The article describes extrusion foaming of poly(lactic acid) (PLA) using carbon dioxide in the supercritical state as foaming agent emphasizing the steps required to establish a stable extrusion process. Low melt strength of PLA plays a role in optimizing processing conditions. The tests included PLA grades of different viscosity in addition to a chain extender. Processing at low temperature is possible due to the plasticizing effect of the CO2 on the PLA melt and a sufficiently low melt temperature is also a prerequisite in production of stable foams due to improved melt strength. Foams were characterized by density, cell structure, crystallinity, and mechanical properties in compression. Low density, microcellular foams with density down to 20–30 kg/m3 were obtained for three different PLA grades. Varying die temperature and pressure drop rate we can explain observed abrupt drops in density with increasing CO2 content by the interplay between cell nucleation and gas diffusivity at given temperatures. An effect on melt strength similar to using a chain extender is achieved by lowering the melt temperature at the die. Observed variations in sample crystallinity do not correlate with foam density. The PLA foams have good energy absorption capability. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Effects of material parameters on the haze of blown films were analyzed. Four linearlow‐density polyethylenes (two metallocene grades and two Ziegler‐Natta grades) were studied in combination with three additives (two sorbitol‐based clarifying agents and a low‐molecular‐weight long‐chain branched polyethylene). One of the sorbitol‐based additives reduced the haze of both the metallocene materials in this study, but did not have any positive effect on the two Ziegler‐Natta materials. The variation in haze among the four base materials was directly related to the root‐mean‐square surface roughness (σ). When considering all 16 material/additive combinations, the link between haze and surface topography was not a simple σ‐haze relationship, but the haze was correlated with the average distance between adjacent surface profile peaks, the average slope, and the power spectral density at high lateral frequencies. Both of the mechanisms referred to in the literature, extrusion‐induced haze and crystallization‐induced haze, were probably active for the films in this study.

Approximate analytic expressions for haze (and gloss) of Gaussian randomly rough surfaces for various types of correlation functions are derived within phase-perturbation theory. The approximations depend on the angle of incidence, polarization of the incident light, the surface roughness, σ, and the average of the power spectrum taken over a small angular interval about the specular direction. In particular it is demonstrated that haze(gloss) increase(decrease) with σ/λ as exp(−A(σ/λ)2 ) and decreases(increase) with a/λ, where a is the correlation length of the surface roughness, in a way that depends on the specific form of the correlation function being considered. These approximations are compared to what can be obtained from a rigorous Monte Carlo simulation approach, and good agreement is found over large regions of parameter space. Some experimental results for the angular distribution of the transmitted light through polymer films, and their haze, are presented and compared to the analytic approximations derived in this paper. A satisfactory agreement is found. In the literature haze of blown polyethylene films has been related to surface roughness. Few authors have quantified the roughness and other have pointed to the difficulty in finding the correct roughness measure.

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Warpage of various semicrystalline polyethylenes (linear low density polyethylene [LLDPE]) has been investigatedunder typical rotational molding conditions, which means slow cooling from only one side. We have developed an experimental technique that is able to quickly rate different materials with respect to warpage under typical process conditions. We have also developed a numerical model simulating the experiments assuming a thermoelastic material including crystallization. As has been observed in practical rotational molding, it has been found in both experiments and simulations that materials with high crystallinity have in general higher warpage. The simulations also showed that the crystallization kinetics has implications on the warpage because the crystallinity gradient during solidification depends on the rate of crystallization. POLYM. ENG. SCI., 45: 945-952, 2005.

The dispersibility of silane-functionalized alumina nanoparticles (NPs) in syndiotactic polypropylene (sPP) was investigated. The Al 2 O 3 NP surface was modified with two different silane coupling agents, (3-chloropropyl)triethoxysilane and (octyl)triethoxysilane, to create hydrocarbon groups, or their halogen derivatives, for the enhanced hydrophobic interaction with the sPP matrix. Transmission electron microscopy studies showed that the presence of hydrophobic groups improved the dispersibility of alumina NPs in the sPP matrix. A good dispersibility was found in both (3-chloropropyl)triethoxysilane-modified Al 2 O 3 /sPP nanocomposites and (octyl)triethoxysilane-modified Al 2 O 3 /sPP nanocomposites.

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The authors present a study of the crystalline properties of selected series of polypropylene (PP) materials in relation to their warpage at conditions relevant for rotational molding. The PP materials have different crystallization temperatures and kinetics. The authors study the crystalline features of the materials using hot-press experiments and differential scanning calorimetry (DSC). In the DSC, the authors do constant cooling rate runs as well as isothermal crystallization and subsequent heating. A multimode crystallization kinetics model was fitted to the DSC cooling runs. Regression analysis was done to relate the results from the crystallization experiments to warpage. The derived empirical model shows that the crystallization temperature, crystallization half-time, and heat of fusion are the most significant parameters influencing warpage. Materials with warpage deviating from the model average are discussed taking aspects of the multimode kinetics into account. The present work could represent a basic platform for understanding and predicting the warpage during rotational molding process.