Synthetic resin methods mainly include addition polymerization and condensation polymerization.
a. Additive polymerization
As a rule, low molecular weight compounds that undergo addition polymerization to form high molecular weight polymers are mainly ethylene derivatives, which are collectively referred to as vinyl monomers. A characteristic of addition polymerization is that high polymer resulting from reaction has same chain structure as monomer, that is, some small molecules are not lost during reaction, for example, ethylene is polymerized to polyethylene.
The addition polymerization method can be divided into bulk polymerization, solution polymerization, suspension polymerization and emulsion polymerization according to different polymerization systems. Resins produced by different processes often have distinct differences in characteristics and are suitable for different purposes.
b. Condensation polymerization
The low molecular weight condensation reaction is a kind of organic reaction that has many types and wide applications. The condensation reaction of bifunctional compounds is skillfully used for synthesis of polymers. The condensation reaction is a stepwise polymerization reaction, which differs from addition polymerization in that low molecular weight compounds resulting from condensation reaction volatilize during reaction. Nylon (polyamide) and polyester (eg polyethylene terephthalate) are resins synthesized by condensation polymerization.
In addition to above two main methods for producing synthetic resins, there are ring-opening polymerization (such as synthesis of nylon 6), polyaddition reaction (such as production of polyurethane foam), etc.② Types of resins and plastics
Resin categories can be divided from different points of view. For example, according to structure of main chain of polymer molecules, it can be divided into carbon chain resins and mixed chain resins. The former include polyethylene, polypropylene, polystyrene, polyvinyl chloride, etc., and most of them use additive polymerization methods. and others, by condensation polymerization. According to polarity of resin molecules, it can be divided into non-polar (or weakly polar) and polar, as well as several types with strong interactions between molecules. If classified by solutionproperties in water, distinguish between water-soluble resins, water-insoluble resins, and so on.
The division of plastics is largely based on their processing characteristics, and they are divided into thermoplastics and thermosetting plastics.
This type of plastic is based on thermoplastic resins. Since thermoplastic resin softens (or melts) after heating and has plasticity, and after cooling, resin hardens and retains shape formed in plasticized state, this process can be repeated many times, therefore it is called thermoplastic. Thermoplastic resins are typically linear polymer compounds such as polyvinyl chloride, polystyrene, and acetate fibers, as well as nylon and polyester.
b. Thermosetting plastics
Plastics based on thermosetting resins. During processing and molding, resin has plasticity, but under temperature conditions of processing, it simultaneously reacts to form a resin that no longer softens (or does not melt), and after cooling and hardening becomes insoluble and infusible, that is, it can be plasticized only once .Chemical molding such as phenolic plastics, amino plastics, etc. are all thermoset plastics.③ Properties of plastic packaging materials
a. Barrier properties of plastic packaging materials
Plastic barrier properties refer to ability of plastic packaging materials or containers to prevent penetration of low molecular weight gases such as O2, CO2, N2, water vapour, fragrances and other organic solvent vapours. An indicator characterizing barrier ability of plastics is transmittance, that is, volume or mass of small molecular substances permeable per unit time and per unit area by a plastic product of a certain thickness under certain conditions of pressure, temperature and humidity. The lower transmittance of plastic, higher its barrier ability.
The barrier properties of plastic packaging materials are not only related to molecular size and physical properties of permeable substances, but also related to internal structure of plastic itself, such as macromolecular structure and state of molecular aggregation, and affinity between plastic and permeable substances related to gender and compatibility. Factors affecting barrier properties of plastic packaging materials:
Comparing molecular polarity of different polymer resins, when degree of crystallinity is constant, polar macromolecules or highly polar macromolecules are larger than non-polar macromolecules or weakly polar macromolecules. Diffusion within them is difficult. The greater polarity of molecule, lower permeability of resin and better gas impermeability. Among commonly used plastic resins, PET and PVA are highly polar resins, PA and PVC are polar resins, PS, etc. are low polar resins, and PE, PP, etc. are non-polar resins. Their gas barrier properties increase with increasing molecular polarity, for example, gas permeability of PET and PE for O2 is very different.
Water vapor is a polar molecule, so rate of dissolution and diffusion of water vapor into polar molecular plastics is higher than that of non-polar plastic molecules, and moisture permeability coefficient is also higher. The high-barrier PET material has a strong molecular polarity, and its moisture permeability coefficient is higher than that of non-polar molecule PE, so PE is an excellent moisture-proof packaging material.
The diffusion energy of gas and water vapor permeating through crystalline polymers is higher than that of non-crystalline polymers, and diffusion coefficient is lower, so crystalline polymers have better gas-tight properties. Other things being equal, higher crystallinity of resin molecules, better barrier performance.
Plastic films and containers undergo varying degrees of macromolecular orientation due to molding stretch. Macromolecules are regularly distributedare arranged close to each other, and barrier properties are improved. The higher degree of orientation of a macromolecule, better its barrier properties. Especially after biaxial stretching of plastic film, not only grain size is greatly reduced, but also crystallinity can be increased. The principle can be explained by fact that when stretched, original crystalline particles break and become smaller; on other hand, stretching increases orientation of macromolecules, and arrangement of macromolecules becomes more regular and ordered, thereby increasing crystallinity and density of macromolecules.
Plastic resins have hydrophilic properties mainly in PVA, PA and other films. Due to strong water absorption, hydrophilic resin swells and distance between molecules increases, which reduces barrier properties. In general, water vapor diffusion coefficient of hydrophilic resin is not constant, it increases with increasing water vapor concentration, resulting in a change in moisture permeability coefficient. The moisture permeability of non-hydrophilic polymers is practically independent of ambient humidity.
Relationship between ambient temperature and barrier properties of plastic resin
Temperature affects molecular structure of plastic resin. Increasing temperature will reduce crystallinity and orientation of resin, increase distance between molecules and decrease density, which will cause barrier properties of polyethylene film to decrease.
The gas transmission rate of conventional plastic films increases and decreases exponentially with temperature. In comparison, gas barrier properties of PVDC are less dependent on temperature, and aluminum foil is less dependent on temperature, so these two types of flexible packaging films are usually chosen as high temperature retort bags. Ultra-high barrier coated polymer film coated with silicon dioxide, barrier properties of which are less dependent on temperature. The oxygen permeability of silica-coated composite material changes little after high temperature cooking, while oxygen permeability of aluminum foil and PVDC composite film changes relatively strongly during high temperature cooking.
In practice, EVOH, PVDC copolymer, PAN, PA, PEN, PET copolymer and other materials are often used as barrier materials, among which EVOH, PVDC, PAN copolymer and MXD6 aromatic nylon are high barrier materials. are medium barrier materials. Although EVOH, PVDC, PEN, and PAN have excellent barrier properties, they are generally not used alone due to poor processability, high cost, or incomplete performance, but are often used for blending, compounding, and modification.coatings.
b. Mechanical properties of plastic packaging materials
Strain curves for thermoplastics
The mechanical behavior of thermoplastics is usually studied experimentally, that is, by means of tensile tests. From stress-strain curve obtained from test, performance parameters that can be obtained include tensile strength, yield strength, Young's modulus, and elongation at break. These parameters can help evaluate strength, softness, toughness and brittleness of plastic packaging materials, and help us choose the right plastic packaging materials for our needs.
There are many types of thermoplastics. The stress-strain curve of a typical glassy polymer under uniaxial tension is shown in fig. 2-2:
Figure 5-1 Typical glassy polymer block
Stress-strain curve in axial tension
(temperature gradually increases from a to d)
When a glassy polymer is stretched, initial section of curve is a straight line, and stress and strain become proportional. From slope of this straight line, Young's modulus of sample can be calculated. Curve a has a high modulus and high tensile strength, but no yield strength. The failure that occurs before material becomes ductile is called brittle failure, and this polymer is both hard and brittle. Examples include polystyrene and polymethyl methacrylate at room temperature.
Fracture after material flow is called ductile failure, such as curves b and c. The material has a large deformation after fluidity, and its molecular mechanism is mainly movement of a macromolecule chain segment, that is, under action of a large external force, frozen chain segment of glassy polymer begins to move, and macromolecular chain stretching provides a large deformation of material.
The stress-strain curve of same polymer obviously differs due to different stretch rates, temperatures, and thermal history. At high temperature, thermal motion of a segment of macromolecular chain is intensified, and deformation is large. For example, polystyrene, which is very brittle at room temperature, will become a ductile material near Tg. With an increase in tensile rate, movement of chain link does not keep pace with action of external force, in order to make material flowable, a greater external force is required, i.e., yield strength of material increases. As tensile rate increases further, material will eventually undergo brittle fracture at higher stresses.
Viscoelasticity of polymers
The phenomenon in which mechanical properties of solid polymeric materials change with time is collectively referred to as mechanical relaxation or viscoelasticity. Various types of mechanical relaxation phenomena can be observed under various external influences on polymeric materials, most basic of which are creep and stress relaxation.
Creep is a phenomenon in which deformation of a material gradually increases with time at a certain temperature and a small constant external force. Metals and other materials exhibit creep phenomena to varying degrees, but creep phenomenon of polymeric materials is most obvious and, of course, depends on material. The creep properties of a polymer reflect dimensional stability of material.
For example, polymers used as fibers must have property of not falling apart at room temperature, otherwise clothes will become longer and longer. Another example is electrical packaging products, in whichThe foam plastic used for docking material is piled up in a warehouse for a long time, and foam plastic, which acts as a buffer at bottom of box, is increasingly deformed under influence of an electric current. products, resulting in a gap at top. The product may vibrate up and down, making shock pack ineffective.
The essence of polymer creep lies in movement of macromolecular chains, so soft macromolecular chains are prone to creep. Polymers with aromatic rings in main chain and rigid molecular chains have better creep resistance.
In addition to internal factors of molecular structure, creep characteristics are also affected by temperature and external force. The higher temperature or greater external force, greater tendency to creep.
Stress relaxation is a phenomenon in which internal stress of a polymer gradually decreases over time at constant temperature and strain. For example, if you use plastic rope to bind things, after a long time plastic rope will loosen and lose its binding function. Secure neck of bottle with a rubber band. When first fastening, it is very tight, and tension is very large. After a long time, it gradually loosens, that is, tension gradually weakens. This will result in loss of packaging.
Stress relaxation is also a result of movement of a polymer chain segment. With a certain deformation of polymer, a large internal stress arises, in order to reduce or eliminate it, stretched molecular chains tend to return to twisted state. Due to constant deformation, external force (stress) acting on polymer decreases accordingly. Stress relaxation and creep are two different manifestations of same thing. The former gradually decays in its stress at constant strain, while latter gradually develops its strain at constant stress.④ Hygiene of plastic packaging materials
Plastic packaging materials made from pure resin have good hygiene properties and can be used directly in food packaging without causing any harm to consumers. However, in order to improve some of physical and mechanical properties, chemical stability, and processing characteristics of resin, people often add a certain amount of plasticizers, stabilizers, antioxidants, fillers, dyes, lubricants, hair dyes, etc. to resin. Foaming agents, adhesives, etc. Some of these substances have a certain toxicity.After being added to resin for manufacture of plastic packaging materials, when in contact with food, they easily migrate into food and pose a health hazard to eater. Therefore, when choosing plastic packaging materials, in addition to meeting basic requirements for food packaging, attention should be paid to hygiene of plasticoutput materials.
Hygienic properties of plastic resins
Most of plastics used for packaging are pure resins that are non-toxic, but most of their monomeric molecules are toxic, and some are highly toxic, and some are clear carcinogens. When used in food packaging, it presents a health and safety issue. From a health and safety standpoint, US Food and Drug Administration (FDA) standards are internationally recognized best standards.
In synthetic resins, PVC and PVDC monomers have obvious mutagenic activity, and content of monomers must be strictly controlled when used in food packaging, so attention should be paid to use of food grade resins.
The styrene monomer in polystyrene resin has a destructive effect on liver cells. The United States, Germany, Belgium and other countries have introduced standards for content of monomer in polystyrene.
For sanitization of acrylonitrile plastics, US and Netherlands stipulate that content of acrylonitrile monomers in their polymers should be <6mg/kg~10mg/kg. The U.S. Food and Drug Administration prohibits use of beverages in plastic acrylonitrile bottles; when packaging other food products, amount of acrylonitrile monomer entering food should be below 0.3 mg/kg. France and Germany have higher standards and monomer migration to food is <0.05 mg/l. Because acrylonitrile monomer is a strong carcinogen, many countries can produce polyacrylonitrile resin without acrylonitrile monomer.
Hygiene plastic additives
Plastic additives tend to have hygiene issues. The choice of non-toxic or low-toxic additives is key to whether plastic can be used in food packaging.
Plasticizers can be divided into five categories based on their chemical composition: phthalates, phosphates, esters of aliphatic dibasic acids, citrates and epoxy resins. Less toxic. Plasticizers can be divided into four categories based on their toxicity: they can be used in food industry; they can be used in food industry with restrictions; there are still doubts about compliance with requirements for use; they cannot be used in food industry. industry. Plastic products with a high plasticizer content are not suitable for packaging liquid foods and are generally not suitable for packaging other foods with a high liquid content, especially foods containing alcohol and oil.
Hygienic properties of stabilizers
When processing PVC and vinyl chloride copolymers into packaging plastic, it is necessary to add theat stabilizers. According to different uses and processing requirements, polyethylene, polypropylene, polystyrene, polyamide, polyethylene terephthalate, etc., also need to add some stabilizers such as antioxidants and UV absorbers. Stabilizers for food packaging plastics must be non-toxic. Many commonly used stabilizers such as lead compounds, barium compounds, cadmium compounds, and most organotin compounds cannot be used in food packaging plastics due to their toxicity. It is now recognized in various countries that heat stabilizers approved for use in plastics for food packaging include calcium and zinc salts and fatty acids.
Dye and ink hygiene
Plastic dyeing or ink printing is a common treatment for plastic packaging. When they are used for food packaging, it inevitably leads to health and safety issues.
In addition to giving different colors, plastic coloring also has function of shading and blocking ultraviolet rays, but most dyes are toxic to varying degrees, and some are also strong carcinogens. Therefore, it is better not to use plastic that is in direct contact with food. For coloring, when coloring is necessary, non-toxic dyes should also be used.
Most inks for printing on plastics are polyamide inks, as well as aniline inks and alcohol-soluble phenolic inks. Polyamide itself is non-toxic, but its solvent contains more toluene and xylene, which are toxic substances. Since ink used for plastic printing has a certain toxicity, printed layer of packaging material should not be in direct contact with food.
Plastic films usually need to undergo a surface active treatment before printing, such as flame or corona treatment, to increase ink adhesion, but this can also cause tiny pores in film to enter packaging through ink solvent contaminating food. Therefore, all printed food packaging materials must be completely dry to allow solvent to completely evaporate and not contaminate food.
Hygiene of other plastic additives such as lubricants, foaming agents, etc.
The hygienic safety of food packaging materials is very important. Before choosing, you should first understand hygienic requirements of food packaging materials. For foods high in fat, care should be taken not to use packaging materials containing fat-soluble substances, especially high-fat plastics.plasticizer content; for alcohol with a high ethanol content should pay attention to content of residual polymer monomers, prevent residual monomers are extracted by ethanol into food.⑤ Chemical stability of plastic packaging materials
In plastic packaging materials during processing, storage and use, phenomenon of deterioration in physical and chemical properties and mechanical properties is called aging. The phenomenon of aging manifests itself in form of hardening of material, brittleness, denaturation or cracking of surface, stickiness, discoloration, etc. Aging factors consist of internal and external factors. The chemical structure and physical state of materials are main determinants of aging resistance. For example, molecular structure of polypropylene has a large number of tertiary carbons, and hydrogen on tertiary carbons is easily oxidized, so heat resistance and photooxidative aging characteristics of polypropylene are relatively poor. Another example is silicon-oxygen chain polymer. Since bonding energy of silicon-oxygen bond is larger than that of carbon-carbon bond, a large energy is required to break silicon-oxygen bond. Therefore, aging resistance of silicon-oxygen chain-structure polymer is relatively low and good. The thermotropic polymer has a network structure and has good resistance to heat aging.
External causes of material aging include physical, chemical, biological and other factors. Physical factors include heat, light, electricity, high-energy radiation and mechanical impact, chemical factors include oxidation, chemical attack of acids, alkalis, solvents, etc.⑥ Other properties
Modern packaging requires materials to have a certain degree of transparency so that packaged goods can be displayed to people. In addition, it is important to understand transparency of polymers. Some products are very sensitive to light of a certain wavelength, which requires selection of plastics that can block this wavelength as packaging materials. The light transmission of polymers is related to their structure, for example, whether it is crystallized, whether grain size is too small, molecular weight, whether additives are added, type and amount of additives, etc. Plastics with high visible light transmission include polyacrylates, cellulose acetate, PC , PS, etc. Transparent films and sheets mainly include polyethylene, polypropylene, PVC, EVA, PET, etc.
Static electricity is a common occurrence in nature. The phenomenon of electrification of polymers is even more common. Because of poor conductivity of polymers, static electricity builds up and it can persist for months with static electricity. Plastic packaging materials charged with statelectric electricity, cause great harm to packaged goods such as electronic devices, meters, explosives, etc.
The electrostatic charge of a polymer is closely related to its chemical composition, crystallinity, orientation, and other structural factors. Generally, polar polymers such as PA, PVC, etc. are easily positively charged. Non-polar high molecular weight polymers are easily negatively charged, such as PE and PP, while high molecular weight polymers that easily absorb moisture have excellent antistatic properties, such as PVA and EVOH. PVA is very suitable for packaging electronic products. The charge ratio of EVOH is very small, and there is no problem of static electricity during use, which can greatly reduce adhesion of dust. Other varieties of plastic are electrostatically charged and are used to package electronics or powders. Antistatic treatment should be carried out for granular and other products.
The PS surface is very comfortable to print on. PS is extremely easy to paint and has vibrant colors. It can be turned into a two-color container with a white inner layer and a red, yellow, green outer layer as needed.
PE and PP are non-polar polymers with high chemical properties that do not adhere well to inks and are not suitable for printing. Before printing, corona, chemical or fire surface treatment is required. By printing PP with polyolefin based inks, a clear printing effect can be achieved, as well as multi-color printing can be realized. Even though polyethylene has undergone surface treatment, printing with high durability is still difficult.
EVA is easy to color and product has a vibrant color. PVA is suitable for gravure and offset printing and has good print adaptability. PVC has good printing properties. PA has good paint adhesion and bright colors. Suitable for gravure printing. However, since polyamide absorbs moisture easily, film will wrinkle after absorbing moisture, so moisture-proof packaging should be done before and after printing. PA sometimes has printing burrs caused by static electricity, so it is necessary to use inks that can prevent burrs and add anti-static devices to printing equipment. PC has poor solvent resistance, so special attention should be paid to ink selection when printing. Despite presence of polar groups in PET molecule, adhesion resistance of paint is low. Therefore, corona treatment of film surface is required before printing.
In plastic packaging materials, most olefin plastics have good heat sealing properties. Such as PE, ionic polymer, PP, EVA, PVC, PVDC. Among them, ionic polymers can still be sealed even if seam is slightly dirty; PA and PET have poor heat sealability, and even when ultrasonic sealing is used, seam strength is also low, and PA and PET are often combined with heat sealing. sealing material PE; sealing temperature is relatively narrow. Commonly used heat sealing methods include pulse and high frequency methods. PCs are sealed by pulsed, ultrasonic and high-frequency methods.
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