فصلنامه بسپارش
سال دوازدهم شماره 4 (پیاپی 45، زمستان 1401)

In recent years, the use of polymers in the packaging industry has increased significantly due to their excellent advantages over conventional materials. In the meantime, polyethylenes (PE) have become one of the most important options in the manufacture of polymer food packaging, due to high structural diversity, low cost, easy processability, and flexibility in the design and chain architecture and their effect on the control of polymer engineering properties. The aim of this article is to introduce the heat sealing process of polymer films with an emphasis on polyethylene polymer as well as methods and mechanisms related to this process. In the heat-sealing process study, the diffusion phenomenon of tangled chains through the interface and the effect of chain structure on improving the adhesion strength and increasing the heat-sealing capability of polyethylene films were studied. Understanding the factors affecting the chain diffusion phenomenon, by controlling these factors, the strength of the interface during the heat-sealing process can be increased, which leads to improvement the adhesion of the two polymers to each other. Also, different methods of modifying the structure of polyethylene chains and placing different side groups on the chain were introduced. Hyper branching in the polymerization stage, hyper branching using high-energy rays, and hyper branching in the reactive melt mixing state using free radical generators are the three main methods for changing the structure of polyethylene chains. Each of these methods has its advantages and limitations, and different methods are used according to the desired properties of the polymer.

One of the challenges in emulsion polymerization over the past two decades has been how to increase the solid content of polymer latex. Compared to conventional latex, high solid content latex (HSC) has a large volume fraction of the monomer dispersed phase, even more than 70% by weight. Increasing the concentration of polymer in latex, has important advantages such as increasing the space-time efficiency of the reactor, reducing the film formation and drying time, as well as reducing its storage and transportation costs. Conventional emulsion polymerization, mini-emulsion polymerization, self-emulsification polymerization, and concentrated emulsion polymerization have all been used to prepare HSC latexes, and good results have been reported in recent years. Production of HSC latexes in a reproducible manner requires precise control of the particle size distribution. To obtain a solid content above 60% or 70% by weight, the particle size distribution must be quite broad or multimodal. In addition, viscosity of HSC latex is highly sensitive to particle size distribution. In this study, the advances of the last few years, which are mainly in the preparation processes of such latexes and their applications are briefly reviewed.

The perishable nature of fruits and vegetables makes their shelf-life limited. Environmental factors, transportation and preservation conditions through postharvest can decrease the storage time and quality. Therefore, prolonging supply period of fruits and vegetables by using safer postharvest treatments lead preservation methods to edible coatings. Active edible coatings include different types of functional substances can be considered as a preservation method to improve strategies for improving the quality, safety and shelf-life of fruits and vegetables after storage. In this article, an overview of the basics and methods of using lignocellulosic materials to create edible coating films and the recent challenges and developments of this type of lignocellulosic material application are presented. A review of the scientific sources shows that cellulose, lignin and chitosan are polysaccharides that have high potential in the production of edible coatings. These types of coatings are commonly odorless, tasteless, non-toxic, non-allergenic, water soluble, transparent, and resistant to oil and fats. In addition, these coatings can be good carriers for active additives, antimicrobial compounds, antioxidants, anti-browning agents, texture enhancers, and nutraceuticals to prevent unwanted reactions and enzymatic or biochemical damage to inhibit microbial decay and enzymatic or biochemical damage and prevent physical textural deterioration in fruits and vegetables during storage.

Due to their unique properties such as proper strength to weight ratio, excellent thermal insulation, sound insulation and good mechanical properties polyethylene foams have received special attention from the scientific and industrial communities. One of the goals of modern research in the field of polyethylene foams is to increase the cell density and reduce the cell size of these materials to achieve the desired properties. Today, it is a well-known fact that the use of nucleating agents is absolutely necessary to control cell morphology (ie, cell density, cell size and distribution) in polyethylene foams. In the present study, first, two theories of nucleation and the characteristics of the nucleating agent have been fully discussed from the point of view of classical nucleation and self-consistent field theories, which are the most important and practical theories of nucleation in polymer foams. Then, the necessary characteristics for the selection of nucleating agent in polyethylene foams were introduced and examined. Based on the investigations carried out in this regard, it has been determined that the optimal concentration, volumetric free energy density, and especially surface tension of the nucleating agent, are the most important criteria and characteristics for the selection of a nucleating agent compared to other parameters such as geometry (F <1, the lowest effect compared to the other two factors) particle size, and dispersibility.

Morphological control is one of the important issues in the development of catalysis industries in the polyethylene production process. New generations of catalysts for the production of polyolefins exhibit different behaviors during the fragmentation process. In the initial stages of polymerization, fragmentation occurs with the rapid penetration of ethylene monomer into the pores of catalyst with low mechanical strength, which causes the formation of fine particles. On the other hand, particles formed during the polymerization process may be broken due to collision with each other, the stirrer or the wall of the reactor, and particles smaller than the standard size can be produced. In the case of catalysts with low mechanical strength, the formation of fine particles would occur considerably. These particles with a smaller size than usual (>63 µm) are known as fine powder. The presence of fine powder particles is one of the important problems in the polyolefin industry, especially polyethylene, and causes problems such as the formation of a layer on the reactor wall and hot spots, clogging in the gas phase reactors and peripheral equipment such as heat exchangers, pumps, dryers, and the production of undesirable and non-standard products. Today, in the polyolefin industry, they are always looking for a way to reduce the amount of fine powder particles in the polymerization process. In this article, due to the importance of this issue, substantial factors of the formation of these particles, including process and catalysis factors are discussed.

Electrospinning is a technique of fabricating nanofibrous layers with wide applications in various biomedicine fields. Electrospun webs have unique properties such as high surface area to volume ratio, improved cellular interactions, protein absorption to improve cell attachment to the web, ease of fabrication and flexibility in relative control of pores, scaffold shape and fibers alignment. For these reasons, their use in the field of tissue engineering and dental applications has received much attention. Extensive research has been done to discover the potential of electrospun nanofibers for dental and oral applications, however, there are limitations related to the electrospinning of various materials that prevent their advanced practical or clinical applications. To overcome these limitations, it is necessary to pay more attention to various aspects of biological materials and to better understand the properties, performance and controlled production of electrospun materials. The purpose of this article is to review the recent advances in electrospun nanofibers in dental applications. Electrospun layers have been used in regeneration of pulp dentin complex, caries prevention, drug delivery, and tissue regeneration for periodontium. Also, nanofiber layers have been used in the modification of dental composites and the surface of implants.