As packaging engineers, it is essential to conduct thorough engineering analyses before proceeding with testing. In this blog, I introduce a straightforward method for calculating the Water Vapor Transmission Rate (WVTR). This simple analysis will assist you in selecting the appropriate materials and suppliers, as well as in designing effective blister tooling. Furthermore, you can utilize the calculated WVTR to predict moisture levels in product stability modeling.
We frequently conduct WVTR tests because bottles and blisters come in various sizes from different suppliers, and testing conditions can vary (25°C, 30°C, and 40°C). These tests are time-consuming as required by USP <671>, and the results often show large variations, making the process quite challenging.
One day, pressed for time, I calculated the WVTR of new blisters instead of performing the actual testing. To my surprise, the calculated values were reasonable, and later, I found that the calculated WVTR was close to the experimentally determined values. This was an eye-opener! I knew that molding and forming processes could change the orientation and crystallinity of polymer structures, which in turn could affect their barrier properties. But how significantly do these properties differ between flat sheets and molded (or formed) containers? I strongly recommend calculating the WVTR and comparing these values.
Every new material’s WVTR is determined and this information is readily available. You can visit MatWeb (https://www.matweb.com/) to find material properties, including barrier properties like WVTR, which are typically determined using flat sheets. I’m sure many of you have used these WVTR values to compare materials and select the right ones for your products. If your calculated WVTR values match the experimentally determined ones, the WVTR data in the MatWeb database can save you time and money.
Let’s delve into some relevant theories. The forming process can align polymer chains in specific directions (orientation), often improving barrier properties. Forming can also influence the crystallinity of the plastic. The heating and cooling cycles during thermoforming can alter the polymer’s crystallinity: rapid cooling post-forming can lead to more amorphous regions, while slower cooling can increase crystallinity. Generally, increased crystallinity enhances barrier properties.
WVTR Values:
- WVTR of Honeywell ACLAR® UltRx 2000 CTFE Homopolymer: 0.0190 g/m²/day at 25°C and 60% RH
- WVTR of 10 mil PVC: Approximately 50 times greater than Aclar (considered only as a dust cover with minimal moisture barrier properties)
- WVTR of 10 mil PVC/2 mil Aclar: 0.0190 g/m²/day at 25°C and 60% RH
Specifications for a blister cavity:
- Area of Unformed Film: determined by CAD
- Thickness of Unformed barrier Aclar Film: 2 mil (0.0000508 m)
- Area of Thermoformed Blister: determined by CAD
Given that the amount of material used for forming remains constant, the overall thickness of the formed blister can be estimated using the following calculation:
Thickness of Formed Blister:
Thickness of formed blister (m) = (Area of unformed film × Thickness of unformed film) / Area of formed blister
WVTR of formed blister:
WVTR of Aclar film based on the area of unformed blister: A mg/(Area of unformed blister)/day at 25°C and 60% RH
WVTR of formed blister: A * (ratio of area between unformed and formed) * (ratio of thickness between unformed and formed)
Key Understanding:
It is important to recognize that the WVTR calculated from the average thickness can represent the WVTR of an uneven thickness blister. WVTR is directly proportional to the area and inversely proportional to the thickness. Therefore, even though the formed blister may have areas that are thinner and thicker than the average thickness, the overall WVTR should remain consistent.
By maintaining the same amount of material through the forming process, the average thickness calculation provides a reliable estimate of the WVTR for the formed blister. This approach ensures that the calculated WVTR accurately reflects the barrier properties, despite variations in thickness across the formed blister.