STUDY ON THE IMPACT OF MINERAL, FIBROUS, AND CARBON-GRAPHITE FILLERS ON THE PHYSICAL AND MECHANICAL PROPERTIES OF COMPOSITE FRICTION RESISTANT POLYMER MATERIALS

ABSTRACT

We conducted research on the development of composite polymer materials for engineering purposes for use as working parts of cotton processing machines and mechanisms. This paper presents studies of the influence of mineral, fibrous and carbon-graphite fillers, such as fiberglass, wollastonite, chalk, kaolin, talc, graphite and carbon black, on the physical and mechanical properties of the developed composite polymer materials, such as breaking stress in bending, impact strength, hardness according to Brinell, modulus of elasticity. Fiberglass and wollastonite are introduced into compositions based on polypropylene and high-density polyethylene to reinforce the material, imparting good strength properties. Graphite and carbon black were used reduce friction coefficient and to improve electrical and thermal conductivity properties. Chalk talc and kaolin were used to increase hardness and reduce the cost of the material. Studies have shown that a polypropylene composition filled with 20-30 m.p. of glass fiber and composition based on high-density polyethylene, filled with 30 m.p. of fiberglass, has the greatest strength.

АННОТАЦИЯ

Проведены исследования по разработке композиционных полимерных материалов машиностроительного назначения для использования в качестве рабочих органов хлопкоперерабатывающих машин и механизмов. В данной работе представлены исследования влияния минеральных, волокнистых и углеграфитовых наполнителей, таких как стекловолокно, волластонит, мел, каолин, тальк, графит и сажа, на физико-механические свойства разработанных композиционных полимерных материалов, такие как разрушающее напряжение при изгибе, ударная вязкость, твердость по Бринеллю, модуль упругости. Стекловолокно и волластонит введены в композиции на основе полипропилена и полиэтилена высокой плотности для армирования материала, придания ему хороших прочностных свойств. Графит и сажа использованы для снижения коэффициента трения и улучшения электро- и теплопроводных свойств. Мел, тальк и каолин использованы для повышения твердости и снижения себестоимости материала. Исследования показали, что полипропиленовая композиция, наполненная 20-30 м.ч. из стекловолокна и композиции на основе полиэтилена высокой плотности, наполненного 30 м.ч. стекловолокна, обладает наибольшей прочностью.

Keywords: Composite polymer materials, polypropylene, polyethylene, mineral fillers, carbon graphite, talc, kaolin, fiberglass, cotton lint, wollastonite, breaking stress in bending, elastic modulus in bending.

Ключевые слова: Композиционные полимерные материалы, полипропилен, полиэтилен, минеральные наполнители, углеграфит, тальк, каолин, стекловолокно, хлопковый линт, волластонит, разрушающее напряжение при изгибе, модуль упругости при изгибе.

Introduction. It is known that raw cotton occupies an important place in the economy of Uzbekistan. In this regard, increasing the efficiency of cotton growing through comprehensive mechanization of all processes, from cotton harvesting to its processing, increasing the performance and productivity of the machines and mechanisms used, preserving the natural characteristics of raw cotton is the main technical and economic task facing scientists and materials scientists and industry designers. This problem can be solved by using composite polymer materials (CPM) in the manufacture of parts for working parts of cotton processing machines [1-4, 6].

In this regard, the development of effective resource-saving technologies for the production of antifriction and wear-resistant composite polymer materials for structural purposes based on local raw materials and the establishment of optimal technological parameters for the manufacture of products and parts from them for mechanical engineering, in particular for machines in the cotton processing industry, is relevant.

It is obvious that a polymer matrix for a composite material operating under friction conditions with raw cotton must have sufficiently high physical-mechanical and antifriction properties and wear resistance [5, 7]. Also, a composition working in interaction with raw cotton should have high thermal and electrical conductivity [18, 22], good rheological [23] properties. In works [8-16], friction of coatings based on polyvinyl butiral (PVB), high-density polyethylene (HDPE), furan-epoxy resin, polycaproamide (PСA) and pentaplast (PNP) with raw cotton under different friction modes and cotton moisture contents.

Methods. The objects of study are polypropylene (PP), high-density polyethylene (HDPE), fillers: fiberglass, cotton lint, wollastonite, chalk, kaolin, talc, graphite, carbon black. As a counterbody, raw cotton of the 1st grade, variety S-6524, machine harvested with a moisture content of 8.2% and a weed content of 7.6% was selected. The following physical and mechanical properties were studied: breaking stress (σ_b) and elastic modulus (E_b) in bending, hardness (H_B) and impact strength (σ_is).

The breaking stress in bending was determined according to State Standard 4648, the modulus of elasticity in bending - according to State Standard 9550-81, Brinell hardness - according to State Standard 9012-58, impact strength - according to State Standard 4647-2015. We investigated the friction of polypropylene (PP), polycarbonate (PC) and polystyrene (PS) with raw cotton of the 1st grade, variety S-6524, machine harvested with a moisture content of 8.2% and a contamination level of 7.6%.

Research Results. To identify the optimal compositions of compositions based on PP and HDPE, studies have been carried out to study the dependence of the basic physical-mechanical and tribological properties of polymers on the type and content of mineral, carbon-graphite and fibrous fillers [17-20]. The main indicators of the properties of a composite polymer material were taken as breaking stress (σ_b) and elastic modulus (E_b) in bending, hardness (H_B) and impact strength (σ_is). This is due to the fact that the performance and durability of composite polymer pegs of the working bodies of cotton machines and mechanisms are mainly determined by these indicators of the peg material, as well as tribological properties during friction with raw cotton.

Using experimental data, curves were constructed depending on the strength properties of composite materials based on PP and HDPE on the type and content of mineral, fibrous and carbon-graphite fillers (Fig. 1-6) [21-24].

As can be seen from the course of the curves in Fig. 1 and 2, the introduction of mineral, fibrous and carbon-graphite fillers - fiberglass, cotton linters, wollastonite and chalk up to 20-30 m.p. lead to an increase in the breaking stress during bending of polypropylene and polyethylene composite materials to a maximum.

A further increase in the content of these fillers is accompanied by a gradual decrease in the values ​​of breaking stress during bending. When mineral ingredients such as talc, kaolin and, especially, carbon-graphite fillers - soot and graphite are introduced into polypropylene and high-density polyethylene, the breaking stress during bending decreases with increasing their content.

However, the magnitude of the breaking stress during bending remains quite high for compositions filled with up to 15-20 m.p. including talc, kaolin and up to 5-10 m.p. including soot and graphite.

In general, the increase in flexural strength up to a certain filler content appears to be associated with its accumulation in the interspherulitic regions where the filler enters during crystallization. The decrease in the strength of compositions with large fillings is apparently caused by the fact that the presence of a large amount of filler between polymer macromolecules somewhat complicates the energy of their intermolecular interaction, accelerating the process of destruction of the composition.

Impact strength of composite materials based on polypropylene and high-density polyethylene with an increase in the content of glass fiber, lint and wollastonite up to 30 m.p., chalk up to 10-15 m.p. also increases and then decreases. With the introduction of other fillers, the value of impact strength gradually decreases with increasing filler content (Fig. 3-4).

The hardness of composite polypropylene materials (Fig. 5) with the introduction of fiberglass, lint, chalk, kaolin is up to 30 m.p., talc, soot and graphite up to 10-15 m.p. increases and then decreases.

The flexural modulus of elasticity of composite polypropylene compositions (Fig. 6) gradually increases with increasing content of mineral fillers.

A comparison of the measurement results of the main indicators of strength of composite materials based on polypropylene and high-density polyethylene shows that the values ​​of fracture stress in bending, impact strength, hardness and elastic modulus in bending depend on the type and content of mineral, fibrous and carbon-graphite fillers. At the same time, the polypropylene composition filled with 20-30 m.p. has the greatest strength. including glass fiber and a composition based on high-density polyethylene, filled with 30 m.p. including fiberglass. So, for example, when adding fillers of glass fiber, talc, carbon black, graphite, kaolin, chalk, wollastonite and cotton linters up to 10 -15 m.p. the breaking stress during bending of the composition practically remains at the level of the unfilled matrix. When containing high concentrations, this indicator decreases monotonically and amounts to 113.8, respectively; 89.9; 85.1; 86.2; 90.0; 91.0; 91.5 and 94.4 MPa.

This increase in flexural strength and impact strength of polypropylene and high-density polyethylene compositions filled with glass fiber (GF) and cotton linters is explained by the reinforcing ability of the fiber filler and the high strength of GF. Reduced properties at a content of more than 30 wt. including dry matter and cotton linters is explained by the lack of wetting of the fibrous fillers by the polymer matrix. In places where the fillers are not wet enough, air bubbles appear, which are local points of concentration of internal stresses in the matrix, i.e. centers of destruction of the polymer composition.

The addition of talc to high-density polyethylene and polypropylene does not significantly change the flexural strength of composite materials. Obviously, this is explained by the scaly shape of this filler and its high specific surface area.

It is known [18] that filling polymers with carbon black leads to an increase in the rigidity of the material and elastic modulus, therefore, this leads to a decrease in the impact strength of the composition. The results of experimental data confirm this position.

In general, the decrease in the strength of composite polymer materials at large fillings is apparently caused by the acceleration of the processes of destruction of the polymer composition. Dangerous defects appear in composite materials.

A change in the hardness value with a low content of mineral, fibrous and carbon-graphite fillers occurs due to a change in the rigidity of the polymer-filler system. An increase in hardness with a small amount of filler is due to a change in the degree of structural order of polypropylene, and a decrease in the hardness of the composition with a further increase in the content of fillers is apparently explained by the fact that during shear deformation the structure of their fillers is destroyed, which, moreover, is not capable of active interaction with the polymer. Therefore, during deformation, contacts between filler particles are easily destroyed, and in the absence of their interaction with the polymer matrix, the entire system is weakened.

The decrease in the impact strength of polypropylene and polyethylene composite materials is obviously explained by the fact that an increase in filler content leads to the emergence of large internal stresses, which reduce the strength of bonds between the polymer matrix and filler particles, the appearance of dangerous defects in the form of microcracks, pores, etc.

The observed increase in flexural strength for fibrous fillers (fiberglass, cotton lint, wollastonite) is associated with the known effect of reinforcing the polymer matrix, while the use of dispersed fillers maintains or slightly increases s_b at low filler contents. This effect is associated with the phenomenon of initiation of the polymer crystallization process on the surface of the filler (crystallization centers, the effect of small additives). Decline s_b at high filler concentrations, in both cases it is associated with the agglomeration of filler particles, the generation of significant internal stresses, which lead to an acceleration of the process of destruction of composites.

The dependence of impact strength on the content and type of fillers is based on the same reasons. The level of dissipated energy is determined by the volume of the polymer medium participating in the process of forced elastic deformation when a load in the form of an impact is applied. In this case, short-fiber fillers perform best.

As expected, the hardness and flexural modulus behave in the same way when varying the concentration and type of filler; the higher the elastic modulus of the filler, the higher this indicator for the composite at the same filler content.

The experimental data obtained are in good agreement with the results of studies by a number of authors who studied the dependence of the physical and mechanical properties of other materials on the filler content.

Conclusion. Thus, analysis of experimental data shows that the nature and structure of fillers has a certain influence on the physical and mechanical properties of composite materials based on polypropylene and high-density polyethylene. At the same time, fiber-filled compositions have the greatest mechanical strength.

The physical and mechanical properties of composite polypropylene and polyethylene materials were studied depending on the type and content of fillers such as fiberglass, wollastonite, chalk, kaolin, talc, graphite and soot. Studies have shown that a polypropylene composition filled with 20-30 m.p. has the greatest strength. including glass fiber and a composition based on high-density polyethylene, filled with 30 m.p. including fiberglass.

It has been established that with an increase in the content of mineral and fibrous fillers, the breaking stress and flexural modulus of elasticity, impact strength and hardness increase, reaching a maximum at their content of 10-30 m.p. At the same time, composite polymer materials filled with fibrous filler have the greatest strength.

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