The opportunities in oriented polypropylene (OPP)

Starting in the early 80s, oriented polyporopylene (OPP) was hailed as a replacement for PET every 5 years or so, soon to be pushed to the margins again and again. The latest revival started about 5 years ago when new clarifiers promised to take care of some of the problems that were formerly associated with it.

Let's review the advantages of OPP that would make it attractive to brand owners:

• OPP's density is 30% lower than PET's (0.9 vs. 1.33 g/cm³)
• OPP's moisture barrier is 3 times better than PET's
• OPP does not need to be dried
• OPP can be hot-filled up to 95 °C (203 °F)
• OPP used to be cheaper than PET by 20%

Looking over this list, many in the industry imagined lightweight containers at a reduced cost, both because they were lighter and the material was cheaper. However, the recent price increases in raw materials have hit the polyolefins group of materials, of which PP is a member, harder than PET.

The reason for this is that polyolefins are C2 materials, i.e., 2 steps removed from the resources they are made of. PET is a C4 material, as the additional solid-stating is independent of raw material prices. Therefore, polyolefins have almost doubled from 0.38 USD per pound to 0.70 USD per pound, whereas PET has gone from 0.55 USD per pound to about par with PP today.

That leaves material savings because of the OPP's lower density. Unfortunately, though, OPP's top load strength is far lower compared to that of PET at the same wall thickness. To be able to pack, store, and transport OPP bottles successfully, the wall thickness has to be increased to the point where the weight is about the same, if not higher than that of a comparable PET bottle.

That leaves OPP's ability to be hot-filled. While PET can be hot-filled to the same temperature, it must undergo a special heat-set process. This requires more costly machinery and molds and is usually associated with cycle time penalties.

Unfortunately, most hot-filled products, such as juices and functional drinks, also require good to excellent gas barriers. OPP's gas barrier is about 30 times less than PET's, thus making it unsuitable for these beverages. The low gas barrier even creates problems for any product that might easily pick up flavors (such as diesel fumes) from the outside. It is for that reason that we will most likely never see water being filled in OPP bottles.

When it comes to processing, there are other disadvantages to OPP. PET processes in the 100 °C (212 °F) range with a process window of about 15 °C (27 °F). Whereas, OPP needs to be heated up to about 135 °C (275 °F) and has a process window of about 4 °C (7 °F). This poses several challenges depending on the process used.

• Single-stage process: It is often difficult to retain enough heat in the thinner parts of the preform as the melting point of OPP is close to 150 °C (302 °F). Some machines feature heaters in the conditioning station to especially heat the inside of the preform before blow. Even so, conditions have to be monitored closely to ensure the process stays in the tight window OPP demands.
• Two-stage process: In this method, the standard machines are not up to the task. OPP does not accept infrared heat the same way as PET. If lamps are turned to high output, OPP's outside skin will burn without penetration to the inside. Therefore, lamps have to be turned to lower voltages (50% and lower), requiring 2 to 3 times as many ovens as PET. While many rotary machines allow some additional ovens to be added, the standard machines can process OPP only at a reduced throughput.

When machines are modified to allow for the required number of ovens, speeds similar to those achievable for PET are possible, if not quite the same. Thin-walled PET preforms and bottles can be run at 1,800 bottles/cavity/hour, and some processors have achieved 1,400 b/c/h with OPP containers.

One caveat here is that the finished container must have very even wall thickness in all parts. Thicker parts, like the bases in soft drink bottles or thick shoulders that some containers have for extra top load strength, pose new challenges. This is because OPP cools at a much slower rate than PET. A 1 mm (0.040) wall section will not cool at the short cooling time that comes with high machine speeds and will then shrink after demolding.

This can be compensated for in the mold, but usually requires extensive testing that only a few brand owners are willing to accept. Machine speeds may have to be dialed down to 600 to 800 b/c/h to accommodate the thicker wall sections.

There are two groups of containers that can benefit from using OPP. These include hot-filled containers and pill bottles. Let's understand each one in detail.

Hot-filled containers

The first group requires OPP to be fitted with a barrier layer of some sort. Internal barriers, such as are used with PET (Nylon or EVOH), have been used successfully with extrusion blow-molded PP but are difficult with OPP. This is because the melting points of OPP and the barrier material are so different that adhesive layers have to be sandwiched between the two. 6-layer technology is used in extrusion blow molding for this purpose, but it seems too costly for injection molding. 

Companies are working to overcome this, but so far without success. The more promising way seems to be to devise a coating. Several companies are working on ways to dip-coat bottles or even preforms to boost gas barrier performance. The challenge is considerable, as even PET with its much better barrier performance is not suitable for many products without additional barrier protection. The coating process can also not cost more than 10 USD per 1000 bottles to stay competitive.
 

Pill bottles

The second application is readily available. Pill bottles require an excellent moisture barrier to guarantee long shelf life in a typical bathroom's humid environment. Here, many of these bottles are stored after purchase. OPP fits the bill nicely. The small bottle size poses little top-load challenges. OPP's excellent moisture barrier allows for some light weighting compared to PET.

In summary, while there are other niche applications for OPP, it will remain barred from large volume beverage applications until the gas barrier challenges have been solved in an economical way.