Amabile Penati

The combined effect of roughness and heterogeneity on contact angles : the case of polymer coating for stone protection

The individual effects of heterogeneity and roughness on contact angles have been repeatedly analysed in the literature, but the application of the accepted models to practical situations is often not correctly performed. In the present paper the combined effects of roughness and heterogeneity on the contact angles of water on stone surfaces protected by a hydrophobic polymer coating are considered. Two different kinds of calcareous stone with different surface roughnesses and porosities were protected against the effect of water absorption by two different polymer coatings. The contact angles of water on the protected stone surfaces were measured by the Wilhelmy and the sessile drop techniques. A comparison of the results obtained shows not only the limits of the static sessile drop technique, but also the combined effect of roughness and heterogeneity. Some considerations are developed on the application of commonly accepted models to surfaces with a combination of roughness and heterogeneity. Some other results obtained with techniques such as roughness measurements, mercury porosimetry, energy dispersive X-ray spectroscopy (EDXS), thermogravimetric analysis (TGA), water absorption by capillarity experiments (WAC), all able to show the structure and properties of the obtained films, are also compared with those obtained from contact angle measurements. It is concluded that the static contact angle is not well correlated with the degree of protection; on the contrary, the receding contact angles are well correlated with the degree of protection actually obtained. An ideal protecting agent should have a receding contact angle greater than 90°.

Polypropylene/cycloolefin copolymer blends: effects of fibrous phase structure on tensile mechanical properties

Tensile mechanical properties of polypropylene (PP)/cycloolefin copolymer (COC) blends were studied using an Instron tensile tester. As COC was expected to impart enhanced mechanical properties to the blends, their modulus, yield strength, tensile strength and tensile energy to break were measured as functions of blend composition. With regard to the reported sensitivity of the COC structure to thermal history, the influence of annealing at two different temperatures was also tested. The attention was primarily concentrated on blends with the volume fraction of COC in the interval 0<v2<0.40, where COC formed (short) fibres almost uniaxially oriented in the direction of injection moulding. In the interval 0.40<v2<0.75, the blends consisted of partially co-continuous components. Two different models were applied in the analysis of mechanical properties, namely (i) the rule of mixtures for fibre composites and (ii) the equivalent box model for isotropic blends (employing the data on the phase continuity of components obtained from modified equations of the percolation theory). Experimental data on the studied mechanical properties were better fitted by the models for fibre composites. Annealing of the samples (75 °C for 45 days; 120 °C for 3 h) did not markedly affect the tensile modulus, yield stress, and stress at break of the blends. On the other hand, the strain at break was markedly reduced by the annealing up to v2=0.2; COC and the blend with 75% of COC ruptured in a brittle manner without yielding.

Hydrolytic resistance of model poly(ether urethane ureas) and poly(ester urethane ureas)

Poly(ester urethane ureas) (PesURUs) and poly(ether urethane ureas) (Pe- tURUs) synthesized from diphenylmethane-4,49-diisocyanate and poly(butylene adi- pate) diol, and poly(tetramethylene oxide) diol or poly(propylene oxide) diol, respec- tively, were hydrolyzed at 70°C for various periods up to 16 weeks. Differences in thermal and mechanical properties of as-received dry samples are correlated with the number and strength of hydrogen bonds formed between urea/urethane groups of hard segments and polyester or polyether groups of soft segments. Gel permeation chroma- tography measurements show that the molar mass of linear PesURUs markedly de- creases with the hydrolysis time, whereas that of linear PetURUs remains almost unaffected. PesURU crosslinked by polymeric isocyanate has lower crystallinity, but shows somewhat better resistance to hydrolysis than its linear counterpart because of its more stable three-dimensional molecular structure. Water uptake at 37°C, dynamic mechanical thermal analysis, and differential scanning calorimetry thermograms de- termined for redried hydrolyzed specimens concurrently show that advancing hydroly- sis accounts for decrease in the crystallinity (if any) of soft polyester segments, in the efficacy of hydrogen bonding and in crosslinking density. Experimental data indicate that hydrolytic resistance of PetURUs is primarily determined by (1) the hydrolytic stability of individual types of present groups, (2) steric hindrances affecting the access of water molecules to these groups, and (3) the hydrophilicity of backbones.

Effect of the polymer―filler interaction on the thermo-mechanical response of polyurethane-clay nanocomposites from blocked prepolymer

Thin transparent films of polyurethane—clay nanocomposites were prepared by dispersing different amounts of commercial organo-modified clay in a mixture of cycloaliphatic amines used as chain extender of a blocking prepolymer, in order to investigate the role of the filler content on the thermo-mechanical behavior of the resulting composites. X-ray diffraction measurements evidenced the formation of an intercalated structure, regardless to the clay loading. Furthermore, the optical clarity of the samples was not substantially compromised by the nanofiller addition even at elevated clay amounts. Interestingly, the relatively strong polymer—filler interaction led to a substantial reduction of the matrix crosslinking degree for high clay loadings. Consequently, the relative thermal lifetime was positively affected by the presence of clay up to a filler content of 7 wt%, while uniaxial tensile tests under quasi-static and impact conditions evidenced an increase of the elastic modulus proportional to the clay concentration, without impairing the original tensile properties at break.

Ternary polymer blends: prediction of mechanical properties for various phase structures

WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW A predictive scheme is proposed for the simultaneous calculation of the modulus and yield (or tensile) strength of ternary polymer systems. According to the continuity or discontinuity of constituting phases, the scheme combines in two steps the models for binary systems: (i) in the interval of phase duality (co-continuity), a twoparameter equivalent box model is used along with the data on the phase continuity rendered by modified equations of the percolation theory; and (ii) the effects of a dispersed phase on the mechanical properties of a continuous phase are treated by using the approach developed earlier for particulate systems. Simultaneously predicted values of the modulus and yield (or tensile) strength of ternary systems are interrelated because they are calculated by using an identical set of input parameters characterizing a specific phase structure. The predictive scheme will allow the experimentalists: (i) to anticipate selected mechanical properties of envisaged blends (for presumed phase structures); (ii) by comparing experimental and theoretical data, to assess to which percentage the potential of a material has been exploited; (iii) to analyze the phase structure of prepared ternary blends; and (iv) to evaluate interfacial adhesion or the extent of interfacial debonding. The versatility of the predictive scheme is demonstrated on three examples of various types of ternary systems. CopyrightÓ 2000 John

Thermal stabilities of different polyurethanes after hydrolytic treatment

Different polyester urethanes and polyether urethanes were exposed to hydrolytic degradation at 70°C for up to 16 weeks. The dried samples were subsequently analysed in thermooxidation tests in the range 250-300°C by measuring the lifetime relating to 5% mass loss. The experimental results showed that polyether urethanes have lower thermal stabilities than those of polyester urethanes; the activation energies determined from the Arrhenius plot are around 65 and 80 kJ mol-1, respectively. The activation energies of polyether urethanes did not change significantly as hydrolysis proceeded. In contrast, the polyester urethanes exhibited a progressive decrease in activation energy, which fell (after 16 weeks of hydrolysis) to the values characterizing polyether urethanes. The entropic parameter of the Arrhenius equation was also evaluated and related to the chemical composition of the as-received and hydrolysed polymers.

Effect of hydrolysis on molar mass and thermal properties of poly (ester urethanes)

Effect of hydrolysis time on molar mass, glass transition temperature, crystallinity, and resistance to thermooxidation at elevated temperatures was analyzed for Estanes 54600, 54610, and 54650. Kinetics of the hydrolysis can be plausibly described in terms of the first-order reaction with an average induction period of about 7 days. Reduction of molar mass induced by hydrolysis brings about an appreciable decrease in glass transition temperature, fraction of crystalline domains of soft segments, and thermooxidative stability. The latter effect is manifested by shortening of the lifetimes (related to 5% mass loss) the temperature dependence of which obeys the Arrhenius plot. The observed differences in hydrolysis resistance of Estanes can be related to their chemical composition.ZusammenfassungFür Estan 54600, 54610 und 54650 wurde der Einfluß der Hydrolysedauer auf die Molmasse, den Glasumwandlungspunkt, die Kristallinität und die Beständigkeit gegenüber Thermooxidation bei erhöhten Temperaturen untersucht. Die Reaktionskinetik der Hydrolyse kann einfach mit den Ausdrücken der Reaktion erster Ordnung, unter Berück-sichtigung einer durchschnittlichen Induktionsperiode von 7 Tagen beschrieben werden. Die hydrolyseinduzierte Abnahme der Molmasse verursacht eine beträchtliche Verminderung des Glasumwandlungspunktes, der Fraktion der kristallinen Bereiche von Soft-Segmenten und der thermooxidativen Beständigkeit. Letzterer Effekt drückt sich in einer Verkürzung der Lebensdauer in Verbindung mit 5% Masseverlust und der Temperaturabhängigkeit aus, der die Arrheniussche Darstellung unterliegt. Die beobachteten Unterschiede bei der Hydrolysebeständigkeit von Estanen können mit ihrer chemischen Zusammensetzung in Verbindung gebracht werden.