Method Requirements for Selecting Organic Chemicals Used in Thermal Fuses

Author: Wang Junyan

Selecting chemicals for organic thermal fuses (organic-type TCO) is essentially a balancing act between two conflicting requirements: reliable melting/breaking at the specified temperature, and long-term stability without degradation below that temperature. The specific requirements for each type of raw material are detailed below.

I. Core Requirements for the Primary Temperature-Sensitive Material

The primary material determines the tripping temperature and long-term reliability, and represents the most demanding part of the formulation in terms of performance thresholds.

A. Melting point must be "adjustable yet consistent"

The melting point of the pure substance itself should fall near the target Tf (typically 5°C to 15°C below the nominal Tf, with compensation provided by spring pressure and housing design).

If a blending approach is adopted (ternary system of acid + anhydride + ester), the phase diagram of the three components must be smooth, and the eutectic region should not be too narrow; otherwise, even slight batch-to-batch variations will cause the Tf to drift out of tolerance.

B. Phase transition must be "brittle fracture" – no stringing

After melting, the material must rapidly lose mechanical support, allowing the spring to push the slider/contacts apart. If the melt viscosity is too high and the material strings like syrup, it will slow down the tripping speed and may even cause re-sticking after opening (reset risk – which would directly fail certification).

Therefore, the primary material should preferably be a low-molecular-weight crystalline substance (such as benzoate esters, imides, or organic acid salts), rather than a high-molecular-weight melt – unless a specially designed low-viscosity system such as a block copolymer fluororesin (e.g., THV) is used.

C. No carbonization or crosslinking during long-term thermal aging

The material must withstand several thousand hours at 0.8 to 0.9 × Tf (as required by UL 60691 Th testing and aging validation).

If the primary material gradually undergoes oxidative crosslinking under prolonged heat exposure – turning brown and becoming rigid – the spring will not be able to actuate, and the fuse may fail to trip even when the Tf is reached. This is the most insidious failure mode for organic-type TCOs. Therefore, the primary material must have good thermo-oxidative stability, or the formulation should include an appropriate antioxidant – provided that the antioxidant does not compromise insulation resistance.

D. Sublimation rate must be controllable

Pure organic substances will slowly sublime at 80–90% of Tf. As the particles shrink, the spring preload decreases, potentially causing premature tripping.

The key requirement for the primary material is that its saturated vapor pressure must not be too high – typically below 10⁻³ Pa at 120°C. Otherwise, a pure-substance approach cannot withstand long-term operation, and a filler system must be employed as a compensatory measure. This is also why high-temperature organic-type TCOs (>200°C) are difficult to produce, and why above 240°C, the technology essentially gives way to ceramic-based sensing (fusible alloy) solutions.

E. Purity ≥98%, with strict impurity limits

Three critical impurity thresholds:

Ionic impurities (Cl⁻, SO₄²⁻, Na⁺, K⁺): Must be controlled to below the ppm level; otherwise, leakage current will surge under high temperature and high humidity, failing UL 1020 requirements.

Moisture: If the primary material absorbs moisture, the Tf will drift. Additionally, at elevated temperatures, hydrolysis can occur, producing acids that corrode the inner wall of the housing (copper or nickel-plated layer).

Metal particles: Ferromagnetic metal particles can cause micro-shorts at room temperature or result in unacceptable resistance readings after tripping.

II. Requirements for Fillers

Fillers may seem like a secondary component, but they ultimately determine whether the product can pass life testing.

Thermal stability > 1.2 × Tf: The filler must not decompose or outgas under long-term load; otherwise, internal pressure may compromise the seal.

Insulation performance: Volume resistivity must exceed 10¹² Ω·cm. The filler itself must be non-conductive – materials such as carbon black or metal powders are to be excluded entirely.

Particle size distribution: D50 is typically in the range of 2–10 μm. If the particles are too coarse, voids will remain in the pressed body; if too fine, agglomeration may occur. Additionally, the particle size should match that of the primary material to ensure uniform density after pressing.

Non-reactivity with the primary material: For example, calcium carbonate can slowly react with acidic primary materials and generate CO₂. Compatibility testing should be conducted in advance.

Low moisture absorption: Calcined kaolin and mica are preferred over regular talc, as moisture absorption leads to reduced insulation resistance and Tf drift.

III. Requirements for Lubricants and Binders

These two additives are used in small quantities, but failure often originates right here.

Lubricants:

Melting point should be above the maximum storage temperature but below Tf (typically 120–160°C; stearates and EBS are generally acceptable).

No surface migration: under long-term thermal aging, if the lubricant migrates to the ends of the housing, it can cause a drop in insulation resistance at room temperature.

Calcium stearate is more commonly used than zinc stearate – zinc salts tend to generate acids under prolonged湿热 (damp-heat) conditions.

Binders:

Must not leave high-viscosity carbonaceous residues after melting that could obstruct the mechanical mechanism (PEG and low-molecular-weight PA are more suitable than epoxy, which tends to carbonize and harden after curing).

Compatibility with the primary material: must provide sufficient "cold bonding" during pressing, but upon heating to Tf, must rapidly lose strength along with the primary material.

IV. Requirements for Pigments / Marking Agents

Only inorganic high-temperature-resistant pigments should be used (e.g., iron blue, chromium green, iron oxide series). Organic pigments will fade under long-term exposure to temperatures above 150°C, which would compromise the identification of different temperature ratings on the production line.

The addition level should be ≤2%. Higher loadings will affect the Tf distribution, since pigments do not melt and effectively act as an additional inert filler.

V. Chemical Qualification Checklist for Complete Formulations

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