ANALYSIS OF TENSILE STRENGTH PROPERTIES OF EXPERIMENTAL KRAFT PAPERS IN ELONGATION

ABSTRACT

This study investigates the tensile strength and anisotropic properties of experimental kraft papers, developed using a combination of primary and secondary fibers, fillers, and adhesives. Given the rising demand for cost-effective and eco-friendly packaging materials, incorporating recycled fibers and waste into kraft paper production presents significant economic benefits. Various formulations of kraft paper were prepared and evaluated under controlled laboratory conditions, examining parameters such as fiber orientation, strength, and elongation at break. Results reveal that incorporating secondary waste fibers improves kraft paper’s structural integrity, mechanical strength, and hydrophobic properties. Sample No. 4, combining cotton cellulose with secondary waste, demonstrated optimal tensile strength and anisotropy, suggesting a viable, sustainable alternative for packaging applications.

АННОТАЦИЯ

В данном исследовании изучаются прочностные и анизотропные свойства экспериментальной крафт-бумаги, разработанной с использованием первичных и вторичных волокон, наполнителей и клеев. Учитывая возрастающий спрос на экономичные и экологически чистые упаковочные материалы, включение переработанных волокон и отходов в производство крафт-бумаги позволяет достичь значительных экономических выгод. Были подготовлены и оценены различные составы крафт-бумаги в лабораторных условиях, исследованы такие параметры, как ориентация волокон, прочность и удлинение при разрыве. Результаты показывают, что использование вторичных волокон повышает структурную целостность, механическую прочность и гидрофобные свойства крафт-бумаги. Образец №4, сочетающий хлопковую целлюлозу и вторичное сырье, продемонстрировал оптимальную прочность на разрыв и анизотропию, предлагая устойчивую альтернативу для упаковочных решений.

Keywords: Kraft paper, tensile strength, anisotropy, recycled fibers, packaging materials, eco-friendly packaging.

Ключевые слова: Крафт-бумага, прочность на разрыв, анизотропия, переработанные волокна, упаковочные материалы, экологичная упаковка.

Introduction. Currently, due to the continuously increasing requirements for packaging products, it is necessary to consistently improve the types and technologies of packaging materials. In our country, significant attention is paid to transforming alternative and secondary raw materials into finished products. Ongoing research in the republic focuses on the production of paper and cardboard products using various  alternative and locally sourced secondary raw materials, including waste from the textile industry [1].

The primary national raw materials used in our country for paper production are cotton and silk, which play a significant role. High-quality paper can be produced from these materials; however, it is not economically viable to manufacture kraft paper for packaging from pure cotton cellulose. Therefore, it is essential to conserve valuable cotton cellulose by incorporating waste paper and fibrous waste generated during the cocooning process, alongside industrial waste, to address challenges in the pulp and paper, wrapping, and packaging industries. This approach not only helps to eliminate shortages but also reduces the cost of kraft paper production. When producing kraft paper by combining bleached cotton cellulose with alternative secondary fibers, the macrostructure of the paper significantly improves due to the reduction in material unevenness during formation. This also enhances the overall characteristics of kraft paper and cardboard. Utilizing alternative and secondary waste in the paper industry substantially lowers production costs [2].

Methods. During the experimental work, kraft papers were prepared using primary and secondary fiber materials, fillers and adhesives. In the conditions of the test laboratory of Global Komsco Daewoo QK, the production of kraft paper was carried out on the basis of the following list, and samples of kraft paper were taken.

Figure. The sequence of obtaining kraft paper in laboratory conditions

In order to achieve the grinding level of the cellulose fibers in the research, they were ground in a PFI mill according to Schopper-Rigler (°ShR) 45–55 (ISO 5267-), at a speed of 3000 revolutions and to a grinding level of 7°ShR for the lower secondary waste paper. Quatsi was used to hold the paper fibers in the sieve. This solution was added to the fiber suspension until the pH reached 6.8, then mixed for 1 min. Modified cationic starch was added to the mass as an adhesive in the amount of 1.5% compared to the usual paper mass. To achieve a good bonding effect, the pH of the paper pulp is required to be 4.5–5.0, for which Al2(SO4)3 was used as a precipitating reagent [2]. To further reduce the cost of kraft paper, in order to save cotton cellulose, secondary MS-3A paper waste was added to the paper pulp in addition to fiber waste generated during cocooning.

Figure 1 shows the chosen process for the production of laboratory kraft paper. For the sample, the mass of 1 m2 of kraft paper was determined to be in the range of 80-100 g. preparation of samples. The sample of kraft paper was made in the "Rapid" (Germany) machine designed for sheet molding in laboratory conditions. Research to determine the optimal composition of the paper composition was carried out based on the existing regulatory documents TU 17.12.14.199-013-00279054-2020. In all options, the percentage composition of the main components of the paper composition has been changed.

Table 1.

Options for the composition of kraft paper

This technology involves separate processing of fiber wastes from domestic raw fiber cocooning at the initial stage, and then paper samples were obtained by mixing the components of a certain ratio of fibers. One of the advantages of using secondary waste from the point of view of the economy is saving electricity consumption and reducing the cost of kraft paper [3-4].

Today, kraft paper is the leader in all packaging. The reason is that not only bags for mass construction materials but also bags, envelopes, and other products for supermarkets and restaurants can be made from it. Due to the need to improve the production technology of packaging materials and maintain their consumer properties, research on determining the dimensions of the surface microstructure and adding strengthening additives to the paper composition is urgent.

Mechanical properties are very important in evaluating the quality of kraft papers. The strength properties of kraft paper are determined by the strength and hardness of inter-fiber and intermolecular hydrogen bonds. The ratio of inter-fiber and intermolecular hydrogen bonds to kraft paper tensile strength can be changed by studying its anisotropy. In this work, the results of studying the anisotropic properties of kraft paper samples obtained in laboratory conditions are analyzed [5-6].

Kraft paper has the property of anisotropy, that is, its properties measured in three perpendicular directions are different. Anisotropy is related, first of all, to the methods of paper production, and secondly, to the properties of the fiber. The properties of the paper on the web side of the paper casting machine are different from the properties on the fabric side. This is because the mass of paper consists of fibers of unequal length. The longest fibers are in the lower layer of the sheet, and the amount of fine substances increases in the upper layer. Also, the underside of the paper is usually less smooth and porous. The side facing the mesh shows a clear mesh mark, and the opposite side remains smooth [7].

Results and Discussion. The anisotropy of kraft paper properties is determined by the degree of fiber orientation in the paper mass, and it is very difficult to quantify it. The simplest measure of fiber orientation is called the anisotropy coefficient, which is the inherent inter-fiber bonding of kraft paper and paperboard properties. In addition, the properties of primary and secondary fibers in the paper pulp are explained by the presence of structural elements that differ from inter-fiber binding forces. There are several variations of K, called the anisotropy coefficient. In this case, the coefficient of anisotropy K can be calculated as follows [7-8].

K¹₌Y_i / Y_CD

Where Y- given character; i- MD is the angle at which the sample is cut relative to the machine direction; MD- machine direction of the sample; CD- cross-sectional direction of the sample.

Table 2.

 Coefficient of anisotropy in elongation of kraft paper samples made on the basis of local raw materials

It is known from the literature that cellulose fibers provide the highest level of strength to paper and cardboard, which was confirmed by the above experimental results. Sample 4 kraft paper has the highest tensile strength value. As expected, sample No. 6, which contains waste paper, has a lower strength index. The machine direction tensile strength of the sample papers varied from 32.1 to 68.9 MPa, and the tensile strength varied from 112 to 163 MPa. The tensile strength of the sample papers in the transverse direction was 22.2 - 36.0 MPa, and the breaking strength was in the range of 57 - 96 MPa. It can be seen from the experimental results that kraft No. 4 paper is the strongest, this situation may be related to the high density that enhances the inter-fiber interaction, and this situation may be related to the high density of the fibers in the pulp that enhances the molecular interaction. This allows us to conclude that the use of CP+CW+MS-3A natural fibers provides increased mechanical strength and hydrophobicity. The anisotropy of the sample kraft paper has a significant effect on the printing process of this product [9].

During the formation of the paper web (kraft paper and cardboard), a large amount of water is retained in the paper at the end of the screen. The process of dewatering the paper canvas is carried out in the pressing part of the equipment. During the pressing process, the canvas, under the influence of external force, the drying, strength, and density of the paper increases, and the cellulose fibers are subjected to intensive hydromechanical effects. This process is controlled by the ability to lose water and is quantitatively characterized by the degree of surface smoothness of the kraft paper [6].

Depending on the degree of chemical and mechanical impact, the deformation properties determine the values ​​of the structure of the fibrous material located on the surface of the paper. The values ​​of deformation and strength properties of cellulose and paper material are affected by its structure, which in turn is formed from fibers that have been chemically and mechanically treated to give them the desired properties [6]. The study of deformation properties is an important indicator in the preparation of kraft packaging materials. In the relative elongation of kraft packaging materials, the fibers are stretched (stretched) in the spaces between the molecular bonds. The interaction of molecular bonds is not the same because it depends on the distance between the fibers [5]. During stretching, it was observed that the integrity of the kraft packaging materials was not damaged. Table 3 presents the results of determining the deformation properties of samples of kraft packaging materials at break.

From the given data, it can be concluded that sample 4 has the highest tension. The mechanical properties of the kraft packaging product depend on its composition and the properties of fiber semi-products. These factors also determine the density of kraft paper [10].

During the pressing and post-press finishing processes, the material is affected by compression. During compression, the structure of the cardboard becomes denser, and the surface of the kraft paper becomes much smoother under the influence of pressure.

In the stretching of kraft paper, the fibers are stretched (pulled) in the spaces between the bonds. The level of interaction between the bonds is not the same, because it depends on the distance between the fibers. During stretching, the integrity of the kraft paper is not damaged, individual bonds may be damaged. Table 3 presents the results of determining the deformation properties of kraft paper samples.

Table 3.

Deformation properties of kraft paper at break

From the given data, it can be concluded that the 4th sample has the highest tension. The 2nd and 6th kraft paper samples have the lowest tension. The mechanical properties of kraft paper depend on its composition and the crushing of fiber semi-products. These factors also determine the density of kraft paper. There is almost no obvious dependence on the density of the kraft paper samples for the transverse direction. Density dependence is observed for values ​​averaged over two directions only when the density of kraft paper samples increases significantly.

Conclusion. The dependence of the effective modulus of elasticity on the density of the printed material is one of the main indicators of the stretching of samples under the influence of pressure in the printing process, especially for post-printing processes on the surface of kraft papers. If there is significant stretching during the pressing process, this may cause the inks to not match. But when printing on the surface of kraft paper, due to the sufficient hardness and strength of the material, such a defect is seldom encountered, because the stress encountered during the passage of kraft paper between the cylinders in the printing equipment is lower than the stress that causes its irreversible deformation.