Nylon 6.6 As Superior Cap Ply
05 April 2017
Sadettin Fidan
Current tire production technology and the need for high performance call for bi-elastic cap ply: Nylon 6.6 fulfills this requirement
Radial tires, with their heavy steel belt package and flexible sidewalls are subjected to strong centrifugal forces under high speed conditions which leads to the diameter increase (tire growth), and belt edge separation failures as a result of intense pantographic (angular) movements of steel cords.
Winding cap ply strips around the circumference of the belt package, so that they cover the whole width of the belt package, serves to reduce tire growth and improve high speed durability by preventing belt edge separations.
Cap ply in a tire ensures comfort and the flexibility or rigidity of the belt package, whilst improving impact resistance and steering stability. It also leads to reduced rolling resistance and improved high speed durability. Polymeric cap ply protects the metallic belt cords against moisture, and the ply separation resistance of the belt edge zone is enhanced thanks to the compressive forces of Nylon 6.6 at high temperatures.
Owing to its superior properties, Nylon 6.6 o‑ers the following advantages when deployed as cap ply:
1- Bielasticity: Whilst Nylon 6.6 promotes high initial extensibility, enabling tire building even with very high lifting ratios(tire expansion during curing process) , it also leads to high final tensile modulus(much higher than in the case of PET), which enhances restraining forces applied to the belt package under high speed conditions.
2- High thermal contraction force: Even at relatively low temperatures, an increase in temperature in line with speed generates a thermal contraction force (shrink force), which compensates for the modulus reductions due to polymer softening at temperatures above Tg (glass transition temperature).
3-Stress relaxation: By adjusting LASE value, Nylon 6.6 makes it possible to control in-tire cord tension after curing process. For superior comfort, the LASE value can be lowered, or for increased belt package rigidity it can be increased.
4- High flex and compression-fatigue resistance: The molecular structure and its morphology enable a high level of bending and axial compression-fatigue resistance. Fatigue resistance can be further increased using the 3T (Time-Temperature-Tension) process. By controlling dip di‑usion between the filament bundles, the cord strength can be maximized.
5- High breaking energy: With the help of Nylon 6.6, impacts under high speed conditions can be easily absorbed without cord breaking (enveloping property), in this way, cap ply also functions as a protective covering for steel belt layers.
6- Fiber surface functionality: Due to the surface activity of the filaments, a su‑icient level of static and dynamic adhesion can be obtained even with conventional RFL dips, without having to resort to additional adhesion promoters, which could negatively a‑ect dynamic properties by increasing cord-bending sti‑ness (decreased potential for the individual filaments to slide on each other).
As mentioned above, owing to its bi-elastic tensile behavior and high thermal shrink force generation, Nylon 6.6 has been widely used for several years as cap ply reinforcement.
Under high speed conditions, Nylon 6.6 can minimize the pushing-out e‑ect of the steel belt layers as well as the pulling-out e‑ect of the tread on the cap ply. This is because of the high e‑ective hot-modulus of Nylon 6.6, which is wound around the circumference of the top belt layer. A higher temperature increase in the belt edge zones raises the thermal contraction force of Nylon 6.6, leading to further compression of the belt edge steel cord layers (sandwich e‑ect) without any resultant ply separations (belt edge separations)
When choosing a cap ply reinforcement, two points must be kept in mind:
1- In-tire cord properties like cord tension and thermal contraction force (shrinkforce) are di‑erent than intial cord properties. . As a result of stress relaxation during the curing process, the initially raised cord tension caused by applied winding tension and process expansion (lifting) will be reduced.
2- The specified thermal contraction force (shrinkforce) of the dipped cap ply cord becomes much higher when the cord is embedded in the tire. This is due to the existing residual cord tension (especially at the crown center). The initial expansion tension during the curing process adds to the amorphous orientation in molecular structure (cold stretching e‑ect in the 160-180° temperature range), which will enhance the cap ply cord’s force-generation potential in response to increased temperature.
In principle, a high bi-elastic modulus can be regarded as the most important property of a cap ply cord. In contrast, simple high modulus (mono-elastic) cords like PET or PEN or aramid cords are in theory not suitable for use as cap ply, due to the extremely high cord tensions generated by process expansion during the curing process. When cap ply cords endure such large forces, they may cut into the skim compound of belt layers, which become soft at cure temperature, and the cap ply cords may therefore come into contact with the steel cords, causing failures due to mutual abrasions. At the same time, a large di‑erence in cord tension between the belt edge and the crown center will give rise to a butterfly-shaped footprint, so that tread wear will not be even.
The second most important property required of a cap ply cord can be said to be medium shrinkage and high shrinkforce (thermal contraction force). Temperature increase in line with speed gives rise to additional forces, which help inprotecting the belt package from tire growth under the e‑ect of centrifugal forces. In this way, durability at high speeds is ensured.
In order to further improve the performance of Nylon 6.6 as cap ply, Kordsa is developing new Nylon 6.6 products with increased bi-elasticity (e‑ective modulus/initial modulus) and thermal contraction modulus (shrink-modulus), all of which makes Nylon 6.6 irreplaceable as a material for cap ply