Microspheres: How to Evaluate New Applications

In order to evaluate the suitability of microspheres in general or specific grades for a new application, the thermal, physical, and chemical compatibility of the microspheres to the manufacturing process and end use of the product must be considered.

a- Thermal Compatibility

Microsphere grades are available in a wide range of operating or expansion temperatures.  The ranges are defined by T start which is the lowest temperature at which the microsphere shells begin to soften to allow expansion, and Tmax, which is the temperature at which the maximum expansion is achieved for very short exposure times  (i.e. 1 – 3 minutes).  Short exposures at temperatures above T max or long exposures at temperatures below Tmax but above T start will cause rupturing of the microspheres.

Microsphere grades can be grouped into several classes for ease of discussion.  In the table below, single point temperatures have been selected for the T start and Tmax ranges just to give a rough idea of the temperatures involved; they are not exact numbers by any means.  A hypothetical example illustrating the use of these numbers is included in the Appendix.

Fig. 1.  Microsphere expansion temperatures / softening temperatures (deg C•)

The table in Fig. 2 shows the melting point of selected thermoplastic materials.  For a microsphere grade and a thermoplastic to be thermally compatible, the melting point of the plastic must be below the T start of the microsphere so that they can be initially mixed without causing expansion.  Since PET, PTFE, PEN, and polyimide all have melting points above the T start of even the highest temperature microsphere, they cannot be used together.   On the other hand EVA, PE, PP, and some PVC materials have melting points below the T start of some medium temperature microspheres and all high temperature grades, so they are compatible.

Fig. 2. Melting temperatures of selected thermoplastic materials

b.Physical Compatibility

This refers to the resistance to physical stresses such as pressure and shear.  There are several stages in which microspheres exist, which greatly influence their resistance to physical forces.

• Unexpanded microspheres (by definition at temperatures below T start). These are by far the strongest microspheres and the most resistant to pressure and shear forces.  However, even these cannot survive the high pressure and shear of twin screw melt extruders.
• Expanded microspheres below T start. These can be handled with low shear equipment.  Ribbon mixers and paddle stirrers work well.  Cowles blades do not.  Some grades of expanded microspheres are stable for short periods to static pressure up to 2000 psi.   Some grades have been added to mixes which have been spray applied with pump pressures in the 1500 – 2000 psi range.
• Expanded or expanding microspheres at temperatures between T start and T max. This is the most sensitive stage since the polymer shells are soft enough to be easily deformable.  Of course typically the microspheres are encased in some matrix resin which tends to reinforce the thin walls of the microspheres.
• Expanded microspheres above T max. No physical force is needed.  The microspheres will rupture from temperature alone.

c. Chemical Compatibility

This refers chiefly to solvent resistance.

In general, microspheres are incompatible with:

• Low molecular weight ketones – MEK, acetone
• Low molecular weight alcohols – methanol, ethanol
• Low molecular weight amide – DMF (dimethyl formamide)

Some grades have some level of incompatibility with:

• Alkyl phthalates – di-Butyl, di-Octyl
• Cyclohexanone
• Ethyl acetate
• Higher temperature grades tend to be more resistant to these types of solvents

Microspheres are generally stable to:

• Water
• Higher molecular weight alcohols and glycols (IPOH, ethylene glycol and above)
• Alkanes
• Aromatic hydrocarbons (toluene, xylene)
• Chlorinated hydrocarbons (chloroform, perchlorethylene)

Other Considerations

• Dispersion Stability

Because of the large difference in density between microspheres and any liquid mixtures to which they are added (0.1 g/ml vs 1.0 g/ml), the microspheres will tend to separate out quickly if the viscosity is below 15,000 – 20,000 cps.

• Flammability

Expanded microspheres coated with 65% or more by weight of CaCO3 are classified as non-flammable by the standard transportation test. They can support a flame but not propagate it quickly.  The same is true for expanded microspheres coated with 35 – 40% of ATH.

• Uncoated expanded microspheres and those coated with less than 65% by weight of CaCO3 are flammable. At lower mineral levels there is a measurable explosion hazard.
• Dualite E130-FR, which is 85% by weight ATH instead of CaCO3, is non-flammable and will not support a flame. However, the fire retardant properties of the ATH coating are largely offset by the inherent flammability of the underlying microspheres, so there is little residual fire retardancy left to impart to the bulk material to which it is added.

Appendix

Example using thermal properties

Assume a hypothetical grade of unexpanded microsphere with T start of 160 – 170 C and a T max of 210 -220 C.   This would fit nicely in a process where the unexpanded microspheres are mixed into a coating solution at temperatures below 160 C.   The coating solution is applied to a moving web which is dried in an oven set at 230 C.  The microspheres see a temperature of 200 – 210 for a minute or so while the coated layer is still pliable just before it dries.    After leaving the oven the web temperature drops to near room temperature with its new coating of well expanded microspheres that are now in a stable temperature range.

Conclusion

Choosing the right microsphere for your specific application involves considering several factors.  Following these guidelines will help you better understand the use of microspheres in your process and products.  This will increase your chances of success and save you time and money by making the right decision up front.

If you have any questions regarding your microsphere project, please contact Chase Corporation for an expert opinion