To evaluate how fast the gas can permeate throught the wall of the plastic container and cap (the cap is often made of PP.), Gas Barrier or Gas Transmission Rate is defined. Because the two main gases to affect the quality or shelf time of a medicine is oxygen and mositure (water vapor), the rates for oxygen and mositure are summaried here.
The name of Water Vapor Transmission Rate is WVTR. The name of Oxygen Transmission Rate is O2TR. The transmission rate depends on materials, but the tests were done based on the whole set of bottle and cap to study the actual packaging using situation. It is found the influence by sealing can be ignored when testing WVTR, and is very minor when testing O2TR. Roughly, the only the materials will determinate the transmission rate under the same condition.
For WVTR, the sort is PET > PE, PP > Glass. Typical condition of testing is 40C temperature and 75% humidity, which can make the difference more obvious. WVTR of PET is around 10 times than WVTR of HDPE in the left picture. Other studies show the increasing is 4 to 8 times. Therefore, we can conclude WVTR of PET is 4 to 10 times of WVTR of HDPE.
WVTRs of PP and HDPE are similar, and WVTR of LDPE is alittle bigger but still much lower than PET. Because low WVTR of PP, the influence of sealing by PP cap can be ignored.
Temperature is a big factor to affect WVTR PET. The WVTR of PET at 40C is 3 times of WVTR at 25C. For HDPE, the difference is not so obvious because the WVTR is small already.
For O2TR, the sort is PP, PE >> PET > Glass. The left upper table shows the O2TR of HDPE is 6 to 15 times of O2TR of PET. However, the value of O2TR under this condition is quite small so the result is not very accurate. The left down table and other studies show the O2TR of PP and PE is 40 to over 100 times of O2TR of PET. Therefore, we conclude the oxygen barrier of PET is much better than PE and PP.
Because the cap is PP and PP’s O2TR big, the influence of PP cap can not be ignored. However, the infulence is not big because the area of cap is relative much smaller than the bottle wall.
Temperature is also a major factor to affect O2TR. Some studies show O2TR at 40C is a few times of O2TR at 25C.
The factors to affect transmission rate include the materials nature, temperature, humidity, crystallinity, thickness and area.
Material nature. PET contains polar chemical groups, so diffusion of water in PET is faster than PP or PE which doesn’t include polar groups.
Temperature influence is obvious as summaried above. The influence of humidity on WVTR can also be observed although the infulence of humidity is smaller than temperature. It is interesting humidity can also affect O2TR. O2TR of PET decreases along with the increasing of humidity. It is explained water molecules occupy the channel of diffusion of oxygen.
Increasing crystallinity will imporve transmission rate. The typical example is glass with best gas barrier and better crystalinity than plastics. Simply understanding is good crystallinity make the molecules be grouped tightly. The O2TR of PP, PE is high because the plastic wall is semi-amorphous and porous.
Under same other condition, it is easy to understand transmission is more if the wall’s thickness is thinner and area is bigger. However, here it is not transmission RATE. It should be transmission VOLUME.
Finally, the exact infulence level of water and oxygen is simply calculated here according to the above results. The above researches show the amount of oxygen is 0.313ml, per 120ml bottle per day. Please note this amount of oxygen is gas volume so the molar quantity of oxygen is not so much. PP or HDPE bottle can also be used to pack those medicines with just a little air sensitivity, if glass or PET bottles can not be used. However, for PET bottle, 9.9mg of water was transmitted per 30ml bottle per day. It is a big quantity. The increasing volume can be 3% of the bottle capacity during 90 days. Therefore, WVTR of PET bottle must be considered carefully when packing the mositure sensitive medicines.