1、1.2 The Plastics Industry,A diagram of the plastics industry is given in Figure 1.3, with each of the major functions represented in a box. The interactions between the various functions are illustrated by the arrows. The heavy arrows are the most common paths. Generally these interactions are sales

2、, and they are made in the directions shown by the arrows, but sometimes the interactions are simply intercompany transfers when a company has integrated more than one function internally. The integration of several steps is quite common in the plastics industry.,Figure 1.3 Diagram of the plastics i

3、ndustry,The resin manufacturers convert chemicals (derived from crude oil, natural gas, coal, and other sources) into the basic polymer materials. Hence, these processes are called polymerization processes.,These resins, which require further processing to be useful, are generally made in large, hig

4、hly integrated manufacturing facilities that resemble oil refineries in their size and scope. The enormous investments required to build such facilities have resulted generally in large petrochemical companies becoming resin manufacturers. Some of the most well-known resin manufacturers are DuPont,

5、ExxonMobile, Dow, Bayer, BASF, and Huntsman, among many others.,There are approximately 40 major resin types, with each resin type differing from all others according to the fundamental chemical nature of the polymer. Most of the resin types are available from more than one resin manufacturer. Some

6、of the most common of these resin types are polyethylene, polypropylene, polystyrene, polyvinyl chloride, nylon, polycarbonate, polyurethane, and polyester.,Most of the processes used to make resins are flow processes that require careful control over temperature, pressure, flow rate, catalyst, and

7、other associated parameters in order to obtain the desired properties and to minimize the production of unwanted side products. This text skips over most of these details and considers only the most basic concepts in polymerization (in the chapter on polymeric materials). This approach allows the re

8、ader to appreciate the general way polymers are made and understand the changes in polymer properties that arise from the polymerization processes. The emphasis of this book is on the molding of the resins after they have been polymerized.00,The resins usually exit from the polymerization reactor in

9、 one of three physical forms: liquids, granules, or flakes. The liquids are roughly the consistency of honey. The granules resemble laundry, soap powders in texture, size, and consistency. The flakes resemble uncooked oatmeal or instant potato flakes in texture and size. If sold directly from the re

10、actor, the liquid resins are generally packaged in 5-gatlon, 55-gallon, or tank car containers. The granules and flakes are generally packaged in 50-pound bags, palletized cartons, or gaylords (1000 pounds), in hopper trucks, or in rail cars.,In many cases, the resin manufacturers send the granules

11、and flakes through one more process in order to make a more consistent product for later processing and/or to remove contaminants, especially solvents, which might be present. In this additional processing step, the granules and flakes are converted into pellets that are shaped like small rods about

12、 0.1inch (2 mm) in diameter and 0. 2 inch (4mm) long (like spaghetti that has been chopped into very short pieces). This additional processing step also gives the resin manufacturers an opportunity to adjust the average pellet properties by blending resins of the same polymer material from different

13、 batches.,Blending is often much easier than trying to fine-tune the resin-making operation itself for each of the minor differences in products that might be offered. Every resin type is available in many varieties, each of which is made by a slightly different set of polymerization process paramet

14、ers or by blending of the materials from different polymerization batches. These varieties differ slightly from each other in physical properties but are similar in overall properties within each resin type. Minor additives such as antioxidants could also be blended into the polymer in the pelletizi

15、ng step.,Most resins are sold from the resin manufacturers directly to the molders. If a molder does not buy in large enough quantities to be supplied directly from the resin manufacturers, the molder might buy from a resin distributor, who typically buys in large quantities and then ships in smalle

16、r lots. The molder may also need services such as color matching, addition of processing aids, or grinding. In these cases the molder would buy from a compounder.,The molders convert the resins (liquids, granules, flakes or pellets) into the desired shapes by using one or more plastic molding proces

17、ses. Typical molding processes are extrusion, injection molding, compression molding, and casting. These and other plastic molding processes are discussed in detail. Molding companies that have plastic parts as their principal products usually have many plastic processing machines and make parts in

18、large quantities. Typical examples of these companies are plastic-pipe extruders, injection molders of automotive parts, or plastic-toy manufacturers.,Other molders are companies whose principal product is something other than a plastic but in which some plastic part or parts are used. These compani

19、es would typically have only a few plastics processing machines. Examples of this type of company might be a manufacturer of medical devices that uses special plastic fittings, airplane manufacturers who use plastic parts within the airplane, or a milk processor who uses plastic milk jugs. These ess

20、entially non-plastics companies may elect to have an outside company do the molding for them. Such outside companies are called job shops or custom molders, which typically would have several injection molding machines, often of different sizes, and would make parts for companies who use these parts

21、 as components in their products. These job shops often provide other services for the companies such as part design assistance, mold making, and, perhaps, some assembly.,Still other molders make standard-shaped parts that are usually sold to companies that perform additional shaping operations. The

22、se shapers of already-molded parts are called fabricators. A typical fabricator might, for instance, buy extruded sheet and vacuum form this sheet into a finished product, such as a case for some instrument. Another typical fabricator buys PVC plastic film and laminates the film to cloth. If the fab

23、ricators choose to buy in small quantities or desire special services, they may also buy from stock parts distributors that warehouse standard plastic shapes, such as sheets, rods, and blocks.,Molders, fabricators, and stock parts distributors may also transfer their parts to a function that is conc

24、erned with finishing, assembling, or integrating the plastic part into a larger assembly. These operations are distinguished from fabricating because they are focused on mechanical operations such as cutting, bonding, and painting rather than forming from standard shapes by secondary molding. Finish

25、ing, assembling, and integrating are often done in-house by the molder or fabricator but can be done by companies dedicated to this type of specialized operation. Companies that use plastic parts in their products and buy the parts from molders are often involved in this functional step. For instanc

26、e, furniture companies will cut and shape foam for use in their products. Likewise, aircraft companies could buy molded parts and then rivet or otherwise join them to major components of the airplane.,Molders, fabricators, stock parts distributors, and finishers may all sell directly to end users, b

27、ut they may sell to some service provider that may perform some finishing step. For instance, a physician might use epoxy resin and fiberglass to mold a cast for a patients broken arm. A landscaper might bond together components of a sprinkler system. A homeowner might install a plastic roof on a ca

28、rport of make repairs to a damaged plastic automobile part.,The entire plastics industry is certainly immense and is getting bigger each year. The number of new jobs being created in the industry has been growing at a rate that is roughly twice the rate of jobs being created in all other parts of th

29、e manufacturing sector. The steady growth of plastics in modern society leaves little doubt that the number of jobs in the plastics industry will continue to outpace the general economy.,UNIT 3 MICROSTRUCTURES IN POLYMERS,3.1 POLYMERIC SOLID STATE: AMORPHOUS AND CRYSTALLINE,The solid state of polyme

30、ric materials is, in some ways, more complex that the solid state of most ionic, metallic, and small covalent molecules. The complexity in polymers arises because solid polymeric materials can exist, in two very distinct types of configurations. In one type, the polymer molecules are randomly coiled

31、 about each other with entanglement, much as cooked spaghetti would intertwine. (See Photo 3.1.) This structure type is called amorphous. In the second type of configuration, the polymer molecules can pack together into regular, repeating structural patterns. These regularly packed regions are calle

32、d crystals or crystalline regions. Although no polymer is completely crystalline (having some amorphous regions), those with large concentrations of crystalline areas are said to be crystalline or, more accurately, semicrystalline.,Photo 3.1 Spaghetti showing entanglement.,A semicrystalline polymer

33、is pictured in Figure. 3.1 in a two-dimensional representation. This representation, called the fringed micelle model, explains many, but not all, of the properties of crystalline polymers.,Figure 3.1 Crystalline and amorphous regions in a polymer structure,One property not well explained by this mo

34、del is the appearance of spherulite structures in the X-ray diffraction spectra.,A model in which the molecules fold into platelike structures (see Figure 3.2), which grow from a central nucleation point, explain the growth pattern that forms the spherulites. This theory is called the laminar/spheru

35、lite model or more commonly, the folded chain model. Although the two models have not been fully reconciled, it may be that the folded chain model is simply the shape that emerges when the crystalline regions of the fringed micelle model are allowed to grow slowly into larger crystalline regions.,Fi

36、gure 3.2 Folded chain model or laminar/spherulite model of polymer crystallization,The most important (but not the only) feature of a polymer that determines whether it will be amorphous or crystalline is the shape of the polymer repeat unit. If the repeat unit is complex, especially with large pend

37、ant groups, the polymer cannot pack tightly together and will be amorphous. Some of the most common amorphous polymers are polystyrene, acrylic, polycarbonate, and most copolymers. Approximately half of the most common commercial plastics are amorphous.,If the polymer repeat unit is simple and the p

38、endant groups are small, the polymer may be able to pack tightly and crystalline regions could be formed. The regions of crystallinity are composed of folded chains held together by crystal bonds (secondary bonds). These bonding forces between the chains are localized to the tightly packed, crystall

39、ine areas and occur because the crystal structures, when they occur, represent structures with lower energy than a random, noncrystalline arrangement of the molecule. The lower energy is the result of the molecules forming bonds, which releases energy. The crystalline sections are scattered througho

40、ut the polymer with some nonstructured (amorphous) regions, between them.,The amount of crystallinity in the polymer depends upon several factors. As already mentioned, the most important is the size and regularity of the pendant groups (the groups attached to the main polymer backbone). If these pe

41、ndant groups are relatively small and are regularly spaced along the polymer chain, they will not interfere with each other and the polymer chains can pack closer together. Forces of attraction and other similar interactions between polymers, such as hydrogen bonding, also increase crystallinity. So

42、me important highly crystalline polymers are polyethylene (HDPE), acetal, and nylon.,In addition to these structural factors, the crystallinity of polymers also depends upon molding or processing conditions. Crystallization in polymers takes time to occur. Therefore, factors such as cooling rate can

43、 have strong influences on the amount of the material that crystallizes, since below certain temperatures there is not sufficient molecular motion to allow the molecules to rearrange into a close packing configuration. In some polymers, mechanically stretching the polymer will draw the chains into c

44、lose proximity and, therefore, induce crystallization. This phenomenon is the basis for the stretch blow molding process that is used to cause crystallization in soft-drink bottles, thus increasing their strength and resistance to gas diffusion over what they would be if amorphous.,Some polymers wil

45、l only form crystalline structures when polymerized under very special conditions. The most important of these, and the classical example of the type, is polypropylene. The relatively large pendant group on polypropylene prevented it from crystallizing and so the polymer lacked the strength and stif

46、fness that would arise from the closely packed crystalline regions. Hence, early polypropylene had only limited applications. However, in the 1940s and 1950s, it was discovered that if polypropylene were polymerized using a special catalyst, crystallization would occur. Recent catalyst developments

47、(especially with metallocenes) have indicated that many polymers once believed to only exist as amorphous polymers could be polymerized in such a way that crystalline structure would form.,The amount of crystallinity, that is, the total number of atoms involved in a crystalline structure as opposed

48、to the number in amorphous regions, can vary widely. In some polymers, no crystallinity takes place. In others, if all of the conditions are favorable, crystallinity can approach 100% but is more likely to be in the 60 to 70% range. As the material becomes more crystalline, it also becomes denser. E

49、specially in polyethylene and other plastics with a wide range of possible crystallinities, the density, is the most common method of expressing crystallinity. For instance, polyethylene with a density of 0.97 grams per centimeter3 would be high-density (HDPE) whereas a density of 0.92 g/cc would be

50、 low-density polyethylene (LDPE).,The following methods are commonly used to measure specific gravity and density. Specific Gravity (ASTM D 792). This test determines the weight of a sample in air and then immersed in water. density-Gradient Technique (ASTM D 1505). This method uses a density-gradie

51、nt column. Bulk Density (ASTM D 1895). The bulk density is the apparent density of the material, that is, the density of the material without compaction or modification. Sieve-Analysis (Particle-Size)Test (ASTM D 1921).,X-ray diffraction is useful in determining the degree of crystallinity because X

52、-rays will develop a characteristic pattern when diffracted through a crystal structure. (This same technique is used to investigate crystal properties in metals and ceramics.) Infrared spectroscopy can also be used to investigate crystallinity, because the vibrations and rotations of the atoms that

53、 are detected by infrared spectroscopy are affected by the crystal structure and, therefore, appear at slightly higher energy levels than do freely rotating and vibrating atoms. Hence, a shift in spectrographic pattern is detected when crystalline regions are present. Differential scanning calorimet

54、ry is another method that has been used, but not as frequently as the others.,Because of the bonding forces within a crystal, crystallinity affects many physical properties in ways that are similar to other intermolecular attractions already discussed. Tensile strength and stiffness, for instance, a

55、re increased by crystallinity because of the high resistance to movement in the crystalline regions and the need to overcome the intermolecular (crystalline) forces. This resistance can be very high in some cases, resulting in a marked increase in these properties over amorphous polymer analogs. For

56、 instance, high-density polyethylene is strong and stiff enough to be self-supporting even when quite thin. It is used extensively for milk bottles. Low-density polyethylene is much more flexible and is used widely for trash bags.,The effect of crystallinity on impact toughness is somewhat more invo

57、lved. The crystalline sections of a polymer are not as effective in absorbing and dissipating impact energy as are the amorphous regions because the atoms in the crystalline regions are not as free to rotate, vibrate, and translate. This restriction on atomic movement causes highly crystalline mater

58、ials to be stiffer and more brittle. Therefore, even though the strength increases, the impact toughness often decreases for highly crystalline materials.,Several other Properties are also affected by crystallinity. For instance, solubility of the polymer is generally reduced in crystalline material

59、s because of the compactness of the crystalline structure compared to the amorphous region. This compactness retards the access of solvent molecules to the bulk of the structure. Crystallinity is a basic property of plastics that should be considered in the selection of a polymer for any application. Many polymers can be obtained in a range of crystallinities, thus allowing the designer a wide choice of material properties.,3.2 POLYMERIC LIQUID STATE,Polymer chains are generally so long that significant intermolecular entanglement exists even in the liquid state. This entanglement in