Polymer, amorphous plasticized

In order for a plasticizer to enter a polymer stmcture the polymer should be highly amorphous. Crystalline nylon retains only a small quantity of plasticizer if it retains its crystallinity. Once it has penetrated the polymer the plasticizer fills free volume and provides polymer chain lubrication, increa sing rotation and movement. 

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. 

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. 

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... 

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. 

A distinction should be made between solvent plasticizers and nonsolvent plasticizers. With an amorphous polymer, any plasticizer is a solvent plasticizer— i.e., under suitable conditions the polymer would eventually dissolve in the plasticizer. With a crystalline or semicrystalline polymer, there are some compounds which enter both the crystalline (ordered) and the amorphous (disordered) regions. These are true plasticizers-sometimes they are called primary plasticizers. If, on the other hand, only the amorphous regions are penetrated, the compound may be considered as a nonsolvent plasticizer, also known as a secondary plasticizer, or softener. Such softeners are used sometimes as diluents for the primary plasticizer. 

Fig. 5.21 Polymer feed temperatures are at or near Tmom. For common amorphous plastics, TV00m < Tg, and for semicrystalline T < Tmom < As disussed in the text, PED, through large solid-state irreversible deformations, makes the solid an active participant in the melting process, rapidly creating a molten state. The modulus of amorphous polymer is higher and less temperature dependent in the region T > rroom. Consequently, the magnitude of amorphous PED is larger and less temperature dependent when compared to semicrystalline PED.

The crystalline plastics (basic polymers) tend to have their molecules arranged in a relatively regular repeating structure such as polyethylene (PE) and polypropylene (PP). This 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 has up to 80% crystalline structure and the rest is amorphous. 

Fig. 1-3. Deformation of various polymer types when stress is applied and unloaded, (a) Cross-linked ideal elastomer, (b) Fiber, (c) Amorphous plastic.

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

Haward, R. N. The Post-Yield Behaviour of Amorphous Plastics, in The Physics of Glassy Polymers, (ed. Haward, R. N.), London, Applied Science Publ., 1973, p. 340... 

There can be no doubt as to the importance of plane strain conditions for the fracture of plastics especially where sharp notches and thick sections are concerned. Such conditions nearly always lead to brittle or semi-brittle fracture. Vincent has shown that the notch sensitivity in a braod range of amorphous and crystalline polymers is increased as the testing temperature is lowered and the loading rate is increased. Before fracture occurs, amorphous plastics often craze under these conditions. The complex questions of craze initiation, propagation and transformation into a crack have been treated extensively for amorphous polymers in the first three chapters of this book (see also The problem becomes more complicated when... 

The load-displacement curves for C(T) tests of the neat EpoxyH were almost linear until the final unstable fracture. The fracture toughness value in 77K-LNj was 210 J/m and that in RT-air was 120 J/m. Thus the toughness increased by 1.8 times by changing the test environment from RT-air to 77K-LN. Brown and co-workers have found that amorphous polymers crazed in 77K-LNj, but not in a helium or vacuum at about 78K [20-22]. They have also reported that the stress-strain behavior of all polymers, amorphous and crystalline, is affected by at low temperatures [22]. Kneifel has reported that the fracture toughness of epoxy in 77K-LNj is higher than that in RT-air and 5K, and that the reason for this is the reduced notch effect by plastic deformation [23]. Then, the increase of the fracture toughness of the neat EpoxyH in this study is probably caused by the similar effect. 

Until now we have considered the basic origin of birefringence and some of the general techniques used for determining this optical parameter. It is necessary, however, to discuss certain limitations when interpreting this parameter. Until now no mention has been made of two or multiphase systems such as semicrystalline polymers, amorphous block copolymers or even plasticized or filled polymers. In such systems the measured birefringence can be expressed as... 

Altogether, polymer solutions show essential differences with normal solutions as well as with colloidal dispersions and need a special treatment. Highly concentrated polymer systems behave differently, again, and tend to form amorphous solids. Typical concentrated synthetic polymers are plastics and rubbers. 

Copolymers also exist such as PETG, offered by Eastman Plastics. The G indicates the use of a second glycol in the polymer, i.e. 1, 4-cyclohexane-dimethanol (CHDM). The resulting copolymer is an amorphous plastic which has a high degree of clarity, low haze, good resistance to acids, alkalis and many oils and excellent fragrance retention. 

Polymers can exhibit a number of different conformational changes with each change accompanied by differences in polymer properties. Two major transitions occur at Tg, which is associated with local, segmental chain mobility in the amorphous regions of a polymer, and the melting point (Tjj), which is associated with whole chain mobility. The Tn is called a first-order transition temperature, and Tg is often referred to as a second-order transition temperature. The values for Tjj are usually 33 to 100% greater than for Tg, and Tg values are typically low for elastomers and flexible polymers and nigher for hard amorphous plastics. 

Amorphous plastics, such as acrylics, are transparent to infrared rays. Some semiciystalline polymers, such as PFA, have surprisingly high levels of infrared transmission. PFA can be welded by infrared under certain circumstances. For example, a transparent tube of PFA was welded to a black sheet of PFA that absorbed the infrared light generated by aNd YAG laser (1,064 nm).P8] a tube composed of natural PFA (6.4 mm outer diameter and 3.2 mm inner diameter) was pressed into a 6.4 mm diameter hole in an aluminum sheet. The aluminum sheet was used to shield the black PFA sheet from stray radiation. The IR light was defocused to a diameter of 6.4 mm and aimed at the end of the tube in the aluminum. A laser power of 30 W with a tube length of 50 mm produced a strong weld in a few seconds. 

As has already been stated, the verified possibility of extending the reduced variables principles to ABS resins makes it possible to treat these typical heterophase systems as blends of amorphous homophase polymers and plasticizers. One possible explanation is that over the experimental y range it is not possible to separate the contributions of the two different phases, and the materials will behave as homophase polymer. In fact, long-time molten polymer rheology experiments measure viscoelastic processes over the entire molecule, and, as a consequence, molecular compatibility is evaluated (13). On the other hand, high frequency and/or low temperature tests involve the main chain as well as the side chains of the polymer system the segmental miscibility of the polymer-polymer system is then evaluated. It is important in experimental measurements of polymer compatibility to evaluate the actual size of the volume subject to the test. 

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