Plastics amorphous

Crystalline behavior identifies its morphology that is the study of the physical form or structure of a material. They are usually translucent or opaque and generally have higher softening points than the amorphous plastics. They can be made transparent with chemical modification. Since commercially perfect crystalline polymers are not produced, they are identified technically as semicrystalline TPs. The crystalline TPs normally have up to 80% crystalline structure and the rest is amorphous. 

The amorphous plastic is the term used that means formless describing a TP having no crystalline plastic structure. They form no pattern whereby their structure tends to form like spaghetti with their molecules going in all different directions. They have a randomly ordered molecular 

---

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

Figure 9.3. Stress-strain curves for (a) rigid amorphous plastics material showing brittle fracture and (b) rubbery polymer. The area under the curve gives a measure of the energy required to break the...

Distortion in mouldings can be worse in crystalline polymers than in amorphous plastics. This is because additional stresses may be set up as a result of varying crystallinity from point to point in the moulding so that the shrinkage on cooling from the melt also varies from point to point. This uneven shrinkage sets up stresses which may lead to distortion. 

In general, plastics have a higher density when they crystallise due to the closer packing of the molecules. Typical characteristics of crystalline and amorphous plastics are shown below. 

As regards the general behaviour of polymers, it is widely recognised that crystalline plastics offer better environmental resistance than amorphous plastics. This is as a direct result of the different structural morphology of these two classes of material (see Appendix A). Therefore engineering plastics which are also crystalline e.g. Nylon 66 are at an immediate advantage because they can offer an attractive combination of load-bearing capability and an inherent chemical resistance. In this respect the arrival of crystalline plastics such as PEEK and polyphenylene sulfide (PPS) has set new standards in environmental resistance, albeit at a price. At room temperature there is no known solvent for PPS, and PEEK is only attacked by 98% sulphuric acid. 

Thermal Properties. Before considering conventional thermal properties such as conductivity it is appropriate to consi r briefly the effect of temperature on the mechanical properties of plastics. It was stated earlier that the properties of plastics are markedly temperature dependent. This is as a result of their molecular structure. Consider first an amorphous plastic in which the molecular chains have a random configuration. Inside the material, even though it is not possible to view them, we loiow that the molecules are in a state of continual motion. As the material is heated up the molecules receive more energy and there is an increase in their relative movement. This makes the material more flexible. Conversely if the material is cooled down then molecular mobility decreases and the material becomes stiffer. 

The relaxation modulus (or any other viscoelastic function) thus obtained is a mean s of characterizing a material. In fact relaxation spectra have been found very useful in understanding molecular motions of plastics. Much of the relation between the molecular structure and the overall behavior of amorphous plastics is now known. 

Mechanical properties of crystalline plastics are much more complex than those of amorphous plastics (Chapter 6, STRUCTURE AND MORPHOLOGY). For... 

Crazing. This develops in such amorphous plastics as acrylics, PVCs, PS, and PCs as creep deformation enters the rupture phase. Crazes start sooner under high stress levels. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plastics, at the time-dependent knee in the stress-strain curve. 

The load or stress has another effect on the creep behavior of most plastics. The volume of isotropic or amorphous plastic increases as it is stretched unless it has a Poisson ratio of 0.50. At least part of this increase in volume manifests itself as an increase in free volume and a simultaneous decrease in viscosity. This decrease in turn shifts the retardation times to being shorter. 

The most desirable annealing temperatures for amorphous plastics, certain blends, and block copolymers is just above their glass transition temperature (Tg) where the relaxation of stress and orientation is the most rapid. However, the required temperatures may cause excessive distortion and warping. 

The amorphous TPs, which have their molecules going in all different directions, are normally transparent. Compared to crystalline types, they undergo only small volumetric changes when melting or solidifying during processing. Tables 6-5 to 6-9 compare the basic performance behaviors of crystalline and amorphous plastics. Exceptions exist, particularly with respect to certain plastic compounds that include additives and reinforcements. 

Amorphous plastics soften gradually as they are heated, but they do not flow as easily during molding as do crystalline materials. 

Typical crystalline plastics are polyethylene, polypropylene, nylon, acetals, and thermoplastic polyesters. Typical amorphous plastics are polystyrene, acrylics, PVC, SAN, and ABS. 

The specific heat of amorphous plastics increases with temperature in an approximately linear fashion below and above Tg, but a steplike change occurs near the Tg. No such stepping occurs with crystalline types. 

Polysulfone It is a high performance amorphous plastic that is tough, highly heat resistant, strong and stiff. Products are transparent and slightly clouded amber in color. Material exhibits notch sensitivity and is attacked by ketones, esters, and aromatic hydrocarbons. Other similar types in this group include polyethersulfone, polyphenyl-sulfone, and polyarylsulfone. Use includes medical equipment, solar-heating applications and other performance applications where flame retardance, autoclavability and transparency are needed. 

Materials such as polyethylene, polypropylene and acetal, which have waxy surfaces, or other crystalline plastics that are very solvent resistant, can be difficult to paint and require special primers or pre-treatments (flame, etc.) for satisfactory adhesion. Many amorphous plastics easily accept a wide variety of paint coatings. 

Glassy state In amorphous plastics, below the Tg, cooperative molecular chain motions are frozen , so that only limited local motions are possible. Material behaves mainly elastically since stress causes only limited bond angle deformations and stretching. Thus, it is hard, rigid, and often brittle. 

Our work with SDIBS demonstrated that TPEs based on amorphous plastic-rubber-plastic blocks do not necessarily require narrow MWD for good phase separation and mechanical properties some of these SDIBS blocks exhibited MWD >2, and irregular phase morphology. This disagrees with earlier conventional wisdom and opens new avenues in TPE research. 

Except for a lew thermoset materials, most plastics soften at some temperatures, At the softening or heat distortion temperature, plastics become easily deformahle and tend to lose their shape and deform quickly under a Load. Above the heat distortion temperature, rigid amorphous plastics become useless as structural materials. Thus the heat distortion test, which defines The approximate upper temperature at which the material can be Safely used, is an important test (4,5.7.24). As expected, lor amorphous materials the heat distortion temperature is closely related to the glass transition temperature, hut tor highly crystalline polymers the heat distortion temperature is generally considerably higher than the glass transition temperature. Fillers also often raise the heat distortion test well above... 

Poly(methyl methacrylate) (PMMA), an amorphous plastic material, is extremely stable to aging and to weathering it is hard and glass clear. 

Crystalline polymers with high melting temperatures and a very narrow melting range are generally difScult to weld ultrasonically, whereas the rigid amorphous plastics (e. g. polycarbonate or polystyrene) are best. 

Conversely, in most observed cases where solidification occurs as a result of continued depletion of solvent (as described in Case B), the highly concentrated polymer layer solidifies as a relatively dense, amorphous, plasticized film. Water diffusion into this highly plasticized layer becomes prevalent (Case A) at a stage where the contraction has gone "too far" to yield even a microporous membrane structure. 

The flexibility of amorphous polymers is reduced drastically when they are cooled below a characteristic transition temperature called the glass transition temperature (Tg). At temperatures below Tg there is no ready segmental motion and any dimensional changes in the polymer chain are the result of temporary distortions of the primary covalent bonds. Amorphous plastics perform best below Tg but elastomers must be used above the brittle point, or they will act as a glass and be brittle and break when bent. 

The most widely used polyvinyl acetal is polyvinyl butyrai (PVB). This transparent amorphous plastic is used as a plasticized polymer in the inner lining of safety windshield glass (Saflex). Because of the presence of hydroxyl groups, the commercial product, which is produced from 75% hydrolyzed PVAc, has a Tg of about 49 C and has excellent adhesion to glass. 

Since the sulfone group is chain-stiffening, these amorphous plastics are rigid. However, some flexibility is provided by the ether and methylene groups. The Tg of these materials is high (>190 °C), but polyalkyl sulfones, which... 

---