Fault-tolerant hybrid epoxy-silane coating for corrosion protection of magnesium alloy AZ31

In this work, a hybrid epoxy-silane coating was developed for corrosion protection of magnesium alloy AZ31. The average thickness of the film produced by dip-coating procedure was 14 μm. The adhesion strength of the epoxy-silane coating to the Mg substrate was evaluated by pull-off tests and was found to be higher than 16 MPa both in dry and wet conditions. The hybrid epoxy-silane coating showed high corrosion resistance both when intact and when punched through by a needle. The low frequency impedance of intact coating was higher than 1 GΩ cm² after one month of immersion in 3.5% NaCl solution. Both, artificially induced defects and corrosion sites that appeared on the metal surface did not propagate. Their passivation behavior, that we call fault-tolerance, was observed by EIS, SVET-SIET and SEM-EDS. It was ascribed to the good adhesion, high coating integrity and corrosion inhibiting effect provided by diethylenetriamine used as epoxy hardener.     

Hollow porous poly(lactic acid) microspheres

On the basis of the water solubility of poly(N‐vinyl‐2‐pyrrolidinone), hollow porous poly(lactic acid) microspheres (HPPLAs) were prepared by a water‐in‐oil‐in‐water multiple‐emulsion solvent evaporation method. The influence of the concentration of the stabilizer Span80 in the oil phase on the morphology was investigated. It was found that when the content of Span80 solutions was 3.5 wt %, most HPPLAs were about 2 μm in diameter. Field scanning electron microscopy results show that the HPPLAs were porous and hollow. The structure and crystal form of the HPPLAs were characterized by Fourier transform infrared spectroscopy and X‐ray diffraction analysis. Using these HPPLAs as degradable templates, we successfully synthesized Litchi‐like polystyrene (PS) microspheres about 2 μm in diameter by the emulsion method. When used as drug carriers, these HPPLAs would be convenient in which to embed drugs, whereas the Litchi‐like PS microspheres may have potential as new materials for polymer modification. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Halochromic poly (lactic acid) film for acid base sensor

A novel strategy to engineer biodegradable halochromic film based on poly (lactic acid) (PLA) for pH sensorial substances was presented. The effect of polyethylene glycol (PEG) compositions and dye amount on the efficiency of halochromic pH film was observed. The interactions between PLA, PEG, and dye was evidence by Fourier transform infrared. UV–VIS results indicated the sensitivity of the halochromic film through color changes. Mechanical performance was investigated by tensile testing to assess the potential of the halochromic film. It was found that the halochromic pH film has a clear color response from yellow to purple at pH 3–11 with a rapid response time within a minute. The results also revealed that varying content of PEG showed a significant effect on response time compared with varying dye compositions. PLA/PEG film and dye formed new interactions where PEG increased the free volume of PLA and made it possible for the PEG and the dye to diffuse and entrap between PLA chain. In term of the film strength, the addition of PEG and dye improved the flexibility of PLA film, which enables great potential in many applications such as medical, textile, and packaging.     

聚（乳酸⁃乙醇酸）薄膜制备及其性能研究

以左旋乳酸（L?LA）和乙醇酸（GA）为原料,利用一步法熔融共聚合成聚（乳酸?乙醇酸）（PLLGA）共聚物,通过差示扫描量热仪（DSC）对共聚物薄膜的结晶性能进行了表征,并利用Avrami方程对其进行了等温结晶动力学研究,通过万能拉伸试验机和压差法气体透过仪对共聚物薄膜的力学性能和气体阻隔性能进行测试。结果表明,PLLGA共聚物薄膜中GA的引入对材料结晶性能有较大影响,在GA含量为4 %（摩尔分数,下同）的PLLGA中,GA表现为成核剂作用,共聚物结晶比纯聚左旋乳酸（PLLA）薄膜快,半结晶时间减少;而在GA含量为8 %的PLLGA中,GA则表现出限制分子链运动的作用,破坏共聚物分子间的规整度,导致材料结晶性能大幅度降低,处于非晶态;随着GA含量的增加,PLLGA薄膜的拉伸强度和弹性模量逐步下降,而断裂伸长率大幅度增加,GA含量为8 %的PLLGA的断裂伸长率达到了130.1 %,是纯PLLA薄膜的21.3倍;同时,PLLGA薄膜的气体阻隔性显著增加,5 ℃时,相比于纯PLLA薄膜,GA含量为8 %的PLLGA薄膜的O₂、CO₂、N₂透过量分别降低了47 %、41 %和39 %。     

Permeability of ethanol solution through poly(lactic acid) film

The permeation properties of ethanol solution through poly(lactic acid) (PLA) films were investigated. The total flux of ethanol solution through PLA films was strongly depended on the flux of water. In addition, the diffusion coefficient of water was 1000 times higher than that of ethanol, and decreased with increasing feed concentration. After the permeation measurement, crystallization (X_C‐DSC = 1–2%) was observed. However, the crystallinity was not dependent on the feed concentration. On the other hand, the mole ratio of ethanol and water molecules in the PLA film strongly depended on the feed concentration. Based on the results, we concluded that the interaction with ethanol molecules caused the decrease in diffusion coefficient of water in PLA film. Thus, the permeation mechanism of the ethanol solution to the PLA film was investigated in detail. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42031.     

Top coating of low-molecular weight polymer MALPB used for enhanced protection on anodized AZ31B Mg alloys

A protective composite coating on an AZ31B magnesium alloy was prepared by anodic oxidation to form an oxide layer, followed by single immersion in maleic anhydride-g-liquid polybutadiene (MALPB) solution to cover a polymer coating on top. As a low-molecular weight polymer with low viscosity, MALPB had a tendency to infiltrate into the pores and cracks in the anodic layer to fill the defects among oxides so that a compact layer could be formed after it was cured by its hardeners, as observed by scanning electron microscopy (SEM). This compact layer, which was composed of anodic oxides integrated with solidified MALPB, possessed thickness around 0.7 μm as detected by energy dispersive X-ray (EDX). The anodized, MALPB-coated AZ31B alloy exhibited enhancement in corrosion resistance superior to that separately coated by anodizing oxides or MALPB, as reflected by its much higher corrosion potentials (E _ccor) and lower corrosion current density (i _corr) in DC polarization tests. Based on electrochemical impedance spectroscopy (EIS) data, we conclude that the anticorrosive performance of the composite coating can be attributed to the barrier effects provided by different layers when the electrolyte passed through the MALPB layer, compact layer, and then the inner anodic layer before it reached the surface of Mg alloy, and that among them, the compact layer acted as a much more effective barrier to the electrolyte. The appearance of damaged areas on the composite coating surface after a much longer duration in a salt spray environment revealed that the life-span of the AZ31B Mg alloy could be greatly prolonged if the pores and cracks on the anodizing films were properly sealed by suitable polymers.     

Corrosion protection behavior of AZ31 magnesium alloy with cathodic electrophoretic coating pretreated by silane

As an alternative process to phosphate and chromate conversion coatings, silane pretreatment was used to improve the performance of cathodic electrophoretic coating (E-coat) on AZ31 Mg alloy in this study. The galvanic corrosion behavior of AZ31 Mg alloy with E-coat coupled with Q235 steel was investigated. Compared to bare Mg alloy and Mg alloy with conventional painting, the corrosion properties of the AZ31 Mg alloy pretreated with silane and subsequently E-coated were studied during salt solution immersion and salt spray testing. The surface morphologies of the Mg alloy were examined in detail after immersion in NaCl solution for different times using digital photography and scanning electron microscopy (SEM). The corrosion current density of the specimens was characterized by DC polarization tests. It was found that silane pretreatment of AZ31 Mg alloy followed by subsequent E-coat led to much better corrosion protection than that without silane treatment. The silane pretreatment and E-coat delayed the galvanic corrosion of Mg alloy coupled with 235 steel bolts.     

Adherability and weldability of poly(lactic acid) and basalt fibre-reinforced poly(lactic acid)

The adherability and weldability of pure poly(lactic acid) (PLA) and basalt fibre-reinforced PLA were investigated in this research. The joining efficiency rate is introduced as a comparative parameter among different joining processes. In the case of adhesive bonding, 16 different adhesives were used to join specimens together. The highest bond strength and joining efficiency rate for both the pure (16 MPa, 78%) and basalt fiber-reinforced (18 MPa, 44%) adhesive-bonded specimens was achieved with acrylate-based two-component adhesives. The bond strength and joining efficiency rates of bonded specimens manufactured with four welding technologies (hot gas welding, friction stir welding, ultrasonic welding, laser welding) were also investigated. The highest bond strength for both pure PLA and basalt fibre-reinforced PLA specimens (51 and 125 MPa, respectively) was attained by laser welding. The highest joining efficiency rate for pure PLA specimens (85%) was attained by ultrasonic welding, while it was achieved by laser welding for basalt-fibre reinforced PLA specimens (70%).     

Improvement in poly(lactic acid) fabric performance via hydrophilic coating

The effects of a hydrophilic coating on poly(lactic acid) (PLA) fabric using polyethylene glycol-dimethyloldihydroxyethyleneurea (PEG-DMDHEU) were studied to obtain highly cross-linked polyethylene glycol (PEG) with acceptable fastness properties owing to the possibility of fixation PEG on the fibres surface at lower temperature than melting point of PLA fibres. PEG as a Phase Change Materials (PCMs) imparts thermal adaptability, which is so important for the comfort of textiles, to the substrate. While there is a good adhesion between the fibre and the PEG polymer for cotton and polyester fibres, polymer adhesion to PLA fibres and its effects on PLA fabrics have never been studied. The effect of hydrophilic coating on the PLA fabric was studied in comparison to polyethylene terephthalate (PET) fabric by measuring the thermal regulating effect, antistatic, air permeability, and mechanical properties. The results exhibit the possibility of multipurpose finishing on both fabrics samples leading to permanent thermal regulating effect and durable antistatic finish.     

Hydroxyapatite-carboxymethyl cellulose-graphene composite coating development on AZ31 magnesium alloy: Corrosion behavior and mechanical properties

In this study, hydroxyapatite (HA) coatings containing carboxymethyl cellulose (CMC) and graphene (Gr) were developed on AZ31 magnesium alloy through two-step electrophoretic deposition method. The morphology and chemical bonding of coatings were characterized and also the phase identification was done using scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction, respectively. Moreover, the corrosion behavior of the applied coatings was compared with the bare AZ31 Mg alloy substrate in the simulated body fluid by the means of potentiodynamic polarization test and electrochemical impedance spectroscopy. Obtained results revealed that the novel HA-CMC-Gr coating possesses the highest corrosion resistance compared to the HA, HA-CMC, and HA-Gr coatings due to its uniform and compact structure. To investigate the mechanical properties and to elucidate the effect of CMC on the adhesion of coating-alloy interface, pull-off test was employed, where results demonstrated that the addition of CMC increases the adhesion force from 1.06 MPa to 1.62 MPa. Besides, the modulus of elasticity and the hardness of HA and HA-Gr composite coatings were compared by applying nanoindentation test. Interestingly, it is detected that the presence of Gr has considerably increased the elastic modulus of the coating by approximately 30% in comparison to the pure HA coating.     

聚乳酸/聚乙二醇共混物的结晶与降解行为

针对聚乳酸（PLLA）亲水性差、降解周期长的问题,利用与亲水性高分子聚乙二醇（PEG）共混的方法对其进行改性。采用转矩流变仪制备了不同组成的PLLA/PEG共混物颗粒,系统研究了PLLA/PEG共混物的结晶和熔融、亲水性和在酸碱介质中的降解行为。结果表明,PEG的加入增强了共混物中PLLA的结晶能力,提高了PLLA在降温过程中的熔融结晶温度。PLLA/PEG共混物在等温结晶中表现出比纯PLLA更快的结晶速度。通过改变PLLA/PEG共混物的组成,可调控材料的表面亲水性和降解速率。随着PEG含量的增多,PLLA/PEG共混物的表面接触角降低。PLLA与PLLA/PEG共混物均可在水溶液中降解,共混物的降解速率高于纯PLLA,随着PEG含量的升高和降解液中酸碱浓度的提高,PLLA/PEG共混物的降解速率加快。     

Incorporation of polyfluorenes into poly(lactic acid) films for sensor and optoelectronics applications

Films of neat and plasticized biodegradable poly(lactic acid) (PLA) matrices containing anionic conjugated polyelectrolytes, poly[9,9‐bis(4‐phenoxybutylsulfonate)]fluorene‐2,7‐diyl‐alt‐arylenes, with 1,4‐phenylene and 4,4″‐p‐terphenylene, respectively, as arylene groups or a neutral poly(9,9‐dialkylfluorene) for comparison were prepared by solution casting. These films were characterized using differential scanning calorimetry, thermogravimetry, scanning electron microscopy and fluorescence spectroscopy. In addition, the effects of plasticizer on the thermal properties and the oxygen permeability of the PLA films were measured through the oxygen transmission rate. Results show that it is possible to obtain thin, optically transparent and luminescent films with potential in oxygen sensing, exhibiting good thermal and photochemical stability. At high polyelectrolyte content, evidence is found for phase separation and aggregate formation and it is no longer possible to obtain completely homogeneous films. The possibility of incorporating the cationic metal complex tris(2,2′‐bipyridyl)ruthenium(II) into plasticized PLA films containing conjugated polyelectrolytes for dual‐wavelength ratiometric luminescence sensing is also discussed. Copyright © 2012 Society of Chemical Industry     

Morphological control and properties of poly(lactic acid) hollow fibers for biomedical applications

Poly(lactic acid) (PLA) hollow‐fiber (HF) membranes were prepared by wet spinning with a phase‐inversion technique. Dimethyl sulfoxide (DMSO), N ‐methyl‐2‐pyrrolidone (NMP), and 1,4‐dioxane (DIO) were applied as solvents (Ss), and water was applied as a nonsolvent in the precipitation bath. The polymer solution viscosities, PLA–S–water miscibility regions, and precipitation onsets were measured and related to the Hansen solubility and Flory–Huggins interaction parameters. We observed a morphological transition from fingerlike to spongelike pores when DIO was applied as the S instead NMP or DMSO. The water permeabilities of these membranes were not detectable at a transmembrane pressure of 1 bar, and higher pressures caused them mechanical damage. However, the addition of 5 wt % poly(vinyl pyrrolidone) (PVP) induced a higher porosity and water permeabilties from 3.14 to 9.38 L m^?2 h^?1 bar^?1. These membranes were characterized by dialysis, and after 6 h, feed concentration reductions of 2% and 17% for bovine serum albumin and lysozyme, respectively, were observed. In vitro degradation tests showed that a 30% mass loss took place after 90 days of incubation, and a faster initial degradation of spongelike membranes occurred. The spongelike membranes presented a higher maximum stress (12.80 MPa) than the fingerlike membranes (～6 MPa). With PVP addition, the HFs were less resistant to axial traction and showed a decreased elongation of break from 58% to 23%. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45494.     

Influence of V-containing species on formation and corrosion resistance of PEO coatings developed on AZ31B Mg alloy

The defects, micropores, and microcracks on the plasma electrolytic oxidation (PEO) coating severely affect the long-term anti-corrosion feature. Inclusion of additives in the electrolyte is a feasible and effective strategy to attain the more intact PEO coating. The PEO coatings are fabricated on AZ31B Mg alloys in an silicon-based electrolyte with Na₃VO₄ (OPEO), NaVO₃ (MPEO), and V₂O₃ (VPEO), respectively, to explore the influence of additives on the corrosion resistance. MPEO sample has the smallest poriness and pore size unfolding the best anti-corrosion resistance, which is also consistent with the electrochemical analysis. Even after 336 h salt spray test, the MPEO coating exhibits the least corrosion. The excellent anti-corrosion performance of MPEO sample is the synergistic effect of inorganic salts and multiple metal oxides generated in the formation of PEO coating. Inclusion of inorganic salt with valence change metal is a feasible pathway to achieve the compact PEO coating since multiple metal oxides could be produced and deposited in the PEO coating during the fabrication.     

Crystallization of poly(lactic acid) enhanced by phthalhydrazide as nucleating agent

The effect of phthalhydrazide compound on the nonisothermal and isothermal crystallization behavior of bio-based and biodegradable poly(lactic acid) (PLA) was investigated by differential scanning calorimetry and polarized optical microscopy. The nonisothermal melt crystallization of PLA started much earlier in the presence of phthalhydrazide even at a phthalhydrazide content as low as 0.1 wt%. The isothermal crystallization kinetics was analyzed by the Avrami model. It was found that the Avrami exponent of the PLA crystallization was not significantly influenced by the addition of phthalhydrazide, indicating that the crystallization mechanism almost did not change in the composites. The crystallization half-time of PLA/phthalhydrazide composites decreased significantly with increase in phthalhydrazide loading. The observation from optical microscopy showed that the presence of phthalhydrazide increased the number of nucleation sites. The above observations indicate that phthalhydrazide is an efficient nucleating agent of PLA.     

The surface, microstructure and corrosion of magnesium alloy AZ31 sheet

This study investigates the effect of some industrial pre-treatments, including heat-treatment, surface grinding and polishing, acid cleaning and sandblasting, on the corrosion performance of Mg alloy AZ31 sheet. It is found that tempering accelerates the corrosion of AZ31. Surface grinding and acid cleaning dramatically improve its corrosion resistance, while sandblasting severely deteriorates its corrosion performance. Surface analysis indicates that the change in corrosion behavior of AZ31 by these pre-treatments can be associated with the alteration of its impurity contamination, surface state, grain size and intermetallic particles. A corrosion model concerning the dissolution and precipitation of tiny Al-Mn-(Fe) particles in grains and grain boundaries is proposed to explain the effect of mechanical deformation and heat-treatment on the corrosion performance of AZ31.     

Novel hybrid sol-gel coatings for corrosion protection of AZ31B magnesium alloy

This work aims to develop and study new anticorrosion films for AZ31B magnesium alloy based on the sol-gel coating approach.Hybrid organic-inorganic sols were synthesized by copolymerization of epoxy-siloxane and titanium or zirconium alkoxides. Tris(trimethylsilyl) phosphate was also used as additive to confer additional corrosion protection to magnesium-based alloy. A sol-gel coating, about 5-μm thick, shows good adhesion to the metal substrate and prevents corrosion attack in 0.005 M NaCl solution for 2 weeks. The sol-gel coating system doped with tris(trimethylsilyl)-phosphate revealed improved corrosion protection of the magnesium alloy due to formation of hydrolytically stable Mg-O-P chemical bonds.The structure and the thickness of the sol-gel film were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The corrosion behaviour of AZ31B substrates pre-treated with the sol-gel derived hybrid coatings was tested by electrochemical impedance spectroscopy (EIS). The chemical composition of the silylphosphate-containing sol-gel film at different depths was investigated by X-ray photoelectron spectroscopy (XPS) with depth profiling.     

Blown film extrusion of poly(lactic acid) without melt strength enhancers

Processing strategies were developed to manufacture poly(lactic acid) (PLA) blown films without melt strength enhancers (MSEs). The effects of processing temperature on PLA's melt properties (shear and elongational viscosities), PLA grades, and other processing conditions [ratio of take‐up roller to extruder's rotational screw speeds or processing speed ratio (PSR) and internal air pressures] on film's blow‐up ratio were examined. Experimental results indicate that extrusion‐blown amorphous and semicrystalline PLA films can be successfully manufactured without MSEs by controlling melt rheology through processing temperature and other extrusion processing conditions. PLA processed at lower extrusion temperature had higher melt viscosities, which favored the formation of stable films depending on the PSR and internal air pressure used. Inappropriate control of PSR and internal air pressure led to unstable films with various processing defects such as melt sag, bubble dancing, or draw resonance, irrespective of the lower extrusion processing temperature. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45212.     

Biodegradable poly(lactic acid)-based scaffolds: synthesis and biomedical applications

Biodegradable polymers are identified as substantial materials for biomedical applications. These polymers have the ability to deteriorate through an unpretentious hydrolysis and eliminated through kidneys’ functions or metabolic processes. Among widely used biodegradable polymers in biomedical applications, poly(lactic acid) (PLA) is becoming one of the most paramount polymers. Synthesizing PLA through melt/solution polycondensation polymerizations makes it relatively easy to tailor properties of final product. However, their synthesis reactions are affected by several parameters such as polymerization time, temperature, pressure, catalysts, and the polarity of the solvent. Moreover, equilibrium reactions are controlled through utilizing a hydrophilic monomer such as ethylene glycol (EG). These factors can strongly impact final properties of PLA. Thus, it is indispensable to comprehend the effect of operating parameters during the polymerization process. Optimizing synthesis conditions can be accomplished through reducing side reactions. Furthermore, this can be achieved through racemization by utilizing chain extenders to build high molecular weight and enhance thermal stability. In this review, the design and fabrication of porous PLA scaffolds and their physicomechanical behavior are reviewed. Different PLA scaffold parameters were investigated thoroughly, which include biocompatibility, biodegradability, and mechanical properties for different porosity and pore sizes to mimic the complex architecture of the natural tissue regeneration.

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Electrochemical corrosion behavior of composite coatings of sealed MAO film on magnesium alloy AZ91D

Protective composite coatings were prepared on magnesium alloy AZ91D by micro-arc oxidation (MAO) treatment plus a top coating with sealing agent using multi-immersion technique under low-pressure conditions. The corrosion resistance of AZ91D alloy with composite coatings was superior evidently to that with merely MAO film. SEM observations revealed that the sealing agent was integrated with MAO film by physically interlocking; therewith covered uniformly the surface as well as penetrated into pores and micro-cracks of MAO film. The anti-corrosion properties in 3.5% NaCl solution of the composite coatings were evaluated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Based on the results of chronopotentiometric (E ∼ t) and EIS measurements for long time immersion in 3.5% NaCl solution, appropriate equivalent circuits for the composite coatings system were proposed. It follows that due to the blocking effect of the sealing agent in pores and cracks in MAO film, the composite coatings can suppress the corrosion process by holding back the transfer or diffusion of electrolyte and corrosion products between the composite coatings and solution during immersion.