Special foam pads have been developed for use in helmets. The foams in these pads have been formulated to obtain a combination of light weight, resistance to oxidation, flame retardance, superior shock-absorbing properties, and physical comfort during long use. In the original application, the pads are intended especially for use in supporting the heads of reclining astronauts during launch, providing a slight upward tilt so that the astronauts can see critical panels, switches, and checklists more easily. Modified versions of the pads and foams may be useful on Earth in helmets for motorcyclists, athletes, and others.
The major ingredient of a foam of this type can be either of two materials: a soft silicone rubber or Viton® (or equivalent) fluoropolymer. In the case of a fluoropolymer, the density of the foam is about 5 lb/ft3 (about 80 kg/m3), which is considerably less than the 12 lb/ft3 (about 192 kg/m3) of standard commercial Viton® foams. The density is reduced by replacing part of the Viton A (or equivalent) resin with Viton LM (or equivalent), decreasing the proportion of carbon-black filler, and using a greater proportion of blowing agent. The temperature-vs.-time schedule followed in molding and curing is also modified to obtain uniform cell size and good foam skin thickness at low density.
In the case of a silicone formulation, a soft silicone rubber is compounded with activators, cross-linking agents, and blowing agents. Increasing the amount of blowing agent decreases the density and hardness, while increasing the amount of both cross-linking and blowing agents decreases the postcuring shrinkage of the foam.
Whether fluoropolymer or silicone, the base rubber is banded and cut off several times on a two-roll rubber mill. The ingredients are added slowly to the milling rubber, one at a time, until the blend is uniform. The blowing agent, Celogen 130 (or equivalent), is added last to prevent activation of it during milling. The temperature of the blend is maintained below 140 °F (60 °C) by water-chilled nip rolls. When the ingredients are uniformly blended, a slab of uncured rubber is sheeted off the mill at a thickness slightly greater than 1/4 in. (0.635 cm).
A slab, typically 4 × 4 in. (about 10 × 10 cm), is cut from the sheet and pressed between flat plates at room temperature for 10 min at a pressure of 300 lb/in.2 (about 2.1 MPa) to produce a preform slightly over 1/4 in. thick. The preform is placed in a preheated mold and held at 350 °F (177 °C) for 15 min, then cooled with water to room temperature. The mold is opened and the preform pops out, expanded to four times its original volume.
This work was done by Frederic S. Dawn and Jean S. Alexander of Johnson Space Center and Richard P. Tschirch and Paul M. Drennan of Arthur D. Little, Inc.