Morphological changes in epoxy resin (DGEBA/TETA) exposed to low temperatures

Abstract

Epoxy resin (DGEBA/TETA) is commonly used as an adhesive material. When this material is subjected to low temperatures for an extended period of time, it undergoes changes in inter/intramolecular vibrations which has been probed in this research through a combination of terahertz spectroscopy, differential scanning calorimetry and infrared spectroscopy. Distinctive morphological changes of the virgin material are observed for the exposed material at cold temperature which can be attributed to incomplete reversibility of segmental as well as entire molecular chain mobility.     

Bulk and nano‐mechanical behavior of silver and silver‐CNT‐reinforced hybrid polymer composites

The present study concerns study of the Bulk and nano‐mechanical properties of Ag‐CNT vis‐à‐vis Ag reinforced polymer composites. The Ag‐CNT hybrid powder blend was prepared by mechanical milling. Variation of the bulk and nano‐mechanical properties has been attributed to the morphological features and the degree of dispersion of the fillers. It has been found that dispersion of Ag‐CNT hybrid particle is beneficial for improvement of both the mechanical properties. POLYM. COMPOS., 37:2581–2587, 2016. © 2015 Society of Plastics Engineers     

Structural changes of starch/polyvinyl alcohol biocomposite films reinforced with microcrystalline cellulose due to biodegradation in simulated aerobic compost environment

Starch/Polyvinyl alcohol (PVA) based biocomposite films reinforced with micro crystalline cellulose (MCC) (10 wt %) particles were prepared by solution casting method, incorporating glycerol as plasticizer. These biocomposite films were subjected to biodegradation at ambient temperature in a simulated aerobic compost pit. The extent of biodegradation of these films was studied in terms of weight loss. The corresponding changes in the structure of the films were observed using scanning electron microscopy, X‐Ray diffraction study, and differential scanning calorimetry. The melting point of PVA component of the biocomposite film shifted from 204 to 223°C with increase in biodegradation time and a remarkable difference was observed in their melt crystallization behavior. The unreinforced films also showed a similar trend, but the increase in the crystallinity of PVA was more pronounced in MCC reinforced films than that observed in the unreinforced ones. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Study of compostable behavior of jute nano fiber reinforced biocopolyester composites in aerobic compost environment

Jute nano fiber (JNF) reinforced biocopolyester‐based composite sheets were prepared with 2% and 10 wt % filler loading and compostability tests were performed in simulated aerobic compost environment at ambient temperature for a period of 50 days. Weight loss study revealed that the incorporation of JNF enhanced the rate of degradation significantly. The unreinforced sample exhibited a steady loss in weight, whereas, the JNF reinforced samples showed three phase degradation. They had a steady weight loss up to 30 days followed by a plateau zone between 30 and 40 days and after that, there was again an increase in weight loss up to 50 days. The biodegraded samples were investigated for their change in molecular weight by Gel Permeation Chromatography (GPC). The change in structure was examined by Differential Scanning Calorimetry (DSC) and morphological change was observed by Scanning Electron Microscopy (SEM). Molecular weight study revealed the fact that Biocopolyester molecules had a significant breakdown in chain length during melt mixing with 10 wt % JNF, which was much less predominant in 2 wt % JNF loaded composites. Such a decrease in chain length and presence of 10 wt % JNF might have facilitated the biodegradation process resulting in highest weight loss. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of alkali treated jute fibres on composite properties

Jute fibres were subjected to a 5% alkali (NaOH) solution treatment for 0, 2, 4, 6 and 8 h at 30°C. An improvement in the crystallinity in the jute fibres increased its modulus by 12%, 68% and 79% after 4, 6 and 8 h of treatment respectively. The tenacity of the fibres improved by 46% after 6 and 8 h treatment and the % breaking strain was reduced by 23% after 8 h treatment. For the 35% composites with 4 h treated fibres, the flexural strength improved from 199.1 MPa to 238.9 MPa by 20%, modulus improved from 11.89 GPa to 14.69 GPa by 23% and laminar shear strength increased from 0.238 MPa to 0.2834 MPa by 19%. On plotting the different values of slopes obtained from the rates of improvement of the flexural strength and modulus, against the NaOH treatment time, two different failure modes were apparent before and after 4 h of treatment. In the first region between 0 and 4 h, fibre pull out was predominant whereas in the second region between 6 and 8 h, transverse fracture occurred with a minimum fibre pull out. This observation was well supported by the SEM investigations of the fracture surfaces.     

Fracture toughness of carbon fiber/polyether ether ketone composites manufactured by autoclave and laser‐assisted automated tape placement

A comparative study is presented on the fracture toughness of carbon fiber/PEEK composites manufactured by autoclave and laser‐assisted automated tape placement (LATP). Formation of a good inter‐laminar bond is always a concern in ATP due to the short time available for intimate contact development and polymer healing, yet our double cantilever beam (DCB) tests reveal 60–80% higher Mode I fracture toughness for the LATP processed specimens than for the autoclave processed specimens. This magnitude of difference was unexpected, so specimens were further examined via differential scanning calorimetry, dynamic mechanical analysis, nano‐indentation, and scanning electron microscopy. The results indicate that the LATP process has been very effective in heating and consolidating the surface of plies, creating an excellent bond. However, it has been less effective in processing the interior of plies, where a low crystallinity and poor fiber–matrix bonding are evident. The higher fracture toughness of the LATP processed specimens is also not solely due to a better bond, but is partially due to significant plastic deformation in the interior of plies during the DCB test. The findings indicate there is still considerable scope for optimizing the laser‐assisted ATP process, before the optimum balance between strength and toughness is achieved at favorable lay‐down speeds. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41643.     

Thermogravimetric and resistivity study of Ex situ and In situ poly(methyl methacrylate)/carboxylic acid group functionalized multiwall carbon nanotubes composites

Polymer composites based on poly(methyl methacrylate) (PMMA)/carboxylic acid group functionalized multiwall carbon nanotubes (MWCNT) were prepared by the ex situ and in situ techniques with 0.05% loading by weight. Composite films were fabricated by solvent casting method. Electrical conductivity of the composites as well as of the neat PMMA polymer was measured in the temperature range 333 K to 423 K. Neat PMMA samples prepared by the same method showed complete insulating behavior. Ex situ technique leads to a lower value of percolation threshold. Infrared spectroscopy was used to analyze the effect of functionalization of MWCNT on the interfacial bonding of PMMA and MWCNT. Thermogravimetric analysis revealed that the maximum degradation temperature has been shifted to higher region for in situ composites compared to PMMA itself—and the ex situ composites indicated better thermal stability. X‐ray diffraction study of composites also indicates that in situ composites functionalization incorporated MWCNT particles in the polymer chain. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Fracture characteristics of vinylester resin under impact fatigue

The impact fatigue behavior of a vinylester resin was studied with a pendulum‐type repeated‐impact tester especially designed and fabricated for the determination of single‐impact and repeated‐impact strengths. A well‐defined energy–endurance impact fatigue curve was obtained with a progressive endurance at values of the impact energy below the critical value, with the endurance limit set at an energy level of 31 N mm, 17.4% of the single‐impact energy. The nature of the crack propagation was investigated for a single impact as well as high, medium, and low impact energy levels with progressively longer endurance. The fracture characteristics varied with the impact energy imparted and the number of cycles endured. The rate of lip growth was high at the higher impact energy levels with a lower number of endurance cycles and low at the lower impact energy levels with longer endurance; the repeated impacts created large and small compressive zones through the bending of specimens with the development of long and short lips, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1995–2001, 2002     

Study of the thermal behavior of alkali‐treated jute fibers

Jute fibers were treated with 5% NaOH solution for 2, 4, 6, and 8 h to study the performance of the fibers as a reinforcing material in the composites. Thermal analysis of the fibers was done by the DTG and DSC technique. The moisture desorption was observed at a lower temperature in the case of all the treated fibers, which might be a result of the increased fineness of the fibers, which provides more surface area for moisture evaporation. The decrease in percentage moisture loss for the fibers treated with alkali for 6 and 8 h could be the result of the increased crystallinity of the fibers. The percentage degradation of the hemicellulose decreased considerably in all the treated fibers, conforming to the fact that the hemicellulose content was lowered on alkali treatment. The decomposition temperature for α‐cellulose was lowered to 348°C from 362.2°C for all the treated fibers, and the residual char formation increased to a significant extent. The enthalpy for the thermal degradation of α‐cellulose showed a decreasing trend for the fibers treated for 2 and 4 h, which could be caused by the initial loosening of the structure, followed by an increase in the enthalpy value in the case of the 6‐ and 8‐h‐alkali‐treated fibers resulting from increased crystallinity, as evident from the X‐ray diffraction. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2594–2599, 2002     

Fracture behavior of vinylester resin matrix composites reinforced with alkali‐treated jute fibers

Jute fibers were treated with 5% NaOH solution for 4 and 8 h, respectively, to study the mechanical and impact fatigue properties of jute‐reinforced vinylester resin matrix composites. Mechanical properties were enhanced in case of fiber composites treated for 4 h, where improved interfacial bonding (as evident from scanning electron microscopy [SEM]) and increased fiber strength properties contributed effectively in load transfer from the matrix to the fiber; but their superior mechanical property was not retained with fatigue, as they showed poor impact fatigue behavior. The fracture surfaces produced under a three‐point bend test and repeated impact loading were examined under SEM to study the nature of failure in the composites. In case of untreated fiber composites, interfacial debonding and extensive fiber pullout were observed, which lowered the mechanical property of the composites but improved their impact fatigue behavior. In composites treated for 4 h under repeated impact loading, interfacial debonding occurred, followed by fiber breakage, producing a sawlike structure at the fracture surface, which lowered the fatigue resistance property of the composites. The composites with fibers treated with alkali for 8 h showed maximum impact fatigue resistance. Here, interfacial debonding was at a minimum, and the fibers, being much stronger and stiffer owing to their increased crystallinity, suffered catastrophic fracture along with some microfibrillar pullout (as evident from the SEM micrographs), absorbing a lot of energy in the process, which increased the fatigue resistance property of the composites. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2588–2593, 2002