1.1 Introduction to Polymers (A Practical Guide to Rubber and Plastic Technology)

1.1 Introduction to Polymers (A Practical Guide to Rubber and Plastic Technology)

Source: "Practical Guide to Rubber and Plastic Technology"

Chapter 1. Introduction

1.1. Introduction to polymers

Rubber and plastic materials are short for two main categories of polymer materials: rubber and plastic. In order to better research and apply rubber and plastic materials, we must first find out general characteristics of polymers.

1.1.1 Characteristics of polymer

What is main mystery of polymers?

When it comes to polymers, we must first have a deep understanding of concept of polymers.

Why is it called a polymer? Macromolecules are small molecules. Here, small molecules are considered as a group of individual balls, and these small molecular loose balls are strung together on a thin and long rope, forming a whole, forming structural characteristics that we call polymers.

Structure determines performance. What kind of performance will such a structure bring? First of all, we compare small organic molecules. Small organic molecules are small in size and have weak intermolecular forces. Even organic solid materials have low cohesive energy and cannot be high-strength structural materials. The force is large, so polymer does not have a gaseous state and can be used as a high-strength structural material with high impact strength. After polymer chains are connected in series to grow into chains, they tend to be in their lowest free energy state - random coils. Under action of an external force, thin chains are oriented along direction of external force, and when external force is removed, they automatically relax and return to random coils, showing unique entropy elasticity of polymer chains. A very long and thin rope is like a long rope in our life. When a certain force is applied to one end of rope to make rope move, other end of rope will wait for a certain period of time before it starts to move. , There is a time delay. Force and motion create a phase difference, exhibiting typical viscoelasticity. Therefore, polymeric materials have unique characteristics of both elasticity and viscosity.

Of course, if we only say that it is both viscous and elastic, this cannot be considered an essential property of polymer. Why did you say that? For example, an aqueous liquid is generally considered only viscous and has no elasticity at all, but when a force parallel to surface of water is applied at a very high speed to a thin tile, thin tile can fly on surface of water, showing elasticity of water, and common solid materials such as glass, will also flow after being placed for hundreds of years. Glass on European churches is that lower part becomes thicker than upper, and upperast flows into lower. In other words, as long as velocity is high enough, a typical viscous substance, water, can also exhibit elasticity; given a sufficiently long time (equivalent to a sufficiently slow action rate), glass can also create a viscous flow. Therefore, water can also be viscous and elastic at same time, and glass can also be viscous and elastic at same time. However, polymeric materials exhibit significant viscoelasticity at normal temperatures (tens of degrees above and below room temperature) and normal time of exposure to an external force. Quantify viscoelastic properties using a viscoelastic mechanics model. Here it is:

Viscoelasticity is main secret (an integral characteristic) of polymers.

The thin rope is original mystery (fundamental truth) of viscoelasticity.

Polymer theory is relatively abstract and usually requires a professional lab to get an intuition. The following is a simple explanation of concept of polymers for most practitioners - understanding flexibility and rigidity of polymers. I hope readers will be able to draw conclusions from one case and deepen it. Understanding a concept or theory.

The concept of flexibility and stiffness in polymers is difficult to understand for people who are not familiar with polymers. Let's use steel wire to explain flexibility and rigidity of polymers, hoping to deepen understanding of those interested in polymers. layman.

We consider each small piece of steel wire as a structural unit of polymer. When countless small pieces of steel wire are connected in series, polymerization process, as it were, combines structural units into a chain. When steel wire grows to a certain extent, steel wire will bend at will, which is flexibility of polymer. If length of steel wire is continued to be extended, steel wire will twist. This is polymer entanglement. When we pull two ends of steel wire, steel wire will gradually change. Straight, this is polymer entanglement.

If we bond two or more steel wires together, they will not arbitrarily bend over range of any bend length or greater than length of a single steel wire. We can consider bonded steel wires as polymers. The benzene ring, heterocycle, and other structures on surface are rigidity of polymer. .When link reaches a certain length, characteristics of a single steel wire reappear, which can be said to be flexible.

It's same: when it's long it gets soft, when it's short it gets hard, when it's thin it gets soft, and when it's thick it gets hard.

In addition to seemingly rigid steel wire mentioned above, there is also seemingly flexible water: falling into water from a great height can kill a person,because duration is short.

1.1.2 Polymer properties

The performance of a material is reflected in its cost of use, which is also basis for market positioning, market development and market value realization. Therefore, performance analysis is very important to improve product performance according to market needs. In terms of structure, we will briefly analyze commonly used properties of polymeric materials one by one, taking polymer layered silicate nanocomposites as an example.

A. Properties of polymeric materials

(a) Rigidity

Rigidity characterizes ability of a material to resist deformation under small deformation, which is represented by tensile modulus or flexural modulus according to different loading methods. The modulus of clay sheet is much higher than modulus of polymer matrix, so modulus of composite is always increased, which is same as traditional polymer filler. However, reason why modulus of polymeric layered silicate nanocomposites greatly increases is that number of clay layers per unit volume increases dramatically, and interface area is huge due to dispersion of clay layers at nanoscale.

(b) Intensity

Toughness is degree of stress to break a material under large strain, usually expressed as yield strength or fracture strength. The strength is more sensitive to defects within material. To obtain high strength, two-phase interfacial bond is better, and aspect ratio of dispersed phase shape must reach a sufficiently high value. The strong combination of ionic bonds at interface between two phases of polymer-layered silicate nanocomposite and high aspect ratio of clay sheet provide a significant increase in strength.

(c) Sustainability

Strength is a measure of energy expended on destruction of a material. This is combined effect of material strength and relative elongation. It is usually expressed in terms of impact strength. Provided that two-phase interface is well bonded, toughness mainly depends on size of dispersed phase. When size of dispersed phase is less than a certain value (relative to composite material system), toughness of material will not decrease, but increase. The two-phase interface of polymer-layered silicate nanocomposite material is strongly bonded with ionic bonds, and clay flakes are dispersed at nanometer scale, so that toughness is not reduced, but improved.

(d) thermal distortion temperature

Polymer materials soften and deform when used at higher temperatures, limiting their use in many applications. Increasing heat resistance has always been a focus of research and development. The thermal deflection temperature is temperature at which a material reaches a given small deformation at a fixed temperature.fixed bending load. In polymer layered silicate nanocomposites, countless nanoscale clay sheets are oriented parallel to spline surface, and movement of polymer chains is hindered, so that its ability to resist bending deformation is greatly improved, so that thermal deformation temperature becomes multiplied.

(e) Barrier property

Barrier is a measure of rate of diffusion, migration and penetration of small molecules in a direction perpendicular to membrane surface. In a polymer-layered silicate nanocomposite, nanosized dispersed clay flakes with a sufficiently large aspect ratio, oriented along film surface, form countless layers of nanocomposite films, which everywhere prevent migration of small molecules that are forced to bypass these obstacles and move in a zigzag pattern. forward, as in a maze, increasing distance of migration, thereby slowing down speed of migration, which is manifested in a significant increase in barrier properties of material.

(f) Machining rheology

The nanosized dispersed clay flakes in polymer layered silicate nanocomposite gradually orient themselves along flow direction under process flow field, so that it has strong shear thinning characteristics, that is, at a relatively low shear rate (assuming relative viscosity is measured), its viscosity is higher than polymer matrix, and at high shear, viscosity is lower than that of polymer matrix, with good fluidity and ease of molding and processing. Moreover, at similar relative viscosities, since polymer chains are firmly bound to clay sheet in form of ionic bonds, melt strength is higher than that of polymer matrix.

(g) Crystallinity

The clay flakes are dispersed in polymer matrix at nanometer scale, and there is a huge interface between clay flakes and matrix, which provides a large number of heterogeneous nucleation points for crystallization of polymer matrix, and makes crystallization of polymer matrix faster. Accelerates, spherulites are crushed and size becomes homogeneous, one end of polymer chain connects with clay sheet in form of ionic bonds, and its movement is difficult, which slows down crystal growth rate. And improve transparency of material.

Based on an understanding of performance, this article analyzes some of them.

B. Performance Analysis

(a) Sustainability

Strength is defined as energy spent on formation of a unit area of ​​destruction, which depends on many factors:

(1) Download method

①High-speed impact: cantilever beam (IZOD), freely supported beam (CHARPY).

②Stretching at medium speed: area under stretching curve = tensile strength * elongation at break.

③Bending at low speed: integral J, etc. (fracture toughness).

(2)Slot Size: Thick, Thin, Serrated, such as ISO, ASTM, DIN, etc.

(3) Loading speed and test temperature: higher speed, lower temperature and lower toughness, and vice versa.

(4) Material composition and internal structure:

Toughness is a general term (eg horse), toughness is a specific term (eg white horse), and elongation at break is part of another specific term (eg black horse tail).

Impact strength is a specific expression for toughness (① high speed impact), and elongation at break does not itself express toughness, it is just another specific expression for toughness (② medium speed tensile). Tensile strength * elongation at break is only one of them. Even if elongation at break is low, as long as tensile strength is high enough, toughness (measured by impact or tension) can be improved, for example, in glass fiber reinforced plastics.

Among them, integral J characterizes absorption of crack propagation energy; Charpy beam characterizes energy absorption caused by bending load; impact of cantilever beam characterizes absorption of energy during destruction; stretching characterizes absorption of fluidity energy. However, in this type of toughness characterization, commonly considered toughness characterization is toughness of a freely supported beam or cantilever beam. The application considers that use of a simple supported beam or cantilever beam for material toughness characterization is of little value.

As we all know, impact strength is calculated based on position of pointer after pendulum hit. If it collides with a material with high flexibility, impact rebound may occur, and even phenomenon of continuous impact may. In this case, under circumstances, there will be a very large discrepancy in calculation of position of pointer, but in fact, its toughness will not necessarily be high.

(b) Physical analysis of heat resistance of plastic

When it comes to heat resistance of thermoplastic materials, we must first ask ourselves, what effect does an increase in temperature have on material? The answer is that material becomes soft and deforms as temperature rises. Then ask, what does it mean that material becomes soft and deforms? The answer is that chaotic thermal motion of units of intramolecular motion (segments) exceeds intermolecular cohesion. Ask again, why does an increase in temperature cause random thermal motion of chain segments to exceed cohesion between molecules? The answer is that as temperature rises, random thermal motion of chain links increases, and distance between molecules increases, which leads to a weakening of intermolecular interactions. Therefore, any material is a unity of memolecular interaction and random thermal motion of chain links. Heat resistance is a balance value for measuring this contradictory unity. The experiment measures temperature at which a material reaches a certain deformation under certain external load conditions. The stronger intermolecular interaction and larger chain section of random thermal motion (the greater rigidity of chain), higher degree of random thermal motion (the temperature indicates degree of random thermal motion) is required for balance, and heat resistance is also higher. Intermolecular interaction is determined both by type of intermolecular force and by distance between molecules, and rigidity of chain is determined by difficulty of intramolecular rotation.

The strength of intermolecular interaction - hydrogen bond, dipole, non-polar van der Waals, etc. The intermolecular distance is mainly determined by three-dimensional packing density of molecule. The more regular molecule, stronger more symmetrical (the more crystallization is facilitated), higher packing density of molecules, smaller distance between molecules. For same type of molecules, density of crystalline phase of crystallization is always higher than density of amorphous phase, so crystallization is useful for improving thermal stability. The introduction of aromatic groups into main chain of molecule increases rigidity of chain and increases size of random thermal motion segment, which requires a higher temperature segment to move (this is also reason why symmetrical rigid segment preferentially crystallizes). ), which is aromatic. The introduction of group is useful for improving heat resistance. In short, stronger intermolecular force, smaller intermolecular distance, greater rigidity of molecular chain, higher degree of random thermal motion (temperature) required for balance, and better heat resistance. According to this conclusion, thermoplastic material with best heat resistance should be that main chain of molecule is fully aromatic (high rigidity), intermolecular force is as many hydrogen bonds as possible (strong force), and main chain of molecule has no substituents (highly symmetrical) materials, closest materials on market are high temperature resistant polymeric materials such as fully aromatic polyheterocycles, fully aromatic polyimides and fully aromatic polyamides. In presence of fillers in polymer, higher modulus of filler, greater aspect ratio, better compatibility with matrix (lower polarity of filler) and greater increase in thermal deformation temperature of material.

Physical entanglement points are similar to chemical crosslink points. Between points of engagement (as well as between points of stitching) are segments of chain. The whole is larger than part, so link segment is larger than chain segment. TOLike cross-link density, mesh density refers to amount of entanglement per unit volume of material. Similarly, longer chain length between stitching points, lower density of stitching points. The longer chain segment between entanglement points, lower density of entanglement points. It shows that at temperature at which modulus first drops, mobile unit is smaller than entire molecular chain, and for same polymer mobile unit is exactly same (does not change with molecular weight). Recalling other post on entanglement molecular weight, a motor unit can be thought of as a segment between entanglement points (called an entanglement segment). Recalling definition of thermal distortion temperature, one can also consider that "the same temperature at which first drop in modulus occurs" is thermal distortion temperature of material. Because entanglement segment is larger than chain segment (the unit of motion in glass transition) and smaller than entire molecular chain, thermal distortion temperature is higher than glass transition temperature and lower than polymer's flow temperature.

The larger engagement segment (the higher thermal deformation temperature), lower density of engagement points (the lower fracture energy of polymer material).

The destruction of materials is associated with slippage of molecular chains. Relatively speaking, destruction energy of materials with more entangled molecular chains should be higher.

Reasons for doubling of heat distortion temperature of fiberglass reinforced:

The thermal deflection temperature is temperature at which a material reaches a given small deformation under a fixed bending load. Although heat distortion temperature is a temperature parameter, it is related to mechanical properties. Its focus is not heat, but deformation. It describes change in flexural modulus (small flexural strain) with temperature and is related to flexural modulus. The modulus of inorganic filler itself is high, so thermal deflection temperature is much higher than that of polymer; larger size ratio of inorganic filler, greater effect of stiffness and greater increase in thermal deflection. temperature. Therefore, with same amount of additives, rod-like and flake inorganic fillers increase rigidity, so that increase in thermal deformation temperature is always greater than that of spherical inorganic fillers. Especially for glass fiber reinforced materials, ability to resist bending deformation is greatly improved, so thermal deformation temperature is doubled for following reasons:

(1) high modulus fiberglass oriented parallel to stressed spline surface;

(2) movement of polymer chain segment caused by crossing of crystal between glass fibers is difficult;

(3) Influence hnucleation of glass fibers on crystallization of matrix.

(c) Statistics on development of mesoscopic lesions of mixed/filled polymers

All damage and destruction is, in fact, a dynamic process, which is a process of damage to microstructure and formation of voids inside material. Therefore, to understand damage and damage, it is necessary to understand both mechanism of damage and law of their dynamic development. The damage and destruction of polymers is a complex multilevel and multistage process. It begins with destruction of structure of network of molecular chains at microscopic level, and then causes nucleation, growth and destruction of silver crazes and/or microshear bands at mesoscopic level, thereby triggering formation and expansion of microcracks until main cracks are formed. causing macroscopic damage to material.

The entire process of damage and destruction is a dynamic process that strongly depends on distribution of defects in material and develops non-linearly in time. To fully understand process of damage and failure, on one hand, it is necessary to understand mechanism and dynamic mechanical behavior of internal damage and failure of polymer at micro- and mesolevels. Polymer physicists and materials scientists are making tireless efforts towards this, on other hand, at macroscopic level, polymeric materials can be considered as continuums. People investigate non-linear mechanical response of materials with a strong dependence on velocity and history in process of external complex dynamic loading and describe it phenomenologically with mathematical methods. Continuum mechanics has made significant progress in this respect. The connection of macroscopic, mesoscopic and microscopic levels and construction of a bridge between continuum mechanics, materials science and polymer physics is mission of non-equilibrium statistical physics based on thermodynamic structure of irreversible processes, and consists in formation of a high concentration of emerging interferences. disciplines and is one of important pillars of dynamic nonequilibrium statistical mechanics of evolution of mesoscopic damage.

In field of polymer materials science, people have studied craze and microshear band. Many studies have been done on mechanism and kinetics of material initiation, growth, and failure, and some useful clues have been given, but they are still limited to observation and induction of mesoscopic damage and failure. material. , and remains at local microscopic and microscopic level of material. However, although continuum mechanics usually describes macroscopic mechanical properties of materials quantitatively, because it is too abstract and does not have a microscopic and mesoscopic physical basis, it is difficult to see characteristics of specific characteristics of damage and ruptureshrinkage, their connection with micro-mesostructure of materials, mechanism of micro-mesodamage and destruction. Thus, how to generalize results of studies of numerous mechanisms of damage to polymeric materials in order to abstract from physical model of dynamic mechanics of mesodamages in materials and, on this basis, obtain a statistical log of mesodamages in polymeric materials. macroscopically damaged materials have been identified, and mesophysical meaning of macroscopic mechanical properties of materials is an actual problem in theory of damage and destruction of polymeric materials.

Recently, significant progress has been made on two fronts. Firstly, dramatic increase in computer simulation has allowed simulation of highly non-linear behavior of materials, and secondly, use of controlled impact experiments and discontinuous quasi-static experiments has made it possible to describe relationship between observed microdamages and known stress. a connection between amplitude and deformation period becomes possible. Therefore, dynamic mechanical process of microdamage can be "frozen" at different stages of development to obtain accurate dynamic mechanical data. Finally, development of models for analysis of microdamage processes makes it possible to combine theory with experimental data and computer simulation to describe damage to materials under conditions of dynamic and quasi-static loading.

Currently, a general mesoscopic dynamic mechanical model of emergence, growth and merging of craters and microshear bands in macroscopic behavior of polymeric materials during damage is being created in order to reveal mesophysical significance of macroscopic behavior during damage and to determine specific velocity, dependence of macroscopic structure and mechanical parameters is introduced into a nonlinear determining dependence depending on speed, and a connection is established between mesoscopic damage mechanism and macroscopic characteristics of damage and destruction, and conditions for connection between materials science and mechanics of continuous damage are gradually established. is approaching. We believe that research in this area is important because it attempts to directly connect micro and macro worlds, and interesting because it allows us to test our understanding of both.

A mixing/filling system is a combination of two or more materials with different properties in a specific process. The properties of each of its components work synergistically to produce superior properties not found in a single material. Comprehensive properties are becoming a new type of high-performance materials. Polymer mixing/filling systems skillfully combine strength, modulus, dimensional stability, and photoelectric activity of inorganic materials with toughness, processability, and dielectric properties of polymer materials. Possibility of full disclosure of excellent complex propertiesin a mixed/filled system mainly depends on two coupling factors: size of dispersed phase and compatibility of two-phase interface. The entire history of development of mixing/filling systems revolves around a deep understanding of these two factors, proposal of new concepts, development of new methods, clarification of new structures, creation of new models and discovery of new laws in order to design in shortest possible time. more perfect level., control and adjust structure of dispersed phase and interface phase, to maximize performance and function of material system to prepare new high-performance, multi-functional materials, and it continues to attract increasing attention in a wide range of scientific fields.

During dynamic failure of polymer blend/filler system, triaxial stress concentration caused by dispersed phase particles near crack front can be divided into two parts: normal static tensile stress and shear stress. Static tensile stress causes bulk expansion processes such as interfacial delamination, matrix cracking, and cavitation, while shear stress causes matrix deformation in shear with shape change. Depending on relative magnitude of stresses required to initiate various processes, interfacial delamination, matrix cracking and cavitation, as well as matrix shear fluidity, can occur in damaged zone around crack front, respectively. For various polymer mixing/filling systems, energy dissipation may be due to debonding between surfaces, matrix cracking and cavitation, matrix shear deformation, or any combination of these. Thus, characteristics of matrix determine way in which mixed/filled system tends to dissipate energy, and way in which energy is actually dissipated is also related to characteristics of dispersed phase and interface conditions.

To start craze for matrices, three conditions must be met simultaneously:

(1) Mechanical conditions: static tensile part of matrix stress concentration caused by dispersed phase particles is greater than stress caused by matrix craze;

(2) Geometric conditions: size of area causing craze is greater than several interfibrillar spaces;

(3) Interface bond conditions: interface bond strength is higher than stress caused by a crack in matrix.

The initiation of matrix fluidity during shear must simultaneously meet two conditions:

(1) Dispersed phase particles cause stress concentration in surrounding matrix and weaken static tensile stress of matrix around particles of dispersed phase in process of interface destruction, formation of cracks in matrix and expansion of cavitation volume;


(2) Local concentration of tangentsstress, established in layer of matrix between particles of dispersed phase, is greater than yield strength of matrix in shear.

The stress concentration caused by dispersed phase particles in surrounding matrix can cause volumetric expansion processes such as interface delamination, matrix cracking and cavitation, as well as weaken static tensile stress that is part of stress concentration, so that matrix is ​​within a certain thickness around particles of dispersed phase. There is a transition from a plane-deformed state to a plane-stressed one. When shear stress fields of neighboring particles of dispersed phase overlap each other, intergranular matrix layer experiences shear deformation.

The degree of overlap of field of interparticle shear stresses depends on size of effective area of ​​concentration of shear stresses caused by particles of dispersed phase. The value of effective area of ​​shear stress concentration is related not only to ratio of modulus of dispersed phase, matrix to dispersed phase and shear yield strength of matrix, but also to particle size d of dispersed phase. phase. At small d, area of ​​effective concentration of shear stresses increases rapidly with increasing d, and at large d it is a characteristic parameter of mixing/filling system that has nothing to do with d.

For a given polymer mixing/filling system, if distance T between two adjacent particles of dispersed phase is less than critical thickness of matrix layer Tc, areas of effective shear stress concentration caused by particles of dispersed phase overlap, matrix layer between particles undergo shear deformation, material undergoes brittle-ductile transition, and impact strength of mixed/filled system is significantly increased, and, conversely, if T>Tc, corresponding matrix layer is not subject to shear fluidity. The critical thickness of matrix layer Tc is a characteristic parameter of mixed/filled system and is not directly related to glass transition temperature of matrix, yield strength of mixed/filled system, particle size and volume fraction of dispersed phase, but is related to shear yield strength of matrix. From point of view of structure of matrix molecular chain, shear yield strength of matrix is ​​directly proportional to flexibility C∞ of matrix molecular chain at microscopic level, so T is inversely proportional to C∞.

(r) Transparency

Different phenomena, such as transparency, color, brilliance, etc., are caused by a common cause - light. Without light, there are no phenomena of transparency, color, brilliance, etc. Different colors of light color objects in different colors. At opening and closing ceremonies of Olympic Games, colorful pennants and stage lighting are striking examples.

Sunlight in natural conditions is white light mixed with red, orange, yellow, green, blue and violet. Various phenomena such asThe brightness, color and brilliance are caused by different interactions between white sunlight and objects. The interaction of light with objects can be divided into: reflection / scattering of light on surface of an object, refraction propagating in an object, absorption by molecules in an object, transparency through an object, etc. Reflection of light on surface of an object is a source of glare, scattering of light on surface of particles is cause sky and white clouds, as well as reason that transparent glass slag is white; refraction of light propagating in an object is a rainbow caused by water droplets in air after rain. The reason; only when light is absorbed by molecules in object is narrow sense dye color rendering principle; remains unchanged, which is called transparency.

(e) Barrier property

At present, multilayer composite is mainly used for barrier materials.

Laminated composite packaging material is most dynamic and fastest growing packaging material in plastic packaging industry, representing today's packaging material development direction. In recent years, this type of material has developed at an annual rate of 20%. The multilayer composite material packaging container can greatly improve performance of barrier, and effect is very good. Molding PET and other high barrier materials into a multilayer composite bottle is main method for improving barrier properties.

Currently, there are three methods for making multi-layer composite plastic beer bottles: one of them is outer coating film, which is mainly spraying high barrier materials (such as epoxyamine latex, PVDC latex, etc.) on outer layer. .). .); Inner coating (such as silicon oxide evaporation, amorphous carbon plasma deposition, etc.); third layer is a multi-layer intermediate layer of polymer material (such as PET/EVOH/PET, PET/MXD6, nylon series 5 layers, etc.).

(1) Outer coating film

PPG provides an epoxy-amine barrier coating with barrier properties 200 times greater than PET that is sprayed onto outer surface of PET bottles. The coating is insensitive to temperature and humidity and can be removed with an alkaline bath containing surfactants. The thickness of coating is only 1% of thickness of bottle, which allows you to extend shelf life of beer in bottle up to 3 months. The coating was first applied by Carlton United Breweries, Australia in 1996 on world's first industrial line for production of disposable beer bottles (500 ml).

(2) Inner lining

The protective properties of silicon oxide are 1500 times higher than those of a PET bottle. If it is sprayed onto innersurface of PET bottle, coating thickness is only 0.13% of bottle thickness, which can improve barrier property by 3 times compared to PET bottles. It is successfully produced and used in developed countries such as USA, Japan, Germany, Italy and Switzerland.

(3) Polymeric interlayer

EVAL reported that barrier properties of new EVOH are 100 times higher than those of PET. Three-layer PET/EVOH/PET composite, thickness of EVOH barrier layer is 4% of bottle thickness, which can extend shelf life of beer in bottle up to 5 months. A beer bottle made of multilayer composite plastic (330 ml) was made in 1997 by British company Bass-Breweries.

(4) Plate mixing

This is a general method for developing new plastics by blending other polymers in a polymer matrix to overcome weakness of one component and obtain polymer materials with ideal complex properties. An example of application of this method is improvement of barrier properties of materials by mixing. High barrier properties correspond to dispersed-phase lamellar mixtures with a large aspect ratio of dispersed phase. Based on this, a new mixing technology has been developed - lamellar mixing. The multi-layer mixing technology realizes layer-by-layer distribution of barrier resin in matrix resin by selecting mixture composition, using appropriate processing equipment and appropriate molding process conditions to achieve goal of improving barrier properties of matrix resin. PET and high barrier rubber are blended with flexible resins (such as liquid crystal polymers LCP, PEN) to form high barrier blends and then blown to form high barrier PET bottles.

(5) Composite with nanosized high barrier layer

The preparation of polymer nanocomposites by method of intercalation polymerization of composites is a modern direction and an advanced direction in science of polymeric materials. In this case, monomers are inserted between layered silicate sheets of montmorillonite, and then polymerized. During polymerization, phyllosilicate is stratified into nanosized sheets and uniformly dispersed in polymer to form a nanocomposite polymer. The method of intercalation polymerization of composite reduces interfacial tension between filler and polymer matrix and provides perfect interfacial adhesion. The obtained nanocomposite polymer material has excellent mechanical strength, heat resistance, solvent resistance and barrier properties. The price-performance ratio of existing large varieties of polymeric materials can be significantly improved.

Countless layers of high-barrier phyllosilicates, dispersed at nanoscale in a polymer matrix, with a significant length-to-thickness ratio and oriented along surface withThe walls of bottle form countless layers of nanocomposite films that prevent gas migration. molecules everywhere When blocked, gas molecules are forced to bypass these obstacles and move forward, like walking in a maze, increasing migration distance, thereby slowing down migration rate, which manifests itself in a significant increase in barrier properties. But in long run, a single-layer barrier is direction of development.

(e) Lubricity

During processing of plastic, you can add required amount of lubricant to reduce friction and increase speed of extrusion. However, its function is different from that of plasticizers, and excessive addition of lubricants will result in poor plasticization. Depending on compatibility with respective resin, greases are divided into two types: internal greases and external greases:

(1) Internal lubrication

The lubricant with good compatibility with resin that stays inside resin is called internal lubricant, which can reduce friction between molecules in resin.

(2) External lubrication

Lubricant with poor resin compatibility that migrates and settles outside resin is called external lubricant, which can reduce friction between resin surface and mold.

(3) Description example

The most common lubricants are polyethylene wax and stearic acid.

①When used in polyolefin resins, polyethylene wax has good compatibility and remains inside resin as an internal lubricant; stearic acid has poor compatibility and migrates and precipitates outside resin as an external lubricant.

②When used in PVC resin, stearic acid has good compatibility and remains inside resin as an internal lubricant; polyethylene wax has poor compatibility and migrates and deposits outside resin as an external lubricant.

1.1.3 Laws of polymer modification

A. The first law of modification process: diffusion control PK shear control

When choosing a screw combination, two competing dispersive mixing processes are possible:

(a) Distribution Management Process

(1) The screw speed is reduced and residence time of material is increased, which promotes uniform diffusion and mixing.

(2) As temperature rises, diffusion rate increases, which also promotes uniform diffusion and mixing.

(b) Shear control process

Shear stress = melt viscosity * shear rate.

(1) As screw speed increases, shear rate increases accordingly, which increases shear stress, which aids in shear control, dispersion, and mixing.

(2) Lowering temperature will increase viscosity of melt, which will also increase shear stress, which will aid in shear control, dispersion, and mixing.

Influence of processing conditions on compatible propertiesx polypropylene nanocomposites.

Since a lower profile temperature and a higher screw speed do not lead to a deterioration in final structure and properties of material, diffusion processes are not a determining factor. Thus, it can be stated that for a compatible polypropylene nanocomposite s, controlling factor is intensity of shear stress transferred to clay layers from polymer melt, and not diffusion of polymer molecules into clay galleries. This statement is also consistent with fact that best results are obtained when using processing conditions that maximize shear stress on polymer, i.e. at higher screw speed and lower barrel profile temperature.

The above text is an excerpt from final part of article. The system used in this article is polypropylene + maleic anhydride grafted polypropylene + commercially available organic montmorillonite, and preparation of polypropylene composites by twin screw melt mixing is investigated. . The effect of mixing process, such as bowl temperature and screw speed, on dispersion of organomontmorillonite in a polypropylene matrix and on properties of polypropylene composites. The experiment showed that combination of a lower bowl temperature and a higher screw speed promotes dispersion of organic montmorillonite, thereby improving performance of composite materials.

A large number of previous studies of a mixture of polystyrene and commercial organomontmorillonite in melt have concluded that diffusion of polymer molecules between clay plates is a controlling factor in process. As we all know, diffusion coefficient of polymer molecules increases with increasing temperature, so higher cylinder temperature promotes diffusion of polymer molecules into intermediate layer, so that organic montmorillonite can be dispersed, thereby improving performance of composite. materials. But this is clearly related to this. The experimental results of this article are opposite.

How to explain this experimental phenomenon? The author of article believes that “since lower temperature of outer wall of barrel and higher speed of screw did not cause a deterioration in final structure and properties of material, diffusion process is not a determining factor. Therefore, it can be considered that for a compatibilized polypropylene nanocomposite material, controlling factor is intensity of shear stress transferred from polymer melt to clay sheets, and not diffusion of polymer molecules between clay sheets.

Shear stress Note. Shear Stress = Melt Viscosity * Shear Rate.

Among them: shear rate increases with screw speed increase, melt viscosity increases with decrease in barrel temperature. Thus, "combinationlower barrel temperature and higher screw speed" means an increase in single shear stress parameter. Under conditions of constant barrel temperature, increasing screw speed increases shear rate, so that shear stress increases.

B. The Second Law of Modification Process: Distributed Distribution of PK

Mixing involves two processes: dispersion and distribution. The dispersion process is controlled by shear stress and distribution process is controlled by diffusion. When a screw combination is selected, there are two competing mixing processes:

(a) Distribution Management Process

(1) The speed of screw is reduced and residence time of material is increased, which promotes even distribution and mixing.

(2) As temperature rises, diffusion rate increases, which also contributes to uniform mixing of diffusion distribution.

(b) Shear control process

Shear stress = melt viscosity * shear rate.

(1) As screw speed increases, shear rate increases accordingly, which increases shear stress, which aids in shear control, dispersion, and mixing.

(2) Lowering temperature will increase viscosity of melt, which will also increase shear stress, which will aid in shear control, dispersion, and mixing.

1.1.4 Comprehensive assessment of modification effect

Plastic modification is mixing of plastics, fillers, additives, etc. within a specific process, with synergistic effect of each component providing excellent performance and low cost. Plastic modification includes a large number of plastics, fillers and their components, and various combinations of performance indicators of interest, such as stiffness, strength, impact strength, etc. does plastic modification affect performance metrics?

A, Comprehensive Performance Index

The effect of plastic modification is considered on basis of a combination of plastic matrix efficiency indicators, and dimensions (units) of efficiency indicators are different. Thus, arithmetic mean of dimensionless performance improvement factor is chosen as composite performance index (PI) to measure effect of modification:

PI= (1)

Where Pm is a certain performance indicator of modified plastic, P0 is corresponding performance indicator of plastic matrix; < > is average of several different performance factors.

B, price index

Given that plastic products are sold in units of volume, price index (CI) for measuring cost of modifying plastic was chosen to be unitless.surcharge:

CI=(Cm*ρm)/(C0*ρ0)-1 (2)

Among them, Cm and C0 are prices of modified plastic and plastic matrix, respectively (10,000 yuan/ton), and ρm and ρ0 are density of modified plastic and plastic matrix, respectively (ton/m3).

C. Universal Cost Effectiveness Index

From formula (1) and formula (2), universal cost effectiveness index (UI) can be obtained as follows:

UI=PI/CI= /[(Cm*ρm)/(C0*ρ0)-1] (3)

The price per cm of modified plastic is:

Sm=C0*(1-W1)+C1*W1 (4)

W1 is amount of filler added (%) and C1 is filler price (10,000 RMB/ton).

Density ρm of modified plastic:

1/ρm=(1-W1)/ρ0+W1/ρ1 (5)

where ρ1 is filler density (t/m3).

Substitute formula (4) and formula (5) into formula (3) and simplify:

UI= /{[(C1*ρ1/CO-ρo)*W1]/[ρ1-(ρ1-ρo)*W1]} (6)

If price per unit volume of filler (C1*ρ1) is much higher than price per unit volume of plastic matrix (C0*ρ0), and amount of filler added (W1) is small, formula (6) can be simplified as:

UI= /(C1*W1/C0) (7)

Equation (7) is a simplified and practical expression for Universal Economic Efficiency Index

D. Estimation example (data collection time - August 2001)

1.1 Introduction to Polymers (A Practical Guide to Rubber and Plastic Technology)

Table 1.1 Comprehensive assessment of effect of modification of nylon-6-montmorillonite nanocomposites

* Control strips and tests are under control of Beijing Chemical Industry Research Institute: GB1040___92, stretching speed 20mm/min, starting temperature 30°C, heating rate 120°C/hour, # Average increase rate of corresponding three characteristics indicators .

** Calculate using formula (7)

1.1.5 Development trend of modified plastics

After more than 100 years of research and development, people have gradually realized that in development of polymer material synthesis technology, research and use of various modification technologies is an effective way to quickly and economically apply polymer materials. Let's look at development trend in field of plastic materials.

A. General plastic engineering.

B. High performance engineering plastics.

C. Low cost high performance engineering plastics.

D. Nanocomposite technology will breathe life into modified plastics.

E. Environmental protection requirements for plastic modification are becoming generally accepted.

F. The development of new high-performance additives has become an important topic for modified plastics.

G. Modification and application of plastic waste.

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