Microhardness and water sorption in injection‐molded starch

ABSTRACT:The microhardness (H) technique is used to characterize the quality of the weld line in injection-molded glassy polystyrene by means of a cylindrical obstacle. In particular, the effect of the indentation location (closer or further from the obstacle edge parallel to the injection direction and across the weld line), both on the surface and in the bulk, was examined. Only for surface measurements close to the obstacle (up to 10 mm) a well-pronounced decrease in H (ϳ30%), followed by a sharp increase in a narrow distance (0.20 -0.25 mm), was observed. For the bulk measurements on the same location a slight decrease in H was detected. Additional H measurements made up to 60 mm from the obstacle for both cases showed that the weld line remains undetectable. The results obtained reveal that the presence of a cylindrical obstacle causes the formation of a weld line on and near the surface only at distances not exceeding the obstacle diameter. At larger distances, because of the effective mutual interdiffusion of polymer chains, the two parallel fronts coming from the two sides of the obstacle developed a homogeneous material without any weld line according to the microhardness test.

Section: Introductionmentioning

confidence: 99%

Effect of an obstacle during processing on the weld line of injection‐molded glassy polystyrene: Microhardness study

Nguyen‐Chung¹,

Mennig²,

Boyanova³

et al. 2004

“…12 Perhaps degradation is the reason why indentations could not be clearly seen at temperatures beyond 70°C.…”

Section: Discussionmentioning

confidence: 97%

“…These results are in agreement with previous data on injection-molded pure starch samples. 12 From the plot of log H versus log t for the series of studied composites, the creep constant, k, was derived as:…”

Section: Microindentation Hardness Measurements: Dependence Of Wood-pmentioning

confidence: 99%

Micromechanical properties of injection‐molded starch–wood particle composites

Ueberschaer

Cagiao

Bayer

et al. 2006

ABSTRACT:The micromechanical properties of injectionmolded starch-wood particle composites were investigated as a function of particle content and humidity conditions. The composite materials were characterized by scanning electron microscopy and X-ray diffraction methods. The microhardness of the composites was shown to increase notably with the concentration of the wood particles. In addition, creep behavior under the indenter and temperature dependence were evaluated in terms of the independent contribution of the starch matrix and the wood microparticles to the hardness value. The influence of drying time on the density and weight uptake of the injection-molded composites was highlighted. The results revealed the role of the mechanism of water evaporation, showing that the dependence of water uptake and temperature was greater for the starch-wood composites than for the pure starch sample. Experiments performed during the drying process at 70°C indicated that the wood in the starch composites did not prevent water loss from the samples.

“…21 Recently, dynamic mechanical spectroscopy has been shown to be very fruitful in the elucidation of the gelatinization and gelation processes in corn starch. 22 In addition, the interaction of water with starch films, 23 the influence of the processing methods on the structure and surface mechanical properties of potato starch, 24,25 and its cement clay composites have been investigated. 26 In a preceding study, "in situ" X-ray scattering results using synchrotron radiation on injection molded starch samples, before and after conditioning in a wet atmosphere, were reported.…”

Section: Introductionmentioning

confidence: 99%

Structural changes of injection molded starch during heat treatment in water atmosphere: Simultaneous wide and small‐angle X‐ray scattering study

Cagiao

Rueda

Bayer

et al. 2004

Self Cite

Native starches with wide varying amylose content were processed by injection molding. The injectionmolded materials were conditioned in water for 20 days and sealed in glass capillaries. Simultaneous wide-and smallangle X-ray scattering (WAXS and SAXS, respectively) were recorded during thermal heating using a synchrotron source. Crystallinity, SAXS invariant, Q, and long period, L, were measured as a function of heating temperature. The injection-molding process provokes a destruction of the crystal forms A (cereal starch) and B (tubercle starch) but favors a development of the crystal form V h . After wet conditioning, WAXS of the injection-molded samples shows again the appearance of the crystal forms A or B, and crystallinity reaches values similar or larger than those of native starch. A constant heating rate (5°C/min) was particularly used for a comparison of potato and corn starch with a similar amylose content. While the crystallinity associated to forms A and B slowly decreases below 55°C and then rapidly decreases until its disappearance at 85-90°C, the invariant shows a maximum around 40°C and rapidly decreases thereafter. The total nanostructure disappearance occurs at temperatures about 10°C higher for the case of potato starch. In addition, a recovery of the WAXS and SAXS maxima during the subsequent cooling process before reaching room temperature was observed only for potato starch. Analysis of WAXS and SAXS for the rest of the starch materials reveals clear differences in the structural parameters of the samples that cannot be easily explained solely on the basis of the amylose content. Thus, for Cerestar and Roquette, it is noteworthy that there was a continuous decrease of L until its total disappearance as well as the persistence of crystallinity (form B), presumably stabilized by the presence of the V h structure (12-15%). Real-time crystallization experiments on two amorphous injection molded samples, waxy maize (free amylose starch) and potato starch, are also discussed. It is shown that the absence of amylose delays the recrystallization of amylopectine during the experiment.