Plates varying in thickness between 1-10 mm (0.04-0.4 in) were molded and subsequently annealed at elevated temperature (halfway between the glass transition temperature Tg and melting temperature Tm, where the crystallization speed is at its maximum). This heat treatment was done for 16 hours, which is believed to represent a substantial amount of crystallinity (although it's known that higher levels of crystallinity can be achieved with additional heat treatment). Nitrogen atmosphere was used to avoid potential oxidation of the samples.

Parametrization of the various commercial polyamide grades is based on extensive water uptake and release experimentation in the temperature regime 23 - 120°C (73-248°F).  The majority of plates was put in a water bath, some were put in a conditioning chamber. Weighing the plates reveals how much moisture was absorbed over time. In dry conditions, desorption was studied in the same way.

Crystallinity

Semi-crystalline plastics typically comprise both an amorphous phase in which the polymer chains are arranged randomly as well as a crystalline (ordered) phase. Only the amorphous phase can take up moisture. However, the level of crystallinity is not a fixed material parameter, but depends on the processing conditions and ageing. After injection/blow-molding and cooling (‘dry as molded’) the plastic has not yet reached its maximum crystallinity. For some applications a state below this maximum offers advantages to the application, in other cases the fully crystalline state is desired. Dry-as-molded samples can absorp more moisture compared to annealed (or aged) materials. Over the lifetime of a part, the crystallinity usually increases (and moisture absorption decreases), but to speed up this process ‘annealing’ (heat treatment for a short period of time) can be applied.

Because the crystallinity depends amongst others on processing conditions, environmental conditions and age, it's impossible to present a single value as absolute truth for our materials. Instead, our tool provides both a prediction for the moisture content in dry-as-molded samples as well as  a prediction for annealed (under the specified conditions) samples. Unless extreme annealing is applied, a part's moisture content will most likely be in between these two predictions.

On data sheets, normally the ‘dry as molded’ values are provided. For PA6, PA66 and PPA the ‘annealed’ equilibrium moisture content can be 10-20% lower, for PA46 even a factor 2 lower compared to the dry-as-molded samples.

This question can be answered by a simple moisture uptake calculation, assuming that the part can be approached by a flat-plate geometry. Start by selecting the requested grade from the dropdown (or search the grade by typing its name) and provide the necessary input:

• Sample geometry = Infinite plate
• Plate/sample thickness = 3.2 mm (0.13 in)
• Initial temperature = 23°C (73°F)
• Initial humidity = 0% (after molding a part does not contain any moisture, this is called ‘dry-as-molded’)
• Imposed temperature = 23°C (73°F)
• Imposed humidity = 50%
• Maximum diffusion time = 100 hrs (start with a small time to speed up the calculation)

The Moisture evolution graph shows that 100 hours is not enough to reach equilibrium in this part. Click “Edit calculation” icon in the legend to increase the maximum diffusion time to 9000 hours and re-calculate by “Update calculation”.

• The graph displays two lines, the solid for ‘dry-as-molded’ samples and the dashed ‘annealed’ line for samples with a higher crystallinity. Higher crystallinity can be achieved by applying a heat/moisture treatment (see the Measurements tab for more info) but naturally also occurs as a part ages. For Stanyl® (PA46) this effect is bigger compared to other polyamides. In practice, a part’s state will probably fall between these two model predictions.
• After 9000 hours the Moisture evolution graph ‘as-molded’ is almost flat, meaning that equilibrium is reached (for annealed samples this is already the case after 4000 hours).
• In theory, the moisture concentration could be anywhere between the two lines. However, because no extreme conditions were applied the part is likely to be close to the ‘as-molded’ line with 2.6wt% moisture.

To add a graph for the accelerated conditioning at 70°C(158°F)/62%RH, the same procedure can be followed: Either select the same grade and fill in the required fields, or start from the previous calculation using the “Edit calculation” button (all input fields are now pre-filled) and click “Add new calculation”.

• Sample geometry = Infinite plate
• Plate/sample thickness = 3.2 mm (0.13 in)
• Initial temperature = 23°C (73°F)
• Initial humidity = 0% (after molding a part does not contain any moisture, this is called ‘dry-as-molded’)
• Imposed temperature = 70°C (158°F)
• Imposed humidity = 62%
• Maximum diffusion time = 9000 hrs

The Moisture evolution graph shows that the equilibrium condition is reached much faster; after 2000 hours for dry-as-molded samples. The equilibrium moisture concentration is slightly higher in this case, around 3.0wt%. Because the conditioning temperature and humidity were higher compared to the standard conditioning method of 23°C(73°F)/50%RH, the crystallinity might be slightly higher (reducing the moisture content a little bit). Although the moisture content is not necessarily identical for both conditioning methods, both will result in similar mechanical properties.

A possible solution to the problem would be to define the following case and calculate the average steady state moisture content within the part. The case represents a situation that during a day (24 hours) the car is driving from 9am to 10am and the rest of the day the car is on the parking lot. During driving conditions, the polymer part is exposed to a temperature of 100˚C and 0% RH and during parking conditions the part is exposed to a temperature of 23˚C and 100% RH. These daily conditions are applied in a repetitive cyclic daily regime for 50 days (1200 hours). If longer time frames are needed, please contact our experts for an off-line calculation (due to time-out limits of the web interface) 

In figure 1a, the water concentration profile is plotted over the thickness of the part at the end of each day (cycle) for specific days. After the first day, the two outer slabs of the polymer part already contain moisture while the inner part of the sample is still dry. With an increase of the number of days, the situation in the outer slabs is not really changed. For the inner part however, it is observed that the moisture content is gradually increasing over the number of cycles and has reached an almost constant value after 50 days.

Figure 1: a) Moisture profile over thickness and b) overall moisture content development of a ForTii® Ace MX53B (glass filled PPA) part after 1, 8 ,27 and 50 days exposition to cyclic conditions. One cycle represents one day. Cyclic conditions are specified in the text.  

Figure 1b shows the overall average moisture content in the part is displayed during the daily rhythm for various days. The drying out effect of the outer slabs each day during driving conditions is observed as well as the water uptake during parking conditions. This daily rhythm of water exchange is maintained during the total period of 50 days, and what is clearly observed is that the overall water content is increased with the number of days and is hardly changing any more for long periods of time.

This overall water content in the polymer part is displayed as a function of time in Figure 2. The daily rhythm is visible as well as the water contents levelling off after approximately 100 days to an end value in the range 1.0-1.3 wt%. It should be noted that this end value is not equal to one of the equilibrium values belonging to parking or driving conditions, but it is a value somewhere in between these two equilibrium values. This end value and its distribution over thickness is relevant since it will affect the mechanical performance of the part.