Effects of moisture

As with any other polyamide, Stanyl absorbs moisture reversibly due to the presence of the amide groups in the molecular chain. Moisture absorption is dependant on the temperature, the relative humidity of the environment, and the wall thickness of the specific part.  In general moisture absorption results in a decrease of the glass transition temperature (see graph below), which may lead to an increase in toughness and reduction in stiffness and strength at room temperature.

This drop in stiffness for Stanyl is small compared to the drop for other polyamides due to Stanyl’s high level of crystallinity. The performance above the glass transition temperature (75°C) is not affected by moisture uptake. As Stanyl is typically used at higher operating temperatures, the effect of moisture will not be noticed.

Competitive materials such as semi-aromatic polyamides have a higher Tg, often in the operating temperature range. A shift in Tg due to moisture uptake will in this case lead to a change in properties at the critical operating temperatures.

In addition, due to this higher Tg, higher mold temperatures are required, resulting in the need for oil or electrically heated molds, with higher safety risks, higher mold and maintenance costs, and more difficult processing.

For prolonged exposure above 100ºC, Stanyl dries out, especially rapidly at higher temperatures, and properties will approach those given by the "dry" curve. This leads to a consistent property profile over a wide temperature range, especially once the effects of annealing are taken into account.

Shear modulus of glass fiber reinforced thermoplastics.

Moisture uptake leads to dimensional changes.  However, because highly filled compounds are used in many applications this dimensional change is limited. Due to glass fiber orientation, dimensional changes mainly take place in the direction perpendicular to the flow direction (thickness of the part, see tables below).

Dimensional change as a function of moisture uptake of non-flame retardant grades.

Note:
orientated part: thickness 2mm
not-orientated part: thickness 3-4 mm

Dimensional change as a function of moisture uptake for flame retardant grades.

This is the direction that in terms of dimensions is often the least critical. The effect of moisture on dimensions is small compared to the dimensional change due to temperature changes (Coefficient of Linear Thermal Expansion - see table below).  Stanyl exhibits excellent performance in many applications where dimensions are very critical, including many small connectors or SMT components. For E/E applications where dimensional stability is very critical special flame retardant, reinforced grades have been developed: 46HF5050 and 46HF5041LW.

Typical CLTE values for Stanyl grades.

Moisture absorption usually takes place at room temperatures. This is a rather slow process taking a long time before equilibrium is reached. When using the application at operating temperatures, which for Stanyl parts is often above 100°C (210°F), drying is extremely fast. This means that full saturation is very often not seen in many applications (see figure below) and effects of moisture uptake are often limited.

Water absorption at 23°C/50%RH followed by desorption at 180°C of Stanyl (3.2 mm thickness sample).

Annealing significantly reduces moisture uptake
Moisture absorption is significantly reduced upon annealing of Stanyl. Annealing results in densification of the amorphous part of Stanyl upon exposure at high temperatures (> 100°C). This phenomenon is unique for Stanyl and is irreversible. Annealing takes place during operation at elevated temperatures in for instance automotive applications. Annealing may result in a moisture uptake reduction by a factor three. Annealing can also be used as a separate step to improve the dimensional stability of Stanyl parts (preferably using a nitrogen atmosphere). Moisture uptake reduction depends on the annealing time and temperature. DSM has developed a model to quantify this, please contact your local sales engineer for further information.

Properties such as stiffness, strength, fatigue, creep and abrasion resistance are generally improved upon annealing while toughness might be slightly reduced although at a level that still outperforms competitive materials. This leads to a strongly improved property profile for applications such as gears.

Reduction of water uptake of Stanyl GF and competitive materials at several anneal conditions.

Blistering  
Moisture uptake at extreme humidities may lead to blistering during soldering at very high peak temperatures close to a material’s HDT. This is a phenomenon not unique to Stanyl but is also observed for other polyamides and even for LCPs. Blistering can be prevented by protection from moisture, and optimized processing.

• Keep parts under dry circumstances as much as possible.
• Record the actual molding date to control the time to re-flow process.

More information on blistering in connectors.

Moisture control during processing
During melt processing, a high moisture content may lead to the occurrence of silvery streaks or splash marks on the surface of the final parts. In extreme cases it may lead to degradation of the base polymer resulting in a drop in viscosity. To prevent this Stanyl granules are supplied dry in airtight, moisture proof bags. Should your Stanyl material come into contact with ambient air for extended periods, moisture will be absorbed and it should be dried prior to processing.

Drying

• Dryer should be a dehumidifying dryer or vacuum dryer (Not hot air oven)
• Material before molding should contain below 0.1% moisture.
• Regrind material also should be dried before molding.
• Regrind material size should be uniform as much as possible.
• Content of regrind material should be controlled.

Molding

• Temperature at the nozzle should be controlled precisely.
• Frequently check the wear of sealing ring at the front.
• Tool temperature should be set between 80 to 120°C (175-250°F).
• Barrel temperature setting should be set 310 +/-10°C.  
  (could be different for High Flow grades, please check processing)
• Keep residence time in screw as short as possible.
• Keep screw RPM as low as possible.
• Backpressure should be set around 5 kg/cm2.
• Injection speed should be set as fast as possible.
• Select suitable holding pressure and time.
• Suckback should be set as low as possible.
• At the start purge should be done sufficiently.

Tool design

• Sprue/runner/gate should not be too small.
• Top of the submarine gate should be “R” design.
• Gas release should be set sufficiently at the edge.
• Cooling the tool should be uniform especially for the core.