ADDITIVES
FOR FLEXIBLE SLABSTOCK FOAM FROM
"AIR PRODUCTS", "SCHILL & SEILACHER "& "YOKE"
1)
Silicone Surfactants:
Silicone surfactants act as emulsifiers
and foam stabilizers in the foam manufacturing
process. They contain both hydrophobic
and hydrophilic groups, enabling them
to retain an affinity for both water
and organic phases and thereby remain
near the interface. In flexible slabstock
foams, surfactants help solubilize formulation
components and stabilize the rapidly
expanding froth.
If the level of silicone surfactant
is incorrect, either foam stability problems
may lead to excessive “sign back” (foam
setting from its maximum height) or closed-cells
and shrinkage may occur. In addition,
the processing window for the tin(II)
catalyst may be narrowed considerably.
Silicon efficiency:
Silicone efficiency is a measure of
surfactants potency at a given use a
level (in pphp). Potency is measured
by rise height of foam (ability to stabilize).
Rise height is inversely proportional to
density (kg/m3).
2)
Catalysts:
Slabstock foam production requires the
use of catalysts. Two major reactions
take place during foam formation:
• Gelling
(or polymerisation) reaction
• Blowing
(or gas-producing) reaction
In the gelling reaction, polyfunctional
isocyanates react with polyols to form
polyurethane. In the blowing reaction,
the isocyanate reacts with water to form
polyurea and carbon dioxide. These reactions
take place at different rates; both reaction
rates are dependent on temperature, catalyst
level, catalyst type and
a variety of other factors. However,
to produce high-quality foam, the rates
of the competing reactions must be properly
balanced.
If the blowing reaction occurs faster
than the gelling reaction, the gas generated
by the reaction may expand before the
polymere is strong enough to contain
it and internal splits or foam collapse
can occur. In contrast, if the gelling
occurs faster than the blowing reaction,
the foam cells will remain closed, causing
the foam to shrink as it cools.
When the two reactions are properly balanced,
open cells dominate in the foam structure.
Open cells offer little resistance to diffusion
and cell-pressures quickly equilibrate without
significant foam shrinkage.
2.1 Tin(II) Catalyst:
Tin(II) compounds, especially
stannous octoate, are usually chosen
as one of the catalysts in flexible slabstock
production (exception : polyester
foams).
Tin(II)
catalyst
strongly
catalyse
the
gelling
reaction.
Insufficient tin catalyst
will lead to foam splits or possibly
collapse if the polymer fails
to gel sufficiently.
Excessive
catalyst will
result
in
closed cells
and
shrinkage.
2.2 Amine Catalysts:
Tertiary amines primarily catalyse
the reaction between the water and isocyanate.
They are often referred to as “blowing” catalysts
however; they do have varying degrees
of activity in promoting the reaction
between the Polyol and the isocyanate
(gelling reaction).
Combinations of various amines are used
in an attempt to balance the gelling
and blowing reactions so that the foaming
process can be adequately controlled,
e.g.:
• Blow
catalyst: gives more open foam
• Gel
catalyst: gives more closed foam
New catalysts are being developed to
meet the environmental concerns of industry
regarding the volatility of amines. Newly
developed reactive catalysts appear to
meet the requirements.
Stabilization:
Stabilization is a balance of silicone
surfactant, tin catalyst and amine catalyst
(of course also the Polyol, the isocyanate,
the amount of water and all the other
remaining parts of the formulation are
having an influence)
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