Underwear testing

Underwear testing is the set of product and safety tests that verify an intimate-apparel garment — knitted underwear, bras, and infant bodysuits — meets the basic textile safety and colorfastness requirements that direct-skin garments owe at the strictest level, plus the physical and functional properties that make it wearable next to the skin: fibre content, burst strength, elasticity, and (where claimed) antibacterial or cool-touch performance. The governing product standards in China are GB/T 8878 (Knitted underwear, the 2023 edition superseding the 2014 Cotton knitted underwear, national standard platform), FZ/T 73012 (Bras), and FZ/T 81001 for intimate apparel; safety is governed by the mandatory GB 18401 (National general safety technical code for textile products, Intertek compliance guide), with infant underwear additionally governed by GB 31701. Underwear testing is not a re-run of general apparel testing — because underwear is in continuous, full-surface contact with skin (and, for infants, with saliva), it owes the strictest chemical and colorfastness limits, plus the saliva colorfastness test that adult garments do not owe. It therefore shares its baseline methods with our Fabric testing — the colorfastness, fibre-content, and strength methods are common across textiles — but the safety limits, the saliva fastness test, and the antibacterial-finish evaluation are underwear-specific and must be reported on the garment's own datasheet.

What Makes Underwear a Distinct Test Subject?

The single fact that defines underwear testing: every square centimetre of an underwear garment is in prolonged contact with skin (underwear background), and for infant underwear, with the mouth as well (infants suck and chew on fabric). This places underwear at the top of the textile safety hierarchy — it is Class B (direct skin contact) for adult underwear and Class A (infant) for baby bodysuits and infant underwear, and the acceptance limits tighten accordingly. Three consequences follow that general apparel testing does not capture:

• Chemical limits are at their strictest. Formaldehyde, pH, and banned aromatic amines (azo dyes) are limited to the Class A or Class B values — never the looser Class C values that outerwear may use. A finish or dye that is acceptable on an overcoat can be illegal on underwear.
• Perspiration colorfastness is mandatory, not optional. Underwear sits against sweating skin; a dye that bleeds in perspiration transfers onto skin and, for infants, into the mouth. The acid- and alkaline-perspiration fastness tests (GB/T 3922) are headline tests for underwear in a way they are not for a coat.
• Saliva colorfastness applies to infant underwear. Class A adds the saliva colorfastness test (GB/T 18886), because infants chew their clothing, and a dye that is fast in water and perspiration can still migrate in saliva. This is the test that no adult garment owes.

The fact the SERP obscures: an underwear datasheet that quotes only fibre content and weight is unverifiable. The safety class (A vs B), the formaldehyde value, the pH, and the perspiration/saliva fastness grades are the properties that decide whether the garment is legal to sell next to skin — and they are the most common failures in market surveillance, where fibre content and pH are the items most frequently found non-compliant.

What Are the Headline Underwear Tests?

The tests that define an underwear product, and that are weighted more heavily than for general apparel:

• Safety to GB 18401 — the mandatory baseline, with the limits tightened for skin-contact (B) and infant (A) use:
  • Formaldehyde (GB/T 2912) — Class A ≤ 20 mg/kg, Class B ≤ 75 mg/kg, Class C ≤ 300 mg/kg. Underwear is Class A or B, never C.
  • pH (GB/T 7573) — Class A 4.0–7.5, Class B 4.0–8.5. A pH outside the skin-compatible range causes irritation and is one of the most common surveillance failures.
  • Banned aromatic amines / azo dyes (GB/T 17592, GB/T 23344) — the 24 cleavable carcinogenic aromatic amines listed in GB 18401 Annex C must be below 20 mg/kg ("not detected"). Azo dyes are the most safety-critical failure because they are carcinogenic, not merely irritating.
  • Odour — none allowed; organoleptic check.
• Colorfastness — the underwear-weighted set — soap washing (GB/T 3921), acid and alkaline perspiration (GB/T 3922), water (GB/T 5713), dry rubbing (GB/T 3920), and for Class A, saliva (GB/T 18886). Class A requires ≥ 3-4 on water and perspiration, ≥ 4 on dry rubbing and saliva. The saliva test is infant-underwear-specific.
• Fibre content (GB/T 2910 / FZ/T 01053, reported per GB/T 29862) — the cotton/modally/polyester/spandex blend must match the label. Fibre content is the single most common failure item in underwear market surveillance — a garment sold as "95 % cotton, 5 % spandex" that contains more polyester or less cotton than labelled is both a labelling violation and a comfort defect.
• Burst strength (GB/T 19976 for knit burst, or GB/T 3923 for woven tensile) — knitted underwear fails by bursting, not by tensile break, so the burst test is the one that predicts whether the garment survives wear and wash.

What Physical and Functional Tests Complete the Profile?

Beyond safety and colorfastness, underwear characterization includes:

• Elasticity and stretch recovery (FZ/T 70006) — the waistband and the fabric's ability to stretch and recover, which decides whether the garment keeps its fit. For bras, FZ/T 73012 adds specific requirements on the elastic recovery of the band and the strap.
• Pilling (GB/T 4802 for knit, or GB/T 21196 for woven) — underwear pills under friction against outer garments, and a pilled inner surface is rough against skin.
• Seam strength and seam flatness — a seam that opens or that sits proud of the skin is a comfort and durability failure; seamless or flat-seam construction is tested for both strength and the absence of skin irritation.
• Antibacterial performance (GB/T 20944.1/.2/.3) — for underwear sold with an "antibacterial" or "anti-odour" claim. GB/T 20944 offers three methods — agar diffusion (plate), absorption, and shaking flask — and the safety of any antibacterial finish is governed separately by GB/T 31713 (Hygienic requirements for antibacterial textile products), which evaluates whether the antibacterial agent is safe for skin contact, not merely whether it kills bacteria.
• Cool-touch / contact instantaneous cooling (GB/T 35263) — for "cool-feel" summer underwear, the contact cooling sensation (measured as a temperature drop on contact with a heated detector) must meet the declared value; a "cool" claim without a GB/T 35263 measurement is marketing.

What Safety Tests Apply to Infant Underwear?

Infant underwear (bodysuits, vests, pants for under-36-month use) carries the heaviest safety load of any textile category, because it combines Class A GB 18401 with the additional requirements of GB 31701 (Safety technical code for infant and children textile products):

• GB 18401 Class A — formaldehyde ≤ 20 mg/kg, pH 4.0–7.5, the 24 banned aromatic amines below 20 mg/kg, plus saliva colorfastness (GB/T 18886) ≥ 4 — the test that exists because infants chew their clothing.
• GB 31701 — adds requirements specific to children's products: cord and drawstring safety (entanglement hazard, restricted on infants), small-parts and attachment tensile strength (buttons, snaps, and decorations must not detach under a defined pull force, because a detached small part is a choking hazard), filling safety (for padded infant garments), and colour restrictions (light colours preferred for infant products to limit dye load).

An infant underwear product must pass both GB 18401 Class A and GB 31701 — passing one without the other is non-compliant. This is the most common reason an infant-underwear test report is rejected: the lab tested GB 18401 but the buyer or regulator also required GB 31701. The same infant-safety logic governs our Infant pacifier testing and baby bottle testing — any article intended for under-36-month use must be evaluated for what an infant will actually do with it (suck, chew, mouthing), not just for adult-use conditions — and the small-parts tensile logic in GB 31701 overlaps with our toy testing, where detachable small components are the choking hazard that both standards exist to prevent.

Frequently Asked Questions

What standard governs underwear testing?
GB/T 8878 (Knitted underwear, 2023 edition, superseding the 2014 cotton-knitted-underwear standard) is the product standard for knitted underwear; FZ/T 73012 for bras; FZ/T 81001 for intimate apparel. Safety is the mandatory GB 18401, with infant underwear additionally governed by GB 31701. Functional claims (antibacterial, cool-touch) are governed by GB/T 20944 and GB/T 35263.

Why is underwear tested to stricter limits than outerwear?
Because every square centimetre of underwear is in prolonged contact with skin (and, for infants, with saliva). GB 18401 therefore places underwear in Class B (direct skin contact) for adults or Class A (infant) for baby products, with the strictest formaldehyde (≤ 20 mg/kg Class A), pH (4.0–7.5), and banned-amine limits. A finish or dye acceptable on an overcoat can be illegal on underwear.

What is the saliva colorfastness test and why does only infant underwear need it?
Saliva colorfastness (GB/T 18886) measures whether dye migrates from the fabric into artificial saliva. It applies only to Class A infant products because infants suck and chew their clothing, and a dye that is fast in water and perspiration can still migrate in saliva. Adult garments do not owe this test.

What is the most common failure in underwear market surveillance?
Fibre content (the cotton/spandex/polyester blend does not match the label) and pH (outside the skin-compatible range) are the items most frequently found non-compliant. Both are labelling- and chemistry-level failures, not physical failures — which is why a test report that covers only strength and colour misses the items most likely to fail in surveillance.

Must an "antibacterial" underwear claim be method-tested?
Yes. An antibacterial claim must be tested to GB/T 20944 (agar diffusion, absorption, or shaking flask) and the safety of the antibacterial finish must be evaluated to GB/T 31713 (Hygienic requirements for antibacterial textile products). The first proves the finish works; the second proves it is safe for skin contact. A claim without both is non-compliant.

Does infant underwear need GB 31701 as well as GB 18401?
Yes. Infant underwear must pass GB 18401 Class A (chemical and colorfastness limits) and GB 31701 (cord and drawstring safety, small-parts tensile strength, filling safety). A report that covers only GB 18401 is incomplete for an infant product, because GB 31701's mechanical and attachment-safety tests are not in GB 18401.

Our Testing Capabilities

Beijing ZKGX Research (ISO/IEC 17025 testing laboratory) provides underwear testing across safety, physical, colorfastness, and functional properties:

• Safety to GB 18401 — formaldehyde (GB/T 2912), pH (GB/T 7573), banned aromatic amines (GB/T 17592 / GB/T 23344), odour; Class A and Class B qualification.
• Colorfastness — soap washing (GB/T 3921), acid/alkaline perspiration (GB/T 3922), water (GB/T 5713), rubbing dry/wet (GB/T 3920), and saliva (GB/T 18886) for Class A infant underwear.
• Product tests to GB/T 8878 (knitted underwear) and FZ/T 73012 (bras) — fibre content (GB/T 2910 / GB/T 29862), burst strength (GB/T 19976), pilling, elasticity and stretch recovery (FZ/T 70006), seam strength.
• Functional claims — antibacterial performance (GB/T 20944.1/.2/.3) with finish-safety evaluation (GB/T 31713); cool-touch / contact instantaneous cooling (GB/T 35263).
• Infant underwear — full GB 18401 Class A plus GB 31701 (cord/drawstring, small-parts tensile, filling safety).

If you have an underwear line to qualify against GB/T 8878, a bra to test to FZ/T 73012, an antibacterial or cool-touch claim to method-test, or an infant-underwear product to verify against GB 18401 Class A and GB 31701, contact our testing team to scope the applicable tests and acceptance criteria.