The differences between HPMC and MC, HEC, CMC
1,
methyl cellulose (MC)
(1)
Methylcellulose is soluble in cold water, and it is difficult to dissolve in
hot water. The aqueous solution is very stable in the range of pH=3~12. It has
good compatibility with starch, guar gum and many other surfactants. When the
temperature reaches the gelation temperature, a gelation phenomenon occurs.
After
the refined cotton is treated with alkali, methyl chloride is used as an
etherifying agent, and a series of reactions are carried out to prepare a
cellulose ether. The degree of substitution is generally 1.6 to 2.0, and the
degree of substitution differs depending on the solubility. It belongs to
nonionic cellulose ether.
(2)
The water retention of methyl cellulose depends on its addition amount,
viscosity, particle fineness and dissolution rate. Generally, the amount of
addition is large, the fineness is small, and the viscosity is large, and the
water retention rate is high. The addition amount has the greatest influence on
the water retention rate, and the viscosity level is not proportional to the
water retention rate. The rate of dissolution depends mainly on the degree of
surface modification of the cellulose particles and the fineness of the
particles. Among the above cellulose ethers, methylcellulose and
hydroxypropylmethylcellulose have a high water retention rate.
(3)
The change of temperature will seriously affect the water retention rate of
methyl cellulose. Generally, the higher the temperature, the worse the water
retention. If the temperature of the mortar exceeds 40 ° C, the water retention
of methyl cellulose will be significantly deteriorated, which seriously affects
the workability of the mortar.
(4)
Methyl cellulose has a significant effect on the workability and adhesion of
the mortar. “Adhesiveness” herein refers to the adhesive force felt
between the worker’s application tool and the wall substrate, that is, the
shear resistance of the mortar. The adhesion is large, the shear resistance of
the mortar is large, the force required by the worker during use is also large,
and the workability of the mortar is poor. Methylcellulose adhesion is
intermediate in cellulose ether products.
2,
hydroxypropyl methylcellulose (HPMC)
Hydroxypropyl
methylcellulose is a cellulose variety that has been rapidly increasing in both
production and dosage in recent years. It is a nonionic cellulose mixed ether
prepared by a series of reactions using propylene oxide and methyl chloride as
an etherifying agent after alkalization of refined cotton. The degree of
substitution is generally 1.2 to 2.0. Its properties are different depending on
the ratio of methoxyl content and hydroxypropyl content.
(1)
Hydroxypropyl methylcellulose is easily soluble in cold water, and it may be
difficult to dissolve in hot water. However, its gelation temperature in hot
water is significantly higher than that of methyl cellulose. The dissolution in
cold water is also much better than methyl cellulose.
(2)
The viscosity of hydroxypropylmethylcellulose is related to the molecular
weight thereof, and the viscosity is high when the molecular weight is large.
The temperature also affects its viscosity, the temperature rises, and the
viscosity decreases. However, its viscosity has a lower temperature than methyl
cellulose. Its solution is stable at room temperature.
(3)
The water retention of hydroxypropylmethylcellulose depends on the amount of
addition, viscosity, etc., and the water retention rate under the same amount
is higher than that of methylcellulose.
(4)
Hydroxypropyl methylcellulose is stable to acids and bases, and its aqueous
solution is very stable in the range of pH=2~12. Caustic soda and lime water do
not have much effect on their properties, but alkali accelerates their
dissolution rate and increases viscosity. Hydroxypropyl methylcellulose has
stability to general salts, but when the concentration of the salt solution is
high, the viscosity of the hydroxypropylmethylcellulose solution tends to
increase.
(5)
Hydroxypropyl methylcellulose can be mixed with a water-soluble polymer
compound to form a uniform, higher viscosity solution. Such as polyvinyl
alcohol, starch ether, vegetable gum and the like.
(6)
Hydroxypropyl methylcellulose has better enzymatic resistance than
methylcellulose, and its solution is less likely to be enzymatically degraded
than methylcellulose. Hydroxypropyl methylcellulose is more adhesive to mortar
than methylcellulose.
3.
Hydroxyethyl cellulose (HEC)
It
is prepared by subjecting refined cotton to alkali treatment and then reacting
with ethylene oxide as an etherifying agent in the presence of acetone. The
degree of substitution is generally 1.5 to 2.0. It has strong hydrophilicity
and is easy to absorb moisture.
(1)
Hydroxyethyl cellulose is soluble in cold water, and it is difficult to
dissolve hot water. The solution is stable at high temperatures and does not
have gel properties. It can be used for a long time in the mortar at high
temperature, but the water retention is lower than that of methyl cellulose.
(2)
Hydroxyethyl cellulose has stability to general acid and alkali, alkali can
accelerate its dissolution, and slightly increase the viscosity, its dispersion
in water is slightly worse than methyl cellulose and hydroxypropyl methyl
cellulose. .
(3)
Hydroxyethyl cellulose has good performance against mortar hanging, but the
retardation time for cement is longer.
(4)
Hydroxyethyl cellulose produced by some domestic enterprises has a performance lower
than that of methyl cellulose due to its high water content and high ash
content.
4,
carboxymethyl cellulose (CMC)
After
the natural fiber (cotton, etc.) is treated with a base, sodium chloroacetate
is used as an etherifying agent, and a series of reaction treatments are
carried out to prepare an ionic cellulose ether. The degree of substitution is
generally 0.4 to 1.4, and its performance is greatly affected by the degree of
substitution.
(1)
Carboxymethyl cellulose has a large hygroscopicity and generally contains a
relatively large amount of water during storage.
(2)
The carboxymethylcellulose aqueous solution does not produce a gel, and the
viscosity decreases as the temperature rises. When the temperature exceeds 50 °
C, the viscosity is irreversible.
(3)
Its stability is greatly affected by pH. Generally used in gypsum-based mortar,
it can not be used in cement-based mortar. When it is highly alkaline, it loses
its viscosity.
(4)
Its water retention is much lower than that of methyl cellulose. It has a
retarding effect on the gypsum-based mortar and reduces its strength. But the
price of carboxymethyl cellulose is significantly lower than that of methyl
cellulose.