Sivkov S.P., Zaitsev A.Ye. «Influence of polymer re-dispergation degree onto properties of polymer-containing dry pack mortars»

It was established with the use of laser granulometry
method, that a speed of redispergation of a polymer
powder in mortars depends on pH value of the mortar
and a type of applied electrolytic additives. Employment
of sodium formiate as an additive intensifying a
process of polymer re-dispergation enables to improve
construction and engineering properties of gypsumbased
dry pack mortars.

Redispersible polymer powders are widely used for
production of dry pack mortars for various applications,
such as facing, flooring, water and heat insulation, etc.
They render hardened mortar such properties as high
elasticity, deformability, impact resistance capability,
provide good adhesivity to various materials, abrasive
hardness, and other.

Technological process of polymer powder production
includes synthesis of polymer in a form of water
dispersion (latex) and following flash drying in a spraytype
drier till water is fully eliminated. The size of polymer
globules in a mother polymer dispersion is 0,5–0,8
my. The size of polymer granules in a dry powder varies
from 120 to 500 my.

According to general notion, at water tempering polymer
powders in a composition of mortar go through spontaneous
re-dispergagion and for the initial polymer dispersion.
At slow elimination of water from a mortar and its following
hardening, polymer globules for a film that penetrates
into a structure of hardened material. Hardened polymermodified
cement mortar is a composite material, at which a
polymer film plays a role of a reinforcing component. This
film ensures formation of additional cohesive contacts and
prevents spread of cracks in a material.

End-use properties of a composite material to a
considerable degree depend on evenness of distribution
and linear expansion of a reinforcing component (polymer
films) in a matrix of hardened mortar. The bigger
a size of polymer clusters and the more even they are
distributed inside a cement brick, the better physical,
mechanical, construction and technical properties of a
composite material are.

Evenness of distribution and linear expansion of
polymer clusters increase together with the decrease
of particle size in the original polymer. It was established
with a help of compute-rized modeling method,
that under a condition of equal degree of dispersion of a
binding material, downsize of original polymer particles
from 10 to 1 my results in the growth in an average size
of polymer clusters in hardened material by 1.88-2.49
times. The increase in linear expansion of polymer clusters,
that are formed at a process of gypsum-polymer
composition hardening, when size of polymer particles
decreases from 890 down to 640 my is experimentally
proved (Pic. 1).

A speed of re-dispergation of a polymer powder on
base of copolymer of vinyl acetate and ethylene in different
solutions has been studied applying a method
of laser granulometry. It was established that a maximum
re-dispergation speed of a polymer powder was
observed in mortars on base of Са(ОН)2 and Portland
cement, pH values of which are high (Pic. 2). The minimum
redispergation of original polymer powder was
observed in a mortar on base of gypsum building plaster
(pH of mortar is 6.86). Even after long-term stirring,
gypsum mortar contain considerable amounts of
relatively large polymer particles of the size 40-80 my,
while a size of polymer globules in an original polymer
dispersion, from which the polymer powder was made,
equals to only 0.53 my. Probably, this can explain a fact
that traditional polymer powders are not sufficiently
effective in gypsum-based dry pack mortars.

Study of factors influential for a degree of redispergation
of polymer powder used in composition of
gypsum-containing dry pack mortars, attracts interest
from scientific and practical points of view. A speed
of a polymer powder re-dispergation process depends on a character of interaction between polymer and solution.
Interaction of polymer powder particles with mortar
usually starts from penetration of water molecules
into a structure of a polymer particle under the effect of
osmotic pressure. The higher the osmotic pressure value,
the higher a speed of polymer dispergation process.
According to Flory-Huggins theory, osmotic pressure in
a polymer solution can be calculated by equation

1

Pic. 1 Polymer clusters, formed at a process of gypsum-polymer mixture hardening (polymer content — 8 mass %): a — original size of polymer particles — 30 my; b — original size of polymer particles — 5 my

Pic. 2 Changes in specific surface area of polymer particles after 10 min of re-dispergation in different solu-tions/mortars

Method of determining value of second virial coefficient
А2 is based on equation that accounts dependence
of optical density of polymer solution τ on its concentration
in the solution с and the resulting value of
osmotic pressure π:

1

The developed method has been applied to study
the influence of different additives (electro-lytes,
surface-active compounds) onto a value of the second
virial coefficient and a speed of po-lymer powder dispergation
in a gypsum solution. The Pic. 3 shows how
the surface area of po-lymer particles changes after 10
min of dispergation in CaSO4•2H2O at presence of different
electrolytes additives taking into account a value
of second virial coefficient. As obtained results show, a
speed of polymer dispergation in gypsum solution actually
grows with an increase in a value of second virial
coefficient. Relation between these two parameters is
close to linear.

It was found that additives of sals – strong electrolytes
reduced the value of А2 and slowed down a
speed of polymer dispergation in a solution. The rate
of dispergation decrease is in proportion with a polarizing
ability of ion (cation or anion), that is expressed as
an ion charge to radius ratio. Ions that are providing a
strong polarizing effect onto a solvent, reduce its activity,
which leads to reduction of osmotic pressure. Additives
of organic acids sals (sodium formiate and acetate)
increase a value of А2 and accelerate s speed of polymer
powder dispergation in a gypsum solution. The maximum
speed is observed when sodium formiate is added.
Based on earlier studies, one can expect that the use of
potassium formiate would be even more effective.

Reasons of such influence of organic acids sals onto
a value of second virial coefficient are not absolutely
clear. It can be supposed, that hydrolysis of these sals
slightly increases a value of solution pH. Probably, anions
of these sals have properties of low-molecular surfactants,
and increase affinity of solution to a polymer.

Pic. 3 Influence of electrolyte additives onto a speed of polymer dispergation: 1 — solution with no additives; 2 — Ca(NO3)2; 3 — NaNO3; 4 — LiCl; 5 — NaCl; 6 — KCl; 7 — Na2SO4; 8 — K2SO4; 9 — НСООNa; 10 — СН3СОО Na Approximation validity coefficient R2 = 0,9584

Introduction of surface-active compounds of different
nature (plasticizers of melamine-formaldehyde
and naphthaleneformaldehyde types, polycarboxylate
hyper plasticizers, sodium oleate) into a gypsum solution
does not increase a value of second virial coefficient
and does not intensify a process of polymer powder redispergation
in a solution. Search of new additives intensifying
a process of polymer powder re-dispergation
is a subject for future research works.

 Thus, introduction of redispersible polymer powders
together is intensifier of their redispergation (sodium
formiate) into a composition of gypsum-based dry
pack mortars, enables to lessen the size of polymer globules
in a mortar, enlarge the size of forming polymer
clusters, im-prove an evenness of distribution of polymer
films in hardened mortar and therefore to improve
physical and mechanical properties of the material.

It was found, that introduction of 5 mass % of redispersible
polymer based on a copolymer of vinyl acetate
and ethylene into gypsum building plaster, increases
bending and impact strength of a hardened mortar
by only 12–21%. If 5 mass % of a polymer and 3 mass
% of sodium formiate are added into gypsum building
plaster at the same time, bending and impact strength
of a hardened mortar increase by 45–78% compared to
those of sample without additives. This shows a high effect
from introduction of polymer into a composition of
mortar. At that, adhesion of gypsum mortar to surfaces
of any type increases, porosity goes down, and waterresisting
properties of hardened gypsum improves.

Observation of hardened gypsum mortar with a
method of scanning electron microscopy shows that
slowly dispergating polymer granules form isolated
pores in the material. From the inside, the pores are covered
with rather thick polymer film (Pic. 4). A structure
of material becomes more homogeneous, when a polymer
and intensifier of its redispergation are introduced together into a composition of dry pack mortar. Thin
polymer films are evenly distributed between crystals
of two-water gypsum and practically unobservable at a
given magnification power.

Based on obtained results, we conclude that intensification
of polymer powder re-dispergation processes
in dry pack mortars leads to formation of polymer cluster
with large linear extension, which increases an effect
from use of polymer in a complex composite material.

A speed of polymer powder redispergation process
depends on pH of a mortar. The maximum speed of
polymer redispergation is observed in lime or Portland
cement mortar, the minimum — in mortar on base of
gypsum building plaster. Properties of water solution

Pic. 4 Microstructure of hardened gypsum: a, b — with no additives; c, d — with 5 mass. % of polymer; e, f — with 5 mass. % of polymer and 3 mass. % of sodium formiate

Pic. 4 Microstructure of hardened gypsum: a, b — with no additives; c, d — with 5 mass. % of polymer; e, f — with 5 mass. % of polymer and 3 mass. % of sodium formiate

provide considerable influence onto a speed of polymer
reispergation process, in particular a presence of
electrolytes with high polarizing ability (they reduce
mobility of water molecules and slow down a process of
redispergation). Additives of surfactants (plasticizers,
super- and hyper- plasticizers) do not provide a considerable
influence onto a speed of polymer re-dispergation
process. Introduction of organic acids sals (sodium
(potassium) formiate or acetate) into gypsum-based dry
pack mortars speeds up a process of polymer powder
redispergation and sufficiently improves properties of
hardened mortar.


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