Brittleness problems with nylon (polyamide) cable ties.
time:2020-2-23 hits:381
Usually, the aging has no effect on the nylon cable ties, only the aesthetics. The natural color cable ties tend to yellow with age, but the effectiveness of the cable tie is unaffected by the changing color (or, you could use the colored cable ties).
For indoor use under a controlled environment, perfect conditions for tie wraps are 70°F and 50% RH, you can get them a very long life. A plastic cable tie will remain supple as a result of the stable temperature and RH in the indoor environment.
Most brittleness problems with plastic cable ties are caused by temperature extremes with very low humidity. When plastic cable zip ties exit the mold, they are in a dry and brittle state. In order to give ties flexibility and suppleness, moisture is added by placing water in a sealed bag with the wire ties. Nylon 6.6 is hydroscopic, i.e. it has the ability to absorb moisture, up to 8% by weight. Making cable ties good is to add 2-3% water.
Just like nylon 6.6 absorbs moisture, under cold and dry conditions the moisture seeks to migrate from the strap. Sealed plastic bags are used to retard this process, but the bags are not a vapor barrier eventually, under these conditions, the wire ties will return to its "as molded” (i.e. brittle) state. This is true of all nylon 6.6 cable ties produced by the various cable tie manufacturers.
Virtually all brittleness problems occur in outdoor applications in very cold weather, or where cable zip ties have been stored for long period in an unheated warehouse. The tie wrap breaks as it is being flexed. Once wire ties are placed around a wire or cable, they are virtually immune to brittleness.
Suggestions for engineering standard are:
・Polyamide cable ties should be stored in the manufacturer's original packaging until ready for use.
・Polyamide cable ties should not be stored in an unheated warehouse.
・Plastic cable ties in stock should be used frequently.
・Nylon cable ties should be used within one year after purchase.
・Bags for partial wire cable ties should be closed or sealed if at all possible.
Nylon is a hygroscopic material, and as such is affected by variations in moisture levels. For peak performance and for the as designed and tested conditions nylon should be stored at a controlled 73° F and at 50% Relative Humidity. This storage results in a moisture conditioning of the nylon of 2.5%. While one realizes the realistic limitation of these conditions, nevertheless these are the optimum as designed and tested conditions.
Nylon can vary from 0% moisture to a fully absorbed moisture maximum condition of about 8%. The process of absorption is reversible and nylon will desorb depending on the environmental conditions. When dry, nylon is stiff and brittle, and when moist it is pliable and tough. So, the moisture is what gives nylon cable ties their toughness.
Brittleness problems with most nylon cable tie come about from exposure to dry conditions and as a result the moisture in the cable tie is fully desorbed. This is accentuated in hot arid and dry cold climates. Nylon is also notch sensitive and dryness amplifies this sensitivity.
Cable ties undergo significant stress during installation. Dry cable ties will exhibit several failure modes. The most common failures under dry desorbed conditions are the pawl breaking out of the head and/or the strap snapping. In addition, any color or UV inhibitors added to the natural nylon resin will compound the problem of brittleness caused by dryness. Dry cable ties can be returned to fully functional and specification condition by storage under proper conditions. The cable ties can be soaked in hot water to rapidly return them to a pliable tough cable tie.
