1. Physical Polymer Chemistry of PET ISBT Bottle and Closure Committee
Eric Morrison
Ecolab Food and Beverage R&D
October 18, 2005

2. Bottle Failure – Cause and Effect (Ichikawa) Diagram

3. Physical Polymer Chemistry of PET
Objective: Characterize physical changes occurring in beverage bottles during filling, processing, and storage
Physical aging
Hydrolysis
Crazing (Volynskii crazing)
Three physical phenomena occur in filled beverage bottles and containers made of PET: physical aging, hydrolysis, and crazing.

4. Physical Polymer Chemistry of PET
Resolution of Physical Aging, Crazing, and Hydrolysis
time, temperature
physical aging
surface active model compounds
crazing
The three physical phenomena can be resolved and studied separately
hydrolysis, stress cracking
alkalinity

5. Physical Polymer Chemistry of PET
Resolution of the Effects of Aging, Hydrolysis, and Crazing
The relationship between crazing and bottle failure needs further investigation
Crazing is sometimes believed to initiate, nucleate, or facilitate hydrolysis (stress cracking), however:
There often is observed a negative correlation between extent of crazing and bottle failure
It is important to understand the relationship between the three physical changes on PET. For example, the role of crazing in bottle failure is not well understood. By separating the effects out, it is possible to determine whether crazing for example promotes or suppresses cracking and failure.

6. Physical Polymer Chemistry of PET
Resolution of the Effects of Aging, Hydrolysis, and Crazing
Jabarin – previous investigations of stress cracking did not separate the effects of crazing and hydrolysis (used alkaline surfactant) work includes investigation of neutral nonionic surfactants and glycol ether compounds.
Moskala – investigated stress cracking under conditions very different than what bottles typically see (0.2% NaOH).
Ecolab – investigation of neutral nonionic surfactants and glycol ether compounds on bottles.
Some effort is now underway to resolve these phenomena. Jabarin’s earlier work did’t separate alkalinity from crazing, because the surfactant studied was by nature alkaline. Currently the PET consortium is investigating nonionic surfactants separately and with NaOH.
Earlier work has included nonionic surfactants with and without caustic. Note the difference between caustic (i.e. 0.2% NaOH) and alkalinity. There is an opportunity to do a related study with conditions that are more coincident with real life bottling conditions.

7. PET - Physical Aging Loss of polymer “free volume” below Tg
Causes PET to become brittle
Occurs in amorphous, unoriented areas
Evaluated by DSC (Differential Scanning Calorimetry )
relaxation endotherm is observed in aged samples
Evaluated by Physical Property measurement
Strain to failure, i.e. elasticity of the polymer

8. DSC Relaxation Endotherm
PET - Physical Aging
DSC Relaxation Endotherm
This DSC trace shows heat flow into and out of a PET specimen. The peaks are due to heat flow into the sample (negative peaks) and heat flow out of the sample (positive peaks). The large positive peak is due to crystallization of amorphous PET. The negative peak at the Tg (about 75C) is the relaxation endotherm. The relaxation endotherm is directly related to the extent of physical aging.

9. PET - Physical Aging Bottle Burst Testing Burst Strength
Percent Expansion

10. PET - Physical Aging
Physical Property Measurement Agr•TopWave Plastic Pressure Tester (PPT/BR3000)
Relaxation Endotherm = 2.2 J/g
Relaxation Endotherm = 0.5 J/g
Physical aging causes brittleness, but not a loss of strength. This slide shows how the strength of bottles changed upon aging for 28 days aging at 100F and 85%RH. There was no change in average burst strength. However, there was a loss of elasticity. Virgin bottles exhibit ductile failure (70 – 100 % volume expansion in the AGR Topwave burst test), while the majority of aged bottles exhibited brittle failure (10-20% volume expansion in the burst test). The extent of physical aging can be assessed by DSC, the relaxation endotherm values for the bottles is shown in labels.
What this slide shows is physical testing on bottles that is analogous to strain to failure values for polymer coupon samples.

11. Alkalinity and PET Hydrolysis
More work on this topic is required
Different types of alkalinity behave differently
Hydroxide solutions will stress crack bottles
It appears that a change must occur for bicarbonate solutions to stress crack bottles (e.g. become more concentrated and/or change chemically)
Stress cracking can occur with bicarbonate containing solutions that have been neutralized to near pH 7
Is stress cracking better understood from a perspective of pH (i.e. concentration of hydroxide ions) or from a perspective of the nucleophilicity of anions (hydroxide > carbonate > bicarbonate)
This understanding is required to relate results from the “caustic stress crack test” (hydroxide alkalinity) to real life (bicarbonate alkalinity)
Hydrolysis is the destructive force that causes bottles to fail. Although the most significant environmental contributor to bottle failure is well know to be alkalinity, a better understanding of the mechanistic details is possible. For example, although alkalinity in the field is primarily bicarbonate, in model lab systems bicarbonate solutions do not cause hydrolysis and stress cracking, so hydroxide solutions are often used instead. For bottle testing it appears that bicarbonate alkalinity even at high levels will not break bottles until a change occurs (increase in concentration and/or chemical change). A better understanding of the relationship of bottle failure rates to lab tests such as ISO 6252 and the caustic stress test is possible.

12. PET - Hydrolysis P P H2O alkalinity P P
Hydrolysis is the destructive force that causes bottles to fail. Hydrolysis of ester bonds in chemistry text books is catalyzed by acids and bases, however in real life bottling environments, only base catalysis is important. The most significant environmental contributor to bottle failure is alkalinity. P P

13. PET - Crazing May be referred to as Volynskii type crazing
Surface active liquid stabilizes colloidal size fibrils of oriented polymer, preventing necking
Can be distinguished from stress cracking (hydrolysis) by SEM analysis
Crazing is a change in the physical appearance of polymers. It results from interaction of surface active liquid with surface area created by strain or expansion of the polymer. It occurs in amorphous regions. Crazing is also referred to as Volynskii crazing to distinguish it from other types of appearance changes. It is characterized by the presence of fibrils in what appear to be cracks in the polymer.
When polymers are stretched, the force aligns polymer molecules into fibrils which have dimensions on the colloidal scale. If there is no surface active liquid present, the fibrils coagulate forming a clearly defined neck. If a surface active liquid is present, the surface of the fibrils are stabilized and they remain separated.
The relationship between Volynskii type crazing and failure is very unclear. It has been suggested that crazing is a precursor to failure in some cases, while in others it has been suggested that it opposes macroscopic failure. Most people believe that crazing simply does not correlate either positively or negatively with failure. An investigation that resolves separate contributions of physical aging, hydrolysis, and crazing is expected to provide important information about the relationship of Volynskii crazing to failure.

14. SEM: Stress Cracking vs Crazing
Experts in PET bottle testing can sometimes tell visually the difference between stress cracking (caused by hydrolysis) and crazing. However, an analytical tool is required to make objective analyses, to provide corroboration for visual inspection, and to allow participation by individuals not expert on visual grading. SEM imaging is one such tool. This SEM photograph shows a stress crack formed by aging a filled CSD bottle in a PET compatibility testing using alkaline water compared to that of crazing caused by the presence of a surfactant mixture.
Stress cracking (hydrolysis)
Crazing

15. Physical Polymer Chemistry of PET
Resolution of Physical Aging, Crazing, and Hydrolysis