Udo Wagenknecht

An investigation of chemical crosslinking effect on properties of high-density polyethylene

High-density polyethylene (HDPE) was chemically crosslinked with various amounts of di-tert butyl cumyl peroxide (BCUP). Crosslink density determined by rubber elasticity theory using hot set test showed an increase with increasing BCUP. Glass transition temperature (Tg), thermal stability, crystallization, melting behavior and tensile properties were studied. The results showed a new finding about decrease in Tg as a consequence of the ‘chemical crosslinking’ of HDPE. This was explained by observed reduction in crystallinity and expected increase in free volume as a result of restriction in chain packing. However, chemical crosslinking had no significant effect on the thermal stability. The stress at break, Youngs modulus yield strength and elongation at break generally decreased with increase in BCUP. By increasing the temperature for slightly crosslinked HDPE, the elongation at break was increased but by increasing the crosslinking level an opposite effect was observed. Crosslinked HDPE showed an decrease in creep strain and an increase in creep modulus with increasing BCUP.

Layered Double Hydroxide Based Polymer Nanocomposites

Nanocomposites based on polymers and inorganic filler materials not only create enormous interest among researchers because of their unique way of preparation and properties, but also promise development of new hybrid materials for specific applications in the field of polymer composites. The present article deals with the application of a relatively new class of inorganic materials, namely layered double hydroxides (LDHs), as nanofiller for synthesizing polymer-based nanocomposites. LDHs are mixed metal hydroxides of di- and trivalent metal ions crystallized in the form similar to mineral brucite or magnesium hydroxide (MH) with the incorporation of interlayer anionic species. Several procedures for the synthesis of LDHs, their organic modification, and the synthesis of polymer/LDH nanocomposites are discussed in detail with reference to work done in very recent years. The potential of LDHs, especially magnesium and aluminum-based LDHs (Mg–Al LDH) as nanofillers for the polymer matrix has been investigated. The important aspects in characterizing such hybrid materials (i.e., morphological analysis and melt rheological behavior) have been reported in detail to understand the nature of LDH particle dispersion and its influence on the melt flow behavior of the nanocomposites. The specialty of LDHs as nanofiller is their thermal decomposition behavior, which makes them potential flame retardants for polymers. This aspect has been reported in detail in the case of polyethylene-based systems, where the flame retarding efficiency of organically modified Mg–Al LDH alone and also in combination with conventional flame retardants has been discussed.

Synthesis of Organo Cobalt−Aluminum Layered Double Hydroxide via a Novel Single-Step Self-Assembling Method and Its Use as Flame Retardant Nanofiller in PP

Synthesis of polypropylene/organo-layered double hydroxide (PP/OLDH) has been carried out based on self-assembled organocobalt-aluminum LDH (O-CoAl-LDH). The novel method of synthesizing self-assembled CoAl-LDH and its characterization have also been reported in details. This method is proven to be very efficient way of producing OLDH in a single step with homogeneous composition and structure. As flame-retardant nanofiller, O-CoAl-LDH shows significant decrease in heat release rate (HRR), the total heat release (THR) and the heat release capacity (HRC) of the PP composites, though the thermal stability of the compounds decreases slightly compared to the base polymer. Morphological analyses show that the LDH particles are dispersed in PP matrix in a partially exfoliated form. The activation energy calculation based on the Kissinger method reveals that O-CoAl-LDH has a positive effect on the activation energy of thermal decomposition of PP. However, in the presence of this filler, decomposition of the composites starts at an earlier stage than that of pure PP.

Structural characteristics and flammability of fire retarding EPDM/layered double hydroxide (LDH) nanocomposites

A high performance elastomeric flame retardant nanocomposite was prepared which was based on maleic anhydride grafted ethylene-propylene-diene terpolymer (mEPDM), a one-step synthesised organo-layered double hydroxide (LDH), and an intumescent flame retardant (FR) comprised of pentaerythritol (PER), ammonium polyphosphate (APP) and methyl cyanoacetate (MCA). The morphology, fire behavior and mechanical properties of the flame-retarded mEPDM/LDH nanocomposite have been studied in detail. Wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS) and TEM observation confirmed an exfoliated structure of LDH in a particular composite containing 2 phr (parts per hundred) LDH and 38 phr FR. As an effective flame retardant synergistic agent, MgAl–LDH shows a significant decrease in the heat release rate (HRR), low mass loss (ML) and low fire growth rate (FIGRA) of the nanocomposite. The flame retardant mechanism has been proposed, which is mainly due to the condensed phase flame retardant mechanism to form reinforced char layers during combustion, leading to the low volatiles produced. Moreover, as far as the mechanical properties of the vulcanizates are concerned, in all cases of flame retardant mEPDM and flame retarded mEPDM/LDH nanocomposites, they exhibit superior values compared to the gum compound.

Thermal and shrinkage behaviour of stretched peroxide-crosslinked high-density polyethylene

Abstract The thermal shrinkage of stretched crosslinked high-density polyethylene (HDPE) was investigated with the aim to produce heat shrinkable materials. The heat shrinkable property was achieved by a process of heating–stretching–cooling by aid of tensile machine on crosslinked HDPE obtained by compounding with various amount of peroxide. Effect of stretching ratio and stretching temperature on thermal and shrinkage behaviour at varying peroxide contents was investigated. The results showed that crosslinking hindered the crystallization process by decreasing the melting and crystallization temperatures as well as the total degree of crystallinity. The stretching ratio had no significant effect on shrink temperature but rather on ultimate shrinkage. The stretching temperature had relatively significant influence on the shrink temperature. Crosslinked HDPE stretched at above melting point (140 °C) had higher shrink temperature as compared to those stretched at lower temperature (90 °C). These effects could be reasonably explained by Hoffman theory and changes in crystallites size and total amount of crystallinity.

A new approach to reducing the flammability of layered double hydroxide (LDH)-based polymer composites: preparation and characterization of dye structure-intercalated LDH and its effect on the flammability of polypropylene-grafted maleic anhydride/d-LDH composites

Dye structure-intercalated layered double hydroxide (d-LDH) was synthesized using a one-step method, and its intercalated behaviors have been characterized by Fourier transform infrared spectroscopy (FTIR), wide angle X-ray scattering (WAXS), scanning electron microscopy, thermogravimetric analysis (TGA), etc. As a novel functional potential fire-retarding nanofiller, it was used to prepare a polypropylene-grafted maleic anhydride (PP-g-MA)/d-LDH composite by refluxing the mixture of d-LDH and PP-g-MA in xylene, aiming to investigate its effect on the flammability of the PP-g-MA composite. The morphological properties, thermal stability, and flame retardant properties of the PP-g-MA/d-LDH composite were determined by FTIR, WAXS, transmission electron microscopy, TGA, and microscale combustion calorimetry. Compared with NO3-LDH (unmodified LDH) and LDH intercalated by sodium dodecylbenzenesulfonate (conventional organo-modified LDH), d-LDH can significantly decrease the heat release rate and the total heat release of the PP-g-MA composite, offering a new approach to imparting low flammability to LDH-based polymer composites.

Preparation of zinc oxide free, transparent rubber nanocomposites using a layered double hydroxide filler

A layered double hydroxide (LDH) mineral filler particle has been designed and employed in rubber vulcanization to prepare a more environmentally friendly rubber composite. The LDH delivers zinc ions in the vulcanization process as accelerators, stearate anions as activators and simultaneously the mineral sheets act as a nanofiller to reinforce the rubber matrix whilst totally replacing the separate zinc oxide (ZnO) and stearic acid conventionally used in the formulation of rubber. This method leads to a significant reduction (nearly 10 times) of the zinc level and yields excellent transparent properties in the final rubber product. The morphological characterization, rheometric curing behaviour, mechanical properties and uniaxial multi-hysteresis behaviours of the resultant rubber/LDH nanocomposite are studied in this paper.

Synthesis, characterization and properties of novel aliphatic–aromatic polyamide/functional carbon nanotube nanocomposites via in situ polymerization

An effective approach relying on in situ direct polycondensation to synthesize polyamide grafted with functional carbon nanotube (PA–FCNT) has been reported and the properties of PA nanocomposites were studied in this paper. The oxidized carbon nanotube (CNT–COOH), which was modified by the treatment of CNTs with mixed concentrated sulfuric acid and nitric acid, were converted into amino-functionalized CNTs (FCNT) by the reaction between CNT–COOH and an excess amount of 1,2-diaminoethan. Then, the aromatic–aliphatic polyamide/FCNT nanocomposites (PA–FCNT) were synthesized by using in situ polycondensation. To prove the two types of grafted CNT (FCNT–sPA with the properties closer to CNT, and FCNT–lPA with the properties closer to the polymer), PA–FCNT had been separated simultaneously after in situ preparation of PA–FCNT nanocomposite. The grafting was confirmed by investigation of FCNT–sPA and FCNT–lPA structures by Fourier transform infrared spectra, thermogravimetric analysis (TGA), nuclear magnetic resonance and transmission electron microscopy and presence of FCNT and PA in both compounds were observed. The effect of FCNT and pristine CNT on the morphology, thermal stability and flammability of PA were studied by TEM, TGA and microscale combustion calorimeter (MCC). TGA analysis illustrated that the grafted FCNT significantly increased the thermal stability and slightly improved char residues of PA. MCC tests showed the incorporation of FCNT in the polyamide resulted in the significant reduction of the heat release rate and total heat release, indicating that the flammability of PA had been reduced greatly.

Method for Simultaneously Improving the Thermal Stability and Mechanical Properties of Poly(lactic acid): Effect of High-Energy Electrons on the Morphological, Mechanical, and Thermal Properties of PLA/MMT Nanocomposites

Nanocomposites derived from poly(lactic acid) (PLA) and organically modified montmorillonite (oMMT) have been cross-linked by high-energy electrons in the presence of triallyl cyanurate (TAC). The morphology of untreated and cross-linked PLA/MMT nanocomposites was characterized by wide-angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). This treatment can improve both the thermal stability and the glass-transition temperatures of the PLA nanocomposites (e.g., PLA-MMT-TAC 30kGy, 50kGy, and 70kGy) because of the formation of cross-linking structures in the nanocomposites that will considerably reduce the mobility of polymers. Interestingly, at relatively low irradiation doses (e.g., 30 and 50 kGy) a good balance between tensile strength and elongation at break for the PLA nanocomposites could be achieved. These mechanical properties are superior to those of pure PLA. Therefore, combining nanotechnology and electron beam cross-linking is a promising new method of simultaneously improving the mechanical properties (toughness and tensile strength) and thermal stability of PLA.

Thermoplastic Composites Based on Renewable Natural Resources: Unplasticized PVC/Olive Husk

The current study explores the potential of eco-friendly biomaterials, namely olive husk (OH) as a reinforcing filler for PVC composite. Thus, composite-based unplasticized poly(vinyl chloride) (u PVC) and olive husk were mixed by a Brabender two-roll mill at 180°C and 25 rpm. The olive husk concentration was progressively varied from 0–20 phr. The fabricated samples were inspected with respect to their tensile properties, impact strength, thermal stability, and density and water uptake. It has been found that stress at peak increased with filler loading up to certain loading. This scenario was related to hydrogen bond formation due to polar-polar interactions. Evidence of the hydrogen bond formation between the polymer matrix and the olive husk was examined with the aid of attenuated reflectance infrared spectra (ATR-IR). Such interactions were cited to justify the improved performance of the composites. Fracture mode and filler dispersion of the composites were compared to the unfilled counterpart by scanning electron microscopy (SEM). The influence of olive husk on the thermal stability of the PVC composites was studied by differential scanning calorimeter (DSC). It has been found that the enthalpy of fusion was improved with OH loading. The observed trend was correlated with the phenolic hydroxyl group of the lignin component used as an antioxidant.