Technical Fabrics: From Protective Gear to Thermal Underwear

Foote Brother (福依蒂) is a personal protective equipment (PPE) brand that originated in the United States. In China, it focuses on technical-fabric thermal underwear and long johns made with care for older adults. In China, Foote Brother is operated by Guangdong Zhuoheng Investment Development Co., Ltd.

Protective clothing is worn next to the body for a full shift. It has to stay clean, move with the body and keep its shape after many washes. Thermal underwear is worn just as close, often every day for a whole winter, so we start from the same requirements. This page explains each fabric property used in 福依蒂 thermal underwear sold in China: how it works, which products use it, and the test results behind the numbers.

One point first: clothing does not make heat. A base layer keeps you warm by slowing down the heat your body loses.

Each test result below belongs only to the product that was tested. We do not carry one product's result over to another.

1. Antibacterial finish

How it works. Underwear worn for many hours collects sweat and skin flakes, and bacteria can grow on it and cause odor. An antibacterial finish holds back that growth. The bacteriostatic rate is the share of bacterial growth that is held back; our results are measured by the shake-flask method in GB/T 20944.3-2008, on samples washed 300 times.

Used in. 福依蒂大红三盾保暖内衣 (Fuyidi Red Triple Shield thermal set), 福依蒂高弹轻盾保暖内衣 (Fuyidi High-Stretch Light Shield thermal set), and the long johns of 福依蒂长绒吸汗盾保暖内衣 (Fuyidi Long-Staple Cotton Shield thermal set).

Test results (after 300 washes).

Antibacterial grade: Red Triple Shield was washed 300 times by the FZ/T 73023-2006 method and rated antibacterial class 10A under T/TTGA 007-2024 (report TTTS-WH25503078-03). High-Stretch Light Shield: antibacterial class 10A (report SQ2502972).

2. Easy on, easy off, keeps its shape

How it works. Spandex (a polyurethane elastic fiber) has soft segments that stretch and hard segments, held in place by hydrogen bonds, that pull the chains back. Necklines and cuffs open wide, then return to shape, and knees and elbows are less likely to bag. Knit fabric without a high spandex share works through its structure: under pull, the knit loops change shape and the yarn shifts; when you let go, the yarn springs back and the loops return.

Used in. High-Stretch Light Shield relies most on stretch: 11% spandex in a light 200 g fabric. The other sets also contain spandex: 福依蒂静气厚盾保暖内衣 (Fuyidi Still-Air Thick Shield thermal set) 10%, the outer layer of 福依蒂肩腰膝烯盾保暖内衣 (Fuyidi Shoulder-Waist-Knee Graphene Shield thermal set) 8%, 福依蒂双绒盾保暖内衣 (Fuyidi Double-Brushed Shield thermal set) 5%, and Long-Staple Cotton Shield 5%. Red Triple Shield works through its knit-loop structure and a polyester body; polyester has an official moisture regain of only 0.4%, so it hardly takes up water or swells, and its size stays stable after washing.

Test results. High-Stretch Light Shield: pilling grade 4 (report SQ2502972). Red Triple Shield: first-grade product under GB/T 8878-2023 (report TTTS-WH25503078-03).

Care. Fabrics with spandex: no chlorine bleach; wash at a low temperature. Red Triple Shield: avoid ironing at high heat.

3. Still air

How it works. Air that cannot move is a poor conductor of heat: about 0.026 W/(m·K), roughly one fifth of polyester fiber. A thicker fabric holds more still air, so it slows heat loss more. A garment's thermal resistance rises roughly in step with its thickness.

Used in. Still-Air Thick Shield: a thick fabric of 90% polyester and 10% spandex, with a mid-height collar that covers the base of the neck. The same principle is behind the air gap in 福依蒂透湿双盾保暖内衣 (Fuyidi Breathable Double Shield thermal set) and the batting in 福依蒂三明治盾保暖内衣 (Fuyidi Sandwich Shield thermal set).

Test results. No published test value for Still-Air Thick Shield yet. See the product page on our China site.

4. Moisture-absorbing warmth

How it works. Fibers with water-attracting (hydrophilic) groups, such as viscose, wool and silk, take up the water vapor that leaves your skin. When water molecules bind to these groups through hydrogen bonds, they release heat of sorption, roughly the latent heat of condensation of water (2.4 kJ/g) plus binding heat. This is a property of the fibers. It stops once the fibers are full of moisture; it is not a source of continuous heat.

Used in. Breathable Double Shield. The outer layer is 100% polyester. The inner layer contains viscose 11.0%, wool 2.5% and mulberry silk 2.4%. The two layers are joined by connecting yarn, which leaves a layer of still air between them.

Test results. Fiber content only (see the product page). No warmth test value is published for this product.

5. Graphene patches: re-radiation, less heat loss

How it works. Graphene is a two-dimensional crystal of sp² carbon atoms. Skin on the torso is about 34℃ and gives off far-infrared energy that peaks around 9.4 micrometers. Graphene absorbs this energy and radiates part of it back toward the skin. The patch is also one more layer of material, so it adds thermal resistance. Both paths of heat loss, radiation and conduction, become narrower. All of the warmth comes from your own body. There is no power supply and nothing heats up on its own.

Used in. Shoulder-Waist-Knee Graphene Shield: patches at the shoulders, waist and knees, the places that tend to feel cold first. Everywhere else is a single layer of 92% polyester and 8% spandex, so the set is not bulky.

Test results. No published test value for this product. See the product page on our China site.

6. Long-staple cotton: fiber length

How it works. Cotton is graded by fiber length. Fine-staple (upland) cotton fibers are 25–32 mm long, with 28 mm as the standard grade. Long-staple cotton fibers are 33 mm or longer. Longer fibers twist together more firmly in the yarn, leaving fewer loose ends, so the fabric is smoother and less likely to pill. Cotton is a cellulose fiber whose hydroxyl groups attract water; its official moisture regain is 8.5%, which is why it absorbs sweat well.

These lengths describe cotton grades in general. They are not measurements of our fabric.

Used in. Long-Staple Cotton Shield: 95% cotton, 5% spandex.

Test results (report M26012809). Measured fiber content: cotton 95.3%, spandex 4.7%. Top: color fastness grade 4–5 (page 3 of the report). Long johns: antibacterial results in section 1. Made to GB/T 8878-2023 (knitted underwear).

7. Double-brushed fabric: how it feels on first touch

How it works. Whether a fabric feels cold the moment it touches you depends on how fast heat flows from your skin into it in that first instant. Brushing raises fine fiber tips on the surface. Your skin touches only the tips, the contact area is small, and less heat is drawn away, so the fabric does not feel icy when you put it on. The raised fibers on both sides also hold air.

Used in. Double-Brushed Shield: brushed on both sides, 220 g fabric.

Test results. No published test value for contact warmth. See the product page on our China site.

8. Three-layer quilted structure

How it works. The outer layer resists wind and wear. The middle batting holds still air in gaps smaller than a millimeter, too small for the air to move. The inner layer sits against the skin. Diamond quilting keeps the batting in place so it does not shift or bunch. Warmth here comes from the still air the layers hold, not from the weight of the garment.

Used in. Sandwich Shield. It is a thick set, best worn under a loose coat. For wear under fitted clothes, Shoulder-Waist-Knee Graphene Shield or High-Stretch Light Shield fits better.

Test results. No published test value for this product. See the product page on our China site.

9. Far infrared

How it works. Far-infrared emissivity describes how well a fabric gives off far-infrared energy; a value closer to 1 means more efficient. The temperature-rise test measures how much warmer the test surface becomes under the fabric, under the fixed conditions of GB/T 30127-2013. Like everything on this page, this works with the heat your body already gives off.

Used in, with test values. Only Red Triple Shield has published far-infrared results: emissivity 0.93 (standard requires ≥0.88) and temperature rise 2.2℃ (standard requires ≥1.4℃), GB/T 30127-2013. The far-infrared test is in subcontracted report TTTS-WF25557461, under main report TTTS-WH25503078-03. These values apply to Red Triple Shield only. Sandwich Shield also uses a far-infrared fabric; no test value is published for it.

Test reports

All reports below are for 福依蒂 thermal underwear sold in China.

Testing body names and report images: shown on the first page of each report (images masked for non-brand information)

Product pages: Shoulder-Waist-Knee Graphene Shield · Still-Air Thick Shield · Breathable Double Shield · Double-Brushed Shield · High-Stretch Light Shield · Sandwich Shield · Long-Staple Cotton Shield · Red Triple Shield

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