Dry ice: how much is too much?

Dry ice is solid carbon dioxide. At normal atmospheric pressure and temperature it sublimates (changes directly from a solid to a gas), resulting in a tremendous increase in volume.

To calculate the amount of pressure that occurs in a 0,5L PET bottle when a specific amount of dry ice is inserted and then heated up, we can apply the  Van der Waals equation, which is a revised version of the ideal gas equation, used to more accurately describe the behavior of real gasses.

Therefore, if, for example, we have a PET bottle of V=0.5L and room temperature 

T= 20° C +273= 293K, then 3gr of dry ice are going to produce 3.23atm pressure.

According to design specifications, a PET bottle for carbonated products can hold up to 13,6 atmospheres of pressure before it bursts. This level of pressure can be reached if 13,3 grams of powdered dry ice are inserted in the bottle.

The maximum level of pressure that a bottle can hold depends of course on the thickness of the PET vessel (ex. 0.5 liter Sprite PET bottle, 0,45 mm thick has burst pressure =13.6atm) and the configuration of the bottle (ex. a five-footed bottle base gets less distorted than a four-footed base). Nonetheless, 13gr of dry ice can be the starting value for the experiments.

Performance of 20g and 22gr 0,5L bottles in 4- and 5-foot base variations

Source: Raising the bar in PET bottle lightweighting - WRAP

 

 

 

 

 

 

 

 

 

    

Blowing up bottles: the procedure

First thing to be tested is the performance of the PET bottles. The procedure of this experiment was defined with the guidance of Dr.Ir. F.A.Veer, Associate Professor of Materials Science in the Faculty of Architecture, TU Delft.

Thus, the experiment can be organised in the following series of steps:

1. Define the dosage of dry ice the PET bottle can support
a. Start with extreme dosages to determine the maximum amount of dry ice the bottle can support 

b. Lower dosage to observe the reaction of the bottle and the form it takes 

    when subjected to different amounts of pressure (if the bottle curves extensively it will 

    cooperate less with its adjacent bottles)

Step 1

2. Test various types of bottles in terms of shape

a. Bottles of rectangular section will probably cooperate better than those with a circular 

b. Bottles' elevation, again in terms of contact surface between the bottles

1. Step 2

3. Check leakage and therefore for how long will the bottles be stable

a. Add dry ice in the bottles and close them normally. 

    Measure daily for a week’s period the diameter of the bottles to estimate possible leakage 

b. Heat-seal the bottles and repeat the experiment

Step 3

 

4. Test the bottles in compression (different types of bottles and different dosages)

Step 4

 5. Place a number bottles together and test them under lateral compression 

Step 5

Prototyping and testing: series of steps and questions to be answered

To investigate the behavior of the floating module, a series of prototypes will be produced and tested in compression. First step is to identify the properties of the core and work towards its optimization in terms of strength and lightness. Once satisfactory results are reached, the optimized core will be laminated with a layer of FRP and, consequently, the properties of the composite module will be defined, through compression tests.

More specifically, the research will be organized as following:

By this stage the questions that need to be answered are summarized as such:

 Core

          

              PET bottles:

o      Grams of dry ice needed

o      Compressive strength of the carbonated bottled containing the dry ice

o      Packing method (vacuum membrane, band, adhesive?)

o      Should the void space between the bottles be filled?

o      Relation/Connection with the FRP layer

              PET Foam:

o      Is the lowest-density version of this foam material (80kg/m³) suitable?

o      Relationship with the exterior FRP layer (needs protection? Infusion holes in foam?)

o      Relationship with the bottles’ caps- should a sheet layer be placed in between?

Then the second phase contains the core with the surrounding FRP layer:

 Issues to be determined by this stage:

FRP layer

            Resin:

o      Resin to be used (in terms of strength, sustainability-recycling, durability)

o      Total thickness of the material

o      Volume shrinkage? Absorption from

             Fiber reinforcement:

o      Fiber reinforcement to be used  (in terms of strength, sustainability-recycling)

o      Type of fabric

o      Volume percentage of reinforcement

o      Direction of reinforcement

             Gelcoat:

o      Is it necessary?

o      If so, what kind of gelcoating should be applied

Floating Component

o      Compressive and bending strength

o      Total mass. How can the optimum strength-mass ratio be achieved?

o      How is the highest part (more foam) behaving in comparison with the lowest

When the stage of prototyping and testing is over, digital simulations will be required, to further understand the behavior of the platform as a whole:

Both for the physical tests and the digital simulations a list of criteria will be made, for the assessment of the different alternatives.