Skin-core effects

Highest thermal performance with PPS compounds requires that parts be molded under conditions leading to a high level of crystallinity. Glass-filled PPS compounds can be molded so that crystalline or amorphous parts are obtained. Mold temperature influences the crystallinity of PPS parts. Mold temperatures below approximately 93°C produce parts with low crystallinity and those above approximately 135°C produce highly crystalline parts. Mold temperatures between 93 and 135°C yield parts with an intermediate level of crystallinity. Part thickness may also influence the level of crystallinity. Thinner parts are more responsive to mold temperature. Thicker parts may have skin-core effects. When thick parts are molded in a cold mold the skin may not develop much crystallinity. The interior of the part, which remains hot for a longer period of time, may develop higher levels of crystallinity. 

The composites injection molded at the lower temperature (180°C) did not exhibit any skin/core effect, but rather contained fibers throughout. 

Figure 13. Dark-field micrograph of me cross section of a Type 4 PA 66 fiber. No silver sulfide deposits notice the skin-core effect and the dimensions of the...

Figure 10.11. Schematics of skin-core effect on phase morphology in injection molded part.

One of the most important features of extruded and moulded articles is the presence of a skin-core structure caused by high shear gradients in the flowing polymer close to the die or mould surface, which induce high orientation. In rods or other extrudates the skin-core effect can be removed by subsequent drawing to increase the overall orientation of the sample this cannot, of course, be done for mouldings. 

Fibers vary in cross-sectional shape, both naturally and by design. Wool fibers are essentially round and cotton fibers are elliptical. Synthetic fibers made by melt spinning can be of a desired shape. Artificial fibers that are spun from solvents in air or from an aqueous medium are usually irregular in shape because of the skin-core effect [2]. Rayon, for example, can have both regular and irregular cylindrical forms composed of hollow as well as solid fibers. 

To study the skin-core effects in more detail, ultrasonic measurements were made on a sample as it was machined progressively thinner from alternate sides, with the removed layer being 0.4 mm thick. Results from successive experiments were compared to obtain the pulse transit times for the layer of material which had been removed. Analyses of these transit times gave the profiles of 3 and for sample 1 shown in Figures 14.8 and 14.9, respectively. Besides the peak in the modulus in the skin region there is another peak about 1.2 mm from the surface of the plaque. This peak has also been observed in static modulus experiments on other injection molded plaques and has been attributed... 

Other issues concerning PLCs are also such that simulations provide answers to questions to which experiments cannot. For instance, what is the skin-core effect oti properties In processing of real materials always a skin which is... 

The transverse radius of curvature of the graphene layers, n, increases from the core of a fiber to its surface, and graphene sheets near the fiber surface usually lie parallel to it. This structure is the cause of the notable skin/core effect and the low reactivity of the fiber surface. The reactivity of aromatic carbons is lower in-plane than at the layer edge. The turbostratic character of carbon in PAN based HM carbon fibers and the occurrence of elongated pores explain their low density (1.8-1.9 g/cm ). 

Figure 3.51 Cross-sections of 1.67 d tex Courtauld s SAF, oxidized in air for periods up to 3 h showing skin/core effect. Source Kind permission of Acordis Ltd (formerly Courtaulds PLC).

Unfortunately this value, determined by elemental analysis, is often established by difference, although Watt and Johnson [88] measured the uptake of O2 with time of a 3 denier Courtelle at 220°C directly using a Coleman O2 analyzer (see Figure 3.5). Later, Watt and Johnson [89] determined the O2 uptake of 1.5 denier Courtelle at 230°C (Figure 5.36). Examination of cross-sections showed a skin core effect after 2 and 4 h of treatment. 

Dunham and Edie [98] established a mathematical model of the stabilization process for 12-60k PAN fiber and checked theory with experiments using 3k and 12k 1.22 d tex Courtauld SAF PAN fiber (6% MA, 1% ITA) by embedding a thermocouple in the fiber bundle. The governing equations for the model are based on the rates of chemical reactions, mass balances on reacting species, radial mass transfer and radial heat transfer within the bundle. They showed that the fiber bundle can be as much as 15°C above the stabilization oven temperature and the model predicted the measured temperatures quite well, except for, as would be expected, run-away reaction conditions. Samples stabilized below 230°C did not exhibit a skin core effect, but above 245°C, exhibited distinct skin core differences which were observed by reflected light microscopy. Hence diffusion appears to limit the stabilization rate above 245°C but not below 230°C. Bundles larger than 12k tended to bum when stabilized much above 230°C. The model would not hold for temperatures above 245°C. 

Examine the section under a microscope for skin core effects (Figure 3.51). 

Effect of Pressure Pressure is one of the physical factors that often influences the glass transition, since polymeric products are frequently prepared by injection molding. Here, the skin-core effect needs to be mentioned that is, a thin surface layer of an injection-molded polymeric product solidified at a temperature different from that of the bulk. If the sample sohdi-fied at an elevated pressure, endothermic peaks would appear at the low-temperature side of the glass transition curve. These peaks shift to lower temperatures with increasing pressure. The DSC curves get rather complex, but little quantitative information can be extracted besides the fact that the sample was cooled under high pressure. 

Combination of these methods provides a useful measure of the differential birefringence, a skin-core effect, if it is present. 

Aromatic polyamides (aramids) split longitudinally due to fibrillation, whereas nylon shows plastic deformation under the same conditions [33], Gupta [34] showed skin-core effects in polyester fibers. Hearle and Wong [35] studied the fatigue properties of nylon 6,6 and PET. 

In the case of less reactive pofymer ems, sudi as the meta-isomer of the PMDA/ODA based pol>(amic ethyl ester), mbmd results were obtained when investigating the pofymer imidization behavior of this pofymer in the presence of various amines as illustrated in Table 3. All spedmens were soft-baked at 80 C for S minutes, treated with the amine for 10 minutes and finalfy cured at 200 for IS minutes. Although imidization levels of base-treated spedmens are significantly higher as compared to the non-treated spedmen, no distinctive trend in the data is observable. This is most likety due to factors sudi as the ability of the amine to effectivety diffuse into the polymer film or the actual residence time of the amine in the film at elevated temperatures as determined by volatility considerations. Another aq[>ect not reflected tty the experimental IR imidization data relates to surface effects. Since the IR measurement determines bulk imidization levels, skin-core effects are completely ne ected. Thus, it is quite conceivable that in case of low imidization levels, the surface of the film could actually be completely imidized with little or no imidization in the underlying material. This skin-effect may then prevent the diffusion of catalyst into the bulk of the polymer film. 

In structural studies, fibres are generally considered to be transversely isotropic. However, it is sometimes found that commercial fibres have a distinct radial differentiation of structure. This is most pronounced in fibres spun from solutions, which show a distinct skin-core effect. A radial nonuniformity has been also found in some melt-spun fibres, e.g. in polyester fibres produced at very high spinning speeds.It should also be noted that in some fibres e.g. aramids" ) there is a preferential radial orientation of certain crystal planes, in contrast to a random radial orientation normally exhibited by conventional commercial fibres. 

---