Polyester Cord Strapping
What is Polyester?
Polyester is a term often defined as “long-chain polymers chemically composed of at least 85% by weight of an ester and a dihydric alcohol and a terephthalic acid”. In other words, it means the linking of several esters within the fibers. Reaction of alcohol with carboxylic acid results in the formation of esters.
Polyester also refers to the various polymers in which the backbones are formed by the “esterification condensation of polyfunctional alcohols and acids”.
Polyester can also be classified as saturated and unsaturated polyesters.
Saturated polyesters refer to that family of polyesters in which the polyester backbones are saturated. They are thus not as reactive as unsaturated polyesters. They consist of low molecular weight liquids used as plasticizers and as reactants in forming urethane polymers, and linear, high molecular weight thermoplastics such as polyethylene terephthalate (Dacron and Mylar). Usual reactants for the saturated polyesters are a glycol and an acid or anhydride.
Unsaturated polyesters refer to that family of polyesters in which the backbone consists of alkyl thermosetting resins characterized by vinyl unsaturation. They are mostly used in reinforced plastics. These are the most widely used and economical family of resins.
Polyester fibre was first produced in USA in the 1950's and used for fabric production and subsequently, it's potential as a substitute for steel strapping was recognised due to its excellent tensile, elongation and recovery properties.
The glue bonded polyester cord strapping was first developed followed by woven polyester shortly after, and the composite strapping was developed in more recent times.
Types of Cord Strapping
There are 3 main types of polyester cordstrap:
Glue-Bonded Cord Strapping
The linear polyester yarns are joined together by hot-melt glue to form a web of the width required, and consequently. has a soft and tacky feel . The strap is joined with metal buckles
Woven Polyester Cordstrap
Manufactured using the same polyester yarns in a wide variety of colours. The woven strapping is soft ,lightweight and flexible and can be produced by non specialist strapping companies i.e manufacturers of braiding who will already have the same weaving equipment.
Composite Cord Strappings
The polyester yarns are co-extruded with polyethylene to produce a stiff strapping band that aids the threading of strap through pallet voids and also prevents the linear polyester strands from parting as sometimes happens - without detriment - to conventional hot glued corded strapping.
All three types of corded strapping will perform the same load restraint functions with the same ability.
Corded strap basic characteristics.
With the exception of the composite cordstrapping which is stiffer, the product is soft, pliable and easy to handle, consequently it is unlikely to damage either personnel or product to which it is being applied. It is available in various widths, deniers and strengths that equal those associated with steel straps and consequently really is a substitute for steel strapping for most applications. Unlike steel it does not rust and is much lighter making handling much easier. There are other significant benefits however when compared with steel strapping as with cold rolled steel strap elongation is only 1.5% at break on average i.e very little margin to absorb shock loading, and with 10% average elongation the corded polyester has a much superior ability to absorb shock loads in conditions where steel strapping would fracture, and then recover.
Strength of the strap when applied to a load is different to the linear strength quoted by the manufacturers for the various specifications as presented to the user i.e unjoined. The figure quoted is that obtained from a tensile test where the strap test length is gripped at both ends and pulled until it fractures and this is the generally accepted comparison figure for all strap types. When the strap is used however and the strap loop around the load has to be joined, in this instance with a metal buckle this becomes the 'weakest link' and the point at which fracture would generally occur. The quality of the buckle and its load bearing capability becomes an integral and important part of the system.
Suppliers and distributers of corded strapping systems quote sytem strengths - not linear strengths of their strapping, and this can mis-lead unless there is an understanding of the term.
System strength is not the break strength established from a straight pull. System strength is calculated by taking the value of twice the linear strength multiplied by the buckle efficiency i.e its load holding ability. This can vary with buckle design and with the specification of the strapping being used, and results in a figure within the range 75-85% being generally quoted. For example using a strap with a linear strength of 500 kgs and assuming a buckle efficiency average of 80% the calculation would be 2 x 500 x .8 = 800 kgs. Although this calculation is only generally used for corded straps it would remain valid for other forms of polyester strapping if the joint efficiency is known, as would be the case with extruded polyester joined with a friction weld tool which produces joint efficiencies with the same range.
it is a natural phenemenon for tensioned strapping systems to lose strength over time and this is called creep . It has been shown
that the corded and extruded polyester strapping has less than 10% creep, and in the case of the corded presentation joined with buckles can be retightened, as can extruded polyester strapping provided a tail of strapping is left on the underside of the joint to enable a friction weld strapping tool be reapplied.