Yekaterinburg, Ekaterinburg, Russian Federation
Yekaterinburg, Ekaterinburg, Russian Federation
Yekaterinburg, Ekaterinburg, Russian Federation
Yekaterinburg, Ekaterinburg, Russian Federation
Yekaterinburg, Ekaterinburg, Russian Federation
Yekaterinburg, Ekaterinburg, Russian Federation
Introduction. Processing agricultural waste into plant biodegradable plastics is a promising way for its recycling. This work featured the main physical-and-mechanical properties of plant plastics without adhesive substances obtained from millet husk and wheat husk and wood plastic obtained from sawdust, as well as their biodegradation potential. Study objects and methods. Objects of the study were plastics without adhesives based on wood sawdust, millet husk, and wheat husk. Results and discussion. We analyzed of the physical-and-mechanical parameters of the plant plastic based on millet husk, wheat husk, as well as wood plastic based on sawdust. The analysis showed that, in general, the strength characteristics of the wood plastics were higher than those of the plastics based on millet husk, especially flexural strength. Thus, the average value of the density of the wood plastic exceeded that of the plant plastic from millet husk by 10%, hardness by 40%, compression elasticity modulus by 50%, and flexural modulus by 3.9 times. It was found that wood and plant plastics obtained from sawdust, millet husk, and wheat husk without adhesives had a high biodegradation potential. Conclusion. The plastics obtained can be used as an insulating, building, and decorative material in the steppe regions experiencing a shortage of wood and wood powder.
Plastic, agricultural waste, grain, husk, biodegradation
INTRODUCTION
The concept of organic agriculture, which first
appeared in European countries, has gained its
popularity in Russia in the last few years. According
to the concept, agricultural industry should ensure the
environmental and biological safety of technologies, raw
materials, and products [1–3]. In Russia, a new federal
law on organic products comes into force in 2020 that
regulates the activity of agricultural enterprises. It
prohibits the use of packaging and transport package
which damages to the environment and encourages
the application of methods and technologies aimed
at ensuring a favorable state of the environment,
strengthening human health, as well as at maintaining
soil fertility.
Besides, one of the problems is the utilization of
agricultural wastes, which are currently mainly stored
or disposed. Only a small part of them is used to
produce coarse low-value feed and bedding for animals,
fertilizers, or fuel [4]. These include straw, flax shive,
coffee grounds, nutshells, plant waste from cereals and
flour manufacture, as well as from fruit and berries
processing. In Russia, efficient agricultural enterprises
generate several million tons of such waste annually.
In the Altai Territory, for example, the amount of grain
husks obtained at elevators is on average 2–3 million
tons per year [5].
One of the alternatives of plant waste recycling
is the production of bioplastics and paper [6–9]. In
the world practice, plant fillers from cereal husks
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Glukhikh V.V. et al. Foods and Raw Materials, 2020, vol. 8, no. 1, pp. 149–154
and agricultural plant fibers are widely used in the
production of biocomposites and reinforced bioplastics.
They can be produced both from plant materials
only and with the addition of classical petrochemical
products [10–12]. The improved technologies and
performance characteristics of bioplastics as well as
production cost reduction make it possible to fill new
niches in the market [13]. Bioplastics without adhesive
substances based on plant materials are environmentally
friendly. Plant materials can be wood powder, sawdust,
cereal husk, as well as flax and hemp fibers [6, 13].
Such plastics are biodegradable materials, and their
degradation is due to microbial enzymes [14–16].
The use of plant biodegradable plastics, including
plastics without adhesives, to produce containers with a
short life cycle, building and packaging materials can be
a promising way for agricultural waste recycling, which
corresponds to the concept of organic agriculture [7,
17, 18].
Taking into consideration the obvious advantages of
biodegradable plastics, the study of the main physicaland-
mechanical properties of plant plastics without
adhesives obtained from millet husk and wheat husk and
analysis of their biodegradation potential are relevant.
STUDY OBJECTS AND METHODS
The objects of the study were plastic samples
without an adhesive substance based on wood and plant
materials. The samples were made at the Ural State
Forestry University. Wood-based plastics made from
industrial sawdust (State Standard 18320-78I) were used
as control samples. Experimental samples were plant
plastics obtained from millet husk and wheat husk,
cereal production wastes.
The plant fillers of 18.0 and 30.0 g in mass were
subjected to pressing to obtain disks of 2.0 and 4.0 mm
in thickness and 90 mm in diameter (moisture content
of the molding material was 12%). The conditions
of pressing were as follows: molding material mass,
10.0 g; pressing pressure, 124.0 MPa; pressing time,
10 min; and cooling time under pressure, 10 min. The
physical-and-mechanical characteristics of the samples
were analyzed both before and after biostability and
biodegradation tests. We determined water absorption
(State Standard 4650-80II) and strength parameters such
as density, flexural strength, hardness, elasticity number,
compression elasticity modulus, elastic modulus in
flexure, breaking stress, yield strength (State Standard
4648-71III, State Standard 4670-77IV, State Standard
I State Standard 18320-78. Technological wooden sawdust for
hydrolysis. Specifications. Moscow: Izdatelʹstvo standartov; 1986. 7 p.
II State Standard 4650-80. Plastics. Methods for the determination of
water absorption. Moscow: Izdatelʹstvo standartov; 2008. 7 p.
III State Standard 4648-71. Plastics. Method of static bending test.
Moscow: Izdatelʹstvo standartov; 1992. 11 p.
IV State Standard 4670-77. Plastics and ebonites. Method for
determination of hardness by ball indentation under a given load.
Moscow: Izdatelʹstvo standartov; 1992. 6 p.
10634-88V).
To study biodegradation potential, the test samples
were kept in soil for 21 days, then the main visual
morphological characteristics of their biodegradation
were evaluated. Soil was prepared in accordance with
State Standard 9.060-75VI. At the beginning of the test,
pH of the soil extract was 7.0 and biological activity
coefficient was 0.8. The soil microbiocenosis was
formed by native field strains of microorganisms of the
initial components of the soil.
We observed such biodegradation signs as splitting,
swelling, loosening, macro- and microcavities
formation, changes in the shape and size of the main
plant component particles, fibrillation and fragmentation
of particles, the local discoloration of the sample,
the presence of colonies of microorganisms, hyphae,
fungal fruit inside or on the sample surface, as well as
its mucilagination. The samples that did not display
biodegradation signs were tested for strength and water
absorption.
In addition, the test with the germination of oat
and clover seeds on a substrate containing the samples
under study was carried out. For this, the substrate was
prepared that included two layers of multi-purpose soil
(60%) alternating with two layers of the samples (40%).
Multi-purpose soil was used as control sample. Oat and
clover seeds were sown in the substrate, germinated
for 21 days, after that growth rate, as well as stem and
leaves formation were evaluated in the experimental
and control samples. The root system of the plants was
determined in visible light and in ultraviolet light.
RESULTS AND DISCUSSION
We analyzed the physical-and-mechanical parameters
of plant plastics based on husks of millet and
wheat and wood plastic based on sawdust. The samples
did not include adhesives. The analysis showed that the
strength characteristics of the wood-based samples were
higher than those of the plastics based on millet husk,
especially flexural strength. Thus, the average density of
wood plastics exceeded that of plant plastics from millet
husk by 10%, hardness by 40%, compression elasticity
modulus by 50%, and flexural modulus by 3.9 times
(Figs. 1 and 2).
A comparative analysis of the physical-andmechanical
properties of the wood plastic samples with
the samples of plant plastics from wheat husk showed
similar results. Thus, the average value of the density
of the wood-based plastics exceeded that of the plant
samples from wheat husk by 15%, hardness by 10%,
compression elasticity modulus by 10%, and flexural
modulus by 2.6 times.
V State Standard 10634-88. Wood particle boards. Methods
for determination of physical properties. Moscow: Izdatelʹstvo
standartov; 1991. 10 p.
VI State Standard 9.060-75. Unified sуstem of corrosion and ageing
protection. Fabrics. Method of laboratory tests for microbiological
destruction stability. Moscow: Izdatelʹstvo standartov; 1994. 9 p.
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Glukhikh V.V. et al. Foods and Raw Materials, 2020, vol. 8, no. 1, pp. 149–154
According to the results of the comparative analysis
of the physical-and-mechanical properties of the two
plant plastic samples, the average values of hardness,
compression elasticity modulus, and elastic modulus in
flexure of the plastic from millet husk were higher than
those of the samples based on wheat husk by 1.3, 1.3, and
1.5 times, respectively.
An increased pressing temperature (from 180 to
170°C) led to an improvement in the physical-andmechanical
properties of the plant plastics compared to
the wood-based samples. Thus, the values of elasticity
number, the elasticity moduli, breaking stress, and
yield strength increased. Presumably, this is due to the
difference in the dynamics of lignin polymerization
reactions in wood and plant plastics. This factor should
be taken into consideration when selecting pressing
conditions.
The study of the biodegradation potential of plastics
without an adhesive component based on wood sawdust,
millet husk, and wheat husk showed that all the samples
studied had a relatively equal high biodegradation
potential.
We analyzed the morphological signs of
biodegradation of the materials kept in active soil
for 21 days. The analysis showed that all the samples
had surface mucilagination, edge swelling, and local
discoloration of the surface (Fig. 3).
60% of the samples based on millet husk, 58% of the
samples from wheat husk, and 47% of the wood plastics
based on sawdust displayed longitudinal and transverse
splitting, loosening, and macrocavities formation
(Fig. 4). The splitting and loosening sites ranged from
1.5 to 5.5 mm in size.
Microscopy was used to assess signs of the
sample destruction. The analysis revealed marginal
fibrous structure; the fragmentation and destruction
of individual particles of the plant component; focal
darkening of particles; and microcavities formation
between the particles of plant material. Moreover, all the
samples under study showed bacterial contamination.
74% of the samples with millet husk, 85% of the samples
from wheat husk, and 62% of the wood samples had
Figure 2 Average values of compression elasticity modulus,
breaking stress, yield strength, and elastic modulus in flexure
of plastics without adhesives
elastic
modulus
in flexure,
MPa 10–3
compression
elasticity modulus
MPa 10–1
breaking stress,
MPa
yield strength,
MPa
(1) wood plastic (2) plant plastics from millet husk
(3) plant plastics without from wheat husk
1 3
2
Figure 3 Plant plastic from millet husk, plant plastic from
wheat husk, and wood plastic kept in active soil for 21 days
(all plastics are made without adhesives)
Figure 4 Wood plastic fragment with signs of splitting,
swelling, marginal and longitudinal fragmentation
Figure 1 Average values of density, hardness,
and elasticity number of plastics without adhesives
(1) wood plastic (2) plant plastics from millet husk
(3) plant plastics without from wheat husk
density, kg/m3 10–2 hardness, MPa elasticity
number, %
1 2 3
80
60
40
20
0
60
40
20
0
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multiple large colonies of mold fungi of different growth
phases (Fig. 5).
The plant plastics demonstrated a more pronounced
biological destruction compared to the wood samples.
Thus, they had changes throughout the sample, while the
wood plastics were characterized by edge and surface
changes.
Further, we evaluated the growth rate and organs
formation of oat and clover grown on control and
experimental substrates. Multi-purpose soil was used as
the control substrate, while the experimental substrate
contained multi-purpose soil and the samples under
study. According to the results of the experiment,
morphological signs of retardation and deviation in
the oat and clover development were not detected. The
root system of the plants did not also have significant
differences. The roots penetrated into the plant and
wood samples, fragmenting them. The soil – root
conglomerate was analyzed in ultraviolet light. The
result was typical of these plant species; no dependences
on the presence of the samples under study in the
substrate were found (Fig. 6).
Based on the sings found, the plastics with wheat
husk had the highest degree of biodegradation among all
the samples under study.
The keeping of the plant and wood samples in
active soil for three weeks led to thier physical-andmechanical
properties deterioration. Then, hardness
decreased by 66, 70, and 62%, elasticity number by
43, 47 and 46%, and compression elasticity modulus
by 76, 80, and 73% for the wood plastics, plant plastics
from millet husk, and plant plastics from wheat husk,
respectively. Breaking stress and yield strength values
decreased by 64% and 63% for the wood-based plastics
and by 60% and 68% for the plant-based samples,
respectively.
A comparative analysis of flexural strength values
of the plastics under study without adhesives showed
that the highest average value of this indicator was for
the wood plastic samples (4 MPa), and the lowest for
the plant plastic based on wheat husk (1 MPa). Waterabsorbing
capacity was 96, 85, and 94% for the samples
with wheat husk, with wheat husk, and with sawdust,
respectively.
CONCLUSION
According to the results of the study, plant plastic
obtained under the same pressing conditions which are
used for wood plastic production had lower strength
characteristics. A decrease in the pressing temperature
by 10°C improved such strength characteristics of the
husk-based samples as elasticity number, moduli of
elasticity in compression and flexure, as well as breaking
stress. Also, the wood and plant plastics without
adhesives based on sawdust, millet husk, and wheat
husk were found to have a high biodegradation potential.
Therefore, it makes it possible to utilize such materials
naturally, i.e. without composting, in contrast with
biodegradable wood-polymer composites.
On the other hand, a high biodegradation potential
also indicates a low biostability of materials. Special
conditions should be applied to use of products from
plant plastics based on husk of millet and wheat, as
well as from wood with sawdust. The conditions
include a low humidity, no contact with water and soil,
or with using antiseptics and waterproofing agents.
The advantages of plant plastics without adhesives
based on agricultural waste ‒ millet husk and wheat
husk ‒ include their ecological safety, relative ease
of production, low cost, raw materials availability, a
high biodegradation potential, as well as performance
characteristics comparable to those of wood plastics.
Plant plastics without adhesives obtained from
agricultural waste can be relevant in steppe regions as an
insulating, building, and decorative material.
CONTRIBUTION
The authors were equally involved in developing the
Figure 5 Fragment of plant plastics from wheat husk (without
adhesives) with signs of mold growth
Figure 6 Root system of oat on a substrate with plant plastic
from wheat husk without adhesives in ultraviolet light with
luminescence foci
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Glukhikh V.V. et al. Foods and Raw Materials, 2020, vol. 8, no. 1, pp. 149–154
research concept, obtaining and analyzing data, as well
as in writing the manuscript.
CONFLICT OF INTEREST
The authors state that there is no conflict of interest.
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