Arguably the most important document for any individual is their passport. As well as serving as a key means of identification and passage from one country to another, the passport also chronicles personal travel history. Whilst this is clearly useful in many ways, unfortunately, it can also create the potential for certain individuals to attempt to alter or conceal details of their travel history, for various undesirable reasons.
Criminals often try to do this via the disassembly of stolen passports so that visa pages can be removed and/or replaced, thereby altering the recorded travel history contained in the passport. This illicit process involves cutting and removing the existing security stitching thread that holds the passport booklet together, followed by sewing the passport back together once the page removal or substitution has been undertaken.
To counteract such unwelcome activities, passport designers have typically incorporated intricate security features into the stitching thread substrate, to make this counterfeiting process more difficult. Additionally, the utilisation of lock-stitching ensures that cutting the thread is necessary for the passport to be unstitched and disassembled.
This is where the composition of the security stitching thread can play a vital role. Until fairly recently, many passport security stitching threads primarily consisted of polyester-cotton thread (cotton enveloping a polyester core). In the case of this material, it is possible for the thread to be unpicked to enable page removal or substitution, and for the thread to then be carefully re-sewn, leaving little or no visible sign in the passport that this activity had taken place.
Polyester-cotton stitching thread
However, stitching threads made from a multi-core/multi-ply polyamide (nylon) substrate, and then subjected to a special under-tension twisting process during the production phase, overcome this potentially serious flaw because it is not possible to unpick them and then re-sew them without leaving a clear and obvious visible sign in the affected passport.
Multi-ply polyamide (nylon) stitching thread
This visible sign can be made even more prevalent and stand-out with the use of a dark colour within the 3-ply colour combination. So, in a sense, a multi-ply polyamide (nylon) thread can act as a marker or highlighter of any illicit passport tampering or interference that may have taken place.
The key differences between polyester-cotton and polyamide (nylon) thread substrates can be summarized as follows:
- Polyester-cotton thread has a softer and more fibrous surface finish, since the outer layer is cotton.
- Polyamide (nylon) substrate is notably stronger for an equivalent thread thickness.
- Polyester-cotton threads are prone to shedding fibres during sewing, potentially resulting in fine hairs—including security features—on the paper surface.
- Polyamide (nylon) threads exhibit greater durability and resistance to abrasive degradation, which is crucial in heavily used or frequently verified passports.
- The special tensioned twist of a polyamide (nylon) thread makes it more distinguishable, with multi-colour ply combinations even more apparent than with multi-ply polyester-cotton threads.
- Advanced chemical expertise is required in order to ensure that polyamide (nylon) threads accept visible and fluorescent colours effectively. This makes counterfeiting and illegitimate/ unauthorized production of the thread highly unlikely.
- The dyeing process for polyester-cotton thread is much easier, due to the cellulose base of the outer layer. This means that counterfeit or illegitimate sewing thread may be more readily available.
- It should be noted that security dyestuffs and other features may behave differently with different substrates, meaning that dye recipes are not directly interchangeable across thread substrate types.
Finally, the integration of polycarbonate data pages in passports has been a significant recent advancement in passport design and manufacturing. These pages require specialized hinge systems due to their increased thickness, stiffness, and abrasiveness, compared to traditional paper/laminate data pages. Consequently, it is highly recommended that polyamide (nylon) security stitching thread is used in passports employing polycarbonate data pages, since this thread is much stronger and will cope with the increased abrasion that these pages cause.
While it is certainly true that some older passport designs still utilize polyester-cotton stitching threads that have historically performed well enough, the increasing mobility of populations and the subsequent increase in the volume of passport checks also makes it prudent to adopt the more durable polyamide (nylon) threads in all new passport design specifications.
Sources/References:
Thijs Boonen is the Operations Director of Liberty Security Threads, based in Rotterdam, Netherlands. Liberty is a global supplier of specialist sewing threads for passports and brand protection applications.
