|
Proper
selection of rheology modifiers
Familiarity
with the rheological nuances of a particular modifier can differentiate
an exceptional formulation from a routine one
Gum
Arabic (Acacia)
Rheology:
Gum Arabic is a very low viscosity gum, with possible concentrations
of upto 50 per cent in water. Below a 40 per cent concentration,
solutions are Newtonian; above 40 per cent they are pseudoplastic.
Solutions
show reversible viscosity loss at elevated temperature and possess
yield value at sufficient concentration.
Compatibilities:
Gum Arabic is compatible with moderate amounts of most salts, acids
and alkalis, as well s with most water-soluble thickeners. Solutions
are stable between pH 1 and pH 14; viscosity peas at pH 6, dropping
sharply below pH5 and abvoe pH7. Electrolytes depress solution viscosity.
Solutions are tolerant of water-miscible solvents to about 50 per
cent of solution weight, and are susceptible to bacterial, heat
and UV degradation.
Gum
Tragacanth
Rheology:
Gum Tragacanth is available in grades of varying quality and refinement
with 1 per cent viscosities of about 300 cps to 3000 cs. Solutions
are pseudoplastic, show a reversible decrease in viscosity at elevated
temperatures and possess good yield value.
Compatibilities:
Gum Tragacanth solution are tolerant of monovalent and divalent
cations, but are precipitated by trivalent species. They show a
limited tolerance of water-miscible solvents, but provide synergistic
viscosity with glycerin. Solutions are stable between pH 2 and pH
11, with some loss in viscosity at pH 5. Gum Tragacanth is compatible
wtih most water-soluble thickener. Solutions are unusually resistant
to bacterial growth and degradation.
Sodium
Alginate
Description:
Sodium alginate is an anionic polysaccharide extracted principally
from the giant kelp Macrocystis pyrifera as alginic acid and neutralized
to the sodium salt. It is soluble in hot or cold water and gives
somewhat hazy solutions of neutral pH.
Rheology:
Sodium alginate is available in grades ranging from about 20 cps
to about 1000 cps at 1 per cent. Solutions are pseudoplastic and
show a reversible decrease in viscosity at elevated temperatures.
Sodium alginate solutions lack yield value.
Compatibilities:
Sodium alginate has limited compatibility with monovalet salts.
Polyvalent cations tend to cause gelation or precipitation. Solutions
show a fair to good tolerance of water miscible solvents (10 to
30 per cent of volatile solvents, 40 to 70 per cent of glycols).
Highy efined sodium alginate shows good stability over the pH 3
to pH 10 range; residual Ca raises the pH minimum to 5. sodium alginate
is compatible with most water soluble thickeners and resins. Its
solutions are more resistant to bacterial and enzyme degradatin
than those of many other organic thickeners.
Polyacrylates
Description:
The polyacrylates are anionic polymers typically produced by emulsion
polymerization of acrylic acid or substituted acrylic acid. They
are most often sold as thin emulsions of poly (acrylic acid) which
upon neutralization give clear solutions.
Rheology:
Sodium polyacrylates usually give pseudolplastic solutions with
some yield value. With the multitude of variatins of sodium polyacrylate
and copolymers containing polyacrylate, almost any desired rheology
can be found. Solutions usually show a reversible dcrease in viscosity
at elevated temperatures.
Compatibilities:
Many organc and inorganic bases can neutralize poly (acrylic acid)
to produce viscous, clear solutions. Neutralized solutions have
limited compatibility with excess alkali or electrolytes before
salting out. Eletrolytes tend to depress solution viscosity. Sodium
polyacrylates are compatible with many water-soluble thickeners
and resins. Solutions are resistant to bacterial degradation, but
susceptible to shear degradation.
Carbomer
Description:
Carbomers are anionic carboxyvinyl polymers made by chemically crosslinking
poly(acrylic acid). They are dispersible in hot or cold water and
give acidic solutions; they must be neutralized to develop desired
rheological properties.
Rheology:
Carbomers are typically neutralized with sodium hydroxide (NaOH)
or triethanolamine (TEA) to give extremely high viscosity, clear,
colorless gels at 0.5 per cent (depending on the grade) or smooth-pouring
viscous solutions at lower levels. Solutions are pseudoplastic,
possess excellent yeild value and show some reversible viscosity
loss at elevated temperatures.
Compatibilities:
Electrolytes depress the viscosity of neutralized solutions to carbomers,
usually in proportion to the valence of the cation. NaOH or TEA
neutralized solutions show good viscosity stability over the range
of approximately pH 5 to pH 11. Carbomer solutions typically show
excellent tolerance of water miscible solvents. A number of polar
liquids can be effectively thickened in the anhydrous state by the
proper coice of neutralizing agent. Water solutions are generally
resistant to shear, heat, bacterial and enzyme degradation, but
are susceptible to UV degradation.
(Concluded)
Courtesy:
R T Vanderbilt, USA
|
