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New Avenues for OPP

PET Planet Insider, No.9/04
Improvements in material and machines make OPP more attractive to brand-owners

Twelve to fifteen years ago Oriented Polypropylene (OPP) was hailed as the future choice of packaging material, replacing glass, polyolefins, and PET. That was when Norman Gottlieb of Container Corporation of Canada (CCC) of Richmond Hill, Ontario, Canada began thinking about what it would take to transform this idea into clear plastic bottles that would be equivalent or better than their PET counterparts. Ten years ago he started working on the concept in earnest and only recently have he and his team put the final touches on the first commercially available reheat stretch blow machines that are able to do exactly that. Named Enviroclear® and marketed by Blowmolding Parts & Services Inc.(BMPS) of Concord, Ontario, these machines could be on their way to make the long promised inroads into the packaging market for OPP.

The Pros and Cons to OPP

Both advantages of and problems with OPP are well known. On the positive side, OPP has a 30% lower density than PET and offers weight savings accordingly. Combined with lower resin costs, brandowners have long been considering this process to lower their costs. OPP does not need to be dried and therefore does not incur all the problems and expenses associated with it. The processing of OPP does not create any acetaldehyde, a major advantage for water bottlers.
 


32 Cavity Enviroclear preform mould


32 Cavity Enviroclear preform mould injection side

OPP can also be hot-filled up to 100ºC without any gram weight increase as is customary with PET bottles. And OPP has a five times better moisture barrier than PET, which can be beneficial to both liquid goods (keep moisture in) and dry goods (keep moisture out). Powerful arguments indeed and the question arises why OPP bottles do not fill the shelves of our supermarkets at this time.

The answers to this questions are many-fold and encompass material, processing techniques, and the very specific requirements of packaged goods. Starting with the latter, PET has a nine times better carbonation retention and we therefore will not see any carbonated drinks in an OPP bottle. Most hot-filled products also need a good oxygen barrier to avoid discoloration and vitamin loss. PET is often not good enough a barrier but is still 30 times better than OPP.

A Difficult Process

Besides the challenges that the packaged goods provide, the stretch blow process for OPP requires very tight control of parameters. While PET offers a rather generous process window of ±8º C, the same variable for OPP drops to ±2º C and lower for some bottles. As Earnest Sollberger, Vice President of Technology, has found out, day and night factory or cooling water temperature cycles affect PET only when it is run at the edge of the process window but can increase the scrap rate during OPP production beyond a tolerable limit. OPP has often worked well in a laboratory type environment only to fail miserably in a standard factory setting.
When it comes to OPP preform processing cycle times can be considerably longer compared to PET for the same bottle. This is partly due to the slower heat transfer rate of OPP and partly because OPP preforms are generally shorter and therefore have thicker walls. A new material additive is poised to increase the output in future machines. This also means that reheat stretch blow machines need longer oven sections to properly reheat the material.

Finding the Right Resin

There have been a fair number of materials over the years but not all of them lend themselves to day-by-day production. Since mechanical strength of OPP is less than 60% of that of PET, the ethylene content of OPP must be carefully controlled to give containers enough top load. Even so, end users should not expect a 30% weight reduction from PET as one would assume looking at the densities of the two materials. It will depend on the application how much lighter, if any, the equivalent OPP bottle will be. Hot-fill bottles however, can always be expected to be lighter when comparing shrinkage values, i.e. if a PET bottle experiences 3% volume shrinkage during and after the hot-fill process, the equivalent OPP bottle with the same shrinkage value will be lighter. Not all materials have organoleptical properties that make them suitable for food packaging and it may take several weeks after production before these are noticed.

To get comparable or even superior clarity than PET bottles, the material contains clarifiers but OPP preforms must also be designed differently from PET preforms. An axial stretch ratio of 3:1 (unusually high for PET) is quite common, making the preforms more stubby. In order not to end up with too heavy a wall, OPP preforms are usually wider in diameter. Unoriented sections of the preform such as the neck and parts of the bottom stay rather opaque but are usually not visible to consumers at the point of purchase.
 

A Customized Machine

CCC was established in 1977 and employs currently 85 people. In order to overcome all the challenges as described above, CCC had to design a special machine, make preforms for other end users, and make bottles as well. It took ten years and the result is a unique opportunity for small and medium-sized applications.

CCC designs the machines but has them built by partners in the Far East to stay cost-competitive. There are currently four machines available, named Enviroclear®. A linear machine features four cavities and a typical output of 3,200 bottles/hour. It contains 7 ovens. Two rotary machines with 10 and 16 cavities are also available with outputs of 10,000 and 16,000 bottles/hour respectively. Last but not least to mention is a 2-cavity, linear wide-mouth machine, ready for testing in the fall. All machines can of course be used to run PET as well OPP, giving molders extra flexibility.

Preforms are heated up rapidly in the first two or three ovens and the other ovens are used to precisely heat condition them up to a temperature of 130ºC. Because OPP is quite opaque in the preform stage, infrared light does not easily penetrate them and reflectors are virtually without a positive impact.

CCC has experimented with a patented hot air heating entering through the preform neck but still experience difficulties with neck overheating. This latter issue is challenged by the fact that OPP’s blowing temperature is only 10º C below the melt point, whereas with PET there is at least a 150ºC difference. The higher blowing temperature also requires a longer cooling cycle in the blow mould, reducing the output of CCC’s machine to less of what can be accomplished with PET. Cooling improvements will narrow that difference in future machines.
 


Precise heating in the oven section is key to success in OPP blow moulding


Heating section


Take-off section

And the Oxygen Barrier

One exciting development for CCC is a coating process currently in the testing phase. It is said to enhance the oxygen barrier to nine times that of PET and would indeed open up a whole new range of applications for OPP. Instead of spraying the solution on, bottles are dipped into it and instantly dried with the help of a UV catalyst. Therefore non-round bottle shapes, that are difficult if not impossible to spray-coat, do not pose any problems. Projected cost is just over US 1¢ per bottle, which places it at the bottom cost range for barrier improvements.

CCC still has a long way ahead to find the best niches for its products and improve machine and process further. Dried goods as well as jams or peanut butter may offer the best opportunities at the moment and the up-coming wide-mouth version of the machine will explore these possibilities. Reduced injection and blow cycles coupled with higher-cavitation machines will create even more markets.

The scope of improvements in machine and process technology as well as material has excited the industry. OPP holds the promise of lighter and lower-cost containers, always a welcome sound to the ears of the packaging industry.


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