Wednesday, July 08, 2015
Films are thin sheet material extruded from synthetic polymers and are typically used along with woven cloth in a laminate (see laminates below).
PET film is the most common film used in laminated sailcloth. It is an extruded and biaxially oriented version of PET fiber. In the US and Britain, the most well-known trade names are Mylar and Melinex.
PEN film is extruded and biaxially oriented version of PEN fiber. Just as PEN fiber is stronger than PET fiber, PEN film is stronger than PET film. However, PEN film is rarely used in standard sailcloth styles because it shrinks more rapidly than PET, is less resistant to abuse, and reduces the working life of the sail.
Scrim and strands
Strands are combined from fibers; these are frequently narrow flat bands or ribbons of high strength material. Scrim is a loose weave or lattice of strands, typically bonded where they cross to maintain the grid pattern. Strands and scrims are used to strengthen or reinforce sailcloth (see laminates below).
In the 1970s sailmakers began to laminate multiple materials with different characteristics to synergize the qualities of each. Using sheets of PET or PEN reduces stretch in all directions, where weaves are most efficient in the direction of the threadlines. Lamination also allow fibers to be placed in a straight, uninterrupted paths. There are four main construction styles:
Film is sandwiched in between two layers of woven taffeta, the film provides most of the stretch resistance and the taffeta enhances tear and abrasion resistance. The high-end versions of this method use a woven Spectra or Kevlar taffeta. In some newer styles, off threadline aramid yarns, are also laid into the laminate. In some cases the second layer of taffeta is eliminated for cost and weight savings
Film-scrim-film or film-insert-film (film-on-film)
In this construction, a scrim or strands (inserts) are sandwiched between layers of film. Thus load-bearing members are laid straight, which maximizes the high modulus of the fibers, where a woven material will have some inherent stretch to the weave. Laminating film to film around the strands creates a very strong and dependable bond reducing the amount of adhesive needed. In high quality cloth, the strands or scrim are tensioned during the lamination process.
The drawbacks are: film is not as abrasion or flex resistant as a weave, it does not protect the structural fibers from UV rays. In some cases UV protection is added.
Woven fabric with high UV and abrasion protection is added to the film-on-film. This combines the best of the above, but is costly, heavy, and stiff. This is an attractive method to combine high modulus fibers with poor UV resistance.
Wovens on both sides of a scrim without the film layer. The problem is getting enough high modulus yarn into the sandwich, and still being able to get a good bond, because, dissimilar fabrics don’t often bond well. This technique is more experimental than practical, but may yield results in time.
Posted by Dave at Wednesday, July 08, 2015