. At this point the band may either stay on the bottle neck or on the cap as a “pigtail”. Both solutions have their merits. Consum- ers usually prefer the band to stay on the bottle neck as it may interfere with the resealing of the bottle. Previously, the band was considered a challenge to recycling, but because relatively simple flotation devices that are able to remove them are part of every recy- cling operation, this no longer seems to be a problem. However, bands do have to stay on caps when it comes to refillable bottles as it is virtually impos- sible to get them off in an economical way. The unruly pigtail may also be a more obvious sign of tampering than the band on the neck. Pigtails differ in how easy they come off the cap. If it is easy enough for consumers to pull them off, they may end up littering up cities or, worse, being pushed into refillable bottles and very difficult to remove from there. There are two ways how a tamper- evident band may be accomplished. It can be either moulded right into the cap at the time of manufacture or slit afterward in a secondary operation. In the former case, the mould parts that form the ring interruptions are under- cuts in the direction of cap ejection and must be retracted inside the tool before ejection can take place. This adds costs to injection tooling. How- ever, these may outweigh the costs and operational headaches associ- ated with unscrambling, orienting, and slitting the finished caps in a special- purpose machine. 4.4.1 Construction methods There are two basic construction methods for pilfer-proof rings. The most common for the one-way PET bottle is the drop away system. For the returnable PET and glass bottles a detachable pilfer-proof ring is needed. In some countries law requires even one-way bottles to have detachable bands. This is because some people are under the misconcep- tion that it is necessary to separate the HDPE or PP closure from the PET bottle before recycling. However, this is not the case because today’s recycling systems are perfectly capable of sepa- rating all elements during the recycling process (Figs 4.20–4.29). Fig. 4.20: 1. closed position Fig. 4.21: 2. after 90° turning Fig. 4.22: 3. completely detached