Gel Content Determination Research Articles

Fabrication of 3D scaffolds based on fully biobased unsaturated polyester resins by microstereo-lithography

Additive Manufacturing (AM) technologies are an effective route to fabricate tailor made scaffolds for tissue engineering (TE) and regenerative medicine, with microstereo-lithography (µSLA) being one of the most promising techniques to produce high quality 3D structures. Here, we report the crosslinking studies of fully biobased unsaturated polyesters (UPs) with 2-hydroxyethyl methacrylate (HEMA) as the unsaturated monomer (UM), using thermal and µSLA crosslinking processes. The resulting resins were fully characterized in terms of their thermal and mechanical properties. Determination of gel content, water contact angle, topography and morphology analysis by atomic force microscopy and scanning electron microscopy were also performed. The results show that the developed UP resins (UPRs) have promising properties for µSLA. In vitro cytotoxicity assays performed with 3T3-L1 cell lines showed that the untreated scaffolds exhibited a maximum cellular viability around 60%, which was attributed to the acidic nature of the UPRs. The treatment of the UPRs and scaffolds with ethanol (EtOH) improved the cellular viability to 100%. The data presented in this manuscript contribute to improve the performance of biobased UPs in AM.

Synthesis and characterization of ricinoleic acid based hyperbranched alkyds for coating application

Hyperbranched alkyds based on ditrimethylolpropane, dimethylolpropionic acid, and ricinoleic acid were synthesized. The synthesized resins were characterized by the determination of hydroxyl and acid numbers, viscosity, average molecular masses, and molecular masses distribution. Compared with conventional alkyd resins of similar molar masses, the synthesized hyperbranched alkyds have significantly lower viscosity values at a given temperature. Therefore, a significantly smaller amount of organic solvents is required for their dissolution and can be used as precursors to obtain eco-friendly coatings. The obtained hyperbranched alkyd resins were cured by two commercial melamine resins (mass ratio 70:30). The curing of the resin mixtures was examined by FTIR spectroscopy, rheometry, and by the determination of gel content. The curing of the resin mixtures proceeds via the reactions between hydroxyl and carboxyl groups of hyperbranched alkyds with isobutoxymethyl and methoxymethyl groups in melamine resins. Hyperbranched alkyd/melamine resin mixtures were cured at 150 °C for 60 min. Thermal properties, dynamic mechanical behavior, physico-mechanical characteristics, and chemical resistance of cured films were determined. The crosslinking densities of films obtained from mixtures using hexamethoxymethyl melamine resin are significantly higher than those based on mixtures using isobutylated melamine resin. The thermal degradation of cured films is a complex process, and their thermal stability increases slightly with the increase of hyperbranched alkyds generation. It is assumed that mixtures of synthesized hyperbranched alkyds with melamine resins would be suitable for obtaining the coatings with good chemical resistance and physical-mechanical properties.

Resilience improvement of an isotactic polypropylene-g-maleic anhydride by crosslinking using polyether triamine agents

The elaboration of new hybrid polymer/metal composite materials depends on the development of innovative polymer matrices with improved mechanical properties and high effective adhesion behaviour with metal surfaces. This way, a new type of polyolefin was developed based on a peculiar crosslinking reaction. For this elaboration, an iPP-g-MAH with 1 wt% of MAH was crosslinked with polyether triamine molecules by twin screw reactive extrusion. The evolution of the crosslinking reaction, the microstructure and the mechanical behaviour of the different materials were studied as a function of the NH2:MAH molar ratio in the range of 0:1 and 1.5:1. Gel content determination and FTIR analysis were performed to characterise the material at the molecular scale, when differential scanning calorimetry, wide angle X-rays scattering and self-successive auto-nucleation techniques were used for a deep microstructure analysis. Brillouin spectroscopy showed an interesting evolution of the elastic constants at the microscopic scale and results were in good agreement with those obtained at the macroscopic scale by a video-controlled uniaxial tensile test. Moreover, it was demonstrated that the true mechanical behaviours switch from brittle to ductile illustrating an important improvement of the resilience ability of the synthetized materials as a function of the NH2:MAH molar ratio even for molar ratio other than an equimolar ratio, with remaining MAH or NH2 chemical groups for further chemical reaction.

The Thermal and Mechanical Properties of Poly(ethylene-co-vinyl acetate) Random Copolymers (PEVA) and its Covalently Crosslinked Analogues (cPEVA).

The thermal and mechanical properties of poly(ethylene-co-vinyl acetate) random copolymers (PEVA) and its covalently crosslinked analogues (cPEVA) were controlled by the overall crystallinity of the polymer networks. The cPEVAs with different VA-content were synthesized by thermally-induced crosslinking of linear PEVA with dicumyl peroxide (DCP). This work was mainly concerned with the effect of vinyl acetate (VA) content on the crosslinking density, thermal and mechanical properties of PEVAs and cPEVAs, respectively. The chemical composition was analyzed by thermogravimetric analysis and 1H-NMR. The thermal and mechanical properties of PEVAs and cPEVAs have been studied through a series of conventional analytical methods, including gel content determination, different scanning calorimetry, thermogravimetric analysis, dynamic mechanical thermal analysis and traditional mechanical measurements. The experimental results show that the thermal and mechanical properties of PEVAs and cPEVAs increase with decreasing the VA-content. A broad melting transition with a ΔTm in the range from 78 °C to 95 °C was observed for all polymer networks.

The study on aging behaviors and critical stress of cross-linked high-density polyethylene during stress and photo-oxidative aging

In order to study the effects of cross-linking and critical stress during stress and photo-oxidative aging, the aging behaviors were studied by making cross-linked HDPE materials through a self-made stress and photo-oxidative aging apparatus in this paper. The cracking and aging behaviors of cross-linked polyethylene under stress was studied through microscopic observation, strain measurement, determination of gel content, breaking time statistics, fitting of stress-breaking time curves and so on. The critical stress was studied through fitting of stress-breaking time curves. It can be found that the surface morphologies were different under different stresses. The growth rate and strain at break decreased as the degree of cross-linking increasing. The breaking time of specimens were different under different stresses, and showed different trends as time increasing due to UV radiation and the network structure. The critical stresses of all kinds of samples ranged from 6.20 MPa to 7.02 MPa, showing an increasing trend as the degree of cross-linking increasing.

Synthesis of Polyurethane from Diphenyl Methane 4,4 Diisocyanate (Mdi) Polymerization with Hydroxilated Avocado Oil Polyol

Polymerization between polyol and isocyanate compounds produces polyurethane in different forms. Polyol used in this study utilized spoiled avocado as the oil source for polyol to produce polyurethane. Avocado oil was epoxidized with formic acid (HCOOH) and H2SO4 catalyst at 40 – 45oC followed by hydrolysis reaction to produce polyol reaction. Next polyol was purified and structure confirmation was done using FT-IR spectroscopy analysis. Polyol was then reacted with diphenylmethane 4,4 diisocyanate (MDI) with ratios of (polyol : MDI) 9:1; 8:2; 7:3; 6:4; and 5:5 (v/v) in a total volume of 10 mL with open air stirring at 40 – 45oC to produce polyurethane. Characterization for the form was observed visually followed by the determination of gel content, density and structure using FT-IR spectroscopy. The results of polyol from avocado oil reacted with diphenylmethane 4,4 diisocyanate for each of the mixing ratio to produce polyurethane is gel content of 74.63% to 99.80% where the higher MDI ratio is, the higher gel content becomes. Density determination from the polyurethane produced is between 0.1341 g/cm3 to 0.7220 g/cm3. FT-IR spectrometer analysis to polyurethane produced is marked with the peaks at 3400 – 3300 cm-1, 2270 – 1940 cm-1, 1700-1600 cm-1 and 1590 – 1540 cm-1 wavelengths which are the characteristic of urethane functional groups.

Switching from brittle to ductile isotactic polypropylene-g-maleic anhydride by crosslinking with capped-end polyether diamine

Ductile functionalized polypropylene is of great interest in multi-layer composites development. A capped-end triblock polyether diamine (H2N-PO2-TMO9-PO3-NH2) was used as a linker of two different maleated isotactic polypropylenes (iPP-g-MAH, at 1 and 3 wt%) by twin-screw reactive extrusion. The molecular architecture, the semi-crystalline microstructure and the mechanical properties of the resulting polymers were analysed as a function of the molar ratio of NH2:MAH (0:1, 0.33:1, 0.5:1, 0.66:1, 1:1, 1.5:1, 2:1). At the molecular scale, the chemically modified polymers were characterized by FTIR and by the determination of gel content following the ASTM D2765-84 standard method. Differential Scanning Calorimetry, Wide Angle X-rays Scattering and Self Successive Auto-nucleation techniques showed the impact of the crosslinking: a decrease of the melting temperature as compared with the unmodified iPP-g-MAH polymer, as well as a decrease of the crystallinity ratio for the iPP-g-MAH at 3 wt% of MAH. Interestingly, Dynamic Mechanical Analysis and video-controlled uniaxial tensile tests showed that the polymers obtained in stoichiometric NH2:MAH molar ratio exhibit the best mechanical properties enlightened by a switch from brittle to ductile mechanical behaviour with few differences between the two iPP-g-MAH.

Characterization of Epoxidized Natural Rubber for Coating Application

Epoxidized natural rubber was prepared by epoxidation of natural rubber with fresh peracetic acid in latex stage. The epoxidized natural rubbers with epoxy group content of 8, 15 and 27 mol% were prepared by adjusting the amount of fresh peracetic. The characterization of resulting materials was carried out through NMR spectroscopy, gel content determination and particle size measurement. The epoxidized natural rubber with various epoxy group contents were coated on the surface of the wood substrate by spin coating technique. The contact angle measurement and bending test were made for wood coated by epoxidized natural rubbers. The results show that the obtained epoxidized natural rubber is liquid resistance as is evident in high contact angle of water and poly(ethylene glycol) solution on the rubber surface. The bending strength of wood is improved by coating epoxidized natural rubber. The material with epoxy group content of 15 mol% is suitable for the protection of the wood surface.

Preparation and Chemical/Microstructural Characterization of Azacrown Ether-Crosslinked Chitosan Films

Chemically stable porous azacrown ether-crosslinked chitosan films were prepared by reacting varying molar amounts of N,N-diallyl-7,16-diaza-1,4,10,13-tetraoxa-dibenzo-18-crown-6 (molar equivalents ranging from 0, 0.125, 0.167, 0.25 and 0.5) with chitosan. Their chemical and structural properties were characterized by solid state-nuclear magnetic resonance (NMR), elemental, Fourier transform infrared (FTIR), microscopy, and X-ray analyses, as well as gel content. NMR and FTIR analyses of the reaction products suggested that new –CH2– crosslink bridges were produced between the amine groups of chitosan (Ch) and the allyl groups of the azacrown (DAC). The crosslinking chemistry between allyl and amine groups of the reactants was further evidenced with solution NMR studies on model compound of glucosamine with the azacrown. X-ray diffraction analysis of the Ch/azacrown films using wide angle X-ray scattering (WAXS), including synchrotron-WAXS, revealed that the crystalline arrangement of chitosan (Ch) was partially destroyed with increasing grafting of azacrown ether proportion on the Ch polymer chain. Solubility and gel content determination confirmed network formation with a gel content as high as 84–95 wt %. Microstructural analysis revealed microporous morphology with high surface area. The morphology and structure of the azacrown ether-crosslinked chitosan films could be tailored by stoichiometry of the reacting species.

The effects of e‐beam crosslinking of LDPE on the permeation of hydrocarbons

ABSTRACTPolyethylene is a useful material in numerous applications, such as packaging, fuel tanks as well as oil and gas pipes. However, it must be considered that polyethylene is permeable to gases and liquids. Small molecules, such as hydrocarbons, may permeate through the material and cause environmental problems. Consequently, a particular aspect of polymer materials is the requirement of a high permeation resistance against low‐molecular‐weight hydrocarbons. Modifications of the commonly used polyethylene material are necessary to achieve adjusted permeation properties. In this contribution, the effect of electron‐beam crosslinking of polyethylene on the resulting permeation characteristics was investigated. Polyethylene sheets were processed and crosslinked by irradiation with high energy electrons. The formation of a network structure was characterized by gel content determination and by rheological measurements. The permeation properties of the polymer sheets were assessed by a gravimetric method. It is demonstrated that—as a result of the crosslinking process—the permeation of low molecular weight hydrocarbon through polyethylene is reduced with respect to the chain length of the aliphatic substances. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44968.

Investigation of the curing state of ethylene/vinyl acetate copolymer (EVA) for photovoltaic applications by gel content determination, rheology, DSC and FTIR

The cure behaviour of a specific ethylene vinyl acetate material as used for encapsulation of photovoltaic modules was analysed by rheometer, differential scanning calorimetry and Fourier transform infrared spectroscopy to test for a suitable replacement for the laborious determination of gel content.The results show that all applied methods are capable of describing the effects of the cross-linking process. Some provide results very similar to those yielded by analysis of the insoluble content, but the question remains as to whether indirect methods should be preferred over the direct measurement of physical properties, e.g. as performed by the curemeter.A material stored for one year was also tested to demonstrate the effect of extended storage on cure behaviour and how this is detected by different methods. This complements the other methods, which were clearly able to detect the different cure behaviour of the aged EVA, whereas determination of the gel content could not.

Fast and Nondestructive Detection on the EVA Gel Content in Photovoltaic Modules by Optical Reflection

Poly(ethylene-co-vinyl acetate) (EVA) has been the dominating material in the photovoltaic (PV) encapsulant market for decades, owing to its superior cost-performance balance. To achieve its desired material properties, EVA undergoes a curing reaction during the module encapsulation process. The resulting EVA gel content after encapsulation is an important criterion for the module encapsulation quality control. Normally, the determination of gel content is achieved using a tedious solvent extraction method. In this paper, a fast and nondestructive detection method on the EVA gel content based on the optical reflection is explored. First, the homogeneity of the EVA gel content distribution after the standard EVA encapsulation process is studied. Then, the feasibility of the proposed optical approach applied to transparent modules is investigated. After that, a method is developed to apply it to opaque modules by incorporating a mirror into the module construction. It was found that the haze factor of the reflected light correlates well with the EVA gel content in the opaque modules. This proof-of-concept work could lead to the development of a fast and nondestructive tool for detecting the EVA gel content in both transparent and opaque PV modules, which is promising for integration as an inline diagnostic tool in the module manufacturing line.

Influence of in situ -generated silica nanoparticles on EPDM morphology, thermal, thermomechanical, and mechanical properties

Silica nanoparticles were synthesized by means of a sol–gel method and generated in ethylene propylene diene monomer rubber (EPDM) by in situ synthesis. The properties were determined using scanning electron microscopy, attenuated total reflectance Fourier-transform infrared spectroscopy, thermogravimetric analysis, tensile testing, dynamic mechanical analysis, swelling tests, and gel content determination. The silica particles were homogenously dispersed in the EPDM matrix, with the presence of agglomerates, especially for high silica contents. The swelling experiments showed a decrease in the crosslinking density of the vulcanized rubber due to the presence of the silica nanoparticles. The mechanical properties, however, were significantly improved by the presence of the stiff silica nanoparticles. The effect of the amount of silica on the thermomechanical properties and thermal degradation of EPDM was also investigated. The presence of silica showed an increase in the storage and loss moduli at high temperatures, probably due to the increasing filler content. The thermal degradation analysis showed that the presence of silica particles incorporated in the EPDM matrix had no significant influence on the thermal stability of the composites. POLYM. COMPOS., 36:825–833, 2015. © 2014 Society of Plastics Engineers

Thermoxidative stabilization of poly(ethylene terephthalate) and alkyd Using dibutyltin diethanoate

The thermo-oxidative degradation of unstabilized and stabilized coatings of each of a saturated polyester and a poly (pentaerythritol-phthalate)based alkyd coatings separately pigmented with syncomine yellow 1630 have been studied. The test stabilizer was dibutyltin diethanoate employed at varying concentrations. The degradation of the coatings was monitored using color development, determination of gel content, stress–strain relationships, gloss retention and optical microscopy. The results showed that incorporating a concentration of 5 or 10 x 10 -4 mole % of the tin ester can effectively stabilize both the polyester and alkyd coatings against thermo oxidative degradation.

Photopolymerizable Synthons from Glycerol Derivatives

AbstractPhotopolymerizable monomers based on monoglycerides were prepared by a convenient two‐step procedure. The first one consists of the synthesis of highly pure monoglycerides by an organocatalyzed solvent‐free route. This process was carried out by condensation of bio‐sourced carboxylic acids (i.e. butyric, decanoic, undecylenic or stearic acids) with glycerol carbonate or glycidol in the presence of a quaternary ammonium salt as a catalyst. The obtained bio‐based monoglycerides were modified in the second step by reaction with acryloyl and methacryloyl chloride leading to a series of new diacrylated and dimethacrylated monomers. The structures of the monoglycerides, diacrylated and dimethacrylated monomers were fully characterized by spectroscopic methods. Photopolymerization investigations monitored by infrared spectroscopy were achieved under ultraviolet radiation in the presence of a photoinitiator. The resulting cross‐linked materials were analyzed by thermal gravimetric analysis, gel content determination and tests of swelling in water, ethanol and methylene chloride. Data relative to the pendulum hardness of these materials are also included.

The use of multiarm star-like polymers in the preparation of epoxy thermosets by UV-cationic photopolymerization. Effect of the arms of the star in the curing process and in the final properties and morphology

New epoxy thermosets have been prepared via cationic UV-photopolymerization introducing two different multiarm star-like polymers. Both stars have a poly(glycidol) core but one has poly(methylmetacrylate) arms and the other poly(ε−caprolactone) ones. The characterization of the curing process has been performed by Real-Time FTIR and photo-DSC, observing a slight reduction in the curing rate on increasing the proportion of star. The thermosets prepared were characterized by gel content determination, DMTA and TGA, and finally the morphology observed by FE-SEM, demonstrating the formation of nanophases in the case of the star with poly(ε−caprolactone) arms. POLYM. ENG. SCI., 54:17–23, 2014. © 2013 Society of Plastics Engineers

Grafting maleic anhydride onto EVA and effect on the physical and rheological properties of PETG/EVA‐g‐MAH blends

AbstractMaleic anhydride (MAH) was grafted onto ethylene vinyl acetate copolymer (EVA) in the internal mixer in the presence of dicumyl peroxide (DCP), and a prepared reactive compatibilizer, MAH‐g‐EVA, was blended with Poly (ethylene glycol‐co‐cyclohexane‐1,4‐dimethanol terephthalate) (PETG). The gel content determination and element analysis (EA) was performed to confirm the grafting reaction. It was found that grafting reaction of MAH on to EVA could compete with crosslinking reaction of DCP during the modification process. In addition, the introduction of small amount of MAH showed a great effect on reducing gel content by decreasing crosslink reaction. As MAH content increased, grafted MAH concentration increased, whereas the grafting yield decreased. It was also confirmed that MAH‐g‐EVA acted as a reactive compatibilizer in the blend with PETG, and enhanced compatibility by reacting with the hydroxyl end groups, OH, of PETG. Addition of EVA in the blend leads a plastic deformation of PETG, and MAH had a great effect on enhancing interfacial adhesion resulting in significant increasing of % strain; however, improved compatibility could not be changed much in low strain tensile strength and Young's modulus. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Kenaf fiber‐reinforced copolyester biocomposites

AbstractIn this study the morphology and properties of a biodegradable aliphatic–aromatic copolyester mixed with kenaf fiber were investigated. Untreated kenaf fiber, as well as kenaf fiber treated with NaOH, and with NaOH followed by silane coupling agent treatment at various concentrations, were used as fillers in the composites. The biocomposites were prepared by melt‐mixing and a 10 wt% fiber loading was used for all the composites. The properties of the biocomposites were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), tensile properties, environmental scanning electron microscopy (ESEM), and biodegradability. The extent of silane initiated grafting was followed by gel content determination. The presence of fiber and fiber treatment influenced the determined properties in a variety of ways, but the best balance of properties were found for the copolyester mixed with alkali‐treated fiber. This composite showed improved thermal, thermomechanical, and mechanical properties. The introduction of alkali treatment caused increased surface roughness in the fiber, which resulted in mechanical interlocking between the filler and the matrix, while silane treatment slightly reduced the properties. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

On gel determination in PA6-glass fiber composites

Engineering plastics are frequently reinforced but the testing procedures are in many cases significantly affected by the presence of the reinforcing agent. A method for gel content determination in PA-6/glass fiber composite crosslinked by irradiation was investigated based on extraction of the sample fraction soluble in 90% formic acid. The sample, wrapped in glass fabric, was treated under ten selected modes to optimize the analysis. Shape of the analyzed sample was shown to be important. Applying statistical tests to the results, an optimal procedure was developed that consisted of sample extraction by the solvent under mild shaking for 24 h and then removing the excessive solvent followed by drying the sample. A correction for the fraction of the glass fabric lubricant extractable by the solvent was necessary for calculation of correct results. A gel content of approximately 60 wt.% was determined with relative standard deviation less than 5%.

Effect of irradiation on mechanical and structural properties of ethylene vinyl acetate copolymers hollow fibers

AbstractEffect of irradiation on mechanical and structural properties of ethylene vinyl acetate copolymers (EVA) hollow fibers was studied by the tests such as determination of gel content, density, tensile, FTIR, SEM, and DMA. These effects were discussed based on dose and irradiation environment. The results of gel content depicted that irradiated EVA in ambient conditions had tendency to chain scission while the crosslinking overcame in irradiated samples under nitrogen. Density insignificantly enhanced with irradiation dose. In tensile test, irradiation induced increase in tensile strength and decrease in elongation at break (especially in samples irradiated in nitrogen). Also, changing in layer orientation could be observed by SEM images. In addition, irradiation caused altering peak intensity in FTIR spectrum. DMA results demonstrated that irradiation broaden the elastic zone. Totally, irradiation enhances features especially in irradiated EVA18 in nitrogen. Since, according to stabilization of induced deformation and improvement of mechanical properties (that created by radiation), the irradiated samples can be used in different applications. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011