Research of Changes of Strength Indicators of Semi-Rigid Covers Glued by Modified Adhesive Compositions

The object of research is the processes for producing modified adhesive compositions based on PVA (polyvinyl acetate) dispersions for use in technological processes for the manufacture of semi-rigid book and magazine covers. The conducted experimental studies are based on the application of a comparative method for determining the tensile strength of glued, without bias, samples of binding materials using modified adhesive compositions and identical compositions without modification. The main assumption of the study is that the use of polymer thickeners and emulsifiers compatible with latex PVA, which are actively used in the technological processes of the paper and printing industries, contribute to the formation of additional strength of semi-rigid covers. This can be achieved without analysis and the selection of possible components, taking into account the mechanical properties that arise after the use of such modifiers that substantially depend on the adhesive strength between the material of the outer part of the cover and the adhesive binder. It is proposed for an experimental study of the use of substances with high adhesive properties to thick coated papers and thin binding boards, from which semi-rigid book and magazine covers are made. The structural features of adhesive films obtained from polyvinyl acetate latex were determined, which have improved flexibility, elasticity, high adhesion and water resistance when water-soluble modifiers are added to them. An experimental determination was made of the possibilities of increasing the structural strength of semi-rigid book and magazine covers after gluing with modified binding adhesives, which did not significantly increase the thickness of the adhesive layer and did not change the contour geometry of the manufactured covers. It was proved that the adhesive mixtures used to modify the PVA dispersion did not affect the structural homogeneity of the glued spatial structure of the semi-rigid covers, and did not affect the appearance of external defects in the form of plane inhomogeneity, or point thickenings of the surface of the covers. The results of studies on the gluing of structural parts of reamers of semi-rigid covers with modified adhesive compositions create additional opportunities in planning the use of consumables in the manufacture of covers for strength indicators, predefined dimensional conditions for the use of modifier substances. Experimental studies have shown discreteness – growth, decrease in dimensional indicators of the strength of semi-rigid covers glued with modified adhesive compositions, contributes to the practical application of the results, both in the planning of technological processes and in the calculation of the cost of book production.<br><br>


Introduction
Semirigid glued covers, as an innovative type of book frames, correspond to the properties of representative ness and hardness inherent in binding, although they are manufactured using simplified edged cover technologies significantly lower than cost [1]. One of the significant advantages of joining materials due to the use of adhesive polymer compositions, as shown in the results of studies [2], is the ability to adhere materials according to the physico chemical parameters without significant changes in their geometric and volume structure. The advantages of using adhesive polymer compositions are confirmed [3] by im proving the strength indicators of glued products, as well as their indicators of resistance to destructive factors, re duce the duration of use and storage. It is possible to note that in the printing industry there is a tendency for a quantitative increase in book titles with a simul taneous reduction in their circulation [4]. Insignificant circulations of books made include the introduction of manufacturing techniques for simplified ergonomic and resourcesaving covers, as well as the widespread use of TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 3/1(53), 2020 ISSN 2664-9969 nonsewn adhesive gluing of book blocks [5]. Let's consider the glued threelayer structure of a semirigid cover as an anisotropic composite medium in which the externalfront and back parts are formed by binding materials from which the blank of the workpiece is carved [6]. And the inner part is filled with an adhesive polymer composi tion, the thickness and physicochemical properties of which, together with binding materials, create a cover, with strength and hardness indicators substantially close to those in binding [7,8]. A change in the thickness of the adhesive layer in the inhomogeneous anisotropic structure of the semirigid cover significantly affects the change in strength indicators, both up and down [9,10]. In the process of gradual wear of materials under the influence of destructive loads, the specific interaction of the adhesive composition with the material of the outer part of the cover significantly affects the strength, structural density and wears resistance of the adhesive composition [11]. The operational stability and resource of use of the adhesive polymer composition depends and can change with fluc tuations in the thickness and structural geometry of the semirigid cover [12]. For the manufacture of semirigid covers, only a part, albeit substantial, of binding materials is used, such as thin cardboards, chrome ersatz cardboards, dense coated papers (250-350 g/m 2 ), etc. Such materials ensure the integrity of the spatial geometry of the manu factured covers, without cracking at the folds. Therefore, to increase the strength and hardness of semirigid covers, the direction of increasing the thickness of binding mate rials is limited and not very promising for experimental studies. At the same time, the technology for modifying adhesive polymer compositions, by which structural parts of semirigid covers are glued, allows to keep the adhe sive thickness unchanged compared to the use of adhesive compositions without modification [13,14]. At the same time, they improve the physicomechanical characteristics of the covers in strength, hardness and operational stability. Thus, the object of research is the processes for producing modified adhesive compositions based on PVA (polyvinyl acetate) dispersions for use in technological processes for the manufacture of semirigid book and magazine covers. The aim of research is to experimentally determine the possibilities of increasing the structural strength of semi rigid book and magazine covers after gluing with modified binding adhesives, which do not significantly increase the thickness of the adhesive layer and do not change the contour geometry of the manufactured covers.

Methods of research
The conducted experimental studies are based on the application of a comparative method [6] for determining the tensile strength of binding materials glued without bias, using modified adhesive compositions and identical compositions without modification. The main assumption of research is that the use of polymer thickeners and emulsifiers compatible with PVA latex, which are actively used in technological processes of the paper and print ing industries, contribute to the formation of additional strength of semirigid covers [15]. This can be achieved after analysis and the selection of possible components, taking into account the mechanical properties that arise after the use of such modifiers, which substantially depend on the adhesive strength between the material of the outer part of the cover and the adhesive binder. Therefore, for experimental use, substances with high adhesive properties should be used for thick coated papers and thin binding boards [16], from which semirigid book and magazine covers are made. When planning an experimental study, in the preparation and testing of modified adhesive com positions, it was taken into account that films obtained from PVA latex have improved flexibility, elasticity, high adhesion and water resistance when watersoluble modifiers are added to them. The list proposed by the authors of thickeners and emulsifiers compatible with the disper sion of PVAD DF 51/15B as the basis for modification for experimental studies of the formulation of polymer mixtures is given in Table 1. Given the promise of improving the physicmechanical properties of adhesive joints, through the modification of the adhesive base PVAD DF 51/15B (N1, Table 1) with highly dispersed particles, the introduction of even a small amount of highly dispersed modifiers leads to a significant change in the properties of the resulting adhesive compo sition. The additives listed in Table 1 lead to a change in the properties of semirigid covers, such as polymer composite structures, and the appearance of properties characteristic of finely dispersed particles. There is an improvement in the strength and cure rates of modified adhesive compositions compared to unmodified ones. This is especially important when using such adhesive com positions in highspeed brochure and palette equipment.   Table 2), by which fragments of semirigid covers corresponding to the strength indices are glued, more than the strength indi ces of glued by PVA dispersion without mo dification. Fig. 1, 2 show the graphical de pendences of the weight constituents used to modify the adhesive polymer compositions based on PVA dispersion in order to increase the strength and hardness of the composite structure of semirigid covers.

Research results and discussion
The effect presented in Table 2, ad hesive polymer compositions for the strength of semirigid covers were in vestigated on a tensile testing machine RMB302M (Ukraine), with a maxi mum load force of 300 N (30.5 kgf). The mechanical system of the tensile testing machine complies with the test require ments of ASTM, ISO, DIN, TAPPI, GB, JIS, ANSI, etc.   For testing, the fragments of glued covers are selected, which are strips of thin cardboard chrome ersatz, 0.6 mm thick, 100 mm long and 15 mm wide [1].
The samples are glued in two directions: using PVAD DF 51/15B dispersion (N1, Table 2) without modification, as well as using modified adhesive polymer compositions based on the specified PVA dispersion. The research results are listed in Table 3. Numbering of options of modified adhesive mixtures in Table 3 corresponds to the number  ing with Table 2.
A comparative analysis of tensile strength indicators demonstrates an increase in the strength glued by fragments of modified adhesives from 4.8 %, in position 2 ( Table 2), to 12.2 %, in position 10 ( Table 2), in contrast to glued fragments by dispersion of PVAD DF 51/15B (N1, Table 2) without modification.
Growth of indicators, according to the graph in Fig. 3, is carried out not in a proportional sequence, but in a dis crete one, with separate sections of growth and fall. This requires further studies of the structural features of semi rigid covers, the details of which are glued with modified adhesive polymer compositions.
The most favorable for use were modified adhesive polymer compositions based on PVAD DF 51/15B, indi cated in Tables 2, 3  The high porosity and friability of powder pulp parti cles (N4, Table 2), which is used in addition to using a mine ral additive in the form of kaolin in the mixture (N5, Table 2), provides a significant decrease in the content of water in the nonaligned state in the adhesive mixture. And this, in turn, contributes to a uniform and rapid thickening of the polymer adhesive composition.
Due to the dispersing component, which is powder cellulose (N4, Table 2), it is possible to ensure stable homo genization and structuring of the constituents of the ad hesive mixture in a bulk mass.
As a result of this, conditions are created for a com pletelyuniform deposition of a layer of adhesive compo sition, without the formation of clots, unjustifiably thin adhesive sections and planar tears.

Conclusions
As a result of experimental studies of glued frag ments of semirigid covers with modified polymer adhesive compositions based on dispersion PVAD DF 51/15B, an increase in strength indicators is noted. Growth is fixed depending on the weight proportionality of the substances used for the modification, compared with the glued fragments of the covers of the compositions without modification. The growth of tensile strength indices for fracture of cover fragments glued by experimental modified adhesives ranges from 4.8 % to 12.2 %, unlike glued fragments by dispersion of PVAD DF 51/15B, without modification.
The features of applying modified adhesive composi tions to the structural elements of semirigid covers are revealed, which indicate the absence of a significant increase in the thickness of the adhesive layer, and the absence of influence on the change in the contour geometry of the manufactured covers.
During experimental studies, it was proved that the adhesive mixtures used to modify the PVAD DF 51/15B dispersion did not affect the structural uniformity of the glued spatial structure of the semi rigid covers. And also did not affect the appearance of external defects in the form of planar inhomogeneity, or point thickenings of the surface of the covers. Modified adhesive com positions are re commended for use based on PVAD DF 51/15B, indicated in the Tables 2, 3 with positions 7,  8, 9, 10, with significant results in volved an increase in the strength of covers, in tension, by 9.8 %, 10.9 %, 11.3 %, 12.2 %. The obtained research results create additional opportunities in planning the use of consumables, in the manufacture of semirigid covers for taking into ac count strength indicators, predefined dimensional conditions for the use of modifier substances.
Experimental studies have shown dis creteness -growth, drop in the strength indicators of semirigid covers glued with modified adhesive compositions, contrib utes to the targeted use of the results, both in the planning of technological processes and in the calculation of the cost of book production.