Gas Balloon Envelope Material

The envelope material of today’s gas balloons consists of one layer of synthetic fabric that is coated on both sides with a linen binding and additional rip stop threads.  For those who have not been close to gas balloon fabric it is significantly heavier than hot air balloon fabric and can easily compared to a yellow slicker or rain suite fabric.

Balloons in the 1800-1900 used gold beaters skin.  Gold beaters skin was a thin membrane taken from the internal organs of the ox.  One animal would give one piece about 6 inches square.  It was almost perfectly gas tight and was very light.

The next development in balloon fabric was the utilization of varnished fabric.  Both cotton and silk were varnished with fair results.  The product was much cheaper and lighter because it was a single ply fabric but not nearly as impervious to hydrogen as gold beaters skin.    The primary problem with varnished fabrics was the varnish cracked each time the balloon was folded up and was known to spontaneously combust and catch fire due to the varnish mixtures of the day.

The next achievement in balloon fabric occurred when rubberized balloon fabric made in various weights, colors and number of ply's was developed by the Goodyear Tire and Rubber Company.  Two plys were normally used and three when added strength was needed.

A description of Goodyear Tires process of manufacturing balloon fabric in the early part of the 1900's  is included here:

The raw fabric is first passed over a glass table which is illuminated from below with electric lights.  Here all defects are removed.  A sample from each roll is tested for strength and the number of threads per inch counted to make sure the fabric is up to standard.  The fabric is now run through the spreader time after time until the required weight of rubber has been spread on the fabric.  Sometimes it is necessary to “spread “ the fabric 30 times before the required weight of the rubber has been applied to a square yard.  The compound spread on the fabric is a secret mixture made of the purest grades of material accurately and carefully mixed and closely resembling thin putty.  Two rolls of this fabric is spread in the manner described.  The one is left as it is while the other is cut into bias pieces 45 degrees to the edge of the fabric.  These bias pieces are all turned around and cemented together again to form a continuous roll of bias fabric.  These two rolls are now run through another machine which rolls the two plys together into the 2 ply fabric.  It may be said that rubber before it is cured or heated is very sticky.  The electrician friction table is a good example of this property of uncured rubber.

The question may be asked as to the use of the bias ply.  Its purpose is to oppose the diagonal stretch of the straight ply and to make the fabric equally strong in every direction to prevent tears from spreading.

During all of these steps of spreading it is necessary to wrap a clean piece of fabric with the spread fabric in order to keep the layers of the spread fabric from sticking together.  After the fabric has been plied up, it is wrapped on a large iron drum with a clean strip of fabric as before and the edges of the roll are tightly bound to prevent moisture from passing into the roll between layers of fabric.  This drum with fabric is now lowered into a “heater” and the heater bolted shut.  Steam is admitted into the heater, which cures the fabric removing all signs of stickiness.  The correct temperature and pressure of steam in the heater and the length of cure are very important factors in the production of balloon fabric.  Should the cure be too great, the rubber becomes too hard and cracks?  If too little the diffusion is bad and the fabric is sticky. 

Today’s modern synthetic balloon fabric uses rip stop construction is a double thread woven into the fabric at even intervals in both direction of the thread. (Horizontal and Vertical directions (Warp and Weft) this is a safety feature that increases the tear resistance of the material.

Some older balloons used a coated cotton material.  The currently synthetic fabric is approximately 50% lighter and is 60% better from a tear resistance standpoint. 

On the external surface the synthetic rubber material “hypalon” offers good coloring properties and relative UV radiation resistance if used.  The black internal surface is coated with butyl rubber, the graphite content of which provides electric conductivity.  The surface resistance of this internal coating must not exceed a value of <10 Ohm.  This is critical for balloons that use hydrogen as it's lifting gas.  All possible static discharge must be eliminated so they hydrogen does not catch fire or explode.

The weight of the balloon material made of the synthetic fiber (e.g. Nylon) is approximately 50% lighter than cotton coated material.   A balloon with a capacity of 1000 m3 weighs approximately 350-375 lbs (160-170 kg).

The synthetic fabric is glued together with a 15 mm overlap..  That seam is sealed with another 40 mm wide cover strip on the external and internal walls.  At the top of the envelope and bottom of the envelope it is protected from tearing by an edge reinforcement line made of steel wire rope.