Summary of Part I

In part I, I discussed the general types of thermoplastic polyesters, noting that polyethylene terephthalate or PET is a slowly crystallizing polymer that has a melting temperature of about 260 oC. It is considered a likely candidate for many high-temperature applications, particularly since it is relatively inexpensive when compared with other high-temperature polymers such as polyamides. Foamed crystalline PET is considered a candidate for higher temperature thermal insulation applications. Part II of this Technical Minute focuses on the desired properties of foamed PET.

Packaging Applications

Crystalline polyethylene terephthalate is called CPET in the written literature. When the polymer crystallinity is in the 20% range, the polymer is tough at room temperature. At temperatures greater than the glass transition temperature of about 70 oC, it is pliable but retains its shape until temperatures in excess of about 200 oC. CPET at 40% crystallization level is quite brittle. Unfoamed CPET has been thermoformed into “TV dinner trays” for more than a decade, where the tray is exposed to “recommended” convection oven temperatures of 175 oC to 180 oC for up to 60 minutes without deterioration. However, the traditional unfoamed tray suffers from several limitations:

  • It is difficult to keep PET from crystallizing during extrusion when the sheet thickness is in excess of 0.060 inches or 1.5 mm. As a result, thermoforming is restricted to relatively shallow shapes. This obviates certain applications, such as bakery containers, which require relatively deep containers.
  • In addition to the restriction on the depth of draw, container stiffness becomes a problem when the side walls of the container becomes too thin. Keep in mind that flexural stiffness, S, is given as:S = EIwhere E is the modulus of the polymer and I is the moment of inertia. For a thin beam, b in width and t in thickness, the moment of inertia is given as:I = bt3/12It is apparent that stiffness is proportional to the container wall thickness to the cube power. For economy, it is desired to keep the wall thickness as thin as possible. Therefore the container wall can become quite flexible, particularly above Tg, where the PET modulus drops dramatically.
  • When unfoamed PET is thermoformed, two types of crystallization occur. The first, thermal crystallization, dominates, effectively locking the shape into its final configuration until the product temperature is raised to the PET melting temperature of about 260 oC. However the PET sheet is also biaxially stretched, and a second type of crystallization, orientation crystallization also occurs. Certainly, the level of orientation increases as the draw depth increases. This added crystallization level may make the CPET product more brittle than tough.
  • And, in a typical food container application, the product placed in the CPET container is flash frozen at temperatures of about -30 oC. PET at this temperature is 100 oC below its glass transition temperature. It is brittle, much like PS is at room temperature. As a result, special care must be taken to minimize “rough handling” such as impact during filling, flash-freezing, and shipping of frozen product. Otherwise, the product may be compromised by broken corners and split sealing regions.

Impact and Foaming

Even though it is well-documented that biaxial orientation of a brittle polymer increases its toughness, it is also well-known that the impact strength of a brittle polymer does not necessarily increase by foaming. However, foaming does alter the nature of flexural impact failure. Foamed CPET fails by crushing under impact, instead of failing catastrophically, as unfoamed CPET does. That is, individual CPET membranes bend, then break under impact, with impact energy being dissipated first to the intersections of the membranes, then to neighboring cell membranes. As a result, even though many membranes rupture under impact, the foam structure remains essentially intact, thereby protecting the product. This effect is well-known for polystyrene foam, where the container of low-density polystyrene foam is used to protect refrigerated eggs, for example.

However, one must be careful to recognize that the transition from catastrophic failure to non-catastrophic, crushing damage occurs at relatively high foaming levels. For example, polymeric structural foam, where density reduction usually does not exceed 50%, fails under impact in much the same way as the unfoamed polymer. Density reduction of more than 50% and usually 90% is needed in order to achieve crushing rather than catastrophic failure.

Stiffness and Foaming

As noted above, container stiffness is proportional to the container wall thickness to the cube power and to the modulus of the polymer to the first power. At the same container weight, foaming increases the effective wall thickness of the container. But foaming decreases the effective modulus of the container. It is well-known that the modulus of a foamed polymer decreases in proportion to the square of the foam density:

Ef = Eof / ρo

At the same container weight, the sidewall thickness increases in inverse proportion to the foam density:

tf = tof / ρo) -1

Therefore, we can easily show that at the same container weight, the container stiffness increases with decreasing foam density:

S = Ef If = Eo Iof / ρo) -1

In short, by foaming, we can improve two critical aspects of container performance – impact strength, particularly at freezer temperature, and sidewall stiffness, particularly for deep-draw containers.

The real question that remains is: Can we produce a CPET foam having up to 90% density reduction?

The Practical Aspects of Foam Density Reduction

The foam industry has long believed that certain polymers foam to what has been called “natural densities”. For example, polystyrene and HDPE seem to foam well at a natural density of about 2.2 lb/ft3 = 35 kg/m3, whereas PP seems to foam well at about 10 to 15 kg/m3. PVC, on the other hand, is difficult to foam to densities less than about 100 kg/m3. And, it is thought, PET should also be difficult to foam to densities less than about 100 kg/m3.

The natural density belief has some basis in fact. The production of low-density foams involves, as the terminal phase of the process, the biaxial stretching of membranes. As discussed in detail elsewhere [JLThrone, Thermoplastic Foams, Sherwood Publishers, 1996 – see this Web page for ordering information], the available stretching force is directly related to the differential pressure between the cell gas and the environment of the forming foam. The resistance to the stretching force is viscoelastic character of the polymer film, which is experiencing strain hardening and rapid cooling. If the stretching force is too high or the resistance to the stretching force is too low, the film may rupture, which may result in catastropic foam collapse. If the stretching force is too low or the resistance is too high, the film may not stretch sufficiently, and the foam density may not achieve a desired low value.

Many polymers can be uniaxially stretched to 4X or more near but above their transition temperatures, before rupturing. This implies that foams of 30X density reduction are achievable before catastrophic collapse. For PS, for example, this predicts a foam density of 35 kg/m3. This further imples that for CPET, and PVC for that matter, foam densities of 45 kg/m3 are achievable, given the correct processing conditions. The key phase here, of course, is “correct processing conditions”. [Some processing conditions were discussed in Part I, but additional discussion will be found in an upcoming Technical Minute.]

So, to summarize this part, foaming of CPET to low densities for commercial container applications is justified by improved performance at low temperatures and improved stiffness for deeply formed containers.

What about thermal insulation? Does CPET have adequate insulating properties? Part III addresses this question.

Jim Throne

29 August 1997

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