The Study of Modification of PVC-materials by Rice Husk Ash Including Activated by Cationic Surfactant and Wollastonite on Its Basis

. This article examines the use of waste rice processing as a promising fillers for PVC finishing materials. Due to the presence of amorphous silicon dioxide these fillers improve the performance properties of filled PVC, while lignin and cellulose in their composition accelerate biodegradation when buried in the soil. The use of these wastes allows solving the problems of "green" chemistry and closed-cycle economy and reducing the cost of finished polymer products. The paper studies the effect of ash of rice husk and synthetic wollastonite on its basis on the diffusion processes occurring during mixing of PVC and plasticizer. Activation of the RHA surface by CATAPAV significantly affects its elemental and particle size distribution and structure porosity, leading to a decrease in the porosity of the filler, reducing its particle size and narrowing the distribution of particles and pores in size. At use of silicate fillers of optimum composition growth of tensile strength, thermal stability and decrease of plasticizer migration from PVC compositions, owing to plasticizer retention in pores of these fillers is reached.


Introduction
The use of fillers derived from the processing of agricultural waste in the composition of PVC-materials is of interest both in terms of "green chemistry" and the economy of the closed cycle [1].
Rice processing wastes are promising as raw materials for the production of fillers, because their content of amorphous silicon dioxide [2] suggests the possibility of improving the performance properties of PVC filled with them, and the content of cellulose and lignin in the organic part can accelerate biodegradation when buried in the soil after the end of their service life.This solves the problem of involvement in the production of secondary material resources, reducing the polymer intensity and the cost of finished products.
In the production of PVC linoleum and other materials, there is a problem of plasticizer migration from the polymer composition [3].The kinetics of this process can be limited by the diffusion of plasticizer in the PVC-matrix.
One of the promising ways to reduce plasticizer migration is the use of porous fillers in the formulation of PVC compositions, the representatives of which are rice husk ash and wollastonite on its basis [4].
The following fillers were used: rusk husk ash (RHA) obtained by combustion at 500°С and synthetic wollastonite (SW) obtained by solid-phase synthesis on its basis [6].
Migration of plasticizer from PVC-compositions was determined by the change in weight of samples, in accordance with the European standard EN 664:1994.
Tensile strength and relative elongation of PVC-compositions were evaluated according to GOST 270-75.
Determination of thermostability of PVC-compositions was carried out according to GOST 14041-91 by "Congo red" method.Thermogravimetric analysis was carried out on TA Q5000 SA device by TA Instruments in the temperature range from 20 to 600°С.
Pore size distribution was determined using the BJH theory (Barrett-Joyner-Halenda method) according to ISO 15901-2 on a Quantachrome Nova 1200e surface area and pore size analyzer.
The bond strength between linoleum layers was determined according to GOST7251-77.
Changes in linear dimensions of linoleum were evaluated according to GOST 11529-86.Influence of fillers on mutual diffusion in system PVC -plasticizer was studied by optical wedge method on diffusiometer ОDА-2 (The Institute of Physical Chemistry and Electrochemistry, Russia) [8].
Infrared spectra of the initial samples of RHA and activated CATAPAV in the ratio filler : surfactant 1:1 by mass, were taken in the reflection mode on the Frontier IR-Fourier spectrometer according to GOST 57941-2017.
Quantitative elemental analysis was performed on a Vario macro Cube CHNS analyzer manufactured by Elementar (Germany).Samples were weighed on Sartorius Cubis II microscales (Germany) in tin capsules.The Vario Macro Software was used for calculations.

Results and discussion
Due to the relatively high volatility, plasticizers can migrate to the surface of PVCmaterials during their production and operation, which has a significant negative impact on their quality [9].Filling with rice husk ash and wollastonite on its basis, due to their porous structure [10], significantly reduces plasticizer migration from PVC-compositions (Table 1), with a greater effect of rice husk ash compared to SW.It may be due to the amorphous structure of silicon dioxide in the composition of RHA, which increases the efficiency of interfacial interactions.Additional growth of adhesive bonds between the filler and polymer matrix can provide activation of the surface of silicates by surfactants, in particular by quaternary ammonium salts [10].To study this process effectively, the method of infrared spectroscopy is used.
Analysis of IR-spectra of pre-dried RHA at 100°C for 4 hours showed the presence of characteristic absorption bands of Si-O bond, appearing in the regions of 1058 cm-1, 797 and 458 cm-1 (Fig. 1).In the IR-spectrum of RHA modified with benzyl dimethylammonium chloride, a shift of Si-O bond bands to lower frequencies was observed (1055, 787, 451 cm-1, respectively).In addition, a peak appears with a maximum of 1466 cm-1 corresponding to the C-H bond vibrations, which is a fact indicating the adsorption of CATAPAV on the surface of RHA.The shift of the C-H vibration peak indicates the donor-acceptor interaction between the molecules of RHA and CATAPAV (formation of the hydrogen bond ORHA.....HCATAPAV).
With increasing plasticizer concentration in the PVC-EDOS system, the interdiffusion coefficients grow from 10-7.7 to 10-6.1 (cm2/sec).At the same time, introduction of the studied fillers into the composition does not lead to their change in the area of PVCconcentrated solutions.At the same time, in the area of dilute (up to 30% vol. of polymer) PVC-solutions, interdiffusion coefficients decrease from 10-6.1 to 10-6.3 (cm2/sec.).

FORM-2023
https://doi.org/10.1051/e3sconf/202341001002 E3S Web of Conferences 410, 01002 (2023) Thus, the effect of rice husk ash and synthetic wollastonite based on it on the diffusion processes at mixing PVC and plasticizer is manifested in a slight decrease in the mutual diffusion coefficients, which falls within two decimal orders.
We can assume that the observed decrease in diffusion constants is associated with a partial penetration of EDOS into the pore space of the fillers [11].This is indicated by a decrease in the porosity of the silicates under study during treatment of their surface with a plasticizer.This effect is caused by the fact that the size of the EDOS molecules, calculated in the Gaussian 16 software complex [12], is not more than 1.7 nm, which is less than the average pore diameter of all the fillers under study (more than 3 nm).Significant growth of nitrogen content in the elemental composition of ash and slag during activation of its surface by CATAPAV indicates the adsorption of this surfactant on the surface of rice husk ash (table 3).This agrees with the results obtained by infrared spectroscopy.
Analysis of infrared spectra of SW (Fig. 2) showed that when treating wollastonite with CATAPAV no new absorption bands appear, which are absent in the original components, indicating that the chemical interaction between quaternary ammonium salts and the surface of synthetic wollastonite does not occur.At the same time, there is a significant decrease in the absorption bands intensity (Fig. 2) of CATAPAV in the 1380 cm-1 and 1460 cm-1 bands, indicating adsorption of the -CH2 -N+-groups responsible for them on the wollastonite surface.In addition, the intensity of asymmetric and symmetric valence oscillations of -CH2-groups, which correspond to the Activation RHA by CATAPAV results in a significant decrease in the average particle size of ash from about 35 mcm to 2 mcm.This is probably due to a decrease in the tendency of filler particles to agglomeration as a result of surface-active agent treatment.At the same time, there is a sharp narrowing of particle size distribution of RHA [11].That is, the homogeneity of the structure of the activated filler increases.
At the same time, activation of the surface of ash from rice husk causes a significant narrowing of its pore size distribution.This curve (Fig. 3) is unimodal with one maximum with an average pore size of about 4 nm.Thus, we can conclude that the activation of the surface of RHA significantly affects its elemental and granulometric composition and structure porosity.Treatment of surface of rice husk ash and wollastonite on its basis with CATAPAV leads to improvement of the main performance characteristics of PVC-compositions modified by them, as compared with the basic composition.RHA and SW significantly increase the strength of PVC-materials at a sufficiently high relative elongation, and increase their thermal stability.This is due to the alkaline nature of the surface of rice husk ash and wollastonite based on it, which promotes the reaction of the filler with hydrogen chloride, released from PVC during high-temperature exposure.
As a result, the degradation of EDOS decreases, due to the fact that hydrogen chloride promotes the destruction of unstable in acidic environments dioxane ring in the plasticizer structure [13].
When activating the surface of RHA of CATAPAV the temperature of 30% weight loss of PVC-compositions is slightly higher compared to the use of non-activated ash.Synthetic calcium silicate has slightly less heat stabilizing effect (Table 3) than RHA, which is probably due to its higher porosity and crystalline structure.Note: 10 % wt.modifier content per 100 % wt. of PVC.At modification of PVC-composition for flooring production by rice husk ash and synthetic wollastonite there is (Table 4) an increase of bond strength between linoleum layers and reduction of its shrinkage (decrease of linear dimensions change value).It is connected with a decrease of plasticizer EDOS migration from PVC (Table 1).This effect grows with activation of surface by CATAPAV.
There is a clear dependence between the antimigration effect of silicates used and the growth value of bond strength between the linoleum layers and reduction of its shrinkage (Tables 1 and 4).
Thus, the application of RHA and wollastonite on its basis, especially organo-activated fillers, is a promising way to improve the performance of their modified PVC materials, in particular floor coverings [10].

Conclusions
The impact of rice husk ash and synthetic wollastonite on the diffusion processes, occurring during mixing of PVC and plasticizer, is manifested by a slight decrease in the coefficients of mutual diffusion.
Activation of the RHA surface by CATAPAV significantly affects its elemental and granulometric composition and structure porosity.There is a decrease in porosity of the filler, a decrease in the size of its particles and narrowing of the distribution of particles and pores in size.
IR spectroscopy data testify to the donor-acceptor interaction of RHA and CATAPAV molecules and the formation of a hydrogen bond between them.The adsorption of CATAPAV on the surface of rice husk ash is indicated by a significant increase in the nitrogen content in the elemental composition of the activated filler.
Modification of PVC-compositions with silicate fillers of optimal composition, obtained from rice husk processing, causes an increase in tensile strength, thermal stability and reduction of plasticizer migration, due to retention of EDOS in pores of these fillers.

Fig. 3 .
Fig. 3. Pore size distribution according to the BJH method in RHA treated with CATAPAV.

Table 1 .
Migration of EDOS from filled PVC-compositions.

Table 2 .
Results of elemental analysis of RHA samples, initial and activated by

Table 3 .
Properties of PVC-compositions modified with 10 wt.% of rice husk ash and synthetic wollastonite on its basis.

Table 4 .
Properties of PVC-linoleum based on paste modified with rice husk ash and wollastonite on its basis.