Manufacturer · Fashion Accessories
Bonded thread is a multifilament synthetic thread – usually polyamide (nylon) or polyester – that is coated with resin after twisting. The coating bonds the individual filaments into one, so the thread does not fray, creates less friction in the needle and withstands the high speed and heat of sewing thick leather, vinyl and upholstery materials.
What bonded thread is and how it is made
The base is a continuous synthetic filament. Several filaments are twisted together, as with ordinary thread. Then comes the key step: the thread passes through a bath of special coating and then through an oven, where the coating cures. The result is a smooth, slightly stiffer thread in which the filaments do not separate, even when its end is cut.
Bulgarian upholsterers also call it "bonded", "glued" or simply "nylon upholstery thread". In English-language catalogues you will find it as bonded nylon or bonded polyester, sometimes with the American V designation (for example V69, V92, V138), which roughly matches the thickness in tex.
Why the coating matters in upholstery
When sewing seats, steering wheels and dashboards, the needle pierces dense material at high speed. This creates two problems for the thread: heavy friction in the needle eye and heat. Ordinary multifilament thread "unravels" under such conditions – individual filaments snag, form loops and eventually the thread breaks. The coating solves exactly this:
- No fraying – the filaments move as one, so the loop under the needle forms consistently and stitches are not skipped.
- Less friction – the smooth surface passes more easily through the needle, guides and material.
- Heat protection – the coating protects the filaments from overheating in the needle during prolonged sewing.
- Better abrasion resistance – the finished seam wears less when the seat is used.
- Easier threading – the end of the thread stays stiff and does not split.
Bonded thread in car interiors
In automotive interior production, bonded thread is the standard for load-bearing seams – joining seat panels, inserting piping, seams on armrests and doors. For visible decorative seams on the dashboard or the top of the seats, bonded polyester is preferred because it tolerates sunlight better. See our selected threads for the automotive industry according to the seam location.
Bonded polyamide or bonded polyester
| Property | Bonded polyamide | Bonded polyester |
|---|---|---|
| Tensile strength | very high | high |
| Elasticity | higher | lower, the seam is more stable |
| UV resistance | moderate | very good |
| Typical location | internal and heavily flexed seams | visible seams, dashboards, convertibles |
When bonded thread is not needed
Bonded thread is stiffer than ordinary thread. On light textile covers, inner linings and seams on thin materials it can make the seam coarse and leave visible holes. There a quality continuous-filament polyester sewing thread of a smaller thickness is often sufficient. Bonded thread is chosen when there is a combination of dense material, high machine speed and high tension in the finished product.
Storage and shelf life
The coating on bonded thread is stable, but not eternal. Cones should be kept in a dry, dark place, away from radiators and direct sunlight. If cones sit for months next to a window, the outer wraps fade and become more brittle, and their colour no longer matches the inner ones. Good practice is to use threads in the order of delivery and keep opened cones in closed boxes or plastic bags, especially in workshops with a lot of leather and foam dust.
Bonded thread and quality tests
Automotive interior manufacturers usually test the finished seam, not just the thread itself. Typical checks include seam tensile strength, abrasion, ageing under light and heat, and colour stability. The specific requirements are written into each car manufacturer's specifications and differ from brand to brand. If you are working to such a specification, send it with your inquiry – we will then select a thread with the right parameters and send you a sample for testing.
How to recognise bonded thread
- Cut the end. Bonded thread keeps a "clean" end; ordinary thread immediately splays into filaments.
- Rub the end between your fingers. With bonded thread the filaments are hard to separate.
- Look at the surface. Bonded thread is smooth and slightly shiny, without fluff.
- Bend it. It is stiffer and springier than unbonded thread of the same thickness.
Setting up the machine for bonded thread
- Use a needle with an eye that matches the thickness – the thread must pass freely without rubbing against the walls of the eye.
- For edge and hem seams with a walking foot, balance the presser foot pressure so the layers do not shift.
- Do not over-tighten the upper tension – bonded thread does not shrink, and excessive tension puckers the material.
- Use cones, not small spools, and a thread stand above the machine so the thread unwinds evenly.
If you also sew leather goods such as bags, belts or shoes, bonded thread is useful there too – see also our threads for footwear and leather.
Bonded threads wholesale from Polkom
Polkom is a Bulgarian manufacturer and wholesaler of sewing accessories with over 30 years of experience. We offer bonded polyamide and polyester threads for upholstery workshops and interior manufacturers, in standard and contrast colours, with colour matching to a sample and fast sampling. Minimum quantities depend on the article. Write to us via our contact form with the material and seam type.
