Why Do Clothing Seams Matter for Personal Protective Clothing?
Typically, when choosing PPE, you select an option based on the type of hazards it protects against (i.e., chemical-resistant, fire-resistant, etc.). But, what about the seams?
Our free seam safety guide takes a deep dive into PPE seam types for protective clothing, so you can understand how they work and why they're essential for certain applications or environments. It includes comparison charts, advantages and disadvantages of each seam construction, as well as best uses for each seam type. To access all seam safety information, download our free guide, below:
Not all seams offer the same level of protection against certain hazards, which is why its important to understand how each type functions. More importantly, understanding how these seams are made can help you make the right decision for your job site and workforce.
Protective clothing seam types are specialized for use in personal protective equipment (PPE). These seams are designed differently than non-protective seams or seams used in “street clothes” for several reasons. Their mechanical strength is higher and protective seams are designed to prevent various substances from entering a garment such as chemicals, blood, or hazardous dry particles.
How Seam Construction Impacts Safety
The most likely route of contamination for a chemical is not permeation through the fabric but rather penetration through small holes in the suit, gaps between the suit and other PPE, or weak construction points of the suit (i.e. seams tearing apart or loosening).
For disposable chemical protective clothing, there are typically four different seam styles:
- Welded (either ultrasonically or via high heat)
- and stitched and taped (includes double-taped and stitched)
Only “welded” and “stitched and taped” seams provide a liquid-tight seal. Even if the fabric offers the highest resistance to liquid permeation, if holes are present in the seam or there are gaps between the protective clothing and other PPE, then a chemical will more readily and easily run through those rather than permeate the fabric.
Due to two principles, even tiny holes can have disproportionately large effects: the bellows effect for dusts and the “wicking” process for liquids.
Understanding Wicking and The Bellows Effect
Wicking, also referred to as “capillary action,” describes the physical process of a liquid being actively drawn through a tiny opening due to its surface tension. As a result, even though a hole may be extremely small— for example, a stitch hole in a serged seam—the volume of liquid that ultimately penetrates may be considerable.
The Bellows effect is a critical factor in the prevention of hazardous dry particles from entering a garment. Dust, unlike liquids, floats freely and will only be able to penetrate a garment through airflows, which are created by the construction or assembly of the garment. Therefore, the tendency of the garment to produce airflows through gaps or holes (e.g. zip holes, seam holes, or gaps between the garment and other PPE) is a key factor in determining how effective a garment is at preventing dust ingress.
According to Bellows Effect, a fabric that is not breathable will create greater airflows through gaps like seam holes than a fabric that is breathable. This is because air flows more easily through a breathable fabric. A breathable fabric has a lower tendency to create these airflows. This means that a garment made of a breathable fabric can provide better overall dust protection compared to a similar garment made of a non-breathable material, although the latter fabric may have better particle filtration.
The combination of these factors demonstrates how seam structure and chemical resistant clothing interact with other PPE and may be as significant as, if not more important than, the fabric’s permeability.
What are Serged or “Stitched” Seams?
A serged seam, also known as an overlocked seam, is the simplest seam type. It is a stitched seam that uses either three or four threads to join two pieces of fabric together with an overlocking stitch.
This type of stitching is more resilient than what you'd find in regular “street clothing.” Although it is the simplest seam type in protective clothing, it can hold fabric together to resist light liquid splashes and dry particulates.
Serged seams are the least protective garment seam type, because the process of stitching creates small needle holes that can allow dust and chemicals to enter the garment. This seam construction does not provide a full barrier to liquids or dry particulates. Serged seams are suitable for basic protective suits that provide light splash and dry-particle protection (EN 943 Type 5 and 6 suits). Serged seams are not sufficient for garments intended for directional jets of spray or spray under pressure.
A stitched seam, whether bound or serged, is not sealed; it has tiny stitch holes that are prone to open under stress (a problem worsened by tight fitting garments) and may allow more liquids or dusts into the garment. Therefore, when an application involves liquids or particulate hazards that can be harmful in small amounts, users should consider whether a garment with sealed seams (not serged or bound) should be used instead.
What are Bound Seams?
A bound seam is similar to a serged seam, however it offers an extra layer of protection by sewing a strip of fabric over the top of the two edges of the fabric to bind it together. This helps increase the level of holdout against leakage of liquids or dust particles. It may also help reinforce the seam for added seam strength.
Bound seams provide a higher degree of liquid and particle repellency compared to serged seams, but they are also not liquid-tight. Although these seams are more resilient, they're not strong enough to warrant exposure to higher CE or fire hazard levels since they're not sealed completely.
Learn about Stitched and Taped Seams
Stitched and taped seams, also known as over-taped seams, are serged seams that are sealed under a layer of tape. This seam construction is liquid-tight, making it suitable for use in Type 3 and Type 5 chemical suits. Of the protective clothing seam types used, stitching and taping a seam provides the optimal mix of a flexible and functional seam while maintaining an impervious seal.
Taped seams act as a barrier to seal off any gaps or holes in the stitching of the suit. When workers are exposed to toxic chemicals and fire, they need sealed PPE that won't allow dust, sparks, or liquid to seep through the seams.
Taped seams prevent seepage by covering the stitching underneath. The tape covers the seam where the two edges of fabric are sewn together to provide the highest level of inward leak resistance. Tape is chosen based on a similar chemical resistance and impermeability to the fabric the coverall is made of.
Single-taped PPE uses one layer on the inside, which fills in the small holes left by serged seams. The tape is applied by using heat or ultrasonic sealing to create a complete seal that won't come undone or peel at the edges.
What are Double-Taped and Stitched Seams?
Rather than using a single layer of tape, this clothing uses two layers - one on the inside of the garment and one on the outside of the garment. This ensures the seam is sealed off on both sides of the garment. The process is still the same, meaning that the tape is heat-sealed so that it will not peel off.
Double-taped suits are used in gas-tight suits (Type 1 and Type 2 suits).
What are Ultrasonically Welded Seams or “Heat Sealed” Seams?
Typically, sealed seams require a two-step process. First, sewing the seam and second, taping it. Welded seams can create a similar seal with just one step. Instead of a "butt" seam, as you'd find with the other varieties, welded seams use a lap stitch.
This stitching is flatter, smoother, and less noticeable. Once finished, the stitching is "welded" together with heat to create a tight seal. This seam construction is also commonly called a “heat-sealed” seam.
Welded or heat-sealed seams are not always 100-percent effective. The problem lies in how much the seam is welded. Too much heat can make the material brittle, meaning the seam can split more easily allowing contaminants to seep in. With too little heat, the sides of the seam won't bond together correctly leading to an improper seal.