Poly crazing

Plastic Sheet. Poly(methyl methacrylate) plastic sheet is manufactured in a wide variety of types, including cleat and colored transparent, cleat and colored translucent, and colored semiopaque. Various surface textures ate also produced. Additionally, grades with improved weatherabiUty (added uv absorbers), mat resistance, crazing resistance, impact resistance, and flame resistance ate available. Selected physical properties of poly(methyl methacrylate) sheet ate Hsted in Table 12 (102). 

As may be expected of an amorphous polymer in the middle range of the solubility parameter table, poly(methyl methacrylate) is soluble in a number of solvents with similar solubility parameters. Some examples were given in the previous section. The polymer is attacked by mineral acids but is resistant to alkalis, water and most aqueous inorganic salt solutions. A number of organic materials although not solvents may cause crazing and cracking, e.g. aliphatic alcohols. 

As an indication of the changes in deformation modes that can be produced in ionomers by increase of ion content, consider poly(styrene-co-sodium methacrylate). In ionomers of low ion content, the only observed deformation mode in strained thin films cast from tetra hydrofuran (THF), a nonpolar solvent, is localized crazing. But for ion contents near to or above the critical value of about 6 mol%, both crazing and shear deformation bands have been observed. This is demonstrated in the transmission electron microscope (TEM) scan of Fig. 3 for an ionomer of 8.2 mol% ion content. Somewhat similar deformation patterns have also been observed in a Na-SPS ionomer having an ion content of 7.5 mol%. Clearly, in both of these ionomers, the presence of a... 

Poly(ethylene terephthalate), (PET), is a thermoplastic polymer widely used in the production of fibers and films on exposure to near ultraviolet light, PET fibers tend to lose their elasticity and break easily PET films become discolored, brittle and develop crazed surfaces. Such deterioration in properties has been attributed to photochemical reactions initiated by the... 

The mechanism how a rubber distributed in a network influences the rupture mechanism is not quite well understood yet. It is known that poly(vinyl chloride) forms shear bands when stress is applied and that parts of the rubber which are located in these shear bands may form crazes.13 It might well be that a network structure is efficient for the delocalization of stress energy only in combination with the formation of shear bands. Experimental work is needed to elucidate this further. 

Crazing inhibition by crosslinking explains the better fatigue resistance of thermosets compared with thermoplastics (this is not valid for semicrystalline thermoplastics such as polypropylene or poly (ether ether ketone). 

FIG. 26.13 Critical strain of poly(2,6-dimethyl-l,4-phenylene oxide) vs. solubility parameter 5 of crazing and cracking liquids. Minimum in cr occurs at 8 equal to that of the polymer. Band at top indicates critical strain of polymer in air (Bernier and Kambour, 1968 reproduced by permission of the American Chemical Society). 

The acrylic plastics use the term acryl such as polymethyl methacrylate (PMMA), polyacrylic acid, polymethacrytic acid, poly-R acrylate, poly-R methacrylate, polymethylacrylate, polyethylmethacrylate, and cyanoacrylate plastics. PMMA is the major and most important homopolymer in the series of acrylics with a sufficient high glass transition temperature to form useful products. Repeat units of the other types are used. Ethylacrylate repeat units form the major component in acrylate rubbers. PMMAs have high optical clarity, excellent weatherability, very broad color range, and hardest surface of any untreated thermoplastic. Chemical, thermal and impact properties are good to fair. Acrylics will fail in a brittle manner, independent of the temperature. They will suffer crazing when loaded at stress about halfway to the failure level. This effect is enhanced by the presence of solvents. 

Marshall GP, Culver LE, Williams JG (1970) Craze growth in poly(methyl methacrylate) a fracture mechanics approach. Proc R Soc Lond A Math Phys Eng Sci 319(1537) 165-187... 

Bernier GA, Kambour RP (1968) The role of organic agents in the stress crazing and cracking of poly(2,6-dimethyl-l,4-phenylene oxide). Macromolecules l(5) 393-400... 

Kawagoe M, Kitagawa M (1987) Craze initiation in poly(methyl methacrylate) exposed to n-alkanes. J Mater Sci 22(8) 3000-3004... 

In the present work the variation in stretch ratio or strain along a craze in polystyrene has been determined experimentally using methods related to those recently described by Brown and Ward (5) for poly-(methyl methacrylate), and the elastic displacements across the craze zone were compared with the values predicted for the Dugdale model (7). 

Surface oxyfluorination was also performed on poly(methyl methacrylate) by Jolet [4] as a method for manufacturing the outer panel of craze-resistant windows. 

Craze formation is a dominant mechanism in the toughening of glassy polymers by elastomers in polyblends. Examples are high-impact polystyrene (HIPS), impact poly(vinyl chloride), and ABS (acrylonitrile-butadiene-styrene) polymers. Polystyrene and styrene-acrylonitrile (SAN) copolymers fracture at strains of 10 , whereas rubber-modified grades of these polymers (e.g., HIPS and ABS) form many crazes before breaking at strains around 0.5. Rubbery particles in... 

Fig. 4. TEM micrograph of a craze tip in poly(styrene-acrylonitrile) (stained with 0s0.t). The film is tilted so the craze front (normal to the film surface) can be seen in projection. Note the stained wedge of plastically deformed polymer ahead of the crtize tip. Courtesy of Dr. A. M. Donald...

Fig. 20a—d. Surface stress profiles for polytertbutylstyrene [PTBS], poly(styrene-26% acrylonitrile) [PSANl] and poly(styrene-65% methylmethacrylate) [PSMMAj. The stress at the craze tip S, is plotted vs. Ve in d. The value of the shear yield stress Y of polycarbonate is indicated... 

The mechanical properties of a craze were first investigated by Kambour who measured the stress-strain curves of crazes in polycarbonate (Lexan, M = 35000) which had first been grown across the whole cross-section of the specimen in a liquid environment and subsequently dried. Figure 25 gives examples of the stress-strain curves of the craze determined after the 1st and 5th tensile loading cycle and in comparison the tensile behavior of the normal polymer. The craze becomes more and more elastic in character with increasing load cycles and its behavior has been characterized as similar to that of an opencell polymer foam. When completely elastic behavior is observed the apparent craze modulus is 25 % that of the normal poly-... 

A transmission electron micrograph of a craze in a thin film of poly(styrene-acrylo-nitrile), shown in Fig. 1 a, will serve to introduce the principal microstructural features of crazes. The direction of the tensile stress is marked and it can be seen that the craze grows with the primary direction of its fibrils parallel to this tensUe stress and with the interfaces between the craze and the nearly undeformed polymer matrix normal to the stress. Since the overwhelming portion of the experiments to be reviewed here rely on the use of thin film deformation and transmission electron microscopy techniques, a brief review of the general methods of these experiments is in order. 

Fig. 8 a, b. Experimental extension ratio X of crazes as a function of the theoretical maximum extension ratio of a single strand in the network (entanglements and crosslinks) a crosslinked polystyrene (open circles) and poly(pmethylstyrene) (closed circles) and b various entangled homopolymers and copolymers (open circles), and polymer blends of PS and PPO (closed circles)... 

Similar craze breakdown morphologies have been observed for dust-free films of polymethylmethacrylate (PMMA), poly(a-methylstyrene) (PaMS) and poly(styrene-acrylonitrile) (PSAN) Large pear-shaped voids nucleate at the craze-bulk polymer interface, never in the craze mid-rib, and thus this mode of craze breakdown seems to be a dominant one for all glassy polymers. 

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