“A newly developed thermal grease started separating right after shipment — and the customer reported a sharp increase in thermal resistance.”
A thermal interface material manufacturer attempted to optimize cost by blending low-cost dimethyl silicone oil with phenyl silicone oil. The result? Severe phase separation: filler sedimentation, paste hardening, and over 30% decline in thermal performance.
The root cause: silicone oils with different backbone structures are not automatically compatible. Blind blending can lead to catastrophic formulation failure.
This is not an isolated case.
In cost-reduction or performance-balancing efforts, some manufacturers attempt to mix Polydimethylsiloxane (PDMS) with Polymethylphenylsiloxane (PMPS). However, these materials differ fundamentally in molecular polarity and solubility parameters.
The introduction of phenyl groups significantly improves heat resistance and refractive index — but it also alters polarity and intermolecular interactions. If the blending ratio is not carefully controlled, common failures include:
Oil–filler separation during storage
Hardening and cracking after high-temperature aging
Reduced interfacial wetting and sharply increased thermal resistance
To minimize such risks, we strongly recommend conducting a basic compatibility screening before blending structurally different silicone oils:
✅ Method: Mix the two silicone oils at a 1:1 ratio. Add a standard filler (e.g., alumina), stir uniformly, and store at 60°C for 7 days.
✅ Evaluation criteria: No phase separation, no oil bleeding, no gelation, and viscosity change <15% — preliminary compatibility confirmed.
✅ Advanced validation: Use DSC or rheometry to analyze glass transition behavior and storage modulus stability.
“Not all ‘silicone oils’ belong to the same phase,” a senior thermal materials engineer emphasized. “PDMS is essentially non-polar, while PMPS gains partial polarity due to phenyl rings. Mixing them improperly is like trying to blend oil and water — unless copolymerization or compatibilizers are involved.”
Today, mainstream high-performance thermal greases typically adopt a single backbone system:
Standard CPU cooling: High-purity PDMS (lower cost, good flowability)
Automotive / power modules: Fully phenyl-based or phenyl-modified silicone oils (higher heat resistance, reduced pump-out effect)
If blending is necessary, pre-copolymerized base oils should be selected — not simple physical mixing.
Thermal conductivity is not just about loading more filler —
It’s about maintaining system stability.
