In today's sport balloons the envelope is generally made from nylon fabric and the mouth of the balloon (closest to the burner flame) is made from fire resistant material such as Nomex.
Modern hot air balloons are usually made of light-weight and strong synthetic fabrics such as ripstop nylon, or dacron (a polyester).
For most envelopes made of nylon fabric, the maximum internal temperature is limited to approximately 120 °C (250 °F).
It should be noted that the melting point of nylon is significantly higher than these maximum operating temperature — about 230 °C (450 °F). However the lower temperatures are generally used because the higher the temperature, the more quickly the strength of the nylon fabric degrades over time. With a maximum operating temperature of 120 °C (250 °F), balloon envelopes can generally be flown for between 400 and 500 hours before the fabric needs to be replaced. Many balloon pilots operate their envelopes at temperatures significantly below the maximum in order to extend the longevity of their envelope fabric.
On a hot day, a balloon cannot be loaded as much as on a cool day, because the temperature required for launch will exceed the maximum sustainable for nylon envelope fabric. Also, in the lower atmosphere, the lift provided by a hot air balloon decreases about 3% for each 1,000 meters (1% per 1,000 ft) of altitude gained.
Rip-stop nylon is a light-weight nylon fabric with inter-woven ripstop reinforcement threads in a crosshatch pattern. It is woven with coarse, strong warp and filling yarns spaced at intervals so that tears will not spread. The same effect can be achieved by weaving two or three of the fine yarns together at intervals.
Rip-stop nylon is often used in yachts for sails and spinnakers hot air balloons, kites, parachutes, camping equipment such as lightweight tents and sleeping bags, flags, banners, and many other applications which require a strong lightweight fabric.
Silnylon, a contraction of Silicone impregnated nylon, is a synthetic fabric used mainly in lightweight outdoor gear. It is made by impregnating (also called coating) a thin woven nylon fabric with liquid silicone from both sides. This makes it strong for its weight as the silicone adds to the fabric's tear strength, it is highly waterproof, but not breathable. Many colours are available.
Some versions are made from the stronger type 66 nylon and are used for parachutes, hot air balloons etc.
For More Information: Hot air Balloons: K-12 Projects, Experiments & Background Information
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)