1. What Does the 85/85 Test Actually Measure?

An insulating part inside a distribution enclosure, a chemical pump or an outdoor meter housing does not fail on the day it is installed. It fails slowly, as moisture works its way in.

The 85/85 damp-heat test — 85 °C at 85 % relative humidity — compresses that slow process into weeks. The industry convention is to run it for 1 000 hours or more. What the test really asks is not whether a material meets its datasheet on day one, but how quickly it stops meeting it.

As-received strength tells you where a material starts. Damp-heat retention tells you where it will be after years in service. That is why insulation parts for humid, condensing or outdoor service are usually selected on wet-state data rather than on catalogue values.

2. How Was the Test Run?

· Conditions: 85 °C, 85 % relative humidity, continuous.

· Sample: one hydrolysis-resistant BMC formulation, UL94 V-0 rated, in two wall thicknesses — 3.0 mm and 1.5 mm.

· Sampling points: as-received, then 240 h, 480 h, 720 h and 960 h.

· Properties tracked: appearance (discolouration, blistering, cracking); mechanical strength (flexural, tensile, impact); electrical performance (insulation resistance, surface resistance).

Two thicknesses were tested in parallel for one reason: to check whether a thinner wall amplifies the effect of damp-heat aging.

3. How Did Each Property Change Over 960 Hours?

Table 1 — 3.0 mm wall (V0), 85 °C / 85 % RH

Table 2 — 1.5 mm wall (V0), 85 °C / 85 % RH

Table 3 — Retention after 960 hours, relative to as-received

4. Reading the Data: What Held, and What Dropped

Flexural strength — essentially unchanged. The 3.0 mm sample moved from 151 MPa to 157 MPa (104 % of as-received); the 1.5 mm sample from 170 MPa to 165 MPa (97 %). Both stayed close to their starting level, and the thinner wall did not behave noticeably worse.

Tensile strength — down about 7 %. 59 → 55 MPa (−6.8 %) at 3.0 mm, and 71 → 65.8 MPa (−7.3 %) at 1.5 mm. The two thicknesses tracked each other closely.

Impact strength — the one property that clearly dropped. 61 → 43 MPa (−29.5 %) at 3.0 mm and 74 → 47 MPa (−36.5 %) at 1.5 mm. The original test reports describe this as a marked decrease, so it is not a rounding artefact.

One caveat matters more than the headline number. The 3.0 mm sample fell to 32 MPa at 720 h and then recovered to 43 MPa at 960 h. Scatter of that size means a single sampling point should not be quoted as a stable value — read the trend, not one reading.

Electrical performance — no order-of-magnitude change. Surface resistance stayed at ≥10¹¹ Ω at every stage. Insulation resistance moved within a 62–96 MΩ band (3.0 mm: 80 → 75 → 96 → 67.3 → 73.6 MΩ; 1.5 mm: 70 → 66 → 62 → 67 → 89 MΩ) with no downward trend. For a grade designed for hydrolysis resistance, this is the core result: the property damp heat usually destroys first is insulation performance, and here it did not move by an order of magnitude.

Appearance — progressive yellowing, no structural damage. Both thicknesses yellowed gradually, more so with longer exposure, but no blistering and no cracking appeared across the full 960 hours.

5. Why Hydrolysis, Not Heat, Drives Damp-Heat Aging

BMC is a thermoset. During curing, the resin forms a three-dimensional cross-linked network: the polymer chains are chemically locked together and cannot flow again, as a thermoplastic can. That dense network is the structural reason water struggles to penetrate the material.

Thermoplastics behave differently. Their chains are not chemically bonded to one another, so water enters more easily — and where the polymer contains ester linkages, as polycarbonate does, water attacks them directly. The result is hydrolysis: chain scission, loss of strength, and a tendency to embrittle.

So the 85/85 test is not really asking whether a material will fail. It is asking which internal structure resists water for longer. For the same reason, an outdoor meter enclosure is usually specified on damp-heat behaviour rather than on as-moulded strength. The material-selection side of this argument is covered in our note on hydrolysis-resistant BMC for chemical pumps, valves and underwater electrical parts.

6. What Does This Mean for Material Selection?

· Insulation-led applications in wet service (condensation, high outdoor humidity, chemical environments): 960 hours of exposure with no order-of-magnitude drop in electrical performance is solid supporting evidence.

· Parts that also take impact (assembly knocks, transport drops, repeated switching): plan a margin. Impact was the weakest of the mechanical properties, retaining 63–70 % over 960 hours.

· Load-bearing and dimensional-critical parts: flexural and tensile behaviour held up over the full exposure. Even so, structural design should follow the relevant design code and safety factors — these are material-level aging data, not design allowables.

· Predicting service life in years: this test cannot do it. It provides 960 hours under an accelerated condition, not a service-life figure.

Selection normally works in two steps: match the application to a material series, then confirm the specific grade and its parameters. Our Material page lists the series and their property ranges.

7. What Does This Data Not Tell You?

① Duration. The test stopped at 960 hours. It says nothing about behaviour beyond that point.

② Conversion. 85 °C / 85 % RH is an accelerated stress. It cannot be converted into a number of service years, indoors or outdoors.

③ Sample scope. One batch, one formulation, one sample per wall thickness, with no replicate statistics. Individual points scatter — the 3.0 mm impact reading at 720 h is the clearest example — so single-point values carry limited weight.

④ Unit convention. In the original reports, impact strength is recorded in MPa, whereas impact is more commonly reported in kJ/m². Quote the unit alongside the value.

The data is taken from Wenzhou Jintong's internal test reports. For supplier qualification or product acceptance, request the original test report and confirm that grade, wall thickness and colour match the parts actually supplied.

8. Frequently Asked Questions

Q1. What is the 85/85 damp-heat test?

A. A constant-condition aging test at 85 °C and 85 % relative humidity, typically run for 1 000 hours or more. It compresses the effect of long-term humid service into a few weeks, and is widely used as a selection criterion for outdoor, humid and condensing applications.

Q2. After 960 hours, how much does insulation performance drop?

A. In this test, surface resistance stayed at ≥10¹¹ Ω with no change, and insulation resistance moved within a 62–96 MΩ band with no downward trend. There was no order-of-magnitude loss.

Q3. How much strength is retained?

A. Flexural strength was essentially unchanged (104 % and 97 % of as-received). Tensile strength fell by about 7 %. Impact strength fell by 29.5 % (3.0 mm) and 36.5 % (1.5 mm) — the only property to show a clear decline.

Q4. Impact strength dropped by about a third. Is the material still usable?

A. It depends on whether the part sees impact in service. Where insulation and dimensional stability govern, the data supports continued use. Where a part takes assembly knocks, transport drops or repeated actuation, impact margin should be designed in.

Q5. Can these figures be used as design allowables?

A. Not directly. They describe how a material ages and which properties weaken first. Structural design should derive its own values from the relevant design code, safety factors and continuous service temperature.

Q6. How is wet-state insulation resistance different from 85/85 aging?

A. Wet-state insulation resistance is a short-term measurement; 85/85 is a long-term accelerated aging test. One answers "is it still insulating now that it is wet"; the other answers "will it degrade over time in permanent damp heat". For outdoor and condensing service, both are worth having.

9. About Wenzhou Jintong

Wenzhou Jintong Complete Electrical Equipment Co., Ltd. has produced SMC and BMC components in Wenzhou, Zhejiang Province since 2001, combining material formulation, mold making and molding under one roof.

Its qualifications can be checked individually:

· National standard: contributed to the revision of GB/T 23641-2018, the national standard for fibre-reinforced unsaturated polyester moulding compounds for electrical purposes (SMC/BMC).

· Standards committee: member of SAC/TC51/SC1, the national sub-committee for thermosetting moulding compounds for electrical use.

· Management systems: ISO 9001:2015, ISO 14001:2015 and ISO 45001:2018 certified.

· Product certification: BMC materials hold a UL Yellow Card (file no. E504820, for grade BMC 1616), searchable in UL Prospector under "Wenzhou Jintong" or "smcbmc".

· Manufacturing: a 10,800 m² main facility integrating mold manufacturing, compression molding and injection molding, plus a 24,000 m² plant brought into use in 2020.

Further detail is on our About Us page, and the three business lines — material, mold and molding — are described under Material.

Conclusion

The value of an 85/85 aging test is not a flattering number. It is that the test exposes strengths and weaknesses at the same time. Over 960 hours, this hydrolysis-resistant BMC held its flexural strength and its electrical performance; impact strength was the first property to soften.

Knowing where a material weakens first is more useful for selection than a general claim of moisture resistance.

📧 wendy.qiu@smcbmc.com 　 📞 +86-13868305300