10.1002/spepro.004339
Starch-filled polymer
composites
Sanghoon Kim
A new method produces robust, degradable polymer composites at
room temperature from starch and alkyl cyanoacrylate.
Degradable polymer composites have long been considered part of the
solution to the environmental and waste management problems posed
by extensive use of non-degradable polymeric materials. One of the
most commonly used approaches to create such degradable composites
is to incorporate starch into degradable synthetic polymers.1 Starch, a
semi-crystalline polymer stored in granules as a reserve in most plants,
is renewable, non-toxic, and completely biodegradable.2 It has received
increased attention as a filler for composites (or as a component of polymer blends) because of its cost effectiveness and high volume availability. Both granular and destructurized starches have been used in
combination with other polymers such as poly(propylene carbonate),
polycaprolactone, poly(butylene succinate), poly(3-hydroxybutyrate),
poly(lactic acid), and poly(vinyl alcohol).3–8
Composite materials are made from two or more constituent materials that are significantly different from each other in physical or
chemical properties. For polymer composites, the polymer needs to be
fluidized during production, which requires application of heat under
pressure, unless the desired product is a film, where organic solvents
can be used and removed later by evaporation.
We have designed a composite production process that does not
require heat because it uses a monomer instead of a polymer. The
monomer molecules spontaneously begin polymerization after mixing with starches. We chose to use the monomer ethyl cyanoacrylate (ECA), which is is a liquid at room temperature and has
low viscosity and excellent wetting properties. It undergoes spontaneous anionic polymerization at room temperature by weak bases to
form poly(ethyl cyanoacrylate) or PECA, a degradable, uncrosslinked
homopolymer.9, 10 ECA will polymerize between two objects, gluing
them together, if their surfaces contain initiators for polymerization and
if the gap between the objects is small enough for the polymers to hold.
We blended ECA with starch. Hydroxyl ions on the surface of the
starch granules act as an initiator and the micrometer-scale gaps between starch granules satisfy the condition for forming strong bonds
between particles. Once initated by the hydroxyl ions, the reaction is
Figure 1. Various samples made from composites with 60% starch: (A)
– (C) with dyed cornstarch (homogeneous coloring), (D) – (F) with
dyed cornstarch (inhomogeneous coloring), (G) with conductive filler
and cornstarch, (H) with potato starch, (I) with plain cornstarch, and
(J) with cornstarch molded using the base of a water bottle.
highly exothermic, self-propagating, and does not require application
of heat. The polymerized ECA acts as a binder between the starch
granules, resulting in robust particle-filled polymer composites (see
Figure 1).
Polymerization time depends on the moisture content of the starch,
which can vary from batch to batch. Consequently, the humidity of the
room also influences the rate of the reaction. The temperature of the
mixture increases as the reaction proceeds. In increases very slowly at
the beginning and then very abruptly toward the end, until eventually it
reaches peak temperature. At that moment, the whole reaction mixture
hardens, indicating that the polymerization process has finished. After
that, the temperature of the mixture begins to drop to room temperature. The polymerization time (cure time) is adjustable by lowering the
moisture content of the starch using a vacuum oven or microwave oven.
The starch concentration needs to be selected with care. At a concentration below 54%, the starch tends to precipitate slowly in the reaction mixture before polymerization is complete (because the density
of starch is slightly higher than that of the cyanoacrylate monomer).
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10.1002/spepro.004339 Page 2/2
When the content of starch is higher than 62%, the reaction mixture
shows shear-thickening behavior, making stirring and homogenization
very difficult. Considering these restrictions, the optimal composition
is between 60% and 62% starch. The compressive strength of 60%
starch composite is about 80 MPa, which is is comparable to that of
polyethylene terephthalate (PET) and is higher than that of most commonly used polymers, such as acrylonitrile butadiene styrene (ABS),
polyethylene, polypropylene, polystyrene, and polycarbonate.
Our starch/ECA polymer composite has many merits. It polymerizes
at room temperature without a catalyst in under 20 minutes and can be
molded to any shape (see Figure. 2). It is degradable, strong, and stable at temperatures up to 150ı C. The composite is machineable, and
its hydrophobic surface makes it stable upon contact with water. It can
also be colored easily. Conductive composites can be manufactured by
adding conductive fillers, such as carbon black (see Figure 1). Meanwhile, additional components such as fiber, sand, or pebbles, can easily
be incorporated to improve mechanical properties.
In summary, our degradable biopolymer composite makes use of
the unique properties of starch granules. The surface of the granules
supplies the initiator for the polymerization of ethyl cyanoacrylates.
The granules act as a filler, and the micrometer-scale gaps between the
starch granules satisfy a necessary condition for the polymerization of
ECA. Although ECA is the most commonly used monomer, other alkyl
cyanoacrylate monomers can also be used for the same purpose.
The most unique feature of the presented composites is that they
can be manufactured in a short time at ambient temperature without
specialized facilities.11 Since the production process is simple and inexpensive, the developed polymer composite has a good potential for
commercialization. The necessity for further research depends on the
specific application field.
Figure 2. Sculptures made using composites with 60% cornstarch and
latex molds.
Author Information
Sanghoon Kim
United States Department of Agriculture/Agricultural Research
Service/National Center for Agricultural Utilization Research (USDA/
ARS/NCAUR)
Peoria, IL
Sanghoon Kim is a research chemist whose main research interests
include degradable polymer composites and nanoparticles containing
agricultural byproducts, such as starches and proteins.
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doi:10.1002/pc.22218
c 2012 Society of Plastics Engineers (SPE)