Subject: 8b.1   Patching Tubes
From: Jobst Brandt <jbrandt@sonic.net>
Date: Thu, 5 Mar 2009 15:47:21 -0800

WHY PATCHES FAIL

Tubes are made in metal molds to which they would stick if mold
release were not sprayed into the mold before making each tube.  The
release agent is designed to prevent adhesion and it can do the same
for patches, much of it having transfered onto the surface of the
tube.  To make a reliable patch, mold release must be removed.  Good
patch kits have sand paper that is not used to roughen the surface but
to remove it.  Failure to remove the "skin" of the tube is a cause
of leaky patches.

Tube surfaces often have ridges at mold seams and near the valve stem
that prevent effective sanding.  Occasionally this presents a problem
if the leak is directly adjacent to a mold ridge.  For most punctures
that do not abut on a mold ridge, rubber glue and plasticity of the
orange (REMA) patch material can make an effective seal.  However, a
plastic disposable (BIC) razor can remove interfering mold ridges and,
with handle removed, makes a handy addition to a patch kit.

Once mold release has been removed, rubber glue 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 until no liquid or jelly
remains, leaving a tacky sheen, the patch can be pressed into place.

For best results, press the patch firmly into place -before- the
rubber glue is fully dried and still tacky to the touch and let the
patch cure (evaporate the solvents) overnight.  Such patches have
shown to be unremovable without heat, while ones placed on dried
rubber adhesive can be pulled off using an ice pick stabbed into their
center.

Although patches can be made from tube material, this does not work
well because butyl tube material, unlike commercial patches, is
impervious to rubber cement solvents and will not adhere reliably.

PATCHES
 
Patches commonly have a metal foil cover on the sticky side and a
cellophane or impervious paper cover on the back.  The foil must be
pulled off to expose the adhesion surface before pressing the patch
into place.  The backing paper or cellophane often has perforations so
that it will split when tube and patch are manually stretched.  This
makes peeling the cover of the patch from its center to the edge
possible and prevents peeling a newly applied patch from the tube.
The cellophane should be removed to allow volatile solvents in the
patch interface to escape and properly cure the patch.

REMA patches, the most commonly available in 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 brown blotches often seen on light colored sidewall tires.

STEM SEPARATION

Stem separations were common about a year ago with many believing the
knurled stem nut was pulling stems out of their tubes.  To show that
this is incorrect, manually screw the stem nut down as tight as
possible before inflation.  After inflation to about 100 psi, the nut
will be loose and not bearing on the rim because inflation pressure is
a greater force than one can muster with thumb and forefinger on the
knurled nut.

As I mentioned in that thread, these are a manufacturing flaw for
which there is no patch repair because the stem alone separates from
the rubber and the only way to fix that is to pull the stem cleanly
from the tube and insert a stem from an old latex tubed tubular tire.
Placing a patch around the stem does not fix the leak because the leak
is between the brass stem and butyl rubber tube.

Fortunately that era is behind us but there are still shops that did
not return the faulty tubes to their supplier and get ones that don't
separate.  So there are still some around.

 http://www.sheldonbrown.com/brandt/tube-failure.html

LEAKY PATCHES 

Assuming a patch was properly installed, it may still leak after a few
miles, if used immediately after patching.  Because tubes are
generally smaller than the inside of the tire to prevent wrinkles on
installation, they stretch on inflation, as does the patch.  The
stretched tube under the patch tries to shrink away from the patch.
Because there is no holding force from inflation pressure at the hole,
the tube can gradually peel away from the patch starting at the hole,
while the tube under the remainder of the patch is pressed against it
by air pressure.

Flexing of rolling bias ply tires also works to loosen patches.
Laying a standard 3.5 x 2 inch paper business card between tire and
tube shows how severe this action is.  After a hundred miles or so,
the card will have been shredded into millimeter size confetti.  That
fresh patches move and delaminate is also visible in that they become
dome shaped although they were flat when first installed.  In
contrast, patches that have cured overnight before use will remain
flat and fully adhered.

If the puncture is a 'snake bite', chances of a leak are greater.
Pinch flats from insufficient inflation or overload are called snake
bites because they usually cause two holes that roughly approximate
the fang marks of a snake.  Although a single patch will usually cover
both holes, these will be closer to the edge of the patch and have a
shorter leakage path.

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.
Proof that patches cure is shown by how easily a patch can be pulled
off shortly after application, while it is practically impossible a
day later.  For reliable patches, the freshly patched tube should be
put in reserve, while a reserve tube is installed.  This allows a new
patch more time to cure before being 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 using the finger
opposite the stem to prevent self re-inflation.  This is not done by
inhaling but by puckering the cheeks.  Although the powders inside
tubes are not hazardous in the mouth, they are less so in the lungs.

PATCH REMOVAL

The best remedy for a leaky patch is to remove it and start over.
However, after several days of curing, even leaky patches are hard to
remove.  With heat supplied by a hot iron or heated frying pan at
moderate temperature, patches can be removed more easily by pressing
the patch against a hot surface with the thumb until the heat is felt
through the tube at which time the patch can be pulled off.  Patch
remnants can be cleaned off with rubber solution (patch glue).

Minutia
 
Separating patches are often hard to find because separation usually
stops at the edge, air pressure preventing further separation.  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 patch separation when cut through the center of
a dome shaped patch with shears to reveal talcum powder from the
inside of the tube under most of the patch.
  
Although talcum powder on the outside of tubes does nothing useful, it
is essential on the inside, where it is found in any butyl tube.
Without it, tubes would adhere to themselves after manufacture and not
inflate properly.  Externally, talcum may prevent adhesion to the
tire, and slight this adhesion is, it helps prevent sudden air loss in
the event of a puncture, but it does nothing for the wellbeing of the
tube.  When inflated, tubes act like an integral part of tire casings
with or without talcum.

Because tires are less flexible at a patch, tread may wear slightly
faster there, but patches have no effect on dynamic balance since
wheels naturally have a greater imbalanced than patches can cause and
have no effect on the heaviest position of the wheel which is either
at the valve stem or the rim joint.