This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
The question often arises whether tubes can be practically and safely
patched. I suppose the question comes up because some people have had
leaky patches or they consider it an arcane exercise. Either way, it
need not be difficult if simple rules are followed.
Tubes are made in metal molds to which they would stick if mold
release were not sprayed into the mold. The release agent is designed
to prevent adhesion and it will do the same for patches because it
remains on and in the surface of the tube. To make a patch stick,
this material must be removed. That means, the sand paper in the
patch kit is not to roughen the surface but to remove it. Not
removing the 'skin' of the tube is a major reason for leaky patches.
Once the mold release has been removed, rubber solution can be applied
with the finger by wiping a thin film over the entire area that the
patch is to cover. After the glue has dried so that no liquid or
jelly remains, leaving the area with a tacky sheen, the patch should
be pressed into place. Patches can be made from tube material but
this must be done carefully following the same procedure as preparing
the tube. The trouble is that butyl tube material, unlike patches, is
impervious to rubber cement solvents and will never cure if the glue
is not completely dry. This presents a substantial problem.
Patches commonly have a metal foil cover on their sticky side and a
cellophane or impervious paper cover on the other. The foil should be
pulled off to expose the adhesion surface and the patch pressed into
place. The backing paper or cellophane often has perforations that
will break if the tube and patch are stretched. This makes peeling
the cover from inside to outside of the patch possible and prevents
peeling a newly installed patch from its position.
REMA patches, the most commonly available in north American bicycle
shops, have a peculiarity that not all have. Their black center
section exudes a brown gas that discolors light colored tire casings
in daylight. This causes the brown blotches often seen on sidewalls
of light colored tires.
Assuming the patch was properly installed, it will still possibly leak
after a few miles, if used immediately after patching. Because the
tube is generally smaller than the space inside the tire, to prevent
wrinkles on installation, it will stretch when inflated as does the
patch. Although it stretches less than the rest of the tube by the
greater thickness, it resists stretch more than the tube alone. Under
the patch, the stretched tube tends to shrink away from the patch, and
because there is no holding force from inflation pressure at the hole,
the tube can peel away from the patch that is held by air pressure.
If the puncture is a 'snake bite', the chances of a leak are even
greater. Pinch flats from insufficient inflation or overload are
called snake bites because they usually causes a pair of holes that
roughly approximate the fang marks of a snake. These holes are near
the rim where the contour of the tube is nearly a sharp fold. This
location is especially susceptible to the tube separation at the hole
closest to the rim.
In a rolling tire, the patch and tube flex, shrink, and stretch making
it easier for the tube to separate from a partially cured patch. To
test how fast patches cure, a patch can be pulled off easily shortly
after application, while it is practically impossible after a day or
so. For best results, the freshly punctured tube should be patched
and put in reserve, while a reserve tube is installed. This allows
a new patch more time to cure before it is put into service.
A tube can be folded into as small a package as when it was new and
practically airless, by sucking the air out while carefully using the
finger opposite the stem to prevent re-inflation. This is not done by
inhaling but by puckering the cheeks. Although the powders inside the
tube are not poisonous in the mouth, they are not good for the lungs,
but then that's obvious.
The difficult part of loose patches is that separation always stops at
the edge of the patch because air pressure prevents further
separation. The annoying intermittent slow leaks that occur, often
close when the tube is inflated outside a tire, so the offending patch
cannot be found. Old tubes to be discarded often reveal partial
failures by cutting through the center of patches with shears.
Tires are less flexible at a patch and will wear slightly faster
there, but patches have no effect on dynamic balance since wheels are
so imbalanced that patches have no effect on the heaviest position of
the wheel. Heat from braking can accelerate separation of a fresh
patch but this generally does not pose a sudden hazard because lifting
patches most often causes only a slow leak.