Tearing relieves the tension but it immediately increases again and, unless corrected, this process gets repeated. An example of melt fracture is also shown in Figure 3. Some of these processes are described in more detail below.
Plastic melt is extruded through a ring-shaped tube die annular gap , usually vertically, to form a thin walled tube. Air is introduced via a hole in the centre of the die to blow up the tube like a balloon. During the process, the ilm is drawn both in radial and longitudinal directions, and the level of drawing the extent of biaxial molecular orientation can be controlled by changing the volume of air inside the bubble and by altering the haul of speed.
It is simply the ratio of the inal diameter of the blown tube relative to the die diameter, typically 1. Increasing the BUR is equivalent to working at a higher frost line height. At a given melt temperature, a high FL allows a greater relaxation of surface irregularities, resulting in a glossier and less hazy ilm.
Typically a gap of 0. It controls the molecular orientation in the MD. It can be much greater during the formation of the bubble than in the die, particularly if the blow-up ratio is high.
Another problem in blown ilms is the entrapped air between ilm layers and rollers — this may cause ilm scratching or wrinkling, or processing problems when winding up the ilm due to reduced friction.
One of the solutions is to use vacuum to remove entrapped air. Large amounts of blown ilm are used as geomembranes Figure 3. Ireland 3. In food packaging ethylene-vinyl alcohol EVAL is included in multilayer co-extruded ilms for its superior barrier properties.
Co-extrusions are commonly used to provide precise properties for speciic applications. Multi head extruders feed into the extrusion die with the difering materials, e. Whereas crystalline PET, contain nucleating agents, crystallise readily under heat during thermoforming. Crystallinity renders the tray rigid and therefore prevents it from deformation when being heated and served.
Almost all the skeletal scrap can be reused. However if this is not acceptable, for reasons of colour matching or contamination, then it can be used as feed stock for other types of productions. During extrusion, when the slots of the inner and outer rings are in register the knots of the mesh are produced, and when they are out of register the strands of the mesh are produced. Some of the panels would have a membrane of UV barrier laminated on, to absorb harmful UV radiation.
In melt spinning a die, known as a spinneret, a lat plate containing hundreds of small holes of a fraction of a millimetre in diameter is used. A bundle of ibres, tow, can be combined with a slight twist to produce ibre yarns. Die and pin should be accurately centralised to achieve a uniform extrudate cross-section. Certain applications require multilayered or striped tubes that require co-extrusion by employing a number of extruders simultaneously extruding into a single die.
However, it is important to note that not all the polymers can be co-extruded. Some products require mechanical stretching of preforms prior to blowing with compressed air. Injection-blow moulding involves injection moulding of the parison, which is then transferred to another machine for blow moulding.
Depending on the application, there are, however, other considerations that need to be met. For example, the bottles for sot drinks should have: - high transparency for appeal - impermeability to gases izzy drinks must not lose CO2 pressure too quickly - creep resistance the container must not lose its shape on the shelf.
In thermoforming, however, a greater use is made of air pressure and plug assisted forming of the sotened sheet. A sheet blank is clamped over the vacuum box thus sealing of the box beneath a heater, which is oten a retractable one. Once the blank becomes suiciently pliable and begins to sag the sag point , it is then sucked down and held tight into the mould cavity by the application of vacuum for shaping.
Ater the part has suiciently cooled, it is removed from the mould. See Strong , p for detailed description. In all these processes the forming operation is followed by cooling, oten with air, and by a secondary operation to trim of any excess material from the moulding to obtain the inished product. A real draw ratio is a measure of the biaxial orientation that the heated blank undergoes during forming.
A linear draw ratio can be determined by scribing a line of known length onto the blank, the draw ratio then becomes the ratio of the length of the scribed line on the formed moulding to that of the scribed line on the sheet blank used to form the product. It is a measure of the overall uniaxial elongation capacity the sotened plastic must have for the forming process. For products that require deep draw ratios, greater than , pre-stretched male forming or plug-assisted female forming is suggested to obtain the most uniform material distribution.
Heating and fusion — once inside the oven, the mould is rotated around two axes. Mould surfaces can be insulated see Figure 3. Shielding to achieve lower wall thicknesses and the openings, as in a wheel barrow mould is shown in the igure , are achieved by a layer of insulation such as rock wool placed inside the relevant mould parts as in the lid of the wheel barrow mould.
When the melt has been consolidated to the desired level, the mould is cooled either by air, water or a combination of both. De-moulding and unloading — when the polymer has cooled suiciently to retain its shape and be easily handled, the mould is opened and the product removed. At this point powder can once again be placed in the mould and the cycle repeated.
For some applications cross-linked PE is also used. Typical products include agricultural products e. As long as there are extra stations available, the machine can be itted by up to six arms of the same type or diferent types. Note that in a service area, the inished product is removed from the mould and then the mould is charged with fresh plastic powder ready for the next run.
On a ixed machine three or four arms move in succession into and out of various stations. Of course the arms cannot rotate past each other but they can move separately from each other and spend diferent lengths of times in diferent stations. Processing with PE, the air temperature inside the mould reaches approximately oC when the air temperature inside the oven stabilises at a set temperature of oC.
When all the powder has melted at Point B, the internal-air temperature begins to increase again rapidly. Following complete solidiication, Point E, the internal air temperature continues to fall at its normal rate. At Point F, the plastic shrinks and detaches itself from the tool face, which is accentuated by the release agent usually silicone solution wiped on the mould , leaving a gap between the part and the tool surface. A multi layer rotational moulding may consist of three layers: e.
Micropellets wash out and do not remain put on lat surfaces. One of the obvious concerns in incorporating engineering ibres such as glass ibre, natural plant ibres or nanoparticles into polymers for rotational moulding is the possibility of distortion of the inished parts, for example the walls of a box-shaped container bowing inwards.
Such distortions to the mouldings are a consequence of the diferences in the coeicient of thermal expansion CTE values for the polymer and the additives, which may result in the generation of un-balanced internal residual stresses. Typical TS polymers include polyesters, alkyds, amino resins, epoxy resins and polyurethanes.
TSs contain cross-links in their structure and in general ofer greater resistance to temperature and creep cf. TPs, however, they sufer from low impact resistance. Most thermosets are available in two forms: resins or as moulding compounds. Moulding compounds in the forms of dough moulding compounds DMC and sheet moulding compounds SMC are a mixture of TS resins with both illers and ibre reinforcements. In SMC longer glass strands are used. Polyester resins unsaturated are produced by condensation polymerisation of a polyfunctional acid with a polyfunctional alcohol.
Polyester resins are usually used with woven glass-ibre cloth or mats of chopped glass-ibre strands to produce strong laminates. Polyester DMC is of a putty-like consistency and has a very low viscosity at moulding temperatures and, therefore, can be compression moulded using low pressures. A resol is formed by using excess formaldehyde phenol to formaldehyde molar ratio up to 1: 2. Early applications were in the electrical industry.
PF impregnated paper and cotton fabric laminates ind application as very durable gear wheels e. PF and MF exhibit good lame resistance. London Transport speciies phenolic glass-ibre reinforced plastic GRP for use in underground rolling stock and as cladding in station escalator wells. Similarly, the Dusseldorf airport ire in , which claimed 16 lives, focused attention on the type of materials used in public places.
Other uses of PF include the weapons and equipment storage boxes on British naval ships, where, following the Falklands conlict, ire retardancy and low smoke emission were found to be essential.
For SMC, the charge is cut into small pieces, weighed and placed in the mould cavity and pressure is applied to the hot mould to force the material to take up the shape of the cavity. Ater compression the mould remains closed until curing is complete and the prepolymer has cross-linked.
Compression and transfer moulding, covered below, are both batch processes. It is better suited than the compression moulding for the applications where delicate inserts or hollow cores are required in a moulding, because the cavity is not being directly pressed. Electrical components are usually produced by transfer moulding.
Cycle times are longer for thermosets than for thermoplastics due to the chemical reaction. Vacuum assisted resin injection VARI , resin transfer moulding RTM or vacuum infusion processes are variants of low-pressure moulding, where the catalysed resin is injected into matched moulds or bagged tools in which the reinforcement has been placed prior to mould closure.
Application of vacuum ahead of resin injection assists mould illing. All these techniques depend on the easy low of the prepolymer and good wetting of the reinforcement. Many thermosetting polymers e. Dispensing units include facilities for metering and mixing of ingredients. While this problem can normally be prevented in TP injection moulding by proper mould construction and processing parameters, it cannot be avoided in TS injection moulding and needs to be removed as a secondary operation.
When possible, the parts are de-lashed in an automatic tumbling operation rather than hand de-lashing to save costs. Cellular plastics ind uses in industries such as furniture and bedding, looring, automotive, building and aerospace sandwich panels, memory foam. By way of an example, the basic chemistry of PU foam is briely described here: a urethane link is produced as a result of the reaction between an isocyanate and a hydroxyl group.
Polyols: there is a wide range of polyether or polyester based polyols. For lexible foam production, high molecular weight, approximately , polyether or polyester prepolymer triols are used.
For rigid foam, the polyols are of higher functionality and lower molecular weight Catalysts: amines or organometallic compounds, e.
Hydrocarbons such as cyclopentane, isopentane, and normal- pentane as well as butane isomers, which are mostly used as co-blowing agents ofer the advantages of being low cost, low GWP and commercially available. Water also reacts with isocyanate and produces CO2, thus, controls the foam density, and contributes to producing a more cross-linked rigid micro-structure.
Surfactant: usually a silicone oil to control foam cell structure and cell homogeneity. For processing purposes, the raw materials of polyols, catalysts, expanding agents and surfactant are mixed together and degassed; making up Part A.
Isocyanate constitutes Part B. During the production Parts A and B are mixed together and the mixture is immediately dispensed either into moulds or onto a moving belt for the slab stock production Figure 3. Other types of foam, besides polyurethane-based rigid and lexible foams, include: Polystyrene and polyethylene foam: these thermoplastic foams are extensively used in packaging and building applications.
Polymerisation of PS beads by suspension polymerisation for expansion includes pentane blowing agent in its formulation. Nucleating agent sodium bicarbonate or citric acid is also used to promote uniform beads with uniform pore size and structure to ensure the encapsulation of pentane gas in beads. PS foam can also be extruded in sheet form: in a post-polymerisation process, the beads and foaming agents hydrocarbons, HCFCs, HFCs and CO2 or their blends are fed into an extruder where the beads expand and incorporate the blowing agent and are extruded through a lat die.
PE foam is mainly produced through extrusion technologies: the foam is produced by dissolving and mixing a gas iso- butane or pentane into the molten PE, where they form small gas bubbles or cells and expand the polymer and inally cooling of the expanded polymer produces the foam.
By using a suitable extrusion die, the foam can be shaped into diferent semi-inished products such as tubes, proiles, sheets and blocks. Urea-formaldehyde UF foam: foaming is achieved by mechanical agitation. It readily absorbs a variety of liquids. Hollow beads may be added to plastics as an alternative to foaming to reduce weight.
Phenol-formaldehyde is used to produce inherently ire retardant foam. Pigment is the obliterating component because of its high refractive index compared with the binder and promotes colour. Drying and hardening processes for the applied coating are air drying or heat drying and hardening.
In air drying the solvent is eliminated from the applied paint by evaporation, and the ilm of coating is hardened by cross-linking reactions under the activation of the atmospheric oxygen.
Whereas in heat drying and hardening, heat accelerates not only solvent evaporation but also cross-linking reactions. Paints that require heat for drying and cross-linking are known as curing or stoving paints.
Coating techniques: - Dipping: the component is heated in an oven to a temperature such that when it is dipped, the polymer adheres to the hot metal, melts, lows, and fuses into a coherent coating. Main stages in the dip coating are 1 metal preparation e. Some of the techniques use an electric ield: electrostatic and electrophoretic coating.
Electrostatic coating involves spraying of an electrically grounded part with charged polymer powder. At some particular gas velocity, the weight of the particles will be slightly less than the buoyant force of the gas, and the powder becomes luidised. Dipping objects into luidised powder is much easier than dipping into stationary powder. Indicate if the viscosity of a polymer decreases with increases in a shear strain rate, b molecular weight, c temperature d pressure.
Does MFI of a polymer increase or decrease with increasing viscosity? How are weld-lines caused in injection mouldings and how do they afect the quality of the components? Indicate true or false: weld lines become a source of weakness because polymer chains difuse very slowly.
Cite factors which determine the choice of fabrication techniques for polymers. What two parameters are used in rating an injection moulding machine? In moulding, what is the purpose of cold-wells in a mould? Excess lashing could be the result of a material being too hot b mould being too hot c injection pressure being too high d all of the above conditions.
Why is it important to have the sections of a moulding as uniform in thickness as possible? Distinguish between the screw types for the efective extrusion of Nylon 6,6 and polyethylenes.
On a standard extruder screw, there are three sections — what are they called? What is the purpose if the tubes or passages under the surface of the feed throat? What will probably happen if plastic melts too early and sticks to the screw in the feed zone? Indicate two important microstructural characteristics for polymers that are considered for ibre production. Why is the inside diameter of the calibrators sizing rings oten bigger than the outside diameter of a tubular extrudate?
What is the magnitude of variation and what does it depend on? Die swelling occurs because of a attempting to extrude a product at too fast a rate b the chains become completely disentangled at high shear rates and expand when they re-entangle c relaxation of shear oriented molecules d the pressure of the polymer melt expands the die. What processing conditions must be met in the production of PET bottles for sot drinks? What processing method would you use to make a rigid plastic pipe that can be laid easily?
Which of the following processing methods would you use for compounding a polymer with colorants and stabilizers a injection moulding b thermoforming c single-screw extrusion d twin-screw extrusion e transfer moulding. Indicate true or false: PMMA glazing for aircrat windows is biaxially stretched because this encourages crystallisation while preventing the formation of large spherulites that can scatter light. Indicate the relationship between the melt temperature and the height of the frost line in blown-ilm production.
Moulds for blow moulding can be made using aluminium whereas moulds for injection moulding are usually made out of tool steel, why? What processing method s would you use to make disposable plastic plates? What are typical thermoformed plastic products? Calculate the length x width x thickness of a blank sheet needed to produce a rectangular container of x 50 x 10 cm of a 2 mm wall thickness by vacuum forming. Assume 2 cm is required all round for clamping. Answer: x 54 x 0.
What processing method would you use to make large, hollow polyethylene playground items? What processing method would you use to make a plastic traic cone? Describe a method for producing a rotational moulded part consisting of two solid layers of diferent materials. Describe the circumstances when transfer moulding would be a better choice than compression moulding for production of polymeric parts.
Describe the injection moulding process and distinguish between the injection moulding of TP and TS polymers. Guess what the following pattern on the loor is, how was it produced, and relate it to one of the topics covered in this chapter.
Describe the luidised-bed coating technique, outlining the parameters which inluence the quality of the coating. Distinguish between electrostatic and electrophoretic coating. Identify and briely describe a method for coating a plastic with metal.
Injection moulding scrap from the sprue and the runner system can be reground and used again for processing, explain if the same is possible with the cull and runner scrap produced in transfer moulding. In this chapter some measurements associated with the morphological and microstructural features of polymeric materials will be briely outlined.
Polymers consist of chain like molecules, where thousands of monomers repeat units are strung together. In thermoplastics the arrangement dictates whether the polymer is crystalline orderly or amorphous disorderly , and in thermosets, particularly in elastomers, it also controls the propensity of the polymer in unvulcanised state to be crystalline or not and, therefore, its suitability for making rubber. In cis coniguration the double bonds and these groups CH3 are on the same side of the chain, and in trans coniguration they are on opposite sides or across from one another Figure 4.
In general, cis-polymers show a lower Tg and Tm values than the trans polymers, as in polyisoprenes and polybutadienes, since regular symmetrical structure of trans polymers enable crystalline formation. Cis-1, 4-polyisoprene natural rubber do not normally crystallise unless highly strained and the molecules coil rather than remain linear, which gives rise to long-range rubber elasticity.
Consequently, three diferent forms of polymer chain results in thermoplastics: atactic, isotactic and syndiotactic. Figure 4. Atactic form describes the random attachment of the side groups about the back-bone chain. Stereoregularity inluences the ability of a polymer to crystallise and also the degree of crystallinity and, in turn, signiicantly inluences properties.
For example, polystyrene has large Download free eBooks at bookboon. But in real production situations the spherulites in thermoplastic ilms are imperfect in shape and much smaller in size 0. Spherulite size and its uniformity signiicantly inluence mechanical and optical properties. X is the axial distance between the sample and the photographic plate, and Y is half the distance between the peaks of the diagonal lobes of the SALS pattern.
Although such a photographic output shows the concept well, the equipment produces digital data in the form of plots of x-ray intensity vs. L and q is the Bragg angle note that 2q is known as the difraction angle. Accordingly the crystalline polymers such as PEs, PP and nylons are more appropriately also referred to as semi-crystalline. Small pendant groups and tacticity favour close packing of the molecules and therefore increase crystallinity.
Time is required for random and entangled chains to become ordered, slow cooling allows time for molecules to arrange themselves into an orderly structure. Intermolecular forces, such as H-bonding, also encourage crystallinity and more importantly leads to increased stability of the crystalline regions. In using all these techniques for the determination of the degree of crystallinity, certain assumptions are made: a polymers consist of two separable phases — crystalline and amorphous, and b the properties of the phases are additive, i.
Calculation of Vc for PE: Five polymer chains can be associated with the unit cell: four at the corners and one through the centre, see Figure 4. By the same approach, the contribution of the central chain to the unit cell is also one monomer unit. Density values for crystalline and amorphous components of various polymers are presented in Table 4. Table 4. SAXS measurements are conducted at very small scattering angles 0.
On IR spectra there are crystalline, amorphous and crystalline-amorphous sensitive bands. For PP, the crystalline sensitive band is at Crosslinking may be achieved by reaction of functional groups; by vulcanisation of elastomers, using sulphur or peroxides; by reaction of unsaturated groups with atmospheric oxygen; by high energy radiation; by photolysis using UV or visible light ; or by ionic bonding.
Oten the nature of the side groups and their distribution will determine which reaction will dominate. PEs crosslink LDPE with high amorphous content is most commonly crosslinked with irradiation, because radiation penetration is easier but, for example, PVC degrade with loss of halogen, PP with methyl side group is susceptible to degradation, similarly PMMA.
Crosslinking improves mechanical, chemical and thermal properties. MFI of crosslinked PE should be lower since the crosslinking holds the material together even at the crystalline melting point of the uncrosslinked phases.
TSs e. X-link density allows control of mechanical Figure 4. While ABS is roughly twice as expensive as PS, it is more superior for its hardness, gloss, toughness, solvent resistance and electrical insulation properties.
Note that copolymers should be distinguished from polymer blends. In Block copolymer chains, the long segment oligomers of each type of monomer are joined together, and similarly in grat copolymers long segments of a monomer string of Monomer B in Figure 4. Normally the random and alternating copolymers produce properties that are averages of the properties of homopolymers, whereas block and grat copolymers can exhibit a combination of properties that are unique to the individual homopolymers as in thermoplastic elastomers, covered in the next section.
Ordinary crosslinked vulcanised elastomers do not melt or dissolve and cannot, therefore, be processed using processing equipment suitable for thermoplastics, and also their waste cannot be reprocessed like thermoplastics.
TPEs are attractive alternatives because they can be processed as a thermoplastic. Orientation is successfully exploited by industry — production of synthetic ibres depends on orientation. Measuring orientation in polymers provides valuable information about micro-structure and therefore properties. It can be measured by birefringence, sonic modulus, X-ray difraction, infrared dichroism, laser-Raman spectroscopy, etc.
Birefringence is a measure of the total molecular orientation of a system i. Anisotropy increases with the increased orientation in a material, and D increases too. To measure D, one method would be the direct measurement of the refractive indices, which is a tedious procedure. A more rapid method is the use of a compensator e.
Birefringence is a suitable technique for transparent samples and requires a polarising microscope itted with a compensator for measurement. As the specimen is oriented, these rings break up into arcs and spots Figure 4. The book takes a multidisciplinary approach to the study of the viscoelasticity of polymers, and is self-contained, including the essential mathematics, continuum mechanics, polymer science and statistical mechanics needed to understand the theories of polymer viscoelasticity.
It covers recent achievements in polymer rheology, such as theoretical and experimental aspects of large amplitude oscillatory shear LAOS , and numerical methods for linear viscoelasticity, as well as new insights into the interpretation of experimental data. Aimed at readers familiar with the mathematics and physics of engineering at an undergraduate level, the multidisciplinary approach employed enables researchers with various scientific backgrounds to expand their knowledge of polymer rheology in a systematic way.
Introductory material on fundamental mechanics is included to provide a continuous baseline for readers from all disciplines. Introductory material on the chemical and molecular basis of polymers is also included, which is essential to the understanding of the thermomechanical response. This self-contained text covers the viscoelastic characterization of polymers including constitutive modeling, experimental methods, thermal response, and stress and failure analysis. Example problems are provided within the text as well as at the end of each chapter.
Polymers the basic materials of the rubber and plastic industries and important to the textile, petroleum, automobile, paper, and pharmaceutical industries as well exhibit viscoelasticity to a pronounced degree.
Their viscoelastic properties determine the mechanical performance of the final products of these industries, and also the success of processing methods at intermediate stages of production. Viscoelastic Properties of Polymers examines, in detail, the effects of the many variables on which the basic viscoelastic properties depend. These include temperature, pressure, and time; polymer chemical composition, molecular weight and weight distribution, branching and crystallinity; dilution with solvents or plasticizers; and mixture with other materials to form composite systems.
With guidance by molecular theory, the dependence of viscoelastic properties on these variables can be simplified by introducing certain ancillary concepts such as the fractional free volume, the monomeric friction coefficient, and the spacing between entanglement loci, to provide a qualitative understanding and in many cases a quantitative prediction of how to achieve desired results.
The phenomenological theory of viscoelasticity which permits interrelation of the results of different types of experiments is presented first, with many useful approximation procedures for calculations given. Polymer compounding plays an important role in the successful use of polymers. It helps to extend the properties of polymers such as durability, stiffness or thermal resistance so that these properties can be incorporated into an improved end-product. Several thousand of compounds currently used incorporate additives such as antioxidants, fillers or lubricants.
Innovation is an essential element in polymer compounding with respect to the manufacture of increasingly sophisticated products such as polymer blends and composites. This book gives an idea of the productive area of polymer compounding.
Volume 2 focusses on manufacturing technology and processing and provides an overview of the basic and fundamental aspects of polymer compounding. This volume should interest students, scientists and engineers, and constitutes a reference text for the experimental polymer technologist.
Written in a simple and accurate style this book can be understood even by the reader who is not familiar with polymer compounding. The book is also very informative and helps give an overall view of compounding.
The figures are well organised with technical and economic considerations, as well as consideration of the problems associated with polymer compounding.
Therefore, the book is distinctly quantitative in nature and designed to inspire a large audience of industrial and academic polymer scientists interested in the technology of polymer compounding. Introduction to Polymers, Second Edition discusses the synthesis, characterization, structure, and mechanical properties of polymers in a single text, giving approximately equal emphasis to each of these major topics.
It has thus been possible to show the interrelationship of the different aspects of the subject in a coherent framework. The book has been written to be self-contained, with most equations fully derived and critically discussed. It is supported by a large number of diagrams and micrographs and is fully referenced for more advanced reading. Problems have been supplied at the end of each chapter so that students can test their understanding and practice the manipulation of data.
Taking the most common polymers this undergraduate textbook describes the basic principles of polymer science. Using polymers with which students will be familiar, for example polystyrene and PVC, this book relates the structure of polymers to their properties, and explains how and their properties can be tailored for a particular use.
The Chemistry of Polymers, 5th Edition, is fully updated with the latest developments in polymer science providing a highly readable textbook for those requiring a broad overview of the subject. Like previous editions, the book continues to explore the subject from an applications point of view, providing a comprehensive introduction to all aspects of polymer science including synthesis, structure, properties, degradation and dendrimers.
Foundations of Analytical Chemistry. This book offers a completely new approach to learning and teaching the fundamentals of analytical chemistry. It summarizes basic concepts of the field on the basis of slides.
Each of the nine chapters offers the following features: - Introduction: Summary. General scheme. Teaching objectives. Lithium-Ion Batteries. This is the first machine-generated scientific book in chemistry published by Springer Nature.
0コメント